Flue Gas system
Air pre Heater
9 April 2015
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Presentation Plan
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Air heaters
Types of air heaters
Materials Used
Sealing arrangement for air heaters
Air heater Performance
Performance tests
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AIR PRE HEATER
APH is the last heat
exchanger in the
boiler flue gas
circuit. To achieve BYPASS SEAL
maximum boiler
efficiency
HOT END
maximum
possible
useful
heat
must
be
removed from the
gas
before
it
leaves the APH.
However certain
minimum
COLD END
temperature has
to be maintained
in the flue gas to
prevent cold end
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corrosion
RADIAL SEAL
AXIAL
SEAL
HOT INTERMEDIATE
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Air Pre-Heater-functions
• An air pre-heater heats the combustion air where it is
economically feasible.
• The pre-heating helps the following:
• Igniting the fuel.
• Improving combustion.
• Drying the pulverized coal in pulverizer.
• Reducing the stack gas temperature and increasing the
boiler efficiency.
• There are three types of air heaters:
• Recuperative
• Rotary regenerative
• Heat pipe
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Advantages by use of APH
•
•
•
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Stability of Combustion is improved by use of hot air.
Intensified and improved combustion.
Permitting to burn poor quality coal.
High heat transfer rate in the furnace and hence lesser heat
transfer area requirement.
• Less un-burnt fuel particle in flue gas thus combustion and
both r efficiency is improved.
• Intensified combustion permits faster load variation and
fluctuation.
• In the case of pulverised coal combustion, hot air can be
used for heating the coal as well as for transporting the
pulverised coal to burners.
• This being a non-pressure part will not warrant shut-down of
unit due to corrosion of heat transfer surface which is
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inherent
with lowering of flue gas temperature.
Types Of Air Preheater
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Recuperative
Regenerative
Plate type Airheater
Steam Air Preheater
Langsdorm type
Rothemuhle type
Tri sector Air Heater
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Tubular Air Heaters
(Recuperative)
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Tubular Air Pre-Heater
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Design Parameters
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Tubes are generally arranged in staggered pattern.
Steel tubes of Dia: 37 – 63 mm.
Transverse pitch: S1/d = 1.5 – 1.9
Longitudinal pitch: S2/d = 1.0 – 1.2
The height of air chamber:1.4 – 4.5 m.
Gas and Air flow velocity : 10 – 16 m/s.
Plate Recuperators:
Instead of tube, parallel plates are used.
The gas passage is 12 – 16 mm wide.
The air passage is 12 mm wide.
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Regenerative air pre heater
BYPASS SEAL
HOT END
AXIAL
SEAL
COLD END
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RADIAL SEAL
HOT INTERMEDIATE
10
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Rotary Plate (Regenerative)
type Pre-Heater
• Rotates with a low speed : 0.75 rpm.
• Weight : 500 tons.
• This consists of : rotor, sealing apparatus, shell
etc.
• Rotor is divided into 12 or 24 sections and 12 or 24
radial divisions.
• Each sector is divided into several trapezoidal
sections with transverse division plates.
• Heat storage pales are placed in these sections.
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The Material used in APH for
heat storage
• Material used Cold end in the basket is a special type of
steel (corten steel (trade name)) which has high resistance
to the low temperature sulphur corrosion, thus prolonging
operational life.
• In the hot end mild steels are used
• The optimal geometric shape is usually corrugated and
sizes are determined based on design modelling and
experimental data. The turbulence of air and gas flow
through the package increases the heat transfer rate.
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Stationary-Plate Type Air Pre-Heater
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Stationary-Plate Type Air Pre-Heater
• The heat storage elements are static but the air/gas flow
section rotates.
• The storage plates are placed in the stator.
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Trisector air Heater
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Heating Elements
• Hot End Baskets
• Hot Intermediate Baskets
• Cold End Baskets
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TYPES OF SEALS
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RADIAL SEAL (HE & CE)
AXIAL SEAL
CIRCUMFERENTIAL SEAL
ROTOR POST SEAL
SECTOR PLATE STATIC SEAL
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PERCENTAGE AIR LEAKAGE
OF TOATAL LEAKAGES
1. HE Radial seal leakage
- 62.21%
2. CE Radial seal leakage
- 11.98%
3. Axial seal leakage
- 08.78%
4. By pass or circumferential seal leakage 0.87%
5. Center post seal leakage
3.17%
_______________________________________________
Total percentage
=
87.01%
Entrapped leakage
=
12.99%
TOTAL
=
100%
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RADAIAL SEALS &
SECTOR PLATE
RADIAL SEALS AND SECTOR PLATES ARE
LOCATED AT THE HOT AND COLD ENDS OF THE AIR
PREHEATER. THE RADIAL SEALS ARE ATTACHED
TO THE DIAPHRAGMS, WHICH SEPARATE THE
INDIVIDUAL ROTOR COMPARTMENT.
