Combustion and Flame

• Introduction

  • Combustion definition (chemical process with oxygen producing heat/light)
  • Fuel definition (substance undergoing combustion)
    • Types: Solid, Liquid, Gas
  • Combustible vs Non-combustible substances

• Conditions Necessary for Combustion

  • Presence of a Combustible Substance
  • Presence of Supporter of Combustion (Oxygen/Air)
  • Attainment of Ignition Temperature
    • Lowest temperature at which a substance catches fire
    • Inflammable substances (low ignition temperature, e.g., LPG)

• Control of Fire

  • Removal of fuel
  • Cutting off supply of oxygen (using fire extinguishers, blanket, sand)
  • Lowering the ignition temperature (using water)
  • Special cases: Water cannot be used for electrical fires or oil/petrol fires

• Types of Combustion

  • Rapid Combustion (e.g., matchstick, LPG)
  • Spontaneous Combustion (e.g., phosphorus catches fire on its own at room temp)
  • Explosion (sudden reaction with heat, light, and sound, e.g., crackers)

• Flame

  • Definition (region where combustion of gases takes place)
  • Substances that form flames (vaporize on heating, e.g., wax, kerosene)
  • Substances that do not form flames (only glow, e.g., charcoal)

• Structure of a Flame

  • Innermost Zone (Dark zone)
    • Unburnt wax vapors
    • Least hot
  • Middle Zone (Luminous zone)
    • Partial combustion of fuel
    • Moderate temperature (yellow color)
  • Outermost Zone (Non-luminous zone)
    • Complete combustion
    • Hottest part (blue color)
    • Used by goldsmiths for melting gold/silver

• Fuel and Fuel Efficiency

  • Ideal fuel characteristics (cheap, readily available, burns easily, moderate rate, no harmful residues)
  • Calorific Value
    • Definition (Amount of heat energy produced on complete combustion of 1 kg of fuel)
    • Unit: Kilojoule per kg ($kJ/kg$)
  • Harmful effects of burning fuels
    • Carbon fuels release unburnt carbon particles (respiratory diseases like asthma)
    • Incomplete combustion produces Carbon Monoxide (toxic, poisonous gas)
    • Increase in Carbon Dioxide leads to Global Warming
    • Oxides of sulphur and nitrogen cause Acid Rain