Influence of SiO2/Al2O3 on crystallization characteristics of synthetic coal slags

Abstract

In entrained flow gasifiers, crystallization inside the liquid slag can increase the viscosity of the slag, affecting flow down the wall and resulting in operational challenges. Chemical composition of the slag is an intrinsic factor of crystallization characteristics. In the study reported here, five synthetic slags with different SiO2/Al2O3 (S/A) ratios from 0.5 to 4.5 were investigated for their crystallization tendency. Differential Scanning Calorimetry (DSC) and the Single Hot Thermocouple Technique (SHTT) were combined to measure the influence of S/A on crystallization temperature and crystalline morphology, and to construct Temperature Time Transformation (TTT) diagrams, which can be an practical operation guide of temperature to a gasifier. For the kinetics calculation, the Kissinger method was used for activation energy based on DSC results under cooling conditions and the JMA equation was applied for crystallization mechanisms under isothermal conditions with SHTT results. Understanding the crystallization temperature is significant to the practically operation of gasifier when using a crystalline slag. It turns out that crystallization of the slag with low S/A (0.5) was so weak that the crystals could hardly be observed while the S/A range from 2.5 to 3.5 favored crystallization around 1100–1250°C. As the S/A increased to 4.5, crystallization was retarded and shifted to a lower temperature range, below 1150°C. Bulk quenching samples were produced in a high temperature quenching furnace and analyzed by XRD to determine phases. The thermodynamic modeling program Factsage was also applied to predict solids in equilibrium and compared with experimental results.