Why most coal avoids a bath

15 July 2014

While metallurgical coal is usually washed, most thermal coal is not. Photo of coal washing by Xstrata Technology via Wikimedia Commons

Coal washing removes impurities in the rock, improving quality and price while reducing eventual emissions. So why is most coal not washed?

Coal washing is known as preparation, processing or beneficiation when it is combined with crushing the rock. Different processes exist for cleaning coal, most of them based on the difference in density between coal and other, heavier rock, although finer-size coal can be cleaned by flotation. The most common way to wash coal is by (usually magnetite-based) dense media separation, in which crushed raw coal is introduced into cyclones or a bath, where the heavier rock falls to the bottom while the lighter coal floats and then is removed for drying.

But whichever form is used, coal washing consumes energy and water and adds to the producer’s cost. In China, for instance, washing contributes to the 18% of total national water use that goes to coal, the second-largest source of water consumption after agriculture.

Why wash coal?

Coal is a sedimentary rock made from buried vegetation, transformed through the action of pressure and temperature over tens or hundreds of millions of years. But not just the organic material becomes coal: the vegetation was usually accompanied by inorganic material, impurities in the form of mineral matter, also commonly known as ash, which forms as part of the coal. Other impurities get added during the stripping process while mining. The proportion of ash in coal is very variable, from less than 10% in high-quality coal to more than 40%.

Ash has several negative effects. It raises transportation costs per energy unit because the ash (which has no useful heating value) gets transported as part of the coal; it cuts power plant efficiency by hampering heat transmission; and it complicates plant operation and maintenance because of corrosion, fly and bottom ash removal, etc. Higher ash contents also lead to a greater variety of pollutants, while the lower coal-burning efficiency increases CO2 emissions.

So removing ash through coal washing improves product quality, and hence prices, and it saves money in transportation and end-use at the consumption point.

The costs and benefits of coal washing

Metallurgical coal, with its higher quality specifications, generally must be washed. But while customers want coal of a certain quality – and consistent quality is as important as quality itself – most thermal coal is not. Why not?

To begin with, coal and ash discrimination is not perfect, so the result is two fractions with higher and lower calorific value than the original raw coal. Unless a nearby power plant can burn the rejected fraction, part of the coal’s energy is lost. While it is difficult to assign a number to that rejection fraction, it tends to range from 5% to 20%. If unburned, this fraction must be disposed of in an environmentally friendly manner, which can be problematic. And for economic and energy-related reasons, especially in places (such as India) with a coal shortage, the energy in the rejected fraction is an issue.

For example, raw coal of 4 000 kilocalories per kilogram (kcal/kg) and 38% ash can produce two fractions: fourth-fifths of the raw coal that now has 4 500 kcal/kg and 30% ash, and the remaining one-fifth with 2 000 kcal/kg and 70% ash. If the poorer fraction is not burned, 10% of the raw coal’s energy is lost. Assuming USD 50 per tonne as variable mining costs and USD 5 per tonne as washing costs, the cost of washed coal is 37% higher on a tonnage basis and 22% higher on an energy basis.

Consequently, washing coal that does not need to travel far is a complex issue. A policy or regulatory framework that requires internalisation of externalities (such as emissions) would help promote the use of cleaner coal, with positive impacts on plant efficiencies, emissions and the environment.

This article originally appeared in IEA Energy: The Journal of the International Energy Agency. Through the end of 2014, the IEA regularly produced IEA Energy, but analysis and views contained in the journal are those of individual IEA analysts and not necessarily those of the IEA Secretariat or IEA member countries, and are not to be construed as advice on any specific issue or situation. Click here to view issues of IEA Energy. Carlos Fernández Alvarez, the author of this article, joined the IEA in 2010 with more than 20 years of experience in the energy sector. He is also the lead author of the annual IEA publication Medium-Term Coal Market Report