Spinel ferrite-contained industrial materials as oxygen carriers in chemical looping combustion

Abstract

Spinel ferrites (MxFe3-xO4) as oxygen carriers (OC) in chemical looping combustion (CLC) process have drawn significant interest owing to unique lattice structure and oxygen transfer properties. However, their employment in practice is likely cost prohibitive due to the complicated synthesis processes compared to natural occurring materials. In the work, several of the low-cost industrial materials naturally containing ferrite spinel structure (MxFe3-xO4, M = Cr and Mn) were, for the first time, investigated to explore the possibility as an alternative of synthetic OCs. The reactivity and their cyclic performance during the chemical looping combustion were examined by lab-scale fluidized bed and TGA. It was demonstrated that all the tested materials contain spinel structures, especially after CLC cycles. They exhibited almost the same reactivity and stability, and less agglomeration occurred when compared to the synthetic materials contain the same kind of spinel structure. But the tested materials presented lower oxygen transport capacity than the synthetic ones. Specifically, the FM1 material containing MnFe2O4 showed best reactivity towards CH4 and syngas conversion, which may attribute to the oxygen uncoupling ability of Mn based species and the formation of spinel ferromanganese structure. But the stability was not good enough. It may because of the cracked particles attributing to the shrinking during the reduction of Mn2O3 to Mn3O4. The Fe-Cr based sample showed more superior stability and improved performance due to the formation of (Fe,Mg)(Cr,Fe)2O4 spinel structure. The Fe-Cr based samples exhibit poor performance for complete combustion of fuel; however, it appears to convert more CH4 to CO which may be desirable for hydrogenation, gasification, and cracking processes. It is worth noting that these industrial materials did not show significant difference in reactivity and stability when compared to the same kind of synthetic materials, presenting a possibility of potential substitution, especially taking its low cost into account.