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Energy & Fuels 2003, 17, 842-849
Catalyst Performance of Rh/CeO2/SiO2 in the Pyrogasification of Biomass Mohammad Asadullah, Tomohisa Miyazawa, Shin-ichi Ito, Kimio Kunimori, and Keiichi Tomishige* Institute of Materials Science, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8573, Japan Received October 15, 2002
The catalytic performance of Rh/CeO2/SiO2, steam reforming catalyst G-91, and dolomite has been evaluated in the pyrolytic, CO2, oxygen, and steam gasification of biomass in a laboratoryscale continuous feeding fluidized bed gasifier. With respect to the biomass conversion to the product gas and selectivity of the useful gases, Rh/CeO2/SiO2 has shown much better results in all gasification systems. In the pyrolytic gasification, about 79% of the carbon in the biomass was converted to the product gas at 923 K. The carbon conversion reached about 95% level when a small amount of oxygen (ER ) 0.18) was introduced with nitrogen. However, under almost the same conditions, the carbon conversion was within 75-80% of the level on G-91 catalysts. Although a small amount of char was formed on the highly active Rh/CeO2/SiO2, there was no tar found in the effluent gas stream; thus, this process has great potential to be utilized at the commercial level. With CO2 and steam in the absence of oxygen, it is also possible to convert char to the product gas.
Introduction Biomass, either agricultural byproducts or forest, which is abundantly available everywhere in the world, can be used as a petroleum substitute for power generation and chemical production via the production of syngassa mixture of carbon monoxide and hydrogens by gasification.1-4 Because plants naturally recycle CO2 and plant-derived biomass contains a small amount of toxic materials, the increasing use of biomass for energy through the production of syngas or hydrogen has a great impact on environmental protection.5-9 However, the process remains challenging due to many drawbacks. The most severe one is the formation of tar, a liquid mixture of higher hydrocarbons, during the gasification processseven when the process is operated at very high temperatures (>1073 K).10,11 The thermal conversion of tar is a function of temperaturesthe lower the temperature, the higher the tar formation. However, * Corresponding author. Tel/Fax: +81-298-53-5030. E-mail: tomi@ tulip.sannet.ne.jp. (1) Demirbas¸ , A. Energy Convers. Manage. 2001, 42, 1239. (2) Mahlia, T. M. I.; Abdulmuin, M. Z.; Alamsyah, T. M. I.; Mukhlishien, D. Energy Convers. Manage. 2001, 42, 2109. (3) Natarajan, E.; Nordin, A.; Rao, A. N. Biomass Bioenergy 1998, 14, 533. (4) Rozakis, S.; Soldatos, P. G.; Papadakis, G.; Kyritsis, S.; Papantonis, D. Energy Policy 1997, 25, 337. (5) Chum, H. L.; Overend, R. P. Fuel Process. Technol. 2001, 71, 187. (6) Scarpellini, S.; Romeo, L. M. Energy Convers. Manage. 1999, 40, 1661. (7) Lynch, M. C. Appl. Energy 1999, 64, 31. (8) Ogden, J. M. Annu. Rev. Energy Environ. 1999, 24, 227. (9) Steinberg, M. Int. J. Hydrogen Energy 1999, 24, 771. (10) Olivares, A.; Aznar, M. P.; Caballero, M. A.; Gil, J.; France´s, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5220. (11) Gil, J.; Corella, J.; Aznar, M. P.; Caballero, M. A. Biomass Bioenergy 1999, 17, 389.
to make the gasification process energy efficient, the process should be developed at low temperature. The tar conversion to gas at a lower temperature than usual must require some secondary activator such as a catalyst. Various types of catalysts, such as nickel-based catalysts, dolomites, zeolites, etc., have been investigated for tar cracking. Most of the studies used a catalyst in the secondary reactor where the volatile tar, generated from the thermal decomposition of biomass in the primary gasifier, cracked down to the product gas on the catalyst surface in the presence of gasifying agents such as steam, oxygen, or air. Dolomites12-14 and steam reforming nickel-based catalysts15-20 are the most conventional and active catalysts for tar cracking in the secondary reactor at 1073-1173 K for dolomite and 973-1073 K for nickel-based catalysts. Surprisingly, about 90 to 99% of the tar (depending on temperature ranging from 973 to 1073 K and catalyst types) has been cracked down to product gas on nickel-based catalysts in the secondary reactor. However, concerning the (12) Pe´rez, P.; Aznar, M. P.; Caballero, M. A.; Gil, J.; Martı´n, J. A.; Corella, J. Energy Fuels 1997, 11, 1194. (13) Corella, J. Fuel Energy Abst. 1997, 38, 163. (14) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 1535. (15) Caballero, M. A.; Corella, J.; Aznar, M. P.; Gil, J. Ind. Eng. Chem. Res. 2000, 39, 1143. (16) Corella, J.; Orio, A.; Toledo, J. M. Energy Fuels 1999, 13, 702. (17) Corella, J.; Orio, A.; Aznar. P. Ind. Eng. Chem. Res. 1998, 37, 4617. (18) Aznar, M. P.; Caballero, M. A.; Gil, J.; Martin, J. A.; Corella, J. Ind. Eng. Chem. Res. 1998, 37, 2668. (19) Caballero, M. A.; Aznar, M. P.; Gil, J.; Martin, J. A.; France´s, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5227. (20) Narva´ez, I.; Corella, J.; Orio, A. Ind. Eng. Chem. Res. 1997, 36, 317.
