1. Page | 1
A REPORT
ON
Fireball Formation
And
Combustion of Coal in Boiler
By:
Tarun Dogra 2013A4PS299P
Animesh Okhade 2013B3A1575G
Shah Zalak 2013A4PS251P
At
Adani Power Maharashtra Limited, Tirora
A Practice School-I station of
Birla Institute of Technology & Science, Pilani
June/July 2015

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A REPORT
ON
Fireball Formation
And
Combustion of Coal in Boiler
By:
Tarun Dogra 2013A4PS299P
Animesh Okhade 2013B3A1575G
Shah Zalak 2013A4PS251P
Prepared in fulfilment of the
Practice School-I Course No.
BITS F221
At
Adani Power Maharashtra Limited, Tirora
A Practice School-I station of
Birla Institute of Technology & Science, Pilani
June/July 2015

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1.Acknowledgement
We would like to express our sincere gratitude to our College, Birla Institute
of Technology and Science for conducting Practice school–I which gives
industrial experience and Adani power for giving me this opportunity to visit
the plant and prepare a report. I would like to thank our PS-1 instructor, Mr.
Abhijit Asati sir for his guidance and support. Also we would like to thank our
co-instructor Mr. Vishal Parwani for his support.
We would like to thank our station head, Mr C P Sahoo sir and Dy. General
Manager (Technical training), Dr. Vijay V Gandhewar and Asst. Manager
(Technical training), Mr Sanjay Kr. Kajuri for their immense efforts made for
us to learn about a Thermal power plant.
We would also like to thank Mr. Pritish Pandey and all other senior engineers
who were our mentors during plant visits for teaching us in spite of their busy
schedule.
Place: Adani Power, Tirora
Date: July 12, 2015

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BIRLA INTITUTE OF TECHNOLOGY & SCIENCE
PILANI (RAJASTHAN)
Practice School Division
Station: Adani Power Maharashtra Limited, Tirora Centre: Nagpur/Tirora
Duration: From: 22 May 2015 to: 16 July 2015
Date of Submission: 12 July 2015
Title of the Project: Fireball Formation and Combustion of Coal in Boiler
ID No. Names of Students
2013A4PS299P Tarun Dogra
2013B3A1575G Animesh Okhade
2013A4PS251P Shah Zalak
Name of the PS Faculty: Dr. Abhijit Asati
Key Words: Combustion, Pulverisation, Burners, Firing, Excess Air, Furnace.
Project Area: Fireball Formation and Combustion of Coal in Boiler
Abstract: This report gives an overview of combustion process and its
optimisation, at Adani Power Maharashtra Ltd., Tirora.
Tarun Dogra
Animesh Okhade
Shah Zalak Dr. Abhijit Asati
Signature of Students Signature of PS Faculty
Date: July 12, 2015 Date: July 12, 2015

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3.Table of Contents
Title Page
1. Acknowledgement 3
2. Abstract 4
3. Table of contents 5
4. Introduction 7
5. Combustion 8
5.1. Principles of combustion 8
5.2. Combustion Reactions 10
5.3. Significance of various elements of coal 10
5.4. Coal Analysis in Tirora 11
5.5. Excess Air 11
5.6. 3T’s of Combustion 13
5.7. Pulverization 14
5.8. Reasons for improper Combustion 14
5.9. Combustion generated Pollutants 15
5.10. Need for Combustion Optimization 17
6. Combustion Analysis 17
6.1. Why perform Combustion Analysis 18
6.2. What’s measured? 19
6.3. Measurement Tools 24
6.4. Using the Measurements 25
7. Fuel Firing System 29
7.1. Introduction 29
7.2. Characteristics of Ideal Firing System 29
7.3. Firing System Concepts 30
7.4. Comparison of single and Multiflame concepts 30
7.5. Methods of Fuel Firing 32
7.6. Components of Fuel Firing System 37
7.7. Oil Firing System 50
8. Boiler 53
9. Furnace 55
10.Burners 58
11.Improving Boiler Efficiency 61
12.Methods for lowering NOX and CO levels 66
13.Conclusion 70
14.References 71

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List of Figures
Title Page
1. Combustion Efficiency vs Excess Air 12
2. Boiler Heat Losses 18
3. Excess Air Supplied 20
4. Too little combustion causes CO to form 21
5. Combustion gas concentrations 22
6. Flue temperature vs efficiency 23
7. Combustion Efficiency vs Excess Air 26
8. Vertical Firing System 32
9. Wall Firing Burner 34
10.Burner for wall firing 34
11.Eddy Plate Ignitor arrangement 39
12.Eddy plate Ignitor 39
13.I.F.M. Ignitor 41
14.H.E.A. Ignitor 42
15.Re-circulating pattern in H.E.A. Ignition system 43
16.Concentric and Parallel Oil Gun 44
17.Mechanical/Pressure Atomiser 46
18.Internal Mixing Atomiser 47
19.Boiler Block Diagram 55
20.Comparison of furnaces for different fuels 57
21.Effect of excess air on efficiency 65
22.Concept of in-flame NOx reduction 66
23.The structure of HT-NR3 Burner 67
24.NOx performance of HT-NR Series 68
25.Concept of new TSC System 79
List of Tables
1. Coal Specifications in Adani Power, Tirora 11
2. Possible savings per year with a
5% improvement in boiler efficiency 19
3. LDO parameters 50
4. HFO parameters 51

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4.Introduction
A thermal power station works on the basic principle that heat liberated by
burning fuel is converted into mechanical work by means of a suitable working
fluid. The mechanical work is converted into electrical energy by means Of
generator. In a steam power station, heat is realized by burning fuel, this heat
is taken by water, which works as the working fuel. Water is converted into
steam as it receives heat in the boiler. The steam then expands in turbine
producing mechanical work, which is then converted into electrical energy
through a generator. The exhaust steam from the turbine is then condensed in
the condenser and the condensate is there after pumped to the boiler where it
again receives heat and the cycle is repeated. The basic theoretical working
cycle is of a steam power plant is ‘THE RANKINE CYCLE’. The modern steam
power plant uses ‘MODIFIED RANKINE CYCLE’, which includes reheating,
superheating and regenerative feed water heating.
There are numerous concerns related to the increasing costs in the thermal
processing industry today. With the globalization of manufacturing creating a
hyper competitive environment and increasing cost on all manufacturing
processes where energy and labour are involved, every aspect related to
efficiency is being reviewed. As energy costs increase the pressure of being
more efficient is even more important.
Environmental friendly and efficient use of capital equipment is a key
component to addressing the rising costs. Providing a more efficient operation,
resource costs decline while output increases. Efficient and environmentally
sound operations can be accomplished by taking advantage of technology.
Energy savings related to equipment remains a critical component. Many
companies are looking at new technology to continuously improve on
operating efficiencies in every aspect of an operation. Controls and technology
related to temperature and gas consumption are practices that companies are
utilizing delivering very short ROI with current energy costs and environmental
concerns. Systems and procedures today can be implemented to ensure that
the most environmental friendly and efficient ratio of air to gas are used to
combust gaseous fuels and measuring the by-products of the exhaust.

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5. Combustion
Everywhere, at all times, oxygen combines with other elements. This
General process is called Oxidation.
Burning, or Combustion, is a special form of oxidation: Oxygen combines
rapidly with certain types of fuels, such as coal, oil, gas or wood, and
substantial amounts of heat are liberated.
The Degree of Flammability depends on how easy it is to turn into a gas,
because nothing truly burns until it is a gas. This in turn depends on the nature
and quantity of the substance, compared with amount of heat available to
start combustion.
The task of burning commercial fuels in actual furnaces includes:
1) Preparing the fuel and air;
2) Converting the complex fuel into elementary fuels;
3) Bringing these fuels and air together in the right propositions and at the
proper temperature for ignition and combustion;
4) Transferring heat from the products of combustion to the boiler or other
surfaces, while retaining enough heat in the combustion zone to maintain
volatilization and ignition.
All these actions occur at the same time in any furnace, and each particle of
fuel traces the entire sequence, in order, in its brief passage through the
furnace.
5.1. Principles of Combustion
The most suitable combustible fuels are those made up primarily of carbon or
hydrocarbons where the main constituents are carbon and hydrogen. All fossil
fuels derived originally from carboniferous matter fall into this category. Other
elements such as sulphur are also combustible and, if present in the fuel,
contribute to the generation of heat but produce undesirable products such as
sulphur dioxide. Pure hydrocarbon fuels have the advantage of producing
carbon dioxide and water vapour, both relatively innocuous products. Oxygen
is readily available from the atmosphere but is not pure. The major constituent
of the air is nitrogen and, although the relatively inert gas passes through the
combustion process largely unreacted, it does produce some undesirable
nitrogen oxides at high temperatures. Generally combustion for large scale
heat generation should proceed continuously under stable conditions. This
implies that the fuel and oxygen must be supplied to the combustion zone
continuously and the products of combustion likewise removed. Within the

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combustion zone the air containing oxygen must be brought into intimate
contact with the fuel, irrespective of whether it is in gaseous, liquid or solid
form. This requires that fuel be well dispersed and the air turbulent in the
combustion zone. Liquid and solid fuels require suitable division into small
particles to ensure intimate mixing with the air.
Solid fuels usually produce significant amounts of ash which must be removed
from the combustion zone but only after sufficient time has been allowed for
the combustible elements within the fuel particles to have fully reacted with
the oxygen of the air.
The purpose of combustion is to produce heat which is radiated from the
combustion zone or carried away by the gaseous products of combustion. This
heat is to be transferred effectively to the working fluid of the thermodynamic
cycle but a certain portion is inevitably lost to the environment when the
exhaust gases are discharged to the atmosphere.
The main function of oil and coal burning system is to convert chemical energy
into heat energy, which is utilized by the components of the boiler to convert
water into the steam, which drives the turbine. The combustion element of
fuel consists of carbon, hydrogen and small amount of sulphur. The exhaust
gases released after combustion contains CO2, SO2 and CO, etc. When the coal
is burnt with oxygen following reaction takes place and large amount of heat is
released.
𝐶 + 𝑂2 → 𝐶𝑂2
𝑆 + 𝑂2 → 𝑆𝑂2
𝐶 + 𝑂2 → 2𝐶O
The average composition of air is
79% nitrogen and 21% oxygen by volume
77% nitrogen and 23% oxygen by weight
 During combustion process nitrogen does not burn but passes through
the chimney.
 The amount of air required to burn any fuel can be calculated if the
amount of the elements present in the fuel are known.
 The amount of air to burn is known as theoretical air. If this quantity is
not sufficient for complete combustion process then extra amount of air
is supplied, known as excess air. In combustion process, turbulence,
time, temperature and combustion efficiency are the important
parameters to be considered.
The maximum combustion efficiency depends on
1. Design of boiler

