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Coal mining in South Africa

After being for centuries man’s main source of heat, and of energy too in the industrial age, coal suffered a major decline when the oil industry displaced it and the world opted for liquid fuel. Gaseous fuels, initially the by-product of oil, also grew in importance; again, coal stuttered. Oil became the substitute for coal and defeated coal in most of its traditional markets. Oil not only dominated the world’s transport scene; it came to have widespread use in domestic and industrial heating and for power generation. In short, in many countries, oil became the prime source of energy

That situation still prevails: on a global basis, oil and gas are still the most widely-used fuels. But since the oil price explosions of the 1970s, which restored coal as the dominant fuel for power stations and cement works, coal has steadily come back into favour.

Geology

Coal is formed when peat - the residue of decomposed vegetation - is subjected to pressure and temperature over a long time period. Generally, the greater the pressure and temperature,the higher the coal rank or maturity: from peat to lignite to bituminous to anthracite.

Coal reserves in South Africa are found in sediments of Permian age which overlie a large area of the country. They generally occur as fairly thick, flat, shallow-lying coal seams. In the Witbank area of Mpumalanga, which contains extensive coal reserves and is the country’s most productive coalfield, five major coal seams occur at intervals within a sedimentary sequence deposited during a 35-million year geological time period.

Occurrence and production

The coalfields in the country are spread over an area of 700 kilometres from north to south and 500 kilometres from east to west. Generally the rank or carbon content of the coals increases eastwards while the number of seams and their thickness decrease. Thus, Mpumalanga and Northern Province coals are usually classified as bituminous, occurring in seams up to several metres thick, while KwaZulu-Natal coals are often anthracitic and are found in relatively thin seams. The recoverable coal reserves in South Africa amount to about 55 billion tons, equivalent to nearly 11 per cent of the world’s total (this figure excludes low-grade, high-ash content coal which could add as much as 25 per cent to the country’s total reserves). Most of South Africa’s coal is of a bituminous thermal grade; only two per cent is anthracite, and 1,6 per cent coal of metallurgical quality. At current production levels, coal reserves are estimated at 200 years.

South Africa is the third biggest coal producer in the world. South African collieries range in size from small operations with output limited to a few thousand tons of coal per year to Secunda, the world’s largest underground coat mining complex, which has an annual production of about 35 million tons. Almost 90 per cent of the country’s saleable coal is mined in the Mpumalanga.

From the beginning of 2000, the domestic coal industry consistently increased production, albeit very slowly. The local industry is in the throes of consolidation. Anglo Coal increased its stake in Colombian Carbones del Cerrejon, and Lonmin sold its Duiker operations to Glencore. In addition, three of the largest locally based companies, BHP Billiton, Anglo Coal and Glencore have acquired 50 per cent of a large coal block in Colombia.

Mining methods

Around 40 per cent of coal comes from opencast operations, some of which have recovery rates approaching 90 per cent. Coal lying less than 70 metres below the surface is extracted from a progressive series of parallel, long, narrow trenches. Overburden rock and soil lying above the coal seams is blasted and scraped out of the currently mined trench and tipped into the mined out void of the previous trench. Walking draglines with large scraper buckets slung beneath long, crane-type boom arms carry out the stripping operation. The exposed underlying coal seams are drilled and blasted loose and hauled out of the pit by heavy duty trucks. When the coal from all viable seams has been removed and the spoil of the next parallel trench has been deposited in the void, the rehabilitation process begins. The overburden is flattened, the previously stored top soil is spread over it and the area is seeded with a mixture of grasses to return the landscape to its ecological balance.

Three different mining methods are used in underground mines:
The most common technique, accounting for just under half of total production, is the ’bord and pillar’ method. Bord and pillar mining is ideal for relatively shallow deposits where overlying rock pressure is low. Seams are mined leaving in situ coal pillars, which are big enough to support the roof indefinitely, and a chequer-board pattern of mined-out ’rooms’. This method currently permits around 65 per cent of the available coal to be extracted. However, the adoption by several collieries of the ’squat-pillar’ method developed by the now defunct Chamber of Mines Research Organisation (COMRO), and approved by the Government Mining Engineer, will increase extraction rates - especially at depth - through the employment in bord-and-pillar mining of smaller pillars than were previously thought necessary.

