CO2 Reforming of Methane in a Molten Carbonate Salt Bath for Use in

Nov 17, 2000 - Due to the specific properties of a concentrated solar radiation energy source, modifications of the conventionally used reactor system...
0 downloads 8 Views 107KB Size
Download PDF
60

Energy & Fuels 2001, 15, 60-65

CO2 Reforming of Methane in a Molten Carbonate Salt Bath for Use in Solar Thermochemical Processes T. Kodama,* T. Koyanagi, T. Shimizu, and Y. Kitayama Department of Chemistry & Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan Received June 12, 2000. Revised Manuscript Received October 5, 2000

CO2 reforming of methane was studied by bubbling a CH4/CO2 mixture in a molten salt bath of alkali-metal carbonate mixture (Na2CO3/K2CO3) containing suspended metal catalyst powder at 1223 K. Ni, Fe, Cu, or W metals, supported on an Al2O3 support, were examined for activity and selectivity. The most active and selective catalyst was the Ni/Al2O3 catalyst. The methane conversion increased with an increase in the W/F ratio (W ) weight of the molten salt bath containing the Ni/Al2O3 catalyst, and F ) flow rate of CH4/CO2 mixture). About 70% of methane conversion was obtained at a W/F ratio of 0.25 g min cm-3, in which the H2/CO ratio in the product gas was approximately the stoichiometric ratio of one. This technique will be applied to the solar thermochemical methane reforming for converting solar high-temperature heat to chemical fuels, to give stable operations under fluctuation of insolation and thermal uniformity in the solar reformer under concentrated solar irradiation.

Introduction

CH4 + H2O f CO + 3H2 ∆H°298K ) 206 kJ (1)

The efficient utilization of high-temperature heat from concentrated solar radiation in regions of the sun belt represents a subject which is of current interest.1-3 The conversion of solar heat to chemical fuels enables solar energy storage and transportation from the sun belt to remote population centers. From the point of view of the chemical pathway for this process, several hightemperature endothermic reactions have been investigated as solar high-temperature thermochemical processes, such as a multistep water-splitting reaction,4-6 coal gasification,7,8 and natural gas reforming. Optimal operating temperatures for converting concentrated solar radiation into chemical-free energy range from 800 to 1300 K for a blackbody solar cavity receiver under peak solar flux intensities between 1000 and 12000 kW m-2.9,10 The reforming process is a catalytic endothermic reaction between low hydrocarbons, such as methane, with steam or carbon dioxide, which produces synthesis gas or syngas containing primarily CO and H2:

CH4 + CO2 f 2CO + 2H2 ∆H°298K ) 247 kJ (2)

* Corresponding author. Fax: +81-25-262-7010. E-mail: tkodama@ eng.niigata-u.ac.jp. (1) Fletcher, E. A. J. Minn. Acad. Sci. 1983/84, 49, 2, 30-34. (2) Grasse, W.; Tyner, C. E.; Steinfeld, A. J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 1999, 9, Pr3-9-3-15. (3) Tamaura, Y. Solar Thermal 2000, Proceedings of the 10th SolarPACES International Symposium on Solar Thermal Concentrating Technologies, 2000, pp 189-192. (4) Nakamura, T. Solar Energy 1977, 19, 467-475. (5) Bilgen, E.; Joels, R. K. Int. J. Hydrogen Energy 1985, 10 (3), 143-155. (6) Lundgerg M. Int. J. Hydrogen Energy 1993, 18 (5), 369-376. (7) Gregg, D. W.; Taylor, R. W.; Campbell, J. H.; Taylor, J. R.; Cotton, A. Solar Energy 1980, 25, 353-364. (8) Flechsenhar, M.; Sasse, C. Energy 1995, 20, 8, 803-810. (9) Fletcher, E. A.; Roger, L. M. Science 1977, 197, 1050-1056. (10) Steinfeld, A.; Schubnell, M. Solar Energy 1993, 50 (1), 19-25.

