Energy Fuels 2010, 24, 108–116 Published on Web 08/26/2009
: DOI:10.1021/ef900513a
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Catalytic and Noncatalytic Mechanisms in Steam Gasification of Char from the Pyrolysis of Biomass† Makiko Kajita,‡ Tokuji Kimura,‡ Koyo Norinaga,§ Chun-Zhu Li, and Jun-ichiro Hayashi*,§ )
‡ Center for Advanced Energy Conversion Materials, Hokkaido University, Sapporo 060-8628, Japan, §Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580, Japan, and Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia
Received May 23, 2009. Revised Manuscript Received August 9, 2009
Steam gasification of chars from the pyrolysis of a Japanese bamboo and cedar was studied using a reactor that enabled experimental definition of the gas composition in the vicinity of gasifying char particles. Intraparticle diffusion of neither steam nor the product gases influenced the kinetics of gasification. The chars underwent noncatalytic and catalytic gasification in parallel. The noncatalytic gasification, in which kinetic parameters were successfully defined by those for the gasification of the acid-washed char, was firstorder with respect to the amount of residual carbon over the entire range of char conversion. In consequence of this, contribution of the catalytic gasification was quantified as a function of the char conversion. Among the inherent alkali and alkaline earth metallic species, potassium (K) played the major catalytic role and its overall activity changed via a maximum in the course of gasification, suggesting the presence of optimum sizes of clusters or particles of K catalyst. The noncatalytic and catalytic reactions obeyed respective Langmuir-Hinshelwood mechanisms that involved inhibition by H2.
gasification mechanism would be to distinguish catalytic gasification from noncatalytic gasification.4-9 General kinetic models for gas-solid reactions, such as uniform conversion models, shrinking-core models, grain models,10 and random-pore models,11 are undoubtedly useful for considering dependency or independency of the rate of gasification on the char conversion. However, applying such models without distinguishing catalytic/noncatalytic reactions from each other may not reach understanding of the mechanism of the gasification, as claimed by Zhang et al.12 The rate of catalytic gasification is, different from the case of noncatalytic gasification, not necessarily a direct function of either the fraction of unconverted char (i.e., 1 - X) or its effective surface area. It may rather be a function of factors related to physical contact or chemical bonding between the catalyst and carbon matrix for any size of the catalyst particles, such as clusters.6-9 Here is an example that will be mentioned in detail later. In the present study, a random pore model described time-dependent changes in the char conversion successfully by optimizing a parameter for the initial porous structure of the char, ψ, of the following equation. It was, however, found that completely different values of ψ were required for the gasification of a parent char and that of an acid-washed char. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dX ¼ kð1-XÞ 1-ψ lnð1-XÞ ð1Þ dt
1. Introduction Steam gasification of char from the biomass pyrolysis is often the rate-determining step in the biomass gasification process,1 and it has therefore been studied extensively.2 On the other hand, the pyrolysis at a temperature much lower than that for the gasification can give char a tar-free (i.e., smokeless) nature and high reactivity with steam. Such reactivity is at least partly attributed to inherent alkali and/or alkaline earth metallic (AAEM) species that play catalytic roles. Thus, biomass char has a great potential of a high-quality solid fuel that is applicable to production of syngas by gasification with no tar removal processes, while quite a few existing biomass-to-gas processes suffer from problems caused by tar. Although there have been a number of technical reports on the steam gasification of biomass-derived char, understanding of the kinetics and mechanism of the gasification is not necessarily enough to design char gasifiers and gasification processes that can take full advantage of properties of biomass char as mentioned above. Catalysis of inherent AAEM species is a particular feature of the gasification of biomass char.3-5 It may be said in many cases that inherent catalysts contribute largely to the overall kinetics of gasification or even determine it. In this sense, the first step toward a quantitative and comprehensive understanding of the
As far as associated with the carbon matrix of char that contains ash precursors, such as silica- and alumina-containing
†
Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. E-mail: junichiro_
[email protected]. (1) Bridgwater, A. V. Fuel 1995, 74, 631–653. (2) Blasi, C. D. Prog. Energy Combust. Sci. 2009, 35, 121–140. (3) Raveendran, K.; Ganesh, A. Fuel 1998, 77, 769–781. (4) Zolin, A.; Jensen, A.; Jensen, P. A.; Frandsen, F.; Dam-Johansen, K. Energy Fuels 2001, 15, 1110–1122. (5) Keown, D.; Favas, G.; Hayashi, J.-i.; Li, C.-Z. Bioresour. Technol. 2005, 96, 1570–1577. (6) Hashimoto, K.; Miura, K.; Xu, J.; Watanabe, A.; Masukami, H. Fuel 1986, 65, 489–494. r 2009 American Chemical Society
(7) Walker, P. L., Jr.; Matsumoto, S.; Hanzawa, T.; Miura, T.; Ismail, M. K. Fuel 1983, 62, 140–149. (8) Hengel, T. D.; Walker, P. L. Fuel 1984, 63, 1214–1220. (9) Bazardorj, B.; Hayashi, J.-i.; Shimada, T.; Sathe, C.; Hatakeyama, K.; Li, C.-Z.; Chiba, T. Fuel 2005, 84, 1612–1621. (10) Szekely, J.; Evans, J.; Sohn, H. Gas-Solid Reactions; Academic Press: New York, 1976. (11) Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1980, 26, 379–386. (12) Zang, Y.; Ashizawa, M.; Kajitani, S.; Miura, K. Fuel 2008, 87, 475–481.
