Experimental study on the formation characteristics of PM0.4 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 PM0.4 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 char-acteristics of PM0.4 from preheating combustion of SS. The effects of preheating temperature (Tp) and combustion temperature (Tc) on PM0.4 were analyzed. The experimental results showed that simultaneous reduction of PM0.4 and NOx from preheating combustion of SS can be achieved with Tp & LE; 1300 degrees C and Tc & LE; 1200 degrees C. The reduction of PM0.4 in preheating combustion compared to conventional combustion is mainly attributed to reduction in vaporization of Ca. Tc has more significant effect on PM0.4 while Tp 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 Tc 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 PM0.4 in preheating combustion of SS than high alkali coal.