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{{Short description|Process for breaking-down polymers}}
'''Thermal depolymerization''' ('''TDP''') is the process of converting a ] into a ] or a mixture of monomers,<ref>{{GoldBookRef|title=Depolymerization|file = D01600}}</ref> by predominately thermal means. It may be ] or un-catalysed and is distinct from other forms of ] which may rely on the use of chemicals or biological action. This process is associated with an increase in ]. '''Thermal depolymerization''' ('''TDP''') is the process of converting a ] into a ] or a mixture of monomers,<ref>{{GoldBookRef|title=Depolymerization|file = D01600}}</ref> by predominantly thermal means. It may be ] or un-catalyzed and is distinct from other forms of ] which may rely on the use of chemicals or biological action. This process is associated with an increase in ].


For most polymers thermal depolymerisation is chaotic process, giving a mixture of volatile compounds. Materials may be depolymerised in this way during ], with the volatile components produced being burnt as a form of ] in a ] 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. For most polymers, thermal depolymerization is chaotic process, giving a mixture of ] compounds. Materials may be depolymerized in this way during ], with the volatile components produced being burnt as a form of ] in a ] process. For other polymers, thermal depolymerization 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 ] technologies.<ref name="Thiounn2020">{{cite journal |last1=Thiounn |first1=Timmy |last2=Smith |first2=Rhett C. |title=Advances and approaches for chemical recycling of plastic waste |journal=Journal of Polymer Science |date=15 May 2020 |volume=58 |issue=10 |pages=1347–1364 |doi=10.1002/pol.20190261|doi-access=free}}</ref>


==Disordered depolymerisation== ==Disordered depolymerization==
For most polymeric materials thermal depolymerisation proceeds in a disordered manner, with random ] giving a mixture of volatile compounds. The result is broadly akin to ], although at higher temperatures ] takes place. These reactions can be seen during ], with the products being burnt as synthetic fuel in a ] process. In comparison to simply ] the starting polymer, depolymerisation gives a material with a higher ] which can be burnt more efficiently and may also be sold. Incineration can also produce harmful ]. However, as the depolymerisation step is energy-consuming the ultimate balance of energy efficiency can be very tight and has been the subject of criticism.<ref>{{cite journal |last1=Rollinson |first1=Andrew Neil |last2=Oladejo |first2=Jumoke Mojisola |title='Patented blunderings', efficiency awareness, and self-sustainability claims in the pyrolysis energy from waste sector |journal=Resources, Conservation and Recycling |date=February 2019 |volume=141 |pages=233–242 |doi=10.1016/j.resconrec.2018.10.038}}</ref> For most polymeric materials, thermal depolymerization proceeds in a disordered manner, with random ] giving a mixture of volatile compounds. The result is broadly akin to ], although at higher temperatures ] takes place. These reactions can be seen during ], with the products being burnt as synthetic fuel in a ] process. In comparison to simply ] the starting polymer, depolymerization gives a material with a higher ], which can be burnt more efficiently and may also be sold. Incineration can also produce harmful ] and requires specially designed reactors and emission control systems in order to be performed safely. As the depolymerization step requires heat, it is energy-consuming; thus, the ultimate balance of ] compared to straight incineration can be very tight and has been the subject of criticism.<ref>{{cite journal |last1=Rollinson |first1=Andrew Neil |last2=Oladejo |first2=Jumoke Mojisola |title='Patented blunderings', efficiency awareness, and self-sustainability claims in the pyrolysis energy from waste sector |journal=Resources, Conservation and Recycling |date=February 2019 |volume=141 |pages=233–242 |doi=10.1016/j.resconrec.2018.10.038|s2cid=115296275 }}</ref>


===Biomass=== ===Biomass===
Many agricultural and animal wastes can be processed, but these are often already used as ], animal feed, and, in some cases, as feedstocks for ]s or as low-quality ] fuel. Thermal depolymerisation can convert these into more economically valuable materials. Numerous ] technologies have been developed. In general, ]s contain oxygen atoms which are retained during pyrolysis, giving liquid products rich in ]s and ]s.<ref>{{cite journal |last1=Collard |first1=François-Xavier |last2=Blin |first2=Joël |title=A review on pyrolysis of biomass constituents: Mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin |journal=Renewable and Sustainable Energy Reviews |date=October 2014 |volume=38 |pages=594–608 |doi=10.1016/j.rser.2014.06.013}}</ref> These can be viewed as partially oxidised and make for low-grade fuels. ] technologies dehydrate the biomass during thermal processing to produce a more energy rich product stream.<ref>{{cite journal |last1=Kumar |first1=Mayank |last2=Olajire Oyedun |first2=Adetoyese |last3=Kumar |first3=Amit |title=A review on the current status of various hydrothermal technologies on biomass feedstock |journal=Renewable and Sustainable Energy Reviews |date=January 2018 |volume=81 |pages=1742–1770 |doi=10.1016/j.rser.2017.05.270}}</ref> Similarly, ] produces hydrogen, a very high energy fuel. Many agricultural and animal wastes can be processed, but these are often already used as ], animal feed, and, in some cases, as feedstocks for ]s or as low-quality ] fuel. Thermal depolymerization can convert these into more economically valuable materials. Numerous ] technologies have been developed. In general, ]s contain oxygen atoms, which are retained during pyrolysis, giving liquid products rich in ]s and ]s.<ref>{{cite journal |last1=Collard |first1=François-Xavier |last2=Blin |first2=Joël |title=A review on pyrolysis of biomass constituents: Mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin |journal=Renewable and Sustainable Energy Reviews |date=October 2014 |volume=38 |pages=594–608 |doi=10.1016/j.rser.2014.06.013}}</ref> These can be viewed as partially oxidized and make for low-grade fuels. ] technologies dehydrate the biomass during thermal processing to produce a more energy-rich product stream.<ref>{{cite journal |last1=Kumar |first1=Mayank |last2=Olajire Oyedun |first2=Adetoyese |last3=Kumar |first3=Amit |title=A review on the current status of various hydrothermal technologies on biomass feedstock |journal=Renewable and Sustainable Energy Reviews |date=January 2018 |volume=81 |pages=1742–1770 |doi=10.1016/j.rser.2017.05.270}}</ref> Similarly, ] produces hydrogen, a very high-energy fuel.


