Effect of K₂O/Na₂O on fusion behavior of coal ash with high silicon and aluminum level

Ash fusion temperatures (AFTs) are widely used for evaluation of fusibility of coal ash for combustion and gasification. K₂O and Na₂O is regarded as the effective component to enhance the ash fusibility for slag tapping. The combination of K₂O and Na₂O may enhance the flux efficiency. In this work, the effect of K₂O/Na₂O mass ratio on fusion behavior of coal ash with high silicon and alumina level was evaluated by the height variation of ash cone during heating. The results showed that AFTs and theoretical fusion range (T_liq-T_ini) decreased quickly and then increased slowly as the K₂O/Na₂O mass ratio in the coal ash was reduced. The sub-liquidus phase changed from mullite into feldspar, which was responsible for change of AFTs. Meanwhile, most of K element occurred in leucite (KAlSi₂O₆) and the rest existed in nepheline (KAlSiO₄), but Na almost existed in nepheline (NaAlSiO₄). The competition effect between K₂O and Na₂O on AFTs was reflected by KAlSiO₄/NaAlSiO₄ in nepheline, so the SiO₂-KAlSi₂O₆-NaAlSi₂O₆ ternary phase diagram was constructed, which well explained the variation of AFTs caused by K₂O/Na₂O mass ratio. Meanwhile, the fusion process was divided into two stages based on variation rate of ash cone height. At first stage, the amount of liquidus phase is limited and the viscosity is high, so only a low shrinkage rate of ash cone exhibits. However, at the second stage, high shrinking rate is caused by not only the rapid increase of slag content but also the sharp decrease of viscosity besides, the addition of K₂O/Na₂O change ash fusion process from “melting-dissolving” into “softening-melting”.