Effect of CO2 and H2O Gasifications on the Burning Behavior and NO Release Process of Pulverized Coal at Low Oxygen Concentrations during Oxy-Fuel Combustion

Abstract

Through the use of a thermogravimetric analyzer and a fixed-bed reactor, the effects of CO2 and H2O gasifications on oxy combustion and NO emission of pulverized coal at low oxygen concentrations were investigated by comparing the burning behaviors and NO release processes in O2/N2, O2/CO2, and O2/CO2/H2O mixtures (the latter two represented oxy-fuel combustion without and with steam addition, respectively). The thermogravimetric analyses show that at 5% O2 concentration, the burning rate of coal samples decreased in the order of 5% O2/95% N2, 5% O2/95% CO2, and 5% O2/20% H2O/75% CO2 (molar concentration). As the oxygen concentration dropped to 2% because of the influence of CO2 and H2O gasification, the overall reaction rate of coal samples increased in the same order at above 800°C. At 2% O2 concentration, because of the effect of CO2 gasification the overall reaction rate of char samples in the 2% O2/98% CO2 mixture was accelerated and exceeded the burning rate in the 2% O2/98% N2 mixture with increasing temperature. And the overall reaction rate of char samples in 2% O2/20%H2O/78% CO2 further increased due to the enhanced CO2 and H2O co-gasification. The fixed-bed reactor tests showed that the NO emissions of pulverized coal in the O2/N2 mixture were always higher than those in the O2/CO2 mixture, regardless of temperature and O2 concentration. At 1,000°C and 2% O2 concentration, the NO emission in 2% O2/20% H2O/78% CO2 (5.64 mg/g) was higher than that in 2% O2/98% CO2 (5.13 mg/g) because the additional nitrogen precursors (HCN and NH3) generated by H2O gasification were oxidized to NO easily and quickly. However, increasing oxygen concentration to 5%, because of the weakened effect of H2O gasification, NO emission in 5% O2/20% H2O/75% CO2 (5.98 mg/g) decreased compared to that in 5% O2/95% CO2 (6.39 mg/g). In addition, because the effect of H2O gasification was enhanced at 1,200°C, regardless of the O2 concentration, the NO emissions in the O2/CO2/H2O mixture were always higher than those in the O2/CO2 mixture.