The circular bioeconomy: Its elements and role in European bioeconomy clusters

Biomass is projected to play a key role in meeting global climate targets. To achieve a resource-e ﬃ cient biomass use, European bioeconomy strategies increasingly consider the concept of a circular bioeconomy (CBE). We de ﬁ ne the term CBE via a literature review and analyze the concept ’ s role in north-west European bioeconomy clusters through interviews. We identify strategies regarding the clusters ’ feedstock and product focus, and investigate what role biore ﬁ neries, circular solutions, recycling and cascading play. Finally, we discuss gaps in CBE literature and the potential contributions of the CBE to sustainability. The analyzed bioeconomy clusters move towards a CBE by increasingly considering residues and wastes as a resource, developing integrated biore ﬁ neries and focusing more on material and high value applications of biomass. However, there is so far only little focus on the end-of-life of bio-based products, i.e. on circular product design, recycling and cascading. Key challenges for implementing circular strategies are policies and regulations, costs and the current small size of bio-based markets. Amongst the product sectors the interviewees identi ﬁ ed as promising for the bioeconomy, plastics and construction & building materials have most recycling and cascading potential. While the CBE could contribute to improving the sustainability of the bioeconomy, the concept is not inherently sustainable and its potential trade-o ﬀ s need to be addressed. Especially social aspects, cascading, circular product design, and aspects related to product use seem to be underrepresented in CBE literature, while the topics biore ﬁ nery, wastes and residues as well as waste management are signi ﬁ cantly covered.


Introduction
Biomass is projected to play an important role in meeting the global climate targets set in the Paris agreement (Creutzig et al., 2015;Daioglou et al., 2019;Rogelj et al., 2018;Rose et al., 2014). For the chemical industry, heavy road transport and marine and aviation sectors, biomass is one of the few options to replace their fossil feedstock with a renewable resource, thereby reducing the sectors' Greenhousegas (GHG) emissions (Bazzanella and Ausfelder, 2017;Carus and Raschka, 2018; International Energy Agency (IEA), 2018; S. de Jong et al., 2018;Mawhood et al., 2016). Hence, the concept of a bioeconomy (BE) has been put forward by the European Union (European Commission, 2012) and by almost 50 countries around the globe (Fund et al., 2018). A bioeconomy can be defined as the "production of renewable biological resources and the conversion of these resources and waste streams into value added products, such as food, feed, bio-based products and bioenergy" (European Commission, 2012).
A strong optimism has been observed regarding the benefits of the bioeconomy (McCormick and Kautto, 2013); some bioeconomy strategies and scientific publications consider the bioeconomy to be inherently sustainable (Hetemäki et al., 2017;Pfau et al., 2014). However, there are also various publications highlighting potential tradeoffs and negative impacts (McCormick and Kautto, 2013;Pfau et al., 2014): They expect an increased pressure on water bodies and natural ecosystems and question the emission reduction potential. Some key issues in this debate evolve around the competition for land, i.e.direct and indirect land-use change, agricultural intensification, eutrophication and risks posed by invasive species (Pfau et al., 2014). While some authors consider the bioeconomy to be "circular by nature" (Carrez and Van Leeuwen, 2015;Sheridan, 2016), Hetemäki et al. (2017) see the risk of following a linear business-as-usual approach if the principles of a circular economy (CE) are not considered. The CE is defined by the European Commission (2015) as minimizing the generation of waste and maintaining the value of products, materials and resources for as long as possible.
As a response to these critical discussions, the updated bioeconomy strategy of the European Commission (2018a) announces that the "European Bioeconomy needs to have sustainability and circularity at its heart" (European Commission, 2018a). Since the publication of the EU action plan for the CE (European Commission, 2015), "practically all of the European bioeconomy(-related) strategies" have increasingly been linked to the CE (Fund et al., 2018).
Merging these two concepts has led to the term 'circular bioeconomy' (CBE), which appeared around 2015 and is increasingly used in scientific publications since 2016 (see section 2.1). However, there have been only few attempts to define the term and to describe what the CBE concept actually entails (see sections 2 and 3.1). Furthermore, a bottom-up perspective on the role of the CBE in regional bioeconomy clusters is missing. This perspective is particularly relevant since many key strategies towards a more resource-efficient and circular bioeconomy, e.g. integrated biorefineries and cascading use of biomass which are both defined in chapter 3.1, depend on a close cooperation of local actors from agriculture, industry, research and regional public institutions, e.g. within bioeconomy clusters 1 . The important role of regional clusters in driving the European bioeconomy is increasingly recognized (Haarich, 2017;Kircher, 2012).
With this paper we aim to contribute to a better understanding of the CBE concept and furthermore investigate to what extent it already plays a role in the strategies of selected regional bioeconomy clusters.
To this end, the paper addresses the following research questions (RQ): 1 What are the defining elements of a CBE? 2 What are the strategies and foci in selected bioeconomy clusters regarding (i) feedstocks; (ii) products; (iii) biorefineries; (iv) circular thinking, cascading and recycling? 3 Do the clusters implement the CBE elements defined in RQ1 in their strategies?
Moreover, we (4) discuss the potential contributions of the CBE to sustainability and (5) identify gaps in literature and cluster strategies that deserve more focus when moving towards a CBE.
The focus of this study is North-West Europe, namely the Netherlands, Belgium, the United Kingdom, Germany and France. These countries represented 51 % of the turnover of the EU bioeconomy in 2015 (European Commission, 2018c). Furthermore, north-west Europe is traditionally a major hub for the European petrochemical industry, a sector that heavily relies on biomass for decarbonization; these five countries alone are still responsible for almost 66 % of the EU chemical sales in 2018 (CEFIC, 2018). This makes this region a relevant case study to explore RQ 2 and 3.
After describing this paper's materials and methods, we define the CBE concept and its elements, based on a literature review (RQ1). The second part of the results presents strategies in the selected bioeconomy clusters (RQ2) based on interviews with their representatives. Finally, we discuss (a) if the clusters implement the CBE elements defined in RQ1 and (b) the potential contributions of the CBE to sustainability.