10.1021/ef020234z CCC: $25.00 © 2003 American Chemical Society Published on Web 06/04/2003
Rh/CeO2/SiO2 in the Pyrogasification of Biomass
catalyst life, whether to use the nickel or dolomite catalyst is the question. These types of catalysts are usually deactivated by the deposition of carbon on the surface. This problem is more severe in the fixed bed catalyst than fluidized bed. The fluidized bed reactor has been found to be always advantageous, either in the gasification or in any heterogeneous catalytic reaction.21 For example, in methane reforming with CO2 and O2, the fluidized bed reactor is very effective in removing the low-reactive carbonaceous species from the catalyst surface.22-24 Moreover, the temperature is quite homogeneous in the fluidized bed catalyst as a result of the high degree of heat transfer. The catalyst deactivation is more significant in the primary gasification reactor than in the secondary tar converter. This is because some primary char is formed and deposited on the catalyst surface during the gasification reaction in the primary reactor. Thus, although the use of a catalyst in the primary bed can improve the quality of the raw gas and simplify the total reactor system, making it possible to convert about 75% of the total carbon in the biomass to the gas at around 973 K,25-30 some authors have demonstrated that the use of nickel and dolomite catalysts in the primary reactor unit resulted in rapid deactivation. Loss of catalyst activity is apparently due to fouling by build-up of carbon, which blocks access to the active sites. For example, a process using the nickel-based catalysts under the conditions of W/mb (wt of catalyst/feeding rate of biomass) ) 0.833 h and throughput ) 37 kg/h m2 at 1023 K gave 90% carbon conversion (C-conv) with 10% solid; however, in this system the Ni-based catalysts were deactivated by carbon deposition on the catalyst surface within a matter of minutes.31,32 On the other hand, it has reported that the catalyst was deactivated within 25 min when W/mb was lower than 0.6 h and throughput ) 17-50 kg biomass/h m2 at 973 K.26-28 The carbon deposition is faster in the pyrolytic and steam gasification of biomass than oxygen and CO2 gasification.28,29 The catalyst deactivation by the carbon deposition is related to the catalyst activity in the carbon conversion. Efficient catalysts must convert the carbonaceous materials at a higher rate than the deposition rate. Such a catalyst for biomass gasification has not yet been (21) Warnecke, R. Biomass Bioenergy 2000, 18, 489. (22) Tomishige, K.; Matsuo, Y.; Sekine, Y.; Fujimoto, K. Catal. Commun. 2001, 2, 11. (23) Tomishige, K.; Matsuo, Y.; Yoshinaga, Y.; Sekine, Y.; Asadullah, M.; Fujimoto, K. Appl. Catal. A: General 2002, 223, 225. (24) Matsuo, Y.; Yoshinaga, Y.; Sekine, Y.; Tomishige, K.; Fujimoto, K. Catal. Today 2000, 63, 439. (25) Narva´ez, I.; Orı´o, A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110. (26) Garcı´a, L.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Energy Fuels 1999, 13, 851. (27) Garcia, L.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Ind. Eng. Chem. Res. 1998, 37, 3812. (28) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Ind. Eng. Chem. Res. 1997, 36, 67. (29) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Energy Fuels 1994, 8, 1192. (30) Rapagna, S.; Jand, N.; Foscolo, P. U. Int. J. Hydrogen Energy 1998, 23, 551. (31) Baker, E. G.; Mudge, L. K.; Brown, M. D. Ind. Eng. Chem. Res. 1987, 26, 1335. (32) Baker, E.; Mudge, L.; Wilcox, W. A. Catalysis of gas-phase reactions in steam gasification of biomass. In Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., et al., Eds.; Elsevier Applied Science: London 1985; p 863.
