What is the role of tertiary air in a biomass-fired boiler, and how does it contribute to NOx control?

In a biomass-fired boiler, tertiary air plays a crucial role in completing combustion and controlling the formation of nitrogen oxides (NOx). Primary air is supplied through the grate or bed to support initial combustion of the fuel. Secondary air is introduced above the fuel bed to provide additional oxygen for more complete combustion of volatile gases and unburned carbon particles. Tertiary air, if present, is injected even further above the secondary air nozzles, often at multiple locations in the upper furnace. Its primary function is to ensure complete oxidation of any remaining unburned gases and to promote mixing of the combustion products. Regarding NOx control, tertiary air can be used to implement staged combustion. Staged combustion involves creating fuel-rich zones in the lower part of the furnace, where less air is available. This reduces the formation of thermal NOx because the lower oxygen concentration inhibits the oxidation of nitrogen in the air. By delaying the introduction of the remaining air (tertiary air) to a higher point in the furnace, the combustion process is completed at a lower overall temperature, further suppressing thermal NOx formation. The precise location and flow rate of the tertiary air are carefully controlled to optimize both combustion efficiency and NOx emissions. Overfire air (OFA) is a common term for tertiary air used for NOx control in boilers.