This endothermic reaction is the basis for upgrading the calorific value of the hydrocarbons by roughly 25%, using solar energy. The calorifically upgraded product of syngas can be stored and transported to be combusted in a conventional gas turbine (GC) or a combined cycle (CC), to generate electricity with a high conversion efficiency (up to 55% in a modern, large CC). The product syngas can be readily converted to liquid fuels such as methanol, which can be more easily transported from the sun belt to remote population centers, e.g., overseas by a conventional oil tanker.3 Another future potential utilization of the product syngas or the methanol is to use them for fuel cells with high conversion efficiency, higher than GC and CC. Due to the specific properties of a concentrated solar radiation energy source, modifications of the conventionally used reactor system are necessary for the solar reforming process. Different types of solar reforming reactors have been developed and tested, e.g., by the German Aerospace Research Center (DLR) in Germany, Sandia National Laboratories in the U.S.A., and the Weizmann Institute of Science (WIS) in Israel.11-18 One (11) Levitan, R.; Rosin, H.; Levy, M. Solar Energy 1989, 42 (3), 267272. (12) Bo¨hmer, M.; Langnickel, U.; Sanchez, M. Solar Energy Mater. 1991, 24, 441-448. (13) Yao, C.; Epstein, M. Solar Energy 1996, 57 (4), 283-290. (14) Epstein, M.; Spiewak, I.; Segal. A., Levy, I.; Liebermann, D.; Meri, M.; Lerner, V. Proceedings of 8th International Symposium on Solar Thermal Concentrating Technology 1997, Vol. 3, pp 1209-1229. (15) Muir, J. F.; Hogan, R. E., Jr.; Skocypec, R. D.; Buck, R. Solar Energy 1994, 52 (6), 467-477. (16) Skocypec, R. D.; Hogan, R. E., Jr.; Muir, J. F. Solar Energy 1994, 52 (6), 479-490.

10.1021/ef000130t CCC: $20.00 © 2001 American Chemical Society Published on Web 11/17/2000

CO2 Reforming of Methane

of them is a conventional tubular reactor type.11-14 This directly irradiated tubular reactor has been scaled up to 480 kW and tested for operation at the WIS in 1994.14 Another is a directly irradiated volumetric reactor type.15-18 Reforming of methane with CO2 using this reactor system was first demonstrated in the “catalytically enhanced solar absorption receiver” experiment conducted by DLR and Sandia National Laboratories (U.S.A.) in 1990.17 A 200-300 kW volumetric reactor was demonstrated also at the WIS for CO2 reforming of methane.18 Typical operating temperatures ranged from 937 to 1133 K, with an absolute pressure of 3.5 bar, reaching methane conversions over 80%. One of the essential problems in the solar reforming of methane, however, is the fluctuating incident solar radiation. The catalytic methane reforming process requires stable operation under the fluctuation of insolation by a cloud passage. Moreover, the solar chemical receiver-reactor to which the concentrated solar radiation is directed requires thermal uniformity inside the reactor. One solution to these problems is to use a molten metal salt with high heat capacity as heat transfer medium in the solar receiver-reactor. Here we propose the new catalytic methane reforming system in a molten salt bath to result in the stable operating conditions and thermal uniformity in the solar receiverreactor. A reactant gas mixture of methane with CO2 or steam is bubbled through the molten salt bath inside the receiver-reactor. Fine catalyst powder is suspended in the molten salt bath. Concentrated solar radiation is directed to the external metal wall of the receiverreactor, and the salt inside the receiver-reactor is heated indirectly by conduction through this metal wall of the receiver-reactor. The gaseous reactants and the catalyst powder are directly heated by the molten salt. Because of the high heat capacity of the molten salt bath, the rapid temperature change of the reactor inside will be circumvented under fluctuation of insolation. Also, this system will be able to achieve the thermal uniformity inside the reactor by natural convection or by forced convection of molten salt. In the present work, CO2 reforming of methane was demonstrated at 1223 K, using a small-scale reactor, by bubbling a CH4/CO2 mixture in a molten salt bath of alkali-metal carbonate mixture (Na2CO3/K2CO3) in which an Al2O3-supported metal catalyst was suspended. Experimental Section Preparation of Metal Catalysts. It is well-known that noble metals such as Rh, Ru, and Ir are very active for methane reforming. However, in the industrial scale, it is very difficult to use these expensive noble metals. Especially in the methane reforming in this molten salt system, recovery and reuse of the catalyst will be technically and economically more difficult in comparison to the conventional fixed-bed or fluidized-bed reaction systems. Thus, we tested cost-effective common metals as the active component of the catalyst in the present work. Ni, Fe, Cu, or W metal catalyst, supported on an Al2O3 support, were prepared by a conventional impregnation method by use of metal nitrate for Ni, Cu,and Fe, and (NH4)10W12O41‚5H2O for W as a precursor. The metal content (17) Buck, R.; Muir, J. F.; Hogan, R. E.; Skocypec, R. D. Solar Energy Mater. 1991, 24, 449-463. (18) Wo¨rner, A.; Tamme, R. Catalysis Today 1998, 46, 165-174.

