It briefly describes about different types of fuels and combustion

1. Boiler Combustion
Theory
Presented by
Nauman Salim Kalshekar
Manager-Operations
Nabha Power Ltd.

2. Agenda
Topic- Boiler Combustion Theory
Fuel and its classification
Fuel characteristics
Properties of fuel
Chemical analysis of coal
Combustion of coal

3. Fuel
• Fuel is a substance, which, at elevated
temperature, if ignited through a source, shall
vigorously react chemically, combining with oxygen
and release enormous, exothermic heat energy.
• An efficient fuel should have low ignition
temperature and high calorific value.
• It should have very less ash content.
• Good fuel shall leave least residue and shall
produce minimum smoke and no toxic fume.

4. Classification of Fuels
Fuels can be classified according to
• The physical state in which they exist in nature
• Solid Fuels (Wood, Peat, Coal, Coke etc.)
• Liquid Fuels (Petroleum, Gasoline, Diesel oil etc.)
• Gaseous Fuels. (Natural gas, Coal gas etc.)
• The mode of their procurement
• Natural or Primary Fuels (Wood, Coal, Petroleum, Natural gas etc.)
• Synthetic or Secondary Fuels (Coke, Charcoal, Gasoline, Coal gas)
• Their calorific value.

5. Solid Fuels Wood
• Wood contains too much moisture, unless it is
thoroughly dried. As such, wood is not suitable for
industrial purpose.
Coal
• Coal is the most widely used solid fuel which is used for
steam generation in power plants and the process
industry.
• It is the result of decay of vegetable matter, which is
buried under the earth since millions of years, and it is
formed by the actions of earth pressure and heat in
absence of Oxygen.

6. Biomass
Bagasse
Bagasse is the refuse of sugarcane from which the juice is
extracted in sugar mills. Bagasse is bulky tenacious and
fibrous. Bagasse contains 30 to 50% wood fiber, 01 to 10%
sucrose, & 40 to 60% moisture. The C.V. of Bagasse with 54%
moisture is about 2222 kcal/kg. For better burning,
preheated air (at 204°C) is used.
Rice Husk
Husk is a bye-product of agriculture crop and which is the
outer covering of rice, groundnut etc. During husking
operation, husk is separated from rice or ground nuts
and hence readily available on site. It has got very little value
except for using it as fuel for Boiler. The C.V. of husk is about
3055 kcal/kg.
Solid Fuels

7. Classification
of Coal
Peat
It is a fibrous brown substance formed by the partial
carbonization of vegetable matter. As an industrial fuel it has
limited use, since it is too bulky and like wood, it contains too
much of moisture.
Soft coal Or Lignite
It is a stage of development between peat and bituminous coal.
The moisture content is as high as 30% to 50%. It also has got
high ash content and less volatile matter. If left exposed it is
liable to crumble to dust and therefore it cannot be stored for
long periods.
Bituminous
This coal makes a flame when burning because it contains
inflammable gases which are known as volatile and are released when
the coal is heated. These gases escape and ignite above the surface of
the coal causing the familiar flame. Soft and bituminous coals are
generally deployed in Power Stations for electricity generation.
Anthracite
It is hard dense coal, difficult to burn because it has to be heated to a
high temperature before it will ignite. It has a high heating value and
is normally used in special Boilers. This fuel burns without smoke.

8. Grade GCV Range (Kcal/kg) Old Grades
G-1 Exceeding 7000 A
G-2 Exceeding 6700 and not exceeding 7000
G-3 Exceeding 6400 and not exceeding 6700
G-4 Exceeding 6100 and not exceeding 6400 B
G-5 Exceeding 5800 and not exceeding 6100
G-6 Exceeding 5500 and not exceeding 5800 C
G-7 Exceeding 5200 and not exceeding 5500 D
G-8 Exceeding 4900 and not exceeding 5200
G-9 Exceeding 4600 and not exceeding 4900 E
G-10 Exceeding 4300 and not exceeding 4600
G-11 Exceeding 4001 and not exceeding 4300 F
G-12 Exceeding 3700 and not exceeding 4000
G-13 Exceeding 3400 and not exceeding 3700 G
G-14 Exceeding 3100 and not exceeding 3400
G-15 Exceeding 2800 and not exceeding 3100
Non Cocking Coal UngradedG-16 Exceeding 2500 and not exceeding 2800
G-17 Exceeding 2200 and not exceeding 2500
Indian Coal Rank

