High Pressure Methane Ignition Delay Measurements under Supercritical Carbon Dioxide Conditions

An inquiry to be investigated for supercritical carbon dioxide (sCO₂) oxy-combustion is how well existing chemical kinetic models perform as no experimental data exists at relevant conditions. Autoignition delays are reported for CH₄/O₂/CO₂ mixtures above the CO₂ critical pressure (up to 200 bar) and for a temperature range of 1139 to 1433 K. Experiments reveal that a widely used kinetic model, GRI 3.0, underpredicts the ignition delay by a factor of 3 at 100 bar. However, kinetic models Aramco 2.0, USC Mech II, HP-Mech, and FFCM1 are capable of predicting autoignition delays though previously not validated at this pressure. At 100 bar, CH₃ recombination to form C₂H₆ through CH₃+CH₃+M=C₂H₆+M becomes dominant compared to direct oxidation to form CH₃O. The branching ratio of these two reaction pathways dictates the autoignition delay. The experimental results at 200 bar however, shows that only one chemical kinetics mechanism could capture the ignition behavior. Simulation results using Aramco 2.0 kinetics mechanism is determined to have a reasonable agreements to predict ignition delay times at 200 bar and a temperature range of 1139 K to 1250 K. No chemical effect from CO₂ on autoignition was observed at supercritical conditions.