Reaction pathways and kinetics of hydrothermal liquefaction of plastics and food waste macromolecules under partially oxidative conditions

This study investigated the reaction mechanisms and kinetics of hydrothermal liquefaction (HTL) enhanced by reactive oxygen species from H₂O₂, aimed at converting mixed plastic and agri-food wastes into valuable products. The HTL research comprised two approaches: (1) mixture experiments designed using a simplex lattice framework for five components of degree two and (2) species-specific kinetic profiling experiments. A representative set of pure compounds was selected to model primary plastic and organic macromolecules: polyethylene (PE), D-(+)-cellobiose (CEL), 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol (DMP), glutamic acid (GLUTA), and linoleic acid (LIN). Mixture experiments, conducted at 400°C for 60 minutes, demonstrated synergistic interactions among the binary mixtures PE-CEL, CEL-GLUTA, GLUTA-DMP, and LIN-DMP in promoting biocrude formation, as well as PE-CEL and CEL-DMP in enhancing the production of aqueous organics and hydrochar. Conversely, antagonistic interactions were observed in CEL-DMP mixtures for biocrude formation, CEL-GLUTA and GLUTA-DMP mixtures for aqueous coproducts, and PE-CEL mixtures for gas formation. The kinetic profiling experiments at 300–400°C with 10-minute sampling intervals provided mechanistic explanations for the observed effects from mixture experiments. Initially, lump kinetic models were employed to describe the time evolution of product formation. These models used pseudo-first-order and pseudo-second-order kinetics for single reactants and binary interactions, respectively. Subsequently, detailed reaction pathways were elucidated by integrating well-established organic chemistry mechanisms with time-resolved concentration data of major chemical species. Species-specific kinetic models, employing nth-order power-law equations and Arrhenius parameters, coupled with thermodynamic models to estimate ΔH based on bond energy, revealed endothermic or exothermic nature of individual pathways. The findings from this study offer valuable insights into the kinetic and thermodynamic factors governing HTL, enabling improved control over product yields and the optimization of their physicochemical properties.