Coal Prep – Art Or Science?
Coal Bin article by Harold Hough 

            To some, coal preparation is simple science – cleaning, blending, and removing waste materials.  However, in reality, given the numerous types of coal, the labyrinth of government regulations, and the differing requirements of coal users, the final product of the coal preparation process more closely resembles a artfully blended bottle of Burgundy wine.
            Contrary to common perceptions, raw coal is a complex mineral that contains a variety of impurities, which the user doesn’t want. Coals may contain from below 3% to over 40% of ash-forming constituents and from below 0.5% to over 5% of sulphur.  Some types of coal also include up to 60% water.  There are also other impurities like mercury and other trace elements that coal powered plants don’t want to release into the atmosphere.
            Coal prep consists of several steps, but usually includes reducing the size of the coal in order to release many of the impurities like ash and pyrites.  The process separates of some of these impurities, primarily by using the differences in density between the impurities and coal, and differences in surface wetting properties. How successful this is depends on the specific nature of the coal.
            Although the technology is old, practical coal prep is a complex process.  The coal coming into a coal preparation plant is heterogeneous, which means the coal that comes in the morning may be quite different than that coming in the afternoon, even if it is from the same mine.  That means a coal that is lower in sulfur and amenable to washing at 11 am may be much harder to prep by 2 pm.  Needless to say, constant testing is necessary.
            Properly prepared coal not only helps reduce emissions, it can improve plant life by reducing wear, tear, and corrosion caused by marginal coal. Therefore, coal prep cuts plant maintenance costs.  In addition, clean coal reduces transportation costs because clean coal weighs less, even though it holds nearly the same BTU value.
            But, there is no single “clean” coal.  Different plants use different coal burning and scrubbing technologies.  There are different government regulations throughout the country.  Add to that hundreds of differing coals from different parts of the country and seams that may be used signally or in blends.  No wonder coal prep is not a cut and dried science, but requires the touch of a master and a good computer.
            The customer has four major considerations when it comes to coal and coal prep.  What is the cost of the coal?  How are costs impacted by coal quality variations and potentially different sources?  How secure is the coal supply?  And, what are the environmental regulations relating to emissions and residues disposal?  The situation may be even more complex as one power company will have several coal powered plants with different needs thanks to age and design.
            At the same time, the coal mine has its own concerns, which include: the cost of the mining, cleaning and transporting the coal; the specifications of the purchaser, and what it will cost to meet those standards; how the coal may vary in the future; and long-term market trends.
            The result is that several blends and coal prep levels can meet the needs of the customer.  For instance, a power plant back east can use local coals with higher sulfur levels and more coal prep, or it can use low sulfur coal from out west that don’t require the same type of prep, but has lower BTU values and more transportation costs.  The best blend becomes the one that is cheaper in terms of immediate cost, impact on the plant, emission controls, and long term availability.
            Although computers have made the process easier, coal prep is dependant on the mine, the coal prep plant, and what the customer needs.  Like wine making, there is no “right” answer – just a complex set of factors that make raw coal the energy source that gives us predictable, inexpensive energy. 
            And, if you see a coal prep worker taking a glass of wine after a day of work, now you know why.