Effect of methane slippage on an indirect internal reforming solid oxide fuel cell

Abstract

Creation of an autothermal system by coupling an endothermic to an exothermic reaction demands matching the thermal requirements of the two reactions. The application studied here is the operation of a solid oxide fuel cell (SOFC) with both direct (DIR) and indirect (IIR) internal reforming of methane. Such internal reforming within a high-temperature fuel cell module can lead to an overall autothermal operation which simplifies the system design and increases efficiency. However, such coupling is not easy to achieve because of the mismatch between the thermal load associated with the rate of steam reforming at typical SOFC temperatures and the local amount of heat available from the fuel cell reactions. Previous results have shown that the use of typical metal-based (e.g. Ni) IIR catalysts leads to full methane consumption but undesirable local cooling at the reformer entrance and the use of less active IIR catalysts (e.g. non-metals or diffusion limited nickel) leads to methane being carried-over into the SOFC anode (methane slippage). In order to evaluate performance in the latter case, a combined DIR and IIR SOFC steady-state model has been developed. Simulation results have shown that, lowering the IIR catalyst activity to prevent local cooling effects at the reformer entrance is not adequate, as the fast kinetics of the direct reforming reaction then lead to full methane conversion and steep temperature gradients in the first 10% of the fuel channel length. It is shown that the simultaneous reduction of the anode DIR reaction rate improves performance considerably. The system behaviour towards changes in current density, operating pressure, and flow configuration (counter-flow vs. co-flow) has been studied. Reduction of both DIR and IIR catalyst activity combined with a counter-flow operation leads to the best performance. System performance with an IIR oxide-based catalyst is also evaluated.