NiMgO Catalyst Driven by Direct Light Irradiation for Catalytically

for Catalytically-Activated Foam Absorber in a Solar. Reforming ... irradiation of the catalyst by high-flux visible light from a sun-simulator. The 8...
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Energy & Fuels 2003, 17, 914-921

Ni-Mg-O Catalyst Driven by Direct Light Irradiation for Catalytically-Activated Foam Absorber in a Solar Reforming Receiver-Reactor T. Kodama,*,† A. Kiyama,† T. Moriyama,† T. Yokoyama,† K-I. Shimizu,† H. Andou,‡ and N. Satou§ Department of Chemistry & Chemical Engineering Faculty of Engineering, and Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan, and Technical Department and Fine Ceramics Division, Krosakiharima Corporation, 1-1 Higashihamamachi, Yawatanishi-ku, Kitakyushu 806-8586, Japan Received November 19, 2002. Revised Manuscript Received April 16, 2003

A nickel-magnesia solid solution Ni-Mg-O was examined as a catalyst for solar CO2 reforming of methane. The activity was tested in a laboratory-scale transparent (quartz) reactor under direct irradiation of the catalyst by high-flux visible light from a sun-simulator. The 8-11 wt % NiMg-O catalyst gave the high reforming activity or about 100% of chemical conversion, with little coking, under a high-flux irradiation of 890 kW m-2 and at a short residence time of about 0.15 s while passing a 1:1 CH4-CO2 gas mixture at 1 atm. The comparison of the activity data with those obtained in a light-irradiated, nontransparent (steel) reactor showed that the intensification of heat supply by the direct light irradiation of the Ni-Mg-O catalyst leads to considerable reaction rate enhancement. Applying this Ni-Mg-O catalyst, a new type of “catalytically activated” ceramic (alumina) foam absorber was prepared and tested on activity in a laboratoryscale volumetric receiver-reactor using the sun-simulator. This new absorber may be applied in solar receiver-reactor systems for converting concentrated solar high fluxes to chemical fuels via endothermic natural-gas reforming.

Introduction

tion. The overall reaction is written by

Efficient utilization of high-temperature heat from concentrated solar radiation represents a subject that is of current interest.1,2 The conversion of the solar heat to chemical fuels has the advantage of producing energy carriers for storing and transporting solar energy from the sun belt to the remote population centers. Direct thermochemical conversion is desired. Steam or CO2 reforming of methane is a highly endothermic and hightemperature process:

CH4 + H2O(l) f CO + 3H2 ∆H298K ) 250 kJ

(1)

CH4 + CO2 f 2CO + 2H2 ∆H298K ) 247 kJ (2) This makes it a candidate for solar high-temperature thermochemical process. The endothermic reaction upgrades the calorific value of the methane feed, ideally by 28%, using solar thermal energy if the process heat is supplied by high-temperature heat from concentrated solar radiation. The solar-processed syngas can be readily converted to hydrogen via water-gas shift reac* Corresponding author. Fax: +81-25-262-7010. E-mail: tkodama@ eng.niigata-u.ac.jp. † Niigata University. ‡ Technical Department, Krosakiharima Corporation. § Fine Ceramics Division, Krosakiharima Corporation. (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 Solar PACES International Symposium on Solar Thermal Concentrating Technologies 1999, 9: Pr3-17-Pr3-22.

CH4 + 2H2O(l) f CO2 + 4H2 ∆H298K ) 253 kJ (3) The solar energy contribution to the product hydrogen can be ideally 22%. 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. Several types of solar-specific reformers have been developed and demonstrated in largescale solar tests, such as tubular reformer with integrated heat exchanger,3 directly irradiated metal tubular reactor4 and volumetric receiver-reactor.5-11 Especially, the volumetric receiver-reactor system is the very unique and effective one, in which the receiver and (3) Bo¨hmer, M.; Langnickel, U.; Sanchez, M. Sol. Energy Mater. 1991, 24, 441-448. (4) Epstein, M.; Spiewak, I.; Segal, A.; Levy, I.; Lieberman, D.; Meri, M.; Lerner, V. Proceedings of the 8th International Symposium on Solar Thermal Concentrating Technology (Cologne, Germany, 1996); Becker, M., Bo¨hmer, M., Eds.; C. F. Muller: Heidelberg, 1997; Vol. 3, pp 12091229. (5) Buck, R.; Muir, J. F.; Hogan, R. E. Sol. Energy Mater. 1991, 24, 449-463. (6) Muir, J. F.; Hogan, R. E., Jr.; Skocypec, R. D.; Buck, R. Sol. Energy 1994, 52 (6), 467-477. (7) Skoypec, R. D.; Hogan, R. E., Jr.; Muir, J. F. Sol. Energy 1994, 52 (6), 479-490. (8) Buck, R.; Abele, M.; Bauer, H.; Seitz, A.; Tamme, R. ASME: J. Sol. Energy Eng. 1994, 116, 73-78. (9) Abele, M.; Bauer, H.; Buck, R.; Tamme, R.; Wo¨rner, A. ASME: J. Sol. Energy Eng. 1996, 118, 339-346. (10) Wo¨rner, A.; Tamme, R. Catal. Today 1998, 46, 165-174.

