In-Situ Confocal Scanning Laser Microscopy and Post-Exposure Analysis of a Cu/Fe-based Oxygen Carrier Material in High-Temperature Oxidation and Reduction Environments for Chemical Looping Combustion Applications
A Cu/Fe-spinel-based oxygen carrier designed at the National Energy Technology Laboratory was studied in order to characterize its physical and chemical behavior with cyclic oxidation and reduction treatments at 800°C for applications in chemical looping combustions. Observations were made on individual particles of the oxygen carrier in-situ using a confocal scanning laser microscope during the high temperature treatments. Data from the confocal laser microscopy indicated particles underwent a highly variable reduction in volume upon the first oxidation exposure. They underwent a significant expansion during the subsequent reduction exposure. Subsequent oxidation and reduction exposures both caused modest increases in volume. Both the pattern of volume changes observed and their relationship to surface area measurements derived from the same confocal microscopy data are difficult to rationalize. A critical assessment of the data is discussed. Post-exposure analysis in the form of x-ray diffraction and scanning electron microscopy with energy-dispersive x-ray spectroscopy showed that the spinel was partially reduced to metallic copper during high temperature reduction. Cu- and Fe-based oxides that comprised the spinel were separating during reduction. These spectra also indicated that the copper was oxidized effectively such that no metallic copper peaks were seen in samples that ended their treatment with an oxidizing exposure. This suggests that, while the chemistry alternated between an oxidized state and a reduced state, the microstructure experiences an irreversible alteration when it was first exposed to the reducing environment.
Funding Statement (additional comments about funding)
This work was performed in support of the US Department of Energy’s Fossil Energy Advanced Combustion Program. The Research was executed through NETL Research and Innovation Center’s Advanced Combustion effort.