Steam Gasification of Cellulose with Cobalt Catalysts in a Fluidized

590

Energy & Fuels 2007, 21, 590-595

Steam Gasification of Cellulose with Cobalt Catalysts in a Fluidized Bed Reactor Kazuhiko Tasaka,† Takeshi Furusawa,‡ and Atsushi Tsutsumi*,† Department of Chemical System Engineering, The UniVersity of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-8656, Japan, and Department of Applied Chemistry, Utsunomiya UniVersity, Yoto 7-1-2, Utsunomiya 321-8585, Japan ReceiVed May 28, 2006. ReVised Manuscript ReceiVed NoVember 29, 2006

The catalytic performance of Co/MgO catalysts for the treatment of tar derived from cellulose steam gasification in a bubbling fluidized bed gasifier of 22 mm i.d. and 500 mm height was investigated by means of two different methods: hot gas cleaning in a fixed bed reactor after the fluidized bed gasifier (secondary method) and treatment inside the gasifier with catalyst as a fluidizing medium (primary method). For steam gasification without catalyst, the tar produced in the gasifier was about 12 wt % of the fed cellulose. The main components of tar were cellotriosan, cellobiosan, and levoglucosan. In the secondary method with 12 wt % Co/MgO catalyst, the tar in effluent gases from the fixed bed reactor were considerably less at 873 K: tar conversion was greater than 80%. The amounts of produced H2 and CO2 increased; however, the amount of CO remained almost at its level without catalyst. In addition, the amount of produced gases remained almost constant during the 100 min of reaction time at 873 K. Therefore, results suggest that steam reforming of tar derived from cellulose gasification proceeds sufficiently with 12 wt % Co/MgO catalyst at 873 K. The primary method showed that tar conversion increases with the Co loading amount. The 36 wt % Co/MgO catalyst showed 84% tar conversion. High carbon conversion to gas (67%) was also attained in the case of the 36 wt % Co/MgO catalyst. The amount of produced gas remained for 2 h. Results showed that the developed Co catalyst has a great potential for tar treatment in both the primary and secondary methods.

Introduction Biomass energy offers great potential for alleviating global environmental problems because biomass is a carbon-neutral resource. Biomass gasification is a promising technology for producing a fuel gas that is useful for power generation systems and synthetic gas applications. The products of biomass gasification are tar, ash, and alkali metals, along with syngas. During the biomass gasification process, tar formation often causes tar troubles in the application of the produced gas (gas engine/turbine). Therefore, the proper tar treatment is required for biomass gasification applications.1-10 In the last few decades, extensive research efforts have been devoted to catalytic tar treatment.1-46 * Corresponding author. Tel.: +81-3-5841-7336. Fax: +81-3-58417270. E-mail: [email protected]. † The University of Tokyo. ‡ Utsunomiya University. (1) Bridgwater, A. V. Appl. Catal. A: General 1994, 116, 5-47. (2) Narvaéz, I.; Orıó, A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110-2120. (3) Gil, J.; Aznar, M. P.; Caballero, M. A.; France´s, E.; Corella, J. Energy Fuels 1997, 11, 1109-1118. (4) Gil, J.; Corella, J.; Aznar, M. P.; Caballero, M. A. Biomass Bioenergy 1999, 17, 389-403. (5) Padban, N.; Wang, W.; Ye, Z.; Bjerle, I.; Odenbrand, I. Energy Fuels 2000, 14, 603-611. (6) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155-173. (7) Dayton, D. C. Milestone Completion Report; Report TP-510-32815, National Renewable Energy Laboratory: Golden, CO, 2002. (8) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125-140. (9) Abu El-Rub, Z.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911-6919. (10) Donnot, A.; Magne, P.; Deglise, X. J. Anal. Appl. Pyrolysis 1991, 21, 265-280.

