Solid oxide fuel cells (SOFCs) can operate by using various fuel species. The triple-combined power generation systems composed of pressurized SOFCs and gas/steam turbine(s) may achieve higher efficiency for next generation thermal power plants [1]. Natural gas (consisting mainly of CH4) and air-blown and oxygen-blown coal gas will be generally used for such power generation. Since these fuel compositions change due to fuel reforming and/or electrochemical reactions, carbon deposition may occur on SOFC anodes, decreasing cell performance. Here, in this study, C-H-O equilibrium diagrams are constructed systematically by applying thermochemical equilibrium calculations to evaluate the effect of fuel compositions and impurities on thermochemical equilibria under extensive conditions e.g. at high pressures, where experimental studies are relatively difficult [2].
Calculation Procedure
Provided that carbon, hydrogen, and oxygen are mixed in the form of CxHyOz as a mixed fuel (x, y, z are the proportions in the mixture), thermochemical calculations were carried out using two approaches: (i) algebratic solution, and (ii) Gibbs free energy minimization using thermochemical equilibrium calculation software HSC Chemistry and FactSage. As the first approach, we considered major reactions within the fuel and solved the system of equations using relational expression of equilibrium constants and elemental ratios by Mathematica. It has been known that the major constituents of equilibrium products are H2(g), O2(g), H2O(g), CO(g), CO2(g), CH4(g), and C(s) (graphite) [3, 4]. Carbon deposition boundaries meaning the equilibrium concentration of C(s) equal to 10-6 of the initial carbon content in the fuel, the molar fractions of each gas, and the theoretical open circuit voltage were derived and described on the C-H-O diagrams between 400 and 1000oC, and between 1 and 30 bar. The influence of impurities was also calculated and described for Ni anodes on the C-H-O-Ni-M (M: impurity) diagrams.
Results and discussion
Fig. 1 shows the carbon deposition region boundaries for various total pressures at 800 oC. With increasing total pressure, the carbon deposition region contracts on the hydrogen-rich side and expands on the oxygen-rich side [2]. The results of the theoretical open circuit voltage (OCV) at 800 oC at 1 bar are also compared with those at 20 bar. The theoretical OCV becomes higher for almost all the fuel compositions with increasing total pressure. Based on these results, the optimal reforming condition to prevent carbon deposition was analyzed for three cases, (a) steam reforming, (b) partial oxidation, and (c) CO2 reforming, for CH4 and air-blown coal gas. Steam reforming is thermochemically most effective for CH4 to avoid carbon deposition, and to obtain high OCV. In contrast, for air-blown coal gas, steam reforming is effective to obtain high theoretical OCV, smaller amount of additives is needed for partial oxidation, and CO2 reforming has an advantage in effectively using CO2. Consequently, reforming procedures for coal gas have to be selected and optimized by considering all these issues.
Fig. 2 shows the C-H-O-Ni-P diagrams for phosphorus impurity in the fuel applied to Ni-based anode, assuming enough amount of C-H-O and much lower concentration of phosphorus impurity supplied. This figure can clearly show the C-H-O ratios for the formation of Ni-P compounds.
References
[1] Y. Kobayashi et al, Mitsubishi Heavy Ind Tech Rev, 48, 9-15 (2011).
[2] A. Muramoto et al, Int J. Hydrogen Energy, 42 (52), 30769-30786 (2017).
[3] K. Sasaki and Y. Teraoka, J. Electrochem. Soc., 150 (7), A878-A884 (2003).
[4] K. Sasaki and Y. Teraoka, J. Electrochem. Soc., 150 (7), A885-A888 (2003).