Comparison of carbon formation boundary in different modes of solid oxide fuel cells fueled by methane

A detailed thermodynamic analysis is employed as a tool for prediction of carbon formation boundary for solid oxide fuel cells (SOFCs) fueled by methane. Three operating modes of SOFCs, i.e. external reforming (ER), indirect internal reforming (IIR) and direct internal reforming (DIR), are considered. The carbon formation boundary is determined by finding the value of inlet steam/methane (H2O/CH4) ratio whose equilibrium gas composition provides the value of carbon activity of one. It was found that the minimum H2O/CH4 ratio requirement for which the carbon formation is thermodynamically unfavorable decreases with increasing temperature. For SOFCs with the oxygen-conducting electrolyte, ER-SOFC and IIR-SOFC show the same values of H2O/CH4 ratio at the carbon formation boundary, independent of the extent of electrochemical reaction of hydrogen. In contrast, due to the presence of extra H2O from the electrochemical reaction at the anode chamber, DIR-SOFC can be operated at lower values of the H2O/CH4 ratio compared to the other modes. The difference becomes more pronounced at higher values of the extent of electrochemical reaction. For comparison purpose, SOFCs with the hydrogen-conducting electrolyte were also investigated. According to the study, they were observed to be impractical for use, regarding to the tendency of carbon formation. Higher values of the H2O/CH4 ratio are required for the hydrogen-conducting electrolyte, which is mainly due to the difference in location of water formed by the electrochemical reaction at the electrodes. In addition, with this type of electrolyte, the required H2O/CH4 ratio is independent on the SOFC operation modes. From the study, DIR-SOFC with the oxygen-conducting electrolyte seems to be the promising choice for operation.