Real Gas Effects on Autoignition Delay and Laminar Flame Speed Predictions for Direct-Fire sCO2 Combustion

Supercritical carbon dioxide (sCO₂) power cycles are promising for improving the carbon footprint of natural gas based electricity generation through sequestration while also offering efficiency and size advantages; however, the cycle conditions pose challenges to in-situ experimental measurements needed for validating models due to extremely high working pressures (around 300 bar). There is still significant uncertainty in the best practices for modeling real gas effects at these conditions and the implications for optimal burner designs and configurations.

In this study, the impact of real gas models on chemical kinetic mechanism predictions for autoignition delay times, laminar flame speeds, and counterflow diffusion flames is assessed for conditions relevant to direct-fire sCO₂ cycles. It is demonstrated that real gas effects result in a 15–20% decrease in ignition delay times at these conditions for the mechanisms studied. Additionally, laminar flame speed simulations were performed in Cantera to investigate the real gas effects and transport modeling with relevance to predicting flashback and blowout limits in combustors. Switching from an ideal gas equation of state to real gas only resulted in a 5–10% increase in flame speed predictions; however, flame speeds increased by a more significant 20–30% when a high-pressure transport model was implemented. The importance of equation of state consideration in chemical kinetic mechanism design is then discussed. The authors conclude that variations in density and transport properties are most important for simulation accuracy.