Coal ash is a byproduct of coal combustion, created when coal is burned to generate energy. Coal-fired power plants are major producers of coal ash worldwide. In some regions, the components of this byproduct can be re-used in a variety of products, including concrete, while the remainder must be isolated and stored to prevent pollution. Failure to contain the ash properly can lead to catastrophic problems; a coal ash spill in Tennessee in 2008 generated over $1 billion United States Dollars (USD) in damage.
The components of the ash vary, depending on the original source of the coal. The major ingredient is minerals which could not be burned, including an assortment of radioactive isotopes. Bottom ash, found at the bottom of boilers, is a thick, coarse assortment of minerals. Fly ash is much finer, composed of fine particles which have bonded together. Boiler slag is another form of coal ash. In facilities where filters and scrubbing systems are in place, a product known as flue gas desulfurization (FGD) gypsum is also generated and needs to be handled with care.
When the ash is removed from a power generation facility, it is classically stored, often in a landfill. Some of the ash can be sold for other uses, with utilization rates varies, depending on the component. Fly ash, for example, can be very useful, and up to 75% of the fly ash from a power plant may be reused in concrete and other products.
Environmental studies have shown that coal ash can be quite dangerous. It is highly radioactive in some cases, with poor environmental controls which may not entirely prevent contamination. Unlike a nuclear facility, which is carefully monitored and must follow a series of laws to limit radiation exposure, a coal fired power plant can release radioactive material into the surrounding environment and handle its ash without safety measures comparable to those used to control nuclear waste. The ash also contains dioxins and other toxins which can be dangerous when released into the environment.
Coal ash disposal is a serious environmental concern in some regions of the world. The problem has been inadvertently complicated by environmental laws mandating more trapping of pollutants; the collection of pollutants in the stacks of power plants prevents them from being released into the environment, but generates more ash which must be properly disposed of. Some nations have initiated government programs to monitor coal fired power plants and develop recommendations for handling byproducts of combustion to keep the surrounding environment cleaner.