Incineration

Municipal solid wastes

Municipal incinerators

Emissions of pollutants

Specialized incinerators

The role of incinerators

Resources

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 yard’s 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 Earth’s 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

KEY TERMS

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.

Resources

BOOKS

Hemond, Harold F. and E.J. Fechner. Chemical Fate and Transport in the Environment. San Diego, CA: Academic Press, 2000.

Hester, R.E., and R.M. Harrison. Global Environmental Change. Cambridge, UK: Royal Society of Chemistry, 2002.

Metcalfe, Sarah E. Atmospheric Pollution and Environmental Change. London, UK: Hodder Arnold; New York: Oxford University Press, 2005.

Molles, Manuel C. Ecology: Concepts and Applications. Boston, MA: McGraw-Hill, 2005.

Niessen, Walter R. Combustion and Incineration Processes. New York: Marcel Dekker, 2002.

Smith, Thomas M. Elements of Ecology. San Francisco, CA: Benjamin Cummings, 2008.

Bill Freedman

Incineration

Incineration is the thermal destruction of waste. It is as old as throwing food wastes on a wood fire, and in many developing nations, garbage is still routinely burned in drums and boxes on city streets. Modern incineration systems use high temperatures, controlled air, and excellent mixing to change the chemical, physical, or biological character or composition of waste materials. The new systems are equipped with state-of-the-art air pollution control devices to capture particulate and gaseous emission contaminates. There are still many health concerns connected with incineration systems, especially for populations living near incinerators. However, the stringent regulations that have been enacted by federal and state regulators ensure that the design, operation, testing, and maintenance of these systems provide maximum safety and minimum risk to the surrounding area and inhabitants.

In 1992 the United States had 190 operating incinerators with a design capacity of 114,339 tons/day and an annual capacity of 35.5 million tons. Germany, which has the highest concentration of incinerators in Europe, has 53 units with an annual capacity of 10.7 million tons.

Incineration can be adapted to the destruction of a wide variety of wastes. This includes but is not limited to household wastes, often referred to as municipal wastes, industrial wastes, medical wastes, sewage, Superfund soils and liquids, and the hazardous wastes (liquids, tars, sludges, solids, and vent fumes) generated by industry. Unlike many other methods of waste disposal, incineration is a permanent solution. The major benefit of incineration is that the process actually destroys most of the waste rather than just disposing of or storing it.

Many local community incinerators were built after World War II. The suburban communities surrounding large urban centers selected incineration as the method of disposal over landfills. There was a lack of consideration of exhaust emissions from these units in the original designs: Tall stacks were used for dispersion rather than proper air pollution controls. The combustion furnaces operated at high excess air levels resulting in lower temperatures, incomplete combustion and high levels of carbon monoxide and unburned hydrocarbons. Typical conditions surrounding these facilities were high soot and odor levels as well as corrosion from acid gas deposition. It was an unhealthy and unsafe environment for the neighbors.

This created the well-known NIMBY syndrome—"Not in My Backyard." In the 1960s and 1970s, more units were shut down than planned for new construction. The Resource Recovery Act (RRA) was passed in 1965, followed by the Clean Air Act (CAA) in 1970, the Resource Conservation and Recovery Act (RCRA) in 1976, the Hazardous and Solid Wastes Amendments (HSWA) in 1984, and the Maximum Achievable Control Act (MACT) in 1999. New and existing systems require the proper controls for combustion and air pollution control to receive a construction, retrofit, and operating permit. This has reduced the past concerns about health and the environment surrounding these facilities. Incinerator regulations in the twenty-first century are considered the most stringent of all types of combustion and energy recovery systems. They are also the most protective for the health and environment of local communities.

Combustion

Waste incineration involves the application of combustion processes under controlled conditions to convert waste materials to inert mineral ash and gases. The three Ts of combustion (temperature, turbulence, and residence time) must be present along with sufficient oxygen for the reaction to occur:

• The burning mixture (air, wastes, and fuel) must be raised to a sufficient temperature to destroy all organic components. The combustion airflow is reduced to the minimum level needed to provide the oxygen for the support fuel (gas, oil, or coal) and the combustible wastes without forming high levels of CO and unburned hydrocarbons. This will raise the temperature to the level needed for good combustion.
• Turbulence refers to the constant mixing of fuel, waste, and oxygen.
• Residence time is the time of exposure to combustion temperatures.
• Oxygen must be available in the combustion zone.

