Doctorat De L'université De Toulouse

Lina Maria Romero Millan

Professeur Ange Nzihou

Professeur Laurence Lecoq

Claire Courson

Professeur Marco Castaldi

Maria Fernanda

Gomez Galindo

Carlos Alberto

Guerrero Fajardo

THÈSE THÈSE en vue de l'obtention du

Keywords: Steam gasiĄcation, GasiĄcation reactivity, Inorganic composition, Lignocellulosic agrowaste, Syngas, Biochar GazéiĄcation sous vapeur d'eau, Réactivité, Composition inorganique, Biomasse lignocellulosique, Syngas, Biochar

Firstly, I would like to

Spanish abstract / Resumen

En el contexto económico de una gran parte de países en vía de desarrollo, la gasiĄcación con vapor de agua de residuos agrícolas y agroindustriales puede ser considerada como un proceso de especial interés para la producción simultanea de energía en regiones aisladas, y productos de valor agregado. Teniendo en cuenta que la disponibilidad de los residuos agrícolas no es constante y depende de la estacionalidad de los cultivos, diferentes tipos de biomasa deben ser utilizados para asegurar el funcionamiento de las instalaciones de gasiĄcación. En este sentido, el presente trabajo está enfocado en la comprensión del impacto de las características de la biomasa en el proceso de gasiĄcación, y en las propiedades de los subproductos sólidos y gaseosos. Tres residuos agrícolas lignocelulósicos con diferente composición macromolecular e inorgánica fueron seleccionados para este estudio: cáscara de coco (CS), bambú guadua (BG) y cuesco de palma (OPS).

La cinética de la descomposición térmica de los materiales seleccionados fue estudiada a escala termogravimétrica en atmosfera inerte y de vapor de agua. A pesar de las diferencias en la estructura macromolecular de las muestras, la composición inorgánica es el parámetro con mayor inĆuencia en la reactividad y la cinética de gasiĄcación. El impacto positivo de los metales alcalinos y alcalino-térreos fue conĄrmado, al igual que el efecto inhibidor del Si y P. En particular, la relación K/(Si+P) es considerada apropiada para describir el comportamiento de los materiales durante el proceso de gasiĄcación con vapor de agua. En consecuencia, un nuevo modelo cinético de gasiĄcación a partir de la composición inorgánica de la biomasa es propuesto.

Igualmente, la validez de la relación K/(Si+P) para clasiĄcar y predecir el comportamiento de las biomasas fue conĄrmada a mayor escala, en un reactor de lecho Ćuidizado a escala de laboratorio. Las muestras con relación K/(Si+P) superior a 1 mostraron una reactividad a la gasiĄcación superior a la de las muestras con relación inferior a 1, y en consecuencia, una mayor producción de gas y un rendimiento energético más elevado. Asimismo, se observó que la composición inorgánica tiene a su vez una importante inĆuencia en las propiedades del sub-producto sólido. En particular, una xi mayor reactividad a la gasiĄcación está relacionada con una mayor área superĄcial y una mayor cantidad de grupos funcionales en los carbonizados.

El modelo de gasiĄcación y los resultados experimentales presentados pueden ser considerados como una importante referencia para aplicaciones reales de gasiĄcación que trabajen con diferentes tipos de residuos. Asimismo, en el contexto de los países en vías de desarrollo, la gasiĄcación con vapor de agua de biomasas lignocelulósicas se presenta como una alternativa para mejorar el acceso a la energía de zonas rurales aisladas, promoviendo a la vez el desarrollo de actividades productivas que generen nuevos ingresos para las comunidades locales.

Palabras claves: GasiĄcación con vapor de agua, Reactividad, Composición inorgánica, Biomasa lignocelulósica, Syngas, Biochar.

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French abstract / Résumé

Dans le contexte économique de la plupart des pays en voie de développement, la gazéiĄcation sous vapeur d'eau de résidus agricoles lignocellulosiques pourrait être un procédé intéressant, à la fois pour la génération d'énergie dans des régions isolées et pour la production des produits à valeur ajoutée. Étant donné que la disponibilité des résidus agricoles est souvent saisonnière, diférents types de biomasse doivent être utilisés pour assurer le fonctionnement des installations de gazéiĄcation. A cet égard, ce travail est axé sur la compréhension de l'impact des caractéristiques de la biomasse sur le procédé de gazéiĄcation et les propriétés des sous-produits gazeux et solides. Trois biomasses lignocellulosiques à composition macromoléculaire et inorganique diférentes ont été sélectionnées pour cette étude : coques de noix de coco (CS), bambou guadua (BG) et coques de palmier à huile (OPS).

La cinétique de décomposition thermique des biomasses a été étudiée une échelle thermogravimétrique sous atmosphère inerte et sous vapeur d'eau. Malgré les diférences dans la structure macromoléculaire des échantillons, la composition inorganique s'est avérée être le paramètre le plus important inĆuençant la réactivité et la cinétique de gazéiĄcation. L'impact bénéĄque des métaux alcalins et alcalino-terreux a été conĄrmé, ainsi que l'efet inhibiteur du Si et du P. Plus précisément, le ratio K/(Si + P) est considéré approprié pour décrire et comparer le comportement des biomasses pendant la gazéiĄcation sous vapeur d'eau. En conséquence, une nouvelle approche pour la modélisation de la cinétique de gazéiĄcation à partir de la composition inorganique de l'échantillon a été proposée.

La validité du ratio K/(Si + P) pour classiĄer et prédire le comportement des biomasses a également été conĄrmée par des expériences dans un réacteur à lit Ćuidisé à l'échelle laboratoire. Les échantillons avec un ratio K/(Si + P) au-dessus de 1 ont montré des réactivités de gazéiĄcation supérieures à celles des échantillons dont le ratio était inférieur à 1, et donc, une production de gaz et un rendement énergétique plus élevés. De plus, la composition inorganique a non seulement impacté le taux de gazéiĄcation des échantillons, mais également les propriétés du sous-produit solide. En particulier, une réactivité de gazéiĄcation plus élevée est liée à des chars avec une surface spéciĄque xiii

Introduction

Agricultural and agroindustrial wastes represent one of the most abundant and inexpensive resources on earth, considering that billion tons of these residues are generated every year all around the world. However, most of them are nor valorized and often remain underexploited. In several locations, agrowastes could even represent an environmental risk, as they are not disposed properly, causing problems such as air and soil pollution. In this regard, considering their low cost and abundance, agroresidues may constitute an interesting and renewable source for the production of biofuels and value-added products.

In the particular case of developing countries, the valorization of agricultural and agroindustrial residues may also represent an opportunity to improve energy security and provide business opportunities for several communities, generally situated in rural areas. Nevertheless, it has been observed that the sustainability of energy supply projects in these areas usually depends on the development of associated productive activities that could improve the income levels perceived by the communities.

In this context, the selection of the appropriate valorization pathways of agrowastes could provide rural communities in developing countries the opportunity to improve their access to electricity, and simultaneously generate additional incomes.

In relation to lignocellulosic agrowastes, pyro-gasiĄcation can be considered as a very interesting process, taking into account that it converts a solid residue in fuel gases that can be used for the production of heat or power, and a solid by-product that could also be valorized in several applications. In particular, steam as gasifying agent produces fuel gases with a considerably high heating value, and a porous carbon material that could have similar properties to activated carbons. These characteristics suggest that steam gasiĄcation could be an interesting process for the simultaneous valorization of gasiĄcation gaseous and solid by-products, as presented in Ągure 6.

xv In this regard, one of the most important challenges is probably the fact that the availability of agrowastes is not constant over the year and often depends on seasonal crops. Consequently, gasiĄcation facilities should work using diferent kind of residues with several characteristics, to ensure a constant operation. Depending on their origin and nature, the composition and structure of lignocellulosic residues varies and should be taken into account for their transformation process and subsequent valorization possibilities. In this context, the main objective of this work is to analyze the steam gasiĄcation behavior of diferent lignocellulosic agrowastes, and understand the impact of their characteristics on the process by-products properties and valorization pathways.

Although the present research project was motivated by the social and energy context of Colombia and other tropical developing countries, the results of this work may be also valid in developed countries with a large production of agrowastes, where the implementation of appropriate strategies for their treatment are also required.

This PhD thesis was developed in the framework of an international cooperation (cotutelle agreement) between the university "Universidad Nacional de Colombia" in Colombia, and the "IMT Mines Albi-Carmaux" institute in France. The research project in Colombia is entitled "Use of gasiĄcation by-products from lignocellulosic biomasses for agroindustrial applications". In France, the project is entitled "Steam gasiĄcation of tropical lignocellulosic agrowaste: Impact of biomass characteristics on the gaseous and solid by-products".

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The present manuscript has been structured in 7 chapters, in the form of a scientiĄc paper, already published, submitted, or in intent of submission. A brief description of the structure, objectives, and main results of each chapter is presented below.

Chapter 1. Literature review

The objective of this Ąrst chapter is to provide the elements required to understand the choices and research directions developed in this work. In this respect, the basic concepts related to the gasiĄcation of lignocellulosic biomasses are described, as well as the progress of the current research on he valorization pathways of gasiĄcation byproducts. Particular attention is given to the process solid residue and its properties.

At Ąrst, Section 1.2 describes the main characteristics of lignocellulosic biomass as a suitable raw material for thermochemical transformation processes. Special attention is given to agricultural and agroindustrial residues. Then, the diferent stages involved in the pyro-gasiĄcation process are detailed, describing the associated chemical reactions and their main by-products. Moreover, the impact of operating parameters like temperature, reaction agent, and biomass composition, on the transformation process is discussed. Finally, the most commonly used technologies for pyro-gasiĄcation are detailed and compared.

For its part, section 1.3 is dedicated to the description of the pyro-gasiĄcation solid by-product, also called char. At Ąrst, the inĆuence of the process parameters on the char yield and physico-chemical properties is discussed. Then, an overview of the main char characteristics and their interest for subsequent applications and valorization pathways is presented. Finally, attention is given to the recent research developments in relation to the valorization of chars in several applications. In particular, their use in environmental and agricultural applications is of particular interest in the context of both developed and developing countries.

Chapter 2. Materials and experimental methods

This chapter summarizes the materials and experimental methods used in this study.

Considering that the main objective of this work is to understand the impact of biomass characteristics on the steam gasiĄcation process and by-products, section 2.2 presents the selection criteria of the lignocellulosic residues to be analyzed and their main characteristics. Coconut shells (CS), oil palm shells (OPS) and bamboo guadua (BG), a bamboo species native from Central and South America were selected for this study, considering their inorganic and macromolecular composition.

For its part, section 2.3 describes the characteristics of two experimental devices initially considered for the development of gasiĄcation tests. From the analysis of the advantages and disadvantages of each technology and device, the most appropriate setup to achieve the objectives of the present work was selected. Thus, the steam gasiĄcation tests of the analyzed biomasses were carried out in a Ćuidized bed reactor at a laboratory scale. The selected device allows the accurate control of the process parameters, in order to make a precise comparison between the feedstocks and their gasiĄcation by-products.

To better analyze the diferences between the selected lignocellulosic residues, a kinetic study of the pyro-gasiĄcation process was carried out at a thermogravimetric scale. In this respect, section 2.4 is dedicated to the description of the experimental methods used xvii to analyze the thermal decomposition behavior of biomasses, under inert atmosphere, and under steam as a gasifying agent.

Finally, Section 2.5 presents the experimental procedure and the operating conditions used for the steam gasiĄcation experiments of the selected feedstocks, and the sampling of their gasiĄcation by-products. The experimental methodology and the techniques used to evaluate the physico-chemical properties of the raw biomass and the produced char are also summarized.

Chapter 3. Kinetic analysis of tropical lignocellulosic agrowaste pyrolysis

In this chapter, the thermal decomposition behavior of coconut shells (CS), oil palm shells (OPS), and bamboo guadua (BG), the three selected biomasses for this work, was analyzed under inert atmosphere at thermogravimetric scale.

A non-isothermal approach using a combined kinetics parallel reaction model was proposed, assuming that the decomposition of the three main macromolecular components of the samples takes place independently. In this respect, the mathematical deconvolution of the derivative thermogravimetric curves (DTG) using Fraser-Suzuki functions was performed. This mathematical treatment allowed the description of the biomass pyrolysis as the sum of the decomposition of its three macromolecular constituents: hemicellulose, cellulose and lignin.

Considering that the calculated apparent activation energy of each biomass component evidenced no dependence on the reaction extent in all the conversion range, master plots approach was used to identify their decomposition mechanism. In this regard, modelfree isoconversional methods proved to be suitable to determine the pyrolysis kinetic parameters of lignocellulosic agrowastes with diferent macromolecular compositions, and H/C and O/C ratios near 1.5 and 0.8 respectively.

In particular, this study has shown that even when the elemental composition of the samples is very similar, their macromolecular constituents can have an important impact on their decomposition rate, and especially on the apparent activation energy associated with the pyrolysis process. In this respect, considering the role of lignin for the stability of the lignocellulosic structure, it has been observed that the activation energy of the samples is in the order BG <CS <OPS , consistent with their lignin content (BG <CS <OPS).

The validation of the kinetic approach proposed in this work showed a very good agreement with the experimental results, with a Ątting error below 10%. Therefore, the kinetic model presented can be considered as a valuable tool for reactor design, and for the development and scale-up of pyrolysis and gasiĄcation processes using tropical lignocellulosic agrowastes as a feedstock.

Chapter 4. Steam gasiĄcation of tropical agrowastes: a new modeling approach based on their inorganic content

In this chapter, the steam gasiĄcation and co-gasiĄcation reactivity and global kinetics of the three selected biomasses was analyzed at a thermogravimetric scale, with temperatures ranging from 750°C to 900°C, and steam partial pressures from 3 to 10 kPa. An isothermal experimental approach was used for this analysis, considering that the beginning of the steam gasiĄcation reactions can be controlled by switching the atmosphere from inert to reactive after the isothermal regime is established.

xviii Moreover, model-free isoconversional methods and the generalized master-plots approach were used to determine the kinetic parameters and the most suitable reaction model that better describe the steam gasiĄcation process of the samples. In this regard, in order to better understand and compare the diferences between the analyzed biomasses, their gasiĄcation behavior was described using a th order reaction model with respect to the reacting solid.

It has been found that despite the diferences in the macromolecular composition of the samples, inorganics is the most important parameter inĆuencing the biomass gasiĄcation reactivity and kinetics. In particular, the experimental results conĄrmed the beneĄcial impact of alkali and alkaline earth metals, principally K, on the gasiĄcation reactivity, as well as the inhibitory efect of Si, Al, and P. In accordance, a linear relationship was found between the reactivity of the samples and their inorganic ratio K/(Si+P), for all the studied experimental conditions.

Furthermore, from the kinetic analysis, it was possible to notice that the identiĄed reaction model that describes the steam gasiĄcation process is also related to the inorganic composition of the samples. More precisely, a linear and inverse relationship between the identiĄed order of reaction and the inorganic ratio K/(Si+P) of the biomass was found.

Accordingly, a new steam gasiĄcation kinetic model that considers the inorganic composition of the feedstocks was proposed. For all the analyzed samples and gasiĄcation conditions, a good agreement was found between the model and the experimental results, with Ątting errors below 10%. This value was considered reasonable, taking into account the heterogeneity of the biomasses and the incertitude in the measurement of their inorganic composition.

As a result, the proposed kinetic model could constitute a valuable tool for reactor design, and for the development and scale-up of steam gasiĄcation facilities using lignocellulosic residues. Moreover, considering that the availability of agrowastes is not constant, this approach could be also useful in applications where diferent kind of residues should be gasiĄed at the same time.

Chapter 5. Impact of feedstock characteristics on the steam gasiĄcation of lignocellulosic agrowastes

The analysis of the steam gasiĄcation process of lignocellulosic agrowastes was also carried out in a Ćuidized bed reactor at a laboratory scale. In this chapter, various operating conditions were evaluated in order to understand their impact on the gasiĄcation process, product yield, and particularly, on syngas production and quality, in terms of composition and caloriĄc value.

At Ąrst, the impact of the steam quantity on the gasiĄcation products distribution was analyzed in order to determine the most suitable conditions for the production of combustible gases for energy applications. In particular, experimental results showed that steam fractions between 15% and 30% in the gasifying agent promote the production of fuel gases. In contrast, higher values were associated with an increase in the tar production, resulting in a negative impact on the process eiciency.

In consequence, the steam gasiĄcation analysis and comparison of the selected samples were performed using 30% of steam in the gasifying agent. For the three studied xix biomasses, the experimental results showed that in the analyzed temperature range, a rise in the process temperature is associated with a higher gasiĄcation reactivity, an increase in the amount of gas produced, and therefore, a higher eiciency of the process.

Even though the process parameters showed similar efects on the gasiĄcation behavior of the three samples, remarkable diferences were identiĄed between them, mainly related to their mineral composition. In accordance to the results of the kinetic analysis of the process, presented in chapter 4, the beneĄcial impact of alkali and alkaline earth metals, as well as the inhibitory efect of Si, Al, and P on the steam gasiĄcation process was conĄrmed. In particular, the validity of the inorganic ratio K/(Si+P) to classify and predict the steam gasiĄcation behavior of lignocellulosic agrowaste was veriĄed.

Under the same experimental conditions, gasiĄcation of samples with K/(Si+P) above 1 resulted in higher gas yields and gas eiciencies compared to samples with K/(Si+P) below 1.

In contrast, no remarkable diferences were observed between the analyzed samples in terms of gas composition and heating value. In this regard, with a high heating value between 10 and 12 MJ/m 3 , and a H 2 /CO ratio between 2.5 and 4, the steam gasiĄcation gas produced from lignocellulosic feedstocks with H/C and O/C ratio near 1.5 and 0.8 respectively, could be considered suitable for energy applications using boilers, gas turbines, or internal combustion engines.

In this regard, the experimental observations presented in this chapter could be an important reference for real gasiĄcation applications working with diferent kind of lignocellulosic residues. In particular, the presented results provide useful information to adapt the process parameters and conditions to the available feedstocks and the application requirements.

Chapter 6. Physico-chemical characterization of steam gasiĄcation chars from tropical lignocellulosic agrowastes

This chapter is dedicated to the analysis of the physico-chemical properties of steam gasiĄcation chars or biochars produced from the three selected biomasses. After each gasiĄcation test described in chapter 5, the resulting chars were recovered and characterized, in order to understand the impact of the process parameters and raw biomass characteristics on their properties.

The biochars chemical composition, speciĄc surface area, structure, and surface chemistry were determined and compared. In accordance with the results presented in chapters 4 and 5, the gasiĄcation chars characterization proved that the inorganic composition of the raw samples impacts also the physico-chemical properties of the gasiĄcation solid by-product. In particular, the experimental results showed that inorganics have an important inĆuence on the gasiĄcation reactivity of the samples, and in consequence, an impact on the porous structure development and surface chemistry of the gasiĄcation chars.

The comparison between chars produced under the same experimental conditions showed that for samples with inorganic ratio K/(Si+P) above 1, the beneĄcial impact of AAEM (specially K) on the steam gasiĄcation reactions resulted also in higher surface area development and oxygen-containing functional groups in the char surface, in comparison to samples with inorganic ratio below 1. In particular, as a result of xx the steam gasiĄcation reaction mechanism, a relationship between the char speciĄc surface area and the total quantity of oxygen-containing functional groups was found. Furthermore, the pore size distribution of gasiĄcation chars seems to be related to the nature and the macromolecular composition of the raw feedstocks.

In general, the analyzed gasiĄcation chars are highly microporous carbonaceous materials, with speciĄc surface areas between 500 and 1000 m 2 /g. A dominance of acidic oxygenated functional groups in the char surface, with pH PZC values below 3 was observed in all cases. In this regard, according to the observed characteristics, similar of those of several activated carbons, steam gasiĄcation chars could be considered as an interesting by-product that may be valorized in applications where the adsorption or retention of cations is required. Moreover, the signiĄcant inorganic content of chars may also suggest their valorization in soil amendment and remediation applications.

The experimental observations presented in this work could be an important reference for real gasiĄcation applications working with diferent kind of residues, when the simultaneous valorization of the gaseous and solid by-products is intended. According to the nature and composition of the samples, the gasiĄcation parameters and conditions could be adapted in order to obtain the by-products proportion required by each application, with the char needed characteristics.

Chapter 7. Characterization of agrowaste steam gasiĄcation chars for soil amendment applications

Considering the characterization results presented in chapter 6 for the steam gasiĄcation solid by-product, this chapter is dedicated to the analysis of the char properties, in order to determine their suitability to be used in soil amendment and remediation applications.

In this regard, steam gasiĄcation chars from coconut shells, oil palm shells, and bamboo guadua, produced at 850°C with a steam fraction of 30% in the gasifying agent were selected for this study. These conditions proved to be adequate for the simultaneous production of syngas with a relatively high process eiciency, and a porous carbon material with desirable properties for several applications.

The experimental results conĄrmed that the three analyzed chars showed an interesting potential to be used in soil amendment applications. In particular, their high pH values between 9 and 11, and their signiĄcant acid neutralization capacity above 62 cmol H + /kg, suggest the char capacity for the improvement of acidic soil characteristics. Moreover, their cation exchange capacity values between 11 and 45 cmol/kg may enhance the nutrient retention and fertility of several kinds of soils.

In general, it has been observed that the inorganic composition of the samples impacts in a very important way the char properties with regard to soil applications. Particularly, BG chars, with an ash content above 30%, exhibited the highest cation exchange and acid neutralization capacities, in comparison to chars with lower ash content (coconut shells and oil palm shells).

Moreover, the mineral release analysis of the three gasiĄcation chars at diferent pH values revealed that these materials could be a non negligible source of macro and micro-nutrients, that may also impact the soil fertility and crop yield. More speciĄcally, xxi thanks to their high mineral content, coconut shells and bamboo guadua chars may supply up to 100% and 20% of the K and P needs of some low-input crops.

Although the behavior of chars in real applications may difer from the one observed during the laboratory characterization, the results presented in this chapter give an useful insight regarding a promising valorization pathway for steam gasiĄcation chars from diferent lignocellulosic agrowastes, adapted to both developed and developing countries.

General conclusion

In the current world energy, environmental, and economic context, agricultural and agroindustrial wastes represent an interesting, low-cost, and renewable resource for the production of biofuels and value-added products in both developing and developed countries.

In particular, considering the great variety of lignocellulosic agrowaste sources, the understanding of the biomass organic and inorganic composition impact on the pyro-gasiĄcation behavior is of great importance to adapt the process parameters to the available feedstocks and application requirements. Moreover, the understanding of the inĆuence of biomass characteristics on the pyro-gasiĄcation by-products is also essential to determine their most suitable valorization pathways.

In general, the results presented in this work have contributed to a better understanding of the steam gasiĄcation process of lignocellulosic agrowastes, and highlighted the important role of inorganics in their gasiĄcation behavior and by-products properties. In accordance, the design and operation of agrowaste gasiĄcation facilities should take into account the organic and inorganic composition of the feedstocks, in order to properly adapt the process parameters to ensure the production of the required by-products yield and properties.

Moreover, the present research showed that steam gasiĄcation could be considered as a suitable technique for the simultaneous valorization of lignocellulosic residues in energy applications, and the production of value-added products, that could represent an opportunity to implement productive activities in the context of both developing and developed countries.

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Introduction

Avec des milliards des tonnes produites chaque année, les déchets agricoles et agroindustriels peuvent être considérés comme l'une des ressources le plus abondantes au monde. Cependant, la plupart de ces résidus ne sont pas utilisés et restent souvent sous-exploités. Dans certains endroits, la mauvaise disposition des déchets agricoles et agroindustriels peut même entrainer la contamination des sols par le ruissellement de lixiviats, et de l'air, par la décomposition des déchets, représentant un risque environnemental.

À cet égard, en raison de leur faible coût et de leur abondance, les résidus agricoles peuvent représenter une source intéressante et renouvelable pour la production de biocarburants et de produits à valeur ajoutée.

Dans le cas particulier des pays en voie de développement, la valorisation énergétique de ces déchets pourrait contribuer à améliorer la sécurité de l'approvisionnement d'énergie dans plusieurs communautés, généralement situés dans des zones rurales. Cependant, il a été constaté que la mise en place de projets productifs associés à l'approvisionnement d'énergie, visant la génération de revenus supplémentaires pour les communautés, est nécessaire pour assurer la pérennité des projets.

Dans ce contexte, le choix d'un procédé de conversion énergétique des déchets adapté, peut ofrir aux communautés rurales des pays en voie de développement la possibilité d'améliorer leur accès à l'énergie, et à la fois, de générer des revenus alternatifs.

En ce qui concerne les résidus lignocellulosiques, la pyro-gazéiĄcation est un procédé particulièrement intéressant, car il permet de produire à la fois un gaz combustible qui peut être utilisé pour la génération d'électricité ou de chaleur, et un sous-produit solide qui peut également être valorisé dans des diverses applications. En particulier, l'utilisation de vapeur d'eau comme atmosphère de gazéiĄcation produit un gaz avec un haut pouvoir caloriĄque, et un solide poreux avec des propriétés similaires à celles des charbons actifs. Ceci suggère que la gazéiĄcation sous vapeur d'eau pourrait être un xxiii procédé d'intérêt pour la valorisation simultanée des sous-produits gazeux et solides, comme présenté dans la Ągure 2. Néanmoins, la valorisation des résidus lignocellulosiques agricoles pour des applications énergétiques présente plusieurs déĄs, car il est nécessaire de tenir compte de diverses conditions, telles que les cultures et leur rotation, les pratiques agricoles, le climat, le volume des résidus, et les besoins énergétiques des communautés locales.

À cet égard, l'un des déĄs les plus importants est probablement le fait que la disponibilité des biomasses n'est pas constante tout au long de l'année et dépend souvent des cultures saisonnières. Par conséquent, aĄn d'assurer leur fonctionnement continu, les installations de gazéiĄcation doivent travailler avec divers types de résidus, présentant diférentes caractéristiques. Selon leur origine et leur nature, la composition et la structure des biomasses varient et peuvent impacter leur comportement au cours de la gazéiĄcation, ainsi que les possibles voies de valorisation de leurs sous-produits.

Dans ce contexte, le principal objectif de la thèse est donc d'étudier la gazéiĄcation sous vapeur d'eau de diférents résidus agricoles lignocellulosiques, et de comprendre l'impact de leurs caractéristiques sur le procédé et les propriétés des sous-produits, ainsi que sur leurs possibles voies de valorisation.

Bien que ce projet de recherche a été motivé par le contexte social et énergétique de la Colombie et d'autres pays tropicales en voie de développement, les résultats de ce travail sont également valides dans le cadre des pays développés avec une production importante de résidus agricoles et agroindustriels, qui ont également besoin de mettre en place des stratégies adaptées pour leur traitement.

Ce travail de thèse a été développé dans le cadre d'une cotutelle internationale entre l'université "Universidad Nacional de Colombia" en Colombie, et l'institut "IMT Mines Albi-Carmaux" en France. Le projet de recherche en Colombie est intitulé "Use of xxiv gasiĄcation by-products from lignocellulosic biomasses for agroindustrial applications". En France, le projet est intitulé "GazéiĄcation sous vapeur d'eau de résidus agricoles: Impact des caractéristiques de la biomasse sur les propriétés des sous-produits gazeux et solides".

La structure générale du manuscrit, composé de sept chapitres est présentée dans la Ągure 3. Les chapitres 3 au 7 ont été rédigés sous la forme d'articles scientiĄques, déjà publiés ou en vue de publication dans des journaux scientiĄques internationaux.

Chapitre 2. Matériels et méthodes expérimentales

Ce chapitre récapitule les matériaux et les méthodes expérimentales utilisés au cours de cette étude.

Étant donné que l'objectif principal de ce travail est de comprendre l'impact des caractéristiques de la biomasse sur le procédé de gazéiĄcation sous vapeur d'eau et les propriétés de ses sous-produits, la section 2.1 présente les critères de sélection des résidus lignocellulosiques à analyser et leurs principales caractéristiques physicochimiques. Les coques de noix de coco (CS), les coques de palmier à huile (OPS), et le bambou guadua (BG), une espèce de bambou originaire de l'Amérique centrale et du sud ont été choisis pour cette étude en tenant compte de leur composition inorganique et macromoléculaire.

Chapitre 3. Analyse cinétique de la pyrolyse de résidus agricoles

Dans ce chapitre, le comportement de la décomposition thermique des coques de noix de coco (CS), coques de palmier à huile (OPS), et bambou guadua (BG), les trois biomasses sélectionnées pour ce travail, a été étudié à l'échelle thermogravimétrique (TGA).

Une approche non-isotherme utilisant un modèle de réactions parallèles est proposée en tenant compte des trois principaux composants macromoléculaires des échantillons: hémicellulose, cellulose et lignine. A cet égard, une méthode de déconvolution des courbes DTG (première dérivée de la perte de masse) a été mise en place avec des fonctions mathématiques de Fraser-Suzuki. Ceci a permis de décrire avec un bon accord le comportement de la pyrolyse de la biomasse, comme étant la somme de la décomposition de ses trois constituants macromoléculaires, ici appelés "pseudo-composants".

L'analyse cinétique est ensuite basée sur de méthodes mathématiques appelées "modelfree methods", adaptées aux donnés thermogravimétriques. Ces méthodes se sont avérées adéquates pour la détermination des paramètres cinétiques de la pyrolyse des biomasses avec diférentes compositions macromoléculaires.

En particulier, cette étude a montré que même lorsque la composition élémentaire des échantillons est très similaire, ses constituants macromoléculaires peuvent avoir un impact sur leur vitesse de décomposition, et notamment sur l'énergie d'activation associée à la pyrolyse. A cet égard, en considérant le rôle de la lignine pour la stabilité de la structure lignocellulosique, il a été observé que l'énergie d'activation des échantillons est dans l'ordre BG < CS < OPS , en cohérence avec leur contenu de lignine (BG < CS < OPS).

La validation de l'approche cinétique proposée dans ce travail montre un très bon accord avec les résultats expérimentaux, avec une erreur inférieure à 10%. Par conséquent, le modèle cinétique présenté peut représenter un outil important dans le futur, pour l'analyse et la conception des procédés de pyrolyse et gazéiĄcation à une plus grande échelle, en utilisant de résidus lignocellulosiques. En conséquence, le modèle cinétique proposé permet de prédire avec un bon accord le comportement de la gazéiĄcation de diférents types de biomasses lignocellulosiques à partir de leur composition inorganique. Ces résultats peuvent être considérés comme un outil important pour la conception des procédés de gazéiĄcation à une plus grande échelle, en utilisant diférents types de résidus lignocellulosiques.

Chapitre 4. Analyse cinétique de la gazéiĄcation sous vapeur d'eau de résidus agricoles

Chapitre 5. Impacts des caractéristiques de la biomasse sur le procédé de gazéiĄcation sous vapeur d'eau

L'étude du procédé de gazéiĄcation sous vapeur d'eau de résidus lignocellulosiques agricoles a également été faite dans un réacteur a lit Ćuidise à l'échelle laboratoire. Dans ce chapitre, diférents paramètres opératoires ont été évalués aĄn de comprendre leur impact sur le procédé, rendement des sous-produits, et notamment, sur la production du gaz et sa qualité, en termes de composition et pouvoir caloriĄque.

Dans un premier temps, la variation de la quantité de vapeur d'eau utilisée pour la gazéiĄcation a permis d'identiĄer les conditions pour lesquelles la production de gaz est privilégiée. En particulier, les résultats expérimentaux ont montré que des fractions de vapeur entre 15% et 30% dans l'agent de gazéiĄcation favorisent la production des gaz combustibles. En revanche, des valeurs supérieures sont associées à une augmentation de la production de goudrons, avec un impact défavorable sur l'eicacité du procédé. En conséquence, l'analyse comparative des trois biomasses d'étude a été faite pour une fraction de vapeur de 30%.

Également, il a été observé que l'augmentation de la température du procédé est associée à des réactivités de gazéiĄcation supérieures, et donc à une augmentation de la quantité de gaz produit, et de l'eicacité du procédé.

Bien que les conditions opératoires aient montré des efets similaires sur la gazéiĄcation des trois biomasses, des diférences importantes ont été identiĄées entre elles, principalement liées avec leur composition minérale. Les observations expérimentales présentées dans ce travail pourraient constituer une référence importante pour les applications réelles de gazéiĄcation utilisant diférents types de résidus, lorsque la valorisation simultanée des sous-produits gazeux et solides est recherché. En fonction de la nature et de la composition des échantillons, les paramètres et les conditions de la gazéiĄcation pourraient être adaptés pour obtenir la proportion requise de sous-produits, avec les caractéristiques nécessaires.

Chapitre 7. Caractérisation des chars vis-à-vis de leur utilisation dans des applications d'amendement de sols

En tenant compte des résultats de caractérisation présentés dans le chapitre 6 pour les chars issus de la gazéiĄcation sous vapeur des biomasses d'étude, ce chapitre est dédié à l'analyse des propriétés des chars vis-à-vis de leur utilisation dans des applications agricoles et d'amendement des sols.

Conclusions

Dans le cadre du contexte énergétique, environnemental et économique actuel, les résidus agricoles at agroindustriels représentent une source intéressante, renouvelable, et à faible cout, pour la production de biocarburants et produits à valeur ajoutée, dans des pays développés et en voie de développement.

En particulier, compte tenu de la grande diversité des sources des résidus lignocellulosiques, la compréhension de l'impact de la composition organique et inorganique de la biomasse sur le comportement de la pyro-gazéiĄcation a une grande importance, aĄn d'adapter les paramètres du procédé aux matériaux disponibles et aux applications envisagées. De plus, la compréhension de l'inĆuence des caractéristiques de la biomasse sur les sous-produits de la pyro-gazéiĄcation est également essentielle pour déterminer les voies de valorisation les plus appropriées.

Social and scientiĄc context of the research

Each year, billion tons of agricultural and agroindustrial wastes are generated all around the world, representing one of the most abundant and inexpensive resources on earth [1]. However, most of these residues are not valorized and often remain underexploited.

In several locations, agrowastes could represent an environmental risk, as they are not always disposed properly, causing problems like air and soil pollution. In this regard, due to their low cost and abundance, agroresidues may represent an interesting and renewable source for the production of biofuels and value-added products. In the particular case of developing countries, the valorization of agricultural and agroindustrial wastes could also represent an opportunity to provide energy security and business opportunities for several communities.

In relation to this, it is worth noting that according to International Energy Agency [2], around 1.1 billion people in the world still lack access to electricity and cooking facilities (15% of the total global population). Most of them live in rural areas in developing countries, as observed in Ągure 4, and may have access to important amounts of agrowastes, particularly in the case of tropical countries, where the climate conditions favor the agricultural and farming activities [3]. In this regard, agrowaste could represent a valuable resource to provide energy solutions to rural communities, considering that crop production and processing usually take place in rural areas.

In the case of Colombia, according to the Colombian Energy and Mining Planning Unit -UPME, even though the global energy access is above 96%, there are still more than 470.000 homes that do not have access to electricity, principally in rural areas [4]. On the other side, considering its geographic location and climate conditions, it has been estimated that the agricultural and agroindustrial activities in the country produce more than 70 million tons of wastes every year, from cofee, banana, coconut, oil palm, rice, sugar cane, corn, and other crops [5]. As observed in Ągure 5, the comparison of the energy needs with the agrowaste resources location reveals that most of the agricultural residues in the country are produced close to areas with low electriĄcation levels.

Figure 5: Comparison between Colombian energy needs and agrowaste available resources

Thus, these observations suggest that agrowaste valorization in energy applications could be a valuable solution for energy supply in most Colombian rural communities.

Moreover, considering the situation of several developing countries located in tropical areas, agrowaste energy valorization may contribute to the global access to electricity.

However, energy access is not the only concern for communities, considering that energy supply projects should be sustainable in time. In this context, the term sustainability refers to whether or not the project services and structures continue working over an extended period of time after their installation. In relation to this, several authors have documented numerous unsuccessful cases related to energy supply projects for poor communities, showing that diferent cultural and societal factors can hinder the implementation of these kind of initiatives. Among these reported unsuccessful cases, nearly 80% correspond to technologies using biomass as energy source [6,7]. One of the most determinant factors that inĆuence the sustainability of energy projects in developing countries is possibly the fact that commonly, no productive activities are associated to the projects and then, no particular improvements of the life quality, and principally, of the income levels are perceived by communities.

In relation to this and considering the characteristics of agrowastes, the selection of the appropriate valorization pathways could provide rural communities the opportunity to improve their access to energy and simultaneously generate additional incomes.

In this regard, agricultural residues can be valorized via physical, chemical, and biological treatments according to their nature and characteristics. Notably, lignocellulosic agrowastes are suitable for thermochemical conversion processes, considering their low moisture and high carbon contents [8]. In relation to this, combustion, pyrolysis, and gasiĄcation are the most common processes used for lignocellulosic biomass valorization. Therefore, the choice of the most suitable process will depend on the Ąnal products required for each application and their intended uses.

In particular, combustion refers to the exothermic oxidation of biomass at relatively high temperatures. In accordance, hot Ćue gases are the main product of this process, that can be used directly or indirectly for cooking, heating applications, or electricity production [9].

For its part, pyrolysis is related to the thermal decomposition of the feedstocks in the absence of oxygen or a reacting atmosphere. This process produces gas, liquid and solid by-products in diferent proportions, depending on its operation parameters. The main product of slow pyrolysis is char or charcoal, while fast pyrolysis produces mainly bio-oils. The process by-products can be easily transported for further use in combustion or gasiĄcation facilities, or reĄned into chemicals in the case of bio-oils [10].

In contrast, gasiĄcation consists in the partial oxidation of biomass under the presence of an oxidant agent like steam, air, oxygen, or carbon dioxide. This process is a very interesting alternative in the presented context, as it converts a solid residue in fuel gases that can be used for heat or power generation, and a solid by-product, usually considered as a residue, that could also be valorized (Ągure 6). In this case, fuel gases can be used in a more eicient way, with less emissions, compared to the combustion of raw biomasses [8]. In particular, steam as gasifying agent produces considerably high heating value fuel gases, and a porous carbon material that could have similar properties to activated carbons [11,12]. These characteristics suggest that steam gasiĄcation could be an interesting process for the simultaneous valorization of gasiĄcation gaseous and solid by-products. Nevertheless, agrowaste valorization for energy applications has several challenges, as it must take into account the speciĄc local conditions, such as the relevant crops and agricultural practices, climate, crop rotation and availability, residues volume, and energy requirements of local communities.

In this regard, one of the most important challenges is probably the fact that the availability of agrowastes is not constant and often depends on seasonal crops. Consequently, gasiĄcation facilities should work with diferent kind of residues with several characteristics, to ensure a constant operation. Depending on their origin and nature (e.g. type of crop, plant part, treatment, harvesting method, etc), the composition and structure of lignocellulosic residues varies and should be taken into account in their transformation process and subsequent valorization possibilities.

In consequence, the understanding of the impact of the feedstock characteristics on the gasiĄcation behavior and by-products is of great importance, in order to properly adapt the process conditions to the speciĄc requirements of each application.

Although this research project was inspired in the social and energy context of Colombia and several developing countries in tropical regions, the results obtained are also applicable to developed countries, where great amounts of agrowastes are generated as well, with the associated treatment and valorization requirements.

In accordance, the main objective of this work is to analyze the steam gasiĄcation behavior of diferent lignocellulosic agrowastes, and understand the impact of their characteristics on the process by-products properties and valorization pathways.

The associated speciĄc objectives are listed below: 2. To identify the inĆuence of biomass characteristics and gasiĄcation parameters on syngas quality and its use as fuel gas.

3. To identify the inĆuence of biomass characteristics and gasiĄcation parameters on the textural properties and structure of biochars.

