Steam Boiler: Fluidized Bed Boiler

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Circulating Fluidized Bed Combustion Boilers (CFBC)

In the circulatory system, the bed parameter is maintained to form solid fly above bed. Solids removed in a relatively dilute phase in the riser, and a down-comer with a cyclone for the solids. There is no steam generator pipes located in the bed. Steam generation and superheating takes place in the convection, water walls, at the output of the riser.

Circulating Fluidized Bed Combustion Boilers (CFBC) is generally more economical than Atmospheric Fluidized Bed Combustion (AFBC) boiler. For application in the industry requires more than 75-100 Ton / hour steam.

For large units, the higher the characteristics of Circulating Fluidized Bed Combustion Boilers (CFBC) furnace will provide a better use of space, a larger fuel particles, the residence time absorbing materials for efficient combustion and the capture of SOx is the greater, and the easier the application of techniques for controlling NOx combustion than Atmospheric Fluidized Bed Combustion (AFBC) steam generator.

Figure 1: Circulating Fluidized Bed Combustion Boiler (CFBC)
(Source: Steam its Generation & Use Babcock & Wilcocx 41th)

Pressurized Fluidized Bed Combustion (PFBC) Boiler

In type Pressurized Fluidized Bed Combustion (PFBC) boiler, a compressor supplying air Force Draft (FD) and the combustor is pressurized tank. The rate of heat release in the bed is proportional to the pressure in the bed so the bed is used for extracting large amounts of heat.

This condition will improve the combustion efficiency and sulfur dioxide absorption in the bed. Steam generated in the two tube bundles, one in the bed and one above it. Hot gas from flue gas drive gas turbine power generator.

System Pressurized Fluidized Bed Combustion (PFBC) Boiler can be used for cogeneration (steam and electricity) or power plant with combined cycle. Operating combined cycle (gas turbine & steam turbine) increase overall conversion efficiency 5% up to 8%.

Atmospheric Fluidized Bed Combustion (AFBC) Boiler

Most fluidized bed combustion boiler in operation for this type is Atmospheric Fluidized Bed Combustion (AFBC) boiler. This equipment is just shell of conventional boiler which is coupled with fluidized bed combustor and the system has been installed together with water wall tube boiler.

Coal is crushed into size 1 – 10 mm depend on the level of coal and type of feed air then distribute them into combustion chamber. Atmospheric air which acts as fluidization and combustion air is inserted by pressure after preheated by exhaust flue gas.

Tubes in the bed carrying water generally act as an evaporator. The flue gas is the product of combustion will pass through superheater and then flow into economizer. From economizer flue gas flow to air preheater then dust collector before being discharged into atmosphere.

Fluidized Bed Combustion Boiler

Fluidized Bed Combustion (FBC) has emerged as a viable alternative and has significant advantages compared to conventional combustion system and provides many advantages such as flexible fuel higher combustion efficiency and reduces emission of harmful pollutant such as SOx and NOx.

Fuels can be burned in fluidized bed combustion are coal, reject washing clothes, rice husks, bagasse and other agricultural wastes. Fluidized bed combustion boiler has wide capacity range between 0.5 ton/hour to more than 100 ton/hour. When air or gas is distributed evenly passed upward through a bed of solid particles such as sand which is supported on a fine mesh, the particles will be undisturbed at low velocity. As air velocity is gradually increased, it becomes a state where particles are suspended in the air stream-bed which is called as “fluidized”.

With the increase in air velocity, there is bubble formation, strong turbulence, rapid mixing and formation of the dense bed surface. Solid bed particles show properties of boiling liquid and looks like a fluid “bubbling fluidized bed”. If sand particles in fluidized sate are heated to the temperature of flame coal and coal is injected continuously into the bed, the coal will burn rapidly and the bed reaches uniform temperature.

Fluidized Bed Combustion (FBC) takes place at temperature about 840 C to 950 C. Because the temperature is much below the ash fusion temperature, the melting of ash and the problem related therein can be avoided. The lower combustion temperature is reached due to high heat transfer coefficient as a result of rapid mixing in the fluidized bed and effective extraction from bed through heat transfer between tubes and wall bed.

The gas velocity is reached between minimum fluidization velocity and particle entrainment velocity. This ensures stable operation of the bed and avoids particle entrainment in the flue gas.

Advantage Fluidized Bed Boiler

Fluidized bed boiler has more some advantageous than other type of boiler such as pulverized coal boiler. The followings below are the characteristic of fluidized bed boiler which has more attractions:

1. Manage emission of sulfur dioxide efficiently because the temperature of furnace can be maintained uniformly so the non combustible particle can be mixed perfectly. Therefore sulfur dioxide will compound with CaCo3 to be calcium sulfate (CaSO4) and then can be discharged into hopper or bin.

2. Less emission of NOx than other type of boiler because the combustion temperature is low (800 – 900 C)

3. Fluidized bed boiler does not have a flame.

4. Easier to operate fluidized bed boiler

5. Has more stability operation than pulverized coal boiler.

6. Eliminate corrosion because fluidized bed boiler generates soft ash which can melt in the furnace temperature relatively low.

7. Does need pulverizer so the preparation of fuel is simpler and less maintenance.

8. Need short time to start up the boiler

9. During start up, fluidized bed can be burnt without oil fuel and can be operated in low load. It is different with pulverized coal boiler.

10. he owner of power plant has more flexibility to choose fuel for combustion in the fluidized bed boiler with some quality of coal.

11. Because of low temperature of furnace, the alkali metal in the ash can not be evaporated, so the possibility of fouling is less because it can not be condensed.

Fluidized Bed Boiler

Fluidized bed boiler use non combustible materials such as sand or ash as medium for taking heat transfer process with fluidize fuel hydro dynamically. A bed is required to burn fuel and combustion air is supplied in high pressure. Fluidized bed boiler can be classified as two types; circulating fluidized bed boiler and bubbling fluidized bed boiler.

1. Circulating fluidized bed boiler
This type supplies higher amount of combustion air than bubbling fluidized bed boiler type through the grid to the furnace. Therefore the combustion produces a bed with lower density and longer dimension up to the top furnace. Loop seal is used to catch solid non combustible particle and recycle it into furnace again. This condition increase boiler efficiency and low emission.

2. Bubbling fluidized bed boiler
This type requires less amount of combustion air than type circulating fluidized bed boiler. The combustion air is supplied through the grid or can be also called as distributor and maintain sand (non combustible particle) at the low level of furnace. This condition is different with the circulating type. Bubbling fluidized bed boiler is usually applied to burn fuel with high volatile matter and low quality such as sludge, waste and wood.