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Test Results and Operation Performance Analysis of a 1-MW Biomass Gasification Electric Power Generation System Zhengshun Wu,*,†,‡ Chuangzhi Wu,† Haitao Huang,† Shunpeng Zheng,† and Xianwen Dai† Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 81 Xianlie Zhong Road, Guangzhou, 510070, China, and Huazhong University of Science and Technology, Wuhan, 430074, China Received June 24, 2002. Revised Manuscript Received February 7, 2003
This paper introduces the tests and operation performance analysis of a 1-MW-scale biomass gasification and electric power generation system. The experimental data indicate that operation temperature of the gasifier and ratios of air to fuel have a strong impact on gasification efficiency, conversion efficiency of carbon, gas productivity, gas composition, and its low heating value (LHV). The optimal operation parameters for wood powder feed are temperature 780 °C, the equivalence ratio (ER): 0.26, 70% gasification efficiency, 80% conversion efficiency of carbon, and LHV 5.8 MJ/m3. During the experiment, it was found that fly ash carried by fuel gas and tar produced during biomass gasification are the main factors affecting continuous operation of the electric generation system; the loss of sensible heat and carbon of fly ash lead to the low efficiency of electric power generation. The pollutant emission and effect of catalyst on tar cracking are also investigated. The results provide information for the scale-up design and development of a biomass gasification and electric power generation system.
1. Introduction With the rapid depletion of fossil fuels and environmental pollution by SO2, NOx, etc., people will confront an energy crisis soon, and have to search for renewable energy sources in nature. Biomass is often considered as an important renewable energy source, and is available in huge quantity as industrial, agricultural, and forest waste per year, and may become a prospective and alternative energy source as a substitute for fossil energy. Using biomass energy sources can alleviate the contradiction of energy scarcity, and decrease the CO2 greenhouse effect with no net CO2 emission and atmosphere pollution of SO2 and NOx because of much lower N and S content in the biomass. It is important to develop effective technologies for biomass utilization that can be economically accepted and extensively commercialized.1-3 Biomass gasification is a thermochemical process that can turn biomass into combustible gas. Biomass gasification with a circulating fluidized bed gasifier can treat biomass in a large quantity with high efficiency and low * Corresponding author. Tel: 86-20-87787140. Fax: 86-20-87608586. E-mail:
[email protected]. † Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences. ‡ Huazhong University of Science and Technology. (1) Beenackers, A. A. C. M. Solar Energy R&D in the European Community. Series E: Advanced gasification. D. Reidel Publishing Company: London, 1986. (2) Reed, T. B. Biomass Gasification Principle and Technology; Noyes Data Corporation: Park Ridge, NJ, 1981. (3) Palz, W.; Chartier, P.; Hall, D. O. Energy from Biomass; Applied Science Publishers Ltd.: London, 1981.
cost, and is one of the most important conversion technologies. However, there are only a few MW-scale biomass gasification and electric generation system installations in China.4-7 Operation performance and results that provide guidance for scale-up design and the development of an electric energy generation system using biomass gasification remain unclear; so an experimental study of the gasification operation is very useful. This paper introduces the testing results of a 1-MW biomass gasification and electric generation plant. The optimal operation parameters were studied, and the factors affecting efficiency of electric energy generation were analyzed. The pollutant emission and effect of catalyst on tar cracking were also investigated. 2. Experimental Method 2.1. Biomass Gasification and Power Generation System. A MW-scale demonstration plant has recently been constructed and come into operation in Sanya Timber Factory (4) Wu, C. Z.; Xu, B. Y.; Luo, Z. F.; Zhou, X. G. Performance Analysis of a Biomass Circulating Fluidized Bed Gasifier. Biomass Bioenergy 1992, 3 (2), 105-110. (5) Xu, B. Y.; Wu, C. Z.; Luo, Z. F.; Huang, H. T.; Zhou, X. Design and operation of a circulating fluidized-bed gasifier for wood powders. In Advances in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackie Professional Publishers: Glasgow, U.K., 1994; pp 365376. (6) Wu, C. Z.; Liu, P.; Luo, Z. F.; Xu, B. Y.; Chen, Y. The scale-up of biomass circulating fluidized bed gasifier. The 6th China-Japan Symposium on Fluidization, Beijing, China, Oct. 9-11, 1997; pp 196200. (7) Wu, C.; Yin, X.; Zheng, S.; Huang, H.; Zhang, W.; Chen, Y. A Demonstration Project for Biomass Gasification and Power Generation in China. Program in Thermo-chemical biomass conversion; Bridgwater, A. V., Ed.; IEA Bioenergy: 2001; pp 465-472.
