Written by Rickdatech
Spontaneous Combustion is one of the leading causes of fires in homebrewing operations according to fire investigators. Oily rags will burst into flames without an apparent ignition source if the conditions are right. Unfortunately the right conditions are fairly easy to replicate. Just pile up some oily rags. The oil will chemically react with the air in a process called oxidation, giving off heat. The rags act like insulation holding in the heat, letting it build up to autoignition temperatures.
The autoignition temperature is the temperature at which a substance will ignite without a spark. The autoignition temperature of vegetable oil is over 600F. The autoignition temperature of dry rags and paper is just over 400F. But like a wick in oil, the autoignition temperature of oily rags and paper is around 200F. Experimentation has shown that the temperature of oily rags with limited oxygen can build up to over 700F just through oxidation.
Spontaneous Combustion does not need oily rags, all you need is some oil and some kind of fibrous material to act as insulation and hold in the heat while letting in oxygen. There have been reports of oil soaked sawdust, oil soaked wool, and even oil soaked fiberglass insulation bursting into flames by spontaneous combustion.
Machine washing and drying your oily rags makes it easier for them to catch fire. The heat from the drier will raise the temperature enough so that it only needs a little oxidation to reach the autoignition temperature. Just like piling your rags up in a warm spot, or in direct sunlight will also make it easier for them to burst into flames.
What you can do to prevent Spontaneous Combustion:
Put your oily rags in a metal can with an airtight metal lid, or
Put them in a bucket of water, or
Spread them out so they can't build up heat as the oil oxidizes.
Wash your oily rags by hand, and air dry on a clothesline.
Spontaneous Combustion of Drying Oils as a Fire Cause - UC Davis pdf