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2. Fuel used
3. Skill in obtaining combustion within the minimum amount of excess air.
5.2. 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
5.3. Significance of Various elements of coal
Fixed carbon:
Solid fuel left after volatile matter is distilled off. It consists of mostly carbon.
•Gives a rough estimate of heating value of coal
Volatile Matter:
It is an index of the gaseous fuels present.
Volatile Matter
•Proportionately increases flame length, and helps in easier ignition of coal.
•Sets minimum limit on the furnace height and volume.
•Influences secondary air requirement and distribution aspects.
•Influences secondary oil support
Ash Content:
Ash is an impurity that will not burn.
•Reduces handling and burning capacity.
•Increases handling costs.
•Affects combustion efficiency and boiler efficiency
•Causes clinkering and slagging.
Moisture Content:
Moisture in coal must be transported, handled and stored. Since it replaces
combustible matter, it decreases the heat content per kg of coal.
•Increases heat loss, due to evaporation and superheating of vapour

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5.4. Coal Analysis in Tirora Plant
Parameters Design coal Best coal Worst coal
Moisture (%) 10 12 11.90
Ash content (%) 37 28.45 41.5
Fixed carbon (%) 25.4 28.05 23.6
Volatile (%) 27.6 31.5 23.5
Carbon (%) 41.6 47.8 35.60
Hydrogen (%) 2.6 2.95 2.23
Sulphur (%) 0.25 0.3 0.45
Nitrogen (%) 1.025 1.20 1.22
Oxygen (%) 7.2 7.3 7.6
Carbonate (%) 0.3 - -
Phosphorus (%) 0.025 - -
Moisture (%) 10 12 11.9
Gross calorific value (kCal/kg)
3927 4527 3331
Hard grove grind ability index
50 53 50
Table 1: Coal Specifications in Adani Power, Tirora
5.5. Excess Air
Insufficient combustion air causes a reduction in fuel efficiency, creates highly
toxic carbon monoxide gas and produces soot. To ensure there is enough
oxygen to completely react with the fuel, extra combustion air is usually
supplied. This extra air, called “Excess Air,” is expressed as the percent air
above the amount theoretically needed for complete combustion. In real-world
combustion, the excess air required for gaseous fuels is typically about 15
percent. Significantly more may be needed for liquid and solid fuels.
Although required, higher excess air comes with a price—it wastes fuel. There
are a number of reasons why this occurs but, stated simply, supply air cools the
combustion system by absorbing heat and transporting it out the exhaust flue.
The more air, the more the cooling. 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.
Figure illustrates how increasing excess air reduces combustion efficiency.

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Fig 1. % Combustion Efficiency vs. % Excess Air (fuel oil)
Using too much excess air is one of the principal causes of poor fuel economy.
For this reason, optimizing excess air usage can be one of the simplest ways to
achieve significant fuel savings.
NOTE: Excess Air and Fuel/Air Mixing
Adding additional excess air is often done to reduce the CO concentration. Too
much excess air can actually have the reverse effect of increasing CO. This
results when fuel and air no longer mix properly in the burner, reducing the
time of contact between oxygen and fuel and inhibiting a complete reaction.
The impact of Excess air on Efficiency
In theory, to have the most efficient combustion in any combustion
process, the quantity of fuel and air would be in a perfect ratio to provide
perfect combustion with no unused fuel or air. This type of theoretical perfect
combustion is called stoichiometric combustion. In practice, however, for
safety and maintenance needs, additional air beyond the theoretical “perfect
ratio” needs to be added to the combustion process—this is referred to as
“excess air.” With boiler combustion, if some excess air is not added to the
combustion process, unburned fuel, soot, smoke, and carbon monoxide
exhaust will create additional emissions and surface fouling. From a safety
standpoint, properly controlling excess air reduces flame instability and other
boiler hazards. Even though excess air is needed from a practical standpoint,
too much excess air can lower boiler efficiency. So a balance must be found
between providing the optimal amount of excess air to achieve ideal
combustion and prevent combustion problems associated with too little excess
air, while not providing too much excess air to reduce boiler efficiency.

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Research has shown that 15% excess air is the optimal amount of excess air to
introduce into the boiler combustion process. While some boilers have been
able to achieve 15% excess air at the top end of a boiler’s firing range, the
challenge presents itself at the lower end of the firing range, or below 60% of
the boiler’s maximum capacity. In general, most boilers tend to increase excess
air requirements as the firing rate of the boiler decreases, leading to lower
efficiency at the lower end of the firing range. To complicate matters, most
boilers operate on the lower end of the firing range—so selecting a boiler that
has low excess air throughout the firing range is important. This will ensure
that you are always operating at high efficiencies.
5.6. 3T’s of Combustion
When coal is burned in the body of a power plant, it is not just set alight like
the coal in a domestic fireplace. It has to burn fast, for power is the energy
converted per second, and it has to burn completely, for efficiency is a
measure of how much energy is usefully harnessed rather than lost. The
efficient combustion of coal needs the “three T’s”,
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.
Time
Incomplete combustion is a problem in fireplaces that largely contributes to
fireplace emission levels. The longer the time combustion gases spend in the
higher temperatures of the combustion zone the more completely they will
combust and subsequently the lower the emissions will be from the fire.
A refractory panel above the fire makes the flames travel a longer distance
before exiting the combustion chamber. This longer path increase the
residence time of the gases thus promoting a better overall burn and a
reduction in emissions.
Temperature
The higher the temperature in the combustion zone the better the overall burn
and the lower the emission.

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Insulating refractory panels placed around the burning helps retain heat in the
combustion zone. This again increases the burn temperature and promotes
better combustion of coal.
Turbulence
The more turbulence that is encountered in the combustion gas path the
better these gases mix with the surrounding oxygen, the more uniform the gas
temperature, and more complete the burn.
5.7. Pulverization
For efficient combustion of coal in a coal fired power plant, pulverizing to a fine
powder for burning is an important requirement.
The basic requirement of coal combustion is that all the carbon particles in coal
should get sufficient air to burn and release the heat.
Because of large size of coal, some of the carbon particles do not come in
contact with the air. These unburnt carbon particles go out with ash, which
causes wastage of fuel and hence loss of efficiency.
The pulverization process coverts coal into powdered form with particle size of
about 75micron. This process increases the surface area of coal and hence fuel-
air contact increases leading to better combustion of coal.
Most coal fired power station boilers use pulverized coal. This technology is
well developed, and there are thousands of units around the world, accounting
for well over 90% of coal fired capacity. The coal is pulverized to a fine powder,
so that less than 2% is +300 micro meter (μm) and 70-75 % is below 75
microns, for a bituminous coal.
This system has many advantages such as ability to fire varying quality of coal,
quick responses to changes in load, use of high pre-heat air temperatures etc.
One of the most popular systems for firing pulverized coal is the tangential
firing using four burners corner to corner to create a fireball at the centre of
the furnace.
5.8. Reasons for improper combustion
 Significant quantities of air in-leakage or “tramp” air into the furnace
 Improper turbulence
 Improper fuel sizing
 Inadequate fuel flows
 Inadequate fuel velocities
 Improper temperatures

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5.9. Combustion generated Pollutants
Nitrogen Oxides (NOX)
The release of oxides of nitrogen (nitrogen oxides and nitrogen dioxides) reacts
with volatile organic compounds in the presence of sunlight to produce ground
level ozone, the primary ingredient in smog. Nitrogen oxide also contributes to
fine particulate matter, or soot. Both smog and soot are linked to a host of
serious health effects. Nitrogen oxides also harms the environment,
contributing to acidification of lakes and streams (acid rain).
Sulphur Dioxide (SO2)
Sulphur dioxide contributes to the formation of microscopic particles
(particulate pollution or soot) that can be inhaled deep into the lungs and
aggravate respiratory conditions such as asthma and chronic bronchitis,
increasing cough and mucous secretion.
Sulphur corrosion
Sulphur corrosion is connected with the presence of alkali metals in coal:
potassium K and sodium Na, which combined with sulphur form alkali
sulphides Na2SO4 and K2SO4 condensing on the tubes’ surface.
They aren’t directly responsible for corrosion because of their melting high-
temperature (Na2 SO4 – 884 °C K2SO4 -1069 °C), however in the presence of
SO3 they form corrosive pirosulfides and trisulfides near the surface of the
evaporator tubes.
Reactions of sulphur corrosion
I. Pirosulfides
Na2SO4 + SO3 ® Na2S2O7 (Tmelt = 389 °C)
K2SO4 + SO3 ® K2S2O7 (Tmelt = 404 °C)
II. Trisulfides: sodium- and potassium-iron:
3Na2SO4 + Fe2O3 + 3SO3 ® 2Na3Fe (SO4)3 (Ttopn= 624 °C)
3K2SO4 + Fe2O3 + 3SO3 ® 2K3Fe (SO4)3 (Ttopn= 618 °C)
Mercury (Hg)
Coal contains trace amounts of mercury that, when burned, enter the
environment and human bodies, effecting intellectual development.
Particulate Matter (PM)
Also known as particle pollution, includes the tiny particles of fly ash and dust
that are expelled from coal burning power plants. Fine particles are a mixture

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of variety of different compounds and pollutants that originate primarily from
combustion sources such as power plants, but also diesel trucks and buses,
cars, etc.
Fine particles are either emitted directly from these combustion sources or are
formed in the atmosphere through complex oxidation reactions involving
gases, such as sulphur dioxide or nitrogen dioxides.
Among particles, fine particles are of particular concern because they are so
tiny that they can be inhaled deeply, thus evading the human lungs.
Smog
It is the chemical reaction of sunlight, nitrogen oxides, and volatile organic
compounds in the atmosphere, which leaves airborne particles and ground
level ozone.
Carbon Dioxide (CO2)
It is the most significant greenhouse gas that contributes to global warming.
The dangers of global warming include disruption of global weather patterns
and ecosystems, flooding, severe storms, and droughts. A warming climate will
also extend the range of infectious diseases.
Corrosion hazard due to co-firing biomass and coal – chlorine corrosion
Chlorine (Cl2) is particularly corrosive for steel at high temperature. It causes
active oxidation of metal removing the protective layer of iron oxides, which
are converted into porous, not protecting deposit.
Sources of molecular chlorine (Cl2) near a tube surface are present in flue gas
hydrogen chloride (HCl) and present in deposit alkali metals chlorides (KCl and
NaCl).
Mechanism of chlorine corrosion
Chlorine diffuses through deposit to metal and reacts with it
Fe + Cl2 = FeCl2(s)
Metal chlorides formed on the metal surface have high pressure at the
temperature of 500 °C, therefore they diffuse through the protecting
magnetite layer (Fe3O4) and damage it. After this iron chloride meets oxygen
and undergoes oxidation:
2FeCl2 (g) + 3/2O2 ® Fe2O3(s) + 2Cl2
3FeCl2 (g) + 2O2 ® Fe3O4(s) + 3Cl2
As a result a new layer of iron oxides is formed, however porous and not
protecting. Moreover released chlorine can return to metal.