===Plastics=== ===Plastics===
] consists mostly of ] and may be actively ] from ]. Pyrolysis of mixed plastics can give a fairly broad mix of chemical products (between about 1 and 15 carbon atoms) including gases and aromatic liquids.<ref>{{cite journal |last1=Kaminsky |first1=W. |last2=Schlesselmann |first2=B. |last3=Simon |first3=C.M. |title=Thermal degradation of mixed plastic waste to aromatics and gas |journal=Polymer Degradation and Stability |date=August 1996 |volume=53 |issue=2 |pages=189–197 |doi=10.1016/0141-3910(96)00087-0}}</ref> Catalysts can give a better defined product with a higher value.<ref>{{cite journal |last1=Aguado |first1=J. |last2=Serrano |first2=D. P. |last3=Escola |first3=J. M. |title=Fuels from Waste Plastics by Thermal and Catalytic Processes: A Review |journal=Industrial & Engineering Chemistry Research |date=5 November 2008 |volume=47 |issue=21 |pages=7982–7992 |doi=10.1021/ie800393w}}</ref> Likewise, ] can be employed to give ] products. ] consists mostly of ] and may be actively ] from ]. Pyrolysis of mixed plastics can give a fairly broad mix of chemical products (between about 1 and 15 carbon atoms), including gases and aromatic liquids.<ref>{{cite journal |last1=Kaminsky |first1=W. |last2=Schlesselmann |first2=B. |last3=Simon |first3=C.M. |title=Thermal degradation of mixed plastic waste to aromatics and gas |journal=Polymer Degradation and Stability |date=August 1996 |volume=53 |issue=2 |pages=189–197 |doi=10.1016/0141-3910(96)00087-0}}</ref> Catalysts can give a better-defined product with a higher value.<ref>{{cite journal |last1=Aguado |first1=J. |last2=Serrano |first2=D. P. |last3=Escola |first3=J. M. |title=Fuels from Waste Plastics by Thermal and Catalytic Processes: A Review |journal=Industrial & Engineering Chemistry Research |date=5 November 2008 |volume=47 |issue=21 |pages=7982–7992 |doi=10.1021/ie800393w}}</ref> Likewise, ] can be employed to give ] products. The presence of ] can be problematic, as its thermal depolymerization generates large amounts of ], which can corrode equipment and cause undesirable chlorination of the products. It must be either excluded or compensated for by installing dechlorination technologies.<ref>{{cite journal |last1=Fukushima |first1=Masaaki |last2=Wu |first2=Beili |last3=Ibe |first3=Hidetoshi |last4=Wakai |first4=Keiji |last5=Sugiyama |first5=Eiichi |last6=Abe |first6=Hironobu |last7=Kitagawa |first7=Kiyohiko |last8=Tsuruga |first8=Shigenori |last9=Shimura |first9=Katsumi |last10=Ono |first10=Eiichi |title=Study on dechlorination technology for municipal waste plastics containing polyvinyl chloride and polyethylene terephthalate |journal=Journal of Material Cycles and Waste Management |date=June 2010 |volume=12 |issue=2 |pages=108–122 |doi=10.1007/s10163-010-0279-8|s2cid=94190060 }}</ref> ] and ] account for just less than half of global plastic production and, being pure ], have a higher potential for conversion to fuel.<ref name=2011CommercialRev>{{cite journal |last1=Butler |first1=E. |last2=Devlin |first2=G. |last3=McDonnell |first3=K. |title=Waste Polyolefins to Liquid Fuels via Pyrolysis: Review of Commercial State-of-the-Art and Recent Laboratory Research |journal=Waste and Biomass Valorization |date=1 August 2011 |volume=2 |issue=3 |pages=227–255 |doi=10.1007/s12649-011-9067-5|hdl=10197/6103 |s2cid=98550187 |hdl-access=free }}</ref> Plastic-to-fuel technologies have historically struggled to be economically viable due to the costs of collecting and sorting the plastic and the relatively low value of the fuel produced.<ref name=2011CommercialRev /> Large plants are seen as being more economical than smaller ones,<ref>{{cite journal |title=Pyrolysis of plastic waste for production of heavy fuel substitute: A techno-economic assessment |journal=Energy |date=15 April 2018 |volume=149 |pages=865–874 |doi=10.1016/j.energy.2018.02.094|last1=Fivga |first1=Antzela |last2=Dimitriou |first2=Ioanna |url=http://eprints.nottingham.ac.uk/50558/3/A.%20Fivga%202018%20Authors%20copy.pdf }}</ref><ref>{{cite journal |last1=Riedewald |first1=Frank |last2=Patel |first2=Yunus |last3=Wilson |first3=Edward |last4=Santos |first4=Silvia |last5=Sousa-Gallagher |first5=Maria |title=Economic assessment of a 40,000 t/y mixed plastic waste pyrolysis plant using direct heat treatment with molten metal: A case study of a plant located in Belgium |journal=Waste Management |date=February 2021 |volume=120 |pages=698–707 |doi=10.1016/j.wasman.2020.10.039|pmid=33191052 |s2cid=226972785 |hdl=10468/12445 |hdl-access=free }}</ref> but require more investment to build.
The presence of ] can be problematic, as its thermal depolymerisation generates large amounts of ], which can corrode equipment and cause undesirable chlorination of the products. It must be either excluded or compensated for by installing dichlorination technologies.<ref>{{cite journal |last1=Fukushima |first1=Masaaki |last2=Wu |first2=Beili |last3=Ibe |first3=Hidetoshi |last4=Wakai |first4=Keiji |last5=Sugiyama |first5=Eiichi |last6=Abe |first6=Hironobu |last7=Kitagawa |first7=Kiyohiko |last8=Tsuruga |first8=Shigenori |last9=Shimura |first9=Katsumi |last10=Ono |first10=Eiichi |title=Study on dechlorination technology for municipal waste plastics containing polyvinyl chloride and polyethylene terephthalate |journal=Journal of Material Cycles and Waste Management |date=June 2010 |volume=12 |issue=2 |pages=108–122 |doi=10.1007/s10163-010-0279-8|s2cid=94190060 }}</ref>
] and ] account for just less than half of global plastic production and being pure ] have a higher potential for conversion to fuel.<ref name=2011CommercialRev>{{cite journal |last1=Butler |first1=E. |last2=Devlin |first2=G. |last3=McDonnell |first3=K. |title=Waste Polyolefins to Liquid Fuels via Pyrolysis: Review of Commercial State-of-the-Art and Recent Laboratory Research |journal=Waste and Biomass Valorization |date=1 August 2011 |volume=2 |issue=3 |pages=227–255 |doi=10.1007/s12649-011-9067-5|hdl=10197/6103 |s2cid=98550187 |hdl-access=free }}</ref> Plastic-to-fuel technologies have historically struggled to be economically viable due to the costs of collecting and sorting the plastic and the relatively low value of the fuel produced.<ref name=2011CommercialRev /> Large plants are seen as being more economical than smaller ones,<ref>{{cite journal |title=Pyrolysis of plastic waste for production of heavy fuel substitute: A techno-economic assessment |journal=Energy |date=15 April 2018 |volume=149 |pages=865–874 |doi=10.1016/j.energy.2018.02.094|last1=Fivga |first1=Antzela |last2=Dimitriou |first2=Ioanna |url=http://eprints.nottingham.ac.uk/50558/3/A.%20Fivga%202018%20Authors%20copy.pdf }}</ref><ref>{{cite journal |last1=Riedewald |first1=Frank |last2=Patel |first2=Yunus |last3=Wilson |first3=Edward |last4=Santos |first4=Silvia |last5=Sousa-Gallagher |first5=Maria |title=Economic assessment of a 40,000 t/y mixed plastic waste pyrolysis plant using direct heat treatment with molten metal: A case study of a plant located in Belgium |journal=Waste Management |date=February 2021 |volume=120 |pages=698–707 |doi=10.1016/j.wasman.2020.10.039|pmid=33191052 }}</ref> but require more investment to build. The approach can however, lead to a mild net-decrease in ] emissions.<ref>{{cite journal |last1=Benavides |first1=Pahola Thathiana |last2=Sun |first2=Pingping |last3=Han |first3=Jeongwoo |last4=Dunn |first4=Jennifer B. |last5=Wang |first5=Michael |title=Life-cycle analysis of fuels from post-use non-recycled plastics |journal=Fuel |date=September 2017 |volume=203 |pages=11–22 |doi=10.1016/j.fuel.2017.04.070}}</ref>


