Coal Geology

Coal is a combustible sedimentary 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 commonly called ‘coalification’. Coal occurs as layers or seams, ranging in thickness from millimetres to many tens of metres. It is composed mostly of carbon (50–98 per cent), hydrogen (3–13 per cent) and oxygen, and smaller amounts of nitrogen, sulphur and other elements. It also contains water and particles of other inorganic matter. When burnt, coal releases energy as heat which has a variety of uses. Coal is broadly separated into brown and black which have different thermal properties and uses.

Brown coal (lignite) has a low energy and high ash content. Brown coal is unsuitable for export and is used to generate electricity in power stations located at or near the mine.

Black coal is harder than brown coal and has a higher energy content. In Australia anthracite, bituminous and sub-bituminous coals are called black coal whereas in Europe, sub-bituminous coal is referred to as brown coal.

Thermal (steaming) coal is black coal that is used mainly for generating electricity in power stations where it is pulverised and burnt to heat steam generating boilers.

Metallurgical (coking) coal is black coal that is suitable for making coke, which is used in the production of pig iron. These coals must also have low sulphur and phosphorus contents, and are relatively scarce and attract a higher price than thermal coals.

Coke is a porous solid composed mainly of carbon and ash and is used in blast furnaces that produce iron.

Coal reserves are discovered through exploration. Modern coal exploration typically involves extensive use of geophysical surveys, including 3D seismic surveys aimed at providing detailed information on the structures with the potential to affect longwall operations, and drilling to determine coal quality and thickness.

Mining

Coal is mined by both surface or ‘opencut’ (or opencast) and underground or ‘deep’ mining methods, depending on the local geology of the deposit. Underground mining currently accounts for about 60 per cent of world coal production but around 80 per cent of Australia’s coal is produced from opencut mines. Opencut mining is only economic when the coal seam(s) is near the sur face. It has the advantage of lower mining costs and it generally recovers a higher proportion of the coal deposit than underground mining, as most seams present are exploited (90 per cent or more of the coal can typically be recovered).

Technological advancements have made coal mining today more productive than it has ever been. Modern large opencut mines can cover many square kilometres in area and commonly use large draglines to remove the overburden and bucket wheel excavators and conveyor belts to transport the coal. Modern equipment and techniques allow opencut mining to around 200m. Many underground coal mines in Australia use longwall mining methods, which enable extraction of most of the coal from a seam using mechanical shearers. The mining ‘face’ can be up to 250m long. Self-advancing, hydraulic powered supports temporarily hold up the roof while the coal is extracted. The roof over the area behind the face, from which the coal has been removed, is then allowed to collapse. Over 75 per cent of the coal in the deposit may be extracted using this method (World Coal Institute 2009).