Iron transformation behavior in coal ash slag in the entrained flow gasifier and the application for Yanzhou coal

•Iron transformations during tapping process are quantified for the first time.•Iron content in the liquid phase decreases as spinel and melilite precipitate.•The model for Fe transformation including Fe2+/Fe3+ and Fe-minerals is built.•The slag in Shell gasifier is more reduced than that in Texaco gasifier.•The models built in this work benefits the utilization of iron rich coal.