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Management of coal combustion wastes

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World Coal,

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.

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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.

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Coal combustion waste management and its environmental impacts are influenced by the characteristics of the waste. Depending on the mineral components of coal, the combustion technique, and the pollution control technology used, one or more types of coal combustion products, such as fly ash, bottom ash, boiler slag, FBC ash or FGD residues, are produced. The physical and chemical properties of each type of ash are influenced by the coal source and its quality, combustion process, co-fired materials, air emission control methods, degree of ash weathering and so on. The texture, particle size, colour and bulk density are different for different type of ashes. While the type of ash and its characteristics vary, all coal combustion waste is likely to include certain amounts of toxic constituents, primarily heavy metals, such as aluminium, arsenic, beryllium, boron, calcium, cadmium, chromium, cobalt, iron, lead, manganese, mercury, molybdenum, selenium and silicon. The waste may also include a certain level of toxic organic compounds, such as dioxins and PAH. These toxic substances pose a risk to human health and the environment when coal combustion wastes are disposed of without proper management.

The US EPA put forward a risk assessment to characterise waste management scenarios and found that the majority of the damage and highest potential health risks associated with coal combustion waste involved its deposition into unlined units and/or with an absence of ground water monitoring systems. With a liner and ground water monitoring system in new coal combustion waste disposal facilities, along with practices to prevent co-disposal of coal refuse with coal combustion waste, future risks from onsite coal combustion waste disposal are likely to be lower.

The dangers of leaching from coal combustion wastes
The primary environmental concern for coal combustion waste disposal is leaching, which may contaminate surface and ground water although the seriousness of the environmental impacts from leaching is still being debated. Leaching is a complex process, which depends on many factors, namely chemical speciation of the constituent, solution pH, availability of the constituent for leaching, and age (weathering) of the ash. In addition, subsequent chemical reactions and secondary mineral formation can further modify leaching characteristics. There is a variety of laboratory leaching test procedures. When these are applied to coal combustion waste testing, field conditions known to influence leaching and initial and final site conditions need to be considered. The US EPA has validated a suite of four leaching test (LEAF methods) in order to provide a comprehensive and sound laboratory protocol for evaluating leaching under a variety of conditions. Liner, leachate collection systems, groundwater monitoring systems or pre-treatment technologies should be employed to prevent leaching.

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The US Kingston disaster provided a ‘wake-up call’ about the danger from the release of coal combustion waste due to the structural failure of surface impoundments. Dam safety and planning requirements must be applied to regulate the construction, operation and maintenance of surface impoundments.

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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Over the last decade, a number of changes have occurred globally in the coal-fired power generation sector that have affected the quality and quantity of coal combustion products. The regulations for coal combustion waste management, which are currently under discussion will also have an impact on the cost of coal combustion waste disposal. Coal combustion waste management is facing new challenges.

Written by Xing Zhang, IEA Clean Coal Centre.

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