Thermal 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.{{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 }}

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.{{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 }} 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.{{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 }} Similarly, gasification produces hydrogen, a very high-energy fuel.

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.{{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}} Catalysts can give a better-defined product with a higher value.{{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}} 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.{{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 }} 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.{{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 }} 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,{{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 }}{{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 }} but require more investment to build.

The method can, however, result in a mild net-decrease in greenhouse gas emissions,{{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 }} 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.{{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}} Other studies have confirmed that plastics pyrolysis to fuel programs are also more energy intensive.{{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}}{{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 }}

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.{{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}}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. The area faces legislative, economic, and marketing obstacles.{{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 }} In most cases, tires are simply incinerated as tire-derived fuel.

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.{{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 }}{{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 }}

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.

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,{{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|bibcode=2019RSERv.11309248L |s2cid=198328225 }} although such processes have not yet been commercialized with any lasting success.{{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 |url-access=subscription }}

{{Main|Plastic recycling}}

Certain polymers like PTFE, Nylon 6, polystyrene, and PMMA{{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}} undergo depolymerization to give their starting monomers. These can be converted back into new plastic, a process called chemical or feedstock recycling.{{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 }}{{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}}{{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 }} 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.

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.{{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}} 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.{{cite journal

| last = Demirba

| first = Ayhan

| title = Thermochemical Conversion of Biomass to Liquid Products in the Aqueous Medium

| journal = Energy Sources

| volume = 27

| issue = 13

| pages = 1235–1243

| publisher = Taylor Francis

| date = 2005-10-07

| doi=10.1080/009083190519357| bibcode = 2005EneSA..27.1235D

| s2cid = 95519993

}}{{cite journal

|first = Yuanhui

|last = Zhang

|author2 = Gerald Riskowski

|author3 = Ted Funk

|title = Thermochemical Conversion of Swine Manure to Produce Fuel and Reduce Waste

|publisher = University of Illinois

|year = 1999

|url = http://www.age.uiuc.edu/bee/RESEARCH/tcc/tccpaper3.htm

|access-date = 2008-02-05

|url-status = dead

|archive-url = https://web.archive.org/web/20080515195211/http://www.age.uiuc.edu/bee/RESEARCH/tcc/tccpaper3.htm

|archive-date = 2008-05-15

}} 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.{{cite web

| last1 = Goudriaan

| first1 = Frans

| last2 = Naber

| first2 = Jaap

| last3 = van den Berg

| first3 = Ed

| 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

| access-date = 2008-01-12

| 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

}} 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.{{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 }} 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}} polyethylene terephthalate has been the most heavily studied polymer.{{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 }} It has been suggested that waste plastic could be converted into other valuable chemicals (not necessarily monomers) by microbial action,{{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 }}{{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 }} but such technology is still in its infancy.

• Thermal treatment
• Mechanical heat treatment
• Wet oxidation
• Staged reforming

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