Bottom Ash Production

The non-combustible fraction of the waste fed to the furnace forms a residue (ash) remaining on the grate at the completion of the combustion cycle. This material is generally referred to as bottom ash, but is also called grate ash, slag or clinkers.
Bottom ash is generated at a rate of approximately 20 to 25 percent by weight of the waste combusted (or more, if there is a high amount of ash or other noncombustible material in the waste), but only 5 to 10 percent by volume. It is similar in appearance to porous greyish silty sand with gravel, and contains small amounts of unburnt organic material and chunks of metal. The bottom ash stream consists primarily of glass, ceramics, ferrous and nonferrous metals, and minerals.

Bottom Ash Treatment

Bottom ash must be taken out of the furnace in such a way that maintains control over the combustion process. A seal on the furnace is normally provided by a column of water. The water bath also serves to extinguish any remaining combustibles and cool the ash. Furthermore large pieces of clinker fracture when quenched, reducing their size. The bottom ash leaves the furnace wet thereby minimizing fugitive dust emissions.

The grain size distribution of bottom ash is largely determined by the composition of the waste and the incinerator type. The grain size distribution is important for the mechanical properties of the bottom ash, and has to imitate the grain size of natural gravel. Screening, which in some cases is followed by crushing, might be carried out in order to make ash suitable for some applications, such as in cement and concrete. Also, lumps of ferrous and non-ferrous metal are separated from the ash through the use of magnets and eddy current separators.

Weathering (Maturation)

Bottom ash is exposed to the atmosphere to allow metal oxides and hydrates to react with water and carbon dioxide to form carbonate. These reactions reduce the leaching ability of the metals and reduce the potential impact on the environment. Reactions with water can cause swelling of the material, therefore weathering is essential. Weathering (maturation) is normally achieved by leaving the ash in a stockpile to allow rainfall and time to complete the reactions. For the new generation of incinerators a weathering time of at least 3-4 months is required. The leachate produced during storing/weathering requires appropriate disposal as it may contain high concentrations of highly soluble salts and minor amounts of metals, especially copper, chlorides and sulfates.

Weathering is sometimes followed by cement stabilization, which is typically carried out on the construction site by mixing the bottom ash with cement or other pozzolanic materials to form a monolithic material that effectively excludes moisture (physical encapsulation). In addition, the cement environment provides a highly buffered framework that limits the solubility of most trace metals by maintaining a high pH. For some trace metals, however, the high pH provides higher solubility.

Bottom Ash Usage

Bottom ash can either be landfilled or utilized. The legislation and boundary conditions regarding the utilization of bottom ash appear to be different in the countries of the European Union, the USA and Canada. Since a large quantity of solid waste incineration bottom ash is generated, the impact of byproduct utilization is great (both financially and environmentally, and as concerns public acceptance).

In many countries there is an increasing shortage of suitable natural aggregate and lack of available landfill space and at the same time, an increase in the amounts of bottom ash. This is the principal motivation for the utilization of bottom ash. In all countries, however, a portion of the bottom ash is landfilled.

If bottom ash is not managed properly it constitutes a potential environmental hazard. Proper legislation for reuse is therefore essential to ensure that the environmental impact of contemplated land uses is restricted to an acceptable level. In Western Europe, the legislation is mainly based upon leachate limit values according to EC decision 2003/33 (in USA TCLP tests).

Mechanical Properties of Bottom Ash

The mechanical properties of bottom ash have been studied in several countries, concluding that bottom ash can replace not only sand, but also natural gravel in unbound layers (sub-base), if the content of organic matter is kept low. The most abundant elements in municipal waste combustor ash are silica, calcium, and iron. Although ash composition can be expected to vary from facility to facility, these elements are present within relatively predictable ranges. The presence of a relatively high salt content and trace metal concentrations, including such elements as lead, cadmium, and zinc, in municipal waste combustor ash (compared with conventional aggregate materials) has raised concerns in recent years regarding the environmental acceptability of using ash as an aggregate substitute material



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