Applied Catalysis A: General 346 (2008) 164–173
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Catalytic decomposition of methane over a wood char concurrently activated by a pyrolysis gas A. Dufour a,*, A. Celzard b, V. Fierro c, E. Martin d, F. Broust d, A. Zoulalian e a
Gaz de France, Research & Development Division, 361 avenue du Pre´sident Wilson, BP 33, 93211 Saint-Denis-la-Plaine Cedex, France LCSM, UMR CNRS 7555, Nancy-Universite´, ENSTIB, 27 rue du Merle Blanc, BP 1041, 88051 Epinal Cedex 9, France c LCSM, UMR CNRS 7555, Nancy-Universite´, Faculte´ des Sciences & Techniques, BP 239, 54506 Vandoeuvre le`s Nancy Cedex, France d CIRAD, PERSYST, UPR Biomasse-Energie, Avenue Agropolis, TA B-42/16, 34398 Montpellier Cedex 5, France e LERMaB, UMR INRA 1093, Nancy-Universite´, Faculte´ des Sciences & Techniques, BP 239, 54506 Vandoeuvre le`s Nancy Cedex, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 April 2008 Received in revised form 15 May 2008 Accepted 16 May 2008 Available online 27 May 2008
The catalytic activity of a wood char towards CH4 decomposition in a pyrolysis gas was investigated in a fixed bed reactor for maximising hydrogen production from biomass gasification. Wood char is suggested to be the cheapest and greenest catalyst for CH4 conversion as it is directly produced in the pyrolysis facility. The conversion of methane reaches 70% for a contact time of 120 ms at 1000 8C. Because steam and CO2 are simultaneously present in the pyrolysis gas, the carbon catalyst is continuously regenerated. Hence the conversion of methane quickly stabilises. Such a phenomenon is shown to be possible through the oxidation of the char by CO2 and H2O at high temperature, which prevents the blocking of the mouth of pores by the concurrent pyrolytic carbon deposition. In the experimental conditions, oxygenated functional surface groups are continuously formed (by steam and CO2 oxidation) and thermally decomposed. The active sites for CH4 chemisorption and decomposition are suggested to be the unsaturated carbon atoms generated by the evolution of the oxygenated functions at high temperature. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Hydrogen production Biomass gasification Methane decomposition Carbon-based catalyst Activation Surface properties
It is now widely accepted that global warming is mainly due to human activities, and especially to the resultant production of greenhouse gases like carbon dioxide. Alternatives to fossil fuels are thus actively looked for, and hydrogen appears to be one of the most promising energy vector, and also the greenest one if produced from renewable resources. Even if its economy is not expected to be mature before at least 10 years, environmentfriendly means of production and storage have to be promptly developed. For several years, much research effort has been devoted to hydrogen production, either from conventional (fossil) fuels, or from biomass. At large scale, steam methane reforming is the most widely used process, however the latter requires much energy and fossil CO2 is produced [1]. This is the reason why thermal decomposition of methane is now considered as a serious alternative [2–7], using various kinds of catalysts. Metal catalysts
like Fe, Co and Ni were shown to be very efficient [8–15], thus considerably lowering the temperature of methane conversion and also increasing the kinetics. Because metals quickly deactivate, are sensitive to sulphur poisoning, and are prone to carbidization, the use of carbon-based catalysts has been proposed [16]. Several kinds of carbonaceous materials, mainly activated carbons and carbon blacks, have been tested for that purpose [16–27]. Generating hydrogen from biomass is highly advantageous from the point of view of the CO2 balance, which is indeed expected to be almost neutral. In the present work, gathering hydrogen production from biomass and thermocatalytic decomposition of methane is proposed into a single process. Biomass (wood) pyrolysis produces both char and gases, mainly CO, CO2, CH4, H2, H2O and tars. The resultant char is used as the catalyst for the thermal decomposition of methane, and is expected to be the greenest and cheapest catalyst as it is directly produced in the pyrolysis facility. Another advantage of the carbon catalyst is the enhanced hydrogen production due to three simultaneous mechanisms:
* Corresponding author at: Present address: LERMaB, UMR INRA 1093, NancyUniversite´, Faculte´ des Sciences & Techniques, BP 239, 54506 Vandoeuvre le`s Nancy Cedex, France. E-mail address:
[email protected] (A. Dufour).
