Fireball Formation and Combustion of Coal in a Boiler
1. Combustion of Coal in
Boiler and Fireball
Formation
Prepared By:
Tarun Dogra
Animesh Okhade
Zalak Shah
2. Combustion
Combustion is a chemical process in which a substance reacts
rapidly with oxygen and gives off heat.
3. Combustion Reactions
C+ O2 = CO2 + 8084 kCals/kg of carbon
2C + O2 = 2CO + 2430kCals/kg of carbon
2H2 + O2 = 2H2O + 28,922 kCals/kg of hydrogen
S + O2 = SO + 2,224 kCals/kg of sulphur
4. Complete vs Incomplete Combustion
Complete Combustion
• Involves complete burning of
fuel.
• It takes place when there is
constant and enough oxygen
supply as well as sufficient
temperature.
• In complete combustion limited
number of products are formed
in contrast to incomplete
combustion. If case of
hydrocarbon, only carbon
dioxide and water is produced.
• Results in more energy.
6. 3T’s of Combustion
1. Temperature high enough to ignite the fuel,
2. Turbulence vigorous enough for the fuel constituents
to be exposed to the oxygen of the air, and
3. Time long enough to assure complete combustion.
The three requirements are best met by pulverized coal,
which is forced into the furnace by an air stream under high
pressure and is ignited as it enters through a nozzle.
8. Excess Air
To ensure there is enough oxygen to completely react with the fuel,
extra combustion air is usually supplied. This extra air, called “Excess
Air”.
• Reduces amount of carbon monoxide, unburned fuel, soot and
smoke in exhaust.
• Prevents surface fouling from emissions.
Too much excess air can lower boiler efficiency.
o Wastes fuel by absorbing combustion heat and transporting it out
the exhaust flue.
o Consider, too, that nitrogen, which makes up about eighty
percent of the air, plays no role chemically to produce heat. It
does, however, add significantly to the weight of gas that absorbs
heat energy.
9. Various Elements of Coal
1. Volatile Matter
2. Fixed Carbon
3. Ash Content, Moisture and other minerals.
10. Pulverization
Pulverization process coverts coal into powdered form with particle
size of about 75micron.
Advantages:
Increases the surface area of coal and hence fuel-air contact area
increases.
Reduces amount of excess air required.
Ability to fire varying quality of coal.
Quick responses to changes in load.
Free from clinker or slagging troubles.
Large amount of heat release possible.
11. Pulverization
Disadvantages:
Capital cost is high as it requires additional pulverization system.
Operating cost is also high.
This system produces fly ash (fine dust), which requires costly fly-
ash removal equipment like ESP.
Flame temperatures are high, therefore it is necessary to provide
water cooled walls for safety of furnace.
The possibility of explosion is more as coal burns like a gas.
The storage of powdered coal requires special attention and high
protection from fire hazards.
13. Why do we need Efficient
Combustion ?
Pulverized unburned coal may fall back into the boiler
Erosion of mill, pipes and burner components
Increased levels of Nitrogen oxides and other harmful gases
Flame instability
14. Combustion Efficiency
14
Combustion Efficiency = 100 - (Stack heat losses/Fuel heating value*100)
Note: Efficiency calculated in percentage.
It is a measure of how effectively energy from the fuel is converted into useful energy.
15. Factors affecting Combustion Efficiency
Amount of excess air
Stack Gas exit temperature
Reheat Stages
Steam temp & pressure
15
19. Step 1. Pulverization of Coal
Coal crushed to the size of 25mm reaches coal mill
Further crushed to the size of 75 micron before firing in the boiler
Coal pulverized to increase fuel-air contact area
Feeding rate of coal according to the boiler demand controlled by computers
19
20. Step 2. Boiler Start-up
20
Oils used for start-up
LDO (Light Diesel
Oil)
HFO (Heavy Fuel Oil)
22. 22
Methods for NOx reduction in the boiler during combustion
NOx emission harmful to environment
SOFA technique used to reduce the emission levels of NOx
Reduces combustion efficiency but helps to maintain NOx emission levels
23. Fuel Firing System
• Provide controlled, efficient conversion of the chemical energy of
the fuel into heat energy which in turn is transferred to heat
absorbing surfaces.
• It introduces the fuel and air for combustion, mix these reactants,
ignite the combustible mixture and distribute the flame envelope
and products of combustion.
24. Characteristics of Ideal Firing System
No excess oxygen or unburned combustibles in the end products of
combustion.
A low rate of auxiliary ignition energy input to initiate combustion.
An economic reaction rate between fuel and oxygen compatible
with acceptable N0x & SOx formation.
Effective Handling and disposal of solid fuel impurities.
Wide and stable firing range.
Fast responses to changes in firing rate.
Low maintenance costs.
25. Firing System Concepts
In the first concept, the fuel and air are divided and
distributed into many similar streams. Each stream is treated
independently to provide multiple flame envelopes called
multi flame envelope concept.
In the second concept a single flame envelope is produced, by
providing interaction between all streams of air and fuel
introduced into the furnace. This is called single flame
envelope concept.
26. Single-flame vs Multi-flame
Single-flame
• Provides interaction between all streams of fuel and air
introduced into the furnace and so precise subdivision of fuel and
air at each point of admission is not required.
• Allows more time for contact between all fuel and air molecules.
• Mechanical turbulence is sustained throughout the furnace.
Multi-flame
• Requires accurate subdivision fuel and air supplied to the furnace.
• Limits the opportunity for sustained mechanical turbulence.
30. Components of Fuel Firing System
Ignition System
Provide ignition energy to the flammable mixture of fuel and air
introduced to the furnace.
Auxiliary Ignition System
For igniting the oil while starting the oil burners, ignitors are used
in the firing system.
31. Components of Fuel Firing System
Oil Guns and Atomisers
• Fuel oils like light diesel oil (LDO), heavy fuel oil (HFO) are used in
boilers as supplement fuel.
• These fuel oils are burnt by spray combustion method wherein the
oil is split into fine droplets (atomised) and distributed into the
furnace in a spray form in a controlled manner.
• Oil guns installed in windbox with atomiser mounted at their tips
provide this oil spray to the furnace.
32. Components of Fuel Firing System
Flame Scanners/Flame Detecting System
- Must be reliable.
- Sensible to discern the minimum flame envelope.
- Reaction time must be minimum.
Flame Scanner Types
- Ultraviolet Scanners
- Visible Light Scanners
- Infrared Scanners
