The viscosity and crystallization behavior of slag from co-gasification of coal and extraction residue from direct coal liquefaction residue at high temperatures

Abstract

Co-gasification of coal and extraction residue (ER) of direct coal liquefaction residue (DCLR) is an efficient route for DCLR utilization. The synthetic ash samples of different composition were designed to investigate the crystallization of slag and its effect on slag viscosity to optimize co-feedstock of coal and ER. FactSage, XRD, SEM and single hot thermocouple technique (SHTT) were used to investigate crystallization behavior of slags. The viscosity-temperature curves were measured by the high temperature rotating viscometer. The results indicated that the slag viscosity and crystallization behavior are dominated by chemical composition. With the ratio of acid/basic oxides (A/B) increasing, slag viscosity at high temperature increased dramatically due to higher polymerization degree. The polymerized structure of poor atomic mobility limits the growth of crystallization, so the initial crystallization temperature increases and crystallization rate decreases. Meanwhile, the change of SiO2/Al2O3 (Si/Al) showed the similar influence on slag viscosity and crystallization. Fe2+ has stronger depolymerization ability than Ca2+ on aluminosilicates structure. The decreasing CaO/Fe2O3 ratio leads to low viscosity and then the slag crystallization tendency is observed increasing. However, Fe2+ does not involve anorthite crystallization reaction, so the change of slag viscosity and crystallization depends on the Ca/Fe of liquid phase. To obtain the smooth slag tapping of coal and ER in the entrained flow gasifier, the chemical composition ranges as 2.10 < A/B < 2.69, 2.0 < Si/Al < 2.5 and 1.0 < Ca/Fe < 1.20 for coal blending with ER as feedstock is proposed. In the end, a TCV prediction model was established (TCV = 180.2 + 0.67*Tliq + 1.33*CaO*Al2O3*(SiO2)2/[(SiO2 + Al2O3)*(88.2–49.68*α)], α = CaO/(CaO + Fe2O3)).