Coal (Batubara)

INTRODUCTION

Coal is a combustible rock of organic origin composed mainly of carbon (50-98 per cent), hydrogen (3-13 per cent) and oxygen, with lesser amounts of nitrogen, sulphur and other elements. Some water is always present, as are grains of inorganic matter that form an incombustible residue known as ash.

Coal is classified by rank, which is a measure of the amount of alteration it has undergone during formation. Consecutive stages in evolution of rank, from an initial peat stage, are brown coal (or lignite), sub-bituminous coal, bituminous coal, and anthracite. Increase in rank is due to a gradual increase in temperature and pressure that results in a decrease in water content and therefore an increase in carbon content. A continuous gradation occurs between these ranks. Sub-bituminous coal, bituminous coal and anthracite are together known as black coal.

The Industrial Revolution that began in Britain in the early 19th century was fuelled by coal. Then, in the 1950s and 1960s, it was eclipsed by petroleum as the world's most used fuel, but the oil shocks of the 1970s resulted in a worldwide resurgence of interest in coal as an energy source because of its relative abundance.

OCCURENCE

Coal is formed from accumulated vegetable matter that has been altered by decay and by various amounts of heat and pressure over millions of years. The progressive transformation of coal is referred to as coalification. Interlayered with other sedimentary rocks, it forms beds ranging from less than a millimetre to many metres thick. Such a bed, or several beds separated by thin layers of shale, siltstone or sandstone (dirt bands or partings), constitute a coal seam. Major coal deposits have been formed in nearly every geological age since the Carboniferous (350-250 million years ago). The considerable diversity of coal type (organic composition), grade (mineral matter content) and rank (degree of coalification) depends on the differences in the mode of formation. Coal is widely distributed in the world being located on every continent and in over 70 countries. In Australia coal occurs in all States and the Northern Territory.

BLACK COAL

Black coal is so called because of its black colour. It varies from having a bright, shiny lustre to being very dull and from being relatively hard to soft. It has higher energy and lower moisture content than brown coal. Most are of Permian age (about 250 million years old), but lower-rank, younger deposits of Triassic, Jurassic and Cretaceous ages are also important. Permian black coal from New South Wales and Queensland is exported in large quantities to Japan, Europe, South-East Asia, and the Americas.

RESOURCES AND DEPOSIT

In New South Wales black coal is mined near the eastern and western edges of the Sydney-Gunnedah Basin, where the seams are relatively close to the surface. Mining centred along the western edge, where most of the production is from underground mines, is in the Wollongong-Appin-Bulli area and in the Lithgow-Mudgee area. Mines near the eastern edge of the basin are spread along the Hunter Valley from Newcastle in the south to Muswellbrook in the north; many of these mines are open-cut. Further north mining also occurs at Yarrawonga near Gunnedah. Southern Coalfield mines such asAppin , Tahmoor and Metropolitan produce mainly coking coal.  Mines such as Ulan and Springvale in the Western Coalfield and Mandalong and Westside in the Newcastle Coalfield produce mainly thermal coal.  In the Hunter Valley Coalfield both soft coking and thermal coal products are produced from mines such as Hunter Valley Operations and Bulga.

In Queensland, the coal industry has grown rapidly from the early 1970s. Most production has been of export coking coal from mines such as Goonyella and Kestrel in the Bowen Basin. However, thermal coal mines such as those at Newlands and Rolleston and mines near Brisbane have increased the proportion of steaming coal that is mined and exported. The Bowen Basin extends south from Collinsvilleto beyond Blackwater and Moura . A locally important thermal coal mine operates in the Callide Basin. Significant thermal coal resources occur in the Surat and Moreton Basins between Wandoan and Millmerran and mines are operating at Wilkie Creek , New Acland and Commodore . During the 1990s Pulverised Coal Injection (PCI) coals increased in importance with a number of PCI mines now operating in the Bowen Basin including Jellinbah East and Coppabella.

Other locally significant black coal mines include Muja and Premier (Western Australia), Leigh Creek (South Australia) and Duncan (Tasmania).

In 2009 economically recoverable black coal resources were reported to be 43.8 billion tonnes with over 96% of these resources in New South Wales and Queensland. Australia has about 7% of the world's economically recoverable black coal and ranks fifth behind USA (31%), Russia (22%), China (14%) and India (8%).

MINING

In open-cut mining, rock covering the coal seam (the overburden) is blasted and removed by large draglines and/or electric or hydraulic shovels and trucks. Modern equipment and techniques allow open-cut mines to be operated to depths much greater than the 60m that was considered a maximum depth for many years. The use of advanced methods is typified in Queensland at the Goonyellamine where the upper section of overburden is removed by bucket wheel excavator followed by a truck and shovel operation that removes more overburden before the deeper overburden is stripped by dragline.

