Grain and Dust Explosions
Name: Krista V.
What makes corn dust combust when stored in a barn?
Even though it will burn, corn dust undisturbed and left in place does not
easily "combust." However, if the dust (or any kind of dust from grain) is
airborne and ignited, it will burn furiously and explosively.
It does not easily burn when left in place because oxygen necessary for
combustion does not make good enough contact with the dust particles -- the
air contacts only the surface of the dust film. When the dust is airborne,
each tiny grain is surrounded by air. If a spark ignites a few of these dust
grains, the energy released from that tiny fire almost instantly ignites
nearby grains until the whole of the dust cloud is ablaze. Thus, the fire
rapidly flashes through the dust cloud with explosive violence.
Now you can better understand why grain elevators sometimes explode and why
all the electrical switches are shielded in explosion-proof, sealed boxes
and why all light fixtures in any part of the elevator which might be
exposed to airborne dust are covered with shielded glass chambers that
protect them from injury.
In silos and processing plants corn (or wheat) is kept dry to prevent
mildew and mold from growing. The mechanics of moving the grain around
produce dust. Now the Surface/Volume ratio of a dust particle (assume it is
a sphere is proportional to 1/R where R is the radius. Since combustion of a
solid begins on the surface, the rate at which it begins increases
dramatically for small dust particle -- explosively fast.
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Update: June 2012