Carbon materials: towards a circular economy through thermochemical recycling of mixed waste
Doctoral thesis, 2024

Carbon-containing materials, such as paper, wood, plastic, and textiles, are essential for our daily lives, being used in everything from clothing to infrastructure. However, their use typically follows a linear pattern, in that we extract carbon resources, create products, and eventually dispose of them, thereby contributing to greenhouse gas (GHG) emissions throughout the supply chain. This linear approach has limitations, especially in terms of the recycling of these materials, with only a small fraction being recycled, often producing a lower quality product. Thermochemical recycling, which breaks down materials into building blocks, is a promising solution to close the loop of carbon materials.

An alternative perspective is to focus on carbon recovery rather than just material recovery, which could significantly change our approach to carbon-containing waste. Analysing the current carbon material system, it is clear that we lose more carbon in the system than we produce, with potential GHG emissions of around 6%. In addition, there is sufficient carbon available from post-consumer waste to produce synthetic materials, potentially reducing emissions and reducing our reliance on fossil resources. However, recycling mixed waste, which contains a variety of materials and heteroatoms, presents various challenges.

The thermochemical conversion of five different mixed wastes was tested in a semi-industrial scale reactor, to determine the product distribution. The experimental results showed that the conversion yielded a mixture of gases and aromatic compounds, with a clear correlation between the olefinic polymer content in the feedstock and the production levels of C2–C3 aliphatic compounds at 730°C and 800°C. The study also examined the correlations between specific bond types and product distributions, finding positive links between COx and C2–C3 and certain C-O and aliphatic bonds, respectively. Aromatics content, while not linearly correlated with the percentage of aromatic bonds, remained consistent at around 20%C, regardless of the aromatics content, suggesting dependence on both aromatics content and the cyclisation of linear hydrocarbons.

Thermochemical recycling emerges as a viable method to recover carbon from mixed waste. However, challenges such as unidentified products and the fate of heteroatoms remain. Higher conversion temperatures can mitigate heteroatom levels but further research is needed to understand nitrogenated compound distributions. While thermochemical recycling holds potential for promoting circularity and emissions reduction, additional efforts are necessary to address challenges and establish it as a viable recycling method for mixed wastes.

Further research should focus on improving sampling and analysis methods for hydrocarbons containing heteroatoms. In addition, exploring the utilization of syngas, PAHs, and other fractions, along with addressing the impact of contaminants like ash on product quality, is crucial for advancing thermochemical recycling as a sustainable waste management solution.

plastic waste

circular economy

thermochemical recycling

mixed waste

recycling

Carbon materials

steam cracking

Lecture hall HC4
Opponent: Tobias Pröll, Professor, BOKU, Institute of Chemical and Energy Engineering (IVET)

Author

Isabel Cañete Vela

Chalmers, Space, Earth and Environment, Energy Technology

Ever wonder what happens to all the plastic that we use? Turns out, most of it doesn't get recycled. Instead, it ends up in landfills or gets burned, which isn't great for the environment. Why? Well, plastic waste is not only plastics. Waste contains everything that is essential for our daily lives – paper, wood, plastics and textiles – and while we try to separate and recycle them, this is not an easy task. Thus, we don't recycle much of these materials, and when we do, the end-product is not always of high quality.

A major challenge is how to recycle mixed waste, which contains all sorts of different materials in. But what if we could turn that waste into something useful? This is where thermochemical recycling comes in. Since plastics, paper, wood and textiles are mostly carbon and some hydrogen. If you heat these materials high enough (700-800°C), you get a few gases, made of carbon and hydrogen. Instead of just recycling the materials, we can break them down into smaller parts and use them again. It's like giving them a second life! The idea is to focus not just on getting the materials back, but on getting the carbon back.

There's hope! Thermochemical recycling could be a game-changer. Most of these gases are useful and can be used to produce plastics. However, it is not so simple, as the type of waste that you put in affects what comes out. In addition, you sometimes have impurities that complicate the process… Do you want to know more about this exciting area? If so, read this thesis!

Komplexa polymerrika materialströmmar och deras roll i en cirkulär materialförsörjning

VINNOVA (1.11VINNVÄXTJSPKPI), 2023-01-01 -- 2025-12-31.

Scandinavian Enviro Systems, 2023-01-01 -- 2025-12-31.

T-Hub

Region Västra Götaland (RUN2022-00058), 2022-03-30 -- 2022-09-30.

The foundation for Swedish textile research, 2022-03-30 -- 2022-09-30.

Steam reforming of plastics for a transformative conversion of petrochemical clusters

Borealis GmbH, 2020-01-01 -- 2024-12-31.

Swedish Energy Agency (49514-1), 2020-01-01 -- 2024-12-31.

Svenskt förgasningscentrum Etapp 3

Swedish Energy Agency (34721-3), 2017-04-20 -- 2021-12-31.

Återvinning av rejektströmmar från textilsortering och kartongåtervinning via termisk omvandling

The foundation for Swedish textile research, 2020-12-01 -- 2021-12-31.

Utvärdering av termokemisk återvinning av engångsprodukter frånsjukvården och utsorterade plastrika materialströmmar från hushållsavfall

VINNOVA (VINNVÄXTUtvtermokemåv), 2022-08-01 -- 2023-05-30.

Driving Forces

Sustainable development

Innovation and entrepreneurship

Areas of Advance

Energy

Materials Science

Subject Categories

Other Environmental Engineering

Infrastructure

Chalmers Power Central

ISBN

978-91-8103-023-5

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5481

Publisher

Chalmers

Lecture hall HC4

Online

Opponent: Tobias Pröll, Professor, BOKU, Institute of Chemical and Energy Engineering (IVET)

More information

Latest update

3/21/2024