Maxime Hervy
email: maxime.hervy@mines-albi.fr

Audrey Villot

Claire Gerente

Doan Pham

Elsa Weiss-Hortala

Ange Nzihou

Laurence Le Coq

Claire Gérente

Catalytic cracking of ethylbenzene as tar surrogate using pyrolysis chars from wastes

Keywords:

Introduction

The pyro-gasification of biomass and waste appears as a promising conversion routes to produce a gaseous energy carrier named syngas, mainly composed of H 2 and CO. The pyro-gasification consists in the reaction between biomass or waste and an oxidizing agent (air, steam, O 2 or CO 2 ) at elevated temperature (800-1000 °C), and leads to the decomposition of the solid fuel in three fractions: syngas, tar, and a solid residue composed of char and/or ash (depending on the carbon content) [START_REF] Arena | Process and technological aspects of municipal solid waste gasification. A review[END_REF]. Syngas could be valorised in energy conversion applications (power and heat generation by combustion in gas engines, gas turbines, fuel cells …), and as precursor in the production of liquid fuel (via the Fischer-Tropsch synthesis) or chemicals [START_REF] Milne | Biomass Gasifier Tars: Their Nature, Formation and Conversion[END_REF]. However, it should be purified prior to subsequent uses in order to remove impurities originally contained in solid fuels (i.e.: sulphur, chlorine, nitrogen compounds, etc.), particles (inorganic elements, soot …), and pollutants generated by incomplete gasification (tar) [START_REF] Le Coq | Syngas treatment unit for small scale gasification-application to IC engine gas quality requirement[END_REF].

The formation of tar is the major issue limiting the development of the pyro-gasification technologies. Indeed, tar can condense at high temperature (350 °C) thus fouling the equipment downstream of the gasifier (pipes, filters, heat exchangers …), or deactivate the catalysts used in the syngas upgrading process [START_REF] Burhenne | Benzene removal over a fixed bed of wood char: the effect of pyrolysis temperature and activation with CO2 on the char reactivity[END_REF]. Tar is a complex mixture of condensable aromatic and oxygenated hydrocarbons having a molecular weight higher than benzene (78 g/mol) [START_REF] Milne | Biomass Gasifier Tars: Their Nature, Formation and Conversion[END_REF], and are divided in several classes depending on their properties [START_REF] El-Rub | Review of catalysts for tar elimination in biomass gasification processes[END_REF][START_REF] Devi | Pretreated olivine as tar removal catalyst for biomass gasifiers: investigation using naphthalene as model biomass tar[END_REF]. Light aromatic compounds are the main class of tar generated by the pyro-gasification of biomass. Their concentration can vary between 5 and 200 g/Nm 3 of syngas [START_REF] Milne | Biomass Gasifier Tars: Their Nature, Formation and Conversion[END_REF], while the standards required for syngas utilization vary between 0.01 and 100 mg/Nm 3 , depending on the end-use [START_REF] Milne | Biomass Gasifier Tars: Their Nature, Formation and Conversion[END_REF][START_REF] Le Coq | Syngas treatment unit for small scale gasification-application to IC engine gas quality requirement[END_REF]. Therefore, tar removal from the syngas is mandatory and several methods have been developed to this end [START_REF] Woolcock | A review of cleaning technologies for biomass-derived syngas[END_REF][START_REF] Valderrama Rios | Reduction of tar generated during biomass gasification: a review[END_REF][START_REF] Villot | Separation of particles from syngas at high-temperatures with an electrostatic precipitator[END_REF][START_REF] Nair | Tar removal from biomass-derived fuel gas by pulsed corona discharges[END_REF]: physical treatment (electrostatic precipitation, inertial separation, wet or dry scrubbing), plasma cracking, thermal cracking and catalytic cracking. The advantages and disadvantages of these methods were summarized by Shen et al. [START_REF] Shen | By-products recycling for syngas cleanup in biomass pyrolysisan overview[END_REF]. The catalytic cracking presents several advantages, such as: high reaction rate, the increase of syngas yield and purity, and the use of moderate temperatures [START_REF] Shen | Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis-a review[END_REF][START_REF] Mani | Catalytic decomposition of toluene using a biomass derived catalyst[END_REF][START_REF] El-Rub | Experimental comparison of biomass chars with other catalysts for tar reduction[END_REF][START_REF] Shen | In-situ catalytic conversion of tar using rice husk char-supported nickel-iron catalysts for biomass pyrolysis/gasification[END_REF][START_REF] Devi | Catalytic decomposition of biomass tars: use of dolomite and untreated olivine[END_REF][START_REF] Guan | Catalytic steam reforming of biomass tar over iron-or nickel-based catalyst supported on calcined scallop shell[END_REF]. Tar cracking aims at converting tar into permanent gases (such as H 2 and CO), thus simultaneously increasing the syngas yield and quality. This reaction allows to transfer the lost energy contained in the tarry-molecules into useful syngas. For these reasons, catalytic cracking of tar appears as a main issue in the development of pyrogasification processes.

