Energy & Fuels 2005, 19, 1619-1623
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Air and Steam Coal Partial Gasification in an Atmospheric Fluidized Bed Hongcang Zhou* Department of Environmental Science & Engineering, Nanjing University of Information Science & Technology, Nanjing210044, China, and Key Laboratory of Clean Coal Power Generation and Combustion Technology of Ministry of Education, Southeast University, Nanjing 210096, China
Baosheng Jing,* Zhaoping Zhong, Yaji Huang, and Rui Xiao Key Laboratory of Clean Coal Power Generation and Combustion Technology of Ministry of Education, Southeast University, Nanjing 210096, China Received September 29, 2004. Revised Manuscript Received March 8, 2005
Using the mixture of air and steam as gasification medium, three different rank coal partial gasification studies were carried out in a bench-scale atmospheric fluidized bed with the various operating parameters. The effects of air/coal (Fa/Fc) ratio, steam/coal (Fs/Fc) ratio, bed temperature, and coal rank on the fuel gas compositions and the high heating value (HHV) were reported in this paper. The results show that there is an optimal Fa/Fc ratio and Fs/Fc ratio for coal partial gasification. A rise of bed temperature favors the semigasification reaction of coal, but the concentrations of carbon monoxide and methane and the HHV decrease with the rise of bed temperature, except hydrogen. In addition, the gas HHVs are between 2.2 and 3.4 MJ/Nm3. The gas yield and carbon conversion increase with Fa/Fc ratio, Fs/Fc ratio, and bed temperature, while they decrease with the rise of the rank of coal.
1. Introduction Coal gasification is a technology that has been around for 200 years. With the recent technology advances in the past 20 years, it has become an option for the clean production of power and other energy forms. China will continue to be the largest producer and user of coal in the world. Coal is the source of energy in almost every area of everyday life in China.1 Coal partial gasification technology can realize the stage conversion of coal, which is difficult to gasify completely at low temperature and low pressure. This technology does not pursue the high heating value (HHV) of fuel gas and carbon conversion. The remaining char is burned to produce heat and electric power in a fluidized bed combustor. The carbonizer fuel gas is filtered to remove particulate, and the cleaned fuel gas is then fired in a combustion turbine to increase the temperature of flue gas at the entrance of gas turbine.2 Coal partial gasification in fluidized bed has been considered as a key technology of the integrated coal gasification combined cycle (IGCC) and the advanced pressurized fluidized bed combustion combined cycle (APFBC-CC) in order to realize the high efficiency and good environmental performance for electricity generation, replacing existing coal-fired power plants.3 * Authors to whom correspondence should be addressed. Tel.: +8625-83794744. E-mail:
[email protected]. (1) Attwood, T.; Fung, V.; Clark, W. W. Market opportunities for coal gasification in China. J. Cleaner Prod. 2003, 11, 473-479. (2) McClung, J. D.; Kastner, C.; Dellefield, R.; Sears, R. Design and operating philosophy for an advanced PFBC facility at Wilsonville. Fluid. Bed Combust. 1993, 2, 1047-1052.
Up to now, Ocampo et al., Tomeczek et al., Watkinson et al., Kawabata et al., and Saffer et al. have studied the coal gasification characteristics in a fluidized bed and reported the gas heating values between 2.9 and 3.5 MJ/Nm3 using air and between 1.6 and 4.5 MJ/Nm3 using air-steam mixtures at atmospheric pressure.4 However, few researchers reported the coal partial gasification in a fluidized bed.5 This paper presents the results obtained in the partial gasification of three different rank coals in a bench-scale fluidized bed at atmosphere pressure with the presence of air and steam. 2. Experimental Section 2.1. Experimental Apparatus. Figure 1 shows a schematic diagram of the fluidized bed system for coal partial gasification in this study. The whole system consists of a start-up burner subsystem, an air supply subsystem, a steam generation subsystem, a coal feed subsystem, a fluidized bed gasifier, and a measurement and control subsystem. The fluidized bed gasifier is made of a 4 mm thick, stainless steel cylinder of 100 mm diameter and 4.4 m height. The coal feed subsystem consists of a frame, a hopper, and a drive subsystem consisting of an electric motor and a speed controller. The cold air from (3) Shinada, O.; Yamada, A.; Koyama, Y. The development of advanced energy technologies in Japan IGCC: A key technology for the 21st century. Energy Convers. Manage. 2002, 43, 1221-1233. (4) Ocampoa, E.; Arenasb, A.; Chejne, F.; Londono, C.; Aguirre; Perez, J. D. An experimental study on gasification of Colombian coal in fluidised bed. Fuel 2003, 82, 161-164. (5) Huang, J. J.; Fang, Y. T.; Chen, H. S.; Wang, Y. Coal gasification characteristic in a pressurized fluidized bed. Energy Fuels 2003, 17, 1474-1479.
