Spontaneous combustion can happen when substances oxidize slowly in confined spaces where there is poor circulation. The heat of the reaction is absorbed and raises the temperature of the material until auto-ignition occurs. This is why you should never throw solvent soaked rags into a broom closet.

The wet hay is ignited because of microorganisms that multiply in the wet hay and their biological processes generate heat. (thermogenesis). This gets the hay to about 160 degrees, which is hot enough to kill the organisms, but the chemical reaction continues and the heat builds faster than it dissipates and POOF! a fire.

Jean
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