Synthesis Gas Production via Biomass Catalytic Gasification with

Feb 3, 2005 - Biomass synthesis gas production via co-reforming of biogas and H2-rich ... with addition of biogas is simple and highly efficient for n...
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Energy & Fuels 2005, 19, 637-644

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Synthesis Gas Production via Biomass Catalytic Gasification with Addition of Biogas Tiejun Wang,* Jie Chang, and Pengmei Lv Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 1 Nengyuan Road, Guangzhou, 510640 China Received June 2, 2004. Revised Manuscript Received December 16, 2004

Biomass synthesis gas production via co-reforming of biogas and H2-rich syngas produced by biomass air-steam gasification has been investigated in a bubbling bed biomass gasifier with a NiO-MgO catalyst packed in a downstream reactor. The composition of synthesis gas was adjusted by a co-reforming reaction to meet the desired stoichiometric factor. The H2/CO ratio of biomass synthesis gas is above 1.5. It contains trace CH4 and CO2. The feeding rate of biogas depends on the composition and flow velocity of the H2-rich syngas produced in the gasifier. Compared with the commercial nickel-based reforming catalyst Z409R, the NiO-MgO catalyst exhibits better catalytic activity and anti-coke ability at high temperature (>750 °C). Compared with conventional pathways for adjustment of the stoichiometric factor, the method of co-reforming with addition of biogas is simple and highly efficient for nearly complete utilization of the carbon contained in the biomass.

Introduction Methanol and dimethyl ether (DME) produced from biomass are promising as a carbon neutral fuel. They are ultra-clean, emitting none of the air pollutants SOx, NOx, VOS, or dust, and they could provide a major alternative for the transport sector worldwide in a greenhouse-gas-constrained world.1 Several authors have reported the syngas stream compositions of benchscale and pilot biomass gasification units.2-13 However, the utilization of biomass for methanol or DME production via gasification faces the problem of a low H2/CO ratio and a large excess of carbon dioxide in the produced syngas. The large CO2 content lowers the overall yield of methanol or DME.14 The H2/CO ratio can be adjusted either by adding hydrogen or by a water gas * Corresponding author. Tel: 86-20-87057751. Fax: 86-20-87057789. E-mail: [email protected]. (1) Hamelinck, C. N.; Faaij, A. P. C. J. Power Sources 2002, 111, 1-22. (2) Aznar, M. P.; Corella, J.; Delgado, J.; Lahoz, J. Ind. Eng. Chem. Res. 1993, 32, 1-10. (3) Narva´ez, I.; Orı´o, A.; Aznar, P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110-2120. (4) Aznar, M. P.; Caballero, M. A.; Gil, J.; Martı´n, J. A.; Corella, J. Ind. Eng. Chem. Res. 1998, 37, 2668-2680. (5) Perez, P.; Aznar, M. P.; Caballero, M. A.; Gil, J.; Martin, J. A.; Corella, J. Energy Fuels 1997, 11, 1194-1203. (6) Gil, J.; Aznar, M. P.; Caballero, M. A.; Frances, E.; Corella, J. Energy Fuels 1997, 11, 1109-1118. (7) Caballero, M. A.; Aznar, M. P., Gil, J.; Martin, J. A.; Frances, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5227-5239. (8) Corella, J.; Orio, A.; Aznar, M. P. Ind. Eng. Chem. Res. 1998, 37, 4617-4624. (9) Corella, J.; Orio, A.; Toledo, J. M. Energy Fuels 1999, 13, 702709. (10) Caballero, M. A.; Corella, J.; Aznar, M. P.; Gil, J. Ind. Eng. Chem. Res. 2000, 39, 1143-1154. (11) Corella, J.; Orio, A.; Aznar, M. P. Ind. Eng. Chem. Res. 2000, 37, 4617-4624. (12) Gil, J.; Caballero, M. A.; Martin, J. A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1999, 38, 4226-4235. (13) France, S. L. Biomass Bioenergy 1998, 15, 233-238.

