cart.general.title

Distinguishing between level 1 and level 2 air of a fluidized bed boiler?

Steam bubbling fluidized bed boiler
Installation of steam bubbling fluidized bed boiler mushrooms

Some basic concepts about bubbling fluidized bed boilers can sometimes be confusing, especially when it comes to primary air and secondary air of the boiler. So, what makes the difference between these two types of air?

The primary air of the bubbling fluidized bed boiler is supplied to the combustion chamber from below, playing a crucial role in maintaining the bed material in the combustion chamber, controlling the temperature, and supplying the necessary oxygen for the initial combustion process. On the other hand, secondary air focuses on supporting combustion in high-density and upper areas of the bubbling fluidized bed boiler.

Let's delve deeper into these concepts and their roles in the operation of the bubbling fluidized bed boiler.

What is primary air of the bubbling fluidized bed boiler? What is its function?

Air holes on the mushroom
The small holes on the mushroom distribute primary air

The primary air of the bubbling fluidized bed boiler is air supplied to the combustion chamber from beneath the boiler mushroom to lift the bed material in the fluidized bed, control the combustion chamber temperature, and ensure the necessary oxygen for the initial combustion process. The primary air needs to overcome the resistance of the fuel layer and boiler mushroom and requires a dedicated fluidized bed blower.

It should be noted that not all the air discharged from the primary blower is primary air; a small portion of the air will become secondary air, recirculation air, or supplementary air for secondary air. Subtracting these additional air amounts, over 90% of the remaining air from the primary blower becomes primary air. In a typical bubbling fluidized bed boiler, the air box pressure is usually 3000 - 6000Pa. The primary air pressure mainly relates to the resistance of the boiler mushroom, the air duct resistance, the resistance of the air preheater, the fuel size and thickness of the fuel layer, as well as the combustion chamber temperature.

The primary air has the following functions:

  1. Creates an adequate air cushion to lift the fuel layer, ensuring rapid heat transfer between the burning red fuel particles and the fresh fuel particles into the combustion chamber, promoting the mixing of hot and cold particles, balancing the temperature of the fluidized bed layer.

  2. Supplies the necessary oxygen for the initial combustion process and the early stage of the combustion process, creating a stable initial combustion state, ensuring combustion efficiency, and igniting a large bed combustion layer.

  3. By adjusting the primary air, it effectively controls the fluidized bed temperature, meeting the combustion requirements of the bubbling fluidized bed boiler, with a fluidized bed temperature ranging from 850-950°C.

  4. Through the even distribution of air by the boiler mushroom, the fuel layer is evenly distributed, minimizing the phenomenon of fuel pile or empty boiler mushroom.

  5. Creates a vigorous fuel combustion process, promoting the fragmentation of large particles in the fuel layer, providing basic small particle groups for high-density areas and upper regions, facilitating the transition from large particles to small particles.

What is the secondary air of the circulating fluidized bed boiler? What is its function?

Secondary air holes of the bubbling fluidized bed boiler
The secondary air holes are arranged above the fuel feed line

The secondary air of the bubbling fluidized bed boiler is supplementary air for combustion, not participating in the fluidized bed combustion process in the lower part of the combustion chamber, used for burning fuel particles in high-density and upper areas.

The secondary air nozzles are usually arranged at the height of the combustion chamber, two or three levels. Fuel blowing air and recirculating air are supplied above the boiler mushroom, at a certain level equivalent to a small portion of the secondary air.

The secondary air has the following functions:

  1. Adjusts the operational oxygen amount, creating a stratified air supply method, under low-temperature combustion conditions, continuously reducing the emission of NOx and SOx.

  2. Enhances the movement, friction, and collision between particles, facilitating the shedding of ash deposits on the surface of large fuel particles, promoting their contact with oxygen, ensuring the combustion efficiency of the fuel.

  3. The good penetration ability of high-speed secondary air ensures sufficient oxygen supply to the particles in the cross-section of the boiler and reduces the oxygen-poor area of high-density fuel particles.

  4. The strong turbulence created by the secondary air accelerates the mass transfer process and enhances the heat transfer capability of the inner material of the boiler, increasing the heat exchange surface's thermal conductivity.