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4
Storage
4.1 Introduction
Coal storage in stockpiles is essential in ensuring continuous supply of feedstock for large capacity power units. In fact, it has been estimated that coal stockpiles at electric power plants in the United States in March 2012 (EIA, 2012) were approximately 196 million tons, almost 18% w/w above the level in March 2011 and above the five-year range. Coal stockpile levels typically decline during summer and winter months as power plants burn through stocks to meet seasonal peak electric demand for cooling and heating load, respectively. However, mild weather during the winter of 2011/2012 combined with decreasing natural gas prices decreased the demand for coal-fired electricity. There are disadvantages to storing large quantities of coal because of the characteristics of coal and important problems may emerge because of its long time storing in open areas.
By way of explanation, there are other forms of coal storage – while coal is typically stored in stockpiles, which are piles or storage locations for the bulk materials. More controlled stockpiles are used in many different areas and are formed using stackers to form piles along the length of a conveyor and reclaimers to retrieve coal when required for product loading. On the other hand, a coal bin or a coal bunker is a storage container for coal awaiting use or transportation. This can be either in domestic, commercial, or industrial premises, or on a ship or locomotive tender, or at a coal mine or processing plant. Domestic coal bunkers are associated with the use of coal in open fires or for solid-fuel central heating. Free-standing bunkers were commonly made of wood or concrete and are currently sold in materials including plastic or galvanized metal. Coal bins or bunkers could also be partly or fully underground. Coal bins formed part of industrial plants and were also used for coal storage on steam ships.
A stockpile (stockpiling machine, commonly referred to in this text as a stacker) is a bulk stockpiling machine that is used to stockpile coal either at the mine site before transportation or at the power plant before use. The stockpiling machines come in different shapes according to specific requirements and the function of the machine is to pile or stack the coal on to a stockpile for later reclamation and use – a reclaimer is generally used to reclaim or recover the stockpiled material. The stockpiling unit typically moves on a rail between stockpiles in a yard and usually has three directions of movement, depending on requirements (i) horizontally along the rail, (ii) vertically by raising or lowering the boom, sometime referred to as luffing, and (iii) slewing, which is rotation of the stacker around its central axis and may not always be a requirement.