April 17, 2023 PMI Revision 00 1
AIR HEATER PERFORMANCE
April 17, 2023 PMI Revision 00 2
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.
April 17, 2023 PMI Revision 00 3
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
April 17, 2023 PMI Revision 00 4
The Performance Indicators are
• Air Leakage• Gas Side Efficiency• X - ratio• Gas & Air side pressure drops
Therefore…..
April 17, 2023 PMI Revision 00 5
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
April 17, 2023 PMI Revision 00 7
Thermocouples
l/6 l/2 5/6 l
Ports
Gas Side Probes
Air Side Probes
Traverse Ports
April 17, 2023 PMI Revision 00 8
GasAnalysers
DatascanBoxes
Vacuum Pump
DesiccantJar
Condenser
FG Samplefrom probes
Bubble Jar
Flue Gas Sampling Train
April 17, 2023 PMI Revision 00 9
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
April 17, 2023 PMI Revision 00 10
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
April 17, 2023 PMI Revision 00 11
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
April 17, 2023 PMI Revision 00 12
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).
April 17, 2023 PMI Revision 00 13
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.
April 17, 2023 PMI Revision 00 14
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
April 17, 2023 PMI Revision 00 15
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
April 17, 2023 PMI Revision 00 16
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
April 17, 2023 PMI Revision 00 17
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
April 17, 2023 PMI Revision 00 18
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)
April 17, 2023 PMI Revision 00 19
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)
April 17, 2023 PMI Revision 00 20
THANK YOU