Freitas Homem De Faria Bruno

Lanvin Charline

Valette Jeremy

Rousset Patrick

Cassia Oliveira

Carneiro Angélica

Caldeira-Pires Armando

Candelier Kévin
email: kevin.candelier@cirad.fr

De Freitas

Homem De Faria Bruno

De Cassia Oliveira

Emulation of field storage conditions for assessment of energy properties of torrefied sugarcane bagasses de

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 Energy, Fungal decay, Leaching, Storage emulation, Bagasse, Torrefaction. 38

INTRODUCTION

Brazil is strongly dependent to the agricultural and forestry production sectors. Brazil is still among the top-countries using the most biomass for energy production with Total Primary Energy Supply (TPES) from solid biomass of 15 GJ/capita [START_REF] Pelkmans | IEA Bioenergy Countries'Report -Update 2018[END_REF]. The Brazilian energy production from solid biomass is outranked by those of Finland (62 GJ/capita), Sweden (39 GJ/capita), Estonia (GJ/capita), Austria (24 GJ/capita) and Denmark (20 GJ/capita). However, in term of global production, Brazil is the first country in the world using the most solid biomass for energy production followed by the United States [START_REF] Pelkmans | IEA Bioenergy Countries'Report -Update 2018[END_REF]. Lignocellulosic biomasses mainly come from plantations dedicated to energy valorization, or as residues from some agricultural production chain. Due to the huge sugarcane planting area in Brazil with 10 millions hectares [START_REF]Instituto Brasileiro de Geografia e Estatística -Sistema IBGE de recuperação automática, SIDRA, Produção agrícola municipal[END_REF], sugarcane ranks first among Brazilian crop wastes (including paddy straw, bean straw, soya straw, maize straw and wheat straw) and agro-industrial residues (including rice husk, coffee husk, sugarcane and fruits bagasse, soybean husk and wood wastes) used for energy conversion [START_REF]UNICA, Sugarcane Industry Association -Ethanol[END_REF]. Sugarcane bagasse has been widely used for energy generation. Amongst the domestic energy alternatives in Brazil, biomass from sugarcane bagasse accounts for 16.1% [START_REF] Costa | Torrefied briquettes of sugar cane bagasse and eucalyptus[END_REF]. A large resource of bagasse is available at low cost [START_REF] Schmitt | From agriculture residue to upgraded product: The thermochemical 25 conversion of sugarcane bagasse for fuel andconversion of sugarcane bagasse for fuel and chemical products[END_REF], providing raw material for energy production [START_REF] Santos Costa | Torrefied briquettes of sugar cane bagasse and eucalyptus[END_REF]. Each ton of sugarcane is estimated to produce 140 kg of dry bagasse, giving a world supply of 200 million tons per year with energy values range from 17 to 20 MJ.kg -1 [START_REF] Anukam | Pre-processing of sugarcane bagasse for gasification in a downdraft biomass gasifier system: A comprehensive review[END_REF].

Even if sugarcane bagasse (SCB) can be burnt directly to produce heat and electricity, the solid residue after pressing, has some characteristics that hinder its direct use as fuel, compared to fossil fuel, such as high fibrous (43-52%, on a bagasse dry weight basis) and low bulk density (80-120 kg.m -3 ), high moisture content (46-52%, on a wet basis), inconsistent particle size, heterogeneous chemical composition, hydrophilic nature, and relatively low calorific value [START_REF] Arias | Influence of Torrefaction on the Grindability and Reactivity of Woody Biomass[END_REF][START_REF] Chew | Recent Advances in Biomass Pretreatment-torrefaction Fundamentals and Technology[END_REF]. Sugarcane bagasse is composed by approximately 32-34% (on SCB dry weight basis) cellulose, 19-24% (on SCB dry weight basis) hemicelluloses, 25-32% (on SCB dry weight basis lignin, 6-12% (on SCB dry weight basis) extractives, and 2-6% (on SCB dry weight basis) ash [START_REF] Haghdan | 1-Source of Lignin[END_REF]. In addition, the storage of the bagasse in humid conditions can cause several damages due to moisture absorption and biological degradation [START_REF] Kymäläinen | Fungal colonisation and moisture uptake of torrefied wood, charcoal, and thermally treated pellets during storage[END_REF]. For example, it has been previously shown that fungal degradation (Trametes versicolor) of eucalyptus, pine and coffee husks samples lead to decreases in their HHV values from 17.429 to 17.069 MJ.kg - 1 , 18.801 to 17.377 MJ.kg -1 and 18.823 to 17.194 MJ.kg -1 , respectively [START_REF] Freitas Homem De Faria | Effect of Leaching and Fungal Attacks During Storage on Chemical Properties of Raw and Torrefied Biomasses, Waste and Biomass Valorization[END_REF]. Indeed SCB is usually stored in outdoor piles [START_REF] Ribeiro | Effect of Calcination Temperature on the Pozzolanic Activity of Brazilian Sugar Cane Bagasse Ash (SCBA)[END_REF] occurring moisture adsorption by wood and the resultant chemical oxidation and/or microbiological activity lead to hydrolysation [14]. This phenomenon increases the internal temperature of the stored biomass stack thereby accelerating the biomass decay degradation [15,[START_REF] Medic | Effect of torrefaction on water vapor adsorption and resistance to microbial degradation of corn stover[END_REF]. Such degradations reduce the biomass potential to be processing and consequently decrease its market value. Some pre-treatment are then necessary like torrefaction, which is a thermal treatment who improves the biomass energetic, physical and mechanical properties in order to make the biofuel more homogenous and stable to biological degradation [START_REF] Batidzirai | Biomass torrefaction technology: Techno-economic status and future prospects[END_REF]. The market potential of torrefied biomass is expected to be huge in Central Europe and the Nordic countries, and also around the world, considering the substitution of coal in large-scale power and heat product [START_REF] Svanberg | Analysing Biomass Torrefaction Supply Chain Costs[END_REF]. Torrefaction means a thermos-chemical treatment of biomass at 200 to 320°C and the process takes place in oxygen free circumstances [START_REF] Föhr | Manufacturing of Torrefied Pellets without a Binder from Different Raw Wood Materials in the Pilot Plant[END_REF]. Although the torrefaction allows reducing the hydrophilic behavior, it is possible that the torrefied wood nevertheless reaches, during its storage, a sufficient moisture content level so that the rots can develop [START_REF] Kymäläinen | Biological degradation of torrefied wood and charcoal[END_REF]. Previous studies showed that such a process can be used in order to improve and preserve the energetic properties of torrefied biomasses (eucalyptus, pine and coffee husks residues) during water and fungal exposure [START_REF] Freitas Homem De Faria | Effect of Leaching and Fungal Attacks During Storage on Chemical Properties of Raw and Torrefied Biomasses, Waste and Biomass Valorization[END_REF]. Other past studies highlighted that the improvement the quality of bagasse as feedstock can be also done with torrefaction pretreatment [START_REF] Daniyanto | Torrefaction of Indonesian sugar-cane bagasse to improve bio-syngas quality for gasification process[END_REF]. The utilization of torrefied bagasse in existing handling and storage facilities and associated issues has been reported recently [START_REF] Hermes | Large-scale tests using refined wood pellets in coal power plants[END_REF][START_REF] Zwart | Description of existing handling and storage facilities and the associated issues, Public report within EU FP7 Project: SECTOR -Production of solid sustainable energy carriers by means of torrefaction[END_REF][START_REF] Kiel | Torrefaction -an enabler for biomass gasification[END_REF] and could be one of the solution to preserve its energetic properties during storage depending on the torrefaction treatment intensity [START_REF] Candelier | Main potential industrial methods about the determination of wood thermal treatment intensity and prediction of conferred durability: a review[END_REF]. Despite recent advances over the world, torrefaction technology is still under development and only a few techno-economic assessments regarding the entire logistic chain, including storage, have been made [START_REF] Kymäläinen | Biological degradation of torrefied wood and charcoal[END_REF].

