Experimental study on the formation characteristics of PM_0.4 from preheating combustion of sewage sludge

Municipal sewage sludge (SS) is gaining increasing attention as a carbon–neutral fuel. Preheating combustion has been applied in coal combustion to reduce both PM_0.4 and NOx. It is a promising technology for SS disposal while PM formation from preheating combustion of SS is still unknown. This study investigated the formation characteristics of PM_0.4 from preheating combustion of SS. The effects of preheating temperature (T_p) and combustion temperature (T_c) on PM_0.4 were analyzed. The experimental results showed that simultaneous reduction of PM_0.4 and NOx from preheating combustion of SS can be achieved with T_p ≤ 1300 °C and T_c ≤ 1200 °C. The reduction of PM_0.4 in preheating combustion compared to conventional combustion is mainly attributed to reduction in vaporization of Ca. T_c has more significant effect on PM_0.4 while T_p can affect NOx more effectively. Because SS has a low content of fixed carbon, vaporization of refractory elements (Ca, Mg, Fe, Si and Al) through reduction reaction of refractory oxides mainly occurs under reductive atmosphere in preheating furnace. Increasing T_c will significantly increase vaporization of alkali metals (Na and K), S and Cl, while has no significant effect on vaporization of refractory elements. These are different with the results from high-alkali coal in our previous research (Reference [28]). Fe-precipitation agent used in waste water treatment results in uniform distribution of Fe in SS particles, which enhanced interaction between Fe and Si-Al containing minerals. This decreases reaction between Si-Al containing minerals and Na containing compounds. Therefore, even SS contains less Na and more Si-Al than high alkali coal, Na is easier to transfer to PM_0.4 in preheating combustion of SS than high alkali coal.