The method can, however, result in a mild net-decrease in ] emissions,<ref>{{cite journal |last1=Benavides |first1=Pahola Thathiana |last2=Sun |first2=Pingping |last3=Han |first3=Jeongwoo |last4=Dunn |first4=Jennifer B. |last5=Wang |first5=Michael |title=Life-cycle analysis of fuels from post-use non-recycled plastics |journal=Fuel |date=September 2017 |volume=203 |pages=11–22 |doi=10.1016/j.fuel.2017.04.070|osti=1353191 |doi-access=free }}</ref> though other studies dispute this. For example, a 2020 study released by Renolds on their own Hefty EnergyBag program shows net greenhouse gas emissions. The study showed then when all cradle-to-grave energy costs are tallied, burning in a cement kiln was far superior. Cement kiln fuel scored a -61.1&nbsp;kg {{CO2}} equivalents compared to +905&nbsp;kg {{CO2}} eq. It also fared far worse in terms of landfill reduction vs. kiln fuel.<ref name="Table ES.1 – End of Life GWP Summary Table">{{cite web |last1=Sustainable Solutions |title=Hefty® EnergyBag® Program Life Cycle Assessment |url=https://www.hefty.com/sites/default/files/2021-01/Hefty-EnergyBag-Program-Life-Cycle-Assessment-Aug-2020.pdf |website=hefty.com |publisher=Reynolds/Sustainable Solutions |access-date=21 June 2022 |ref=Table E51}}</ref> Other studies have confirmed that plastics pyrolysis to fuel programs are also more energy intensive.<ref>{{cite news |last1=Brock |first1=Joe |last2=VOLCOVICI |first2=VALERIE |last3=Geddie |first3=John |title=The Recycling Myth |url=https://www.reuters.com/investigates/special-report/environment-plastic-oil-recycling/ |access-date=21 June 2022 |work=Reuters}}</ref><ref>{{cite web | url=https://www.theatlantic.com/ideas/archive/2022/05/single-use-plastic-chemical-recycling-disposal/661141/ | title=Plastic Recycling Doesn't Work and Will Never Work | website=] | date=30 May 2022 }}</ref>
⚫ In tire waste management, ] is also an option. Oil derived from tire rubber pyrolysis contains high sulfur content, which gives it high potential as a pollutant and requires ] before use.<ref>{{cite journal|author1=Choi, G.-G. |author2=Jung, S.-H. |author3=Oh, S.-J. |author4=Kim, J.-S. |title=Total utilization of waste tire rubber through pyrolysis to obtain oils and CO2 activation of pyrolysis char|journal=Fuel Processing Technology|volume=123|pages=57–64|doi=10.1016/j.fuproc.2014.02.007|year=2014}}</ref><ref>Ringer, M.; Putsche, V.; Scahill, J. (2006) {{webarchive|url=https://web.archive.org/web/20161230234405/http://www.nrel.gov/docs/fy07osti/37779.pdf |date=2016-12-30 }}; NREL/TP-510-37779; National Renewable Energy Laboratory (NREL), Golden, CO.</ref> The area faces legislative, economic, and marketing obstacles.<ref name='j.rser.2013.02.038'>{{cite journal | year = 2013 | title = Waste tyre pyrolysis – A review, Renewable and Sustainable | journal = Energy Reviews | volume = 23 | pages = 179–213 | doi = 10.1016/j.rser.2013.02.038 | last1 = Martínez | first1 = Juan Daniel | last2 = Puy | first2 = Neus | last3 = Murillo | first3 = Ramón | last4 = García | first4 = Tomás | last5 = Navarro | first5 = María Victoria | last6 = Mastral | first6 = Ana Maria }}</ref> In most cases tires are simply incinerated as ].

