The most favorable condition for NOx reduction in the swirl burner of wall-fired type boilers is to increase the residence time of pulverized coal particles in the internal recirculation zone which should be fuel-rich environments. The purpose of this study is to elucidate fundamental mechanisms for NOx reduction in pulverized coal swirling flames by means of the detailed observation of the flow field of the oxygen lean internal recirculation zone. The structure of pulverized coal swirling flames with secondary swirl intensity is studied experimentally by particle image velocimetry and local flame colors based on OH*, CH*, and C-2(*) chemiluminescence intensities. The results show that the internal recirculation zone is enlarged with increasing the outer swirl intensity because the location of the stagnation point moves toward downstream. Also, the exhaust tube vortex is observed along the centerline in the flames, and started from near the stagnation point. For the combustion characteristics, the maximum temperature reduces as the outer swirl intensity increases, and the flame then moves downstream. The higher outer swirl intensity would be more effective in the low NOx swirl burner systems because of enhancing a dimension of the internal recirculation zone and reducing a local rate of chemical reaction. (C) 2015 Elsevier Ltd. All rights reserved.