Numerical investigation on the lifetime decline of burners in a wall-fired dual-fuel utility boiler

This paper reports the results of the numerical investigation on the causes of burners lifetime decline in a dual-fuel, front-wall-fired utility boiler. For this purpose, a three-dimensional modeling was performed using Computational Fluid Dynamics (CFD) to simulate a 150 MW boiler. The whole boiler, including the water-wall, low-NOx swirling burners with both types of nozzles, etc., was modeled in the real dimensions. Furthermore, the heat exchangers, like superheaters and economizers, were treated as porous media. Major attention was paid to the furnace zone, and especially region close to the burners where the failures have occasionally occurred due to the metal overheating. In order to achieve the objective, effects of the fuel type, the burners swirl ratio, droplet diameter of the fuel, and air-fuel ratio were investigated. Comparisons between numerical results, field measurements and plant observations showed the simulation can predict the flow and temperature fields in the boiler with acceptable accuracy for both fuels. It was found that, the most damage occurs while the plant works with natural gas, since the flame establishes closer to the burners. In this situation, the average heat flux received by the rear wall is about 0.25% higher than the received heat flux due to heavy fuel oil. Additionally, the rise in the swirl ratio of the burners can be another parameter, which strongly influences the flow and temperature field within the boiler and causes overheating damages to the burners and water-wall. Although variations in other investigated parameters have dramatic effects on performance characteristics of the boiler, they cannot lead to the overheating failure.