Energy & Fuels 2006, 20, 2727-2731
2727
Steam Reforming of Woody Biomass in a Fluidized Bed of Iron Oxide-Impregnated Porous Alumina Koichi Matsuoka,* Takaaki Shimbori, Koji Kuramoto, Hiroyuki Hatano, and Yoshizo Suzuki National Institute of AdVanced Industrial Science and Technology (AIST), Energy Technology Research Institute, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ReceiVed June 28, 2006. ReVised Manuscript ReceiVed August 15, 2006
Steam reforming of woody biomass in a fluidized bed was performed at 773, 873, or 973 K. Nonporous silica sand, porous γ-alumina and iron oxide-impregnated porous γ-alumina (Fe-impregnated alumina) were used as bed materials. The addition of iron oxide to the alumina promoted H2 production at all the temperatures. Larger amounts of H2 were produced at higher temperatures. Tar evacuated during steam reforming was captured on the Fe-impregnated alumina, and the captured tar (referred to as coke) was reformed with steam to form H2. In addition to the reforming, a redox reaction occurred on the iron oxide: CO produced during steam reforming was consumed to reduce the iron oxide, and the reduced iron oxide came into contact with steam to form H2. The redox reaction, rather than reforming of the coke, was the predominant pathway of H2 formation at higher temperatures.
Introduction Fluidized bed gasification of woody biomass is a promising method for the large-scale production of synthesis gas. However, this method produces large amounts of undesirable tarry materials (condensable organic compounds). When the product gas is used for gas turbines and engines, the tar eventually fouls the processing equipment and causes some troubles. To eliminate tar from the product gas, gas cleaning after the gasifier has been widely investigated, but it is not economically viable in some cases. Therefore, catalytic elimination of the tar inside the gasifier using cheap catalyst is gaining much attention and several types of catalysts for tar elimination have been attempted.1-11 In our previous study,12 we clarified the mechanism of steam reforming of oak sawdust on porous γ-alumina by using a two-stage bubbling fluidized bed reactor. Tar formed * To whom correspondence should be addressed. Fax: +81-29-8618209. E-mail address:
[email protected]. (1) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125-140. (2) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155-173. (3) El-Rub, Z. A.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911-6919. (4) Bridgwater, A. V. Appl. Catal., A 1994, 116, 5-47. (5) Wang, T.; Chang, J.; Lv, P.; Zhu, J. Energy Fuels 2005, 19, 22-27. (6) Orio, A.; Corella, J.; Narvaez, I. Ind. Eng. Chem. Res. 1997, 36, 3800-3808. (7) Ito, K.; Moritomi, H.; Yoshiie, R.; Uemiya, S.; Nishimura, M. J. Chem. Eng. Jpn. 2003, 6, 840-845. (8) Shimizu, T. Proceedings of FLUIDIZATION 2003 Science and Technology, Fluid and Particle Processing Division, Society of Chemical Engineers: Japan, 2003; pp 450-457. (9) Hosokai, S.; Hayashi, J.; Shimada, T.; Kobayashi, Y.; Kuramoto, K.; Li, C.-Z.; Chiba, T. Chem. Eng. Res. Des. 2005, 83, 1093-1102. (10) Corella, J.; Herguido, J.; Gonzalez-Saiz, J.; Alday, F. J.; RodriguezTrujillo, J. L. In Research in Thermochemical Biomass ConVersion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: London, 1988; pp 754-765. (11) Corella, J.; Toledo, J. M.; Padilla, R. Energy Fuels 2004, 18, 713720. (12) Matsuoka, K.; Shinbori, T.; Kuramoto, K.; Nanba, T.; Morita, A.; Hatano, H.; Suzuki Y. Energy Fuels 2006, 20, 1315-1320.
