Jun Dong

Ange Nzihou

Yong Chi
email: chiyong@zju.edu.cn

Elsa Weiss-Hortala

Mingjiang Ni

Nathalie Lyczko

Yuanjun Tang

Marion Ducousso

Hydrogen-Rich Gas Production from Steam Gasification of Bio-char in the Presence of CaO

Keywords: Calcium oxide, Steam gasification of biochar, CO 2 absorption, Hydrogen production, Fluidized bed

The steam gasification of bio-char to produce hydrogen-rich gas with CaO as in-bed catalyst is experimentally investigated in a fluidized bed reactor. The purpose of the study is twofold: (1) evaluate the optimal operating conditions of catalytic gasification for hydrogenrich gas production, by examining catalyst activity and steam flowrate; (2) optimize the process by investigating the potential for reducing the gasification temperature. Results reveal that increasing the CaO/wood mass ratio from 0 to 1.0 is effective for both H 2 improvement and tar reduction; this is attributed to enhanced CO 2 absorption and reforming reactions. Increasing the steam flowrate is beneficial towards H 2 production via the water gas shift reactions. A maximum H 2 concentration is achieved at a steam flowrate of 160 g/h; however, excessive steam injection will lead to an opposite effect, due to decreased available heat. The reaction temperature should be balanced between the gasification performance and CaO carbonation ability, and the optimal temperature is found to be 700 °C in this study. Comparing the results with non-catalytic high-temperature gasification, 50% CaO addition at 700 °C could reach as high H 2 yield as that gasified at 800 °C without catalyst. Meanwhile, the use of CaO also optimizes the gasification performance with a much higher H 2 concentration, a higher H 2 /CO ratio and reduced tar yield. Therefore, it is concluded that 50% CaO addition as catalyst at 700 °C is effective to reduce the steam gasification reaction temperature by 14% (ca. 100 °C).

Introduction

With increasing demand of energy and the environmental impact from fossil fuel use, energy recovery from waste biomass such as forestry residue and agricultural waste is gaining increasing attentions. Thermal treatment is the most common conversion method to upgrade biomass energy quality [START_REF] Zhong | A comparative experimental study on the liquefaction of wood[END_REF], among which pyrolysis is regarded as an economical and promising process to obtain high energy efficiency and low emissions [START_REF] Bridgwater | A techno-economic comparison of power production by biomass fast pyrolysis with gasification and combustion[END_REF]. By pyrolysis, biomass is converted into bio-oil, bio-char and gaseous fraction. In this regard, gaseous and liquid products are suitable to be used as fuels for feeding efficient energy generation devices, or as chemicals. On the other hand, bio-char could also achieve utilization rather than being disposed of, such as burned for energy recovery [START_REF] Scala | Fluidized bed combustion of a biomass char (Robinia pseudoacacia)[END_REF], prepared as activated carbon [START_REF] Tay | Preparation and characterization of activated carbon from waste biomass[END_REF], or used in soil amendment [START_REF] Lee | Characterization of biochars produced from cornstovers for soil amendment[END_REF].

Recently, gasification of bio-char for hydrogen-rich gas production has gained increasing interest as a potential energy utilization route of the bio-char. With steam introduced as gasification agent, bio-char exhibits effective ability to produce syngas with a high hydrogen content, which is regarded as a clean, low carbon future source of energy. Compared with raw biomass or wastes, the char produced from pyrolysis significantly exhibits a reduced volatile and oxygen content; in which fixed carbon content is largely accumulated [START_REF] Beis | Fixed-bed pyrolysis of safflower seed: influence of pyrolysis parameters on product yields and compositions[END_REF]. Such property makes char highly reactive towards steam gasification. Thus H 2 -rich gas can be obtained by adjusting the steam to char ratio [START_REF] Klaas | Optimisation of hydrogen production by steam reforming of chars derived from lumber and agricultural residues[END_REF], offering possibilities for a higher H 2 /CO ratio to satisfy its further utilization [START_REF] Yan | Hydrogenrich gas production by steam gasification of char from biomass fast pyrolysis in a fixed-bed reactor: influence of temperature and steam on hydrogen yield and syngas composition[END_REF].

Calcium oxide (CaO) has been widely used as an in-bed catalyst in biomass gasification, thanks to its low cost and abundant availability, in order to further improve hydrogen production from syngas [START_REF] Acharya | An investigation into steam gasification of biomass for hydrogen enriched gas production in presence of CaO[END_REF][START_REF] Widyawati | Hydrogen synthesis from biomass pyrolysis with in situ carbon dioxide capture using calcium oxide[END_REF]. The loaded CaO to the gasifier takes effect via several pathways. Firstly, CaO acts as a sorbent for CO 2 absorption to form CaCO 3 . Following the decrease in CO 2 partial pressure, the water gas shift reaction is enhanced towards hydrogen production. Secondly, since the CaO carbonation reaction is exothermic, the releasing heat could also provide necessary energy to facilitate the endothermic gasification process [START_REF] Han | Influence of CaO additives on wheat-straw pyrolysis as determined by TG-FTIR analysis[END_REF].

Tar formation is a major issue concerning biomass gasification since it may decrease the system efficiency and even block and damage downstream gasifier equipment [START_REF] Kuo | Woody waste air gasification in fluidized bed with Ca-and Mg-modified bed materials and additives[END_REF]. Another potential advantage of the CaO additives is the possibility to reduce tar. As it has been reported [START_REF] Zhou | Effect of calcium oxide on high-temperature steam gasification of municipal solid waste[END_REF], CaO could catalytically contribute to tar cracking and accelerate the reaction between tar and H 2 O, therefore it significantly reduces the tar content of the product gas from a gasifier.

Considering the catalytic effect of CaO on gasification, it is thus proposed that in situ CaO additive could serve as an efficient approach to reduce the steam gasification temperature compared with the non-catalytic process. Actually, the current steam gasification is mostly carried out at a relatively high temperature above 1000 °C to ensure the gasification performance [START_REF] Shuai | The synergistic effect of Ca(OH)2 on the process of lignite steam gasification to produce hydrogen-rich gas[END_REF]. Such high reaction temperature leads to a lower energy efficiency and poor economy. On the other hand, due to the ability on H 2 enhancement and tar reduction, the CaO catalytic gasification offers the possibility to obtain as high quality as the syngas produced by non-catalytic gasification, thus substantially lowers the required gasification temperature. Ohtsuka [START_REF] Ohtsuka | Steam gasification of coals with calcium hydroxide[END_REF] studied the reactivity of coal by steam gasification, indicating that the reaction temperature could be decreased by 110-150 °C in the presence of Ca(OH) 2 . This value was obtained at ca. 150 °C by the study of Dalai et al [START_REF] Dalai | Catalytic gasification of sawdust derived from various biomass[END_REF], when using CaO as catalyst on biomass steam gasification. However in the field of bio-char catalytic steam gasification, study on the reaction temperature reduction potential is still limited; therefore conducting such an assessment is significantly meaningful.

Therefore, steam gasification of bio-char in a fluidized bed reactor has been investigated in this study, using CaO as in-bed catalyst. The purpose of the study is twofolds: [START_REF] Zhong | A comparative experimental study on the liquefaction of wood[END_REF] to evaluate the optimal working condition of catalytic gasification for hydrogen-rich gas production; (2) investigate the potential of reducing the gasification temperature by using catalyst. The influence of CaO catalyst activity, steam flowrate and reaction temperature is studied. The knowledge acquired would assist in the industrial application bio-char gasification technique and develop a more effective and economical steam gasification process in converting biomass to energy.

Materials and Methods

Samples and Catalyst

Poplar wood chip, which is one of the most representative types of biomass, is used as the feedstock in this study. It is produced by grounding the raw poplar wood into sizes of ca. 4 mm 9 4 mm 9 1 mm. This results in a relatively homogeneous starting material for experiments. The proximate and ultimate analysis of the wood are shown in Table 1. Compared with other commonly used biomass such as corn stover or switchgrass, poplar wood has a high calorific value (17.87 MJ/kg, lower heating value, LHV) and a low sulfur content (0.64% of dried wt) [START_REF] Klinghoffer | Utilization of char from biomass gasification in catalytic applications[END_REF]. Calcium oxide (CaO) is adopted as in-bed catalyst, which is purchased according to the Chinese national standard ''GB/T 15897-1995'' with 99% purity. The CaO reagent is sieved beforehand and a same fraction of 0.28-0.45 mm is used. Its specific surface area is 2.38 m 2 /g, as measured in an ASAP 2010 apparatus.

Experimental Apparatus and Procedures

Gasification experiments are performed in a laboratory batch-scale fluidized bed reactor. Schematic diagram of the apparatus is depicted in Fig. 1. The furnace is made of stainless steel, with the main reaction chamber 600 mm in height and 60 mm in inner diameter [START_REF] Klinghoffer | Catalyst properties and catalytic performance of char from biomass gasification[END_REF]. Ten thermocouples are employed to monitor the temperature profile throughout the reactor in the vertical direction. The reactor is heated externally up to 1200 °C; and the temperature is controlled by a thermocouple placed close to the middle of the reactor. Two frits are equipped on the bottom and top of the furnace, respectively. The bottom one is to hold the feedstock; while the top one is to prevent char escaping from the reactor. Steam acts as the gasifying agent, which is provided by steam generator from the bottom of the furnace at controlled flowrate. To prevent tar condensing from the flue gas, a heating tape is installed in outlet pipe to maintain the temperature above 200 °C.

