Dry Reforming of Methane Using a Solar-Thermal Aerosol Flow

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Ind. Eng. Chem. Res. 2004, 43, 5489-5495

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Dry Reforming of Methane Using a Solar-Thermal Aerosol Flow Reactor Jaimee K. Dahl and Alan W. Weimer* Department of Chemical Engineering, ECCH 111, Campus Box 424, University of Colorado, Boulder, Colorado 80309-0424

Allan Lewandowski and Carl Bingham National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401-3393

Fabian Bruetsch and Aldo Steinfeld Department of Mechanical and Process Engineering, Institute of Energy Technology, ETH-Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092, Zurich, Switzerland

Research has focused on dry reforming because it offers a sink for CO2 and relative to steam methane reforming produces a more desirable ratio of H2 to CO for feed to a Fischer-Tropsch synthesis process. To form synthesis gas by dry reforming in a rapid, environmentally benign manner, a fluid-wall aerosol flow reactor powered by concentrated sunlight has been designed. Operating with residence times on the order of 10 ms and temperatures of approximately 2000 K, CH4 and CO2 conversions of 70% and 65%, respectively, have been achieved in the absence of any added catalysts. Methane to carbon dioxide feed ratios greater than unity were fed in order to prevent reaction of CO2 with the graphite tube. The carbon black particles formed by reaction were amorphous carbon black with a primary particle size of approximately 20-40 nm. 1. Introduction Reforming of methane is done commercially as either a method to produce hydrogen or for the production of synthesis gas (a mixture of hydrogen and carbon monoxide). Synthesis gas is then used to produce higher hydrocarbons via Fischer-Tropsch synthesis. Currently, steam reforming of methane is most widely used, although dry reforming is gaining in popularity. Both steam and dry reforming are highly endothermic reactions that require high temperatures to achieve high conversions of methane. As shown in reaction 1, steam reforming is the reacting of stoichiometic quantities of methane and steam to form a 3:1 H2:CO ratio.

CH4 + H2O f CO + 3H2

(1)

Typically, carbon deposition is an issue for reactors containing catalysts so excess steam is used to minimize deposition.1 Bohmer et al.2 added water vapor in a 3:1 ratio to CH4 to minimize soot deposition on the catalyst. However, addition of excess water vapor reduces significantly the energy conversion efficiency due to the power wasted to produce excess steam at the operating temperature. It further increases the H2/CO ratio above 3 and ratios of 3 or greater are too high for FischerTropsch synthesis.3 Therefore, many studies have been conducted to find methods of reducing the H2O/CH4 ratio. Yokota et al.4 investigated steam reforming at 923-1223 K and 1 atm pressure using a Ni-Al2O3 * To whom correspondence should be addressed. Telephone: 303-492-3759. Fax: 303-492-4341. E-mail: [email protected].

catalyst. Carbon deposition was reduced by carefully maintaining a 1:1 H2O:CH4 ratio at all times. Above 1123 K, 85% of methane conversion efficiency was achieved. Studies have also concentrated on increasing methane conversion during steam reforming. Researchers have found that removing hydrogen significantly increases conversion.5,6 Solh et al.6 used a fluidizable catalyst in a circulating fluidized-bed reactor to carry out the steam reforming of methane combined with a hydrogenpermeable membrane. Some of the catalyst did have to be regenerated as a result of coke deposition. Oklany et al.5 developed a model to study steam reforming in a catalytic membrane reactor and focused on both Pd/Ag dense composite membranes as well as microporous membranes. It was found that removing hydrogen from the system greatly increased methane conversion for both membranes but that increasing the pressure only increased conversion for the Pd/Ag membrane. Their model predicted 100% methane conversion at 973 K and 100% hydrogen removal. Hou et al.7 found two issues that arise as a result of hydrogen removal. Both carbon formation and poisoning of the catalyst by sulfur compounds are increased when hydrogen is removed from the system. However, if the reactor is operated at higher temperatures, the effects of H2S poisoning can be reduced. Operating at higher pressures can reduce carbon deposition. An alternative method for producing synthesis gas is dry reforming:

