Fuels and Energy Sources – NDA Chemistry Notes

Every machine needs fuel. The chulha in your kitchen, the bus you travel in, the power station that lights your home, all need fuel to work. Understanding fuels and energy sources is important both for daily life and for competitive exams.

This chapter covers what fuels are, how they are classified, the properties of major fuels, and the basics of energy sources both conventional and renewable.

1. What is a Fuel?

A fuel is a substance that releases energy in the form of heat and light when it burns in air (oxygen). The burning of a fuel is called combustion.

A good fuel has the following characteristics. It has a high calorific value. It is easy to store and transport. It produces less harmful gases when burned. It burns at a moderate and controllable rate. It is cheap and easily available.

2. Calorific Value

Calorific value is the amount of heat energy produced when one gram of a fuel is completely burned.

The unit of calorific value is kilojoules per gram (kJ/g) or kilocalories per kilogram (kcal/kg). A higher calorific value means the fuel gives more heat per gram. It is a better fuel in that sense.

Fuel	Approximate Calorific Value (kJ/g)
Hydrogen gas	~143 (highest of all common fuels)
Natural gas (methane)	~55
LPG	~50
Petrol (gasoline)	~47
Diesel	~45
Coal (anthracite)	~35
Coal (bituminous)	~30
Wood	~17
Biogas	~35

Hydrogen has the highest calorific value among all common fuels.

3. Classification of Fuels

Fuels are classified in two ways.

By physical state:

State	Examples
Solid fuels	Wood, coal, charcoal, coke, dung cakes
Liquid fuels	Petrol, diesel, kerosene, alcohol
Gaseous fuels	LPG, CNG, natural gas, biogas, hydrogen

By origin:

Type	Description	Examples
Fossil fuels	Formed from remains of ancient organisms over millions of years	Coal, petroleum, natural gas
Biofuels	Made from recently living plant or animal material	Biogas, ethanol from sugarcane, biodiesel
Nuclear fuels	Energy released from nuclear reactions	Uranium, thorium, plutonium

4. Coal

Coal is a solid fossil fuel. It is formed over millions of years from the remains of trees and plants that were buried under the earth and subjected to heat and pressure. Coal is one of the most important sources of energy for electricity generation.

Grades of Coal: Coal comes in different grades depending on the percentage of carbon and the amount of heat it produces.

Grade	Carbon Content	Heat Value	Notes
Peat	Lowest (~50-60%)	Lowest	Youngest form, partly decomposed wood
Lignite	~60-70%	Low	Also called brown coal
Bituminous	~70-80%	Moderate-High	Most common type, used in power plants
Anthracite	Highest (~90-95%)	Highest among coals	Hardest, shiniest, best quality coal

The rank of coal increases in this order: Peat → Lignite → Bituminous → Anthracite.

Anthracite is the highest quality coal. It has the highest carbon content and highest calorific value among coal types.

Graphite is NOT a grade of coal. Graphite is a pure crystalline allotrope of carbon. The statement “graphite is the highest grade of coal” is FALSE. [NDA 2010-II]

5. Petroleum

Petroleum means “rock oil” in Latin. It is a dark, thick liquid found underground. It is also called crude oil.

Petroleum is a mixture of many hydrocarbons. It was formed millions of years ago from the remains of tiny marine organisms (plankton and algae) that were buried under the sea floor and subjected to heat and pressure.

Petroleum is refined by fractional distillation at a petroleum refinery. Different fractions come out at different temperatures.

Fraction	Boiling Range	Use
Petroleum gas (LPG)	Below 30°C	Cooking fuel
Gasoline (Petrol)	30-200°C	Motor vehicles, lowest boiling point liquid fraction
Naphtha	70-120°C	Making chemicals
Kerosene	150-300°C	Lamps, jet fuel, cooking
Diesel	250-350°C	Trucks, buses, trains
Lubricating oil	300-400°C	Machine lubrication
Paraffin wax	Solid	Candles, polish
Bitumen (Tar)	Residue	Road surfacing

Gasoline has the lowest boiling point among the liquid fractions of petroleum.

Cracking is the process of breaking large hydrocarbon molecules from heavy fractions into smaller, more useful molecules (like petrol) by heating at high temperature with a catalyst. This increases the yield of petrol from crude oil.

6. Petrol and Diesel Engines

Petrol Engines: Petrol (gasoline) is used in cars, motorcycles, and small engines. It burns at a lower temperature than diesel.

Knocking (or engine knock) is an irregular combustion that happens in petrol engines. It causes a knocking or pinging sound. It damages the engine and reduces efficiency.

The octane number (octane rating) measures the anti-knocking quality of petrol. A higher octane number means the petrol is more resistant to knocking. Good quality petrol has a higher octane number. Isooctane (2,2,4-trimethylpentane) is assigned octane number 100 (best). Normal heptane is assigned octane number 0 (worst).

Tetraethyl Lead (TEL): Tetraethyl lead (Pb(C₂H₅)₄) was previously added to petrol to increase its octane number and prevent knocking. It is called an anti-knock agent. However, TEL releases toxic lead compounds in exhaust gases. Lead causes serious neurological damage, especially in children. So TEL has been banned in most countries.

