Decoding the Hydrogen-Transfer Synergy in Camellia oleifera Shell Hydrochar/HDPE Co-Pyrolysis: A Kinetic and Thermodynamic Perspective

This study examined the co-pyrolysis of high-density polyethylene (HDPE) and hydrochar from Camellia oleifera shell (HCS), focusing on thermal decomposition behavior, non-isothermal kinetics, thermodynamics, and the interaction mechanism between the two feedstocks. Hydrothermal carbonization raised the heating value of Camellia oleifera shell from 18.07 MJ/kg (raw CS) to 20.80 MJ/kg (HCS), confirming its fuel-upgrading effect, while HDPE alone reached 46.40 MJ/kg. TG-DTG analysis of HDPE, HCS, and their blends at five HCS:HDPE mass ratios (1:9, 3:7, 5:5, 7:3, and 9:1, denoted 1HCS9HDPE through 9HCS1HDPE) at 5–25 °C/min showed a single sharp decomposition stage for HDPE, a broader profile with much higher char yield for HCS, and non-additive DTG behavior in the blends, pointing to a synergistic interaction. Isoconversional kinetics (FWO, KAS, Starink, Friedman) showed activation energy rising steadily with conversion for HDPE (170–240 kJ/mol), a plateau-then-sharp-rise pattern for HCS (180–400 kJ/mol), and marked fluctuations at intermediate conversion for the balanced blends, a signature of composition-dependent synergy. Thermodynamic analysis (ΔH, ΔG, ΔS) confirmed an endothermic, non-spontaneous process throughout, with entropy trends mirroring the kinetic fluctuations. A hydrogen-radical transfer mechanism between HDPE- and hydrochar-derived intermediates is proposed to explain the reduced char yield and non-additive behavior of the blends. Overall, the results support hydrochar–HDPE co-pyrolysis as a viable route for combined agricultural and plastic waste valorization and provide kinetic and mechanistic data to guide co-pyrolysis process design.