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Hydrogen-rich syngas production through coal and charcoal gasification using microwave steam and air plasma torch
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Hydrogen-rich syngas production through coal and charcoal gasification using microwave steam and air plasma torch Sang Jun Yoon, Jae-Goo Lee* High Efficiency and Clean Energy Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
In the present study, microwave plasma gasification of two kinds of coal and one kind of
Received 4 June 2012
charcoal was performed with various O2/fuel ratios of 0e0.544. Plasma-forming gases used
Received in revised form
under 5 kW microwave plasma power were steam and air. The changes in the syngas
26 July 2012
composition and gasification efficiency in relation to the location of the coal supply to the
Accepted 11 August 2012
reactor were also compared. As the O2/fuel ratio was increased, the H2 and CH4 contents in
Available online 16 September 2012
the syngas decreased, and CO and CO2 increased. When steam plasma was used to gasify the fuel with the O2/fuel ratio being zero, it was possible to produce syngas with a high
Keywords:
content of hydrogen in excess of 60% with an H2/CO ratio greater than 3. Depending on the
Microwave plasma
O2/fuel ratio, the composition of the syngas varied widely, and the H2/CO ratio necessary
Gasification
for using the syngas to produce synthetic fuel could be adjusted by changing the O2/fuel
Hydrogen
ratio alone. Carbon conversion increased as the O2/fuel ratio was increased, and cold gas
Syngas
efficiency was maximized when the O2/fuel ratio was 0.272. Charcoal with high carbon and
Coal
fixed carbon content had a lower carbon conversion and cold gas efficiency than the coals used in this study. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The limited reserves of existing fossil energy resources and environmental pollution due to greenhouse emissions have motivated active research on efficient and clean utilization of energy resources. Coal is the most abundant energy resource available and therefore is the extensively used fuel source globally. Accordingly, interest in finding ways to use it more efficiently and cleanly is rapidly rising. As a bridge leading to the hydrogen and renewable energy economy, efficient utilization of fossil fuels is very important. As gasification technology, a clean coal utilization technology, is considered to have strong potential, a great deal of research is currently being conducted and this technology is being widely utilized [1e6]. Diverse gasification technologies are being developed to
expand the types of fuel available, including cheaper lowgrade fossil fuels. Among these is gasification technology using a plasma torch. The active particles of the radicals and ions contained in the plasma flame, which is thousands of degrees higher than the operating temperature of the existing entrained bed gasification, can catalyze chemical reactions and reduce reaction time [7,8]. On the basis of this advantage, many studies have recently been conducted in this area [9e11]. Previous studies focused on the combustion [7], pyrolysis [11,12], and gasification [13e17] of fuels such as waste, biomass and coal using an arc plasma torch. However, arc plasma generator electrodes have limited lifetime and are vulnerable to steam, which is often used as a gasification agent [7,18]. If microwaves are used as the plasma-forming energy source, no electrode is used, and thus steam can be
* Corresponding author. Tel.: þ82 42 8603353; fax: þ82 42 8603134. E-mail address: [email protected] (J.-G. Lee). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.054
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used not only as a gasification agent, but also as a plasmaforming gas. Also, as a microwave plasma source has a higher power transfer efficiency from the generator to the plasma relative to that of arc plasma technology [19,20], it can improve the efficiency of the entire gasification process. Gases such as nitrogen and air, which are typically used as plasma-forming gases, are mixed with the syngas generated from the gasification of the fuel, and emitted from the gasifier. The large quantity of plasma-forming gas, as used at this time, dilutes the syngas and reduces calories, and hinders the utilization of the syngas in producing electricity, synthetic fuel and chemical products. If steam is used as the plasmaforming gas, it can be removed simply through condensation in the process after the gasifier. This can improve the calories of the syngas, and simplify the gas separation process for its utilization. Also, use of a steam plasma torch makes it possible to generate hydrogen-rich syngas with a great deal of steam in the gasification reaction. In the case of plasma gasification, which exploits the high temperature of the plasma flame, the length of the flame is closely related with its contact time with the fuel as well as efficiency. The length of the plasma flame varies according to the plasma-forming gas species. Accordingly, with regard to gasification of various fuels using a plasma torch, research must be carried out on the optimal mixing ratio between plasma-forming gases, according to the uses of the syngas for hydrogen, manufacturing of synthetic fuel and power generation. Most previous research concerns the use of arc plasma to handle waste and biomass, and most studies on the use of microwave plasma for fuel gasification have employed nitrogen as a plasma-forming gas. In this study, gasification of fuels based on a microwave plasma torch, which uses steam and air as the plasma-forming gases, was conducted for three kinds of fuels (two kinds of coal and one charcoal) under various O2/fuel ratios. In fuel gasification using a plasma flame, efficiency is closely related to the contact method between the high-temperature plasma flame and fuel use. Therefore, changes in the syngas composition and gasification efficiency with the coal supply location to the reactor were compared as well.
2.
