Coal mines vary widely in the amount of impurities in the coal they produce. Low-impurity coal requires less scrubbing to meet a give emissions target. Sulfur is the worst of the impurities -- it becomes sulfuric acid in the atmosphere and is precipitated by rain, causing concrete to erode.

The cleaner coal is called Anthracite [0]. The dirtier coal is Lignite [1]. Lignite is usually cleaned, by heating it up to drive out the water so it burns better (and weighs less during shipping.)

[0] https://en.wikipedia.org/wiki/Anthracite [1] https://en.wikipedia.org/wiki/Lignite