+61 7 4943 8444

What is Coal?

Coal Basics

Coal is a combustible, sedimentary, organic rock formed from ancient vegetation, which has been consolidated between other rock strata and transformed by the combined effects of microbial action, pressure and heat over a considerable time period. This process is referred to as 'coalification'.

Layered between other sedimentary rocks, coal is found in seams ranging from less than a millimetre in thickness to many metres.

Coal is composed mainly of carbon (50-98%), hydrogen (3-13%) and oxygen, and smaller amounts of nitrogen, sulphur and other elements. It also contains a little water and grains of inorganic matter that remain as a residue known as ash when coal is burnt.

How was Coal Formed?

Initially peat, the precursor of coal, was converted into lignite or brown coal - coal types with low organic 'maturity'. Over many more millions of years, the continuing effects of temperature and pressure produced additional changes in the lignite, progressively increasing its maturity and transforming it into the range known as sub-bituminous coals.

Coal Rank

The degree of 'metamorphisrn' or coalification undergone by a coal, as it matures from peat to anthracite, has an important bearing on its physical and chemical properties, and is referred to as the 'rank' of the coal.

Low rank coals, such as lignite and sub-bituminous coals, are typically softer, friable materials with a dull, earthy appearance; they are characterised by high moisture levels and a low carbon content, and hence a low energy content.

Higher rank coals are typically harder and stronger and often have a black vitreous lustre. Increasing rank is accompanied by a rise in the carbon and energy contents and a decrease in the moisture content of the coal. Anthracite is at the top of the rank scale and has a correspondingly higher carbon and energy content and a lower level of moisture. Between anthracites and peat there are three broad coal rankings.

Bituminous coals are dense black solids, frequently containing bands with a brilliant lustre. The carbon content of these coals ranges from 78 to 91 percent and the water content from 1.5 to 7 percent.

The major NSW and Queensland deposits are bituminous and many are suited to the production of metallurgical coke. Non-coking bituminous coals are used for power generation, cement making and to provide heat and steam in industry.

Sub-bituminous coals usually appear dull black and waxy. They have a carbon content between 71 and 77 percent and a moisture content of up to 10 percent and are used for electricity generation or can be converted to liquid and gaseous fuels.

Queensland, NSW, Tasmania, South Australia and Western Australia have deposits of sub-bituminous coals.

The lower ranked coals are browner and softer. Brown coal or lignite has a high oxygen content (up to 30 percent), a relatively low carbon content (60-75 percent on a dry basis), and a high moisture content (30-70 percent).

Brown coals, found in Australia in Victoria's Latrobe Valley, are used for power generation but generally are uneconomic to transport because of their high moisture content. These coals are also susceptible to spontaneous combustion.

Coal Types

Geologists also classify coal types according to the organic debris, called macerals, from which the coal is formed. Macerals are identified (microscopically) by reflected light - the reflective or translucent properties of the coal indicating the individual component macerals and the way they have combined to form the coal.

The purpose of classifying coal in this way is to determine its best uses. There is a finite supply of the resource and therefore, type and chemical composition must be matched to the most suitable end use.

The mineral or inorganic content of coal is another significant factor affecting end use. Mineral content is assessed by burning coal and measuring the amount of incombustible material remaining, referred to as the ash content of coal.

Source - http://www.worldcoal.org/coal/what-is-coal/
Source: Coal, Power for Progress - World Coal Institute