Keramische Sauerstoffmembranen in 4-End-Oxyfuel Prozessen

To curb global warming in the coming decades the current CO2 emissions have to be re-duced considerably. A large portion of the CO2 emitted into the earth's atmosphere stems from the intensive use of fossil fuels in the energy and industrial sector. A promising solution for the reduction of CO2 emissions of power plants and in the industrial sector is the retrofitting with the oxyfuel technology that allows the direct capture and storage of CO2. Ceramic oxygen membrane modules are being developed to cover the high oxygen demand of oxyfuel plants. In the 4-end oxyfuel mode Membrane modules utilize the heat and low oxygen partial pressure of the flue gases to efficiently separate oxygen from air. This requires membrane modules that retain a stable and high oxygen flux during direct contact with aggressive process gas components such as CO2 and SO2. In this research work the suitability of ceramic oxygen membranes for the use in 4-end oxyfuel plants is studied. For this purpose the stability of single and dual phase membrane materials in SO2-containing atmospheres was investigated and the oxygen flux of the membranes in contact with SO2 was determined.In the first part of this research single-phase membranes are studied. It was shown that the membrane material La1-xSrxCo0.2Fe0.8O3-δ (LSCF) degrades at 850 °C in the presence of as few as 2 ppm of SO2, leading to the formation of SrSO4. During the presence of 100 and 500 ppm SO2 in sweep gas the oxygen flux of a LSCF membrane was reduced by 88% and 93% respectively, due to the adsorption of SO2 on the membrane surface. In addition after the SO2- contact, the flux was irreversibly lower due to the formation of SrSO4. The single-phase membrane material La0.6Ca0.4Co0.8Fe0.2O3-δ (LCCF) also reacted with SO2, forming CaSO4.In the second part the dual phase membrane material Ce0.8Gd0.2O2-δ-Fe1Co2O4 (CGO-FCO) was investigated. No SO2-related degradation and sulfate formation could be observed in contact with SO2. A cation diffusion of cobalt and iron towards the side with the higher oxygen partial pressure was observed during the permeation experiments in an oxygen partial gradient. The higher mobility of cobalt led to the segregation of the spinel phase, whereby cobalt rich crystals were formed on the surface of the feed side and pores were formed on the sweep side. As a result the effective exchange surface increased consequently, leading to a higher oxygen flux of the membrane. The addition of 50-500 ppm SO2 to the sweep gas resulted in an abrupt reduction of the oxygen flux by up to 64 %, due to the adsorption of SO2. The higher the SO2 concentration, the lower was the oxygen flux of the CGO-FCO membranes.This work shows that CGO-FCO is a possible membrane material for 4-end oxyfuel plants. In contrast, the investigated single-phase membranes of LSCF and LCCF proved to be less suitable