Modeling of Pressurized Electrochemistry and Steam-Methane Reforming in Solid Oxide Fuel Cells and the Effects on Thermal and Electrical Stack Performance

Abstract

Executive SummaryThis report summarizes work done to extend the electrochemical performance and methane reforming submodels to include the effects of pressurization and to demonstrate this new modeling capability by simulating large stacks operating on methane-rich fuel under pressurized and nonpressurized conditions. Pressurized operation boosts electrochemical performance, alters the kinetics of methane reforming, and affects the equilibrium composition of methane fuels. This work developed constitutive submodels that couple the electrochemistry, reforming, and pressurization to yield an increased capability of the modeling tool for prediction of solid oxide fuel cell (SOFC) stack performance.The electrochemistry model was advanced to characterize the increased SOFC performance due to diminished activation polarization. The activation polarization depends upon the exchange current density between the electrodes and electrolyte. The exchange current density is increased by elevated pressure resulting in decreased polarization loss. The Nernst potential is also increased with pressure, and, when augmented by the decreased polarization, provides increased cell voltage and stack power.The operating pressure has competing effects on reformation; it increases the methane conversion rate while also increasing the equilibrium methane concentration. The methane conversion rate expression developed by this work accounted for these effects. The rate expression used equilibrium gas compositions for temperatures ranging from 650° to 850°C and pressures of 1 to 10 atmospheres, and incorporated the steam-methane reaction equation. The gas compositions were used to calculate the equilibrium constant, and the reforming reaction equation established the form of the forward and backward terms. The combination of these elements resulted in an expression of the overall conversion rate that correctly accounts for the pressurization effect on the kinetics and the reverse reaction.The models accounting for the pressure effects on the electrochemistry and methane reforming were applied in stack simulations to examine how they can potentially change the distributions of fuel, air, current density, temperature, and reforming rates within large area stacks. An ordinary 20x20-cm cross-flow stack model geometry was used as a generic example of a "large" planar stack to demonstrate how the modeling tool can be applied to any proposed stack design for a similar analysis. We expect differences in the scalar distributions depending upon the size and configuration of the stack. We also expect the trends of pressurization effects on the reforming rates, temperatures, current density, gas concentrations, and pressure drops would be the same for any other planar stack under pressurized operation. Cases of operating pressures ranging from 1 to 10 atmospheres and 2 reformation rates differing by a factor of 10 were simulated. The predictions showed consistently increased electrical performance for both reforming rate cases with increased operating p...