What should we do with CO ₂ from biogas upgrading?

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Introduction
Climate change is the most well-known environmental problem globally, contributing to the generation of extreme weather events and irreversible effects on ecosystems [1].The World Meteorological Organization has issued a warning that the probability of surpassing a 1.5 • C rise in global temperatures compared to pre-industrial levels within the next five years has increased [2,3] necessitating further urgent action.International and local strategies intend to reduce greenhouse gas (GHG) emissions (e. g. [4] and [5]) but additional efforts might be necessary.According to the International Energy Agency (IEA), carbon capture, use, and storage (CCU and CCS) technologies are essential in net zero GHG emissions strategies [6].The IEA [7] also points out that CO₂ can be a valuable commodity in the future due to its capacity to provide products such as fuels, chemicals, and building materials.Indeed, some carbon-based products are essential to cover human needs and are thus difficult or even impossible to decarbonize.Hence, society requires defossilization rather than decarbonization [8] to reduce the dependency on fossil fuels and mitigate the climate crisis.
Biogenic sources of CO₂ can potentially support bioeconomy but are not commonly explored.Biogas upgrading, for instance, is a highly concentrated source of CO₂ with a potential increase at regional and country levels [9,10].Biogas upgrading technologies separate the CO₂ from raw biogas to deliver a pure biomethane flow with high energy content.Although this practice resembles carbon capture technologies, CCU is not common practice in biogas systems, and CO₂ is usually released into the atmosphere because it does not represent an increase in net climate change due to the use of short-life materials [11].The implementation of CCU in biomethane plants requires information on how it can influence biogas systems considering the variation of geographical location and scales of biogas plants [10].Moreover, using biomethane production as a source of CO₂ will be increasingly relevant in the future, considering, on the one hand, the demand for renewable carbon and, on the other hand, the demand and potential for increasing biomethane production.Through the REPowerEU strategy [9], the EU aims to increase the energy sector's resilience and reduce the dependence on fossil fuels, partly by ramping up biomethane production from 3.5 to 35 billion cubic meter (corresponding to about 350 TWh) by 2030.This strategy would simultaneously increase the availability of biogenic CO₂.
The sustainability of CO₂ utilization options can be assessed in different ways.Previous approaches to asses CCU options have selected criteria to evaluate the development of chemical reactions to synthesize substances from CO₂ [12] and the technical deployment of alternatives [13,14].Furthermore, some researchers highlighted the importance of harmonized methods for the techno-economic and environmental assessment of carbon capture and CO₂ conversion technologies to offer comparable results among different studies for technological development [15][16][17][18].Hence, techno-economic assessments (TEA) [17] and life cycle assessment (LCA) guidelines [16,17,19] for CCU have been elaborated for the assessment of technologies.Moreover, the multi-criteria analysis (MCA) framework offers the possibility to integrate qualitative and quantitative indicators from diverse disciplines for decision making [20][21][22].For instance, Chauvy et al. [13] employed an MCA to evaluate CCU alternatives using key performance indicators from earlier assessments in three criteria: engineering, economic, and environment.The MCA included two sets of different weights, one for the relative importance of the indicators and the other for the criteria for obtaining a ranking of the alternatives [13].The authors reflected on the limited data available to assess the alternatives, but using two levels of weighting made it difficult to identify points for improvement.The method includes valuable indicators for the available technologies, yet it does not consider indicators for potential integration with sources of CO₂ or a stakeholder perspective.Zhaurova et al. [23] recognized the limitations of using complex weighting methods to assess CO₂ alternatives and tested the assessment for cement production.The authors mainly used the principles of safety, reduction of CO₂ emissions, and economic viability to propose indicators of assessment using simple equal weighting due to its simplicity and transparency, without including stakeholder perspective.Nevertheless, the authors highlight that they based the study on the scale, purity of CO₂, and process emissions that are characteristic of the cement industry [23], making the results inapplicable to biogas systems.
