Upper Flammability Limits of Methane/Air Mixtures at Elevated Temperature and Pressure

Methane/air mixtures in preheated and pressurized process equipment may exhibit a broadened flammability envelope, which is critical for explosion-hazard assessment and safety-envelope definition. Reliable upper flammability limit (UFL) data under elevated-temperature and elevated-pressure conditions are therefore needed for methane-related industrial systems. In this work, the UFLs of methane/air mixtures were measured in a 20 L closed spherical vessel at initial temperatures of 25–100 °C and pressures of 0.1–1.0 MPa in accordance with EN 17624:2022 and GB/T 42368–2023. Within the investigated range, the UFL increases approximately linearly with temperature and logarithmically with pressure. Under simultaneous temperature and pressure elevation, the increase in UFL is greater than that estimated from the simple superposition of their individual effects, indicating a non-negligible interaction between the two parameters. Based on the full experimental matrix, an empirical correlation incorporating a temperature–pressure interaction term is proposed to represent the UFL surface and shows good agreement with the experimental data. Chemical-kinetic simulations using USC Mech II were performed to assist interpretation of the experimental trends. The normalized sensitivity analysis identifies temperature-promoted branching pathways and pressure-enhanced radical stabilization/recombination pathways as the principal competing kinetic tendencies near the UFL. The present work provides a protocol-compliant methane/air UFL data set and a practical correlation for preheated and pressurized methane systems.