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Coal is a valuable and plentiful natural global resource
Not only does coal provide electricity, it is also an essential fuel for steel and cement production, and other industrial activities.
Coal is a combustible, sedimentary, organic rock, which is composed mainly of carbon, hydrogen and oxygen. It is formed from vegetation, which has been consolidated between other rock strata and altered by the combined effects of pressure and heat over millions of years to form coal seams. Coal is a fossil fuel and is far more plentiful than oil or gas, with around 118 years of coal remaining worldwide.
WHAT IS COAL?
Coal is a fossil fuel and is the altered remains of prehistoric vegetation that originally accumulated in swamps and peat bogs.
The energy we get from coal today comes from the energy that plants absorbed from the sun millions of years ago. All living plants store solar energy through a process known as photosynthesis. When plants die, this energy is usually released as the plants decay. Under conditions favourable to coal formation, the decaying process is interrupted, preventing the release of the stored solar energy. The energy is locked into the coal.
Coal formation began during the Carboniferous Period - known as the first coal age - which spanned 360 million to 290 million years ago. The build-up of silt and other sediments, together with movements in the earth's crust - known as tectonic movements - buried swamps and peat bogs, often to great depths. With burial, the plant material was subjected to high temperatures and pressures. This caused physical and chemical changes in the vegetation, transforming it into peat and then into coal.
Coalification
The quality of each coal deposit is determined by:
- varying types of vegetation from which the coal originated
- depths of burial
- temperatures and pressures at those depths
- length of time the coal has been forming in the deposit
The degree of change undergone by a coal as it matures from peat to anthracite is known as coalification. Coalification has an important bearing on coal's physical and chemical properties and is referred to as the 'rank' of the coal. Ranking is determined by the degree of transformation of the original plant material to carbon. The ranks of coals, from those with the least carbon to those with the most carbon, are lignite, sub-bituminous, bituminous and anthracite (see 'Coal Types' diagram).
Types of Coal
Initially the peat is converted into lignite or 'brown coal' - these are coal-types with low organic maturity. In comparison to other coals, lignite is quite soft and its colour can range from dark black to various shades of brown.
Over many more millions of years, the continuing effects of temperature and pressure produces further change in the lignite, progressively increasing its organic maturity and transforming it into the range known as 'sub-bituminous' coals.
Further chemical and physical changes occur until these coals became harder and blacker, forming the 'bituminous' or 'hard coals'. Under the right conditions, the progressive increase in the organic maturity can continue, finally forming anthracite.
In addition to carbon, coals contain hydrogen, oxygen, nitrogen and varying amounts of sulphur. High-rank coals are high in carbon and therefore heat value, but low in hydrogen and oxygen. Low-rank coals are low in carbon but high in hydrogen and oxygen content.
Different types of coal also have different uses, as shown in the diagram below.
It has been estimated that there are over 847 billion tonnes of proven coal reserves worldwide. This means that there is enough coal to last us around 118 years at current rates of production. In contrast, proven oil and gas reserves are equivalent to around 46 and 59 years at current production levels.
Coal reserves are available in almost every country worldwide, with recoverable reserves in around 70 countries. The biggest reserves are in the USA, Russia, China and India. After centuries of mineral exploration, the location, size and characteristics of most countries' coal resources are quite well known. What tends to vary much more than the assessed level of the resource - i.e. the potentially accessible coal in the ground - is the level classified as proved recoverable reserves. Proved recoverable reserves is the tonnage of coal that has been proved by drilling etc. and is economically and technically extractable.
Definitions
| Resource | The amount of coal that may be present in a deposit or coalfield. This does not take into account the feasibility of mining the coal economically. Not all resources are recoverable using current technology. |
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| Reserves | Reserves can be defined in terms of proved (or measured) reserves and probable (or indicated) reserves. Probable results have been estimated with a lower degree of confidence than proved reserves. |
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| Proved Reserves | Reserves that are not only considered to be recoverable but can also be recovered economically. This means they take into account what current mining technology can achieve and the economics of recovery. Proved reserves will therefore change according to the price of coal; if the price of coal is low proved reserves will decrease. |
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Over recent years there has been a fall in the reserves to production (RP) ratio, which has prompted questions over whether we have reached 'peak coal'. Peak coal is the point in time at which the maximum global coal production rate is reached after which the rate of production will enter irreversible decline. However, recent falls in the RP ratio can be attributed to the lack of incentives to prove up reserves, rather than a lack of coal resources. Exploration activity is typically carried out by mining companies with short planning horizons rather than state-funded geological surveys. There is no economic need for companies to prove long-term reserves.
