MadSci Network: Physics

Re: What causes dust explosions?

Date: Sun Mar 15 15:06:01 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Physics
ID: 889718653.Ph

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.


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