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COMBUSTION IN A CFBC BOILER

TAPASH NAG

CFBC CHARACTERISTICS

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Operates under special fluid dynamic conditionSolid particles are mixed through furnace at a velocity

exceeding the average terminal velocity of the particlesMajor fractions of solids are captured by cycloneHigh recycle rate intensifies solid mixing and evens out

combustion temperatureHigh turbulence, solid mixing and absence of a defined

bed levelSolids are distributed through out the furnace with

steadily decreasing density from bottom to top of furnace

COMBUSTION FACTORS

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COMBUSTION

PA SA

COAL QUALITY

COAL SIZE

BED HEIGHT

BED TEMP

ASH RECFURNACE VOLUME

Conveys coalUniform distribution of coal in

furnaceProvides air for combustionFluidizes the bed ashEnsures proper mixingProvides sealing air to feeders

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PRIMARY AIR

Staged CombustionMaintains bed temperatureBurner airProvides excess air requirementControls overboard temperatureEliminates NOx formation

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SECONDARY AIR

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The primary air shall be adjusted in such a manner that it provides minimum fluidization and better combustion

The secondary air shall be adjusted in such a manner that it has maximum penetration so that there is adequate reaction of oxygen with fuel.

High pressure for bottom SA is required, to guarantee high penetration and better mixing of air with bed material

If PA quantity is high and SA quantity is not adequate, the fuel combustion in dense phase of furnace, i.e., the lower portion will decrease but increase in the lean phase or the upper portion resulting in more water wall erosion.

Secondary combustion shall occur in boiler cyclone and seal pot due to less SA quantity.

The temperature of flue gas at cyclone outlet will rise and there will be wide variation between cyclone inlet and outlet temperatures The heat transfer in back pass will increase due to high flue gas temperature. Attemperation will increase and heating surface will be damaged

AIR DISTRIBUTION CRITERIA

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AIR ADJUSTMENT TABLE

SRLOAD(MW)

SA DAMPER POSITION (%) SA

HEADER PRESSURE

(kPa)

PA QUANTITY

BED HEIGHT

(kPa)

BED TEMP( )℃

OXYGEN(%)

TOP BOTTOM

1 ≤50 45 25 ≥6.0≥Critical

fluidizing air10.0 Around

7004.0-5.0

2 50-80 40-50 30-40 6.0-7.5≥Critical

fluidizing air11.0

700-850 3.5-4.2

3 80-100 60-70 40-50 7.5-8.0Adjust bed

temperature ≥11.5

800-850 3.5-4.0

4 100-120 70-80 50-60 8.0-9.0Adjust bed

temperature≥12.0 800-900 ~ 3.5

5 120-135 80-90 60-70 ≥9.5Adjust bed

temperature≥12.5 800-910 3.0-3.5

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SECONDARY AIR

POOR

OXYGEN

CORE

OXYGEN PROFILE IN CFBC FURNACE

1. Poor oxygen core formed due to the combustion in reducing atmosphere due to insufficient mixing of air

2. Results in bad combustion efficiency when burning coals with low volatile content

3. Staged air supply to be provided for better combustion

4. Proper PA to SA ratio should be maintained

5. Proper air velocities should be maintained

6. Angle and size of SA duct should be such that the penetration length of air jet is deep in to the furnace

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OPERATION PRACTICES

FUEL VOLATILE CONTENT(%) RATIO OF PA TO TOTAL AIR

ANTHRACITE <20 ~65

BITUMINOUS 20~40 ~60

LIGNITE >40 ~50

RATIO OF PRIMARY AIR

VELOCITY OF AIR (m/s)

PA(Based on throat area of Nozzle) SA

~40 55~85

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Coefficient of Excess air is a function of the fuelReasonable air supply ensures better temperature

distribution in the furnace and better combustion efficiency

Ensures lower NOX generation

EXCESS AIR REQUIREMENTS

EXCESS AIR AT FURNACE OUTLETFUEL VOLATILE CONTENT(%) COEFFICIENT OF EXCESS AIR

ANTHRACITE <20 1.23~1.25

BITUMINOUS 20~40 1.22~1.24

LIGNITE >40 1.20~1.22

GCVMoistureAsh ContentVolatile materialCarbon content

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COAL QUALITY

FinesOversizeDistributionVolatile materialCarbon content

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COAL SIZE

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Hard Coal with less volatile don’t burn effectively in CFBC boiler