The experimental procedures are divided into two steps: bio-char production and steam gasification, which is consecutive throughout the whole reaction. At the beginning of each test, a total of 80 g poplar wood chip is mixed with CaO catalyst at specified mass ratios and put into the furnace. Afterwards the furnace is heated to the desired temperature at a constant heating rate of 20 °C/min. Nitrogen is introduced as the purge gas at a steady flowrate of 0.6 Nm 3 /h. Therefore the bio-char is actually produced under pyrolysis condition. The properties of the obtained bio-char (at 700 °C) are shown in Table 1. It could be found that compared with the raw poplar wood, the volatile matter of the bio-char is decreased to a large extent, accompanied by a significantly increased ratio of fixed carbon content. This effectively results in an increased energy density of the material, i.e., the LHV of the bio-char is increased from 17.87 to 30.52 MJ/kg. And the yield of bio-char at this reaction temperature is 0.24 g/g-wood.

Once the desired temperature is reached, steam is injected to react with the pyrolytic bio-char. From this point the steam gasification step starts, and the product gas begins to be collected by gas sampling bag. The raw flue gas passes through a series of three impingers immersed in an icewater bath for tar removal and excess steam condensation. The first and second bottles are filled with water; and the third one is empty for moisture removal. The gasification is maintained for a period of 15 min. After that, the steam and the electrical heater are turned off but N 2 is still supplied until the furnace reaching room temperature.

The effect of steam CaO additive, flowrate and temperature on gasification characteristics is investigated. The amount of CaO additive loaded is 0, 16, 40 and 80 g, corresponding to the CaO/wood mass ratio of 0, 0.2, 0.5 and 1. Steam flowrate varies from 80 to 320 g/h, and a temperature series of 600, 650, 700, 750 and 800 °C is examined. In order to verify the potential effect of CaO on reducing the gasification operating temperature, non-catalyst high-temperature steam gasification at 800 and 900 °C is carried out as comparison. For each trial, critical mass balance calculation is conducted to ensure the reliability of test data.

Methods of Sampling and Analysis

Syngas components including H 2 , CO, CO 2 , CH 4 and N 2 are measured. The gas sampling is analyzed using gas chromatography (micro GC-3000A analyzer, Agilent). The total gas yield (Nm 3 /kg-wood) is quantified according to a Wood bio-char is produced through pyrolysis in N 2 atmosphere to a temperature of 700 °C at a constant heating rate of 20 °C/min b For proximate analysis, M, V, FC and A represents moisture, volatiles, fixed carbon and ash, respectively N 2 balance, based on the known N 2 flowrate and the molar ratio of N 2 in the produced gas [START_REF] Zhou | Effect of calcium oxide on high-temperature steam gasification of municipal solid waste[END_REF]. The liquid in the impingers are extracted by dichloromethane solution and separated into organic and inorganic fractions. The organic part is then heated for dichloromethane evaporation and the remaining fraction is recorded as the tar yield [START_REF] Kihedu | Performance indicators for air and air-steam auto-thermal updraft gasification of biomass in packed bed reactor[END_REF][START_REF] Van Paasen | Guideline for sampling and analysis of tar and particles in biomass producer gases. Final report documenting the Guideline[END_REF]. The produced solid residue is collected from the bottom of the furnace and weighted. X-ray diffraction (XRD) measurements of the solid product are carried out to determine main components and investigate the catalytic performance of the experiment.

Results and Discussion

According to the aim of the study, this section is divided into two parts. ''Effect of Operating Conditions on Catalytic Steam Gasification'' section investigates the effect of operating conditions on the performance of catalytic steam gasification, where the catalyst activity and steam flowrate is examined. Based on the results obtained, ''Catalytic Steam Gasification Process Optimization'' section mainly focuses on process optimization. Catalytic steam gasification under different temperatures is conducted, and the results are further compared with non-catalyst gasification cases to verify the potential of reducing the gasification temperature.

Effect of Operating Conditions on Catalytic Steam Gasification

Catalyst Activity

The first operating conditions investigated are focused on the effect of catalyst loading. Figure 2 illustrates the effect of CaO/wood mass ratio (as x axis) as a function of the gas composition and H 2 yield (as y axis), respectively; which is studied at steam flowrate of 160 g/h and temperature of 700 °C. The CaO/wood mass ratio studied ranges from 0 to 1, which actually corresponds to CaO/char mass ratio at 0, 0.84, 2.11 and 4.21, as illustrated by another x axis in Fig. 2. Results from Fig. 2 show that the H 2 yield is increased rapidly from 0.06 Nm 3 /kg of wood in the absence of CaO to 0.12 Nm 3 /kg of wood at CaO/wood mass ratio of 1.0. It could be also noticed that the H 2 yield is increased quite linearly with the increase of the ratio up to 0.5. At the same time, the H 2 concentration is elevated to 64.2%, following a linear trend. The results indicate that CaO catalyst enhances the H 2 production (volume produced) and increases the quality of the syngas (higher H 2 concentration in the gas), as expected.

It has been proven that during steam gasification process, the final gas composition is a result of several complex and competing reactions. The important reactions involved in steam gasification are listed as follows, considering the actual elemental composition of the bio-char according to Table 1.

Stoichiometric steam reforming reaction:

C 6:60 H 2:77 O 0:56 þ 6:04H 2 O $ 6:60CO þ 7:425H 2 ð1Þ
Water gas reaction:

C þ H 2 O $ CO þ H 2 À 131 kJ/mol ð2Þ
Water gas shift reaction:

CO þ H 2 O $ CO 2 þ H 2 þ 41 kJ/mol ð3Þ
Boudouard reaction: C þ CO 2 $ 2CO À 162:4 kJ/mol ð4Þ
Tar reforming reaction:

Tar þ H 2 O À! CaO H 2 þ CO 2 þ CO þ hydrocarbon þ Á Á Á ð5Þ
According to the stoichiometric reaction from Eq. 1, a complete steam gasification of the bio-char should result in a high amount of CO and H 2 , and a H 2 /CO ratio of 1.125. Figure 2 verifies that the high H 2 concentration reaches 43.6% without catalyst. Nevertheless the CO concentration is quite low, representing one third of CO 2 value. This is mainly attributed to the water gas shift reaction (Eq. 3). In the presence of excessive water during steam gasification, CO 2 could be produced in higher amount as a result of CO oxidation. Meanwhile, Fig. 2 shows also that with the increasing CaO loading, the increase in H 2 concentration is accompanied with a significant decrease in CO 2 concentration. This decrease is also quite linear, indicating that the CaO concentration is involved in the amount of CO 2 measured in the gas phase.

To better verify the catalytic mechanism, CO 2 equilibrium is conducted. According to the involved reactions, the production of CO 2 could be attributed to the water gas shift reaction (Eq. 3) and tar reforming reaction (Eq. 5); while Boudouard reaction (Eq. 4) is the main contributor to CO 2 consumption. The mechanism of CO 2 absorption by CaO is via carbonation reaction Eq. 6, which could further lower the CO 2 partial pressure and enhance the water gas shift reaction to produce more H 2 [START_REF] Acharya | An investigation into steam gasification of biomass for hydrogen enriched gas production in presence of CaO[END_REF]. Previous studies have indicated that the catalytic effect of CaO is via two sides: to serve as CO 2 acceptor, as well as to participate in the tar reforming reactions [START_REF] Jordan | Effect of CaO on tar production and dew point depression during gasification of fuel cane bagasse in a novel downdraft gasifier[END_REF]. From results in Fig. 2, the concentration of CO and CH 4 exhibits an increasing trend at higher CaO/wood ratios, which could be attributed to the facilitated tar reforming reaction. Nevertheless, the changed tendency is not as obvious as the linearly decreased CO 2 concentration. Thus, it could be speculated that the dominant catalytic mechanism of CaO at the reaction conditions is via CO 2 absorption, mainly due to that the contact frequency between CO 2 gas and CaO sorbents is improved with the increasing CaO amount, and in the fluidized bed.Carbonation reaction

CaO þ CO 2 ! CaCO 3 þ 178 kJ/mol ð6Þ
The gasification performance under different CaO/wood mass ratios is presented in Fig. 3. Increasing the CaO ratio, the H 2 /CO shows an increasing trend from 3.36 to 5.99. The peak value of 6.16 appears at CaO/wood mass ratio of 0.5; thereafter the H 2 /CO ratio exhibits a slight decrease since the increase in H 2 production is not as obvious as CO. This is companied by the decline in the tar yield from 22.4 to 3.2 g/Nm 3 . Especially, the concentration of tar could be effectively reduced by 60% under CaO/wood ratio of 0.2 compared with that in the absence of CaO. It is also observed that the color of the collected tars turns from deep brown into light yellow when increasing CaO/wood ratio from 0 to 1, indicating that the tar reforming reaction (Eq. 5) is indeed enhanced by CaO catalyst. Research by Han [START_REF] Han | Influence of CaO additives on wheat-straw pyrolysis as determined by TG-FTIR analysis[END_REF][START_REF] Han | Hydrogen production via CaO sorption enhanced anaerobic gasification of sawdust in a bubbling fluidized bed[END_REF] proved that CaO could catalyze the reduction reactions of tar species such as toluene, phenol and formic acid. Besides, recent studies also reported that the O 2-ions formed as the active sites on CaO could disrupt the stability of the p-electron cloud of tar species and finally leads to the splitting of the aromatic rings [START_REF] Shuai | The synergistic effect of Ca(OH)2 on the process of lignite steam gasification to produce hydrogen-rich gas[END_REF][START_REF] Udomsirichakorn | Effect of CaO on tar reforming to hydrogen-enriched gas with in-process CO 2 capture in a bubbling fluidized bed biomass steam gasifier[END_REF]. For practical application, it is evident that the tar content should be down to a lower level to avoid the damage the downstream energy conversion device. Elimination of tar inside the gasifier seems to be an optimal method, since removing the tar from the syngas by means of a wet physical method owes the drawback of losing the chemical energy [START_REF] Arena | Process and technological aspects of municipal solid waste gasification[END_REF]. In this sense, using CaO as catalyst could be served as a potential approach for both H 2 -rich and tar-free gas production.