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reported. To find an efficient catalyst we previously investigated various types of oxide catalysts and supported noble metal catalysts. At the beginning of our work, we used cellulose as a model compound for real biomass and gasified in the batch feeding reactor on various kinds of catalysts. Among the oxide catalysts, we found CeO2 to be a much better catalyst for hydrogen and syngas production than the so-called basic oxides such as MgO or CaO at a temperature as low as 823 K. The loading of Rh metal on CeO2 efficiently converted the total carbon in the cellulose at this temperature.33,34 In this system, we also found catalyst deactivation as a result of sintering of CeO2 under the reaction conditions. The catalyst surface area was decreased because of sintering, and it was more severe when Rh/CeO2 catalyst was used in the continuous feeding system.35 Thus, to prevent the CeO2 sintering, we have further developed the catalyst by loading the CeO2 on the highsurface-area SiO2 (380 m2/g), followed by the Rh loading. The Rh/CeO2/SiO2 catalyst thus formed with 35 mass % of CeO2 and 1% of Rh showed excellent performance in the cellulose gasification at very low temperature, and the TEM images of the used catalyst showed no sintering of the CeO2 and Rh.36,37 The Rh/CeO2/SiO2 catalyst was also used in the real biomass (cedar wood) gasification where about 98% of the carbon in the wood was converted to the product gas at 923 K.38 In this system, air was used as the gasifying agent. We have further evaluated our catalyst in the pyrolytic, CO2, and steam gasification of cedar wood. The catalyst performance was quite excellent in these systems that we have reported in this paper. We have compared Rh/CeO2/SiO2 and conventional methods in pyrogasification of biomass. Experimental Section Materials. Biomass. In this process the gasification of cedar wood was carried out. The sawdust of this wood was collected and was ground with a ball mill to about 0.1-0.3 mm size. At the useful level, the granule of the wood contained about 10 mass % of moisture. The composition of the wood granules was H2O, 10 mass %; C, 45.99 mass %; H, 5.31 mass %; O, 38.25 mass %; N, 0.11 mass %; Cl, 0.01 mass %; S, 0.02 mass %; and ash, 0.3 mass %. Catalyst. The Rh/CeO2/SiO2 with 60 mass % of CeO2 and 1.2 × 10-4 mol Rh/g-catalyst, commercial steam reforming catalyst G-91 ((TOYO CCI, composition: 14 mass % Ni, 6570 mass % Al2O3, 10-14 mass % CaO, and 1.4-1.8 mass % K2O), and natural dolomite (21.0 mass % MgO, 30.0 mass % CaO, 0.7 mass % SiO2, 0.1 mass % Fe2O3, and 0.5 mass % Al2O3), have been used in this process. The dolomite was calcined at 973 K for 3 h before reaction. The CeO2/SiO2 was prepared by the incipient wetness method using the aqueous solution of Ce(NH4)2(NO3)6 and SiO2 (Aerosil, 380 m2/g). After loading the Ce salt on SiO2, it was dried at 383 K for 12 h following the calcination at 773 K for 3 h under air atmosphere. (33) Asadullah, M.; Tomishige, K.; Fujimoto, K. Catal. Commun. 2001, 2, 63. (34) Asadullah, M.; Tomishige, K.; Fujimoto, K. Ind. Eng. Chem. Res. 2001, 25, 5894. (35) Asadullah, M.; Ito, S.; Kunimori, K.; Tomishige, K. Ind. Eng. Chem. Res. 2002, 41, 4567. (36) Asadullah, M.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. J. Catal. 2002, 208, 255. (37) Asadullah, M.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. Environ. Sci. Technol. 2002, 36, 4476. (38) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. Green Chem. 2002, 4, 385.
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Figure 1. Schematic diagram of the experimental setup. Then the Rh was loaded on CeO2/SiO2 by the simple impregnation method of the support with acetone solution of Rh(C5H7O2)3. The acetone solvent was then evaporated at around 333 K with constant stirring. Then the catalyst thus produced was dried at 383 K for 12 h. The final catalyst was pressed, crushed, and sieved to 45-150 µm particle size. The mass % of CeO2 in the Rh/CeO2/SiO2 is denoted in parentheses, like Rh/CeO2/SiO2(60). Before the gasification reaction, the catalyst was pretreated by a hydrogen flow at 773 K for 0.5 h. The fresh (after H2 treatment) and used catalysts were characterized by a Brunauer-Emmett-Teller (BET) analysis. Apparatus. The experimental setup of this gasification process is shown in Figure 1. The quartz-made gasifier with a height of 66 cm is constructed with a distributor at the middle of the reactor. The internal diameter at the distributor is 15 mm. The internal diameter of the upper part of the reactor just after the fluidized bed section is larger (30 mm) so as to decrease the gas turbulence and the catalyst particles can easily return to the fluidized bed section. The biomass was fed by vibrating the feeder from the top of the reactor. The feeder consisted of a conical glass vessel with a screw valve at the bottom, allowing continuous feeding of biomass particles by