The aim of the study was to determine how the various raw and torrefied SCB materials tolerate moisture and fungal decay exposure, simulating by various storage conditions. SCB was torrefied at 290°C with 5, 7.5 and 10 min residence time then leached and exposed to different fungal decay (brown and white rots) during five different exposure durations (from 2, 4, 6, 8, 12 and 16 weeks). Both effects of the torrefaction duration and the storage conditions of SCB were investigated. The impact of water-leaching (without decay exposure) and the fungal degradation impact on the chemical (elemental composition) and energy properties (High Heating Values) of raw and torrefied SCB for an energy conversion were thus evaluated. Finally, importance of the torrefaction intensity, depending on the process duration, on these different mechanisms was highlighted. The results obtained through this study could be a great additional data potentially enhancing the models used to predict the energy properties and the degradation level of torrefied biomass during storage.

MATERIALS AND METHODS

Biomass samples

Sugarcane bagasse residues (Saccharum spp. hybrids) (SCB) were obtained from Jatiboca Sugar and Ethanol industry (Ponte nova, Minas Gerais, Brazil). SCB was first dried outdoors in a drying yard during 30 days, until they reached hygroscopic equilibrium moisture (about 20%, on wet basis). Afterwards, milling was done in a knife mill and a classification in screen sieves between 6 and 2 mm, collecting the fraction retained on the 2 mm sieves to diminish the presence of fine particles, which might compromise subsequent analysis. SCB particles, with long size ranged from 10 to 20 mm (and 2-6 mm diameter), were oven-dried at 103 ± 2 °C for 48 hours with constant air circulation to reach a moisture content near to 0% and avoiding the influence of water on treatments.

Torrefaction process

SCB was then torrefied at 290° C using 5 kg of biomass for each sample. The three following residence times were evaluated: 5, 7.5, 10 minutes. Two torrefaction replicates were performed for each modality. Torrefaction processes were carried out in an endless screw type reactor, developed in the Panels and Energy Laboratory of the Federal University of Viçosa (Fig. 1). The heating section (with length of 1.375 m) was built in carbon steel main structure with an indirect heating exchange system with a combustion gas followed by an indirect water cooling system. Two timers controlling the gear motor were used to control the residence time of the biomass sample within the reactor. The first of one was used to activate the gear motor for approximately 4 seconds (time required for one complete revolution of the worm screw), and the second timer was used to stop the gear motor for a predetermined time. The sum of these two durations recorded but the both timers, multiplied by the number of revolutions of the worm screw (without end with the roasting section) then defined the residence time of the biomass samples it torrefaction process. A Gulton Gulterm 700-10S digital thermometer, using eight type J thermocouples (Figure 1) was used to continuously record and control the temperature of the torrefaction chamber. More information of the biomasses torrefaction process and reactor can be found in Magalhães et al. 2018 [26].

Mass loss due to thermal degradation

The Mass Loss (ML%) due to the thermal degradation was determined according to the Equation 1 (Eq. 1):

%, = × 100 (1)
Where :

MLtt = mass loss of the sample due to torrefaction process (in %, dry basis) m0 = initial oven-dried mass before torrefaction m1 = final oven-dried mass after torrefaction

Chemical composition of raw and torrefied SCB samples

The Chemical Composition was obtained for the determination of the Extractive, total Lignin (insoluble and soluble), and Holocellulose contents. The samples were ground and sieved with mesh of 40 (420 µm) and 60 (250 µm).

The total extractive content was determined according to TAPPI 204 cm-97 (TAPPI 1996).After a drying step performed at 103 °C until mass stabilization, the sawdust (10 g) was extracted in a Soxhlet with a solution of 1/3 ethanol and 2/3 benzene [from Carlo Erba Reagents Group (France)] during 6 h, and then dried at 103 °C for 48 h to obtain the oven-dried mass of the free extractives sample. Extractive contents were determined by the ovendried mass difference.

