blr ah perf indices

7/31/2019 Blr AH Perf Indices

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Factors affecting Boiler Performance


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Boiler Performance Characterisation

Combustion / Thermal Efficiency- Conversionof chemical heat in fuel to production of steam

adequate Time / Temperature / Turbulence

Auxiliary Power Consumption The total powerbeing consumed by ID, FD, PA fans and the mills.


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OFF Design/Optimum Conditions

Parameter Deviation Effect on Heat

RateExcess Air (O2) per % 7.4 Kcal/kWhExit Gas Temp per

oC 1.2 Kcal/kWh

Unburnt Carbon per % 10-15 Kcal/kWhCoal moisture per % 2-3 Kcal/kWh

Boiler Efficiency per % 25 Kcal/kWh

Effect of Boiler side Parameters (Approx.)


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Boiler Control Volume


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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.


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Efficiency Vs HHV of

Coal

AssumptionsExit Gas Temp - Constt.

Fuel Moisture - Constt

Fuel Hydrogen - ConsttExcess Air - 20 %

GCV - 3700 kal/kg


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Proximate Analysis, Ultimate Analysis, CalorificValue, 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 %, Ash30.0 %, 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 electricalresistivity of ash,

Problem

The Coal


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CAsh H O N S Mi M

As fired basis

Air dry basis

Dry basis

Dry & Ash free basis

A FC VM MCoke Volatile

Ultimate

Proximate

Coal Composition -

Different bases of representation


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Coal characteristics decide the heat release rates,furnace wall conditions and consequently the

furnace heat transfer

Deterioration in Coal quality affects boiler capabilityto operate at rated parameters.

Change in coal quality affects capacity, efficiencyand combustion stability.

Increase in moisture affects mill drying, temperingair requirement, gas velocities, ESP & Boilerefficiency.

Ash quality / quantity affects boiler erosion, millwear, slagging and fouling propensity, ashhandling system, sprays, sootblowingrequirements etc.

Change in coal characteristics affects mill wearparts life & throughput of Pulverizers.


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PF fineness

Typical recommended value of pulverised fuel

fineness through 200 mesh Sieve is 70% and 1%

retention on 50 mesh sieve.

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 indicates the number of openings per

linear inch.


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Excessive PF fineness would cause

Reduction in mill capacity

Increased mill component wear

Increased mill and fan power combustion

Excessive PF fineness may not necessarily result

in improved combustion


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Control Room

Boiler

1

2 3

4

Mills

Mill discharge pipes offer different resistance to the flows dueto 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


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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.


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Burner Balance

Balanced PF flows are an essential pre-requisite to an

optimized combustion. Usually the imbalance gets

camouflaged by additional excess air, thereby losing out on

boiler efficiency and operating flexibility.


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Excess Air

Low excess air operation can lead to unstable combustion (furnace puffs)

increased 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


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Boiler Air Ingress

Cold air leaks into the boiler from openings in the furnace andconvective 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 zones

causes 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.


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

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

Air Ingress


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Boiler operation under adverse conditions continues as inmajority of units On line CO feedback is not available.

All boilers need to be equipped with On line CO monitors

at Eco Outlet / ID fan discharge.

Air ingress across AH outlet to ID suction observed to begenerally in the range of 5 to 9%.

Flue gas ducts & expansion joints at Eco outlet and APH

inlet / outlet inspected thoroughly during O/H

Replacement of Metallic / Fabric Expansion joints in 10

years / 5 years cycle

Air Ingress


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Air Heaters

Factors 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


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Air Heaters

Factors affecting performance include

PA Header Pressure

High 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 practices

Condition of heating elements, seals / seal setting,

sector plates / axial seal plates, diaphragm plates,

casing / enclosure, insulation


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Boiler Exit Gas Temperature

Ideal flue gas temperature at stack outlet should be just above thedew point to avoid corrosion; Higher gas temperatures reduce

efficiency; Possible causes of temperature deviations are

Dirty heat transfer surfaces High Excess air

Excessive casing air ingress

Fouled/corroded/eroded Air heaterbaskets

Non - representative measurement

Contd..


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Air Heaters - Exit Gas Temperatures

Factors affecting EGT include

Entering air temperature - Any changes would

change exit gas temperature in same direction

Entering Gas Temperature - Any changes wouldchange exit gas temperature in same direction

X-ratio - An increase in X-ratio would decrease exit

gas temperatures & vice versa

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

AH leakage - An increase in AH leakage causes

dilution of flue gas & a drop in As read exit gas

temperatures


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AUXILIARY POWER CONSUMPTION

Major auxiliaries Consuming Power in a Boiler areFD fans, PA fans, ID fans and mills. Reasons forhigher 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


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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.


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Testing Techniques & Performance Optimisation


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Boiler & Air Heater Tests

Tests to be conducted underdefined operating regime (O2

level / PA Header Pressure / no. of mills) at nominal load


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Pre Test Stabilisation Period

Prior to the test run, equipment must be operated at steadystate conditions to ensure that there is no net change in

energy stored in steam generator envelope.

Minimum Stabilisation Time - 1 hour


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Test Duration

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

When point by point traverse of Flue gas ducts is done, test

should be long enough o complete 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.


