MSW Combustor Ash

Municipal solid waste (MSW) combustor ash is the by-product that is produced during the combustion of municipal solid waste in solid waste combustor facilities. In most modern mass burn sohd waste combustors, several individual ash streams are produced. They include grate ash, siftings,boiler ash, scrubber ash and precipitator or baghouse ash [113-118]. 

At the present time in the United States, all of the ash streams are typically combined. This combined stream is referred to as combined ash. The term bottom ash is commonly used to refer to the grate ash, siftings and, in some cases, the boiler ash stream. The term fly ash is also used and refers to the ash collected in the air pollution control system, which includes the scrubber ash and precipitator or baghouse ash. In Europe, most facilities separate and separately manage the bottom ash and fly ash streams. Table 6 summarized the different types of MSW combustor ash [113-128]. 

There are two basic types of solid waste combustors currently in operation in the United States, mass burn facilities and refuse derived-fuel (RDF) facilities. Mass burn facilities manage over 90% of the solid waste that is combusted in the United States. Mass burn facilities are designed to handle unsorted sohd waste, whereas RDF facilities are designed to handle preprocessed trash. The ash produced by RDF facilities, where the incoming municipal solid waste stream is shredded and presorted to remove ferrous metal and in certain cases nonferrous metal prior to combustion, can be expected to have different physical and chemical properties from ash generated at mass burn facilities [115-118]. 

Waste rock - Large amounts of waste rock are produced from surface mining 

Mill tailings - Mill tailings consist predominantly of extremely fine particles that are rejected from the grinding, screening, or processing of the raw material 

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MSW combustor ash is a relatively lightweight material compared to natural sands and aggregate. The bulk specific gravities that were reported range from 1.5-2.2 for sand-size or fine particles and 1.9-2.4 for coarse particles, compared to approximately 2.6-2.8 for conventional aggregate materials. 

Figure 4 shows the major elemental chemical constituents present in MSW combustor ash. The most abundant elements in municipal waste combustor ash... 

The presence of calcium as the oxide, as well as other oxides, in relatively high concentrations in MSW combustor ash makes the ash susceptible to hydration and/or cementation reactions with subsequent swelhng. The presence of elemental aluminum in the ash when combined with water can also result in the formation of hydrogen gas. In addition, the high oxide (salt) content also suggests that ash could be corrosive if placed in contact with metal structures, and that it would likely interfere with curing and strength development if used in Portland cement concrete. 

MSW combustor ash has been tested for use as an aggregate substitute in asphalt 113-121 paving mixes, where it has performed in a satisfactory manner, particularly in base or binder course applications... 

MSW waste combustor ash (grate ash) has been used as a granular base in road construction, as a fill material, and as an embankment material in Europe for almost two decades... 

During MSW combustion in a modern mass burn combustor with reciprocating grates, 11 of MSW is converted to heat gases such as C02, NOx, SOx, and H20 and about 350 kg of ash residuals partitioned into the various ash streams. For every tonne of MSW combusted, about 5 kg of grate siftings, 295 kg of grate ash, 5 kg of boiler/economizer ash, 20 kg of ESP ash, and 12 kg of dry scrubber solids are produced (Chandler et al. 1997). 

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