Ash formation characteristics in co-combusting coagulation sludge and Zhundong coal

Abstract The co-combustion of coagulation sludge (CS) and Zhundong coal (ZDc) can simultaneously convert waste to energy and mitigate fouling and slagging problems during circulating fluidized bed (CFB) combustion of ZDc. In order to explore the ash forming characteristics after blending CS with ZDc, the ash melting curve method quantified the real-time ash fusion characteristics, and alkali metal sodium was analyzed. The results revealed that the ash fusion process of ZDc was divided into two stages. During the first stage, shrinkage was caused by the release of Na and the low temperature eutectic of Na2SO4 and CaSO4. The aluminosilicates in CS effectively decreased the release of alkali metal sodium and inhibited the low temperature eutectic, so the shrinkage rate of ZDc ash was effectively decreased by blending CS during the first stage. During the second stage of ZDc ash shrinkage, the ash shrinkage temperature decreased and the shrinkage rate increased by blending CS. It was attributed that the iron salts in the blends generated low melting point substances such as SiO2-Al2O3-Fe2O3 (MP: 1073 ℃), which caused the blends ash to begin to shrink at around 1000 ℃. As the blending ratio increased, iron-bearing minerals formed low temperature eutectic with the minerals previously produced, resulting in the rapid melting of the blends ash at around 1200 °C. In connection with the sodium retention and the ash melting curve, ash formation characteristics were optimized by blending CS with ZDc. The co-combustion of CS and ZDc is suitable for the boiler that operation temperature under 1000 ℃.