25-Questions & Answers on AFBC Boilers


1-What do you mean by AFBC Boiler?

Atmospheric Fluidized Bed Combustion (AFBC) boilers are those, which have the potential to use alternative fuel sources such as coal, wood, or waste or any other low grade fuel, and are able to reduce and control nitrogen oxide (NOx) and sulphur dioxide (SO2) emissions.

2-List down the advantages of FBC boilers over other boilers?

  •          FBC boilers have higher combustion efficiency.
  •          Any low grade fuel can be burnt in FBC Boilers
  •          Boiler size is compact as compared to travelling grate & pulverized boilers.
  •          Different variety of fuels can be burnt
  •          Very high ash content fuel can be burnt
  •          Over & under feed fuel feeding system availability
  •          Higher turn down ratio

3-Briefly explain the process of fluidisation?

When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized.” With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid “bubbling fluidized bed” desirable for good heat transfer and intimate contact. If sand particles in a fluidized state is heated to the ignition temperatures of coal, and coal is injected continuously into the bed, the coal will burn rapidly and bed attains a uniform temperature. The fluidized bed combustion (FBC) takes place at about 840 °C to 950 °C.

4-Explain the different types of Fluidization in FBC boilers

Boiler Gauge Glass Line Up Procedure

Fluidization: At low velocity air flows through the porosity of bed. Once the velocity of air increases, the air starts moving in a turbulent state & causes particles of bed to attain turbulence along with the air. Here bed materials move like fluid, so called Fluidization.

Different types of fluidization are:

When gas flow introduced through the bottom of the bed of solid particles, it moves upward through the bed via the empty spaces between the particles.

During low air velocity bed remains under stable fixed state, this condition is called Pseudo Fluidization

Minimum Fluidization:

At higher air velocity, the aerodynamic drag forces begins to counteract the gravitational forces causing the bed to expand in volume as the particles move away from each other. Further increasing the air velocity, it reaches the critical value at which upward drag forces exactly equal to the gravitational forces causing particles to remain in suspended within the fluid, this is called minimum fluidization.

Maximum fluidization or bubbling fluidization:

Further increasing the air velocity, the bulk density of the bed continues to decrease and its fluidization becomes more violent, until the particles no longer form a bed and are conveyed upwards by the gas flow. This is called bubbling fluidization.

Questions & Answers on Boiler Troubleshooting

5-What the bed plate is consisting of?

Bed plate is the partition plate between wind box & furnace. It consists of Bed nozzle, coal feed nozzle & ash drain pipes. Generally bed plates are made up of stainless steel materials.

6-What are the different types of nozzles used in AFBC Boilers

Fluidizing nozzles:

These are the stainless steel or alloy steel nozzles fitted on bed plate. It has 2 to 5mm holes around it, through which FD air enters rom wind box to furnace. It is manufactured by machining a solid SS/alloy steel bars.

Coal feeding Nozzles:

Coal with primary air enters into the furnace through these nozzles.4 to 6 nozzles or depending on Boiler capacity are fitted in each compartments. Coal feeding nozzles are fitted in bed plate to distribute coal uniformly

7-What is the function of Ash drain pipes?

Generally Ash drain pipes are fitted in bed plates & extended outside. These are terminated to bed ash cooler where ash is getting cooled by air before discharges through ash conveying system.

How to convert air /gas flow from M3/hr to Nm3/hr or Sm3/hr??

What do you mean by IBE & IBSH in FBC Boilers?

IBE: In Bed Evaporating Coils

In some FBC Boilers, complete furnace is covered with evaporator coils. Coils are generally fitted in 2 nos inner & outer. To prevent erosion due to fluidizing materials, coils are covered with refractory materials.

IBSH: In bed Super heater coils

One SH coil is placed inside the bed, it is just placed right angle of IB coil & generally installed from last compartment & extended up to second compartment. To get rated main steam temperature at lower loads, sometime these are installed in 1st compartment also.

Why & How these in Boilers??

8-What do you mean by DP test in AFBC Boilers?

It is the test carried out before the light up of Boiler to ensure the healthiness of air nozzle & bed plate.

9-Write down the potential reasons to carryout DP test

To check the condition of air nozzles that is to identify the nozzles are clean, choke free & correct hole size

  • To identify the leakages in bed plate
  • To check the maximum FD air flow for each compartment
  • To identify air leakages in APH
  • To check problems associated with each compartment

10-Briefly explain the procedure of DP test

DP test is done on clean bed


  • Ensure bed is clean
  • Ensure all maintenance works of air nozzles, bed etc are completed
  • Ensure FD fans are healthy & can be run at its full flow capacity (Air flow is allowed in each compartment from 25% to 100% of its capacity)


  • Start ID fan & then FD fan at minimum RPM initially
  • Take APH in line
  • Increase air flow gradually from 25% to 100%
  • Note down wind box pressure at each stage
  • Repeat this procedure for remaining all compartments
  • The wind box pressure values should match commissioning or design values
  • Now take all the compartments together & increase FD air flow up to 100% & again check wind box pressure & FD fan discharge pressure


  • If the DP test results (wind box pressure) are 10 mmwc more or less than commissioning values, nozzles& bed plates are said to be in good condition
  • If result data shows higher value, then nozzles are blocked
  • If the results show less value, then nozzles or damaged or holes are enlarged
  • Higher pressure drop in APH indicates tubes leakage

11-What is the fluidization air velocity in AFBC Boilers?

