EFFICIENT AND SAFE OPERATION OF POWERPLANT AND PROCESS BOILERS

PARAMETERS REQUIRED TO BE MAINTAINED STRICTLY  FOR EFFICIENT OPERATION OF AFBC BOILERS

—Bed Height/FD Air Pressure

—Primary Air Pressure

—Bed Temp./Furnace Temperature

—Fuel Size

—Bed Material size & Specifications

—Air & Fuel ratio

—BED HEIGHT/FD AIR PRESSURE

—Static  height for AFBC Boiler is to be maintained 300 to 325 mm during initial light up of boiler.

—Expand bed height for AFBC boiler is to be maintained 500 mmwc to 600 mmwc.

—FD Air discharge Pressure is to be maintained 600 to 650 mmwc.

—Wind box or Air Box pressure is to be maintained 500 to 600 mmwc.

NOTE: High W.B. pressure increases the erosion of bed coils & low W.B. stops fluidization.

PRIMARY AIR PRESSURE

—P.A. fan air header pressure for AFBC boiler is to be maintained 1000 to 1150 mmwc.

—Difference of primary air pressure & F.D. wind box pressure is to be maintained approximately 500 mmwc.

NOTE: High P.A. pressure increases the bed coil erosion and carryover to enhance secondary combustion.

BED TEMPERATURE/FURNACE TEMPERATURE

Depends upon the type of fuel & firing method.

For(Indian coal) bed temp. in AFBC/CFBC is to be maintained  between 850 to 900 Deg.C.

For Imported coal bed temp. in  AFBC/CFBC is to be maintained between 875 to 925 Deg.C.

For Lignite fuel bed temp. in AFBC/CFBC is to be maintained 800 to 850 Deg.C.

For Pet coke bed temp. in AFBC/CFBC is to be maintained 875 to 950 Deg.C.

BED TEMPERATURE/FURNACE TEMPERATURE CONT’D

For Rice husk and other biomass fuel bed. In AFBC/CFBC is to be maintained 850 to 900 Deg.C.

For Stoker firing furnace temp. is to maintained 1100 to 1200 Deg.C.

For Pulverized fuel firing furnace temp.is to be maintained 1100 to 1300 Deg.C.

For oil & gas firing furnace temperature is to be maintained 1100 to 1500 Deg.C.

FUEL SIZE

When firing with Indian/Imported coal following sizes are to strictly maintained as it will affect on performance of boiler.

—For AFBC/CFBC

 Size : 0 to 8 mm

Less than 1 mm not allowed more than 20%.

Distribution: 1 to 5 mm – 70%

                       5 to 8 mm -20%

                       0 to 1 mm – 10%

NOTE: Lower the size of coal higher the unburnt in fly ash, higher the size of coal higher the erosion of bed coils and blockage of fuel air pipe & clinkerization in boiler. 

—For stoker fired boiler

Size: 5 to 25 mm ,

Less than 5 mm not allowed more than 5%

Distribution: 5 mm to 15 mm-25%

                        15 mm to 25 mm-70%

                           0 mm to 5 mm  -5%

NOTE: Lower the size of coal higher unburnt in flyash & carryover through boiler.

—For Pulverized fired boiler

Pulverized coal size: 200 mesh powder

Size Distribution: 150 to 200- 60%

                                100 to 150- 30%

                                 75 to  100 – 10%

  

NOTE: Higher the size of pulverized coal below 100 mesh results unburnt in bottom ash.

—

BED MATERIAL SIZE & SPECIFICATION

—Bed Material Size: 0.85 mm to 2.36 mm

Distribution : 0.85 mm to 1.00 mm – 10%

                         1.00 mm to 1.50 mm -  50%

                          1.5 mm to 2.36  mm  - 40 %

—Bed Material Specifications:

Crushed fire bricks castables IS8 grade bricks or river silica sand

Fusion Temp.   1300 Deg.C     Shape: Spherical Angular

Bulk Density    1050 Kg/M3    Silica : 65%

Al2O3                28%                 Fe2O3: 1.05%

PbO2                  1.67 %             MnO  : Trace

MgO                   0.23 %             P2O5  : 0.08 %

V2O                     0.22 %            K2O    : 0.45 %

                      

                      AIR TO FUEL RATIO

—Theoretical Air for combustion:

Theoretical Air Required: 4.31(8/3C+8(H-O/8)+S) Kg/Kg of fuel burnt.

—To Understand the basics of efficient boiler Operation, the combustion process must be understood. Stable combustion condition requires the right amount of fuels and Oxygen, combustion products are heat energy,CO2,water vapour, N2,Sox,Nox and O2. In theory there is a specific amount of O2 needed to completely burn a given amount of fuel. In practice , burning conditions are never ideal, Therefore excess air must be supplied to burn the fuel completely depending upon the type of fuel.

