In a recent study, a team of researchers from Ghent University laid out what they believe to be the most efficient route to gaining high-quality petrochemical feedstocks from mixed plastic packaging waste via pyrolysis. But what does that mean for packaging? Marvin Kusenberg, PhD student at the university’s Laboratory for Chemical Technology and a researcher on the project, tells us more about the present and future applications of this process when it comes to recycling.
For the sake of our readers, could you explain in layman’s terms how pyrolysis works and why you’d recommend it as a sustainable way to close the material loop?
In very simple terms, pyrolysis is the chemical decomposition of a material using thermal energy in the absence of oxygen. When applied to plastic waste, pyrolysis yields an oil-like liquid hydrocarbon product that contains a wide variety of chemical compounds, which are highly useful for further chemical processing.
The composition of pyrolysis oils is dependent on the plastic waste type that was pyrolyzed. Polyolefins (i.e., PE and PP) contain only carbon and hydrogen in their molecular structure and are thus similar to hydrocarbon feedstocks of fossil origin. Therefore, pyrolysis oils are a promising feedstock to substitute crude oil-derived feedstocks such as fossil naphtha within large-scale petrochemical processes.
This is especially relevant for steam cracking, which is the most important process in the production of base chemicals such as ethylene, propylene, butadiene, aromatics, and others. By going down to the base chemicals, virgin-quality polymers can be produced, keeping the carbon in the loop.
Hence, current disposal methods for post-consumer plastic waste, such as incineration or landfill – which are still applied for around 42% and 24% of the collected plastic waste, respectively, in the EU27+3 – can be reduced substantially.
Obviously, connecting the waste collection and management schemes with the petrochemical production processes of virgin plastics would be an elegant way to close the material loop. As well as evading waste-to-landfill, chemical recycling has vast greenhouse gas emission-saving potential by avoiding incineration and reducing the need for crude oil-derived feedstocks, as shown in a recent study by Eschenbacher et al.
When combined with electrification to avoid using conventional gas-fired processes, more than 90% of the CO2 equivalents could be saved.
You mention that the EU does not consider chemical processes that result in pyrolysis oils a form of recycling. Do you believe that it should? Why, or why not?
In my opinion, it is essential that all techniques that lead to a second life for waste materials be considered recycling, as it means that waste incineration and landfill can be avoided, and materials kept in the loop. However, it is important to evaluate the fate of pyrolysis oils throughout the process chain.
While in mechanical recycling, sorted plastics are re-shaped into recycled products, in chemical (or, as it is often called advanced) recycling, waste streams are chemically or thermally decomposed into a chemical feedstock. This can be fed into the petrochemical infrastructure, leading to a large variety of products that can be used for multiple purposes.
For instance, the most important products of steam cracking are ethylene and propylene, which are typically used to produce virgin-quality PE and PP. Therefore, if the pyrolysis oils are used to produce new plastics, we are essentially talking about recycling. Obviously, if pyrolysis oils are used as fuels for energy recovery, this pathway should not be considered recycling, but rather secondary incineration.
Food Safety Magazine published an article last year about chemical recycling in relation to food safety. If plastic that has undergone pyrolysis is reused to package food, should there be any concern surrounding contamination?
When comparing mechanical and chemical recycling, one of the advantages of chemical recycling is that the product is a chemical feedstock, which can be used to produce base chemicals. In chemical recycling, we are going all the way down to the molecule and there is no longer any distinguishable difference between ethylene of fossil or of waste origin. Therefore, the polymers produced from waste-derived ethylene are essentially the same as fossil-derived polymers and thus no concern should be there in terms of food safety.
Contamination of waste streams, however, is a large concern when it comes to the technical applicability of these pyrolysis oils in large-scale industrial processes such as steam cracking. As these units are designed for fossil feedstocks, it is still uncertain if pyrolysis oils are feasible feedstocks in the long run from a technical point of view, as we have recently elaborated upon.
Certainly, thorough upgrading of pyrolysis oils is needed before they can be used to (partly) substitute fossil feedstocks in industrial steam cracking units without disturbing running processes.
BOPET Films Europe recently wrote an article for us in which it was claimed that plastic to plastic yield through pyrolysis can be as low as 10%, which would not hit 30% recycled targets at industry level. How would you respond to this?
In terms of pyrolysis, it must be considered that, in going all the way down to the molecule, one obtains a large variety of valuable products, rather than a single recycled plastic product. Furthermore, the fact that the products of steam cracking of pyrolysis oils are not exclusively being used for polymer production, but also other chemical products shows that pyrolysis needs to be evaluated from a broader point of view.
In fact, the efficiency of pyrolysis and subsequent steam cracking should not be evaluated by the ethylene and propylene yields alone, but by a number of products – the so-called high-value-chemicals, which are defined as ethylene, propylene, hydrogen, benzene, and butadiene. In fact, a recent study by Eschenbacher et al. showed that a light olefin yield of 75% could be achieved using catalytic pyrolysis, which would pose a significant plastic to plastic yield.
The Consumer Goods Forum previously released a Life Cycle Assessment study that claims pyrolysis “can be considered complimentary technology to produce food-grade recycled PE and PP packaging”, but do you believe we’ll ever reach a point where pyrolysis could stand alone as its own recycling technology?
In my opinion, chemical and mechanical recycling should be complementary rather than competing. As there are efficient large-scale mechanical recycling schemes in place for packaging waste streams such as PET, the main focus of chemical recycling should be on the streams that mechanical recycling struggles with.
This mainly applies to polyolefin waste, which makes up around 60% of collected plastic waste in Europe. Such waste streams are often in the form of foils, which contain multiple layers and are thus highly difficult to separate in waste sorting and separation plants.
As I’ve already mentioned, polyolefins are ideal feedstocks for chemical recycling, and considering the high and still increasing demand for virgin-grade PE and PP, pyrolysis has a very good chance of becoming a highly relevant large-scale recycling technology. This can also be seen through the fact that several large industrial companies in the petrochemical sector are starting up pilot or small industrial-scale chemical recycling units.
In your opinion, is it realistic to expect the industry to embrace pyrolysis on a wider scale anytime soon?
There are two very important aspects when it comes to industrial adaption of pyrolysis as a recycling technique. The first aspect is the technical side, which shows promising results when using plastic waste pyrolysis oils for chemicals production.
The second aspect is the legislative side, which incorporates more and more strict regulations in terms of sustainable processes – making conventional fossil-based manufacture of polymers more and more expensive. Hence, companies are forced to adapt chemical recycling into their production schemes.
Given the potential reduction of greenhouse gas emissions when using waste-derived feedstocks in steam cracking compared to conventional fossil fuels, it is very likely that the industry will embrace this technology at a growing scale.