Coal Education
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 Americal 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, Inda, Antactica and Australia.
Plants are most made of carbon, hydrogen amd 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 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.
Coal Types
Vast specifications from locations all over the world.
There are several different types of coal, that different properties usually dependent on their age and the depth to which they have been buried under other rocks.
In some parts of the world, coal development is accelerated by volcanic heat or crustal stresses. The degree of coal development is referred to as a coal's "rank," with peat being the lowest rank coal and anthracite the highest.
Peat
Peat is the layer of vegetable material directly underlying the growing zone of a coal forming environment. The vegetable material shows very little alteration and contains the roots of living planets. Peat is widely used as a domestic fuel in rural parts of Scotland and Ireland.
Lignite
Lignite is geologically very young (upwards of around 40,000 years). It is brown and can be soft and fibrous, containing discernible plant material. It also contains large amounts of moisture (around 70%) and so has a low energy content: around 8 to 10 megajoules per kilogram. As the coal developes it loses its fibrous character and darkens in color.
Black coal
In Australia, black coal ranges from Cretaceous age (65 to 105 million year ago) to mid Permian age (up to 260 million years ago). They are all black; some are sooty and still quite high in moisture (sub-bituminous coal), which is sometimes termed a "black lignite."
Coals that get more deeply buried by other rocks lose more moisture and start to lose their oxygen and hydrogen. These coals are harder and shinier (bituminous coal), which have energy contents around 24 to 28 megajoules per kilogram. These coals generally have less than 3% moisture.
Anthracite
Anthracite is a hard, black, shiny form of coal that contains virtually no moisture and very low volatile content. Becuase of this, it burns with little or no smoke and is sold as a "smokeless fuel". Anthracites can have energy contents up to about 32 megajoules per kilogram, depending on the ash content.
Coal rank has little to do with quality; as a coal matures its ash content actually increases as a proportion because of the loss of moisture and volatiles. Lower rank coals may have lower energy contents, but they tend to be more reactive (they burn faster) because of their porosity and resultant higher surface area.