9. Coal
Characteristic
s

10. Liquid Fuels • The liquid fuel used for firing in boilers is mineral oil obtained
from many hundreds of feet beneath the earth’s surface.
• Coal is, generally, regarded, as having been formed from
vegetable matter, trees and other plant life.
• Mineral Oil is believed to have basically originated from marine
life.
• As oil comes from the well it is known as crude petroleum or
Crude.
• It varies considerably in its composition and characteristics
according to the part of the world from which it is obtained.
• Crude is a mixture of many constituent oils. Some of these oils
evaporate at a low temperature and under ordinary atmospheric
conditions would evaporate into the air to form an explosive
mixture.
• Crude oil is taken to refinery where it is refined or distilled.
• In this process the lighter or thinner oils such as benzene, petrol,
paraffin, gas oil etc. are successively boiled off and separated.
• The heavy residue or so-called black oils are used as fuel oils in
boilers. These are the oils, which are used for steam generation
in power stations.

11. Liquid Fuels Naphtha
Naphtha is a petroleum product. It is very light and easily
evaporates even in atmospheric conditions. Naphtha firing is to be
done with utmost care, as it is susceptible for explosions. It has
density of about 0.7 and a calorific value of about 11000 kcal/kg air.
Light diesel oil
It is heavier than Naphtha. It does not evaporate in atmospheric
conditions as Naphtha. It does not require any preheating since its
viscosity is low and is very light and flowable even in atmospheric
conditions. Because of high cost it is not economical to fire in Boiler
on a continuous basis.
High speed diesel oil
High speed diesel oil is lighter and cleaner than LDO. Since it is
clean and lighter, HSD is used for firing of igniters. It is not usual
practice to fire HSD in boilers. It is exclusively used for surface
transport and agricultural purpose. It is also not economical since
the cost of HSD is more than LDO.

12. Liquid Fuels
Heavy Furnace oil
• HFO is a heavy residual oil. The viscosity is very high at room
temperature conditions. Hence it requires preheating to ensure
its flowability.
• It is difficult to handle. Heavy oil storage tanks are provided
with preheating arrangement and the tank heating is done by
steam coil heaters to maintain oil temperature around 65 to
70°C. The tanks are to be insulated.
• Further preheating is required to be done before it is fired in
boilers, in order to ensure proper firing of oil. HFO is cheaper
compared to all other petroleum products and is economical for
continuous firing in a liquid fuel fired boiler.

13. Properties of
Liquid Fuel
Specific gravity
This is defined as the ratio of the weight of a
given volume of oil to the weight of the same
volume of water at a given temperature. The
density of fuel, relative to water, is called
specific gravity
Viscosity
The viscosity of a fluid is a measure of its
internal resistance to flow. Viscosity
depends on temperature and decreases as
the temperature increases.
Flash Point
The flash point of a fuel is the lowest
temperature at which the fuel can be heated
so that the vapor gives off flashes
momentarily when an open flame is passed
over it. Flash point for furnace oil is 66oC.
Density
This is defined as the ratio of the mass of the fuel to the
volume of the fuel at a reference temperature of 15°C.
Density is measured by an instrument called hydrometer.
Specific gravity
This is defined as the ratio of the weight of a given volume
of oil to the weight of the same volume of water at a given
temperature. The density of fuel, relative to water, is called
specific gravity
Viscosity
The viscosity of a fluid is a measure of its internal resistance
to flow. Viscosity depends on temperature and decreases as
the temperature increases.
Flash Point
The flash point of a fuel is the lowest temperature at which
the fuel can be heated so that the vapor gives off flashes
momentarily when an open flame is passed over it. Flash
point for furnace oil is 66oC.

14. Properties of
Liquid Fuel
Fire Point
This is the temperature at which oil gives off
sufficient vapor to burn continuously. This is
usually 10⁰ C higher than the flash point. Oils
with a low flash point constitute a special
hazard during handling and storage.
Auto Ignition Point
It is the temperature at which a
flammable liquid burns without any fire
source. It is different for different types
of liquids.
Pour Point
This is the temperature at which the oil starts flowing on its
own with out any force. It indicates the extent to which the
oil resists congealing at low temperatures.
Fire Point
This is the temperature at which oil gives off sufficient vapor
to burn continuously. This is usually 10⁰ C higher than the
flash point. Oils with a low flash point constitute a special
hazard during handling and storage.
Auto Ignition Point
It is the temperature at which a flammable liquid burns
without any fire source. It is different for different types of
liquids.
Settling Point
Settling paint is the temperature at which oil passes from
liquid to solid state.

