Potential for the valorization of carbon dioxide from biogas production in Sweden

Biogas solutions offer many advantages to improve sustainable development, but there is still untapped potential in its environmental performance. During biogas upgrading, CO 2 is separated from the gas to deliver a flow with high methane concentration and thus high energy content. In this practice, CO 2 is commonly emitted to the atmosphere without contributing to a net addition of climate gases because of its biological origin, being a missed opportunity for carbon capture. In this paper, CO 2 valorization is an option that has been evaluated using a qualitative and quantitative approach, taking Sweden as an example. Results showed that around 140 kt of CO 2 can potentially be captured and utilized from biogas upgrading, which can significantly increase in future scenarios. If CO 2 were turned into methane using power-to-gas technology, an additional 35% of biogas could be produced in the short term, meaning up to additional 0.7 TWh in 2020. By 2050, around 600 to 1600 kt of CO 2 could be available, depending on how well the biogas production develops and how much of the biogas is upgraded, resulting in up to 6.2 TWh of biomethane. The qualitative assessment suggested that only minor modifications in the upgrading process are required for this practice. Biogas actors are interested in getting involved in valorization projects that enhance their circular business and avoid carbon lock-in mainly to improve the environmental performance of biomethane. Moreover, the application of CO 2 valorization requires collaboration with different actors to integrate current CO 2 demand or innovative transformation technologies.


