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Last edited 18 Feb 2019
Fly ash (or flue ash or pulverised flue ash PFA) is a by-product of the coal combustion process, particularly in electricity generating power stations. It is formed when fine particulates of burned coal fuse in suspension and are driven out with flue gases. As they are driven out of the combustion chamber, the fused material cools and forms glassy particles called fly ash. Modern coal-fired power plants use particle filtration equipment to catch the fly ash before it reaches the chimneys.
(NB: Coal ash is the term given to the mixture of fly ash (which rises) and the bottom ash which falls to the bottom of the boiler).
The chemical components of fly ash will depend on the type of coal being burnt and its origin. Typically, if the coal derives from coal-bearing rock strata, the fly ash will include substantial quantities of silicon dioxide (SiO2), aluminium oxide (Al2O3) and calcium oxide (CaO). Other constituents may be present as trace quantities and may include (alphabetical order) arsenic, beryllium, boron, lead, mercury, thallium and vanadium – to name a few. In appearance similar to Portland cement, fly ash is a fine powder with particles that are largely spherical-shaped, ranging in size from 0.5µm to 300 µm.
Before the advent of modern air pollution control standards, much of the smog and air pollution in early to mid-20th century European cities was partly the result of releasing unfiltered fly ash directly into the atmosphere.
Fortunately, environmental and health legislation has reduced the emissions to less than 1% of the fly ash produced. Modern filtration processes may use mechanical collectors (such as dust cyclones that rely on vortex separation); electrostatic precipitators or filter bags. This has resulted in huge quantities of fly ash that must be disposed of: today, around 65% of the global fly ash production is sent to landfill. This has increased landfill costs globally which has led to more efforts to find recycling uses.
 Classes of fly ash
There are two classes of fly ash:
- Class F (high calcium) largely produced by burning bituminous or anthracite coals. Pozzolanic in nature.
- Class C (low calcium) largely from burning sub-bituminous or lignite coal.
- Concrete block paving and other concrete products, such as pipes.
- Clay brick production.
- Blended cements.
- Mineral filler for asphalt roads.
- Soil stabilisation.
- Structural fill.
- Waste stabilisation and treatment.
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