15. HFO
Characteristics

16. LDO
Characteristics

17. Oil Firing
FUEL OIL SYSTEM:
• Fuel oil system consists of storage tanks, fuel oil pumps, piping system, oil heaters,
oil strainers and associated instrumentation such as fuel oil meters, various
pressure, temperature gauges etc.
• From fuel storage tanks, oil is transferred by pump oil pre-heaters before it is
supplied to burner manifold, for firing in an oil burner.
• Preheating of fuel oil is required to lower its viscosity to a required level so that it
could be easily atomized.
• Without preheat, fuel combustion will be bad, since oil will not be properly
atomized.
• There will be severe deposit of carbon particles, choking the burner nozzles.
• Due to improper combustion the flame shape will not be good and there will be
unburnt particle.
• The flame will become smoky and black at the tail.

18. CORNER – 1 VIEW
From LDO
Boiler Area
From Service Air
Boiler Area
BURNER INLET
OIL VALVE
BURNER INLET
ATOMIZATION VALVE
BURNER PURGE
VALVE
AB Oil Burner
EF Oil Burner
CD Oil Burner
Burner Rack Valves
LDO Firing
Overview

19. CORNER – 1 VIEWHFO / ATOMIZATION STEAM FLOW DIAGRAM
Atomization steam
From Aux Steam
Header
AB Oil Burner
HFO
Supply
Heade
r
HFO
Return
Heade
r
CD Oil Burner
EF Oil Burner
BURNER INLET
OIL VALVE
BURNER INLET
ATOMIZATION VALVE
BURNER PURGE
VALVE
Burner Rack Valves
HFO Firing
Overview

20. Fuel Atomization
• Just by preheating and lowering viscosity of oil to flowability level alone cannot help in firing fuel oil.
• For efficient firing the oil is to be broken to minute particles and sprayed into the furnace, where already
sufficient ignition atmosphere is available.
• Only when it is broken to minute particles, the fuel oil can get thoroughly mixed with combustion air and
can form a good air fuel combination.
• By systematically breaking the fuel into minute particles and spraying, the fuel oil surface contact to
combustion air is enormously increased and every particle can get required oxygen and burn efficiently.

21. Turn-down ratio:
Turn down ratio is the ratio between maximum and
minimum flow at which an oil burner can sustain good
and efficient combustion as compared to the design
maximum burning capacity of the oil burner.
If a turndown ratio is of 4:1, it means that even at 25%
of the burner rating, atomization of oil will be efficient
and the good combustion will sustain.
Turn-Down
Ratio

22. Gaseous Fuel
Natural Gas:
It is mostly found under the earth crust along with the crude oil. It is a mixture of
hydrocarbon (80-95% methane and 5-20% ethane and other hydrocarbon.
Calorific Value: 12000-14000 Kcal/m3
Use: Largely used as domestic fuel.
A. CNG (Compressed Natural Gas):
It is natural gas compressed at high pressure.
Use: Largely used as alternative fuel for motor vehicles.
B. LNG (Liquified Natural Gas):
It is natural gas liquified by advanced refrigeration system at -260⁰F.
Use: Since it is more dense than CNG, it is used in heavy vehicles as fuel.
LPG (Liquified Petroleum Gas):
It is a mixture of hydrocarbons (80-95% propane and 5-20% ethane and other
hydrocarbons.
Calorific value: 27800 Kcal / m3
Use: Used as domestic or industrial fuel.

23. Chemical
Analysis of
Coal
• Analysis of coal sample is required to access the quality of
coal.
• This analysis is important in designing various boiler
components.
• Proximate Analysis:
It is the process of determination of moisture, volatile matter, ash
and fixed carbon content.
• Ultimate Analysis:
It is the process of determination of elemental composition of
various components of coal which includes determination of % of
C, H, S, N and O.
C 65-95 %
H 2-7 %
O < 20%
S < 10%
N 1-2 %
FC 20-70 %
ASH 5-40 %
TM 2-20 %
VM 20-45 %
ProximateAnalysisUltimateAnalysis

24. Proximate
Analysis of
Coal
•Proximate analysis, an important property of the solid fuels is the
name given to the estimation of moisture, volatiles, chars and ashes etc.
in a sample of a solid fuel.
•The volatiles and chars roughly give the distribution of the original
constituents into components partaking in flaming and glowing
combustion.
•While ash contents in wood may be negligible, coal, (for example
some varieties of Indian coal) may contain considerable amounts of ash.
•The sum of moisture, volatiles and ash percentages when subtracted
from 100 give the proportion of the so called fixed carbon.
•The constituents given by the proximate analysis is generally used in
design of a boiler and coal handling plant.

