Rotary-Type Solar Reactor for Solar Hydrogen Production with Two

May 25, 2007 - A single solar energy converter has been developed by Roeb et al.23 using metal oxide redox systems of mixed iron oxides coated upon mu...
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Energy & Fuels 2007, 21, 2287-2293

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Rotary-Type Solar Reactor for Solar Hydrogen Production with Two-step Water Splitting Process Hiroshi Kaneko, Takao Miura, Akinori Fuse, Hideyuki Ishihara, Shunpei Taku, Hiroaki Fukuzumi, Yuuki Naganuma, and Yutaka Tamaura* Research Center for Carbon Recycling and Energy, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo, 152-8552, Japan ReceiVed NoVember 17, 2006. ReVised Manuscript ReceiVed February 5, 2007

The rotary-type solar reactor has been developed and fabricated for solar hydrogen production by a twostep water-splitting process using the reactive ceramics of CeO2 and Ni,Mn-ferrite (Ni0.5Mn0.5Fe2O4). It has a cylindrical rotor and dual cells for discharging O2 and for the H2O splitting reaction. A detailed specification and the efficiency of the rotary-type solar reactor were examined for the two-step water-splitting process. The maximum temperature of the reactive ceramics mounted on the cylindrical rotor was ca. 1623 K by irradiation with a solar simulator of an infrared imaging lamp. Repetition of the two-step water-splitting process using the rotary-type solar reactor with CeO2 was achieved, and successive evolution of H2 was observed in the H2O-splitting reaction cell at the optimum reaction temperatures of the O2-releasing reaction cell (T ) 1623 K) and H2O-splitting reaction cell (T ) 1273 K). Also, repetition of the two-step water-splitting process was achieved in the case of using the reactive ceramics of Ni,Mn-ferrite, and its optimum reaction temperatures of the O2-releasing and H2-generation reactions were 1473 and 1173 K, respectively. It was confirmed that the higher O2-releasing reaction temperature of above 1800 K was achieved with the about 10-times scaled-up rotary-type solar reactor.

1. Introduction Hydrogen is becoming an important fuel of fuel cells for automobiles and cogeneration systems. Considering the importance of development of renewable energy, solar hydrogen would be an ultimate hydrogen fuel for fuel cells. Solar hydrogen can be produced by the two-step water-splitting process with metal oxides (redox materials) using concentrated solar energy; thermochemical reactions (eqs 1 and 2) which consist of the O2-releasing and H2-generation reactions proceed at lower temperatures than the direct water splitting.1,2 The redox reactions of the two-step water-splitting process are given by

MOox ) MOred + 1/2O2 (g)

O2-releasing step

(1)

and

MOred + H2O (g) ) MOox + H2 (g)

H2-generation step (2)

where MOox and MOred denote the oxidized and reduced states of metal oxide, respectively. H2 and O2 can be successively obtained by alternately repeating the two steps of the reaction eqs 1 and 2. Since the O2-releasing step is an endothermic reaction, the chemical energy of the H2 produced by eq 2 is derived from the thermal energy of concentrated solar heat absorbed in eq 1. The two-step water-splitting process is a promising thermochemical energy-conversion process to produce solar hydrogen, and the ZnO system has been studied as a * Corresponding author. Tel.: +81-3-5734-3292. Fax: +81-3-57343436. E-mail: [email protected]. (1) Steinfeld, A. Solar Energy 2005, 78 (5), 603-615. (2) Fletcher, E. A.; Moen, R. L. Science 1977, 197, 1050-1056.