shift (WGS) reaction. The addition of hydrogen allows better utilization of the carbon contained in the biomass, with a high fuel production rate. But hydrogen admixture to the syngas requires large supplementary investments. The WGS reaction converts the large amount of CO to CO2, and the stoichiometric adjustment can be accomplished by removing excess carbon dioxide. However, due to the extremely low carbon conversion efficiency of about 20% of the biomass carbon content, the methanol or DME production costs become very high. Reforming is a logical subsequent step in a downstream reactor to maximize CO and H2 production. The most significant body of literature published on the area of hot gas cleaning for biomass gasification concerns dolomite, cheap calcined minerals, and nickel catalysts.2-21 Several authors4,7-10 have investigated a system of raw gas cleaning that involves dolomite for the removal of tar, followed by the adjustment of the gas composition (reforming of CH4 and the other hydrocarbons) using nickel steam reforming catalysts. Using these catalysts, there is generally an increase in the hydrogen and carbon monoxide content of the effluent gas, with elimination or reduction of the hydrocarbons and methane content. Several groups22-25 investigated the bio(14) Ouellette, N.; Rogner, H. H.; Scott, D. S. J. Hydrogen Energy 1995, 20, 873-880. (15) Baker, E. G.; Mudge, L. K.; Brown, M. D. Ind. Eng. Chem. Res. 1987, 26, 1335-1339. (16) Roy, C.; Pakdel, H.; Zhang, H. G.; Elliott, D. C. Can. J. Chem. Eng. 1994, 72, 98-105. (17) Corella, J.; Toledo, J. M.; Aznar, M. P. Ind. Eng. Chem. Res. 2002, 41, 3351-3356. (18) Kinoshita, C. M.; Wang, Y.; Zhou, J. Ind. Eng. Chem. Res. 1995, 34, 2949-2954. (19) Orio, A.; Corella, J.; Narvaez, I. Ind. Eng. Chem. Res. 1997, 36, 3800-3808. (20) Corella, J.; Toledo, J. M.; Aznar, M. P. Ind. Eng. Chem. Res. 2002, 41, 3351-3356. (21) Corella, J.; Caballero, M. A.; Aznar, M. P.; Brage, C. Ind. Eng. Chem. Res. 2003, 42, 3001-3011.

10.1021/ef0400518 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/03/2005

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Table 1. Proximate and Ultimate Analysis of Pine Sawdust moisture content (wt % wet basis) higher heating value (kJ/kg) proximate analysis (wt % dry basis) volatile matter fixed carbon ash ultimate analysis (wt % dry basis) C H O N S

9.1 19483 84.1 15.2 0.7 48.28 7.31 43.41 0.25 0.23

mass air-steam gasification for the production of H2rich fuel gas. However, the concentration of CO2 in the fuel gas is very high. To avoid removing carbon dioxide from syngas for the desired stoichiometric factor (H2, CO, CO2), an acceptable way is reforming the excess carbon dioxide by adding CH4 to the syngas stream for stoichiometric adjustment due to the low content of methane (4-8 mol %) inherent in the syngas. The source of CH4 can be provided by either natural gas or biogas from the biomass by the method of anabatic digestion. Generally, the biomass gasification plant is far from the natural gas field. The transportation cost of natural gas will be very high. However, the biogas can be produced by biomass anabatic digestion near the gasification plant. It is promising to use biogas as a methane source for low-cost production of biomass synthesis gas. In this work, a bubbling fluidized bed biomass gasification system was developed, and the reforming of excess carbon dioxide in the biomass syngas produced by pine sawdust air-steam gasification with biogas was investigated over the NiO-MgO catalyst placed in a downstream reactor. The influence of operation parameters in the gasifier and reforming reactor, biogas feeding rate, and activity of NiO-MgO catalyst on the performance of this system was characterized. Experimental Section Feed Materials and Catalysts. Pine sawdust obtained from a timber mill in Guangzhou City, China, was used as the feedstock for experimental runs. The particle size of this pine sawdust is between 0.3 and 0.45 mm. Its proximate and ultimate analysis is reported in Table 1. A mixed gas of CH4 (68 vol %), CO2 (30 vol %), and N2 (2 vol %) was used as a clean model biogas similar in composition to that from anaerobic digestion processes of biomass. Calcined dolomite, iron-magnesium-based oxide catalyst, and NiO-MgO catalyst were used in experiments. The dolomite was crushed and sieved to a particle size 0.3-0.45 mm, and it was then calcined in air at 900 °C for 4 h. It contains 30 wt % CaO, 21 wt % MgO, 45 wt % CO2, and trace minerals SiO2, Fe2O3, and Al2O3. A commercial iron-magnesium-based oxide catalyst (B104, zhijiang nitrogenous feitilizer Company, China) was crushed and sieved to a particle size 0.3-0.45 mm. It contains 57 wt % Fe2O3, 18 wt % MgO, 8 wt % Cr2O3, and trace minerals Al2O3 and K2O. The NiO-MgO catalyst was prepared by two-step coprecipitation (Ni/Mg ) 15:85, atomic (22) Lv, P. M.; Chang, J.; Xiong, Z. H.; Huang, H. T.; Wu, C. Z.; Chen, Y. Zhu, J. Energy Fuels 2003, 17, 677-682. (23) Hanaoka, T.; Inoue, S.; Uno, S.; Ogi, T.; Minowa, T. Biomass Bioenergy 2005, 28, 69-76. (24) Lv, P. M.; Chang, J.; Wang, T. J.; Fu, Y.; Chen, Y. Energy Fuels 2004, 18, 228-233. (25) Lv, P. M.; Xiong, Z. H.; Chang, J.; Wu, C. Z.; Chen, Y.; Zhu, J. Bioresour. Technol. 2004, 95, 95-101.

