AFBC ( Atmospheric Fluidized Bed Combustion ) BOILER COLD START UP PROCEDURE:-

AFBC boiler is a very developing technology now these days this technology is widely used in the industrial boiler for the purpose of efficient and clean burning of coal and other fuel for steam generation. afbc boiler  is suitable for combustion of low quality fuel . In afbc boiler fluidization bed combustion is method of burning of solids fuel in which the fuel is continuous fed into hot fluidized bed of inert bed material. Inert fluidized bed is heated to the ignition temperature of fuel and fuel is supplied continuously into the bed. The fuel burn rapidly and the bed attain a uniform temperature. In afbc boiler combustion is take place at about 850- 950 degree Celsius. In this article we discussed about the afbc boiler cold start up and what the precautions are take before start up the afbc boiler light up ? What are the things should be inspect during cold start up of afbc boiler ? And also know about how to Preparing for start up of AFBC boiler ? and how to do Cold startup procedure of AFBC boiler  ? and compartment transfer in afbc boiler?

PRE STARTUP INSPECTION OF AFBC BOILER:-

1.All main hole doors are closed
2. all drives id , fd , pa fans , drag chain , pocket feeders are proper running condition.
3. All pneumatic damper id ,fd ,pa suction/ discharged damper and FD,PA compartmental damper are in proper working condition.
4. All interlock & protection are in line.
5. Check availability of coal of specifications size ( 4mm to 6mm) & c.v : 6000-6600 kcal/kg
6. Check stock availability of bed material.
7. Check availability of bed material of below specification :
Size :0.85 mm to 2.36 mm
Partical density : 2000 kg/cu.m.
Bulk density: 1000 kg/cu.m
chemical composition:
Alumina: 35% to 45 %
Silica 65% to 55%
Fusion temperature : 1400 deg. C
8.Check availability of kerosene Approx. 25 liters required per start up.
9. Check availability of dm water and boiler feed pump running.

Preparing for start up of AFBC boiler:-

1. Before filling bed material into the furnace fd nozzles must be cleaned by admitting maximum fd air into the furnace by opening fd compartmental damper ( pa damper remaining closed ) for 10 -15 minutes.
2. Fill bed material into the furnace so that bed height should be about 220-240 mm above air nozzles or 280-300above db plate. The bed material below the air nozzles remains static all the time and should be counted as bed height.
3. Uniformly spread the bed material 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 remains closed.
4. After uniform spreading of bed material and maintaining required bed height starting the start up compartment by varying the air flow gradually so that bed material begins to fluidize.
5. Slowly increase the air flow  so that small bubble formation take 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.
6. 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.
7. Now stopped the fans and boiler can be taken for start up.
8. Maintain the drum level about 40%
9. Drum vent, super heater vent , startup vent and main steam line drain should be kept open.

Cold startup procedure of AFBC boiler  :-

1. Desired quantity of dry charcoal is spreading uniformly over the bed in the start up compartment.
2. Desired quality of kerosene mixed charcoal is spreaded over the dry charcoal.
3. Initiate the fire using kerosene soaked cotton.
4. After spreading of fire over the total bed area start ID, PA,FD fans as per sequenceand  open their discharge dampers then suction damper ( assuming vfd control) and open their discharged damper then suction dampers.
5. Open compartmental PA and FD damper and maintaining furnace draft at 0 to -1 mmwc.
6. Now check physically and ensure when the flame is in bluish condition mixing should be done with mixing air flow for 20 – 30 second maintaining furnace draft at 0 to – 1 mmwc.
7. After giving the mixing air flow should be reduced to bed minimum fluidization condition.
8. Now bed temperature will start rising and after getting bed temperature more than fuel ignition temperature, ( at about 550 * C for Indian coal ) the fuel feeding can be start with minimum feed rate .
9. Before starting of feeding ensure PA lines are clear and PA header pressure is maintained at 800 mmWC (slightly more than wind box pressure)
10. Keep Wind box pressure at about 280 to 300 mmWC.
11. Regulate fuel feeding rate depending upon the temperature of the bed so that the temperature become stable at 850 to 900 degree Celsius.
12. Close drum air vent at 2.5 kg CM square pressure.
13. Close super heater drain at 18 kg/cm2 pressure.
14. Open MSSV (main steam stop valve) after the prior & proper charging through equalizer valve.
15.  Closed startup vent after MSSV fully open.

Precautions during start up of AFBC boiler :-

1. If mixing is done at higher airflow the burning charcoal and bed material will be spill into the adjacent compartment and bed height will reduce in the startup compartment.
2. If mixing is done at lower floor airflow the burning charcoal and bed material will not mix properly the charcoal will burn top layer of bed material a leading to clinker formation.
3. If fuel feeding is started to earlier before attending fuel ignition temperature income fuel will not be able to burn and accumulate in the furnace and may lead to clinker formation.
4. Lower bed height and less air flow can lead to higher temperatures and bed material can be charged to limit the rising temperature.
5. In any case bad temperature should not be alone beyond 950 degree Celsius So charcoal mixing , the time of feeding and air flow should be done properly.
6.  During startup Drum level will be fluctuate due to thermal action in water and should be maintained accordingly.

COMPARTMENT TRANSFER IN AFBC BOILER.

1. After stabilizing the first compartment open that PA line damper of second compartment and ensure the lines are clear.
2. Open the FD line damper of second compartment and increasing the air up to fluidization level. Compartmental mixing can be done at 55 to 60 mmWC. Airflow and higher according to requirement as cold bed would require more to fluidized.
3. At the time of mixing the bed temperature of the running compartment will go down and that of mixing compartment of will rise. at the time of mixing the bed temperature in the running compartment should not be allowed to go below fuel ignition temperature and mixing should be stopped at accordingly and air flow to be normalized .
5. The mixing compartment cannot be taken in line by studying the feeding with sufficient airflow air flow can be maintained at 45 to 55 mm WC and wind box pressure at 370 to 380 mm WC
6. Adjust the full feeding such that the bedtime pictures in both compartments become stable at 850 to 900 degree Celsius.

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joginder chauhan

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