CFD Systems Level Modeling of a Protonic Ceramic Fuel Cell

Abstract

There is a critical need for intermediate-temperature fuel cells that offer low-cost, distributed generation both at the system and device levels. Current development work at the Colorado School of Mines is being carried out for a mixed proton and oxygen-ion-conducting electrolyte that enables fuel-cell operation at temperatures less than 500°C. By using a proton- and oxygen-ion-conducting electrolyte, the fuel cell stack is able to reduce coking which clogs anodes with carbon deposits and enhance the process of turning hydrocarbon fuels into hydrogen. Today's solid-oxide fuel cells (SOFCs) are based on oxygen-ion conducting electrolytes and operate at high temperatures. Advanced mixed proton- and oxygen-ion-conducting fuel cells operate at lower temperatures, and have the capacity to utilize hydrogen, ethanol, methanol, or methane fuels. Additionally, an advanced single-step sintering process has been developed to reduce manufacturing costs of these protonic-ceramic fuel cells (PCFC). “Solid-state reactive sintering” reduces the number of manufacturing steps from 15 to 3, drastically decreasing costs. Understanding the systems-level behavior of the PCFC is critical to enable its emergence as a roll-out technology. The team at Calif. State Polytechnic Univ. at Pomona is performing numerical modeling of the PCFC system. The goal of this poster is to present results of Computational Fluid Dynamic (CFD) systems level modeling of a typical PCFC stack having a cylindrical shape factor. The configuration is modeled using commercial CFD software which can easily be benchmarked using experimental data. The flow is H2 on anode side and air on the cathode side. Flow rates are on the order of 100 sccm for each side. The interconnect between each cell is made of ferritic stainless steel (70% Fe, 27% Cr, 1% Y2O3). The operation of a PCFC is fundamentally different than SOFCs due to their ability to conduct protons as well as oxygen ions across a dense ceramic electrolyte. Two global electrochemical reactions are needed since the material is capable of conducting two charged defects. One global reaction is the result of proton conduction across the electrolyte, and the other is the result of oxygen-ion transport across the electrolyte. Similar to SOFCs, heterogeneous internal reforming of hydrocarbons such as methane and water-gas-shift reactions still occur on the nickel phase of the anode. However, since minimal water is generated in the anode from the electrochemical reactions, the reforming reactions must be supplied with an external water source, such as an external waste heat recovery boiler or via anode gas recycle. Additionally, the low temperature operation tends to shift the equilibrium composition of the fuel species toward higher methane content (i.e., lower conversion of internal reforming reaction) where hydrogen partial pressures in the anode will be limited by thermodynamics. The difference in operating temperatures and the removal of hydrogen from the anode via electrochemical oxidation can also influence the optimal steam-to-carbon ratio (S/C) required to avoid solid carbon deposition when compared to SOFCs. The above-described electromechanical kinetics are incorporated into the systems-level CFD model of the PCFC presented in this poster. Results for CFD based Multi-physics of the electro-mechanical PCFC behavior for mole fraction vs. temperature, fuel utilization vs. temperature, voltage potential vs. current density, and power density vs. current density are presented in the poster. Figure 1