Sylvain Rodat
email: srodat@gmail.com

Stéphane Abanades

Eusebiu Grivei

Giorgos Patrianakos

Alexandra Zygogianni

Athanasios G Konstandopoulos

Gilles Flamant

Characterisation of carbon blacks produced by solar thermal dissociation of methane

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

compared to a commercial conductive grade carbon black. Transmission electron microscopy showed primary particles of 10-70 nm diameter. The crystallinity of the samples was characterised by Raman spectroscopy. It was also possible to correlate the specific surface area with the reaction temperature and with the concentrations of residual methane and of the acetylene by-product. The resistivity and the structure of the agglomerates were determined by simultaneous measurements of the conductivity and the density under compression.

1.Introduction

Carbon black is a well-known additive for rubbers, inks, conductive polymers and batteries. It has many applications depending on its physical and chemical properties. It is mainly produced by the furnace process [START_REF] Lockwood | Parametric Study of a Carbon Black Oil Furnace[END_REF]. The world production is about 10 Mt per year [START_REF] Adams | Booming world carbon black demand but price rises fail to keep pace with rising gas & feedstock costs[END_REF]. This process releases 5.7 kg CO2 -equivalent per kg of carbon black [START_REF] Wyss | Rapid Solarthermal Decarbonization of Methane in a Fluid-wall Aerosol Flow Reactor -Fundamentals and Application[END_REF] and thus contributes significantly to global warming. Typical yields of carbon black, depending on feedstock quality, is about 0.5 ton per ton of feedstock (for non porous carbons) and values as low as 0.35 are also reported (the other part being transformed into CO2 and H2O) [START_REF] Fulcheri | From methane to hydrogen, carbon and water[END_REF]. In order to avoid CO2 emission, the heat supply should be achieved by non-conventional processes (no combustion). Consequently, alternative solutions have to be investigated. Plasma reactors have been proposed [START_REF] Fulcheri | Plasma processing: a step towards the production of new grades of carbon black[END_REF][START_REF] Gonzalez-Aguilar | Comparison of simple particle-radiation coupling models applied on a plasma black process[END_REF][START_REF] Moreno-Couranjou | A non-thermal plasma process for the gas phase synthesis of carbon nanoparticles[END_REF]. However, this process requires electricity, i.e. high-grade energy, whereas solar high temperature heat could be used. Solar methane dissociation was studied with two possible reactor configurations: either direct heating reactors with particle seeding or indirect heating reactors. In the case of seeded reactors, particles catalyse the reaction but carbon blacks grow on the catalyst surface leading to a blocking of active sites [START_REF] Maag | Solar thermal cracking of methane in a particle-flow reactor for the co-production of hydrogen and carbon[END_REF][START_REF] Lee | Catalytic decomposition of methane over carbon blacks for CO2-free hydrogen production[END_REF]. In this paper, the solar methane dissociation is proposed in an indirect heating reactor. The solar reactor design and the experimental results about chemical reactor performances were published previously in [START_REF] Rodat | A pilot-scale solar reactor for the production of hydrogen and carbon black from methane splitting[END_REF] and this paper focuses on the thorough analysis of the carbon black properties produced in the solar reactor. Little work has been proposed in the literature concerning carbon black properties in case of solar methane dissociation. XRD and microscopy analysis were essentially employed [START_REF] Wyss | Rapid Solarthermal Decarbonization of Methane in a Fluid-wall Aerosol Flow Reactor -Fundamentals and Application[END_REF][START_REF] Hirsch | Solar hydrogen production by thermal decomposition of natural gas using a vortex-flow reactor[END_REF] but this kind of characterisation is not sufficient. Brunauer, Emmett and Teller method (BET), Raman spectroscopy, evaluation of carbon blacks by the assessment of pressure to volume relation along with conductivity measurements have to be carried out. The specific surface area is an important property of carbon blacks that clearly influences the end-use application [START_REF] Donnet | Carbon Black, Second edition, Revised and expanded, Science and Technology[END_REF]. Raman spectroscopy permits to estimate the crystallinity of nanostructured carbons [START_REF] Larouche | Classifying nanostructured carbons using graphitic indices derived from Raman spectra[END_REF]. Also, the curves giving the conductivity of a carbon black sample versus density can be considered as an ideal way to characterize the so-called conductive blacks [START_REF] Probst | Structure and electrical properties of carbon black[END_REF]. In a first part, the experimental set-up for solar methane dissociation will be briefly described, and then the carbon black properties will be discussed with respect to Transmission Electron Microscopy (TEM) imaging, Raman spectroscopy, specific surface area measurements (BET) and tests under compression.

Comparison with commercial samples (conductive carbon black E250G from Timcal company 1 ) and correlations between solar reactor operating conditions and carbon black properties are also addressed, bringing new information for potential applications.

2.Experimental

2.1.Experimental set-up

The solar reactor for methane dissociation is displayed in Figure 1. It is composed of a cubic graphite cavity (about 40 cm side) serving as radiant absorber. It is crossed by 7 horizontal tubes (800 mm length, 26 mm o.d., 18 mm i.d.) heated up by both in-coming concentrated 1 Timcal Ltd. Technical Data Sheet. ENSACO 250G. Conductive Carbon Black. Version 02/09. Bodio, Switzerland, http://www.timcal.com. solar radiations and IR radiations from the hot cavity walls. The concentrated solar power entered the cavity via a 13 cm-diameter opening in the water-cooled front face. A domed quartz window and an aluminium box separate the graphite absorber from the oxidizing atmosphere. Methane thermal dissociation occurs in the tubes. Additional information can be found in reference [START_REF] Rodat | A pilot-scale solar reactor for the production of hydrogen and carbon black from methane splitting[END_REF]. Once the desired temperature reached, the mixture of argon and methane was injected with a controlled composition. Two mass-flow controllers were dedicated to each tube to control accurately the Ar and CH4 flow-rates. The temperature was measured by a solar blind optical pyrometer (wavelength: 5.14 µm) pointing toward the outer wall of a graphite tube inside the cavity through a CaF2 window. Pt-Rh thermocouples were also used. At the exit of each tube, the exhaust gas-solid flows were collected, cooled and mixed together. The solid and gaseous products were cooled down and flowed through a filter bag to separate carbon particles. The pressure was monitored by pressure sensors placed at each tube entrance and was regulated thanks to the use of a vacuum pump (absolute operating pressure of about 40 kPa). The filtered gas was then analyzed to determine the gas composition. A continuous analyzer permitted to monitor the concentration of H2 and CH4. The methods used for H2 and CH4 analysis were thermal conductivity and non-dispersive infrared detections, respectively. A gas chromatograph also measured online the outlet concentrations of CH4, C2H6, C2H4, C2H2, and H2. The chromatograph (Varian CP 4900) was equipped with 2 columns: MolSieve 5A PLOT for H2 and CH4, and PoraPLOT U for light hydrocarbons (C2Hy). The chromatography analysis was based on thermal conductivity detection and the carrier gas was argon, also used as buffer gas during methane cracking experiments. The experiments were carried out at the 1 MW solar furnace (CNRS-PROMES, Odeillo, France). This furnace is composed of a field of 63 heliostats for full power (45 m² per heliostat) and of a parabolic concentrator (1830 m 2 ) delivering up to 9000 suns at the focal plane. During experiments, only a fraction of the parabola was used by limiting the number of heliostats tracking the sun and by using a shutter.

The resulting power density varies in the range 1.7 -2.8 MW.m -2 .

2.2.Experimental conditions

The experimental conditions tested are summarized in Table 1. Total methane dissociation was reached with a hydrogen yield greater than 80%. The carbon yield is defined as the ratio between the quantity of solid carbon obtained (calculated from a mass balance on the gaseous species) and the initial carbon content in the feed:

4 6 2 4 2 2 2 4 4 CH 0, H C H C H C CH CH 0, C F ) F.y . 2 F.y . 2 F.y . 2 -(F.y F Y     (1)
where CH4 0,

F

is the inlet molar flow-rate of CH4, yi is the mole fraction of species i, and F is the total outlet gas flow-rate (including argon as buffer gas) obtained from:

. ) y y y y (y - 1 F F 6 2 4 2 2 2 2 4 H C H C H C H CH Ar      (2)
FAr is the molar flow-rate of Ar. In equations ( 1) and ( 2), polycyclic aromatic hydrocarbons are neglected.

It was strongly affected by the amount of by-product acetylene. Nevertheless, in each experiment, the mass of carbon black recovered in the filter (given in Table 1) was high enough to perform representative sample characterizations, regardless of the experimental conditions. The carbon black deposited on the walls was recovered separately and was not mixed with the carbon black recovered in the filter.The samples from the filter were analyzed and the results are presented below. Raman scattering is a unique characterization tool for nanostructured carbon materials, such as carbon black, because of its ability to determine the structural basis for the emission and propagation of optical phonons, which thus leads to information regarding the nanostructure of these materials [START_REF] Larouche | Classifying nanostructured carbons using graphitic indices derived from Raman spectra[END_REF][START_REF] Sadezky | Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information[END_REF][START_REF] Lorentzou | Catalytic nano-structured materials for next generation Diesel particulate filters[END_REF] such as the degree of graphitization and crystallite size.

Furthermore, TEM is a very useful tool for directly observing the aggregate morphology of particulate materials, such as carbon black, as well as the primary particle nanostructure in the case of high resolution TEM. Four solar particle samples and one sample of the commercial carbon black E250G were characterized using Raman spectroscopy and high resolution TEM.