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: DOI:10.1021/ef900513a
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species, it is implausible that the activity of catalytic species is maintained until complete char conversion.9,13,14 In addition to this, volatilization3,13-15 is another factor lowering the overall catalytic activity. Catalytic roles of AAEM species were examined in previous studies on the steam gasification of biomass chars. However, in most of those studies, quantitative evaluation was made only at specific levels of char conversion of nearly 0 or 50%.14 The way of measuring the rate of gasification is very important. Even if the gasification is performed using char particles of sizes are small enough to assume a kinetically controlled process inside the particles, diffusion of steam and product gases across the char bed often may limit the overall mass transport, causing a difference in the gas composition between inside and outside the bed.9,17-21 Such a difference would make it difficult or even impossible to experimentally define the composition of gas surrounding gasifying char particles. Inhibition of steam gasification by H222-24 is wellknown, and it often decelerates the gasification tremendously. It is thus needed to provide bulk gas flow with a sufficient velocity around individual char particles for defining the gas composition there. In the present study, steam gasification of chars from two different biomass resources was investigated by means of a method that enabled us to experimentally define the gas composition around gasifying char particles. The catalytic gasification was distinguished from noncatalytic gasification by comparing the characteristics of gasification between the parent char and acid-washed char. Detailed kinetic analyses were carried out with attempts to develop a dual LangmuirHinshelwood (L-H) model, assuming progress of the catalytic and noncatalytic gasification in parallel, and to investigate behavior and catalytic roles of the inherent AAEM species.
Table 1. Carbon, Hydrogen, and Ash Contents of Parent Biomass and Char Samples biomass sample ID
C (wt % daf)
H (wt % daf)
ash (wt % db)
cedar bamboo cedar char (CC) bamboo char (BC)
50.9 49.0 79.9 81.7
6.4 6.1 3.9 3.7
0.4 0.9 1.6 3.9
CC and BC were washed with 3.0 N aqueous solution of HCl at 40 °C for 24 h for removal of AAEM species. After the acid washing, the char was washed with 60 °C deionized water repeatedly until no Cl- was detected in the filtrate. The acidwashed CC and BC are hereafter referred to as A-CC and A-BC, respectively. Contents of AAEM species were measured by means of a sequence of ashing under flow of air, digestion of resulting ash in HCl-HNO3 mixed acids, evaporation of the acids, dissolution of the ash into a CH3SO3H solution, and then analysis of the solution by ion chromatography.25 As shown in Table 2, Na and K were removed from both chars nearly completely, while the Ca and Mg removals were much lower. Every char sample was crushed to sizes within a range from 1.4 to 2.3 mm and subjected to gasification runs unless otherwise noted. As mentioned later, it was confirmed that the gasification was performed within a chemical-reaction-controlled regime even with the as-prepared char particles with rectangular shapes and sizes of ca. 772 mm. 2.2. Char Gasification. Steam gasification was performed with a reactor designed originally. It was made of a transparent quartz glass tube with an inner diameter of 15 mm, and its bottom was closed by employing a sheet of quartz-made wire mesh. The char sample with a mass of 35-45 mg was placed over the wire mesh in a form of monolayer. The char was heated up to 850 °C at a rate of 27.3 °C min-1 under a downward flow of N2 (purity >99.9998 vol %), and then the temperature was maintained. After a 15 min interval that was long enough for devolatilization of the char, the N2 flow was switched by that of a H2O-N2 or H2O-H2-N2 mixture. The losses of carbon until devolatilization were about 10% for CC and A-CC and 7% for BC and A-BC. CO, CO2, CH4, and H2 were formed because of the devolatilization, while the total yield of aromatics, except benzene and toluene, i.e., that of phenols and polyaromatic hydrocarbons with 2-6-fused rings, was well below 0.01 wt % of the initial mass of the char. The H2O-N2 or H2O-H2-N2 mixture was forced at a rate of 0.20 N L min-1 to pass through the monolayer of char particles and sent to a gas analysis system via a water condenser that was cooled at -73 °C. The dry gas was analyzed by gas chromatography at every 160-170 s. The concentration of H2O in the feeding gas was 15, 30, or 60 vol %, while that of H2 was 0, 10, or 20 vol %. The gas flow forced to pass through was in fact necessary to measure the real kinetics of steam gasification. Employment of commercially available thermogravimetric reactors (TGAs) is popular in studies on the kinetics of