Overview
This lecture covers the topic of combustion and flames, including types of combustion, fire extinguishing methods, flame structure, fuels, and their environmental impact.
Combustion: Basics and Types
- Combustion is a chemical reaction where a combustible substance reacts with oxygen to produce heat, light, and products.
- Burning is a type of combustion with visible flames; if only a glow appears, it's called smoldering (flameless combustion).
- All burning is combustion, but not all combustion is burning (e.g., smoldering incense sticks).
- Types of combustion: rapid (needs external ignition, e.g., LPG), spontaneous (no external ignition needed, e.g., sodium), and explosive (rapid with sound, e.g., crackers).
Fire and Extinguishing Methods
- Fire needs three things: fuel, oxygen, and heat (fire triangle).
- Removing any of these extinguishes the fire: cooling (removes heat), smothering (removes oxygen), starvation (removes fuel).
- Fire classification:
- Class A: paper, wood, cloth.
- Class B: flammable liquids (petrol, oil).
- Class C: flammable gases.
- Class D: reactive metals (sodium, magnesium).
- Class E: electrical fires.
- Class F/K: cooking oils/fats.
Fire Extinguishers
- Water is used only for Class A fires (never use on oil, electrical, or metal fires).
- Foam and carbon dioxide extinguishers are used for Class B and E fires.
- Dry powders (like sodium chloride) are best for Class D fires.
- Wet chemical extinguishers are used for cooking oil fires.
Flame Structure and Mechanism
- Candle flame points upward because hot air rises (lower density), setting up convection currents.
- Candle's flame zones:
- Outer (blue): complete combustion, highest temperature.
- Middle (yellow): partial/incomplete combustion, hot unburnt carbon (luminous).
- Inner (dark): unburnt wax vapors, least hot.
- Capillary action draws melted wax up the wick.
Fuels and Calorific Value
- Calorific value is the heat produced by burning 1 kg of fuel (measured in kJ/kg).
- Hydrogen has the highest calorific value (~50,000 kJ/kg).
- Ideal fuel: high calorific value, cheap, easily available, and environment-friendly (though no real fuel is ideal).
- Wood is popular in villages due to low cost and easy availability.
Harmful Products of Combustion
- Incomplete combustion produces carbon monoxide (CO) and soot (carbon particles), causing health hazards.
- CO is dangerous as it binds to hemoglobin better than oxygen, reducing oxygen transport, and can cause hypoxia.
Key Terms & Definitions
- Combustion — chemical reaction with oxygen that produces heat and usually light.
- Burning — combustion with flames.
- Smoldering — flameless, glowing form of combustion.
- Calorific Value — amount of heat produced by burning 1 kg of fuel.
- Fire Triangle — the three essentials: fuel, heat, oxygen.
- Capillary Action — upward movement of liquid through narrow spaces.
Action Items / Next Steps
- Review NCERT chapter on Combustion and Flames.
- Practice identifying fire classes and suitable extinguishers.
- Study the structure of a candle flame and zones.
- Prepare for questions on fuel properties and harmful combustion products.