Using MCFC for high efficiency CO_2 capture from natural gas combined cycles: Comparison of internal and external reforming

摘要

In recent years, several research groups have proposed the combination of Molten Carbonate Fuel Cells (MCFCs) and gas turbine cycles for the application to CO_2 capture. One of the most promising configuration relies on the use of MCFCs as "active CO_2 concentrator" in combined cycles (CCs): the fuel cell is placed downstream the gas turbine and ahead the heat recovery steam generator (HRSC), to concentrate the CO_2 from the gas turbine exhaust feeding the cathode, to the anode (where CO_2 is transferred together with oxygen) and generate electricity; while exhaust heat released by the cell effluents is recovered by the steam cycle. It has been shown that such plant configuration can capture 70-85% of CO_2 with small efficiency penalties compared to the combined cycle, and increasing by about 20% the overall power output (mainly given by the MCFC section); hence, this configuration could have relevant advantages with respect to competitive carbon capture technologies. This work presents a comprehensive discussion of the results of a modeling activity developed at Politecnico di Milano regarding the possible use of MCFCs for high efficiency CO_2 capture from combined cycles. The work discusses different types of MCFC-CC cycles, focusing on the comparison of two families of MCFC and corresponding power plants which have been discussed only separately in the past. The MCFC can be fed with natural gas according to an internal reforming (IR) or external reforming (ER) process, according to the technological proposals of different MCFC manufacturers. Then, the anode exhaust stream of the MCFC, where is concentrated the majority of CO_2, is sent to a CO_2 purification section which can be based on (ⅰ) a cryogenic CO_2 separation section, or (ⅱ) an oxy-combustion of residual fuel components, followed by cooling, condensation of water and separation of CO_2 In both cases, a high purity CO_2 stream is obtained and pumped to liquid form for storage. Detailed results are presented in terms of energy and mass balances of the different proposed cycles, evidencing pros and cons of the different layout and pointing out the role of relevant FC operating parameters (CO_2 utilization, operating current density and voltage) on the overall balances. Moreover, it is presented a comparison between the best proposed cycles and conventional NGCC-CCS systems.