Raphael:
There are literally thousands of articles on the subject of "spontaneous combustion of coal stockpiles". Use your favorite search engine to find them.
One such article is -
http://www.saftek.net/worksafe/bull94.txt The issues is compounded when the coal gets wet. This is especially true with Bituminous coal since the moisture results in heating and the oxidation process.
Niel Leon
engineering.com