Fuels

Engineering Chemistry (revised edition)
ISBN: 978-81-265-4475-2
Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Introduction
A fuel is defined as any substance used to produce heat or power by combustion.
Any chemical process accompanied by the evolution of light and heat is called
combustion. It is simply the reaction of substances with oxygen and converts
chemical energy into heat and light.
ISBN: 978-81-265-4475-2

Classifications of
Fuels
Based on Physical
State
Solid fuel
(e.g., wood, coal)
Liquid fuel (e.g., crude
petroleum, natural
gasoline)
Gaseous fuel
(e.g., natural gas)
Based on
occurrence
Primary or natural
fuels (e.g., wood, coal)
Secondary or prepared
fuels
(e.g., charcoal, petrole
um coke).
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Characteristics of a Good Fuel
1. It should ignite easily. The temperature of the fuel at which ignition starts
and continues to burn without further addition of heat is called ignition
temperature. It should be moderate for a good fuel. Very low ignition
temperature leads to fi re hazard and very high ignition temperature
disfavors the starting of fi re.
2. It should give out a lot of heat, that is, its specific heat should be high.
3. It should have low smoke and combustible matter such as ash. It should not
give out harmful combustion products. This property depends on the nature
of elements present in the fuel.
4. It should be inexpensive and readily available.
5. It should be easy to store and transport.
6. It should have low ash content. Ash reduces the calorific value of the fuel,
causes hindrance to the flow of air and heat, reduces the specific heat and
leads to unwanted disposable problems.
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Calorific Value
Units
1. Mean British Thermal Unit (BTU)
2. Calorie (cal)
Gross and Net Calorific Values
1. Higher calorific value (HCV) or gross calorific value
2. Lower calorific value (LCV) or net calorific value
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Determination of Calorific Value
Theoretically Determination
Dulong’s formula for calculating the calorific value from the chemical
composition of the fuel may be written as follows:
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Experimentally Determination
Bomb calorimeter
1. For calorific values of solid and
liquid fuels
2. Known amount of fuel is burnt at
constant volume
3. Temperature of surrounding water
increases as heat is produced.
4. Quantity of heat and calorific values
are calculated.
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Boy’s calorimeter
1. Gas or volatile liquid burns at constant rate.
2. Water flowing at constant rate absorbs the heat produced.
3. Calorific value is calculated from volume of water, increase in temperature
and volume of gas/liquid burnt.
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Junker’s calorimeter
1. Control of rate of burning of gaseous/liquid fuel and water circulation is
maintained.
2. The combustible products are released at nearly the atmospheric pressure.
3. Calorific value is calculated from amount of water passed, volume of gas
burnt, the steady rise in temperature and mass of the condensed water
flowing out.
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Combustion
Combustion is a chemical process accompanied by evolution of light and heat.
Concepts
Ignition
temperature
Explosive
range
Flame
Surface
combustion
Primary
combustion
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Calculations
Calculation of theoretical air for combustion of a fuel requires the following
points:
1. Percentage of oxygen in air by volume is 21% and 23.2% by weight.
2. Stoichiometric equations involved in combustion
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Flue Gas Analysis
It comprises the gaseous products of combustion of fuel. Its analysis helps in
finding out the correct quantity of air to be supplied in a furnace.
Orsat’s apparatus
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Solid Fuels – Coal
Formed from dead plants buried for several million years.
Types of
Coal
Peat The lowest carbon content
Lignite
Brown variety containing 25-
30% carbon and 60% moisture
content.
Sub-bituminous
coal
Black variety, 35-45% carbon
Bituminous coal
Hard black variety, 45-86%
carbon.
Anthracite coal
The highest ranking
coal, carbon content 86-97%.
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Uses of Coal
1. As a primary fuel: Coal is used to produce steam through heat and
combustion, which is again used for running turbines to generate electricity
in power plants.
2. As a secondary fuel: The product of burning coal in the absence of air is
of metallurgical importance. The byproducts are useful in making plastics,
tar and synthetic fibers and also used making steel in industries.
Pulverized Coal
Pulverized coal generally refers to coal in powdered form obtained by crushing,
