A smooth slag flow out of gasifier bottom is key to a reliable operation of entrained flow gasifier. Therefore, avoiding slag blockage at the bottom of gasifier is very essential. Under the gasifier reduction operation conditions, precipitation of metallic iron from the liquid slag sometimes occurred. This solidified slag resulted in the gasifier outlet blockage. Consequently, gasifier operation has to stop. In this study, the thermodynamic software FactSage was used to predict the behavior of iron in the liquid slag within the temperature range of gasifier normal operation. From the results of this study, the mechanism of iron precipitation was revealed. The study showed that due to the higher reducing gas ratio in the syngas, the precipitation of metallic iron only occurred in the pulverized coal gasifiers instead of the coal water slurry gasifiers. When weight ratio of SiO2/Al2O3 is too low, or CaO content is too high in the slag, Fe2+ is not able to be saturated in the liquid phase. As a result, precipitation of metallic iron from slag was observed. A beta value (beta = SO4+ / (Ca2+ + Al3+), mole basis) of 1.25 was regarded as the boundary of metallic iron precipitation when 18.4% Fe2O3 content in coal ash. Finally, a metallic iron precipitation predicting model was given by using ternary phase diagram to guide the feedstock selection for the coal application in the entrained flow gasifiers. (C) 2017 Elsevier B.V. All rights reserved.