Investigation on the interaction mechanism during co-combustion of sewage sludge and coal slime: The effect of coal slime type and pretreatment method.

Co-combustion of sewage sludge (SS) and coal slime (CS) is the preferred method for mitigating their environmental impact and increasing their added value. However, the interaction mechanism between SS and CS during the co-combustion process has not yet developed a unified understanding. This work aims to obtain the effect of CS types on SS-CS co-combustion and reveal the interaction mechanism between SS and CS based on the influence of pretreatment methods on the interaction. The results showed that during co-combustion, SS reduced the ignition and burnout temperatures, and CS with high fixed carbon content (e.g., XCS) improved the comprehensive combustion characteristics. Principal component analysis showed that the effect of CS on co-combustion was more significant. The interaction between SS and CS mainly occurred within 100-700 °C, in which inhibition and synergism coexisted. The large differences in the interactions before and after de-volatilization and pickling treatments revealed that the volatiles and ash in SS were the main interaction factors. The analysis of the interaction mechanisms showed that the free radicals and heat released from the SS volatiles combustion accelerated the weight loss of CS, but the formation of tars from its incomplete combustion may inhibit the decomposition of CS. The interaction in the fixed carbon combustion stage was mainly caused by SS ash, which can catalyze the combustion of CS fixed carbon, but for the high ash CS (e.g., QCS), the combustion of fixed carbon was hindered by the addition of SS ash higher than 10 %. The final manifestation (synergy or inhibition) of SS and CS interactions was the result of the competitive balance of the above interactive behaviors. This work provides a more comprehensive understanding of the interaction between SS and CS during co-combustion.