Thermodynamic analysis of a methane fed SOFC system based on a protonic conductor

doi:10.1016/S0167-2738(02)00363-6

Solid State Ionics

Volumes 152–153, December 2002, Pages 555-560

https://doi.org/10.1016/S0167-2738(02)00363-6 Get rights and content

Abstract

Possibility of utilization of CO-containing fuel in a fuel cell based on a solid oxide protonic conductor (SOFC(H⁺)) is demonstrated. Thermodynamic analysis of a methane fed SOFC(H⁺) system is performed. It is stated that the fuel utilization depends on H₂O/CH₄ mole ratio in the feeding fuel mixture and has a maximum at the ratio of about 2.6. It is stated that average EMF of the SOFC(H⁺) is higher than that of an SOFC based on oxygen ion conductor under the same conditions and, hence, the maximum efficiency of the SOFC(H⁺) system is about 15% higher than of the SOFC system based on oxygen ion conductor.

Introduction

Solid oxide fuel cells (SOFCs) are electric power generators having high efficiency of transformation of fuel chemical energy into electricity. Thermodynamic analysis of a methane fed SOFC system based on oxygen ion conductor (SOFC(O²⁻) system) showed that its maximum efficiency varies from 82% to 68% in the temperature region 900–1300 K [1]. As it was discovered during last two decades, some oxides have acceptable level of protonic conductivity [2]. These oxides can be used as electrolytes in the SOFC. Comparison of a hydrogen fed SOFC based on protonic conductor (SOFC(H⁺)) with a hydrogen fed SOFC(O²⁻) showed that the former has higher efficiency than the latter [3]. It is of great interest to consider possibility of utilization of carbon containing fuel, for instance, methane, in the SOFC(H⁺) system and to compare its efficiency in this case with that of the SOFC(O²⁻) system.

Section snippets

Theoretical model

Generally, an SOFC system transforms chemical energy of the fuel into electrical energy. In the case of organic fuel or coal, they have to be firstly transformed into gas mixture containing hydrogen and carbon monoxide. As a rule, a device for fuel transformation, a reformer, operates under endothermic condition. In the case, when the reformer utilizes heat from an SOFC electrochemical section, the process is called “internal reforming”. In the case when the reformer utilizes side heat, i.e.

Results and discussion

Results of calculation of equilibrium output anode gas are presented in the form of dependencies of main SOFC system characteristics on operation parameters. Dependencies of the fuel utilization on steam/methane mole ratio, m, and on partial pressure of residual hydrogen in the output anode gas, p_H₂, at 900 and 1300 K are shown in Fig. 1a and b, respectively. It is very interesting that the dependence has a maximum at p_H₂(L)=const within the steam/methane mole ratio interval from 2.4 to 2.9.

Conclusion

The SOFC system based on protonic conductor can utilize CO-containing fuel, in particular, products of methane reforming. Efficiency of such system is evidently higher than that of the SOFC system based on oxygen ion conductor. Search for solid oxides with high protonic conductivity and stability in carbon dioxide containing atmospheres seems to be very important task for creation of highly efficient electrochemical generators.

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