Bottom ash is the coarse, granular, incombustible by-product of coal combustion that is collected from the bottom of furnaces. Most bottom ash is produced at coal-fired power plants. Below is a comparison of fly ash and bottom ash, to show the size difference of the particles and the difference in texture.
When pulverized coal is burned in a bottom boiler, most of the unburned material is caught in the flue gas and captured as fly ash. Only about 10-20% of this ash is bottom ash. This ash is dark grey in colour, and is about the size of sand. This ash is collected in a water-filled hopper at the bottom of the furnace and removed by high-pressure water jets. It is deposited in a collection pond and stored for disposal or later use after recycling.
Along with the health and environmental impacts of bottom ash, one issue with it is that it exists in massive quantities. Although the total coal ash generation varies from year to year depending on the amount and ash content of the coal burned, the amount of coal ash has been increasing since 1996.
Coal ash - which includes bottom ash - can have a different chemical makeup depending on where the coal was mined. Broadly speaking, coal ash is a pollutant, and it contains acidic, toxic, and radioactive matter. This ash can contain lead, arsenic, mercury, cadmium, and uranium. The EPA found that significant exposure to bottom ash and other components of coal ash increases a persons risk of developing cancer and other respiratory diseases. As well, storage lagoons can pollute groundwater and if ingested, the arsenic contaminated water increases a persons risk of developing cancer. Inhalation is not as much of an issue with bottom ash as it is much heavier than fly ash, but ingestion of bottom ash can have nervous system impacts, cognitive defects, developmental delays, and behavioral problems along with increasing a persons chance of developing lung disease, kidney disease, and gastrointestinal illness.
When fly ash and bottom ash are disposed in lagoons, there are associated environmental effects. These wet lagoons can be an issue, because if they do not have proper liners to prevent leaking and leaching then groundwater contamination is much more common. Leaching is a process that occurs when coal ash is wet, and it simply means that the toxic components of the ash dissolve out and percolate through water. This groundwater contamination can be harmful to human heath if the groundwater is a source of drinking water.
About 30% of bottom ash is re-purposed and recycled in a variety of ways. Most recycled bottom ash is used for snow and ice control, as a road base or a structural fill material, or as a raw feed material for some cements. Bottom ash can also be added into hot asphalt, although it is a fairly fine powder and has a low durability. Because of this there is usually a sifting process to collect the larger particles when used in asphalt.
Bottom ash that is not to be recycled is discarded in landfills or storage lagoons. If the bottom ash goes to a storage lagoon, it is generally mixed with fly ash and referred to collectively as ponded ash. About 30% of all coal ash is disposed of when wet as ponded ash. This ash is potentially usable even after it is put in a storage lagoon. Because fly ash and bottom ash have different weights, the heavier bottom ash settles first and the fly ash remains suspended. This ponded ash can be reclaimed if the bottom ash is scraped up and dewatered. The more bottom ash that is in ponded ash, the easier it is to dewater and the more potential it has for reuse. This reclaimed ash can be used in embankment construction or as a filler.