The development of CCU requires integrated assessments for a future implementation [24], along with aspects for a feasible interconnection between CO₂ producers and users.This aspect is relevant, considering that CO₂ is commonly seen as a waste flow in isolated systems and requires innovative technologies or links with external actors for its valorization.The MCA framework developed by Feiz [25] includes a collaborative process to deal with the active participation of stakeholders during its application in order to provide relevant information to support decision making instead of weighting and scoring options [21,26].Feiz [25] emphasized that this method becomes relevant for the use of waste flows in industrial applications like biogas systems.According to the author, the method prioritizes the discussion of the performance and trade-offs of waste flows utilization rather than a ranking due to the inherent complexity of this practice.Moreover, the methodology proposes the inclusion of uncertainty evaluation to determine points that require further research for its implementation.
Therefore, this research aims to identify and assess possible CO₂ utilization alternatives for possible integration with biomethane production, employing an MCA framework.The MCA framework is developed in this research to incorporate stakeholders' perspectives, using indicators for implementing CCU in biomethane production systems.Due to the novelty of the CCU in biogas upgrading, an uncertainty analysis is also included in the research to identify possible improvement points and research gaps that will require further learning.

Methodology
This research follows the soft MCA framework developed by Feiz [25] and further elaborated by Lindfors et al. [27].The authors used a framework consisting of five steps (Fig. 1), including i) definition of the goal, ii) identification of alternatives, iii) definition of a multi-criteria framework, iv) assessment of alternatives, and v) interpretation of results.Even though these five steps can be recognized in other MCAs, Feiz Fig. 1.Overview of the literature review and multi-criteria analysis methodology.

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[25] and Lindfors et al. [27] emphasize the third step, the definition of the multi-criteria framework, where the criteria, key areas, and indicators are defined with the active participation of stakeholders.
A literature review was incorporated into the MCA methodology to identify potential utilization alternatives of CO₂ and the most representative assessment criteria.The review focused on indicators apart from technical aspects and mature technologies.The results of this step were shared with the stakeholders to allow active participation during the process.

Literature review
A literature review was used to identify scientific publications that involve the assessment of carbon capture and utilization options.This literature review had three main objectives: i) to identify available carbon dioxide utilization options; ii) to identify the most used criteria in research to assess the utilization options; and iii) to obtain data for assessment.
The literature review was performed in the Scopus search engine using the keywords carbon capture and use, carbon capture and utilization, CCU, and carbon dioxide valorization for the title, abstract, and keywords of articles and review papers published until 2021.A first exclusion criterion was required for articles where the CCU abbreviation stands for other phrases.A second exclusion criterion was employed for duplicated papers, conference papers, or book chapters and papers not published in English.After that, a screening of the abstract of the papers was made to exclude the articles not relevant to this research.The exclusion criteria included publications describing and assessing only technical parameters of CO₂ chemical conversion or limited to capture or storage.Moreover, some additional sources were manually incorporated, employing a snowball method by looking into relevant publications on the reference lists of the papers [28,29].Additionally, other sources known by the authors were included to obtain a representative sample of publications to identify relevant utilization options and criteria for evaluation.The manual inclusion considered outcomes from international agencies, project reports, or other scientific articles.

Multi-criteria analysis
Multi-criteria analysis (MCA) integrate qualitative and quantitative aspects for decision making in complex problems [22,30].The methods can vary from a qualitative assessment of criteria, or soft MCA, to a complex numerical weighting of the criteria, or hard MCA, depending on the availability of resources, time, and information [21,22,31].Hard MCAs focus on the interpretation steps with different methodologies allowing a ranking or aggregation of results [21,22] using criteria weights to show their relative importance [20].In contrast, soft MCAs prioritize the definition of criteria for evaluation to allow transparency and simplicity for complex decisions where there is no optimal alternative [22].Soft MCAs allow the identification of aspects of the alternatives that require learning or development for an implementation, avoiding the loss of valuable information.
