It is generally assumed that the coalification of terrestrial kerogens proceeds along an orderly, progressive path from lignitic to anthracitic coal. However, when information concerning the volatile compounds present in coal (the average volatilization rate [AVR] determined by thermogravimetry) is compared with the hydrogen to carbon (H/C) ratio, some subbituminous coal samples that have relatively low H/C ratios (≤0.75) appear to contain insufficient amounts of hydrogen to be transformed into high-volatile bituminous coal. These samples also have relatively high oxygen to carbon (O/C) ratios (>0.14). The positions of these samples on an AVR vs. H/C plot suggests that their transformation to the high-volatile bituminous rank would not be possible unless hydrogen in some form were added from an external source.
The results suggest that all terrestrial coal may not necessarily pass through all preceding ranks. Alternative coalification paths may be possible as a result of differences in the composition of precursor plant material, the conditions prevalent during peatification, and the temperature, pressure, and confining-pressure conditions during coalification.