Effect of Alkali and Alkaline Earth Metallic Species on Biochar

Jul 24, 2009 - Curtin Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University of. Technology, GPO Bo...
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Energy Fuels 2010, 24, 173–181 Published on Web 07/24/2009

: DOI:10.1021/ef900534n

Effect of Alkali and Alkaline Earth Metallic Species on Biochar Reactivity and Syngas Compositions during Steam Gasification† Kongvui Yip,‡ Fujun Tian,‡ Jun-ichiro Hayashi,§ and Hongwei Wu*,‡ Curtin Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia, and Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, 816-8580, Japan. ‡Curtin University of Technology. §Kyushu University. Received May 26, 2009. Revised Manuscript Received June 30, 2009

Biochars were prepared from the pyrolysis of the wood, leaf, and bark components of mallee biomass in a fixed-bed reactor at 750 °C. The results show that the volatilization of inherent alkali and alkaline earth metallic (AAEM) species is 10-20% during the pyrolysis of raw wood, bark, and leaf samples. Acid treatment of the biochar samples was also carried out to prepare a set of acid-treated biochar samples. Although the majority of the inherent AAEM species were removed by acid-treatment, there are always some AAEM species that could not be removed for all biochars. Steam gasification experiments of both the raw and acid-treated biochar samples were carried out in a fixed-bed reactor at 750 °C and a steam concentration of 8.2 vol %. Data on the instantaneous syngas composition were obtained as a function of biochar conversion during steam gasification. Our data illustrated the importance of, in the study of steam gasification reaction mechanisms and kinetics of solid fuels such as biochars, optimizing the reaction conditions to minimize steam consumption so that the steam partial pressure in the reactor is kept reasonably constant during the whole course of gasification. The results indicate that Na, K, and Ca retained in the biochars are the key catalytic species, with the catalytic effect appearing to be in the order K > Na > Ca during the steam gasification reaction of these biochars. During steam gasification, almost all of the inherent AAEM species in biochar are retained in the reacting biochar, throughout the course of conversion. Steam gasification of both the raw and acid-treated biochars produces high-quality syngas products that contain little methane. Further analysis shows that during the course of biochar conversion, the primary gasification product is most likely CO, and overall the water-gas-shift reaction is primarily responsible for the CO2 formation. It is found that the inherent AAEM species, although catalyzing the biochar gasification significantly, appear to have insignificant catalytic effect on the water-gas-shift reaction under the current gasification conditions.

and the synthesis of chemicals or liquid fuels.7,8 Biomass pyrolysis is the first step of biomass gasification, therefore gasification of biochar, which is produced from biomass pyrolysis,9-11 can play an important role in overall biomass gasification reactions. Additionally, biomass as a fuel has several undesired characteristics, including the bulky nature, high moisture, and low energy density.1,9,11 A recent study11 has shown that biochar produced from biomass via pyrolysis is of high energy density and also has excellent grindability therefore can be attractive for biomass utilization. Therefore, there is a strong need to understand the behavior of biochar steam gasification. Biomass may contain a significant amount of inherent alkali and alkaline metallic (AAEM) species as a result of nutrients uptake during growth. It is well-known that AAEM species can be good catalysts for the combustion

1. Introduction Biomass is one of the most important renewable energy sources.1 In Australia, mallee biomass is projected to be a major biomass source for bioenergy production, as byproduct of managing dryland salinity to prevent the loss of quality agricultural land.2-4 Mallee biomass production in WA achieves excellent energy performance and is close to carbon neutral.5,6 Therefore, mallee biomass can contribute significantly to local energy security and regional development in Australia. Gasification is an important process for utilization of biomass in power generation, production of hydrogen, † Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. Telephone: þ61-892667592. Fax: þ61-8-92662681. E-mail: [email protected]. (1) Varma, A.; Behera, B. Green Energy: Biomass Processing and Technology; Capital Publishing Company: New Delhi, 2003. (2) Cooper, D.; Olsen, G.; Bartle, J. Aust. J. Exp. Agric. 2005, 45, 1369. (3) Bartle, J.; Olsen, G.; Don, C.; Trevor, H. Int. J. Global Energy Issues 2007, 27, 115–137. (4) Yu, Y.; Bartle, J.; Li, C.-Z.; Wu, H. Energy Fuels 2009, 23, 3290– 3299. (5) Wu, H.; Qiang, F.; Rick, G.; Bartle, J. Energy Fuels 2008, 22, 190– 198. (6) Yu, Y.; Wu, H. Life cycle greenhouse gas emission from mallee biomass production. CHEMECA 2008; Newcastle, Australia, 2008.