10.1021/ef0201376 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/22/2003
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Figure 1. 1-MW biomass gasification technology flowchart. Table 1. Particle Size Distribution of Biomass Fuel particle size distribution (mm) wood powder wt % average diameter (mm)
0.28
0.16
0.28
0.15
0.41
0.21
particle size distribution (mm) leftover wt % average diameter (mm)
3.2
0.39
0.24
0.26
0.11
1.85
Table 2. Ultimate Analysis of Biomass Fuel
ultimate analysis
proximate analysis
Figure 2. Tar and gas sampling system sketch. (STF), Hainan Island of China. STF is a large-scale timber processing enterprise that leaves, every day, about 100 tons of wood residue such as wood powder, leftover material which is coarse wood residue. On the other hand, timber processing is a high electricity consumption industry. About 5 MW of electric power is needed in STF. Therefore, to convert the wood powder residue into electricity for the factory self-consumption through biomass gasification-power generation technology has benefits to the factory economy and environmental protection which is the aim of the demonstration projects. The main equipment of the biomass gasification-power generation system includes a biomass conveyer, a fuel feeder, a CFB gasifier, a Venturi purifying tube, water scrubbers, a blower, 5 sets of 200 KW gas engine/generator sets (made by Hongyan Machine Works, China), a wastewater treatment facility, etc. The flow diagram is shown in Figure 1. The wood residue is fed to the CFB gasifier, and the product gases are released by the gasification reaction. Most solids that are entrained with the product gases leaving the CFB gasifier are separated and removed by a cyclone separator and returned to the CFB gasifier. Before passing to the gas engines, the product gas is subjected to a series of conditioning steps to remove tars, ammonia and fine particulate using a Venturi tube, and water scrubbing towers. 2.2. Data Measurement. A data acquisition system is installed for registering temperature, pressure, air volume, and feeding rate during operation. There are seven sampling pointssfive along the gasifier at the height of 1.2 m (point 1), 2.6 m (point 2), 4.1 m (point 3), 5.5 m (point 4), and 6.9 m (point 5) above the bottom of gasifier, and the other two are located at the outlet of the secondary cyclone (point 6) and gas tank (point 7). Figure 2 presents the tar and gas sampling system. The gas composition is analyzed by a HP 5890 gas chromatograph with tcd and fid detectors. NH3 is absorbed in 0.1 M H2SO4 and the concentration is determined by means
C H O N S fixed carbon volatile ash moisture HHV (kJ/kg) LHV (kJ/kg)
wood powder
leftover material
47.0 6.9 42.2 3.4 0.1 17.7 77.3 0.4 4.6 19070 17662
49.3 6.8 40.1 3.0 0.2 16.7 79.5 0.6 3.2 19718 18310
of a pH/ISE meter, using the ammonium ion-selective electrodes. The tar content in the gas product is measured following the method provided by Biomass Technology Group.8 2.3. Particle Size Distribution and Element Analysis of Biomass Fuel. Table 1 and Table 2 list particle size distribution and element analysis results of biomass fuel.
3. Results and Discussion Operation parameters of the CFB gasifier, including temperature, pressure, air flow volume, and feeder rate, were automatically recorded by a data-collecting system with 2-s interval. 3.1. Operation Parameters. 3.1.1. Temperature. The normal operation temperature of the gasifier is about 750 °C. If the temperature is too high, it leads to poor fuel gas quality; on the other hand, if the temperature is too low, a large quantity of tar is produced. The temperature curves of measurement points 1 and 2 are shown in Figure 3. It can be seen from Figure 3 that the temperature of the gasifier has some fluctuation, because the feeder rate, the water content, and particle size distribution of wood powder are not uniform. 3.1.2. Pressure. Figure 4 illustrates pressure curves of the gasifier for points 3 and 5. It can be seen from Figure 4 that there is evidently a large pressure (8) Knoef, H. A. M. The UNDP/Word Bank Monitoring Program On Small Scale Biomass Gasifiers (BTG’s Experience On Tar Measurements). Biomass Bioenergy 2000, 18, 39-54.
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Figure 3. The typical temperature curve of gasifier.
