Pyrolysis: the secret weapon in the fight against plastic waste?

Iranpolymer/Baspar The scourge of plastic pollution has ravaged our natural habitats for decades and the problem shows no signs of slowing down. As we approach the conclusion of the United Nations’ global plastic treaty at the end of this year, now is the time for countries across the globe to take stock of their waste management systems and consider the optimal pathways to advancing circularity through a combination of reduction, re-use and recycling strategies.

A variety of factors, from rising global population and a growing demand for consumer goods, continue to fuel plastic production, which is forecast to nearly triple from around 460 million tonnes (Mt) in 2019 to 1,231 Mt in 2060.  We are only just beginning to understand the full extent of the damage, with a research report revealing that out of the 16,000 chemicals discovered in common plastic products, 4,000 are hazardous to the environment and human health.
There are many international policies aimed at tackling the plastic problem. As part of the Biden-Harris Administration’s comprehensive strategy to tackle plastic pollution in the US, the country aims to phase out single-use plastics in all federal operations by 2035 and eliminate plastic waste from land-based sources by 2040. Across the Atlantic, the UK’s Plastic Packaging Tax charges companies placing plastic into the market that contains less than 30% recycled content. Europe’s Packaging and Packaging Waste Directive mandates that 50% of plastic packaging waste must be recycled by 2025.
The number of policy initiatives underway is promising. Still, the statistics around plastic waste and the world’s failed attempts to address the issue are damning: global plastic recycling rates currently sit at a paltry 12-13%, according to the latest data. It is clear that current solutions to tackle plastic waste are not sufficient on their own and there is a need for a more diverse approach to waste management.

Mechanical recycling limitations
Traditional recycling, otherwise known as mechanical recycling, is a vital means by which municipal authorities have managed plastic waste in recent years. However, it is constrained by a number of crucial limitations and can therefore not be relied upon as the sole answer to building a circular economy.
When plastic is melted and reformed into pellets in the final stage of the mechanical recycling process, the polymer chains are often weakened, reducing the quality of plastic compared to virgin material. This makes the recycled plastic less suitable for some applications.
The quality of recycled plastic is also negatively affected by the inability of mechanical recycling systems to effectively process contaminants such as food residue, labels, multi-layer and other non-plastic materials. This is one of the main reasons for woefully low rates of recycling. In the UK for example, only 7% of waste such as packaging films and flexible materials are recycled, compared to 63% of plastic bottles.
Ultimately, lower quality recycled plastic has a shorter life cycle and is likely to end up in landfills or incinerators, resulting in harmful greenhouse gas emissions. They are also more prone to breaking down into microplastics, which can leak into ecosystems, harming wildlife and damaging natural habitats, both on land and in the oceans.

Strong bonds in chemical recycling
Due to greater versatility in terms of what types of plastic can be processed, chemical recycling through pyrolysis (often referred to as ‘advanced recycling’) has emerged as a powerful complement to mechanical recycling. Pyrolysis involves heating mixed plastic waste materials to temperatures of 400-600°C in the absence of oxygen, with or without catalyst, to convert polymers into a mixture of liquid hydrocarbons.
The initial steps are similar to mechanical recycling with sorting, pre-treatment (acid washing) and shredding before the material is transferred to a reactor to be melted. The high temperatures cause the complex hydrocarbon chains to break into smaller molecules. The resulting oil-gas mixture is transferred to a condenser to be cooled into ‘pyrolysis oil’. This can be further refined to produce approximately 80% liquid, 15% gas and 5% carbon black (ash).
The resulting products can be used in various ways. The gas can be fed back into the system to heat the reactor’s furnace, and the carbon black can be used for various purposes, such as the production of rubber goods, automotive parts and coatings, batteries, cables, and printer inks.
The oil, as the majority product by volume of pyrolysis, can be used as feedstock for the chemical and petrochemical industry to produce new plastics which have the same chemical structure as first-generation plastics with virgin quality. Moreover, research by the US Department of Energy’s Argonne National Laboratory shows that the production of plastic with just 5% of pyrolysis oil reduces greenhouse gas emissions by up to 23% compared to plastic made by crude oil.

