Molecular structural evolution during coal oxidation based on in situ FTIR and Raman spectroscopy

Abstract

The elucidation of surface functional groups and microcrystalline structures in coal is fundamental for exploring the advanced coal conversion processes and elaborate pollutant formation mechanisms. In this work, in situ Fourier transform infrared (FTIR) and in situ Raman spectroscopy were employed to characterize the evolution of surface functional groups and microcrystalline structures throughout the coal oxidation process. The results indicate that oxidation causes the decomposition of -OH and -COOH, an increase in C-O functional group content, and the cleavage of CC bonds at high temperatures. As the temperature rises, aromaticity parameters (f_a, I) and condensation degree (DOC) generally increase, while the structural parameter ‘C’ for oxygen-containing groups decreases. Furthermore, Raman parameters show an increase in A_D/A_G (proportion of large disordered aromatic rings) and a decrease in A_(GR+VL+VR)/A_D (relative content of 3–5 ring aromatic hydrocarbons), which corroborate the FTIR aromaticity parameters. These results suggest that during the oxidation process, condensation reactions and aromatization of aromatic rings occur, resulting in a general rise in aromaticity. In addition, the anthracite HN coal undergoes significant graphitization at 500°C. Notably, during the comminution process, mechanical forces introduce an increase in -OH, CC, and CO content on coal surfaces, while C-O content decreases. Additionally, the reduction in particle size partially inhibits coal oxidation reactions in the initial stages, whilst accelerates the maturation of the aromatic system and the trend toward graphitization. These findings offer a theoretical foundation for analyzing the homogeneous and heterogeneous reduction mechanisms of NOx, and developing low-NOx combustion technologies.