Air Preheat Flue Gas System

Flue Gas system Air 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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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 29 May 2013 PMI Revision 00 corrosion
RADIAL SEAL
AXIAL SEAL
HOT INTERMEDIATE
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
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 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 pulverized coal combustion, hot air can be used for heating the coal as well as for transporting the pulverized 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.
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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 heater

BYPASS SEAL RADIAL SEAL
HOT END
AXIAL SEAL
COLD 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, 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.
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
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 Entrapped leakage = = 87.01% 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
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.
Axial Seal Arrangement
BHEL APH has better accessibility of axial seal adjustment as compared to Russian design
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
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H.E. ROTOR FLANGE
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ROTOR FLANGE
AXIAL SEAL
CIRCUMFERENTIAL SEAL
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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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DIAPHRAGM
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CE ROTOR POST SEAL
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AIR SEAL HOUSING

ROTOR POST SEAL

THICKNESS OF RADIAL SEAL STRIPS

RUSSIAN MODEL BHEL DESIGN SOFT SEAL : 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 29 May 2013 Revision 00 CONTACT SEAL, THE PMI 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
Boiler efficiency decreases generally on account of APH performance degradation. This also affects ESP, ID & FD fan loadings & at times unit 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 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
10 8
Inlet O2 %
Oxygen in Flue Gas at AH B Inlet / Outlet

12 10
Outlet O2 %
10 8
Inlet O2
10 8 6 4 2 0 A B C Inlet O2 Probe D E Outlet O2 F

Outlet O2
6 4 2 0 A B C Inlet O2 Probe D E Outlet O2 F
8 6 4 2 0
6 4 2 0
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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 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
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) PMI Revision 00
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FG TEMP (Corr.) Vs BLR EFF & GAS SIDE EFF

86.2
BOILER EFFICIENCY (%)
66
GAS SIDE EFFICIENCY (%)
52
86 85.8
Boiler Efficinecy
65 64 63 62
Gas Side Efficiency
85.6 85.4 85.2 85 150
61 60 59 58 180
155
160
165
170
175
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
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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Air Preheat Flue Gas System

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Air Preheat Flue Gas System

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Flue Gas system Air pre Heater

29 May 2013 PMI Revision 00 1

AIR PRE HEATER

Advantages by use of APH

Types Of Air Preheater

Tubular Air Heaters (Recuperative)

Tubular Air Pre-Heater

Regenerative air pre heater

Rotary Plate (Regenerative) type Pre-Heater

The Material used in APH for heat storage

Stationary-Plate Type Air Pre-Heater

Stationary-Plate Type Air Pre-Heater

Trisector air Heater

PERCENTAGE AIR LEAKAGE OF TOATAL LEAKAGES

RADAIAL SEALS & SECTOR PLATE

AXIAL SEALS AND SEALING PLATES

AXIAL SEAL DIAPHRAGM

COG RIM PIN

AXIAL SEAL PLATE

AXIAL SEAL INSPECTION DOOR

Axial Seal Arrangement

2.5MM RADIAL SEAL

H.E. ROTOR FLANGE

CIRCUMFERENTIAL SEALS ACTUATING MECHANISMRUSSIAN Actuating Bolt

ROTOR POST SEALS

SECTOR PLATE STATIC SEAL

CE ROTOR POST SEAL

AIR SEAL HOUSING

ROTOR POST SEAL

THICKNESS OF RADIAL SEAL STRIPS

FLEXIBLE / SOFT SEALS

COST OF SOFT SEALS

MODULAR APH - BHEL

BY PASS SEAL RUSSIAN

Factors affecting APH performance

Procedure for cleaning, soot blowing & regular maintenance etc.

FLUE GAS EXIT TEMP AT APH OUTLET

Typical Oxygen Levels at APH Inlet / Outlet

Oxygen in Flue Gas at AH B Inlet / Outlet

10 8 6 4 2 0 A B C Inlet O2 Probe D E Outlet O2 F

6 4 2 0 A B C Inlet O2 Probe D E Outlet O2 F

Hot End / Cold End / Entrained Leakage

CO2in - CO2out * 0.9 * 100 CO2out

PERFORMANCE DEGRADATION OF APH

APH Performance Test

GAS SIDE EFFICIENCY = (Temp drop / Temperature head) * 100

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

85.6 85.4 85.2 85 150

CORRECTED FLUE GAS TEMP (Deg C)

Flue Gas Exit Temperature

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