The Basic Study Of The Thermochemical Quasi Cycle For Hydrogen Production Based On The Zn/ZnO Redox Pair

A new promising thermochemical quasi-cycle for hydrogen production based on the Zn/ZnO redox pair has been studied basically in this paper. The study is mainly focused on two key steps of the cycle:the carboreduction of zinc oxide and the hydrogen production via the hydrolysis of zinc. And a new kind of hydrogen production system with coal gasification and a new near zero emission clean coal energy utilization system based on Zn/ZnO redox pair via the two-step thermochemical quasi cycle are both constructed and evaluated. The research is mainly taken as follows:Firstly, the thermodynamic study is taken on the carboreduction of zinc oxide. The result shows that the carboreduction of zinc oxide is a highly endothermic process. The reaction starts when the temperature is higher than 910℃. Increasing temperature and decreasing pressure are all beneficial to the carboreduction of zinc oxide. Excess carbon will be taken in reaction for a higher conversion of zinc oxide.The thermodynamic study of ZnO reduction by coal demonstrats that the reduction of ZnO by coal starts at a lower temperature, compared to the reduction by solid carbon. The zinc yield increases as the temperature increases. In the reduction of ZnO by coal, the main Occurrence pattern of each trace element in coal depends on the reaction temperature. Though there will be no gas pollutant producing in the reduction of ZnO by coal, the coal with high Sulfur and high moisture is not fit for the ZnO reduction. There is a best ZnO-input-per-coal-mass for ZnO reduction. It is reasonable to control the temperature in a range of 1000-1200℃for ZnO reduction with coal.The cooling process for the gas products of ZnO reduction is also studied thermodynamically. The results show that the reoxidation of product zinc should be prevented in the cooling process. It is advantageous for zinc condensation to increase the temperature of the ZnO reduction, and decrease the cooling temperature or increase the initial concentration of gas zinc in product gases.The thermodynamic study is also taken for the hydrolysis of zinc. It is shows that the zinc hydrolysis can proceed auto-thermally. It is reasonable to keep the temperature of zinc hydrolysis under 900℃. The system pressure has no notable thermodynamic influences on the hydrolysis reaction. The initial H2O/Zn molar ratio should be controlled in a reasonable range. The steam concentration in carrying gas in the experiments should better be kept above 50%. From the perspective of thermodynamics, the impurities of steam or zinc are all disadvantageous to the hydrolysis process.The mechanism and characteristic studies are taken for ZnO carboreduction by thermogravimetric and TG-FTIR analyses. The result indicates that the mechanism of ZnO carboreduction fits the R3 nuclear contraction model function. Its activation energy decreases as the conversion increases, and its initial activation energy is about 315.3kJ/mol. The initial temperature of ZnO carboreduction is about 950℃. The initial temperature increases as the flow rate of carry gas increases. The final temperature increases as the heating rate increases, as the flow rate of carry gas decreases and as the initial C/ZnO molar ratio decreases. The increase of the initial C/ZnO molar ratio improves the overall reaction performance. But when the initial C/ZnO molar ratio is higher than 4, the influence of C/ZnO molar ratio becomes quite small. Compared to the solid carbon, coal reduces the zinc oxide at a lower temperature, but the final temperature is higher for coal. The activation energy of ZnO reduction by Zaozhuang coal is about 416.2 kJ/mol, the best fitting model is the third-order model F3. The temperature range for ZnO reduction by Zaozhuang coal is 820-1264℃. The main contents of product gas are Zn, CO and CO2.The mechanism and characteristic studies are taken for zinc hydrolysis by pressure thermogravimetric and fixed-bed-experimental analyses. The results demonstrate that there are two kinds of reaction mechanism in zinc hydrolysis. One is the surface reaction mechanism, the other one is the heterogeneous reaction mechanism. The later mechanism is major for the zinc with small particle size, or for the reaction at a low temperature. As the particle size of zinc increases and the temperature increases, the surface reaction mechanism becomes more and more important. The kinetic studies conclude that the activation energy of zinc hydrolysis is about 109.1 kJ/mol, the best fitting model is one-dimensional diffusion controlling model D1. In the fixed bed experiments, when the accumulating thickness of zinc powder is too large, the inner diffusion of steam becomes the controlling step of hydrolysis reaction at low temperature. Altogether, the zinc conversion increases as the heating rate increases, as the system pressure decreases, as the particle size of zinc powder decreases, as the water partial pressure increases, and as the temperature increases in iso-temperature reaction cases. The dispersed zinc powder system is more favorable than accumulated zinc powder system for zinc hydrolysis process. A zinc conversion of more than 99% could be got in the experiment. The results of iso-temperature experiments show that the initial temperature of zinc hydrolysis is about 410℃. And the initial temperature increases as the heating rate increases and as the water partial pressure decreases. The impurities have great influences on the zinc hydrolysis reaction. Generally speaking, low concentrations of Al2O3, CuO and Zn(OH)2 could increase the zinc conversion, while Fe2O3, CaO, ZnCl2, Zn(NO)3 and Zn(SO4)2 could suppress the zinc hydrolysis. The MnO2 and MgO seems have little influences on the zinc conversion.A new kind of hydrogen production system with coal gasification and a new near zero emission clean coal energy utilization system based on Zn/ZnO redox pair via a two-step thermochemical quasi-cycle are both constructed and evaluated on energy efficiency and environmental influence. Results show that the hydrogen production quasi-cycle based on Zn/ZnO can get a thermal efficiency as high as 95%, and an exergy efficience as high as 90%(based on the low heating value of product gases). The hydrogen production system can get a thermal efficiency as high as 89% for auto-thermal system, and 67% for solar heating system, and the relative exergy efficiencies for which are 80% and 66% respectively. For the new near zero emission clean coal energy utilization system, ignoring the energy penalty of the CO2 sequestration, the new energy utilization system can get a thermal electric efficiency as high as 65% for auto-thermal system and 62% for solar heating system. And overall, for both two kinds of studied systems in this paper, the solar heating system seems more environmentally favorable than the others, but the heat collection efficiency of the solar heat collection system should be promoted for the improvement of the whole system efficiency.