Experimental
The results of ultimate, proximate and higher heating value analyses of the two kinds of coal and oak charcoal used in the present study are shown in Table 1. The samples were pulverized to 75 mm in size before use. In addition to bituminous coal, generally used in conventional gasifiers, coal with high moisture content, typically found in low-grade coal, and charcoal, which is a biomass fuel, were used in the experiment. The plasma gasification system with a 6 kW microwave generator (2.45 GHz, SM1280, Richardson Electronics) used in this study is schematically illustrated in Fig. 1. The system consists of a fuel feeder, a microwave generator, a steam supplier, a gasification agent and plasma-forming gas feeding system, a gasification reactor, a gas purifier and analyzer, and a data collector. The fuel feeding system uses a screw feeder to feed a 1.26 kg/h of fuel to the reactor with an accuracy of
Table 1 e Proximate, ultimate, and higher heating value analyses of coals and charcoal on air dried basis. Shenhua coal ECO coal Charcoal (China) (Indonesia) (South Korea) Proximate analysis (wt%) Moisture 5.17 Volatile matter 31.71 Ash 5.80 Fixed carbon 57.32 Ultimate analysis (wt%) Carbon 67.46 Hydrogen 4.96 Nitrogen 1.03 Oxygen 14.84 Sulfur 0.71 Higher heating 27,067 value (kJ/kg)
24.34 37.40 3.48 34.78
0.60 27.61 1.42 70.37
61.30 4.85 1.15 29.18 0.04 25,288
83.33 3.63 0.43 11.19 0.004 30,312
0.01 kg/h. All experiments were conducted under a plasma power condition of 5 kW. Air and steam were used as the plasma-forming gases, and a mass flow controller (MFC) and steam generator were used for quantitatively controlling the supply. The plasma-forming gas supply conditions, applied in this study, are shown in Table 2. The plasma-forming gas supply line is kept over 100 C with a band heater, with the steam phase provided to the gasifier. The plasma-forming gas supplied to a reactor with dimensions of 5.8 cm diameter and 100 cm height; this gas is fed as a swirl-flow, in order to condense the plasma flame and increase the residence time in the reactor. Fuel is supplied after the plasma-forming gas meets the microwave and forms the plasma flame. The gasification characteristics were compared in three directions depending on the fuel supply locations as illustrated in Fig. 2. To measure the temperature at each location of the reactor, two R-type and four K-type thermocouples with 0.1 C accuracy were evenly installed throughout the reactor. The syngas produced as a result of plasma gasification passes through the cyclone and filter to remove unburned carbon, ash, and moisture, and is supplied to a gas chromatograph (HP 6890). A thermal conductivity detector (TCD, Carbosphere 80/100 Packed column, Alltech) is used for quantitative and qualitative analyses of H2, N2, CO, CH4, and CO2. The temperature, flux and syngas composition were monitored and acquired through a computer in real time. At least two experiments were conducted for each condition and the results were averaged for plotting.
3.
Results and discussion
3.1.
Effect of fuel feeding location
In previous studies, gasification of a fuel using a plasma flame shows a low conversion rate (particularly for coal with more fixed carbon contents than biomass). This is thought to be because the fuel is not supplied to the center of the hightemperature plasma flame, or the residence time of the fuel in the reactor is reduced due to the high speed of the plasma flame [7,10,21]. The temperature in the center of the plasma
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Fig. 1 e Schematic diagram of the microwave plasma gasification system.
flame is very high, i.e. thousands of degrees, whereas the temperature drops rapidly moving farther away from the center. Accordingly, supplying fuel to the center of the plasma flame and increasing the residence time in the flame are important factors that can improve the efficiency of gasification. In this study, as depicted in Fig. 2, the gasification of Shenhua coal at three fuel supply locations was conducted and compared. A mixture of 1.1 kg/h of steam and 20 L/min of air was used as a plasma-forming gas, with an O2/fuel ratio of 0.272 and a steam/fuel ratio of 0.873. Fig. 3 shows a comparison of the syngas composition produced after gasification of Shenhua coal at three coal supply locations. When coal is supplied at location #1, the H2, CO, and CO2 contents in the syngas were approximately 32%, 32%, and 35%, respectively, and a small quantity of CH4, i.e. about 1%, was produced. Meanwhile, when coal was supplied at location #2, the contents of H2, CO, and CO2 in the product gas changed to 45%, 29%, and 25%, respectively. In other words, syngas with an increased heating value was obtained with 13% increased H2 and 10% decreased CO2 content. When coal was supplied at fuel supply location #3, the syngas composition was similar to that of location #2, H2 content was somewhat high, and CO and CO2 contents were slightly low. CH4 content was almost the same regardless of the coal supply location.
Fig. 4 illustrates changes in carbon conversion and cold gas efficiency according to the coal supply locations under the same operating conditions. Carbon conversion is calculated according to the mass flow rate of the carbon in the product gas over mass flow rate of carbon in the feedstock. Cold gas efficiency as an indicator of the thermodynamic efficiency of the gasification process is calculated according to the heating value of the product gas over the heating value of the feedstock. When coal was supplied at location #1 in Fig. 2, carbon conversion and cold gas efficiency were the lowest, and when coal was supplied at location #3, efficiencies were the highest. In Fig. 3, at coal supply locations #2 and #3, the syngas composition was almost identical, but carbon conversion and cold gas efficiency were higher at location #3. If the same reactions occur when the coal reacts under the same operating conditions, the composition of the syngas must be similar. When coal is supplied at location #1, however, syngas with relatively low H2 and high CO2 content was obtained. As the coal was devolatilized, it was partially gasified, and the combustion reaction occurred at the same time, whereas little gasification of the char occurred due to the short residence time. Coal supply locations #2 and #3 showed similar gas compositions, and location #3 showed an improved efficiency value. When coal is supplied at location #3, reactions take
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50
Case Case Case Case Case
1 2 3 4 5
Steam (kg/h)
Air (L/min)
O2/fuel ratio
2.2 1.65 1.1 0.6 0
0 10 20 30 40
0 0.136 0.272 0.408 0.544
place while the coal particles remain in the high temperature area of the plasma flame, and thus the amount of product gas is increased. The results in Figs. 3 and 4 show that coal must be supplied at locations #3, as shown in Fig. 2, to generate syngas with high H2 content and a high heating value. Coal supply location #3 is where coal and the plasma flame meet in a space narrower than the diameter of the reactor, and very close to where the plasma flame is formed. These results confirmed that increasing contact between the plasma flame and coal as well as the residence time will make it possible to generate syngas with high H2 content and heating value, and to increase carbon conversion and cold gas efficiency. Accordingly, location #3 was used as the fuel supply location in the following experiments.
3.2.