Materials & methods
This research consists of two work-streams (see Fig. 1): Work-stream A is a literature review of publications on the CBE concept to answer RQ1, and work-stream B covers interviews with bioeconomy cluster-representatives and their subsequent analysis to answer RQ2. In the discussion (C) both work streams are linked to answer RQ3.

CBE literature review
We searched for the term CBE and its synonyms (Boolean string: "circular bioeconomy" OR "circular bio-based economy" OR "circular bio-economy") within titles, abstracts and keywords on Elsevier's scientific search engine Scopus and found 84 peer-reviewed publications in English dating from 2016 to 2019.

Qualitative analysis
The titles, abstracts and -where necessary -the full texts of these 84 documents were screened to identify those publications that do not only mention the term CBE but also define or explain the concept. This limited the results to five publications. Analyzing the bibliography of these five publications, e.g. identifying frequently mentioned authors, led to the inclusion of four additional documents, of which three are not peer-reviewed. We analyzed these nine documents (Appendix A) in detail to identify key elements of the CBE.

Analysis of keywords
In a second step we used the software VOSviewer to compare the author and index keywords of the initial 84 CBE documents to 1275 bioeconomy (BE) publications found on Scopus for the same period 2016-2019; Boolean string: ("bioeconomy" OR "bio-based economy" OR "bio-economy") AND NOT ("circular bioeconomy" OR "circular biobased economy" OR "circular bio-economy"). The goal was to observe differences in the occurrences of keywords, in particular on topics identified as important CBE elements in the qualitative analysis of the nine documents (see Appendix A). We clustered similar keywords and treated them as synonyms (see Appendix B). To compare the relative importance of a keyword in CBE and BE documents, the number of keyword occurrences in CBE and BE documents was divided by the total number of CBE and BE publications respectively.
Eventually, we used the CBE elements identified in this literature review to derive a CBE definition (RQ1).

Limitations
Comparing the share of keywords of the only 84 available CBE documents to keywords of the 1275 BE documents might lead to an exaggeration of some findings; the results may show high differences in relative terms, while being derived from a small number of publications. Moreover, when clustering similar keywords, a few subjective choices have been made. By attaching the list of the clustered keywords in Appendix B, these choices are made transparent.
For the detailed qualitative analysis, only nine documents were identified and selected. This sample is not very diverse as four publications are closely linked to the forest-based bioeconomy (Dalia D'Amato et al., 2018;Falcone et al., 2020;Hetemäki et al., 2017;Temmes and Peck, 2019).

Interviews with representatives of bioeconomy clusters
We conducted seven semi-structured interviews with representatives of seven north-west European bioeconomy clusters (see Fig. 2). The interviews were complemented by an analysis of overview documents and web pages of the clusters, which provided additional insights into the strategies and focus areas of the clusters (see Appendix C).

Selection of clusters and interviewees
The clusters were selected in consultation with the project stakeholders (see acknowledgments). Following the relatively open definition of clusters by Su and Hung (2009) (see footnote 1), clusters of different size and degree of organization were considered. According to 1 While clusters greatly vary in their size and degree of organization, they can be generally defined as a "geographically proximate group of interconnected companies and associated institutions in a particular field, including product manufacturers, service providers, suppliers, universities, and trade associations" (Su and Hung, 2009). the project focus (see introduction), only clusters from north-west Europe covering a wider range of bioeconomy sectors and involving both public and private actors were selected. Another important factor for consideration was international visibility and impact, signaled by their involvement in international cooperation and projects: Four of the seven selected clusters closely collaborate within the 3BI intercluster 2 and three within the BIG-C, the Bio Innovation Growth mega Cluster 3 , highlighting their role as key actors of the bioeconomy in north-west Europe. The bioeconomy region Northern Netherlands was added following recommendations of the project stakeholders.
For each of the clusters we interviewed a key knowledge holder with a good overview of the activities in the respective cluster and region: Managing directors (4) and in three cases other cluster representatives that were recommended by project partners or interviewees. The interviews were conducted between February and April 2018 (2 in person, 2 via Skype, 3 via phone).

Interview approach
The interview approach applied in this paper can be classified as a semi-structured interview according to Bryman (2012), allowing a change of sequence in asking the prepared questions and allowing for more general, open questions as well as for further questions not defined in the interview schedule. This relatively flexible approach can help identifying new priorities and foci of the interviewees that were not yet considered by the interviewer (Bryman, 2012) and was therefore considered suitable for the explorative nature of this study. However, there were also pre-coded questions, meaning that a range of answers was provided. But also in this case, the schedule allowed for   adding additional options not foreseen by the interviewer. The interviews focused on strategies and activities in the clusters regarding the following topics: 1 Feedstocks Based on the interviews and supporting documents the different types of primary biomass feedstocks used in the clusters (structured in lignocellulosic crops, starch crops, sugar crops, oil crops and algae) were classified in a qualitative manner according to their importance within the cluster as main, complementary and prospective feedstock. The same method was applied for assessing the role of agricultural and forestry residues, wastes (from industry and from consumers) and CO2 as a resource. The results were then sent to the interview partners for correction and confirmation. Only the feedstocks that were at least named twice were considered in the results.

Product sectors
The interviewees were asked about the focus of their cluster's research and innovation programs regarding the uses of biomass, namely energetic (e.g. for biogas and biofuels) and material uses. Material biomass use is referring to all non-energetic uses of biomass, ranging from applications in the chemical industry to food & feed as well as for construction materials. Furthermore, the interviewees were asked to name bioeconomy product sectors with a high or low growth potential in the future (up to 2030).

Biorefineries
The questions aimed at identifying the type of biorefineries (according to Cherubini et al., 2009) the clusters are working on and at what stage these projects are (research, pilot and demonstration, commercial).

