Performance Indices & Computations

Factors affecting AH & Boiler Performance

Performance Evaluation by Field Tests

Boiler & Air Heater Performance

Presentation Coverage

Air Heater - Performance Indicators

Air-in-Leakage (~13%) Gas Side Efficiency (~ 68 %) X ratio (~ 0.76) Flue gas temperature drop (~220 C) Air side temperature rise (~260C) Gas & Air side pressure drops

(The indices are affected by changes in entering air or gas temperatures, their flow quantities and coal moisture)

Air heater Air-in-leakageThe leakage of the high pressure air to the low pressure flue gas is due to the Differential Pressure between fluids, increased seal clearances in hot condition, seal erosion / improper seal settings.

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 inefficient operation and at

times restricting unit loading

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

Air Heater Leakage (%) Direct - Hot End / Cold End

(~ 60% through radial seals + 30% through Circumferential bypass)Air leakage occurring at the hot end of the air heater affects its thermal and hydraulic performance while cold end leakage increases fans loading.

Entrained Leakage due to entrapped air between the heating elements (depends on speed of rotation & volume of rotor air space)

Rotor Turndown- Hot end grows radially more than cold end, rotor goes outward and downward

Seals - close the gaps & minimize leakage

Leakage Assessment

Leakage assessment must be done by a grid survey using a portable gas analyser.

Calculation of leakage using CO2 values is preferred because of higher absolute values and lower errors.

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

Single point O2 measurement feedback using orsat is on dry basis while zirconia measurement is on wet basis.

Leakage assessment is impacted by air ingress from expansion joints upstream of measurement sections.

Air Heater Leakage - Calculation

This leakage is assumed to occur entirely between air inlet and gas outlet; Empirical relationship using the change in concentration of O2 or CO2 in the flue gas

= CO2in - CO2out * 0.9 * 100CO2out

= O2out - O2in * 0.9 * 100 = 5.7 2.8 * 90(21- O2out) (21-5.7)

= 17.1 %

Gas Side Efficiency

Ratio 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 -Tgas out (no leakage)Temp head = Tgasin - T air in

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

Tgas out (no leakage) = The temperature at which the gas would have left the air heater if there were no AH leakage

= AL * Cpa * (Tgas out - Tair in) + Tgas outCpg * 100

Say AH leakage 17.1%, Gas In Temp 333.5 C, Gas Out Temp 133.8 C, Air In Temp 36.1 CTgasnl = 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 * 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

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.

Air Heaters - Exit Gas Temperatures Factors affecting EGT include Entering air temperature - Any changes would change gas temperature in

same direction. (10C rise in air temp ~ 10*0.7(Efficiency) = 7C rise in EGT)

Entering Gas Temperature - Any changes would change exit gas temperature in same direction (10C rise in gas temp ~ 10*0.3 = 3C rise in EGT)

X-ratio - An increase in X-ratio would decrease exit gas temperatures & vice versa

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

AH leakage - An increase in AH leakage causes dilution of flue gas & a drop in As read exit gas temperatures; So, Exit gas temperatures need to be corrected to a reference ambient and to no leakage conditions for comparison.

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.

Boiler Performance

Boiler Efficiency

The % of heat input to the boiler absorbed by the working fluid (Typically 85-88%)

Direct Method

BoilerFuel+ Air

Stea

m

Efficiency = Heat addition to Steam x 100Gross Heat in Fuel

Flue Gas

Wa

ter

100 valuecalorific Gross x rate firing Fuel

enthalpy) water feed enthalpy (steam x rate flow SteamxEfficiencyBoiler =

Boiler EfficiencyDirect method or Input / Output method measures the heat absorbed by water & steam & compares it with the total energy input based on HHV of fuel. Direct method is based on fuel flow, GCV, steam flow

pressure & temperature measurements. For coal fired boilers, its difficult to accurately measure coal flow and heating value on real time basis.

Another problem with direct method is that the extent and nature of the individual components losses is not quantified.

Boiler Efficiency

Indirect method or Loss method

For utility boilers efficiency is generally calculated by heat loss method wherein the component losses are calculated and subtracted from 100.

