Thermodynamic analysis of high temperature steam and carbon dioxide systems in solid oxide cells

Abstract

A thermodynamic analysis of the process in solid oxide cells with H₂O and CO₂ (SOCc) was performed based on the data available in the open literature. This analysis identified a range of operating parameters (temperature and pressure) and the composition of the feed gas mixture ensuring the stability of the system and important reactions with respect to the desired process. Primarily, the thermodynamic equilibrium in the system was determined on the basis of minimizing the Gibbs free energy of the reaction system. Temperature and pressure were the operational parameters studied in the range of 600 °C to 1000 °C and 1 bar to 50 bar, respectively. The admissible components in the reaction system were: oxygen, hydrogen, water vapor, carbon dioxide, carbon monoxide, methane, ethane, ethene, formic acid, formaldehyde, methanol and solid carbon (graphitic form). The model predicts the equilibrium mixture composition from which the equilibrium voltage of the cell is computed. The composition of the equilibrium mixture is expressed by means of ternary diagrams as a function of the elemental ratio of carbon, hydrogen and oxygen, known as the C : H : O ratio. The following species appear in negligible amounts in the aforementioned condition ranges in the equilibrium mixture: methanol, formaldehyde, formic acid and oxygen. A strong possibility of solid carbon forming was identified at high conversion of water vapor and carbon dioxide under co-electrolysis conditions. The results of this thermodynamic analysis serve for an evaluation of the degree of conversion with respect to the equilibrium of the outlet gas mixture obtained during the experimental characterization of the solid oxide cell.