Solar Rotary Reactor for Continuous H2 Production Using Two-

WHEC 16 / 13-16 June 2006 – Lyon FranceSolar Rotary Reactor for Continuous H2 Production Using Two-Step Water Splitting ProcessH.Kaneko, T.Miura, H.Ishihara, T.Yokoyama, M.Chen and Y.Tamaura Research Center for Carbon Recycling and Energy, Tokyo Institute of Technology Ookayama 2-12-1, Meguro-ku, Tokyo, 152-8552, JAPAN, ytamaura@chem.titech.ac.jpABSTRACT: The rotary-type solar furnace was developed and fabricated for solar hydrogen production by a two-step water splitting reaction using the special reactive ceramic. The rotary-type solar furnace is the dual cell solar reactor, which has two different type reaction rooms, one is for oxygen releasing reaction and the other is for hydrogen generation reaction. The detailed specification and the efficiency of the rotary-type solar furnace were examined. The reactive ceramics mounted on the cylindrical rotor in the rotary-type solar furnace was heated up to ca. 1623K by a solar simulator. Successive evolutions of oxygen and hydrogen were observed in the oxygen releasing and water splitting reaction cells, respectively. Two-step water splitting process using newly developed rotary-type solar furnace was achieved. The optimum reaction temperatures of the oxygen releasing and hydrogen generation reactions with Ni,Mn-ferrite were 1473 K and 1173 K, respectively. KEYWORDS : Solar furnace, Rotary reactor, Concentrated solar heat, Two-step water splitting, Solar hydrogen production.1. Introduction The utilization of hydrogen energy as a clean energy greatly becomes important to solve global environmental problems, especially grovel warming caused by CO2 emission from fossil fuel combustion. Hydrogen can be produced by two-step water splitting with metal oxides as a redox pair in a thermochemical reaction. In the solar hydrogen production development with concentrated solar thermal energy, the key factor is to develop the solar reactor, in which solar hydrogen will be produced by a two-step water splitting reaction using the special reactive ceramics (reactive type at around 1473-1673K).1-4 The two-step water splitting cycle, which consists of the oxygen releasing reaction and hydrogen generation reaction, using metal oxides and thermo-chemical methods have been proposed on account of the lower reaction temperature than the direct water-splitting.5, 6 They can be represented as: Oxygen releasing step (1) MOox + thermal energy = MOred + 1/2O2(g) Hydrogen generation step MOred + H2O(g) = MOox + H2(g) (2) where MOox and MOred denote the oxidized and reduced states of metal oxide, respectively. Hydrogen and oxygen are obtained by repeating two steps alternately in the two-step water splitting cycle. Since the oxygen releasing step is an endothermic reaction, hydrogen obtained in Eq.(2) is solar hydrogen, 100% renewable energy, when thermal energy in Eq.(1) is supplied with the collector of concentrated solar heat. Two-step water splitting process is promising thermo/chemical energy conversion process to

produce solar hydrogen. The high temperatures of the oxygen releasing reaction with MO systems form a barrier to integration of a two-step water splitting cycle and construction of a solar concentrating system for hydrogen production. The redox materials with MxFe3-xO4 (M = Mn, Co, Ni, Zn) have been investigated to lower the temperature of the oxygen releasing reaction.7-12 In the case of x = 1, MFe2O4 (M = Ni, Zn) is also possible to lower the temperature of the oxygen releasing reaction, the oxygen releasing reaction proceeds at 1800K under an air flow with both Zn ferrite (ZnFe2O4) and Ni ferrite (NiFe2O4) systems.13,14 Moreover, CeO2-MOx (M = Mn, Fe, Ni, Cu) solid solution has been prepared for lowering the temperatures of the oxygen releasing reaction and preventing from sintering of the material in the oxygen releasing reaction.15 The various types of solar furnace have been introduced as an efficient thermo/chemical energy conversion device to produce solar hydrogen. The two-step water splitting process has been carried out in a single solar energy converter.16 The metal oxide redox systems such as mixed iron oxides coated upon 1/6WHEC 16 / 13-16 June 2006 – Lyon France multi-channeled honeycomb ceramic which supports capable of absorbing solar irradiation were utilized in the whole process. The gaseous products of oxygen and hydrogen are obtained alternately in the cyclic twostep water splitting process with the solar furnace having a single reaction cell. A solar chemical reactor for conducting the thermal dissociation of ZnO into zinc and oxygen at above 2000K has been developed for the following hydrogen generation reaction.17 With the reactor configuration, ZnO is directly exposed to high-flux solar irradiation and serves simultaneously the functions of radiant absorber, thermal insulator, and chemical reactant. The hydrolyser is necessary for achievement of the water-splitting solar thermochemical cycle, separately.18,19 We develop and fabricate the solar furnace with cylindrical rotor coated by reactive ceramics and two different type reaction cells, one is for oxygen releasing reaction and the other is for hydrogen generation reaction.1-4 The evolutions of oxygen and hydrogen in the two-step water splitting cycle can be effected simultaneously and continuously in the oxygen releasing reaction cell and hydrogen generation cell of the rotary-type solar furnace, respectively. A conceptual outline of the rotary-type solar furnace is illustrated in Fig.1. The cylindrical rotor coated with reactive ceramics is mounted in the middle of two reaction cells and turned around between the oxygen releasing Reduced reactive and hydrogen generation reaction cells Oxygen ceramics alternately. The reactive ceramics reduced releasing in the oxygen releasing reaction cell can reaction cell continuously split water and produce O2 hydrogen with the oxidation in the hydrogen H O 2 generation reaction cell where steam is flowing. The aim of this stu

