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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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APH is the last heat exchanger in the boiler flue gas circuit. To achieve maximum boiler efficiency maximum possible useful heat must be removed from the gas before it leaves the APH. However certain minimum temperature has to be maintained in the flue gas to prevent cold end corrosion

AIR PRE HEATER

RADIAL 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 the boiler 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 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 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

RADIAL 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, 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

•RADIAL SEAL (HE & CE)•AXIAL SEAL•CIRCUMFERENTIAL SEAL•ROTOR POST SEAL•SECTOR PLATE STATIC SEAL

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•HE Radial seal leakage - 62.21%•CE Radial seal leakage - 11.98%•Axial seal leakage - 08.78%•By pass or circumferential seal leakage - 0.87%•Center post seal leakage - 3.17%____________________________________________

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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 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 SEALPLATE

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 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 SEAL-RUSSIAN

CIRCUMFERENTIAL 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 BETWEEN THE HOT & COLD END SECTOR PLATES AND THE HOT AND COLD END CENTER SECTIONS.

* *HE ROTOR POST SEAL

SECTOR PLATE STATIC SEAL

* *CE 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 SEAL STRIPS

•RUSSIAN MODEL : 6 MM

•BHEL DESIGN : 2.5 MM

•SOFT SEAL : 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 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•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

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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 = 5.7 – 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.

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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)

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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 * CpaWgas 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 TemperatureFlue 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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