Thermodynamic analysis of supercritical co₂ systems

In this study, we carried out the thermodynamic analysis for the mixture of CO2/H2O (a multi-component real-fluid system) in the relevant conditions of sCO2 compressor and combustor. We firstly report the recommended the thermal conditions for the recuperater based on the thermal performance of compressor, in order to keep the highest thermal efficiency of system. It is found that the thermodynamic state for the CO2/H2O mixture under compressor operating conditions is in liquid phase, and there will be corrosion. the compressibility of this mixture is still very low and close to the critical point of CO2, but it is due to the liquid state instead of supercritical state. Within the same framework, we also analyzed the thermodynamic state for the mixture of CH4/CO2/O2 in the combustor with and without H2O to understand its influence on the real thermodynamics for this type mixture. For the cases without H2O, when 2₂₂₂ ≥ 2. 2, the influence of N2 is very small; but when 2₂₂₂ ≤ 2. 2, N2 can significantly increase the mixture critical pressure. For typical operating conditions, the mixture without H2O is in pure gas-like state. For the cases with H2O, even a small amount of H2O can significantly increase the mixture critical point such that a big portion of the operating condition is within the two-phase zone with liquid state. However, when temperature is high (e.g., 900 K for autoignition), even with H2O, the mixing process does not pass the two-phase zone, so it is a supercritical mixing process.