Coal, oil and natural gas are collectively known as fossil fuels. Fossil fuels are produced from the remains of dead animals, buried, compressed and heated over millions or hundreds of millions of years.
Coal is formed from dead plant matter that once grew in swamps. Layers of decaying matter formed upon w=one another, eventually becoming overlaid with rock and soil, sometimes hundreds of metres or more deep. The type and quality of coal – how much energy per kg may be extracted, which depends on the carbon content, and how much it has been heated and compressed - depends on the depth at which it is formed (and found. Peat is close to the surface. Lignite is found further from the surface. It is a low quality coal. Bituminous coal is found deeper underground and is a higher quality coal Anthracite is a high quality coal and is found deepest. The percentages of carbon in peat, lignite, bituminous coal and anthracite are about 60, 70, 80 and 95% respectively. Because the combustion reaction is
a higher percentage of carbon in coal translates directly into more energy per kg.
Oil is formed from the remains of microscopic marine life. When the organisms died, they sank to the sea floor and layers of rotting, dead organisms formed. These were buried, heated, and over millions of years they turned into oil. This process means that oil is often found under the sea floor, or in areas which were once sea – and are now deserts. Often natural gas formed as pockets near the top of the oil.
There are different grades of oil as there are coal. The type and quality of oil is determined mainly by the lengths of the carbon chains, with the most volatile types – petrol and diesel – having shorter chains of carbon atoms than the heavier, less volatile types of oil – bitumin and fuel oil.