Measurements of swirling coal combustion and NO formation

Abstract

The effect of secondary-air on NO formation in swirling pulverized-coal combustion is an important problem in practical burners and furnaces. It was argued by some investigators whether changing the secondary-air flow rate can control NO emission. In this paper, the PIV (particle image velocimetry)technique, together with temperature and species concentration measurements are used to study the NO formation in swirling pulverized coal combustion under different secondary-air ratios. It is found that when the secondary-air ratio is larger (Case 1), small eddies are immediately produced in the shear layers, the vortex structures spread earlier. When the secondary-air ratio is smaller (Case 2), the vorticity is stronger, but the formation of vortex structures is delayed. For Case 1, near the inlet of the combustor, the near-axis gas temperature is higher than that in Case 2. For case 2, more intense vortex structures in the central region result in the higher near-wall gas temperature than that in Case 1. The CO concentration is higher in Case 1 than that in Case 2. Although initially the NO concentration is higher for Case 1 than that for Case 2, but due to subsequent stronger reduction of NO by CO for Case 1, finally, the conclusion is that different secondary-air ratios almost cannot affect the NO concentration at the exit of the combustor.