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Municipal solid wastes

Municipal incinerators

Emissions of pollutants

Specialized incinerators

The role of incinerators


Incineration is the process of burning substances to ashes with the use of facilities called incinerators. Thus, incinerators are industrial facilities used for the controlled burning of waste materials. The largest incinerators are used to burn municipal solid wastes, often in concert with a technology that utilizes the heat produced during combustion to generate electricity. Smaller, more specialized incinerators are used to burn medical wastes, general chemical wastes such as organic solvents, and toxic wastes such as polychlori-nated biphenyls and other chlorinated hydrocarbons. Incineration is popular in countries with limited natural resources, such as Sweden, Japan, and Denmark.

Municipal solid wastes

Municipal solid waste comes from a wide range of sources in cities and suburban areas, including residences, businesses, educational and government institutions, industries, and construction sites. Municipal solid waste is typically composed of a wide range of materials, including food wastes, paper products, plastics, metals, glass, demolition debris, and household hazardous wastes (the latter assumes that hazardous wastes from industries, hospitals, laboratories, and other institutions are disposed as a separate waste stream).

Depending on the municipality, some of this solid waste may be recycled, reused, or composted. More typically, however, most of the wastes are disposed in some central facility, generally some sort of sanitary landfill. These facilities are regulated, engineered disposal sites to which the wastes are hauled, dumped on land, compacted, and covered with earth. The basin of a modern sanitary landfill is generally lined with an impermeable material, such as heavy plastic or clay. This process allows the collection of water that has percolated through the wastes, so it can be treated to reduce the concentrations of pollutants to acceptable levels, prior to discharge to the environment.

However, in many places large, sanitary landfills are no longer considered a preferable option for the disposal of general solid wastes. In some cases, this is because land is locally scarce for the development of a large landfill. More usually, however, local opposition to these facilities is the constraining factor, because people living in the vicinity of operating or proposed disposal sites object to these facilities. These people may be variously worried about odors, local pollution, truck traffic, poor aesthetics, effects on property values, or other problems potentially associated with large, solid-waste disposal sites.

Everyone, including these people, recognizes that municipalities need large facilities for the disposal of solid wastes. However, no one wants to have such a facility located in their particular neighborhood. This popularly held view about solid waste disposal sites, and about other large, industrial facilities, is known as the not in my back yards or NIMBY syndrome, and sometimes as the locally unacceptable land use or LULU syndrome.

Municipal incinerators

Incinerators are an alternative option to the disposal of general municipal garbage in solid-waste disposal sites. Municipal incinerators accept organic wastes and combust them under controlled conditions. The major benefit of using incinerators for this purpose is the large reductions that are achieved in the mass and volume of wastes.

In addition, municipal incinerators can be engineered as waste-to-energy facilities, which couple incineration with the generation of electricity. For example, a medium-sized waste-to-energy facility can typically take 550 tons (500 tonnes) per day of municipal solid wastes, and use the heat produced during combustion to generate about 16 megawatts of electricity. About 2 to 3 megawatts would be used to operate the facility, including its energy demanding air-pollution control systems, and the rest could be sold to recover some of the costs of waste disposal.

Among the major drawbacks of incinerators is the fact that these facilities have their own problems with NIMBY, mostly associated with the fears of people about exposures to air pollutants. As is discussed in the next section, incinerators emit a wide range of potentially toxic chemicals to the environment.

In addition, municipal incinerators produce large quantities of residual materials, which contain many toxic chemicals, especially metals. The wastes of incineration include bottom ash that remains after the organic matter in the waste stream has been combusted, as well as finer fly ash that is removed from the waste gases of the incineration process by pollution control devices. These toxic materials must be disposed in sanitary landfills, but the overall amounts are much smaller than that of the unburned garbage.

Incinerators are also opposed by many people because they detract from concerted efforts to reduce the amounts of municipal wastes by more intensive reducing, recycling, and reusing of waste materials. Incinerators require large quantities of organic garbage as fuel, especially if they are waste-to-energy facilities that are contracted to deliver certain quantities of electricity. Because of the large fuel demands by these facilities, it can be difficult to implement other mechanisms of refuse management. Efforts to reduce the amounts of waste produced, to recycle, or to compost organic debris can suffer if minimal loads of fuels must be delivered to a large incinerator to keep it operating efficiently. These problems are best met by ensuring that incinerators are used within the context of an integrated scheme of solid waste management, which would include vigorous efforts to reduce wastes, reuse, recycle, and compost, with incineration as a balanced component of the larger system.