r 2009 American Chemical Society

(7) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106 (9), 4044. (8) Yung, M. M.; Jablonski, W. S.; Magrini-Bair, K. A. Energy Fuels 2009, 23, 1874. (9) Das, P.; Ganesh, A.; Wangikar, P. Biomass Bioenergy 2004, 27, 445–457. (10) Ozcimen, D.; Karaosmanoglu, F. Renewable Energy 2004, 29, 779–787. (11) Abdullah, H.; Wu, H., Energy Fuels, DOI: 10.1021/ef900494t, in press.

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and gasification of solid carbonaceous fuels. During gasification/combustion, these species are also responsible for various notorious ash-related operational problems and erosion of combustors/gasifiers and downstream gas turbines.15 It is known that volatilization of AAEM species during pyrolysis and gasification of Victorian brown coal can occur to a varying extent,16-19 which can in turn affect the coal char gasification reactivity significantly.16,20-27 Even during the gasification of some other low-rank fuels such as sub-bituminous coal chars,28,29 the inherent AAEM species can play an important role in the gasification behaviors. Previous studies30-36 have also shown that the AAEM species in biomass, are good catalysts for biomass steam gasification reactions. After biomass pyrolysis, at least part of the inherent AAEM species may be retained in the biochar. To use biochar as a fuel, a thorough understanding on the transformation of the inherent AAEM species during gasification is required. The understanding on the composition of syngas produced from biochar steam gasification is also essential to the understanding of gasification reaction mechanisms and the tailoring and adjusting of syngas37 for suitable applications in practice. While previous studies were mainly concerned on coal chars,38-41 insufficient work has been done so far to study the catalytic effects of inherent AAEM species on the syngas compositions during biochar steam gasification. It is expected that the inherent AAEM species in the reacting biochar and

the biochar reactivity evolves with the progress of gasification reactions. However little is known on how such evolution will affect the instantaneous syngas compositions as a function of biochar gasification conversion. There are also debates in the literature on the possible reaction routes or pathways for catalyzed char steam gasification, which account for the gaseous compositions observed. For instance, some studies38,39 assumed or reported that CO is the primary product from the catalyzed char gasification, whereas other workers40,41 proposed that CO2 is possibly the primary one. This is an important fundamental question to be answered for utilization of biochars as a fuel in practice. Typically, for studying the catalytic effect of AAEM species on char gasification reactions, experiments are carried out using chars produced from the pyrolysis of the so-called “Hform” samples which have been prepared from acid-washing of the raw coal or biomass samples to remove the majority of inherent AAEM species in the coal or biomass.15,23,26,39,42-46 However, comparisons between such chars with those of the raw coal or biomass for studying the effect of AAEM species on char steam gasification are complicated by the possible effect of the inherent AAEM species in the raw coal or biomass on pyrolysis reactions and hence on the structure of the resultant chars. Therefore, acid treatment of biochars after biomass pyrolysis, instead of raw biomass before pyrolysis, would be a better approach to prepare acid-treated biochar samples for investigating the effect of AAEM species on biochar steam gasification. In real applications, biomass produced from harvested mallee trees is a mixture containing various components (e.g., wood, leaf, and bark), which will be delivered to bioenergy plant as gasification feedstock as separation of these biomass components is impractical. Individual biomass components may have significantly different fuel properties and characteristics, which should also be important considerations in the design and operation of practical gasifiers. Therefore, the present study aims to investigate steam gasification of biochars produced from the wood, leaf, and bark fractions of mallee biomass (E.loxophleba lissophloia). The study specially focuses on investigating the effect of inherent AAEM species on the instantaneous gasification reactivity and evolution of syngas compositions during the course of biochar steam gasification. The objectives are to understand the gasification behavior of different biomass components, the key inherent catalytic species affecting biochar reactivity, and the most probable steam gasification reaction route that dictates the gaseous yields and selectivity, all of which are essential to gasification process design and development.

