Learning Objectives
- Understand the process of combustion and its requirements
- Learn about types of combustion
- Study the structure and zones of a flame
- Understand fuel efficiency and calorific value
Key Concepts
Combustion
Combustion is a chemical process in which a substance reacts with oxygen to give heat and light. The substance that burns is called a fuel (e.g., wood, charcoal, LPG). The lowest temperature at which a substance catches fire is called its ignition temperature.
Conditions for Combustion
Three conditions are necessary for combustion (fire triangle):
- Presence of a combustible substance (fuel)
- Supply of air (oxygen, the supporter of combustion)
- Heating the fuel to its ignition temperature
Fire extinguisher principle: Remove any one of the three conditions — cut off fuel supply, cut off air supply, or cool the burning substance below its ignition temperature. Water cools and cuts off oxygen. CO₂ is heavier than air and blankets the fire.
Types of Combustion
- Rapid combustion: Burns quickly producing heat and light (e.g., burning LPG).
- Spontaneous combustion: Occurs on its own without external heating (e.g., forest fires, white phosphorus igniting in air).
- Explosion: Very rapid combustion with large amounts of gas, heat, light, and sound (e.g., firecrackers).
Structure of a Flame
A candle flame has three zones:
- Dark zone (innermost): Least hot, unburnt vapours of wax present. Black in colour.
- Luminous zone (middle): Moderately hot, partial combustion occurs. Yellow, luminous region due to glowing carbon particles.
- Non-luminous zone (outermost): Hottest part, complete combustion occurs. Blue in colour. Goldsmith uses this zone for melting gold.
Fuel Efficiency
Calorific value: The amount of heat energy produced by complete combustion of 1 kg of fuel. Measured in kJ/kg. Higher calorific value means more efficient fuel.
Examples: LPG: 55000 kJ/kg, Kerosene: 45000 kJ/kg, Wood: 17000 kJ/kg, Cow dung cake: 6000 kJ/kg.
An ideal fuel should have high calorific value, be cheap, easily available, easy to transport and store, and produce less pollution.
Harmful Effects of Burning Fuels
Burning fuels produces CO₂ (greenhouse gas), CO (poisonous), SO₂ (acid rain), and particulate matter (smog). Incomplete combustion produces carbon monoxide, which is highly toxic.
Summary
Combustion requires fuel, oxygen, and ignition temperature. It can be rapid, spontaneous, or explosive. A flame has three zones with the outermost being the hottest. Fuel efficiency is measured by calorific value. Burning fuels causes air pollution, contributing to global warming and acid rain.
Important Terms
- Combustion: Burning of a substance in the presence of oxygen with release of heat and light
- Ignition Temperature: Minimum temperature at which a substance catches fire
- Inflammable: Substances with very low ignition temperature that catch fire easily (e.g., petrol, LPG)
- Calorific Value: Heat produced by complete combustion of 1 kg of fuel (kJ/kg)
- Spontaneous Combustion: Combustion that occurs without external ignition
Quick Revision
- Fire triangle: fuel + oxygen + ignition temperature
- Fire extinguishing: remove any one element of the fire triangle
- Flame zones (inside to outside): dark → luminous → non-luminous (hottest)
- Calorific value: LPG > kerosene > wood > cow dung
- CO₂ → global warming; SO₂ → acid rain; CO → poisonous
- Ideal fuel: high calorific value, low pollution, easy storage