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Energy & Fuels 2008, 22, 1980–1985
Steam Gasification of Woody Biomass in a Circulating Dual Bubbling Fluidized Bed System Koichi Matsuoka,* Koji Kuramoto, Takahiro Murakami, and Yoshizo Suzuki Energy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ReceiVed December 3, 2007. ReVised Manuscript ReceiVed February 4, 2008
Partial oxidation is generally applied for woody biomass gasification. However, the calorific value of gaseous products is relatively low. To increase the calorific value of the gaseous product, a circulating dual bubbling fluidized bed (CDBFB) system for woody biomass gasification based on a concept of separation of combustion zone from gasification zone was developed in this study. The CDBFB consisted of two bubbling fluidized beds, with one serving as a gasifier and the other as a combustor. Two kinds of sawdusts were gasified with steam in the CDBFB at 773, 873, 973, and 1073 K. Almost all the tar evolved from the sawdust samples was captured by the porous γ-alumina particle bed material in the gasifier. The tar deposited on the alumina, referred to herein as coke, and the resulting char were gasified with steam in the gasifier. Since the residence time of solid in the gasifier can be controlled, coke as well as char was effectively gasified with steam in comparison with the conventional circulating fluidized bed. A high carbon conversion and hydrogen yield were achieved using the CDBFB, resulting in high cold gas efficiency.
Introduction Among biomass conversion processes, gasification is one of the important and promising technologies. Large-scale biomass gasification typically employs a partial oxidation process using air. However, the calorific value of the gaseous product is relatively low, because of the following two reasons: the major one is the dilution of gaseous product by nitrogen in air, and the other is the partial oxidation of volatiles such as H2, CO, and CH4, which are formed by pyrolysis prior to the gasification. To increase the calorific value of the gaseous product by avoiding the dilution of the gaseous product by nitrogen, a concept of separation of combustion zone from gasification zone has been demonstrated.1 By separating the two zones, gaseous products with relatively high calorific value can be obtained by steam gasification in the gasifier, and thus the total efficiency of the gasification process can be increased.2 Dual fluidized bed gasification systems have been used to successfully separate the gasification zone from the combustion zone,2 and some researchers have reported utilizing such systems for the gasification of biomass.3–5 In this process, biomass is gasified in the gasifier, and unconverted char is circulated with bed material to the combustor. The bed material heated by combustion of the char is recirculated to the gasifier to supply heat for gasification; that is, heat from the exothermic combustion * To whom correspondence should be addressed. Fax: +81-29-861-8209. E-mail:
[email protected]. (1) Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Stoneham, UK, 1991; Chapter 2. (2) Hayashi, J.-i.; Hosokai, S.; Sonoyama, N. Trans. I ChemE, Part B 2006, 84 (B6), 409–419. (3) Hofbauer, H.; Fleck, T.; Veronik, G.; Rauch, R.; Macinger, H.; Fercher, E. The FICFB gasification process. In DeVelopments in Thermochemical Biomass ConVersion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie: London, 1997; pp 1016–1025. (4) Pfeifer, C.; Rauch, R.; Hofbauer, H. Ind. Eng. Chem. Res. 2004, 43, 1634–1640. (5) Xu, G.; Murakami, T.; Suda, T.; Matsuzawa, Y.; Tani, H. Energy Fuels 2006, 20, 2695–2704.
process is used to drive the endothermic gasification process. Another important aspect for efficient biomass conversion is the effective elimination of tar in the gasifier of the dual fluidized bed,6 and many researchers have attempted to catalytically eliminate tar during biomass gasification.7–14 Previous reports have shown that nascent tar produced during gasification can be almost completely eliminated by using Ni-based catalysts.4,13,14 Ni-based catalysts are a potential bed material for dual bed gasification, but they are too costly to be used on a large-scale basis. Additionally, deactivation of the Ni-based catalysts due to carbon deposition (known as coke formation) is also a severe problem. Coke deposited on the Ni-based catalysts can be removed by oxidation, but it is quite difficult to avoid sintering. Pfeifer et al. examined steam gasification of woody biomass using a Ni/olivine catalyst in a dual-bed gasification system.4 Tar could be eliminated in the gasifier, but a large amount of expensive Ni was needed to lower the tar emission to a satisfactory level. Alternatively, Xu et al. attempted gasification of coffee grounds in a dual-bed gasification system without using any catalyst.5 Inexpensive silica sand was used as a bed material in their study, but the silica sand had no tar cracking activity, and tar elimination in the gasifier was quite difficult even when the operating variables were widely varied. (6) Devi, L.; Ptasinski, K. J.; Jansen, F. J. J. G. Biomass Bioenergy 2003, 24, 125–140. (7) Bridgwater, A. V. Fuel 1995, 74, 631–653. (8) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155–173. (9) El-RubZA.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911–6919. (10) Bridgwater, A. V. Appl. Catal., A 1994, 116, 5–47. (11) Wang, T.; Chang, J.; Lv, P.; Zhu, J. Energy Fuels 2005, 19, 22– 27. (12) Orio, A.; Corella, J.; Narvaez, I. Ind. Eng. Chem. Res. 1997, 36, 3800–3808. (13) Aznar, M. P.; Caballero, M. A.; Gil, J.; Martin, J. A.; Corella, J. Ind. Eng. Chem. Res. 1994, 34, 703–717. (14) Rapagna, S.; Jand, N.; Foscolo, P. U. Int. J. Hydrogen Energy 1998, 23, 551–557.
