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Managing Coal waste


centuries, coal has been a reliable source of energy; however, despite terms such as “clean coal,” coal is considered “dirty.” Unlike other fossil fuels, coal is a solid and in its rawest form contains significant quantities of incombustible impurities, which must be dealt with before, during, and after mining. Overburden waste, processing refuse, and coal combustion residuals constitute large volumes of material that must be handled and managed. Handling segregates waste material into coarse and fine fractions. Coarse fractions are used to construct impoundments into which fine fractions are pumped. Storage piles quickly become part of the “natural” landscape, but unless properly constructed and maintained, they can have long-term negative impacts on the surrounding environment including generation of geochemical pollutants and creation of geotechnical safety hazards. After defining coal waste and sustainability, traditional coal waste disposal practices are briefly summarized. The remainder of the chapter focuses on sustainable coal waste disposal practices of the future. The first is eliminating slurry impoundments by codisposal of coarse processing waste and dewatered fine processing waste. This practice requires low-cost, efficient dewatering technology for fine waste material, and two contemporary options are discussed. Next, backfilling is explored as an alternative to stowing coal waste in expansive surface landfills. Finally, the sustainability principles of reduce, reuse, and recycle are examined from the coal mining perspective, an approach that can seem counterintuitive unless a holistic viewpoint is taken.

As the organic compounds of coal combust, a variety of solid wastes are generated inside the boiler from unburnt inorganic components and unburnt carbon. These solid wastes are known as coal combustion products. Some coal combustion products can be used beneficially: for example, fly ash is used in cement or concrete production. Coal combustion products that cannot be used will have to be stored or disposed of. These are often referred to as coal combustion wastes.

Coal demand is expected to continue to grow in the emerging economies for the next ten years, driven in particular by the power generation industries in China and India. As a result, the production of coal combustion products will inevitably increase. From 2009 – 2011, only around 53% of coal combustion products were utilized; the rest went to storage or disposal sites. The world total production of coal combustion products in 2010 was approximately 780 million t. If the estimate for 2010 coal combustion products production was correct, there were approximately 360 million t of coal combustion products that needed to be disposed of in 2010 – and this figure is expected to continue (if not increase) for many years.

Coal combustion waste disposal
Coal combustion wastes are disposed of in surface impoundments, landfills, mine and quarry fills and oceans. Ocean disposal is not commonly carried out today. Mine and quarry fill is a way of using the material but only takes a small proportion of coal combustion products production. Coal combustion waste is generally managed in two ways: landfills or surface impoundments. More than half of the coal-fired power plants manage their coal combustion wastes onsite, mostly using surface impoundments. Landfills are commonly used offsite. Onsite surface impoundment has low delivery costs but requires large amounts of water and more land than landfill disposal. It also has a bigger potential for generating leachate. After the catastrophic ash spill from the US Kingston power plant, more attention has been given to dry ash landfills.

Fine dry fly ash can be airborne under windy conditions. However, little has been published about the environmental effects of dust generated at CCW disposal sites. Windblown ash can be controlled by spraying water onto the open ash areas and covering inactive areas at the disposal sites. Although there are concerns about the risks from radioactivity and mercury, no significant damage has been reported.

Future challenges for coal combustion waste management
Coal combustion waste management needs to be guided by legislation. There are no finalised regulations in place although lively debates have taken place over the years in the USA, Europe and South Africa about regulating coal ashes as hazardous waste. At the moment, coal combustion products are generally classified as non-hazardous waste. While the decision on whether coal combustion products are hazardous or not is very important, for coal combustion waste management, the impact is not as big as for coal combustion product utilisation because the liner requirements for hazardous or non-hazardous wastes (Subtitle D and Subtitle C) are identical. From a technical perspective, both classifications demand the same technologies to preventing leaching.

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