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FUELS ANALYSES AND COMBUSTION
Syllabus
Fuels Analyses: Ultimate and proximate analysis of coal, Determination of calorific value
of solid and gaseous fuels by bomb and Junker’s Calorimeter respectively
...
Combustion, requirement of oxygen/ air in
combustion process
...


2
...
The
composition of fuels and hence the calorific value may vary widely
...
Fuel characterization refers to
“chemical composition” and “energy content” of the fuel
...
2

ANALYSIS OF COAL

There are two methods to analyze coal:
1
...


Ultimate analysis
...
Ultimate analysis
gives information about elemental composition of various constituents present in the coal
...
2
...
Proximate analysis is a quick analysis which gives information about the
suitability of coal for industrial use
...
It is
heated in an electric hot air maintained at 105-110˚C for an hour
...
The loss in weight represents moisture
and calculated on percentage basis
...
The crucible is
covered with a lid, and heated in an electric furnace at 950˚C for seven minutes
...
Loss of weight represents volatile
matter
...
The crucible is then taken
out, cooled in air, then in a desiccator and weighed
...

Percentage of ash content in coal

=

Weight of ash formed x 100%
Weight of dried coal
d) Determination of percentage of fixed carbon
Fixed carbon is the quantity of carbon in coal that can be burnt by primary current of air
drawn through the hot bed of fuel
...
Greater the percentage of fixed carbon,
greater is the calorific value of the coal
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

Moisture in coal adds to handling, storage and transportation costs
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

Heat is wasted on evaporating the moisture
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

More volatile matter increases flame length and smoke
...




Higher the volatile matter, larger the combustion space
...


Desirable: Ideally a fuel should have less volatile matter content
...




Ash causes hindrance to the flow of air and heat, thereby lowering the
temperature
...




Disposal of ash is an environmental problem
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Fixed carbon


Fixed carbon gives a rough estimate of the calorific value of coal
...




It is this carbon that is burnt in the solid state
...




Percentage of fixed carbon helps in designing the furnace and shape of the firebox because it is the fixed carbon that burns in the solid state
...


2
...
2 ULTIMATE ANALYSIS OF COAL
Ultimate analysis includes the determination of carbon, hydrogen, oxygen, sulphur etc in
coal
...
The analyses give exact results and are useful in calculating the calorific value
of coal using Dulong’s formula
...
Determination of percentage of Carbon and Hydrogen
A known amount of the coal sample is burnt in a current of O2 in a combustion
apparatus shown below
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1: Estimation of carbon and hydrogen
The carbon and hydrogen present in the coal sample are converted into CO2 and H2O
respectively according to the following equations
...
The increase in weights of KOH tube is due to the
absorption of CO2 while increase in weight of CaCl2 tube is due to the absorption of
H2O
...

Calculations
2 KOH + CO2 

K2CO3 + H2O

CaCl2 + 7 H2O 
Percentage of Carbon in coal

CaCl2
...
Determination of percentage of Nitrogen
The determination of nitrogen content is carried out by Kjeldahl’s method
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H2SO4 in presence of K2SO4
(catalyst) in a long necked flask (called Kjeldahl’s flask)
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N2 + 3H2 + H2SO4  (NH4)2SO4
The clear solution is then heated with excess of NaOH and the librated ammonia is
distilled over
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(NH4)2SO4 + 2NaOH  2NH3 + Na2SO4 + 2H2O
NH3 + HClNH4Cl
The volume of unused HCl is then determined by back titrating it against standard
NaOH
...
From this the percentage of nitrogen is calculated as follows
...
4 X Volume of acid consumed X Normality of acid
Weight of coal sample

Figure 2
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Determination of percentage of Sulphur
A known amount of coal sample is burnt completely in a bomb calorimeter in a current
of oxygen
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Solid residue
obtained from the calorimeter is extracted with water
...
The precipitate is filtered,
washed, dried and heated to constant weight
...

233 grams of BaSO4 contains 32 grams of sulphur
Percentage of Sulphur in coal

=

32 X Weight of BaSO4 obtained
233 Weight of coal sample

X 100

4
...
(From proximate analysis)
Percentage of ash content in coal

=

Weight of ash formed x 100%
Weight of dried coal

5
...