Types of Incinerators

Waste incinerators are used to destroy solids, sludges, liquids, and tars. Depending upon the physical, chemical characteristics of the waste and the handling they require, different incinerator designs will be applied. Solids, sludges, and tars are incinerated in fixed-hearth and rotary kiln incinerators. Liquids may also be burned in these systems and used as support fuel. In many plants where liquids are the primary wastes, liquid injection incinerators are used. Boilers, process furnaces, cement kilns, and lightweight aggregate kilns also utilize the energy available from liquid wastes and burn liquid wastes as well as the fossil fuels (natural gas and oil).

Fixed-Hearth Incinerators. Fixed-hearth incinerators are used extensively for medical and municipal waste incineration. Fixed hearths can handle bulk solids and liquids. A controlled flow of "underfire" combustion air (70 to 80 percent of the theoretical air required) is introduced up through the hearth on which the waste sits. Bottom ash is removed by dumping into a water bath.

Unburned combustibles and high levels of carbon monoxide and hydrogen exit above the hearth. These volatiles are oxidized in the combustion zone where overfire air provides sufficient excess air and residence time at temperature to ensure complete burnout. The three Ts of combustion and oxygen provide high combustion efficiency. Natural gas or oil is supplied to maintain temperatures as high as 2,000°F. In some large municipal waste combustors, called waste-to-energy plants, heat recovery boilers are used to generate steam for electric generation. These plants are also referred to as trash-to-steam plants. All incinerator systems are now regulated by exhaust emissions. Air pollution control systems are installed to control emissions of particulate matter including metals and ash, hydrocarbons including dioxins and furans, and acid gases created from the combustion of wastes containing chlorine, sulfur, phosphorous, and nitrogen compounds.

Rotary Kiln Incineration. Solid wastes as well as liquid wastes generated by industry are destroyed by on-site and commercial-site rotary kiln incinerator systems. The rotary kiln is a cylindrical refractory -lined shell that is rotated to provide a tumbling and lifting action to the solid waste materials. This exposes the waste surface to the flames from fuel burning as well as liquid waste burning in the rotating kiln. Flames will also be generated over the surface of waste solids exposed to the heat and incoming air. Pumpable sludges and slurries are injected into the kiln through nozzles. Temperatures for burning vary from 1,300 to 2,400°F. Lower temperatures are often necessary to prevent slagging of certain waste materials.

The rotary kiln provides excellent mixing through a rotating-tumbling action that distributes heat evenly to all the waste materials contained within it. The kiln is the primary combustion chamber (PCC) where organic compounds in the wastes are volatilized and oxidized as air is introduced into the kiln. The unburned volatiles enter the secondary combustion chamber (SCC) along with the hot products of combustion from the PCC where additional oxygen is introduced and ignitable liquid wastes or fuel can be burned. Complete combustion of the volatized waste from the PCC, liquid wastes and fuel occurs in the SCC.

Liquid Injection. The chemical industries generate liquid wastes that contain toxic organics. Typical wastes from the agricultural and pharmaceutical plants may contain compounds such as chlorinated benzenes, vinyl chloride, toluene, phosphorous, and naphthalene. On-site liquid injection incinerators are used to destroy these wastes. Liquid injection incinerators are refractory-lined
chambers, generally cylindrical in shape and equipped with a primary combustor and often secondary injection nozzles for high-water-content waste materials. The liquids are atomized through nozzles, exposed to high temperature fuel burner flames, vaporized, superheated, and when combined with air in a turbulent zone attain temperature levels from 1,800 to 3,000°F. Residence time in the chamber is based on the flow volume of these combined products of combustion (fuel, air, and liquid wastes) in actual cubic feet per second. The physical volume of the chamber in cubic feet determines the total time of these gases in the chamber. This time may vary from 0.5 seconds up to 2.5 seconds. The toxic organic components of the liquid waste are oxidized to carbon dioxide, water vapor, oxygen, nitrogen, and acid gases. Acid gases formed are cleaned from the exhaust stream by wet scrubbers , thus allowing clean products to leave the exhaust stack. Incineration has resulted in the ultimate answer to the disposal of these waste materials.