The four solar samples were obtained, respectively, from experiments 1, 2, 3 and 5, corresponding to four different reactor temperatures but approximately similar gas residence times in the reactor.

The Raman spectra of the samples were recorded in the 100-3200 cm -1 region using a

Renishaw inVia Reflex micro-Raman spectrometer equipped with an air-cooled Argon ion laser and CCD detector. A 514.5 nm (50 mW) laser line was used as an excitation source. The samples were examined at room temperature. Backscattering illumination and collection of the scattered light were made through a Leica confocal microscope. The diameter of the laser spot on the sample surface was 1 μm for the fully focused laser beam and 40 μm for the fully defocused beam. A five band fitting procedure [START_REF] Sadezky | Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information[END_REF] was applied to the Raman spectra (Lorentzian shape for D1, D2, D4 and G bands, Gaussian shape for the D3 band). The integrated area ratio AD1/AG of the D1 and G bands was interpreted as a measure of the crystallinity of the primary particles, in other words as a graphitization index (decreasing ratio corresponding to increasing graphitization), while an estimation of the average crystallite length (a parameter influencing the applicative properties of the material [START_REF] Fulcheri | Comparison of new carbon nanostructures produced by thermal plasma with industrial Carbon black grades[END_REF]) was derived from the peak intensity ratio ID1/IG of the same bands using the correlation of Knight and White for a 514.5 nm laser [START_REF] Larouche | Classifying nanostructured carbons using graphitic indices derived from Raman spectra[END_REF][START_REF] Knight | Characterization of diamond films by Raman spectroscopy[END_REF][START_REF] Tuinstra | Raman spectrum of graphite[END_REF]:

D1 a G 4.4 I L I         (3)
The results from Raman spectroscopy are summarized in Table 2 where it can be seen that all samples exhibit a similar degree of graphitization (AD1/AG) and similar average crystallite lengths (La). In the case of the solar particles, therefore, the 320 K variation in reactor temperature does not appear to have significantly affected the crystallinity, apart for the high temperature sample 5 which is slightly more graphitic. TEM analysis was performed using a high-resolution JEOL JEM 2011 microscope. Prior to imaging, the powder samples were de-agglomerated by sonication in a high purity isopropanol solution in order to isolate the aggregates as much as possible. During imaging, the copper grid was browsed in order to identify, by visual inspection, the various morphologies exhibited by the particles, and to qualitatively estimate which of these were dominant or exceptional in the population. Images were subsequently taken of particles whose morphology was found to be representative of the dominant morphologies observed, combining where possible two or more different particle morphologies in the same image for contrast. Images were also taken of exceptional particle morphologies in order to consider their possible origin.

TEM images of particles having morphologies that are representative of the dominant morphologies observed in each of the samples analyzed are shown in Figures 2345678. On the basis of the adopted TEM analysis methodology, qualitative observations and conclusions derived from visual inspection of these figures apply, in the most part, to all the particles in the respective sample. Inspection of Figures 2 -7 reveals greater variability in the primary particle size of the solar aggregates compared to the commercial aggregates; nevertheless, the fractal structure of the solar aggregates and the commercial aggregates appears to be similar.

It is also interesting to note that in the case of the solar samples, aggregates consisting entirely of smaller primary particles or larger primary particles are present in the same sample (Figures 4 and6), as are aggregates consisting of various primary particle sizes (Figure 7); however, in the case of the commercial sample, greater uniformity in primary particle size is observed (Figure 2). This is indicative of the existence of an extensive reaction boundary layer in the solar reactor, which results in the production of aggregates of different age and also of mixed age. Indeed, the solar reactor consists of a number of reactor tubes of large aspect ratio (length to diameter ratio); the flow in the tubes is laminar (no turbulent mixing) and a reaction boundary layer develops along the irradiated portion of the tubes [START_REF] Rodat | A pilot-scale solar reactor for the production of hydrogen and carbon black from methane splitting[END_REF]. Particles formed at the beginning of the reaction boundary layer are subject to significant growth via heterogeneous reaction on their surface and aggregation with smaller, newly formed particles; however, this does not apply as much to particles formed further along the reaction boundary layer. TEM can also reveal the length of the crystallites in the primary particles; for example, visual inspection of the primary particles from the sample from Run 5 in Figure 8 indicates crystallite lengths in the range of 1 -5 nm approximately. However, a statistical image analysis, sampling several primary particles from each sample, would be necessary in order to verify the crystallite lengths estimated from Raman spectroscopy.

3.2.Specific surface area: BET measurements

The specific surface area of the samples was evaluated with the BET method. The measurement accuracy is +/-1 m²/g. The results obtained are reported in Figure 9. The specific surface area can reach 100 m²/g and increases when increasing the reactor temperature. The higher the methane flow-rate, the smaller the specific surface area.

According to the EMSA (Electron Microscope Surface Area) formula, a specific surface area of 100 m²/g corresponds to a mean particle diameter of 33 nm.

) . /( 6000

p d S   (4)
with S the specific surface area (m²/g), ρ the carbon black density (= 1.8 g/cm 3 ) and dp the mean particle diameter (nm). The assumption behind this formula is that the particles are not porous (indeed, there is no oxidizing treatment) so that the specific surface area determined from the BET method can be ascribed to the specific surface area S of spherical particles with a mean diameter dp. Carbon black with EMSA ranging from 10 to 140 m²/g are notably used for tyres; for pigment application, very fine particles are required (9-16 nm) [START_REF] Parkash | Petroleum Fuels Manufacturing Handbook[END_REF]. The BET of the conductive grade E250G is 65 m²/g. The specific surface area can also be correlated with the residual concentration of methane (Figure 10) or acetylene (not shown) in the outlet gas. The trend shows that the specific surface area increases when the residual concentration of CH4 decreases. Let us note that CH4 concentration at the reactor outlet decreases with a temperature increase. Also, a similar trend is obtained when the C2H2 off-gas concentration decreases. In conclusion, there is a strong dependence of the specific surface area on the reactor temperature. Increasing the temperature results in an increase of the nucleation rate, which leads to smaller particles with a higher specific surface area [START_REF] Buxbaum | Industrial Inorganic Pigments, Third, Completely Revised Edidtion[END_REF]. The carbon black structure refers to the three-dimensional organization of the carbon black. A carbon black is defined as a high structure carbon black (numerous catenary agglomerates) or as a low structure carbon black. The present work considers the void-volume technique under a broader angle addressed in [START_REF] Probst | Structure and electrical properties of carbon black[END_REF]. The evolution of the volume of a given amount of carbon black subjected to increasing compression as well as the evolution of the electrical resistivity is recorded and analysed. The structure can be evaluated by measuring the mechanical work required to bring a carbon black sample to a given density [START_REF] Probst | Structure and electrical properties of carbon black[END_REF] (Figure 11). The accuracy for both mechanical work and resistivity is +/-2% of the measurement. The slopes of the curves increase with carbon black structure. Indeed, high structure blacks require more energy to increase their apparent density. However, the curves of the mechanical energy versus density approach a linear behaviour for a given carbon black, which means that the amount of energy required to increase the apparent density is constant over the range of pressure applied. Thus, for E250G, an energy of 160 kg.cm is required to reach 0.9 g/cm 3 whereas only 105 kg.cm is required to reach the same density for the carbon black obtained at 1928K. E250G has a higher structure than the carbon black obtained at the highest temperature but this last sample approaches the behaviour of the commercial reference. This is confirmed by DiButyl Pthalate Absorption measurements (DBPA). The DBPA of E250G is 190 mL/100g whereas it is 144 mL/100g for the sample produced at 1928K. The higher the DBPA, the higher the structure. It is also interesting to measure the conductivity of the carbon black samples under pressure (Figure 12). This can inform about the conductive capability of the carbon black [START_REF] Pantea | Electrical conductivity of conductive carbon blacks: influence of surface chemistry and topology[END_REF]. Three experimental samples are represented for three temperatures (1928K, 1778K, 1693K) along with E250G. For a given carbon black, increasing the density improves the conductivity because the contact area between the particles increases. E250G shows the best conductivity for a given density: the resistiviy is 0.055 Ohm.cm for a density of 0.8 g/cm 3 whereas it is 0.11 Ohm.cm for the sample from run 5. A conductive carbon black has a good conductivity even for low density. The experimental sample from run 5, obtained at the highest temperature, shows the closest behaviour with the commercial reference. For lower temperatures, the resistivity is even higher. It seems that high temperatures favour the conductivity. Poly-aromatic hydrocarbons adsorbed at the carbon black surface in the case of the experimental samples could also explain why a lower conductivity is measured. E250G has the highest conductivity and the best structure. A high structure should improve the conductivity since the probability of contact between the carbon particles is greater [START_REF] Sanchez-Gonzalez | Electrical conductivity of carbon blacks under compression[END_REF]. Conductive carbon black is used in polymers to decrease their electrical resistivity. The higher the structure, the smaller the required load of carbon black to reach a given conductivity.

Compounding tests [START_REF] Probst | Compounding with conductive carbon black[END_REF] have been carried out for the carbon black sample from run 5. For a 25% load (mass) in High Density Poly Ethylene, the matrix resistivity was 4200 Ohm.cm compared to 6 Ohm.cm for E250G. Both a lower structure and conductivity thus lead to weaker performances of the compounded polymer when a conductive polymer application is targeted.

