Article pubs.acs.org/EF
Syngas Production from Coal through Microwave Plasma Gasification: Influence of Oxygen, Steam, and Coal Particle Size Sang Jun Yoon and Jae Goo Lee* Climate Change Technology Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea ABSTRACT: Plasma gasification is widely applied because of its clean syngas production performance and high chemical reactivity accelerated by the free radicals produced by plasma. In the present study, the plasma gasification of four kinds of coal with various conditions of O2/fuel ratio, steam/fuel ratio, and coal particle sizes was performed by using a 2 kW microwave plasma unit. Nitrogen was used as a plasma forming gas. Gasification was conducted with an O2/fuel ratio of 0−1.3, a steam/fuel ratio of 0−1.5, and a coal particle size of 45−150 μm. With an increasing O2/fuel ratio, the H2 composition in the syngas decreased while the CO2 content increased. The CO composition increased until reaching an O2/fuel ratio of approximately 0.6, depending on the coal type, and decreased thereafter. CH4 was not observed in a syngas, which is typical of high temperature plasma gasification. The H2 and CO2 contents tend to increase, and the CO content in the product gas decreased with increasing steam/fuel ratio. The syngas composition produced from plasma gasification at same conditions is affected by the physicochemical properties of coals. The Shenhua coal shows highest H2 production of other coals by plasma gasification. Gasification of coal with high moisture content generates high CO2 composition in the syngas.
1. INTRODUCTION Coal is a cheaper and more plentifully and widely distributed fossil fuel than petroleum. There has been rising interest in clean coal technologies as a result of emerging issues related to serious climate change caused by greenhouse gas emissions. Gasification, a clean energy technology, refers to a process that generates a syngas consisting of hydrogen and carbon monoxide through the partial oxidation of a fuel source. However, the major shortcomings of the existing gasification processes include the following: It is operated at high pressure, requires a long time to heat up during start-up and an expensive air separation unit, and it is not suitable for use with low grade coal. Plasma gasification can compensate for these weaknesses. It is operated under atmospheric pressure and requires a shorter time to elevate to higher temperatures than a conventional entrained-flow gasifier, using external electric energy. Conventional gasification technologies maintain the high temperature required for gasification through the partial oxidation of fuels. Plasma gasification technology, however, achieves a gasification reaction temperature by using a hightemperature plasma flame generated using external electric energy, so that there is less cost and no need for an air separation unit. For the latter, oxidation rarely occurs, which leads to generation of tiny amounts of CO2. According to a thermodynamic analysis of the equilibrium composition in reaction to coal gasification, the resulting gas mainly consists of H2 and CO, with less than 1% being CO2.1 Plasma gasification technology is commonly referred to as “true gasification” or “pure gasification” because it leads to a pure gasification reaction with a rare occurrence of combustion.2,3 Using this technology promotes chemical reactions due to the generation of active particles, including radicals and ions to reduce reaction times.4,5 © 2011 American Chemical Society
Coal is an abundant fuel resource, but, high grade coal is not as abundant as low grade coal throughout the world. A large amount of moisture and ash contributes to the low calorific value of low grade coal. For high moisture coal, various methods to improve its quality by drying are currently being studied.6−8 For high ash coal, a variety of physiochemical deashing methods are being tried.9,10 Using these methods, however, results in time and energy consumption, which is required for drying and deashing, which reduces the efficiency of the overall process. Entrained-flow gasification technology burns some fuel to generate heat and keep gasification temperatures high. For this reason, high quality coal with high calorific value is needed to be used. Plasma gasification technology, however, uses electricity to maintain a temperature higher than that in entrained-flow gasification. Therefore, the technology can be applied to low grade coal, for which keeping gasification temperatures high is difficult as a result of its low calorific value. For fuel technology using plasma torch, researchers have carried out studies of combustion,4,11 pyrolysis,12−14 and gasification15−20 of wastes, biomass, and coal through the generation of a high-temperature plasma flame by using an arc electrode. For gasification technology, supplying the steam should be done to generate syngas containing H2 and CO. Using the arc plasma torch reduces the life expectancy of the electrodes because arc electrodes are vulnerable to moisture.4 Microwave plasma technology provides a better method because it is more resistant to moisture. Using microwaves as an energy source for plasma generation can form a plasma flame by using pure steam21,22 to generate a gasification Received: September 8, 2011 Revised: November 22, 2011 Published: November 23, 2011 524
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Nitrogen was used as a plasma forming gas and was supplied by an mass flow controller (MFC) at the rate of 15 L/min. Oxygen used as a oxidizer was injected using an MFC within the range from 0 to 1.0 L/ min under experimental conditions. Steam used for a gasifying agent was fed to the gasifier by a syringe pump at a rate from 0 to 1.5 mL/ min. The temperature of the feeding line was kept above 100 °C with a band heater, with steam provided to the reactor. The plasma forming gas, oxygen, and steam were fed to the gasifier (2.54 cm in diameter) via a swirl-flow that reduced the thermal impact of the reactor and extended residence times. Two R-type and five K-type thermocouples with an accuracies of ±0.1 °C were spaced 5 cm apart from the coal feeding region to measure the temperature at each reactor position. Syngas generated by the plasma gasification reaction passes the cyclone and filter by removal of unburned carbon, ash, and moisture to go to the gas chromatograph (HP 6890) for quantitative and qualitative analysis. The temperature at each reactor position and the flow and composition of the syngas were monitored and acquired by computer in real time.
