Pyrolysis behavior of tar-rich coal with various coal-forming environments: A TGA and in-situ transmission FTIR study

Tar-rich coal, abundant in northwestern China, undergoes variations in the coal-forming environment, resulting in diverse occurrences of tar-rich coal and its molecular structures across regions and layers. The pyrolysis process of these coals also differs accordingly. This study investigated four coal samples with different tar yields and indicative of various coal facies using thermogravimetric analysis and in-situ transmission FTIR, and revealed that coal facies significantly affect tar yield by controlling the molecular structure. In particular, coal samples formed in a stronger reducing environment contain higher levels of aliphatic and oxygen-containing functional groups. Consequently, during pyrolysis, a substantial proportion of C_al-C_al, C_al-H, C_al-O, and C_al-N bonds were broken, contributing to approximately 30–60 % of the total cracked bonds, and X18 exceeded Z52 by a significant 21.55 %, leading to higher tar yields. In-situ transmission FTIR observations indicated that coal samples formed in a stronger reducing environment exhibit higher contents of –CH₂–, –CH–, C–O, and C[dbnd]O structures, which decompose more rapidly after 400 °C, and X18 with the highest decomposition of 18.51 % in C–O, while Z52 had the minimum. Moreover, coal samples formed in a stronger reducing environment exhibit a more significant reduction in –CH– structures compared to –CH₂– and free-ends –CH₃ structures beyond 400 °C, while coal samples formed in a weaker reducing environment display higher reductions in free-ends –CH₃ and –CH₂– structures than –CH– structures. The formation of gaseous products during coal pyrolysis is closely linked to the decomposition and evolution of residual coal functional groups, with increased accumulation observed under more enhancing and reducing conditions. These findings offer valuable insights into optimizing tar-rich coal resource utilization and the development of effective pyrolysis techniques.