V-shaped effect of primary and secondary airflow momentum ratio on performance of a 660 MW tangentially fired lignite boiler: Identification and verification

Abstract

To explore how the primary and secondary air ratios affect the overall performance of lignite-fired power generation boilers, a 660 MW wall-tangentially fired lignite boiler was chosen as research subject. 17 combustion scenarios were designed and then numerically simulated through a validated model, and the in-furnace coal combustion behavior, heat transfer process, and NOx conversion characteristics under various conditions were analyzed in detail. Results show that, except for the monotonous increase in NOx emission, the overall boiler performance follows a V-shaped trend with the continuous increase in primary air ratio (PAR). Initially, as PAR is slightly increased, the in-furnace combustion temperature, heat transfer intensity and coal utilization rate notably decrease, followed by a subsequent increase in these parameters as PAR is significantly increased. The separated over fire air (SOFA) ratio doesn’t alter this V-shaped pattern; however, a higher SOFA ratio does reduce the critical PAR at which the overall boiler performance is most compromised. This is attributed to changes in the momentum ratio between primary and secondary airflow caused by varying PAR. Boiler performance deteriorate significantly when their momentum ratios are nearly equal, and a higher SOFA ratio makes it easier. These observations are confirmed under different thermal load conditions, where the close proximity of primary and secondary air momentum is avoided by reducing SOFA ratio. Based on these findings, a substantial increase in PAR should be avoided in practice, if necessary, lowering the SOFA ratio helps mitigate significant deterioration in boiler performance.