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Frequency of Observations

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


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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)


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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


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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 notaccurately 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


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Need for Off line Grid Measurement

On Line Instruments are adequate to monitor airheater performance but not good for assessing

degradation. PG tests also necessitates installation of

grid in air and flue gas ducts.

a) Flue gas O2 measurement at AH outlet is not

available

b) Single point Orsat can be misleading due to

stratification in flue gas

c) The grid also validates & cross checks

representative ness of online feedback


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FG

Economizer

FG

APH

Sampling

Locations

APH

Expansion

Bellow

Test Locations - AH Inlet & Outlet

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

AH hopper / Manhole air ingress can influence test data


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Sampling Ports in Flue Gas Ducts (Typical )

Sampling Point for Flue Gas Temperature & Composition

100mm

Flue Gas Duct is divided into equal cross-sectional areas and

gas samples are drawn from each center


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Typical problems

High Economiser / AH exit gas temperature Air ingress from furnace bottom, penthouse and

second pass

Boiler operation at high excess air

Metal temperature excursions

High Unburnt carbon in ashes

Uneven Flyash Erosion

Flame failures

Shortfall in steam temperatures

Imbalance in Left - Right steam temperatures


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Deterioration of Boiler efficiency and increase in auxiliary poweris generally on account ofAir Heater performancedegradation from O/H to O/H.

Major symptoms of this degradation include the following

Increased flue gas volume - Affects ESP performance

Lower flue gas exit temperatures due to high air heater leakage- An

erroneous boiler efficiency feedback generates complacency

Lower fan margins - Limit the unit output at times

Boileroperation at less than optimum excess air- Specially in units

where in ID fans are running at maximum loading

Air Heaters


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Air Heater - PerformanceIndicators

Air-in-Leakage

Gas Side Efficiency

X - ratio

Flue gas temperature drop Air side temperature rise

Gas & Air side pressure drops

(The indices are affected by changes in entering

air or gas temperatures, their flow quantities and

coal moisture)


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AH Performance Monitoring

O2 & CO2 in FG at AH Inlet

O2 & CO2 in FG at AH Outlet

Temperature of gas entering / leaving air heater Temperature of air entering / leaving air heater

Diff. Pressure across AH on air & gas side

(Above data is tracked to monitor AH performance)


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Air Heater Leakage (%)

The leakage of the high pressure air to the lowpressure flue gas is due to the Differential Pressure between

fluids, increased seal clearances in hot condition, seal erosion /

improper seal settings. Typically air heater starts with a

baseline leakage of 6 to 10% after an overhaul.

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 attimes restricting unit loading


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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 airheater 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)


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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 * 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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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 %


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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 out

Cpg * 100

Say AH leakage 17.1%, Gas In Temp 333.5 C, Gas Out Temp

133.8 C, Air In Temp 36.1 C

Tgasnl = 17.1 * (133.8 36.1) + 133.8 = 150.5 C100


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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 boilershutdown - 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 four hours after basket washing.


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Air Heaters

Baskets cleaning with HP water jet cleaning during

Overhauls after removal from position

Heating elements to be covered with templates during

maintenance of air heaters.

Gaps between diaphragms & baskets to be closed for

better heat recovery & lower erosion rate at edges.

Ensuring healthiness offlushing apparatus of Eco &

AH ash hoppers


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Boiler Performance

Boiler Efficiency

The % of heat input to the boiler absorbed by the

working fluid (Typically 85-88%)


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Boiler Efficiency

Boiler Efficiency can be determined by

a) Direct method or Input / Output method

b) Indirect method or Loss method


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Boiler Efficiency

Direct method or Input / Output method measures theheat 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 coalfired boilers, its difficult to accurately measure coal

flow and heating value on real time basis.

Another problem with direct method is that the extentand nature of the individual components losses is not

quantified.


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Boiler Efficiency

Indirect method or Loss method

For utility boilers efficiency is generally calculated by heat loss

method wherein the component losses are calculated andsubtracted from 100.

Boiler Efficiency = 100 - Losses in %


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Indirect or Loss method

In Heat Loss method 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.

Efficiency = 100 (L/Hf) * 100L losses

Hf heat input


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Indirect or Loss method

This method also requires accurate determination of

heating value, but since the total losses make a relatively

small portion of the total heat input (~ 13 %), an error in

measurement does not appreciably affect the efficiency

calculations.

In addition to being more accurate for field testing, the

heat loss method identifies exactly where the heat

losses are occurring.


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Boiler Efficiency

Commonly used standards for boiler performance testing are

ASME PTC 4 (1998)

BS 2885 (1974)IS: 8753: 1977

DIN standards


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rameters required for computing Boiler Efficiency

AH flue gas outlet O2 / CO2 / CO

AH flue gas inlet and outlet temp C

Primary / Secondary air temp at AH inlet / outlet C

Total Airflow / Secondary Air Flow t/hr

Dry/Wet bulb temperatures C

Ambient pressure bar a

Proximate Analysis & GCV of Coal kcal / kgCombustibles in Bottom Ash and Flyash


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Boiler Losses Typical values

Dry Gas Loss 5.21Unburnt Loss 0.63

Hydrogen Loss 4.22

Moisture in Fuel Loss 2.00

Moisture in Air Loss 0.19

Carbon Monoxide Loss 0.11

Radiation/Unaccounted Loss 1.00

Boiler Efficiency 86.63


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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 andFD fan inlet air temperature

Typically 20 C increase in exit gas temperature ~ 1%

reduction in boiler efficiency.


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Dry Gas Loss

Sensible 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


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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


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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

Burnerbalance / worn orifices

Primar Air Flow / Pressure


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Computation - Moisture Loss

Total Moisture Loss

= (9H+M) * Sw / GCV of Coal

Sw 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 tocorrosion considerations in the cold end of air heater, gas

ducts etc.


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