Reactivity of iron-based oxygen carriers with coal ash in pressurized chemical looping gasification

Coal ash is one of the significant factors for the operating efficiency of pressurized coal chemical looping gasification (CLG). To explore the influence mechanism of physical deposition and chemical combination of coal ash on iron-based oxygen carrier (OC) under pressurized conditions, the effects of coal ash content, coal ash components and pressure on the reduction performance of Fe2O3/Al2O3 OC are investigated. The micro-structure and phase transformation of OC particles under pressurized conditions are characterized by SEM, BET and XRD. It shows that coal ash deposition only has a significant inhibition on the reduction process of Fe2O3 to Fe3O4. The deposition inhibition is the resistance of internal diffusion, which is generated by coal ash coating the surface of OC particles and blocking pores. Then a modified kinetic model is developed to analyze inhibitory mechanism of coal ash deposition at different pressure. There is a fairly strong coating ability of new composite metal compounds ((MgAl0.74Fe1.26)O4, Ca2Fe2O5 and CaAl2SiO6) on OC particles. However, when pressure is greater than 7 bar, (MgAl0.74Fe1.26)O4 is reduced to (MgO)0.239(FeO)0.761, which can weaken coating effect. Additionally, when pressure is greater than 10 bar, Fe3O4 is largely reduced and converted into CaFe5O7, which can inhibit formation of Ca2Fe2O5.