Thermal Use of SOFC Exhaust Gases: Practical Experiences with Non-Catalytic Systems

摘要

Within the last years major steps have been done to improve the technical progress of solid oxide fuel cells. Nevertheless, fuel utilisation of SOFC - stacks is limited by chemical and technical reasons. As a consequence a solution is needed to use the SOFC exhaust gases to increase the overall efficiency and to avoid emissions. The type of the solution is depending on the application of the SOFC - system, taking into account emission limits, operational cycle, system lifetime, space demands and cost aspects. At the Oel-Waerme- Institut investigations have been done for thermal use of SOFC exhaust gases of a small scale mobile or stationary system using liquid fuels, e.g. Diesel or heating oil for single households or as an APU. In this application cost aspects and system lifetime have a high influence on the system design and on the commercial success of a product. Therefore a simple system design using as few as possible components is a good assumption. Consequently, the exhaust gas treatment unit should be used for start up of the system including the regenerative use of heat energy in the flue gas. These requirements are leading to a wide field of operation for the exhaust gas treatment unit starting at ambivalent conditions and with high heating value and ending with high temperatured combustibles from 950 K to 1150 K and very low heating value in operation. Several possibilities for combustion of SOFC exhaust gases have been studied and tested. As result a diffusion flame burner design has shown the best compromise. The design demonstrates a stable operation for anode off-gas with the main components hydrogen, carbon monoxide, nitrogen and water with a heating value from 0,9 MJ/m3 to 4,7 MJ/m3 and was investigated for thermal load from 0,5 MW/m3 to 20 MW/m3. The requirements for exhaust gas treatment of a SOFC are described in detail, corresponding on a fuel processor for Diesel. Combustion parameters like ignition limit, burning velocity and flue gas temperature are discussed in relation to fuel utilisation and exhaust gas temperature.