Energy & Fuels 2006, 20, 1315-1320
1315
Mechanism of Woody Biomass Pyrolysis and Gasification in a Fluidized Bed of Porous Alumina Particles Koichi Matsuoka,*,† Takaaki Shinbori,† Koji Kuramoto,† Tetsuya Nanba,‡ Atsuko Morita,† Hiroyuki Hatano,† and Yoshizo Suzuki† Energy Technology Research Institute, and Research Institute for EnVironmental Management Technology, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ReceiVed January 15, 2006. ReVised Manuscript ReceiVed March 19, 2006
To understand the mechanism of steam gasification of woody biomass, pyrolysis and gasification were carried out at 773 and 973 K on porous γ-alumina in a two-stage fluidized bed reactor. Significant amounts of volatiles, including gas and tarry materials, were formed upon devolatilization, but the tar was mostly captured by the alumina. The tar captured on the alumina was subsequently reformed with the steam to H2 and CO. However, little of the char was gasified under the present conditions. CO and water molecules penetrated into the large pores of the char, allowing the water-gas shift reaction to occur on the active sites on the surface of the char. Although the char itself was not gasified at these relatively low temperatures, it played an important role in the water-gas shift reaction.
Introduction The production of hydrogen from biomass is attracting more and more attention. Various studies on the gasification of biomass to produce hydrogen show that the type of gasification process depends on the type of biomass, or more specifically, the moisture content of the biomass. Relatively dry biomass (e.g., woody biomass) has frequently been employed in thermochemical gasification processes,1 whereas wet biomass (e.g., sewage sludge) has sometimes been treated in supercritical water.2 Woody biomass gasified at relatively low temperatures (e.g., 773 K-1073 K) in the thermochemical process forms greater amounts of undesirable tarry materials (condensable organic compounds) that eventually cause trouble in the processing equipment as well as in the turbines and engines that use the product gas. Much work has been conducted to avoid this trouble with tar.3 Catalytic elimination over catalysts such as nickel, alkali metals, dolomite, olivine, or alumina is one of the promising attempts.4-8 Porous γ-alumina can quickly capture and then catalytically decompose a large amount of tar.9-11 For example, Ito et al.9 pyrolyzed woody biomass in a * To whom correspondence should be addressed. Fax: +81-29-8618209. E-mail:
[email protected]. † Energy Technology Research Institute. ‡ Research Institute for Environmental Management Technology. (1) Bridgwater, A. V. Fuel 1995, 74, 631-653. (2) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J. Biomass Bioenergy 2005, 29, 269-292. (3) Devi, L.; Ptasinski, K. J.; Janseen, F. J. J. G. Biomass Bioenergy 2003, 24, 125-140. (4) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155-173. (5) El-Rub, Z. A.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911-6919. (6) Bridgwater, A. V. Appl. Catal., A 1994, 116, 5-47. (7) Wang, T.; Chang, J.; Lv, P.; Zhu, J. Energy Fuels 2005, 19, 22-27. (8) Orio, A.; Corella, J.; Narvaez, I. Ind. Eng. Chem. Res. 1997, 36, 3800-3808. (9) Ito, K.; Moritomi, H.; Yoshiie, R.; Uemiya, S.; Nishimura, M. J. Chem. Eng. Jpn. 2003, 6, 840-845.
