Jet
Anthracite Coal

Coal is a sedimentary rock formed from plants that flourished millions of years ago when tropical swamps covered large areas of the world. Lush vegetation thrived in these swamps, vegetation such as sedges (grasslike flowering plants), reeds, and enormous ferns. Generations of this vegetation died and settled to the swamp bottom, and over time the organic material lost oxygen and hydrogen, leaving the material with a high percentage of carbon. Layers of mud and sand accumulated over the decomposed plant matter, compressing and hardening the organic material as the sediments deepened. Over millions of years, deepening sediment layers exerted tremendous heat and pressure on the underlying plant matter, which eventually became high-grade coal.

Before decayed plant material forms coal, the plant material forms a dark brown, compact organic material known as peat. Although peat will burn when dried, it has a low carbon and high moisture content relative to coal. Most of coal's heating value comes from carbon, while inorganic materials, such as moisture and minerals, detract from its heating value. For this reason, peat is a less efficient fuel source than coal. Over time, as layers of sediment accumulate over the peat, this organic material forms lignite, the lowest rank of coal. As the thickening geologic overburden gradually drives moisture from the coal and increases its fixed carbon content, coal evolves from lignite into successively higher-ranked coals: subbituminous coal, bituminous coal, and anthracite. Anthracite, the highest rank of coal, has nearly twice the heating value of lignite.