Spontaneous Combustion Heat
I am curious about spontaneous combustion when oily rags are discarded improperly. What causes heat build up?
In order for something to burn (oxidize), ignition heat and oxygen are necessary. As you know, such reactions release heat. Burning may be a slow oxidation like that which occurs when a iron rusts, or it may occur quickly in a very fast oxidation like that which occurs when grain dust explodes.
If the rags are soaked in a combustible substance such as gasoline or oil, a very gradual, slow oxidation may occur. As it does, the pile of rags serves as a kind of insulating blanket that traps and allows a build-up of heat and flammable vapors from the reaction. When the temperature rises far enough, the whole pile may catch fire very quickly in a "spontaneous combustion."
If the rags are hung up so that they are exposed to the air but not piled up, they are far less likely to ignite on their own. Nevertheless, If soiled rags (like shop towels) must be stored prior to washing, they should be stored in an air-tight container, thus limiting the initial amount and supply of oxygen available to enable combustion.
One additional caution: Certain drying oils used in wood preservation are so chemically reactive that they may ignite in circumstances under which a common oily rag would not. Whenever those kinds of oils are used, one must be extra careful to follow instructions on the proper disposal of any rags or paper towels that may be contaminated with those materials.
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Update: June 2012