Effect of O2/fuel ratio on syngas composition
#1 #2 #3
40
30
20
10
0
H2
CO
CH4
CO 2
Fig. 3 e Comparison of syngas composition with coal feeding location.
that are impossible in conventional gasification, i.e. where the O2/fuel ratio is zero, is possible, and it was possible to produce syngas with high H2 content in excess of 60%. When the calories supplied by the plasma flame are less than the level necessary for fuel gasification, it is necessary to supply oxygen to induce partial oxidation. The greater amount of oxygen supplied reduces the electric energy consumption needed for plasma formation by utilizing the heat from the partial oxidation of the fuel for the gasification reaction [21]. Fig. 5 shows that, as oxygen supply increases, CO and CO2 in the syngas increase due to partial oxidation. ECO coal shows the highest content of CO in the syngas followed by Shenhua coal and charcoal. This tendency was similar to that of the volatile matter contents, listed in Table 1. In other words, the higher the volatile matter contents in the fuel, the higher the CO content in the syngas. However, when the O2/fuel ratio was 0.544, the three kinds of fuels showed similar CO contents. This is thought to be due to the characteristics of the volatile matters gasified at a lower temperature than that required for char. As for the CO2 contents in the syngas,
Carbon conversion and cold gas efficiency (%)
Microwave plasma gasification was conducted for two kinds of coal and one kind of charcoal for the condition where steam and air were mixed as plasma-forming gases, as shown in Table 2. The variation of the syngas composition according to the O2/fuel ratio is illustrated in Fig. 5. The H2 contents in the product gas decreased as the O2/fuel ratio increased. The H2 contents in the syngas of the three kinds of fuels used in this study were almost identical. In conventional gasification, as the temperature necessary for gasification is maintained through partial oxidation of the fuel, a certain quantity of oxygen must be supplied. In the meantime, as plasma gasification uses electric energy to form plasma, and uses a high temperature plasma flame to gasify the fuel, oxygen does not need to be supplied. Accordingly, operation under conditions
Syngas composition (vol %, dry, N2 free)
Table 2 e Plasma-forming gas supply conditions.
60
50
Carbon conversion Cold gas efficiency
40
30
20
10
0 #1
Fig. 2 e Schematic diagram of the coal feeding location to the reactor.
#2
#3
Fig. 4 e Comparison of carbon conversion and cold gas efficiency with coal feeding location.
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c
70 Shenhua ECO Charcoal
60
CO2 content (vol.%, dry, N2 free basis)
H2 content (vol.%, dry, N2 free basis)
a
50
40
30
40 Shenhua ECO Charcoal
35
30
25
20
15
20 0.0
0.1
0.2
0.3
0.4
0.5
0.6
10 0.0
O2/fuel ratio
d
60 Shenhua ECO Charcoal
50
CH4 content (vol.%, dry, N2 free basis)
CO content (vol.%, dry, N2 free basis)
b
40
30
20
0.1
0.2
0.3
0.4
0.5
0.6
O2/fuel ratio 6
5
Shenhua ECO Charcoal
4
3
2
1
0
10 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.0
O2/fuel ratio
0.1
0.2
0.3
0.4
0.5
0.6
O2/fuel ratio
Fig. 5 e Effect of O2/fuel ratio on the variation of H2 (a), CO (b), CO2 (c), and CH4 (d) contents in syngas.
contrary to the CO content, charcoal showed the highest level, followed by Shenhua coal and ECO coal. The carbon content in the fuel is limited, and as more carbon is converted into CO, there remain less carbon content that can be converted into CO2. In particular, for charcoal, which has high carbon and fixed carbon content, the CO2 content in the syngas was the highest. The CH4 content in the syngas was less than 4%, and as the O2/fuel ratio increased, the CH4 content decreased as in the general high-temperature gasification reaction. Looking at the CH4 content in the syngas by fuel according to the O2/fuel ratio, Shenhua coal and ECO coal showed almost identical results, and charcoal showed the lowest value. The syngas heating value decreased with increasing O2/fuel ratio. The heating values of syngas produced from Shenhua coal, ECO coal, and charcoal varied from 11,200 to 8,950 kJ/Nm3, 10,700 to 9,590 kJ/Nm3, and 10,400 to 8220 kJ/Nm3, respectively, with an O2/fuel ratio of 0e0.544. As shown in Table 2, when the O2/fuel ratio is 0, pure steam plasma is used, and if the O2/fuel ratio is 0.544, pure air plasma is used. That is, if pure steam plasma is used without an oxygen supply, it is possible to make syngas with the highest H2 and CH4 contents. As the contents of combustible gas are high at this time, the heating value of the syngas is the
highest. If steam is used as the plasma-forming gas, the steam in the syngas can be separated simply by lowering the temperature below 100 C after the gasifier. This indicates that a simple separation process can produce high-purity syngas. In the case where pure air plasma is used, the oxygen contained in the air activates partial oxidation of the fuel, and the CO and CO2 contents in the syngas tended to increase while the H2 and CH4 contents tended to decrease. However, the nitrogen in air not participating in the reaction is emitted from the gasifier along with the syngas. This lowers the heating value, as it dilutes the syngas, which is a disadvantage.
3.3.
Variation of H2/CO ratio
Fig. 6 illustrates the changes in the H2/CO ratio in the syngas produced through plasma gasification according to the O2/fuel ratio of the three kinds of fuel used in this study. The results show that there were some differences between the fuels, but as the O2/fuel ratio changed from 0.0 to 0.544, the H2/CO ratio decreased from 3.5 to 0.5. The syngas can be utilized widely, such as in power generation with the use of an engine and turbine, and the production of hydrogen and synthetic materials. As oil prices have been volatile and generally rising
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4.0
70
3.5
Shenhua ECO Charcoal
Carbon conversion (%)
3.0 2.5
H2/CO
Shenhua ECO Charcoal
60
2.0 1.5 1.0
50
40
30
20
0.5 0.0 0.0
0.1
0.2
0.3
0.4
0.5
10
0.6
0.0
0.1
O2/fuel ratio
0.2
0.3
0.4
0.5
0.6
O2/fuel ratio
Fig. 6 e Variation of H2/CO ratio with O2/fuel ratio.
