Flue Gas System-2

7/31/2019 Flue Gas System-2

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May 24, 2012 PMI Revision 00 1

• Air heaters

• Types of air heaters

• Materials Used

• Sealing arrangement for air heaters

• Air heater Performance

• Performance tests

APH is the last heatexchanger in theboiler flue gascircuit. To achievemaximum boiler efficiencymaximumpossible usefulheat must beremoved from thegas before itleaves the APH.However certainminimumtemperature hasto be maintainedin the flue gas toprevent cold endcorrosion

RADIAL SEAL

BYPASS SEAL

COLD END

HOT END

HOT INTERMEDIATE

• An air pre-heater heats the combustion air where it iseconomically 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

• 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

efficiency is improved.• Intensified combustion permits faster load variation and

fluctuation.

• In the case of pulverised coal combustion, hot air can beused for heating the coal as well as for transporting thepulverised 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 inherent

with lowering of flue gas temperature.

Recuperative Regenerative Plate type Airheater

Steam Air Preheater Langsdorm type Rothemuhle type Tri sector Air Heater

• 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.

RADIAL SEAL

BYPASS SEAL

COLD END

HOT END

HOT INTERMEDIATE

• 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.

• Material used Cold end in the basket is a special type of steel(corten steel (trade name)) which has high resistance to thelow temperature sulphur corrosion, thus prolongingoperational life.

• In the hot end mild steels are used

• The optimal geometric shape is usually corrugated andsizes are determined based on design modelling andexperimental data. The turbulence of air and gas flow throughthe package increases the heat transfer rate.

• The heat storage elements are static but the air/gas flow

section rotates.

• The storage plates are placed in the stator.

• Hot End Baskets

• Hot Intermediate Baskets

• Cold End Baskets

• RADIAL SEAL (HE & CE)

• AXIAL SEAL

• CIRCUMFERENTIAL SEAL

• ROTOR POST SEAL

• SECTOR PLATE STATIC SEAL

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%

PERCENTAGE AIR LEAKAGE

OF TOATAL LEAKAGES

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 THEINDIVIDUAL ROTOR COMPARTMENT.

• PURPOSE: - THE PURPOSE OF RADIAL SEALS IS TO

REDUCE THE AREA AVAILABLE FOR LEAKAGE

FROM THE AIR TO THE GAS SIDE BETWEEN THEDIAPHRAGM AND THE SECTOR PLATE

AXIAL SEALS AND

SEALING PLATES

• AXIAL SEALS MINIMIZE LEAKAGE PASSING

RADIALLY AROUND THE ROTOR SHELL. THE AXIALSEALS ARE MOUNTED ON THE OUT SIDE OF THE

ROTOR SHELL AND SEAL AGAINST THE AXIAL SEAL

PLATES MOUNTED ON THE AIR PREHEATER

HOUSING.

AXIAL SEAL DIAPHRAGMAXIAL SEAL DIAPHRAGM

COG RIM PINCOG RIM PIN

JACK BOLTJACK BOLT

AXIAL SEAL INSPECTION DOOR AXIAL SEAL INSPECTION DOOR

ADJUSTABLEADJUSTABLEBOLTBOLT

TURN-BUCKLETURN-BUCKLE

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 APH has better accessibility of axialseal adjustment as compared to Russian

design

CIRCUMFERENTIAL

• 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.

CIRCUMFERENTIAL SEAL-

RUSSIANCIRCUMFERENTIAL SEAL

H.E. ROTOR FLANGEH.E. ROTOR FLANGE2.5MM RADIAL SEAL2.5MM RADIAL SEALH.E. DIAPHRAGMH.E. DIAPHRAGM

ADJUSTABLE BOLTADJUSTABLE BOLTCIRCUMFERENTIAL SEALCIRCUMFERENTIAL SEAL

ROTOR FLANGEROTOR FLANGEAXIAL SEALAXIAL SEAL

CIRCUMFERENTIAL SEALS

ACTUATING MECHANISM-

RUSSIANActuating Bolt

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 BETWEENTHE HOT & COLD END SECTOR PLATES AND THE

HOT AND COLD END CENTER SECTIONS.