Cascading, recycling and other circular strategies
After asking the interviewees for their understanding of cascading, the further questions aimed at identifying cluster activities regarding the end of life of bio-based products, e.g. strategies or projects on cascading, recycling and circular product design.
The interviews were transcribed and the relevant answers coded according to the interview foci (as described in e.g. Gorden, 1992). Together with the complementing documents providing an overview of the cluster's strategies, the coded answers were analyzed and compared to identify common trends, key differences and the underlying reasons for the cluster's choices.

Limitations
The small sample size of bioeconomy clusters does not allow for general conclusions on the status of the CBE in north-west Europe. Furthermore, the clusters are not comparable in size, degree of organization, feedstock or product focus. Nonetheless, the selected seven clusters provide interesting case studies that give valuable insights into the strategies and challenges of some of Europe's leading bioeconomy clusters. Due to the diversity of the sample, the analysis covers a wide range of bioeconomy developments in north-west Europe, by representing different local circumstances and focus areas. However, it still excludes certain bioeconomy regions, in particular the forest-rich Scandinavian countries. Moreover, the clusters are from countries that have comparably strong CE-policies and well-established waste management sectors in place. Therefore, conclusions from this study are region-specific and might not apply to regions with a different feedstock and industry focus or a less developed CE. Finally, basing the cluster assessment solely on interviews and strategy/overview documents just allows for indicative conclusions on the foci and trends in clusters, as not the whole range of projects and reports of each cluster was analyzed.

Analyzing the cluster activities and strategies in the context of the CBE
The CBE elements identified in the literature review (RQ1, Appendix A) then served as a framework for analyzing the strategies of the bioeconomy clusters (RQ2) to answer RQ3: Do the clusters implement the CBE elements in their strategies. Combining both research streams in the discussion helps us to see in how far the still largely academic concept of a CBE is already a reality in practice and allows us to identify potential shortcomings in literature and industry that deserve more focus when moving towards a CBE.

Defining the CBE and its elements
In this section we identify and define key elements of the CBE based on a literature review (Appendix A) and an analysis of keywords used in scientific publications (Appendix D). From this analysis we eventually derive a definition of the term CBE and discuss its implication on biomass use.

CBE elements in literature
We identified three overarching perspectives on the CBE in relation to the bioeconomy and CE (see Fig. 3): Before the term CBE appeared, the Ellen MacArthur Foundation (2013) implied that the bioeconomy is an integral part of the CE by including the biological cycle into their CE illustration. Similarly, Temmes and Peck (2019) see the CBE as a CE where "non-renewable […] inputs to industrial systems are replaced by renewable biological resources". The European Commission (2017a) defined the CBE as the application of the CE concept to biological resources, products and materials. We analyzed nine publications Fig. 3. Perspectives on the circular bioeconomy (CBE) in relation bioeconomy (BE) and circular economy (CE). P. Stegmann, et al. Resources, Conservation & Recycling: X 6 (2020) 100029 explaining the CBE concept. Four of them define the CBE as the intersection of BE and CE (Carus and Dammer, 2018;Falcone et al., 2020;Philp and Winickoff, 2018;Venkata Mohan et al., 2019) while Hetemäki et al. (2017) and Dalia D'Amato et al. (2018) argue for a more comprehensive vision and see the CBE as "more than bioeconomy or circular economy alone". While the perspectives on the term differ, the analysis of the CBE publications showed that they often refer to similar CBE elements: All of the nine analyzed publications highlight the use of wastes and residues as a resource. Furthermore, keywords relating to wastes & residues are used 3.5 times more in CBE compared to BE documents (see Appendix D).
The efficient use of biomass is considered part of the CBE by all nine authors, even though their definitions of efficiency vary or are not given. As Ekins et al. (2017), we argue for a definition of resourceefficiency that considers technical efficiency (material output/material input), resource productivity (economic output/material input) and emission intensity (emission output/material input). Considering these three interpretations of resource-efficiency allows for a more balanced approach; maximizing for only one might negatively influence the others, e.g. maximizing the technical efficiency might be very energy intensive and thus increase emissions and costs. Resource-efficiency as a keyword does only play an insignificant role (see Appendix D); although keywords referring to efficiency strategies are frequent (see the following paragraphs).
(Integrated) biorefineries are considered an important part of the CBE by seven of nine CBE publications and are seen as a measure to improve the resource-efficiency and total value of the biomass (Hetemäki et al., 2017;Temmes and Peck, 2019;Zabaniotou, 2018). Moreover, keywords relating to biorefinery are used almost three times more in CBE documents compared to BE ones. E. De Jong et al. (2012) define a biorefinery as "the sustainable processing of biomass into a spectrum of marketable products (food, feed, materials, chemicals) and energy (fuels, power, heat)"(E. De Jong et al., 2012). Integrated biorefineries are a combination of several biomass conversion technologies that allow for more flexibility and cost reduction (McCormick and Kautto, 2013). They do thus ease the use of side-streams and wastes and facilitate the combined production of high value products (e.g. fine chemicals) with lower value products (e.g. bioenergy).
Maintaining the value of products, materials and resources for as long as possible and the waste hierarchy 4 are two of the key principles of the CE concept of the European Commission (2015) and therefore also apply to biological resources in a CBE (European Commission, 2017). Five of the nine analyzed CBE publications also referred to those principles. However, these principles do not necessarily result in the most economical or environmentally friendly solution (see discussion section). We therefore rather suggest the optimization of the value of biomass over time as a key characteristic of the CBE. Such an optimization can focus on economic (e.g. for profit), environmental (e.g. for GHG emissions) or also social aspects (e.g. for job creation) and ideally considers all three pillars of sustainability.
The cascading use of biomass could facilitate such an optimization over time. Six of nine publications considered cascading as an element of the CBE, while its use in keywords is insignificant. Cascading has various definitions in literature but usually the common theme is the "sequential use of resources for different purposes" (Olsson et al., 2018). Within the bioeconomy literature cascading use of biomass is mostly defined similarly; e.g. Fehrenbach et al. (2017) define it as the processing of biomass into a bio-based final product which is used at least once more either for material or energy purposes. We also adopt this definition, as it is in line with the European Commission (2017b) and further literature (Kosmol et al. 2012;Meyer 2017;Odegard et al., 2012). However, Olsson et al. (2018) pointed out that cascading is also sometimes interpreted as an order of priority, aiming for the highest added value (Odegard et al., 2012;Zabaniotou, 2018). Fig. 4 contrasts these two interpretations of cascading. Furthermore, cascading in the context of biorefineries is often used to describe co-production 5 and factory-internal recycling and recovery loops (Odegard et al., 2012;Zabaniotou, 2018). In recent years economic aspects have been highlighted when talking about value optimization (Olsson et al., 2018), even though there are various ways of defining the resource quality or the value of cascading choices; some of them are intrinsic (e.g. chemical structure, embodied energy) and others are based on human value judgements (e.g. economic, environmental, cultural) (Sirkin and Houten, 1994).
Waste management is an important topic in CBE publications; keywords related to this theme are used 4.2 times more in CBE publications compared to BE documents. Recycling and other circular waste management strategies are considered part of the CBE by all nine analyzed publications.
Circular product design was mentioned in five publications, while it only has a marginal share in keywords for both CBE and BE publications.
Four publications also advocate for an increased product utilization in the CBE by sharing and see prolonged use or durability of bio-based products as an element of the CBE. However, the concept of the sharing economy (see Curtis and Lehner, 2019) as well as prolonged use/durability are ignored by the remaining publications and are not existent within the keywords.
Looking at the keywords, sustainability, climate change and other environmental impacts seem to play a slightly more salient role in CBE publications compared to BE ones (see Appendix D). All nine analyzed publications highlight sustainability issues when discussing the CBE. However, the social aspects seem to fall short in the CBE discourse, both in keywords as well as in the conceptual discussions as only three of nine highlight them. Fig. 5 illustrates the CBE and its elements and Appendix A shows their coverage in literature.