Percentage of Oxygen in coal =

100 - [ % of (C + H + N + S + ash)]

Significance of ultimate analysis
1
...




Higher percentage of carbon in coal reduces the size of combustion
chamber required
...




Sometimes, hydrogen is present in the form of water which lowers the
calorific value
...
Nitrogen


Nitrogen has no calorific value and is an unwanted constituent in coal
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

Nitrogen produces corrosive oxides when burning of coal
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

When coal is converted to coke, nitrogen combines with hydrogen to form
ammonia which could be recovered as a byproduct
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Sulphur


Though sulphur increases the calorific value, presence of sulphur is
undesirable
...
It causes atmospheric pollution and can cause acid rain
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4
...




Ash causes hindrance to the flow of air and heat, thereby lowering the
temperature
...




Disposal of ash is an environmental problem
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Oxygen



Oxygen present in coal lowers the calorific value as it is usually associated
with moisture thereby decreasing available hydrogen for combustion process
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7%
...
3

CALORIFIC VALUE OF A FUEL

An important parameter of fuels is calorific value
...
The performance of a fuel is expressed in terms of its calorific value
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Calorific value is normally expressed in calories per gram (cal/g) in cgs units
...

Types of Calorific value

2
...
1 GROSS OR HIGHER CALORIFIC VALUE (HCV)
Gross calorific value is the amount of heat energy released when a unit mass of fuel is
burnt completely in air and the products of combustion are cooled down to room
temperature (15C or 60 F)
...
On combustion, carbon and hydrogen
present in the fuel are converted to CO2 and steam respectively
...
The
measured gross calorific value includes the heat of combustion of the fuel and latent
heat of steam
...
Calorific value can be calculated using Dulong’s formula
...
3
...

In actual practice, the comubustion products are not cooled to room temperature, but
simply let off to atmosphere
...
So, net calorific value is always
less than gross calorific value
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09 H g
...
09 H X 587 since
latent heat of steam, which is 587 Kcal/kg
...
09 H X 587) Kcal/kg

2
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4 DETERMINATION OF CALORIFIC VALUE
Calorific values of solid and non-volatile liquid fuels are determined by Bomb
caloriemeter where as calorific value of a gaseous fuel is determined by Junker’s
caloriemeter
...
3
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1 BOMB CALORIEMETER
Principle:
A known weight of the sample( solid or liquid fuel) is burnt completely in excess of
oxygen
...
Thus
heat liberated during the combustion of fuel is equal to the heat absorbed by water and
the copper caloriemeter
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It is provided with a screw type lid to make it air tight
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A small ring is attached to one of the
electrodes
...
The bomb is placed in a copper
caloriemeter which is surrounded by an air jacket and an outer water jacket to prevent
heat loss due to radiation
...

Working:
Accurately weighed amount (m g) of the solid/liquid fuel is taken and placed in a
crucible which is supported over the ring
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The oxygen supply is forced into the
bomb with the help of oxygen inlet valve till pressure reaches upto 25 atmospheres
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The
initial temperature of water is noted after thorough stirring
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Sample is burnt and heat energy is
liberated which is transferred into water
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The maximum temperature shown by the Beckmann’s
thermometer is recorded
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Calculations
Weight of the fuel taken in the crucible

=mg

Weight of the water taken in the caloriemeter = W g
Water equivalent of calorimeter, stirrer, thermometer, bomb etc
...
09 H x 587) cal/g

Figure 2
...

(ii) Acid correction: Fuels containing sulphur and nitrogen are oxidized under high
pressure and temperature of ignition to sulphuric acid and nitric acid
...


(iii) Cooling correction: Time taken to cool water from maximum temperature to room
temperature is noted
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HCV = (W+w) xS x (t2–t1+cooling correction)–[(Acid+fuse wire correction)]
m
Example 1: Calculate the calorific value of a coal sample with the following data
...
60 g, Water equivalent of calorimeter 2200 g, specific heat of
water = 4
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Increase in temperature = 6
...

