At present, the research on the co-gasification of IS and coal was principally focused on the ash melting behavior [
22,
23]. For example, Schwitalla et al. [
24] investigated the slagging characteristics of lignite and its mixture with sludge. It was demonstrated that when the content of sludge ash was increased to 50% (mass percentage), FT could be significantly reduced. At the same time, it was found that the FactSage thermodynamic calculations were very consistent with the experimental ash melting behavior. Similarly, Folgueras et al. [
25] studied the slagging behavior of three types of sludge on lignite and found that the addition of sludge could reduce the AFTs. Meanwhile, based on the ternary phase diagram of SiO
2-Al
2O
3-CaO, it was illustrated that most of the minerals in the mixed ash were located in the low temperature region and close to eutectics. In addition, Li et al. [
26] researched the effects of industrial sludge on AFTs of three kinds of high-melting-point coal. It was considered that the addition of iron-bearing industrial sludge reacted with mullite in high-melting-point coal ash, and eventually transformed into low-melting-point hercynite. However, in some cases, the assessment method of AFTs was feasible [
27,
28]. However, in actual industrial gasification operation, for some coal, although the FT was lower than the operating temperature of the gasifier, the fluidity of molten slag was still very poor [
29]. Therefore, to achieve the optimal slag-tapping conditions, it was quite necessary to consider the viscosity-temperature characteristic parameters during the co-gasification process [
30]. However, up to the present, the research on co-gasification of industrial sludge and coal has mainly been focused on the ash melting characteristics, while little research has been conducted on the crystallization and viscosity-temperature characteristics. Since the ash melting characteristics and viscosity-temperature characteristics jointly guide the stable operation of gasifier, it is particularly important to study the viscosity-temperature characteristics in the process of co-gasification of IS and coal.