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Exploring the potential environmental performance of pyrolytic chemical recycling for waste plastics

Martin Skelton (UGent)
(2024)
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(UGent) , (UGent) , (UGent) and (UGent)
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Abstract
The use and disposal of plastics have grown tremendously in the decades since they first became commercially available. This has been accompanied by a proliferation in new plastic materials and utilization in a wide variety of applications, including many short-lived products. Development of waste management methods for these materials has lagged, resulting in a range of environmental and resource management concerns around plastics use. At the same time, increasing societal attention to specific environmental concerns like global warming, and more generally to the need for efficient use of resources have brought increased scrutiny to the current production and waste management systems for plastics. At a broader level, the circular economy concept has been of growing importance, both within academia and among policy-makers, for the past few decades. This has prompted the formation of ambitious targets to close material loops and increase the circularity of production systems, including that of plastics through a range of strategies, one of which is recycling of waste material. While the recyclability of plastics has long been identified as one of their positive attributes, conventional recycling methods suffer from significant limitations in terms of the types of input material they can accept and the performance level of the secondary materials they produce. As a result, even with the existence of current recycling technology and infrastructure, the majority of plastic waste is still incinerated or land-filled around the world, including within Europe. Pyrolytic chemical recycling is a family of techniques under development in which waste plastics are broken down at elevated temperatures in an oxygen-free environment in order to produce a range of secondary products, including base and specialty chemicals, waxes, petrochemical feedstocks, and fuel substitutes. As these technologies are being researched and developed, it is important to look at how they might address the varied environmental and resource management concerns associated with plastics use. This thesis seeks to explore the environmental performance potential of these pyrolytic chemical recycling methods. Chapter 1 lays the groundwork for this exploration process, describing the fundamentals of plastics as a class of material, the current waste treatment situation for plastics around the world, and some of the environmental and resource management concerns associated with plastics use. Next, the overall landscape of recycling and energy recovery from waste plastics is described including pyrolytic chemical recycling and the varied potential product strategies found therein. The chapter continues by providing background on some of the current methods to assess the merit of recycling processes, including quality metrics for the secondary materials produced, exergy analysis as a means to measure process efficiency, life cycle assessment (LCA) as a means to measure environmental performance of a waste management or material production system, and the varied proposed metrics for measuring circular economy performance. The chapter concludes with a description of the overall structure of the thesis. In Chapter 2, the issue of quality of secondary materials, and more broadly quality of any material mixture, is explored with the extension of a prior method using statistical entropy as a quality proxy. In this work, a multi-level statistical entropy is proposed, using a categorization tree for material enumeration. With this approach, it is possible to visualize this quality proxy as a pseudo-time series through a product life cycle by looking at snapshots of material mixtures encountered through the life cycle. In addition, by assessing these values at the multiple levels found within the material categorization tree, the evolution of the shape of this pseudo-time series can be explored as one shifts from low to high specificity of the materials enumerated. Within the case study used in this chapter, a key finding was that the largest increase in statistical entropy (decrease in quality) occurs at the beginning of the end-of-life phase when plastic packaging items are mixed together with other recyclable materials for curb-side collection. Following a route through mechanical recycling steps, this high entropy state is systematically decreased until secondary plastic pellets are produced, showing a comparable but slightly higher entropy value than the primary plastic pellets found earlier in the life cycle. In contrast, when following a pyrolytic chemical recycling pathway, a second peak is encountered just after pyrolysis occurs. The measured quality of the finally-produced secondary naphtha is actually higher than the primary naphtha found earlier in the life cylcle, according to this metric. Chapter 3 uses exergy analysis as a way to explore the potential efficiency of individual process steps which might be found in a pyrolytic chemical recycling process chain. This modular process-level approach is necessary in the absence of fully developed and integrated process chains, which is generally the case for nascent pyrolytic chemical recycling technologies. Based on laboratory-scale experimental results, the exergetic efficiency of pyrolysis and membrane separation under a few different process conditions is calculated. Exergetic efficiencies of around 95% were found for the described pyrolysis conditions, with essentially no variability found between catalytic and non-catalytic process conditions. For membrane separation, in which no chemical transformation (only physical separation) of the input material occurs, very high exergetic efficiencies of greater than 99% were calculated. However for one of the cases, this efficiency drops to around 94% if the exergetic