Lower Flammability Limits of Hydrogen and Light Hydrocarbons at Subatmospheric Pressures

The knowledge of flammability limits is essential in the prevention of fire/explosion when handling combustible gases and vapors. In this study, the lower flammability limits (LFLs) of hydrogen-air, methane-air, ethane-air, n-butane-air, and ethylene-air were measured in a closed cylindrical vessel (inner diameter 10.22 cm, length 100 cm) with upward flame propagation, at room temperature (20 degrees C) and initial pressure of 1.0, 0.7, 0.5, 0.3, and 0.1 atm. The LFL of hydrogen initially decreased with pressure from 1.0 to 0.3 atm, and then the LFL increased with the further decrease of pressure. In contrast, the LFLs of the hydrocarbons increased when the pressure decreased from 1.0 to 0.1 atm, except for methane for which the LFL did not change with pressure. The adiabatic flame temperatures (AFTs) at the obtained LFL concentrations of hydrogen and the hydrocarbons were also calculated at subatmospheric pressure conditions. The behaviors of the AFTs of hydrogen and the hydrocarbons were similar to those of the LFLs under the influence of low pressures. On average, at initial pressures from 1.0 to 0.1 atm and LFL concentration, the AFT of hydrogen was 730K, of the alkanes was 1900K, and of ethylene was 1800 K. On the basis of the LFLs and AFTs, the risk/hazard associated with fire/explosion of hydrogen and the hydrocarbons at subatmospheric pressures was also discussed.