Modeling issues for solid oxide fuel cells operating with coal syngas

Fuel cells which directly convert the chemical energy into electricity are considered one of the most promising technologies to support energy needs in the near future. Solid Oxide Fuel Cells (SOFC) can run with alternative fuels such as synthesis coal gas due to their high operating temperature. Utilization of coal syngas in fuel cells would enable cleaner coal energy. A better understanding of the chemical and transport processes when running on syngas will allow researchers to find solutions to problems currently delaying the introduction of the technology into the market. A comprehensive modeling tool is developed for simulation of SOFCs operating on a wide range of fuel mixtures such as syngas, natural gas, etc. The new code takes into account the methane reforming and water gas shift reactions occurring on a common Ni-YSZ electrode using two alternative mechanisms namely, a global mechanism and a detailed surface mechanism. Simultaneous electrochemical oxidation of hydrogen and carbon monoxide at the anode-electrolyte active interface is accounted for using a new electrochemistry model. Model validation was performed by comparing the results with data published in the literature and available experimental data obtained at West Virginia University. Also, the associated numerical uncertainty was assessed and found that this was small in general, except for the ohmic heating. A parametric analysis considering the effect of temperature, fuel composition, and activation overpotential parameters for carbon monoxide oxidation was also performed. It was found that the increase of cell performance with temperature increase was caused by a decrease in all the overpotentials. The cell performance also increases when the concentration of the fuel in the anode stream increases. The operating temperature, as well as the composition of the fuel stream have a significant effect on the direction of the water gas shift reaction as well as on the current supported by hydrogen and carbon monoxide. However, when the ratio of hydrogen concentration to carbon monoxide concentration is kept constant, the splitting of the total current between these two fuels is not affected considerably. The cell showed one limiting current even when two fuels, hydrogen and carbon monoxide, are directly and simultaneously oxidized irrespective of the inlet composition. The new computational tool is applied to a more realistic case of planar SOFC with a surface of circa 100 cm2.

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