Martin Høj KT

Introducing e-plastic

mandag 15 aug 22
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af Morten Andersen

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Martin Høj
Lektor
DTU Kemiteknik
45 25 28 42
Many polymer waste fractions are hard to recycle mechanically. For these fractions, a relevant recycling technique could be incineration followed by CO2 capture with the aim of manufacturing new polymer materials by reacting CO2 with hydrogen (H2). This is the focus of the project ‘CO2 to Monomers’ at the CHEC research centre, DTU Chemical Engineering.

“Using CO2 as a raw material for making new polymer building blocks is not in itself a new idea. Several industrial trials are already taking place. However, low economic costs will be crucial for the practical feasibility of the concept. Our ambition is to eliminate some of the steps of the current efforts. If we succeed in developing a onestep process, chances of minimizing costs will be good,” explains Associate Professor Martin Høj, Project Manager.

The project develops a process for direct conversion of CO2 and H2 into light olefins and/or aromatics as plastic monomers or monomer precursors. The CO2 could be obtained by carbon capture from incineration of polymer waste fractions, which are difficult or overly costly to recycle. However, the CO2 could also come from other sources, such as biomass gasification or biogas upgrading. The H2 could be obtained from water electrolysis, where the power for the electrolysis should be renewable energy such a wind or solar power, possibly combined with biomass gasification. When using electrolytic hydrogen, the concept issometimes referred to as e-plastic.

Products are high-value polymers
“The e-plastic concept is related to the concept of e-fuels and more broadly to Powerto-X. The idea of producing monomers fromCO2/CO/H2 (syngas) mixtures dates back to the 1980’ies but has only gained momentum within the last three years or so, as Power-to-X has become the centre of attention,” notes Martin Høj.

The idea is not to incinerate all polymer waste. “For certain fractions, direct recycling is possible. However, 100 per cent polymer recycling by purely mechanical techniques can never be achieved. It will only be possible to keep the recycling going by adding some amount of pristine polymer while also removing some amount of the degraded polymer. This is where chemical recycling becomes relevant,” comments Martin Høj.

E-plastic is a chemical recycling technique. The incineration results in CO2, which is then reacted with H2 to form methanol (CH3OH). The methanol can be further converted into olefins and/or aromatics as plastic monomers or precursors for monomers.

“Here we see a big advantage of chemical recycling. The end product will be a high-value polymer, or virgin polymer if you like. It will not be possible to distinguish between a polymer produced in this way and a polymer produced for instance from fossil raw material. Both can act as the high-value polymer necessary for mechanical recycling of other polymer fractions,” says Martin Høj.

Circumvents equilibrium limitation
“Admittedly, if an evaluation of the economic feasibility was to be carried out right now, it would point to e-plastic as very expensive. However, this would largely be because CO2 can presently be emitted into the atmosphere at very low cost. As we are seeing upcoming demands for CO2 capture and storage, the relative costs of making use of CO2 as raw material are due to be reduced.”

In current industrial trials, the processes will initially yield so-called syngas which is a mixture of CO2, CO, and H2. The syngas is then converted into methanol, which is later processed into monomers. In ‘CO2 to Monomers’, methanol will also be formed but will only exist shortly in the reactor as the methanol will be reacted directly into the desired end products. Taking this short-cut is enabled by use of a tailor-made catalyst system.

“Besides the improved economy of having a direct process, another advantage is that we circumvent the equilibrium limitation related to the methanol synthesis. Thereby, we can achieve a higher yield of the desired end products,” explains Martin Høj.

Still fundamental research
The goal of the three-year project is to achieve 80 per cent selectivity for production of olefins with 15 per cent conversion per catalytic cycle.

“We still have plenty of work to do. Producing our end products in a direct process involves several trade-offs. We need to manage the field of reactions and find a compromise which results in the best economic feasibility possible,” notes Martin Høj.

Several projects on chemical polymer recycling

The incineration concept of the ‘CO2 to Monomers’ project is not the only technique for chemical recycling of polymers. Another technique is pyrolysis. Here, plastic is cracked down to fractions of shorter carbon chains. The end product is a fluid which resembles petrol. The project ‘Bi-cycle’, headed by Professor Anker D. Jensen, investigates this technique. The two projects run in parallel. Both were initiated in autumn 2020, run for three years, and are financed by the Independent Research Fund Denmark, Technology and Production Sciences. A third CHEC project, titled ‘Reburn’, addresses the energy aspect of polymer waste incineration. When polymer waste is incinerated to produce CO2 for production of e-plastic, heat is also produced. This heat should obviously be exploited. Also, some issues regarding prophylactic measures to ensure a clean exhaust gas stream from this incineration needs to be addressed. The ‘Reburn’ project is coordinated by Associate Professor Hao Wu. The projects on polymer recycling in CHEC are coordinated with another centre at DTU Chemical Engineering, the Danish Polymer Centre (DPC).

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