Energy & Fuels 2007, 21, 2335-2339
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Characteristics of Carbon Dioxide Reforming of Methane via Alternating Current (AC) Corona Plasma Reactions Ming-Wei Li,*,†,‡ Yi-Ling Tian,† and Gen-Hui Xu§ Department of Chemistry and School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, and Center for Energy and Processes, Ecole des Mines de Paris, 06904-Sophia Antipolis, France ReceiVed March 24, 2007. ReVised Manuscript ReceiVed May 18, 2007
Carbon dioxide reforming of methane via plasma reactions has the double advantage of realizing energy conversion and protecting the environment. We studied the characteristics of AC corona plasma CO2-reforming reactions at atmospheric pressure. The effects of the CH4/CO2 ratios in the feeds, discharge power, and flow rates were investigated. The H2/CO ratios in the products mainly depended upon the CH4/CO2 ratios, and they increased from 0.21 at 0.2 CH4/CO2 to 2.15 at 2.0 CH4/CO2. The conversions of CH4 and CO2 increased with increasing discharge power, whereas they decreased with increasing flow rates. Within the tested range, the conversions of CH4 changed from 67.5 to 90.5% and the conversions of CO2 changed from 45.7 to 78.5%. The conversions obtained in the reactor below 380 K were higher than the calculated data, resulting from equilibrium thermodynamics at 890 K. Comparing the experimental results with the calculated values, we speculate that corona plasma possibly has a high-temperature character and that the plasma reactions possess an inherent quenching mechanism; i.e., the gases were rapidly cooled as they flowed out of the corona plasma area. This quenching mechanism resulted in the corona plasma reactions possessing higher conversions. The H2/CO ratios in the products could be used to support this hypothesis.
Introduction Synthesis gas can be transformed into liquid fuels via Fischer-Tropsch synthesis that has been applied in industry.1 Three major processes, including partial oxidation, steam reforming, and carbon dioxide reforming of methane, may be used to produce synthesis gas.2-4 Moreover, they are also the main routes for producing hydrogen, which may become an important energy carrier in the future. However, from the viewpoint of energy conversion, partial oxidation of methane
CH4 + 1/2O2 f 2H2 + CO ∆Ho ) -35.7 kJ/mol (1) is an exothermic reaction. According to equilibrium thermodynamics, reaction 1 should have a lower H2 yield at higher temperature (>1000 K), which is often used to accelerate the reactions. In addition, there always exists a competing reaction in which CH4 is completely oxidized into CO2, and it should result in the loss of CH4. To increase the energy efficiency, H2O or CO2 may be used to react with CH4 and they form steam reforming (reaction 2) or CO2 reforming (reaction 3).
CH4 + H2O f 3H2 + CO ∆Ho ) 250.2 kJ/mol
(2)
CH4 + CO2 f 2H2 + 2CO ∆Ho ) 247.3 kJ/mol (3) * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry, Tianjin University. ‡ Ecole des Mines de Paris. § School of Chemical Engineering and Technology, Tianjin University. (1) van der Laan, G. P.; Beenackers, A. A. C. M. Catal. ReV.sSci. Eng. 1999, 41, 255-318. (2) Hu, Y. H.; Ruckenstein, E. AdV. Catal. 2004, 48, 297-345. (3) Rostrup-Nielsen, J. R.; Sehested, J.; Noerskov, J. K. AdV. Catal. 2002, 47, 65-139. (4) Bradford, M. C. J.; Vannice, M. A. Catal. ReV.sSci. Eng. 1999, 41, 1-42.
