劉 醇一

Pure and mixed rare-earth hydroxides are expected to be new materials for chemical heat pumps. In this study, the dehydration and hydration behavior of these materials was investigated by thermogravimetry. The dehydration temperature could be lowered by increasing the Nd or Gd ion content in the hydroxides. La0.50Gd0.50(OH)(3) is an ideal candidate material for chemical heat pumps that can store thermal energy at 300 degrees C; further, heat output can be generated from the heat source at 40 degrees C.

Lithium chloride (LiCl)-modified magnesium hydroxide (Mg(OH)(2)) is a material used for chemical heat pumps. LiCl-modified Mg(OH)(2) is able to store thermal energy at around 250 degrees C. LiCl-modified Mg(OH)(2) was examined using 1, 7, and 105 runs of a cyclic dehydration and hydration operation. The sample was dehydrated at 250 degrees C and hydrated at 110 degrees C. It was found that the hydration conversion, which corresponds to the heat output performance, decreased from 81.1 to 65.7%. There were no LiCl needle-like crystals on the sample before the cyclic operation. On the other hand, after 105 runs of the cyclic operation, the sample showed LiCl needle-like crystals. The reason for the decrease in the reactivity of the LiCl-modified Mg(OH)(2) could be a change in the surface state.

In order to enhance the CO2 decomposition efficiency of plasma chemical reactions, a hybrid reactor consisting of a dielectric barrier discharge (DBD) reactor and a solid oxide electrolyzer cell (SOEC) was fabricated. The SOEC, composed of a yttria-stabilized zirconia (YSZ) tube (electrolyte: o.d. is 15 mm and thickness is 2 mm) with lanthanum strontium manganite (LSM) thin films (electrodes), was inserted into a quartz tube of 18 mm in inner diameter. The outer surface of the quartz tube was wrapped with SUS mesh, which was connected to a high voltage supply. CO2 was fed into the space between the quartz tube and the SOEC where the dielectric barrier discharge was formed. The composition of product gas was analyzed by gas chromatography. The inside of the SOEC was evacuated with a rotary pump in order to enhance desorption of permeated oxygen. When the permeation current of the SOEC was high enough to remove the oxygen from the CO2 plasma region completely, the synergistic effect of the hybridization became apparent, and CO2 conversion increased rapidly with increasing SOEC permeation current. When the plasma reactor and SOEC were operated independently, the maximum CO2 conversions were 40% for the DBD plasma reactor and 6% for the SOEC. However, the maximum CO2 conversion by the hybrid reactor was about 80%, much higher than the sum of the conversions of the plasma reactor and the SOEC. Possible reasons are discussed for the synergistic effect of hybridization of the DBD reactor and the SOEC on the CO2 decomposition.

Fuel reforming with carbon dioxide recovery for hydrogen supply to fuel cell vehicles is discussed experimentally. Ethanol water solution was chosen as the fuel, and carbonation of calcium oxide was performed for carbon dioxide recovery. A packed bed reactor containing calcium oxide and a reforming catalyst was used to produce hydrogen from the ethanol solution under 0.10-0.30 MPa at 450-550 degrees C. Thus, the production of hydrogen with a concentration of over 95% and with less than 0.1% of ethanol, carbon dioxide, and carbon monoxide was demonstrated. On the basis of the experimental results, the hydrogen storage capacity of state of the reforming reactor for a fuel cell vehicle was evaluated and compared with that of other hydrogen storage methods. In comparison with pressurized and liquid hydrogen cylinder storages, the reforming system was more advantageous from the standpoint of storage density, and the need for a compression system explosion risk. The reforming was also found to be better than metal hydrides from the standpoint of material cost. It is expected that a carbon recycle hydrogen system based on the proposed reforming system using ethanol as the fuel can be effectively used for hydrogen storage and transportation for fuel cell vehicles.

The dehydration behaviors of metal-salt-added magnesium hydroxide as chemical heat storage media were studied. Dehydration temperature of magnesium hydroxide, which was corresponding to heat storage temperature, was dropped from 277 to 233 degrees C by addition of lithium chloride. The heat storage capacity of 6.8 wt % LiCl/Mg(OH)(2) (816 MJ m(-3)) was 11 times as high as one of authentic Mg(OH)(2) under the heat storage operation at 280 degrees C.