char gasification. However, as the present authors demonstrated in previous reports,9,18 diffusion of steam and/or product gases often limits the overall rate of char gasification, causing a clear difference in the gas composition between inside and outside the char bed. The instantaneous rate of gasification was calculated on a carbon basis from the flow rates of CO, CO2, and trace CH4 and integrated with respect to steam feeding time for determining the carbon-based char conversion, X, as a function of the time. Carbon-based selectivity to CH4 was around 0.5%. Although CH4 was not a direct product from the steam gasification, involving its yield to the overall char conversion caused insignificant error in the rate analysis of the steam gasification of the char. An example of time-dependent change in the char conversion is illustrated in Figure 1. Gas formation because of the
2. Experimental Section 2.1. Char Preparation. Chipped Japanese cedar and bamboo with average sizes of 10 10 2 mm and 10 5 2 mm, respectively, were used as the biomass samples. These were pyrolyzed in a horizontal screw-conveyor reactor for preparing char samples under the same conditions with a feeding rate, an average residence times of chips, a peak temperature, and a gas pressure inside the reactor of 270 g h-1, 47 s, 500 °C, and 1.0 atm, respectively. The heating rate of chips was estimated to be 5.0-5.5 °C s-1. Yields of the cedar and bamboo chars were 25.4 and 24.4 wt % on respective dry biomass bases. Elemental compositions and ash contents of the biomass and char samples are shown in Table 1. The cedar and bamboo chars are hereafter abbreviated to CC and BC, respectively. (13) Sonoyama, N.; Okuno, T.; Masek, O.; Hosokai, H.; Li, C.-Z.; Hayashi, J.-i. Energy Fuels 2006, 20, 1294–1297. (14) Okuno, T.; Sonoyama, N.; Sathe, C.; Li, C.-Z.; Hayashi, J.-i.; Chiba, T. Energy Fuels 2005, 19, 2164–2171. (15) Keown, D. M.; Hayashi, J.-i.; Li, C.-Z. Fuel 2008, 87, 1187–1194. (16) Mermoud, F.; Salvador, S.; Van de Steene, L.; Golfier, F. Fuel 2006, 85, 1473–1482. (17) G omez-Barea, A.; Ollero, P.; Arjona, R. Fuel 2005, 84, 1695– 1704. (18) Kitsuka, T.; Bazardorj, B.; Sonoyama, N.; Hosokai, S.; Li, C.-Z.; Norinaga, K.; Hayashi, J.-i. Energy Fuels 2007, 21, 387–394. (19) Messenbock, R. C.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1999, 13, 122–129. (20) Jamil, K.; Hayashi, J.-i.; Li, C.-Z. Fuel 2004, 83, 833–843. (21) Liu, H.; Luo, C.; Kaneko, M.; Kato, S.; Kojima, T. Energy Fuels 2003, 17, 961–970. (22) Yang, R. T.; Duan, R. Z. Carbon 1985, 23, 325. (23) Huttinger, K. J. Carbon 1988, 26, 79. (24) Lussier, M. G.; Zhang, Z.; Miller, D. J. Carbon 1998, 36, 1361.
(25) Li, C.-Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fuel 2000, 79, 427– 438.
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Figure 3. Effects of acid washing on the characteristics of gasification of (a) CC and (b) BC. PH2O=0.30 atm; PH2=0 atm.
Figure 1. Cumulative amount of carbon-containing gases as a function of time. Char sample, CC. Steam/hydrogen pressures at reactor inlet, PH2O = 0.60 atm; PH2 = 0 atm. X indicates the char conversion by steam gasification.
Figure 2. Effect of the particle size of CC on its conversion by steam gasification as a function of time. Steam/hydrogen pressures at reactor inlet, PH2O=0.60 atm; PH2=0 atm.
steam gasification was distinguished from that during the devolatilization clearly. Forced flow of steam-containing gas through the monolayer of char particles successfully made the reactor a differential one with steam conversion below 5%. H2O and H2 concentrations in the particle vicinities were thus defined by averaging concentrations of respective gases up- and downstream of the reactor. The concentration of steam in the product gas stream was not measured but calculated on the basis of an oxygen balance, i.e., from the flow rates of CO and CO2 and the atomic O/C ratio (0.06-0.08) of the char just before the steam gasification started. The O/C ratio was given as that of char prepared by a blank experiment, in which the char was heated in a flow of N2 in the same way as mentioned above and cooled to ambient temperature without supplying either H2O or H2. In preliminary experiments, char samples with carbon-based conversions of about 0.5 and 0.7 were prepared and it was found that the chars had an O/C ratio of 0.05-0.08, i.e., similar to those just before the steam gasification. In calculating the steam concentration, it was assumed that the O/C ratio of the char was maintained at 0.07 during the gasification. Namely, ΔrH2 O ¼ rCO þ2rCO2 -aðrCO þrCO2 þrCH4 Þ ð2Þ
Figure 4. Time-dependent changes in (1 - X) for gasification of A-CC and A-BC. (a) PH2O=0.60 atm; PH2=0 atm. (b) PH2O=0.15 atm; PH2=0 atm. (c) PH2O=0.60 atm; PH2=0.20 atm. Table 2. Contents of AAEM Species in the Char Samples char sample ID CC A-CC BC A-BC
Na (wt % db)
K (wt % db)
Ca (wt % db)
Mg (wt % db)
0.02
