Numerical Optimization of Combustion and Nox Emission in a Retrofitted 600mwe Tangentially-Fired Pulverized-Coal Boiler with Deep-Air-Staging

Abstract

In order to meet the ultralow emission standard of NOx, the combustion system of a 600 MWe tangentially-fired pulverized-coal boiler was retrofitted using deep-air-staged combustion technology. The air nozzles in primary combustion zone of the boiler are arranged in upper and lower groups. Flow, combustion, temperature, and NOx emission characteristics of the original and retrofitted boiler are numerically studied and compared. Optimization simulations for retrofitted boiler are carried out under different over-fired air ratios. The results show that the NOx emission at the furnace outlet is reduced by approximately 20% through retrofitting. The upper and lower arrangement of primary and secondary air nozzles has significant effect on the combustion and NOx emission. The highest temperature, the lowest O 2  concentration, and the most NOx emission simultaneously appear in the junction region of upper and lower primary combustion zones. Taking NOx emission and boiler thermal efficiency into account, the OFA ratio of 25% is found to be the optimal design for retrofitted boiler, which can further reduce the NOx emission by about 49%, and the corresponding unburned carbon in fly ash is 2.78%.

Keywords: Tangentially fired boiler, Numerical Simulation, Deep-air-staging, NOx emission, Over-fired air ratio