Methane Conversion and Reforming by Nonthermal Plasma on Pins

High-Efficient Conversion of CO2 in AC-Pulsed Tornado Gliding Arc Plasma. Jing-Lin Liu , Hyun-Woo Park .... Qi Wang , Bin-Hang Yan , Yong Jin , Yi Che...
2 downloads 0 Views 192KB Size
Download PDF
5918

Ind. Eng. Chem. Res. 2002, 41, 5918-5926

Methane Conversion and Reforming by Nonthermal Plasma on Pins Yun Yang† Institute of Low-temperature Plasma-physics, Friedrich-Ludwig-Jahn-Strasse 19, D-17489 Greifswald, Germany

Nonthermal plasma on pins means nonthermal or nonequilibrium plasmas produced between two pin electrodes, pin-to-plane, or multipin configurations working at atmospheric pressure. In this paper, it is shown that nonthermal plasma on pins can produce not only the corona discharges but also the homogeneous glow discharge and pseudo glow discharge by using a 1-2 kHz ac power supply. The electrode configurations and input power control the amplitude and duration of the discharge current pulses. It is found that the discharge conditions determined the product selectivity in direct nonoxidative methane conversion. The glow discharge produces acetylene, ethylene, and ethane in approximately equal quantities, while the corona discharge is mainly selective to acetylene. In the pseudo glow discharge, the product distribution can be controlled by the specific energy input. With lower specific energy input, the product distribution is almost the same as that in glow discharge, while with higher specific energy input, it is mainly selective to acetylene. Pseudo glow discharges have been studied for the CO2 reforming of methane to synthesis gas. With one pair of pins and 60-70 kJ/L specific energy input, CH4 and CO2 conversions were higher than 70% and 60%, respectively. The H2/CO ratio could be controlled and depended on the concentration of CO2 in the feed gas. No carbon or soot formation was detected in the CO2-rich feed (CO2 content > 50%). Introduction The conversion or reforming of methane into more valuable higher hydrocarbons, hydrogen, or syngas (CO + H2) represents a great chemical and technological challenge for both the chemical engineering and petrochemical industries and has been studied intensively. Gaseous plasma acting as a catalyst is a good source for generating active species, including electrons, ions, and radicals. A well-established process is the direct conversion of methane into acetylene in thermal arc plasma; this process has been developed and practiced for more than 40 years by Hu¨ls1 and has been developed for hydrogen productions.2 Nonthermal plasmas are currently being investigated as a promising alternative room temperature method to convert methane to higher hydrocarbons.3 Dielectric barrier discharge (DBD) is the most commonly used method of atmospheric pressure, nonthermal or nonequilibrium plasma. The basic principle of this technique is that the major part of electrical energy is transferred to energetic electrons and active radical species rather than to heat gas. Many studies now also combine the DBD with some certain catalysts to reform methane; for example, Kraus et al.4 studied the CO2 reforming of methane by combination of solidstate catalysts with DBD, Matsumoto et al.5 studied selective oxidation of methane with nitrous oxide in a DBD reactor, etc. Liu et al. studied the methane conversion by using corona discharge (CD) and by combining the plasma with heterogeneous catalysts such as zeolite.6,7 It has been found that methane was converted mainly to acetylene over NaY zeolite in a dc CD. A recent study has shown direct conversion of methane to acetylene by using dc pulse CD at atmospheric pressure with a †

Present address: Mechanical & Manufacturing Engineering Department, Loughborough University, Loughborough LE11 3TU, U.K. E-mail: [email protected].

