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Air Heater Performance Presentation Coverage Performance Indices & Assessment AH
Performance Enhancement Options Calculation of Boiler Efficiency - Sample
Calculations

2. Air Heaters · Boiler efficiency and APC deteriorate with Air Heater performance
degradation fr om O/H to O/H. · The symptoms include Lower fan margins ± (ID
amperes 95 to 135A) Lo wer gas exit temperatures due to high AH leakage
Increased flue gas volume - aff ects ESP performance Boiler operation at less
than optimum excess air - Speciall y in units where in ID fans are running at
maximum loading

3. Air Heater - Performance Indicators · Air-in-Leakage (~13%) · Gas Side
Efficiency (~ 68 %) · X ± ratio (~ 0.76) · Flue gas tem perature 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)

4. AH Performance Monitoring · · · · · O2 & CO2 in FG at AH Inlet O2 & CO2 in
FG at AH Outlet Temperature of gas enteri ng / leaving air heater Temperature of
air entering / leaving air heater Diff. P ressure across AH on air & gas side
(Above data is tracked to monitor AH performance)

5. Air heater Air-in-leakage All units that operate with a rotary type regenerative
air heater experience som e 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. Typicall y air heater starts with a baseline leakage of 6 to
10% after an overhaul.

6. 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 A H efficiency · Increased fan power consumption
· Higher gas velocities that affect E SP performance · Loss of fan margins
leading to inefficient operation and at times restricting unit loading

7. 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 h ydraulic 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)

8. Rotary Air heater BYPASS SEAL RADIAL SEAL HOT END AXIAL SEAL COLD END HOT
INTERMEDIATE

9. SEALS ARRANGEMENT

10. 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 u sing orsat is on dry basis while
zirconia measurement is on wet basis. Leakage a ssessment is impacted by air
ingress from expansion joints upstream of measureme nt sections.

11. Air Heater Leakage - Calculation This leakage is assumed to occur entirely
between air inlet and gas outlet; Empi rical relationship using the change in
concentration of O2 or CO2 in the flue ga s = CO2in - CO2out * 0.9 * 100 CO2out
= O2out - O2in * 0.9 * 100 (21- O2out) = 5.7 ± 2.8 * 90 (21-5.7) = 17.1 % CO2
measurement is preferred due to high absolute values; In case of any measure
ment errors, the resultant influence on leakage calculation is small.

12. Gas Side Efficiency Ratio of Gas Temperature drop across the air heater,
corrected for no leakage, t o 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 %

13. 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

14. X ± Ratio Ratio of heat capacity of air passing through the air heater to the
heat capacit y of flue gas passing through the air heater. = Wair out * Cpa Wgas
in * Cpg Tga s 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

15. X-Ratio depends on · · · moisture in coal, air infiltration, air & gas mass
flow rates leakage from the s etting specific heats of air & flue gas X-ratio
does not provide a measure of th ermal performance of the air heater, but is a
measure of the operating condition s. 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 e xcessive tempering air to the mills or
excessive boiler setting infiltration.

16. Pressure drops across air heater · Air & gas side pressure drops change
approximately in proportion to the square o f the gas & air weights through the
air heaters. If excess air is greater than e xpected, 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 thr ough the air heaters due to
changes in density. · · ·

17. Air Heaters - Exit Gas Temperatures Factors affecting EGT include · Entering
air temperature - Any changes would chang e 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 i n same direction (10C rise in
gas temp ~ 10*0.3 = 3C rise in EGT) · X-ratio - An i ncrease 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

18. 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 a ir heaters after boiler shutdown -
flue gas temperature ~ 180 to 150 C with draf t fans in stopped condition.
(Ideally pH value can verify effective cleaning) Ba sket drying to be ensured by
running draft fans for atleast four hours after bas ket washing ·

19. Air Heaters ± Good Practices ¼contd · · · · 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

20. Air Heaters ± Good Practices ¼contd · Replacement of baskets recommended when
Weight loss of heating element baskets > 20-30 % Thinning of element thickness >
one-third Erosion of heating elements i s > 50 mm depth Trends of Gas side and
air side efficiency before and after Over haul may also supplement the
replacement decision. · Reversal of baskets not recom mended

