Flammable limits dust suspensions

Suspensions of finely divided combustible solids (flammable dusts) can explode in much the same fashion as flammable gases. It is significant that, in a dust suspension in air, small concentrations of flammable gas, even well below the lower flammable limit of the gas, can contribute to a more severe explosion than that of the dust alone. Such mixtures are called hybrid mixtures. 

A fundamental solution to the dust explosion problem is to use a wet process so that dust suspensions do not occur at all. If a wet process can be used, it is one of the most satisfactory methods. However, the process must be wet enough to be effective. Some dusts with a high moisture content can still ignite. Dust concentrations in major equipment can be designed below the lower flammable limit, but this often cannot be counted on in operation. Dust concentrations cannot be safely designed to be above an upper flammable limit, because such a limit is ill-defined (Lees, 1980). For a large number of flammable dusts, the lower explosion limit lies between 20 and 60 g/m3. The... 

A flammable dust is suspended in air at a concentration within the flammable limits and with an oxygen concentration above the minimum oxygen for combustion. Sparks generated by a grinding wheel pass through the suspension at high speed, but no fire or explosion results. Explain why. 

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. 

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