thermobalance with porous alumina. Tar was effectively captured by the alumina, and very little was evacuated from the reactor. Fluidized bed gasification of woody biomass is a promising method for large-scale clean gas production. The porous alumina is a suitable material for the refractory bed in a fluidized bed reactor because in addition to its tar-capturing ability, it has a higher mechanical strength than other catalysts. However, to design a reactor that uses the alumina, the gasification mechanism needs to be elucidated. Recently, Hosokai et al.11 clarified the cracking and steam reforming mechanism of woody biomass tar over the alumina. However, because they used a two-stage pyrolysis/reforming fixed bed reactor, they eliminated the effect of char on the cracking and steam reforming process. Therefore, the overall steam gasification mechanism in the presence of char was not clarified. When biomass is gasified in the conventional bubbling fluidized bed reactor using alumina, the gasifying agent comes in contact with the char formed as a result of devolatilization as well as with the tar captured by the alumina, and both are simultaneously gasified. Therefore, it is difficult to fully understand the gasification mechanism. In this study, we developed a two-stage fluidized bed reactor that separates the reaction zone of the char from the reaction zone of the gasifying agent and the tar deposited on the alumina. We then attempted to understand the gasification mechanism by comparing the gasification characteristics in this reactor with the characteristics in a single-stage fluidized bed reactor. (10) Shimizu, T. In Capacitance effect of porous bed materialssAn approach to improVe the performance of fluidized bed combustors and gasifiers, Proceedings of Fluidization 2003 Science and Technology, Gifu, Japan, Dec 3-5, 2003; Organizing Committee of CFJ-8, Ed.; Fluid and Particle Processing Division, Society of Chemical Engineers: Japan, 2003; pp 450-457. (11) Hosokai, S.; Hayashi, J.; Shimada, T.; Kobayashi, Y.; Kuramoto, K.; Li, C.-Z.; Chiba, T. Chem. Eng. Res. Des. 2005, 83, 1093-1102.
10.1021/ef0600210 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006
1316 Energy & Fuels, Vol. 20, No. 3, 2006
Matsuoka et al.
Figure 1. Schematic diagram of (a) the experimental apparatus and (b) details of the reactor. Table 1. Properties of Japanese Oak Sawdust (0.71-1.00 mm) C 49.4
ultimate analysis [wt %, dry] H N S 5.5
0.2
0.0
O (diff)
ash [wt %, dry]
43.8
1.1
Experimental Section Sample. Japanese oak sawdust (BM) with particle size ranging from 0.71 to 1.00 mm was used; its properties are listed in Table 1. The sample was dried at 353 K for 8 h under vacuum prior to the experiment. Gasification Procedure. A fluidized bed reactor made of quartz was used in this study. A schematic diagram of the apparatus is shown in Figure 1. Porous alumina particles (Mizusawa Chemicals) were used as the bed material. From the N2 adsorption isotherm at 77 K, we determined the BET specific surface area to be 200 m2/g and the pore volume to be 0.4 cm3/g. As a reference, nonporous silica sand was also used as a bed material. The sizes of both types of particle were in the range of 180-250 µm. The static bed height in the reactor was 2.0 cm. A screw feeder fed BM at about 0.08 g/min into the fluidized bed heated to a prescribed temperature (773 or 973 K), as shown in Figure 1b (cases I, II, and III). Argon gas was used as the fluidizing gas; its flow rate was 2 L (STP)/min. The gasifying agent, steam, was supplied through a liquid chromatography pump at a rate of 0.1 mL/min. Outlet gas was analyzed by high-speed gas chromatography (GC). In addition to the above experiments (Figure 1b; cases I, II, and III), we developed a twostage bed reactor (Figure 1b; case IV). The inner diameters of the lower bed and upper bed were 36 and 32 mm, respectively. BM was introduced in the same manner as in the above experiments. BM was pyrolyzed in the lower bed where the resultant char gradually accumulated. Volatiles (tar and gas) formed in the lower bed were transported upward and came in contact with alumina in the upper bed. Then steam was allowed to react with the tar captured by the alumina but was not allowed to come in contact with the char. After the above gasification or pyrolysis experiments, the amounts of carbon in the char and tar captured on alumina were determined by the temperature-programmed combustion. Tar evacuated from the reactor was trapped by two cold traps controlled at 273 and 200 K and also by a thimble filter. We attempted to recover the tar by washing the tar in the traps with acetone and then vaporizing the acetone with an evaporator. The residual tar was then dried at 353 K under vacuum for 8 h. Because a portion of the tar would be vaporized during the drying process,