Ethylene Dibromide and Petrol: When TEL was used in petrol, ethylene dibromide was also added. Ethylene dibromide reacted with the lead oxide formed during combustion to convert it into volatile lead bromide. This lead bromide was expelled through the exhaust. This prevented lead oxide deposits from building up inside the engine.

Diesel Engines: Diesel is used in trucks, buses, trains, and generators. Diesel engines burn fuel at a higher temperature than petrol engines.

Diesel burns at a high enough temperature for nitrogen and oxygen from air to combine and form nitric oxide (NO). [NDA 2010-II] This is why diesel engines produce more NOₓ (nitrogen oxide) pollutants than petrol engines.

7. LPG (Liquefied Petroleum Gas)

LPG is used for cooking in millions of homes across India.

Composition: LPG is mainly butane (C₄H₁₀) and propane (C₃H₈).

Why is it liquid in the cylinder? LPG components have fairly high critical temperatures. They can be liquefied at room temperature by applying moderate pressure. So LPG is stored as liquid under pressure in cylinders.

Foul-smelling agent in LPG: Pure LPG is colourless and odourless. If there is a gas leak, you would not be able to smell it. This is dangerous. So a foul-smelling compound called thioethanol (ethanethiol, C₂H₅SH) is added to LPG. It has a very strong, unpleasant smell. If LPG leaks, you can immediately detect it by smell.

8. CNG (Compressed Natural Gas)

CNG is used in buses, autos, and cars in cities.

Composition: CNG is mainly methane (CH₄).

CNG cannot be liquefied at room temperature because methane has a very low critical temperature (about -82°C). Below this temperature, methane cannot be liquefied no matter how much pressure is applied. So CNG is stored as a highly compressed gas (not liquid) in heavy steel cylinders.

CNG is a cleaner fuel than petrol and diesel. It produces fewer pollutants.

9. Natural Gas

Natural gas is found underground along with petroleum deposits. It is mainly methane (CH₄) with small amounts of ethane, propane, and butane.

Natural gas is used for cooking, heating, and electricity generation. It is transported through pipelines.

10. Biogas

Biogas is produced by the anaerobic (without air) decomposition of organic waste (cow dung, agricultural waste, sewage) by bacteria.

Composition: Biogas is mainly methane (CH₄), about 55-65%. The rest is mainly carbon dioxide (CO₂).

Gobar gas is biogas made specifically from cow dung (gobar = cow dung in Hindi).

Biogas is a renewable and clean fuel. It is used in rural areas for cooking and lighting.

11. Hydrogen as a Fuel

Hydrogen gas has the highest calorific value of any common fuel. It produces only water when burned. It causes zero pollution.

H₂ + ½O₂ → H₂O (only product is water)

The challenge is storage. Hydrogen gas is very light and needs large tanks or must be stored at very high pressure or very low temperature.

Hydrogen is used as a rocket fuel. Liquid hydrogen is used in space rockets combined with liquid oxygen.

12. Fuel Cells

A fuel cell is a device that converts chemical energy directly into electrical energy through an electrochemical reaction, without burning the fuel. [NDA 2012-II]

A fuel cell is NOT rechargeable. It is a primary cell in the sense that it works as long as fuel is supplied. [NDA 2012-I]

The most common type is the hydrogen-oxygen fuel cell. In a hydrogen-oxygen fuel cell, hydrogen is fed at the anode, oxygen is fed at the cathode, and the reaction produces electricity and water.

Fuel cells are highly efficient and produce no harmful emissions. They are used in spacecraft, electric vehicles, and some industrial applications.

A fuel cell works like a galvanic cell but continuously. It does not need recharging. It needs fuel (hydrogen) to be continuously supplied.

13. Helium-3 and Lunar Energy

Helium-3 (He-3) is a rare isotope of helium. It is present on the Moon’s surface in significant quantities.

Helium-3 is valuable as a potential fuel for nuclear fusion reactors. If fusion reactors using He-3 can be built, they would provide clean, virtually unlimited energy. This is why space agencies are interested in mining the Moon.

14. Renewable vs Non-Renewable Energy Sources

Non-Renewable Energy Sources: These sources will eventually run out because they take millions of years to form. Once used, they are gone. Examples: Coal, petroleum, natural gas (fossil fuels), nuclear fuels (uranium, thorium).

Renewable Energy Sources: These sources are continuously replenished by nature. They will not run out.

Renewable Source	How Energy is Obtained
Solar energy	Sunlight converted to electricity (solar cells/photovoltaic cells) or heat
Wind energy	Wind turbines convert wind motion to electricity
Hydropower	Flowing water spins turbines to generate electricity
Tidal energy	Tides drive turbines
Geothermal energy	Heat from inside the Earth
Biomass energy	Burning wood, agricultural waste, biogas

15. Ignition Temperature

The ignition temperature (or kindling temperature) is the minimum temperature at which a substance catches fire and continues to burn on its own.

If the temperature of a fuel is below its ignition temperature, it will not catch fire even if a spark is applied and then removed.

Different fuels have different ignition temperatures. Petrol has a lower ignition temperature than diesel. This is why petrol engines use a spark plug to ignite the fuel, while diesel engines use compression (which heats the air enough to ignite diesel).

White phosphorus has a very low ignition temperature (about 30-35°C). It catches fire spontaneously in air at room temperature. [Covered in Chapter 14]