Energy & Fuels 2008, 22, 693–694
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Communication High-Efficiency Dry Reforming of Biomethane Directly Using Pulsed Electric Discharge at Ambient Condition Yasushi Sekine,* Junya Yamadera, Shigeru Kado, Masahiko Matsukata, and Eiichi Kikuchi Department of Applied Chemistry, Waseda UniVersity, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan ReceiVed September 15, 2007. ReVised Manuscript ReceiVed NoVember 22, 2007 Biomethane is produced by the fermentation of biomassderived organic waste, such as waste from stock breeding. From the viewpoint of better energy use, biomethane, which is a kind of biomass, is an efficient energy source. Biomethane contains about 40% carbon dioxide, 60% methane, and highly concentrated hydrogen sulfide (H2S) [a few thousand parts per million (ppm)]. Existing processes for biomethane use involve predesulfurization and CO2 separation because of the high concentrations of H2S and CO2 in biomethane. Dry reforming of biomethane (eq 1) is a promising process for using biomethane; however, catalytic reforming requires the removal of impurities, such as H2S to prevent deactivation of the catalyst. dry reforming: CH4 + CO2 f 2CO + 2H2
(1)
In addition, catalytic dry reforming of biomethane requires temperatures as high as 1000 K because of thermodynamic equilibrium limitations. Many papers have described catalytic dry reforming of methane,1 but almost all indicated similar problems, such as, the requirement of expensive catalysts, deactivation of the catalyst because of the presence of small amounts of sulfur or metal sintering, coke formation, and the need for a robust system that enables high-temperature operation.1–5 Our previous study6 showed that a nonequilibrium pulsed discharge was a good means of converting methane by dry reforming; the process could be performed at ambient temperature and atmospheric pressure with high-energy efficiency. Here, we report on the successful direct conversion of biomethane into syngas at ambient temperature and atmospheric pressure without gas cleaning. Experiments were performed using a flow-type reaction apparatus equipped with two oppositely charged electrodes. The reactor configuration is described in the literature.7 A pulsed, high-voltage electric discharge was applied within the electrode gap using a high-voltage power supply. The reaction gas was a mixture of 59.7% methane and 40.2% carbon dioxide, and the * To whom correspondence should be addressed. E-mail: ysekine@ waseda.jp. (1) Fisher, F.; Tropsch, H. Brennst. Chem. 1928, 9, 39. (2) Rostrupnielsen, J. R.; Hansen, J. H. B. J. Catal. 1993, 144, 38–49. (3) Nagaoka, K.; Takanabe, K.; Aika, K. Chem. Commun. 2002, 1006– 1007. (4) Wei, J.; Iglesia, E. Phys. Chem. Chem. Phys. 2004, 13, 3754–3759. (5) Liu, C. J.; Xue, B.; Eliasson, B.; He, F.; Li, Y.; Xu, G. H. Plasma Chem. Plasma Process. 2001, 21, 301–310. (6) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K. Chem. Commun. 2001, 415–416. (7) Sekine, Y.; Urasaki, K.; Kado, S.; Matsukata, M.; Kikuchi, E. Energy Fuels 2004, 18, 455–459.
Figure 1. Effect of H2S on dry reforming of methane: current, 4 mA; electrode gap distance, 8 mm; ambient temperature; and atmospheric pressure.
total flow rate of the reaction gas was 20 cc/min. The experiments were performed for two conditions of H2S, i.e., with and without 960 ppm of H2S. Input electric energy required for discharge was only 1.2 W. Waveforms of discharge and energy consumption were examined using a digital oscilloscope and probes. The produced gas was analyzed by gas chromatography (GC); sulfur-containing compounds such as H2S and carbon disulphide were analyzed by GC and sulfur detectors. Both gases of the mixture, i.e., CH4 and CO2, reacted after being activated by the nonequilibrium discharge, irrespective of the addition of H2S, and formed syngas, as shown in eq 1. However, the conversion percentage for both gases was slightly higher with H2S than without it. The effect of H2S on dry reforming of methane is shown in Figure 1; the conversion of methane and carbon dioxide was 10% higher for the gas containing H2S compared to that without H2S, under identical reaction conditions. No problems, such as coking or corrosion of reactor/electrodes by sulfuric compounds, were encountered in either case. Thus, inclusion of H2S in biomethane positively affects dry reforming by nonequilibrium electric discharge. We infer that the enhancement in conversion was caused by the promotional effect of the HS radical. The relationship between input current and the conversion of methane and carbon dioxide is shown in Figure 2. Methane and carbon dioxide conversion increased proportionally with the input current. The ratio of hydrogen/carbon monoxide was about 1.7–1.8; the value decreased slightly with an increased input current caused by a slight increase in the selectivity to C2 hydrocarbon. The relationship among input current and concentrations of sulfur-containing products and the dry reform-
10.1021/ef700552u CCC: $40.75 2008 American Chemical Society Published on Web 12/12/2007
694 Energy & Fuels, Vol. 22, No. 1, 2008
Communications Table 1. Energy Balance and Efficiency of Dry Reforming of Biomethane by Nonequilibrium Electric Discharge at Ambient Temperature (298 K) consumption rate (mmol min-1) CH4 CO2 electricity (W) (a) E input CO H2 C2H4 C2H6 C2H2 (b) E output
Figure 2. Conversion ratios of methane and carbon dioxide and hydrogen/carbon monoxide ratio during dry reforming of biomethane, in the presence of 960 ppm of H2S, 4 mm electrode gap, atmospheric pressure, and ambient temperature.
Figure 3. Concentrations of sulfur-containing compounds for simultaneous desulfurization during dry reforming of biomethane, in the presence of 960 ppm of H2S, 4 mm electrode gap, atmospheric pressure, and ambient temperature.
ing of biomethane is shown in Figure 3. H2S decreased uniformly with an increased input current from 960 to 120 ppm; transient behavior was observed in the formation of CS2 from
formation rate (mmol min-1)
0.305 0.149 1.2 0.297 0.484 0.004 0.001 0.058 energy efficiency (%) f (b)/(a) × 100
heating value (kJ min-1) 0.271 0.000 0.072 0.343 0.071 0.135 0.006 0.001 0.075 0.289 84.1
the reaction between H2S and CH4. The yield of solid sulfur gradually increased by the following series of reactions: H2S f CS2 f solid sulfur These results show that H2S in the biomethane gradually decomposed by the electric discharge and formed sulfurcontaining radicals. The radicals reacted with carbon atoms to form CS2. Subsequently, CS2 was also decomposed by the electric discharge, converted into solid sulfur, and deposited on and around the electrodes. We evaluated the energy efficiency of this novel reaction, wherein the following two processes occur simultaneously: (1) dry reforming of biomethane and (2) desulfurization. Calculation was made based on the heating value of reactants/products and energy consumption by electric discharge. The calculated energy efficiency based on the lower heating value of methane and the products and input electric power are shown in Table 1. In this table, the amounts of sulfur compounds are not included in the calculation because they are negligibly small (only a few hundred ppm). Energy efficiency for this process was as high as 84.1%. In conclusion, we developed a novel process for converting biomethane to syngas accompanied by desulfurization under ambient conditions without pretreatment or the presence of a catalyst. Total energy efficiency was very high (84.1%), and H2S was directly converted to solid sulfur. EF700552U
