Editing Thermal depolymerization (section)

==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 [[Thermal efficiency|energy efficiency]] 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|bibcode=2019RCR...141..233R |s2cid=115296275 }}</ref>

===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 mill]]s 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, [[biochemical]]s contain oxygen atoms, which are retained during pyrolysis, giving liquid products rich in [[phenol]]s and [[furan]]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|bibcode=2014RSERv..38..594C }}</ref> 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.<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|bibcode=2018RSERv..81.1742K }}</ref> Similarly, [[gasification]] produces hydrogen, a very high-energy fuel.

===Plastics===
[[Plastic waste]] consists mostly of [[commodity plastics]] and may be actively [[waste sorting|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.<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, [[hydrocracking]] can be employed to give [[Liquefied petroleum gas|LPG]] products. The presence of [[PVC]] can be problematic, as its thermal depolymerization generates large amounts of [[hydrogen chloride|HCl]], 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|bibcode=2010JMCWM..12..108F |s2cid=94190060 }}</ref> [[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.<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|bibcode=2011WBioV...2..227B |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 |bibcode=2018Ene...149..865F |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 |bibcode=2021WaMan.120..698R |s2cid=226972785 |hdl=10468/12445 |hdl-access=free }}</ref> but require more investment to build.

The method can, however, result in a mild net-decrease in [[greenhouse gas]] 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 |bibcode=2017Fuel..203...11B }}</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=[[The Atlantic]] | date=30 May 2022 }}</ref>

For tire waste management, [[Tire recycling#Tire pyrolysis|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.<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) [http://www.nrel.gov/docs/fy07osti/37779.pdf Large-Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis] {{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 | bibcode = 2013RSERv..23..179M }}</ref> 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 [[landfill]]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 |bibcode=2020RSERv.11909512M }}</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 |bibcode=2017WaMan..67..360F }}</ref>