Composition of Coal


A) Coal originates from vegetal matter and forms from the compression of peat over millions of years (Figure 1).

B) It is a stratified, organic, biochemical sedimentary rock.

C) Coal is a heterogeneous mixture of plant materials and minerals.

        -Ex. wood, bark, leaves and sap.

D) The calorific value (which is the calories or thermal units contained in one unit of a substance and released when the substance is burned) of coal and lignite varies tremendously; ranging from 15,000 BTUs/lbs to under 6,300 BTUs/lbs.  These values, however, are the extremes.  Most ranks of coal are between these two levels.

Figure 1: Formation of Coal

  Source: Coal Exploration, Mine Planning, and Development

E) The organic component of coal consists of chemical compounds from carbon, hydrogen, oxygen, sulfur, and nitrogen. (Figure 2)

F) With the exception of nitrogen, these elements are also found in the mineral species that occur in coal.

         - Ex. carbonates, sulfides, sulfates and hydrous clay minerals.

         - The ash in coal is made of decomposed residues of silicate,     

          carbonates and sulfides.

Figure 2: Composition of Coal

             Source: imic/combust.htm              

G) Coal is ranked in six different categories:

 1. Peat - not really a coal; it is undeveloped, soft and spongy. It is 20-25% carbon, and was deposited during the last 100 years by decomposed plant remains.    

 2. Lignite- the first "real" coal.  It has a low heating value and is often found near the earth's surface. Lignite coal loses its moisture when exposed to air.

 3. Subbituminous - lower in carbon content than bituminous coal.  It also loses its moisture when exposed to air.  When exposed to wetness it

 4. Bituminous - has a high carbon content (65-85%), and a low oxygen content (5-15%).  It usually burns with a yellow flame.

 5. Semianthracite - a hard coal but softer than anthracite.  It has a fixed carbon percentage of 80-85%.  It contains more volatile matter than anthracite and also burns more freely.

 6. Anthracite - hard, durable, high density.  It has low volatile matter and is 85-98% fixed carbon.  It ignites slowly and burns with little flame or smoke.

H) Hydrogen and Carbon Composition

1. Low-rank coals typically have relatively low carbon contents and relatively high oxygen contents (Figure 3).

2. Lignite, subbituminous, and low rank bituminous coals have hydrogen    contents of 5-6% (Figure 3).

3. High rank bituminous and anthracite coal are 3-4% hydrogen (Figure 3).

Figure 3: Percent Hydrogen, Percent Carbon

   Source: Coal Exploration, Mine Planning, and Development


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