Effect of alkali metals and alkaline earth metals on promotion and agglomeration of Fe-based oxygen carrier during chemical looping gasification

Alkali metals and alkaline earth metals (AAEMs) are the most abundant trace elements in biomass, which will provide different effects of catalytic or inhibitory at different time during chemical looping gasification (CLG). However, the role of AAEMs in the catalysis or agglomeration at different time remains unclear in biomass CLG. In this study, the promotion and agglomeration effects of AAEMs on CLG performance is studied under different time and different AAEMs. The results show that K plays a major role in promoting CLG before 20 min and inhibiting CLG at 30 min. Moreover, K promotes the conversion from Fe3+ to Fe2+ and the earlier existence of more FeAl2O4 and Fe2Al4Si5O18, causing obvious agglomeration. Na and Mg provide obvious catalytic and promotional effects after the excitation. Further, more FeO is converted from Fe3+ at 3-8 min in Na-AH, leading to the earlier formation of Fe2Al4Si5O18 and agglomeration between particles. Ca obviously promotes the whole CLG process. Furthermore, Ca helps to promote the conversion of Fe2O3 to Fe3O4, but delay the conversion to FeO and a slower generation of FeAl2O4, resulting in less adhesion and obvious inhibition of agglomeration. Mg slightly slows down the conversion of Fe2O3 to Fe3O4, but earlier generates FeO with a lower strength than other samples, leading to low FeAl2O4 and slight agglomeration. In addition, the effect of Mg on reducing agglomeration is weaker than that of Ca, but better than that of K and Na.