Introduction
The interest in the bioeconomy has increased strongly during the last years.Technical advances in production based on biotic resources biomass transformation and economic incentives offer the opportunity to potentially reduce environmental impacts and dependency on fossil energy sources (Sonnenberg et al., 2007).The biogas sector is one clear example because it has evolved from being an organic waste treatment option providing energy and nutrients (Fallde and Eklund, 2015;Winquist et al., 2019).Biogas is considered a good energy source for electricity or heat production due to the high calorific value of methane and can provide additional benefits when upgraded (Hagman et al., 2018;Dahlgren et al., 2019).Also, it can reach larger geographical areas when compressed or liquefied with a good environmental performance (Gustafsson et al., 2020).Furthermore, the semi-solid product of anaerobic digestion, or digestate, is an interesting source of nutrients when used as biofertilizer (Seadi and Lukehurst, 2012;Drosg et al., 2015).Moreover, biogas solutions can even contribute to some extent to all of the UN sustainability goals using organic waste as feedstock (Hagman and Eklund, 2016).
The benefits biogas can potentially provide have helped production in Europe grow to 193 TWh in 2019 (Sainz Arnau et al., 2021).Yet, the growth of biogas production in Europe has decelerated compared with the fast development up to 2015 (Brémond et al., 2021).In contrast, the share of biogas upgraded into biomethane has increased over the years (Sainz Arnau et al., 2021).The European Commission has declared a goal to increase biomethane production to 35 bcm by 2030 in the EU to aid a rapid, sustainable, and secure energy transition in the region due to the important need for imported gas (European Commission, 2022).A relevant application of biomethane is in gas vehicles, where it easily substitutes for natural gas.For instance, in Europe, 25% of vehicle gas on average is renewable biomethane (NGVA Europe, 2022), while the corresponding figure for Sweden is 95% (Klackenberg, 2021b).Like many other European countries, Sweden has a large untapped potential for biogas production.Indeed, Westlund et al. (2019) proposed a goal to increase biogas production from 2.1 TWh in 2019 to 7 TWh in 2030, with total energy from renewable gases of 10 TWh.It has not been determined how much biogas will be upgraded, but according to Energigas Sverige (2021), the use of biomethane has rapidly increased during the last 10 years, and it is expected to continue.By 2020, around 65% of the biogas produced in Sweden was upgraded and used as a renewable alternative to natural gas, mainly in the transport sector (Klackenberg, 2021a).
Biomethane can potentially reduce GHG emissions and improve environmental performance compared with other energy carriers (e.g., Bargiacchi et al., 2018;Lyng and Brekke, 2019;Cignini et al., 2020;Gustafsson and Svensson, 2021;Gustafsson et al., 2021).Nevertheless, EU regulations and directives provide a narrow framework to assess environmental performance, and thus, only a part of the benefits that biomethane can provide during its production is reported.For instance, negative emissions could be achieved when other methodologies and systems boundaries are considered (Feiz et al., 2020;Gustafsson and Svensson, 2021).According to Gustafsson and Svensson (2021), the climate impact of liquefied biomethane is negative when the life cycle assessment incorporates system expansion, like substitution of mineral fertilizer by digestate, following ISO guidelines (ISO, 2006(ISO, , 2020)).Hence, additional strategies for GHG emissions reduction focused on biomethane production could provide biomethane an extra competitive advantage.
When looking into the biomethane production process, carbon capture technologies can reduce GHG emissions.During biogas upgrading, biogenic carbon dioxide (CO 2 ) is separated from the gas flow to deliver a high energy content gas comparable to natural gas called biomethane.Raw biogas commonly contains 50-70% methane (CH 4 ) and around 30-50% CO 2 and traces of other contaminants (Luo and Angelidaki, 2013;Ullah Khan et al., 2017).Existing biogas upgrading technologies commonly first remove small impurities from the gas and then extract the CO 2 , which resembles technologies for carbon capture.Since this CO 2 originates from the digestion of biomass, it does not contribute to the increased emission of climate gases.Therefore, its capture and further utilization or storage could contribute to reducing the overall GHG emissions of the biomethane production process.Particularly, carbon capture and utilization (CCU) concepts could potentially reduce GHG emissions in two ways: first, by capturing the CO 2 that is otherwise emitted, and second, by substituting fossil-based products and consequently avoiding the emission of fossil GHG (Xiaoding and Moulijn, 1996;Styring et al., 2011;Chauvy et al., 2019;IEA, 2019).
Moreover, the utilization of this CO 2 as raw material for different applications provides economic value to a waste flow.Traditional sources of CO 2 include chemical industries like ammonia and hydrogen production, but also biomass fermentation and underground deposits (Naims, 2016;Hansson et al., 2017;IEA, 2019).From them, biomass-based sources call attention to the bioeconomy for being a source of renewable or green CO 2 .The CO 2 can be used in direct applications like cooling systems, fire extinguishers, the food industry, and greenhouses (Dammer and Carus, 2019;Esposito et al., 2019;IEA, 2019;Zhang et al., 2020).In connection with biomethane production facilities, there are some implemented examples of CO 2 direct use in the Scandinavia region.One includes the biomethane Korskro plant, located in Denmark, where 16.25 kt of CO 2 is purified into food-grade CO 2 from a co-digestion plant (IEA Bioenergy, 2020).
Further innovations in CCU employ CO 2 as a raw material in the production of fuels, plastics, chemicals, and other products based on carbon structures (Xiaoding and Moulijn, 1996;Styring et al., 2011;Alberici et al., 2017;Aresta and Dibenedetto, 2020;Saeidi et al., 2021).Some techniques combine biogas upgrading with carbon capture in minerals to provide biomethane with high purity in a single process (Baciocchi et al., 2011;Baena-Moreno et al., 2018).Moreover, the conversion of CO 2 from biogas production into fuels or chemicals is possible through the Fischer-Tropsch process, CO 2 hydrogenation, or biological conversion utilizing only CO 2 or taking advantage of both the CO 2 and methane content of biogas (e.g., Ahmadi Moghaddam et al., 2015;Dimitriou et al., 2015;Deutz et al., 2018;Hernandez and Martin, 2018;Giuliano et al., 2019;Cuéllar-Franca et al., 2019;Rodin et al., 2020;Naresh Kumar et al., 2022).To provide a few examples, the Fischer-Tropsch process with a prior conversion of CO 2 into CO delivers compounds like diesel, methanol, and dimethyl ether, among other hydrocarbons (Dimitriou et al., 2015).This process can also use the methane fraction of biogas to produce syngas (a mix of CO and hydrogen) and increase the yield of the final desired product (Hernandez and Martin, 2018).In addition, hydrogenation can be applied for the use of CO 2 from biomethane production, where hydrogen is used to produce compounds like methane, methanol, and alcohols (e.g., Götz et al., 2016;Santosdos et al., 2018;Chein et al., 2021;Tozlu, 2022) or other chemicals using the CO 2 as a building block.Furthermore, additional alternatives for the integration of biomethane production and CO 2 utilization have been researched in recent studies.This includes, for instance, the production of formic acid (Baena-Moreno et al., 2020a) and urea (Baena-Moreno et al., 2020b).Formic acid is obtained from CO 2 and hydrogen and utilizing methanol and a tertiary amine, while urea can be obtained from ammonia CO 2 (Baena-Moreno et al., 2020a;Baena-Moreno et al., 2020b).
Methanation of CO 2 is certainly attractive for the biogas sector because it can increase the overall methane production.The conversion of CO 2 into methane is performed by the Sabatier reaction with hydrogen (Hashimoto et al., 1999;Müller et al., 2013) with the help of a biological (Voelklein et al., 2019) or metal oxide catalyst (Müller et al., 2013).This concept is commonly known as power-to-gas due to its connection with the electricity grid to obtain the required hydrogen enabling the chemical reaction (Götz et al., 2016).Some technologies for methanation are being studied for the biogas sector after upgrading (ex-situ) or inside the anaerobic digestion reactor (in-situ), reducing the energy consumption of the system (Lorenzi, 2017;Voelklein et al., 2019;Entesari et al., 2020).Regardless of the application of CO 2 , only a handful of projects have been initiated in the biogas sector, and there is a limited understanding of the total potential and what aspects could influence such projects.In Sweden, for instance, biogas plants have not incorporated CO 2 valorization in their processes when it could potentially improve the business case for biogas producers and contribute towards the 7 TWh goal in Sweden.
This research aims to provide an overview of the opportunities and challenges of incorporating CO 2 valorization in biogas solutions, using Sweden as an example.Therefore, it addresses the theoretical availability of CO 2 from biogas production in the present and future scenarios, including the effects of policy measures.Moreover, it highlights the requirements for incorporating its use inside current biogas systems and the potential benefits from this practice.To illustrate these benefits, it includes an estimation of the theoretical amount of additional biomethane that can be obtained from CO 2 through methanation.Hence, this study addresses the following research questions: • What is the current and future production potential for CO 2 from biogas in Sweden?• How are potential sources of CO 2 from biogas production distributed across Sweden and between different types of production plants?
• To what extent could CO 2 from biogas be turned into biomethane to improve the overall carbon conversion efficiency of biogas production?• What aspects could promote or detain the valorization of CO 2 from biogas production in Sweden regarding different upgrading technologies and actors' business considerations?

