The factors on metallic iron crystallization from slag of direct coal liquefaction residue SiO2-Al2O3-Fe2O3-CaO-MgO-TiO2-Na2O-K2O system in the entrained flow gasification condition

Abstract

The gasification of direct coal liquefaction residue (DCLR) enhances the overall economy of the direct coal liquefaction process. However, the DCLR ash is rich in iron due to pyrite as the catalyst during liquefaction. Coal blending plan with DCLR for gasification should consider the possibility of metallic iron crystallization and agglomeration ahead of suitable viscosity range. In this study, the iron behavior in the DCLR slag in the gasification condition was discussed by the thermodynamic modelling of SiO2-Al2O3-Fe2O3-CaO-MgO-TiO2-Na2O-K2O. Results show that the metallic iron forms only in the dry feed entrained flow gasifier for the high content of reducing gases. The metallic iron crystallizes when Fe2+ of the liquid phase fails to be stabilized by -Si-O. The competition reaction with -Si-O between Ca2+ and Fe2+ and the formation of calcium aluminum-silicate also contribute the metallic iron crystallization. In the gasification condition, the metallic iron crystallization can be inhibited by decreasing the iron content/α value (α = CaO/(SiO2 + Al2O3), mass ratio) or by increasing S/A (S/A = SiO2/Al2O3, mass ratio) of DCLR ash. Finally, the DCLR blends with 3.5 wt% sand/8.0 wt% mullite/50.0 wt% Dalai Nur coal/90.0 wt% Duolun coal is the feasible feedstock for pulverized coal gasifier, because the temperature of iron crystallization is below the tapping temperature of 1350 °C and the viscosity is less than 25 Pa·s.