Hydrogen Production by Direct Contact Pyrolysis of Natural Gas

Energy & Fuels 2003, 17, 705-713

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Hydrogen Production by Direct Contact Pyrolysis of Natural Gas Manuela Serban,† Michele A. Lewis,† Christopher L. Marshall,*,† and Richard D. Doctor‡ Chemical Technology Division, Energy Systems Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 Received November 19, 2002

In this paper, we propose the concept of utilizing the heat generated in Generation IV nuclear reactors to produce hydrogen and carbon from methane or natural gas by direct contact pyrolysis, a process that features zero greenhouse gas emissions. Methane or natural gas was bubbled through a bed of either low-melting-point metals (e.g., lead or tin), granular or catalytic materials (e.g., silicon carbide, R-alumina, NiMo/γ-alumina), or a mechanical mixture of molten metal and solid media. The methane conversions were found to be dependent upon the contact time between the methane and the heat transfer media, as well as on the methane bubble size. The most efficient systems used for the pyrolysis process were found to be the ones in which natural gas was bubbled through Mott porous metal filters, in a bed of either 4-in. Sn + SiC or Sn, with the product stream comprised of almost 80 and 70 vol % of hydrogen at 750 °C, respectively. The main advantage of this proposed system is the ease of buoyant separation of the generated carbon byproduct from the liquid heat transfer media. These experiments lay the groundwork for developing technical expertise in producing pure hydrogen cost-effectively by utilizing the heat energy contained in the liquid metal coolant in Generation IV nuclear reactors.

Introduction Currently, petroleum refining is by far the biggest consumer of pure hydrogen, but the major petroleum companies and vehicle manufacturers are orienting their industries toward a hydrogen economy. Motivations for exploring hydrogen as the leading alternative energy source are to ensure a future energy supply through a secure and abundant domestic fuel and to solve environmental problems through reducing CO2 emissions and improving air quality. It is predicted that world demand for hydrogen will grow by 10% per year over the next five years, from the present level of 1012 billion scf/day.1 For decades, the major technology for hydrogen production has been steam methane reforming (SMR) coupled with pressure-swing adsorption (PSA), which accounts for more than 90% of the hydrogen made for petroleum refining and other large-scale uses. Other conventional processes for hydrogen production from fossil fuels (partial oxidation of residual oil and coal gasification) are significantly more expensive than steam reforming.2 The SMR reaction is equilibriumlimited and highly endothermic, requiring heat input of 251 kJ/mol CH4, including the heat needed to produce steam from liquid water. A considerable part of the * Author to whom correspondence should be addressed. E-mail: [email protected]. † Chemical Technology Division. ‡ Energy Systems Division. (1) Parkinson, G. Chem. Eng., 2001, Sept., 29-35. (2) Steinberg, M.; Cheng, H. Int. J. Hydrogen Energy 1989, 14 (11), 797-820.

natural gas required by the process (up to 30-35% of the total amount) is used as a fuel in feedstock desulfurization and the production of steam. Thus, the thermal efficiency of the process, defined as the energy in the hydrogen produced divided by the energy in the natural gas feedstock, is less than 75%.3 However, SMR systems that can market the byproduct steam produced will show higher efficiencies, hence cycles of 80-85% efficiency have been reported.4 Most importantly, this technology generates unwanted byproducts, i.e., CO, which must be managed in conformance with the increasingly strict EPA regulations,5 and CO2. Future regulations on CO2 emissions might require that CO2 be captured and sequestered, resulting in additional costs and risks. The total CO2 emission from the SMR process could reach up to 0.32-0.42 m3 per m3 of hydrogen produced.6,7 The capture, transportation, and sequestration of CO2 are energy intensive processes. According to Audus et al.,8 the capture and disposal of CO2 (80-85% of CO2 captured from the concentrated streams of the SMR process) add about 25-30% to the cost of the hydrogen that is produced. The world average for CO2 emissions associated with the production of electricity is 0.153 kg (3) Steinberg, M. Int. J. Hydrogen Energy 1999, 24, 771-777. (4) Patel, N. Oil Gas J. 1994, Oct., 54. (5) Alkabbani, A. S. Hydrocarbon Process. 1999, July, 61. (6) Muradov, N. Z. Energy Fuels 1998, 12, 41-48. (7) Muradov, N. Z. Int. J. Hydrogen Energy 2001, 26, 1165-1175. (8) Audus, H.; Kaarstad, O.; Kowal, M. In Proceedings of 11th World Hydrogen Conference; Stuttgart, Schon & Wetzel GmBH: Frankfurt am Main, 1996; pp 525-534.

