Energy & Fuels 2001, 15, 299-302
299
Co-generation of Syngas and Higher Hydrocarbons from CO2 and CH4 Using Dielectric-Barrier Discharge: Effect of Electrode Materials Yang Li,† Gen-hui Xu,† Chang-jun Liu,*,† Baldur Eliasson,‡ and Bing-zhang Xue‡ State Key Laboratory of C1 Chemical Technology, Tianjin University, Tianjin 300072, P. R. China, and Energy and Global Change, ABB Corporate Research Ltd. CH5405, Baden, Switzerland Received October 25, 2000. Revised Manuscript Received January 4, 2001
The effect of electrode materials on the co-generation of syngas and higher hydrocarbons from CO2 and CH4 using dielectric-barrier discharge has been investigated. The electrode materials tested here include aluminum, copper, steel, and titanium. For the feed of methane in the absence of CO2, the order of activity of methane conversion from high to low was Ti ≈ Al > Fe > Cu, while the order of the activity of CO2 conversion was Al > Cu > Ti > Fe for the case of CO2 feed without methane. Regarding the co-feed of methane and CO2, the titanium electrode shows the best activity for the conversions, while the other three materials show a similar performance for the conversions. The effect of dilution gas, helium, on the conversions has also been discussed.
Introduction There are two major objectives for the utilization of carbon dioxide. One is to reduce the greenhouse effect.1-3 The other is to use CO2 as carbon sources to manufacture chemicals. The most important reactions involving CO2 are as follows:3,4 (1) the incorporation of CO2 into the C-C, C-H, and C-N bonds with formation of carboxy- and carbonyl compounds; (2) the oxidation of other hydrocarbons using CO2 as an oxidant. However, there are some difficulties in conventional catalytic conversion of CO2. The principal difficulty is from the intensive energy consumption and the expensive hydrogen source.5 The most direct way for CO2 utilization seems to be the production of CO and O2:
CO2 f CO + 1/2O2
∆G1000K ) 190.5 kJ/mol (1)
Obviously, there are some difficulties thermodynamically. But this reaction can be realized with the plasma technology.3,6-11 The corona discharge has been considered as the most effective technique for the production of CO6 among all the plasma technologies applied. * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Tianjin University. ‡ ABB Corporate Research Ltd. (1) Bradford, M. C. J.; Vannice, M. A. Catal. Rev.-Sci. Eng. 1999, 41 (1), 1-42. (2) Malik, M. A.; Malik, S. A. J. Nat. Gas Chem. 1999, 8 (2), 166178. (3) Liu, C. J.; Xu, G. H.; Wang, T. M. Fuel Process Technol. 1999, 58, 119-134. (4) Krylov, O. V.; Mamedov, A. K. Russ. Chem. Rev. 1995, 64 (9), 877-900. (5) Xu, X.; Moulijn, J. A. Energy Fuels 1996, 10 (2), 305-325. (6) Jogan, K.; Mizuno, A.; Yamamoto, T.; et al. IEEE Trans. 1nd. Appl. 1993, 29 (5), 876-881. (7) Sigmond, R. S. Electrical Breakdown of Gases; Wiley: New York, 1978; pp 319-384.