Energy & Fuels, Vol. 15, No. 1, 2001 61

Figure 1. Experimental apparatus for CO2 reforming of methane in a molten salt bath. of the catalysts was 20 wt %. The R-Al2O3 support with a surface area of 6.3 m2 g-1 was suspended in the metal salt solution. The suspended solution was evaporated to dryness. After graining the dried powder in a mortar, it was calcined at 1023 K for 3 h in air and then reduced by H2 at 723 K for 0.5 h. The Al2O3-supported metal catalysts thus prepared were characterized by X-ray diffractometry (XRD) with CuKR radiation (Rigaku, RAD-γA diffractometer). In the XRD pattern, only the peaks due to the metallic Ni, Fe, Cu, or W appeared along with those of R-Al2O3 support. The BET surface area of the Ni/Al2O3 catalyst was determined by nitrogen adsorption (Shimadzu, Micromeritics Flow Sorb II 2300) to be 7.5 m2 g-1. Reaction System. An alkali-metal carbonate mixture of Na2CO3 and K2CO3 (weight ratio ) 1:1) was used as a molten salt bath because these molten carbonate salts have high heat capacities (Na2CO3: melting point 1131 K, heat capacity 1.819 kJ kg-1 K-1; K2CO3: melting point 1171 K, heat capacity 1.496 kJ kg-1 K-1) and the melting point of the mixture is around 973 K. The experimental apparatus is shown in Figure 1. The Na2CO3/K2CO3 mixture was mixed with the Al2O3-supported metal catalyst at a desired weight ratio. A mixture of catalyst and carbonate salt (30-50 g) was placed in a cylindrical stainless steel reactor (SUS-310S, 34 mm i.d., 2 mm thickness, and 150 mm long). A CH4/CO2 mixture (CH4/CO2 mole ratio ) 1:1) was introduced to the bottom of the inside of the reactor through the stainless steel tube (φ 3 mm). The flow rates of CH4 and CO2 were controlled by mass flowmeters. The flow rate of the CH4/CO2 feed ranged from 200 to 800 Ncm3 min-1 at atmospheric pressure. The reactor was externally heated by an infrared furnace (ULVAC, E45) up to 1223 K within 10 min to melt the carbonate salt. Afterward, the CH4/CO2 mixture was bubbled through the molten salt bath. The temperature was controlled using a K-type thermocouple in contact with the external stainless wall of the reactor. The steam in the effluent gases from the reactor was condensed in a cooling trap connected to the outlet of the reactor. The dry effluent gases were analyzed by gas chromatography equipment (Shimadzu, GC-4C) with a TCD detector. The conversion of methane during the CO2 reforming was calculated by the following equation:

conversion of methane )

yCO + yH2 4yCH4 + yCO + yH2

(3)

62

Energy & Fuels, Vol. 15, No. 1, 2001

Figure 2. Equilibrium composition of major components of the system CH4 + CO2 at 1 atm as a function of temperature. where yCH4, yCO, and yH2 are the dry mole fractions for CH4, CO, and H2, respectively, in the effluent.