grinding or pulverizing coal. As the surface area of pulverized coal is large, the
volatile matter present in it comes quickly in contact with air and is released,
enabling the combustion of fixed carbon. This increases the calorific value of the
coal and enhances its quality.
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The advantages of pulverized coal are as follows:
1. It provides easy transportation and storage.
2. Even high grade coal having high ash content, such as found in India, could
be used satisfactorily.
3. It can easily undergo combustion in small percentage of excess air with less
wastage of heat, and has high thermal efficiency.
4. It does not form clinkers (incombustible residue) and provides an oxidizing
and reducing atmosphere inside the furnace for metallurgical purposes.
The disadvantages of pulverized coal are as follows :
1. It involves the extra cost of pulverizing and sieving.
2. Special types of burners are required for good mixing of air and fuel.
3. During the process of grinding, crushing and pulverizing, finely divided ash
is generated in form of fly-ash causing air pollution and causing problems
associated with its disposal.
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Proximate and Ultimate Analyses of Coal
It includes the determination of moisture content, volatile matter, ash and fixed
carbon. In this analysis, the data varies with the procedure adopted for study. The
content of moisture, volatile matter and ash are experimentally determined, while
that of fixed carbon is calculated.
Proximate Analysis
1. Moisture content: Lesser the moisture content, better is the quality of coal.
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2. Volatile matter: Lesser the volatile matter, better is the rank of coal.
3. Ash content: Lower the ash content, better is the quality of coal.
4. Fixed carbon: Higher fixed carbon content, better is the quality of coal.
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Ultimate Analysis
1. Carbon and hydrogen: Greater the percentage of carbon and hydrogen,
better is the quality of coal.
2. Nitrogen: Presence of nitrogen is undesirable.
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3. Sulphur: Presence of sulphur is highly undesirable.
4. Ash: Determined the same way as in proximate analysis.
5. Oxygen: Lower the percentage of oxygen, better is the coal.
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Significance of Ultimate Analysis
1. Greater the percentage of carbon and hydrogen, better is the coal in quality
and calorific value. However, hydrogen is mostly associated with the volatile
matter and hence, it affects the use to which the coal is put to.
2. Nitrogen has no calorific value and hence its presence in coal is undesirable.
Thus, a good quality coal has very little nitrogen content.
3. Sulphur is usually present to the extent of 0.5–3.0% and is derived from ores
like iron pyrite and gypsum, etc., mines along with the coal. Sulphur,
although contributes to the heating value of coal, on combustion it produces
acids which have harmful effects of corroding the equipment and also
causes atmospheric pollution.
4. Oxygen content decreases the calorific value of coal. High oxygen content
coals are characterized by high inherent moisture content, low calorific value
and low coking power. Moreover, oxygen is in combined form with
hydrogen in coal and thus, hydrogen available for combustion is lesser than
actually present.
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Coke
Coke is obtained when coal is heated strongly out of contact with the air, the
process is called carbonization or coking.
1. Low-temperature carbonization: 500 C-700 C; low temperature coke.
2. High-temperature carbonization: 900 C-1100 C; metallurgical coke.
Comparison of high and low temperature carbonization processes
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Percentage analysis of a coke and the coal
Coke as Metallurgical Fuel
Metallurgical coke is preferred over coal due to the following reasons:
1. It is hard, porous and of good mechanical strength.
2. It does not contain much sulphur content.
3. It burns with a short flame.
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Caking and Coking Coals
Coals that produce soft and plastic mass at around 400 C that re-solidifies to
form a porous solid are called caking coals. Caking coals that produce solid
product (coke) of useful grade are called coking coals. A good coking coal will
produce a bright gray, strongly coherent, porous coke. All coking coals are caking
coals but not vice versa.
High volatile coals are mixed with poor or non-coking low-volatile coals, to yield
a denser, stronger coke. Where mixing is practiced, the mixture used depends on
the coal used. Usually high- and low volatile coals are mixed to give a mixture
having about 30% of volatile matter.
ISBN: 978-81-265-4475-2

