At this time, the correlation between the existing coal slagging indices and the actual observations made in most conventional boilers is poor. Some of the conventional coal slagging test procedures and the empirical relations they generated, such as, coal fusibility, ash viscosity, silica ratio, percentage of iron oxide in ash, and the ratio of basic to acidic oxides, frequently offer misleading information, especially, when their use is extended to other coals or blends. In this investigation, a CFD code to predict coal ash deposition rates, and a numerical slagging index (NSI) based on the ash fusibility, viscosity and content in coals, have been developed. The two codes have been validated using experimental data on Australian coals. In this paper, the NSI has been tested on wide range of coals and blends, ranging from Indian low-rank coals, to bituminous coals from Australia, Columbia, Indonesia, South Africa, the UK and the US. The results of predictions correlate well with the reported field performance of the coals and blends, while the results of predictions using some of the existing slagging indices show significant inconsistencies with the experimental data on most of the tested coals. The findings of this numerical investigation provide an encouraging development towards the search for a generic numerical technique of assessing the slagging potential of pulverized coals/blends in boilers. (C) 2012 Elsevier Ltd. All rights reserved.