Co-Combustion of Sewage Sludge and Coal Slime: Kinetic Mechanisms and Slagging Characteristics Affected by Coal Slime Diversity

Co-combustion enables the resource recovery of sewage sludge (SS) and coal slime (CS). This study examined how CS variability affects SS–CS combustion kinetics and slagging. SS and CS exhibited multistep thermal decomposition kinetics. The activation energy for SS combustion followed an “N-shaped” distribution with respect to the conversion rate, whereas the activation energy for the combustion of CS decreased gradually with increasing conversion rate and was positively correlated with ash content. Blending with CS reduced the apparent activation energy, notably by 20.0 kJ/mol at a 50% ratio, accompanied by a shift in the kinetic mechanism from f(α) = (1 − α) to f(α) = (1 − α)1.3 during fixed-carbon combustion. SS had a much higher slagging tendency than CS, consistent with the predicted results for the evaluation indices except Fe/Ca. The incorporation of CS markedly improved the ash-fusion characteristics of the blends. The sulfur slagging index and comprehensive ash index were reliable indices for predicting co-combustion slagging. Effective mitigation of slagging was achievable when the CS proportion exceeded 40% and the combustion temperature remained below 900 °C. These findings inform CS type selection, optimization of blending ratios and combustion temperatures, and slagging-risk assessment, providing an experimental basis for parameter selection and stable SS–CS co-combustion operation.