Coal Combustion Products (CCPs) is the term used to describe fly ash, bottom ash, boiler slags, cenospheres and flue gas desulfurization materials, being by-products from the production of power within coal-fired power stations. Today, significant industry revenues are generated for use of these materials.
When coal is used as fuel in coal-fired power stations, it is crushed, pulverised, and blown into a combustion chamber where it immediately ignites and burns to heat boiler tubes. The inorganic components, known commonly as coal ash, either remain in the combustion chamber or are carried away by the flue gas stream.
Larger ash particles that accumulate at the bottom of the boiler are defined as furnace bottom ash (FBA). Boiler slag is the molten inorganic material that is collected at the bottom of the boilers and discharged into a water-filled pit where it is quenched and removed as glassy particles (resembling sand).
Fly ash is the fine fraction that remains suspended in the flue gas stream and carried out of the boiler by the flue gases. Fly ash comprises of up to 90% of the total CCPs produced. Prior to the flue gas leaving the stack, the fly ash is removed by electrostatic precipitators or other scrubbing systems such as a mechanical dust collector.
CCPs can potentially be collected at two points; as dry conditioned materials after collection at the power station via mechanical separation systems or once transported to the storage dam, as a wet ash product.
Dry CCPs which have been collected and stored in silos ready for dispatch to processors or customers is preferred. Dry CCPs have fewer material emissions (water and energy) than wet ash. When combined with large quantities of water, CCPs are pumped via pipelines into large collection ponds. Unused CCP's can also be placed in a dry condition. Once these ponds are full, the materials are recovered and transported to a final storage destination within the site, adding more costs and associated emissions.