vibrating the vessel with an electric vibrator. The biomass feeding was controlled by adjusting the vibration rate and also the valve opening. An inner tube of 11 mm i.d. made of quartz is inserted from the feeder to the catalyst bed. In the pyrolytic gasification, N2 gas was introduced from both the upper and bottom parts of the reactor. The N2 stream from the upper part transported the biomass particles into the catalyst bed and the N2 stream that was introduced from the bottom part of the reactor kept the catalyst fluidized. In the CO2 gasification experiment, both of the streams were CO2; however, in the steam gasification experiment, the bottom stream was a mixture of N2 and steam. The water was supplied by a syringe pump through the capillary tube, which was inserted from the bottom of the reactor into the hot zone of the reactor. The water was vaporized at the hot zone, and the steam thus produced was mixed with N2 at the bottom of the distributor. The temperature at the outside of the reactor was measured by thermocouples. The process was carried out under atmospheric pressure. The product gas was successively entered through a filter and an ice water condenser so as to remove any solid or liquid materials from the product gas stream. The clean sample of the product gas was collected from the
Asadullah et al. sampling port by the syringe and analyzed by a gas chromatograph (GC). CO, CO2, CH4, H2, and H2O were formed as the products. In the noncatalyzed and dolomite-catalyzed reactions, a small amount of C2 product was also formed. The concentration of CO, CO2, CH4, and C2 products was determined by FID-GC equipped with a methanator using a stainless steel column packed with Gasukuropack 54, and the concentration of hydrogen was determined by TCD-GC using a stainless steel column packed with a molecular sieve 13X. The flow rate of the inlet gas flow was measured by a thermal mass flow controller; however, the flow rate of the product gas flow out of the reactor was measured by a soap membrane meter. The formation rate of the gas products was calculated from the GC analysis in the unit of µmol/min. The carbon-based conversion to gas (C-conv) was calculated by “A/B × 100”, where A represents the forming rate of CO + CO2 + CH4 + C2 and B represents the total carbon supplying rate of biomass. In this experiment, solid carbonaceous materials were observed in the catalyst bed during the reaction test. This kind of carbon materials is the carbon that originated from the pyrolysis of biomass, mainly lignin, and the carbon deposited on the catalyst surface derived from the decomposition of hydrocarbons. In the analysis, it is difficult to distinguish between these two kinds of carbon. However, it is easy to determine the total amount by combustion. Here, char contains two kinds of carbon, and the amount of carbon-based char was determined by the amount of the gas (mainly CO2) formed under the air stream at the reaction temperature after the supply of biomass was stopped. The yield of char is calculated by (total CO2 + CO)/(total carbon amount in fed biomass). Tars are volatile and a complex mixture of hydrocarbons and oxygenates. In our reaction system, it is difficult to collect tars and to determine the amount of tars directly. Therefore, the yield of tar is calculated by (100 - C-conv (%) - char yield (%)). The feeding rates of biomass, N2, and air are described in each result. The equivalence ratio can be calculated by (wt of oxygen/wt of dry wood)/(stoichiometric oxygen /wood ratio).
Results Performance of Catalysts in the Pyrolytic Gasification. The pyrolytic gasification of cedar wood on Rh/CeO2/SiO2(60), G-91, dolomite, and in the noncatalyst system in the nitrogen atmosphere was carried out in the fluidized bed reactor at various temperatures. In each run, 3 g of catalyst was used. The catalyst was pretreated by means of hydrogen flow of 40 mL/min at 773 K for 0.5 h. Table 1 provides the performance of different catalysts with respect to the carbon conversion (C-conv), product distribution, char, and tar formation. The results are average values in 20 min of activity test. The inlet gas flow rate was 50 mL/min in all reaction systems; however, the gas residence time was varied, depending on the reaction temperature and the activity of the catalysts. In the noncatalytic system the C-conv was only 32% at 873 K and reaches about 60% at 1023 K. The rest of the carbon was related to the tar and char, which was measured as listed in Table 1. Under this low temperature range, the carbon conversion hardly increased on dolomite; however, it increased remarkably on G-91. Actually, the dolomite is more or less active catalyst, depending on the physical properties at relatively high temperatures (1073-1173 K).39 The C-conv attained about 78% on G-91 catalyst at 1023 K. Surprisingly, this value is 79% even at 923 K and goes to about 93% at (39) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155.
