Energy & Fuels 1998, 12, 11-18
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Plasma Reforming of Methane L. Bromberg,* D. R. Cohn, and A. Rabinovich Plasma Science and Fusion Center, Massachusetts Institute of Technology, NW16-108, 77 Massachusetts Ave, Cambridge, Massachusetts 02139
C. O’Brien and S. Hochgreb Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Received July 7, 1997. Revised Manuscript Received October 15, 1997X
Thermal plasma technology can be used in the production of hydrogen and hydrogen-rich gases from a variety of fuels. This paper describes experiments and calculations of high-temperature conversion of methane using homogeneous and heterogeneous processes. The thermal plasma is a highly energetic state of matter that is characterized by extremely high temperatures (several thousand degrees Celsius) and high degree of ionization. The high temperatures accelerate the reactions involved in the reforming process. Plasma reformers can be operated with a broad range of fuels, are very compact and are very light (because of high power density), have fast response time (fraction of a second), can be manufactured with minimal cost (they use simple metallic or carbon electrodes and simple power supplies), and have high conversion efficiencies. Hydrogen-rich gas (50-75% H2, with 25-50% CO for steam reforming) can be efficiently made in compact plasma reformers. Experiments have been carried out in a small device (2-3 kW) and without the use of efficient heat regeneration. For partial oxidation it was determined that the specific energy consumption in the plasma reforming processes is 40 MJ/kg H2 (without the energy consumption reduction that can be obtained from heat regeneration from an efficient heat exchanger). Larger plasmatrons, better reactor thermal insulation, efficient heat regeneration, and improved plasma catalysis could also play a major role in specific energy consumption reduction. With an appropriate heat exchanger to provide a high degree of heat regeneration, the projected specific energy consumption is expected to be ∼15-20 MJ/kg H2. In addition, a system has been demonstrated for hydrogen production with low CO content (∼2%) with power densities of ∼10 kW (H2 HHV)/L of reactor, or ∼4 m3/h H2 per liter of reactor. Power density should increase further with power and improved design.
I. Introduction Manufacturing of hydrogen from hydrocarbon fuels is needed for a variety of applications. These applications include fuel cells used in stationary electric power production and in vehicular propulsion. Hydrogen can also be used for various combustion engine systems. Hydrogen manufacturing is also needed for industrial applications and could be used in refueling stations for hydrogen-powered vehicles. There is a wide range of requirements on the capacity of the hydrogen manufacturing system, the purity of the hydrogen fuel, and capability for rapid response. The overall objectives of a hydrogen manufacturing facility are to operate with high availability at the lowest possible cost and to have minimal adverse environmental impact. II. Plasma Reforming Plasma technology has potential to significantly alleviate shortcomings of conventional means of manu* To whom correspondence should be addressed. E-mail: BROM@ PFC.MIT.EDU. Telephone: 617-253-6919. Fax: 617-253-0700. X Abstract published in Advance ACS Abstracts, November 15, 1997.
facturing hydrogen. These shortcomings include cost and deterioration of catalysts, size and weight requirements, limitations on rapid response, and limitations on hydrogen production from heavy hydrocarbons. In addition, use of plasma technology could provide for a greater variety of operating modes including the possibility of virtual elimination of CO2 production by pyrolytic operation. This mode of hydrogen production may be of increasing importance because of recent additional evidence of global warming. Disadvantages of plasma reforming are the difficulty of high-pressure operation and the dependence on electrical energy. High pressure, while achievable, increases electrode erosion and decreases electrode lifetime. Dependence on electrical energy results in energetics that are less favorable than the energetics of purely thermal processes, especially for endothermic reforming reactions. Plasma devices referred to as plasmatrons can generate very high temperatures (>2000 °C) with a high degree of control, using electricity. The heat generation is independent of reaction chemistry, and optimum operating conditions can be maintained over a wide range of feed rates and gas composition. Compactness
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of the plasma reformer is ensured by high-energy density associated with the plasma itself and by the reduced reaction times, resulting in a short residence time. Hydrogen-rich gas (50-75% H2, with 25-50% CO for steam reforming) can be efficiently produced in plasma reformers with a variety of hydrocarbon fuels (gasoline, diesel, oil, biomass, natural gas, jet fuel, etc.) with conversion efficiencies to hydrogen-rich gas close to 100%.1,2 Plasma reformers are relatively inexpensive (they use relatively simple metallic or carbon electrodes). The plasma conditions (high temperatures and a high degree of ionization) can be used to accelerate thermodynamically favorable chemical reactions without a catalyst or to provide the energy required for endothermic reforming processes. Plasma reformers can provide a number of advantages: (1) compactness and low weight (due to high power density); (2) high conversion efficiencies; (3) minimal cost (simple metallic or carbon electrodes and simple power supplies); (4) fast response time (fraction of a second); (5) operation with a broad range of fuels, including heavy hydrocarbons; (6) no need for catalysts and therefore no problems of catalyst sensitivity and deterioration; (7) operation with a broad range of fuels, including heavy hydrocarbons. The technology could be used to manufacture hydrogen for a variety of stationary applications, e.g., distributed, low-pollution electricity generation from fuel cells.3 It could also be used for mobile applications (e.g., on-board generation of hydrogen for fuel cell-powered vehicles) and for refueling applications (stationary sources of hydrogen for vehicles). Plasma reformers also offer a unique potential for pyrolytic generation of hydrogen without production of CO2, thus providing an option for reduction of gases that could contribute to global warming. This paper describes the investigation of plasma reforming with use of different oxidizing feedstocks: (1) pyrolytic decomposition (no oxidizer); (2) partial oxidation (air); (3) modified partial oxidation (air/water); (4) heterogeneous partial oxidation. The overall objective of this work is to investigate the dependence of the hydrogen yield and the specific energy input of thermal, noncatalytic processes and to preliminarily explore plasma catalysis.