promising metal oxide for practical applications.3 However, the O2-releasing reaction in the ZnO system requires a high temperature of around 2000 K. We have been studying the development of another reactive ceramic which can work at a lower temperature than that for the ZnO system. The redox materials of ferrites (MxFe3-xO4; M ) Mn, Co, Ni, Zn) with spinel-type structure have been investigated with the aim of lowering the temperature of the O2-releasing reaction.4-9 In the case of x ) 1, the O2-releasing reaction of Zn ferrite (ZnFe2O4) and Ni ferrite (NiFe2O4) proceeds at 1800 K in the air.10,11 In the Zn and Ni ferrite systems (ZnFe2O4 and NiFe2O4), the reduced forms after the O2-releasing reaction are ZnO + Fe3O4 (two phases of wurtzite structure and spinel structure) and Ni1/3Fe2/3O [NaCl (rock salt) structure], which are different solid phases from a single spinel structure. These reduced forms, however, can produce H2 with H2O, but the O2 and H2 gas volumes produced by eqs 1 and 2 decrease with the running of the two-step water-splitting process because of the sintering/ (3) Palumbo, R.; Le´dea, J.; Boutina, O.; Elorza Ricarta, E.; Steinfeld, A.; Mo¨llera1, S.; Weidenkaff, A.; Fletcher, E. A.; Bielicki, J. Chem. Eng. Sci. 1998, 53, 2503-2517. (4) Lundberg, M. Int. J. Hydrogen Energy 1993, 18, 369-376. (5) Tamaura, Y.; Steinfeld, A.; Kuhn, P.; Ehrensberger, K. Energy 1995, 20, 325-330. (6) Ehrensberger, K.; Frei, A.; Kuhn, P.; Oswald, H. R.; Hug, P. Solid State Ionics 1995, 78 (1-2), 151-160. (7) Ehrensberger, K.; Kuhn, P.; Shklover, V.; Oswald, H. R. Solid State Ionics 1996, 90 (1-4), 75-81. (8) Alvani, C.; Ennas, G.; Barbera, A. L.; Marongiu, G.; Padella, F.; Varsano, F. Int. J. Hydrogen Energy 2005, 30 (13-14), 1407-1411. (9) Agrafiotis, C.; Roeb, M.; Konstandopoulos, A. G.; Nalbandian, L.; Zaspalis, V. T.; Sattler, C.; Stobbe, P.; Steele, A. M. Solar Energy 2005, 79 (4), 409-421. (10) Tamaura, Y.; Kaneko, H. Solar Energy 2005, 78 (5), 616-622. (11) Kaneko, H.; Hasegawa, N.; Aoki, H.; Suzuki, A.; Tamaura, Y. The XVth International Symposium on the Reactivity of Solids, Kyoto, Japan, Nov. 2003, C026.

10.1021/ef060581z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/25/2007

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melting of the ferrite at the high temperature of 1800 K in the O2-releasing reaction. In order to gain a constant production of the O2 and H2 gas volumes in the two-step water-splitting process, the heat-resistant reactive ceramics (YSZ-NiFe2O4 solid solution,12-15 YSZ-iron oxide solid solution,16-21 and CeO217) were synthesized and examined for practical applications. Moreover, the CeO2-MOx (M ) Mn, Fe, Ni, Cu) solid solution has been studied, and it was confirmed that sintering of the material in the O2-releasing reaction can be prevented at the high temperature of 1700 K.22 Another approach by us for lowering the temperature in the O2-releasing reaction is usage of the reactivity of the oxygen deficiency of the ferrite with a spinel structure. The mixed ferrite of Fe3O4, NiO, and MnO (Ni0.5Mn0.5Fe2O4) can form the oxygen deficiency in the O2-releasing reaction at 1473 K and decompose H2O to generate H2 at 1073 K.5 This reaction system can be operated for the O2-releasing reaction at a lower temperature (T ) 1473 K), but the generated H2 volume is limited to the magnitude of the oxygen deficiency (δ ) 0.053). This H2 evolution amount derived from the oxygen deficiency (δ ) 0.053), however, is theoretically enough to absorb the high flux of the concentrated solar heat, if the oxygen deficiency can be successively regenerated by the repetition of the two-step watersplitting process using a device such as the solar reactor described in this paper. For a practical application of solar hydrogen production by the two-step water-splitting process using concentrated solar thermal energy, we have to develop the solar reactor, in which the two-step water-splitting process can be successively repeated using the reactive ceramics. A single solar energy converter has been developed by Roeb et al.23 using metal oxide redox systems of mixed iron oxides coated upon multichanneled honeycomb ceramic supports capable of absorbing solar irradiation. Another type of solar chemical reactor for conducting the thermal dissociation of ZnO into metal zinc and O2 above 2000 K has been developed by Steinfeld et al.;24-26 ZnO is directly exposed (12) Kaneko, H.; Aoki, H.; Ishihara, H.; Takahashi, Y.; Hasegawa, N.; Suzuki, A.; Tamaura, Y. Proceedings of the Chemical Society of Japan 84th Spring Meeting, Nishinomiya, Japan, March, 2004, 2D4-46. (13) Aoki, H.; Takahashi, Y.; Kaneko, H.; Hasegawa, N.; Tamaura, Y. Proceedings of 2004 SEE Conference, Utsunomiya, Japan, 2004, 27-29. (14) Ishihara, H.; Chen, M.; Yokoyama, T.; Kaneko, H.; Ihara, M.; Tamaura, Y. Proceedings of the Chemical Society of Japan 85th Spring Meeting, Yokohama, Japan, Mar 26-29, 2005, 1H2-48. (15) Ishihara, H.; Kaneko, H.; Yokoyama, T.; Fuse, A.; Hasegawa, N.; Tamaura, Y. Proceedings of 2005 International Solar Energy Conference, Orlando, Florida, Aug 6-12, 2005, ISEC2005-76151. (16) Ishihara, H.; Kaneko, H.; Fuse, A.; Hasegawa, N.; Tamaura, Y. Solar Energy Submitted. (17) Ishihara, H.; Chen, M.; Yokoyama, T.; Fuse, A.; Kaneko, H.; Hasegawa, N.; Tamaura, Y. Proceedings of JSES/JWEA Joint Conference 2005, Chino, Japan, 2005, 109-112. (18) Ishihara, H.; Yokoyama, T.; Kaneko, H.; Miura, T.; Imaeda, S.; Tamaura, Y. Proceedings of the 22nd Conference on Energy, Economy, and Environment, Tokyo, Japan, 2006, 405-406. (19) Ishihara, H.; Yokoyama, T.; Kaneko, H.; Nakajima, H.; Tamaura, Y. Proceedings of the Chemical Society of Japan 86th Spring Meeting, Funabashi, Japan, 2006, 3D3-10. (20) Ishihara, H.; Chen, M.; Kaneko, H.; Hasegawa, N.; Tamaura, Y. Proceedings of 13th International Symposium on Concentrating Solar Power and Chemical Energy Technologies, Seville, Spain, 2006, FB2-S10. (21) Ishihara, H.; Kaneko, H.; Nakajima, H.; Miura, T.; Tamaura, Y. Energy Submitted. (22) Kaneko, H.; Miura, T.; Ishihara, H.; Yokoyama, T.; Taku, S.; Nakajima, H.; Tamaura, Y. Energy 2007, 32, 656-663. (23) Roeb, M.; Sattler, C.; Klu¨ser, R. K.; Oliveira, M. L.; Konstandopoulos, A. G.; Agrafiotis, C.; Zaspalis, V. T.; Nalbandian, L.; Steele, A.; Stobbe, P. Proceedings of 2005 International Solar Energy Conference, Orlando, Florida, Aug 6-12, 2005, ISEC2005-76126. (24) Steinfeld, A. Int. J. Hydrogen Energy 2002, 27, 611-619. (25) Haueter, P.; Moeller, S.; Palumbo, R.; Steinfeld, A. Solar Energy 1999, 67 (1-3), 161-167.

