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Final Audit :Utilization of Flue Gas Energy
P M V Subbarao
Professor
Mechanical Engineering Department
Minimize Final Exhaust Gas Temperature….Properly Utilize Enthalpy of Flue Gas….
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s
1
2
3
4
5
6
2f
2s
4523 hhmhhm mreheatsteafmainsteam
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DPNL SH
Platen SHT
R
RHTR
LTSH
Economiser
APH ESP ID Fan
drum
Furnace
BCWpump
Bottom ash
stack
screentubes
Thermal Structure of A Boiler Furnace
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Furnace ater wall absorption
Platen SH
Pendent SHCSH
Reheater
Combustion Losses C & R losses
Hot Exhaust Gaslosses
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Sequence of Energy Exchange from Flue Gas to Steam
PLATEN SH
FLUE GAS
EVAPORATORWater Wall
COVECTIVE LTSH
RH PENDENT SH
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Fuel Power
Furnace absorption
Platen SH
Final SHLTSH
Reheater
Combustion Losses C & R losses Hot Exhaust Gaslosses
~4000C
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Details of 500 MW(e) Capacity Indian Power Unit
• Main steam Flow rate: 425 kg/s.
• Main Steam Temperature: 5400C
• Reheat Steam Flow rate: 38.7 kg/s.
• Air Flow Rate: 577.06 kg/s.
• Coal Flow Rate: 73.8 Kg/s.
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500 MW
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Gas Temperatures
• Platen Super Heater:• Inlet Temperature: 1236.4 0C• Outlet Temperature: 1077 0C• Final Super Heater:• Inlet Temperature: 1077 0C• Outlet Temperature: 962.4 0C• Reheater:• Inlet Temperature: 962.4 0C• Outlet Temperature: 724.3 0C• Low Temperature Super
Heater:• Inlet Temperature: 724.30C• Outlet Temperature: 481.3 0C
Steam Temperatures
• Platen Super Heater:• Inlet Temperature: 404 0C• Outlet Temperature: 475 0C• Final Super Heater:• Inlet Temperature: 475 0C• Outlet Temperature: 540 0C• Reheater:• Inlet Temperature: 345 0C• Outlet Temperature: 5400C• Low Temperature Super
Heater:• Inlet Temperature: 3590C• Outlet Temperature: 404 0C
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Design Calculated
1 Adiabatic Flame Temp (K) 1957 1966
2 FEGT (0C) 1102 1117
3 Platen SH-I Outlet (0C) 932 951
4Platen SH-II Outlet-I outlet (0C) 859 878
5 RH 3rd & 2nd outlet (0C) 595 604
6 RH 1st Stage outlet (0C) 510 531
7 Economiser outlet (0C) 385 398
8 APH Outlet (0C) 138 151
Flue Gas Temperature At different regions of Furnace:210 MWe)
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Steam and Gas Paths
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Suggested Fluid Velocities
• Flue gas velocities: 10 – 18 m/s.
• Steam in super heaters & reheaters: 10 – 25 m/s.
• Water Wall circulation : 0.35 – 3.5 m/s.
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500 MW
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LMTD for various Devices
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Surface Area of Heat Exchangers: 500 MW
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Economizer
• The economizer preheats the feed water by utilizing the residual heat of the flue gas.
• It reduces the exhaust gas temperature and saves the fuel.
• Modern power plants use steel-tube-type economizers.
• Design Configuration: divided into several sections : 0.6 – 0.8 m gap
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Tube Bank Arrangement
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Thermal Structure of Economizer
• Out side diameter : 25 – 38 mm.
• Tube thinckness: 3 – 5 mm
• Transverse spacing : 2.5 – 3.0
• Longitudinal spacing : 1.5 – 2.0
• The water flow velocity : 600 – 800 kg/m2 s
• The waterside resistance should not exceed 5 – 8 %. Of drum pressure.
• Flue gas velocity : 7 – 13 m/s.
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Thermal Balance in Economizer.
• The energy absorbed by steam
• The convective heat lost by flue gas
• Overall Coefficient of Heat Transfer, U
)( ,,, inecooutecosteamconabs hhmQ
TUAQ ecolosscon
,
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Mean Temperature Difference
• The average temperature difference for parallel flow and counter flow is expressed as
• It is also called log mean temperature difference• When tmax /tmin > 1.7, the average temperature may be expressed as:
• Generally, the flow direction of the flue gas is perpendicular to the axes of tubes.
• If number of bends are more than four, the flow can be treated as counter or parallel flow.
min
max
minmax
log3.2t
t
ttt
2minmax tt
t
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Complex Flow
• Parallel flow and counter flow may simultaneously exist in one section of an economizer.
• This is called complex flow.
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• For a given set of inlet and outlet temperatures of the fluids,
• The temperature difference of parallel flow is the greatest,
• The temperature difference of counter flow is the lowest
• And that of complex flow is in between.
• The average temperature difference in a complex flow can be calculated as:
• When
• Otherwise, the temperature difference is determined by
• The value of Ktd is determined by flow type and the thermal parameters.
copa ttt 5.0
copa tt 92.0
cotd tKt
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Gas Temperatures
• Platen Super Heater:• Inlet Temperature: 1236.4 0C• Outlet Temperature: 1077 0C• Final Super Heater:• Inlet Temperature: 1077 0C• Outlet Temperature: 962.4 0C• Reheater:• Inlet Temperature: 962.4 0C• Outlet Temperature: 724.3 0C• Low Temperature Super Heater:• Inlet Temperature: 724.30C• Outlet Temperature: 481.3 0C• Economizer:• Inlet Temperature: 481.3 0C• Outlet Temperature: 328.5 0C
Steam Temperatures
• Platen Super Heater:• Inlet Temperature: 404 0C• Outlet Temperature: 475 0C• Final Super Heater:• Inlet Temperature: 475 0C• Outlet Temperature: 540 0C• Reheater:• Inlet Temperature: 345 0C• Outlet Temperature: 5400C• Low Temperature Super Heater:• Inlet Temperature: 3590C• Outlet Temperature: 404 0C• Economizer:• Inlet Temperature: 254 0C• Outlet Temperature: 302 0C
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Furnace absorption
Platen SH
Pendent SHCSH
Reheater
Economizer APH
Combustion Losses C & R losses Hot Exhaust Gaslosses