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Anti-corrosion protecting measures in pulverized coal-fired boilers
1. Maintenance of oxidizing atmosphere in the boundary layer at the furnace
walls.
2. Reduction of temperature of steam approximately to 537 °C.
3. Application of protecting coatings.
4. Additives to flue gas neutralizing some corrosive agents.
5. Reduction of sulphur, chlorine and alkali metals in fuels.
5.10. Need for combustion optimisation
Operating a boiler that is not optimised, or tuned, can
 fallout of pulverised fuel, blocked pipes, or high mill pressure
 erosion of mill, pipes and burner components
 poor burner ignition, and flame instability and dislocation
 incorrect primary and secondary air-to-fuel ratios
 increased nitrous oxide production
 increased levels of unburnt carbon
 increased excess-air requirements
 increased erosion between furnace and boiler exit
 reduced boiler efficiency
 localised furnace problems that can include inappropriate superheaters and
reheater temperature profiles,
 Increased slagging and greater water-wall wastage.
6. Combustion Analysis
Combustion analysis is part of a process intended to improve fuel economy,
reduce undesirable exhaust emissions and improve the safety of fuel burning
equipment. Combustion analysis begins with the measurement of flue gas
concentrations and gas temperature, and may include the measurement of
draft pressure and soot level.
To measure gas concentration, a probe is inserted into the exhaust flue and a
gas sample drawn out. Exhaust gas temperature is measured using a
thermocouple positioned to measure the highest exhaust gas temperature.
Soot is measured from a gas sample drawn off the exhaust flue. Draft is the
differential pressure between the inside and outside of the exhaust flue.
Once these measurements are made, the data is interpreted using calculated
combustion parameters such as combustion efficiency and excess air. A more in

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depth analysis will examine the concentration of the undesirable products
described earlier.
6.1. Why Perform Combustion Analysis?
Improve Fuel Efficiency
The largest sources of boiler heat losses are shown Figure. Heat energy leaving
the system exhaust flue (or stack) is often the largest single source of lost fuel
energy and is made up of the Dry Gas loss and Latent Heat Loss. Although
some flue loss is unavoidable, an equipment tune-up using combustion analysis
data can often significantly reduce this source of heat loss and save fuel costs
by improving fuel efficiency. Table 1 gives examples of yearly cost savings that
can be realized by improving equipment efficiency by five percent.
Fig 2. Boiler Heat Losses
Boiler HP
Fuel Cost
$0.75 $1.00 $1.50 $2.00
100 $3,635 $4,847 $7,271 $9,694
200 $7,271 $9,694 $14,541 $19,389
300 $10,906 $14,541 $21,812 $29,083
500 $18,177 $24,236 $36,354 $48,471
800 $29,083 $38,777 $58,166 $77,554
Table 2. Possible savings per year with a 5% improvement in boiler efficiency (based on
3000 hours per year of operation).
Reduce Emissions

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Carbon monoxide, sulphur dioxide, nitrogen oxides and particles are
undesirable emissions associated with burning fossil fuels. These compounds
are toxic, contribute to acid rain and smog and can ultimately cause respiratory
problems. Federal and state laws govern the permissible emission rates for
these pollutants under the guidance of the Clean Air Act and oversight of the
federal Environmental Protection Agency (EPA). State and local environmental
agencies also exert authority in regulating the emissions of these pollutants.
Combustion analysis is performed to monitor toxic and acid rain forming
emissions in order to meet these federal, state and local regulations.
For specific information on emissions as they relate to a particular region,
contact the local regulatory agency.
Improve Safety
Good equipment maintenance practice, which includes combustion analysis,
enables the boiler technician to fully verify and maintain the equipment
operating specifications for safe and efficient operation.
Many boiler manufacturers suggest that flue gas analysis be performed at least
monthly. Boiler adjustments that affect combustion will tend to drift with time.
Wind conditions and seasonal changes in temperature and barometric
pressure can cause the excess air in a system to fluctuate several percent. A
reduction in excess air can cause, in turn, a rapid increase of highly toxic
carbon monoxide and explosive gases, resulting in rapid deterioration in
system safety and efficiency. Low draft pressures in the flue can further result
in these combustion gases building up in the combustion chamber or being
vented indoors. Excessive draft pressures in the flue also can cause turbulence
in the system. This can prevent complete combustion and pull explosive gases
into the flue or cause flame impingement and damage in the combustion
chamber and to the heat exchanger material.
6.2. What’s measured?
Combustion analysis involves the measurement of gas concentrations,
temperatures and pressure for boiler tune-ups, emissions checks and safety
improvements. Parameters that are commonly examined include:
• Oxygen (O2)
• Carbon Monoxide (CO)
• Carbon Dioxide (CO2)
• Exhaust gas temperature
• Supplied combustion air temperature
• Draft
• Nitric Oxide (NO)

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• Nitrogen Dioxide (NO2)
• Sulphur Dioxide (SO2)
Oxygen, Carbon Monoxide and Carbon Dioxide
As described earlier, simple combustion involves the reaction of oxygen in the
air with carbon and hydrogen in the fuel, to form carbon dioxide and water
and produce heat. Under ideal conditions, the only gases in the exhaust flue
are CO2, water vapour and nitrogen from the combustion air.
When O2 appears in the flue exhaust, it usually means that more air (20.9
percent of which is O2) was supplied than was needed for complete
combustion to occur. Some O2 is left over. In other words, the measurement
of O2 gas in the flue indicates that extra combustion air, or Excess Air, was
supplied to the combustion reaction. This is demonstrated in Figure where the
bar on the right represents the exhaust gas composition.
Fig 3. Excess Air Supplied
When too little air is supplied to the burner, there is not enough oxygen to
completely form CO2 with all the carbon in the fuel. Instead, some oxygen
combines with carbon to form carbon monoxide (CO). CO is a highly toxic gas
associated with incomplete combustion and efforts must be made to minimize
its formation. This effort goes hand-in-hand with improving fuel efficiency and
reducing soot generation. This formation of CO gas is illustrated in Figure.
Fig 4. Too little combustion causes carbon monoxide to form.
As a rule, the most efficient and cost-effective use of fuel takes place when the
CO2 concentration in the exhaust is maximized. Theoretically, this occurs when

21. Page | 21
there is just enough O2 in the supplied air to react with all the carbon in the
fuel supplied. This quantity of supplied air is often referred to as the theoretical
air.
The theoretical air required for the combustion reaction depends on fuel
composition and the rate at which the fuel is used (e.g. pounds per hour, cubic
feet per minute, etc.). In real-world combustion, factors such as the condition
of the burner and the burner design also influence the amount of air that is
needed. The theoretical air is rarely enough.
The general relationship between the O2 supplied and the concentration of
CO2 and CO in the exhaust is illustrated in Figure. As the air level is increased
and approaches 100% of the theoretical air, the concentration of CO molecules
decreases rapidly as they pick up additional oxygen atoms and form CO2.
Still more combustion air and CO2 reaches a maximum value. After that, air
begins to dilute the exhaust gases, causing the CO2 concentration to drop. The
maximum value of CO2 is dependent on the type of fuel used.
Fig 5. Combustion Gas Concentrations at Percent
Temperature and Draft
Exhaust Gas Temperature and Supplied Combustion Air Temperature
Heat leaving the exhaust flue with the hot gases is not transferred to do useful
work, such as producing steam. This heat loss becomes a major cause of lower
fuel efficiency. Because the heat content of the exhaust gas is proportional to

22. Page | 22
its temperature, the fuel efficiency drops as the temperature increases. An
example of efficiency loss due to the increase in stack gas temperature is
shown in Figure.
When determining heat loss from the flue, the temperature of the supply air is
subtracted from the flue gas temperature. This gives the net temperature and
accounts for the heat supplied to the system by the supply air.
Some heat loss is unavoidable. The temperature in the flue needs to remain
high enough to avoid condensation inside the stack. One process for
recovering some of the heat loss in the exhaust is to use the hot flue gases to
preheat the supply air before it is introduced into the combustion chamber.
Fig 6. Flue Temperature vs. % Efficiency (fuel oil)
Draft
Draft refers to the flow of gases through the heat generating equipment,
beginning with the introduction of air at the back of the burner. Once
combustion occurs, the heated gas leaves the combustion chamber, passes
heat exchangers and exits the exhaust stack. Depending upon the design of the
equipment, draft is natural, meaning combustion air is pulled in by buoyant
heated gases venting up the stack, or it is mechanical, meaning air is pushed or
pulled through the system by a fan. Often, draft relies on a combination of
both natural and mechanical means.
Adequate draft is typically verified by measuring the pressure in the exhaust
stack. The manufacturer of the fuel burning equipment provides specifications
for the required draft pressure and locations for making the draft
measurement. Measurement is important since environmental influences such
as changes in barometric pressure and ambient temperature can influence the
flow. Typical draft pressures are in the range of –0.5 to 0.5 inches of water
column.

