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Nonequilibrium Pulsed Discharge: A Novel Method for Steam Reforming of Hydrocarbons or Alcohols Yasushi Sekine,† Kohei Urasaki,*,† Shigeru Kado,‡ Masahiko Matsukata,† and Eiichi Kikuchi† Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan, and Department of Mechanical Engineering and Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo 152-8552, Japan Received July 11, 2003. Revised Manuscript Received November 18, 2003
Steam reforming of methane, propane, hexane, cyclohexane, methanol, and ethanol using a nonequilibrium pulsed discharge was investigated under the conditions of atmospheric pressure and low temperature (393 K) without the use of catalyst. In each case, steam reforming proceeded efficiently and selectively and hydrogen was formed as a main product. Although the steam/ carbon ratio was 1, there were trace amounts of carbon deposition or wax formation. The energy injection for the discharge region calculated by current and voltage waveforms measured by a digital signal oscilloscope was very small. As compared with the conventional catalytic steam reforming process, this method has some advantages such as fast start-up, quick response, and miniaturization and simplification of a hydrogen production system. Therefore we consider that the hydrogen production system employing a nonequilibrium pulsed discharge has a potential for being an effective way for supplying hydrogen or syngas.
Introduction In recent years, proton-exchange membrane fuel cell (PEMFC) technology has attracted considerable attention on a worldwide level because of its high energy efficiency and clean exhaust gas. Due to its small size, lightweight, fast start-up, and rapid response, PEMFC is suitable for transportation applications or small-scale power generation. One of the problems of commercializing PEMFC is the difficulty regarding hydrogen supply. Because hydrogen is a gaseous fuel and the energy per weight is low, its storage and transportation are very difficult. Thus, the on-site steam reforming of hydrocarbons1-3 such as natural gas, LPG, or gasoline (eqs 1 and 2) or of alcohols such as methanol4,5 or ethanol6,7 (eqs 3, 4, and 5) is considered the best available method for supplying hydrogen to PEMFC. Unfortunately, conventional catalytic steam reforming entails some difficulties. Steam reforming of hydrocarbons has the merit of coordination with existing infrastructures. However, as shown in eqs 1 and 2, steam reforming of hydrocarbons is strongly endother* Corresponding author. Tel. & Fax: +81-3-5286-3850. E-mail:
[email protected]. † Waseda University. ‡ Tokyo Institute of Technology. (1) Rostrup-Nielsen, J. R. Phys. Chem. Chem. Phys. 2001, 3, 283288. (2) Pettersson, L. J. Int. J. Hydrogen Energy 2001, 26, 243-264. (3) Armor, J. N. Appl. Catal. A, Gen. 1999, 176, 159-176. (4) Schmidt, V. M.; Brockerhoff, P.; Hohlein, B.; Menzer, R.; Stimming, U. J. Power Sources 1994, 49, 299-313. (5) Takezawa, N.; Iwasa, N. Catal. Today 1997, 36, 45-56. (6) Haga, F.; Nakajima, T.; Miya, H.; Mishima, S. Catal. Lett. 1997, 48, 223-227. (7) Cavallaro, S.; Freni, S. Int. J. Hydrogen Energy 1996, 21, 465469.
mic. Therefore high temperature (700-1000 K) is required. This restricts the selection of reactor materials, causes slow start-up, and enlarges and complicates the PEMFC systems. The drawback concerning slow start-up is fatal in the case of transportation applications. Moreover, coking8,9 and deactivation10 of the catalyst decrease the operating time of PEMFC systems and make it difficult to quickly adjust the amount of hydrogen produced.
CH4 + H2O f CO + 3H2
∆H° ) 206 kJ mol-1 (1)
C3H8 + 3H2O f 3CO + 7H2 ∆H° ) 498 kJ mol-1 (2) CH3OH + H2O f CO2 + 3H2 ∆H° ) 50 kJ mol-1
(3)
C2H5OH + H2O f 2CO + 4H2 ∆H° ) 256 kJ mol-1 (4) CH3OH f CO + 2H2
∆H° ) 91 kJ mol-1
(5)
On the other hand, steam reforming of alcohols has some merits. For example, alcohols contain no sulfur, which deactivates the catalyst. Ethanol has the added benefit of renewability, as it can be produced from a variety of crops.11,12 However, the catalytic steam reforming of ethanol has the same problems caused by (8) Trimm, D. L. Catal. Today 1999, 49, 3-10. (9) Rostrup-Nielsen, J. R. Catal. Today 1997, 37, 225-232. (10) Lindstrom, B.; Pettersson, L. J. Catal. Lett. 2001, 74, 27-30. (11) Sun, Y.; Cheng, J. Y. Bioresour. Technol. 2002, 83, 1-11.
