Entrained flow gasification technology was received widely application in the field of energy conversion. 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 a high temperature (above the ash flow temperature) to ensure a good slagging condition for the stable operation [2,3]. The slag viscosity is the key factor in determining whether the slagging condition is stable and smooth. The measurement of the viscosity of molten slag is significant not only for the theoretical research on melt, but also for the industrial application. Slag type with suitable fluidity at the low operation temperature for saving energy consumption is required to study [4]. The viscosity of slag with low melting point should be investigated to develop an improved gasification operation at low temperature. Molten slag is a solid-liquid two-phase mixture and solid particles do exist in slag in gasification processes. The viscosity of fully liquid slag increased with decreasing temperature, and the temperature dependence of the viscosity was usually expressed in the form of the Arrhenius relationship. With the precipitation of crystals from molten slag below the melting point, the slag behaved as a non-Newtonian fluid and the viscosity showed a sharp increase near the melting point [5]. Therefore, the characteristics of crystal precipitation during the cooling process of the molten slag were studied. Xuan et al. [6–۹] studied the influences of CaO, Fe2O3, SiO2/Al2O3 on crystallization characteristics of synthetic coal slags. With the increase of CaO, crystallization of the slags became obvious, especially in which with a calcium content range between 15% and 35%. 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 (Critical viscosity temperature) in viscosity. The kinetics under isothermal 1100 °C show that the growth rate of crystals increases with the addition of iron oxide. As the S/A ratio increased between the range of 1.5–۳٫۵, the crystal was clear and energy barrier was significantly lowered and the crystallization ratio increased. Shen et al. [10] studied the effect of cooling process on the generation and growth of crystals in coal slag. Low cooling rate was beneficial to the generation of the crystals and long residence time below the initial crystallization temperature promoted the generation and growth of crystal. The influence of internal and external factors on slag viscosity was studied by Kong et al. [11–۱۳]. They found that viscosities of glassy and crystalline slag both decreased with increasing cooling rate. The viscosity of the molten slag increased with the increasing the mass fraction of residual carbon. Kondratiev et al. [14] proposed chemical components such as P2O5, B2O3 possessed strong, highly covalent metaloxygen bonds, leading to high liquid viscosities. Ilyushechkin et al. [15] concluded that slag viscosity depended on the slag bulk composition above the liquid temperature line. Zhang et al. [16] found that the viscosity of molten slag increased as increasing the volume fraction of TiC. Many researchers also proposed other factors (constant temperature, continuous cooling and different kinds of mineral) that affect the crystallization [17–۲۱]. Owing to the difficulties in measurement, the viscosity of two-phase mixtures was hardly investigated. In view of the importance of slag viscosity in the optimization of gasification processes, researchers have used different models to estimate the viscosity of solid-liquid mixtures.