10.1021/ef700726s CCC: $40.75 2008 American Chemical Society Published on Web 03/25/2008
Steam Gasification of Woody Biomass
The yield of volatiles during woody biomass pyrolysis/ gasification generally exceeded 70% at low temperatures ( CO2 > CH4 > C2H4, and this trend was independent of the temperature. By contrast, the yield of hydrogen gas was strongly dependent on the temperature: at 773 K, the yield of H2 was smaller than the CO2 yield, whereas at 1073 K, the H2 yield was much larger than that of CO2. We have previously reported that coke deposited on the alumina cracks (dehydrogenated) to form H2 during pyrolysis and that the H2 formation rate is enhanced at higher temperatures.20 On the basis of our previous findings, we believe that the temperature dependency of the H2 yield in the present study is due to the cracking of the coke deposited on the alumina. Steam Gasification in CDBFB. The sawdust sample was gasified with steam under a wide range of operating variables. The conversion of carbon into carbonaceous gases and H2 as a function of steam-to-carbon (S/C) molar ratio in the gasification zone is plotted in Figure 5. The carbon conversion into carbonaceous gases as well as the yield of H2 monotonically increased with S/C at both 973 and 1073 K until a ratio of 1.0 was reached. Carbon conversion as well as H2 yield is expected to increase with S/C, but the increase of carbon conversion with S/C was not observed at higher S/C (1 < S/C < 3, i.e., steam concentration 12–36 vol %). One of the reasons might be that H2 and volatiles formed by gasification play an inhibitor of gasification of coke and char as is reported in steam gasification of coal.23,24 The yields of carbonaceous gases formed during gasification at different temperatures using an S/C molar ratio of 2.0 are summarized in Figure 6. The yields obtained under pyrolysis conditions at the same temperatures are also shown for reference. The difference in carbon conversion into carbonaceous gases (sum of CO, CO2, CH4, and C2H4) between pyrolysis and gasification was larger at higher temperatures. More specifically, the differences in CO2 and H2 yields between pyrolysis and steam gasification were most substantial. The steam gasification behavior depended on the biomass type. Gasification characteristics would be influenced by sample properties such as cellulose content, carbon content, ash content, etc. Since only two samples were used in the present study, it is insufficient to discuss details of the sample dependency on the gasification characteristics only from the present results. During gasification, the coke deposited on the alumina and the char should be simultaneously gasified, but the contribution of coke and char gasification to the yield of carbonaceous gases and H2 is unclear (23) Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Shimada, T.; Hayashi, J. I.; Li, C.-Z.; Chiba, Fuel 2006, 85, 340–349. (24) Huttinger, K. J.; Merdes, W. F. Carbon 1992, 30, 883–894.
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Figure 4. Yields of outlet gas components as a function of time as measured from the CDBFB gasifier during pyrolysis of white oak sawdust at (a) 773 K, (b) 873 K, (c) 973 K, and (d) 1073 K.
Figure 5. Effect of steam-to-carbon molar ratio (S/C) on carbon conversion to carbonaceous gases (closed symbols) and H2 yield (open symbols) during gasification of white oak at 973 (circles) and 1073 K (squares).
because the coke and char gasification rates cannot be estimated from the results shown in Figure 6. To more fully understand the increase in carbon conversion, as well as the H2 yield, with increasing temperature and steam content, we investigated the gasification rates of coke and char. These results are discussed in the following section. Gasification Rates of Coke and Char. To determine the gasification rates of the coke deposited on the alumina and the char in the gasification zone of the CDBFB, a series of experiments using a batch bubbling fluidized bed reactor was conducted using the conditions described above for gasification in the CDBFB. The gas formation rates during steam gasification of cedar sawdust coke and char in the bubbling fluidized bed at 1073 K are shown in Figure 7. H2 was the most abundant gas in both cases. The CO:CO2 ratios for the coke and the char differed. We have previously reported that the water-gas shift reaction (CO + H2O T CO2 + H2) occurred to a substantial extent during the gasification of biomass char, though the reaction did not proceed as substantially during coke gasification.20 As shown in Figure 7, the amount of CO2 formed during the gasification of char was larger than that observed during coke gasification. Judging from Figure 7 and our previous report,20 CO was formed in the gasifier during the coke gasification, and then it was converted to CO2 through the water-
Figure 6. Yield of carbonaceous gases and H2 during pyrolysis and gasification of (a) cedar and (b) white oak sawdust at different temperatures.
gas shift reaction on the surface of coexisting char in the gasifier. Such a process would explain the difference in H2 and CO2 between gasification and pyrolysis (Figure 6).
