Thermochemical stability of LaxSr1-xCoyFe1-yO3-δ and NiFe2O4-Ce0.8Tb0.2O2-δ under real conditions for its application in oxygen transport membranes for oxyfuel combustion

This work addresses the thermochemical stability of ceramic materials –typically used in oxygen transport membranes– under the harsh gas environments found in oxyfuel combustion processes. Specifically, a dual-phase NiFe2O4-Ce0.8Tb0.2O2-δ (NFO-CTO) composite and a single-phase La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were studied. The effect of the main contaminants present in this kind of processes (CO2, SO2 and H2O) has been tested. NFO-CTO composite remains stable under all the conditions studied whereas LSCF presents a poor stability in the presence of CO2 and SO2. Regardless of the treatment, NFO-CTO conserves its crystalline structure, without giving rise to new species due to segregation or incorporation of sulphur and/or carbon. On the contrary, LSCF is prone to degradation in contact with CO2 and SO2, segregating Sr in the form of SrCO3 and SrSO4 and Co and Fe in the form of CoO and Fe3O4. It is also shown that SO2 interaction with LSCF is stronger than in the case of CO2. A concentration of just 2000 ppm of SO2 in CO2 is enough to subdue the formation of SrCO3, promoting the segregation of Sr only in the form of SrSO4. With the results presented in this work, it is possible to conclude that the NFO-CTO is a suitable candidate from the thermochemical viewpoint to be used as membrane material in 4-end modules for oxygen generation integrated into oxyfuel combustion processes whereas the use of LSCF should be dismissed.