CBE definition
Considering these elements, we suggest the following CBE definition: The circular bioeconomy focuses on the sustainable, resource-efficient valorization of biomass in integrated, multi-output production chains (e.g. biorefineries) while also making use of residues and wastes and optimizing the value of biomass over time via cascading. Such an optimization can focus on economic, environmental or social aspects and ideally considers all three pillars of sustainability. The cascading steps aim at retaining the resource quality by adhering to the bio-based value pyramid 6 and the waste hierarchy where possible and adequate.

CBE impact on biomass use
The analysis of keywords used in CBE documents showed an almost 50 % stronger focus on material biomass uses (bio-based chemicals and materials, food & feed) compared to energy and fuels, while this ratio is balanced in BE documents (see Appendix D). Using biomass directly for energy or fuels makes it impossible to maintain its value via reuse or recycling (Carus and Dammer, 2018). Hence, the CBE could induce a reduction of direct energetic use of biomass or a reallocation of biomass resources with biomass suitable for high-value applications going to materials while lower quality biomass is used for energetic purposes. In a CBE, more biomass would ideally first be delegated to a material use beforeafter one or potentially multiple cascading stepsit would be delegated to a final energetic use or composting. In theory, this cascading would follow a movement down the bio-based value pyramid (Fig. 6) and the waste hierarchy, moving from high value to lower value biomass applications. Moving from the upper part of the bio-based value pyramid and the waste hierarchy to the lower part theoretically goes along with decreasing options for further uses and cascading opportunities, due to the lowering of the resource quality. Staying on the upper part of both hierarchies would therefore theoretically be desirable in a CBE. However, in practice, applications on the lower part might still be preferable from an environmental and economic perspective (see discussion section).
Those publications advocating for a comprehensive vision of the CBE (Dalia D' Amato et al., 2018;Hetemäki et al., 2017) highlight sustainable sourcing of biomass. However, the majority of the CBE documents seems to focus on how the feedstock is used and on the role   of residues and waste. Consequentially, the literature review did not reveal a clear preference regarding the types of primary feedstocks or the origin of feedstocks (import or local). However, supporters of a European circular (bio) economy often highlight the argument of reducing the dependency on imports by keeping resources in the loop. Thus, one might expect a slight preference in a CBE for locally sourced feedstocks (see e.g. Bio-based Industries Consortium, 2017b).

Strategies in European bioeconomy clusters
The interviews with representatives from seven north-west European bioeconomy clusters aimed at identifying current strategies in their respective clusters and regions. The analysis of the interviews is structured in the clusters' strategies regarding a) feedstocks, b) products, c) the role of biorefineries, and d) the role of circular thinking, cascading and recycling.