Fig. 7 e Effect of O2/fuel ratio on carbon conversion.
in recent years, interest in producing various synthetic fuels from syngas has been increasing. To produce synthetic fuels such as methanol, dimethyl ether (DME), synthetic natural gas (SNG), and liquid fuels through FischereTropsch (FT) reactions, the H2/CO ratio must be 1 or greater [22e25], although it varies depending on the catalyst used and materials for production. As for syngas generated through a conventional gasifier without using a plasma torch, the H2/CO ratio does not vary greatly depending on the O2/fuel ratio. Furthermore, the H2/CO ratio in the syngas produced by using the entrained bed gasifier, which one of the widely-used gasifier types, is generally less than 1. Accordingly, to produce synthetic fuels, a wateregas shift reactor needs to be additionally installed behind the gasifier to adjust the H2/CO ratio in the syngas. On the other hand, as shown in Fig. 6, the H2/CO ratio in plasma gasification can be easily adjusted to fit the applications of syngas by changing the O2/fuel ratio. In particular, it was possible to generate syngas with higher H2 content than in conventional gasification without using a plasma flame when steam is used as the plasma-forming gas and the O2/fuel ratio is 0. Accordingly, it was found that plasma gasification of coal and charcoal can be effectively used not only for manufacturing synthetic fuel, but also for fuel cells and chemical processes requiring hydrogen.
As seen in Fig. 7, carbon conversion of Shenhua coal and ECO coal shows almost identical values, whereas charcoal shows very low carbon conversion. This is likely due to the high fixed carbon and carbon content of the charcoal. In gasification using a plasma torch, the residence time of the fuel in the high-temperature plasma flame area greatly affects the conversion of fuel into syngas. When fuel is supplied to the plasma flame, the moisture and volatile materials in the fuel are volatilized first and react, and then the char reacts. At this time, sufficient residence time for the reaction of the char (fixed carbon) in the high-temperature plasma flame area is necessary. Accordingly, charcoal with a high amount of fixed carbon of low reactivity needs a longer residence time than the coals used in this study. As the samples were gasified under the same conditions in the same reactor, a relatively small amount of syngas was produced, and thus the carbon conversion of charcoal was low. In the results presented in Fig. 8, as with carbon conversion, cold gas efficiency of Shenhua coal and ECO coal showed a similar trend and value, whereas charcoal showed a low value. Charcoal had a high heating value, while according to
40
Effect of O2/fuel ratio on gasification efficiency
Figs. 7 and 8 show the variation of carbon conversion and cold gas efficiency during microwave plasma gasification of Shenhua coal, ECO coal and charcoal depending on the O2/fuel ratios. As the O2/fuel ratio was increased, carbon conversion increased due to increased CO and CO2 content in the syngas. On the other hand, cold gas efficiency shows the maximum value when the O2/fuel ratio is 0.272. When the O2/fuel ratio was lower than 0.272, the heating value of the syngas was high, but a small amount of syngas was produced; the cold gas efficiency was consequently low. When the O2/fuel ratio was higher than 0.272, more syngas was generated, but the heating value of the syngas was low; the cold gas efficiency was consequently low.
Cold gas efficiency (%)
3.4.
45 Shenhua ECO Charcoal
35
30
25
20
15 0.0
0.1
0.2
0.3
0.4
0.5
O2/fuel ratio
Fig. 8 e Effect of O2/fuel ratio on cold gas efficiency.
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the syngas composition in Fig. 5, syngas produced from charcoal gasification has high CO2 content and low CO and CH4 contents, resulting in a low heating value. Also, as a large amount of low-reactivity char in the charcoal was not sufficiently gasified, a small amount of syngas was produced, and thus the cold gas efficiency was low. As for fuel gasification using steam plasma when the O2/fuel ratio is 0, it was possible to produce syngas with high H2 and CH4 contents, and a high heating value. On the other hand, as a small amount of syngas was generated, the carbon conversion and cold gas efficiency were low. When pure air with an O2/fuel ratio condition of 0.544 was used as the plasma-forming gas, the oxygen in the air led to partial oxidation of the fuel, and the CO and CO2 contents in the syngas increased. On the other hand, as H2 and CH4 contents decreased, the heating value of the syngas decreased. However, as the amount of syngas production increased, improved carbon conversion and cold gas efficiency value were shown relative to the case of steam plasma using gasification.
4.
Conclusions
In this study, the microwave plasma gasification of two kinds of coal and one kind of charcoal was conducted using 5 kW steam and air plasma torch. First, variation of the syngas composition, carbon conversion, and cold gas efficiency were measured and compared depending on the location of the fuel supply to the reactor. When coal was supplied near where the plasma flame is formed using microwaves, the contact and residence times between the plasma flame and the fuel increase, thereby making it possible to produce syngas with high hydrogen content and heating value, resulting in improved carbon conversion and cold gas efficiency. At the optimal fuel supply location identified in this study, the gasification characteristics of the fuels were studied at different mixing ratios of steam and air, which are the plasma-forming gases. When pure steam was used as the plasma-forming gas, it was possible to produce a high heating value syngas with high H2 content exceeding 60%. On the other hand, the carbon conversion and cold gas efficiency were low. When air was used as the plasma-forming gas, syngas with low H2 content and high CO and CO2 contents was produced. However, the carbon conversion and cold gas efficiency were higher than when steam was used as the plasmaforming gas. In the case of where a mixture of steam and air was used as the plasma-forming gas, the maximum cold gas efficiency was shown when the O2/fuel ratio was 0.272. The carbon conversion and cold gas efficiency of charcoal with high fixed carbon and carbon contents were lower than the values obtained with the coals used in this study.
Acknowledgments This research was supported by the Korea Micro Energy Grid project of the Ministry of Knowledge Economy and was partially funded by Korea Institute of Energy Research.
