Coal gasification was carried out in order to verify the coal fouling tendency in the drop tube furnace (DTF), which can simulate an entrained-bed gasifier. Thirteen pulverized coal samples, in the range of bituminous and sub-bituminous, were utilized, and compared, with predictions using a wide range of empirical indices. Large discrepancies were noted in most cases, with respect to experimental results, ash deposition rates, and different indices. Some indices used for anticipating fouling tendency are not co-related with the results of this experiment. A newly approached index, which is especially important considering the diffusivity parameter, looks at the ratio of acid and alkali mineral matters injected into gasifier and heat flux, indicates that traditional expressions are modified to account for other parameters which influence the ash deposition phenomenon. The alternative index resulted in improved correlations between predictions and experimental observations. (c) 2012 Elsevier Ltd. All rights reserved.