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.