Energy & Fuels 2006, 20, 455-462
455
A New Catalyst System for High-Temperature Solar Reforming of Methane Alexander Berman,* Rakesh K. Karn, and Michael Epstein Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, RehoVot 76100, Israel ReceiVed July 5, 2005. ReVised Manuscript ReceiVed NoVember 9, 2005
A new type of solar-heated reformer, called the volumetric reformer, has been recently developed. In this reformer, the concentrated solar radiation directly illuminates the catalyst through a transparent window. This new type of solar reformer can operate at a temperature range of 1273-1373 K, higher than that common in industrial reformers. Different configurations of the catalyst substrate in these volumetric reformers have been reported in the literature. This article describes a catalytic system specifically applied in the directly irradiated annular pressurized receiver (DIAPR), which was developed at the Weizmann Institute of Science for operation at high temperatures and pressures. The catalytic system based on the DIAPR concept was constructed on an array of ceramic pins loaded with catalyst. Cost-effective catalytic elements that will be implemented in this type of volumetric reformer were developed in the present work. A Ru/Al2O3 catalyst promoted with Mn oxides was supported on ceramic pins (made of alumina) by wash coating. The catalyst was characterized by SEM, XRD, and BET. The fresh catalyst contains mainly R-alumina and Mn2O3. Al2O3, Mn3O4, and MnAl2O4 were observed in the XRD pattern after reaction in the temperature range of 773-1473 K. Activity tests were conducted in a tube flow reactor made of sintered alumina (99.7% Al2O3). The results show that the activity of the new catalyst is similar to the activity of the commercial Engelhard 1%Ru/γ-Al2O3 catalyst. However, while the activity of the commercial catalyst decreases drastically when approaching to 1273 K because of phase transformation in the support, the new catalyst is stable even after calcinations at 1373 K for 500 h under argon flow.
CH4 ) C + 2H2
1. Introduction The sun is an enormous source of energy; in one year, the amount of solar energy incident on the earth is about 15 000 times the world’s energy use. However, because the solar energy intensity is low, the challenge is to find an efficient way of converting the solar radiation into an energy form that one can readily utilize. One promising concept is using the concentrated solar energy to upgrade fuels such as methane into an energyrich synthesis gas (a mixture of hydrogen and CO called syngas), which has been commonly used in industry, for example, for hydrogen production. Syngas can be burned in a conventional gas turbine to produce electricity. It can also be employed as a starting material for a variety of chemical products, from ammonia and its fertilizer derivations to methanol and different types of alcohols, acids, and other chemicals. The solar process for making syngas (solar reforming) is a catalytic reaction between methane and steam (or carbon dioxide) which takes place in the solar reactor. Steam reforming of methane is a highly endothermic reaction accompanied mainly by the side reaction of the water-gas shift (WGS), which is slightly exothermic:
CH4 + H2O ) CO + 3H2 ∆H ) 206 kJ/mol
(1)
CO + H2O ) CO2 + H2 ∆H ) -36 kJ/mol
(2)
One of the less favorable possible side reactions is the cracking of methane, resulting in carbon deposition on the catalyst: * Corresponding author. Phone: +972-8-934-3763. Fax: +972-8-9344117. E-mail:
[email protected].
(3)
Excess steam is usually used to reduce the tendency for carbon formation via this reaction. Numerous studies have focused on developing solar-powered reactors with direct solar irradiation of the catalyst. Levy et al.1 operated a solar volumetric reactor with Rh catalyst supported on alumina honeycomb in a solar furnace. The maximum working temperature was 1173 K. Worner and Tamme2 described a solar-powered reactor also using Rh catalyst supported on ceramic foam made of alumina and SiC. The reactor was operated at temperatures of 973-1133 K with an absolute pressure of 3.5 atm. More recently, a volumetric solar reformer, using a directly irradiated catalytic ceramic foam, was designed to operate at 1098 K and 9 atm with a solar power input of 400 kW.3 A large-scale volumetric receiver-reformer placed on a parabolic solar concentrator named the catalytically enhanced solar absorption receiver (CAESAR) was successfully tested by Muir et al.4 for reforming methane with CO2. The reactor’s catalytic absorber was built of multilayered alumina foam coated with Rh catalyst. The maximum methane conversion was 70%. Problems with cracking and degradation of the (1) Levy, M.; Levitan, R.; Rosin, H.; Rubin R. Sol. Energy 1993, 50, 179-189. (2) Worner, A.; Tamme, R. Catal. Today 1998, 46, 165-174. (3) Moeller, S.; Buck, R.; Tamme, R.; Epstein, M.; Libermann, D.; Mery, M.; Fisher, U.; Rotstein, A.; Sugarmen, C. 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. (4) Muir, J F.; Hogan, R. E.; Scocipes, R. D.; Buck R. Sol. Energy 1994, 52, 467-477.
