No abstract

A significant challenge for operation of entrained-flow coal gasifiers is the crystallization that occurs inside the liquid slag, which results in an increase in viscosity and slower flow of the slag along the wall. The chemical composition of slag is an intrinsic factor in determining crystallization characteristics. In this study, nine synthetic slags with various CaO concentrations (5−45%) were investigated for their crystallization tendency. Differential scanning calorimetry (DSC) and observations using the single hot thermocouple technique (SHTT) were combined to measure the influence of CaO on the crystallization temperature and crystalline morphology and to construct temperature−time transformation (TTT) diagrams. The crystalline phases were determined using scanning electron microscopy (SEM) with an energy-dispersive X-ray detector (EDX) and were compared to phases predicted by FactSage simulations. For the prepared slags, the wide range of CaO concentrations considered crossed through several stable crystalline phases. The resulting influence on crystallization behavior and degree of crystallization is consequently not linear or proportional to the CaO content. Slags with 15−35% CaO had a higher crystallization tendency and lower crystallization temperature than slags with lower CaO contents. Diopside and anorthite were the two main crystalline phases for slags with less than 35% CaO. However, when the content of CaO exceeded 40%, calcium tended to specifically combine with silica to form Ca 2 SiO 4 , which resulted in high crystallization temperatures.

No abstract

Crystallization in the molten slag of entrained-flow coal gasifiers can increase its viscosity and affect the flow of the slag layer along the wall. The chemical composition of the slag plays an important role in crystallization behavior and crystal characteristics. The aim of the work reported here is to study the influence of Fe 2 O 3 on crystallization tendency. Six synthetic slags with different Fe 2 O 3 concentrations, ranging from 5 to 30 wt %, were investigated using differential scanning calorimetry (DSC) and the single hot thermocouple technique (SHTT). In addition, three of the slags were reduced in a 60:40 CO/CO 2 atmosphere, and their crystallization characteristics were compared to the original slags. The time−temperature−transformation (TTT) diagrams for each slag were constructed. The Kissinger method was used to calculate the activation energy based on DSC results under cooling conditions, and the Johnson−Mehl−Avrami (JMA) equation was applied to calculate crystallization mechanisms under isothermal conditions. As the iron oxide concentration increased, a higher crystallization temperature and lower activation energy were observed, which indicates a stronger crystallization tendency. However, when the slags were in reducing conditions, the crystallization tendency declined slightly with a higher activation energy and lower crystallization temperature. The thermodynamic modeling program FactSage was applied to predict the equilibrium composition of the system. The modeling results indicate that reduced slags have a lower liquidus temperature and less species of solids, which provided further evidence of the comparative weakness of crystallization in a reduced atmosphere. The upward shift of the crystallization temperature caused by iron oxides will significantly influence the temperature of critical viscosity (T cv ) of crystalline slag.

No abstract