The first step of performing the MCA is the definition of the goal (Fig. 1).The MCA aimed to identify suitable alternatives for CO₂ utilization that support a sustainable biomethane production system.Environmental, economic, social, and technical criteria are employed to evaluate the alternatives.The former criteria correspond to the three dimensions of sustainable development used for decision making [32] and are broadly applied in comparing alternatives.The inclusion of technical criteria aims to evaluate the feasibility of the CO₂ of different technological routes in biomethane production [18], considering the early stage of technological integration.
The second step, identifying alternatives, was performed based on the literature review and workshops with stakeholders from the biogas sector.The participation of stakeholders consisted of two workshops as part of a research project on the valorization of CO₂ from biogas production.Prior to the workshops, researchers and practitioners shared knowledge about CCU and experiences with its application in various meetings that also helped define the study's goal.Therefore, stakeholders' engagement was ensured by their active participation in the research project.The group of stakeholders consisted of experts from five companies representing different parts of the biogas sector, including production, distribution, technology provider, and municipal company.The first workshop (W1) performed in May 2021, intended to present the results of the available CCU alternatives and criteria of assessment from the literature review.During W1, a preliminary framework of MCA was presented to stakeholders, including possible criteria, indicators, and scales for the assessment.To facilitate the assessment, groups of alternatives were presented to the stakeholders.During the second workshop (W2), performed in September 2021, the indicators and groups of alternatives were revised and refined with the help of the stakeholders.
The third step involves defining the relevant criteria, key aspects, and indicators for the assessment.The criteria were defined from the literature review and stakeholder participation to provide relevant information for decision making in the biogas system.This also allowed us to define the key aspects, indicators, and scale for assessment.
The fourth step, the assessment of alternatives, was performed based on information gathered through the literature review and the participation of stakeholders.Here, a qualitative uncertainty assessment was included based on Lindfors et al. [27] and Feiz and Ammenberg [26] for each result.The methodology suggests assigning a simple three-level scale (*, **, ***) from high to low uncertainty.Therefore, a low certainty was assigned when the literature review showed a high variety of results, poor information on alternatives with biogas upgrading as sources of CO₂, and low knowledge regarding the alternative among the workshop participants.For instance, a high certainty was defined for consistent results among the literature and information obtained from real cases.
The fifth step, the interpretation of results, was made based on horizontal or soft MCA interpretation [22,33].For that reason, it is limited to a qualitative interpretation of results, avoiding weighting of different criteria, and providing information for a potential incorporation of this practice of CO₂ utilization in biomethane production systems.

Literature review
The literature search resulted in 2242 scientific articles after a first exclusion.The exclusion of duplicates, conference papers, book chapters, and publications not in English as a first language gave 1434 articles for the literature screening.Screening through the title and abstract of the papers gave a total of 144 scientific articles relevant to this research; in addition, 19 articles were incorporated manually due to their relevance to the topic.The 163 articles were considered a significant sample for the literature review.Biogenic sources of CO₂ considered for assessment were bioethanol, biomass combustion, biogas upgrading and combustion, and gasification-based biofuels, of which 25 studied biogas.Most articles considered heat and power plants using fossil fuels or industries like cement, iron and steel, oil refineries and petrochemical industries, and ammonia plants.

Criteria of assessment of CO₂ utilization alternatives
The literature review showed different types of frameworks and criteria for the assessment of alternatives for CO₂ utilization.Some studies mentioned, for instance, the 3E performance framework, which includes i) engineering, ii) economic, and iii) environmental health and safety performance [13,[34][35][36][37], using some key aspects for criteria.For instance, Pan et al. [35] incorporated a life cycle approach for the economic and environmental criteria of the 3E framework using LCA and S.S. Cordova et al. life cycle costing (LCC).Another study used a 4 A framework that included i) availability, ii) acceptability, iii) applicability, and iv) affordability to evaluate the security of supply [38].Some studies also mentioned optimization or network integration to minimize costs or maximize the use of CO₂ [39][40][41][42][43][44][45][46][47].Therefore, the main indicators employed for the methodologies were summarized as part of the literature review.