Figure 1. Schematic diagram of biomass catalytic conversion. Table 2. Averaged Parameters for the Pore Structure of the NiO-MgO Catalyst Calcined at 950 °C N2 adsorption BET surface area, m2/g micropore area, m2/g area 17-3000 Å, m2/g pore volume, 17-3000 Å, m2/g micropore volume, cm3/g average micropore diameter, Å Hg porosimetry pore area, m2/g total pore volume, cm3/g average pore diameter (4V/A), Å bulk density, g/cm3 apparent (skeletal) density, g/cm3

18 1.15 25-27 0.103 6.9 × 10-4 164 19 0.47 1140 1.26 2.78

ratio) from an aqueous solution of Ni(CH3COO)2‚4H2O (>98.0%, Haotian Chemical Co. Ltd., China) and Mg(NO3)2‚6H2O (>99.2%, Haotian Chemical Co. Ltd., China) using K2CO3 (99.5%, Haotian Chemical Co. Ltd., China) as the precipitant. After being filtered and washed with hot water, the precipitate was dried at 120 °C for 12 h; it was then calcined in air at 950 °C for 10 h. The catalyst was pressed into a disk, then crushed and sieved to a particle size 0.3-0.45 mm. It was characterized for its pore structure by nitrogen adsorption (in an ASAP 2000 apparatus) and mercury porosimetry (in a 9320 pore-sizer apparatus).5 The results are listed in Table 2. Its BET surface area is 18 m2/g, and the total pore volume is 0.47 cm3/g. The average pore diameter is 1140 Å. Apparatus. The tests were performed in an atmospheric pressure, indirectly heated, fluidized-bed gasification system, which is shown schematically in Figure 1. It is composed of a fluidized-bed gasifier, a biomass feeder, a steam generator, an air compressor, a high-efficiency cyclone, a biogas feeder, a metallic filter (also for the gas mixture), and a catalytic reforming reactor. The fluidized-bed biomass gasifier and downstream reforming reactor are made of 1Cr18Ni9Ti stainless steel pipe and are externally heated by an electric furnace. The total height