Boiler Efficiency = 100 - Losses in %

Indirect Method

Boiler Flue gas

Efficiency = 100 (1+2+3+4+5+6+7+8)

Fuel + Air

1. Dry Flue gas loss2. H2 loss3. Moisture in fuel4. Moisture in air5. CO loss

7. Fly ash loss

6. Radiation

8. Bottom ash lossW

ater

Stea

m

The unit of heat input is the higher heating value per kg of fuel. Heat losses from various sources are

summed & expressed per kg of fuel fired.

Boiler Efficiency

Commonly used standards for boiler performance testing are

ASME PTC 4 (1998)BS 2885 (1974)IS: 8753: 1977DIN standards

Boiler Losses Typical valuesDry Gas Loss 5.21Unburnt Loss 0.63Hydrogen Loss 4.22Moisture in Fuel Loss 2.00Moisture in Air Loss 0.19Carbon Monoxide Loss 0.11Radiation/Unaccounted Loss 1.00

Boiler Efficiency 86.63

Dry Gas Loss (Controllable)

This is the heat carried away by flue gas at AH outlet

Its a function of flue gas quantity and the temperature difference between air heater exit gas temperature and FD fan inlet air temperature

Typically 20 C increase in exit gas temperature ~ 1% reduction in boiler efficiency.

Dry Gas LossSensible Heat of flue gas (Sh)

Sh = Mass of dry flue gas X Sp. Heat X (Tfg Tair)

Dry Flue Gas Loss % = (Sh / GCV of Fuel) * 100

Dry Gas loss (DFG) reduction requires Operation at optimum excess air Hi O2 ~ Hi DFG

Cleanliness of boiler surfaces Dirty tubes ~ Hi EGT

Good combustion of fuel

Reduction of tempering air to mill.

Reduction in air ingress

Representative Measurements

Cleaning of air heater surfaces and proper heating elements / surface area

Unburnt Carbon Loss (Controllable)

The amount of unburnt is a measure of effectiveness of combustion process in general and mills / burners in particular.

Unburnt carbon includes the unburned constituents in flyash as well as bottom ash.

Ratio of Flyash to Bottom ash is around 80:20 Focus to be on flyash due to uncertainty in repeatability and

representative ness of unburnt carbon in bottom ash +50 PF fineness fractions to be < 1-1.5%

Unburnt Carbon Loss (Controllable)

Loss due to Unburnt Carbon= U * CVc * 100 / GCV of Coal

CVc CV of Carbon 8077.8 kcal/kg

U = Carbon in ash / kg of coal = Ash * C (Carbon in coal)

100 100 - C

Influencing Factors - Unburnt Carbon Loss Type of mills and firing system Furnace size

Coal FC/VM ratio, coal reactivity Burners design / condition PF fineness (Pulveriser problems) Insufficient excess air in combustion zone Air damper / register settings Burner balance / worn orifices Primary Air Flow / Pressure

Moisture Loss

Fuel Hydrogen LossThis loss is due to combustion of H present in fuel. H is burnt and converted in water, which gets evaporated.

Fuel Moisture LossThis loss is due to evaporation and heating of inherent and surface moisture present in fuel. (Can be reduced by judicious sprays in coal yards)

Computation - Moisture Loss

Total Moisture Loss= (9H+M) * Sw / GCV of CoalSw Sensible Heat of water vapour= 1.88 (Tgo 25) + 2442 + 4.2 (25 - Trai)

The moisture in flue gases (along with Sulphur in fuel) limits the temperature to which the flue gases may be cooled due to corrosion considerations in the cold end of air heater, gas ducts etc.

Other Losses1. Sensible Heat Loss of ash

Bottom Ash Hoppers Eco Hoppers AH Hoppers ESP hoppers

Sensible Heat Loss (%) = (X / GCV) *100 (~0.5-0.6 %)

X = [{Ash * Pflyash * C pash * (T go - T rai)} + {Ash * Pahash * C pash * (T go - T rai)} + {Ash * Peash * C pash * (T gi -T rai )}+ {Ash * Pba * C pash * (T ba - T rai )}]

Other Losses2. Radiation Loss through Bottom Ash Hopper

Coal Flow Rate 135 Tons/Hr GCV of Coal 3300 Kcal/Kg Eqv. Heat Flux thro Bottom opening 27090 Kcal/hr/m2

Bottom opening area of S-Panel 15.85 m2

Radiation Loss through Bottom Ash Hopper =[H BOTTOM * A S-PANEL *100 ] / [Coal Flow * GCV * 1000] = 0.096 %

