Ash melting characteristics of five coals up to 1873 K were characterized using Thermo-mechanical analysis (TMA) to predict their ash deposition tendency during coal combustion in a rotary kiln. Conventional coal indices commonly used for the assessment of the ash deposition in pulverized coal fired boiler did not provide a satisfactory distinction for rotary kiln. However, the ratio of combined amounts of silica and alumina levels to the relative proportion of silica to alumina provided a satisfactory distinction of their possible ash deposition potential particularly for the refractory ashes. A new index based on temperatures corresponding to 25% and 50% penetration of the TMA ram in ash is proposed to predict the ash deposition potential. The differences in the TMA and the ash-based indices were related to the clay minerals of the parent coals. The TMA based ash deposition indices provided a good association with the ash deposition trend observed in the temperature regions of kiln entrance during combustion in the Experimental Combustion Furnace (ECF) tests. The study suggests that the TMA index could be used to distinguish the ash deposition tendency of coals near the kiln entrance regions. The study has implications for the coal selection criterion to minimize the adverse impact of coal ash chemistry on the process efficiency of rotary kiln. (C) 2014 Elsevier Ltd. All rights reserved.