The European Union strongly emphasizes the concept of bio-economy and member state countries, and other countries around the World have adapted this concept in their biomass policies nationally (Fund et al., 2018). The definition of BE is “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 bio-energy” (European Commission (EC), 2012). The concept has however received critique for not being circular and sustainable and merely applying business-as-usual (Hetemäki, et al., 2017; Pfau et al., 2014), where circular economy (CE) is defined as “minimizing the generation of waste and maintaining the value of products, materials and resources for as long as possible.” Thus, the two concepts above have been merged by the EU to Circular Bio-Economy, and the European Commission have stated that BE to a larger extent must have a circular focus to become fruitful (European Commission (EC), 2018).

The CBE concept is thus relatively new and the role of the concept in a future green transition is not fully defined or exemplified at the local and regional levels (Stegmann et al., 2020). This research will contribute to the lack of knowledge of how to adopt and apply this concept in a local context emphasizing the opportunity for using biomass residual from the Danish agricultural sector to speed up the pathway for green transition. By using cereal and beet tops residues, as well as animal manure, it illustrates and exemplifies how to utilize biomass residues from the agricultural sector, which otherwise could be wasted. The article will highlight how these residual waste streams can be valorized and utilized, by producing nutrients, renewable energy, and bio-products through upgrading and cascading methods (Sirkin, 1990; Sirkin and ten Houten, 1994). Such contributions are, according to Stegmann et al. (2020), underrepresented in the current CBE literature.

The thinking of this approach is depicted in Figure 1, showing the CBE system (green part of the triangle), “disrupting” the traditional carbon cycle system with a long timeframe (gray part of the triangle). The orange arrows illustrate the focal point of investigation in this article—a future green transition pathway—in which upgrading and cascading production methods (orange bend arrows) are intensified and developed further within the energy sector using agricultural residues. This is facilitated by the biogas technology that acts as an “engine” for CBE activities within the local community. Enhancing the biomass carbon cycle system, by using residues from the agricultural sector currently being treated as waste, to provide bio-product extraction activities and upcycled materials and energy, is thus the overall emphasis of this article. These resources must be utilized more efficiently, together with other local natural resources such as for example solar and wind energy, as well as geothermal energy, however not being a part of this investigation.

Resource cascading—normally just referred to as cascading—is a central method for optimizing the use of resources in the CBE. Cascading can be described as the use of outputs from one process, as an input to a sequential process at another level in the cascade chain, with the aim of extending the overall utilization time and maintaining the resource quality (Sirkin, 1990). At every level in the cascading chain, three options need to be the addressed 1) Upgrade or upcycle the resource to a higher level in the same cascading chain or in a new cycle, 2) Cascade the resource to a next (lower) level in the cascading chain, or 3) Maintain the resource quality at the same utility level (Sirkin and ten Houten, 1994). This study mainly provides suggestions of how to apply upgrading/upcycle to higher levels—or to new cycles—in the cascading chain, thus being option number one.

When applying the cascading concept to the utilization of cereal straw and beet tops, bio-products will provide an upgrading of the agricultural residues to a higher level, just as the production of upgraded biogas will enable cascading of the generated energy to new and higher cycles, compared with the production and utilization of traditional biogas only. Also, upcycling of manure and crop residues as fertilizer will for example happen, as the substrate becomes a higher value digestate when leaving the reactor tank due to a mineralization of the substrate (Jørgensen, 2009). Thus, maintaining the quality of the resources at the same utility level is, as mentioned, not pursued in this case, as the analysis mainly is connected to upgrading/upcycling. An example could however be to utilize cereal straw as building materials, and hence make use of the residual straw for energy services when these buildings are demolished.

Below, the article shortly introduces wax and furfural that potentially can be extracted as bio-products from straw, as well as introduces, how proteins can be extracted from the green top of sugar beet, here relying on Danish experiences with grass clover.

Wax from straw: Waxes are a broad term and cover substrates as fatty alcohols, fatty acids, sterols, wax esters, alkanes, etc. Waxes can be produced from biomass residues such as palm leaves and cereal straw and are extracted from the greasy content on the cereal straw surface (Bulushi et al., 2018), and can serve various purposes as for example food supplements, cosmetics, flavorings, fragrances, and coatings (Sin et al., 2014). Wax made from biomass residues (natural wax) can substitute environmental unfriendly petroleum-based (fossil fuel) wax, which currently are limited in production scale (Bulushi et al., 2018). Natural wax can be extracted from cereal straw, by using supercritical CO₂ acting as an extract-fluid/solvent (Hyatt, 1984; Hunt et al., 2010), which facilitates extraction and fractionation of wax from biomass, and can be carried out in an internal one step process (Deswarte et al., 2006; Sin, 2012). This method is thus a viable alternative to more traditional methods applying organic solvents, which are prohibited in substrates used in, e.g., cosmetics and food supplements, due to the fact that no solvent residues are produced when using supercritical CO₂ (Hunt et al., 2010; Sin et al., 2014). According to Attard et al. (2015), Attard et al. (2016) is the use of supercritical CO₂ as a first step in applying a bio-refinery where upcycling of downstream residues—thus low-value cascading’s (see Figure 2 further below)—can be applied.