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to high-flux solar irradiation and serves simultaneously the functions of radiant absorber, thermal insulator, and chemical reactant. The hydrolyzer, which is provided near the radiant absorber separately, is necessary for achievement of the watersplitting solar thermochemical cycle.25,26 We have been developing and fabricating a rotary-type solar reactor consisting of a cylindrical rotor coated with reactive ceramics and two different types of reaction cells; one is for discharging O2 (O2-releasing reaction cell) and the other for H2O splitting (H2-generation reaction cell). The concept of the rotary-type solar reactor and development of the prototype reactor configuration have been reported by us so far elsewhere.27-32 The simultaneous and continuous evolutions of O2 and H2 in the two-step water-splitting process can be readily achieved in the O2-releasing reaction cell and H2-generation reaction cell of the rotary-type solar reactor; the reactive ceramics are reduced in the O2-releasing reaction cell at every time of the rotation and, by the rotation, can continuously split water and produce H2 in the H2-generation reaction cell where steam is flowing. A conceptual outline of the rotary-type solar reactor combined with the beam-down solar concentrating system33 is illustrated in Figure 1. The beam-down solar concentrating system can realize 1000-2000 suns of concentration of insolation, and a simulation study suggests that a reaction temperature of 1000-2000 K can be obtained on the surface of the reactive ceramics coated on the cylindrical rotor.33 The objectives of the present paper, based on the concept of the rotary-type solar reaction, are to fabricate the rotary-type solar reactor having dual reaction cells for the production of solar hydrogen with the two-step water-splitting process and to demonstrate simultaneous and continuous H2 generation by the rotary-type solar reactor concept. CeO2 (O2-releasing temperature ) 1673 K) and Ni0.5Mn0.5Fe2O4 (1473 K) were selected as reactive ceramics for the present work. The design of the scaled-up reactor and its capability to proceed with the twostep water-splitting process are investigated. 2. Experimental Methods 2.1. Preparation of Reactive Ceramics. The reactive ceramics of CeO2 and Ni,Mn-ferrite (Ni0.5Mn0.5Fe2O4) were used. The CeO2 was purchased from Rare Metallic Co., Ltd. and used without further purification. The Ni,Mn-ferrite was prepared by a solidstate reaction at 1273 K.5 In the preparation by a solid-state process, required amounts of NiO, MnO, and Fe2O3 were mixed with an agate mortar, and the mixed powder was heated in an electric furnace in the air for 8 h. The synthesized sample was characterized (26) Palumbo, R.; Keunecke, M.; Moeller, S.; Steinfeld, A. Energy 2004, 29, 727-744. (27) Tamura, Y.; Fuse, A.; Kaneko, H.; Hasegawa, N.; Ihara, M.; Tamaura, Y. Proceedings of the Chemical Society of Japan 85th Spring Meeting, Yokohama, Japan, Mar 26-29, 2005, 2H2-26. (28) Kaneko, H.; Fuse, A; Miura, T.; Imaeda, S.; Ishihara, H.; Tamura, Y.; Tamaura, Y. Proceedings of JSES/JWEA Joint Conference 2005, Chino, Japan, 2005, 487-489. (29) Miura, T.; Kaneko, H.; Fuse, A.; Imaeda, S.; Chen, M.; Tamaura, Y. Proceedings of 25th Hydrogen Energy Systems Society of Japan Conference, Tokyo, Japan, 2005, 69-72. (30) Kaneko, H.; Miura, T.; Fuse, A.; Yokoyama, T.; Chen, M.; Tamaura, Y. Proceedings of the 22nd Conference on Energy, Economy, and Environment, Tokyo, Japan, 2006, 407-410. (31) Kaneko, H.; Miura, T.; Ishihara, H.; Yokoyama, T.; Chen, M.; Tamaura, Y. Proceedings of 16th World Hydrogen Energy Conference, Lyon, France, Jun 13-16, 2006, 550. (32) Kaneko, H.; Fuse, A.; Miura, T.; Ishihara, H.; Tamaura, Y. Proceedings of 13th International Symposium on Concentrating Solar Power and Chemical Energy Technologies, Seville, Spain, 2006, FB2-S10. (33) Kaneko, H.; Ishihara, H.; Yuasa, M.; Nakajima, H.; Utamura, M.; Tamaura, Y. Solar Energy Submitted.