Concerning the assessment indicators, the most common in the literature review were categorized and grouped according to technical, economic, environmental, and social criteria.Fig. 2 shows the indicators mentioned five or more times in the literature, while other indicators are presented in the supplementary material.The figure also shows the number of mentions of an indicator to evaluate alternatives using CO₂ with biogas as a source.In Fig. 2, similar indicators were arranged into big groups due to the variation of methodology.Profitability, for instance, was grouped with indicators like return on investment (ROI), return on equity (ROE), annualized profit, an account of costs versus sales, profit penalty, and internal rate of return (IRR).Other profitability indicators, like net present value and payback time [48], were presented in different categories due to the high number of mentions in the literature and because the methodology was consistent.
In the technical criteria, energy aspects were prominent in the literature (39 mentionssee Fig. 2), including energy use, efficiency or storage, heat integration, and possible energy retrieved by the product.In the biogas sector, energy was also the most common technical aspect for assessment, as mentioned in nine publications.Energy and exergy analysis is widely implemented for a techno-economic assessment due to the high energy requirement in conversion pathways that could increase the reliance on fossil fuels if renewables are not available.Moreover, technical maturity level (TRL) based on Horizon 2020 program from the European Commission and the US Department of Energy [19,49,50] was a concept mentioned in technical assessments (15 mentions).The TRL proposed for the Horizon 2020 program [49,50] consists on a nine-level framework to rank innovations and grant funding for research and development.Levels 1-3 indicate the basic idea, technology concept, and experimental proof.Level 4 indicates validation on a laboratory scale, and Levels 5 and 6 refer to validation and demonstration of the technology.Finally, Level 7 includes a prototype demonstration, Level 8 a complete system application, and Level 9 a complete technological system in an operational environment, which means a fully commercial application.Zimmermann and Schomäcker [18] suggest determining the indicators for assessment based on the TRL to make fair comparisons due to potential issues with data availability of early-stage technologies.Additional relevant technical aspects are also related to the quantity of CO₂ that can be used per alternative (15 mentions), but also how much of that CO₂ is converted (18 mentions).
Regarding the economic criteria, costs, including capital and operational costs, as well as indicators related to economic gain, were also evaluated in 42 publications (Fig. 2).In some cases, the costs were shown per kilogram of product, also called levelized costs, to be consistent with the levelized cost of energy (LCOE), which is commonly used in energy production plants [51,52].A similar indicator called price competitiveness, product price, or minimum selling price was also used to estimate the minimum price on which the product needs to be commercialized to ensure a breakeven over the project lifetime.The former indicator allows for evaluating the difference in the final price with products on the market.Another mentioned aspect was related to CO₂ costs expressed per unit of mass of CO₂ to evaluate a possible economic penalty compared with carbon credits or the price of CO₂ in the market.This group includes CO₂ avoidance costs (e.g., [53][54][55]), willingness to pay (e.g., [56]), costs of capturing, and costs per unit of utilized CO₂.Moreover, the market size was also mentioned in the literature to identify the future demand for products.This aspect includes other criteria like potential market, business cases, and revenue potential of the market size that share the same objective.
Among the environmental criteria, life cycle assessment (LCA) was mentioned in the literature to evaluate possible environmental impacts in the life cycle and compare technologies and products.The methodology includes impact categories, of which climate change is the most used.Different methodologies were identified for climate change impacts, as noted in previous research [57].Even though there are standardized methodologies [58] and guidelines (e.g.[17,19,57]), there are some variations in the goal and scope and methodological choices that decrease the comparability of the studies.Other methodologies for climate impact include climate return of investment [59], emission saving potential [52], total GHG reduction potential of the emissions considering the product demand, carbon balance, global warming ratio, CO₂ not produced, and levelized GHG emissions [60], among others.Fig. 2. Common indicators for the assessment of CO₂ utilization options and number of mentions in the literature review (criteria with five or more mentions).