⚫ For tire waste management, ] is also an option. Oil derived from tire rubber pyrolysis contains high sulfur content, which gives it high potential as a pollutant and requires ] before use.<ref>{{cite journal|author1=Choi, G.-G. |author2=Jung, S.-H. |author3=Oh, S.-J. |author4=Kim, J.-S. |title=Total utilization of waste tire rubber through pyrolysis to obtain oils and {{CO2}} activation of pyrolysis char|journal=Fuel Processing Technology|volume=123|pages=57–64|doi=10.1016/j.fuproc.2014.02.007|year=2014}}</ref><ref>Ringer, M.; Putsche, V.; Scahill, J. (2006) {{webarchive|url=https://web.archive.org/web/20161230234405/http://www.nrel.gov/docs/fy07osti/37779.pdf |date=2016-12-30 }}; NREL/TP-510-37779; National Renewable Energy Laboratory (NREL), Golden, CO.</ref> The area faces legislative, economic, and marketing obstacles.<ref name='j.rser.2013.02.038'>{{cite journal | year = 2013 | title = Waste tyre pyrolysis – A review, Renewable and Sustainable | journal = Energy Reviews | volume = 23 | pages = 179–213 | doi = 10.1016/j.rser.2013.02.038 | last1 = Martínez | first1 = Juan Daniel | last2 = Puy | first2 = Neus | last3 = Murillo | first3 = Ramón | last4 = García | first4 = Tomás | last5 = Navarro | first5 = María Victoria | last6 = Mastral | first6 = Ana Maria }}</ref> In most cases, tires are simply incinerated as ].


===Municipal waste=== ===Municipal waste===
Thermal treatment of ] can involve the depolymerisation of a very wide range of compounds including plastics and biomass. Technologies can include simple incineration, as well as pyrolysis, ] and ]. All of these are able to accommodate mixed and contaminated feedstocks. The main advantage is the reduction in volume of the waste, particularly in densely populated area lacking suitable sites for new ]s. In many countries incineration with energy recovery remains the most common method, with more advanced technologies being hindered by technical and cost hurdles.<ref>{{cite journal |title=A review on municipal solid waste-to-energy trends in the USA |journal=Renewable and Sustainable Energy Reviews |date=1 March 2020 |volume=119 |pages=109512 |doi=10.1016/j.rser.2019.109512|last1=Mukherjee |first1=C. |last2=Denney |first2=J. |last3=Mbonimpa |first3=E.G. |last4=Slagley |first4=J. |last5=Bhowmik |first5=R. }}</ref><ref>{{cite journal |last1=Fernández-González |first1=J.M. |last2=Grindlay |first2=A.L. |last3=Serrano-Bernardo |first3=F. |last4=Rodríguez-Rojas |first4=M.I. |last5=Zamorano |first5=M. |title=Economic and environmental review of Waste-to-Energy systems for municipal solid waste management in medium and small municipalities |journal=Waste Management |date=September 2017 |volume=67 |pages=360–374 |doi=10.1016/j.wasman.2017.05.003|pmid=28501263 }}</ref> Thermal treatment of ] can involve the depolymerization of a very wide range of compounds, including plastics and biomass. Technologies can include simple incineration as well as pyrolysis, ], and ]. All of these are able to accommodate mixed and contaminated feedstocks. The main advantage is the reduction in volume of the waste, particularly in densely populated areas lacking suitable sites for new ]s. In many countries, incineration with energy recovery remains the most common method, with more advanced technologies being hindered by technical and cost hurdles.<ref>{{cite journal |title=A review on municipal solid waste-to-energy trends in the USA |journal=Renewable and Sustainable Energy Reviews |date=1 March 2020 |volume=119 |pages=109512 |doi=10.1016/j.rser.2019.109512|last1=Mukherjee |first1=C. |last2=Denney |first2=J. |last3=Mbonimpa |first3=E.G. |last4=Slagley |first4=J. |last5=Bhowmik |first5=R. |s2cid=209798113 |doi-access=free }}</ref><ref>{{cite journal |last1=Fernández-González |first1=J.M. |last2=Grindlay |first2=A.L. |last3=Serrano-Bernardo |first3=F. |last4=Rodríguez-Rojas |first4=M.I. |last5=Zamorano |first5=M. |title=Economic and environmental review of Waste-to-Energy systems for municipal solid waste management in medium and small municipalities |journal=Waste Management |date=September 2017 |volume=67 |pages=360–374 |doi=10.1016/j.wasman.2017.05.003|pmid=28501263 }}</ref>


