The crystallization behavior of anorthite in coal ash slag under gasification condition

Abstract

The major reason for gasifier blockage is the crystallization of coal ash slag. To prevent the intensive crystallization during slag tapping under gasification condition, crystallization behavior of anorthite in slags with different composition was investigated in this work. The crystallization thermodynamics and kinetics of 40 slag samples were studied by the FactSage software and single hot thermocouple technology (SHTT), respectively. Chemical composition plays a key role on anorthite crystallization behavior. Slag with SiO2/Al2O3 = 2.5 has the least Si-O-Al structure and then weak thermodynamic driving force (Ran < 0.25 wt%/K), so anorthite shows the weak crystallization tendency in slag and crystallization was not observed by SHTT. When SiO2/Al2O3 < 2.5 and SiO2 + Al2O3 ≤ 70 wt%, anorthite crystallization in slag can be observed by SHTT. Sufficient nucleus (n = m) and rapid precipitate crystals occurs in the early stage of crystallization of slag with SiO2 + Al2O3 < 70 wt%. Crystallization activation energy (Ec) of anorthite in coal ash slag is from 263 kJ/mol to 835 kJ/mol. With decreasing CaO/FexO, reduced Ec values were observed in slags with SiO2/Al2O3 = 1.5, while Ec values of slags with SiO2/Al2O3 = 2.0 increased. Besides, Ec increased significantly with increasing SiO2 + Al2O3. The increasing SiO2 + Al2O3 leads to more complex cluster structure and lager cluster size and complex anion structure makes it difficult to bond between clusters. Then, the rate-determining step of anorthite formation changes from diffusion controlled crystallization to reaction controlled crystallization. The samples with α (SiO2/CaO + FexO) ≥ 1.1 and SiO2 > 20 wt% showed weak crystallization tendency. The results can provide theoretical guidance for stable operation of gasifier.