Investigation of combustion and NOx emission characteristics in 300 MW subcritical CFB boilers under low-load conditions

The low-load operation of circulating fluidized bed (CFB) boilers poses challenges in balancing combustion efficiency and nitrogen oxide (NOx) emissions. To minimize the initial generation of NOx in CFB boilers, researchers conducted a computational analysis using the CPFD (computational particle fluid dynamics) approach. The study focused on a 300 MW subcritical CFB boiler operating under low-load conditions, analyzing gas-solid flow dynamics and combustion behavior. Key variables, including the influence of primary and secondary air volumes, as well as the upper-to-lower secondary air ratio, were systematically evaluated for their effects on combustion efficiency and NOx formation. The findings provide valuable insights into improving operational parameters for cleaner emissions while maintaining stable low-load performance. The simulation results reveal that the annulus-core flow structure is a characteristic feature of the boiler. A reduction in primary air supply enhances the reducing atmosphere, thereby effectively inhibiting NOx formation. Under conditions of constant total secondary air volume, the optimal operational strategy involves moderately increasing the proportion of upper secondary air volume ratio. This adjustment promotes incomplete combustion and strengthens the reducing atmosphere, which in turn suppresses NO formation. Meanwhile, the combustion temperature is only marginally influenced. The peak temperature in the dense-phase zone decreases slightly from 1155 K to 1143 K. Concurrently, NO emissions at the furnace outlet are reduced from 196 mg/m3 to 153 mg/m3, corresponding to a 21.9 % decrease. Therefore, NOx formation can be effectively suppressed by appropriately reducing the primary airflow and increasing the upper secondary air ratio. This adjustment does not compromise furnace fluidization or combustion efficiency.