It is an academic question. You know that the fuel/air mixture or dust cloud had an ignition source above the MIE, or you wouldn't be there. The more important issue, in my opinion, is the source of the fuel.

Ignition occurs more easily, that is, with the addition of less energy, in mixtures of vapors or dusts in air than on solids. Fuel/air mixtures just slightly richer than stoichiometric are the most easily ignited, with minimum ignition
energies (MIEs) on the order of 0.2 millijoules (mJ) for hydrocarbons and 0.01 mJ for hydrogen [1]. Depending on the particle size distribution, MIEs for dust clouds are on the order of tens to hundreds of millijoules [2].

1. Drysdale, D. (1985) An Introduction to Fire Dynamics, Wiley-Interscience, New York, 80.
2. Eckhoff, R. K. (1991) Dust Explosions in the Process Industries, Butterworth-Heinemann, Oxford, UK, p. 562.

So, when you consider that 1 joule (J) is the amount of energy required to run a one-watt lightbulb for one second, these MIEs are really tiny. The ignition source could be just about anything. The fuel source is the most important factor.

John J. Lentini, CFI
Scientific Fire Analysis
Islamorada, FL
www.firescientist.com
scientific.fire@yahoo.com