Coupled evolution of macromolecular structure, gas generation, and carbon isotopic fractionation during lignite pyrolysis

Pyrolysis is a critical approach for the clean and efficient utilization of low-rank coals. In this study, lignite samples collected from the Huolinhe Depression, Erlian Basin, China, were subjected to confined gold-tube pyrolysis experiments within a temperature range of 120–600 ℃. The evolution of functional groups, gaseous product yields, and carbon isotopic compositions (δ 13 C) of gases were characterized by FTIR, gas chromatography, compound-specific carbon isotope analysis, and kinetic modeling. The results demonstrate that hydrocarbon generation of lignite follows a three-stage evolution pattern governed by the dynamic competition between thermal cracking and condensation reactions. The early stage (R o,max < 0.6%) is dominated by the removal of oxygen-containing functional groups generating substantial CO 2 and H 2 O. The middle stage (0.6% < R o,max < 1.3%) is characterized by intensive cleavage of aliphatic side chains, during which C 2 –C 5 heavy hydrocarbon yields increase rapidly and CH 4 generation rises markedly. The late stage (R o,max > 1.3%) is governed by aromatization and polycondensation, during which CH 4 production surpasses CO 2 to become the dominant gaseous component. Hydrocarbon yields are controlled by maceral composition, with higher huminite content corresponding to greater gas generation potential. Kinetic modeling reveals that C 6 –C 14 and C 6+ exhibit the lowest activation energies (34–48 kcal/mol) and the narrowest distributions, whereas CH₄ shows the broadest activation energy distribution (38–62 kcal/mol), indicating a dual origin from both primary cracking of kerogen and secondary cracking of liquid hydrocarbons and heavy gases. The δ 13 C values of CH 4 and C 2 H 6 show a "first decreasing and then increasing " trend with increasing R o,max , with inflection points at ~1.3% and ~0.9%, respectively, while C 3 H 8 displays a monotonic enrichment trend throughout the entire thermal process. In contrast, δ 13 C (CO 2 ) becoming heavier at R o,max < 0.6% and then continuously lighter—which is collectively governed by decarboxylation, carbon isotopic exchange between CH 4 and CO 2 , and CO 2 reduction. These findings provide experimental and theoretical constraints for understanding the coupling mechanism between structural evolution and isotopic fractionation during lignite pyrolysis.