Fluidized Bed Combustion (FBC): Types, Working Principle & Advantages and Disadvantages

In this article, I will share the types of fluidized bed combustion systems, their working principles and their advantages and disadvantages.

Introduction to Fluidized Bed Combustion (FBC)

The fluidized bed combustion system was introduced to solve the problems that couldn’t be solved by the pulverized coal firing, stokers and hand firing.

If we talk about using pulverized fuel burners, they can only burn certain types of coal.

You might like this: Pulverized Fuel Burner: Components, Types, Requirements and Working Principle

In most cases, the coal that the pulverized fuel burner can burn is limited from 70 to 100 microns.

So, this means if the pulverized coal is produced slightly lower or above the required sizes, then it won’t be useful to the furnace and this will result in the wastage of the fuel.

Another problem with pulverized fuel burners is that they can only burn certain types of coal and produce the required efficiency.

The type of coal that isn’t supposed to be used in the pulverized fuel burner, won’t burn with the same efficiency.

I talked deeply about the disadvantages of using pulverized coal in a power plant in the article linked below.

You might like this: Disadvantages Of Using Pulverized Coal

The best efficiency of the furnace can only be achieved by a particular type of coal that is best suited for the pulverized fuel burner.

So, this leaves the power plant with limited options when it comes to the selection of the type of coal that will be used in the power plant.

You might like this: Properties Of Coal Selected For Steam Power Plants

If the type of coal that is chosen to be used in the power plant runs out, the power generation process will have to stop.

Another problem that the fluidized bed combustion system is trying to solve is the extreme temperatures of the furnace.

Pulverized coal produces extremely high temperatures when it burns and a sufficient cooling system needs to be designed.

Water walls will have to be created around the furnace to ensure that the furnace is at safe operating temperatures.

Also, extreme temperatures in the furnace result in the formation of slag on the superheater; the evaporation of alkali metals in ash which will end up deposited on the surfaces of the fireside and heat transfer surfaces thus decreasing the efficiency of the entire furnace and the formation of SO₂and NO_xin large amounts.

The formation of sulphides and nitrates in large amounts contributes to pollution of the environment.

With the fluidized bed combustion system, any size of pulverized coal can be used, and any grade can be burnt completely without any issues even the coal that contains 70% ash.

Additionally, a fluidized bed combustion system comes with the capability of burning oil, gas and even municipal waste thus making it the best when different types of fuels need to be used to keep the power plant processes going.

Also with the use of a fluidized bed combustion system, the production of sulphides and nitrates is completely reduced thus protecting the environment.

And these are just some of the main reasons why fluidized bed combustion system is considered for use in many power plants.

With the decrease in finding the required fuel, the power plants now can resolve to use any type of fuel they will find available to lower the operation cost of the power plant and also keep the energy generation processes going.

Types Of Fluidized Bed Combustion Systems (FBC)

The fluidized bed combustion systems are of the following types;

Atmospheric fluidized bed combustion system is further divided into overfeed and underfeed systems just like the stokers.
Pressurized fluidized bed combustion

Pressurized fluidized bed combustion and Atmospheric fluidized bed combustion system

If you know something to do with the underfeed and overfeed stokers then you have an idea what the overfeed and underfeed systems are.

You might like this: Overfeed & Underfeed Stokers: Classification and Working Principles

The main difference between the atmospheric fluidized bed combustion system and the pressurized fluidized bed combustion system is that the atmospheric one uses atmospheric pressure while the pressurized one uses generated pressure provided by a machine e.g. pump.

With the pressurized fluidized bed combustion system, the fluidization and combustion are improved because of the higher pressures compared to the atmospheric fluidized bed combustion system.

The advantages of using pressurized fluidized bed combustion are as follows;

Due to the higher pressure generated higher burning rates are experienced.
It completely reduces the formation of sulphides (desulphurization) and emission of the nitrates.
The cost of the pressurized fluidized bed combustion system is a little bit cheaper.

Working Principle Of Fluidized Bed Combustion System

The fluidized bed combustion system consists of one big chamber with several openings on the sides that are used for feeding the fuel, water and air while other openings are for letting the ash, steam and flue gases out of the combustion chamber.

The fuel and dolomite or limestone are fed from the side opening that is closer to the bottom of the chamber, while the fresh air enters the chamber from the centre bottom.

Limestone or dolomite is added intentionally with the fuel to reduce the formation of SO_{2 by}about a level of 15% compared to the other firing methods used.

The dolomite reduces SO₂by absorbing it thus preventing it from escaping with the flue gasses out of the chamber.

Dolomite and limestone are inert materials (they don’t break down under extremely high temperatures) and that is what makes them amazing for the absorption of SO_2.

The air supplied enters at a high velocity thus leaving the fuel suspended in the air while it burns.

Inside the combustion chambers, tubes of water are spread throughout.