Influence of Steam Flowrate

Figure 4 presents the influence of steam flowrate on H 2 production and syngas composition at CaO/wood of 0.2 and 700 °C, using the varied steam flowrate of 80-320 g/h (equal to 1.05-4.21 g/g-char) as x axis and the gas composition and H 2 yield as y axis, respectively. Under such test conditions, the speed of steam is in a range of 0.09-0.20 m/s. Results indicate that with the initial increase in steam flowrate from 80 to 160 g/h, the H 2 yield is remarkably increased from 0.059 to 0.079 Nm 3 /kg of wood. However the H 2 yield will drop to 0.063 and 0.047 Nm 3 /kg of wood when further increase the steam flowrate to 240 and 320 g/h, respectively; corresponding to a decreased rate of 20 and 41%. In regard to the syngas composition, the H 2 concentration exhibits a similar trend of first increase and then decrease; the maximum H 2 content reaches 50.9% as steam flowrate is 160 g/h. The evolution of CH 4 is opposite although in a relatively low concentration.

The increased steam quality could effectively enhance the concentration of steam in reactor to facilitate steam reforming reactions (Eqs. 1-3), which improves the H 2 yield and concentration in the syngas. It could be verified by the decreased concentration of CH 4 . It is also suggested that CaO reveals better catalytic performance at the adequate presence of steam. Manovic et al. [START_REF] Manovic | Steam hydration of sorbents from a dual fluidized bed CO 2 looping cycle reactor[END_REF] concluded that steam hydration would efficiently promote the carbonation reactivity of CaO, which is helpful to further shift the thermodynamic equilibriums of water gas (Eq. 2) and water gas shift (Eq. 3) reactions towards the formation of H 2 . Nevertheless when the steam flowrate further exceeds 160 g/h, a declined trend of H 2 yield and concentration is obtained. Acharya et al. [START_REF] Acharya | An investigation into steam gasification of biomass for hydrogen enriched gas production in presence of CaO[END_REF] and Li et al. [START_REF] Li | Experimental study on catalytic steam gasification of municipal solid waste for bioenergy production in a combined fixed bed reactor[END_REF] also observed a decreased H 2 production at higher steam flowrates (H 2 O/ MSW mass ratio of 1.33-2.66 and H 2 O/C mass ratio of 0.83-1.58 in their studies, respectively). This phenomenon could probably due to that at higher steam flowrate, the amount of steam is sufficient inside the furnace and is no longer the rate limiting step. Then the further increase in steam flowrate would not facilitate the reaction; on the contrary this will result in the decreased available heat in the reactor by the absorption of excessive steam.

The products characteristics as a function of steam flowrate are shown in Table 2. The product distribution of syngas, tar and char exceeds 100% due to the reaction of steam. With steam flowrate increasing from 80 to 320 g/h, the char yield decreases gradually from 86.7 to 85.4%, which is explained by Paviet [27] that a higher steam molar fraction has resulted in a lower diffusion resistance of the bio-char. However, the production of char turns to have a slightly rise when further increase the steam flowrate to 240 and 320 g/h, since the primary decomposition of char is inhibited at a lower reaction temperature. Similarly, the tar concentration reaches its minimum of 8.9 g/Nm 3 at steam flowrate of 160 g/h; and then slightly increases to 9.6 g/Nm 3 .

The H 2 /CO molar ratio shows an increasing trend, in agreement with the results of He [START_REF] He | Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): influence of steam to MSW ratios and weight hourly space velocity on gas production and composition[END_REF]. It could be attributed to the gradually decreased CO concentration as well as the increased H 2 content at lower steam flowrate. As a consequence, the maximum H 2 /CO value of 5.30 appears under steam flowrate of 160 g/h. Previous researches have concluded that syngas with different levels of H 2 /CO ratio are suitable for different applications. The high range of H 2 /CO ratio could open application for syngas in chemical industry, for ammonia synthesis or for fuel cell applications [START_REF] De Filippis | Prediction of syngas quality for two-stage gasification of selected waste feedstocks[END_REF].

Catalytic Steam Gasification Process Optimization

Based on the results obtained, the next step of using CaO as catalyst is focused on the process optimization.

Considering the combined effects of catalyst loading and steam flowrate on increasing both the H 2 yield and concentration, it is worth investigating if catalytic steam gasification could be served as potential approach to reduce the gasification temperature. To verify the possibility, firstly, catalytic steam gasification under different temperatures is examined.

Figure 5 gives the influences of temperature on H 2 production at CaO/wood of 0.2 and steam flowrate of 160 g/h. The amount of CaO loading and steam flowrate is chosen based on the results obtained and discussed in the previous sections. It is obvious that temperature has played a crucial impact on the gasification process. The H 2 concentration and yield in the syngas is significantly climbed from 0.006 Nm 3 /kg of wood and 18.3% at 600 °C to 0.079 Nm 3 /kg of wood and 50.9% at 700 °C, respectively. According to Le Chatelier's principle, higher temperature provides more favorable conditions for the endothermic reactions as water gas reaction (Eq. 2) and steam reforming reaction (Eq. 1), which are responsible for the accelerated H 2 production and concentration output. This speculation could be evidenced by the gradually increased CO concentration with temperature. The enhanced tar cracking (Eq. 5) could be the second reason for the promoted H 2 production at a higher temperature, which is effectively indicated by the decreasing concentration of CH 4 in the syngas.

However, it is found that the H 2 concentration decrease when further rises the reaction temperature from 700 to 800 °C. This is accompanied by a gradual increase of CO 2 concentration to 39.5% at 800 °C. Previous studies have revealed that the absorption ability of CaO catalyst is related to the reaction temperature and CO 2 equilibrium partial pressure, i.e., carbonation reaction and its reverse calcination reaction occurs under different thermodynamic limitations of the system [START_REF] Florin | Enhanced hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents[END_REF][START_REF] Florin | Hydrogen production from biomass coupled with carbon dioxide capture: the implications of thermodynamic equilibrium[END_REF][START_REF] Blamey | The calcium looping cycle for large-scale CO 2 capture[END_REF]. Figure 6 exhibits the chemical equilibrium between CO 2 equilibrium partial pressure and temperature, by the research of Baker [START_REF] Baker | The calcium oxide-carbon dioxide system in the pressure range 1-300 atmospheres[END_REF]. It is proposed that for a given partial pressure of CO 2 , CaO carbonation takes place when the temperature is lower than the corresponding equilibrium value. But if the temperature continuously increases and exceeds the equilibrium value, carbonation reaction is inhibited and CaCO 3 calcination reaction occurs. With regard to the present study, the average CO 2 concentration in the reactor is around 1-2%, corresponding to the CaCO 3 calcination temperature of ca. 650-700 °C. The theoretical calculation accords well with the experimental results. When the temperature reaches 750 or 800 °C, the catalytic effect of CaO sorbent is unfavorable and little CO 2 would be absorbed. As a consequence, H 2 concentration drops when the temperature is higher than 700 °C.

Figure 6 shows also that the H 2 yield keeps rising; however, its rate of increase is declined. XRD analysis of the solid product at 600/700/800 °C is conducted. It reveals that the catalytic activity of CaO is greatly affected by the reaction temperature. As shown in Fig. 7, the main peaks at 600 °C correspond to CaCO 3 , CaO and Ca(OH) 2 , with the intensity of CaCO 3 dominants over other compounds. The diffraction peaks of CaCO 3 are significantly dropped when the temperature is increased to 700 °C, indicating that the decomposition of CaCO 3 starts to take place and less CO 2 is captured. It is also found that no CaCO 3 is observed at 800 °C; on the contrary, it is converted to CaO or Ca(OH) 2 under the presence of steam, resulting in quite strong peaks of these compounds. Literature further revealed that CaO obtained at high temperature is of more perfect crystal phase, and this will cause a poor catalytic activity due to fewer crystal defects [START_REF] Feng | Variation of the crystalline structure of coal char during gasification[END_REF]. As a result, it could be concluded that at temperature higher than 700 °C, CaO exhibits basically the catalytic effect on tar cracking only, which leads the secondary cracking reactions into the gas fraction. However, its catalytic reactivity on H 2 -rich gas improvement is quite limited. Therefore, the reaction temperature should be balanced to an adequate level by considering both the CO 2 absorption ability of CaO catalyst and the gasification performance. Besides, sintering could also be another reason for the reduced catalytic reactivity of CaO at higher temperature. Figure 8 gives the SEM images of CaO before and after reaction. It is found that the size of the particles increase, some small particles as observed in Fig. 8a disappear, or they are agglomerated into larger particles. Also, the particle surface is divided into small cracks. Since the surface area of the catalyst will decrease due to sintering, this will cause the loss of small pores necessary for the storage of the produced CaCO 3 [START_REF] Grasa | CO 2 capture capacity of CaO in long series of carbonation/calcination cycles[END_REF]. Therefore the catalytic effect of CaO is inferred to decrease, which seems to be severe under higher temperatures. Regarding the present data, the optimal operating temperature is 700 °C so that the formation of CaCO 3 could be enhanced at the presence of CaO catalyst to improve the H 2 production. Then the results are further compared with non-catalyst steam gasification cases. The comparison under the same reaction temperature (700 °C) has already been presented Fig. 5 Effect of temperature on H 2 production and syngas composition (CaO/wood = 0.2; steam flowrate = 160 g/h) Fig. 6 CO 2 equilibrium partial pressure versus temperature for carbonation/calcination reactions in ''Catalyst Activity'' section; and non-catalyst hightemperature steam gasification cases are further discussed to investigate the possibility of reducing the gasification operating temperature by using CaO as catalyst. Figure 9 depicts the syngas composition of both catalytic and noncatalytic steam gasification; with products characteristics illustrated in Table 3. Comparing the syngas composition, it can be seen that 20% CaO addition at 700 °C could reach as high H 2 content (ca. 50%) as that gasified at 900 °C without catalyst. This value is higher than that obtained by non-catalyst gasification at 800 °C; and obviously, the further increase in CaO addition amount will continue to raise the H 2 concentration. As a result, a much higher H 2 / CO ratio could be obtained under catalytic gasification cases, which is about 1.4-2 times higher than the cases under non-catalyst gasification (Table 3). Meanwhile, as illustrated in Table 3, the tar yield after adding the CaO catalyst could be declined significantly. The tar yield after 20% CaO addition at 700 °C drops to 8.9 g/Nm 3 , which is 39% lower than the non-catalyst gasification at 900 °C (14.5 g/Nm 3 ), mainly attributed to the great activity of CaO on tar cracking. With regard to H 2 yield, 50% CaO addition at 700 °C reaches H 2 yield of 0.11 m 3 /kg of fed wood, equivalent to that gasified at 800 °C without catalyst (0.10 m 3 /kg-wood). By further investigating the syngas properties, it is found that the H 2 concentration could be improved by 16% with 50% CaO addition at 700 °C compared with that under 800 °C of non-catalyst gasification. Meanwhile, CO and CO 2 content has a marked decrease by adding CaO catalyst, and the H 2 / CO ratio is thus elevated to a higher level (6.16 under 50% CaO addition at 700 °C vs. 3.76 under 800 °C without catalyst). Therefore, it is concluded that using CaO as catalyst is an effective approach to reduce the reaction temperature of steam gasification. 50% CaO addition could effectively reduce the reaction temperature by 14% (ca. 100 °C).