25. •Coal normally has some moisture in it. This moisture is known as “inherent
moisture”.
•On storage also coal might have absorbed some moisture due to various reasons
and some moisture may be present on coal surface, called “surface moisture”.
•These moistures will turn to steam taking latent heat from combustible products
on burning. The Hydrogen in coal also will burn to become water and evaporate to
steam.
•Thus the heat given for conversion water into steam is wasted heat in the process
of combustion.
•The water vapor and sulfur dioxide gas may combine into a corrosive product called
Sulfuric Acid, which is harmful and pollute the atmosphere. Carbon mono-oxide as
well as Nitrogen Oxides are also polluting gases and are undesirable.
Moisture
Content in
Coal
Determination of Moisture:
1 gm of finely powdered coal, taken in a crucible, is heated in an electric oven at 105⁰C-107⁰C
for 1 Hour. % Moisture can be calculated from loss of weight.
% Moisture =
loss of weight in coal
Weight of initial coal taken
X 100

26. Volatile
Matter in
Coal
• Volatile matter consists of mainly Hydro-Carbon, a combination of Carbon (C) with
Hydrogen (H2), in different proportions, along with other gas forming constituents.
• These are liberated in gaseous form when the fuel gets heated up initially and starts
burning above the bed of fuel with long yellow flame.
• Maintains flame stability and accelerates char burn-out.
• Coals with minimal volatile matter like anthracite are difficult to ignite.
• The heat value of volatile matter depends upon the quality of components. High rank
coals have considerable amount of hydrocarbons and high heat value. Whereas, with low
rank coals these are predominantly CO and moisture which have lower heat values.
• High VM coal loses its VM on prolonged storage.
• Lower mill outlet temperature to avoid mill fires.
Determination of Volatile Matter:
1 gm of finely powdered moisture free coal, taken in a covered crucible, is heated in a muffle
furnace at 950⁰ C for 7 minutes. % volatile content can be calculated from loss of weight.
% volatile matter =
loss in weight of moisture free coal
Weight of moisture free coal taken
X 100

27. Ash
Content in
Coal
•Coal consists of 15 to 50 % of ash in it, which cannot burn.
•The ash will get collected over the grate or bottom ash hopper, which is
to be removed periodically.
•It is also a waste product and has to be disposed off.
•If the temperature goes beyond certain limits, the ash may start fusing
and form clinker.
Determination of Ash content:
It is the residue obtained after burning of moisture and VM free coal, in
a muffle furnace under constant current of air at 700-750⁰ C for 1 Hr.

28. Fixed
Carbon in
Coal
•Fixed carbon is a black carbonaious product, which is hard and is
difficult to get ignited as compared to volatile matter.
•Fixed carbon is the solid fuel left in the furnace after volatile matter is
distilled off.
•Fixed carbon gives a rough estimate of heating value of coal.
Determination of Fixed Carbon:
% Fixed carbon = 100-( % moisture + % VM + % Ash)

29. Ultimate
Analysis of
Coal
•Solid fuels like coal, wood etc. are graded by their detailed
composition as well as the results of proximate analysis.
•In the case of liquid fuels like fuel oil etc. additional
properties, for example viscosity, pour point, flash point,
volatility, octane number etc. become relevant.
•Ultimate analysis or elemental analysis is the name given to
the straight forward analysis involving the estimation of the
important elements in the fuel.
•The ultimate analysis of coal gives the composition as
percentage by weight of various elements viz. C, H, N, O, S and
Ash.

30. Ultimate
Analysis of
Coal
Determination of % C and
HThis is usually done in single experiment when a known quantity of
accurately weighed coal is burned in presence of O2 in the apparatus
shown below.
• Carbon burns to form CO2 and hydrogen forms H2O.
• CaCl2 and KOH absorbs H2O and CO2 respectively.
• Increase in weight of tubes noted.
% C =
increase in weight of KOH tube X 12
Weight of coal sample taken X 44
X 100
% H =
increase in weight of Cacl2 tube X 2
Weight of coal sample taken X 18
X 100

31. Ultimate
Analysis of
Coal
Determination of % N
• 1 gm of accurately weighed powdered coal is heated with conc H2SO4
solution in kjeldahl’s flask.
• It is treated with excess KOH solution.
• Liberated ammonia is distilled over and absorbed in a known volume of
standard solution of acid.
• Unused acid is determined by back titration with standard NaOH.
% of N =
volume of acid used X normality X 1.4
Weight of coal sample taken

