What is the role of secondary air in a grate incinerator?
Secondary air in a grate incinerator is injected into the furnace at high speed from the side or from above. Its main roles are to strengthen turbulence, adjust the temperature distribution, control the flame position and reduce pollutant formation.
Stronger gas turbulence in the furnace
Waste burning on the grate produces rising hot flue gas, and the gas may carry unburned particles and combustible gas. Secondary air collides and mixes with the hot gas, breaking the original laminar flow into turbulence, greatly increasing the chance of contact between unburned matter and oxygen and lengthening their residence time in the furnace, which promotes complete combustion. When volatile organic compounds produced by part of the waste rise quickly with the gas, they may fail to meet enough oxygen and burn incompletely without the turbulence from secondary air.
Adjusting the furnace temperature distribution
Reasonably arranged and controlled secondary air can carry heat from high-temperature zones to cooler zones and make the furnace temperature more even. Waste has different temperature requirements in the drying, combustion and burnout stages, and secondary air can carry part of the heat from the combustion zone to waste that is not yet fully burning or is still drying, helping it burn afterwards. When the flame core is hot while the area near the furnace wall or the feed end is cooler, secondary air can guide the hot gas from the core to other zones, avoiding local overheating or local incomplete combustion.
Controlling flame shape and position
A flame too close to the furnace wall or roof damages the furnace body. By adjusting the direction and flow of secondary air, the flame can be held in a suitable position, preventing it from washing the furnace wall directly and reducing thermal stress and corrosion risk on the furnace body. When the flame tends to rise towards the roof, an appropriate amount of secondary air injected from the side or below presses the flame down, lengthening the flame path in the furnace so combustible matter has more time to burn while the roof structure is protected.
Reducing pollutant formation
Secondary air strengthens the oxygen supply and the degree of mixing during combustion, so organic matter in the waste burns more fully and emissions of incomplete-combustion products such as carbon monoxide and hydrocarbons fall. At 850-1100℃ with sufficient oxygen supply, organic matter breaks down quickly and the precursors of dioxin formation are reduced. Proper use of secondary air also lets the small amount of dioxin already formed decompose further at high temperature, lowering the emission concentration.
