CO_2 SEPARATION FROM COMBINED CYCLES USING MOLTEN CARBONATE FUEL CELLS

摘要

This paper presents an analysis of advanced cycles with limited CO_2 emissions based on the integration of Molten Carbonate Fuel Cells (MCFC) in natural gas fired combined cycles (NGCC) in order to efficiently capture CO_2 from the exhaust of the gas turbine. In the proposed cycles, the gas turbine flue gases are used as cathode feeding for a MCFC, where CO_2 is transferred from the cathode to anode side, concentrating the CO_2 in the anode exhaust. At the anode side the MCFCs are fed with natural gas, processed by an external reformer which is thermally integrated within the FC module; the corresponding CO_2 production is completely concentrated at the anode. The resulting anode exhaust stream is then sent to a CO_2 removal section which is based on a cryogenic CO_2 removal process, based on internal or external refrigeration cycles, cooling the exhaust stream in the heat recovery steam generator, and recycling residual fuel compounds to the power cycle. In all cases, a high purity CO_2 stream is obtained after condensation of water and pumped in liquid form for subsequent storage. It is investigated the possibility to arrange the MCFC section with different configurations and operating parameters of the fuel cell modules, and the option to include two fuel cell modules in series connection, with intermediate cooling of the cathode stream, in order to enhance the plant CO_2 separation effectiveness. The MCFC section behavior is simulated taking into account Ansaldo Fuel Cells experience and reference data, based on a dedicated simulation tool. Detailed energy and material balances of the most promising cycle configurations are presented; fuel cell and conventional components working parameters are described and discussed, carrying out a sensitivity analysis on the fuel cell CO_2 utilization factor. The plant shows the potential to achieve a CO_2 avoided fraction approaching 70-80%, depending on the CO_2 concentration limit at cathode outlet, with an overall electric efficiency only 1-2% point lower than reference combined cycle. The plant power output increases by over 40%, thanks to the contribute of the MCFC section which acts as an active CO_2 concentrator, giving a potentially relevant advantage with respect to competitive carbon capture technologies.