Catalytic tar removal technologies can be broadly classified into two approaches: hot gas cleaning after gasification (secondary method) and treatments inside the gasifier (primary method). Most studies have particularly examined dolomite (CaMg(CO3)2)8-15 and Ni catalysts16-27 as effective catalysts (11) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 3637-3643. (12) Myreń, C.; Ho¨rnell, C.; Bjo¨rnbom, E.; Sjo¨stro¨m, K. Biomass Bioenergy 2002, 23, 217-227. (13) Corella, J.; Toledo, J. M.; Padilla, R. Energy Fuels 2004, 18, 713720. (14) Corella, J.; Toledo, J. M.; Padilla, R. Ind. Eng. Chem. Res. 2004, 43, 2433-2445. (15) Devi, L.; Ptasinski, K. J.; Janssen, F. J. G.; Paasen, S. V. B.; Bergman, P. C. A.; Kiel, J. H. A. Renewable Energy 2005, 30, 565-587. (16) Baker, E. G.; Mudge, L. K.; Brown, M. D. Ind. Eng. Chem. Res. 1987, 26, 1335-1339. (17) Aznar, M. P.; Corella, J.; Delgado, J.; Lahoz, J. Ind. Eng. Chem. Res. 1993, 32, 1-10. (18) Narvaéz, I.; Corella, J.; Orıó, A. Ind. Eng. Chem. Res. 1997, 36, 317-327. (19) Caballero, M. P.; Aznar, M. P.; Gil, J.; Martıń, J. A.; France´s, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5227-5239. (20) Aznar, M. P.; Caballero, M. A.; Gil, J.; Martıń, J. A.; Corella, J. Ind. Eng. Chem. Res. 1998, 37, 2668-2680. (21) Corella, J.; Orıó, A.; Aznar, M. P. Ind. Eng. Chem. Res. 1998, 37, 4617-4624. (22) Corella, J.; Orıó, A.; Toledo, J. M. Energy Fuels 1999, 13, 702709. (23) Caballero, M. A.; Corella, J.; Aznar, M. P.; Gil, J. Ind. Eng. Chem. Res. 2000, 39, 1143-1154. (24) Rapagna´, S.; Provendier, H.; Petit, C.; Kiennemann, A.; Foscolo, P. U. Biomass Bioenergy 2002, 22, 377-388. (25) Engelen, K.; Zhang, Y.; Draelants, D. J.; Baron, G. V. Chem. Eng. Sci. 2003, 58, 665-670. (26) Pfeifer, C.; Rauch, R.; Hofbauer, H. Ind. Eng. Chem. Res. 2004, 43, 1634-1640. (27) Ma, L.; Verelst, H.; Baron, G. V. Catal. Today 2005, 105, 729734.

10.1021/ef060241d CCC: $37.00 © 2007 American Chemical Society Published on Web 01/19/2007