CH4 + CO2 f 2H2 + 2CO

(2)

The advantage of using dry reforming over steam reforming is that H2/CO ratios lower than 3 are easily

10.1021/ie030307h CCC: $27.50 © 2004 American Chemical Society Published on Web 10/11/2003

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attained, which is desirable for Fischer-Tropsch synthesis. Another advantage of dry reforming is that it is highly selective to CO, whereas steam reforming has poor selectivity to CO and also forms CO2.3 In addition, this chemistry offers a way to fix CO2 derived from CO2emitting processes (e.g., combustion, cement, and metallurgical processes) and to utilize “contaminated” natural gas wells that also contain large quantities of CO2. Catalysts are frequently used to carry out dry reforming in order to lower the reactor temperatures needed for high conversion of methane. However, the two side reactions shown in reactions 3 and 4 often take place and the carbon produced fouls the catalyst.8,9

CH4 f C + 2H2

(3)

2CO f C + CO2

(4)

Therefore, numerous studies have focused on synthesizing catalysts that result in little or no coking. Pen˜a et al.10 summarize work done by others and discuss some catalysts, such as noble metal-based catalysts, that have operated successfully under certain conditions with no carbon formation. Levy et al.9 operated a solar-powered reactor with a supported rhodium catalyst. At a maximum achievable temperature of 1233 K, a methane conversion of 84% was obtained with CO2:CH4 ratios of about 1.1:1.3. Carbon deposition became an issue when the gas flows were stopped, so a small circulating pump was installed to keep the gases circulating when the reactor was off-sun. Wo¨rner and Tamme11 also operated a solar-powered reactor but used two different ceramic foam structures as the catalysts. The reactor was operated at temperatures of 973-1133 K with an absolute pressure of 3.5 atm. A methane conversion of 80% was achieved, but coking did occur which caused a reduction in the methane conversion. To simulate solar heating, Kodama et al.12 directly irradiated Ni catalysts using a Xe-arc lamplight in a packed bed reactor. Greater than 90% methane conversion was obtained but issues arose with carbon deposition on the catalyst surface. Kodama et al.13 did some earlier work on bubbling methane and carbon dioxide through a molten salt bath that contained an Al2O3-supported metal catalyst. The reactor was operated at 1223 K and methane conversions of 6098% were achieved. The solar reforming reactor technology has been scaled-up to power levels of 300-500 kW and tested in a solar tower facility using a high-pressure (8-10 bar) indirectly irradiated tubular reactor14 and a low-pressure (1-3 bar) directly irradiated volumetric reactor.15 More recently, a volumetric solar reformer reactor, featuring a directly irradiated catalytic ceramic foam, has been designed to operate at 1098 K and 9 atm with a solar power input of 400 kW.16 NiO-MgO catalysts have been widely used. Ruckenstein and Hu3 obtained 91% CH4 conversion and 98% CO2 conversion with 100% selectivity to CO and H2 using a NiO-MgO solid-solution catalyst, which was reported to be very stable. However, Choudhary et al.17 found significant deposition of carbon on unsupported NiO-MgO as well as on NiO-CaO catalysts. Therefore, to reduce carbon deposition, a supported NiO-MgO catalyst was used in their studies as well as simultaneous steam and dry reforming of methane with and without oxygen present. Although these measures reduced carbon deposition, some coking was still observed.

