The crystallization kinetics analysis and the new prediction index of melilite-bearing slag under solid fuel gasification condition

Abstract

Melilite represents a prevalent crystalline phase precipitated from the gasification slag produced by various solid fuels, including coal and biomass. The crystallization of melilite can lead to gasifier blockage, posing significant challenges. This study investigated the crystallization behavior of melilite-bearing slags during slag tapping. To achieve this, FactSage, XRD, SEM-EDS, and DSC were employed to investigate the crystallization thermodynamics and kinetics of 12 synthetic melilite-bearing slag samples. The results revealed that the crystallization temperatures of slags with the mass fraction of SiO2 + Al2O3 is 50 wt% (SiO2 + Al2O3 = 50 wt%) ranged between 1150 and 1100 °C. Slags with the SiO2 + Al2O3 = 60 wt% and mass ratio of SiO2 to Al2O3 > 1.0 (Si/Al > 1.0) showed no crystals precipitate, which is conducive to slag tapping. Thermodynamics plays a crucial role in melilite crystallization, as it involved the uphill diffusion of Ca and Al. To quantify the thermodynamic crystallization ability, a parameter called Thermodynamic Crystallization Ability (TCA) is defined. Increasing Si/Al contributes significantly to reducing TCA. When the TCA is less than 0.33 wt%/K, the melilite-bearing slag will not crystallize. Furthermore, the kinetic analysis results demonstrated that the crystallization activation energy (EC) of melilite-bearing slag ranged from 310.64 to 458.54 kJ/mol. Moreover, the rate-determining step is the surface reaction, which was influenced by cluster structure. The essential short-range ordered structure that composes the melilite crystal is Al-O-Al; however, due to higher Si/Al, the concentration of the Al-O-Al structure in the slag decreases, resulting in higher EC value. These findings provide theoretical guidance for the stable operation of the gasifier.