Sigmond,7 Chang,8 Maezono,9 and Suib et al.10 have developed the relative model for the production of CO from carbon dioxide via corona discharges. Methane is another major greenhouse gas. Scientists pay more attention to the utilization of methane since 1980s. Due to its high H/C ratio, methane is a very good co-reactant for CO2 utilization using plasmas.3,12-15 Most of these research interests focus on the syngas (H2 and CO) formation from CH4 and CO2 via reaction 2:
CH4 + CO2 f 2H2 + 2CO
∆G1073K ) -254 kJ/mol (2)
This reaction can be also performed catalytically. The undesirable carbon deposits, however, are not avoidable with the present catalyst design in CO2 reforming of CH4. Eliasson et al.13 recently reported a direct conversion of CO2 and CH4 into higher hydrocarbons using the catalytic dielectric-barrier discharges. The major byproduct is syngas. Yao et al.16 suggested that a pulsed plasma with a high frequency is useful for oxidative (8) Chang, J. S. Proceedings of the International Symposium on High-Pressure Low-Temperature Plasma Processing. IEE Japan Press: 1987; 2 (ED-87-75), pp 45-54. (9) Maezono, I.; Chang, J. S. IEEE Trans. Ind. Appl. 1990, 26 (4), 651-655. (10) Brock, S.; Marquez, M.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. J. Catal. 1998, 180, 225-233. (11) Savinov, S. Y.; Lee, H.; Song, H. K.; Na, B. K. Ind. Eng. Chem. Res. 1999, 38, 2540-2547. (12) Huang, A.; Xia, G.; Wang, J.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. J. Catal. 2000, 189, 349-359. (13) Eliasson, B.; Liu, C. J.; Kogelschatz, U. Ind. Eng. Chem. Res. 2000, 39 (5), 1221-1227. (14) Liu, C. J.; Mallinson, R.; Lobban, L. Appl. Catal. A 1999, 178, 17-27. (15) Gesser, H. D.; Hunter, N. R.; Probawono, D. Plasma Chem. Plasma Processing 1998, 18, 241-245. (16) Yao, S. L.; Ouyang, F.; Nakayama, A.; Suzuki, E.; Okumoto, M.; Mizuno, A. Energy Fuels 2000, 14, 910-914.
10.1021/ef0002445 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/20/2001
300
Energy & Fuels, Vol. 15, No. 2, 2001
Li et al.
Figure 2. Schematics of DBD reactor. Figure 1. Schematic diagram of the experimental setup.
coupling and reforming of CH4 with CO2. The products are C2H4 and syngas. Eliasson17 and Mizuno18 investigated the hydrogenation of CO2 to methanol by H2 or CH4 with dielectric-barrier discharge plasma and pulsed plasma, respectively. In addition to syngas, different hydrocarbon products have been produced with the same feed of CH4 and CO2 using different plasma technologies. The plasma utilization of methane and carbon dioxide are mostly related to the plasma properties, co-reactant, and catalyst applied.12-18 As to the energy efficiency in the plasma reactions, Yao16 has achieved a high-energy efficiency of methane conversion using plasmas. Mallinson19 also reported the energy efficiency of plasma methane conversion has reached a practical level. Some industrial tests of plasma methane conversion are being conducted in the United States and Canada. However, few investigations have been conducted to evaluate the effect of electrode materials. In this paper, we investigated the influence of the electrode materials on the plasma utilization of methane and carbon dioxide using the dielectric barrier discharges.
Table 1. Comparison of Conversion of CH4 and Selectivity of C2H6 with All Four Metal Electrodes electrode material conversion of CH4(%) selectivity of C2H6(%) aluminum steel copper titanium
33.45 ( 0.51 31.38 ( 0.48 24.09 ( 0.46 33.65 ( 0.49
11.9 12.7 20.9 12.8
All the experiments were started at room temperature and atmospheric pressure. The overall conversions and selectivities are defined as
conversion of CH4 ) [(moles of CH4 in the feed) (moles of CH4 in the products)]/ (moles of CH4 in the feed) × 100% conversion of CO2 ) [(moles of CO2 in the feed) (moles of CO2 in the products)]/ (moles of CO2 in the feed) × 100% selectivity of CO ) (moles of CO produced)/ [(moles of CH4 converted) + (moles of CO2 converted)]
Experimental Section The experimental setup is schematically shown in Figure 1. The high voltage generator supplied about 8 kV (peak to peak) and 25 kHz sinuous signals to the reactor. The voltage and current measurements were conducted with a digital oscilloscope (Tektronix TDS 210) through a high voltage probe (Tektronix P6015) and a current probe (Tektronix CT-2). The input power was fixed at about 15 W. The DBD reactor contains a quartz disk (φ84 × 3 mm) and a ground metal electrode, which was made of copper, titanium, aluminum or steel. A transparent conductive membrane (φ50 mm) was deposited on the one side of the quartz. The disk was served as the high voltage electrode and the discharge gap was 1 mm. The reactor was schematically shown in Figure 2. The micro-DBD reactor was originally designed for the optic emission analysis with low applied power that the conversions were not as high as the reported previousely.13 The feed gas, carbon dioxide or methane or a mixture, was introduced into the DBD reactor via digital mass flow controllers. Helium was used as a dilution gas for all the cases. The total flow rate of the feed was 20 mL/min, while the flow rate of helium was fixed at 15 mL/min. The feed ratio of CH4 and CO2 can be adjusted during experiments. The products were analyzed with an online gas chromatograph (HP-4890D) equipped with a 3m Porapak N and a 25 m × 0.32 mm Pora PLOT Q, which connected with a TCD (thermal conductivity detector) and a FID (flame ionization detector), respectively. (17) Eliasson, B.; Kogelschatz, U.; Xue, B. Z.; Zhou, L. M. Ind. Eng. Chem. Res. 1998, 37 (5), 3350-3357. (18) Okumoto, M.; Takashima, K.; Katsura, S.; Mizuno, A. Thermal Sci. Eng. 1999, 7 (3), 23-31. (19) Mallinson, R. G. Personal communication, Dec. 2000.