The soluble lignin, insoluble lignin and holocellulose contents were determined on freeextractives SCB samples, with some adjustments, according to the guidelines from the methodology used by Pereira et al. [START_REF] Pereira | Influence of Chemical Composition of Eucalyptus Wood on Gravimetric Yield and Charcoal Properties[END_REF].

Soluble and insoluble lignin contents were determined as following: 500 mg of the three native and torrefied biomasses sawdust, previously extracted (by the 1/3 ethanol: 2/3 benzene solvent extraction process), were separately mixed with 72% H2SO4 (10 mL) for 4 h at 30 °C.

The mixture was then diluted with distilled water until a H2SO4 concentration of 3%, submitted to another hydrolysis at 2 atm pressure and 121 °C for 1 h. The insoluble lignin content was determined by filtration and the soluble lignin content by UV spectrometry. Total lignin was obtained by the sum of insoluble and soluble lignin fractions.

Holocellulose content was determined as following: 500 mg of the three native and torrefied biomasses sawdust, previously extracted (by the 1/3 ethanol: 2/3 benzene solvent extraction process), were separately placed in a 100 mL flask containing 30 mL of distilled water and heated at 75 °C. Acetic acid (0.1 mL) and 15% aqueous sodium chlorite (2 mL) were then added each hour for 7 h. The mixture was filtered on a Büchner funnel and the residue washed with water, soxhlet extracted for 2 h with ethanol and dried at 103 °C to a constant mass.

Fungal decay exposure

All samples (raw and torrefied) were firstly submitted to leaching process according to the NFX 41-568 [START_REF]Wood preservatives -laboratory method for obtaining samples for analysis to measure losses by leaching into water or synthetic sea water[END_REF] standard. This standardized procedure for accelerated ageing is commonly used for natural durability wood testing. Samples were immersed in distilled water (1 volume of biomass for 5 volumes of water) and subjected for six leaching periods of increasing duration under continuous shaking at 20°C. Water was replaced by fresh water after each leaching period. A first cycle of 3 leaching periods of 1, 2 and 4 hours was performed.

Samples were then kept air drying for 16 hours. Leaching was pursued for 3 additional periods of 8, 16 and 48 hours, with change of water between each period. The leached samples were then oven-dried at 103°C and weighed (m2). Percentage of leaching was calculated following the Equation 2 (Eq.2):

%, = ! × 100 (2)
Where :

MLleach = Mass loss of the sample due to leaching process (in %, dry basis) m1 = initial oven-dried mass of sample before leaching m2 = final oven-dried mass of leached sample Then, raw and treated biomasses were place into plastic grid bags (dimensions of 10 x 3 x 1.5 cm) in order to expose themselves to basidiomycete attacks. Before starting the fungal exposure experiment, the bags were oven dried 103 °C for 24 h and the initial dry mass (m3) was measured for further determination of the weight loss of the sample due to the fungal degradation. Each sample was crafted to have an initial dried mass of sample around 3 grams.

All samples were then placed in conditioned room (22°C, 70% Relative Humidity) in order to keep humidity prior to the decay exposure tests. After that, all samples were sterilized by xray process with a dose of 25.0 (± 5.6%) kGy, as recommended by the NFX 41-568 [START_REF]Wood preservatives -laboratory method for obtaining samples for analysis to measure losses by leaching into water or synthetic sea water[END_REF] standard, by Ionisos Compagny (France). Contrary to autoclave sterilization, such a Gammaradiation does not cause significant changes in wood properties and not affect the wood natural durability. In addition x-ray process is more efficient than oven drying to sterilized wood samples.

Decay resistance of raw (leached and not leached) and torrefied biomasses after leaching was tested according to an adaptation of XP CEN/TS 15083-1 [START_REF]Durability of wood and wood-based products -Determination of the natural durability of solid wood against wood destroying fungi -Test methods -Part 1: basidiomycetes, European Committee for Standardization (CEN)[END_REF] standard criteria, on both fungi species required by the standard: Trametes versicolor (white rot) and Coniophora puteana (brown rot). The following fungal exposure duration were tested: 2, 4, 8, 12 and 16 weeks.

During these fungal exposures, all test devices were place in dark conditioned chamber (at 22°C, 70% Relative Humidity). After the different decay exposure duration, mycelia were removed and each sample was weight (m4) and then was dried at 103 °C for 24 h and the final dried mass was determined (m5).

The final moisture content (MC) after fungal exposure of the sample was determined according to the following formula (Eq. 3):

MC %, = " # # × 100 (3)
Where :

MC = Moisture content of the sample due to fungal exposure (in %, dry basis) m4 = wet mass of fungal decayed sample sample m5 = oven-dried mass of fungal decayed sample

The Weight Loss was determined according to the following formula (Eq. 4):

$ %, = % # % × 100 (4)
Where :

WL = Weight Loss of the sample due to fungal degradation (in %, dry basis) m3 = initial oven-dried mass of sample before fungal exposure m5 = final oven-dried mass of fungal decayed sample

The decay exposure tests were conducted as follows:

-The three torrefaction intensities for sugarcane bagasse, after leaching, were tested in order to evaluate the impact of the heat treatment intensity on the biomasses durability;

-For both each fungus and fungal exposure duration, 4 replicates of raw biomasses (after leaching) and for each torrefied biomasses (after leaching) were done.

-Beech (Fagus sylvatica) and Pine (Pinus sylvestris) sapwood samples of the dimensions 50x25x15mm 3 (L, R, T) were used as controls for the virulence of the strains (8 tested blocks for each fungus and for each fungal exposure duration).