(12) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221–270. (13) Miura, K.; Hashimoto, K.; Silveston, L. Fuel 1989, 68, 1461– 1475. (14) Wen, W.-Y. Catal. Rev. - Sci. Eng. 1980, 22, 1–28. (15) Li, C.-Z. Fuel 2007, 86, 1664–1683. (16) Li, X.; Wu, H.; Hayashi, J.; Li, C.-Z. Fuel 2004, 83, 1273–1279. (17) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 143–149. (18) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 151–158. (19) Wu, H.; Quyn, D. M.; Li, C.-Z. Fuel 2002, 81, 1033–1039. (20) Quyn, D. M.; Wu, H.; Hayashi, J.-i.; Li, C.-Z. Fuel 2003, 82, 587– 593. (21) Wu, H.; Hayashi, J.-i.; Chiba, T.; Takarada, T.; Li, C.-Z. Fuel 2004, 83, 23–30. (22) Wu, H.; Li, X.; Hayashi, J.; Chiba, T.; Li, C.-Z. Fuel 2005, 84, 1221–1228. (23) Bayarsaikhan, B.; Hayashi, J.; Shinada, T.; Sathe, C.; Li, C.-Z.; Tsutsumi, A.; Chiba, T. Fuel 2005, 84, 1612–1621. (24) Quyn, D. M.; Hayashi, J.; Li, C.-Z. Fuel Process. Technol. 2005, 86, 1241–1251. (25) Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Shinada, T.; Hayashi, J.; Li, C.-Z.; Chiba, T. Fuel 2006, 85, 340–349. (26) Kitsuka, T.; Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Li, C.-Z.; Norinaga, K.; Hayashi, J. Energy Fuels 2007, 21, 387–394. (27) Hayashi, J.; Takahashi, H.; Iwatsuki, M.; Essaki, K.; Tsutsumi, A.; Chiba, T. Fuel 2000, 79, 439. (28) Yip, K.; Wu, H.; Zhang, D.-k. Energy Fuels 2007, 21, 419–425. (29) Yip, K.; Wu, H.; Zhang, D.-k. Energy Fuels 2007, 21, 2883–2891. (30) Garca, L.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Energy Fuels 1999, 13, 851–859. (31) Moghtaderi, B. Fuel 2007, 86, 2422–2430. (32) Dastidar, M. G.; Sarkar, M. K. Fuel Process. Technol. 1983, 7, 261–275. (33) Baker, E. G.; Mudge, L. K. J. Anal. Appl. Pyrolysis 1984, 6, 285– 297. (34) Mudge, L. K.; Sealock, L. J.; Weber, S. L. J. Anal. Appl. Pyrolysis 1979, 1, 165–175. (35) Elliott, D. C.; Hallen, R. T.; Sealock, L. J. J. Anal. Appl. Pyrolysis 1984, 6, 299–316. (36) Encinar, J. M.; Gonzalez, J. F.; Rodriguez, J. J.; Ramiro, M. J. Fuel 2001, 80, 2025–2036. (37) Moulijn, J. A.; Makkee, M.; Van Diepen, A. V. Chemical Process Technology; John Wiley & Sons: England, 2001. (38) Kayembe, N.; Pulsifer, A. H. Fuel 1976, 55, 211–216. (39) Miura, K.; Aimi, M.; Naito, T.; Hashimoto, K. Fuel 1986, 65, 407–411. (40) Saber, J. M.; Kester, K. B.; Falconer, J. L.; Brown, L. F. J. Catal. 1988, 109, 329–346. (41) Wigmans, T.; Elfring, R.; Moulijn, J. A. Carbon 1983, 21, 1–12.

2. Experimental Section 2.1. Samples. The green mallee trees (E.loxophleba lissophloia) were harvested from Narrogin, Western Australia and then fractioned into various biomass components, with the three major components being wood, leaf, and bark. Samples of these biomass components were dried at 40 °C in a large lab oven before being cut using a cutting mill into sizes (85%) among all inherent inorganic species in the wood, bark, and leaf biomass samples, therefore this study focuses on AAEM species. The data in Table 2 also provide some guidance that may assist in evaluating the performance of different biomass components in the use of biomass as feedstock for production of biochar and for subsequent biochar gasification. The content of AAEM species in wood is the lowest compared to leaf and bark.

(50) Wiedenhoeft, A.; Miller, R. Structure and Function of Wood. In Handbook of Wood Chemistry and Wood Composites; Rowell, R. M., Ed.; Taylor & Francis: New York, 2005; Ch 2.

(49) Li, C.-Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fuel 2000, 79, 427– 438.

