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Energy & Fuels 2003, 17, 1062-1067
Production of Hydrogen and/or Syngas (H2 + CO) via Steam Gasification of Biomass-Derived Chars S. T. Chaudhari, A. K. Dalai,* and N. N. Bakhshi Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, 110 Science Place, University of Saskatchewan, Saskatoon SK S7N 5C9, Canada Received January 22, 2003. Revised Manuscript Received May 16, 2003
Steam gasification of two biomass-derived chars was studied at 700, 750, and 800 °C in a fixed bed microreactor at different steam flow rates in the range of 1.25 to 10 g/h/g of char. The chars used in the present study were (i) bagasse charsobtained from Natural Resources Canada, CANMET Energy Technology, Ontario (produced by Dynamotive Technologies Corp., Vancouver, BC), and (ii) commercial charsobtained from ENSYN Technologies Inc., Ontario (produced during the fast pyrolysis of biomass using their RTI process). Both chars were highly reactive, particularly at 800 °C with steam flow rate of 5 and 10 g/h/g of char. In the case of bagasse char, maximum conversion of 81% was achieved at 800 °C with a steam flow rate of 10 g/h/g of char, whereas maximum conversion of 69% for commercial char was obtained at 800 °C with steam flow rates of 5 and 10 g/h/g of char. The product gas obtained was mainly a mixture of H2, CO, CO2, and CH4 with a high H2/CO molar ratio (about 4:7 for bagasse char and 9:15 for commercial char) at a temperature of 800 °C and a steam flow rate of about 10 g/h/g of char. Under the present reaction conditions, synthesis gas (H2 + CO) produced by steam gasification of bagasse char and commercial char was in the range of 80-88 mol % and 77-84 mol %, respectively. The heating value of the product gas was in the range of 270-290 Btu/scf for bagasse char and 250-280 Btu/scf for commercial char. The results suggest that there is a strong potential for producing hydrogen and syngas from biomass-derived chars by a simple steam gasification process.
Introduction Large quantities of agricultural and forestry residual wastes are generated through out the world annually. A judicious use of these biomass resources could play an important role in mitigating the environmental impacts such as global warming and acid rain, which occur due to the use of nonrenewable energy. Pyrolysis and gasification of these waste materials have been identified as the most favorable thermo-chemical biomass conversion processes to renewable energy and are environmental friendly as a result of their low sulfur and nitrogen content. Among all the renewable energy sources, biomass represents the highest potential and will play a vital role in the near future.1 It is well-known that biomass can be converted to a liquid product (called bio-oil or biofuel) using fast pyrolysis. This technique has now reached the commercial stage.2 For example, two commercial plants using this technique (ENSYN Technologies Inc., Ottawa, ON, Canada) are being operated by Red Arrow Products Co., WI (50 tonnes/day). Also, a pilot demonstration unit (3 tonnes/day) is being operated in Galcia, Spain, using a bubbling fluidized bed3 (using the * Corresponding author. Tel.: (306) 966 4771. Fax: (306) 966-4777. E-mail:
[email protected]. (1) Maniatis, K.; Millich, E. Biomass Bioenergy 1998, 15 (3), 195. (2) Freel, B. A.; Grahm, R. G.; Huffman, D. R.; Vogiatzis, A. J. Proceeding of Energy from Biomass and Wastes XIV; Klass, D. L., Ed.; 1993; pp 811-826. (3) Narvaez, I.; Orio, A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110-2120.
technology developed at the University of Waterloo, Canada). In these processes, the product slate consists of approximately 70 wt % liquid, 15 wt % char, and 15 wt % gas. Thus, a certain amount of char is produced which needs to be utilized somehow. However, this char produced during pyrolysis of biomass is highly reactive and can be gasified with gasifying agents such as steam, CO2, oxygen, and hydrogen to gaseous fuels. The char produced during the pyrolysis of biomass can also be used separately for various applications.4 Recently, there is lot of interest in utilizing the char for steam gasification to produce gaseous fuels.6,9,10,12-14 Biomass-derived chars have promising applications in the production of hydrogen and synthesis gas (H2 + CO), which are used as feedstock for methanol and ammonia synthesis. Synthesis gas can also be converted to liquid fuels using Fischer-Tropsch technology. Also, the product gas could be directly used in the production (4) Kumar, M.; Gupta, R. C. Energy Sources 1998, 20 (7), 575. (5) Kumar, M.; Gupta, R. C.; Sharma, T. Fuel Process. Technol. 1992, 3269-3276. (6) Kumar, M.; Gupta, R. C. Fuel 1994, 73, 1922-1925. (7) DeGroot, W. F.; Richards, G. N. Fuel 1988, 67, 345-351. (8) Ponder, G. R.; Richards, G. N. Energy Fuels 1994, 8, 705-713. (9) Chen, G.; Yu, Q.; Sjostrom, K. J. Anal. Appl. Pyrol. 1997, 40/ 41, 491-499. (10) Chen, G.; Sjostrom, K.; Bjornbom, E. Ind. Eng Chem. Res. 1992, 31, 2764-2768. (11) Geoffery, N. R.; Guangcheng, Z. Energy Fuels 1995, 9, 136140. (12) Kojima, T.; Assavadakorn, A.; Furusawa, T. Fuel Process. Technol. 1993, 36, 201. (13) Standish, N.; TanJung, A. Fuel 1988, 67, 666. (14) Slaghuis, J. H.; Van der Walt, T. J. Fuel 1991, 70, 831.
