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Thermal treatment is most often a term that refers to a method of processing waste that cannot be recycled back into the consumer products sector. By applying heat to the waste at specific levels, or, in fact, incinerating it, the volume of unrecoverable materials is reduced dramatically and combustible materials are burned for the waste-to-energy generation of electrical power. A related field of thermal treatment is the processing of contaminated soil or groundwater to remove pollutants. The primary goal of such treatments is to separate hydrocarbons and other organic compounds from inorganic materials like heavy metals and metallic salts. Some of the industries that rely on thermal treatment for economic efficiency and compliance with environmental laws are the municipal waste management sector, cement kilns, and the emerging industry of thermal depolymerization (TDP) that creates crude oil from waste.
Waste management through thermal treatment is not always a final stage process for the waste. In some cases, the process produces waste compounds that are in compliance with environmental laws and can be disposed of in landfills. In situations where high amounts of heavy-metal compounds are produced, these materials must be processed further or shipped to facilities that can use the waste in some sort of industrial production. The cost for treating waste with thermal methods is considered to be fairly low, however, and is mostly a factor of labor expenses.
There are two main types of thermal treatment for waste as of 2011. Incineration is used in cement kilns at temperatures of 2,552° to 2,732° Fahrenheit (1,400° to 1,500° Celsius), where hydrocarbon compounds are destroyed or burned for fuel, and it is also applied to dangerous forms of biological waste such as that produced in the medical field. Other approaches to incineration include pyrolysis involving the breakdown of organic compounds without oxygen present, and gasification that reacts the same compounds with oxygen and steam to produce syngas, a fuel composed of mostly carbon monoxide and hydrogen.
Thermal desorption is the second method of thermal treatment available as of 2011, where compounds are vaporized but not burned. The methodology can be used to treat polluted water and soil on site, to remove volatile organic compounds that are vaporized, and collected for further use or disposal. Treating soil or water in this manner is done through various methods, including electrical resistance and radio frequency heating or the injection of hot compounds such as air, water, or steam. Soil and water that have extreme levels of contamination such as from radioactive waste are treated through a thermal desorption process known as vitrification, where the materials are reformed into a type of glass that removes organic compounds and traps metals and radionuclides. Vitrification is an expensive process, however, that must be conducted at temperatures of 2,912° to 3,632° Fahrenheit (1,600° to 2,000° Celsius).
Thermal depolymerization is another form of thermal treatment of waste, which uses the feedstock of waste biomass and plastic in an accelerated version of the natural process that generates fossil fuels. Pressure and heat are applied to the waste over the course of several hours to break down the molecular structure of the compounds into simpler hydrocarbon chains. Initially, thermal depolymerization required more energy to create the fuel than the fuel could itself provide until 1996 when refinements in the process made it economically viable.
It is estimated that, as of 2007, at least 3,198,916 tons of municipal solid waste (MSW) was being converted to energy on an annual basis by the top three western companies in the field. This is only a very small amount of the solid waste actually produced around the globe annually, however, with China alone producing about 211,000,000 tons of MSW in 2007 alone. Japan is estimated to be leading the world as of 2007 in thermal treatment of MSW, where in excess of 40,000,000 tons were processed. The major drawback to thermal treatment is that, despite strict controls, the process generates significant amounts of highly toxic air pollutants, such as dioxin compounds, mercury, and carbon monoxide.