Methodology
This research methodology included a qualitative and a quantitative assessment (Fig. 1).Firstly, the quantitative analysis was performed to estimate how much CO 2 could be obtained from biogas production in Sweden in both present and future scenarios.The amount of CO 2 was then translated into the additional biomethane that can be obtained through methanation to improve the conversion efficiency of biogas production plants.Secondly, the qualitative analysis focused on the aspects that could promote or detain CO 2 utilization from biogas production in Sweden, focusing on the biogas system.This step included a literature analysis on potential technical limitations that could influence the application of carbon capture and use in biogas plants and characteristics of successful projects related to CO 2 utilization in the biogas sector.This study was complemented with workshops and interviews with actors of the biogas system to incorporate their perceptions of the factors that could influence CO 2 valorization in Sweden.

Quantitative assessment
The calculation of potential CO 2 in Sweden from biogas production for 2020 was made from the database gathered by the Swedish Energy Agency (Energimyndigheten).Whilst this research used the raw data, the processed data can be obtained in yearly reports from 2005 (Energimyndigheten, 2021; Klackenberg, 2021a).The database provides information reported by biogas upgrading facilities, including the total energy delivered per facility, type of substrate, upgrading technology, and geographical location.The information related to upgrading facilities was correlated with information regarding anaerobic digestion (AD) plants and geographical location, making it possible to estimate the methane content of raw biogas.In the case that two or more AD plants supply the upgrading facility, the methane content was a weighted average with respect to the total amount of energy produced.
With this information, the theoretical CO 2 production for 2020 per plant was estimated.First, the theoretical volume of biogas treated in each upgrading facility was calculated using the lower heating value of pure methane of 9.97 kWh/Nm 3 (Swedish Gas Technology Centre, 2012) and the fraction of methane before and after upgrading.The final biomethane purity was assumed based on the upgrading technology of each plant.A biomethane purity of 97% was assigned for upgrading facilities utilizing water scrubbers, membranes, and pressure swing adsorption, and 99% for plants employing chemical scrubbers (Adna-nOng et al., 2019;Lombardi and Francini, 2020).Even though some research suggests that upgrading technologies can reach higher efficiencies (Lombardi and Francini, 2020), a conservative approach was taken using the lower values from the literature.Moreover, the lower methane content assumed was 97% due to national vehicle gas regulations requiring less than 3% contaminants (Swedish Standards Institute, 2017).Other types of upgrading technologies were not considered in the study because they are not applied in Sweden (Klackenberg, 2021a).Furthermore, 1% of impurities was assumed for raw biogas (Schüwer et al., 2015;Andersson et al., 2021) and a CO 2 density of 1.7 kg/Nm 3 (Engineering ToolBox, 2018).Losses of methane or CO 2 during upgrading were considered negligible in this model.Finally, the results were used to analyze the distribution of CO 2 by geographical area, type of substrate, and upgrading technology.
The potential future CO₂ production for 2030, 2040, and 2050 was based on estimates of the potential future production of biogas in Sweden.Previous studies on theoretical and practical biogas production potential gave a low and high estimate for the years 2030, 2040, and 2050 for different substrates (Table 1).The potential substrates were grouped according to the literature (Dahlgren et al., 2013;Ekstrand et al., 2013;Tufvesson et al., 2013;Björn et al., 2016;Lönnqvist, 2017;Prade et al., 2017;Swedish Gas Association, 2018;Klackenberg, 2021a).The substrates include the municipal solid organic waste (MSOW), sewage sludge from wastewater treatment plants (WWTP), manure, food industry waste, non-indirect land-use change (non-ILUC) crops, energy crops and industrial sludge.Table 1 also presents a differentiation between energy and non-ILUC crops, the first being crops with the purpose of energy generation (Lönnqvist, 2017), and the second includes agricultural residues without economic value or crops from formerly used land (Prade et al., 2017).Biogas from food industry waste was estimated by Lönnqvist (2017) from potential residues from meat, diary, brewery and other industries in Sweden.For MSOW and WWTP, the estimated population growth was taken into account (Statistics Sweden, 2021).
The potential volume of biomethane and CO₂ for future scenarios was calculated based on the substrates used in the different types of biogas plants and the average methane content of each type of plant according to statistics on Swedish biogas plants (Klackenberg, 2021a).The division of substrates per type of biogas plant (Table 2) was based on the same statistics report, with an increased share of crops digested at agricultural plants based on Westlund et al. (2019).
Besides the scenarios of biogas production, this study also incorporated the potential CO₂ production based on the share of biogas upgrading.Therefore, a low and a high scenario for biogas upgrading was used to identify the potential CO₂ production.The first scenario assumes that the share of biogas upgraded in 2020 (65%) remains constant in the future (Klackenberg, 2021a).This rate translated into the type of biogas plant corresponds to 69%, 92%, and 38% from wastewater treatment, co-digestion, and agricultural biogas plants, respectively.In contrast, the 100% upgrading scenario considers that a full upgrade is possible in the future.
A third alternative scenario for potential CO₂ utilization was incorporated in this research considering ranges of CO₂ capture.This aims to provide an overview of potential effects on strategies to incentivize CO₂ capture or the development and application of new technologies for carbon utilization.Currently, the capture and utilization of CO₂ from biogas production is not a common practice in Sweden, and hence, the range started with 0% of capture, assuming that no actions are taken in 2030, 2040, and 2050.The maximum capture potential was set to 100%, considering that current technologies allow this practice in biogas production.
Furthermore, to illustrate further effects in the utilization of CO₂ from biogas production and show the potential benefits of its valorization, the results from the different scenarios were used to estimate the range of extra biomethane that could be obtained through methanation.Methanation (or PtG) refers to CO 2 hydrogenation into methane (CH 4 ).The process is based on the Sabatier reaction, where 1 kg of CO 2 can produce 2.74 kg of CH 4 (Eq.( 1)).Due to the low reactivity of CO 2, the reaction requires the use of a chemical or biological catalyst (Rittmann et al., 2015;Su et al., 2016;Angelidaki et al., 2018).When applied in the biogas sector, methanation can be either ex-situ or in-situ.Ex-situ methanation refers to the conversion of CO 2 into methane after anaerobic digestion in a separate reactor, while in-situ methanation incorporates hydrogen into the digester, making additional upgrading redundant (Götz et al., 2016;Angelidaki et al., 2018).In both cases, the poor solubility of hydrogen in water can challenge a conversion beyond 90% (Voelklein et al., 2019).Nevertheless, some sources suggest that it is possible to achieve a total conversion of CO 2 (Vo et al., 2017;Hoppe et al., 2018).This research evaluates the theoretical potential of CO 2 valorization, and thus up to 100% conversion of CO 2 is considered.

