Numerical simulation of the combustion characteristics of a low NOx swirl burner: Influence of the primary air pipe

Numerical investigations of the combustion process and NOx emission were conducted for two swirl burners (a prototype swirl burner and an optimised swirl burner) and a 600 MWe wall-fired boiler. The predicted results and the measured information from the full-scale boiler were found to be in good agreement. The flow field, particle distribution, temperature pattern, and species concentration of two burners were discussed and analysed in detail. The species distributions in a furnace equipped with the prototype swirl burners were compared with those for optimised swirl burners. Simulation results showed that particles ejected from the prototype swirl burner diffused to the secondary air quickly, whereas particles of the optimised swirl burner recirculated for a long distance and then diffused into the secondary air passing through the recirculation zone. The gas particle field of the optimised swirl burner favours the formation of a large reducing atmosphere zone, contributing to NOx reduction. When the prototype swirl burners were replaced by optimised swirl burners, NOx emission decreased from 440 to 265 mg/Nm3 at 6% O2, but the unburnt carbon content of the fly ash increased from 1.02% to 1.33%. The detailed results of this work promotes a full understanding of NOx formation characteristics, and helps to reduce NOx emission in pulverised-coal furnaces.