Feedstock strategies
We examined the following three aspects of the cluster's feedstock strategies: the type of primary biomass used, the use of residues and wastes as a resource, and the origin of the biomass feedstocks (import or local). Table 1 shows the type of primary biomass feedstocks used in the clusters and their importance. We see that lignocellulosic feedstocks (mainly wood chips but also grasses) play an important role in almost all clusters, either as main (4) or complementary feedstocks (2), while it is seen as a prospective feedstock in only one case. Starch and sugar crops are also relevant feedstocks for the majority of the clusters; for three clusters they are a main feedstock. Oil crops on the other hand play a major role in only two of the clusters. Algae were mentioned as a prospective feedstock by two interviewees but do not (yet) play a relevant role.
Even though primary feedstocks are dominant in most clusters, Table 2 shows that residues and wastes are already playing a relevant role as well. Especially agricultural and/or forestry residues are considered a feedstock by all clusters. Six interviewees classify them as either main (2) or complementary feedstock (4), and one considers them as a prospective feedstock. Industry and household wastes do not play such an important role. Only three clusters consider them as main or complementary feedstock and one as a prospective feedstock. Lastly, three interviewees see CO 2 as a prospective feedstock for their cluster. Fig. 7 compares the importance of residues and wastes between the clusters (Y-Axis) and provides some insights into different strategies regarding the origins of the feedstocks used in the clusters (X-Axis). Based on the discussions with the interviewees, the clusters were positioned according to their focus on imports or local feedstocks.
Only BioVale (United Kingdom) and BIO.NRW / CLIB2021 (Germany) consider residues or wastes as a key resource, while these are just complementary or prospective feedstocks for the remaining five clusters (see Y-axis of Fig. 7). However, the clusters still seem to struggle with the practical implementation of using wastes and residues on an industrial scale. Most projects are still on the level of feasibility studies and research and development.
The results show that three of the seven clusters have a strong strategic focus on locally produced feedstocks (BIO.NRW/CLIB2021, IAR and Bioeconomy cluster) and use only very small amounts of imported biomass. For BioVale, local feedstocks play an important role (focus on local wastes and residues) but the cluster also makes use of the biomass supply (mainly wood chips) coming in from the regional Humber seaports. Biobased Delta and Northern Netherlands are much more inclined to import biomass, but they also have an interest in using locally available resources, e.g. from local sugar beet farmers (Biobased Delta). Flanders Biobased Valley has the strongest focus on imports.
Reasons for the diverging feedstock strategies between the clusters are manifold. The cluster location plays a key role. The four clusters with a stronger focus on imports are all situated close to major European harbors (see Fig. 7). However, also the importance of certain actors within the clusters is a relevant factor. Having a strong role of farmers associations within the cluster will most likely result in a much stronger role of local feedstocks as it is e.g. the case with IAR. If the cluster evolves more around industry stakeholders, it is more likely that using local resources is less of a priority if their price and stability of supply is outperformed by imports. Harbor vicinity in combination with strong agricultural stakeholders within the cluster will likely result in dual strategies: the Biobased Delta focuses on the one hand on wood chips and pellets import (Redefinery program) and on the other hand on using locally available sugar beets (Sugar delta programme) (Biobased Delta Foundation, 2017b).
The figure also shows that no cluster has a strong focus on imports and at the same time considers residues and wastes as a main or complementary feedstock, while all clusters with a strong focus on local feedstocks (the French and German clusters) consider residues and waste at least as complementary feedstock. The clusters seem to not consider importing wastes and residues via their harbors but only refer to locally available wastes and residues. However, the presence of a harbor does not exclude a key role of wastes and residues, as the example of BioVale shows. The available local farmland also plays an important role in the feedstock strategy. A limited amount of agricultural areas in the region can either result in a feedstock strategy that heavily relies on imports (e.g. Flanders Biobased Valley) or in a stronger focus on making use of residues and wastes (e.g. BIO.NRW / CLIB2021).
The interviews provided insights into the diverse feedstock strategies of the clusters, which are the result of their different local circumstances. This diversity in strategies and regions is also reflected in publications providing an overview of Europe's bioeconomy regions (Bio-based Industries Consortium, 2017a;Haarich, 2017) and makes it difficult to design a "one fits all" policy on EU level.

Product strategies: Energetic vs material biomass use and promising bioeconomy sectors
All interviewees described a clear shift in their research and innovation programs from energetic to material biomass use. A reason for this development could be concerns that the support for e.g. biofuels and bioenergy might be reduced. We can indeed notice some negative signals for certain energetic biomass sectors, e.g. that the revised Renewable Energy Directive (RED II) caps the contribution of biofuels produced from food and feed crops to the Renewable Energy goals (European Commission, 2018d). However, the RED II also contains a target for advanced biofuels and an overall renewables target of 32 % for 2030, to which bioenergy could significantly contribute if it meets the sustainability criteria. However, NGOs increasingly pressure the sector to reduce direct biomass use for energy and to focus more on cascading and material applications of biomass (see e.g. Greenpeace Table 1 Primary biomass feedstock types and their role in the clusters (n = 7 clusters).

Feedstock role in cluster
Lignocellulosic Starch Sugar Oil Algae  Natuur and Milieu, 2018;WWF, 2017). Also on national level we can observe some tendencies towards the cascading principle and material and high value-applications, such as in the discussions on the climate agreement of the Netherlands (van Veldhoven-van der Meer, 2019). Two cluster representatives mentioned that they prefer a business model that works without subsidies and thus see a more stable business environment and better chances to compete on the market for high quality products, especially if the bio-based products offer additional features that fossil competitors do not offer, such as biodegradability. Another interviewee explained the trend to material biomass uses in research & innovation with the higher research needs in these sectors, while there are already more mature technologies for biofuels and bioenergy. Furthermore, bioenergy usually requires larger feedstock volumes to be profitable, while e.g. fine chemicals require lower biomass inputs. Densely populated bioeconomy regions without harbor access and small agricultural areas as North-Rhine Westphalia thus showed a strong focus on pharmaceuticals, fine and specialty chemicals.
The cluster's focus on materials refers to research and innovation projects and does therefore not mean that the energetic use of biomass only plays a minor role. The clusters could still have large installed capacities of e.g. biogas or biofuel plants 7 , while their research and innovation increasingly evolves around material biomass use. Furthermore, some local policies support energetic biomass uses, such as in the Haut de France region (IAR cluster) for biogas.
Looking at the turnover of bioeconomy sectors in the countries the clusters are located in (Belgium, France, Germany, Netherlands, United Kingdom), we observe a rising turnover of bio-based chemicals, pharmaceuticals, plastics and rubber (NACE classification) by 6.4 % between 2011 and 2015, while the turnover of biofuels decreased by 44 % in the same period (European Commission, 2018c). However, bio-based electricity demonstrated the by far highest percentage growth with almost 123 % (European Commission, 2018c). The International Energy Agency (IEA) projects a decline for conventional biofuels until 2023 but an increase for novel advanced biofuels from non-food crops (Bahar et al., 2018). Furthermore, bioenergy capacities are expected to increase in Europe from 41.7 GW in 2017 up to 49.9 GW in 2023, while co-firing biomass is increasingly questioned (Bahar et al., 2018), e.g. the Netherlands is planning not to issue any new grants for co-firing biomass (Government of The Netherlands, 2017).
Hence, looking at commercial developments we see a mixed picture and cannot identify a clear trend towards material biomass uses. Nevertheless, the shift in the cluster's research and innovation towards material biomass use could be a relevant indicator for the future focus of the bioeconomy clusters.
We asked the cluster experts which product sectors they see as promising (or not) for the future bioeconomy. Not all felt comfortable in providing an encompassing answer as such an assessment is rather speculative and potentially sensitive concerning their own strategy. Fig. 8 shows that the experts mostly mentioned rather higher value applications of biomass as promising sectors. Especially bioplastics, pharmaceuticals and food and feed additives are considered to have a significant potential for the bioeconomy. Bio-composites and (innovative) construction and building materials were also named, which could be classified as lower to medium value on the bio-based value pyramid (see Fig. 6). High volume but low value applications like bioenergy, biofuels and bulk chemicals were even assessed negatively by the cluster experts.
In general, most of the experts see potential mainly in highly functionalized biomass applications with high economic value such as fine and specialty chemicals. They do not see great opportunities in competing with fossil alternatives in lower value applications like bulk chemicals under current circumstances, due to their low price and lacking policy support. This suggests a change in the bioeconomy, as in particular bulk chemicals received attention in the past as a promising sector (Dornburg et al., 2008;Patel et al., 2006).
To conclude, the interviews revealed a product strategy of the clusters that demonstrates a shift in the clusters' research and innovation programmes from energetic to material use of biomass and an upward movement on the bio-based value pyramid towards low volume but high value biomass applications. The implications of these results for the CBE are discussed in section 4.1.