Emissions of pollutants

Incinerators are often located in or near urban areas. Consequently, there is intense concern about the emissions of chemicals from incinerators, and possible effects on humans and other organisms that result from exposure to potentially toxic substances. Consequently, modern incinerators are equipped with rigorous pollution control technologies to decrease the emissions of potentially toxic chemicals. The use of these systems greatly reduces, but does not eliminate, the emissions of chemicals from incinerators. Also, as with any technology, there is always the risk of accidents of various sorts, which in the case of an incinerator could result in a relatively uncontrolled emission of pollutants for some period of time.

Uncertainty about the effects of potentially toxic chemicals emitted from incinerators is the major reason for the intense controversy that accompanies any plans to build these facilities. Even the best pollution-control systems cannot eliminate the emissions of potentially toxic chemicals, and this is the major reason for incinerator-related NIMBY. In fact, some opponents of incinerators believe that the technology is unacceptable anywhere, a syndrome that environmental regulators have dubbed by the acronym BANANA, for build absolutely nothing near anybody or anything. During the incineration process, small particulates are entrained into the flue gases; that is, the stream of waste gases that vents from the combustion chamber. These particulates typically contain large concentrations of metals and organic compounds, which can be toxic in large exposures.

To reduce the emissions of particulates, the flue gases of incinerators are treated in various ways. There are three commonly used systems of particulate removal. Electrostatic precipitators are devices that confer an electrical charge onto the particulates, and then collect them at a charged electrode. A baghouse is a physical filter, which collects particulates, as flue gases are forced through a fine fabric. Cyclone filters cause flue gases to swirl energetically, so that particles can be separated by physical impaction at the periphery of the device. For incinerators located in or near urban areas, where concerns about emissions are especially acute, these devices may be used in series to achieve especially efficient removals, typically greater than 99% of the particulate mass. Virtually all partic-ulates that are not removed by these systems are very tiny, and therefore behave aerodynamically as gases. Consequently, these emitted particulates are widely dispersed in the environment, and do not deposit locally in significant amounts.

The most important waste gases produced by incinerators are carbon dioxide (CO2), sulfur dioxide (SO2), and oxides of nitrogen (NO and NO2, together known as NOx). The major problem with carbon dioxide is through its contribution to the enhancement of Earths greenhouse effect. However, because incinerators are a relatively small contributor to the total emissions of carbon dioxide from any municipal area, no attempts are made to reduce emissions from this particular source.

Sulfur dioxide and oxides of nitrogen are important in the development of urban smog, and are directly toxic to vegetation. These gases also contribute to the deposition of acidifying substances from the atmosphere, for example, as acidic precipitation. Within limits, sulfur dioxide and oxides of nitrogen can be removed from the waste gases of incinerators. There are various technologies for flue-gas desulfurization, but most rely on the reaction of sulfur dioxide with finely powdered limestone (CaCO3) or lime [(Ca(OH)2)] to form a sludge containing gypsum (CaSO4), which is collected and discarded in a solid-waste disposal site. This method is also effective at reducing emissions of hydrogen chloride (HCl), an acidic gas. Emissions of oxides of nitrogen can be controlled in various ways, for example, by reacting this gas with ammonia. Because urban areas typically have many other, much larger sources of atmospheric emissions of sulfur dioxide and oxides of nitrogen, emissions of these gases from incinerators are not always controlled using the technologies just described.

Various solid wastes can contain substantial concentrations of mercury, including thermometers, electrical switches, batteries, and certain types of electronic equipment. The mercury in these wastes is vaporized during incineration and enters the flue-gas stream. Pollution control for mercury vapor can include various technologies, including the injection of fine activated carbon into the flue gases. This material absorbs the mercury, and is then removed from the waste gases by the particulate control technology.

