Energy & Fuels, Vol.34, No.12, 16132-16146, 2020
Investigation on Co-Gasification Characteristics of Semicoke and Bituminous Coal in a CO2 Atmosphere at High Temperatures
Co-combustion of semicoke and coal under oxy-fuel conditions is a promising approach for the utilization of semicoke with a low volatile fraction. The devolatilization and gasification of fuels at high temperatures in a CO(2 )atmosphere is the preliminary stage of oxy-fuel combustion and significantly impacts the following processes of ignition, burnout, and fuel nitrogen release. In this paper, CO2 co-gasification of semicoke and bituminous coal was performed using a high-temperature drop tube furnace with a focus on interactions between the two fuels on the porous structure of resultant chars and fuel nitrogen transformation. The results indicated that blending semicoke and coal resulted in the lower Brunauer-Emmett-Teller (BET) surface area (S-BET) of mixture char than semicoke char at 900-1100 degrees C but led to the higher S-BET of mixture chars at 1300-1500 degrees C. The SBET of resultant chars in a CO2 atmosphere was evidently higher than those of the chars in a N-2 atmosphere. The interactions between the two fuels tended to promote the release of NOx and its precursors during CO2 co-gasification and were enhanced by the more content of semicoke. The O/N ratio had a more remarkable correlation with NO yield than the H/N ratio and the fuel ratio. The interaction between the semicoke and coal promoted the CH4 release at 900-1100 degrees C but turned to show inhibition with a further increase in temperature. Less CH4 was yielded in a CO2 atmosphere than in a N-2 atmosphere due to the reactions of CO2 and hydrocarbon. The high temperature and CO2 atmosphere promoted the release of char-N, which was hardly impacted by the interaction between the two fuels. The release of hydrogen was much faster than that of carbon during CO2 gasification. The release rate of nitrogen was lower than carbon at 900-1400 degrees C, while the similar release rates of carbon and nitrogen were found at 1500 degrees C.