Enabling Informed Decisions on Pyrolysis: A Key to Turn the Tide on Plastics Recycling

Abstract

The rapid expansion of the plastic industry has led to significant environmental challenges, prompting the exploration of alternative recycling methods. While mechanical recycling has limitations, chemical recycling, particularly pyrolysis, presents a promising solution. However, it faces contention regarding its environmental impacts and economic feasibility. In this perspective, we analyze both supporting and opposing viewpoints of plastic pyrolysis, highlighting the need for transparent, comprehensive, and measurement-informed life cycle assessments (LCAs) of pyrolysis systems to inform decisions. We also present a case study of literature-reported greenhouse gas (GHG) emissions from pyrolysis-derived ultralow sulfur diesel (ULSD) in the United States, showing that depending on plant capacity and co-product allocation methods, emissions can range from 28% lower to 30% higher than fossil-derived ULSD. Similarly, when viewed as a waste management strategy, net GHG emissions from plastic pyrolysis can range from 220% lower to 60% higher than those from current U.S. plastic waste management practices, depending on system conditions. These findings underscore the variability of results and the need for currently missing, robust, and contextualized LCAs. Finally, we discuss regulatory and social challenges and opportunities for the wider adoption of chemical recycling, emphasizing the critical role of public support in realizing the potential of pyrolysis for a circular economy.

Keywords: chemical recycling; circularity; life cycle assessment; plastic waste; public support; pyrolysis; stakeholder engagement.

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Petrochemical life cycle with and without pyrolysis. The petrochemical life cycle begins with crude oil exploration and extraction, followed by processing at oil refineries to produce fuels and chemicals. Some of these chemicals are used to manufacture polymers, which are then converted into plastic products. Today, the most common plastic waste management practices are landfilling and incineration for electricity production (gray blocks). A small fraction of plastic waste is mechanically recycled (green block and arrows) substituting some of the virgin plastics produced with recycled ones (r-Plastics). Chemical recycling via pyrolysis can provide a more flexible and versatile alternative to mechanical recycling (pink block and arrows). The main products of plastic pyrolysis are diesel oil and naphtha, while fuel gas (noncondensable gases), paraffin wax, and char (solid residue) are common pyrolysis co-products. These products and co-products can be processed further to potentially displace manufacturing of equivalent virgin energy (fuels) or polymer products (chemicals) from fossil resources or can be treated as wastes (W).

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Comparative analysis of GHG emissions associated with pyrolysis of plastic waste. Reported GHG emissions , of plastic pyrolysis-derived ULSD (x axis) compared to those of fossil-derived ULSD (vertical dashed line); net GHG emissions of recycling plastic waste through pyrolysis to produce ULSD (y axis) compared to the net GHG emissions of current plastic waste management practices in the United States (U.S.) (horizontal dashed line). Green plot area (C): Emissions lower than fossil-derived ULSD and current U.S. waste management practices; red plot area (B): Emissions higher than fossil-derived ULSD and current U.S. waste management practices; gray plot areas (A, D): Emissions higher than fossil-derived ULSD or current U.S. waste management practices. Different scales of plastic pyrolysis plants concern different plastic waste processing capacities in kilotonnes/year (kt/yr), with each scale represented by a distinct color. Co-product handling methods are depicted with different shapes, i.e., circles: energy allocation; triangles: market allocation; and hexagons: displacement method.