Conclusions

The catalytic steam gasification of bio-char for hydrogenrich gas production is experimentally investigated in a fluidized-bed reactor, using poplar wood as raw material and CaO as catalyst. The influence of operating conditions (CaO activity and steam flowrate) on the H 2 yield and tar reduction potential is studied. The catalytic effect of CaO has been examined: as CO 2 acceptor, as well as to participate in the tar reforming reactions. Increasing the mass ratio of CaO/wood is effective for H 2 improvement and simultaneously reduce the tar yield. Increasing the steam flowrate is beneficial to shift the water gas shift reactions towards H 2 production. However, an excessive steam injection leads to an opposite effect, and a maximum H 2 concentration and yield is achieved at steam flowrate of 160 g/h.

Based on the results obtained, process optimization is conducted to verify the potential of reducing the gasification temperature by using CaO as catalyst. Gasification at different temperatures reveals that the H 2 concentration is decreased when temperature exceeds 700 °C. CaO exhibits only the catalytic effect on tar cracking at higher temperature. The results are further compared with non-catalyst gasification cases. 50% CaO addition at 700 °C could reach as high H 2 yield as that gasified at 800 °C without catalyst. Meanwhile, a much higher H 2 concentration and H 2 /CO ratio could be obtained under catalytic gasification cases. Therefore, it is concluded that 50% CaO addition as cata- lyst at 700 °C is effective to reduce the steam gasification reaction temperature of 14% (ca. 100 °C).

In conclusion, the results indicate that there is a strong potential for producing hydrogen-rich gas from bio-char by catalytic steam gasification with inexpensive and abundant CaO as catalyst. For the forthcoming research, the effect of CaO during raw biomass pyrolysis step should be studied, since it would catalyze pyrolysis process and the decrease in catalyst reactivity might occur. Besides, it is also worth investigating CaO reactivity with the aim to reduce its decay rate during gasification process. It is expected that the results could be helpful for the better utilization of biochar towards a more effective route. Catalytic steam gasification represents the working conditions under temperature = 700 °C, steam flowrate = 160 g/h while varying the CaO/wood mass ratios a The case of '700 °C, non-catalyst' stands for the same working condition as '700 °C, CaO/wood = 0'

Fig. 1

 1 Fig. 1 Schematic diagram of the furnace. 1 reaction chamber; 2 thermocouples; 3 steam generator; 4 controller; 5 outlet pipe with heating tape; 6 absorption solution with water; 7 empty impinger; 8 ice-water bath

Fig. 2 Fig. 3

 23 Fig. 2 Effect of CaO/wood mass ratio on H 2 production and syngas composition (steam flowrate = 160 g/h; temperature = 700 °C). *Note CaO/wood = 0 stands for noncatalyst gasification case

Fig. 4

 4 Fig. 4 Effect of steam flowrate on H 2 production and syngas composition (CaO/wood = 0.2; temperature = 700 °C)

Fig. 7

 7 Fig. 7 XRD analysis of the gasification char at different temperature (CaO/wood = 0.2; steam flowrate = 160 g/h)

Fig. 8 Fig. 9

 89 Fig. 8 SEM images of CaO catalyst. a Original CaO particles before experiment; b CaO particles after experiment, tested at CaO/wood = 1.0, steam flowrate = 160 g/h and temperature = 800 °C

Table 1

 1 Proximate and ultimate analysis of poplar wood and its pyrolytic bio-char

		Proximate analysis (ad, wt%)	Ultimate analysis (ad, wt%)	LHV (MJ/kg)
		M b	V b	FC b	A b	C	H	O	N	S
	Poplar wood 9.87	77.03	12.57	0.53	44.41 5.07 39.36 0.12 0.64 17.87
	Wood char a	6.33	10.84	80.42	2.41	79.23 2.77	9.02 0.24 0	30.52

Table 2

 2 Characteristics of products at different steam flowrate (CaO/wood = 0.2; temperature = 700 °C)

	Steam flowrate (g/h)	80	160	240	320
	Product distribution (wt%)
	Syngas	24.80	29.93	26.35	22.15
	Tar	7.99	4.90	5.12	5.17
	Char	86.68	85.36	86.13	86.59
	Product characteristics
	Syngas yield (Nm 3 /kg-wood)	0.126	0.156	0.133	0.105
	Tar yield (g/Nm 3 )	14.7	8.9	9.4	9.6
	H 2 /CO molar ratio	3.41	5.30	4.24	3.73

Table 3

 3 Comparison products characteristics between catalytic and non-catalytic steam gasification

	Condition	700 °C, non-	800 °C,	900 °C,	700 °C,	700 °C,	700 °C,
		catalyst a	non-catalyst	non-catalyst	CaO/wood = 0.2	CaO/wood = 0.5	CaO/wood = 1.0
	H 2 yield (Nm 3 /kg-wood)	0.06	0.10	0.16	0.08	0.11	0.12
	Tar yield (g/Nm 3 )	22.4	20.9	14.5	8.9	5.5	3.2
	H 2 /CO	3.36	3.76	3.02	5.30	6.16	5.99

Acknowledgements This project is supported by the National Natural Science Foundation of China (No. 51276168) and the Program of Introducing Talents of Discipline to University (B08026).


Hélène Boucard
email: boucard.h@gmail.com

Masaru Watanabe

Seiichi Takami

Elsa Weiss-Hortala

Radu Barna

Tadafumi Adschiri

Beneficial use of CeO 2 nanocatalyst for black liquor conversion under sub and supercritical conditions

Keywords:

Black liquor is an alkaline liquid residue from paper industry, containing ∼80 wt% of water and ∼20 wt% of organics and minerals. Hydrothermal conversion of black liquor in a batch reactor is particularly interesting towards hydrogen production, as the carbon dioxide is highly solubilized in the basic aqueous media. Thus hydrogen with higher purity than other processes is obtained. Cerium oxide nanocatalyst (cubic CeO 2 ) is used to improve the amount of hydrogen produced and to decrease the coke formation, based on the water splitting into active species. Experiments have been carried out at sub and supercritical conditions 350 • C or 450 • C, 25 MPa) in a batch reactor for 15 min or 60 min. CeO 2 nanocatalyst improved the conversion of black liquor at sub and supercritical conditions as expected. The amount of carbonaceous solid recovered was decreased due to the catalytic effect. Recovered hydrogen was not significantly affected by the catalyst as it was involved in secondary reactions such as hydrogenation. Oxidations reactions, as well as O 2 production, were also improved using catalyst and supercritical media. Supercritical water media combined with catalyst clearly affects the fragmentation of dissolved lignin compared to subcritical conditions. Then it was assumed that water splitting occurs and produces active hydrogen and oxygen species. Thus these actives species produced using catalyst are able to improve hydrogenation and oxidation reactions, as well as recombination into H 2 and O 2 .