Coking Processes
Bee-Hive Oven
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Otto-Hoffmann or Byproduct Oven
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There are two methods of cooling the coke: dry and wet quenching:
1. Dry quenching: The insulated wall of the oven is removed, so that the inert
gases like nitrogen present in the atmosphere are able to come in contact
with the coke and cool it naturally over a period of time.
2. Wet quenching: Water is sprayed over the red hot coke for cooling it down.
As a result of this, some gases escape from the hot surface of the coke in
form of steam and cause pollution. This method also generates a large
amount of coke dust.
Dry quenching is preferred over wet quenching due to the following reasons:
1. The method is dust free and dry without causing pollution to the
environment.
2. The heated gases could be used to recover heat by circulating them through
the boilers for steam generation.
3. The coke obtained is strong, dense and free from moisture.
ISBN: 978-81-265-4475-2

Various byproducts of dry
quenching are
Coal tar
Ammonia
Naphthalene
Light oil or crude
benzol
H2S
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Advantages of Byproduct oven.
1. The byproduct coke oven, the valuable byproducts are saved; whereas in the
beehive oven, they are allowed to escape into the atmosphere.
2. The byproducts are well worth saving and have many important uses. Their
recovery helps to make the process economical.
3. In the purification of the byproduct coke-oven gas by means of a liquid-
contact process, pure sulphur can be obtained in a form much finer than the
commercial precipitated sulphur.
4. This finely divided sulphur has been found to process superior qualities as a
fungicide.
5. Among the other interesting and valuable byproducts recovered by the
liquid-purification processes are the thiocyanates of ammonium, sodium,
and calcium.
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Biofuels
Biofuels are solid, liquid or gaseous
fuels that are derived from living
organisms and their waste matter.
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Biomass
Waste material produced by living things There are methods of utilization of
biomass in the form of energy.
1. Burn the biomass directly and obtain energy.
2. Convert the matter into ethanol and methanol or it is fermented
anaerobically to obtain gaseous fuel, biogas.
Biodiesel
Produced from transesterification of vegetable oils that contain triglycerides.
Biogas
The advantages of using gobar gas over heating cattle dung directly in dried state
are as follows:
1. The gobar gas produces more useful energy.
2. It provides high device efficiency.
3. It is free from dust and smoke, and environment-friendly.
4. It is used as a domestic fuel as well as an illuminant.
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Anaerobic digestion tank
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Semi-Solid Fuels – Some Recent Advances
The term semi-solid fuel in modern context refers to non-volatile substances that
are environmentally safe and produce no hazardous waste on burning. These
have long shelf-life and their ignition can be easily started and stopped. The
conventional solid and liquid fuels are now being used as formulations in semi-
solid form to circumvent the problems associated with their use in the regular
form.
The use of coal is associated with many environmental problems, starting from
its excavation from mines to gaseous and fly ash emissions. To overcome these
problems, developing countries are transforming coal into gaseous or liquid fuel
formulations or converting it into low ash and low sulphur varieties. For example,
a solvent-refined, semi-solid form of coal has been prepared by suspending
pulverized coal in a solvent and treating it with hydrogen gas at high temperature
and pressure. The product compares well with high grade anthracite in
combustion properties, is free from ash and has high calorific value of 16000
BTU per pound.
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Liquid fuels – Petroleum
1. Petroleum is made from the remains of plants and animals buried millions
of years ago.
2. It is a non-renewable resource.
3. It contains straight or cycloparaffins.
4. Olefins
5. Aromatics
6. Other organic compounds containing N, O, S.
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Composition
Petroleum is a dark, greenish brown, viscous liquid that is found underground. It
comprises hydrocarbons such as:
1. Straight paraffins or cycloparaffins such as methane, ethane, propane,
butane, isobutane, pentane, hexane.
2. Olefins such as ethylene, butene, isobutene and acetylene, butadienes.
3. Aromatics such as benzene, naphthalene, cyclohexane, methyl cyclopentane.
4. Some organic compounds containing nitrogen, oxygen and sulphur.
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Production from Refining of Crude Oil
The petroleum obtained by mining is viscous and dark colored liquid. Due to the
presence of sulphur, it has an unpleasant smell. It also contains impurities of
sand, brine or sea water. Hence it is called crude oil.
The important steps involved are:
1. Fractional distillation to give various fractions.
2. Conversion of less desirable fractions to valuable products by processes like
cracking.
3. Treatment of fractions to remove undesirable substances.
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Fractional Distillation
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Fractions of petroleum
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Cracking
Cracking is decomposition of high molecular weight compounds (with high
boiling points) to low molecular weigh compounds with low boiling points).
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Methods of
Cracking
Thermal Cracking
Liquid phase
thermal cracking
Vapor phase
thermal cracking
Catalytic Cracking
Fixed bed cracking
Fluidized (moving)
bed catalytic
cracking (FCC)
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Fixed bed catalytic cracking
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Fluidized-bed catalytic cracking
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The fluidized bed cracking has the following advantages over fixed-bed cracking:
1. Better contact with the feed and the catalyst, enabling uniform temperature
and efficient heat transfer.
2. The catalyst can be regenerated and used again for the cracking process.
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Catalytic Reforming
Reforming is a process of converting low octane naphthas into high octane
gasoline.
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Compounds
Isolated from
Crude Oil
Gasoline (petrol)
Diesel
Aviation gasoline
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Knocking
Knocking in Spark Ignition Engines and Octane Number
Petrol is used in spark ignition engines. The rapid compression of the fuel-air
mixture heats the engine, and it detonates without the spark being passed. This
causes a violent jerk to the piston giving a metallic sound called knocking.
Octane number is the percentage volume of isooctane in the isooctane-heptane
mixture that matches the knocking characteristics of the fuel being tested is
called the octane number.
Molecular structure affects the octane number.
ISBN: 978-81-265-4475-2

