Coal cleaning offers a way of improving the quality of a coal, both economically and environmentally. The most well-established methods of coal-cleaning focus on removing excess moisture from the coal and reducing the amount of uncombustible material which will remain as ash after combustion. Moisture removal reduces the weight and volume of the coal, rendering it more economical to transport. Ash removal improves its combustion properties and aids power plant performance.
Moisture can be removed from coal by drying. This can simply be solar drying, leaving the coal in the open before transporting it. The alternative, producing heat to dry the coal, is a more expensive option.
Drying coal by heating is most often carried out at the power station, utilising surplus energy in the plant flue gases. Such a procedure is absolutely essential when burning high-moisture lignites such as brown coal. It does not, however, affect the transportation costs because the fuel has, by this stage, already reached the power station.
Ash removal is carried out by crushing the coal into small particles. Incombustible mineral particles are more dense than the coal and can be separated using a gravity-based method. Such treatment will remove some minerals containing sulphur, and can result in a reduction of up to 40% in sulphur dioxide emissions during combustion. (Some sulphur is bound to the carbon in the coal. Such sulphur is not affected by this type of cleaning.)
There have been attempts to develop more advanced methods for coal treatment employing either higher-temperature processing of the coal or chemical rather than physical processes. These have not, so far, found commercial application.
According to the World Bank,2 the cost of cleaning coal using existing technology is between US$1/tonne and US$10/tonne depending on the degree of cleaning required. This type of technology is simple and can be deployed in most parts of the world.
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