10.1021/ef034029a CCC: $27.50 © 2004 American Chemical Society Published on Web 01/24/2004
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Figure 2. Steam reforming or decomposition of hydrocarbons or alcohols. Figure 1. Experimental setup for nonequilibrium pulsed discharge.
high reaction temperature (∼800 K). Therefore, the obstacles of slow start-up and responsiveness, and deactivation and coking of the catalysts remain. In this study, we investigated how nonequilibrium pulsed discharge can be used in the steam reforming of hydrocarbons or alcohols under conditions of atmospheric pressure and low temperature (393 K) without the use of any catalyst. As its name suggests, this discharge is one of the nonequilibrium plasmas.13-16 When exposed to a pulsed discharge, a large amount of electrons can be irradiated (30-40 A) in a very short time (less than 1 µs) at constant intervals (in the order of milliseconds). Molecules are decomposed by electron impact even if they are very stable molecules such as methane. Because of the very short irradiation time, discharge can be kept stable under atmospheric pressure and the supplied energy can be held down as compared with thermal plasmas. Moreover, the temperature of the gas phase does not increase despite very high electron temperature; thus, useless side-reactions can be suppressed. So far, this method can be utilized effectively for acetylene synthesis, dry reforming, and other applications.17-21 Experimental Section In all experiments a quartz tube was used as a flow-type reactor as shown in Figure 1. Two stainless steel rods inserted from each end of the quartz tube were used as electrodes. Water, higher hydrocarbons, or alcohols were injected using a microsyringe pump (Bioanalytical Systems Inc. MF-9090) or a mini chemical pump (Nihon Seimitsu Kagaku Co. Ltd. NP-KX-1005), and evaporated at 413 K in a preheating zone. The reaction temperature was set at 393 K for the sole purpose
of preventing condensation of reactant gases such as steam, higher hydrocarbons, or alcohols. No catalyst was used. A DC power supply (Matsusada Precision Inc. HARb-40R30) was used to produce the nonequilibrium pulsed discharge. The output polarity was variable (negative or positive) and the capacity was (30 kV, 0-10 mA. The output from the power supply was controlled by varying the fixed current value. When the discharge occurred, the output from the power supply was cut off due to the protection circuit against over-current and the voltage became 0 V within 100 ns. After the current became 0 A, the output system was restored immediately to repeat the discharge periodically. The higher the output from the power supply, the higher the pulse discharge frequency. Thus, the pulse discharge frequency was controlled by changing the output from the power supply. All products were analyzed using gas chromatography equipped FID and TCD (Shimadzu GC-14B), and GC-MS (Shimadzu QP1100EX). The waveforms of current and voltage were observed by means of a digital signal oscilloscope (LeCroy Japan Corp. 9314C, 400 MHz bandwidth) using a voltage probe (PMK-20 kV, 100 MHz, LeCroy) and current transformer (AP015, 50 MHz, LeCroy). The conversion of hydrocarbons or alcohols, selectivity to products, and product yield are defined as follow:
Conversion ) (1 - nr/n0r) × 100%
(6)
Selectivity ) [carbon number × nx/(carbon number × mole number of reactant molecule converted)] × 100% (7) Product yield ) [nx /
∑(mole number of each product)] ×
100% (8)
where nr, n0r, and nx are the mole number of reactant in the outlet gas, the mole number of reactant in the inlet gas, and the mole number of products, respectively.
