Carbon materials: towards a circular economy through thermochemical recycling of mixed waste

Licentiate thesis, 2022

Carbon materials, such as paper, wood, plastic and textiles, play an important role in our everyday life, from clothes and packaging to infrastructure. However, the use of those materials follows a linear way. We take carbon resources, we make products, and we discharge them in a short amount of time, producing GHG emissions along its supply chain. From its extraction, manufacture and, unlike other materials, also at its end of life, releasing its embedded carbon into the ecosphere.
One approach to reduce emissions and resource extraction is to move towards a circular economy, by recirculating waste to produce new materials. However, today's material recycling process fall short, only a small fraction is recycled and often to a lower quality. As an alternative, this work shows that emphasizing carbon recovery, instead of material recovery, changes the perspective on carbon-containing waste flows.
Consequently, material flow analysis of the current carbon material system was set, illustrating that the system losses are greater than the carbon material produced. If those carbon losses are assumed to be released as CO2, they will equal 6% of the current GHG. The flow analysis also showed that there is enough carbon in the waste for producing synthetic materials and that carbon can help to reduce the emissions and decouple from fossil extraction.
This analysis also displayed that the carbon available in post-consumer waste consists of a mix of synthetic and natural carbon materials, together with heteroatoms such as oxygen, nitrogen, and chlorine. A potential way to recover all carbon is thermochemical recycling, which can break down materials into building blocks, similar to the chemicals employed in the petrochemical industry. As mixed waste comprises a wide variety of materials, the thermal conversion poses a variety of challenges ranging from the unknown product distribution to the fate of heteroatoms.
The thermochemical conversion of three different mixed wastes was tested in a pilot-scale reactor to understand the product distribution. The experimental results showed that the conversion yielded a mixture of gases and aromatics compounds, together with a high share of unconverted. While some of these products can be used directly, such as olefins and benzene, others require further recovery and processing. Another finding is that a higher conversion temperature helps to limit heteroatoms in the hydrocarbons. Increasing the temperature to 800°C reduced the Chlorine-content in aromatics under ppm levels, but Oxygen and Nitrogen content are higher than ppm level and that may affect the carbon recovery and may require further separation steps.
While thermochemical recycling has the potential to go towards a circular economy and reduce emissions, further efforts are required to tackle the different challenges to make thermochemical conversion a viable recycling method for mixed wastes.