Coal self-heating and spontaneous combustion, how does this happen? I finally broke down and read the Wikipedia article on spontaneous combustion and I was surprised to see how serious of a problem this can be. Wet hay, oily rags, there really are exothermic reactions in these combinations that speed up as the temperature increases.
Then I saw the following:

Coal
Self-heating in coal has been extensively studied. The tendency to self-heat decreases with increasing rank of the coal. Lignite coals are more active than bituminous coals, which are more active than anthracite coals. Freshly mined coal consumes oxygen more rapidly than weathered coal, and freshly mined coal self-heats to a greater extent than weathered coal. The presence of water vapor may also be important, as the rate of heat generation accompanying the absorption of water in dry coal from saturated air can be an order of magnitude or more than the same amount of dry air.

I understand that coal is a complex material, it's more than a lump of carbon. But what are the types of chemical reactions within coal that will cause a pile of it to self-ignite? Is moisture always required? Is there moisture in freshly mined coal naturally, or is it absorbed hygroscopically from the atmosphere or is a liquid source necessary (getting wet)?
Sulphur/sulphides in coal may be one of the causes of self combustion.

A slight diversion - in some metal sulphide mines, such as copper, at high level of sulphides in the ore, the sulphides can oxidize thus creating sulphide fires. If a lot of dry dust is produced during stope blasts, during the mining process, sometimes the  sulphide dust can cause a secondary, unwanted, explosion.

Most coal deposits contain sulphides/sulphur. Most lignite (brown coal, also low quality or low grade coal) deposits are nearer the surface, thus if lignite has lots of sulphur &amp; it oxidizes with atmospheric oxygen combustion can occur.