Evolution and formation pathways of volatiles during co-pyrolysis process of coal and various polymers based on ReaxFF MD simulation and in-situ Py-TOF-MS

Integrating ReaxFF MD with TGA and in-situ Py-TOF-MS reveals synergistic mechanisms in co-pyrolysis of coal with HDPE, PP, and PS. • PP promotes C 5 -C 6 high-calorific gases, while PS enhances aromatics by cross-linking pathways. • PP promotes C C and C H cleavage while PS inhibits C O breaking and weakly promotes C H cleavage. • Coal/HDPE and coal/PP co-pyrolysis drive PAHs formation via Diels-Alder, alkylation, and aromatization reactions. • The polymer type critically determines the nature of the dominant reactive intermediates. Co -pyrolysis of low-rank coal and waste polymers offers a promising approach for plastic valorization while enhancing coal utilization and oil quality. In this work, the polymer types on the synergistic mechanism governing the co -pyrolysis of Zhundong low-rank coal and three typical polymers of HDPE, PP and PS were systematically investigated by integrating ReaxFF MD simulation with experiments of TG and in-situ Py-TOF-MS. It is observed that the temperatures of maximum weight loss ( T max ) follow the order of HDPE > PP > PS, consistently reproduced across three methods. PP addition promotes C 5 -C 6 high-caloric-value gases via long-chain cleavage while inhibiting the MAHs production. In contrast, PS facilitates phenolic hydroxyl removal from coal, promoting aromatic products. PS preferentially enhances light tar yield from 17.81% to 22.14%, whereas HDPE suppresses heavy tar yields from 39.90% to 35.56%. The ReaxFF MD simulation results further demonstrate the underlying molecular-scale mechanisms that are hardly accessible by experiments. HDPE and PP inhibit C O bond formation while promoting C H bond breaking during the co-pyrolysis. PP exhibits the strongest promotion of C-C/ C H cleavage, whereas PS shows the weakest effect on C H dissociation. During coal/HDPE and coal/PP co-pyrolysis, the formation of PAHs is promoted through Diels-Alder, alkylation and aromatization reactions. For coal/PS, the phenyl-alkyl radicals from PS combine with light tar, heavy tar, and char intermediates from coal to form cross-linked intermediates, facilitating cleavage of α -C ar -C al , β -C al -C al , and C ar -O bonds toward aromatics. The integrated strategy provides a comprehensive perspective for understanding co-pyrolysis mechanisms in other solid fuels and waste treatments. Download: Download high-res image (87KB) Download: Download full-size image