• PURPOSE: - THE PURPOSE OF RADIAL SEALS IS
TO REDUCE THE AREA AVAILABLE FOR LEAKAGE
FROM THE AIR TO THE GAS SIDE BETWEEN THE
DIAPHRAGM AND THE SECTOR PLATE
•
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AXIAL SEALS AND
SEALING PLATES
•
AXIAL SEALS MINIMIZE LEAKAGE PASSING
RADIALLY AROUND THE ROTOR SHELL. THE AXIAL
SEALS ARE MOUNTED ON THE OUT SIDE OF THE
ROTOR SHELL AND SEAL AGAINST THE AXIAL SEAL
PLATES MOUNTED ON THE AIR PREHEATER
HOUSING.
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AXIAL SEAL DIAPHRAGM
COG RIM PIN
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AXIAL
SEAL
PLATE
TURN-BUCKLE
AXIAL SEAL INSPECTION DOOR
ADJUSTABLE
BOLT
JACK BOLT
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Axial Seal Arrangement
• Curved axial sector plate adjustable from
outside
• Seal strips are attached to the rotor.
• The thickness of seal strips :
6 MM straight strips in Russian.
2.5 mm thick and bend backward in
BHEL.
BHEL APH has better accessibility of axial
seal adjustment as compared to Russian
design
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CIRCUMFERENTIAL
SEALS
• THE CIRCUMFERENTIAL SEALS PREVENT
AIR AND GAS FROM BYPASSING THE
HEATING SURFACE THROUGH THE SPACE
BETWEEN THE ROTOR AND THE HOUSING
SHELL. THEY ALSO PREVENT AIR AND GAS
FROM FLOWING AXIALLY AROUND THE
ROTOR.
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CIRCUMFERENTIAL SEALRUSSIAN
CIRCUMFERENTIAL SEAL
H.E. DIAPHRAGM
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2.5MM RADIAL SEAL
H.E. ROTOR FLANGE
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ROTOR FLANGE
CIRCUMFERENTIAL SEAL
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AXIAL SEAL
ADJUSTABLE BOLT
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CIRCUMFERENTIAL SEALS
ACTUATING MECHANISMRUSSIAN
Actuating Bolt
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ROTOR POST SEALS
• ROTOR POST SEALS PREVENT LEAKAGE
BETWEEN THE ENDS OF THE ROTOR POST AND
THE AIR PREHEATER HOUSING.
• THE STATIC SEALS PREVENT LEAKAGE BETWEEN
THE HOT & COLD END SECTOR PLATES AND THE
HOT AND COLD END CENTER SECTIONS.
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SECTOR PLATE STATIC SEAL
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HE ROTOR POST SEAL
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DIAPHRAGM
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CE ROTOR POST SEAL
35
AIR SEAL HOUSING
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ROTOR POST SEAL
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THICKNESS OF RADIAL SEAL
STRIPS
• RUSSIAN MODEL
• BHEL DESIGN
• SOFT SEAL
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: 6 MM
: 2.5 MM
: 0.1 MM
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FLEXIBLE / SOFT
SEALS
• THE FLEXIBLE SEALS WAS DEVELOPED TO
REDUCE NORMAL LEAKAGE CAUSED BY THE
THERMAL EXPANSION OF THE ROTOR WHILE THE
UNIT IS OPERTAING. THE ROTOR EXPANSION
OPENS UP AREAS OF DIRECT AIR TO GAS
LEAKAGE THAT CAN BE GREATLY REDUCED BY
INSTALLING FLEXIBLE SEALS.
• MERIT : - SOFT SEAL IS SET TO A NEGATIVE
CLEARANCE IN COLD CONDITION, AND WHICH
WILL EXTEND IN THE HOT CONDITION TO
OPERATE AS A STANDARD PROXIMITY SEAL.
• DEMERIT : - SINCE THIS IS AN INTERFERENCE OR
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CONTACT SEAL, THE WEAR LIFE IS VERY LOW.
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COST OF SOFT
SEALS
• RADIAL SOFT SEAL HE/ SET :- 1 LAC
• RADIAL SOFT SEAL CE/ SET :- 0.95 LAC
• AXIAL SOFT SEAL/ SET :- 0.4 LAC
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MODULAR APH - BHEL
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RADIAL SEAL
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BY PASS SEAL
RUSSIAN
BY PASS SEAL RUSSIAN
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APH PERFORMANCE
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Boiler efficiency decreases generally on account of APH performance
degradation. This also affects ESP, ID & FD fan loadings & at times
unit capability
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Factors affecting APH performance
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Excess air level / No of Mills in service
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Primary Air to Secondary Air ratio
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Moisture in coal/ Air ingress level
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Performance of upstream ash evacuation system
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Procedure for cleaning, soot blowing & regular maintenance etc.
