Conversion of Methane to Hydrogen via Pulsed Corona Discharge

Transcription

1 Journal of Natural Gas Chemistry 13(2004)82 86 Conversion of Methane to Hydrogen via Pulsed Corona Discharge Lekha Nath Mishra, Kanetoshi Shibata, Hiroaki Ito, Noboru Yugami and Yasushi Nishida Energy and Environmental Science, Graduate School of Engineering, Utsunomiya University, Yoto, Utsunomiya, Tochigi , Japan [Manuscript received December 24, 2003; revised January 16, 2004] Abstract: Experiments are performed to develop a pulsed corona discharge system for the conversion of methane to hydrogen at atmospheric pressure ( 760 Torr) without using a catalyst. The corona discharge was energized by µs wide voltage pulses ( 7 kv) at a repetition rate of about khz. The residual gases were characterized by mass spectrometry. The conversion of methane is as high as 50.8% producing the 70% yield of hydrogen. The influences of argon on the discharge of methane were studied. This result could be useful for the mass production of hydrogen in both academic and industrial point of view. Key words: pulsed corona discharge, mass spectrometer, methane, hydrogen 1. Introduction The world needs a consumption of low price energy which increases day by day. Some studies predict that the global energy consumption will at least be tripled in the next 30 years [1]. Consumption of energy depends evidently on the increase in population and economic situation. The environmental contaminations, such as carbon dioxide production by day to day technologies led to an anxious level [2,3], that is why refining or filtering methods have to be developed. We believe that the future source of energy will be the controlled thermonuclear fusion for producing electric power, but it is very far to be economically feasible in the present world. The hydrogen fuel is believed to be an alternative means for the near future, because it is the most abundant element on the earth (more than 75% of the environmental elements), and renewable. Electrical discharges play an important role in the technologies, such as, semiconductor fabrication, high efficiency lighting, arc welding, high voltage switch-gear, plasma production and many chemical treatments of gases, liquids and surfaces. Due to the complexity of the plasma production by such discharges, their application is still largely based on experimental knowledge. Recently, the conversion of methane using both thermal [4,5] and non-thermal [6,7] plasmas has attracted considerable attention. In the former case, plasma replaces catalysis and accelerates chemical reactions due to the high temperature effect. But in the latter case, plasma can accelerate chemical reactions at the low temperature as well due to active species generation by fast electrons. Considerable research efforts have been made on dielectric barrier discharge (DBD) conversion of methane into more useful chemicals including synthesis gas, gaseous and liquid hydrocarbons [8 10]. In addition to this, the by-product of this conversion, such as the growth of carbon nanotubes [11] and diamond-like carbon [12] at atmospheric pressure glow discharge is often taken into account. Some researchers paid much of their atten- Corresponding author. Permanent address: Department of Physics; Patan Multiple Campus, Patangate, Lalitipur, Nepal; mishra@plasma.ees.utsunomiya-u.ac.jp; Permanent address: Kanto Polytechnic College, Mitake Yokokura, Oyama, Tochigi , Japan

2 Journal of Natural Gas Chemistry Vol. 13 No tion to the field of microwave plasma in utilizing different parameters such as broad discharge spectrum, high energy density, and the ratio of electron temperature to ion for the conversion of methane to hydrogen [13]. Formation of microwave plasma field needs very high electric field intensity under the normal pressure. Meanwhile, the experiment indicated that this methane plasma was easily converted into arc plasma which made serious carbon deposition and thus continuous operations for the above conversion could not be obtained. Besides this, a high power pulsed ultraviolet laser [14], solar-thermal process [15], ferroelectric packed-bed reactor and silent discharge plasma reactor [16] have been used for the conversion of methane to produce hydrogen gas as well. It is worth mentioning here that there are no such related works dealing with the production of hydrogen using brush electrode in pulsed corona discharge. Hence, keeping in mind, we have developed a new technique in our laboratory for the conversion of methane to hydrogen at atmospheric pressure and ambient temperature. The present paper is organized as follows. In Sec.2, the experimental setup is introduced; Sec.3 gives the results and its discussion and the paper is concluded in Sec Experimental setup Figure 1 shows the experimental setup. The corona discharge was performed in Chamber I (φ 150 mm 360 mm), called the discharge chamber. This discharge chamber was connected with a narrow stainless steel tube (φ 7 mm 1500 mm) to Chamber II (φ 150 mm 300 mm) through a needle valve. This chamber II, called the detection chamber, was fitted to the mass spectrometer analyzer head (model Pfeiffer vacuum Prisma T M 80 quadrapole mass Spectrometer QMS 200). The vacuum system in the discharge chamber side consisted of a 1200 L/s diffusion pump backed by a 300 L/min rotary pump. The detection chamber side contains a 550 L/min Turbo Molecular pump backed by a 300 L/min rotary pump. The base pressure of both the chambers were kept below 10 7 Torr. After the completion of evacuating process, methane gas was introduced into the discharge chamber at atmospheric pressure. Once the corona discharge was finished, the needle valve between the chambers was adjusted to maintain the required operating pressure range for the mass spectrometer. The operating pressure of mass spectrometer is less than Torr. Figure 1. Experimental Setup The pulsed discharge reactor was operated with a high pulse voltage power supply. By adjusting the high pulse voltage power supply, the electric discharge could be energized at desired conditions such as 10 µs duration voltage pulse ( 7 kv) with a variable repetition rate of pulses per second. The discharge power is evaluated by measuring the applied voltages, discharge current and the pulse repetition rate. A four channel digital storage oscilloscope is used to record voltage waveforms through a voltage divider. Plasma-chemical reactions were performed in a gas reactor, which is located in the discharge chamber. This reactor was made of stainless steel mesh (φ 26 mm 200 mm) with a metallic electrode such as brush electrode made of iron (φ 6 mm 200 mm). The inner electrode and the outer mesh electrode were separated from each other by a ceramic like insulator (mica) to avoid undesired electrical connection. The high voltage pulse was applied to the inner electrode, while the outer electrode was grounded with a connecting

