Heat transfer and parameter optimization for medical waste pyrolysis in an indirectly heated rotary kiln

This study investigated the pyrolysis of mixed medical waste (MMW) in an indirectly heated rotary kiln, focusing on the effects of operating parameters (filling ratio, heat source temperature, and rotation speed) on the heat transfer performance and product distribution. The pyrolysis behaviors of individual components (cotton swabs, paper, bandages, and plastics) and their composite mixtures were characterized using thermogravimetric-differential thermal analysis (TG-DTA). The heat transfer characteristics, chemical reaction properties, kiln operating parameters, and interactions between the processes were also investigated. It was found that endothermic reactions in MMW irradiation retard the heating rate, and the operating parameters can modulate this effect. Lower filling ratios improved temperature uniformity during drying and the first pyrolysis(25-371 ℃). The heat transfer coefficient decreased from 182.2 to 153.5 W/(m²·K) as the filling ratio increased from 11.3 % to 37.5 %, with an decelerated reduction rate observed beyond 22.5 %. Elevated heat source temperatures (750-850 ℃) accelerated pyrolysis completion (5.04 h-2.28 h) and enhanced the heat transfer coefficient from 147.6 to 241.8 W/(m²·K), while also increasing the gas yield (35.58-39.30 %). A rotation speed threshold (1.5 rpm) governs the solid transport regime transitions, with higher speeds (0.5-3 rpm) improving the heat transfer coefficient from 169.2 to 186.5 W/(m²·K). These findings provide actionable insights for optimizing the energy efficiency and throughput of industrial-scale MMW pyrolysis systems. Environmental Implications The pyrolysis of medical waste offers a sustainable solution for reducing hazardous waste volumes and recovering valuable resources. This study demonstrates that kiln optimizing parameters (e.g., filling ratio, heat source temperature, and rotation speed) can enhance energy efficiency and product yield while minimizing environmental pollution. By converting medical waste into usable products, such as syngas and biochar, this process reduces landfill dependency and greenhouse gas emissions, thereby promoting scalable and eco-friendly waste management practices. This study contributes to the advancement of circular economy strategies for healthcare waste treatment, aligning with the global sustainability goals.