Role of methyldioxy radical chemistry in high-pressure methane combustion in CO2

The combustion chambers of direct-fired supercritical CO₂ power plants operate at pressures of approximately 300 bar and CO₂ dilutions of up to 96%. The rate coefficients used in existing chemical kinetic mechanisms are validated for much lower pressures and much smaller concentrations of CO₂. Recently, the UoS sCO₂ 1.0 and UoS sCO₂ 2.0 mechanisms have been developed to better predict ignition delay time (IDT) data from shock tube studies at pressures from 1 to 260 bar in various CO₂-containing bath gas compositions. The chemistry of the methyldioxy radical (CH₃O₂) has been identified as an essential combustion intermediate for methane combustion above 100 bar, where mechanisms missing this species begin to vastly overpredict the IDT. The current literature available on CH₃O₂ is very limited and often concerned with atmospheric chemistry and low-pressure, low-temperature combustion. This means that the rate coefficients used in UoS sCO₂ 2.0 are commonly determined at sub-atmospheric pressures and temperatures below 1000 K with some rate coefficients being over 30 years old. In this work, the rate coefficients of new potential CH₃O₂ reactions are added to the current mechanism to create UoS sCO₂ 2.1 It is shown that the influence of CH₃O₂ on the IDT is greatest at high pressures and low temperatures. It has also been demonstrated that CO₂ has very little effect on the chemistry of CH₃O₂ at 300 bar meaning that CH₃O₂ rate coefficients can be determined in other bath gases, reducing the impact of non-ideal effects such as bifurcation when studying in a CO₂ bath gas. The updated UoS sCO₂ 2.1 mechanism is then compared to high-pressure IDT data and the most important reactions which require reinvestigation have been identified as the essential next steps in understanding high-pressure methane combustion.