Numerical Simulation of Coal Combustion in a Tangential Pulverized Boiler: Effect of Burner Vertical Tilt Angle

Understanding of the flow field and heat transfer in a boiler is important to meet with the often-conflicting objectives of efficient steam generation, safe operation and minimization of pollutant emissions. Steam generation requires high gas temperatures, which often lead to high NOx emissions. High gas temperatures inside the boiler may also lead to slagging and fouling problems, which adversely affect the heat transfer to the steam side. One of the principal ways of finding a compromise in tangentially fired pulverized coal (PC) boilers, which constitute the majority of the utility boilers, is through tilting the burners. The current work seeks to obtain a detailed understanding of the effect of burner tilt on the flow and heat transfer inside the boiler by utilizing numerical simulations. Computational fluid dynamics (CFD) simulations were performed on a 210 MWe tangentially fired pulverized coal boiler to understand the impact of two vertical burner tilt angles, - 15 degrees and + 15 degrees. The effect of burner tilt was investigated by analyzing the effect on gas flow, temperature distribution, coal particle trajectories and NOx emissions. The results suggest that a downward tilt of the burner (- 15 degrees) has a significant effect on the particle trajectories and the residence time of the particle in the high temperature burner zone. The temperature of the exit gas was reduced by 33 K due to the downward tilt. NOx emissions were reduced by about 5.5% when the burners were tilted downward by 15 degrees which be attributed to two factors, namely the relatively insignificant contribution of thermal NOx mechanism in utility boilers and the role of NOx reburning in reducing conditions.

Automated summary

Burner tilt has a significant effect on the flow and heat transfer inside a boiler, reducing nitrous oxide emissions and residence time of coal particles in high temperature zones while lowering the exit gas temperature. The thermal NOx mechanism and NOx reburning play key roles in reducing NOx emissions in utility boilers.