reaction at a temperature several thousand degrees Celsius above the operation temperature of conventional entrained flow coal gasification. Most studies using a plasma torch in the literature are concerned with utilization of wastes and biomass. There are few studies of the microwave plasma gasification of various types of coals, including high moisture content coal, with injections of a variety of gasifying agents. In this study, the characteristics of the gasification of various kinds of coals subjected to microwave plasma, used according to the properties of the coals and the operating conditions, were studied. Nitrogen was used as the plasma forming gas. Conventional gasification technology supplies the heat for gasification reaction by partial oxidation of feedstock. Plasma gasification technology, however, provides the heat required for gasification reaction by generating high-temperature plasma flames using external electric energy. Therefore, there is no need to supply oxygen in the reaction. However, when the number of calories supplied by plasma flame is lower than that required for coal gasification, there is a need for partial oxidation through providing oxygen. The more oxygen supplied, the less electric energy consumed.1 Therefore, the changes in the composition of the syngas produced with the varying conditions of the supplied steam and oxygen used as gasifying agents were investigated. Also, the changes in the characteristics of syngas composition and carbon conversion with the size of the coal particles were verified.
3. RESULTS AND DISCUSSION 3.1. Effect of O2/Fuel Ratio. As mentioned earlier, the supplementing of oxygen as a gasification agent affects the syngas composition and gasification performance. Therefore, in this study, microwave plasma gasification experiments for each of the four types of coal under various oxygen supply conditions were performed. Figure 2 shows the compositional changes of the syngas generated after gasification of the four different types of coal (ground to 75 μm in size), according to the O2/fuel ratio. A 1.6 kW nitrogen microwave plasma flame was used, and steam was constantly supplied at a rate of 1 mL/min. There were slight differences in the syngas composition of each type of coal. However, as the O2/fuel ratio increased, the CO2 content of the syngas increased, while H2 content had a tendency to decrease. There was no CH4 in syngas. The CO content had a tendency to increase up to an O2/fuel ratio of 0.6, and it decreased with an increase in the O2/fuel ratio. The H2 content decreased as the O2/fuel ratio increased because H2 is easily burned in an oxygen atmosphere. Therefore, it is adequate to operate without supplying oxygen as a gasifying agent in a process such as that of an integrated gasification fuel cell (IGFC) or in a hydrocracking process in which H2 is needed. To produce synthetic fuel using syngas generated by gasification, it is good to operate within the range of 0.2−0.6 in O2/fuel ratio. The amount of CO in the syngas increased up to an O2/fuel ratio of approximately 0.6, at which partial oxidation occurs. However, it decreased as the ratio rose above 0.8, where complete oxidation occurs and CO is converted to CO2. The variation tendency of syngas composition according to the O2/fuel ratio in plasma gasification shows a similarity to the trends in conventional gasification.23−26 Figure 2a reveals the variation of H2 content in the syngas for each coal with the O2/fuel ratio. WIRA coal showed the highest H2 content, followed by Shenhua coal, Mengtai coal, and Mt. Arthur coal, in which the contents are almost the same. This result is similar to the order of volatile contents shown in the proximate analysis results in Table 1, and it is inversely related to the order of ash contents. Therefore, volatile and ash contents in coal can be considered to be parameters that affect the H2 content in syngas. Experiments on coal with higher volatile content and less ash prove high H2 contents in syngas. Figure 2b shows the variation of the CO content within plasma gasification syngas in each of the four types of coal with the O2/fuel ratio. The results demonstrate that the gasification
2. EXPERIMENTAL SECTION Table 1 shows the results of ultimate, proximate, and higher heating value analysis of coals used in this study. To study the characteristics of
Table 1. Proximate, Ultimate, and Higher Heating Value Analysis of Coals on Air Dried Basis Shenhua coal (China) moisture volatile matter ash fixed carbon carbon hydrogen nitrogen oxygen sulfur HHV
