AFBC boiler: 17 Answers You Should Know


afbc boiler flow diagram | afbc boiler diagram

afbc boiler
Schematic diagram of FBC boiler credit Wikipedia

advantages of cfbc boiler over afbc boiler | difference between afbc and cfbc boiler

AFBC means atmospheric – fluidized – bed – combustion. The speciality in this boiler is to keep furnace pressure at atmospheric conditions. The burnt gases developed in the combustion chamber are passes through the cyclone and are discharged into the atmosphere.

CFBC means circulating – fluidized – bed – combustion. In this kind of boiler, the furnace gas is pressurized to recirculate in the chamber. This recirculation of gases captures the unburnt carbon. Thus, the thermal efficiency of a boiler is increasing because of gas recirculation.

The comparison of the AFBC and CFBC boiler can be made based on the following parameters and criteria.

The velocity of the gases

In AFBC, It is around the 1.2 – 3.7 m/s

In CFBC, It is around 3.7 – 9 m/s

Heat Transfer Surface

In AFBC, The heat extraction can be done from bed only

In CFBC, The heat transfer was carried out bed and other surfaces of combustion chamber. It is known as the convective zone. The heat transfer from water walls is possible.

Fuel size

Coal is a widely used fuel in both boilers, but the size of coal particles in CFBC is 2-3 mm more.

Supply of air in combustion chamber

In AFBC boiler , The range  is  3 to 5 PSI gauge

In CFBC boiler, The range  1.5 to 2 PSI gauge

From above comparison we can say that the Performance characteristics of the CFBC boiler are better than the AFBC. It is an advanced technology of circulating gases. It is developed to solve some difficulties that arise in the AFBC. Some of them are discussed as below,

  • 1. The utilization space in the CFBC boiler is adequate than AFBC.
  • 2. The combustion of fuel is efficient more in the CFBC boiler.
  • 3. SO2 and NOx can be controlled more in the CFBC boiler.
  • 4. The control of temperature is more in the CFBC boiler.
  • 5. It can work effectively even with the low calorific value of the fuel.
  • 6. the turndown ratio is higher
  • 7. Both over & under feed systems can be used

afbc boiler working principle | afbc boiler start-up procedure

  • Following are the steps to be followed to start the afbc boiler
  • The air nozzles are cleaned by entering the full Fd air into the combustion furnace. Open the air compartment door for 10 to 15 minutes to complete this task
  • Enter the bed material into the combustion furnace. The bed height is to be around 250 to 300 mm above the nozzles.
  • The material below the nozzle is to be a static fix. It is also considered in the bed height
  • Enter the fluidizing air through the complete bed to uniformly distribute the bed materials over the bed. The PA damper is kept close during this operation
  • Open the startup compartment when you feel the uniformity in bed material and bed height.
  • Now increase the flow of air to develop tiny bubbles all over the bed material. This stage is called the bubbling stage. Read the airflow rate and note it down at this stage.
  • In the next step, Increase the furthermore the flow of air to make the bed turbulent. It is helpful for proper mixing of the upper and lower layer of the bed material. This is called the mixing stage. Read the airflow reading at this stage.
  • Switch off the fan and airflow rate. Now boiler is ready to start
  • The drum is level is to be maintained at around 40 %
  • Initiate firing
  • Keep initializing for few minutes, start All three fans as per sequence (ID, FD and PA)

afbc boiler design | accepted range of afbc boilers

The construction and working of AFBC with an explanation of various parts involved,

Main systems in the AFBC

  • Fuel supply system
  • Air Distribution
  • Heat transfer in Bed and surface
  • Ash Control system

Generally, these four main systems are included in every FBC boiler.

1. Fuel Supply System

There are two types of fuel supply systems in FBC boilers. Under-bed pneumatic supply and over bed supply.

The absorbent is used for fuel supply—examples: dolomite, limestone etc.

In under bed pneumatic supply, the coal is crushed and pulverized in 1 to 6 mm in size. This coal is supplied from the inlet hopper to the combustion chamber with the pneumatic arrangement. This system is developed according to capacity.

