The effect that ambient oxygen concentration has on NO that is released during coal char combustion was studied under particle packed-layer conditions at combustion temperatures of 700-1100 degrees C. The results show that the addition of quartz sand in the packed-layer mixture can effectively minimize the interaction of the char particles by inhibiting the secondary reaction of char and NO. During the char combustion tests, char-N/NO conversion decreased with an increasing ambient oxygen concentration at low temperatures (700-900 degrees C). At this temperature range, with coal char generally burning in zones I or II, more oxygen penetrated into the pores and less NO formed because of a more accessible pore surface area and an increased NO reduction time; a weak increasing trend was observed at a high temperature (similar to 1100 degrees C). At this temperature and above, more char-N/NO conversion was observed because less O-2 diffused into the char particles, and NO was reduced less during its formation close to the char particle external surface. A change in the oxidization from kinetic to transition or diffusion control affected the conversion of char nitrogen to NO during combustion. A quantified description of the diffusion of oxygen into the pores of char and the effect that the original coal rank has on NO release is presented in this paper.