23. Page | 23
Excessive draft can prevent heat transfer to the system and increase the flue
temperature if the excess air percentage is not elevated. If the excess air
increases from the high draft, the flue temperature will decrease. Low draft
pressures can cause temperatures in the flue to decrease, allowing water
vapour to condense in the flue, forming acid and damaging the system.
Nitrogen Oxides (NOx)
Nitrogen oxides, principally nitric oxide (NO) and nitrogen dioxide (NO2), are
pollutant gases that contribute to the formation of acid rain, ozone and smog.
Nitrogen oxides result when oxygen combines with nitrogen in the air or in the
fuel. NO is generated first at high flame temperatures, then oxidizes further to
form NO2 at cooler temperatures in the stack or after being exhausted.
The NO concentration is often measured alone, and the NO2 concentration is
generally assumed to comprise an additional five percent of the total nitrogen
oxides. The nitrogen oxide gas concentrations are sometimes combined and
referred to as the NOX concentration.
Sulphur Dioxide (SO2)
Sulphur dioxide combines with water vapour in the exhaust to form a sulphuric
acid mist. Airborne sulphuric acid is a pollutant in fog, smog, acid rain and
snow, ending up in the soil and ground water. Sulphur dioxide itself is corrosive
and harmful to the environment.
Sulphur dioxide occurs when the fuel contains sulphur and where the emission
levels are directly related to the amount of sulphur in the fuel. The most cost-
effective way to reduce sulphur emissions is to select a Low-sulphur or de-
sulphured fuel.
Hydrocarbons (HCs)/Volatile Organic Compounds (VOCs)
Organic compounds are sometimes present in the combustion exhaust
products because of incomplete combustion. Hydrocarbons (HCs), or volatile
organic compounds (VOCs), are best reduced through proper burner
maintenance and by maintaining the proper air/fuel mixture.
Soot
Soot is the black smoke commonly seen in the exhaust of diesel trucks, and is
present whenever fuel oils or solid fuels are burned. Excessive soot is
undesirable because it indicates poor combustion and is responsible for
coating internal heat transfer surfaces, preventing good thermal conductivity.
Over time, serious damage to the heat exchanger can occur.

24. Page | 24
Soot is primarily unburned carbon, and is formed for the same reasons CO is
formed—insufficient combustion air, poor mixing and low flame temperature.
As with CO, it is usually impossible or impractical to entirely eliminate soot
formation for some fuel types.
6.3. Measurement Tools
Manual Gas Measurements
The Orsat analyser is a gas concentration analysis tool typically used to
manually sample CO2, O2 and CO from the flue of a combustion system. The
Orsat analyser determines the gas concentrations from a sample of gas
extracted from the flue and bubbled through solutions of reagents that
selectively absorb each gas. By measuring the decrease in gas volume over the
liquid reagents, the amount of gas absorbed is indicated. From this
information, stack gas concentration is calculated. Manual gas measurements
are time consuming and do not accurately reflect real-time adjustments made
to a system.
Portable Electronic Instruments
In recent years, electronic instruments such as the CA-CALC™ Combustion
Analyser from TSI Incorporated have been developed to analyse combustion
routinely for tune-ups, maintenance and emissions monitoring. These
instruments are extractive. They remove a sample from the stack or flue with a
vacuum pump and then analyse the sample using electrochemical gas sensors.
Thermocouples are used for stack and combustion air temperature
measurements, and a pressure transducer is used for the draft measurement.
An on-board computer performs the common combustion calculations,
eliminating the need to use tables or perform tedious calculations. Electronic
instruments show the results of boiler adjustments in real-time and give more
accurate information to help ensure that a system has been tuned properly.
Continuous Emission Monitors
Continuous emission monitors, or CEMS, are a class of electronic instruments
designed to measure exhaust stack gases and temperature continuously. CEMs
are sometimes used for combustion control, but typically are used for
monitoring pollutant gas emissions as required by government regulations.
CEMs can use both extractive and in-situ (sensors in the stack) sampling
methods, and employ a variety of electronic sensor technologies for gas

25. Page | 25
detection. CEMs are used most often on larger installations or when required
by regulatory agencies.
6.4. Using the Measurements
Once flue gas and temperature measurements are made, combustion
parameters are calculated to help evaluate the operating performance of the
furnace or boiler. Typical combustion parameters include:
• Excess Air
• Carbon Dioxide
• Combustion Efficiency
• O2 Reference
• Emissions Conversions
Excess Air
A good estimate of excess air can be determined using the following formula.
This calculation uses the oxygen concentration measured in the exhaust. If the
CO concentration is very high, it may also be included in the excess air
calculation.
%Excess Air = %O2 measured / (20.9-%O2 measured) *100
Although required, higher excess air comes with a price—it wastes fuel. There
are a number of reasons why this occurs but, stated simply, supply air cools the
combustion system by absorbing heat and transporting it out the exhaust flue.
The more air, the more the cooling. 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.
Figure illustrates how increasing excess air reduces combustion efficiency.

26. Page | 26
Fig 7. Combustion Efficiency vs Excess Air
Using too much excess air is one of the principal causes of poor fuel economy.
For this reason, optimizing excess air usage can be one of the simplest ways to
achieve significant fuel savings.
Calculating the Carbon Dioxide Concentration
Carbon dioxide (CO2) forms when carbon in the fuel combines with O2 in the
combustion air. When there is just enough O2 supplied to react with the
carbon in the fuel, the CO2 concentration in the stack exhaust is at its highest
level. This is generally at or close to the ideal operating condition for the heat
generating equipment. The maximum possible CO2 exhaust concentration
depends ultimately on the carbon content of the fuel being burned. This
number, the CO2 maximum, appears often in combustion calculations, such as
the one below for determining the percent of CO2 in the exhaust.
%CO2 (by volume) = CO2 (maximum) * [(20.9-%O2 measured)/20.9]
NOTE: Using Measured O2 to Determine CO2
Using the O2 concentration to determine the concentration of CO2 has
advantages compared to measuring CO2 directly. As indicated in Figure 5, the
same CO2 concentration is possible when there is too little air supplied (less
than 100% theoretical air) or too much air (greater than 100% theoretical air).
When CO2 is measured alone, it is not possible to tell if the mix of exhaust
gases is represented by those to the left of the CO2 peak or those to the right
of the CO2 peak. When to the left of the peak, high levels of toxic CO are

27. Page | 27
present, resulting in a potentially dangerous operating condition. By detecting
sufficient O2 in the exhaust, the combustion reaction stays on the right side of
the CO2 peak, minimizing the formation of CO.
Determining Combustion Efficiency
Combustion efficiency is a measure of how effectively energy from the fuel is
converted into useful energy (e.g. to create steam). Combustion efficiency is
determined by subtracting the heat content of the exhaust gases, expressed as
a percentage of the fuel’s heating value, from the total fuel-heat potential, or
100%, as shown in the formula below.
%Combustion Efficiency = 100% - (Stack heat losses/Fuel heating value * 100)
Stack heat losses are calculated using gas concentration and temperature
measurements from combustion analysis, and using the fuel’s specifications
for chemical composition and heat content. These fuel specifications are
unique properties of the fuel, determined from chemical analysis by the fuel
supplier.
Stack heat losses are primarily from the heated dry exhaust gases (CO2, N2,
O2) and from water vapour formed from the reaction of hydrogen in the fuel
with O2 in the air. When water goes through a phase change from liquid to
vapour, it absorbs a tremendous amount of heat energy in the process. This
“heat of vaporization,” or latent heat, is usually not recovered. The white cloud
seen exiting a stack on a cold day is mostly condensing water vapour giving up
its latent heat to the atmosphere.
NOTE: Other Definitions of Efficiency
It is important to recognize that other definitions of efficiency are often used
to describe furnace or boiler performance in addition to combustion efficiency.
Thermal efficiency or boiler efficiency are examples.
These may include or exclude sources of heat loss as part of their calculation.
Combustion efficiency, for example, does not include losses from radiation or
steam leaks. Sometimes the latent heat of water formation is not included.
There are also fundamental differences between calculations performed in the
U.S. and those performed in some European countries. When comparing the
performance of equipment from different manufacturers, it is important to
know how an efficiency number is calculated.
NOTE: Why use Combustion Efficiency?

28. Page | 28
Combustion efficiency, determined from combustion analysis, is a cost-
effective way to improve equipment operation and reduce fuel expense. The
stack losses used in combustion efficiency calculations are simple to determine
using combustion analysis. Other losses, such as those from steam leaks,
radiation or boiler blow-down, are much more difficult to assess. Stack losses
are typically the largest source of energy waste. If the equipment is properly
maintained, losses such as those from steam leaks are minimal. Convection
and radiation losses are often small also, and usually unavoidable.
The O2 Reference
As discussed earlier, excess air is supplied to the combustion process to ensure
that there is enough oxygen to completely react with the fuel. Excess air is
measured in the flue as a percentage of O2. This excess air dilutes the
concentration of other gases measured.
Local regulatory agencies have guidelines for monitoring NO, NO2, CO, and
SO2 gases. Generally, it is required that the concentration of these gases be
corrected for the diluting effects of excess air. The amount of excess air is
determined from the O2 concentration measured in the flue. The measured O2
concentration, together with the O2 reference value is used in the equation
below to obtain the corrected gas concentration. O2 reference values of 3 and
6 percent are often used, giving a corrected gas concentration equivalent to
that at oxygen concentrations of 3 or 6 percent. When an O2 reference of zero
is used, the gas concentration is referred to as undiluted or air free.
To obtain the O2 referenced concentration of gasses in the flue, the following
equation is used:
Corrected PPM = measured PPM * [(20.9 – O2 reference) / (20.9 – O2 measured)]
Emission Conversions
A measure of the toxic gas concentration in parts per million (PPM) or percent
does not indicate the actual weight of pollutant entering the atmosphere. The
EPA requires the conversion of pollutant concentrations to pounds per million
Btu of fuel consumed (lb/MBtu). This is done so the weight of pollutants can be
readily determined from the pollutant concentration and the rate of fuel
usage. EPA Method 19 has equations for performing the conversions and
presents fuel-specific conversion factors for use in performing the calculations.

29. Page | 29
7. Fuel Firing System
7.1. Introduction
The fuel firing system of the boiler provide controlled, efficient conversion of
the chemical energy of the fuel into heat energy which in turn is transferred to heat
absorbing surfaces. Any fuel burning system must introduce the fuel and air for
combustion, mix these reactants, ignite the combustible mixture and distribute the
flame envelope and products of combustion.
7.2. Characteristics of Ideal Firing System
An ideal fuel firing system fulfilling the above functions will have the following
characteristics
- 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.
- An effective method of handling and disposing of the solid impurities
introduced with the fuel.
- Uniform distribution of product weight and temperature in relation to the
parallel circuits of heat absorbing surface.
- A wide and stable firing range.
- Fast responses to changes in firing rate.
- High equipment availability with low maintenance.
In actual practice, compromises must be made to achieve balance between
combustion efficiency and cost. For example with stoichiometric air infinite
residence time will be required at temperature above ignition temperature for
complete combustion and so excess air is used resulting in unconsumed
oxygen in the product gas.

