Dynamic effects in autothermal systems - Application to a coated-wall internally reformed solid oxide fuel cell

Creation of an autothermal system by coupling endothermic and exothermic reactions demands matching the thermal requirements of two reactions. Autothermal operation is applied here to a solid oxide fuel cell with internal methane steam reforming (IR-SOFC), for which the excess heat generated in the cell provides the heat required by the endothermic reforming reactions. However, such coupling is not easy to achieve because of the mismatch between the thermal load associated with the reforming rate at typical SOFC temperatures and the local amount of heat available from the fuel cell reactions. This paper compares the performance of a SOFC with two different internal reformers and investigates the dynamic response of a coated-wall indirect IR-SOFC to changes in current density. It is shown that a coated-wall indirect IR-SOFC design could be effective in reducing the IR reaction rate, preventing steep temperature gradients, and increasing efficiency. Dynamic simulations show that, after a positive current density step-change, the intermediate period between the disturbance imposed and the new steady-state is characterized by cell potential undershoot and hydrogen conversion and temperature increase. The SOFC unit studied has a total fuel residence time of 0.22 s, for which the response is essentially complete in 320 s.