higher selectivity8 without using any catalysts. It remains unclear whether methane is converted to acetylene mainly through the catalytic effects of discharge in a volume process or through a surface reaction on the catalysts inside the plasma or both. Nonthermal plasma on pins means nonthermal plasma produced between two pin electrodes or a pins-to-plane or even multipin configurations working at atmospheric pressure and ambient room temperature. When the electrical fields between two electrodes are strong enough, the discharge breaks down and produces the current streamers. Usually it is a CD and is one kind of nonthermal plasma working at atmospheric pressure. Recently, these kinds of discharges have attracted much attention; not only can it directly convert the methane to acetylene,8 but also it can use CO2 reforming of CH4 to produce syngas.9 The oxidative coupling and reforming of CH4 with CO2 can produce C2H4, CO, and H2.10 Yao et al. studied the important factors and discharge features in methane conversion using a high-frequency pulsed CD.11,12 In this paper we report that the nonthermal plasma on pins not only can produce streamer-like CD but also can produce the homogeneous glow discharge (GD) and pseudo glow discharge (PD). The different types of nonthermal plasmas, e.g., DBD, CD, homogeneous GD, and PD stabilized in one reactor at atmospheric pressure and a 1-2 kHz ac supply, were studied and applied to the nonoxidative CH4 conversion and CO2 reforming of CH4. The most important features of this study are that we can realize these four kinds of discharges, especially those of GD and PD in any gases including air and at atmospheric pressure, and can study the catalytic effects of these different discharges. As is known, the main drawbacks of nonthermal plasma technology for methane conversion account for higher energy consumption or low-energy efficiency and low selectivity toward desired hydrocarbons. An alter-

10.1021/ie0202322 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/31/2002

Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 5919

Figure 1. Schematic diagram of the experiment.

native approach to this problem implies a deep understanding of the basic physical and chemical processes, the status or properties of the plasma itself with relation to catalytic effects, and the development of a quantitative description for the system under consideration. Only then may it become possible to choose the most effective chemical pathway toward the target product, to distinguish the limiting elementary steps, and to try to promote or replace them. Within the framework of such a general strategy, we undertake an experimental and theoretical study and emphasize a systematic analysis and comparative studies of methane converted to higher hydrocarbons in different types of discharges. The effects of methane conversion, selective excitation of one reactant, input specific energy, and product distributions and yields were investigated. In the end, the CO2 reforming of methane to synthesis gas (CO + H2) was also studied by PD and a comparison with CD was also presented. Experimental Section 1. Experimental Arrangement. A schematic diagram of the experiment is shown in Figure 1. The experiment was carried out in a quartz glass tube with an internal diameter of 18 mm. An ac high-voltage power supply was done by means of a transformer. The power supply provided 0-480 V rectangular-like voltage waveforms to the transformer primary. In the transformer secondary, a high voltage (1-10 kV) with a frequency between 1 and 2 kHz was produced and applied to the electrodes. A transmission line (TL) section was utilized which was helpful in stabilizing the discharge. A digital oscilloscope (400 MHz, 2 GS/s) recorded the voltages and current signals. The current was measured with a grounded resistor in series with the electrode and was also measured by a fast current probe. The power input to the discharge was calculated by integrating the product of voltage and current as a function of time. The reactant gas or mixture was introduced using electronic mass flow controllers (MKS Mass Flo). In the experiments of direct methane conversion, the pure methane were fed, and in the experiment of CO2 reforming of methane, the different mixtures of methane and CO2 were controlled by mass flow controllers. The feed gas and products from the reactor were analyzed online by a GC-MS (HP G1800C GCD) and a quadruple

mass spectrometer (Balzers QMS 200). The flow gas was sampled and passed through a split to a CP-porabond Qcapillarycolumn(32m×0.25mmi.d.byCHROMPACK), which can separate CO, CO2, CH4, C2H2, C2H4, C2H6, H2O, C3H6, C3H8, C4H2, C6H6, etc., all high hydrocarbons. An uncoated silica capillary inlet was utilized to match the pressure difference between the gas flow and QMS and heated to about 100 °C. The total gas inlet and outlet flow rates were also carefully measured by digital flowmeters (Optiflow 520). All of the experiments were operated at atmospheric pressure and ambient room temperature. For each condition, at least three experiments were performed to get the result. 2. Calculations. The concentrations of each species were calculated using calibrated gas mixtures as external standards. The errors of the concentration measurements by this external standard gas method were less than 0.1%. The hydrogen concentration was calculated by two methods: one was estimated by moles of H2 formed equal to moles of the total gas outlet minus moles of the CH4 outlet and the all products outlet. The uncertainty from this method can reach up to 8%. In the experiment of direct methane conversion, the hydrogen concentration was calculated by this method. In the experiment of CO2 reforming of methane, another method was used by recording the mass spectrum from QMS and calibrating to the gas flow rate. The relative error from this method is less than 3%. The methane and CO2 conversions were defined based on the carbon conversion:

