Combustion stability and NOx emission characteristics of three combustion modes of pulverized coal boilers under low or ultra-low loads

Under low or ultra-low loads, poor combustion stability and high NOx emissions hinder the stable operation of pulverized coal boilers. Numerical simulation was used to investigate the impact of boiler load rate and operation oxygen on combustion stability and NOx emission characteristics in tangentially fired, swirl-opposed fired, and W-flame boilers. To quantitatively assess combustion stability, a combustion stability index was introduced using ignition distance and average temperature of the main combustion zone as parameters. The results indicate that as the load decreased, the temperature and combustion stability decreased rapidly below the 50% load; ignition distance and NOx emission concentration (@6% O2) increased. Moreover, under stable low loads, reduced operation oxygen enhanced temperature and combustion stability while reducing ignition distance and NOx emissions (@6% O2), with minimal impact on CO concentration. Under stable low loads in all three boilers, reducing operation oxygen to 3.5% led to an average temperature increase of 56.8 °C. Additionally, the W-flame boiler exhibits the highest average temperature (1371 °C), oxygen concentration, and NOx concentration, and the lowest temperature-decrease rate. Under equal conditions, the average combustion stability index (Ics) decrease rate of the three boilers under low loads was 2.3 times that under mid–high loads. The tangentially fired boiler demonstrates the highest combustion stability capacity (Ics = 0.78), possibly owing to the close distance between each direct-current burner layer, and the mutual support and ignition between each corner of the same layer.