Sample characterization

The impacts of leaching and fungal degradation, according to the duration of fungal exposure were evaluated on the following raw and treated biomass properties according to 3 characterizations: (i) the Weight Losses (WL) caused by fungal degradation, the Elemental Compositions (EC); and (iii) the High Heating Values (HHV). For each tested modality, the four replicated used for decay test were mixed, ground using a cutting mill Retsch SM 100 and sieved. Particle sizes fraction of 0.1 to 0.2 mm was retained. Sawdust was then conditioned at 103 °C for 24 h and stored in air-tight bottle before analysis.

Elemental composition

Elemental analyses were carried out with an ElementarVario Macro tube CHN analyzer according to the BS EN 15104 [START_REF] Bs En | Solid biofuels. Determination of total content of carbon, hydrogen and nitrogen, Instrumental methods[END_REF]. Prior to analysis, the samples were packed in a tin foil and two tests were performed for each sample. Atomic H/C (Eq. 4) ratios of samples were determined following Patch et al. [START_REF] Pach | Torrified biomass a substitute for wood and charcoal[END_REF] and Ascough et al. [START_REF] Ascough | Charcoal reflectance measurements: implications for structural characterization and assessment of diagenetic alteration[END_REF]: (4)

High Heating Values

The calorific value was measured using an Automated Isoperibol Fixed Bomb Parr 6200 bomb calorimeter, following the CEN/TS 14918 [START_REF]Solid bio fuels-method for the determination of calorific value[END_REF].

1 g of oven-dried biomass, with a particle size < 0.2 mm, were required to perform the calorific test. 2 replicates have been done for each sample and all the results are given with an accuracy of ± 5.00%. -The impact of torrefaction duration and leaching process on elemental composition and HHv values of SCB samples. Results were then ranked into several categories from "A" to "D" to evaluate the influence torrefaction duration and from "a" to b" to compare unleached and leached SCB samples (Tab.1).

Statistical analysis

-The impact of torrefaction duration on the chemical composition of SCB samples . Results

were then ranked into several categories from "A" to "D" (Figure 5).

-The impact of torrefaction duration on MLtt (in %, dry basis) and MLleach (in %, dry basis) of SCB samples . Results were then ranked into several categories from "A" to "D" (Fig. 4).

-The impact of torrefaction duration on the chemical composition of SCB samples. Results

were then ranked into several categories from "A" to "D" (Fig. 5).

-The impact of fungal exposure duration on MC (in %, dry basis) (Fig. 6a) and WL (in %, dry basis) (Fig. 7a) of pine and beech control samples. Results were then ranked into several categories from "A" to "E", "A'" to "E'", "a" to "e" and "a'" to "e'" for beech control (CP), pine control (CP) , beech control (TV) and pine control (TV), respectively.

-The impact of torrefaction duration on MC (in %, dry basis) (Fig. 6b) and WL (in %, dry basis) (Fig. 7b), according to the fungal exposure duration of SCB samples. Results were then ranked into several categories from "A" to "D", "a" to "d", for CP and TV, respectively.

The assumption of homogeneity of variance and normal distribution of the sample required for the use of parametric statistics are satisfied. The impact of a parameter on a system not connected by the same letter was considered as no significant at the 5% level.

For a better comprehension of the study approach, a scheme of the different analysis protocols are presented in Fig. 2. In addition, the sampling and analytical devices used in this study are illustrated in Fig. 3.

Mass loss determination (MLtt %)

According to the exponential kinetic of wood thermal degradation, the treatment duration has an impact on treatment intensity as well as on mass loss kinetic [START_REF] Candelier | Main potential industrial methods about the determination of wood thermal treatment intensity and prediction of conferred durability: a review[END_REF]. The results presented in Fig. 4, are in accordance with this statement. For heat treatments performed at 290 °C, MLtt (%) of SCB samples increased with the increasing of the process duration, from 12 ± 0.20 % (t=5 min.) to 23,5 ± 0.21 % (t=10 min.). This phenomenon is due to the fact of for a given temperature, the quantity of each degradation product increases progressively as the treatment duration increases [START_REF] Candelier | Thermodesorption coupled to GC-MS to characterize volatiles formation kinetic during wood thermodegradation[END_REF].

[Figure 4]

Chemical composition changes after torrefaction processes

According to the Fig. 5, raw sugarcane bagasse sample (SCB-Ref) was composed of 69.15 % holocellulose, 25.49 % lignin (including 24.47 % insoluble lignin and 1.02% soluble lignin) and 5.36 % extractive compounds. Fig. 5 indicated that extractives content of SCB increased at 290 °C -5 and 7.5 minutes and seemed to decrease at 290°C -10 minutes. These results are consistent with other past studies [START_REF] Rosa | The effect of long term treatment at 100°C-150°C on structure, chemical composition and com-pression behaviour of cork[END_REF][START_REF] Esteves | Extractive composition and summative chemical analysis of thermally treated eucalypt wood[END_REF], highlighting that the major increase at low intensity was due to water and ethanol extractives because of polysaccharide degradation. Almost all of the original extractives released and new compounds were generated by hemicelluloses and lignin thermal degradation. These mainly included monosaccharides and their dehydratation products, as well as syringaldehyde, syringic acid and sinapaldehyde, as the most prominent lignin derived compounds [START_REF] Esteves | Extractive composition and summative chemical analysis of thermally treated eucalypt wood[END_REF][START_REF] Rousset | Effect of severe thermal treatment on spruce and beech wood lignins[END_REF].

