Effect of CaO/Fe2O3 ratio on slag viscosity behavior under entrained flow gasification conditions

Abstract

The key factor for smooth slag tapping of entrained flow gasifiers was slag viscosity behavior. Shanxi coals can’t meet the requirement for discharging slag due to its high SiO2 + Al2O3 contents. The single fluxing agent (CaO or Fe2O3) was also inoperative for Shanxi coals. Hence, the development of binary composite flux was urgent and necessary. The effect of CaO-Fe2O3 binary composite flux on slag viscosity temperature behavior was investigated in this work. As the CaO/Fe2O3 ratio decreased, the slag viscosity at same temperatures decreased. Furthermore, a linear relationship between the slag viscosity and CaO/Fe2O3 ratio was established at high temperatures. Characterization of slag structure showed that the polymerization degree decreased with the decreasing CaO/Fe2O3 ratio. The calculated BO/(BO + NBO) (fraction of bridging oxygen) also presented an excellent linear relationship with CaO/Fe2O3 ratio. This indicated that CaO/Fe2O3 ratio affected the slag structure, leading to the variation of slag viscosity. During the cooling step, the crystallization activation energy Ec was used to describe the effect of CaO/Fe2O3 ratio on slag crystallization behavior quantitatively. Besides, the growth pattern of crystal phases transformed from the surface crystallization to the bulk crystallization with the decreasing CaO/Fe2O3 ratio, and the bulk crystallization was more easily to promote the crystallization behavior. In brief, slag viscosity and crystallization behavior could be modified via adjusting CaO/Fe2O3 ratio. These results can provide a guide for using CaO-Fe2O3 binary composite flux to improve the slag viscosity behavior in entrained flow gasification.