Combustion and Flame

1.1 Combustion

This chapter Combustion and Flame, focuses on the principles of combustion, types of flames, fire control methods, characteristics of fuels, and the environmental impact of burning fuels.

Introduction to Fuels

• Different fuels are used at home, in industry, and for running automobiles.
• Examples of fuels:
  • Home: Cow dung, wood, kerosene
  • Industry: Coal, charcoal
  • Automobiles: Petrol, diesel, compressed natural gas (CNG)

Burning with and without Flame

• Candle burns with a flame.
• Coal burns without a flame.
• Many materials burn without producing a flame.

What is Combustion?

• Combustion: A chemical process where a substance reacts with oxygen (air) to produce heat.
• Example: Burning of magnesium ribbon (forms magnesium oxide, produces heat and light).
• Charcoal and coal also burn in air, producing heat and light.
• Combustible substances: Substances that undergo combustion (e.g., magnesium, charcoal).
  • Combustible substances = Fuel.

Conditions for Combustion

For combustion to occur, several conditions must be met:

• Fuel: Something that can burn, like wood, paper, or wax.
• Oxygen: Air that helps the fuel burn.
• Heat: Enough warmth to make the fuel start burning, like a spark or flame.

Ignition Temperature

• Ignition temperature: The lowest temperature at which a substance catches fire.
• A matchstick doesn’t burn on its own because room temperature is below its ignition temperature.
• Example:
  • Cooking oil can catch fire if overheated.
  • Kerosene oil needs less heat to catch fire compared to wood.
• Care with storage: Kerosene oil requires careful storage due to its lower ignition temperature.

Key Points to Remember

• Combustion requires oxygen (air).
• Different materials have different ignition temperatures.
• A substance must reach its ignition temperature to burn.

Interesting Facts about Fire and Matches

Inflammable Substances

• Definition: Substances with very low ignition temperatures that catch fire easily.
• Examples: Petrol, alcohol, Liquefied Petroleum Gas (LPG).
• Can you think of more inflammable substances?

1.2 Controlling Fire

Fire Accidents

• Fires can break out in homes, shops, and factories.
• Fire brigade helps by pouring water on the fire.

How Water Helps to Control Fire.

• Cooling: Water cools the combustible material, lowering its temperature below ignition point.
• Cutting off Air: Water vapors surround the material, reducing air supply.

Essential Requirements for Fire

1. Fuel
2. Air (oxygen)
3. Heat (ignition temperature)

Methods to Control Fire

1. Remove Fuel:
2. Remove Air: Using fire extinguishers to cut off the oxygen supply.
3. Reduce Temperature: Cooling the fuel below its ignition temperature.

Fire Extinguishers

Water as a Fire Extinguisher

• Common Use: Water is the most common fire extinguisher.
• Suitable For: Works well on fires involving wood and paper.
• Limitations:
  • Electrical Fires: Water can conduct electricity and harm people.
  • Oil and Petrol Fires: Water is heavier than oil, so it sinks below the oil, and the oil keeps burning on top.

Carbon Dioxide (CO2) as a Fire Extinguisher

• Best For:
  • Electrical equipment fires.
  • Fires involving inflammable materials like petrol.
• How It Works:
  • CO2 is heavier than oxygen and covers the fire like a blanket.
  • This cuts off the contact between the fuel and oxygen, controlling the fire.
• Advantages:
  • Does not harm electrical equipment.
  • Cools down the temperature of the fuel when released.

Getting CO2

• Stored in Cylinders: CO2 can be stored as a liquid at high pressure in cylinders.
• Expansion: When released, it expands a lot and cools down.
• Another Method: Using dry powder of chemicals like baking soda (sodium bicarbonate) or potassium bicarbonate, which give off CO2 near the fire.

Key Points

• Water: Good for wood and paper fires, not for electrical or oil fires.
• CO2: Best for electrical and oil fires, cools down the fuel and does not damage electrical equipment.
• Storage: CO2 can be stored in high-pressure cylinders or produced by using dry chemical powders.

1.3 Types of Combustion

Rapid Combustion

• Example: Burning gas stove when lit with a matchstick.
• Rapid burning, producing heat and light.

Spontaneous Combustion

• Example: Phosphorus burning in air at room temperature.
• Material bursts into flames without an apparent cause.

Explosion

• Example: Firecrackers.
• Sudden reaction producing heat, light, sound, and gas.
• Can occur from pressure or ignition.

Interesting Facts

• Spontaneous Combustion: Coal dust in mines can cause fires.
• Forest Fires: Can start from sun heat or lightning, but often due to human carelessness.
• Safety Tip: Always extinguish campfires completely before leaving a forest.

Important Points

• Different types of fire require different methods to control.
• Understanding combustion helps in preventing and controlling fires effectively.

1.4 Flame

Definition: A flame is the bright, hot, glowing gas that you see when something is burning.

Observing Flames

• LPG Flame: Observe its color.
• Candle Flame: Compare its color to the LPG flame.
• Burning Magnesium Ribbon: Recall the bright flame of Magnesium from Class VII.

1.5 Structure of a Flame

Zones of a Flame

• Outermost Zone: Blue, hottest part, where complete combustion occurs.
• Middle Zone: Yellow, moderately hot, with partial combustion.
• Innermost Zone: Black, least hot, with no combustion, containing unburned wax vapors.

