"Four maturation/coalification stages are distinguidsh. Stage 1 (75 50 80% by
wt. C) occurs when modifications mainly concern the oxygen-bearing functions
of the kerogen. Stage 2 (82 to 85% by wt C) is characterized by the decrease
of the aliphaticity and the primary cracking of the coal. Stage 3 (86 to
89% by wt C) corresponds to the production of methane and the condensation
of aromatic rings in the solid residue" P. Landais and L. Gerard
"Coalification Stages from Confined Pyrolysis of an immature Humic Coal"
Int. Jour. Coal Geol 30:4:285-301 (Aug. 1996)
The compression of coal from plant matter requires the removal of most of the
oxygen and hydrogen. To do this it is required that some Carbon go with
them in the form of CO2 and CH4