Combustion of Finely Divided Particles

Finely divided particles of combustible materials are somewhat analogous to vapors with respect to flammability. One snch example is a spray or mist of hydrocarbon liquid in which oxygen has the opportunity for intimate contact with the liquid particles causing the liquid to ignite at a temperature below its flash point. 

FIGURE 15.2 Schematic of the Cleveland open-cup method for the determination of flash point and Are point. 

lower flammability limit UFL, upper flammability limit at 25°C. 

Dust explosions can occur with a large variety of solids that have been ground to a finely divided state. Many metal dusts, particularly those of magnesium and its alloys, zirconium, titanium, and aluminum, can burn explosively in air. In the case of aluminum, for example, the reaction is as follows  

Coal dust and grain dusts have caused many fatal fires and explosions in coal mines and grain elevators respectively. Dusts of polymers such as cellulose acetate, polyethylene, and polystyrene can also be explosive. 

---