Emission-Control Systems

A great amount of effort has gone into the proper design of air pollution control systems associated with incinerators. Most liquid injection incinerators generate acid gases: hydrogen chloride, sulfur oxides, nitrogen oxides, and others. A proper scrubber is required for the absorption of acid gases.

In systems burning solid and liquid wastes, the wastes may contain toxic metals such as arsenic, beryllium, cadmium, chromium, lead, and mercury.

Particulates that form may be submicron in size and carried in the combustion gases. These particulates are removed in high-efficiency scrubbers. Wet scrubbers are also used to neutralize acid gases formed from burning wastes containing chlorine, sulfur, phosphorous, and nitrogen compounds. Dry scrubbers are typically bag filters. Most recent larger systems incorporate spray dryers for acid-gas removal followed by baghouses for ash-particulate removal. When wet packed tower absorber scrubbers are used for HCl, SOx, and NOx scrubbing, a lean acid solution is generated that is then delivered to a lagoon for neutralization with caustic or lime solutions prior to discharge to the plant's wastewater treatment system. See the table for a comparison of scrubber types used for waste incineration.

see also Hazardous Waste; Medical Waste; Solid Waste; Waste to Energy.

Bibliography

American Society of Mechanical Engineers. (1984). Hazardous Waste Incineration: What Engineering Experts Say, Vol. 32. New York.

Oppelt, E.T. (1987). "Incineration of Hazardous Waste—A Critical Review." Journal of the Air Pollution Control Association 37(5):558–586.

Santoleri, J.J. (1985). "Design and Operating Problems of Hazardous Waste Incinerators." Environmental Progress 4(4):246–251.

Joseph J. Santoleri

Incineration

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 polychlorinated biphenyls and other chlorinated hydrocarbons .

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 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 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 yard" 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 2-3 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. As a result 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 particulates 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 Earth's 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 572–932°F (300–500°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 1,742–2,102°F (950–1,150°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 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.

See also Air pollution.

Resources

books

Dennison, R.A., and J. Rushton, eds. Recycling and Incineration: Evaluating the Choices. Washington, DC: Island Press, 1990.

Freedman, B. Environmental Ecology. 2nd ed. San Diego, Academic Press, 1994.

Hemond, H.F. and E.J. Fechner. Chemical Fate and Transport in the Environment. San Diego Academic Press, 1994.

McConnell, Robert, and Daniel Abel. Environmental Issues: Measuring, Analyzing, Evaluating. 2nd ed. Englewood Cliffs, NJ: Prentice Hall, 2002.

Bill Freedman

KEY TERMS

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."

Incineration

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 type—usually of a horizontal burning grate type—a 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 ]

RESOURCES

BOOKS

Brunner, C. R. Handbook of Incineration Systems. New York: McGraw-Hill, 1991.

Edwards, B. H., et al. Emerging Technologies for the Control of Hazardous Wastes. Park Ridge, NJ: Noyes Data Corporation, 1983.

Hickman Jr., H. L., et al. Thermal Conversion Systems for Municipal Solid Waste. Park Ridge, NJ: Noyes Publications, 1984.

Vesilind, R. A., and A. E. Rimer. Unit Operations in Resource Recovery Engineering. Englewood Cliffs, NJ: Prentice-Hall, 1981.

Wentz, C. A. Hazardous Waste Management. New York: McGraw-Hill, 1989.

in·cin·er·ate / inˈsinəˌrāt/ • v. [tr.] (often be incinerated) destroy (something, esp. waste material) by burning: such garbage must be incinerated at the hospital.DERIVATIVES: in·cin·er·a·tion / -ˌsinəˈrāshən/ n.