Selection of Suitable Oxygen Carriers for Chemical Looping Air Separation: A Thermodynamic Approach

The increasing demand for oxygen combined with the need for improved economic performance necessitates the search for alternative methods of oxygen production. Chemical looping air separation (CLAS) is one of these alternatives with a relatively small energy footprint. The present paper describes the results of a comprehensive thermodynamic study conducted by our group to identify suitable oxygen carriers for CLAS at medium to low temperatures. The thermodynamic simulations were carried out using Fact-Sage 6.1 for 20 different metal oxides forming 40 oxygen carrier systems. An Ellingham diagram was developed to relate the Gibbs free energy of the relevant reactions to the temperature for all metal oxide systems. Furthermore, the equilibrium partial pressure of oxygen was calculated at elevated temperatures. The mass balance calculations were also performed for identifying the steam/CO2 requirements for the reduction reactor. On the basis of the comprehensive thermodynamic study, oxides of manganese, cobalt, and copper have been found most suitable for the CLAS process. Additionally, the possibility of carbonate and hydroxide formation during the reduction with CO2 and steam, respectively, was calculated. The formation of the mixed oxide phases or the spinel structures between the metal oxides and various supports (such as SiO2 and Al2O3) has also been thermodynamically investigated. Several other important factors were also qualitatively assessed. The Cu oxides with SiO2 and the Co oxides with Al2O3 were found to be the most suitable oxygen carriers for CLAS.