We burn fuels every day. We use them to cook food, run vehicles and make electricity. Exams ask a fixed set of questions on this topic. They ask about calorific value, the zones of a flame, LPG and CNG, and fire extinguishers. This post explains fuels and combustion in simple words. It ends with 15 previous year questions.
Key Points (Quick Revision)
- Combustion is a chemical process. A substance reacts with oxygen and gives out heat.
- Ignition temperature is the lowest temperature at which a substance catches fire.
- Hydrogen has the highest calorific value, about 150,000 kJ/kg.
- LPG is mainly butane and propane. CNG and biogas are mainly methane.
- The outermost zone of a flame is the hottest. Carbon dioxide is used on electrical and oil fires.
1. What combustion is
Combustion is a chemical process. In it, a substance reacts with oxygen. It gives out heat, and often light too. The substance that burns is called a combustible substance. Wood, paper, kerosene and LPG are combustible. Stone, glass and iron nails are not.
Three things are needed for combustion:
- A fuel - something that can burn.
- Oxygen - usually from the air.
- Heat - enough to raise the fuel to its ignition temperature.
Together, these three are called the fire triangle. Remove any one, and the fire stops. This idea is the base of all fire control.
The ignition temperature is the lowest temperature at which a substance catches fire. Different substances have different ignition temperatures. Substances with a very low ignition temperature catch fire easily. They are called inflammable substances. Petrol, alcohol and LPG are examples.
A safety match uses this idea. The match head has antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorus. Striking the match produces enough heat to start the fire.
2. Types of combustion
| Type | What happens | Example |
|---|---|---|
| Rapid combustion | The substance burns quickly and gives heat and light | LPG burning on a gas stove |
| Spontaneous combustion | The substance catches fire on its own, with no outside flame | White phosphorus burning in air at room temperature |
| Explosion | Sudden burning with a lot of heat, light and sound, and gas is released | Firecrackers |
Spontaneous combustion also causes some real disasters. Coal dust in mines can catch fire this way. Some forest fires also start on their own, in very hot weather.
3. The flame and its three zones
Not every fuel burns with a flame. Wax, kerosene and LPG form a flame. Charcoal only glows. A substance forms a flame only when it turns into vapour while burning.
A candle flame has three zones:
| Zone | Burning | Colour | Heat |
|---|---|---|---|
| Outermost zone | Complete combustion | Blue (non-luminous) | Hottest |
| Middle zone | Partial combustion | Yellow (luminous) | Moderately hot |
| Innermost zone | No combustion (unburnt wax vapour) | Dark | Least hot |
The outermost zone is the hottest because the fuel burns fully there. It gets the most oxygen from the air. Goldsmiths use this zone. They blow air through a metal pipe to melt gold and silver.
4. Calorific value - measuring a fuel
The calorific value of a fuel is the heat produced when 1 kg of it burns completely. It is measured in kilojoule per kg (kJ/kg). A higher calorific value means a better fuel.
| Fuel | Calorific value (kJ/kg) |
|---|---|
| Cow dung cake | 6,000 - 8,000 |
| Wood | 17,000 - 22,000 |
| Coal | 25,000 - 33,000 |
| Biogas | 35,000 - 40,000 |
| Petrol, Kerosene, Diesel | 45,000 |
| Methane, CNG | 50,000 |
| LPG | 55,000 |
| Hydrogen | 150,000 |
These are the standard school textbook values. Hydrogen has the highest value. But it is hard to store and carry safely, so it is not yet a common fuel.
An ideal fuel has these qualities:
- It is cheap and easily available.
- It burns easily in air at a steady rate.
- It gives a large amount of heat.
- It leaves no harmful substances behind.
5. Common fuels and what they contain
| Fuel | Main component(s) |
|---|---|
| LPG (Liquefied Petroleum Gas) | Butane and propane |
| CNG (Compressed Natural Gas) | Methane |
| Biogas | Methane, with carbon dioxide |
| Water gas | Carbon monoxide and hydrogen |
| Producer gas | Carbon monoxide and nitrogen |
| Coal gas | Hydrogen, methane and carbon monoxide |
LPG has no smell of its own. A strong-smelling compound, ethyl mercaptan, is added to it. This helps people notice a gas leak quickly.
Methane is also called marsh gas. It forms when plant matter rots in marshy places without air.
Coal comes in four main types. They are ranked by carbon content, from lowest to highest:
- Peat - lowest carbon, lowest quality.
