1. Combustion Theory
Lecture 12
Combustion in Boilers

2. Learning Outcomes
1. Overview of general boiler designs and applications
– Heat balance/heat transfer/ heat availability/excess air/size and shape of
the combustor
– Boiler configurations: Smoke tube (Fire tube) and water tube/ Water wall
designs Natural Circulation and Forced Circulation
2. Type of fuels used and different combustion configurations
– Gas, liquid and solid fuels burn in boilers
3. Types of burners used and the related emission control
methods in boiler designs
– Gas burner designs/Liquid fuel burner designs/Solid fuel PC burners,
burning beds etc.
4. Emission cleaning methods in boiler operation
– Species of concerns and appropriate techniques/layouts

3. Overview of Boiler Operations
• Boilers are being used since 18th century
• Applications range from:
small hot water/steam
applications in
domestic/industrial use
large scale steam
production for power
generation
• Improvements in the quality of steam produced for
industrial applications
A couple of Bars Pressure
& a few 100’s °C
Temperature
(Saturated Steam)
~ 200-300 Bars pressure &
600 °C Temperature
(Superheated Steam)
[Large power generation
systems]

4. Heat Balances – Condensing Power
Flue gas heat
loss ~ 15 – 20%
Heat balance: Boiler
Energy in fuel
100%
Condensing
power plant
Electricity
(25 - 40%)
Heat distribution
loss ~ 10 – 15%
Heat balance: Steam plant
Heat rejection
by condenser
~ 35 – 45%

5. Available Heat
Available Heat = Gross heating value of the fuel – Enthalpy lost from the
process by hot exhaust
gases

7. Optimum Excess Air
Depends on
• Fuel type - coal, gas, oil, and rate of combustion
• Furnace/ combustor design - influences
– Combustion time
– Mixing
– Temperature
• Type and number of burners which affect fuel/air
mixing
• Burner turndown
• Air preheat, oxidant
– enhances combustion rate

8. Excess Air
Air rate too high ⇨ Increased sensible heat loss
Air rate too low ⇨ Increased potential heat loss
Flue gas temp. too high ⇨ Increased sensible heat loss

9. Operating Excess Air Range fro
Common Fuels

10. Estimation of the Shape & Size of a
Combustion Chamber
• Useful criteria:
– Cross sectional surface loading QC = Qfuel/Ac [MW/m2]
– Volumetric Loading QV = Qfuel/V [MW/m3]
Where
AC – Cross sectional area of the furnace [m2]
V – volume of furnace [m3]
Qfuel – fuel power [MW]; This is related to electrical power Qe by
Qe = Qfuel · ηboiler · ηcycle
(ηboiler -Boiler efficiency; ηcycle- efficiency of the thermodynamic cycle)
• At a given air excess,
– QC - associated with cross sectional average gas velocity
– QV - connected to gas residence time

11. Volume and Surface Loadings of
Suspension Fired Boilers

12. Burn-up Limit and Exit Temperature
Limit for Tangentially Fired Pulverized
Coal Boilers

13. Types of Fuels Used and Different
Combustion Systems
Size comparison:
Combustion chamber of same thermal power boiler with
different types of fuels, Gas, liquid and solid

14. Fire Tube (Smoke Tube) Boilers
•
•
•
•
Maximum pressure 20 Bar
No superheated steam
Gas/oil fired
Industrial applications
Chimney
Steam
Water
Burner
Water
Steam
at 150°C
Chimney
Water
350°C
Burner
200°C
Water

15. Water Tube Boilers
Water tube boiler- drum boiler
•
•
•
•
•
•
Pressures - up to 80 - 100 Bar
Steam - Superheated steam
Fuel - Gas/oil/solid
Water circulation: Natural or forced
Steam flow through tubes
Applications - Industrial and power plants

16. Water Wall Boilers –Large Scale
Water tube boiler - once
Installations
through boiler
• Pressure - High, up to 80 200 Bar
• Steam - Superheated/
Supercritical steam
• Fuel - Gas/oil/solid
• Forced water /steam flow
through tubes
• Applications - Power plants

17. Gas/Liquid/Solid Powder Fuel
Burning Systems
• Use different types of
burners as appropriate for
the fuel
• Gas - least complicated
o lean premixed methods are
employed
• Liquid – Lean premixed/prevaporized
o Low excess air burners and
staged burning employed
• Solid – premixed with air
o Staged burning and reburning techniques are
applied

18. Solid Fuel Burning Systems
• Grate Furnace
– Solid fuel low burning rate
– No internal emission control
• Pulverized Coal
– High burning rate
• Fluidized Bed
– Solid fuel
– emission control
• Bubbling Circulation Bed
– Solid fuel
– High burning rate & emission control