10.1021/ef020270y CCC: $25.00 © 2003 American Chemical Society Published on Web 06/06/2003

Ni-Mg-O Catalyst Driven by Direct Light Irradiation

reformer are the same unit.5-11 The concentrated solar radiation passes through a transparent quartz window and is absorbed by an absorber of catalytically activated ceramic foams which is mounted behind the window. Reticulated ceramic foams, combining high gas permeability and turbulence of flow with a geometry suitable for effective and uniform absorption of solar radiation, are preferably used instead of conventional honeycomb structures. The solar volumetric receiver-reactors with good performances have been successfully demonstrated in large-scale solar tests, which could absorb solar power approaching 300 kW.8-10 Chemical storage efficiencies up to 66% and chemical methane conversion up to 88% were observed in the large scale demonstration.9,10 The developments of the different types of solarspecific reformers have aimed to increase the capability for converting concentrated solar thermal energy. Therefore, there have been many efforts to operate the solar reformer under a higher solar flux level and, hence, at a higher temperature because high solar fluxes are available in the sun belt. In the regions of the so-called sun-belt in which the maximum insolation reaches about 1 kW m-2, solar volumetric reactors can be operated under irradiation of the absorber by high-level solar fluxes (e.g., the average flux densities more than 300 kW m-2 on the solar absorber surface8,9) and, hence, at high temperatures (e.g., the average absorber temperature > 1130 K and a local maximum temperature > 1250 K10). For the volumetric receiver-reactors, Rh catalysts, supported on γ-Al2O3, were usually used for coating the absorber matrix ceramic material (alumina or silicon carbide) in order to catalytically activate the solar absorber.8-10 However, the price of Rh increased sharply twelve years ago and its availability then became scarce. It was therefore necessary to develop alternatives to the Rh catalyst for use in the solar reforming process. In addition, at temperatures higher than 1273 K and in the presence of water, the specific area of the γ form of alumina is strongly decreased. This phenomenon is associated with the transformation of γ-alumina into the R phase. Therefore, Berman and Epstein12 examined the promoted Ru-Ce catalyst to improve the activity and thermal stability of the Ru/Al2O3 catalyst for solar CO2 reforming of methane. The Ni-based catalyst is usually used for the industrial reforming processes, which is the best one for the industrial use from an economical point of view in comparison to active noble metal catalysts. The main problem of the Ni-based catalyst is, however, that the deactivation occurs more rapidly as a result of the deposition of carbon on an Ni catalyst surface in comparison to noble metal catalysts such as Rh- and Ru-based ones. However the recent catalyst developments have shown that the activity and stability of the Ni-based catalyst may be improved by rare-earth-based promoters or alkaline-earth metal ions.13-22 Especially, (11) Mo¨ller, S.; Buck, R.; Tamme, R.; Epstein, M.; Liebermann, D.; Meri, M.; Fisher, U.; Rotstein, A.; Sugarmen, C. Proceedings of 11th Solar PACES International Symposium; Steinfeld, A., Ed.; Paul Scherrer Institute: Villgen PSI, 2002; pp 231-237. (12) Berman, A.; Epstein, M. Hydrogen Power: Theoretical and Engineering Solutions; Saetre, T., Ed.; Kluwer Academic Publisher: The Netherlands, 1998; pp 213-218. (13) Gadalla, A.; Sommer, M. J. Am. Ceram. Soc. 1989, 72 (4), 683687. (14) Gadalla, A.; Sommer, M. Chem. Eng. Sci. 1989, 44 (12), 28252829.