perfect recovery of carbon was not easy, especially in case I (Figure 1b), where a significant amount of tar was emitted. On the other hand, in case II (Figure 1b), because most of the tar was captured by the alumina, evacuation of tar from the reactor was so small that the carbon material balance was about 90-95%. Reproducibility of the gas formation characteristics was checked and was found to be fairly good under each of the operating conditions. Therefore, we discuss the gasification mechanism later by comparing the gas formation characteristics under each of the operating conditions. Water-Gas Shift Reaction on Carbon. To examine the possibility that the BM char and the coke deposited on the alumina participated in the water-gas shift reaction, a CO/steam mixture (Ar balance) was allowed to independently come in contact with the char and the coke deposited on the alumina in case IV (Figure 1b). Acid-washed char was also used to check the effect of metals remained in the char on the water-gas shift reaction. The acidwashed char was prepared by washing raw char with 6 wt % HNO3 under ultrasonic irradiation for 1 h. An ICP-AES analysis of the aqueous solution after the washing confirmed that almost all of the potassium, sodium, calcium, magnesium, and iron was removed. Ash-free commercial carbons A and B, which are prepared from phenol resin and carbon black, respectively, were also used as references. The sample was first heated at 973 K under steam/Ar gas flow for 20 min in the reactor used for the gasification experiment; the gas was then switched to CO/steam/Ar flow, and then the gas was switched back to steam/Ar flow. The gas flow rate was the same as that of the above-mentioned gasification experiments. The pore structure of the carbon samples was determined from the N2 adsorption isotherm at 77 K. Pore size distribution (PSD) was determined by the BJH method.12 The amount of active sites in the carbon was determined by a temperature-programmed desorption (TPD) technique as follows.13 The sample was heated under Ar flow from ambient temperature up to 973 K to remove oxygen-containing functional groups until no CO or CO2 was desorbed, and then the Ar gas was switched to a steam/Ar flow for 20 min. The sample was cooled quickly to ambient temperature under Ar flow. Then the sample was again heated under Ar flow at 3 K/min to 973 K, and the amounts of CO and CO2 desorbed (12) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (13) Klose, W.; Wolki, M. Fuel 2005, 84, 885-892.
Woody Biomass Pyrolysis and Gasification Mechanism
Energy & Fuels, Vol. 20, No. 3, 2006 1317
Figure 2. Yields of gas formed during pyrolysis of biomass using the single-stage reactor. (a) Bed material, silica sand; temperature, 773 K. (b) Bed material, alumina; temperature, 773 K. (c) Bed material, silica sand; temperature, 973 K. (d) Bed material, alumina; temperature, 973 K.
from the sample were determined by GC. These amounts corresponded to sites newly formed by gasification at 973 K.
Results and Discussion Gas Formation Characteristics. We first carried out pyrolysis of BM in the single-stage fluidized bed reactor shown in Figure 1b on a bed of either alumina or silica sand (cases I and II, respectively). Although the data are not shown here, a significant amount of tar was formed and evacuated from the reactor when pyrolysis was carried out on silica sand (case I). On the other hand, when pyrolysis was carried out on alumina, most of the tar was captured by the alumina, and the amount of evacuated tar was quite small (case II). From TPD using NH3 as a basic adsorbate, Hosokai et al.11 found that the density of acidic sites in alumina was lower than that of representative zeolites (H-YZ and H-ZSM-5) but that the acidity of alumina was as strong as that of H-YZ and H-ZSM-5. In addition to the acidity, the pore structure of the alumina was well developed in comparison to that of silica sand; therefore, tar was converted into coke and then thermally cracked on the surface. Thus, little tar was evacuated from the reactor when alumina was used as the reactor bed material. The tar deposited on the alumina is hereafter defined as coke. As mentioned in the Experimental Section, it was difficult to quantitatively determine the tar in the present study. In this study, we examined how the tar was cracked by comparing the yields of gas formed on silica sand and on alumina during pyrolysis of BM at 773 and 973 K. CO and CO2 were the dominant gases at 773 K (Figure 2parts a and b). The amounts of CO and CO2 formed at 773 K on the alumina were slightly larger than the amounts formed on silica sand. At 973 K, the amount of CO formed on the silica sand (Figure 2c) was slightly larger than that on the alumina, while yields of H2 and CO2 were significant on the alumina (Figure 2d). This result suggests that the tar was cracked on the alumina surface to form H2 and CO2 owing to the acidity of the alumina and that the cracking was enhanced at higher temperature. Carbon conversion into gas and the yield of H2 formed under pyrolysis and steam gasification using the single-stage fluidized
Figure 3. Yields of gas formed upon pyrolysis and gasification on a bed of silica sand or alumina at 773 or 973 K.
