UNIT-4

FUELS AND COMBUSTION

• Fuels: Introduction: Classification of fuels; Coal and coke: Analysis of
coal (proximate and ultimate), Carbonization, Manufacture of
metallurgical coke (Otto Hoffmann method). Petroleum and Diesel:
Manufacture of synthetic petrol (Bergius process), Knocking – octane
number, diesel oil – cetane number; Power alcohol and biodiesel.

• Combustion of fuels: Introduction: Calorific value – higher and lower

calorific values, Theoretical calculation of calorific value; Ignition
temperature: spontaneous ignition temperature, Explosive range;
Flue gas analysis – ORSAT Method. CO2 emission and carbon foot
print.
 Definition of a Fuel
• Fuel is a combustible substance, containing carbon as the main
constituent, which on proper burning gives large amount of heat
which can be used economically for domestic and industrial purposes
eg. Wood, charcoal, kerosene, diesel, petrol etc.
• During combustion atoms present in the fuel such as carbon,
hydrogen, etc. combine with oxygen with the simultaneous liberation
of heat at a rapid rate.
 Classification of Fuels

• Fossil fuels have been classified according to

1)Occurrence
2) State of aggregation
 Classification based on occurrence
1) Natural or primary fuels which occur in nature as such e.g. wood,
peat, coal, petroleum, natural gas etc.
2) Artificial or secondary fuels are those which are prepared from the
primary fuels e.g. charcoal, coke, kerosene oil, diesel oil, petrol, coal
gas, producer gas et
Classification based on state of aggregation
• The second classification is based on the state of aggregation like
• a) Solid fuels
• b) Liquid fuels
• c) Gaseous fuels
 Requirements of a good fuel
• High calorific value
• Moderate ignition temperature
• Should not undergo combustion
• Moderate velocity of combustion
• Combustion should be easily controlled
• Low moisture content
• Low non-combustible matter content
• Products of combustion should not burn
• Should burn in air with efficiency without much smoke
• Low cost
• Easy to transport
• Storage cost in bulk should be minimum
 PRIMARY SOLID FUEL - COAL
• Coal is an important primary solid fuel that has been formed as a
result of alteration of vegetable matter under some favorable
conditions.

1.Coalification (or) Metamorphism

• The process of conversion (or alteration) of vegetable matter to
anthracite (coal) is called coalification or metamorphism of coal.
 COAL
• Coal is a highly carbonaceous matter that has been formed as a result
of alteration of vegetable matter (eg. plants) under certain favorable
conditions. It is chiefly composed of C, H, N and O besides non-
combustible inorganic matter.
• Types:
Peat
Lignite
Bituminous
Anthracite
 On moving from peat to anthracite
• decrease in the moisture content
• decrease in hydrogen, oxygen, nitrogen &sulphur contents
• decrease in volatile matter content
• increase in carbon content from 57% to 93%
• Increase in calorific value
• Increase in hardness
 Analysis of Coal

• The quality of coal can be assessed by

(a)Proximate analysis
(b)Ultimate analysis
 PROXIMATE ANALYSIS
It involves the determination of:
i. Moisture
ii. Volatile matter
iii. Ash content
iv. Fixed carbon
 1. Moisture Content

• About 1 gm of powdered coal sample is taken in a crucible, and is

heated at 100 − 105°C in an electric hot-air oven for 1 hour. The loss
in weight of the sample is found out and the percentage of moisture
is calculated as
 2. Volatile matter
• After the analysis of moisture content the crucible with residual coal sample is
covered with a lid, and is heated at 950 ± 20°C for 7 minutes in a muffle
furnace.
• The loss in weight of the sample is found out and the % of volatile matter is
calculated as loss in weight of the coal.
 3. Ash content

• After the analysis of volatile matter, the crucible with residual coal sample is
heated without lid at 700 ± 50°C for half an hour in a muffle furnace.
• The loss in weight of the sample is found out and the % of ash content is
calculated as
 4. Fixed carbon

