Effect of Char Particle Size on NO Release during Coal Char Combustion

In this study, the effect of particle size on the release of NO during the char combustion under particle packed-layer conditions was investigated at combustion temperatures of 900-1400 °C. The results show that, after pyrolysis, the small size char favored more pores and specific surface area. No significant variations of the intrinsic reactivity of O₂ and NO with char particle size were measured. By increasing char particle size, more NO conversion from nitrogen in char (char-N) was found regardless of the combustion temperature and bulk oxygen concentration. This can be explained as follows. By increasing char particle size, less O₂ penetrated into the pores and then more NO formed which was attributed to a less accessible pore surface area and a decreased NO reduction time. The effects of reaction temperature and bulk oxygen concentration were also discussed. A quantified description using a first-order reaction model shows that the char particle size will finally influence the accessible pore surface area and depth of O₂ diffusion, which will directly determine the char-N/NO conversion. The evolution of nitrogen functionality with particle size and burnoff were also presented to demonstrate migration tendency of the char-N. The larger ratio of O/C in smaller size particle as determined using XPS (X-ray photoelectron spectroscopy) further confirmed higher pore diffusion. In addition, nitrogen was found to be preferentially retained in the char when carbon is oxidized.