It is around to 1.5 to 3.7 m/sec

12-What do you mean by Elutriation in AFBC Boilers?

Process of separation & escaping of fine bed materials with the air during fluidsation is called Elutriation.

13-What do you mean bed level?

It is the height of the bed above the bed plate

Power plant maintenance Calculations

14-How do you calculate the Bed level?

Bed level = wind box pressure-DP across the bed nozzles

What is the function of Over Fire Air (OFA) in AFBC Boilers?

OFA is given at height around 5 to 8 meter above the bed to achieve complete combustion

15-What is the standard size of Bed materials used in AFBC Boilers?

Bed material is Crushed refractory of size around 0.85 to 2.36 mm & having fusion temperature > 1300 Deg Cdf

16-What is the density of refractory material?

It is around 1100 kg/m3

17-What are the standard compositions of Bed materials?

Sl No.

Chemicals Compositions






Al2O3 37-40 , Min 30% required

37-40 , Min 30% required









Small traces











Na2O + K2O

< 3.0

 Questions & Answers on Spent wash fired Boilers

18-What are the startup fuels used for AFBC boiler light up?Write down the procedure for AFBC boiler start up

Charcoal & Light Diesel Oil (LDO) are used as start fuels for AFBC boiler light up

Light up & Start up procedures

  • Before filling bed material into the furnace FD air nozzles must be cleaned by admitting maximum FD air into the furnace by opening FD air compartmental damper (PA air damper remaining closed) for 10–15 minutes.
  • Fill bed material into the furnace so that bed height should be about 250–300 mm above air nozzles or 280–300 above the distribution plate.
  • The bed material below the air nozzles remains static all the time and should be counted as bed height.
  • Spread the bed material uniformly by admitting sufficient fluidized air through the bed and bed height can now be measured physically after putting off air. During fluidizing PA dampers should remain closed.
  • After uniform spreading of bed material and maintaining required bed height start the startup compartment by varying the air flow gradually so that bed material begins to fluidize.
  • Slowly increase the air flow so that small bubble formation takes place over the bed material and incoming air escape the bed as small bubbles. Note down the air flow at the stage known as bubbling stage.
  • Again increase the air flow so that the bed become turbulent and complete mixing of top and bottom layer of bed material take place. Note down the air flow at this stage. This is the amount of air required fir mixing during start up.
  • Now stop the fans and boiler can be taken for startup.
  • Maintain the drum level about 40%.
  • Drum vent, super heater vent, startup vent and main steam line drain should be kept open.
  • Desired quantity of dry charcoal is to be spreaded uniformly over the bed in the start up compartment. Generally 1 to 3% of bed material
  • Desired quality of Diesel mixed charcoal is spreaded over the dry charcoal. Generally Diesel must be 10-15% of total charcoal. Out of total charcoal 70-75% should be dry & spreaded at the bottom & 25-30% should be wet & spreaded over top of dry charcoal
  • Initiate the fire using diesel soaked cotton. Ensure fire is spreaded uniformly
  • After some times start ID, FD & PA fans as per sequence and open their discharge dampers then suction damper. Keep at minimum RPM if there is VFD if not then control the flow by discharge & suction dampers.
  • Maintain PA header pressure up to 500 MMWC initially
  • Now check physically and ensure when the flame is in bluish condition mixing should be done with mixing air flow for 10–15 second
  • Now bed temperature will start rising and after getting bed temperature up to 450 Deg C start the coal feeding by increasing PA header pressure up to 900-1000 mmwc
  • Now slowly increase fuel feeding & FD air
  • Close drum air vent at 2.5 kg/cm2.
  • Close super heater drain at 20 kg/cm2 pressure and open start up vent control valve.
  • Open MSSV (main steam stop valve) after the prior and proper charging through equalizer valve (MS bypass valve).
  • In order to increase the load on Boilers take 2nd, 3rd etc compartments as per requirement

19-What is the size & GCV of Charcoal used for light up

Charcoal size is around 15 to 25mm & GCV is around d 6500 to 7000 Kcal/kg

20-What are the important parameters in AFBC boilers?

  • Fuel size
  • Bed material size and specification
  • Bed height and FD air pressure
  • Bed temperature and furnace temperature

21-What action you will take if bed temperature increases?

  • Increase bed material
  • Reduce load
  • Control the bulk density of fuel

22-What are the reasons for drop in bed temperature?

Poor quality of bed material

Sudden reduction of boiler load

Moisture in fuel

More excess air

23-Why do you add Lime in AFBC boilers?

Lime is added to absorb the moisture from coal

24-Explain the process of sulphur dioxide absorption by lime

Lime stone (CaCo3) on heating gets converted to slaked lime

CaCo3 + Heat = Cao + Co2

Sulphur on heating gets converted into Sulphur di-oxide

S + O2 = SO2

Slaked lime reacts with Sulphur dioxide & converts into Calcium sulphur

Cao + SO2 + ½ O2 = CaSO4

25-How do you decide the quantity of sulphur required for desulphurization of coal?

Around 3.3 kg of lime is required for 1 kg of sulphur in coal or 2 kg of sulphur dioxide

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