Results and Discussion The MALT219 software program was used to compute the equilibrium composition of the system CH4 + CO2 at 1 atm and over the range of temperatures of interest, shown in Figure 2. The equilibrium conversion of methane for CO2 reforming at 1223 K exceeds 96%. Table 1 shows the methane conversion and the H2/CO mole ratio in the effluent gas when bubbling the CH4/ CO2 mixture at a flow rate of 400 Ncm3 min-1 through the molten carbonate salt bath containing the Al2O3supported metal catalyst at 1223 K; the weight ratio of the catalyst to the molten carbonate salt in the mixture (the Cat/MS weight ratio) was 1.0 (20 g of the catalyst and 20 g of the carbonate salt). The CH4/CO2 mixture was also bubbled through the reactor, containing only the molten carbonate salt bath (20 g) without any catalyst under similar conditions, giving only 7% of CH4 conversion. In this case the H2/CO ratio in the effluent gas (the H2/CO production ratio) was 0.23, which is much smaller than the stoichiometric ratio of one. This indicates that a significant amount of H2O was produced. From the material balances of H, C, and O, the overall reaction occurring in the gas phase could be estimated and is given in Table 1. When the catalyst was suspended in the molten salt bath at a Cat/MS weight ratio of 1, the methane conversion increased as shown in Table 1. The order of the activity was Ni > Fe > Cu > W. The H2/CO production ratio for the Ni/Al2O3 catalyst was close to the stoichiometric ratio of one, while the H2/CO production ratios were much smaller for the Fe, Cu, and W/Al2O3 catalysts. The overall gasphase reactions, estimated from the material balance, are listed in Table 1. Therefore, the most promising catalyst among the Al2O3-supported metal catalysts studied here is the Ni/Al2O3. Figure 3 shows the methane conversion and the H2/ CO production ratio as a function of the Cat/MS weight ratio with a fixed overall amount of the mixture (50 g) used in the reactor. A flow rate of the CH4/CO2 feed was fixed at 200 Ncm3 min-1. The methane conversion (19) Yamauchi, S. Netsu Sokutei 1985, 12 (3), 142-144.

Kodama et al.

linearly increased with the Cat/MS ratio. The experiments on the Cat/MS ratio were also done by changing the amount of the Ni/Al2O3 catalyst while keeping the amount (25 g) of the carbonate salt fixed (Figure 4). In this case, the catalytic effect of the Ni/Al2O3 strongly appeared when the Cat/MS weight ratio exceeded 0.8. In both cases, high Cat/MS ratios raised the H2/CO production ratio toward the stoichiometric ratio of 1.0. The experiments on a W/F dependence were carried out, at the Cat/MS weight ratio of 1.0, by changing the amount of the Ni/Al2O3-carbonate mixture in the reactor and the flow rate of the reactant gas, where W and F represent weight of the catalyst-carbonate mixture and flow rate of CH4/CO2 mixture, respectively (W ) 30-50 g and F ) 200-800 Ncm3 min-1). Figure 5 shows the methane conversion and the H2/CO production ratio as a function of W/F ratio. The methane conversion increased linearly with W/F ratio. At low W/F ratio, the H2/CO production ratio was smaller than the stoichiometric ratio of 1.0, but it approached one at higher W/F ratio. At W/F ratio of 0.25 g min Ncm-3, the methane conversion reached about 70%, resulting in a H2/CO production ratio nearly equal to 1.0. If this reaction in the carbonate salt bath proceeds via CO2 reforming of methane followed by reverse water gas shift reaction, it is likely that the H2/CO production ratio decreases with increasing residence time (the W/F ratio). However, the observed H2/CO production ratio increased with an increase in the residence time. This may indicate that the first step of the CO2 reforming of methane in the carbonate salt bath is the formation of CO and H2O:

CH4 + 3CO2 f 4CO + 2H2O

(4)

This step may be followed by steam reforming of eq 1. This could explain the results that the H2/CO production ratio and methane conversion increased when the residence time increased. The activity of the Ni/Al2O3-carbonate mixture with the Cat/MS of 1.0 was tested at W/F ratio of 0.25 g min Ncm-3 (50 g of the Ni/Al2O3-carbonate mixture and the CH4/CO2 flow rate ) 200 Ncm3 min-1) for the simulation of seven-day solar operation (49-h operation in total). The period in which solar high-temperature operation is possible was assumed to be 7 h per day. After the 7-h continuous operation at 1223 K, the CH4/CO2 feeding was stopped and the reactor was cooled to room temperature to freeze the molten salt containing the catalyst. At the beginning of the 7-h operation in the next day, the frozen salt was melted again and heated at 1223 K and the CH4/CO2 feed was introduced into the reactor to commence the methane reforming. Figure 6 shows the changes in the methane conversion and H2/ CO production ratio during this intermittent operation. A methane conversion of 60-98% was retained during the operation. The H2/CO production ratio was close to the stoichiometric ratio of 1.0 during the first 4 days. However, the H2/CO value exceeded the stoichiometric ratio during the next 3 days, indicating that methane decomposition reaction occurred to a significant extent:

CH4 f C + 2H2

(5)

Many researchers suggested that the reaction sequence

CO2 Reforming of Methane

Energy & Fuels, Vol. 15, No. 1, 2001 63

Table 1. Methane Conversions and H2/CO Mole Ratios When Bubbling the CH4/CO2 Mixture in the Molten Carbonate Salt Bath at 1223 Ka catalyst MSc Ni/Al2O3 Fe/Al2O3 Cu/Al2O3 W/Al2O3

conversion(%)b

H2/COb

overall reaction

6.58 29.08 25.30 15.19 10.57

0.23 0.87 0.43 0.29 0.24

CH4 + 2.24CO2 f 3.24CO + 0.76H2 + 1.24H2O CH4 + 1.14CO2 f 2.14CO + 1.86H2 + 0.14H2O CH4 + 1.79CO2 f 2.79CO + 1.21H2 + 0.79H2O CH4 + 2.09CO2 f 3.09CO + 0.91H2 + 1.09H2O CH4 + 2.22CO2 f 3.22CO + 0.78H2 + 1.22H2O

a A CH /CO mixture (CH /CO mole ratio ) 1:1) was fed in the molten carbonate salt bath with/without the Al O -supported metal 4 2 4 2 2 3 catalysts at a flow rate of 400 Ncm3 min-1. The Cat/MS weight ratio was 1.0 (20 g of the catalyst and 20 g of the carbonate salt). b Data c were taken from the values after 4 h of the reaction. The molten carbonate salt bath without any catalysts.

Figure 3. Methane conversion and H2/CO ratio in the effluent (H2/CO production ratio) as a function of the weight ratio of Ni/Al2O3 catalyst and molten carbonate salt in the mixture (Cat/MS weight ratio) used in the reactor, under the fixed amount of the Ni/Al2O3-carbonate salt mixture (50 g). Data were taken from the values after 4 h of the reaction. A flow rate of the CH4/CO2 feed was 200 Ncm3 min-1.

Figure 5. Methane conversion and H2/CO ratio in the effluent (H2/CO production ratio) as a function of W/F, where W and F represent weight of the Ni/Al2O3-carbonate mixture and flow rate of CH4/CO2 feed, respectively. W and F ranged from 30 to 50 g and 200-800 Ncm3 min-1, respectively. The Cat/ MS weight ratio was fixed to be 1.0. Data were taken from the values after 4 h of the reaction.

