Synergistic evolution of covalent bonds in coal-oil co-pyrolysis via13C NMR and in-situ transmission FTIR analysis

A comprehensive understanding of the structural characteristics of heavy oil and coal, as well as the synergistic effects of their co-pyrolysis at the covalent bond level, is crucial for the investigation of the coal-oil co-pyrolysis. This study investigated the carbon structural parameters and the concentrations of relevant covalent bonds in Baishihu coal (BSH) and the heavy fractions of oil sand oil (YSY) using 13C NMR. The evolution of seven representative functional groups in pyrolysis residue and relevant gases of BSH, YSY, and their mixture (BSH-YSY) was monitored during the heating stage (100–430 °C) and the constant temperature stage (430 °C for 30 min) with in-situ transmission FTIR. The results showed that BSH and YSY exhibit “archipelago-type” and “island-type” structures, respectively. Specifically, the pyrolysis of BSH predominantly involved the cleavage of weak bonds such as C_al–C_al and C_al–O. In contrast, YSY possessed nearly twice the concentration of easily cleavable bonds (e.g., C_al–C_al and C_al–H), resulting in the formation of a substantial amount of small alkyl radicals. These radicals from YSY promoted the cleavage of C_al–C_al and C_al–O bonds in BSH, as well as stabilized the small aromatic and oxygen-containing radicals generated from BSH. This synergistic interaction facilitated the release of aliphatic, aromatic, and oxygen-containing organics, while suppressing the emission of CO and CO₂. These findings provide a scientific basis for feedstock selection and process optimization in coal-oil co-pyrolysis by elucidating its synergistic mechanism from the perspectives of covalent bond cleavage and radical stabilization.