Chemists Unveil Plastic Pyrolysis to Combat Waste

In 1950, global plastic production was about 2 million tons. It's now about 400 million tons - an increase of nearly 20,000%.

As a material, it has seemingly limitless potential. Plastic is inexpensive to produce while being lightweight and sturdy. Its applications range from food and beverage packing to clothing and health care.

When a plastic item ends its useful life, it can take a very long time to decompose, up to 500 years in some cases. Even then, the plastic pieces don't disappear entirely - instead, they break down into smaller and smaller pieces, eventually becoming microplastics that end up in the soil where we grow food, the water we drink and the air we breathe.

Research has linked these microplastics to health issues such as diabetes, heart disease and low male fertility.

For years, local governments and manufacturers have relied on recycling as the answer to keep plastic waste from accumulating. However, despite their efforts to sort and separate recyclables, most plastics still end up in landfills - or worse, in green spaces and waterways .

According to the U.S. Environmental Protection Agency, the overall recycling rate for plastics is 8.7% . About a third of milk jugs and plastic bottles are recycled - a higher rate than other types of plastic.

Because plastic is so commonly used, finding new ways to manage and recycle plastic waste is becoming ever more important. Plastic waste pyrolysis is one technology that could help address this issue.

This is a relatively new technique, so researchers still have only a limited knowledge of the pyrolysis process. As analytical chemists , we strive to understand the composition of complex mixtures, especially new creations from sources such as plastic waste pyrolysis.

What is plastic pyrolysis?

Plastic pyrolysis is a chemical process that involves chemically breaking down plastics into other molecules by heating the plastics to extremely high temperatures in the absence of oxygen.

Unlike traditional plastic recycling, pyrolysis theoretically isn't limited to specific types of plastic. It could be made to accommodate many of them, although current technology is limited to a few types - polyethylene and polypropylene, used in food containers and bottles - at an industrial scale.

So, plastic pyrolysis could help handle the waste from consumer products such as plastic bags, bottles, milk jugs, packaging materials, wet wipes and even discarded children's toys. Pyrolysis can also handle more complex plastic waste such as tires and discarded electronics, although solid waste handlers and recyclers avoid certain plastic types in pyrolysis, such as polyvinyl chloride - or PVC, which is found in pipes and roofing products - and polystyrene, used in packaging, as these can create harmful byproducts .

During pyrolysis, the plastic polymers are broken down into smaller molecules, resulting in the production of liquid oil, fuel source gases such as methane, propane and butane , and char .

Char is the solid residue left at the end of the pyrolysis process. It can be used as a carbon-rich material for various applications, including adding it to soil to make it healthier for farming, as it increases soil moisture and pH, benefiting nutrient absorption. Char also has the ability to absorb harmful carbon gases from the air , which can help prevent climate change.

The main downside of char is if it's used too much it can increase soil alkalinity , which may hinder plant growth.

How pyrolosis works

The plastic pyrolysis process typically involves several key steps.

In the first step of pyrolysis, community recyclers collect the plastic waste and clean it to remove any contaminants. The plastic then gets shredded into smaller pieces to facilitate the pyrolysis process. Unlike traditional recycling, it needs only minimal sorting .

Chemical recyclers operating pyrolysis plants feed the shredded plastic into a pyrolysis reactor, where they heat it to temperatures ranging from 600 to 1,600 degrees Fahrenheit (315 to 871 degrees Celsius). Without oxygen, plastics in the reactor don't catch fire and emit fumes into the air. Instead, this high-temperature environment causes the plastic polymers to break down into smaller hydrocarbon molecules . These smaller molecules can be further refined.

The high temperature turns some molecules into vapors, which condense into liquid oil. Chemical companies can further refine this oil to be used as fuel or as a raw material to make other chemicals or plastics.

In addition to liquid oil, the pyrolysis process generates natural gases, such as methane, ethane, butane and propane. Pyrolysis operators then capture these gases, and they can sometimes use them as a source of energy to power the pyrolysis reactor or other industrial processes.

Benefits of pyrolysis

When done effectively, plastic pyrolysis offers several benefits.

By expanding recycling beyond just plastic bottles and milk jugs, pyrolysis could reduce the amount of plastic waste pollution that ends up in landfills and oceans.

Additionally, converting plastic waste into usable products could help lower the production demand for new plastics from petroleum hydrocarbons. The byproducts could get used in recycled plastics.

Some researchers are also testing pyrolysis oils to see whether they can use them instead of gasoline to fuel vehicles . The gases produced during pyrolysis can even generate energy that fuels the pyrolysis reactor, making the process more self-sustaining and reducing the need for external energy sources.

Currently, about 15% to 20% of the pyrolysis products are recycled into new propylene and ethylene, while most - about 80% to 85% - becomes diesel fuel, hydrogen, methane and other chemicals.

While plastic pyrolysis holds some promise, it also faces challenges. The cost of setting up and operating pyrolysis plants is high. How profitable the process is depends on the availability of suitable plastic waste, the market demand for the oils and gases produced, and the costs of energy and staff necessary to operate the reactor.

Another issue is quality control. Most plastic types can undergo pyrolysis, but different plastics create oils with different chemical makeups. Scientists will need to understand the composition of these oils before industry can determine which plastic types to focus on and how each oil could create new materials.

Researchers like us at The University of Texas at Arlington and our international colleagues are studying new chromatography-based oil-separation techniques that can successfully identify some types of pyrolysis oils. Chromatography is the process of separating components in a mixture by passing them through a stiff material.

Different components in the mixture are attracted to this material to different degrees. So, they exit the chromatography system at different times, which separates them from one another.

With more research into the technique's efficiency and technological advancements to scale up pyrolysis, this technique could be one part of a sustainable solution to plastic waste management. In the meantime, pyrolysis is being used now, with one report estimating the market for pyrolysis plants at US$40 billion in 2024 and predicting it to grow to $1.2 billion by 2033.

Kevin A. Schug receives funding from the National Science Foundation, the National Institutes for Health, ExxonMobil, and Weaver Consultants Group. He is affiliated with VUV Analytics, Inc. and Infinity Water Solutions as a member of their scientific advisory boards. Lummus Technology, LLC provided the funding for research on plastic waste pyrolysis oils at UT Arlington.

Alexander Kaplitz does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.