A methane conversion of 97% and 100% selectivity for both CO and H2 were reported. Gadalla and Sommer18 used a Ni catalyst supported on CaO-TiO2-Al2O3 and obtained ∼100% conversion for a CO2/CH4 ratio of 2.64. However, carbon deposition on the catalyst was observed, which decreased the surface area of the catalyst. Operating a flow-type reactor with a Rh-modified Ni-Ce2O3-Pt catalyst, Inui et al.19 obtained 86.2% and 88.2% CH4 and CO2 conversions, respectively. A feed stream diluted with N2 was fed into a reactor operating at 673-773 K and atmospheric pressure. One heating method that avoids the use of catalysts is dielectric-barrier discharges. Because of the nature of the heating method, these reactors can be operated at low temperatures and pressures. Zhou et al.8 fed CO2rich mixtures into their reactor to prevent carbon and wax deposition. Maximum conversions of 64% and 54% for CH4 and CO2, respectively, were obtained with an 80/20 CH4/CO2 feed mixture. This paper proposes another reactor configuration and heating method that avoids the use of catalysts for the dry reforming of methane. The reaction is carried out in a solar-powered fluid-wall aerosol flow reactor. Temperatures on the order of 2000 K and residence times of 10 ms can easily be obtained. Besides the ability to produce significant quantities of product rapidly, there are numerous advantages to this process. Because sunlight is a renewable energy source, the cost and environmental consequences of achieving extremely high temperatures are avoided. Since high temperatures can easily be achieved, catalysts are not necessary, so coking is not an issue. In fact, the presence of carbon in the system is desirable because the fine particles enhance the radiative heat transfer in the process. The process can also be operated in remote locations, so it can easily be utilized at remote natural gas fields. Studies were conducted to determine the effect of reactor wall temperature as well as CH4/CO2 feed ratios on conversion. Three different feed ratios were investigated:

CH4 + CO2 f 2H2 + 2CO

(5)

1.5CH4 + CO2 f 3H2 + 2CO + 0.5C

(6)

2CH4 + CO2 f 4H2 + 2CO + C

(7)

The three feed ratios yield H2/CO ratios ranging from 1 to 2, which are reasonable values for feed to a Fischer-Tropsch synthesis process. 2. Experimental Apparatus The High Flux Solar Furnace (HFSF) facility at the National Renewable Energy Laboratory (NREL) was used to carry out the experiments. The reader is referred to Dahl et al.20 for a description of the HFSF. The furnace delivered a concentrated sunlight beam approximately 0.1 m in diameter to the reactor. The sunlight was reflected toward the reactor using a secondary concentrator. The cone section of the secondary concentrator, which had an orthogonal opening narrowing to a rectangular opening, reflected the sunlight toward the reactor. The trough section, which wrapped around the backside of the reactor tubes, enabled 360° heating of the reactor tube. The inner surface of both sections was highly reflective. The

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Figure 1. Photograph of reactor heated to approximately 1650 K, showing secondary concentrator, cooling zones, quartz tube, and solid graphite tube.

heated length was 0.094 m. The graphite reactor tube absorbed 76% of the energy from the primary concentrator. The reactor itself was composed of three concentric vertical tubes. The innermost tube was a 0.012-m i.d., 0.31-m long porous graphite with two solid graphite end caps on either end of the tube. The center tube was a 0.021-m i.d. solid graphite tube that was 0.36 m in length. The outer tube was quartz with an i.d. of 0.048 m and a length of 0.26 m. The sunlight heated the center solid graphite tube, which then radiated to the porous graphite tube. Argon was fed into the annular region between the two graphite tubes and the gas was forced through the porous tube wall. It served to protect the inner tube wall from carbon particle deposition. Argon was fed during these studies in order to simplify the product gas analysis for hydrogen, but the gas used in the full-scale system will be hydrogen in order to eliminate separation requirements. The methane and carbon dioxide were fed into the top of the porous graphite tube, and the reaction products as well as the fluid-wall gas exited the bottom of the reactor. Argon was fed counter currently into the annular region between the solid graphite tube and the quartz tube in order to provide an inert atmosphere to prevent oxidation of the graphite tube. A photograph of the reactor is shown in Figure 1. Each of the three tubes was sealed using O-rings in order to isolate the annular argon stream from the methane, carbon dioxide, and fluid-wall gas streams. Therefore, cooling zones were integrated into the upper and lower fittings in order to rapidly cool the gas streams before they passed by the O-rings. The temperature of the solid graphite tube was measured using an optical pyrometer. The measurement was made through a vertically centered hole in the side of the trough section of the secondary concentrator, as can be seen in Figure 1. Modeling studies indicate that the temperature of the porous graphite tube is nearly identical to the temperature of the solid graphite tube. Therefore, the temperature of the two tubes is consid-