Steam Gasification of Cellulose

for use with the secondary method. Corella et al. reported that, using a guard bed with a calcined dolomite as a pretreatment to decrease the tar content at the inlet of the Ni catalyst bed below 1-2 g m-3, the tar content at the outlet of the Ni catalyst bed can be reduced to a level of 2-10 mg m-3. Additionally, no deactivation was observed with a time-on-stream of up to 65 h.22,23 However, the secondary method with two different reactors made the gasification plant more expensive and complex. From this perspective, developing a more effective catalyst is essential for scaling down the plant size while still facilitating high and stable catalytic performance. Instead of Ni catalysts, some researchers have investigated rare metal catalysts that are known to have higher catalytic activity than natural ore or alkali and alkaline earth metals (AAEM).6-9 Tomishige et al. extensively explored the use of the Rh catalyst for biomass gasification.28-41 The effect of support material on the catalytic performance for the decomposition of cellulose tar was investigated in a fluidized bed gasifier with the catalyst as a fluidizing material.28-31,34,36 Tomishige et al. reported that Rh/CeO2/SiO2 catalyst showed complete conversion of cellulose tar at 823 K.31 Moreover, when Rh/CeO2/SiO2 catalyst was applied to actual biomass (jute stick, bagasse, rice straw, and cedar wood sawdust), the catalyst was observed to be effective for tar reduction.32-33,35-41 In our previous studies, several kinds of Co/MgO catalysts were prepared, and their catalytic performance was investigated for steam reforming of naphthalene.42,43 Naphthalene is a main biomass tar compound that is difficult to decompose. Therefore, naphthalene reforming is commonly used as a test reaction for developing a catalyst applied for biomass tar decomposition.25,27,44-47 The effects of calcination temperature and the amount of Co loading on catalytic performance were investi(28) Asadullah, M.; Tomishige, K.; Fujimoto, K. Catal. Commun. 2001, 2, 63-68. (29) Asadullah, M.; Fujimoto, K.; Tomishige, K. Ind. Eng. Chem. Res. 2001, 40, 5894-5900. (30) Asadullah, M.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. EnViron. Sci. Technol. 2002, 36, 4476-4481. (31) Asadullah, M.; Ito, S.; Kunimori, K.; Tomishige, K. Ind. Eng. Chem. Res. 2002, 41, 4567-4575. (32) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. Appl. Catal. A: General 2003, 255, 169-180. (33) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Tomishige, K. Appl. Catal. A: General 2003, 246, 103-116. (34) Asadullah, M.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. J. Catal. 2003, 208, 255-259. (35) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Tomishige, K. Energy Fuels 2003, 17, 842-849. (36) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. Appl. Catal. A: General 2004, 267, 95-102. (37) Tomishige, K.; Asadullah, M.; Kunimori, K. Catal. Today 2004, 89, 389-403. (38) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Koyama, S.; Tomishige, K. Biomass Bioenergy 2004, 26, 269-279. (39) Tomishige, K.; Miyazawa, T.; Kimura, T.; Kunimori, K.; Koizumi, N.; Yamada, M. Appl. Catal. B: EnVironmental 2005, 60, 299-307. (40) Miyazawa, T.; Kimura, T.; Nishikawa, J.; Kunimori, K.; Tomishige, K. Sci. Technol. Appl. Mater. 2005, 6, 604-614. (41) Tomishige, K.; Miyazawa, T.; Kimura, T.; Kunimori, K. Catal. Commun. 2005, 6, 37-40. (42) Furusawa, T.; Tsutsumi, A. Appl. Catal. A: General 2005, 278, 195-205. (43) Furusawa, T.; Tsutsumi, A. Appl. Catal. A: General 2005, 278, 207-212. (44) Zhao, H.; Draelants, D. J.; Baron, G. V. Ind. Eng. Chem. Res. 2000, 39, 3195-3201. (45) Wang, T. J.; Chang, J.; Wu, C. Z.; Fu, Y.; Chen, Y. Biomass Bioenergy 2005, 28, 508-514. (46) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Fuel Process. Technol. 2005, 86, 707-730. (47) Devi, L.; Craje, M.; Thune, P.; Ptasinski, K. J.; Janssen, F. J. J. G. Appl. Catal. A: General 2005, 294, 68-79.

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Figure 1. Experimental setup: (1) water and pump, (2) steam generator, (3) feeder, (4) pressure gauge, (5) fluidized bed gasifier, (6) quartz wool filter, (7) catalytic fixed bed reactor, (8) ice-cold traps, (9) CaCl2 column, (10) pump, (11) gas bag, and (12) vent line.

