Thermodynamic Analysis of Integrated Molten Carbon Fuel Cell–Gas Turbine Cycles for Sub-MW and Multi-MW Scale Power Generation

[+] Author and Article Information
S. Campanari, P. Silva, E. Macchi

Dipartimento di Energetica, Politecnico di Milano, Milan, Italy

P. Iora

Dipartimento di Ingegneria Meccanica, Università di Brescia, Brescia, Italy

Natural gas is assumed for simplicity to contain 100% methane, available at low pressure (1.2bar). Eventual sulphur compounds (H2S, odorizers) are captured by a desulphurizing filter to reach the very low levels (e.g., below 1ppm) generally required by MCFCs.

Separate works (not reported here) have anyway shown that considering with more detail such aspects yields only limited effects on plant simulated performances.

As it happens on small scales, large hybrid cycles always rely on fuel cells as the prevailing power generator, leaving to gas turbines a limited role in the overall cycle energy balances (where the gas turbine typically covers 20–30% of the total power output). Being high-temperature fuel cells, rather low energy density devices, the issue of integrating a large number of high-temperature fuel cell modules in a wide-footprint power plant, with inevitably long high-temperature pipes for the connection to large gas turbines is not completely straightforward.

LHV reference. Because of this assumption, in both cases the plant percentage efficiency is numerically equal to the cycle power output (in megawatts), with the exception of the cases with post-firing, where additional fuel is introduced in the combustor.

The considered steam quantity reflects a steam-to-carbon ratio variable from a lowest of 2.7:1 in moles, generally considered sufficient to still prevent carbon deposition problems, up to a maximum of 4.8:1. The central value used for basic simulations corresponds to a steam-to-carbon ratio equal to 3.5:1.

In any case, the CO2 content at cathode inlet is higher than the minimum fraction (4%) required by this FC technology (22).

A partial counterpart to this advantages is related to the cost of heat exchangers, which can be reduced by pressurization.

J. Fuel Cell Sci. Technol 4(3), 308-316 (Nov 02, 2006) (9 pages) doi:10.1115/1.2744051 History: Received December 22, 2005; Revised November 02, 2006

This paper investigates the thermodynamic potential of the integration of molten carbon fuel cell (MCFC) technology with gas turbine systems for small-scale (sub-megawatt or sub-MW) as well as large-scale (multi-MW) hybrid cycles. Following the proposals of two important MCFC manufacturers, two plant layouts are discussed, the first based on a pressurized, externally reformed MCFC and a recuperated gas turbine cycle and the second based on an atmospheric MCFC, with internal reforming integrated within an externally fired gas turbine cycle. Different levels of components quality are considered, with an analysis of the effects of variable pressure ratios, different fuel mixture compositions (variable steam-to-carbon ratio) and turbine inlet temperature levels, together with potential advantages brought about by an intercooled compression process. The analysis shows interesting effects due to the peculiarity of the mutual interactions between gas turbine cycle and fuel cells, evidencing the importance of a careful thermodynamic optimization of such cycles. Results show the possibility to achieve a net electrical efficiency of about 57–58% for a small plant size (with a difference of 1.5–2 percentage points between the two layouts), with the potential to reach a 65% net electrical efficiency when integrated in advanced cycles featuring high-efficiency, large-scale equipment (multi-MW scale cycles).

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Results of plant A in case of sub-MW scale analysis; calculation assumptions as per Table 1

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Figure 2

Results of plant B in case of sub-MW scale analysis; calculation assumptions as per Table 1

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Figure 3

Plant A and plant B, sub-MW scale analysis: efficiencies as a function of the pressure ratio of the bottoming cycle

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Figure 4

Sub-MW scale analysis: gas turbine and fuel cell power output as a function of the pressure ratio of the bottoming cycle in case of plants A and B

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Figure 5

Comparison of different cycle conditions for plant A

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Figure 6

Multi-MW plant A, case with 72.7%H2O-27.3%CH4 fuel mixture

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Figure 7

Power output of FC and GT and overall efficiency in case of plant A with pressure ratio of 3.5 and fuel composition at reformer inlet 77.7%H2O-22.3%CH4

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Figure 8

Comparison of different cycle conditions for plant B

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Figure 9

Multi-MW plant B, case with fuel mixture 72.7%H2O-27.3%CH4

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Figure 10

Power output of FC and GT and overall efficiency in case of plant B with fuel composition at reformer inlet 77.7%H2O-22.3%CH4 and optimized pressure ratio β=5.3




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