Possibility of a carbon recycle hydrogen carrier system driven by nuclear power for transportation field was discussed. The hydrogen carrier system, which was zero carbon dioxide emission and small hydrogen compression work and explosion risk, was examined for fuel cell vehicles. The combination of the hydrogen carrier system with a high-temperature gas reactor was proposed. It was expected that the carrier system realized lower pressure and safe storage of hydrogen consisting with the similar energy efficiency of conventional water electrolysis hydrogen system. Carbon neutral bio-mass energy system and the carrier system were compared for the next generation vehicle energy system. Biomass energy had the limitation of quantity and would take small part of vehicle market in Japan. Nuclear assisted bio-mass hydrogen system could enhance yield of bio-fuels under carbon neutral. The carbon recycle nuclear hydrogen carrier system was expected to be applicable to the demand of much more larger number of vehicles because it was free from the limitation of bio-mass carbon quantity. (C) 2007 Elsevier Ltd. All rights reserved.

Durable reaction material for repetitive reaction operation of a chemical heat pump that used a reversible magnesium oxide/water reaction system was discussed to enhance the heat pump performance. Because a material for the heat pump use was required to fit multi-production process, a molding method was introduced for material preparation. Some molded samples under different preparation conditions were evaluated kinetically by a thermo-balance analysis. Magnesium hydroxide prepared from ultra fine particle magnesium oxide and purified water showed enough reactivity, and also stable durability to 70 of repetitive reaction cycles. Removal or impurity was effective for durability enhancement of magnesium hydroxide made from seawater precipitated precursor. The cost of the precipitated hydroxide was less than one several tenths of the ultra line particle material. It was demonstrated that the molded reactant made from precipitated hydroxide was applicable for multi production and cost reduction of magnesium hydroxide material for the heat pump. The thermal performance of magnesium oxide/water chemical heat pump was evaluated from experimental results.

Durable reaction material for repetitive reaction operation of a chemical heat pump that used a reversible magnesium oxide/water reaction system was discussed to enhance the heat pump performance. Because a material for the heat pump use was required to fit multi-production process, a molding method was introduced for material preparation. Some molded samples under different preparation conditions were evaluated kinetically by a thermo-balance analysis. Magnesium hydroxide prepared from ultra fine particle magnesium oxide and purified water showed enough reactivity, and also stable durability to 70 of repetitive reaction cycles. Removal or impurity was effective for durability enhancement of magnesium hydroxide made from seawater precipitated precursor. The cost of the precipitated hydroxide was less than one several tenths of the ultra line particle material. It was demonstrated that the molded reactant made from precipitated hydroxide was applicable for multi production and cost reduction of magnesium hydroxide material for the heat pump. The thermal performance of magnesium oxide/water chemical heat pump was evaluated from experimental results.

The mixing effect of transition metal ion into magnesium hydroxide on dehydration and hydration reactivity was studied to develop a new material for chemical heat-storage, because the mixing effect was expected to reduce dehydration-temperature, corresponding to heat-storage temperature, of authentic magnesium hydroxide. Two-components composite materials mixed with some content of nickel ion or cobalt ion into magnesium hydroxide were tested, respectively. It was demonstrated that the dehydration-temperatures of the composites were shifted to lower temperature below 300 degrees C with increase of nickel or cobalt content in comparison with dehydration-temperature of authentic magnesium oxide of 350 degrees C. These composites showed higher hydration reactivity than that for authentic magnesium oxide under the same reaction condition, and were expected to be applicable to heat utilisation of middle-temperature waste heat less than 300 degrees C.