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Table 1. Performance of Various Catalysts in the Pyrolytic Gasification of Cedar Wooda catalyst none
dolomite
G-91
Rh/CeO2/SiO2(60)
formation rate (µmol/min) H2 CH4 C2b
T (K)
CO
873 923 973 1023 873 923 973 1023
408 530 636 616 375 528 604 697
152 250 220 264 160 316 432 526
92 116 153 232 141 137 147 237
873 923 973 1023 873 923 973 1023
690 750 1071 1329 961 1107 1285 1592
985 1116 1300 1367 1270 1313 1314 1467
244 237 227 194 290 348 321 328
81 120 145 215 55 74 86 148
CO2
H2/CO
C-conv (%)
char (%)
tar (%)
150 239 244 305 185 299 374 314
0.4 0.4 0.3 0.4 0.4 0.6 0.7 0.8
32 44 51 60 33 45 53 61
23 21 18 14 39 33 25 21
45 35 31 26 28 22 22 18
375 365 333 272 368 364 339 227
1.4 1.5 1.2 1.0 1.3 1.2 1.0 0.9
57 59 71 78 70 79 85 93
22 24 22 21 21 19 15 7
21 17 7 1 9 2 0 0
surface area (m2/g) fresh used
1.1
0.9
33
20
125
118
a
Conditions: Catalyst, 3 g; biomass 60 mg/min (H2O, 10%; C, 2299 µmol/min; total H, 3852 µmol/min; O, 1767 µmol/min); N2, 25 mL/min from the top and 25 mL/min from the bottom (total 2046 µmol/min). b Carbon-based. Table 2. Effect of Oxygen Addition in the Pyrolytic Gasification of Biomass on Rh/CeO2/SiO2(60) and G-91a catalyst
ERb
Rh/CeO2/SiO2(60) 0.09 0.13 0.18 0.3 G-91 0.07 0.13 0.21 0.3
O2 feeding rate (mL/min)
CO
0.0 5.1 7.9 10.7 17.3 0.0 4.0 7.9 12.2 17.3
1107 1100 952 884 681 750 847 943 902 838
formation rate (µmol/min) H2 CH4 CO2 1313 1070 925 845 700 1116 1220 1362 1299 1122
348 364 389 433 314 237 193 185 155 182
364 554 715 880 1233 365 524 608 780 1040
H2/CO
C-conv (%)
char (%)
tar (%)
1.2 1.0 1.0 1.0 0.9 1.5 1.4 1.4 1.4 1.3
79 88 89 95 97 59 68 75 80 89
19 12 11 5 3 26 28 21 19 11
2 0 0 0 0 15 4 4 1 0
a Conditions: Catalyst ,3 g; biomass, 60 mg/min (H O, 10%; C, 2299 µmol/min; total H, 3852 µmol/min; O, 1767 µmol/min); N , 25 2 2 mL/min from the top and 25 mL/min from the bottom (total 2046 µmol/min). b ER ) (wt of oxygen/wt of dry wood)/(stoichiometric oxygen/ wood ratio).
1023 K on Rh/CeO2/SiO2(60). The char formation was almost unchanged (21-24%) at different temperatures, and the tar formation decreased at high temperature to about 1% at 1023 K on G-91 catalyst. However, on the Rh/CeO2/SiO2(60), the char was decreased to 7% at 973 K. Tar was not formed on this catalyst even at 873 K. The useful gas formation rate is much higher on this catalyst than on the other catalysts. The BET surface area of the fresh and used catalysts was measured and was found almost unchanged. The product gas from the biomass gasification can either be used in the gas turbine system for power generation or can be used for chemical production. In the power generation system, the CO, H2, and CH4 are the useful gases. Consequently, their total formation as a function of temperature has been considered and is shown in Figure 2. Their total formation is very low in the noncatalyst systems, but is remarkably increased when any type of catalyst is used. The dolomite catalyst usually works to form gas product at very high temperatures (>1073 K), thus the formation rate of the useful gases within this low-temperature range is low. However, on the G-91 catalyst, it is quite high and increases with increasing temperature. The Rh/CeO2/SiO2(60) shows much better results than other catalysts in the entire temperature range. Effect of the Gasifying Agents. Table 2 shows the C-conv, product distribution, H2/CO ratio, char, and tar formation as a function of equivalence ratio on Rh/CeO2/
Figure 2. Effect of temperature in the pyrolytic gasification of biomass on different catalysts. b Rh/CeO2/SiO2(60), 9 G-91, 2 dolomite, and O noncatalyst. Conditions: catalyst, 3 g; biomass, 60 mg/min (H2O, 10%, C, 2299 µmol/min; total H, 3852 µmol/min; O, 1767 µmol/min); N2, 25 mL/min from the top and 25 mL/min from the bottom (total 2046 µmol/min).
SiO2(60) and G-91 catalysts at 923 K. On Rh/CeO2/ SiO2(60) catalyst, C-conv is about 79% when oxygen is absent. A small amount of oxygen (ER ) 0.09) remarkably decreased (from 19 to 12%) the char and coke deposited on the catalyst surface. The C-conv increases with increasing ER and finally about 97% conversion was achieved when ER ) 0.3 was used. However, on
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Table 3. Performance of the Rh/CeO2/SiO2(60) and G-91 Catalyst in the CO2 Gasification of Biomassa catalyst Rh/CeO2/SiO2(60)
G-91
formation rate (µmol/min)
CO2 flow rate (µmol/min)
CO
H2
CH4
H2/CO
C-conv (%)
char (%)
tar (%)
0 3151 4861 6670 0 3273 4910 6711 8757
1107 1894 2583 2482 750 1775 2340 2527 2751
1313 1201 1209 1236 1116 1032 1096 1208 969
348 253 118 187 237 91 48 44 42
1.2 0.6 0.5 0.5 1.5 0.6 0.5 0.5 0.4
79 85 88 89 59 60 76 78 79
19 15 12 11 26 24 22 22 21
2 0 0 0 15 16 2 0 0
a Conditions: Catalyst, 3 g; biomass, 60 mg/min (H O, 10%; C, 2299 µmol/min; total H, 3852 µmol/min; O, 1767 µmol/min); N , 25 2 2 mL/min from the top and 25 mL/min from the bottom (total 2046 µmol/min). CO2 conversion or formation is not mentioned here due to the high concentration in the product gas.