content of diluent is not included in the calculation. Some possible catalytic upgrade steps are described on the basis of laboratory-scale experiments and it is shown in general that chemical exergetic content of the pyrolyzate is preserved under processes like isomerization and oligomerization. Overall, this exercise serves as a back-of-the-envelope calculation to suggest that these particular process steps do not appear to be hot-spots for exergy loss, though this result should be treated with some caution since it relies on laboratory-scale experimental results in combination with simplified process modeling approaches. In Chapter 4, the relationship between pyrolytic chemical recycling and the circular economy is explored through the proposal and application of a multi-dimensional life cycle circularity indicator. The still ill-defined nature of the circular economy concept is discussed and three important dimensions are suggested for assessing the circular economy performance of a product or material life cycle: (1) the amount of material remaining within the economy; (2) the state (or quality) of that material when it enters a subsequent life cycle, and; (3) the environmental impact of the cycle. The inclusion of environmental impact as one important dimension, while perhaps considered unconventional by some, is intentionally chosen due to the predominance of an assumed link between circularity and greater sustainability of production systems. The proposed indicator is applied to the life cycle of a plastic packaging product under varying end-of-life scenarios, including open burning, incineration with energy recovery, chemical recycling, and mechanical recycling. The results showed that the overall indicator value was comparable between the life cycles containing mechanical and chemical recycling, while the open burning and incineration with energy recovery routes showed lower circularity performance. As one advantage of this approach, the three dimensions of the overall indicator can be plotted to easily visualize trade-offs which occur between dimensions. For example, the mechanical recycling route showed slightly higher mass efficiency and better environmental performance than the chemical recycling route, but could not manage to achieve the same quality level. While this serves as an interesting first demonstration of the composite indicator, the chapter also discusses how the operationalization of the three dimensions should be further refined. The environmental performance of a model pyrolytic chemical recycling system is explored in Chapter 5 through the use of LCA. More specifically, a comparative product-focused LCA was carried out, comparing the production of a basket of chemical and fuel products via catalytic pyrolysis of mixed polyolefin (MPO) waste with the conventional production mix for this same product basket. The catalytic pyrolysis case started from laboratory-scale experimental results in a micropyrolyzer using real post-consumer MPO waste. Process modeling was performed including a heat-integrated separation train to produce final products from the whole pyrolysis effluent which essentially match conventionally derived chemicals, fuels, and feedstocks. The full set of midpoint indicators from the ReCiPe impact assessment method were explored in order to identify in which impact categories the catalytic pyrolysis system performed better or worse compared to conventional production. From this first screening, contribution analysis was performed to understand which process steps in the catalytic pyrolysis process should be in focus for process developers. Interestingly, within those categories where the catalytic pyrolysis process performed worse, the major contributors were adjacent processes like incineration of residue materials, transportation of MPO waste, and land-filling of pyrolysis residues. The global warming impact of the catalytic pyrolysis process was around 60% of the total impact of conventionally producing the product basket. While these results should also be treated cautiously due to uncertainties in modeling and some lack of process information which required estimation or assumptions, the overall results suggest that a catalytic route directly toward high yield of propylene and ethylene might be a good strategy to pursue from an environmental performance perspective. Taken together, Chapters 2 through 5 touch on multiple aspects of the central aim of the thesis to explore the environmental performance of pyrolytic chemical recycling. Chapter 6 provides a synthesis of the overall results from these prior chapters, followed by a higher-level look at what these findings might mean for the development and possible future place of pyrolytic chemical recycling in the broader plastic waste management landscape. This includes a discussion through a few different lenses, including the industrial and economic context, theories of innovation and technology diffusion, and potential shifts in policy and attitudes. Additionally, some suggestions for future research into pyrolysis of plastics as well as the assessment methods used in this work are given.
Keywords
Chemical recycling, Plastic waste, Circular economy, Pyrolysis, Environmental sustainability

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MLA
Skelton, Martin. Exploring the Potential Environmental Performance of Pyrolytic Chemical Recycling for Waste Plastics. Ghent University. Faculty of Bioscience Engineering, 2024.
APA
Skelton, M. (2024). Exploring the potential environmental performance of pyrolytic chemical recycling for waste plastics. Ghent University. Faculty of Bioscience Engineering, Ghent, Belgium.
Chicago author-date
Skelton, Martin. 2024. “Exploring the Potential Environmental Performance of Pyrolytic Chemical Recycling for Waste Plastics.” Ghent, Belgium: Ghent University. Faculty of Bioscience Engineering.
Chicago author-date (all authors)
Skelton, Martin. 2024. “Exploring the Potential Environmental Performance of Pyrolytic Chemical Recycling for Waste Plastics.” Ghent, Belgium: Ghent University. Faculty of Bioscience Engineering.