Figure 6. Effect of bed temperature (a) 750 °C, (b) 820 °C, and (c) 900 °C on the composition of gas. Figure 4. The pressure curve of gasifier without bed material.
3.1.3. Air and Feeding Rate. According to load and gasifier temperature, the air volume of flow can be adjusted; the normal volume of flow is 1000 Nm3/h, corresponding to 900 kg/h of feeding rate. During operation, the less the amount of air added, the less the amount of heat that is produced by wood powder oxidation, which does not meet the demand of endothermic reduction reaction in the process of biomass fuel gasification. On the other hand, if amount of air added is too much, the heat value of fuel gas decreases, because the combustible gas of fuel gas, such as CO, H2, etc., is completely oxidized, and the fuel gas is diluted by N2 of air. By testing operation, the optimal equivalence ratio (ER) is 0.26, which is defined as the oxidant-to-fuel weight ratio divided by the stoichiometric ratio as in ref 2, namely:
ER ) {weight of oxidant}/{weight of dry biomass fuel} {stoichiometric oxidant}/{biomass fuel ratio} Figure 5. The pressure curve of gasifier with bed material.
fluctuation without sand as the bed material in the gasifier, which results from different bed pressure drop. This phenomenon disappears with sand as the bed material in the gasifier (see Figure 5). Therefore, during operation, it is important for the gasifier to maintain a suitable height of bed material.
3.2. Effect of Operation Temperature on Gas Composition. Figure 6 illustrates the effect of different bed temperaturess750 °C (a), 820 °C (b), and 900 °C (c)son gas composition at the equivalence ratio (ER) 0.26. It can be seen from Figure 7 that when temperature increases, gas output rate increases, the CO content of fuel gas decreases significantly, and the H2, CH4 content of fuel gas decreases slightly; the heat value of
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Figure 7. Effect of bed temperature on tar formation. Table 3. Effects of Temperature on Gas Output Rate, Gasification Efficiency, and Carbon Conversion Efficiency at Different ER temperature (°C) ER production rate of fuel gas (Nm3/kg fuel) gasification efficiency (%) HHV of fuel gas (MJ/Nm3) carbon conversion efficiency (%)
750 0.25 1.9 70 5.83 79.56
820 0.32 2.4 72 4.3 81.4
900 0.37 2.5 75 3.32 85
fuel gas also decreases. At the same temperature, the heat value of fuel gas increases along the bed height, while N2 content of the fuel gas decreases, because more fuel gas is produced. Production rate of fuel gas, gasification efficiency, HHV of fuel gas, and carbon conversion efficiency in different temperatures are listed in Table 3. The definitions of gasification efficiency and carbon conversion efficiency are as follow:
gasification efficiency ) heat value of producing fuel gas at unit time × chemical heat value of feed fuel at unit time 100% carbon conversion efficiency ) carbon mol of reaction at unit time × 100% carbon total mol of feed fuel at unit time It can be found from Table 3 that when temperature increases, gas output rate, gasification efficiency, and carbon conversion efficiency of fuel increase, whereas the heat value of fuel gas decreases. Therefore, as far as the gas engine can be normally operated, the temperature should be as high as possible. For wood powder, suitable temperature is about 780 °C. 3.3. Effect of Operation Temperature on Tar Content of Fuel Gas. 3.3.1. Tar Content of the Wood Powder Gasification Process. During wood powder gasification, gas output rate and the heat value of fuel gas decrease because of tar production; furthermore, fuel gas piping and continuous operation of the electric generation system are hindered by tar in fuel gas. How to decrease tar content of fuel gas needs extensive attention.9 Increasing gasification temperature is one way of decreasing tar content. Figure 7 illustrates the relation between tar content and operation temperature. It can be seen from Figure 7 that tar content decreases as temperature increases. However, a too-high operation temperature can lead to fuel gas quality deterioration, because CO, H2, and CH4 of fuel gas are oxidized to CO2 (9) Gil, J.; Caballero, M. A.; Martin, J. A.; Aznar, M. P.; Corella, J. Biomass Gasification With Air In A Fluidized Bed; Effect of The InBed Use of Dolomite Under Different Operation Conditions. Ind. Eng. Chem. Res. 1999, 38, 4226-4235.
