Pyrolysis 'not suitable' for effective plastic recycling

By heating up plastic in the absence of oxygen, the pyrolysis industry claims that the process can produce a type of oil that can be converted back into ‘virgin-like’ plastic, offering a recycling alternative to mechanical recycling of plastic. However, ZWE’s report identifies several issues with the technology.

The report argues that pyrolysis suffers from numerous constraints, including incompatibility with various plastic types, low yield, and the need to heavily dilute the oil with a petroleum-based mixture due to contamination, in many cases by a ratio of over 40:1.

For pyrolysis to be used in the production of recycled plastic, it should either apply multiple energy-intensive purification steps or highly dilute the oil with virgin petroleum naphtha. ZWE highlights that neither of these options align with the targets of the EU Green Deal.

The report also emphasises the formation of unwanted and toxic hydrocarbons during the pyrolysis process, including toxic polycyclic aromatic hydrocarbon (PAH) compounds and dioxins. These contaminants pose significant health and safety concerns.

Furthermore, the report points out a disparity between industry claims about pyrolysis oil quality and findings from independent research. It suggests that the only way to reconcile these claims is through intermediate stages of pyrolysis oil upgrading and/or blending with petroleum.

Ongoing discussions to establish unified European standards for end-of-waste criteria for plastics are expected to continue until 2025 when the review is scheduled for approval. This is particularly significant for pyrolysis, as there is a desire within the industry to categorise pyrolysis oil as a product.

However, it is essential for decision-makers to prioritise addressing the presence of toxic substances and the necessary purification processes for pyrolysis oil. Lowering safety standards in the pursuit of a circular economy should be avoided.

ZWE’s report also delves into the technical limitations and process instability of pyrolysis. It describes pyrolysis as a highly sensitive and delicately balanced process, prone to producing new 'pyrosynthetic' products that significantly lower the usable oil yield and cause downstream issues like fouling and corrosion. These new products also make the oil toxic and create new hazardous waste streams.

Engineering challenges are highlighted, with no consensus on the best reactor setup or designs for pyrolysis. Managing heat transfer is key, but this becomes increasingly difficult at an industrial scale due to heat losses and internal temperature variations.

Additionally, the report discusses the problematic nature of plastic as a feedstock for pyrolysis. Plastic exacerbates the inherent problems of pyrolysis due to its low thermal conductivity, leading to uneven heating and further complicating the process. The contamination of pyrolysis oil with various elements originating from the multitude of additives present in plastic is also a significant concern.

Commenting on the findings of the report, Lauriane Veillard, Chemical Recycling and Plastics-to-Fuels Officer at ZWE said: “If pyrolysis oil is reclassified as a product instead of being classified as waste, as industry demands, it must meet EU requirements. We cannot afford to accept haphazard legislation that undermines a true circular economy. Our north star should be the protection of the environment, human health, and public trust. These values should form the basis of different discussions on plastic recycling-related issues, like calculating recycled content and defining end-of-waste criteria.”

The author of the report, Andrew Rollinson, added: “Physical laws dictate that the nature of pyrolysis is to synthesise new molecules, not just decompose plastic polymers. Since these laws are unlikely to yield in response to policy goals or alter as a consequence of marketing pressure, decision-makers would be sensible to accept the fact that pyrolysis will not somehow miraculously step up to the task required. Encouragement alone will not be enough to make pyrolysis solve the problem of plastic waste created by linear thinking in plastic production.”