Kinetic analysis and structural evolution during the co-pyrolysis of coal gangue and coix seed straw

Co-pyrolysis is an effective technology for the simultaneous and efficient utilisation of biomass and coal gangue (CG). However, the key factors influencing the properties of co-pyrolysis char at different stages remain unclear. In this study, TG and multiple kinetic models (FWO, KAS, Friedman, and DAEM) were employed to analyse the synergistic effects and stage division of the co-pyrolysis process of biomass (coix seed straw, CSS) and CG. The results indicate that CSS8CG2(CSS: CG = 8:2) exhibited optimal performance, and the pyrolysis process can be divided into three distinct stages: drying (<150°C), rapid pyrolysis (150–450°C), and carbonisation (>450°C). By integrating characterisation techniques such as GC, TG-FTIR, XRD, BET and SEM-EDS, the main structural evolution during each stage was systematically revealed: the drying stage primarily involved the removal of moisture and light volatiles; the rapid pyrolysis stage was accompanied by intense decomposition of cellulose/hemicellulose and significant pore structure development (20.12 nm); the carbonisation stage involved aromatisation reorganisation of the carbon matrix and mineral phase transformation, with XRD confirming the formation of new potassium aluminosilicate crystalline phases (KAlSiO₄, KalSi₃O₈). These phases catalysed deoxygenation and aromatisation reactions, thereby stabilising the char structure. CSS8CG2 demonstrated optimal pyrolysis characteristics during the rapid pyrolysis stage (400°C): the syngas exhibited a high heating value of 49.5 MJ·kg^-1 (analysed by GC), and the apparent activation energy reached a minimum of 104.83 kJ·mol^-1. Life cycle assessment revealed that processing per kilogram of the mixed sample achieved a carbon reduction benefit of −2356.62 × 10^-4 kg CO₂ equivalent, demonstrating strong alignment with national climate targets.