They are both endothermic. In comparison to steam reforming, CO2 reforming is of particular interest because it can convert the main greenhouse gas CO2. Additionally, CH4 and CO2 exist together in many natural gas fields.5 Therefore, the research on CO2 reforming has attracted increasing attention. CO2 reforming via conventional thermal reactions, regardless of whether catalysts were used, however, often had a serious problem involving coke formation at high temperature.4-7 In recent years, nonequilibrium plasma methods have been applied in various chemical reactions.8,9 Within nonequilibrium plasma, there are many energetic free electrons, which can initiate various reactions via inelastic collisions. Simultaneously, the nonequilibrium plasma systems often show a lower temperature.8,10 Therefore, it is possible to realize CO2 reforming via plasma reactions, which often have the advantages of less coke formation, higher reaction rates, and higher energy efficiency. Dielectric barrier discharge (DBD) and corona discharge are the two most attractive methods to form nonequilibrium plasma applied in CO2-reforming reactions in industry, because they could be generated at atmospheric or higher pressure. The characteristic of DBD is that one or both of the electrodes are covered by a dielectric layer.8 When enough high alternating current (AC) voltage is applied, a large number of microdischarges should be formed between the electrodes. The dielectric layer not only limits the amount of charge transported by a single microdischarge but also distributes the microdischarges over the entire electrode area. In CO2-reforming reactions via DBD, (5) Suhartanto, T.; York, A. P. E.; Hanif, A.; Al-Megren, H.; Green, M. L. H. Catal. Lett. 2001, 71, 49-54. (6) Inui, T. Appl. Organometal. Chem. 2001, 15, 87-94. (7) Souza, M. M. V. M.; Aranda, D. A. G.; Schmal, M. Ind. Eng. Chem. Res. 2002, 41, 4681-4685. (8) Eliasson, B.; Kogelschatz, U. IEEE Trans. Plasma Sci. 1991, 19, 1063-1077. (9) Ostrikov, K. ReV. Mod. Phys. 2005, 77, 489-511. (10) Denysenko, I. B.; Xu, S.; Long, J. D.; Rutkevych, P. P.; Azarenkov, N. A.; Ostrikov, K. J. Appl. Phys. 2004, 95, 2713-2724.
10.1021/ef070146k CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
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the products contained not only synthesis gas but also higher hydrocarbons and oxygenates.11-14 Corona discharge could be initiated using a pair of inhomogeneous electrodes by direct current (DC) or AC high-voltage electric field. In comparison to DBD, corona discharge is more easily established. Actually, in the famous Miller experiment, which successfully synthesized amino acids under the conditions simulating the atmosphere of primitive Earth, corona discharge was used to initiate the plasma reactions.15,16 The widely applied electrostatic precipitator is a device that uses corona discharge to control pollution.9 Corona plasma technologies were also applied in the investigations of oxidative coupling of methane,17 decomposition of CO2,18,19 and CO2 reforming.20,21 We have previously investigated the CO2 reforming using DC corona discharge plasma reactions,21 and it was found that the plasma reactions had higher conversions of reactants. The corona discharge polarity, being negative or positive, influenced the conversions, and the reactions using the positive corona always had higher conversions than the negative corona. When the CH4/CO2 ratios in the feeds were 1:2, the synthesis gas in the products had lower H2/CO ratios around 0.56, which is a potential feedstock for directly synthesizing liquid fuels via Fischer-Tropsch synthesis. Through the CO2-reforming plasma reactions, electric energy from clean energy, such as hydroelectricity, solar energy, and nuclear energy, could be converted into chemical energy stored in synthesis gas. However, the energy efficiency of the plasma reactions was below 16% in our tested range. Herein, we investigated the CO2 reforming via AC corona plasma reactions and compared the results