Fuel cell (FC) offers the possibility of expanding new vehicle market. One of the key technologies that will make the widespread use of FCs possible is efficient and safety hydrogen supply system. The possibility of a hydrogen carrier system for FC vehicles, which utilized chemical reactants and capable to supply hydrogen safety with no carbon dioxide emission, was discussed in this study. The system uses a portable thermally regenerative fuel reformer of carbon dioxide fixation type. The reactivity of metal oxide to carbon dioxide was used for the carbon dioxide fixation, hydrogen purification and also for heat source of fuel reforming. In the experimental study, calcium oxide was used as the first candidate for carbon dioxide fixation and methane was chosen as a candidate reactant for steam reforming. Fuel reforming, carbon dioxide fixation and pure-hydrogen production were examined by a laboratory scale packed-bed reactor. High-purity hydrogen production under mild operation conditions was demonstrated. The fixed carbon dioxide can be decarbonated thermally by consuming high-temperature heat source. Nuclear power, unstable energy sources such as renewable energy, surplus industrial process heat and so forth are applicable on the system. The contribution of nuclear power and other thermal energy sources on the zero-emission hydrogen career system was evaluated based on the experimental results. The reactor size was more compact than conventional hydrogen career systems. Because the reforming system supplies hydrogen safety on-board to a vehicle FC and the reformer is thermally regenerative, the proposed system was expected to develop new hydrogen market for FC vehicle.

The possibility of a hydrogen production system for Fuel Cell (FC) vehicles, which was zero carbon dioxide emission based on nuclear power, was investigated. The reactivity of calcium oxide to carbon dioxide was used for the carbon dioxide fixation and also for heat source of fuel reforming in experimental discussion. Methane was chosen as the first candidate reactant for steam reforming. Simultaneous reaction of methane reforming and carbon dioxide fixation by calcium oxide was demonstrated in a reactor packed with a reforming catalyst and calcium oxide. High-purity hydrogen, of which the concentration was higher than one at reaction equilibrium of conventional reforming, was generated from the reactor under mild operation conditions at temperature of 500-600 degrees C and under pressure of 101 MPa. The efficiency of the fuel reforming system was estimated from the experimental results. The proposed system was expected to be applicable as a hydrogen carrier system in carbon dioxide zero-emission FC vehicles based on nuclear power. (C) 2005 Elsevier Ltd. All rights reserved.

The mixing effect of alkaline earth metal halides on ammonia absorption-desorption behavior was studied to develop a new ammonia storage material to be used for the pressure-swing absorption method. The halide mixtures with different cations (having a common anion) did not form the solid solution after drying of the mixed aqueous solution. The ammonia absorption isotherms on those samples were identical with those of the single-phase halide having the higher ammonia affinity among the two. The halide mixtures with a common cation (having different anions) formed a solid solution through the same mixing method. The isotherms on those samples showed an intermediate profile between the components. Among these kinds of combinations, CaCl2-CaBr2 (actually CaClBr) showed a pronounced ammonia storage capacity (16.4 mmol g(-1)) under the pressure-swing conditions of 60 and 10 kPa at 298 K, which was 12.6 times as high as that of the Na form of the Y-zeolite.

The behavior of ammonia absorption and desorption was studied for CaCl2-CaBr2 halide mixtures with various molar ratios prepared via the aqueous solution method. The CaCl2-CaBr2 halide mixture was revealed to form solid solutions in any molar contents. Ammonia absorption abruptly increased at some pressure, forming the ammine complex, and the (step) pressures were different for the samples with different molar contents. The step pressure decreased when the Cl/Br ratio was decreased for both absorption and desorption cycles. All samples absorbed ammonia irreversibly, up to two molecules of ammonia coordination. Three samples-CaCl1.33Br0.67, CaClBr, and CaCl0.67Br1.33-proved to separate a great deal of ammonia at pressures of 60-10 kPa at 298 K. These samples are promising candidates for ammonia storage material to be used for pressure swing separation in a new ammonia synthesis process. Especially, CaCl1.33Br0.67 treated at 523 K showed the best separation capacity (27.1 mmol/g).

The water sorption behavior of CaCl2-containing materials as heat transfer media was studied. Amount of the water sorption on CaCl2-containing materials was higher than those on the zeolites at the higher humidity region. Heat storage capacity of 33 wt % CaCl2/FSM16 (1200 MJ m(-3)) was 3.5 times as high as Na form Y-zeolite at 18.7 Torr of water vapor.

