Investigation of Chemical Looping Combustion of Coal with CuFe₂O₄ Oxygen Carrier

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Abstract

Chemical looping combustion (CLC) has great advantages to obtain pure CO2 from coal combustion flue gas at a manageable cost. CuFe2O4 was put forward as a novel oxygen carrier, which integrated Cu and Fe metals into one oxide matrix with superior characteristics over single metal oxide of either CuO or Fe2O3 and had a high potential to be used in CLC. In this study, the reaction of CuFe2O4 with two Chinese coals of different ranks [Liu Pan Shui (LPS) sub-bituminous coal and Yang Quang (YQ) anthracite] was performed in a thermogravimetric analyzer (TGA). Fourier transform infrared (FTIR) spectroscopy was used to detect in situ the emitted gases from the TGA. Field scanning electron microscopy/energy-dispersive X-ray spectrometry (FSEM/EDX) was used to study the morphology and elemental compositions present in the solid residues, and the related phases were further identified by X-ray diffraction (XRD). Meanwhile, to explore the reaction mechanisms involved for the reaction of CuFe2O4 with coal, a more realistic simulation system with 376 species was designed for thermodynamic analysis. Through all of these measures, it was found that the reaction of LPS lean coal with CuFe2O4 underwent two distinct reaction stages at 300–600 and 600–850 °C, respectively. At these two reaction stages, CuFe2O4 was dominantly reduced into Cu and Fe3O4 by transfer of the lattice oxygen [O] in CuFe2O4, and then the formed Fe3O4 was further reduced into Fe2.962O4. However, above 800 °C, CuFeO2 and Cu2O were produced through direct decomposition of CuFe2O4 into CuFeO2 and then further partial decomposition of CuFeO2 into Cu2O. Especially, O2 generated was greatly beneficial to the full conversion of the remaining coal. Different from LPS, the reaction of YQ with CuFe2O4 presented only one discernible reaction stage above 600 °C. Besides Cu and CuFeO2, Fe2.957O4 was also generated. Furthermore, four cycles of reduction of CuFe2O4 with H2 and then oxidation with air displayed a good reaction stability of synthesized CuFe2O4. However, if coal was used, iron silicates were formed from the interaction of the reduced CuFe2O4 with ash and resulted in the insufficient reoxidation of reduced CuFe2O4. As such, effective separation of coal ash should be included in the CLC process to ensure the full regeneration of reduced CuFe2O4.

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