==Ordered depolymerisation== ==Ordered depolymerization==
Some materials thermally decompose in an ordered manner to give a single or limited range of products. By virtue of being pure materials they are usually more valuable than the mixtures produced by disordered thermal depolymerisation. For plastics this is usually the starting ] and when this is recycled back into fresh polymer it is called feedstock recycling. In practice, not all depolymerisation reactions are completely efficient and some competitive pyrolysis is often observed. Some materials thermally decompose in an ordered manner to give a single or limited range of products. By virtue of being pure materials, they are usually more valuable than the mixtures produced by disordered thermal depolymerization. For plastics this is usually the starting ], and when this is recycled back into fresh polymer, it is called feedstock recycling. In practice, not all depolymerization reactions are completely efficient, and some competitive pyrolysis is often observed.


===Biomass=== ===Biomass===
] convert low-value agricultural and animal waste into useful chemicals. The industrial production of ] by the acid catalysed thermal treatment of ] has been in operation for over a century. ] has been the subject of significant research for the potential production of ] and other aromatics compounds,<ref>{{cite journal |last1=Lok |first1=C.M. |last2=Van Doorn |first2=J. |last3=Aranda Almansa |first3=G. |title=Promoted ZSM-5 catalysts for the production of bio-aromatics, a review |journal=Renewable and Sustainable Energy Reviews |date=October 2019 |volume=113 |pages=109248 |doi=10.1016/j.rser.2019.109248}}</ref> although such processes have yet to be fully commercialised.<ref>{{cite journal |last1=Wong |first1=Sie Shing |last2=Shu |first2=Riyang |last3=Zhang |first3=Jiaguang |last4=Liu |first4=Haichao |last5=Yan |first5=Ning |title=Downstream processing of lignin derived feedstock into end products |journal=Chemical Society Reviews |date=2020 |volume=49 |issue=15 |pages=5510–5560 |doi=10.1039/D0CS00134A|pmid=32639496 }}</ref> ] convert low-value agricultural and animal waste into useful chemicals. The industrial production of ] by the acid-catalyzed thermal treatment of ] has been in operation for over a century. ] has been the subject of significant research for the potential production of ] and other aromatic compounds,<ref>{{cite journal |last1=Lok |first1=C.M. |last2=Van Doorn |first2=J. |last3=Aranda Almansa |first3=G. |title=Promoted ZSM-5 catalysts for the production of bio-aromatics, a review |journal=Renewable and Sustainable Energy Reviews |date=October 2019 |volume=113 |pages=109248 |doi=10.1016/j.rser.2019.109248|s2cid=198328225 }}</ref> although such processes have not yet been commercialized with any lasting success.<ref>{{cite journal |last1=Wong |first1=Sie Shing |last2=Shu |first2=Riyang |last3=Zhang |first3=Jiaguang |last4=Liu |first4=Haichao |last5=Yan |first5=Ning |title=Downstream processing of lignin derived feedstock into end products |journal=Chemical Society Reviews |date=2020 |volume=49 |issue=15 |pages=5510–5560 |doi=10.1039/D0CS00134A|pmid=32639496 |s2cid=220405457 |url=https://eprints.lincoln.ac.uk/id/eprint/46399/1/Lignin%20Valorisation%20Review-Chem.%20Soc.%20Rev.%20Final%20version.docx }}</ref>


===Plastics=== ===Plastics===
{{Main|Plastic recycling}} {{Main|Plastic recycling}}


Certain polymers like ], ], ] and ]<ref>{{cite journal |last1=Kaminsky |first1=W |last2=Predel |first2=M |last3=Sadiki |first3=A |title=Feedstock recycling of polymers by pyrolysis in a fluidised bed |journal=Polymer Degradation and Stability |date=September 2004 |volume=85 |issue=3 |pages=1045–1050 |doi=10.1016/j.polymdegradstab.2003.05.002}}</ref> undergo ] to give their starting ]. These can be converted back into new plastic, a process called chemical or feedstock recycling.<ref>{{cite journal |last1=Kumagai |first1=Shogo |last2=Yoshioka |first2=Toshiaki |title=Feedstock Recycling via Waste Plastic Pyrolysis |journal=Journal of the Japan Petroleum Institute |date=1 November 2016 |volume=59 |issue=6 |pages=243–253 |doi=10.1627/jpi.59.243 |url=https://www.jstage.jst.go.jp/article/jpi/59/6/59_243/_article/-char/en|doi-access=free }}</ref><ref>{{cite journal |last1=Rahimi |first1=AliReza |last2=García |first2=Jeannette M. |title=Chemical recycling of waste plastics for new materials production |journal=Nature Reviews Chemistry |date=June 2017 |volume=1 |issue=6 |pages=0046 |doi=10.1038/s41570-017-0046}}</ref><ref>{{cite journal |last1=Coates |first1=Geoffrey W. |last2=Getzler |first2=Yutan D. Y. L. |title=Chemical recycling to monomer for an ideal, circular polymer economy |journal=Nature Reviews Materials |date=July 2020 |volume=5 |issue=7 |pages=501–516 |doi=10.1038/s41578-020-0190-4|bibcode=2020NatRM...5..501C |s2cid=215760966 }}</ref> Certain polymers like ], ], ], and ]<ref>{{cite journal |last1=Kaminsky |first1=W |last2=Predel |first2=M |last3=Sadiki |first3=A |title=Feedstock recycling of polymers by pyrolysis in a fluidised bed |journal=Polymer Degradation and Stability |date=September 2004 |volume=85 |issue=3 |pages=1045–1050 |doi=10.1016/j.polymdegradstab.2003.05.002}}</ref> undergo ] to give their starting ]. These can be converted back into new plastic, a process called chemical or feedstock recycling.<ref>{{cite journal |last1=Kumagai |first1=Shogo |last2=Yoshioka |first2=Toshiaki |title=Feedstock Recycling via Waste Plastic Pyrolysis |journal=Journal of the Japan Petroleum Institute |date=1 November 2016 |volume=59 |issue=6 |pages=243–253 |doi=10.1627/jpi.59.243 |url=https://www.jstage.jst.go.jp/article/jpi/59/6/59_243/_article/-char/en|doi-access=free }}</ref><ref>{{cite journal |last1=Rahimi |first1=AliReza |last2=García |first2=Jeannette M. |title=Chemical recycling of waste plastics for new materials production |journal=Nature Reviews Chemistry |date=June 2017 |volume=1 |issue=6 |pages=0046 |doi=10.1038/s41570-017-0046}}</ref><ref>{{cite journal |last1=Coates |first1=Geoffrey W. |last2=Getzler |first2=Yutan D. Y. L. |title=Chemical recycling to monomer for an ideal, circular polymer economy |journal=Nature Reviews Materials |date=July 2020 |volume=5 |issue=7 |pages=501–516 |doi=10.1038/s41578-020-0190-4|bibcode=2020NatRM...5..501C |s2cid=215760966 }}</ref> In theory, this offers infinite recyclability, but it is also more expensive and has a higher ] than other forms of plastic recycling; however, in practice, this still yields an inferior product at higher energy costs than virgin polymer production in the real world because of contamination.
In theory this offers infinite recyclability but it is also more expensive and has a higher ] than other forms of plastic recycling.


