Ind. Eng. Chem. Res. 2004, 43, 4043-4047
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New Approach for Methane Conversion Using an rf Discharge Reactor. 1. Influences of Operating Conditions on Syngas Production Cheng-Hsien Tsai* and Tsung-Hua Hsieh Department of Chemical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan
Traditionally, the process for converting CH4 into syngas (CO and H2) is usually carried out at high temperature with catalysts. In this study, a new approach using a lab-scale radio frequency (rf) plasma reactor is made at room temperature with a single-step process. The major factors that affected the conversion and selectivity were examined by the fractional factorial experimental design method. Experimental results indicated that the higher methane conversion was achieved at either a higher applied power or inlet O2/CH4 molar ratio (R), while the simultaneous higher selectivities of H2 and CO were obtained at a lower R. At [CH4]in ) 33.3% and 4000 N/m2, the better operating conditions were at R ) 1 with 130 W. The methane conversion equaled 80.1%, and the selectivities of H2 and CO reached 92.6% and 74.0%, respectively, with a H2/CO molar ratio of 2.5. Introduction Methane (CH4) is the major component of natural gas, which is the most abundant and clean energy source. To convert CH4 directly into more valuable compounds, such as synthesis gas (syngas, H2 + CO), ethylene, methanol (CH3OH), and formaldehyde (CH2O), many promising processes are proposed.1-3 Traditionally, the catalytic conversion of CH4 to syngas and H2, which are important for use in methanol and ammonia synthesis, the Fischer-Tropsch reaction, and fuel cells, etc., has attracted great attention because the processes are wellestablished. The postulated reaction pathways involve initial partial oxidation of CH4 and O2 (eq 1) with feed to CO, CO2 and H2O, followed by steam reforming (eq 2), and CO2 reforming (eq 3).4
CH4 + 0.5O2 f CO + 2H2 (∆H°298 ) -36 kJ/mol) (1) CH4 + H2O f CO + 3H2 (∆H°298 ) +206 kJ/mol) (2) CH4 + CO2 f 2CO + 2H2 (∆H°298 ) +247 kJ/mol) (3) A number of studies have been made on the CH4 conversion process using highly active catalysts based on Ni, Co, Ir, Pd, Pt, and Ru group metals supported on Al2O3, TiO2, or oxidized diamond.4,5 However, the problems, including how to reduce the operating temperature, carbon depositions, and catalyst poisons, as well as elevating the conversion and the selectivity, need to be overcome. More studies are in process in order to develop a more economic, environmentally friendly alternative. * To whom correspondence should be addressed. Tel.: +8867-381-4526 ext. 5110. Fax: +886-7-383-0674. E-mail: chtsai@ cc.kuas.edu.tw.
Several discharge methods have been performed for the CH4 or CH4/O2 conversion, including high pulse frequency, pulsed corona, rf, microwave, dielectricbarrier discharge (DBDs), and AC plasmas, which are used to yield C2 hydrocarbons, methanol, or the diamondlike carbon film, not designed for the production of syngas.6-16 As for the syngas, it has been studied via the DBDs approach by converting CH4 with CO2 (eq 3). The results showed that the yields of H2 and CO were 52% and 14%, respectively, and the conversions of CH4 and CO2 reached 64% and 54%, respectively, at inlet CH4/CO2 ) 4, which seemed to improve the performance.17 So far, the partial oxidation of methane into syngas by the rf plasma-driven technology, which can generate homogeneous discharges at room temperature and at low-pressure environments, has not been examined and the rf plasma has potential with a single-stage, noncatalytic conversion process. The comprehensive ranges of design conditions are used to probe for the identification of the key variables in affecting the methane conversion and the selectivities of H2 and CO in this study. Experimental Section Similar experimental setup was described in detail elsewhere.18,19 Briefly, high-purity CH4, O2, and N2 (carrier gas) were supplied from compressed gas cylinders, their flow rate was adjusted with a calibrated mass flow controller, respectively, and was introduced into a gas mixer and then entered a 4.14-cm-i.d., 15-cm-long, laboratory-scale, vertical cylindrical-type, glass tube reactor. The plasma reactor was wrapped in two outer, symmetrical, 5.4-cm-height copper electrodes coupled to a 13.56 MHz rf generator (PFG 600, Fritz Huttinger Elektronik Gmbh) with a matching network (Matchbox PFM) to generate a physical discharge zone. The reactants and final mixtures were analyzed by a gas chromatograph (Varian, GC3800) equipped with a thermal conductivity detector for identifying CH4, H2,
