Mechanistic investigation of chemical looping combustion of coal with Fe2O3 oxygen carrier

The reaction of three Chinese coals with Fe2O3 oxygen carrier (OC) was performed in a thermogravimetric analyzer (TGA), with special focuses on the effects of varying heating rate and coal rank on reactivity. Fourier transform infrared spectroscopy (FTIR) was used to in situ detect the emitted gases from TGA. Field scanning electron microscopy/energy-dispersive X-ray spectrometry (FSEM–EDX) was used to study the morphology and elemental compositions of the reaction residues collected from TGA and the related phase evaluation was further identified by X-ray diffraction (XRD). Through all these experiments, it was found that the pyrolysis of coal samples without Fe2O3 OC under N2 atmosphere underwent the dehydration and the ensuing primary and secondary pyrolysis stages. The increasing heating rate shifted the characteristic temperature (Tm) of the primary pyrolysis to a higher temperature and favored a more rapid generation of volatile matters. When the three coals reacting with Fe2O3 OC, TGA results demonstrated even over 200 °C, the reaction still experienced the partial pyrolysis at the relatively low temperature and the ensuing two reactions of Fe2O3 with the pyrolysis products at the primary and secondary stages. The coal of low rank with high volatile content should be preferred for the full conversion of coal into CO2. Furthermore, the activation energy of Fe2O3 OC reacting with PDS at its primary pyrolysis stage was the largest, more than 70 kJ/mol. Finally, SEM–EDX and further XRD analysis of the residues from the reaction of PDS with Fe2O3 OC indicated the reduced counterpart of Fe2O3 was Fe3O4, and some inert iron compounds such as Fe2SiO4 and FeAl2O4 were also generated, which might deteriorate the reactivity of Fe2O3 OC.

► Coal of three different ranks was combusted with Fe2O3 and Fe2O3/Al2O3 oxygen carriers in TGA.
► The results were compared with coal pyrolysis.
► Reaction kinetics of Fe2O3 based oxygen carriers with coal were calculated.
► The reaction mechanisms involved were illuminated.