Figure 2. Schematic of fluid-wall reactor system.

ered identical for the purposes of this study. A schematic of the reactor system is shown in Figure 2. Any carbon particles formed by reaction were collected downstream of the reactor exit. The composition of the product gas stream was then measured by an in-line NOVA thermal conductivity/infrared detector (TC/IR) that outputs volume percent of methane, carbon dioxide, carbon monoxide, and hydrogen. Conversion values reported in succeeding sections are calculated using output from that instrument. 3. Experimental Design The experiments were designed such that the effect of both reactor wall temperature and CH4:CO2 ratio could be studied. This resulted in 16 runs, plus a single repeat of each run for a total of 32 tests. To study the effect of reactor wall temperature, the CH4:CO2 ratio was set at 2:1 and total (methane and carbon dioxide) flow rates of both 1.67 × 10-5 and 3.33 × 10-5 m3/s (1 and 2 slpm) were investigated. The temperature was increased from 1873 K to the maximum attainable temperature, which was approximately 2100 K. In addition, at the highest temperature (≈2100 K), three

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Figure 3. Real time data from test #16 in which 2.0 × 10-5 m3/s (1.2 slpm) of CH4 and 1.33 × 10-5 m3/s (0.8 slpm) of CO2 were fed.

Figure 4. Real time data from test #14 in which 1.67 × 10-5 m3/s (1.0 slpm) of CH4 and 1.67 × 10-5 m3/s (1.0 slpm) of CO2 were fed.

Table 1. Experimental Design

run number

methane flow rate (m3/s)

carbon dioxide flow rate (m3/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1.12 × 10-5 1.12 × 10-5 1.12 × 10-5 1.12 × 10-5 1.12 × 10-5 2.22 × 10-5 2.22 × 10-5 2.22 × 10-5 2.22 × 10-5 2.22 × 10-5 8.33 × 10-6 1.12 × 10-5 1.0 × 10-5 1.67 × 10-5 2.22 × 10-5 2.0 × 10-5

5.5 × 10-6 5.5 × 10-6 5.5 × 10-6 5.5 × 10-6 5.5 × 10-6 1.12 × 10-5 1.12 × 10-5 1.12 × 10-5 1.12 × 10-5 1.12 × 10-5 8.33 × 10-6 5.5 × 10-6 6.67 × 10-6 1.67 × 10-5 1.12 × 10-5 1.33 × 10-5

total flow rate reactor wall of CH4 and CO2 temperature (K) (m3/s) 1.67 × 10-5 1.67 × 10-5 1.67 × 10-5 1.67 × 10-5 1.67 × 10-5 3.33 × 10-5 3.33 × 10-5 3.33 × 10-5 3.33 × 10-5 3.33 × 10-5 1.67 × 10-5 1.67 × 10-5 1.67 × 10-5 3.33 × 10-5 3.33 × 10-5 3.33 × 10-5

1873 1923 1973 2023 2073 1873 1923 1973 2023 2073 2123 2123 2123 2123 2123 2123

CH4:CO2 ratios were studied: 1:1, 1.5:1, and 2:1. For all runs, the annular argon flow rate was maintained at 1.67 × 10-4 m3/s (10 slpm), and argon was fed through the porous tube wall at 6.67 × 10-5 m3/s (4 slpm). Table 1 shows one replicate of the experimental design. All experiments were run in a random order, except for the 1:1 CH4:CO2 runs, which were run last because of their effect on the reactor tube material. 4. Results and Discussion Real time data from a run in which 2.0 × 10-5 m3/s (1.2 slpm) and 1.33 × 10-5 m3/s (0.8 slpm) of CH4 and CO2, respectively, were fed are shown in Figure 3. The reactor was put on-sun at 132 s and a power level of approximately 9800 W was obtained, which is almost at the maximum attainable level for the furnace, which is 10000 W. The test was conducted on a relatively clear day so the power level did not have to be varied significantly to maintain a steady reactor wall temperature. As is seen in Figure 3, it took approximately 60 s for the reactor wall temperature to reach 2108 K, which was maintained for the remainder of the run. There was another delay time of about 60 s before the exit gas stream concentrations (as outputted by the thermal conductivity and infrared detector located downstream of the particle collection area) began to change. The CH4 and H2 concentration values changed

