Interestingly - I've been told that coal can indeed spoil! I know a lot of folks who use coal for their large scale model trains (Look up live steam trains - I'm building one)

It seems with 'soft' coal (bituminious) there are volates that are fiven off once you mine the coal/the coal gets repeatedly wet. I understand that lignite is worse, and that the problem does NOT happen with Anthracite

That said, it supposidly only happens VERY slowly, and mostly to the top few inches of your pile - and to the surface of each lump.

According to the folks I talk to the answer is to store the coal in fairly large size lumps (say, 'lump' size), and breack it down to the 'nut', 'pea' or 'rice' size you need as you need it - and to either cover your pile with a tarp, or actually bury it!

Supposidly only a real issue for multi year - say decade storage

BTW I've lumped (Pun is intended) coal into 2 broad classes above. Lignite, Bituminious, and Anthracite (from softest to hardest). There are lots and LOTS of subgrades. When you are looking for peak performance - say you are running a power plant or steel Mill - or, on the opposite side of the size scale - the guys who run the model trains, you design your grate, flue and other parts of the system around a particular coal, and try not to switch

The Live Steam guys swear by Welch Steam Coal - but it's VERY hard to get and expensive (hey - you have to bring it in from Wales), and they USED to love Pocohantas #7 coal - but I understand #7 mine is shut, and the closest you can get is #14 (yes, different mines have different coals - so do different seams from the same mine)