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Regarding other types of environmental impacts, the indicator related to the use of fossil fuels and other non-renewable energy sources was also mentioned in the literature to identify if CO₂ utilization requires fossil fuels at any point of the life cycle.Moreover, some publications considered land occupation [61] and water use [61][62][63][64][65], which can be relevant for algae or alternatives that require hydrogen as raw material for electrolysis.The last two indicators could be more relevant in geographical areas where water or land resources are limited.
Social indicators were less common in the assessment of CO₂ utilization options.Some key aspects mentioned in the literature are public acceptance, health and safety, and policy.Public acceptance is related to attitudes and perceptions of products, processes, or industrial plants [66][67][68].Lutzke et al. [69] highlight that consumer acceptance of products from CO₂ utilization is vital to creating and maintaining a market.Health and safety were used mainly to complement comparative studies, including potential environmental risks and hazards, public health, and work-related issues due to product handling [13].Arning et al. [67] mentioned that the perceptions of health and sustainability risks of the alternatives and environmental awareness influence public acceptance of CO₂-based products.Meanwhile, Fraga and Ng [38] assessed acceptability as dependent on life cycle emissions and storage time in the final product.Policy was part of qualitative studies of the potential programs, roadmaps, investment frameworks, and research and development support.Furthermore, when the study considered biogas as the source, policy and health and safety were evaluated.For instance, Meylan et al. [70] analyzed the European Energy Directive in power-to-gas systems, and Peters et al. [71] qualitatively analyzed safety and health regarding product handling.
The technical, economic, social, and environmental criteria described in the literature are presented in the supplementary material.

Alternatives for CO₂ utilization
Regarding the CO₂ utilization alternatives, Fig. 3 shows a summary of the different options assessed in the literature with high technical maturity.Synthetic methanol (70 mentions) and methane (43 mentions) were dominant in the literature review.Likewise, these alternatives were also the most dominant when biogas was used as the source, with 9 and 14 mentions, respectively.One common benefit of producing methanol and methane is their versatility in serving as fuels or as chemical intermediaries of valuable compounds.Methanol is commonly obtained by steam methane reforming and is used for the production of other chemicals like acetic acid, dimethyl ether, and formaldehyde [37], but it can also be used as fuel or a hydrogen (H 2 ) carrier for transport [72].Possible routes to produce methanol from CO₂ include catalytic hydrogenation, electrochemical conversion with water oxidation, and reverse water gas shift.In connection to biogas plants, methanol can be produced with both the methane and CO₂ fraction [73,74] using dry reforming or only with CO₂ using catalytic hydrogenation [73][74][75] that can also incorporate heat recovery to improve CO₂ capture [76].In case raw biogas is used to produce methanol, it might require purification of impurities like H 2 S to avoid catalyst poisoning [74].Other routes are being investigated and have lower technical maturity [60].Methane can be produced by the Sabatier reaction, where CO₂ and H 2 react with the presence of catalysts.This process is known as power-to-gas because it can be used to convert electricity, via H 2 from electrolysis, to methane [77].Other technological routes for methane include solid oxide electrolysis cells [78], photocatalytic conversion [79], and biological methanation [80], like microbial electrolysis cells [80,81].In the biogas field, biological methanation is possible within the anaerobic digestor or with an external reactor [81,82], and it is considered to be flexible for small-scale applications compared to catalytic methanation [75].
