Designing Air Staging Systems with Mathematical Modeling

Combustion systems that include air staging ports are being applied to utility boilers to meet legislated NOx emission limits. The systems must be designed for effective emission control while minimizing the impact on combustion efficiency and boiler performance. Traditional methods for applying overfire air have worked well in the past and continue to work well in some cases. However, as low NOx retrofits are applied to a more diverse range of boiler sizes and configurations, these traditional methods cannot be extrapolated readily. A design tool - mathematical modeling - has been developed for extending current and developing new methodologies for applying overfire air systems. The applications include single wall-fired, opposed wall-fired, roof-fired, cyclone-fired, and cell-fired boilers. At Babcock & Wilcox, mathematical modeling is playing a vital role in the design and application of air staging systems. Three-dimensional models have been developed to accurately predict flow, combustion, and heat transfer in a wide variety of fossil-fuel fired furnaces. The models provide the means to evaluate alternate designs and to optimize system performance. Consequently, tuning a system to match the unique requirements of a specific boiler can be accomplished rapidly with confidence. This paper discusses the role that mathematical modeling played in the sorting of options for low NOx retrofit designs that included air staging ports.