Analysis of a Solid Oxide Fuel Cell System with Low Temperature Anode Off-Gas Recirculation

The daily growing global energy consumption has to be provided in the future by technologies which emit increasingly less greenhouse gases. This requires highly efficient energy systems. Methane fueled SOFC systems are already able to produce electricity with an efficiency up to 60 %. Through the combined use of produced electricity and thermal energy the overall efficiency can be above 90 %. One possibility for a further improvement of the electrical efficiency is the implementation of an anode off-gas recirculation loop. Challenging is the high anode off-gas temperature of at least 700 °C, which prohibits the use of commercial components. Therefore, the use of ejectors and blowers with or without anode off-gas cooling are discussed in literature. At Forschungszentrum Jülich an anode off-gas recirculation loop operating at temperatures up to 200 °C is favored. To analyze the effect of anode off-gas recirculation, a dynamical model of the system including the recirculation loop was implemented in Matlab/Simulink®. The results show, that the recirculation rate has a huge effect on the electrical efficiency (both positive and negative). In principle, at constant current density high recirculation rates decrease the cell voltage and increase the power consumption of the recirculation blower. Therefore, the highest electrical efficiency can be reached with high systems fuel utilizations, low recirculation rates and in consequence high stack fuel utilizations. On the other hand less amount of steam is available for the reforming reaction at low recirculation rates, therefore the minimum recirculation rate is determined by the formation of carbon. An optimal operation range to avoid carbon formation and to ensure a high electrical efficiency requires recirculation rates between 65 and 70 % and system fuel utilizations above 90 %. This leads to acceptable stack fuel utilizations in the range of 70 to 75 % and electrical efficiencies above 60 %.