Bromberg et al.
Figure 1. Schematic diagram of research plasmatron used in the experiments. cooled cathode with zirconium tip and water-cooled copper tubular anode. The plasma-forming gas is air or nitrogen. The length of the arc is stabilized by the ledge in the internal diameter of the anode. The hydrocarbons (methane) are injected radially downstream from the anode root of the arc. This type of plasmatron has a long lifetime (1000-2000 h) at currents less than 100 A (corresponding to ∼10 kW). The zirconium tip provides thermionic emission in air or nitrogen atmosphere without excessive cathode erosion. The reactor consists of steel tube with an outer diameter of 0.09 m (3.5 in.) and a length of 0.2 m (8 in.). The inside of the steel tube was covered with a 0.012 m (0.5 in.) ceramic lining (Sauereisen No. 8), leaving a 0.06 m (2.25 in.) inner diameter channel. The plasmatron was connected to the top of reactor. An inefficient heat exchanger was used in some of the experiments, with the air and methane being preheated in tubing wrapped around the reactor. The size of the plasmatron is 0.075 m (3 in.) long and 0.04 m (2 in.) outer diameter. The weight is 0.7 kg (1.5 lbs). The gas chromatograph (GC) used (MTI Model M200) consists of a sample loop, vacuum pump, and two columns, each with an injector valve and thermal conductivity detector (TCD). Gas samples are provided directly from the reactor by means of a water-cooled copper tube. The analyzed components are H2, O2, N2, CO, CH4, CO2, C2H2, C2H4, and C2H6. Bottled gas was used for calibration. Water content in the reformate is not monitored.
III. Experimental Facility
IV. Methane Pyrolysis
The experimental facility includes research plasmatrons, reactors, and GC diagnostic. A plasma system has been developed based on Air Plasma Cutting System PAK MASTER 100. The power supply provides 75 A of maximum current at arc voltages of ∼120160 V. The standard system has been customized in order to control and measure the arc current and voltage. The system was equipped with gas and water rotameters, pressure transducers, and a digital thermocouple meter. A schematic of the research plasmatron used in these studies is shown in Figure 1. The plasmatron has a water-
In pyrolysis reforming, the goal is to manufacture hydrogen gas and solid carbon (soot) in an oxygen-free environment. Equilibrium calculations indicate that at low temperatures, methane decomposes into hydrogen and soot. In practice, the process is dominated by kinetics and nucleation and particle growth, and it is very difficult to model. High temperatures, such as those achievable with plasmas, are required to drive the process. Research in the area of methane plasma reforming has in the past been driven by industrial production of acetylene. Acetylene is produced at high temperatures, and in order to freeze its concentration and optimize its production, fast quenching of the reformate is needed, as in the Huls and DuPont processes.4,5 More recent experiments for hydrogen/carbon conversion in
(1) Rudiak, E. M.; Rabinovich, A.; Tul, N. A. USSR Patent 700935, August 1979. (2) Kaske, G.; Kerke, L.; Muller, R. Hydrogen Production in the Huls Plasma-Reforming Process. Hydrogen Energy Prog. 1986, VI (1). (3) Bromberg, L.; Cohn, D. R.; Rabinovich, A. Plasma Reformer/ Fuel Cell Systems for Decentralized Power Applications. Int. J. Hydrogen Energy 1997, 22, 83-94.
