If not used on a regular basis, the piled up coal would start to spontaneously combust in the center of the pile and start smoking.
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I’ve heard the same about piles of oily rags and leaves, too. I think any big pile of organic material can smolder.
There was a big compost and chicken s*** fire a number of years ago in Albuquerque's south valley.
Saw a huge pile of wood chips in the parking lot of a local golf course a few years ago that were smoking.......