CH4 conversion ) (moles of CH4 consumed/moles of CH4 introduced) × 100% CO2 conversion ) (moles of CO2 consumed/moles of CO2 introduced) × 100% The selectivities and yields of products, if not specified, are calculated based on carbon conversion and are also calculated based on hydrogen, if specified. In addition, we defined the energy cost as the input specific energy or energy per liter input: inlet energy per liter input ) Pin/FCH (kJ/L) 4

(1)

inlet where Pin is the power input to the discharge and FCH 4 is the inlet CH4 flow rate. The energy cost that is required to convert the methane per liter is defined as

energy required per liter converted ) inlet outlet Pin/(FCH - FCH ) (kJ/L) (2) 4 4 The superscripts indicate the inlet or outlet gas flow. 3. Electrode Configurations and Electrical Characteristics. The electrode configuration was varied in order to produce different kinds of nonthermal plasmas and to control the current pulse waveform. Figure 2 shows the electrode configurations, and Figure 3 shows the correspondent electrical characteristics of the plasma. Figure 2a is the configuration used for the CD, which was sustained between two 2.0 mm diameter metal electrodes where one of them was pointed. Figure 3a shows a typical applied voltage and discharge current waveform. The inset in the Figure 3a shows the applied voltage and discharge current waveform of one pulse,

5920

Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002

Figure 2. Two kinds of electrode configurations to produce: (a) CD; (b) GD or PD at atmospheric pressure.

in which the time scale was reduced to 50 ns. The discharge current is a short time pulse with a rising time of less than 50 ns, reaching a maximum value of 80 A or more. The electrode configurations shown in Figure 2b could produce GD or PD. These discharges stabilized by inserting a semiconductor (CuO) acting as a ballasting resistor into both of the electrodes symmetrically. In the tip area a small length of tungsten (W; ∼2 mm) was attached to the electrodes. The function of the tungsten was not only to resist the high temperature that possibly occurred in the front surface but also to conduct the electricity that led to the oscilloscope. The diameters of the Cu electrodes, CuO, and W were about 1 mm, and the length of CuO was about 10 mm. The semiconductor material CuO that was inserted into the electrodes had the effect of ballasting the individual pin resistively to prevent the onset of the glow-to-arc transition. Fluctuations in the current as a result of instability were quenched by equal and opposite changes in the field of the section because of the voltage changes across the series resistor, as was done in transversely excited atmospheric pressure CO2 lasers.13 The resistance of CuO calculated by ac voltage and current across CuO in peak-to-peak values was about 108 kΩ. The voltage across CuO was calculated and measured by two highvoltage probes (Tektronix P6015A) that were connected to the end points of CuO. Figure 3b shows the voltage and current waveforms of the homogeneous or true GD. It is of a pulseless nature and extends over a whole ac cycle, visually appearing as a true glow over the entire interelectrode space. Although it is shown here in methane, it can also be realized in any other gases. Increasing the input voltage will usually change it to PD, which has been studied by Bartnikas14 in helium. The typical applied voltage and discharge current of the PD in methane is shown in Figure 3c, having features common to both CD and GD. The current pulses are longer (∼40 µs) compared to that of the CD, and also a continuous component of the current is observed, in the example of Figure 3c, during the negative half-wave. In a half cycle, one pulse was observed, and at a high power input, several pulses were observed, with the pulse current ranging between 40 and 200 mA. Results and Discussion 1. Direct Methane Conversion. a. Experimental Results. The results of methane conversion are plotted

