Just a side note. Coal is graded by checking it for sulphur content, how much ash it produces and BTU output.

Low-sulphur, low-ash high BTU coal is more expensive since it burns hotter, burns more completely and puts out less raw material for the formation of acid rain.

If you go to buy some coal, be sure to ask for the content ratings.
_________________________
Knowing where you're going is NOT the same as knowing how to get there.