bed reactor are summarized in Figure 3. The yield of gas formed in the case of silica sand was independent of the presence of steam at either temperature. On the other hand, the yield of gas formed in the case of alumina was increased by the presence of steam, especially at 973 K. The yield of H2 formed at 773 K was independent of both the bed material and the presence of steam. At 973 K, the yield of H2 was increased to some extent by the presence of steam, even in the case of silica sand. Larger quantities of H2 were formed by pyrolysis on alumina than on silica sand. Furthermore, the yield of H2 in the case of alumina was increased by the presence of steam at 973 K, indicating that coke on the alumina surface was not only cracked but also reformed. Although the results shown in Figures 2 and 3 roughly clarify the characteristics of gas formation over alumina, the mechanisms of gasification are still unclear. The reason is that the gasifying agent, steam, can come in contact with the resultant char as well as with the coke deposited on the alumina to form gas. To clarify the extent to which steam-char and steamcoke reactions contribute to the final gas composition of the steam gasification, we attempted to separate the reaction zones by using the two-stage bed reactor developed here (Figure 1b; case IV). The yields of gas formed upon gasification using the two-stage bed reactor, where steam comes in contact only with coke on the alumina, are shown in Figure 4a. The yields from
1318 Energy & Fuels, Vol. 20, No. 3, 2006
Matsuoka et al.
Figure 4. Yields of gas formed during steam gasification of biomass: (a) steam in contact only with coke on alumina and (b) steam in contact with both coke on alumina and char.
the single-bed reactor (Figure 1b; case III), where both the char and the coke-deposited alumina are exposed to the steam, are shown in Figure 4b. The yields of H2 and CO formed by the contact of steam with only the coke (Figure 4a) were significantly larger than those of pyrolysis using the alumina (Figure 2d), whereas CO2 formation was independent of the presence of steam. These results suggests that coke on the alumina was reformed with steam through the following reaction:
Figure 5. Step response reaction on switching the gas from steam/Ar to steam/CO/Ar at 973 K: (a) coke-deposited alumina and (b) char/ silica sand mixture.
C(coke on Al2O3) + H2O f CO + H2 When steam came in contact with the char as well as the coke on the alumina (Figure 4b), the yields of H2 and CO2 were larger than those shown in Figure 4a, whereas the yield of CO decreased, suggesting that the water-gas shift reaction
CO + H2O ) CO2 + H2 occurred in the presence of the char. Water-Gas Shift Reaction. We examined the possibility that the water-gas shift reaction occurred on the char by injecting gas stepwise into a particle bed of coke-deposited alumina or a char/silica mixture. The char and coke-deposited alumina obtained in case IV (Figure 1b) were used as the samples. The results of the step response reaction are shown in Figure 5. Note that H2 and CO were detected in the first period (H2O/Ar flow) in Figure 5a because further reforming of the coke on the alumina took place to some extent. The formation of gas (H2, CO, and CO2) occurred continuously even after the injected gas was switched from H2O/Ar to H2O/CO/Ar. CO introduced as the reactant gas was not consumed in the bed of coke-deposited alumina during the second period (Figure 5a). In contrast to the coke-deposited alumina, CO introduced into the char/silica mixture was apparently consumed during the second period (Figure 5b), and the amount consumed as well as the increase in H2 and CO2 during the second period supported the stoichiometric variation of those components in the shift reaction. Preliminary experiments showed that the water-gas shift reaction did not occur on silica sand alone. These results indicate that the shift reaction occurs on char but
Figure 6. PSD of char, coke-deposited on alumina, carbon A, and carbon B as determined by the BJH method.