10.1021/ef020271q CCC: $25.00 © 2003 American Chemical Society Published on Web 04/16/2003

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of CO2 per kW‚h produced.9 Thus, the amount of CO2 produced as a result of the capture of CO2 from the concentrated streams generated in the steam reforming process and its sequestration reaches up to 0.09-0.1 kg per kg of sequestrated CO2.6 It is generally accepted that in the near-to-medium term, hydrogen production will rely on fossil fuels, primarily natural gas. The thermal decomposition of methane (TDM) is regarded as an alternate route to producing hydrogen and elemental carbon, and the only fossil fuel-based process completely free of CO2 emissions. TDM is less endothermic than steam methane reforming; thus, it is possible to crack methane into carbon and hydrogen using less energy than existing commercial processes. It should be noted here that the thermal decomposition of natural gas is not a new process; it has long been practiced for the production of carbon black, in the so-called thermal black process.10 The hydrogen produced is used as fuel to heat the furnace and the methane feedstock. The conversion of methane/natural gas to hydrogen and elemental carbon by direct contact pyrolysis could be commercially attractive, because of the readily available supply of natural gas and because the hydrogen thus produced is clean and free of contaminants, i.e., CO and CO2. This makes it directly usable in proton exchange membrane (PEM) fuel cells, eliminating the necessary cleanup step required in conventional reformers. Also, the carbon byproduct can be marketed as a commodity material or used in direct carbon/air fuel cells. However, a continuous TDM process for hydrogen production is not yet commercially available, and is in a relatively immature state of development compared to the SMR process. There are several reports of catalytic thermal decomposition of methane over a series of catalysts, such as Ni11-14 supported on alumina, SiO2, TiO2, or MgO, or nanoscale binary catalysts Fe-M (M ) Pd, Mo, Ni) on alumina.15 These studies have been conducted mainly to reduce the maximum temperature required for the decomposition of methane. Over a 16.4 wt % Ni/SiO2 catalyst, Zhang and Amiridis11 reported an initial conversion of 35% methane at 550 °C, while Ermakova et al.13 reported 15% methane conversion over Ni-rich catalysts (85-90 wt %). The lifetime of all these catalysts is very short; complete deactivation is observed usually after several hours on stream. The catalytic activity, however, can be fully restored either by burning the carbon off the catalyst surface or by steam gasification.6,11 In this regard, the catalytic decomposition of methane displays no significant advantages over the conventional steam reforming process, because of the large CO2 emissions. The thermal decomposition of methane has great (9) Blok, K.; Williams, R.; Katofsky, R.; Hendriks, C. Energy 1997, 22, 161-168. (10) Donnet, J. B. Carbon Black; Marcel Dekker: New York, 1976; pp 16-18. (11) Zhang, T.; Amiridis, M. D. Appl. Catal., A 1998, 167, 161-172. (12) Kuvshinov, G. C.; Mogilnykh, Y. I.; Kuvshinov, D. G. Catal. Today 1998, 42, 357 (13) Ermakova, M. A.; Ermakov, D. Y.; Kuvshinov, G. C. Appl. Catal., A 2000, 201, 61-70. (14) Takenaka, S.; Ogihara, H.; Yamanaka, I.; Otsuka, K. Appl. Catal., A 2001, 217, 101-110. (15) Shah, N.; Panjala, D.; Huffman, G. P. Energy Fuels 2001, 15, 1528-1534.