Introduction

Black liquor is an alkaline wastewater coming from Kraft process in paper industry. It results from the step of cooking wood chips with white liquor (Na 2 S and NaOH mixture). This step is used for delignifying the wood; corresponding to the dissolution of lignin from the fibrous part (cellulose, hemicelluloses) to kraft lignin [START_REF] Leask | Mechanical Pulping published by Joint Textbook Committee of the Paper Industry TAPPI (Atlanta), CPPA[END_REF]. The cooking juice recovered, namely black liquor, is an aqueous media containing dissolved lignin, fragments of cellulose and hemicelluloses, and several minerals and salts (Na, K, Ca, S. . .. carbonates, sulfates, sulfides. . .). Black liquor (BL) is a basic aqueous solution (pH ∼ 13), as well as white liquor, whose the organic content presents a strong use potential. The composition of black liquor is varying as function of wood and ratio of alkaline in the white liquor. However, Demirbas reported that the molar composition could be assumed as C 10 H 12.5 O 7 Na 2.4 S 0.36 [START_REF] Demirbas | Pyrolysis and steam gasification processes of black liquor[END_REF]. In industrial facilities of the Kraft process a large space is devoted to black liquor treatment. Black liquor is concentrated and incinerated for heat production (due to its high heating value) and white liquor recovery [START_REF] Demirbas | Pyrolysis and steam gasification processes of black liquor[END_REF]. Nevertheless an extra volume of black liquor is produced, and thus useful material recovery could be investigated. Considering the amounts of water in black (∼80 wt%), hydrothermal processes could be possible to use its high water content to convert its significant organic content (∼100 gC L -1 ). Indeed, hydrothermal process uses the properties of water at high pressure and temperature to convert biomass. It has been firstly used to achieve total oxidation of waste and more recently it is used to recycle waste by generating energetic gas, building-block molecules or added-value solids [START_REF] Titirici | Hydrothermal Carbonisation: A Sustainable Alternative to Versatile Carbon Materials[END_REF]. Indeed, water over its critical point (T > 374 • C and P > 22.1 MPa) is a solvent, as well as a reactant [START_REF] Kruse | Hot compressed water as reaction medium and reactant[END_REF]. Water is a polar solvent where salts are miscible and organic molecules immiscible. However water becomes non polar at the supercritical state, and the overall physico-chemical properties change drastically. Indeed water molecules form nonpolar clusters [START_REF] Kruse | Hot compressed water as reaction medium and reactant[END_REF] and its density sharply decreases thus free radical reactions are favored. Furthermore, supercritical water and gases are miscible and the diffusion of the molecules in this phase is then improved [START_REF] Kruse | Hot compressed water as reaction medium and reactant[END_REF].

Preliminary hydrothermal conversion of black liquor have been made in batch reactor and revealed the formation of three phases [START_REF] Boucard | Catalytic conversion of black liquor under sub-/supercritical conditions[END_REF]: gas with a high proportion of H 2 (∼80% at 600 • C), a rich bouquet of phenolic compounds in the liquid phase and an non negligible proportion of carbon converted into solid phase (∼20 wt%). This preliminary study shows the interest of black liquor conversion by hydrothermal process without catalyst such as previous work about gasification [START_REF] Cao | Hydrogen production from supercritical water gasification of alkaline wheat straw pulping black liquor in continuous flow system[END_REF], liquefaction [START_REF] Rönnlund | Waste to energy by industrially integrated supercritical water gasification-effects of alkali salts in residual by-products from the pulp and paper industry[END_REF] or carbonization [START_REF] Titirici | A generalized synthesis of metal oxide hollow spheres using a hydrothermal approach[END_REF]. Moreover, the alkaline salts, such as those in black liquor, are commonly used in hydrothermal processes to improve hydrogen production [START_REF] Guo | Catalytic effects of NaOH and Na2CO3 additives on alkali lignin pyrolysis and gasification[END_REF].

These results have been obtained without catalyst, however a previous study about cracking bitumen [START_REF] Dejhosseini | Catalytic cracking reaction of heavy oil in the presence of cerium oxide nanoparticles in supercritical water[END_REF] using CeO 2 has shown that cerium oxide is an interesting nanocatalyst to improve hydrogen production and to reduce coke formation. Indeed, CeO 2 has a "redox cycle" that allows fixing and releasing easily the oxygen generated by the water at supercritical conditions. During the reaction, CeO 2 acts as a catalyst and splits water into two actives species: H and O [START_REF] Dejhosseini | Catalytic cracking reaction of heavy oil in the presence of cerium oxide nanoparticles in supercritical water[END_REF], which react during hydrothermal process. CeO 2 exists with various crystalline structures (cubic, octahedral. . ..); however the catalytic activity of cubic CeO 2 is higher than octahedral CeO 2 [START_REF] Dejhosseini | Catalytic cracking reaction of heavy oil in the presence of cerium oxide nanoparticles in supercritical water[END_REF].

Considering alkali contains and CeO 2 properties, this work is on the catalytic conversion of black liquor using CeO 2 ; the objective is to study the amount and the proportion of H 2 , as well as the limitation of coke formation. Sub and supercritical conditions were selected (350 • C and 450 • C, 25 MPa) at various reaction times in a batch reactor. In addition, the last section discusses the catalytic role of the catalyst by comparing the medias (L/V mixture vs. supercritical).

Materials and methods

Reagents and reactors

Feedstock: Black liquor

Black liquor used in this study was supplied by Smurfit Kappa Cellulose du Pin in Facture, France. It was obtained from the digester after the recovery of tall oil in Kraft process. The organic and mineral contents dissolved in water represent 23 wt%. The organic compounds amounted to 70 wt% of the dry matter, which equals to 100 gC L -1 . In this study, the original feedstock was diluted to 10 wt% in pure water.

Catalyst: Cerium oxide

CeO 2 nanoparticles were synthesized by hydrothermal process [START_REF] Dejhosseini | Catalytic cracking reaction of heavy oil in the presence of cerium oxide nanoparticles in supercritical water[END_REF][START_REF] Byrappa | Nanoparticles synthesis using supercritical fluid technology-towards biomedical applications[END_REF]. The average size of the nanoparticles was around 8 nm. The catalytic activity of CeO 2 arises from its redox activity via Ce 4+ /Ce 3+ cycle, which is accompanied with capturing and releasing oxygen. CeO 2 is known to have high oxygen storage capacity (OSC). The OSC of cubic CeO 2 is 340 g-O/g-cat and that is 3.4 times higher than octahedral CeO 2 [START_REF] Dejhosseini | Catalytic cracking reaction of heavy oil in the presence of cerium oxide nanoparticles in supercritical water[END_REF]. Due to this high OSC, cubic CeO 2 nanoparticles were used in this study. The ratio of catalyst to the reactants was defined in the following equation: R = mass of catalyst mass of organic content (1)

2.1.2.1. Expected action of CeO 2 . During reaction CeO 2 acts as a catalyst and splits water into two actives species: H and O [START_REF] Dejhosseini | Catalytic cracking reaction of heavy oil in the presence of cerium oxide nanoparticles in supercritical water[END_REF]. These hydrogen active species can form directly H 2 or stabilize the reactive intermediate species reacting with organic molecules to form smaller molecules. The amount of H 2 formed is also linked to hydrogenation/dehydrogenation reaction that occurred for big molecules observed in liquid phase after reaction. The role of active oxygen species were mainly involved in oxidation reactions. The more the oxidation is achieved, the more CO 2 is produced. The concentration of these active species increases in the presence of CeO 2 catalyst and thus the carbon conversion was enhanced.

Reactor and experimental protocol

Experiments were performed at either 350 • C or 450 • C in subor super-critical conditions using a pressure-resistant batch reactor with an inner volume of 5.0 mL. The diluted black liquor was introduced into the reactor with the cubic CeO 2 nanoparticles (R = 0; 5 or 20); then the reactor was capped tightly. The volume of the reactant for the reactions at 350 and 450 • C was 3.128 mL and 0.449 mL, respectively. The reactor was initially pressured with N 2 (∼0.26 MPa), and then placed in an electric oven whose temperature was set to either 350 • C or 450 • C. After the reaction time (15 or 60 min), reactor was cooled in an iced water to stop the reaction immediately, such as a quenching process. Liquid and solid products were collected after gas analysis by rinsing the reactor with tetrahydrofuran (THF). THF is purchased from Wako Pure Chemical Industries (Japan) with a purity of 99.0%. Solid and liquid phases were separated by filtration using PTFE membrane filter with pore diameter of 0.1 m. The THF-insoluble fraction was called solid residue and the THF-soluble fraction form the liquid phase.

Analytic procedures

Gaseous residue

After quenching, the gas from the reactor was directly analyzed by -gas chromatography (GC) (Agilent GC-3000) to identify and quantify following gases: H 2 , O 2 , N 2 , CO, CO 2 , CH 4 and others hydrocarbons; with a carrier gas of Argon. Columns used are: Molesieve 5A and PoraPLOT Q.

Gas volume is estimated with a calibration curve linking pressure reads by the manometer and the volume introduced.

Liquid residue

The composition of the liquid phase was analyzed by gel permeation chromatography (GPC) (HP1100) and gas chromatography coupled with mass spectroscopy (GC-MS) (GC: Agilent 7890A; MS: Agilent 5975C), the column used is HP-5MS.

Solid residue

The solid residue is dried at 60 • C under vacuum during 12 h and then weighted. The weight of coke was evaluated as the weight loss after solid calcination at 600 • C, because the solid products contained CeO 2 nanocatalyst as well as coke. Morphology of solid residue (including coke, inorganics and CeO 2 nanoparticles) was observed via transmission electron microscopy (TEM, Hitachi H7650) on a copper mesh. Moreover microanalysis of solid using an energy dispersive X-ray spectroscope (EDX) was performed. The proportion of coke formed in the solid phase is calculated with the following equation:

%C = m Coke m solide recovered (2)
An XRD analysis is made before and after reaction to chick the composition of the catalyst (Fig. 1).

Results and discussion

Black liquor is a waste alkaline solution containing valuable organic compounds (thiolignin) and a large amount of water. Hydrothermal conversion of black liquor was studied at sub (350 • C, 25 MPa) and supercritical (450 • C, 25 MPa) conditions. The catalytic effect was evaluated as regards to hydrogen production and carbon conversion using either or not CeO 2 nanocatalyst. Liquid phase is also investigated to elucidate secondary reactions occurred between gas phase and molecules formed in liquid phase. Then in a third part, catalytic effect of CeO 2 is discussed by comparing sub and supercritical media.

Supercritical conditions

Experiments at supercritical conditions were carried out in 5 mL batch autoclaves at 450 • C, 25 MPa. Reaction time of 15 min and 60 min, as well as the amount of catalyst, were investigated. The catalytic effect was evaluated towards coke formation and hydrogen production.

To highlight the reactions involved, the liquid is also analyzed and compared according to the operating conditions.

