Qiang Wang, Yu Cong, Qianyu Li, Yukun Zhu, Huan Ma, Fengqi Si
Driven by deep peak-shaving demand, utility circulating fluidized bed (CFB) boilers increasingly operate at ultra-low loads, posing challenges to stable nitrogen oxides (NOx) control. Flue gas recirculation (FGR) offers a potential in-furnace NOx control strategy by sustaining bed-material fluidization and weakening the dense-phase oxidizing atmosphere, but its coupled effects on combustion and emissions remain unclear. This study developed a three-dimensional full-loop computational particle fluid dynamics model for a 300 MW utility CFB boiler and validated it against industrial measurements. To improve radiative heat-transfer prediction in the CO 2 /H 2 O-enriched atmosphere induced by FGR, non-gray gas radiation was incorporated through an improved weighted sum of gray gases model. Results show that, under 25% load operation, a large-scale asymmetric gas-solid circulation pattern develops, producing pronounced lower-furnace thermal non-uniformity. Increasing the FGR flow rate reshapes the temperature field by lowering the overall furnace temperature, suppressing local hot spots, shifting the main heat-release region upward, and delaying coal ignition. Meanwhile, outlet NO and SO₂ concentrations decrease by approximately 21% and 16%, respectively. However, excessive FGR further cools the coal-inlet region and increases the risk of ignition instability, indicating that a moderate FGR flow rate provides a better balance among pollutant reduction, temperature-field regulation, and combustion stability.