A Brief Overview of Coal: Part Three – Reserves & Production, Mining & Preparation
It is estimated that, in 2011, there were around 850 billion tonnes of coal reserves (of all types) worldwide. The regions with the biggest coal reserves are, in order: North America, Russia and countries of the former USSR, China, Asia Pacific and India.
Total global coal production in 2011 is estimated to have been 7.8 billion tonnes. Using these numbers we get a global reserves to production ratio (“R/P”) of 109; that is, coal reserves are sufficient to last 109 years at current rates of production. The R/P ratios are much higher for North America, Europe and Eurasia at >200 years, than for China and the Asia Pacific at around 50 years.
R/P should be taken as a minimum as it does not take account of: improvements in technology that allow more efficient extraction; improvements in the efficiency of electricity production; conversion of resources to reserves and new exploration discoveries.
A probable or proven coal reserve is a coal deposit that can be economically recovered. A coal resource, on the other hand, is a measure of the deposit, but insufficient work has been completed to determine whether the coal can be extracted economically.
It is expensive to convert resources to reserves so generally only enough of a deposit is taken to reserve status to enable financing and/or to provide a pre-determined mine life. In many cases it is expected that resources will be able to be converted to reserves when required. In 2011 global coal resources amounted to 17 trillion tonnes, 20 times larger than resources.
Coal seams that are not part of a resource or reserve are known to exist in many parts of the world. In time, resources and reserves can be expected to be defined for some of these seams.
In conclusion, the global R/P ratio is likely to be very much higher than 109.
In 2011 the top coal producers were: China (3,471Mt), USA (1,004Mt), India (585Mt), Australia (424Mt), Indonesia (376Mt), Russia (334Mt), South Africa (253Mt), Germany (189Mt), Poland (139Mt), and Kazakhastan (117Mt).
Steaming coal formed the largest proportion of production in China, US, India, Indonesia, South Africa, Russia and Kazakhastan. Australian production was evenly spread between steaming and coking coal. Other significant coking coal producers were China, USA and Russia. The largest brown coal producers were Germany, China and Russia.
Out of the total coal production of 7.8 billion tonnes in 2011, only about 1.1 billion tonnes was traded, of which 861Mt was steaming coal and 276Mt coking coal. The biggest exporters are Indonesia, Australia, Russia and USA. The biggest importers were China, Japan, South Korea and India.
Coal, because it is often nearly flat-lying, is amenable to mining methods that often cannot be applied to other types of mineral deposit. Coal is mined in three ways; open cut, highwall and underground.
When coal is near the surface it is generally mined by an open pit method known as strip mining. In strip mining, overburden is removed to expose the coal. The coal is then removed and the pit moves forward. As more overburden is removed to expose the coal seam, it is placed into the area from which the coal has already been removed. This overburden is then covered with topsoil and rehabilitated.
Open pit mining recovers the highest proportion of coal, often up to 90%. Worldwide, about 40% of coal is mined by open pit, in Australia it is closer to 80%.
Highwall mining is a relatively recent method that is used to mine coal that would be uneconomic to mine by other methods. It is used where a coal seam outcrops, such as in a cliff face or at the edge of a completed open pit.
A remote-controlled highwall mining machine is driven into the coal seam, creating a series of “tunnels”, separated by “pillars” that are left to ensure the roof does not collapse. Highwall mining machines can mine well over 1,000 metres into a coal seam and can mine seams less than a metre thick. Modern machines can be very productive, capable of mining around 250,000 tonnes per month.
The two main underground methods of coal mining are “longwall and “room and pillar”.
In long wall mining a large “panel” of the coal bed, up to a million tonnes or so, is first surrounded by drives or passageways. A continuous mining machine is protected by hydraulic roof supports that automatically move with the machine as it advances. The machine shears the coal off the active face of the panel, the coal then passes onto conveyors and thence to the surface. As the machine and its supports advance the roof collapses behind them.
Longwall mining is often thought to be a modern method of coal mining but was actually developed in England in the 17th century. It is low cost and efficient, with up to 80% of the coal being recovered, compared with 60% or less with room and pillar.
Room and pillar mining is considered “conventional”, but as we saw above longwall has been around for several hundred years. When the coal has been extracted, the void that remains is known as a “room” and the blocks of coal left behind to support the coal seam are known as “pillars”. At completion of mining the pillars can be extracted by “retreat” mining – as they are removed the roof collapses. This is a precision operation and can be very dangerous.
The objective of coal preparation is to remove all rock and other contaminants and to size the coal to specifications required by the customer. Coal preparation plants are also known as coal washing or coal beneficiation plants. The first stage of processing is crushing run of mine ore (“ROM”) to around 50mm and then screening to separate the very fine coal particles.
Coal has a relatively low density compared with the contaminants. Therefore, by creating a water/magnetite medium (a suspension of dense magnetite [an iron oxide] in water) with a density between the two, coal floats to the top of the medium and the contaminants sink, thus separating the coal from the contaminants. Finally, the coal is dewatered and stockpiled.