gated under severe conditions (T ) 1173 K, 100 times higher naphthalene concentration than that in the biomass tar, low steam/carbon molar ratio of 0.6). Results of that study showed that 12 wt % Co/MgO catalyst precalcined at 873 K demonstrated the best performance (conversion of 23% in 3 h), and the activity of the best catalyst is higher than that of Ni/MgO catalyst (conversion of 8%). Additionally, the characterization results indicated that the high Co metal surface area had a small amount of coke deposition, which accounted for the high and stable activity.42,43 In this study, the developed Co/MgO catalyst is applied for steam gasification of cellulose using primary and secondary methods. In the secondary method, steam gasification of cellulose was conducted in a fluidized bed gasifier. The produced gas, including tar, was introduced into a fixed bed reactor of Co/MgO catalyst. To recuperate the waste heat energy from the gas turbine (around 1073 K in current available gas turbines), its catalytic performance was investigated at a lower temperature (773 and 873 K) corresponding to heat recovered from the gas turbine.48-50 In the primary method, Co/MgO catalyst was used as a bed material in the gasifier for steam gasification of cellulose at 873 K. Catalytic activities of gas yield and tar decomposition were investigated. Finally, the effect of Co loading on catalytic performance in the primary and secondary methods was discussed. Experimental Details Catalyst Preparation. Cobalt-supported-on-MgO catalysts were prepared by impregnating MgO with aqueous solutions of Co(NO3)2‚6H2O, followed by calcination at 873 K for 8 h in air. The details of the preparation of the catalysts are described in the previous papers.42,43,51 The catalysts were pelletized, crushed, and sieved to 0.35-0.5 mm before use. (48) Kuchonthara, P.; Bhattacharya, S.; Tsutsumi, A. J. Power Sources 2003, 117, 7-13. (49) Kuchonthara, P.; Bhattacharya, S.; Tsutsumi, A. J. Power Sources 2003, 124, 65-75. (50) Kuchonthara, P.; Bhattacharya, S.; Tsutsumi, A. Fuel 2005, 84, 1019-1021. (51) Tasaka, K.; Furusawa, T.; Ujimine, K.; Tsutsumi, A. Stud. Surf. Sci. Catal. 2006, 159, 517-520.

592 Energy & Fuels, Vol. 21, No. 2, 2007 Apparatus. A schematic diagram of the bubbling fluidized bed gasifier and catalytic fixed bed reactor is shown in Figure 1. The setup consisted of a fluidized bed for biomass gasification, a biomass feeder, a steam generator, a catalytic bed for decomposition of biomass tar, ice-cold traps, and an analysis section. A 500 mm high, vertical stainless steel tube reactor with 22 mm i.d. and an attached distributor was used for biomass gasification. The bed material was fluidized with the mixture of Ar and steam. The gasifier was heated using an infrared gold image furnace (RHLP616; Ulvac). The bed temperature was controlled at 873 K using an N-type thermocouple inserted into the bed. Samples were stocked in a special vibrating feeder made of an acrylic tube (i.d. 15 mm) and fed from the top of the gasifier through a stainless steel feeding tube inserted in a bed while checking the feed rate. The exit of the feeding tube was positioned near the bottom of the bed material. Respective flows of Ar gas were 0.5 and 1.0 L min-1 from the bottom and top of the fluidized bed gasifier. The superficial gas velocity was 230 mm s-1; minimum fluidization velocity of the catalysts was 51 mm s-1. The gas was also used to pressurize the feeder box. The catalytic fixed bed reactor was a stainless steel tube (i.d. 22 mm, length 300 mm), which was also heated using an infrared gold image furnace (RHL-E48; Ulvac). The catalyst bed temperature was controlled using an N-type thermocouple inserted into the bed. The flexible tube between the fluidized bed gasifier and catalytic fixed bed reactor was heated to 623 K using a ribbon heater to prevent condensation of tar compounds. Effluent gases from the fluidized bed gasifier or catalytic fixed bed reactor were fed through two cotton filters set inside ice-cold traps. Then, a CaCl2 column was connected in series. Unreacted tar and water were recovered in the two ice-cold traps. The noncondensable gases were analyzed using an on-line mass spectrometer (Standam; Ulvac) to obtain gas profiles. They were also collected in gas bags for analysis by off-line gas chromatograph (HP3000; Agilent column, molecular sieve 5A for separation of H2, N2, CH4, CO, and Pora Plot Q column for separation of CO2 and C2). Procedure. Cellulose was selected as a feed sample in this study to avoid the effects of AAEM and sulfur included in the original biomass. First, 60 g of the cellulose microcrystalline (Avicel; MERCK Inc.) was set into the feeder and oxygen in the feeder box was purged with Ar. All reactions were conducted under atmospheric pressure. Since our present interest is developing a new catalyst for catalytic tar decomposition, the experiments were conducted for only 100 min to screen the catalyst easily. In the secondary method, quartz sand particles (0.35-0.5 mm) were used as the bed material of the gasifier; 3 g of catalyst was supported by two plugs of quartz wool in the catalytic fixed bed reactor (GHSV ) 5.9 × 104 h-1, residence time ) 0.06 s). All catalysts were reduced in a 50% H2/Ar mixture with heating rates of 10 K min-1 to 1173 K. They were held at 1173 K for 30 min. After reduction, the temperature was decreased to the reaction temperature (773 or 873 K) under Ar flow. Oxygen in the fluidized bed gasifier was also purged with Ar. Then, steam (0.1 cm3 min-1) and cellulose (0.12-0.21 g min-1) were fed into the gasifier. The flue gas (including tar) was passed through a heated flexible tube (623 K) and introduced into the catalytic fixed bed reactor. In the primary method, a mixture of catalyst (5 g) and quartz sand (5 g) was introduced into the fluidized bed gasifier (residence time ) 0.10 s). All catalysts were reduced and cooled under identical conditions to those mentioned above. Then, steam (0.1 cm3 min-1) and cellulose (0.12-0.21 g min-1) were fed into the gasifier. The effluent gas flowed directly to ice-cold traps. Unreacted tar and water were recovered in two ice-cold traps. They were washed with distilled water and acetone to produce a solution. The obtained solution and cotton filters were dried at 383 K overnight according to the method of Simell et al.52 (52) Simell, P.; Ståhlberg, P.; Kurkela, E.; Albrecht, J.; Deutsch, S.; Sjo¨stro¨m, K. Biomass Bioenergy 2000, 18, 19-38.