Solution:
Mass of the fuel = 0
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6 X10-3 kg
Water equivalent mass of the calorimeter = 2200 g = 2
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187 kJkg-1 °C-1
Increase in temperature = 6
...
2 x 4
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52
0
...
2 kJ kg-1
Example 2: Calculate the gross and net calorific value of a coal sample from the
following data obtained from bomb calorimeter experiment
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73 g
Weight of water taken in the calorimeter (W)= 1500 g
Water equivalent of calorimeter (w) = 470 g
Initial temperature = 25
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3C
Percentage of hydrogen in coal sample = 2
...
187 kJkg-1 °C-1
Solution:
HCV of the fuel = (W+w) x S x (t2-t1)
m

cal/g

HCV

= (1500+470) x 4
...
3-25
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73 x10-3
= 2
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09H x 587)
= 2
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09 x 2
...
187 x 103)
= = 2
...

Mass of coal (m) = 0
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0C
Final temperature= 27
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0 cal
Acid correction = 50
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42)-[50+10]
0
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48 cal/g
LCV = [GCV- 0
...
48 – 0
...
28 cal/g

2
...
4
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Principle:
Calorimeter works on the principle of absorption of heat by water
...
From the volume of the gas burnt and the
volume of water, the mean rise of temperature of water in its passage through the jacket
is measured
...


Figure 2
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Bunsen burner is used for the combustion of fuel
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Gas flow meter is used to measure the volume of gas burning

per unit time
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Gas calorimeter includes a vertical cylinder in which the fuel is burnt
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Three thermometers are used, two of them are used to
measure the temperature of inlet water and outlet water
...

Working:
A known volume of gas is burnt in excess of air at a constant rate in combustion
chamber
...
Circulation
of water and the burning of gases are performed continuously about 15 minutes at
uniform rates
...
Water formed by the condensation of steam is also collected in a
graduated cylinder through a small outlet at the bottom
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1 m3
Volume of water collected in certain time = 20 kg

Volume of steam collected in certain time = 0
...
1
3
= 2000 Kcal/m

HCV
HCV

Latent heat of steam = 587 K
Latent Heat of steam = [HCV- m x 587 ] = 2000- [0
...
6 Kcal/m3
V
0
...
02 m3
Volume of water collected in certain time = 5 litre
Volume of steam collected in certain time = 2
...
1
3
= 2500 Kcal/m

Latent heat of steam = 587 K
Latent Heat of steam = [HCV- m x 587 ] = 2500- [0
...
6 Kcal/m3
V
0
...
4

GROSS(HIGHER) CALORIFIC VALUE (GCV OR HCV)

Gross calorific value is the amount of heat energy released when a unit mass of fuel is
burnt completely in air and the products of combustion are cooled down to room
temperature (15 C or 60 F)
...
On combustion, carbon and hydrogen
present in the fuel are converted to CO2 and steam respectively
...
The

measured gross calorific value includes the heat of combustion of the fuel and latent
heat of steam
...
Calorific value can be calculated using Dulong’s formula
...
4
...

If the combustion products like steam, CO2 are allowed to escape into the atmosphere,
the calorific value measured under these conditions does not include the latent heat of
condensation of steam
...
09 H g
...
09 H X 587 since
latent heat of steam, which is 587 cal/g
...
09 H X 587 cal/g
1
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C= 84
...
5%; O= 8
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5% and N=
0
...
0%; H= 5
...
4%; S= 1
...
6 %

Substituting the values we get,

2
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0%; H= 5
...
5%; S=
1
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5 % and ash=3
...
Calculate the gross and net calorific
values of the sample
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0%; H= 5
...
5%; S= 1
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5 %; ash= 3
...
5

COMBUSTION

Combustion is the rapid oxidation in which a fuel burns with the evolution of heat and
light
...
For example, combustion of carbon in oxygen
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Rate of combustion can be increased by a) pre-heating the
fuel and air b) increasing the surface area of the fuel (powdering of solid fuels) and c)
increasing the pressure of air or oxygen used for combustion
...


2
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1 COMBUSTION CALCULATIONS
The amount (volume or weight) of oxygen and air required for the complete combustion
of a given quantity of fuel can be calculated by considering following points
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Substances always combine in definite proportions by their molecular weights or

volume
...

Most commonly involved combustion reactions are

N2, ash and CO2 (if any) present in the fuel or air are incombustible matters and hence
they do not take any O2 during combustion
...