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APH PERFORMANCE
• Higher than expected leakage would decrease the flue
gas exit temperature, resulting in false sense of
improved working.
• Higher inlet flue gas temperature is rather rare, but this
could be one reason for high exit temperature.
• Optimum flue gas temperature is required for effective
ESP performance
• Unequal temperature at air heater exit should be
investigated.
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FLUE GAS EXIT TEMP AT
APH OUTLET
• FLUE GAS TEMP AT AH OUTLET IS INDICATIVE OF HEAT
LEAVING THE UNIT .THIS IS LOWERED ON ACCOUNT OF
AH LEAKAGES.
• FGET TO BE MEASURED AT A LOCATION SLIGHTLY
AWAY FROM AIR HEATERS.
• NO OF TEMPERATURE SENSOR PROVIDED SHOULD
COVER THE DUCT ADEQUATELY.
• CORRECTED TEMP SHOULD BE USED FOR
COMPARISION.
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Typical Oxygen Levels at
APH Inlet / Outlet
Oxygen in Flue Gas at AH A Inlet / Outlet
Oxygen in Flue Gas at AH B Inlet / Outlet
8
10
8
8
6
6
4
4
2
2
0
0
8
6
6
4
4
2
2
0
0
A
B
C
Inlet O2
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D
Probe
E
Outlet O2
F
A
B
C
Inlet O2 Probe
D
E
Outlet O2
10
Inlet O2
10
Outlet O2 %
12
Inlet O2 %
10
F
Outlet O2
47
CO2 measurement is preferred due to high
absolute values; In case of any measurement
errors, the resultant influence on leakage
calculation is small.
Air Leakage
Weight of air passing from air side to gas side; This
leakage is assumed to occur entirely between air inlet and
gas outlet
Hot End / Cold End / Entrained Leakage
Calculation
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Empirical relationship using the change in
concentration of O2 or CO2 in the flue gas
=
CO2in - CO2out * 0.9 * 100
CO2out
=
O2out - O2in * 0.9 * 100
(21- O2out)
= 5.7 – 2.8 * 90
(21-5.7)
= 17.1 %
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PERFORMANCE
DEGRADATION OF APH
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Seal Leakage
Erosion
Corrosion
High Press Drop Across APH
APH Fire
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APH Performance Test
• APH Leakage
• Gas Side Efficiency
• X-Ratio
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• GAS SIDE EFFICIENCY
= (Temp drop / Temperature head) * 100
• X- RATIO = T (gas in) – T (gas out) (no lkg)/ T(air out)
–T (air in)
• Air Leakage = CO2in - CO2out * 0.9 * 100
CO2out
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= O2out - O2in * 0.9 * 100
(21- O2out)
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FG TEMP (Corr.) Vs BLR EFF & GAS SIDE EFF
66
65
86
64
85.8
63
Boiler Efficinecy
85.6
62
Gas Side Efficiency
61
85.4
60
85.2
85
150
59
155
160
165
170
175
GAS SIDE EFFICIENCY (%)
BOILER EFFICIENCY (%)
86.2
58
180
CORRECTED FLUE GAS TEMP (Deg C)
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X – Ratio
Ratio of heat capacity of air passing through the air heater
to the heat capacity of flue gas passing through the air
heater.
=
Wair out * Cpa
Wgas in * Cpg
=
Tgas in - Tgas out (no leakage)
Tair out - Tair in
Say AH leakage – 17.1%, Gas In Temp – 333.5 C, Gas Out
Temp – 133.8 C , Air In Temp – 36.1 C, Air Out Temp – 288 C
X ratio = (333.5 – 150.5) / (288 –36.1) = 0.73
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X-Ratio depends on
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moisture in coal, air infiltration, air & gas mass flow rates
leakage from the setting
specific heats of air & flue gas
X-ratio does not provide a measure of thermal
performance of the air heater, but is a measure of the
operating conditions.
A low X-ratio indicates either excessive gas weight
through the air heater or that air flow is bypassing the air
heater.
A lower than design X-ratio leads to a higher than
design gas outlet temperature & can be used as an
indication of excessive tempering air to the mills or
excessive boiler setting infiltration.
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Flue Gas Exit Temperature
Flue Gas Exit Temperature is corrected for inlet air temperature
X1 = Reference Air Temp * (Gas Temp In – Gas Temp Out) +
Gas Temp In * (Gas Temp Out - Air Temp In)
X2 = Gas Temp In - Air Temp In
EGTcor.
= X1/ X2
= 35 * (345-143.9) + 345 (143.9 – 41.6) / (345 – 41.6)
= 139.5 C
EGT Corrected for inlet air temp and for AH leakage
= AL * Cpa * (EGTcor - Tair in) + EGTcor
Cpg * 100
= [13.7* (139.5 – 41.6)] / 100 + 139.5 = 152.9
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THANK YOU
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