Energy & Fuels, Vol. 17, No. 4, 2003 915 Table 1. BET Surface Areas of Some of Catalysts Used

catalyst Ni-Mg-O

Ni/MgO Ni/Al2O3 MgO

metal loading/ wt % nickel 4 8 11 17 17 17

preparation temp/K calcination H 2reduction in air 1273 1273 1273 1273 1023 1023 1273

723 723

BET surface area/ m2 g-1 22.2 16.3 22.3 19.5 33.8 7.5 14.4

Tomishige et al.21 reported that the nickel-magnesia solid solution Ni0.03Mg0.97O has high and stable activity without carbon deposition for a long time (100 days) in the CO2 reforming of methane. These stable and costeffective Ni-based catalysts may be used as a solarreforming catalyst. First, in the present work, a Ni-Mg-O catalyst was examined on the basis of its activity for solar CO2 reforming of methane under direct irradiation of the catalyst by solar-simulated, high-flux visible light. Second, the Ni-Mg-O-activated alumina foam absorber was prepared and tested in a laboratoryscale volumetric receiver-reactor under its direct irradiation by the solar-simulated light. Experimental Section Materials. The chemicals Ni(NO3)2‚6H2O (purity 98%) and RuCl3‚nH2O (n ) 1-3, purity 99.9%) used for the catalyst preparation were purchased from Kanto Chemical Co., Inc., and Wako Pure Chemical Industries, Ltd., respectively. The MgO (Grade 500A, purchased from Ube Material Industries Ltd.) and γ-Al2O3 (CAS registration No. 1344-28-1, purchased from Nippon Aerosil Co. Ltd.) were also used for the catalyst preparation. Distilled water was used for preparation of the solutions. Preparation of Catalysts. Ni-Mg-O catalyst was prepared as follows. The powder of MgO support (3.0 g) was suspended in a nickel nitrate solution (50 cm3). The suspended solution was evaporated to dryness. The dried powder was ground in a mortar, and then it was calcined at 1273-1473 K in air (at the flow rate of 0.3 N dm3 min-1) for 3 h. For comparison, Ni/Al2O3 and Ni/MgO catalysts were also prepared by a conventional impregnation method. The γ-Al2O3 (3.0 g) or MgO (3.0 g) support powder was suspended in the nickel nitrate solution (50 cm3), and then the suspended solution was evaporated to dryness. After grinding in a mortar, it was calcined at 1023 K in air (at the flow rate of 0.3 N dm3 min-1) for 3 h and then reduced by H2 (at the flow rate of 50 N cm3 min-1) at 723 K for 0.5 h. The BET surface areas of some of the used catalysts were determined by nitrogen adsorption (Shimadzu, Micromeritics Flow Sorb II 2300) and are listed in Table 1. Preparation of Ni-Mg-O-Activated Ceramic Foam Absorber. A ceramic or alumina foam disk (supplied by Krosakiharima Co.) was used for preparation of the Ni-Mg(15) Yamazaki, O.; Nozaki, T.; Omata, K.; Fujimoto, K. Chem. Lett. 1992, 1953-1954. (16) Takayasu, O.; Soman, C.; Takegahara, Y.; Matsuura, I. Stud. Surf. Sci. Catal. 1994, 88, 281-288. (17) Choudhary, V.; Uphade, B.; Mamman, A. Catal. Lett. 1995, 32, 387-390. (18) Gronchi, P.; Fumagalli, D.; Del Rosso, R.; Centola, P. J. Thermal Anal. 1996, 47, 227-234. (19) Hariuchi, T.; Sakuma, K.; Fukui, T.; Kubo, Y.; Osaki, T.; Mori, T. Appl. Catal. A: General 1996, 144, 111-120. (20) Hu, Y.-H.; Ruckenstein, E. Catal. Lett. 1997, 43, 71-77. (21) Tomishige, K.; Yamazaki, O.; Chen, Y.; Yokoyama, K.; Li, X.; Fujimoto, K. Catal. Today 1998, 45, 35-39. (22) Choudhary, V.; Mamman, A. Appl. Energy 2000, 66, 161-175.

