Der Einfluss von Wasserdampf auf den Sauerstofftransport in keramischen Hochtemperaturmembranen

Ceramic high temperature oxygen transport membranes (OTMs) represent a promising alternative for the extraction of oxygen from ambient air compared to conventional high-energy intensive processes like cryogenic air separation. Especially materials with a mixed ionic and electronic conductivity (MIEC) are of current interest as O2- -ion conductors. The high number of vacancies and the high mobility for electrons at sufficient high temperature and partial pressure gradient, enables these materials to transport oxygen via the crystal lattice and achieve a 100 % selectivity. Due to their high conductivities, mostly perovskites or a combination of fluorite- and spinel-phases are used as OTM-materials. Depending on the application, OTMs can be used either directly for the oxygen production or in a so-called membrane reactor where chemical reactions are controlled by selective oxidation of particular reactants. This work examines the so-called “oxyfuel-combustion”, where fossil fuels like coal are combusted under pure oxygen atmosphere in a power plant, to reduce \NOx-emissions and make the combustion more efficient. A OTM-module implemented in the power plant should provide the needed amount of oxygen. The flue gas of such a oxyfuel power plant consist, except for H2O and traces of SO2, of a pure CO2-stream which can be directly used for Carbon Dioxide Capture and Storage. To reduce the high temperatures occurring in the oxyfuel-combustion and to flush the oxygen from the membrane module, usually recycled flue gas is used. Unfortunately some of the most promising OTM-materials show degradations facing CO2 and CO from the flue gas. Therefore a new concept was invented, where water vapor is used to sweep the membrane instead of the aggressive flue gas. On this point the current work deals with lab-scale experiments on different membrane materials in a permeation measurement setup with humidified sweep gas. Several OTM-materials are investigated concerning their long-term stability and the degradation of the permeation performance while using different amounts of water vapor in the sweep. All tested materials show a decreasing permeation rate for oxygen with increasing water content. The reason for this declined performance can be explained by miscellaneous materials corrosion and degradation mechanisms, which are discussed in the results part