Coke formation

Black liquor is mainly composed of dissolved lignin. During hydrothermal conversion lignin is expected to be degraded into aromatics molecules [START_REF] Sasaki | Recovery of phenolic compounds through the decomposition of lignin in near and supercritical water[END_REF] such as phenolic compounds and smaller molecules such as little aldehydes, glyceraldehyde, etc. Polymerization of the phenolic molecules takes place thanks to these smaller molecules, which connect aromatics components together. Coke formation was the result of this polymerization and was formed since the splitting of lignin molecule began [START_REF] Saisu | Conversion of lignin with supercritical water-phenol mixtures[END_REF].

The solid recovered was observed with TEM. As shown in Fig. 2, the solid is shapeless (Fig. 2). An EDX analysis was simultaneously realized showing the composition was carbon, oxygen and minerals; particularly sulfur (darkest spots on Fig. 2) but minerals recovered in the solid phase are not involved in the morphology of the carbonaceous material as seen previously by Wu [START_REF] Wu | Etude de procédés de conversion de biomasse en eau supercritique pour l'obtention d'hydrogène. Application au glucose[END_REF]. So, the coke recovered after reaction is composed of organic and mineral compounds that could influence the mass recovered. However as the proportion of minerals is low, the contribution of coke in the weight of solid residue is assumed to be the totality. The amount of coke was evaluated for the set of experiments and summarized in Table 1. The mass of coke produced during the reaction is about two times lower with using CeO 2 catalyst. This result confirms that the use of catalyst reduces the coke formation. However, the amount of coke is ten times higher by increasing the reaction time. This same order of magnitude is observed using either or not the catalyst.

Using supercritical conditions, radical reactions are favored and result in the efficient degradation of lignin into phenolic compounds. As a result, several condensation reactions [START_REF] Kang | Solid fuel production by hydrothermal carbonization of black liquor[END_REF] are expected to take place and explain this non negligible amount of coke formed. Reaction time increased these phenomena; results Table 1 confirmed that point.

Indeed, as described previously, CeO 2 catalyst is able to oxidize organics more efficiently or rapidly than supercritical water. Thus, the whole organics (aromatic and aliphatic molecules) are subjected to efficient oxidation reactions. Small molecules such as aldehyde, are known as promoter for polymerization of phenolic compounds [START_REF] Saisu | Conversion of lignin with supercritical water-phenol mixtures[END_REF]. In presence of catalyst, the kinetics of aldehyde oxidation is expected to be enhanced that results in a decrease of polymerization reactions if the oxidation is complete and then a decrease of coke formation. Simultaneously, this complete oxidation of aliphatic compounds would increase the amount of CO 2 released in the gas phase.

Gas formation

The gas phase is analyzed directly after reaction. For each condition, gas phase is composed of H 2 , CO, CO 2 , and light hydrocarbons. However the proportion changes slightly regarding parameters studied (reaction time and catalyst). Indeed, reaction time, as well as the catalysts, played an important role in the carbon conversion and enhanced the gasification particularly H 2 production. Moreover a large amount of CO 2 is dissolved as carbonates due to the pH of the remaining solutions which are equal to 10. Fig. 3 presents the proportion of carbon recovered in gas phase versus reaction time. The main part of carbon in the gas phase was CO and CO 2 , which were produced from the oxidation and water-gas shift reactions. Fig. 3 shows that the amount of carbon in gas phase increases with reaction time using or not the catalyst.

At long reaction time, the analyses of the gaseous phases obtained by micro chromatography indicated that the compositions of both gaseous products are almost similar. The main difference concerns the amount of carbon dioxide, and to a lesser extent carbon dioxide and methane, in the catalytic experiment. CO, CH 4 and CO 2 are thus increasing the amount of carbon recovered in the gaseous phase as shown in Fig. 3.

As regards to the catalytic effect, Fig. 3 shows that the amount of carbon converted to the gaseous phase is slightly higher at long reaction time (60 min). At short reaction time, carbon conversion to gas phase is surprisingly lower using catalyst. Indeed the gas produced using catalyst contains six times fewer methane and three times fewer CO 2 and light hydrocarbons. Moreover the amount of CO was under the detection limit and was not quantified using catalyst. This means that the oxidation is quite achieved in gas phase. As the volume of gas produced is almost the same in both experiments, the amount of carbon is thus lower using catalyst and would be due to concurrent reactions. Indeed the amount of oxygen in gas phase is triple using catalyst and reaches 145 mol. Taking into account the role of catalyst on water splitting, these results would indicate that oxygen active species are recombined into O 2 . These results indicate that the carbon gasification efficiency using catalyst is affected by reactions between oxygen actives species. Kinetics of reaction between oxygen active species is expected to be higher than that of oxygen active species on organics, for short reaction time. This result underscores that the recombination of oxygen activated species could be a preliminary reaction in the global oxidation reaction on organics. Nevertheless, for a higher reaction time (450 • C-60 min), competition occurs between water gas shift reaction and extreme oxidation of organic molecules that released of CO 2 also. This double production generates more CO 2 . After reaction equilibrium between liquid and gas should occurs and limits the capture of CO 2 in the liquid phase, which explain the existence of CO in gas phase and more amount of CO 2 in the presence of catalyst.

Considering the global reaction of CeO 2 on hydrothermal conversion, this catalyst increased the conversion of carbon into CO 2 by promoting oxidation and water gas-shift reactions.

During the catalytic hydrothermal conversion of black liquor, hydrogen activated species is also expected. Fig. 4 shows the amount of H 2 produced for same operating conditions as previously. For a long reaction time (60 min), the amount of H 2 was slightly affected by the catalyst ratio (R = 0 or 5). The amount of H 2 obtained for this reaction time reached 345 mol and 322 mol using or not catalyst, respectively. Demirbas reported that more than 10 mol of H 2 was produced from 1 mol of black liquor if the formula C 10 H 12.5 O 7 Na 2.4 S 0.36 is considered [START_REF] Demirbas | Pyrolysis and steam gasification processes of black liquor[END_REF]. Applying this calculation to our case, the maximal value of H 2 from black liquor gasification would be 360 mol. Considering this value and CeO 2 activity, H 2 is expected to be higher. Nevertheless the amount of H 2 measured was related to its production and its consumption. Active hydrogen species that was produced from water splitting would easily and continuously react with another active hydrogen to form H 2 ; as the amount of light hydrocarbons is lower using catalyst; the consumption of H 2 could be related to hydrogenation or/and capping reactions with organic molecules to form smaller molecules in liquid phase.

For short reaction time, the amount of H 2 is 5 times lesser using catalyst. This low amount of H 2 , while the amount of O 2 is high, indicated its consumption in other reactions. As described previously, the amount of light hydrocarbons is also lower using catalyst meaning that H 2 produced could be consumed in reactions such as hydrogenation in liquid phase.

To conclude, at long reaction time the catalyst improves carbon conversion, mainly for CO 2 , and H 2 production. At short reaction time, the catalyst has a strong effect on O 2 production while H 2 and C amounts in gas phase are lower than without catalyst. The proportion of carbon converted into gas phase suggested that exchange can occur between gaseous and liquid phase. In this case, hydrogenation reactions with liquid phase are supposed to play an important role.

Liquid phase

The original black liquor is dark brown. The color of the liquid phase obtained after reaction is linked to its conversion and the effectiveness of the catalyst.

At 450 • C, the liquid was yellow, almost transparent. The color became lighter with increased reaction time. The observed color variation was caused by the conversion of oligomer molecules to either smaller colorless molecules (such as acids, aldehydes, alcohols. . .), polycyclic (2 or 3) aromatics molecules and/or to solid residues. This observation was supported by gel permeation chromatography (GPC). GPC separates molecules towards their molecular weight. The profiles of the curves are plotted in Fig. 5, where the intensity is reported as function of the molecular weight. The profile obtained for black liquor indicates that molecules have mainly a molecular weight centered at log M = 2.05. Moreover, a significant amount of molecules presents higher molecular weight up to log M = 3.5. First, the profiles of liquids recovered after reaction is the sharp decrease of the number of molecules between a log M of 2.7 and 3.5. Hydrothermal conversion, whatever the conditions used, is efficient to either convert organics into smaller structures (C removal) or hydrogenate molecules (O removal and H addition). Catalyst and reaction time are improving this phenomenon. Nevertheless this strong decrease could be also attributed to coke formation.

For experiments performed with catalyst, intensity at low molecular weight is higher, meaning that CeO 2 improved degradation of black liquor. At longer reaction time and using catalyst, the number of heavy molecules decreases while that of medium molecules increases with a molecular weight center at log M = 1.77 and explain the higher release of carbon to gas phase. Simultaneously, GC-MS analyses were carried out and confirmed that smaller molecules were detected in the presence of catalyst.

At 15 min: 3-butanoic acid; propane; 3-methyl butanal; phenol, etc. At 60 min: acetone; cyclopropane, 1-ethyl-2-methyl-, cis-; butanal, 3-methyl; phenol, etc.

The degradation of black liquor during reaction is confirmed by the presence of these smaller molecules. No molecules more oxidized were detected meaning that oxidation by CeO 2 on the black liquor is a complete oxidation leading to the release of CO 2 into gas phase.

The basicity of the liquid phase after reaction is preserved (pH > 10), CO 2 is captured by liquid phase until equilibrium between liquid and gas is achieved. The saturation of molecules in liquid phase does not change that it suggested no hydrogenation but capping reaction occurred.

Conclusions about the catalytic effect of CeO 2 under supercritical conditions

CeO 2 nanocatalyst was selected to decrease coke formation and improve hydrogen production. The objectives have been achieved considering gas and liquid analysis. Under supercritical conditions splitting of water occurred efficiently, with the use of cubic CeO 2 ; forming oxygen and hydrogen activated species. On the one hand, complete oxidation happened for smaller molecules to release CO 2 . A first step of oxygen activated species recombination into O 2 has to be considered. On the other hand, the hydrogen actives species seemed to react into homolytics reaction to form H 2 and also into capping reaction to form smaller aromatics molecules. CeO 2 improved water-gas shift reaction that can explain also the higher amount of H 2 , higher amount of CO 2 and lower amount of CO.

