Solid oxide fuel cell systems with an anode off-gas recirculation have the potential for high electrical efficiency. The anode off-gas recirculation within an SOFC system has two significant advantages. Firstly, a part of the unused fuel at the stack outlet is returned to the stack inlet, where it can again participate in the electrochemical reaction. Therefore, the amount of fresh fuel fed into the system can be reduced. This increases the system fuel utilization and leads to a significant increase in efficiency. Secondly, the recirculated electrochemically produced steam can be used for the steam reforming process. Thus, during operation an external steam supply is no longer necessary. A challenge is the high anode off-gas temperature of at least 700°C, which prohibits the use of commercially available blower units.

At Forschungszentrum Jülich GmbH an SOFC subsystem with an anode off-gas recirculation loop was developed and tested using methane as the fuel. The off-gas recirculation loop consists of two heat exchangers in combination with a low temperature blower, which operates at temperatures up to 200°C. With this setup, tests were carried out to study the influence of e.g. recirculation rate and fuel utilization on the system operation behavior and performance. During the tests the system fuel utilization was driven up to 93%. The test results indicate that changes in recirculation rate affect the cell voltage, the amount of excess air for cooling and the electrical efficiency. At constant current density high recirculation rates decrease the cell voltage, but also the amount of excess air. Therefore, high electrical efficiency could be achieved with high system fuel utilization at moderate recirculation rates.