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Table 2. Typical Catalytically Activated Ceramic Foam Absorbers Used and Their Results of the Methane-Reforming Testsa reforming resultsa loadingc

d

catalyst used for activation

washcoat/supportb

metal wt % Ni or Ru

Tm K

Xe %

ηchemf%

coke depositiong wt %

Ni-Mg-O Ni/Al2O3 Ru/Al2O3

10 wt % MgO 10 wt % γ-Al2O3 10 wt % γ-Al2O3

8 8 26

1323 1299 1323

75.1 81.3 79.3

23.6 25.7 24.9

0.09 0.30 0.12

a The absorber was irradiated by the solar-simulated light with an AFD ) 380 kW m-2 (PFD ) 590 kW m-2) while passing the CH 4 CO2 mixture at a GHSV ) 8500 h-1. The data were taken from the values after 3-h irradiation. b With respect to the mass of the ceramic c d or alumina foam. With respect to the mass of the washcoat or support. Temperature at the center of the irradiated surface of the absorber (maximum absorber temperature). e Methane conversion. f Chemical storage efficiency. g With respect to mass of the ceramic foam.

O-activated ceramic foam absorber. The disk was a 30-mmdiameter and 10-mm-long foam with about 30 cells per linear inch (cpi). The geometric surface area of the foam disk was about 25 cm2/cm3. The ceramic foam disk was first wash-coated with fine MgO. The ceramic foam was soaked into an aqueous slurry of the MgO support (1 g per 100 cm3 of distilled water), dried at room-temperature overnight, and calcined at 1273 K for 1 h in air at the flow rate of 0.3 N dm3 min-1. The density of this solution has to be kept low enough to prevent a clogging of the pores of the foam structure. This magnesia coating process was repeated until the magnesia coating reached up to 10 wt %: the magnesia loadings were calculated from the masses of coated and uncoated foams. Afterward, an Ni2+ was applied at 8-11 wt % nickel with respect to the mass of magnesia support. An Ni(NO3)2 ethanol solution (about 0.3 g per 10 cm3 of ethanol) was added dropwise to the magnesiacoated ceramic foam disk, allowed to dry overnight at room temperature, and calcined at 1273 K in air at the flow rate of 0.3 N dm3 min-1 for 3 h. For comparison, the ceramic foam absorbers activated with Ru/Al2O3 and Ni/Al2O3 catalysts were also prepared using the same alumina foam disk. The ceramic foam disk was first wash-coated with γ-Al2O3 slurry solution (2 g per 100 cm3 of distilled water) and then calcined at 1273 K for 1 h in air at the flow rate of 0.3 N dm3 min-1. The alumina coating process was repeated until the alumina loading reached up to 10 wt %. Afterward, Ni(NO3)2- or RuCl3-ethanol solution (about 0.1-0.3 g per 10 cm3 of ethanol) was added dropwise to the alumina-coated foam disk, allowed to dry overnight at room temperature, calcined at 1023 K (or 823 K) in air at the flow rate of 0.3 N dm3 min-1 for 3 h, and then reduced with a 10% H2-90% N2 gas mixture (at the flow rate of 50 N cm3 min-1) at 723 K (or 873 K) for 1 h. Typical absorbers of the Ni-Mg-O-, Ru/Al2O3-, and Ni/Al2O3-activated ceramic absorbers are listed in Table 2. Activity Tests of Catalysts. The experimental setup is illustrated in Figure 1a. The catalyst (0.1-0.35 g) was packed in the reactor of a transparent quartz tube with an inner diameter of about 6 mm. The length of the catalyst bed was set to 10 mm. A CH4-CO2 gas mixture (pCH4 ) 50% and pCO2 ) 50%) was fed to the reactor at a flow rate of 40-160 N cm3 min-1 and at 1 atm, and then the catalyst in the reactor was directly irradiated by solar-simulated, high-flux visible light in order to carry out the methane reforming. A so-called sunsimulator consisting basically of a Xe-arc lamp placed in the focus of a parabolic mirror (Ushio U-Tech, 3 kW XEBEX HIBEAM IIIR, Tokyo, Japan, or CINEMECCANICA, 5 kW ZX8000H, Milano, Italy) was used to simulate concentrated solar or visible light. The circular diameter of the focal area was fitted to the catalyst-bed length (10 mm) to irradiate the bed throughout. The flux density (FD) of the incident solarsimulated light at the position of the catalyst bed was previously measured by a heat flux transducer with sapphire window attachment (Medherm, 64-100-20/SW-1C-150). The circular diameter of the sapphire window of the heat transducer was about 10 mm. The temperature of the catalyst bed was measured using a K-type thermocouple placed at the center of the catalyst bed in contact with it packed inside the