One of the most contentious pollution issues concerning incinerators involves the fact that various chlorinated hydrocarbons are synthesized during the incineration process, including the highly toxic chemicals known as dioxins and furans. These are formed during combustions involving chlorine-containing organic materials, at a rate influenced by the temperature of the combustion and the types of material being burned, including the presence of metallic catalysts. The synthesis of dioxins and furans is especially efficient at 572 to 932°F (300 to 500°C), when copper, aluminum, and iron are present as catalysts. These reactions are an important consideration when incineration is used to dispose of chlorinated plastics such as polyvinyl chloride (PVC, commonly used to manufacture piping and other rigid plastic products) and polychlorinated biphenyls (PCBs).

Attention to combustion conditions during incineration can greatly reduce the rate of synthesis of dioxins and furans. For example, temperatures during incineration are much hotter, typically about 1,742 to 2,102°F (950 to 150°C), than those required for efficient synthesis of dioxins and furans. However, the synthesis of these chemicals cannot be eliminated, so emissions of trace quantities of these chemicals from incinerators are always a concern, and a major focus of NIMBY and BANANA protests to this technology.

Specialized incinerators

Relatively small, specialized incinerators are used for the disposal of other types of wastes, particularly hazardous wastes. For example, hospitals and research facilities generally use incinerators to dispose of biological tissues, blood-contaminated materials, and other medical wastes such as disposable hypodermic needles and tubing. These are all considered to be hazardous organic wastes, because of the possibilities of spreading pathogenic microorganisms.

Incinerators may also be used to dispose of general chemical wastes from industries and research facilities, for example, various types of organic solvents such as alcohol. More specialized incinerators are used to dispose of more toxic chemical wastes, for example, chlorinated hydrocarbons such as PBCs, and various types of synthetic pesticides. For these latter purposes, the incineration technology includes especially rigorous attention to combustion conditions and pollution control. However, emissions of potentially toxic chemicals are never eliminated.

The role of incinerators

Industrialized and urbanized humans have a serious problem with solid wastes. These materials must


Flue gas The waste gases of a combustion. These may be treated to reduce the concentrations of toxic chemicals, prior to emission of the flue gases to the atmosphere.

Incinerator An industrial facility used for the controlled burning of waste materials.

NIMBY Acronym for not in my back yard.

be dealt with by society in a safe and effective manner, and incineration is one option that should be considered. However, incinerators have some drawbacks, including the fact that they invariably emit some quantities of potentially toxic chemicals. The role of incinerators in waste disposal would best be determined by an objective consideration of the best available scientific information.

Environmental damages have been caused in the past by the use of less efficient technologies to dispose of the wastes of society, including incinerators without modern combustion and pollution-control systems. In large part, these damages were associated with industries, politicians, and societies that were not sufficiently aware of the potential environmental damages, or did not care about them to the degree that is common today. Modern incineration uses a technology called waste-to-energy plant (WtE), or sometimes energy-from-waste (EfW), which is a process that incinerates wastes in high-efficiency furnaces/boilers that use continuous emission monitors and air pollution control systems. Such systems are incorporated in Germany where, as of 2005, dioxin emissions from incineration plants generated only 1% of the pollution in the country. According to German government reports, this percentage is down from 33% in 1990, at a time when less efficient incinerators were used.

See also Air pollution.



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Bill Freedman

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As a method of waste management , incineration refers to the burning of waste. It helps reduce the volume of landfill material and can render toxic substances non-hazardous, provided certain strict guidelines are followed. There are two basic types of incineration: municipal and hazardous waste incineration.

Municipal waste incineration

The process of incineration involves the combination of organic compounds in solid wastes with oxygen at high temperature to convert them to ash and gaseous products. A municipal incinerator consists of a series of unit operations which include a loading area under slightly negative pressure to avoid the escape of odors, a refuse bin which is loaded by a grappling bucket, a charging hopper leading to an inclined feeder and a furnace of varying typeusually of a horizontal burning grate typea combustion chamber equipped with a bottom ash and clinker discharge , followed by a gas flue system to an expansion chamber. If byproduct stream is to be produced either for heating or power generation purposes, then the downstream flue system includes heat exchanger tubing as well. After the heat has been exchanged, the flue gas proceeds to a series of gas cleanup systems which neutralizes the acid gases (sulfur dioxide and hydrochloric acid, the latter resulting from burning chlorinated plastic products), followed by gas scrubbers and then solid/gas separation systems such as baghouses before dischargement to tall stacks . The stack system contains a variety of sensing and control devices to enable the furnace to operate at maximum efficiency consistent with minimal particulate emissions. A continuous log of monitoring systems is also required for compliance with county and state environmental quality regulations.

