Incineration

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 NO_x). 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.