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Figure 7. Gas formation rates during gasification of cedar sawdust (a) coke and (b) char at 1073 K.
Matsuoka et al.
Figure 9. Temperature dependency of the gasification rates of coke and char for (a) cedar and (b) white oak sawdusts.
unconverted fraction of carbon, denoted 1 - X, meaning that the kinetics of steam gasification of the coke and char can be described as dX/dt ) kapp(1 - X)
Figure 8. Extent of carbon conversion as a function of time during gasification of coke (open symbols) and char (closed symbols) at different temperatures for (a) cedar and (b) white oak sawdusts.
The amount of carbon conversion as a function of time during the gasification of the char and coke at different temperatures for both sawdust samples is shown in Figure 8. As seen in the figure, the gasification rate of the coke was larger than that of the char at all temperatures for both types of sawdust. The gasification of the coke and char did not occur substantially at 873 K for either type of sawdust. Though not shown here, good linearity was obtained for first-order kinetic plots of the
(1)
The first-order apparent reaction rate constants (kapp, min-1) were determined for both the coke and char and are summarized in Figure 9. The gasification rate of the coke was higher than that of the char under the present conditions. Furthermore, our preliminary experiments confirmed that the yields of coke and char during the pyrolysis of cedar sawdust at 973 K were about 40 and 20 wt %, respectively. Considering the observed yields and the gasification rates of the coke and char, the contribution of coke gasification to gas formation in the gasifier of the CDBFB was more dominant than the contribution of char gasification, leading to the differences in carbon conversion and H2 yield between gasification and pyrolysis shown in Figure 6. Performance of CDBFB. As mentioned in the Introduction, we have reported the gasification of cedar sawdust using a conventional circulating fluidized bed (CFB) system.22 The riser of the CFB system was used as a gasifier in the previous study,22 and the configurations of other units, such as the cyclone and the combustor, were the same as those used for the CDBFB developed in the present study. In the previous study using the CFB system, only a small extent of biomass gasification was observed, although the circulation of the bed material (porous alumina) and other system operating parameters were quite stable. In the present study, a bubbling fluidized bed reactor was used as a substitute for the riser-type reactor to extend the residence time of solids in the gasification zone of the CDBFB. The residence time of the bed material in the previous CFB gasifier was about 1 s, and the residence time in the CDBFB was about 10 min. In Figure 10, comparisons of gaseous yields between the previous CFB and the present CDBFB are shown. The carbon conversion and H2 yield from the CDBFB were larger than those from the CFB, most notably at higher temperatures. As mentioned above in the discussion of Figures 8 and 9, the gasification of the coke deposited on the alumina was more dominant than the char gasification. Apparently,
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Figure 11. Cold gas efficiency during gasification of cedar sawdust at different temperatures using the CDBFB and CFB systems.
To develop an efficient dual bed gasification system for biomass, heat balance and material flow between the gasifier and the combustor are quite important. In a future study, the experimental results obtained here, such as carbon conversion and CGE, will be compared with results obtained with a system simulation considering heat balance and material flow. The gasification conditions will be further optimized to design a stand-alone gasification plant. Figure 10. Comparison of (a) H2 yield and (b) carbon conversion during pyrolysis and gasification of cedar sawdust between the CDBFB and CFB systems.
control of the residence time of the alumina in the gasifier enhanced coke gasification and cracking, thus increasing the gaseous yield. Cold gas efficiency (CGE) was employed as a parameter to judge the improvement of performance of the CDBFB over the CFB. The CGE was determined as follows: CGE )
∑((heating value of gas formed by gasification, LHV basis) × (gas yield)) ⁄ (heating value of the biomass sample, HHV basis) (2) Figure 11 shows the CGE of cedar sawdust during steam gasification at different temperatures. The CGE of the CDBFB increased with increasing temperature because H2 and CO yields were larger at higher temperatures (Figure 6). Notably, the CGE of the CDBFB was higher than that of the CFB at all temperatures.
Conclusions A method of high-efficiency biomass gasification based on the concept of separation of combustion zone from gasification zone was verified using a circulating dual bubbling fluidized bed system (CDBFB) with porous γ-alumina particle bed material. Steam gasification and pyrolysis of two kinds of sawdusts were performed in the CDBFB at several temperatures. Since the residence time of the bed material can be controlled in the gasifier of the CDBFB, tar captured by the porous alumina particles (coke) as well as char was effectively gasified. Especially, coke was preferentially gasified compared with the char. Therefore, higher carbon conversion and H2 yield could be achieved in the CDBFB in comparison with the conventional circulating fluidized bed. Acknowledgment. The sawdust samples were provided by Dr. T. Yoshida of the Forestry and Forest Products Research Institute, Japan. EF700726S