30. Page | 30
7.3. Firing System Concepts
Successful molecular contact of reactants of combustion through turbulence
can be achieved by producing two methods of flow pattern in the furnace.
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.
7.4. Comparison of Single and Multiflame Concept
The single flame envelope 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 and mechanical turbulence is sustained throughout the furnace.
The multi flame envelope requires accurate subdivision fuel and air supplied to
the furnace. This concept limits the opportunity for sustained mechanical turbulence
particularly in the early stages of combustion.
7.5. Firing Systems:
This can be broadly classified into direct firing system and indirect firing or
intermediate bunker system. Both the systems can use any type of mill. Either
hot gas or air can be used for drying and transporting the coal.
Direct Firing System
 In this type of firing system, coal is fed to the mill at controlled quantity.
Hot air whose temperature can be controlled with the help of cold air is
permitted to flow through the mill. The air dries the coal and picks up the
milled product and flows through the classifier where higher size particle is
rejected back to the mill.
 The fine coal is carried by the air through the coal burner to the combustion
chamber. The flow through the system is carried out by primary air fan or
by exhauster. In case of hammer mills forced draft itself can do the purpose
of primary air fan.

31. Page | 31
 This system is simple involving minimum equipment hence minimum initial
cost and maintenance cost. As there is no fine coal storage the mill load is
varied according to the boiler load. Hence part load operation of mill is
essential and this means increase in power consumption and maintenance
per tonne of coal.
 Mill outage will result in reduction of boiler output if spare mills are not
provided or available. This is best suited to use with high speed and
medium speed mills as the mill power consumption varies in direct
proportion to the mill load. Tube ball mills with this system is also used but
to a limited extent.
Indirect Firing System
 In this system, mills are operated independent of boiler loading and
pulverised coal is stored in the intermediate bunker.
 From the bunker it is taken to combustion chamber with the help of
primary air fan. Boiler loading is controlled by the amount of pulverised fuel
fed to boiler. Hot air or gas is used for drying and transporting.
 Cyclone type separators are used to separate the fine coal from coal,
air/gas mixture for storing in fine coal bunker. As fine coal dust cannot be
completely removed by cyclone type separators, a certain portion of very
fine particle is carried along by air/vapour. This necessitates admissions of
vapour/air into the combustion chamber to utilise the heating value of fine
coal dust carried along with vapour/air. This can be done by providing
separate vapour burners or the air can be used as primary air for carrying
the fine coal.
 This system using gas as drying medium requires one additional fan called
vapour fan for each mill. If air is used for drying, one fan called mill fan can
be designed to carry out both the functions of primary air fan and vapour
fan.
This system favours the following advantages:
a) Mill can be operated always at full load, thus saving in power, maintenance
cost per tonne of coal for the selected mill. Hence this system is adopted
normally for tube ball mill.
b) Separate spare mill is not necessary for carrying out the maintenance.
Certain percentage as spare capacity on total basis is normally enough.
c) Mills can be operated during off peak hours only and hence higher power
out during the peak period.

32. Page | 32
7.6. Methods of Fuel Firing
There are many ways of firing the coal in furnace. They are:
a) Vertical firing
b) Horizontal firing
c) Impact firing
d) Corner or tangential firing
Vertical Firing
Vertically fired systems are used only to fire solid fuels that are difficult to ignite
such as coals with moisture, ash, and free volatile matter less than 13 percent. They
require less supplementary fuel than the horizontal or tangentially fired system but
have more complex firing equipment and therefore more complex operating
characteristics.
The firing concept and the arrangement of the burners in this system are
shown in Fig. Pulverised coal is discharged through the nozzles in the furnace
arch. A portion of the heated combustion air is introduced around the fuel
nozzles and though adjacent auxiliary parts. High-pressure air jets are used to
avoid short-circuiting of fuel-air streams to the furnace discharge. Tertiary air
ports are located in a row along the front and rear walls of the lower furnace.
This firing system produces a long looping flame in the lower furnace, with the hot
gases discharging up the center. A portion of the total combustion air is withheld from
the fuel stream until it projects well down into the furnace. This arrangement has
the advantage of heating the fuel stream separately from a significant portion of its
Fig 8. Vertical Firing System

33. Page | 33
combustion air to provide good ignition stability. The delayed introduction of tertiary
air provides needed turbulence at a point in the flame where partial dilution from the
products of combustion has occurred. The furnace flow pattern passes the hot
product gases immediately in front of the fuel nozzles to provide a ready source of
inherent ignition energy, which raises the primary fuel stream to ignition
temperature. The flow pattern also ensures that the largest entrained solid fuel
particles with the lowest surface area to weight ratio, have the largest residence time
in the combustion chamber.
Horizontal Firing (Front Firing)
In horizontally fired systems the fuel is mixed with combustion air in individual
burner registers. The burners are located in rows through a wind box receiving hot
secondary air either on the front wall only or on both front and rear walls. A typical
burner is shown in Fig.
The coal and primary air are introduced tangentially to the coal nozzle, thus
imparting strong rotation within the nozzle. Secondary air from the windbox is
admitted to the burner through adjustable radial or axial in flow swirl vanes called air
registers. These vanes impart rotation to the secondary air. The degree of air swirl,
coupled with the flow-shaping contour of the burner throat, establishes a re-
circulation pattern extending several throat diameters into the furnace. Once the
coal is ignited hot products of combustion are directed back towards the nozzle to
provide the ignition energy necessary for stable combustion.
This type of coal burners is fitted with an oil burner mounted in a central
support tube for coal burner ignition.

34. Page | 34
Because the major portion of the combustion process must take place within
the re-circulation zone, it is imperative that the air fuel ratio to each burner is within
close tolerances. The rate of combustion drops of rapidly as the reactants leave the
re-circulation zone and interaction between flames occurs only after that point. The
degree of interaction depends on burner and furnace configurations.
Fig 9. Wall firing Burner
Fig 10. Burner for Wall Firing

35. Page | 35
Impact Firing
This is the arrangement with the type of burner used with slag tap furnaces
where the ash is kept in a molten state on the furnace floor and tapped off as
and when necessary.
Corner or Tangential Firing
The tangentially fired system is based on the concept of a single flame envelope.
Both fuel and combustion air are projected from the corners of the furnace along a
line tangent to a small circle lying in a horizontal plane at the centre of the furnace.
Intensive mixing occurs where these streams meet. A rotated motion similar to that
of a cyclone is imparted to the flame body, which spreads out and fills the furnace
area.
In this system at each corner of the furnace a wind box assembly is installed. Fig.
shows a typical wind box. The wind box is vertically divided into number of
compartments. Each compartment receives hot secondary air from the secondary
air duct through a damper called secondary air damper.
Alternate compartments of the wind box are provided with coal nozzles
through which the pulverised coal from the mill is delivered to the furnace. Four
corner nozzles of one elevation are generally connected to one mill. In these
compartments secondary air is admitted to the furnace surrounding the coal nozzles.
These compartments are called fuel compartments and the secondary air dampers of
these compartments are termed fuel air dampers.
The other compartments are known as Auxiliary air compartments. Each
auxiliary air compartment between two fuel compartments is provided with oil gun
for firing the oil. The secondary air dampers of the auxiliary air compartments are
called auxiliary air dampers.
With this arrangement the fuel and air are admitted to the furnace from
corners in vertical layers. The secondary air dampers control the air to each
compartment, making it possible to vary the distribution of air over the height of the
wind box, the velocities of the air stream, change the mixing rate of fuel and air and
control the distance from the nozzle at which the coal ignites.
It is customary that the auxiliary air dampers are set to modulate to maintain
a fixed wind box to furnace differential pressure to have sufficient air stream velocity
to achieve sustained turbulence in the furnace. The fuel air dampers are set to

36. Page | 36
modulate according to the speed of the raw coal feeder corresponding to that
elevation. This ensures the distribution of secondary air in the furnace in accordance
to the elevation loading.
In this arrangement, there will be a facility for tilting the fuel and air nozzles of
the wind box compartments. All these nozzles (Elevations and corners) tilt in unison
to raise and lower the flame in the furnace to control furnace heat absorption and
thus heat absorption in the super heater and reheater sections.
The most effective method for producing intense turbulence is by the
impingement of one flame on another. This action is secured through the use
of burners located in each of the four corners of the furnace, close to the floor
or the water-screen. The burner nozzles are so directed that the streams of
coal and air are projected along a line tangent to a small circle, lying in a
horizontal plane, at the centre of the furnace. Intensive mixing occurs where
these streams meet. A scrubbing action is present which assures contact
between the combustible and oxygen, thus promoting rapid combustion and
reducing carbon loss. The ignition at each burner is aided by the flame from
the preceding one.
With tangential firing the furnace is essentially the burner, consequently
air and coal quantities need not be accurately proportional to the individual
fuel nozzle assemblies. Turbulence produced in the furnace cavity is sufficient
to combine all the fuel and air. This continuously insures uniform and complete
combustion so that test performance can be maintained throughout daily
operation. With other types of firing the fuel and air must be accurately
proportioned to individual burners making it difficult to always equal test
results.
With this type of firing, combustion is extremely rapid and short flame
length results. The mixing is so intense that combustion rates exceeding 35,000
Btu/ (ft3·h) or 360 kW/m3 are practical under certain conditions. However,
since there is considerable impingement of flame over the furnace walls it is
absolutely necessary that they be fully water-cooled. This sweeping of the
water-cooled surfaces, in the furnace, by the gas increases the evaporation
rate. Thus, in addition to absorption by radiation from the flame envelope,
there is transfer by convection, and the resulting furnace temperatures are
lower than with other types of burners, even though the heat liberation rates
may be somewhat higher. Tangentially-fired furnaces are usually clean in the
upper zone and, as a result, both the furnace and the boiler are comparatively
free from objectionable slag deposits.