dy is to develop Endothermic Concentrated reaction the rotary-type solar furnace having dual solar heat reaction cells for the production of solar H2 hydrogen using the two-step water splitting Ar carrier gas cycle with reactive ceramics. CeO2 (oxygen Hydrogen releasing temperature of around 1673K) and generation Ni,Mn-ferrite (that of around 1473K) were Oxidized reactive reaction cell selected to be materials as reactive ceramics ceramics for the rotary-type solar furnace. Fig.1 Conceptual outline of the rotary-type solar furnace. 2. Experimental Methods 2.1. Preparation of reactive ceramics CeO2 purchased from RARE METALLIC Co., LTD. was used without further purification in the two-step water splitting reaction with the rotary-type solar furnace. Ni,Mn-ferrite (Ni0.5Mn0.5Fe2O4) was prepared by a solid state reaction at 1273 K.8 For a solid state preparation, required amounts of NiO, MnO and Fe2O3 was mixed with an agate mortar and heated in an electric furnace for 8h. The synthesized sample was characterized by an X-ray diffractometry. 2.2. Rotary-type solar furnace To achieve the two-step water splitting cycle using concentrated solar heat the rotary-type solar furnace was designed and built up with heat-stable materials. A schematic outline of the rotary-type solar furnace is demonstrated in Fig.2. The reactor was made of SUS304 Sealing gas (Ar) stainless steel with quartz Infrared image lamp windows for the irradiation of H2 O2 (Preliminary heater) concentrated solar beam from a Quartz glass solar simulator to the reactive H2O ceramics. The graphite packing Direct gas mass spectrometer material, Grafoil, was used as an airtight seal between the stainless steel vessel and the quartz glass. The inside of the reactor was Thermocouple covered with a ceramic block as Infrared image lamp Rotary cylinder (Solar simulator) an insulator. The infrared image Quartz glass Reactive ceramics lamps were used as a solar Sealing gas (Ar) simulator for heating the reactive ceramics up to the oxygen Hydrogen generation Oxygen releasing releasing temperature in the reaction cell reaction cell oxygen releasing reaction cell and Fig.2 Schematic outline of the rotary-type solar furnace preliminary heater for keeping the for two-step water splitting reaction. 2/6WHEC 16 / 13-16 June 2006 – Lyon France reaction temperature in the hydrogen generation reaction cell. Ar gas was passed through the oxygen releasing reaction cell for a carrier of evolved gaseous product. A steam was generated by dropping demineralized water with a micro pump (0.044mol min-1) in an electric furnace at 673K. The mixture of Ar gas (250cm3 min-1) and steam was made to flow through the hydrogen generation reaction cell for the water splitting reaction and a carrier of evolved hydrogen. The contents of H2 and O2 in evolved gases from the oxygen releasing reaction and the H2 generation reaction cells were determined by the direct gas mass spectrometer (BRUKER axs, MS9600). The cylindrical r