during the steam reforming was quickly captured on the alumina but could not be efficiently reformed into gases such as H2 and CO. These results suggest that the alumina alone is not effective to decompose tar. However, if the alumina is impregnated with some catalyst such as iron, tar may be decomposed effectively. The steam-iron reaction has been reported as a possible method for H2 production:13-15
FexOy-1 + H2O f FexOy + H2
(1)
Furthermore, lattice oxygen in metal oxides can be utilized to reform hydrocarbons.16 In the case of iron oxide, the stoichiometric equation can be expressed as follows:
(2n + m)FexOy + CnH2m f (2n + m)FexOy-1 + nCO2 + mH2O (2) On the basis of these literature reports, we expected that if porous alumina impregnated with iron oxide were used as a bed material for the steam reforming of woody biomass in a fluidized bed, tar could be captured on the alumina and then reformed to produce H2 by a combination of the reactions shown in eqs 1 and 2. We therefore examined the effects of loading iron oxide on porous alumina on H2 production by steam reforming in a fluidized bed. Furthermore, the role played by iron oxide in H2 formation during steam reforming was clarified using a two-stage fluidized bed reactor. Experimental Procedures Sample. Japanese oak sawdust with particle sizes ranging from 0.71 to 1.00 mm was used. Its analysis on a dry basis was C 49.4, H 5.5, N 0.2, O (by diff.) 43.8, and ash 1.1 wt %. The sample was dried at 353 K for 8 h under vacuum before the experiment. (13) Fukase, S.; Suzuka, T. Appl. Catal., A: Gen. 1993, 100, 1-17. (14) Fukase, S.; Suzuka, T. Can. J. Chem. Eng. 1994, 72, 272-278. (15) Urasaki, K.; Tanimoto, N.; Hayashi, T.; Sekine, Y.; Kikuchi, E.; Matsukata, M. Appl. Catal., A: Gen. 2005, 288, 143-148. (16) Xu, W. C.; Tomita, A. Fuel 1989, 68, 673-676.
10.1021/ef060301f CCC: $33.50 © 2006 American Chemical Society Published on Web 09/09/2006
2728 Energy & Fuels, Vol. 20, No. 6, 2006
Matsuoka et al.
Figure 2. Gas yields upon pyrolysis of oak sawdust at 773, 873, or 973 K.
Figure 1. Schematic of experimental apparatus: (a) single-stage fluidized bed reactor and (b) two-stage reactor.
Steam Reforming Procedure. A fluidized bed reactor made of quartz was used. A schematic diagram of the apparatus is shown in Figure 1. Nonporous silica sand, porous γ-alumina particles (Mizusawa Chemicals), and porous γ-alumina impregnated with iron oxide (Fe-impregnated alumina) were used as the bed materials. The sizes of all the particles were in the 180-250 µm range. The Fe-impregnated alumina was prepared by immersion of the alumina in 45 wt % aqueous Fe(NO3)2 followed by heating at 873 K in air. The form of the iron was determined by X-ray diffraction to be hematite (Fe2O3). The amounts of Fe2O3 on the alumina was 5.4 wt %. The BET surface areas of the alumina and the Fe-impregnated alumina were 200 and 180 m2/g, respectively. The pore volumes of the alumina and the Fe-impregnated alumina were 0.44 and 0.37 cm3/g, respectively. Steam reforming and pyrolysis experiments were conducted at 773, 873, or 973 K. The oak sawdust sample was fed at about 0.08 g/min into the fluidized bed heated to the prescribed temperature (Figure 1a). Argon gas was used as the fluidizing gas. The gasifying agent (steam) was supplied through a liquid chromatography pump. The outlet gas was analyzed by highspeed gas chromatography. In addition to the apparatus shown in Figure 1a, we used a twostage bed reactor (Figure 1b) to determine the role of the iron in the reforming of the tar deposited on the alumina (hereafter referred to as coke). The oak sawdust was devolatilized in the lower bed, where char gradually accumulated. As the volatiles (tar and gas) formed in the lower bed were transported upward, they came into contact with the alumina in the upper bed. Then, steam was allowed to react with the coke on the alumina but was not allowed to come into contact with the char. The details of the experimental procedure were described in a previous report.12 In preliminary experiments, we confirmed that the reproducibility of gas-formation yields upon pyrolysis and steam reforming, as determined by analysis of the outlet gas, was fairly good. Therefore, we discuss the mechanism of pyrolysis and steam reforming on the basis of the gas-formation yields. Step Response Reaction. To study the effect of the iron oxide on the composition of the gas formed during steam reforming, CO, which was formed abundantly during steam reforming, was allowed to come into contact independently with the Fe-impregnated alumina onto which the coke had been deposited. The sample was first
Figure 3. Gas yields upon steam reforming of oak sawdust at 773, 873, or 973 K.
