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Research Papers

# High Efficiency SOFC Power Cycles With Indirect Natural Gas Reforming and CO2 Capture

[+] Author and Article Information
Stefano Campanari

Energy Department,
Politecnico di Milano,
Via Lambruschini 4,
Milano 20156, Italy
e-mail: stefano.campanari@polimi.it

Matteo Gazzani

Energy Department,
Politecnico di Milano,
Via Lambruschini 4,
Milano 20156, Italy

This component, recovering energy from the expansion of natural gas taken from high pressure pipelines, is sometimes considered in plant efficiency analysis. His elimination would entail a 0.5% reduction of efficiency.

1Corresponding author.

2Present address: Institute of Process Engineering, ETHZ, Sonneggstrasse 3, Zurich 8092, Switzerland.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF FUEL CELL SCIENCE AND TECHNOLOGY. Manuscript received December 3, 2014; final manuscript received December 14, 2014; published online January 13, 2015. Editor: Nigel M. Sammes.

J. Fuel Cell Sci. Technol 12(2), 021008 (Apr 01, 2015) (10 pages) Paper No: FC-14-1142; doi: 10.1115/1.4029425 History: Received December 03, 2014; Revised December 14, 2014; Online January 13, 2015

## Abstract

Driven by the search for the highest theoretical efficiency, several studies have investigated in the last years the adoption of fuel cells (FCs) in the field of power production from natural gas with CO2 capture. Most of the proposed power cycles rely on high temperature FCs, namely, solid oxide FCs (SOFCs) and molten carbonate FCs (MCFCs), based on the concept of hybrid FC plus gas turbine cycles. Accordingly, high temperature FCs are integrated with a simple or modified Brayton cycle. As far as SOFCs are concerned, CO2 can be separated downstream the FC via a range of available technologies, e.g., chemical or physical separation processes, oxy-combustion, and cryogenic methods. Following a literature review on promising plant configurations, this work investigates the potential of adopting an external natural gas conversion section with respect to the plant efficiency. As a reference plant, we considered a power cycle proposed by Adams and Barton (2010, “High-Efficiency Power Production From Natural Gas With Carbon Capture,” J. Power Sources, 195(7), pp. 1971–1983), whose performance is the highest found in literature for SOFC-based power cycles, with 82% LHV electrical efficiency. It is based on a prereforming concept where fuel is reformed ahead the SOFC, which thus works with a high hydrogen content fuel. After reproducing the power cycle with the ideal assumptions proposed by the original authors, as second step, the simulations were focused on revising the power cycle, implementing a complete set of assumptions about component losses and more conservative operating conditions about FC voltage, heat exchangers minimum temperature differences (which were previously neglected), maximum steam temperature (set according to heat recovery steam generator (HRSG) practice), turbomachinery efficiency, component pressure losses, and other adjustments. The simulation also required to design an appropriate heat exchangers network, which turned out to be very complex, instead of relying on the free allocation of heat transfer among all components. Considering the consequent modifications with respect to the original layout, the net electric efficiency changes to around 63% LHV with nearly complete (95%+) CO2 capture, a still remarkable but less attractive value. On the other hand, the power cycle requires a complicated and demanding heat exchangers network and heavily relies on the SOFC performances, not generating a positive power output from the gas turbine loop. Detailed results are presented in terms of energy and material balances of the proposed cycles. All simulations have been carried out with the proprietary code GS, developed by the GECOS group at Politecnico di Milano.

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## Figures

Fig. 1

Plant layout for the investigated SOFC cycle, adapted from its original proposal [8]. Details about the flow arrangement are discussed in the text. For the colored version refer to the digital copy. Blue and red lines around the HRSG section (bottom right in figure) are related to water and steam flows. Each of the other colored line represents one or more than one parallel nitrogen streams, flowing from one to another heat exchanger. Points where a N2 stream is split from a mainstream are represented with dots. Other crossing of N2 lines do not indicate a mixing process, which instead occurs ahead heat exchangers in the points represented with squares (this representation is used to avoid drawing too many parallel lines). Some heat exchangers work with a number of parallel flows indicated in the figure and collapsed in a single line for simplicity and compactness of representation.

Fig. 3

Thermal integration of the SOFC with the reformer and the tail heat recovery: on the left, the ideal case where the SOFC does not provide all the required heat to the reformer. On the right, the closer to reality case: the lower cell voltage increases the heat production.

Fig. 4

Temperature heat exchanged diagram for the heat exchangers reported in Table. 6

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