Under subcritical conditions properties of water are intermediate between atmospheric conditions and supercritical condition. In addition, most of the reactions occurred into liquid phase.

Subcritical conditions

Experiments at subcritical conditions were carried out in 5 mL batch autoclaves at 350 • C, 25 MPa. Reaction time of 15 min and 60 min, as well as the amount of catalyst, were investigated. The catalytic effect was evaluated towards coke formation and hydrogen production as previously.

Coke formation

At 350 • C, under subcritical conditions, reaction media is made by the mixture of three phases: aqueous phase, oily phase and gaseous phase. Due to the pressure, it assumed that the gaseous phase is minority. During the conversion, polymerization of phenolic compounds occurred, and formed at macroscopic scale a nuclei non miscible with aqueous phase. Because of non-miscibility oily drops appeared into aqueous phase to give the morphology of coke obtained under subcritical conditions: spherical micro-particles (Fig. 6). Coke from black liquor at 350 • C is still composed by carbon, oxygen and minerals. Sulfur (darkest spots on pictures) is still detected. Minerals are observed as independent crystals to the coke. However, in the presence of catalyst, spherical particles disappeared and the amount of coke formed is less. Coke is not visible by TEM observation, only the nanoparticles of catalyst as shown in Fig. 7. Thus the coke is not recovered as particles but as deposit on the catalyst surface. Simultaneously, the color of the virgin catalyst changed from yellow to dark brown after reaction.

The mass of coke was also evaluated after experiments and summarized in Table 2. At both reaction times, the amount of coke is almost halved using the catalyst. The proportion of coke formed decreased in the presence of catalyst. A second experiment was carried out using higher catalyst ratio (R = 20) for 60 min reaction time. The total amount of coke formed was higher in mass (15.4 mg at R = 20 compared to 5.1 mg at R = 5) but lower in mass percentage in the solid (1.56 wt% compared to 3.81 wt% at R = 20 and 5, respectively). A hypothesis was an antagonist effect of CeO 2 as catalyst and as carrier of coke formation. In addition, probably oxidation is not complete because catalyst efficiency is low under subcritical conditions and water properties are less optimal for reactivity.

Analyze of gas phase can give first information on the level of oxidation made by CeO 2 on aromatics molecules regarding in particular carbon conversion.

Gas formation

After the experiments carried out at 350 • C, 25 MPa the volumes of the gaseous phases are quite similar meaning that it is not strongly affected by reaction time and catalyst. Fig. 8, shows that even if the amount of carbon species under subcritical conditions is low, the amount is higher with catalyst and confirmed that more carbon is converted into gas in the presence of catalyst. The low quantity conversion is due to subcritical temperature which improves reactions in liquid phase exclusively. Furthermore in these conditions water is enough reactive to react with CeO 2 , which was confirmed by the rising amount of carbon converted into gas phase with increasing the ratio of catalyst.

Considering only the proportion of carbon converted (Fig. 8) in the gaseous phase, it is impossible to conclude significantly so the proportions are low.

Indeed, in subcritical conditions with catalyst, as coke is decreased, higher conversion of carbon into gas phase could be expected. The main compound is H 2 , and to a lesser extent CO 2 and CH 4 are produced. Some light hydrocarbons were also detected in the gas produced. This tendency was in accordance with the lower quantity of gaseous products. Proportion of carbon at 350 • C can be also explained by the remaining basic pH of the solution that dissolves a large amount of CO 2 in the liquid phase.

So to understand the catalytic conversion of black liquor under subcritical conditions, we choose to consider the percentage of initial carbon transformed and recovered in the gaseous phase, as well as dissolved carbonates (inorganic carbon in liquid phase) and presented Fig. 9. For both reaction times, the amount of organic carbon converted increased using the catalyst.

Under subcritical conditions, gas composition changes a little; depending on operating conditions but it is mainly composed of H 2 , CO, CO 2 , and light hydrocarbons. For example, concerning the total amount of CO 2 (in gas phase and dissolved as carbonates), it is significantly higher using catalyst at higher reaction time. For short reaction time using the catalyst, the amounts of light hydrocarbons are 2 to 3 times higher while at higher reaction time, the amount of CH 4 increased significantly in presence of catalyst (10 times higher). For short reaction time, carbon monoxide was also detected. Comparing Figs. 8 and9, it is evident that under subcritical conditions, CO 2 is formed but stored in liquid phase. CO 2 stays in liquid phase as carbonates due to solubility equilibrium between gas and liquid phase. Indeed, at 350 • C, all the reactor volume is filled by black liquor solution. As with catalyst, the proportion of carbon is higher, the catalyst enhances the reactivity of the media.

Probably, the action of CeO 2 on water under subcritical conditions is the same as supercritical conditions with lower effectiveness. Into subcritical conditions, in the same way as supercritical conditions, hydrogen active species can form directly H 2 or react with organic molecules. Simultaneously, others probably reactions occurred in subcritical conditions like alkenes are formed and release H 2 in gas phase by dehydrogenation reaction.

Fig. 10 shows that the amount of produced H 2 significantly increased in the presence of the catalysts at 350 • C and 60 min. The other experimental conditions demonstrate a moderate effect of the catalysts.

Active hydrogen species that was produced from water splitting would easily and continuously react in gas phase to form H 2 . H 2 was also produced by water-gas-shift reaction and by hydrogenation reaction from alkane. However the increase in active hydrogen species would simultaneously increase reactions with organic molecules. Concerning the experiments carried out at 350 • C and 60 min, the amount of H 2 was multiplied by 10 using CeO 2 as a catalyst. This means that H 2 consumption was higher at long reaction time without catalyst while H 2 production would be higher using catalyst. As the gas-liquid equilibrium was expected for these experiments due to sub-critical water conditions, limitative diffusion of hydrogen from gas to liquid phase could explain its low consumption.

The amount of H 2 was not affected by the catalyst ratio (R = 5 or 20, 350 • C and 60 min). As the amount of H 2 produced from water splitting would be higher especially for R = 20, the mass transfer at the interface would be also enhanced by this high concentration. As a result, produced H 2 would be used for capping reaction in liquid phase to produce smaller molecules; also committed into hydrogenation/dehydrogenation reaction.

Conversion of carbon is not high at long reaction time that can suggested oxidation is not complete to release CO 2 . H 2 seemed to be limited or used with fast kinetics into reactions. The analysis of liquid phase should help to better understand.

Liquid composition

From gas phase analysis, black liquor conversion is expected to be less than under supercritical conditions. As the proportion of converted carbon is less also, partial oxidation of liquid molecules is expected using catalyst. Regarding catalyst action, as previously, the focus is made on hydrogenation and oxidation reactions.

The oxidation of molecules is noticed; by GC-MS in the presence of catalyst, especially at 60 min; R = 20 and R = 5; with oxidized molecules like 1(3H)-isobenzofuranone, 6,7-dimethoxy-3-[2-(2-methoxyphenyl)-2-oxoethyl]. Alkenes were also detected (2-Methylphenylacetylene; 3,4,5-trimethyl-2-cyclopenten-1-one; 10,18-bisnorabieta-8,11,13-triene, etc) so reaction of dehydrogenation occurred in the same time as retroaldol reactions. As a result, these reactions compete with capping reactions. Probably kinetics (at 350 • C) for dehydrogenation reaction is higher than kinetics for capping reaction. This tendency can explain why the amount of H 2 remains the same. A high amount of aromatic compound was obtained into liquid and less coke was formed.

Some heavy molecules were remained into liquid phase (cf.: GC-MS results). This result was supported by liquid color. The deep color is due to the presence of phenolic compounds [START_REF] Rönnlund | Waste to energy by industrially integrated supercritical water gasification-effects of alkali salts in residual by-products from the pulp and paper industry[END_REF].

The liquid was still brown and no color variation was observed within the reaction time. The color became lighter with increased reaction time and for a high ratio (R = 20). These results mean that degradation is slow under subcritical conditions and most of molecules were still polycyclic molecules. This observation was supported by gas permeation chromatography (Fig. 11).

The main peak of log M observed for original black liquor is slightly decreased using catalyst. The curve obtained after 15 min reaction time without catalyst presents a shift of the main peak to higher molecular weights. This significant increase of M would be due to O or C addition rather than H addition. For the whole set of conditions, the molecules in the region of log M comprised between 2.2 and 2.5 seem to be particularly reactive as the peaks of the original BL are smoothed. However the intensity of log M in the region of 2.5 to 3 suggests recovery of polycyclic molecules.

The shift of the curve for R = 20 means other populations of molecules were more represented. For this temperature, all curves are stackable to log M = 2; so no smaller molecules are formed.

Action of CeO 2 in the conversion under subcritical conditions

Under subcritical conditions, splitting of water occurred by CeO 2 making water more reactive. From results obtained, most of reactions were in liquid phase. On the one hand, H is used into hydrogenation/dehydrogenation reaction to form alkene and saturated cycle like indane. On the other hand, O is used to oxidize molecules but under these conditions oxidation is partial. That can explain the low percentage of carbon that was converted into gas phase and into coke. Under subcritical conditions retro-aldol reactions occurred also that decreased the availability of small aldehyde to connect phenolic compounds into coke.