Different types of catalysts have been studied for tar removal, such as calcined rocks (dolomite, magnesite and calcite) [START_REF] González | Reduction of tars by dolomite cracking during two-stage gasification of olive cake[END_REF][START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H 2 production over natural calcite, dolomite and olivine materials[END_REF][START_REF] Tennant | Steam-pyrolysis activation of wood char for superior odorant removal[END_REF], olivine [START_REF] Kuhn | Olivine catalysts for methane-and tar-steam reforming[END_REF][START_REF] Virginie | Characterization and reactivity in toluene reforming of a Fe/olivine catalyst designed for gas cleanup in biomass gasification[END_REF], clay minerals [START_REF] El-Rub | Review of catalysts for tar elimination in biomass gasification processes[END_REF], zeolites [START_REF] Buchireddy | Biomass gasification: catalytic removal of tars over zeolites and nickel supported zeolites[END_REF], iron oxides [START_REF] Azhar Uddin | Catalytic decomposition of biomass tars with iron oxide catalysts[END_REF], fluid catalytic cracking catalysts (FCC) [START_REF] Sutton | Review of literature on catalysts for biomass gasification[END_REF], noble metals [START_REF] Chen | Promoting effect of trace Pd on hydrotalcite-derived Ni/Mg/Al catalyst in oxidative steam reforming of biomass tar[END_REF][START_REF] Juutilainen | Zirconia: selective oxidation catalyst for removal of tar and ammonia from biomass gasification gas[END_REF][START_REF] Cheah | Nickel cerium olivine catalyst for catalytic gasification of biomass[END_REF][START_REF] Constantinou | Low-temperature conversion of phenol into CO, CO2 and H2 by steam reforming over Lacontaining supported Rh catalysts[END_REF][START_REF] Wang | Catalyst property of Co-Fe alloy particles in the steam reforming of biomass tar and toluene[END_REF], alkaline and alkaline earth metals (AAEM) [START_REF] Raveendran | Influence of mineral matter on biomass pyrolysis characteristics[END_REF][START_REF] Klinghoffer | Influence of char composition and inorganics on catalytic activity of char from biomass gasification[END_REF], nickel catalysts [START_REF] Blanco | Characterization and evaluation of Ni/SiO2 catalysts for hydrogen production and tar reduction from catalytic steam pyrolysis-reforming of refuse derived fuel[END_REF] and chars [START_REF] Park | Reduction of primary tar vapor from biomass by hot char particles in fixed bed gasification[END_REF][START_REF] Feng | In-situ steam reforming of biomass tar over sawdust biochar in mild catalytic temperature[END_REF][START_REF] Assima | Utilization of MSW-derived Char for Catalytic Reforming of Tars and Light Hydrocarbons in the Primary Syngas Produced during Wood Chips and MSW-rdf Air Gasification[END_REF]. As summarized by Shen et al. [START_REF] Shen | By-products recycling for syngas cleanup in biomass pyrolysisan overview[END_REF], each catalyst presents some pro and cons but metal catalysts (such as alumina or activated carbon supported Fe, Ni and Co) appear as the most efficient [START_REF] Liu | Catalytic cracking of tar derived from rice hull gasification over palygorskite-supported Fe and Ni[END_REF][START_REF] Min | Catalytic reforming of tar during gasification. Part I. Steam reforming of biomass tar using ilmenite as a catalyst[END_REF][START_REF] Fermoso | Sorption enhanced catalytic steam gasification process: a direct route from lignocellulosic biomass to high purity hydrogen[END_REF]. However, these catalysts suffer from rapid deactivation by coking [START_REF] Zhang | Tar destruction and coke formation during rapid pyrolysis and gasification of biomass in a drop-tube furnace[END_REF][START_REF] Hosokai | Reforming of volatiles from the biomass pyrolysis over charcoal in a sequence of coke deposition and steam gasification of coke[END_REF], and elevated production costs. The development of low-cost and eco-friendly catalysts remains a main issue to reduce the cost of the syngas cleaning process.

Chars are co-products of the pyro-gasification process and represent 12 to 30 wt% of the initial biomass [START_REF] Bridgwater | Review of fast pyrolysis of biomass and product upgrading[END_REF]. This large amount of solid residues should be valorised. Since the costs associated with the char landfilling compromise the economic sustainability of the pyro-gasification units, the valorisation of chars as catalysts for syngas cleaning appears as an attractive approach. This topic has been increasingly investigated over the past few years. Previous studies reported that four characteristics of the chars determined their activity even if their role remains not clear: the porous structures [START_REF] Gilbert | Tar reduction in pyrolysis vapours from biomass over a hot char bed[END_REF][START_REF] Liu | Fundamentals of catalytic tar removal over in situ and ex situ chars in two-stage gasification of coal[END_REF], the presence of Ocontaining groups on the char surface [START_REF] Wang | Tar reforming using char as catalyst during pyrolysis and gasification of Shengli brown coal[END_REF][START_REF] Ducousso | Reactivity enhancement of gasification biochars for catalytic applications[END_REF][START_REF] Lu | Preparation of high catalytic activity biochar from biomass waste for tar conversion[END_REF][START_REF] Klinghoffer | Catalyst properties and catalytic performance of char from biomass gasification[END_REF][START_REF] Zhang | An advanced biomass gasification technology with integrated catalytic hot gas cleaning. Part III: effects of inorganic species in char on the reforming of tars from wood and agricultural wastes[END_REF], the structure of the carbonaceous matrix [START_REF] Keown | Drastic changes in biomass char structure and reactivity upon contact with steam[END_REF][START_REF] Fu | Structural evolution of maize stalk/char particles during pyrolysis[END_REF], and the active sites formed by the inherent alkaline (i.e.: Na, K) and alkaline earth (i.e.: Mg, Ca) species (AAEM) distributed in the char matrix [START_REF] Min | Catalytic reforming of tar during gasification. Part II. Char as a catalyst or as a catalyst support for tar reforming[END_REF][START_REF] Yildiz | Effect of biomass ash in catalytic fast pyrolysis of pine wood[END_REF][START_REF] Mašek | Examination of catalytic roles of inherent metallic species in steam reforming of nascent volatiles from the rapid pyrolysis of a brown coal[END_REF]. In addition, these AAEM species can slow the deactivation rate by coke deposition since they can catalyse the gasification reactions of coke [START_REF] Sueyasu | Simultaneous steam reforming of tar and steam gasification of char from the pyrolysis of potassiumloaded woody biomass[END_REF]. However, chars from biomass usually have low mineral content, and metal impregnation is necessary to enhance their catalytic activity by increasing active sites [START_REF] Feng | Effects of K and Ca on reforming of model tar compounds with pyrolysis biochars under H 2 O or CO 2[END_REF]. This impregnation increases the production cost and the environmental footprint of the resulting catalyst, thus reducing the benefits of using char as catalyst. On the contrary, chars from waste (such as sludge) can be rich in minerals but suffer from low specific surface area and carbonaceous structure, resulting in a fast deactivation by coke deposition [START_REF] Lu | Catalytic cracking of toluene as a tar model compound using sewage-sludge-derived char[END_REF].

The improvement of the catalytic activity of waste-based pyrolysis chars by applying low-cost and eco-friendly modification processes has scarcely been explored. In the literature, there is also a lack of study focused on the use of chars for the catalytic cracking of light aromatic hydrocarbon compounds other than toluene. This paper aims at studying the catalytic cracking of ethylbenzene (C 8 H 10 ) as a tar model compound over different types of pyrolysis chars from wastes, and functionalised with low-cost processes. A comprehensive set of characterisations (elemental analysis, XRFS, Raman, XRD, N 2 adsorptiondesorption, TPD, SEM-EDX) was used to understand the relationships between the modification process, the physicochemical properties of the chars, and their catalytic activity for ethylbenzene cracking. The selectivity of the ethylbenzene cracking products was investigated in order to identify the reactions involved in the decomposition mechanisms. In order to determine the performance of each char in increasing the syngas production and quality, three parameters were calculated: the volume of syngas produced by tar cracking reaction, the lower heating value (LHV) of the produced syngas, and the energy transferred from tar to syngas during the ethylbenzene decomposition.

Materials and methods

Catalysts production

Pyrolysis treatment

The waste materials used in this study were obtained from cruise ships. However, they are generated in large amount by modern societies: Used Wood Pallets (UWP), Food Waste (FW) and Coagulation-Flocculation Sludge (CFS). UWP was made of softwood, previously employed in the production of pallets for food transportation. Food Waste was composed of a mixture of vegetables and animal wastes. Coagulation-Flocculation Sludge was recovered from a wastewater treatment plant present on board.

