Thermal depolymerization (TDP) is the process of converting a polymer into a monomer or a mixture of monomers, by predominately 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.
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. 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.
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. 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 oxidised 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.
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 depolymerisation 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 dichlorination 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 approach can however, lead to a mild net-decrease in greenhouse gas emissions.
In 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.
Thermal treatment of municipal waste can involve the depolymerisation 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 area 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.
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 monomer 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.
Biorefineries convert low-value agricultural and animal waste into useful chemicals. The industrial production of furfural by the acid catalysed 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 aromatics compounds, although such processes have yet to be fully commercialised.
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.
Although rarely employed today, coal gasification has historically been performed on a large scale. Thermal depolymerisation is similar to other processes which use superheated water as a major step 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. 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. The Hydro Thermal Upgrading (HTU) process uses superheated water to produce oil from domestic waste. 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 depolymerisation differs in that it contains a hydrous process followed by an anhydrous cracking / distillation process.
Condensation polymers baring cleavable groups such as esters and amides can also be completely depolymerised 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 depolymerisation but have the potential for lower energy costs. Thus far polyethylene terephthalate has been the most heavily studied polymer. Alternatively, waste plastic may be converted into other valuable chemicals (not necessarily monomers) by microbial action.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Depolymerization". doi:10.1351/goldbook.D01600
- Rollinson, Andrew Neil; Oladejo, Jumoke Mojisola (February 2019). "'Patented blunderings', efficiency awareness, and self-sustainability claims in the pyrolysis energy from waste sector". Resources, Conservation and Recycling. 141: 233–242. doi:10.1016/j.resconrec.2018.10.038.
- Collard, François-Xavier; Blin, Joël (October 2014). "A review on pyrolysis of biomass constituents: Mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin". Renewable and Sustainable Energy Reviews. 38: 594–608. doi:10.1016/j.rser.2014.06.013.
- Kumar, Mayank; Olajire Oyedun, Adetoyese; Kumar, Amit (January 2018). "A review on the current status of various hydrothermal technologies on biomass feedstock". Renewable and Sustainable Energy Reviews. 81: 1742–1770. doi:10.1016/j.rser.2017.05.270.
- Kaminsky, W.; Schlesselmann, B.; Simon, C.M. (August 1996). "Thermal degradation of mixed plastic waste to aromatics and gas". Polymer Degradation and Stability. 53 (2): 189–197. doi:10.1016/0141-3910(96)00087-0.
- Aguado, J.; Serrano, D. P.; Escola, J. M. (5 November 2008). "Fuels from Waste Plastics by Thermal and Catalytic Processes: A Review". Industrial & Engineering Chemistry Research. 47 (21): 7982–7992. doi:10.1021/ie800393w.
- Fukushima, Masaaki; Wu, Beili; Ibe, Hidetoshi; Wakai, Keiji; Sugiyama, Eiichi; Abe, Hironobu; Kitagawa, Kiyohiko; Tsuruga, Shigenori; Shimura, Katsumi; Ono, Eiichi (June 2010). "Study on dechlorination technology for municipal waste plastics containing polyvinyl chloride and polyethylene terephthalate". Journal of Material Cycles and Waste Management. 12 (2): 108–122. doi:10.1007/s10163-010-0279-8. S2CID 94190060.
- Butler, E.; Devlin, G.; McDonnell, K. (1 August 2011). "Waste Polyolefins to Liquid Fuels via Pyrolysis: Review of Commercial State-of-the-Art and Recent Laboratory Research". Waste and Biomass Valorization. 2 (3): 227–255. doi:10.1007/s12649-011-9067-5. hdl:10197/6103. S2CID 98550187.
- Fivga, Antzela; Dimitriou, Ioanna (15 April 2018). "Pyrolysis of plastic waste for production of heavy fuel substitute: A techno-economic assessment" (PDF). Energy. 149: 865–874. doi:10.1016/j.energy.2018.02.094.
- Riedewald, Frank; Patel, Yunus; Wilson, Edward; Santos, Silvia; Sousa-Gallagher, Maria (February 2021). "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". Waste Management. 120: 698–707. doi:10.1016/j.wasman.2020.10.039. PMID 33191052.
- Benavides, Pahola Thathiana; Sun, Pingping; Han, Jeongwoo; Dunn, Jennifer B.; Wang, Michael (September 2017). "Life-cycle analysis of fuels from post-use non-recycled plastics". Fuel. 203: 11–22. doi:10.1016/j.fuel.2017.04.070.
- Choi, G.-G.; Jung, S.-H.; Oh, S.-J.; Kim, J.-S. (2014). "Total utilization of waste tire rubber through pyrolysis to obtain oils and CO2 activation of pyrolysis char". Fuel Processing Technology. 123: 57–64. doi:10.1016/j.fuproc.2014.02.007.
- Ringer, M.; Putsche, V.; Scahill, J. (2006) Large-Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis Archived 2016-12-30 at the Wayback Machine; NREL/TP-510-37779; National Renewable Energy Laboratory (NREL), Golden, CO.