When the overlying strata impose no restrictions, ’total-extraction’ mining can take place (though, in reality, somewhat less than 90 per cent is recovered on average) There are two major underground total extraction systems employed in South Africa.

In rib-pillar extraction, a continuous miner machine cuts a roadway up to 1,5 kilometres in length through the coal and five metres in from the edge of the area to be mined. This leaves a five metre-wide band of coal in the form of a long, isolated rib pillar along one side of the tunnel. With the aid of timber or hydraulic props to hold up the now unstable roof, the continuous miner cuts away the rib pillar in a series of curved cutting sweeps. The machine repeats the cycle by mining into the remaining coal area, again cutting a tunnel and leaving a rib pillar.

The other total extraction method employed is longwall mining. Longwalls are usually several hundred metres long and essentially consist of a corridor in which one wall and the roof are formed by steel supports capable of resisting hundreds of tons of pressure from the subsiding mine roof above. The second side of the corridor is formed of coal and is the actual face from which coal is cut. A mechanical coal cutter, bearing two large revolving shearing drums with steel picks, runs the whole length of the coal face on rafts. This cuts into the coal and widens the corridor during each sweep, thus advancing the coal face. The hewn coal falls on to a conveyor and is drawn out of the longwall face.

Hydraulic rams linked to the line of props push the conveyor and coal cutter forward into the newly-mined-out space in the face. In turn, each hydraulic support is then released from its position and hauls itself forward after the advancing face, reinstalling its steel canopy against the recently exposed area of face roof.

Domestic uses

Coal is South Africa’s primary energy source and has been a major stimulus to economic growth and a significant factor in the country’s industrialisation. It provides 88 per cent of commercial energy needs and plays a vital role in meeting liquid-fuel requirements. The South African coal mining industry is fortunate to be supported by a large domestic market as well as having a strong international position. Domestic sales are dominated by electricity generation (53 per cent), synfuels and petro-chemicals (33 per cent), with metallurgical and other users accounting for the remaining 12 per cent.

About 40 per cent of all bituminous coal produced in South Africa is used in the generation of electricity, making this industry the largest single user of coal in the country.

With power generation demand likely to go on rising there is the inevitable question of how long South Africa’s coal reserves will last. Assuming growth rates of between three and five per cent, the coal used by 2030 - when estimates suggest electricity demand is likely to peak at 75 000 megawatts - together with coal dedicated to future use by power stations either operating or under construction in 40 years’ time, amounts to a staggering 30 billion tons, about half of the country’s reserves. By that time today’s power stations will have reached the end of their useful lives and consumed most of the coal reserves of the dedicated collieries.

Inevitably, alternative methods of generating electricity will need to be phased in alongside South Africa’s coal-fired programme. More nuclear power stations are likely to come on stream in the early years of the 21st century and by the year 2030 fast-breeder reactors will probably take up the base-load of electricity production. In the meantime. Eskom has demonstrated with its Lethabo power station, which at times uses coal with an ash content as high as 40 per cent, that low grade coal can be used in power generation, so adding up to 25 per cent to the total of the Republic’s proven coal reserves.

The second most important coal user in the home market is Sasol - the only successful commercial oil-from-coal plant in the world. Sasol uses the Fischer-Tropsch indirect liquefaction method, which has the added benefit of being able to process high-ash content coal - to convert coal into petrol and diesel fuels, and provides raw materials for the petro chemical industries and other important by-products such as fertiliser.

Other significant domestic users are Iscor’s metallurgical plants (the steel industry requires coking coal to be prepared in coke ovens to provide metallurgical coke capable of reducing and melting iron ore to liquid iron in blast furnaces), the cement industry and large municipalities.

In the last few years, offtake by Eskom, the railways and the mining industry has shrunk, while demand from Sasol and the metallurgical sector is flat. Any capacity for local consumption added to the South African coal mining industry in the foreseeable future, therefore, is likely to be replacement, rather than additional, capacity.