A numerical study to determine proper steam-to-fuel ratio in a biogas-fueled solid oxide fuel cell for different levels of biogas methane content

Abstract

It is vital to find the optimal quantity of water added to the fuel in a biogas-fueled Solid Oxide Fuel Cell (SOFC). An under-optimal water quantity can impede steam reforming reactions, while an over-optimal quantity of water (which is equivalent to reduced biogas portion) may reduce the performance of the fuel cell due to a shortage of fuel. Water production in electrochemical reactions and the carbon deposition constraint add to the complexity of water quantity optimization. The present work develops a 3D numerical model to simulate a direct biogas-fueled SOFC to find the optimal steam/biogas (S/C) ratio. Several biogas mixtures (CH4/CO2 ratios) are analyzed. For each mixture, a wide range of S/C ratios is evaluated in a wide range of voltages. It is found that the polarization curves corresponding to different S/C ratios intersect each other for a given biogas mixture. In other words, an S/C ratio can be optimal at some voltages and non-optimal at others. Based on power density maximization as an explicit criterion, the optimal S/C ratio is found to be 0.75 at low CH4 molar fractions (~50%) and 1.25 at high CH4 molar fractions (~75%).