Thanks to pyrolysis, even problematic plastics can be recycled

How can heavily contaminated or problematic plastics be recycled? CCPE scientists in the Research Department Advanced Recycling of the Systems Division are working on this question. Their research focuses on materials such as carbon fiber reinforced plastics (CFRP) and glass fiber reinforced plastics (GRP), which are used to manufacture wind turbines and rotor blades, for example, but also thermosets, resins and residues from the processing of electronic scrap (printed circuit boards) and end-of-life vehicles (brake pads or air filters) or sorting residues. Dr. Alexander Hofmann from the Fraunhofer UMSICHT in Sulzbach-Rosenberg explains in an interview why these material flows are difficult to recycle and what options pyrolysis offers for recycling.

© Fraunhofer UMSICHT
Dr. rer. Nat. Alexander Hofmann

Why are materials like CFRP, GFRP, thermoset or resin so difficult to recycle?

Alexander Hofmann: Depending on the material flow, different problems arise. GFRP and CFRP, for example, are fiber-reinforced plastics. To be able to recycle them, these fibers must first be separated from the plastics. This is quite difficult due to mechanical processes. Materials made of thermosets or resins, on the other hand, cannot be recycled once they have melted down. The challenge is therefore to separate the materials in order to reuse individual components.

What happens when rotor blades, brake pads, etc. have reached the end of their useful life?
Alexander Hofmann: GRP material usually ends up in the waste incineration plant. Some cement plants use the plastic content as an energy source and the glass or fiber content as a filler for the cement itself. Of course, this downcycling is not an optimal utilization.

With CFRP, the situation is even more problematic. Some fibers are suspected of being carcinogenic and must therefore not be released into the environment under any circumstances. Cement works sometimes do not accept the material, and waste incineration plants also face a problem: the carbon fibers contained in the material can cause short circuits in the flue gas cleaning system. In the worst case, all that remains is disposal in a landfill.

Is it possible to quantify how many of these contaminated or problematic plastics have to be disposed of in Germany each year?
Alexander Hofmann: Unfortunately, I don’t have any exact figures on this. But the production volume of GFRP and CFRP in Germany is around 250,000 tons per year. Europe-wide we are talking about one million tons per year. And this material has to be disposed of eventually.

You said that the challenge lies in separating these plastics so that individual components can be recycled. How can this be achieved?
Alexander Hofmann: This can be achieved, for example, by chemical recycling – one of our main research areas in the context of Fraunhofer CCPE. In this process, we separate the inorganic part from the plastics by breaking them down into their individual chemical components – the technical term is depolymerization – and use them to produce plastics in virgin material quality again.

How can I imagine this practically?
Alexander Hofmann: A plastic consists of a long chain of basic building blocks called monomers. This is what we use for pyrolysis. In other words, we bring the material to about 650 degrees under exclusion of oxygen. Under these conditions it does not burn to CO2, but splits up into smaller and smaller building blocks. And we can recover these building blocks as chemicals and use them to start polymerization again.

Has this process already reached a stage where it can be applied as a company?
Alexander Hofmann: That depends entirely on the starting material. If we take a material flow with polystyrene or lightweight packaging material made of PE and PP, the process can be carried out industrially and at the end there would be a material that can be used. However, if we think of material flows such as GFK, CFK or even electronic scrap, things become more complicated. These materials contain residues such as metals or even PVC contaminants through which pollutants can enter the pyrolysis products – for example the pyrolysis oil. You have to make sure that you comply with the limit values. This means that if you want to use this material flow industrially, the pollutants must be removed from the oil.

And this is precisely where our current research comes in: We are looking for a way to reduce the proportion of pollutants in the pyrolysis products – for example, by preventing them from getting in at all or by removing them from the oil. For example, we carry out distillation to produce fractions that are less contaminated. We are also working on a kind of pyrolysis oil post-treatment to remove impurities.

By when will these solutions be ready for the market? And for which industries are they interesting?
Alexander Hofmann: I expect pyrolysis to be generally established on the market within the next ten years. And then we want to be at the start with our solutions.

Pyrolysis is particularly interesting for the chemical industry. Many companies there already use pyrolysis oil as a substitute for crude oil. This works via so-called steam crackers. The crude oil is processed and crushed in these crackers to produce plastics or chemicals. The companies add the pyrolysis oil to this oil. The mixing process means that impurities or differences between crude oil and pyrolysis oil are negligible, and energy and mass balances can be used to calculate a recycling proportion in the new material.


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