5.17 31.71 5.80 57.32
Mengtai coal (China)
WIRA coal (Indonesia)
Mt. Arthur coal (Australia)
Proximate Analysis (wt %) 22.58 1.29 29.06 44.49 13.10 35.16
10.99 43.23
Ultimate Analysis (wt %) 67.46 57.00 65.46 4.96 4.16 5.83 1.03 0.83 1.33 14.84 24.79 12.49 0.71 0.12 1.05 Higher Heating Value (HHV) Analysis (kcal/kg) 6,465 5,710 6,766
1.04 30.27 15.17 53.52
65.50 4.58 1.79 8.64 0.86 6,433
plasma gasification upon each type of coal, four different grades of coals produced in China, Australia, and Indonesia were used. Samples were pulverized to 45 to 150 μm under experimental conditions. One of the low grade coals, Mengtai coal, which has a high moisture content, and bituminous grade coals, which are generally used in conventional gasifiers, were tested. Figure 1 shows a schematic diagram of the plasma gasifier with a 2 kW microwave generator (2.45 GHZ, SM745, Richardson Electronics) used in this study. The gasifier consists of a coal feeder, microwave generator, feeder for the gasifying agent and plasma forming gas, gasification reactor, gas purifier and analyzer, steam supplier, and data collector. For the coal feeder, 1 g/min of coal was provided to reactor quantitatively using a screw feeder with the accuracy of ±0.01 g/min. 525
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Figure 1. Schematic diagram of the plasma gasification system.
flame, which results in a temperature decrease in the reactor. This lower reaction temperature induces a combustion reaction rather than a gasification reaction. 3.2. Variation of H2/CO Ratio. For the syngas composition produced from gasification of carbonaceous feedstock using conventional gasifiers, there is not a significant change in H2/CO ratio with O2/fuel ratio when the operation of gasifier is carried out where cold gas efficiency is the optimum point.24,25,31 Therefore, a water−gas shift reactor at the rear of the gasifier is additionally used to adjust the H2/CO ratio required for producing synthetic fuels, such as dimethyl ether (DME), methanol, and synthetic liquid fuel, by using syngas. Figure 3 shows the variation of the H2/CO ratio in the plasma gasification syngas of four coals applied in this study, with the O2/fuel ratio. As seen in Figure 3, the H2/CO ratio in syngas changed widely according to the O2/fuel ratio, as compared to that according to the H2/CO ratio in syngas produced from conventional coal gasification. This indicates that syngas composition can be adjusted depending on the application of the syngas. For the production of DME, methanol, and synthetic liquid fuel, the H2/CO ratio should be 1, 2, and 2, respectively. Except for WIRA coal, it is possible to control the H2/CO ratio in a syngas required for synthetic fuel production at O2/fuel ratios from 0.2 to 0.7. Also, in an anaerobic reaction condition, plasma gasification can generate syngas with a much higher hydrogen content compared to that generated with conventional gasification. Therefore, plasma gasification of coal is found to be very effective when using coal as the producing source of the hydrogen required for fuel cell and chemical processes. 3.3. Decomposition of Steam. For plasma gasification when steam is fed, H2 can be produced by decomposition of steam. To find the H2 content generated by the decomposition of steam out of the total syngas, the amount of H2 formed by passing a plasma flame without injecting coal was measured. The steam injection rate was 1 mL/min (the same amount as in
of Shenhua coal released the highest CO content in the syngas, followed by Mt. Arthur coal, WIRA coal, and Mengtai coal, according to the average of the O2/fuel ratio conditions studied in this work, especially the O2/fuel conditions above 0.7. This trend is similar to that of the carbon contents from the ultimate analysis results. It indicates that more carbon contents of coal led to more CO content within the syngas. As for WIRA coal and Mt. Arthur coal, WIRA coal had a higher CO content when the O2/fuel ratio was between 0 and 0.7. and Mt. Arthur coal, in which the volatile content is relatively low and which had almost the same carbon content as WIRA coal, showed higher CO content when the O2/fuel ratio was above 0.7. It is considered that the difference between the volatiles gasified at a relatively low temperature led to this result. The tendency for H2 and CO to increase or decrease according to the amount of O2 in the plasma gasification agrees with the results of previous studies that reported results on calculating the equilibrium composition for producing gas