The role of biorefineries in the clusters
IEA's biorefinery definition (see chapter 3.1) includes more "traditional" biorefineries combining e.g. biofuel with food and feed production as well as more recent biorefinery concepts, e.g. lignocellulosic biorefineries producing a range of chemicals together with bioenergy and/or biofuels. While current biorefineries are mainly based on a single conversion technology, the goal is to move towards integrated biorefineries, i.e. a combination of several conversion technologies that allow for more flexibility and cost reduction ( provide an overview of the wide range of biorefinery types, which can be classified according to their feedstocks (e.g. lignocellulosic, oil or starch crops), their platforms (intermediates like syngas or sugars) and their products (e.g. fuels, chemicals). All seven clusters are working on biorefinery concepts for their region and five of them have at least one biorefinery (pilot) plant in place or under construction. Biorefineries on commercial scale only exist in three clusters and they are producing a combination of biofuels and animal feed 8 . All clusters are working towards lignocellulosic biorefineries and three of them have already pilot plants in place or under construction. Examples are listed in Appendix E.
Without going into detail on the projects, we can conclude that there are significant efforts towards biorefineries and the integrated production of bio-based energy, fuel, chemicals and materials throughout all of the considered clusters. The more advanced biorefinery projects with a high technology readiness level still evolve around bioenergy and biofuels production. However, the trend in research and development seems to go towards a more integrated coproduction of a range of bio-based products with an increased importance of chemical applications such as in the pilot plants of the clusters IAR, Bioeconomy cluster and Northern Netherlands (Avantium, 2018;Ernhofer, 2012;Procethol 2G, 2010).

The role of circular solutions, cascading and recycling
When asked to define the term cascading, the interviewees revealed differences in their understanding of the concept. Six interviewees referred to the term as (1) favoring the highest value-added use of biomass and three also see (2) making use of the entire feedstocks including all by-products (i.e. via co-production) as part of cascading. Three of the interview partners defined cascading as (3) the sequential use of biomass. All three definitions are common interpretations of the term and all three approaches also have its role to play in the CBE. As discussed in chapter 3.1, we define cascading as the sequential use of bio-based products and refer to the other two interpretations as value optimization (1) and co-production in integrated biorefineries (2). Implementing the CBE requires a proper communication of the concept and a clarification of the term cascading across the bioeconomy sector. The same is true for other important CBE concepts as Näyhä (2019) indicated.
The interviews and the analysis of publicly available cluster strategies and business plans showed that there is so far no coherent concept or strategic focus within the clusters on cascading use, recycling and generally on how to deal with bio-based products at the end of their life. Only Bio-based Delta articulated the goal of updating its strategy to incorporate the national policy goals regarding the CE (Biobased Delta Foundation, 2017b). Three representatives stated that they have at least one project dedicated to end-of-life options within their clusters but said that the topic is not in the focus of most projects. Two interviewees referred to rather small-scale activities of individual companies in their region in establishing closed-loop recycling schemes for their products, e.g. for bio-based carpets.
Two experts see one of the reasons for this comparably small role of the topic in their cluster in the fact that the bioeconomy is still quite nascent and that the focus is thus more on the preceding steps, i.e. developing and improving the technologies and products before thinking about potential end-of-life scenarios. Fig. 9 shows challenges and drivers seen by the interviewed experts for implementing circular solutions. Impeding policies and regulations were mentioned most, e.g. the classification of a material as waste often limits its further use. According to the interviews, two challenges for implementing recycling schemes on a bigger scale are the comparably still small share of biobased products on the market and the difficulty of tracing bio-based materials flows throughout value chains. Furthermore, three interviewees see higher costs of circular business models, e.g. because the low oil price facilitates the use of virgin feedstocks compared to recyclables. The interviewees also refer to technological challenges and difficulties in recycling composite materials. Moreover, inconsistent waste management practices across Europe were mentioned as a hurdle as well as the lack of waste management companies accepting and adapting to bio-based products. One interviewee also feared an unstable supply of wastes and residues as a resource and a dependency on production changes of companies supplying these wastes and residues. Lastly, also a lack of funding for circular business cases was seen as a challenge by one expert.
As a driver for circular business cases three interviewees named that investors increasingly ask for end-of-life solutions. Two experts also simply see a business case for using wastes and residues as complementary feedstock to reduce costs. Three interviewees see policies and regulations as a way to foster circular business models, e.g. by taxing GHG-emissions, fostering demand via public procurement and by introducing regulations that prevent the contamination of products hampering recycling. Fostering circular product design is seen as key, e.g. by educating product designers accordingly. Furthermore, efforts towards enhancing the cooperation along the supply chain and amongst regions have been mentioned as well as better showcasing the benefits of cascading biomass use.