Figure 3. Effect of equivalence ratio on the carbon conversion and product distribution in the biomass gasification at 923 K. / C-conversion, b H2, 9 CO, 2 CO2, and O CH4. (a) Rh/CeO2/ SiO2(60) and (b) G-91. Conditions: catalyst, 3 g; biomass 60 mg/min (H2O, 10%; C, 2299 µmol/min; total H, 3852 µmol/min; O, 1767 µmol/min); N2, 25 mL/min from the top and 25 mL/ min from the bottom (total 2046 µmol/min).
the G-91 catalyst the maximum C-conv even at ER ) 0.3 is about 89%. The H2/CO ratios slightly decreased on both Rh/CeO2/SiO2(60) and G-91 catalysts. The decrease of char formation on Rh/CeO2/SiO2(60) catalyst with increasing ER is remarkable and there was no tar formed on this catalyst within the temperature range applied here. However, a substantial amount of char and tar was formed on the G-91 catalyst as a result of low cracking reactivity of this catalyst. Figure 3 shows the trend of the product distribution and C-conv on Rh/CeO2/SiO2(60) (a) and G-91 (b) catalysts at different ER. As shown in Figure 3a, both the CO and hydrogen formation rates constantly de-
creased on the Rh/CeO2/SiO2(60) catalyst and consequently the CO2 formation rate increased. The methane formation rate slightly decreased. However, on the G-91 catalyst, the formation of hydrogen and CO initially increased and finally decreased with increasing ER. Since the C-conv is lower on the G-91 catalyst, a substantial amount of char always remained on the catalyst surface, which gradually takes part in the combustion reaction to form CO2, and thus the CO2 formation rate constantly increased. The methane formation was almost constant. In the biomass gasification, substantial amounts of CO2 formed are completely useless; however, it is very effective in the hydrocarbon reforming reaction on the nickel catalysts.40 The tar and char formed in the biomass gasification are a mixture of higher hydrocarbons, which can also take part in the reforming reaction with CO2 on the suitable catalysts. This possibility can direct the recycling of the product CO2 in the gasification reaction, which improves the carbon utilization by the reforming reaction with tar and char to form CO and hydrogen. The performance of Rh/CeO2/SiO2(60) and G-91 catalysts in CO2 gasification of biomass is shown in Table 3. Increasing concentration of CO2 in the fluidizing medium, increases the amount of CO2 conversion with the fixed biomass feeding rate on both of the catalysts. The tar was completely converted and also the char conversion remarkably increased on Rh/CeO2/ SiO2(60) catalyst so as to increase the total C-conv to gas. However, although the tar formation decreased to almost zero with increasing the concentration of CO2 on the G-91 catalyst, the char formation was almost unchanged. The H2/CO ratio gradually decreased due to the increase of CO2 concentration on both of the catalysts. As shown in Figure 4, the formation of CO is largely affected by the concentration of CO2 and increased with increasing CO2 flow rate. The hydrogen formation remained almost unchanged, but methane formation gradually decreased with the concentration of CO2. Biomass always contains low to high moisture depending on the drying conditions. This moisture content should be low when it is subjected to gasification. Table 4 shows the results of the steam gasification of biomass on Rh and G-91 catalysts. To evaluate the effect of steam within the moisture content level from low to dry biomass in the catalytic gasification, the steam flow rate was kept low. At 923 K the char was gasified in the (40) Tomishige, K.; Chen, Y.; Fujimoto, K. J. Catal. 1999, 181, 91.
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Table 4. Performance of the Rh/CeO2/SiO2(60) and G-91 Catalyst in the Steam Gasification of Biomassa catalyst Rh/CeO2/SiO2(60)
Rh/CeO2/SiO2(60) G-91
temp (K)
steam flow rate (µmol/min)
CO
923 923 923 923 923 923 823 873 973 823 873 923 973
0 555 1111 1666 4440 5550 1666 1666 1666 1666 1666 1666 1666
1107 498 292 340 148 77 210 166 450 144 306 373 553
formation rate (µmol/min) H2 CH4 CO2 1313 1131 1461 1931 2742 2983 2003 1942 1884 2493 2502 2448 2807
348 560 580 587 330 188 446 555 433 204 234 158 86
363 807 1048 1227 1521 1435 1215 1208 1291 1232 1068 1313 1253
H2/CO
C-conv (%)
char (%)
tar (%)
1.9 2.3 5.0 5.7 18.5 38.0 9.5 11.7 4.2 17.3 8.2 6.6 5.1
79 81 84 93 87 74 82 84 95 69 70 80 83
21 19 16 7 12 17 15 14 5 20 11 11 9
0 0 0 0 1 9 3 2 0 11 19 9 8
a Conditions: Catalyst, 3 g; biomass, 60 mg/min (H O, 10%; C, 2299 µmol/min; total H, 3852 µmol/min; O, 1767 µmol/min); N , 25 2 2 mL/min from the top and 25 mL/min from the bottom (total 2046 µmol/min).
