In my world, there could be no Carboniferous, during which the overwhelming majority of coal was deposited on our planet. No swamps probably exist either, cutting off that way of generating coal as well. The world is basically an artificial creation in form of a planet-sized cliff, and though this can justify the "just put coal in there anyway, then!" solution, it's handwaving, and I'd like to avoid that.
Yet, I need coal or similar substance (Rich in carbon, good for burning and steelmaking, mined from underground, although the last one is just desirable one for authenticity to the real coal mines) to propel the steampunk industrial revolution phase of the world's history. Is this possible? Maybe some form of carbon-accumulating plant that for some reason can grow deep underground ("Coalroot" sounds like a cool name for such thing)?