Experimental set-up, which is used to quickly heat plastic together with hydrogen and catalyst, so that it is converted into petrol. Photo taken by Thorkild Christensen

Closing the plastic recycle loop

Tuesday 01 Dec 20
by Vibeke Lykke Stenstrop


Anker Degn Jensen
DTU Chemical Engineering
+45 45 25 28 41


Martin Høj
Associate Professor
DTU Chemical Engineering
+45 45 25 28 42

What is plastic?

Plastics are made from hydrocarbon molecules called monomers, primarily ethene and propylene (called olefins) as well as aromatics, which are organic molecules based on the basic chemical structure benzene (C6H6). During a chemical process called polymerization, the monomers form long chains of molecules, called polymers, which are plastics.

What is a catalyst?

The catalyst is a material that increases the rate of reaction. In practice, good catalysts are essential for making chemical reactions happen. The better the catalyst, the easier (under milder conditions) the reaction takes place.

Only 16 per cent of the more than 250 million tons of plastic we use every year globally is recycled. If, however, we could recycle more plastic and at the same time get something useful out of the CO2 emitted by incineration of residual plastic waste, we would optimally be able to achieve full recycling of all plastic. This is the goal for two new research projects at DTU Chemical Engineering.

From plastic to new plastic or gasoline

Plastics are made from chemicals made from crude oil, primarily molecules found in or made from gasoline.

“If we could transform plastic into gasoline, which can then be processed into new plastic, we could achieve what is called chemical recycling of the plastic that cannot be recycled by simply melting it. This can be because the quality of the plastics is low due to previous recycling, it is commingled with other types of plastics or simply is not sorted, says Anker Degn Jensen, who is a professor and researcher at the Center for Combustion and Harmful Emission Control (CHEC) at DTU Chemical Engineering.

It is precisely the most commonly used types of plastic that are the focal point of his project.

“In my project, we develop and investigate a process in which the most commonly used types of plastic are flash heated together with hydrogen and a catalyst under pressure to convert it into high-yield gasoline, which can subsequently be turned into new plastic or used as fuel. We investigate the effect of catalyst, pressure and temperature and the effect of different types of plastic in pure form as well as in mixtures,” says Anker Degn Jensen.

The most commonly used types of plastic are polyethylene and polypropylene, which we all know from our households in the form of e.g. freezer or waste bags. Therefore, there is really something to be gained in terms of sustainability if we can turn up the recycling of the plastic we use the most.

Chemical recycling of CO2 from incinerators

Even if we wanted to, it is not possible to recycle all plastic. There will always be a residual fraction left which ends up in incinerators. It is the CO2 from here that is the focus of the second research project.

“When a residual product of plastic ends up in the waste incineration, it is either because it is mixed with other materials, is dirty or perhaps even potentially contaminated, e.g. face masks. Crisps bags, which have aluminum foil on one side and plastic on the other, also end up in the incineration, because the layers in these bags cannot be separated", says Martin Høj, who is associate professor and researcher at CHEC.

The energy from the combustion of the residual product is recovered as electricity and district heating, but at the same time, fossil greenhouse gases, primarily CO2, are emitted from the process. So, what if you could collect and convert this CO2 into new plastic?

“In my project we look at the chemical recycling of CO2. It is already possible to produce methanol, which can subsequently be converted to olefins and aromatics if you combine CO2 with hydrogen from electrolysis of water. However, it will be much more efficient, both in terms of energy and economy, to convert CO2 and H2 directly into olefins and aromatics which is why we develop and investigate special catalyst mixtures for this very purpose ", says Martin Høj.

The goal is to develop a simple and efficient process for the production of new plastic building blocks from CO2 to increase the overall recycling of plastic.

Plastic recycling figureCircular plastic use as a joint goal

Common to Anker Degn Jensen’s and Martin Høj's projects is that they have catalysts as a focal point when it comes to recycling plastic, but with their own research approach.

They also contribute to the same goal, namely to ensure that the plastic we all use in our everyday lives is eventually recycled to the highest possible extent, thereby closing the plastic recycle loop.

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