2. Air distribution system

The air distributor is the main component of any FBC boiler. It is utilized to pass or distribute the fluidized air from the furnace bed. The air distributor is keeping the solid particle motion at a constant rate. The air distributor is made from a metallic plate. The pattern geometry is made with the perforation in it. The nozzles are installed with perforation in it. This perforation is not allowing the solid particles to go back into space.

There are some arrangements made to protect the distributor from the temperature of the bed.

i) Refractory material Lining brick

ii) Fix layer of the materials in bed

iii Cooling tubes

3. heat transfer in bed and other surfaces

a)  heat transfer in Bed

The bed is made with particular kinds of materials like crushed limestone, refractory, ash and sand. The size of material particles is around 1 mm. There are two types of bed widely used beds in the FBC boilers.

(1) Shallow bed

(2) Deep bed

(1) Shallow bed

The power consumption of the fan is low in the shallow bed. In addition, the resistance of the bed is less in shallow beds, so the pressure drop is also lower.

(2) Deep bed

The power consumption of the fan is high in the deep bed. The resistance of the bed is more in a deep bed. The gas velocity is raising because of  pressure drop.

b)  Heat Transfer on surface

In the FBC boiler, the heat transfer should be sufficient within the bed material and the bundle of the tube or coils. The heat transfer is more superior in the horizontal orientation of the heat exchanger in shallow bed air distribution. There are few  parameters on which the heat transfer is depending,

Temperature of bed

Solid fuel particle size

Design and the layout of heat exchanger

Structure of the air distributor

Velocity of gas

4. Ash Handling System

a) Bottom Ash drain

There are two types of ash present in the FBC boiler. One is the fly ash, and the other is the bottom ash. Generally, the sediment ash is nearby 30 to 40 %. This ash is taken out when exceed limit to control bed height.

b) Removal of fly ash

The fly ash of the FBC boiler is more than other boilers. The combustion efficiency can be increased by utilizing the fly ash in some units. It is happening because the speed of the particle is very high. The fly ash travels with the flue gases, which is taken out at various stages from the FBC unit. There are three stages for the removal of the fly ash. (1) Convection part of FBC (2) Before air preheater or economizer (3) dust collector

There are many types of dust collectors available in the FBC boilers. (1) Cyclone (2) electrostatic precipitator (3) bagfiler (4) Combination of dust collectors

afbc boiler bed height calculation | afbc boiler bed height

The height of bed is calculated with the following equation in boiler,

level  of bed = Pressure in wind box – Differential pressure in the bed nozzles

DP test in afbc boiler | dp test procedure for afbc boiler

  • The first step is to precheck the bed with the following steps:
  • Make bed properly clean
  • Complete the maintenance of air nozzle and bed
  • The FD fan should work with higher efficiency
  • Steps of Procedure for DP test:
  • Initially start the ID fan, Start FD fan with minimum speed
  • Keep Air preheater inline
  • Raise the speed of airflow (Increase from 25% – 100%)
  • Read and note the pressure in the wind box at every stage
  • For all other compartments, repeat the same procedure
  • The value of pressure in the wind box needs to be nearby with designed values
  • Interpretation of DP test
  • The nozzle, bed plates are in good condition if the value of wind box pressure is nearby the designed value
  • The nozzles may be blocked if the value of wind box pressure is exceeded the design value
  • The nozzles may be damaged or defected hole if the value of wind box pressure is less than the design value

bed material size for afbc boiler |  afbc boiler bed coil

In the AFBC boiler, there are many grades of solid fuel (coal) available. The size of the coal particle is varied from 1 -10 mm. The size of coal particle is chosen based on the type of coal, quality of coal etc. The atmospheric air is used for two purposes, fluidizing air and air for combustion. First, this air is provided with sufficient pressure over the bed. Second, this is preheated by an air preheater in the boiler.

The velocity of this fluidizing air can be in the range of 1.2 – 3.7 m/s in the AFBC boiler. The flow rate of air passes through the bed can be utilized to determine fuel reaction. The bed temperature can be maintained by installing the bed evaporator tube to construct a limestone or sand bed. The bed evaporator tube helps reject the heat from the bed to maintain the temperature of the bed.

The bed is made up of depth  0.9 m to 1.5 m. the pressure drop across the bed is expected around 1 Inch per inch of depth of the bed.

