Long term stability and permeability of mixed ion conducting membranes under oxyfuel conditions

The thermochemical properties, especially the long-term behaviour of mixed electronion conducting materials for oxygen separation, were investigated under oxyfuel condition in this thesis. Amongst those oxygen-permeable materials, perovskite-type oxides demonstrate remarkably high oxygen fluxes. Nevertheless, great effort has been put into the investigation of the long-term sustainable problem occurring in the intermediate temperature (IT) range of 500-800 °C in some perovskite membranes, which is caused by thermodynamic decomposition. This decay of membrane properties during operation becomes a serious obstacle for applications in coal-fired power plants. Besides, a relatively low expansion coefficient of membrane materials is also suggested to avoid compatibility issues. Perovskite-structured BaxSr1-xCoyFe1-yO3-d (BSCF) materials were synthesized by the method of solid state reaction. The sintering behaviour of BSCF powders was studied prior to the production of the gastight membranes. In view of future application, characteristic membrane properties like melting temperature, oxygen nonstoichiometry and thermal expansion behaviour were measured in synthetic air accordingly. The association between the thermochemical properties and the doping compositions of BSCF materials as well as temperature effect was investigated. Moreover, the degradation process in BSCF membranes during long-term operation was studied under oxyfuel condition. In the case of oxygen permeation measurements, the variation of oxygen flux through the membranes under the air/He pressure gradient was recorded. A slow exponential decay of the permeate flux was observed at 800 °C compared to the more stabilized oxygen permeability of membranes at higher temperatures. The reason for the deterioration of membrane permeability is mainly ascribed to the phase decomposition from cubic into hexagonal polymorph. Unexpectedly, an increment of oxygen flux was found in the Ba0.4Sr0.6Co0.2Fe0.8O3-d membrane measured at 850 °C, though kinetic decomposition still occurs in this material. Based on the permeation behaviour of BSCF membranes during cyclic permeation tests, it is confirmed that the cubic-hexagonal phase transition is reversible and membrane performance could be retrieved by the periodical variation of temperatures above 850 °C. The driving force as well as kinetics for the phase transition is discussed in this study according to the long-term annealing measurements.