Fly ash is the solid, disperse (particulate, particulate, dust-like) residue from burns, which is discharged with the flue gases due to its high dispersion (fine distribution). Fly ash is produced in large quantities in thermal power plants and waste incineration plants , where it has to be separated from the flue gases using dust extractors . The particle size ranges from about 1 µm to 1 mm. Particle shapes include smooth, massive spheres as well as hollow spheres (so-called cenospheres ), platelets, fibers and agglomerates . The density is 2.2 to 2.4 kg / l, the bulk density is between 0.9 and 1.1 kg / l.
The composition of the fly ash depends heavily on the fuel (e.g. lignite or hard coal ) and ranges from residual carbon and minerals ( quartz , aluminum silicate ) to toxic substances such as heavy metals ( arsenic to zinc ) and dioxins . The fly ash also acts as a carrier for adsorbed pollutants. While pure, uniform, constant fuels such as hard coal result in easily usable fly ash, lignite fly ash (BFA) is made up of many different substances.
In the past, the exhaust gases from the stationary combustion of fossil fuels were emitted untreated into the atmosphere - including those from combustion in large plants such as power plants and industrial furnaces. (This is still the case with gas today; gas burns less pollutants than other fuels because it has fewer impurities.) The emissions resulted in visible environmental pollution, especially in densely populated or heavily industrialized regions: the chimneys emitted gray smoke ; Laundry hung up to dry became dirty after a short time; Depending on the weather and the distance to the emitter, visible layers of dust were deposited.
In Germany, the first TA Luft came into force in 1964, followed by the Federal Immission Control Act and the ordinance on combustion systems in 1974 . As a result, the cleaning of exhaust gases was promoted on an industrial scale. In the 1980s, forest dieback prompted efforts to be intensified; the initially existing dioxin pollution in the vicinity of waste incineration plants was another reason to improve the exhaust gas cleaning technology.
The higher the degree of separation of the exhaust gas cleaning system and the higher its degree of dispersion, the higher the amount of fly ash collected annually. Lignite produces around three times as much ash (namely around 63 grams) as hard coal (20 grams) per kilowatt hour generated. In the USA there are 61 million tons annually, 10 million tons in Turkey. Fly ash that cannot be used in industry or construction is dumped.
Due to its chemical and physical properties, such as the pozzolanic reactivity , the spherical grain shape and the grain distribution, hard coal fly ash (SFA) in particular is a high-quality secondary raw material and can be used in a variety of applications in construction.
Pollutant-free fly ash is used in the building materials industry in accordance with DIN EN 450 as an additive in cement and concrete . Furthermore, the fly ash can be used to manufacture masonry blocks from sand-lime brick or aerated concrete. In road construction and earthworks, fly ash is used together with aggregates as a building material for unbound base courses.
In the meantime, processes for separating the fly ash into its components and thus into higher-quality products are also used commercially. The spheres and hollow spheres made of aluminum silicate and silicon dioxide are used as fillers in the rubber and plastics industry, the residual carbon as fuel in power plants.
Radioactive metal emission
Radioactive metals naturally occurring in the coal can be emitted from the power plant via the fly ash. The combustion of the combustible components leads to a concentration of the metals. This means a higher specific radioactivity of the fly ash compared to the fuel used. The BUND demands that the emitted radioactivity be taken into account in approval procedures under immission control law.
- Research Society for Roads and Transportation e. V. -FGSV-, Working Group Aggregates, Unbound Building Methods (Ed.): M KNP - Information sheet on the use of power plant by-products in road construction . FGSV Verlag, Cologne 2009, ISBN 978-3-941790-16-2 .
- Federal Ministry of Transport -BMV-, Road Construction Department, Bonn; Research Society for Roads and Transportation e. V. -FGSV-, Working Group Minerals in Road Construction, Cologne (ed.): Leaflet on the use of hard coal fly ash in road construction . BMV, Bonn 1993.
- Construction Chemicals Web Directory - Fly Ash
- Innovative geopolymer concrete based on fly ash
- Applications of fly ash in construction
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