Bottom ash is a byproduct of coal burning, with disposal options ranging from recycling to landfilling. It can be used as an aggregate in building products, but may contain dangerous partial combustion products like dioxins. Careful handling and disposal is necessary to prevent environmental risks.
Bottom ash is a byproduct of coal burning. The largest producers of bottom ash are coal-fired power plants, which burn a very large volume of coal each year to generate electricity. There are several disposal options for this substance, ranging from recycling to landfilling, and various industry advisory councils and advocacy groups around the world promote disposal methods considered more environmentally friendly.
Many combustion by-products are generated when coal is burned. Bottom ash and a component known as fly ash are made up of coal components that have not burned during the combustion stage. The fly ash is light enough to be carried up the flue with the flue gases and ideally trapped in the filters before reaching the environment. The bottom ash forms clinkers on the kiln wall, which eventually fall to the kiln floor.
The composition of the ash left after the coal has been burned depends on the impurities present in the coal when it was mined and processed for use as fuel. As a general rule, power companies try to use as pure coal as possible as fuel, so that it burns efficiently. Impurities that do not burn reduce overall efficiency and generate more bottom and fly ash that must be disposed of, increasing plant operating costs.
Some kilns grind the ash and pump it out using a water system, while others are designed for dry ash handling. Once removed, the ashes can simply be disposed of in a landfill equipped to manage them, or they can be sold to companies that deal with recycling and reclamation activities. Bottom ash can be used as an aggregate in building products such as concrete, for example, and in the production of concrete blocks.
As long as bottom ash is handled with care, it shouldn’t pose an environmental risk. However, it may contain partial combustion products such as dioxins, making it dangerous. In the event of landfill, it is important to dispose of it in a facility equipped to prevent leaching, ensuring that toxins do not enter the environment. If used as aggregate it should only be used in materials for which it has been deemed an appropriate aggregate by building authorities and should be tested to confirm that it will be safe for use.
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