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Figure 1. Conceptual outline of the solar furnace with the rotary-type solar reactor and beam-down concentrating system.

Figure 2. Schematic outline of the rotary-type solar reactor for the two-step water-splitting process.

by X-ray diffractometry, and the X-ray diffraction pattern of the synthesized material (Ni,Mn-ferrite) showed good agreement with data reported previously.5 2.2. Rotary-Type Solar Reactor. A schematic outline of the rotary-type solar reactor is illustrated in Figure 2. The reactor made of SUS304 stainless steel (5 mm thickness) has a quartz glass window (A) for the irradiation of the reactive ceramics coated on the cylindrical rotor by a concentrated solar beam. The graphite packing material, Grafoil, was used as an airtight seal between the stainless steel vessel and the quartz glass. The inside of the reactor was covered with a ceramic block as an insulator. The infrared imaging lamps (ULVAC-RIKO, Inc., RHL-Pss34VP) were used as a solar simulator [lamp (A) in Figure 2] for heating the reactive ceramics up to the O2-releasing reaction temperature (O2-releasing reaction cell) and as a heat controller [lamp (B) in Figure 2] for maintaining the reaction temperature in the H2-generation reaction cell. Irradiation from the infrared imaging lamps was regulated by

the temperature program controller (ULVAC-RIKO, Inc., TPC1000) to obtain a definite reaction temperature. Ar gas was passed through the O2-releasing reaction cell to carry out the evolved gaseous product. The steam, which had been generated by feeding water with a micro pump (0.044 mol min-1) into an electric furnace at 673 K, was mixed with Ar gas (250 cm3 min-1) and flowed through the H2-generation reaction cell for the H2-generation reaction of eq 2. The gas contents of H2 and O2 evolved in the O2-releasing and the H2-generation reaction cells were determined by direct gas mass spectrometer (Bruker Axs, MS9600). The calibration of peak area of the H2 or O2 gas was made using the standard gases for quantitative analysis by the direct gas mass spectrometer. The cylindrical rotor (40 mm diameter) coated with reactive ceramics was fabricated as follows: The SUS304 tube which had a stem in the center was divided into eight fractions, and each was packed with a supporting bed composed of quartz wool and an

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Figure 3. H2 generation and O2 releasing profiles observed by mass spectrometer in the H2-generation reaction cell and O2-releasing reaction cell, respectively, with CeO2.

