What is the role of secondary air in a grate incinerator?

Enhancing in-furnace gas turbulence

Secondary air produces strong turbulence in the flue gas and incompletely burned matter inside the furnace. In a grate incinerator, waste burning on the grate generates rising hot flue gas that may contain unburned particles and combustible gases. Secondary air is injected at high speed from the side or from above, colliding with and mixing into the hot flue gas. This breaks up the original laminar flow and creates turbulence. That turbulence greatly increases the contact between unburned matter and oxygen, allowing it to stay in the furnace longer and promoting complete combustion.

For example, during combustion, some of the volatile organic compounds in the waste may rise quickly with the flue gas. Without the disturbance of secondary air, these compounds may not come into full contact with enough oxygen and may not burn completely. Secondary air acts like a “stirrer” for the combustion process, letting these compounds mix better with oxygen and raising combustion efficiency.

Regulating the temperature distribution in the furnace

Proper placement and control of secondary air can effectively regulate the temperature distribution in the furnace. It can carry heat from the high-temperature zone to cooler areas, making the temperature inside the furnace more even. In a grate incinerator, waste has different temperature requirements at different combustion stages (such as drying, combustion, and burnout). Secondary air can carry some of the heat generated in the combustion zone to the waste that has not yet fully burned or is still drying, helping that waste undergo the subsequent combustion more effectively.

For instance, during the combustion stage the core of the flame is very hot, while the areas near the furnace wall or the waste feed end are relatively cooler. With properly directed secondary air injection, the high-temperature flue gas from the flame core can be guided to other areas, making the temperature environment across the furnace more favorable for complete waste combustion and avoiding localized overheating or incomplete burning.

Controlling the shape and position of the flame

Secondary air can effectively control the shape and position of the flame. In an incinerator, if the flame comes too close to the furnace wall or roof, it can damage the furnace body. By adjusting the direction and flow rate of the secondary air, the flame can be kept in the right position, preventing it from washing directly against the furnace wall and reducing thermal stress and the risk of corrosion on the furnace body.

For example, when the flame shows a tendency to shoot upward and approach the furnace roof, injecting an appropriate amount of secondary air from the side or from below can push the flame down, keeping it in a more reasonable position in the furnace, lengthening its travel, and giving the combustible components in the waste more time and opportunity to burn, while protecting structural parts such as the furnace roof from excessive erosion by the high-temperature flame.

Reducing the formation of pollutants

Secondary air plays an important role in reducing the formation of products of incomplete combustion (such as carbon monoxide and hydrocarbons) and harmful substances such as dioxins. Because secondary air increases the oxygen supply and the degree of mixing during combustion, the organic matter in the waste burns more completely, lowering emissions of incomplete-combustion products such as carbon monoxide.

For the control of dioxins, the good turbulence provided by secondary air helps maintain suitable combustion conditions in the furnace. At high temperatures (850–1100°C) with an ample oxygen supply, the organic matter in the waste breaks down quickly, reducing the precursors of dioxin formation and lowering the likelihood of dioxins being produced at the source. At the same time, the proper use of secondary air also allows the small amount of dioxins that has already formed to break down further in the high-temperature environment, reducing the concentration of dioxin emissions.

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