Energy & Fuels, Vol.34, No.3, 3449-3457, 2020
Improvement of the Oxygen Uncoupling Properties of Copper-Based Composite Oxygen Carriers for Chemical Looping Air Separation
To increase the oxygen concentration in the product gas of a chemical looping air separation (CLAS) system, it is important to reduce the oxygen uncoupling temperature of the oxygen carrier (OC). In this paper, it is investigated whether and to what extent the addition of Mn2O3, Co3O4, Pb3O4, and Cr2O3, all of which have oxygen uncoupling propensity according to thermodynamics, to CuO improves the oxygen uncoupling behavior of the mixed-oxide OC at lower temperatures. Using thermogravimetric analyses, it is shown that the oxygen uncoupling temperature of a Cu-Mn composite OC is decreased by approximately 200 degrees C relative to a Cu OC. In addition, the oxygen transport capacity of this composite OC can reach up to 0.056 g O-2/g OC. Density functional theory calculations have been carried out to analyze the changes in reactivity and stability of the mixedoxide OC. When a Mn3O4(001) surface adsorbs a Cu4O4 cluster, the average bond length of Cu-O in the Cu4O4 cluster increases from 1.983 to 2.047 angstrom and the charge is transferred from Cu4O4 to Mn3O4(001). Chemical reaction between the cluster and the surface occurs, and Cu-Mn composite oxides are formed after adsorption. Moreover, the adsorption energy is -3.749 eV, which is higher than that of Cu4O4 on a CuO(111) surface (-2.92 eV), and the composite OC has better stability and sintering resistance than the Cu OC. The stability of the composite OC was also investigated at 900 degrees C under reducing conditions and at 700 degrees C under oxidizing conditions. The results show that the Cu-Mn/Zr composite OC remains stable over 15 consecutive cycles. Moreover, the Cu-Mn composite does not only reduce the oxygen uncoupling temperature of CuO but it also overcomes thermodynamic limitations of the oxidation of Mn3O4 to Mn2O3, where the redox reaction of the Cu-Mn composite OC can be expressed as follows: CuxMn3-xO4 reversible arrow CuxMn2-x + O-2(g). From the results, it can be concluded that Mn2O3 is the most suitable composite oxide to improve the oxygen uncoupling properties of a Cu OC for oxygen uncoupling in the CLAS system.