from the AC and DC corona plasma reactions. According to the characteristics of the reactions, we suggest that corona plasma reactions had an inherent quenching mechanism and it resulted in higher conversions and corresponding H2/CO ratios. This mechanism possibly helps us better understand the corona plasma reactions. Experimental Section Experimental Apparatus. The schematic diagram of the experiment is shown in Figure 1, which is similar to that reported previously.21 A quartz tube with an inner diameter of 13.2 mm containing a pair of pin-plate stainless-steel electrodes was used as the plasma reactor. The upper pin electrode was positioned with its top 10 mm above the plate electrode. The flow rates of the two reactants, CH4 (>99.9%) and CO2 (>99.5%), were controlled by a mass-flow controller, respectively. The reactants were well-mixed in different molar ratios from 0.2 to 2.0 CH4/CO2 and flowed through the reactor at room temperature and atmospheric pressure. A high-voltage AC power supply with a frequency of 20 kHz was used to initiate corona discharge. The (11) Huang, A.; Xia, G.; Wang, J.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. J. Catal. 2000, 189, 349-359. (12) Zhang, K.; Kogelschatz, U.; Eliasson, B. Energy Fuels 2001, 15, 395-402. (13) Li, Y.; Liu, C.-J.; Eliasson, B.; Wang, Y. Energy Fuels 2002, 16, 864-870. (14) Zou, J.-J.; Zhang, Y.-P.; Liu, C.-J.; Li, Y.; Eliasson, B. Plasma Chem. Plasma Process. 2003, 23, 69-82. (15) Miller, S. L. Science 1953, 117, 528-529. (16) Bada, J. L.; Lazcano, A. Science 2003, 300, 745-746. (17) Liu, C.; Marafee, A.; Hill, B.; Xu, G.; Mallinson, R.; Lobban, L. Ind. Eng. Chem. Res. 1996, 35, 3295-3301. (18) Wang, J.-Y.; Xia, G.-G.; Huang, A.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. J. Catal. 1999, 185, 152-159. (19) Xu, W.; Li, M.-W.; Xu, G.-H.; Tian, Y.-L. Jpn. J. Appl. Phys., Part 1 2004, 43, 8310-8311. (20) Yang, Y. Ind. Eng. Chem. Res. 2002, 41, 5918-5926. (21) Li, M.-W.; Xu, G.-H.; Tian, Y.-L.; Chen, L.; Fu, H.-F. J. Phys. Chem. A 2004, 108, 1687-1693.
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Figure 1. Schematic diagram of the experimental apparatus: A, highvoltage AC power supply; B, pin electrode; C, plate electrode; D, plasma reactor; E, AC corona discharge; F, quadrupole mass spectrometer; G, cold trap; H, flow meter; I, gas chromatograph.
discharge voltage and current were read from the display of the power supply showing both current and voltage, and they were calibrated by a high-voltage probe (Tektronix P6015) and a current probe (Tektronix CT-2) with a digital oscilloscope (Tektronix TDS 210) during the plasma reactions. The effects of the CH4/CO2 ratios in the feeds, discharge power, and flow rates of the feeds on the products were investigated. Under each set of conditions, 30 min was needed for stabilizing the reaction before quantitative analyses. Analyses and Calculations. The compounds of the effluents from the reactor were first qualitatively analyzed by an online quadrupole mass spectrometer (QMS) (Balzers MSC 200). Then, the effluents were introduced into a cold trap to remove the liquid products, and the gas products were quantitatively analyzed by an online gas chromatograph equipped with a thermal conductivity detector. The flow rates of the gas effluents were measured by a soap-film flow meter to carry out the conversion calculations and the balance calculations of carbon. The conversions (X) of CH4 and CO2, selectivities (S) of H2 and CO, and balance calculations (B) of carbon were defined as X(CH4) (%) ) ([CH4]in - [CH4]out)/[CH4]in × 100% X(CO2) (%) ) ([CO2]in - [CO2]out)/[CO2]in × 100% S(H2) (%) ) 0.5[H2]out/([CH4]in - [CH4]out) × 100% S(CO) (%) ) [CO]out/([CH4]in - [CH4]out + [CO2]in [CO2]out) × 100% B(C) (%) ) (1- ([CH4]out + [CO2]out + [CO]out)/ ([CH4]in + [CO2]in)) × 100% where [CH4]in and [CO2]in are the flow rates of the introduced reactants and [CH4]out, [CO2]out, [H2]out, and [CO]out are the flow rates of the corresponding compositions in the gas effluents.