4.

To analyze the efect of surface properties and structure of chars in their capacity to be used in agroindustrial applications

This work was developed in the framework of an international cooperation between Colombia (Universidad Nacional de Colombia), and France (IMT -Mines Albi -Carmaux)

Manuscript structure

The present manuscript has been structured in 7 chapters, as summarized in Ągure 7. Each chapter is in the form of a scientiĄc paper, already published, submitted or in intent of submission.

Chapter 1 is dedicated to the literature review, presenting the relevant background related to the gasiĄcation of lignocellulosic biomass, and the current research progress on the valorization pathways of its gaseous and solid by-products. Attention is given to lignocellulosic agrowastes.

Chapter 2 describes the materials used in the present study and summarizes the experimental methods employed. At Ąrst, the procedure and criteria considered for the choice of the most appropriate feedstocks to be analyzed is described. Hence, three lignocellulosic agrowastes with diferent macromolecular and inorganic composition were selected: coconut shells (CS), bamboo guadua (BG), and oil palm shells (OPS). Moreover, the evaluation of two diferent experimental devices for gasiĄcation experiments is discussed, as well as the selection criteria of the most appropriated one. Finally, the methodology and experimental plan for the analysis of steam gasiĄcation process is presented, as well as the procedures applied for the characterization of the process gaseous and solid by-products.

Chapter 3 is dedicated to the analysis of the thermal decomposition behavior of the selected feedstocks under inert atmosphere, based on non-isothermal thermogravimetric analysis. An approach using a combined kinetics parallel reaction model and modelfree isoconversional methods was employed. The inĆuence of the biomass nature in the thermal decomposition behavior was discussed, comparing the calculated kinetic parameters of the studied materials.

Chapter 4 analyses the steam gasiĄcation behavior of the selected feedstocks and their blends, using an isothermal thermogravimetric analysis. The inĆuence of the gasiĄcation temperature and steam partial pressure on the gasiĄcation reactivity of biomasses was discussed. Also, the impact of biomass and blends composition on the gasiĄcation reactivity and kinetics was analyzed. An approach using model-free isoconversional methods and generalized master plots was employed to determine the gasiĄcation kinetic parameters of the samples and compare their decomposition behavior. A new steam gasiĄcation kinetic model based on the inorganic composition of the samples is proposed.

Chapter 5 focuses on the steam gasiĄcation behavior and product distribution of the three selected samples and their blends. The mass and energy distribution of the gasiĄcation by-products was compared for the analyzed feedstocks. Also, the inĆuence of the biomass organic and inorganic composition on the gasiĄcation product yield and gas eiciency is discussed. A focus on the permanent gaseous by-products is done, analyzing its composition, heating value, and possible suitable applications in the presented context.

Chapter 6 is dedicated to the analysis of the physico-chemical properties of the steam gasiĄcation chars produced from the selected feedstocks. The organic and inorganic composition of the chars were analyzed, as well as the distribution of minerals in their surface. In addition, the surface area, pore structure, and surface chemistry were also evaluated and compared. Finally, the relationship between the char structure and surface chemistry was discussed for the three samples.

Finally, chapter 7 presents the evaluation of the physico-chemical properties of selected steam gasiĄcation chars produced from coconut shells, oil palm shells and bamboo guadua, in order to determine their suitability to be used in soil amendment and remediation applications. The pH and bufering capacities of chars were analyzed, as well as their cation exchange capacity (CEC) and mineral release at diferent pH conditions. Accordingly, the agronomical implications of the observed gasiĄcation char properties were discussed, providing new insights to steam gasiĄcation chars valorization pathways.

The general conclusions related to the present work are summarized, and perspectives are proposed for future research work in order to complement the current investigations.

1

Chapter Background and literature review

Introduction

The aim of this chapter is to provide the relevant background information related to the gasiĄcation of lignocellulosic biomass, and the current research progress on the valorization pathways of its gaseous and solid by-products. This literature review is essential to understand the choices and research directions developed in the present work. In this regard, the chapter has been structured in two main sections.

Section 1.2 is dedicated to the biomass pyro-gasiĄcation principal concepts. At Ąrst, the main characteristics of lignocellulosic biomass, as a suitable feedstock for pyro-gasiĄcation process are described. Attention is given to agricultural and agroindustrial residues. Then, the diferent processes involved in biomass pyro-gasiĄcation, as well as their principal by-products are discussed. The most common technologies used are also described and compared.

For its part, section 1.3 is dedicated to the description of the pyro-gasiĄcation solid by-product. At Ąrst, the inĆuence of the process parameters on the char yield and physico-chemical properties is discussed. Then, an overview of the char main characteristics and their interest for subsequent applications is presented. Finally, attention is given to the current research progress on char valorization in several applications.

Biomass pyro-gasiĄcation basics 1.2.1 Lignocellulosic biomass

A unique deĄnition of the term biomass does not exist in the literature. However, it generally refers to any organic material derived from plants or animals. Biomass is usually considered as a sustainable and renewable energy resource, as it is constantly being formed by the interactions of CO 2 , air, water, soil, and sunlight with animals and plants [1].

Among the ones coming from botanical sources (plants), lignocellulosic biomass is the most abundant resource. Lignocellulosic material is the non-starch, Ąbrous part of plants, constituted mainly by hemicellulose, cellulose and lignin. A brief description of these three constituents is presented below [2]:

Cellulose

As the primary structural component of plant cell walls, cellulose is a long chain polymer with a high degree of polymerization (∼10.000) and a large molecular weight. Due to its substantial degree of crystallinity, it functions as a rigid and high strength component of the cell wall, providing the skeletal structure of the plants. Cellulose can be represented by the generic formula (C 6 H 10 O 5 )n.

Hemicellulose

Unlike cellulose, which is crystalline and strong, hemicellulose has a random and amorphous structure with little strength. In general, it is a group of carbohydrates with a low degree of polymerization (∼80-200). The composition of hemicellulose varies depending on the plant source, but may be represented by the generic formula

(C 5 H 8 O 4 )n.

Lignin

Lignin is a very complex molecule containing cross-linked polymers of phenolic monomers. It is located in the layers of cell walls, forming with hemicellulose, an amorphous matrix around cellulose Ąbers, protecting them against biodegradation. In accordance, lignin acts as a cementing agent of plant constituents, giving structural support and resistance against microbial and chemical attack.

The proportion of these polymers varies from one plant species to another, based on the biomass type and origin, as observed in table 1.1.

In general, lignocellulosic biomass may be broadly classiĄed as follows:

• Virgin biomass: Related to forest biomass, grasses and plants without particular transformation (e.g. wood).

• Waste biomass: Harvest related residues or waste generated after processing harvested materials, or from any other productive activity (e.g. bagasse, rice husk, coconut shells, demolition wood, etc). • Energy crops: Plants cultivated for the purpose of producing energy.

In this work, special attention is given to waste biomass, considering that these lignocellulosic materials have an interesting valorization potential in energy applications, or for the production of value-added products. In particular, agricultural and agroindustrial residues are largely available and cheap, and their valorization may imply not only an economic gain but also an environmental beneĄt.

Agricultural and agroindustrial waste

Agricultural and agroindustrial waste can be deĄned as the residues from the growing and processing of raw agricultural products. They are mostly available in the countries with large agricultural sector and food production. In the European Union, the production of crop residues is estimated at 258 million tons per year [4], while in the case of developing countries, agroresidues potential is estimated at more than 2900 million tons per year [5]. Residues like rice straw and husk, sugarcane bagasse, maize straw and cobs, wheat straw, coconut shells and coir, and oil palm residues are some of the most abundant.

The elemental composition of some agrowastes is presented in table 1.2. The production of residues depends on several factors like the type of crop, crop rotation, climate conditions, cultivated area, and agricultural practices. These variables must be taken into account to determine the valorization potential of agrowastes in each location.

In this regard, lignocellulosic agrowastes may constitute a very interesting resource for energy applications taking into account their low cost, availability, and the fact that they do not compete with food production. Moreover, considering that the wrong disposal of these residues could cause problems like air and soil pollution, the energy valorization of agrowastes may also represent an environmental beneĄt.

Pyro-gasiĄcation process and reactions

Among the alternatives available for lignocellulosic biomass conversion and valorization, pyro-gasiĄcation is a thermochemical process, where a carbon-based material is converted into fuel gases, accompanied by a small fraction of liquid and solid by-products [8].

Unlike combustion, where the sample is completely oxidized, pyro-gasiĄcation uses non-stoichiometric quantities of oxidizing agents like steam, air, or oxygen to rearrange the molecular structure of the feedstock by means of diferent heterogeneous reactions [9].

Pyro-gasiĄcation, usually referred as gasiĄcation, is a very complex sequence of processes, which can be classiĄed in three diferent individual steps: drying, pyrolysis and gasiĄcation. The main chemical reactions occurring during the pyro-gasiĄcation process are summarized in table 1.3. Each individual step is brieĆy described below:

Pyrolysis

This step corresponds to the thermal decomposition or devolatilization of the organic material in the absence of oxygen, at temperatures between 300°C and 800°C. The pyrolysis process produces gas, liquid and solid by-products in diferent proportions, depending on the feedstock composition (e.g. hemicellulose, cellulose and lignin), heating rate, temperature, and time [11]. The produced gases are mainly composed of H 2 , CO 2 , CO,CH 4 , and C n H m , while the liquid phase is mainly constituted of water and condensable hydrocarbons, also named tars. For its part, the solid by-product or char consist of almost pure carbon with a fraction of inorganic material originally present in the feedstock. The production of char is maximized at low temperatures and slow heating rates [12]. In contrast, fast heating rate conditions favor the production of liquids [13].

The composition of the biomass and particularly its macromolecular constituents have an important impact on the pyrolysis product yield, considering that each component has a diferent decomposition behavior. From thermogravimetric analysis, several authors have identiĄed the decomposition temperature range for hemicellulose, cellulose and lignin, as the mains constituents of lignocellulosic biomass [14].

Hemicellulose has been reported as the less stable constituent, with low decomposition temperatures between 200°C and 350°C, followed by cellulose, with a decomposition range between 300°C and 400°C. In contrast, lignin pyrolysis occurs in a wide temperature range, from 200°C to 800°C, due to its high aromaticity [14]. In accordance, several authors describe the biomass decomposition behavior as the superposition of the three main components.

Nevertheless, considering that the hemicellulose, cellulose, and lignin are not isolated in the biomass structure, interactions between them may occur during the pyrolysis process. In relation to this, Lui et al. [15], studied the interactions between the biomass components, concluding that lignin impacts in an important extent the hemicellulose and cellulose decomposition rates. Similarly, Mendu et al. [16] found that high lignin biomasses show well-diferentiated peaks for hemicellulose and cellulose decomposition, unlike low lignin samples.

Regarding the product yield, hemicellulose and cellulose are the main source of volatiles during pyrolysis. Hemicellulose releases mainly non-condensable gases, while cellulose is the primary source of tars. In relation to its aromatic nature, lignin degrades slowly and contributes to the char yield [11]. In this regard, the macromolecular composition of the samples is probably the main factor responsible for the diferences observed between the pyrolysis behavior and products of diferent feedstocks. Nevertheless, the interactions between macromolecular components during the thermochemical decomposition processes still need further investigations.

GasiĄcation

GasiĄcation consists in the partial oxidation of char under the presence of an oxidant agent like steam, air, oxygen, or carbon dioxide. GasiĄcation process typically occurs at high temperatures, in the range of 700°C to 1300°C [17]. If air is used, the nitrogen will dilute the produced gas, resulting in a lower heating value. In contrast, oxygen gasiĄcation has the highest heating value followed by steam gasiĄcation. However, the high cost of oxygen production limits the use of this agent in most gasiĄcation facilities [18,19]. Despite the fact that several reactions occur simultaneously during a gasiĄcation process, R4 and R5 are considered as the main reactions in the case of CO 2 or steam gasiĄcation respectively.

Besides the diferences in the produced fuel gas heating value, the reacting atmosphere also impacts the gas composition and product yield of the gasiĄcation process. For instance, steam gasiĄcation enhances the production of hydrogen, while CO 2 favors the production of CO and reduces the H 2 and CH 4 yields [10]. Moreover, in several applications, the gasiĄcation is carried out using a mixture of air, oxygen, steam, or carbon dioxide.

Some technical challenges associated to the use of steam and CO 2 as reacting atmospheres should be taken into account. In particular, energy requirements associated to the production of steam, or the CO 2 separation and storage, may increase the Ąnal energy costs of the process. In consequence, the gasiĄcation parameters should be analyzed for each particular application, and adapted to the required gas composition and product yield. In this regard, the main advantages and technical challenges associated to air, steam and CO 2 as gasifying agents are presented in table 1.5.

Furthermore, as observed in table 1.3, most of the reactions involved in the gasiĄcation process are endothermic and then, require an energy supply to maintain the temperature of the process. This energy may be provided internally in the reactor, by a controlled partial oxidation of the feedstock, considering that oxidation reactions (R2 and R3) are exothermic. In this case, the process is known as direct or autothermal gasiĄcation. In contrast, if the heat is externally supplied, the process is called indirect or allothermal gasiĄcation [20,21].

Several authors have investigated the efect of operating parameters and conditions on the performance of the biomass gasiĄcation process. Parameters like temperature and gasifying agent have shown an important impact on the produced gas composition and product yield [22,23]. Moreover, the impact of the indigenous inorganic elements of the raw materials on the steam gasiĄcation reactivity has been also reported [24,25] Table 1.5: Advantages and technical challenges of diferent gasifying agents.

Adapted from [10] Lignocellulosic biomass from agricultural and agroindustrial activities exhibit diferent amounts and compositions of inorganic elements that inĆuence its gasiĄcation behavior.

In particular, alkali and alkaline earth metals (AAEM) such as K, Na, Ca and Mg, could have a catalytic efect on the gasiĄcation reactions [24].

In this regard, several authors have proposed diferent reaction mechanisms to describe the catalytic efect of alkali carbonates during CO 2 gasiĄcation [26]. The R4 reaction sequence via the metal M proposed by McKee is presented below [27]:

M 2 CO 3 + 2 C --→ 2 M + 3 CO 2 M + CO 2 --→ M 2 O + CO, or M 2 O + CO 2 --→ M 2 CO 3 C + CO 2 --→ 2 CO
In the case of steam gasiĄcation, the R5 reaction sequence is also considered as an oxygen transfer mechanism via the metal M [27,28]:

M 2 CO 3 + 2 C --→ 2 M + 3 CO 2 M + 2 H 2 O --→ 2 MOH + H 2 2 MOH + CO --→ M 2 CO 3 + H 2 2 C + 2 H 2 O --→ 2 CO + 2 H 2
Among the AAEM, K has been reported to be the most active species in steam and carbon dioxide gasiĄcation [28]. In contrast, elements like Si, Al and P may inhibit this beneĄcial efect, as they tend to react with AAEM to form alkali silicates [29].

In this regard, it is important to point out that the biomass inorganic content may inĆuence in a signiĄcant way the biomass combustion and gasiĄcation behavior [24]. However, a better description and understanding of the interactions between the biomass mineral constituents and their inĆuence in the gasiĄcation process are still required.

Pyro-gasiĄcation by-products

As aforementioned, combustible gases are the main product of pyro-gasiĄcation process. However, a liquid and a solid phase are also produced in diferent proportions, depending on the process parameters and feedstocks. The main characteristics of the gasiĄcation by-products are presented below:

Gases

Pyro-gasiĄcation gas is principally composed of CO and H 2 . CO 2 is the third component followed by CH 4 and light hydrocarbons, present in minor concentrations. The gasifying agent impacts in an important way the composition and heating value of the produced gas [30]. For instance, the H 2 content can be increased if steam is used as the gasifying agent. In contrast, the use of CO 2 enhances the production of CO and reduces the H 2 and CH 4 fractions [31]. Alternatively, mixtures of steam or CO 2 with air or O 2 can be also used as gasifying atmosphere, impacting also the component fractions in gas [32].

In accordance, produced gas from lignocellulosic biomass can be used in boilers, internal combustion engines, or gas turbines for power generation [18]. Moreover, depending on its composition, it can be also converted to advanced chemicals through Fisher-Tropsh synthesis and oxo-synthesis processes [33]. In particular, the H 2 /CO ratio of the gas gives an insight to its possible valorization pathways.

Figure 1.1: Gas valorization pathways according to the H 2 /CO ratio. Adapted from [31] As presented in Ągure 1.1, H 2 /CO ratios between 4 and 6 are suitable for solid oxide fuel cells (SOFC) operation, while values between 4 and 2.5 are mostly recommended in gas turbines and internal combustion engines applications. Finally, H 2 /CO ratios below 2 are more suited for Fisher-Tropsh (FT) synthesis processes [31,34].

In this regard, depending on the intended application, the process gasifying agent is a very important parameter to consider that may be adapted to obtain the required H 2 /CO ratio.

Liquids

GasiĄcation liquid by-products, generally known as tars, are deĄned as a complex mixture of condensable hydrocarbons with molecular weight higher than benzene [35]. These compounds are a main concern for gasiĄcation process, as they can condense and clog pipes and equipment, and produce metallic corrosion. In accordance, the presence of tars in the gas is associated to its usefulness. In applications where the raw gas is burned directly without cooling, there is no need of gas cleaning. However, if a use in internal combustion engines or gas turbines is intended, tar conversion or removal is compulsory [36].

For energy applications, internal combustion engines require syngas with tar levels of maximum 100 mg/m 3 , while gas turbines require less than 10 mg/m 3 , in order to avoid equipment damages [37]. Moreover, in the case of advanced chemicals production applications, the tar limits are even more stringent, with upper values as low as 0.02 mg/m 3 .

To meet these requirements and face tar problems, several syngas conditioning methods have been developed. They are mainly classiĄed in primary or secondary methods depending on the location where tars are removed. Primary methods are related with the gas treatment inside the gasiĄcation reactor, whereas secondary methods are performed outside the gasiĄer, and can be physical or chemical [38].

Solids

Char or biochar is the solid residue from biomass pyro-gasiĄcation process. Pyrolysis chars are mainly composed of carbon, while gasiĄcation chars may contain important amounts of inorganic material, depending on the progress of the gasiĄcation reactions [39]. Biochars could have several valorization pathways according to their physicochemical characteristics: combustion for energy recovery, catalyst or catalyst supports, soil amendment materials, adsorbents, and activated carbon precursors [40,41]. A focus on the pyrolysis and gasiĄcation chars physico-chemical properties and possible valorization pathways is presented in section 1.3.

Pyro-gasiĄcation technologies

The sequence of pyro-gasiĄcation reactions and the characteristics of the process byproducts depend in an important extent on the reactor type used. Various works on design improvements have been done for enhancing the performance of gasiĄcation devices. However, they are generally classiĄed in Ąxed-bed and Ćuidized-bed reactors [17]:

Fixed-bed reactors

Fixed-bed reactors are also known as moving-bed reactors and can be classiĄed in updraft (counter-current) gasiĄers and downdraft (co-current) gasiĄers (Figure 1.2).

• In updraft gasiĄers, the feedstock is fed by the top, while the oxidizing agent is introduced from the bottom, as observed in 1.2a. Then, the gases rise through the bed and are recovered at the top of the reactor. The biomass is moving in the opposite direction and passes Ąrst through the drying zone, then, the pyrolysis zone, the reduction zone, and Ąnally, the combustion zone. These reactors are easy to operate, but may produce gases with an important content of tars (typically 10% to 20% by weight of the feed) [1].

• In a downdraft gasiĄer, the biomass is fed from the top, while the oxidizing agent is fed at the sides. In this reactor, the gas is moving in the same direction as the biomass and is recovered at the bottom of the reactor as observed in 1.2b. Since the pyrolysis products pass through the combustion high-temperature zone, the tar cracking reactions are favored, resulting in a low tar gas production (<1 g/Nm 3 ) [1].

Fluidized-bed reactors

The Ćuidized bed reactors have some advantages over Ąxed-beds, regarding the mixing and temperature uniformity, the high reaction rates, high heat and mass transfer, and the possibility to be built in large scales [42]. A Ćuidized bed is composed of Ąne solids that are kept suspended in a liquid-like state by the action of the gasifying agent, injected from the bottom of the reactor with a suicient pressure. Fluidized-bed gasiĄers can be mainly classiĄed in bubbling and circulating reactors.

• In a bubbling Ćuidized bed reactor, the velocity of the upward Ćowing gasifying agent is around 1-3 m/s and the expansion of the bed occurs only at the lower part of the gasiĄer, as presented in Ągure 1.3a. Considering the low gas velocities, the bed materials remain in the reactor. GasiĄers operating at temperatures below 900°C are preferred in order to avoid ash agglomeration. However, bubbling Ćuidized bed reactor can work at temperatures above 1000°C in the case of low ash feedstocks.

• In a circulating Ćuidized bed, the solids circulate around a loop that is characterized by intense mixing, as shown in Ągure 1.3b. This design exhibits great gasiĄcation eiciencies and high carbon conversions, thanks to the increase in the residence time of the particles and gases inside the reactor. The velocity of the upward Ćowing gasifying agent is around 5-10 m/s to ensure the circulation of the solids. Moreover, the operation temperatures are between 800°C and 1000°C in order to avoid ash agglomeration. The advantages and technical challenges identiĄed for the described gasiĄer designs are summarized in table 1.6.

Focus on pyro-gasiĄcation chars properties

As extensively discussed in the literature, pyro-gasiĄcation process operating conditions inĆuence in a great extent the physico-chemical properties of the solid by-products. More speciĄcally, parameters like heating rate, temperature, and reacting atmosphere should be carefully chosen in accordance to the required char yield and properties.

InĆuence of process parameters

Heating rate

In a pyro-gasiĄcation process, the heating rate has an important impact on the quantity of char recovered and its properties. In particular, pyrolysis can be classiĄed in slow and fast, according to the heating rate. It has been widely observed in the literature, that low heating rates (∼20°C/min) favor the production of char, while high heating rates (>10°C/s) maximize the production of liquids [24,43]. These diferences may be explained by the fact that a slow heating allows the slow release of volatiles and the occurrence of secondary char formation reactions that increase the resulting solid yield [40]. In this regard, it has been reported that the highest recovered char quantities, between 20% and 40% are obtained for slow pyrolysis process, followed by fast pyrolysis, and Ąnally by gasiĄcation, with approximately 15% and 10%, respectively [41].

The diferences between chars prepared under both conditions has been widely described in the literature. In general, it has been observed that high heating rate chars have higher speciĄc surface areas, as a result of the extremely fast release of volatiles from the carbon structure, that breaks the carbon matrix [44]. In consequence, these chars have shown higher reactivities to gasiĄcation or combustion reactions, mostly related to their open structure [45].

Temperature

The pyro-gasiĄcation temperature impacts in a very important way the biochar quantity and quality. Some of the char properties that are widely inĆuenced by the process temperature are the elemental composition, Ąxed carbon content, surface area, pore volume, heating value, pH, and cation exchange capacity [40].

Considering that higher pyrolysis temperatures are associated to greater biomass decomposition extents, pyrolysis chars show a decrease in the H/C and O/C ratios with the temperature increase, indicating higher carbon percentages, but lower surface functionalities in the char surface. Likewise, greater surface areas and pore volumes have been observed by several authors for high temperature chars.

Furthermore, the amount of char that can be obtained from pyro-gasiĄcation depends mainly on the process temperature and residence time. Higher temperatures result higher decomposition extents and lower char yields, as well as longer residence times. In this regard, the efect of the temperature on some of the most relevant properties of pyrolysis biochars is summarized in Ągure 1.4.

Figure 1.4:

Efects of the temperature on biochar properties. Adapted from [40,46] As observed, depending on the intended applications of the chars, some properties can be improved with the temperature rise, while others may be deteriorated. In consequence, the choice of the process temperature may be done considering the expected applications and the required char characteristics. Moreover, the quantity of char recovered from a process is also an important point to consider for the economic design of a particular char valorization activity.

Reacting atmosphere

The reacting atmosphere is a very important parameter to consider in pyro-gasiĄcation processes, as it has a signiĄcant impact on the chars structure and surface chemistry.

As stated previously, under inert atmosphere, the biomass decomposition occurs only as a result of the temperature increase, and no particular reactions take place.

In contrast, in the presence of a reacting agent, heterogeneous reactions between the char and the gaseous phase create a porous network in the char and integrates surface functionalities [47]. In this regard, several authors have reported that gasiĄcation chars produced with steam or CO 2 exhibit considerably higher surface areas in comparison to pyrolysis-only chars [48]. Values between 2 and 400 m 2 /g have been reported in the literature for pyrolysis chars produced at diferent temperatures, while values near 700 m 2 /g or higher have been observed for CO 2 and steam gasiĄcation chars [49,50].

More speciĄcally, Klinghofer et al. [51] investigated in detail the physico-chemical properties of woody chars produced with air, CO 2 , and steam as gasifying agents. Diferent characteristics were observed for the three samples, considering that the reaction mechanism difers from one agent to another. CO 2 chars exhibited a higher microporosity in comparison to steam gasiĄcation chars. In this regard, Guizani et al. [52] suggested that steam molecules difuse more deeply in the biochar matrix and enhance the formation of larger pores.

Regarding the char surface chemistry, pyrolysis chars produced at high temperatures does not exhibit signiĄcant amounts of surface functional groups, as a result of the sample devolatilization. In contrast, when a reacting atmosphere is present, chemisorption of oxygen and hydrogen may occur on the char structure, creating surface complexes [47].

In this regard, gasiĄcation chars may have interesting properties that could suggest their valorization in several applications.

As presented in this section, the process parameters impact in a very important way the physico-chemical properties of the resulting chars. Therefore, gasiĄcation conditions should be carefully selected according to the required by-products characteristics and intended applications.

Pyro-gasiĄcation chars physico-chemical properties

This section gives a broad overview of the main physico-chemical properties of pyro-gasiĄcation chars and their interest for subsequent applications:

Elemental composition

The knowledge of the C,H,N,S and O content of the char gives a Ąrst insight to its possible applications. In particular, the atomic ratios H/C and O/C are used as a measure of the aromaticity and decomposition extent in the case of pyrolysis chars [53]. Higher gasiĄcation temperatures are associated to an increase in the char carbon content, and a decrease in the H/C and O/C ratios, due to the devolatilization of functional structures containing oxygen and hydrogen.

In contrast, steam and CO 2 gasiĄcation chars may exhibit higher hydrogen and oxygen contents in comparison to pyrolysis chars produced at the same temperature, considering that gasiĄcation reactions could result in the functionalization of the char surface, as aforementioned [49].

In relation to the inorganic composition, chars could contain diferent amounts of minerals, depending on the indigenous inorganic content of the parent materials. As a large part of the ash remains in the char after the pyro-gasiĄcation process, the increase of the temperature and residence time will be also associated to an increase in the char inorganic content [40]. For some applications, the presence of minerals in the char structure could be beneĄcial, as in the case of soil remediation and heavy metal adsorption [54], while for others, the ash content could generate operational problems like slagging, fouling and corrosion [55]. In this regard, the organic and inorganic composition of pyro-gasiĄcation chars are important parameters to consider in the choice of their most suitable valorization pathways.

SpeciĄc surface area and porosity

The char speciĄc surface area is usually determined by the BET analysis, assessed using N 2 adsorption at 77K. This property is a very important parameter to identify the possible uses of a char. High speciĄc surface areas are required for char applications as adsorbent, catalyst, or catalyst support [56].

In addition to the surface area, the pore size distribution and pore volume of chars play and important role in their performance in several applications. Biochars may contain diferent proportions of micropores (<2nm), mesopores (2-50nm), and macropores (>50nm) depending on the process parameters, as aforementioned, and on the parent material [47]. In adsorption applications, favorable pore size distribution for a targeted molecule, makes the internal surface accessible and enhances the adsorption process [57]. For instance, according to the pore size, microporous carbons can be used for gas applications, like gas separation and puriĄcation. In contrast, mesoporous and macroporous materials are suitable for liquid phase applications, such as water puriĄcation [57].

Surface oxygen-containing functional groups

As a result of the thermal decomposition of the biomass structure, high temperature pyrolysis chars show low H/C and O/C ratios, indicating a more aromatic structure and less surface functional groups in comparison to chars produced at low temperatures [58]. Aromatic structures have a great thermodynamic stability, and are associated to a signiĄcant char recalcitrance, useful in soil amendment and carbon sequestration applications [49].

However, the content of oxygen and hydrogen in the char structure also plays an important role in the char performance in several applications. More speciĄcally, oxygencontaining functional groups in the char structure are highly reactive and could afect their adsorption ability. Moreover, the type and amount of functional groups inĆuence the char alkalinity and their ability to exchange cations and neutralize acidity in soil amendment applications [59]. In relation to the character of these groups, carboxylic acids, hydroxyl, lactone and phenol groups are known as acidic surface functional groups [61,62], while carbonyl, quinone and pyrone-like functional groups are reported to be neutral or basic [63].

More speciĄcally, oxygen-containing functional groups, specially the acidic ones, have shown to be responsible for the amphoteric character of chars. In particular, in aqueous phase, the char surface may be positively or negatively charged, depending on the pH of the surrounding solution. Then, the carbon may have the capacity to interact with positive or negative charges. The pH at the point of zero charge or pH PCZ is a measure of this ability. The pH PCZ is deĄned as the pH at which the char surface has zero charge. In this regard, if the pH PCZ of a carbon is higher than the pH of the solution, its surface will be positively charged and will enhance interactions with anions. In contrast, if the pH PCZ is lower than the solution pH, the surface will be negatively charged and will favor cationic interactions [57].

Surface functional groups can be analyzed qualitatively and quantitatively using different techniques like Boehm titration, Fourier transform spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), or thermal programmed desorption (TPD). The latter one is particularly interesting as it is relatively simple and allows a quantitative determination based on the thermal stability of each functionality. It consist on the heating of the sample in an inert atmosphere, analyzing the CO and CO 2 released. As observed in Ągure 1.5, the nature of each group can be determined according to the emitted gas and its desorption temperature.

In this regard, the accurate characterization of surface functionalities on chars is of great importance to determine their possible uses and applications.

pH-value

The char pH is also a very important parameter to consider, particularly for agricultural applications, as it can help to adjust the soil pH and increase its bufering capacity. The alkalinity of chars depends mainly on the raw material and the treatment temperature. Generally, pH increases with the pyro-gasiĄcation temperature, due to the material devolatilization and the loss of acidic complexes existing in the raw biomass [46].

Moreover, the increase in the ash content, usually basic in nature, results in higher pH values [64]. The existence of alkaline minerals in the char matrix also enhance the char acid neutralization capacity, making this material suitable for the amendment of acidic soils [65].

Cation exchange capacity

The cation exchange capacity (CEC) is a relevant char property usually evaluated for environmental applications. It is related to the char capacity to hold and exchange cations (e.g. K + , Na + , Ca 2+ , Mg 2+ and NH 4 + ), and is usually associated to soil fertility [66]. CEC depends directly on the char surface structure, oxygen containing functional groups (e.g. Ű COOH, Ű OH), and mineral content [64]. In particular, it is the result of the negative charges on the char surface.

High cation exchange capacities have been observed for low-temperature pyrolysis chars, considering their low devolatilization extent. In contrast, a decrease in the CEC with the increase of the pyrolysis temperature has been reported by several authors [START_REF] Banik | ŞImpact of pyrolysis temperature and feedstock on surface charge and functional group chemistry of biochars[END_REF]. In the particular case of steam and CO 2 gasiĄcation chars, as the heterogeneous gasiĄcation reactions could result in the functionalization of the char surface, CEC comparable to low-temperature pyrolysis chars may be observed [START_REF] Banik | ŞImpact of pyrolysis temperature and feedstock on surface charge and functional group chemistry of biochars[END_REF][START_REF] Lee | ŞCharacterization of Biochars Produced from Cornstovers for Soil Amendment[END_REF].

Pyro-gasiĄcation chars applications

As observed in section 1.3.2, depending on the production parameters, biochars from agrowastes have a wide range of physico-chemical properties that could be interesting for several applications. The environmental and industrial applications of biochar in the areas of energy production, carbon sequestration, soil amendment, and advanced materials production have been widely discussed in the literature in the recent years, and will depend on the material properties. The most common pyro-gasiĄcation chars applications are summarized in Ągure 1.6

Figure 1.6: Common applications of pyro-gasiĄcation chars

Adsorbent

The inherent high speciĄc surface area of biochars is of great interest in adsorption applications. According to their pore size distribution, biochars could be used in gas or liquid phase applications, for the removal of several kinds of pollutants. Diferent studies have investigated the efectiveness of biochars in the removal of organic contaminants from water [START_REF] Tan | ŞApplication of biochar for the removal of pollutants from aqueous solutions[END_REF], Ćue gases treatment [START_REF] Abdulrasheed | ŞSurface modiĄcation of activated carbon for adsorption of SO 2 and NO X : A review of existing and emerging technologies[END_REF], and syngas tar removal [START_REF] Shen | ŞChars as carbonaceous adsorbents / catalysts for tar elimination during biomass pyrolysis or gasi Ą cation[END_REF].

According to diferent authors, the adsorption performance of biochars depend on the parent material used for their production. Xu el al. [START_REF] Xu | ŞAdsorption of methyl violet from aqueous solutions by the biochars derived from crop residues[END_REF] compared the methyl violet adsorption behavior of diferent biochars, suggesting that the adsorption characteristics of biochar are inĆuenced by the number of negative charges in its surface. Moreover, the mineral content and composition of the chars played an important role on their adsorption ability. In particular, mineral-rich biochars proved to be suitable for heavy metal adsorption in water and soils [64], considering the interactions between metals and the charged minerals in the char surface. The higher the negative charge of the char surface, the more pronounced the electrostatic adsorption of heavy metals.

The ability of biochars to adsorb organic contaminants has also been highlighted by diferent authors. Several studies have reported that for this application the presence of minerals is not beneĄcial, and thus, de-ashing treatments could be required. The removal of minerals can greatly inĆuence the surface characteristics of the biochar and then, impact its adsorption properties [64].

Similarly, biochars have proved to be suitable for the adsorption of light aromatic hydrocarbons and light polycyclic aromatic tars from syngas. Reported studies showed that biochars can achieve an adsorption performance of almost 87% for some tars compounds, and almost 95% if combined with other techniques [START_REF] Paethanom | ŞPerformance of tar removal by absorption and adsorption for biomass gasiĄcation[END_REF][START_REF] Paethanom | ŞInĆuence of Pyrolysis Temperature on Rice Husk Char Characteristics and Its Tar Adsorption Capability[END_REF]. In relation to this, Hu et al [START_REF] Hu | ŞRemoval of Tar Model Compounds Produced from Biomass GasiĄcation Using Activated Carbons[END_REF], found that higher surface areas, smaller particle sizes and an important proportion of micropores, are related to better adsorption performances. Also, the average pore diameter inĆuenced the tar adsorption capacity, depending on the type of model compound.

Catalyst precursor

The use of biochar from agrowastes as catalyst precursor or catalyst support has also been widely discussed in the literature, considering the physico-chemical properties of biochar and its relative low cost [40]. In catalysis applications, both the surface area and the functional groups are important criteria. In this regard, the proper choice of pyro-gasiĄcation conditions may allow the production of biochars with the required characteristics for catalytic applications.

In particular, several authors have reported the promising behavior of biochar as a catalyst for gasiĄcation syngas cleaning applications [START_REF] Phuphuakrat | ŞTar removal from biomass pyrolysis gas in two-step function of decomposition and adsorption[END_REF]. El-Rub et al. [START_REF] El-Rub | ŞExperimental comparison of biomass chars with other catalysts for tar reduction[END_REF] compared biomass chars with other typical catalysts used for tar decomposition, concluding that char has a great potential to convert diferent tar compounds like phenol and naphthalene. For their part, Chen at al. [START_REF] Chen | ŞExperimental Investigation on Tar Formation and Destruction in a Lab-Scale Two-Stage Reactor[END_REF] analyzed the catalytic performance of biochars from diferent agrowastes, varying the temperature, residence time and particle size. They concluded that the tar conversion rate was improved at higher temperatures and with smaller particle sizes. In contrast, no signiĄcant diferences were observed between the materials analyzed. Tar conversion percentages up to 98% have been reported for some tar compounds using biochar from diferent feedstocks [START_REF] Maria | ŞArtículo de revisión Materiales carbonosos para el acondicionamiento de gas de síntesis y remoción de alquitranes Carbon materials for syngas conditioning and tar removal Este documento posee una licencia Creative Commons Reconocimiento-No Comercial 4.0 I[END_REF].

Recent applications of biochars as catalysts for biodiesel production have been also reported. Dehkhoda et al. [START_REF] Amir Mehdi Dehkhoda | ŞBiochar based solid acid catalyst for biodiesel production[END_REF] studied the esteriĄcation and transesteriĄcation of vegetable oils over sulfonated biochar, concluding that chars with high speciĄc surface areas lead to high biodiesel production yields. The observed conversion percentages demonstrated the char potential as solid acid catalyst for biodiesel production. Moreover, biochars have also been used as catalysis supports for diferent applications. In relation to syngas conditioning and tar removal, diferent authors have studied the performance of char supported Ni and Fe-based catalyst. Conversions up to 97% at 800°C have been reported for real syngas using wood and coal chars [START_REF] Shen | ŞIn-situ catalytic conversion of tar using rice husk char-supported nickel-iron catalysts for biomass pyrolysis/gasiĄcation[END_REF]. The carbon structure and porosity have also shown a signiĄcant role in the tar conversion process, improving the interactions between the syngas and the catalyst surface.

Activated carbon precursor

For some applications, the biochar pore structure and surface functionalities may not be suicient to ensure their outstanding performance. In this case, biochar could be used as a renewable and low cost precursor for activated carbon production. According to the intended applications, activated carbons could be produced by physical or chemical activation, enhancing the material porous structure and including the desired functional groups in its surface.

For physical activation, steam or CO 2 are generally used as activating agents. Other gases like NH 3 , O 2 or their mixtures may also be employed. Typical activation temperatures have been reported near between 800°C and 1000°C [START_REF] Tan | ŞBiochar as potential sustainable precursors for activated carbon production: Multiple applications in environmental protection and energy storage[END_REF]. The physical activation process of biochars from agrowastes produces carbon porous materials with surface areas up to 1000 m 2 /g and a well developed microporous structure.

For its part, chemical activation uses diferent chemicals as activating agents. Reagents like HCl, HNO 3 , KOH, K 2 CO 3 , H 3 PO are generally used [41], resulting also in the production of highly porous materials, with high speciĄc surface areas and functional complexes [START_REF] Tan | ŞBiochar as potential sustainable precursors for activated carbon production: Multiple applications in environmental protection and energy storage[END_REF]. Although chemical activation is less time and energy intensive, environmental concerns related to the use of chemicals should be considered.

Physical and chemical biochar-based activated carbons have found several applications for gases and liquids treatments. Diferent authors have analyzed the use of activated biochars for air pollution control and NO x removal [START_REF] Abdulrasheed | ŞSurface modiĄcation of activated carbon for adsorption of SO 2 and NO X : A review of existing and emerging technologies[END_REF], water treatment and puriĄcation [START_REF] Menya | ŞProduction and performance of activated carbon from rice husks for removal of natural organic matter from water: A review[END_REF], gas storage [START_REF] Madzaki | ŞScienceDirect Carbon Dioxide Adsorption on Sawdust Biochar[END_REF][START_REF] Ramesh | ŞActivated carbons derived from tamarind seeds for hydrogen storage[END_REF], capacitors [START_REF] Guo | ŞPerformance of electrical double layer capacitors with porous carbons derived from rice husk[END_REF] and medical applications [START_REF] Sinan | ŞEvaluation of activated charcoal and lipid emulsion treatment in model of acute rivaroxaban toxicity[END_REF][START_REF] Brahmi | ŞInĆuence of activated charcoal on the pharmacokinetics and the clinical features of carbamazepine poisoning[END_REF]. The applications are determined by the characteristics of the materials such as surface area, pore structure, pore volume and chemical composition.