selectivity of C2H6 ) (moles of C2H6 produced)/ [(moles of CH4 converted) + (moles of CO2 converted)] H2/CO ) (moles of H2 produced)/(moles of CO produced)
Results and Discussion Results with Methane Feed Only. Table 1 shows the conversions of methane for plasma methane conversion in the absence of CO2. The order of the activity for methane conversion from high to low is Ti ≈ Al > Fe > Cu. The selectivity of C2H6 was low when the conversion of CH4 was high with all four electrodes. A little C2H2 and C2H4 were detected with the GC, but the concentrations were less than 0.1%. In gas-phase reactions, C2H6 is formed mainly via reaction 3:
CH3 + CH3 f C2H6
(3)
The discharge voltage was decreased with the addition of helium in the feed of CH4, but the current was increased. The increase of the current suggested that more free electrons were supplied by helium. These electrons with high energy may enhance the generation of the radicals via reaction 4:
CH4 + (e, H,...) f CH3 + (H + e, H2,...)
(4)
Co-generation of Syngas and Higher Hydrocarbons
Energy & Fuels, Vol. 15, No. 2, 2001 301
Table 2. Comparison of Conversion of CO2 with All Four Metal Electrodes electrode material aluminum steel copper titanium
conversion of CO2(%) 9.24 ( 0.42 2.18 ( 0.45 8.28 ( 0.50 4.98 ( 0.40
Then the radical chain reactions may continuously take place.
C2H6 + e f C2H5 + H + e
(5)
C2H5 + CH3 f C3H8
(6)
C2H5 + C2H5 f C4H10
(7)
and so forth. Only small amount of C3H8 was detected with the GC because the residence time was short in this reactor. More C3H8, C4, C5, and so on were detected when we increased the residence time by decreasing the flow rate or changing with a larger reactor, as reported previously.13 These results indicated that more CH4 was converted to higher hydrocarbons with the electrode made of titanium, aluminum, and steel compared to that of the copper electrode. The former three metals seem to be much easier to lose the electrons than copper does. It has been considered that the free electrons with high energy played an important role in the process of methane conversion under the plasmas condition. Results with CO2 Feed Only. The conversions of CO2 with the four kinds of metal electrodes are shown in Table 2. CO and O2 were detected in the products of the decomposition of CO2 with the GC. The order of the activity in the decomposition of CO2 is Al > Cu > Ti > Fe. Suib et al.10 reported that they have found no correlation between the conversion and the work function of the metal. The change of the reactivity with different metal electrodes may be from the involvement of the dilution gas, helium. The addition of the diluent gases will change the route of the decomposition of CO2.9
He + e- f He+ + 2e-
(8)
He+ + 2He f He2+ + He
(9)
He+(or He2+) + CO2 f CO2+ + He(or 2He) CO2+ + e- f CO + O
(10) (11)
In this pathway, coke was not formed as a byproduct, as shown in the experiments with the feed of CO2 and helium. The diluent gas played an important role in the decomposition of CO2. The order of the activity of the metals is reflected in the excitation temperature obtained for pure He plasmas in the reactors with the metals.10 Results with the Mixture of CH4 and CO2. Figure 3 shows the change of conversion of CH4 with the methane concentration in the feed. The conversions of CH4 decreased when the concentration of CH4 in the feed increased. Similar results were observed with all
Figure 3. Conversion of CH4 with 75% He at the flow rate of 20 N mL/min.