Figure 3. Voltage and current waveform across the discharge corresponding to different cases: (a) CD (note that the current streamers are very narrow and an expanded profile is shown in the inset); (b) homogeneous GD; (c) PD and an expanded current profile is also showed in the inset. The methane gas flow rate was 30 mL/min.

against the input energy per liter in Figure 4. In these experiments the gas flow rate was 30 mL/min. The homogeneous GD may only be operated at a specific energy input below 26 kJ/L, upon increasing power; in most cases the discharge switches to a PD. In another words the glow-to-streamer transaction occurred around the specific energy input of 26 kJ/L. The methane conversion increases with the specific energy input. When PD, DBD, GD, and CD are compared at about 20 kJ/L, PD and GD have almost the same methane conversion for similar specific energy input, but CD gives a higher conversion; this means that the streamer

Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 5921

Figure 4. Methane conversion against the input energy density in different kinds of nonthermal plasmas at atmospheric pressure. The methane gas flow rate was 30 mL/min.

is more efficient in converting methane even at a lower average power input. At about 82 kJ/L specific energy input, the methane conversion can reach up to 60% in a CD and even up to 70% in a PD. PD is more efficient than CD in a higher power input. Table 1 lists the C2 hydrocarbon selectivities and yields. The types of discharges investigated have their own product selectivities and yields. In a GD (e.g., at 24.6 kJ/L specific energy input), the selectivities to C2H2, C2H4, and C2H6 are almost the same. The behavior of the PD changes with the energy input; at a low power input, product selectivity similar to that in a GD is found but with higher C2H6 yields. Long-time (∼40 µs), low-current pulses (∼40 mA) with an energy per pulse of about 1.1 mJ as in PD produce preferentially all C2 products. Upon further increasing the power or energy input, the reactions become mainly selective toward acetylene with small amounts of C2H4, C2H6, and other hydrocarbons such as C4H2 and C6H6. A long-time (∼40 µs) current pulse larger than ∼80 mA with energy per pulse ∼ 3.2 mJ as in PD or a short-time (∼50 ns) and higher current pulse (∼80 A) with energy per pulse ∼ 3.3 mJ as in CD produce preferentially C2H2. The selectivity to acetylene was significant; for example, in CD or PD with a specific energy input of about 81 kJ/L, 60 or 73% methane was converted, yielding 31 or 43% acetylene and 8 or 11% other hydrocarbons, as well as soot or carbon deposition in the reactor. The PD is even more efficient than a CD in methane conversion at about 81 kJ/L specific energy input. In DBD the reaction mainly selected C2H6 and C3H8 with small amounts of C2H2, C2H4, and other hydrocarbons such as nC4H10. Short-time (60%, respectively). Plasma-assisted conversion of CH4 and CO2 can take place at low temperature and without equilibrium limitation. Syngas production was an active radicaldominated reaction. The initial step is assumed to be the dissociation of CO2 and CH4 by electron collisions. The electrons in the primary streamer head of the CD possess an average energy of 10-20 eV.26 We estimated that the mean electron energy of the PD is in the same quantity that is efficient for CH4 molecules decomposed into CH3, CH2, CH, and C radicals by electron collisions. The reaction mechanism has been discussed in the former section, although more reactions of CO2 should be added in the simulation. In PD, the electron energy may be lower than that in CD, but the active time is longer, spreading across the whole cycle. In this case the step decomposition of CH4 (CH4 f CH3 f CH2 f CH f C) by electron collisions is thought to be the predominate reaction. Introducing CO2 can decrease the soot produced in pure methane conversions. In the present experiments with the CH4/CO2 mixture, we observed that carbon or soot formation strongly depends on the feed gas composition. No carbon or soot formation was found in the CO2-rich feed (CO2 content > 50%), and no O2 was detected in feed mixtures containing less than 80% CO2. This means that the deposited carbon that was produced in the reaction is an active species; it can react with chemically active oxygen atoms to form CO (C + O f CO). Meanwhile, H2 production is reduced, while H2O formation is enhanced. A thermodynamic equilibrium calculation showed that carbon deposition could be prevented by introducing additional water to the CO2/ CH4 mixtures.28 It is expected that reactions of carbon and water also play a certain role in avoiding carbon deposition or soot under plasma conditions. Producing syngas from a CH4/CO2 mixture is a highly endothermic process which requires external energy. Concerning the energy cost, take for an example the experiment shown in Figure 8a. In this experiment, the total feed gas flow rate was about 42.1 mL/min and produced H2 and CO with flow rates of 31 and 10.6 mL/ min, respectively, or when an input of 100 mol of feed gas was used, it produced 73.1 mol of H2 and 25.2 mol of CO. The energy per liter input is 67.2 kJ/L, and the required specific energy for syngas production in this case is 67.6 kJ/L. Compared with CD, the feed gas flow rate was 40.7 mL/min and produced H2 and CO with flow rates of 24.6 and 9.9 mL/min, respectively, or when an input of 100 mol of feed gas was used, it produced 60.4 mol of H2 and 24.5 mol of CO. The energy per liter input is 66 kJ/L, and the required specific energy for syngas production in this case is 82.2 kJ/L. Compared with DBD, 52 mol of H2 and 14 mol of CO from 100 mol of the feed gas were obtained at an energy input of 315 kJ/L (87 kW‚h/m3) and the required specific energy for syngas production was as high as 474 kJ/L.29 This fact indicates that energy transformation from electricity to chemicals is more efficient in PD than in CD, and DBD is the lowest one.

Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 5925 Table 2. Summary of the Results of PD and CD in the CO2 Reforming of Methane CO2 CH4 CO2 admixture (%) conversion (%) conversion (%) PD CD PD CD PD

20 20 50 50 60

72. 58.8 75.8 62.4 85.9

62.2 45.6 68 47.8 76.1

selectivity based on carbon (%) CO C2H2 C2H4 37.2 37.2 80.3 66.8 95.3

The main drawbacks of nonthermal plasma technology for methane conversion and reforming account for higher energy consumption or low-energy efficiency, for example, by DBD,29,30 the same for the present studies of one pair of pins. For example, in Figure 8a, the feed gas flow rate was about 42.1 mL/min and the power input was 47.2 W and produced H2 and CO with flow rates of 31 mL/min and 10.6 mL, respectively. In this condition the resulting energy cost was as high as 91.4 kJ/L (1024 MJ/kg) of H2. As we know, the hydrogen’s lower heating value is about 120 MJ/kg. By the thermal plasma method with 2-3 kW power input, the energy cost can be as low as 16 MJ/kg of H2,31 and by a classic gliding arc reactor, it was about 97 MJ/kg of H2.32 However, by using the multipin configuration, we expected that the energy cost in PDs would be reduced greatly. For example, we have employed three pairs of pins in one tube, with an inlet gas mixture of CH4:H2O ) 1:1, a total gas flow rate of 400 mL/min, a total power input of 65 W. In an outlet of produced H2 with a flow rate of 318 mL/min, the resulting energy cost was 137.4 MJ/kg of H2. More results on multipair pins will be reported in later studies. The basic idea of this research originated from the thought that in CD the energy in every period was only concentrated on several points of time, while in GD and PD the energy was spread over the whole period. We expected that this advantage could also be utilized in the other fields such as waste-gas treatment, detoxification of gaseous pollution, instantly activated reflectors and absorbers for electromagnetic radiation, surface treatment, or thin film deposition, etc.; especially by using multipin or multitube configurations, it will have latent potentials in practical use. Conclusions In ac nonthermal plasma, the current pulses have been controlled, giving a transaction between GD, PD, and CD. Each kind of plasma has its own plasma chemistry and product selectivity. A homogeneous GD produces almost the same amount of ethane, ethylene, and acetylene, while high methane conversions were found in CD and PD at energy densities above certain values, and products mainly selective to acetylene, and also produce a large amount of hydrogen. The nonthermal plasma method may open a new way of acetylene synthesis and hydrogen fuel production by direct methane conversion at low temperature. GD and PD are more efficient than the DBD concerning energy costs and have a latent potential in practical use. Experiments were also performed to investigate the conversion of the greenhouse gases CH4 and CO2 to syngas in ac PD and CD at atmospheric pressure and low temperature without using any catalysts. The results demonstrate that PDs are suitable for producing syngas. The selectivity strongly depended on the composition of the feed gas, and a desired H2/CO ratio could

