In a grate furnace, proper air distribution is critical for complete combustion, low emissions, and high thermal efficiency. The air supply is typically divided into primary air and secondary air, with a small amount sometimes used as tertiary air or overfire air.
Primary air is introduced from below the grate, passing through the fuel bed. Its main functions are to dry the fuel, drive off volatiles, and supply oxygen for the initial char combustion. Primary air usually accounts for 60 to 80 percent of the total combustion air. A higher primary air ratio strengthens the fire on the grate but may increase fly ash carryover and NOx formation. Too little primary air leads to incomplete combustion and high carbon content in the bottom ash.
Secondary air is injected above the grate, into the freeboard region. It serves to burn the volatile gases released from the fuel, create turbulence, and complete the combustion of any unburned hydrocarbons and carbon monoxide. Secondary air typically makes up 20 to 40 percent of the total air. A well tuned secondary air supply reduces emissions of CO, dioxins, and soot. It also helps control furnace temperature and minimize NOx by staging the combustion.
The total air flow is usually set at 1.4 to 1.8 times the stoichiometric air requirement, corresponding to an excess air ratio of 1.4 to 1.8. Lower excess air improves efficiency but risks incomplete combustion; higher excess air reduces flame temperature and increases heat loss through flue gases.
The optimal primary to secondary air ratio depends on fuel properties, grate design, and load. For typical waste to energy plants, a common starting point is 70 percent primary air and 30 percent secondary air. During the drying and ignition zones, primary air may be reduced to prevent cooling the fire. In the burnout zone, primary air can be lowered while secondary air is increased to ensure complete gas phase combustion.
Modern grate furnaces use continuous oxygen monitoring and feedback control to adjust the air flow ratio in real time, balancing combustion quality and emission compliance. Proper air staging can achieve high burnout efficiency with low NOx and CO emissions.
