Heat treatment is a method of treating waste by applying heat to reduce volume and generate energy. It’s used in municipal waste management, cement kilns, and thermal depolymerization. Thermal desorption vaporizes compounds without burning, while vitrification turns materials into glass. Thermal depolymerization breaks down waste biomass and plastic feedstock into hydrocarbon chains. However, the process generates toxic air pollutants. Japan is the leading country in thermal treatment of municipal solid waste.
Heat treatment is most often a term that refers to a method of treating waste that cannot be recycled into the consumer products industry. By applying heat to waste at specific levels, or, in effect, incinerating it, the volume of stranded materials is drastically reduced and combustible materials are burned for electrical energy. A related field of thermal treatment is the treatment of contaminated soil or groundwater to remove pollutants. The main objective of such treatments is to separate hydrocarbons and other organic compounds from inorganic materials such as heavy metals and metallic salts. Some of the industries that rely on heat treatment for economic efficiency and environmental compliance are the municipal waste management sector, cement kilns, and the emerging thermal depolymerization (TDP) industry that creates crude oil from waste .
Waste management through thermal treatment is not always a final step in the waste process. In some cases, the process produces waste compounds that comply with environmental laws and can be landfilled. In situations where large quantities of heavy metal compounds are produced, these materials need to be further processed or shipped to facilities that can use the waste in some sort of industrial production. However, the cost for waste treatment with thermal methods is considered to be quite low and is mainly a labor cost factor.
There are two main types of heat treatment for waste as of 2011. Incineration is used in cement kilns at temperatures between 2,552° and 2,732° Fahrenheit (1,400° to 1,500° Celsius), where the hydrocarbon compounds are destroyed or burned as fuel, and also applies to hazardous forms of biological waste such as that produced in the medical field. Other approaches to incineration include pyrolysis which involves the breakdown of organic compounds with no oxygen present and gasification which reacts the same compounds with oxygen and steam to produce syngas, a fuel composed primarily of carbon monoxide and hydrogen.
Thermal desorption is the second heat treatment method 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 which are vaporized and collected for further use or disposal. Treating soil or water in this way 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 materials are reformed into a type of glass that removes organic compounds and traps metals and radionuclides. Vitrification is an expensive process, however, and must be conducted at temperatures between 2,912° and 3,632° Fahrenheit (1,600° to 2,000° Celsius).
Thermal depolymerization is another form of thermal waste treatment, using waste biomass and plastic feedstock 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 itself could supply until 1996, when refinements to the process made it economically viable.
It is estimated that, as of 2007, at least 3,198,916 tons of municipal solid waste (MSW) have been converted into energy on an annual basis by the top three Western companies in the sector. However, this is only a very small amount of the solid waste actually produced worldwide each year, with China alone producing around 211,000,000 tons of municipal solid waste in 2007 alone. As of 2007, Japan is estimated to be the leading in the thermal treatment of municipal solid waste, where over 40,000,000 tons have been treated. The main disadvantage of thermal treatment is that, despite strict controls, the process generates significant quantities of highly toxic air pollutants, such as dioxin, mercury and carbon monoxide compounds.
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