The tar conversion is catalysed by the char [28–30]. The kinetic of water gas shift reaction is increased by the char [28].
1. Introduction
0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.05.023
A. Dufour et al. / Applied Catalysis A: General 346 (2008) 164–173
The steam also contributes to the continuous regeneration of the carbon catalyst through its progressive gasification, which is an additional source of hydrogen production. It is not the aim of the present paper to investigate the proposed process; however, for the sake of clarity, we have chosen to present it in Fig. 1. It is the only process, among the other ones already described in the open literature for the conversion of natural gas [16–27] or biogas [31], which continuously produces and regenerates the carbon catalyst. Mass and energy balances have been investigated elsewhere [32], and were shown to be favourable for such a process with an energy balance of 50%, and a hydrogen production of 85 g H2/kg of dry wood. To the authors’ best knowledge, methane conversion on such a wood char concurrently activated by a wet pyrolysis gas has never been investigated so far. Related works indeed dealt with a carbon bed (lignite and Saran char) submitted to simultaneous streams of H2O or CO2 and CH4 [33,34], which behaviour was investigated by thermogravimetric methods. Rather than the conversion of methane, the weight changes of the carbon were mainly studied in these papers, wherein the contact times were not controlled. These two studies offered considerable insight into the relationship between carbon deposition and physical properties of char; however, wood char was neither investigated for itself, nor from the point of view of the rate of methane conversion. The present paper demonstrates that a simple wood char may be a very efficient catalyst for methane conversion in various conditions. The article is organised as follows. Section 2 is devoted to the description of the used materials and experimental devices for preparing the wood chars and testing them as catalysts for the conversion of methane. The decomposition of methane on such
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wood chars in various experimental conditions is then investigated in Section 3, and the effects of several parameters (gas composition, presence of inorganic matter, pore texture of the char and surface chemistry) are reviewed and discussed. 2. Experimental 2.1. Preparation of the wood char samples Pinewood particles (2–5 mm) were introduced in a pyrolysis pilot plant (CIRAD Pyrotar facility [35]), consisting of a rotating screw reactor externally heated at its walls, throughout which the wood chips were simultaneously moved forward and heated up to the pyrolysis temperature. The heating rate of the particles was about 20 8C min1 up to the final temperature of 750 8C, and the residence time of the wood chips/char was about 1 h. After pyrolysis, the resultant char was sieved in order to collect the granulometric fraction ranging from 0.2 to 0.4 mm. In order to separate the influence on the catalytic properties of the surface properties on one hand, and of the presence of inorganic matter on the other hand, several samples of wood char were prepared. The so-called sample 1 corresponds to the raw char directly obtained at the outlet of the pyrolysis screw reactor, and next sieved (0.2–0.4 mm). Sample 2 is the same material after having been heat-treated half an hour in nitrogen at the final temperature at which the thermocatalytic process was subsequently carried out, i.e., 1000 8C. This heating step was added because the char was seen to release volatile matter when heated from 750 to 1000 8C. After such a heat-treatment, any modification of the resultant material was then expected to be only due to the subsequent catalytic process, and not to the temperature-induced devolatilisation. Sample 3 is similar to sample 2 but, before the heat-treatment at 1000 8C, was submitted to a demineralisation treatment achieved as follows. The material was first soaked into 5 N hydrochloric acid and stirred for 24 h, next vacuum-filtered and acetone-washed, then soaked into 5 N hydrofluoric acid and stirred for 24 h, and again into 5 N hydrochloric acid, after filtering and washing ([36,37] and references therein). Such a way of doing allowed first the dissolution of the major part of the metallic oxides, followed by that of the silica, then finally that of the remaining oxides not available before the removal of the silica. This demineralisation process was rather soft for the carbonaceous material, by comparison with a treatment using an oxidising acid, such as nitric acid, which produces a number of new surface functions. The demineralised char was finally thoroughly washed with distilled water, thus leading to the socalled sample 3. Finally, mesoporous silica (Aerolyst 3030 supplied by Degussa) was used for comparing its catalytic activity with that of wood char; such a ceramic material had a surface area of 250 m2 g1, and was mainly mesoporous (total pore volume: 0.9–1 cm3 g1, poresize distribution ranging from 6 to 25 nm, with an average pore width of 12 nm) [38]. 2.2. Experimental device for studying the thermocatalytic decomposition of methane on the as-prepared wood chars
Fig. 1. Schematic view of the process for producing hydrogen from wood pyrolysis. The resultant char is used both as a catalyst for methane conversion in the presence of steam and carbon dioxide (among other gases), and as a fuel for heating the process.