Qualitative assessment
The qualitative assessment of the valorization potential of CO 2 from biogas included research on motivations, interests, and potential barriers from a literature review and the perspective of actors from both the biogas sector and the distribution of CO 2 in Sweden.
First, a literature review was done on technological aspects relevant to CO 2 capture from biogas production.Current technologies are designed to deliver a methane flow that complies with national standards for commercialization.Thus, the literature search aimed to identify possible modifications required to optimize CO 2 capture in biogas upgrading.Second, the assessment also incorporated an analysis of potential barriers or drivers for the valorization of CO 2 from biogas from the literature review and analysis of CO 2 capture and utilization cases.Since CO 2 valorization is not a common practice in Sweden, the analysis focused on relevant aspects that could further influence the application of this practice in the future, like scale, target market for CO 2, and motivations.Also, it included potential influences from CCUS looking into projects in Sweden in that field.The sources of information included a literature review and a search on social media; other projects were also acknowledged from workshops.
The research also included a series of workshops with relevant actors from the biogas system and interviews with representatives of CO 2 distribution in Sweden to complement the qualitative assessment.According to Ottosson et al. (2020), a recurrent interaction with sector actors is relevant to improving the learning process and maintaining a transdisciplinary approach in the research.The workshops were conducted on the occasions listed in (Table 3) with the participation of actors involved in biomethane production (A1, A2, A3, A4) and distribution (A1, A4), a technology provider (A5), and a municipal company (A4) in Sweden.Biogas producers, distributors, and technology providers that participated in the workshops also have active involvement in the energy sector in Nordic countries.The representatives involved were mainly in charge of research and development tasks for each company and thus have some background knowledge regarding Based on.a (Prade et al., 2017).b (Klackenberg, 2021a).c (Tufvesson et al., 2013).d (Dahlgren et al., 2013).e (Lönnqvist, 2017).f (Swedish Gas Association, 2018).g (Ekstrand et al., 2013); and.h (Björn et al., 2016).biomethane and CO 2 utilization.In addition, two guest actors were also invited to the last workshop.One of them was an additional technology provider (A6), and the second was a representative of a research organization with experience in bioenergy carbon capture and storage, also known as BECCS (A7).During the workshops, the participants discussed perspectives and potential drivers and barriers to the implementation of CO 2 valorization.The workshops took place at different times, which allowed the participants to share experiences with the development of the CO 2 utilization technologies in their business areas.
In addition, semi-structured interviews were performed involving two main distributors of CO 2 in Sweden (A8 and A9).A third actor was contacted but did not participate in an interview.The interviews covered aspects like CO 2 commercialization and transportation requirements, sources of CO 2 in Sweden, and perceived willingness to utilize CO 2 from biogas production.

Potential CO 2 production and distribution across Sweden and between different types of production plants
In Sweden, by 2020, 68 upgrading units produced approximately 1.34 TWh of biomethane.The calculation showed that the theoretical production of CO 2 from biogas upgrading facilities reached around 138 ktonnes in 2020.Moreover, the sources are scattered throughout the country.Fig. 2 shows the geographical distribution of the potential production of CO 2 grouped in 4 groups of provinces to maintain the confidentiality of the data.The first group includes the potential biomethane production from facilities located the provinces of Dalarna, Stockholm, Södermanland, Uppsala, Värmland, Västmanland and

Table 3
Summary of events for the qualitative assessment of valorization of CO 2 from biogas production.Fig. 2. Theoretical CO 2 production by group of provinces.

