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Energy & Fuels 2000, 14, 910-914
Oxidative Coupling and Reforming of Methane with Carbon Dioxide Using a High-Frequency Pulsed Plasma† S. L. Yao, F. Ouyang, A. Nakayama, E. Suzuki,* M. Okumoto,‡ and A. Mizuno‡ Catalysis Science Laboratory, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan, and Department of Ecological Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan Received February 4, 2000. Revised Manuscript Received April 26, 2000
We studied the oxidative coupling and reforming of CH4 with CO2 to C2H4, CO, and H2 using a pulsed plasma with a pulse frequency ranging from 166 to 3050 PPS. The largest selectivity of C2H4 was 64% with 31% CH4 conversion and 24% CO2 conversion at 2920 PPS and 500 °C. Selectivities of CO and H2 were about 20% and 100%, respectively. Ratios of H2 to CO and (CO + CO2) were, respectively, 7.1 and 2.5, which are acceptable for methanol production from (CO + H2) and (CO2 + H2) over a catalyst. The results indicated that the pulsed plasma with a high frequency can promote conversion of CH4 and CO2. The energy efficiency of the pulsed plasma was improved using a high pulse frequency and a high reaction temperature. We suggested that a pulsed plasma with a high pulse frequency is useful for oxidative coupling and reforming of CH4 with CO2 in industry.
Introduction Oxidative coupling and selective oxidation of methane to produce ethylene and higher hydrocarbons have received worldwide attention as a potentially interesting process for upgrading natural gas since 1982.1 Oxidative coupling of methane to ethane2 and ethylene3 requires a high reaction temperature and high pressure due to the similar high dissociation energy of activation of most C-H bonds. Methane reforming with steam or with carbon dioxide conventionally requires a high reaction temperature, even in the presence of catalysts.4 Such high-temperature processes have low energy efficiencies. For oxidative coupling of methane using a plasma process to produce ethylene and carbon oxygenates such as methanol and formaldehyde, ac and dc corona discharges, dielectric-barrier discharge, arc plasma, and the combination of microwave plasma and catalysts have been reported.5-11 Okumoto and co-workers re† A part of this paper was published at AIChE 2000 Spring National Meeting, Atlanta, GA, 2000. * To whom correspondence should be addressed. Tel.: (+81)77475-2305. Fax: (+81)774-75-2318. E-mail:
[email protected]. ‡ Toyohashi University. (1) Keller, G. E.; Bhasin, M. M. J. Catal. 1982, 73, 9. (2) Matherne, J. L.; Culp, G. AIChE, Annual Meeting, Chicago, 1990, paper 59f. (3) Lee, L. L.; Aitani, A. M. Fuel Sci. Int. 1991, 9 (2), 137. (4) Tsang, S. C.; Claridge, J. B.; Green, M. L. H. Catal. Today 1995, 23, 3. (5) Mallinson, R. G.; Sliepcevich, C. M.; Rusek, S. Am. Chem. Soc., Div. Fuel Chem. 1987, 32, 266. (6) Suib, S. L.; Zerger, R. P. J. Catal. 1993, 139, 383. (7) Bhatnagar, R.; Mallison, R. G. Methane and Alkane Conversion Chemistry; Bhasin, M. M., Slocum, D. W., Eds.; Plenum Press: New York, 1995, p 249. (8) Liu, C. G.; Marafee, A.; Hill, B. J.; Xu, G. H.; Mallinson, R.; Lobban, L. Ind. Eng. Chem. Res. 1996, 35 (10), 3295.
