|MadSci Network: Physics|
Dust explosions typically occur in nearly empty wheat silos, or in flour mills. They can also occur in coal mines or metal works. They are usually triggered by a flame or spark. An explosion occurs when a chemical reaction suddenly accelerates and goes at an extremely fast speed. There are two main types of explosion: thermal feedback explosion, and free radical explosion. With a thermal feedback explosion, a reaction gives out a lot of heat, which heats up the reactants, which makes the reaction go faster, which heats up the reactants even more, and so on. It happens when there is no opportunity for the heat of reaction to escape, or for the reactants to expand and move away. A free radical explosion is different, and a bit harder to explain. Some chemical reactions are much more complicated than they seem to be. All combustion (burning) reactions are like this. The reaction seems to be fuel + oxygen --> oxides + heat But when you study the reaction carefully, you find that there are a lot of intermediate steps. These steps involve very reactive molecules called free radicals. Molecules react by colliding with one another. Most molecules react on about 1 collision in a million; free radicals typically react on 1 collision in 10, or even on nearly every collision. The rate of a burning reaction depends on the numbers of free radicals around, and usually there are not very many. Particle surfaces are often good places for extra free radicals to be formed. And dust has a surprisingly large surface area, because the particles are so small. A thimblefull of dust might have a surface area about the same as the floor area of your house! So if there is a dust, you can sometimes get a very large number of free radicals, and the burning reaction can go extremely fast -- an explosion! It works particularly well if the dust is made of something that can burn, like flour or coal or metal. It can also happen if the dust does not burn, but the fuel is around for some other reason -- like rock dust and natural gas, for example. John.
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