In order to get a more fundamental understanding of the conversion of pyrolysis gases over wood char, an artificial syngas, representative of real pyrolysis gases, was generated and injected throughout a fixed bed of wood char in the experimental set-up shown in Fig. 2. This way of doing allowed varying the gas composition, for instance by removing or adding one component,
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2.3. Characterisation of the catalysts before/after the conversion of methane
Fig. 2. Experimental set-up for thermocatalytic decomposition of methane.
and observing its influence on the conversion of methane, as it will be shown below. All the individual gases were supplied by Air Liquide (Marseille, France), with a purity of 99.995%, in the form of three separate cylinders: one of nitrogen (used as tracer), one of methane, and the last one containing, for simplicity of use, a tailormade mixture of H2/CO/CO2 according to the respective molar percents: 33.09/37.78/29.13. The resultant syngas was prepared by using three mass flow rate regulators (Brooks Instrument, Veenendaal, The Netherlands), see Fig. 2. Depending on the desired experimental conditions, water could be additionally injected with a syringe pump (Avantec, Illkirch, France). The nominal molar gas composition was for the corresponding wet syngas: CH4 14%, CO 19%, CO2 14%, H2 16%, H2O 30%, N2 7%. In any case, the partial pressures of the gases involved in the thermocatalytic decomposition were always kept invariable by adjusting the nitrogen flow. A total gas flow of 430 N mL min1 was maintained constant for all experiments. The syngas was injected through a heated line (250 8C) toward the char bed, as shown in Fig. 2. The contact time between gas and char was controlled as a function of the char bed volume. The bed of wood char was placed inside a vertical quartz tubular reactor (see Fig. 2). Because metal is known to have a possible catalytic activity toward the decomposition of methane, no metallic parts were present inside the reactor. Thus, the bed of carbon particles was supported by a plate of sintered silica, and the thermocouple used for controlling the temperature was systematically removed before the syngas enters the quartz tube. At the outlet of the reactor, the gases CH4, CO, CO2, N2 and H2 were analysed with a micro gas chromatograph equipped with 2 columns (5A molecular sieve and Poraplot Q) and 2 thermal conductivity detectors (Varian, Palo Alto, CA, USA). The amount of condensed water, collected by two impingers in series filled with isopropanol at 0 8C and–20 8C, was measured by Karl Fischer volumetric method.
The chars used as catalysts in the proposed process needed to be characterised before and after methane conversion. Such woodderived carbonaceous materials were studied according to different aspects: particle morphology and pore texture, surface area before and after methane conversion in various atmospheres, amount and composition of mineral matter (also called ‘‘ashes’’), and surface chemistry. Qualitative pore texture and dispersion of ashes were first evaluated using a scanning electronic microscope (MEB HITACHI, S 4800, Berkshire, UK) equipped with an EDX instrument. The ash content, expressed as a weight% of the initial mass of char, was measured by weighing the char before and after complete oxidation of the latter at 710 8C in air, according to the standard AFNOR NF EN 1860-2. Each experiment was reproduced 4 times, and the scattering of the results was found to be not higher than 0.5% of the average values. The composition of ashes was determined using an ICP-MS (Perkin-Elmer, Shelton, CT, USA) apparatus, after the sample was burnt in air [39]. The corresponding surface areas of the initial chars, and those resulting after the thermocatalytic decomposition of methane, were measured by the BET calculation method [40] applied to the sorption isotherms of nitrogen at 77 K. For that purpose, an automatic device Sorptomatic 1990 (Thermo Finnigan, Waltham, MA, USA) was used, and all the samples were previously outgassed at 523 K for several hours. The micropore