Other Losses3. Coal Mill Reject Loss

Coal Flow 135 T/hr Coal Mill Rejects 200 kg/hr GCV of Coal 3300 kcal/Kg CV of Rejects 900 kcal/Kg Mill Outlet Temp Tmillout 90 C Reference Temperature Trai 30 C Specific Heat of Rejects CpREJECT 0.16 kcal/Kg/C

Loss due to Mill Rejects = X / (Coal Flow * GCV * 1000)X = [Rejects * (CVREJECT + CpREJECT (Tmillout Trai))* 100 ]

= (0.0408 %)

Other Losses4. Radiation Loss

Actual radiation and convection losses are difficult to assess because of particular emissivity of various surfaces.

HEAT CREDIT

Heat Credit due to Coal Mill Power = [MP * 859.86 * 100] / [Coal Flow * GCV * 1000]

Coal Flow Rate Coal FLOW Tons/HrTotal Coal Mill Power MP kWhGCV of Coal Kcal/Kg

Factors affecting AH & Boiler Performance

OFF Design/Optimum Conditions

Parameter Deviation Effect on Heat Rate

Excess Air (O2) per % 7.4 Kcal/kWh Exit Gas Temp per oC 1.2 Kcal/kWh Unburnt Carbon per % 10-15 Kcal/kWh Coal moisture per % 2-3 Kcal/kWh Boiler Efficiency per % 25 Kcal/kWh

Effect of Boiler side Parameters (Approx.)

Boiler Control Volume

Factors affecting Boiler efficiency include

Design Coal Quality Mill Performance - PF Fineness Burner-to-burner PF balance Excess Air Level Boiler Air Ingress AH Performance Furnace / Convective section Cleanliness Quality of Overhauls Water Chemistry, boiler loading, insulation etc.

Efficiency Vs Moisture in Coal

AssumptionsExit Gas Temp - Constt.Fuel Moisture - 20.5 %Excess Air - 20 %GCV - 3700 kal/kg

Efficiency Vs Hydrogen in Coal

AssumptionsExit Gas Temp - Constt.Fuel Hydrogen - 2.33 %Excess Air - 20 %GCV - 3700 kal/kg

Efficiency Vs HHV of Coal

AssumptionsExit Gas Temp - Constt.Fuel Moisture - ConsttFuel Hydrogen - ConsttExcess Air - 20 %GCV - 3700 kal/kg

Efficiency Vs Excess Air

AssumptionsExit Gas Temp - Constt.Ambient Temp - 27 CGCV - 3700 kal/kg

Efficiency Vs Ambient Temp / RH

AssumptionsExit Gas Temp - Constt.Excess Air - 20 %GCV - 3700 kal/kg

Proximate Analysis, Ultimate Analysis, Calorific Value, Ash

Constituents, Ash Fusion Temperatures, FC/VM ratio, Hard Grove

Index, YGP (Yeer Geer Price) Index

Typical Proximate Coal Analysis - Fixed Carbon - 32.4 %, Volatile

matter - 21.6 %, Moisture 16.0 %, Ash 30.0 %, GCV 4050 kcal/kg

+ve aspects - Low Sulfur, Low chlorine, Low iron content and High Ash

fusion temp

-ve aspects - High ash, moisture, high silica / alumina ratio, low calorific

value, high electrical resistivity of ash,

Problem

Variation in heating values, moisture, ash content and volatile matter

The Coal

FACTORS AFFECTING MILL PERFORMANCE

0

0.4

0.8

1.2

1.6

60 65 70 75 80 85 90 95 100

FINENESS - % THRU 200 MESH

C

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0.85

0.9

0.95

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0 4 8 12 16 20% MOISTURE

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HARDGROOVE INDEX (HGI)

M

I

L

L

O

U

T

P

U

T

X

1

0

0

%

GRINDABILITY (HGI)

FINENESS

MOISTURE

SIZE OF RAW COAL

MILL WEAR (YGP)

MTC PRACTICES

PF fineness

Fineness is expressed as the percentage pass through a 200-mesh screen (74m).

Coarseness is expressed as the percentage retained on a 50-mesh screen (297m).

Screen mesh - number of openings per linear inch.

