Doosan Babcock is a world expert in the design, manufacture and installation of staged combustion systems, both OFA and BOFA.

The Overfire Air (OFA) process involves diverting a portion of the combustion air from the primary zone and channelling it through a number of ports above the top row of burners. This creates two zones, an oxygen lean primary zone and a second oxidising burnout zone where the combustion is completed. For standard OFA systems the injection velocity through the ports is typically similar to that through the burners as the air is taken at the windbox or secondary air pressure. OFA has the advantage of needing no additional power consumption as the FD fan supplies the overfire air.

Due to the higher air velocities, reduced residence times are required before the injection of the overfire air, to allow complete burnout of the coal particles.

Boosted Overfire Air (BOFA) is similar to OFA but utilises booster fans to allow the injection of the overfire air at a much higher velocity. The increased velocity gives an improved penetration of the air into the furnace and allows for lower achievable Carbon in Ash or a greater NOx reduction for a given burnout.