Figure 5. Real time data from test #2 in which 1.12 × 10-5 m3/s (0.67 slpm) and 5.5 × 10-6 m3/s (0.33 slpm) of CH4 and CO2, respectively, were fed.

at a faster rate than the CO and CO2 values. It took approximately 600 s for the gas concentrations to settle. Traces from a test in which 1.67 × 10-5 m3/s (1.0 slpm) of CH4 and 1.67 × 10-5 m3/s (1.0 slpm) of CO2 were fed are shown in Figure 4. A reactor wall temperature of approximately 2060 K was maintained. Once again, there was about 60 s of delay time from the time when the reactor was placed on-sun until the reactor wall temperature reached the steady-state value. Due to hazy weather conditions, the power level had to be reduced during the final portion of the test in order to maintain a reactor wall temperature of approximately 2060 K. Figure 5 contains data from a run at a reactor wall temperature of 1923 K and initial CH4 and CO2 flow rates of 1.12 × 10-5 m3/s (0.67 slpm) and 5.5 × 10-6 m3/s (0.33 slpm), respectively. Since this is a lower temperature test, the power level was at a value of approximately 6000-6500 W. As can be seen by the trace of the power level, it was a hazy day with varying irradiance. Therefore, the power level was adjusted twice in order to maintain the reactor wall temperature at approximately 1923 K. It took about 700 s for the exit gas concentrations to reach steady-state values. Both methane conversion and carbon dioxide conversion were calculated for all runs. The methane conversions for all 2:1 CH4:CO2 runs are shown in Figure 6. A portable gas chromatograph and mass spectrometer

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Figure 6. Sensitivity of methane conversion to reactor wall temperature for all 2:1 CH4:CO2 runs.

Figure 8. Sensitivity of carbon dioxide conversion to reactor wall temperature for all 2:1 CH4:CO2 runs.

Figure 7. Rate of methane dissociation as a function of reactor wall temperature.

Figure 9. Rate of carbon dioxide dissociation as a function of reactor wall temperature.

analysis indicated that virtually all hydrogen exited the reactor as unreacted CH4 or as H2. With the exception of the tests run at 1873 K, increasing the residence time increased conversion. The residence times for all runs were on the order of 10 ms. In addition, increasing the reactor wall temperature increased conversion, although an asymptotic behavior appeared to be emerging at temperatures of 2073 K or higher. A methane conversion of approximately 70% was obtained at a wall temperature of 2100 K and a total CH4 and CO2 flow rate of 1.67 × 10-5 m3/s (1.0 slpm). The rate at which CH4 is converted is shown in Figure 7. The rate increases with temperature until a reactor wall temperature of 2073 K is reached. Above 2073 K, the rate appears to be constant with increasing temperature. Similarly, conversions of CO2 were calculated and those values are shown in Figure 8 for all 2:1 CH4:CO2 runs. The conversions were calculated assuming that all oxygen exited the reactor as unreacted CO2 or as CO. There is more variability in the CO2 conversion values than was observed for the CH4 conversions. This is due to the fact that the instrument measuring the gas stream outlet concentrations had a somewhat unstable percent CO2 reading. However, the same trends (increase of conversion with reactor wall temperature and with increased residence time) were still observed. The maximum CO2 conversion obtained was approximately 65% at 2073 K. Increasing the reactor wall temperature above 2073 K did not appear to increase the conversion. The CO2 reaction rate is shown as a function of reactor wall temperature in Figure 9. There appears to be an exponential increase in rate with temperature.

