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Abstract:
Due to their high operating temperature some fuel cells like the SOFC and the MCFC offer the possibility to perform steam reforming within the cell stack (fig. 1). This option reduces the required size of an external reforming vessel or even makes this reactor redundant. It also implies the possibility to reach higher degrees of conversion compared to an external reformer/ fuel cell combination, which is equilibrium limited [1]. The trade-off is a more complex behaviour of the stack and therefore a more difficult system design. In Germany, the company MTU Friedrichshafen has developed a 250 kW MCFC system called HotModule. This system consists of a 340 cell stack and is operated with external, indirect internal and direct internal reforming [2]. At the University and the MPI in Magdeburg the HotModule is investigated using dynamic mathematical models. Among other things the models allow to simulate the temperatures in the gas and solid phase of the fuel cell [3]. Thereby the reforming plays an important role. One aspect is the distribution of the reforming catalyst inside the reforming channel and the anode channel. This distribution is strongly related to the distribution of heat sources and sinks and therefore determining the temperature field within the cell. This again is crucial to efficiency and life time of an MCFC. On the one hand, as the steam reforming reaction is endothermic, cold spots must be avoided, while on the other hand hydrogen concentration should not be too low at any location within the anode channel. Model based optimisation offers the possibility to improve the catalyst proportioning within the fuel cell.