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Table 3. Contents of AAEM Species (Expressed as Mol of Species/Mol of C) in the Biochars Produced from the Pyrolysis of Raw Biomass and in the Acid-treated Biochars samples

Na, mol of species/mol of C

K, mol of species/mol of C

Mg, mol of species/mol of C

Ca, mol of species/mol of C

raw wood char acid-treated wood char raw leaf char acid-treated leaf char raw bark char acid-treated bark char

0.000795 0.000055 0.014800 0.004170 0.004502 0.000786

0.001130 0.000031 0.005510 0.000271 0.001317 0.000125

0.000686 0.000266 0.003930 0.002980 0.001570 0.001490

0.002340 0.001520 0.010300 0.001890 0.017955 0.002250

Figure 3. AAEM retention in the biochars following pyrolysis of raw biomass.

Figure 5. Specific gasification reactivity as a function of carbon conversion for various biochars, gasification at 750 °C with a steam concentration of 8.2 vol %.

species can be good catalysts for gasification reactions; the other is that the volatilization of AAEM species, and hence the ash-related problems, are minimized. For the retention of AAEM species in the acid-treated biochars, a similar trend was also observed. 3.2. Biochar Steam Gasification Reactivity. Figure 5 shows the specific gasification reactivity of biochars prepared from various biomass samples. It can be seen that the acid-treated biochars have much lower gasification reactivity than that of the biochars from raw biomass, suggesting the expected significant catalysis of inherent AAEM species in biochars. For the steam gasification of raw biochars, the specific reactivity follows the order: the leaf char > the bark char > the wood char, and their reactivity increases as the carbon conversion increases. On the other hand, the acid-treated biochars appear to have rather similar reactivity, and their reactivity appear to remain constant throughout the whole level of conversion studied. As aforementioned, all experimental data presented in this paper were obtained from gasification experiments that were optimized to have minimal total steam conversion during biochar steam gasification. Figure 6 shows the effect of total steam conversion on the biochar gasification. As shown in Figure 6b, the total steam consumptions are minimal in all experiments (1-5%). This is critical to the interpretation of experimental data. Experiments were conducted to illustrate the importance of this consideration. Figure 6a shows the comparison between the reactivity of the raw leaf char and the raw bark char for the cases with sufficiently low total steam conversion (1-5 vol %, the reactivity curves are identical to those shown in Figure 5) and high total steam conversion. The corresponding steam concentrations after reactions (after the char bed) are shown in Figure 6b. Here, for each biochar, the variation of total steam conversion was achieved by the use of different amounts of biochar (i.e., different char bed depth in the reactor). It can be seen that for

Figure 4. Retention of AAEM species (Na, K, and Ca, as labeled in the graphs, respectively) in raw biochars as a function of carbon conversion during steam gasification at 750 °C with a steam concentration of 8.2 vol % (0% carbon conversion refers to the biochar from pyrolysis).

The contents of AAEM species in various biochars were quantified, and the retentions at various gasification conversions, with respect to the biochar from pyrolysis (“0%” gasification conversion), were determined from mass balance, as shown in Figure 4. It can be seen in Figure 4 that there is insignificant loss/volatilization of AAEM species during the course of steam gasification for all cases under the current conditions. The insignificant AAEM species volatilization is mainly due to the absence of volatile-char interactions in the current experimental conditions in a fixedreaction system and the little effect of entrainment by product gas (as compared to that in fluidized-bed systems51). It is important to retain the AAEM species within the reacting biochars for two reasons. One is that these AAEM (51) Li, X.; Li, C.-Z. Fuel 2006, 85, 1518–1525.

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Figure 6. (a) Comparison between reactivity of raw leaf char and raw bark char for the cases with low and high total steam conversion during the gasification experiment. (b) Steam concentration after the char bed for various biochars during gasification.

Figure 7. AAEM species (Na, K, and Ca, as labeled in the graphs, respectively) contents, expressed as mol of species/mol of C, in the raw biochars as a function of carbon conversion during the steam gasification of the raw wood char (9); raw leaf char (b); raw bark char (2); acid-treated wood char (0); acid-treated leaf char; (O) and acid-treated bark (4).

