Effect of elevated pressure on the explosion and flammability limits of methane-air mixtures

Air injection into oil reservoirs for improved oil recovery is a proven technique in oilfields, while the safety of the process has been concerned over the possible explosion of natural gas at high pressure conditions. The combustion and explosion characteristics of natural gas-air mixture are also important for natural gas engines. In this study, ignited explosion experiments of methane-air mixtures have been conducted using a cylinder chamber under pressure up to 32 MPa, in which the influences of pressure on the explosion and flammability limits have been investigated. The experimental results indicate that the lower flammability limit of methane decreases slightly at elevated pressures, while its upper flammability limit increases significantly over 3 MPa, and the theoretical limiting oxygen concentration required for explosion is gradually reduced, posing greater explosion risks. At ambient temperature, the measured explosion limit range for methane in air is 2.93%–60.75%vol at 30 MPa, in contrast to 4.95% and 15.51%vol at atmospheric pressure, and the corresponding theoretical limiting oxygen concentration at 30 MPa can be reduced to as low as 5.86% from around 10% at 0.1 MPa when ignited using heated tungsten wire. A high pressure explosion limits model for methane-air mixture has been proposed based on the experimental data.