Flammability limit behavior of methane with the addition of gaseous fuel at various relative humidities

Abstract The lower flammability limit (LFL) of methane is 5 % in volume. Usually, in the relevant industry, the methane leakage monitoring threshold is set on the basis of the LFL from the perspective of safety precautions. When small amounts of other combustible gases coexist with methane, setting the monitoring threshold according to 5% vol may cause a large deviation and lead to a potential explosion accident. To investigate the effects of flammable gases and the relative humidity (RH) on the methane flammability limit behavior, the flammability limits of methane-air mixtures with the addition of gaseous fuels were experimentally measured in a standard cylindrical setup at various relative humidities. A theoretical model based on the adiabatic flame temperature method was employed to obtain reasonable estimates of the flammability limits of binary mixtures of methane and vapor. The combustion hazards of methane-air mixtures were evaluated. The results showed that both the LFL and the upper flammability limit (UFL) of methane were diminished with the addition of gaseous fuel at the same RH, while these parameters increased with an increase in the RH. The flammable range of methane-air mixtures was slightly narrowed by increasing the RH but was extended by the addition of gaseous fuel. The combustion hazard of methane increases as the volume fraction of added gases increases but drops with an increase in the RH. These phenomena were ascribed to the dual roles of water vapor and the influence of the added gaseous fuel on the initial stage of the methane-air chain reactions. The obtained data could supplement the flammability limits database and help to prevent potential explosion hazards.