Abstract: The experiments to simulate the slagging processes in the power plant boilers were carried out in a drop tube furnace with Shenhua coal, Zhungar coal and the blends of these two coals which are of different ash fusion temperatures. SEM and XRD techniques were used to analyze the surface morphology as well as the crystalline compositions of the residue. The result shows that a great mass of kaolinite and boehmite in Zhungar coal are turned ioto mullite. There is no boehmite and less kaolinite found in Shenhua coal, so there won't be too much mullite formed. Because the newly formed mullite can react with CaO at high temperature, so the mullite is not detected in the residue. Mullite with higher fusion temperature (1850℃) can significantly improve the coal ash fusion temperature, and the residue of Shenhua coal combustion does not contain any mullite, so it lacks the minerals as a “skeleton”, which is an important reason for that Shenhua coal has a relatively low ash fusion temperature.