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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
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 solid waste combustors, several individual ash streams are produced. They include grate ash, siftings, boiler ash, scrubber ash and precipitator or baghouse ash. Figure 10-1 presents a cross-section of a modern mass burn waste combustor with energy recovery, showing where each of the ash streams is produced.
At the present time in the United States, typically all of the ash streams are 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.
Approximately 90 percent of the bottom ash stream consists of grate ash, which is the ash fraction that remains on the stoker or grate at the completion of the combustion cycle. It is similar in appearance to a porous, grayish, silty sand with gravel, and contains small amounts of unburnt organic material and chunks of metal. The grate ash stream consists primarily of glass, ceramics, ferrous and nonferrous metals, and minerals. It comprises approximately 75 to 80 percent of the total combined ash stream.
Boiler Ash and Fly Ash
Boiler ash, scrubber ash, and precipitator or baghouse ash consist of particulates that originate in the primary combustion zone area and are subsequently entrained in the combustion gas stream and carried into the boiler and air pollution control system. As the combustion gas passes through the boiler, scrubber, and precipitator or baghouse, the entrained particulates stick to the boiler tubes and walls (i.e., boiler ash) or are collected in the air pollution control equipment (i.e., fly ash), which consists of the scrubber, electrostatic precipitator, or baghouse. Ash extracted from the combustion gas consists of very fine particles, with a significant fraction measuring less than 0.1 mm (No. 140 sieve) in diameter. The baghouse or precipitator ash comprises approximately 10 to 15 percent of the total combined ash stream.
Approximately 29.5 million metric tons (32.5 million tons) of solid waste is combusted annually at approximately 160 municipal waste combustor plants in the United States,(1) generating approximately 8 million metric tons (9 million tons) of residual or ash.(2)
Figure 10-1. Mass burn waste-to-energy facility- typical cross section and ash streams.(2)
There are two basic types of solid waste combustors in operation in the United States today - mass burn facilities and refuse derived-fuel (RDF) facilities.
Mass burn facilities manage over 90 percent of the solid waste that is combusted in the United States. Mass burn facilities are designed to handle unsorted solid waste, whereas RDF facilities are designed to handle preprocessed trash. The as produced 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.
There are also significant differences between ash generated at modern waste-to-energy facilities and that generated at older facilities. Newer facilities, with improved furnace designs, generally achieve better burnout and have reduced organic content in the ash product. Due to air pollution control requirements in newer facilities, lime or a lime-based reagent is introduced into the pollution control system to scrub out acid gases from the combustion gas stream. This produces a fly ash that contains both reacted and unreacted lime. Older facilities without acid gas scrubbing do not have lime in their fly ash. Finally, newer facilities with improved air pollution control equipment (e.g., baghouses) are better able to capture the finer particulate materials and trace contaminants, which many of the older facilities usually release into the air. It also is likely that, in the future, more stringent air pollution control requirements (e.g., mercury and NOx control) will further alter both the physical and chemical properties of fly ash streams.
There are no specific trade or industrial groups associated with incinerator ash recycling in the United States, but additional information can be obtained from the following municipal waste combustor trade association:
Integrated Waste Services Association
1133 21st Street, N.W., #205
Washington, D.C. 20036
CURRENT MANAGEMENT OPTIONS
At the present time most operating facilities in the United States recover the ferrous metal fraction present in MSW combustor ash, which can comprise up to 15 percent of the total ash fraction. Only a very small fraction (less than 5 percent) of the nonferrous fraction of the ash generated in the United States is recovered and utilized. Most of the ash is used as a landfill cover material. There is some commercial use of ash in road paving applications presently ongoing in Tennessee.
In some European nations (e.g., The Netherlands and Denmark), more than one-half of the bottom ash generated by municipal waste combustors is used in construction applications. Lesser percentages are used in West Germany and France. In Europe the most common application is the use of ash as a granular road base material.(3)
In the United States and Japan, numerous studies in recent years have focused on the potential for using processed bottom ash and combined ash as an aggregate substitute in asphalt concrete, Portland cement concrete, and as an aggregate in stabilized base applications.
Although neither federal nor most state regulations categorically restrict the use of MSW combustor ash (as long as the ash is determined to be nonhazardous in accordance with regulatory testing criteria), the presence of trace metals, such as lead and cadmium, in MSW combustor ash, and concern over leaching of these metals, as well as the presence of dioxins and furans in selected ash fractions (fly ash), has led many regulatory agencies to take a cautious approach in approving the use of MSW combustor ash as a substitute aggregate material.
At the present time in the United States almost all of the annual 8 million metric tons (9 million tons) of ash produced is landfilled. This is in sharp contrast to the aforementioned European practice.
MSW combustor ash is generally managed by the waste-to-energy facility operator on behalf of the local municipality or authority that owns the facility. The final disposition of ash is usually the responsibility of the municipality or authority. As a result, ash for recycling purposes could normally be obtained by contacting the municipality or authority responsible for the facility.
The properties of the ash that may be made available for market will depend on the ash stream (e.g., combined, bottom or grate ash) that is proposed for use. In most cases combined ash contains excess unreacted lime that has been added as an acid gas treatment reagent and as a treatment additive to reduce the leachability of trace metals that are present in the residue. Combined ash, which contains the air pollution control system residues, typically has higher concentrations of volatile metals (e.g., lead, cadmium, zinc) than bottom ash. Bottom ash is usually lime free and contains fewer fines. It is the preferred material for recycling in Europe where the boiler ash is also segregated from the bottom ash prior to bottom ash use.