not on coke under the present conditions. There are several plausible mechanisms for the acceleration of the water-gas shift reaction on the char: (1) metals inherent in BM, such as alkali and alkaline earth metals, were retained in the char and behaved as shift catalysts;14 (2) CO and water molecules diffused into the pores of the char, and the shift reaction occurred in the pores; and (3) the shift reaction occurred on the active sites15 of the char surface. We examined the above three possible mechanisms. Although the data are not shown here, the gas formation characteristics of acid-washed char were similar to those of nontreated char. Therefore, metal retained in the char was not a dominant factor influencing the shift reaction. Next, we carried out the step response reaction with ash-free commercial carbons with different pore structures. Figure 6 shows the PSD of the commercial carbons, char, and coke(14) Cannon, F. S.; Snoeyink, V. L.; Lee, R. G.; Dagois, G. Carbon 1994, 32, 1285-1301. (15) Laine, N. R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67, 2030-2034.
Woody Biomass Pyrolysis and Gasification Mechanism
Energy & Fuels, Vol. 20, No. 3, 2006 1319
Cf + H2O f C(O) + H2 C(O) f CO + Cf
Figure 7. Step response reaction for coke-deposited alumina at 973 K by decreasing the gas flow rate from 2 L (STP)/min to 0.4 L (STP)/ min. Table 2. BET Specific Surface Area and Pore Volume of the Samples sample coke-deposited alumina char carbon A carbon B
BET surface area [m2/g]
pore volume [cm3/g]
180
0.41
90 700 150
0.09 0.29 0.69
deposited alumina as determined by the BJH method. The PSD of coke-deposited alumina was very sharp. Two peaks were seen in the PSD of the char: one was at around 2 nm and the other was a broad peak at around 40 nm. The PSD of carbon A was very sharp, and its peak was similar to the first peak of the char (2 nm). On the other hand, the pore sizes of carbon B ranged from 10 to 50 nm, and the volume of pores smaller than 50 nm was much larger than that of char. BET specific surface area and pore volume of the samples as determined from the volume of N2 adsorbed at P/P0 ) 0.99 are listed in Table 2. BET surface area and pore volume of the char were lowest among the four samples. We conducted the step response reaction with carbons A and B under the conditions used for the char/silica sand mixture (Figure 5b). No shift reaction was observed with either carbon A or carbon B. We then examined the effect of gas flow rate on the shift reaction. When the gas flow rate was decreased from 2 L (STP)/ min to 0.4 L (STP)/min, the shift reaction was observed on cokedeposited alumina in the step response reaction (Figure 7). Although the data are not shown here, no shift reaction was observed on commercial carbons A and B even when the gas flow rate was decreased until 0.4 L (STP)/min. From these results, we can infer that CO and water molecules penetrate into the narrow pores of coke-deposited alumina under slow gas flow rates and that the shift reaction then occurs in the pores. However, no shift reaction was observed on the commercial carbons where the pore structures are more developed than those of the char. Therefore, the pores of the carbon are not the only factor influencing the shift reaction. Surface state of the sample would also contribute to the shift reaction. For this reason, active sites15 on the carbon after steam gasification at 973 K for 20 min were evaluated by the TPD technique13 (Figure 8). The amounts desorbed from coke on alumina, char, carbon A, and carbon B after heating at 973 K for 1 h were 10.5, 4.7, 0.8, and 0.6 mmol/g C, respectively. The amounts desorbed here correspond to the sites newly formed by steam gasification at 973 K. The steam gasification mechanism has been examined by many researchers and following one is widely accepted:16,17
