Fluidized Bed Combustion – Parts, Working, Advantages and Disadvantages

Fluidized bed combustion is the process of combustion of solid fuels. In this process, the solid fuel is fluidized for better combustion.

Fluidization is the process in which a granular material is converted from a static solid state to a dynamic fluid like state.
In this process, the solid fuel i.e coal is fluidized using pressurized air by putting the solid fuel in a bed that has some gaps for the air to pass.

In this combustion system, low-grade fuel can be used. Low-grade fuel which is difficult to burn otherwise can be easily burned in the fluidized bed combustion system.

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Parts of Fluidized Bed Combustion:

1) Bed:
The coal, limestone, ash, and sand are mixed on this bed. After mixing, the mixture formed is burnt on this bed.

2) Combustion Chamber:
The fuel which is coal and the mixture of sand, ash and limestone is burnt inside this combustion chamber.

3) Water tubes:
It is present above the bed. The water tubes get heat from the burning fuel.

4) Air Preheater:
The preheater is used to preheat the air before entering the combustion chamber. It preheats the air using flue gases.

5) Blower:
The blower is used to blow high-pressure air to the fluidized bed combustion system and this air is used to fluidize the mixture of coal, limestone, sand, and ash.

6) Cyclone Separator:
A cyclone separator is used to separate the mixture of ash and the flue gases using cyclone waves generated in it.

7) Flying Ash Recycling Chamber:
This recycling chamber is used to recycle the ash produced during the combustion and then delivering it to the bed where it mixes with coal, limestone, and sand.

List Of granular components present on the bed:

1) Coal:
Coal is the fuel in the fluidized bed combustion and it will be burnt in the combustion chamber.

2) Limestone:
Limestone is used to remove the sulphate from the fuel during combustion. It removes the sulphate by the process of precipitation.
By removing sulphate from fuel during combustion, it reduces the emission of harmful gases into the atmosphere.

3) Sand:
Sand is used for optimizing the fluidization process. It is mixed with coal, ash, and limestone for a better fluidization process.

4) Ash:
At the starting of the combustion cycle, ash is not present on the bed. After some time, the ash is supplied to the bed from the flying ash recycling chamber.

Working Of Fluidized Bed Combustion:

The water flowing in the water tubes gets heat from the burning fuel and gets converted to steam while flowing.
This steam generated is sent to the area where it is required. The steam may be used to rotate the turbine or to do any other thing.

At first, the blower is started and high-pressure air starts flowing towards the bed after passing through the air preheater.
High pressurized air will flow through the yellow region. This air flowing in the yellow region is preheated slightly.
It is preheated because it flows through the chamber which is connected to the preheater.

When high pressurized air is transferred to the mixture of coal, limestone, ash, and sand, then this mixture will act as a fluid.

When the air pressure is applied to the solid fuel particles from the bottom, there is no change in fuel particles. But as the air pressure is increased gradually then after increasing the air pressure by a certain amount a stage is reached when the fuel particles get suspended in the air.
After that, if the air pressure is increased above that level then the bed becomes highly turbulent and mixing of particles occurs which appears like the formation of a bubble in a boiling liquid, and this point the mixture of coal, limestone, sand, and ash is said to be fluidized.
The point at which granular particles start acting as fluid is called the critical point

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In this state when the fluidized mixture of granular particles with air is burnt, then it is called the fluidized bed combustion.

When this mixture will be burnt, then some ash particles will flow up in the air as these particles will become very lightweight due to a decrease in density. Also, the hot flue gases will flow up along with the ash.
When these ash particles and flue gases will flow in an upward direction, they both will come in contact with the water tubes and will transfer heat to the water tubes. After supplying heat to the water tubes both ash and flue gases will enter the cyclone separator.
In the Cyclone separator, the mixture of flue gases and ash is separated. In this conical cyclone separator, cyclone waves are generated and when the mixture of ash and hot gases are tangentially entered in these cyclone waves, the ash will move towards the center and will move towards the flying ash recycling chamber and the remaining hot gases will move towards the air preheater and will help to preheat the air coming from the blower.
After supplying heat to the preheater, the flue gases are released into the atmosphere.
After recycling the ash in the flying ash recycling chamber, the recycled ash will be supplied to the bed again.
The flue gases generated after the combustion of fuel will come in contact with the water tubes and will help in converting water to steam and then after passing through the water tubes the flue gases will be directed toward the air preheater and will be used to preheat the air coming from the blower.
This preheated air will help in the combustion of the mixture of coal, limestone, ash, and sand.

Advantages Of Fluidized Bed Combustion:

1) This process has high thermal efficiency.
2) It has short commissioning and erection period.
3) No need for coal crushing as pulverized coal is not necessary here.
4) It reduces the emission of harmful gases to the atmosphere by removing the sulphate from fuel by precipitating using limestone.
5) Ash can be easily removed and can be sent to made cement.
6) In this combustion system, municipal waste, sewage plant sludge, biomass, agricultural waste, and other high moisture fuels can be easily used as fuel in this process.
7) Boiler Size can be reduced using this process.

Disadvantages Of Fluidized Bed Combustion:

1) In this combustion system, the fan speed has to be maintained at a high value. It has to be maintained at a high value because there is a continuous requirement of high-pressure air for supporting the bed.
2) Due to the requirement of continuous high-pressure air, the cost of the auxiliary units of the plant increases.

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