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[23] Kaltschmitt M, Hartmann H, Hofbauer H. Energie aus biomasse. Berlin, Heidelberg: Springer-Verlag; 2009. [24] Garcia-Trenco A, Martinez A. Direct synthesis of DME from syngas on hybrid CuZnAl/ZSM-5 catalysts: new insights into the role of zeolite acidity. Appl Catal A-Gen 2012;411: 170e9. [25] Lee S, Gogate MR, Kulik CJ. A novel single-step dimethyl ether (DME) synthesis in a three-phase slurry reactor from CO-rich syngas. Chem Eng Sci 1992;47:3769e76.
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Hydrogen-rich syngas production through coal and charcoal gasification using microwave steam and air plasma torch Sang Jun Yoon, Jae-Goo Lee* High Efficiency and Clean Energy Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
In the present study, microwave plasma gasification of two kinds of coal and one kind of
Received 4 June 2012
charcoal was performed with various O2/fuel ratios of 0e0.544. Plasma-forming gases used
Received in revised form
under 5 kW microwave plasma power were steam and air. The changes in the syngas
26 July 2012
composition and gasification efficiency in relation to the location of the coal supply to the
Accepted 11 August 2012
reactor were also compared. As the O2/fuel ratio was increased, the H2 and CH4 contents in
Available online 16 September 2012
the syngas decreased, and CO and CO2 increased. When steam plasma was used to gasify the fuel with the O2/fuel ratio being zero, it was possible to produce syngas with a high
Keywords:
content of hydrogen in excess of 60% with an H2/CO ratio greater than 3. Depending on the
Microwave plasma
O2/fuel ratio, the composition of the syngas varied widely, and the H2/CO ratio necessary
Gasification
for using the syngas to produce synthetic fuel could be adjusted by changing the O2/fuel
Hydrogen
ratio alone. Carbon conversion increased as the O2/fuel ratio was increased, and cold gas
Syngas
efficiency was maximized when the O2/fuel ratio was 0.272. Charcoal with high carbon and
Coal
fixed carbon content had a lower carbon conversion and cold gas efficiency than the coals used in this study. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The limited reserves of existing fossil energy resources and environmental pollution due to greenhouse emissions have motivated active research on efficient and clean utilization of energy resources. Coal is the most abundant energy resource available and therefore is the extensively used fuel source globally. Accordingly, interest in finding ways to use it more efficiently and cleanly is rapidly rising. As a bridge leading to the hydrogen and renewable energy economy, efficient utilization of fossil fuels is very important. As gasification technology, a clean coal utilization technology, is considered to have strong potential, a great deal of research is currently being conducted and this technology is being widely utilized [1e6]. Diverse gasification technologies are being developed to
expand the types of fuel available, including cheaper lowgrade fossil fuels. Among these is gasification technology using a plasma torch. The active particles of the radicals and ions contained in the plasma flame, which is thousands of degrees higher than the operating temperature of the existing entrained bed gasification, can catalyze chemical reactions and reduce reaction time [7,8]. On the basis of this advantage, many studies have recently been conducted in this area [9e11]. Previous studies focused on the combustion [7], pyrolysis [11,12], and gasification [13e17] of fuels such as waste, biomass and coal using an arc plasma torch. However, arc plasma generator electrodes have limited lifetime and are vulnerable to steam, which is often used as a gasification agent [7,18]. If microwaves are used as the plasma-forming energy source, no electrode is used, and thus steam can be
* Corresponding author. Tel.: þ82 42 8603353; fax: þ82 42 8603134. E-mail address: [email protected] (J.-G. Lee). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.054
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used not only as a gasification agent, but also as a plasmaforming gas. Also, as a microwave plasma source has a higher power transfer efficiency from the generator to the plasma relative to that of arc plasma technology [19,20], it can improve the efficiency of the entire gasification process. Gases such as nitrogen and air, which are typically used as plasma-forming gases, are mixed with the syngas generated from the gasification of the fuel, and emitted from the gasifier. The large quantity of plasma-forming gas, as used at this time, dilutes the syngas and reduces calories, and hinders the utilization of the syngas in producing electricity, synthetic fuel and chemical products. If steam is used as the plasmaforming gas, it can be removed simply through condensation in the process after the gasifier. This can improve the calories of the syngas, and simplify the gas separation process for its utilization. Also, use of a steam plasma torch makes it possible to generate hydrogen-rich syngas with a great deal of steam in the gasification reaction. In the case of plasma gasification, which exploits the high temperature of the plasma flame, the length of the flame is closely related with its contact time with the fuel as well as efficiency. The length of the plasma flame varies according to the plasma-forming gas species. Accordingly, with regard to gasification of various fuels using a plasma torch, research must be carried out on the optimal mixing ratio between plasma-forming gases, according to the uses of the syngas for hydrogen, manufacturing of synthetic fuel and power generation. Most previous research concerns the use of arc plasma to handle waste and biomass, and most studies on the use of microwave plasma for fuel gasification have employed nitrogen as a plasma-forming gas. In this study, gasification of fuels based on a microwave plasma torch, which uses steam and air as the plasma-forming gases, was conducted for three kinds of fuels (two kinds of coal and one charcoal) under various O2/fuel ratios. In fuel gasification using a plasma flame, efficiency is closely related to the contact method between the high-temperature plasma flame and fuel use. Therefore, changes in the syngas composition and gasification efficiency with the coal supply location to the reactor were compared as well.
2.
