Air Heater & Air Heater Performance Indices

AIR PREHEATER

Air Preheater

HEAT EXCHANGER

HEAT TRANSFER FROM FLUE GAS TO AIR

HEAT REJECTED TO ATMOSPHERE REDUCED

INCREASE BOILER EFFICIENCY BY STABILIZING COMBUSTION WITH HELP OF HOT AIR.

HOT AIR USED FOR DRYING THE COAL AS WELL AS FOR TRANSPORTING.

FOR EVERY 20°C DROP IN FLUE GAS EXIT TEMPERATURE THE BOILER EFFICIENCY INCREASE BY ABOUT 1%.

10% IMPROVEMENT IN BOILER EFFICIENCY WHEN COMPARED TO AN IDENTICAL UNIT WITHOUT AN APH.

TYPES OF AIR PREHEATERS:

AIR PREHEATERS CAN BE CLASSIFIED AS RECUPERATIVE AND REGENERATIVE TYPES BASED ON THEIR OPERATING PRINCIPLE.

IN RECUPERATIVE TYPE HEATING MEDIUM FLUE GAS IS ON ONE SIDE AND AIR IS ON THE OTHER SIDE OF TUBE OR PLATE AND THE HEAT TRANSFER IS BY CONDUCTION THROUGH THE MATERIAL WHICH SEPARATES THE MEDIA. THESE ARE OF STATIC CONSTRUCTION AND HENCE THERE IS ONLY NOMINAL LEAKAGE THROUGH EXPANSION.

IN REGENERATIVE TYPE THE HEATING MEDIUM FLOWS THROUGH A CLOSELY PACKED MATRIX TO RAISE ITS TEMPERATURE AND THEN AIR IS PASSED THROUGH THE MATRIX TO PICK-UP THE HEAT. EITHER THE MATRIX OR THE HOODS ARE ROTATED TO ACHIEVE THIS AND HENCE THERE IS SLIGHT LEAKAGE THROUGH SEALING ARRANGEMENTS AT THE MOVING SURFACES.

TYPES

RECUPERATIVE RE-GENERATIVE

STATIC CONSTRUCTION ROTARY BY CONSTRUCTION

TUBULAR TYPE

PLATE TYPE

TRI-SECTORBI-SECTOR

LJUNGSTROM TYPE

MATRIX ELEMENT ROTATING

HEAT PIPE or THERMOSYPHONE

TUBULAR AIR PREHEATER (RECUPERATIVE):

•LARGE NUMBER OF STEEL TUBES OF 40 TO 65 MM DIA.•EITHER WELDED OR EXPANDED INTO THE TUBE PLATES.•EITHER GAS OR AIR FLOW THROUGH THE TUBE.•GAS THROUGH THE TUBE NORMALLY REQUIRES HIGHER SIZE TUBE AND VERTICAL FLOW TO REDUCE FOULING.•SINGLE OR MORE PASSES ON THE GAS SIDE AND MULTIPASS CROSS FLOW ON THE AIR SIDE USUALLY FITS IN WITH THE OVERALL PLANT DESIGN.•THE PORTION OF AIRHEATER AT LOW TEMPERATURE ZONE IS DESIGNED NORMALLY WITH A SHORTER TUBE LENGTH SO AS TO FACILITATE MAINTENANCE OF SURFACES DUE TO CORROSION AND FOULING.

PLATE TYPE AIR PREHEATER (RECUPERATIVE):

•THESE COMPRISE OF PARALLEL PLATES.• WHICH PROVIDE ALTERNATE PASSAGE FOR GAS AND AIR.•THIS TYPE IS SIMPLE AND COMPACT COMPARED TO THAT OF TUBULAR TYPE.•THE NARROW PASSES BETWEEN PLATES MAKE THE CLEANING TEDIOUS BUT WITH SHOT CLEANING METHOD IT IS IMPROVED.•REPLACEMENT IS A MAJOR TASK.