Typical recommended value of pulverised fuel fineness through 200 mesh Sieve is 70% and 1% retention on 50 mesh sieve.

Flyash is over 80% of total ash, So its important to test for unburntcarbon; For monitoring unburnts in bottom ash, a visual in shift beginning or after mill change overs is good enough.

PF fineness is influenced by

Coal Quality, Mill settings, mill problems

PA flows / velocities

Sampling Techniques

Conventional Cyclone / ASME Sampler

64 point rotary sampler

Sampling location Preferably near burner from all the pipes

Testing Preferably using a motorised sieve shaker

Mill loading Always at Nominal / defined loading of the mill.

Control Room

Boiler

1

2 3

4

Mills

Mill discharge pipes offer different resistance to the flows due to unequal lengths and different geometry layouts.

Fixed orifices are put in shorter pipes to balance velocities / dirty air flow / coal flows. The sizes of the orifices are specified by equipment supplier.

A B C D E F

Burner Imbalance

TANGENTIAL FIRINGUneven fuel and air distribution can result in

High unburnt carbon in flyash Non - uniform release and

absorption of heat across the furnace resulting in temperature imbalance

Reducing furnace leading to slagging and fouling

High furnace and boiler exit gas temperatures

Water wall wastage and tube metal overheating

Burner Imbalance

Primary Air Flow

Coal Flow

Dirty air flow distribution should be with in +/- 5.0%of the average of fuel pipes

Coal distribution should be with in +/-10% of the average of fuel pipes

Balanced Clean air flows do not necessarily result in balanced Dirty air flows.

Excess Air

Typically 20 % excess air is recommended for boiler operation; Actual optimal value would vary from boiler to boiler depending on coal quality, fineness and other operating practices.

Optimum level of oxygen could be less than value specified by OEM.

O2 instruments are installed at the economizer exit, where they can be influenced by air infiltration. The O2 reading in control room may not be necessarily representative of the actual O2 in furnace.

Excess Air - CO monitors a must for boilers

C + O2 = CO2 + 8084 kcal / kg of Carbon2C+ O2 = 2CO + 2430 kcal / kg of Carbon2H2+ O2 = 2H2O + 28922 kcal / kg of HS + O2 = SO2 + 2224 kcal / kg of Sulphur

All boilers need to be equipped with On line CO monitors at Eco Outlet / ID fan discharge. We lose 5654 kcal for each kg of CO formed.

Ideally, average CO at gooseneck after combustion completion should be below 100 ppm and no single value over 200 ppm

Excess Air

Low excess air operation can lead to unstable combustion (furnace puffs) slagging of waterwalls and SH sections loss in boiler efficiency due to increased CO / unburnt

combustibles

High excess air operation can lead to Increased boiler losses High SH / RH temperatures Higher component erosion

Boiler Air Ingress Cold air leaks into the boiler from openings in the furnace and

convective pass and through open observation doors. Some of the boiler leakage air aids the combustion process;

some air that leaks into the boiler in the low temperature zonescauses only a dilution of the flue gas.

This portion of air appears as a difference in O2 level between the furnace exit and oxygen analysers at economizer exit. Actual oxygen in the furnace could be much less.

Also, boiler casing and ducting air ingress affects ID fans power consumption and margins in a major way.

Air-in-leakage

Furnace Outlet

Zirconia O2 Probe

AH Seal Lkg

ESP

Expansion Joints

Air Ingress Points Furnace Roof , Expansion joints, Air heaters, Ducts, ESP Hoppers, Peep Holes, Manholes, Furnace Bottom

Typical Air ingressPenthouse & 2nd pass ~ 0-5%Air heaters ~ 12-20% (tri sector)AH outlet to ID suction ~ 5 to 9%.

The difference between oxygen at furnace outlet (HVT) and economizer outlet (zirconia) was in the range of 1.0 to 2.5 % in many boilers.

Apart from degradation of AH baskets performance, another reason for lower heat recovery across air heaters is boiler operation at lesser SA flows due to high air-in-leakage.

Replacement of Metallic / Fabric Expansion joints in 10 years / 5 years cycle recommended.

Boiler Air Ingress

Boiler Air Ingress Track ID Amperages from OH to OH

Air ingress can be quantified by the increase in oxygen % in flue gas; The temperature drop of the flue gas from air heater outlet to ID fan discharge also provides an indication of the same.