heated at a given temperature (773 or 973 K) under a steam-Ar gas for 20 min in the reactor used for the steam reforming experiment; the gas was switched to a CO-steam-Ar gas and then switched back to a steam-Ar gas. The coke-bearing alumina was also used as a reference. Details of the step response reaction have been described in a previous paper.12
Results and Discussion Effect of Iron Loading on Gas Composition. Pyrolysis and steam reforming of the oak sawdust were carried out at 773, 873, or 973 K. Figure 2 shows the yield of each gas under pyrolysis conditions. To know the carbon mass balance, the gas yield was represented on the basis of moles of carbon in the oak sawdust. At all three temperatures, carbon conversion into gas (as measured by the cumulative yield of CO, CO2, CH4, and C2H4) increased in the order silica sand < alumina < Feimpregnated alumina, and the increases were due mainly to increases in the CO2 yields. At 773 K, the yield of H2 obtained with the Fe-impregnated alumina was much larger than the yield obtained with alumina or with silica sand. The H2 yield with Fe-impregnated alumina at 773 K was larger than that of the silica sand as high as at 973 K. The iron oxide on the alumina behaved as a cracking catalyst, and its activity was high at 773 K. The difference between the H2 yields obtained with Feimpregnated alumina and alumina decreased with increasing temperature, which suggests that cracking of the coke by the alumina alone occurred to a lesser extent at higher temperatures and that the effect of the iron oxide was relatively small. Figure 3 shows the gas yields under steam reforming conditions. Carbon conversion into gas increased in the order
Steam Reforming of Woody Biomass
silica sand < alumina < Fe-impregnated alumina, as was the case under pyrolysis conditions. However, under steam reforming conditions, the difference in carbon conversion observed with the Fe-impregnated alumina and the alumina at all temperatures was more marked than under pyrolysis conditions. This result was due to the enhancement of CO2 formation by the iron oxide loading. At 773 K, H2 formation with Feimpregnated alumina was much larger than with alumina, and this trend was similar to that observed for pyrolysis (Figure 2). At 973 K, the H2 yield obtained with Fe-impregnated alumina was about 1.8 times that obtained under pyrolysis conditions. Under steam reforming conditions, the difference between the H2 yields with Fe-impregnated alumina and alumina was more marked, unlike that under pyrolysis. This result suggests that the catalytic activity of the iron oxide was obvious in the presence of steam at higher temperatures. Role of Iron Oxide in Hydrogen Formation. The iron oxide played an important role in H2 formation during steam reforming, but it is difficult to determine the H2 formation pathway during steam reforming on Fe-impregnated alumina. There are many possible reaction pathways, including the following: (1) devolatilization, (2) steam reforming of char, (3) reforming of tar in the gas phase, (4) reforming or cracking of coke on the alumina, (5) reforming or cracking of coke on the iron oxide, (6) the water-gas shift reaction (WGSR) on the active site of the char surface,12 (7) reforming of char by contact with the iron oxide, and (8) the steam-iron reaction.13-15 The steam reforming of char (path 2) and the WGSR on char (path 6) can be eliminated by using a two-stage fluidized bed reactor, because in this reactor steam can come into contact with the coke on the Fe-impregnated alumina, but not with the char. In the single-stage reactor, char is gradually accumulated in the bed and then steam reforming is promoted by contact between the iron oxide and the char (path 7). Although the catalytic activity of the iron oxide for the steam reforming of char is not as high as the activity of transition elements such as rhodium,19 the possibility of path 7 cannot be ignored. Use of the twostage reactor can also eliminate the possibility of contact between the iron oxide and the char. Reaction paths 1 and 3 are independent of the bed material, and paths 2, 6, and 7 can be ignored if the two-stage reactor is used. Therefore, by using the two-stage reactor to compare the gas-formation yields of the alumina with those of the Fe-impregnated alumina, we should be able to elucidate the role of iron oxide in the reforming of the coke. Yields of gas formed during pyrolysis and steam reforming in the two-stage reactor at 773 K are shown in Figure 4. The panels labeled “Pyrolysis” refer to experiments in which coke was deposited on the sample in the upper bed (Figure 1b) and no steam came into contact with the coke or the char accumulated in the lower bed. The panels labeled “Reforming” refer to experiments in which coke was deposited on the sample in the upper bed and then steam came into contact with the coke but not with the char. The yields of the gases obtained with Fe-impregnated alumina under steam reforming conditions were similar to the yields under pyrolysis conditions, suggesting that the iron oxide had only a small effect on the steam reforming of the coke at 773 K. The yields of CO2 obtained with Fe-impregnated alumina were slightly larger than with (17) Simell, P. A.; Leppalahti, J. K.; Bredenberg, J. B. Fuel 1992, 71, 211-218. (18) Tamhnkar, S. S.; Tsuchiya, K.; Riggs, J. Appl. Catal. 1985, 16, 103-121. (19) Asadullah, M.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. EnViron. Sci. Technol. 2002, 36, 4479-4481.