4.Conclusion

A pilot-scale solar reactor (50 kWth) was developed and operated for the co-production of carbon black and hydrogen. Methane was dissociated in tubular sections at temperatures ranging from 1608K to 1928K. A detailed analysis of carbon blacks obtained was presented in this paper. TEM imaging, Raman spectroscopy, BET analysis and measurements of conductivity under compression were carried out.

Crystallite lengths in the 1-4 nm range were observed by TEM imaging, which is in agreement with estimated values from Raman spectroscopy. The temperature of methane dissociation is the main parameter that influences the specific surface area of carbon (in the range 60-100m²/g). The comparison with the properties of a commercial conductive carbon black shows that the sample obtained at the highest temperature (1928K) has a structure and a conductivity approaching this reference.

According to the obtained specific surface area, applications of the solar-synthesized carbon black in the tyre industry appear possible. Further improvements of carbon properties can also be anticipated by adjusting and optimising the operating conditions and by applying pre-or post-treatments (e.g. oxidation). With special attention on the carbon black production process, applications related to conductive carbon black seem reachable. Tests should be carried out in the targeted end use applications to get more feedback. Finally, the solar process should be seriously considered as an environmentally friendly option for a clean coproduction of valuable carbon black and hydrogen.

Figure 1 :

 1 Figure 1: 50 kW solar reactor

Figure 2 :

 2 Figure 2: TEM image of representative commercial E250G particles.

Figure 3 :

 3 Figure 3: TEM image of representative solar particles from run 1, produced at 1608K.

Figure 4 :

 4 Figure 4: TEM image of representative solar particles from run 2, produced at 1693K.

Figure 5 :

 5 Figure 5: TEM image of representative solar particles from run 3, produced at 1778K.

Figure 6 :

 6 Figure 6: TEM image of representative solar particles from run 5, produced at 1928K.

Figure 7 :

 7 Figure 7: TEM image of representative solar particles from run 5, produced at 1928K.

Figure 8 :

 8 Figure 8: High magnification TEM image of representative solar primary particles from

Figure 9 :

 9 Figure 9: Specific surface area of carbon blacks as a function of the reactor temperature

Figure 10 : 3 . 3 .

 1033 Figure 10: Specific surface area of carbon blacks as a function of the off-gas methane

Figure 11 :

 11 Figure 11: Mechanical work versus density for various experimental carbon black

Figure 12 :

 12 Figure 12: Resistivity versus density for various experimental carbon black samples

Table 1 :

 1 Summary of the experimental conditions tested (pressure is related to conditions inside the reaction tubes)

										Carbon
	Run	Ar (NL/min)	CH4 (NL/min)	CH4 mole fraction	Pressure (Pa)	Tpyrometer (K)	Residence time (s)	Duration (min)	Carbon yield	mass collected in the
										filter (g)
	1	31.5	10.5	0.25	43000	1608	0.070	72	0.43	54.4
	2	31.5	10.5	0.25	46000	1693	0.071	63	0.50	77.8
	3	31.5	10.5	0.25	43000	1778	0.063	24	0.50	13.9
	4	31.5	10.5	0.25	42000	1793	0.061	33	0.47	24.8
	5	31.5	10.5	0.25	42000	1928	0.057	57	0.63	72.7
	6	49	21	0.3	47000	1698	0.043	46	0.41	53.0
	7	21	21	0.5	41000	1798	0.059	25	0.49	41.1
	8	49	21	0.3	43000	1808	0.037	53	0.42	106.5
	9	49	21	0.3	45000	1873	0.038	33	0.45	69.1
	3.Carbon black properties
	3.1.Particle morphology and crystallinity: Raman spectroscopy and TEM imaging

Table 2 :

 2 Raman spectroscopy results

	Sample Temperature	AD1/AG	ID1/IG La (nm)
		of reactor (K)
	E250G -	3.7	1.6	2.8
	Run 1	1608	3.8	1.6	2.8
	Run 2	1693	3.6	1.6	2.7
	Run 3	1778	3.6	1.6	2.7
	Run 5	1928	3.4	1.7	2.6

Acknowledgements:

This study was funded by the European Project Solhycarb (2006-2010, Contract SES-CT2006-19770). The authors wish to thank Jean-Louis Sans, Olivier Prévost and Marc Garrabos for their technical support during the solar reactor manufacturing and operation.


Quentin Bellouard

Stéphane Abanades
email: stephane.abanades@promes.cnrs.fr

Sylvain Rodat

Biomass gasification in an innovative spouted-bed solar reactor: experimental proof of concept and parametric study

Solar thermochemical gasification of lignocellulosic biomass promises a new path for the production of alternative fuels as well as storage and transport of solar energy as a convertible and transportable fuel. The use of concentrated solar energy as the external heat source for the high-temperature reaction allows producing high-value syngas with both higher energy conversion efficiency and reduced cost of gas cleaning and separation, while saving biomass feedstock. A newly designed solar reactor based on the principle of spouted bed reactor was used for continuous solar-driven gasification of biomass particles. The reliable operation of this 1.5 kW reactor was experimentally demonstrated under real solar irradiation using a parabolic dish solar concentrator. Several types of biomass particles were continuously fed in the reactor at temperatures ranging from 1100°C to 1400°C. The injected particles consisted of beech wood or a mix of resinous wood with size ranging from 0.3 mm to 2 mm. The aim of this study was to achieve a proof of concept for the novel solar reactor applied to biomass gasification. A parametric study of the gasification conditions was realized to optimize the syngas production. The influence of temperature, oxidizing agent nature (H2O or CO2) and flow rate, heating configuration (direct or indirect irradiation), biomass type, particles size and feeding rate on gas yield and composition was investigated. The syngas yield increased drastically with the temperature for both steam and CO2 gasification, while increasing the steam content favored H2 and reduced CO production. Maximum amounts of produced syngas over 70 mmol/gbiomass and carbon conversion rates over 90% were achieved. The biomass energy content was solar-upgraded by a factor of 1.10 at 1400°C.

Introduction

With the fossil fuel resource depletion and the growing concerns about climate change, renewable energies, and particularly biomass, are considered with a growing interest. Biomass presents the advantage of being environmentally clean and available worldwide. A promising and sustainable way to draw benefits from this resource is the thermochemical gasification of biomass as it produces a high quality syngas suitable for power generation or biofuel synthesis.

Through a complex process involving hundreds of chemical species and thousands of reactions, biomass gasification intends to produce a syngas composed of hydrogen and carbon monoxide only. The first step is the pyrolysis of the biomass, which occurs at temperatures above 300°C. During this phase the wood is decomposed into volatile products either condensable (steam and primary tars) or non-condensable gases (H2, CO, CO2, CH4 and C2Hx), and a solid residue called char. The quantity of char and tars produced can be reduced with higher temperatures and heat transfer rates [START_REF] Deglise | Bois énergie[END_REF][START_REF] Dufour | Synthesis gas production by biomass pyrolysis: Effect of reactor temperature on product distribution[END_REF] . The tars are then cracked to produce more gas as well as secondary and tertiary tars. The char is gasified using an oxidizing agent, H2O or CO2, to produce syngas according to the following Many reactions occur in the gas phase and the main ones are the following:

Water-gas shift reaction:

𝐶𝑂 + 𝐻 𝑂 → 𝐶𝑂 + 𝐻 ∆H°4 = -42 kJ/mol (4)

Steam methane reforming:

𝐶𝐻 + 𝐻 𝑂 → 𝐶𝑂 + 3𝐻 ∆H°5 = 206 kJ/mol (5) Dry methane reforming:

𝐶𝐻 + 𝐶𝑂 → 2𝐶𝑂 + 2𝐻 ∆H°6 = 247 kJ/mol (6)
The wood molecular formula can be written approximately as C6H9O4 [START_REF] Ballerini | Les biocarburants, état des lieux, perspectives et enjeux du développement[END_REF] , thus in the specific case of wood gasification, the whole process can be represented by the following equation for gasification with H2O and CO2:

𝐶 𝐻 𝑂 + 2𝐻 𝑂 → 6𝐶𝑂 + 6.5𝐻 ∆H°7 = 0.8 MJ/mol (7) 𝐶 𝐻 𝑂 + 2𝐶𝑂 → 8𝐶𝑂 + 4.5𝐻 ∆H°8 = 0.9 MJ/mol (8) The ideal syngas composition can only be obtained if the thermodynamic equilibrium above 900°C is reached. In the case of a real gasification process, higher temperatures will be preferable as they reduce the gas residence time required to reach the thermodynamic equilibrium [START_REF] Higman | Gasification[END_REF] . Besides, biomass gasification is a very endothermic process and requires a massive energy input. To provide this energy input, two different kinds of technologies have been developed [START_REF] Heidenreich | New concepts in biomass gasification[END_REF] , namely autothermal and allothermal reactors. The first one uses both the heat and H2O and CO2 obtained from the combustion of a part of the biomass feedstock (at least 30%) to sustain the process and to gasify the rest of the feedstock. The second one relies on an external heat source for the process needs. In this context, the use of concentrated solar power to provide the necessary heat to the reaction is a promising way to save biomass or fossil resources and it has already been investigated [START_REF] Puig-Arnavat | State of the art on reactor designs for solar gasification of carbonaceous feedstock[END_REF][START_REF] Nzihou | Synthetic fuels from biomass using concentrated solar energy -A review[END_REF][START_REF] Piatkowski | Solar-driven gasification of carbonaceous feedstock-a review[END_REF] . Thanks to the absence of combustion, a solar process produces a syngas with a higher calorific value and no combustion product contamination. Besides, this also eliminates the need for upstream air separation and reduces costly downstream gas cleaning and separation requirements. The whole feedstock being gasified, the syngas output per unit of feedstock is increased. Thanks to the use of highly concentrated solar power, higher gasification temperatures (>1200°C) are reachable thus improving the reaction kinetics and the syngas quality and greatly reducing the tar content. This process thus represents an efficient way to store the solar energy in a chemical form and improves the dispatchability of the initial resource.

