Dust explosion

A dust explosion refers to the explosive combustion of a dust suspended in air in an enclosed location.

Conditions for dust explosion
There are five necessary conditions for a dust explosion:
 * 1) A combustible dust;
 * 2) The dust is suspended in the air at a sufficient concentration;
 * 3) There is an oxidant (typically atmospheric oxygen);
 * 4) The dust is confined;
 * 5) There is an ignition source.

If any of these five conditions is missing there can be no dust explosion.

Sources of dust
Many materials which are commonly known to combust can generate a dust explosion, 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 aluminium, can form explosive suspensions in air.

The dust can arise from activities such as transporting grain and indeed grain silos do regularly have explosions. Mining of coal leads to coal dust and flour mills likewise have large amounts of flour dust as a result of milling. A similar problem occurs in saw mills and other places dedicated to carpentry. Thermobaric weapons, depending upon their fuel, are also a potential and intentional source of dust. Dust filling a volume of 10 litres is sufficient to produce an explosion.

The dust must also consist of very small particles, where the surface area is very large, and so will support combustion. Dust is defined as Powder (substance)s with particles less than about 500 micrometres in diameter, but finer dust will present a much greater hazard than coarse particles by virtue of the larger surface area.

Sources of ignition
There are many sources of ignition and a naked flame need not be one: Over one-half of the dust explosions in Germany in 2005 were from non-flame sources. Common sources of ignition include:
 * electrostatic discharge;
 * friction;
 * sparks from machinery;
 * sparks from electrical equipment;
 * hot surfaces;
 * fire.

Concentration
Below a certain value, the lower explosive limit (LEL), there is simply insufficient dust to support the combustion at the rate required for an explosion. A figure 20% lower than the LEL is considered safe. Similarly, if the fuel/air ratio increases above the upper explosive limit there is insufficient oxidant to permit combustion to continue at the necessary rate.

Mechanism of dust explosions
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. Due to the thermal expansion of the gas, the pressure increases. In an enclosed space this leads to the condition called overpressure.

Protection from dust explosions
Much research has been carried out in Europe and elsewhere to understand how to control these dangers, but explosions still occur. The alternatives for making processes and plants safer depend on the industry. In the coal mining industry, stone dust is spread along mine roadways, or suspended from trays in the roof, so as to dilute the coal dust raised ahead of the combustion zone by the shock wave, to the point where it cannot burn. Mines may also be sprayed with water to inhibit ignition. Some industries exclude air from the process, known as inerting. Typically this uses nitrogen or carbon dioxide, and if this is done properly nothing can burn.