Formation of agglomerates with core–shell structure in a large-scale CFB boiler

Abstract

Cylindrical agglomerates with cross-sectional diameters over 200 mm are found in the bottom ash discharging chute of a 550 MWe large-scale circulating fluidized bed (CFB) boiler, which results in disfunction of the bottom ash discharging system due to the blockage of the bottom ash discharging chute. The agglomerates with core–shell structure are found to be significant to understand the mechanisms of the agglomeration, and agglomerates with similar structure are also found inside the furnace. The X-ray diffraction analysis on the core–shell structured agglomerate particle shows both the core and the shell comprise a significant amount of mullite; meanwhile, cristobalite is found in the shell, which is supposed to be formed at temperature higher than the bed temperature. The results of the transmission electron microscopy analysis show that mullite crystal in the shell is distributed inside the amorphous SiO2. Sintering experiments show that the formation temperature for mullite and cristobalite is ca. 1050 ℃, while the transformation temperature of quartz to amorphous silica is ca. 1150 ℃, which is much higher than the bed temperature. Thus, the agglomeration is supposed to be resulted from the high-temperature condition caused by the burning char inside the furnace, and the core–shell structure of the agglomerates is supposed to be formed due to the temperature gradient along the particle diameter of the burning char.