10.1021/ie049958j CCC: $27.50 © 2004 American Chemical Society Published on Web 06/26/2004
4044 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 Table 1. Factors and Levels for the 26-2 Fractional Factorial Experimental Design with Defining Relation I ) ECRPQT level factor E: C: R: P: Q: T:
applied rf power (W) CH4 feeding concentration (%) inlet O2/CH4 molar ratio operational pressure (N/m2) total flow rate (sccm) temperature of feeds (K)
-
+
30 5 0.25 1333 50 303
130 33.3 2.0 4000 200 423
CO, CO2, C2H2, C2H4, and C2H6 and an online Fourier transform infrared spectrometer (Bio-Rad, model FTS7) for qualitative analyses of gaseous products. Furthermore, the accuracy of the results was checked by carbon mass balance. For cleaning the contaminants and residues as well as checking if any leaks occurred in this system after any experiment was performed, the pressure was pumped to below 1.3 N/m2 by attaching a mechanical pump and then switching to a dry pump to operate at the assigned system pressure. After the plasma conversion process was at steady state, N2 was passed through the overall system to clean up the reactor for at least 15 min. Finally, the pressure was reduced to 1.3 N/m2 again until the next run was conducted. The compositions of products are based on mole fraction of the gaseous effluent. The conversion of CH4 (V) and the selectivities of H2 (SH2, included H2O, and SH2*, excluded H2O) and CO (SCO) are calculated by the equations
V) (CH4 consumed)/(CH4 fed to the reactor) × 100% SH2 ) (H2 produced × 2)/(CH4 converted × 4) × 100% SH 2 * ) [H2]/([H2] + [C2H2] + 2[C2H4] + 3[C2H6]) × 100% SCO ) (CO produced)/(CH4 converted) × 100% Results and Discussion Identification of Key Factors by Fractional Factorial Design. In this study, the experimental design approach used the 26-2 fractional factorial design (FFD) method20 to reduce the experiments from 64 runs (by full factorial design method) to 16 runs and to discuss the influences of individual factors or the interaction of two factors on V, SH2, and SCO. The factors and the levels (-means low level, +means high level) for the FFD with defining the relation I ) ECRPQT are presented in Table 1. The design matrix and the experimental data are listed in Table 2. The results indicated that V could reach 100% by runs 8 and 16 while with the poor SH2 and SCO. In addition, the simultaneous maximum SH2 and SCO were observed at runs 4 and 11 to reach 98.2% and 81.7%, and 98.0% and 79.8%, respectively, but with only the poor V. Therefore, simultaneously reaching the maximum V, SH2, and SCO seemed to be in conflict. The quantitative graphical presentation by the statistical evaluation of the influences of factor showed that E, C, or R apparently affected V (Figure 1A), R dominated SH2 and SCO (Figure
Figure 1. Graphical presentation of the statistical evaluation of the influences of one factor and the combined effects of two factors on V (A), SH2 (B), and SCO (C). For details, refer to the text. Table 2. Design Matrix and the Experimental Data from the 26-2 Fractional Factorial Design factor run
E
C
R
P
Q
T
V (%)
SH2 (%)
SCO (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
+ + + + + + + +
+ + + + + + + +
+ + + + + + + +
+ + + + + + + +
+ + + + + + + +
+ + + + + + + +
61.1 72.8 96.2 13.9 67.9 99.9 69.0 100.0 71.4 63.6 15.6 94.8 99.6 68.8 32.5 100.0
80.2 93.9 26.4 98.2 37.6 4.5 92.0 15.9 95.9 28.5 98.0 91.2 19.4 93.1 34.6 3.2
62.8 74.1 46.1 81.7 61.0 19.5 71.7 38.5 77.2 47.7 79.8 69.3 45.2 74.0 58.2 18.9
1B,C). Also, P, Q, and T had only a smalll effect on V, SH2, and SCO. The combined effects of two factors indicated that only a low level of R favored the elevation of SH2 and SCO (Figure 1B,C), while it fairly decreased V (Figure 1A). A high level of C with a lower E also favored the increase of SH2 and SCO (Figure 1B,C), while resulting in a lower V (Figure 1A). Clearly, the effects of factors revealed that conflicts took place between E and C and R for simultaneously reaching the higher V, SH2, and SCO and must be compromised. Conversion at Theoretical Stoichiometric Ratio (R ) 0.5). The theoretical reaction for partial oxidation of CH4 can be written as CH4 + 0.5O2 ) CO + 2H2 (eq 1) with the stoichiometric ratio of O2/CH4 ) 0.5. Hence, the experiments were carried out at R ) 0.5 first. Figure
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4045
Figure 2. At inlet O2/CH4 ratio ) 0.5, the conversions of CH4 at different inlet concentrations of CH4 and the temperatures of feeds for various applied powers.