the case with a thick char bed (i.e., a high total steam conversion, as high as 25 vol %), the specific biochar reactivity is considerably lower than that with a thin char bed (i.e., low steam conversion), due to the reduced steam partial pressure throughout the char bed. Furthermore, this discrepancy in reactivity, due to the variation in steam concentration, appears not to obey the simple m-order power law kinetic equation for steam gasification where the reactivity is a function of (steam concentration)m, with m mostly 0.4-0.6 for various chars.12,52 For instance, this effect varies with carbon conversion and is also more significant for the raw leaf char (where the catalytic effect of AAEM species is more dominant) compared to the raw bark char. Therefore, in order to obtain useful experimental data for understanding gasification reaction mechanisms and kinetics of solid fuels such as biochars, the total steam conversion must to be kept minimal throughout the char bed during the whole carbon conversion level. The results in Table 3 and Figure 5 together reveal some insights into the importance of various AAEM species on the biochar reactivity. The raw leaf char, which has the highest reactivity, contains high amount of Na and K. Ca is notably high in the raw bark char, and it is also high in the raw leaf char. These AAEM species are considerably removed through acid treatment, which reduces the biochar reactivity drastically. Mg, on the other hand, is not considerably removed through acid treatment. Therefore, it appears that Na, K, and Ca are probably the key species playing catalytic roles on the raw biochar reactivity. Figure 7 shows the AAEM species contents, expressed as mol of species per mol of C, in the raw biochars and the acidtreated biochars as a function of carbon conversion during gasification. The curves in Figure 7 were constructed based on the trend obtained on the retention of these species in the biochars shown in Figure 4, taking the volatilization of

AAEM species to be negligible for all biochars during gasification. The contents of AAEM species in biochar increase with carbon conversion during steam gasification. Careful examination into Figures 5 and 7 provides some insights on the relation between biochar reactivity and the AAEM species in biochars. The two figures clearly show that the biochar reactivity generally increases with the concentration of AAEM species in the biochars. The biochar from raw wood, having the lowest contents of AAEM species among the biochars from raw biomass, also has the lowest reactivity among these biochars. The biochar from raw leaf, containing high amounts of Na and K relative to the biochars from raw bark and raw wood, shows the highest reactivity. The biochar from raw bark, having the highest Ca content but lower contents of Na and K than the biochar from raw leaf, has intermediate reactivity. These results suggest that Na and K in the samples lead to higher catalytic activity than Ca. This is consistent with the previous findings in the literature on the relative catalytic effects of AAEM species during the steam gasification of coal chars.42,43 Moreover, from Table 3, it can be seen that the raw wood char contains lower or comparable amounts of Na and Ca but a higher amount of K, compared to the acid-treated leaf char and the acidtreated bark char. However, the raw wood char still has noticeably higher reactivity than the acid-treated biochars. This further suggests that K probably contributes to higher catalytic activity than Na. Another important observation from Figure 5 is the trend of variation of reactivity with conversion. The reactivity of raw wood char remains rather constant up to conversion level around 55-60% and then increases drastically. This may be due to the fact that the AAEM species concentration in the biochar at an early conversion level is too low, but

(52) Yip, K.; Wu, H.; Zhang, D.-k. Proc. Combust. Inst. 2009, 32, 2675–2683.

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Figure 8. Specific formation rate of gaseous products, including CO (0), CO2 (O), and H2(4), during the gasification of: raw wood char, raw leaf char, raw bark char, acid-treated wood char, acid-treated leaf char, and acid-treated bark char, respectively. (Little CH4 was formed during the steam gasification of all biochars under the reaction conditions.)

when the concentration reached a “critical” level the catalytic effect, and hence the reactivity, is increased remarkably. Such a trend on the existence of a critical AAEM concentration has also been observed in previous studies, such as those on brown coal char oxidation at low temperatures.21 The reactivity of the raw bark char increases considerably throughout the conversion level. On the other hand, the reactivity of the raw leaf char increases considerably at low conversion, then its increase becomes less significant at high conversion. The above observations indicate that, apart from the catalyst contents, other factors such as the biochar structure, which can affect the dispersion of AAEM species and the interactions between carbon and the catalysts,15,21,51,53 are also important. Also, from Figure 7, generally the AAEM contents in the acid-treated biochars are significantly lower than that in the raw biochars. One interesting observation can be noted on the behavior of acid-treated leaf char, which has a relatively high content of AAEM species (compared to acid-treated wood char and acid-treated bark char). The contents of AAEM species in the acid-treated leaf char at 80% conversion are higher (for Na and Ca) or at least comparable (for K) to those in the raw wood char at 55-60% conversion (where the critical concentrations of AAEM species are reached and the reactivity starts to increase considerably, as aforementioned). However, for the acid-treated leaf char, its reactivity at 80% conversion still does not show any increasing trend, possibly due to that at least part of the AAEM species has been deactivated by reaction with other mineral matter to form catalytically inactive silicate or aluminosilicate;13,23 or these AAEM species, which are originally present in the biochar and are difficult to wash by acid treatment, may exist in some forms that are catalytically inactive. Further study is required to understand this phenomenon. 3.3. Instantaneous Syngas Compositions and Possible Primary Reaction Route of Biochar Steam Gasification. Figure 8