where Cf represents free (active) carbon sites and C(O) represents oxygen-containing surface complexes. The larger the amount of COx desorbed during TPD experiment, the larger the number of free carbon sites. The pores of the char were wide, and a large amount of free carbon sites was present on the char surface; therefore, CO and water molecules can easily penetrate into the pores of char even at high gas flow rates and then react at the free sites. In the case of coke-deposited alumina, decreasing the gas flow rate allowed CO and water molecules to penetrate into the pores and then react on the free sites. On the other hand, although CO and water molecules can penetrate into the pores of commercial carbons, they would not react because of the few free carbon sites. It is clear that both the pore structure and the surface state were important for the shift reaction. The diffusion of CO and water molecules into the pores of the char and alumina is affected by the hydrodynamic conditions (e.g., flow rate of fluidizing gas) as well as temperature, which is the subject to be concerned. Gasification Mechanism. Yields of gas formed at 973 K under different operating conditions are summarized in Figure 9. Under the pyrolysis conditions of cases I and II (see Figure 1b), the yields of CO were independent of the bed material, but the yields of CO2 and H2 were increased by using alumina. This result indicates that tar was first deposited in the pores of the alumina and then thermally cracked into CO2 and H2. In the presence of steam, the total amounts of CO and CO2 in case III (steam in contact with both coke-deposited alumina and char) and case IV (steam in contact with coke-deposited alumina) were similar because little char is reformed under the present conditions. The difference between the amounts of CO in cases III and IV is the amount contributed to the water-gas shift reaction on char as described above. Therefore, the difference in H2 between cases III and IV should be the same as the amount involved in the shift reaction. However, the actual difference was a little larger than the theoretical difference. Here, in case IV, the amount of methane was larger than that in case I (pyrolysis in silica sand). If no reaction involving methane formation occurs in the upper bed (case IV), then the amount of methane (case IV) should be the same as in case I. However, the amount was larger than that of case I, which means that H2 formed by the devolatilization in the bottom bed was consumed to form methane, and the formation of this methane enlarged the difference in H2 yield between cases III and IV. We summarize the gasification mechanism of BM on a bed of porous alumina in Figure 10. BM is first converted into gas, tar, and char upon devolatilization. The tar is quickly captured by the porous alumina and then cracked on the alumina surface and reformed with steam. A portion of the H2 formed upon devolatilization reacts with the coke deposited on the alumina to form methane. CO is formed by the reforming of tar as well as during the devolatilization; CO diffuses into the pores of the resultant char along with water molecules, and then the shift reaction occurs on the active sites. Conclusions We carried out steam gasification of woody biomass in a twostage fluidized bed reactor using porous γ-alumina. In addition (16) Ergun, S.; Menster, M. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1965; Vol. 1, pp 203-263. (17) Huttinger, K. J. Carbon 1988, 26, 79-87.
1320 Energy & Fuels, Vol. 20, No. 3, 2006
Matsuoka et al.
Figure 8. Profiles of TPD after the steam gasification at 973 K for 20 min: (a) coke-deposited alumina, (b) char, (c) carbon A, and (d) carbon B.
Figure 10. Schematic of biomass gasification in a reactor using porous alumina.
Figure 9. Yields of gas formed during pyrolysis and gasification at 973 K under different conditions. (Each experimental condition corresponds to that of Figure 1.)
to the gasification, a step response reaction was also carried out to determine the site of the water-gas shift reaction. The woody biomass was first devolatilized to form volatiles (gas and tar) and char. The amount of tar was significant at lower temperatures but was mostly captured by the porous alumina. In the subsequent gasification process, the tar captured on the alumina was cracked and reformed with steam. On the
other hand, the resultant char was not gasified with steam under the present conditions. CO formed by the devolatilization and steam reforming diffused into the pores of the resultant char along with water molecules, allowing the water-gas shift reaction to occur on the active surface sites. Acknowledgment. We are grateful to Professor Jun-ichiro Hayashi of Hokkaido University for his helpful discussion. EF0600210
