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 (ΔG) 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.