All fossil fuels will eventually run out and it is essential that we use them as efficiently as possible. Coal reserves could be extended further through a number of developments including:
- the discovery of new reserves through ongoing and improved exploration activities;
- advances in mining techniques, which will allow previously inaccessible reserves to be reached.
Additionally, significant improvements continue to be made in how efficiently coal is used so that more energy can be generated from each tonne of coal produced.
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| Exploration drilling
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Coal Exploration
Coal reserves are discovered through exploration activities. The process usually involves creating a geological map of the area, then carrying out geochemical and geophysical surveys, followed by exploration drilling. This allows an accurate picture of the area to be developed. The area will only ever become a mine if it is large enough and of sufficient quality that the coal can be economically recovered. Once this has been confirmed, mining operations begin.
Surface Coal Mining Operations & Mine Rehabilitation
Over 6185 million tonnes (Mt) of hard coal is currently produced worldwide and 1042 Mt of brown coal/lignite. The largest coal producing countries are not confined to one region - the top five hard coal producers are China, the USA, India, Australia and South Africa. Much of global coal production is used in the country in which it was produced; only around 15% of hard coal production is destined for the international coal market.
| Top Ten Hard Coal Producers (2010e) |
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| PR China | 3162Mt | Russia | 248Mt |
| USA | 932Mt | Indonesia | 173Mt |
| India | 538Mt | Kazakhstan | 105Mt |
| Australia | 353Mt | Poland | 77Mt |
| South Africa | 255Mt | Colombia | 74Mt |
Source: International Energy Agency 2011
Mining Methods
Coal is mined by two methods:
- surface or 'opencast' mining
- underground or 'deep' mining
The choice of mining method is largely determined by the geology of the coal deposit. Underground mining currently accounts for a bigger share of world coal production than opencast; although in several important coal producing countries surface mining is more common. For example, surface mining accounts for around 80% of production in Australia; while in the USA it is used for about 67% of production.
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| Surface or 'opencast' mining
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Surface Mining
Surface mining - also known as opencast or opencut mining - is only economic when the coal seam is near the surface. This method recovers a higher proportion of the coal deposit than underground mining as all coal seams are exploited - 90% or more of the coal can be recovered.
Large opencast mines can cover an area of many square kilometres and use very large pieces of equipment, including:
- draglines, which remove the overburden
- power shovels
- large trucks, which transport overburden and coal
- bucket wheel excavators
- conveyors
The overburden of soil and rock is first broken up by explosives; it is then removed by draglines or by shovel and truck. Once the coal seam is exposed, it is drilled, fractured and systematically mined in strips. The coal is then loaded on to large trucks or conveyors for transport to either the coal preparation plant or direct to where it will be used.
Underground Mining
There are two main methods of underground mining: room-and-pillar and longwall mining.
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| Room & pillar mining
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Room & Pillar Mining
In room-and-pillar mining, coal deposits are mined by cutting a network of 'rooms' into the coal seam and leaving behind 'pillars' of coal to support the roof of the mine. These pillars can be up to 40% of the total coal in the seam - although this coal can sometimes be recovered at a later stage.
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| Longwall mining
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Longwall Mining
Longwall mining involves the full extraction of coal from a section of the seam, or 'face' using mechanical shearers. A longwall face requires careful planning to ensure favourable geology exists throughout the section before development work begins. The coal 'face' can vary in length from 100-350m. Self-advancing, hydraulically-powered supports temporarily hold up the roof while coal is extracted. When coal has been extracted from the area, the roof is allowed to collapse. Over 75% of the coal in the deposit can be extracted from panels of coal that can extend 3km through the coal seam.
Technological advancements have made coal mining today more productive than it has ever been. To keep up with technology and to extract coal as efficiently as possible modern mining personnel must be highly skilled and well-trained in the use of complex, state-of-the-art instruments and equipment.
Article by: World Coal Association
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