Tasaka et al. Table 1. Characterization Results of Reduced Catalysts reduction exposed Co-metal BET surface area surface area degree of dispersion Co [%] [%] [m2 g-1 × 100] [m2 g-2]

catalyst MgO 4 wt % Co/MgO 12 wt % Co/MgO 36 wt % Co/MgO

19.3 29.2 30.0 19.7

55 73 66

0.50 0.37 0.35

23.4 50.8 145.7

Table 2. Effect of S/C Molar Ratio on the Amount of Recovered Tar and Elemental Composition of Recovered Tar for the Steam Gasification of Cellulose in the Fluidized Bed Gasifier without Catalyst S/C ratio [-]

recovered tar [wt %]

1.3 1.0 0.6

12.3 11.9 11.4

elemental composition of recovered tar [wt %] C H O 44.4 46.2 44.8

7.1 6.6 7.1

48.5 47.2 48.1

Then, the amount of unreacted tar was calculated from the weight increment of cotton filters and the weight of dried material obtained from the solution described above. In this paper, the tar remained after drying was defined as “recovered tar”. The composition of dried tar was analyzed using a CHNS elemental analyzer (2400II CHNS/O; Perkin-Elmer Inc.). The molecular weight of tar was also measured using a gel filtration chromatograph (CTO-10A, column, SB-802.5HQ; detector, RID-10A; Shimadzu Corp.). Characterization. An AMI-200 apparatus (ALTAMIRA Instruments) was used to analyze the BET surface area of the catalyst with flowing He and N2. The amount of N2 adsorbed on the catalyst at 77 K was determined quantitatively using a thermal conductivity detector (TCD). The exposed Co metal surface area of the reduced catalyst was determined by CO pulse adsorption at room temperature using an AMI-200 apparatus (ALTAMIRA Instruments), assuming a 1/1 stoichiometry. A 0.2 g portion of calcined catalyst powder was loaded into the U-shaped reactor (i.d. 4 mm). Its reduction was carried out in a 10% H2/Ar mixture with a heating rate at 10 K min-1 to 1173 K, and it was held at 1173 K for 30 min. After reduction, the reduced catalyst was cooled to room temperature with flowing Ar (30 mL min-1) and CO was pulsed (58 µL per pulse) over the catalyst until no further adsorption of CO was observed. The CO that remained after CO adsorption was determined quantitatively using a TCD. For estimation of char and coke on the catalyst, temperature programmed oxidation measurements (TPO) were performed on the tested catalyst after cellulose gasification using an AMI-200 apparatus. The catalyst was transferred to the U-shaped reactor and was supported using two plugs of quartz wool. A 30 mL min-1 feed of 5% O2/He was used for oxidation, and the temperature was ramped at 10 K min-1 to 1273 K. The deposited coke or remaining char was oxidized to CO or CO2, and the O2 consumption was detected using TCD. The characterization results of reduced catalysts are summarized in Table 1.

