This work aims to study the ash slagging behaviors of two specific coals, on account of a blocking problem of the slag discharge encountered when these coals were used in an industrial entrained-flow gasifier. Through the measurements of the ash fusion temperatures (AFTs) and the slag viscosities of the two coal samples, it was found that while the coal ashes had sufficiently low AFTs, they exhibited a strong viscosity fluctuation as well as a slag foaming and expansion. By comparing to the slagging behaviors of some artificial ash mixtures, it was ascertained that while the enrichment of sulfate in the coals ashes was responsible for the slag foaming and expansion, the viscosity fluctuation was attributed to a high content of calcium together with a low Content of silicon in the coal ashes. With the help of X-ray diffraction (XRD) analysis and FactSage thermochemical modeling, it was found that the presence of a crystalline gehlenite phase during the slagging process for the two coal ashes was closely related to the slag viscosity fluctuation. The addition of silica to the coal ashes could virtually eliminate the slag viscosity fluctuation. The sand (a silica rich mineral) addition method had also been successfully used in the industrial gasifier. (C) 2016 Elsevier B.V. All rights reserved.