Fossil Fuels

Fossil fuels are energy sources formed from ancient plants and animals buried for millions of years. The three main fossil fuels are coal, natural gas, and petroleum (crude oil).

Hydrocarbons

Most fossil fuels are hydrocarbons. Hydrocarbons are compounds made of hydrogen and carbon only. For example, methane is a simple hydrocarbon with formula $CH_4$.

Natural Gas

Natural gas is mainly methane. We say methane is the main constituent of natural gas. It is used for heating, cooking, and electricity generation.

Petroleum and Fractional Distillation

Petroleum (crude oil) is a mixture of many hydrocarbons. To separate it, refineries use fractional distillation. The crude oil is heated and vapours rise up a tall column. Different hydrocarbons condense at different heights because they have different boiling points. This is like sorting people by height on different floors.

• From bottom to top of the column: chain length decreases
• Volatility increases (they evaporate more easily)
• Boiling point decreases
• Viscosity decreases (thick, sticky at bottom; runny at top)

Main Fractions and Uses

• Refinery gas: gas for heating and cooking
• Gasoline/Petrol: fuel for cars
• Naphtha: chemical feedstock (to make plastics and other chemicals)
• Kerosene/Paraffin: jet fuel
• Diesel oil/Gas oil: fuel for diesel engines
• Fuel oil: ships and home heating systems
• Lubricating oil: lubricants, waxes, polishes
• Bitumen: road surfacing

Combustion and Air Quality

Complete combustion of hydrocarbons produces carbon dioxide and water: $\text{hydrocarbon} + O_2 \to CO_2 + H_2O$. Incomplete combustion (not enough oxygen) can produce carbon monoxide (a toxic gas) and soot (particulates).

• $CO_2$ contributes to global warming
• Carbon monoxide is poisonous
• Particulates can harm lungs
• Sulfur in fuels forms $SO_2$ on burning, leading to acid rain
• High engine temperatures form nitrogen oxides (NOx), which cause smog and acid rain; catalytic converters help reduce NOx

Common Misconceptions

• Crude oil is a mixture, not a single substance
• Fractional distillation separates by boiling point; it does not break molecules
• Natural gas is mostly methane, not “just any gas”