Fuels

4.1.2 Hydrocarbons

4.2 Organic compounds

Fuels

  1. Fuels

    1. Most fuels, such as coal, contain carbon or hydrogen
    2. Fuels may also contain sulfur

  2. Combustion

    1. The combustion of hydrocarbon fuels releases energy
    2. During combustion, the carbon and hydrogen in the fuels are oxidised
    3. The products of combustion depend on the elements in the fuel and whether the combustion is complete or incomplete
    4. In complete combustion, there is enough oxygen present for the fuel to break down completely, forming carbon dioxide and water (vapour)
    5. In incomplete combustion, there is not enough oxygen present for the fuel to react completely, instead producing carbon monoxide and particulates of soot (carbon) & unburnt fuel
    6. At high temperatures, oxides of nitrogen may be produced
    7. If sulfur is present, sulfur dioxide may be produced
    8. Sulfur can be removed from fuels before they are burned, such as in vehicles
    9. Sulfur dioxide can be removed from the waste gases after combustion, such as in power stations

  3. Biofuels

    1. Biofuels, including biodiesel and ethanol, are produced from plant material
    2. They are possible alternatives to hydrocarbon fuels
    3. Ethanol fuel is produced from a dilute solution of ethanol obtained by fermentation of plant material, such as glucose: C6H12O6 (aq) → 2C2H5OH(aq) + 2CO2 (g) at a temperature between 20-35 °C

    4. The use of biofuels is controversial:

      Advantages Disadvantages
      Biodiesel is less harmful to animals and plants if spilled than the diesel from crude oil People are worried about endangered species since habitats, such as tropical forests, are cleared to grow biofuel crops
      Biodiesel burns more cleanly, reducing the amount of particulates and sulfur dioxide released The use of large areas of farmland to produce biofuel instead of food could pose problems
      As non-renewable fuels such as crude oil run out, their price will increase and biofuels will become cheaper to use This could result in famine in poorer countries if the price of staple food crops rises as demand overtakes supply
      The carbon dioxide produced from combustion is balanced by the carbon dioxide absorbed by the plants during photosynthesis, so biofuels are, in theory, carbon neutral Forests, which absorb lots of carbon dioxide, may be cleared to grow the biofuel crops if they get more popular

  4. Hydrogen

    1. Hydrogen can be burned as a fuel in combustion engines or in fuel cells that produce electricity to power vehicles
    2. It burns very cleanly: 2H2 (g) + O2 (g) → 2H2O(g), and the water can be used to produce more hydrogen by electrolysis
    3. However, there are safety concerns in case of leaks or accidents, since hydrogen is very explosive when mixed with air and ignited
    4. Since it is a gas, it requires a large storage volume or high pressure, both of which causes problems