Coal Fired Circulating Fluidized Bed

Coal Fired Circulating Fluidized Bed

To understand the principle of circulating fluidized bed boiler, it is necessary to understand the theory of bubbling bed and fast bed as well as the dynamic characteristics, burning characteristics and heat transfer characteristics of materials in various states from bubbling bed to turbulent bed to fast bed.

I. fluidization:
When the solid particles in the fluid through, as the fluid velocity increases gradually, solid particles began to exercise, and the friction between solid particles is becoming more and more big, when the velocity reaches a certain value, the friction between solid particles and their gravity is equal, each particle can freedom of movement, all solid particles showed similar flow state of the phenomenon, this phenomenon is called fluidization.
For liquid-solid fluidized solid particles, the particles are evenly distributed in the bed, which is called “scattered” fluidization. However, for gas-solid fluidized solid particles, the gas does not flow evenly through the bed, and the solid particles are divided into groups for turbulent motion, and the voidage in the bed varies with the position and time. This fluidization is called “poly” fluidization. Circulating fluidized bed boiler belongs to “poly” fluidization.

Solid particles (bed), fluids (fluidized air), and the equipment that completes the fluidized process are called fluidized beds.

Critical fluidization velocity

1. For the uniform particle size of particles in bed, in a fixed bed by gas flow rate is very low, with the increase of wind speed, the pressure drop of the bed into a direct ratio increases, and when the wind speed reaches a certain value, the bed pressure drop reached maximum value, the value is slightly larger than static bed layer, if continue to increase the wind speed, fixed bed suddenly unlock, bed pressure drop down to the bed of the static pressure. If the bed is composed of wide screening particles, its characteristics are as follows: before the large particles have moved, the small particles in the bed have been partially shelled, and the unlocking phenomenon of the bed from fixed bed to fluidized bed is not obvious, but the phenomenon of stratified fluidization often occurs. The minimum velocity required for fluidization of particle bed from static state is called critical fluidization velocity. With the further increase of wind speed, the pressure drop of the bed is almost unchanged. The general fluidization velocity of CFB boiler is 2-3 times the critical fluidization velocity.

2. Factors affecting the critical fluidization velocity:

(1) the thickness of material layer has little influence on the critical velocity.

(2) when the average equivalent diameter of the material layer increases, the critical velocity increases.

(3) when the density of solid particles increases, the critical velocity increases.

(4) when the fluid kinematic viscosity increases, the critical velocity decreases: for example, when the bed temperature increases, the critical velocity decreases.

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