Chemical looping combustion of high-sulfur coal with NiFe2O4-combined oxygen carrier

Chemical looping combustion (CLC) of coal has gained increasing attention as a novel combustion technology for its attractive advantage in the inherent separation of CO2. In relative to the single metal oxide-based oxygen carrier (OC), combined OC owned superiority for CLC of coal. In this research, combined NiFe2O4 OC was synthesized using sol–gel combustion synthesis method, and its reaction with a typical Chinese high-sulfur coal as Liuzhi (LZ) coal was performed in a thermogravimetric analyzer (TG). And then, systematic investigation was carried out to explore the evolution of sulfur species and minerals involved in coal and their interaction with the reduced NiFe2O4 OC through different means, including fourier transform infrared (FTIR), field scanning electron microscopy/energy-dispersive X-ray spectrometry, X-ray diffraction, and thermodynamic simulation. TG–FTIR analysis of LZ reaction with NiFe2O4 indicated that two reaction stages were experienced at 350–550 and 800–900 °C, respectively, far different from LZ pyrolysis, and SO2 occurred mainly related to oxidization of H2S with NiFe2O4 over 550 °C. Meanwhile, lattice oxygen transfer rates of NiFe2O4 involved at the two reaction stages were higher than that of directly mixed NiO with Fe2O3 OC and thus more beneficial for LZ coal conversion. Both experimental means and thermodynamic simulation of the solid-reduced residues of NiFe2O4 with LZ coal indicated that the main-reduced counterparts of NiFe2O4 were Ni and Fe3O4. In addition, though good regeneration of the reduced NiFe2O4 was reached, the side products Ni3S2 and Ni2SiO4 should be noted as well for its detrimental effect on the reactivity of NiFe2O4 OC.