• It is determined by subtracting the sum total of moisture, volatile and

ash contents from 100.
• % of fixed carbon in coal =
100-% of (moisture + volatile matter + ash content)
Significance of Proximate Analysis
 ULTIMATE ANALYSIS
It involves the determination of percentage of
• carbon and hydrogen contents,
• nitrogen content
• sulphur content
• ash content
• oxygen content
 1. Carbon and Hydrogen contents
• A known amount of the coal sample is burnt in a current of O2 in a
combustion apparatus.
• The carbon and hydrogen, present in the coal sample, are converted
into CO2 and H2O respectively according to the following equations.
• The liberated CO2 and H2O vapors are absorbed respectively in KOH and
anhydrous CaCl2 tubes of known weights.
• The increase in weight of KOH tube is due to the formation of CO2 while
increase in weight of CaCl2 tube is due to the formation of H2O.
• From the weights of CO2 and H2O formed, the % of carbon and hydrogen
present in the coal can be calculated as follows.
 2. Nitrogen content

• The determination of nitrogen content is carried out by Kjeldahl’s method.

• A known amount of powdered coal sample is heated with con. H2SO4 in
presence of K2SO4 (catalyst) in a long necked flask (called Kjeldahl’s flask).
• Nitrogen in the coal is converted into ammonium sulphate and a clear
solution is obtained.
• The clear solution is then heated with excess of NaOH and the liberated
ammonia is distilled over and is absorbed in a known volume of standard
N/10 HCl.

• The volume of unused N/10 HCl is then determined by titrating it against

standard N/10 NaOH. Thus the amount of acid neutralized by liberated
ammonia from coal is determined.
 3. Sulphur content

• A known amount of coal sample is burnt completely in a bomb calorimeter.

During this process sulphur is converted into sulphate, which is extracted
with water.
• The extract is then treated with BaCl2 solution so that sulphates are
precipitated as BaSO4.
• The precipitate is filtered, dried and weighed. From the weight of BaSO4
obtained, the sulphur present in the coal is calculated as follows.
3. Ash content
• Determination of ash content is carried out as in proximate analysis
4. Oxygen content
• The percentage of oxygen is calculated as follows.
 Carbonization

• When coal is heated strongly in the absence of air (called destructive

distillation) it is converted into lustrous, dense, porous and coherent mass
known as coke.
• This process of converting coal into coke is known as Carbonization.
• Types of carbonization
(i) Low temperature carbonization
(ii) High temperature carbonization
 Caking coals and coking
coals

• When coals are heated strongly, the mass becomes soft, plastic and fuses to
give a coherent mass.
• Such type of coals are called Caking Coals.
• But if the mass so produced is hard, porous and strong then the coals are
called Coking Coals.
• Coking coals possess lower volatile matter and are used for the manufacture
of metallurgical coke. Thus all coking coals are caking coals but all caking
coals are not coking coals.
 SECONDARY SOLID FUEL - METALLURGICAL COKE
• When bituminous coal is heated strongly in the absence of air, the volatile
matter escapes out and the mass becomes hard, strong, porous and
coherent which is called Metallurgical Coke.
 MANUFACTURE OF METALLURGICAL COKE
(Otto-Hoffman’s by-product oven)
• There are so many types of ovens used for the manufacture of metallurgical
coke.
• But the important one is Otto-Hoffman’s by product oven.
• Objectives & Advantages
increase the thermal efficiency of the carbonization process and,
recover the valuable by products (like coal gas, ammonia, benzol oil, etc.,
heating is done externally by producer gas hence we can save fossil fuel
the carbonization time is less.
1. The oven consists of a number of silica chambers. Each chamber is
about 10 − 12 m long, 3 − 4 m height and 0.4 − 0.45 m wide. Each
chamber is provided with a charging hole at the top, it is also provided
with a gas off take valve and iron door at each end for discharging
coke.