- Martínez, Juan Daniel; Puy, Neus; Murillo, Ramón; García, Tomás; Navarro, María Victoria; Mastral, Ana Maria (2013). "Waste tyre pyrolysis – A review, Renewable and Sustainable". Energy Reviews. 23: 179–213. doi:10.1016/j.rser.2013.02.038.
- Mukherjee, C.; Denney, J.; Mbonimpa, E.G.; Slagley, J.; Bhowmik, R. (1 March 2020). "A review on municipal solid waste-to-energy trends in the USA". Renewable and Sustainable Energy Reviews. 119: 109512. doi:10.1016/j.rser.2019.109512.
- Fernández-González, J.M.; Grindlay, A.L.; Serrano-Bernardo, F.; Rodríguez-Rojas, M.I.; Zamorano, M. (September 2017). "Economic and environmental review of Waste-to-Energy systems for municipal solid waste management in medium and small municipalities". Waste Management. 67: 360–374. doi:10.1016/j.wasman.2017.05.003. PMID 28501263.
- Lok, C.M.; Van Doorn, J.; Aranda Almansa, G. (October 2019). "Promoted ZSM-5 catalysts for the production of bio-aromatics, a review". Renewable and Sustainable Energy Reviews. 113: 109248. doi:10.1016/j.rser.2019.109248.
- Wong, Sie Shing; Shu, Riyang; Zhang, Jiaguang; Liu, Haichao; Yan, Ning (2020). "Downstream processing of lignin derived feedstock into end products". Chemical Society Reviews. 49 (15): 5510–5560. doi:10.1039/D0CS00134A. PMID 32639496.
- Kaminsky, W; Predel, M; Sadiki, A (September 2004). "Feedstock recycling of polymers by pyrolysis in a fluidised bed". Polymer Degradation and Stability. 85 (3): 1045–1050. doi:10.1016/j.polymdegradstab.2003.05.002.
- Kumagai, Shogo; Yoshioka, Toshiaki (1 November 2016). "Feedstock Recycling via Waste Plastic Pyrolysis". Journal of the Japan Petroleum Institute. 59 (6): 243–253. doi:10.1627/jpi.59.243.
- Rahimi, AliReza; García, Jeannette M. (June 2017). "Chemical recycling of waste plastics for new materials production". Nature Reviews Chemistry. 1 (6): 0046. doi:10.1038/s41570-017-0046.
- Coates, Geoffrey W.; Getzler, Yutan D. Y. L. (July 2020). "Chemical recycling to monomer for an ideal, circular polymer economy". Nature Reviews Materials. 5 (7): 501–516. Bibcode:2020NatRM...5..501C. doi:10.1038/s41578-020-0190-4. S2CID 215760966.
- "Biomass Program, direct Hydrothermal Liquefaction". US Department of Energy. Energy Efficiency and Renewable Energy. 2005-10-13. Archived from the original on 2007-03-12. Retrieved 2008-01-12.
- Demirba, Ayhan (2005-10-07). "Thermochemical Conversion of Biomass to Liquid Products in the Aqueous Medium". Energy Sources. Taylor Francis. 27 (13): 1235–1243. doi:10.1080/009083190519357. S2CID 95519993.
- Zhang, Yuanhui; Gerald Riskowski; Ted Funk (1999). "Thermochemical Conversion of Swine Manure to Produce Fuel and Reduce Waste". University of Illinois. Archived from the original on 2008-05-15. Retrieved 2008-02-05. Cite journal requires
- Sforza, Teri (2007-03-14). "New plan replaces sewage sludge fiasco". Orange county register. Retrieved 2008-01-27.
- de Swaan Arons, Jakob; H. van derKooi; Wei Feng. "Hydrothermal Upgrading of Biomass". University of Delft. Retrieved 2008-02-05.
- Goudriaan, Frans; Naber, Jaap; van den Berg, Ed. "Conversion of Biomass Residues to Transportation Fuels with th HTU Process". Retrieved 2008-01-12.
- Wei, Ren; Zimmermann, Wolfgang (November 2017). "Microbial enzymes for the recycling of recalcitrant petroleum‐based plastics: how far are we?". Microbial Biotechnology. 10 (6): 1308–1322. doi:10.1111/1751-7915.12710. PMC 5658625. PMID 28371373.
- Geyer, B.; Lorenz, G.; Kandelbauer, A. (2016). "Recycling of poly(ethylene terephthalate) – A review focusing on chemical methods". Express Polymer Letters. 10 (7): 559–586. doi:10.3144/expresspolymlett.2016.53.
- Ru, Jiakang; Huo, Yixin; Yang, Yu (21 April 2020). "Microbial Degradation and Valorization of Plastic Wastes". Frontiers in Microbiology. 11: 442. doi:10.3389/fmicb.2020.00442. PMC 7186362. PMID 32373075.
- Wierckx, Nick; Prieto, M. Auxiliadora; Pomposiello, Pablo; Lorenzo, Victor; O'Connor, Kevin; Blank, Lars M. (November 2015). "Plastic waste as a novel substrate for industrial biotechnology". Microbial Biotechnology. 8 (6): 900–903. doi:10.1111/1751-7915.12312. PMC 4621443. PMID 26482561.