shows the specific gas formation rates during gasification of the various biochars. Generally, the specific gas formation rates during gasification of the raw biochars are higher than that of the acid-treated biochars. This is consistent with the fact that the AAEM species catalyze the biochar gasification reaction, as already discussed in the previous sections. It is interesting to note that little CH4 was formed during the gasification of all biochars. This is in accordance with the fact that at temperature above 900 K and at atmospheric pressure the production of CH4 in carbon steam gasification reaction is thermodynamically unfavored.41 Such an observation has significant practical implications, particularly when the biochar, instead of the biomass, is adopted as the feedstock to the gasifier in practice, hence no methane will be further produced from the pyrolysis of biomass in the gasifier, as syngas free of methane is considered to be of good quality for subsequent utilization. For example, methane is undesirable in the syngas because it can cause the soot formation and deactivation of catalysts37 in catalytic synthesis processes (e.g., F-T synthesis) for liquid fuels. In fuel cell application, CH4 will have to either be reformed onboard or recycled for energy recovery. In fact, in the present work, the H2 production rate, at any reaction time, can be calculated from the measured CO and CO2 production rates by: H2 production rate = CO production rate þ 2 (CO2 production rate). It is found that the calculated H2 production rate is in good agreement with the measured H2 production rate, for all cases. This is in accordance with the observation that CH4 formation is negligible (therefore the CH4 formation rate is not plotted in Figure 8). As aforementioned, there has been controversy in the literature on whether CO or CO2 is the primary product during catalyzed carbon steam gasification reaction. Therefore, analysis was then carried out to investigate the possible routes of biochar steam gasification. The approach to equilibrium of the water-gas-shift reaction can provide insights into the possible routes for the production of CO2 and CO

(53) Li, X.; Hayashi, J.; Li, C.-Z. Fuel 2006, 85, 1509–1517.

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Figure 9. Comparison between the actual CO2/CO and H2/CO ratios and the theoretical equilibrium ratios, for the wood chars (raw and acidtreated), the leaf chars (raw and acid-treated), and the bark chars (raw and acid-treated), respectively. Legend: 9 - raw biochars, actual values; b - raw biochars, equilibrium values; 0 - acid-treated biochars, actual values; O - acid-treated biochars, equilibrium values.

and whether CO and/or CO2 is the primary product of the gasification.41 If the water-gas shift reaction is at equilibrium, no distinction between the routes is possible. Conversely, if no equilibrium is attained, the gaseous product ratios provide information on the reaction route. Lower CO2/CO and H2/CO ratios than the equilibrium values dictate that CO is the primary product, whereas ratios higher than the equilibrium values indicate the formation of CO2 as the primary product. Furthermore, product ratios CO2/CO and H2/CO affect the thermal efficiency in biomass conversion and utilization processes54 and are also important to syngas applications. Therefore, the (actual) various gas products ratios of syngas that is instantaneously produced at a given reaction time is thus determined from the measured H2, CO, and CO2 formation and then compared to the theoretical equilibrium gaseous product ratios as a function of biochar conversion level (see Figure 9). The theoretical equilibrium gaseous product ratios are estimated as follows. For reaction

