Steam cracking plastic to transform the waste system

Materials World magazine
,
27 Nov 2019

A thermochemical recycling process that uses steam cracking can transform plastic waste back to new virgin-quality plastics. Shardell Joseph investigates.

A new plastic recycling process has been developed that can break down plastics to molecular level and convert the gases into material matching its original quality. Researchers at Chalmers University of Technology, Sweden, have adapted the steam cracking process, usually used for crude oil, claiming the process could contribute to more sustainable manufacturing and waste management systems.

The researchers experimented with chemical recovery through the steam cracking of plastic at the Chalmers power central facility in Gothenburg. After testing the system, they believe that turning used plastic waste into plastics of virgin quality can retain its value, making it a genuinely circular system.

‘Through finding the right temperature – which is around 850°C – and the right heating rate and residence time, we have been able to demonstrate the proposed method at a scale where we turn 200kg of plastic waste an hour into a useful gas mixture. That can then be recycled at the molecular level to become new plastic materials of virgin quality,’ said Chalmers University Head of the Division of Energy Technology at the Department of Space, Henrik Thunman.

Because the process is applicable to all types of plastics in our waste systems, including those that end up in landfill or at sea, the researchers envisage the system being part of a circular economy, reducing the need for oil and fossil-derived gases to produce high quality materials. The technique is also expected to create net negative emissions by capturing carbon dioxide in the process.

Waste hierarchy

According to the researchers, In 2015, around 350 million tonnes of plastic waste were generated worldwide, 14% of which was collected for material recovery. Out of this, 8% was recycled into plastic of lower quality, 4% was lost in the process, and 2% was of plastics was restored to of similar quality.

The EU Waste Framework Directive, revised in 2008, introduced a five-step waste hierarchy with prevention at the top as the best option, followed by preparing for re-use, recycling, other recovery and disposal. Prevention also resonates in the United Nations’ 17 sustainable goals, stating that the current linear use of resources needs transforming to a circular form.

‘As we take back the molecules to the original quality, we actually put something in the recycling back on the top of the hierarchy. So, we start again with the initial quality product,’ Thunman told Materials World. ‘This means we actually go from prevention and reuse, and when it comes down to recycling, we take it back up again to the highest level, closing the loop for carbon-based materials.’

According to the research paper, Circular use of plastics-transformation of existing petrochemical clusters into thermochemical recycling plants with 100% plastic recovery, published in Sustainable Materials and Technologies this year, the system has the capacity to transform existing petrochemical clusters that uses naphtha/alkanes, together with natural gas as feedstocks, into a petrochemical cluster that is based on 100% recycled or renewable feedstocks. The paper also stated it had a carbon utilisation rate that is close to 100%.

What’s the crack?

As a process used in petrochemistry, steam cracking breaks down saturated carbons into smaller, often unsaturated, hydrocarbons. Principally, the method is used in industrial applications used from fossil feedstock. Inside the cracker, building blocks consisting of simple molecules are created, which can be combined in many different combinations resulting in a wide range of plastics that are used daily.

‘You take this hydrocarbon when it comes as a very clean feedstock and then you put it into a pipe or you mix it with some steam,’ said Thunman. ‘On the outside of the pipe you heat them up with burners to around 800°C. You then smash these molecules into new molecules.’

Thunman explained the step, which requires the separation of these molecules with the aim of producing three types of molecules – ethylene, benzene and propylene. The new process uses the same technique, applicable to all types of collected plastics from waste systems.

‘This is the starting point, and if you look at plastics, it is also hydrocarbon like a crude oil but processed in another way,’ Thunman explained. ‘By doing this process in the steam environment, you create a similar or identical molecule mix as you do from the fossil feedstock.’

The researchers believe that by further developing this process, it has the potential to enable a hugely significant transformation of petrochemical plants into recycling refineries of the future.

‘We are now moving on from the initial trials, which aimed to demonstrate the feasibility of the process, to focusing on developing more detailed understanding,’ said Thunman. ‘This knowledge is needed to scale up the process from a few tonnes of plastic a day, to hundreds of tonnes. That is when it becomes commercially interesting.’