The characteristics of burning pulverized coal motion and the combustion process are investigated by numerical simulation to analyze the relations between the NOx formation and the gas flow and burning pulverized coal moving in a swirl pulverized coal burner. The depth of particle penetration into the IRZ L-dav and the effective time of IRZ tau(eav) are obtained under three swirl numbers s=0.5, 1.03 and 1.2, respectively. The results show that the minimum outlet NO, emission appears at the maximum L-dav and tau(eav) and the minimum outlet NO emission, the maximum L-dav and the maximum tau(eav) occur at the same time (s= 1.03). It is also found that, the diameter of pulverized coal first increases, but then decreases the particle residence time tau(int), in IRZ. (c) 2005 Elsevier Ltd. All rights reserved.