32. • Fuel and oxygen are essentially required for creation
of fire.
• When fuel burns with bright flame and heat it is
called the combustion.
• For good combustion three entities, each spelt to
begin with a “T”, are essential and they are
‘Temperature’, ‘Time’ and ‘Turbulence’.
Combustion

33. Combustion

35. Three Ts of
Combustio
n
• In order to ignite the fuel, it is to be heated to a required minimum
Temperature. This temperature at which the fuel can catch fire is
called the Ignition Temperature of the fuel.
• There should be sufficient Time for the combustible matter to react
with oxygen and complete the process of chemical reaction of
oxidation to take place.
• To enable perfect burning, the fuel is to thoroughly get mixed with
combustion air, where from it can get required oxygen. Unless the
combustion air creates a sort of Turbulence this kind of molecule to
molecule mixing can not take place.
• It is customary to call ‘Temperature’, ‘Time’ and ‘Turbulence” as
“three Ts” necessary for efficient combustion. A good contact
between fuel particles and air is also necessary and hence some
people consider ‘touch’ (Contact)’ as a fourth ‘T’ necessary for
efficient combustion.

36. Combustio
n as a
chemical
process
A few equations on chemistry of combustion
Burning of carbon to carbon dioxide (Complete combustion)
C+O2=CO2
1+2.67=3.67 + 33800 kJ per kg of carbon
i.e. 1 kg of carbon needs 2.67 kg of oxygen to produce 3.67 kg of
carbon dioxide OR in other way, 12 grams of carbon needs 32
grams of oxygen, producing 44 grams of carbon dioxide.
Burning of carbon to carbon monoxide (Incomplete
combustion)
2C+O2=2CO+10130 kJ per kg of carbon.
1+1.33=2.33
i.e. 1 kg of carbon takes 1.33 kg of oxygen and produces 2.33 kg
of CO, and evolves only 10130 kJ per kg of carbon burnt.

37. Combustio
n as a
chemical
process
Burning of sulphur to sulphur dioxide
S+O2=SO2+9300 kJ/ kg of sulphur
1+1=2
i.e. 1 kg of S needs 1 kg of O2 and produces 2 kg of SO2 and
evolves 9300 kJ per kg of sulphur burnt.
Burning of carbon monoxide to carbon dioxide.
2CO+O2=2CO2+23670 kJ/ kg of carbon monoxide.
1+4/7=14/7
i.e. 1 kg of CO needs 4/7 kg of O2 and produces 14/7kg of CO2
and evolves 23670 kJ per kg of CO burnt.

38. Combustio
n as a
chemical
process
Burning of Methane (CH4)
CH4+2O2=CO2+2H2O+605800 kJ/ kg
Burning of Ethylene (C2H4)
C2H4 + 3O2 = 2CO2+2H2O + 639600 kJ/ kg
1+3.43=3.14+1.29
Burning of Acetylene (C2H2)
2C2H2 + 5O2 =4CO2 + 2H2O + 707200 kJ/ kg
1 + 3.08 = 3.39 + 0.69

39. Theoretical
air required
for
combustion
of fuel
• Let us consider 1 kg of a fuel, the ultimate analysis of which
shows that carbon is C kg, hydrogen H kg, oxygen O kg and
sulphur S kg.
• 1 kg of carbon requires 2.67 kg of oxygen for its complete
combustion; therefore C kg of carbon will require C x 2.67
kg of oxygen which is equivalent to 2.67 C kg of oxygen.
• 1 kg of hydrogen requires 8 kg of oxygen; therefore H kg of
hydrogen requires 8 H kg of oxygen.
• 1 kg of sulphur requires 1 kg of oxygen; therefore S kg of
sulphur requires S kg of oxygen.
The quantity of oxygen required for combustion of 1 kg of the
fuel is;
(2.67 C + 8 H + S) kg.