Knocking in Compression Ignition Engines and Cetane Number
Diesel is used in compression ignition engines.
Cetane number represents the spontaneous ignition temperature of a particular
diesel fuel. It is the percentage of cetane present in a mixture of cetane and
alpha-methylnaphthalene which matches the fuel under test in ignition property.
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Power Alcohol and Synthetic Petrol
Power Alcohol
Ethyl alcohol is used as additive to motor fuels. When blended with petrol at
concentrations of 5–10%, it is called power alcohol. The addition of alcohol to
petrol increases its octane number.
Manufacture of Ethanol
1. By fermentation
2. Using Molasses as raw material
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The advantages of power alcohol are as follows:
1. Ethyl alcohol is a good antiknocking agent and power alcohol has octane
number 90, while that of petrol is 65.
2. Any moisture content present is absorbed by alcohol.
3. Ethyl alcohol contains oxygen atoms, which help for complete combustion
of power alcohol and the polluting emissions of CO, hydrocarbon,
particulates are largely reduced.
4. Power alcohol is cheaper than petrol.
ISBN: 978-81-265-4475-2

The disadvantages of power alcohol are as follows:
1. Ethyl alcohol has calorific value 7000 cal/g much lower than that of petrol
which has 11500 cal/g. However, this problem could be overcome by using a
specially designed engine with higher compression ratio.
2. The output of the power generated is reduced up to 35%.
3. It has high surface tension and its atomization is difficult, especially at lower
temperature, thereby causing starting trouble.
4. It may undergo oxidation reaction to form acetic acid, which corrodes
engine parts.
5. As it contains oxygen atoms, the amount of air require for complete
combustion is less, therefore, the carburetor and engine need to be modified.
ISBN: 978-81-265-4475-2

Synthetic Petrol
Synthetic petrol is a mixture of alkanes with composition resembling that of
petrol, obtained artificially from coal.
Bergius Process
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Fischer–Tropsch Process
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Gaseous Fuels
The ease and flexibility of application of gaseous fuels give them advantages
over solid or liquid fuels
AdvantagesofGaseousFuels
Combustion may be readily controlled
Greater thermal efficiency
Smoke and ash are eliminated
Economical utilization of low grade
solid-fuels after gasification
Use in internal combustion engines for
the direct production of power
ISBN: 978-81-265-4475-2