The proportion of lignin increases considerably after thermal treatment, mainly due to the increase of insoluble lignin content, whereas soluble lignin content decreases according to the heat treatment duration. The ether linkages of lignins are cleaved and new free phenolic hydroxyl groups and a-and b-carbonyl groups arise, which are responsible for crosslinking via formation of methylene bridges [38][START_REF] Wikberg | Characterisation of thermally modified hard-and softwoodsby13C CPMAS NMR[END_REF][START_REF] Tjeerdsma | Chemical changes in hydrothermal treated wood: FTIR analysis of combined hydrothermal and dry heat-treated wood[END_REF]. The methoxyl content decreases and the new reactive sites on the aromatic ring can lead to further condensation reactions [START_REF] Wikberg | Characterisation of thermally modified hard-and softwoodsby13C CPMAS NMR[END_REF]. The decomposition of the holocellulose, mainly due to hemicellulose degradation [START_REF] Haseli | Modeling biomass particle pyrolysis with temperature-dependent heat of reactions[END_REF] is statistically significant as the residence time is increasing. The holocellulose is decomposed greatly and the proportion of lignin in the material increases, leaving a highly lignified material [START_REF] Drummond | Pyrolysis of sugarcane bagasse in awire-mesh reactor[END_REF][START_REF] Rezende | Chemical and morphological characterization of sugarcane bagasse submitted to a delignification process for enhanced enzymatic digestibility[END_REF][START_REF] Sluiter | Evaluation of Brazilian Sugarcane Bagasse Characterization: An Interlaboratory Comparison Study[END_REF].

Figure 5 FT-IR spectra analyses, performed on raw and torrefied sugarcane bagasse by Manatura (2020) [START_REF] Manatura | Inert torrefaction of sugarcane bagasse to improve its fuel properties[END_REF] indicated the changes in the chemical structures of the hemicellulose, cellulose and lignin, which were associated with the functional groups such as C-O, C=O, O-H, C=C, and C-C-O. Even if the thermal degradation kinetics are different than those of other wood species, the undergone chemicals changes are similar for the sugarcane than those of woods, in despite of their difference in chemical compositions and anatomical structures.

Elemental Composition and High Heating Values of raw and heat-treated SCB samples before and after leaching

The objectives of this section are to (i) evaluate the influence of leaching step on the elemental composition and energy properties (HHV) of sugarcane bagasse changes, and then to (ii) determine if torrefaction process, according to the curing temperature and residence time, improves the SCB energy properties and limit the impact of leaching for an energy use (mainly for combustion conversion).

As shown in Tab. 1, torrefaction duration affects the element compositions of SCB materials.

The carbon and nitrogen in the material increased with residence time, while the hydrogen decreased. Hydrogen and oxygen are released as water vapor and carbon dioxide emissions [START_REF] Granados | Biomass Torrefaction in a Two-Stage Rotary Reactor: Modeling and Experimental Validation[END_REF]. Hemicelluloses mainly generate carbon monoxide and carbon dioxide through decarboxylation reactions of acid groups linked to hemicelluloses [START_REF] Werner | Thermal decomposition of hemicelluloses[END_REF], while cellulose generates small amounts of carbon dioxide [START_REF] Meija-Feldmane | Leachates of Thermally Modified Pine (Pinus sylvestris L.) Wood[END_REF]. In addition, small volatile molecules (as acetic acid, furfural, methylfurfural, vanillin, syringaldehyde, etc.) are released during biomass torrefaction [START_REF] Candelier | Thermodesorption coupled to GC-MS to characterize volatiles formation kinetic during wood thermodegradation[END_REF].

Table 1 As shown in Tab. 1, HHV increased with the residence time. Overall, Tab. 1 and Fig. 4 indicate that the HHV increased with an increase of Mass Loss due to thermal degradation in SCB samples.

One of the processes in material usage outdoors is leaching. Leachates of thermally modified wood consist mainly of wood constituents' degradation products (tannic acid, acetic acid, furfural and furfural derivatives), but also extractives [START_REF] Meija-Feldmane | Leachates of Thermally Modified Pine (Pinus sylvestris L.) Wood[END_REF], that may explain the higher MLleach% of SCB-HT-5 than those of the three other samples (Fig. 4). In addition, due to the thermal degradation of SCB biomass components, the content of hydroxyl groups decreases leading to increased hydrophobity and wettability that is more difficult. According to obtained results of wood mass loss during the thermal treatment, degradation process in SCB-HT-10 (MLleach= 2.16%) occurs at greater degree than in SCB-Ref (MLleach= 3.54%), SCB-HT-5 (MLleach= 4.88%) and SCB-HT-7.5 (MLleach= 2.78%), thereby reducing the amount of substances accessible to leach (Fig. 5). These results are in accordance with other past studies showing that thermally modified pine and eucalyptus woods (temperature 190, 200 and 210°C; durations: 2, 6 and 12 hours are less sensitive to water depending on the treatment intensity, than their respective native woods [START_REF] Esteves | Influence of steam heating on the properties of pine (Pinus pinaster) and eucalypt (Eucalyptys globulus) wood[END_REF][START_REF] Brito | Chemical composition changes in eucalyptus and pinus woods submitted to heat treatment[END_REF]. Finally, thermally modified SCB treated during 10 minutes (SCB-HT-10) is more environmentally friendly, than the other treatment, due to less water leachable substances (Fig. 4).

Tab. 1 also highlights that the HHV of raw and heat-treated SCB samples was decreased by leaching process. In view of the fact that the elemental composition is not significantly modified for all SCB samples following leaching, the decreases of HHV values could be explained by the removal of some extractives compounds during this leaching process. In fact, previous studies shown that extractives content and composition are strongly correlated to the HHV of woody biomass [START_REF] White | Effect of lignin content and extractives on the higher heating value of wood[END_REF][START_REF] Howard | Heat of combustion of various southern pine materials[END_REF].

Decay degradation of raw and heat-treated SCB samples according to the fungus exposure duration

The objectives of this section is to simulate the storage of raw and torrefied SCB in order to (i) evaluate the impact of SCB decay degradation on its energy properties and then to (ii) determine if torrefaction process, according to the curing temperature and residence time, allows to limit the impact of decay degradation on the energy properties.