40. Theoretical
air required
for
combustion
of fuel
If the fuel contains O kg of oxygen, then the oxygen required
from air for the complete combustion of fuel will be;
(2.67 C + 8 H + S – O)
which can be otherwise written as;
2.67 C + 8 (H – O/8) + S
the term in the bracket being known as the available
hydrogen.
Minimum oxygen required for complete combustion of a fuel
2.67C + 8 (H – ( O/8)) + S kg
Since air contains 23 per cent by mass of oxygen, 1 kg of oxygen
is contained in 100 /23 kg and thus the minimum quantity of air
required for complete combustion of one kg of fuel will be
100/23 [ 2.67C + 8 {H – (O/8)} + S] kg

41. Effects of
Excess Air
If more excess air is supplied than required
• Lowering the furnace temperature
• Increasing heat loss through the chimney gases
• Additional auxiliary power, required to handle excess air.
• Less heat absorption in radiant zone super heaters.
If less excess air is supplied than required
• Increasing furnace temperature, which may have bad
effect on the structures and components of furnace.
• Incomplete combustion.
Excess Air
In practice air or oxygen in excess of the theoretical amount has
to be supplied to approach conditions of Stoichiometry.

42. Effects of
Nitrogen in
Air
• Nitrogen being an inert gas, it usually takes no useful part in
chemical reaction.
• Since it is present in the air in larger proportion, it helps to
keep the combustion temperature very low.
• Oxygen being very less in proportion compared to nitrogen,
presence of nitrogen retards the intimate mixing of the fuel
with oxygen.
• Being larger constituent in air and not taking part in
combustion, carries away substantial heat along with it as
waste energy through the chimney of the boiler.
• In certain combustion conditions it reacts with oxygen and
produces NOX(Nitrogen Oxides such as N2O, NO, NO2, NO3
etc. which pollutes the air.

43. Efficient or
perfect
combustion
The basic requirements necessary to be fulfilled for efficient and perfect
combustion installations are:
• Thorough mixing of fuel and air.
• Optimum fuel-air ratio leading to most complete combustion possible
and maintained over full load range.
• Ready and accurate response of rate of fuel feed to load demand.
• Continuous and reliable ignition of fuel.
• Practical distillation of volatile components of coal followed by
adequate action covered as above.
• Adequate control over point of formation and accumulation of ash,
when coal is the fuel.
• Excess air should be as minimum as possible without CO in flue gas.
CO2 in flue gas should be maximum and O2 in flue gases should be
minimum.
For Furnace oil/LSHS fuel oil etc. liquid fuel, the flame should be bright
golden yellow of appropriate length, without sparkles/fire at the end.

44. The term combustion efficiency measures the effectiveness of
the combustion reaction and is defined as follows:
Combustion efficiency = (Heat released during the combustion /
Heat contained in the fuel).
The combustion efficiency can also be expressed in terms of
ultimate CO2 of the fuel (the ultimate CO2 represents 100%
combustion efficiency) in the following way,
Efficiency η = (CO2 x 100) / U
Where CO2 is the percentage in the flue gas and U is the ultimate
CO2 of the fuel.
Combustio
n
Efficiency

45. To control the air pollution following measures may be taken while
operating boiler.
• Sufficient air supply (with required excess air) should be provided to
ensure complete combustion i.e. No formation of carbon-mono-
oxide (CO).
• Fuel should be properly preheated (or pulverised to requirement
and dried) before firing.
• Stack height should be adequate enough to ensure the emission out
of flue gases gets spread over a wide area and gets diluted in air at
that height.
• Leakages of gaseous fuel should be prevented or minimized.
• Burners should be in proper condition and well maintained. Fuel oil
guns should be regularly cleaned.
• Electrostatic precipitator or cyclone separator should be provided in
flue gas path to chimney so that suspended particles may be
collected before it is let out of chimney and disposed off to a
secluded area.
• Water vapour may be' mixed with flue gas to reduce NOX level.
• Sulphur should be minimum in fuel to reduce SO2 level otherwise,
desulphurisation devices should be kept in the flue gas path.
Measures
to control
air pollution

46. Calorific value
Calorific value of fuel is amount of heat liberated when one
unit of fuel is burnt completely.
Gross or higher calorific value ( GCV or HCV):
• It is defined as amount of heat liberated when one unit of
fuel is burnt completely and product of combustion has
been cooled to room temperature.
• Latent heat of vaporization of water in the combustion
products also considered as usable energy.
Net or Lower Calorific Value (NCV or LCV):
• Net calorific value (NCV) or lower calorific value (LCV) is
determined by subtracting the heat of vaporization of the
water vapor from the higher heating value.
• The energy required to vaporize the water therefore is not
considered as usable energy.

47. Relationship
between HCV
and LCV
Where,
C= % of carbon in fuel
H= % of hydrogen in fuel
O= % of oxygen in fuel
S= % of Sulphur in the fuel
And
Calorific value of carbon= 8080 kcal/kg
Calorific value of hydrogen = 34500 kcal/Kg
Calorific value of Sulphur = 2240 Kcal/Kg