Interaction of manganese with aluminosilicate support during high temperature (1100 °C) chemical looping combustion of the Fe-Mn-based oxygen carrier

Abstract

Investigation of the redox reactivity of mixed-metal oxides in Chemical-Looping Combustion (CLC) can improve our understanding of the associated reaction mechanisms that are related to this technology. The Fe-Mn-based oxygen carrier supported on a spent fluid catalytic cracking catalyst (FCC) was characterized during CLC of CH4 by fixed bed reactor studies coupled with mass spectrometry, X-ray diffraction, and Raman spectroscopic analysis. Research was carried out to investigate Mn interaction with the FCC support during the high temperature (1100 °C) CLC and the potential impact on performance. The addition of Mn to Fe/FCC led to an increase in the oxygen transfer capacity at 900 °C and a decrease in oxygen transfer capacity at 1100 °C. Following 15 redox cycles, the average methane conversion was 85% at 900 °C and 68% at 1100 °C. The Fe-Mn/FCC carriers were selective for CO2 at 900 °C, and selective for CO at 1100 °C, which is consistent with the thermodynamic limitation of CO2 production at high-temperature. The Fe-Mn/FCC oxygen carrier’s crystal structure remained stable at 900 °C, whereas at 1100 °C, XRD and Raman spectroscopic analysis revealed formation of MnAl2O4 and Al2Mn3Si3O12 phases due to reduced Mn metal alloying with the aluminosilicate. The decrease in reactivity and oxygen transfer capacity at 1100 °C was attributed to the formation of new Fe-Mn phases and alloying with the aluminosilicate, sintering, and agglomeration at high-temperature. These results demonstrate the need to stabilize the Mn-active phase of oxygen carriers on aluminosilicate supports being used for the high-temperature CLC application.