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

Creation of an autothermal system by coupling an endothermic reaction (such as steam reforming) to an exothermic oxidation reaction requires the match ing of the thermal requirements of the two reactions. The application under study is a solid oxide fuel cell (SOFC) with indirect internal steam refor ming of methane, whereby the endothermic reforming reaction in thermally co upled 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 temperatu res and the local amount of heat available for this purpose from the fuel c ell reactions. Two possible methods of achieving such coupling at SOFC oper ating conditions are the use of catalysts with non-uniform distribution of active metal within the inert support, and/or the introduction of a diffusi ve barrier placed near the outer surface of the catalyst. Optimum distribut ions of active catalyst and effective pore sizes within a reforming catalys t were determined which maintain a desired rate of reaction despite a halvi ng of the intrinsic catalyst activity because of coking. It is found that f or a spherical pellet, an "egg-yolk" distribution of active catalyst couple d with a diffusion barrier placed in the outer regions of the pellet lead t o the desired performance. Catalyst pellets with this formulation have been fabricated and tested. (C) 2001 Elsevier Science Ltd. All rights reserved.