2. Coal is introduced into the silica chamber and the chambers are closed.
The chambers are heated to 1200°C by burning the preheated air and
the producer gas mixture in the interspaces between the chambers.
3.The air and gas are preheated by sending them through 2nd and 3rd
hot regenerators.
4. Hot flue gases produced during carbonization are allowed to pass
through 1st and 4th regenerators until the temperature has been
raised to 1000°C.
5. While 1st and 4th regenerators are heated by hot flue gases, the 2nd
and 3rd regenerators are used for heating the incoming air and gas
mixture.
6.For economical heating, the direction of inlet gases and flue gases are
changed frequently.

7.The above system of recycling the flue gases to produce heat energy
is known as the regenerative system of heat economy. When the
process is complete, the coke is removed and quenched with water.

8. Time taken for complete carbonization is about 12-20 hours. The

yield of coke is about 70%. The valuable by products like coal gas,
tar, ammonia, H2S and benzol, etc. can be recovered from flue gas.
 PRIMARY LIQUID FUELS – PETROLEUM /
CRUDE OIL
• Crude oil is a mixture of paraffinic, olefinic, and aromatic
hydrocarbons with small amounts of organic compounds like N, O,
and S. The approximate composition of petroleum is:
• Petroleum may be of three types:
1. Paraffinic based – straight-chain hydrocarbons
2. Naphthenic based – Cycloparaffins and aromatic
3. Mixed based – both paraffinic and naphthenic
 REFINING OF PETROLEUM (or) CRUDE OIL
• The process of removing impurities like water, sulphur, dissolved
salts like MgCl2 and separating the crude oil into various fractions
having different boiling points is called refining of petroleum. This
process of refining involves the following four steps.
• When the vapors of the oil go up in the fractionating column, they
become cooler and get condensed at different trays.
• The fractions having higher boiling points condense at lower trays
whereas the fractions having lower boiling points condense at higher
trays.
• The gasoline obtained by this fractional distillation is called straight-
run gasoline.
• Various fractions obtained at different trays are given in the table.
 MANUFACTURE OF SYNTHETIC PETROL
• The gasoline, obtained from the fractional distillation of crude
petroleum oil, is called straight-run petrol.
• As the use of gasoline is increased, the amount of straight-run
gasoline is not enough to meet the requirement of the present
community.
• Hence, we are in need of finding out a method of synthesizing petrol.
 Hydrogenation of coal (or) Synthetic petrol
• Coal contains about 4.5% hydrogen compared to about 18% in
petroleum. So coal is a hydrogen deficient compound. If coal is heated
with hydrogen to high temperature under high pressure, it is
converted to gasoline. The preparation of liquid fuels from solid coal
is called the Hydrogenation of coal (or) synthetic petrol.
• There are two methods available for the hydrogenation of coal
1. Bergius process (or direct method).
2. Fischer-Tropsch process (or indirect method).
 Bergius process (or) direct method
1. In this process, the finely powdered coal is made into a paste with
heavy oil, and a catalyst powder (tin or nickel oleate) is mixed with it.
2. The paste is pumped along with hydrogen gas into the converter,
where the paste is heated to 400 − 450°C under a pressure of 200 −
250 atm.
3. During this process hydrogen combines with coal to form saturated
higher hydrocarbons, which undergo further decomposition at higher
temperatures to yield a mixture of lower hydrocarbons.
4. The mixture is led to a condenser, where the crude oil is obtained.
5. The crude oil is then fractionated to yield (i) Gasoline (ii) Middle oil
(iii) Heavy oil.
6. The middle oil is further hydrogenated in the vapor phase to yield
more gasoline. The heavy oil is recycled for making paste with fresh
coal dust.
7.The yield of gasoline is about 60% of the coal used.
 KNOCKING
• The sudden explosion (unwanted sound) due to a rapid rise in pressure inside
the engine is known as knocking. It is common in petrol and diesel engine.
1. Causes of knocking in S.I (Spark Ignition) Engine [Petrol engines]