In addition to the type of ash, the quality of ash received from a given facility will depend in great part on the processing equipment at the facility. A wide range of processing systems have been used to produce an ash-aggregate material from unprocessed residue. The most basic process consists of ferrous metal removal followed by screening of the residue, generally to a minus 19 mm (3/4 in) material, to create a sandy, granular product. Many waste combustors are presently incorporating not only ferrous removal equipment, but nonferrous removal to recover valuable aluminum and copper from the residue.
Ferrous and nonferrous metal removal, screening and, in some cases, air classification can assist in producing a sandlike, metal-free, low organic content product.
To reduce the fines content of combustor residues, pelletizing processes using Portland cement as a binding agent have been proposed and applied (4, 5) to improve the engineering and environmental characteristics of incinerator ash.(6) Vitrification, which is a high-temperature process designed to melt and subsequently cool the ash, has been proposed as a potential processing strategy. There are a number of commercial-sized ash vitrification systems currently in operation in Japan.(7) The high cost of vitrification, due primarily to the energy and facilities required to heat and melt the ash, tends to discourage the use of this technology in the United States.
HIGHWAY USES AND PROCESSING REQUIREMENTS
Municipal waste combustor ash has been tested for use as an aggregate substitute in asphalt paving mixes, where it has performed in a satisfactory manner, particularly in base or binder course applications. In this application, the ash is used to replace the sand-size or fine aggregate portion of the mix. In most cases, processed ash that is screened to less than 19 mm (3/4 in) with ferrous and nonferrous metal removal can be introduced to replace anywhere from 10 to 25 percent of the natural aggregate normally present in the mix for surface course applications and up to 50 percent for base course applications.
Granular Base, Fill, and Embankments
Municipal 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. The use of ash in granular base and fill applications in the United States has been limited primarily to demonstrations.
In granular base or embankment applications, properly processed ash (i.e., screened to less than 25 to 38 mm (1 to 1-1/2 in) and metal removed) can be either blended or used alone in these applications. Ash can also be stabilized with Portland cement or lime to produce a stabilized base material.
During the 1970's and 1980's a number of comprehensive investigations were undertaken to characterize the properties of municipal waste combustor ash. (See References 8,9,10,11.) Most of the data from these early investigations reflect the characteristics of ash from batch-fed or older continuous flow grate designs that are unlike the modern, high-efficiency energy recovery combustors in operation today. During the past few years there have been a number of comprehensive investigations that have characterized the properties of combined and bottom ash residues generated from these newer facilities.(12,13)
Table 10-1 presents a list of data for some typical physical properties of municipal waste combustor combined and bottom ash that were obtained from two recent comprehensive investigations in which the physical properties of both bottom and combined ash were characterized.
Table 10-1. Selected physical properties of municipal solid waste combustor ash.
The data indicate that municipal solid waste combustor ash is a relatively lightweight material compared with natural sands and aggregate. The bulk specific gravities that were reported range from 1.5 to 2.2 for sand-size or fine particles and 1.9 to 2.4 for coarse particles, compared with approximately 2.6 to 2.8 for conventional aggregate materials.
Combustor ash is highly absorptive with absorption values ranging from 5 to 17 percent for fine particles and from about 4 to 10 percent for coarse particles. Conventional aggregates typically exhibit absorption values of less than 2 percent.
Prior to exiting a municipal solid waste combustor, the ash is quenched, resulting in the high moisture content values listed in Table 10-2. This high moisture content is due the quenching and relatively high porosity and absorptive nature of combustor ash.
The relatively low unit weights further underscore the lightweight nature of combustor ash, and the loss on ignition values suggest that the ash can contain relatively high levels of organics compared with conventional aggregates.
Combustor ash is primarily a sandy material with the major fraction passing a 4.75 mm (No. 4) sieve. Ash also contains a relatively high minus 0.075 mm (No. 200 sieve) silt fraction.
Table 10-2 lists the major chemical constituents present in MSW combustor ash.
Table 10-2. Typical chemical composition (percent).
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. This is reflected in the results presented in Table 10-2.
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.
The presence of calcium and other salts in relatively high concentrations in MSW combustor ash makes the ash susceptible to hydration and/or cementitious reactions (particularly in the combined ash, which contains unreacted lime) and subsequent swelling. The presence of free aluminum in the ash when combined with water can also result in the formation of hydrogen gas. In addition, the high 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.
Table 10-3 lists some typical mechanical properties associated with municipal waste combustor ash.
The data suggest that MSW combustor ash has relatively poor durability characteristics as measured by the Los Angeles. Abrasion Test. Los Angeles abrasion values presented are above or at the limits normally specified for the use of coarse aggregates in asphalt paving applications. Reported soundness values are generally within typical specifications, suggesting that ash is not highly susceptible to freeze-thaw cycles. California Bearing Ratio (CBR) results for both bottom and combined ash are similar to those that could be anticipated for a well-graded crushed stone, suggesting excellent bearing capacity: reported friction angle data suggest a material with good lateral stability.
Table 10-3. Typical mechanical properties of MSW combustor ash.