Moreover, the production of construction materials and aggregates like mineral carbonates (31 mentions) or concrete curing (7 mentions) was mentioned in the literature.The production of mineral carbonates requires a source of sodium or magnesium to form carbonates with CO₂.Sources of minerals are in the form of silicate materials [83], although waste like fly ash from coal combustion or waste incineration, steel slags [84], or brine can be treated [85,86].The products can be used as aggregate materials for construction, like light blocks or isolation materials [87], or if the product contains high amounts of silicon dioxide, it can be used to substitute cement [84].In the biogas sector, innovative technologies combine biogas upgrading with CO₂ sequestration in minerals in a single step using sodium hydroxide as a solvent [88].
Other products were mentioned in the literature, including polymers, alcohols, dimethyl ether, dry ice, waxes, syngas, urea, and other fuels, but only a few considered biogas as a source of CO₂.Polymer production could absorb high quantities of CO₂ in the future [7].Nevertheless, von der Assen et al. [57], Fernández-Dacosta et al. [89], and Otto et al. [12] mentioned that the most studied routes for polymer production, and with higher technical maturity, are still dependent on oil.Syngas was also assessed in the literature (11 mentions) but is not usually sold on the market [14].Syngas can be used as building blocks in chemical processes for producing methanol, dimethyl ether, and other fuels.Ethanol and butanol, for instance, were mentioned as products from syngas biological fermentation or catalytic conversion with biogas CO₂ as the source [74].Dimethyl ether can also be obtained from syngas, with methanol as an intermediate product [89].Dry ice was assessed by Horschig et al. [90] as a co-product of raw biogas cryogenic upgrading and liquefaction and compared with the production of methanol by CO₂ hydrogenation, and the production of waxes and other high-value products via Fischer-Tropsch (FT).In FT, alkane products compatible with liquid fuels or hydrocarbons with lower or higher molecular weight can be produced from CO hydrogenation with the help of catalysts [71,75].In biogas systems, FT has been studied in different settings with raw biogas, CO₂ from biogas upgrading, or flue gases from biogas CHP plants, and in combination with additional hydrogen if required for a desired H 2 :CO ratio.Regarding urea production, Ghavam et al. [91], for instance, investigated the environmental impacts of ammonia production from black fermentation and anaerobic digestion.

Multicriteria analysis 3.4.1. Identification of alternatives
The high number of available CO₂ utilization alternatives required some filtering criteria to narrow the number of alternatives for assessment.The first filtering criteria was the maturity level of the technologies (TRL).Zimmermann et al. [17] highlighted that a high maturity level also decreases the data uncertainty and allows a higher understanding of the alternatives and their markets and costs.Hence, higher maturity levels suggest that the technology is likely to be applied in the short term and that more data is available for assessment.In this study, technologies with Level 7 [13,24,87] were considered as potential short-term alternatives for biomethane production.Therefore, the excluded alternatives with low maturity were acetic acid [13], specialized chemicals and high-value waxes [13,92], dimethyl ether [13,93], carbamic acids [13], formic acid [13,[92][93][94][95], and succinic acid (TRL assumed from specialized chemicals).
Furthermore, a second filtering criterion excluded alternatives that require fossil-based raw materials or are connected to the fossil supply chain [10].In this sense, enhanced oil recovery (EOR), polyols, polymers, and urea were excluded.While it is evident that EOR relies on oil extraction, mature technologies for polymers and polyols production require epoxides, which are obtained from fossil fuels [12,57,89].Urea is a carbamic acid whose production route has a high maturity level but an indirect dependency on fossil fuels [12].Urea production typically occurs near units producing ammonia, conventionally obtained from steam methane reforming of natural gas or gasification of coal [96].Ammonia plants produce CO₂ as a by-product used in the same facility [97].
The filtered alternatives were grouped according to the literature and validated in workshops.The groups include fuels, chemicals, building materials, and direct use.Fig. 3 includes examples of the alternatives that could be applied in biogas production.From the groups, some representative examples were chosen for assessment, such as methane, methanol, FT fuels, mineral carbonation, liquid CO₂ for the food industry or refrigerant, CO₂ for pH control, and CO₂ for yield boost in greenhouses.

