The presentation deals with the most complex and fundamental process in a CFBC boiler. i.e., Combustion. Provides an insight into the various features in a CFBC boilers which are incorporated to enhance cpmbustion.
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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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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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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