Kinetic studies of NO formation of methane at elevated pressures using a novel high-pressure well-stirred turbulent combustor

In this study, NO formation in methane combustion at elevated temperatures and pressures was measured using a newly developed High-Pressure Well-Stirred Turbulent Combustor (HP-WSTC), which enables kinetic characterization at millisecond flow residence times under the assumption of a zero-dimensional homogeneous reactor at elevated pressures. Methane combustion and NO kinetic experiments were conducted at 1 to 10 atm over equivalence ratios between 0.8 and 1.2. Major species concentrations and NO formation were measured and compared with predictions of five kinetic mechanisms. Significant differences in predictions of NO formation were found among these mechanisms, particularly at higher pressure. Reaction-path analysis revealed that CH + N₂ = NCN + H, NCN + H = HCN + N, and the recently proposed NCN + CH₃ = CH₂NH + CN are key pathways governing prompt NO formation. However, these pathways remain insufficiently constrained under the present high-pressure, prompt NO-dominant conditions. The results also show that additional contributions from third-body reactions such as NCN + H (+M) = HNCN (+M) and CH + N 2 (+M) = HNCN (+M) become increasingly important at elevated pressures. Furthermore, CH formation and the competing oxidation pathways leading to CH₂O were identified as the key processes influencing NO production. These findings demonstrate that the HP-WSTC provides a new validation platform for prompt NO chemistry under elevated pressures and short residence times, a regime that has not been thoroughly investigated. Novelty and significance statement This study investigates NO formation kinetics in methane combustion at elevated temperatures and pressures up to 10 atm using a new High-Pressure Well-Stirred Turbulent Combustor (HP-WSTC). The HP-WSTC enables kinetic measurements at high temperatures, elevated pressures, and short residence times over a wide range of equivalence ratios relevant to gas-turbine combustion, thereby extending prompt NO validation data to higher pressures at near-flame temperatures. The results identify several key reactions that remain poorly constrained at elevated pressures, including NCN + CH₃ = CH₂NH + CN, the competition between CH + N₂ = NCN + H and CH + N₂ (+M) = HNCN (+M), and the competition between NCN + H = HCN + N and NCN + H (+M) = HNCN (+M). Pathways governing CH formation and competing CH₂O-forming oxidation routes are also shown to influence prompt NO predictions. These findings clarify the pressure dependence of prompt NO chemistry and support improved predictive capabilities for NO formation in methane-fueled gas-turbine systems.