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Table 3. Operating Condition and Gas Composition run no. biomass feeding rate (kg/h) air (Nm3/h) steam (kg/h) calcined dolomite feeding rate (g/h) calcined dolomite in the gasifier (g) WGS catalyst in the gasifier (g) gasifier bed temperature (°C) equivalence ratio steam-to-biomass ratio exit syngas composition from gasifier (dry, inert-free, mol %) H2 CO CO2 CH4 C2 N2 H2 + CO syngas yield (Nm3/h, dry basis) model biogas feeding rate (Nm3/h) biogas/syngas ratio (V/V) mixed gas composition prior to reforming reactor (dry, inert-free, mol %, calculated) H2 CO CO2 CH4 C2 N2 CH4/CO2 (mol/mol) reforming reactor temperature (°C) exit synthesis gas composition from reforming reactor (dry, inert-free, mol %) H2 CO CO2 CH4 C2 N2 H2 + CO synthesis gas yield (Nm3/h, dry basis)

of the gasifier is 1400 mm, the bed diameter is 40 mm, and the freeboard diameter is 60 mm. An air distributor is installed below the gasifier for better fluidization of biomass particles. The biomass is fed into the gasifier through a screw feeder driven by a variable-speed metering motor. The air (as the gasification agent) from the air compressor is preheated to 105 °C before entering the gasifier to improve performance. The steam from a stream generator is fed into the gasifier at a height of 400 mm. After the gasifier, there is a high-efficiency cyclone to remove most particles in the fuel gas. The cyclone is heated to 260 °C to prevent the tar from condensing. After the cyclone, the metallic filter is used to remove the small size particles. It was operated at about 600 °C. This filter is also used to mix and preheat the syngas and biogas before the mixture of gases enters into the reforming reactor. The reforming reactor (length, 400 mm; inner diameter, 38.5 mm) is installed vertically, with an external electrical oven to maintain the temperature at the desired level in each experiment. A mixture of 120 g of calcined dolomite, 30 g of silica sand (0.2-0.3 mm), and 72 g of commercial B104 iron-magnesiumbased oxide catalyst was put in the gasifier before the test. Calcined dolomite erodes in the gasifier and is easily eluted out of the gasifier with the exit gas stream. Therefore, additional calcined dolomite was mixed carefully with the pine sawdust and continuously fed into the gasifier to attain a steady state. The feeding rate of calcined dolomite depended on the results of the preliminary test. After the test, the calcined dolomite left in the gasifier was separated and measured to calculate the weight percent of calcined dolomite in the gasifier during gasification. In the initial test, the reforming reactor was loaded with the NiO-MgO catalyst, and then the gasifier and reforming reactor were preheated to the reaction temperature. When the