TYPICAL EXCESS AIR TO ACHIEVE HIGHEST EFFICIENCY FOR DIFFERENT FUELS

— Captive Power plant(Coal) boilers normally run about 15 to 20 %.

—Fuel oil fired boilers may run as low as 5 to 10 %.

—Natural gas fired boilers may run as low 5 to 8%.

—Pulverized coal fired boilers may run about 10 to 15%.

 

—OXYGEN AT BOILER OUTLET

—To Ensure complete combustion of the fuel used, combustion chambers are supplied with excess air. Excess air increase the amount of oxygen and the probability of combustion of all fuels. When fuel and Oxygen in the air are in perfectly balance- the combustion is said to be stoichiometric . Combustion efficiency with increased excess air, until the heat loss in the excess air is larger than the heat provided by more efficient combustion. CO2% or  O2% in flue gas is an important indication of the combustion efficiency. 

BOILER EMERGENCIES

—Various Emergencies situations during Operation with a special emphasis on the safety aspect like boiler protection systems controls and interlocks.

—Drum level low and low-low.

—Drum level high and high-high.

—Furnace draught high and high-high.

—Bed Temp. high.

—Bed Temp low.

—Water wall/screen tube/Evaporator tube failure.

—Super heater  tube failure.

—High Super heater Temp.

—Low Super heater Temp.

—Flame failure.

—Furnace Explosion.

—Boiler pressure high.

—Coal feeder failure.

—PAH/SAH tube failure.

—Boiler feed pump failure.

—Fan Failure.

          DRUM LEVEL LOW AND LOW-LOW

—(A) CAUSES:

—Failure of BFP.

—Failure of drum level controllers.

—Excess opening of CBD/IBD.

—Sudden change in load(sudden red’n in load)

—Water tube failure

—(B) EFFECT:

—Boiler pressure parts may damage badly.

—(C) ACTIONS:

—Run the boiler if drum level is with in safe limit otherwise allow boiler to trip when the water level goes low-low limit to protect the boiler pressure parts.

DRUM LEVEL HIGH AND HIGH HIGH

(A) CAUSES:

— Failure of drum level controller.

—Sudden increase in load.

—Sudden increase in firing rate.

 

(B) EFFECT:

— Water may enter in to the turbine and serious damage of turbine blades and thrust pads.

—Carry over in Super heater abd sharp fall S.H. temp.

—Flange gasket may be failure.

(C) ACTIONS 

Run the boiler if drum level within safe limit and control the FCV.

—Open the CBD to maintain drum level in safe limit.

—Open the TG side main steam & Turbine drains to avoid the water entering into TG.

—Trip the TG when steam temp below safe limit.

FURNACE DRAUGHT HIGH AND HIGH HIGH

(A) CAUSE:

—Due to faulty operation of fan control.

—Disturbed combustion .

—Uncontrolled fuel entry.

(B) EFFECT:

—Boiler may damage due to high furnace pressure.

—Weak part of furnace(ducting & Enclosure)may explode high furnace pressue.

(C) ACTION: If it is due to faulty operation of ID/FD/PA/SA fan control,take it on manual mode and maintain furnace in suction.

—If furnace pressure has increased beyond limit allow boiler to trip on furnace draught high-high.

BED TEMP. HIGH

(A) CAUSE:

—High CV & low ash coal

—Low PA/FD/SA flow

—Sudden change in load

—Ash recirculation  system trouble.

—Faulty bed thermocouple

(B) EFFECT: 

Chances of clinker formation

—Chances of refractory failure.

—Chances of Screen tube failure.

(C) ACTION: 

Control bed temp.by recirculation of  ash.

—Increased PA & SA flow and reduce the load by cutting coal feeder.

—Coal feeder should be trip if bed temp increases beyond 975 deg.c

—If bed temp. exceeds further then allow boiler to trip to avoid clinker formation.

—Check the bed thermocouple.

BED TEMP. LOW

(A) CAUSE:

—High PA/FD/SA flow w.r.to load.

—Low CV & high Ash content coal used.

—Coal feeder trips or overfeeding of coal in to furnace.

—Faulty bed thermocouples.

—Water /screen/evaporater  tube leakage.

(B) EFFECT: 

Boiler steam flow reduce.

—Super heater temp. drops.

—Furnace draught fluctuates.

(C)ACTIONS: 

Boiler PA/FD/SA flow reduced if excessive.

—Check bed thermocouple.

—Stop bed material supply, if running.

—Check any  leakage sound from furnace.

WATER WALL/SCREEN TUBE/EVAPORATOR TUBE FAILURE(A) CAUSE:

—Starved water wall.

—Block tube ,erode tube, pitted tube, salt deposits.

(B) EFFECT:

—Hissing steam leakage noise from boilers.

—Unstable flame fluctuating draught.

—Bed temp. drops sharply.

