How is coal formed?

The environments or conditions under which these coals were formed: anthracite coal, bituminous coal,
lignite?

Coal formed millions of years ago 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 to 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.  In the process of transformation (coalification), peat is altered to lignite, lignite is altered to sub-bituminous, sub-bituminous coal is altered to bituminous coal, and bituminous coal is altered to anthracite.

Lignite - is the lowest rank of coal - which means that it has the lowest heating value and lowest carbon content.  Although lignite is more solid than peat it crumbles when shipped long distances.  Most lignite in the U.S. is in North and South Dakota, Montana, and Texas. Lignite is used to generate electricity. Other uses include generating synthetic natural gas and producing fertilizer products.

Bituminous - is intermediate in rank and sometimes called soft coal. It appears smooth when you first see it, but look closer and you'll find it has many layers.  It is the most abundant kind of  coal.  It has a high heating value, but it also has a high sulfur content.   More than 80% of the bituminous coal produce in the U.S. is burned to generate electricity.  Other major coal users are the cement, food, paper, automobile, textile and plastic industries.  Another important industrial use is to provide coke for iron and steel industries.  Bituminous coal derivatives, or by-products can be changed into many different chemicals form which we can make paint, nylon, aspirin and many other items.

Anthracite - is the highest rank of coal which means that it has the highest heating value and highest carbon content.  It is very hard, deep black, and looks almost metallic because it is brilliantly glossy.  Anthracite burns longer, with more heat and with less dust and soot than other types of coal.  The primary market for anthracite is for heating homes.  Nearly all of the anthracite in the U.S. is in Pennsylvania, but there are some small beds in other states.

For more info on coal for your research check out the Kentucky Coal Facts Book, it has lots of information about coal from how it was formed to how it's used to make electricity.  Another good sight is the Kentucky Geological Survey.

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