Soil amendment

Recent studies have highlighted the beneĄcial efects of the use of pyro-gasiĄcation chars in soil amendment applications. In particular, biochars show high stability and an important ability to retain plant nutrients compared to several kind of soils [40].

Application of biochars to low-quality soils is mainly associated to an improvement in their soil cation exchange capacity (CEC), microbial activity and nutrient retention [START_REF] Lee | ŞCharacterization of Biochars Produced from Cornstovers for Soil Amendment[END_REF]. In fact, an increase in the cation exchange capacity of soils is related to the improvement of the soil ability to retain nutrients, most of them cations (e.g. K + , Na + , Ca 2+ , and Mg 2+ ), and avoid their leachability [START_REF] Donald | Environmental soil chemistry[END_REF]. Moreover, considering that chars from agrowastes usually have considerable amounts of minerals, these materials could be a source of nutrients to soils (e.g. K, Ca, Na, Mg, and P), increasing their availability and reducing the traditional fertilizers requirements [START_REF] Lee | ŞCharacterization of Biochars Produced from Cornstovers for Soil Amendment[END_REF]. The bio-availability of biochar nutrients to plants depends on two principal factors: the occurrence form and distribution of mineral species, and the morphological structure of chars [START_REF] Kong | ŞLeaching characteristics of inherent inorganic nutrients in biochars from the slow and fast pyrolysis of mallee biomass[END_REF].

Furthermore, mineral-rich biochars have also proved their capacity to immobilize heavy metals in soils, reducing their availability for plant uptake [64]. In the literature, several mechanisms have been proposed in relation to metal sorption by biochars. They include electrostatic interactions, cation exchange, metal complexation, and metal precipitation [START_REF] Li | ŞMechanisms of metal sorption by biochars: Biochar characteristics and modiĄcations[END_REF]. Diferent studies have shown that both carbon and mineral fractions of biochar contribute to heavy metals retention. More speciĄcally, for biochars with low mineral content, the sorption of heavy metals is mainly associated to surface oxygen containing functional groups. In contrast, for mineral-rich biochars, interactions between heavy metals and the char mineral fraction are the dominant factor, and could be associated to up to 90% of the metal retention and removal [64].

In relation to organic pollutants, several authors have also underlined the capacity of pyro-gasiĄcation chars to reduce contaminant exposure in soils. Generally, soil pollutants can accumulate and produce negative efects on soil organisms and other living beings [92]. Recent studies have found that the reduction of bioaccessible polycyclic aromatic hydrocarbons (PAHs) in soils after the application of biochars varied from 27% to 80%, depending on the char precursor and quantity applied [93]. Similarly, Kotowski et al.

[94] also validated the suitability of biochars and biochar-based activated carbons for the immobilization of freely dissolved and bioaccessible PAHs. Higher efectiveness were observed for materials with higher speciĄc surface areas and pore volumes.

Moreover, in agricultural applications, the soil pH is an important parameter to consider. Acidic soils may present several problems to plant growth and crop yields, mainly related to aluminum (Al) and manganese (Mn) toxicity to plants, and an important deĄciency of essential nutrients like phosphorous (P), potassium (K), calcium (Ca) and magnesium (Mg) [95]. In this regard, considering that most biochars have a basic nature and a signiĄcant acid neutralization capacity, their application to acidic soils may result in an increase in the pH value and an improvement in the soil ability to resist acidiĄcation [65].

Conclusion

From the literature review, it was possible to conclude that pyro-gasiĄcation is a very interesting process for the valorization of agricultural and agroindustrial wastes. In particular, this process produces fuel gases that can be used for the generation of heat or power, and a solid by-product with interesting properties for several applications. Notably, steam gasiĄcation is of particular interest, as it produces high heating value fuel gases, and a carbon-rich highly porous material, with similar characteristics to activated carbons.

Despite the fact that diferent process parameters impact in an important way the gasiĄcation by-products yield and properties, biomass is possibly the most important one to consider when the valorization of lignocellulosic agrowastes is intended. In fact, considering that agroresidues come from a wide variety of sources and are often seasonal, gasiĄcation facilities should deal with a large range of materials with diferent characteristics and chemical compositions.

[92] B J Reid, K C Jones, and K T Semple.

Introduction

This chapter aims to describe the materials used in the present study and the experimental procedures employed. Considering that the main objective of this research work is to understand the impact of the biomass characteristics on the steam gasiĄcation process and by-products, the section 2.2 presents the selection criteria of the lignocellulosic residues to be analyzed, and their main physico-chemical characteristics. For its part, in the framework of the international cooperation between Colombia and France, the section 2.3 describes the evaluation of two diferent experimental devices considered for the gasiĄcation experiments, and the selection criteria of the most appropriate setup to achieve the objectives of the present work.

Section 2.4 is dedicated to the description of the experimental methods used to analyze the thermal decomposition behavior and kinetics of the selected biomasses, under inert atmosphere, and under steam as gasifying agent. Finally, section 2.5 presents the experimental procedure followed for the steam gasiĄcation tests of the selected feedstocks, and the sampling of the gasiĄcation by-products. Also, the experimental methodology used to evaluate the physico-chemical properties of the raw biomass and the produced char is summarized.

Selection of gasiĄcation feedstocks

In order to understand the inĆuence of the biomass characteristics in the gasiĄcation process and the properties of its gaseous and solid by-products, three lignocellulosic agrowastes with diferent physico-chemical characteristics were selected for this study, among several residual biomasses from Colombian agricultural and agroindustrial activities.

From the literature review, the organic and inorganic composition of diferent lignocellulosic agrowastes were analyzed and compared, in order to determine the most suitable feedstocks for the present study. The characteristics of some of the most representative Colombian crop residues compared are presented in tables 2.1 and 2.2. In accordance, from the analyzed feedstocks, coconut shells (CS), bamboo guadua (BG), and oil palm shells (OPS) were chosen, taking into account their organic and inorganic composition and their high heating value. Moreover, the great availability of these residues in Colombia and several other developing countries was also considered.

Regarding the composition of the selected samples, the reported elemental analysis shows very similar results for the three feedstocks. In contrast, the comparison of their macromolecular and inorganic composition reveals some remarkable diferences.

In relation to the macromolecular composition, it can be noted that coconut shells (CS) and oil palm shells (OPS), as endocarp residues, have high lignin contents in comparison to bamboo guadua (BG), considered as a grass. In particular, OPS exhibit the highest lignin content with nearly 50%, followed by CS and BG, with 36.5% and 25% respectively. For its part, BG shows the highest cellulose percentage with a value of 54%. Moreover, it can be observed that bamboo guadua (BG) has a higher ash percentage in comparison to coconut shells (CS) and oil palm shells (OPS), indicating an important amount of mineral content in this biomass. From table 2.2 it can be noted that CS contain mainly K, Ca and Na, while OPS and BG are principally composed of Si, P, and also K.

As already stated, the inorganic elements present in the biomass are reported as an inĆuential parameter in the steam gasiĄcation reactivity [9]. In particular, alkali and alkaline earth metals (AAEM) like K, Ca, Na and Mg, could have a catalytic impact on biomass gasiĄcation [10]. In contrast, elements like Si, P and Al may react with AAEM and inhibit their beneĄcial efect [11]. In this regard, two of the selected samples are Si rich, while the other one contains mainly AAEM. More speciĄcally, among the high lignin content samples, coconut shells (CS) exhibit an important amount of AAEM, contrary to oil palm shells (OPS). For its part, bamboo guadua (BG) shows the highest AAEM content of the three samples, but also an important amount of Si and P.

In consequence, as the selected feedstocks show interesting diferences in their macromolecular and inorganic content, the comparison of their gasiĄcation behavior will contribute to the understanding of the inĆuence of the biomass characteristics on the gasiĄcation process of lignocellulosic agrowastes, and the properties of their gaseous and solid by-products.

According to the Colombian context, a brief description of the selected samples (Ągure 2.1) is presented below:

Coconut shells (CS):

Cocos nucifera palms are grown in tropical countries, as they require a warm climate and high humidity for successful development. Generally, coconut palm is considered a very versatile plantation, as almost every part of the plant can be used for diferent applications. Nevertheless, in Colombia, coconut shells are considered a material with little or non-economic value and have been traditionally used for handicraft making or discharged in soils. The amount of this residue is not negligible in the country, considering that about 18.400 hectares of land are being used for coconut palm plantations, with a production near 116.000 tons, according to the Colombian Ministry of Agriculture and the Food and Agriculture Organization of the united Nations [12].

Bamboo guadua (BG):

Bamboo guadua (Guadua Angustifolia Kunth) is a woody bamboo species native from Central and South America, considered as the largest bamboo in America and the third largest in the world [6]. In Colombia, Bamboo guadua grows in the Andean region, between 500 and 1500 meters above the sea level, and ambient temperature between 17°C and 26°C [13,14].

This biomass is a sustainable natural resource that has been used by native and rural communities as construction material or Ąrewood. Taking into account its high growing rate, Bamboo Guadua could represent an interesting material for energy purposes in Colombian rural areas. However, energetic applications of this biomass have not been studied in detail. Reported studies related with bamboo as energy source have been mainly performed with other bamboo species [15Ű17].

Oil palm shells (OPS):

Palm kernel shells are the shell fractions left after the crushing of the kernel nut in the oil palm extraction process from the Eleais Guineensis palm. Taking into account that Colombia has become the largest palm oil producer in Latin America and the fourth largest producer in the world after Indonesia, Malaysia and Thailand [18], the amount of residual wastes produced is not negligible. In Colombia, there are about 475.000 hectares planted with oil palms, with a production of near one million tons of crude palm oil per year (1.85% of the world production).

Accordingly, near 220.000 tons of palm kernel shells are generated as a residue in the country [19]. Due to the huge quantities of residual biomass generated, one of the biggest problems of oil production industry is the management of wastes, which can be extremely harmful if discharged on the land, as they can degrade and pollute soil, water sources, and air. In Colombia, raw palm kernel shells are usually burnt in boilers for steam production, or disposed as a cover for plantation roads; without giving any value to this residue [20].

Evaluation of the gasiĄcation experimental setup

To analyze the gasiĄcation behavior of the selected samples, and the properties of their gaseous and solid by-products, two diferent experimental reactors were considered in a Ąrst stage to perform the gasiĄcation tests:

• An autothermal pilot-scale downdraft reactor at the Colombian National University (Universidad Nacional de Colombia).

• A lab-scale Ćuidized bed reactor at RAPSODEE research center (IMT Mines-Albi).

Both experimental devices are equipped with the required control and monitoring systems to analyze the gasiĄcation process. However, considering that the reactor technologies and scales are diferent, the most appropriate experimental setup should be selected. For this purpose, preliminary tests were performed with both reactors using the three selected feedstocks.

A brief description of both experimental devices is presented below, as well as the advantages and disadvantages identiĄed for each one:

Autothermal pilot scale downdraft reactor

With a height of 2.28 m and an external diameter of 0.34 m, the Ąxed bed downdraft gasiĄer is constructed in steel with an internal refractory lining acting as thermal insulation. Ten K-type thermocouples along the gasiĄer wall allow the monitoring of the temperature proĄle inside the reactor. All the setup is supported by three load cells for weight measurement, indicating the biomass consumption during the process. The temperature and mass change readings are collected using a National Instruments data acquisition system.

According to the design of the gasiĄer, the produced gas outlet is at the bottom of the reactor. A gas conditioning unit is provided downstream to clean the syngas, including a wet cyclone and two Ąber Ąlters that remove tars and particulate matter. A Ćare point on the outlet piping is used to burn gases before being released into the atmosphere and also to check their combustibility. An infrared Cubic Gasboard 3100P gas analyzer is used to determine the content of CO, CO 2 , H 2 , CH 4 and C n H m in the gas. The Ąxed bed downdraft gasiĄer setup and the characteristic temperature proĄle inside the reactor during a gasiĄcation test is presented in Ągure 2.2.

It was possible to observe that unlike the pyrolysis and combustion zone, the temperature in the reduction area, is relatively constant during all the gasiĄcation experiment. From the preliminary experimental tests, the advantages and disadvantages identiĄed for this experimental device are listed below:

Advantages

• Pilot scale reactor: experimental conditions close to those of a real gasiĄcation application.

• Autothermal reactor: No electrical input is required to reach the gasiĄcation temperature and operate the reactor. • Temperature monitoring: The reactor setup allows the complete monitoring of the temperature proĄle along the reactor height, and consequently, the distinction of the diferent reaction zones.

Disadvantages

• Reactor capacity: SigniĄcant amounts of biomass are required for each experimental test (>15 kg).

• Control of gasiĄcation parameters: GasiĄcation parameters like temperature and heating rate cannot be directly controlled and depend mainly on the equivalence ratio (ER) of the process.

• GasiĄcation agent: The steam gasiĄcation atmosphere cannot be accurately controlled, even with a steam feeding point in the gasiĄcation zone.

• Char recovery: The complete reactor disassembly is required for the recovery of char. Additionally, as the reactor zones are not well diferentiated in this kind of reactor, heterogeneous chars from diferent zones may be recovered.

Laboratory scale Ćuidized bed reactor

The Ćuidized bed reactor with a height of 60 cm and the internal diameter 6 cm, is made of stainless steel and is externally heated with an electrical furnance composed of two radiative covers. A porous disk in the bottom holds the biomass sample and allows the gasiĄcation atmosphere to pass through. 10 K-type thermocouples along the gasiĄer height allow the monitoring of the temperature proĄle inside the reactor. The thermal regulation is ensured by an Eurotherm controller connected to a thermocouple in the center of the reactor. The reacting agent and Ćuidizing inert gas are regulated by calibrated Ćow mass controllers.

At the exit of the gasiĄer, a gas conditioning unit including 7 impinger bottles with isopropanol allows the condensation of tars and water. For its part, the volume of produced permanent gases is determined with a gas meter, before being released to the atmosphere. Tedlar®. bags are used for gas sampling in order to analyze their composition (CO, CO 2 , H 2 , CH 4 and C n H m ). For this purpose, a micro-GC (MyGC Agilent) is used.

The Ćuidized bed experimental setup is presented in Ągure 2.3, as well as the temperature proĄle inside the reactor. According to the reactor technology, separate reaction zones cannot be distinguished, unlike the Ąxed bed downdraft gasiĄer. Thus, an homogeneous temperature is observed along the reactor height. The advantages and disadvantages identiĄed for this experimental device are summarized below:

Advantages

• Reactor capacity: Reduced amounts of biomass are required for gasiĄcation experiments (<100 g)

• Control of gasiĄcation parameters: GasiĄcation parameters like temperature, heating rate, and gasifying agent can be accurately controlled, considering that the reactor is externally heated.

• Temperature monitoring: The reactor setup allows the complete monitoring of the temperature proĄle along the reactor height.

• Experimental preparation time: short biomass and reactor preparation times are required prior to the gasiĄcation tests, in comparison to the pilot-scale reactor.

• Char recovery: The recovered gasiĄcation char could be considered homogeneous, as there are not diferent reaction zones inside the reactor.

Disadvantages

• Laboratory scale reactor: Experimental results cannot be directly extrapolated to pilot or industrial facilities.

• Char recovery: Only small char quantities can be recovered from each experiment.

• Mass measurement: The mass consumption during the gasiĄcation experiments cannot be monitored.

From the preliminary tests it was observed that both gasiĄcation systems allow the complete analysis of the gasiĄcation process and resulting by-products. However, in addition to the reactor scale, the main diferences identiĄed were related to the control of the process parameters and the recovery of chars for further analysis.

In particular, several diiculties were encountered to control the gasiĄcation temperature in the autothermal reactor, considering that it depends on the development of the reactions in the combustion zone, and then, on the air quantity supplied to the system. In accordance, a precise air regulation is required. Moreover, in a downdraft reactor the combustion gases act also as gasifying agents, making the accurate control of the reaction atmosphere diicult, even with a steam feeding point in the reduction zone. In contrast, as the Ćuidized bed reactor is externally heated, the gasifying atmosphere and temperature can be easily controlled and modiĄed.

In this regard, taking into account that the accurate control of the process parameters is required for comparison purposes between the feedstocks, the experimental gasiĄcation tests for this study will be performed using the laboratory-scale Ćuidized bed reactor. Moreover, as the selected reactor is smaller than the Ąxed bed gasiĄer, lower biomass quantities are required for the experimental analysis, reducing the time and costs associated to the biomass transport and preparation.

Biomass thermal decomposition behavior and kinetics

Thermogravimetric analysis (TGA) of the studied samples were performed in order to determine their pyrolysis and steam gasiĄcation behavior and kinetics. Regarding the pyrolysis analysis, non-isothermal tests were carried out from 25°C to 800°C, according to the experimental program summarized in Ągure 2.4a. The non-isothermal approach allowed the evaluation of the pyrolysis behavior of the samples in a wide temperature range. In the case of steam gasiĄcation analysis, isothermal tests were performed at diferent temperatures, with three steam partial pressures, as presented in the experimental program in Ągure 2.4b. The heat-up stage is carried out under nitrogen in order to better control the beginning of the gasiĄcation reactions by switching the atmosphere from inert to reactive, when the isothermal regime is established.

In accordance, the analysis procedure of the experimental results should be adapted to the isothermal or non-isothermal approach. In this regard, the kinetic analysis of the pyrolysis and gasiĄcation process is developed using model-free isoconversional methods, described in detail in chapter 3 for biomass pyrolysis, and chapter 6 for steam gasiĄcation.

Steam gasiĄcation experiments 2.5.1 Methodology and experimental plan

As already stated, pyrolysis and steam gasiĄcation experiments were carried out in the semicontinous lab-scale Ćuidized bed gasiĄer described in section 2.3, in order to analyze the gasiĄcation process of the selected feedstocks, and the resulting by-products. Especial attention was given to fuel gases and char. Before the experimental tests, the raw biomasses were ground and sieved to a size range between 2 mm and 4 mm. 80 g of biomass were placed inside the reactor and heated to the gasiĄcation temperature at a heating rate of 20 ◇ C/min, under nitrogen. When the gasiĄcation temperature was reached, the atmosphere was switched to a mixture of H 2 O/N 2 and was maintained during all the gasiĄcation stage. A total Ćow rate of 0.7 m 3 /h was used for all the experiments. Preliminary tests were performed to ensure that particle Ćuidization is attained during the experiments. Information regarding the characterization of Ćuidizing velocity in this reactor could be found by Ducousso [21] and Klinghofer [22]. It is important to note that in this work no inert bed materials were used. After each test, the reactor was cooled down to room temperature under nitrogen. The remaining char was collected and weighted, as well as the impinger bottles and pipes, in order to determine the quantity of water and tars condensed. Considering that the nitrogen Ćow supplied to the process is known and constant for each experiment, the total volume of permanent gases produced can be deduced from the gas volume measured using the gas meter. The experimental program and conditions are presented in Ągure 2.5.

Figure 2.5: Pyrolysis and steam gasiĄcation experimental program

Pyrolysis-only tests were also performed in order to determine the gas, solid, and liquid yields at the end of the heating period (pyrolysis stage) for each gasiĄcation temperature. For all the experimental conditions, a mass balance was established in order to determine the product yield associated to the pyrolysis and gasiĄcation stage, according to Ągure 5.3. More speciĄcally, the gas, liquid, and solid mass measured for the pyrolysis tests are subtracted from the gasiĄcation experiments to obtain the product yields associated only to the gasiĄcation stage.

For all the experimental conditions, gas and char samples were collected for further analysis. In particular, a calibrated MyGC Agilent micro-GC was used to determine the produced gas composition and heating value.

Regarding the gasiĄcation chars, the characterization plan and techniques used to determine their physico-chemical characteristics are presented in section 2.5.2. The analysis procedure of the steam gasiĄcation experimental results is detailed in chapter 5

Biomass and steam gasiĄcation char characterization

In order to understand the impact of biomass characteristics on the steam gasiĄcation chars properties and possible valorization pathways, the complete physico-chemical characterization of both raw biomass and char is needed. For this purpose, diferent analytical techniques were used.

In the case of the raw feedstocks, the knowledge of their organic and inorganic composition is of great interest, as well as their caloriĄc value. Accordingly, the biomass characterization methods are presented in Ągure 2.7.

Likewise, regarding the gasiĄcation chars, their organic and inorganic composition, as well as their textural properties, structure, and surface chemistry were analyzed and compared. The characterization plan and techniques used are summarized in table 2.3.

In accordance to the manuscript structure, the experimental procedures and devices used for each presented technique are precisely detailed in the corresponding chapter, indicated in the last column of the table. In particular, the techniques described in chapter 6 give a general insight to the properties of the steam gasiĄcation chars produced from lignocellulosic agrowastes, and contribute to the understanding of the impact of the process parameters and biomass characteristics on the gasiĄcation chars. For their part, the techniques applied in chapter 7 are mostly associated to the establishment of the potential of the analyzed materials to be used in environmental and agricultural applications.

Summary

This chapter presented the materials and experimental techniques used in this research study. In accordance, the general methodology employed for the development of the experimental activities is illustrated in Ągure 2.8. The precise procedures and devices are detailed in the corresponding chapter, as well as the analytical methods used to treat and exploit the obtained results.

Abstract

The thermal behavior of three Colombian agricultural residues was studied by nonisothermal thermogravimetric analysis (TGA) at various heating rates. An approach using a combined kinetics parallel reaction model and model-free isoconversional methods proved to be suitable to determine the pyrolysis kinetic parameters of biomasses with diferent macromolecular composition and H/C and O/C ratios near 1.5 and 0.8 respectively. Fraser-Suzuki functions representing the derivative TGA (DTG) of hemicellulose, cellulose and lignin showed a very good agreement with the experimental data. The calculated apparent activation energy of biomass pseudo-components evidenced no de-pendence on the reaction extent in all the conversion range, validating the use of master plots for decomposition mechanism identiĄcation. Pseudo-hemicellulose, pseudo-cellulose and pseudo-lignin showed to be close to a second order kinetic model, a random scission or an Avrami-Erofeev model, and a high order kinetic model, respectively. Comparing the three feedstocks, the apparent activation energy of the pseudo-components was in the order: bamboo guadua < coconut shells < oil palm shells . The results show that even when samples elemental composition is very similar, macromolecular constituents, in particular lignin, could have an impact in the biomass decomposition rate and apparent activation energy. For the three studied materials, the model Ątting error below 10% showed that the calculated kinetic parameters are suitable for the description and prediction of the biomasses thermal decomposition.

Introduction

Thanks to their climate variety, tropical regions have a great biodiversity and the appropriate conditions for the development of agroindustrial activities. Agroindustry produces large amounts of low cost residues, which can be used for biofuels production or transformed in value added products. However, in most tropical developing countries, these residues are not valorized and represent an environmental risk, as they are not always disposed properly. According to diferent studies, the agro-residues potential in developing countries is higher than 2900 million tons per year [1]. In Colombia, according to the Colombian Energy and Mining Planning Unit -UPME, more than 70 million tons of wastes are produced every year from agroindustrial activities [2]. In this context, the valorization of these residues from an energetic point of view or for the production of value-added products represents an alternative for their treatment and disposal, and for strengthening the local economy.

Three Colombian lignocellulosic biomasses were chosen for this study: oil palm shells (OPS), coconut shells (CS) and bamboo guadua (BG). OPS are the shell fractions left after the crushing of the kernel nut in the oil palm extraction process from the Eleais guineensis palm. This biomass is considered an important waste of the Colombian oil palm extraction industry, with near 220.000 tons generated every year [2]. In the country, raw oil palm shells are usually burnt in boilers for steam production in palm oil facilities, or disposed as a cover for palm plantation roads, without giving any value to this residue [3].

Furthermore, CS from cocos nucifera palm, are considered a material with little or non-economic value from the Colombian alimentary industry, and have been traditionally used for handicraft making or discharged in soils. The amount of this residue is not negligible in the country, considering that near 119.000 tons of this fruit are produced in Colombia every year, according to the statistics from the Food and Agriculture Organization of the United Nations [4].

Finally, BG Ű Guadua angustifolia Kunth, is a native woody bamboo species from South and Central America [5,6]. It has been traditionally used in Colombia as a construction material or Ąrewood. With a high growing rate, BG could represent an interesting material for energy purposes in Colombia and other tropical countries, either from crops or as a residue from the construction industry. Energetic applications of this biomass have not been studied in detail. Reported studies related to bamboo as energy source have been mainly performed with other bamboo species [7Ű9].

Considering that the three selected biomasses have a heating value above 18 MJ/kg and a great availability [5,10,11], it could be interesting to explore their potential to be used as a source of energy, or as a feedstock for the production of value-added materials in Colombia and other tropical countries.

Pyrolysis is the Ąrst step in the conversion of biomass into biofuels, adsorbent materials, and value added chemicals [12Ű15]. This thermochemical conversion process, constitutes the previous stage of combustion and gasiĄcation, and should be properly understood for the valorization of diferent materials through thermochemical conversion processes. In particular, the knowledge of the pyrolysis kinetic parameters is very important to analyze the thermal decomposition process of residual biomasses, predict their conversion, and determine their possible valorization pathways.

Thermogravimetric analysis (TGA) has been widely used for the study of lignocellulosic biomass decomposition kinetics, either from isothermal and non-isothermal approaches.

Several pyrolysis kinetic studies using non-isothermal data have focused on model-free methods to estimate the biomass kinetic parameters, as they allow the evaluation of the activation energy without knowing the decomposition reaction model [16,17]. However, considering that the biomass pyrolysis is an extremely complex process due to the morphologies of its components and their interaction, the model free-procedure is not always suicient for the identiĄcation of the complete kinetic triplet [18].

Most biomass pyrolysis analysis have reported diferent activation energy values for several kinds of lignocellulosic biomasses, usually considering only a global decomposition approach [19Ű22]. Works based on model-free isoconversional methods, showed that there is a clear dependence of activation energy on the reaction extent, suggesting that the pyrolysis process includes many diferent reactions occurring at the same time. Under these circumstances, an interesting alternative for the analysis of complex conversions is a parallel reaction approach, separating the individual processes by derivative TGA (DTG) peak deconvolution [23], followed by the kinetic analysis of the resulting individual curves. Few studies have been reported for biomass pyrolysis kinetic analysis using DTG deconvolution [24Ű26]. In particular, bamboo guadua, coconut shells and oil palm shells kinetic analysis using this alternative approach has not been reported yet.

According to this, the aim of this work is to study the pyrolysis characteristics and kinetics of three tropical lignocellulosic biomasses, from a non-isothermal thermogravimetric analysis. A three parallel reaction model was employed, using a DTG peak deconvolution procedure, followed by a model-free isoconversional approach and generalized master plots. The inĆuence of the biomass nature in its thermal decomposition characteristics was discussed, comparing the calculated kinetic parameters of the selected materials.

Materials and methods

Materials

Tropical feedstocks used in this study were collected in Colombia, South America. Oil palm shells were provided by a palm oil extraction plant in the Meta region (N4°16 ′ 0 ′′ O73°29 ′ 0 ′′ , 500 meters above sea level, average temperature 27°C and 2858 mm of precipitation throughout the year). Coconut shells from coconuts coming from the Nariijo region (N1°10 ′ 0 ′′ O77°16 ′ 0 ′′ , 0 to 400 meters above the sea level and 28°C of average temperature) were provided by a food processing industry. Both OPS and CS are considered as an industrial residue. For its part, 3 to 4 years old bamboo guadua coming from a bamboo forest in the Quindío region (N4°32 ′ 0 ′′ O75°42 ′ 0 ′′ , 800 to 1200 meters above sea level and 23°C of average temperature) was obtained from a furniture and handicraft construction site, where biomass was indoor stored.

Raw materials were milled and sieved to a size range of 1mm to 2mm before thermogravimetric analysis. CHNS composition of samples was determined using a Themoquest NA 2000 elemental analyzer. Moisture, volatiles and ash content were calculated according to the standards EN ISO 18134-3, EN ISO 18123 and EN ISO 18122, respectively. The high heating value was calculated using an IKA C 5000 automated bomb calorimeter. The samples chemical composition and heating value are presented in table 3.1 as an average of three replicates. Molecular composition of the biomasses is referred to literature reported values [27Ű29].

Thermogravimetric analysis

Thermogravimetric analysis (TGA) of selected materials were performed using a Mettler Toledo TGA 2 SF analyzer. Approximately 30 mg of each sample were placed in an alumina crucible and heated from 25°C to 800°C, with 2, 5, 10 and 20°C/min as heating rates. Experiments were conducted under a nitrogen atmosphere, using a Ćow rate of 50 ml/min. To verify the repeatability of TGA experiments, each run was conducted two times and averaged; then, the mean standard deviation was calculated. A blank experiment was made for each heating rate to exclude buoyancy efects. Experimental runs were Ąrst performed at 2°C/min, followed by 5°C/min, 10°C/min and 20°C/min. Duplicates were done following the same order described. For each experimental condition, the standard deviation was below 0.6% for the investigated temperature range. This value was considered reasonable to ensure the repeatability of the obtained mass loss curves.

Kinetic study

Theoretical background

Thermogravimetric analysis has been extensively used to study the kinetics of biomass pyrolysis and to determine the reaction mechanisms controlling this process. Generally, the basis of a kinetic study is a series of experiments where the degree of conversion of a material is measured as a function of time and temperature. According to this, the degree of conversion or reaction extent is usually deĄned as in Eq. 3.2.1.

Ð = 0 - 0 - (3.2.1)
Where 0 is the initial mass of the sample, the Ąnal mass, and the current mass at a given temperature or time. The reaction rate Ð/ depends on the temperature and the degree of conversion Ð, as shown in Eq. 3.2.2. In this expression, (Ð) is the reaction model function representing how the solid state decomposition process occurs, and ( ) the Arrhenius equation, representing the temperature dependence of the process.

Ð = ( ) (Ð) (3.2.2) ( ) = exp ⎤ - ⎣ (3.2.3)
From the Arrhenius equation (Eq. (3.2.3)), the reaction rate expression can be rearranged as follows:

Ð = exp ⎤ - ⎣ (Ð) (3.2.4)
Finally, for linear non-isothermal thermogravimetric analysis, the reaction rate is expressed as in Eq. 3.2.5, where Ñ is the heating rate used to perform the experiment in K/min.

Ð = Ñ exp ⎤ - ⎣ (Ð) (3.2.5)
In the case of biomass decomposition, the most commonly accepted reaction model functions (Ð) and their integral forms are presented in table 3.2.

Three parallel reaction model

Thermal decomposition of lignocellulosic biomass is quite complex, considering that it is constituted by diferent components, mainly hemicellulose, cellulose and lignin.

Model (Ð) (Ð)
Order based

Or1 -First order 1 -Ð -ln(1 -Ð) Or2 -Second order (1 -Ð) 2 [1/(1 -Ð)] -1 Or3 -Third order (1 -Ð) 3 (1/2)  1/(1 -Ð) 2  -1 Difusion D1 Ű One dimensional 1/(2Ð) Ð 2 D2 ŰTwo dimensional [-ln(1 -Ð)] ⊗1 Ð + (1 -Ð) ln(1 -Ð) D3 Ű Three dimensional (3/2)(1 -Ð) 2/3  1 -(1 -Ð) 1/3  ⊗1  1 -(1 -Ð) 1/3  2 D5 Ű Three dimensional (3/2)(1 -Ð) 4/3  (1 -Ð) ⊗1/3 -1  ⊗1  (1 -Ð) ⊗1/3 -1  2 Power law Pn Ű Power law (Ð) (⊗1)/ Ð 1/

Nucleation and growth

An Ű Avrami Erofeev (1 -Ð) [-ln(1 -Ð)] (⊗1)/ [-ln(1 -Ð)] 1/ Geometrical contraction R2 Ű Contracting area 2(1 -Ð) 1/2 1 -(1 -Ð) 1/2 R3 Ű Contracting volume 3(1 -Ð) 2/3 1 -(1 -Ð) 1/3 Random scission L2 Ű Random scission L=2 2(Ð 1/2 -Ð) -
Accordingly, pyrolysis can be described with a parallel reaction model (PRM), assuming that the three main components of the biomass react independently [31]. Three pseudocomponents representing the hemicellulose, cellulose and lignin are then considered in this approach. The total reaction rate is expressed as the addition of each pseudocomponents reaction rate, as in Eq. 3.2.6.

Ð = 3  =1 Ð (3.2.6)
It is important to note that modeled pseudo-components do not represent the real proportion of biomass constituents, as interactions between them can exist [32]. However, the pseudo-components proportion should be in accordance with each biomass molecular composition.

In order to model these three parallel reactions, the biomass DTG curves were deconvoluted, representing each pseudo-component reaction rate as a mathematical function of temperature. Global DTG curves were then considered as the addition or overlap of pseudo-components DTG proĄle. Gaussian, Lorentzian and Fraser-Suzuki functions were used in this study.

Gaussian function [26]:

⎤ Ð ⎣ = exp  - 1 2 ⎤ - ⎣ 2 ⟨ (3.2.7)
Where is the amplitude in 1/K; is the peak temperature in K, and the width of the curve in K.

Lorentzian function [26]:

⎤ Ð ⎣ = 1 +  ⊗ ⎡ 2 (3.2.8)
Where is the amplitude in 1/K; is the peak temperature in K, and the width of the curve in K.

Fraser-Suzuki function [33]:

⎤ Ð ⎣ = exp  - ln 2 2 ln ⎦ 1 + 2 ⎤ - ⎣⎢ 2 ⟨ (3.2.9)
Where is the amplitude in 1/K; is the peak temperature in K, the half width of the curve in K, and is the asymmetry of the curve.

Least squares method and an optimization algorithm were used to determine the function parameters that best Ąt each biomass experimental decomposition proĄle. The Ąt error was determined with equation 3.2.10, where Ð/ are the experimental and calculated values of the decomposition rate, and is the total number of experimental points [32].

According to this, the smaller the Ąt error, the better Ąt to the experimental data.

(%) = 100

        =1 ⎤  Ð i ⎡ -  Ð i ⎡ ⎣ 2 √  Ð i ⎡ ,       (3.2.10)
Once the best Ąt was determined, isoconversional model-free methods were used for kinetic analysis, as presented in section 3.2.3.

Isoconversional model-free approach

In this study, diferent isoconversional model-free methods were used to determine the Arrhenius parameters from experiments performed with four diferent heating rates. In particular, three isoconversional methods were compared: Friedman, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS). Friedman is deĄned as a diferential method, while FWO and KAS are integral methods [18,34]. All of them are well known and widely used for thermal decomposition kinetics analysis. The activation energy value at a constant Ð can be calculated from the slope of the isoconversional Arrhenius plots, according to the expressions summarized in table 4.3.

Ú(Ð) = (Ð) (Ð) Ð=0.5 = Ð/ (Ð/) 0.5 exp( / ) exp( / 0.5 ) (3.2.11)
If the apparent value of does not vary in a signiĄcant way with Ð, the process can be described by a single step kinetics and then, generalized master plots can be used to determine the most suitable reaction mechanism associated and as a result, the pre-exponential factor . The reduced-generalized reaction rate expression in equation 3.2.11, needs the previous knowledge of activation energy value [35]. The most suitable (Ð) model can be identiĄed as the best match between the experimental Ú(Ð) values and the theoretical master plots presented in Figure 3.1 [36].

Table 3.3: Kinetic model-free methods used in this study

Method Expression

Results and discussion

Biomass composition and thermal decomposition characteristics

In reference to the biomasses chemical composition presented in table 3.1, it is possible to observe that the three selected materials have similar C and H contents, with H/C ratio between 1.5 and 1.7. In contrast, the molecular composition has remarkable diferences, particularly related to lignin content. OPS and CS are endocarp biomasses with relatively high lignin content (50% and 36% respectively). In contrast, BG has lignin content below 25% and a high proportion of cellulose (> 50%).

Regarding the thermal decomposition behavior, the TG and DTG curves of biomasses are presented in Fig. 3.2. For the three samples, it is possible to distinguish four decomposition stages. The Ąrst one (stage I), registered before 200°C is related to the moisture and light volatiles release. The second and the third one (stage II and stage III), from 200°C to 330°C, and from 300°C to 380°C, correspond mainly to the hemicellulose and cellulose decomposition; and the last one (stage IV), from 380°C, is mainly related to lignin decomposition. These decomposition ranges found are in accordance with reported values for cellulose, hemicellulose, lignin, and other lignocellulosic biomasses [12,37]. It is possible to notice from the TG curves that the mass loss in each stage is in agreement with the fraction of hemicellulose, cellulose and lignin of the samples. In a dry basis, the three biomasses, with similar hemicellulose content, showed a similar mass loss during the second decomposition stage. This value, higher than their reported hemicellulose composition was expected, taking into account that during this stage some light volatiles can also be released. Regarding the third stage, BG showed the greatest mass loss, in accordance with its high cellulose content (53.9%). In the same way, the comparison of the TG curves, presented in Fig. 3.2d showed that the char yield at the end of the test was diferent for each biomass. OPS solid residue was 33%, while CS and BG solid yield was 28% and 19% respectively. These diferences could be related to the molecular composition of the samples. It is known that lignin contributes in an important way to the solid yield in biomass decomposition [12], explaining the fact that OPS, with the highest lignin content, registered the lowest mass loss, followed by CS and BG.

Moreover, signiĄcant diferences can be observed between the DTG curves. In the case of OPS and CS, two distinct peaks can be identiĄed for hemicellulose and cellulose decomposition; while for BG, the hemicellulose decomposition is represented by a shoulder next to the cellulose peak. It should also be noted that as lignin decomposition range occurs over a wide temperature range from 150°C to 800°C [38], no speciĄc lignin peak could be distinguished. Regarding the cellulose decomposition rate, endocarp biomasses showed lower values compared with BG. Taking into account that lignin is the binding agent of biomass Ąbers, higher lignin contents could be related to lower cellulose decomposition rates, and with the well diferentiated decomposition peaks for hemicellulose and cellulose. In relation to this, Lui et al [39], studied the interaction between biomass components during pyrolysis. They concluded that lignin has a strong efect in hemicellulose and cellulose decomposition. Mendu et al. [40] also found that high lignin biomasses show well diferentiated peaks for hemicellulose and cellulose decomposition.

DTG curves deconvolution

Biomass DTG curves were deconvoluted, representing each pseudo-component with Gaussian, Lorentzian and Fraser Suzuki functions. As Gaussian and Lorentzian functions are symmetric, they were particularly inadequate to Ąt the OPS and CS decomposition patterns, with an error greater than 13% and 15% respectively.

In contrast, Fraser-Suzuki function allowed the Ątting of asymmetric curves, giving a good agreement with experimental data. In all cases, the Ąt error was lower than 3%. Parejon et al [23], found that Fraser-Suzuki function is the mathematical algorithm that better Ąts the decomposition rate patterns of complex processes. Fig. 3.2 presents the results of the Fraser-Suzuki deconvolution Ątting of BG, CS and OPS at a heating rate of 10 °C/min. Table 3.4 summarizes the Ąnal parameters that better Ątted each experimental data set.

From the deconvolution results, the pseudo-hemicellulose, pseudo-cellulose and pseudolignin fractions of biomass samples were calculated. BG values were 32%, 49% and 19%, respectively. CS values were 28%, 29% and 42%; and Ąnally, OPS values were 26%, 24% and 50%. It should be noted that even when modeled pseudo-components do not represent the real proportion of biomass constituents, their fractions show the diferent nature of the studied samples in terms of their molecular composition.

