Overfire Air Systems

Overfire air systems are used to divert a portion of the total combustion air away from the primary combustion zone. Overfire air lowers the air-to-fuel ratio at the burners, lowering the oxygen in the primary flame zone, which reduces NOx conversion of fuel-nitrogen in volatiles and reduces thermal NOx formed in the primary flame zone.

Steag Energy Services (formerly ESA) has conceived, designed and installed overfire air systems of various types. Overfire air systems range in complexity of design, cost of installation and effectiveness of NOx reduction. In the design of overfire air systems, Steag Energy Services (formerly ESA) uses CFD Modeling to design overfire air flow, duct size, nozzle size, flow orientation and various other system design parameters as well as to predict performance.

A simple but cost-effective overfire air design Steag Energy Services (formerly ESA) has implemented involves direct flow of air from the windbox to the main furnace above the burner zone. This is accomplished by removal of refractory webbing between waterwall tubes between furnace and windbox. The image at the right shows where refractory has been removed from wall tubes at the windbox. While overfire airflow control with such a design is limited, the resulting combustion staging reduces NOx.

Another simple and effective overfire overfire air system Steag Energy Services (formerly ESA) has tested and implemented applies to wall fired boilers with multiple burner elevations. By removing the uppermost burners from service (stopping coal flow while allowing combustion air flow) staged combustion can be created. Steag Energy Services (formerly ESA) has employed this technique in combination with split flame burners to achieve significant NOx reductions (over 40% below baseline levels for a 200 MW wall fired unit).

Steag Energy Services (formerly ESA) has designed and installed more complex overfire air systems involving additional controls, fans, duct work, dampers and ports. Systems of this complexity offer greater control of air distribution and combustion staging for more optimized firing.

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