10.1021/ef0501997 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/31/2005
456 Energy & Fuels, Vol. 20, No. 2, 2006
Berman et al.
Figure 1. Schematic view of a DIAPR solar reformer.
contribution, known as solar gain, which can be defined as follows: Solar Gain ) (Heat of Combustion of Products - Heat of Combustion of Feed)/ Heat of Combustion of Feed
Figure 2. Concept of a solar volumetric reformer.
porous matrix and catalyst deactivation due to sintering have been reported. Buck et al.5 and Anikeev et al.6 describe other studies on developing volumetric reformers based on a foam type of absorber which was coated with catalysts. A directly irradiated annular pressurized receiver (DIAPR) with ceramic pins absorber7 was developed recently at the Weizmann Institute of Science for operation at high temperatures and pressures.8 The test results for heating air at various irradiation conditions and flow rates are reported by Kribus et al.9 Exit air temperatures of up to 1473 K and operating pressure of 17-20 bar were obtained. The catalytic solar reformer based on the DIAPR conception is under construction. Figure 1 shows schematically a DIAPR-type reformer.10 An array of ceramic pins loaded with the catalyst (Figure 2) is used for both absorbing the solar radiation, effective heat transfer to the flowing gases, and performing the chemical reaction. Solar reforming can upgrade fossil fuels by 20-30% with respect to their calorific values.2 The upgraded fuel can then be combusted in a gas turbine, thus introducing a solar (5) Buck, R.; Muir, J F.; Hogan, R. E.; Scocipes, R. D. Sol. Energy Mater. Sol. Cells 1991, 24, 449-463. (6) Anikeev, V. I.; Parmon, V. N.; Kirillov, V. A.; Zamaraev, K. I. Int. J. Hydrogen Energy 1990, 15, 275-286. (7) Karni, J.; Kribus, A.; Rubin, R.; Sagie, D.; Doron, P.; Fiterman, A. ASME J. Sol. Energy Eng. 1997, 119, 74-78. (8) Karni, J.; Kribus, A.; Rubin, R.; Doron, P. ASME J. Sol. Energy Eng. 1998, 120, 85-95. (9) Kribus, A.; Doron, P.; Rubin, R.; Reuven, R.; Taragan, E.; Duchan, S.; Karni, J. ASME J. Sol. Energy Eng. 2001, 123, 10-17. (10) Rubin, R.; Karni, J.; Yeheskel, J. ASME J. Sol. Energy Eng. 2004, 126, 858-866.
The equilibrium product compositions of steam reforming of methane were calculated for the temperature range of 7731473 K and pressures of 1, 10, and 20 bar. Steam to methane molar ratio was 2.5. The solar energy gain has been calculated for these conditions. Figure 3 shows the solar energy gain as a function of temperature for different pressures. It can be seen that the operation at temperatures of 1173-1273 K is desirable for obtaining higher chemical conversions and increasing the solar contribution to the reforming process. The high efficiency can be achieved, in principle, using solar volumetric reformers where the solar radiation illuminates the catalyst directly through a transparent window. This type of solar reactors can operate at high temperatures (1273-1373 K) since the limit of the surface temperature of a metal tube in regular industrial reformers is eliminated. The main challenge is the chemical and thermal stability of the catalysts at these high operating temperatures. The activity of the catalysts decreases at high temperatures due to thermal sintering of the metallic particles and phase transformations in the support. For example, γ-alumina with surface area of 100-200 m2/g is used often for catalyst
Figure 3. Solar energy gain as a function of temperature calculated for pressures of 1, 10, and 20 atm.