Figure 4. Effect of CO2 flow rate on the carbon conversion and product distribution in the biomass gasification at 923 K. / C-conversion, b H2, 9 CO, and O CH4. (a) Rh/CeO2/SiO2(60) and (b) G-91. Conditions: catalyst, 3 g; biomass, 60 mg/min (H2O, 10%; C, 2299 µmol/min; total H, 3852 µmol/min; O, 1767 µmol/min); N2, 25 mL/min from the top and 25 mL/min from the bottom (total 2046 µmol/min).
presence of external steam and it increased until 1666 µmol/min of steam supply, so as to increase the total C-conv. The higher steam flow did not further improve the char gasification. This can be a result of the higher flowing rate and short contact time. However, with a constant steam supply, the increase of temperature further increases the C-conv. Although the C-conv also increased with increasing temperature at a constant steam supply on G-91 catalyst, it is much lower than
Figure 5. Effect of steam at different temperatures on the carbon conversion and product distribution in the biomass gasification. / C-conversion, b H2, 9 CO, 2 CO2, and O CH4. (a) Rh/CeO2/SiO2(60) and (b) G-91. Conditions: catalyst, 3 g; biomass, 60 mg/min (H2O, 10%; C, 2299 µmol/min; total H, 3852 µmol/min; O, 1767 µmol/min); N2, 25 mL/min from the top and 25 mL/min from the bottom (total 2046 µmol/min).
that on Rh/CeO2/SiO2(60) catalyst. The H2/CO ratio dramatically increased with increasing concentration of steam in the gasification medium, suggesting that the water gas shift reaction efficiently proceeded on Rh/ CeO2/SiO2(60) catalyst. Figure 5 shows the product distribution and C-conv as a function of temperature on the Rh/CeO2/SiO2(60) and G-91 catalysts. The steam supply was kept constant at 1666 µmol/min in this experiment. As shown in Figure 5a, although the elevation of temperature slightly improved the CO
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formation, the hydrogen formation slightly decreased on Rh catalyst. However, both of them gradually improved on G-91 catalyst under the same conditions. At high temperature the methane formation was suppressed. Discussion The overall results of the gasification of cedar wood without and with different gasifying agents on different catalysts demonstrate that the Rh/CeO2/SiO2(60) is a multifunctional and efficient catalyst for biomass gasification below 973 K. In our gasification system the biomass is first subjected to the fast thermal pyrolysis in the feeding tube to form tar, char, steam, and small fraction of gases such as CO, H2, CO2, CH4, and CnHm. In this zone the main chemical process could be described as
biomass f char + tar + H2O + gas
(1)
The composition of the products in the pyrolysis zone greatly varies, depending on temperature. Low temperature favors the formation of tar and char, whereas the high temperature favors the formation of gas products. In this work, the gasification process is operated at considerably low temperatures and thus formed the tar and char as major products in the pyrolysis zone. The pyrolytic products then are exposed to the cracking zone where tar, char, and gases take part in the number of secondary reactions41 that can be described below so as to produce more product gas.
tar f CH4 + H2O + CnHm + H2
(2)
C + H2O f CO + H2
(3)
C + CO2 f 2CO
(4)
CH4 + H2O T CO + 3H2
(5)
CO + H2O T CO2 + H2
(6)
In the absence of any catalyst and gasifying agent in the cracking zone, most of the primary char formed in the pyrolysis zone accumulates in the cracking zone and volatile tar exits the hot zone of the reactor and condenses in the cooling zone. Thus the total C-conv to gas is quite low in this process, as shown in Table 1 and Figure 2. In our case, most of the heavy tar accumulated at the top of the reactor and the light tar was accumulated in the condenser. The total char accumulated at the cracking zone was estimated by measuring the CO2 formed by burning of them as listed in Table 1. The tar amounts 14-23%, depending on temperature, is quite consistent with the reported results of polar wood under almost the same temperature.42 The presence of catalysts in the cracking zone under nitrogen atmosphere could promote reactions 2-6. The Rh/CeO2/SiO2(60) promoted the reactions efficiently, especially reactions 2-4, so as to improve the C-conv from 32% to about 70% at 873 K and 93% at 1023 K. In (41) Xianwen, D.; Chuangzhi, W.; Haibin, L.; Yong, C. Energy Fuels 2000, 14, 552. (42) Gu¨llu¨, D.; Demirba¸ , A. Energy Convers. Manage. 2001, 42, 1349.
this pyrolytic gasification in the presence of catalysts, CO formation improved greatly; however, the H2 formation hardly improved with increasing temperature. This suggests that reaction 4 is greatly promoted with elevated temperature. In addition, reaction 3 is supposed to improve; however, reactions 5 and 6 are supposed to proceed at high temperature. Although substantial amounts of C2 hydrocarbon formed in the noncatalyzed and dolomite-catalyzed reactions, it was undetectable in the G-91- and Rh/CeO2/SiO2(60)-catalyzed reactions. This suggests that the reforming of C2 hydrocarbons on these catalysts by a reaction such as that shown in eq 5 becomes very fast on the catalysts. Char formation on dolomite and G-91 catalysts is higher than in noncatalyst systems at above 923 K. This may be a result of secondary char formation from the tar adsorbed on the catalyst surface. This indicates that the nickel and dolomite catalysts are less effective for char gasification through the reactions 3 and 4. The presence of an external gasifying agent such as oxygen can promote the C-conv through the equation below and consequently decrease the char formation.
