Air Preheat Flue Gas System

7/28/2019 Air Preheat Flue Gas System

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29 May 2013 PMI Revision 00 1

Flue Gas systemAir pre Heater


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Presentation Plan Air heaters

Types of air heaters

Materials Used

Sealing arrangement for air heaters

Air heater Performance

Performance tests


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

AIR PRE HEATERRADIAL SEAL

AXIAL

SEAL

BYPASS SEAL

COLD END

HOT END

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 theboiler efficiency.

There are three types of air heaters:Recuperative

Rotary regenerative

Heat pipe


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Advantages by use of APH

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 heattransfer area requirement.

Less un-burnt fuel particle in flue gas thus combustion andefficiency is improved.

Intensified combustion permits faster load variation andfluctuation.

In the case of pulverized coal combustion, hot air can beused for heating the coal as well as for transporting thepulverized coal to burners.

This being a non-pressure part will not warrant shut-down ofunit due to corrosion of heat transfer surface which isinherent with lowering of flue gas temperature.


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Types Of Air Preheater 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 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 heaterRADIAL SEAL

AXIAL

SEAL

BYPASS SEAL

COLD END

HOT END

HOT INTERMEDIATE


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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, shelletc.

Rotor is divided into 12 or 24 sections and 12 or 24

radial divisions.

Each sector is divided into several trapezoidalsections with transverse division plates.

Heat storage pales are placed in these sections.


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The Material used in APH forheat storage

Material used Cold end in the basket is a special type ofsteel (corten steel (trade name)) which has high resistanceto the low 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 flowthrough 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

RADIAL SEAL (HE & CE)

AXIAL SEAL

CIRCUMFERENTIAL SEAL

ROTOR POST SEAL

SECTOR PLATE STATIC SEAL


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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


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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 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


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AXIAL SEALS ANDSEALING 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.


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AXIAL SEAL DIAPHRAGM

COG RIM PIN


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AXIALSEALPLATE

JACK BOLT

AXIAL SEAL INSPECTION DOOR

ADJUSTABLEBOLT

TURN-BUCKLE


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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 axialseal adjustment as compared to Russian

design


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CIRCUMFERENTIALSEALS 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 SEAL-

RUSSIANCIRCUMFERENTIAL SEAL

H.E. ROTOR FLANGE2.5MM RADIAL SEALH.E. DIAPHRAGM


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ADJUSTABLE BOLTCIRCUMFERENTIAL SEAL

ROTOR FLANGE AXIAL SEAL


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CIRCUMFERENTIAL SEALS

ACTUATING MECHANISM-

RUSSIANActuating 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 BETWEENTHE HOT & COLD END SECTOR PLATES AND THE

HOT AND COLD END CENTER SECTIONS.


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29 May 2013 PMI Revision 00 34HE ROTOR POST SEAL

SECTOR PLATE STATIC SEAL


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29 May 2013 PMI Revision 00 35CE ROTOR POST SEAL

DIAPHRAGM


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AIR SEAL HOUSING


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ROTOR POST SEAL


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THICKNESS OF RADIAL SEALSTRIPS

RUSSIAN MODEL : 6 MM

BHEL DESIGN : 2.5 MM

SOFT SEAL : 0.1 MM


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FLEXIBLE / SOFTSEALS 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 GASLEAKAGE THAT CAN BE GREATLY REDUCED BY

INSTALLING FLEXIBLE SEALS.

MERIT : - SOFT SEAL IS SET TO A NEGATIVE

CLEARANCE IN COLD CONDITION, AND WHICHWILL EXTEND 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.


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COST OF SOFTSEALS 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 SEALRUSSIAN


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APH PERFORMANCE Boiler efficiency decreases generally on account of APH performance

degradation. This also affects ESP, ID & FD fan loadings & at timesunit capability

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.


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APH PERFORMANCE Higher than expected leakage would decrease the flue

gas exit temperature, resulting in false sense ofimproved working.

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.


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FLUE GAS EXIT TEMP ATAPH OUTLET FLUE GAS TEMP AT AH OUTLET IS INDICATIVE OF HEAT

LEAVING 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.


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Oxygen in Flue Gas at AH A Inlet / Outlet

0

2

4

6

8

10

A B C D E F

Probe

InletO2%

0

2

4

6

8

10

12

OutletO2

%

Inlet O2 Outlet O2

Oxygen in Flue Gas at AH B Inlet / Outlet

0

2

4

6

8

10

A B C D E F

Probe

InletO2

0

2

4

6

8

10

OutletO2

Inlet O2 Outlet O2

Typical Oxygen Levels at

APH Inlet / Outlet

CO t i f d d t hi h


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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 = 5.72.8 * 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.


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PERFORMANCEDEGRADATION OF APH

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

= O2out - O2in * 0.9 * 100

(21- O2out)


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FG TEMP (Corr.) Vs BLR EFF & GAS SIDE EFF

85

85.2

85.4

85.6

85.8

86

86.2

150 155 160 165 170 175 180

CORRECTED FLUE GAS TEMP (Deg C)

BOILEREFFICI

ENCY(%)

58

59

60

61

62

63

64

65

66

G

ASSIDEEFFIC

IENCY(%)

Boiler Efficinecy

Gas Side Efficiency


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XRatioRatio 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 leakage17.1%, Gas I n Temp333.5 C, Gas Out

Temp133.8 C , Air I n Temp36.1 C, Air Out Temp288 C

X ratio = (333.5150.5) / (28836.1) = 0.73


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X-Ratiodepends 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.


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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 (143.941.6) / (34541.6)

= 139.5 C

EGT Corrected for inlet air temp and for AH leakage

= AL * Cpa * (EGTcor - Tair in) + EGTcorCpg * 100

= [13.7* (139.541.6)] / 100 + 139.5 = 152.9


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Air Preheat Flue Gas System