The feeding point of water into these tubes is located at the bottom side of the chamber.

This is to allow the water to have sufficient time to be heated by the burning fuel thus increasing the steam quantity and quality.

When the fuel is burning, the water in these tubes is heated converting into steam that leaves the fluidized bed combustion system through an opening on the side.

The flue gasses due to the combustion are emitted out through the top of the chamber while the ash is removed by the ash opening on the side that is located at the top.

The ash outlet is located at the top because the fuel is burnt while suspended and the supply of air at high velocity doesn’t give a chance for the ash to go down the chamber.

The molten slag as a result of the combustion of the fuel is tapped at the top of the bed since the high-velocity air supplied doesn’t allow it to flow down the chamber also.

The bed temperature can range from 800 to 900^oC and that is what makes it suitable for the addition of limestone or dolomite to lower the emission of SO₂.

The production of Nitrates is also significantly reduced because of the lower temperatures of the bed (the high temperatures are lowered by the continuous addition of water in the tubes) and the amount of excess air is reduced compared to the pulverized fuel firing methods.

Advantages Of Fluidized Bed Combustion Systems

1. It’s a cheaper method of power generation.

As I said earlier the introduction of a fluidized bed combustion system is that it doesn’t have a specific type of fuel that it can operate with.

With the fluidized bed combustion system, pulverized coal, gas, oil and even municipal waste can be used to generate heat energy that will be used to heat the water in the system.

It doesn’t matter whether the fuel is of a lower or higher grade; its operation efficiency is always going to be on top.

This type of system provides the means of a cheaper method of power generation in the power plants.

This is because the largest cost when it comes to power generation after the erection of the power plant, it’s the fuel.

Fuel will be needed in large quantities to be able to generate power.

If cheaper and lower grades of fuel can be used effectively in the power plant, the power generation tends to be even cheaper.

2. It lowers the emission of SO₂and NO_x.

The operating temperatures of the fluidized bed combustion system range from 800 to 900⁰C which allows the addition of dolomite and limestone in the combustion chamber which allows absorption of the SO₂.

Also, since the temperature of the combustion chamber is lower, the emission of Nitrates is reduced significantly.

These are some of the characteristics that make the fluidized bed combustion systems even better since they reduce atmospheric pollution.

Instead of the power plant coming up with ways to treat the flue gasses, which will end up adding to the cost of operation, the fluidized bed combustion system takes care of that.

You might like this: Methods Of Controlling Gaseous Pollutants (Ultimate Guide)

The only thing that will be needed to be done is the recovery of heat from the flue gases, which can easily be done by the economizer.

You might like this: Boiler Economizers: Function, Types, Design And Operation

3. It reduces the operating and capital cost of the power plant.

When it comes to the operating cost of the power plant, it will be lower because lower grades of fuel can be used.

Which I have already talked about in the first point of this section.

Another thing that you should know is that with the use of the fluidized bed combustion system, a boiler unit can be eliminated from the power plant.

Since the fluidized bed combustion system directly heats the water in the tubes and converts it into steam.

The operating costs that were to be allocated for the maintenance of the boiler unit will be saved.

There will be no calculation of the depreciation cost of the boiler at the end of every year.

The capital cost will also be reduced because of this issue I am talking about thus saving the power plant about 10% of the operating cost and 15% of the capital cost.

4. It uses any size of fuel.

With pulverized coal firing, you need a specific size of pulverized coal to be used in the furnace.

Any size that is not required be it smaller or bigger, will only result in the waste of the fuel.

And that is why it’s very essential that the pulverized mill selected can operate at the highest level of accuracy.

You might like this: Requirement That Pulverized Mill Should Satisfy

With the fluidized bed combustion systems, any size of pulverized coal can be used without any issues and bring about a higher efficiency that is expected.

The particle size of the pulverized coal preferred in the fluidized bed combustion system is about 6mm to 13mm but even 50mm can be used, you see the wide range of sizes that can be used here.

But with the pulverized coal firing system, 70 to 100 microns is the range of size of pulverized coal that can be used in this system.

If you look at the range of sizes, the fluidized bed combustion system allows an additional 7 units in size ranges of fuel that can be used in the chamber.

Lastly…

We can see that the fluidized bed combustion systems are amazing because of the various advantages they offer compared to the other conventional types of firing methods.

The FBCs are preferred in areas where it’s hard to locate quality fuel needed to be used in the power plant.

It saves the power plant lots of money and increases the operating efficiency since even lower grades of fuel are used.

On the other hand, atmospheric pollution is largely reduced because of the lower emission of SO₂and Nitrates; this is a positive thing to see in any power plant.

Thanks for reading; hopefully you have understood how I have explained the entire FBC system. If you have any additional information just don’t be afraid to reach out to me so that I can add it up here.

See you in my next article.