COAL CLEANING AND PROCESSING
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 incombustible material, which will remain as ash after combustion. Moisture removal reduces the weight and volume of the coal, rendering it more economical to transport and easier to burn. Ash removal improves its combustion properties and aids power plant performance.
Moisture is removed from coal by drying. This can simply be solar drying— leaving the coal in the open before transporting it. Drying coal in this way can reduce its mass and increase its energy density, making it relatively cheaper to transport.
The alternative—drying coal by heating—is most often carried out at the power station, utilizing waste heat in the plant flue gases. Such a procedure is absolutely essential when burning high-moisture lignites such as brown coal.
In this case, drying does not affect 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 sulfur, and can result in a reduction of up to 40% in sulfur dioxide emissions during combustion. (Some sulfur is bound to the carbon in the coal; such sulfur 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 processes, which are aimed at removing polluting impurities in the coal to make it a cleaner fuel to burn, have not so far found commercial application. Coal-cleaning processes are also being developed to treat coal wastes that have previously been discarded to make them suitable for combustion.