Coal is a sedimentary rock [1] formed from plants that flourished millions of years ago when tropical swamps [2] covered large areas of the world. Lush vegetation [3], such as early club mosses [4], horsetails [5], and enormous ferns, thrived in these swamps. 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 [6] accumulated over the decomposed plant matter, compressing and hardening the organic material as the sediments deepened. Over millions of years, deepening sediment layers, known as overburden, exerted tremendous heat and pressure on the underlying plant matter, which eventually became coal.

Before decayed plant material [7] forms coal, the plant material forms a dark brown, compact organic material known as peat [8]. 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, whereas inorganic materials, such as moisture and minerals [9], 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 [10], the lowest grade 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-graded coals: subbituminous coal [11], bituminous coal [12], and anthracite [13]. Anthracite, the highest rank of coal, has nearly twice the heating value of lignite.

Coal formation began during the Carboniferous Period (known as the first coal age), which spanned 360 million to 290 million years ago. Coal formation continued throughout the Permian [14], Triassic [15], Jurassic [16], Cretaceous [17], and Tertiary [18] Periods, which spanned 290 million to 1.6 million years ago. Coals formed during the first coal age are older, so they are generally located deeper in Earth��s crust. The greater heat and pressures at these depths produce higher-grade coals such as anthracite and bituminous coals. Conversely, coals formed during the second coal age under less intense heat and pressure are generally located at shallower depths. Consequently, these coals tend to be lower-grade subbituminous and lignite coals.

Coal contains organic (carbon-containing) compounds transformed from ancient plant material. The original plant material was composed of cellulose [19], the reinforcing material [20] in plant cell walls [21]; lignin [22], the substance that cements plant cells together; tannins [23], a class of compounds in leaves and stems; and other organic compounds, such as fats and waxes. In addition to carbon, these organic compounds contain hydrogen, oxygen, nitrogen, and sulfur. After a plant dies and begins to decay on a swamp bottom, hydrogen and oxygen (and

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