Dust explosion

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

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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.^[1]

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. A few metals, like aluminium, can also create a suspension of sufficient concentration.

This dust can arise from activities such as transporting grain and indeed grain silos do regularly have explosions. Mining of coal leads to coal dust as a matter of course and flour mills likewise have large amounts of flour with a similar principle for 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.

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.^[2] Common sources of ignition include:

• static charge;
• friction;
• sparks from machinery;
• sparks from electrical equipment;
• hot surfaces;
• fire.

Concentration

Below a certain value, the lower explosive limit (LEL),^[2] 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

There are three classes of explosion protection:

Primary: Do not use combustible material thereby precluding any risk of explosion.

Secondary: Prevent ignition by

1. eliminating ignition sources including the use of
   1. dust proof machinery or machines with small cavities,
   2. non-sparking machinery (or fabrics, such as happened with women's nylon knickers in World War II), or
   3. infrared or thermal imaging to detect heat sources which can then be cooled or removed,
2. diluting fuel concentration,
3. absorbing thermal energy from combustion (such as by suspension of an inert dust), or
4. depleting oxidant by replacing with an inert gas.

Tertiary: Mitigate the effects of an explosion through existence of

1. pressure resistant vessels,
2. overpressure/explosion vents (channelling overpressure to where it will cause less damage),
3. escape routes for humans, or
4. explosion suppressants (the equivalent of an extremely fast-acting fire-extinguisher linked to a special pressure sensor which snuffs out the flames before dangerous pressures are reached).

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 depend on the industry. In the coal mining industry, stone dust is spread along mine roadways, literally to dilute the coal to the point where it does not burn. 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.

See also

• Air to fuel ratio
• Flour bomb

References