LJUNGSTROM REGENERATIVE AIR - HEATER

•THE HEAT TRANSFER ELEMENTS ARE ROTATED AT A CONSTANT SPEED AND THEY PASS ALTERNATELY THROUGH GAS AND AIR PASSES.

•THE AXIS OF ROTATION MAY BE HORIZONTAL OR VERTICAL. •THE DRIVE IS NORMALLY ELECTRICAL OPERATED THROUGH REDUCTION GEAR WITH COMPRESSED AIR MOTOR AS STAND-BY.

•THE PLATES FORMING THE ELEMENTS (MATRIX) MAY BE VARIED IN SPACING AND THICKNESS.

•COLD ENDS ARE MADE OF SPECIAL CORROSION RESISTANCE ALLOY SUCH AS CORTEN OR ENAMELED TO ACHIEVE CORROSION RESISTANCE.

•THIS TYPE IS VERY COMPACT AND LENDS EASILY FOR DUCTING ARRANGEMENT EFFECTIVE CLEANING OF HEAT-TRANSFER SURFACE BY SOOT BLOWING IS POSSIBLE.

• THE BASIC COMPONENT OF THE CONTINUOUSLY ROTATING CYLINDER, CALLED THE ROTOR, THAT IS PACKED WITH THOUSANDS OF SQUARE FEET OF SPECIALLY FORMED SHEETS OF HEAT TRANSFER SURFACES.

• AS THE ROTOR REVOLVES, WASTE HEAT IS ABSORBED FROM THE HOT EXHAUST GAS PASSING THROUGH ONE HALF OF THE SURFACE.

• THIS ACCUMULATED HEAT IS RELEASED TO THE INCOMING AIR AS THE SAME SURFACES PASS THROUGH THE OTHER HALF OF THE STRUCTURE. THE HEAT TRANSFER CYCLE IS CONTINUOUS AS THE SURFACES ARE ALTERNATELY EXPOSED TO THE OUTGOING GAS AND INCOMING AIR STREAMS.

• FUEL SAVINGS WITH THE LJUNGSTRÖM AIR PREHEATER ARE ABOUT 1-1½% FOR EVERY 4.4°C TO 10°C INCREASE IN COMBUSTION AIR TEMPERATURE, DEPENDING ON THE APPLICATION.

•THEIR SIMPLIFIED DESIGN AND OPERATING INTEGRITY ASSURE CONTINUOUS RELIABLE SERVICE THROUGHOUT THE LIFE OF THE PLANT.

BI-SECTOR

THE MAJORITY OF LJUNGSTRÖM AIR PREHEATERS SUPPLIED ARE IN THE BI- SECTOR DESIGN. THESE HEATERS HAVE TWO BASIC STREAMS, ONE OF GAS AND ONE OF AIR.

TRI-SECTOR TRI-SECTOR AIR PREHEATER PERMITS A SINGLE HEAT EXCHANGER TO PERFORM TWO FUNCTIONS: COAL DRYING AND COMBUSTION AIR HEATING.

THE DESIGN HAS THREE SECTORS – ONE FOR THE FLUE GAS, ONE FOR THE PRIMARY AIR THAT DRIES THE COAL IN THE PULVERIZER, AND ONE FOR SECONDARY AIR THAT GOES TO THE BOILER FOR COMBUSTION

BI-SECTOR TRI-SECTOR

HEATING ELEMENTS:

They are packed in containers called baskets, are placed in rotor in three tiers: - Hot, Intermediate and Cold.

The notches are used for maintaining the spaces between the elements and minimizing the pressure drop across the air pre-heater.

• HOT END BASKETS & HOT INTERMEDIATE : -

Hot End is the first layer & Hot Intermediate is second layer of heating element packing from hot end side. The elements are usually made from 24 gauge / 22 gauge(0.5 – 0.8MM) open hearth steel (IS 513 Gr. DD). They are having a profile called “Double Undulation. The notches run parallel with the rotor axis and provide the correct spacing of sheets and the undulations run at 60° to the notches to impart turbulence. Open-channel element, where the notches, which provide the required plate spacing, rest on a series of point contacts on the adjacent sheet. Flow can move across the element pair because there are openings between the two sheets along the flow length between point contacts.