Moisture content

Fig. 6 shows the final moisture content in % of control beech and pine samples, raw and torrefied SCB samples, after Trametes versicolor and Coniophora puteana fungi exposures and according to the duration of these decay expositions.

Whatever the fungi exposed to the SCB samples, the final moisture content of SCB sample was directly proportional to its hemicellulose content. By comparing Fig. 6 and Tab. 1, it clearly appears that the amount of water absorbed by sugarcane bagasse during fungi test increased when its hemicellulose content increased. The hydrophilic nature of bagasse is mainly due to the high content of hemicellulose. Hemicellulose polymers have many hydroxyl groups (-OH) which drive sugarcane bagasse to be polar and to make easily hydrogen bonds with water molecules [START_REF] Tumuluru | A Review on Biomass Torrefaction Process and Product Properties for Energy Applications[END_REF]. Hemicellulose degradation during torrefaction causes the loss of hydroxyl groups in sugarcane bagasse [START_REF] White | Effect of lignin content and extractives on the higher heating value of wood[END_REF]. Thus, torrefied sugarcane bagasse loses its hydrophilic nature, since there are no hydroxyl groups, which tend to be polar and easily make hydrogen bonds with water molecules. In addition, the changes of lignocellulosic polymers during torrefaction lead to the formation of unsaturated non-polar structures in sugarcane bagasse.

The degradation of hemicellulose, cellulose, and lignin in sugarcane bagasse during torrefaction causes shortening of polymer chains. The changes of sugarcane bagasse properties into less fibrous, soft, and brittle is mainly caused by the degradation of hemicellulose and cellulose [START_REF] Supramono | Effect of heating rate of torrefaction of sugarcane bagasse on its physical characteristics[END_REF].

Figure 6

The hydrophobic behavior of raw SCB is very similar than those of beech and pine control samples. Their moisture content levels increase highly during the first four fungi exposition weeks, and then continue to increase more slowly for longer tests durations (Fig. 6).

The moisture content of torrefied SCB samples, after decay exposure, is lower than those of beech and pine control samples and raw SCB samples, whatever the decay test duration and the fungi type. In addition, we can observe a significant moisture content decrease for SCB-HT-7.5 and SCB-HT-10 samples compared to raw and SCB-HT-5 samples. The moisture content of SCB-HT-7.5 and SCB-HT-10 seems to be increased highly during the four first weeks and tend to stabilize for longer fungal exposure duration.

Decay degradation

The overall Weight Loss (WL %) due to Trametes versicolor and Coniophora puteana exposure of beech and pine control samples, raw and torrefied SCB samples, is shown in Fig. 7a.

According to the weight loss values obtained concerning the fungal degradation of beech and pine wood control sample after 16 weeks, the decay resistance test performed through this study has been validated. Indeed, the following minimal degradation levels (according to XP CEN/TS 15083-1 [START_REF]Durability of wood and wood-based products -Determination of the natural durability of solid wood against wood destroying fungi -Test methods -Part 1: basidiomycetes, European Committee for Standardization (CEN)[END_REF]) of control samples were reached:

-Coniophora puteana (Brown rot) on Pine: WL = 57.12 % > 30% -Coniophora puteana (Brown rot) on Beech: WL = 48.18 % > 30 %; -Trametes versicolor (White rot) on Pine: WL = 24.18 % -no requirement; -Trametes versicolor (White rot) on beech: WL = 44.24 % > 20 %.

For the both tested rots, a similar tendency concerning the decay resistance improvement of SCB was observed after torrefaction. In addition, as for the results observed previously with respect to the moisture content levels, conferred durability to SCB by thermal modification depends on the heat treatment duration.

Average values of weight loss due to fungal degradation showed raw and torrefied SCB samples are generally similar to Coniophora puteana (Brown rot) than Trametes versicolor (White rot). In addition, it clearly appears that SCB-HT-7.5 and SCB-HT-10 samples are significantly more resistant to the tow fungal degradations than SCB-Ref and SCB-HT-5.

According to the torrefaction duration, the torrefied SCB samples suffered a destruction of their cell wall [START_REF] Alfredsen | A step towards a better understanding of fungal colonization of modified wood -QRT-PCR studies[END_REF], a largely hemicelluloses degradation than the other macromolecular components [START_REF] Shafizadeh | Thermal deterioration of wood[END_REF], occurring moisture exclusion [START_REF] Ringman | Possible targets of wood modification in brown rot degradation[END_REF][START_REF] Ringman | Mode of action of brown rot decay resistance in modified wood: a review[END_REF][START_REF] Jellison | What molecular biology can tell us about the biodegradation of lignocellulose: the 30 utilization of molecular techniques for the detection, identification and enhanced understanding of wood degrading organisms[END_REF] and a limited feed stock, then resulting in a high capacity to limit the colonization and the damages caused by the both fungi.

According to the Fig. 7a, the both fungal degradation of beech and pine control samples appears to begin as early as the fourth week of rot exposure. The fungal degradation (TV and CP) of SCB-HT-5 starts as early as the second week of rot exposure, while those of SCB-HT-7.5 and SCB-HT-10 samples is delayed and begins only from the eighth week of fungal contact and then increase very slowly. Finally, SCB-HT-7.5 (WL < 5 %) and SCB-HT-10 (WL < 2%) seem to be very promising treatment ways to preserve SCB properties during the biomass storage (Fig. 7b).

Degradation by Trametes versicolor (TV) -White rot

Loss of carbon was recorded in all samples with white rotter Trametes versicolor (TV) (Fig. 8a). According to the literature, white rot mainly degrades the lignin polymer, causing loss of carbon [START_REF] Brito | Chemical composition changes in eucalyptus and pinus woods submitted to heat treatment[END_REF]. Indeed, fungal activity was expected to reduce the carbon content as carbon compounds serve as nutrients and are respired as CO2 by fungi [60,[START_REF] Rytioja | Plant polysaccharide degrading enzymes from basidiomycetes[END_REF].

