section_path: Formation of coal/Chemistry of coalification
section_name: Chemistry of coalification
paragraph_text: Chemistry of coalification
  
  The woody tissue of plants is composed mainly of cellulose, hemicellulose, and lignin. Modern peat is mostly lignin, with a content of cellulose and hemicellulose ranging from 5% to 40%. Various other organic compounds, such as waxes and nitrogen- and sulfur-containing compounds, are also present. Lignin has a weight composition of about 54% carbon, 6% hydrogen, and 30% oxygen, while cellulose has a weight composition of about 44% carbon, 6% hydrogen, and 49% oxygen. Bituminous coal has a composition of about 84.4% carbon, 5.4% hydrogen, 6.7% oxygen, 1.7% nitrogen, and 1.8% sulfur, on a weight basis. The low oxygen content of coal shows that coalification removed most of the oxygen and much of the hydrogen a process called carbonization.
  
  Carbonization proceeds primarily by dehydration, decarboxylation, and demethanation. Dehydration removes water molecules from the maturing coal via reactions such as
  
  Decarboxylation removes carbon dioxide from the maturing coal:
  
  while demethanation proceeds by reaction such as
  
  In these formulas, R represents the remainder of a cellulose or lignin molecule to which the reacting groups are attached.
  
  Dehydration and decarboxylation take place early in coalification, while demethanation begins only after the coal has already reached bituminous rank. The effect of decarboxylation is to reduce the percentage of oxygen, while demethanation reduces the percentage of hydrogen. Dehydration does both, and (together with demethanation) reduces the saturation of the carbon backbone (increasing the number of double bonds between carbon).
  
  As carbonization proceeds, aliphatic compounds convert to aromatic compounds. Similarly, aromatic rings fuse into polyaromatic compounds (linked rings of carbon atoms). The structure increasingly resembles graphene, the structural element of graphite.
  
  Chemical changes are accompanied by physical changes, such as decrease in average pore size.