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Research Papers

CO2 Separation From Combined Cycles Using Molten Carbonate Fuel Cells

[+] Author and Article Information
G. Manzolini1

 Politecnico di Milano, Dipartimento di Energia, Via Lambruschini 4, 20156 Milano, Italygiampaolo.manzolini@polimi.it Ansaldo Fuel Cells S.p.A, C.so Perrone 118, Genova, Italygiampaolo.manzolini@polimi.it

S. Campanari, P. Chiesa, A. Giannotti, P. Bedont, F. Parodi

 Politecnico di Milano, Dipartimento di Energia, Via Lambruschini 4, 20156 Milano, Italy Ansaldo Fuel Cells S.p.A, C.so Perrone 118, Genova, Italy

Natural gas can be desulphurized in advance by means of proper treatment since reformer catalysts and MCFCs do not tolerate the presence of sulfur compounds, including the typical NG odorizers, above 0.5-1 ppmv.

In this case, the anode exhaust oxidation is simply carried out burning the residual fuel together with the cathode exhaust stream; the hot stream resulting from this combustion process is then cooled down in the HRSG and vented. Besides, the MCFC operates with a cathode stream having the same composition of the GT exhaust gas, without the post-firing allowed by spent fuel recycling from the cryogenic process. Apart from these differences, the cycle layout is equal to the one of Fig. 1.

Typically, voltages below 0.65–0.70 V/cell are not considered proper operating conditions due to the fact that it forces the cell to operate under gas diffusion limiting conditions, outside of the straight region of the cell characteristic curve and reducing its lifetime.

1

Corresponding author.

J. Fuel Cell Sci. Technol 9(1), 011018 (Dec 27, 2011) (8 pages) doi:10.1115/1.4005125 History: Received September 06, 2011; Revised September 15, 2011; Published December 27, 2011; Online December 27, 2011

This paper presents an analysis of advanced cycles with limited CO2 emissions based onthe integration of molten carbonate fuel cells (MCFCs) in natural gas fired combined cycles (NGCC) in order to efficiently capture CO2 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 CO2 is transferred from the cathode to anode side, concentrating the CO2 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 CO2 production is completely concentrated at the anode. The resulting anode exhaust stream is then sent to a CO2 removal section which is based on a cryogenic CO2 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 CO2 stream is obtained after condensation of water and pumped in liquid form for subsequent storage. The possibility to arrange the MCFC section with different configurations and operating parameters of the fuel cell modules is investigated, 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 CO2 separation effectiveness, is also examined. 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 CO2 utilization factor. The plant shows the potential to achieve a CO2 avoided fraction approaching 70–80%, depending on the CO2 concentration limit at cathode outlet, with overall electric efficiency only 1–2% points lower than the reference combined cycle. The plant power output increases by over 40%, thanks to the contributions of the MCFC section which acts as an active CO2 concentrator, giving a potentially relevant advantage with respect to competitive carbon capture technologies.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Plant layout with integration of MCFCs in a combined cycle, with cryogenic CO2 separation and double fuel cell configuration

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Figure 2

Schematic of double cell concept with 2 + 1 configuration

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Figure 3

Layout of the low temperature CO2 separation and compression section. Flowsheet of the refrigerating unit shown in the bottom is reported in Fig. 4.

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Figure 4

Detailed flowsheet of the cascaded propane/ethane loops refrigerating unit

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Figure 5

(a) Temperature maps inside the first MCFC. (b) Temperature maps inside the second MCFC.

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Figure 6

Cell voltage and CO2 avoided as a function of the CO2 concentration at cathode outlet

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