10.1021/ef0497558 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/26/2005
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Figure 1. Schematic diagram of the test facility. Table 1. Proximate and Ultimate Analysis of Three Different Rank Coals Cad Bitumite 1 Bitumite 2 Anthracite
60.36 70.40 68.22
ultimate analysis (wt%) Had Oad Nad
Sad
Qnet.ad (MJ/kg)
Aad
3.72 4.54 2.64
0.44 0.63 0.93
24.27 28.91 26.24
22.90 12.62 20.85
7.24 7.86 2.77
0.92 1.24 0.92
Table 2. Distribution of Particle Size of Three Different Rank Coals distribution of particle size (wt %) 0.3-0.4 0.4-0.6 0.6-0.8 0.8-1.0 mean particle mm mm mm mm size (mm) Bitumite 1 Bitumite 2 Anthracite
24 17 12
29 19 26
19 10 24
28 54 38
0.54 0.63 0.61
the blast blower is divided into two pathways, one supply the oxygen for the combustion of diesel oil in the start-up burner and then enter the interlayer to heat the gasifier, the other is preheated by the heat exchanger placed in the start-up burner and then enter the gasifier to fluidize the bed material and supply the reagents for coal gasification after mixing with the steam generated from the steam boiler. At the top of the gasifier, the primary cyclone allows the recovery of entrained particles. The temperature probes and pressure gauges are placed along the height of the gasifier and fuel gas pipe. There is a sampling port of fuel gas downstream of the secondary cyclone. 2.2. Experimental Materials. Three different rank coals were used in the fluidized bed gasifier in this study. Their properties are shown in Tables 1 and 2, respectively. 2.3. Experimental Procedure. Each run was started with the filling of the bed of quartz sand up to the required height. The screw feeder was turned on, and the minimum fluidizing air flow rate required to fluidize the bed materials in the gasifier was supplied through the diesel oil start-up burner. The start-up period was necessary to preheat the bed up to the required temperature before the commencement of coal feeding. When the bed temperature reached 600 °C, coal was added into the fluidized bed gasifier by a screw feeder; the coal feed rate was adjusted to allow a certain excess air in order to achieve complete combustion of coal. When the bed
proximate analysis (wt%) Mad Vad 4.42 2.72 3.67
25.24 30.57 7.97
Cfix 47.44 54.09 67.51
temperature in the gasifier reached the demand of coal gasification, the air, steam, and coal flow rates were adjusted to give the desired equivalence ratio. The fluidized bed gasifier operated at the steady-state condition for an hour. The pressure loss and the temperature were monitored and registered at a 10 min interval. After the bed temperature stabilized, the fuel gas sample was collected for analysis. The coal feeder was stopped once the sample collection and data recording were finished. The whole fluidized bed coal gasification system was shut down after the temperature dropped below the safe temperature. 2.4. Method of Analysis. The fuel gas sampled from the downstream of the secondary cyclone was sent to analyze. The composition of fuel gas was analyzed by a gas chromatography (Shanghai Analytical Instrument Overall Factory Model GC1102). Chromatography calibration was done with standard gas, and the standard deviation curve of the typical component was drawn. Argon was used as the carrier gas at a flow rate of 44 mL/min. The temperature of the chromatography column was 80 °C, and the temperature of the thermal conductivity detector (TCD) was 120 °C. 2.5. Methods of Data Processing. The HHV of fuel gas is defined as the following
HHV ) (XCO × 3018 + XH2 × 3052 + XCH4 × 9500) × 0.01 × 4.1868 (kJ/Nm3) (1) Where, XCO, XH2, XCH4, are the volumetric percentages of CO, H2, CH4 in fuel gas, respectively. The dry gas yield, Y, is figured out from the material balance of nitrogen:
Y)
Qa × 79% WCXN2%
(2)
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Figure 2. Effect of Fa/Fc ratio on gas composition and HHV.
Figure 3. Effect of Fa/Fc ratio on gas yield and carbon conversion.
where, Qa is the flow rate of air (Nm3/h), Wc is the coal feed rate (kg/h), and XN2 is the volumetric percentage of N2 in fuel gas. The carbon conversion is calculated by,
XC )
12Y(CO% + CO2% + CH4%) × 100% 22.4 × C%
(3)
where, Y is the dry gas yield (Nm3/h), C% is the mass percentage of carbon in coal ultimate analysis, and the other symbols are the volumetric percentage of fuel gas compositions.