Figure 3. Conversions of CH4 (A) and the selectivities of H2 and CO (B) for various inlet O2/CH4 molar ratios at 130 W, 303 K, and [CH4]in ) 33.3%.
2 shows that V apparently increases from 58.6% to 94.1% for C ) 5% and elevates from 15.9% to 60.0% at C ) 33.3%, by increasing E from 30 to 130 W, at 303 K and 4000 N/m2. The results reveal that a larger applied rf power or a lower feeding concentration of CH4 is favored for the CH4 conversion. A higher feeding temperature of reactants, such as 423 K, did not affect the conversion obviously (Figure 2). Hence, the rf plasma system can be operated at room temperature (303 K) to achieve a close performance with a higher temperature of feeds. Selectivity and Conversion at 130 W. Though a larger E improved V, the condition at R ) 0.5 was unfavorable for the elevation of conversion of CH4. Hence, the experiments were operated with increased R from 0.25, 0.5, 1, 1.5 to 2, to elevate V significantly from 54.2%, 60.0%, 80.1%, 99.4% to 99.95%, respectively, at 130 W, 303 K, and 4000 N/m2 (Figure 3A), to denote that the contents of oxygen improve apparently V. The radicals or ions generated from O2 plasmalysis oxidize CH4 to not only enhance the conversion but also
Figure 4. Conversions of CH4 for various applied powers at different inlet O2/CH4 ratios, 303 K, and [CH4]in ) 33.3%.
inhibit the recombination of CH4 and the formation of C2 compounds, which are the major products of reforming pure CH4 in an rf plasma environment.21 In addition, a slightly higher V was observed at 4000 N/m2 than at 1333 N/m2 because the relative density of electrons available for effective impact dissociation with CH4 did not change significantly. Moreover, the greater gas density at a higher P elevated the probability of collisions, through a shorter mean free path of electrons with a lower mean electron temperature in the plasma. Figure 3B shows that the simultaneous higher SH2 and SCO can be obtained at R ) 1, to reach 92.6% and 74.0%, respectively, with V ) 80.1%, while decreasing to 46.1% and 57.2%, respectively, at R ) 1.5 with a high V of 99.4%. Hence, the better operating conditions shall be compromised between R ) 1 and R ) 1.5. The reasons may be that at a higher R only a little C2 hydrocarbons with abundant H2O and CO2 are generated, to result in SH2 and SCO decreasing rapidly, just like the oxygen-rich combustion reactions, though V is improved. Comparisons between O2/CH4 Ratios ) 1 and 1.5. The former results revealed that the better conversion conditions were at either R ) 1 or 1.5. Hence, V, SH2, SCO, and H2/CO molar ratios were compared for different E at C ) 33.3%, 4000 N/m2, and 303 K. First, Figure 4 shows that a greater V is achieved when enough power is supplied, reaching 98.5% for R ) 1.5 at only 110 W, and 80.1%, for R ) 1 at 130 W. The results reveal that the greater V is achieved at not only a high R, but also a high E, due to the increased E elevated the plasma density to result in the elevation of probability of the impact-dissociation reactions. In addition, Figure 5 shows that SH2 and SCO were apparently greater at R ) 1 than at R ) 1.5, especially at a high E. SH2 was highly dependent on E and R, varying between 87.9% and 92.6% at R ) 1, and was in the range of 77.3% and 46.1% at R ) 1.5 (Figure 5A). The higher SH2 (92.6%) at R ) 1 with 130 W than that at R ) 1.5 with 110 W (SH2 ) 50.7%) was observed because of the fewer H2O formed at lower oxygen feeds. SH2 increased with increased E at R ) 1 due to the C2 compounds that easily formed at a lower E, reacted with O2 into H2, H2O, and CO2 at a higher E. However, the calculation of SH2 was based on the consideration of H2O as the gaseous products. If the new selectivity of H2 (SH2*) was calculated excluding H2O as defined. SH2* was as high as 98.0% for R ) 1.5 at 110 W, to higher than 96.0% for R ) 1 at 130 W (Figure 5A). A little higher SH2 at R ) 1.5 than that at R ) 1 was observed
4046 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
for R ) 1.5 at 110 W. The higher H2/CO ratio provides more evidence that operating this rf plasma reactor at R ) 1 is a better choice. The H2/CO ratios decreased in accompaniment with the increased R to indicate that the H atoms decomposing from CH4 formed H2O easier than the decomposing of C atoms of CH4 to yield CO2. Conclusion
Figure 5. Selectivities of H2 and CO for various applied powers at different inlet O2/CH4 molar ratios, 303 K, and [CH4]in ) 33.3%.