21. O2 Stratification at AH Outlet FG Duct 8 7 6 7-8 % 6-7 5-6 4-5 3-4
Stratification in Gas ducts at AH outlet 5 4 S3 3 S1 A B C D E F Probe
Temperature Stratification in AH Outlet FG Duct (Trisector Air heater) 170 160
Temp C 150 140 130 A B S2 S3 Grid sampling is needed for correct assessment of
gas temperature & composition at AH outlet due to stratification in flue gas
160.0-170.0 150.0-160.0 140.0-150.0 130.0-140.0 C Probes D E S1 F

22. Air Heaters · · · · Thermocouples for flue gas temperatures at AH inlet as
well as exit are generall y 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 ou tlet 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 th ermal performance of a tri-sector air heater
in a very major way; performance ev aluation is difficult. ·

23. It's worthwhile to re-look at all the instrumentation around Air heaters for air
t emperatures / Flue gas composition & temperature measurement. The unit
operation , equipment efficiency assessments and maintenance decisions are based
on the sa me.

24. Case Study Air Heaters · · · · High air temp rise Low gas temp drop High AH
leakages Low X-ratio Unit 2 A B 222 217 166 155 15.4 16.9 182 195 0.67 0.61 50.2
45.4 Unit 3 A B 219 222 155 158 16.5 18.4 185 188 0.62 0.61 47.9 47.5 Unit 1
Design PGT A B Air Temp Rise C 230 228 228 221 Gas Temp Drop C 200 185 16 5 162
Leakage % 8.8 6.6 15.9 16.6 Gas Out Temp (NL) C 146.8 164.5 190 188 X rati o %
0.83 0.73 0.64 0.64 Gas Side Efficiency % 62.6 56.1 49.1 49.1 · 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 ingr ess

25. Air heater Performance Enhancement through Up gradations Double sealing
retrofits with Fixed sealing plates Before After

26. Double Sealing

27. Rotor modifications Before Typical 24 sector rotor design New axial seal
carrying bars fitted After Rotor modified to 48 sectors

28. Flexible seal assembly - Cold Condition

29. Flexible seal assembly - Hot Condition

30. Heating Surface Element retrofits · All our air heaters have DU & NF profile at
Hot end & Cold end · Potential for imp rovement by changing basket profiles ·
Reduction in Air heater exit gas temperatur es to 125C

31. Additional Surface area & 150mm height HE baskets Minimum Basket Hot End Hot
Intermediate Cold End

32. Boiler Performance Boiler Efficiency The % of heat input to the boiler absorbed
by the working fluid (Typically 85-88 %)

33. Boiler Efficiency¼ Boiler Efficiency can be determined by a) Direct method or
Input / Output method b) Indirect method or Loss method

34. Direct Method Steam Flue Gas Fuel + Air Boiler Efficiency = Heat addition to
Steam x 100 Gross Heat in Fuel Boiler Efficiency = Steam flow rate x (steam
enthalpy - feed water enthalpy) x 100 Fuel firing rate x Gross calorific value
Water

35. Boiler Efficiency¼ Direct method or Input / Output method measures the heat
absorbed by water & ste am & 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 va lue on real time basis. Another problem with
direct method is that the extent an d nature of the individual components losses
is not quantified. ·

36. Boiler Efficiency¼ Indirect method or Loss method For utility boilers
efficiency is generally calcu lated by heat loss method wherein the component
losses are calculated and subtra cted from 100. Boiler Efficiency = 100 - Losses
in %

37. Indirect Method 1. Dry Flue gas loss 2. H2 loss 3. Moisture in fuel 4. Moisture
in air 5. CO los s Steam 6. Radiation 7. Fly ash loss Fuel + Air Boiler Flue gas
Water 8. Bottom ash loss Efficiency = 100 ± (1+2+3+4+5+6+7+8) The unit of heat
input is the higher heating value per kg of fuel. Heat losses f rom various
sources are summed & expressed per kg of fuel fired.

