Types of Combustion, Zones of Flame and Fuels
Combustion & Flame: Types of Combustion, Zones of Flame and Fuels
Types of Combustion, Zones of Flame and Fuels
Combustion and Flame
What you'll learn
- What combustion is and what conditions are needed.
- Types of combustion: rapid, spontaneous, explosive.
- Structure of a candle flame — three zones.
- What makes a good fuel — calorific value, pollution.
- Fire triangle and fire extinguishing methods.
Key concepts
What is combustion?
- Combustion: a chemical reaction in which a substance reacts with oxygen to produce heat and light.
- The burning substance is called the fuel.
- Combustion is always exothermic (releases energy).
General equation:
Fuel + Oxygen → Carbon dioxide + Water + Heat + Light
(For complete combustion of a hydrocarbon fuel)
Conditions for combustion
All three must be present — the Fire Triangle:
| Condition | Role |
|---|---|
| Fuel | Substance that burns |
| Oxygen (air) | Supporter of combustion |
| Heat (ignition temperature) | Minimum temperature needed to start burning |
- Ignition temperature: the lowest temperature at which a substance catches fire.
- Remove ANY one side of the triangle → fire goes out.
Types of combustion
| Type | Description | Example |
|---|---|---|
| Rapid combustion | Burns quickly with flame; produces heat and light | LPG burning on stove; matchstick |
| Spontaneous combustion | Ignites without external heat source; substance reaches ignition temp on its own | White phosphorus catches fire in air at room temp; haystacks catching fire from bacterial heat inside |
| Explosive combustion | Extremely rapid; produces large amounts of gas suddenly; accompanied by loud sound | Crackers; dynamite; petrol vapour + spark |
| Slow combustion | Occurs at low temperature; no flame or light | Rusting of iron; respiration in cells |
Inflammable substances
- Substances with very low ignition temperatures — catch fire easily.
- Examples: petrol, alcohol, LPG, kerosene, CNG, acetone.
- Must be stored carefully, away from flames.
- Non-inflammable: water, stone, sand — do not burn.
Structure of a flame
A candle flame has three zones:
| Zone | Colour | Temperature | What happens |
|---|---|---|---|
| Innermost zone | Dark/black | Lowest | Unburnt wax vapours; no combustion here |
| Middle (luminous) zone | Yellow/orange | Medium | Partial combustion; carbon particles glow yellow (luminescence) |
| Outermost zone | Blue | Highest | Complete combustion; enough O₂ |
- Most useful zone for heating: outermost (blue) zone — highest temperature.
- Goldsmith uses the outermost zone of flame to melt gold (hottest part).
- Yellow flame = incomplete combustion (soot produced).
- Blue flame = complete combustion (no soot).
Complete vs Incomplete Combustion
| Complete combustion | Incomplete combustion | |
|---|---|---|
| Oxygen supply | Sufficient | Insufficient |
| Products | CO₂ + H₂O | CO (carbon monoxide) + soot (carbon) |
| Flame colour | Blue | Yellow/orange |
| Heat produced | More | Less |
| Pollution | Less | More (CO is toxic; soot causes air pollution) |
- Carbon monoxide (CO) produced in incomplete combustion is colourless, odourless, and highly toxic — binds to haemoglobin 200× more strongly than O₂ → prevents O₂ transport → can be fatal.
- Reason to never burn fuel in an enclosed room without ventilation.
Fuels
A fuel is a substance that produces energy (heat/light) on combustion.
Types of fuels
| State | Examples |
|---|---|
| Solid fuels | Wood, coal, charcoal, coke, cow dung cake |
| Liquid fuels | Petrol, diesel, kerosene, alcohol |
| Gaseous fuels | LPG (liquefied petroleum gas), CNG (compressed natural gas), biogas, hydrogen |
Calorific value
- Calorific value (CV): amount of heat energy produced by 1 kg of fuel on complete combustion.
- Unit: kJ/kg (kilojoules per kilogram).
- Higher CV = better fuel (more energy per kg).
| Fuel | Calorific value (kJ/kg) |
|---|---|
| Wood | ~17,000 |
| Coal | ~25,000–33,000 |
| Charcoal | ~33,000 |
| Kerosene | ~45,000 |
| Diesel | ~45,000 |
| Petrol | ~47,000 |
| LPG | ~50,000 |
| Biogas | ~35,000–40,000 |
| Hydrogen | ~150,000 (highest known) |
| Natural gas (CNG) | ~50,000 |
Ideal fuel characteristics
A good fuel should:
- Have high calorific value.
- Burn at moderate ignition temperature.
- Produce no harmful products (no CO, SO₂, ash).
- Be easy to store and transport — stable, not too heavy.
- Be affordable and available.
- No fuel is ideal — all have trade-offs.
Why hydrogen is considered the fuel of the future
- Highest calorific value (~150,000 kJ/kg).
- Product of combustion: only water (H₂ + O₂ → H₂O) — zero pollution.
- But: difficult to store (must be very cold or at high pressure); explosive; currently expensive to produce.
Fire extinguishing methods
To put out a fire, remove one side of the fire triangle:
| Method | What it removes | Example |
|---|---|---|
| Pouring water | Heat (cools below ignition temp) | Wood, paper fires |
| CO₂ extinguisher | Oxygen (blankets the fire) | Electrical fires; oil fires |
| Sand | Oxygen (covers fire) | Small fires |
| Fire blanket | Oxygen | Clothing fires; small kitchen fires |
| Dry powder | Oxygen + heat | Most fires |
- Never use water on: electrical fires (conducts electricity → electric shock), oil/petrol fires (water makes burning oil splatter).
- CO₂ extinguisher: releases CO₂ gas which is heavier than air → settles around fire → cuts off oxygen → fire goes out.
Quick check
- What are the three conditions needed for combustion? What is the fire triangle?
- Describe the three zones of a candle flame and their temperatures.
- What is calorific value? Which common fuel has the highest calorific value?
- Why should you never use water to put out an electrical fire?
- What makes hydrogen a promising future fuel? What is its main challenge?
Open the Practice tab for graded questions on Combustion and Flame.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Quick check
Master this topic with Drishti OS
Get unlimited mock tests, AI-powered mentorship, and complete video courses when you join.
Start Free Practice