overfire

Overfire refers to the introduction of air or gases above the main combustion zone in a furnace or boiler. The most common form is overfire air (OFA), which uses ports or dampers positioned above the primary flame to supply a portion of the combustion air after fuel-rich combustion has begun in the lower furnace.

The purpose of overfire air is to achieve staged combustion. Primary air supplies the fuel near the burners, creating a fuel-rich zone, while the overfire air completes combustion higher in the furnace. This arrangement lowers peak flame temperatures, improves combustion efficiency, and reduces nitrogen oxide (NOx) formation by limiting oxygen availability in the hottest region. OFA can also aid in burnout and reduce emissions of carbon monoxide when properly controlled.

Implementation and operation typically involve dampers and fans managed by the boiler’s control system. The amount and timing of overfire air are adjusted based on operating load, fuel characteristics, and emission goals. In facilities such as waste-to-energy plants and some utility boilers, OFA is part of broader NOx reduction strategies and helps meet regulatory limits.

Limitations and considerations include the risk that improper OFA settings can cause incomplete combustion, increased CO or unburned carbon, and potential furnace temperature imbalances. Successful use requires combustion modeling, adequate instrumentation, and careful commissioning to harmonize OFA with primary air and fuel distribution.