Fuel flexibility in power generation by solid oxide fuel cells

Power generation characteristics of solid oxide fuel cell (SOFC) with internal steam reforming of hydrocarbons were investigated. Steam reforming reaction over a Ni-YSZ cermet catalyst attained almost the equilibrium conversion and selectivity in the fixed bed reactor at 1000 °C. The conversion of internal reforming of hydrocarbons was incomplete because of the limited contact time with a thick layer of the Ni cermet electrode. Therefore, the fuel cell supplied with pre-reforming gas to the anode always gave rise to a lower terminal voltage because of the insufficient conversion of fuel compared with that supplied with post-reforming gas at a given current density. Methane internal reforming proceeded without deterioration with time, whereas the power generation with ethane and ethylene suffered from carbon deposition even at high steam-to-carbon ratio. Carbon deposition region and equilibrium partial pressure of oxygen in the C-H-O diagram were estimated from the thermodynamic data. The effect of the gas composition in the power generation characteristics, especially, difference in reactivity between H₂ and CO, was investigated. The H₂-H₂O and CO-CO₂ fuel systems led to almost the same open circuit voltage at the same H₂/H₂O and CO/CO₂ ratios at 1000 °C, as expected from the thermodynamic equilibrium. The output voltage in a discharge condition was always higher for H₂-H₂O than for CO-CO₂ at every current density.