Action of CeO 2 on water is the same under sub and supercritical reaction but main reactions differ between these both conditions 3.3. Comparison between the catalytic conversion of black liquor under sub-/supercritical conditions

Carbon conversion

The quantity of coke was increased in the supercritical conditions; due to the efficiency of water under these conditions on the high degradation of lignin in phenolic compounds. Radical reactions were increased in supercritical conditions so a high level of condensation reaction is expected and explain that no negligible weight of coke is formed. Reaction time increased this phenomenon. The degradation of organic carbon to gas phase (or inorganic carbon) is higher at 450 • C as expected. So gasification is favored under supercritical conditions. Indeed ∼20 wt% of initial carbon is converted into gas phase at 450 • C whereas only 5 wt% max is converted into gas phase under subcritical conditions. This observation suggested that efficiency of catalyst is higher under supercritical conditions especially due to oxidation reactions. At the same time, the analyses of liquid phases (GPC, GC-MS and color) confirm the production of molecules with lower molecular weights at high temperature as reported in Fig. 12 (GPC results). As a result, partial oxidation was obtained at subcritical conditions while the process is more achieved at high temperature.

As regards to coke formation, for both temperatures, coke was formed (Tables 1 and2); its quantity was increased in the supercritical conditions. The proportion of coke formed decreased when R of catalyst increases at 350 • C. The efficiency of catalyst seems to be higher at low temperature (subcritical conditions). Indeed the mass of coke formed after 15 min reaction time is in the same order of magnitude for both temperatures (around 10% of initial organic content). On the contrary, the amount of solid at 450 • C is higher for 60 min reaction time using the same calculation. For example, the mass of solid using catalyst represents 10% and 43% of initial organic content at 350 and 450 • C, respectively. Under subcritical conditions, amount of coke would be lower due to a combined effect of catalyst and retro-aldol reaction. Thus the efficiency of the catalyst towards the reduction of coke formation seems to be higher at subcritical conditions. The analysis of the solid morphologies reveals some differences. Indeed, Fig. 13a presents TEM images of solid recovered at 450 • C (60 min, using catalyst). A micro-analysis with EDX detector was performed and showed the presence of mineral content in the solid formation (Fig. 13b-f).

The pictures from Fig. 13a and b shows a rod-shape solid. However this rod-shape is not attributed to the carbonaceous solid (i.e. coke) as regards to C distribution (Fig. 13a andb). Oxygen is a main element recovered in the catalyst (Fig. 13c andd), as well as in the solid formed (Fig. 11b andd). However this solid formed is not only composed of carbon as minerals, such as Na, Mn, P (Fig. 13e-g) or Ca, S (not showed), are also recovered. These analyses highlight the high use of oxygen active species for oxidation or complexation reactions, either with organics or minerals. To conclude, the solid obtained from catalytic conversion at supercritical conditions seems to contain higher amounts of minerals compared to carbonaceous material.

On the contrary, in the presence of cubic CeO 2 , carbonaceous solid was not visible by TEM but the cartography was able to measure some carbon (Fig. 14). Fig. 14 presents the TEM pictures and micro-analysis of solid residue recovered at subcritical conditions (350 • C, t = 60 min, R = 5). The shape of the solid recovered is different from that obtained at supercritical conditions. Fig. 14b andc shows that carbonaceous solid is dispersed on the catalyst. The amount of oxygen is still high but seems to be more distributed between organic and mineral solids. The amount of minerals, especially oxygenated compounds, seems to be lower at subcritical conditions. Aside from the quenching that would lead to high amounts of precipitate salts at supercritical conditions, oxygen species from water splitting seems to be less available for mineral oxidation at subcritical conditions. Thus the effect of catalyst towards oxygen release seems to be higher at 450 • C as regards to oxidation reactions in solid phase. In addition, the carbon conversion to solid phase requires an elemental analysis to be calculated. As regards to the local micro-analyses, the amount of solid recovered contains the mass of coke and minerals, and thus the value of 43% calculated previously is manifestly excessive.

H 2 production

As mentioned previously the effect of catalyst has to be evaluated towards coke inhibition and hydrogen production. To compare the results, the amount of H 2 produced (in mol) was divided by the mass of organic content introduced (mg). The values obtained are close to 2 at subcritical conditions while this ratio reaches 10 to 40 at supercritical conditions. So the amount of hydrogen produced is clearly enhanced by temperature. Concerning the gaseous phase composition, light hydrocarbons with single bonds (C 2 and C 3 ) are in higher concentration at supercritical conditions compared to C 2 and C 3 compounds with double bonds. On the contrary, concentrations of alkenes are higher than alkanes at 350 • C. So these results indicate that hydrogenation is enhanced at 450 • C. Hydrogenation reactions would be favored as the amount of hydrogen, such as hydrogen active species or dihydrogen, is higher at 450 • C.

Hydrogen formed by water splitting is used to react with organic molecules. Reactions pathways are guides by operating conditions. Under subcritical conditions, hydrogenation/dehydrogenation is favored whereas, under supercritical conditions, capping reaction is predominant.

Conclusion

The goal of the study was to investigate the catalytic effect of cerium oxide nanocatalyst towards hydrothermal conversion of black liquor. The use of catalyst would lead to an increase of hydrogen production and an inhibition of the coke formation. First the analyses of gaseous phases reveal that catalyst seems to improve hydrogen production, as well as the temperature and the reaction time. Although this amount at 450 • C, short reaction time and using catalyst was lower than without catalyst, the amount of hydrogenated products was increased. The produced coke without catalyst was lower than that with catalyst.

As expected, cerium oxide nanocatalyst is efficient to improve black liquor conversion.

As the trends observed for hydrogen concentration was not clearly defined, the mechanism or action of catalyst was also investigated. The hypothetic action was suggested through water splitting into hydrogen and oxygen active species. This active species are either able to react with organic and mineral compounds (i.e. hydrogenation, oxidation) or with other actives species. Indeed, H 2 molecules can be formed by hydrogenation reaction from alkane or is released when dehydrogenation occurred. A part of these active species can react also with liquid molecules by capping to form smaller molecules (fewer complexes). GPC and color of liquid attested this degradation. Thus active hydrogen species leads to H 2 molecules, which could be involved in hydrogenation reaction to form alkane at high temperature or in the liquid at subcritical conditions. H 2 could be also released from dehydrogenation.

Active oxygen species are able to oxidize organic molecules that release CO 2 and CO at the final stages. The low amounts of CO suggested its consumption in water gas shift reaction; which was promoted by alkaline salts and CeO 2 catalyst. As the oxidation would be almost complete using catalyst, the amount of alcohols or aldehydes was limited compared to acids. When oxidation was extreme, some CO 2 was released to gas mixture; CO 2 was also due to the consumption of CO by the water gas shift reaction; which was promoted by alkaline salts and CeO 2 catalyst. Thus polymerization of phenolic compounds and aldehydes is limited.

We also noticed that the amount of O 2 and H 2 at 450 • C, short reaction time and using catalyst was significantly modified compared to the experiment without catalyst. Indeed, a high O 2 content was measured and hydrogenated products (such as alkanes) were identified. By increasing reaction time, O 2 would be used for oxidation reaction that increases CO, CO 2 and intermediates of oxidation. In the same way the amount of mineral oxide in the solid phase seems to be favored by catalyst and temperature increase. Moreover the ratios between oxygen (or hydrogen) and initial organic content showed that the reaction were improved at high temperature where the splitting would be favored. So the water splitting into active species would be suitable to describe the system.

Fig. 1 .

 1 Fig. 1. XRD analysis of CeO2 before and after reaction.

Fig. 2 .

 2 Fig. 2. solid obtained after 450 • C, 25 MPa, 15 min of reaction time, without catalyst.

Fig. 3 .

 3 Fig. 3. Proportion of carbon converted into gas phase at 450 • C, 25 MPa.

Fig. 4 .

 4 Fig. 4. H2 produced into gas phase after reaction at 450 • C, 25 MPa.

Fig. 5 .

 5 Fig. 5. catalyst influence on molecular weight at 450 • C, 25 MPa.

Fig. 6 .

 6 Fig. 6. Solid obtained after 350 • C, 25 MPa, 60 min of reaction time, without catalyst.

Fig. 7 .

 7 Fig. 7. Solid obtained after 350 • C, 25 MPa, 60 min of reaction time, with catalyst.

Fig. 8 .

 8 Fig. 8. Proportion of carbon converted into gas phase at 350 • C, 25 MPa.

Fig. 9 .

 9 Fig. 9. Distribution of initial organic carbon in gas phase and dissolved carbonates at 350 • C, 25 MPa.

Fig. 10 .

 10 Fig. 10. H2 produced into gas phase after reaction at 350 • C, 25 MPa.

Fig. 11 .

 11 Fig. 11. Catalyst influence on molecular weight at 450 • C and 350 • C.

Fig. 12 .

 12 Fig. 12. Comparison of GPC results at 350 and 450 • C. t = 15 min and R = 5.

Fig. 13 .

 13 Fig. 13. TEM image (a) and micro-analysis (b)-(g) of solid recovered after experiment (450 • C, t = 60 min, R = 5).

Fig. 14 .

 14 Fig. 14. TEM image (a) and micro-analysis (b)-(g) of solid recovered after experiment (350 • C, t = 60 min, R = 5).

Table 1

 1 Proportion and weight of coke formed after reaction at 450 • C.

	13.87%	4.59

m coke+cat = (g) % coke formed m coke = (mg) 60 min R = 5 3.31E -02

Table 2

 2 Proportion and weight of coke formed after reaction at 350 • C.

		m coke+cat = (g)	% coke formed	m coke = (mg)
	60 min R = 20 60 min R = 5 60 min 15 min R = 5 15 min	9.85E -01 1.34E -01 9.30E -03 2.51E -01 1.01E -02	1.56% 3.81% 2.70%	15.4 5.1 9.3 6.8 10.1

Acknowledgement

Authors want to thank Marie Curie Project of the European commission.