1

2

3

4

5

0.471 0.65 0.4 14 56 72 800 0.30 0.85

0.536 0.70 0.4 15 65 72 800 0.28 0.75

0.556 0.70 0.5 16 65 72 800 0.26 0.90

0.574 0.65 0.5 17 67 72 800 0.24 0.87

0.608 0.60 0.5 18 72 72 800 0.22 0.82

33.07 12.31 25.60 6.10 1.78 21.07 45.38 1.42 0.53 0.37

33.23 16.46 20.27 7.24 2.37 20.23 49.69 1.23 0.3 0.24

36.16 12.25 24.50 4.88 1.73 20.09 48.41 1.34 0.5 0.38

35.25 13.38 23.24 6.23 1.80 19.78 48.63 1.83 0.58 0.32

34.76 14.09 24.53 5.53 1.46 19.43 48.85 2.49 0.89 0.36

24.14 8.99 26.79 22.82 1.30 15.38 0.85 750

26.80 13.27 22.15 19.00 1.91 16.32 0.86 750

26.20 8.88 26.02 22.26 1.25 15.15 0.86 750

26.71 10.14 24.87 21.20 1.36 14.98 0.85 750

25.56 10.36 25.97 22.07 1.07 14.29 0.86 750

46.32 32.27 2.67 2.23 0.26 15.15 78.58 2.03

47.68 30.29 2.54 1.97 0.24 16.28 77.97 1.62

47.66 32.23 2.38 2.11 0.19 15.07 79.89 1.97

47.14 32.78 2.19 1.89 0.22 14.67 79.92 2.53

46.82 32.68 2.41 2.37 0.23 14.08 79.50 3.40

reaction temperature reached the desired lever and remained steady, the air compressor forced the air through the preheater, air distributor, and into the gasifier. Simultaneously, the pine sawdust was fed into the gasifier by a screw feeder operating at the desired rotation speed, and the steam was fed into gasifier after controlling the flow rate by a hightemperature valve and steam flowmeter. The NiO-MgO catalyst in the reforming reactor was reduced by the syngas from the gasifier under the conditions of 750 °C, atmospheric pressure, and a time period of 30 min. Then the biogas was fed and mixed with the syngas from gasifier at the desired volume ratio. The feeding rate of biogas was determined by the composition analysis of the syngas from the gasifier. Periodically, gas samples were taken from points A, B, and C for composition analysis. Analysis. The cool, dry, clean gas was sampled by gas bag and analyzed on a gas chromatograph (model GC-2010, Shimadzu, Japan), which is fitted with a GS-carbon plot column (30 m × 0.530 mm × 3.00 µm), FID, and TCD detectors; helium was used as the carrier gas.

Results and Discussion Typical Operating Conditions and Gas Composition. Some typical operating conditions and test results are listed in Table 3. The table indicates that with the use of dolomite and B104 iron-magnesiumbased oxide catalyst in the gasifier, the H2-rich syngas was produced in the gasifier. Compared with the data obtained under similar gasification conditions by other authors,22,24-25 the H2 content of syngas increases by 12 mol % while CO content decreases by 19 mol %. These effects may be due to the contribution of the WGS

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reaction by addition of B104 WGS catalyst in the gasifier. The stoichiometric factor (ratio of components H2, CO, and CO2) of syngas was adjusted by a reforming reaction with addition of biogas in the downstream reforming reactor. The content of CO2 and CH4 is below 3 mol % in the outlet gas stream of the reforming reactor; the content of CO increases greatly from 13 to 31 mol %; the H2 content increases slightly from 33 to 46 mol %; C2 content is below 0.3 mol %. It can be concluded that the CH4-CO2 dry reforming reaction (eq 1) and reverse water gas shift (RWGS) reaction (eq 2) occurred in the reforming reactor simultaneously. Under the reforming reaction conditions, the steam is also reformed with CH4 to produce synthesis gas (eq 3). The elimination of the CO2 is also achieved by the dry reforming of the hydrocarbons present in the syngas according to eq 4.

CH4 + CO2 f 2CO + 2H2

(1)

CO2 +H2 f CO + H2O

(2)

H2O + CH4 f CO + 3H2

(3)

CnHm + nCO2 f 2CO + (m/2)H2

(4)

B104 iron-magnesium-based oxide catalyst is a hightemperature (550 °C) WGS reaction catalyst. The equilibrium constant for the WGS increases as the temperature decreases. However, to achieve the necessary reaction kinetics, a higher temperature is required. For methanol or DME synthesis, the WGS reaction of the syngas needs not to be complete due to the stoichiometry requirement (H2/CO ratio of 2 for methanol, H2/CO ratio of 1.5 for DME). With B104 catalyst in the gasifier, the WGS reaction occurred in the gasifier and the kinetics of high temperature was profitable. The pine sawdust gasification and WGS reaction occurred simultaneously. The contents of H2 and CO2 increased greatly while the CO content of syngas from the gasifier was lower than that of other groups.22-25 The carbon content on the surface of B104 catalyst was less than 1.1 wt % after a 10-h test. After this period test, 95 wt % of the B104 catalyst was collected in the gasifier. It indicated a little attrition. The large amounts of CO2 in the syngas stream were consumed mostly via a dry reforming reaction with the addition of biogas in the downstream reforming reactor, and the exit gas from the reforming reactor contained large amounts of gases (H2 + CO) and trace CH4 and CO2. The compositions of synthesis gas produced by all of these runs were apparently similar: 45-47 mol % H2, 30-32 mol % CO, 1.9-2.4 mol % CH4, 2.1-2.7 mol % CO2, 0.2-0.3 mol % C2, and 14-16 mol % N2, dry basis. The H2/CO ratio was above 1.5. It contained trace CH4 and CO2. Almost 95% of carbon content in the biomass was converted to synthesis gas. With the biogas addition of 0.56-1.00 Nm3 (dry basis)/Kg of biomass, the yield of gases (H2 + CO) was 2.33-4.40 Nm3 (dry basis)/Kg of biomass. Effect of Biogas Feed Rate on Synthesis Gas Composition. The syngas from the gasifier contains large amounts of CO2 (20-25 mol %), small amounts of CH4 (4-7 mol %), and trace amounts of light hydrocarbons. It is unfit for the production of methanol or FT

