Combustion characteristics of lignite char in a fluidized bed under O2/N2, O2/CO2 and O2/H2O atmospheres

As a possible new focus of oxy-fuel work, O-2/H2O combustion has many advantages over O-2/CO2 combustion, and has gradually gained increasing attention. The unique physicochemical properties (thermal capacity, diffusivity, reactivity) of H2O significantly influence the char combustion characteristics. In the present work, the combustion and kinetics characteristics of lignite char particle were studied in a fluidized bed (FB) reactor under N-2, CO2 and H2O atmospheres with different O-2 concentrations (15%-27%) and bed temperatures (T-b, 837-937 degrees C). Results indicated that the average reaction rate (r(average)) and the peak reaction rate (r(peak)) of lignite char in H2O atmospheres were slower than those in CO2 atmospheres at low O-2 concentrations. However, as the O-2 concentration increases, the r(peak) and r(average) of lignite char in H2O atmospheres significantly improved and exceeded those under CO2 atmospheres. The calculation result for the activation energy based on the shrinking core model showed that the order of activation energy under different atmospheres is: O-2/CO2 (28.96 kJ/mol) > O-2/H2O (26.11 kJ/mol) > O-2/N-2 (23.31 kJ/mol). Furthermore, gasification reactions play an important role in both O-2/CO2 and O-2/H2O combustion, and should not be ignored. As the T-b increased, the active sites occupied by gasification agent were significantly increased, while the active sites occupied by oxygen decreased correspondingly.