Furfural from straw: Furfural are regarded as important future bio-products and thus a platform for green chemicals and biofuels (Li et al., 2016), and can be utilized as a “biobased alternative for the production of everything from antacids and fertilizers to plastics and paints” (Biomass Furfural, 2021). Traditional furfural is made from fossil fuel petroleum products where natural furfural is made from lignocellulosic comprising of lignin, hemicellulose, and cellulose. Xylose or xylan (hemicellulose) are often utilized to produce furfural using an acid catalyst, whereby hydrolysis of xylan into xylose appears, and a successive dehydration of xylose (pentose) then leads to the production of furfural. Xylan is found in lignocelluloses biomass, as for example cereal straw, maize cob, rice husk, bagasse, etc. (Matsagar et al., 2017). It is important to separate, hence pretreat, the lignocelluloses to be able to extract the content of hemicellulose, which then enables this substrate to be converted into C5 sugars and then finally into furfural. The production of furfur is, as mentioned, a chemical platform—or building blocks—for producing various other types of green chemical and products and can also be facilitated by using supercritical CO₂ (Sangarunlert et al., 2008). Hence, as for the case of wax, supercritical CO₂ can be utilized as a solvent to extract furfural instead of environmental unfriendly mineral acid, where the recyclability of this solvent also is a major advantage.

Protein extraction from beet tops: Today, it is possible to use different technologies to extract edible protein concentrates from biomass residues to be used as food and fodder, in this case also from the green tops of sugar beet (Beta vulgaris L.). Experiments show that proteins can be extracted and fractionated from sugar beet tops by using a wet process to obtain two types of leaf protein concentrates. One, obtained by heat coagulation of the green juice with protein contents of approximately 34%, and another, obtained from the brown juice with protein contents of 44%, in both cases the results depend on the temperature and time used for extraction (Jwanny et al., 1993). Another method applied is to utilize enzymes to facilitate the process of extracting protein (Akyüz and Ersus, 2021).

Beet tops has historically been utilized for animal feed, but nowadays mostly left on the field to decompose or in some cases plowed into the soil for fertilizing, which is the same situation for residual straw (BioBIGG, 2020a). Literature state that beet-tops hold a protein content of 19–23% (dry matter, DM) (Akyüz and Ersus, 2021). Little knowledge, however, exists on the extraction of proteins specifically from beet tops, whereas protein extraction—from within a Danish context—mostly has revolved around grass clover and extraction of proteins by a screw-pressers. This knowledge can most likely be transferred to extraction of proteins from beet-tops (University of Århus, 2015). Below, the article shortly presents the methods of extracting protein from green stuff, here exemplified by grass clover for animal fodder.

Newer research highlights (University of Århus, 2015; Santamaria-Fernandez et al., 2019; Frandsen, 2020) that the fiber/pulp fraction (70% of the DM)—when feeding green stuff to a screw-presser—can be utilized for dairy cattle fodder and might thus improve the economy of utilizing green stuff, such as grass clover and beet tops, as protein fodder. Owing to a splitting up of the green fibers within the screw-presser, the cattle tend to be able to utilize the energy in the processed green stuff—the fiber/pulp—better than ordinary green fodder, which they normally are feed by. A study further suggests that dairy cattle increase their milk yield production by as much as 37% when feed by fiber/pulp from green stuff, compared to ordinary non-processed green stuff (Damborg et al., 2019).

From the screw-presser, a juice (30% of the DM)—being rich in proteins—is also extracted, which can be heated and turned into fodder pellets (25% of the DM), and usually utilized for one-stomach livestock animals. The remaining part (5% of the DM), which is a brown juice, can be utilized for biogas production, hence another upcycling in the cascading chain as far as energy production. See Figure 3 above:

Having outlived the theoretical lens and presented the bio-products etc., being the main focus of this article, next is to present the data retrieval methods applied in this study, here also emphasizing our case study area.

The data retrieval in this article is based on case study analysis, journal articles, and various literature studies. This will be detailed below.

