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| Fig. 1: Women Pulling a Bag of Recycled Plastic. (Image source: M. Tanni through a Pexels license.) |
The invention of plastics revolutionized both industry and daily life within a decade of their emergence at the beginning of the 20th century. Plastics have become ubiquitous in modern households, making it hard to imagine life without them. [1] However, despite the benefits they bring, the large accumulation of plastic litter negatively impacts the flora, fauna, the economy, our health, and our well-being (Fig. 1). [2] Halting plastic production might could be a solution, but given their widespread use in nearly every industry, complete eradication is unrealistic at this stage. A more viable alternative is recycling plastics for energy production. Below, I discuss two methods for converting plastic into different types of fuel.
Pyrolysis offers a promising avenue for converting plastic waste into diverse fuels. [3] This process involves the thermal degradation of plastics without oxygen, setting it apart from incineration and gasification, which involve oxygen and can lead to CO2 emissions and potentially harmful byproducts. Pyrolysis aims to break down the long carbon chains in plastics into shorter ones. For instance, when polyethylene undergoes pyrolysis at high temperatures, the resulting products include approximately 65%wt of gases (C1-C4), 25%wt of liquids (C5-C10), and 10%wt of waxes and aromatics. [3,4] Although pyrolysis requires high temperatures (650-700°C) and yields low octane fuels, the energy content of these liquid fuels varies from 40 MJ/kg in PVC to 44 MJ/kg in PP comparable to gasoline. [5] The process's temperature can be lowered through catalytic cracking, which involves mixing plastic waste with catalysts to reduce the activation energy required. This allows the thermal reaction to initiate at lower temperatures (240-450°C), producing higher carbon chain hydrocarbons, including C5-C15 aliphatics and C8-C18 aromatics
In plastic oxidation, the polymers are oxidized into low atomic weight molecules. This process uses a catalyst to generate a hydroxyl radical (OH), which then breaks the C-C bonds in the plastic. [5] Common catalysts in this method include semiconductor photocatalysts, Fenton, and electrocatalysts. The fuel products derived from this process include hydrogen (H2) and C2-C4 carboxylic acids. While these fuels have a lower calorific value than pyrolysis byproducts, the advantage of this process lies in its ability to operate at lower temperatures (25-50°C). [5]
In conclusion, pyrolysis and plastic oxidation are two promising areas of active research for converting plastic waste into usable fuels. However, there is much work to be done in the sorting and collection of plastic waste (Fig. 1) since only 5% of plastic produce globally is recycled. [6]
© Carlos Kometter. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. J. Jacobs, "Trying to Live a Day Without Plastic," New York Times, 20 Jan 23.
[2] I. E. Napper and R. C. Thompson, "Plastics and the Environment," Annu. Rev. Environ. Resour. 48, 55 (2023).
[3] L. Dai et al., "Pyrolysis Technology For Plastic Waste Recycling: A State-of-the-Art Review," Prog. Energy Combust. Sci. 93, 101021 (2022).
[4] S. M. Al-Salem, P.Lettieri, andJ. Baeyens, "The Valorization of Plastic Solid Waste (PSW) by Primary to Quaternary Routes: From Re-Use to Energy and Chemicals," Prog. Energy Combust. Sci. 36, 103 (2010).
[5] N. Li et al., "Conversion of Plastic Waste into Fuels: A Critical Review," J. Hazard. Mater. 424, 127460 (2022).
[6] A. Rahimi and J. M. Garcia, "Chemical Recycling of Waste Plastics For New Materials Production," Nat. Rev. Chem. 1, 0046 (2017).