The total or theoretical amount of O2 required by the fuel will be given by the sum

of the amounts of O2 required by the individual combustible constituent present in the
fuel
...

Excess air for combustion: It is essential to supply excess air for complete
combustion of the fuel
...


Theoretical amount of air X (100 + % excess air)
100
Air contains 21% of O2 by volume and 23% of O2 by weight
...

Minimum weight of air required = 100 X Minimum O2

23
Minimum volume of air required = 100 X Minimum O2
21
4
...
92 (32 x 0
...
77 = 28
...


The mass of 22
...

ie, 22
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The volume (V1) of air/O2 required at a particular temperature (T1) and pressure

(P1) can be converted to some other volume (V2) at other temperature (T2) and pressure
(P2) using the following relationship
P1V1 = P2V2
T1
T2
PROBLEMS BASED ON COMBUSTION (VOLUME)
1
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2
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We know that 21 litres of O2 is supplied by 100 litres of air
Volume of air required = 0
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238 m3

3
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Find the air requirement for the
complete completion of 1m3 of this gas if 40% excess air is used
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A fuel has following composition by volume
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5% ; C4H8 = 2
...
0%; N2 = 14% and O2=2%
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PROBLEMS BASED ON COMBUSTION (MASS)
5
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6
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Calculate the
minimum quantity of air required for the complete combustion of fuel
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Calculate the minimum weight of air required for the complete combustion of 1
kg of coal having the following percentage of composition by weight
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5% ; S= 0
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5% ; O2=3% and ash= 2
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Calculate the percentage
composition by weight of dry products of combustion
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A coal sample on analysis gave the following composition by weight
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2%; H2= 4
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9%; N2 = 1
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1% and ash= 19
...
Calculate the
percentage composition by weight of dry products of combustion if 1 kg of coal
is burnt with 25% excess of air
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6

FLUE GAS ANALYSIS (ORSAT PROCESS)

The mixture of gases such as CO2, O2, CO etc
...
The analysis of a flue gas
would give an idea about the efficiency of the combustion process
...
It consists of a horizontal
tube provided with a three way stop-cock and a 100 ml graduated burette at the other
end
...
One end of the burette is connected to a leveling bottle filled with water
(water reservoir)
...

The horizontal tube is connected with three different absorption bulbs A, B, C for
absorbing CO2, O2 and CO
...


Working:
Water leveling bottle is adjusted such that the entire gas burette is filled with water
...
Now entire gas burette will be filled with
flue gas (100 ml)
Absorption of CO2
Now first stopcock is opened such that flue gas will pass thorough solution A, which
absorbs CO2
...
The difference between
initial volume and volume after CO2 absorption give the volume of CO2
...
Solution B absorbs O2
...

Absorption of CO

After complete absorption of O2, third stopcock is opened and CO is made to absorb
completely in solution C
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Decrease in volume of flue gas by first bulb = volume of CO2
Decrease in volume of flue gas by second bulb = volume of O2
Decrease in volume of flue gas by first bulb = volume of CO
The gas remaining in the burette after the absorption of CO, CO2 and O2 is taken as
nitrogen
...

It is essential to follow the order of absorbing the gases: CO2 first, O2 second and third
CO
...
This will resulted in reporting CO2 less and O2 more than actual
...

Significance of flue gas analysis
From the flue gas analysis, it is possible to know whether the fuel has been completely
burnt or not
...

b) If there is high percentage of O2 in a flue gas, it indicates complete combustion of
fuel and the oxygen supply is more than required amount
...

c) By the analysis of flue gas, one can increase or decrease the oxygen supply to
ensure complete combustion of fuel with little or no wastage of fuel
...
Write note on estimation of nitrogen in a coal sample
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2
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What is the calorific value of coal?
of coal by bomb calorimeter
Refer section 2
...
4
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What is proximate analysis of coal? Explain the steps involved in the proximate
analysis of coal
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2
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What is ultimate and proximate analysis of coal? Discuss their significance
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2
5
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Describe giving a neat labelled diagram
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3
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2
6
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Refer section 2
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The ultimate analysis of a coal sample gives the following data: C = 84
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5 %, H = 6 %, O= 8
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Calculate the high and low calorific
values by using Dulong’s formula

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