Fluidized bed boiler

Circulating fluidized bed boilers are developed on the basis of bubbling bed boilers (boiling furnaces), so some theories and concepts of bubbling bed can be used in circulating fluidized bed boilers. But they are quite different. The early circulating fluidized bed boiler has a relatively high fluidization speed, so it is called a fast circulating bed boiler. The basic theory of fast bed can also be used in circulating fluidized bed boiler. The basic theory of bubbling bed and fast bed has been studied for a long time. To understand the principle of circulating fluidized bed, it is necessary to understand the theory of bubbling bed and fast bed, as well as the dynamic characteristics, combustion characteristics and heat transfer characteristics of materials in various states of bubbling bed-turbulent bed-fast bed.

1) Fluidization

When a fluid passes through the solid particles, as the fluid velocity gradually increases, the solid particles begin to move, and the friction between the solid particles becomes larger and larger. When the flow velocity reaches a certain value, the friction between the solid particles and Their gravity is equal, each particle can move freely, and all solid particles show a phenomenon similar to a fluid state, which is called fluidization.

For liquid-solid fluidized solid particles, the particles are evenly distributed in the bed, which is called “discrete” fluidization. For gas-solid fluidized solid particles, the gas does not flow through the bed uniformly, and the solid particles are divided into groups for turbulent movement. The void ratio in the bed changes with position and time. Stateization is called “poly” fluidization. The circulating fluidized bed boiler belongs to the “poly” fluidization. The solid particles (bed material), fluid (fluidization wind) and the equipment that completes the fluidization process are called fluidized beds.

2) Critical fluidization speed

For a bed composed of particles of uniform size, when the gas flow rate through the fixed bed is very low, as the wind speed increases, the bed pressure drop increases proportionally, and when the wind speed reaches a certain value, the bed pressure drop reaches the maximum This value is slightly larger than the static pressure of the bed. If you continue to increase the wind speed, the fixed bed will suddenly unlock and the bed pressure will drop to the static pressure of the bed. If the bed is composed of wide-screened particles, its characteristics are: before the large particles move, the small particles in the bed have been partially fluidized, and the unlocking of the bed from a fixed bed to a fluidized bed is not obvious. The phenomenon of stratified fluidization often occurs. The minimum velocity required for the bed of particles to transform from a static state to a fluidized state is called the critical fluidization velocity. As the wind speed increases further, the bed pressure drop is almost unchanged. The general fluidization wind speed of circulating fluidized bed boiler is 2 to 3 times the critical fluidization speed.

The factors affecting the critical fluidization velocity are as follows:

(1) The thickness of the material layer has little effect on the critical flow rate.

(2) As the equivalent average diameter of the material layer increases, the critical flow rate increases.

(3) The critical flow rate increases when the density of solid particles increases.

(4) The critical flow rate decreases when the kinematic viscosity of the fluid increases: as the bed temperature increases, the critical flow rate decreases.