3. Results and Discussion 3.1. Effect of Fa/Fc Ratio. The effect of the air/coal (Fa/Fc) ratio on the gas composition and the HHV is plotted in Figure 2 for a steam/coal (Fs/Fc) ratio of 0.29. It can be seen that, with the rise of Fa/Fc ratio, the concentrations of carbon monoxide and hydrogen and the HHV increase at first and then decrease, while the concentration of methane decreased from 2.4% to 2.15%. With the rise of air, more oxygen is offered to enhance the combustion of coal and the bed temperature become higher. Therefore, the increase of air favors the oxidation reaction (C + 0.5O2 S CO) leading to the increase of carbon monoxide, the boudouard reaction (C + CO2 S 2CO) leading to the increase of carbon monoxide, the water gas reaction (C + H2O S CO + H2) leading to the increasing of carbon monoxide and hydrogen, and the steam decomposed reaction. In general, the amount of methane produced from chemical reactions is low at low temperature and atmospheric pressure. It is mainly produced from the pyrolysis of volatile content contained in coal during coal gasification.6 With the rise of air, the fluidized velocity increases and the residence time of coal particles in gasifier becomes short. At the same time, the decrease of the C/O molar ratio may lead a small part of carbon monoxide and hydrogen produces in dense phase zone to be oxidized in the lean phase zone. In addition, some more nitrogen brought into the gasifer by fluidizing air will drag out a number of caloric and reduce the concentration of the burnable compositions in fuel gas. The gas HHV is between 2.86 and 3.05 MJ/Nm3. The low HHV obtained is mainly due to a high entrainment of particles as a consequence of a small size of coal used and the short residence time in the gasifier. (6) Fang, Y. T.; Huang, J. J.; Wang, Y.; Zhang, B. J. Experiment and mathematical modeling of a bench-scale circulating fluidized bed gasifier. Fuel Process. Technol. 2001, 69, 29-44.
Figure 4. Effect of Fs/Fc ratio on gas composition and HHV.
The heat losses through the walls and the bare flanges are also important. Figure 3 shows the effect of the Fa/ Fc ratio on gas yield and carbon conversion. It can be noted that the gas yield and carbon conversion increase with the rise of the Fa/Fc ratio. At the fixed coal feed rate and Fs/Fc ratio, the rise of Fa/Fc means more oxygen will react with the carbon of coal, which can provide more heat and favor the increase of gas yield and carbon conversion during coal partial gasification. 3.2. Effect of Fs/Fc Ratio. The effect of the Fs/Fc ratio on the gas composition and HHV is shown in Figure 4 for a Fa/Fc ratio of 4.04. It can be found that the concentration of hydrogen increases with the increase of Fs/Fc ratio, while the concentrations of carbon monoxide, methane, and HHV decrease following a slight increase. With the increase of Fs/Fc ratio, the amount of steam decomposed absolutely increase, but the relative amount of steam decrease. Thus, the efficiency in use of steam decreases. The decrease of bed temperature due to the increase of steam consumed prevents the formation of carbon monoxide. In contrast, the hydrogen content in the gas increases because the steam decomposes more easily than carbon dioxide. The present of steam first favors the water gas reaction (C + H2O S CO + H2) leading to the increase of carbon monoxide and hydrogen, and then promotes the watergas shift reaction (CO + H2O S CO2 + H2) leading to the increase of hydrogen and the decrease of carbon monoxide. Therefore, the change of HHV is shown as above. Due to the increasing rate of hydrogen being
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Figure 5. Effect of Fs/Fc ratio on gas yield and carbon conversion.
Figure 6. Effect of temperature on gas composition and HHV.
smaller than the decreasing rate of carbon monoxide, the HHV takes on a dropping trend. At the same time, the undecomposed steam will lead to the loss of caloric, which also make the HHV become lower. Figure 5 compares the various Fs/Fc ratios for the gas yield and carbon conversion. As the Fs/Fc ratio increases, the gas yield and carbon conversion have a same increasing trend. The water-gas reaction (C + H2O S CO + H2) leads to the increase of the volume of fuel gas. Accordingly, the carbon conversion will be improved at the fixed coal feed rate. 3.3. Effect of Bed Temperature. The effect of bed temperature on the gasification reaction is the most important because the gasification reaction in fluidized bed is mainly controlled by the chemical reaction rate, which is directly correlative with bed temperature. The effect of the bed temperature on the fuel gas composition and HHV is presented in Figure 6. It can be concluded that the concentration of hydrogen increases almost linearly with the bed temperature, while carbon monoxide, methane, and HHV decrease. The composition of fuel gas can be regarded as a function of the temperature for coal partial gasification in a fluidized bed gasifier. The process of coal gasification usually undergoes three steps, which are the initial devolatilization or pyrolysis step that produces volatile matter and a char residue, the secondary reactions involving the volatile products, and finally, the gasification reactions of the remaining carbonaceous residue with steam and
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Figure 7. Effect of temperature on gas yield and carbon conversion.
