0
Research Papers

Exergy Analysis of a Gas Turbine Cycle With Steam Generation for Methane Conversion Within Solid Oxide Fuel Cells

[+] Author and Article Information
Mikhail Granovskii

Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, ON, L1H 7K4, Canada

Ibrahim Dincer1

Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, ON, L1H 7K4, Canadaibrahim.dincer@uoit.ca

Marc A. Rosen

Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, ON, L1H 7K4, Canadamarc.rosen@uoit.ca

1

Corresponding author.

J. Fuel Cell Sci. Technol 5(3), 031005 (May 23, 2008) (9 pages) doi:10.1115/1.2894469 History: Received March 14, 2006; Revised March 07, 2007; Published May 23, 2008

The combination of fuel cells with conventional mechanical power generation technologies (heat engines) promotes effective transformation of the chemical energy of fuels into electrical work. The implementation of solid oxide fuel cells (SOFCs) within gas turbine systems powered by natural gas (methane) requires an intermediate step of methane conversion to a mixture of hydrogen and carbon monoxide. State-of-the-art Ni-YSZ (yttria-stabilized zirconia) anodes permit methane conversion directly on anode surfaces, and contemporary designs of SOFC stacks allow this reaction to occur at elevated pressures. An exergy analysis of a gas turbine cycle integrated with SOFCs with internal reforming is conducted. As the efficiency of a gas turbine cycle is mainly defined by the maximum temperature at the turbine inlet, this temperature is fixed at 1573K for the analysis. In the cycle considered, the high-temperature gaseous flow from the turbine heats the input flows of natural gas and air, and is used to generate pressurized steam, which is mixed with natural gas at the SOFC stack inlet to facilitate its conversion. This technological design permits avoidance of the generally accepted recirculation of the reaction products around the anodes of SOFCs, which reduces the capacity of the SOFC stack and the entire combined power generation system correspondingly. At the same time, the thermal efficiency of the combined cycle is shown to remain high and reach 65–85% depending on the SOFC stack efficiency. The thermodynamic efficiency of the SOFC stack is defined as the ratio of electrical work generated to the methane oxidized (through the intermediate conversion). For a given design and operating condition of the SOFC stack, an increase in the thermodynamic efficiency of a SOFC is attained by increasing the fuel cell active area. Achieving the highest thermodynamic efficiency of the SOFC stack leads to a significant and nonproportional increase in the stack size and cost. For the proposed steam generating scheme, increasing the load of the SOFC stack is accompanied by a decrease in steam generation, a reduction in the steam to methane ratio at the anode inlet, and an increased possibility of catalyst coking. Accounting for these factors, the range of appropriate operating conditions of the SOFC stack in combination with steam generation within a gas turbine cycle is presented.

FIGURES IN THIS ARTICLE
<>
Copyright © 2008 by American Society of Mechanical Engineers
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Figure 1

Operation of a SOFC with internal methane reforming into hydrogen and carbon monoxide

Grahic Jump Location
Figure 2

Gas turbine cycle with steam generation for methane conversion inside SOFCs. Numbers indicate devices, as follows: 1, SOFC stack; 2, combustion chamber; 3, turbine; 4,5, compressors; 6,7,8, heat exchangers (6, recuperator; 7, fuel preheater; 8, steam superheater); 9, evaporator; 10, condenser; 11, pump; 12, separator; 13,14, mixers; 15,16, flow divider valves; a, and b, anode and cathode of SOFC stack, respectively.

Grahic Jump Location
Figure 3

Typical temperature profiles of countercurrent flows in the heat exchangers (Devices 6–8 in Fig. 2)

Grahic Jump Location
Figure 4

Typical temperature profiles of countercurrent flows in the evaporator (Device 9 in Fig. 2)

Grahic Jump Location
Figure 5

Variation of operational-circuit fuel cell voltage Vs and number of moles of oxygen nO2 that penetrate through the ion-conductive membrane (electrolyte) of the fuel cell with efficiency ηs of the fuel cell stack. Values are for 1mol of methane consumed in the combined SOFC-gas turbine cycle.

Grahic Jump Location
Figure 6

Variation of fuel (Uf) and oxygen (UO2) utilization factors with operational-circuit fuel cell voltage Vs

Grahic Jump Location
Figure 7

Variations of molar flow rate of air (Nair) (a) and circulating water (nH2O) (b) with operational-circuit fuel cell voltage Vs. Values are for 1mol of methane consumed in the combined SOFC-gas turbine cycle.

Grahic Jump Location
Figure 8

Variations of electrical work We, mechanical work W, total exergy losses, and the exergy losses in the SOFC stack and combustion chamber with operational-circuit fuel cell voltage Vs. Values are for 1mol of methane consumed in the combined SOFC-gas turbine cycle.

Grahic Jump Location
Figure 9

Exergy losses for the devices in the system considered in the illustrative example (Vs=0.61V(ηs=0.20)). Numerical column labels denote devices in Fig. 2. Values are for 1mol of methane consumed in the combined SOFC-gas turbine cycle.

Grahic Jump Location
Figure 10

Dependence of the thermal efficiency ηT of the considered combined SOFC-gas turbine cycle on operational-circuit fuel cell voltage Vs. Values are for 1mol of methane consumed in the combined SOFC-gas turbine cycle.

Grahic Jump Location
Figure 11

Fuel cell polarization (voltage versus current density) and power density curves (modified from Ref. 21)

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In