• In a petrol engine, a mixture of gasoline vapor and air at a 1:17 ratio is used
as fuel. This mixture is compressed and ignited by an electric spark.
• The product of oxidation reaction (combustion) increases the pressure and
pushes the piston down the cylinder.
• If the combustion proceeds in a regular way, there is no problem in knocking.
• But in some cases, the rate of combustion (oxidation) will not be uniform due
to unwanted chemical constituents of gasoline.
• The rate of ignition of the fuel gradually increases and the final portion of the
fuel-air mixture gets ignited instantaneously producing an explosive sound
known as “Knocking”.
• Knocking in petrol engine is rated by “ Octane number”
 OCTANE NUMBER OR OCTANE RATING

• The octane number is defined as, the percentage of iso-octane

present in a mixture of iso-octane and n-heptane.
2.Knocking property based on the Chemical structure
• The knocking tendency of fuel hydrocarbons mainly depends on their
chemical structures.
• Knocking property of the fuel reduces the efficiency of the engine. So
good gasoline should resist knocking. The knocking tendency
decreases in the following order.
• Straight chain paraffins > Branched chain paraffins > Cycloparaffins >
Olefins > Aromatics.
• Thus olefins of the same carbon-chain length possess better anti-
knock properties than the corresponding paraffin.
• Reduction of knocking (or) Improvement of Anti knocking
Characteristics
(i) by adding anti-knock agents like Tetra-Ethyl Lead (TEL)
(ii) by blending low octane numbered fuel with high octane number
fuel.
(iii) Nowadays aromatic phosphates are used as an antiknock agent
because it avoids lead pollution.
(iv) by proper cracking
3. LEADED PETROL (ANTI – KNOCKING AGENT)

• The anti-knock properties of gasoline can be improved by the

addition of suitable additives.
• Tetraethyl lead (TEL) (C2H5)4Pb is an important additive added to
petrol. Thus the petrol containing tetraethyl lead is called leaded
petrol.
4. Mechanism of Knocking

• TEL reduces the knocking tendency of hydrocarbon.

• Knocking follows a free radical mechanism, leading to a chain growth
which results in an explosion.
• If the chains are terminated before their growth, knocking will cease.
• TEL decomposes thermally to form ethyl free radicals which combine
with the growing free radicals of knocking process and thus the chain
growth is stopped.
Disadvantages of using TEL