COLD END BASKETS

The elements are made of 18 gauge / 22 gauge carton steel. Enameled elements are also used in severe corrosive conditions like for more percentage of sulphur in fuel or for low gas duct temperature. All the heating surface elements are packed into reversible containers called baskets to facilitate easy removal and handling. Cold end baskets are arranged for removal through the basket removal door in the housing. A closed element profile is the notched flat 6-mm element (NF6), the element pair is formed by a series of notches that rest on an adjacent flat sheet with contact along the total flow length. They provide the necessary spacing and form discrete individual flow channels of fixed cross-sectional area along the flow length or element depth. There is no flow communication from one channel to the adjacent one.

Sealing System Usually air leaks in to the gas in the air preheater due to pressure differences. This leakage air decreases the flue gas temperature without extracting the heat. It is an implied requirement that the rotating parts should have some working clearance between the static parts to avoid any interference between them.

Here, in air preheaters, rotors are constructed to have high clearance to take care of thermal expansion and these gaps are closed with the flexible seal leaves .

• Radial seals • Axial seals • Bypass seals • Circumferential seals

. Major types of seals used in power plant.

The main purpose of these seals is to reduce leakage between the gas and air

• Air in Leakage‐ ‐ (~13%)

• Gas Side Efficiency (~ 68 %)

• • X – ratio (~ 0.76)

• Flue gas temperature drop (~220 C)

• Air side temperature rise (~260C)

Air Heater - Performance Indicators (Indices)

REGERATIVE AIR PREHEATER

Increased AH leakage leads to

• Reduced AH efficiency

• Increased fan power consumption

• Higher gas velocities that affect ESP performance

• Loss of fan margins leading to in efficient operation and at times restricting unit loading

Leakage Paths

What is leakage ?

The leakage of the high pressure air to the lowpressure flue gas due to the Differential Pressure, increased seal clearances in hot condition, seal erosion/ improper seal settings.

➢ Direct – flow of air through gaps between rotating and fixed structure;

Leakage ≈ gap area x (density x ∆P)1/2;

➢ Entrained – volume of air in porous elements carried via rotation from air side to gas side

Rotor Turndown – HE grows radially more than the CE, rotor goes outward

and downward

Leakage whether direct or entrained has no effect on the heat transfer efficiency of air heater; the gas temperature leaving air heater decreases by mixing of the

cooler air with the flue gas.

Air heater Air-in-leakage

Typically air heater starts with a baseline leakage of 6 to 10% after an overhaul.

What we measure is mainly leakage through radial seals at

hot & cold end.

Leakage through circumferential seals is substantial and has a

major effect on heat transfer but nominal effect on APC.

Leakage is expressed as a % of inlet gas flow and not a % of fan input flow

Leakage

% Leakage

Weight of Air passing from Air Side to Gas Side (Gas Out Flow Gas In‐ Flow)

(Air Leakage / Gas In Flow) x 100

(%O2Gas Out %O‐ 2Gas In) / (21 %O‐ 2Gas out) x

90Air Temp Rise

Gas Temp Drop

Temperature Head

Increase in Temp of Air in passing through the AH = Tao Tai ‐Decrease in Temp of Gas in passing through the AH = Tgi Tgo ‐Temp of gas entering minus Temp of Air entering AH = Tgi ‐ Tai

Tgas out (no leakage) (AL * Cpa * (Tgas out Tair in) / Cpg*100) + Tgas‐ out

Gas Side Efficiency Ratio of Gas Temp Drop to Temperature Head. (Tgi Tgo) / (Tgi ‐ ‐ Tai)

Air side efficiency Ratio of Air Temp Rise to Temperature Head. (Tao ‐ Tai) / (Tgi ‐ Tai)

X ‐ Ratio Ratio of the Heat Capacity of air passing through the AH to Heat Capacity of the Gas passing through the AH

(Wao x CpA) / (Wgi x CpG) ; (Tgi Tgo) / (Tao ‐ ‐ Tai)

AH Performance Indices

11

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 * DTa = Wgas in * Cpg * DTg

X ratio = Wair out * Cpa = Tgas in Tgas out (no ‐ lkge) Wgas in * Cpg Tair out Tair‐ in

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 rato does ‐ not provide a measure of thermal performance of the air heater, but is a measure of the operatng conditons. A low X rato indicates ‐ either excessive gas weight through the air heater or that air flow is bypassing the air heater.