52.6 47.7 15.7 15.8 3.9

2.3 2.3 1.1 1.5

selectivity based on hydrogen (%) H2 C2H2 C2H4 71.4 68.2 79.5 70. 70.7

16.4 14.7 7.5 7. 2.2

0.7 0.7 0.5 0.7

kJ/L H2/CO 67.0 68.9 66.3 64.6 60.3

3.1 3.0 1.0 1.2 0.7

be obtained by controlling the content of the feed gas. CO2-rich mixtures prevent carbon and soot formation. Acknowledgment The author thanks Dr. M. Schmidt, Dr. K. Zhang, and Dr. M. Heintze for their useful discussions and suggestions. He also expresses his thanks to Dr. R. Basner and Ms. K. Anklam for their continuous encouragement and support. Literature Cited (1) Gladish, H. Hydrocarbon processing. Pet. Refin. 1962, 41, 159-180. (2) Kaske, G.; Kerke, L.; Muller, R. Hydrogen production by the Hu¨ls plasma-reforming process. In Hydrogen Energy Progress VI; Veziroglu, T. N., Ed.; Pergamon Press: 1986; pp 185-190. (3) Thanyachotpaiboon, K.; Chavadej, S.; Caldwell, T. A.; Lobban, L. L.; Mallinson, R. G. Conversion of methane to higher hydrocarbons in AC nonequilibrium plasmas. AIChE J. 1998, 44, 2252-2257. (4) Krause, M.; Eliasson, B.; Kogelschatz, U.; Wokaun, A. CO2 reforming of methane by the combination of dielectric-barrier discharges and catalysis. Phys. Chem. Chem. Phys. 2001, 3, 249300. (5) Matsumoto, H.; Tanabe, S.; Okitsu, K.; Hayashi, Y.; Suib, S. L. Selective oxidation of methane to methanol and formaldehyde with nitrous oxide in a dielectric-barrier discharge-plasma reactor. J. Phys. Chem. A 2001, 105, 5304-5308. (6) Liu, C. J.; Mallinson, R.; Lobban, L. Non-oxidative methane coupling to acetylene over zeolites in a low-temperature plasma. J. Catal. 1998, 179, 326. (7) Liu, C. J.; Mallinson, R.; Lobban, L. Plasma catalytic methane conversion to higher hydrocarbons over zeolites: 1. comparative investigation with different zeolites and co- reactants. Appl. Catal. A 1999, 178, 17. (8) Kado, S.; Sekine, Y.; Fujimoto, K. Direct synthesis of acetylene from methane by direct current pulse discharge. J. Chem. Soc., Chem. Commun. 1999, 2485-2486. (9) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K. Lowtemperature reforming of methane to synthesis gas with direct current pulse discharge method. J. Chem. Soc., Chem. Commun. 2001, 415-416. (10) Yao, S. L.; Ouyang, F.; Nakayama, A.; Suzuki, E.; Okumoto, M.; Mizuno, A. Oxidative coupling and reforming of methane with carbon dioxide using a high-frequency pulsed plasma Energy Fuels 2000, 14, 910-914. (11) Yao, S. L.; Nakayama, A.; Suzuki, E. Methane conversion using a high-frequency pulsed plasma: Important factors. AIChE J. 2001, 47, 413-418. (12) Yao, S. L.; Nakayama, A.; Suzuki, E. Methane conversion using a high-frequency pulsed plasma: Discharge features. AIChE J. 2001, 47, 419-426. (13) Beaulieu, A. J. Transversely excited atmospheric pressure CO2 lasers. Appl. Phys. Lett. 1970, 16, 504. (14) Bartnikas, R. Note on discharges in helium under a.c. conditions. J. Appl. Phys. 1969, 40, 1974-1976. (15) Yang, Y.; Anklam, A.; Heintze, M. An experimental and modeling study on the non-oxidative methane conversion in a dielectric barrier discharge. International Symposium on HighPressure Low-temperature Plasma Chemistry, Greifswald, Germany, Sept 10-13, 2000; pp 267-271. (16) Meeks, E.; Shon, J. W. Modeling of plasma-etch processes using well stirred reactor approximations and including complex

