The slagging of heat transfer surfaces during the combustion of pulverized coal in power stations is a major problem as power generators strive to improve the efficiency of their plants. In this paper, a numerical model to predict ash deposition rates in boilers has been developed. The deposition model is based on the sticking tendency of ash particles on impaction on boiler walls. A numerical slagging index (NSI) that correlates with the practical performance of different coals has also been developed. The index is based on ash viscosity, ash fusibility and the ash loading. Experimental data from the Australian Coal Industries Research Laboratories (ACIRL) furnace have been used to validate the models. The predicted results from the ash deposition model on four different coals are in good agreement with the experimental measurements, and the new slagging index has successfully ranked the investigated coals according to their field performance. In general, the predicted results from both the ash deposition model and the new NSI are in good agreement with the experimental data.