Interesting comments. Coal, as you point out, ranges from "packed peat" to "pure carbon."

I don't know about "spoilage" exactly, but I can tell you that our local coal (sub-/semi- bituminous) will fracture into smaller and smaller "fines" if exposed to the weather. Pretty soon it's approaching dust in its consistency. This is mildly problematic for use as stove coal; I may need to mix it with sawdust and pack it in old tin cans in order to burn it.

Meanwhile, coal stored in a dry outbuilding is exactly as it came from the colliery. So your point about storage is quite valid.

I'd love to get a quarter ton of that Welsh Steam Coal to play with. Wonder how much it would cost to FedEx it here? (Kidding, I'm a kidder. ;-)