Plasma Reforming of Methane
Figure 2. Hydrogen yield in plasma pyrolysis of methane as a function of the power consumption per unit mass of methane for several reactor setups.
a plasma (without quench) with large plasma devices have been performed by Kvaerner in Norway and Sweden.6 Thermal cracking of methane has in the past been used for carbon black manufacturing and has been proposed for hydrogen manufacturing.7 In France, Fulcheri8 performed calculations on methane pyrolysis in a plasma and conducted experiments whose main objective was that of carbon black manufacturing.9 Both the Kvaerner and French work use graphite electrodes, operating at near-atmospheric pressure. These experiments indicate high-energy consumption requirements. The goal of the present experiments is to investigate the means of decreasing specific energy consumption, both through plasmatron improvements and reactor modifications. The methane pyrolysis experiments were carried out with a plasmatron operating with nitrogen gas. The methane gas was injected downstream from the anode and allowed to cool slowly (no quenching, to minimize the acetylene production). The ratio of flow rates of CH4 to N2 has been varied from 1:1 to 1:3. Several arrangements of the plasmatron and the reactor were tested. These included inverted vertical injection into reactor chamber (plasma jet facing down), side injection into the reactor chamber with extracting spout (cyclone configuration), and fluidized bed. The cyclone configuration was motivated by the hope of removing the condensed carbon (soot) from the flow. The fluidized bed was done with activated charcoal particles to investigate if providing high surface area for the deposition of the carbon atoms resulted in a decrease in the specific energy consumption. The results of experiments are shown in Figures 2 and 3. The cyclone, downstream injection, and fluidized bed setups were described above. The CH4 preheat (4) Muller, R. The Use of Hydrogen Plasma Processes in the Petrochemical and Iron-Smelting Industries. Hydrogen Energy Prog. 1988, YII (2), 885-900. (5) Holmes, J. M.Evaluation of DUPONT Arc Process for Acetylene and Vinyl Chloride Monomer Production. U.S. Department of Commerce/National Bureau of Standards, 1969 (30 pages). (6) Gaudernack, B.; Lynum, S. Hydrogen from Natural Gas without Release of CO2 to the Atmosphere. Hydrogen Energy Prog., Proc. World Hydrogen Energy Conf., 11th 1996, 511-523. (7) Steinber, M. Production of Hydrogen and Methanol from Natural Gas with Reduced CO2 Emissions. Hydrogen Energy Prog., Proc. World Hydrogen Energy Conf., 11th 1996, 499-510, (8) Fulcheri, L.; Schwob, Y. From Methane to Hydrogen, Carbon Black and Water. Int. J. Hydrogen Energy 1995, 20, 197-202. (9) Lynum, S. Private communication.
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Figure 3. Specific energy consumption for hydrogen for the same conditions as in Figure 2.
refers to a simple heat regeneration system consisting of a few turns around the outside of the steel reactor; it did not operate effectively as a heat exchanger. Straight refers to the conventional setup, with vertical plasma facing up. Big anode refers to a larger anode and a longer residence time. For all of the experimental setups, the average specific power consumption was in the range 200-250 MJ/kg H2. The hydrogen yield, defined as the ratio of measured hydrogen concentration to that of complete conversion, increases from ∼30% to 70% when the specific power input increases from 250 to 650 kJ/mol CH4. The product gas composition in the case of the highest hydrogen yield was 33.3 vol % H2, 54.4 vol % N2, 6.8 vol % CH4, 3.2 vol % C2H2, and 1.2 vol % C2H4. No other peaks were observed in the GC. Future research will concentrate on development of different types of plasmatron (with uncooled graphite electrodes) and reactor chambers. In a plasmatron with these characteristics, methane can be used as the plasma gas, eliminating the diluent (nitrogen in the present experiments) and decreasing the specific energy consumption. Specific energy consumption could also be reduced by heat regeneration and by operation with conditions that result in a longer residence time, such as increased reactor volume. V. Partial Oxidation/Steam Reforming of Methane The process explored in the experiments involved using a combination of air and steam as the oxidizer. The plasmatron was operated with air, and methane and water were injected downstream from the anode. Partial oxidation is an exothermic reaction, while steam reforming is strongly endothermic. The purpose of the reagent mixture was to explore the effectiveness of near-energy-neutral processes. In addition, it may be possible to achieve one-step reforming/water shift reactions, with a compact design. The results of plasma reforming of methane by airwater mixture at constant power are shown in Figures 4 and 5. Kair is the amount of air normalized to the air required for stoichiometric partial oxidation of the methane. Similarly, Kwater is the amount of water normalized to the water required for stoichiometric steam reforming of the methane. As could be seen from Figure 4 at 0.4 < Kair < 0.7, the maximum H2 yield (∼60%) is achieved at water excess Kwater close to 0 and decreased to 40% as the water excess increases to 0.7. At this air-to-methane ratio (close to pyrolysis) the
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Figure 4. Contours of constant hydrogen yield as a function of Kair and Kwater for partial oxidation/steam reforming of methane.
