Study On Cu-based Oxygen Carriers For Chemical Looping With Oxygen Uncoupling

The Chemical-Looping with Oxygen Uncoupling (CLOU) process is a novel Chemical-Looping Combustion (CLC) technology that allows the combustion of solid fuels with inherent CO2separation using oxygen carriers. The selection of suitable oxygen carriers, which has the ability to release O2at suitable temperatures, is a key factor for the CLOU process. The basic concept of CLOU and its application in O2-CO2production and pure O2production are reviewed. The research progress of oxygen carriers for CLOU has been systematically summarized and the new research advances in recent years are highlighted. Six Cu-based oxygen carriers were prepared by the sol-gel method and the co-precipitation method. XRD was used to study their compositions and SEM was used to observe their micro-structures.The reaction rates for reduction and oxidation of ARCu, COCu, SGCu prepared by different methods were carried out successive cycles in a TGA system at900℃. CuO prepared by sol-gel process was identified as a suitable material for CLOU process, which exhibits better reaction characteristics and sustainable circulating capacity than the others. TGA experiments were conducted to study the absorption and desorption characteristics of5%CeO2/CuO,5%Y2O3/CuO,5%ZrO2/CuO oxygen carriers prepared by sol-gel method. The research results indicate that the reaction rates and the sustainable circulating capacities of the oxygen carriers with5wt.%CeO2/Y2O3/ZrO2dropped in CuO particles increase by a large margin, which are identified as suitable materials for CLOU process.The mechanism functions and kinetics parameters of three different CuO particals and three kinds of composite Cu-based oxygen carriers were calculated and characterized by the non-isothermal kinetics method. The kinetic analysis shows that the oxygen release of CuO can be described by the nucleation and nuclei growth model, while the oxidation of Cu2O can be described by the shrinking core model. The mechanism function of oxygen decomposition is the J-M-A function, which described as G(a)=[-In(1-a)]1/2,f(a)=2(1-a)[-In(1-a)]1/2. The activation energy values of ARCu, COCu, SGCu oxygen carriers are208.96kJ/mol,176.61kJ/mol,171.32kJ/mol, respectively, while the pre-exponential factor values,InA, are11.84s-1,9.21s-1,10.06s-1of oxygen generation respectively. In the stage of oxygen regeneration, the mechanism function is the phase interface function, which described as G(a)=1-(1-a)1/3, f(a)=3(1-a)2/3. The activation energy values of ARCu, COCu, SGCu oxygen carriers are147.93kJ/mol,106.56kJ/mol,91.87kJ/mol, respectively, while the pre-exponential factor values,InA, are12.13s-1,7.91s-1,5.76s-1respectively in the stage of oxygen regeneration. The activation energy values of composite Cu-based oxygen carriers5%CeO2/CuO,5%Y2O3/CuO,5%ZrO2/CuO are in the range110.33～152.62kJ/mol,155.95～167.46kJ/mol,102.06～151.32kJ/mol, respectively. The activation energy values of composite Cu-based oxygen carriers are lower than pure CuO, which indicates that composite oxygen carriers show excellent reaction characteristics and are suitable materials for CLOU process. The mechanism functions and kinetics parameters of CuO/Cu2O system were calculated and characterized by the isothermal kinetics method in the end. The activation energy and the pre-exponential factor are250.71kJ/mol and1.84×107s-1of oxygen generation, respectively, while in the stage of oxygen regeneration, the activation energy and the pre-exponential factor are76.23kJ/mol and2.05×105s-1, respectively. The mechanism functions and kinetics parameters of Cu-based oxygen carriers calculated by the isothermal kinetics method are the same as the results calculated by non-isothermal kinetics method.Several experiments were carried out in a fixed-bed reactor at different temperatures. In this way, Cu-based oxygen carriers are identified as a suitable material of CLOU process with O2generation under CO2atmosphere. The reactions between5%ZrO2/CuO and Qinghua coal coke were researched in the fixed-bed reactor. During the combustion process, the conversion rate of reduction of oxygen carriers is higher than that with no fuel in the fixed-bed reactor, which increases with the increase of reaction temperature. The results show that Cu-based oxygen carriers are approximate to CLOU for the combustion of solid fuels.