Underground coal mining in Australia is done by either the bord and pillar or longwall method. In bord and pillar mining, coal is extracted in a series of parallel tunnels (bords) cut at right angles by another series (cut-throughs). This leaves blocks of coal known as pillars which may be extracted in a second stage of mining. Longwall mining results in large blocks of coal being totally extracted and the mine roof allowed to collapse behind the working face. Generally, longwall techniques result in higher productivity and higher recovery of coal than does the bord and pillar method.

Highwall mining was introduced into Australia in the early 1990s and uses the void left by open-cut mining to employ remote underground mining equipment to extract coal. Australia's first punch longwall mining operation commenced in the late 1990s using conventional longwall equipment to mine coal from blocks developed directly from an open-cut final highwall.

In 2009 economically recoverable black coal resources were reported to be 43.8 billion tonnes with over 96% of these resources in New South Wales and Queensland. Australia has about 7% of the world's economically recoverable black coal and ranks fifth behind USA (31%), Russia (22%), China (14%) and India (8%).

PROCESSING

Black coal may be used without any processing other than crushing and screening to reduce the rock to a useable and consistent size. However, it is often washed to remove pieces of rock or mineral that may be present. This reduces ash and improves overall quality. Washing involves immersing the crushed coal in a liquid of high specific gravity in which coal floats and can be recovered while the heavier rock and minerals sink and are discarded.

USES

The major use of black coal is for generating electricity in power stations, where it is pulverised and burnt to heat steam-generating boilers. Coal used for this process is called steaming coal. In 2008 77% of the electricity generated in Australia was produced by coal fired power stations (includes black and brown coals).

Some types of black coal are suitable for coke-making. These coals are called coking coal and when heated in the absence of air produce gases and coke. Coke is a porous solid composed mainly of carbon and ash. Good quality coke is hard, has a high crushing strength, and is mainly used in blast furnaces that produce iron. Many organic chemicals, including tars and feedstocks for making various plastics, can be prepared from the by-products of coke and gas production. Some coal is used primarily to obtain these products.

PCI coal is used in blast furnaces as a way of lowering operating costs and extending the life of coke ovens. Black coal is also used as a heat source in the manufacture of cement and food processing.

BROWN COAL

Brown coal, sometimes called lignite, is a relatively soft material which has a heating value only about one-quarter of that for black coal. It has a much lower carbon content than black coal and a higher moisture content. Where found near the surface in thick seams, it can be mined economically on a large scale by open-cut methods.

RESOURCES AND DEPOSIT

Australian brown coal deposits are Tertiary in age and range from about 15 million to about 50 million years old. The main deposits are in Victoria, the only State that produces brown coal. In the Latrobe Valley in Gippsland, thick seams, up to 165 m, form part of a sequence of brown coal measures that is the basis of Victoria's electric power industry. Smaller deposits occur in the Bacchus Marsh, Altona and Anglesea areas of Victoria, in the St Vincents and Murray Basins and around Pidinga in South Australia, in the Murray Basin in New South Wales and Victoria, at Waterpark Creek near Rockhampton in Queensland, and at Scaddan, O'Sullivans and Balladonia in the south east of Western Australia.

In 2009 Australia's economically recoverable brown coal resources were reported to be 37 billion tonnes, all of which is in Victoria and with over 90% in the La Trobe Valley. Australia has about 24% of the world's economic resources and is ranked first.

MINING

The La Trobe Valley mines of Yallourn, Hazelwood and Loy Yang extract brown coal from large open-cut mines utilising giant bucket-wheel excavators, or dredgers, which may weigh several thousand tonnes. The coal is loaded onto conveyor belts for delivery to power stations. However, in a recent development the dredgers at Yallourn have been replaced by four large dozers. At Anglesea, Alcoa of Australia Ltd operates an open-cut mine to provide brown coal for its power station. This power station provides most of the electricity for the company's aluminium smelter at Point Henry. The small Maddingley mine near Bacchus Marsh produces a horticultural product.

Annual brown coal production is about 68 million tonnes, all from Victoria and with over 98% from the La Trobe Valley. Australia produces about 7% of the world's brown coal and is ranked fifth largest after Germany (21%), Russia (10%), Turkey (9%) and USA (8%).

USES

In Victoria, almost all of the brown coal extracted is burnt to heat steam-generating boilers in electrical power stations located near the coal mines. It is also made into briquettes, which are used for industrial and domestic heating in Australia and are also exported. Brown coal can also be used to produce water gas, which is used in the production of ammonia, solvents, and liquid fuels, and can be a source of industrial carbon, used to decolourise and purify solutions and (as char) in iron, glass, and cement manufacture.