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potential in producing hydrogen without CO2 or CO emissions, but it can hardly be of practical interest unless a thermally efficient process is developed and an efficient way of disposing of the carbon byproduct generated during the reaction is designed. In this paper, we propose the concept of utilizing the heat generated in Generation IV nuclear reactors16-18 to produce hydrogen and carbon from methane or natural gas by direct contact pyrolysis, an approach that features zero greenhouse gas emissions. Methane or natural gas was bubbled through a bed of either low-melting-point metals (e.g., lead or tin) or through a mechanical mixture of metal and solid media (e.g., silicon carbide). A similar method has been proposed by Steinberg et al.19 for the production of methanol from natural gas; it is comprised of the thermal decomposition of methane and the subsequent reaction of the resulting hydrogen with carbon dioxide. The main advantage of this proposed system is the ease of separation of the generated carbon byproduct from the heat transfer media, by buoyancy due to density differences. These experiments lay the groundwork for developing technical expertise in producing pure hydrogen cost-effectively by utilizing the heat energy contained in the liquid metal coolant in the Generation IV nuclear reactors. In consequence, this method takes advantage of synergies between nuclear power generation and hydrogen generation from methane or natural gas. Experimental Section Reactor Apparatus and Materials. All experiments were conducted in a 1-in. by 14-in. 304 stainless steel vertical microreactor. Methane (99.995% research grade) or commercial grade natural gas (95% methane and 5% ethane), from AGA Gas Central, was fed to the microreactor using Brooks 5850 mass flow controllers connected to appropriate power supply/ readout units (Brooks 5876). Typical volumetric flow rates were between 2 and 15 cm3/min, corresponding to nominal gas hourly space velocities (GHSVs) of 18 and 133 h-1, respectively. The methane/natural gas was forced to bubble into the heated media through either a 1/4- or 1/16-in. stainless steel feed tube or a Mott (Mott Corp.) porous metal filter extending all of the way to the bottom of the reactor. The porous metal filter creates small methane bubbles, thus improving the surface area for effective heat transfer between the gas and the heated media. The heat transfer media, which was either low melting point metals (e.g., lead or tin), granular or catalytic materials (e.g., silicon carbide, alumina, or NiMo/alumina), or a mechanical mixture of tin and solid media, was placed in a 1/2-in. stainless steel cup, inside the 1-in. microreactor. The temperature was controlled using four Omega CN375-type temperature controllers connected to a Thermcraft 4-zone heated furnace. The temperature inside the reactor was monitored with a K-type thermocouple placed in a thermowell adjacent to the 1/2 in. cup. The reaction was run isothermally at temperatures of 600 °C and higher, above the melting points of Pb (mp ) 327 °C) or Sn (mp ) 232 °C) such that the metal was in the liquid state at all times during the reaction. The effluent gas (16) Lewis, D.; Lewis, M. A., Patent Disclosure ANL IN# 00-070. (17) Marshall, C. L.; Lewis, M. A.; Leibowitz, L.; Lewis, D. Proceedings of the Nuclear Production of Hydrogen Meeting; Paris, France, October 2000. (18) Wade, D. C.; Doctor, R. D.; Spencer, B. W.; Peddicord, K. L.; Boardman, C.; Marucci, G. Nuclear Production of Hydrogen; First Information Exchange Meeting, Paris, France, October 2000. (19) Steinberg, M.; Dong, Y. U.S. Patent 5,767,165, 1998.