38. 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 iden tifies exactly where the heat losses are
occurring. ·

39. Boiler Efficiency¼ Commonly used standards for boiler performance testing are
ASME PTC 4 (1998) BS ± 2885 (1974) IS: 8753: 1977 DIN standards

40. Parameters required for computing Boiler Efficiency · AH flue gas outlet O2 /
CO2 / CO · AH flue gas inlet and outlet temp C · Primary / S econdary 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

41. 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

42. 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 temperatur e ~ 1% reduction in boiler
efficiency. ·

43. Dry Gas Loss¼ 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

44. Dry Gas loss reduction requires · · · · · · Boiler operation at optimum
excess air Cleanliness of boiler surfaces Good combu stion of fuel Reduction of
tempering air to mill. Reduction in air ingress Clean ing of air heater surfaces
and proper heating elements / surface area

45. Unburnt Carbon Loss (Controllable) · · · · · The amount of unburnt is a
measure of effectiveness of combustion process in gen eral and mills / burners
in particular. Unburnt carbon includes the unburned con stituents in flyash as
well as bottom ash. Ratio of Flyash to Bottom ash is arou nd 80:20 Focus to be
on flyash due to uncertainty in repeatability and represent ative ness of
unburnt carbon in bottom ash +50 PF fineness fractions to be < 1-1 .5%

46. 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 *
C (Carbon in coal) 100 - C = Ash 100

47. Influencing Factors - Unburnt Carbon Loss · · · · · · · · · Type of
mills and firing system Furnace size Coal FC/VM ratio, coal reactiv urners
design / condition PF fineness (Pulveriser problems) Insufficient excess air in
combustion zone Air damper / register settings Burner balance / worn orif ices
Primary Air Flow / Pressure

48. Moisture Loss Fuel Hydrogen Loss This loss is due to combustion of H present in
fuel. H is bur nt and converted in water, which gets evaporated. Fuel Moisture
Loss This loss i s due to evaporation and heating of inherent and surface
moisture present in fue l. (Can be reduced by judicious sprays in coal yards)

49. 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 d ue to corrosion considerations in the cold
end of air heater, gas ducts etc.

50. Other Losses 1. Sensible Heat Loss of ash · · · · Bottom Ash Hoppers Eco
Hoppers AH Hoppers ESP hoppe rs (~0.5-0.6 %) Sensible Heat Loss (%) = (X / GCV)
*100 X = [{Ash * Pflyash * C pash * (T go - T rai)} + {Ash * Pahash * C pash *
(T go - T rai)} + {Ash * Peash * C pash * (T g i -T rai )} + {Ash * Pba * C pash
* (T ba - T rai )}]

51. 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 op ening
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 %

52. Other Losses 3. Coal Mill Reject Loss · · · · · · · Coal Flow Coal Mill
Rejects GCV of Coal CV of Rejects ill Outlet Temp Tmillout Reference Temperature
Trai Specific Heat of Rejects CpR EJECT 135 T/hr 200 kg/hr 3300 kcal/Kg 900
kcal/Kg 90 C 30 C 0.16 kcal/Kg/C Loss due to Mill Rejects = X / (Coal Flow * GCV
* 1000) X = [Rejects * (CVREJECT + CpREJECT (Tmillout ± Trai))* 100 ] = (0.0408
%)

53. Other Losses 4. Radiation Loss Actual radiation and convection losses are
difficult to assess because of particular emissivity of various surfaces.

54. HEAT CREDIT Heat Credit due to Coal Mill Power = [MP * 859.86 * 100] / [Coal
Flow * GCV * 10 00] Coal Flow Rate Coal FLOW Tons/Hr Total Coal Mill Power MP
kWh GCV of Coal Kc al/Kg

55. Computations · Two Excel spreadsheets for determination of Boiler and Air
Heater performance in dices are being provided with this presentation. These
also include methodology for correcting these indices for deviation in coal
quality and ambient temperatu re from design. The operating equipment
performance should be corrected for boun dary conditions before comparison with
design parameters. · ·

56. THANKS