ported the reforming of methane with oxygen using a nonthermal pulsed plasma at a fixed pulse frequency of 250 PPS.12 Oxidative coupling and reforming of methane using a pulsed plasma with a high frequency is relatively unexplored. Since carbon dioxide is present in many natural gas resources, Larkin et al. studied the oxidative coupling of methane with carbon dioxide using an ac dielectricbarrier plasma reactor. The selectivities of oxygenates (methanol, formaldehyde, formic acid, methyl formate) amount to 50-65%.13 Zhou et al. studied methane reforming with carbon dioxide using a high-frequency AC nonthermal plasma. They found that most of methane can be converted to carbon monoxide and hydrogen.10 Huang et al. found that methane reforming with carbon dioxide using an AC arc plasma has a higher energy efficiency than that using an AC glow plasma.14 The oxidative coupling and reforming of methane with carbon dioxide in a pulsed plasma are the subject of this paper. This plasma system involves a pulsed plasma operated at room temperature and 500 °C with pulse frequencies ranging from 166 to 3050 PPS. The characteristics of pulsed plasma, conversions, selectivities, and energy efficiencies are reported. (9) Bromberg, L.; Cohn, D. R.; Rabinovich, A.; O’Brien, C.; Hochgreb, S. Energy Fuels 1998, 12, 11. (10) Zhou, L. M.; Xue, B.; Kogelshatz, U.; Eliasson, B. Energy Fuels 1998, 12, 1191. (11) Yao, S. L.; Takemoto, T.; Ouyang, F.; Nakayama, A.; Suzuki, E.; Mizuno, A.; Okumoto, M. Energy Fuels 2000, 14 (2), 459. (12) Okumoto, M.; Takashima, K.; Katsura, S.; Mizuno, A. Thermal Sci. Eng. 1999, 7 (3), 23. (13) Larkin, D. W.; Caldwell, T. A.; Lobban, L. L.; Mallinson, R. G. Energy Fuels 1998, 12 (4), 740. (14) Huang, A.; Xia, G.; Wang, J.; Suib, S.; Hayashi, Y.; Matsumoto, H. J. Catal. 2000, 189, 349.
10.1021/ef000016a CCC: $19.00 © 2000 American Chemical Society Published on Web 06/21/2000
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Experimental Section The experiment setup is schematically shown in Figure 1. The pulse power (DP-15K35, Pulse electronic engineering) was generated with a hydrogen thyratron. The pulse frequency was modulated by controlling the hydrogen thyratron. The pulse voltage can be adjusted via adjustment of the input voltage of the charging system. The input voltage was fixed at 160 V. Discharge currents through anode and cathode were measured with two current transformers (CT1, Model 2-1.0, 35 MHz; CT2, Model 0.5-1.0, 20 MHz; Strangenes Industries) and the discharge voltage was measured with a voltage probe (V-probe, EP-50K, 50 MHz; Pulse electronic engineering). The signals both from the voltage probe and current transformers were recorded with a digital oscilloscope (TDS754D, Tektronix) having an analogue bandwidth of 500 MHz and a maximum sample rate of 2 GS/s. The reactor was composed mainly of a Pyrex tube (i.d. 12 mm, o.d. 15 mm, length 500 mm), a SUS tube (SUS 316, cathode, i.d. 7.7 mm, o.d. 9.7 mm, length 100 mm) and a SUS wire (SUS 316, anode, 0.5 mm diameter). The SUS tube and SUS wire were set in the central part of the Pyrex tube. The reactor was heated with a tubular electric furnace, which is capable of maintaining a temperature up to 1200 °C, with a heating zone length of 30 cm and uniformly heated zone of at least 20 cm. A mixture of methane (83.4 mL/min) and carbon dioxide (16.6 mL/min) was introduced into the upper part of the vertically placed reactor. Carbon compounds in the products from the lower part of the reactor were analyzed with an online gas chromatograph (GC 103, Okura Riken; FID) equipped with a 2 m Porapak Q. The products were pretreated with a methanizer (MT-221, GL Science) to convert carbon compounds to related alkane prior to detection by FID. Concentrations of H2 and O2 were measured with another online gas chromatograph (AGC 280, Okura Riken, TCD) equipped with a 2 m activated carbon. All experiments were carried out at atmospheric pressure. The overall conversions are defined as
Figure 1. Schematic diagram of the pulse generator DP15K35. C1 and C2, high voltage capacitors, (C1, 200 pF; C2, 100 pF); CT1 and CT2, current transformers; EP-50K: high voltage probe; L1 and L2, inductors, (L1, 100 µH; L2, 200 µH); R0, R1, and R3: high-voltage resistors (R0, 200 Ω; R1, 110 Ω; R2, 1.3 kΩ; R3, 100 Ω); THY, high-voltage hydrogen thyratron.
