Role of Secondary Air in a Grate Incinerator

Strengthening In-furnace Airflow Disturbance

Secondary air causes strong disturbance of the flue gas and incompletely combusted materials in the furnace. In a grate-type incinerator, waste burning on the grate produces rising hot flue gas, which may contain unburned particles and combustible gases. Secondary air is injected at high speed from the side or top, colliding and mixing with the hot flue gas, breaking the original laminar state and forming turbulence. This turbulence greatly increases the contact opportunity between unburned materials and oxygen, allowing them to stay longer in the furnace and promoting complete combustion.

For example, during combustion, some volatile organic compounds from the waste may rise rapidly with the flue gas. Without secondary air disturbance, these compounds may not fully contact enough oxygen to burn completely. Secondary air acts like “stirring” the combustion process, helping these organic compounds mix better with oxygen and improving combustion efficiency.

Regulating In-furnace Temperature Distribution

Reasonable placement and control of secondary air can effectively regulate the temperature distribution in the furnace. It can carry heat from high-temperature zones to lower-temperature zones, making the furnace temperature more uniform. In a grate-type incinerator, waste at different combustion stages (drying, combustion, burnout) has different temperature requirements. Secondary air can carry part of the heat from the combustion zone to waste areas that are not yet fully burned or are still drying, helping them burn better.

For example, during combustion, the flame center temperature is high while the temperature near the furnace wall or feed end is relatively low. Through proper secondary air injection, the high-temperature flue gas at the flame center can be guided to other zones, making the overall furnace temperature more favorable for complete combustion and avoiding local overheating or incomplete combustion.

Controlling Flame Shape and Position

Secondary air can effectively control the shape and position of the flame. If the flame gets too close to the furnace wall or roof, it may damage the furnace body. By adjusting the direction and flow of secondary air, the flame can be controlled in a proper position, preventing the flame from directly scouring the wall and reducing thermal stress and corrosion risk.

For example, when the flame tends to rise toward the furnace roof, injecting an appropriate amount of secondary air from the side or below can press the flame down, making its position more reasonable, extending the flame path in the furnace and giving the combustible components more time and opportunity to burn, while protecting structural parts such as the roof from excessive erosion by the high-temperature flame.

Reducing Pollutant Formation

Secondary air plays an important role in reducing incomplete-combustion products (such as carbon monoxide and hydrocarbons) and harmful substances like dioxins. By enhancing oxygen supply and mixing during combustion, organic matter in the waste burns more completely, reducing CO and other incomplete-combustion emissions.

For dioxin control, good secondary air disturbance helps maintain suitable combustion conditions. At high temperature (850-1100°C) with sufficient oxygen, organic matter in the waste decomposes rapidly, reducing the precursors of dioxin formation and lowering the possibility of dioxin generation at the source. Meanwhile, reasonable use of secondary air can further decompose the small amount of dioxin already formed in the high-temperature environment, reducing dioxin emission concentration.