CO_2 capture from combined cycles integrated with Molten Carbonate Fuel Cells

摘要

In this paper Molten Carbonate Fuel Cells (MCFCs) are considered for their potential application in carbon dioxide separation when integrated into natural gas fired combined cycles. The MCFC performs on the anode side an electrochemical oxidation of natural gas by means of CO_3~(2-) ions which, as far as carbon capture is concerned, results in a twofold advantage: the cell removes CO_2 fed at the cathode to promote carbonate ion transport across the electrolyte and any dilution of the oxidized products is avoided. The MCFC can be "retrofitted" into a combined cycle, giving the opportunity to remove most of the CO_2 contained in the gas turbine exhaust gases before they enter the heat recovery steam generator (HRSG), and allowing to exploit the heatrecovery steam cycle in an efficient "hybrid" fuel cell + steam turbine configuration. The carbon dioxide can be easily recovered from the cell anode exhaust after combustion with pure oxygen (supplied by an air separation unit) of the residual fuel, cooling of the combustion products in the HRSG and water separation. The resulting power cycle has the potential to keep the overall cycle electrical efficiency approximately unchanged with respect to the original combined cycle, while separating 80% of theCO_2 otherwise vented and limiting the size of the fuel cell, which contributes to about 17% of the total power output so that most of the power capacity relies on conventional low cost turbo-machinery. The calculated specific energy for CO_2 avoided isabout 4 times lower than average values for conventional post-combustion capture technology. A sensitivity analysis shows that positive results hold also changing significantly a number of MCFC and plant design parameters.