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. Total gas side pressure dropPa
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

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

5. prest = pexit + pgas –Dpnd
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

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

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

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

9. Modeling of 210 MW Draught SystemFD
Fan
Duct
APH
Furnace
Back
pass
ESP ID
Chimney
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 Duct FD Fan SCAPH APH Duct Wind Box Boiler APH ESP
ID Fan
Duct

14. Off Design Pressure Variation
Pressure Variation in Air & Gas Path at Part Load
-2000
-1500
-1000
-500
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
SCAPH
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
The net effect is saving in energy of kW due to increase in furnace vacuum from 58.9 Pa to Pa.

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

23. Analysis of Flue Gas at the ID Fan Inlet
Partial pressure of each constituent in flue gas,
 pCO2 = kPa
 pO2 = kPa
 PN2 = kPa
 pSO2 = kPa
 pH2O = kPa
Mass flow rate of each constituent in tons/hour is:
 Mass flow rate of O2 in the flue gas = tph
 Mass flow rate of CO2 in the flue gas = tph
 Mass flow rate of N2 in the flue gas = tph
 Mass flow rate of SO2 in the flue gas = tph
 Mass flow rate of H20 in the flue gas = 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 bar) 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 C KJ/kg
Mass flow rate of each constituent at C ( kg/s)
Total thermal power released (MW)
CO2
16.37
606.32
527.85
3.69
0.2895 O2
1.11
374.43
294
72.9
5.8678 N2
68.14
425
335.09
193.3
S02
0.036
487
430.55
0.2341
0.0132
H20
13.36
2752
2591
30.444
11.576

26. 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
606.32
485.83
3.69 O2
374.43
248.35
72.95 N2
425
283.32
193.3
S02
487
399.58
0.2341
H20
2752
2501

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 = 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 OUTSIDE ID DUCT
I/L TO HEAT EXCHANGER
O/L TO HEAT EXCHANGER
WATER
I/L TO HEAT EXCHANGER
O/L TO HEAT EXCHANGER DP
WATER FLOW
QTY OF WATER CONDENSED
Temp °C
cm WC
LPM
lt. /Hr.
1.2.10
103 60 30 29 5 12
1.1
105 65 32 31 10
0.9
2.2.10
121 69 82
4.2 1

30. Calculation of Flue Gas Flow Rate
 Dp (cm)
Tin 0C 
Density (kg/m3) 
 Flow rate (kg/sec) 5 60 65 69
4.2 82
Calculation of Condensate Flow rate
Gas Flow rate (kg/sec)
Mesured condensate
kg/hr
g/sec
Condensate loading (gms/kg of gas)
1.1
0.9
0.25 1
Design rate of condensate loading using present heat exchanger = 37.85gms/kg of flue gas.

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