reactor. For comparison, instead of the transparent quartz tube, a nontransparent stainless steel tube reactor with the same inner diameter was used for the catalyst reforming tests. The external surface of the steel tube was coated with a black paint with a high-temperature resistance to absorb more than 95% of the incident solar-simulated light. 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 (Simadzu, GC-4C) with a TCD detector to determine the gas composition. Activity Tests of Ni-Mg-O-Activated Foam Absorbers. A double-walled quartz reactor was used for the catalytic activity tests of the absorbers. The experimental setup is illustrated in Figure 1b. The inner diameter of the outer quartz tube was 39 mm. The inner quartz tube had an inner diameter of 31 mm. The thickness of the quartz tubes was about 2 mm. The catalytically activated absorber disk was placed on a porous quartz plate in the inner tube to be fixed. The reactor was insulated with refractory bricks. The reaction feed gas was fed to the outer annulus and allowed to flow through the foam absorber disk. A 50% CH4-50% CO2 gas mixture was fed to the reactor at a flow rate of 1.0-2.0 N dm3 min-1. The GHSV was 8500-17000 h-1. Then, the absorber was irradiated with the solar-simulated Xe light from the sun-simulator. The fluxdensity (FD) distribution of the incident solar-simulated light was previously measured using a heat flux transducer placed at the positions of the irradiated surface of the absorber. The maximum absorber temperature (Tm) under the irradiation was measured using a K-type thermocouple placed at the center of the irradiated surface of the absorber. The dried effluent gases were analyzed by gas chromatography equipment. Calculation of Efficiencies. The methane conversion (X) was estimated by the following equation:

X)

yCO + yH2 4yCH4 + yCO + yH2

(4)

where yCH4, yCO, and yH2 are the dry mole fractions for CH4, CO, and H2, respectively, in the effluent. The light-to-chemical energy conversion or chemical storage efficiency (ηchem) was estimated as follows. First, the overall reaction occurring in the gas phase was determined from the experimental data. CO2 reforming of methane of eq 2 is frequently associated with reverse water-gas shift reaction:

H2 + CO2 f CO + H2O(l) ∆H298K ) -3 kJ

(5)

and the H2/CO ratio in the product gas becomes lower than the stoichiometric ratio of one. In this case, the overall reaction in the gas phase is written by

CH4 + (1 + x)CO2 f (2 + x)CO + (2 - x)H2+ xH2O(l) ∆H298K(overall) ) 247 - 3x kJ

(6) (7)

where x indicates the contribution of the reverse water-gas

Ni-Mg-O Catalyst Driven by Direct Light Irradiation

Energy & Fuels, Vol. 17, No. 4, 2003 917

Figure 1. Schematic of experimental setups for methane-reforming under direct light irradiation of (a) the powder catalyst, and (b) the catalytically activated foam absorber. shift reaction. The x value was determined from the experimental H2/CO ratio in the product gas. Thus, the energy stored as chemical enthalpy by the overall reaction (Wchem) can be experimentally estimated by

Wchem ) FCH4‚ X ‚∆H298K(overall)

(8)

where FCH4 represents the molar flow rate of CH4 to the reactor inlet. The chemical storage efficiency, ηchem, of the incident light to chemical enthalpy is defined by

ηchem ) Wchem/Winc

(9)

where Winc is the total energy of the incident light onto the absorber.

Figure 2. XRD pattern of the Ni-Mg-O catalyst (17 wt % nickel loading, the calcination temperature in preparation ) 1273 K).

Results and Discussion

solution.23 This Ni-Mg-O catalyst was compared with the Ni/Al2O3 (17 wt % Ni), Ni/MgO (17 wt % Ni), and pure MgO-support, on activity and coking for the CO2 reforming performed under direct irradiation of the catalyst by the solar-simulated, high-flux light (FD ) 890 kW m-2). Compared with these normal metallic Ni

Activity of Ni-Mg-O under Direct Light Irradiation. Figure 2 shows the XRD pattern of the initial Ni-Mg-O (17 wt % Ni) catalyst. The XRD peaks due to metallic Ni were not initially observed. The hightemperature calcination of 1273 K in the preparation makes it possible that the nickel ions were incorporated in the lattice of MgO support to form a NiO-MgO solid

(23) Parmaliana, A.; Arena, F.; Frusteri, F.; Giordano, N. J. Chem. Soc., Faraday Trans. 1990, 86 (14), 2663-2669.

