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Show Introduction Deposition in coal-fired steam generators, leading to reduced efficiency and availability, is an area still requiring much investigation, both to understand the exact mechanisms involved and to improve on the existing predictive techniques. With the coal industry's rapid expansion, an increased proportion of the world fuel market is of coals from untried sources with unknown operational properties. This has left boiler manufacturers in the position of having to base designs for new fuels on laboratory evaluation techniques rather than upon their field experience of a fuel's behavior. Current methods for assessing the deposition propensity of a fuel are based on a variety of parameters. Some empirical methods are based on the ash fusion temperature, the base-to-acid ratio, or the silica percentage. However, methods which relate deposit formation to ash viscosity have the advantage that they relate deposition to a property of the ash at actual furnace temperatures. A commonly used method for predicting the viscosity of an ash as a function of temperature is the use of an empirical formula, such as the Watt-Fereday formula (1). This method relates the viscosity behavior of the ash to the ash composition. A limitation of this method, however, is that it is valid only for temperatures which are above the liquidus, temperature of the ash. Below the liquidus temperature, solids crystallize out of the liquid slag, causing a change in the liquid composition which is not accounted for by the Watt-Fereday formula. This viscosity prediction formula could be extended to lower temperatures, though, if the changing composition of the mixture could be taken into consideration. This paper reports the results of an effort to extend viscosity prediction formulas to lower temperatures by using a chemical equilibrium code to predict the composition of coal slags at temperatures below the liquidus temperature for either oxidizing or reducing environments. The methodology used to extend current viscosity formulas is given in the next section. A presentation of the results of the effort and a discussion then follow. Viscosity Prediction Methodology The extension of the Watt-Fereday and other viscosity prediction formulas to sub-liquidus temperatures is a relatively straightforward 3-2 |