19. Solid Fuel Burning Systems Applications
1. Bubbling Fluidized Bed
(BFB)

20. Components of an FBC
(Fluidized Bed Combustion) Boiler

21. Gas/Liquid Fuel Burning Systems
• Oil burning
– high burning rate
– Emission control
– Expensive
• Gas
– Very good fuel
– Uneconomical to burn in
boilers for steam power

22. Solid Fuel Burning Systems
Applications Contd…
Circulating Fluidized
Bed (CFB)

23. Solid Fuel Burning Systems Applications
Contd… (Pressurized Fluidized Bed Combustion)

24. Oil Burners - Stabilized
by Flame Holder
Blockage ratio :
A disc
B=
A ex. air nozzle
Swirl number :
S0 =
GΦ
G x ⋅ R throat

25. Oil Burners Stabilized by Swirl

26. Solid Fuel Burning Systems
Burners for Powder
• Factors influencing ignition
– Size distribution of the fuel
– Properties of the fuel
– Burner design and interaction with the
combustion chamber
• Two main groups of pulverized fuel burners
1. Jet burners
2. Swirl Burners

27. Solid Fuel Burning Systems
Burners for Powder Contd…
Direct injection through
a pipe (Arrows ⇨
entrainment of gas)
Jet burner arranged
for pulverized fuel

28. Solid Fuel Burning Systems
Burners for Powder Contd…
Tangential Burners for
large coal fired boilers

29. Solid Fuel Burning Systems
(Pulverized Coal Combustion)
courtesy of Hitachi Ltd. Group
• 1st generation: Low-NOx burner ‘HT-NR burner’
– based on "In-flame NOx reduction” concept
• 2nd generation: Low-NOx burner ‘HT-NR2’
– with further enhanced NOx decomposition capability
– extremely reliable and has many applications in both the domestic and
foreign industrial and power boiler markets
• 3rd generation: Low-NOx burner ‘HT-NR3’
– further reduced NOx emission levels, improved combustion efficiency
and ease of maintenance
– a wider and shorter, highly stable flame for excellent fuel combustion
characteristics with extremely Low NOx levels.

30. Solid Fuel Burning Systems
(Pulverized Coal Combustion) Contd…
courtesy of Hitachi Ltd. Group
• Features of the 3rd generation HT-NR3
– Rapid ignition with Flame Stabilizing Ring
– Effective separation of outer air to produce reducing
conditions in the center zone
– PC concentration for higher combustion efficiency in
reducing condition

31. Emission Control Methods in
Boiler Designs
Nox Reduction
• Nonpremixed
combustion
(Furnace)
– Re-burning
technique for NOx
reduction

32. Solid Fuel Burning Systems
(Pulverized Coal Combustion -Hitachi)
History of Low NOx Burners

33. In-Flame Nox Redcution Mechanism
(Courtesy: Hitachi Ltd. Group)
Reaction in a high
temperature
reducing flame
Characteristics of
NOx Emission
Flame
structure
Oxidizing
Zone
NOx
Reduction
zone

34. Premixed/Nonpremixed Combustion
Exhaust Treatment for NOx Reduction in
Industrial Applications
• Selective Catalytic Reduction (SCR)
• Selective Non Catalytic Reduction (SNCR)

35. Emission Control in Boiler Designs Reduction
of CO2 Emissions by Improving the Efficiency of Coalfired Power Plants
• Subcritical
– E.g. FBC’s
• Supercritical &
Ultrasupercritical
– (New) pulverized
coal combustion
systems
• IGCC
– Integrated
Gasification &
combined cycle

36. Emission Control in Boiler Designs
Nonpremixed Combustion (Furnace)
Oxy-Fuel Combustion
Steam Turbine
Electricity
CO2
Boiler
Cooling
Water
Mechanical Energy
Cooler &
Condenser
Steam
Condenser
CO2
Compression
Sulfur
Removal
Cooler &
Compressor
Particle
Removal
Heat
Nitrogen
Heat
Water
Sulfur
Water
Air Separation
fuel
Fly Ash
• Extra cost of oxygen
Oxygen
Recycle (mainly
CO2 & water vapor)
• Leak tight operation
requirement
Energy
Air
Bottom
Ash
• CO2 capture

37. Emission Control in Boiler Designs
Pre-combustion Capture (IGCC)

38. Emission Control in Boiler Designs
Post-combustion Capture

40. Summary
This section was focused on different aspects on
combustion in boilers such as,
• An overview - general boiler designs and applications
– Heat balance/heat transfer/ heat availability/excess air/size and shape of
the combustor
– Boiler configurations: Smoke tube (Fire tube) and water tube/ Water wall
designs Natural Circulation and Forced Circulation
• Types of fuels used and different combustion configurations
– Gas, liquid and solid fuels
• Types of burners used and the related emission control
– Gas burner designs/Liquid fuel burner designs/Solid fuel PC burners,
burning beds etc.
• Emission cleaning methods in boiler operation
– Species of concerns and appropriate techniques/layouts