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Figure 3. Methane conversion (X) and coke deposition for the CO2 reforming performed under direct light irradiation of the powder catalysts of Ni/Al2O3, Ni/MgO, and Ni-Mg-O and the pure MgO support. The nickel loadings were fixed to be 17 wt % nickel. A 0.1-0.35 g sample of the catalyst was irradiated by the solar-simulated light with a FD of 890 kW m-2 while passing the CH4-CO2 mixture at a GHSV of 24000 h-1. The data were taken from the values after 4 h of the irradiation.

Kodama et al.

Figure 5. Effect of the calcination temperature of Ni-Mg-O catalyst, during preparation, on methane conversion (X) and coke deposition for the CO2 reforming under direct light irradiation. The nickel loadings were fixed to be 17 wt % nickel. A 0.1 g sample of the catalyst was irradiated by the solarsimulated light with a FD of 780 kW m-2 while passing the CH4-CO2 mixture at a GHSV of 16000 h-1. The data were taken from the value after 4-h irradiation.

Figure 6. Effect of nickel loading of the Ni-Mg-O catalyst on methane conversion (X) and coke deposition for the CO2 reforming under direct light irradiation. A 0.1 g sample of the catalyst was irradiated by the solar-simulated light with a FD of 890 kW m-2 while passing the CH4-CO2 mixture at a GHSV of 16000 h-1. The data were taken from the values after 4-h irradiation. Figure 4. XRD pattern of (a) Ni/MgO and (b) Ni-Mg-O catalyst after 4 h reforming under direct light irradiation. The nickel loadings were fixed to be 17 wt % nickel. A 0.1 g sample of the catalyst was irradiated by the solar-simulated light with a FD of 890 kW m-2 while passing the CH4-CO2 mixture at a GHSV of 24000 h-1.

catalysts, supported by alumina or magnesia, the NiMg-O catalyst showed a slightly greater activity, and quite a smaller coking at a high GHSV value of 25000 h-1 (Figure 3). Figure 4 shows the comparison of the XRD patterns of the normal MgO-supported Ni-metal catalyst (Ni/MgO) and the Ni-Mg-O catalyst after 4-h reforming. The reflection peaks due to metallic Ni appeared in the XRD pattern of the used Ni-Mg-O catalyst, but they were much smaller than those observed in the used Ni/MgO with the same nickel loading. This indicates that crystal growth of metallic Ni particles was suppressed in the Ni-Mg-O catalyst. Many investigators indicated that the clustering of metallic Ni, which stimulates carbon formation from methane, is inhabited or suppressed on the reduction of NiOMgO solid solution.20,22

Effects of the calcination temperature of Ni-Mg-O catalyst on activity and coking were studied (Figure 5). Increased calcination temperature up to 1473 K improved the methane conversion by 9% (from 87 to 96%) but the coke deposition also increased 2-fold. Thus, we chose the calcination temperature of 1273 K in the present work. Figure 6 shows the nickel loading effects of the Ni-Mg-O catalyst on the activity and coking. The greatest activity as well as the smallest coking was observed in the range of 8-11 wt % nickel for magnesia support. In the XRD patterns of the Ni-Mg-O catalysts after use of the 4-h reforming, the (111) reflection (main) peak due to metallic Ni was hardly observed at the nickel loading values smaller than 11 wt % (Figure 7), indicating effective suppression of the clustering of metallic Ni on the Ni-Mg-O catalyst. Hence this nickel-loading range (8-11 wt %) was used for experiments hereafter. Here we compared the methane-reforming performances by the Ni-Mg-O catalyst in the transparent (quartz) reactor and in the nontransparent (blackcolored steel) reactor of the same size under the same reaction condition. In an ordinary nontransparent reac-

Ni-Mg-O Catalyst Driven by Direct Light Irradiation

Figure 7. Change in the (111) XRD-reflection peak due to metallic Ni on the Ni-Mg-O catalyst after use of 4-h reforming under direct light irradiation. A 0.1 g sample of the catalyst was irradiated by the solar-simulated light with a FD of 890 kW m-2 while passing the CH4-CO2 mixture at a GHSV of 24000 h-1. The nickel loadings were (a) 17 wt %, (b) 8 wt %, and (c) 4 wt %.

Energy & Fuels, Vol. 17, No. 4, 2003 919

Figure 9. Flux-density distribution of the incident, solarsimulated light on the irradiated surface of the absorber. The AFD was 380 kW m-2 (PFD ) 590 kW m-2) and the Winc was 270 W.