==Related processes== ==Related processes==
Although rarely employed today, ] has historically been performed on a large scale. Thermal depolymerisation is similar to other processes which use ] as a major step to produce fuels, such as direct ].<ref>{{cite web| title =Biomass Program, direct Hydrothermal Liquefaction| publisher =US Department of Energy. Energy Efficiency and Renewable Energy| date =2005-10-13| url =http://www1.eere.energy.gov/biomass/pyrolysis.html#thermal| access-date =2008-01-12| url-status =dead| archive-url =https://web.archive.org/web/20070312025649/http://www1.eere.energy.gov/biomass/pyrolysis.html#thermal| archive-date =2007-03-12}}</ref> Although rarely employed presently, ] has historically been performed on a large scale. Thermal depolymerization is similar to other processes which use ] as a major phase to produce fuels, such as direct ].<ref>{{cite web| title =Biomass Program, direct Hydrothermal Liquefaction| publisher =US Department of Energy. Energy Efficiency and Renewable Energy| date =2005-10-13| url =http://www1.eere.energy.gov/biomass/pyrolysis.html#thermal| access-date =2008-01-12| url-status =dead| archive-url =https://web.archive.org/web/20070312025649/http://www1.eere.energy.gov/biomass/pyrolysis.html#thermal| archive-date =2007-03-12}}</ref> These are distinct from processes using dry materials to depolymerize, such as ]. The term ''thermochemical conversion'' (TCC) has also been used for conversion of biomass to oils, using superheated water, although it is more usually applied to fuel production via pyrolysis.<ref>{{cite journal
These are distinct from processes using dry materials to depolymerize, such as ]. The term Thermochemical Conversion (TCC) has also been used for conversion of biomass to oils, using superheated water, although it is more usually applied to fuel production via pyrolysis.<ref>{{cite journal
| last = Demirba | last = Demirba
| first = Ayhan | first = Ayhan
Line 57: Line 56:
|archive-url = https://web.archive.org/web/20080515195211/http://www.age.uiuc.edu/bee/RESEARCH/tcc/tccpaper3.htm |archive-url = https://web.archive.org/web/20080515195211/http://www.age.uiuc.edu/bee/RESEARCH/tcc/tccpaper3.htm
|archive-date = 2008-05-15 |archive-date = 2008-05-15
⚫ }}</ref> A demonstration plant due to start up in the Netherlands is said to be capable of processing 64 tons of biomass (]) per day into oil.<ref>{{cite web
}}</ref>
Other commercial scale processes include the "SlurryCarb" process operated by EnerTech, which uses similar technology to decarboxylate wet solid biowaste, which can then be physically dewatered and used as a solid fuel called E-Fuel. The plant in Rialto, California, was designed to process 683 tons of waste per day. However, it failed to perform to design standards and was closed down. The Rialto facility defaulted on its bond payments and is in the process of being liquidated.<ref>{{cite web
| last = Sforza
| first = Teri
| title = New plan replaces sewage sludge fiasco
| publisher = Orange county register
⚫ | date = 2007-03-14
| url = http://www.ocregister.com/ocregister/news/local/article_1620010.php
| access-date = 2008-01-27 }}</ref>
The Hydro Thermal Upgrading (HTU) process uses superheated water to produce oil from domestic waste.<ref>{{cite web
| last = de Swaan Arons
| first = Jakob |author2=H. van derKooi |author3=Wei Feng
| title = Hydrothermal Upgrading of Biomass
| publisher = University of Delft
| url = http://www.dct.tudelft.nl/ttf/biomass.htm
| access-date = 2008-02-05}}</ref>
⚫ A demonstration plant is due to start up in The Netherlands said to be capable of processing 64 tons of biomass (]) per day into oil.<ref>{{cite web
| last1 = Goudriaan | last1 = Goudriaan
| first1 = Frans | first1 = Frans
| last2 = Naber | last2 = Naber
| first2= Jaap | first2 = Jaap
| last3= van den Berg | last3 = van den Berg
| first3= Ed | first3 = Ed
| title = Conversion of Biomass Residues to Transportation Fuels with th HTU Process | title = Conversion of Biomass Residues to Transportation Fuels with th HTU Process
| url = http://www.nvrd.nl/nvrd/proceedings/downloadProceedings.asp?filename=618085%20Paper.pdf&filesize=85441 | url = http://www.nvrd.nl/nvrd/proceedings/downloadProceedings.asp?filename=618085%20Paper.pdf&filesize=85441
⚫ | access-date = 2008-01-12
⚫ | access-date = 2008-01-12}}</ref> Thermal depolymerisation differs in that it contains a hydrous process followed by an anhydrous cracking / distillation process.
| archive-date = 2020-06-16
| archive-url = https://web.archive.org/web/20200616024142/https://www.nvrd.nl/cms/nonexistingpage.aspx?404%3Bhttp%3A%2F%2Fwww.nvrd.nl%3A80%2Fnvrd%2Fproceedings%2FdownloadProceedings.asp%3Ffilename=618085%20Paper.pdf&filesize=85441%2F
| url-status = dead
⚫ }}</ref> Thermal depolymerization differs in that it contains a hydrous process followed by an anhydrous cracking / distillation process.