volume, VDR, corresponding to pores narrower than 2 nm, was also calculated according to the Dubinin–Radushkevitch method ([41] and references therein). The pore-size distribution was estimated by the DFT (Density Functional Theory) model formerly developed by Tarazona [42], considering that the porosity of carbons is based on slit-shaped pores. The total pore volume, sometimes referred to as the socalled Gurvitch volume V0.99, was defined as the volume of liquid nitrogen corresponding to the amount adsorbed at a relative pressure p/p0 = 0.99 [43]; the Gurvitch volume is assumed to be the sum micro + mesopores volumes. The mesopore volume, Vme, was calculated as the difference between V0.99 and VDR. The oxygenated functions of the carbons was investigated both by thermogravimetry (TG) and by Fourier-transform infra-red (FTIR) spectroscopy. TG was performed in a SETARAM (Ecully, France) 92–16.18 apparatus: carbon samples were placed in an alumina crucible and heated under argon flow (1 bar, 2 L h1) at 20 8C min1 from room temperature to 1000 8C, then at 5 8C min1 from 1000 to 1500 8C. Transmission IR spectra were obtained with a Perkin Elmer Spectrum 100 instrument, using discs of dry KBr in which 0.1 wt.% of carbon powder were previously dispersed. 3. Results and discussion 3.1. Homogeneous versus heterogeneous conversion of CH4, and role of wood char The catalytic effect of the carbon surface on the conversion of methane at high temperature was already clearly demonstrated in other previous works [16–27]. However, several preliminary trials were made in the present work, in order to be completely sure that the same happens in our conditions. First, homogeneous conversion (i.e., without solid inside the quartz reactor) at 1000 8C of the wet syngas nominal mixture (CH4 14%/CO 19%/CO2 14%/H2 16%/ H2O 30%/N2 7 mol%) was attempted. Methane conversion was found to be lower than 5% for a gas residence time close to 2 s. Consequently, in the following, the homogeneous reaction at 1000 8C will thus be considered as negligible.
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3.2. Effect of ashes on methane decomposition
Fig. 3. Inlet and outlet molar flow rates (sample 2, 1000 8C, contact time 120 ms).
Other trials were carried out at the same temperature on a silica bed made of Aerolyst. Silica is considered to present an inert surface towards methane conversion [17], and the latter was indeed seen to be, again, lower than 5% at 1000 8C and for a contact time of 120 ms. Now, when a wood char was present, having a pore texture and a particle size (0.2–0.4 mm) similar to that of Aerolyst silica, the measured conversion of methane was found to be close to 70% for a contact time of 120 ms (equivalent to a volumetric hourly space velocity of 3 N L g1 h1). The corresponding results will be detailed in Section 3.2.2. Such a result undoubtedly corroborates the catalytic activity of wood char. In other words, not only the presence of a surface is required, but its chemical nature is of highest importance. As an example, mesoporous alumina of similar surface area was shown to be slightly catalytically active [22], while silica is not, and while the activity of carbon is much higher, although below that of Ni or Fe. The corresponding molar flow rates of each gas, compared at the inlet and at the outlet of the reactor, are presented in Fig. 3. These results of methane conversion were obtained with the following experimental: temperature 1000 8C, initial contact time 120 ms, duration of experiment 20 min, using the aforementioned nominal (i.e., wet) syngas composition. Fig. 3 evidences that CH4, H2O and CO2 were consumed, whereas H2 and CO were produced. Given these observations, the three independent, basic, following reactions are worth considering: C þ H2 O ! CO þ H2
(1)
C þ CO2 ! 2CO
(2)
CH4 ! C þ 2H2
(3)
In each of these equations, the solid carbon (wood char and pyrolytic carbon from Eq. (3)) plays a major role. In the following, the catalytic activity of the wood char will be discussed in relation with several experimental parameters: ash content, pore texture and surface functions composition.
