Co-combustion of sewage sludge and high ash coal: Thermal behavior, ash formation behavior, interaction mechanisms and economic analysis

Abstract

Co-combustion of sewage sludge with high ash coal enables both waste valorization and sustainable disposal, yet its combined impacts on boiler efficiency, slagging risks, and economic viability require systematic investigation. This study developed a comprehensive evaluation system including combustion characteristics, operational safety, and techno-economic analysis to reveal the interactions between components and mineral evolution during sewage sludge-high ash coal co-combustion. The results demonstrated that blending sewage sludge significantly improved the ignition performance of high ash coal. When the sewage sludge blending ratio exceeded 50 %, combustion transitioned from a fixed carbon-dominant to a volatile-dominated mode. A 50 % sewage sludge blend (sewage sludge to high ash coal mass ratio of 5:5) achieved a comprehensive combustion characteristic index of 4.80 × 10⁻⁸, representing a 100.58 % enhancement compared with coal. Synergistic interactions enhanced the combustion efficiency of fixed carbon, as evidenced by a 25.12 % increase in the fixed carbon weight-loss rate for the 60 % sewage sludge blend, compared to theoretical predictions. Minerals (CaO, Fe₂O₃, AlPO₄) exhibited dual catalytic-inhibitory regulation: optimal concentrations enhanced combustion via catalytic cracking and oxygen storage-release mechanisms, while excessive minerals promoted slag formation impeding oxygen diffusion, with the efficacy hierarchy following CaO > AlPO₄ > Fe₂O₃. Ash chemistry analysis revealed that the addition of sewage sludge increased the alkaline oxide content, promoting silicate network depolymerization. At blending ratios exceeding 50 %, the ash deformation temperature sharply decreased to 1157 °C, increasing the risks of slagging. Economic evaluation indicated dynamic payback periods of 2.91 years for a 50 % blend and 7.67 years for a 60 % blend. Multi-objective optimization identified the optimal sludge-to-coal ratio as 5:5, achieving balanced improvements in combustion efficiency, ash fusion control, and economic viability. This study explores the feasibility of sewage sludge-high ash coal co-combustion, providing a new approach for intensive sewage sludge treatment.