S.S. Cordova et al.
Örebro; group 2 includes Gotland, Västra Götaland, Östergötland provinces; group 3 includes Blekinge, Jönköping, Kalmar, Kronoberg provinces; and group 4 includes Halland and Skåne provinces.The northern provinces (Norrbotten, Västerbotten, Jämtland, Västernorrland, Gävleborg) were not included in the results due to low number of producers to maintain confidentiality of the data and to provide a clear visualization scale.The figure shows that potential sources of CO 2 are concentrated in the central and southern parts of the country.The location of producers can be relevant when identifying potential users of CO 2, any source of renewable energy for integration, or integration of CO 2 sources to provide higher volumes and provide economies of scale.
The distribution across the different types of plants displays a large diversity regarding the substrate and scale.Regarding scale, the potential production of CO 2 of individual upgrading plants ranges from around 36 t to 15,600 t per year.From the 68 facilities, 7 can theoretically produce between 5 and 15.6 kt of CO 2 , 32 between 1 and 5 kt, and 29 below 1 kt.Larger producers are linked mainly with co-digestion facilities and some sewage treatment facilities (Fig. 3).The number and scale of biogas plants and upgrading facilities treating agricultural waste are low, and hence the potential CO 2 accounts for only 1.4% of the total potential production.Moreover, water scrubber (43 facilities) with and without water recirculation is the most applied biogas upgrading technology.Less common upgrading technologies include chemical scrubber (12 facilities), pressure swing adsorption (PSA) (8 facilities), and membrane separation (5 facilities), as shown in Fig. 3.

Future potential of CO 2 utilization from biogas production
By 2050, the production of CO 2 can potentially increase from 0.58 to 1 Mt for the low and high scenarios, respectively, mainly from codigestion sources (Fig. 4).Moreover, this amount can increase up to 1.6 times if all the biogas in Sweden is upgraded.In future scenarios, it is expected that co-digestion plants are still the main source of CO 2 .In addition, the high scenario with 100% upgrading also incorporates industrial sources for CO 2 available for utilization.Fig. 4 shows that biogas production leads to a considerable increase in the potential production of CO 2 from 2030.The CO 2 is of biogenic origin, so it does not count for the net GHG emissions.Nevertheless, if actions for CO 2 utilization are applied, the values in Fig. 4 can be translated into avoided emissions, contributing to carbon neutrality or negative emissions from the biogas sector.
In addition, to illustrate the potential of CO 2 utilization, the total CO production was calculated for different capture rates (Fig. 5).The values in Fig. 5 show a high potential production of CO 2 .In future scenarios, even a minimum capture rate could be comparable with the total CO produced in 2020.This shows that strategies or the development of technologies for CCU could have a long-term beneficial effect in the biogas sector.
By transforming the CO 2 into additional biomethane (using Eq. 1), the theoretical results show that a potential increase of up to 35% in biogas production could have been possible if methanation was employed in 2020 (Fig. 6).In future scenarios, the share increases to 50%, being higher with full biogas upgrading.Therefore, this practice could theoretically help reach the suggested Swedish goal of 10 TWh from renewable gases in 2030 if all the biogas is upgraded.In practice, the full potential could be difficult to reach in the short term because PtG requires integration with electricity systems, especially renewable energy sources, to support the decrease of GHG emissions in the whole system.

Relevant technological aspects for CO 2 capture from biogas production
Biogas upgrading technologies resemble those employed in carbon capture, so CO 2 can be obtained with high purity, including few modifications.First, raw biogas can contain trace substances like hydrogen sulfide (H 2 S), volatile organic compounds (VOCs), siloxanes, and others in different compositions, depending on the substrate (Rasi et al., 2007;Arnold and Kajolinna, 2010;Piechota et al., 2013;F. M. Baena-Moreno et al., 2019).For this reason, it is expected that some valorization options of CO 2 require composition analyses, treatment, and monitoring before implementation.Also, additional CO 2 purification might be required, depending on the upgrading technology.The common technologies for biogas upgrading in Sweden are membranes, pressure swing adsorption (PSA), absorption through water (water scrubber), and chemical absorption (amine scrubber) (Bauer et al., 2013;AdnanOng et al., 2019;European Biogas Association, 2021).The generic implications of the technologies from the valorization of CO 2 perspective from the literature are summarized in this section.• Water scrubbers: This relatively simple process produces CH 4 with high purity (96-98%) at low maintenance costs (AdnanOng et al., 2019;Lombardi and Francini, 2020).First, H 2 S is removed from biogas, and then the CO 2 is absorbed by a water column at high pressure.Next, the CO 2 can be released in a desorption column by airflow under lower pressure conditions.The resulting CO 2 flow can contain an air mix but also up to 1.5% of CH 4 (Andersson et al., 2021).Thus, the technology might require modifications to avoid the air mixture and include purification steps.In addition, some upgrading plants do not include the CO 2 desorption column, which should be incorporated for carbon capture.• Pressure swing adsorption (PSA): This technology includes a pretreatment step for H 2 S removal and biogas drying.Then, CO 2 is absorbed employing materials like zeolites, activated carbon, polyimide, or silica in multiple adsorption tanks to deliver a CH 4 of 96-98% purity (AdnanOng et al., 2019;Lombardi and Francini, 2020).Common practices in Sweden leave the CO 2 flow with around 4% CH 4 to utilize as a heating source at the biogas plant (Andersson et al., 2021).Additionally, the CO 2 flow usually contains nitrogen and oxygen (Andersson et al., 2021); therefore, CO 2 utilization might require modifications in the plant to cover heating requirements and purification steps.
• Membrane: This technology uses membranes acting as dense filters that retain CH 4 while CO 2 permeates the membrane, and it can reach a methane purity of 96-98% (AdnanOng et al., 2019;Lombardi and Francini, 2020).This technology delivers CO 2 with a similar composition to water scrubbers, including a low methane slip (Andersson et al., 2021).This means CO 2 utilization might require additional purification steps but does not require changes in the upgrading facility.• Amine scrubber: This uses chemical absorption of CO 2 with chemicals like ethanolamine (MEA) or dimethylethanolamine (DMEA).These amines react selectively with CO 2 and can produce methane of more than 99% purity (AdnanOng et al., 2019;Lombardi and Francini, 2020).After being captured, the CO 2 is removed in a stripper by heat.Before the biogas enters the scrubber, it is usually purified, reducing the H 2 S and oxygen content (Andersson et al., 2021).Therefore, the resulting CO 2 flow has high purity and only requires drying steps for utilization.

