Ignition Energy of Hybrid Mixtures

Britton, L.G., Estimating the minimum ignition energy of hybrid mixtures, Process Safety Progress, 17, No. 2, Summer 1998. 

Hybrid mixture A suspension of dust in air/vapor. Such mixtures may be flammable below the lower explosive limit of the vapor and can be ignited by low energy sparks. 

An analysis of static ignition hazard should start with data on the ignition sensitivity of the particular flammable material at its most flammable concentration in air, i.e., its MIE. This is especially important for dusts. It is prudent to determine this value on fines of the specific dust of interest, rather than to rely on published data. Hybrid mixtures, i.e., mixtures of dust and vapor for which vapor concentrations may be below their lower flammable limit, can be ignited by smaller discharge energies than might be expected. 

A small amount of flammable gas or vapor mixed in with a flammable dust can cause an explosive mixture to be formed even if both are at concentrations below the explosive range by themselves. These mixtures are called hybrid mixtures. The ignition energy to ignite a hybrid mixture is often less than that required for the flammable dust by itself. 

Hybrid explosible mixtures can be composed of substances which on their own are not flammable dust-fuel for example. In general, the strength of an explosion increases and the minimum ignition energy decreases in relation to the single explosible substance. The following is true for all hybrid systems ... 

Low energy ignition incapable of igniting a given dust-air mixture may easily cause an ignition of a hybrid mixture... 

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