Kinetic analysis

For the selected biomasses, decomposition rate curves (Ð/ Vs ) of each pseudocomponent were analyzed using isoconversional model-free methods, in order to determine their kinetic triplet , and (Ð). Friedman, KAS and FWO Arrhenius plots for OPS, CS and BG pseudo-components showed a good linear Ąt in all the conversion range between 0.1 and 0.9. In all cases, 2 values were higher than 0.9871, as presented in table 3.5, where the maximum and minimum 2 values are summarized. The high correlation coeicients 2 obtained, suggest that the three isoconversional methods used are reliable and accurate for the apparent activation energy calculation. It is possible to observe that in all cases the dependence of on Ð is quite low. The apparent activation energy remained almost constant in all the conversion range, indicating that pseudo-components thermal decomposition follow a single stage process and no complex reactions occur [30]. In relation to this, table 3.6 summarizes the mean activation energy value found for the three biomasses pseudo-components, using the described isoconversional methods. It should be noted that the relative standard deviation of the activation energy was always lower than 8% (à = 13.5 kJ/mol), with values even below 2% (à = 3.2 kJ/mol). Friedman, KAS and FWO approaches gave similar apparent activation energy values with absolute deviation below 11% in all cases. These results show that all the three methods are convenient for the calculation of the activation energy of the samples pseudo-components decomposition. In particular, it can be said that for biomasses with H/C and O/C near 1.6 and 0.8, any of the presented methods is suitable for the determination of pyrolysis kinetic parameters, despite the diferences in hemicellulose, cellulose and lignin fractions and their decomposition behavior. The highest absolute deviation between the methods was found for the mean Ea calculated with Friedman and FWO for OPS pseudo-hemicellulose (22.4 kJ/mol Ű 10.5%). In contrast, the results obtained with KAS and FWO were very close, with deviations below 1%. These diferences are related to the mathematical approach of the isoconversional methods and the treatment of the experimental data, considering that Friedman is a diferential method while FWO and KAS are integral.

From table 3.6 it is also possible to notice that for the three biomasses, pseudocomponents decomposition followed nearly the same behavior, as is in the order: pseudo-hemicellulose < pseudo-cellulose < pseudo-lignin. Keeping in mind that is the minimum energy required to start a reaction, the lower pseudo-hemicellulose value means that this component degrades easier than the two others.

Accordingly, it is possible to see from Fig. 3.4 that pseudo-hemicellulose decomposition starts at a lower temperature than that of pseudo-cellulose and pseudo-lignin. For its part, pseudo-lignin decomposition over a large temperature range indicates that this component degrades slowly, and is harder to decompose than pseudo-hemicellulose and pseudo-cellulose. The high values associated with pseudo-lignin could be related to its aromatic nature and the fact that this component is the cementing agent of biomass Ąbers. Lignin is a complex three-dimensional polymer with a large variety of chemical functions which difer in thermal stability and decompose in a broad temperature range [41], interacting with cellulose and hemicellulose. These interactions during biomass decomposition may explain the fact that calculated pseudo-lignin activation energy is higher than isolated lignin reported values, which can be in the range of 37 kJ/mol to 160 kJ/mol, depending on the analyzed lignin type [42,43].

The apparent activation energies obtained with the three employed methods were in accordance with diferent biomass pseudo-component values in the literature. Reported pseudo-hemicellulose activation energy is between 86 kJ/mol and 180 kJ/mol; pseudocellulose between 140 kJ/mol and 210 kJ/mol; and pseudo-lignin between 62 kJ/mol and 230 kJ/mol [24, 44Ű47]. However, it can be observed that there are some diferences between the three studied biomass samples. In particular, OPS is the material that presented the highest activation energy for the three pseudo-components, followed by CS and then by BG. OPS pseudo-hemicellulose is near 15% and 20% higher than CS and BG in that order. OPS pseudo-cellulose value is greater than CS and BG in around 10% and 17%; and pseudo-lignin value in around 5% and 10% respectively.

This behavior could be possibly explained by the interactions between the biomass components and structure, during thermal decomposition. As the binding agent for biomass structure, lignin could have an impact in the required energy to decompose hemicellulose and cellulose. In relation to this, OPS have the highest lignin content between the studied biomasses and presented the highest values; while BG has the lowest lignin content and the lowest for the three pseudo-components. Thus, it is possible to infer that even when the three studied materials are mainly constituted by the same components, biomass structure plays a role in their decomposition characteristics [48,49].

According to this, the most suitable decomposition reaction model for each biomass pseudo-component was determined using the generalized master plots procedure, which is valid only for single stage process analysis, where there is no dependence of on Ð [18]. Experimental data were normalized to Ð=0.5 using Eq. 3.2.6, and compared with theoretical master plots in Fig. 3.1. Activation energy in Eq. 3.2.6 is the mean value calculated for each pseudo-component using the mentioned three isoconversional methods.

Fig. 3.5a, shows that for the three biomasses, pseudo-hemicellulose matches closely the theoretical plot of a second order kinetic model (Or2), except at low conversion (Ð < 0.2), where the decomposition model of OPS and CS pseudo-hemicellulose is between Ąrst order and second order. For its part, BG is close to a second order kinetic model in all the decomposition range. These diferences at low conversion could be possibly due to interactions between the hemicellulose and the other biomass components. Similar approaches to other types of lignocellulosic biomasses have concluded that pseudohemicellulose decomposition follows an order based kinetic model with between 1.5 and 4 [50].

Master plots in Fig. 3.5b, show that pseudo-cellulose decomposition is in agreement with a random scission or an Avrami Erofeev kinetic model, for the three studied biomasses. Notably, BG pseudo-cellulose matches better with an A1.5 Avrami Erofeev nucleation and growth model, while OPS and CS pseudo-cellulose are closer to a L2 random scission model. Theoretical master plots of both models are close and are related to narrow reaction proĄles, as seen in Fig. 3.4, where pseudo-cellulose decomposition range is narrower compared with pseudo-hemicellulose and pseudo-lignin. In general, Avrami Erofeev models assume that reaction or decomposition is due to the appearance of random nuclei and their subsequent growth; while random scission is related to the arbitrary break of polymer chains into smaller segments [30,51]. As the shape of both models is very close, it is not easy to consistently distinguish between them, in spite of the diferences in their theoretical background. According to this, from a mathematical point of view, both of these models could describe the pseudo-cellulose decomposition mechanism. Similar studies reported for cellulose in the literature, have concluded that both Avrami-Erofeev and random scission models could be suitable for the description of the cellulose thermal decomposition [22,36,52].

Finally, as seen in Fig. 3.5c, pseudo-lignin decomposition does not match with any known theoretical kinetic model. Due to the complexity of the lignin structure and its interactions with the other biomass components [45,49,50], it is not easy to fully understand its decomposition mechanism. As the cementing agent of biomass, lignin could interact with hemicellulose and cellulose in diferent ways according to the biomass structure and operating conditions. Moreover, lignin decomposes in a wide range of temperature and with low decomposition rate, making it diicult to completely model its corresponding reaction mechanism. At low conversion (Ð<0.5), pseudo-lignin decomposition could be modeled by an order reaction mechanism, with n between 11 and 12 (Fig. 3.5c). From Ð=0.5 to Ð=0.9, decomposition is near a three dimension difusion mechanism. Particularly, a D5 model (Zhuravlev, Lesokin, Tempelman), as presented in Fig. 3.5d. Other reported studies have described pseudo-lignin decomposition with a third order reaction model [24], or even a high order model with > 12 [22]. The proposed models, however, do not Ąt completely the pseudo-lignin the decomposition, due to its complexity.

With the knowledge of the most suitable reaction model for each pseudo-component, pre-exponential factor A values were calculated. Taking into account that the diference between the apparent activation energy calculated with the Friedman, KAS and FWO methods is not signiĄcant, of each biomass pseudo-component was deĄned as the mean value of the activation energy estimated with the three methods. Furthermore, Friedman method was chosen for the evaluation of the pre-exponential factor.

The dependence of ln() on Ð, presented in Fig. 3.6, is similar to that of the activation energy, remaining almost constant during all the conversion range (0.1 < Ð < 0.9). The relative standard deviation was in all cases inferior to 8% with values of even 2%. Accordingly, the mean values of and for the three biomasses are summarized in table 3.7. These kinetic parameters were validated and used to reproduce the experimental decomposition curves of each biomass from 2 to 20°C/min. For the three studied materials, a good agreement was found between the experimental data and the computed decomposition behavior, with Ątting errors below 10% in all cases. Fig. 3.7 shows the modeled decomposition rate of the three biomasses and pseudo-components at a heating rate of 10°C/min. 3.7 for biomasses and their pseudo-components.

It is possible to observe from this Ągure that there is a gap between experimental and modeled data at temperatures above 400°C, for the three biomasses. Comparing these results with Fraser-Suzuki Ątting presented in Fig. 3.4, it can be noticed that the behavior of modeled pseudo-hemicellulose and pseudo-cellulose is in good agreement with the deconvolution patterns initially proposed. As a result, a good agreement is also obtained between modeled and experimental decomposition at temperatures below 400°C. In contrast, it is clear that the main diferences observed are related to pseudo-lignin. This is principally due to the complexity of modeling the pseudo-lignin decomposition behavior in the investigated temperature range. In spite of this, the Ątting error below 10% for the three materials showed that the calculated kinetic parameters using model-free isoconversional methods are suitable for the description of the biomasses thermal decomposition.

Conclusion

In this study, the thermal decomposition of three lignocellulosic biomasses with diferent macromolecular composition but nearly the same H/C and O/C fraction was investigated. Bamboo guadua (BG), coconut shells (CS) and oil palm shells (OPS) were used.

In general, the approach presented in this paper using a parallel reaction model using Fraser-Suzuki deconvolution and model-free isoconversional methods proved to be suitable to determine the pyrolysis kinetic parameters of biomasses with H/C and O/C near 1.5 and 0.8. Despite the diferences in the hemicellulose, cellulose and lignin fraction of biomasses, any of the presented isoconversional methods can be used to calculate and predict the pseudo-components activation energy and pre-exponential factor, and to estimate their decomposition mechanism. This information could constitute a valuable tool for reactors design and for the development and scale-up of pyrolysis and gasiĄcation processes using tropical lignocellulosic agrowastes as a feedstock.

The apparent activation energy of biomass pseudo-components, followed the same behavior for the three studied materials: pseudo-hemicellulose < pseudo-cellulose < pseudo-lignin . Regarding the decomposition mechanism, pseudo-hemicellulose and pseudo-cellulose were well described by a second order model, and a random-scission or an A1.5 Avrami-Erofeev model, respectively. For its part, pseudo-lignin decomposition did not completely match with any known theoretical model, and was described by a combination of a high order model with n between 11 and 12, and a third dimension difusion model. Lignin behavior was possibly due to the complexity of its structure and to the interactions with the other biomass components.

Diferences between the calculated kinetic parameters of BG, CS and OPS showed that biomass structure and molecular composition play a role in the biomass decomposition characteristics. Considering the nature of lignin, it could have an impact in the required energy to decompose hemicellulose and cellulose. The apparent activation energy for the three biomass pseudo-components followed the order BG < CS < OPS ; with OPS and BG being the highest lignin and lowest lignin content biomasses analyzed in this study, respectively. For the three studied materials, the model Ątting error below 10% showed that the calculated kinetic parameters using model-free isoconversional methods are suitable for the description and prediction of the biomasses thermal decomposition.

4

Chapter

Steam gasiĄcation behavior of tropical agrowastes: a new modeling approach based on the inorganic composition

Abstract

The steam gasiĄcation and co-gasiĄcation reactivity and kinetics of coconut shells, oil palm shells and bamboo guadua were studied from an isothermal thermogravimetric analysis, with temperatures ranging from 750°C to 900°C, and steam partial pressures from 3 to 10 kPa. In the analyzed experimental range, inorganics were identiĄed as the most inĆuential parameter in biomass reactivity and kinetics. Accordingly, a new modeling approach is proposed to predict the gasiĄcation behavior of lignocellulosic agrowastes based on their inorganic composition. A good agreement between the experimental and modeled data was found, showing that the proposed approach is suitable for the description and prediction of the gasiĄcation behavior of biomasses with diferent macromolecular structure and within a wide range of inorganic composition, and H/C and O/C ratios near 1.5 and 0.8 respectively. This kinetic model could constitute a valuable tool for reactor design and scale-up of steam gasiĄcation facilities using tropical lignocellulosic feedstocks.

Introduction

Tropical regions are rich in biodiversity thanks to their geographic location and climate conditions. In this regard, they are appropriate for the development of agricultural activities and cultivation of a great variety of crops. Several developing countries in tropical regions base their economy in agriculture and farming and produce great amounts of agro-wastes that usually remain under-exploited. These residues could be valorized as biofuels or transformed into value-added products, giving new development opportunities for local communities.

In this regard, gasiĄcation is a very interesting thermochemical process for the recovery of energy from agrowastes. In particular, steam gasiĄcation produces high heating value fuel gases that can be used for the generation of heat and power [1Ű4]. However, the valorization of agrowastes could have some restrictions. One of the most important is probably the fact that agricultural residues availability often depends on seasonal crops. Consequently, most gasiĄcation facilities should operate intermittently, or work with diferent kind of residues or even blends.

Several authors have highlighted the diferences in the gasiĄcation behavior of chars from diferent biomasses. In particular, the inorganic elements are reported to be the most inĆuential parameter in the steam gasiĄcation reactivity and conversion proĄle [5,6]. Alkali and alkali earth metals (AAEM) like K, Na, Ca and Mg, which are present in indigenous biomass could have a catalytic efect on biomass gasiĄcation [7]. Among these elements, K has been reported to be the most active species for steam and CO 2 gasiĄcation of charcoal and biomass [8,9]. In contrast, Si, Al or P may inhibit this catalytic efect, as they tend to react with AAEM [10,11].

Most studies related with the impact of inorganics in the gasiĄcation behavior of biomasses use impregnation techniques to modify the composition of the samples [8, 12Ű15]. In general, these treatments allow a good understanding of the efects of inorganics on gasiĄcation. However, the behavior of impregnated inorganic elements may difer from the one of inorganic elements in their natural form and distribution in biomass. In this regard, Dupont et al. [16] studied 21 samples of woody biomasses, conĄrming the beneĄcial impact of K and the inhibitory efect of Si in the indigenous biomass. Generally, authors analyze the gasiĄcation behavior of biomasses individually. Nevertheless, most gasiĄcation applications in developing countries should work with diferent kind of residues or blends depending on their availability. In this context, the understanding of the impact of biomass characteristics and blends interactions in their gasiĄcation behavior is important in order to properly adapt the process parameters and conditions to the application.

To describe the steam gasiĄcation behavior of biomasses, several authors have proposed diferent kinetic approaches, some of them incorporating the impact of inorganic elements. In this regard, Kajita et al. [17] considered that gasiĄcation occurs in a parallel reaction scheme and developed a dual Langmuir-Hinshelwood model, identifying K as the main catalytic element. This study highlighted the beneĄcial impact of K, but did not deal with other inorganic elements present in biomasses. For their part, Zhang et al. [10] analyzed the inĆuence of AAEM and Si in the gasiĄcation behavior of diferent biomasses. They proposed a modiĄed random pore model, introducing two dimensionless parameters that depends on the inorganic content of samples. Even when the calculation method of these parameters was not explicitly presented, the authors concluded that the gasiĄcation reactivities of biomasses are governed mainly by the amount of inorganic species. More recently, Dupont et al. [6] proposed a kinetic approach with two diferent kinetic laws for biomasses with inorganic ratio K/(Si+P) > 1 and K/(Si+P) < 1. A zeroth-order and a volumetric Ąrst-order model were found to describe the behavior of samples, respectively. Furthermore, in the case of biomasses with K/(Si+P) > 1, the authors suggest that the gasiĄcation kinetics can be predicted simply through the knowledge of the K content of the sample. Even when the proposed approach in this work can satisfactorily estimate the behavior of biomasses within a wide range of inorganic compositions, the conversion of samples with intermediate values of inorganic ratio (K/(Si+P) ∼ 1) is not completely described. The authors stated that the decomposition of these biomasses can be estimated either with a zeroth or a Ąrst order law. Additionally, concerning the analysis of the simultaneous steam gasiĄcation of diferent kind of biomasses and their kinetic modeling, no studies have been found in the literature until now.

Accordingly, the aim of this work is to study the steam gasiĄcation behavior of three tropical lignocellulosic biomasses and their blends, from an isothermal thermogravimetric analysis. The inĆuence of the gasiĄcation temperature and steam partial pressure on the gasiĄcation reactivity of biomasses was discussed. Also, the impact of biomasses and blends composition on the gasiĄcation reactivity and kinetics was analyzed. An approach using model-free isoconversional methods and generalized master plots was used to determine the gasiĄcation kinetic parameters of the samples and compare their decomposition behavior. As a result, a new model with a unique kinetic equation is proposed to describe and predict the steam gasiĄcation behavior of tropical lignocellulosic agrowastes based on their inorganic composition.

Materials and methods

Biomass samples

Three tropical lignocellulosic feedstocks were selected for this study: oil palm shells (OPS), coconut shells (CS) and bamboo guadua (BG). The samples were collected in Colombia, South America, and were provided by a palm oil extraction plant, a food processing industry, and a furniture and handicraft construction site, respectively. The origin of selected samples has been detailed on a previous work [18]. The chemical composition of the biomasses was determined according to the standards of solid biofuels with at least three replicates, and is presented in tables 4.1 and 4.2. The raw biomasses were milled and sieved to a size range between 100 µm and 150 µm before TGA. The characteristic time analysis of the experiments showed that under the presented conditions, limitations by heat or mass transfer can be neglected. Bicomponent biomass blends were prepared after milling and sieving using diferent proportions.

Isothermal TGA gasiĄcation experiments

Raw biomasses and biomass blends gasiĄcation under steam was performed using a Seratam TG-ATD 92 thermal analyzer, coupled with a Wetsys humid gas generator. Approximately 20 mg of each sample were placed in an aluminum crucible and heated from 25°C to the Ąnal gasiĄcation temperature (750°C, 800°C and 900°C) at a heating rate of 10 °C/min, under an inert atmosphere. After 10 minutes, the measured mass loss of the sample was below 0.01%/min and then, it was considered that the pyrolysis stage has Ąnished. The atmosphere was then switched to a mixture of H 2 O/N 2 (steam partial pressure from 3.7 kPa to 10 kPa). The total Ćow rate was 4 l/h for all the experiments.

TGA experiments were conducted twice and averaged to verify their repeatability. A blank test was made for each experimental condition to exclude buoyancy efects. For each experimental condition, the repeatability was found to be satisfactory, as the calculated standard deviation of the mass loss was below 1%.

Reactivity and kinetic study Theoretical background

The isothermal gasiĄcation experiments described are the basis of the kinetic analysis of biomasses decomposition under a steam atmosphere. The degree of conversion or reaction extent during gasiĄcation is deĄned as in Eq. 4.2.1:

Ð = 0 -() 0 - (4.2.1)
Where 0 is the mass of the sample at the beginning of the gasiĄcation stage, the Ąnal mass, and the current mass at a given time. According to this, the apparent gasiĄcation reactivity can be deĄned as a function of the conversion degree Ð:

(Ð) = 1 1 -Ð Ð (4.2.2)
Moreover, the gasiĄcation reaction rate Ð/ can described as in Eq. 4.2.3:

Ð = ( )ℎ( H2O ) (Ð) (4.2.3)
Where ( ) is the Arrhenius equation, representing the temperature dependence of the process, ℎ( ) the relation expressing the gasiĄcation agent partial pressure dependence, and (Ð) the reaction model function representing how the solid state decomposition process occurs. Diferent authors have suggested that the inĆuence of the reactive atmosphere partial pressure in the reaction rate is described by a power law with a constant exponent value, for almost all conversion levels [22Ű24].

According to this, the reaction rate can be then written as follows:

Ð = exp ⎤ - ⎣ H2O (Ð) (4.2.4)

Isoconversional model free approach

In this study, an isoconversional model-free approach was used to determine the Arrhenius parameters of steam gasiĄcation of biomasses. Isoconversional methods are based on the hypothesis that the reaction rate at a constant degree of conversion is only a function of temperature and pressure, and then, the activation energy of the process can be calculated without a previous assumption of the reaction model [24,25]. Also, the dependency of with Ð can give information about the existence of a single-step or multi-step kinetics [26].

Besides the calculation of the apparent activation energy, the appropriate determination of the decomposition model describing the process is also important, as the wrong assumption of the decomposition mechanism could cause the misestimation of the kinetic parameters and an inaccurate interpretation of the process.

For biomass and coal gasiĄcation, three models are generally used in the literature for the description of char conversion. The volumetric model (VM), the shrinking core model (SCM), and the random pore model (RPM). Several studies have shown good agreement between experimental results and the three presented models, regardless of the diferences in their theoretical background [27Ű30]. However, as there is not any relationship established between the biomass characteristics and the decomposition mechanism, no theoretical models were presupposed in this study. Isoconversional modelfree methods and the generalized master-plots approach were used for the selection of the reaction expression that better describes the steam gasiĄcation process [31,32]. Some solid-state reaction models used for char gasiĄcation or combustion description are presented in table 4.3.

Model (Ð) (Ð)
Order based

Or1 -First order 1 -Ð -ln(1 -Ð) Or2 -Second order (1 -Ð) 2 [1/(1 -Ð)] -1 Or3 -Third order (1 -Ð) 3 (1/2)  1/(1 -Ð) 2  -1 Or -nth order (1 -Ð) [1/( -1)]  (1 -Ð) (1⊗) -1  Difusion D1 Ű One dimensional 1/(2Ð) Ð 2 D2 ŰTtwo dimensional [-ln(1 -Ð)] ⊗1 Ð + (1 -Ð) ln(1 -Ð) D3 Ű Three dimensional (3/2)(1 -Ð) 2/3  1 -(1 -Ð) 1/3  ⊗1  1 -(1 -Ð) 1/3  2 Geometrical contraction R2 Ű Contracting area 2(1 -Ð) 1/2 1 -(1 -Ð) 1/2 R3 Ű Contracting volume 3(1 -Ð) 2/3 1 -(1 -Ð) 1/3
For isothermal experiments, the reduced-generalized reaction rate expression Ú(Ð) in Eq. 4.2.5 can be simpliĄed and calculated without the previous knowledge of the activation energy .

Ú(Ð) = (Ð) (Ð) Ð=0.5 = Ð/ (Ð/) 0.5 exp( / ) exp( / 0.5 ) (4.2.5)
The most suitable (Ð) model can be then identiĄed as the best match between the experimental Ú(Ð) values and the master-plots calculated from theoretical models. The generalized master-plots of the reaction models presented in table 3 and calculated according to Eq. 4.2.5, are presented in Ąg 4.1. In this regard, table 4.4 summarizes the steam gasiĄcation reactivity and reaction time at a conversion degree of 50% for the three raw biomasses analyzed. For all the samples, it is possible to observe that higher gasiĄcation temperatures are associated to higher reactivities and shorter reaction times. For instance, it is possible to notice that a temperature rise from 750°C to 800°C, increases twice the steam gasiĄcation reactivity of CS. Moreover, at 900°C, its reactivity is 6.6 times higher compared to 800°C. The same trend was observed for BG and OPS. The results obtained in the analyzed temperature range are in accordance with those presented in the literature. As already described by several authors for CO 2 and steam atmospheres, at higher temperatures, the gasiĄcation process requires shorter times to be completed [33Ű36].

Results and discussion

Impact of temperature and steam partial pressure on gasiĄcation reactivity

Regarding steam partial pressure, the conversion proĄle of coconut shells (CS) can be observed in Ągure 4.3 at three diferent steam partial pressures and an intermediate gasiĄcation temperature of 800°C. It can be noticed that an augmentation of this parameter is also associated with a decrease in the reaction time. Indeed, a higher steam partial pressure indicates a greater steam quantity available to react during the gasiĄcation stage, explaining the increase in the gasiĄcation rate of biomasses. At 800°C, CS reactivity at a conversion degree of 50% increased from 3.2 min -1 to 4.2 min -1 with a steam partial pressure rise from 3.7 kPa to 10 kPa. For its part, reaction time decreased from 67 min to 32 min, respectively. As observed with temperature, at higher steam partial pressures, the gasiĄcation process requires shorter times to be completed. According to this and considering eq. 4.2.4, it is possible to determine the exponent n of the power law describing the inĆuence of the steam partial pressure 2 on the reaction rate. For each conversion degree, this value was calculated from the slope of the plot ln(Ð/) vs ln( 2), using at least three gasiĄcation conditions. For the three raw biomasses analyzed, remained nearly constant during all the conversion range, with a mean value of 0.5 and a standard deviation of 0.1. This result is in agreement with the values reported by diferent authors between 0.4 and 0.8 for steam gasiĄcation of biomass and coal [16,22,37].

Impact of feedstock characteristics on steam gasiĄcation reactivity

The impact of the feedstock characteristics on the steam gasiĄcation reactivity was studied comparing the behavior of the three selected materials and their blends under diferent gasiĄcation conditions. As an example, the conversion proĄle of the raw biomasses at a gasiĄcation temperature of 800°C and a steam partial pressure of 3.7 kPa is presented in Ągure 4.4. It is possible to notice that the behavior of the three samples is notably diferent. In particular, CS conversion rate is nearly constant and tends to increase at high conversion levels, while OPS and BG conversion rate seems to decrease with time. This behavior was the same under all the analyzed experimental conditions. Therefore, the gasiĄcation behavior does not seem to be related to the macromolecular composition of biomasses. Even when the hemicellulose, cellulose and lignin content of coconut shells and oil palm shells are similar, their gasiĄcation conversion proĄle are clearly diferent. Moreover, the behavior of oil palm shells and bamboo guadua are quite similar.

To better understand this, the Ągure 4.5 shows the conversion proĄle of coconut shells (CS) and bamboo guadua (BG), at three diferent gasiĄcation temperatures and a steam partial pressures of 10 kPa.

For the analyzed experimental conditions, it can be noticed that CS conversion rate is nearly constant and tends to increase at high conversion levels, as described previously.

In contrast, BG conversion rate seems to decrease with time. Also, it can be observed that for a speciĄc conversion degree, there is an important diference in the reaction time of both biomasses. For instance, at 800°C and 10 kPa, CS total conversion was reached after 55 minutes, while BG took more than 300 minutes. This behavior shows that CS have a higher gasiĄcation reactivity compared to BG. The reactivity of the three selected biomasses was already highlighted in table 4. It was possible to observe that CS reactivity is always higher compared with BG and OPS. These diferences can be related to the inherent inorganic content of raw biomasses. In particular, it is possible to notice from table 2, that coconut shells contain mainly K, Ca, Al and P, while oil palm shells and bamboo guadua are composed mainly of Si, Al, Ca and a little K. Diferent authors have identiĄed the impact of inorganic elements in biomass gasiĄcation. Particularly, the catalytic efect of alkaline and alkaline earth metals (AAEM), and the inhibitory impact of Si, Al and P [9,17,38]. In this regard, Dupont et al [6], proposed a relevant correlation between the biomass gasiĄcation behavior and the inorganic ratio K/(Si+P). They found that biomasses with K/(Si+P) above 1 show a constant decomposition rate with a slightly increase at high conversion, while biomasses with K/(Si+P) below 1 have a decreasing rate along the whole conversion. According to this and taking into account that K has been reported to have the most beneĄcial impact on gasiĄcation reactivity among AAEM [39], the inorganic ratio K/(Si+P) will be used in this study to characterize and compare the inorganic composition of the selected biomasses.

The observed gasiĄcation behavior of the three analyzed biomasses may suggest that despite the diferences in the macromolecular constituents, the inorganic composition of samples is the most important parameter that inĆuences their steam gasiĄcation behavior. To better understand this impact, the gasiĄcation of diferent biomass blends was also studied. The inorganic ratio K/(Si+P) was calculated for each sample and is presented in table 4.5.

From experiments, it was possible to notice that the behavior of biomass blends is not clearly related to the mass blending ratio, but to the inorganic composition of the blend, calculated from the inorganic content of the individual samples.

The analysis of the gasiĄcation conversion proĄles, showed that the behavior of blends is not additive, if compared to raw biomasses. Moreover, it is not possible to describe synergistic or inhibitory efects as a function of the biomass blend ratio. Since limitations by heat or mass transfer can be neglected under the presented experimental conditions, this behavior suggests that the interactions between biomasses are notably related to their inorganic content. For the analyzed biomasses and blends, it was noted that the conversion rate increased with the K/(Si+P) ratio of the sample. In this regard, a linear relationship was found between the biomass reactivity and the inorganic ratio K/(Si+P) of samples, in the analyzed temperature range, as observed in Ąg. 4.6. This result, conĄrms the fact that the inorganic species of the biomasses have an impact on their gasiĄcation behavior. The gasiĄcation reactivity increases with the inorganic ratio, highlighting the beneĄcial efect of K. In contrast, higher quantities of Si and P could react with K and other AAEM, inhibiting their impact in gasiĄcation reactions, and reducing the biomass gasiĄcation reactivity. This observation may have an important impact in real gasiĄcation applications. As already discussed, biomasses with lower reactivities require longer gasiĄcation times or higher working temperatures to achieve a desired conversion level, impacting the energy consumption of the process. Thus, according to the feedstock characteristics, the process parameters and conditions should be properly adapted.

Steam gasiĄcation kinetic analysis

To better understand the impact of the inorganic composition on the biomass gasiĄcation behavior, a kinetic analysis was performed. As already presented, the decomposition curves (Ð Vs ) of biomasses or blends were analyzed using isoconversional methods in order to determine their kinetic triplet , and (Ð).

Firstly, the most suitable reaction model that describes the biomasses gasiĄcation was determined using the generalized master plots approach. In general, it was observed that for all the analyzed materials, the decomposition model that best Ąt the gasiĄcation behavior is independent of the temperature and the steam partial pressure. This is the case for many solid-state reactions and was expected for steam gasiĄcation [26]. To better understand and compare the diferences in the identiĄed reaction models, the gasiĄcation behavior of the samples was described as a reaction order model, where is the reaction order with respect to the reacting solid, as presented in table 3.

As already stated, for the analyzed biomasses and blends, it was observed that the identiĄed reaction model depends on the inorganic composition. Figure 4.7a shows that the feedstocks with inorganic ratio closer or higher than 1 match closely the theoretical plot of a zeroth-order model (Or0) in almost all the conversion range. This result is in accordance with diferent authors that found that catalytic gasiĄcation can be properly described by a reaction order 0 with respect to the reactive solid [17,40,41]. The divergences observed at conversion levels above 60% are possibly related to the catalytic impact of K that becomes more evident at the end of the gasiĄcation, when the relative proportion of inorganics compared to carbon is higher. In contrast, it can be noticed that for the biomasses and blends with inorganic ratio below 1 the most suitable reaction model identiĄed with generalized master plots varies. With the decrease of the inorganic ratio, the reaction model moves away from a zeroth-order model and goes towards a second-order model. From Ągure 4.7b, c and d it is possible to observe that BG behavior is close to a reaction order 1.5, while the blends 50%CS -50%BG and 75%CS -25%BG approach a reaction order 1 and 0.7 respectively. From these results, it can be noticed that there is a linear and inverse relationship between the identiĄed order of reaction and the inorganic ratio of the sample, as presented in Ągure 4.8.

Figure 4.8: IdentiĄed reaction order vs. inorganic ratio of analyzed samples

The observed increase in the reaction order with the decrease in the inorganic ratio could be related to the inhibition of K catalytic impact by Si and P. The higher the Si and P content, the stronger the inhibition impact, and then, the lower the gasiĄcation reaction rate. This trend is in accordance with related studies which found that Si and P tend to react with AAEM reducing their catalytic impact in gasiĄcation behavior [39].

Similarly, in the case of samples with K/(Si+P) higher that one, the catalytic impact of steam gasiĄcation also depends on the inorganic ratio. Figure 4.9 shows that even when all the samples seem to follow a zeroth order reaction, the higher the K/(Si+P) content, the stronger the catalytic efect. Consequently, the total gasiĄcation time was the lowest for the biomass with the highest inorganic ratio. Moreover, the acceleration of gasiĄcation rate at the end of the conversion is more important for the biomass with the highest inorganic ratio. As already discussed, this behavior is possibly related to the catalytic impact of K that becomes more evident when the relative proportion of inorganics compared to carbon is higher.

According to these results and in agreement to previous work in the literature, it is possible to say that the steam gasiĄcation behavior of biomasses with K/(Si+P) below one follow a non-catalytic mechanism, contrary to biomasses with K/(Si+P) higher than one. The variation in the reaction order identiĄed could be due to the interactions between the inorganic constituents of the biomass. In particular, Si and P could react with K and other AAEM to form alkali silicates, restraining their catalytic impact [10].

After the identiĄcation of the appropriate reaction model, the apparent activation energy Ea was determined from the slope of the isoconversional plots regression lines. For all Figure 4.9: Conversion proĄle of diferent samples with inorganic ratio higher than 1 at 800°C and 3.7 kPa. The calculated inorganic ratio of the samples is indicated next to each curve.

the analyzed samples, the mean apparent calculated was 134 kJ/mol, with a variation coeicient of 6% between the samples. The apparent activation energy calculated in this study is in agreement with diferent values reported in the literature for biomass steam gasiĄcation [16,36,42,43]. According to Ągure 4.10, no relationship was found between the apparent activation energy and the inorganic coeicient of biomasses and blends. For all the analyzed samples, the calculated remained almost constant with the reaction extent Ð, with variation coeicients below 15%. This result indicates that the and the reaction model selected using master-plots approach are suitable for the description of the samples gasiĄcation during all the conversion range. Finally, the pre-exponential factor was determined from Eq. 4.2.4. A mean value of 13 500 min -1 kPa -0.5 was calculated for all the analyzed biomasses and blends. From these results, a kinetic equation that describes the behavior of biomasses and blends as a function of their inorganic content is proposed:

Ð = 13500 exp ⎤ -134000 ⎣ 0.5 (1 -Ð) 1 (4.3.1)
Where is the steam partial pressure in kPa, the temperature in K, the ideal gas constant in J mol -1 K -1 , the theoretical model reaction order, and 1 a coeicient that takes into account the diferences observed between the gasiĄcation reaction rate of the biomasses that follow a zeroth-order model. Thus, the reaction order and the coeicient 1 depends on the inorganic ratio K/(Si+P) as follows:

If /( + ) ≥ 1 = 0 (4.3.2) 1 = 0.15 ⎤ + ⎣ + 0.7 (4.3.3) If /( + ) < 1 = -1.62 ⎤ + ⎣ + 1.64 (4.3.4) 1 = 1 (4.3.5)
Unlike the existing models that propose diferent kinetic laws for catalytic and noncatalytic gasiĄcation [6,17], it is worth mentioning that the presented kinetic equation (Eq. 4.3.1) allows the description of the steam gasiĄcation behavior of biomasses within a wide range of inorganic compositions, including those with an inorganic ratio close to 1. Moreover, the coeicient 1 was found to be proportional to the inorganic ratio K/(Si+P) and not to the K content, for the samples that follow a zeroth-order model. In this regard, the presented kinetic approach facilitates the understanding of the inĆuence of biomass inorganic species and their interactions, on the steam gasiĄcation process. Furthermore, it conĄrms the validity of the inorganic ratio K/Si+P proposed by Hognon et al. [38] to describe biomasses and predict their steam gasiĄcation behavior.

The proposed kinetic equation was validated and used to reproduce the gasiĄcation behavior of the studied biomasses and their blends. For all the analyzed samples and gasiĄcation conditions, a good agreement was found between the experimental and the calculated data. The Ąt error was determined according to Eq. 4.3.6, Where Ð/ are the experimental and calculated values of the decomposition rate, and is the total number of experimental points [44].

(%) = 100        ⎤  Ð i ⎡ -  Ð i ⎡ ⎣ 2 √  Ð i ⎡ ,       (4.3.6)
The calculated Ątting error was below 7% in all cases. This value was considered reasonable, taking into account the heterogeneity of the biomasses and the incertitude in the measurement of their inorganic composition. Figure 4.11 shows the comparison between the experimental results and the model predictions of some analyzed samples.

From this Ągure, it can be noticed that the conversion behavior of biomasses and blends is well described by the proposed kinetic model, highlighting the impact of inorganic composition on the steam gasiĄcation kinetics of lignocellulosic biomasses. The low Ątting error showed that the presented kinetic equation (eq. 4.3.1) is suitable for the description and prediction of the gasiĄcation behavior of biomasses or blends based on their inorganic composition.

Conclusion

The steam gasiĄcation of coconut shells (CS), oil palms shells (OPS) and bamboo guadua (BG) was analyzed under diferent experimental conditions. Despite the diferences in their macromolecular composition, inorganics showed to be the most important parameter inĆuencing the biomass gasiĄcation reactivity and kinetics. The results conĄrmed the beneĄcial impact of K in gasiĄcation and the inhibitory efect of Si and P. Accordingly, a new kinetic equation considering the inorganic composition of feedstocks was proposed. This approach proved to be suitable for the description and prediction of the gasiĄcation behavior of lignocellulosic biomasses within a wide range of inorganic compositions, and H/C and O/C ratios near 1.5 and 0.8 respectively. Furthermore, it was evidenced that the co-gasiĄcation behavior of biomasses in the presented range can be predicted from the inorganic composition of the individual feedstocks. In future work, the presented approach can be expanded to other lignocellulosic residues and a wider range of blends.

As a result, the proposed kinetic model could constitute a valuable tool for reactor design and for the development and scale-up of steam gasiĄcation facilities using tropical lignocellulosic feedstocks. In particular, taking into account that residual biomass availability is variable, this approach could be useful in applications where diferent kind of residues should be gasiĄed at the same time.

Introduction

The diverse climate conditions in tropical regions are associated with a great biodiversity and the possibility to develop agricultural and agroindustrial activities. In accordance, most developing countries in these areas base their economy in agriculture and related activities, producing great amounts of agrowastes. These residues are nevertheless barely valorized, and remain under-exploited. Taking into account that developing countries usually have particular energy needs in isolated areas, a possible valorization pathway for this materials could be their transformation in biofuels.

In this context, gasiĄcation is a very interesting process for the recovery of energy from agricultural and agroindustrial residues, as it produces fuel gases that can be used for the generation of heat or power [1Ű3]. In particular, steam gasiĄcation fuel gases have a higher heating value in comparison to air gasiĄcation [4Ű6], and then, the analysis and understanding of this process is of great importance in the presented context.

The main reactions involved in the steam gasiĄcation process are listed below [5]:

Water gas reaction:

C + H 2 O --→ CO + H 2 +131 kJ/kmol (R5.1)
Water-gas shift reaction:

CO + H 2 O --→ CO 2 + H 2 -
CH 4 + H 2 O --→ CO + 3 H 2 +206 kJ/kmol (R5.8)
The Ąnal gas composition of the steam gasiĄcation process is the result of the combination of the presented reactions. In general, steam gasiĄcation produces gases with heating value between 10 and 18 MJ/m 3 , and high H 2 concentrations [5].

Despite the great potential of the steam gasiĄcation process, the valorization of agrowastes could have some restrictions associated with the availability of the feedstocks. Agricultural and agroindustrial activities are generally seasonal and then, the availability of the residues is not constant. In most cases, gasiĄcation facilities should operate intermittently, or work with diferent kind of residues with diferent characteristics, or even blends. In consequence, the impact of the feedstock on the gasiĄcation behavior and products should be understood, in order to properly adapt the process parameters, according to the application requirements.

Several authors have investigated the efect of operating parameters and conditions on the performance of the gasiĄcation process, with diferent types of reactors and feedstocks [7Ű9]. Parameters like gasiĄcation temperature and steam quantity, have shown an important impact in the produced gas composition and quantity. Moreover, the impact of the indigenous inorganic elements of the raw materials on the steam gasiĄcation reactivity has been also reported [10,11], and conĄrmed in a previous work [12].