otor with reactive ceramics was fabricated as follows: SUS304 tube which had a stem in the center was pided 8 fractions and was covered with quartz wool and YSZ powder mixed with inorganic heat-stable adhesive, then emulsion of reactive ceramics (CeO2 or Ni,Mn-ferrite) and ethanol was heaped up on the mixtures of YSZ powder and inorganic adhesive. The cylindrical rotor with reactive ceramics was dried at 393K and heated at 1473K in the solar furnace using an infrared image furnace. The cylindrical rotor was rotated by an electric motor at the rate of 360/n (n=2) degrees a 10 min intermittently. The sealing gas of Ar was exhausted from the outlet equipped on top and bottom of the furnace owing to separate the gases evolved in the oxygen releasing reaction and hydrogen generation reaction cells. The temperatures of reactive ceramics in the reaction cells were measured by the R-type thermocouples covered with ferrite (Fe3O4) and located near the surface of the cylindrical rotor. 3. Theoretical Calculations Nomenclature ρ density (kg m-3) C specific heat (kJ kg-1 K-1) µ thick (m)The maximum temperature of the reactive ceramics loaded on a cylindrical rotor was estimated by the calculation of heat transfer from the infrared image lamps to the oxygen releasing reaction cell in the rotarytype solar furnace. It was supposed that thermal energy was transferred by conduction in order of quartz glasses, the reactive ceramics, insulation (YSZ powder and inorganic heat-stable adhesive), quartz wool and a stainless steel rotor (Fig.3). The reactive ceramics were considered to be in contact with the quartz glass of the cylindrical rotor’s side. An Ar gas was passed through the oxygen releasing reaction cell at the flow rate of 1100cm3 min-1 and took thermal energy away from the heated materials of solar furnace at the rate of specific heat at constant pressure (Cp=2kJ kg-1 K-1).Reactive ceramics Insulation Quartz wool Stainless steel Heat transfer by Ar gasDensity (kg m-3) Specific heat (kJ kg-1 K-1) Thick Thermal conductivity (m) (W m-1 K-1) Table 1 The density, specific heat, thick, and thermal conductivity for the materials employed to fabricate the solar furnace.Ty erg l en ma herQuartz glass 2700 0.8 0.6 0.4 0.4 0.46 0.033 0.001 0.005 0.005 0.003 0.8 0.2 0.04 0.04 16.3Reactive ceramics 2000 Insulation 2000 200 7800Heat transferQuartz glassQuartz wool Stainless steelFig.3 Schematic outline of heat transfer for the cylindrical rotor with reactive ceramics.The rise in temperatures of the quartz glass and materials constituting the cylindrical rotor in a unit time were evaluated with Eq. (3). T = net heat / (1000 x ρ x C x µ) (3) The net heat was estimated from conduction of heat from/to neighbor materials, radiation loss, heat transfer by Ar gas and irradiation of infrared image lamps. The parameters of the density, specific heat, thick, and thermal conductivity for the materials employed to fabricate the solar furnace are summ

