Thermodynamics of Hydrogen Generation from Methane for Domestic Polymer Electrolyte Fuel Cell Systems

Low temperature fuel cells such as the Polymer Electrolyte Fuel Cell (PEFC) are preferably used for domestic applications because of their moderate operating conditions. Using the existing distribution system, natural gas is used as a source for a hydrogen rich gas to power this fuel cell type. The high requirements on the fuel gas quality as well as high conversion efficiencies for the small local gas processing units are critical aspects in the evaluation of decentralized fuel cell systems. In the present paper, three typical gas processing methods evaluated for the supply of a hydrogen rich gas for PEFCs: steam reforming, partial oxidation, and autothermic conversion. All three processes are studied in detail by varying the relevant process parameters: temperature, pressure, steam to fuel ratio, and oxygen to fuel ratio. The results are graphically displayed in numerous nomograms. With the help of these graphs, regions of stable operation and the sensitivity to the operational parameters are discussed. For all three gas processing methods, the graphs generated display methane conversion, the hydrogen yield, and the yields of unwanted components, i.e., carbon monoxide and solid carbon. Although only steady-state operating conditions were simulated, critical modes of operation, which might occur during start-up or transient operation can easily be identified. For instance, operating conditions where soot is generated have to be avoided under all circumstances. All simulations were done with the Gibbs reactor model of a commercial simulation program. The Gibb's reactor model was found to be a suitable tool, since the simulated results compared well with reported literature data. According to the simulation results, the methane-steam-reforming process appears to be favorable for application to PEFCs. Methane conversion and hydrogen yields are highest for this process while the yield of CO is relatively low.