Results and Discussion Effects of the CH4/CO2 Ratios. At a certain flow rate (60 mL/min) and discharge power (45 W), the effects of CH4/CO2 ratios in the feeds changing from 0.2 to 2.0 were investigated. As shown in Figure 2a, the conversions of CO2 increased from 45.7 to 64.9% with increasing CH4/CO2 ratios, whereas the conversions of CH4 decreased from 90.5 to 68.9%. The conversions of CH4 were always higher than that of CO2. Figure 2b indicates that increasing CH4/CO2 ratios in the feeds resulted
AC Corona Plasma CO2-Reforming Reactions
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Figure 3. Effects of the discharge power on the conversions of reactants via AC corona plasma CO2-reforming reactions. Flow rate, 60 mL/min; CH4/CO2 ratio, 1:2. Figure 2. Effects of the CH4/CO2 ratios on the (a) conversions of reactants and (b) H2/CO ratios in the products obtained via AC corona plasma CO2-reforming reactions. Flow rate, 60 mL/min; discharge power, 45 W.
Table 2. Effects of the Discharge Power on the Plasma Reactionsa discharge power (W)
H2/CO ratio
S(H2) (%)
S(CO) (%)
B(C) (%)
27 36 45 54 63
0.61 0.59 0.57 0.58 0.57
71.7 70.8 70.6 72.8 70.9
90.1 93.4 96.0 94.5 95.1
6.0 4.2 2.6 3.8 3.9
Table 1. Effects of the CH4/CO2 Ratios in the Feeds on the Plasma Reactionsa
a
CH4/CO2 ratio
S(H2) (%)
S(CO) (%)
B(C) (%)
1:5 1:4 1:3 1:2 1:1 2:1
35.3 43.8 57.0 70.6 92.0 98.1
97.7 97.7 95.8 96.0 83.5 62.0
1.2 1.4 2.6 2.6 11.1 25.70
a
Flow rate, 60 mL/min; CH4/CO2 ratio, 1:2.
Flow rate, 60 mL/min; discharge power, 45 W.
in the proportional increase of H2/CO ratios in the products. With the CH4/CO2 ratios increasing from 0.2 to 2.0, the H2/CO ratios increased from 0.21 to 2.15. The H2/CO ratios were always a little higher than the CH4/CO2 ratios. As listed in Table 1, the selectivities of H2 in the products increased with increasing CH4/CO2 ratios and they increased from 35.3% and reached the maximum 98.1% at 2:1 CH4/CO2. However, the selectivities of CO decreased from 97.7 to 62.0%, simultaneously. Besides main products of H2 and CO, QMS detected that other chemicals including C2-C6 hydrocarbons and water were also produced. The balance calculations of carbon can be used to estimate the yields of carbon-containing chemicals except CO. The data of B(C) listed in Table 1 indicate that the concentration of carbon-containing chemicals except CO increased with increasing CH4/CO2 ratios. This implies that higher hydrocarbons, oxygenates, or coke was easily formed at higher CH4/CO2 ratios. Effects of the Discharge Power. As shown in Figure 3, for a mixture of 1:2 CH4/CO2, flow rate of 60 mL/min, and discharge power increasing from 27 to 63 W, the conversions of CH4 increased from 69.8 to 89.7%, and the conversions of CO2 increased from 55.4 to 73.4%. The conversions of CH4 were always higher than that of CO2. Table 2 shows that the H2/CO ratios varied between 0.57 and 0.61 with increasing discharge power and the selectivities of H2 and CO changed within narrow ranges from 70.6 to 72.8% and from 90.1 to 96.0%, respectively. Noticeably, the H2/CO ratios and both selectivities of H2 and CO hardly changed with
Figure 4. Effects of the flow rates on the conversions of reactants via AC corona plasma CO2-reforming reactions. CH4/CO2 ratio, 1:2; discharge power, 45 W.