Aspects for CO 2 utilization from CCUS projects in the Nordics
In Sweden, CO 2 capture and utilization are currently applied in a bioethanol plant.There, biogenic CO 2 is purified, liquefied, and distributed as food-grade carbonic acid from bioethanol production.Hansson et al. (2017) mentioned that the plant produces around 100 kt of CO 2 per year, and the company planned to increase production to 150 kt by 2021 (Aspenberg, n.d.).According to A8, bioethanol can be seen as a good source of CO 2 because it provides a stable flow during the year.Also, both A8 and A9 mentioned that a high-scale plant of CO 2 capture could potentially decrease transportation costs.Nevertheless, biogas producers agree that the scale of AD plants is lower than other sources of biogenic CO 2 , but they can provide continuous production of CO 2 with a high purity level during the year.Therefore, the valorization of CO 2 could require collaboration and engagement of producers and users to obtain the maximum benefit.
In the Nordics, one successful project of CO2 valorization from biogas production is the Korskro plant, located in Denmark.A5 mentioned that some drivers of the project included i) the availability to account for the reduction of GHG emissions and thus provide a competitive advantage over other fuels; ii) to provide a second revenue stream with CO 2 sales; and iii) to cover the required supply of CO 2 in the country.Indeed, Korskro covers 25% of the CO 2 consumption in Denmark, corresponding to 16.25 kt CO 2 /year (IEA Bioenergy, 2020).The capacity of this plant is comparable to some of the larger Swedish plants mentioned in the previous section.However, some of the Swedish food industry is partly covered by bioethanol production.Hence, other alternatives for CO 2 utilization could be required for further development and to increase business options.
Carbon capture projects in Sweden focus mainly on storage (CCS and BECCS).One example is the Preem CCS plant that will capture and export CO 2 from energy production to a storage site in Norway (Chalmers Tekniska Högskola, 2020;IOGP, 2019).Another example is Stockholm Exergi, which aims to capture around 800 kt of CO 2 per year from a combined heat and power plant (Winter Mortensen et al., 2019;Stockholm Exergi, 2019).Hansson et al. (2017) mentioned that by 2015, recoverable sources of CO 2 in Sweden reached 45 Mt, the pulp and paper plants and heat and power plants being the biggest sources of CO 2 with 68% of the total, which shows a high potential in the country.Moreover, BECCS can become relevant in Sweden, where almost 70% of CO 2 emissions come from biogenic sources, but the implementation of the technology is rather slow (Hansson et al., 2017;Lefvert et al., 2022).According to Fuss and Johnsson (2021) and Lefvert et al. (2022), reasons for the slow development of BECCS include high implementation costs and the absence of clear policies.Other aspects in the Swedish context, according to the authors, could be related to an interest in prioritizing the storage of biogenic CO 2 instead of fossil sources and in supporting biomass cascading systems.Both are difficult to address with BECCS because fossil and biogenic CO 2 are usually present in flue gases, and CO 2 storage inherently breaks the carbon cycle.Nevertheless, Lefvert et al. (2022) stated that actors involved in the field indicated that BECCS is a promising technology to achieve climate neutrality in Sweden, and governmental support can be expected soon in this regard.
In the biogas sector, the Research Institute of Sweden (RISE) made a techno-economic assessment (TEA) of CO 2 storage from plants with medium and large potential CO 2 (Andersson et al., 2021).The model assumed the sequestration site in Norway, named Northern Lights, to be the first concept in the Baltic Sea.Therefore, it included operational costs, for example, purification, liquefaction, land transport to temporal CO 2 storage terminals onshore, and ship transport for the storage site.Results showed that costs were related to the scale of the biogas plant, but in all cases, they were competitive with the current CO 2 market for CCS.The higher costs were related to the transport and storage of CO 2 .Also, some additional treatment and modifications are required for most upgrading technologies, including drying, purification, cooling, and liquefaction.Regarding climate performance, the authors suggest that CCS could provide climate neutrality in the biogas sector, and in some cases, negative emissions are possible.
Furthermore, the development of CCS and BECCS could positively influence CCU.Instead of being competing strategies for GHG mitigation, Fagerström et al. (2021) highlight that CO 2 capture projects can give some learning experiences for capture units in CCU and provide platforms for CO 2 value chains.In this aspect, biogas plants have a clear advantage for providing pure biogenic CO 2 , which, combined with its utilization, can support carbon closed-loop systems.Furthermore, actors A1-A5 agreed that utilization of CO 2 is more attractive than BECCS because, besides reducing GHG emissions, it broadens the spectrum of benefits from biogas solutions.

