Effect of the addition of oily sludge on Zhundong coal combustion characteristics: Thermal behavior and synergistic effect

Oily sludge (OS) is characterized by a high moisture content, a high ash content, and a complex composition, making its direct and efficient utilization challenging. Zhundong coal, a typical high-alkali coal, exhibits considerable combustion potential; however, its high alkali-metal content in the ash can readily induce slagging and fouling, thereby limiting its clean and extensive utilization. In this study, OS and Zhundong coal were blended at different proportions, and their co-combustion characteristics were systematically investigated using multiple analytical techniques. The results indicated a pronounced synergistic interaction between the two fuels during co-combustion. An appropriate addition of OS improved ignition stability, with the blend containing 20 % OS exhibiting comparatively favorable overall combustion performance. Compared with the combustion of Zhundong coal alone, OS co-combustion generally increased the ash fusion temperatures, which contributed to mitigating the slagging tendency associated with the high-alkali ash of Zhundong coal. Increasing the proportion of OS intensified ash-particle agglomeration and localized sintering. Kinetic analysis showed that the activation energy decreased significantly with increasing OS content. At high blending ratios the reaction mechanism became more complex, reducing the applicability of conventional single reaction-order models. These findings provide a theoretical basis for the synergistic and cleaner utilization of OS and Zhundong coal, as well as for the regulation of ash-related behavior during co-combustion.