Oxygen in Flue Gas (%) - 200 MW (Nov'07)

0123456789

Furnace Exit Eco Outlet AH Inlet AH Outlet ID Fan Outlet

Penthouse Air-In-Leakage 210 MW Unit

Penthouse Ash deposit in some boilers are reportedly as high as 5 feet and it takes up a substantial time of overhaul duration for this ash to be

cooled and removed for inspection work to start.

Ash Level

One possible reason of Penthouse Ash

Many Ceiling Tubes (binder tubes for Platen & Pendants) sag causing the roof seals to fail and ash to infiltrate the penthouse.

Ceiling Tubes

Sagging tube

Gas ducts downstream AHs Eroded Metallic Expansion Joints

Eroded Ducts & Expansion Joints

210 MW Bottom Hopper Seal Trough

Large cracks in seal plates and trough connections to hoppers

Air HeatersFactors affecting performance include Operating excess air levels PA/SA ratio Inlet air / gas temperature Coal moisture Air ingress levels Sootblowing No. of mills in service

(Contd)

Air HeatersFactors affecting performance include

PA Header PressureHigh pressure results in increased AH leakage, higher ID fan loading, higher PA fan power consumption, deteriorates PF fineness & can increase mechanical erosion

Upstream ash evacuation

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

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 ~ 180 to 150 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 Heating elements to be covered with templates during

maintenance of air heaters Timely basket replacements; Reversal of baskets not

recommended Gaps between diaphragms & baskets to be closed

for better heat recovery & lower erosion rate at edges Ensuring healthiness of flushing apparatus of Eco &

AH ash hoppers

Flue Gas Bypassing Air heaters

Excessive gaps between AH baskets

& diaphragms, leading to gas

bypass and erosion

Cold End BasketsIntermediate Baskets

HOT END

COLD END

HOT END

COLD END

Correct Design Small Baskets

Double sealing retrofits with Fixed sealing platesBefore

After

Air heater Performance Enhancement through Up gradations

(Slide Howden)

Double Sealing

(Slide Howden)

Rotor modifications

BeforeTypical 24 sector rotor design

AfterRotor modified to 48 sectors

New axial seal carrying bars fitted

(Slide Howden)

Flexible seal assembly - Cold Condition

Flexible seal assembly - Hot Condition

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 have been recommended by a consultant

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

Gaps

Hot End

Hot Intermediate

Cold End

Additional Surface area in 150mm gaps in Hot End

AUXILIARY POWER CONSUMPTION

Major auxiliaries Consuming Power in a Boiler are FD fans, PA fans, ID fans and mills. Reasons for higher APC include

* Boiler air ingress* Air heater air-in-leakage* High PA fan outlet pressure* Degree of Pulverisation* Operation at higher than optimum excess air

Main Steam/ Reheated Steam Temperature

While an increase in steam temperatures is beneficial to Turbine Cycle Heat Rate, theres no benefit to boiler efficiency, infact it affects reliability adversely.

Performance Evaluation by Field Tests

Test Objective To generate feedback for opn & mtc.

To determine current AH & boiler efficiency levels To determine each component of the heat loss to find

the reasons for deterioration To establish the cost / benefit of annual boiler O/H To establish baseline performance data on the

boiler after major equipment modifications To build a database for problem solving and

diagnosis

Suggested Frequency of Testing

QuarterlyBoiler Efficiency

Pre/Post O/H & Six monthly

FG Path O2mapping

QuarterlyAH Perf. Test

Pre/Post O/HDirty Air FlowFrequency

Boiler & Air Heater TestsTests to be conducted under defined operating regime (O2level / PA Header Pressure / no. of mills) at nominal load

Pre Test Stabilisation Period

Prior to the test run, equipment must be operated at steady state conditions to ensure that there is no net change in energy stored in steam generator envelope.

Minimum Stabilisation Time - 1 hour

Pre Test Checks

Sootblowing completed at least one hour before start of the test Steam coil air preheaters steam supply kept isolated All feedwater heaters in service with normal levels, vent settings

and with normal drain cascading No sootblowing or mill change over during the test. In case oil guns

are used, the test shall be repeated Air heater gas outlet dampers are modulated to ensure minimum

opening of cold air dampers to mills Auxiliary steam flow control kept isolated or defined during the test. CBD / IBD blowdowns kept isolated for the test duration Bottom hopper deashing after completion of test and not during the

tests

Test Duration

Should be sufficient to take care of deviations in parameters due to controls, fuel variations & other operating conditions.