Aspects of CO 2 valorization from the actors' perspective
From the biogas actor's perspective (A1 to A5), CO 2 valorization can be relevant for improving the economic and environmental performance of biomethane.First, this practice can reduce the carbon footprint at the production stage.Climate neutrality or negative emissions provide a competitive advantage for biomethane compared to other fuels.Also, policies like the Renewable Energy Directive (RED) will require to explicitly include guidelines for the account of CO 2 reduction in products or fuels for commercialization.Nevertheless, current plants might require integration with renewable electricity sources, depending on the valorization option of CO 2 .For instance, Reiter and Lindorfer (2015) and Vo et al. (2018) observed a wide variety in the environmental performance of PtG technologies with different energy sources.Also, biogas actors expect that turning CO 2 into a product with economic S.S. Cordova et al. value can potentially provide an additional source of income for the biogas plant or cover expenses.Moreover, it is important from the biogas actor's perspective to get involved in products supporting the bioeconomy and replacing fossil-based products or avoiding carbon lock-in.According to Alberici et al. (2017), some technologies are still under development, and it is difficult to identify their economic performance in an early stage.Still, IEA (2019) mentions that it is possible to expect a higher willingness to pay for some CO 2 -based products, such as specialized chemicals.
According to A8, most of the CO 2 sold in Sweden is still produced from fossil fuels, particularly ammonia production.Moreover, ammonia is conventionally obtained from steam methane reforming of natural gas or gasification of coal (Peters et al., 2011), so its production could be affected by the natural gas market or geopolitical situations due to the dependency on imports in Europe.Nevertheless, an increasing interest in green CO 2 is expected.Furthermore, there are some aspects to consider before its commercialization.The demand for CO 2 requires a steady amount over the year, increasing before summer to cover beverage demand.In current practices, the requirement is to deliver food-grade CO 2 regardless of the application to avoid changes in logistics and equipment, which increases costs (A9).For that reason, in the short term, a food purity level is likely to be required for CO 2 valorization from biogas production.The European Industrial Gases Association (EIGA) provides standards for CO 2 in food industries and beverages (EIGA Doc 70/17).Besides limiting characteristics of impurities in the gas, the standards require a risk assessment for CO 2 from AD, especially when the plant uses waste substrates.This could make users hesitant to use this CO 2 source since it would require additional monitoring methods in practice.However, the Korskro case implies that purification steps are technologically available that would enable CO 2 utilization (IEA Bioenergy, 2020).Nevertheless, A5 to A7 mentioned that small-scale sources like farm-scale AD could require constant monitoring to reach food-grade CO 2 , which could increase operational costs.For that reason, its application could require the collaboration of different actors to divide operational costs or alternative uses for CO 2 .
Immediate markets for CO 2 in Sweden could include its use as pH stabilizers and building materials or in PtG technologies, as shown in the previous section.The former gives the possibility to boost methane production with alternative renewable energy sources.This practice offers the advantage of reducing costs related to biogas upgrading and increasing the methane yield of the plants.Nevertheless, according to Reiter and Lindorfer (2015), the current plants will require integration with sources of renewable electricity to reduce emissions.Nevertheless, according to actors in the field (A1 to A5), it can be expected that newer biogas upgrading facilities consider CCU in their operations to avoid emitting CO 2 into the atmosphere due to the benefits it can provide.