] polymers baring cleavable groups such as ]s and ] can also be completely depolymerised by ] or ], this can be a purely chemical process but may also be promoted by enzymes.<ref>{{cite journal |last1=Wei |first1=Ren |last2=Zimmermann |first2=Wolfgang |title=Microbial enzymes for the recycling of recalcitrant petroleum‐based plastics: how far are we? |journal=Microbial Biotechnology |date=November 2017 |volume=10 |issue=6 |pages=1308–1322 |doi=10.1111/1751-7915.12710|pmid=28371373 |pmc=5658625 }}</ref> Such technologies are less well developed than those of thermal depolymerisation but have the potential for lower energy costs. Thus far ] has been the most heavily studied polymer.<ref>{{cite journal |last1=Geyer |first1=B. |last2=Lorenz |first2=G. |last3=Kandelbauer |first3=A. |title=Recycling of poly(ethylene terephthalate) – A review focusing on chemical methods |journal=Express Polymer Letters |date=2016 |volume=10 |issue=7 |pages=559–586 |doi=10.3144/expresspolymlett.2016.53|doi-access=free }}</ref> Alternatively, waste plastic may be converted into other valuable chemicals (not necessarily monomers) by microbial action.<ref>{{cite journal |last1=Ru |first1=Jiakang |last2=Huo |first2=Yixin |last3=Yang |first3=Yu |title=Microbial Degradation and Valorization of Plastic Wastes |journal=Frontiers in Microbiology |date=21 April 2020 |volume=11 |pages=442 |doi=10.3389/fmicb.2020.00442|pmid=32373075 |pmc=7186362 }}</ref><ref>{{cite journal |last1=Wierckx |first1=Nick |last2=Prieto |first2=M. Auxiliadora |last3=Pomposiello |first3=Pablo |last4=Lorenzo |first4=Victor |last5=O'Connor |first5=Kevin |last6=Blank |first6=Lars M. |title=Plastic waste as a novel substrate for industrial biotechnology |journal=Microbial Biotechnology |date=November 2015 |volume=8 |issue=6 |pages=900–903 |doi=10.1111/1751-7915.12312|pmid=26482561 |pmc=4621443 }}</ref> ] polymers bearing cleavable groups such as ]s and ] can also be completely depolymerized by ] or ]; this can be a purely chemical process but may also be promoted by enzymes.<ref>{{cite journal |last1=Wei |first1=Ren |last2=Zimmermann |first2=Wolfgang |title=Microbial enzymes for the recycling of recalcitrant petroleum-based plastics: how far are we? |journal=Microbial Biotechnology |date=November 2017 |volume=10 |issue=6 |pages=1308–1322 |doi=10.1111/1751-7915.12710|pmid=28371373 |pmc=5658625 }}</ref> Such technologies are less well developed than those of thermal depolymerization but have the potential for lower energy costs. Thus far,{{As of when|date=June 2024}} ] has been the most heavily studied polymer.<ref>{{cite journal |last1=Geyer |first1=B. |last2=Lorenz |first2=G. |last3=Kandelbauer |first3=A. |title=Recycling of poly(ethylene terephthalate) – A review focusing on chemical methods |journal=Express Polymer Letters |date=2016 |volume=10 |issue=7 |pages=559–586 |doi=10.3144/expresspolymlett.2016.53|doi-access=free }}</ref> It has been suggested that waste plastic could be converted into other valuable chemicals (not necessarily monomers) by microbial action,<ref>{{cite journal |last1=Ru |first1=Jiakang |last2=Huo |first2=Yixin |last3=Yang |first3=Yu |title=Microbial Degradation and Valorization of Plastic Wastes |journal=Frontiers in Microbiology |date=21 April 2020 |volume=11 |pages=442 |doi=10.3389/fmicb.2020.00442|pmid=32373075 |pmc=7186362 |doi-access=free }}</ref><ref>{{cite journal |last1=Wierckx |first1=Nick |last2=Prieto |first2=M. Auxiliadora |last3=Pomposiello |first3=Pablo |last4=Lorenzo |first4=Victor |last5=O'Connor |first5=Kevin |last6=Blank |first6=Lars M. |title=Plastic waste as a novel substrate for industrial biotechnology |journal=Microbial Biotechnology |date=November 2015 |volume=8 |issue=6 |pages=900–903 |doi=10.1111/1751-7915.12312|pmid=26482561 |pmc=4621443 }}</ref> but such technology is still in its infancy.