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Pressure drops across air heater

• Air & gas side pressure drops change approximately in proportion to the square of the gas & air weights through the air heaters.

• If excess air is greater than expected, the pressure drops will be greater than expected.

• Deposits / choking of the basket elements would lead to an increase in pressure drops

• Pressure drops also vary directly with the mean absolute temperatures of the fluids passing through the air heaters due to changes in density.

Air Heaters

Factors affecting performance include

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•

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Operating excess air levels & PA/SA ratio

Inlet air / gas temperature

Coal moisture

Air ingress

levels Soot

blowing

No. of mills in service & PA Header Pressure

Upstream ash evacuation

Maintenance practices - Condition of heating elements, seals / seal setting, sector plates / axial seal plates, diaphragm plates, casing / enclosure, insulation

Air Heater Exit Gas Temperatures

An increase in AH leakage causes dilution of flue gas & a drop in ‘As read’ exit gas temperatures; Gas temperatures need to be corrected to a reference ambient and to no leakage conditions for comparison

Other factors affecting exit gas temperatures are

• Entering air temperature - Any changes would change gas temp in same direction. (~10C rise in air temp ~ 10*0.7(Efficiency) = 7C rise in EGT)

• Entering Gas Temperature - Exit gas temp changes in the same direction (~10C rise in gas temp ~ 10*0.3 (1 - Efficiency) = 3C rise in EGT)

• X-ratio - An increase in X-ratio would decrease gas temps & vice versa

• Gas Weight - Increase in gas weight would result in higher exit gas temperatures

Air Heater Performance Assessment

Objectives

1.To determine Air heater performance indices – Leakage, Gas-side Efficiency & X-ratio

2. To provide information for performance analysis & identify the causes of performance degradation, if any

3. To cross-check the readings of online instruments around air heaters

• Leakage assessment must be done by a grid survey using a portable gas analyser. Online Leakage values should be used for relative comparison only; absolute values may not be correct.

• The O2 readings need to be weighted with velocity pressures taken from

a pitot traverse.

• Method of determination of O2 or CO2 should be the same at inlet and outlet wet or dry‐ (Orsat)

(O2 dry = O2 wet / (1 FG‐ Moisture))

Air Heater Leakage Assessment

• Gas analysis & Temp at APH inlet, outlet & at ID Fan Outlet

• Temperature of Primary & Secondary Air at AH in/outlet

• Power Measurement – Mills & Fans

• Control Room Data

AH Tests –Measurements

Measurement Locations

The number and type of instruments required for conducting this test depend on the unit being tested. The following table lists the measurement locations.

Measurement Temperature Gas Analysers Pressure

AH Gas Inlet Yes Yes Yes

AH Gas Outlet Yes Yes Yes

AH Air Inlet Yes Yes

AH Air Outlet Yes Yes

Test set up - Operating Conditions of Test Runs

Test runs are conducted at an easily repeatable level at defined baseline conditions at full load with same number of mills in service and same total air levels as previous tests. The operating conditions for each test run are as follows.

1. No furnace or air heater soot blowing is done during the test.

2. Unit operation is kept steady for at least 60 minutes prior to the test.

3. No mill change over is done during the test.

4. All air and gas side damper positions should be checked and recorded

5.The test is abandoned in case of any oil support during the test period.

