Slag critical viscosity

Sanyal and Gumming [1985] point out that it is the viscosity of the ash that determines its slagging propensity. The indices based on chemical analysis are really measures of viscosity since viscosity depends on the chemical composition and the ratio of acidic to basic oxides. Measurements of viscosity change with temperature and show a rapid rise in viscosity (a departure from Newtonian behaviour) when crystallisation occurs. The temperature at which the viscosity rises steeply has been called the temperature of critical viscosity, The value of... 

For crystalline slags, the temperature at which the viscosity abruptly changes is called critical viscosity temperature Tcv- It can be identified and indicates the start temperature of slag freezing. 

Because the temperature of critical viscosity is, of course, related to the ash fusion behavior, it is suggested to be in the range of the ST up to the HT plus lllK [98,99]. Watt refined these estimations involving the main constituents of the slag on a wt% basis, where the sum of all indicated components yields... 

Ash yield between 6% and 25% for bituminous coal, or 4% and 25% for subbituminous coal—the ash must have particular slag viscosity characteristics since ash slag behavior is critical to satisfactory operation, and the maximum temperature at which the slag has a viscosity of 250 cP is 1340°C (2445°F) for bituminous coal and 1260°C (2300°F) for sub-bituminous coal. 

As shown in Fig. 6.24, increases with an increase in air fiow rate Qg (cm /s). Also, Foo is 4n increasing function of the vessel diameter D (cm) and the kinematic viscosity of molten slag, Vg, (see Fig. 6.25). The effects of the thickness of molten metal layer, H n (cm), and of molten slag layer, Hs (cm), on the total volume of molten metal droplets, Foo, are shown in Figs. 6.26 and 6.27, respectively. In Fig. 6.26, Foo increases monotonically with the thickness of the molten oil layer. On the other hand, in Fig. 6.27 Foo becomes independent of // when //m exceeds a certain critical value. This value was found to be closely related to the boundary between the momentum-dominant and buoyancy-dominant regions [17]. 

---