New Catalyst System for Solar Reforming of Methane
preparation. At temperatures of 1223-1323 K, phase transformation to R-alumina occurs, resulting in a sharp drop of the surface area to 5-20 m2/g and therefore decreasing the activity of the supported catalyst. Another example is industrial Ni/Ralumina catalyst. At high temperatures, the spinel structure NiAl2O4 is formed, causing a decrease of the activity.11 The industrial steam reforming using Ni catalyst is performed around 1023 K with steam to carbon molar ratio in the range of 2-5 to preserve stable catalyst activity. Coke formation is a major problem. It destroys the catalyst structure and spoils its activity. It was found that the rate of carbon deposition can be suppressed considerably by using noble metals,12 which is ascribed to a smaller dissolution of carbon into these metals. Worner and Tamme2 used the Rh catalyst for the mixed reforming of methane with steam and CO2. Rostrup-Nielsen and Hansen studied the steam reforming of methane on Ni and noble metal catalysts supported on MgO.13 The results show the following sequence of activity: Ru, Rh > Ir > Ni, Pt, Pd. The activities of the Rh and Ru catalysts are close to each other, but the cost of Ru is significantly lower. The Ru catalysts are promising candidates for solar reformers. An additional advantage of the Ru catalyst is the possibility to work at a lower steam-to-methane ratio without carbon deposition.14 It was found also that the thermal stability of the Ru/γ-Al2O3 catalysts can be improved by adding Ce oxide.15 Rare earth-based promoters and alkaline earth metal ions have been described in the literature with respect to this objective.16,17 The Ru (or Ni, Pt, Rh) catalysts supported on monolithic porous support containing alumina and Mn oxide were recently developed.18,19 These catalysts are durable and effective for steam reforming of lower hydrocarbons. The use of catalysts containing alkali metals, alkali earth metals, and rare earth metals on the monolithic carrier was also studied.18 Promoters such as La2O3, MnOx, MgO, and others can be added to R-Al2O3 (as in the case of Ni catalyst) to enhance the activity of the Ru catalyst, to suppress carbon deposition, and to improve the stability at high temperature through diminishing of the growth of the metallic particles and the oxidation of the active component.20 In the present article, a cost-effective catalytic system for the DIAPR reformer was developed. The Ru/Al2O3 catalyst promoted with Mn oxides was supported on a ceramic pin by wash coating. The catalyst was characterized by XRD, SEM, and BET. Results of the stability test are presented. (11) Gadalla, A. M.; Bower, B. Chem. Eng. Sci. 1988, 43, 3049-3062. (12) Lobo, L. S.; Trimm, D. L.; Figueiredo, J. L. In Catalysis: Proceedings of the Fifth International Congress on Catalysis, International Congress on Catalysis, Miami Beach, FL, Aug 20-26, 1972; Hightower, J. W., Ed.; North-Holland/American Elsevier: Amsterdam/New York, 1973; Vol. 2, p 1125. (13) Rostrup-Nielsen, J. R.; Hansen, J. H. J. Catal. 1993, 144, 38-49. (14) Berman, A.; Levitan, R.; Epstein, M.; Levy, M. In Solar Engineering 1996, Proceedings of the 1996 International Solar Energy Conference, San Antonio, TX, March 31-April 3, 1996; American Society of Mechanical Engineers: New York, 1996; p 61. (15) Berman, A.; Epstein, M. In Hydrogen Power: Theoretical and Engineering Solutions, Hypothesis II, Grimstad, Norway, Aug 18-22, 1997; Kluwer Academic: Dordrecht, The Netherlands, 1998; p 11. (16) Sauvion, G. N.; Ducros, P. J. Less-Common Met. 1985, 111, 2335. (17) Machida, M.; Eguchi, K.; Arai, H. J. Catal. 1987, 103, 385-393. (18) Fukunaga, T.; Yanagino, T.; Takatzu, K.; Umeki, T. (Idemitsu Kosan Co., Ltd., Japan). Patent WO 2002078840, 2002. (19) Fukunaga, T.; Takazu, K.; Yanagino, T. (Idemitsu Kosan Co., Ltd., Japan). Jpn. Kokai Tokkyo Koho Patent JP 2003265963, 2003. (20) Berman, A.; Karn, R. K.; Epstein, M. Appl. Catal., A 2005, 282, 73-83.
Energy & Fuels, Vol. 20, No. 2, 2006 457
2. Experimental Section 2.1. Preparation of the Catalytic Elements. In the experiments, pins made of sintered alumina (99.7% Al2O3) were used. Standard sintered alumina (produced for kiln construction) fired at 2073 K has good mechanical properties. The disadvantage of the standard material for catalytic applications is its very low porosity of about 0.2%. The samples made of the porous alumina pins were supplied by Haldenwanger Company. The samples were fabricated by lowering the firing temperature. The samples were cylindrical rods (OD ) 3.0 mm, length ) 40 mm or 70 mm). Water adsorption was 11-12.5%, surface area of 1-2 m2/g, and pore size was about 0.25 µm. In some experiments, pins made of Pythagoras were used. The Pythagoras material (Al2O3*SiO2) was produced by Haldenwanger, type C610 with Al2O3 content of approximately 60% and alkali content of 3%. The physical properties were: maximum working temperature of 1673 K, flexural strength at 293 K was 120 MPa, water adsorption was