Figure 2. Effect of model biogas feed rate on contents of CH4 and CO2 in the outlet gas stream of the reforming reactor (catalyst: NiO-MgO; reforming temperature: 750 °C).

synthesis fuel. The dry reforming reactions (eqs 1 and 4) are used to convert CO2 to CO with the addition of biogas as the CH4 resource. The effect of the biogas/ syngas ratio before the reforming reactor on the composition of synthesis gas was investigated under the condition referring to run 2 in Table 3. The results are shown in Figure 2. On increasing the biogas/syngas ratio, there is an increase of CH4 content in the inlet gas stream of the reforming reactor. In the interval studied for the biogas/syngas ratio (from 0.1 to 0.45), on increasing the biogas/syngas ratio, the CH4 content in the outlet gas stream of the reforming reactor increased and the CO2 content decreased. The cross point of curves (∆) and (0) indicated that the contents of both CH4 and CO2 in the outlet gas stream of the reforming reactor are less than 3 mol % at the biogas/ syngas ratio of 0.33 (CH4/CO2 ratio of 0.85 in the inlet gas stream). Above 95%, conversion of CH4 and CO2 present in the inlet gas stream could be obtained simultaneously at the CH4/CO2 ratio of 0.85. The RWGS reaction took place as well and brought the reformer products to chemical equilibrium. It gave almost 100% conversion of light hydrocarbons present in the inlet gas stream in this interval studied, since light hydrocarbons are more easily activated than CH4. The dry reforming reactions resulted in an increase in the H2 and CO content of synthesis gas. Effect of the Reforming Temperature on Synthesis Gas Composition. The effect of temperature in the reforming reactor on the composition of the synthesis gas was investigated under the condition referring to run 2 in Table 3 with the reforming temperature from 600 to 800 °C. The results are shown in Figure 3. There was a continual increase of the H2 and CO contents and a continual decrease of the CO2, CH4, and C2 hydrocarbon contents in the outlet gas stream over the whole temperature range. The contents of CH4 and CO2 in the outlet gas stream of the reforming reactor were less than 3 mol % when the reforming temperature was above 750 °C. No tar was observed, and the C2 content was below 0.3 mol %. A higher temperature did not lead to better product composition. Several authors2,4 have suggested that the catalytic reactor for the upgrading of biomass product gas should

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Figure 3. Synthesis gas composition vs reforming temperature (catalyst: NiO-MgO; GHSV: 2325 h-1).

Figure 4. Synthesis gas composition vs GHSV (catalyst: NiO-MgO; reforming temperature: 750 °C).

operate at or above 730 °C for removal of CH4 and other hydrocarbons with commercial steam reforming catalysts for a long time. The results in our tests are in agreement with their results and trends. The reforming temperature was close to the biomass gasification temperature. With the development of the hot gas clean technologies, the biogas gas stream can be heated by the outlet gas stream from the gasifier up to nearly the reforming temperature in order to match the reforming heat demand and supply. In this case, it eventually favors a lower cost of synthesis gas production. Performance of the NiO-MgO Catalyst. The composition of synthesis gas in Figure 3 indicates the activity of the NiO-MgO catalyst. It exhibits little catalytic activity of the reforming reaction under conditions of 600 °C, 0.1 MPa, and gas hourly space velocity (GHSV) ) 2325 h-1. However, it exhibits excellent catalytic activity (91% of CH4 conversion and 93% of CO2 conversion) at the temperature of 750 °C, which is close to the equilibrium (95%). The effect of GHSV on the activity of NiO-MgO catalyst was also investigated by decreasing the weight of loading catalyst in the reforming reactor while other conditions were kept constant. The results are shown in Figure 4. The composition of the effluent gas stream remained constant as GHSV increased from 1400 h-1 to 11000 h-1.The conversions of both CH4 and CO2 were above 90%. However, the contents of H2 and CO decreased, and contents of CH4 and CO2 increased as GHSV increased from 11000 to 16000 h-1. This value is a little less than the result (14000 h-1 for 98% tar conversion) obtained by Aznar et al.4 The difference may be the high CH4 content (20 mol %) in the inlet gas stream of the reforming reactor in our tests and the different nickel-based catalysts used. To investigate the effect of the reducing method on catalyst activity, the NiO-MgO catalyst was prereduced at 750 °C by mixed gas (5% H2, 95% N2) for 30 min and passivated before the test. The reaction conditions were the same as those of run 2 in Table 3. The results indicate that the catalytic activity of the catalyst prereduced for 30 min is almost the same as that of the catalyst reduced in situ by syngas from the gasifier for 30 min. It can be concluded that the biomass gasification syngas plays a role of the mixed gas (5% H2, 95% N2) to