Several solar reactors have already been designed for carbonaceous materials gasification, the most represented concepts being packed bed and fluidized bed solar reactors. They can be classified into two categories, (i) directly irradiated solar reactors (windowed) allowing optimal radiative transfer to the feedstock and (ii) indirectly irradiated solar reactors offering the possibility to remove the window at the expense of an additional heat transfer resistance through an opaque heat transfer wall serving as absorber and heat conductor.

Regarding packed bed solar reactors, either they are directly or indirectly irradiated, the core is the same. They consist of a cavity receiver filled with a batch of feedstock which receives the concentrated solar power directly through a window [START_REF] Gregg | Solar gasification of coal, activated carbon, coke and coal and biomass mixtures[END_REF][START_REF] Taylor | Solar gasification of carbonaceous materials[END_REF][START_REF] Flechsenhar | Solar gasification of biomass using oil shale and coal as candidate materials[END_REF] or indirectly through an emitter plate reemitting energy via IR radiation [START_REF] Piatkowski | Solar-Driven Coal Gasification in a Thermally Irradiated Packed-Bed Reactor[END_REF][START_REF] Piatkowski | Experimental investigation of a packed-bed solar reactor for the steam-gasification of carbonaceous feedstocks[END_REF] . Such reactors offer high reaction extent thanks to the presence of a large quantity of feedstock particles absorbing the radiation and the long solid residence time. Those reactors can also accept a large variety of feedstock with different compositions and large particle sizes. However the scaling up of those reactors is limited for the same reason, the thermal inertia of the bed leads to an important temperature gradient and non-homogenous reactions, thus impacting the reaction rate and the syngas quality [START_REF] Piatkowski | Solar-Driven Coal Gasification in a Thermally Irradiated Packed-Bed Reactor[END_REF] . To avoid such problem, fluidized bed solar reactors were proposed in which the feedstock is continuously stirred with inert particles by a neutral gas and/or the oxidizing agent. Several concepts of solar fluidized bed reactors have already been tested [START_REF] Taylor | Solar gasification of carbonaceous materials[END_REF][START_REF] Murray | Reaction of steam with cellulose in a fluidized bed using concentrated sunlight[END_REF][START_REF] Zedtwitz | Steam-Gasification of Coal in a Fluidized-Bed/Packed-Bed Reactor Exposed to Concentrated Thermal RadiationModeling and Experimental Validation[END_REF][START_REF] Müller | Kinetic investigation on steam gasification of charcoal under direct high-flux irradiation[END_REF][START_REF] Kodama | Fluidized Bed Coal Gasification with CO2 under Direct Irradiation with Concentrated Visible Light[END_REF][START_REF] Kodama | Application of an Internally Circulating Fluidized Bed for Windowed Solar Chemical Reactor with Direct Irradiation of Reacting Particles[END_REF][START_REF] Kodama | Coal Coke Gasification in a Windowed Solar Chemical Reactor for Beam-Down Optics[END_REF][START_REF] Gokon | CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification[END_REF][START_REF] Gokon | Steam gasification of coal cokes in an internally circulating fluidized bed of thermal storage material for solar thermochemical processes[END_REF][START_REF] Gokon | Steam gasification of coal cokes by internally circulating fluidized-bed reactor by concentrated Xe-light radiation for solar syngas production[END_REF] , however the proper operation of those reactors requires small particle sizes (<1 mm) with preliminary shredding and sorting steps for the feedstock. For both packed and fluidized bed solar reactors, no suitable concept was proposed for continuous gasification process. Indirectly irradiated entrained flow solar reactors were developed for continuous gasification process with horizontal [START_REF] Melchior | Solar-driven biochar gasification in a particle-flow reactor[END_REF] or vertical [START_REF] Lichty | Rapid High Temperature Solar Thermal Biomass Gasification in a Prototype Cavity Reactor[END_REF] circulation of the particles. Because of the short particle residence times in those reactors, the particle sizes must be limited and only feedstock with high reactivity can be gasified. Even so the short residence time of particles and gas has an adverse impact on carbon conversion rate and syngas composition. In order to offer both long residence time and continuous process, some alternative concepts were developed. One of them is based on the principle of an indirectly irradiated drop-tube reactor combined with a packed bed. A tubular cavity receives the solar irradiation while a porous material in the heated zone retains the particles until their complete gasification while allowing the gas to pass through [START_REF] Kruesi | Solar-driven steam-based gasification of sugarcane bagasse in a combined drop-tube and fixed-bed reactor -Thermodynamic, kinetic, and experimental analyses[END_REF][START_REF] Kruesi | A two-zone solar-driven gasifier concept: Reactor design and experimental evaluation with bagasse particles[END_REF][START_REF] Kruesi | Analysis of solar-driven gasification of biochar trickling through an interconnected porous structure[END_REF][START_REF] Bellouard | Solar thermochemical gasification of wood biomass for syngas production in a high-temperature continuously-fed tubular reactor[END_REF] . Another alternative concept is a directly irradiated horizontal cylinder in which the feedstock and the gas are injected in a way to form a vortex flow inside the reactor, thus extending the residence time of the particles inside the heated zone [START_REF] Z'graggen | Hydrogen Production by Steam-Gasification of Petroleum Coke Using Concentrated Solar Power: Reactor Experimentation With Slurry Feeding[END_REF][START_REF] Z'graggen | Hydrogen production by steam-gasification of petroleum coke using concentrated solar power-II Reactor design, testing, and modeling[END_REF][START_REF] Z'graggen | Hydrogen production by steam-gasification of petroleum coke using concentrated solar power-III. Reactor experimentation with slurry feeding[END_REF][START_REF] Z'graggen | Hydrogen production by steam-gasification of carbonaceous materials using concentrated solar energy-IV. Reactor experimentation with vacuum residue[END_REF] . This reactor was recently modified into an indirectly irradiated reactor and the two configurations were compared [START_REF] Müller | A pressurized high-flux solar reactor for the efficient thermochemical gasification of carbonaceous feedstock[END_REF] .

This study follows an experimental campaign realized with a drop tube/packed bed reactor [START_REF] Bellouard | Solar thermochemical gasification of wood biomass for syngas production in a high-temperature continuously-fed tubular reactor[END_REF] and focuses on the parametric study of a newly developed solar reactor under real solar irradiation, based on the concept of spouted bed reactor. Spouted bed reactors have already been used for nonsolar biomass pyrolysis and gasification [START_REF] Fernandez-Akarregi | Design and operation of a conical spouted bed reactor pilot plant (25 kg/h) for biomass fast pyrolysis[END_REF][START_REF] Erkiaga | Steam gasification of biomass in a conical spouted bed reactor with olivine and γ-alumina as primary catalysts[END_REF][START_REF] Erkiaga | Influence of operating conditions on the steam gasification of biomass in a conical spouted bed reactor[END_REF] and present the advantages to offer a high solid residence time as well as a continuous stirring of the feedstock. However, previously developed reactors show different characteristics and operating principles. For example, packed-bed [START_REF] Piatkowski | Solar-Driven Coal Gasification in a Thermally Irradiated Packed-Bed Reactor[END_REF][START_REF] Piatkowski | Experimental investigation of a packed-bed solar reactor for the steam-gasification of carbonaceous feedstocks[END_REF] and fluidized bed [START_REF] Gokon | CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification[END_REF][START_REF] Gokon | Steam gasification of coal cokes in an internally circulating fluidized bed of thermal storage material for solar thermochemical processes[END_REF][START_REF] Gokon | Steam gasification of coal cokes by internally circulating fluidized-bed reactor by concentrated Xe-light radiation for solar syngas production[END_REF] reactors were chiefly operated in batch as the reacting particles were preloaded in the reactor before heating and most reactors used artificial light instead of concentrated solar radiation. In this study, the developed solar-heated reactor also offers the possibility to continuously feed the reactant and to work with both directly and indirectly irradiated modes thanks to a removable emissive plate. Such continuous operation with particles injection directly at high temperatures represents a major innovation when compared with previous reactors. Finally, in previous works [START_REF] Gokon | Steam gasification of coal cokes by internally circulating fluidized-bed reactor by concentrated Xe-light radiation for solar syngas production[END_REF] , the fluidized bed was mostly heated from the top surface with high radiation losses and temperature gradient through the bed from top to bottom (also the case for packed beds [START_REF] Piatkowski | Solar-Driven Coal Gasification in a Thermally Irradiated Packed-Bed Reactor[END_REF][START_REF] Piatkowski | Experimental investigation of a packed-bed solar reactor for the steam-gasification of carbonaceous feedstocks[END_REF] ). The new developed reactor in this study features a cavity receiver with a small aperture on top to maximize energy absorption while minimizing radiation losses, thereby achieving homogeneous temperatures inside the cavity.

The concept validation, experimental demonstration and performance assessment of the solar reactor was conducted under real solar irradiation.

