1. Anatomy & Organs of Power Plant Steam GeneratorsBy
P M V Subbarao
Professor
Mechanical Engineering Department
I I T Delhi
Structure of A Two in One Device– Combustor and Heat Exchanger……

2. Details of Basic Processes
Generation of Thermal Energy – Using Natural Resources.
Chemical Energy: Microscopic Potential Energy
Conversion of MPE to MKE
Combustion Sciences.
Transfer of Thermal Energy to working fluid.
Transfer MKE of gas to MKE of steam.
Science of Heat Transfer.

3. A True Design Reason behind Geometry of Natural Systems…..
Indian Elephant
African Elephant
Mammoths

4. An Evolution to Perfection of Heat Transfer is Sustainable

5. A Reason behind Survival of Electonics

6. Global Geometry of A Steam Generator

7. Phenomenological Model
Hot Flue
Gas
Thermal Structure SH
Steam
Convection &
Radiation HT
Convection HT
Drop in Enthalpy
of Flue Gas
Rise in Enthalpy of
Steam
Mechanism of Heat Transfer
Source/Supply
Thermal Structure
Sink /Demand

8. Components of A Steam Generator System : Heat Transfer
DPNL SH
Platen SHTR R H T
LTSH
Economiser
APH ESP ID Fan
drum
Furnace
BCW
pump
Bottom ash
stack
screen
tubes

9. Steam Generator Furnace : Function -1
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.

10. Steam Generator Furnace : Function - 2
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.

11. Furnace Exit
Hot Exhaust gases
Heat Radiation & Convection
Flame
Burner

12. Structure of Furnace Wall

13. General Design Principles
The furnace should provide the required physical environment and the time to complete the combustion of fuel.
The furnace should have adequate radiative heating surfaces to cool the flue gas sufficiently to ensure safe operation of the downstream convective heating surface.
Aerodynamics in the furnace should prevent impingement of flames on the water wall and ensure uniform distribution of heat flux on the water wall.
The furnace should provide conditions favoring reliable natural circulation of water through water wall tubes.
The configuration of the furnace should be compact enough to minimize the amount of steel and other construction material.

14. Further Geometrical Details of A Furnace

15. Determination of Furnace Size
What is the boundary of a furnace?
The boundary of a furnace is defined by
Central plane of water wall and roof tubes
Central lines of the first row super heater tubes.
 = 30 to 50O
 > 30O
 = 50 to 55O
E = 0.8 to 1.6 m
d = 0.25 b to 0.33 b

16. Sizing of A Furnace

17. Furnace Depth & Height
Depth (a) to breadth (b)ratio is an important parameter from both combustion and heat absorption standpoint.
Following factors influence the minimum value of breadth.
Capacity of the boiler
Type of fuel
Arrangement of burners
Heat release rate per unit furnace area
Capacity of each burner
The furnace should be sufficiently high so that the flame does not hit the super heater tubes.
The minimum height depends on type of coal and capacity of burner.
Lower the value of height the worse the natural circulation.

18. Heat Available to The Furnace Walls
Incomplete combustion loss
Unburned Carbon loss
Loss due to slag
Energy brought in by preheated air & fuel.
An exact fraction of this total heat should be absorbed in furnace.
The designer provides an environment for the same.

19. Burner Arrangement & Flame Shapes
An array of burner installed on walls or at corners of 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
Tangential fired furnace*
Down fired furnace
Opposed wall fired furnace

20. Radiative HT from Flame to Water Wall
Emitted Radiation heat flux of flame:
Emitted Radiation = Available Heat
Heat flux absorbed by walls :
Thermal efficiency factor, y.
The rate of heat absorption

21. Furnace Exit Gas Temperature
The temperature of products of combustion at the exit of the furnace is called FEGT.
Defines the ratio of furnace heat absorption to outside heat absorption.
High FEGT – Compact furnace & Large secondary section
FEGT < Ash Deformation Temperature.
Generally FEGT = Ash Softening Temperature – 100.
FEGT is a Right Measure of Steam Generator Health !
A Healthy SG will Always Perform Better !!!
A SG Always Healthy will Perform Better !

