Mechanism of Dust Explosions
Dusts have a very large surface area compared to their mass. Since burning can only occur at the surface of a solid or liquid, where it can react with oxygen, this causes dusts to be much more flammable than bulk materials. For example, a 1 kg sphere of a material with a density of 1 g/cm3 would be about 12.4 cm across and have a surface area of 0.05 m2. However, if it were broken up into spherical dust particles 50 µm in diameter (about the size of flour particles), it would have a surface area of 120 m². This greatly increased surface area allows the material to burn much faster, and the extremely small mass of each particle allows it to catch on fire with much less energy than the bulk material, as there is no heat loss to conduction within the material. When this mixture of fuel and air is ignited, especially in a confined space such as a warehouse or silo, a significant increase in pressure is created, often more than sufficient to demolish the structure. Even materials that are traditionally thought of as non-flammable, such as aluminum or iron, or slow burning, such as wood, can produce a powerful explosion when finely divided and can be ignited by even a small spark. Such metal powders are widely used in fireworks for their dramatic effects.
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