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What is Coal?

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Published Date Written by Sharon Banks

Coal is a combustible sedimentary rock composed mostly of carbon and hydrocarbons. It is a natural resource that is used to produce energy, both in the form of heat and electricity, but it is a non renewable resource.

The energy in coal comes from the energy stored by decayed matter. Over time, millions of years, dead plant matter is covered by layers of water and dirt that traps this energy. The heat and pressure from the top layers, undergoing chemical and physical changes, transforms these remains into coal. Coal is a complex resource and can vary in composition even within the same deposit. Each type or ranking level of coal has different energy outputs, generally as a result of increased pressurization, heat, and time. The main coal categories are, low-rank (lignite), low-rank (sub-bituminous), medium-rank (bituminous) and high-rank (anthracite).

Two coal types are representative of South African coal deposits, i.e. multiple seam and thick inter-bedded seam deposit types. South African coal resources are predominantly bituminous in rank, followed by some anthracitic coals and negligible lignites.

Lignite – This is a brownish-black coal with high moisture and ash content, it has the lowest heating value of the coal types. It is considered an “immature” coal that is still soft.

Sub-bituminous coal – This is a dull black coal with a higher heating value than lignite.

Bituminous coal – It is the most commonly used type of coal for electric power generation. It is a dark, hard coal that has a higher heating value than lignite and sub-bituminous coal, but a lower heating value than anthracite.

Anthracite – This is also known as "hard coal" that was formed from bituminous coal under increased pressures in rock strata during the creation of mountain ranges.  It is very hard and shiny. This type of coal is the most compact and therefore has the highest energy content of the coal ranks.

Thermal coal is used in power generation whilst coking coal is used in steel making. Thermal (steam) coal and coking (metallurgical) coal have similar geologic origins.

Thirty six percent of electricity generated worldwide is produced from coal. In modern highly efficient systems the coal is pulverised, increasing its surface area and hence the rate of combustion. The powdered coal is blown into a boiler, where water is converted into steam. The high pressure steam is passed into a turbine. At the end of the turbine shaft is the rotor, consisting of carefully wound wire coils, electricity is generated when these coils are rapidly rotated in a strong magnetic field (the stator). The electricity is transformed into the higher voltages used for economic transmission via power line grids (400,000 volts and above). Near the point of consumption, the electricity is stepped down to the safer 100-250 voltage systems used in the domestic market.

Coke is made by baking a blend of selected bituminous coals in special high temperature ovens without contact with air until almost all of the volatile matter is driven off. The resulting product, coke, consists principally of carbon.  Some 70% of total steel production is based on the smelting of iron ore in blast furnaces and the subsequent refining of the iron into steel, mainly in Basic Oxygen Furnaces (BOF). A blast furnace typically uses iron ore, coke (made from coal), small quantities of limestone, and, where Pulverised Coal Injection (PCI) is employed, pulverised or granulated thermal coal.

South Africa is the fifth largest coal producing country in the world. Presently, about 77% of our country's primary energy needs are provided by coal. South Africa's coal reserves are estimated at 53 billion tonnes, and with our present production rate there should be almost 200 years of coal supply left. Waste problems of coal include sulphur and nitrogen oxides, organic compounds, heavy metals, radioactive elements, greenhouse gases and a lot of ash.

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