Experimental section

Feedstock

Several feedstocks were used for the experiments to assess the impact of the wood composition and particles size on syngas production and biomass injection. While the biomasses are all grinded wood, they present some differences in composition, humidity content, low heating value (LHV) and particles size. The characteristics of the four different types of feedstocks are presented in Table 1.

Experimental set-up

The entire experimental system is depicted in Figure 1. It is mainly composed of a solar concentrating system, the 1.5 kW solar reactor associated with a particle feeding system, a filtering unit and a gas analysis unit. The reactor is located at the 6 th floor to the south of the CNRS-PROMES building (Odeillo, France), in a room above ground as described in Figure 2. A sun tracking heliostat located 30 m below the trapdoor reflects the solar radiation vertically to a parabolic mirror (2 m diameter). This parabolic mirror then concentrates the flux on the focal point (2 cm diameter) where the reactor aperture is settled. The flux density at the focal point is up to 10 MW/m 2 (for a Direct Normal Irradiation (DNI) of 1 kW/m 2 ) with a Gaussian distribution. The trapdoor activated via a motor can be partially closed or opened to adjust the incident power and thus the reactor temperature.

The design of the solar reactor is based on the concept of conical spouted bed reactor [START_REF] Olazar | Hydrodynamics of Sawdust and Mixtures of Wood Residues in Conical Spouted Beds[END_REF] , for which the feasibility for biomass gasification has already been proven [START_REF] Erkiaga | Steam gasification of biomass in a conical spouted bed reactor with olivine and γ-alumina as primary catalysts[END_REF][START_REF] Erkiaga | Influence of operating conditions on the steam gasification of biomass in a conical spouted bed reactor[END_REF] . The cavity of the reactor, which receives the solar flux and shelters the reaction, is a 78 mm inside diameter cylinder (47 mm height, 3 mm wall thickness) whose bottom is a cone (68 mm height) with a 60° angle, for a total height of 11.5 cm. This cavity is made of a FeCrAl alloy whose melting point is around 1500°C. A gas flow comprising Ar carrier gas (0.3 NL/min) and an oxidizing agent (CO2 or H2O) is injected via an alumina tube (4 mm o.d., 2 mm i.d.) from the bottom of the conical part of the cavity to (i) propel the biomass particles to the top of the cavity where they can absorb highly concentrated solar power, and

(ii) ensure proper mixing of the biomass with the oxidizing agent. The trajectory of the biomass particles is schemed in Figure 3.

All the gas flows are injected via mass-flow controllers (model Brooks SLA5850S, scale: 0-5 NL/min, precision: ± 0.2% of full scale). Steam is injected via a stainless steel capillary exiting 5 mm below the bottom of the conical cavity and placed inside the gas injection tube, water is vaporized inside the capillary and then entrained by the Ar flow before entering the cavity at the tip of the cone, thus providing continuous steam flow inside the system. The liquid water flow rate is controlled by a mass flow controller (scale 0-30 g/h, precision: ± 1% of full scale). To broaden the gas stream with an enlarged effective gas flow diameter and prevent biomass particles from being propelled outside the cavity, a fixed bed (about 15 mm high) of SiC particles (1-2.3 mm diameter), which does not move with the gas flow, is set at the bottom of the cavity. This bottom layer of particles also improves the reactor thermal inertia by absorbing radiation and storing energy, and further protects the bottom injection alumina tube from solid or liquid reaction products clogging.

The cavity is supported by a shaped piece of porous ceramic that ensures a thermal insulation on its side and bottom (~30 mm thickness around the cavity). To limit the thermal losses on top of the cavity and infrared re-radiation, an alumina cap with a 20 mm aperture is placed above it. This The biomass is mechanically injected directly in the cylindrical part of the cavity using a screw feeding device going through the reactor wall and the insulating material. The particles exit the tilted screw feeder and then fall down by gravity inside the cavity where they are gasified thanks to the oxidizing gas injected from the cavity bottom. An Ar flow (1 NL/min) is injected in the hopper in which the biomass is stored (capacity 1.15 L) and then goes through the screw path (screw dimensions: 14 mm o.d., 10 mm core diameter) all the way to the cavity, thus preventing backflow of the hot gases from the cavity up to the screw. Instead the gas exits the reactor via an alumina tube (8 mm o.d., 5 mm i.d.) passing through the insulating material and the reactor wall. This outlet gas comprises the gas products from the gasification reaction occurring in the cavity as well as the carrier gas injected at the bottom, and the protective gas injected both at the window and through the particle feeding system. Then the exiting gas flows into a gas washing unit composed of a bubbler and two cartridge filters (filtration threshold: 99.99998% for 0.1 µm particle size) to be cooled and to separate the remaining steam, ashes and tars before entering the gas analyzer.

Metrology and gas analysis

Temperature inside the cavity was measured at two locations using B-type thermocouples (Pt-Rh). Those thermocouples were covered by an alumina sheath and inserted in the cavity through the insulating ceramic and the reactor shell as shown in Figure 1 (T2 & T3). Another B-type thermocouple (T1) was placed inside the insulating ceramic to measure the temperature of the upper part of the cavity wall. A solar-blind pyrometer (operating at 4.8-5.2 µm in a H2O absorption band) placed at the center of the parabolic mirror was used to measure the temperature inside the cavity (direct heating configuration) or at the surface of the emissive plate (indirect heating configuration). Three pressure gauges were used to monitor the pressure in the gas injection tube at the window (P1), in the cavity (P2) and in the gas outlet pipe (P3). The pressure in the cavity was especially controlled during the gasification process to monitor the gas expansion.

An in-line syngas analyzer (GEIT 3100) was used to measure the concentration of the main species in the produced gas. The concentrations of CO, CO2, CH4 and CnHm are measured using NDIR cells and the H2 concentration using a thermal conductivity detector. For some experiments the gas was collected in a sampling bag to be analyzed with a gas chromatograph (GC, Varian CP49000) in order to double-check the results. The gas chromatograph and the in-line analyzer showed similar results with a 5% relative error. Only the data from the in-line analyzer are reported here for the data exploitation.

The amount of syngas produced was measured by a volumetric gas counter (drum-type gas meter, Ritter TG20, range 40-2800 NL/h) and calculated from the known Ar flow. The composition and the amount of syngas being known, it was possible to determine its Low Heating Value (LHV).

One of the main interests of solar-driven biomass gasification relates to the improvement of the energy content of the initial biomass. A performance indicator used to assess this solar upgrade, called Cold Gas Efficiency (CGE), is the ratio of the solar gasification products energy content (LHV) to the energy content of the initial biomass feedstock and is defined as follows:

𝐶𝐺𝐸 = × × (9)
A ratio superior to 1 thus indicates an upgrade of the initial biomass energy content through gasification and the efficient storage of solar energy.

The biomass composition being known, it is possible to calculate the carbon conversion rate defined as the ratio of carbon contained in the gaseous product (in the form of CO, CO2, CH4 and CnHm) to the injected carbon in the biomass.

Experimental protocol and conditions

The reactor was first flushed with inert gas to remove air and then the trapdoor was progressively opened (speed 19 mm/s, 105 s for full opening) in several steps to avoid thermal shocks of the ceramic parts. The reactor was heated up to the targeted temperature inside the cavity with continuous Ar flow (total duration of 30 min to reach 1200°C in the direct heating configuration and 60 min in the indirect heating configuration). The temperature was controlled by adjusting the trapdoor opening. Then the oxidant (either steam or CO2) was injected at the cavity bottom before starting the continuous biomass particle feeding in the cavity at high temperature. Initially the biomass amount was precisely weighted and preloaded inside the hopper, and the feeding was operated until complete biomass particle load injection (the total amount of feedstock fed during each run was thus precisely known). The particles fall inside the cavity by gravity and are pushed by the upward particles from the screw and by the Ar injected via the screw path along with the particles. The gasification then proceeds inside the cavity where the oxidant (steam or CO2) is injected (Fig. 3). Because the biomass injection induces a slight temperature decrease in the cavity (energy consumed for particle heating and endothermal reaction), the reactor was initially overheated by about 30-50°C above the temperature set-point. The gas species concentrations at the reactor outlet were analyzed continuously to determine the syngas yield and chemical composition, as well as the total amount of each gas produced by time integration of the production rates. After the test, the different reactor components were cleaned and the remaining char/ash in the cavity and outlet parts (tubes, bubbler, filters) was weighted. After each run, a mass balance was performed from the amounts of solid residues and gas products compared to the initial amounts of injected biomass and gaseous oxidant. The mass balance was closed at over 95% in every case. The gas velocity at the bottom cavity entrance for the spouted bed (mixture of Ar and oxidizing gas) was settled above the minimum spouting gas velocity (2.91 m/s at the exit of the alumina tube for a required minimum gas velocity of roughly 1 m/s at normal conditions). Cold pretests with visual observation of preloaded wood particle movement confirmed the particle lifting and stirring inside the cavity at room temperature, thus providing evidence of the spouting process.

However, it is likely that upon continuous particles injection at high temperature, the gasification reaction rate is high enough to consume rapidly and entirely the whole particles during their feeding inside the cavity and that the spouting process may differ from cold tests because of particles consumption and shrinkage. It thus appears possible that most of the reactions occur fast in the gas volume and that the formation of a bed just comes from the remaining char particles. It was not possible to visually observe the real process of particle movement and gasification inside the reactor because of the harsh temperature conditions.