Energy & Fuels, Vol. 20, No. 6, 2006 2729
Figure 4. Gas yields upon pyrolysis and steam reforming of oak sawdust at 773 K in the two-stage fluidized bed reactor: (lower bed) SiO2; (upper bed) γ-Al2O3 or iron oxide-impregnated γ-Al2O3.
Figure 5. Gas yields upon pyrolysis and steam reforming of oak sawdust at 973 K in the two-stage fluidized bed reactor: (lower bed) SiO2; (upper bed) γ-Al2O3 or iron oxide-impregnated γ-Al2O3.
alumina under steam reforming conditions, whereas the yields of other gases (CO and CH4) were independent of the presence of the iron oxide. This result indicates that lattice oxygen in the iron oxide was used to reform the coke on the alumina16 (eq 2), but only to a small extent. On the other hand, the H2 yield with Fe-impregnated alumina was much larger than with alumina, and the yield difference was larger than that for CO2. There have been several studies on the decomposition of tarry materials on iron-containing materials.17,18 Although the activity of iron oxide with respect to the cracking of tar is lower than that of metallic iron,17 tarry materials can be decomposed with iron oxide.17,18 Judging from these literature reports and from the results shown in Figure 6, we can say that the extent of cracking of the coke on the iron oxide particles was marked and that it was the dominant pathway of H2 formation at 773 K. Figure 5 illustrates gas yields during pyrolysis and steam reforming experiments using the two-stage reactor at 973 K. Under both conditions (pyrolysis and reforming), the sum of
2730 Energy & Fuels, Vol. 20, No. 6, 2006
Matsuoka et al.
Figure 6. Step response reaction on switching gas from steam-Ar to steam-CO-Ar: (a) coke-bearing γ-Al2O3 at 773 K, (b) coke-bearing iron oxide-impregnated γ-Al2O3 at 773 K, (c) coke-bearing γ-Al2O3 at 973 K, and (d) coke-bearing iron oxide-impregnated γ-Al2O3 at 973 K.
the yields of CO, CO2, CH4, and C2H4 obtained with Feimpregnated alumina was slightly larger than with alumina, and the difference in the yields was not enlarged by the presence of steam. This result indicates that steam reforming of coke on the iron oxide (eq 2) occurred and that it was not promoted by either increasing the temperature or adding steam. Under pyrolysis conditions at 973 K, the CO yield was independent of the presence of the iron oxide, whereas the yields of CO2 and H2 were slightly increased by the iron oxide loading. In contrast, under steam reforming conditions, the yield of CO was decreased by the presence of the iron oxide and the yield of CO2 was increased. The difference between the H2 yields obtained with alumina and Fe-impregnated alumina under steam reforming conditions was similar to the increase in CO2 yield obtained by iron oxide loading. On the basis of these results, we can infer that the WGSR (CO + H2O ) CO2 + H2) occurred on the iron oxide particles. To verify this inference, we carried out a step response reaction by injecting the gas stepwise into a particle bed containing the coke-bearing Fe-impregnated alumina after the steam reforming reactions shown in Figures 4 and 5. Figure 6 shows the step response reaction on the coke-bearing Feimpregnated alumina. For reference, coke-bearing alumina was also used. At 773 K, little gas was formed with the coke-bearing alumina either before or after the gas was switched (Figure 6a), which suggests that little coke was reformed with steam. This result agrees with the results shown in Figure 4. At 973 K, H2, CO, and CO2 were formed before the gas was switched (Figure 6c) and H2 and CO2 continued to form even after the gas was switched.10 This result indicates that the coke was reformed with steam before the gas was switched and that the WGSR did not occur on the coke-bearing alumina under these conditions. In the case of Fe-impregnated alumina at 773 K (Figure 6b), small amounts of H2 and CO2 were formed under the steam flow until 20 min, which suggests that steam reforming of the coke was promoted on the iron oxide. Little CO was consumed after the gas was switched, which indicates that the WGSR did not occur.
