The amount of impurities (ash and trace elements) in the coal determines its GRADE:
1. Ash. Ash is the unburnable part of coal. It is most often sand and clay blown into the swamp or brought in by river or tides. Most commercial coals range from 3% to 9% ash.
2. Trace Elements. A number of elements can affect either the combustion process or add to possible atmospheric pollution through emissions from the smoke stacks of power plants including sodium, sulfur, phosphorous, chlorides, nitrates, sulfates, and arsenic.
Sodium in the coal causes ash to precipitate on the boilers reducing the efficiency of the boiler and raising the cost of generating electricity. In one test, a lignite with 8% sodium fouled the boiler such that it had to be shut down for cleaning after only three days.
Sulfur in the coal is released as sulfur dioxide (SO2) upon burning. If it is not scrubbed out of the emissions, it will combine with moisture in the air producing sulfuric acid (H2SO4) which makes up some 60% of acid rain. (The other 40% is nitric acid from automobiles and trucks). Coals range from 0.5% to 8% or more sulfur. See next article on acid rain.
Phosphorus in coal causes slagging in boilers reducing their efficiency. In addition, steel made with a phosphorus rich coal as a heat source tends to be brittle.
Chlorides, Nitrates, and Sulfates cause corrosion in the boilers.
Arsenic is present in most coals at the ppb (parts per billion) level.
(From the Internet)