Since a wood char always contains a non-negligible fraction of mineral matter, the influence of the latter on the observed catalytic activity of carbon was checked. For that purpose, the features of raw and demineralised chars, and their impact on the conversion of methane, were compared. The composition of ashes is reported in Table 1. In sample 1, the total amount of inorganic matter is close to 7 wt.%, which is a typical value for wood char. The main compounds are silica and oxides of alkaline and alkaline-earth elements: K, Ca and Mg, as expected for such a kind of carbonaceous material. The amount of Fe is surprisingly high (below but close to 1 wt.%); such a value was checked and indeed recovered in another experiment. Since the presence of iron may influence the catalytic activity of the carbon (although only metallic iron is supposed to be active), its content was worth to be known. 3.2.1. Effects of demineralisation on the physico-chemical properties of the char In order to separate a possible catalytic effect of the ashes, it was necessary that the demineralisation, achieved according to the method described in the previous section, did not change the pore texture of the wood char. Scanning electron microscope (SEM) observations and adsorption studies were thus carried out. The dispersion of ashes may be seen in Fig. 4, which shows some granules of raw wood char (sample 2). The pore morphology is typical of a carbon derived from wood: a well-open pore structure corresponding to the former wood cells and channels is evidenced. The images obtained using backscattered electrons suggest that some carbon grains contain more ashes than other ones: ashes are seen as bright zones unevenly distributed on the char grains. Various star-shaped particles of ash are sometimes evidenced, whose composition is mainly based on Ca and K, according to the EDX analysis. Such a shape is assumed to derive from temperatureinduced sintering and re-crystallisation of formerly present oxides and carbonates. After demineralisation, no particle of inorganic matter could be observed (see Fig. 5), suggesting the efficiency of the acid washing process. However, ICP analyses have shown that a low fraction of hetero-elements still remained. Table 1 shows that the demineralisation process removed typically 70% of the initial ash content. Such a washing allowed removing about 75% of the elements Fe and K, which are possibly active towards the conversion of methane. Heat-treatment at 1000 8C (sample 2) led to higher ash content because 18% of volatile matters were released, thus increasing the inorganic fraction. It is then easy to calculate that, after heattreatment at 1000 8C, the demineralised char should have an ash content of 2.6 wt.%. Pore textures and surface areas of sample 2 and its demineralised counterpart are given in Table 2. Since the pristine wood char (sample 1) was never used in the thermocatalytic process, its pore texture was not investigated. Sample 2 presents a rather low surface area: 142 m2 g1 and, as expected, the demineralisation has a low effect of the textural
Table 1 Mass fraction of ashes present in char samples, and their ICP analyses Kind of wood char
Ash content (wt.%)
SiO2 (wt.%)
N (wt.%)
P (wt.%)
K (wt.%)
Ca (wt.%)
Mg (wt.%)
Na (wt.%)
Fe (wt.%)
S (wt.%)
Sample 1 (=wood char just pyrolysed at 750 8C) Sample 2 (=sample 1 heat-treated at 1000 8C in nitrogen) Demineralised sample 1
6.98 8.50 2.16
1.860 2.200 0.840
0.510 0.450 0.310
0.044 0.050 0.020
0.398 0.512 0.109
1.048 1.239 0.305
0.157 0.176 0.092
0.044 0.057 0.010
0.952 1.238 0.195
0.022 0.031 0.004
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3.2.2. Effects of ashes on the conversion of methane The catalytic activities of the two kinds of char: raw (sample 2) and demineralised (sample 3), both previously pre-treated at 1000 8C, are compared in Fig. 6. In this plot, the conversion of methane is presented as a function of time for identical experimental conditions: 1000 8C, initial contact time of 120 ms, flow rate 0.43 N L min1 of the wet syngas (i.e., having the aforementioned nominal composition). It can be seen that the methane conversion is very high (70%), as compared with the data reported in the literature for the catalytic decomposition of pure methane, even if the reaction temperature used in the present study (1000 8C) is indeed somewhat higher than in other works (typically ranging from 700 8C to 950 8C [16– 27]). As already observed by other authors [18,21,23], the negligible role played by ashes may also be pointed out from Fig. 6, since both raw and demineralised materials led, within the experimental uncertainty, to the same conversion of methane. Especially, the presence of Fe in the carbonaceous material does not influence the conversion of methane, in agreement with the conclusions given in [18,21]. 3.3. Effect of the composition of the syngas: role of steam and CO2 The effect of both steam and carbon dioxide, two gases always present in pyrolysis atmospheres, is now discussed. Their influence on the physico-chemical properties of the char on one hand, and on the conversion of methane on the other hand, are considered consecutively.
