Combustion and Flame Notes

Introduction

  • The teacher greets the students and asks how they are doing.
  • She acknowledges students from different classes (6th, 7th, 8th, 10th, and even 12th).
  • The topic for the day is Combustion and Flame.
  • The lecture will include:
    • combustion, zones of flame
    • fuels, pollution, fire control, combustible and non-combustible substances, flammable and inflammable substances, and types of combustion

What is Combustion?

  • Combustion is defined as a chemical process where new substances are formed.
  • The primary products of combustion are heat and require the presence of oxygen.
  • Conditions necessary for combustion:
    • Oxygen: Presence of oxygen is essential.
    • Substance/Fuel: A combustible substance or fuel is required (e.g., wood, paper).
    • Heat/Temperature: Applying heat or reaching a specific ignition temperature is necessary.
  • If any of these three components is missing the combustion process will not occur
  • Triangle of Combustion: Illustrates the necessity of oxygen, fuel, and heat for combustion.

Poll Question

  • Question: Which of the following is a chemical reaction that produces heat when a substance comes in contact with oxygen?
  • Answer: Combustion.

Types of Combustion

  • Two types of combustion based on the presence of oxygen:
    • Incomplete Combustion: Occurs when there is not enough oxygen available.
      • Produces carbon monoxide (CO), which is harmful.
    • Complete Combustion: Occurs when there is sufficient oxygen.
      • Produces carbon dioxide CO2CO_2.
  • Completeness of combustion can be indicated by flame color
    • Blue Flame: Complete combustion
    • Orange/Yellow Flame: Incomplete combustion

Complete vs Incomplete Combustion

  • Complete Combustion:
    • Adequate oxygen supply.
    • Products: Carbon dioxide (CO2)(CO_2) and water.
    • Also known as clean combustion.
  • Incomplete Combustion:
    • Insufficient oxygen supply.
    • Products: Carbon monoxide (CO) and water.

Ignition Temperature

  • Definition: The lowest temperature at which a substance catches fire.
    • Example: White phosphorus catches fire at 35°C.
    • Different substances have different ignition temperatures
      • Gasoline: 246°C246 °C
      • Kerosene: 220°C220 °C
      • Diesel: 210°C210 °C
      • Wood: 300°C300 °C
    • Raising the ambient temperature can promote combustion

Poll Question

  • Question: Ignition temperature is the _ temperature at which a substance catches fire.
  • Answer: Lowest.

Combustible vs. Non-Combustible Substances

  • Combustible Substances: Easily catch fire (e.g., paper, coal, wood).
  • Non-Combustible Substances: Do not easily catch fire (e.g., water, sand, glass).
  • Within combustible substances, there are:
    • Flammable
    • Inflammable
  • Both flammable and inflammable materials catch fire easily, the difference lies in their ignition temperature.
    • Inflammable substances have very low ignition temperatures and catch fire very quickly.
      • Examples: diesel, LPG, acetone.
    • Flammable Substances: Require an ignition source to catch fire.
      • Example: wood.
  • Both flammable and inflammable are warning signs to indicate dangerous substances.

Poll Question

  • Question: Which of the following is NOT an inflammable substance?
  • Answer: Wood

Types of Combustion

  • Three types of combustion:
    • Rapid Combustion: Occurs quickly with an ignition source.
      • Example: Burning a stove.
    • Spontaneous Combustion: Occurs without an external ignition source due to low ignition temperature.
      • Example: Forest fires due to rising temperatures.
    • Explosion: Involves a sudden reaction with heat, light, and sound.

Complete Combustion Descriptions

  • Rapid Combustion:
    • High-speed combustion that produces heat and light.
      • Example: Lighting a stove.
  • Spontaneous Combustion:
    • Material suddenly bursts into flames without an apparent cause.
    • Due to low ignition temperatures.
  • Explosion:
    • Sudden reaction with the release of gases, heat, light, and sound.

Poll Question

  • Question: Which of the following describes a type of combustion in which a substance suddenly bursts into flames without any apparent cause?
  • Answer: Spontaneous combustion.

Zones of a Flame

  • Candle has different zones:
    • Outermost Zone:
      • Blue in color; complete combustion.
      • Hottest part of the flame.
      • Non-luminous.
    • Middle Zone:
      • Yellow in color; partial combustion (incomplete combustion).
      • Luminous - provides the most light.
      • Moderately hot.
    • Innermost Zone:
      • Black in color.
      • Unburnt wax particles/vapors.
      • Dark zone; least hot.

Zones of a Flame Break Down

  • Non-Luminous Zone:
    • Blue in color and the hottest part of the flame.
  • Luminous Zone:
    • Yellowish-orange and moderately hot.
  • Dark Zone:
    • Contains unburnt carbon particles.

Poll Question

  • Question: Which zone of the candle flame burns with a blue flame?
  • (No Answer)

What is Fuel?

  • Definition: A substance that is burned to produce heat.
    • Should provide considerable heat without undesirable byproducts.

Calorific Value

  • The amount of heat energy produced on the complete combustion of 1 kg of fuel.
    • Expressed in kilojoules per kilogram (kJ/kg).
  • Heat is expressed in kilojoules (kJ)
  • Fuel is calculated in kilograms (kg)

Poll Question

  • Question: Which of the following units expresses fuel efficiency in terms of calorific value?
  • Answer: kJ/kg.

Types of Fuels

  • Three types of fuels:
    • Solid Fuels:
      • Examples: coal, wood, and charcoal
    • Liquid Fuels:
      • Examples: Kerosene, petrol, and diesel
    • Gaseous Fuels:
      • Examples: CNG and biogas

Ideal Fuel

  • A fuel that meets all requirements for a particular use.
  • Characteristics:
    • Cheap
    • Easily Available
    • Readily Combustible
    • High Calorific Value (produces a lot of heat when burned).
    • Does Not Produce Harmful Gases: (lowers pollution levels)

Poll Question

  • Question: Which of the following is NOT an ideal fuel characteristic?
  • Answer: Costly

Pollution

  • Caused by fuels like wood or coal
  • Unburnt carbon particles cause respiratory diseases
  • Incomplete combustion leads to the production of carbon monoxide, which is harmful
  • Use of non-renewable fuels increase carbon dioxide which in turn leads to global warming (increase average temperature in our environment).
  • Burning of fuels such as coal release sulfur oxides, that cause acid rain ( rain becomes very acidy and corrosive )
  • CNG causes less pollution than petrol or diesel
    • Cleaner burning
    • Less pollution

Pollution Key Points

  • Burning fuels releases unburnt particles, causing respiratory problems.
  • Incomplete combustion produces poisonous gases like carbon monoxide.
  • Burning fossil fuels increases carbon dioxide levels, leading to global warming.
  • Acid rain is caused by oxides dissolving in rainwater.
  • CNG is a cleaner and less polluting fuel.

Fire Control

  • A fire can be controlled by removing any element of the combustion triangle ( oxygen, fuel, and heat).
  • Other methods of fire control:
    • Fire extinguishers
    • Sand can be used for oil or electrical based fires
    • Note: Electrical fires are not to be extinguished with water (spreads the charges, or oil).
  • For oil fires, water cannot be used because water is denser than oil, and oil floats on top causing more distribution of fire over the newly spread liquid..Use sand!

Poll Question

  • Question: Which of the following is the best fire extinguisher for electrical equipment and inflammable substances?
  • Answer: CO2CO_2.