Optimization of operating conditions to achieve combustion stability and reduce NOx emission at half-load for a 550-MW tangentially fired pulverized coal boiler

•Achieving combustion stability while reducing gas emissions under half-load conditions in boiler.•Furnace suffers from asymmetrical combustion due to various burner group arrangements.•The optimal CCOFA/SOFA ratio is 5% CCOFA and 15% SOFA.•The influence of excess air ratio on NOx/UBC is discussed.•Field tests are performed under optimal conditions. The off-peak period of the grid load (i.e., deep peak load) prevents a power generation boiler from operating at full load. To achieve stable combustion of 550-MW tangentially fired pulverized coal boiler and ultra-low pollutant emissions (nitrogen oxides and unburned carbon) under half-load conditions, different operating parameters have been analyzed and optimized. In this study, under half-load conditions, the numerical method was used to simulate the flow field characteristics, combustion stability, and pollutant emissions of the boiler under various operating conditions. The operating conditions included various burner group arrangements, close-coupled overfire air (CCOFA)/separated overfire air (SOFA) distributions, and excess air ratios. The simulation prediction results showed that the middle burner group (BCDE) arrangement has a good flow field distribution. Compared with the upper burner group (CDEF) arrangement, this reduces NOX by approximately 62 ppm and also maintains a higher pulverized coal burnout rate than the lower burner group (ABCD) arrangement. Considering the stability of the combustion and lowest emissions, the ratio of CCOFA of 5% and SOFA of 15% were preferred as the operating conditions for the air-staging distribution. The high excess air caused an increase in NOX while the combustion temperature significantly reduced in the furnace, made the furnace exit gas temperature (FEGT) too low, and affected the steam temperature. In addition, the simulation results of the optimized scheme were in good agreement with the field test results.