3 84 Lekha Nath Mishra et al./ Journal of Natural Gas Chemistry Vol. 13 No wire. In a brush electrode, a number of brush-like needles made of stainless steel (φ 1 mm 3 mm) were placed on the electrode with a gap of 1 cm between one another (Figure 1). 3. Results and discussion When an electric field ( DC, AC or pulse) is applied to a gas, the energetic electrons transfer their energy to gas molecules by collision, leading to excitation, attachment, dissociation, or ionization of molecules. Figure 2 shows a typical mass spectrum of the residual gas, (a) before gas discharge, and (b) after discharge, recorded on a computer, through a mass spectrometer. Figure 2(a) shows that, at 0 V, the partial pressures of the hydrogen and methane are Torr and Torr respectively. But a comparable increase and decrease in the partial pressures of hydrogen and methane were found in Figure 2(b) after the discharge for five minutes at 7 kv with the repetition frequency of khz. From the two firgures it is inferred that the pressure of hydrogen increases up to Torr and methane decreases down to Torr. Hence it is clear that in order to produce hydrogen gas, the input pulse voltage must be increased. In this case, radicals such as CH3 and H produced by plasma from source gas, will undergo chain reaction leading to re-formation of methane and hydrogen gas (Figure 2). Figure 2. Typical gas spectrum of the residual gas observed by mass spectrometer at (a) before and (b) after gas discharge Figure 3 shows the conversion of methane and the relative yield of hydrogen with different input energies, at constant frequency (1.5 khz) and pulse width of (10 µs) for the duration of five minutes. In this case, a brush electrode (6 mm diameter and 200 mm length) is used for discharging the methane gas. This figure infers that the relative yield of hydrogen increases rapidly up to 300 J, and after that it increases gradually. This result indicates that at low input energy, the production rate of hydrogen is more than that of high input energy consumption. In this case, we employed the discharge voltage from 300 V to 7000 V at atmospheric pressure. However, the discharge current varies from 8.37 ma to 74.5 ma for those corresponding discharge voltages. Figure 3. Conversion of methane and relative hydrogen yield with variation of input energy for pulse discharge