insulator (a mixed solid material with a YSZ powder and inorganic heat-stable adhesive ceramics). The emulsion of the reactive ceramics of CeO2 or Ni,Mn-ferrite (approximately 2 g) with ethanol was painted on the surface of the supporting bed. The reactive ceramics painted on the supporting bed were dried at 393 K and fixed on the surface of the supporting bed by heating at 1473 K using an infrared imaging lamp. The cylindrical rotor was rotated by an electric motor at a rate of 360/n (n ) 2) degrees every 10 min intermittently. The sealing Ar gas was exhausted from the outlet tubes equipped on the top and the bottom of the reactor for separation of the evolved O2 and H2 gases. The temperature of reactive ceramics in the reaction cells was measured by the R-type thermocouples coated with the ferrite (Fe3O4), which had been fixed near the surface of the cylindrical rotor. The temperature of the surface of the reactive ceramics was estimated, without rotating the cylindrical rotor, by comparing the two temperatures measured at the same time by the thermocouple in the reactor and the other one fixed to the surface of the reactive ceramics on the cylindrical rotor. The estimated temperatures of the reactive ceramics in the O2-releasing and H2generation reaction cells were higher by 400 and 200 K, respectively, than those measured by practically rotating the cylindrical rotor in the two-step water-splitting reaction experiment.

3. Results and Discussion 3.1. Two-Step Water-Splitting Cycle with Rotary-Type Solar Reactor. The two-step water-splitting process test was carried out using the rotary-type solar reactor with CeO2 and Ni,Mn-ferrite (Ni0.5Mn0.5Fe2O4). The cylindrical rotor coated with the reactive ceramics (CeO2) was revolved by 180° intermittently, and each fraction with the reactive ceramics (the eight divided fractions of the cylindrical rotor) was alternately passed through the O2-releasing reaction cell and the H2-generation reaction cell. The profiles determined by the mass spectrometer for the evolutions of O2 and H2 are shown in Figure 3. The value in the x axis of Figure 3 represents the time (in minutes) after attainment of the estimated temperature of the reactive ceramics at 1623 K, and the evolution of H2 is indicated by a dotted circle in Figure 3. The cylindrical rotor was rotated by 180° at the points shown by the dotted straight lines. As can be seen from Figure 3, the peaks of H2 evolution were observed every 10 min with the rotation of the cylindrical rotor. The volume of the evolved H2 gas was estimated to be 0.34 cm,3 and the H2-generation reactivity of the CeO2 in the two-step water-splitting process was 0.68 cm3/g of sample. Thus, H2 generation was observed repeatedly under a flow of a mixed gas of steam and Ar gas, indicating that the

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Figure 4. O2 releasing profiles observed by mass spectrometer in the O2-releasing reaction cell at the temperatures of (a) 873 K, (b) 973 K, and (c,d) 1073 K for the H2-generation reaction (a-c, with steam; d, without steam).

Figure 5. H2 generation profiles observed by mass spectrometer in the H2-generation reaction cell at the temperatures of (a) 1073, (b) 1123, and (c) 1173 K.

two-step water-splitting process with the CeO2 system can be repeated at temperatures of 1623 and 1273 K for the O2releasing and H2-generation reactions, respectively, using the rotary-type solar reactor fabricated in the present paper. As seen in Figure 3, no peak of the O2 evolution was observed, and it seems that no O2-releasing reaction proceeds. This is due to the lower reaction rate in the O2-releasing reaction cell, as mentioned below: The maximum temperature of the surface of the cylindrical rotor coated with the reactive ceramics was estimated to be 1623 K in the O2-releasing reaction cell (the maximum temperature for the rotary-type solar reactor fabricated in the present study), as mentioned above. Since the temperature required for the O2-releasing reaction with CeO2 is above 1673 K,22,34,35 the reaction rate for the O2-releasing reaction was not large enough to determine the content of evolved O2 in the effluent gas from the O2-releasing reaction cell at the temperature of 1623 K. This resulted in a slightly lower evolution of the H2 gas (0.68 cm3/g of CeO2) in Figure 3 compared with the 0.76 cm3/g of CeO2 obtained for the two-step water-splitting reaction with CeO2 using the batchwise experimental setup at a temperature of 1773 K.22 The O2-releasing reaction and the H2-generation reaction were also carried out using the same rotary-type solar reactor with (34) Kaneko, H.; Ishihara, H.; Miura, T.; Nakajima, H.; Hasegawa, N.; Tamaura, Y. Proceedings of 2006 International Solar Energy Conference, Denver, Colorado, 2006, ISEC2006-99065. (35) Abanades, S.; Flamant G. Solar Energy 2006, 80, 1611-1623.

