Effect of CO2 and H2O on the combustion characteristics and ash formation of pulverized coal in oxy-fuel conditions

The combustion characteristics and ash formation of pulverized coal under different atmospheres (O2/N2, O2/CO2 and O2/H2O/CO2) were investigated using an isothermal thermal analysis system. The burning rate in O2/N2 was faster than that in O2/CO2 due to differences in the physical properties between N2 and CO2 at 800 °C. The effect of CO2 gasification was enhanced with increasing temperature; as a result, the overall reaction rate in O2/CO2 approached and exceeded the combustion rate in O2/N2. However, the effect of gasification was weakened by increasing O2 concentration, and the overall reaction rate in O2/CO2 decreased and eventually lagged the combustion rate in O2/N2. Because of the distinct physical properties of H2O, the burning rate in O2/CO2/H2O was lower than that in O2/N2 but higher than that in O2/CO2 at 800 °C. At a low O2 concentration, the influence of CO2 and H2O gasification was enhanced with increasing temperature; thus, the overall reaction rate in O2/H2O/CO2 was higher than that in O2/CO2. Moreover, the burning rate in O2/H2O/CO2 increased slightly with increasing H2O concentration. However, higher O2 concentrations weakened the effect of gasification, leading to similar burning processes under different atmospheres. The XRD analysis of the ash samples showed that CO2 and H2O had no significant effect on the transformation of main minerals.