Pyrolysis of waste is a multi-million-pound concept which claims to be a sustainable solution to reusing domestic refuse by transforming organic matter into valuable biofuel.
Its environmental merits are based on creating gas, oil or char (biocoal) from waste and offsetting the burning of fossil fuels.
But a new study by Loughborough University has found the fundamental principles on which the recycling process is based are not physically possible.
And, the technology, which uses more energy than it generates, is being globally mis-sold as a self-sustaining system.
Dr Andrew Rollinson, who led the study, said: &#;It is misrepresentative to describe pyrolysis as being a sustainable energy-from-waste concept.
&#;The energy needed to extract the chemicals from the waste is more than the chemicals themselves produce, leaving the system in negative energy equity &#; a violation of the laws of thermodynamics.
&#;There seems to be collective amnesia to the extensive library of research that describes the technology's thermodynamic limitations.
&#;The advantages are based on freeing useful energy from waste and offsetting the burning of fossil fuels.
&#;But if system efficiency is low, for example where much of the waste's energy density is lost in order to stabilise the process, the benefits are undermined.
&#;And where the system is so ill that more fossil fuel-derived energy is input than can practically be extracted from the feedstock as waste-derived energy (negative efficiency), the notion of energy-from-waste is completely refuted.&#;
&#;Pyrolysis is one option for converting waste oil into useable fuel
The paper, 'Patented blunderings', efficiency awareness, and self-sustainability claims in the pyrolysis energy from waste sector, reviewed current studies and methods related to pyrolysis.
It says that while machines which violate physical laws are not new, in a modern context this phenomenon is found to be driven by financial rewards, and a need to reduce the mountains of waste we produce.
Dr Rollinson said: &#;Companies describe themselves as experts in their field, their patented systems being able to convert any waste into renewable energy, and their technologies offering an opportunity for a 'high return on investment'.
&#;However, this paper shows that self-sustaining pyrolysis, creating energy from waste, is thermodynamically unproven, practically implausible, and environmentally unsound.
But we are creating huge amounts of waste which need to be reduced.
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&#;The other option, incineration, which is more efficient, has become more expensive because of fighting planning and permit applications due to the negative public perception of burning waste.
&#;The result is we are putting more CO2 into the atmosphere by recycling waste via pyrolysis than if we simply burnt it.&#;
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A new Zero Waste Europe (ZWE) report, " Leaky Loop Recycling: A Technical Correction on the Quality of Pyrolysis Oil made from Plastic Waste ," has set out a critical analysis of the quality of pyrolysis oil derived from plastic waste, challenging the scope of the technology to play a significant role in meeting European targets.
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 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.&#;
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