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

Abstract

CO 2 /O 2 combustion of pulverized coal is one of the promising new technology in order to reduce the emission of CO 2 and NO x 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. CO 2 has distinctive thermodynamic and optical property compared with that of other gas, and it is important to know the effect of CO 2 on the flame stability of pulverized coal. In this study, effect of CO 2 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 CO 2 /O 2 mixture gas decreases to about 1/3–1/5 of that in N 2 /O 2 mixture gas at the same oxygen concentration. By using Ar/O 2 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 CO 2 gas is proven to be relatively small compared with that of thermodynamic property especially for heat capacity of CO 2 . Consequently, it is clarified that reduction of flame stability in CO 2 /O 2 combustion is mainly due to the larger heat capacity of CO 2 gas.