Minimum ignition energy (MIE) is the minimum amount of energy required to ignite a combustible vapor, gas or dust cloud, for example by means of an electrostatic discharge. Ignition of a fuel/air mixture is possible only when the rate of liberation of heat near the ignition zone is greater than the heat loss by conduction. Heat loss due to radiation is not considered, as it is assumed to be negligible, and the ignition process is assumed to be steady and one-dimensional. MIE is measured in joules (J).
Explosives, hydrogen, unsaturated hydrocarbons and alkanes in oxygen have the lowest MIE: in the range of 1 to 100 μJ. Alkanes in air, distillate fuels, hybrid mixtures and extremely sensitive dusts have a MIE range of 0.1 to 10 mJ. Combustible dusts have a MIE range of 0.01 to 10 J.
For most materials the stoichiometric concentration can be calculated from the carbon and hydrogen content and is typically about twice the lower flammable limit (LFL). The lowest ignition energy value rarely occurs at the stoichiometric combustion concentration. For heavier gasses, the minimum usually occurs slightly above stoichiometric concentration and for lighter gasses such as Hydrogen, slightly below.