Coal is not a homogenous substance meaning it has no fixed chemical formula.  It was formed when the earth was covered with huge swampy forests where plants - giant ferns, reeds and mosses - grew.  As the plants grew, some died and fell into the swamp waters.  New plants grew up to take their places and when these died still more grew.  In time, there was thick layer of dead plants rotting in the swamp.  The surface of the earth changed and water and dirt washed in, stopping the decaying process.  More plants grew up, but they too died and fell, forming separate layers.  After millions of years many layers had formed, one on top of the other.  The weight of the top layers and the water and dirt packed down the lower layers of plant matter.  Heat and pressure produced chemical and physical changes in the plant layers which forced out oxygen and left rich carbon deposits. In time, material that had been plants became coal.

Coals are classified into three main ranks, or types: lignite, bituminous coal, and anthracite.  These classifications are based on the amount of carbon, oxygen, and hydrogen present in the coal.  Coal is defined as a readily combustible rock containing more than 50% by weight of carbon.  Coals other constituents include hydrogen, oxygen, nitrogen, ash, and sulfur.  Some of the undesirable chemical constituents include chorine and sodium.

For more information check out the Online Kentucky Coal Facts Book, and go to Coal Resources then click on Coal Origin and Properties and Coal Properties/Improvements.

Also the Kentucky Geological Survey has a great website - www.uky.edu/KGS/home.htm