Vancouver
1.
Skelton M. Exploring the potential environmental performance of pyrolytic chemical recycling for waste plastics. [Ghent, Belgium]: Ghent University. Faculty of Bioscience Engineering; 2024.
IEEE
[1]
M. Skelton, “Exploring the potential environmental performance of pyrolytic chemical recycling for waste plastics,” Ghent University. Faculty of Bioscience Engineering, Ghent, Belgium, 2024.
@phdthesis{01HZHW7TZASRK3XD87ZE24C0EM,
  abstract     = {{The use and disposal of plastics have grown tremendously in the decades since they first became commercially available. This has been accompanied by a proliferation in new plastic materials and utilization in a wide variety of applications, including many short-lived products. Development of waste management methods for these materials has lagged, resulting in a range of environmental and resource management concerns around plastics use. At the same time, increasing societal attention to specific environmental concerns like global warming, and more generally to the need for efficient use of resources have brought increased scrutiny to the current production and waste management systems for plastics. At a broader level, the circular economy concept has been of growing importance, both within academia and among policy-makers, for the past few decades. This has prompted the formation of ambitious targets to close material loops and increase the circularity of production systems, including that of plastics through a range of strategies, one of which is recycling of waste material.
While the recyclability of plastics has long been identified as one of their positive attributes, conventional recycling methods suffer from significant limitations in terms of the types of input material they can accept and the performance level of the secondary materials they produce. As a result, even with the existence of current recycling technology and infrastructure, the majority of plastic waste is still incinerated or land-filled around the world, including within Europe. Pyrolytic chemical recycling is a family of techniques under development in which waste plastics are broken down at elevated temperatures in an oxygen-free environment in order to produce a range of secondary products, including base and specialty chemicals, waxes, petrochemical feedstocks, and fuel substitutes. As these technologies are being researched and developed, it is important to look at how they might address the varied environmental and resource management concerns associated with plastics use. This thesis seeks to explore the environmental performance potential of these pyrolytic chemical recycling methods. 
Chapter 1 lays the groundwork for this exploration process, describing the fundamentals of plastics as a class of material, the current waste treatment situation for plastics around the world, and some of the environmental and resource management concerns associated with plastics use. Next, the overall landscape of recycling and energy recovery from waste plastics is described including pyrolytic chemical recycling and the varied potential product strategies found therein. The chapter continues by providing background on some of the current methods to assess the merit of recycling processes, including quality metrics for the secondary materials produced, exergy analysis as a means to measure process efficiency, life cycle assessment (LCA) as a means to measure environmental performance of a waste management or material production system, and the varied proposed metrics for measuring circular economy performance. The chapter concludes with a description of the overall structure of the thesis.
In Chapter 2, the issue of quality of secondary materials, and more broadly quality of any material mixture, is explored with the extension of a prior method using statistical entropy as a quality proxy. In this work, a multi-level statistical entropy is proposed, using a categorization tree for material enumeration. With this approach, it is possible to visualize this quality proxy as a pseudo-time series through a product life cycle by looking at snapshots of material mixtures encountered through the life cycle. In addition, by assessing these values at the multiple levels found within the material categorization tree, the evolution of the shape of this pseudo-time series can be explored as one shifts from low to high specificity of the materials enumerated. Within the case study used in this chapter, a key finding was that the largest increase in statistical entropy (decrease in quality) occurs at the beginning of the end-of-life phase when plastic packaging items are mixed together with other recyclable materials for curb-side collection. Following a route through mechanical recycling steps, this high entropy state is systematically decreased until secondary plastic pellets are produced, showing a comparable but slightly higher entropy value than the primary plastic pellets found earlier in the life cycle. In contrast, when following a pyrolytic chemical recycling pathway, a second peak  is encountered just after pyrolysis occurs. The measured quality of the finally-produced secondary naphtha is actually higher than the primary naphtha found earlier in the life cylcle, according to this metric.
