Comprehensive investigation of the mechanisms for pyrolyzing macromolecular networks in Hecaogou subbituminous coal by comparing the ethanolysis and flash pyrolysis

The organic macromolecular networks (MMNs) from Hecaogou subbituminous coal (HSBC) were investigated by multiple technical strategies, including SEM, H-1 and C-13 NMR, FTIR, XPS, TG/DTG, and flash pyrolysis (FP) technology. The pyrolysis mechanisms of MMNs were effectively investigated to release organic volatile species between ethanolyzed (E) inextractable portion (IEPE) and FP of IEP (IEPFP) at molecular level. In detail, solid-state C-13 NMR analysis shows that carbon skeleton of IEP mainly consists of aliphatic (59.6%) and aromatic carbons (39.3%). XPS analysis shows that the >C-OH groups, pyrrolic, and sulfonic sulfurs in IEP surface are the most abundant O-, N-, and S-containing species. FTIR analysis proves that the weak- and medium >C-al-X (X denotes C-al<, H, O-, N<, and S-) in IEP could be cleaved during ethanolysis. Based on the destructive analysis with TG/DTG, the structure of MMNs in IEP features dissociating small molecular groups with different types >C-al-X bridge bonds, which facilitates the release of organic volatile species and then prevents the thermal condensation during pyrolysis process. Additionally, the relative content (RC) in IEPE of various organic species are as follows: chain alkanes (CAs, 18.5%), alkenes (0.9%), arenes (11.9%), and oxygen-containing organic compounds (OCOCs, 58.4%), while the RC of CAs (46.8%), alkenes (17.5%), arenes (16.8%), and OCOCs (13.8%) in IEPFP. Comprehensive investigation and detailed analysis the distribution of organic species further prove that the active hydrogen (H center dot) cleave the >C-al-X of MMNs at different positions during the pyrolysis process to obtain more CAs, alkenes, alkanols, and phenols at molecular level.