Reactivity of carbon decreases while in circulation for a long time

Coals having low fusion temperature can be burnt as temperature in dense bed and free board is around 850-900oC

Fuel grain size distribution depends upon the material balance, the rate of combustion in the dense phase and free board and combustion efficiency

The biggest grain size will be fluidized in dense phase and will start burning

The finer grains shall be entrained in the flue gas and burn in free board zone

The finest grains up to 90 microns shall be returned by the separator

The coal of size higher than 6 mm may be permitted, if the coal has fragmentation characteristics during heating

FEW FACTS

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COAL INDEX

0 5 10 15 20 25 30 350

5

10

15

20

25

30

I (MJ/kg)

%C

IN A

SH L

EAVI

NG

FU

RNAC

E

COAL INDEX

I = Vdaf

QLHV

Vdaf = Volatile content in

coal on dry basis

QLHV = Lower heating

value of coal

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SIZE DISTRIBUTION

0 100 200 300 400 500 600 700 8000

10

20

30

40

50

60SIZE DISTRIBUTION IN A CFBC BOILER

FLY ASH

CIRCULATING ASH

DRAIN ASH

PARTICLE SIZE (µm)

SIZE

FRA

CTIO

N (%

µm)

Should be sufficient enough to burn the large size particles in dense phase

Bed height should be such that it provides enough resistance to the PA supplied to the furnace

Lower bed height will increase the PA velocity resulting in erosion of water walls and entrainment of high size particles in lean phase

Depends on the coal quality and size

Bed height higher than normal will reduce the bed temperature and will reduce combustion efficiency

Bed height is a function of load 16TN - 28 Nov 2013 V0

BED HEIGHT

Should be around 850-900oC

Too low the temperature will result in inefficient combustion, will increase the cyclone and seal pot temperatures. Back pass temperatures will increase. Will increase attemperation.

Too high the temperature will result in clinker formation and agglomeration

Depends on coal quality, bed height, air velocity and quantity

Maintaining bed temperatures within the specified range is very important for ensuring that the heat transfer takes place in the boiler as per the design specifications

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BED TEMPERATURE

Cross section determines the velocity(5 to 6m/s) of air in the furnace

If cyclone separators are arranged in one direction, the ratio of furnace width to depth is equal to the no of cyclones

If cyclone separators are arranged in side, the ratio of furnace width to depth is equal to ¼th of no of cyclones

Ratio of width and depth determines the SA penetration in the furnace

Minimum 20m height is required for a CFBC boiler for ensuring better combustion efficiency

Furnace height is too high for the water wall length required18TN - 28 Nov 2013 V0

FURNACE VOLUME

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RECOMMENDED FURNACE HEIGHT

FUEL BOILER CAPACITY(TPH)

35 75 130 220 410 680 1000

ANTHRACITE 20 21 26 32 36 43 54

BITUMINOUS 20 20 25 29 34 39 50

LIGNITE 20 20 24 28 30 37 47

IT IS OF PRIME IMPORTANCE THAT THE FURNACE HEIGHT SHOULD BALANCE THE COMBUSTION EFFICIENCY AND THE HEATING SURFACE. BELOW IS THE RECOMMENDATIONS FOR DIFFERENT CAPACITY CFBC BOILERS

If Combustion is the heart, loop seal is the valve of the heart

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ASH RECIRCULATION

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Returns solids captured by the cyclone to the furnace

Prevents backflow of flue gas from furnace to cyclone

Failure of loop seal to transfer the solids at required rate will result in an entrained bed conditions

Large temperature gradient along the furnace, low heat absorption & higher back pass temperature

Loss in steam output, excessive steam temperatures & high stack temperature

Height of dip leg is very important. It shall be always more than riser leg

Pressure difference between dip leg and riser shall be around 2kPa

The air velocity in dip leg shall be around 0.3m/s and in riser leg shall be around 1.3 m/s

The velocity of circulating material in loop seal shall be around 0.2m/s

High air velocity may result in combustion in loop seal causing agglomeration

LOOP SEAL – FUNCTION & ADVANTAGES

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QUESTIONS PLEASE