Air Heater Performance

Presentation Coverage

Performance Indices & Assessment

AH Performance Enhancement Options

Calculation of Boiler Efficiency - Sample Calculations

• Boiler efficiency and APC deteriorate with Air Heater performance degradation from O/H to O/H.

• The symptoms include

�Lower fan margins – (ID amperes 95 to 135A)

�Lower gas exit temperatures due to high AH leakage

� Increased flue gas volume - affects ESP performance

�Boiler operation at less than optimum excess air - Specially in units where in ID fans are running at maximum loading

Air Heaters

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)

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)

Air heater Air-in-leakage

All units that operate with a rotary type regenerative air

heater experience some degree of air leakage across the air heater seals.

An increase in air leakage across the seals of an AH results

in increased ID and FD fan power and flow rate of flue gas. Sometimes it can put limitations on unit loading as well.

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

Air Heater Leakage (%)

The 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

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)

Rotary Air heater

RADIAL SEAL

AXIAL

SEAL

BYPASS SEAL

COLD END

HOT END

HOT INTERMEDIATE

SEALS ARRANGEMENT

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 %

CO2 measurement is preferred due to high absolute values; In case of any measurement errors, the resultant influence on

leakage calculation is small.

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 C

Tgasnl = 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

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

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.

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 - 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 = 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

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 four 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

• 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

Air Heaters – Good Practices …contd

• Replacement of baskets recommended when

Weight loss of heating element baskets > 20-30 %

Thinning of element thickness > one-thirdErosion of heating elements is > 50 mm depth

Trends of Gas side and air side efficiency before and

after Overhaul may also supplement the replacement decision.

• Reversal of baskets not recommended

A B C D E FS1

S2

S3130

140

150

160

170

Te

mp

C

Probes

Temperature Stratification in AH Outlet

FG Duct (Trisector Air heater)

160.0-170.0

150.0-160.0

140.0-150.0

130.0-140.0

A B C D E FS1

S3

3

4

5

6

7

8

%

Probe

O2 Stratification at AH Outlet FG Duct

7-8

6-7

5-6

4-5

3-4

Grid sampling is needed for

correct assessment of gas

temperature & composition at AH outlet due to

stratification in flue gas

Stratification in Gas

ducts at AH outlet

Air Heaters

• Thermocouples for flue gas temperatures at AH inlet as well

as exit are generally clustered on one side.

• A grid survey is needed for representative values.

• Exit gas temperatures need to be corrected to a reference

ambient and to no leakage conditions for comparison.

• Thermocouples for SA temperature measurement at AH

outlet are mounted too close to air heaters and need to be

relocated downstream to avoid duct stratification.

• Additional mill or changes in coal quality change thermal

performance of a tri-sector air heater in a very major way;

performance evaluation is difficult.

It’s worthwhile to re-look at all the

instrumentation around Air heaters for air

temperatures / Flue gas composition &

temperature measurement.

The unit operation, equipment efficiency

assessments and maintenance decisions are

based on the same.

Design PGT A B A B A B

Air Temp Rise C 230 228 228 221 222 217 219 222

Gas Temp Drop C 200 185 165 162 166 155 155 158

Leakage % 8.8 6.6 15.9 16.6 15.4 16.9 16.5 18.4

Gas Out Temp (NL) C 146.8 164.5 190 188 182 195 185 188

X ratio % 0.83 0.73 0.64 0.64 0.67 0.61 0.62 0.61

Gas 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

Double sealing retrofits with Fixed sealing plates

Before

After

Air heater Performance Enhancement

through Up gradations

Double Sealing

Rotor modifications

Before

Typical 24 sector rotor design

After

Rotor modified to 48 sectors

New axial seal carrying bars fitted

Flexible seal assembly - Cold Condition

Flexible seal assembly - Hot Condition

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

Minimum Basket

Hot End

Hot Intermediate

Cold End

Additional Surface area & 150mm height HE baskets

Boiler Performance

Boiler Efficiency

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

Boiler Efficiency…

Boiler Efficiency can be determined by

a) Direct method or Input / Output method

b) Indirect method or Loss method

Direct Method

BoilerFuel

+ Air

Ste

am

Efficiency = Heat addition to Steam x 100

Gross Heat in Fuel

Flue

Gas

Wa

ter

100 valuecalorific Gross x rate firing Fuel

enthalpy) water feed enthalpy (steam x rate flow SteamxEfficiencyBoiler

−

=

Boiler Efficiency…

Direct 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, it’s 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 loss

2. H2 loss

3. Moisture in fuel

4. Moisture in air

5. CO loss

7. Fly ash loss

6. Radiation

8. Bottom ash loss

Wat

erS

tea

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.

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.

Boiler Efficiency…

Commonly used standards for boiler performance testing are

ASME PTC 4 (1998)

BS – 2885 (1974)

IS: 8753: 1977

DIN standards

Parameters 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 / kg

• Combustibles in Bottom Ash and Flyash

Boiler Losses Typical values

Dry Gas Loss 5.21

Unburnt 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

Dry Gas Loss (Controllable)

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

• It’s 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 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

Dry Gas loss reduction requires

• Boiler operation at optimum excess air

• Cleanliness of boiler surfaces

• Good combustion of fuel

• Reduction of tempering air to mill.

• Reduction in air ingress

• 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 Loss

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

Fuel Moisture Loss

This 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 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 to corrosion

considerations in the cold end of air heater, gas ducts etc.

Other Losses

1. 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 Losses

2. 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 Losses

3. 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 Losses

4. 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/Hr

Total Coal Mill Power MP kWh

GCV of Coal Kcal/Kg

Computations

• Two Excel spreadsheets for determination of Boiler and Air Heater performance indices are being

provided with this presentation.

• These also include methodology for correcting these

indices for deviation in coal quality and ambient temperature from design.

• The operating equipment performance should be

corrected for boundary conditions before comparison with design parameters.

THANKS