Numerical investigation on combustion and NOx formation characteristics under deep-staging conditions within a cyclone barrel

Numerical investigation on combustion and NOx distribution characteristics of coal within a cyclone barrel under deep-staging conditions was conducted. Char conversion model and NOx formation model with refined NO heterogeneous reduction mechanism and oxidation rate of HCN suitable for cyclone barrel were implemented and validated with experimental data. The maximum deviations between the predicted and experimental values of NO and CO concentrations are 12.1% and 18.6% respectively under various stoichiometric ratios. Based on the refined models, the NO distribution in cyclone barrel under deep-staging condition is obtained. In the cyclone barrel, the concentration of NOx is higher in the primary-air-controlled and near-wall zones, which hinders further NOx reduction. The appropriately delayed secondary air supply can avoid the centralized supply of O2 and form a reducing atmosphere region in the primary-air-controlled zone, which greatly reduces the initial formation of NOx. In addition, the central zone of the cyclone barrel is the core region for NOx reduction. The area of the central zone with a strong reducing atmosphere can be significantly enlarged by increasing the velocity of secondary air. The NOx formation in cyclone barrel drops by 26.5% under deep-staging condition when the secondary air velocity increases from 20 m/s to 50 m/s. Finally, the oxygen-enriched and deep-staging technology is applied to the cyclone barrel. An ultra-low NOx formation in cyclone barrel is achieved. When the O2 concentration of secondary air increases from 0.21 to 0.40, the NOx formation drops by 64.1% under deep-staging condition. Moreover, the elevated flue gas temperature with O2 enriched in cyclone barrel makes it more suitable for the safe operation of slag tapping cyclone barrel.