Analysis of direct carbon fuel cell (DCFC) based coal fired power cycles with CO2 capture

Abstract

This work presents an analysis of the application of direct carbon fuel cells (DCFC) to large scale, coal fueled power cycles. DCFCs are a type of high temperature fuel cell featuring the possibility of being fed directly with coal or other heavy fuels, with high tolerance to impurities and contaminants (e.g., sulfur) contained in the fuel. Different DCFC technologies of this type are developed in laboratories, research centers or new startup companies, although at kW-scale, showing promising results for their possible future application to stationary power generation. This work investigates the potential application of two DCFC categories, both using a “molten anode medium” which can be (i) a mixture of molten carbonates or (ii) a molten metal (liquid tin) flowing at the anode of a fuel cell belonging to the solid oxide electrolyte family. Both technologies can be considered particularly interesting for the possible future application to large scale, coal fueled power cycles with CO2 capture, since they both have the advantage of oxidizing coal without mixing the oxidized products with nitrogen; thus releasing a high CO2 concentration exhaust gas. After a description of the operating principles of the two DCFCs, it is presented a lumped-volume thermodynamic model which reproduces the DCFC behavior in terms of energy and material balances, calibrated over available literature data. We consider then two plant layouts, using a hundred-MW scale coal feeding, where the DCFC generates electricity and heat recovered by a bottoming steam cycle, while the exhaust gases are sent to a CO2 compression train, after purification in appropriate cleaning processes. Detailed results are presented in terms of energy and material balances of the proposed cycles, showing how the complete system may surpass 65% lower heating value electrical efficiency with nearly complete (95%+) CO2 capture, making the system very attractive, although evidencing a number of technologically critical issues.