Energy recovery and critical element immobilization in medical waste-derived fuel: hydrothermal vs. vapor thermal carbonization

In response to the rapidly increasing production of medical waste (MW) and the lack of fuel utilization, this study comparatively analyzes hydrothermal carbonization (HTC) and vapor thermal carbonization (VTC) as reaction pathways, examining the differences in fuel characteristics, combustion behavior, and the distribution of key elements in the solid products (high-carbon fuels). The results show that the VTC process homogenizes and improves the quality of MW. In contrast, the HTC process has a stronger carbonization ability, converting MW into high-performance solid fuels. The HTC products have higher energy density and carbon content (78.9 %). The VTC process, on the other hand, shows better performance in devolatilization and fixed carbon retention at temperatures below 280 °C. SEM analysis indicates that VTC products have higher surface roughness and larger pores at lower temperatures, demonstrating effective homogenization and improvement in the quality of MW. The HTC process exhibits superior performance in combustion behavior. ICP-OES and XPS results indicate that both processes effectively remove sodium, potassium, and chlorine from MW, with HTC achieving the highest removal rates for sodium and potassium at 260 °C and VTC achieving a higher proportion of inorganic chlorine, exceeding 85 %. These findings provide important references for the fuel utilization of MW and valuable insights for industrial applications.