There are two ways to measure coal quality. Coal rank is a measure of the heat content and coal grade is a measure of the coal purity.
The major ranks of coal from lowest to highest are lignite, subbituminous, bituminous, semi-anthracite and anthracite. The higher the coal rank the higher the temperature and pressure of coal formation. The higher coal ranks have a higher percent carbon. As moisture and volatiles are driven off during coal maturation carbon is left behind. With an increase in carbon content there is an increase in the heat content (Btu/lb) of the coal.
This diagram shows the distribution of coal by rank. The majority of U.S. coal is bituminous. The highest rank coal anthracite only makes up 2 percent of coal reserves. This coal is too valuable to be burned as a fuel and is used mainly for coking steel.
One of the major contaminants that lowers the grade of coal is the sulfur content. Sulfur occurs as organic sulfur or in sulfide minerals such as pyrite. The form of sulfur becomes important when trying to remove it from coal to improve its grade. Pyrite is relatively easy to remove but organic sulfur is difficult to remove. The major problem with sulfur is that when coal is burned, it emits the sulfur which lowers the air quality and causes acid rain.
Sulfur dioxide which is emitted due to the burning of high sulfur coal reacts with water in the atmosphere forming sulfuric acid. The resulting precipitation is more acidic than normal precipitation. Acid rain has been cited as the cause of the decline in forest of the eastern U.S. and Canada, which are located downwind of major industrial areas along the Great Lakes. When acid rain falls in areas, such as Ohio, that have soils with a high carbonate content it is neutralized and does not cause any problems. Problems arise in areas such as the Adirondack Mountains and New England that are underlain by igneous and metamorphic rocks and have soils that are naturally acidic. These soils lack carbonate minerals which act as buffers to neutralize the acid rain. A small increase in the acidity of these soils can have a dramatic effect on the vegetation.
The ash content consist of noncombustible minerals and metals. It may be rock and mineral matter that was included when the coal was deposited or sedimentary rocks associated with the coal that were mixed into the coal when it was mined. The ash content lowers the heat content of the coal because this material does not burn. The ash residue left after the coal is burned must be discarded and the large volumes of ash present disposal problems.