Commentary: Pyrolysis offers hope to tackle the plastics crisis

Images of plastic mountains at landfills and islands of floating plastics in our oceans demonstrably show that we are living in an age of immense consumer consumption. A rising worldwide population coupled with an increasing desire for goods mean that the global production of plastic is set to nearly triple in the coming decades from around 460 million metric tons in 2019 to 1,231 million metric tons in 2060, according to the Organization for Economic Co-operation and Development (OECD).

Despite international concern that more waste generated equals more harm to the environment, the problem continues to grow. Additionally, as increasing volumes of plastic enter the food chain via animals and fish feeding on discarded materials, the threat to public health increases. In short, we are in the middle of a plastics crisis.

Various international policies aimed at tackling the issues have been adopted or proposed, including the U.S. Environmental Protection Agency’s (EPA’s) draft National Strategy to Prevent Plastic Pollution, the European Commission’s ban on single use plastics and its commitment that 55 percent of end-of-life plastic be recycled by 2030 via the Packaging and Packaging Waste Directive. The U.K. government recently increased its Plastic Packaging Tax which is levied on components with less than 30 percent recycled plastic that are roduced in or imported into the U.K.

We must consider, though, whether these measures adequately deal with the issues. One of the main factors is that less than one-fifth of plastic is recycled. Households and businesses diligently might put out their refuse for recycling collections, but if the majority of this material is not converted into new products, environmental and health problems will increase exponentially.

Mechanical recycling has certainly been useful as the prevalent mechanism employed by municipal authorities to manage plastics in recent years. However, mechanical recycling technologies are facing challenges in treating plastic streams, limiting their scope. In contrast, chemical recycling technologies can have higher tolerances to treat contaminated and complex mixed plastics streams. Those highly contaminated mixed plastic streams are not recycled yet given the limitations of current mechanical recycling technologies.

Good chemistry

Chemical recycling through pyrolysis (often referred to as “advanced recycling”) is gaining traction as an alternative to mechanical recycling and incineration because of the wider scope of what can be processed. Pyrolysis involves heating mixed plastics to temperatures of 400 to 600 C (750 to 1,110 F) in the absence of oxygen, with or without a catalyst, to convert polymers into a mixture of liquid hydrocarbons.

The initial steps are similar to mechanical recycling with sorting, pretreatment (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 oil can be further refined to produce approximately 80 percent liquid, 15 percent gas and 5 percent carbon black (ash).

The resulting products from pyrolysis can be used in a number of ways. The gas can be fed back into the system to heat the reactor’s furnace, and the carbon black can be used for a variety of purposes, such as the production of rubber goods, automotive parts and coatings, batteries, cables and printer inks.

The oil, the majority product by volume, can be used as feedstock for the chemical and petrochemical industries to produce new plastics that have the same chemical structure as first-generation plastics with virgin quality. Moreover, research by the U.S. Department of Energy’s Argonne National Laboratory shows that production of plastic using just 5 percent pyrolysis oil reduces greenhouse gas emissions by up to 23 percent compared with plastic made using crude oil.

Overcoming contamination challenges

The pyrolysis process is not without its challenges, however. With respect to the pyrolysis technology and the plastic feedstock used, the concentration of downstream contamination and its nature can differ significantly. Numerous types of plastics and nonpolymeric sources are combined in mixed plastic feedstocks. Those feedstocks contain coarse to fine particles (e.g., filler, flame retardants, etc.) and other materials that are detected in the oil produced 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, can be found in pyrolysis oil. This complex mixture of contaminants must 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 fouling of equipment and unnecessary downtime for maintenance.

Creating the desired product

To refine the pyrolysis oil further for use as fuel or a feedstock for plastic production, it must be transferred to a steam cracker to convert the oil into lighter olefins. The presence of particles and metal contaminants in crude pyrolysis oils could have significant negative impacts on the steam cracker‘s furnace and recovery section such as furnace run-length reduction from the coking increase.

However, using depth filtration can be an effective method to remove harmful contaminants and reduce the contamination in 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.

Recently published work by Kevin M. van Geem (et. al., including me) highlights that when the filtered pyrolysis oils were subjected to steam cracking, radiant coil coke formation was reduced by 40 percent to 60 percent compared with 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 occur in the plastic oil production site, in a separated oil upgrade unit or directly in the steam cracker, before blending the oil 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 could occur from degradation and polymerization.

Searching for sustainability

We know that we need to minimize our use of the earth’s natural resources and reduce the amount of waste generated to prevent environmental damage. Recycling of mixed plastics via pyrolysis and subsequent steam cracking toward light olefins is a promising solution for the ever-growing plastic waste crisis. It can be understood as a substituent of crude fossil oil.

The more that plastics and other items are chemically recycled, the less pollution there will be in waterways and oceans. Consequently, this should reduce harm to wildlife and minimize the volume of microplastics entering the food chain that pose a threat to human health.

Universal collection, sorting, pretreatment and design of plastic products for recycling are keys to using mechanical recycling methods as the most established technology in industry. However, many end-of-life plastic streams remain unsuitable for mechanical recycling. To apply circularity to an increased share of end-of-life plastics, chemical recycling must be scaled up.

International government perspectives on the role of chemical recycling technology should be reviewed and acknowledged as being crucial to improve plastic circularity and recycling rates. Pyrolysis providers’ R&D also must clarify the role, performance and use of these technologies at an industrial scale.

If these elements are in place, the price of pyrolysis oil production could come down. If it falls to a level equal to the cost of current liquid fossil feedstocks, there will be less impetus to create first-generation plastics from fossil fuels. As such, chemical recycling could become the default option in the plastics value chain, waste and pollution would be reduced, and we would all be living in a more sustainable world.

Emmanuelle Biadi is a petrochemicals and recycling expert at Pall Corp., Port Washington, New York. She is a co-author of the scientific paper “Contaminant removal from plastic waste pyrolysis oil via depth filtration and the impact on chemical recycling: A simple solution with significant impact.”