There are several products from a municipal incinerator system: items which are removed before combustion such as large metal pieces; grate or bottom ash (which is usually water-sprayed after removal from the furnace for safe storage); fly (or top ash) which is removed from the flue system generally mixed with products from the acid neutralization process; and finally the flue gases which are expelled to the environment . If the system is operating optimally, the flue gases will meet emission requirements, and the heavy metals from the wastes will be concentrated in the fly ash . (Typically these heavy metals, which originate from volatile metallic constituents, are lead and arsenic.) The fly ash typically is then stored in a suitable landfill to avoid future problems of leaching of heavy metals. Some municipal systems blend the bottom ash with the top ash in the plant in order to reduce the level of heavy metals by dilution. This practice is undesirable from an ultimate environmental viewpoint.

There are many advantages and disadvantages to municipal waste incineration. Some of the advantages are as follows: 1) The waste volume is reduced to a small fraction of the original. 2) Reduction is rapid and does not require semi-infinite residence times in a landfill. 3) For a large metropolitan area, waste can be incinerated on site, minimizing transportation costs. 4) The ash residue is generally sterile, although it may require special disposal methods. 5) By use of gas clean-up equipment, discharges of flue gases to the environment can meet stringent requirements and be readily monitored. 6) Incinerators are much more compact than landfills and can have minimal odor and vermin problems if properly designed. 7) Some of the costs of operation can be reduced by heat-recovery techniques such as the sale of steam to municipalities or electrical energy generation.

There are disadvantages to municipal waste incineration as well. For example: 1) Generally the capital cost is high and is escalating as emission standards change. 2) Permitting requirements are becoming increasingly more difficult to obtain. 3) Supplemental fuel may be required to burn municipal wastes, especially if yard waste is not removed prior to collection. 4) Certain items such as mercury-containing batteries can produce emissions of mercury which the gas cleanup system may not be designed to remove. 5) Continuous skilled operation and close maintenance of process control is required, especially since stack monitoring equipment reports any failure of the equipment which could result in mandated shut down. 6) Certain materials are not burnable and must be removed at the source. 7) Traffic to and from the incinerator can be a problem unless timing and routing are carefully managed. 8) The incinerator, like a landfill, also has a limited life, although its lifetime can be increased by capital expenditures. 9) Incinerators also require landfills for the ash. The ash usually contains heavy metals and must be placed in a specially-designed landfill to avoid leaching.

Hazardous waste incineration

For the incineration of hazardous waste, a greater degree of control, higher temperatures, and a more rigorous monitoring system are required. An incinerator burning hazardous waste must be designed, constructed, and maintained to meet Resource Conservation and Recovery Act (RCRA) standards. An incinerator burning hazardous waste must achieve a destruction and removal efficiency of at least 99.99 percent for each principal organic hazardous constituent. For certain listed constituents such as polychlorinated biphenyl (PCB), mass air emissions from an incinerator are required to be greater than 99.9999%. The Toxic Substances Control Act requires certain standards for the incineration of PCBs. For example, the flow of PCB to the incinerator must stop automatically whenever the combustion temperature drops below the specified value; there must be continuous monitoring of the stack for a list of emissions; scrubbers must be used for hydrochloric acid control; among others.

Recently medical wastes have been treated by steam sterilization, followed by incineration with treatment of the flue gases with activated carbon for maximum absorption of organic constituents. The latter system is being installed at the Mayo Clinic in Rochester, Minnesota, as a model medical disposal system.

See also Fugitive emissions; Solid waste incineration; Solid waste volume reduction; Stack emissions

[Malcolm T. Hepworth ]



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