Sustainable circular economy: turns mixed waste into premium plastics with no climate impact

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Only a fraction of the material that could be turned into new plastic is currently recycled. Researchers at Chalmers have now demonstrated how the carbon atoms in mixed waste can replace all fossil raw materials in the production of new plastic. The recycling method is inspired by the natural carbon cycle and could eliminate the climate impact of plastic materials, or even clean the air of carbon dioxide.

“There are enough carbon atoms in waste to meet the needs of all global plastic production. Using these atoms, we can decouple new plastic products from the supply of virgin fossil raw materials. If the process is powered by renewable energy, we also get plastic products with more than 95% lower climate impact than those produced today, which effectively means negative emissions for the entire system,” says Henrik Thunman, Professor of Energy Technology at Chalmers University of Technology and one of the authors of the study published in the Journal of Cleaner Production.

To achieve circular cycles, we need to make better use of the resources already in use in society. Henrik Thunman and his research team want to focus on an important resource that often goes up in smoke today: the carbon atoms in our waste, which are currently incinerated or end up in landfills instead of being recycled.

This is made possible with technologies targeting the carbon contained in plastic, paper and wood wastes, with or without food residues, to create a raw material for the production of plastics with the same variety and quality as those currently produced from fossil raw materials.

reference link: https://doi.org/10.1016/j.jclepro.2022.132674

Carbon-based fuels (mainly fossil fuels but also biomass) account for about 90% of the current global primary energy supply. Carbon is also a building block in a wide range of materials (carbon-based materials; C-materials) used in society. While fossil fuel use is the main cause of anthropogenic greenhouse gas (GHG) emissions, and a transition away from the use of such fuels is essential to limit the global temperature increase to 1.5 °C (IPCC et al., 2018), the production and use of materials such as plastics, cement and steel entail significant GHG emissions (IEA, 2018; Jambeck et al., 2015).

The use of biomass-based products can effectively reduce the use of fossil fuels and GHG-intensive materials. There is also scope for substituting existing biobased products with more benign products. For example, cellulose-based textiles can replace cotton which is associated with soil and water depletion, as well as harmful impacts on human health and biodiversity due to excessive use of pesticides and fertilizers (IPCC et al., 2019c; Niinimäki et al., 2020).

However, the biomass supply potential is limited by resource constraints (Gerten, 2018) and implications of expanded biomass use for mitigation and other objectives depend on many factors, including soil and climate conditions, biomass type, land management system, scale and pace of deployment, and influence on land use (Calvin et al., 2021). For instance, cropland expansion for energy crop production may cause deforestation, with consequent GHG emissions and negative impacts on biodiversity (IPCC et al., 2019b).

C-materials also entail GHG emissions and environmental impacts at the end-of-life. Landfilling is the most common end-of-life management strategy, and in countries that avoid landfilling the waste is instead often incinerated (Kaza et al., 2018). Much of the material becomes litter, polluting the environment (Kaza et al., 2018). While only a fraction of C-material waste is recycled into new products, not all waste can be used today to produce new C-materials.

Furthermore, C-materials manufactured from waste often are of lower quality than the original product. For instance, recycled paper pulp is commonly used for newsprint and packaging, which require lower paper grades. Repeated recycling degrades the fibre quality, eventually making the fibres unsuitable for material purposes (Ormondroyd et al., 2016; Van Ewijk, Stegemann and Ekins, 2018), as depicted in Fig. 1. Mechanical recycling of plastics is similarly associated with downgrading (Hahladakis and Iacovidou, 2018), i.e., use in lower-quality products. Thus, today’s recycling systems do not achieve full circularity, but merely slow down a linear resource flow that is characterized by losses and quality degradation during reprocessing.

Fig. 1. Current downgrading schemes and losses associated with recycling of C-materials (left-hand side) and proposed use of advanced thermochemical recycling to produce high-value C-materials (right-hand side).

Circular economy (CE) approaches are commonly depicted as two cycles, where the biological cycle focuses on regeneration in the biosphere and the technical cycle focuses on reuse, refurbishment and recycling to maintain value and maximize material recovery (MacArthur, 2019; Mayer et al., 2019). Biogenic carbon flows and resources are part of the biological cycle (Carus and Dammer, 2018; Velenturf et al., 2019). However, C-materials are technical products that can be included in and affect, both the biological and the technical carbon cycles (Kirchherr et al., 2017; Winans et al., 2017). The integration of CE and bio-economy concepts has been discussed in the context of waste management (Teigiserova et al., 2020) and current policy development (Carus and Dammer, 2018; EC, 2018), as well as strategies for reaching targets set in the 2030 Agenda for Sustainable Development and the Paris Agreement on climate change (EFI et al., 2017; 2020). A circular bio-economy emphasizes the use of renewable energy sources and sustainable management of ecosystems, setting limits on biomass usage in society.

Biomass scarcity is an argument for adopting CE principles for the management of biomass that are similar to those for non-renewable resources, i.e., minimize virgin resource use through reuse, recycling and waste avoidance, which also helps to reduce the negative impacts caused by losses, e.g., littering of plastic waste. However, it needs to be considered that reuse and recycling are not always feasible, e.g., when biofuels are used for transport and biobased biodegradable chemicals are used to reduce ecological impacts. Thus, a balanced framework could take its departure in the carbon cycle from a value-preservation perspective and the possible routes for carbon, considering a carbon budget defined by the Paris Agreement and principles for ecosystem protection.

Conventional recycling is commonly investigated and applied separating biomass-based materials (e.g., wood, paper) from fossil-based materials (e.g., plastics, synthetic fibers). In contrast, this work aims to investigate the global potential of a co-recycling system where carbon from the biomass-based and fossil-based waste streams are used as feedstock to produce new synthetic C-materials. Pyrolysis, gasification and combustion with carbon dioxide (CO2) capture and utilization (CCU) are advanced thermochemical recycling processes (ATCR) that allow the recovery and utilization of carbon, regardless of its origin.

These thermochemical recycling routes and combinations of them are key enablers for a co-recycling system for converting discarded C-materials, residues, and processing losses into synthetic products of high quality, as depicted in Fig. 1. Thunman et al. (2019) show that by using these three complementary ATCR routes in an existing chemical cluster, it is possible to achieve 100% carbon recovery using today’s available technologies. In addition, several authors showed the need and potential of using CCU for a circular carbon economy (Saygin and Gielen, 2021; Meys et al. 2021; Kähler et al., 2021). In all, thermochemical routes can create new opportunities for increasing the recovery of C-materials, promoting circularity, and avoiding carbon losses such as CO2 emissions and waste accumulation in landfills and nature.

In this work, the production and management of carbon materials (C-materials) are conceptualized and modelled, whereby synthetic and natural C-materials are co-recycled into high-quality synthetic C-materials, such as plastics and synthetic fibers. The global carbon flows in a co-recycling scheme are described, including ACTR, which provides a novel approach to recycling with a focus on carbon reuse rather than on traditional material recycling.

A co-recycling scenario is used to envision how the introduction of these ATCR technologies and changes in carbon flow management facilitate reduced waste generation and decoupling of the Carbon Material System (CMS) from the extraction of fossil resources. Prospective reductions in resource extraction and emissions within the CMS are estimated. In addition, the energy requirement for ATCR is estimated and compared to today’s system, including the energy-related emissions given different energy sources.

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