Figure 8. Effect of dolomite catalyst on tar content.
and H2O in the air gasification process. So, there exists an optimal operation temperature; at this temperature, tar content of fuel gas is low and the heat value of fuel gas can meet the demand of the electric generation system. 3.3.2. Effect of Tar Cracking Catalyst on Fuel Gas Content. Besides increasing the operation temperature to decrease the tar content of fuel gas, another way to decrease tar content is to add a tar-cracking catalyst. The usual catalyst for tar cracking has a nickel group catalyst, charcoal, and dolomite catalysts. Catalyst activity of a nickel group catalyst is high, but the surface of this type of catalyst can easily be covered by coke produced by the gasification process, so it is not suitable for in-bed industrial application.10-14 In addition, the nickel group catalyst is very expensive. During our testing process, dolomite was used as the tar-cracking catalyst; dolomite is added to feed in proportion of 1:10. After acting as catalyst, a part of the dolomite was carried out of the gasifier and separated by a cyclone separator, the residue dolomite was also periodically removed from the bottom of gasifier. The dolomite concentration in the gasifier can be approximately constant. Figure 8 illustrates the effect of dolomite catalyst on tar cracking. It is shown in Figure 8 that tar content evidently decreases with the addition of dolomite cracking catalyst. According to calculation, compared with the same temperature, before and after tar catalyst cracking, the cracking efficiency of tar is 25.4% at 750 °C, and 55.2% at 900 °C. The fuel gas content variation in Figure 9 indicates that H2 content increases, especially at 900 °C, while CH4, C2H6, C2H2 content decreases, demonstrating that tar cracking deepens. The lab-scale test of tar cracking by Gil et al.9 reported similar trends of results. 3.4. Pollutant Emission of the Gasification Process. 3.4.1. SO2 Content of Fuel Gas. Sulfur element (10) Orio, A.; Corella, J.; Narvaez, I. Performance Of Different Dolomite On Hot Raw Gas Cleaning From Biomass Gasification With Air. Ind. Eng. Chem. Res. 1997, 36, 3800-3808. (11) Kinoshita, C. M.; Wang, Y.; Zhou, J. Effect Of Reformer Conditions On Catalytic Reforming Of Biomass-Gasification Tars. Ind. Eng. Chem. Res. 1995, 34, 2949-2954. (12) Sjostrom, K.; Taralas, G.; Liinanki, L. Dolomite-Catalyzed Conversion Of Tar From Biomass Pyrolysis. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Science: London, 1988. (13) Morris, M.; Waldheim, L. Efficient Power Generation Gasification From Wood Gasification. Gasification For The Future; Noordwijk, The Netherlands, 11-13 April, 2000. (14) Corella, J.; Aznar, M. P.; Gil, J.; Caballero, M. A. Biomass gasification in fluidized bed: Where to locate the dolomite to improve gasification? Energy Fuels 1999, 13, 1122-1127.
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Figure 11. The distribution of NOx composition along gasifier height.
Figure 12. The disposal process of wastewater. Figure 9. Effect of dolomite catalyst on fuel gas composition.
Table 4. Typical Wastewater Treatment Results analysis item chemical oxygen demand (COD) (mg/L) biochemical oxygen demand (BOD) (mg/L) pH solid suspension tar (mg/L)
Figure 10. Effects of bed temperature and height of gasifier on NH3 emission.
content of wood powder is very low (0.1%). After fuel gas from the CFB gasifier passes water scrubbers, the SO2 content of fuel gas is below the detective limits of our own analysis procedure. 3.4.2. NH3 Content of Fuel Gas. Due to low gasification temperature, nitrogen in air cannot be transformed into a thermal nitrogen compound.15,16 The nitrogen compounds produced by gasification process come mainly from fuel nitrogen; among them, NH3 is a main compound, because of the reduction conditions. NH3 content increases when temperature increases, and nitrogen element conversion efficiency changes from 6% to 70%, as illustrated in Figure 10. 3.4.3. NOx Content of Fuel Gas. NOx content of fuel gas is very low (about 10-20 ppm). This is attributed mainly to the low nitrogen element content of wood powder feed and reduction conditions of the gasification process. Figure 11 illustrates NOx content of fuel gas (15) Leppa¨lahti, J. Formation of NH3 and HCN in slow-heating-rate inert pyrolysis of peat, coal and bark. Fuel 1995, 74 (9), 1363-1368. (16) Leppalahti, J.; Simell, P.; Kurkela, E. Catalytic conversion of nitrogen compounds in gasification gas. Fuel Process. Technol. 1991, 29 (1), 43-56.
before after treatment treatment 800-1000 320-400 8.08 700-1100 35