• When the leaded petrol is used as a fuel, the TEL is converted to lead
oxide and metallic lead.
• This lead deposits on the spark plug and on cylinder walls which is
harmful to engine life.
• To avoid this, a small amount of ethylene dibromide is added along
with TEL. This ethylene dibromide reacts
• with Pb and PbO to give volatile lead bromide, which goes out along
with exhaust gases.
• But this creates atmospheric pollution. So nowadays aromatic
phosphates are used instead of TEL.
 KNOCKING IN DIESEL ENGINE:
• In the diesel (Compression) engine, the diesel and air are not sent at
same time.
• The compressed air is sent first. The compression raises the temperature
around 500 C. Now, the diesel oil is sprayed. This further increases the
temperature and pressure. The expanding gases push the piston and
power stroke begins.
• The time difference between the diesel injection and its ignition is
known as “ignition lag”.
• If diesel contains any impurity, the ignition will be delayed. This delayed
ignition lag increases the accumulate pion of vapour, hence increases the
pressure rapidly and knocking occurs.
• Knocking of diesel engine is rated by “cetane number”.
 CETANE NUMBER OR CETANE RATING
• The cetane number is defined as "the percentage of hexa decane
present in a mixture of hexa decane and α-methyl naphthalene,
which has the same ignition lag as the fuel under test".
• Cetane number is introduced to express the knocking characteristics
of diesel.
• Cetane (hexa decane) (C16H34) has a very short ignition lag and
hence its cetane number is taken as 100. On the other
• hand α-methyl naphthalene has a long ignition lag and hence its
cetane number is taken as zero.
• 7. The cetane number decreases in the following order.
• Straight chain paraffins > Cycloparaffins > Olefins > Branched paraffins
> Aromatics.
• 8. The cetane number of a diesel oil can be increased by adding
additives called dopes. Ex:Ethyl nitrate, Iso-amyl nitrate.
 Power alcohol
• When ethyl alcohol is blended with petrol at a concentration of
5-10%. It is called power alcohol in other words absolute alcohol
(100% ethyl alcohol) is also called power alcohol.
• Ethyl alcohol is used in internal combustion (IC) engines. The addition
of ethyl alcohol to petrol increases its octane number. When ethyl
alcohol is blended with diesel it is called E Diesel.
• Manufacture
• Step:1
• Manufacture of ethyl alcohol
• Ethyl alcohol can be synthesized by fermentation of carbohydrates
(sugar materials).
• Fermentation of molecules which is the residue left after the
crystallization of sugar, with yeast generates alcohol. This
fermentation yields only about 20 % alcohol.
• Concentration of alcohol can be increased up to 97.6% by fractional
distillation yields rectified spirit.
• The concentration of alcohol cannot be increased by distillation above
97.6 %. Because it forms a constant boiling mixture with water.
• The constant boiling point has a lower boiling point than alcohol.
• Step:2
• Conversion of ethyl alcohol into power alcohol.
• But for the IC engine, 100% alcohol (absolute alcohol) is prepared by
removing the last trace of water from rectified spirit. It can be done
by the following two methods.
i) Alcohol-containing trace of water, is distilled with benzene. When
benzene passes over with a portion of alcohol and water, it leaves
behind absolute (power)alcohol.
ii) alcohol is distilled in the presence of the dehydrating agent, which
holds water. Finally absolute alcohol is mixed with petrol at a
concentration of 5-10% to get power alcohol.
• Properties
• Power alcohol has lower calorific value (7000K.Cal/Kg)
• It has high octane number(90)
• Its anti-knocking properties are good.
• It generates 10% more power than the gasoline of the same quality.
• Its compression ratio is also high.
• Uses: It is used as a very good fuel in motors.
• Advantages and disadvantages of power alcohol
• Advantages
• 1. It is cheaper than petrol
• 2. If any moisture is present, power alcohol absorbs it.
• 3. As ethyl alcohol contains oxygen atoms complete combustion
occurs, so the emission of CO2, hydrocarbon particulates are reduced.
• Disadvantages
1. As the calorific value of power alcohol (7000 cal/gm) is lower than
petrol (11,500cal/gm), a specially designed engine is required.
2. Output power is reduced up to 35%
3. Due to its high surface tension, atomization of power alcohol is
difficult, so it causes starting trouble.
4. It may undergo oxidation to give acetic acid, which corrodes the
engine part.
5. As it contains oxygen atoms, the amount of air required for
combustion is less therefore the engine and carburetor need to be
modified.
• BIO-DIESEL
• Vegetable oils comprise 90-95% triglycerides with a small number of
diglycerides, free fatty acids, phospholipids, etc. triglycerides are
esters of long-chain fatty acids, like stearic acids and palmitic acids.
• The viscosity of vegetable oils is higher and their molecular weight is
in the range of 600 to 900 which is about 3 times higher than those of
the diesel fuels.
• Problem in using vegetable oils directly.
1. As the viscosity of vegetable oils are high, atomization is very poor,
and hence inefficient mixing of oil with air leads to incomplete
combustion.
2. Oxidation and thermal polymerization of vegetable oil cause deposit
formation.
3. Their high viscosity causes misfire and ignition delay.
4. Their high volatility and consequent high flash point leads to more
deposit formation.
5. The use of vegetable oils as direct fuel requires modification of the
conventional diesel engine design.
• Manufacture: trans-etherification (or) alcoholysis.
• The above problems are overcome by reducing the viscosity of the
vegetable oils by the process known as trans-etherification or
alcoholysis. Alcoholysis is nothing but displacement of alcohol from
an ester by anther alcohol.
• It involves treatment of vegetable oil (sunflower oil, palm oil, soybean
oil mustard oil, etc) with excess of methanol in presence of catalyst to
give mono ethyl ester of long-chain fatty acid and glycerin. It is
allowed to stand for some time and glycerin is separated.
• Methyl ester of fatty acid, thus formed are called Bio-Diesel is defined
as a mono-alkyl ester of long chain fatty acid derived from vegetable
oils or fats.
• It is a pure fuel before blending with conventional diesel fuel. Bio-
diesel can be blended with petroleum diesel.
Advantages and Disadvantages of Bio diesel.

