Numerical investigations on overfire air design for improved boiler operation and lower NOx emission in commercial wall-firing coal power plants

Abstract

Air staging using overfire air (OFA) is a common practice in a pulverized coal combustion for reduction of NOx emission without deteriorating the combustion efficiency. In opposed wall-firing boilers, a simple single-level arrangement of OFA nozzles aligned with swirl burners is typically employed. In this study, an ideal OFA design was developed using computational fluid dynamics for efficient combustion, heat transfer, and emission reduction in a 595-MWe industrial-scale boiler. Using the modeling methodology validated using boiler design data, various OFA designs were evaluated for detailed impact on the boiler performance including char burnout, NOx emission, furnace exit gas temperature (FEGT), and heat absorption including convective heat exchangers. Comparing with single-level arrangements at various heights or two-level arrangements with an increased number of nozzles, the most favorable results were achieved for a two-level staggered arrangement with wide spacings between nozzles and a doubled flow ratio in the upper level. Stronger upper-level jets penetrated deeper into the central region, whereas weaker lower-level jets covered the near-wall region. This enhanced mixing with hot gas from the burner zone resulted in a uniform flow and temperature distributions in the upper furnace. In addition to the reduction of NOx emission, a detailed analysis of the results revealed various benefits of an ideal OFA design that have not been identified in existing studies, including i) reduced ash deposition potential in the tube bundles because of the lower FEGT, ii) reduced attemperating water spray requirement for steam because of the increased heat absorption in the evaporator, and iii) alleviated tube metal overheating because of a lower peak heat flux in the platen superheater. The proposed OFA design can be applied to other large-scale wall-firing boilers.