The document discusses the characteristics and factors affecting combustion in a Circulating Fluidized Bed Combustion (CFBC) boiler. It emphasizes the importance of air distribution, coal quality, bed temperature, and furnace volume for achieving efficient combustion and minimizing emissions. Key operational practices and recommended parameters for maintaining combustion efficiency are also outlined.

1. COMBUSTION IN A CFBC
BOILER
TAPASH NAG
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2. CFBC CHARACTERISTICS
Operates under special fluid dynamic condition
Solid particles are mixed through furnace at a velocity
exceeding the average terminal velocity of the particles
Major fractions of solids are captured by cyclone
High recycle rate intensifies solid mixing and evens out
combustion temperature
High turbulence, solid mixing and absence of a defined
bed level
Solids are distributed through out the furnace with
steadily decreasing density from bottom to top of
furnace
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3. COMBUSTION FACTORS
COAL
QUALITY
SA
PA
FURNACE
VOLUME
COMBUSTION
BED
TEMP
ASH REC
BED
HEIGHT
COAL
SIZE
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4. PRIMARY AIR
Conveys coal
 Uniform distribution of coal in
furnace
 Provides air for combustion
Fluidizes the bed ash
Ensures proper mixing
Provides sealing air to feeders
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5. SECONDARY AIR
Staged Combustion
 Maintains bed temperature
Burner air
Provides excess air requirement
Controls overboard temperature
Eliminates NOx formation
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6. AIR DISTRIBUTION CRITERIA
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
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7. AIR ADJUSTMENT TABLE
SA DAMPER POSITION
(%)
TOP
SR
LOAD
(MW)
BOTTOM
SA
HEADER
PRESSURE
(kPa)
PA
QUANTITY
BED
HEIGHT
(kPa)
BED
TEMP
(℃)
OXYGEN
(%)
Around
700
4.0-5.0
700-850
3.5-4.2
800-850
3.5-4.0
1
≤50
45
25
≥6.0
≥Critical
fluidizing air
10.0
2
50-80
40-50
30-40
6.0-7.5
≥Critical
fluidizing air
11.0
3
80-100
60-70
40-50
7.5-8.0
Adjust bed
temperature
≥11.5
4
100-120
70-80
50-60
8.0-9.0
Adjust bed
temperature
≥12.0
800-900
~ 3.5
5
120-135
80-90
60-70
≥9.5
Adjust bed
temperature
≥12.5
800-910
3.0-3.5
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8. OXYGEN PROFILE IN CFBC FURNACE
POOR
OXYGEN
CORE
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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
SECONDARY 5. Proper air velocities should be
AIR
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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9. OPERATION PRACTICES
RATIO OF PRIMARY AIR
FUEL
VOLATILE CONTENT(%)
RATIO OF PA TO TOTAL AIR
ANTHRACITE
<20
~65
BITUMINOUS
20~40
~60
LIGNITE
>40
~50
VELOCITY OF AIR (m/s)
PA(Based on throat area of Nozzle)
SA
~40
55~85
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10. EXCESS AIR REQUIREMENTS
Coefficient of Excess air is a function of the fuel
Reasonable air supply ensures better temperature
distribution in the furnace and better combustion
efficiency
Ensures lower NOX generation
EXCESS AIR AT FURNACE OUTLET
FUEL
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
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11. COAL QUALITY
 GCV
 Moisture
Ash Content
Volatile material
Carbon content
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12. COAL SIZE
 Fines
 Oversize
Distribution
Volatile material
Carbon content
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13. FEW FACTS
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
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14. COAL INDEX
COAL INDEX
%C IN ASH LEAVING FURNACE
30
25
I = Vdaf
QLHV
20
15
Vdaf = Volatile content in
coal on dry basis
10
5
0
0
5
10
15
20
I (MJ/kg)
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30
35
QLHV = Lower heating
value of coal
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15. SIZE DISTRIBUTION
SIZE DISTRIBUTION IN A CFBC BOILER
140
FLY ASH
120
CIRCULATING ASH
SIZE FRACTION (%µm)
100
DRAIN ASH
80
60
40
20
0
0
100
200
300
400
500
600
700
800
PARTICLE SIZE (µm)
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16. BED HEIGHT
 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
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17. BED TEMPERATURE
 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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18. FURNACE VOLUME
 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 required
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19. RECOMMENDED FURNACE HEIGHT
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
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
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20. ASH RECIRCULATION
If Combustion is the heart, loop seal is the valve of the heart
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21. LOOP SEAL – FUNCTION & ADVANTAGES
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
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22. QUESTIONS PLEASE
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