Implementation of CBE elements in the clusters
The interviews showed that the clusters are mostly addressing elements of the CBE related to the production phase (top of Fig. 5): They increasingly consider residues and wastes as a feedstock and work on resource-efficient biomass use in integrated biorefineries. Using residues and wastes has the potential to achieve higher GHG-emission reductions compared to primary biomass use (Creutzig et al., 2015) and could reduce feedstock costs (see Fig. 9). Residues and wastes could potentially meet a significant part of global biomass demand by 2050 but sustainability constraints should be acknowledged (see e.g. Hanssen et al., 2019 on biomass residues).
The clusters seem to address CBE elements aiming at the use and end-of-life of bio-based products (i.e. product design, recycling and cascading) only to a limited extent. Nonetheless, the interviews confirmed a stronger focus on material biomass uses within the clusters' research & innovation programs, which we described as a likely development in a CBE (chapter 3.1.3). Furthermore, the interviewees see more opportunities in products with a high economic value (Fig. 8) and thus indicate a movement up the bio-based value pyramid (Fig. 6). This movement theoretically allows for more recycling and cascading options as a higher resource quality of biomass is maintained. However, in practice, this cascading potential is difficult to realize.
According to Braungart et al. (2007), product groups can be structured in biological nutrients (i.e. biodegradable materials) or technical nutrients, i.e. materials with the potential to stay in the technical cycle via reuse and recycling. The latter requires collection, separation and recycling schemes for bio-based products that are often not in place yet, and which are probably only profitable when implemented on a large scale; but most bio-based products still have a comparably small market share (Aeschelmann and Carus, 2015;Panchaksharam et al., 2019).
Moreover, many products are difficult to collect, separate and recycle (Carus and Dammer, 2018), amongst them several named as promising in Fig. 8: i.e. cosmetics, pharmaceuticals, lubricants, additives, biocomposites. For these materials aiming for their integration into the biological cycle, i.e. by making them biodegradable, seem like more promising approaches. From the sectors in Fig. 8, only bioplastics and construction & building materials have a higher recycling and cascading potential because they are produced on larger scale and are easier recoverable compared to the other product groups. However, also for these sectors, this potential strongly depends on (1) the availability of suitable collection, separation and recycling systems and (2) on the design of the products, which should avoid the use of composites or incompatible and hazardous materials (see de Aguiar et al., 2017) and meet other waste management requirements. Despite being highlighted as a driver in the interviews, the analysis of CBE publications and cluster interviews/strategies showed that circular product design does not seem to play a significant role so far. Moreover, design for durability and a prolonged and shared product use did not appear to be a major topic in the analyzed CBE publications.

Towards a sustainable, circular bioeconomy?
Implementing the overarching CBE principles, i.e. sustainability and optimizing the value of biomass over time, in practice is a challenge, as it requires perfect foresight and cooperation across value chains. Furthermore, various potential trade-offs need to be considered, e.g. between sustainability dimensions or optimizing product design for either prolonged use or easy repair or recycling. The benefits of CBE strategies like using wastes and residues, recycling and cascading still have to be proven in practice and are probably case specific. A paper by Daioglou et al. (2014) indicated that recycling and recovery options "do not necessarily reduce energy demand or carbon emissions". Case studies on cascading pathways of bioplastics, textiles, paper and wood conducted Fig. 9. Challenges, hurdles and drivers for implementing circular solutions according to the 7 interviewed experts. P. Stegmann, et al. Resources, Conservation & Recycling: X 6 (2020) 100029 by Fehrenbach et al. (2017) showed environmental benefits for intelligently designed cascading pathways, even though not overwhelmingly large for some cases. Bais-Moleman et al. (2018) showed significant GHG emissions reductions for cascading wood; however, they also acknowledge short-term trade-offs with the energy sector. By focusing on material biomass uses and aiming for high-value biomass applications, the interviewees indicated a movement up the bio-based value pyramid. While this theoretically increases the recycling and cascading potential, it might lead to a trade-off with energetic biomass use: Using biomass to displace fossil-based electricity, heat and transport fuels at a large scale might offer a higher absolute GHG mitigation potential than using biomass for material applications. For example, Daioglou et al. (2015) showed that biomass use in the electricity sector promises the highest GHG-mitigation potential when replacing coal. However, as other renewables such as solar and wind are reaching higher shares in the future energy mix (Rogelj et al., 2018), this mitigation potential will likely diminish over time. While the electricity sector could use other renewables for decarbonization, biomass is one of the few short term decarbonization options for the chemical industry, heavy road transport and marine and aviation sectors (Bazzanella and Ausfelder, 2017;Carus and Raschka, 2018; International Energy Agency (IEA), 2018; S. de Jong et al., 2018;Mawhood et al., 2016). The CBE could reduce the competition between biomass uses by using biomass resource-efficiently and cascade it over potentially multiple material applications before delegating it to an energetic use.
The EU bioeconomy strategy sees a sustainable and circular bioeconomy as a key contributor to a GHG-neutral Europe (European Commission, 2018b). The importance of GHG-emissions in the CBE discourse is also reflected in the frequent use of keywords related to climate change in the analyzed CBE publications. However, there are also various other sustainability aspects to be considered, which are not highlighted as much in the analyzed CBE literature (see 3.1.1). Amongst them are social aspects, and impacts related to feedstock production like land-use change and eutrophication. By keeping biomass in the loop, the CBE has the potential to reduce primary feedstock demand and its related emissions. However, this only holds true if the CBE does not follow the paradigm of continuous economic growth (Giampietro, 2019), offsetting its potential benefits with excessive biomass use and rebound effects (Dalia D'Amato et al., 2018). After all, both CE and bioeconomy are resource-focused concepts that do not address degrowth topics (D. D'Amato et al., 2017). Criticism of the CE and bioeconomy also largely applies to the CBE, even though the ambition is to be better, to be "more than bioeconomy or circular economy alone" (Dalia D'Amato et al., 2018;Hetemäki et al., 2017).
To achieve this goal, the CBE will need to address its critics claiming a lack of evidence regarding its environmental and social value (Dalia D'Amato et al., 2018;Temmes and Peck, 2019) by following a comprehensive sustainability vision and showing its benefits case by case. However, it is a complex question which biomass uses, cascading chains and end-of-life strategies are most beneficial in terms of reducing emissions. To what extent the current cluster strategies support a sustainable, circular bioeconomy can thus not be finally answered.

