I've heard east coast (US) coal is dirtier than west coast (US) coal, particularly in sulfides. The acid rain fix known as the Clean Air Act of 1990 was heavily lobbied by the east coast coal producers and bears some marks to show for it. Burning so called "Clean Coal" has to do with how much gross weight of sulfur you pull out of the coal, which makes sense for coal heavy in sulfides, but less so for west coast fuel. As it stands, to burn west coast coal in the US requires the addition of sulfides to the coal so it can then be pulled back out.
As for other impurities... there are a lot of radioactive crap in coal that is aerosolized during the combustion process. The most notable of these is uranium, which is carried away with the rest of the fly ash. There's a "shadow" around coal smoke stacks where such fly ash ends up accumulating. If it makes it into the water table or onto arable land that is used to grow crops, BAM, you've irradiated a small chunk of your population.
Depending on the source of the coal, there are various other heavy metals that can end up in your fly ash.
As for other impurities... there are a lot of radioactive crap in coal that is aerosolized during the combustion process. The most notable of these is uranium, which is carried away with the rest of the fly ash. There's a "shadow" around coal smoke stacks where such fly ash ends up accumulating. If it makes it into the water table or onto arable land that is used to grow crops, BAM, you've irradiated a small chunk of your population.
Depending on the source of the coal, there are various other heavy metals that can end up in your fly ash.