Improving catalyst structures and reactor configurations for autothermal reaction systems: application to solid oxide fuel cells

Abstract

Creation of an autothermal system by coupling an endothermic reaction (such as steam reforming) to an exothermic oxidation reaction requires the matching of the thermal requirements of the two reactions. The application under study is a solid oxide fuel cell (SOFC) with indirect internal steam reforming of methane, whereby the endothermic reforming reaction in thermally coupled to the exothermic oxidation reaction in a single unit. 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 for this purpose from the fuel cell reactions. Two possible methods of achieving such coupling at SOFC operating conditions are the use of catalysts with non-uniform distribution of active metal within the inert support, and/or the introduction of a diffusive barrier placed near the outer surface of the catalyst. Optimum distributions of active catalyst and effective pore sizes within a reforming catalyst were determined which maintain a desired rate of reaction despite a halving of the intrinsic catalyst activity because of coking. It is found that for a spherical pellet, an “egg-yolk” distribution of active catalyst coupled with a diffusion barrier placed in the outer regions of the pellet lead to the desired performance. Catalyst pellets with this formulation have been fabricated and tested.