reduce the catalyst. It eventually simplifies the process for the catalyst preparation. The catalytic reforming of CH4 with CO2 to synthesis gas faces the problems of the deactivation or the destruction of catalyst by carbon deposition via the Boudouard reaction and methane decomposition.26-28 The catalyst deactivation due to coke formation may be more serious in the process of biomass hot gas cleanup because there are also some other light hydrocarbons and tars in the inlet gas stream of the reforming reactor and there is always coke formation by thermal and catalytic cracking and reforming of these hydrocarbons and tars.2 The particulates, or dust, and sulfur are also a main cause for catalyst deactivation.10 In our process of biomass synthesis gas production, the content of particulates in the inlet gas stream of the catalytic bed is low because of the high-efficiency cyclone, metallic filter, and quartz wool at the entrance of catalytic bed. However, we still detect trace particles with diameters up to 3.5 µm. The sulfur content in the inlet gas stream is lower than 15 ppm by our test. Its influence on the catalyst deactivation is not notable in a short time. The stability and activity of the NiO-MgO reforming catalyst is vital to the total efficiency of the system. Z409R commercial nickel-based catalyst (Qilu Petrochemical Company, China) was used to compare and evaluate the performance of the NiO-MgO catalyst in this process. The reaction conditions were the same as that of run 2 in Table 3. For catalyst lifetime studies, the running of the facility during one test lasted on average 15 h, of which about 10 h were under the stationary state. It was stopped overnight and operated the next day. Data reported above refers only to stationary states. After one test every day, the gasifier, cyclone, and filter were opened for cleaning of the remaining particles. We did not unload the catalysts in the catalytic bed. The results of a lifetime test over the NiO-MgO catalyst for 1 day are shown in Figure 5. It includes five stages (A, B, C, D, and E). At stage A, the conditions of (26) Tomishige, K.; Asadullah, M.; Kunimori, K. Catal. Today 2004, 89, 389-403. (27) Chen, Y. G.; Tomishege, K.; Fujimoto, K. Appl. Catal. A: Gen. 1997, 161, 11-17. (28) Chen, Y. G.; Tomishege, K.; Yokoyama, K.; Fujimoto, K. Appl. Catal. A: Gen. 1997, 165, 335-347.

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Figure 5. Synthesis gas composition vs time on stream (initial test) (reforming temperature: 750 °C; GHSV: 2325 h-1; catalyst: NiO-MgO; 0 H2, O CO, 4 CO2, 3 CH4, + N2, × C2, and ] O2).

Figure 6. Synthesis gas composition vs time on stream (initial test) (reforming temperature: 750 °C; GHSV: 2325 h-1; catalyst: Z409R; 0 H2, O CO, 4 CO2, 3 CH4, + N2, × C2, and ] O2).

