Effect of CaO/Fe2O3 ratio on fusibility of coal ashes with high silica and alumina levels and prediction

•AFTs generally increased with the increasing of CaO/Fe2O3 mass ratio.•Liquidus temperature cannot be used to predict FT for all ashes.•Fusion behavior of ash in mullite primary phase was the “melting-dissolution” mechanism.•A Tmullite model was proposed to predict FT of the ash in mullite primary phase. Ash fusibility which is usually investigated and evaluated by the ash chemical compositions is widely used to guide the coal selection in boiler and gasifier. Calcium and iron are the main basic oxides in coal ash, which tend to decrease ash fusion temperatures (AFTs). However, the change of AFTs varied with CaO/Fe2O3 mass ratio is not yet revealed. In this work, effect of CaO/Fe2O3 ratio on the fusibility of ash with high silica and alumina levels was explored under weak reducing atmosphere (CO: CO2 = 3/2, volume ratio). Thermodynamic calculations were applied to investigate the fusion behavior. A general rise of AFTs with the increasing CaO/Fe2O3 ratio was verified, especially for the coal ash with low SiO2 + Al2O3 level and SiO2/Al2O3 mass ratio. Mullite and anorthite are main refractory minerals phase of the ash samples with high SiO2 and Al2O3 levels. The fusion of the ash in anorthite primary phase is the “soft-melting” mechanism, and liquidus temperature was well used to predict flow temperature (FT). However, the liquidus temperature should not be used to predict FT of the ash in mullite primary phase due to the “melting-dissolve” mechanism. A Tmullite model was proposed to predict FT for the ashes in mullite primary phase. The deviation of predicted and measured FT was within the measuring error range (±40 °C), which was supported by 25 real coal ashes.