The chars were produced by pyrolyzing wastes at 700 °C during 30 min (heating rate of 22 °C/min) in a semi-continuous horizontal screw reactor (internal diameter of 0.167 m and 2 m in length). The details on the experimental procedure were described in a previous papers [START_REF] Mura | Pyrolysis of biomass in a semi-industrial scale reactor: study of the fuel-nitrogen oxidation during combustion of volatiles[END_REF]. Two pyrolysis chars were produced: (1) c. UWP (only from UWP), and (2) c. FW/CFS (from a mixture of 50 wt% FW and 50 wt% CFS). The char yields were 22 wt% for c. UWP, and 23 wt% for c. FW/ CFS. Due to the internal diameter of the tar cracking reactor, chars were sieved to particle size varying from 0.5 to 1.6 mm to avoid edge-effects. To modify the physicochemical properties of the chars and in an attempt to improve their tar cracking efficiency, two chemical-free modification processes were applied: oxygenation by gas-phase treatment, and steam activation.

Oxygenation of the pyrolysis chars by O 2 /N 2 gas-phase treatment

To increase the amount of oxygenated groups on the char surface, an O 2 /N 2 gas-phase treatment was applied to the two pyrolysis chars. The dry chars were loaded in a vertical fixed-bed quartz reactor (bed height of 15 cm, internal diameter of 2.4 cm) and heated at 20 °C/min under pure nitrogen flow up to the oxygenation temperature (280 °C). Then, the nitrogen flow was replaced by a mixture of 8 v% O 2 /92 v% N 2 at a flow rate of 2.0 L/min. Based on previous study [START_REF] Ducousso | Reactivity enhancement of gasification biochars for catalytic applications[END_REF] tests were performed to determine the optimal conditions of oxygenation (temperature, duration). The pyrolysis chars were oxygenated at 280 °C during 4 h for c. UWP, and only 2 h for c. FW/CFS in order to limit the mass loss. Indeed, for modification processes, high solid yield is an important parameter in order to produce significant amount of catalyst from the available amount of char. Under the selected conditions, the solid yield of this treatment was 98 wt% for ox. UWP and 97 wt% for ox. FW/CFS, whereas after 4 h of oxygenation, the solid yield of ox. FW/CFS was only 57.4 wt%. The oxygenated chars were identified using the prefix "ox".

Steam activation of the pyrolysis chars

A steam activation was applied to the two pyrolysis chars as this process is known to favour the development of micro, meso and macropores [START_REF] Molina-Sabio | Effect of steam and carbon dioxide activation in the micropore size distribution of activated carbon[END_REF][START_REF] Nor | Synthesis of activated carbon from lignocellulosic biomass and its applications in air pollution control-a review[END_REF]. A 100 g sample of dry char was loaded in a semi-rotary quartz reactor (Carbolite HTR 11/150) and was heated to the activation temperature (850 °C) at a 10 °C/min heating rate in an inert atmosphere (N 2 flow rate of 0.5 L/min). At 850 °C, the activation started with the addition of steam (activating agent) to the nitrogen flow at a concentration of 15 v% during 80 min. The cooling of the reactor took place under inert atmosphere in order to preserve the catalysts properties for further characterisations. The resulting activated chars were named with the prefix "ac". The solid yield of the steam activation was 77 wt% for ac. UWP, and 69 wt% for ac. FW/CFS. Table S.1 lists the different materials used in this study.

Characterization of feedstock and chars

Elemental composition

First, the moisture of the samples was eliminated with a drying step (105 °C in a furnace). The elemental composition (C, H, N, S) of the samples was determined with a Thermo Finnigan AE1112 Series Flash. The ash content of the chars was determined by measuring the residual mass after the combustion of 7.0 g of the sample for 15 h in a muffle furnace (Nabertherm P330) at 650 °C.

Mineral species

The chemical composition of the resulting ash was analysed by Xray fluorescence spectroscopy (SHIMADZU EDX-800HS). Contrary to ICP-MS, the XRFS method allows determining the chemical composition of a large sample (about 2 g against 20 mg with ICP-MS). The analyses were performed under vacuum using powdered samples, with an acquisition time of 100 s. The ash was obtained from the combustion of around 20 g of each material. This significant amount of ash was used to analyse a sample representative of the materials.

The surfaces of the chars were observed by scanning electron microscopy (SEM) using a ZEISS DSM982 microscope equipped with a high-resolution Gemini column, operated at 10-15 kV. The local chemical composition at micro-scale was assessed by energy dispersive Xray spectroscopy (EDX) using a Noran Voyager IV microanalysis system. For each char, several zones were analysed in order to obtain a statistical dataset representative of the materials.

O-functional groups

TPD analysis (coupled with μ-GC) was performed to quantify the Ofunctional groups on the char surface. The decomposition of O-containing groups occurs at a given temperature and is accompanied by a release of CO or CO 2 [START_REF] Ducousso | Reactivity enhancement of gasification biochars for catalytic applications[END_REF][START_REF] Zhou | Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and FT-IR[END_REF]. A 150 mg sample of char was introduced in a thermogravimetric analyser (Labsys T6 Evo) and flushed during 20 min under a pure nitrogen flow. The char was then heated from 30 to 1100 °C in an inert atmosphere (N 2 flow rate of 33 mL/min) with a heating rate of 5 °C/min. The gas produced (CO and CO 2 ) was analysed online with a μ-GC (My-GC, Agilent). Although the assignment of the peaks during TPD analysis is still discussed in the literature, global trends have emerged in previous studies [START_REF] Ducousso | Reactivity enhancement of gasification biochars for catalytic applications[END_REF][START_REF] Zhou | Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and FT-IR[END_REF]. The correlations are given in Table S.2. The deconvolution of the experimental data, performed with Matlab ® , was used to quantify the amount of CO or CO 2 released for each desorption peak. Thus, the different O-containing groups can be identified and quantified.

Nature of the carbonaceous matrix

Raman spectroscopy was used to determine the different carbon structures of the chars. Raman spectra were acquired for each sample at room temperature and in air using a Confocal Raman -AFM WITEC Alpha 300 A R microscope equipped with a CCD camera detector. Spectra were recorded using a 50× lens (Na = 0.75) and an excitation laser at 532 nm in the 175-4000 cm -1 Raman shift region. For each sample, at least two areas of 25 μm 2 of three different particles were analysed. For ash-rich materials, a post treatment was applied to discriminate the mineral and the carbonaceous contributions. Then, the carbonaceous matrix was analysed according to a deconvolution method described elsewhere [START_REF] Hervy | Multi-scale characterisation of chars mineral species for tar cracking[END_REF].

Fig. 1. Scheme of the device used for the tests of ethylbenzene decomposition.

Textural properties

The specific surface area, the pore size distribution and volume of the chars were studied by means of nitrogen adsorption-desorption at 77 K using a Micromeritic 3Flex apparatus. Prior to each measurement, the samples were outgassed under vacuum (1-30 μm Hg) at 30 °C during at least 16 h. The specific surface area was determined using the BET method, while the HK (Horvath-Kawazoe) and BJH (Barrett, Joyner, Halenda) methods were used to characterise the microporosity and mesoporosity, respectively. Textural properties of each sample were studied by at least three nitrogen adsorption-desorption isotherms.

