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.
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
Pre-checks
- 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)
Procedure:
- 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
Interpretation
- 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 |
Limits |
|
1 |
SiO2 |
55-60 |
|
2 |
Al2O3 37-40 , Min 30% required |
37-40
, Min 30% required |
|
3 |
FeO |
<2.0 |
|
4 |
TiO2 |
1.67 |
|
5 |
MnO |
Small
traces |
|
6 |
CaO |
0.54 |
|
7 |
MgO |
0.23 |
|
8 |
P2O5 |
0.08 |
|
9 |
Na2O + K2O |
<
3.0 |
18-What
are the startup fuels used for AFBC boiler light 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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