Experimental
The results of ultimate, proximate and higher heating value analyses of the two kinds of coal and oak charcoal used in the present study are shown in Table 1. The samples were pulverized to 75 mm in size before use. In addition to bituminous coal, generally used in conventional gasifiers, coal with high moisture content, typically found in low-grade coal, and charcoal, which is a biomass fuel, were used in the experiment. The plasma gasification system with a 6 kW microwave generator (2.45 GHz, SM1280, Richardson Electronics) used in this study is schematically illustrated in Fig. 1. The system consists of a fuel feeder, a microwave generator, a steam supplier, a gasification agent and plasma-forming gas feeding system, a gasification reactor, a gas purifier and analyzer, and a data collector. The fuel feeding system uses a screw feeder to feed a 1.26 kg/h of fuel to the reactor with an accuracy of
Table 1 e Proximate, ultimate, and higher heating value analyses of coals and charcoal on air dried basis. Shenhua coal ECO coal Charcoal (China) (Indonesia) (South Korea) Proximate analysis (wt%) Moisture 5.17 Volatile matter 31.71 Ash 5.80 Fixed carbon 57.32 Ultimate analysis (wt%) Carbon 67.46 Hydrogen 4.96 Nitrogen 1.03 Oxygen 14.84 Sulfur 0.71 Higher heating 27,067 value (kJ/kg)
24.34 37.40 3.48 34.78
0.60 27.61 1.42 70.37
61.30 4.85 1.15 29.18 0.04 25,288
83.33 3.63 0.43 11.19 0.004 30,312
0.01 kg/h. All experiments were conducted under a plasma power condition of 5 kW. Air and steam were used as the plasma-forming gases, and a mass flow controller (MFC) and steam generator were used for quantitatively controlling the supply. The plasma-forming gas supply conditions, applied in this study, are shown in Table 2. The plasma-forming gas supply line is kept over 100 C with a band heater, with the steam phase provided to the gasifier. The plasma-forming gas supplied to a reactor with dimensions of 5.8 cm diameter and 100 cm height; this gas is fed as a swirl-flow, in order to condense the plasma flame and increase the residence time in the reactor. Fuel is supplied after the plasma-forming gas meets the microwave and forms the plasma flame. The gasification characteristics were compared in three directions depending on the fuel supply locations as illustrated in Fig. 2. To measure the temperature at each location of the reactor, two R-type and four K-type thermocouples with 0.1 C accuracy were evenly installed throughout the reactor. The syngas produced as a result of plasma gasification passes through the cyclone and filter to remove unburned carbon, ash, and moisture, and is supplied to a gas chromatograph (HP 6890). A thermal conductivity detector (TCD, Carbosphere 80/100 Packed column, Alltech) is used for quantitative and qualitative analyses of H2, N2, CO, CH4, and CO2. The temperature, flux and syngas composition were monitored and acquired through a computer in real time. At least two experiments were conducted for each condition and the results were averaged for plotting.
3.
Results and discussion
3.1.
Effect of fuel feeding location
In previous studies, gasification of a fuel using a plasma flame shows a low conversion rate (particularly for coal with more fixed carbon contents than biomass). This is thought to be because the fuel is not supplied to the center of the hightemperature plasma flame, or the residence time of the fuel in the reactor is reduced due to the high speed of the plasma flame [7,10,21]. The temperature in the center of the plasma
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Fig. 1 e Schematic diagram of the microwave plasma gasification system.
flame is very high, i.e. thousands of degrees, whereas the temperature drops rapidly moving farther away from the center. Accordingly, supplying fuel to the center of the plasma flame and increasing the residence time in the flame are important factors that can improve the efficiency of gasification. In this study, as depicted in Fig. 2, the gasification of Shenhua coal at three fuel supply locations was conducted and compared. A mixture of 1.1 kg/h of steam and 20 L/min of air was used as a plasma-forming gas, with an O2/fuel ratio of 0.272 and a steam/fuel ratio of 0.873. Fig. 3 shows a comparison of the syngas composition produced after gasification of Shenhua coal at three coal supply locations. When coal is supplied at location #1, the H2, CO, and CO2 contents in the syngas were approximately 32%, 32%, and 35%, respectively, and a small quantity of CH4, i.e. about 1%, was produced. Meanwhile, when coal was supplied at location #2, the contents of H2, CO, and CO2 in the product gas changed to 45%, 29%, and 25%, respectively. In other words, syngas with an increased heating value was obtained with 13% increased H2 and 10% decreased CO2 content. When coal was supplied at fuel supply location #3, the syngas composition was similar to that of location #2, H2 content was somewhat high, and CO and CO2 contents were slightly low. CH4 content was almost the same regardless of the coal supply location.
Fig. 4 illustrates changes in carbon conversion and cold gas efficiency according to the coal supply locations under the same operating conditions. Carbon conversion is calculated according to the mass flow rate of the carbon in the product gas over mass flow rate of carbon in the feedstock. Cold gas efficiency as an indicator of the thermodynamic efficiency of the gasification process is calculated according to the heating value of the product gas over the heating value of the feedstock. When coal was supplied at location #1 in Fig. 2, carbon conversion and cold gas efficiency were the lowest, and when coal was supplied at location #3, efficiencies were the highest. In Fig. 3, at coal supply locations #2 and #3, the syngas composition was almost identical, but carbon conversion and cold gas efficiency were higher at location #3. If the same reactions occur when the coal reacts under the same operating conditions, the composition of the syngas must be similar. When coal is supplied at location #1, however, syngas with relatively low H2 and high CO2 content was obtained. As the coal was devolatilized, it was partially gasified, and the combustion reaction occurred at the same time, whereas little gasification of the char occurred due to the short residence time. Coal supply locations #2 and #3 showed similar gas compositions, and location #3 showed an improved efficiency value. When coal is supplied at location #3, reactions take
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50
Case Case Case Case Case
1 2 3 4 5
Steam (kg/h)
Air (L/min)
O2/fuel ratio
2.2 1.65 1.1 0.6 0
0 10 20 30 40
0 0.136 0.272 0.408 0.544
place while the coal particles remain in the high temperature area of the plasma flame, and thus the amount of product gas is increased. The results in Figs. 3 and 4 show that coal must be supplied at locations #3, as shown in Fig. 2, to generate syngas with high H2 content and a high heating value. Coal supply location #3 is where coal and the plasma flame meet in a space narrower than the diameter of the reactor, and very close to where the plasma flame is formed. These results confirmed that increasing contact between the plasma flame and coal as well as the residence time will make it possible to generate syngas with high H2 content and heating value, and to increase carbon conversion and cold gas efficiency. Accordingly, location #3 was used as the fuel supply location in the following experiments.
3.2.