Catalytic tests 2.3.1. Ethylbenzene cracking reactor

Ethylbenzene was chosen as tar surrogate and the catalytic cracking tests were carried out in a stainless-steel reactor (internal diameter 2.4 cm) placed in an electric furnace in which the heating was monitored by three thermocouples (Fig. 1).

The model syngas composition and flow rate were controlled by mass flow controllers connected to individual gas cylinders with a purity of 99.995%. Ethylbenzene was introduced in the gas flow by a syringe pump. The cracking tests were performed with an ethylbenzene concentration of 40 g/Nm 3 representative to the tar concentrations in real syngas (5-200 g/Nm 3 ) [START_REF] Milne | Biomass Gasifier Tars: Their Nature, Formation and Conversion[END_REF]. The mixture flowed in a preheater placed in the electric furnace, and entered on the bottom of the reactor. The exhaust gas was analysed online with a μ-GC (R-3000, SRA Instruments) and tar cracking products were identified using GC-MS (GC-MS Perkin Elmer Auto System XL). Details on the nature of the chromatographic columns and the analysis parameters are given on Table S. [START_REF] Le Coq | Syngas treatment unit for small scale gasification-application to IC engine gas quality requirement[END_REF].

The catalytic tests were performed at 650 °C, corresponding to a conventional syngas temperature at the gasifier outlet [START_REF] Arena | Process and technological aspects of municipal solid waste gasification. A review[END_REF]. In these conditions, no supplementary heat source would be required to heat the syngas in the tar cracking process.

In an empty reactor set at 650 °C, the ethylbenzene conversion due to homogeneous reactions in a model syngas matrix (composed of H 2 (30%), CO (40%), CO 2 (15%) and N 2 (15%)) reached 68.4%. This conversion can also be explained by the catalytic activity of the stainless-steel reactor. To reduce the homogeneous cracking reactions, a less reactive gas composition was selected: CO (40 v%) and N 2 (60 %v). Thus, this gas matrix permitted to quantify the amount of permanent gases (H 2 , CO 2 …) produced by the tar cracking reactions, and to evaluate the catalytic effect of the chars based on the ethylbenzene conversion. The bed height was 4.5 cm for all chars. Since the gas velocity was kept constant (9.6 cm/s), the residence time of the gas in empty column was 0.47 s at 650 °C. The degradation of the closure system of this reactor did not allow measuring precisely the bed weight evolution.

Experimental data evaluation

During the tar cracking reactions, the volumetric flow rate at the reactor outlet could vary. Thus, the nitrogen initially present in the model syngas was used as an internal standard, allowing the calculation of the real output volumetric flow rate of syngas.

The average ethylbenzene conversion (X EB ) was defined as the molar difference of ethylbenzene leaving the reactor (n EB out ) to the ethylbenzene amount fed into the reactor (n in EB ) over a given time, according to Eq. ( 1).

= - X n n n .100 EB EB in EB out EB in (1)
The composition of the tar cracking products (named i) was discussed based on the molar selectivity (Sel i ), calculated with Eq. ( 2):

= ∑ Sel n n .100 i prod,i out prod out (2)
where n prod,i out is the molar amount of the tar cracking product i at the reactor outlet.

The catalytic effect of the chars on ethylbenzene cracking was also evaluated through the atomic balances in carbon (η C ), reflecting the percentage of C atoms entering in the catalytic system that were in vapour phase at the reactor outlet.

= * η n n 100 C C out C in (3)
where:

-n C in is the amount of carbon atoms entering in the reactor, -n C out is the amount of carbon atoms in the vapour phase at the reactor outlet. bdl: below the detection limit (< 0.2 wt%); n.m: non-measured (< 0.001 wt%).

Results and discussion

3.1. Effect of the modification process on the physicochemical properties of the chars 3.1.1. Chemical differences between UWP-and FW/CFS-based chars Due to the different chemical composition of the initial wastes [START_REF] Hervy | Multi-scale characterisation of chars mineral species for tar cracking[END_REF], two types of chars were obtained: chars from UWP were carbonaceous materials having carbon content higher than 84 wt%, while chars from FW/CFS were hybrid carbon/mineral materials (Table 1). Indeed, the materials obtained from FW/CFS presented a relatively low carbon content (32.6-44.1 wt%) counterbalanced by an important ash content (> 45.9 wt%). The composition of the ash is presented in Table 1 and was discussed in a previous communication [START_REF] Hervy | Multi-scale characterisation of chars mineral species for tar cracking[END_REF].

The presence of small particles of calcium (0.5-5 μm) well distributed on the surface of FW/CFS-based chars was highlighted by the SEM observations, as shown in a previous study [START_REF] Hervy | Multi-scale characterisation of chars mineral species for tar cracking[END_REF] and in Figure S.1.

The minerals speciation was characterised with Raman spectroscopy. A post treatment was applied to the Raman spectra obtained with FW/CFS-based chars in view to separate the mineral and the carbonaceous contributions, as described in a previous article [START_REF] Hervy | Multi-scale characterisation of chars mineral species for tar cracking[END_REF]. Based on the RRUF database [START_REF] Downs | The RRUFF Project: an Integrated Study of the Chemistry, Crystallography, Raman and Infrared Spectroscopy of minerals[END_REF], the mineral contribution of FW/CFS Raman spectra was identified as hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ). The chemical structure of the hydroxyapatite was confirmed by XRD analyses (Figure S.2).

The following paragraphs describe the physicochemical changes of the chars occurring during the modification processes.

Influence of the oxygenation process

It can be noticed from Table 1 that the oxygenation step did not significantly modified the elemental composition of the carbonaceous char (ox.UWP), while it slightly increased the oxygen content of the char rich in mineral species (ox.FW/CFS) from 4.5 to 7.8 wt%. However, as the oxygen content was calculated by difference, the values must be considered carefully.

Raman spectrum of the carbonaceous matrix of each char was analysed after a deconvolution treatment. The composition of the carbonaceous matrix is presented in Fig. 2. For ox. UWP and ox. FW/CFS, the oxygenation increased the proportion graphene-like sheets structures (+4.5 and + 2%, respectively) and reduced the defects in graphene-like sheets (-0.4% and -3.1%, respectively).

The precise catalytic role of O-functional groups on tar cracking reactions is not clear. These groups can interact with tar through hydrogen bonds [START_REF] Shen | By-products recycling for syngas cleanup in biomass pyrolysisan overview[END_REF] or π -π* stacking interactions promoting the multilayer adsorption of tar [START_REF] Liu | Preparation of high adsorption capacity bio-chars from waste biomass[END_REF]. However, O-functional groups are thermally unstable and can be decomposed. This decomposition leads to the formation of free carbon sites on the char matrix that can catalyse the tar cracking reactions [START_REF] Dufour | Catalytic decomposition of methane over a wood char concurrently activated by a pyrolysis gas[END_REF].

