Entrained flow gasification technology is a type of clean coal conversion technology, which plays an important role in reducing carbon dioxide emissions and improving energy efficiency. It allows for various combinations of electricity, liquid fuels, hydrogen, chemicals and heat with the characters of high efficiency and fuel flexibility [1]. Entrained flow gasifier usually operates at high temperature (above the ash flow temperature) to ensure a suitable slagging condition for the stable operation [2,3]. The viscosity of molten slag is the key factor in determining whether the slagging condition is smooth and stable. The viscosity of molten slag exhibited a rapid increase because of the increase of crystals in the molten slag when temperature was lowered below a certain temperature which was referred as the temperature of critical viscosity (Tcv) [4,5]. Many factors affect the viscosity of molten slag, including composition, cooling rate, residence time and so on. And these factors influence the viscosity of molten slag by changing the volume fraction of crystal phase in the molten slag. A number of scholars have studied the factors that affect the crystallization of slag. Fredericci et al. [6] considered the crystallization mechanism of an unaltered blast-furnace slag composition. They suggested that most crystallization was on surface and this suggestion was confirmed by the study of nucleation kinetics. Xuan et al. [7–۱۰] studied the influences of CaO, Fe2O3, SiO2/Al2O3 on crystallization characteristics of synthetic coal slags. With the increase of CaO, crystallization of the slag became significant, especially in those with a calcium range between 15% and 35%. The crystallization temperature increased but only slightly. With a higher ratio of Fe2O3, more crystallization heat was released and the crystallization shifted to a higher temperature, potentially leading to a higher Tcv in viscosity. The kinetics under isothermal 1100 °C showed that the growth rate of crystals increased with the addition of iron oxide. As the S/A ratio increased in the range from 1.5 to 3.5, energy barrier was significantly lowered which increased the crystallization ratio. Shen et al. [11] studied the effect of cooling process on the generation and growth of crystals in coal slag. The variation of the cooling process obviously affected the crystallization behavior of molten slag. Low cooling rate benefited the generation of the crystals and long residence time below the initial crystallization temperature promoted the generation and growth of crystal. Louhich et al. [12] reproduced the experimental T dependence of the crystallization temperature with numerical calculations based on standard models for the nucleation and growth of hardsphere crystals, classical nucleation theory and the Johnson-Mehl-Avrami-Kolmogorov theory. These results suggested that deep analogies existed between hard-sphere colloidal crystals and pluronics micellar crystals, in spite of the difference in particle softness. Studies about glassy slag are relatively rich in the glass industry. Karamanov et al. [13] summarized results of the crystallization of iron-rich glasses. The results indicated that magnetite and pyroxene were the main crystal phases and that the kinetics of pyroxene formation could be explained as growth on a fixed number of magnetite nuclei. Pacurariu et al. [14] studied non-isothermal crystallization kinetics of some glass-ceramics with pyroxene structure. Many other scholars [15–۱۹] have also studied the effect of other factors on the slag crystallization process including residual carbon, trace elements and so on. About the viscosity of molten slag, Kondratiev et al. [20], Ilyushechkin et al. [21] and Zhang et al. [22] studied the characters of flowability and Tcv of coal ash slag. Kong et al. [23–۲۵] proposed the internal and external factors influencing the slag viscosity at high temperature, including CaO content, residual carbon and cooling rate. It was found that the viscosities of both the glassy and the crystalline slag were declining when the cooling rate increased.