22. Limits on Furnace Exit Gas Temperature
FEGT is an important design parameter.
Defines the ratio of furnace heat absorption to Convection heat absorption.
FEGT < Ash Deformation Temperature.
FEGT = AST – 100
FEGT < C – Strong slag
FEGT < C – Moderate slag
FEGT < C -- Weak slag
Any design procedure can be used but it should satisfy the requirements of FEGT.

23. Effect of Coal Quality on Furnace Size

24. Daily Variation of FEGT
1225
1200
1175
1150
1125
1100 0C

25. Instantaneous Dirtiness of Water walls
During the operation of steam generator, ash and slag deposits on the boiler water walls and other heat transfer sections.
Thermal conductivity of the deposits are much less than steel.
Furnace dirtiness decreases radiation heat transfer to water walls.
FD Enhances the Furnace Exit Gas Temperature.
Less steam is generated in the furnace.
More heat is transferred to superheaters.
Leads to increase in turbine inlet temperature.
Special water spray is automatically activated to attemparate the Turbine inlet steam.
This leads to increased chimney gas temperatures.

26. Impact of Dirtiness on SG Performance
Dirtiness of furnace leads to increased chimney gas temperatures.
Enhances the metal temperatures of superheaters.
Increases the probability of tune leakage/failure.
Furnace and convective pass slagging and fouling have a detrimental effect on boiler performance and emissions.
High FD is responsible for the primary cause of reduced operating efficiency in coal-fired boilers.

27. The Act of Sootblowing
Sootblowing is used to control the level of ash and slag deposits on the boiler heat transfer sections.
On-line cleaning of localized areas is performed by sootblowers using high pressure steam or air.
Wall blowers are used to remove slag from furnace water walls.
Retractable blowers are used to clean the convective pass of the boiler.
Furnace cleaning increases radiation heat transfer to water walls and reduces the Furnace Exit Gas Temperautre.

28. Positive Impact of Sootblowing
Regular sootblowing results in :
higher furnace exit gas temperature (FEGT),
higher steam temperature,
higher desuperheating spray flows, and
higher Nox emissions.
As the convective pass of a boiler slags and fouls,
heat transfer decreases,
resulting in a decrease in steam temperature,
decrease in desuperheating spray flows, and
an increase in flue gas temperature at the boiler exit.
For best boiler performance it is important to maintain an optimal balance between furnace and convection pass heat transfer.

29. Typical 500MW plant Soot Blowing system:
TYPES OF SOOT BLOWERS
TYPE OF BLOWERS APPLICATIONS
- LONG RETRACTABLE CONVECTION ZONE
SOOT BLOWER (SH, RH, LTSH, ECO)
WALL DESLAGGER FURNACE WALL
ROTARY SOOT BLOWERS AIR HEATERS
Typical 500MW plant Soot Blowing system:
Wall Blowers - 88 Nos + (16 No. FP.)
Long Retractable Blowers - 36 Nos + (28 No.FP )
Air Heater Blowers - 4 Nos.

30. Arrangement of Soot Blowers
in A 500 MW Steam Generator

31. Schematic Layout of Sootblowers

32. Effects of Sootblowing on Unit Operation and Emissions
While ash and slag deposition is a gradual process, sootblowing results in an abrupt change in local heat transfer.
This can result in unfavorable energy distributions among different heat transfer sections, loss of thermal performance and increased unit heat rate.
Figure shows the effect of sootblowing on furnace cleanliness (expressed as FEGT).
Next Figure shows the effect of cleaning of a platen superheater on its surface cleanliness.

33. Pre & Post-Sootblowing Variations in FEGT

34. Effect of SB on A Superheater Cleanliness

35. Effect of SB on A Superheater Scaled Cleanliness Factor