TGA-FTIR investigation of chemical looping combustion by coal with CoFe2O4 combined oxygen carrier

• Combined oxygen carrier CoFe2O4 were prepared using the novel sol–gel combustion and synthesis (SGCS).
• Reaction of the synthesized CoFe2O4 OC with a Chinese typical high sulfur coal was performed in TGA.
• Fate of various gaseous and solid sulfur species was investigated.
• Reaction mechanisms of CoFe2O4 combined OC involved were explored.
• The limiting factor for incomplete conversion of coal was determined from the atomic scale.

Chemical looping combustion (CLC) of coal has received great interest for its pronounced advantage in CO2 capture. Sulfur evolution from coal causes great harm to CLC system. In this research, combined CoFe2O4 oxygen carrier (OC) was synthesized and its reaction with a typical Chinese high sulfur coal as Liuzhi (LZ) coal was investigated with the research focus on the sulfur evolution from LZ coal during its reaction with CoFe2O4 OC. Analysis of LZ coal reaction with CoFe2O4 OC through thermogravimetric analyzer (TGA)-Fourier transform infrared (FTIR) indicated that CoFe2O4 owned the desired reaction superiority over its reference oxide CoO or Fe2O3, and SO2 produced was mainly related to oxidization of H2S with CoFe2O4. Field emission scanning electron microscopy/energy-dispersive X-ray spectrometry (FESEM-EDX) and X-ray diffraction (XRD) analysis of the solid product from LZ coal reaction with CoFe2O4 indicated that CoFe2O4 were mainly reduced to Fe3O4 and Co, as verified by the related thermodynamic simulation. But most of sulfur present in LZ coal was converted to solid Co3S4 during LZ coal reaction with CoFe2O4. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis of various carbon functional groups distributed in the solid product of LZ reaction with CoFe2O4 revealed that CC or CH functional group was the lowest reactive to limit the full conversion of LZ coal on the atomic scale. Finally, such sulfides as CoS and Co3S4 and various silicates, including Co2SiO4 and Fe2SiO4, should be well addressed and separated during reaction of LZ with CoFe2O4 to ensure the full regeneration of the reduced CoFe2O4.