Results and Discussion Steam Gasification of Cellulose in a Fluidized Bed Gasifier (Bypass Test). The tar produced from steam gasification of cellulose in the fluidized bed gasifier without catalyst bypassed the catalytic fixed bed reactor and was introduced directly into the ice-cold traps to analyze the tar properties. By changing the feed rate, the steam/carbon molar ratios (S/C ratios) were varied and the effects of S/C ratios on the amount of produced tar were investigated. Experiments under each condition were performed three times, and the deviation was around 5%. Table 2 shows the effect of the S/C ratio on the amount of recovered tar (in grams per gram cellulose × 100) and the averaged elemental composi-



Figure 2. Time profiles of gas evolution for the steam gasification of cellulose.

tion of recovered tar (in weight percent). To evaluate the catalytic performances for tar decomposition, the amounts of produced tar should be enough to clarify the difference between each catalyst. For this point of view, the operative condition in this study is far from the optimized experimental condition for tar reduction. The amount of recovered tar was about 12 wt % of the fed cellulose and did not change for different S/C ratios. The elemental composition of recovered tar is similar to that of the original cellulose (C 44.4, H 6.1, O 49.4 wt %), independent of the S/C ratio. According to the GFC analysis, the main components of the recovered tar were cellotriosan, cellobiosan, and levoglucosan.53 These results suggest that secondary reactions of produced tar in the freeboard region are negligible under this experimental condition. The time profiles of gas evolution for the steam gasification of cellulose in the fluidized bed are shown in Figure 2 (S/C ratio 1.3). To eliminate the effect of the fluctuation of the feed rate, the amounts of produced gases in each 10 min interval were divided by the amount of fed cellulose in each 10 min interval, thus the units are moles per gram of cellulose. The main produced gases were CO and H2; the amounts of produced CO2 and CH4 were less than 10% of CO. Gas evolution became stable after 20 min of induction. The induction period stems from the accumulation of char in the fluidized bed. The amount of char that remained in the fluidized bed gasifier was estimated by TPO analysis as 3-5 C-mol % of the fed cellulose. The calculated carbon balance is about 70% in the bypass test. The rest of the carbon is attributable to the char accumulated in the bed of the gasifier and/or remaining in the quartz wool filter. In addition, for analysis of tar after experiments, the solution including tar was dried at 383 K. Some of the produced tar was lost because the vaporization temperatures for some tar were lower than 383 K. Catalytic Performance of Co/MgO Catalysts for the Steam Reforming of Tar Derived from Cellulose Gasification (Secondary Method). In the secondary method, in which the fluidized bed gasifier is connected to the catalytic fixed bed reactor, steam reforming of tar derived from cellulose gasification was conducted with 12 wt % Co/MgO catalyst, at either 773 or 873 K. Blank tests were also conducted with quartz wool plugged into the catalytic reactor. Assuming that all recovered tar (12 wt % of fed cellulose) was introduced into the catalytic bed reactor, tar conversion was defined as

Figure 3. Amount of recovered tar and calculated tar conversion in the secondary method with and without catalyst.

Figure 4. Gas evolution at reaction times of 20 and 100 min in the secondary method with 12 wt % Co/MgO catalyst at 773 and 873 K: (a) 773; (b) 873 K.