arized in Table 1. The temperature of the reactive ceramics and other materials heated by infrared image lamps were calculated with an integration of the increases introduced by Eq.(3). 3/6WHEC 16 / 13-16 June 2006 – Lyon France4. Results and Discussion X-ray diffraction pattern of the synthetic material (Ni,Mn-ferrite) showed good agreement with data reported previously.4 The prepared Ni,Mn-ferrite was provided for the two-step water splitting reaction using the rotary-type solar furnace in addition to the purchased CeO2. The variations of temperatures 1673 for all materials of the cylindrical rotor are presented in Fig.4 as the reactive ceramics results of the theoretical calculation 1473 of heat transfer properties. The temperature of the reactive 1273 ceramics in the rotary-type solar furnace was estimated to be insulation 1073 attained up to 1573K by the infrared image lamp with output of 4x105 W 873 m-2. The cylindrical rotor coated with 673 the reactive ceramics (CeO2) was quartz wool revolved 180 degrees intermittently quartz glasses 473 and each fraction of reactive ceramics was alternately located in stainless steel the oxygen releasing reaction cell 0 100 200 300 400 500 600 and the hydrogen generation reaction cell. The profiles Time (s) determined by the direct gas mass Fig. 4 The variation of temperatures for all materials of spectrometer for the evolutions of the cylindrical rotor and the quartz glass. oxygen and hydrogen with the rotary-type solar furnace are shown in Fig.5. The value in x-axis of Fig.5 is represented H2 time-course after the attainment of estimated temperature of reactive ceramics at 1623K, and the evolution of hydrogen is indicated by a dotted circle. The peaks of hydrogen evolution were confirmed every 10 Rotation of min with the rotation of cylindrical rotor. On 180 degrees the other hand, no peak of oxygen evolution was observed in the two-step water splitting cycle. The surface of the cylindrical rotor O2 coated with the reactive ceramics was heated by the infrared image lamp and its maximum temperature was approximately 10 15 20 25 estimated at 1623K by means of an observation of the reactive ceramics Time (min) established on the cylindrical rotor. Since Fig. 5 H2 generation and O2 releasing profiles observed the appropriate temperature of the oxygen by mass spectrometry in the hydrogen generation releasing reaction with CeO2 is above reaction cell and oxygen releasing reaction cell, 1673K, the reaction rate for the oxygen respectively, with CeO2. releasing was not large enough to determine the content of evolved oxygen in the effluent gas from the oxygen releasing reaction cell at 1623K by the mass spectrometer. The reactive ceramics which exhibited the evolution of oxygen efficiently at the low temperature were provided to confirm the oxygen releasing reaction with the rotary-type solar furnace. The oxygen releasing reaction and the hydrogen generation reaction were carried out using the rotary-ty

pe solar furnace with Ni,Mn-ferrite at the estimated temperatures of 1473 and 1073 K, respectively. The cylindrical rotor was rotated 180 degrees every 10 min between the oxygen releasing reaction cell and the hydrogen generation reaction cell. In case of a steam with an Ar carrier gas was passed through the hydrogen reaction cell continuously, the evolution of oxygen was observed every 10 min (Fig.6 (a)). In the hydrogen generation reaction cell without steam flow, the oxygen releasing reaction didn’t proceed (Fig.6 (b)). It was indicated that the two-step water splitting reaction with the rotary-type solar furnace was repeatedly accomplished. The dependence of hydrogen generation temperature for the reproduction of oxidized reactive ceramics in the two-step water splitting process with Ni,Mn-ferrite was investigated to reveal the optimum temperature ofTemperature (K) (K)Intensity (a.u.)4/6WHEC 16 / 13-16 June 2006 – Lyon France the hydrogen generation reaction using the rotary-type solar furnace. The oxygen releasing reaction and the hydrogen generation reaction were carried out at the estimated temperatures of around 1473 and 873-1073 K, respectively. The cylindrical rotor was rotated 180 degrees every 10 min. The peaks of oxygen evolved in the oxygen releasing reaction cell were observed as the cylindrical rotor turned around at the estimated temperature of 1073 K in the hydrogen generation reaction cell. On the other hand, the hydrogen generation reaction with Ni,Mn-ferrite didn’t proceed at the estimated temperature of 873 K in the hydrogen generation reaction cell. The amounts of oxygen evolved in the oxygen reaction cell increased with an increase in the reaction temperature of hydrogen generation reaction proceeding in the other cell (Fig.7). The optimum estimated temperature for the hydrogen generation reaction with Ni,Mn-ferrite was 1073 K with the rotarytype solar furnace in the temperature range between 873 K and 1073 K.Rotation of 180 degrees Intensity (a.u.)Rotation of 180 degreesIntensity (a.u.)(c)(a)(b)(b)(a)5101520 Time (min)2530350102030405060Time (min)Fig.6 O2 generation profile observed by mass spectrometry in the oxygen releasing reaction cell, (a) steam on and (b) steam off.Fig.7 O2 generation profile observed by mass spectrometry in the oxygen releasing reaction cell at the temperatures of 1073, 973 and 873 K for the hydrogen generation reaction.The two-step water splitting cycle with Ni,Mn-ferrite was repeated at different Rotation of (c) temperature of the hydrogen generation 180 degrees reaction on the rotation of the cylindrical rotor at 180 degrees every 10 min in order to confirm the hydrogen generation reaction using the rotary-type solar furnace. The oxygen releasing reactions were carried out at (b) the estimated temperatures of around 1473 K, and the hydrogen generation reactions were performed at the estimated temperatures of (a) 1073, 1123 and 1173

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