SOFC Modeling Considering Internal Reforming by a Global Kinetics Approach

Abstract

Fuel cells (FCs) are promising for future energy systems, since they are energy efficient and fuel can be produced locally. When hydrogen is used as fuel, there are no emissions of greenhouse gases. In this study a two dimensional CFD (COMSOL Multiphysics) is employed to study the effect from porous material surface area ratio on reforming reaction rates and gas species distributions for an anode-supported solid oxide fuel cell (SOFC). FCs can be considered as multifunctional energy devises, combining (electro-) chemical reactions, heat exchange, gas- and ionic transport. All these functions are strongly integrated, making modeling an important tool to understand the couplings between mass-, heat-, momentum transport and chemical reactions. Steam reforming takes place at the nickel material surfaces in the anode and water-gas shift reaction occurs where fuel gas is present. Benefit from the internal reforming is that the energy conversion efficiency will be higher, compared to the case of pure hydrogen as fuel.

Analysis of Microscopic Anode Structure Effects on an Anode-Supported SOFC Including Knudsen Diffusion

“…λ is the second viscosity, and for gases it is normally assumed as: λ=-2μ/3 (11). The densities and viscosities for the participating gases are dependent on local concentration and temperature, as described in (7)(8). The gas inlet velocities are defined as a laminar flow profile.…”

“…In this case, the intermolecular collisions will be dominated. For a multi-component gas mixture system, the diffusion coefficients are calculated by the expressions in (7)(8).…”

“…S i , the source term by the chemical reactions, is only defined for the internal reforming reactions, because the electrochemical reactions are assumed to take place at the interfaces between the electrolyte and the electrodes. The ordinary diffusion coefficients are also dependent on the local temperature, as described in (7)(8).…”

“…Heat is transferred between the phases at the channel walls and in the porous electrodes. The governing equations for the transport of heat are defined and explained in (7)(8). The inlet gas temperature is defined by the operating temperature (1000 K) and the outlet one is defined as a convective flux.…”

“…The charge transfer equations are not solved in this study, however effects of the ohmic-, the concentration-and the activation polarization losses are included in the governing equations for heat transport by interface or source terms. The equations for polarization losses and exchange current density are described in (7)(8).…”