Study on molecular structural heterogeneity of tar-rich coal based on micro-FTIR

The coal molecular structure in micro-areas plays a critical role in matrix thermal conduction and volatile generation during the pyrolysis of tar-rich coal. However, as a major maceral contributing to hydrocarbon generation, the molecular structures of different micro-areas in vitrinite show heterogeneity, which still lacks research. Micro-FTIR technology was used in this study to characterize the molecular structure in different micro-areas of tar-rich coal with varying tar yields. It exhibits a significant advantage in obtaining the molecular structure of the coal surface at the micrometer scale compared to transmission FTIR. The results have shown: Even within the homogeneous vitrinite under a microscope, the molecular structural heterogeneity in different micro-areas is also marked. Specifically, the functional groups in the 1100–1800 cm⁻¹ band show a greater heterogeneity than the 2800–3000 cm⁻¹ and 700–900 cm⁻¹ bands. In the former, the variation of the C=C, C-O, and –CH₂- contents is particularly pronounced, indicating that aromatic structures, ether bonds, and alkyl structures are the key factors leading to heterogeneity. Furthermore, samples with higher tar yields exhibit weaker molecular structural heterogeneity. The above research provides theoretical guidance for analyzing the pyrolysis behavior of tar-rich coal.