The Problem of Waste Plastic and Why Pyrolysis Oil Might Just Contain the Answer

For Earth Day, Aniqah Majid speaks to chemical engineer George Huber who is looking to simplify the process of integrating pyrolysis oil back into the plastics production chain

ONE OF the few technologies that can break down unrecyclable post-consumer waste plastic, pyrolysis is fast becoming a potential recycling route for companies trying to reduce their waste output.

The world produces around 450m t/y of plastic, but only 9% is recycled, with most waste ending up in landfill. Pyrolysis, which involves heating the plastic at extremely high temperatures in the absence of oxygen, breaks down the molecules to produce pyrolysis oil or gas. The oil can then be used to develop new products.

George Huber, a professor of chemical engineering at the University of Wisconsin-Madison, is leading a research team that is investigating the chemistry of pyrolysis oil and its use in polyolefin recycling.

Huber said: “Waste plastic should be viewed as a resource we can use to make plastics and other chemicals. We should not be landfilling or burning it, we should be reusing the carbon in waste plastics.”

In the last few years, Dow, BASF, and Shell have launched initiatives that involve processing between 20,000 and 100,000 t of waste plastic to make pyrolysis oil. The aim is to integrate the oil into their existing chemical production processes, replacing the crude oil-derived naphtha used to produce olefins like ethylene and propylene, essential building blocks needed to make food packaging, tyres, and batteries.

However, the use of pyrolysis oil to make new plastics is currently difficult due to quality requirements. As plastic waste, particularly plastic packaging can be mixed in with other residual waste and contaminants, the pyrolysis oil cannot be used on its own as feedstock to develop the same results as naphtha without it being upgraded.

Upgrading removes impurities like oxygen, nitrogen, and sulfur. However, to help it reach the quality necessary for use as a feedstock, the complicated process involves separating the high-quality waste, pyrolysing it, upgrading it, and then blending it with naphtha.

A different route

Huber is taking a different approach, circumventing the upgrading and steam cracking process to produce olefins, and instead separating the olefins already present in pyrolysis oil.

He said: “What we noticed is that in the pyrolysis oils there are large amounts of olefins, anywhere between 50 to 60%.”

The Huber Research Group is currently testing the use of pyrolysis oil in the production of aldehydes through hydroformylation.

This process involves adding CO and H2 to olefins to create aldehydes, which can then be reduced through homogenous and heterogenous catalysis to make alcohols, carboxylic acids, and amines. The research team is testing this process to make surfactants, polymers, and other new materials.

Huber estimates that the use of hydroformylation to create new plastics instead of using steam cracking could reduce CO2 emissions from the conventional production of industrial chemicals by roughly 60%.

On why industry has not taken advantage of the olefin content of pyrolysis oil yet, Huber said: “The production of pyrolysis oil is rather new. So, a lot of people asked the question of how they can use their existing assets to do something with that oil.”

Similar quality issues

After difficulty perfecting the pyrolysis and hydroformylation processes on a lab-scale, the last year has seen them successfully develop alcohols and amines through the technologies.

Now, the challenge is chemical separation, and finding which cuts of pyrolysis oil can produce different products like pepfactants and polymers.

Huber is currently working with several companies in industry and scaling up the hydroformylation process to develop samples of different cuts that can be used in plastic production.

He acknowledges that sourcing high-quality plastics to make products will be an issue, as well as transporting materials to a localised facility to support the process. But, due to the high olefin content of the pyrolysis oil, Huber stresses that the process could cut plastic production costs by 50%.

He added: “In this project we have tried to make higher value chemicals that are difficult to make from oil. The whole chemical industry is around taking a molecule with low functionality and functionalising it into an olefin. With pyrolysis, you already have olefins, so let us take advantage of that.”