Fig. 4. SEM pictures of wood char before demineralisation (sample 2) as seen by back-scattered electrons, at magnifications of (a) 120 and (b) 11,000.
properties (185 m2 g1). Such surface areas and corresponding pore volumes (below 0.1 cm3 g1) are typical of wood chars [44]. They are much lower than those of activated carbons, having BET surface areas and micropore volumes most of the times higher than 1000 m2 g1 and 1 cm3 g1, respectively [45], but are close to those of porous ceramics used as catalyst supports, like alumina, titania, or silica like the one we used for comparing its catalytic activity with that of our wood chars.
Fig. 5. SEM picture of wood char after demineralisation of sample 1, as seen by backscattered electrons.
3.3.1. Modification of the chars’ characteristics as a function of syngas composition Heterogeneous conversion of methane results in decomposing the CH4 molecule as pyrolytic carbon: C, at the surface of the catalyst, thus releasing gaseous hydrogen: 2H2. The structure of the deposit is known to be different from that of the support, at least as far as metal catalysts are concerned [46]. Moreover, when deposited onto a carbon substrate, a dense pyrolytic graphite is obtained in the present experimental conditions, i.e., at high temperature and low pressure ([47] and references therein), thus resembling those of the isothermal process of pyrolytic carbon infiltration in black ceramics [48]. Changes of pore texture were then expected after thermocatalytic decomposition of methane on wood char, and the results are reported in Table 3 and in Fig. 7. These data were obtained using sample 2 with different compositions of the syngas, an initial contact time of 100 ms, and after 40 min of experiment. Due to its very low surface area after reaction, the char submitted to methane alone (14 mol% in nitrogen) was not represented in Fig. 7. This material is the only one which weight increased during the experiment (see Table 3): +6 wt.% after 40 min. It is rather difficult to question such a value, since no comparable data (based on similar carbon catalyst, temperature and pressure conditions) may be found in the literature. Carbon deposition at 825 8C inducing weight uptakes of 3.6% and 13.3% were reported by Kamishita et al. [33] on lignite coal and on activated carbon, respectively, using a feed containing 6.5 vol.% of methane. However, the corresponding experiments were made in a thermobalance, and the contact times were consequently not controlled. The results of [33] agree with those of [24], who showed that the mass uptake was a function of the initial surface area of the carbon catalyst. If the deposited carbon is assumed to fill the porosity of the wood char, a maximum uptake of +16.8% would be possible, given the Gurvitch volume of 0.084 cm3 g1 reported in Table 3, and taking a density of 2 g cm3 [47] for the pyrolytic carbon. If the
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Table 2 Pore texture of raw and demineralised wood char, both having been heat-treated at 1000 8C Kind of wood char
BET surface area (m2 g1)
Micropore volume (cm3 g1)
Gurvitch volume (cm3 g1)
Sample 2 Sample 3 (=demineralised sample 2)
142 185
0.054 0.066
0.084 0.094
Table 3 BET surface areas, micropore and Gurvitch volumes of wood chars (sample 2) submitted to various atmospheres at 1000 8C: contact time = 100 ms; duration of experiment = 40 min Sample/gas composition
Weight changes (%)
BET surface area (m2 g1)
Micropore volume (cm3 g1)
Gurvitch volume (cm3 g1)
Reference = sample 2 Sample 2/CH4 (14%) in N2 (86%) Sample 2/CH4 (14%), H2O (30%), in N2 (56%) Sample 2/CH4 (14%), CO2 (14%), H2 (16%), CO (19%), in N2 (37%) Sample 2/nominal wet syngas = CH4 (14%), H2O (30%), CO2 (14%), H2 (16%), CO (19%), in N2 (7%)
0 +6 25 8 43
142 20 150 145 297