Results and Discussion (12) Beguin, P.; Aubert, J. P. FEMS Microbiol. Rev. 1994, 13, 2558. (13) Eliasson, B.; Kogelschatz, U. IEEE Trans. Plasma Sci. 1991, 19, 1063-1077. (14) Li, Y.; Xu, G. H.; Liu, C. J.; Eliasson, B.; Xue, B. Z. Energy Fuels 2001, 15, 299-302. (15) Yao, S. L.; Okumoto, M.; Nakayama, A.; Suzuki, E. Energy Fuels 2001, 15, 1295-1299. (16) Okazaki, K.; Kishida, T.; Ogawa, K.; Nozaki, T. Energy Convers. Manage. 2002, 43, 9-12. (17) Kado, S.; Sekine, Y.; Fujimoto, K. Chem. Commun. 1999, 24852486. (18) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K. Chem. Commun. 2001, 415-416. (19) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K. Therm. Sci. Eng. 2003, 11, 1-8. (20) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K. Fuel 2003, 82, 1377-1385. (21) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K.; Nozaki, T.; Okazaki, K. Fuel 2003, 82, 2291-2297.
Figure 2 shows the results of the steam reforming of methane, propane, n-hexane, cyclohexane, methanol, and ethanol and the decomposition of methanol and ethanol using nonequilibrium pulsed discharge. Methane and propane served as models for natural gas and LPG, respectively, while n-hexane and cyclohexane served as models for gasoline. Reaction conditions were as follows: conversion, 30%; molar steam/carbon ratio ) 1 (except for the case of decomposition of alcohols); gap distance, 2.1 mm; flow rate, 20-35 cm3 min-1; temperature, 393 K; pressure, 0.1 MPa. Argon was added as carrier gas for n-hexane, cyclohexane, methanol, and ethanol. In each case, steam reforming proceeded effectively and selectively, and
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Figure 3. Effect of pulse discharge frequency on steam reforming of methane: ((A) O: methane conversion, 2: carbon monoxide, 1: C2 compounds, [: hydrogen, (B) b: hydrogen formation rate, 0: H2/CO ratio). Conditions: total flow rate, 20 cm3 min-1; molar CH4/H2O ratio ) 1; gap distance, 2.1 mm; temperature, 393 K; atmospheric pressure.
Figure 4. Effect of partial pressure of steam on steam reforming of methane: ((A) b: carbon monoxide, 2: C2 compounds, 1: carbon dioxide, O: pulse discharge frequency, (B) O: acetylene, 0: ethylene, ]: ethane). Conditions: methane conversion rate, 19%; total flow rate, 35 cm3 min-1; methane flow rate, 10 cm3 min-1; gap distance, 2.1 mm; temperature, 393 K; atmospheric pressure.
hydrogen was a main product. In addition to hydrogen, carbon monoxide, C2 compounds (mainly acetylene), carbon dioxide, and methanesexcept in case of methane steam reformingswere produced. In case of the steam reforming of n-hexane and cyclohexane, small amounts of C3 and C4 compounds were produced. Other products such as higher hydrocarbons or oxygenated compounds were not observed. The formation of methane, which lowers the hydrogen yield, can be limited by further adjustments to the process. The results of our experiment demonstrated the subtle advantages to the nonequilibrium pulsed discharge method along with the more obvious ones. Carbon deposition and wax formation are serious problems in the steam reforming process because these cause a blockage in the reactor or deactivation of the catalyst. To prevent these problems, the catalytic steam reforming process requires a surplus of steam, thereby lowering the net energy efficiency of the PEMFC systems. Using nonequilibrium pulsed discharge, carbon deposition and wax formation were prevented under the condition of molar steam/carbon ) 1; discharge remained stable for a relatively long period of time. In addition, there was much greater responsiveness to changes in the pulse discharge frequency in nonequilibrium pulsed discharge than in catalytic steam reforming. Of course, operating temperature has a great significance. Compared to the temperature demand of catalytic steam reforming, nonequilibrium pulsed discharge operated at a very reasonable temperature (393 K), resulting in a shortened start-up time. Moreover, the low operating temperature simplifies the overall PEMFC systems, reduces its size by eliminating the
