Fuels

1. Naming the Fossil Fuels

• Fossil fuels are energy sources formed from the remains of dead plants and animals over millions of years under high pressure and temperature in the Earth’s crust.
• Main types:
  • Coal
    • Solid fossil fuel.
    • Formed mainly from plant matter buried in swampy areas.
    • Composed mostly of carbon, with some hydrogen, oxygen, nitrogen, and sulfur.
  • Natural Gas
    • Gaseous fossil fuel.
    • Found in underground deposits, often with petroleum.
    • Main component: methane (CH₄).
  • Petroleum (Crude Oil)
    • Liquid fossil fuel.
    • Formed from microscopic marine organisms.
    • Composed of a complex mixture of hydrocarbons.

2. Methane as the Main Constituent of Natural Gas

• Methane (CH₄): simplest hydrocarbon, one carbon atom bonded to four hydrogen atoms.
• Properties of methane:
  • Colourless, odourless gas (odour is added commercially for safety detection).
  • Burns with a clean blue flame, producing carbon dioxide and water:
    • CH₄ + 2O₂ → CO₂ + 2H₂O
  • High calorific value (releases a lot of heat energy when burned).

3. Hydrocarbons – Definition

• Hydrocarbons: compounds made of only hydrogen (H) and carbon (C) atoms.
• Found in fossil fuels such as petroleum and natural gas.
• General categories:
  • Alkanes: saturated hydrocarbons (single bonds only, formula CₙH₂ₙ₊₂).
  • Alkenes: unsaturated hydrocarbons (contain C=C double bonds, formula CₙH₂ₙ).
  • Aromatic hydrocarbons: contain benzene rings (C₆H₆ structure).

4. Petroleum as a Mixture of Hydrocarbons

• Petroleum is not a single substance; it contains many different hydrocarbons with different chain lengths.
• Can contain straight-chain, branched-chain, and cyclic hydrocarbons.
• Composition depends on the source location but typically has both alkanes and cycloalkanes.

5. Separation of Petroleum by Fractional Distillation

• Process: separates petroleum into fractions based on different boiling points.
• Steps:
  1. Crude oil is heated in a furnace until most of it vaporises.
  2. Vapours enter a fractionating column – tall column with trays at different heights.
  3. Temperature gradient: hottest at the bottom, coolest at the top.
  4. Vapours rise; as they cool, components condense at different heights depending on boiling points.
  5. Each condensed liquid is collected as a fraction.
• Key principle:
  • Shorter hydrocarbon chains have lower boiling points → condense higher up.
  • Longer chains have higher boiling points → condense lower down.

6. Changes in Properties from Bottom to Top of Fractionating Column

• As you move from bottom to top:
  1. Chain length decreases – molecules have fewer carbon atoms.
  2. Volatility increases – evaporates more easily.
  3. Boiling point decreases – due to weaker intermolecular forces in shorter chains.
  4. Viscosity decreases – liquids become runnier and flow more easily.

7. Uses of Petroleum Fractions