Conditions of Explosion
If combustible material is mixed homogeneously with the oxidizing agent (e.g., to create a homogenous dust cloud), the burning rate increases significantly, and the combustion process can be completed in a 10-millisecond order. This phenomenon is responsible for the significant difference between burning a piece of coal and the rapid deflagration of a coal dust cloud. If the third condition – the ignition source – is also provided, then all of the conditions exist for the rapid release of the stored energy, i.e. the dust explosion. In a closed vessel, under certain conditions, this rapid reaction can usually cause a pressure rise of 10-12 barg (although some powders can reach higher pressures). By definition, an explosion is an exothermic chemical process that occurs in a finite volume by a sudden and significant increase in pressure.
So, the explosion properties of dust significantly differ from the original materials from which are formed. The main cause of this phenomenon is the following: the same material has larger order of magnitude specific surface after dust formation. In this bigger area, the intensity of oxidation is much higher, and after glowing of one particle, a chain reaction maybe start.
Figure 2.: Conditions of explosion
The minimum condition of an explosion of dust-air and gas-air systems are the simultaneous presence of an explosive substance, and an oxidizing atmosphere, and an ignition source. An additional requirement is a sufficiently small particle size of the hazardous substance in an appropriate concentration. The two endpoints of this concentration range are the lower and upper explosion limits, within which the substance is explosive. Due to the existence of these five conditions at the same time and place, an explosion can occur. The common name of the system of conditions is the "explosion pentagon" (Figure 2).
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