The Temperature Programmed Desorption analyses (Table 2) revealed the spectacular difference in the amount of O-functional groups between c. FW/CFS (2.2 mmol/g) and the other chars (< 0.45 mmol/ g). While the oxygen content of ox. FW/CFS was higher than that of c. FW/CFS (7.8 and 4.5 wt%, respectively) (Table 1), the amount of Ofunctional groups was 4.9 times lower for ox. FW/CFS after the oxygenation. These results showed that the oxygenation step did not produce O-functional groups at the carbonaceous surface, but mainly oxidised the inorganic species of ox. FW/CFS, initially in metallic form due to the reductive atmosphere of the pyrolysis. Simultaneously, oxidation reactions of the carbonaceous matrix occurred during the oxygenation step resulting in the destruction of O-functional groups from the ox. FW/CFS surface (Table 2), and in the decrease of the graphene-like sheets structures containing defects (Fig. 2). The composition of Ofunctional groups is presented in Table S. [START_REF] Burhenne | Benzene removal over a fixed bed of wood char: the effect of pyrolysis temperature and activation with CO2 on the char reactivity[END_REF].

The effect of the oxygenation process on the textural properties of the chars was discussed in a previous article [START_REF] Hervy | H 2 S removal from syngas using wastes pyrolysis chars[END_REF]. Whatever the chemical nature of the pyrolysis char, the specific surface area was not significantly modified by the oxygenation process. Both pyrolysis and oxygenated chars presented low porosity, with S BET lower than 10 m 2 /g for FW/CFS-based chars, and lower than 80 m 2 /g for UWP-based chars [START_REF] Hervy | H 2 S removal from syngas using wastes pyrolysis chars[END_REF].

Influence of the steam activation process

The chemical composition of ac. UWP was not drastically modified by steam activation at 850 °C (Table 1). On the contrary, activation increased the ac. FW/CFS ash content (relative increase of 12.4 wt%) and decreased its carbon content (relative drop of 11.5 wt%). These evolutions resulted from the catalytic effect of the mineral species of c. FW/CFS on the gasification reactions of the carbonaceous matrix occurring during the activation step [START_REF] Dupont | How inorganic elements of biomass influence char steam gasification kinetics[END_REF].

The effect of steam activation on the O-functional groups was similar for both types of chars (FW/CFS and UWP) (Table 2). Indeed, steam activation reduced the amount of O-functional groups, but raised the proportion of basic O-functional groups (+19%). The higher thermal stability of basic groups was mainly responsible for this evolution (Table S.2), as acidic groups were decomposed during the steam activation at 850 °C.

Contrary to oxygenation, steam activation strongly developed the specific surface area (S BET ) of the chars [START_REF] Hervy | H 2 S removal from syngas using wastes pyrolysis chars[END_REF]. Microporous and mesoporous volumes were both increased by this process, which is in line with the literature results [START_REF] Molina-Sabio | Effect of steam and carbon dioxide activation in the micropore size distribution of activated carbon[END_REF]. The S BET of ac. UWP (625 m 2 /g) was more intensively developed than that of ac. FW/CFS (221 m 2 /g) during steam activation due to its superior carbon content (87.4 vs 32.6 wt%, respectively).

Steam activation also modified the carbonaceous matrix of the chars. Contrary to oxygenation, defects in graphene-like sheets (such as vacancy, Stone-Wales defects) significantly increased after steam activation, especially for ac. UWP (+14.5%). This evolution is explained by the interactions between the O atoms of steam and the carbonaceous matrix of the char.

The catalytic activity of these materials in ethylbenzene cracking reaction was studied in the following sections.

Effect of tar cracking over chars on the syngas production

Ethylbenzene conversion

The ethylbenzene conversion versus time of experiment for the different catalysts and for thermal cracking are presented in Fig. 3. For thermal cracking tests, the conversion of EB was initially 100% due to the catalytic activity of the stainless-steel reactor walls, and decays with time due to the deposition of non-active coke on the walls. Compared to thermal cracking, each char improved the ethylbenzene conversion. While ox. UWP, c. UWP and ox. FW/CFS were rapidly deactivated, three catalysts had higher catalytic activity but different catalytic behaviour: ac. FW/CFS, ac. UWP, and c. FW/CFS. During the first 30 min, the initial catalytic activity of c. FW/CFS and ac. UWP was similar and higher than that of ac. FW/CFS. Between 30 and 60 min, ac. FW/CFS was the most active catalyst while c. FW/CFS was more efficient than ac. UWP. For test duration longer than 60 min, the two activated chars (ac.FW/CFS and ac/UWP) remained substantially more efficient than the pyrolysis char c. FW/CFS. The char ox. UWP was the less reactive catalyst and was completely deactivated after 85 min of reaction. Thus, the average conversion of ethylbenzene (X EB ) achieved within 85 min was calculated and the results are presented in Fig. The horizontal line shows the result obtained from the non-catalytic thermal cracking test, with an average conversion of 37.2%. Two repetitions were performed with ac. UWP (X EB = 77.3%) and ac. FW/CFS (X EB = 85.8%) and the relative deviation was lower than 8% for ac. UWP, and lower than 2% for ac. FW/ CFS, which was acceptable, as previously shown in the literature [START_REF] Nestler | Catalytic decomposition of biomass tars: the impact of wood char surface characteristics on the catalytic performance for naphthalene removal[END_REF]. This deviation can be explained by the heterogeneity of waste-based chars, and by the catalytic contribution of the stainless-steel walls of the reactor.

The results revealed the antagonist effect of the two modification processes on the char activity. On the one hand, the oxygenation step negatively affected the catalytic activity of both chars, as reflected by the drastic drop of ethylbenzene conversion (from 53.2 to 41.3% for ox. UWP, and from 77.2 to 48.9% for ox. FW/CFS). On the other hand, the steam activation significantly improved the catalytic performances of the chars. The ethylbenzene conversion reached 77.3% with ac. UWP, and 85.8% with ac. FW/CFS. It can be noticed that for similar conditions of production, the FW/CFS-based chars reached higher ethylbenzene conversion than UWP-based chars.

Within 85 min, the pyrolysis char c. FW/CFS appeared as an attractive catalyst due to its significant catalytic activity without the need for additional steam activation. However, above this time, the catalytic performance of c. FW/CFS decreased drastically compared to the activated chars (Fig. 3). After 170 min of test, the average ethylbenzene conversion was 57.0% for c. FW/CFS, 65.4% for ac. UWP, and 78.5% for ac. FW/CFS.

The effect of the catalytic activity of the chars on the composition of the produced gas was studied by investigating the reaction products selectivity.

