Abstract: During the process of coal combustion, the ash depositionmay change the wall boundary conditions, which in turn will changeheat transfer and burning condition in the coal-fired furnaces. Without considering the diversity and chemistry of coal and coal ash, and the effect of burning conditions on the ash deposition, theconventional prediction methods, such as Clinkering-index and ASTM methods,do not have the necessary precision. Recently, many researchers apply the methodof computational fluid dynamics (CFD) to the coal combustion simulation,together with a physical or numerical model for every branch process, to simulate the coal combustion and ash deposition. Their researchresults provide very important guidance to the design and operation ofcoal-fired furnaces or boilers. This paper presents a review onthe researches of slag models, proposed during recent several decades, including ash formation,ash transportation, ash impaction and sticking, deposit growth and strengthdevelopment, heat transfer through the slag layer and the effect ofdeposition on the burning condition. Because most existing numerical methodsdo not consider the intense phase change rigorously,these models should be modified. As an addition to existing methods, thewall-burning model, whose numerical description is given at the end of thispaper, might be used to simulate the ash deposition in the pulverized coalliquid-bath combustion.