C + O2 f CO2
(7)
2C + O2 f 2CO
(8)
Since these two reactions are promoted with increasing concentration (equivalence ratio) of oxygen in the gasifying medium, the C-conv improved until 97% with improving the equivalence ratio to 0.3 on the Rh/CeO2/ SiO2(60) catalyst. However, the maximum C-conv on G-91 is 89% under the same equivalence ratio. The cokelike carbonaceous materials formed on the catalyst surface is very refractive to the chemical conversion. Consequently, only up to 80% of the C-conv can be realized on the conventional nickel-based or dolomite catalysts at 923 K,43 even when the pure model compound of tar such as benzene, toluene, and naphthalene are used at higher temperature (1050 K).44 The redox properties of CeO2 and the high lability of lattice oxygen45 may be the factors which contribute to the conversion of char, tar, or coke-like materials to gas by their partial oxidation on Rh/CeO2/SiO2(60) catalyst. On the other hand, the reforming and methanation reactions, which give CO, H2, and CH4, are known to proceed on the reduced surface of the metal. The high yield of CO, H2, and CH4 on Rh/CeO2/SiO2(60) catalyst in the presence of oxygen at low temperature suggests that the redox properties of CeO2 also contribute to the reduction of Rh/CeO2/SiO2(60) during reaction. Under the high concentration of CO2 in the fluidizing medium, reaction 4 was especially promoted on the Rh/ CeO2/SiO2 catalyst and thus the CO concentration constantly increased in the product gas with increasing the CO2 supply rate, as shown in Table 3 and Figure 4. The C-conv has gone to about 89% when 6670 µmol/ min of CO2 is supplied in the catalyst bed. However, the char formation is a little higher in the CO2 gasification than that in the O2 gasification and this is due to (43) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Energy Fuels 1994, 8, 1192. (44) Coll, R.; Salvado´, J.; Farriol, X.; Montane´, D. Fuel Process. Technol. 2001, 74, 19. (45) Trovarelli, A. Catal. Rev. 1996, 38, 439.
Rh/CeO2/SiO2 in the Pyrogasification of Biomass
much lower reactivity of CO2 than O2 in the char conversion. The C-conv on G-91 catalyst is about 79% even when 8757 µmol/min of CO2 was used, and the char formation remained in the range of pyrolytic gasification and also in the range of the noncatalytic system. This suggests that the primary char formed in the pyrolysis zone does not take part in reaction 4 on the G-91 catalyst. As Table 4 shows, the char formation is lower in the steam gasification compared with the char formation in CO2 gasification (Table 3) on both of the catalysts. Of course, on Rh/CeO2/SiO2(60), it is much lower than on the G-91 catalyst at different temperatures. However, it is contrary to fact that the C-conv decreased on Rh/CeO2/SiO2(60) catalyst when a flow rate of steam higher than 1666 µmol/min was used. Under the high flow rate of steam, some tar was also formed. It might be due to a decrease of the residence time. The water gas shift reaction 6 was promoted greatly on Rh/CeO2/ SiO2(60) catalyst, and consequently the formation rate of H2 and CO2 constantly increased with an increase in the flow rate of steam and the CO formation rate decreased simultaneously, as shown in Figure 5. The biomass (cedar wood) used here contains 0.02 mass % sulfur. It is known that sulfur is a poisonous material for catalysts. At present, we have not analyzed the compounds (H2S, COS, and so on) containing sulfur, and carried out surface analysis of the used catalysts. In the results of the activity test, the deactivation was
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not observed. This is because the reaction time (20 min) was not so long. Further investigation on the effect of sulfur on the catalyst performance is necessary. Conclusions The Rh/CeO2/SiO2(60) catalyst appeared to be an effective catalyst for biomass gasification even in the absence of any gasifying agent within the temperature range of 873-973 K. The secondary char or coke as well as primary char was remarkably reduced on Rh/CeO2/ SiO2(60) catalyst in the presence of any type of gasifying agent such as O2, CO2, and steam. A great deal of char or coke conversion with CO2 on this catalyst within this low-temperature range emphasized the recycling of CO2 from the product gas in order to increase the carbon utilization from the biomass. The external steam supply with the rate of 1666 µmol/min increased both the char and tar conversion, which suggested that the biomass with high moisture content is also feasible for the gasification on Rh/CeO2/SiO2(60) catalyst. Acknowledgment. This research was supported by the Future Program of Japan Society for the Promotion of Sciences under the Project “Synthesis of Ecological High Quality of Transportation Fuels” (JSPS-RFTF98P01001). EF020234Z