6. Eco hopper de-ashing or Bottom hopper de-ashing is not done during the test.

7. Regenerative heaters should be in service.

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Sampling grid in Gas Ducts at AH inlet / outlet

100mm Gas Duct is divided into equal cross- sectional areas and gas samples are drawn to analyzers from each center using multi point probes or

point by point traverseSampling Points

3

5

4

6

8

7

A B C D E F A B C D E F

0

40

80

160

120

Variation of Oxygen & Temperature across AH Exit FG Ducts 200 MW (Nov '07)

9

200

Oxygen %Temperature C

Single point sampling for gas composition / temp may or

may not be representative of the conditions in flue gas

ducts

FLUE GAS COMPOSITION &TEMPERATURE

A representative value of flue gas composition (O2 / CO2 /CO) is obtained by grid sampling of the flue gas at multiple points in a plane perpendicular to the flow at air heater inlet and outlet using a portable gas analyser. Two complete sets of data are collected for each traverse plane during each test run to ensure data repeatability

• Flue gas samples are drawn by a vacuum pump from the test grid probes and sent to a portable gas analyzer through a gas conditioning system.

• Typically gas-conditioning system consists of a wash bottle, partially filled with water for cleaning the gas sample, a condenser to condense the water vapor out of the gas sample and a desiccant column to remove any water vapor that got through the condenser.

• Similarly a representative value of temperature is obtained by grid measurement of flue gas temperatures at multiple points in a plane perpendicular to the flow at air heater inlet and outlet using multi point probes.

• A single tube probe with a portable analyzer can also be used for traversing duct cross-section. Marking / etching is done on the sampling tube at d/6, d/2 & 5d/6, if d is the duct depth. The probe is inserted in each port & samples are drawn at different depths as per markings

Leakage Calculation

Air heater leakage is expressed as a percentage of gas flow entering the air heater. It’s determined by following equation.

AL = (CO2 ge – CO2gl) x 0.9 x 100

CO2 gl

AL = air heater leakage

CO2ge = percent CO2 in gas entering air heater

CO2gl = percent CO2 in gas leaving air heater

Alternatively, the air heater leakage may also be determined from the following equation:

AL = (O2 gl – O2 ge) x 0.9 x 100

(21 - O 2 gl)

AL = ? From following measured data

O2ge = percent O 2 in gas entering air heater (2.8 %)

O2gl = percent O 2 in gas leaving air heater (5.7 %)

Leakage Calculation

= (5.7 – 2.8) * 90 (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.

EXIT TEMPERATUR CORRECTION

Air heater leakage dilutes the flue gas and lowers the as measured exit gas temperatures. Gas outlet temperature corrected to no leakage condition is calculated using the following formula.

Tgnl = AL x Cpa x (Tgl – Tae) + Tgl

100 x Cpg

AL= Measured Air heater leakage (17.1%)

Tgnl = gas outlet temperature corrected for no leakage

Cpa = the mean specific heat between Tae and Tgl

Tae = temperature of air entering air heater (36.1 C)

Tgl = temp of gas leaving air heater (133.8 C)

Cpg = mean specific heat between Tgl and Tgnl

What is gas outlet temperature corrected to no leakage condition ?

Gas Side EfficiencyRatio of Gas Temperature drop across the air heater, corrected for no leakage, to the temperature head.

= (Temp drop / Temperature head) * 100

where Temp drop = Tgas in(333.5C) -Tgas out (no leakage)

Temp head = Tgasin - T air in(36.1) What is the gas side efficiency ?

Tgnl = 17.1 * (133.8 – 36.1) + 133.8 = 150.5 C 100

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 * DTa = Wgas in * Cpg * DTg

X ratio = Wair out * Cpa =Tgas in - Tgas out (no lkge) Wgas in * Cpg 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. What will be X-ratio ?

