Generation and Control of Vacuum in Furnace

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Generation and Control of Vacuum in Furnace

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Safe and Efficient Combustion Needs Appropriate
Furnace Pressure
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Development of Air Flow Circuits
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Total gas side pressure drop


Pa where ??p1 total pressure drop from the
furnace outlet to the dust collector, Pa
??p2 pressure drop after the
dust collector, Pa ?
ash content in the glue gas, kg/kg
pa v average pressure of the gas, Pa
pg o flue gas density
at standard conditions, kg/Nm3
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The ash fraction of the flue gas calculated as,
where ?f h ratio of fly ash in flue gas
to total ash in the fuel A ash content of
working mass, Vg average volume of gas from
furnace to dust collector calculated from the
average excess air ratio, Nm3/kg of fuel
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The pressure drop from the balance point of the
furnace to the chimney base is ?prest ?pexit
?pgas Dpnd where ?pexit pressure drop up to
the boiler outlet
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Draught Losses
Total losses
Dp
Furnace, SH RH Losses
Economizer Losses
Ducts dampers losses
Percent Boiler Rating
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ID fan power calculation
ID fan power is calculated as
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Air Pressure Losses
Total losses
Dp
Burner Losses
APH Losses
Ducts dampers losses
Percent Boiler Rating
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Modeling of 210 MW Draught System

• Pressure drop calculation in air gas path and
  its comparison with design value.
• Assessment of ID and FD fan power as a function
  of furnace pressure.

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Important variables along air and gas path
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Pressure Variation
Duct
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Off Design Pressure Variation
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Operational Data of 210 MW plant
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Effect of Furnace Vacuum on Boiler Efficiency
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The net effect is saving in energy of 117.32 kW
due to increase in furnace vacuum from 58.9 Pa to
230.6 Pa.
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New Ideas for Future Research
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Analysis of Flue Gas at the ID Fan Inlet

• Partial pressure of each constituent in flue gas,
  
•  pCO2 16.366209 kPa
•  pO2 1.138404 kPa
•  PN2 68.142138 kPa
•  pSO2 0.036081 kPa
•  pH2O 13.363218 kPa
• Mass flow rate of each constituent in tons/hour
  is
•  Mass flow rate of O2 in the flue gas 13.2867
  tph
•  Mass flow rate of CO2 in the flue gas
  262.646 tph
•  Mass flow rate of N2 in the flue gas 695.893
  tph
•  Mass flow rate of SO2 in the flue gas 0.84219
  tph
•  Mass flow rate of H20 in the flue gas 118.33
  tph

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Energy Audit of Flue Gas

• Temperature of flue gas 136 ºC 150oC
• Dew point of water is (obtained based on partial
  pressure of 0.1336 bar) 51.59 ºC
• Cooling of the exhaust gas below the dew point
  will lead to continuous condensation of water
  vapour and reduction of flue gas volume and mass.
  
• The temperature of the flue gas in order to
  remove x of the available moisture can be
  obtained using partial pressures of water.

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Energy Potential of Flue Gas with 10 water
Recovery
Flue gas constituents Partial pressure at 136 C in kPa Enthalpy at 136 C (KJ/kg) Mass flow rate of each constituent at 136 C ( kg/s) Enthalpyat 49.74 C KJ/kg Mass flow rate of each constituent at 49.74 C ( kg/s) Total thermal power released (MW)
CO2 16.37 606.32 3.69075 527.85 3.69 0.2895
O2 1.11 374.43 72.9572 294 72.9 5.8678
N2 68.14 425 193.303 335.09 193.3 17.3797
S02 0.036 487 0.23413 430.55 0.2341 0.0132
H20 13.36 2752 32.8694 2591 30.444 11.576
            35.1270
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Energy Potential of Flue Gas with 100 water
Recovery
Flue gas constituents Partial pressure at 136 C in kPa Enthalpy at 136 C (KJ/kg) Mass flow rate of each constituent at 136 C ( kg/s) Enthalpy at 0 C (kJ/Kg) Mass flow rate at 0 C ( kg/s) Total thermal power released (MW)
CO2 16.366209 606.32 3.69075 485.83 3.69 0.444698
O2 1.138404 374.43 72.9572 248.35 72.95 9.198452
N2 68.142138 425 193.303 283.32 193.3 27.38828
S02 0.036081 487 0.23413 399.58 0.2341 0.020468
H20 13.363218 2752 32.8694 2501 0 90.45671
            127.5086
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Model Experimentation
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Expected Performance of the heat exchanger
Cooling capacity of the heat exchanger 10
kW Cooling load available with the heat exchanger
115.3 kJ/kg of flue gas Available rate of
condensation of the present heat exchanger
37.85gms/kg of flue gas.
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Experimental validation
Flue Gas heat exchanger measured data

DATE FLUE GAS I/L JUST OUTSIDE ID DUCT FLUE GAS I/L TO HEAT EXCHANGER FLUE GAS O/L TO HEAT EXCHANGER WATER I/L TO HEAT EXCHANGER WATER O/L TO HEAT EXCHANGER DP WATER FLOW QTY. OF WATER CONDENSED
Temp C Temp C Temp C Temp C Temp C cm WC LPM lt. /Hr.
1.2.10 103 60 30 29 30 5 12 1.1
1.2.10 105 65 32 31 32 5 10 0.9
2.2.10 121 69 31 30 31 5 12 1.1
2.2.10 121 82 32 31 32 4.2 12 1
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Calculation of Flue Gas Flow Rate
 Dp (cm) Tin 0C  Density (kg/m3)   Flow rate (kg/sec)
5 60 1.051754 0.007159
5 65 1.036203 0.007106
5 69 1.024089 0.007065
4.2 82 0.986604 0.006355
Calculation of Condensate Flow rate
Gas Flow rate (kg/sec) Mesured condensate kg/hr Mesured condensate g/sec Condensate loading (gms/kg of gas)
0.007159 1.1 0.305556 42.67864
0.007106 0.9 0.25 35.17994
0.007065 1.1 0.305556 43.25126
0.006355 1 0.277778 43.70829
Design rate of condensate loading using present
heat exchanger 37.85gms/kg of flue gas.
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Combustion and Draught Control

• The control of combustion in a steam generator is
  extremely critical.
• Maximization of operational efficiency requires
  accurate combustion.
• Fuel consumption rate should exactly match the
  demand for steam.
• The variation of fuel flow rate should be
  executed safely.
• The rate of energy release should occur without
  any risk to the plant, personal or environment.

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Furnace Draught
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The Control

• Furnace (draft) pressure control is used in
  balanced draft furnaces in order to regulate
  draft pressure.
• Draft pressure is affected by both the FD and ID
  fans.
• The FD fan is regulated by the combustion control
  loop, and its sole function is to provide
  combustion air to satisfy the firing rate.
• The ID fan is regulated by the furnace pressure
  control loop and its function is to remove
  combustion gases at a controlled rate such that
  draft pressure remains constant.

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Furnace Draught Control
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Windbox Pressure Control
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Combustion Prediction Control
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The Model for Combustion Control
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Parallel Control of Fuel Air Flow Rate
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Flow Ratio Control Fuel Lead
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Flow Ratio Control Fuel Lead
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Cross-limited Control System
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Oxygen Trimming of Fuel/air ratio Control
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Combined CO O2 Trimming of Fuel/Air Ratio
Control
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Resistance to Air Gas Flow Through Steam
Generator System