What you're really discussing here is the difference between white ash and red ash.
The red ash has a higher ash content, sometimes up around 16% I think for superior. Because of the higher volatile matter it still produces a lot of heat. One of the problems is if you try and burn it too hot, you'll get clinkers.
The white ash has lower ash content, typically around 10 or 11%. As the ash content goes up with white ash the BTU's drop. I've seen some numbers where it has dropped into the 5 and 6% range. This coal is rock hard and not exactly easy to burn. It is so dense it would appear that half a ton was missing over a 5 ton load. While this is harder to burn it produces a lot of heat for a very long time once you get it roilling, the added benefit is picking up ash tubs that feel like they are filled with feathers.
You can get coal like that burn up to nothing but powder especially in a hand fired stove.
I picked up a lot of customers over the years that were using red ash and most preferred the white ash. Of course they weren't getting crap white ash which is a whole other ball game.