Hugh – at least two paper mills losing their entire wood chip inventory to spontaneous combustion. At least one chip car that was loaded with chips from a log that had not been permitted to drain its excess water, got bad orderd, sat for several weeks, and then repair became a non issue. Fire department poured water on it for several days and finally let it burn itself out.
Even coal will do this. Windjammers that hauled coal from England to various parts of the world for British Empire use kept fire watchs, measured coal temperatures at least once each watch (every 4 hours more or less), because coal loaded wet and stored for an extended period of time in a closed environment (watertight ships hold) will catch fire.
Used tires (tyres in England) that have been chipped up and used as road fill have caught fire under the road. Now, that is an interesting fire response excercise!