Kinetic and Thermodynamic Characteristics of Pyrolysis for Typical Medical Waste: Individual and Co-Pyrolysis Analysis

In this study, thermogravimetric experiments combined with kinetic and thermodynamic analyses were performed to investigate the pyrolysis and co-pyrolysis mechanisms of four typical medical wastes, including swab sticks, polypropylene (PP), textiles, and paper. The results indicated a distinct difference in pyrolysis behaviors between biomass-based and plastic medical wastes. Biomass samples exhibited a wide pyrolysis temperature range and high char yield, while PP presented concentrated and complete thermal decomposition. When the heating rate increased from 10 °C/min to 40 °C/min, the characteristic pyrolysis temperature rose and the reaction rate accelerated, whereas the inherent pyrolysis mechanism of each material remained unchanged. Kinetic analysis demonstrated that swab sticks and paper had activation energies of 110–208 kJ/mol and 42.7–194.0 kJ/mol, respectively, and their experimental data were best fitted by the first-order reaction model. Textiles possessed activation energies of 121–135 kJ/mol, and their pyrolysis process was well described by the three-dimensional diffusion control model. PP showed the highest activation energy of 204–240 kJ/mol with favorable reaction homogeneity. The co-pyrolysis of mixed medical wastes was a multi-stage reaction, and its mechanism transformed sequentially from the three-dimensional diffusion model to the first-order reaction and then to the three-dimensional diffusion model with the increase in conversion rate. Radical interactions between biomass and plastic components exerted an obvious inhibitory effect on the co-pyrolysis process. Thermodynamic analysis verified that the pyrolysis of all tested samples was an endothermic process. Different from single-component systems, the mixed system exhibited stepwise abrupt changes in thermodynamic parameters, indicating that the thermodynamic characteristics of individual components cannot accurately reflect the coupled reaction behaviors of mixed medical waste systems. Therefore, the optimization of medical waste pyrolysis processes should be based on the intrinsic laws of co-pyrolysis rather than the pyrolysis properties of single components.