==See also== ==See also==

Latest revision as of 10:03, 3 July 2024

Process for breaking-down polymers

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 catalyzed or un-catalyzed and is distinct from other forms of depolymerization which may rely on the use of chemicals or biological action. This process is associated with an increase in entropy.

For most polymers, thermal depolymerization is chaotic process, giving a mixture of volatile compounds. Materials may be depolymerized 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 depolymerization 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.

Disordered depolymerization

For most polymeric materials, thermal depolymerization 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, depolymerization 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 depolymerization 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.

Biomass

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 depolymerization can convert these into more economically valuable materials. Numerous biomass to liquid technologies have been developed. In general, biochemicals contain oxygen atoms, which are retained during pyrolysis, giving liquid products rich in phenols and furans. These can be viewed as partially oxidized and make for low-grade fuels. Hydrothermal liquefaction technologies dehydrate the biomass during thermal processing to produce a more energy-rich product stream. Similarly, gasification produces hydrogen, a very high-energy fuel.

Plastics

Plastic waste consists mostly of commodity plastics and may be actively sorted from municipal waste. Pyrolysis of mixed plastics can give a fairly broad mix of chemical products (between about 1 and 15 carbon atoms), including gases and aromatic liquids. Catalysts can give a better-defined product with a higher value. Likewise, hydrocracking can be employed to give LPG products. The presence of PVC can be problematic, as its thermal depolymerization generates large amounts of HCl, which can corrode equipment and cause undesirable chlorination of the products. It must be either excluded or compensated for by installing dechlorination technologies. Polyethylene and polypropylene account for just less than half of global plastic production and, being pure hydrocarbons, have a higher potential for conversion to fuel. Plastic-to-fuel technologies have historically struggled to be economically viable due to the costs of collecting and sorting the plastic and the relatively low value of the fuel produced. Large plants are seen as being more economical than smaller ones, but require more investment to build.

The method can, however, result in a mild net-decrease in greenhouse gas emissions, though other studies dispute this. For example, a 2020 study released by Renolds on their own Hefty EnergyBag program shows net greenhouse gas emissions. The study showed then when all cradle-to-grave energy costs are tallied, burning in a cement kiln was far superior. Cement kiln fuel scored a -61.1 kg CO2 equivalents compared to +905 kg CO2 eq. It also fared far worse in terms of landfill reduction vs. kiln fuel. Other studies have confirmed that plastics pyrolysis to fuel programs are also more energy intensive.

For tire waste management, tire pyrolysis is also an option. Oil derived from tire rubber pyrolysis contains high sulfur content, which gives it high potential as a pollutant and requires hydrodesulfurization before use. The area faces legislative, economic, and marketing obstacles. In most cases, tires are simply incinerated as tire-derived fuel.

Municipal waste

Thermal treatment of municipal waste can involve the depolymerization of a very wide range of compounds, including plastics and biomass. Technologies can include simple incineration as well as pyrolysis, gasification, and plasma gasification. All of these are able to accommodate mixed and contaminated feedstocks. The main advantage is the reduction in volume of the waste, particularly in densely populated areas lacking suitable sites for new landfills. In many countries, incineration with energy recovery remains the most common method, with more advanced technologies being hindered by technical and cost hurdles.

Ordered depolymerization

Some materials thermally decompose in an ordered manner to give a single or limited range of products. By virtue of being pure materials, they are usually more valuable than the mixtures produced by disordered thermal depolymerization. For plastics this is usually the starting monomer, and when this is recycled back into fresh polymer, it is called feedstock recycling. In practice, not all depolymerization reactions are completely efficient, and some competitive pyrolysis is often observed.

Biomass

Biorefineries convert low-value agricultural and animal waste into useful chemicals. The industrial production of furfural by the acid-catalyzed thermal treatment of hemicellulose has been in operation for over a century. Lignin has been the subject of significant research for the potential production of BTX and other aromatic compounds, although such processes have not yet been commercialized with any lasting success.

Plastics

Main article: Plastic recycling

Certain polymers like PTFE, Nylon 6, polystyrene, and PMMA undergo depolymerization to give their starting monomers. These can be converted back into new plastic, a process called chemical or feedstock recycling. In theory, this offers infinite recyclability, but it is also more expensive and has a higher carbon footprint than other forms of plastic recycling; however, in practice, this still yields an inferior product at higher energy costs than virgin polymer production in the real world because of contamination.

Related processes

Although rarely employed presently, coal gasification has historically been performed on a large scale. Thermal depolymerization is similar to other processes which use superheated water as a major phase to produce fuels, such as direct hydrothermal liquefaction. These are distinct from processes using dry materials to depolymerize, such as pyrolysis. The term thermochemical conversion (TCC) has also been used for conversion of biomass to oils, using superheated water, although it is more usually applied to fuel production via pyrolysis. A demonstration plant due to start up in the Netherlands is said to be capable of processing 64 tons of biomass (dry basis) per day into oil. Thermal depolymerization differs in that it contains a hydrous process followed by an anhydrous cracking / distillation process.

Condensation polymers bearing cleavable groups such as esters and amides can also be completely depolymerized by hydrolysis or solvolysis; this can be a purely chemical process but may also be promoted by enzymes. Such technologies are less well developed than those of thermal depolymerization but have the potential for lower energy costs. Thus far, polyethylene terephthalate has been the most heavily studied polymer. It has been suggested that waste plastic could be converted into other valuable chemicals (not necessarily monomers) by microbial action, but such technology is still in its infancy.

See also

Portals:
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Biofuels
Energy from
foodstock
Non-food
energy crops
Technology
Concepts

References

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