I checked out the compost thing a couple of years ago when I was planning to put a compost pile near a building. All the studies indicated that any pile less than 7'x'7'x7' would not ignite.

Apparently, the biggest offender was green hay stored in a barn. When too green, it starts to compost and the internal temperature rises high enough to spontaneously combust, and there goes another barn. Not to mention the hay.

Sue