Characterization of hematite oxygen carrier in chemical-looping combustion at high reduction temperature

Using H₂ as a reactant gas, the reaction characterization of hematite oxygen carrier was investigated in a thermogravimetric analyzer (TGA). The solid reduction products were characterized by XRD (X-ray diffraction) and SEM-EDS (Scanning Electron Microscope-Energy Dispersive Spectrometer). The results show that there is a maximum reaction rate when the conversion is 0.11. In this stage, Fe₂O₃ as an active phase was converted to Fe₃O₄ and the reduction reaction was easy to happen. Then, there is a decrease of reaction rate. When the conversion degree of hematite oxygen carrier was 0.178, the solid reduction products were composed of Fe₃O₄ and FeO. When the conversion is 0.477, Fe₃O₄, FeO and Fe were all found in the reduction products. SEM-EDS results show that the grains at the surface of hematite oxygen carrier were gathered and grown up, and the particle volume shrinkage and sintering effect are observed, especially in the region rich in Fe element. However, a relative stable structure was seen in the region rich in SiO₂ or Al₂O₃ contents. Further, compared with the results based on interconnected beds, the sintering of hematite was alleviated due to a good inert material distribution, which kept a good long-term reactivity of hematite oxygen carrier.