For this study, all the experiments were conducted with a continuous biomass injection (total amount of 10 g) with a feeding rate set to 2 g/min and using the Type A biomass (unless stated otherwise). However due to some issues with the motor driving the screw feeder and the biomass rheology, the actual biomass feeding rate was somewhat lower and varied between 1 and 2 g/min. All the experiments were carried out using either H2O or CO2 as an oxidizing agent. Most of them were realized with an oxidizing agent flow rate such that the biomass/oxidant ratio was close to the stoichiometric ratio presented in equation ( 7) and ( 8). The biomass humidity rate was included in the calculation to determine the required oxidant flow rate for both H2O and CO2. For a biomass feeding rate of 2 g/min, the calculated stoichiometric flow rates are 234 mg/min for H2O and 0.29 NL/min for CO2. Due to the lower actual biomass injection rate, these calculated flow rates were rounded down to 200 mg/min for H2O and 0.2 NL/min for CO2. All the parameters of the experiments are presented in Table 2.

Results and discussion

Temperature influence

The influence of the temperature on syngas composition from biomass gasification with H2O and CO2 was studied. Injections of biomass (type A) were realized in the reactor heated at 1100°C, 1200°C, 1300°C and 1400°C while injecting either H2O or CO2. The quoted nominal temperatures correspond to T3 measured inside the cavity. As the values given by T2 and T3 were very close (less than 10°C discrepancy), the temperature could be considered uniform in the reaction zone where the particles are injected. The quantity of gas produced per gram of dry biomass [mmol/gbiomass] during gasification with H2O at the different temperatures is presented in Figure 4 along with the evolution of the cold gas efficiency. These gaseous productions are obtained by time integration over an entire run.

The theoretical values attained at the thermodynamic equilibrium (TE) reached above 1000°C are also represented. The presence of light hydrocarbons (mainly C2H2) was detected in the syngas but in very small quantity (< 0.3 %) and was thus not represented. The mean residence time of the gas inside the cavity is about 0.6±0.1 s at the considered operating conditions (gas flow rates calculated at the actual reactor temperature and pressure). The quantity of produced syngas increases with temperature (Fig. 4) approaching the thermodynamic equilibrium, but it is not reached because of kinetic limitations.

With the increase of the temperature, the quantity of H2 and CO produced from the same mass of biomass also increases, from 22 and 22.7 mmol/gbiomass at 1100°C to 28.9 and 35.8 mmol/gbiomass at 1400°C for CO and H2, respectively. On the opposite, the quantity of CH4 decreases with the rise of temperature, from 5.2 mmol/gbiomass at 1100°C to 3 mmol/gbiomass at 1400°C. These variations can be explained by the improvement of the kinetic rate of gasification (Eq. 1) and steam methane reforming (Eq. 5). The quantity of CO2 produced remains steady between 3.3 and 4.1 mmol/gbiomass regardless of the temperature. The H2/CO ratio reaches a maximum of 1.4 at 1300°C. The overall quantity of gas produced increases with the temperature, but this growth seems to be lowered above 1300°C. It is also interesting to notice that the overall quantity of carbon-containing gas increases with the temperature too, leading to an improvement of the carbon conversion rate from 78.8% at 1100°C to 91.5% at 1400°C. These variations induce an improvement of the cold gas efficiency (CGE) with the rise of the temperature. It appears that the energy content of the syngas exceeds the one of biomass beyond 1250°C, with the CGE reaching a maximum of 1.10 at 1400°C. However this value remains lower than the one obtained at the thermochemical equilibrium mostly because of the production of CO2 that cannot be valorized during combustion. Independently of the reaction temperature, small amounts of char were observed in the outlet tubes and the filtration devices (less than 0.5 g for 10 g of injected biomass). A small amount of char (< 0.1 g) was also occasionally found in the biomass injection tube at the end of the experiment.

The same experiments were realized with a flow of CO2 (0.2 NL/min mixed with 0.3 NL/min Ar) as oxidizing agent and the quantity of gas produced per gram of dry biomass is shown in Figure 5 along with the CGE. Those values are also compared to those obtained at the thermodynamic equilibrium. Once again the presence of light hydrocarbons was noted but in low quantities (< 0.3%).

The total CO2 measured by the on-line syngas analyzer at the reactor exit corresponds mainly to the excess of the CO2 injected as oxidizing agent and the CO2 produced by the reaction (the only fraction of CO2 in the evolved syngas related to the reaction cannot be quantified in the case of CO2 gasification), and CO2 is thus not represented in Figure 5.

As expected from Eqs. ( 1) and ( 2), the quantity of H2 produced is much lower and the quantity of CO much higher for gasification with CO2 than for steam gasification, leading to low H2/CO ratios (ranging from 0.51 to 0.69). However it is interesting to notice that the quantity of H2 is rather low at 1100°C (16.6 mmol/gbiomass) while it is almost constant for higher temperatures, reaching a maximum of 25.5 mmol/gbiomass at 1400°C. As for steam gasification, the quantity of CH4 decreases with the rise of temperature thanks to the improvement of the CH4 dry reforming (Eq. 6) reaction rate, ranging from 4.8 mmol/gbiomass at 1100°C to almost 1.2 mmol/gbiomass at 1400°C. The temperature has also an influence on the CGE. Compared to steam gasification, the calculated CGE shows a smaller fluctuation range, rising from almost 0.98 at 1100°C up to 1.04 at 1300°C and then stabilizing.

Similarly to steam gasification experiments, small amounts of char were found in the outlet tubes and the filtration devices (about 0.8 g for 10 g of injected biomass).

The results obtained here are consistent with those obtained in an indirectly irradiated tubular reactor [START_REF] Bellouard | Solar thermochemical gasification of wood biomass for syngas production in a high-temperature continuously-fed tubular reactor[END_REF] .

Comparison of the reactor heating configurations

The indirectly-irradiated reactor configuration was investigated to assess the performances according to the reactor heating concept. The reactor heating is longer for the indirectly-irradiated configuration than for the directly-irradiated configuration because of the presence of the emissive plate. The maximum temperature reached was thus 1200°C (temperature inside the cavity). The Figure 6 presents the evolution of temperature inside the cavity (T3) during heating phase for both direct and 3).

The total amount of gas produced (thus carbon conversion rate) is higher for the indirectlyirradiated configuration. As a result, the carbon conversion rate ranges from 78.8% at 1100°C to 83.4% at 1200°C for direct heating, whereas it is improved with indirect heating and goes from 88.1% at 1100°C to 97% at 1200°C. This increase of the amount of produced syngas appears to come from chiefly the increase of the amount of evolved CH4 at the expense of a lower H2 production. Regarding CO, the quantities produced are the same for both direct and indirect heating configurations. The quantity of H2 is slightly lower with indirect heating configuration at 1150°C and 1200°C. The CO2 quantity is slightly higher for indirect heating, while the CH4 quantity is much higher for indirect heating. The quantity of produced H2 is lower with indirect heating configuration, certainly due to the increase of CH4 production.

The CGE in indirect heating configuration is always higher than 1 and reaches 1.15 at 1200°C chiefly because of the high calorific value of CH4 contained in the syngas. However the syngas composition is not ideal for a Fisher-Tropsch process due to the important presence of CH4. This means the particle conversion is enhanced at the expense of a lower syngas quality in the case of indirect heating. It is likely that the favored CH4 formation is the result of a lower gas residence time in the indirect heating configuration. This could be explained by a different gas flow hydrodynamic inside the cavity. Indeed, the Ar flow injected at the window cannot enter directly the cavity because of the inserted emitter plate in indirect heating configuration. Thus, it goes through the interstitial space between the cavity and the insulating material to enter the cavity preferably via the bottom and then exits the reactor, thereby increasing the gas velocity inside the cavity and reducing the residence time. This change in the gas flow hydrodynamic might have an adverse impact on the kinetic rate of the gasification process because of a lowered gas residence time. Furthermore, the use of the emissive plate as the heat transfer wall induces additional energy losses and thereby does not allow the reactor to operate above 1200°C, which represents a strong limitation for the gasification process.

Effect of oxidizing agent and stoichiometry

To completely gasify biomass feedstock with steam, a H2O/biomass molar ratio of 2 or higher is necessary (Eq. 7). The humidity content of the initial biomass feedstocks (Table 1) was included to determine the flow rates and the H2O/biomass ratio. For this study, two steam flow rates were used (mixed with 0.3 NL/min Ar as carrier gas): 200 mg/min for stoichiometric conditions and 500 mg/min for excess steam conditions (for a biomass feeding rate of 2 g/min, this second steam flow rate would be equivalent to a H2O/biomass molar ratio of 3.3). The steam was continuously injected even after the end of the biomass injection (10 g of biomass type A) to completely gasify the char.

According to Figure 7, the quantity of CO produced is higher with the lowest steam flow rate, whatever the temperature. In contrast, the quantity of CO2 produced increases with the steam flow rate due to the excess of oxygen supplied, which favors the formation of CO2 instead of CO (Water-gas shift (Eq. 4)). Likewise, the higher the steam flow rate, the higher the amount of H2 produced, thus increasing its proportion in the final syngas. As a result, the H2/CO ratio is always higher with the highest steam flow rate. At the lowest temperature, the quantity of syngas produced is larger with a lower flow rate, but a reverse trend is observed when increasing the temperature. As a result, the quantity of syngas produced is larger with the highest steam flow rate at 1300°C and the difference becomes significant at 1400°C. At a given temperature, the carbon conversion rate is approximately the same for both steam flow rates, with a maximum of 91.5% at 1400°C. Regarding CH4, no significant impact of the steam flow rate can be observed. Overall the temperature has a more significant impact on syngas production and composition than the steam flow rate. The maximum obtained CGE was 1.1 at 1400°C with a steam flow rate of 200 mg/min. At the same temperature but for a steam flow rate of 500 mg/min, the CGE value is quite close and reaches 1.09.