H2 Production by Direct Contact Pyrolysis of Natural Gas

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Figure 1. Schematic of the experimental apparatus used in the natural gas pyrolysis studies. composition was analyzed continuously using a QMS 200 mass spectrometer. A schematic of the experimental setup is given in Figure 1. Methane conversion was calculated in the conventional manner, according to

XCH4 )

(CH4)in - (CH4)out (CH4)in

× 100

where (CH4)in represents the partial pressure of methane

before the reaction, measured through the bypass line (Figure 1), and (CH4)out represents the partial pressure of methane after the reaction. Ethane conversion was calculated similarly. The mass spectrometer was routinely calibrated over a wide range of compositions with a standard mixture of CH4, C2H6, H2, and Ar (AGA Gas Central). Mass spectrometer analysis was performed at 30-s time intervals, and the average of numerous (frequently several hundreds) measurements was used to characterize the partial pressures of methane or ethane at a given reaction condition. All data reported here are steady-

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Figure 2. Methane (solid lines) and ethane conversions (dashed lines) in the natural gas pyrolysis over 4- (O) and 8-in. ([) molten lead as a function of flow rate. T ) 750 °C.

Figure 3. Methane (solid lines) and ethane conversions (dashed lines) in the natural gas pyrolysis over 4-in. molten Pb (O) and SiC ([) as a function of flow rate. T ) 750 °C.

state values. Hydrogen atom balance was checked after each reaction, and the runs with an error bigger than 7% in the hydrogen balance were discarded. Lead (99.5+%) and tin (99.5+%) were procured from Alfa Aesar and Aldrich Chemicals, respectively; silicon carbide came from Electro Abrasives; R-alumina, γ-alumina, and NiOMo2O3/alumina came from Crosfield Catalysts. Materials Characterization. SEM/EDS analyses of the carbon samples collected after pyrolysis experiments were performed on a JEOL JSM-6400 scanning microscope, while elemental analysis of the carbon was done with a LECO CHN900 analyzer with combustion at 1000 °C. The instrument’s detection limits are 0.05 wt % for carbon and 0.02 wt % for hydrogen. The specific surface area and pore volume were measured in a Micrometrics ASAP 2010 apparatus by adsorption and desorption of nitrogen at liquid nitrogen temperature. XRD spectra were measured with a Rigaku D/max-2400 V diffractometer using Cu KR radiation.

hydrogen formation were found to be approximately in the ratio of 1:2 and 1:3, respectively, thus verifying the reaction stoichiometry. Methane and ethane conversions increased with decreasing flow rates and increasing the bed height, both of which are equivalent to increasing the contact time between the natural gas and the molten metal. Due to a relatively weaker C-H bond (423.0 kJ/mol) in the ethane molecule compared to methane (440.0 kJ/ mol), ethane decomposes preferentially to hydrogen and carbon. At standard conditions, 28.2 kJ/mol are required to produce one mole of hydrogen from ethane, compared to 37.8 kJ/mol for methane. As a result, under all conditions tested, ethane conversion was always higher than methane conversion. To further confirm the importance of the contact time between the natural gas and the heat transfer media in the natural gas pyrolysis process, comparison experiments were performed by passing natural gas through 4-in. molten Pb or 4-in. silicon carbide (SiC) bed, 30 mesh size (Figure 3). The thermal decomposition was found to be more efficient when natural gas was passed through a bed of 4-in. SiC. The higher conversions achieved in the experiments with SiC are mainly attributed to the longer contact times between natural gas and SiC granules versus natural gas and molten Pb. Our calculations indicate that the residence time in the 4-in. SiC bed is 40 times longer than that in the 4-in. molten Pb pool. Methane and ethane conversions were found to be similar under conditions of bubbling natural gas through 4-in. Pb or Sn pools. Since the vapor pressure of Pb at 750 °C (1.96 × 10-2 Torr) is much higher than that of Sn (1.20 × 10-7 Torr), and since Pb is known to be a toxic metal, it is only logical to opt for a less volatile and less toxic metal. Figure 4 shows the results obtained with a bed of 4-in. Sn and 4 in. of a mechanical mixture of 80 wt % Sn + 20 wt % SiC. Again, the conversions increased with decreasing flow rates and with the use of the mechanical mixture as the heat transfer media, which is a direct consequence of increasing the contact time. R-Alumina (80 mesh size), with low surface area (∼0.1

