The functions and differences of the primary, secondary and tertiary air of the boiler burner

The primary air, secondary air and tertiary air of the boiler burner play different roles in the combustion process. The differences mainly lie in the function, wind speed, wind temperature and the stage of participation in combustion. The following are the specific functions and differences:

1. Primary air

Function:

Transport fuel: send pulverized coal (or fuel oil/gas) from the burner into the furnace to ensure the initial mixing of fuel and air.

Volatile combustion: provide a small amount of oxygen to help the volatile matter (combustible gas) in the fuel burn at the beginning of ignition.

Features:

Small air volume: 15%~30% of the total air volume (pulverized coal furnace), the air volume must meet the fuel transportation requirements.

Moderate wind speed: usually 20~30 m/s (pulverized coal furnace), to ensure that the pulverized coal is not deposited.

Low wind temperature: if it is a pulverized coal furnace, the primary air needs to dry the pulverized coal but the temperature should not be too high (to prevent deflagration).

Application scenarios:

In pulverized coal furnaces, primary air is mixed with pulverized coal to form "primary air-powder airflow";

In oil/gas boilers, primary air mainly provides initial combustion oxygen.

2. Secondary air

Function:

Combustion support: Provide the main oxygen required for combustion to ensure the full combustion of coke and residual volatiles.

Enhance turbulence: Enhance the mixing with fuel through high-speed airflow to improve combustion efficiency.

Adjust the flame shape: Control the flame length and temperature distribution through swirl or direct flow design.

Features:

Large air volume: 60%~80% of the total air volume, which is the main oxygen source for combustion.

High wind speed: up to 40~60 m/s, enhancing penetration and mixing effect.

High wind temperature: often heated by air preheater to increase combustion temperature.

Application scenarios:

In pulverized coal furnaces, secondary air is fed in layers or swirls at the burner outlet;

In layer-burning furnaces, secondary air is sprayed in from above the grate to help combustion of suspended particles.

3. Tertiary air

Function:

Burn out the residual carbon: further burn the unburned coke particles to reduce the mechanical incomplete combustion loss.

Use the exhaust gas: in the pulverized coal furnace, the tertiary air may carry the dry exhaust gas (containing a small amount of pulverized coal) of the pulverizing system to recover energy.

Regulate the furnace temperature: affect the furnace temperature distribution through air volume distribution (such as reducing NOx generation).

Features:

Flexible air volume: 10%~15% of the total air volume, some boilers may not have tertiary air.

Low wind speed: about 30~45 m/s, to avoid interference with the main combustion zone.

Low wind temperature: if it is the exhaust gas of the pulverizing system, the temperature may be lower than the primary air/secondary air.

Application scenario:

Mainly used in pulverized coal furnaces, especially the intermediate storage silo type pulverizing system (when hot air is used to deliver pulverized coal, dry exhaust gas is used as tertiary air);

Some boilers arrange the tertiary air on the upper part of the burner to form staged combustion.

Summary of main differences

Supplementary explanation

Stage combustion: Modern boilers may reduce pollutant emissions by distributing air in stages (such as low NOx burners). In this case, the proportion and position design of secondary and tertiary air is more complicated.

Fuel difference: Oil/gas boilers usually do not have tertiary air, and the boundary between primary and secondary air may also be blurred (such as when using swirl burners).

Through reasonable air distribution, the boiler can balance combustion efficiency, emission control and stability.