Methane is successfully converted mainly into syngas via the noncatalytic plasma-oxidized process in an rf plasma reactor at room temperature. The key operating parameters, identified via the experiment design method, were the applied power, inlet O2/CH4 molar ratio, and the CH4 feeding concentration. At [CH4]in of 33.3%, 303 K, and 4000 N/m2, the results revealed that better conversion conditions could be achieved at either R ) 1 (at 130 W) or at R ) 1.5 (at 110 W). Though at R ) 1.5, the CH4 conversion, selectivities of H2 and CO, and energy utility rate of H2 and CO reached 98.5%, 50.7%, 59.1%, 4.1 g/kW‚h, and 22.8 g/kW‚h, respectively. These were consistent performances with R ) 1, to reach 80.1%, 92.6%, 74.0%, 4.1 g/kW‚h, and 22.8 g/kW‚h, respectively. However, at R ) 1 with 130 W, a higher H2/CO ratio of 2.5 was achieved than at R ) 1.5 with 110 W (H2/CO ) 1.7), so as to suggest the optimal operating conditions. Some calculations indicate that on a mole basis for methane the cost of such a conversion is ∼4-8 U.S. cents/mol while the price of natural gas (chiefly methane) is on the order of 0.4 U.S. cents/mol. Though it should be noted that the rf plasma technology is not practical at this stage, because of its relative low pressure for generating glow discharge with a lower effluent temperature; this shall be elevated as high as possible in the future. The results still reveal that the rf approach has potential for producing syngas from methane/oxygen mixtures when elevating the pressure in the future. Acknowledgment
Figure 6. H2/CO molar ratios for various applied powers at different inlet O2/CH4 ratios, 303 K, and [CH4]in ) 33.3%.
because more hydrocarbons formed at a lower R that had fewer oxygen. When E increased from 30 to 130 W, SCO decreased slightly from 76.7% to 74.0% for R ) 1, while SCO reduced obviously to 59.1% for R ) 1.5 at 110 W (Figure 5B). This may be caused by the greater E leading to a larger plasma density and a higher R providing more oxygen radicals, both to elevate the formation rate of CO2 to conduct to the reduced SCO. By calculation, the results show that the rate of H2 and CO production is 0.53 and 2.96 g/h, respectively, for R ) 1 at 130 W, higher than these for R ) 1.5 at 110 W (0.36 and 2.91 g/h, respectively). However, for R ) 1 at 130 W, the energy utility yield of 4.1 and 22.8 g/kW‚h for H2 and CO production, respectively, is almost equal to the conditions of R ) 1.5 at 110 W (reaching 3.2 and 26.4 g/kW‚h, respectively). Finally, the H2/CO molar ratios in effluents were presented at different E. Figure 6 shows that the mean values of H2/CO ratios were 2.68, 2.42, 1.82, and 1.24 for R ) 0.5, 1.0, 1.5, and 2.0, respectively. At R ) 1 and 130 W, the H2/CO ratio was 2.5, while it was only 1.7
We thank the National Science Council in Taiwan for the financial support of this research work (Grant NSC 91-2218-E-151-001). Literature Cited (1) Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F Selective oxidation of methane to synthesis gas using transition metal catalysts. Nature 1990, 344, 319. (2) Spencer, N. D.; Pereira, C. J. Partial oxidation of CH4 to HCHO over a MoO3-SiO2 catalyst: a kinetic study. AIChE J. 1987, 33, 1808. (3) Periana, R. A.; Taube, D. J.; Evitt, E. R.; Loffler, D. G.; Wentrcek, P. R.; Voss, G.; Masuda, T. A mercury-catalyzed, highyield system for the oxidation of methane to methanol. Science 1993, 259, 340. (4) Nakagawa, K.; Nishimoto, H.; Kikuchi, M.; Egashira, S.; Enoki, Y.; Ikenaga, N.; Suzuki, T.; Nishitani-Gamo, M.; Kobayashi, T.; Ando, T. Synthesis gas production from methane using oxidized-diamond-supported group VIII metal catalysts. Energy Fuels 2003, 17, 971. (5) Zaman, J. Oxidative processes in natural gas conversion. Fuel Process Technol. 1999, 58, 61. (6) Yao, S.; Nalayama, A.; Suzuki, E. Methane conversion using a high-frequency pulsed plasma: important factors. AIChE J. 2001, 47, 413. (7) Yao, S.; Nalayama, A.; Suzuki, E. Methane conversion using a high-frequency pulsed plasma: discharge features. AIChE J. 2001, 47, 419.
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Received for review January 10, 2004 Revised manuscript received April 22, 2004 Accepted May 23, 2004 IE049958J