Liquefied Petroleum Gas
Liquefied petroleum gas (LPG) is obtained as a byproduct during cracking of
heavy oil or from natural gas. It contains hydrocarbons that are highly volatile but
are easily liquefiable under high pressures. The components that form LPG are n-
butane, isobutane, butylenes and propane with traces of propylene and ethane. It
is generally used as a domestic fuel for cooking and in industry. Though it is
cheaper fuel and knock resistant, yet, it has limited uses as a motor fuel owing to
its following disadvantages over gasoline:
1. Has a faint odor and its leakage almost goes unnoticed.
2. Pressure needs to be monitored for effective handling.
3. Has low octane number and high load sensitivity.
4. Has high compression ratio.
5. Less responsive towards blending unlike power alcohol.
ISBN: 978-81-265-4475-2

Natural Gas
The origin of natural gas is closely associated with that of petroleum; it is always
found in or near the petroleum fields. Because of this fact, it is not available over
large parts of the country, although pipelines are being constantly extended to
furnish gas to large-consuming centers, where it may be used either alone or
mixed with artificial gas.
Natural gas is a cheap and convenient fuel consisting of methane and other
saturated hydrocarbons. Its calorific value is 12000–14000 kcal/m2. The
composition of a natural gas is as follows:
Methane = 85%, Ethane = 8%, Propane = 4%, Butane = 1.5% and
Hydrocarbons = 1.5%
About 14% of the gas produced is used for making carbon black. It is also used
as a fuel for industrial and is domestic constituent.
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Compressed Natural Gas
This gaseous fuel is compressed natural gas (CNG) which mainly contains
methane. It is obtained by applying high pressure, generally, 1000 atm to reduce
the original volume at standard atmospheric pressure to less than 1%. A steel
cylinder can fill 15 kg of this fuel gas with a volume of 20 m3 at a pressure of 1
atm. CNG is used as automobile fuel, especially, in cars and its use is increasing
progressively. Its advantages are as follows:
1. It is a safer fuel since its ignition temperature is higher than conventional IC
engine liquid fuels.
2. Combustion of CNG produces lesser number of pollutants.
However, its disadvantages are as follows:
1. It requires a lot of space for storage and hence is not suitable for smaller
vehicles.
2. It has high ignition temperature 550 C and requires more air for ignition.
ISBN: 978-81-265-4475-2

Coal Gas
Made up of hydrogen, methane and carbon monoxide.
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Producer gas
Formed from the incomplete combustion of a solid fuel (coal or coke) in a
partly closed generator.
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Water Gas
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Rocket Propellants
Rocket propellants: A chemical mixture (fuel + oxidizer) burned to produce
thrust in rockets.CharacteristicofRocketPropellant
It should ignite readily at low temperature and catch
fi re in the presence of an oxidant in a short time.
It should burn slowly and steadily rise to a high
temperature.
It should be safe to handle and store. It should not
leave any solid residue on burning.
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The chemical propellants are classified into solid and liquid propellants.
1. Solid propellants: These consist of a fuel, usually a hydrocarbon and an
oxidizer which contains a large percentage of oxygen. These substances are
mixed so as to produce a solid of desired chemical or physical
characteristics. The finished product is called a grain or stick. One or more
grains constitute a charge.
2. Liquid propellants: These possess many advantages over solid propellants.
They are more versatile and the engine using them can be checked and
calibrated more easily. However, unlike solid propellants, the engine using
liquid propellant is quite delicate and cannot withstand any rough handling.
Liquid propellants may be monopropellants or bipropellants.
ISBN: 978-81-265-4475-2

Explosives
A material which is capable of producing an explosion by its own energy. It can
be a single compound or a mixture of compounds.
1. Chemical explosives: Compounds containing –NO2, -ONO and –NHNO2
on detonation release gases.
Examples: nitroglycerin, trinitrotoluene (TNT), octogen (HMX),
pentaerythritol (PETN) and nitrocellulose.
2. Mechanical explosives: Materials whose explosion is based on physical
methods.
Examples: nuclear explosives.
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Classification of
Chemical Explosives
On the Basis of
Sensitivity
Primary
explosives
Secondary
explosives
Tertiary
explosives
On the Basis of
Velocity
Low explosives
High explosives
On the Basis of
Physical Form
pressings, castings, polymer
bonded, rubberized, extrudable, binary,
blasting agents, slurries and gels.
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Preparation of Explosives
1. Lead azide
2. Nitrocellulose
3. PETN
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4. TNT (2,4,6-trinitrotoluene):
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5. Picric acid
6. Nitroglycerine
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