May 24, 2012 PMI Revision 00 34HE ROTOR POST SEALHE ROTOR POST SEAL

SECTOR PLATE STATIC SEALSECTOR PLATE STATIC SEAL

May 24, 2012 PMI Revision 00 35CE ROTOR POST SEALCE ROTOR POST SEAL

DIAPHRAGMDIAPHRAGM

AIR SEAL HOUSINGAIR SEAL HOUSING

ROTOR POST SEALROTOR POST SEAL

THICKNESS OF RADIAL SEALSTRIPS

• RUSSIAN MODEL : 6 MM

• BHEL DESIGN : 2.5 MM

• SOFT SEAL : 0.1 MM

• 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 CANBE GREATLY REDUCED BY INSTALLING FLEXIBLE

SEALS.

• MERIT : - SOFT SEAL IS SET TO A NEGATIVE

CLEARANCE IN COLD CONDITION, AND WHICH WILLEXTEND IN THE HOT CONDITION TO OPERATE AS A

STANDARD PROXIMITY SEAL.

• DEMERIT : - SINCE THIS IS AN INTERFERENCE OR

CONTACT SEAL, THE WEAR LIFE IS VERY LOW.

• RADIAL SOFT SEAL HE/ SET :- 1 LAC

• RADIAL SOFT SEAL CE/ SET :- 0.95 LAC• AXIAL SOFT SEAL/ SET :- 0.4 LAC

BY PASS SEAL RUSSIAN

• Boiler efficiency decreases generally on account of APH performancedegradation. This also affects ESP, ID & FD fan loadings & at times unitcapability

• Factors affecting APH performance

• Excess air level / No of Mills in service

• Primary Air to Secondary Air ratio

• Moisture in coal/ Air ingress level

• Performance of upstream ash evacuation system• Procedure for cleaning, soot blowing & regular maintenance etc.

• Higher than expected leakage would decrease the fluegas exit temperature, resulting in false sense of improvedworking.

• Higher inlet flue gas temperature is rather rare, but thiscould be one reason for high exit temperature.

• Optimum flue gas temperature is required for effectiveESP performance

• Unequal temperature at air heater exit should beinvestigated.

• FLUE GAS TEMP AT AH OUTLET IS INDICATIVE OF HEATLEAVING THE UNIT .THIS IS LOWERED ON ACCOUNT OFAH LEAKAGES.

• FGET TO BE MEASURED AT A LOCATION SLIGHTLY

AWAY FROM AIR HEATERS.

• NO OF TEMPERATURE SENSOR PROVIDED SHOULDCOVER THE DUCT ADEQUATELY.

• CORRECTED TEMP SHOULD BE USED FORCOMPARISION.

O x y g e n i n F lu e G a s a t A H A I

A B C D E F

P r o b

I n l e t O

O u t l e t O

In let O0 O u t l e t O0

O x y g e n i n F lu e G a s a t A H B I

A B C D E F

P r o b

I n l e t O 0

O u t l e t O 0

In le t O0 O u t le t O0

Typical Oxygen Levels at

APH Inlet / Outlet

CO t i f d d t hi h

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 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 = 5.7 – 2.8 * 90

(21- O2out) (21-5.7)

= 17.1 %

CO2 measurement is preferred due to high

absolute values; In case of any measurement

errors, the resultant influence on leakage

calculation is small.

• Seal Leakage• Erosion

• Corrosion

• High Press Drop Across APH• APH Fire

• APH Leakage

• Gas Side Efficiency

• X-Ratio

• 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

= O2out - O2in * 0.9 * 100

(21- O2out)

FG TE M P (Cor r.) V s BL R E F F & G AS

000 000 000 000 888 000 000

C O R R E C T E D F L U E G A S T E M P

B O I L E R E F F I C I E N C

Y ( % )

A S S I D E E F F I C I E N C Y ( % )

Boiler Eff icin

G as S ide E f fi c ie

X – RatioRatio 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 –

X ratio = (333.5 – 150.5) / (288 –36.1) = 0.73

X-Ratio depends on

•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 thandesign gas outlet temperature & can be used as an

indication of excessive tempering air to the mills or

excessive boiler setting infiltration.

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) + EGTcorCpg * 100

= [13.7* (139.5 – 41.6)] / 100 + 139.5 = 152.9

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