We Must Accelerate Advanced Recycling to Reach Circularity

The need for better methods to tackle the plague of waste that blights our landscapes has never been more acute. A study has shown that nearly 220 million tonnes of plastic waste are expected to be generated in 2024 — an increase of more than 7% since 2021. Yet when it comes to disposing of waste, around 50% goes to landfill, 19% is incinerated, 22% is discarded as litter, and only 9% is recycled. 

It is vital that we do more, globally, to tackle the critical issue of how to dispose of waste at the end of an item’s use by drastically increasing recycling capabilities. If not, we risk causing continued harm to wildlife and the environment and letting more microplastics enter the food chain, potentially damaging our own health. With the right technologies we can turn trash into treasure. The challenges of implementation are manifold, however.

Expanding possibilities with chemical recycling

Mechanical recycling has certainly been useful as the prevalent mechanism employed by municipal authorities to manage waste in recent years, yet these technologies face obstacles in treating mixed plastic waste streams. By contrast, chemical recycling technologies hold greater potential to treat highly contaminated and more complex mixed waste streams. 

Related:Is Chemical Recycling a Silver Bullet or a Lead Balloon?

Chemical recycling through pyrolysis — often referred to as advanced recycling — is gaining traction as an alternative to mechanical recycling and incineration. This method enables the processing of a wider scope of materials, including multi-layer, multi-material items such as packaging, as well as plastics and vehicle tires. Pyrolysis involves heating mixed plastic and other waste materials to temperatures of 400° to 600°C 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 and involve 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 of the pyrolysis process 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 in the production of rubber goods, automotive parts and coatings, batteries, cables, and printer inks.

Pyrolysis oil. Image courtesy of Pall Corp.

The oil — the majority product by volume of pyrolysis — can be used as feedstock by the chemicals and petrochemicals industries to produce new plastics with the same chemical structure and virgin quality as first-generation plastics. Research by the US Department of Energy’s Argonne National Laboratory shows that production of plastic with just 5% of pyrolysis oil reduces greenhouse gas emissions by up to 23% compared with plastic made entirely from crude oil. 

Purity in pyrolysis is key

The pyrolysis process is not without its challenges, as the plastic feedstock and concentration level of downstream contamination can vary significantly. Numerous types of plastics and non-polymeric sources are combined in mixed plastic waste feedstocks. Those feedstocks contain coarse to fine particles — fillers, flame retardants, and so forth — and other materials, such as coke, which are detected in the oil downstream in the pyrolysis process. 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 fouling of equipment and unnecessary downtime for maintenance.

To refine the pyrolysis oil further for use as fuel or feed 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 within acceptable thresholds for crude naphtha feed in steam crackers. It is an efficient and cost-effective way to remove particle content from the oils.

A paper published by scientists from Ghent University and colleagues at Pall Corp. highlighted that when the filtered pyrolysis oils were subjected to steam cracking, there was a significant 40 to 60% reduction in radiant coil coke formation compared with unfiltered oil. Additionally, this reduction occurred without any changes in product selectivity, 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 the oil blends 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 polymerization.

A unified approach is needed

We see then that there are technical challenges with the pyrolysis process and solutions to optimize the technologies. There are also several other barriers to overcome, with the need for investment a big factor. The capital costs for setting up a pyrolysis plant, including reactor systems, feedstock preprocessing equipment, and pollution control systems, can be substantial. In terms of operational costs, the high energy requirements, need for catalysts, maintenance, and labor can make this form of chemical recycling economically challenging.

Market forces naturally play their part in the relative appetite for pyrolysis. The competition and advanced scale of fossil fuel production mean that oil and gas are cheaper to produce than pyrolysis oil and its subsequent products. At the end of the product's life cycle, mechanical recycling is often cheaper and more established, making it a preferred option for dealing with many types of plastic waste.

On the policy front, there are differing international regulations and incentives, making adoption of pyrolysis variable across the world. In the United States, for example, the Renewable Fuel Standard (RFS) program mandates the use of renewable fuels in transportation, providing market opportunities for pyrolysis-derived biofuels. California has implemented aggressive waste diversion and recycling goals, such as SB 1383, which encourage the adoption of advanced recycling technologies.

The European Union (EU) has been a leader in promoting circular economy principles and waste-to-energy technologies, including pyrolysis. Key policies include the: 

Countries including Japan, South Korea, Canada, and Australia also have policies, frameworks and financial incentives that support waste management and renewable energy production, including the development of pyrolysis infrastructure. For other countries that are behind on these issues — whether due to lack of infrastructure, finance, or political will — the hope is that progress will be made as the growing body of evidence of the benefits of advanced recycling becomes increasingly apparent. 

There is a huge opportunity for pyrolysis to be scaled up to meet the global issue of waste management and provide more options for renewable energy creation. We must minimize our use of the Earth’s natural resources and reduce the amount of waste generated to prevent environmental damage. Recycling of mixed waste via pyrolysis and subsequent steam cracking toward light olefins is a promising solution for the ever-growing plastic waste crisis. 

The more that plastics and other items are chemically recycled, the less pollution there will be of landfill sites, waterways, and oceans. Yet without a cohesive drive to put in place the financing, infrastructure, and legislation needed to increase the adoption of pyrolysis technologies, there will be more damage to the environment and ultimately more risk to our own lives.