Selectivity of the reaction products

Ethylbenzene (C 8 H 10 ) was mostly decomposed to six molecules (Table 3): hydrogen (H 2 ), carbon dioxide (CO 2 ), styrene (C 8 H 8 ), ethylene (C 2 H 4 ), benzene (C 6 H 6 ), and toluene (C 7 H 8 ). It can be noticed that the carbon monoxide (CO) balance was slightly negative in all tests (except for ox. UWP), i.e. that the quantity consumed was higher than the quantity produced (Table S.5). This can be due to: (1) the reaction of CO with metal species of the char to form various types of carbonyls [START_REF] Arena | Energy generation by air gasification of two industrial plastic wastes in a pilot scale fluidized bed reactor[END_REF]; (2) the CO oxidation with the O-containing groups on the char surface producing CO 2 , or (3) the Boudouard reaction (R6 in the next discussion). Indeed, the Boudouard reaction consumes CO to form CO 2 and C at temperature lower than 700 °C [START_REF] Lahijani | Conversion of the greenhouse gas CO2 to the fuel gas CO via the Boudouard reaction: a review[END_REF].

Based on the literature data dealing with ethylbenzene pyrolysis and tar cracking reactions, a set of reactions expected to be involved in the ethylbenzene decomposition has been identified. The styrene can be generated by the cracking reaction (R1) and the oxidative dehydrogenation of ethylbenzene "ODH" (R2) [START_REF] Pereira | Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts. I. Influence of surface chemical groups[END_REF][START_REF] Gomez Sanz | A new perspective on catalytic dehydrogenation of ethylbenzene: the influence of side-reactions on catalytic performance[END_REF]. The ethylbenzene cracking reactions could also produce a wide range of molecules, such as toluene, benzene, ethylene, carbon dioxide or coke [START_REF] Chen | Thermal cracking of substituted aromatic hydrocarbons. II. Kinetic study of the thermal cracking of n-propylbenzene and ethylbenzene[END_REF][START_REF] Hermann | Mathematical simulation of catalytic dehydrogenation of ethylbenzene to styrene in a composite palladium membrane reactor[END_REF]. Although the experimental device did not allow a precise quantification of the bed weight evolution, coke formation was clearly observed.

The oxidative dehydrogenation of ethylbenzene (R2) also generates hydrogen. Hydrogen can then react with ethylbenzene through hydrogenolysis reactions (R3, R4), producing toluene and benzene [START_REF] Hoang-Van | Hydrogenolysis of ethylbenzene over a supported nickel catalyst derived from nickel hydroaluminate[END_REF]. Methane (CH 4 ) is produced by hydrogenolysis reactions, but it was not detected as an ethylbenzene cracking product in this study. This could result from its reaction with carbon dioxide by dry reforming reaction (R5), and/or by methane cracking reaction. Indeed, these reactions can Fig. 3. Ethylbenzene conversion obtained with the different chars and during thermal cracking. occur in the experimental conditions since they are initiated at 640 and 557 °C, respectively [START_REF] Wang | Carbon dioxide reforming of methane to produce synthesis gas over metal-supported catalysts: state of the art[END_REF]. These reactions could also contribute to the coke production observed in our study. Ethylene was detected as an ethylbenzene cracking product and can be formed by aromatic ringopening reactions from ethylbenzene, styrene, toluene, or benzene. (R5)

→

↔ + Boudouard reaction:2CO CO C 2 (R6)
The catalytic effect of the chars was also discussed based on the carbon balance, reflecting the percentage of C atoms entering in the catalytic system that were in vapour phase at the reactor outlet. The atomic balance of carbon ranged between 85.2 and 98.9% (Table 4), confirming that ethylbenzene was catalytically decomposed and not adsorbed on the char surface.

Assuming that the lack of carbon at the reactor outlet is explained by the production of solid coke by ethylbenzene cracking reactions, and assuming this solid coke is pure carbon, the coke production over the 85 min of test (Coke prod ) was estimated according to the Eq. ( 4):

= * - * * ( ) η Coke n 1 100 M 1000 prod C C in C (4) where:
-Coke prod is the weight of coke theoretically produced (mg of coke/ 85 min of test), -n C in is the amount of carbon atoms entering in the reactor (mol/ 85 min of test), -η C is the atomic balance in carbon defined by Eq.( 3), -M C is the molecular weight of carbon (12 g/mol).

The results evidenced that pyrolysis chars generated significant amount of coke, compared to other materials (Table 4). Indeed, the maximum theoretical coke production was obtained with c. FW/CFS and reached 2447 mg over the 85 min of test. Moreover, the coke production was almost similar for c. UWP and ac. UWP (1298 and 1174 mg, respectively), while the ethylbenzene conversion was significantly lower for c. UWP (53.2 vs 77.3%). This result confirms that the reactions promoted by the pyrolysis chars generated higher amount of coke. In addition, the evolutions of CO presented in Table S.5 revealed that the CO consumption was higher with the pyrolysis chars: -5.48% with c. FW/CFS, and -3.16% with c. UWP. All these results revealed that the catalytic activity of pyrolysis chars promoted the Boudouard reaction, leading to CO consumption and production of CO 2 and coke.

The objective of tar cracking consists in converting tar molecules into permanent gases in order to increase the syngas yield in a pyrogasification process. However, Table 3 showed that ethylbenzene was also decomposed into other tar molecules such as styrene, and toluene. The conversion of ethylbenzene into these tarry by-products does not increase the syngas production. The next paragraph studies the efficiency of the chars in converting ethylbenzene into syngas.

Production of syngas from tar cracking reactions

In order to determine the performance of each char for increasing the syngas production and quality, three parameters were calculated. As benzene can be considered as an interesting molecule for syngas valorisation as fuel in gas engine, these three parameters were calculated both including and excluding benzene as a syngas product.

First, for 100 mol of ethylbenzene entering into the catalytic reactor, the volume of syngas produced by cracking reactions over each char (Prod syngas ) was determined (Eq. ( 5)). This latter takes into account both the ethylbenzene conversion of the chars presented in Fig. 4, and the selectivity in tar cracking products (Table 3).

⎜ ⎟ ⎜ ⎟ = * ⎛ ⎝ -⎛ ⎝ ∑ ⎞ ⎠ ⎞ ⎠ * S V Prod X 1 100
syngas EB tar prod m [START_REF] El-Rub | Review of catalysts for tar elimination in biomass gasification processes[END_REF] where S tar prod is the selectivity in tarry reaction products (including or excluding benzene), and V m = 22.4 NL/mol is the molar volume of ideal gas. The lower heating value of the syngas produced (LHV syngas ) was also calculated to assess the quality of the syngas produced by tar cracking reactions:

= ∑ * ∑ S LHV S LHV syngas j j j ( 6
)
where j is a permanent gas (H 2 , CO 2 , C 2 H 4 , with or without C 6 H 6 ), S j is the molar selectivity in permanent gas, and LHV j is the lower heating value of the gas j (in kJ/mol). Then, the energy transferred from tar to syngas by the conversion of ethylbenzene in permanent gases (ΔE syngas , expressed in kJ) was calculated with Eq. ( 7):

= * Prod LHV ∆E 22.4
syngas syngas syngas [START_REF] Woolcock | A review of cleaning technologies for biomass-derived syngas[END_REF] Results are presented in Table 5. First, it can be noticed that including benzene as a syngas compound strongly contribute to increase of the LHV syngas , and thus to ΔE syngas . For example, the selectivity of benzene was only 4.3% with c. FW/CFS. Including benzene as a syngas compound increases the LHV syngas from 221 to 368 kJ/mol of syngas produced, resulting in an increase of 75% of the energy transferred from tar to syngas (from 10.1 to 17.8 MJ/100 mol of initial tar). In the following discussion, the values excluding benzene will be discussed as in many studies, benzene is considered as a tar compound.