increasing discharge power. The balance calculations of carbon and QMS analyses indicated that a small amount of carboncontaining chemicals except CO was formed in the reaction. Effects of the Flow Rates. As depicted in Figure 4, at 1:2 CH4/CO2 and a discharge power of 45 W, both the conversions of CH4 and CO2 decreased with increasing flow rates from 30 to 90 mL/min. The conversions of CH4 decreased from 87.3 to 67.5%, and the conversions of CO2 decreased from 78.5 to 47.9%. Table 3 shows that the H2/CO ratios increased from 0.56 to 0.69 with increasing flow rates. Increasing flow rates also resulted in some changes for the selectivities of both CO and H2. Effects of the Corona Types. In comparison to our previous investigation involving DC corona plasma CO2-reforming reactions,21 the conversions and H2/CO ratios via the AC corona type were between the values obtained in the negative and
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Table 3. Effects of the Flow Rates of Reactants on the Plasma Reactionsa flow rate (mL/min)
H2/CO ratio
S(H2) (%)
S(CO) (%)
B(C) (%)
30 45 60 75 90
0.57 0.56 0.57 0.62 0.69
76.2 69.8 70.6 72.0 75.8
95.8 92.7 96.0 88.3 90.5
3.4 5.5 2.6 7.5 5.2
a
CH4/CO2 ratio, 1:2; discharge power, 45 W.
Figure 6. Effects of the temperature on the (a) conversions of reactants and (b) H2/CO ratios in the products calculated according to equilibrium thermodynamics. CH4/CO2 ratio, 1:2; pressure, 1.0 atm.
Figure 5. Equilibrium H2/CO ratios at 1000 and 2000 K and the measured values in the plasma CO2-reforming reactions changed with the CH4/CO2 ratios (the data of negative and positive corona come from the literature21). Flow rate, 60 mL/min; discharge power, 45 W.
positive corona types. Illustratively, the H2/CO ratios varying with CH4/CO2 ratios are shown in Figure 5. For all three types, the H2/CO ratios almost linearly increased with increasing CH4/ CO2 ratios and the H2/CO ratios were always slightly higher than that of CH4/CO2. The H2/CO ratios obtained using corona plasma reactions in different corona types increased in the order negative corona > AC corona > positive corona. We speculate that this result is related to the different generation mechanisms of corona discharge, and it will be discussed later. Energy efficiency reflects the ability of converting electric energy to chemical energy in the plasma reactions.21 The energy efficiency via AC corona was generally below the value via positive corona; i.e., it was below 16%. Therefore, much work needs to be done to make the corona plasma CO2-reforming reaction an economical method. We speculate that light and heat produced during corona discharge decreased the energy efficiency. Experimentally, higher energy efficiency was obtained at larger flow rates, and in this case, there was less heat accumulated in the reactor. Characteristics of the Corona Plasma Reactions. In our experiments, AC corona discharge was initiated at room temperature, but the temperature of the reactor increased because of the conversion of electricity into heat and became stable about 10 min after the reactions started. Temperature is an important condition for chemical reactions. However, for the corona discharge area, its temperature is nonuniform and is difficult to measure because of both the high-voltage electric field and higher pressure. The measurement of temperature is complicated by the discharge probably occurring between the electrode and the thermocouple. Using the method described in the literature,22 we measured and estimated the gas temperature in the reactor below 380 K during the reaction.
Remarkably, the plasma reactions had far higher conversions than the equilibrium values at 380 K. These experimental results are puzzling. Herein, the equilibrium calculations of CH4 plus CO2 systems were carried out by the method based on the minimization of Gibbs free energy in closed systems.23 The assumed products include CH4, CO2, H2, CO, H2O, benzene (C6H6), and C2-C3 hydrocarbons. According to the calculated results, the total equilibrium concentration of C6H6 and C2-C3 hydrocarbons is always below 0.1% and may be ignored. The main calculated results are shown in Figures 5 and 6. At 1:2 CH4/CO2 and a temperature increasing from 400 to 1500 K, the equilibrium conversions of CH4 and CO2 increase with increasing temperature, as shown in Figure 6a. The equilibrium conversions of CH4 increase from 64.9 to 95.8% with increasing temperature from 890 to 1000 K, and the values of CO2 increase from 46.5 to 79.2% with increasing temperature from 890 to 1500 K. Experimentally, as shown in Figure 4, with decreasing flow rates, the conversions of CH4 increased from 67.5 to 87.3% and the conversions of CO2 increased from 47.9 to 78.5%. It indicates, within the reactor below 380 K, that the plasma reactions had the conversion only possibly reached at above 890 K if via conventional thermal reactions. Moreover, the corona discharge formed a small plasma volume in the reactor, and the measured conversions probably did not reach their maximum. Therefore, through optimizing reaction conditions and designing new reactors, conversions and energy efficiency should be increased via the plasma reactions. As shown in Figure 6b, the equilibrium H2/CO ratios increases from 0.08 to the maximum at about 0.70 with increasing temperature from 400 to about 1000 K and then they decrease with increasing temperature. Because of the endothermic reaction, CO2 + H2 f H2O (g) + CO (∆Ho ) 41.2 kJ/mol), the concentration of H2 decreases but the concentration of CO increases with increasing temperature; therefore, the equilibrium H2/CO ratios decrease from about 1000 K. The H2/CO ratios (22) Marafee, A.; Liu, C.; Xu, G.; Mallinson, R.; Lobban, L. Ind. Eng. Chem. Res. 1997, 36, 632-637. (23) Heuze´, O.; Presles, H.-N.; Bauer, P. J. Chem. Phys. 1985, 83, 47344737.