10.1021/ef030017d CCC: $25.00 © 2003 American Chemical Society Published on Web 06/11/2003
Hydrogen and/or Syngas from Biomass-Derived Chars
of electrical power in fuel cells or by combustion in gas turbines. There are only a few reports on biomass char gasification in recent years. Kumar et al.5,6 have studied the influence of carbonization temperature on the gasification of acacia and eucalyptus wood chars by using carbon dioxide. The results showed that the reactivity of acacia wood char is higher than that of eucalyptus wood chars. Also, the wood carbonization temperature had a noticeable influence on reactivity of chars. DeGroot and Richards7 have studied the effect of ion-exchanged cobalt catalysts on the CO2 gasification of wood chars and found that maximum catalytic activity was observed beyond a reaction temperature of 600 °C. Ponder and Richards8 have reported CO2 and O2 gasification of wood chars treated with iron sulfates and found that sorbed ferrous and ferric sulfates were superior catalysts at low-temperature gasification (∼ 400 °C) but were less effective at high temperature (∼ 850 °C). Chen et al.9,10 have performed steam and CO2 gasification of char obtained from pyrolysis of birch wood. The results showed that the reactivity of these chars was strongly affected by the time-temperature history of the char formation during the wood pyrolysis. For example, the char produced under rapid heating rate was more active than that produced under a slow heating rate. Geoffrey and Guangcheng11 studied the pyrolytic gasification of chars from wood containing iron and copper sulfates in the temperature range of 500950 °C. Also, in some cases, chars were treated with (NH4)2SO4 and H2SO4 to study the influence of copper and iron sulfates. Their results indicated that gasification produced good yields of CO and hydrocarbons with little tar. These investigations demonstrated the feasibility of the gasification process. Kojima et al.12 have studied the gasification kinetics of sawdust char with steam. They have derived the overall gasification rate of the char in a continuous bed with various residence times and conversions. Ross and Fikis15 have studied the gasification reactions of char and modified chars produced from Jack and Pine Bark. The product gases contained CO, CO2, CH4, and H2. They have observed that above the char preparation temperature (i.e., 350 °C), the yields of gaseous products increased with temperature until 550-600 °C when the production of CH4 and CO decreased. From the above discussion, it is evident that the char can be gasified and the composition of the gas produced during steam gasification depends on the nature of char and the process conditions used during the steam gasification. Depending on the composition of the gas produced, it can be used for a variety of purposes. However, a particular set of process conditions during steam gasification may not be optimum for producing gases for different applications. The thrust of this paper is an attempt to convert biomass-derived chars that are essentially waste materials to useful and value-added products such as H2 or syngas (H2 + CO) using a simple steam gasification process. Experimental Section The biomass-derived chars used in the present investigation were (i) bagasse char - obtained from Natural Resources (15) Ross, R. A.; Fikis, D. V. Can. J. Chem. Eng. 1980, 58, 230.