Regarding gasification with CO2, CO2 flow rates of 0.2 NL/min (close to the stoichiometry) and 0.6 NL/min were investigated using biomass type A as feedstock. Accounting for the biomass humidity as an oxidant agent, this second CO2 flow rate corresponds to an oxidant/biomass molar ratio of 3.2. The quantities of syngas produced during experiments at three different temperatures are presented in Figure 8 (CO2 is not represented because in excess in the feed gas).

The influence of the CO2 flow rate is much more significant than the influence of the steam flow rate. The increase of the CO2 flow rate results in enhancing the total amount of syngas produced at a given temperature. The quantity of CO produced strongly increases with the CO2 flow rate thanks to the gasification of the chars by the CO2 (Eq. 3). The carbon conversion rate is thus also increased, ranging between 81.1% at 1200°C and 85.2% at 1400°C at the lowest CO2 flow rate, and between 86.3% at 1200°C and 97.1% at 1400°C at the highest CO2 flow rate. However, the quantity of H2 produced decreases sharply when increasing the CO2 flow rate because of the reverse water-gas shift reaction (Eq. 4). A part of the initial hydrogen content of the biomass is thus converted into steam water (not measured by the gas analyzer) and additional CO is produced. To confirm this statement, thermodynamic equilibrium calculations were realized using the GEMINI software (Gibbs Energy MINImizer) in order to compute the system composition at the equilibrium state (Table 4).

According to the equilibrium composition (Table 4), the formation of H2O is favored to the detriment of H2 when an excess of CO2 is used. The water is condensed in the bubbler after exiting the reactor and it is not detected in the syngas, hence the total quantity of hydrogen in the syngas drops with the increase of the CO2 flow rate. In the case of gasification with CO2, hydrogen is brought to the reaction only through the biomass and its humidity content, and it is interesting to notice that the total amount of hydrogen (H) in the syngas (H2 and CH4 included) is constant around 56 mmol/gbiomass independently of the temperature and varies only with the increase of the inlet CO2 flow. The total input of hydrogen (H) in the reactor being 77.8 mmol/gbiomass (biomass humidity included), a large part of the input hydrogen (28%) seems to react with CO2 to form H2O and CO even with the lowest CO2 flow rate. The amounts of tar and char produced at the end of the experiments were reduced by the increase of the CO2 flow rate (about 0.3 g for 10 g of injected biomass with 0.6 NL/min of CO2 flow rate regardless of the temperature).

The type of oxidant is an important parameter influencing the reactor performances. The gasifying agent affects not only the gasification rates but also the syngas composition and the solar upgrade factor. Moreover CO2 gasification can be a way to valorize CO2 and it was thus necessary to compare the differences in syngas yield and composition when gasifying with either H2O or CO2. The results show drastically different reactor performances according to the type of oxidant. The amount of syngas is higher in the case of steam gasification regardless of the amount of oxidant and temperature.

This comes from the ability of steam to react with CO forming CO2 and H2. A part of the steam is thus consumed and integrated as H2 in the syngas. This is even more pronounced with excess steam resulting in higher CO2 production. Conversely, for CO2 gasification, the increase of the oxidant amount favors the steam generation that is not included in the syngas, and the CO production is favored over the H2 production.

Effect of biomass type and particle size

The four different types of biomass (Table 1) were steam gasified with a steam flow rate of 200 mg/min at 1100°C, 1200°C and 1300°C to compare the amounts of gas produced (Figure 9).

The influence of temperature on the syngas production is confirmed for all the biomasses. However, the effect of temperature is less pronounced for the largest particles (type B), which may be more sensitive to heat and mass transfer limitations. Besides, the observed differences in syngas productions are not very significant for the range of particle size studied. The particle size and the temperature influence both the kinetic rate of gasification and the hydrodynamic of the reactor. These parameters are essential to the gasification process as they determine the required reaction duration for complete gasification. However the effect of particle size on the gas-solid flow hydrodynamic in spouted bed reactors is not well studied, especially for the present size of particles [START_REF] Cui | Spouting of biomass particles: A review[END_REF] . Moreover, the particle size is drastically reduced as soon as the biomass is injected in the hot cavity because of the rapid reactions, which explains to a large extent the low sensitivity of the process to the initial particle size. Similar experiments were conducted (except for biomass type C) using CO2 (0.2 NL/min) as oxidizing agent (Figure 10), confirming again the low sensitivity of syngas yield and composition to the type of biomass and particle size, but the high sensitivity to the temperature.

A steep increase of the syngas production and CGE is obtained when increasing the temperature whatever the type of biomass considered for both steam and CO2 gasification, but the effect of particle size on the syngas production is negligible. This points out that increasing the particle size (thus reducing the pre-processing costs of the feedstock) is a possible option for future optimization of the process.

Biomass feeding rate influence

For a given solar power input, a trade-off in the biomass feeding rate may exist since a too high feeding rate would cause a drop of the cavity temperature because of insufficient energy input, thus reducing the kinetic rate of the gasification reaction and eventually leading to solid reactant accumulation inside the reactor. On the contrary a too low feeding rate would result in complete conversion but limited syngas production, thereby leading to incomplete utilization of the solar energy input. The effect of the biomass feeding mass flow rate on the syngas quality was thus investigated. A given amount of biomass (10 g of biomass type B) was injected continuously and gasified at 1300°C using steam (200 mg/min) as oxidizing agent. During the first test (Figure 11a), the feeding rate was ~0.65 g/min and the feedstock was completely injected after less than 16 min. For the second test (Figure 11b), a higher feeding rate was used (close to 1 g/min).

Despite a clear difference in the syngas production rate, a similar amount of syngas is produced regardless of the biomass feeding rate (Table 5) and the carbon conversion rate remains identical (80.4%).

However, the amount of each gas and the syngas composition are changed significantly. A higher feeding rate favors H2 and CO production to the detriment of the other gas species. The higher production of CO2 at low feeding rate can be explained by the excess of steam with respect to biomass induced by the lower feeding rate of biomass. This induces a higher quantity of CO2 produced at the expense of the CO production, as previously observed on the effect of oxidizing agent flow rate (Figure 7). With the highest feeding rate the quantity of CH4 also diminishes, which might explain the increase of H2 production. The question of the energy efficiency of the reactor is raised. The solar-tofuel energy conversion efficiency of the reactor represents the fraction of the energy input (both solar and calorific content of feedstock) converted into the chemical energy of the syngas produced. It was increased from 10.2% to 16.5% when increasing the biomass feeding rate from 0.65 to 1 g/min (same total amount injected: 10 g), because the solar energy input was inherently lowered when reducing the injection duration. For 2 g/min of fed biomass (610W of power content), the gas power output would be 671W for a typical CGE of 1.10, yielding an energy conversion efficiency of 32% for the 1.5 kW scale reactor. A low feeding rate reduces the solar-to-fuel energy conversion efficiency, but a too high feeding rate would cause a drop of reactor temperature, thus lowering both the particle conversion to syngas and the syngas quality. Consequently, the reactant feed flow rate must be optimized to match the rate of the chemical reaction. A trade-off may thus exist between maximum allowable amount of injected feedstock and maximum chemical conversion to syngas.

Effect of delayed injection of gasifying agent

Biomass pyrolysis followed by char gasification (delayed oxidizing agent injection) was performed to quantify separately the amount of syngas produced during pyrolysis and gasification phases. Biomass (10 g of type A) was injected at 1300°C during 7 min in Ar atmosphere (pyrolysis step). Once no more gas was produced from the flash pyrolysis, the oxidizing agent (either H2O or CO2) was then injected to gasify the char. The time evolution of the syngas composition obtained with steam is shown in Figure 12 and the averaged time-integrated syngas composition in Figure 13a.