CH4 f CH3(ad) f CH2(ad) f CH(ad) f C(ad) (6) CO2 f CO + O(ad)

(7)

The CHx intermediate may react with O(ad) to produce CO, and H2 or H2O:

CHx(ad) + O(ad) f CO + H2, H2O

Figure 4. Methane conversion and H2/CO ratio in the effluent (H2/CO production ratio) as a function of the weight ratio of Ni/Al2O3 catalyst and molten carbonate salt in the mixture (Cat/MS weight ratio) used in the reactor, under the fixed amount of the carbonate salt (25 g). Data were taken from the values after 4 h of the reaction. A flow rate of the CH4/CO2 feed was 200 Ncm3 min-1.

for CO2 reforming of methane over metal catalysts involves the activation of methane and the dissociation of carbon dioxide:20-23 (20) Solymosi, F.; Kusta´n, Gy.; Erdo¨helyi, A. Catal. Lett. 1991, 11, 149-156. (21) Alstrup, I.; Tavares, M. T. J. Catal. 1992, 135, 147-155.

(8)

After the first 4 days in Figure 6, the methane activation of eq 6 might be enhanced to facilitate the C(ad) formation because of some structural or morphological changes of the catalyst surface. It is likely that C(ad) is less reactive than the CHx intermediates formed by activation of methane. This could enhance the methane decomposition of eq 5. It is very interesting that the catalyst was not deactivated despite the coke deposition, as shown by Figure 6. After the seven-day operation, many soot particles were found in the reactor. The soot particles existed, separately from the catalyst particles, in the upper side of the frozen molten salt containing the catalyst particles. The soot particles might be removed from the catalyst surface, probably, by the vigorous convection of the molten salt medium and the gas bubbling. That is why the catalyst was not deactivated by the coke deposition. But there is also another (22) Rostrup-Nielsen, J. R.; Bak Hansen, J.-H. J. Catal. 1993, 144, 38-49. (23) Yu, C.-C.; Lu, Y.; Ding, X.-J.; Shen, S.-K. Stud. Surf. Sci. Catal. 1997, 107, 503-510.

64

Energy & Fuels, Vol. 15, No. 1, 2001

Figure 6. Changes in methane conversion and H2/CO ratio in the effluent (H2/CO production ratio) during the intermittent seven-day operation (49-h operation in total). Data were taken from the values after 7 h of the reaction (final point) in the day. Fifty grams of the Ni/Al2O3-carbonate mixture was used at the Cat/MS ) 1.0. The flow rate of CH4/CO2 feed was 200 Ncm3 min-1. The W/F value was 0.25 g min Ncm-3.

possibility that the coke deposition mainly occurred on the surface of the stainless steel wall of the reactor at the upper empty spacing in the reactor. Further investigations are needed to clarify the reaction mechanism in the molten salt system. To apply this molten salt system for a solar methane reforming process in an industrial scale, a large metal vessel for the high-temperature chemical reaction (>1223 K) should be used. There are three main concepts of concentrating solar radiation: the parabolic dish, the parabolic trough, and the central receiver.24-26 For use of a large vessel for chemical reaction, the central receiver system is suitable. For example, the Solar Two plant, located east of Barstow, CA, was the molten-salt power tower for the large demonstration of secondgeneration power tower technology, being comprised of 1926 heliostats (1818 heliostats of 39.1 m2 each plus 108 of 95 m2 each), a receiver (24 panels forming a cylindrical shell, 5.1 m diameter by 6.2 m high), a thermal storage system, and a steam generation system.26 The large receiver of the Solar Two plant (24 panels forming a cylindrical shell, 5.1 m diameter by 6.2 m high) was rated to absorb 42 MW of thermal energy at an average solar energy flux of 430 kW m-2, while accommodating peak fluxes up to 800 kW m-2. Thus, it is technically possible that the large vessel is directly irradiated with the high-energy solar flux in the existing central receiver system. Levitan et al.11 demonstrated CO2 reforming of methane using a 50-cm long Inconel Ushaped tube reactor (1.5 mm thickness). They reported that the reactor wall and the receiver temperatures (24) Tyner, C. E.; Kolb, G. J.; Meinecke, W.; Trieb, F. J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 1999, 9, Pr3-17-Pr322. (25) Becker, M.; Meinecke, W. J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 1999, 9, Pr3-23-Pr3-34. (26) Pacheco, J. E.; Reilly, H. E.; Kolb, G. J.; Tyner, C. E. Solar Thermal 2000, Proceedings of the 10th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 2000, pp 1-11.

