Modeling of a Solid Oxide Fuel Cell Fueled by Methane: Analysis of Carbon Deposition

Natural gas appears to be a fuel of great interest for solid oxide fuel cell (SOFC) systems. It mainly consists of methane, which can be converted into hydrogen by direct internal reforming (DIR) within the SOFC anode. However, a major limitation to DIR is carbon formation within the ceramic layers at intermediate temperatures. This paper proposes a model solution using the CFD-ACE software package to simulate the behavior of a tubular SOFC. A detailed thermodynamic analysis is carried out to predict the boundary of carbon formation for SOFCs fueled by methane. Thermodynamic equilibrium calculations that take into account Boudouard and methane cracking reactions allow us to investigate the occurrence of carbon formation. This possibility is discussed from the values of driving forces for carbon deposition defined as $α=PCO2∕(KBPCO2)$ and $β=PH22∕(KCPCH4)$⁠, from the equilibrium constants $KB$ and $KC$ of the Boudouard and cracking reactions, and from the partial pressure $Pi$ of species $i$⁠. Simulations allow the calculation of the distributions of partial pressures for all the gas species (⁠$CH4$⁠, $H2$⁠, CO, $CO2$⁠, and $H2O$⁠), current densities, and potentials of both electronic and ionic phases within the anode part (i.e., gas channel and Cermet anode). Finally, a mapping of $α$ and $β$ values enables us to predict the predominant zones where carbon formation is favorable (⁠$α$ or $β<1$⁠) or unfavorable (⁠$α$ or $β>1$⁠) according to the calculation based on thermodynamic equilibrium. With regard to the values of these different coefficients, we can say that a carbon formation can be supposed for temperature less than $800°C$ and for ratios $xH2O∕xCH4$ smaller than 1.