Many people are unaware that the most mundane of powdery materials (dusts) can ignite and even explode under the right set of conditions: A dust explosion is the fast combustion (deflagration) of dust particles suspended in the air in an enclosed location. Coal dust explosions are a frequent hazard in underground coal mines, but dust explosions can occur where any powdered combustible material is present in an enclosed atmosphere. There are four necessary conditions for a dust explosion: A combustible dust; the dust is suspended in the air at a high concentration; there is an oxidant (typically atmospheric oxygen); and there is an ignition source. A fifth favorable, but not strictly necessary condition is that the dust be confined.
Many materials which are commonly known to oxidize can generate a dust fires or explosions, such as coal, sawdust, and magnesium. However, many otherwise mundane materials can also lead to a dangerous dust cloud such as grain, flour, sugar, powdered milk and pollen. Many powdered metals (like aluminum and titanium), can form ignitable/explosive suspensions in air. On Feb 8, 2008, a dust explosion in Port Wentworth, GA, leveled a sugar refinery, killed 6 workers, and injured 44 others. In May of 2009 a mostly empty jar containing metal and resin powders caught fire in the hand of a Georgia Tech researcher while she shook it to loosen the powder stuck to the side of the jar; she was not injured.
For ignition to occur, dust must also consist of very small particles, presenting a large surface area, allowing it to support combustion. Dust is defined as powders with particles less than about 500 micrometers in diameter, but finer dust will present a much greater hazard than coarse particles by virtue of the larger surface area.
Different dusts will have different combustion temperatures and dust of various types will either suppress or elevate this temperature in relation to the stoichiometric concentration of the dusts. It is necessary that sufficient energy, generally either thermal or electrical, be applied to trigger combustion. Due to the small volume in relation to the large surface area, combustion can then proceed very rapidly and the flame front can also travel quickly. For example, 1 kg of powder, 120 µm in diameter will have a surface area of 50 m² or 540 sq ft. Due to the thermal expansion of the gas, the pressure increases. In an enclosed space this leads to the over pressurization of the “container” which could be a jar in a laboratory or an entire building, causing it to burst or “explode”.