37. Page | 37
7.7. Components of Fuel Firing System
The essential components for a pulverised coal fired boiler-firing system are
- Ignition system
- Oil guns and atomisers
- Flame scanner
Ignition System
Any boiler firing system needs a suitable ignition system to provide ignition
energy to the flammable mixture of fuel and air introduced to the furnace.
Combustion reaction starts only when the flammable mixture is heated to its ignition
temperature.
To initiate combustion of any fuel and to keep the flame stable, continuous
supply of ignition energy is required which is supplied in the form of heat. This
ignition energy can be called as the total ignition energy. This total ignition energy
can be derived from the fuel itself when it is burning in stable condition called as
inherent ignition energy, supplied by external sources as auxiliary ignition energy or
combination of both. Hence
Total ignition Energy = Inherent Ignition Energy + Auxiliary Ignition Energy
When a fuel is to be lighted up, the inherent ignition energy available from it is
at zero level so the entire amount of ignition energy required has to be obtained from
auxiliary ignition energy sources only. Also, immediately after light up of the fuel the
heat available from combustion may not be equivalent to the Total ignition energy.
Hence under low firing rates still the auxiliary ignition energy will be required.
When the firing becomes stable and heat available from the combustion of main fuel
is greater than total ignition energy no more auxiliary ignition energy will be required.
This condition will be stated as self-sustainable.
The ignition energy required at any given instant depend on many factors such as
- Location
- Fuel quality

38. Page | 38
- Fuel parameters
- Combustion air parameter
- Fuel air distribution
- Total fuel air ratio
- Mass burner flow rate
In a coal-fired boiler, the auxiliary ignition energy for coal firing is provided by
suitably located oil burners. For igniting the oil while starting the oil burners,
ignitors are used in the firing system.
Requirements of an Auxiliary Ignition System
An auxiliary ignition system provided for igniting the oil burner of a boiler should
meet the following criteria.
It should be one capable of measuring the ignition energy required and
supplied. Ignition energy should be located very close relative to main fuel admission
so as to readily ignite main fuel as it enters the furnace.
Ignition energy equipment should contain a fuel quantity control arrangement,
with a self-resetting or zero run back of the scanning or proving loop. The
quantitative and qualitative feedback of the scanning or proving loop should totalize
both flame presence and actual heat input.
Interlocking with main fuel admission should be used.
An ignition system, which meets these design criteria also, has the inherent
capability of being used as a flame proven for its associated main fuel burner. When
the ignition system is proven in service, fuel supply through the main oil burners can
safely be made.
Ignitor Types
The ignitors that are available now can use any of the load carrying fuels
available to the boiler. Such fuels include natural gas, all grades of fuel oil etc.
however the application of the various ignitor types is largely a function of user
preference, ignitor fuel availability and ignitor fuel economics.

39. Page | 39
Some of the oil ignitors used in Indian Thermal Power Stations especially with
corner firing system are
- Eddy plate ignitor
- Ion Flame Monitoring Ignitor (IFM)
- High Energy Arc Ignitor (HEA)
The first of the above two use an ignition fuel normally light diesel oil and
produce a pilot flame for igniting the main oil burner. The third one uses only a
high intensity electric arc for ignition.
Eddy Plate Ignitor
This ignitor consists of an oil atomiser and a spark rod mounted through an
eddy plate in the ignitor horn. The oil atomiser receives light oil and compressed air
for atomisation through a shut off valve located in the control cabinet. The spark rod
is connected by cables to a transformer in the control cabinet.
The eddy plate ignitor utilises a metered, controlled flow of combustion air
directed through the ignition chamber or horn to create eddy currents about the
down-stream face of the eddy plate. Static pressure measurements are monitored
at the face of the eddy plate and at the nozzle discharge. The flow of combustion air
through the nozzle with ‘no fire’ gives a static pressure differential in which the
furnace tap is higher than the eddy plate tap. When the ignitor is ‘fired’ the energy
conversion and combustion reaction (with the considerable gaseous expansion due
Fig 11. Eddy Plate Ignitor Arrangement
Fig 12. Eddy Plate Ignitor

40. Page | 40
to both the temperature rise and energy release) creates a backpressure on the eddy
plate pressure tap. This results in a positive reversal of differential measurement
between the two taps. The differential is rather significant and can be easily
measured with relatively simple diaphragm meters. At low inputs, below 2 million
Btu per hour, the ignitor differential switch can be set up to be quantitative in
determination of the quantity of ignition energy and therefore, the ignitor is both
quantitative, indicating the flame, and quantitative, indicating the level of igniting
energy. When the device is used for quantities 2 million Btu and above an
independent flow measurement is made of the actual fuel flow through the ignition
system. This is accomplished through a unitized flow controller with a flow switch,
which maintains a constant differential across an orifice. This differential is
continuously monitored and is a permissive in the logic of the ignitor. To release the
external interlocks both flame indication and quantitative fuel flow must be proven.
The eddy plate ignitor has been proven to be an exceptionally stable and
reliable device. The eddy plate gas ignitor has been used even as a main monitoring
device without any optical scanners for the main fuel burner because of its thorough
reliability.
IFM Ignitor
The Ionic flame monitoring side ignitor (IFM) can spark ignite high calorific value gases
or distillate oil. This is having a similar arrangement like eddy plate ignitor with an
exception for proving ignition. The IFM design follows the traditional philosophy of
providing an ignitor with both qualitative and quantitative indications of flame. The
system incorporates the principle of flame ionisation, which is present in all turbulent
hydrocarbon flames, to deduct the presence of combustion. In the burning process
energy is liberated by the combination of two or more reactants to form a product
with lower energy level.

41. Page | 41
During this burning process many ions (charged particles) are liberated taking the
form of electrons and charged nuclei. When a D.C. potential is placed across the
flame a varying current is generated due to the variable resistance the flame
presents to the rod.
The system operates by imposing a DC potential on the rod that is in contact with
the flame. The DC voltage is modulated plus or minus around the imposed level
by the flame and the imposed signal is then filtered out. The variance is then
amplified, changed to a pulse shape and used to drive a flame indication relay.
The circuit is designed to be fail-safe. If there is a component failure, a short
circuit in the flame rod or lead wire or a direct AC interference, a “no flame
indication” will occur.
High Energy Electric Arc (HEA) Ignitors
The High Energy Electric Arc Ignitor is developed to offset the decreasing
availability and rising cost of ignition fuels such a natural gas, HSD and LDO. The
HEA ignitor effectively eliminates dependence on these fuels by igniting Heavy Oil
and L.S.H.S. directly. The HEA ignitor is used with a discriminating scanner, which
proves the operation of the main burner.
Fig 13. I.F.M. Ignitor

42. Page | 42
The HEA ignition system consists of
- A high energy arc ignitor
- An oil compartment capable of producing a stable flame at all loads.
- A flame detecting system sensitive only to its associated oil guns.
- A control system to co-ordinate all the components and provide for unit
safety.
The HEA ignitor can ignite fuel oils ranging from distillate to Heavy oils and crude
oils. The ignitor is a self-contained electrical discharge device for producing a
high intensity spark. Use of high resistance transformer to produce a full wave
charging circuit and to control spark rate enables the sealed power supply unit to
store maximum energy and to deliver a greater percentage of this energy through
insulated cables to the ignitor tip on a very short time, in terms of micro seconds.
A high spark energy also eliminates coking of the ignitor tip. The high-energy arc
ignitor consists of four basic components, the exciter, and flexible cable, spark
tube and guide pipe and retractor assembly (Fig.7.6.)
A key to the successful application of spark ignition is the presence of a strong re
circulation pattern in the primary combustion zone as shown in Fig.7.7.by proper
selection of stabiliser and location of HEA. The re circulation provides the source
of energy required to vaporise the oil and to heat the vapour to its ignition point,
thus maintaining stable ignition after the spark has been deactivated.
Fig 14. H.E.A. Ignitor

43. Page | 43
The discriminating scanners are UV Scanners with a reduced sensitivity, which
permits each scanner to see only the flame from the associated oil gun and not
the flame of the adjacent oil guns.
Oil Guns and Atomisers
Fuel oils like light diesel oil, (LDO), heavy fuel oil (HFO) or low sulphur heavy
stock (LSHS) are used in boilers either as main fuel or supplement fuel. In coal
fired boilers fuel oils are used to ignite the coal as well as to supplement the coal
firing under various exigencies. 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 either concentric
tubes or parallel tubes as shown in Fig Installed through the furnace walls or
Fig 15. Re-Circulating Pattern in H.E.A. Ignition System

44. Page | 44
windbox with atomiser mounted at their tips provide this oil spray to the furnace.
The oil guns at their rear end are connected through flexible hoses to the fuel oil
pipe and atomising agent (either steam or compressed air)
Fig 16. Concentric and parallel oil gun
Pipelines. For each oil gun, in the fuel oil and atomising medium pipelines slow
opening and quick shut off valves are provided.
The oil guns may be provided with a pneumatic gun advance and retract
mechanism. With this the oil gun can be retracted in its housing when it is not in
service. By this the atomiser will be kept at a safe distance from the furnace heat
radiation thereby ensuring long life and reliability of the atomiser.
As this oil gun advance-retract mechanism is more expensive, cumbersome
and prone for increased maintenance many boilers use air-cooled oil guns. These
guns are always stationed at their service position. It is cooled by a low-pressure
stream of air passing through a specially designed gun cap with finned surfaces as
shown in Fig. The low pressure air passing around the atomiser shields and
reduce the furnace radiation falling on atomiser parts by extracting heat through
the finned surfaces. The air fan gun cooling is obtained from the discharge of the
F.D. fans.
Atomisation
The exact mechanism of atomisation (splitting the liquid into fine droplets)
depends on the type of atomiser used and the nature of liquid being atomised
but the basic mechanism involves the formation of unstable columns of liquid
which breakdown into rows of droplets

45. Page | 45
The process of atomisation can be accomplished in a number of ways in practice,
which are usually grouped according to the source of energy used. In boilers fuel
oils are atomised mainly by adopting any one of the following two methods.
i) By forcing the oil at high pressure through an orifice. This is called
mechanical atomisation or pressure atomisation.
ii) In twin fluid atomisation (Steam or Air atomisation) a stream of gas at
high velocity is passed over the liquid surface so that waves are
generated which become extended into thin films.
Mechanical/Pressure Atomisation
In a typical pressure atomiser as shown in Fig. oil at a high pressure flows in the
centre tube and is discharged through tangential slots in the sprayer plate
swirling chamber where the oil rotate at high speed. The swirling oil then passes
with undiminished energy through the sprayer plate orifice and escapes as a
spray. In this case the hollow conical sheet that is produced emerges from the
orifice with a tangential velocity which is sufficiently high to cause an air core
throughout the nozzle so produce a hollow cone spray. The advantage of
pressure atomisation is it does not require any atomising medium. Hence it may
be suitable for lighting up cold boilers. However this atomisers need oil pressure
at a range of 40 to 70 kg/cm2
and so need suitable oil pumps. The turn down
ratio (the ratio of maximum to minimum fuel flow rate required to produce a
stable self-sustained flame) of these atomisers is less compared to other type of
atomiser. In power plants these atomisers are used for light oil start up burners
only.

