This document discusses modeling and optimization of the draught system in a 210 MW steam generator. It includes: 1. Calculations of pressure drops along the air and flue gas paths and comparison to design values. 2. Assessment of induced draft (ID) and forced draft (FD) fan power requirements as a function of furnace pressure. 3. Analysis of operational data and the effect of increasing furnace vacuum on boiler efficiency, finding a potential savings of 117.32 kW from increasing vacuum. 4. Various ideas are proposed for further research including flue gas analysis, energy recovery from flue gas, and experimental validation of a flue gas heat exchanger. 5. Comb

1. Generation and Control of Vacuum in Furnace
P M V Subbarao
Professor
Mechanical Engineering Department
Safe and Efficient Combustion Needs Appropriate
Furnace Pressure…

2. Development of Air & Flow Circuits

3. 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
  

















av
o
g
sy
P
p
p
p
101325
293
.
1
)
1
( 2
1


Total gas side pressure drop

4. 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
g
o
g
h
f
V
A



100


5. The pressure drop from the balance point of the furnace to the chimney
base is
prest = pexit + pgas –pnd
where pexit = pressure drop up to the boiler outlet

6. p
Percent Boiler Rating
Furnace, SH & RH Losses
Economizer Losses
Ducts & dampers losses
Draught Losses
Total losses

7. ID fan power calculation
ID fan power is calculated as:

8. p
Percent Boiler Rating
Burner Losses
APH Losses
Ducts & dampers losses
Air Pressure Losses
Total losses

9. FD
Fan
Duct APH Duct Furnace Duct APH
Back
pass
ESP ID
Fan
Chimney
Duct
Duct
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.

10. Important variables along air and gas path

13. Pressure Variation
Pressure Variation in Air & Gas Path at Full Load
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
3000
1 2 3 4 5 6 7 8 9 10 11 12
Path Element
Pressure
(Pa)
Calculated (215 MW) Design (210 MW)
Duct
FD Fan Duct SCAPH APH Duct Wind
Box
Boiler APH ESP ID Fan

14. Off Design Pressure Variation
Pressure Variation in Air & Gas Path at Part Load
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
1 2 3 4 5 6 7 8 9 10 11 12
Path Element
Pressure
(Pa)
Calculated (168 MW) Design (168 MW)
ID Fan
ESP
Boiler APH
Wind
Box
Duct
APH
Duct
SCAPH
Duct
FD Fan

17. Operational Data of 210 MW plant

19. Effect of Furnace Vacuum on Boiler Efficiency

21. 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.

22. New Ideas for Future Research
FD
Fan
Duct APH Duct Furnace Duct APH
Back
pass
ESP ID
Fan
Chimney
Duct
Duct

23. 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

24. 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.

25. 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)
Enthalpy*at
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

26. Energy Potential of Flue Gas with 100% water Recovery
Flue gas
constitue
nts
Partial
pressure at
136 C in kPa
Enthalpy* at
136 C
(KJ/kg)
Mass flow
rate of
each
constituen
t 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

27. Model Experimentation

28. 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.

29. Experimental validation
Flue Gas heat exchanger measured data:
DATE FLUE
GAS
I/L JUST
OUTSID
E ID
DUCT
FLUE
GAS
I/L
TO HEAT
EXCHAN
GER
FLUE
GAS
O/L
TO HEAT
EXCHAN
GER
WATER
I/L
TO HEAT
EXCHAN
GER
WATER
O/L TO
HEAT
EXCHAN
GER
DP WATE
R
FLOW
QTY.
OF
WATER
CONDEN
SED
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

30. Calculation of Flue Gas Flow Rate
p (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
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.
Calculation of Condensate Flow rate

31. 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.

32. Furnace Draught

33. 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.

34. Furnace Draught Control

35. Windbox Pressure Control

36. Combustion Prediction & Control

37. The Model for Combustion Control

38. Parallel Control of Fuel & Air Flow Rate

39. Flow Ratio Control : Fuel Lead

40. Flow Ratio Control : Fuel Lead

41. Cross-limited Control System

42. Oxygen Trimming of Fuel/air ratio Control

43. Combined CO & O2 Trimming of Fuel/Air Ratio Control

44. Resistance to Air & Gas Flow Through Steam
Generator System