In-situ synchrotron ftir study on functional group evolution during pyrolysis of tar-rich coal de-mineralized by acid washing

Tar-rich coal is a significant coal-based resource for oil and gas, and its pyrolysis for oil production is a crucial direction for its efficient utilization. Inherent minerals in coal can alter the characteristics of pyrolysis products by affecting the migration and release of functional groups during pyrolysis. Acid washing is an effective method for removing these inherent minerals. Understanding the influence mechanism of inherent minerals (removed via acid washing) on the evolution of functional groups during the pyrolysis of tar-rich coal is a key scientific issue in elucidating the pyrolysis process and mechanism of tar-rich coal. Using in-situ infrared spectroscopy technology at the National Synchrotron Radiation Laboratory in Hefei, in-situ characterization was performed on the migration and release of functional groups during the pyrolysis of tar-rich coal and its acid-washed variant (0‒500℃). The absorption peaks of aliphatic (3200‒2800 cm^‒1) and aromatic (1750‒1400 cm^‒1) functional groups in the in-situ infrared spectra were quantitatively analyzed, and combined with thermogravimetric (TG) experiments, to reveal the impact of inherent minerals on the evolution of functional groups during the pyrolysis of tar-rich coal. The study results indicate that after acid washing, the influence of the Si—O absorption peak is removed in the in-situ infrared spectra of tar-rich coal, revealing more characteristic peaks of organic functional groups. In the 3200‒2800 cm⁻¹ range, the removal of minerals significantly accelerates the cleavage of methyl (—CH₃) and methylene (—CH₂—) groups during pyrolysis, allowing the cracking of aliphatic side chains to produce hydrocarbons at lower temperatures. For the aromatic functional groups in the 1750‒1400 cm^‒1 range, acid washing introduces more oxygen-containing functional groups and conjugated structures into the tar-rich coal, promoting aromatization and the condensation of aromatic compounds. Compared to the raw tar-rich coal, the mass loss of acid-washed coal during pyrolysis is 9.7% higher, and the initial pyrolysis temperature is approximately 150 °C lower. The research enhances the understanding of functional group migration and evolution during the pyrolysis of tar-rich coal and the influence mechanism of inherent minerals. It provides a solid theoretical foundation for the development of tar-rich coal pyrolysis technology.