Combustion of fuels and the fire triangle

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  • Combustion of fuels and the fire triangle
    • Combustion of fuels
      • Fuels are substances that react with oxygen to release useful energy (exothermic). Most of the energy is released as heat, but light energy is also released
      • Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen.
      • About 21 per cent of air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion
      • Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only. When they burn completely:
        • the carbon oxidises to carbon dioxidethe hydrogen oxidises to water (remember that water, H2O, is an oxide of hydrogen)
    • The fire triangle
      • The fire triangle is a symbol that shows the three things that are required for a fire to burn, namely:
        • oxygen    heat            fuel
          • Remove just one of these, and the fire can no longer burn. This makes the fire triangle very useful in fire prevention and firefighting.
      • Removing oxygen
        • Removal of oxygen from the area around a fire can be achieved with a carbon dioxide extinguisher The carbon dioxide extinguisher pushes oxygen away from the fire and replaces it with carbon dioxide, which is inflammable
      • Removing heat
        • A useful method of removing heat from a fire is to use water, which absorbs the heat from a fire very effectively. However, water should never be used on electrical fires – as oil and water do not mix, meaning the water only makes the oil form smaller droplets and so makes the fire even worse .
      • Removing fuel
        • By using fire-resistant materials, any fire will not have an adequate fuel source to keep burning and so this is a very effective method of fire prevention
        • Forest fires can also be managed more effectively if the trees around a fire can be removed to stop the fire from spreading. This method is known as a firebreak.
    • Combustion of hydrogen
      • Hydrogen is used as fuel for rockets as well as the fuel for hydrogen fuel cells in cars. It burns in oxygen to form water.
      • hydrogen + oxygen ? water      2H2 + O2 ? 2H2O
      • The flame is almost colourless. Mixtures of hydrogen and oxygen – or hydrogen and air – can be explosive when the two gases are present in a particular ratio, so hydrogen must be handled very carefully.
      • Many people believe that because no carbon dioxide is released when it burns, hydrogen could be a clean fuel for use in the future – to replace fossil fuels that are causing global warming.


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