CO introduced into the system with coke-bearing Feimpregnated alumina was apparently consumed during the second period at 973 K (Figure 6d), and the amount of CO consumed was almost the same as the amounts of the increases in H2 and CO2. This result suggests that the WGSR occurred on the iron oxide particles. To determine the form of the iron oxide during the steam reforming reaction, we performed X-ray diffraction (XRD) analysis of the Fe-impregnated alumina before and after the steam reforming (Figures 4 and 5). Though not shown here, strong γ-alumina peaks and a hematite (Fe2O3) peak were observed in the raw sample (before the steam reforming). After the steam reforming reactions, the hematite peak disappeared and a small magnetite peak (Fe3O4) was observed. No distinct peaks for wustite (FeO) or metallic iron (Fe) were detected after the steam reforming. However, because the peaks for γ-alumina were quite strong and the peak positions for wustite and metallic iron were close to the alumina peaks, we cannot confirm whether wustite and/or metallic iron coexisted with magnetite. Therefore, we can say only that magnetite was a stable iron-containing species after the steam reforming reaction, but we cannot say whether magnetite was further reduced. Fukase and Suzuka14 studied the cracking of residual oil combined with H2 production by the steam-iron reaction using iron oxide. They reported that the following reactions occurred at 830 °C:
Fe3O4 + CO f 3FeO + CO2
(3)
3FeO + H2O f Fe3O4 + H2
(4)
Their experimental conditions were different from ours, but the reactions shown in eqs 3 and 4 can be expected to occurr also under the present steam reforming conditions. Also, the WGSR (CO + H2O ) CO2 + H2) seems have occurred on the ironoxide particles, as shown in Figure 6d. The consumption of CO formed by the steam reforming for the apparent WGSR is the main reason for the increase in H2 caused by the iron oxide
Steam Reforming of Woody Biomass
Energy & Fuels, Vol. 20, No. 6, 2006 2731
loading during the steam reforming conditions at higher temperatures. On the other hand, the H2 yield obtained with Fe-impregnated alumina at 773 K exceeded that obtained with silica sand at 973 K under pyrolysis and steam reforming conditions (Figures 2 and 3). Thus, the activity of the iron oxide for H2 formation was significant at lower temperatures and it was independent of the presence of steam. As shown in Figure 6, the WGSR that apparently occurred at 973 K did not occur at 773 K. Therefore, cracking of the coke on the iron-oxide particles was the main pathway for the increase in H2 yield due to iron oxide loading at lower temperatures.
rather small, whereas a substantial amount of H2 was formed by cracking of the coke on the iron oxide. At 973 K, H2 production under steam reforming conditions was enhanced by the iron oxide loading. This effect was confirmed by means of experiments in a two-stage fluidized bed reactor, in which steam could come into contact with the coke but not with the resultant char. Steam reforming of the coke to form H2 on the iron oxide was not significant. CO, formed by devolatilization and by steam reforming of the coke on the alumina, reduced the iron oxide, and the reduced iron species was then oxidized with steam to form H2.
Conclusions
Acknowledgment. The authors thank Dr. H. Orikasa of the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, for carrying out the XRD analysis. This study was supported partly by the Steel Industry Foundation for the Advancement of Environmental Protection Technology, Japan.
Steam reforming of woody biomass was carried out in a fluidized bed of iron oxide-impregnated porous γ-alumina. During steam reforming, the iron oxide-impregnated alumina captured tarry materials to form coke. At 773 K, the extent of steam reforming of the coke on the iron oxide to form H2 was
EF060301F
