Study on the kinetic characteristics of polyoxymethylene (POM) pyrolysis and hydrothermal conversion

Plastic production has grown exponentially, proposing efficient utilization method of plastic waste demands urgent attention. Direct pyrolysis and hydrothermal processing are prospective technology. Researching the plastic kinetic properties of both pyrolysis and hydrothermal process is necessary to verify the mechanism and promote efficient utilization of plastic. In this work, thermal analysis kinetics of polyoxymethylene (POM) is employed under different atmosphere and heating rate to research the pyrolysis characteristics. Firstly, model-free iso-conversional methods were performed to study the kinetic performance of pyrolysis. Subsequently, the mechanism function and segmentation analysis method are applied to study the kinetic characteristic and pyrolysis mechanism. Finally, different methods were used to investigate the hydrothermal transformation kinetic performance of POM, and compared with pyrolysis. The result showed that the activity energy of direct pyrolysis calculated by both the model-free method and the C-R method was lower than 100 kJ mol⁻¹ in the initial stage, and in the range of 200–260 kJ mol⁻¹ in the middle and later stages. The most probable mechanism function of direct pyrolysis and hydrothermal reaction was Avrami-Erofeev equation, and the reaction mechanism was random nucleation followed by growth. The activation energy decreased by approximately 30 %, 36 %, and 43 %, respectively, with the vapor content of 25 %, 50 %, and 75 % compared with pyrolysis under dry atmosphere, proving the advantages of hydrothermal conditions from kinetic perspective.