Table 2. Conversions Obtained for Three Different CH4 to CO2 Ratios CH4 flow rate (m3/s)

CO2 flow rate (m3/s)

8.33 × 10-6 8.33 × 10-6 1.0 × 10-5 1.0 × 10-5 1.12 × 10-5 1.12 × 10-5 1.67 × 10-5 1.67 × 10-5 2.0 × 10-5 2.0 × 10-5 2.22 × 10-5 2.22 × 10-5

8.33 × 10-6 8.33 × 10-6 6.67 × 10-6 6.67 × 10-6 5.5 × 10-6 5.5 × 10-6 1.67 × 10-5 1.67 × 10-5 1.33 × 10-5 1.33 × 10-5 1.12 × 10-5 1.12 × 10-5

total flow reactor CO2 rate of CH4 wall CH4 and CO2 temp conversion conversion 3 (m /s) (%) (%) (K) 1.67 × 10-5 1.67 × 10-5 1.67 × 10-5 1.67 × 10-5 1.67 × 10-5 1.67 × 10-5 3.33 × 10-5 3.33 × 10-5 3.33 × 10-5 3.33 × 10-5 3.33 × 10-5 3.33 × 10-5

2043 2083 2098 2130 2104 2111 2059 2106 2107 2123 2103 2104

59 68 74 77 66 73 48 52 56 60 48 61

29 38 58 69 44 67 19 21 30 38 24 46

The sensitivity of conversion to changes in the CH4: CO2 feed ratio was studied at the maximum attainable temperature during a given day. That value ranged from about 2040 K to 2130 K but was on average approximately 2100 K. These results are shown in Table 2. For a given total flow rate, changing the ratio of CH4 to CO2 did not significantly change the conversion. However, once again, increasing the residence time appeared to increase the conversion for a given CH4 to CO2 feed ratio. Another interesting result arose during the running of the four 1 to 1 CH4 to CO2 tests. The porous graphite tube was removed after the completion of these tests and large holes were observed on the wall of the tube, as shown in Figure 10. The holes were located at the bottom of the hot zone. It was theorized that the CO2 present reacted with the graphite to form CO, as thermodynamically favored by the Boudouard equilib-

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Figure 10. Photograph of porous graphite tube taken after the completion of the 1:1 CH4:CO2 experiments.

Figure 12. TEM image of dry reforming carbon black showing an amorphous product with a primary particle size of 20-40 nm.

Figure 11. X-ray diffraction of dry reforming carbon black product as well as Shawinigan acetylene carbon black for comparison.

rium above 1000 K. Therefore, it is advantageous for this reactor design to run a CH4 to CO2 ratio that is greater than 1, and as discussed in the previous paragraph, similar conversions were obtained regardless of the CH4:CO2 ratio run for a given total flow rate. The carbon black produced as a result of feeding methane-rich feed streams was analyzed to determine its quality. The product analyzed was collected during all tests in the downstream filter. As shown in Figure 11, an X-ray diffraction pattern of the carbon black product indicated that its crystallinity was very similar to that of Shawinigan acetylene black, which is a highquality amorphous carbon black sold on the world market with a primary particle size of 30 nm. Figure 12 also shows that the carbon was amorphous, as opposed to graphitic, and indicates that the particles had a primary particle size of approximately 20-40 nm. To verify the particle size, surface area measurements were made on the carbon product as well as the Shawinigan acetylene black for comparison. The acetylene black had a surface area of 55 m2/g, which corresponded to a primary particle size of approximately 48 nm. The dry reforming carbon product had a surface area of 33 m2/g, corresponding to an average particle diameter of 81 nm. This reduced surface area indicates that primary carbon black particles are necked, which is a desirable property increasing electrical conductivity. 5. Conclusions A solar-thermal fluid-wall aerosol flow reactor was used for dry reforming of methane to produce synthesis gas. Methane and carbon dioxide conversions of 70% and 65%, respectively, were obtained at a reactor wall temperature of 2100 K and an average residence time of 10 ms. It was found that conversions of both methane

