Dual Phase Oxygen Transport Membrane for Efficient Oxyfuel Combustion

Oxygen transport membranes (OTMs) are attracting great interest for the separation of oxygenfrom air in an energy efficient way. A variety of solid oxide ceramic materials that possess mixed ionic and electronic conductivity (MIEC) are being investigated for efficient oxygen separation (Betz '10, Skinner '03). Unfortunately these materials do not exhibit high degradation stability under harsh ambient conditions such as flue gas containing C$_{2}$, SO$_{x}$, H$_{2}$O and dust, pressure gradients and high temperatures that are typical in fossil fuel power plants. For this reason, dual phase composite membranes are developed to combine the best characteristics of different compounds to achieve high oxygen permeability and sufficient chemical and mechanical stability at elevated temperatures. In this thesis, the dual phase membrane Ce$_{0.8}$Gd$_{0.2}$O$_{2-\delta}$ - FeCo$_{2}$O$_{4}$ (CGO-FCO) was developed after systematic investigation of various combinations of ionic and electronic conductors. The phase distribution of the composite was investigated in detail using electron microscopes and this analysis revealed the phase interaction leading to grain boundary rock salt phase and formation of perovskite secondary phase. A systematic study explored the onset of phase interactions to form perovskite phase and the role of this unintended phase as pure electronic conductor was identified. Additionally optimization of conventional sintering process to eliminate spinel phase decomposition into rock salt was identified. An elaborate study on the absolute minimum electronic conductor requirement for efficient percolation network was carried out and its influence on oxygen flux value was measured. Oxygen permeation measurements in the temperature range of 600 °C - 1000°C under partial pressure gradient provided by air and argonas feed and sweep gases are used to identify limiting transport processes. The dual phase membranes are much more prone to surface exchange limitations because of the limited length of the active triple phase boundaries. A porous catalytic layer made of a single phase MIEC material, i.e. LSCF, showed evidence of these limitations even when using 1 mm thick samples. The dual phase composites were also subjected to thermo-chemical stability in flue gas conditions and mechanical stability under high pressure applications. Microstructure variation based on different powder synthesis routes of the composite impacting oxygen permeation has been investigated. On the other hand, microstructure variation via alternate densification/ sintering techniques such as hot pressing and SPS/FAST were also explored. The finalized dual phase composition was developed into thin film supported membrane layers. An oxygen flux of 1.08 ml cm$^{-2}$ min$^{-1}$ was achieved on an asymmetric membrane at 1015 °C successfully. However, impregnation of catalysts into the porous support can significantly improve the oxygen flux at lower temperatures, overcoming the surface limitations at the interface between the support and dense membrane