4 Journal of Natural Gas Chemistry Vol. 13 No Figure 4 shows the variation of conversion of methane and relative yield of hydrogen gas with a variation of discharge time. Here, (10 µs) duration of pulse voltage 5 kv with a repetition rate of 1.5 khz is applied to decompose methane at atmospheric pressure. We have found that for longer discharge time, the relative yield of hydrogen reaches a maximum as 70%, while conversion of methane reaches 50.8%. We note that the discharge time is one of the key factors for producing hydrogen gas as well as conversion of methane. Figure 5 shows the variation of conversion of methane and relative yield of hydrogen, corresponding to different input voltage of pulse width (10 µs) with a repetition rate of 1.5 khz, in the presence and absence of argon. We have found that the relative yield of hydrogen gas increases in the presence of argon. Hence, it is clear that argon acts as a promoter. Argon atoms also have certain influences on the density and energy distribution of electrons, and thus assist the production of various active species during the formation of plasmas. More specifically, the injection of argon atoms into the input gas enhances the rate of CH 4 conversion to its radicals. For this purpose, the optimal relative pressure of argon to methane was chosen as 1:760. From the above discussion, it is obvious that even at low input energy, we may have some enhancement in the hydrogen gas production. Furthermore, narrow pulse width could be more efficient for a better discharge rate. However, there is some variation of atmospheric pressure related to the ambient conditions during the experiment. Figure 5. Relative hydrogen yield with/without immersing argon gas as a function of different input voltage. The content ratio of argon and methane is 1:760. (1) H 2 without Ar, (2) H 2 with Ar Figure 4. Conversion of methane and relative hydrogen yield with a variation of discharge time (1) Conversion of methane, (2) Relative hydrogen yield Under pulse corona plasma production, the activation and conversion of methane could be realized via collision of methane molecule with energetic electrons. CH 4 + e CH 3 + H (1) This reaction will very likely be followed by the formation of H 2 through the reaction of another molecule of methane with H-radical CH 4 + H CH 3 + H 2 (2) When argon gas is introduced into methane, the charge exchange between argon and methane molecules cannot be expressed in a general way. This is because it depends on the concentration of these molecules, which is, for instance, determined by the deliberate introduction of molecule into the plasma. One important point of attention is the internal energy of the argon atoms produced by the dissociative recombination (RD) reaction via the following equation Ar + e Ar Ar + + e Ar+hν (3) where h is the Plank s constant and ν is the frequency of argon. In this case, if one excited atom remains, the chance of re-ionization is very large leading to further dissociation [17].

5 86 Lekha Nath Mishra et al./ Journal of Natural Gas Chemistry Vol. 13 No The ionization potential (IP) of methane (13.2 ev) is lower than that of argon (15.75 ev). Ionic processes can supply enough energy to produce CH + 4 or more probably CH 3 through the following equation Ar + + CH 4 = Ar + CH 3 + H (4) The above ionic process is however expected to be slow since it involves Ar + with high IP. The formation of hydrocarbon radicals and ions is, therefore, attributed to an energy-transfer process associated with the metastable state of argon gas. Ar + CH 4 = Ar + CH + 4 = Ar + CH 2 + 2H (5) The threshold energy for the above reaction is 9.3 ev. 4. Conclusions We have investigated the characteristics of hydrogen production by the direct conversion of methane inside a reactor. The improvement of conversion process of methane gas due to corona discharge under several conditions has been experimentally observed. Electric field and pulse repetition rates could be varied by a pulse generator and the resultant discharge of methane gas was observed. The mechanism for an efficient production of hydrogen may be attributed to the strong intensity of electric field near the surface of inner electrode. Acknowledgements We gratefully acknowledge Daiko Shoji, Co., for providing us with a high voltage generator. This work is partly supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Sports, Culture, Science and Technology, Japan. References [1] Dahl C A, Donald L M. Energy Sources, 1998, 20: 875 [2] Isenberg G. Journals of Power Sources, 1999, 84: 214 [3] Diner I. Energy Sources, 1998, 20: 427 [4] Bromberg L, Cohn D R, Rabinovich A et al. Int J Hydrogen Energy, 2000, 25: 1167 [5] Brown L F. Int J Hydrogen Energy, : 381 [6] Mallinson R G. AIChE Journal, 1998, 44: 2252 [7] Okazaki K. Thermal Science and Engineering, 1999, 7: 109. [8] Bromberg L, Cohn D R, Rabinovich A et al. Int J Hydrogen Energy, 1999, 24: 1131 [9] Czernichowski A. Oil gas Sci Technol Rev IFP, 2001, 56(2): 181 [10] Kogeschatz U. Plasma Chem Plasma Process, 1998, 18: 375 [11] Nozaki T, Kimura Y, Okazaki K. J Phys D: Appl Phys, 2002, 35: 2779 [12] Liu D P, Yu S J, Ma T C et al. Jpn J Appl Phys, Part 1, 2000, 39: 3359 [13] Steven L S, Zerger R P. J Catal, 1993, 139: 383 [14] Gondal M A, Yamani Z H, Dastgeer A et al. Spectroscopy Lett, 2003, 36: 313 [15] Weimer A W, Dahl Jaimee, Tamburini J et al. Proceedings of the 2000 DOE Hydrogen Program Review NREL/CP [16] Kabashima H, Futamura S. Chem Lett, 2002, 31: 1108 [17] Frommhold L, Biondi M A. Phys Rev, 1969, 195: 244