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Figure 6. Schematic outline of the scaled-up rotary-type solar reactor with the 500 mm of the diameter of the cylindrical rotor for two-step watersplitting process. Table 1. Density, Specific Heat, Thickness, and Thermal Conductivity for the Materials Employed to Fabricate the Solar Reactor material nth layer

density F (kg m-3)

specific heat C (kJ kg-1 K-1)

thickness µ (m)

heat conductivity Λ (W m-1 K-1)

1. reactive ceramics 2. insulator (a) 3. insulator (b) 4. stainless steel

2000 200 500 7800

0.6 0.4 0.4 0.46

0.001 0.005 0.005 0.003

0.2 0.04 0.04 16.3

Ni,Mn-ferrite. First, the temperature dependence of the H2generation reaction was examined to determine the optimum temperature of the H2-generation reaction (Figure 4). The cylindrical rotor was rotated by 180° every 10 min between the O2-releasing reaction cell and the H2-generation reaction cell. In the case of flowing a mixed gas of steam and Ar carrier gas through the H2-generation reaction cell continuously, the peaks of the evolved O2 gas appeared after every 10 min rotation of the cylindrical rotor at T ) 1073 K in the H2-generation reaction cell (Figure 4c). On the other hand, the water-splitting process with Ni,Mn-ferrite did not proceed at T ) 873 K in the H2-generation reaction cell (Figure 4a). However, the O2 gas volume evolved in the O2-releasing reaction cell increased with an increase in the reaction temperature of the H2-generation reaction, which proceeded at the same time in the other side cell (Figure 4b,c). The optimum temperature for the H2generation reaction with Ni,Mn-ferrite was determined to be above 1073 K, which gives a maximum evolved volume of 1.0 cm3 (2.1 cm3/g of sample) of the O2 gas for 10 min at T ) 1473 K (O2-releasing reaction cell). In the case of the run without flowing the steam through the H2-generation reaction cell, no peak of the O2-releasing reaction was observed, because the reduced form of the reactive ceramics formed in the O2releasing reaction cell cannot be reoxidized with H2O in the H2-generation reaction cell (Figure 4d). These results indicate that the Ni,Mn-ferrite (Ni0.5Mn0.5Fe2O4) reduced at T ) 1473 K in the O2-releasing reaction cell was repeatedly oxidized with

steam in the H2-generation reaction cell at T ) 1073 K. Thus, it can be concluded that the rotary-type solar reactor concept for the two-step water-splitting process was experimentally demonstrated on a laboratory scale. Figure 5 shows the H2-generation profile in the two-step water-splitting process with Ni,Mn-ferrite (O2-releasing reaction cell T ) 1473 K) at T ) 1073 K (curve A), 1123 K (curve B), and 1173 K (curve C) (H2-generation reaction cell). The cylindrical rotor was rotated by 180° every 10 min between the O2-releasing and H2-generation reaction cells. The thin solid lines and thick dotted lines in Figure 5 refer to the experimental curves and smoothed profiles, respectively. As can be seen from the smoothed profiles in Figure 5, the H2 gas generation gradually increased with the elapsing of time at every temperature because of an accumulation of the H2 gas in the H2-generation reaction cell. The volume of H2 evolution was calculated by the peak area, which was the difference between the experimental curve (thin solid line in Figure 5) and the baseline of H2 intensity (thick solid line in Figure 5) observed with the mass spectrometer. The volumes of the evolved H2 gas over 30 min were estimated to be 0.68, 0.85, and 2.1 cm3 at H2-generation temperatures of 1073, 1123, and 1173 K, respectively. Thus, the H2-generation reactivity of Ni,Mn-ferrite in one cycle (10 min) of the two-step water-splitting process was 0.46, 0.56, and 1.4 cm3/g of the sample at T ) 1073, 1123, and 1173K, respectively. The ideal mole ratio of H2/O2 equals 2 in the stoichiometric two-step water-splitting process; however,

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the volume of H2 gas evolution at 1073 K, 0.46 cm3/g, was small in comparison with the volume of O2 gas evolution (O2releasing reaction, T ) 1473 K; H2-generation reaction, T ) 1073 K) of 2.1 cm3/g. The evolved H2 gas volume in the H2generation reaction cell increased with an increase in the reaction temperature of the H2-generation reaction. The highest peak of the evolution of H2 was observed at a reaction temperature of 1173 K in the H2-generation reaction cell. 3.2. Scaled-Up Rotary-Type Solar Reactor. The rotary-type solar furnace developed in the present study is not large enough to realize the H2 production on an industrial scale. In the next step, a scaled-up rotary-type solar reactor is planned to manufacture at 300 cm2 of the irradiation area with concentrated solar heat (Figure 6). In the layout of the beam-down concentrating system, the configuration of the reactor in the vicinity of the ground is suitable for the arrangement of piping, since the reactor and piping system are heavy equipment. Therefore, the beamdown concentrating system can adapt to the scaled-up (heavier) rotary-type solar reactor. The outer vessel of the cylindrical rotor is a cylinder shape in order to prevent an accumulation of evolved H2 gas in the H2-generation reaction cell. The maximum temperature of the reactive ceramics coated on a cylindrical rotor was estimated by the calculation of heat transfer from the infrared imaging lamps to the O2-releasing reaction cell in the scaled-up rotary-type solar reactor. Thermal energy with the infrared imaging lamps is transferred to neighboring material with a heat-transfer coefficient and raises the temperature of the materials depending upon their density, specific heat, and thickness. The heat-transfer coefficients for materials of the nth and (n + 1)th layer are introduced by eq 3.