—Increase ID fan loading.

—Flue gas outlet temp. decreased.

(ACTIONS):Take shut down the boiler when boiler tube leakage noticed and maintain the drum level.

      

           SUPER HEATER TUBE FAILURE

(A) CAUSE: Inadequate steam flow and high gas temp. during hot start-up.

—Erosion of tube due to high excess air.

—Blocked tube.

—Starvation of tube.

—Salt deposition due to high water level in drum.

(B) EFFECT: 

Hissing noise noticed.

—Flue gas temp drops & high FW consumption than steam..

—Overloading of ID fan.

(C) ACTIONS:

As soon as leakage noticed start reducing the load and trip the boiler.

—Try to locate leakage through manholes before the boiler depressurized.

—Boiler is to be forced cooled when S.H.  leakage noticed.

—

HIGH SUPER HEATER TEMP.

—(A) CAUSE: High Excess air.

—Low feed water temp or HP heater not in service at constant firing /load.

—Sudden increase in firing rate to increase steam pressure.

—Inadequate spray water.

—(B) EFFECT: 

+ve turbine expansion.

—Creep rate increase in tube metal ,turbine parts & steam piping.

(C) ACTIONS:

 Always keep HP heaters in line when optimum loading of TG.

—Slow down firing rate to limit the S.H. Temp.

—Reduce excess air if more.

—Check Spray control.

LOW SUPER HEATER TEMP

(A) CAUSE:

Soot deposit on super heater tube.

—Inadequate air flow.

—High spray.

—Sudden increase in load and pressure drops.

—High Drum level.

(B) EFFECT:

—Turbine expansion may be –ve.

—May induce thermal stresses in S.H.

(C) ACTIONS:

 Check air flow, increase, if necessary.

—Reduce spray, if more.

—Avoid sudden rise in load to boiler pressure drop.

—Check feed water temp.

—

FLAME FAILURE

(A) CAUSE: 

Dirty Oil/gas burner.

—Faulty flame sensor.

—Furnace pressure high.

—Low combustion Air.

(B) EFFECT:

—Boiler will trip on flame failure.

—Chances of furnace explosion if unburnt oil/gas/coal moisture entered in furnace.

—Steam pressure & temp. may fall.

—Variation in drum level.

(ACTIONS):

Purge the boiler putting burner back and purge burner as per cycle time( minimum 5 minutes).

—Check the flame sensor & clean the photocell if found dirty.

—Check the igniter circuit & H.V. transformer .

—Clean the burner tip & nozzles regularly.

—Ensure the healthiness of  explosion vent & door.

FURNACE EXPLOSIONS

(A) CAUSE: 

Accumulation of unburnt fuel during lit up /start up of boiler.

—Improper burning.

—Inadequate air.

—Secondary combustion.

(B) EFFECT: 

Furnace explosion can cause extensive damage.

(C) ACTION: 

Always purge the boiler with  min 40% full load air for about 5 minutes. No cut short in purging allowed.

—Adjust fuel air ratio.

BOILER PRESSURE HIGH

(A) CAUSE:

Sudden drop in load/steam flow.

—Uncontrolled fuel entry.

—Turbine/prime mover trips.

(B) EFFECT: 

Disturbance in drum water level.

—Safety valve may disturbed if pressure rise in frequent way.

—Boiler may trip at high high pressure.

—

(C) ACTIONS: 

Open start up vent to control the pressure.

—Control fuel ,air  input & drum level.

—If TG /prime mover has tripped first, allow boiler to trip but safety valve may lift.

—TG warm up vent put in auto, if pressure exceeds then it will open accordingly.

—Use Electromagnetic safety valve to limit the frequent operation of  spring loaded safety valve.

PAH/SAH TUBE FAILURE

(A) CAUSE:

 Erosion of Air heater tubes.

—Corrosion of  Air heater tubes.

(B) EFFECT:

 Flue gas temp. after APH will fall down.

—Increase in O2% in at Air heater  I/L.

—  Air heater completely in line during initial start up.

(C) ACTIONS:

—Control flue gas temp. bypasses PAH.

—Reduce coal feeding /air to maintain O2%.

—If leakages of tubes are more then stop the boiler and plug that tubes.

—

COAL FEEDER FAILURE

(A) CAUSE:

Electrical supply failed.

—VFD faulty.

—Bed temp.high.

—Furnace draught low.

—Drum level low.

(B) EFFECT:

 Boiler pressure  may  fall down.

—Steam temp. fall sharply.

—Bed temp.will decrease.

—Variation in furnace pressure.

—Variation in drum level.

(C)ACTION: 

Control boiler pressure by reducing the TG load & control S.H. steam temp. by closing the spray CV.

—Reduce PA/SA air to control bed temp.

—Control furnace draught & drum level.

—Check the electrical fault or emergency stop button.

