Fuel Cell Systems Explained - from and for SET students

minimum difference. This so-called pinch temperature defines the target for the optimumprocess design, since in a real system heat cannot transferred from above or below thistemperature.Other considerations taken into account are the materials of the BOP components and theirmechanical layout.7.4 The molten carbonate fuel cell MCFCThe electrolyte is a molten mixture of alkali metal carbonates, usually lithium and potassium orlithium and sodium, which is retained in a ceramic matrix of LiAlO 2 . At the operation temperatureof 600°-700°C the alkali carbonates form a highly conductive molten salt, with CO 3 2- as mobile ion.Unlike other fuel cells, CO 2 and O 2 needs to be supplied at the cathode. Figure 7.9 shows theprincipal function of a MCFC with the electrode reactions.The Nernst reversible potential, taken the CO 2 transfer into account, is given byIn practice the CO 2 produced at the anode is externally recycled to the cathode. The anod exhaustgas is fed to a burner, which converts any unused fuel into water and CO 2 . The exit flow of theburner is then supplied with fresh air to the cathode inlet. The process also pre-heat the reactant air.Another method of reuse the CO 2 is a membrane seperator. The advantage of this method is unusedfuel gas can be recycled to the anode or used for other purposes. If an external supply of CO 2 isalready existing this can also be used.MCFC doesn't need nobel metal catalysts, due to the high operating temperature. Nickel (anode)and nickel oxide (cathode) is used.Furthermore it is also able to convert CO directly and reform hydrocarbon fuels directly. The highoperating temperature of MCFCs provides the opportunity for achieving higher overall systemefficiencies and greater flexibility in use of available fuels compared to low temperature cells.Unfortunately, the higher temperatures and the aggresive medium of molten carbonate electrolytealso place severe demands on the corosion stability and life of the cell components.7.4.2 Implications of using a molten carbonate electrolyteMCFC uses a liquid electrolyte that is immobilised in a porous ceramic matrix. The electrolyteinterfacial boundaries are established by a balance in capillar pressures in the MCFC. By properlyco-ordinating the pore diameters in the electrodes and in the electrolyte matrix, the electrolytedistribution is established. The smaller pores in the matrix remain completely filled, while thebigger pores in the electrodes are partially filled. The electrolyte management is a critical point toachieve hich performance and endurance.