The generated flue gases from the FBC combustion chamber is passed through the superheater section, economizer and air preheater. After air preheaters, the flue gases are exhausted from the atmosphere.

The AFBC boiler is famous for its temperature range. The temperature range of the AFBC boiler is 800 °C to 950 ° C. If the temperature exceeds this range, the boiler’s performance is decreased.

afbc boiler air nozzle

Two types of nozzles are widely used in the AFBC boiler.

Fluidizing nozzle:

This type of nozzle is made up of the S S or alloy steel. It is fitted on the bedplate. The hole size is about 2 – 5 mm. The air of the FD fan is entering from the wind box to the combustion furnace.

Coal feed Nozzle:

This nozzle is used to enter coal with air into the combustion furnace. The total number of nozzle is taken according to the size and capacity of the boiler. It may be 4 -6 nozzles. It is also fitted on the bedplate.

afbc boiler bed material density

The material density of the bed is around 1100 kg/m3

afbc boiler efficiency | afbc boiler efficiency increase

The combustion efficiency is depending on the following parameters :

(1) Reaction properties of fuel

(2) Volatility of fuel

(3) Size of the fuel particle

Coals like sub-bituminous or lignite are highly efficient in burning. There is no fly ash recycling or under a bed; feeding requires if the coal quality is good. The combustion efficiency of the AFBC boiler is in order of 70 to 99 %. The combustion efficiency is decreased. The efficiency of the AFBC boiler can be achieved by system improvement. The coal-like anthracite burns with low efficiency in the AFBC boiler. It can burn with higher efficiency in the CFBC boiler with the applications of fly ash recycling and the under bed feeding.

afbc standard boiler parameters

The following are the standard parameters which the of the AFBC

  • Size of the coal particles
  • Specification and size of the bed material
  • Air pressure from the FD fan
  • Height of the bed
  • Temperature of furnace
  • The temperature of the bed

afbc boiler velocity of flue gas | flue gas velocity in afbc boiler

The velocity of this fluidizing air can be in the range of 1.2 – 3.7 m/s in AFBC boiler.

afbc boiler interview questions and answers

What steps will you follow if the temperature of the bed increases?

The following are the points to be considered if the temperature of the bed is raising,

  • Load reduction
  • Maintain the density of the coal
  • Increase the material of the bed

What are the probable facts for the decrement in bed temperature?

  • The following are the probable facts for the decrement in the bed temperature,
  • The quality of material used in bed is poor
  • Suddenly action of the boiler load reduction
  • Excessive air entered the furnace
  • Fuel contains moisture

What is the reason behind the use of lime in the AFBC?

The coal contains some moisture, which has to be removed before combustion for better combustion efficiency. The purpose using lime is to absorb and remove the moisture from the fuel.

Which one is superior CFBC or AFBC ? Explain why?

We can conclude that the Performance of the CFBC is more superior to the AFBC. It is an advanced technology of circulating gases. It is developed to solve some difficulties that arise in the AFBC. Some of them are discussed as below,

1. The utilization space in the CFBC boiler is adequate than AFBC.

2. The combustion of fuel is efficient more in the CFBC boiler.

3. SO2 and NOx can be controlled more in the CFBC boiler.

4. The control of temperature is more in the CFBC boiler.

5. It can work effectively even with the low calorific value of the fuel.

6. the turndown ratio is higher

7. Both over & under feed systems can be used

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Deepakkumar Jani

I am Deepak Kumar Jani, Pursuing PhD in Mechanical- Renewable energy. I have five years of teaching and two-year research experience. My subject area of interest are thermal engineering, automobile engineering, Mechanical measurement, Engineering Drawing, Fluid mechanics etc. I have filed a patent on "Hybridization of green energy for power production". I have published 17 research papers and two books. I am glad to be part of Lambdageeks and would like to present some of my expertise in a simplistic way with the readers. Apart from academics and research, I like wandering in nature, capturing nature and creating awareness about nature among people. Let's connect through LinkedIn - https://www.linkedin.com/in/jani-deepak-b0558748/. Also refer my You-tube Channel regarding “Invitation from Nature”

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