Regarding the steam gasiĄcation product distribution and gas composition, several studies have been found in the literature associated with diferent kind of feedstocks like rice husk and straw [13], bagasse [14], wood [15], walnut and pistachio shells [16], or waste water sludge [17]. However, most of these studies deal with residues individually, without comparing diferent materials. A few references working with several biomasses, described some diferences in the gas production rate and composition, which were attributed to the samples ash content and composition, volatile matter, particle porosity, or particle size [7,18,19]. Nevertheless, as the aim of these works was not the comparison of the gasiĄcation behavior of diferent feedstocks, the justiĄcation of these diferences is not extensively detailed. In accordance, the understanding of the inĆuence of the feedstock characteristics on the steam gasiĄcation behavior and product distribution still needs to be improved.

In this regard, the aim of the work developed in this chapter is to understand the impact of the biomass composition on the steam gasiĄcation behavior and product distribution. Three tropical lignocellulosic biomasses with diferent macromolecular and inorganic composition were analyzed and compared. A focus on the permanent gaseous products is made, analyzing its composition, heating value, and possible suitable applications in the presented context.

Materials and methods

Biomass samples

Three tropical lignocellulosic biomasses were selected for this study: oil palm shells (OPS), coconut shells (CS) and bamboo guadua (BG). The detailed origin of the samples, collected in Colombia-South America is described in a previous work [20]. The organic and inorganic chemical composition of the samples is presented in tables 5.1 and 5.2, as the average of three replicates.

The CHNS composition was determined using a Thermoquest NA 2000 elemental analyzer, while for the high heating value an IKA C 5000 automated bomb calorimeter was used. The inorganic composition was determined using an HORIBA Jobin Yon Ultima 2 inductively coupled plasma optical emission spectrometer (ICP-OES), base on the standard EN 16967. The proximate analysis was calculated according to the standards EN ISO 18134-3, EN ISO 18123, and EN ISO18122. Before the experimental tests the raw biomasses were ground and sieved to a size range between 2 mm and 4 mm.

Experimental setup

Pyrolysis and steam gasiĄcation experiments were carried out in a semicontinuous lab-scale Ćuidized bed gasiĄer. The reactor is made of stainless steel and is externally heated with an electrical furnance composed of two radiative covers. The height of the reactor is 60 cm and the internal diameter 6 cm. A porous disk in the bottom holds the biomass sample and allows the gasiĄcation atmosphere to pass through. The thermal regulation is ensured by an Eurotherm controller connected to a K-thermocouple in the center of the reactor.

The nitrogen gas Ćow supplied to the process is regulated by a mass Ćow controller calibrated between 0-5 Nm 3 /h. Steam is supplied by a steam generator equipped with a water mass Ćow controller and a heating device producing superheated steam at 180 ◇ C. At the exit of the reactor, a gas conditioning unit based on the tar protocol [21] allowed the condensation of tars and steam. A series of 7 impinger bottles were used as presented in the experimental setup scheme 5.1. After the condensation train, a gas meter was used to measure the volume of produced permanent gases, before being released to the atmosphere. Every 5 minutes, the gas volume was registered and gas samples were taken using Tedlar®. bags in order to analyze their composition. For this For all the experiments, 80 g of biomass were placed inside the reactor and heated to the gasiĄcation temperature at a heating rate of 20 ◇ C/min, under nitrogen. When the gasiĄcation temperature was reached (from 750 ◇ C to 850 ◇ C), the atmosphere was switched to a mixture of H 2 O/N 2 (from 50 to 300 g/h of steam -15% to 90% of steam in the gasifying agent) and was maintained during all the gasiĄcation stage ( from 1h to 3h). The total Ćow rate was 0.7 m 3 /h for all the experiments. After each test, the reactor was cooled down to room temperature under nitrogen. The remaining char was collected and weighted, as well as the impinger bottles and pipes, in order to determine the quantity of water and tars condensed and perform mass balances. For this purpose, prior to each test the condensation train impinger bottles and pipes were also weighted. The experimental program is presented in Ągure 5.2.

Figure 5.2: Pyrolysis and steam gasiĄcation experimental program

Where, and are the mass of the initial biomass used and the total steam fed to the gasiĄcation process, and are the high heating value of the produced gas and the raw biomass, and ℎ (r) is the enthalpy of the steam at the reactor temperature during the gasiĄcation process.

GasiĄcation reactivity analysis

To analyze the impact of the raw samples in their gasiĄcation behavior, an average reactivity study was performed for diferent gasiĄcation times. The degree of conversion of the char after the gasiĄcation stage is deĄned as in equation (5.2.12):

Ð() = 0 -() 0 -ℎ (5.2.12)
Where 0 is the mass of the sample at the beginning of the gasiĄcation stage, () the mass at the end of the gasiĄcation period , and ℎ the mass of ash in the sample. 0 for each experimental condition is the mass of char in the reactor before the steam injection, determined from the pyrolysis-only tests.

The apparent gasiĄcation reactivity can be deĄned as a function of the conversion degree Ð, as presented in equation 5.2.13. Generally, reactivity comparisons are referred to a speciĄc char conversion level. However, as the continuous monitoring of the mass loss of each biomass in not possible for the presented experimental setup, the gasiĄcation reactivity is presented in this study as an average for a deĄned gasiĄcation time, and is calculated according to the equation (5.2.14), where is the time of the steam gasiĄcation stage, from 1h to 3h.

(Ð) () = 1 1 -Ð() Ð
(5.2.13)

() = 1  =1 1 1 -Ð() Ð
(5.2.14)

Results and discussion

Mass and energy distribution of gasiĄcation products

Impact of steam fraction in the gasifying agent

To understand the impact of the biomass characteristics in its steam gasiĄcation behavior and product yield, diferent process conditions were analyzed and compared for the three selected materials: CS, BG, and OPS.

Firstly, the impact of the steam quantity on the gasiĄcation products distribution was studied at a process temperature of 750°C. Figure 5.4 presents the solid, gas, and liquid yield of the analyzed samples, in grams of by-product per grams of biomass initially fed to the process (g/g bio ). For the three samples, it was observed that the recovered char quantity at the end of each gasiĄcation experiment was reduced with the increase of the steam percentage, as presented in Ągure 5.4a. This trend was expected, considering that higher steam quantities available to react with carbon lead to higher carbon conversions and lower solid yields. For its part, the maximum gas production, of about 0.4 to 0.5 g/g bio (0.6 to 0.8 Nl/g bio ) was observed between 15% and 30% of steam in the gasifying agent (Ągure 5.4c). Above these values, the produced gas quantity was reduced and an increase in the liquid fraction was noticed (Ągure 5.4c). In accordance, high steam quantities showed a negative impact in the gas production. This behavior is possibly related to a reduction in the reaction temperature that favors tars at the expense of permanent gases, and has been also observed by several authors [6,22]. Furthermore, regarding the energy distribution of the gasiĄcation products, it is possible to observe from Ągure 5.5 that for the analyzed samples, the energy fraction contained in the liquid products increases with the steam percentage in the gasifying agent. Excess steam does not react with carbon and is recovered at the reactor exit, involving a greater energy consumption related to steam production and overheating. Accordingly, the energy fraction contained in the liquid products (water+tars) increases, being inconvenient for energy applications.

In accordance to the results presented in Ągure 5.5, the highest energy fraction and gas eiciency gas was also obtained for steam percentages between 15% and 30%, conĄrming that for the presented experimental setup, these conditions may be the most suitable for the study of the selected biomasses for energy applications. In consequence, the impact of temperature and time in the samples gasiĄcation will be analyzed thereafter for 30% of steam in the gasifying agent.

Impact of gasiĄcation temperature and feedstock

For the three analyzed biomasses, an rise in the gasiĄcation temperature resulted in an increase in the gas production and a decrease in the solid and liquid yields, as presented in Ągure 5.6. Indeed, with the temperature change from 750°C to 850°C, the gas yield increased from 0.50 to 0.94 g/g bio (0.66 to 1.41 Nl/g bio ) for CS, from 0.47 to 0.54 g/g bio (0.75 to 0.81 Nl/g bio ) for BG, and from 0.44 to 0.56 g/g bio (0.79 to 0.84 Nl/g bio ) for OPS. This is an augmentation of 18, 3 and 5 percentage points respectively, while the liquid and solid yields decreased for the three samples. More speciĄcally, the liquid yield showed a decrease of 19, 5 and 10 percentage points for CS, BG and OPS, with the temperature increase.

These results were expected, taking into account that endothermic steam gasiĄcation reactions are promoted by the temperature increase, resulting in a greater char and steam consumption, and an enhanced gas production. The described behavior has also been reported by diferent authors working with diferent kinds of biomass [8,19,23]. Furthermore, the rise in the gas yield observed for the same gasiĄcation time indicates an increase in the steam gasiĄcation reactivity of the samples with the temperature. From the average reactivity values calculated for the three analyzed feedstocks and summarized in table 5.3, a reactivity increase of more than 4 times is observed with the temperature change from 750°C to 850°C. This trend was already highlighted in a previous work, at a thermogravimetric scale [12] (Chapter 4). However, even when the described trend is the same for the three samples, some diferences can be noticed. For the same gasiĄcation time, the increase in the gas production was greater for CS in comparison to OPS and BG when the gasiĄcation temperature rose from 750°C to 850°C. Similarly, as shown in Ągure 5.7, the product energy fraction contained in the permanent gases augmented 28 percentage points for CS compared to only 10 and 5 for BG and OPS respectively. Moreover, the decrease in the energy fraction associated with the recovered solid and liquids is more important for CS. Likewise, from the comparison of the gas production as a function of time, it can be observed that under the same gasiĄcation conditions the gas production evolution is diferent for the three samples (Figure 5.8). In particular, CS exhibited the highest gas yield in comparison to BG and OPS. This behavior can be better understood from the analysis of the average gasiĄcation reactivity values, already presented in table 5.3. Regardless the gasiĄcation temperature, the reactivity values of CS were always higher compared to BG and OPS, explaining the observed diferences in the gas yield evolution with the gasiĄcation temperature and time. For instance, at 850°C, the BG and OPS reactivities correspond to only 51% and 14% of the CS value respectively. Accordingly, two diferent trends are identiĄed: in one hand, CS with a high gasiĄcation reactivity, and in the other hand, BG and OPS with a lower reactivity. These diferences may be related to the inherent inorganic content of the raw materials, considering that CS contain mainly K and Ca, while OPS and BG are principally composed of Si, Al, P. The catalytic efect of AAEM and the inhibitory impact of Si, Al and P on the steam gasiĄcation behavior of carbon materials has already been highlighted by diferent authors [24Ű26].

More speciĄcally, the catalytic impact of alkali and alkali earth metals (AAEM) elements on the steam gasiĄcation reaction R5.1 (page 94) has been described as an oxygen transfer mechanism via the metal M, as presented below [26,27]:

M 2 CO 3 + 2 C --→ 2 M + 3 CO 2 M + 2 H 2 O --→ 2 MOH + H 2 2 MOH + CO --→ M 2 CO 3 + H 2 2 C + 2 H 2 O --→ 2 CO + 2 H 2
The enhanced oxygen transfer between the gasifying atmosphere and the reacting solid increases the carbon conversion rate, resulting in a higher reactivity and gas quantity produced under the same gasiĄcation conditions. Among the AAEM, K has been reported to be the most active element that promotes the steam gasiĄcation reactions in carbon materials [28,29]. However, the described catalytic mechanism may be inhibited by elements like Si, Al, and P, as they tend to react with AAEM, reducing their beneĄcial efect in the gasiĄcation behavior [24,30].

In this regard, several authors have proposed diferent reaction mechanisms between AAEM and Si, Al, and P compounds during biomass thermochemical conversion, as presented below [31Ű33]:

2 MOH + P 2 O 5 --→ 2 MPO 3 + H 2 O 2 MOH + nSiO 2 --→ M 2 O • nSiO 2 + H 2 O 2 MOH + Al 2 O 3 2 SiO 2 --→ 2 MAlSiO 4 + H 2 O
In particular, the formation of alkali phosphates, silicates, and aluminosilicates may inhibit the oxygen transfer mechanism via the metal M, and then, may hinder the catalytic impact of AAEM on the steam gasiĄcation reactions.

In this respect, the inorganic ratio K/(Si+P) proposed by Hognon et al. [34] is adopted in this work to compare the inorganic composition of the analyzed samples and its impact on product yield. The validity of this ratio has been conĄrmed in a previous study at a thermogravimetric scale, where a kinetic expression was proposed to describe and predict the steam gasiĄcation behavior of lignocellulosic biomass as a function of its inorganic content (Chapter 4) [12]. More speciĄcally, samples with K/(Si+P) higher than 1 showed a catalytic gasiĄcation behavior, contrary to samples with K/(Si+P) below 1, conĄrming the beneĄcial efect of AAEM (especially K), and the inhibitory impact of Si and P.

Regarding the analyzed samples, CS inorganic ratio is 3.9, while BG and OPS values are 0.2 and 0.17 respectively. These diferences can explain the higher increase in the product energy fraction contained in permanent gases for CS, in comparison to the other two samples, as observed in Ągure 5.7. Therefore, the similarities between OPS and BG, and their diferences with CS may be related to their K/(Si+P) ratio.

Similarly, the comparison of the three analyzed samples showed that the process gas eiciency calculated for gasiĄcation at 850°C, during 1 hour, and 30% of steam in the gasifying agent, is 20 percentage points higher for CS, in comparison to the values obtained for BG and OPS (55% in comparison to 33% and 35% respectively).

In this regard, to further illustrate the impact of the inorganic content of the raw biomass, the gas eiciency calculated for the analyzed samples and selected blends is 2 Sample name nomenclature: Biomass_Steam fraction (%)_Temperature (°C)_Time (h) * Blends were gasiĄed at 850°C, during 1 hour, with a 30% steam atmosphere presented in Ągure 5.9. Under the same process conditions, two diferent trends were clearly distinguished. At Ąrst, it can be noted that the gas eiciency of the process increases with K/(Si+P) for ratios below 1. In contrast, the highest gas eiciencies were observed for samples with K/(Si+P) close or higher than 1. It is worth noting that for samples with inorganic ratio higher than 1, the calculated gas eiciency remains almost constant.

From the analysis of the gas yield and gas eiciency obtained for each sample, it is not possible to evidence synergistic or inhibitory efects as a function of the biomass blend ratio. However, the experimental results highlight the evolution of the gasiĄcation behavior of samples as a function of their inorganic ratio. These Ąndings conĄrm the validity of the inorganic ratio K/(Si+P) adopted in this work to classify and predict the behavior of lignocellulosic agrowaste.

From a practical point of view, this observation is important for real applications, as the process parameters and conditions should be adapted according to the feedstock Figure 5.9: Calculated gas eiciency for pure biomass and selected blends at the same gasiĄcation conditions: 850°C, 1 hour, 30% of steam in the gasifying agent characteristics. In particular, longer gasiĄcation times or higher temperatures are required for biomasses with low gasiĄcation reactivity, identiĄed by an inorganic ratio K/(Si+P) lower than 1. Moreover, the behavior of biomass blends may be estimated from the knowledge of the inorganic composition of the individual feedstocks. The discussed experimental results are summarized in table 5.4 for all the analyzed samples. In the table, the sample name gives information about the raw biomass and the gasiĄcation conditions in the following order: steam fraction in the gasifying agent, gasiĄcation temperature, and gasiĄcation time.

Gas composition and heating value

For all the experimental conditions, no signiĄcant diferences were identiĄed between the three analyzed samples in terms of gas composition. The results obtained for gasiĄcation experiments with 30% of steam in the gasifying agent are presented in Ągure 5.10. As observed, the most representative gas is H 2 , with an average fraction near 60%, followed by CO and CO 2 with 19% and 18% respectively, as expected for a steam gasiĄcation process [5]. Concentrations below 0.6% of CH 4 were also observed for the three samples, and below 0.02% of C 2 H 4 for OPS. In all cases, the obtained gas composition is in accordance with literature reported values for the steam gasiĄcation of lignocellulosic materials under similar experimental conditions [13,15,35,36]. For the three samples, a slight decrease between 2 and 5 percentage points in the H 2 fraction was observed with the temperature rise from 750°C to 850°C. This behavior may be related to the water-gas shift reaction (R5.2 -page 94) that could be disfavored with the temperature increase. In fact, this reaction is slightly exothermic, and has a higher equilibrium constant at lower temperatures, implying lower H 2 yields when the temperature increases [37Ű39]. In the same way, the CH 4 content in the gas slightly decreases with the gasiĄcation temperature, as it is produced by exothermic methanation reactions (R5.5 to R5.7 -page 94), whose equilibrium constant decreases with the temperature. Endothermic methane-reforming reaction (R5.8 -page 94) could also contribute to this result.

Concerning the average H 2 /CO ratio of the produced gas, values between 2.5 and 4.5 were found for gasiĄcation experiments performed with a steam fraction of 30% in the gasifying agent. From table 5.5, it can be observed that the H 2 /CO ratio decreases in a little extent with the increase in the biomass gasiĄcation temperature, in relation with the slight decrease in the H 2 fraction, mentioned previously. Moreover, an increase is also observed in the gas H 2 /CO ratio with the rise in the steam fraction in the gasifying agent. In fact, the water-gas shift reaction (R5.2 -page 94) is favored with greater steam quantities available to react, resulting in higher H 2 concentrations and a decrease in the CO fraction. This behavior was more pronounced for CS, taking into account that the gasiĄcation reactivity of this sample is higher, in comparison to BG and OPS. Despite these little changes, no remarkable diferences were observed in the produced gas heating value with the temperature. For the three analyzed biomasses and under all the tested conditions, the calculated gas HHV was between 10 and 12 MJ/m 3 , as presented in table 5.5, in accordance to the reported values for a steam gasiĄcation process [5]. OPS heating values are slightly higher compared to the other two samples, possibly due to the higher carbon content of the raw feedstock.

The similarities between the gas composition obtained from the steam gasiĄcation of coconut shells, bamboo guadua, and oil palm shells are possibly related to their close organic content, with H/C and O/C ratios near 1.5 and 0.8 respectively. Cao et al [40] found a correlation between the gasiĄcation produced gas H 2 /CO ratio and the H/C ratio of diferent range coals. Particularly, in the case of the analyzed raw biomasses, the H/C ratio is very close; as well as the H/C ratio of the pyrolysis chars at the beginning of the gasiĄcation stage (e.g. 0.15 for CS pyrolysis chars at 850°C, 0.16 for BG chars, and 0.18 for OPS chars).

The required properties of the produced gas in terms of H 2 /CO ratio are diferent for diferent applications and synthesis processes. According to several authors, ratios between 0.5 and 2 are suitable for Fischer Tropsch applications, while ratios between 2.5 and 4 are recommended for use as fuel in gas turbines or internal combustion engines [41Ű43]. Accordingly, the produced gas from the steam gasiĄcation of the selected feedstocks may be appropriate for energy applications in the presented context.

From the experimental results in the present work, it can be observed that the main diferences identiĄed in the produced gas from the three analyzed samples are principally associated with the obtained yield and not to the gas composition. Under the same gasiĄcation conditions, the gas production showed to be related to the sample reactivity, determined mainly by its inorganic content. For its part, the product gas composition and heating value may be related to the organic composition of the samples. In general Sample name nomenclature: Biomass_Steam fraction (%)_Temperature (°C)_Time (h) *Blends were gasiĄed at 850°C, during 1 hour, with a 30% steam atmosphere terms, it can be considered that according to its composition, the produced gas from the steam gasiĄcation of the analyzed samples is suitable for energy applications such as boilers, gas turbines, or internal combustion engines.

Conclusion

Coconut shells (CS), oil palm shells (OPS) and bamboo guadua (BG) were gasiĄed in this work under diferent experimental conditions, in order to understand the impact of the biomass characteristics in their steam gasiĄcation behavior and product yield. The experimental results showed that the inorganic content impact in a very important way the gasiĄcation reactivity and product yield of the samples. For its part, the organic composition, especially the H/C and O/C ratios seem to be related to the composition and heating value of the produced gas.

The beneĄcial impact of AAEM, particularly K, in the steam gasiĄcation reactivity and gas yield was conĄrmed, as well as the inhibitory efect of Si and P. In this regard, the validity of the inorganic ratio K/(Si+P) to classify and predict the steam gasiĄcation behavior of lignocellulosic waste was veriĄed. GasiĄcation of samples with K/(Si+P) above 1 resulted in higher gas yields and gas eiciencies compared to samples with K/(Si+P) below 1.

Moreover, with similar H/C and O/C ratios, no signiĄcant diferences were observed between the analyzed samples in terms of gas composition and heating value. In this regard, with a high heating value between 10 and 12 MJ/m 3 , and a H 2 /CO ratio between 2.5 and 4, the steam gasiĄcation produced gas from lignocellulosic feedstocks with H/C and O/C ratio near 1.5 and 0.8 respectively, is considered suitable for energy applications using boilers, gas turbines or internal combustion engines.

The experimental observations presented in this work could be an important reference for real gasiĄcation applications working with diferent kind of lignocellulosic residues, as the process parameters and conditions should be adapted according to the feedstock characteristics, and particularly, its inorganic content.

6

Chapter

Physico-chemical characterization of steam gasiĄcation chars from tropical lignocellulosic agrowaste

in the gasifying agent. The principal diferences observed between the three analyzed samples can be mainly attributed to their inorganic composition, and more speciĄcally, to their inorganic ratio K/(Si+P). Under the same gasiĄcation conditions, samples with inorganic ratio above 1 showed higher surface areas, with greater proportions of surface oxygen-containing functional groups, in comparison to samples with ratios below 1.

For its part, the macromolecular composition seems to have an inĆuence in the pore size distribution of the gasiĄcation chars. The observations in this work could be an important reference for gasiĄcation applications working with several kind of residues, where the valorization of the solid by-product is intended.

Introduction

Tropical regions have a great potential for the development of agricultural and agroindustrial activities, thanks to their climate and geographic location. Developing countries in these areas have particular energy needs in rural and isolated zones, as several communities still lack access to reliable energy services. These populations base their economy in agricultural and related activities, and usually produce signiĄcant amounts of agrowastes that remain under-exploited. These residues could be valorized as biofuels in local applications to meet the communities energy needs. However, in most cases, the sustainability of biomass valorization projects in isolated areas depends on the creation of related productive activities that go beyond the energy supply, generating new incomes for local communities [1,2]. Considering the existence of important amounts of agrowastes, simultaneous energy and business development opportunities around biomass valorization could be considered.

In this context, steam gasiĄcation seems to be a very interesting thermochemical process, as it produces high heating value fuel gases that can be used for heat or power generation [3Ű5], and a solid porous by-product that could also be valorized [6,7]. Nevertheless, as the availability of agrowastes is often seasonal, gasiĄcation facilities should work with diverse residues, having diferent characteristics. In this regard, the understanding of the impact of the feedstock characteristics on the gasiĄcation behavior and by-products is of great importance, in order to properly adapt the process conditions to the application requirements.

Diferent authors have investigated the inĆuence of the process conditions on the main properties of pyrolysis and gasiĄcation chars. The process temperature and time, as well as the reaction atmosphere, are reported to have a very important impact on the char physico-chemical characteristics [8,9]. Among these parameters, the reaction atmosphere has probably the most important efect in the char porous structure and surface chemistry [10]. Air gasiĄcation chars show considerably lower speciĄc surface areas in comparison to steam, CO 2 , or air/steam gasiĄcation solid by-products. In particular, steam gasiĄcation chars are comparable to physical activated carbons, with high surface areas and pore volumes, and an important amount of surface functional groups. In the literature, the reported surface areas of steam gasiĄcation chars from diferent feedstocks are between 400 and 1000 m 2 /g [11,12], comparable to activated carbons with surfaces areas around 1000 m 2 /g or higher. In contrast, air gasiĄcation chars, usually need to be upgraded by subsequent activation, to be used in adsorption or catalytic applications [13Ű15].

Moreover, the inĆuence of the process conditions in the char properties have been widely analyzed for several lignocellulosic agrowastes like rice husk and straw [8], wood [16], coconut shells [17], switchgrass [18], and others. In contrast, the impact of the raw feedstocks in the properties of the gasiĄcation chars still needs to be investigated. Diferent authors have suggested that the macromolecular composition and the ash content of the samples may have an inĆuence in the properties of activated carbons and chars [19]. However, as most studies deal with only one kind of feedstock, the understanding of this question needs to be improved.

In this regard, previous works have proven that the composition of the samples impact in a very important way their gasiĄcation reactivity [20,21]. In particular, the inorganic content of the biomass could play a catalytic or an inhibitory role in the progress of the steam gasiĄcation reactions. More speciĄcally, alkali and alkali earth metals (AAEM) like K, Ca, Na and Mg have a beneĄcial efect on biomass gasiĄcation [22,23], that could be inhibited by elements like Al, Si and P [24,25]. Nevertheless, the inĆuence of inorganics in the gasiĄcation char properties is not clear. In the same way, regarding the macromolecular composition of the samples, a controversy exists in the literature in relation to the impact of the hemicellulose, cellulose and lignin content of the raw materials in the pore development and structure of chars and activated carbons [26Ű28].

In this regard, the aim of this work is to understand the inĆuence of the biomass characteristics and composition on the physico-chemical properties of steam gasiĄcation chars. The steam gasiĄcation solid by-products from various lignocellulosic agrowastes with diferent macromolecular structure and within a wide range of inorganic composition were analyzed. Coconut shells (CS), bamboo guadua (BG) and oil palm shells (OPS) were used as feedstocks. The bulk organic and inorganic composition of the chars were analyzed, as well as the distribution of minerals in their surface. In addition, the surface area, pore structure, and surface chemistry were also evaluated and compared. Finally, the relationship between the char structure and surface chemistry was discussed for the three samples.

Materials and methods

Materials

The chars obtained from the steam gasiĄcation of three tropical lignocellulosic biomasses under diferent experimental conditions were characterized and analyzed in this study. Oil palm shells (OPS), coconut shells (CS) and bamboo guadua (BG) were selected as raw materials for the gasiĄcation experiments. The detailed origin of the samples, collected in Colombia-South America, as well as their organic and inorganic chemical composition are presented in a previous work [20,29].

Experimental setup

Steam gasiĄcation experiments were carried out in a semicontinuous lab-scale Ćuidized bed gasiĄer. An externally heated stainless steel reactor of 60 cm height and 6 cm internal diameter, equipped with a porous disk in the bottom to hold the sample and allow the gasiĄcation atmosphere to pass through was used. The thermal regulation was ensured by an Eurotherm controller connected to a K-thermocouple in the center of the reactor. A steam generator equipped of a water mass Ćow controller and a heating device producing superheated steam at 180 ◇ C, supplied the required steam for the gasiĄcation process. The detailed experimental setup has been presented in chapter 3.

For all the experiments, 80 g of biomass were placed inside the reactor and heated to the gasiĄcation temperature at a heating rate of 20 ◇ C/min, under nitrogen. When the gasiĄcation temperature was reached (from 750 ◇ C to 850 ◇ C), the atmosphere was switched to a mixture of H 2 O/N 2 (from 15% to 90% of steam in the gasifying agent -50 to 300 g/h of steam) and was maintained during all the gasiĄcation stage ( from 1h to 3h). The total Ćow rate was 0.7 m/h 3 for all the experiments. After each test, the reactor was cooled down to room temperature under nitrogen. The remaining char was collected, weighted, and stored for subsequent characterization. Pyrolysis tests were performed in order to determine the solid yield at the end of the heating period (pyrolysis stage) for each gasiĄcation temperature. In order to verify the reproducibility, several tests were carried out twice. The repeatability was found to be satisfactory as the standard deviation of the solid yield was below 3%

Char burn-off and gasiĄcation reactivity

For each steam gasiĄcation test, the char burn-of achieved at the end of the experiment was calculated according to equation 6.2.1:

-(%) = 0 - 0 × 100 (6.2.1)
Where 0 is the mass of the sample at the beginning of the gasiĄcation stage, determined from the pyrolysis-only tests; and the mass of the remaining char at the end of the experiment, on a dry ash free basis.

Likewise, from the measurement of the remaining char at the end of each gasiĄcation experiment, the average gasiĄcation reactiviy of the samples can be calculated for diferent gasiĄcation times. The degree of conversion of the char after the gasiĄcation stage is deĄned as in equation (6.2.2):

Ð() = 0 -() 0 -ℎ (6.2.2)
Where 0 is the mass of the sample at the beginning of the gasiĄcation stage, () the mass at the end of the gasiĄcation period , and ℎ the mass of ash in the sample.

The apparent gasiĄcation reactivity can be then deĄned as a function of the conversion degree Ð, as presented in equation 6.2.4. Generally, reactivity comparisons are referred to a speciĄc char conversion level. However, as the continuous monitoring of the mass loss of each biomass in not possible for the presented experimental setup, the gasiĄcation reactivity is presented in this study as an average for a deĄned gasiĄcation time, and is calculated according to the equation (6.2.4), Where is the time of the steam gasiĄcation stage, from 1h to 3h.

(Ð) () = 1 1 -Ð() Ð (6.2.3) () = 1  =1 1 1 -Ð() Ð (6.2.4)

Char characterization

Proximate analysis and elemental composition

The elemental analysis (CHNS) of the collected chars was determined using a Thermoquest NA 2000 elemental analyzer. To determine the inorganic composition, 150mg of ground char (with a particle size below 500µm) were acid digested in close vessels at 220°C during 4h. Acid reagents H 2 O 2 , HNO 3 , HF and H 3 BO 3 were used according to EN 16967. Acid solutions were diluted with demineralized water to 50 ml and analyzed using an HORIBA Jobin Yvol Ultima 2 inductively coupled plasma optical emission spectrometer (ICP-OES). For its part, the ash content of the samples was calculated according to the standard EN ISO 18122. All the analysis were performed with at least three replicates.

Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM)

The morphology of the chars was observed using a Hitachi TM3030 Plus tabletop scanning electron microscope (SEM), with an accelerating voltage of 15kV. The local chemical composition at a micro-scale was determined by using the energy dispersive X-ray spectroscopy (EDX) module in the same apparatus.

For its part, the carbon structure of the gasiĄcation chars at a nanometric scale was observed using a JEOL JEM-ARM200F transmission electron microscope (TEM). Elemental mapping of chemical species in the sample was performed using and EDX module in the same microscope. Powder char samples with particle size below 250 µm were dispersed in ethanol, then mixed in an ultrasonic bath. The samples were collected from the surface of the solution and deposited onto carbon support Ąlms.

X-Ray Diffraction (XRD)

X-ray difraction (XRD) analyses of char powder samples (particle size below 250 µm) were carried out to analyze their global carbon structure. Experiments were performed using a Philips Panalytical X'pert Pro MPD difractometer using a Cu Ka radiation source (1.543 Å) with a current of 45kV and an intensity of 40 mA. The difraction patterns were collected between 2=10 and 2=70 with a step size of 0.05°.

Char pore structure and speciĄc surface area

The pore structure of gasiĄcation chars was determined by nitrogen adsorption at 77K using a Micromeritics 3Flex high-resolution analyzer. Prior to measurements, the samples were degassed in vacuum at 90°C during 1h and then at 150°C during 10h. The surface area was calculated from the adsorption isotherms using the Brunauer-Emmett-Teller (BET) model. The t-plot model was used to determine the micropore volume of samples, and the Non-local Density Functional Theory (NLDFT) to determine the micropore size distribution.

Surface oxygen-containing functional groups (SOFG)

The oxygen-containing functional groups in the char surface were determined using the temperature programmed desorption (TPD) technique. Experiments were carried out in a Micromeritics AutoChem II chemisorption analyzer. Approximately, 150mg of char were placed in a quartz U-tube and heated under an helium atmosphere. The sample was kept at 150 ◇ C during 1 hour and then was heated to 1000 ◇ C with a heating rate of 5 ◇ C/min. The concentration of CO and CO 2 released was quantitatively analyzed using a MyGC Agilent micro-GC. Preliminary tests were performed to evaluate the impact of the sample mass and bed height on the TPD results, showing that under the presented experimental conditions, the results are repeatable and the difusion limitations due to sample bed height can be neglected. In general, the repeatability of the TPD tests was found to be satisfactory as the maximum calculated standard deviation between two experiments was below 10%

The obtained CO and CO 2 desorption curves were deconvoluted to determine the contribution of each type of oxygen complex. The procedure presented in this work is based on the TPD spectra analysis proposed by Zhou et al [30], using 6 Gaussian peaks for the deconvolution of the CO desorption curve and 6 peaks for the CO 2 curve. Regarding the peak assignment, even though the reported decomposition temperatures for SOFG in carbon materials vary widely depending on the heating rate and the geometry of the experimental system used, some general trends have been identiĄed in the literature [31].

It is generally stated that the CO 2 desorption at low temperatures is associated to carboxylic acids, while lactones decomposition results in CO 2 desorption at high temperatures. Also, carboxylic anhydrides decomposition is related to both CO and CO 2 , while phenols, ethers and quinones result in CO desorption. Despite these trends, diferent authors have demonstrated that the desorption temperatures of the functional groups and their associated TPD peaks can also vary with their chemical and geometric local environment. Consequently, it may be diicult to clearly determine the content of each particular functional group from TPD patterns. Although, it is possible to quantify the global CO and CO 2 yielding groups and estimate their proportion in the char surface.

Accordingly, the CO desorption curves obtained were decomposed using Gaussian functions centered at 250±50°C and 380±50°C for carboxylic groups, 500±50°C for peroxydes, and 620±50°C, 750±50°C, and 800±50°C for lactones at diferent energetic sites. In the same way, the CO 2 curves were deconvoluted using functions centered at 400±50°C for acid anhydrides, 620±50°C for hydroxyl groups, 740±50°C for phenol groups, 860±50°C for ether groups, and 900±50°C and 970±50°C for quinone and pyrone groups respectively. The mentioned peak positions and a constraint over the peak width (55±5°C) were used for the optimization of the curve Ątting. The Ątting error between the experimental and the deconvoluted curve was always below 8%.

pH at the point of zero-charge (pH PZC )

The pH value at which the net charge of the chars surface is 0, or point of zerocharge (pH PZC ), was measured using a Malvern Pananalytical Zetasizer NanoZS. Char samples were ground into powder, with a particle size below 250 µm. About 15 mg of sample were used for each test with an approximate sample concentration of 0.5 mg/ml. Measurements were done at room temperature within the pH range from 2 to 11, adjusted by the addition of a 0.25M HCl or 0.1M NaOH solution. The carbon pH PZC was then deĄned as the pH at which the electrical charge density measured on the surface was zero.

Results and discussion

Composition of gasiĄcation chars

Table 6.1 reports the elemental composition of the gasiĄcation chars analyzed in the present study. In the table, the sample name gives information about the raw biomass and the gasiĄcation conditions in the following order: steam fraction in the gasifying agent, gasiĄcation temperature, and gasiĄcation time. In general, CS and OPS gasiĄcation chars are highly carbonaceous materials with carbon content above 80%. BG chars, with higher ash content exhibit lower carbon percentages from 70% to 36%. The mineral composition of the chars is in accordance with the composition of the raw materials. CS chars are mainly composed of K and Na, while BG and OPS chars show greater amounts of Si. The dispersion of minerals in the char surface are presented in the SEM-EDX micrographs in Ągure 6.1, for 1 hour gasiĄcation chars obtained at 850°C with 30% of steam in the gasifying agent.

SEM-EDX images are in accordance with the information obtained from the ICP-OES results and show that minerals in the CS chars are well dispersed in the surface. In general, K and Na were the dominant elements detected, with some traces of Ca. Furthermore, Si was the main element observed in OPS chars, as well as some Ca and K. Finally, SEM-EDX analyzes conĄrmed the higher mineral content of BG chars. In this case, Si was detected in almost all the char surface, probably as SiO 2 , or associated to K.

Char yield and burn-off

To understand the impact of the biomass characteristics in the gasiĄcation char properties, diferent process conditions were analyzed and compared.

For the three biomasses, the recovered char at the end of the gasiĄcation process was reduced with the increase of the steam fraction in the gasifying agent, the temperature and the gasiĄcation time. The impact of these parameters in the gasiĄcation behavior of CS, BG and OPS was analyzed previously in a thermogravimetric scale [20], and is in accordance with the experimental results obtained in this work. Higher gasiĄcation temperatures and steam quantities are related to higher reactivities, an then, to higher carbon conversions and lower char yields. In the same way, longer gasiĄcation times also allow higher carbon conversions, resulting in less recovered char at the end of the experiment.

In this regard, from a general point of view, the impact of the process parameters on the steam gasiĄcation solid yield and burn-of, deĄned in equation 6.2.1, is the same for the three studied biomasses. From Ągure 6.2, it can be observed that the three materials exhibit a linear correlation between the gasiĄcation time and the char burn-of.

However, for the same experimental conditions, the calculated char burn-of is always considerably higher for CS compared to BG and OPS. These results are in accordance with Daud and Ali [26], who analyzed the activation behavior of CS and OPS chars under a steam atmosphere, and observed a greater burn-of evolution of CS compared to OPS, with the activation time. This experimental observation is supported by the results of the calculated average steam gasiĄcation reactivity of the three samples, summarized in table 6.2. In fact, regardless of the gasiĄcation temperature, the reactivity values of CS were always higher compared to BG and OPS. For instance, at 850°C, the BG and OPS reactivities correspond to only 51% and 14% of the CS value respectively, explaining the lower solid yield and higher char burn-of observed for CS under the same experimental conditions. As discussed in a previous work, the diferences in the steam gasiĄcation reactivities of the samples can be mainly attributed to the inorganic composition of the raw materials [20]. Coconut shells main inorganic constituents are K and Ca, that have a catalytic impact on the steam gasiĄcation reactions [24,32,33], while oil palm shells and bamboo are mainly composed of Si and P, that tend to react with AAEM and inhibit their beneĄcial efect [34,35].

More speciĄcally, the inorganic ratio K/(Si+P) proposed by Hognon et al. [21] was employed in this work to analyze and compare the impact of the inorganic composition of the samples in their gasiĄcation reactivity. Coconut shells, with an inorganic ratio of 3.9 (K/(Si+P) > 1), have a higher proportion of K in relation to Si and P, and then, follow a catalytic gasiĄcation behavior. In contrast, BG and OPS, with inorganic ratios 0.2 and 0.17 respectively (K/(Si+P) < 1), have a higher proportion of Si and P that inhibit the beneĄcial impact of AAEM (principally K). In accordance to this, the TEM-EDX cartography in Ągure 6.3, shows the distribution of inorganic elements in BG chars produced at 850°C. It is possible to observe that K and Ca, with potential catalytic impact on steam gasiĄcation, may be associated with Si, explaining the inhibition of their beneĄcial efect. In this regard, the inorganic ratio K/(Si+P) can explain the diferences observed between the char burn-of evolution of CS in comparison to BG and OPS. TEM-EDX results are in accordance with SEM-EDX micrographs presented in Ąg.6.1. From the N 2 adsorption experiments, the speciĄc surface area of the gasiĄcation chars was determined. As expected, an increase in the BET char surface area was observed with the char burn-of. Indeed, the progress of the gasiĄcation reactions is related to carbon atoms consumption with the subsequent formation of pores in char structure [36]. This is in fact, the basis of the production of activated carbon through physical or thermal activation, attributed mainly to reaction R6.1, in the case of steam as the reacting atmosphere:

C + H 2 O --→ CO + H 2 (R6.1)
This reaction generates CO and H 2 as gasiĄcation fuel gases and creates a porous network in the char. GasiĄcation can also result in the chemisorption of oxygen and hydrogen in the char structure, creating surface oxygen complexes that inĆuence in an important way the physico-chemical properties of chars [37].

