Coal is the most abundant and sustainable fossil fuel in the world. Its reserves are widely distributed making it the cheapest of the fossil fuels, both to mine and to use.
What is coal?
Coal is a sedimentary rock composed of the altered remains of plants. These may form thick layers in wet environments where the rate of growth exceeds the rate of rotting and decay; either in warm climate swamps like Australia’s Kakadu or the Florida Everglades, or cool climate forests and marshes like much of present day Canada and Russia.
Most European and North American coals are of the warm climate type and date from a period when a great southern continent (Gondwana) was enveloped in ice. This ice melted about 300 million years ago, allowing cool climate coals like most Australian coals to develop over large tracts of Gondwana. Gondwana later broke up to form South America, Africa, India, Antactica and Australia.
Plants are mostly made of carbon, hydrogen and oxygen; these elements are preserved in the organic part of coal. The physical and chemical structure undergoes massive changes in the coal development process, caused by the pressure of later sediments from above, and by geothermal heat from below. Coal also contains impurities in the form of water from plants and swamp, and mineral grains from sand and silt.
Can you comprehend a period of 300 million years? Imagine walking back through time to a place 300 kilometres away, and think of this distance as representing that time. Each millimetre of your journey would represent a year and the width of your hand more than most people’s lifespan.
Within your first half dozen steps civilization would disappear; within a few hundred metres there would be no sign of mankind itself. You would have to walk for 65 kilometres before you would see a dinosaur!
Why coal burns?
Coal burns because it contains large amounts of carbon and hydrogen, the same elements that make wood, oil and natural gas burn. During combustion, these elements combine with oxygen from the air releasing large amounts of energy as heat.
A megajoule (MJ) is a unit of energy, about equivalent to that needed to boil three litres of water. More energy is available if the steam is condensed, but this is rarely practical.
What is coal used for?
Coal can be considered an ore of carbon, which has always been mankind’s major source of “manufactured” heat. Coal can be burnt just for its heat value. When used for this purpose, it is called thermal coal. Coal can also be useful in its own right in some metallurgical processes that also require heat. When used for this purpose it is called metallurgical coal.
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