The thermodynamic method for selecting oxygen carriers used for chemical looping air separation

Chemical looping air separation (CLAS) has been suggested as a new and energy saving method for producing oxygen from air. The selection of suitable oxygen carriers is the key issue for CLAS system. This paper shows a comprehensive thermodynamic method for selecting oxygen carriers used for CLAS through studying the properties of 34 different oxygen releasing reactions referring to 18 elements at different temperatures. The research mainly includes analysis of oxygen releasing capacity by calculating the Gibbs free energy change (Delta G) and the equilibrium partial pressure of oxygen of the reduction or oxidation reaction at different temperatures. Oxygen content and transport capacity were calculated. The spontaneous reaction temperatures for oxygen releasing reactions were presented to determine the operating temperatures. Also, the minimum demand of the steam for the reduction reaction was discussed. On the basis of the comprehensive thermodynamic study, the oxide systems of CrO2/Cr2O3, PbO2/Pb3O4, PbO2/PbO, Pb3O4/PbO, MnO2/Mn2O3, and Ag2O/Ag have been found suitable for the CLAS process in low temperatures (500-800 K). The systems of PdO2/PdO, PdO2/Pd, PdO/Pd, MnO2/MnO, and MnO2/Mn3O4 were suitable for medium temperatures (800-1100 K) CLAS process. And Co3O4/CoO, CuO/Cu2O, Mn2O3/Mn3O4, and OsO2/Os systems only worked successfully in high temperatures (1100-1400 K). In addition, the CaO2/CaO system was not suitable for CLAS because of the reaction with steam. The various binders such as SiO2, TiO2, Al2O3, Y2O3, ZrO2, and YSZ which have been used for CLC could also be the supports for CLAS oxygen carriers.