Accordingly, in Ągure 6.4, it is possible to observe that for the three biomasses, there is a linear increase in the BET surface area with the char burn-of, for values below 60%. Then, the BET surface area begins to decrease. The maximum BET surface area attained was 1041.8 m 2 /g for CS, 1014.0 m 2 /g for OPS and 807.7 m 2 /g for BG. Then, the subsequent decline may be due to the weakening and collapse of the char structure due to the progress of the gasiĄcation reactions [38Ű41]. For burn-of degrees below 30%, CS chars developed nearly 35% more surface area compared to BG and OPS chars. In contrast, above 30%, OPS surface area approached the CS char values. The behavior observed at low burn-of degrees may be attributed to the diferences in the gasiĄcation reactivity of the samples, while the results obtained for higher burn-of degrees could be related to the raw biomass structure and the ash content of chars. In particular, the high ash content of BG chars (higher than 30%), may explain the lower surface area development in comparison to CS and OPS, for burn-of degrees above 30% [42].

Nevertheless, it is worth mentioning that due to the diferences in the gasiĄcation reactivity of the samples, the burn-of degree attained is not the same under identical experimental conditions and then, the developed char surface area is also diferent. From table 6.3, it can be observed that CS surface area is always higher in comparison to similarly produced BG and OPS chars. In this regard and as stated previously, two trends can be identiĄed related to the inorganic composition of the samples. Under the same gasiĄcation conditions, feedstocks with inorganic ratio K/(Si+P) above 1 follow a catalytic gasiĄcation behavior and then, develop higher surface areas compared to samples with K/(Si+P) below 1.

From the analysis of N 2 adsorption tests, it was also found that for the three samples, the gasiĄcation chars obtained for burn-of degrees below 30% are highly microporous, with at least 85% of their surface area attributed to micropores. In contrast, for higher burn-of degrees, the microporosity of BG is reduced to 70%, while CS and OPS continue to show microporosity percentages above 87% and 82%, respectively. In fact, BG chars isotherms showed a higher development of narrow mesopores with the progress of the gasiĄcation process, as presented in Ągure 6.5.

To better understand the porous structure of the gasiĄcation chars, the pore-size distribution (PSD) calculated using the NLDFT model is presented in Ągure 6.6 for two diferent burn-of degrees. In both cases, it can be observed that the chars obtained from the three feedstocks are highly microporous. At a burn-of near 28% (Ągure 6.6a), it can be noticed that the surface area of CS and OPS chars can be mainly attributed to narrow microporosity (<1.0nm), while BG chars show also the existence of incipient mesopores. With the increase in the burn-of degree (Ągure 6.6b), the BG char pore-size distribution becomes larger, and the surface area attributed to narrow microporosity decreases considerably. In contrast, there is an increase in the proportion of micropores between 1-2 nm, and mesopores. For its part, CS still show a high proportion of narrow micropores and pores between 1-2 nm, with no visible development of mesoporosity. Finally, the OPS chars surface area is mostly associated to narrow micropores, with a lower proportion of pores between 1-2 nm, compared to the other feedstocks. These diferences may be related to the structure of the raw materials and their initial macromolecular composition. In fact, BG is a Ąbrous grass with high hemicellulose and cellulose content (>50%) [43]. Therefore, the thermal decomposition of these components during the pyrolysis stage may generate a more open char structure, resulting in a higher proportion of mesopores in the char. In contrast, CS and OPS are high-lignin endocarp feedstocks [44,45].

In accordance to this, the micrographs in Ągure 6.7, present the surface morphology of the obtained chars at a burn-of degree near 55%. It is possible to observe that the structure of BG is visibly diferent from CS and OPS. In particular, BG shows

an irregular and open structure with visible macroporosity, while CS and OPS have a relatively smooth surface with some macropores, and a more compact structure. As reported by Rodriguez-Reinoso [46], the pore development process during char gasiĄcation begins with the creation of micropores followed by their subsequent widening, and then the creation of new pores. In this regard, a less compact structure as in the case of BG, facilitates the access of steam into the char structure and the widening of the existing pores, developing meso and macropores. Unlike some reported studies that attribute the development of macro and mesopores to the lignin content of the feedstock [26,47], in the present study, the highest development of mesoporosity was observed for the samples with the highest hemicellulose and cellulose content.

In general terms, it can be said that the steam gasiĄcation of the analyzed feedstocks produces a highly microporous solid by-product with surface areas from 500 to 1000 m 2 /g, close to literature reported activated carbon values [47], or gasiĄcation chars produced under similar conditions [11]. The main characteristics of the analyzed gasiĄcation chars are summarized in table 6.3.

Char global carbon structure

The global carbon structure of the gasiĄcation chars was analyzed using X-ray difraction (XRD) and transmission electron microscopy (TEM). The difraction proĄles of all the analyzed chars exhibited two broad peaks at 2 ∼ 23°and 2 ∼ 44°, attributed to graphite bands (0 0 2) and (1 0 0) respectively. The Ąrst peak is associated to the stacking of graphitic basal planes, and the latter one to graphite-like atomic order within The observed broad peaks indicate that the analyzed chars have a short-range graphitelike structure, also denoted as turbostratic structure by several authors. Likewise, the background intensity suggests the existence of non-aromatic or amorphous carbon in the carbon matrix [48].

In this regard, the gasiĄcation chars from the three analyzed feedstocks have a similar disordered carbon structure. Some diferences are observed in the peak intensity of the difraction proĄles, probably related to the carbon content of the samples. OPS char, with a carbon content of 85.2% shows the highest (0 0 2) and (1 0 0) peak intensity, followed by CS and BG, with carbon contents of 79.2% and 60.8% respectively.

The quantitative analysis of the XRD patterns of carbon materials has been widely discussed in the literature. In the case of biochars and activated carbons, the assessment of structural parameters of the carbon structure is not easy due to the low crystallinity of the samples and their porous nature [49]. Several authors suggest that the analysis of the height, width and symmetry of the (0 0 2) and (1 0 0) peaks can give information about the crystalline structure of turbostratic carbons. In particular, the interlayer spacing of aromatic layers and the "crystallite" size [48, 50Ű52]. However, several discrepancies In the case of the analyzed chars, the quantitative analysis and comparison of the carbon structure from the height, width and symmetry of the (0 0 2) peak is not appropriate, as errors related to the inorganic content of the samples can be introduced. In particular, for BG chars, the existence of amorphous silica can also show a broad peak near 22°t hat can not be separated from the carbon peak [53,54].

In general, the XRD results are in accordance with the high resolution TEM micrographs of the samples. From the images in Ągure 6.9, it can be observed that the carbon structure of the three chars is very similar, with visible randomly oriented graphene-like layers. This short-range order has been reported by several authors as one of the characteristics of microporous chars, and is in agreement with the porous structure assessed using N 2 adsorption [50,55]. Due to the high inorganic content of BG char, some mineral peaks can be observed in the XRD difraction patterns. According to the JCPDS data base, these peaks correspond to SiO 2 with a tetragonal crystal arrangement (01-082-0512). In fact, for chars treated at temperatures below 600°C, the Si in the structure is mainly amorphous, while at higher temperatures, crystalline polymorphs can be formed [53,56,57]. In the case of CS and OPS chars, carbon peaks may hide XRD peaks of existing crystalline structures.

Char surface oxygen-containing functional groups

The thermal programmed desorption analysis conĄrmed the existence of a certain amount of oxygen-containing functional groups in the surface of chars. This result was expected, taking into account that the char gasiĄcation reactions can also result in the chemisorption of oxygen and hydrogen in the carbon matrix. For all the gasiĄcation conditions, the CO and CO 2 desorption of CS chars was higher compared to OPS and BG, suggesting the presence of a higher amount of surface oxygen complexes. This result is consistent with the steam gasiĄcation reactivity of the samples, since CS reactivity is considerably higher than OPS and BG. In accordance, under the same conditions, the steam gasiĄcation reactions proceed faster, resulting in higher char conversions and activation degrees.

The observed diferences between the three char samples were reduced with the increase of the steam percentage in the gasifying agent, as a greater steam quantity was available to react with the carbon matrix. As presented in Ągure 6.10, with 30% of steam in the gasifying agent, BG and OPS chars CO and CO 2 desorption was 53% and 64% lower than CS respectively. In contrast, with 90% of steam this diference was reduced to 29% and 31%. The diferences between the samples are still visible with the increase in the gasiĄcation temperature. The Ągure 6.11 shows the evolution of the total CO and CO 2 desorption with the gasiĄcation time for processes at 750°C and 850°C with 30% of steam in the gasifying agent. In all cases, the amount of oxygen-containing functional groups in the char surface is higher for CS, as well as the amount increase with the gasiĄcation time.

For CS, an augmentation in the gasiĄcation time from 1h to 2h at 850°C resulted in an increase of 60% in the CO and CO 2 desorption. In contrast, this trend is not observed for BG and OPS. Likewise, for 1h gasiĄcation, the gas desorption of CS is 30% and 65% higher in comparison to BG and OPS respectively. In Ągure 6.11b, it should be noted that no CS data are presented for 3 hours, as under these experimental conditions all the sample was consumed and no char was recovered. As presented in table 6.3, for all the gasiĄcation chars analyzed, the maximum total CO and CO 2 desorption observed was 2.32 and 0.98 mmol/g for CS and BG at 850°C and 30% of steam in the gasifying agent, and 0.8 mmol/g for OPS at 750°C and 90% of steam. These values, in the same range of literature reported amounts for steam activated carbons from lignocellulosic precursors [58,59], conĄrm the potential of steam gasiĄcation chars as value-added by-products for diferent applications.

It is important to note that for the analyzed conditions, even at the same burn-of degree, the observed CO and CO 2 desorption of chars was always in the order CS < BG < OPS. This trend is most likely associated to the reactivity of the samples, and particularly, to the inorganic composition of the raw materials. As discussed earlier, CS is an AAEM rich feedstock while BG and OPS main inorganic constituents are Si and P. More precisely, the deĄned inorganic ratio K/(Si+P) of the raw samples is 3.9, 0.2 and 0.17 respectively, following the order CS < BG < OPS.

A more detailed analysis of the CO and CO 2 desorption curves showed that the surface of the gasiĄcation chars contains an important proportion of acidic oxygen functional groups. In particular, the presence of carboxylic acids, peroxides, lactones and phenols was identiĄed. Furthermore, ether, quinone and pyrone-like functional groups, reported to have a basic and neutral character [60Ű62], were also identiĄed. These characteristics of chars are in line with the gasiĄcation conditions. The introduction of acidic groups (e.g., carboxylic acids) in the char surface has been associated by several authors to their steam treatment or activation [63].

In agreement with this observation, the measured pH PZC of the analyzed chars is in the acidic region, with values between 2 and 3, suggesting also the dominance of acidic groups over basic groups in the char surface [64,65]. No particular trends were observed with the gasiĄcation conditions, and in all cases, the measured values were in the described range. The low pH PZC values observed indicate that the samples surface will be negatively charged in almost all the pH range (pH values above pH PZC ), suggesting that steam gasiĄcation chars may be suitable for applications where the adsorption or retention of cations is required.

Relationship between char structure and surface chemistry

Finally, comparing the BET surface area and the TPD results of the analyzed chars, it was possible to notice that under the same gasiĄcation conditions, higher surface areas are also associated to a greater content of oxygen-containing functional groups in the char surface, as presented in Ągure 6.12. The observed relationship between the total CO and CO 2 desorption and the BET surface of the analyzed chars can be explained by the fact that the progress of the steam gasiĄcation reactions remove carbon atoms from the char matrix, developing a porous structure and including at the same time, oxygen-containing surface complexes [50,66]. Once gain, the inorganic composition of the raw sample could play an important role in the development of oxygen-containing surface groups, as it impacts the char steam gasiĄcation reactivity. According to Marsh [36], the enhanced rate of gasiĄcation reactions produced by AAEM and some transition metals may be attributed to the enhanced rate of formation of chemisorbed oxygen in the char matrix. This could explain the fact that for all the analyzed experimental conditions, CS chars showed the highest CO and CO 2 desorption. In this regard, the inorganic content of the raw biomass impacts not only its gasiĄcation rate, but also the properties of the gasiĄcation solid by-product.

In the context of developing countries, possible char applications related to soil amendment, carbon sequestration or waste water treatment could be considered according to the observed characteristics of gasiĄcation chars. Notably, soil applications may be of particular interest, taking into account the surface structure and chemistry of the analyzed chars and their important inorganic content.

Conclusion

The steam gasiĄcation chars produced from three diferent lignocellulosic agrowastes were analyzed in this work in order to understand the impact of the raw biomass characteristics in the properties of the gasiĄcation solid by-product. Coconut shells (CS), bamboo guadua (BG) and oil palm shells (OPS) were used as feedstocks.

The experimental results showed that the inorganic composition has an important impact on the gasiĄcation reactivity of the samples, and in consequence, an inĆuence on the porous structure development and surface chemistry of the gasiĄcation chars.

For samples with inorganic ratio K/(Si+P) above 1, the beneĄcial impact of AAEM (specially K) in the steam gasiĄcation reactions resulted also in higher surface area development and oxygen-containing functional groups in the char surface, in comparison to samples with inorganic ratio below 1. For its part, the pore size distribution of the chars seems to have a relation with the nature and the macromolecular composition of the raw feedstocks.

In general, the analyzed gasiĄcation chars are highly microporous carbonaceous materials, with speciĄc surface areas between 500 and 1000 m 2 /g. A dominance of acidic oxygenated functional groups in the char surface, with pH PZC values below 3 was observed in all cases. Accordingly, chars could be considered an interesting gasiĄcation by-product that could be valorized in applications where the adsorption or retention of cations is required. Moreover, the important inorganic content of chars suggests also their valorization in soil applications.

The experimental observations presented in this work could be an important reference for real gasiĄcation applications working with diferent kind of residues, when a simultaneous valorization of the gaseous and solid by-products is searched. According to the nature and composition of the samples, the gasiĄcation parameters and conditions could be adapted to obtain the application required by-products proportion, with the char needed characteristics. Future work will include the analysis of possible char applications according to their production conditions and properties. cation exchange and neutralization capacities. In particular, char from bamboo guadua, with an ash content above 30%, exhibited the most notable properties with regard to soil applications, with a cation exchange capacity (CEC) of 45 cmol c /kg and an acid neutralization capacity (ANC) of 125 cmol H + /kg. The observations in this work give an useful insight regarding a promising valorization pathway for steam gasiĄcation chars from lignocellulosic agrowastes.

Introduction

In the context of most developing countries, steam gasiĄcation of agricultural and agroindustrial residues could be an interesting thermochemical process for the simultaneous production of energy [1Ű3], and the development of business opportunities for local communities. In particular, considering the importance of agroindustrial activities for the economy of developing countries, the use of gasiĄcation chars in soil amendment and remediation applications may be an efective strategy to enhance soil fertility and improve crop yield.

Biochars are carbon-rich and porous materials that have been widely recognized as beneĄcial for soil amendment and remediation applications [4,5]. In particular, biochars usually show a high cation exchange capacity (CEC), associated to an important ability to adsorb and retain cations, and in consequence, to the improvement of nutrient retention if applied to soils [6]. Furthermore, the CEC of chars may also be important to the removal and immobilization of heavy metals in soils, making them unavailable for leaching or plant uptake [7].

The high alkalinity of most biochars, may also suggest the use of these materials in acidic soils, to improve their physico-chemical properties and productivity. Several authors have found that the incorporation of biochars in acidic soils increased their acid neutralization or pH bufering capacity (ANC) and enhanced the availability of nutrients for plant uptake, improving their retention in soils and avoiding their leaching [8Ű10].

Nevertheless, the impact of biochars in soils largely depends on their physico-chemical characteristics, generally determined by the raw materials and the production process [11,12]. For instance, in the case of pyrolysis chars, high cation exchange capacities have been observed for low temperature treatments, considering the low devolatilization extent attained. In contrast, a decrease in the cation exchange capacity with the increase of the pyrolysis temperature has been reported. In consequence, low temperature pyrolysis biochars usually show a better performance in soil applications, in comparison to high temperature pyrolysis samples [13].

Contrary to pyrolysis chars, steam gasiĄcation chars are generally produced at higher temperatures and under a reactive atmosphere, and thus, may have diferent physicochemical properties that should be analyzed with care in order to determine their suitability for soil amendment applications.

The use of gasiĄcation chars is not often reported in the literature, considering that gasiĄcation process is usually focused on gas production for energy applications. However, a higher soil amendment and remediation efectiveness has been observed for diferent activated carbons in comparison to pyrolysis biochars [14Ű17], suggesting that steam gasiĄcation chars could also have interesting properties in this Ąeld. In particular, steam gasiĄcation chars develop large speciĄc surface areas and may have an important amount of oxygen-containing functional groups, that could enhance properties like their cation exchange and acid neutralization capacities [18Ű20]. Moreover, due to the high mineral content of agrowastes, gasiĄcation chars from these feedstocks could be also a direct source of nutrients, promoting soil fertility and plant growth [21,22]. More speciĄcally, according to the raw biomass composition, biochar application to soils may supply diferent amounts of macro and micronutrients (e.g. N, P, S, Ca, Mg, Zn, Mn, Cu, B, Mo, Co, Fe, and Ni), considered essential for plant nutrition, and other beneĄcial elements for plant growth (e.g. Na, Co, Si, Se, and V). In the particular case of micronutrients (Zn, Mn, Cu, B, Mo, Co, Fe, and Ni), it is worth noting that even though they are essential for plant growth, they may become toxic beyond a threshold concentration [23].

In this regard, the aim of this work is to evaluate the physico-chemical properties of steam gasiĄcation chars produced from three diferent agrowastes, in order to determine their suitability to be used in soil amendment and remediation applications. Coconut shells (CS), bamboo guadua (BG) and oil palm shells (OPS) have been chosen as raw materials. The pH and bufering capacities of chars were analyzed, as well as their cation exchange capacity (CEC) and mineral release at diferent pH conditions. Accordingly, the agronomical implications of the observed gasiĄcation char properties were discussed, providing new insights to steam gasiĄcation chars valorization pathways.

Materials and methods

Materials

The steam gasiĄcation chars obtained from three tropical lignocellulosic biomasses were characterized and analyzed in this study, in order to determine their potential to be used in soil amendment applications. Oil palm shells (OPS), coconut shells (CS) and bamboo guadua (BG) were selected as raw materials. The detailed origin of the raw biomass, collected in Colombia-South America, as well as its organic and inorganic chemical composition are presented in a previous work [24].

GasiĄcation experiments

GasiĄcation experiments were performed at 850°C with a steam fraction of 30% in the gasifying atmosphere, considering that these conditions allow the production of high heating value fuel gases, with a relatively high gas eiciency, as presented in chapter 5.

The gasiĄcation process was stopped after 1 hour, in order to maximize the quantity of recovered char for further analysis.

The steam gasiĄcation experimental setup was described previously in chapter 3. BrieĆy, 80g of biomass were placed inside a semicontinuous lab-scale Ćuidized bed reactor and heated to 850°C at 20°C/min, under nitrogen. When the temperature was reached, the atmosphere was switched to a mixture of 30% steam/70%N 2 (vol.%), with a steam mass Ćow rate of 100 g/h at 180°C. After 1 hour, the reactor was cooled down to room temperature under nitrogen. Finally, the remaining char was collected, weighted and stored in a desiccator for subsequent analysis. The collected char particle size was between 1mm and 3mm.

Char characterization Elemental composition and ash content

The elemental analysis (CHNS) of the collected chars was determined using a Thermoquest NA 2000 elemental analyzer. To determine the inorganic composition, 150mg of ground char (with a particle size below 250µm) were acid digested in close vessels at 220°C during 4h. Acid reagents H 2 O 2 , HNO 3 , HF and H 3 BO 3 were used according to EN 16967. Acid solutions were diluted with demineralized water to 50 ml and analyzed using an HORIBA Jobin Yvol Ultima 2 inductively coupled plasma optical emission spectrometer (ICP-OES). For its part, the ash content of the samples was calculated according to the standard EN ISO 18122. All the analysis were performed with at least three replicates.

The local chemical composition of the samples at a micro-scale was also analyzed using a Hitachi TM3030 Plus tabletop scanning electron microscope (SEM), with an energy dispersive X-ray spectroscopy (EDX) module in the same apparatus.

pH

The pH of chars in aqueous solution was measured following the experimental protocol proposed by Denyes et al [25], by adding 0.25g of char to 25ml of distilled water. The suspension was shaken for two minutes and then centrifuged to collect the supernatant. The pH was measured using a calibrated Hach PHC725 probe.

pH at the point of zero-charge pH PZC

The pH value at which the net charge of the chars surface is 0, or point of zero-charge (pH PZC ), was measured using a Malvern Pananalytical Zetasizer NanoZS. Char samples were ground into powder, with a particle size below 250 µm. About 15 mg of sample were used for each test with an approximate sample concentration of 0.5 mg/ml of deionized water. Measurements were done at room temperature within the pH range from 2 to 11, adjusted by the addition of a 0.25M HCl or 0.1M NaOH solution. The carbon pH PZC was then deĄned as the pH at which the electrical charge density measured on the surface was zero.

Cation exchange capacity

In order to determine the ability of chars to retain nutrients in soils, their cation exchange capacity (CEC) was measured according to the sodium acetate method described by Laird and Fleming [26]. In general terms, this method involves the saturation of the char samples with Na + ions that replace the exchangeable cations in the char surface, followed by the subsequent displacement of Na + ions by a replacement solution. The amount of displaced Na + ions is measured using inductively coupled plasma-atomic emission spectrometry (ICP-OES). SpeciĄcally, 0.2 g of raw char (not ground) were placed in a 50ml centrifuge tube and washed three times with a 1M NaOAc solution with a pH adjusted to 8.2. The samples were then rinsed with a 80% isopropanol solution followed by a 100% isopropanol solution, until the measured electrical conductivity of the supernatant was below µS/cm. Finally, the retained Na + is displaced washing the sample 3 times with a 0.1M NH 4 Cl replacing solution. The extracted sodium content was analyzed, and the CEC of the samples in cmol/kg was calculated as presented in equation 7.

2.1 = 100 • • • (7.2.1)
Where is the measured Na concentration in mg/L, is the dilution volume of the analyzed solution in L, is the sample mass in g, and is the sodium molar mass in g/mol.

Acid neutralization capacity (ANC) and mineral release

In order to establish the char potential to resist acidiĄcation and to supply nutrients to soils, the char acid neutralization capacity (ANC) and the leaching behavior of inorganic constituents were determined using the protocol described in the NF EN 14429 standard [27]. Char samples were leached with water solutions containing pre-selected amounts of acid or base, in order to reach speciĄc pH values at the end of a 48h extraction period, within the pH range from 2 to 12. Preliminary titration tests were performed to determine the acid and base quantities required to obtain at least, 8 Ąnal pH values within the analyzed range. For this purpose, the maximum acid and base consumption to attain a Ąnal pH value of 2 and 12 where determined and then, divided by the number of pH intended to be tested. 5M HNO 3 and 2.5M NaOH solutions were used.

For each test, 750 mg of raw char (not ground) were leached with a liquid to solid ratio of 10 L/kg. The suspensions were mixed using a GLF 3040 rotating shaker at 6 rpm during 48h. At the end of the test, the pH of the suspensions was measured using a Hach PHC725 probe. The liquid phases were separated from the solid by Ąltration, and analyzed using inductively coupled plasma-atomic emission spectrometry (ICP-OES).

Results and discussion

Composition of gasiĄcation chars

Table 7.1 reports the organic composition and ash content of the gasiĄcation chars analyzed in the present study. It can be observed that the three samples are highly carbonaceous materials with carbon content between 60% and 85%. BG char, with the highest ash content (above 30%) exhibits also the lowest carbon percentage, in comparison to CS and OPS chars. In relation to inorganic composition, table 7.2 presents the main mineral constituents of the analyzed chars, relevant for agricultural applications and plant growth. It can be observed that the three samples contain non-negligible amounts of macro and micronutrients, and other beneĄcial elements for plants. Moreover, it can be noticed that the mineral constituents of the chars are in accordance with the composition of the raw materials. In particular, CS char is mainly composed of K, Ca and Na, while BG and OPS chars main constituents are Si, P, and K.

Char pH and pH PZC

The table 7.3 shows the pH and pH PZC values measured for the analyzed chars. It can be noted that the three samples have very similar pH values in the alkaline region, between 9 and 11. The high observed pH range could be mainly related to the inorganic composition of the chars, and particularly to the presence of alkali and alkaline earth metals as principal inorganic constituents [28,29]. However, no particular correlations were found between the alkalinity of the samples and their ash content. Regarding the pH PZC , the three chars showed similar values in the acidic region, between 1.5 and 3. Considering that the pH PZC represents the pH value at which the net surface of the char is 0, cationic adsorption will be favored at pH values above pH PZC , and anionic adsorption at values below. Accordingly, the low pH PZC observed indicates that the surface of the chars is negatively charged in almost all the pH range, and then, the samples may favor the adsorption or retention of cations in soil applications.

The low pH PZC measured values are in agreement with the TPD analysis of the chars, performed in a previous work (chapter 3), that conĄrmed the existence of a certain amount of acid oxygen-containing functional groups in the char surface. The presence of carboxylic acid, peroxides, lactones and phenols was identiĄed, and may contribute to the negative charge in the char surface at pH > pH PZC .

Cation exchange capacity (CEC)

The cation exchange capacity (CEC) of the steam gasiĄcation chars was also analyzed in order to determine their suitability to be used in soil amendment applications. Generally, CEC may represent the ability of chars to retain nutrients in the soil matrix and avoid their leaching. The three analyzed samples showed CEC values between 10 cmol/kg and 45 cmol/kg, as observed in Ągure 7.1. BG char exhibited the highest value with 44.2 cmol/kg, followed by CS and OPS chars with 15.8 and 11.5 cmol/kg respectively. Nevertheless, it can be observed that the cation exchange capacity of BG char is three times and four times higher than CS and OPS chars respectively. These diferences could be mainly related to the inorganic content of the samples, considering that BG char has a high ash content (>30%). In particular, from the SEM-EDX analysis of the samples, it was possible to determine that SiO 2 is one of the main mineral constituents of BG char, as presented in Ągure 7.2. SiO 2 may contribute in an important way to the cation exchange capacity, considering that this compound is negatively charged even at low pH, due to its very low pH PZC value (∼ 2.0) [30,31] For their part, considering the low ash content of CS and OPS chars (<10%), their cation exchange capacity may be mainly related to the oxygen-containing functional groups in their surface [18,32]. Weakly acid carboxylic and phenolic functional groups can dissociate and form negatively charged sites in the char surface, contributing to the cation exchange capacity. In this regard, as CS char has a considerably higher amount of oxygen-containing functional groups than OPS char, the former one exhibits also a higher CEC.

Although CEC is not an intrinsic property of chars and strongly depends on the measurement method used, the obtained results for the analyzed samples are in accordance with the values presented in the literature for several biochars from diferent feedstocks. From table 7.4, it can be observed that the analyzed samples and in particular BG char, show similar CEC values than those observed for low-temperature pyrolysis chars.

Several authors have reported that low-temperature biochars exhibit higher CEC values than high-temperature biochars, as most of the oxygen-containing functionalities in the char surface are lost with the thermal treatment. However, considering that steam gasiĄcation process can result in the creation of oxygen-containing complexes in the char surface, the analyzed samples show a cation exchange capacity comparable to low-temperature pyrolysis biochars.

Moreover, from the comparison of the pH and CEC values obtained for the analyzed chars and some reported acidic soils, it is possible to observe that gasiĄcation chars could have an interesting potential to be used in soil amendment applications. More speciĄcally, it can be noticed from table 7.5, that the three analyzed char samples have higher pH and CEC values in comparison to the ones reported for most acidic soils. In accordance, their application may result in the increase of the soil pH and CEC.

In particular, acidic soils may present several problems to plant growth and crop yields, mainly related to aluminum (Al) and manganese (Mn) toxicity to plants, and an important deĄciency of essential nutrients like phosphorous (P), potassium (K), calcium (Ca) and magnesium (Mg) [34]. Therefore, it is important to prevent soil acidiĄcation in agricultural activities, or remediate already degraded soils. Several authors have reported an increase in the CEC and pH of soils in the presence of biochar, providing beneĄts to soil fertility and crop yield [35,36].

Although the validation of the beneĄcial efect of the analyzed chars in soil pH and CEC needs further agronomic analysis, the present work gives a useful insight to a possible valorization pathway for the steam gasiĄcation solid by-product, in line with most developing countries context.

In addition to the presented beneĄts to soil fertility, the relatively high CEC of the analyzed chars may be also important to the removal of inorganic contaminants from soils. In particular, the adsorption of cationic heavy metals could be favored, making them unavailable for leaching or plant uptake [20,40]. Considering the low pH PZC values of steam gasiĄcation chars from agrowastes, heavy metal adsorption may be associated to electrostatic attraction between cationic metals and the negatively charged surface, as well as cation exchange between metal ions and mineral ions (K + , Na + , Ca 2+ and Mg 2+ ).

In the particular case of BG char, its high mineral content could signiĄcantly favor heavy metal removal or immobilization through electrostatic adsorption, as reported by diferent authors [41Ű43].

Acid neutralization capacity (ANC)

The acid neutralization capacity of chars was analyzed in order to establish their capability to resist acidiĄcation, if potentially applied to soils. The acid-base titration curves for the three samples, presented in Ągure 7.3, indicate the amount of acid consumed by each char to decrease its pH to a value of 2. In accordance, a higher acid consumption is related to a greater char bufering capacity. Likewise, in all the analyzed pH range, from 2 to 12, BG chars also showed the highest acid consumption, with 170 cmol/kg of H + protons, followed by CS and OPS, with 120 cmol/kg and 90 cmol/kg respectively.

The acid neutralization capacity of chars may be attributed to the acidic oxygencontaining functional groups in the char surface, and their mineral content, existing either as discrete phases or associated with functional groups [9,45]. In this regard, organic anions of weak acid functional groups like carboxyl ( Ű COO -) or hydroxyl ( Ű O -) groups may accept protons, increasing the char resistance to acidiĄcation. Moreover, alkali salts dissolution may also contribute in a very important extent to the char bufering capacity, as well as soluble silicate minerals [8].

Despite the fact that CS char has 30% more oxygen-containing functional groups than BG char, and 66% more than OPS char, BG char showed the highest acid neutralization capacity. In relation to this, it can be noted that BG char has a considerably higher inorganic content (>30% ash), in comparison to CS and OPS (<10% ash).

To better understand this behavior, the release of minerals during the acid-base titration experiments was analyzed and compared for the three samples. From Ągure 7.4, it can be observed that in all cases, the total sum of K + , Na + , Ca 2+ and Mg 2+ (base cations) released remained almost constant with the acid addition, while the soluble Si in the suspensions decreased with the pH. Despite the observed common trends, some diferences were noticed between the three samples. In particular, it can be noted that the amount of base cations released by CS and BG chars was approximately 4 times higher than OPS. Moreover, the Si release was higher for BG in almost all the pH range. at pH higher than 7. Moreover, no signiĄcant release of Zn, Mn, Cu, Ni, or Fe was measured during the leaching tests, even though these elements were identiĄed in the char composition.

In relation to the mineral release behavior, it can be noticed that for the three samples, the K and Na release was pH independent, while Ca and Mg release exhibited an increase with the solution acidity. The opposite trend was observed for Si, whose availability decreased with the pH. In the case of P, the leaching behavior of CS and BG was pH independent while a slight increase was observed for OPS with the pH decrease.

Furthermore, the amount leached of each element is in agreement with the initial composition of the chars. For instance, the highest K content in the leaching solution was observed for BG char in all the pH range, considering that this sample has the highest K content. The same trend was observed for the other analyzed elements with the three samples. Similar observations were reported by Ding et al. [49], suggesting that the composition of the chars could give a Ąrst insight to their mineral leaching potential and nutrient value.

Nevertheless, it is worth noting that the recovered amount of minerals is not necessarily the total initial content present in the chars. The mineral leachability may depend on diferent parameters related to the char morphology, and the occurrence forms and distribution of minerals [50]. On one side, the char morphological structure (e.g. pore structure, surface area) may afect the mineral accessibility, impacting also the observed release. On the other side, the solubility of minerals depends on their occurrence form (chemical form) and bonding to the carbon matrix [51].

In order to better understand the nutrient leaching potential of the analyzed chars, the amount released of each element is presented in Ągure 7.6, as a percentage of the total amount originally available in the char. A neutral pH has been chosen as a reference, considering that the optimum soil pH range is between 5.5 and 7.5 [52]. It can be observed that under the present conditions, K, P, and Na are the minerals that exhibit the highest availability, with a recovery percentage from 40% to nearly 100%. In particular, the recovered fraction of K was above 85% for the three samples, indicating that this element is mostly present in the char structure as soluble K-containing salts [51]. In the case of Si, CS char showed a higher availability compared to BG and OPS.

Considering that the solubility of amorphous silica is higher than that of crystalline silica [48], the leached amounts may correspond principally to the amorphous fraction present in chars. Taking into account the considerably higher amount of Si in BG and OPS chars in comparison to CS, the low leaching percentage observed for BG and OPS could indicate that an important amount of crystalline Si is present in their structure.

The results obtained in the present study are in accordance with some literature reported values, showing that the recyclability of Na and K is relatively high for biochars from diferent agrowastes [21,50]. Nevertheless, the recovered percentages cannot be directly compared, as diferent solid to liquid ratios have been used for leaching studies, resulting in deviations caused by solubility and saturation limitations.

In this regard, despite the fact that the leaching behavior of the analyzed biochars in real soil applications may difer from the presented values, this study gives an useful insight regarding the possible nutrient contribution of gasiĄcation chars from agrowastes.

Agronomical implications of char properties

The observed leaching behavior suggests that the gasiĄcation chars from the analyzed feedstocks may be a signiĄcant source of plant nutrients to soils. In the particular case of primary macronutrients (e.g. N, K, and P), the studied chars showed a high K and P availability, considering their leaching percentages. In the same way, secondary macronutrients such as Ca and Mg may be also supplied in some extent. Regarding micronutrients, also essential for plant metabolism and growth, only a slight release of Mo was observed for the analyzes chars.

Similarly, even though they are not essential for some plants, beneĄcial nutrients such Na and Si showed also a non-negligible release. In particular, Na is essential for the metabolism of some plants, and may improve the taste and texture of several crops. For its part, Si provides protection to plants from biotic and abiotic stress, and may improve their strength and productivity [23].

Considering that the recommended rates of biochar amendments in soils are between 5-50 ton char/ha [53,54], gasiĄcation chars could reduce the consumption of conventional fertilizers, notably regarding primary macronutrients (P and K). As an example, with a mean char application rate of 10 tons/ha, and assuming similar conditions to those of the leaching tests performed in this work, the average available mineral quantities that could be supplied to soil for plant uptake at a pH of 7, are presented in table 7.6. As observed, the amount of K released is signiĄcant for the three samples, as well as the P released by BG char. In this regard, considering the average macronutrients consumption reported for diferent crops, the analyzed gasiĄcation chars could partially replace conventional K and P fertilizers. In particular, CS and BG chars may supply up to 100% and 60% of the K and P needs of some low-input crops, presented in table 7.7. Besides the potential nutrient supply of gasiĄcation chars, their beneĄcial efects are also related to their high pH and their interesting cation exchange capacity (CEC) and acid neutralization capacity (ANC). In particular, the signiĄcant CEC measured for the samples indicates that gasiĄcation chars may increase the retention capacity of plant nutrients in soils, when supplied by conventional procedures (e.g. traditional fertilizers). This behavior can improve the fertilizer use eiciency, ameliorating the soil fertility. Moreover, the high pH observed for chars, as well as their signiĄcant acid neutralization capacity, suggest that the analyzed samples could be used for acidic soils amendment and remediation.

Conclusion

Steam gasiĄcation chars from diferent lignocellulosic agrowastes were analyzed in this study, in order to determine their suitability to be used in soil amendment and remediation applications. Coconut shells (CS), bamboo guadua (BG) and oil palm shells (OPS) were used as feedstocks.

The three analyzed chars showed an interesting potential to be used in soil amendment applications. In particular, their high pH values between 9 and 11, and their signiĄcant acid neutralization capacity above 62 cmol H + /kg, suggest the char capacity for the improvement of acidic soil characteristics. Moreover, their cation exchange capacity values between 11 and 45 cmol/kg may enhance the nutrient retention and fertility of several kinds of soils.

Experimental results showed that the inorganic content of the samples plays a primary role on the char properties with regard to soil applications. In particular, BG chars, with an ash content above 30%, exhibited the highest cation exchange and acid neutralization capacities, in comparison to chars with lower ash content.

The mineral release analysis of the three gasiĄcation chars revealed that these materials could be a source of essential and beneĄcial plant nutrients that could also impact the soil fertility and crop yield. Thanks to their high mineral content, CS and BG chars may supply up to 100% and 60% of the K and P needs of some low-input crops. In this regard, even when the inorganic composition of the chars does not necessarily reĆect their total mineral availability to plants, this information could give a Ąrst approach to their nutrient supply potential.

Although the behavior of chars in real applications may difer from the one observed during the laboratory characterization, the present work gives an useful insight regarding a promising valorization pathway for steam gasiĄcation chars from diferent lignocellulosic agrowastes.

General conclusion and prospects

Conclusion

In the current world energy, environmental, and economic context, agricultural and agroindustrial wastes represent an interesting, low-cost, and renewable resource for the production of biofuels and value-added products in both developing and developed countries.

In particular, considering the great variety of lignocellulosic agrowaste sources, the understanding of the impact of the biomass organic and inorganic composition on the pyro-gasiĄcation behavior is of great importance to adapt the process parameters to the available feedstocks and application requirements. Moreover, the understanding of the inĆuence of biomass characteristics on the pyro-gasiĄcation by-products is also essential to determine their most suitable valorization pathways.

The pyrolysis analysis performed at a thermogravimetric scale, revealed that the macromolecular composition of the samples is the main factor that inĆuences their pyrolysis behavior and product yield. Notably, the kinetic analysis performed showed that the lignin content may have an impact on the energy required to decompose the samples, considering that lignin is the binding agent of the biomass Ąbers. In this regard, oil palm shells, with the highest lignin content showed also the highest pyrolysis activation energy.

In contrast, in the case of gasiĄcation, despite the diferences in their macromolecular composition, inorganics showed to be the most important parameter inĆuencing the biomass reactivity and kinetics. Principally, the inorganic ratio K/(Si+P) proved to be suitable to describe the biomass steam gasiĄcation behavior. Samples with an inorganic ration below 1 exhibited a low gasiĄcation reactivity and a non-catalytic behavior, contrary to biomasses with inorganic ratio higher than 1. This observation is related to the interactions between the inorganic constituents of the samples. In particular, Si and P could react with K and other alkali and alkali earth metals (AAEM) to form silicates, restraining the known catalytic impact of the latter ones.

Regarding the produced gas, the diferences observed between the analyzed samples were mainly related to the gas yield and not to the gas composition. In fact, under the same experimental conditions, biomasses with higher reactivities produced also greater gas quantities, impacting the gas eiciency of the process. In this regard, it can be said that the inorganic composition of the samples also impacts the steam gasiĄcation gas eiciency, and then, is an important parameter to take into account in the design and operation of gasiĄcation facilities.

In relation to this, a new kinetic model was proposed to describe the steam gasiĄcation behavior of lignocellulosic biomass from the knowledge of its inorganic composition. This model could constitute a valuable tool for reactor design, and for the development and scale-up of gasiĄcation facilities working with diferent kind of agrowastes or blends.

