Applications for Molten Carbonate Fuel Cells

Sammanfattning: Molten Carbonate Fuel cells are high temperature fuel cells suitable for distributed generation and combined heat and power, and are today being installed on commercial basis in sizes from 100kW to several MW. Novel applications for MCFC which have attracted interest lately are MCFC used for CO2 separation from combustion flue gas, and high temperature electrolysis with reversible fuel cells. In the first application, the intrinsic capability of the MCFC to concentrate CO2 from the cathode to the anode side through the cell reaction is utilized. In the second application, the high operating temperature and relatively simple design of the MCFC is utilized in electrolysis, with the aim to produce a syngas mix which can be further processed into hydrogen of synthetic fuels.In this thesis, the effect on fuel cell performance of operating a small lab-scale molten carbonate fuel cell in conditions which simulate those that would apply if the fuel cell was used for CO2 separation in combustion flue gas was studied. Such operating conditions are characterized especially by a low CO2 concentration at the cathode compared to normal operating conditions. Sulfur contaminants in fuel gas, especially H2S, are known poisoning agents which cause premature degradation of the MCFC. Furthermore, combustion flue gas often contains sulfur dioxide which, if entering the cathode, causes performance degradation by corrosion and by poisoning of the fuel cell. This makes poisoning by sulfur contaminants of great concern for MCFC development. In this thesis, the effect of sulfur contaminants at both anode and cathode on fuel cell degradation was evaluated in both normal and in low CO2 simulated flue gas conditions.     The results suggested that the poisoning effect of SO2 at the cathode is similar to that of H2S at the anode, and that it is possibly due to a transfer of sulfur from cathode to anode. Furthermore, in combination with low CO2 conditions at the cathode, SO2 contaminants cause fuel cell poisoning and electrolyte degradation, causing high internal resistance.By using a small lab-scale MCFC with commercial materials and standard fuel cell operating conditions, the reversible MCFC was demonstrated to be feasible. The electrochemical performance was investigated in both fuel cell (MCFC) and electrolysis cell (MCEC) modes. The separate electrodes were studied in fuel cell and electrolysis modes under different operating conditions. It was shown that the fuel cell exhibited lower polarization in MCEC mode than in MCFC mode, and a high CO2 concentration at the fuel cell anode reduced the polarization in electrolysis mode, which suggested that CO2 is reduced to produce CO or carbonate.