Pressure dependence of the auto-ignition temperature of methane/air mixtures

The results of experiments conducted to determine the auto-ignition temperature (AIT) and the cool flame temperature (CFT) of methane/air mixtures at elevated pressures (200-4700 kPa) and for concentrations from 30 up to 83 vol.% are reported. The experiments were performed in a closed spherical vessel with a volume of 8 dm(3). It is shown that methane/air mixtures can react spontaneously for methane concentrations far outside the flammability limits. Cool flames are observed for methane concentrations higher than 40 vol.%. The AIT and CFT are strongly pressure-dependent and decrease with increasing pressures. A Semenov correlation is developed for the CFT as a function of the pressure. The upper explosion limit (UEL) of methane/air mixtures at elevated pressure and temperature is linked with the CFT. It is shown that for certain conditions of pressure and temperature, the flammability range increases considerably even for a small temperature rise. (C) 1999 Elsevier Science B.V. All rights reserved.