• Advantages
• Bio-diesel is biodegradable.
• It is from renewable resources.
• The gaseous pollutant is lesser as compared to the vegetable oils.
• Bio-diesel can be produced from different types of vegetable oils.
• Best engine performance and less smoke emission are achieved.
• Disadvantages.
1. Bio-diesel gels in cold weather.
2. As Bio-diesel is hygroscopic, Bio-Diesel can absorb water from the
atmosphere.
3. Bio-diesel decreases the horsepower of the engine.
4. Bio-diesel degrades and softens the rubber and plastics that are used
in some old cars.
5. Bio-diesel has about 10% higher nitrogen-oxide (NOx) emissions than
conventional petroleum.
 COMBUSTION OF FUELS
INTRODUCTION
• Combustion is a process of rapid exothermic oxidation in which a fuel burns
in the presence of oxygen with the evolution of heat and light.
• Aim of combustion is to get the maximum amount of heat from a
combustible substance in the shortest time. Most of the combustible
substances are enriched with carbon and hydrogen. During combustion they
undergo thermal decomposition to give simpler products, which are oxidised
to CO2, H2O, etc.
• Since the above reactions are exothermic, large quantity of heat is
given out.
 CALORIFIC VALUE
• The efficiency of a fuel can be understood by its calorific value. The
calorific value of fuel is defined as the total amount of heat liberated,
when a unit mass of fuel is burnt completely.
UNITS OF CALORIFIC VALUES
• The quantity of heat can be measured by the following units:
• (i) Calorie.
• (ii) Kilocalorie.
• (iii) British Thermal Unit (B.T.U).
• (iv) Centigrade Heat Unit (C.H.U).
• Calorie: It is defined as the amount of heat required to raise the
temperature of 1 gram of water through 1°C (15 to 16°C).
HIGHER AND LOWER CALORIFIC VALUES
( Dulong’s Formula)
• Higher (or) Gross calorific value (GCV)
• It is defined as the total amount of heat produced, when a unit
quantity of the fuel is completely burnt and the products of
combustion are cooled to room temperature. GCV (or) HCV
• Lower (or) Net Calorific Value (NCV)
• It is defined as the net heat produced, when a unit quantity of the
fuel is completely burnt and the products of combustion are allowed
to escape.
 PROBLEMS BASED ON CALORIFIC VALUE :
• Calculate the gross and net calorific value of coal having the following
composition carbon - 85%, hydrogen – 8%, sulphur – 1%, nitrogen –
2%, ash – 4%, latent heat of steam – 587 cals/g
Solution:
 FLUE GAS ANALYSIS (ORSAT’S METHOD)
• The mixture of gases (like CO2, O2, CO, etc.) coming out from the
combustion chamber is called flue gases.
• The analysis of flue gas would give an idea about the complete or
incomplete combustion process.
• The analysis of flue gases is carried out by using Orsat’s apparatus.
 DESCRIPTION
• It consists of a horizontal tube. At one end of this tube, U-tube
containing fused CaCl2 is connected through a 3-way stop cock.
• The other end of this tube is connected with a graduated burette. The
burette is surrounded by a water jacket to keep the temperature of
gas constant.
• The lower end of the burette is connected to a water reservoir by
means of a rubber tube. The level of water in the burette can be
raised or lowered by raising or lowering the reservoir.
• The horizontal tube is also connected with three different absorption
bulbs I, II, and III for absorbing CO2, O2, and CO.
• I-Bulb: It contains potassium hydroxide solution, and it absorbs only
CO2.
• II-Bulb: It contains alkaline pyrogallol solution, and it absorbs CO2 and
O2.
• III-Bulb: It contains ammoniacal cuprous chloride solution and it
absorbs CO2, O2, and CO.
• Precautions
1. Care must be taken in such a way that, the reagents in the
absorption bulb 1, 2, and 3 should be brought to the etched marked
level one by one by raising and lowering reservoir bottle.
2. All the air from the reservoir bottle is expelled to atmosphere by
