Incineration
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Incineration is the process of burning waste streams under controlled industrial conditions. There are two major categories of incineration: hazardous waste and municipal solid waste. The differences in incineration arise from both the different characteristics of the waste stream and their concomitant regulatory contexts. In addition, there are other waste streams that undergo specialized incineration, such as medical waste and chemical weapons.
The primary purpose of incineration is the disposal of waste. Incineration often functions as an alternative to other disposal methods, especially landfilling. Incineration reduces the overall volume of the waste stream and, especially for hazardous wastes, is intended to reduce the wastes' toxicity and other hazardous characeristics. As a disposal method, all forms of incineration are less desirable than more preventive means of waste management, such as source reduction and the recycling of municipal waste. Nevertheless, incineration can be operated in a manner that the combustion of fuels and other materials generates useable energy. As a result, incineration is sometimes characterized as "energy recovery", though the merits of such energy generation are disputed. Although most incinerators are dedicated facilities for the purposes of incineration, cement kilns are also used for incineration purposes (see below).
Incineration generates several forms of waste itself, such as the emission of unburned gases and metals, the hazardous secondary products of combustion, and leftover ash. (The ash is often regulated as a hazardous waste itself and must be landfilled.) The health hazards associated with these emissions and incinerator wastes are the subject of intense controversy. Among the unintended pollutants caused by incineration are dioxins and furans, which are the subject of ongoing study and debate. In addition, the transportation of waste -- especially hazardous waste -- may be pose accident risks to neighboring communities.
The siting of new combustion facilities is often a trigger for local disputes over incineration. In addition, there are often political disputes over the expansion, regulation, and management of incinerators.
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History
The history of municipal waste incineration is linked intimately to the history of landfills and other disposal alternatives. The merits of incineration are inevitably judged in relation to those of the alternatives. Since the 1970s, recycling and other prevention measures have changed the context for such judgements.
The history of hazardous waste incineration is more recent. It is marked by major milestones in environmental regulation, e.g., RCRA, and the ebb and flow of local disputes over the siting and management of incinerator facilities.
The debate over incineration
Use of incinerators for waste management is controversial. The debate over incinerators typically involves business interests (representing both waste generators and incinerator firms), government regulators, and local citizens who must weigh the economic appeal of local industrial activity with their concerns over health and environmental risk.
People and organizations professionally involved in this issue include EPA and AQMD.
Much of the debate over incineration has focused on the traces of residual by-products of incineration, some of which are highly toxic in minute amounts. As a result, attention often focuses on non-combustible by-products, such as mercury and dioxin, that are equivalent to less than 1% of the overall waste stream.
The debate over "energy recovery"
Incinerators can be used for generating electricity or provide energy in other ways such as generating steam for heat. Such a use is known as waste to energy or energy recovery. However, a significant amount of energy is lost due to "scrubbers", and other methods used to clean up the exhaust.
Special waste streams
Medical waste incineration entails the combustion of a waste stream with special biological contamination risks. One of the major problems with medical waste is that there are many relatively-small and under-regulated, under-supervised combustion sites.
Chemical weapons are also incinerated. For example, the U.S. has thousands of tons of sarin stockpiled during the Cold War era, as a countermeasure against even larger stockpiles of the Cold War enemies. Having signed the international ban, US Government has been trying its best to destroy that sarin by the best method, that is by incineration, but the destruction is hampered by the environmental concerns about incineration.
The economics and scope of hazardous waste incineration
The use of incinerators has been on the decline in the United States due in part to high disposal costs. There were 98 such plants in 2002 and 89 in 2004, so it is often cheaper to take waste to a landfill, with obvious harm to the Earth. The difficulties and costs of disposing of harmful chemical wastes in America are high due to the regulation of waste transport, storage, managment and incineration.
Cement Kilns
A versatile type of incinerator is the cement kiln, whose main product is Portland cement. A cement kiln is a rotating cylinder, almost horizontal but slightly inclined, with the upper end continuously fed with a mixture of clay and lime or limestone, and the lower end fed with burning fuel. The temperature of thousands of degrees causes the lime and clay react chemically, and a continuous stream of the white-hot molten portland cement flows from the lower end of the cylinder.
Cement kilns benefit from this fact in their auxiliary role of incinerators, since by their nature, they combine incineration with scrubbing those inorganic gases. Kilns convert and lock those gases and ashes into mineral products. This is done by the molten cement and lime covering the entire walls of the rotating kiln. The alkaline properties of the hot, molten mixture neutralize those gases. A conventional incinerator would produce sulfur dioxide, or when equipped with a scrubber, harmless but cumbersome by-products. A cement kiln converts that sulfur dioxide into the mineral of gypsum, later locked in the portland concrete. The chlorine becomes less toxic calcium chloride, the phosphorus becomes the mineral of apatite, fluorine becomes the mineral of spat, and other similar reactions occur. The only remaining gaseous contaminants that leave a cement kiln are small amounts of nitrogen oxide, and residues of dioxins.
Potential technologies
A process that is hoped to end up supplanting incineration of plastics (if proved to be efficient) is thermal depolymerisation. The reality however is, that (unlike the high-temperature incineration), the intermediate-temperature thermal depolymerisation produces almost equal mass of hundreds of chemical compounds, for which there is no application, and many of them are hazardous and harmful.
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