(1)

introduced into the catalytic bed reactor (12 wt % of fed cellulose) and the amount of unreacted tar contained in flue gas from the catalytic bed reactor (in weight percent). Figure 3 shows the amount of recovered tar (in grams per gram cellulose × 100) and calculated tar conversion in the secondary method of the catalytic bed reactor with and without catalyst. In both blank tests at 773 and 873 K, about 85% of produced tar in the fluidized bed gasifier remained in the effluent gases from the catalytic bed reactor without catalyst. The remaining

where Wi and Wr respectively represent the amount of tar

(53) Yamaguchi, Y.; Fushimi, C.; Tasaka, K.; Furusawa, T.; Tsutsumi, A. Energy Fuels 2006, 20, 2681-2685.

( )

tar conversion[%] ) 1 -

Wr × 100[%] Wi

594 Energy & Fuels, Vol. 21, No. 2, 2007

Tasaka et al.

Figure 5. Amount of recovered tar and calculated tar conversion using the primary method with Co/MgO catalyst. Table 3. Amount of Recovered Tar and Elemental Composition in the Primary Method with Co/MgO Catalyst

catalyst

recovered tar [wt %]

MgO 4 wt % Co/MgO 12 wt % Co/MgO 36 wt % Co/MgO

14.0 9.7 6.9 2.2

elemental composition of recovered tar [wt %] C H O 44.6 43.5 44.7 46.3

5.6 6.4 6.2 6.1

49.8 50.1 49.1 47.6

tar in the effluent gases decreased drastically with 12 wt % Co/ MgO catalyst at both reaction temperatures. Tar conversion was calculated as greater than 80%. Considering the reaction conditions (gasifying agent steam only, residence time 0.06 s), it can be said that the developed Co/MgO catalyst is sufficiently effective for tar reduction, even at 773 K. The gas evolution changed with time-on-stream and came to steady state. The gas evolutions at reaction times of 20 min (initial stage after the induction period) and 100 min (steady state) were shown in Figure 4 with the bypass test results obtained at 120 min. In the case of the secondary method with 12 wt % Co/MgO catalyst, the amount of produced H2 and CO2 increased in the early stage (20 min) at both reaction temperatures because of tar decomposition to light gases. At 873 K, the amount of produced gas remained almost constant during the 100 min reaction time. On the other hand, the amounts of produced CO and H2 decreased with reaction time at 773 K. Considering high tar reduction (89%) and low carbon conversion to gas, tar seems to accumulate on the catalyst surface as coke and is not removed by oxidation using steam at 773 K. Therefore, it can be concluded that reaction temperature should be higher than 773 K to maintain catalytic activity for long reaction times to thereby avoid carbon deposition on the catalyst surface. Catalytic Performance of Co/MgO Catalysts for the Steam Gasification of Cellulose in a Fluidized Bed Gasifier (Primary Method). In the primary method, in which a catalyst is used as a bed material in the fluidized bed gasifier, the bed temperature was set at 873 K, at which 12 wt % Co/MgO catalyst exhibits stable activity. Table 3 shows the amount of recovered tar (in grams per gram cellulose × 100) and elemental composition of tar (in weight percent) in the primary method. In all cases, no marked

Figure 6. Gas evolution at reaction times of 20 and 100 min using the primary method with Co/MgO catalyst: (a) 4 wt % Co/MgO catalyst; (b) 12 wt % Co/MgO catalyst; (c) 36 wt % Co/MgO catalyst.

change in the elemental composition of recovered tar was observed. In addition, the main components of recovered tar were also confirmed as cellotriosan, cellobiosan, and levoglucosan using GFC analysis, indicating that secondary reaction of tar such as polymerization and polycondensation over the catalyst is negligible. The amount of recovered tar (14 wt %) was more than that of recovered tar (12 wt %) in the steam gasification of cellulose with inert bed material when MgO was added to the inert bed material. These results indicate that MgO has less activity for both cellulose gasification and steam reforming of tar derived from cellulose than quartz sand. On the basis of the amount of recovered tar by the primary method with MgO, tar conversion was defined as