Kodama et al.

Figure 7. Temperature distribution of the molten salt containing the catalyst inside the reactor while bubbling with gas. Fifty grams of the Ni/Al2O3-carbonate mixture with the Cat/MS weight ratio of 1.0 was used. CO2 was bubbled through the molten salt at a flow rate of 200 Ncm3 min-1. The temperature of the outside wall of the reactor was heated to 1223 K.

reached 1333 and 1373 K, respectively, by the power flux of 30-120 kW m-2. The reaction temperatures of 953-1303 K could be obtained inside the reactor tube. Therefore, the wall temperature of the metal vessel can be readily heated over 1273 K even by the relatively low power flux of 30-120 kW m-2. The problems may arise from the heat transfer, especially between the metal wall and molten salt medium. If the heat transfer is poor, the metal surface must be at higher temperatures, and even if one uses heat resistance alloys, this is going to be a crucial point. Thus, for our small reactor used in the present work, we measured the temperature of the molten salt inside the reactor at the center positions. Fifty grams of the Ni/Al2O3-carbonate mixture with the Cat/MS weight ratio of 1.0 was placed in the reactor. The static bed height before melting was about 70 mm. The thermocouple was inserted into the reactor at the positions very close to the central stainless steel tube. The reactor was heated with an infrared furnace while bubbling CO2 through the molten salt at a flow rate of 200 Ncm3 min-1. The temperature of the outside wall of the reactor was kept constant at 1223 K. The temperatures were measured at five positions in 0.5-8.5 mm from the bottom with 20 mm spacings. Figure 7 shows the temperature distribution inside the reactor. The temperatures of the molten salt medium at the center of the reactor ranged from 1253 to 1261 K, which were rather slightly higher than the outside wall of the reactor. This result shows that the heat transfer between metal wall and molten salt was good enough by the convection with bubbling in our small-scale metal vessel. However, as the reactor becomes larger, the heat transfer will be poorer. To enhance the heat transfer from metal wall to molten salt, extended internal surface of the metal vessel or forced convection in molten salt may be applied for a large-scale metal vessel

CO2 Reforming of Methane

reactor. For example, to extend the internal surface of the reactor, the inside wall is furnished with rows of fins at certain spacing and height.25,27 The hot fins efficiently transfer the heat to molten salt. Conclusions This paper describes the first attempt to convert methane with CO2 to CO and H2 by bubbling CH4/CO2 (27) Epstein, M.; Segal, A.; Yogev, A. J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 1999, 9, Pr3-95-Pr3-104.

Energy & Fuels, Vol. 15, No. 1, 2001 65

reactant gas through a molten salt bath with a high heat capacity to be used in a solar high-temperature reforming process. The Ni/Al2O3 catalyst was found to be active for the CO2 reforming of methane even in the molten Na2CO3/K2CO3 mixed salt at 1223 K. Methane conversions of 60-98% were obtained during the intermittent seven-day operation at the W/F ratio of 0.25 g min Ncm-3. Further investigations to find more catalysts that are active and stable in a molten salt bath are needed. EF000130T