Migration and Redistribution of Sulfur Species during Chemical Looping Combustion of Coal with CuFe2O4 Combined Oxygen Carrier

摘要

Chemical looping combustion (CLC) by direct use of coal as fuel has gained great recognition for the great advantage for CO, capture, but sulfur occurrence and evolution in the CLC system is always a great concern. In order to gain a comprehensive insight into the migration and redistribution of various sulfur species in the CLC system, a typical Chinese coal (designated as LZ) with large size range around 180-400 mu m (as frequently used in the real CLC system) was selected and its reaction with CuFe2O4 combined oxygen carrier (OC) was investigated using thermogravimetric analysis (TGA), which indicated that the reaction behavior of CuFe2O4 with LZ coal of large size changed greatly due to the maceral enrichment and mineral segregation in the LZ coal. At the two main reaction stages, the characteristic temperatures of CuFe2O4 reaction with LZ coal of large size range shifted to higher temperatures and the reaction rates increased relative to the reaction of CuFe2O4 with LZ coal of small size range (63-106 mu m). Furthermore, the migration and redistribution of various sulfur species formed from reaction of LZ coal with CuFe2O4 at its oxygen excess number Phi = 1.0 were studied through gaseous Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), which revealed that the SO2 mainly resulted from oxidation of H2S by CuFe2O4 combined with direct emission through pyrolysis of LZ coal at the peak temperature of 413.6 degrees C, while the solid Cu2S was formed through the gaseous sulfur liberated out by LZ coal and further reaction with the reduced CuFe2O4. Finally, thermodynamic simulation of LZ coal reaction with CuFe2O4 OC was conducted, and among all the four factors considered the CuFe2O4 oxygen excess number O, reaction temperature, steam concentration, and the system pressure the CuFe2O4 oxygen excess number 0 was ascertained as the most significant to migrate and converting most of the sulfur involved in LZ coal to the solid Cu2S is suggested as a good option for its ensuing separation out of the reduced OC. In addition, in order to simultaneously realize the CO, capture and effective in situ desulfurization during reaction of LZ coal with CuFe2O4 OC, the optimized condition was preliminarily explored and the CuFe2O4 oxygen excess number 0 was suggested to fix around 1.5.