The experimental results also showed that even though the inorganic content of the samples impact in a very important way the gasiĄcation reactivity and gas yield, the organic composition, and specially the H/C and O/C ratios seem to be related to the composition and heating value of the producer gas. With similar H/C and O/C ratios, no remarkable diferences were observed between the analyzed samples in terms of gas composition. In all cases, the producer gas H 2 /CO ratio between 2.5 and 4, suggests that steam gasiĄcation gas may be used in electricity generation applications using boilers, gas turbines, or internal combustion engines.

For its part, the analysis of the char produced from the steam gasiĄcation process revealed that the biomass characteristics also impact the char physico-chemical properties. For samples with inorganic ratio K/(Si+P) above 1, the beneĄcial impact of AAEM (specially K) on the steam gasiĄcation reactions resulted also in a higher speciĄc surface area development and a greater content of oxygen-containing functional groups in the char surface, in comparison to samples with inorganic ratio below 1. For its part, the pore size distribution of the chars seems to have a relation with the nature and the macromolecular composition of the raw feedstocks.

In all cases, the produced steam gasiĄcation chars exhibited a high microporosity, with surface areas between 500 and 1000 m 2 /g, similar to several activated carbons. Moreover, the signiĄcant mineral content of chars and the dominance of acidic oxygen-containing functional groups in their surface, with low pH PZC values, suggest the possibility to use these materials in applications where the adsorption or retention of cations is required, such as soil amendment or remediation applications.

In accordance, the speciĄc characterization of three selected steam gasiĄcation chars for soil applications revealed that these materials could have an interesting potential, particularly in the case of bamboo guadua. In this regard, the experimental results showed that the inorganic content of the samples plays a primary role on the char properties with regard to soil applications. In particular, bamboo guadua chars, with an ash content above 30%, exhibited the highest cation exchange and acid neutralization capacities, in comparison to chars with lower ash content. Moreover, the mineral release analysis of the three samples revealed that these materials could be a source of nutrients that could also impact the soil fertility and crop yield. Thanks to their high mineral content, CS and BG chars may supply up to 100% and 20% of the K and P needs of some low-input crops.

In general, the results obtained in the present work contributed to a better understanding of the steam gasiĄcation process of lignocellulosic agrowastes, and highlighted the important role of inorganics in their gasiĄcation behavior and by-products properties. Moreover, the present research showed that steam gasiĄcation could be considered as a suitable technique for the simultaneous valorization of lignocellulosic residues in energy applications, and the production of value-added products, that could represent an opportunity to implement productive activities in the context of both developing and developed countries.

In accordance, the design and operation of agrowaste gasiĄcation facilities should take into account the organic and inorganic composition of the feedstocks, in order to properly adapt the process parameters to ensure the production of the required by-products yield and properties.

Prospects

Steam gasiĄcation has proved to be an interesting alternative for the energy valorization of agrowastes and the simultaneous production of value-added products. The results obtained in the present work brought to light new challenges for further studies, as discussed below:

• Co-gasiĄcation experiments at a thermogravimetric scale could be extended to other agrowastes within a wider range of H/C and O/C ratios, in order to extend the application limits of the proposed kinetic model. In this study, only steam as gasifying agent was analyzed. However, in several applications, mixed atmospheres with air, steam, and CO 2 may be required or imposed by the reactor design (e.g. autothermal reactors). In accordance, the analysis of the impact of the biomass composition on the gasiĄcation behavior under mixed atmospheres should also be explored. In this regard, the gasiĄcation kinetic model proposed could be extended to take into account a reacting agent composed of diferent proportions of CO 2 or CO 2 /steam mixtures.

Moreover, in order to understand the impact of mixed reacting atmospheres on the yield and properties of the gasiĄcation by-products from lignocellulosic agrowastes, gasiĄcation experiments could also be performed at a laboratory scale, using the described Ćuidized bed reactor. A tar analysis and comparison between the selected samples could be considered. Likewise, the detailed physico-chemical characterization of the resulting chars will be required to determine their suitability in soil amendment and remediation applications, or other uses.

• The analysis of the steam gasiĄcation behavior of lignocellulosic agrowastes at a laboratory scale is extremely important to fully understand the physico-chemical mechanisms involved in the process. In this regard, the results obtained in the present work could constitute a valuable tool for the future scale-up of the steam gasiĄcation process using tropical lignocellulosic agrowastes. In particular, the analysis of diferent gasiĄcation technologies (e.g. updraft Ąxed bed gasiĄer, downdraft Ąxed bed gasiĄer, etc) may also be an interesting work to develop in the future, in order to determine the most suitable conditions adapted to real applications in developing countries. A life cycle analysis approach could be considered for the comparison of gasiĄcation technologies and by-products valorization pathways.

Considering other gasiĄcation technologies, the evaluation of the steam gasiĄcation process using the autothermal downdraft reactor described at the beginning of the present document could also be an interesting work in the future, taking into account that this technology is widely employed for small scale gasiĄcation facilities. In this regard, the development of the present work has been the starting point of several Ąnancing possibilities at the Colombian National University to redesign, adapt, and adjust the existing reactor for steam gasiĄcation tests and subsequent char recovery.

• In relation to the evaluation of the steam gasiĄcation char potential to be used in soil amendment applications, the mineral release analysis of the selected samples revealed that chars could be a non-negligible source of nutrients that could impact the soil fertility. The nutrient extraction tests in the present work were performed using a Ąxed solid to liquid ratio, and extraction time. Even though these conditions gave a Ąrst insight of the char nutrient availability, their release dynamics could not be established. In order to determine the long term availability of char nutrients to soils, various leaching times and solid to liquid ratios should be tested. Moreover, a leaching kinetic analysis could also be performed in order to better predict the char mineral release in real soil applications.

Moreover, an analysis concerning the inĆuence of char morphology and inorganic chemical forms in the mineral leaching potential and nutrient availability could be performed.

• Considering their relatively high mineral content and cation exchange capacity, the suitability of steam gasiĄcation chars from lignocellulosic residues to be used in soil remediation applications could be also an interesting research approach for future work. Principally, their ability to adsorb and retain inorganic contaminants from soils, such as heavy metals. In the particular case of Colombia, several agricultural soils are polluted by Cu, Ni, Pb, Cd, Hg, and Zn, principally due to mining activities, validating the interest of the proposed approach. Moreover, in developed countries, heavy metal pollution may also be important in some locations due to anthropogenic activities.

• The physico-chemical characterization of gasiĄcation chars revealed their potential to be used in soil amendment applications. Despite the fact that incubation experiments to evaluate the efect of chars on real soils are not under the scope of the present work, future interdisciplinary projects including agronomic research specialists may help to determine the real impact of chars in soils. In particular, a recent research cooperation accord with the agronomy school of the National University of Colombia may allow the laboratory characterization and analysis of diferent steam gasiĄcation chars and acidic soils blends. Moreover, incubation plant growth experiments could be also performed.

Steam gasiĄcation of tropical lignocellulosic agrowaste: impact of biomass characteristics on the gaseous and solid by-products

In the context of most developing countries, steam gasiĄcation could be a very interesting process for both energy generation in isolated areas and the production of value-added products from lignocellulosic agrowaste. Considering that the availability of agricultural residues is often seasonal, gasiĄcation facilities should operate with diferent feedstocks. In consequence, this work is focused on the understanding of the impact of biomass characteristics on the gasiĄcation process and the properties of the gaseous and solid by-products. Three lignocellulosic agrowastes with diferent macromolecular structure and inorganic composition were selected for this study: Coconut shells (CS), bamboo guadua (BG) and oil palm shells (OPS). The thermal decomposition kinetics of the selected feedstocks was analyzed in a thermogravimetric scale under inert and steam atmosphere. Despite the diferences in their macromolecular composition, inorganics showed to be the most important parameter inĆuencing the steam gasiĄcation reactivity and kinetics of the samples. The beneĄcial impact of AAEM was conĄrmed, as well as the inhibitory efect of Si and P. More speciĄcally, the ratio K/(Si+P) proved to be suitable to describe and compare the steam gasiĄcation behavior of lignocellulosic agrowastes. In accordance, a new kinetic modeling approach was proposed to predict the gasiĄcation behavior of samples, from the knowledge of their inorganic composition. The validity of the ratio K/(Si+P) to classify and predict the biomass steam gasiĄcation behavior was also conĄrmed from experiments in a lab-scale Ćuidized bed gasiĄer. Samples with K/(Si+P) above 1 exhibited higher gasiĄcation reactivities compared to samples with ratios below 1, resulting in greater gas yields and higher gas efficiencies. Moreover, inorganics impacted not only the gasiĄcation rate of the samples, but also the properties of the gasiĄcation solid by-products. In particular, higher gasiĄcation reactivities were related to greater char surface areas and contents of oxygenated surface functional groups. A temperature of 850°C and a steam fraction of 30% in the reacting atmosphere proved to be the most suitable gasiĄcation conditions for the simultaneous production of fuel gases for energy applications, and a valuable char that could be valorized in soil amendment applications. The gasiĄcation model and experimental results presented in this work might be an important reference for real gasiĄcation applications working with diferent kind of residues, when both the gaseous and solid by-products valorization is intended. Moreover, in the presented context, steam gasiĄcation of lignocellulosic agrowaste may improve the energy access in rural isolated areas, and simultaneously promote the development of productive projects that could generate new incomes for local communities.

Keywords: Steam gasiĄcation, GasiĄcation reactivity, Lignocellulosic agrowaste, Inorganic composition, Biochar.

GazéiĄcation sous vapeur d'eau de résidus agricoles : impact des caractéristiques de la biomasse sur les propriétés des sous-produits gazeux et solides

Dans le contexte économique de la plupart des pays en voie de développement, la gazéiĄcation sous vapeur d'eau de résidus agricoles lignocellulosiques pourrait être un procédé intéressant, à la fois pour la génération d'énergie dans des régions isolées et pour la production des produits à valeur ajoutée. Étant donné que la disponibilité des résidus agricoles est souvent saisonnière, diférents types de biomasse doivent être utilisés pour assurer le fonctionnement des installations de gazéiĄcation. A cet égard, ce travail est axé sur la compréhension de l'impact des caractéristiques de la biomasse sur le procédé de gazéiĄcation et les propriétés des sous-produits gazeux et solides. Trois biomasses lignocellulosiques à composition macromoléculaire et inorganique diférentes ont été sélectionnées pour cette étude : coques de noix de coco (CS), bambou guadua (BG) et coques de palmier à huile (OPS). La cinétique de décomposition thermique des biomasses a été étudiée une échelle thermogravimétrique sous atmosphère inerte et sous vapeur d'eau. Malgré les diférences dans la structure macromoléculaire des échantillons, la composition inorganique s'est avérée être le paramètre le plus important inĆuençant la réactivité et la cinétique de gazéiĄcation. L'impact bénéĄque des métaux alcalins et alcalino-terreux a été conĄrmé, ainsi que l'efet inhibiteur du Si et du P. Plus précisément, le ratio K/(Si + P) est considéré approprié pour décrire et comparer le comportement des biomasses pendant la gazéiĄcation sous vapeur d'eau. En conséquence, une nouvelle approche pour la modélisation de la cinétique de gazéiĄcation à partir de la composition inorganique de l'échantillon a été proposée. La validité du ratio K/(Si + P) pour classiĄer et prédire le comportement des biomasses a également été conĄrmée par des expériences dans un réacteur à lit Ćuidisé à l'échelle laboratoire. Les échantillons avec un ratio K/(Si + P) au-dessus de 1 ont montré des réactivités de gazéiĄcation supérieures à celles des échantillons dont le ratio était inférieur à 1, et donc, une production de gaz et un rendement énergétique plus élevés. De plus, la composition inorganique a non seulement impacté le taux de gazéiĄcation des échantillons, mais également les propriétés du sous-produit solide. En particulier, une réactivité de gazéiĄcation plus élevée est liée à des chars avec une surface spéciĄque et un nombre de groupes fonctionnels plus importants. Une température de 850°C et une fraction de vapeur de 30% dans l'agent de réaction ont été identiĄées comme les conditions les plus adaptées à la production simultanée de gaz combustible et de char pouvant être valorisé dans des applications agricoles. Le modèle de gazéiĄcation sous vapeur d'eau et les résultats expérimentaux présentés dans ce travail peuvent être une référence pour des applications réelles de gazéiĄcation travaillant avec diférents types de résidus. Par ailleurs, dans le contexte présenté, la gazéiĄcation sous vapeur d'eau de déchets lignocellulosiques peut améliorer l'accès à l'énergie des zones rurales isolées, en promouvant simultanément le développement de projets productifs susceptibles de générer de nouveaux revenus pour les communautés locales.

Mots-clés : GazéiĄcation sous vapeur d'eau, Réactivité, Biomasse lignocellulosique, Composition inorganique, Biochar.

Figure 1 :

 1 Figure 1: Possible thermochemical transformation processes for lignocellulosic agrowastes. GasiĄcation by-products and valorization pathways

Figure 2 :

 2 Figure 2: Procédés de transformation thermochimique des résidus agricoles lignocellulosiques. Sous-produits de la gazéiĄcation et voies de valorisation.

Figure 3 :

 3 Figure 3: Structure du manuscrit

Figure 4 :

 4 Figure 4: Population without access to electricity (millions). Data: IEA, 2017.

Figure 6 :

 6 Figure 6: Possible thermochemical transformation processes for lignocellulosic agrowastes. GasiĄcation by-products and valorization pathways.

Figure 7 :

 7 Figure 7: Structure of the manuscript

Figure 1 . 2 :

 12 Figure 1.2: Fixed-bed reactors designs. a) Updraft (counter-current), b) Downdraft (co-current). Adapted from[8]

Figure 1 . 3 :

 13 Figure 1.3: Fluidized-bed reactors design. a) Bubbling bed gasiĄer, b) Circulating Ćuidized gasiĄer. Adapted from[42]

Figure 1 .

 1 Figure 1.5 shows an example of oxygenated functional groups that can potentially exist on the char surface.

Figure 1 . 5 :

 15 Figure 1.5: Oxygen functional groups and thermal stability. Adapted from[60]

Figure 2 . 1 :

 21 Figure 2.1: Selected gasiĄcation feedstocks. a) Coconut shells, b) Bamboo guadua, c) Oil palm shells

Figure 2 . 2 :

 22 Figure 2.2: Fixed bed downdraft reactor experimental setup and characteristic temperature proĄle during a gasiĄcation test.

Figure 2 . 3 :

 23 Figure 2.3: Fluidized bed gasiĄer experimental setup and characteristic temperature proĄle inside the reactor.

Figure 2 . 4 :

 24 Figure 2.4: Thermogravimetric analysis experimental program. a) pyrolysis, b) steam gasiĄcation

Figure 2 . 6 :

 26 Figure 2.6: Inlet and outlet streams to and from the reactor during the gasiĄcation stage.

Figure 2 . 7 :

 27 Figure 2.7: Raw biomass characterization methods [23Ű28]

Figure 2 . 8 :

 28 Figure 2.8: General methodology used for the development of the experimental activities and associated chapters

  Friedman ln(Ð/) = ln [ (Ð)] -/ (Ñ Ð/ ) Vs 1/ KAS ln(Ñ/ 2 ) = ln [ / (Ð)] -/ ln(Ñ/ 2 ) Vs 1/ FWO Ñ = [ / (Ð)] -2.315 -0.4567 / Ñ Vs 1/ KAS -Kissinger-Akahira-Sunose, FWO -Flynn-Wall-Ozawa

Figure 3 . 1 :

 31 Figure 3.1: Generalized master plots of the diferent kinetic models in table, constructed according to Eq.

Figure 3 . 2 :

 32 Figure 3.2: TG and DTG curves at 2, 5, 10, and 20°C/min. a) Oil palm shells (OPS), b) Coconut shells (CS), c) Bamboo guadua (BG), d) Comparison of TG and DTG curves at 5°C/min. Mass loss values and char yield are presented in a dry basis

Figure 3 . 3 :

 33 Figure 3.3: Apparent activation energy values of BG, CS and OPS pseudocomponents, as a function of reaction extent

Figure 3 . 4 :

 34 Figure 3.4: Comparison between experimental curves and Fraser-Suzuki deconvolution results of a) BG at 10°C/min. b) CS at 10°C/min. c) OPS at 10°C/min. P-HC pseudo-hemicellulose; PŰC pseudoŰcellulose; PŰL pseudoŰlignin

Figure 3 . 5 :

 35 Figure 3.5: Comparison between the experimental and theoretical master plots for the three biomass pseudo-components. a) pseudo-hemicellulose, b) pseudo-cellulose, c) pseudo-lignin, d) pseudo-lignin (0.5<Ð<1.0)

Figure 3 . 6 :

 36 Figure 3.6: Pseudo-component dependence of ln() on the reaction extent. a) pseudo-hemicellulose, b) pseudo-cellulose, c) pseudo-lignin

Figure 3 . 7 :

 37 Figure 3.7: Comparison between experimental and modelled decomposition curves of a) BG at 10°C/min. b) CS at 10°C/min. c) OPS at 10°C/min. Modelled curves generated using the calculated and values presented in Table3.7 for biomasses and their pseudo-components.

Figure 4 . 1 :

 41 Figure 4.1: Generalized master-plots of the reaction models presented in table 4.1, calculated according to Eq. 4.2.5.

Figure 4 .

 4 Figure 4.2 presents the conversion degree Ð as a function of time of coconut shells (CS), at three diferent gasiĄcation temperatures and a steam partial pressure of 3.7 kPa. It is possible to observe that the gasiĄcation conversion rate is highly dependent on the temperature. An increase in the gasiĄcation temperature is associated with a reduction of the time required to reach a speciĄc conversion degree. Accordingly, shorter reaction times are related to higher gasiĄcation reactivities.

Figure 4 . 2 :

 42 Figure 4.2: Conversion degree Ð vs. time of coconut shells (CS) at three diferent gasiĄcation temperatures and a steam partial pressure of 3.7 kPa.

Figure 4 . 3 :

 43 Figure 4.3: Conversion degree Ð vs. time of coconut shells (CS) at three diferent steam partial pressures and 800°C.

Figure 4 . 4 :

 44 Figure 4.4: Conversion proĄle of the selected biomasses at 800°C and a steam partial pressure of 3.7 kPa.

Figure 4 . 5 :

 45 Figure 4.5: Conversion degree vs. time of CS and BG at diferent gasiĄcation temperatures. a) CS -10 kPa, b) BG -10 kPa

Figure 4 . 6 :

 46 Figure 4.6: Steam gasiĄcation reactivity at a conversion degree of 50% of biomasses and blends at diferent temperatures and a steam partial pressure of 3.7 kPa

Figure 4 . 7 :

 47 Figure 4.7: Comparison between the theoretical and experimental master plots of some analyzed samples and blends

Figure 4 . 10 :

 410 Figure 4.10: Apparent activation energy of analyzed samples as a function of the inorganic ratio.

Figure 4 . 11 :

 411 Figure 4.11: Comparison between the experimental and modeled decomposition curves of analyzed samples at 800°C and 3.7kPa.

Figure 5 . 1 :

 51 Figure 5.1: Schematic diagram of the gasiĄcation experimental setup

Figure 5 . 4 :

 54 Figure 5.4: Evolution of the solid, gas and liquid products with the steam percentage in the gasiĄcation agent. GasiĄcation temperature and time: 750°C, 1 hour. a) char, b) gas, c) liquid (water+tars)

Figure 5 . 5 :

 55 Figure 5.5: Energy distribution in the gasiĄcation products at 750°C as a function of the steam percentage in the gasifying agent. a) Coconut shells. b) Bamboo guadua

Figure 5 . 6 :

 56 Figure 5.6: Evolution of the solid, liquid and gas yield with the gasiĄcation temperature. 1 hour gasiĄcation with 30% of steam in the gasifying agent. a) Coconut shells, b) Bamboo guadua, c) Oil palm shells

Figure 5 . 7 :

 57 Figure 5.7: Evolution of the energy distribution in the gasiĄcation products with the temperature. 1 hour gasiĄcation with 30% of steam in the gasifying agent. a) Coconut shells, b) Bamboo guadua, c) Oil palm shells

Figure 5 . 8 :

 58 Figure 5.8: Gas production evolution with the gasiĄcation time. a) GasiĄcation at 750°C and 30% of steam in the gasifying agent, b) GasiĄcation at 850°C and 30% of steam in the gasifying agent.

Figure 5 . 10 :

 510 Figure 5.10: Produced gas average composition. 1 hour gasiĄcation with 30% of steam in the gasifying agent. a) Coconut shells, b) Bamboo guadua, c) Oil palm shells

Figure 6 . 1 :

 61 Figure 6.1: SEM-EDX micrographs of analyzed chars. GasiĄcation conditions: 1 hour, 850°C, 30% steam in the gasifying agent

Figure 6 . 2 :

 62 Figure 6.2: Char burn-of Vs. time of studied biomasses gasiĄed in a 30% steam atmosphere. a) 750°C, b) 850°C

Figure 6 . 3 :

 63 Figure 6.3: TEM-EDX cartographic images of BG char. GasiĄcation conditions: 1 hour, 850°C, 30% steam in the gasifying agent

Figure 6 . 4 :

 64 Figure 6.4: BET surface area of gasiĄcation chars in relation to burn-of

Figure 6 . 5 :

 65 Figure 6.5: N 2 adsorption isotherms of chars at ∼ 55% burn-of. a) Coconut shells (CS_30_850_1), b) Oil palm shells (OPS_30_850_4.5), c) Bamboo guadua (BG_30_850_1)

Figure 6 . 6 :

 66 Figure 6.6: Pore size distribution of gasiĄcation chars. a) ∼ 28% burn-of. b) ∼ 55% burn-of

Figure 6 . 7 :

 67 Figure 6.7: SEM micrographs of gasiĄcation chars at a burn-of degree near 55%. a) coconut shells, b) oil palm shells, c) bamboo guadua -cross-section view, d) bamboo guadua -longitudinal view

Figure 6 . 8 :

 68 Figure 6.8: Difraction proĄles of chars produced at 850°C during 1h gasiĄcation with 30% of steam in the gasifying agent

Figure 6 . 9 :

 69 Figure 6.9: High-resolution transmission electron microscopy of chars produced at 850°C during 1h gasiĄcation with 30% of steam in the gasifying agent. a) CS, b) BG, c) OPS.

Figure 6 . 10 :

 610 Figure 6.10: Total CO and CO 2 desorption of steam gasiĄcation chars during TPD analysis. GasiĄcation conditions: a) 1 hour at 750°C with 30% of steam in the gasifying agent. b) 1 hour at 750°C with 90% of steam in the gasifying agent.

Figure 6 . 11 :

 611 Figure 6.11: Evolution of total CO and CO 2 desorption with time of steam gasiĄcation chars during TPD analysis. a) gasiĄcation at 750°C with 30% of steam in the gasifying agent. b) gasiĄcation at 850°C with 30% of steam in the gasifying agent.

Figure 6 . 12 :

 612 Figure 6.12: Total CO and CO 2 desorption Vs. BET surface area of chars at the same gasiĄcation conditions. a) 30% steam in the gasiĄcation agent, 750°C and 1h of gasiĄcation time. b) 30% steam in the gasiĄcation agent, 850°C and 1h of gasiĄcation time.

Figure 7 . 1 :

 71 Figure 7.1: Cation exchange capacity (CEC) and pH PZC of the analyzed chars

Figure 7 . 2 :

 72 Figure 7.2: SEM-EDX analysis of BG steam gasiĄcation chars.

Figure 7 . 3 :

 73 Figure 7.3: Changes in biochar suspension pH as a function of acid-base titration

Figure 7 . 5 :

 75 Figure 7.5: Efect of pH on the K, Ca, Mg, Na, P, and Si release from gasiĄcation chars.

Figure 7 . 6 :

 76 Figure 7.6: Release percentage of mineral species from chars at a pH = 7, during a 48h leaching test with a liquid to solid ratio of 10 L/kg

Figure 7 . 7 :

 77 Figure 7.7: Steam gasiĄcation chars properties and associated soil beneĄcial efects

6. 10

 10 Total CO and CO 2 desorption of steam gasiĄcation chars during TPD analysis. GasiĄcation conditions: a) 1 hour at 750°C with 30% of steam in the gasifying agent. b) 1 hour at 750°C with 90% of steam in the gasifying agent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11 Evolution of total CO and CO 2 desorption with time of steam gasiĄcation chars during TPD analysis. a) gasiĄcation at 750°C with 30% of steam in the gasifying agent. b) gasiĄcation at 850°C with 30% of steam in the gasifying agent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12 Total CO and CO 2 desorption Vs. BET surface area of chars at the same gasiĄcation conditions. a) 30% steam in the gasiĄcation agent, 750°C and 1h of gasiĄcation time. b) 30% steam in the gasiĄcation agent, 850°C and 1h of gasiĄcation time. . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Cation exchange capacity (CEC) and pH PZC of the analyzed chars . . . 7.2 SEM-EDX analysis of BG steam gasiĄcation chars. . . . . . . . . . . . . 7.3 Changes in biochar suspension pH as a function of acid-base titration . 7.4 Mineral release during the acid-base titration experiments. a) Sum of base cations (K + , Na + , Ca 2+ and Mg 2+ ), b) Soluble Si. . . . . . . . . . 7.5 Efect of pH on the K, Ca, Mg, Na, P, and Si release from gasiĄcation chars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Release percentage of mineral species from chars at a pH = 7, during a 48h leaching test with a liquid to solid ratio of 10 L/kg . . . . . . . . . . 7.7 Steam gasiĄcation chars properties and associated soil beneĄcial efects . B.1 TPD spectra and deconvolution of CS chars produced at 850°C with a steam fraction of 30% in the gasifying agent. 1 hour gasiĄcation time. a) CO 2 desorption, b) CO desorption. . . . . . . . . . . . . . . . . . . . . . B.2 TPD spectra and deconvolution of BG chars produced at 850°C with a steam fraction of 30% in the gasifying agent. 1 hour gasiĄcation time. a) CO 2 desorption, b) CO desorption. . . . . . . . . . . . . . . . . . . . . . B.3 TPD spectra and deconvolution of OPS chars produced at 850°C with a steam fraction of 30% in the gasifying agent. 1 hour gasiĄcation time. a) CO 2 desorption, b) CO desorption. . . . . . . . . . . . . . . . . . . . . . List of Tables 1.1 Cellulose, hemicellulose and lignin content of lignocellulosic biomass. . . 1.2 Composition of some lignocellulosic agrowastes from literature. . . . . . 1.3 Main chemical reactions involved in biomass gasiĄcation process. . . . . 1.4 Heating value of produced gas based on the gasifying agent. . . . . . . . 1.5 Advantages and technical challenges of diferent gasifying agents. . . . . 1.6 Advantages and technical challenges of the principal reactor designs. . . 2.1 Organic composition of some representative crop residues reported in the literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Major inorganic composition of some representative crop residues reported in the literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 GasiĄcation chars characterization methods . . . . . . . . . . . . . . . . 3.1 Heating value and chemical composition of studied biomasses. . . . . . . 3.2 Most common reaction mechanisms used in solid-state kinetic analysis related to biomass thermal decomposition . . . . . . . . . . . . . . . . . 3.3 Kinetic model-free methods used in this study . . . . . . . . . . . . . . . 3.4 Fitting results of Fraser-Suzuki deconvolution of selected biomasses. (P-HC pseudo-hemicellulose; P-C pseudo-cellulose; P-L pseudo-lignin) . . . 3.5 R 2 correlation coeicient of isoconversional Arrhenius plots for the three studied biomasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Mean activation energy values of OPS, CS and BG pseudo-components, calculated using Friedman, KAS and FWO model-free methods. . . . . . 3.7 Average and values calculated for BG, CS and OPS pseudo-components 4.1 Organic composition of studied biomasses . . . . . . . . . . . . . . . . . 4.2 Major inorganic composition of studied biomasses . . . . . . . . . . . . . 4.3 Most common reaction mechanisms used in solid state kinetic analysis [24,26] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Steam gasiĄcation reactivity and reaction time at 50% conversion. Steam partial pressure 3.7 kPa . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 Background and literature review

  . . . . . . . . . . . . . . . . . . . . . . . .

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Biomass pyro-gasiĄcation basics . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Lignocellulosic biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Pyro-gasiĄcation process and reactions . . . . . . . . . . . . . . . . . . 1.2.3 Pyro-gasiĄcation by-products . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 Pyro-gasiĄcation technologies . . . . . . . . . . . . . . . . . . . . . . . .

  Pour sa part, la section 1.3 est dédiée à la description du sous-produit solide issu du procédé de pyro-gazéiĄcation, aussi appelé char. Particulièrement, l'inĆuence des paramètres opératoires du procédé sur son rendement et ses propriétés physico-chimiques est discutée dans un premier temps. Ensuite, une description des principales caractéristiques des chars et de leur intérêt vis-à-vis de diférentes applications et voies de valorisation est présentée. EnĄn, l'attention est portée sur les progrès récents de la recherche en relation avec la valorisation des chars dans plusieurs applications. Notamment, leur utilisation dans des applications environnementales et agricoles s'avère particulièrement intéressante dans le cadre des besoins des pays en voie de développement.

L'objectif de ce premier chapitre est de donner les éléments nécessaires pour comprendre les choix et les orientations de recherche développés dans le présent travail. A cet égard, les concepts de base relatifs à la gazéiĄcation de biomasses lignocellulosiques sont décrits, ainsi que les progrès de la recherche actuelle sur les voies de valorisation des sous-produits. Une attention particulière est adressée au résidu solide de la gazéiĄcation et ses propriétés.

Dans un premier temps, la section 1.2 décrit les principales caractéristiques de la biomasse lignocellulosique en tant que matière première approprié pour les procédés de transformation thermochimique. Une attention particulière est accordée aux résidus agricoles et agro-industriels. Ensuite, les diférentes étapes du procédé de pyro-gazéiĄcation sont détaillées, en décrivant les réactions chimiques associées et leurs principaux sous-produits. Un accent spécial est porté sur les diférents paramètres opératoires qui impactent le procédé, tels que la température, l'agent de réaction, et la composition la biomasse. xxv Également, les technologies les plus couramment utilisées pour la pyro-gazéiĄcation sont décrites et comparées.

  En relation avec ceci, les résultats expérimentaux ont conĄrmé l'efet bénéĄque des métaux alcalins et alcalino-terreux, principalement celui du K, sur la réactivité à la gazéiĄcation sous vapeur d'eau des biomasses, ainsi que l'efet inhibiteur d'éléments comme le Si, l'Al et le P. En particulier, une relation linéaire a été observé entre la réactivité des échantillons et leur ratio inorganique K/(Si+P), pour toutes les conditions expérimentales étudiées. D'autre part, l'étude de la cinétique du procédé a également permis d'observer une relation entre le mécanisme réactionnel décrivant la gazéiĄcation sous vapeur d'eau des biomasses et leur composition inorganique. Particulièrement, une relation linéaire inverse a été constaté être l'ordre de la réaction par rapport au solide, et le ratio inorganique (K/Si+P) de chaque biomasse ou mélange de biomasses.De ce fait, un nouveau modèle cinétique tenant compte de la composition inorganique de biomasses a été proposé pour décrire le procédé de gazéiĄcation sous vapeur d'eau. La validation expérimentale du modèle a montré des erreurs inférieures à 10%, considérées raisonnables en tenant compte de l'hétérogénéité des biomasses et des incertitudes dans la détermination de leur composition inorganique.

	La détermination des paramètres cinétiques du procédé est basée sur des méthodes
	d'isoconversion ou "model-free isoconversional methods", permettant de calculer l'énergie
	d'activation sans une hypothèse préalable du modèle de réaction, qui pourrait conduire
	au calcul des paramètres cinétiques inexacts. A cet égard, l'approche "master-plots" a
	été utilisé pour l'identiĄcation du modèle de réaction le plus approprié pour chaque
	condition expérimentale. Ainsi, aĄn de comparer les échantillons, le modèle de réaction

Dans ce chapitre, l'analyse de la gazéiĄcation sous vapeur d'eau des biomasses d'étude et leurs mélanges a été faite à une échelle thermogravimétrique, avec des températures entre 750°C et 900°C, et pressions partielles de vapeur entre 3 kPa et 10 kPa. Une approche expérimentale isotherme a été mise en place pour cette étude, aĄn de contrôler de manière précise le début de la phase de gazéiĄcation des échantillons aux températures d'étude, et de comparer leur comportement. est décrit en fonction d'un ordre de par rapport au solide.

Il a été constaté que malgré les diférences dans la composition macromoléculaire des échantillons, leur composition inorganique est le paramètre principal qui inĆuence xxvii leur réactivité.

Propriétés physico-chimiques de chars issus du procédé de gazéiĄcation sous vapeur d'eau contenu

  Si+P) pour classiĄer et décrire le comportement de la gazéiĄcation des biomasses lignocellulosiques a été conĄrmée. Les échantillons avec un ratio inorganique K/(Si+P) supérieur à 1 ont montré une production de gaz et une eicacité du procédé supérieure en comparaison avec les échantillons avec un ratio inférieur a 1, sous les mêmes conditions expérimentales. D'autre part, l'analyse de la composition du gaz produit n'a pas montré des diférences importantes entre les trois biomasses d'étude. Avec un pouvoir caloriĄque entre 10 et 12 MJ/m 3 et un ratio H 2 /CO entre 2.5 et 4, le gaz produit à partir de la gazéiĄcation sous vapeur des résidus agricoles avec des ratios H/C et O/C proches de 1.5 et 0.8 respectivement, est compatible avec des applications de production d'énergie utilisant des chaudières, moteurs à combustion interne, ou turbines à gaz.Les observations expérimentales présentées dans ce chapitre peuvent être une référence importante pour des installations réelles de gazéiĄcation, permettant d'adapter les paramètres opératoires aux biomasses de travail, et au rendement des sous-produits souhaité en fonction de l'application envisagée. inorganique signiĄcatif des chars suggère également leur valorisation dans des applications agricoles, pour l'amendement des sols.

	Chapitre 6.

En cohérence avec les résultats de l'étude cinétique, l'efet bénéĄque des métaux alcalins et alcalino-terreux, ainsi que l'efet inhibiteur du Si, Al et P sur les réactions de gazéiĄcation a été mis en évidence. xxviii Particulièrement, la validité du ratio K/(

  1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2 Biomass pyro-gasiĄcation basics . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.2.1 Lignocellulosic biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.2.2 Pyro-gasiĄcation process and reactions . . . . . . . . . . . . . . . . . . . 12 1.2.3 Pyro-gasiĄcation by-products . . . . . . . . . . . . . . . . . . . . . . . . .

		16
	1.2.4 Pyro-gasiĄcation technologies . . . . . . . . . . . . . . . . . . . . . . . . .	17
	1.3 Focus on pyro-gasiĄcation chars properties . . . . . . . . . . . . . . . . . . . .	19
	1.3.1 InĆuence of process parameters . . . . . . . . . . . . . . . . . . . . . . . .	19
	1.3.2 Pyro-gasiĄcation chars physico-chemical properties . . . . . . . . . .	22
	1.3.3 Pyro-gasiĄcation chars applications . . . . . . . . . . . . . . . . . . . . .	24
	1.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
	Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28

Table 1 .1: Cellulose

 1 , hemicellulose and lignin content of lignocellulosic biomass[3]

		Cellulose	Hemicellulose	Lignin
		(%)	(%)	(%)
	Hardwood	45-47	25-40	20-25
	Softwood	40-45	25-29	30-60
	Grasses	25-40	35-50	10-30
	Corn cobs	45	35	15
	Switchgrass	45	31	12
	Wheat straw	30	50	15

Table 1 .2: Composition of some lignocellulosic agrowastes from literature [6, 7]. Wheat straw Rice straw Corn stover Rape stalk Cotton stalk Sugarcane bagasse Hazelnut shell Elemental composition (wt.%)

 1

	C	42.4	40.6	44.1	43.1	45.8	43.8	52.3
	H	5.4	5.3	5.6	5.8	5.9	5.8	6.5
	O	41.7	40.5	41.6	42.4	41.7	47.1	26.8
	N	0.6	0.9	1.0	0.8	1.1	0.4	5.2
	Ash	9.2	11.9	7.4	6.8	4.9	2.9	4.3
	Macromolecular composition (wt.%)
	Hemicellulose	21.8	18.2	17.0	14.2	13.0	22.6	15.7
	Cellulose	38.4	41.3	37.2	41.9	38.3	41.3	22.9
	Lignin	21.1	18.1	22.8	19.9	27.5	18.3	51.5

Table 1 .3: Main

 1 chemical reactions involved in biomass gasiĄcation process[8, 10]

	Reaction type	Reaction	Heat of reaction
	Pyrolysis
	R1 (Devolatilization)	Biomass --→ Char + tars + H 2 O + light gases	Endothermic
	Char combustion
	R2 (Partial oxidation) C + 0.5 O 2 --→ CO R3 (Total oxidation) C + O 2 --→ CO 2	-111 kJ/kmol -394 kJ/kmol
	Char gasiĄcation
	R4 (Boudouard) R5 (Water-gas) R6 (HydrogasiĄcation) C + 2 H 2 ← -→ CH 4 C + CO 2 ← -→ 2 CO C + H 2 O ← -→ CO + H 2	+173 kJ/kmol +131 kJ/kmol -78.4 kJ/kmol
	Homogeneous reactions
	R7 (Shift reaction) R8 (Methanation) R9 (Methanation) Tar conversion reactions a CO + H 2 O ← -→ CO 2 + H 2 2 CO + 2 H 2 --→ CH 4 + CO 2 CO + 3 H 2 ← -→ CH 4 + H 2 O	-41.2 kJ/kmol -247 kJ/kmol -206 kJ/kmol
	+ n)H 2 + n CO 2 C n H m + n CO 2 --→ 2 n CO 2 + /2H 2 R10 (Steam reforming) C n H m + n H 2 O --→ (m/ 2 R11 (Dry reforming) a C n H m represents light hydrocarbons	Endothermic Endothermic
	Drying
	In this Ąrst stage, the water contained in the biomass (free water and bound water)
	is released by efect of heat. This endothermic process occurs at temperatures below
	200°C.

  The gasiĄcation agent reacts with the char, converting it into gas by means of diferent heterogeneous reactions, summarized in table 1.3. In accordance, the main product of gasiĄcation process is a fuel gas composed of H 2 , CO 2 , CO,CH 4 ,and C n H m . For their part, tars and char are the liquid and solid by-products of the process, respectively.The heating value of the produced gas is signiĄcantly afected by the choice of the gasifying agent, as presented in table1.4.

	Table 1.4: Heating value of produced gas based on the gasifying agent [8]
	Gasifying agent	Heating value (MJ/Nm3)
	Air	4-7
	Steam	10-18
	Oxygen	12-28

Table 1 . 6 :

 16 Advantages and technical challenges of the principal reactor designs. Adapted from[10]

		GasiĄer design
	1. Fixed-bed
	Advantages	1. Simple and reliable desing
		2. Suitable for wet biomass gasiĄcation
		3. Economically viable at small scales
		4. Downdraft reactor yields gases with low tar content
	Technical challenges 1. Operates at longer residence times
		2. Non-uniform temperature distribution
		3. Low cold-gas eiciency
		4. Limited to low biomass processing capacity
		5. Updraft reactor yields gases with high tar contents
	2. Fluidized-bed

Advantages 1. Short biomass residence time 2. High biomass processing capacity 3. Uniform temperature distribution 4. Yields low char contents 5. High cold-gas eiciencies Technical challenges 1. Yields high particulate dust contents in syngas 2. Yields high tar contents 3. Economically viable only at medium and large scales

  ŞBioavailability of persistent organic pollutants in soils and sediments: a perspective on mechanisms, consequences and assessment.Ť In: Environmental Pollution 108 (2000), pp. 103Ű112 (cit. on p. 27). [93] Sardar Khan, Muhammad Waqas, Fenghua Ding, Isha Shamshad, Hans Peter, H Arp, and Gang Li. ŞThe inĆuence of various biochars on the bioaccessibility and bioaccumulation of PAHs and potentially toxic elements to turnips (Brassica rapa L.)Ť In: Journal of Hazardous Materials 300 (2015), pp. 243Ű253 (cit. on p. 27). [94] Michaş Koştowski, Isabel Hilber, Thomas D. Bucheli, Barbara Charmas, Jadwiga Skubiszewska-Zięba, and Patryk Oleszczuk. ŞActivated Biochars Reduce the Exposure of Polycyclic Aromatic Hydrocarbons in Industrially Contaminated Soils.Ť In: Chemical Engineering Journal 310 (Feb. 15, 2017), pp. 33Ű40 (cit. on p. 27). [95] Somchai Butnan, Jonathan L Deenik, Banyong Toomsan, Michael J Antal, and Patma Vityakon. ŞBiochar characteristics and application rates afecting corn growth and properties of soils contrasting in texture and mineralogy.