Figure 4. Conversion of CO2 with 75% He at the flow rate of 20 N mL/min.
four electrodes. The highest conversion of CH4 was achieved with the electrode made of titanium among all four metals. Furthermore, all the conversions were higher than 35%. As shown in Table 1, the conversions of CH4 were less than 35% when the feed was methane and helium in the absence of CO2. It suggested that the increasing conversions of CH4 were due to the addition of CO2. As an oxidant in this reaction, CO2 was first decomposed to CO and O with high activity. The reaction of methane conversion was deeply influenced by the new reactive oxygen so that the conversion of methane was much higher with the feed of CO2 and methane than that without CO2. The possible mechanism was that the reactive oxygen deprived of H atom in methane to generate the hydrocarbon radicals. Then the chain reactions took place. According to the experimental results, the effect of the electrode materials we used was different in this reaction mechanism. The highest conversion of methane was reached with the titanium electrode. Figure 4 shows that the CO2 conversion was increased with the increasing CH4. Moreover, all the CO2 conversions were higher than those without CH4 as the coreactant. This indicated that CH4 promoted the conversion of CO2. The rates of the decomposition of CO2 and of the reverse reaction depend on the concentrations of CO2, CO, and O2 in the discharge zone. These excited species will react with the hydrocarbon radicals from
302
Energy & Fuels, Vol. 15, No. 2, 2001
Figure 5. Selectivity of CO with 75% He at the flow rate of 20 N mL/min.
Li et al.
Figure 7. The ratio of H2/CO with 75% He at the flow rate of 20 N mL/min.
than the values without CO2 as co-reactant for all four metal electrodes. The ratio of H2 to CO is shown in Figure 7. The increasing H2/CO ratio was obviously due to an increase in the concentration of H2 and a decrease in the concentration of CO. Eliasson et al.20 reported a similar curve of the ratio of H2/CO versus the concentration of CO2. However, the ratio in our experiments was much higher than the results reported by them.20 It has been considered that the conversion of CH4 was increased due to the addition of helium in this work. Therefore the production of hydrogen was promoted with the increasing conversion of CH4 that leads to a high ratio of H2/ CO. Conclusions
Figure 6. Selectivity of C2H6 with 75% He at the flow rate of 20 N mL/min.
CH4 in the plasma zone. More oxygen was consumed with the increasing of methane in the feed, which led to the decreasing of the rate of the reverse reaction of CO2 decomposition. So the conversion of CO2 was obviously increased with the increasing of the concentration of methane in the feed. It was concluded that the co-feed of CH4 and CO2 promoted the conversion of each other. The effect of the electrodes we used was little in the reaction of CO2 decomposition in the presence of methane according to the results. The concentration of CO2 in the feed influenced the conversion of CO2 much more than the materials of the electrode did. However, the selectivity of CO was decreased when the conversion of CO2 increased, as shown in Figure 5. This suggested that more oxygen atoms were converted into oxygenates. The results of all four metal electrodes were quite similar. Figure 6 shows that the selectivity of C2H6 was increased with the increasing concentration of CH4 in the feed. But the selectivity of C2H6 was less
Experiments were performed to investigate the effect of metal electrodes on methane and CO2 conversions in DBD reactor. The order of the activity to convert CH4 was Ti ≈ Al > Fe > Cu. The order of the activity to decompose CO2 was Al > Cu > Ti > Fe. When the feed gas was a mixture of CH4 and CO2, all the conversions of CH4 and CO2 increased. For the conversion of CH4 in the presence of CO2, titanium had the highest activity, while the other three metals had similar activity. The effect of metal electrodes on the conversion of CO2 in the presence of CH4 was very little. In addition, the influence of metal electrodes on the product distribution was nearly the same. Acknowledgment. Supports from Ministry of Science and Technology of China, ABB Corporate Research Ltd., Switzerland, Natural Science Foundation of China (No. 29806011), and Foundation for University Key Teacher by Ministry of Education of China are very appreciated. The assistance from Dr. Tao Jiang is also appreciated. EF0002445 (20) Zhou, L. M.; Xue, B. Z.; Kogelschatz, U.; Eliasson, B. Energy Fuels 1998, 12 (6), 1191-1199.