Results and Discussion The Generation IV nuclear reactor, a fast neutron spectrum, fissile self-sufficient converter reactor using heavy liquid metal coolant, is proposed as the nuclear heat source.18,20-22 These heavy-liquid-metal coolants (lead, lead-bismuth, tin) facilitate reaching the high temperature (∼900 °C) required for driving thermochemical hydrogen production. High-temperature gas (He, N2, CO2) is used for heat transport from the nuclear reactor to the thermal cracking unit.18,20-22 In this study, the choice of Pb and Sn as the heat transfer media for the methane pyrolysis process is independent of their use in the nuclear reactors; they were selected mainly due to their low melting points and absence of carbide formation. Natural Gas Decomposition over Different Heat Transfer Media. Natural gas (95% CH4 and 5% C2H6) was converted selectively to carbon and hydrogen at temperatures ranging from 600 to 900 °C and nominal gas hourly space velocities between 18 and 223 h-1. No gaseous products other than hydrogen were observed in the effluent stream. Figure 2 shows a typical result for the thermal decomposition of natural gas bubbled through a 4- or 8-in.-high pool of molten lead at 750 °C. The rates of methane and ethane conversion and



Figure 4. Methane (solid lines) and ethane conversions (dashed lines) in the natural gas pyrolysis over 4-in. molten Sn (2) and 4-in. of 80 wt % Sn + 20 wt % SiC (9) as a function of flow rate. T ) 750 °C. The open symbols indicate repeated experiments.

Figure 6. Comparison between the concentrations of hydrogen (vol %) produced in the natural gas pyrolysis bubbled through different heat transfer media at 750 °C. Feed tube ) 1/4-in. open bore tube.

Figure 5. Methane (solid lines) and ethane conversions (dashed lines) in the natural gas pyrolysis over 4 in. of R-Al2O3 (2), γ-Al2O3 (0), and NiMo/γ-Al2O3 ([) as a function of flow rate. T ) 750 °C.

m2/g) and no pore structure, and γ-alumina (35 mesh size), with a surface area of 284 m2/g, were also tested for the decomposition of natural gas at 750 °C (Figure 5). The bed of 4-in. R-Al2O3 behaved similarly to that of SiC (30 mesh size). At 750 °C, methane conversions ranged from 14 to 5% for flow rates between 1 and 9 cm3/min. After 45 h on stream, the methane conversion remained unchanged. The thermal decomposition of natural gas was found to be more efficient when methane was passed through a bed of 4-in. γ-Al2O3. Thus, at 750 °C and a flow rate of 5 cm3/min, the conversion of methane was 26%, while ethane converted almost completely, i.e., 98%. For comparative purposes, the catalytic thermal decomposition of natural gas was tested over a binary Ni-Mo/γ-Al2O3 with a surface area of 177 m2/g (Figure 5). The catalyst NiO-Mo2O3/γ-Al2O3 (2-8% NiO, 2030% Mo2O3, balance alumina) was crushed, sieved to 30-40 mesh size, and reduced overnight in a flow of 4% hydrogen in helium at 700 °C. The pyrolytic reactor

Figure 7. Methane conversions in the natural gas pyrolysis as a function of flow rate and bubble size, at 750 °C. Heat transfer media: 4-in. Sn (solid lines), or blank runs (dashed lines).

was loaded with 5.31 g catalyst (Fbulk ) 0.789 g/cm3), corresponding to a bed height of 4 in. It is evident that the highest methane conversions were achieved during the catalytic decomposition, with 45% conversion for a flow rate of 5 cm3/min. However, the catalyst clearly deactivates at a rate of approximately 3% per hour for the first 10 h and 1% per hour after 10 h, such that after 80 h on stream, the measured conversion was approximately 9%. The measurements of surface area and pore volume after 80 h on stream indicate a loss of 34% in surface area and 38.5% in pore volume. The limited lifetime of porous or catalytic materials for thermal decomposition applications represents their main disadvantage. There is no other mechanical or chemical method for removing the carbon accumulated inside the porous structure other than by burning it. Figure 6 depicts a comparison of the hydrogen concentrations (vol %) obtained in natural gas pyrolysis as bubbled through a 1/4-in. feed tube and with a range of catalytic and noncatalytic materials at 750 °C and