Figure 10. Time variation of methane conversion (X) with the Ni-Mg-O-activated absorber for CO2 reforming under direct light irradiation. The absorber was irradiated by the solar-simulated light with an AFD of 380 kW m-2 (PFD ) 590 kW m-2) while passing the CH4-CO2 mixture at a GHSV of 8500 h-1. The MgO loading for mass of the foam was 10 wt %, and the nickel loading for mass of the MgO support was 8 wt %. Figure 8. Comparison of the Ni-Mg-O catalyst-bed temperature and methane conversion (X) for the CO2 reforming between using (2) a nontransparent (steel) reactor and (0) a transparent (quartz) reactor, under light irradiation. The nickel loadings were fixed to be 8 wt % nickel. A 0.1 g sample of each of the catalysts was used. The data were taken from the value after 4-h irradiation with FD of 890 kW m-2.

tor, the light energy is first absorbed by a steel wall and then transferred to catalyst bed inside the reactor. The sufficient thermal resistance between the wall and the catalyst bed, together with the low thermal conductivity of the catalyst, leads to a limitation of the heat flux. Many investigators previously indicated that the direct heating of catalyst with high-flux light in the transparent reactor could minimize the limitation.5,24,25 Experimental data shown in Figure 8 clearly showed that the intensification of heat supply by the direct heating with high-flux light effectively enhances the reforming rate on the Ni-Mg-O catalyst. Under the high-flux irradiation (FD ) 890 kW m-2) of the steel reactor, the NiMg-O catalyst inside the reactor was heated to 1124 K at the GHSV value of 8000 h-1, yielding about 80% (24) Tanashev, Y. Y.; Fedoseev, V. I.; Aristov, Y. I. Catal. Today 1997, 39, 251-260. (25) Aristov, Y. I.; Fedoseev, V. I.; Parmon, V. N. Int. J. Hydrogen Energy 1997, 22 (9), 869-874.

of methane conversion. However, the catalyst temperature in the steel reactor decreased below 1073 K with increased GHSV up to 16000 h-1 and the methane conversion rapidly decreased to zero. On the other hand, the Ni-Mg-O catalyst in the transparent reactor could have the catalyst temperature above 1273 K and yielded about 100% of methane conversion at the GHSV of 16000 h-1. The temperature of the directly irradiated catalyst remained around 1250 K at a GHSV value up to 24000 h-1 or at a residence time down to 0.15 s, yielding about 100% of methane conversion. Activity of Ni-Mg-O-Activated Ceramic Foam Absorber. Figure 9 shows a typical flux-density distribution of incident light for irradiation of the absorber. Here, the central peak flux density (PFD) of irradiation was 590 kW m-2 and the average flux densities (AFD) on the irradiated surface of the absorber was 380 kW m-2. The total energy of the incident light onto the absorber (Winc) was 270 W. In this work, the AFD value was changed from 180 to 380 kW m-2 while the PFD varied from 270 to 590 kW m-2. The Winc varied from 130 to 270 W. Figure 10 shows a typical time variation of the methane conversion when irradiating the Ni-Mg-Oactivated absorber by high-flux visible light, in which the stable activity was observed during irradiation.

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Figure 11. AFD dependences of methane conversion (X), chemical storage efficiency (ηchem), and maximum absorber temperature (Tm) for CO2 reforming using the Ni-Mg-Oactivated absorber under direct light irradiation. The GHSV was fixed to be 8500 h-1. The MgO loading for mass of the foam was 10 wt % and the nickel loadings for mass of the MgO support were in 8-11 wt %. The data were taken from the values after 3-h irradiation.

Figure 12. GHSV dependences of methane conversion (X), chemical storage efficiency (ηchem), and maximum absorber temperature (Tm) for CO2 reforming using the Ni-Mg-Oactivated absorber under direct light irradiation. The absorber was irradiated by the solar-simulated light with an AFD of 310 kW m-2 (PFD ) 470 kW m-2) while passing the CH4CO2 mixture. The MgO loading for mass of the foam was 10 wt %, and the nickel loading for mass of the MgO support was 11 wt %. The data were taken from the values after 3-h irradiation.

