Effect of iron valence distribution on ash fusion behavior under Ar atmosphere by a metallic iron addition in the synthetic coal ash

Abstract The impurities in coal can cause serious ash-related problems at high temperature, threatening the stable operation of the coal gasifier. The effect of iron on the ash fusion properties is still unclear due to its multiple valences (Fe3+, Fe2+ and Fe0). In this study, we investigated the effect of iron valence on the fusion behavior of synthetic coal ash by the metallic iron addition. The ash fusibility was characterized by the ash fusion temperature (AFT) analyzer and thermomechanical analysis (TMA), and the corresponding ash fusion mechanism was revealed from the crystallized phase evolution, heat change, and the thermodynamic modelling. Results demonstrated that the initial deformation temperature of coal ash fails to reflect the initial ash shrinkage/melting event. The addition of metallic iron leads to the formation of Fe2+-containing minerals which facilitates the low-temperature eutectic reaction, decreasing the initial melting temperature of ash. However, the initial shrinkage temperature depends on the liquid content and is influenced by the CaO content. For the low-calcium ash, the metallic iron addition results in the formation of Fe2+ which advantages the eutectic reaction of Si-Fe-O system. The initial shrinkage temperature of low-calcium ash decreases for the massive formation of liquid. In contrast, the abundant calcium of high-calcium ash prevents the transition of Fe3+ to Fe2+, and combined with Fe2+ to form hedenbergite, retarding the eutectic reaction of both Si-Al-Ca-O system and Si-Fe-O system. Therefore, the initial shrinkage temperature of high-calcium ash increases with the metallic iron addition. Finally, the initial melting temperature can be correlated with the initial liquid temperature which is obtained by thermodynamic modelling, potentially acting as the guide to predict the initial ash melting event.