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Figure 8. Argon bubble distribution through (a) a 1/4-in. open bore tube and (b) a 0.5-µm Mott porous filter.

different volumetric flow rates. In the noncatalytic region, we included the runs over molten metals (Pb and Sn), SiC, 80 wt % Sn + 20 wt % SiC, and R-Al2O3. The concentrations of hydrogen (vol %) were calculated according to

[H2] ) (0.95)

200XCH4 100 + XCH4

+ (0.05)

300XC2H6 200 + XC2H6

where XCH4 and XC2H6 are the methane and ethane conversions, respectively. The composition of the natural gas used in this study is 95% methane and 5% ethane. It is apparent that the highest hydrogen concentration was obtained during the catalytic decomposition of natural gas over the bimetallic catalyst, with almost 70 vol % H2 for a flow rate of 5 cm3/min, corresponding to a nominal GHSV of 45 h-1. Decreasing the bubble size is another method of increasing the methane conversion. Figure 7 shows a comparison of the methane conversions obtained by bubbling natural gas through 4-in. molten Sn with two different open-bore feed tubes, i.e., 1/4-in. O.D. (0.21in. I.D.) and 1/16-in. O.D. (0.02-in. I.D.), respectively. As can be seen, a 10-fold decrease in the methane bubble size causes a 5-fold increase in the methane conversion, as a result of better heat transfer between the natural gas and the molten tin pool. However, the carbon generated during the reaction plugged the 1/16-in. O.D. (0.02-in. I.D.) feed tube after only 8 h on stream. A direct consequence of decreasing the methane bubble size can also be seen in the higher methane conversions obtained in a blank reactor, i.e., by bubbling methane through either a 1/4-in. or a 1/16 -in. O.D. feed tube without any heat transfer media (molten metal). An even more dramatic increase in the methane conversion, due to decreasing the methane bubble size, is observed in the experiments performed with Mott

Figure 9. Methane conversions in natural gas pyrolysis over 4-in. molten Sn (solid lines) or blank runs (dashed lines) bubbled through a 10 (2) or 0.5 µm (9) Mott sparger at 750 °C. The open symbols indicate repeated experiments.

porous metal filters as feed tubes. Mock-up experiments with water and argon (Figure 8), performed to visualize the distribution of bubbles across the porous metal filter, indicate that Mott spargers far exceed the performance of open-bore or drilled pipes; with thousands of pores over the surface, the spargers provide fine bubble propagation for an optimal gas/liquid contact and effective mass/heat transfer. A dramatic increase (Figure 9) in methane conversion is achieved as a result of using Mott spargers as feed tubes, with 57% methane conversion obtained for a flow rate of 15 cm3/min, corresponding to a nominal GHSV of 133 h-1. The temperature dependency of the natural gas thermal decomposition bubbled through a 0.5 µm Mott sparger and a pool of


Energy & Fuels, Vol. 17, No. 3, 2003 711 Table 1. EDS Analysis of Carbon Generated during Experiments with Sn as the Heat Transfer Media element

atom %

element wt %

96.25 0.34 1.41 1.28 0.15

79.84 2.8 5.06 4.95 0.61

C Sn Cr Fe Ni

Table 2. Carbon/Hydrogen Molar Ratios for the Carbon By-product Collected after Experiments with Different Heat Transfer Media heat transfer media 4-in. Pb 4-in. Sn 4-in. SiC 4-in. γ-Al2O3

Figure 10. Methane (solid line) and ethane conversions (dashed line) in natural gas pyrolysis over 4-in. molten Sn bubbled through a 0.5-µm Mott sparger as a function of temperature. Flow rate ) 15 cm3/min. The open symbols indicate repeated experiments.

