Environmental Performance of Pyrolysis-Based Chemical Recycling: Effects of Feedstock Composition and Pyrolysis Oil Utilization

The transition towards a sustainable and climate-neutral plastics economy requires the development of recycling technologies capable of recovering valuable resources from mixed plastic waste (MPW) that cannot be effectively processed by conventional mechanical recycling. Pyrolysis-based chemical recycling has emerged as a promising complementary technology by converting heterogeneous plastic waste into secondary chemical feedstocks suitable for the production of new chemicals and plastics. However, the environmental performance of pyrolysis strongly depends on feedstock characteristics, process configuration, and the integration of downstream utilization pathways, while these interdependencies have only been investigated to a limited extent. This thesis therefore evaluates the influence of waste composition on the climate change impact of pyrolysis-based chemical recycling, investigates the effect of pyrolysis oil composition on downstream conversion routes, and assesses its potential to reduce climate change impact relative to waste incineration with energy recovery.
The study combines experimental pyrolysis data from the literature, obtained using well-defined model feedstocks and two different reactor technologies, with process modeling of downstream conversion pathways to generate consistent mass and energy balances and life cycle inventories. ... mehrThe environmental assessment is performed using life cycle assessment (LCA) methodology for seven representative feedstocks ranging from virgin polyolefins to heterogeneous post-consumer plastic waste. Two waste-to-chemicals pathways are investigated: pyrolysis followed by upgrading and steam cracking for the production of base chemicals, and pyrolysis followed by entrained-flow gasification (EFG) for syngas production suitable for methanol or Oxo synthesis.
The results demonstrate that feedstock composition is the dominant factor governing the environmental performance of pyrolysis-based chemical recycling. Heteroatom-free polymers such as polyethylene, polypropylene, and polystyrene consistently achieve high pyrolysis oil yields and consequently provide greater reductions in climate change impact through increased recovery of valuable chemical products. In contrast, feedstocks containing polymers such as polyethylene terephthalate, polyamide, and polyvinyl chloride exhibit lower product yields due to reduced carbon recovery in pyrolysis oil.
Across all analyzed feedstocks, carbon content correlates with higher pyrolysis oil yields, carbon recycling effiency, and climate change impact reduction potential, defined as the difference in net climate change impact between the respective chemical recycling route and conventional energy recovery. This relationship reflects the central role of carbon recovery during pyrolysis, which links feedstock composition to downstream product yields and ultimately to environmental performance. Consequently, heteroatom-containing feedstocks such as MPW exhibit a lower climate change impact reduction potential due to their lower carbon content. Nevertheless, even these feedstocks achieve lower climate change impacts than conventional energy recovery within the investigated scenarios.
Pyrolysis contributes the largest share of total process burdens, primarily because carbon lost to pyrolysis gas and residues is emitted as CO2 during their treatment. Consequently, maximizing carbon recovery through appropriate feedstock selection and reactor design is key to achieving high climate change impact reduction potential. The remaining burdens can be reduced through efficient utilization of pyrolysis side products, particularly by material utilization of pyrolysis gas in EFG or, to a lesser extent, by energy recovery. Flaring of pyrolysis gas should be avoided due to the high associated climate change impacts.
The comparison of downstream utilization pathways reveals that steam cracking and EFG exhibit comparable performance in terms of climate change impact for most feedstocks. Although pyrolysis oil composition influences the preferred downstream utilization pathway, boundary conditions such as low-carbon hydrogen availability, opportunities for heat integration, and specifically the substituted syngas production route – based on heavy fuel oil or natural gas – have a greater influence on environmental performance.
A comparison of perspectives in LCA yielded seemingly contradictory conclusions regarding the environmental performance of pyrolysis-based chemical recycling. While the input-based waste management perspective consistently indicates environmental benefits in comparison to energy recovery, the product-based perspective only identifies advantages for heteroatom-free feed-stocks with high yields in pyrolysis. The contradiction can be resolved in a combined system-wide perspective assessment, attributing the differences to diverging system boundaries and the underlying background system. The combined perspective results support the preferability of pyrolysis-based chemical recycling over energy recovery. Additionally, the combined perspective highlights that the environmental performance of pyrolysis-based chemical recycling cannot be evaluated in isolation but must be assessed within the broader context of the surrounding waste management and chemical production systems.
Overall, carbon recovery of pyrolysis represents the key process variable governing the environ-mental performance of pyrolysis-based chemical recycling. Feedstock composition, reactor design, and process integration influence climate change impacts primarily through their influ-ence on carbon recovery, while the overall environmental benefit depends on the surrounding waste management and chemical production systems.