For the low pressure ammonia synthesis (∼1 MPa, 573-623 K), 40-80 kPa of ammonia produced must be separated. For this purpose, the absorption behavior of five kinds of alkaline earth metal halides (MgCl2, CaCl2, CaBr2, SrCl2, and SrBr2) and their hydrated forms were studied under 0 to 80 kPa of ammonia at 298 to 473 K. Although the equilibrium data of the ammine complex formation of these materials are known, the absorption started at higher pressure than the equilibrium data, due to the presence of enough chemical potential Δμ for ammonia absorption. Initial absorption (to low numbers of coordinated ammonia) was slow and depended on the sample specific surface area. The successive absorption (beyond 1 or 2 of ammonia coordination) occurs easily and reversibly. The "quasi" absorption and desorption isotherms studied here were useful for the design of ammonia separation materials. MgCl(OH), CaCl2, and CaBr, were found to be practical for TSA (Temperature Swing Absorption) material working between 298 and 473 K at 40 kPa. Especially, the ammonia separation capacity of MgCl(OH) (26.1 mmol g-1) was 5.5 times as high as that of Na exchanged Y-zeolite.

Ammonia adsorption isotherm on ion exchanged Y-zeolites (Na-Y, H-Y, Co-Y, Cu-Y, K-Y, Rb-Y, and Cs-Y) was investigated to asses the potential for use in ammonia separation and storage, by measuring the adsorption isotherm at 323 to 473 K and below I atm. Ammonia adsorption on Y-zeolite was increased by exchanging the cation with transition metal ions due to the increase in the number of ammonia adsorption sites with ammine complex formation, but was decreased by exchanging with alkali metal ions due to the decreased electrostatic attraction between ammonia and the zeolite surface. Irreversible ammonia adsorption sites on the ion exchanged Y-zeolite were classified into 3 types by IR (infrared) and TPD (temperature programmed desorption) techniques; M(OH)(+) (NI: divalent cation), H+, and M+ (M: alkali metal ion Na+, K+, Rb+, Cs+). The first type of site bonds by ammine complex formation, the second type of site bonds by ammonium ion formation, and the third type of sites bonds by ammonia adsorption with electrostatic attraction. Cu2+ exchanged Y-zeolite provided the best ammonia separation (4.92 mmol.g(-1)) with the temperature swing adsorption method (323-473 K, 40 kPa).

The behavior of ammonia adsorption was studied on various types of surface-treated active carbon (HNO3-AC, H2SO 4-AC, HCl-AC, NaOH-AC, CuO-AC, and H2-AC) under 0-75 kPa and 323-473 K. Amount of ammonia adsorption on active carbon was increased by the surface oxidation through acid treatment or metal nitrate decomposition. It was decreased by the hydrogen treatment at 1173 K. The equilibrium amount of ammonia adsorption was found to relate with the amount of surface organic oxygen. Ammonia adsorption was analyzed through the isotherm and classified into three types: (1) strong adsorption relating to surface organic oxygen content and being independent of pressure, (2) weak adsorption relating to surface organic oxygen content and depending upon the pressure, (3) van der Waals adsorption. The model well represented the data, and the average relative error was 10%. When the samples are assumed to be used for ammonia separation with temperature swing adsorption method (323-473 K, 40 kPa), H 2SO4-treated active carbon was found to have the highest ammonia separation capacity (2.58 mmolg-1).

Alkali earth metal halides were studied for the use as ammonia storage material operated below 60 kPa. Reversible and irreversible absorption were observed for each samples. MgCl2, CaCl2, and CaBr2 were found to be adequate for TSA method (between 298 and 473 K), and these storage capacities were much higher than that of Na-Y zeolite. CaCl2-CaBr2 mixed halides proved to be a useful material with large absorption for PSA cycling between 10 and 60 kPa of ammonia.

Ammonia adsorption and desorption behavior of surface treated active carbon (AC) and ion-exchanged Y zeolite, as ammonia separation and storage materials for a new de-NOX process with ammonia on-site synthesis, were studied. Surface oxidized AC adsorbed more ammonia than non-treated AC due to ammonium ion formation. These materials were found to increase weak adsorption of ammonia and to be useful for pressure swing adsorption. Transition metal ion exchanged Y-zeolite adsorbed more ammonia (both weak and strong form) than Na Y-zeolite due to ammine complex formation, These materials adsorb and desorb more ammonia than treated AC when used for temperature swing adsorption.