Numerical investigation of the effects of air–coal ratio on combustion characteristics of a 600 MW w-shaped flame boiler under low-load conditions

Abstract

Low-load operation has become increasingly common for large coal-fired power units due to the growing penetration of renewable energy. However, deviations of the air–coal ratio (ACR) from design values under low-load conditions can significantly affect combustion stability and pollutant formation. In this study, a three-dimensional numerical simulation was conducted for a 600 MW supercritical W-shaped flame boiler to investigate the effects of ACR (1.3–1.9) on in-furnace combustion characteristics and NOx formation at 35%–50% THA. The results show that increasing the ACR enhances the rigidity of the primary air while weakening secondary air penetration, leading to delayed ignition and a downward shift of the high-temperature zone. At 35% THA, a stable W-shaped flame cannot be fully established; however, an ACR of 1.5 promotes the formation of a relatively stable lower-furnace high-temperature core, which is favorable for low-load flame stabilization. With increasing load, both the overall temperature level and the extent of high-temperature regions increase significantly. At 45% THA, an ACR of 1.5 provides the most uniform temperature distribution. NOx emissions exhibit a strong positive correlation with the intensity and spatial distribution of high-temperature regions. Fuel-NOx dominates at 35% THA, whereas NOx increases markedly above 40% THA, indicating a transition in the dominant NOx formation mechanism. These findings provide guidance for optimizing the air–coal ratio during low-load operation of W-shaped flame boilers.