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Over-Fire Air Systems

How Fuel Tech's OFA System Will Reduce NOx Emissions

Combustion air is diverted from the burners to create a fuel rich zone in the lower furnace. Fuel bound nitrogen conversion to NO is inhibited on coal and oil fired boilers. Peak flame temperatures are reduced on gas fired boilers to limit thermal NOx formation. A high momentum air stream intersects burner flames in the upper furnace to complete combustion prior to the boiler convective section.


The OFA port is shaped by refractory "boot" with combustion air fed through a 310 SS duct for long life. Automated damper controls allow for tuning through the load range. Full furnace Computational Fluid Dynamics modeling is performed on each project. OFA port sizes and shaping are determined to maximize combustion air penetration and furnace coverage.

The port utilizes a venturi design with zone disk to accurately control airflow through the OFA port.

Over-fire Air Graph Image

Fuel Tech Low NOx Burners & OFA Wall-Fired Coal Boiler NOx vs. BZHR

This graph illustrates NOx reduction capabilities for both eastern bituminous and sub-bituminous coal. Fuel Tech’s Low NOx burners and OFA systems provide maximum NOx reduction across a wide range of Burner Zone Heat Release (BZHR) values.

Deep Staging for Maximum NOx Reduction


CFD Model of Fuel Tech re-design of customers OFA system (left), to SOFA system (right).

Over-fire air is injected into the furnace above the top row of burners and completes the combustion of the unburned CO prior to the gases passing through the superheater. Fuel Tech’s modeling expertise and proven field results allow us to guarantee OFA system performance.

Related Documents

Over-Fire Air Brochure