5926

Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002

gas-phase and surface reactions. IEEE Trans. Plasma Sci. 1995, 23, 539-549. (17) Kee, R. J.; Rupley, F. M.; Meeks, E.; Miller, J. A. ChemkinIII: a fortran chemical kinetics package for the analysis of gasphase chemical and plasma kinetics; Report No. SAND96-8216; Sandia National Laboratories: Albuquerque, NM, 1996. (18) Morgan, W. L.; Penetrante, B. W. ELENDIF: a timedependent Boltzmann solver for partially ionized plasmas. Comput. Phys. Commun. 1990, 58, 127-147. (19) Morgan, W. L. A critical-evaluation of low-energy electronimpact cross-sections for plasma processing modeling. 2. CF4, SIH4, and CH4. Plasma Chem. Plasma Process. 1992, 12, 477-493. (20) Kline, L. E.; Kushner, M. Computer simulation of materials processing plasma discharges. Solid State Mater. Sci. 1989, 16, 1-35. (21) Kline, L. E.; Partlow, W. D.; Bies, W. E. Electron and chemical-kinetics in methane RF glow-discharge deposition plasmas. J. Appl. Phys. 1989, 65, 70-78. (22) Frenklach, M.; Wang, H. Detailed surface and gas-phase chemical-kinetics of diamond deposition. Phys. Rev. 1991, B43, 1520-1545. (23) Tsang, W.; Hampson, R. F. Chemical kinetic database for combustion chemistry. 1. methane and related-compounds. J. Phys. Chem. Ref. Data 1986, 15, 1087-1279. (24) Tsang, W. Chemical kinetic database for combustion chemistry. 3. propane. J. Phys. Chem. Ref. Data 1988, 17, 887952. (25) Eliasson, B.; Kogelschatz, U. Modeling and applications of silent discharge plasmas. IEEE Trans. Plasma Sci. 1991, 19, 309-323.

(26) Creyghton, Y. L. M. Pulsed Positive Corona Dischargess fundamental study and application to flue gas treatment. Ph.D. Thesis, Eindhoven University, Eindhoven, The Netherlands, 1994. (27) Zhang, Z. L.; Verykios, X. E. Carbon-dioxide reforming of methane to synthesis gas over supported Ni catalysts. Catal. Today 1994, 21, 589-595. (28) Dibbern, H. C.; Olesen, P.; Rostrup-Nielsen, J. R.; Tottrup, P. B.; Udengaard, N. R. Make low H2/CO syngas using sulfur passivated reforming. Hydrocarbon Process. 1986, 65, 71-74. (29) Zhou, L. M.; Xue, B.; Kogelschatz, U.; Eliasson, B. Nonequilibrium plasma reforming of greenhouse gases to synthesis gas. Energy Fuels 1998, 12, 1191-1199. (30) Huang, A.; Xia, G.; Wang, J.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. CO2 reforming of CH4 by atmospheric pressure ac discharge plasmas. J. Catal. 2000, 189, 349-359. (31) Bromberg, L.; Cohn, D. R.; Rabinovich, A.; Alexeev, N. Plasma catalytic reforming of methane. Int. J. Hydrogen Energy 1999, 24, 1131-1137. (32) Cormier, J. M.; Rusu, I. Syngas production via methane steam reforming with oxygen: plasma reactors versus chemical reactors. J. Phys. D: Appl. Phys. 2001, 34, 2798-2803.

Received for review March 26, 2002 Revised manuscript received September 16, 2002 Accepted September 21, 2002 IE0202322