High-pressure C-H-O diagrams: Fuel composition, carbon deposition, and open circuit voltage of pressurized SOFCs

Solid oxide fuel cells (SOFCs) can operate by using various fuel species. Pressurized SOFCs with gas/steam turbine(s) may achieve higher power generation efficiency as hybrid or triple-combined power generation systems. In this study, fuel gas composition is systematically investigated by thermochemical equilibrium calculations on the anode side of SOFCs, pressurized up to 30 bar over a wide temperature range, up to 1000 °C. Since conventional hydrogen-containing fuel gas consists mainly of carbon, hydrogen, and oxygen, high-pressure C-H-O equilibrium diagrams are numerically obtained. It is revealed that the carbon deposition region contracts in the hydrogen-rich area and expands in the oxygen-rich area with increasing total pressure. The molar fraction of each gas component, described in such C-H-O diagrams, also depends on the total pressure. The theoretical open circuit voltage (OCV) increases by pressurization. The effect of nitrogen in high-pressure SOFC fuels is also considered, which is important especially for air-blown coal gas. The minimum amount of H2O, O2, and CO2 required to prevent carbon deposition in steam reforming, partial oxidation, and CO2 (dry) reforming, respectively, is also derived up to 30 bar. The high-pressure C-H-O diagrams are also applicable to various high-temperature/high-pressure energy systems such as solid oxide electrolyzer cells (SOECs) and reversible fuel cells.