By and | May 23, 2023

Whether on land or in the world’s oceans, plastic pollution is causing a global waste crisis. The problem is clear: Plastic production has grown exponentially, yet only a small fraction of plastic waste is ever recycled. This is associated with corresponding burdens on the environment. To address this issue, chemical recycling of plastics has emerged as a promising technology.  

Unlike mechanical recycling, which involves melting and reforming plastic waste, chemical recycling breaks down the plastic at a molecular level. Furthermore, chemical recycling can handle contaminated and mixed plastics, which are difficult to recycle by traditional methods. 

Sphera has joined the search for a solution. We regularly discuss the usefulness of the life cycle assessment (LCA) methodology, which evaluates the environmental impact of a product or process through its entire life cycle, from raw material extraction to disposal. In a whitepaper, we explain the need to harmonize approaches to life cycle assessment of chemical recycling technologies. And in an earlier blog, we explore the importance of closing the loop on chemical recycling. We have also conducted a webinar on the LCA of chemical recycling.

Life Cycle Assessment (LCA) of Chemical Recycling

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Among Sphera’s most recent contributions to the field is a master’s thesis on LCA, “Analysis of Critical Modeling Aspects and Practical Implementation of a Life Cycle Assessment of the Chemical Recycling of Plastics”. This looks at the chemical recycling of post-consumer, mixed plastic waste through pyrolysis—heating in the absence of oxygen to break it down into smaller molecules—and related thermal technologies. The chemical recycling system was compared to current conventional plastic waste processing. The thesis was supervised by international experts in the field of LCA, Dr.-Ing. Martin Baitz of Sphera and Prof. Dr.-Ing. Jan Paul Lindner of University of Augsburg, Germany.

Key findings of the thesis are highlighted below. We also provide insight into how this work will benefit the overall LCA of chemical recycling through integration in Sphera’s Managed LCA Content (former GaBi Databases).

Assessing Recycling Methods and Generating LCA Data

The thesis focuses on the environmental assessment of pyrolysis in Germany as part of the plastics value chain compared to conventional waste management methods and the virgin production of plastics from fossil resources. In addition, this thesis investigates the share of environmental burden on selected impact categories along the life cycle of chemical recycling. These impact categories include acidification, climate change, eutrophication, particulate matter, photochemical ozone formation, resource use and water use, among others.

The intended application of this thesis is twofold. First, it will generate datasets that can be included in Sphera’s Managed LCA Content (former GaBi Databases). Secondly, this investigation serves as a contribution to the discussion on the basic procedure for performing life cycle assessments for the chemical recycling of plastics. Overall, this thesis provides a wealth of data for stakeholders who deal with the topics of waste management, plastics production or the circular economy.

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Understanding Environmental Impacts and the Potential for Technical Optimization

The first step to understanding environmental impacts and the potential for technical optimization is to identify the hotspots in chemical recycling of plastics. Here, the relative environmental burden of the sub-processes of chemical recycling on individual impact categories is important. These processes involve steam crackers, which “crack” hydrocarbon feedstocks into building blocks including ethylene, propylene, butadiene, aromatics and acetylene. Polymerization serves to chemically combine molecules (monomers) to produce large network molecules called polymers. 

As illustrated by the following graph (Figure 1), pre-treatment has the lowest share across all impact categories (13.1% without weighting). The other sub-processes—pyrolysis (26.0%), steam cracker (30.4%) and polymerization (30.6%)—show higher values that are roughly similar.

Chemical Recycling: Significance of Sub-Processes in the Environmental Burden

Figure 1: Relative shares of the sub-processes to the respective impact categories

Due to the established use of steam cracker and polymerization processes, no significant optimization potential is expected here. However, environmental impacts will shift if renewable energy sources are used instead of fossil fuels. Consequently, a shift in the burdens can be expected, as the electrical and thermal energy input for the steam cracker and polymerization is decisive for the environmental impact of the overall process.

In addition, the pyrolysis process itself has significant environmental impacts compared to pre-treatment. A possible technical optimization of the chemical recycling process should therefore focus on this area.