CH4 conversion ) [(moles of CH4 before reaction) (moles of CH4 after reaction)]/ (moles of CH4 before reaction) × 100% CO2 conversion ) [(moles of CO2 before reaction) (moles of CO2 after reaction)]/ (moles of CO2 before reaction) × 100% Selectivity of CO is defined as
CO selectivity ) (moles of CO produced)/ [(moles of CH4 converted) + (moles of CO2 converted)] × 100% Selectivity of C2H4 or C2H6 is defined as
C2H4 or C2H6 selectivity ) (2 × moles of C2H4 or C2H6 produced)/[(moles of CH4 converted) + (moles of CO2 converted)] × 100% H2 selectivity ) (moles of H2 produced × 100%)/ [(moles of H in CH4 before reaction) (moles of H in carbon compounds after reaction)] × 2 The input power for discharge is calculated from waveforms of voltage and anode current using the following integration formula
Vi+1 + Vi Ii+1 + Ii (ti+1 - ti) 2 2
∑
P)F
Figure 2. Waveforms of voltage and current at various pulse frequencies at room temperature and 500 °C. where, P, F, V, I, and t are, respectively, input power (in W), pulse frequency (in PPS), discharge voltage (in V), anode discharge current (in A), and discharge time (in s). Conversion ability of a pulsed plasma is given (in µmol of (CH4 + CO2)/J) as
Conversion ability ) (moles of CH4 and CO2 converted during one minute)/60P
Results and Discussion We first measured the waveforms of discharge voltage and discharge current at various frequencies. The
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Figure 3. Conversions of CH4 and CO2 at room temperature.
Figure 5. Selectivity of each product at room temperature.
Figure 4. Conversions of CH4 and CO2 at 500 °C.
Figure 6. Selectivity of each product at 500 °C.
results are shown in Figure 2. At room temperature, the anode current had two peaks at low frequencies of 166 and 410 PPS. The first peak represented a corona discharge and the second peak a streamer discharge. A large amount of positive ions formed in the corona discharge in the plasma zone, and these positive ions contributed to the formation of the coming streamer discharge via increasing secondary electron emission from the cathode and/or decreasing the electric resistance of the background methane. The corona discharge disappeared at 1522 and 3050 PPS. This indicated that positive ions still existed in the plasma zone in the short interval of two pulse waves and could induce the next streamer discharge. The waveform of current at 500 °C had only one peak even at a low pulse frequency. This indicated that the streamer discharge between hot electrodes occurred more easily than that between cold electrodes and did not require a corona discharge for its induction. Conversions of CH4 and CO2 at various frequencies and at room temperature is shown in Figure 3. Conversions of CH4 and CO2 increased when the pulse frequency increased. Similar results were observed when the reaction temperature was 500 °C (Figure 4). The increases in conversion of CH4 and CO2 were obviously
due to the increase in input power. The selectivity of each product at room temperature is shown in Figure 5. The selectivity of C2H4 increased and that of C2H6 decreased as the pulse frequency increased. The selectivity of CO did not change significantly. Figure 6 shows the selectivity of each product at 500 °C. The selectivity of CO was at the same level as that at room temperature. However, the selectivity of C2H4 increased, but that of C2H6 decreased more widely than at room temperature as the pulse frequency increased. This difference was obviously due to an increase in reaction temperature. In a gas-phase reaction, C2H6 is formed mainly via reaction 1
CH3 + CH3 ) C2H6
(1)
since its reaction rate is highest in all C1 related reactions even at room temperature.15 However, in a plasma zone, CH4 is possibly activated to CH3+, CH2+, and other radicals, such as CH3 and CH2. The main product of the reaction (eq 2) is C2H4, not C2H6.16 As
CH2+ + CH4 ) C2H4+ (70%) + C2H5+ (30%) (2) kinetic data of ionic reactions of CH4 and CO2 are very scarce, it is still unknown why selectivities of C2H4 and
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Energy & Fuels, Vol. 14, No. 4, 2000 913
Table 1. Comparison of Conversion Ability to Other Plasmas
plasma
feed
flow (mL/min)
pressure (Torr)
P (W)
T (°C)
microwave microwave/catalyst silent silent
CH4 CH4 CH4 + O2 CH4 + O2/C2H6
100 500 27.5 430
23 50 760 760
80 80 8 118
110 28
pulsed ac dielectric-barrier
CH4 + O2/Ar CH4 + CO2
190 500
760 760
21 500
room 80
AC arcd
CH4 + CO2
75
760
30
pulsed 3050 PPS pulsed 2920 PPS
CH4 + CO2
100
760
29.5
room
CH4 + CO2
100
760
30.6
500
CH4 conversion (%) 26 8 2.5 CH4, 31.8 C2H6, 20 O2, 33 17 CH4, 40 CO2, 20 CH4, 88.9 CO2, 53.2 CH4, 26.7 CO2, 17.2 CH4, 31.8 CO2, 23.9
CH3OH
conversion ability (µmol/J)
21.3 18a
20 36b
0.22 0.34 0.059 0.37
6 6 18 17
54a 88.5
24
0.36 0.18
12 10
81.5
0.15
14
61.6
19.8
0.58
this study
63.7
22.1
0.68
this study
selectivity (%) C 2H 2
C 2H 4
24 23.5
25.3 0 7.1
16c
11.2
2.26
a (CO + CO ). b (CH OH + HCHO + HCOOH + C H OH + HCOOCH ). c (C H + C H ). 2 3 2 5 3 2 4 2 6 energy efficiency.
