Thermochemical recycling of end-of-life and virgin HDPE: A pilot-scale study

• Published in: Journal of analytical and applied pyrolysis 2022-09, Vol.166, p.105614, Article 105614
• Main Authors: Abbas-Abadi, Mehrdad Seifali, Zayoud, Azd, Kusenberg, Marvin, Roosen, Martijn, Vermeire, Florence, Yazdani, Parviz, Van Waeyenberg, Jonathan, Eschenbacher, Andreas, Hernandez, Francisco Jose Arraez, Kuzmanović, Maja, Dao Thi, Hang, Kresovic, Uros, Sels, Bert, Van Puyvelde, Peter, De Meester, Steven, Saeys, Mark, Van Geem, Kevin M.
• Format: Article
• Language: English
• Subjects: Kinetic modeling; Pilot plant experimentation; Pyrolysis; Thermochemical recycling; Two-dimensional GC×GC; Virgin and end-of-life HDPE
• ISSN: 0165-2370; 1873-250X
• DOI: 10.1016/j.jaap.2022.105614

Industrial-scale application of end-of-life plastic pyrolysis still faces significant problems, such as a lack of detailed knowledge on degradation mechanisms, parameters affecting the degradation, and formation pathways of the primary pyrolysis products. Today, the degradation mechanisms based on radical chain scission are insufficiently understood to explain the pyrolysis chemistry from feedstock-to-product comprehensively. In this study, the impact of operating conditions on the degradation mechanism is evaluated by pyrolyzing end-of-life and virgin high-density polyethylene (HDPE) in a continuous pilot-scale unit at temperature and pressure ranges of 450–504 ºC and 0.1–2 bara. The pyrolysis products were analyzed based on the detailed product composition obtained using comprehensive two-dimensional gas chromatography (GC×GC). A simplified kinetic mechanism was proposed to describe the main production pathways of the various components by considering the weakest points along the polymer chain. The results showed that the chain-end scission mechanism is the main mechanism in the HDPE pyrolysis process, even at low temperatures and pressures in the studied ranges. The pyrolysis of virgin HDPE under sub-atmospheric pressure, 0.1 bara, at 464 ºC reactor temperature, yields the highest concentration of linear hydrocarbons in the pyrolysis oil (93.2 wt%). At higher pressure and temperature, the cyclic and branched hydrocarbons had a higher share of up to 17.4 wt% compared to 6.8 wt% at vacuum pressure and lower temperature. Interestingly, the pyrolysis of end-of-life HDPE at atmospheric pressure and 450 ºC led to more cyclic and branched hydrocarbons (sum: 22.1 wt%), as opposed to that of virgin HDPE which is more prone to the production of linear hydrocarbons at the studied conditions. Regarding the additives and contaminants, a large amount of different metals and halogen atoms in the ppm range were detected in end-of-life HDPE, of which a small amount was still found in the pyrolysis oil. [Display omitted] •Pyrolysis of virgin and waste high density polyethylene.•Comprehensive two-dimensional gas chromatography coupled to various detectors.•Providing a comprehensive kinetic model using a unique quantum study.•Double bond and tertiary carbon as effective structural parameters.•Investigating the influential mechanisms in polyethylene pyrolysis.