CaCO₃/CaO Thermochemical Heat Storage Performance Of Ca-based Materials Under High Carbonation Pressure

Due to the characteristics of high energy density,low energy loss,and inexpensive material,high temperature thermochemical heat storage based on CaCO5/CaO cycles has been deemed as one of the most promising technology used in concentrated solar power plants.It is the main drawback of this technology that the heat storage capacities of conventional Ca-based materials decrease gradually with cycle numbers.The heat storage capacities of Ca-based materials,as well as the carbonation pressure and temperature are key factors associated with the overall efficiency of concentrated solar power plant.A lot of investigations have been performed for improving the heat storage performance and cyclic stabilities of Ca-based materials.The usual method is doping inert supports into Ca-based materials by some sample mixing methods,and the heat storage performance of modified Ca-based materials still has much room for improvement.In addition,most of the studies are carried out under atmospheric carbonation pressure,which is different from the actual industrial operation condition.This work is devoted to investigating the heat storage performance of the conventional and modified high-active Ca-based materials under high carbonation pressure by experimental approaches and microstructure analysis.The using of industrial waste Ca-based material,carbide slag,as heat storage materials in concentrated solar power plants was proposed,and a pressurized dual fixed-bed reactor was established.Additionally,the heat storage performances of the limestone and the carbide slag under high carbonation pressure(>1.0 MPa)during CaCO3/CaO cycles were studied.The heat storage capacities of the limestone and carbide slag are improved significantly with increasing the carbonation pressure.The effective conversion and heat density of the limestone carbonated under 1.3 MPa are about 0.79 and 2511 kJ/kg after 10 cycles,respectively,which are 1.76 times as high as those carbonated under 0.1 MPa.The high carbonation pressure can restrain the growth of CaO grains and mitigate the sintering and pore-plugging of CaO,and thus the Ca-based material possesses a more porous microstructure during cyclic heat storage under high carbonation pressure,which is beneficial for heat storage.Although the heat storage performance of carbide slag is lower than that of limestone,the carbide slag has more stable heat storage performance during long-term cycles.Carbide slag is a promising Ca-based heat storage material from the perspectives of economics and environmental protection.In order to retard the decline in heat storage capacity of conventional Ca-based material during the multiple CaCO3/CaO heat storage cycles,the idea of synthesizing high-performance Ca-based heat storage material by co-doping inert support and catalyst was proposed.A CaO/Al2O3/CeO2 composite was fabricated by wet-mixing method.The main compositions of the synthetic material are CaO,Ca12Al14O33 and CeO2.After 30 heat storage cycles under the carbonation pressure of 1.3 MPa,the effective conversion and the heat density of the composite with the addition of 5 wt.%Al2O3 and 5 wt.%CeO2 are 0.79 and 2500 kJ/kg,respectively,which are 33%higher than those of the limestone.The excellent heat storage performance of CaO/Al2O3/CeO2 composite is attributed to the good support of Ca12Al14O33,which improves the sintering resistance of Ca-based materials;the catalytic function of CeO2,which improves the carbonation reactivity of Ca-based materials;and the facilitation from high carbonation pressure.When the CaO/Al2O3/CeO2 composite is used as heat storage material,the operation cost of concentrated solar power plant can be reduced to 321 RMB/MWth.The CaO/Al2O3/CeO2 composite is a promising material for heat storage in concentrated solar power plants.In order to further improve the heat storage performance of CaO/Al2O3/CeO2 composite,the microstructure of the composite was optimized,and a microtubular CaO/Al2O3/CeO2 composite with high specific surface areas and porosity was synthesized by template method.The obtained material features a hollow microtubular structure,and the diameter of the microtubule is approximately 5 μm,as well as the thickness of the microtubule wall is approximately 1.5 μm.The microtubular structure increases the contact area between CaO and CO2,as well as reduces CO2 diffusion resistance in Ca-based material,which is beneficial for heat storage.After 30 heat storage cycles under the carbonation pressure of 1.3 MPa,the effective conversion and the heat density of the microtubular composite with the addition of 2.5 wt.%Al2O3 and 1 wt.%CeO2 are 0.92 and 2924 kJ/kg,respectively,which are 24%and 56%higher than those of the composite without the addition of template and the limestone,respectively.Cal2Al14O33 and CeO2 can not only improve the sintering resistance of CaO,but also stabilize the microtubular structure,so as to maintain a high specific surface areas and pore volumes of the CaO/Al2O3/CeO2 composites,which contributes to the efficient heat storage capacity and high cyclic stability of synthetic Ca-based materials.