Enhancing sustainable waste management: Hydrothermal carbonization of polyethylene terephthalate and polystyrene plastics for energy recovery

Hydrothermal carbonization (HTC) of single plastic polymers such as polyethylene terephthalate (PET) and polystyrene (PS) has not yet been explored on a large scale, particularly their thermal behavior, chemical transformations under subcritical conditions, and the energy properties of the resultant hydrochar. This study investigated these aspects by employing techniques, such as thermogravimetric analysis (TGA), Fourier transformed infrared spectroscopy (FTIR), elemental and calorific analysis. The results show that PET hydrochar has a superior energy densification (1.37) and energy yield (89 %) compared to PS hydrochar (1.13, 54 %). Hydrothermal carbonization modifies the chemical structure of the polymers by increasing the number of carbonyl groups (CO) in PET and forming new ones in PS, and by enhancing hydroxyl groups (OH) in PET while retaining them in PS. Both materials preserve their aromatic and aliphatic structures, with the introduction of alkenes groups (CC) in the PET hydrochar. PET hydrochar begins to decompose at lower temperatures (150-270 °C) than PS hydrochar (242-283 °C) but reaches higher peak temperatures (420-585 °C vs. 390-470 °C), with both types achieving similar burnout temperatures (650-800 °C). PET hydrochar recorded a higher activation energy (121-126 kJ/mol) than PS hydrochar (67-74 kJ/mol) with the Mampel first-order reaction model as the best fit.