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Concept
Thermal depolymerization
Summary
Thermal depolymerization (TDP) is the process of converting a polymer into a monomer or a mixture of monomers, by predominantly thermal means. It may be catalysed or un-catalysed and is distinct from other forms of depolymerisation which may rely on the use of chemicals or biological action. This process is associated with an increase in entropy.
For most polymers thermal depolymerisation is chaotic process, giving a mixture of volatile compounds. Materials may be depolymerised in this way during waste management, with the volatile components produced being burnt as a form of synthetic fuel in a waste-to-energy process. For other polymers thermal depolymerisation is an ordered process giving a single product, or limited range of products, these transformations are usually more valuable and form the basis of some plastic recycling technologies.
For most polymeric materials thermal depolymerisation proceeds in a disordered manner, with random chain scission giving a mixture of volatile compounds. The result is broadly akin to pyrolysis, although at higher temperatures gasification takes place. These reactions can be seen during waste management, with the products being burnt as synthetic fuel in a waste-to-energy process. In comparison to simply incinerating the starting polymer, depolymerisation gives a material with a higher heating value which can be burnt more efficiently and may also be sold. Incineration can also produce harmful dioxins and dioxin-like compounds and requires specially designed reactors and emission control systems in order to be performed safely. As the depolymerisation step requires heat it is energy-consuming, thus the ultimate balance of energy efficiency compared to straight incineration can be very tight and has been the subject of criticism.
Many agricultural and animal wastes can be processed, but these are often already used as fertilizer, animal feed, and, in some cases, as feedstocks for paper mills or as low-quality boiler fuel. Thermal depolymerisation can convert these into more economically valuable materials.
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Related concepts (7)
Synthetic fuel
Synthetic fuel or synfuel is a liquid fuel, or sometimes gaseous fuel, obtained from syngas, a mixture of carbon monoxide and hydrogen, in which the syngas was derived from gasification of solid feedstocks such as coal or biomass or by reforming of natural gas. Common ways for refining synthetic fuels include the Fischer–Tropsch conversion, methanol to gasoline conversion, or direct coal liquefaction. The term 'synthetic fuel' or 'synfuel' has several different meanings and it may include different types of fuels.
Waste-to-energy
Waste-to-energy (WtE) or energy-from-waste (EfW) is the process of generating energy in the form of electricity and/or heat from the primary treatment of waste, or the processing of waste into a fuel source. WtE is a form of energy recovery. Most WtE processes generate electricity and/or heat directly through combustion, or produce a combustible fuel commodity, such as methane, methanol, ethanol or synthetic fuels. The first incinerator or "Destructor" was built in Nottingham, UK, in 1874 by Manlove, Alliott & Co.
Pyrolysis oil
Pyrolysis oil, sometimes also known as bio-crude or bio-oil, is a synthetic fuel under investigation as substitute for petroleum. It is obtained by heating dried biomass without oxygen in a reactor at a temperature of about with subsequent cooling. Pyrolysis oil is a kind of tar and normally contains levels of oxygen too high to be considered a pure hydrocarbon. This high oxygen content results in non-volatility, corrosiveness, immiscibility with fossil fuels, thermal instability, and a tendency to polymerize when exposed to air.