The Nitrogen content increased according to the fungal exposure duration, in all tested materials (Fig. 8c), whereas Hydrogen content decreased for SCB-HT and SCB-HT-5 and increased for SCB-HT-7.5 and SCB-HT-10 (Fig. 8b). Previous study highlighted that fungi have the capacity to scavenge and import nitrogen to decaying wood via a translocating network of mycelium [START_REF] Bebber | Simulated nitrogen deposition affects wood decomposition by cord-forming fungi[END_REF]. It results that, for all tested samples, H/C ratio increases according to the Trametes versicolor (TV) exposure duration, where the most significant evolution occurs in the first four weeks of fungal exposure (Fig. 8d).

As observed in Figures 8e and8f, the decreases of HHV values, for each SCB samples (raw and heat-treated), according the fungal exposure duration are in agreement with the litterature.

Depletion of carbon will lower the calorific value and here, statistically significant changes were detected in carbon content. The heat content is related to the oxidation state of the natural fuels in which carbon atoms generally dominate and overshadow small variations of hydrogen and nitrogen content [START_REF] Susott | Heat content of natural fuels[END_REF]. Susott et al. [START_REF] Susott | Heat content of natural fuels[END_REF] and Tillman [START_REF] Tillman | Wood as an energy resource[END_REF] found a linear relationship between the higher heating value and the carbon content of the natural fuels, chars, and volatiles. Huang et al. [START_REF] Huang | Ultimate analysis and heating value prediction of straw by near infrared spectroscopy[END_REF] and Protàsio et al. [START_REF] Protàsio | Canonical correlations analysis between characteristics of Eucalyptus wood and charcoals[END_REF] have also put highlight that Carbon and Hydrogen are the elements that contribute most to the Heating Values of charcoal. Therefore, low O/C ratio is good to have high HHV values and low H/C ratio is not really desired for the use of wood and charcoals for energy, by combustion process. However high H/C ratios could be suitable for gasification conversion, making more reactive the torrefied biomass.

Degradation by Coniophora puteana (CP) -Brown rot

Concerning the changes in Hydrogen (Fig. 9b) and Nitrogen (Fig. 9c) contents during fungal exposure, similar results were observed for Coniophora puteana (CP) and Trametes versicolor (TV). Regarding the Carbon content, different behavior were observed during Coniophora puteana (CP) exposure, according to the heat treatment intensity. It results that Carbon content decreases for SCB-HT-7.5 and SCB-HT-10 samples relating to the Coniophora puteana (CP) exposition duration, whereas Carbon content increases for SCB-Ref and SCB-HT-5 samples (Fig. 9a). These results could be explained by the remaining cellulose and the hemicelluloses after torrefaction at low temperature (SCB-Ref and SCB-HT-5) are degraded by Coniophora puteana (CP), increasing the relative carbon content. In fact, brown-rot fungus (C. puteana) causes more weight loss than the white-rot (T. versicolor), because it removes the cellulosic fraction and leaves lignin structurally modified. Brown rotter Coniophora puteana (CP) prefers coniferous wood species, but seems to cause also carbon loss in SCB. It uses endoglucanases to degrade cellulose and hemicelluloses, but has also been reported to produce cellobiohydrolases that erode crystalline cellulose and could therefore also utilize quite severely torrefied wood [START_REF] Mansfield | Characterization of endoglucanases from the brown rot fungi Gloephyllum sepiarium and Gloephyllum trabeum[END_REF]. We observe a decrease of H/C ratio and an increase HHV value for SCB-Ref and SCB-HT-5 samples according to the T. versicolor exposure duration, whereas an increase of H/C ratio and a decrease HHV value for SCB-7.5 and SCB-HT-10 samples were observed in the same conditions (Fig. 9d and Fig. 9e).

The increase of HHV in that case (SCB-Ref and SCB-HT-5), may be associated with the degradation of the low-energy content components (e.g., leaf materials or agro-food residues) that were easily accessible to the enzymatic community during storage condition [START_REF] Therasme | Overhead Protection Increases Fuel Quality and Natural Drying of Leaf-On Woody Biomass Storage Piles[END_REF]. Krigstin and Wetzel [69] explained in their review that changes to the proportion of components in the biomass will affect energy content. For example, the preferential decomposition of hemicellulose will result in increased heating values, but the loss of extractives or the preferential fungal decomposition of lignin would tend to reduce heating values.

Balance sheet

The High Heating Value of SCB was significantly increased according to the torrefaction duration. However, HHVs of raw and torrefied SCB were affected by the decay exposure duration. There was a relationship between the HHV and decay exposure time across all the thermal treatments and fungi, but the change over time was small (Fig. 8e and Fig. 9e).

Trametes versicolor is a white rot fungus, degrading mainly the lignin polymer, therefore occurred to all SCB samples a decrease in HHV according to the fungal exposure duration. At the opposite, Coniophora puteana, a brown rot fungi degrading mainly the hemicelluloses and cellulose fractions, brings an increase in HHV of SCB-Ref and SCB-HT-5 samples and only a small decrease in HHV of SCB-HT-7.5 and SCB-HT-10 samples. Numerous studies on wood storage have recently reported no significant change or a decrease of the higher heating value [START_REF] Pari | Poplar wood chip storage: Effect of particle size and breathable covering on drying dynamics and biofuel quality[END_REF][START_REF] Lenz | Open-air storage of fine and coarse wood chips of poplar from short rotation coppice in covered piles[END_REF][72], while others have reported a slight increase of heating value [START_REF] Brand | Storage as a tool to improve wood fuel quality[END_REF][START_REF] Barontini | Storage dynamics and fuel quality of poplar chips[END_REF][START_REF] Krzyzaniak | Effect of storage methods on willow chips quality[END_REF] at the end of the storage.