Energy & Fuels, Vol. 17, No. 4, 2003 1063 Canada, CANMET Energy Technology, Ontario (produced by Dynamotive Technologies Corp. Vancouver, BC) and (ii) commercial char- obtained from ENSYN Technologies Inc., Ontario (produced during the fast pyrolysis of biomass using their RTI process). The various physical properties of these chars such as density, ash content, elemental composition, and BET surface area were measured. Steam gasification experiments were carried out at atmospheric pressure in a fixed bed down-flow reactor. A schematic diagram of the experimental setup used for the steam gasification reaction is shown in Figure 1. It consists of a preheater to vaporize water into steam, an inconell reactor (450 mm long and 11 mm i.d.) with temperature controller, a metering pump (to feed the desired amount of water), a liquid collection trap, and a gas collection system. The reactor temperature was controlled by a temperature controller and was measured with a thermocouple placed in the center of the char bed. The procedure for a typical steam gasification experiment is described below. A mixture of 1 g of char and an equal amount of quartz chips (of 2-3 mm mesh size) was held on a quartz wool plug, which was placed on a supporting perforated disk inside the reactor. The top of the char sample was also covered by another plug of quartz wool. About 3 g of quartz chips (2-3 mm size) was kept on top of this quartz wool for uniform distribution of steam through the char bed. After loading these materials in the reactor, the sample was weighed and then placed in the furnace. The furnace and the preheater were then turned on. The reactor was brought to the desired reaction temperature by passing nitrogen gas (∼50 mL/min) through the reactor. When the desired reactor temperature reached, the nitrogen gas flow was turned off and steam feed was started by pumping water at the desired flow rate by means of a metering pump. The steam gasification reaction was continued for 45 min at the desired reaction temperature. The products leaving the reactor were passed through a water condenser and the gas product was collected over saturated brine solution in the gas collection system. The condensate was mostly unreacted water. Then heating of the furnace and the preheater was stopped. The water pump was turned off, and the reactor was cooled to the ambient temperature. The reactor was removed and weighed to determine the amount of unconverted char. The product gas was analyzed for its composition. The gas analysis was done using two gas chromatographs (Carle GC 500 and Hewlett-Packard 5890). The hydrocarbons were analyzed with a FID using a combination of packed and capillary columns (Stabilwax, 30 m long and 0.25 mm i.d.). Hydrogen, CO, and CO2 were analyzed with a TCD using a Chromosorb 102 column (1.8 m length and 3.175 mm i.d.). Temperature programming of the oven was used for analysis of hydrocarbons and permanent gases. In the present study, each experiment was performed three or four times at each experimental condition, and reproducibility of the experimental data was calculated to be within (3.0%.
Results and Discussion The chars used in the present study were characterized for their physical properties such as density, ash content, elemental analysis, and BET surface area. The results obtained are presented in Table 1. It can be seen from the Table 1 that the ash content is very high in bagasse char (25.9 wt %) compared to ash content in commercial char (3.1 wt %). However, the BET surface area for both the chars was very low (60%). Also, under these reaction conditions the formation of total product gas is large and rich in synthesis gas highly suitable for hydrogen production. The effect of reaction temperature on heating value of product gas obtained for bagasse char and commercial char at different steam flow rates is shown in Figures 6 and 7, respectively. Figure 6 shows that the heating value of the product gas obtained for bagasse char was maximum for a lower steam flow rate (1.25 g/h/g char) for all the temperatures studied; however, the heating value of the product gas did not change much, and it ranged between 270 and 290 Btu/scf. In the case of commercial char also, the maximum heating value of the product gas was observed at low steam flow rate
Chaudhari et al.
Figure 7. Effects of reaction temperature on heating value of the product gas obtained during steam gasification of commercial char at different steam flow rates.
(1.25 g/h/g char) for 700, 750, and 800 °C (Figure 7). The heating value of the product gas obtained did not change much and ranged between 250 and 280 Btu/scf. Bagasse char produced slightly more methane than commercial char, causing the difference in heating values (see Figures 6 and 7). Steam Gasification Mechanism. In general, steam gasification reactions include reactions of O2, CO2, H2, and H2O with the combustible fraction of carbon in biomass or coal, thereby producing gaseous products.16,17 The essential features involve chemisorption of the reacting gas species on the carbon surface. It is reported that all gasification/pyrolysis reactions of biomass and coal are endothermic. Therefore, the supply of thermal energy and thus the reactor temperature have a significant effect on the product gas compositions.18 The important reactions in this case may be given as follows:
C + H2O f CO + H2 (carbon gasification)
(i)
CO + H2O f CO2 + H2 (shift reaction)
(ii)
C + CO2 f 2CO (Boudouard reaction)
(iii)
C + 2H2 f CH4
(iv)
At higher temperatures, reactive carbon may react with oxygen, producing CO and CO2 as follows.
C + 1/2O2 f CO
(v)
C + O2 f CO2
(vi)
Thus, in the steam-carbon reaction, the chemisorption step involves dissociation of water at the carbon surface into hydrogen atom and a hydroxyl radical, which adsorbs on adjacent carbon sites.19 The dissociation of water normally proceeds with the formation of hydroxyl species that are extremely active oxidizing agents. This mechanism is similar to that of the water gas shift (16) Berkowitz, N. Coal gasification: a state of the art review; Fuel gases from coal; MSS Information Corporation: New York, 1976; pp 30-82. (17) Walker, P. L., Jr. Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science Publishers: London, 1985; pp 485-509. (18) Lu, G. Q.; Do, D. D. Fuel Process. Technol. 1991, 28, 35-48. (19) Buekens, A. G.; Schoeters, J. G. Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science Publishers: London, 1985; pp 619689.