Effect of O2/fuel ratio on syngas composition
#1 #2 #3
40
30
20
10
0
H2
CO
CH4
CO 2
Fig. 3 e Comparison of syngas composition with coal feeding location.
that are impossible in conventional gasification, i.e. where the O2/fuel ratio is zero, is possible, and it was possible to produce syngas with high H2 content in excess of 60%. When the calories supplied by the plasma flame are less than the level necessary for fuel gasification, it is necessary to supply oxygen to induce partial oxidation. The greater amount of oxygen supplied reduces the electric energy consumption needed for plasma formation by utilizing the heat from the partial oxidation of the fuel for the gasification reaction [21]. Fig. 5 shows that, as oxygen supply increases, CO and CO2 in the syngas increase due to partial oxidation. ECO coal shows the highest content of CO in the syngas followed by Shenhua coal and charcoal. This tendency was similar to that of the volatile matter contents, listed in Table 1. In other words, the higher the volatile matter contents in the fuel, the higher the CO content in the syngas. However, when the O2/fuel ratio was 0.544, the three kinds of fuels showed similar CO contents. This is thought to be due to the characteristics of the volatile matters gasified at a lower temperature than that required for char. As for the CO2 contents in the syngas,
Carbon conversion and cold gas efficiency (%)
Microwave plasma gasification was conducted for two kinds of coal and one kind of charcoal for the condition where steam and air were mixed as plasma-forming gases, as shown in Table 2. The variation of the syngas composition according to the O2/fuel ratio is illustrated in Fig. 5. The H2 contents in the product gas decreased as the O2/fuel ratio increased. The H2 contents in the syngas of the three kinds of fuels used in this study were almost identical. In conventional gasification, as the temperature necessary for gasification is maintained through partial oxidation of the fuel, a certain quantity of oxygen must be supplied. In the meantime, as plasma gasification uses electric energy to form plasma, and uses a high temperature plasma flame to gasify the fuel, oxygen does not need to be supplied. Accordingly, operation under conditions
Syngas composition (vol %, dry, N2 free)
Table 2 e Plasma-forming gas supply conditions.
60
50
Carbon conversion Cold gas efficiency
40
30
20
10
0 #1
Fig. 2 e Schematic diagram of the coal feeding location to the reactor.
#2
#3
Fig. 4 e Comparison of carbon conversion and cold gas efficiency with coal feeding location.
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c
70 Shenhua ECO Charcoal
60
CO2 content (vol.%, dry, N2 free basis)
H2 content (vol.%, dry, N2 free basis)
a
50
40
30
40 Shenhua ECO Charcoal
35
30
25
20
15
20 0.0
0.1
0.2
0.3
0.4
0.5
0.6
10 0.0
O2/fuel ratio
d
60 Shenhua ECO Charcoal
50
CH4 content (vol.%, dry, N2 free basis)
CO content (vol.%, dry, N2 free basis)
b
40
30
20
0.1
0.2
0.3
0.4
0.5
0.6
O2/fuel ratio 6
5
Shenhua ECO Charcoal
4
3
2
1
0
10 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.0
O2/fuel ratio
0.1
0.2
0.3
0.4
0.5
0.6
O2/fuel ratio
Fig. 5 e Effect of O2/fuel ratio on the variation of H2 (a), CO (b), CO2 (c), and CH4 (d) contents in syngas.
contrary to the CO content, charcoal showed the highest level, followed by Shenhua coal and ECO coal. The carbon content in the fuel is limited, and as more carbon is converted into CO, there remain less carbon content that can be converted into CO2. In particular, for charcoal, which has high carbon and fixed carbon content, the CO2 content in the syngas was the highest. The CH4 content in the syngas was less than 4%, and as the O2/fuel ratio increased, the CH4 content decreased as in the general high-temperature gasification reaction. Looking at the CH4 content in the syngas by fuel according to the O2/fuel ratio, Shenhua coal and ECO coal showed almost identical results, and charcoal showed the lowest value. The syngas heating value decreased with increasing O2/fuel ratio. The heating values of syngas produced from Shenhua coal, ECO coal, and charcoal varied from 11,200 to 8,950 kJ/Nm3, 10,700 to 9,590 kJ/Nm3, and 10,400 to 8220 kJ/Nm3, respectively, with an O2/fuel ratio of 0e0.544. As shown in Table 2, when the O2/fuel ratio is 0, pure steam plasma is used, and if the O2/fuel ratio is 0.544, pure air plasma is used. That is, if pure steam plasma is used without an oxygen supply, it is possible to make syngas with the highest H2 and CH4 contents. As the contents of combustible gas are high at this time, the heating value of the syngas is the
highest. If steam is used as the plasma-forming gas, the steam in the syngas can be separated simply by lowering the temperature below 100 C after the gasifier. This indicates that a simple separation process can produce high-purity syngas. In the case where pure air plasma is used, the oxygen contained in the air activates partial oxidation of the fuel, and the CO and CO2 contents in the syngas tended to increase while the H2 and CH4 contents tended to decrease. However, the nitrogen in air not participating in the reaction is emitted from the gasifier along with the syngas. This lowers the heating value, as it dilutes the syngas, which is a disadvantage.
3.3.
Variation of H2/CO ratio
Fig. 6 illustrates the changes in the H2/CO ratio in the syngas produced through plasma gasification according to the O2/fuel ratio of the three kinds of fuel used in this study. The results show that there were some differences between the fuels, but as the O2/fuel ratio changed from 0.0 to 0.544, the H2/CO ratio decreased from 3.5 to 0.5. The syngas can be utilized widely, such as in power generation with the use of an engine and turbine, and the production of hydrogen and synthetic materials. As oil prices have been volatile and generally rising
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4.0
70
3.5
Shenhua ECO Charcoal
Carbon conversion (%)
3.0 2.5
H2/CO
Shenhua ECO Charcoal
60
2.0 1.5 1.0
50
40
30
20
0.5 0.0 0.0
0.1
0.2
0.3
0.4
0.5
10
0.6
0.0
0.1
O2/fuel ratio
0.2
0.3
0.4
0.5
0.6
O2/fuel ratio
Fig. 6 e Variation of H2/CO ratio with O2/fuel ratio.
