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Name: Krista V.
Status: student
Age: 12
Location: N/A
Country: N/A
Date: 16-11-2002


Question:
What makes corn dust combust when stored in a barn?


Replies:
Krista,

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.

Regards,
ProfHoff 515


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.

Vince Calder



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