Interaction mechanisms of coal macerals during pyrolysis: Insights from TG and Py-GC/MS experiments combined with ReaxFF MD simulations

Abstract

The mechanisms of interaction among coal macerals during pyrolysis processes remain uncertain. This study investigates the interactions between vitrinite and inertinite in three Chinese coals of varying ranks using Thermogravimetric Analysis (TGA), Pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS), and reactive force field molecular dynamics (ReaxFF MD) simulations. Experimental results show that the interactions between vitrinite and inertinite consistently inhibit volatile matter release and increase pyrolysis activation energy across different coal types and mixing ratios. Furthermore, these interactions reduce the yield of polycyclic aromatic compounds while promoting the formation of light volatiles. ReaxFF MD simulations further reveal that the interactions between vitrinite and inertinite significantly enhance polycondensation reactions. The key mechanism involves larger inertinite molecules possessing numerous and persistent binding sites due to steric hindrance. These molecules can fix substantial amounts of smaller vitrinite fragments, particularly polycyclic aromatic molecules, resulting in reduced aromatic product yields. Further analysis reveals that interaction strength decreases with increasing coal rank. At equivalent coal ranks, the 1:1 mixing ratio exhibits stronger interactions compared to other ratios. Additionally, different mixing ratios demonstrate distinct interaction patterns across various temperature ranges. Subsequently, an interaction strength index and a comprehensive coal property index were developed for quantitative characterization. The results show a good positive correlation between the two indices, establishing that interaction strength is fundamentally governed by both coal rank and structural heterogeneity. This study provides new insights into complex coal pyrolysis mechanisms and offers valuable guidance for optimizing industrial pyrolysis processes.