gasification and reforming were prepared at the desired value. It took about 2.5 h to increase the temperature. Gasification took place when biomass and steam were added into the gasifier at stage B. It produced H2-rich fuel gas (H2, 33-35 mol %) to reduce the catalyst. The variations of the exit gas composition indicated that the activity of the NiO-MgO catalyst increased rapidly at the beginning of the reforming reaction due to reduction by biomass H2-rich syngas. At stage C, biogas was added and mixed with the raw syngas from the gasifier. After filtration, the mixture gases were fed into the catalytic bed. It took about 35 min to reach the stationary state. During the 10-h test at stage D, the composition fluctuated a little. However, we observed a small increase of CH4 content in the exit gas stream at the end of stage D, which indicated the catalyst deactivation to some extent. At stage E, the feeding of biomass and steam were stopped, but the air stream continued for 25 min for gasification or combustion of the remaining biomass. At this stage, the contents of H2 and CO

decreased and the content of CO2 increased notably, which can be explained by two main causes: combustion of the remaining biomass or char in the gasifier and combustion of the carbon deposited on the surface of the catalyst in the catalytic bed. The latter would result in restoring catalyst activity. It was confirmed by similar results of a test repeated the next day. The lifetime test ran 10 days by the repeated test (10-h stationary state every day, stopping at night), and we did not observe a notable variation of synthesis gas composition. Blockage of the catalytic bed was not observed. The inlet gas stream of the catalytic bed was upward, and the NiOMgO catalysts remained fluidized (low velocity) during the lifetime test. The trace particles in the inlet gas stream with diameters below 3.5 µm can flow out of the catalytic bed, and they did not accumulate and cause blockage of the catalytic bed. As shown in Figure 6, Z409R commercial nickel-based catalyst exhibited an initial deactivation due to the carbon deposition after 5 h. Then, its activity remained constant from 5 to 10 h.

Syngas Production via Biomass Gasification

Energy & Fuels, Vol. 19, No. 2, 2005 643

Table 4. Different Pathways for Stoichiometric Adjustment of Synthesis Gas process gasification agents composition of raw syngas (mol %, dry) H2 CO CO2 CH4 N2 others adjusting method removing of total CO2 (%) composition of synthesis gas (mol %, dry) H2 CO CO2 CH4 N2 others H2/CO ratio

A

B

C

D

air-steam

electrolysis-O2

electrolysis-O2

PSA-O2

33.5 12.8 24.6 5.4 21.3 2.4 dry reforming by addition of biogas, 750 °C 0

37.3 15.8 34.7 11.4 0.3 2.5 addition of electrolysis-H2

37.3 15.8 34.7 11.4 0.3 2.5 addition of electrolysis-H2

37.3 15.8 34.7 11.4 0.3 2.5

61

0

95

46.8 31.2 2.4 2.7 15.1 0.4 1.5

63.5 13.6 11.7 9.8 0.2 1.2 4.6

68.7 7.8 17.3 5.6 0.15 0.45 8.8

55.6 23.6 2.6 17.0 0.5 0.7 2.3

We observed that the activity of the catalyst in the test repeated on the next day was lower than that of initial test on the first day. It indicated that the carbon deposited on the surface of Z409R was not eliminated completely at stage E. The carbon accumulated, and the activity of the catalyst decreased slightly again after 30 h. Large amounts of coke formed by steam and dry reforming reactions were deposited on the surface of Z409R catalyst, and the catalyst could not be kept fluidized. Once the catalyst was in a fixed bed state, the particles in the inlet gas stream would accumulate in the catalyst bed. The test stopped due to the blockage of the reforming reactor by the considerable amount of coke and particles after 40 h. The commercial Z409R catalyst runs usually under a steam-to-carbon ratio of 1.4 or higher in order to maintain the catalyst activity in the industrial application. In our tests, the steam/ carbon ratio of the reforming reaction is less than 1.0. The coke formation occurred between the domain of the nickel crystal and metal-support interface of the Z409R catalyst, leaving large amounts of coke and destroying the catalyst. The suppressed coke formation of the NiOMgO catalyst may be caused by its low NiO content, small nickel particles, and the difference in the composition of surface carbonaceous species on the nickel particles as the reaction intermediate.26-28 The lifetime test of the NiO-MgO catalyst for a longer period of time will be done in the future. Comparison of Different Pathways for the Stoichiometric Adjustment of Synthesis Gas. Several authors29-31 choose a 10-megawatt biomass gasifier to meet the goal of decentralized fuel production from biomass with a capacity of