Fig. 5A shows the effect of the secondary gas flow rate on the pollutant (CO and NOx). The CO emission goes from 46 to 40 ppm, while NOx emission goes from 91 to 107 ppm with increasing secondary gas flow rate, which can also be attributed
to the vortexing effect. At a higher secondary gas flow rate, more fine particles and unburned char are brought back to bed zone for re-burning, resulting in lower CO emission. The reduction in CO level can also be ascribed to high turbulence created by vortex, which results in better combustion of volatile and unburned char. Despite NOx emission can be reduced by the reduction reaction with unburned char (Chaiklangmuang et al., 2002; Okazaki and Ando, 1997),