From our laboratory experiment, it was clear that fungi are able to settle on seemingly unfavorable material, which is said to be hydrophobic and suitable for outside storage. The more mildly treated samples experienced more carbon loss, but took in less moisture in outside storage, contrary to the more severely treated samples [START_REF] Kymäläinen | Biological degradation of torrefied wood and charcoal[END_REF]. The loss of dry matter (WL%) and carbon (C%) as well as the increasing moisture content decrease the calorific value of the fuel and thus increase associated costs [START_REF] Kymäläinen | Biological degradation of torrefied wood and charcoal[END_REF]. Therefore, torrefaction could be a viable method to eliminate some of the disadvantages of raw biomass as it significantly improves energy content and prevents absorption of moisture during storage [START_REF] Patel | Improved fuel characteristics of cotton stalk, prosopis and sugarcane bagasse through torrefaction[END_REF]. Even if HHV values decrease during fungal exposure, HHV values are still higher for high-intensity treated SCB samples (SCB-HT-7.5 and SCB-HT-10) than those in the raw and or slightly thermally modified (SCB-Ref and SCB-HT-5) (Fig. 8f and Fig. 9f).

CONCLUSION

The torrefaction residence time has an important impact on sugarcane bagasse (SCB) mass loss kinetic during torrefaction process, undergoing chemical and elemental composition modifications, High Heating Values and decay resistance improvements of torrefied SCB.

The storage emulation tests developed in laboratory show that raw and torrefied SCB are still subjected to biological degradation and dry matter losses. It can be stated that in the most cases, the two tested fungi use mainly the carbon fraction of native and torrefied SCB, which leads to a decrease in calorific value. However, the recorded carbon loss due to fungal decay decreased towards the torrefaction performed with a long residence time (290°C during 7.5 or 10 min).Torrefaction pre-treatment performed at severe intensity is therefore a good way to improve SCB's energy properties while limiting lose torrefied biomass quality during storage. Future studies will be needed to simulate the exact fungal degradation dynamics and evaluate the properties changes of different torrefied biomasses during real-life storage.

AKNOWLEDGMENT

to : Thermocouples used in order to control the homogeneity of the temperature in the entire torrefaction chamber Fig. 1 : Layout in side view of the reactor for biomass torrefaction in semi-continuous flow [START_REF] Svanberg | Analysing Biomass Torrefaction Supply Chain Costs[END_REF].

1 8
Fig. 2: Synthesis of the overall approach and the different analysis protocols used for this study. * Each analysis has been duplicate and all the results are given with an accuracy of ± 0.20 %. ** Each analysis has been duplicate and all the results are given with an accuracy of ± 5.00 %.

Letters: bar followed by the same letter are not significantly different.

Here, the letters from "A" to "D" indicate the influence of the influence torrefaction duration on the elemental composition and HHV values (comparing rows). The letters from "a" to "b" indicate the influence of the water leaching process on the elemental composition and HHV values (comparing columns).

Emulation of field storage conditions for assessment of energy properties of torrefied sugarcane bagasses GRAPHICAL ABSTRACT

Figure 1

 1 Figure 1

Figure 2 Figure 3 3 .

 233 Figure 2Figure3

Figure 7 3 . 5 .

 35 Figure 7

Fig. 3 :

 3 Fig. 3: The various sampling and analysis apparatus used in this study: (a) cutting mill Retsch SM 100, (b) ElementarVario Macro tube CHN analyzer, (c) an Automated Isoperibol Fixed Bomb Parr 6200 bomb calorimeter.

Fig. 4 :

 4 Fig. 4: Average values of raw and torrefied SCB Mass Losses (MLtt %) due to thermal degradation and Mass Losses (MLleach %) due to leaching process.

Fig. 5 :

 5 Fig. 5: Chemical composition of raw and torrefied sugarcane bagasse (SCB). For each group, the means with the same letter were not significantly different at 5% (a = 0.05).

Fig. 6 :

 6 Fig. 6: Final moisture content (%) after Coniophora puteana (CP) and Trametes versicolor (TV) expossure of pine and beech control samples (a) and native and torrefied sugarcane bagasse (SCB) (b), according to the fungal exposure duration.

Fig. 7 :

 7 Fig. 7 : Weight losses (WL%) due to Coniophora puteana (CP) and Trametes versicolor (TV) degradation of pine and beech control samples (a) and raw and torrefied sugarcane bagasse (SCB) (b), according to the fungal exposure duration.

Fig. 8 :

 8 Fig. 8 : (a) Carbon, (b) Hydrogen and (c) Nitrogen contents (%), (d) H/C molar ratio and (e) HHV0 (MJ.kg -1 ) of raw and torrefied sugarcane bagasse (SCB), according to Trametes versicolor exposure duration; (f) Correlation between HHV0 (MJ.kg -1 ) and atomic H/C ratio of the different sugarcane bagasse (SCB) samples.

Fig. 9 :Tab. 1 :

 91 Fig. 9: (a) Carbon, (b) Hydrogen and (c) Nitrogen contents (%), (d) H/C molar ratio and (e) HHV0 (MJ.kg -1 ) of raw and torrefied sugarcane bagasse (SCB), according to Coniophora puteana exposure duration; (f) Correlation between HHV0 (MJ.kg -1 ) and atomic H/C ratio of the different sugarcane bagasse (SCB) samples.

  ANOVA (one-way analysis of variance) and Duncan's comparison test, carried out by the JMP 10.0.2 program (SAS Institute Inc., Cary, NC, USA) by applying the Fisher test, were used to evaluate:

, oven dry basis)

The authors gratefully acknowledge the National Council for Scientific and Technological Development (CNPq) for the financial support granted to Bruno De Freitas Homem De Faria allowing him to carry out this PhD in collaboration with BioWooEB-CIRAD Institute.