Recently, the Laboratory of Distributed Energy and Renewable Energy of the Institute of Engineering Thermophysics of the Chinese Academy of Sciences has discovered an endothermic method for recovering waste heat and unreacted gases downstream of a thermochemical cycle reactor by using an endothermic reaction of fossil fuels (such as CH4). For example, the method of H2O or CO2) and the solar thermal chemical polygeneration system based on methane reforming are proposed.
The use of concentrating solar heat to drive the thermochemical cycle of the endothermic reaction is an important solar energy storage technology. The two-step thermochemical cycle catalyzed by ceria and its derivatives is able to effectively decompose H2O and CO2 into H2 and CO have received extensive attention, and both are important raw materials for the production of ammonia, methanol, and liquid hydrocarbon fuels. The decomposition process of hydrazine-based H2O and CO2 is as follows: First, the valence of the metal oxide is reduced by solar energy at a high temperature to release oxygen atoms; then water or CO2 is reduced at a low temperature by a reduced metal or metal oxide to generate H2 and CO . Solar thermal chemical isothermal oxidation reaction and reduction reaction temperature are the same. In comparison, since there is no temperature difference, the use of gas phase heat recovery is simpler than solid heat recovery. Efficient heat recovery and reduced energy consumption are key to improving the thermochemical cycle efficiency of the isothermal process. The gas phase mixture downstream of the oxidation process contains a large amount of sensible heat, and this heat recovery can be used to heat the CO2 and H2O feed on the feed side.
At present, the heat exchanger technology above 1100°C is not yet mature, and the energy of the gas phase mixture downstream of the isothermal oxidation process cannot be fully utilized. Researchers at the Institute of Engineering Thermophysics have studied the direct exchange of heat exchangers in alternative heat exchangers and have proposed an endothermic and unreacted gas (eg, H2O) recovered from the downstream of an isothermal thermochemical reactor by using an endothermic reaction with fossil fuels (such as CH4). Or CO2) method. The temperature of the mixed gas after the isothermal oxidation reaction can be reduced to 600-850° C. and the calorific value of the mixed gas can be increased. The integration of fossil fuels downstream of an isothermal thermochemical reactor can further increase syngas production and use solar energy. Compared to direct fossil fuel reforming or direct combustion, the new method has the advantage of producing low carbon emissions per calorific fuel due to the combination of upstream isothermal thermochemical cycle reactions. On this basis, researchers have proposed a methanol-based polygeneration system based on methane reforming, further demonstrating the advantages of chemical heat recovery methods. For the decomposition process of CO2 and H2O at 1600 °C, the solar-to-syngas conversion efficiency of the new system is 45.7% and 38.1%, respectively, while the efficiency of the isothermal thermochemical cycle without methane integration and without heat recovery is only 21.0. % and 6.4%. Simultaneous decomposition of CO2 and H2O in combination with methane reforming can achieve a tunable CO/H2 ratio to meet different chemical synthesis processes. The fossil fuel consumption required for the production of unit mass of methanol is approximately 22 GJ/ton, which is lower than the energy consumption of typical industrial production processes. The best conversion efficiency from solar energy to methanol can be as high as 44%.
The above work has been supported by the Youth 1000 Project and the Innovation Cross-Team project. The relevant research results have been published in the international journal Applied Thermal Engineering (Applied Thermal Engineering).
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