Figure 11. Methane equilibrium conversion versus maximum methane conversions obtained with different heat transfer media and 1/4-in. open bore feed tube (0) or 0.5-µm Mott sparger (9).

4-in. molten Sn is shown in Figure 10. As expected, the methane and ethane conversions increased with temperature. The use of Mott porous metal filters as feed tubes creates methane conversions approaching the equilibrium conversion limit, i.e., 87% at 750 °C (Figure 11). The most efficient systems tested for the pyrolysis process are the ones using the Mott porous filters, with 57 and 51% methane conversion obtained when natural gas was passed through a bed of 4-in. Sn + SiC and Sn, respectively. Most importantly, the potential ease of separation of carbon byproduct from the reaction mixture makes the technique proposed in this paper more advantageous than the catalytic process. Carbon Formation. The byproduct value of SMR is very low, since there are no large-scale applications for CO2. Carbon, on the other hand, represents an energy source, and could be stored until carbon management

C:H molar ratio 0.0492 0.0530 0.0690 0.0336

is better developed. One possible promising market for the pyrolytic carbon is the carbon fuel cell, a technology currently under development at Lawrence Livermore National Laboratory.23,24 The fuel in such a cell consists of fine particulates (∼100 nm) of carbon produced by controlled pyrolysis of hydrocarbons extracted from coal, petroleum, or natural gas. These forms of carbon can be converted to electricity at efficiencies of 80% at rates practical for the utility application. However, CO2 is produced during operation of the carbon fuel cell, and technologies for handling the CO2 need to be developed. It is expected that the CO2 produced in the carbon fuel cell is concentrated and undiluted with nitrogen or water, and may therefore be more easily sequestered. Using the carbon byproduct for generating electricity could significantly improve the overall efficiency of the thermal decomposition of methane and reduce the cost of hydrogen obtained from the process. Figure 12 shows SEM images of the carbon collected after experiments with Pb, SiC, and Sn. In the pyrolysis of methane, two forms of carbon are generated, namely, finely divided carbon (soot) (Figure 12a,d), and pyrocarbon (Figure 12b) as solid carbon deposits as a result of surface reactions.25,26 Pyrocarbon was found to cover the entire heated section of the reactor, irrespective of the heat transfer media used in the pyrolysis experiments. In the case of methane pyrolysis under conditions of filtering through a porous or granular medium, pyrocarbon is deposited in the pores of the granules and on their surface, and soot is formed in the free space between granules.27 SEM images of the SiC granules (SABET < 0.1 m2/g) confirmed that the particles were covered with filamentous carbon (Figure 12c). SEM/EDS analyses of sections of the interior cup reactor walls (20) Wade, D. C. The Technology of the Integral Fast Reactor and Its Associated Fuel Cycle. Prog. Nucl. Energy, Special Issue, 1997; Vol. 31, Chapter 4, No. 1/2. (21) Hill, D. J. Reliab. Eng. Syst. Saf. 1998, 62, 43-50. (22) Spencer, B. W. Proceedings of the 8th International Conference on Nuclear Engineering, ICONE-8, Baltimore, MD, 2000. (23) Cooper, J., F.; Cherepy, N.; Berry, G.; Pasternak, A.; Surles, T.; Steinberg, M. Paper No. 50, Fall Meeting of the Electrochemical Society, Phoenix, AZ, October 2000. (24) Cooper, J. F.; Cherepy, N.; Berry, G.; Pasternak, A.; Surles, T. S&TR 2001, June, 4-12. (25) Shurupov, S. V.; Tesner, P. A. Combust., Explos., Shock Waves 1999, 35 (4), 386-392. (26) Popov, R. G.; Shpilrain, E. E.; Zaytchenko, V. M. Int. J. Hydrogen Energy 1999, 24, 335-339. (27) Direktor, L. B.; Zaichenko, V. M.; Maikov, I. L.; Sokol, G. F.; Shekhter, Yu. L.; Shpilrain, E. E. High Temperature 2001, 39 (1), 8592.