Hydrogen and/or Syngas from Biomass-Derived Chars
reaction. Carbon-oxide intermediate is involved in the reaction and hydrogen atoms readily diffuse across carbon at typically high reaction temperatures (>600 °C). The relative rates of these reactions, which are influenced by the reaction conditions, determine the gaseous product composition. Because of the complex interdependence of these gasification reactions, there exists, a considerable potential for varying syngas compositions through changes in reactor design and operating conditions. Effect of Steam Flow Rate. The effects of steam flow rate on char conversion, total amount of gas produced, and its composition were studied by changing the steam flow rate from 1.25 to 10 g/h/g of char at 700, 750, and 800 °C. The results obtained for bagasse char (Figure 2) show that, as expected, conversion increased from 27 to 54% at 700 °C, from 40 to 67% at 750 °C, and from 54 to 81% at 800 °C when the steam flow rate was increased from 1.25 to 10 g/h/g of char for the reaction time of 45 min at the desired temperature. As a result, the formation of product gas was increased from 368 to 656 L/kg of char at 700 °C, from 584 to 1072 L/kg of char at 750 °C, and from 680 to 1424 L/kg of char at 800 °C with increasing steam flow rate from 1.25 to 10 g/h/g of char (see Figure 4). This is quite expected since increasing the amount of steam, being one of the reactants, in the reaction leads to higher conversion as well as higher gas production. For commercial char, the results obtained (Figure 3) on conversion and formation of gas product were not consistent with the steam flow rate. For example, at 700 °C, when the steam flow rate was increased from 1.25 to 2.5 g/h/g of char, the conversion increased from 31 to 42% while it decreased to 36 and 29% with increasing the steam flow rate to 5 and 10 g/h/g of char, respectively. However, at higher temperatures, the conversion was increased from 45 to 66% at 750 °C and from 61 to 69% at 800 °C with increasing the steam flow rate from 1.25 to 10 g/h/g of char, which is expected. The steam flow rate (for bagasse and commercial char) did not influence the product gas composition much (see Tables 2 and 3). This is possibly because of the attainment of equilibrium in gas composition. Since, the steam flow rate did not have much effect on the product gas composition, the production of synthesis gas was not changed much with steam flow rate and it ranged between 80 and 88 mol % for bagasse char and 77 to 84 mol % for commercial char. However, the H2/CO ratio in the synthesis gas obtained for bagasse char (Table 2) was decreased while it was increased for commercial char (Table 3) with increasing
Energy & Fuels, Vol. 17, No. 4, 2003 1067
the steam flow rate from 1.25 to 10 g/h/g of char. Also, the results show that the heating value of the product gas did not change much and ranged between 270 and 290 Btu/scf for bagasse char and from 250 to 280 Btu/ Scf for commercial char. It is well-known that synthesis gas having different H2/CO ratios is suitable for different applications. For example, synthesis gas having a H2/CO molar ratio in the higher range is desirable for producing hydrogen for ammonia synthesis. Also, this gas can be used to produce pure hydrogen for fuel cell applications. The production of synthesis gas with higher H2/CO ratios is generally not a problem. Besides the high levels of H2/ CO produced during the commercial steam gasification, this ratio is increased further during the water gas shift reaction for the removal of CO. On the other hand, it is generally rather costly to produce synthesis gas having lower H2/CO ratio.20 Thus, by controlling the input of steam, which is the predominant source of hydrogen, the molar ratio of H2/CO in the synthesis gas during steam gasification of these chars can be adjusted to the desired value. Besides above-mentioned applications, the product gas may be used directly as a fuel. Conclusions The following conclusions are drawn from the present study: 1. Biomass-derived chars can be converted to hydrogen and/or synthesis gas by steam gasification in the temperature range of 700 to 800 °C and steam flow rates of 1.25 to 10 g/h/g of char. 2. The production of hydrogen up to 75 mol % can be obtained from the biomass-derived chars by steam gasification at 700 °C and steam flow rate in the range of 1.25-10 g/h/g of char. 3. Synthesis gas (H2 + CO) produced by steam gasification of bagasse char and commercial char was in the range of 80-88 mol % and 77-84 mol %, respectively. 4. The synthesis gas having very high H2/CO molar ratio ∼ 4-7 for bagasse char and ∼ 4-15 for commercial char can be produced via steam gasification in the temperature range of 700-800 °C and steam flow rates of 1.25 to 10 g/h/g of char. 5.The results indicate that there is a strong potential for producing hydrogen and syngas from biomassderived chars by a simple steam gasification process. EF030017D (20) Wender, I. Fuel Process. Technol. 1996, 48, 189.