in a plasma environment based on thermodynamic analysis.1 The results of the changes in H2 and CO compositions of syngas with the changes in volatiles, ash, and carbon contents of fuel, are similar to those from gasification in conventional gasifiers.27−30 Therefore, it can be considered that gasification in gasifiers using the microwave plasma flame has the same reaction mechanism as that in conventional gasifiers without using a plasma flame. Figure 2c shows the variation in CO2 content in the syngas for each of the four types of coal with O2/fuel ratio. Mengtai coal had the highest value, followed by Mt. Arthur coal, WIRA coal, and Shenhua coal. This result is contrary to the order of previous CO content results. The larger amount converted to CO in limited carbon contents within coal leads to less carbon that can be converted to CO2. Mengtai coal, which has a high moisture content, shows the highest CO2 content in the syngas. To evaporate the moisture contained in the coal, heat is needed, and this consumes the calories of the given plasma 526
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Figure 3. Variation of H2/CO ratio with O2/fuel ratio.
Figure 4. Variation of syngas composition by decomposition of steam (O2/fuel ratio: 0; steam/fuel ratio: 1).
high-temperature plasma environment. However, as the temperature decreases, the decomposed particles get together to form a stable water molecule. Therefore, there is a rare possibility of generating hydrogen after steam passes through a high-temperature region of a plasma flame. However, as in Figure 4, the decomposition of the steam supplied as a gasifying agent increases by a small amount the H2 content in the syngas. This is one of the features of plasma gasification. 3.4. Effect of Steam/Fuel Ratio. In Figure 5, effect of steam injected as gasifying agent on syngas composition produced from plasma gasification of WIRA coal and Mt. Arthur coal was shown. Generally, for these two types of coal, as the amount of steam injected increased, the H2 and CO2 contents increased, while that of CO decreased. This indicates that a water−gas shift reaction occurred with the injection of steam, and CO reacted with the steam (H2O) to form H2 and CO2. Therefore, the amount of steam injected as a gasifying agent should be increased to increase the H2 content within the syngas. This result can also be found in conventional gasification without using a plasma flame.23,32,33 As mentioned earlier, this demonstrates that the reaction mechanism of coal gasification by using plasma flame is similar to that without using it. In a comparison of the variation of each type of gas in the syngas, it was found that less than 8 vol % of the gas composition changed with the variation of the steam/fuel ratio,
Figure 2. Effects of O2/fuel ratio on the variation of H2 (a), CO (b), and CO2 (c) contents in syngas.
Figure 2), and the O2/fuel ratio was zero. The composition of syngas (case 1) formed by the gasification of Shenhua coal in Figure 4 was compared with that of case 2, except that the H2 content was generated by steam decomposition. It was found that H2 is generated by steam decomposition, but H2 is produced in much large amounts by the gasification of coal. Generally, water molecules can be decomposed into hydrogen, hydrogen ions, and OH radicals by putting them through a 527
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Figure 6. Effect of coal particle size on syngas composition and carbon conversion (O2/fuel ratio: 0.39; steam/fuel ratio: 1).
temperature plasma environment because of the rapid plasma flame, and this leads to low carbon conversion.1,4 Therefore, to overcome this, studies changing the reactor structure are being performed. When designing a plasma coal gasification process for a large plant, there is much energy and cost required for pulverizing coal. Finally, ways of enhancing the residence time of coal under high-temperature plasma flame conditions should be considered to improve coal conversion and gasification efficiency. Some researchers have reported high carbon conversion by using high plasma power with a small amount of injected coal.1 It is possible to increase the residence time of coal by changing the structure within the reactor and controlling the hydraulic conditions within the reactor to improve the efficiency of plasma gasification. Also, it may be possible to extend the residence time of coal in the hightemperature range of the flame by installing several plasma torches within the reactor and using a multistep plasma flame.
