Effect of carbon dioxide on flame propagation of pulverized coal clouds in CO2/O2 combustion

CO2/O2 combustion of pulverized coal is one of the promising new technology in order to reduce the emission of CO2 and NOx from coal combustion furnaces. However, several experiments with pulverized coal burners show that temperature and stability of pulverized coal flame is reduced in this condition. CO2 has distinctive thermodynamic and optical property compared with that of other gas, and it is important to know the effect of CO2 on the flame stability of pulverized coal. In this study, effect of CO2 on flame propagation velocity of pulverized coal clouds were studied experimentally using micro-gravity condition, and also numerically considering detailed radiation heat exchange using Monte Carlo method.Experiments were made by using spherical chamber with inner diameter of 200 mm. Micro-gravity condition was used in order to achieve uniform pulverized coal cloud in a chamber. Flame propagation velocity was measured from the photographic image of the flame front by using high speed camera. Results show that flame propagation velocity of pulverized coal cloud in CO2/O2 mixture gas decreases to about 1/3–1/5 of that in N2/O2 mixture gas at the same oxygen concentration. By using Ar/O2 mixture gas, it is revealed that thermal diffusivity of gas seems to have a large effect on flame propagation velocity. From the numerical analysis using Monte Carlo method, effect of absorption of radiation by CO2 gas is proven to be relatively small compared with that of thermodynamic property especially for heat capacity of CO2. Consequently, it is clarified that reduction of flame stability in CO2/O2 combustion is mainly due to the larger heat capacity of CO2 gas.