After oxygenation, the catalytic effect of both types of char on syngas production drastically decreased, with a drop of 25% for ox. UWP, and 37% for ox. FW/CFS (Table 5). It is interesting to note that steam activation decreased the LHV syngas for both types of char (from 225 to 221 kJ/mol for UWP-based chars, and from 221 to 208 kJ/mol for FW/CFS-based chars) but increased the syngas production (Prod syngas ). This latter increased by 26% for UWP-based chars, and by 4% for FW/CFS-based chars.

Although the highest ethylbenzene conversion was obtained with ac. FW/CFS (85.8%) (Fig. 4), the most efficient material for transferring energy from tar to syngas by the conversion of ethylbenzene (ΔE syngas ) was c. FW/CFS (10.14 MJ/100 mol of tar in). While steam activation slightly reduced the efficiency of ac. FW/CFS (9.86 MJ/100 of tar in), this modification process had a beneficial impact on the catalytic activity of ac. UWP. The ΔE syngas of ac. UWP increased by 26% after activation. However, for both types of char, the oxygenation process dramatically decreased the catalytic efficiency. After oxygenation, ΔE syngas was decreased by 26% and 30% for ox. UWP and ox. FW/CFS, respectively.

These results proved that the value of tar conversion is not sufficient to precisely assess the catalytic efficiency of a material.

Based on the characterization performed, the relationships between the char properties and their catalytic efficiency is discussed in the following paragraph.

Catalytic activity related to physicochemical properties

The lower carbon balance was obtained with c. FW/CFS (85.2%), together with the higher selectivity in hydrogen (53.8 mol%) and carbon dioxide (23.0 mol%). Table S.5 demonstrates that the higher consumption of CO during the catalytic tests also occurred with c. FW/ CFS. These results suggested that c. FW/CFS strongly catalysed the aromatic ring-opening reactions (producing H 2 and CO 2 ) resulting in the significant ΔE syngas obtained with this char (Table 5). This catalytic activity was explained by the metallic form of the mineral species present on c. FW/CFS surface, due to the reductive atmosphere of the pyrolysis gas [START_REF] Guan | Catalytic steam reforming of biomass tar over iron-or nickel-based catalyst supported on calcined scallop shell[END_REF]. The calcium particles well distributed on the surface of c. FW/CFS, and the presence of hydroxyapatite particles were expected to increase the catalytic activity of this material [START_REF] De Vasconcelos | Catalytic transformation of carbon dioxide and methane into syngas over ruthenium and platinum supported hydroxyapatites[END_REF][START_REF] Phan | Hydroxyapatite supported bimetallic cobalt and nickel catalysts for syngas production from dry reforming of methane[END_REF]. The strong activity of reduced metallic sites for the above-mentioned reactions (especially the Boudouard reaction) was confirmed by the significant consumption of CO and the high selectivity in CO 2 obtained with the two pyrolysis char (c.UWP and c. FW/CFS).

The oxidation of the mineral species of FW/CFS-based chars during the modification process (oxygenation, or steam activation) strongly changed the catalytic behaviour of these materials. Indeed, the activity of ox. FW/CFS and ac. FW/CFS for aromatic ring-opening reactions and Boudouard reaction strongly decreased, as evidenced by the lower selectivity in H 2 and CO 2 . In addition, these modification processes increased the proportion of basic O-functional groups (Table 2 and Table S.4), resulting in the promotion of oxidative dehydrogenation (ODH) of ethylbenzene responsible for the increase in styrene selectivity (Table 3). Indeed, basic groups, such as quinone, are known to have a catalytic activity for the ethylbenzene dehydrogenation reaction [START_REF] Pereira | Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts. I. Influence of surface chemical groups[END_REF][START_REF] Schraut | Composition and structure of active coke in the oxydehydrogenation of ethylbenzene[END_REF][START_REF] Iwasawa | Reaction mechanism for styrene synthesis over polynaphthoquinone[END_REF]. As basic groups decompose at temperature higher than 840 °C (Table S.2) [START_REF] Ducousso | Reactivity enhancement of gasification biochars for catalytic applications[END_REF], they were stable under the experimental conditions (650 °C) and were thus available to catalyse ODH reactions.

For UWP-based chars, the beneficial effect of the high specific surface area of ac. UWP (625 m 2 /g) after steam activation balanced the detrimental effect of the metallic sites oxidation. The drastic increase of ethylbenzene conversion with ac. UWP compared to c. UWP (from 53.2 to 77.3%) improved the energy transferred from tar to syngas (Table 5). This efficiency could also result from the higher proportion of defects in graphene-like sheets in the carbonaceous matrix (Fig. 2) which were demonstrated to play a catalytic role in tar cracking reactions [START_REF] Min | Catalytic reforming of tar during gasification. Part II. Char as a catalyst or as a catalyst support for tar reforming[END_REF][START_REF] Dufour | Catalytic decomposition of methane over a wood char concurrently activated by a pyrolysis gas[END_REF][START_REF] Li | Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VIII. Catalysis and changes in char structure during gasification in steam[END_REF][START_REF] Serrano | Hydrogen production by methane decomposition: origin of the catalytic activity of carbon materials[END_REF].

While steam activation had a positive impact on the UWP-based char reactivity, the oxygenation step had a detrimental effect on the catalytic activity of both types of char, by oxidizing the mineral active sites and decreasing the defects in graphene-like structures.

Conclusions

This paper investigated the catalytic activity of waste-derived chars in the cracking of ethylbenzene as a model tar molecule (surrogate of the light aromatic compounds), and aimed at studying the influence of chemical-free modification processes (steam activation, or gas-phase oxygenation) on the catalytic performance of the chars. This article is in line with two main issues in the development of the pyrogasification process: optimizing syngas yield and quality; and finding new valorisation routes for residual chars.

Two chars were produced by pyrolysis: (1) used wood pallets (UWP), and (2) a mixture of food waste (FW, 50 wt%) and coagulationflocculation sludge (CFS, 50 wt%). Catalytic tests were performed at 650 °C with an ethylbenzene concentration of 40 g/Nm 3 . Within the first 85 min of test, the average ethylbenzene conversion was 37.2% without catalyst due to the catalytic activity of the reactor walls, and reached 41.3-85.8% by using the chars. The higher ethylbenzene conversions were reached with the activated chars ac. FW/CFS and ac. UWP. Cracking, oxidative dehydrogenation, and hydrogenolysis reactions were involved in the decomposition mechanism of ethylbenzene. The objective of tar cracking consists in converting tar into gaseous products to increase the syngas yield. However, ethylbenzene conversion generated gaseous products (hydrogen, carbon dioxide, and ethylene) but also two tarry molecules (styrene, benzene and toluene) and coke.