Figure 5. Effect of steam/fuel ratio on syngas composition produced from plasma gasification of WIRA coal (a) and Mt. Arthur coal (b) (O2/fuel ratio: 0.39).
4. CONCLUSION In this study, the gasification characteristics of four different types of coal in relation to various coal particle sizes and oxygen and steam injection amounts were studied, using microwave plasma. As the O2/fuel ratio increased for all four types of coals, the H2 content in the syngas decreased, while the CO2 content increased. The CO content increased up to the O2/fuel ratio of 0.6; however, it decreased afterward. As the steam/fuel ratio increased, the H2 and CO2 contents increased, and the CO content decreased. It is estimated that a water−gas shift reaction occurred as the amount of steam fed to the gasifier increased. As the particle size of coal supplied to the gasifier decreased, the H2 content in the syngas decreased, and CO and CO2 contents increased. Varying the plasma gasification conditions of the O2/fuel ratio and steam/fuel ratio, the trends of changing syngas compositions of H2, CO, and CO2 were similar to the trends seen in conventional gasification. This indicates that the reaction mechanism in plasma gasification is similar to that in gasification without using plasma. Plasma gasification technology uses external electric energy for the gasification of coal. Conventional gasification technologies use heat generated by partial oxidation of coal to supply the heat required for gasification. Therefore, there was a difference between the two technologies with regards to the composition of syngas and percentages of constituents. Also, for plasma gasification, a plasma flame whose temperature is over 3000 °C is used and the same reaction can occur in a short residence
while a maximum level of 80 vol % variation was shown for the O2/fuel ratio conditions studied in the present study. Therefore, it is proven that supplying oxygen has a greater influence than providing steam on changing the composition of syngas, where both oxygen and steam are used as gasifying agents. 3.5. Effect of Coal Particle Size. Figure 6 shows changes in the composition of syngas generated by microwave plasma gasification of Shenhua coal and carbon conversion in relation to the particle size of coal. In general, as the particle size of the coal fed into the plasma gasifier decreased, the H2 content in the syngas decreased, while both CO and CO2 increased, except for 75 μm coal particle size condition. Also, as the particle size of the coal decreased, carbon conversion increased from 65% to more than 99%. Carbon conversion was less than 80% where the size of the coal particles was above 75 μm. It is considered that there was not enough detention of devolatilized coal char at the high-temperature plasma flame area required for it to be completely gasified in the area. The center of the microwave plasma flame has a temperature above 3000 °C, while the temperature rapidly drops as the distance from the center increases. Therefore, as coal is fed into the center of the plasma flame and residence time increases, gasification efficiency increases. Many results reported in previous studies reveal that coal has a low residence time under the high528
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time. However, the results reveal that the carbon conversion is less than 90%, even when using 75 μm of coal particles. For microwave plasma flame, the temperature at the center is over 3000 °C, but the temperature rapidly decreases as the distance from the center increases. Supplying coal to the center of the plasma flame and keeping it there is favorable to the gasification of coal. However, for the facility used in this study, coal particles fed to the reactor rapidly escaped from the hightemperature plasma flame along the swirl flow in a short residence time, so the conversion is considered low. It is especially expected that it would be difficult to get a high conversion for coal containing high fixed carbon, and plasma gasification is easy for fuels containing relatively high volatiles. Therefore, for gasification using a plasma torch, realization of a facility for supplying coal to the center of the plasma flame and its detention in it is required.
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AUTHOR INFORMATION Corresponding Author *Telephone: +82428603353. Fax: +82428603134. E-mail:
[email protected].
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ACKNOWLEDGMENTS This research was supported by the National Agenda project of the Ministry of Educational Science and Technology and the Korea Micro Energy Grid project of the Ministry of Knowledge Economy of the Republic of Korea
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