There exists blocking in real gasification process of Yulin coal in whose ash the content of calcium and sulfur is high.The large fluctuation of viscosity-temperature curve measured in laboratory can be observed.FactSage 6.2 software was used to calculate ternary phase equilibrium and phase change rules during ash melting of Yulin coal under different ranges of high temperature.Based on these results and XRD measurement

the characteristics of ash fusion and viscosity-temperature changes caused by the addition of SiO2 as well as the reason of viscosity fluctuation were analyzed.The results show that the fusion temperature of Yulin coal ash decreased initially then increased with the increasing values of SiO2/Al2O3（S/A） and acid/base（A/B）.There is a strong correlation between the formation of gehlenite and viscosity fluctuation within certain temperature ranges.The viscosity fluctuation of Yulin coal ash can be alleviated by adding appropriate SiO2(to the point when S/A=2.48)

its amount can be obtained from binary phase diagrams calculated by FactSage.The numerical calculation results are in agreement with the experimental data

which proves that the chemical thermodynamic equilibrium analysis method is suitable for investigating the characteristics of ash melting.