Ultra-Low NO_x Combustion in W-Flame Boilers: A Physics-Driven Framework via Dimensionless Peak-Temperature Height and Aerodynamic Optimization

W-flame boilers are widely used to burn low-volatile coals such as anthracite but often face high nitrogen oxides (NO_x) emissions due to intense combustion near the lower furnace. This study investigates NOx formation in a 660 MW supercritical W-flame boiler using validated high-fidelity simulations, with a focus on how secondary air injection beneath the arch influences combustion behavior. A key outcome of this work is the development of a quantitative and reliable control method based on the dimensionless height of the peak-temperature zone (h). This parameter captures the spatial position of the high-temperature region and shows a strong correlation with NO_x emissions. Lower or negative h values, indicating a shift of peak-temperature zone below the throat, result in more uniform combustion and lower NO_x formation. The study further demonstrates that adjusting the Y-direction momentum, especially through sub-arch airflow and injection angle, provides a practical means to control h, linking airflow regulation with emission outcomes. Together, these insights form a physics-based control tool that bridges operational parameters and combustion dynamics. It offers a structured, real-time approach for optimizing flame structure and reducing NOx, enabling more efficient and cleaner operation of W-flame boilers.