Rn,n+1 ) µn/2/Λn + µn+1/2/Λn+1

(3)

R1,amb and R4,amb were taken as 0.2. It was supposed that thermal energy was transferred by conduction in order of the reactive ceramics (n ) 1), insulator (a) (n ) 2, a quartz wool YSZ powder and inorganic heat-stable adhesive), insulator (b) (n ) 3), and a stainless steel rotor (n ) 4) (Figure 6). Quartz glass on the window of the reactor vessel was taken as material transparent to an infrared ray. The heat balance for the material of nth layer is calculated with eqs 4-7.

H1 ) (Tamb - T1)/R1,amb + (T1 - T2)/R1,2 + φ radiation loss - heat loss by Ar gas (4) where radiation loss ) σ(T14 - Tamb4). And Ar gas was passed through the O2-releasing reaction cell at a flow rate of 1100 cm3 min-1 and took thermal energy away from the heated materials of the solar reactor at the rate of specific heat at constant pressure (Cp ) 2 kJ kg-1 K-1), then the heat loss by Ar gas ) mArCp(Tamb - T3)/2. H2, H3, and H4 are given by

H2 ) (T1 - T2)/R1,2 + (T3 - T2)/R2,3 - heat loss by Ar gas (5) and

H3 ) (T2 - T3)/R2,3 + (T4 - T3)/R3,4 - heat loss by Ar gas (6) H4 ) (T3 - T4)/R3,4 + (Tamb - T4)/R4,amb

(7)

The reactive ceramics were considered to be in contact with the quartz glass of the cylindrical rotor’s side. The rise in

Figure 7. Time variation of temperatures for all materials of the cylindrical rotor with a simulation using heat transfer theory.

temperatures of the materials constituting the cylindrical rotor at a definite time (1 s) were evaluated with eq 8.

∆Tn ) Hn/(1000FnCn µn)

(8)

The net heat was estimated from the conduction of heat from and to neighbor materials, radiation loss, heat transfer by Ar gas, and irradiation of infrared imaging lamps. The parameters of density, specific heat, thickness, and heat conductivity for the materials employed to fabricate the solar reactor are summarized in Table 1. The temperatures of the reactive ceramics and other materials heated by infrared imaging lamps were calculated with an integration of the increases introduced by eq 8. The time variations of the temperature of the materials (n ) 1-4) composing the cylindrical rotor are shown in Figure 7, which were obtained by theoretical calculation of the heattransfer properties. The temperature of the reactive ceramics in the rotary-type solar reactor was evaluated to reach up to 1800 K (the maximum temperature) by use of an infrared imaging lamp with an output of 6 × 105 W/m2. The maximum temperature of the reactive ceramics for the O2-releasing reaction cell is expected to go up to above 1800 K using the scaled-up rotary-type solar reactor with the beam-down solar concentrating system which is capable of 1000-2000 suns of concentration of insolation (input power of 1000-2000 kW/m2). Ni-ferrite, which exhibits a large amount of O2 evolution at a temperature of 1823 K in the O2releasing reaction, is effective for the two-step waterreleasing process using the scaled-up rotary-type solar reactor as reactive ceramics. The capacities of the O2 and H2 evolutions with Ni-ferrite at 1823 and 1273 K were 1.8 and 3.7 cm3/g of sample per cycle of the two-step water-splitting process for 5 min of O2-releasing reaction time, respectively.36 The O2 and H2 gas volumes with the two-step water-splitting process for 90 s of the O2-releasing reaction time are approximately half of those for 5 min. The Ni-ferrite coated on the scaled-up cylindrical rotor is passed through the O2-releasing reaction cell for 90 s with a rotation of 0.07 rpm. The evolved H2 gas volume in the H2-generation reaction cell is estimated to be 10 cm3/min by rotation of the scaled-up cylindrical rotor with a diameter of 500 mm (Ni-ferrite, 45 g) at a rate of 0.07 (36) Naganuma, Y.; Kaneko, H.; Ishihara, H.; Taku, S.; Imaeda, S.; Fukuzumi, H.; Hasegawa, N.; Tamaura, Y. Proceedings of JSES/JWEA Joint Conference 2006, Chiba, Japan, 2006, 417-419.

