1. Thermo-hydraulics of Power Plant Steam Generators
P M V Subbarao
Professor
Mechanical Engineering Department
I I T Delhi
Design for Performance…
Better Combustion & Heat Transfer …

2. Global Geometry of A Steam Generator

3. A Comprehensive Thermodynamic Cycle of A Steam Generator

4. Steam Generator Furnace
Structurally boiler furnace consists of the combustion space surrounded by water walls.
The furnace is designed to perform two functions simultaneously, namely:
Release of the chemical energy of fuel by combustion
The first task of combustion technology is
to burn the fuel efficiently and steadily,
to inhale controlled excess air (as little as possible),
To generate a flame with controlled shape which will generate lowest amount of pollutants.
Transfer of heat from the furnace to the working fluid inside the water walls.
The important task of furnace heat removal is to produce a controlled Furnace Exit Gas Temperature (FEGT).
FEGT is an important aspect of boiler safety & Efficient Operation.

5. Industrial Level Geometric Specifications of Furnace
 = 30 to 50O
 > 30O
 = 50 to 55O
E = 0.8 to 1.6 m
d = 0.25 b to 0.33 b

6. Basic Geometry of A Furnace

7. Heat Transfer in A Furnace
The flame transfers its thermal energy to the water walls in the furnace by Radiation.
Convective Heat Transfer < 5%.
Only Radiation Heat Transfer is Considered for Design & Performance Analysis!

8. Simplified Approach Radiation Effectiveness of Furnace Wall, y.
Emitted Radiation heat flux of flame:
Emitted Radiation = Available Heat
Heat flux absorbed by walls :
Radiation Effectiveness of Furnace Wall, y.
The rate of heat absorption

9. Furnace Characterization Criteria
M Temperature Field Coefficient
Tad or Tfl Theoretical combustion temperature
TFEGT Furnace Exit Gas Temperature
Afur Total surface area of furnace
mgas Flow rate of fuel+air
 Radiation Effectiveness of Furnace Wall

10. Burner arrangement & flame shapes
Total fuel and air are divided into many stream.
Each stream of fuel and air handled by a device called burner.
An array of burner installed on walls or at corners of furnace
Tangential fired furnace*
Opposed wall fired furnace
Fuel and combustion air projected from the each burner create a complex shape of flame.
Intense mixing of fuel and air stream at the centre
Down fired furnace

11. Effect of Coal Quality on Furnace Size

12. Burner Zone in A Furnace
Coal or any solid fuel is pulverized to generate uniform heat flux with efficient combustion.
98% of Pulverized fuel particles are in general <100 mm).
These particles are carried by primary air entering into furnace.
All these particles are continuously dragged by furnace gas+air.
During their travel, they are subjected to drying, devolatilization and char combustion sequentially.

13. Major Components of Coal Feeding System

14. Drying of Coal Particle: Energy Balance
The rate of Change of internal Energy of the particle + Rate of Energy loss due to evaporation of moisture =
Energy gain due to convection +Radiation
Energy gain: Qin = Qconv + Qrad
Qconv
Qrad
Moisture

15. Drying of Coal Particle
The rate of change of internal energy of the particle
Energy loss due to evaporation =

16. Time taken by coal Particle for drying
Qconv
Qrad
Moisture

17. Themo-physics of Traveling Coal Particle in A Furnace
Coal Particles are dragged into furnace by air and continuously moving towards furnace exit.
The hot gas environment changes the thermo-physics of the coal particle.
Drying
Devolatization & Pyrolysis
Char Ignition and combustion.

18. Drying Time for Coal Particle & Furnace Size
Qconv
Qrad
Moisture

19. Major Components of Coal Feeding System

20. Coal Pulverizers A Inlet Duct; B Bowl Orifice; C Grinding Mill;
D Transfer Duct to Exhauster;
E Exit Duct.

21. Aerodynamic Lifting of Coal Particles

22. Energy Balance in A Pulverizer
Hot primary air
Heat loss
Puliverizer frictional
dissipation
Pulverized coal +
Air + Moisture
Coal
Motor Power Input

23. Air Distribution System

24. Reduced Drying time & Compact Furnace
Qconv
Qrad
Moisture

25. Devolatization & Pyrolysis
Temperature of the particle rises fast after the completion of particle drying.
Start of Pyrolysis:
Terpens : 225 0C
Hemi cellulose : 225 – 325 0C
Cellulose : 325 – 375 0C
Lignin : 300 – 500 0C

26. Time Taken by Devolatization & Pyrolysis
The rate of devolatization of solid fuel
Where
As the dried particle heats up, volatile gases containing hydrocarbons, CO, CH4 and other gaseous components are released.
The released combustible gases get ignite and burn outside the particle.
In a combustion process, these gases contribute about 70% of the heating value of the biomass.
Finally, char oxidizes and ash remains.
Completion of volatization generates char.

27. Char Combustion Char is a highly porous solid carbon.
Wood char , f = 0.9
Coal char, f = 0.7
Internal surface area : 100 sq. m. per gm. – coal char.
: 10,000 sq.m per gm – Wood char.
Oxygen is first absorbed from the gas volume on the surface of particles.
Absorbed oxygen reacts with carbon to from complex carbon-oxygen compounds : CxOy.
These complex compounds dissociated into CO2 & CO.

28. Mechanism of Coal Combustion
Oxygen reacts with char to produce CO in the lower portion of the furnace.
The CO reacts rapidly inn the gas to form CO2.
The CO2 in turn is reduced by the char.
The latter reaction causes CO buildup when oxygen is depleted.
The resulting reactions:
C +1/2 O2 → CO
CO+ 1/2O2 → CO2
C+CO2 → 2CO
C+H2O → H2O + CO

29. Energy Balance Water walls Economizer Furnace
The rate of change of enthalpy of gas is equal to rate of generation of thermal energy due to combustion of several volatile hydro carbon compounds and solid carbon.
Water walls
Economizer
Furnace

30. Diabatic Furnace Wall : Water Wall
Furnace Exit
Hot Exhaust gases
Flame
Heat Radiation & Convection
Burners