Humic coals are formed by peatification and coalification of plant materials.
Diagenesis comprises peatification and early coalification with the main degradation mechanism being biological. The main reactions are depolymerization of polysaccharides, aromatization and removal of methoxy groups from lignin. About 5-15% of bitumen is derived from minor plant component lipids, which are concentrated in the peat. With increasing depth and compaction, bacterial activity decreasing as temperature and pressure increase and catagenesis commences. At the end of diagenesis, the resulting coal contains no carbohydrates and <10% relatively unaltered lignin.
Catagenesis comprises the late coalification stage and leads to a further decrease in oxygen content, accompanied by decarboxylation and aromatization of alkyl side chains. Randomly oriented sheet-like clusters of aromatics are formed. Finally, after further condensation, anthracite is formed with only a few phenolic groups and with a parallel sheet arrangement. The types of coal formed from peat during these stages are defined in order of decreasing rank as: lignite, sub-bituminous coal, bituminous coal and anthracite. During diagenesis, the peat undergoes the loss of oxygen-containing functional groups until the O/C ratio decreases to 0.1. After this, the diagenesis stage ends and the categenisis starts resulting in a decline in H/C ratio.
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