and carbon dioxide were dependent on reactor wall temperature as well as residence time. Changing the CH4/CO2 feed ratio did not affect the conversion for a given residence time. The carbon black particles produced by reaction, which enhanced the heat transfer to the gas phase and potentially acted as a catalyst, were amorphous particles with a primary particle size of ∼30 nm. The high quality of the carbon particles indicates that they may be used to produce further energy and/ or may be a marketable product. This process provides a promising method to produce synthesis gas from the dry reforming of methane without added catalysts, while minimizing the impact on the environment and avoiding major issues such as coking and/or maximum operating temperature limitation of a catalyst. Acknowledgment The authors thank the U.S. Department of Energy Hydrogen Program, the University of Colorado, BP, Chevron-Phillips, Chevron-Texaco, Electric Power Research Institute, General Motors, Harper International, Pinnacle West, PlugPower, and Siemens for financially supporting this work under Grants DEFC36-99GO10454, DE-PS36-99GO10383, and cost-sharing requirements. In addition, we thank the U.S. Department of Education for support through the Graduate Assistantships in Areas of National Need (GAANN) program and the Swiss Federal Office of Energy for support through the SolarPACES program. Literature Cited (1) Roh, H.-S.; Jun, K.-W.; Dong, W.-S.; Park, S.-E.; Baek, Y.S. Highly Stable Ni Catalyst Supported on Ce-ZrO2 for Oxy-Steam Reforming of Methane. Catal. Lett. 2001, 74, 31-36. (2) Bo¨hmer, M.; Langnickel, U.; Sanchez, M. Solar Steam Reforming of Methane. Sol. Energy Mater. 1991, 24, 441-448. (3) Ruckenstein, E.; Hu, Y. H. Synthetic Fuels From Greenhouse Gases. Chem. Innovation 2000, 30, 39-43. (4) Yokota, O.; Oku, Y.; Arakawa, M.; Hasegawa, N.; Matsunami, J.; Kaneko, H.; Tamaura, Y.; Kitamura, M. Steam Reforming of Methane Using a Solar Simulator Controlled by H2O/CH4 ) 1/1. Appl. Organomet. Chem. 2000, 14, 867-870.

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Aspects of Further Application of this Technology. In Proceedings of the 8th International Symposium on Solar Thermal Concentrating Technologies, Cologne, Germany, Oct 6-11, 1996; pp 11851204. (16) Moeller, S.; Buck, R.; Tamme, R.; Epstein, M.; Liebermann, D.; Meri, M.; Fisher, U.; Rotstein, A.; Sugarmen, C. Solar Production of Syngas for Electricity Generation: SOLASYS Project TestPhase. In Proceedings of the 11th SolarPACES International Symposium on Concentrating Solar Power and Chemical Energy Technologies, Zurich, Switzerland, Sept 4-6, 2002; Steinfeld, A., Ed.; pp 231-237. (17) Choudhary, V. R.; Uphade, B. S.; Mamman, A. S. Simultaneous Steam and CO2 Reforming of Methane to Syngas Over NiO/MgO/SA-5205 in the Presence and Absence of Oxygen. Appl. Catal. A 1998, 168, 33-46. (18) Gadalla, A. M.; Sommer, M. E. Carbon Dioxide Reforming of Methane on Nickel Catalysts. Chem. Eng. Sci. 1989, 44, 28252829. (19) Inui, T.; Hara, H.; Takeguchi, T.; Ichino, K.; Kim, J. B.; Iwamoto, S.; Pu, S. B. Highly Effective Gasoline Synthesis From Carbon Dioxide. Energy Convers. Manage. 1997, 38, S385-S390. (20) Dahl, J. K.; Tamburini, J.; Weimer, A. W.; Lewandowski, A.; Pitts, R.; Bingham, C. Solar-Thermal Processing of Methane to Produce Hydrogen and Syngas. Energy Fuels 2001, 15, 12271232.

Received for review April 11, 2003 Revised manuscript received June 30, 2003 Accepted July 2, 2003 IE030307H