Conclusions
A combination of a quantitative and qualitative assessment was performed to evaluate the potential of CO 2 valorization from biogas production in Sweden.Both methodologies were used to provide a complementary overview of the status of CCU from biogas in Sweden to potential application in future plant designs or as additional steps during biomethane production.
• The qualitative assessment showed that current biogas production could theoretically provide around 140 kt of CO 2 .The available amount of CO 2 from biomethane production will increase considering the potential availability of substrates for biogas production and the ambitious goals of the country and the EU.By 2030, scenarios suggest that CO 2 potential will reach up to 400 kt per year and 1 Mt by 2050.Whereas current practices continue, this CO 2 will still be emitted into the atmosphere.Although it does not account for climate impact due to its biogenic origin, its capture and utilization could reduce the emissions and increase the contribution of biogas solutions to sustainability goals.• The sources of CO 2 from biogas production are spread over the country at different scales, leading to significant transport needs or a connection with renewable energy sources in the case of CO 2 transformation.The largest source of CO 2 from biogas corresponds to codigestion plants, a share that is expected to be maintained in the coming years.The current CO 2 market requires meeting food-grade standards and risk assessments for CO 2 from AD.This factor could influence the collaboration among actors to identify new business opportunities for CO 2 , especially for the regions with higher potential for CO 2 , like the central and southern parts of the country.Also, innovative applications could be required for medium and small sources of CO 2 , which together also account for significant CO 2 potential but where constant monitoring of food-grade CO 2 can significantly increase operating costs.• Methanation of CO 2 is attractive to biogas actors for increasing the plant's overall efficiency.In the short term, an additional 35% of biogas can be produced with this technology.The share can rise if biomethane is the final product of the AD plants, which could increase the revenue of biogas plants.The technology is still at an early stage of development.Thus, more studies are required to evaluate the technological pathways of methanation that deliver the best environmental and economic performance before its implementation, considering the characteristics of biogas plants.Its application will require the connection with renewable sources of electricity, which could not be available for all plants.Hence, technological developments in other alternatives for the use of CO 2 need to be considered in a future assessment.• There is an increasing interest in green CO 2 to reduce the dependency on fossil fuels, and technological advances suggest that CO 2 transformation processes can be applied in the future.The current CO 2 market requires food-grade CO 2 for commercialization, but innovative applications could have different purity requirements.Nevertheless, there are no technologies for the valorization of CO 2 presently installed in connection with biogas plants in Sweden, but actors expect that more biogas plants will incorporate CO 2 capture and utilization in the future.Taking a long-term perspective, it could even develop into an established "best available technology" (BAT) within the biogas sector.From the biogas actor's perspective, CCU becomes interesting given the possibility of improving the system's environmental performance, which in turn also can provide a competitive advantage over other renewable fuels.
The biogas sector is open to collaboration for different applications of CO 2 prioritizing options that avoid perpetuating the use of fossilbased materials or energy.
Technological innovations and the increase in the potential CO 2 production allow the application of CCU in biogas production systems.Nevertheless, future research should be done, including environmental and economic assessments of different CO 2 utilization alternatives.Also, considering that CO 2 is a waste, the implementation of CCU requires research on business models and policy required to sustain its implementation.Moreover, the location of the plants and scales can help identify future collaborations for the use of CO 2 with actors from the biogas sector and other types of industries that distribute and utilize CO 2 .Given that biogas solutions interact with socio-technical systems such as the energy, water, or waste systems, the influence of CO 2 valorization practices could be considered in future studies.1.The authors regret that an error was made in the selection of value of density of carbon dioxide employed for the calculation of its theoretical amount for 2020.Therefore, an underestimation of the theoretical production of CO 2 was reported.On the basis of this mistake, the Abstract was revised as "Results showed that around 160 kt of CO 2 can potentially be captured and utilized from biogas upgrading, which can significantly increase in future scenarios."In section 2.1., the 8th sentence of the second paragraph is revised as "Furthermore, 1% of impurities was assumed for raw biogas (Schüwer et al., 2015;Andersson et al., 2021) and a CO 2 density of 1.976 kg/Nm 3 (Pubchem, 2005)" In section 3.1.1., the 2nd sentence is revised as "The calculation showed that the theoretical production of CO 2 from biogas upgrading facilities reached around 160 kt in 2020", and the 2nd sentence in the second paragraph as "Regarding scale, the potential production of CO 2 of individual upgrading plants ranges from around 42 t to 18,240 t per year.From the 68 facilities, 8 can theoretically produce between 5 and 18 kt of CO 2 , 32 between 1 and 5 kt, and 28 below 1 kt."

CRediT authorship contribution statement
The 1st sentence in the second paragraph of the Conclusion section is revised as "The qualitative assessment showed that current biogas production could theoretically provide around 160 kt of CO 2 ".The values in Fig. 2, 3 and 4 are revised.In Supplementary material, the value of CO 2 density and example of calculation are revised.2. The authors regret that a typographical error was made in the 3rd sentence of the last paragraph of section 2.1.without affecting the calculations.The sentence is revised as "The process is based on the Sabatier reaction, where 2.74 kg of CO 2 can produce 1 kg of CH 4 ."

Fig. 3 .
Fig. 3. Potential CO 2 production from biogas in Sweden, divided by the size of biogas upgrading plants and type of feedstock and upgrading technology.

Fig. 4 .Fig. 5 .Fig. 6 .
Fig. 4. Scenarios for future production of CO 2 from biogas in Sweden, assuming either the same share of upgrading to biomethane as in 2020 or 100% upgrading.

Fig. 2 .
Fig. 2. Potential CO 2 production from biogas in Sweden, divided by the size of biogas upgrading plants and type of feedstock and upgrading technology.

Fig. 3 .
Fig. 3. Scenarios for future production of CO 2 from biogas in Sweden, assuming either the same share of upgrading to biomethane as in 2020 or 100% upgrading.

Table 1
Estimated potential biogas production per substrate type in Sweden for the years2030, 2040, and 2050.

Table 2
Assumed division of substrates on biogas plant types and methane content in produced biogas.