FTIR characteristics of charcoal with different combustion degrees as an indication of the genesis by and their significances for formation of fusinite in coal

Fourier transform infrared spectroscopy (FTIR), as a non-destructive method, is widely used for the identification of compounds and the characterization of molecular structures. In order to characterize the changes in the chemical structure of charcoal under different combustion temperatures, and thus to provide a theoretical basis for the formation of fusinite in coal, plant samples (charcoal) from modern wildfires with different degrees of combustion were selected to quantify their chemical structures using FTIR. The results shown that the sample reflectance was positively proportional to the combustion temperature. The sample No. 1 with maximum combustion temperature had the highest degree of combustion, which was measured to reach 518 ℃. The aromatic structure was dominated by tri-substituted benzene rings in all samples except the highest combustion sample No. 1, but dehydrocondensation occurred with increasing combustion temperature, resulting in a reduction of tri-substituted content of benzene rings to 20.5%. The tetra-substituted content was elevated due to dehydroaromatization of the naphthenic structure, while the change in the penta-substituted content was related to the cyclization of aliphatic chain and the decarboxylation of benzene ring. With the increase of combustion temperature, the CC content gradually increased due to the formation of aromatic hydrocarbons or the shedding of molecular side chains after dehydrogenation of cycloalkanes, reached 32% in the sample No. 1. The content of C－O first decreased and then increased. In the sample No. 1, the content of alkyl ether and aryl ether was the lowest, and the content of phenolic hydroxyl group was the highest, which may be the generation of phenolic substances by thermal breakage of ether bond under high temperature combustion. The CO content increased and then decreased to as low as 5.6% in the sample No. 1, which was due to the poor stability of the bond. Due to the influence of combustion temperature, the content of fatty substances varied greatly, with an overall gradual increase in methylene content, a decrease in methyl group, and an increase in branching degree. There were five types of hydrogen bonds in the samples, with ether-oxygen hydrogen bonds predominating in samples affected by low temperature (>55%). Cyclic hydrogen bonds and hydroxyl-N hydrogen bonds appeared in sample No. 1, while the content of ether-oxygen hydrogen bonds decreased significantly to 13.2%, which was attributed to the reduction of oxygen-containing functional groups caused by the increasing temperature. Comparison of reflectance and FTIR characteristics of fusinite in coal revealed that the characteristics of fusinite (semifusinite) in coal were very similar to those of charcoal, which might be produced mainly by wildfires. These changes indicated the effect of combustion temperature on the chemical structure in charcoal, reflecting the process of organic molecular structure changed with temperature in charcoal, and providing a theoretical basis for the evolution of organic matter and the formation of fusinite in coal.