Table 2 shows the comparison of the methane conversion and the coking (after 3-h reaction) by the Ni-MgO-activated absorber with those by the Ni/Al2O3- and the Ru/Al2O3-activated ceramic absorbers. (The Ni loading was set to 8 wt % Ni for the mass of support material, while the Ru loading was 26 wt % Ru for γ-alumina support. The magnesia or alumina support loadings were fixed to 10 wt % for the mass of the foam.) The activity levels of these absorbers were almost the same, but our new Ni-Mg-O-activated ceramic absorber most suppressed the coking at a low level of less than 0.09 wt % carbon with respect to the mass of the foam. As shown in Figure 11, the methane conversion with the Ni-Mg-O-activated absorber (8-11 wt % Ni) was effectively increased from 40 to 80% with increased flux density of irradiation. The coking ranged in only 0.020.09 wt % carbon for mass of the foam. The chemical storage efficiencies (ηchem) were also plotted against the incident flux densities of irradiation in Figure 11. The similar ηchem levels of 23-32% were observed for various average flux densities of irradiation. The maximum absorber temperature (Tm) significantly increased from 1100 to 1400 K with increasing flux density (Figure 11). In the small-scale solar reformer used here, the fraction of the heat losses due to conduction and convection would be very large in comparison to that in a largescale reformer. At a higher temperature of the absorber, the effect of the heat losses on the energy efficiency effectively becomes larger in the small reformer. This is the reason the chemical storage efficiency did not increase with an increase of flux density of irradiation although the chemical methane conversion was effectively increased. The GHSV variations of methane conversion and chemical storage efficiency as well as the maximum absorber temperature were shown in Figure 12 (The Ni loading of 11 wt % Ni for the mass of magnesia was used

for absorbers here). The maximum absorber temperature decreased from 1330 to 1170 K with increasing GHSV from 8500 up to 17000 h-1. The methane conversion decreased from 80 to 50% with the increasing GHSV, but the chemical storage efficiency was rather improved from 30 to 40%. This would be because, with increasing GHSV, the heat losses due to natural convection, conduction, and reradiation were reduced due to the lower absorber temperatures. In the previous literature in which volumetric receiverreactors were demonstrated, the maximum chemical storage efficiencies of 54-66% were reported by using the ceramic foam absorbers activated with Rh/γ-Al2O3 catalyst.5-10 The energy performances of these previous ceramic absorbers were, however, demonstrated in much larger reactor scales and higher input powers of solar energy than those used in the present work. Thus, it is not fair to directly compare their energy performances with ours by the new Ni-Mg-O-activated absorber. The relatively poor energy performances were observed on our Ni-Mg-O-activated absorber, but this is due in large part to the large heat-loss fraction of the small reformer used, as discussed above. The large heatloss fraction will be circumvented in a large-scale solar reformer, resulting in the increased chemical storage efficiency. Tanashev et al.24 and Aristov et al.25 studied the solar steam reforming of methane under direct irradiation of a commercial Ru-based catalyst by concentrated Xe-arclamp or visible light in the experimental scale (the reactor scale and input power levels) similar to the one we used here. They reported the chemical storage efficiencies of 36-74% and methane conversions of 2283% for the average flux densities (AFD) of irradiation from 130 to 670 kW m-2 using a commercial Ru-based catalyst bed with a 20-mm diameter and a 4-20 mm thickness. We compared here their values of the chemi-

Ni-Mg-O Catalyst Driven by Direct Light Irradiation

cal storage rate (r) per 1 m2 of the irradiated surface of the catalyst bed, which is defined by r ) FCH4‚X‚∆H298K(reaction)/S where the ∆H298K(reaction) indicates an enthalpy change of the reaction occurring per mole of CH4 and S is the light-irradiated surface area of the catalyst bed. Their maximum value obtained at the AFD level of around 300 kW m-2 for irradiation was about 140 kW m-2. Our Ni-Mg-O-activated absorber could reveal a similar maximum r value of 130 kW m-2 at the same AFD level. The greater absorber activity may be realized by using promoters of cheaper noble metals than conventionally used metal of Rh, such as Ru and Pt, on this cost-effective Ni-Mg-O-activated foam absorber.

Energy & Fuels, Vol. 17, No. 4, 2003 921

Conclusion For absorbing and chemically converting concentrated solar high-flux radiation into fuels, the Ni-Mg-Oactivated ceramic foam disk was found to be the promising absorber for use in the solar reforming receiver-reactors. The low level of coking on the Ni-Mg-O-activated absorber was superior to that of the Ru/Al2O3-activated absorber, which would promise the relatively long-life activity of the absorber. This costeffective, Ni-based ceramic foam may be applied as a basic matrix of a cheaper-noble-metal-activated absorber that has a greater/comparable activity than/to the conventional rhodium-activated absorber. EF020270Y