Comparing the Performance of Recycling Systems

The next step is to compare the environmental impact of the recycling systems, with two end-of-life (EoL) allocation approaches for chemical recycling: End-of-Life Recycling and Substitution (Net Scrap). The following graph (Figure 2) shows the relative environmental burden to the individual impact category, scaled to 100% for the highest burden per impact category in each case.

Chemical recycling has two functions: It processes waste and produces a product. The virgin production of plastics from fossil resources is only one function. So, to have a fair comparison of the processes, conventional waste treatment was included to fulfill the second function.

Conventional Versus Chemical Plastics Recycling: What Are the Impacts?

Figure 2: Scaled contributions to the impact categories of the cradle-to-grave comparison system and the base system with the EoL allocation methods EoL Recycling and Net Scrap

The comparison shows that across the various impact categories, no system clearly performs better than the others. A general overall assessment depends on the significance and relevance attributed to individual impact categories. A few, fairly new impact categories were excluded from this figure, as they are less relevant for this analysis and would lead to the same findings. Details on these new impact categories can be found in the International Life Cycle Data (ILCD) or Environmental Footprint (EF) guides.

When applying the normalization and weighting factors based on EF 3.0, chemical recycling shows an advantage over the conventional system. The variation between chemical recycling systems with different EoL allocation methods is proportionately high. The Net Scrap method of chemical recycling has the lowest impact value.

Accommodating for Variations and Dependencies in the Analysis

Within the chemical recycling system evaluated, a sensitivity analysis shows that a change in processes can lead to corresponding changes in the results. The greatest sensitivities exist with regard to the material distribution at the end of life of plastic, as it can be landfilled, incinerated, mechanically recycled or chemically recycled. The share of material that goes to either mechanical or chemical recycling has a big impact on overall results. This predominantly affects the impact categories of freshwater ecotoxicity, human toxicity and resource use.

Chemical recycling is also sensitive to changes in pyrolysis parameters, particularly regarding their efficiency in terms of pyrolysis oil yield, which can be used to produce secondary plastic. Variations in the efficiency of the steam cracker also affect the resulting environmental burden across the various impact categories, primarily resource use, human toxicity and climate change.

The investigation showed that the environmental burden of any plastics recycling methodology depends on many factors, but some recommendations for the approach could be given. In a comparative analysis, the consideration of the entire life cycle rather than individual life cycle phases is highly relevant. It is advisable to include several impact categories, such as on the basis of the EF 3.0 recommendations, to ensure that all environmental impacts can be identified.

Recommendation for Further Research

For a more comprehensive comparison of chemical recycling, it is useful to investigate different options for pyrolysis and subsequent processing. The comparison could be extended to high-density polyethylene (HDPE) or polypropylene (PP) – in the best case with a realistic combination of the different polymer types.

Furthermore, the investigation of regional conditions seems relevant. In many European countries, a considerable proportion of plastic waste is landfilled at the end of its life cycle; in Germany, this happens only in exceptional cases. As a result, a significant shift of the burden can be expected, which justifies further analysis.

Once several pyrolysis plants are operating, a review of the inventory data should be conducted to increase the robustness of the results. At that time, it could also be investigated whether catalysts are used in industrial plants and thus whether catalytic cracking is occurring. This process offers the potential to increase efficiency and reduce energy requirements, which would have an impact on environmental performance.

It should be noted that generic datasets with an overall assessment can be created by using multiple data sources. In individual cases, results can vary based on the waste input, the technology at investigation or how one is dealing with by-products.

Integrating Knowledge and Expanding Databases

In summary, the consideration of the entire life cycle and not only individual life cycle phases is highly relevant in a comparative analysis. Attention should be paid to the material distribution at the end of life as well as the pyrolysis parameters.

The datasets and insights generated by the thesis are integrated in Sphera Managed LCA Content Databases. This represents a valuable extension of data for our clients, as there is a growing demand and requirement to incorporate chemically recycled materials into LCA models.

Building on the experience from many projects with chemical recycling technology developers and the chemical industry, Sphera supports the industry association Chemical Recycling Europe on developing guidelines for the LCA of chemical recycling.

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