0.054 0.005 0.061 0.056 0.157
0.084 0.006 0.129 0.115 0.333
macropores of the wood char were taken into account, the carbon uptake could be even higher, much higher than the actual uptake of 6%. Consequently, given the strong drop of surface area (down to 20 m2 g1) obtained with a so low amount of deposited carbon, it is more likely that the deposit is not homogeneous, and especially that it accumulates at the entrance of the pores. In these conditions, only a small fraction of the initial char porosity becomes available for catalysis, and the closure of the pore mouths quickly hinders the diffusion of methane throughout these pores. An additional, shorter, experiment was made, in which the carbon deposit was close to 2 wt.% only. The analysis of the pore-size distribution (not presented here) showed that the micropore volume decreases much faster than the Gurvitch volume. In other words, micropores are closed first. Such a phenomenon was already evidenced in [33,34]. Two main mechanisms could explain why carbon especially settles at the pore mouths. Firstly, hydrogen produced by cracking of methane may impede its diffusion and hence restrict its access to the deeper pores. Secondly, the more reactive is the gaseous molecule towards the porous solid, the less this molecule can diffuse throughout the porosity, so the easier the reaction takes place on the outer surface. Besides, the wider are the pore entrances, the easier they can be entered. Deposition in narrow pores would require a low conversion of methane at very low pressure [47]. Moreover, various authors [24,33] have shown
that increasing the temperature leads to an increased carbon uptake. The diffusion rate of methane indeed increases with temperature, so a higher fraction of inner surface becomes available for cracking inside the porosity [24]. SEM pictures of the surface of sample 2(a) before, and (b) after having been submitted to methane (14 vol.% in nitrogen), are presented in Fig. 8 at the same magnification. If the carbon was assumed to be deposited in the form of a homogeneous layer, the expected thickness of the latter would be extremely small. Given that the surface area of the pristine char is 142 m2 g1, that the density of pyrolytic carbon formed at 1000 8C and 100 Torr (760 14%) is close to 2 g cm3 [47], and that the mass uptake after the experiment is 6% of the initial weight, the thickness of the deposited layer should be of the order of 0.21 nm. In the present experimental conditions, at high temperature and low pressure, the carbon formed by cracking of methane is expected to have the so-called ‘‘laminar aromatic’’ structure [47], different from that of the char substrate. However, even if this structure was certainly formed, observing with an electron microscope a thin layer of such a carbon onto another kind of carbon is extremely difficult, especially if the expected thickness is considered. Moreover, it should be stressed that, because they derive from wood fibres, the carbon grains are highly anisotropic. Thus, depending on the area magnified by the microscope, very different surfaces, from rather smooth to highly rough, may be observed. The gritty surface exhibited in Fig. 8(a) is a typical example. The pristine surface is
Fig. 6. Effect of the presence of ashes on the methane conversion (in wet syngas) as a function of time (1000 8C, contact time 120 ms).
Fig. 7. Pore-size distribution of sample 2 submitted to various atmospheres at 1000 8C during 40 min and a contact time of 100 ms.
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Fig. 8. SEM pictures of the surface of (a) the initial char, sample 2, (b) sample 2 after carbon deposition by action of methane alone (diluted in nitrogen), and (c) sample 2 submitted to wet syngas. Magnification: 35,000.
