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

•Distribution of iron states of coal ash is adjusted by metallic iron addition.•The initial shrinkage temperature of high-calcium ash increases with Fe2+.•The initial melting temperature decreases with metallic iron addition.•Deformation/shrinkage temperature fails to reflect initial ash melting event.•The initial melting event of coal ash can be predicted by thermodynamic modelling.