Conclusion

An innovative spouted bed solar reactor was successfully operated for the gasification of wood at high temperature. The influence of each operating parameter on the syngas yield was investigated during continuous injection of given amounts of biomass. The gas yields were improved with the rise of temperature (from 1100°C to 1400°C) and to a lesser extent with the injection of the oxidizing agent (H2O or CO2) in over-stoichiometric proportion. The size of the particles did not show any significant impact on the gas yields for the range of particle size considered (0.3 to 2 mm). High carbon conversion rates, up to 97.1 %, were achieved. During this experimental study, a high-quality syngas was produced with a typical energy upgrade factor of 1.10, meaning a 10% improvement of the initial biomass energy content by the solar energy input. The reactor was operated according to different heating modes with either direct heating of the reacting particles or indirect heating via an opaque heat transfer wall. The indirectly-irradiated configuration requires further investigation to reach higher upper operating temperatures and optimize the gas residence time inside the cavity. This parametric study conducted on a new solar reactor concept provides several insights for further optimization, which should be focused on determining the optimal biomass feed mass flow rate and gas flow rate (residence time) to maximize the thermochemical reactor performances during continuous solar operation. 33 (Table 1)

Figure 10: Quantity of gas produced during gasification with CO2 (CO2 flow rate: 0.2 NL/min) of different types of feedstock (Table 1)

  reactions: 𝐶 + 𝐻 𝑂 → 𝐶𝑂 + 𝐻 ∆H°1 = 131.3 kJ/mol (1) 𝐶 + 2𝐻 → 𝐶𝐻 ∆H°2 = -74.6 kJ/mol (

  aperture is positioned at the focal point of the parabolic mirror to maximize the incoming flux. The alumina cap and the top of the insulating ceramic (which are at the same level) are protected by a 2 mm layer of zirconium fiber felt. An additional layer of graphite (2 mm thick) with a 15 mm aperture is placed around the alumina cap aperture to protect the rest from the highly concentrated flux during the adjustment of the reactor position. The stainless steel reactor shell (15 cm i.d., 17 cm o.d.) is watercooled at the bottom and the sides, and a transparent hemispherical glass window is used to operate in controlled atmosphere. The window is hermetically attached to the shell, thus separating the cavity from the ambient air. An argon protective flow (2 NL/min) is continuously injected inside the window area to prevent it from being soiled by gasification residues. It is also possible to operate the reactor in an indirectly-irradiated configuration to prevent particle deposition on the window. A graphite plate on top of a SiC plate (2 mm thick) is inserted between the cavity and the alumina cap (17 mm from the aperture) to absorb the solar power and heat the cavity by re-radiation. The graphite plate (not in contact with the oxidant injected in the lower cavity) offers a good protection against thermal shocks and separates the upper cavity receiving solar radiation from the lower cavity receiving biomass feedstock.

14 indirect

 14 heating configuration. Temperature below 200°C could not be measured by the thermocouple and the 0 on the time axis corresponds to the moment when the trap door is opened. It takes less than 35 min to reach 1200°C with direct heating and 52 min to reach 1400°C, which is the maximum operating temperature of the reactor in this configuration. With indirect heating, reaching the maximum operating temperature (1200°C) takes 61 min. Experiments of steam gasification (steam flow rate: 200 mg/min) were carried out at 1100°C, 1150°C and 1200°C with the indirectly-irradiated configuration and compared with the directly-irradiated configuration under the same conditions (Table

Figure 1 :

 1 Figure 1: Experimental set-up: 1 Cavity; 2 Alumina cap; 3 Emissive plate; 4 Insulating material; 5 Cooled hull; 6 Window; 7 Pyrometer; 8 Gas and H2O mass flow controller ; 9 Tilted hopper and feeding screw; 10 Driving motor; 11 Bubbler; 12 Cartridge filter; 13 In-line gas analyzer; 14 Gas volume counter; 15 Sampling bag; 16 Chromatograph

Figure 4 :

 4 Figure 4: Comparison of the quantity of gas produced per gram of dry biomass during steam gasification (steam flow rate: 200 mg/min) at 1100°C, 1200°C, 1300°C and 1400°C and the evolution of the Cold Gas Efficiency.

Figure 5 :

 5 Figure 5: Comparison of the quantity of gas produced per gram of dry biomass during gasification with CO2 (CO2 flow rate: 0.2 NL/min) at 1100°C, 1200°C, 1300°C and 1400°C and the evolution of the Cold Gas Efficiency

Figure 8 :

 8 Figure 8: Influence of CO2 flow rate (0.2 NL/min in 0.3 NL/min Ar-40% CO2, and 0.6 NL/min-100% CO2) and temperature on syngas production.

Figure 9 :

 9 Figure 9: Quantity of gas produced during steam gasification (steam flow rate: 200 mg/min) of different types of feedstock

Figure 11 :

 11 Figure 11: Influence of biomass feeding rate on gas species mole fraction during steam gasification at 1300°C for biomass feeding rates of (a) 0.65 g/min and (b) 1 g/min.

Figure 13 :

 13 Figure 13: Comparison of the quantities of gas produced during steam gasification with continuous and delayed steam injection at 1300°C and 1200°C.

Figure 15 :

 15 Figure 15: Comparison of the quantities of gas produced during gasification with CO2 at 1300°C with continuous and delayed CO2 injection

Table 1 :

 1 Dry composition and characteristics of the different biomasses used for the experiments

		Wood specie	C (w%)	H (w%)	Dry composition O N (w%) (w%)	S (w%)	Ashes (w%)	Humidity (w%)	LHV (MJ/kg)	Mean particle size (mm)
	Type A	Beech	48.3	6.7	44.4 0.11 <0.1 0.46	8.9	18.29	1
	Type B Resinous mix 52.3	7.2	40.1 0.09 <0.1 0.28	9.2	17.4	2
	Type C Resinous mix 49.9	7.1	42.4 0.12 <0.1 0.46	7.3	17.66	0.55
	Type D	Beech	52.8	7.1	40.7 0.14 <0.1 0.29	9.3	18.3	0.3

Table 3 :

 3 Comparison of the quantity of gas obtained from steam gasification (200 mg/min) in the directly and indirectlyirradiated reactor.

	Temperature	Heating configuration	CO	Production (mmol/gbiomass) CO2 CH4 H2	Total	Cold Gas Efficiency
	1100°C	Direct Indirect	22.05 22.46	3.79 5.75	5.21 6.89	22.70 24.99	53.75 60.08	0.90 1.02
	1150°C	Direct Indirect	23.34 24.47	3.90 4.58	4.43 8.98	26.73 23.28	58.40 61.31	0.94 1.12
	1200°C	Direct Indirect	25.81 25.94	3.64 4.74	2.21 8.04	31.69 27.22	63.35 65.94	0.96 1.15

Table 4 :

 4 Calculated thermodynamic equilibrium composition of the syngas obtained at 1300°C (similar results were obtained at 1200°C and 1400°C)

	Low CO2 flow rate	High CO2 flow rate
	(0.2 NL/min)	(0.6 NL/min)
	CO	55.09 %mol	CO	56.25 %mol
	H2	44.89 %mol	H2	31.06 %mol
	Other < 0.02 %mol	H2O	7.82 %mol
			CO2	4.83 %mol
			Other	< 0.05 %mol

Table 5 :

 5 Quantities of gas produced during steam gasification at different biomass feeding rates (same total amount injected:

	10 g)
			Quantity of gas produced (mmole)
		CO	H2	CO2	CH4	CnHm	Total
	Low feed rate 228.7	287.0	41.3	19.8	2.0	578.8
	High feed rate 248.1	307.2	26.9	17.0	1.4	600.6
	Difference	7.85%	6.55% -53.45% -16.63% -41.10% 3.63%

Figure 12: Comparison of the evolution of the mole fraction of H2, CO, CO2 and CH4 in the syngas (argon included) during steam gasification (1300°C) at different steam flow rates and during pyrolysis with delayed steam injection.

Figure 14: Comparison of the evolution of the mole fraction of H2, CO, CO2 and CH4 in the syngas (argon included) during gasification with CO2 (1300°C) at different CO2 flow rates and during pyrolysis with delayed CO2 injection.

Acknowledgments

This study was financially supported by ADEME (The French Environment and Energy Management Agency) and CEA (French Alternative Energy and Atomic Energy Commission). The study was also partly funded by the Carnot Institute "Energies du Futur" (REACSOL project 2014-2016). The authors thank S. Ravel and M. Grateau for her helpful expertise and support on biomass characterization, P.E. Frayssines and J. Cigna for the manufacturing of the reactor and R. Joubert and O. Prevost for their technical expertise and help during the experimental campaign.

The quantities of syngas produced during pyrolysis are lower than those produced during steam gasification (with continuous injection of steam). When steam is injected to gasify char (case of delayed steam injection), a second phase of gas production is observed with an almost equimolar CO and H2 production (Eq. 1). CH4 production only occurs during pyrolysis phase. Regarding CO2, even if a small quantity is produced after delayed steam injection, the total amount of CO2 produced is smaller than for steam gasification with continuous steam injection (Figure 13a). Moreover, the total quantity of syngas produced is smaller in the case of delayed steam injection. This difference is explained by a lower carbon conversion rate in the case of the delayed steam injection (80%) when compared with continuous gasification (about 85%). The same experiment was realized at 1200°C and similar results and trends were observed (Figure 13b).

A further experiment was carried out at 1300°C using CO2 and the comparative results for delayed CO2 injection (0.2 NL/min) and continuous injections (0.2 NL/min and 0.6 NL/min) are represented in Figure 14 and Figure 15. Similarly to delayed steam injection, the production of CH4 only occurs during pyrolysis phase. This is also the case of H2 production for delayed CO2 injection, which denotes that all the hydrogen contained in the biomass is volatilized during the pyrolysis phase.

Only CO is produced after delayed CO2 injection (according to Eq. 3). Regarding the peak of CO production (at time = 14 min) of the continuous CO2 injection experiment (0.6 NL/min), it is due to the sudden fall of some biomass particles that were stuck at the tip of the screw. The fact that no H2 is produced at this moment shows that the biomass had already undergone pyrolysis and was reduced to char in the screw. Regarding CH4, the highest quantity of CH4 is produced during the experiment with delayed CO2 injection, as opposed to the case of steam. This is due to the fact that without CO2, the dry reforming of methane (Eq. 6) cannot occur. In contrast, steam reforming could occur because of the humidity contained in the biomass. Meanwhile, the absence of CO2 allows reaching a higher production of H2 because the reverse water-gas shift reaction (Eq. 4) cannot occur. These experiments bring interesting insights into the chemical mechanism of gasification process.