Pressurized SOFC system combined with gas and steam turbine will achieve high power generation efficiency approaching 70% as the triple combined cycle power generation system. It has been reported that operating SOFCs under high pressure can make cell power output higher ^(1,2). However, there are only a limited number of studies available on the fuel composition for pressurized SOFCs. In general, fuel gas consists of carbon, hydrogen and oxygen, so that the C-H-O equilibrium composition determines anode performance. In this study, fuel gas composition on the anode side of pressurized SOCFs was derived by thermochemical equilibrium calculation for considering various operating conditions in terms of carbon deposition and each gas molar fraction affecting power generation characteristics, all described in the C-H-O diagrams.

Calculation Procedure

We calculated carbon deposition region and each gas molar fraction under various equilibrium conditions upon suppling fuels composed of carbon, hydrogen and oxygen. Thermochemical calculations were carried out using three approaches: (i) calculation with equilibrium constants, using (ii) HSC Chemistry (Version 9.0.5, Outotec Research Oy, Finland) and (iii) FactSage (Version 6.3.1, Thermfact Ltd., Canada). As the first approach, we defined main reactions within the fuel and solved the system of equations using relational expression of equilibrium constants and elements ratios by Mathematica (Version 10.4.1, Hulinks Inc., Japan). HSC Chemistry and FactSage are both thermochemical equilibrium calculation software based on Gibbs free energy minimization, containing various databases. It has been known that the major constituents of equilibrium products are H₂(g), H₂O(g), CO(g), CO₂(g), CH₄(g), and C(s) (graphite) ⁽³⁾. The line specifying the carbon deposition region means the equilibrium concentration of C(s) equal to 10^-6 of the initial carbon content in the fuel. The C-H-O diagrams are described in such a way between 100 and 1000^oC, and between 1 and 30 bar.

Results and discussion

Comparing Fig. 1 with Fig. 2, carbon deposition region boundaries vary with respect to temperature and pressure. The C-H-O diagrams clearly show that carbon deposition region expands on the oxygen-rich side and contracts on the hydrogen-rich side with decreasing temperature and/or increasing total pressure. It can be explained by the chemical equilibrium reactions (I) and (II):

CH₄↔C(s)+2H₂ (I)

2CO↔C(s)+CO₂ (II)

Reaction (I) is an endothermal reaction; on the hydrogen-rich side, equilibrium reaction (I) shifts to the left side, preventing carbon deposition at lower temperatures. With increasing total pressure, the equilibrium also shifts to the left side due to the decrease in the number of molecules. In the same way, Reaction (II) is an exothermal reaction and the number of molecules is lower on the right side, so that the equilibrium reaction shifts to the right side to promote carbon deposition.

The C-H-O diagrams of various gases and the corresponding theoretical open circuit voltage are also calculated. These results suggest, for example, a decrease in the fraction of H₂and CO under higher total pressure.

References

1. S. C. Singhal, Solid State Ionics, 135(1-4), 305-313 (2000).

2. Y. Kobayashi et al, ECS Trans., 51(1), 79-86 (2013).

3. K. Sasaki and Y. Teraoka, J. Electrochem. Soc., 150 (7), A885-A888 (2003).