Green Field Research

Fluidized Bed Boiler Designs

GRI’s primary strength is in the design of fluidized bed (bubbling and circulating) boilers. It can offer complete process design and dimensions of critical components allowing most engineering companies to include this fuel-flexible clean option in their product line.

Fluidization is defined as

"an operation through which fine solids are transformed into a fluid-like state through contact with either a gas or a liquid."

Under the fluidized state, the gravitational pull on granular solid particles is offset by the fluid drag on them. Thus the particles remain in a semi-suspended condition. A fluidized bed displays characteristics similar to those of a liquid, as explained below with the help of Figure 1.

Figure 1: A fluidized bed demonstrates all the characteristics of a fluid.

1. The static pressure at any height is approximately equal to the weight of bed solids per unit cross section above that level.
2. The bed surface maintains a horizontal level, irrespective of how the bed is titled; also the bed assumes the shape of the vessel.
3. The solids from the bed may be drained like a liquid through an orifice at the bottom or on the side.
4. An object denser than the bulk of the bed will sink, while one lighter than the bed will float. Thus, a steel ball sinks in the bed, while a light shuttlecock floats on the surface.
5. Particles are well mixed, and the bed maintains a nearly uniform temperature throughout its body when heated.

An increase in the gas velocity through a bed of granular solids brings about changes in the mode of gas-solid contact in many ways. With changes in gas velocity the bed moves from one state or regime to another.

These regimes arranged tentatively in order of increasing velocities are

• packed bed (fixed)
• bubbling bed
• turbulent bed
• fast bed
• transport bed (pneumatic or entrained bed)

Under certain conditions one may also attain slugging or dense phase suspension flows.

Figure 2 presents a regime diagram illustrating the presence of those regimes in different types of boilers. It shows that the volume fraction of solids in the combustion zone decreases continuously with superficial gas velocity in the furnace or the combustion zone. Stoker-fired boilers use the densest combustion zone, while pulverized boilers use the leanest. The furnace of a Bubbling or Circulating Fluidized Bed (CFB) boiler lies between these two extremes.

Figure 2: Different commercial combustion systems operate under different gas-solid flow regimes.

Each of the above gas-solid process has its distinct character. Table 1 presents a comparison of some characteristic features of different gas-solid processes used in various types of boilers. It should be noted that the term bed has been used loosely in the Table 1 and elsewhere in this site. It refers to a body of gas-solid in one of the above contacting modes.

Table 1. Comparison of principal gas-solid contacting combustion processes.

Property	Packed Bed	Fluidized Bed	Fast Bed	Pneumatic Transport
Application in boilers	Stoker Fired	Bubbling Fluidized	Circulating Fluidized	Pulverized Coal fired
Mean particle diameter (mm)	<300	0.03-3	0. 05-0.5	0.02-0.08
Gas velocity through combustor zone (m/s)	1-3	0.5-3	3-12	15-30
Typical U/Ut	0.01	0.3	2	40
Gas motion	Up	Up	Up	Up
Gas mixing	Near plug flow	Complex Two phases	Dispersed Plug flow	Near plug flow
Solids motion	Static	Up & down	Mostly up, some down	Up
Solid-solids mixing	Negligible	Usually near Perfect	Near Perfect	Near Plug flow
Overall voidage	0.4-0.5	0.5-0.85	0.85-0.99	0.98-0.998
Temperature Gradient	Large	Very Small	Small	Maybe Significant
Typical bed-to-surface. Heat transfer coefficient (W/m2K)	50-150	200-550	100-250	50-100
Attrition	Little	Some	Some	Considerable
Agglomeration	Considerable	Some	No problem	No problem