(

tar conversion[%] ) 1 -

)

Wr × 100[%] WrMgO

(2)


where WrMgO and Wr respectively represent the amount of tar produced with MgO as a fluidized bed material (14 wt % of fed cellulose) and the amount of tar produced with Co/MgO catalyst as a fluidized bed material (in weight percent). Figure 5 shows the amount of recovered tar (in grams per gram cellulose × 100) and calculated tar conversion in the primary method with Co/MgO catalyst. Tar conversion increases with an increase in Co loading amount, and 36 wt % Co/MgO catalyst showed 84% tar conversion. The 36 wt % Co/MgO catalyst also showed the highest carbon conversion to gas (67%) among the catalysts tested in this study. This result differs from those of previous studies,42-43 in which 12 wt % Co/MgO catalyst showed high activity for steam reforming of naphthalene. Further research for investigating this difference is now in progress. The gas evolution at reaction times of 20 min (initial stage) and 100 min (steady state) is shown in Figure 6. In all tests with Co/MgO catalysts, higher amounts of CO, H2, and CO2 were obtained than the amounts of these gases in steam gasification with MgO. In the primary method, catalysts directly contacted with char that accumulated in the fluidized bed gasifier. For that reason, the produced gases increased because of tar decomposition and/or char gasification. In the case of the 4 and 12 wt % Co/MgO catalysts, the amount of produced H2 and CO2 increased in the early stage (20 min) and decreased with reaction time to 100 min. In contrast, in the case of 36 wt % Co/MgO catalyst, the amount of produced gas remained almost constant during the 100 min reaction time, indicating that 36 wt % Co/MgO catalyst is stable during the steam gasification of cellulose in the fluidized bed gasifier. As a result, 36 wt % Co/MgO catalyst showed high activity without catalyst regeneration. Although the primary method requires more catalyst than the secondary method to obtain comparable tar reduction,8 this activity (84%) is approximately the same as that for tar conversion on 12 wt % Co/MgO catalyst in the secondary method at 873 K (Figure 3). Therefore, it can be said that Co/ MgO catalyst is also effective for catalytic steam gasification


of cellulose in the primary method. Tar remaining in the effluent gas was reduced from 14.7 to 2.1 g Nm-3 using 36 wt % Co/ MgO catalyst with a short residence time (0.1 s) and was comparable with that of the dolomite guard bed.22-23 Further experimentation with longer contact times is required. Conclusion The respective catalytic performances of Co/MgO catalysts for tar treatment were investigated using hot gas cleaning in a fixed bed reactor after the gasifier (secondary method) and simultaneous tar decomposition and gasification in the fluidized bed gasifier with catalyst as the fluidizing medium (primary method). For steam reforming of tar derived from cellulose gasification using the secondary method, the remaining tar in the effluent gases from the catalytic bed reactor drastically decreased with 12 wt % Co/MgO catalyst at 873 K. Tar conversion was calculated as more than 80% with a short residence time (0.06 s). The amount of produced gases remained almost constant during 100 min of reaction time at 873 K. These results imply that steam reforming of tar derived from cellulose gasification proceeds sufficiently over 12 wt % Co/MgO catalyst. For steam gasification of cellulose using the primary method, results showed that tar conversion increases with the Co loading amount: 36 wt % Co/MgO catalyst showed 84% tar conversion and high carbon conversion to gas (67%). The amount of produced gas after processing for 100 min indicated that 36 wt % Co/MgO catalyst is stable during the steam gasification of cellulose in a fluidized bed gasifier. These results lead us to conclude that these Co catalysts have potential for steam gasification of cellulose in both the primary and secondary methods. Acknowledgment. This study was financially supported by a “Core Research for Evolutional Science and Technology” grant from the Japan Science and Technology Agency (JST). The authors appreciate this financial support. EF060241D

Steam Gasification of Cellulose with Cobalt Catalysts in a Fluidized

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