AC Corona Plasma CO2-Reforming Reactions
are higher than 0.52 when the temperature is above 800 K at 1:2 CH4/CO2. To realize the measured H2/CO ratios between 0.57 and 0.61 via thermal reactions, the reaction temperature must be higher than 800 K. To explain the high conversions obtained in experiments, we suggest that there existed a quenching mechanism during the corona plasma reactions. It has been accepted by most researchers that there are free electrons with high kinetic energy or temperature in the corona plasma area and that these energetic electrons play an important role in plasma reactions. Therefore, the corona plasma possibly has a high-temperature character, which is endowed by either the energetic free electrons or the nonequilibrium corona plasma. Literature reported that there was an increase of about 100 K from room temperature at 5 mm from DC air-glow discharge, which was generated between a pair of wire-plate electrodes, but the emission spectroscopy revealed the plasma possessing rotational temperature of 1550 K.24 Therefore, the similar corona discharge used in our experiments probably also had a high-temperature character. When the reactants flowed into the corona plasma area, they reacted and realized higher conversions at high temperature and then they were rapidly cooled, i.e., quenched, as they flowed out of the plasma area. Thus, the higher conversions were realized and kept in the products. It was reported that a small hole was formed on a piece of silicon wafer used as a plate cathode electrode during the needle-plate corona discharge.25 In our previous experiments, it was found that the tip of the stainless-steel needle anode became red and blunted during the corona discharge.21 However, these phenomena had not been found when a tungsten needle anode was used in the preparation of carbon nanotubes.26 The melting points of silicon and tungsten are 1693 and 3683 K, respectively. Therefore, the corona plasma in these systems possibly had a high-temperature character. According to this hypothesis, the H2/CO ratios in the products should have no considerable change during the quenching process and could reveal the characteristic of the temperature of corona plasma. The measured H2/CO ratios were compared with the equilibrium H2/CO ratios calculated at 1000 and 2000 K, as shown in Figure 5. Noticeably, although the equilibrium H2/CO ratios increase with increasing CH4/CO2 ratios at both 1000 and 2000 K, at lower CH4/CO2 ratios, the equilibrium H2/ CO ratios at 1000 K are higher than the corresponding values at 2000 K, whereas it is the other way around at higher CH4/ CO2 ratios. There is an intersection at ≈0.8 CH4/CO2 between the two calculated lines. It indicates that there is the same H2/ CO ratio at 1000 and 2000 K. A similar situation occurs in Figure 6b, and there are the same H2/CO ratios at two different temperatures at the two sides of the temperature (∼1000 K), where the H2/CO ratio has the highest value. It is found that the equilibrium H2/CO ratios are appreciably less than the CH4/ CO2 ratios, and it is different with the measured results; i.e., the measured H2/CO ratios were appreciably higher than the CH4/CO2 ratios. The possible reason is that there were more chemicals than our assumption formed in the reactions. However, because of the total yields of these uncalculated chemicals being less than 0.1%, the calculated results still can be used to discuss the characteristics of the corona plasma reactions. Comparing the data shown in Figure 5, we speculate the plasma temperature always being higher than 1000 K and possibly being higher than 2000 K at high CH4/CO2 ratios, such as 2.0. At low CH4/CO2 ratios, such as 0.2, the H2/CO ratios are between the equilibrium values at 1000 and 2000 K, and it implies that (24) Staack, D.; Farouk, B.; Gutsol, A.; Fridman, A. Plasma Source Sci. Technol. 2005, 14, 700-711. (25) Hesamzadeh, H.; Ganjipour, B.; Mohajerzadeh, S.; Khodadadi, A.; Mortazavi, Y.; Kiani, S. Carbon 2004, 42, 1043-1047. (26) Li, M.-W.; Hu, Z.; Wang, X.-Z.; Wu, Q.; Chen, Y.; Tian, Y.-L. J. Mater. Sci. 2004, 39, 283-284.