46. Page | 46
Fig 17. Mechanical/Pressure Atomisers
Twin Fluid Atomiser
In this type of atomisers, the atomisation consists of the following stages
i) Formation of thin liquid sheets along the inner walls of an internal mixed
atomiser or of free sheets.
ii) Disintegration of these sheets by aerodynamic forces to form ligaments
and large droplets to form spray.
Either compressed air or superheated steam will be used as atomising medium to
apply aerodynamic force on the oil sheet.
Two types of these atomisers, a) internal mixing b) external mixing are shown in
Fig. The atomiser is screwed to a concentric tube oil gun. With an internal mixing
atomiser oil flow through the inner tube and atomising medium through outer
type. In internal mixing atomiser, the oil and atomising medium impinge within
the atomiser and comes out as a spray. Atomisation is accomplished by
projecting atomising medium tangentially across the jets of oil and results in the
formation of conical spray of finely divided oil after the mixture has left the
orifice plate. Here a constant differential pressure is to be maintained between
oil and atomising medium. In external mixing atomisers the oil is released into
the gas stream at the outlet from the atomiser. A constant pressure of atomising
medium is to be maintained for the proper operation of external mixing type
atomiser.

47. Page | 47
In Indian power stations normally compressed air is used for atomising light
oil and steam is used for atomising heavy oil.
Flame Scanners
In any boiler, a large quantity of fuel is being admitted to the furnace, the
flammable mixture formed in the furnace must be converted into inert at the
earliest available time before any appreciable quantity builds up in the furnace.
This needs a sustained healthy intensified flame in the furnace. Admission of fuel
without flame in furnace leads to furnace explosions. Hence detection of
absence of flame is required for any control action to prevent furnace explosion.
Firing systems of modern boilers incorporate flame sensing scanners to detect
Fig 18. Internal mixing Atomiser

48. Page | 48
the flame in the furnace and in the loss of flame to initiate control actions to
prevent furnace explosions.
Fundamental Requirements of a Flame Detecting System
Any flame detection system provided in the boiler must meet the following
requirements.
- Must be reliable
- Sensible to discern the minimum flame envelope
- Fail safe characteristic to avoid frequent trips.
- Reaction time must be minimum
Flame Scanner Types
The burning process exhibits many characteristics, which can be sensed as
indicators of existing flame. In large furnace installations like power boiler
furnace, the most practical characteristic to sense the proof of flame is the light
emitted by the burning process. The light emission covers a broad spectrum
including infrared, visible and ultraviolet spectrums. Sensing any one of these
spectrums can be used for flame proving.
Accordingly three basic types of flame scanners are used.
1. Ultraviolet scanners
2. Visible light scanners
3. Infrared scanners.
Further these scanners also utilise the fluctuating intensity (frequency)
character of the furnace flames to prove the flame.
Ultra Violet Scanners
In this type, the scanner head located near the burner consists of ultraviolet tube.
The special glass envelope has a low attenuation for ultra violet rays. The tube is
filled up with helium gas at low pressure. Pure tungsten electrodes are placed
inside the tube. When ultraviolet rays emitted by the flame strike these
electrodes, they emit electrons proportionate to the ultraviolet radiation. The
electron flow through the helium gas is utilised by the scanner electronics to

49. Page | 49
prove the flame. Each scanner tube is provided with a shutter mechanism, which
close the tube at periodical intervals. When the shutter closes the ultraviolet rays
are not passed to the tube and so the scanner should sense no flame. This is
used for self-testing of the healthiness of the scanner.
As ultraviolet radiation which is abundant in fossil flames is not emitted in
significant quantities from other hot bodies of the furnace such as hot refractory,
hot metal and hot gases at the temperatures encountered in boiler furnaces,
ultraviolet detection will not cause any false detection of flame. However one
problem with ultraviolet detection in coal fired boilers is a considerable portion of
ultraviolet radiation is absorbed by intervening coal dust before it can reach the
scanner. This may result in a false indication of no flame.
Visible Light Scanners
This flame scanner is an optical scanner utilising a fibre optic light guide. It
recognises particular characteristics of visible light radiated from the furnace
flames. Visible light is directly related to the burning process. It is given off in
substantial quantities so that a stable flame cannot be masked by unburnt coal.
The scanner head with a fibre optic light guide is mounted in the windbox. Visible
light from the area of combustion as monitored is transmitted through this fibre
optic light guide to the electronics system kept outside the boiler. A photodiode
in this system converts the light to an electric signal. A blue enhanced silicon
photodiode with an integral optical I.R. filter is used as the primary sensor. This
boiler mounted components integrated with a remote signal processing card
form the flame detection system. They recognise the frequency and intensity
levels of the visible light emitted by the furnace flame along with a self-
diagnosing no fault in the system to prove the furnace flame
Infrared Scanners
These flame scanners are working on the principle of sensing infrared rays. The
flame controller in conjunction with the flame detector monitors coal, oil and gas
flames selectively in the near infrared spectral range. The flame monitoring
circuit is sensitive only to the evaluation of the flicker frequency and not to the
steady infrared radiation from the combustion chamber and glowing boiler wall
refractory. It is insensitive to daylight, the system consists of an infrared light
receiver located near the burner and a control unit. Infrared scanners are rarely
used in coal fired boilers.

50. Page | 50
Scanner Air
In any type of scanner, the scanner head is located through the wind box and is
viewing the furnace, but the components of the scanner head are temperature
sensitive and fail normally if this temperature increase above 150o
C. Hence to
keep the scanner head at a low temperature cold air supplied by scanner air fan
will be continuously passed through the scanner guide pipe.
7.8. Oil Firing System
Fuel Oil Preparation
Filtering the oil, Pumping the oil and heating it are the major preparatory
functions.
Filtration of oil in one or more stages to remove any dust, dirt, sediments,
sledge etc. also forms part of preparation. This renders long trouble free
service life to pumps, valves and oil gun atomiser nozzles.
Pumping the oil to overcome pressure drop in the long oil supply lines and
deliver the oil at required pressure at oil gun tips.
LDO:
Sr No Parameters Values
1 Total sulphur 500 mg/kg
2 Ash (% by mass) 0.01
3 Relative density at 15 0
C 0.82-0.86
4 Kinematic viscosity (centistokes at 40 0
C) 2.0/5.0
5 Max. pour point 3 0
C (winter)/ 15 0
C (summer)
6 Water (volume percent) 0.10
7 Gross calorific value 10,000 kCal/kg
8 Min. flash point 66 0
C
HFO:
Sl.
No.
Parameters
Heavy Furnace Oil IS1593-71
Grade HV
Low Sulphur Heavy Stock
(LSHS)
1. Total Sulphur content 4.5% max 1.0% max
2. Gross calorific value (kg) Of the order of 10,000 Of the order of 10,000
3. Flash point (min) 66 o
C 93 o
C
4 Water content by volume (max) 1.0% 1.0%
Table 3: LDO Parameters

51. Page | 51
5. Sediment by weight (max) 0.25% 0.25%
6. Asphaltene content by weight (max.) 2.5% 2.5
7. Kinematic viscosity in centistokes at
50 0
C (max)
370 500
8. Ash content by weight (max) 0.1% 0.1%
9 Acid(Inorganic) Nil Nil
10. Pour point (max) 24 0
C 57 0
C
Fuel Oil Atomisation
Atomisation is the process of spraying the fuel oil into fine mist, for better
mixing of the fuel with the combustion air. While passing through the spray
nozzles of the oil gun, the pressure energy of the steam converts into velocity
energy, which breaks up the oil stream into fine particles. Poorly atomised fuel
oil would mean bigger spray particles, which takes longer burning time, results
in carryovers and makes the flame unstable due to low rate of heat liberation
and incomplete combustion.
Other than pressure, viscosity of the oil is the major parameter which decides
upon the atomisation level. For satisfactory atomisation the viscosity shall be
15 to 20 centistokes.
Oil Recirculation
Before putting in the first burner into service, it is necessary to warm up the
long oil supply lines to the burners, so that the oil does not get cooled in the
colder pipes and that the oil at correct atomising temperature becomes
available at the burners. To achieve this the heated oil is circulated up to the
burners and back to the oil tank through oil return lines till adequate oil
temperature is reached near the burners.
System Vents
Fuel oil heaters, strainers and lines are provided with vent cocks or valves on
oil and steam sides to get rid of air locks while charging system.
System Drains
All oil lines are run with a slope of about 0.3 to 0.5% towards drain. Each
section of oil line is provided with a drain valve at the lowest point. All drain
valves are normally kept closed during operation.
Table 4: HFO Parameters

52. Page | 52
Oil Temperature Control
The result of inadequate heating of oil is "higher oil viscosity" which impairs
atomisation at the oil gun, leading to poor flames, sooting, increased carbon
loss and finally flame failures.
Overheating the oil is also not desirable from the point of thermal cracking of
oil. The formed solid carbon particles build up over the heater tubes, and plug
up the strainer, intricate passages of control and shut off valves and atomisers.
The carbon particles also cause faster atomiser wear out and sparkling in the
flame.
Oil Burner Design Considerations
The essential requirements for an oil burner design are:
a) It must completely atomise the oil without -drooling, fouling or clogging.
b) The jet must be so shaped that it will completely mix with the air necessary
for combustion.
c) Maintenance of atomisation over a comparatively wide capacity range.
d) Combustion must be complete and excess air at a minimum over the entire
operating range.
e) A ready accessibility for effecting repairs, thereby minimising burner outage
as well as maintenance costs.
Air Atomising System
The air atomising system uses compressed air at 7 kg/ sq.cm. The quantity of
air required depends on design of burner, degree of atomisation required,
grade of oil, its pressure and temperature. The air used in the system should
be free of condensate. Air atomising systems are not recommended for heavy
oil system as they tend to chill the oil and decrease atomisation quality. The
guns used in this system have two main components for atomisation. They are:
a) Mixing plate b) Spray Plate. The role of mixing plate is to mix the air and oil
properly and the role of spray plate is to inject this mixture uniformly into the
furnace.
Mechanical Atomisation
Oil under pressure is supply to the burner gun and enters the atomiser tip
through slots cut tangentially to the periphery of the Whirling. The drop
pressure is converted into velocity causing the oil to enter the whirling
chamber at a very high speed, thus establishing a rapid rotation of the oil in
the whirling chamber. Since the mechanical atomisation oil guns are limited in
range they are generally selected for boiler operating at fixed loads.

53. Page | 53
Steam Atomisation
This system uses auxiliary steam to assist in the atomisation of the oil. The
steam used in this method should be slightly superheated and free of
moisture. As in the air atomising system, the steam used for atomising as well
as the fuel pass through the tip and into the furnace. The main advantages of
steam atomising burners over other are: a) Simplicity of its design b) Initial cost
of installation is low c) Low pumping pressure d) Low preheating temperature.
The only disadvantage is the steam consumption in burners.
Refer the figure below for air / steam atomised oil gun compartment with wind
box assembly.
8. Boiler
Boiler or the steam generator is the main part in the power generation
process. Boiler acts as a medium in which the water is converted into the
steam by using the heat released in the process of combustion of coal in the
presence of oxygen.
Categorization of Boilers
Boilers are generally categorized as follows:
Steel boilers
Fire Tube type
Water tube type
Horizontal Straight tube
Bent tube
Natural Circulation
Positive Circulation
Shell type
Cast Iron Boilers
Special Design Boilers
Nuclear Reactors
 The steam generator is a natural circulation, single drum type, corner
fired, and natural draft unit of water tube type.
 In water tube boiler, boiler feed water flows through the tubes and
enters the boiler drum. The circulated water is heated by the
combustion of gases and converted into steam at the vapour space in
the drum.