Rotary-Type Solar Reactor

rpm. The H2 production ability of the scaled-up rotarytype solar reactor is 10 times larger than that of the small one. The volume of H2 gas evolved in the H2-generation cell was small in the case of the estimation based on the batchwise experimental data; on the other hand, the evolution of H2 gas was much larger than that taking into account the complete absorption of thermal energy flux irradiated by the solar simulator (infrared imaging lamp). The maximum irradiation of the infrared imaging lamp was 600 kW/m2, and the thermal energy flux received in the irradiation area of 300 cm2 was 18 kW for 1 s. The Ni-ferrite decomposes and releases O2 gas according to eq 9 at a high temperature with an endothermic reaction (hypothetical ∆H of ca. 700 kJ/mol for the 1 mol of O2 in eq 9).

6NiFe2O4 ) a(Nib,Fec)O + Ni1-dFe2-eO4-f-δ + 1/2δO2 (9) When 30% of the thermal energy flux is used for the endothermic O2-releasing reaction, the volume of O2 gas evolved for 1 min is estimated to be 0.01 m.3 Since the irradiation area of 300 cm2 is approximately 1/12 of the surface area of the cylindrical rotor and the reactive ceramics (Ni-ferrite) coated on the cylindrical rotor are heated within 5 s up to the maximum temperature, an O2 evolution of 0.01 m3/min is capable of being achieved at a rotation rate of 1 rpm. Then, the thickness of the Ni-ferrite layer coated on the cylindrical rotor is 0.33 mm; consequently, the thin layer of Ni-ferrite is fully heated by thermal energy flux and entirely releases the O2 gas according to eq 9. Therefore, the volume of H2 gas evolved at a rotation rate of 1 rpm with a thermal energy flux of 600 kW/m2 is 0.02 m3/min, and the capability of the scaled-up rotary-type solar reactor for H2 production is large enough for the recovery of evolved H2 gas on a practical scale. The more scaled-up rotary-type solar reactor will make it possible to produce solar hydrogen on an industrial scale. 4. Conclusions The rotary-type solar reactor has been newly fabricated, and we have succeeded in demonstrating the two-step water-splitting

Energy & Fuels, Vol. 21, No. 4, 2007 2293

process by our new concept of solar hydrogen production using the rotary-type solar reactor. The maximum temperature of the reactive ceramics mounted on the cylindrical rotor was approximately 1623 K with a maximum output power of the infrared imaging lamp (in the case of a CeO2 system). The twostep water-splitting process using the rotary-type solar reactor was achieved with reactive ceramics of CeO2 and Ni,Mn-ferrite. A successive evolution of the H2 gas was observed under the conditions of the O2-releasing reaction cell (T ) 1623 K) and H2-generation reaction cell (T ) 1273 K) for the reactive ceramics of CeO2. Also, repetition of the two-step water-splitting process was achieved in the case of using the reactive ceramics of Ni,Mn-ferrite, and the optimum reaction temperatures of the O2-releasing and H2-generation reactions were 1473 and 1173 K, respectively. It was indicated that the scaled-up rotary-type solar reactor attained a sufficient temperature for the O2releasing reaction and larger amount of H2 gas evolution than the prototype reactor. The rotary-type solar reactor concept is proved on a laboratory scale and seems to be one of the most promising thermochemical energy conversion devices to produce solar hydrogen. Nomenclature Fn ) Density of nth layer material (kg m-3) Cn ) Specific heat of nth layer material (kJ kg-1 K-1) µn ) Thickness of nth layer material (m) Λn ) Heat conductivity (W m-1 K-1)  ) Emmisivity of reactive ceramics σ ) Stefan Boltzman coefficient (W m-2 K-4) Hn ) Net heat flux into nth layer Tn ) Temperature of nth layer material (K) Tamb ) Ambient temperature (K) Rn,n+1 ) Heat transfer coefficients between n and n + 1 layers φ ) Radiant flux of infrared imaging lamps (W m-2) mAr ) Mass flow of Ar gas (kg m-2 s-1) Acknowledgment. The authors thank Mr. Hiroshi Asakawa (Asakawa Plating Industry Inc., Tokyo, Japan) for manufacturing the gold plating reflector to concentrate infrared imaging lamp beam. This research is funded by Grant-in-Aid for Scientific research (A) No. 17206100 from Japan Society for the Promotion of Science. EF060581Z