Molecular Structure Modification Mechanism during Biodegradation of Coal with Different Degrees of Metamorphism

To explore the process of molecular structure change during the biodegradation of coal, Proximate and ultimate analysis, 16S rRNA sequencing analysis and Liquid chromatography-mass spectrometry (LC-MS) were used to analyze the evolution process of molecular structure of different coal ranks (bituminous coal, anthracite coal, coking coal) in the process of biodegradation. The results indicate that the biodegradation of coal is a process of enriching carbon elements (increasing C element by 1.28%~4.04%) and removing hydrogen, oxygen, nitrogen, and sulfur elements. Biodegradation destroys the ether bond (C-O-C) and carbonyl (C=O) structures in coal, causing the breakage of its aliphatic side chains and the detachment of surface functional groups. In addition, biodegradation breaks the weak bridging bonds between aromatic layers in coal, leading to the cracking of polycyclic aromatic hydrocarbon structures and an increase in aliphatic side chains, resulting in a slight improvement in the aromaticity of coal samples. These results suggest that biodegradation contributes to the aromaticity and maturity of coal, which is similar to the peatization process of coal.