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Figure 12. SEM images of carbon samples collected after experiments with Pb (a, b), SiC (c, d), and Sn (e, f).

(Figure 1) were performed to determine the structure and composition of the carbon formed during the pyrolysis reaction. An analysis of the top layer of a crosssectional area of the cup 4 in. from the bottom reveals that carbon fibers cover the interior walls (Figure 12e,f, and Table 1).

Elemental analyses of carbon samples generated during the pyrolysis experiments indicate that the C:H atomic ratios (on a molar basis) are within about 1 C atom per 0.06 H atom for all samples analyzed (Table 2). This corresponds to the following reaction stoichiometry:



of iron, iron carbide, iron chromium oxide, and nickel chromium oxide indicates that 304 stainless steel is incompatible with the molten metal/CH4/carbon/H2 environment at elevated temperatures. Conclusions

Figure 13. XRD spectra of carbon collected after experiments with NiMo/γ-Al2O3, γ-Al2O3, and Sn. (b) Mo2C; (]) Fe3C; (9) NiCr2O4; (2) (Fe0.6Cr0.4)2O3; (0) Fe; Fe1.86C0.14; (O) Carbolite; ([) Sn.

CH4 + C2H6 ) 3[C(H)0.02] + 4.97H2 The hydrogen atom balance calculated after each experiment confirms the above reaction stoichiometry. The BET surface area of a carbon sample collected after experiments with Sn as the heat transfer media was found to be 64 m2/g, with an average particle size (by TEM) of around 100 nm. XRD spectra (Figure 13) reveal that the carbon byproduct is graphitic in nature. The presence of molybdenum carbide, in the case of the catalytic runs performed with the bimetallic catalyst, is clearly indicated by the strong peak at 2θ ) 39.8°, as well as the weaker one at 52.2°. The peaks at 2θ ) 30.6° and 44.9°, characteristic for Sn, indicate that the sample designated as C/Sn is contaminated with Sn, most probably because the sample was collected by scratching the reactor walls just above the molten Sn pool. A similar analysis performed on carbon collected from the reactor volume is free of Sn contamination. The presence

Direct decomposition of methane is regarded as an alternate route for hydrogen and elemental carbon production and the only fossil fuel-based process completely free of CO2 emissions. Natural gas was converted selectively to carbon and hydrogen at temperatures ranging from 600 to 900 °C under conditions of bubbling natural gas through a bed of either low-melting-point metals (e.g., lead or tin), a heated solid media (e.g., silicon carbide or alumina), or a mechanical mixture of metal and solid media. The methane conversions were found to be dependent upon the contact time between the methane and the heat transfer media, as well as on the methane bubble size. The most efficient systems used for the pyrolysis process were the ones using Mott porous filters as feed tubes, with almost 57 and 51% methane conversions obtained when passing natural gas through a bed of 4-in. Sn + SiC and Sn, respectively. One of the reasons the proposed technique is believed to be very advantageous is the ease of buoyant separation of the generated carbon byproduct from the liquid heat transfer media. It is envisioned that, in an industrial process, the separation can be made either by skimming or filtering the carbon from the heat transfer media surface. Also, technologies such as cyclones and bag filters, developed in carbon black manufacture, can be readily adapted for carbon collection in the direct contact pyrolysis process. Acknowledgment. The work was performed using an Argonne LDRD grant under the auspices of the U.S. Department of Energy, under contract number W-31109-ENG-38. The authors are grateful to Carl A. Udovich for his helpful and stimulating discussions. We also acknowledge Leonard Leibowitz for the thermodynamic analysis and Paul L. Johnson and Don G. Graczyk for the SEM, XRD, and the C:H ratio measurements. EF020271Q

Hydrogen Production by Direct Contact Pyrolysis of Natural Gas

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