already very rough, just like the one after the decomposition process, see Fig. 8(b). Such a characteristic will prevent the observation of pyrolytic carbon deposited onto the original char during the thermocatalytic process. However, it is more likely that the deposit is not homogeneous, as explained above for pore mouth blocking, so carbon aggregates were looked for. Agglomerates were indeed found on both kinds of char, i.e., before and after having been submitted to methane diluted in nitrogen, see Fig. 9. Local chemical EDX analysis showed that such granules are made of pure carbon, but it cannot be claimed that they were produced by the cracking of methane, since very similar structures were already observed at the same magnification in the raw char. The granules should indeed rather be ascribed to the cracking of tars, a phenomenon which occurred during the (slow) pyrolysis of the wood. Tars having a rather long residence time within the char grains, their cracking as graphitelike aggregates is favoured [33]. As expected in the absence of metallic particles, neither filamental nor carbon whiskers could be observed. When the wood char was submitted to the thermocatalytic process using wet syngas (nominal composition), its surface was found to be strongly eroded (see Fig. 8c), evidencing that an important part of the initial matter was removed by the chemical attack, which was confirmed by the important weight loss of the sample (see Table 3). Such a phenomenon is well known and corresponds to the socalled ‘‘physical activation’’ (summarized by Eqs. (1) and (2)), which is very efficient when steam and CO2 are present at such a high temperature (1000 8C) [49]. The porosity of the carbonaceous
material is consequently both open and developed, with a related widening of all kinds of pores. Table 3 thus shows that the pore volumes of the material submitted to wet syngas are much higher than those of sample 2, and so is the corresponding BET surface area close to 300 m2/g, i.e., two times higher than that of the pristine material. Activation of the wood char with H2O and CO2 thus increased the pore volume and the surface area available for methane conversion, despite the concomitant deposition of pyrolytic carbon. The weight loss of the char increases with the partial pressure of oxidising gases (CO2 and H2O), see the gas compositions reported in Table 3. This finding is in agreement with the results obtained in [33,34] for a lignite char, but differs from what was seen with a char made from vinyl polymer, with which steam and CO2 did not react once pyrolytic carbon was deposited [34]. The micropore volumes and the BET surface areas of the initial char (sample 2) on one hand, and of the same material submitted to CH4 + H2O and to CH4 + CO2/CO/H2 on the other hand, are rather similar to each other (see Table 3). Both CO2 and steam, alone at 1000 8C in our partial pressure conditions, have a very low effect on the pore texture of the char undergoing catalytic conversion of methane, even if the pore volumes are slightly increased. Our present study is consequently different from other formerly published works, which mainly dealt either with pure methane alone or with methane followed by intermittent regeneration [19,25]. The action of a flow of methane and either H2O or CO2 on a char bed was also studied in [33,34] in a thermobalance, so only the weight changes of the bed (including char + pyrolytic carbon deposit) were investigated in detail; the conversion of CH4 was not
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Fig. 9. SEM pictures of the surface of (a) the initial char, sample 2 and (b) sample 2 after carbon deposition by action of methane alone (14 vol.% in nitrogen): view of the graphite-like aggregates.
studied, and the contact times were not controlled either. By contrast, in the present article, the simultaneous activation of the carbon catalyst was considered, i.e., during methane conversion and its resulting pyrolytic carbon deposition. 3.3.2. Effect of the presence of steam and CO2 on the conversion of methane Fig. 10 presents the conversion of methane plotted as a function of time, for the different syngas compositions already investigated above and reported in Table 3 and Fig. 7. The experimental conditions were thus, again: initial contact time 100 ms, temperature 1000 8C, duration of experiment 40 min. Dry methane, diluted in an inert gas (14 vol.% in N2), leads to the rapid deactivation of the carbon catalyst, such that the conversion drops down to 10% after 40 min. This finding is interpreted by the blocking of the microporosity by pyrolytic carbon, since a very low surface area is obtained for low amounts
Fig. 10. Conversion of methane on wood char (sample 2), as a function of time, for different atmospheres at 1000 8C (contact time 100 ms).
of carbon deposit (20 m2/g for only 6% of mass increase, see again Table 3). Thus, without the action of oxidising gases like H2O or CO2, a very low conversion is quickly reached, in agreement with what was reported in the literature for a number of different types of carbons [16–27]. In the presence of either CO2, or H2O, or both at once, the deactivation occurs at the same speed, but the conversion stabilises after 10 min to values as high as 45–60%, depending on the atmosphere. These gases were indeed seen to increase or at least maintain the surface area and pore texture of the catalyst (Table 3). CO2, in agreement with the low porosity development seen in Table 3 and Fig. 7, is the gas leading to the lowest conversion, slightly below 50%, whereas steam leads to better results, and finally both gases together, which were shown to duplicate the surface area, lead to the highest conversion, close to 60%. Table 3 has shown that both CO2 and H2O, when used separately, more develop the mesopore volume (i.e., the difference Gurvitch volume micropore volume) than the micropore volume itself, hence a low increase of the surface area was measured accordingly. The stabilisation of the methane conversion can be correlated to such a development of the mesoporosity, thus maintaining the availability of micropores. This result seems to contradict that of [24], who showed that the long-term char activity for methane conversion is related to the presence of mesopores. However, these authors worked with pure methane, without simultaneous activation of their catalyst, unlike what was done here. Therefore, the pyrolytic carbon was expected to block the micropores (