Figure 5. Contours of specific energy consumption for the same conditions as in Figure 4.
reaction is strongly endothermic and addition of water decreases gas temperature and consequently the rate of reaction. The same behavior is observed at Kair ≈ 0.9, which is close to stoichiometric partial oxidation. Although the reaction is slightly exothermic, the H2 yield (80%) is as high as can be achieved with these operational conditions. As in the case with lower Kair, water addition only decreases the temperature and H2 yield. At Kair ≈ 1.1-1.4 and water excess close to 0, the H2 yield (60%) is less than at stoichiometric air/methane ratio. Since the reaction is exothermic, water addition increases H2 yield to 80%. Figure 5 shows the specific energy consumption (for hydrogen) as a function of Kair and Kwater. The minimum power consumption without the use of heat regeneration (∼100 MJ/kg H2) is achieved at ratio air-to-methane close to 1 (stoichiometric partial oxidation) where H2 yield is maximum. Decreasing Kair toward the pyrolysis mode or increasing to complete combustion leads to increased specific power consumption. This value is reduced from previously obtained values of the partial oxidation of methane5 with a reactor with reduced volume (and enhanced increased space velocity) and without ceramic lining. VI. Plasma Catalytic Reforming To decrease the specific power consumption further, experiments on plasma catalytic reforming of methane by the air-water mixture have been carried out.
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Figure 6. Contours of constant hydrogen yield as a function of Kair and Kwater for plasma catalysis of methane.
Figure 7. Contours of specific energy consumption for the same conditions as in Figure 6.
The reactor was filled with NiO catalyst on an Al2O3 support. The air/methane ratio in all experiments was Kair ≈ 1 (partial oxidation), and water excess has been changed to be in the range 0 < Kwater < 4. In the first set of experiments the catalyst volume was 2.5 × 10-4 m3. With this volume, the H2 yield was 6570% and the specific power consumption was ∼72-75 MJ/kg H2. The results of doubling the volume of catalyst to 5 × 10-4 m3 are shown in Figures 6 and 7 as functions of Kair and Kwater. The larger catalyst volume increases the hydrogen yield to 90-100% at specific power consumptions of -45 to 55 MJ/kg H2. As shown in Figures 6 and 7, the optimum operating conditions are Kair = 1 and Kwater ≈ 4. In this regime the hydrogen yield reaches 100% at specific power consumptions of ∼45 MJ/kg H2. Increasing water excess beyond Kwater ≈ 4 significantly decreases the gas temperature and reaction rate, with a corresponding decrease of the hydrogen yield to 70% at a specific power consumption of ∼70 MJ/kg H2. VII. Discussion Comparison of homogeneous plasma and plasma catalytic modes of operation shows the significant advantage of the later regime. Figures 8 and 9 show the hydrogen yield and the specific energy consumption as a function of the energy input per unit mass of fuel for partial oxidation (previous results), partial oxidation/ steam reforming (present results), and plasma catalysis (present results). The results of partial oxidation had
Plasma Reforming of Methane
Figure 8. Hydrogen yields as a function of the energy consumption per unit mass for partial oxidation (previous results), partial oxidation/steam reforming, and for plasma catalysis of methane.
Figure 9. Specific energy consumption for the same conditions as in Figure 8.
been obtained previously,10 while the results labeled partial oxidation/steam reforming were discussed above. The H2 yield for partial oxidation has been increased by ∼20-50% from previous results, owing to improved plasmatron and reactor designs. The specific energy consumption has also been decreased by about 20-30%, mostly owing to increased yield. The plasma catalytic regime results are substantially better than previous results and most recent partial oxidation/steam reforming results. The hydrogen yields are 2-3 times better at significantly lower (1/3) specific power consumption. In addition to an increase of the yields and a decrease of the energy consumption, there is an added advantage of plasma catalytic reforming vs homogeneous plasma reforming. With water addition, it is possible to accomplish in one stage both reforming and water shift processes. It is possible to produce hydrogen with sufficiently low CO concentrations for use in highperformance PEM fuel cells. Conventional process of hydrogen production includes stages of partial oxidation (steam reforming), water shift reaction, and preferential oxidation. In plasma catalytic reforming, the composition of the reformate is 40 vol % H2, 38 vol % N2, 3.4 vol % CH4, 3.4 vol % CO, and 13.5 vol % CO2. Trace (10) Bromberg, L.; Cohn, D. R.; Rabinovich, A.; O’Brien, C.; Hochgreb, S. In Proceedings of the 1996 U.S. DOE Hydrogen Program Review. National Renewable Energy Laboratory Report NREL/CP-43021968; National Renewable Energy Laboratory: Golden, CO, 1996; p 553.
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amounts of light hydrocarbons were observed (
