Numerical study on optimization of secondary air box in a 600 MW opposed wall-fired boiler

The opposed wall-fired boiler has been widely employed in power plants in China due to its adaptability. However, the airflow in the same layer burners from the large air box is uneven, affecting the combustion characteristics of pulverized coal and causing the corrosion and slagging of the water-cooled wall. In this study, the numerical simulation is performed on a 600 MW wall-fired boiler to investigate the air distribution and flow characteristics of each burner and the secondary air duct under variable boiler loads (100%, 75%, 50%, and 30%), which would provide the basis for optimization of the secondary air box. The effectiveness of the modified burner, over-fire air (OFA) nozzle, and secondary air duct is demonstrated through the numerical simulation. The results indicate that the burner and OFA nozzle exhibit distinct flow deviation characteristics within the original air box. Specifically, the burner demonstrates a flow deviation ranging from 2% to 4%, whereas the OFA nozzle exhibits a flow deviation ranging from 10% to 35%. Integrating poly air rings into the burners and OFA nozzles on the sidewall separately addresses the issue of non-uniform flow distribution of the OFA nozzle while concurrently improving the air flow rate on both sides of the burners and OFA nozzles. The main air duct could be divided into three smaller air ducts by installing two deflectors at the corner of the secondary air duct. This scheme effectively decreases the pressure drops at the corner interface from 337.4 to 254.3 Pa. The findings of this study could have the potential to offer scientific insights into and recommendations for the optimal functioning of the boiler system.