carbon dioxide.7 The rise of temperature favors the oxidation reaction (C + O2 S CO2 and C + 0.5O2 S CO) leading to the increase of carbon dioxide and carbon monoxide, the boudouard reaction (C + CO2 S 2CO) leading to the increase of carbon monoxide, the water gas reaction (C + H2O S CO + H2) leading to the increasing of carbon monoxide and hydrogen, and the water-gas shift reaction (CO + H2O S CO2 + H2) leading to the increase of hydrogen and the decrease of carbon monoxide. At high temperatures, the water-gas shift reaction may become more dominant. However, to offer the number of caloric needed by the gasification reaction of coal is the main disadvantage of using higher temperature. These energies are only supplied by the partial combustion of carbon or volatiles when the mixtures of steam and oxygen or air are used as gasification medium. Therefore, the concentration of hydrogen in gas increases, while that of carbon monoxide decreases. The increase of gas yield, caused by the rise of temperature, leads to the relative decrease of methane content in fuel gas. In Figure 6, the gas HHV decreases with temperature because the rise of hydrogen is lower than the decrease of carbon monoxide and methane. Figure 7 shows a linear increase in gas yield and carbon conversion with bed temperature. The increase of bed temperature favors the reactions, such as C + 0.5O2 S CO, C + CO2 S 2CO, and C + H2O S CO + H2, which can lead to the increase of the volume of fuel gas. The rise of the combustible compositions in fuel gas, such as monoxide carbon, means more fixed carbons in coal have been converted. At the same time, a rise of bed temperature needs more carbons to combust to offer enough caloric, which implies that more fixed carbons in coal convert into carbon dioxide. Therefore, the carbon conversion increases with bed temperature. 3.4. Effect of Coal Rank. In Figure 8, the effect of the coal rank on the fuel gas composition and HHV is presented. It can be observed that with the increase of coal rank, there has an increase in carbon monoxide, hydrogen, and methane, while a decrease is detected in the HHV. The higher coal rank, the higher volatile contents easily enter the fuel gas during coal pyrolysis. It is proved that about 60-65% volatile contents con(7) Franco, C.; Pinto, F.; Gulyurtlu, I.; Cabrita, I. The study of reactions influencing the biomass steam gasification process. Fuel 2003, 82, 835-842.
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Figure 8. Effect of coal rank on gas composition and HHV.
Figure 9. Effect of coal rank on gas yield and carbon conversion.
tained in coal enter the fuel gas and enhance the HHV during coal gasification. The gas HHV is in direct proportion to the volatile content in coal. The gasification activity of the low rank coal is higher than that of the high rank coal, which favors the partial and complete gasification of coal. In general, the content of fixed carbon in coal increases with the coal rank, it can offer more carbon to react with air and steam and improve the fuel gas yield. The anthracitic coal is very low in volatile matter and very high in fixed carbon so most of reactions of gasification involve the boudouard reaction (C + CO2 S 2CO) leading to the increase of carbon monoxide and the water gas reaction (C + H2O S CO + H2) leading to the increasing of carbon monoxide and hydrogen. In a word, the effect of coal rank on the fuel gas composition and HHV is very complex. The drop of the combustible compositions (include CO, H2, and CH4) with the rank of coal reduces the yield of fuel gas and the conversion of carbon in coal during coal partial gasification (shown in Figure 9). The fluidized bed gasifier is more suitable for high volatile and high reactivity coal gasification. The reactivity of the carbon of low rank coal is higher than that of high rank coal.6
4. Conclusions The atmospheric fluidized bed coal partial gasification experimental results show that the concentration of carbon monoxide in the fuel gas decreases with the increase of Fa/Fc ratio, Fs/Fc ratio, temperature, and coal rank, except a slight increase due to a rise of Fa/ Fc ratio and Fs/Fc ratio at beginning. Hydrogen concentration of fuel gas increases almost linearly with the Fs/Fc ratio and bed temperature, while it almost decreases with the increase of the Fa/Fc ratio and coal rank. Except the Fs/Fc ratio, the rise of the Fa/Fc ratio, bed temperature, and coal rank can reduce the amount of methane in the fuel gas. The gas HHV is between 2.2 and 3.4 MJ/Nm3. It decreases with the increase of bed temperature and coal rank. The gas yield and carbon conversion increase with Fa/Fc ratio, Fs/Fc ratio, and bed temperature, while they decrease with the rise of the rank of coal. Acknowledgment. This work is subsidized by the Chinese Academy of Sciences and the Special Funds for Major (China) State Basic Research Projects (G19990221053). EF0497558
