Hi bustedwing,, read up on the many 'quality of coal' threads.. there are several highly recommended suppliers, breakers,, and also several to stay away from..
When I visited Jerry's shop a few years ago, he showed me some of the 'not so good' coal, it had about 10-15% unburnt in the ash pan.. Jerry had experimented with sorting out the 'unburnt' coal, and putting it back on the grate to 'reburn',, which it didn't... The unburnt 'coal' was either shale, or old refuse coal that had sat in the sun and rain in culm piles for decades... and the stuff just won't burn...
Buy from a known good breaker and preferably a deep-mine source.. it generally is the best with the least unburnt coal in the ash.
hope this helps.. Greg L