Table 5

Lower heating value (LHV syngas ), volume of syngas produced from the cracking of 100 mol of ethylbenzene fed into the reactor (Prod syngas ) over each char samples. ΔE syngas represents the energy transferred from tar to syngas by tar cracking reactions. These parameters were calculated both including and excluding benzene as a syngas product. In order to determine the efficiency of each char in increasing the syngas production and quality, three parameters were calculated: the volume of syngas produced by tar cracking reaction (Prod syngas ), the lower heating value of the produced syngas (LHV syngas ), and the energy transferred from tar to syngas during the ethylbenzene decomposition (ΔE syngas ).

Despite the lower ethylbenzene conversion obtained with c. FW/ CFS compared to ac. FW/CFS (77.2 vs 85.8%, respectively), c. FW/CFS was the most efficient material to increase the syngas yield and quality. For 100 mol of initial ethylbenzene entering in the system, the maximum energy transferred from tar to syngas was 10.14 MJ with c. FW/ CFS. The significant efficiency of this char was explained by its catalytic activity for aromatic-ring opening reactions, resulting from its high content of mineral species in the metallic form.

The oxygenation process drastically decreased the catalytic activity of both types of char, due to the oxidation of the mineral active sites and the decrease of the graphene-like structures containing defects. On the contrary, steam sactivation increased the catalytic activity of ac. UWP, owing to the significant increase in specific surface area, and to the high proportion of defects in graphene-like sheets in the carbonaceous matrix.

This study demonstrated that waste-based chars were promising catalysts to convert the lost energy contained in tar into useful syngas.

Fig. 2 .

 2 Fig. 2. Composition of the carbonaceous matrix of the chars determined with a deconvolution method of Raman spectra.

Fig. 4 .

 4 Fig. 4. Average conversion of ethylbenzene achieved with the different chars at 650 °C within 85 min of reaction (gas matrix: CO: 40%, N 2 : 60%, EB: 40 g/ Nm 3 ).

Table 1

 1 Solid yield, elemental analysis, and ash composition of the chars.

	Solid yield (wt.%)	c.UWP	ox.UWP	ac.UWP	c.FW/CFS	ox.FW/CFS	ac.FW/CFS
		22.0	98.3	76.7	23.0	96.8	68.7
	Elemental analysis (wt%)
	C	87.2 ± 1.0	84.3 ± 0.4	87.4 ± 1.7	44.1 ± 1.6	42.2 ± 1.6	32.6 ± 3.1
	H	1.8 ± 0.2	1.8 ± 0.0	0.8 ± 0.1	1.3 ± 0.2	1.1 ± 0.0	0.9 ± 0.0
	N	0.6 ± 0.2	0.7 ± 0.0	0.4 ± 0.0	3.1 ± 0.1	2.9 ± 0.1	1.4 ± 0.1
	S	bdl	bdl	bdl	bdl	bdl	bdl
	O (by difference)	8.3	9.7	8.9	4.5	7.8	5.7
	Ash	2.1 ± 0.1	3.5 ± 0.4	2.5 ± 0.2	47.0 ± 0.0	45.9 ± 1.0	59.4 ± 3.4
	Ash composition (wt.%)
	CaO	42.5	44.6	42.2	39.2	42.5	34.3
	P 2 O 5	4.7	5.0	4.2	26.2	19.9	26.2
	K 2 O	11.0	12.9	13.2	6.4	6.7	7.2
	Al 2 O 3	3.0	4.3	2.0	13.8	14.3	18.2
	Cl	n.m	3.3	n.m	7.5	9.5	9.2
	Fe 2 O 3	6.5	4.6	3.6	1.6	0.8	0.6
	SO 3	6.6	4.0	6.7	2.7	1.6	1.3
	SiO 2	8.0	6.8	4.4	2.0	1.8	2.2
	MgO	11.0	8.4	13.4	n.m	2.1	n.m
	Others (TiO 2 , MnO, ZnO, CuO, SrO …)	6.7	6.1	10.3	0.6	0.8	0.8

Table 2

 2 Analysis of the O-functional groups present on the char surface using TPD-μ-GC technique.

	Chars	Total amount	Basic Groups	Acidic groups
		(mmol/g)	(mol%)	(mol%)
	c.UWP	0.28	39.8	60.2
	ox.UWP	0.38	28.7	71.3
	ac.UWP	0.16	59.1	40.9
	c.FW/CFS	2.20	68.3	31.7
	ox.FW/CFS	0.45	72.2	27.8
	ac.FW/CFS	0.39	87.8	12.2

Table 3

 3 Molar selectivity of the products formed during the ethylbenzene cracking over the different chars and during thermal cracking within 85 min of test.

	Materials	Selectivity of EB cracking products (mol%)
		H 2	CO 2	C 2 H 4	C 6 H 6	C 7 H 8	C 8 H 8
	Thermal cracking	34.4	14.8	7.6	0.0	1.4	41.8
	c.UWP	41.7	20.3	3.5	5.1	2.2	27.2
	ox.UWP	40.6	19.5	3.2	0.0	1.5	35.2
	ac.UWP	52.3	13.4	1.7	3.6	2.8	26.2
	c.FW/CFS	53.8	23.0	3.6	4.3	1.5	13.8
	ox.FW/CFS	40.3	15.6	3.6	0.0	1.1	39.4
	ac.FW/CFS	46.3	13.1	1.0	2.1	1.3	36.2

Table 4

 4 Carbon balance during the ethylbenzene cracking tests at 650 °C.

	Materials	η C (mol%)	Coke prod(mg/85 min of test)
	Thermal cracking	97.3	453
	c.UWP	92.1	1298
	c.FW/CFS	85.2	2447
	ox.UWP	98.9	175
	ox.FW/CFS	95.1	809
	ac.UWP	92.9	1174
	ac.FW/CFS	97.0

Acknowledgement

This research was founded by the ANR Carnot M.I.N.E.S. The authors acknowledge the assistance for experiments and analyses from IMT Atlantique (Yvan Gouriou, Eric Chevrel, François-Xavier Blanchet, Jérôme Martin), IMT Mines Albi (Nathalie Lyczko, Laurène Haurie, Sylvie Del-Confetto, Mickaël Ribeiro, Pierre Bertorelle and Denis Marty) and Mines ParisTech' (Alain Thorel, Anthony Chesnaud and Sarah Berhanu).

Appendix A. Supplementary data

Supplementary data related to this article can be found at https:// doi.org/10.1016/j.biombioe.2018.07.020.