need for a large heat exchanger, and holds down the cost of reactor materials. In our recent investigation of acetylene synthesis from methane using nonequilibrium pulsed discharge,21 we suggested a reaction scheme in which methane is not dissociated hydrogen-by-hydrogen (i.e., CH4 f CH3 f CH2 f CH) but is directly dissociated into a CHx fragment due to electron collision. In the case of the steam reforming of methane, we supposed that methane was directly dissociated into CHx fragments. CHx fragments reacting with steam produced carbon monoxide, while CHx fragments reacting with each other produced acetylene. Moreover, we supposed that hydrocarbons and ethanol were dissociated into CHx as well; this inference was suggested by the fact that the main products in the steam reforming of hydrocarbons were hydrogen, carbon monoxide, and acetylene. On the other hand, we supposed that the reaction scheme of the steam reforming of methanol differed from that of hydrocarbons and ethanol because it produced much more carbon monoxide. In the steam reforming of methanol, the amount of CHx fragment produced by electron impact was very small because the amount of acetylene was negligible. Thus carbon monoxide and hydrogen were produced by the decomposition of methanol, and steam did not play an important role. Further experiments revealed that the parameters of pulse discharge frequency, gap distance, flow rate, and partial pressure of steam influenced the results of the steam reforming of hydrocarbons or alcohols using nonequilibrium pulsed discharge. Figures 3 and 4 show the results under various pulse discharge frequencies and partial pressures of steam in experiments involving
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Figure 5. Current and voltage waveforms: (a) 35 Hz, (b) 86 Hz, (c) 210 Hz. Conditions: total flow rate, 20 cm3 min-1; molar CH4/H2O ratio ) 1; gap distance, 2.1 mm; temperature, 393 K; atmospheric pressure.
Sekine et al.
Figure 7. Energy efficiency and hydrogen production energy on steam reforming of methane: (b: energy efficiency, 0: hydrogen production energy). Conditions: total flow rate, 20 cm3 min-1; molar CH4/H2O ratio ) 1; gap distance, 2.1 mm; temperature, 393 K; atmospheric pressure.
nearly in proportion to pulse discharge frequency. By controlling pulse discharge frequency, energy injection for discharge region could be controlled. Although Figure 5 shows the waveforms when the reactant gas was a mixture of methane and steam, the waveforms in the case of other reactant gases mirrored this phenomenon: Figure 6. Energy injection and energy per one pulse on various pulse discharge frequencies: (O: energy injection for discharge region, 9: energy injection per one pulse). Conditions; total flow rate, 20 cm3 min-1; molar CH4/H2O ratio ) 1; gap distance, 2.1 mm; temperature, 393 K; atmospheric pressure.
methane, respectively. In the case of pulse discharge frequency, the conversion of methane and formation of hydrogen increased with an increase in pulse discharge frequency, while selectivity to products other than hydrogen was independent of pulse discharge frequency. The results of changing electrode distance and flow rate mirrored this phenomenon, affecting hydrogen formation but not the other products. As mentioned above, C2 compounds lower the amount of hydrogen obtained. In case of steam reforming of methane, the ratio of acetylene in C2 compounds was higher than 70% and the loss of hydrogen was very small. Therefore, selectivity to hydrogen was very high, more than 85%. The stoichiometric H2/CO ratio of the steam reforming reaction of methane is 3 as shown in eq 1 and the experimental value was relatively close to this, at approximately 4. Because of the formation of C2 compounds and carbon dioxide, the experimental value was slightly larger than the stoichiometric value. In addition to formation of C2 compounds and carbon dioxide, H2/ CO ratio can be larger due to carbon deposition and wax formation. However, as mentioned above, there were trace amounts of carbon deposition and wax formation and H2/CO ratio was larger than stoiciometric ratio only due to formation of C2 compounds and carbon dioxide. The waveforms of current and voltage under some pulse discharge frequencies are shown in Figure 5. The waveforms of current and voltage were not influenced dramatically by changes of pulse discharge frequency. We calculated the energy injection for the discharge region according to eqs 9 and 10. The results are shown in Figure 6. The energy injection of one pulse was not changed dramatically and decreased slightly with an increase in pulse discharge frequency. Therefore, the energy injection for the discharge region was increased
Pi )
∑{Vi +1 + Vi}/2} × {(Ii+1 + Ii)/2} × (ti+1 - ti)
(9)
where Pi is the injection energy per one pulse (J), V is voltage (V), I is current (A), t is time (s).