54. Page | 54
 These boilers are selected when the steam demands as well as the
steam pressure requirements are high as in the case of process cum
power boilers.
The features of water tube boilers are:
 Forced, induced and balanced draft provisions help to improve
combustion efficiency.
 Less tolerance for water quality calls for water treatment plant.
 Higher thermal efficiency levels are possible.
There are many mountings to the boiler, which are compulsory for the safe run
of the boiler. Also, there are accessories, which increase the efficiency of the
boiler.
Arrangement of Main Boiler
The illustration given in figure below shows a symbolic arrangement of various
accessories of a Boiler. These accessories include
1. Economizer
2. Boiler drum
3. Down Comers
4. Water walls
5. Water wall platen (used for Low Pressure Boilers)
6. Primary super heater
7. Platen super heater
8. Final super heater
9. Reheater
10. Burner
11. Igniters
Boiler Combustion Details
32 direct air burners in 8 layers
Elevation of top row burners: 39780 mm and 20020 mm away from platen
bottom
Elevation of bottom row burner: 26150 mm and 5131 mm from furnace corner
U-OFA air nozzle: 3 layers in the upper part of wind box
17 groups of secondary air damper, 3 groups of over fire damper
20 sets of ignition devices in 5 layers (16 sets HFO (steam atomised) + 4 sets
(air atomised) LDO air igniters)

55. Page | 55
Fig 19. Boiler Block Diagram
9. Furnace
In a boiler heat energy is released from the fuel by the combustion reaction.
The furnace provides the enclosure for combustion confining and isolating it so that
combustion is a controlled activity. The furnace is designed for efficient and
complete combustion. Though the furnace enclosure can be built in many ways in
large capacity power boilers the furnace enclosure is formed by water wall system
which has many advantages over the other arrangements.
FURNACE DESIGN
The furnace design is influenced by the following factors:
i) Fuel
ii) Allowable heat loading

56. Page | 56
a) Surface (EPRS)
b) Plan area
c) Volumetric
iii) Burner clearance
EFFECT OF FUELS ON FURNACE DESIGN
The major fuels used in the steam generators are coal, oil and gas. The furnace
designed for firing pulverised fuels are basically sized to ensure complete
combustion with minimum formation of objectionable slag deposits. For coal
fired boilers this requirement generally results in a relatively low furnaces wall
absorption rates with safe metal temperatures.
In the oil-fired boilers, the combustion can be achieved in a much smaller volume
of furnace than a coal fired unit. However this intense combustion process
results in very high-localised heat absorption rate within the active burning zone
of the furnace. In order to avoid these high heat absorption rates, the furnaces
selected for oil firing are increased in size above the minimum required to
complete combustion only, to a size that will produce safe furnace wall
temperatures.
The combustion characteristics of gas produce a more uniform heat release
pattern within the furnace. This allows the use of even smaller furnaces than for
oil.
The Fig. illustrates the relative size of units designed for these three fuels at high
sub critical pressure levels.
All coals have certain characteristics (Table) which may be used on a relative basis
to compare their effect on furnace sizing.
TABLE: COAL CHARACTERISTICS
Total moisture : 15
Ash : 30
Sulphur : 0.5
HHV kcal/Kg : 4500
Ash softening temperature : 1250o
C
(At reducing atmosphere)
Sodium content in ash as Na20 : 01

57. Page | 57
The moisture content in coal affects the design of the boiler in many ways such as
combustion gas weight, flue-gas velocities, boiler efficiency, heat transfer rates and
low temperature corrosion.
The ash quality and quantity can affect the furnace slagging rate, fouling of super
heater, re heater and air heater surfaces, the unburnt carbon loss, the amount of
particulate emission discharged to the atmosphere and the capacity of ash handling
system.
Fig 20. Comparison of Furnaces for different Fuels

58. Page | 58
ALLOWABLE HEAT LOADING
The furnace heat loading is a characteristic requirement for each fuel. Normally
three basic heat loadings are considered for furnace designs. The EPRS (Effective
Projected Radiant Surface) heat loading can be related to the gas temperature
leaving the furnace. This gas temperature is very much significant in the case of
coal fired boilers. This value should be considerably lower than the initial ash
deformation temperature to protect superheaters and re heaters from fouling due
to ash deposition. In the case of oil and gas fired boilers this loading will be high,
however taking care of the metal temperatures in the water wall and maintaining
an optimum gas temperature for the heat transfer in super heater/re heater. This
value change from 200,000 to 325,000 kCal/hr/m2
for coal fired boilers and a
maximum of 550,000 kCal/hr/m2
for oil and gas fired boilers.
The plan area heat loading can be correlated to the maximum localised heat
absorption rate and maximum temperature of products of combustion. In other
words, a very high plan area loading may tend the furnace to slag in the case of coal
fired boilers and increase the furnace wall temperature in the case of oil and gas
fired boilers. The normal value for coal ranges from 3 to 4 million kCal/hr/m2
and
for oil and gas a maximum value of 5 million kCal/hr/m2
shall be used.
The volumetric heat loading is an important criterion especially for fuels critical
about stabilisation for combustion and low heating value gases. A low value of
volumetric heat loading signifies an increased residence time in the furnace for the
combustion to complete. The values used for the normal fuels range from 130 to
300 x 103
kcal/hr/m3
.
10. Burners
Burners undertake the task of delivering coal and air in a proper proportion,
facilitate ignition energy to the coal air stream, sustain the ignition and provide
a stable flame during the operation, complete the task of combustion and
delivering heat to the intended purpose.
Burners are broadly classified as follows:
(1) Tangential Burners
(2) Wall Burners
(3) Down shot or fan tail burners
Modern Burners are equipped with:
(a)Separate flame envelope ports for coal, oil and gas.

59. Page | 59
(b)Secondary air control to adjust the flame envelops.
(c)Ignitors.
(d)Flame Scanners to detect the distinct flames in an enclosure.
(e) Flame Stabilisers.
(f) Flame Analysers
The proportioning of air flow is done based on boiler load, individual burner
load, by a series of air dampers. Each of the auxiliary and end air nozzles are
provided with louver type regulating dampers, at the air entry to individual air
compartment. The damper regulates on elevation basis, in unison, at all
corners.
The burners maybe tilt able about horizontal, in unison at all elevations and
corners. This shifts the flame zone across the furnace height and enables
control over steam temperature.
Combustion Air Distribution
The Combustion air, referred to as Secondary Air, is provided from FD Fans. A
portion of secondary air called `Fuel Air', is admitted immediately around the
burners (annular space around the oil/gas burners) into the furnace. The rest of the
secondary air, called 'Auxiliary Air', is admitted through the auxiliary air nozzles and
end air nozzles. The quantity of secondary air (fuel air + auxiliary air) is dictated by
boiler load and controlled by FD Fan inlet guide vane regulation.
Burner Clearance
Sufficient height between the top row of fuel nozzles and the furnace outlet
must be provided especially in the case of coal fired boilers to obtain proper furnace
retention time. This value ranges from 15 to 20 meters for large utility Boiler. In
the case of horizontal firing, which is adopted mostly for industrial boilers, the
clearances between the burners, between the side walls and burner nose portions
should all be properly sized to avoid flame impingement on the walls and also flame
embracing which is not desirable from the combustion and heat absorption point of
view.
Burner Arrangement
There are 36 pulverised coal burners arranged on the corners at a height of 17
to 38 meters and 20 oil burners. The pulverised coal burners are arranged in
such a way that eight mills supply the coal to burners at 4 corners, of the
furnace, all the nozzle of the burners are inter linked and cannot be tilted. The
oil burners are fed with light diesel oil and heavy fuel oil till boiler load reaches

60. Page | 60
to about 25%. There are four wind boxes fixed at 4 corners of the furnace.
There are 17 nozzles in each wind box 8 for coal and 9 for air. In tangentially
fired boilers, four tall wind boxes are arranged at each corner of the furnace.
The coal burners are located at different elevations of wind boxes. The
numbers of coal nozzles elevations are equivalent to the number of coal mills.
The same elevations of coal nozzles at four corners are fed from a single coal
mill.
 The coal nozzle is sandwiched between air nozzles or compartments. That
is, nozzles are arranged between coal nozzles, one below the bottom coal
nozzle and above the top coal nozzle. If there are ‘n’ numbers of coal
nozzles per corner there will be (n+1) number of air nozzles per corner.
 The coal fuel and combustion air streams from these nozzles directed
tangential to imaginary circle at the centre of the furnace. This creates
turbulent vortex motion of the fuel, air and the hot gases which promotes
the mixing, ignition energy availability and thus combustion efficiency.
 The air nozzles in between are termed as “Auxiliary air nozzles”, and the
topmost and the bottommost air nozzles are termed as “End air nozzles”.
 The coal nozzles elevations are designed as A, B, C, D, E, F, G, and H from
bottom to top, the bottom end air nozzles as AA, and the top end air
nozzles as HH. The auxiliary air nozzles are designated by the adjacent coal
nozzles as, like AB, BC, CD, DE, EF, FG, GH and HH from bottom to top.
 The four furnace corners are designated as 1, 2, 3 and 4 in clockwise
direction looking from the top, and counting front water wall left corner as
‘1’, as shown in the figure below.
 Each pair of coal nozzle elevation is served by one elevation of oil burners
located in between the auxiliary air nozzles. In this with the eight mills or
eight elevations of coal nozzles, there are 20 oil guns arranged in four
elevations at auxiliary air nozzles at AB, CD, EF and GH.
 Heavy fuel oil can be fired at the oil gun elevations and L.D.O can be fired
only in AB elevation. Each oil gun is associated with an igniter arranged at
the side.
11. Improving Boiler Efficiency
Your boiler probably accounts for the majority, if not all, of your heating fuel
bill. If you tend to budget a fixed dollar amount for this fuel bill, you may be
interested in knowing that most boiler systems are, at best, 85% efficient with
an average efficiency of only 65% to 75%. This means from 25 to 35% of your
heating bill is going up the chimney with no appreciable benefit to you. There