When point by point traverse of Flue gas ducts is done, test should be long enough for atleast two traverses.

In case of continuous Data Acquisition System & use of composite sampling grids, shall be based on collection of representative coal & ash samples.

Could be 1/2 to 2 hours in case of parametric optimisation tests or 4 hours for Acceptance Tests.

Frequency of Observations

Parameter readings to be taken at a maximum interval of 15 minutes & a preferred interval of 2 minutes or less

Measurements during a Boiler Test Coal Sample for Proximate analysis & GCV Bottom Ash and Flyash Samples Flue Gas Composition at AH Outlet Flue Gas Temperature at AH Inlet / Outlet Primary / Secondary air temp at AH inlet / outlet Dry / Wet bulb temperatures Control Room Parameters

(All measurements / sampling to be done simultaneously)

Coal Sampling

Coal Samples are drawn from all individual running feeders from sampling ports in feeder inlet chutes

Composite sample is collected from all running feeders

One sample is sealed in an air tight container for total moisture determination

Flyash Sampling

Flyash is collected in several hoppers as Flue Gas goes to stack; Heavier particles fall out first due to turns in gas stream

Relative distribution of ash to various hoppers is not accurately known

Preferred way to collect a) a representative sample b) sample of the test period is to use High Volume Sampler probes on both sides of boiler

High Volume Sampler

This sampler uses 2-3 ksc air through an aspirator to create vacuum to pull out a large volume of flue gas & ash into probes canister; A filter catches the ash but allows the gas to pass through.

Bottom Ash Sampling

Bottom ash samples are collected every 15 minutes from the scrappers system during the test

In case of impounded hoppers, incremental samples are collected from bottom ash hoppers disposal line at slurry discharge end

Unburnt carbon is determined as LOI (Loss on Ignition)

FG

Economizer

FG

APHSamplingLocations

APH

ExpansionBellow

Test Locations - AH Inlet & Outlet

Inlet Sampling plane to be as close to AH as possible; Outlet grid to be a little away to reduce stratification

AH hopper / Manhole air ingress can influence test data

Sampling Ports in Flue Gas Ducts (Typical )

Sampling Point for Flue Gas Temperature & Composition

100mm

Gas Duct is divided into equal cross-sectional areas and gas samples are drawn from each center using multi point

probes or point by point traverse

HVT - High Velocity Thermocouple Probe - A Diagnostic Tool

To establish furnace gas exit temp profile

To establish CO & O2 profile at furnace outlet

To confirm proper distribution of fuel and air

To quantify air ingress between furnace outlet and AH inlet

Variation of Oxygen & Temperature across the Furnace at 203 MW (Nov'07)

0

1

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1400

Oxygen %Temperatrue C

Variation of Oxygen & Temperature across the Furnace at 197 MW (Nov'07)

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Oxygen %Temperatrue C

HVT Feedback 200 MW (Nov07)

Variation of Oxygen & Temp across at RH Inlet Left & Right side 210 MW (May'07)

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UCB O2 (L/R): 1.8/2.1 %After Zir. calibration: 3.4/3.35 %

Excess air is amongst the most important factors affecting boiler performance; Cross-check with HVT

Design PGT A B A B A BAir Temp Rise C 230 228 228 221 222 217 219 222Gas Temp Drop C 200 185 165 162 166 155 155 158Leakage % 8.8 6.6 15.9 16.6 15.4 16.9 16.5 18.4Gas Out Temp (NL) C 146.8 164.5 190 188 182 195 185 188X ratio % 0.83 0.73 0.64 0.64 0.67 0.61 0.62 0.61Gas Side Efficiency % 62.6 56.1 49.1 49.1 50.2 45.4 47.9 47.5

Unit 1 Unit 2 Unit 3

High air temp rise Low gas temp drop High AH leakages Low X-ratio

Increased air flows ~ better heat recovery across Air Heaters Constraint ID fan margins - reduction in AH leakage

boiler casing air-in-leakagegas ducts air ingress

Case Study Air Heaters

THANKS