Real-time high temperature investigations of an individual natural hematite ore particle for chemical looping oxygen exchange

In chemical looping reactors, oxygen carrier’s physical and chemical properties may be continuously modified in response to reduction-oxidation (redox) cycling, affecting the oxygen exchange ability and therefore overall energy efficiency. Reports currently available on hematite’s high temperature redox behaviors have been based primarily on ‘bulk’ material testing in thermogravimetric units or fixed-bed/fluidized bed reactors, followed by post analysis. While ‘bulk’ based analysis effectively provides valuable information on oxygen exchange efficiency as a whole, interpretation of oxygen carrying behaviors occurring in an ‘individual’ particle caused by local particle alterations would be complicated. The present work discusses oxygen exchange behaviors of an ‘individual’ hematite particle based on real-time surface morphology changes subjected to redox cycling. In-operando observations were made possible by utilizing a custom-made high temperature fixed-bed reactor equipped to the confocal scanning laser microscope. The present technique revealed a unique particle expansion behavior that followed a zig-zag (up-down) pattern corresponding to each reduction and oxidation. After 10 redox cycles with 20 consecutive gas exposures, the overall volume was more than doubled, the surface area increased by 25%, and roughness increased by 4 times, resulting in meaningful alterations in the oxygen exchange capability. Reduction-oxidation transformation kinetics was accerelated at each cycle with a directional growth of the product layer over the particle surface; 1 s was long enough for a surface oxidation boundary to travel ≈385 μm after the 1st cycle. The surface transformation kinetics was of 2 dimensional growth in early cycles, which quickly became 3 dimensional growth in later cycles.