Conclusions and recommendations
The concept of a CBE has increasingly found its way into European bioeconomy strategies and reports. A CBE as defined in this paper focuses on (a) the sustainable, resource-efficient valorization of biomass in integrated production chains (e.g. biorefineries) while making use of residues and wastes and (b) on optimizing the value of biomass over time via cascading steps. This will likely reduce the direct energetic use of biomass in favor of material applications to enable a prolonged and more resource-efficient biomass use (e.g. via reuse, recycling and cascading).
The analyzed bioeconomy clusters seem to move towards a more circular bioeconomy, by increasingly considering wastes and residues as a resource, investigating options for integrated biorefineries and putting more focus on material and high value applications of biomass. However, there is only little focus on solutions concerning product design and the end of life of bio-based products. The interviewees highlighted as key challenges for implementing circular strategies impeding policies and regulations, costs and the small size of bio-based markets.
While the CBE has the potential to improve the sustainability of the current bioeconomy, the discussion showed that the concept is not inherently sustainable and needs to address its potential drawbacks and trade-offs. Especially social aspects, cascading, product design, and aspects related to product use (durability, sharing) seem to be underrepresented in CBE literature, while the topics biorefinery, wastes and residues as well as waste management are significantly covered.
Practitioners in bioeconomy clusters can support the development towards a CBE by (1) facilitating cooperation between stakeholders along and across supply chains; (2) fostering bio-based product design that facilitates durability, reuse, repair, recycling or biodegradability; (3) fostering the use of residues and wastes as resource; (4) intensifying the cooperation with the waste management sector to ensure that the bio-based products can be integrated in collection, separation, recycling and composting schemes.
However, to move towards a sustainable CBE clear guidance for practitioners in bioeconomy clusters is needed. This and other papers (see e.g. Näyhä, 2019)revealed different understandings of key CBE concepts amongst practitioners. This calls for an alignment of CBE terminology. Secondly, all the benefits and trade-offs of the CBE have to be laid out: An integrated assessment of the CBE in the context of the wider economy is needed to analyze the aggregated impacts of different biomass uses and the potential benefits of different end-of-life strategies. Such an analysis could contribute to defining pathways and overall policy goals, but it needs to be complemented by case studies developing specific recommendations for implementing these goals. For instance, research needs to show under which circumstances multiple cascading steps are actually beneficial from an environmental, social and economic point of view but also how these cascading chains can be implemented in practice. For a successful transition to a sustainable CBE many more actors need to be involved; e.g. consumers, investors, as well as architects and engineers need guidance towards the implementation of CBE principles. Moreover, current efforts in establishing a monitoring system for the bioeconomy also need to include indicators measuring circularity.
The diversity observed in the clusters' circumstances highlights the importance of designing specific regional CBE-strategies, taking the local strengths and weaknesses into account while avoiding "one fits all" solutions. Furthermore, policies and research programs should focus more on product design and end-of-life strategies for bio-based products as there are only few initiatives addressing it within the clusters. Focusing on the recycling and cascading potential of bio-based plastics and construction & building materials is recommended, as they are produced on larger scale and are easier recoverable compared to the other named sectors. For the other sectors the interviewees considered promising, i.e. cosmetics, detergents and lubricants, efforts improving their biodegradability seem more promising. Moreover, policies increasing the CO 2 price would foster the CBE by increasing the economic competitiveness of resource-efficiency measures and using wastes & residues over primary resources. For optimizing the emission mitigation potential of the CBE, clear policy incentives are needed that do not just foster the bioeconomy as a whole but focus on those biomass uses and cascading pathways that promise the highest emission mitigation potential.

Declaration of Competing Interest
The authors declare that they have no known competing financial P. Stegmann, et al. Resources, Conservation & Recycling: X 6 (2020) 100029 interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement
We would like to thank all the interview partners for their time and valuable insights. Without them this paper would not have been possible.
The funding of this research is supported by the Topconsortia voor Kennis en Innovatie programma BioBased Economy (Netherlands), awarded by the Dutch Ministry of Economic Affairs (Project Reference TKI-BBE-1601 Impact assessment BBE economy). The project partners include Utrecht University, TNO (Netherlands Organization for applied scientific research, ECN division), AkzoNobel, Avantium, RWE and Staatsbosbeheer.

Appendix A. Publications on the CBE concept
This table shows the nine documents chosen for the literature review and their coverage of the CBE elements presented in chapter 3.1.1: 1 = Use of wastes and residues as resource; 2 = Resource-efficiency; 3 = biorefinery; 4 = Maintaining the value of products, materials and resources for as long as possible and/or the waste hierarchy; 5 = Cascading use of biomass; 6 = Waste management (e.g. reuse, recycling); 7 = Circular product design; 8 = Prolonged use / durability; 9 = Sharing economy; 10 = Sustainability; 11 = Social aspects.