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

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.