P ) pulse discharge frequency (Hz) × injection energy of one pulse (J) (10) where P is the injection energy per one pulse (W). On the other hand, the partial pressure of steam influenced the selectivity to products other than hydrogen, as shown in Figure 4. The higher the partial pressure of steam, the larger the selectivity to carbon monoxide and the smaller the selectivity to C2 compounds. Presumably this is because the amount of CHx fragments produced from methane by electron impact increased, as did their reactions with steam. Selectivity to carbon dioxide increased slightly with the increased concentration of carbon monoxide, as the water-gas-shift reaction proceeded. Figure 4B shows the concentration of acetylene, ethylene, and ethane (described “C2Hy” in the vertical axis of Figure 4B) in C2 compounds. As in the selectivity to carbon monoxide or carbon dioxide, the concentration of C2Hy in C2 compounds was influenced by the partial pressure of steam. The higher the partial pressure of steam, the smaller the selectivity to acetylene and the larger the selectivity to ethylene. The concentration of ethane increased slightly, too. We calculated the energy efficiency of the steam reforming of methane using a nonequilibrium pulsed discharge according to eq 11. In addition to energy efficiency, hydrogen formation energy which shows the energy required for hydrogen production was calculated according to eq 12, too. The results are shown in Figure 7. Although the energy efficiency decreased slightly with an increase in pulse discharge frequency, the energy efficiency was very high, about 30-60%. Because the energy efficiency decreased with an increase in pulse discharge frequency, the hydrogen formation energy increased slightly with an increase in pulse discharge frequency. These results were in no way inferior to the energy efficiency of the catalytic steam reforming of
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methane. We would now like to go on to develop a new power supply suitable for a nonequilibrium pulsed discharge, to control the current and voltage waveforms, and improve energy efficiency and hydrogen formation energy.
∑∆H° × FR)/P} × 100
η ) {(
(11)
where η is energy efficiency (%), ∆H° is standard enthalpy of formation (J mol-1), FR is formation rate of product (mol min-1), and P is energy injection for the discharge region (W).
PH2 ) P/FRH2
(12)
where PH2 is hydrogen formation energy (kJ L-1), P is energy injection for the discharge region (W), and FRH2 is the formation rate of hydrogen (L s-1). Conclusions In conclusion, we found that a nonequilibrium pulsed discharge was a very effective method for hydrogen production by methods of the steam reforming of hydrocarbons or alcohols under the conditions of low temperature (393 K), molar steam/carbon ratio ) 1, and atmospheric pressure without the use of a catalyst. In any case, hydrogen was produced as a main product. Carbon deposition and wax formation, which caused the cessation or destabilization of the discharge, were not observed, and the discharge could be kept stable. Products other than hydrogen included carbon monoxide and carbon dioxide, which are produced by steam
reforming or the water-gas-shift reaction, methane, C2 compounds, and C3 and C4 compounds in the case of n-hexane and cyclohexane. The formation of methane and C2, C3, and C4 compounds lowers the hydrogen output. While the production of methane and C3 and C4 compounds was very small regardless of reaction conditions, the volume of C2 compounds, comprised mostly of acetylene, was larger. Even so, the influence of these compounds on hydrogen formation was low, and selectivity to C2 compounds could be controlled by changing the partial pressure of steam. Moreover, we consider that it is possible to increase the selectivity to carbon monoxide and decrease the selectivity to C2 compounds by combining the discharge with a catalyst because the selectivity to carbon monoxide could be increased by using a nickel catalyst in the case of carbon dioxide reforming.22 Low reaction temperature (393 K) was a very attractive advantage because start-up could be shortened to less than 1 s; also, cheaper, more durable reactor materials could be used. The hydrogen formation rate could be controlled very quickly and easily by controlling certain parameters of pulse discharge frequency, the gap distance, flow rate, and the partial pressure of steam. The numerous advantages of nonequilibrium pulsed discharge cannot easily be duplicated in the catalytic process. Therefore, we believe that steam reforming using nonequilibrium pulsed discharge is a promising candidate as a hydrogen generator in PEMFC systems. EF034029A (22) Kado, S.; Urasaki, K.; Nakagawa, H.; Miura, K.; Sekine, Y. ACS Symp. Ser. 2003, 852, 302-313.
