(403g) Pyrolysis of Plastic Waste: Investigating the Kinetic Behavior and the Effects of Process Parameters on Product Yields

In 2021, EU plastics production was 57.2 Mt (87.6% fossil-based). Only 35% of 29.5 Mt plastic waste is recycled; 65% is incinerated/landfilled [1]. This low recycling rate poses a big challenge for waste management and European governments have increasingly focused on improving waste collection and conversion technologies, aiming for a “zero landfilling” scenario. Plastic waste can be managed through four primary approaches: mechanical recycling, chemical recycling (e.g., pyrolysis and gasification), biological processes (for biodegradable polymers only), and energy recovery. Among these, chemical recycling offers a solution for mixed/contaminated plastics that cannot be mechanically recycled, converting them into chemicals and fuels. [2].

Among chemical recycling method, pyrolysis is an endothermic thermochemical process that breaks down plastic waste at high temperatures (300°C-1300°C) in an oxygen-free atmosphere, producing solid residue (char), gaseous products (H₂, CH₄, CO, CO₂, and other hydrocarbons), and pyrolysis oil, minimizing the release of harmful gases (thanks to the absence of oxygen). The yields and composition of products depend on factors like temperature, heating rate, reactor, catalyst, residence time as well as the plastic type fed into the reactor. Generally, high temperatures and long residence times promote the formation of gas, while low temperatures and long residence times promote the formation of char. Finally, medium temperatures, short residence times, and high heating rates are desirable to produce oils [3]. Due to its unique ability to process non-recyclable plastics, pyrolysis is a promising method for converting waste into valuable chemicals and fuels, with the possibility of industrial scalability.

This work focuses on the pyrolysis of Plasmix, a non-recyclable plastic blend derived from mechanical sorting processes in plastic waste management. Plasmix is a highly heterogeneous mixture of various plastics (e.g., PE, PP, PET, PS, PVC) and non-plastic materials (e.g., paper, wood, textiles) [4]. Its composition varies based on local waste management practices. Due to its heterogeneous nature, separating Plasmix into individual components is technically complex and economically unfeasible. As a result, it is often excluded from traditional recycling processes. Instead, it is primarily used for energy recovery in incinerators or cement kilns or sent to landfills. For all these reasons pyrolysis represents a viable solution for valorizing Plasmix, converting it into chemicals and fuels with a low environmental impact.

The pyrolysis of Plasmix is primarily analyzed from two perspectives:

1. Study of the thermal degradation behaviour and kinetics via thermogravimetric analysis (TGA). With kinetic analysis, the reaction system and mechanism during pyrolysis can be discussed, and some fundamental data of thermal chemical conversion can be provided. Knowing the kinetic expression is indeed fundamental for pyrolysis reactor design.
2. Lab-scale vacuum batch pyrolysis tests are carried out at temperatures ranging from 500°C to 1000°C, with residence times varying from seconds to several minutes to evaluate how these parameters affect the distribution and composition of the Plasmix pyrolysis products. This type of analysis can be useful for selecting the optimal process conditions based on the desired product (char, oil or gas).

In this way this study can provide a useful reference for the valorisation of hard-to-recycle waste, based on a sustainable waste-to-energy conversion process with low environmental impact, offering significant ecological and economic advantages.

REFERENCES

[1] “Plastics Europe. Plastics—The Facts 2022: An Analysis of European Latest Plastics Production, Demand and Waste Data; Plastics Europe: Wemmel, Belgium, 2022.”.

[2] M. S. Qureshi et al., “Pyrolysis of plastic waste: Opportunities and challenges,” J Anal Appl Pyrolysis, vol. 152, Nov. 2020, doi: 10.1016/j.jaap.2020.104804.

[3] M. Van de Velden, J. Baeyens, A. Brems, B. Janssens, and R. Dewil, “Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction,” Renew Energy, vol. 35, no. 1, pp. 232–242, Jan. 2010, doi: 10.1016/j.renene.2009.04.019.

[4] R. Cossu, F. Garbo, F. Girotto, F. Simion, and A. Pivato, “PLASMIX management: LCA of six possible scenarios,” Waste Management, vol. 69, pp. 567–576, Nov. 2017, doi: 10.1016/j.wasman.2017.08.007.