lifting the reservoir bottle.
3. It is essential that CO2, O2 and CO are absorbed in that order only.
4. As the CO content in flue gas is very small, it should be measured
quite carefully.
• Working:
• The 3-way stop-cock is opened to the atmosphere and the reservoir is
raised, till the burette is completely filled with water and air is
excluded from the burette.
• The 3-way stop-cock is now connected to the flue gas supply and the
flue gas is sucked into the burette and the volume of flue gas is
adjusted to 100 cc by raising and lowering the reservoir. Then the 3-
way stop cock is closed.
(a) Absorption of CO2: The stopper of the absorption bulb-I,
containing KOH solution, is opened and all the gas is passed into the
bulb-I by raising the level of water in the burette. The gas enters into
the bulb-I, where CO2 present in the flue gas is absorbed by KOH.
• The gas is again sent to the burette. This process is repeated several
times to ensure the complete absorption of CO2. The decrease in
volume of the flue gas in the burette indicates the volume of CO2 in
100 cc of the flue gas.
(b) Absorption of O2:
Stop-cock of bulb-I is closed and stop cock of bulb-II is opened.
The gas is again sent into the absorption bulb-II, where O2 present in
the flue gas is absorbed by alkaline pyrogallol.
The decrease in volume of the flue gas in the burette indicates the
volume of O2.
• (c) Absorption of CO:
• Now stop-cock of bulb-II is closed and stop-cock of bulb-III is opened.
• The remaining gas is sent into the absorption bulb-III, where CO
present in the flue gas is absorbed by ammoniacal cuprous chloride.
• The decrease in volume of the flue gas in the burette indicates the
volume of CO.
• The remaining gas in the burette after the absorption of CO2, O2 &
CO is taken as nitrogen.
 Significance (or) uses of flue gas analysis
1. Flue gas analysis gives an idea about the complete or incomplete
combustion process.
2. If the flue gases contain a considerable amount of CO, it indicates
that incomplete combustion is occurring and it also indicates the short
supply of O2.
3. If the flue gases contain a considerable amount of O2, it indicates
that complete combustion is occurring and also it indicates that the
excess of O2 is supplied.
 IGNITION TEMPERATURE
• The lowest temperature to which the fuel must be heated for its
smooth burning is known as ignition temperature. In the case of
liquid fuels, ignition temperature is known as flashpoint.
• Examples
 Spontaneous Ignition Temperature (SIT)
• The minimum temperature at which the fuel catches fire
spontaneously without any external heating is known as Spontaneous
Ignition Temperature (SIT).
• Pulverized coal, oil rags, cotton wastes get oxidized slowly. If the heat
evolved is unable to escape, the temperature in the system goes on
increasing and when SIT is reached, the system catches fire on its
own.
• Significance of SIT:
• A low SIT value indicates the ready ignition of the fuel and also means
a fire hazard
EXPLOSIVE RANGE (LIMITS OF INFLAMMABILITY)
• For fuel to burn, it should be mixed with air in proper ratio.

• The fuel should be present in the fuel–air mixture in a particular

range.

• The gaseous fuels have two extreme limits in the mixture – a) Upper
limit b) Lower limit

• The range covered by these two extreme (Upper and lower) limits are
known as explosive range or limits of inflammability.
• Significance:
• For continuous burning, the amount of fuel present in fuel – air
mixture should not go below the lower limit or above the upper limit.
• For example, the explosive range of petrol is 2 – 4.5.
• This means, when the concentration of petrol vapor in the petrol-air
mixture is between 2 – 4.5 by volume, the mixture will burn on
ignition.
• When it is below 2% or above 4.5% by volume, the burning will not be
proper.
EXPLOSIVE RANGE (LIMITS OF INFLAMMABILITY)