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Author: Subject: Coal and hay
Zinc
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[*] posted on 1-5-2007 at 09:27
Coal and hay


I have many times heared that large piles of coal and hay cane ignite by it self and that such accidents have happened. The hay can ignite itself if moist. For coal I don't know. Does anyone know why it happens?



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[*] posted on 1-5-2007 at 09:59


Yes, someone knows how it happens and they wrote this about it.
http://en.wikipedia.org/wiki/Spontaneous_combustion_%28combu...
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[*] posted on 1-5-2007 at 11:07


It doesn't explain why coal ignites by itself. It says that spontaneous combustion starts when oxidation occurs within a substance, which releases heat. I don't think that coal can oxidise at room temperature. Perhaps it can. I don't know.



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FrankRizzo
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[*] posted on 1-5-2007 at 12:11


Coal fires usually start from lightning strikes or ignition from roots that have burned through seams as a result of stump burning.

As for wet hay autoignition in rolled bales, I'm truly at a loss. I understand that bacterial decomposition generates heat, but wouldn't the activity be pretty self-limiting as pasteurization temperatures are reached?

EDIT: I found my answer here:

Compost Fires

[Edited on 5/1/2007 by FrankRizzo]
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Mr. Wizard
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[*] posted on 1-5-2007 at 12:15


Coal and just about everything that can oxidize, will do so at room temperature and below. The rates are slow, the surface area effects the rate, as does the oxygen content, catalysts, and the rate of heat loss. Most 'spontaneous combustion' problems are caused when the heat from the reaction exceeds the rate of heat loss. Other factors include freshly exposed surface such as newly ground or mined coal, bacterial action, ambient temperature rise, and fresh oxygen supplies.

The surface of coal is not an inert substance, but a complex mixture containing carbon, sulfur, nitrogen, hydrogen, alkali metals, and everything else you can think of. The fresh surfaces react with the air, water, bacteria and give off energy. If the energy released is trapped the reaction may speed up and become self limited or it may not. I have read about warehouses full of rubber (latex) gloves catching fire, piles of metal turnings with a trace of cutting oil catching fire, and almost any organic oil or fat (as from plants or animals) will spontaneously ignite under the right circumstances. Even freshly ground charcoal is subject to heating if it was ground in an inert atmosphere, and then exposed to air or oxygen.

Everything wants to 'roll downhill' if given a chance.

The human body is supposed to generate more heat per pound than the sun, but we obviously are not as hot. The difference is the heat escapes from us, due to our large surface area, keeping our temperature low.
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