Figure 9 shows that, under the current experimental conditions, the CO2/CO ratio and the H2/CO ratio from the gasification of the raw biochars and acid-treated biochars are all lower than the corresponding equilibrium values. In this study, the steam gasification was verified to be under the chemical-reaction-controlled regime, where mass transport effects are minimized. Therefore, the considerably lower gaseous product ratios compared to the equilibrium values, as shown in Figure 9 for all cases, suggest that CO is likely to be the primary reaction product for the gasification of biochars used in the present study. It follows that overall, it is the water-gasshift reaction that is primarily responsible for the CO2 formation. The reaction scheme C þ H2O f CO þ H2 and CO þ H2O T CO2 þ H2 may thus primarily contribute to the gaseous compositions observed. Hence, it appears that the catalyzed steam gasification reaction for the raw biochars primarily follow the similar reaction scheme with the acid-treated biochars. It is possible that the catalysis possibly occurs through the redox mechanism as proposed by various workers,14,42,55 which results in the above overall reaction scheme. However, it should be pointed out that this does not exclude the possibility of CO2 being one of the direct products of the biochar gasification reactions (i.e., through the reaction C þ 2H2O f CO2 þ 2H2), although we can deduce that CO is the primary one relative to CO2. The relative extent of C þ 2H2O f CO2 þ 2H2 and C þ 2H2O T CO2 þ 2H2 can be determined only if the contribution of the water-gas-shift reaction (CO þ H2O T CO2 þ H2) can be separated from that of the gasification reaction, which, however, is difficult using the current experimental setup (even though a differential reactor has been used, with a thin char bed, hence the contact of the gasification product with the reacting char bed is expected to be small). It is also interesting to see in Figure 9 that the CO2/CO ratio and the H2/CO ratio from the gasification of the raw biochars are similar to those from and acid-treated biochars.

CO þ H2 O T CO2 þ H2 with Keq (the equilibrium constant) = (pCO2pH2)/(pCOpH2O), where p is the partial pressure, the CO2/CO ratio at equilibrium is !   pCO2 pH2 O ¼ Keq pCO eq pH2 actual

The H2/CO ratio at equilibrium is thus determined as !   pH2 pH2 O ¼ Keq pCO eq pCO2 actual

The total H2O (steam) conversion is calculated by considering the amount of steam consumed by the biochar gasification as well as the water-gas-shift reaction. (54) Tamai, Y.; Watanabe, H.; Tomita, A. Carbon 1977, 15, 103–106.

(55) McKee, D. W.; Chatterji, D. Carbon 1978, 16, 53–57.

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4. Conclusions The present work has provided insights into the importance and roles of catalytic species in the steam gasification of wood, leaf and bark chars. Under the current experimental conditions in the fixed-bed reaction system, the loss/volatilization of AAEM species from the biochars is not appreciable throughout the course of steam gasification, and is only marginal during pyrolysis. Our data demonstrate the great importance of minimizing the steam consumption during the whole course of gasification for keeping the steam partial pressure in the reactor as reasonably constant, in order to obtain useful experimental data for understanding gasification reaction mechanisms and kinetics of solid fuels such as biochars. The inherent AAEM species in biochars play an important role in catalyzing the biochar gasification reaction. Na, K, and Ca are most probably the species responsible for the catalytic effect, with the catalytic effect in the order K > Na > Ca for these bichars. Acid-treatment of the biochar can significantly reduce the contents of AAEM species and the reactivity of biochars. The remaining AAEM species in the acid-treated biochars have little catalytic effect during the course of steam gasification. Biochar steam gasification also produces highquality syngas product that contains little methane. It also seems that the primary product from the steam gasification of biochars in the present study is carbon monoxide. The inherent AAEM species in biochars appear to have insignificant catalytic effect on the water-gas-shift reaction under the current experimental conditions.

Figure 10. Amount of steam consumed by the water-gas-shift (WGS) reaction normalized to a unit mol of carbon gasified by steam, for the various biochars during gasification.

To understand this phenomenon, data are therefore derived and plotted in Figure 10 on the amount of steam consumed by the water-gas-shift reaction normalized to a unit mol of carbon gasified, assuming the reaction scheme C þ H2O f CO þ H2 and CO þ H2O T CO2 þ H2. The data in Figure 10 reveals the extent of the water-gas-shift reaction relative to the biochar gasification reaction, hence giving an indication on the extent of catalysis of AAEM species on the water-gasshift reaction. Figure 10 shows a similar trend for both the raw and acid-treated biochars during the course of steam gasification. Taking Figures 9 and 10 together, it appears that the inherent AAEM species have insignificant catalytic effect on the water-gas-shift reaction. This may be due to the reaction conditions used in this study. In the current fixedbed reaction system, the gasification products are continuously swept away by the flowing gas, and a thin char bed is used for all cases. The results suggest that under such conditions, the catalysis of the AAEM species on the water-gas-shift reaction is insignificant.

Acknowledgment. This research is partially supported by the Australian Commonwealth Government as part of the AsiaPacific Partnership on Clean Development and Climate. Support from Australian Commonwealth Government’s Department of Innovation Industry, Science and Research through the Australia-China Special Fund for S&T Cooperation (CH070008) is also acknowledged.

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