Gas Side Efficiency = (333.5-150.5) / (333.5-36.1) = 61.5 %

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

AIR HEATER PERFORMANCE TEST REPORT-200 MW

Station: Report date:25.10.10Unit: 2 Test Date:21.10.10

S.N RESULTS UNITS Design Last Test Current Test APH A/B APH-A APH-B

1 Air Preheater Leakage % 8.8 0.0 17.8 16.72 APH Gas Side Efficiency % 64.3 0.0 59.5 57.53 Air Preheater X-Ratio --- 0.77 0.00 0.71 0.674 EGT-Corr to ref air temp and lkge C 141.0 0.0 172.5 176.65 AH to ID fan Approx. Duct Leakage % 0.0 0.0 13.1 12.1

PARAMETERS Temperatures

1 Ambient Air C 50 0 33 332 Primary Air Inlet C 56 0 46.9 463 Secondary Air Inlet C 53 0 34.2 35.04 Primary Air Outlet C 318 0 298.8 303.55 Secondary Air Outlet C 313 0 297.3 296.36 Flue Gas Inlet C 343 0 347 342.37 Flue Gas Outlet C 134 0 144.8 149.88 EGT - Corr. for AH lkge C 140.7 162.9 167.59 EGT - Corr. for ref air inlet temp C 134.0 155.4 159.8

Avg. Oxygen in Flue Gas 10 AH Inlet % 3.87 0 2.84 3.511 AH Outlet % 5.26 0 5.84 6.212 ID fan Outlet % --- 0 7.8 8.0

DPs across AH 13 Pri Air Side mmwcl 43 0 45.5 44.814 Sec Air Side mmwcl 93 0 58.3 5515 Gas Side mmwcl 108 0 89 87.5

Test Condition 1 Load MW 200 0 2012 Coal Flow T/Hr 131.6 0 1323 Total Air Flow T/Hr 793 0 7144 Secondary Air Flow T/Hr 533 0 5105 Mills In Service No. 4 nos. 0 ABCE6 FD Fan Current A 0 0 25 207 PA fan Current A 0 0 96 968 ID Fan Current A 0 0 85 84

Air Heaters – Some Issues

• Replacement of baskets – No change in exit temperatures?

• Basket Replacement Criterion?• In situ cleaning of baskets?• Fly-ash erosion of peripheral HE Baskets?• Choking of CE Baskets?• Corrections to gas side efficiency?• SCAPH?• Restoration of inlet / outlet gas dampers?

Air Heater Performance Improvement Initiative

• Improved thermal performance of air heaters by Reduction in Air ingress from upstream ducts & expansion joints.

• Use of new design profile basket elements Use of new basket profiles can be explored for better heat recovery in air heaters

• Utlizaton of additonal heat transfer area: Also, both Primary & Secondary air heaters may have a space above hot end baskets, which can be utilized for increasing the height of the basket elements.

• Good practices

Air Heaters – Good Practices

• AH sootblowing immediately after boiler light up

• Monitoring of Lub oil of Guide & Support bearings through Quarterly wear-debris analysis

• Hot water washing of air heaters after boiler shutdown - flue gas temperature ~ 150 to 180 C with draft fans in stopped condition. (Ideally pH value can verify effective cleaning)

• Basket drying to be ensured by running draft fans for atleast six hours after basket washing

Air Heaters – Good Practices …contd

• Baskets cleaning with HP water jet during Overhauls after removal from position

• Timely basket replacements; Reversal of baskets not recommended

• Replace circumferential seals in every overhaul

• Hastealloy flexible seals sustain low leakages for 4-5 months; Use opportunity shutdowns for replacement

• Ensuring healthiness of flushing apparatus of Eco & AH ash hoppers

Flue Gas Bypass - Gaps between diaphragms & baskets to be closed for better heat recovery & lower erosion rate at edges

Excessive gaps between AH baskets & diaphragms, leading to gas bypass and erosionCold End BasketsIntermediate Baskets

HOT END

COLD END

HOT END

COLD END

Correct Design

Small Baskets

➢ Gas side circumferential seals erode with time from OH to OH

➢ Better to replace all the segments every overhaul

➢ New seals of fish scale design is recommended

New Type of seals

Heating Surface Element retrofits

• All our air heaters have DU & NF profile at Hot end & Cold end

• Potential for improvement by changing basket profiles

• Reduction in Air heater exit gas temperatures to 125C

Thank You