C2H6 at 500 °C and a high pulse frequency differ from those at room temperature and a low pulse frequency. The selectivity of H2 was close to 100% both at room temperature and at 500 °C. The H2 concentration in the products was as high as 32.5% at 2920 PPS and 500 °C. A trace of O2 (less than 0.01%) was found. Since the selectivity of C2H4 at high pulse frequencies became as high as 60%, this suggested that a pulsed plasma with a high pulse frequency can be used for oxidative coupling of CH4 with CO2. Similarly, selectivities of CO and H2 were about 20% and 100%, respectively. The ratios of H2 to CO and to (CO + CO2) at 500 °C and 2920 PPS were, respectively, 7.1 and 2.5, which are acceptable for methanol production from CO/H2 and CO2/H2 over a catalyst. These also suggested that a pulsed plasma can be used for reforming of CH4 with CO2. Almost all of the plasma experiments indicated that the energy required by the process is the highest in comparison with the others such as a thermal chemical process. The energy efficiency of a pulsed plasma is represented by a conversion ability of CH4 and CO2 (Figure 7). The results indicated that when the pulse frequency was lower than 1000 PPS, the conversion ability of methane and CO2 at 500 °C was lower than that at room temperature. However, a significant increase in conversion ability of methane and CO2 was obtained when the pulse frequency was higher than 1000 PPS. These findings implied that the energy efficiency can be improved by using a high temperature and a high pulse frequency. A comparison of our results with those reported by Larkin,17 Okumoto,12 Shepelev,18 Suib,6 and Zhou10 is given in Table 1. The energy efficiency using a pulsed plasma is better than that using silent and microwave discharges. The selectivity of ethylene is also higher than those using other kinds of plasmas. (15) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Frank, P.; Hayman, G.; Just, TH.; Kerr, J. A.; Murrells, T.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. Combust. Flame 1994, 98, 59. (16) Chang, J. S.; Hobson, R. M.; Ichikawa, S.; Kaneda, T. Atomic and molecular processes of ionized gases; Tokyo Denki University Press: Tokyo, 1989, p 243. (17) Larkin, D. W.; Lobban, L. L.; Mallinson, R. G. 1st International Conference on Gas Processing, AIChE Spring National Meeting, Atlanta, GA, March 5-9, 2000, p 10. (18) Shepelev, S. S.; Gesser, H. D.; Hunter, N. R. Plasma Chem. Plasma Proc. 1993, 13 (3), 479.
d
CO
ref
CH4 and CO2 (5/5) both excited; maximum
Figure 7. Conversion ability of CH4 and CO2 at room temperature and 500 °C.
The selectivity of CO is as high as 88.5% when CO2 concentration in the feed gas is 70% using an AC dielectric-barrier discharge; The selectivities of ethylene and ethane decrease with the increase of CO2 concentration.10 These facts indicate that not only the plasma itself but also CO2 concentration in the feed gas can influence the reforming result. On the other hand, addition of ethane to the reaction system17 and use of arc AC plasma14 can improve the energy efficiency. These findings together with ours can believingly promote the application of plasma for methane conversion. Conclusions This study demonstrates the following: (1) The pulsed plasma with a high frequency is an effective method for oxidative coupling and reforming of CH4 with CO2. (2) The largest selectivity of C2H4 is 64% with CH4 conversion 31% and CO2 conversion 24% at 2920 PPS and at 500 °C. The selectivities of H2 and CO are about 100% and 20%, respectively. The ratios of H2 to CO and (CO + CO2) are, respectively, 7.1 and 2.5, which are acceptable for methanol production from (CO + H2) and (CO2 + H2) over a catalyst.
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(3) The energy efficiency is improved by increasing the pulse frequency and using a high reaction temperature. (4) This high-frequency pulsed plasma is desirable to promote the application of oxidative coupling and reforming of CH4/CO2 to C2H4, CO, and H2 in industry.
Yao et al.
Acknowledgment. We thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support. EF000016A