Fig. 7 e Effect of O2/fuel ratio on carbon conversion.
in recent years, interest in producing various synthetic fuels from syngas has been increasing. To produce synthetic fuels such as methanol, dimethyl ether (DME), synthetic natural gas (SNG), and liquid fuels through FischereTropsch (FT) reactions, the H2/CO ratio must be 1 or greater [22e25], although it varies depending on the catalyst used and materials for production. As for syngas generated through a conventional gasifier without using a plasma torch, the H2/CO ratio does not vary greatly depending on the O2/fuel ratio. Furthermore, the H2/CO ratio in the syngas produced by using the entrained bed gasifier, which one of the widely-used gasifier types, is generally less than 1. Accordingly, to produce synthetic fuels, a wateregas shift reactor needs to be additionally installed behind the gasifier to adjust the H2/CO ratio in the syngas. On the other hand, as shown in Fig. 6, the H2/CO ratio in plasma gasification can be easily adjusted to fit the applications of syngas by changing the O2/fuel ratio. In particular, it was possible to generate syngas with higher H2 content than in conventional gasification without using a plasma flame when steam is used as the plasma-forming gas and the O2/fuel ratio is 0. Accordingly, it was found that plasma gasification of coal and charcoal can be effectively used not only for manufacturing synthetic fuel, but also for fuel cells and chemical processes requiring hydrogen.
As seen in Fig. 7, carbon conversion of Shenhua coal and ECO coal shows almost identical values, whereas charcoal shows very low carbon conversion. This is likely due to the high fixed carbon and carbon content of the charcoal. In gasification using a plasma torch, the residence time of the fuel in the high-temperature plasma flame area greatly affects the conversion of fuel into syngas. When fuel is supplied to the plasma flame, the moisture and volatile materials in the fuel are volatilized first and react, and then the char reacts. At this time, sufficient residence time for the reaction of the char (fixed carbon) in the high-temperature plasma flame area is necessary. Accordingly, charcoal with a high amount of fixed carbon of low reactivity needs a longer residence time than the coals used in this study. As the samples were gasified under the same conditions in the same reactor, a relatively small amount of syngas was produced, and thus the carbon conversion of charcoal was low. In the results presented in Fig. 8, as with carbon conversion, cold gas efficiency of Shenhua coal and ECO coal showed a similar trend and value, whereas charcoal showed a low value. Charcoal had a high heating value, while according to
40
Effect of O2/fuel ratio on gasification efficiency
Figs. 7 and 8 show the variation of carbon conversion and cold gas efficiency during microwave plasma gasification of Shenhua coal, ECO coal and charcoal depending on the O2/fuel ratios. As the O2/fuel ratio was increased, carbon conversion increased due to increased CO and CO2 content in the syngas. On the other hand, cold gas efficiency shows the maximum value when the O2/fuel ratio is 0.272. When the O2/fuel ratio was lower than 0.272, the heating value of the syngas was high, but a small amount of syngas was produced; the cold gas efficiency was consequently low. When the O2/fuel ratio was higher than 0.272, more syngas was generated, but the heating value of the syngas was low; the cold gas efficiency was consequently low.
Cold gas efficiency (%)
3.4.
45 Shenhua ECO Charcoal
35
30
25
20
15 0.0
0.1
0.2
0.3
0.4
0.5
O2/fuel ratio
Fig. 8 e Effect of O2/fuel ratio on cold gas efficiency.
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the syngas composition in Fig. 5, syngas produced from charcoal gasification has high CO2 content and low CO and CH4 contents, resulting in a low heating value. Also, as a large amount of low-reactivity char in the charcoal was not sufficiently gasified, a small amount of syngas was produced, and thus the cold gas efficiency was low. As for fuel gasification using steam plasma when the O2/fuel ratio is 0, it was possible to produce syngas with high H2 and CH4 contents, and a high heating value. On the other hand, as a small amount of syngas was generated, the carbon conversion and cold gas efficiency were low. When pure air with an O2/fuel ratio condition of 0.544 was used as the plasma-forming gas, the oxygen in the air led to partial oxidation of the fuel, and the CO and CO2 contents in the syngas increased. On the other hand, as H2 and CH4 contents decreased, the heating value of the syngas decreased. However, as the amount of syngas production increased, improved carbon conversion and cold gas efficiency value were shown relative to the case of steam plasma using gasification.
4.
Conclusions
In this study, the microwave plasma gasification of two kinds of coal and one kind of charcoal was conducted using 5 kW steam and air plasma torch. First, variation of the syngas composition, carbon conversion, and cold gas efficiency were measured and compared depending on the location of the fuel supply to the reactor. When coal was supplied near where the plasma flame is formed using microwaves, the contact and residence times between the plasma flame and the fuel increase, thereby making it possible to produce syngas with high hydrogen content and heating value, resulting in improved carbon conversion and cold gas efficiency. At the optimal fuel supply location identified in this study, the gasification characteristics of the fuels were studied at different mixing ratios of steam and air, which are the plasma-forming gases. When pure steam was used as the plasma-forming gas, it was possible to produce a high heating value syngas with high H2 content exceeding 60%. On the other hand, the carbon conversion and cold gas efficiency were low. When air was used as the plasma-forming gas, syngas with low H2 content and high CO and CO2 contents was produced. However, the carbon conversion and cold gas efficiency were higher than when steam was used as the plasmaforming gas. In the case of where a mixture of steam and air was used as the plasma-forming gas, the maximum cold gas efficiency was shown when the O2/fuel ratio was 0.272. The carbon conversion and cold gas efficiency of charcoal with high fixed carbon and carbon contents were lower than the values obtained with the coals used in this study.
Acknowledgments This research was supported by the Korea Micro Energy Grid project of the Ministry of Knowledge Economy and was partially funded by Korea Institute of Energy Research.
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