Within Region Zealand approximately 7.3% of the arable land is cultivated with beet, which equals approximately 35.000 acres. The amount of sugar beet tops to reach 170.000 tons of Dry Matter (DM), with an average of 5 tons per acres, is identified (BioBIGG, 2020a). Besides this, 2.8 million tons of animal manure are produced on an annual basis within Region Zealand, of only which a minor part is being digested on the existing biogas plants, thus approximately 240.000 tons (Gaarsmand and Kjær, 2019). Cereal straw (winter wheat and other cereals) covers approximately 71% of the arable land and take up 310.000 acres at Region Zealand, providing cereal crop yields of approximately 1.4 million tons per year. It is estimated that approximately 35% of the residual straw currently left on the farmland are collected and utilized for combined heat and power production (CHP) on decentralized CHP plant within the Region (BioBIGG, 2020b). Small fractions are also utilized for animal bedding materials, fodder and in individual straw fired farm boilers, but the largest part is yet to find other means of usage, as simply decomposing on farmland.

Within Falster & Lolland Municipalities specifically, it is identified that 800.000 tons of animal manure are readily available for biogas production, within a distance of 30 km from the two suggested plant location sites. Besides this, 880.000 tons of unused beet tops and 220.000 tons of residual straw from the community are assessed to theoretically be feed to the biogas facilities. For the planned biogas plants within Lolland & Falster Municipalities, the following biomass resources have been estimated to be readily available: The full manure potential equal to 800.000 tons; 230.000 tons sugar beet tops; 100.000 tons of residual straw, thus approximately 1.130.000 tons (Gaarsmand and Kjær, 2019; Prade et al., 2019).

The potential gas yield from the two biogas plants that are scheduled to be operated as manure-based biogas plants, mixed with green substrates to utilize the available biomass resources from the community, is assessed as follows. The total energy output from the two plants is estimated to be 40 million m³ biogas (methane) which equals 397.000 MWh (estimated by the lower calorific value of 9.94 kWh/m³ (Danish Energy Agency DEA, 2020a). The fertilizer yield is assessed to reach 5.500 tons of nitrogen (N) and the total greenhouse gas (GHG) reductions to 169,220 tons of CO₂ eq. (Prade et al., 2019). The technical specifications related to the plant layout at the Abed Biogas Plant are exemplified in Table 1.

The biogas concept presented above is, as mentioned earlier, one of two plants schedules to be implemented within Lolland & Falster Municipalities. The technical concepts rely on mature and reliable biogas technology that has been operated in a Danish context for several years (Holm-Nielsen et al., 2009; Jørgensen, 2009; Food and Bio Cluster Denmark, 2020). As seen from the technical concept in Table 1, Lolland Municipality emphasize on upgrading biogas to natural gas standards, as a consequence of the natural gas pipeline that will be in operation in 2024 (SEAS-NVE, 2021). It is also clear that the municipality plan to use a little more than half of the biomass residues that was found appropriate, hence 600.000 tons annually.

There are, however, no current plans of applying solutions that emphasize the production of bio-products in the technical concept presented, nor any plans of how to utilize the specific residues from the local area more efficiently than simply as a bio-substrate to be mixed with manure. In the following, the article discusses how the biogas plant can be optimized, applying the normative lens of CBE adopting applicable cascading opportunities offered by the specific context. This is detailed in the following.

Below, it is identified how bio-product “extraction-activities” for cereal straw and beet tops can be obtained when connecting and integrating such resources to a biogas “engine” technology, where various cascading’s move the system to a higher level. The approach is, as mentioned, to look at options for extracting protein from beet tops, and furfural (from C5 sugars) and wax (fatty alcohols, alkanes) from residual straw (Sin, 2012; Matsagar et al., 2017), as well as to obtain higher value of materials and energy. The notion of using the biogas technology as an “engine” is understood as the technology’s capability to become a “hub” for activities, like thermal, mechanical, biological, and chemical treatment, enabling production of bio-products and higher value materials and energy. “Engine” is thus interpreted as biogas technology being a “multiple treatment/production facility.”

As illustrated in Figure 2, the aim is to develop a method in which biomass residues and side streams are utilized efficiently, and where the development of the system happens in steps according to whether or not they can be deployed based on internal CBE processes at the initial stage, without drawing on “imports” of other types of external biomass resources or highly advanced and costly technology. Having developed such internal CBE processes, emphasis could then be to develop CBE processes relying on additional residues and the production of other types of bio-products. This is emphasized in the following and depicted in Figure 2, showing the cascading chain.

The lower step (1) in the cascading chain is the recirculation of nutrients, such as nitrogen, phosphorous, and potassium, which is pivotal to sustain farmland fertility and hence the cultivation of crops etc. This is based on the agricultural sectors production of animal manure, where this substrate is recirculated as a fertilizer. At this level, however, it is possible not only to utilize the nutrients in the animal manure, but also to include local waste streams such as various crop residues to provide valuable soil nutrient, in this case straw and beet tops. The biomass resources (manure and crops residues) should preferably not be utilized as fertilizer on farmland being un-digested, emphasized in the following step. Hence, the next step (2) includes the benefits of the biogas technology as far as mineralization of nutrients, odor reduction, and the ease of distributing the digestate on farmland after the substrate has been digested and thus becomes more liquid (Jørgensen, 2009; Al Seadi et al., 2013b). This step increases the cascading value of both manure and biomass crop residues. Thus, higher value materials are obtained.