—Check the VFD fault, if any.

—Restart the coal feeder after detecting the cause of failure.

               

                 BOILER FEED PUMP FAILURE

—(A) CAUSE:

—Motor protection relay operates.

—Lube oil temp. high.

—Discharge flow less.

—Motor bearing temp. high

—Deaerator level low.

—BFP Suction DP high.

(B) EFFECT: 

Stand by pump will start in Auto/manual.

(C)Actions: 

Start the stand by pump ,if it does not start on auto ,adjust the load to maintain the drum.

—Analyze and rectify the fault in the main feed pump & put it in auto.

—Check the BFP suction strainer & clean it ,if found chocked.

LOSS OF FANS

—(A)CAUSE: 

Electrical motor protection relay operated.

—Fan bearing temp. becomes high-high.

—Motor bearing temp. becomes very high.

—Drive fault.

—Boiler trip.

—ID fan trips.

—SA trip.

—(B) EFFECT:

 Boiler will trip on.

—Furnace draft either  low or high.

—(C)ACTIONS: Rectify electrical fault, if any.

—Check cause for boiler trip and normalize it.

—Check fan/motor bearing RTD.

—Restart the fan(ID/FD/SA) after checking the cause of tripping and taking corrective actions.

                

                  EXPLOSIVE POWER OF BOILER

—It will not be false to state that power librated by the explosion of Lancashire boiler 7.5’ dia x 30’ length, working at 7 Kg/CM^2 is sufficient to project it to a height of 3.29 KMS. Therefore hazards of boiler explosion may well be imagined.

—As a thumb rule, it could be stated that destruction hazards of 28.3 liters of water  at 4.23 Kg/CM2 and sat.temp in a steam boiler is equivalent to 0.45 Kg of gun powder.

—Introduction  to  furnace explosions In CFBC Boilers

—Many CFBC Boilers have suffered/reported furnace explosion in the past, Apart from causing severe losses to the business concerned , the occurrences have shaken the confidence of CPP professionals , however PF boilers are more prone to the such type explosions than CFBC boiler but the Operation philosophy of PF boiler is  clearly understood and established due to history of centuries.

—As regards of CFBC boilers, these are completely newer generation of technology and explosions avoidance measures are not clearly understood by the operating Engineers.

       TYPE OF EXPLOSIONS IN CFBC BOILERS

—Most of the explosions faced in CFBC boilers are dust explosions caused by small particles of coal in the bed and in the free board kept under suspension by fluidizing air fans.

—Explosions due to FO/HSD/LDO used in duct burners and /or load carrying burners has also been reported.

DUST EXPLOSIONS

—A dust explosions is the rapid combustion of a dust cloud . In a confined or nearly confined space, the explosions is characterized by relatively rapid development of pressure with flame propagation and the evolution of large quantities of heat(coal) and reaction products. The required oxygen for this combustion is mostly supplied by the combustion air.

—The condition necessary for a dust explosions is a simultaneous presence of a dust cloud of proper concentration in air that will support combustion and suitable ignition source.(Coal/HSD/LDO/FO).

—Minor flue gas explosions are called puffs or blow backs.

—FIRE TRIANGLE AND EXPLOSION PENTAGON

—Ignition Source(Coal/LDO/HSD/FO)

—Air or Oxygen

—Heat (Temp.)

—There are three necessary elements which must occur simultaneously to cause a fire: Fuel, heat and Oxygen. By removing any one of these elements , a fire

—On the other hand, for an explosions to occur, there are five.

—Air or Oxygen, Suspension necessary elements which must occur simultaneously : Fuel, heat, Oxygen, Suspension and confinement. These form the five sides of the explosions pentagon like fire triangle, removing any ignition source one of these requirements would prevent an explosions.

—Remembering the three sides of the fire triangle (Fuel, heat & Oxygen) and five sides of the explosion pentagon(Fuel,heat,Oxygen,Suspension & Confinement) is important in preventing fires and explosions at any facility. By eliminating the possibility of either suspension or confinement  , an explosion can not occur, but a fire may  occur. By eliminating the fuel, the heat ,or the Oxygen requirements , neither a fire nor an explosion can occur.

BASIC PHILOSPHY OF EXPLOSIONS PREVENTION

—Basic Principles of avoidance of explosions are:

—Fuel should never fed in to the furnace continuously for than 12 seconds when there is no fire and coal should be added in a small quantity  at ignition temp of coal.

—Furnace is completely purged of the explosive mixture and then fired.

—Fuel supply should be  stopped immediately if fire/flame is not established and repurging is surely done before restart.

—Correct air fuel ratio is to be maintained so that dust concentration with explosive limits is never achieved.

—Explosion doors/vents/bleed valve (IN AFBC) must be perfectly operational and all protections and interlocks and  fan drives sequence to be check in each shutdown as per OEM schedule/recommendations.