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the plasma temperature decreased with decreasing CH4/CO2 ratios. Our explanation is that the reactants with low CH4/CO2 ratios consumed more energy to convert more CO2, thus resulting in the temperature decreasing. As mentioned above, the H2/CO ratios increased in the order negative corona > AC corona > positive corona, indicating that the polarity of corona influenced the composition of the products. Positive and negative corona have different generation mechanisms.27,28 The positive corona possessed higher conversions because it had a larger active volume and higher temperature than negative corona. More CO2 molecules can be dissociated into CO via positive corona; therefore, positive corona reactions had lower H2/CO ratios. If AC corona is deemed to be a combination of a half-period negative corona and a half-period positive corona,29,30 it is reasonable that the H2/CO ratios and conversions obtained via AC corona plasma reactions were between the values obtained via the other two DC corona types. The quenching mechanism can well explain the experimental results of the corona plasma CO2-reforming reactions. Actually, quenching technology had even been applied in high-temperature arc plasma reactions to influence the distribution of the final products.31,32 As early as the 1940s, an arc plasma process with water spray quenching, the Hu¨els process, had even been used to produce acetylene from various hydrocarbons.33 In comparison to the complicated and energy-wasted quenching arc plasma process, the corona plasma reaction undoubtedly has its advantages. Conclusions In this work, the CO2 reforming via AC corona plasma was investigated. The experimental results were compared with our previous report involving DC corona plasma CO2 reforming. In all three corona types, including AC, DC negative, and DC positive corona, the higher conversions of methane and carbon dioxide were realized in a simple plasma reactor and the H2/ CO ratios in the products strongly depended upon the CH4/ CO2 ratios in the feeds. The conversions increased in the order positive corona > AC corona > negative corona, whereas H2/ CO ratios in the products exhibited the opposite order. It was found that the corona plasma reactions often had higher conversions than the calculated values according to equilibrium thermodynamics. We suggest that the corona plasma has a hightemperature character and that the corona plasma reactions have an inherent quenching mechanism, which made the reactions realize higher conversion and corresponding H2/CO ratios in the products. EF070146K (27) Chang, J.-S.; Lawless, P. A.; Yamamoto, T. IEEE Trans. Plasma Sci. 1991, 19, 1152-1166. (28) Grange´, F.; Soulem, N.; Loiseau, J. F.; Spyrou, N. J. Phys. D: Appl. Phys. 1995, 28, 1619-1629. (29) Zhang, C. H.; MacAlpine, J. M. K. IEEE Trans. Dielectr. Electr. Insul. 2003, 10, 312-319. (30) van Brunt, R. J. IEEE Trans. Dielectr. Electr. Insul. 1994, 1, 761784. (31) Baddour, R. F.; Iwasyk, J. M. Ind. Eng. Chem. Process Des. DeV. 1962, 1, 169-176. (32) Nishimura, Y.; Takenouchi, T. Ind. Eng. Chem. Fundam. 1976, 15, 266-269. (33) Fincke, J. R.; Anderson, R. P.; Hyde, T.; Detering, B. A.; Wright, R.; Bewley, R. L.; Haggard, D. C.; Swank, W. D. Plasma Chem. Plasma Process. 2002, 22, 105-136.