Ť In: Geoderma 237-238 (2015), pp. 105Ű116 (cit. on p. 28).

Table 2 . 1 :

 21 Organic composition of some representative crop residues reported in the literature[1Ű6]

		Rice husk	Corncob	Sugarcane bagasse	Coconut shells	Oil palm shells	Bamboo guadua
	Elemental composition (wt. % dry basis)
	C	36.9	43.6	47.1	50.2	49.6	47
	H	5.0	5.8	6.1	5.7	6.02	5.6
	O	37.9	48.6	0.5	43.4	41.9	43
	N	0.4	0.7	42.5	0.0	0.4	0.3
	S	0.0	0.0	1.0	0.0	0.1	0.1
	Ash	20.5	1.2	2.6	1.1	1.8	5.7

High heating value (MJ/Kg dry basis)

	HHV	15.3	16.9	14.7	20.5	20.1	18.8

Molecular composition (wt. % dry basis)

	Cellulose	31.3	42.5	41.3	32.5	30.4	53.9
	Hemicellulose 24.3	31.5	22.6	20.5	12.7	13.5
	Lignin	14.3	15.5	18.3	36.5	49.8	25.1

Table 2 . 2 :

 22 Major inorganic composition of some representative crop residues reported in the literature [1Ű5,7, 8]

			Inorganic composition (mg/kg biomass)
	Major elements	Rice husk	Corncob	Sugarcane bagasse	Coconut shells	Oil palm shells	Bamboo guadua
	Al	-	162.1	-	73.0	31.6	10.0
	Ca	416.9	249.3	1518.0	1501.0	173.7	252.0
	K	2107.2	5408.4	2682.0	1965.0	1477.7	9902.0
	Mg	374.88	404.7	6261.0	389.0	262.9	218.0
	Mn	25.11	9.9	9.0	1.1	15.8	2.0
	Na	30.6	29.3	93.0	1243.0	10.7	3.0
	P	78.3	-	284.0	94.0	115.4	482.0
	Si	51323.3	-	17340.0	256.0	5510.0	12731.0
	-Not reported

Table 2 . 3 :

 23 GasiĄcation chars characterization methods

		Information	Technique	Chapter
	Organic composition	C, H, N, S and O content Moisture, volatile matter, Ąxed carbon, and ash	Elemental analysis accord-ing to ISO 29541 Proximate analysis accord-ing to EN ISO 18134-3, EN ISO 18123, EN ISO 18122	Chapter
	CaloriĄc value	High heating value	ISO 18125	Chapter
	Inorganic	Quantitative mineral com-position of the samples	ICP-OES
	composition	Mineral distribution in the char surface	SEM-EDX	Chapter
		Speciation of crystalline minerals	XRD
	Textural properties	SpeciĄc surface area Pore size distribution	N 2 adsorption	Chapter
		Pore volume
	Carbon structure	Global carbon structure Carbon nanostructure	XRD TEM	Chapter
	Surface chemistry	Surface oxygen-containing functional groups pH PZC	TPD Acid titration	Chapter
		Cation exchange capacity	Sodium acetate displace-ment method	Chapter
		Acid neutralization capac-ity	Acid-base titration accord-ing to NF EN 14429 stan-dard
	pH	pH in water solution		Chapter

3 Chapter Kinetic analysis of tropical lignocellulosic agrowaste pyrolysis 3

  .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.2 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3.2.2 Thermogravimetric analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.2.3 Kinetic study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 3.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.3.1 Biomass composition and thermal decomposition characteristics . 58 3.3.2 DTG curves deconvolution . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.3.3 Kinetic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69 This chapter was published as a research paper in an international journal. Reference: Romero Millan LM, Sierra Vargas FE, Nzihou A. Kinetic Analysis of Tropical Lignocellulosic Agrowaste Pyrolysis. BioEnergy Res 2017. doi:10.1007/s12155-017-9844-5.. The Ąnal publication is available at Springer via http://dx.doi.org/10.1007/s12155-017-9844-5

Table 3 . 1 :

 31 Heating value and chemical composition of studied biomasses.

	OPS	CS	BG

* Calculated by diference

Table 3 . 2 :

 32 Most common reaction mechanisms used in solid-state kinetic analysis related to biomass thermal decomposition[18, 30]

Table 3 . 4 :

 34 Fitting results of Fraser-Suzuki deconvolution of selected biomasses. (P-HC pseudo-hemicellulose; P-C pseudo-cellulose; P-L pseudo-lignin)

				BG			CS			OPS
	°C/min	Parameters	P-HC P-C	P-L P-HC P-C	P-L P-HC P-C	P-L
		a (1/°K)	-0.450 -1.460 -0.110 -0.620 -0.795 -0.169 -0.520 -0.840 -0.220
		b (°K)	283.0	337.7 359.0	251.0	315.0 293.0	251.5	328.5 301.0
	2	c (°K)	57.0	23.0	180.0	36.5	26.0	175.0	35.8	20.5	182.0
		d (-)	-0.200 -0.420 0.210	0.350 -0.300 0.555	0.360 -0.400 0.630
		Fit error (%)		1.8			2.0			2.1
		a (1/°K)	-0.428 -1.266 -0.073 -0.620 -0.795 -0.165 -0.550 -0.788 -0.184
		b (°K)	297.5	350.5 370.5	262.5	327.5 304.0	262.0	339.2 309.0
	5	c (°K)	56.0	25.5	186.0	37.0	26.0	180.0	34.7	22.0	184.0
		d (-)	-0.200 -0.400 0.200	0.350 -0.300 0.555	0.390 -0.345 0.630
		Fit error (%)		1.6			1.2			1.5
		a (1/°K)	-0.440 -1.180 -0.075 -0.620 -0.797 -0.165 -0.600 -0.776 -0.185
		b (°K)	308.5	360.8 380.5	272.5	338.0 312.5	269.8	347.1 319.5
	10	c (°K)	56.0	26.5	190.0	37.0	26.0	185.0	33.5	21.0	190.0
		d (-)	-0.200 -0.400 0.200	0.330 -0.300 0.560	0.420 -0.295 0.620
		Fit error (%)		1.9			1.0			1.8
		a (1/°K)	-0.420 -1.040 -0.075 -0.660 -0.734 -0.165 -0.630 -0.682 -0.180
		b (°K)	318.8	371.7 391.0	281.5	347.8 319.0	279.5	356.4 325.7
	20	c (°K)	56.0	29.0	196.0	38.0	26.0	190.0	36.0	24.0	196.0
		d (-)	-0.200 -0.370 0.210	0.350 -0.300 0.570	0.360 -0.280 0.630
		Fit error (%)		2.0			1.7			2.1

Table 3 . 5 :

 35 R 2 correlation coeicient of isoconversional Arrhenius plots for the three studied biomasses Max 1.0000 1.0000 1.0000 0.9991 0.9997 1.0000 0.9999 0.9999 0.9999 Biomass pseudo-components was determined from the slope of the isoconversional plots regression lines, according to the Friedman, KAS and FWO methods, presented in table 4.3. The calculated values are shown in Fig.3.3, as a function of the reaction extent.

		Pseudo-hemicellulose	Pseudo-cellulose	Pseudo-lignin
		Fried. KAS FWO Fried. KAS FWO Fried. KAS FWO
	BG	R 2 Min 0.9977 0.9982 0.9982 0.9942 0.9986 0.9988 0.9906 0.9871 0.9981 R 2 Max 0.9997 1.0000 1.0000 1.0000 1.0000 1.0000 0.9976 0.9992 0.9993
	CS	R 2 Min 0.9971 0.9990 0.9984 0.9977 0.9989 0.9989 0.9950 0.9946 0.9950 R 2 Max 0.9997 1.0000 1.0000 0.9996 0.9997 0.9998 0.9991 0.9994 0.9995
	OPS	R 2 Min 0.9973 0.9983 0.9984 0.9940 0.9977 0.9971 0.9903 0.9918 0.9938 R 2

Table 3 . 6 :

 36 Mean activation energy values of OPS, CS and BG pseudo-components, calculated using Friedman, KAS and FWO model-free methods.

			Apparent activation energy -(kJ/mol), à(kJ/mol)
		Pseudo-hemicellulose	Pseudo-cellulose	Pseudo-lignin
		Friedman KAS	FWO Friedman KAS FWO Friedman KAS FWO
	BG	167.8 à=1.1	163.2 à=12.2	164.2 à=11.9	191.0 à=7.1	211.3 à=6.2	210.7 à=5.8	198.4 à=5.8	221.2 à=7.0	221.0 à=7.5
	CS	189.9 à=10.0	175.9 à=8.2	175.9 à=8.0	222.5 à=6.8	219.8 à=6.5	218.5 à=6.3	235.8 à=4.4	228.3 à=3.9	226.9 à=4.8
	OPS	217.0 à=10.5	195.8 à=11.4	194.6 à=10.7	234.1 à=7.8	240.0 à=3.3	240.9 à=3.2	237.3 à=7.6	249.5 à=5.8	248.2 à=6.5

Table 3 . 7 :

 37 Average and values calculated for BG, CS and OPS pseudocomponents

		Pseudo-hemicellulose	Pseudo-cellulose	Pseudo-lignin
	Biomass	Ea (kJ/mol)	A (s -1 )	Ea (kJ/mol)	A (s -1 )	Ea (kJ/mol)	A (s -1 )
	BG	165.1	2.62E+14	204.3	2.30E+16	214.5	1.02E+18
	CS	180.6	5.83E+16	220.3	2.30E+18	230.3	1.47E+21
	OPS	202.4	6.33E+18	238.1	3.50E+19	245.0	8.83E+21

  Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 4.2.1 Biomass samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 4.2.2 Isothermal TGA gasiĄcation experiments . . . . . . . . . . . . . . . . . 76 4.2.3 Reactivity and kinetic study . . . . . . . . . . . . . . . . . . . . . . . . . . 77 4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.3.1 Impact of temperature and steam partial pressure on gasiĄcation reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.3.2 Impact of feedstock characteristics on steam gasiĄcation reactivity . 81 4.3.3 Steam gasiĄcation kinetic analysis . . . . . . . . . . . . . . . . . . . . . . 84 4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

	This chapter was published as a research paper in an international journal, reference:
	Romero Millán LM, Sierra Vargas FE, Nzihou A. Steam gasiĄcation behavior of trop-
	ical agrowaste: A new modeling approach based on the inorganic composition. Fuel
	2019;235:45Ű53. doi:10.1016/j.fuel.2018.07.053.

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 4.2

Table 4 . 1 :

 41 Organic composition of studied biomasses

	OPS	CS	BG

Table 4 . 2 :

 42 Major inorganic composition of studied biomasses

			OPS	CS	BG
	Inorganic composition Al 1 500±22	262±8	243±34
	(mg/kg dry basis)	Ca	54±6	391±73	441±99
		Fe	107 ±4	160±28	116±17
		K	1 006±15 2 808±44	5 360±85
		Mg	135±3	170±15	173±10
		Na	1.5±0.5	33±11	2±0.8
		P	270±7	397±40	829±62
		Si	5 600±39	309±43	19 372±354
	Elemental composition (C, H, N, S, and O) was determined using a Themoquest NA
	2000 elemental analyzer, while inorganic speciation was determined using an HORIBA
	Jobin Yvon Ultima 2 inductively coupled plasma optical emission spectrometer (ICP-
	OES), based on EN 16967 standard. Proximate analysis was calculated according to the
	standards EN ISO 18134-3, EN ISO 18123 and EN ISO 18122, respectively. Molecular
	composition of the biomasses is referred to literature reported values [19Ű21].

Table 4 .

 4

3:

Most common reaction mechanisms used in solid state kinetic analysis

[24,26]

Table 4 . 4 :

 44 Steam gasiĄcation reactivity and reaction time at 50% conversion.

	Steam partial pressure 3.7 kPa
		750°C	800°C	900°C
	Sample	Reactivity (% min ⊗1 )	React. time (min)	Reactivity (% min ⊗1 )	React. time (min)	Reactivity (% min ⊗1 )	React. time (min)
	100%CS	0.5	142.8	1.3	66.7	8.7	21.0
	100%BG	0.2	234.6	0.7	83.4	4.1	21.5
	100%OPS	0.2	236.4	0.5	127.8	2.5	24.8

Table 4 . 5 :

 45 Calculated inorganic ratio K/Si+P of analyzed biomasses and blends

	Biomass / blend	K/(Si+P) ( -)
	100% CS	3.98
	100% BG	0.20
	100% OPS	0.17
	90%CS -10%BG	1.18
	85%CS -15%BG	0.92
	75%CS -25%BG	0.61
	50%CS -50%BG	0.39
	90%CS -10%OPS	2.15
	85%CS -15%OPS	1.71
	75%CS -25%OPS	1.03
	50%CS -50%OPS	0.58

Table 5 . 1 :

 51 Organic composition and proximate analysis of studied biomasses

	OPS	CS	BG

Table 5 . 2 :

 52 Major inorganic composition of studied biomasses

			OPS	CS	BG
	Inorganic composition Al 1 500±22	262±8	243±34
	(mg/kg dry basis)	Ca	54±6	391±73	441±99
		Fe	107 ±4	160±28	116±17
		K	1 006±15 2 808±44	5 360±85
		Mg	135±3	170±15	173±10
		Na	1.5±0.5	33±11	2±0.8
		P	270±7	397±40	829±62
		Si	5 600±39	309±43	19 372±354

Table 5 . 3 :

 53 Calculated mean gasiĄcation reactivity during 3 hours in a 30% steam atmosphere

		Mean reactivity
		(%/min)
	Temperature CS BG OPS
	750°C	0.35 0.25	0.08
	850°C	2.28 1.18	0.32

Table 5 . 4 :

 54 GasiĄcation products distribution and energy fraction for the analyzed experimental conditions

		Product distribution	Product energy fraction	Gas
	Raw sample	Solid (g/g bio )	Liquid (g/g bio )	Gas (g/g bio )	Solid (%)	Liquid (%)	Gas (%)	Efficiency (%)
	CS_30_750_1	0.23	0.92	0.50	37.1	18.4	45.0	33.2
	CS_90_750_1	0.20	3.68	0.30	24.3	56.0	18.9	11.7
	CS_30_750_2	0.20	1.98	0.75	23.3	30.1	46.2	37.1
	CS_30_750_3	0.16	2.97	1.26	13.0	32.0	54.4	52.0
	CS_30_850_1	0.12	0.48	0.94	16.9	10.1	73.0	54.6
	CS_30_850_2	0.04	1.18	1.62	3.9	15.4	80.7	78.3
	BG_30_750_1	0.23	0.97	0.47	33.4	21.9	44.7	31.0
	BG_90_750_1	0.20	2.87	0.28	24.1	54.5	21.4	11.1
	BG_30_750_2	0.21	2.11	0.65	21.4	34.4	44.2	34.4
	BG_30_750_3	0.20	3.40	0.79	15.7	43.9	40.5	33.6
	BG_30_850_1	0.15	0.85	0.54	21.8	22.9	55.4	33.3
	BG_30_850_2	0.12	1.84	0.92	8.9	31.4	59.7	45.3
	BG_30_850_3	0.08	2.76	1.34	3.4	34.5	62.0	53.8
	OPS_30_750_1	0.25	0.97	0.44	37.1	18.2	44.8	35.5
	OPS_90_750_1	0.23	3.17	0.26	29.2	50.8	20.0	11.9
	OPS_30_750_2	0.24	2.12	0.65	25.1	27.1	47.8	45.9
	OPS_30_750_3	0.24	3.11	0.88	19.4	31.9	48.7	49.3
	OPS_30_850_1	0.23	0.73	0.56	35.3	15.5	49.2	35.1
	OPS_30_850_2	0.20	1.62	1.00	19.8	21.7	58.5	54.5
	OPS_30_850_3	0.16	2.62	1.35	12.6	27.4	60.0	60.3
	Blends *
	50%CS_50%BG	0.16	0.65	0.76	24.2	13.7	62.1	46.5
	50%CS_50%OPS	0.17	0.72	0.73	27.4	14.8	57.8	44.6
	90%CS_10%BG	0.15	0.46	0.89	22.6	9.3	68.1	52.7
	90%CS_10%OPS	0.12	0.68	0.86	19.0	14.1	66.9	51.

Table 5 . 5 :

 55 Average gas composition and heating value obtained for the three analyzed samples

			Gas composition		Gas	Gas
	Raw sample	CO (% vol.)	CO 2 (% vol.)	CH 4 (% vol.)	H 2 (% vol.)	H 2 /CO (-)	HHV (MJ/m 3 )
	CS_30_750_1	18.19	18.95	1.07	61.78	3.40	10.85
	CS_90_750_1	13.32	22.78	0.71	63.20	4.75	10.02
	CS_30_750_2	16.94	21.69	0.94	60.42	3.57	10.22
	CS_30_750_3	13.80	23.17	0.49	62.53	4.53	9.91
	CS_30_850_1	18.29	21.48	0.37	59.85	3.27	10.09
	CS_30_850_2	16.98	21.58	0.42	61.02	3.59	10.09
	BG_30_750_1	17.66	19.95	1.05	61.35	3.47	10.05
	BG_90_750_1	15.20	22.51	0.82	61.56	4.05	10.08
	BG_30_750_2	17.73	20.35	0.98	60.94	3.44	10.40
	BG_30_750_3	15.56	22.44	0.77	61.22	3.93	10.08
	BG_30_850_1	21.28	19.63	0.51	58.58	2.75	10.76
	BG_30_850_2	17.17	21.71	0.55	60.55	3.53	10.11
	BG_30_850_3	17.04	22.33	0.43	60.18	3.53	10.20
	OPS_30_750_1	20.65	12.69	2.53	64.12	3.11	11.50
	OPS_90_750_1	19.90	14.28	2.72	62.99	3.21	11.65
	OPS_30_750_2	21.04	11.38	4.27	63.30	3.01	12.43
	OPS_30_750_3	21.75	12.57	3.24	62.39	2.87	12.02
	OPS_30_850_1	21.90	17.51	0.88	59.70	2.73	10.73
	OPS_30_850_2	23.42	15.23	1.30	60.02	2.56	11.18
	OPS_30_850_3	24.01	15.10	1.25	59.62	2.50	11.12
	Blends*
	50%CS_50%BG	19.33	20.14	0.56	59.97	3.10	10.31
	50%CS_50%OPS	19.29	20.35	0.64	59.70	3.09	10.84
	90%CS_10%BG	17.45	21.98	0.45	60.12	3.45	10.73
	90%CS_10%OPS	16.47	22.06	0.28	61.19	3.72	10.57

Table 6 . 1 :

 61 Organic and inorganic composition of the analyzed chars

			Elemental analysis		Main inorganic constituents
			(wt.% dry basis)		(mg/kg dry basis)
	Sample	C	H N O* Ash	Ca	K	Na	P	Si

Table 6 . 2 :

 62 Calculated mean gasiĄcation reactivity during 3 hours in a 30% steam atmosphere

		Mean reactivity
		(%/min)
	Temperature CS BG OPS
	750°C	0.35 0.25	0.08
	850°C	2.28 1.18	0.32

Table 6 . 3 :

 63 Surface and structure characteristics of the analyzed gasiĄcation chars N 2 adsorption test did not yield acceptable results, possibly due to the high ash content of the sample a single plane. To illustrate this, the X-ray difraction patterns of CS, BG and OPS gasiĄcation chars obtained at 850°C are presented in Ągure 6.8

		Char		Pore		TPD CO and CO 2
		burn-of		structure			desorption
	Sample	(%)	S BET (m 2 /g)	S micro (m 2 /g)	V micro (cm 3 /g)	CO 2 (mmol/g)	CO (mmol/g)	Total (mmol/g)
	CS_30_750_1	14.7	631.5	550.2	0.21	0.57	0.57	1.14
	CS_90_750_1	25.5	725.6	603.1	0.23	0.50	0.78	1.28
	CS_30_750_2	28.4	772.3	684.2	0.26	0.47	0.76	1.22
	CS_30_750_3	43.2	884.0	779.0	0.30	0.69	1.04	1.73
	CS_30_850_1	57.0	1041.8	898.9	0.34	0.83	0.57	1.41
	CS_30_850_2	84.6	1032.6	952.0	0.38	0.76	1.56	2.32
	BG_30_750_1	16.5	544.9	468.9	0.18	0.36	0.18	0.53
	BG_90_750_1	25.3	560.0	500.0	0.16	0.35	0.55	0.90
	BG_30_750_2	29.1	648.2	539.4	0.21	0.31	0.30	0.62
	BG_30_750_3	30.4	605.4	497.8	0.20	0.37	0.61	0.98
	BG_30_850_1	52.0	807.7	592.8	0.24	0.39	0.59	0.98
	BG_30_850_2	68.4	723.5	508.1	0.21	0.35	0.54	0.90
	BG_30_850_3	78.5	a	-	-	0.25	0.15	0.41
	OPS_30_750_1	8.9	490.0	450.9	0.17	0.20	0.21	0.40
	OPS_90_750_1	15.9	553.7	479.0	0.18	0.30	0.64	0.80
	OPS_30_750_2	10.8	504.2	476.4	0.18	0.23	0.45	0.68
	OPS_30_750_3	14.3	529.9	503.1	0.19	0.23	0.48	0.71
	OPS_30_850_1	27.7	667.4	550.0	0.25	0.19	0.38	0.58
	OPS_30_850_2	34.5	776.0	687.6	0.26	0.17	0.33	0.50
	OPS_30_850_3	47.2	931.0	830.0	0.32	0.17	0.35	0.51
	OPS_30_850_4.5	56.0	1014.0	883.0	0.35	0.18	0.36	0.55

a

Table 7 . 1 :

 71 Organic composition of analyzed chars

		Elemental analysis
			(wt.% dry basis)
	Sample	C	H N O* Ash
	CS char 79.2 1.3 0.0 11.3 8.2
	BG char 60.8 0.8 0.3 6.6 31.5
	OPS char 85.2 0.8 0.5 8.6	4.9

*Calculated by diference

Table 7 . 2 :

 72 Mineral constituents of the analyzed chars

	CS char BG char OPS char

Table 7 . 3 :

 73 Biochar pH in deionized water and pH PZC

		CS BG OPS
	pH (H 2 O) 10.9 10.2	9.8
	pH PZC	2.2	1.6	3.1

Table 7 . 4 :

 74 Cation exchange capacity (CEC) and pH of agricultural residues biochar

	Char feedstock	Production process	Temperature (°C)	pH (-)	CEC (cmol/kg)	Ref.
	Corn stover	Pyrolysis	450°C	-	26.4	[32]
	Corn stover	Air gasiĄcation	700°C	-	10.2	[32]
	Peanut straw	Pyrolysis	400	10.28	146.8	[9]
	Corn straw	Pyrolysis	500	-	50	[33]
	Corn stover	Pyrolysis	400	-	23.9	[13]
	Corn stover	Pyrolysis	700	-	6.5	[13]
	Sawdust	Pyrolysis	500	10.5	41.7	[12]
	Peanut shell	Pyrolysis	500	10.5	44.5	[12]
	Wheat straw	Pyrolysis	500	10.2	95.5	[12]
	Coconut shells Steam gasiĄcation	850	10.9	15.8	This work
	Oil palm shells Steam gasiĄcation	850	9.8	11.5	This work
	Bamboo guadua Steam gasiĄcation	850	10.2	44.2	This work

Table 7 . 5 :

 75 Cation exchange capacity (CEC) and pH of some acidic soils

	Soil	Location	pH (-)	CEC (cmol/kg)	Ref.
	Ultisol -Ternary red sandstone	China	4.96	6.34	[9]
	Ultisol -Quaternary red earth	China	4.78	12.4	[9]
	Oxisol -Basalt	China	4.62	6	[37]
	Ultisol -Granite	China	4.52	4.4	[37]
	Andisol -Sandy loam	Colombia 4.8	19	[38]
	Inceptisol -Sandy loam	Colombia 4.8	25	[38]
	Entisol -Sandy loam	Colombia 5.3	9	[38]
	Ferrosol -Sandy loam	Australia 6.14	11.6	[39]

Table 7 . 6 :

 76 Average mineral release available for plant uptake with a rate of biochar amendment of 10 tons/ha

Average supply potential (kg/ha) Inorganic elements CS char BG char OPS char

	Primary	P	2.3	15.2	8.3
	macronutrients	K	201.0	259.8	56.5
	Secondary	Ca	0.7	3.1	5.7
	macronutrients	Mg	0.4	4.3	8.0
	Micronutrients	Mn Mo	0.0 0.1	0.0 0.1	0.2 0.1
	BeneĄcial	Na	31.7	1.3	0.6
	nutrients	Si	2.9	4.6	2.3

Table 7 . 7 :

 77 Primary macroutrients consumption of selected crops[55, 56]

			Macronutrient consumption*
				(kg/ha)
	Crop	Plants ha -1 N	K	P
	Fresh tomatoes	12 000	250 370	24
	Eggplant	12 500	290 380	33
	Lettuce	100 000	110 250	22
	Sweet corn	75 000	240 320	40
	Carrot	400 000	279 604	73
	Celery	90 000	150 224	36
	Broccoli	33 000	202 165	26
	*Throughout the growing season

Ce chapitre est dédié à l'analyse des propriétés physico-chimiques du char issu de la gazéiĄcation sous vapeur d'eau des biomasses d'étude. A la Ąn des tests de gazéiĄcation décrits dans le chapitre 5, le char résultant a été récupère et caractérisé aĄn de comprendre l'impact des conditions opératoires et des caractéristiques de la biomasse d'origine sur ses propriétés.La composition chimique des chars, ainsi que leur surface spéciĄque, structure, et chimie de surface ont été déterminés. En accord avec les résultats présentés dans les chapitres 4 et 5, la caractérisation des chars a montré que la composition inorganique des échantillons impacte également les propriétés du sous-produit solide de la gazéiĄcation. Particulièrement, la réactivité de la biomasse inĆuence d'une manière importante le développement de la surface spéciĄque des chars et leur chimie de surface.La comparaison des chars produits avec les mêmes conditions expérimentales montre que pour les échantillons avec un ratio inorganique K/(Si+P) supérieure à 1, l'efet bénéĄque du K et des métaux alcalins et alcalino-terreux est associé avec un développement supérieur de la surface spéciĄque des chars, ainsi qu'avec une quantité plus importante de groupes oxygénés dans leur surface, en comparaison avec les échantillons avec un ratio inférieur a 1. Également, du fait du mécanisme réactionnel entre la vapeur d'eau et l'échantillon en cours de la gazéiĄcation, une relation a été observée entre la surface spéciĄque et la quantité totale mesurée des groupes oxygénés dans la surface des chars. D'autre part, la distribution de la taille des pores des échantillons a montré être en relation avec la nature et la composition macromoléculaire de la biomasse d'origine.En général, les chars analysés sont des matériaux carbonés hautement microporeux, avec des surfaces spéciĄques comprises entre 500 et 1000 m 2 /g. Une dominance de groupes fonctionnels oxygénés acides dans la surface du charbon, avec des valeurs de pH PZC inférieures à 3, a été observée dans tous les cas. En conséquence, en fonction des propriétés observées, proches de celles de certains charbons actifs, les chars issus de la gazéiĄcation sous vapeur d'eau peuvent être considérés comme un sous-produit intéressant qui pourrait être valorisé dans des applications d'adsorption. De plus, le xxix

Pour cette étude, des chars de coques de noix de coco, coques de palmier à huile, et bambou, produits à 850°C avec une fraction de 30% de vapeur d'eau dans l'agent de gazéiĄcation ont été choisis. Comme observé dans les chapitres précédants, ces conditions opératoires se sont avérées adéquates pour la production simultanée de syngaz, avec une eicacité de procédé élevée, et de char avec des propriétés physico-chimiques d'intérêt pour des nombreuses applications.Les résultats expérimentaux ont conĄrmé que les trois échantillons analysés ont un potentiel intéressant pour des applications agricoles. Notamment, leur pH élevé, compris entre 9 et 11, et leur capacité de neutralisation acide supérieure à 62 cmolH + /kg, suggèrent leur possible utilisation pour le traitement des sols acides. De plus, leur capacité d'échange cationique comprise entre 11 et 45 cmol/kg, supérieure a celle de plusieurs types de sol, pourrait également contribuer à l'amélioration de la rétention des nutriments et la fertilité des sols.En général, il a été constaté que le contenu inorganique des chars impacte de manière très importante leurs propriétés. En particulier, les chars de bambou guadua, avec un taux de cendres supérieur à 30%, ont montré des capacités d'échange cationique et de neutralisation acide supérieures a celles des chars avec des taux des cendres inférieurs (coques de noix de coco et coques de palmier a huile). D'autre part, l'analyse des tests de lixiviation des minéraux contenus dans la matrice des chars à diférentes valeurs de pH, a révélé que ces matériaux pourraient également constituer une source non négligeable de nutriments, pouvant améliorer la fertilité des sols et le rendement de diférents types de cultures. SpéciĄquement, grâce à leur important contenu inorganique, les chars de bambou et coques de noix de coco, pourraient fournir jusqu'à 100% et 20% des besoins en K et P pour certains cultures.Malgré le fait que le comportement des chars dans des applications réelles peut être diférent de celui observé en cours des caractérisations, le présent travail donne un aperçu très utile sur une voie de valorisation prometteuse pour les sous-produits solides de la gazéiĄcation sous vapeur d'eau des résidus agricoles. xxx

As highlighted with the literature review, the chemical composition of the feedstocks may inĆuence their pyrolysis and gasiĄcation behavior, and the physico-chemical properties of the solid by-products. Hence, a better understanding of the impact of biomass characteristics on the steam gasiĄcation behavior and by-products is essential for process design, operation, and optimization using agrowastes. Moreover, it could be of particular interest to adapt the process parameters to each feedstock in order to obtain the required gas and char properties for the intended applications. Nevertheless, even if previous studies have provided good insights regarding the impact of biomass composition on its pyrolysis and gasiĄcation behavior, further investigations are still required. Particularly, the yield and by-product properties obtained from the gasiĄcation of diferent feedstocks requires in-depth investigation and comparison, as well as the co-gasiĄcation of diferent biomasses.In relation to the solid by-products, the valorization pathways may depend on their physico-chemical properties. In this regard, considering the important inorganic content of most lignocellulosic agrowastes, the analysis of char use in soil amendment applications may be an interesting alternative that requires further investigations.

framework of a cotutelle agreement between the National University of Colombia (Colombia) and the "IMT Mines Albi-Carmaux" institute (France). gratitude to COLCIENCIAS and the Colombian national program for doctoral formation, for the doctoral scholarship granted for the development of my Ph.D. to the research, technical, and administrative staf of the National University of Colombia, and the RAPSODEE research laboratory. Thanks for

5

Chapter

Impact of feedstock characteristics on the steam gasiĄcation of lignocellulosic agrowaste Abstract

The steam gasiĄcation of coconut shells, oil palm shells and bamboo guadua was studied under diferent experimental conditions in order to understand the impact of the biomass characteristics on the steam gasiĄcation product yield and gas composition. GasiĄcation tests were performed in a Ćuidized bed reactor, with temperatures ranging from 750°C and 850°C. The experimental results showed that the inorganic content impact in a very important way the gasiĄcation reactivity and product yield of the samples. GasiĄcation of samples with K/(Si+P) above 1 resulted in higher gas yields and gas eiciencies compared to samples with K/(Si+P) below 1. For its part, the organic composition, especially the H/C and O/C ratios seem to be related to the composition and heating value of the gaseous products. The produced gas obtained, with a heating value between 10 and 12 MJ/m 3 , and a H 2 /CO ratio between 2.5 and 4, is considered suitable for energy applications using gas turbines or internal combustion engines.

Pyrolysis-only tests were performed in order to determine the gas, solid and liquid yield at the end of the heating period (pyrolysis stage) for each gasiĄcation temperature. In order to verify the reproducibility, several tests were carried out twice. The repeatability was found to be satisfactory as the standard deviation of the gas composition and gas production was below 3% and 10% respectively.

Product distribution and reactivity analysis

Product mass yield and energy fraction

The mass balance for each experimental condition was calculated from the gravimetrically measured solid (char) and liquid yields (tar+water), and the registered gas volume production and composition. As the last bottles of the condensation train are placed in a -20 ◇ C cold bath, it is considered that the 2-propanol in the impingers is not volatilized during the experiment, and then, the liquid yield is determined from the mass diference between the condensation train before and after the test.

In the same way, considering that the nitrogen Ćow supplied to the thermo-conversion process is known and constant for each experiment, the total volume of permanent gases produced can be deduced, from the gas volume measured. The volume and mass of each gas species (H 2 , CO, CO 2 , CH 4 ) can be then determined from the measured gas composition. Taking into account that the repeatability was found to be satisfactory, the gas, liquid and solid mass measured for the pyrolysis-only tests are subtracted from the gasiĄcation experiments to obtain the product yield associated only to the gasiĄcation stage. Under all the tested conditions in this study, the mass balance closure for the gasiĄcation stage is in the range 88.6% and 103%, considered acceptable for the presented experimental setup. The schema of the inlet and outlet streams to and from the reactor during the gasiĄcation stage is presented in Ągure 5.3. The gasiĄcation product yield as a function of the initial biomass and steam used in the pyro-gasiĄcation process was calculated according to equations 5.2.1 to 5.2.3

Where ℎ is the mass of the recovered char at the end of the experiment, and and are the mass of liquid and gas produced during the gasiĄcation stage. For their part, and are the mass of the initial biomass used and the steam fed to the gasiĄcation process. For the present study, the gasiĄcation total product yield with respect to the initial biomass is below 100%, as only the gasiĄcation products are taken into account in the calculation procedure.

For all the experimental conditions, the energy fraction associated with the gas, liquid and solid by-products was determined. Their chemical energy, as well as their sensible or latent heat at the reactor temperature were taken into account, as presented below:

Where ℎ and are the mass of the recovered char and gas at the end of the gasiĄcation process. ℎ is the speciĄc heat of the char, and is the reactor temperature during the gasiĄcation stage. For their part, ℎ is the enthalpy of the produced gas at temperature leaving the gasiĄer, and ℎ is the enthalpy of the unreacted steam also at temperature . Finally, ℎ and are the high heating value of the recovered char and gas. As the mass of tars could not be accurately determined during the gasiĄcation stage, it is assumed for the liquid energy fraction calculation that the recovered liquids correspond only to unreacted steam.

In accordance, the energy fraction of each gasiĄcation product is calculated according to equations 5.2.8 to 5.2.10.

Finally, considering the permanent gases as the targeted product, the gas eiciency of the gasiĄcation process is deĄned as follows:

Abstract

In this study, the steam gasiĄcation chars from three lignocellulosic agrowastes with diferent macromolecular structure and inorganic composition were analyzed, in order to understand the impact of the raw biomass characteristics on the char physico-chemical properties. The experimental results showed that the produced chars are highly microporous materials with surface areas between 500 and 1000 m 2 /g, comparable to activated carbons. The char properties can be modiĄed by changing the gasiĄcation process conditions, including gasiĄcation temperature and time, as well as the steam fraction

7

Chapter

Characterization of steam gasiĄcation chars for soil amendment applications Abstract

In this study, steam gasiĄcation chars from coconut shells (CS), bamboo guadua (BG), and oil palm shells (OPS), were analyzed in order to determine their suitability to be used in soil amendment and remediation applications. In general, the three analyzed chars showed high pH values between 9 and 11, and an interesting potential for the improvement of acidic soils characteristics. Moreover, the relatively high inorganic content of gasiĄcation chars from agrowastes suggests that these materials may also be a source of nutrients, that could impact soil fertility and crop yield. The experimental results showed that the inorganic content of the samples impacts in a great extent their The base cations release may be related to the dissolution of alkali salts and oxides present in the char as discrete phases or associated with functional groups. The contribution of base cations release to the bufering capacity has been already described by several authors [8,46], and may explain the fact that OPS char, with the lowest amount of K + , Na + , Ca 2+ and Mg 2+ released, is also the sample that exhibits the lowest proton consumption during tests.

Moreover, the greater Si release observed for BG in comparison to CS, may be related to its higher acid neutralization capacity. In fact, at high pH, soluble Si is present in the solution as H 3 SiO 4 -, which is transformed into H 4 SiO 4 and precipitates when the pH decreases, consuming protons [47,48].

These results suggest that inorganics play a primary role on bufering capacity of the analyzed gasiĄcation chars. The acid neutralization capacity of the analyzed samples is in accordance with some reported values in the literature for pyrolysis biochars from crop residues (100 -200 cmol H + /kg) [6,8], showing the interesting potential of steam gasiĄcation chars for acidic soil amendment applications.

pH dependent mineral release

In addition to the cation exchange and acid neutralization capacity, the signiĄcant inorganic content of steam gasiĄcation chars from agrowastes is also a relevant characteristic that suggests their use in soil amendment applications. More speciĄcally, the elemental analysis of the samples revealed the presence of non-negligible amounts of essential nutrients that could be available for plant uptake.

However, the mineral availability is not immediate, and depends on the environmental conditions, soil type, and char characteristics. In this regard, the analysis of the chars mineral release at diferent pH values may give an insight to their associated nutrient availability at diferent soil conditions. The measured mineral release of the three samples, for a 48 hours leaching test and pH values between 2 and 12, is presented in Ągure 7.5.

In all cases, the release of a certain amount of K, P, Ca, Mg, Na, and Si from the char matrix was observed, suggesting the potential increase of nutrient availability in soils, if treated with the analyzed chars. In contrast, only a slight liberation of Mo was observed

Annexes

A

Appendix

Publications related to this work

Publications in international journals

Appendix

TPD spectra deconvolution of selected samples

In this section, an example of the TPD curves deconvolution process is presented for three selected gasiĄcation chars obtained from coconut shells (CS), bamboo guadua (BG), and oil palm shells (OPS). As previously described in chapter 6, CO and CO 2 desorption patterns were deconvoluted using 6 Gaussian functions respectively (Equation B.0.1), in order to determine the contribution of each type of oxygen complex.

Gaussian function:

Regarding the parameters of the Gaussian function, is the amplitude of the peak in molar percentage ; is the peak central temperature in °C, and the width of the peak in °C.

In accordance, the CO 2 desorption curves were decomposed using 6 peaks centered at 250±50°C and 380±50°C for carboxylic groups, 500±50°C for peroxydes, and 620±50°C, 750±50°C, and 850±50°C for lactones at diferent energetic sites.

In the case of the CO curves, peaks centered at 400±50°C for acid anhydrides, 620±50°C for hydroxyl groups, 740±50°C for phenol groups, 860±50°C for ether groups, and 900±50°C and 970±50°C for quinone and pyrone groups were used respectively. A constraint over the peak width (55±5°C) was Ąxed for the optimization of the curve Ątting.

In this regard,