Tackling contamination issues
There are, however, numerous technical hurdles associated with the pyrolysis process, particularly when it comes to contamination. Numerous types of plastics and non-polymeric sources are combined in mixed plastic waste feedstocks. Those feedstocks contain coarse to fine particles (e.g. filler, flame retardants, etc.) and other materials which are detected in the oil downstream in the pyrolysis process (e.g. coke). Besides the particulate matter, a variety of additional contaminants such as organic gels, dissolved metals and dispersed liquids are found as a side product in pyrolysis oil. The complex mixture of those contaminants needs to be extracted from the oil.

Appropriate filtration media and coalescer technologies are key at various stages of the process to remove particles and separate water from pyrolysis oil or liquids from gas. The retention and separation of contaminants not only purifies the oil and gas, making them more suitable for downstream processing, but also helps prevent equipment fouling and unnecessary maintenance downtime.

To further refine the pyrolysis oil for use as fuel or feedstock to produce plastic again, it must be transferred to a steam cracker to convert the oil into lighter olefins. The presence of particles and metal contaminants in crude plastic waste pyrolysis oils may have significant negative impacts on the steam cracker‘s furnace and recovery section such as furnace run-length reduction due to coking increase.
However, there is potential for using depth filtration as an effective method to remove harmful contaminants and reduce the contamination levels of plastic waste pyrolysis oils to the thresholds accepted for crude naphtha feed in steam crackers. It is an efficient and cost-effective way to remove particle content from the oils.
Recent published work by scientists from Ghent University and colleagues at Pall Corporation highlighted that when the filtered pyrolysis oils were subjected to steam cracking, there was a 40-60% reduction in radiant coil coke formation compared to unfiltered oil. Additionally, this reduction occurred without any changes in product selectivity, thus confirming the significant impact of particulate contamination on coke formation during steam cracking.
This filtration step can take place at the plastic oil production site, in a separate oil upgrade unit, or directly in the steam cracker before the oil is blended with naphtha. This technology can accommodate different filtration grades to mitigate the potential evolution of the pyrolysis oil with an increase in solid contamination that may occur due to degradation and polymerisation.

Building a circular economy
Alongside reduction and re-use strategies, recycling plays a pivotal role in driving circularity and limiting the environmental damage caused by plastic waste. Despite its flaws, mechanical recycling will continue to play a crucial role in waste management systems, especially if it can be further optimised by overhauling packaging design, raising consumer awareness and improving stronger collection, sorting and pre-treatment infrastructure.

Pyrolysis technology, however, is capable of excelling in areas where traditional recycling methods are weak, particularly when it comes to mixed plastic waste. This requires modern, effective filtration systems to enhance operational efficiency, otherwise contaminants can ruin the quality of the pyrolysis oil, making it unsuitable for producing new plastics. Contaminants can also damage critical equipment used in the pyrolysis process, leading to increased downtime for repairs and higher operational costs.
It is imperative that international governments more strongly acknowledge the crucial role of chemical recycling in advancing the circularity of plastic production and reflect this through supportive policy measures and greater investment. This will allow the technology to reach its full potential and scale up to a level that significantly enhances the circular economy and disincentivises the production of virgin plastic from fossil fuels. A greener future is a better proposition for us all.

Serhat Oezeren is the Global Market Manager for the chemical, polymer and recycling industries at Pall Corporation. He joined Pall Corporation in 2007 and has worked in various roles in commercial, business development and marketing departments. Serhat holds a Bachelor of Science degree in Chemical Engineering from Istanbul Technical University and a Master’s degree in Chemical Engineering from KIT Karlsruhe Institute of Technology.

sustainableplastics