ABSTRACT
Coal is a heterogeneous mixture of organic material, moisture, and mineral matter in various compositions. The composition of coal varies widely, depending on the time history under which the coal has undergone exposure to heat and pressure (the process is called coalification). Coals are ranked according to the coalification stage achieved and primarily are grouped into four major types in ascending order of their rank: lignite, subbituminous, bituminous, and anthracite.
