(348f) The Effect of Plastic Feedstock Composition on Heteroatom Formation in Pyrolysis Oil

The depletion of fossil energy resources and the increasing global plastic waste crisis necessitate advanced recycling technologies for sustainable waste management and energy recovery^{1, 2}. Polyolefins (PE, PP) constitute most of the plastic waste, along with heteroatom-containing polymers such as PET, PA, and PVC^{3, 4}. Among recycling methods, pyrolysis presents a promising thermochemical conversion route, operating at 400–700 °C in an inert atmosphere, to generate energy-rich pyrolysis oil (Py-oil)^2-5. However, heteroatoms and additives introduce impurities, influencing Py-oil composition and quality^{1, 6, 7}. To obtain a convenient feedstock for steam crackers in petrochemistry, heteroatom contamination limits should be considered. Nitrogen can be included in the naphtha fraction up to 100 ppm, while 2000 ppm for gas oils. The industrial threshold for sulfur is 500 ppm, oxygen is 100 ppm, chlorine is 3 ppm, and phosphorus is 0.5 ppm⁷. There are ways to solve this situation, i.e. blending pyrolysis oil with real naphtha⁷ or upgrading via saturating the alkenes in Py-oil with hydroprocessing (hydrotreatment and hydrocracking)^7-9. Hydrotreatment removes heteroatoms in pyrolysis oil, while hydrocracking is used to decrease the carbon number distribution to the oil range (C₅-C₂₁) by cracking waxy hydrocarbons (C₂₁₊)^7-9. This study investigates the effect of heteroatom-containing plastics on pyrolysis oil composition.

Pyrolysis experiments were conducted at 600 °C with a carrier gas flow rate of 20 mL/min using a double-shot tandem micro-pyrolysis system coupled with comprehensive two-dimensional gas chromatography (GC×GC-FID/ToF-MS). Polyethylene (PE) was first pyrolyzed individually, followed by systematic additions of polyvinyl chloride (PVC), polyethylene terephthalate (PET), and Nylon 6,6 to form a representative plastic mixture (65% PE, 10% PVC, 10% PET, 15% Nylon 6,6). The powdered plastics (0.1–0.3 mm) underwent pyrolysis for 5 minutes, with volatile products trapped at -170 °C and analyzed via GC×GC. Chromatographic separation was achieved based on boiling point (1D) and polarity (2D).

The inclusion of heteroatom-containing plastics resulted in the formation of chlorinated, oxygenated, and nitrogenated species in the pyrolysis oil, with a carbon number distribution ranging from C₂ to C₄₈. The resulting chromatograms revealed complex mixtures of aromatics, naphthenes, (iso-)olefins, (iso-)paraffins, chlorinates, oxygenates, and nitrogenates with detailed heteroatom characterization visualized via GC Image analysis. Mass spectrometry (ToF-MS) and flame ionization detection (FID) enabled compound identification and quantification using the molar response factor (MRF) method¹⁰. While chlorinated hydrocarbons were formed in the 10% PVC addition, light olefin production decreased. Subsequently, oxygenated hydrocarbons appeared in the pyrolysis oil by adding 10% PET to the feedstock, and aromatic (benzene, toluene, xylene) production changed remarkably. 15% Nylon6,6 was included in the plastic mixture to observe nitrogenated compounds. Final mixture produced 7.22 wt.% oxygenates, 1.17% wt.% nitrogenates, and 3.01 wt.% chlorinates. Understanding the extent of heteroatom contamination in pyrolysis oil derived from mixed plastic waste is crucial for optimizing recycling strategies. This study provides key insights into the transfer of contaminants during pyrolysis, informing approaches for further processing to minimize reliance on fossil feedstocks. Our findings demonstrate that plastic-derived oil presents a viable and sustainable alternative for fossil naphtha, while contributing to the circular economy and reducing carbon footprint.

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