Chapter 3 uses exergy analysis as a way to explore the potential efficiency of individual process steps which might be found in a pyrolytic chemical recycling process chain. This modular process-level approach is necessary in the absence of fully developed and integrated process chains, which is generally the case for nascent pyrolytic chemical recycling technologies. Based on laboratory-scale experimental results, the exergetic efficiency of pyrolysis and membrane separation under a few different process conditions is calculated. Exergetic efficiencies of around 95% were found for the described pyrolysis conditions, with essentially no variability found between catalytic and non-catalytic process conditions. For membrane separation, in which no chemical transformation (only physical separation) of the input material occurs, very high exergetic efficiencies of greater than 99% were calculated. However for one of the cases, this efficiency drops to around 94% if the exergetic content of diluent is not included in the calculation. Some possible catalytic upgrade steps are described on the basis of laboratory-scale experiments and it is shown in general that chemical exergetic content of the pyrolyzate is preserved under processes like isomerization and oligomerization. Overall, this exercise serves as a back-of-the-envelope calculation to suggest that these particular process steps do not appear to be hot-spots for exergy loss, though this result should be treated with some caution since it relies on laboratory-scale experimental results in combination with simplified process modeling approaches.
In Chapter 4, the relationship between pyrolytic chemical recycling and the circular economy is explored through the proposal and application of a multi-dimensional life cycle circularity indicator.  The still ill-defined nature of the circular economy concept is discussed and three important dimensions are suggested for assessing the circular economy performance of a product or material life cycle: (1) the amount of material remaining within the economy; (2) the state (or quality) of that material when it enters a subsequent life cycle, and; (3) the environmental impact of the cycle. The inclusion of environmental impact as one important dimension, while perhaps considered unconventional by some, is intentionally chosen due to the predominance of an assumed link between circularity and greater sustainability of production systems. The proposed indicator is applied to the life cycle of a plastic packaging product under varying end-of-life scenarios, including open burning, incineration with energy recovery, chemical recycling, and mechanical recycling. The results showed that the overall indicator value was comparable between the life cycles containing mechanical and chemical recycling, while the open burning and incineration with energy recovery routes showed lower circularity performance. As one advantage of this approach, the three dimensions of the overall indicator can be plotted to easily visualize trade-offs which occur between dimensions. For example, the mechanical recycling route showed slightly higher mass efficiency and better environmental performance than the chemical recycling route, but could not manage to achieve the same quality level. While this serves as an interesting first demonstration of the composite indicator, the chapter also discusses how the operationalization of the three dimensions should be further refined.
The environmental performance of a model pyrolytic chemical recycling system is explored in Chapter 5 through the use of LCA. More specifically, a comparative product-focused LCA was carried out, comparing the production of a basket of chemical and fuel products via catalytic pyrolysis of mixed polyolefin (MPO) waste with the conventional production mix for this same product basket. The catalytic pyrolysis case started from laboratory-scale experimental results in a micropyrolyzer using real post-consumer MPO waste. Process modeling was performed including a heat-integrated separation train to produce final products from the whole pyrolysis effluent which essentially match conventionally derived chemicals, fuels, and feedstocks. The full set of midpoint indicators from the ReCiPe impact assessment method were explored in order to identify in which impact categories the catalytic pyrolysis system performed better or worse compared to conventional production. From this first screening, contribution analysis was performed to understand which process steps in the catalytic pyrolysis process should be in focus for process developers. Interestingly, within those categories where the catalytic pyrolysis process performed worse, the major contributors were adjacent processes like incineration of residue materials, transportation of MPO waste, and land-filling of pyrolysis residues. The global warming impact of the catalytic pyrolysis process was around 60% of the total impact of conventionally producing the product basket. While these results should also be treated cautiously due to uncertainties in modeling and some lack of process information which required estimation or assumptions, the overall results suggest that a catalytic route directly toward high yield of propylene and ethylene might be a good strategy to pursue from an environmental performance perspective.
Taken together, Chapters 2 through 5 touch on multiple aspects of the central aim of the thesis to explore the environmental performance of pyrolytic chemical recycling. Chapter 6 provides a synthesis of the overall results from these prior chapters, followed by a higher-level look at what these findings might mean for the development and possible future place of pyrolytic chemical recycling in the broader plastic waste management landscape. This includes a discussion through a few different lenses, including the industrial and economic context, theories of innovation and technology diffusion, and potential shifts in policy and attitudes. Additionally, some suggestions for future research into pyrolysis of plastics as well as the assessment methods used in this work are given.}},
  author       = {{Skelton, Martin}},
  isbn         = {{9789463577427}},
  keywords     = {{Chemical recycling,Plastic waste,Circular economy,Pyrolysis,Environmental sustainability}},
  language     = {{eng}},
  pages        = {{XXV, 186}},
  publisher    = {{Ghent University. Faculty of Bioscience Engineering}},
  school       = {{Ghent University}},
  title        = {{Exploring the potential environmental performance of pyrolytic chemical recycling for waste plastics}},
  year         = {{2024}},
}