Chemical looping combustion characteristics of coal with Fe2O3 oxygen carrier

Abstract

Chemical looping combustion (CLC) by direct use of coal as fuel is promising with its prominent advantages, but insufficient conversion of coal in the CLC system is a great limitation. In this research, in order to explore the limiting factor inherent for coal conversion in the CLC system, from the perspective of chemical structure of coal, reaction of a selected Chinese typical coal (designated as LZ) with Fe2O3 was systematically investigated. Thermogravimetric investigation of LZ coal reaction with Fe2O3 at the oxygen excess number Φ = 1.0 indicated that after dehydration, there existed three discernible reaction stages as observed, which were attributed to the combined reactions of Fe2O3 with the primary and secondary gaseous products evolved from LZ coal. Meanwhile, the Fe2O3 provided should be controlled around Φ = 1.0 aiming at effective conversion of LZ coal and simultaneous proper utilization of Fe2O3. And then, both gaseous Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy analysis of the gaseous and solid products formed from reaction of LZ coal with Fe2O3 at Φ = 1.0 indicated that full conversion of LZ coal was not reached with a little unconverted CO occurring, though partial Fe2O3 was over reduced to lower valence of oxides than Fe3O4. Furthermore, in order to explore the insufficient conversion of LZ coal at the molecular scale, X-ray photoelectron spectroscopy analysis revealed the distribution and evolution of the carbon functional groups involved in LZ coal after its reaction with Fe2O3 and further found that effective conversion of the aromatic/aliphatic C=C/C–H groups in LZ coal was the rate-limited step at the molecular scale with the relative content of these groups still dominated around 59% after LZ coal reaction with Fe2O3. Finally, solid IR (infrared) analysis and quantitative evaluation of the solid products of LZ coal reaction with Fe2O3 indicated that the length of aliphatic C–H groups decreased due to its partial disintegration, while the aromatization of the residual char was aggravated with the higher relative IR intensity ratio of the aromatic C=C groups, which reduced the reactivity of LZ residual char and hindered the full conversion of LZ coal.