- Lignite - also called brown coal.
- Bituminous - the most common type.
- Anthracite - highest carbon, best quality.
Petroleum is separated into petrol, diesel, kerosene and other products. This is done in a refinery by fractional distillation. Petrol quality is shown by its octane number. Diesel quality is shown by its cetane number. A higher octane number means less knocking in the engine.
6. Harmful effects of burning fuels
- Carbon monoxide (CO) forms when fuel burns without enough air. It is a very poisonous gas. That is why you should never sleep in a closed room with a burning coal stove.
- Carbon dioxide (CO2) is a greenhouse gas. Too much of it adds to global warming.
- Sulphur dioxide and nitrogen oxides dissolve in rainwater. They cause acid rain, which harms crops, buildings and water bodies.
- Unburnt carbon particles float in the air. They cause breathing problems like asthma.
This is why many cities moved buses and autos to CNG. It burns more cleanly and gives out much smaller amounts of harmful gases.
7. Controlling fire
Every method of fire control breaks the fire triangle. It either cuts off the air, cools the fuel, or removes the fuel.
- Water is the common extinguisher. It cools the burning material below its ignition temperature. It works well on wood and paper fires.
- Water must not be used on electrical fires. Water conducts electricity and can give a shock.
- Water must not be used on oil or petrol fires. Oil is lighter than water. It floats on top and keeps burning.
- Carbon dioxide is the best choice for electrical and oil fires. It is heavier than oxygen. It covers the fire like a blanket and cuts off the air. It also cools the fuel.
Some extinguishers use powders like sodium bicarbonate or potassium bicarbonate. Near a fire, these powders release carbon dioxide.
8. Previous year questions
- Which fuel has the highest calorific value? – Hydrogen
- What is the unit of calorific value? – kJ/kg
- LPG is mainly a mixture of which gases? – Butane and propane
- What is the main component of CNG? – Methane
- Which compound is added to LPG to detect leaks by smell? – Ethyl mercaptan
- Which zone of a candle flame is the hottest? – The outermost (non-luminous) zone
- Which substance catches fire in air at room temperature? – White phosphorus
- Which poisonous gas forms when fuel burns in too little air? – Carbon monoxide
- Which extinguisher is best for fires in electrical equipment? – Carbon dioxide
- Water gas is a mixture of which two gases? – Carbon monoxide and hydrogen
- Producer gas is a mixture of which two gases? – Carbon monoxide and nitrogen
- Which type of coal has the highest carbon content? – Anthracite
- What is the main gas in biogas? – Methane
- Octane number is used to rate which fuel? – Petrol
- What is the lowest temperature at which a substance catches fire called? – Ignition temperature
9. How to revise this topic
Start with the fire triangle: fuel, oxygen and heat. Most questions on fire control come from this one idea.
Next, learn the calorific value table in order. Cow dung cake is the lowest. Hydrogen is the highest. LPG is the highest among common household fuels.
Then make a small list of fuels and their main gases: LPG, CNG, biogas, water gas and producer gas. Exams ask these directly.
Keep the three flame zones in a table - burning, colour and heat. Finally, note which extinguisher suits which fire.
10. Frequently Asked Questions
What is combustion?
Combustion is a chemical process in which a substance reacts with oxygen and gives out heat, and often light. It needs three things: a fuel, oxygen and enough heat to reach the ignition temperature.
What is calorific value?
Calorific value is the amount of heat produced when 1 kg of a fuel burns completely. It is measured in kilojoule per kg (kJ/kg). Hydrogen has the highest calorific value, about 150,000 kJ/kg.
Which is the hottest part of a candle flame?
The outermost zone. It gets the most oxygen, so the fuel burns completely there. This zone is blue and non-luminous.
Why is water not used on electrical and oil fires?
Water conducts electricity, so it can give a shock in an electrical fire. In an oil fire, oil is lighter than water and floats on top, so it keeps burning. Carbon dioxide is used on these fires instead.
What is the difference between LPG and CNG?
LPG is mainly butane and propane, stored as a liquid under pressure, and is used for cooking. CNG is mainly methane, stored as a gas under high pressure, and is used in vehicles. CNG burns more cleanly.
For more General Science topics in simple language, see our Carbon and Its Compounds, Chemical Reactions and Equations and Heat and Temperature posts, or browse the General Science section. Preparing for a specific exam? Start with the SSC CGL guide or join the 100 Hour GS Course.