Thermochemical stability of sulfur compounds in fuel cell gases related to fuel impurity poisoning

Since sulfur compounds are the major poisons of fuel cell systems, equilibrium concentrations of minor sulfur-based impurities in fuel cell fuels are thermochemically calculated in the temperature range between 400°C and 1000°C. As sulfur-based impurities in fuel cell gases, H₂S(g), elementary sulfur, inorganic sulfur compounds, mercaptans, alkyl (di-)sulfides, thiophenes, and related compounds have been taken into account. Various types of fuels are also considered, including H₂, H₂,-CO, CO, CH₄, biogas, liquidified petroleum gas, gasoline, kerosene, and diesel fuel. Among the 21 kinds of sulfur-based typical impurities considered, H₂S(g) is the most stable sulfur-based species. COS(g) can also coexist, but even in CO-rich gases and in hydrocarbon-based fuels, COS concentration in equilibrium is one order or a few orders of magnitude lower than H₂S concentration. Other sulfur compounds, such as CH₄S(g) at intermediate temperatures and HS(g) and SO₂(g) at high temperatures, are also expected to coexist but their concentrations are less than 1 ppb (parts per billion) assuming thermochemical equilibrium.