In step (3) the substrate—and consequently gas production from the reactor tanks—is additionally utilized for renewable energy production as Combined Heat and Power (CHP), which faces out/reduces the consumption of fossil fuels within the local energy system, hereby increasing the level of cascading even further (upcycling). In Step (4) some, or all, of the biogas is upgraded to bio-methane, by the extraction of the content of CO₂. This enables the bio-methane (upgraded biogas) to be distributed to other parts of Region Zealand through the natural gas network, and hence to supply more flexible and higher value energy services with better storage options as a benefit (Dansk Gasteknisk Center, 2019; 2020; SEAS-NVE, 2021). This is also in line with the EC’s ambitions of developing more integrated energy systems within the EU (European Commission, 2020). Thus, higher value energy services are obtained. The scheduled biogas plant at Abed entails until step (4).

In step (5) the use of the biomass residues, and the production of CO₂ from the upgrading of biogas, now enables the use of straw and beet tops residues in a completely other order, which increases the cascading value of the entire process. It is suggested to utilizing the straw for extraction of bio-products, as for example wax and furfural, where the processed/extracted residual straw are feed to the biogas plant as feedstock afterward (see Figure 4). This process can be applied as an internal process using the extracted CO₂ from the upgrading of biogas, as supercritical CO₂ (scCO₂), a solvent, to extract furfural and wax. Being an internal process, not relying on outside “import” of highly advanced technology, such bio-products could be produced before other types of bio-products. During the extraction of wax and furfural the residual straw undergoes a pretreatment in which it becomes highly appropriate for biogas production. Hence, it turns to a higher value substrate in the cascading process, with gas yields surpassing the level of the non-processed straw residues. As a consequence of this, expenses connected to previous pretreatment technologies of straw, such as macerator, choppers etc. might be avoided (Møller and Nielsen, 2016). Synergies will hence be obtained.

Besides recirculation of nutrients (step 1), digestion of manure and biomass residues within the biogas technology (step 2), renewable energy production as CHP to local community (step 3), upgraded biogas to the natural gas network within Region Zealand (step 4), and furfural and wax extraction from straw (step 5), the next step (6)—in this specific case—is to utilize the residual beet tops for production of proteins before feed to the biogas plant. Thus, the green top can be used for high value protein extraction for human and animal feed, by using a screw-pressers, as exemplified earlier in this article (see Figure 2). By extracting the protein from beet tops the crop is utilized at the highest cascading level as possible, where the remaining residues can be utilized together with straw for biogas production, preferably as silage materials, emphasized shortly below. The brown juice from the protein production can, however, be feed directly to the biogas plant, also pointed out earlier in Figure 2.

To move the use of beet-tops and straw to an even higher cascading level, as far as gas yield, the protein extracted beet tops can be mixed and stored as silage together with residual straw, as the straw stabilizes the beet tops whereby the gas yield increases, as opposed to a separate storage and silage. Besides this, the green beet tops will soften the lignin content in the straw, whereby microorganisms within the reactor tank have better access to the hemicelluloses (Møller and Nielsen, 2016; Lybæk et al., 2020). Data regarding the use of straw for the production of wax and furfurals, as well as pretreatment costs and gas yield etc. connected to the use of residual straw at the Abed Biogas Plant, are exemplified in Figure 4.

Applying any further steps in the cascading chain could be to produce second-generation bioethanol with the use of enzymes, which however require the implementation of additional technology and facilities. Such extraction-activity should however happen before any residual substrates are feed to the biogas plant as feedstock. Another option is to include additional types of biomass residues from the local community than those presented previously. This could for example be forest wood waste for the production, e.g., polymers, which again will require implementation of new advanced technology and facilities. Besides this, methanization of CO₂ can be applied if sufficient quantities are available, using hydrogen (H₂) from the production of wind and solar energy, and hence strengthen a more integrated energy system within Region Zealand. The latter suggestions require highly advanced and expensive technologies but could be viably in a later stage of the production of bio-products/building blocks.

Thus, the important aspect here is to apply a method, which step-by-step increases the use of side streams, and constantly seeks to upcycle biomass residues to bio-product and achieve higher value materials and energy. The extraction activities must be applied before other types of activities, as for example biogas production. Thus, the hierarchy of using the biomass residues for bio-products must thus be prioritized, before using it as feedstock for the production of biogas. Moreover, biomass residues from within the local community should, hierarchically, be utilized before “imports” of other types of biomass from outside the area to enhance the circularity within the local community. Finally, priority should be given to develop internal CBE processes, which the existing biogas technology can enable and sustain, before later on emphasizing on development of processes that entails implementation of more advanced and costly technologies and facilities.