FTIR study of hydrocarbon generation reactions of coal

The thermal evolution characteristics of bulk coal as well as chemical structural characteristics of aliphatic and aromatic groups within an artificially matured coal series were characterized and analyzed combining the FTIR analysis with a computer curve-fitting method. The results showed that except for thermal cracking reactions, the aromatization reactions of aliphatic structures and the condensation reactions of aromatic structures also played important roles in the hydrocarbon formation of coal. Both contents of aliphatic hydrogen and aromatic hydrogen were subjected to more than one type of chemical reaction. Functional group analysis alone cannot accurately reflect the mechanism of hydrocarbon generation reactions and the hydrocarbon generation potential of coal. Further investigations on the thermal evolution characteristics of aliphatic and aromatic chemical structures indicated that the initial phase of hydrocarbon generation of coal is dominated by the formation of oil products due to the prior cleavage of long-chain aliphatic groups with lower bond energies. The highest yield of oil products is reached at 325℃. Except for generating gaseous hydrocarbons via further thermal cracking, a significant proportion of long chain aliphatic structures are involved in aromatization reactions in the range of 325-400℃, leading to the increased content of aromatic hydrogen of coal. With increasing temperature, the short chain aliphatic substituents begin to cleave from the β-position to aromatic rings, indicating the methylation of aliphatic substituents. At higher temperature stage further demethylation takes place with the breaking of methyl and bridged bonds connected to aromatic rings. The condensation reactions during hydrocarbon formation of coal take place in stages. The lower temperature phase in the range of 300-400℃ is dominated by the condensation of aromatization products. Due to the steric hindrance of substituents and bridged bonds, the condensation between the primary aromatic structures of coal mainly takes place along with the demethylation effect in the range of 500-600℃.