Understanding Flame Production

• Flame-producing Substances: Materials that vaporize during burning (e.g., kerosene oil, molten wax).
• Non-flame-producing Substances: Materials that do not vaporize (e.g., charcoal).

Goldsmiths and Flame Zones

• Goldsmiths use the outermost zone of the flame with a metallic blow-pipe to melt gold and silver because it is the hottest part of the flame.

Key Points

• Flame Colors: Different flames have different colors.
• Wax Vapors: Vapors from materials like wax produce flames.
  • Kerosene and Wax: Vaporize and form flames when burning.
  • Charcoal: Burns without forming a flame as it doesn’t vaporize.
• Zones of Flame:
  • Luminous zone contains unburnt carbon particles.
  • Non-luminous zone is the hottest part.
• Practical Use: The outermost zone is used for high-temperature applications like melting metals.
  • Goldsmiths: Use the outermost, hottest zone of a flame to melt gold and silver quickly due to their high melting points.
• A circular black ring on the back of a cooking vessel shows unburnt carbon particles from the flame’s luminous zone.

1.6 What is a Fuel?

Definition and Characteristics

• Fuels: Substances like wood, charcoal, petrol, kerosene used for heat energy or undergoes combustion.
• Good Fuel Characteristics:
  • Readily available
  • Cheap
  • Burns easily in air at a moderate rate
  • Produces a large amount of heat
  • Leaves no undesirable substances (non – polluting).

Ideal Fuel

• No perfect fuel exists, but some come close for specific uses.
• Fuels vary in cost and efficiency.

Types of Fuels

• Solid Fuels: Wood, coal, charcoal
• Liquid Fuels: Petrol, kerosene
• Gaseous Fuels: LPG, CNG

1.7 Fuel Efficiency

Calorific Value

• Definition: Amount of heat energy produced by burning 1 kg of fuel.
• Unit: Kilojoule per kg (kJ/kg)
• Example: Comparing cow dung, coal, and LPG to boil water; LPG is preferred for higher calorific value.

Fuel	Calorific Value (kJ/kg)
Cow dung cake	6000-8000
Wood	17000-22000
Coal	25000-33000
Petrol	45000
Kerosene	45000
Diesel	45000
Methane	50000
CNG	50000
LPG	55000
Biogas	35000-40000
Hydrogen	150000

Harmful Effects of Burning Fuels

Pollution and Health Hazards

1. Unburnt Carbon Particles:
   • Burning wood, coal, and petroleum releases tiny carbon particles.
   • Cause respiratory diseases like asthma.
2. Carbon Monoxide:
   • Produced by incomplete combustion.
   • Poisonous gas, can be fatal in closed rooms.
3. Carbon Dioxide:
   • Emitted during combustion.
   • Contributes to global warming (Earth’s temperature rise,).
4. Sulphur Dioxide and Nitrogen Oxides:
   • Released by burning coal, diesel, and petrol.
   • These gases mix with rainwater to form acid rain.
   • Acid rain harms crops, buildings, and soil.

Cleaner Fuels

• CNG (Compressed Natural Gas):
  • Produces fewer harmful products.
  • Preferred for automobiles over diesel and petrol.

Environmental Impact

Historical and Current Use

• Wood: Traditionally used but now replaced by coal and LPG due to harmful smoke and deforestation.
• Rural Areas: Wood still used due to availability and cost but causes respiratory issues and environmental damage.
• Using wood as fuel leads to deforestation, which is bad for the environment.

Global Warming

• Definition: Rise in Earth’s atmospheric temperature.
• Effects: Melting polar glaciers, rising sea levels, coastal floods, potential submersion of low-lying areas.

Chapter Summary:

• Substances that burn in air are called combustible.
• Oxygen (in air) is essential for combustion.
• During combustion, heat and light are given out.
• Ignition temperature is the lowest temperature at which a combustible substance catches fire.
• Inflammable substances have very low ignition temperature.
• Fire can be controlled by removing one or more requirements essential for producing fire.
• Water is commonly used to control fires.
• Water cannot be used to control fires involving electrical equipment or oils.
• There are various types of combustion such as rapid combustion, spontaneous combustion, explosion, etc.
• There are three different zones of a flame – dark zone, luminous zone, and non-luminous zone.
• An ideal fuel is cheap, readily available, easily combustible, and easy to transport. It has high calorific value. It does not produce gases or residues that pollute the environment.
• Fuels differ in their efficiency and cost.
• Fuel efficiency is expressed in terms of its calorific value, which is expressed in units of kilojoule per kg.
• Unburnt carbon particles in air are dangerous pollutants causing respiratory problems.
• Incomplete combustion of a fuel gives poisonous carbon monoxide gas.
• Increased percentage of carbon dioxide in air has been linked to global warming.
• Oxides of sulphur and nitrogen produced by the burning of coal, diesel, and petrol cause acid rain, which is harmful for crops, buildings, and soil.

KEYWORDS

1. ACID RAIN
2. CALORIFIC VALUE
3. COMBUSTION
4. DEFORESTATION
5. EXPLOSION
6. FLAME
7. FIRE EXTINGUISHER
8. FUEL
9. FUEL EFFICIENCY
10. GLOBAL WARMING
11. IDEAL FUEL
12. IGNITION TEMPERATURE
13. INFLAMMABLE SUBSTANCES