There can be quite a difference in coals depending on what vein they're mined from. They vary in BTU content, % ash, how long or how quickly it burns, the color of the ash remaining and the texture of the ash. The ash could be crumbly, sandy or maintain its original shape and be a bit crunchy which is a result of the vien's mineral content. Then there's how it was sorted which might mean the size distribution is wide or narrow not to mention garbage like wood, rock etc that might be present. Look around this forum and you'll find quite a few threads about this topic.
2112 edit: There's a thread in the New Jersey froum about NJ coal dealers.
Check it out and check your PM