air heater performance

April 17, 2023 PMI Revision 00 1

AIR HEATER PERFORMANCE


Role of Regenerative Air Heaters

• To recycle the heat of exit flue gas back in the combustion process

• Enabling efficient coal combustion by raising secondary air temperature.

• Enabling efficient pulverization process by optimizing pulverized output temperature.


What can go wrong ?• Partly ingress of air in flue gas at air heater entry

due to leakage causing lowering flue gas temperature and thereby inefficient heat transfer. It increases the required fan power and can eat away the fan margin.

• Choking of air heater basket resulting high dp across air heater, low heat transfer and high exit temperature.

• Low gas side efficiency.

• Low X-ratio indicating bad operating condition


The Performance Indicators are

• Air Leakage• Gas Side Efficiency• X - ratio• Gas & Air side pressure drops

Therefore…..


Data required

O2 & CO2 in FG at APH Inlet

O2 & CO2 in FG at APH Outlet

Temperature of air entering air heater

Temperature of air leaving air heater

Diff. Prsr. Across APH on air & gas side

April 17, 2023 PMI Revision 00 6Sampling Point for Flue Gas Temperature & Composition

Sampling Ports in Flue Gas Ducts (Typical )100mm


Thermocouples

l/6 l/2 5/6 l

Ports

Gas Side Probes

Air Side Probes

Traverse Ports


GasAnalysers

DatascanBoxes

Vacuum Pump

DesiccantJar

Condenser

FG Samplefrom probes

Bubble Jar

Flue Gas Sampling Train


A B C D E FS1

S2

S3130

140

150

160

170

Tem

p C

Probes

Temperature Stratification in APH Outlet FG Duct

160.0-170.0

150.0-160.0

140.0-150.0

130.0-140.0


A B C D E FS1

S2

S3

3

4

5

6

7

8

%

Probe

O2 Stratification at APH Outlet FG Duct

7-8

6-7

5-6

4-5

3-4


D E F G2.8 2.8 2.9 2.7

C 2.3 2.7 H

B 2.8 2.6 I

A 2.5 2.6 J2.0 1.9 2.3 2.6N M L K

D E F G3.3 2.7 3.1 3.2

C 2.8 3.0 H

B 2.8 3.0 I

A 3.0 2.9 J2.5 2.1 2.4 2.6

Oxygen in Flue Gas at AH inlet(Test x/xx)

Average Oxygen values in Test xx from the fourteen probe grid in Flue Gas Duct at Air Heater Inlet

Average Oxygen values in Test x from the fourteen probe grid in Flue Gas Duct at Air Heater Inlet

4.2 4.2

4.24.6

'On line' Zirconia probe


Air Leakage Weight of air passing from air side to gas side. This leakage is assumed to occur entirely between air inlet and gas outlet. Almost 65% leakage is from radial seals, 30 % from circumference bypass seals and rest from axial, rotor post seals Hot End / Cold End / Entrained Leakage

Calculation Empirical relationship using the change in concentration of O2 or CO2 in the flue gas

% AL = CO2in - CO2out * 0.9 * 100 CO2out

= O2out - O2in * 0.9 * 100 (21- O2out)

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

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


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 air heater leakage

Now Tgas out (no leakage) = AL * Cpa * (Tgas out - Tair in) + Tgas out

Cpg * 100Where Cpa – Specific heat of air & Cpg- Specific heat of gas.


Flue Gas Exit Temperature

Flue Gas Exit Temperature is corrected for deviations of inlet air and inlet gas temperatures from design values.

Temp corrected for leakage and temperature of entering air is

Tgasoutla = Tairind (Tgasin –Tgasoutnl) + Tgasin(Tgasoutnl – Tairin) Tgasin – Tairin

Temp corrected for leakage and temperature of entering flue gas is

Tgasoutlg = Tgasind (Tgasoutnl –Tairin) + Tairin(Tgasin – Tgasoutnl) Tgasin – Tairin

Flue gas exit temp corrected for deviations from design= Tgasoutla + Tgasoutlg - Tgasoutnl


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


4) Air In / Air Out Temperatures

For air inlet and outlet temperatures, weighted averages of primary and secondary air temperatures are used as below.

Fraction of PA flow = F1 = PA flow / Total Air Flow

Fraction of SA flow = F2 = SA flow / Total Air Flow

Weighted Air Temp In = F1 * PA Temp In + F2 * SA Temp In

Wghted Air Temp Out = F1 * PA Temp Out + F2 * SA Temp out


5) The specific heat of the air is calculated at the average air temperature in the AH (Inlet Temperature + Outlet Temperature) / 2. This is done by using ASME’s PTC 4.1 Steam Generating Units, Figure 3. = CpAIR (kcal/kg/K)

Formula: Cp_air = 9.798958E-19 * X5 – 2.76786E–15 * X4 – 7.42842E-12 * X3 +0.00000002654212 * X2 +.000006479326 * X +0.2389161Where X = Average air temperature in AH. 6) If a CO2 analyser is not available, the amount of CO2 in

the flue gas can be calculated by subtracting the O2

percentage from the constant for (O2 + CO2).

CO2 (%) = (O2 + CO2) - O2 = BO2CO2 - O2


7) The specific heat of the flue gas is calculated using average gas temperature (Gas inlet and no-leakage gas outlet), carbon content of fuel, hydrogen content of fuel, and amount of CO2 in the flue gas. See ASME’s PTC 4.1 Steam

Generating Units, Figure 7. = CpGAS (kcal/kg/K)

8) Air Heater Leakage (%) is defined as 100*the mass of wet air leaking to the gas side divided by the mass of wet gas entering the air heater. However, it is usually approximated by one of the two equations using measurements of gas analysis, by volume, on the gas side:

% Leakage = 90* (%CO2 entering - %CO2 leaving) /

%CO2 leaving

% Leakage = 90* (%O2 leaving - %O2 entering) / (21-

%O2 leaving)


9) AH Gas Side Efficiency is calculated as below.

AHE = 100 * (Gas Temp In – NL Gas Temp Out) / (Gas Temp In – Air Temp In)

Or AHE = 100 * (AHGIT - EGTCorTNL) / (AHGIT

– AirT)


THANK YOU

air heater performance

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