APH Performance Improvements

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 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

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 (%)

(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

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

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

= CO_2in - CO_2out * 0.9 * 100 CO_2out

= O_2out - O_2in * 0.9 * 100 = 5.7 – 2.8 * 90

(21- O_2out) (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

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 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 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

• 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 &

Air Heaters – Good Practices

• Replacement of baskets recommended when

Weight loss of heating element baskets > 20-30 % Thinning of element thickness > one-third

Erosion 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.

A _B C _D E _F S1 S2 S3 130 140 150 160 170 T e m p 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 F S1 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

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-leakage gas ducts’ air ingress

Double sealing retrofits with Fixed sealing plates

Before

After

Air heater Performance Enhancement

through Up gradations

Rotor modifications

Before

Typical 24 sector rotor design

After Rotor modified to 48 sectors

New axial seal

Heating Surface Element retrofits

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

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

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

Flue Gas W a te r 100 value calorific Gross x rate firing Fuel enthalpy) water feed enthalpy (steam x rate flow Steam x Efficiency Boiler ₌ −

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.

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

W at er S te a m

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

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

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

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 (S

)

S

= Mass of dry flue gas X Sp. Heat X (T

– T

)

Dry Gas loss reduction requires

• 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

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)

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 T_millout 90 C • Reference Temperature T_rai 30 C

• Specific Heat of Rejects Cp_REJECT 0.16 kcal/Kg/C Loss due to Mill Rejects = X / (Coal Flow * GCV * 1000) X = [Rejects * (CV_REJECT + Cp_REJECT (T_millout – T_rai))* 100 ]

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.