Industrial electric heating furnaces – Environmental control – Arc furnace
Patent
1996-05-08
1998-05-05
Hoang, Tu B.
Industrial electric heating furnaces
Environmental control
Arc furnace
373 82, 373108, F27D 1700
Patent
active
057486662
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a method for thermal treatment of an ash, such as an ash from waste incineration which at least includes pollutants in the form of metals, salts and/or organic compounds, in an electric melting furnace, in the form of a direct current arc furnace. The invention also relates to a furnace for carrying out the invented method.
BACKGROUND ART
During combustion or pyrolysis of fuels and waste, ash arises which contains varying quantities of undesired constituents, i.e., pollutants, which constitute a problem when depositing or reusing the ash. This applies both to that part of the ash which remains in the combustion chamber, i.e., the bottom ash or the slag, and that part of the ash which accompanies the flue gas out of the combustion chamber and is separated in various cyclones and filters, i.e., the fly ash.
The contents of heavy metals in the ash originate from the fuel or the waste which is burnt and released to a varying extent from the fuel at the high temperatures which prevail in the combustion chamber.
The contents of organic substances in the ash are dependent on the efficiency of the combustion process, to what extent the organic contents in the fuel or the burnt waste can be completely broken down into carbon dioxide and water, and to what extent reformation of organic compounds in the fuel gases is prevented. This undesired reformation of organic compounds, among them greatly toxic compounds such as dioxine, is catalyzed by the ash and influenced by the composition of the ash, above all the metal contents of the ash.
Due to its contents of metals, salts and organic substances, the ash constitutes an environmental problem with effects both in the short run and in the long run. Deposition is made difficult since these pollutants can be released and spread in an uncontrolled manner by leaching into the groundwater. This means that deposition is surrounded by rigorous regulations regarding design, embankment, drainage, etc., and that extensive supervision must be carried out for a long time ahead. In addition, leachate from the deposit must in many cases be treated, that is, cleaned.
Reuse of ash is rendered difficult and is limited since there is a risk of leakage of metals, salts and organic substances. Thus, a contaminated product runs the risk of influencing the environment where the product is used. Examples of areas where attempts are being made with reuse of ash is gravel replacement in road embankments or other landfills and filler in cement.
Waste from ash incineration poses special problems, both because of the relatively large ash quantity and because of its relatively high and greatly varying contents of both metals, salts and organic substances. Both the ash quantity and the ash composition may be influenced by pretreatment of the waste, such as sorting out of non-combustible material; however, the influence on the amount of toxic or otherwise harmful substances is generally relatively insignificant. Typical ash quantities for combustion or other thermal treatment of waste is 10-30 kg fly ash per ton of waste and 200-300 kg bottom ash per ton of waste.
It is known, using different forms of further treatment of the ash, such as embedment and thermal treatment, to try to reduce the environmental effect during deposition or reuse.
Known methods for thermal treatment can be divided according to the process temperature, that is to say, the temperature to which the ash is heated, and if the ash is melted or not.
In low-temperature processes where the ash is not melted, that part of the pollutants which becomes volatile at the process temperature in question is driven off without the shape of the ash being influenced. This volatile part is separated from the flue gases and is then taken care of by filtering, wet separation and/or recondensation.
In processes with a treatment temperature sufficiently high to completely or partially melt the ash, in addition to the driving off of the volatile components, there is also obtained an at least partially vitri
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Andersson Karl-Hugo
Stenkvist Sven-Einar
Wikstrom Goran
Asea Brown Boveri AG
Hoang Tu B.
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