Definition of the multicriteria assessment framework
This section includes defining the indicators and parameters for each criterion: environmental, economic, technical, and social.A summary of the indicators and definition of the scale for assessment is included in Table 1.For each key aspect, a three-level scale was defined considering the relative performance of each criterion in the biogas system ranging from good to poor, and moderate as intermediate scale.
The environmental criteria include key aspects like net reduction of climate impact, delay in CO₂ emissions, and other environmental impacts.Besides being the most assessed impact in the literature review, climate change is also of importance to biogas producers due to a possible competitive advantage compared with other renewable fuels Regulations and standards to meet [10].In addition, from a systems perspective, substituting products aims to reduce the overall CO₂ in the atmosphere by reducing oil and gas extraction from the ground.This indicator is independent of the possibility of delaying the CO₂ emissions that some products could provide.
For that reason, a second aspect was introduced that includes the potential storage of CO₂ given by the product's life.The third aspect intends to provide information regarding additional potential impacts that can arise with the application of CCU.This information can also be used to increase research on the potential impacts and technical solutions to avoid them.The technical criteria include five indicators, namely, TRL, energy requirements, CO₂ uptake potential, sensitivity to impurities, and transport of CO₂.The TRL was used for the first filtering of options, but it was also incorporated into the assessment to provide decision makers with more information regarding the technology [19].The second indicator is related to energy requirements, which can be associated with purification, liquefaction, storage or hydrogeneration of CO₂, and production of the alternative.The energy requirement does not consider energy for CO₂ transport.In biogas systems, transportation of CO₂ could depend on the alternative and the plant setting and provides additional information about the requirement of collaboration with other actors or the internal use of CO₂.Hence, it is included as an indicator.Moreover, the uptake potential is related to the scale suitability for biomethane production.The too-large or small scale of the alternative requires more than one source to meet the CO₂; conversely, one single biogas plant requires more than one CCU alternative.Furthermore, the sensitivity to impurities provides decision makers insights of additional steps that might be required in the biogas plant for CO₂ supply.
The economic criteria include two aspects: capital and operational costs and possible market.Cost was the most mentioned economic aspect in both the literature sample and the workshops.Furthermore, the market size indicates the potential demand for the final product.Since some CO₂ products target a different type of consumer, biogas actors can benefit from larger market sizes to cover existing demand.
The social criteria include one indicator related to potential standards or policy that could introduce additional requirements in the technology applied or final product.Regulations compliance could increase costs or require additional permits before implementation.
According to the workshop discussions, the most relevant indicators are the size of the market, reduction of climate impact, and capital and operational cost.Those indicators provide some information to analyze a potential business case.For instance, the cost of implementing and operating CCU should be reasonable and in accordance with the potential climate impact reduction.Other indicators like facility boundaries, profitability, and public acceptance were mentioned.The facility boundaries indicator was considered of low relevance because the stakeholders are open to collaborating with other companies that are interested in sustainability aspects, and thus this indicator is directly dependent on the requirements of CO₂ transport.Profitability was suggested for defined settings with enough information about the product and the plant design.Public acceptance is also relevant to support the biogas narrative as an enabler of the circular economy and supporting sustainability.Nevertheless, there is little information regarding the acceptance of each product due to the lack of cases that include biogas as the source of CO₂.Also, the parameters for public acceptance vary in the literature, making it difficult to establish a scale that can provide relevant information to stakeholders.

Assessment of alternatives
Table 2 presents the results of the assessment for each indicator and alternative.The information was obtained from the literature review, stakeholder discussions, and additional relevant literature.Detailed motivations for the results can be found in the supplementary material.A color scale is used to visualize the result, with green representing + ++ , yellow + +, and red + .
The literature presented some variations, including the difference in system boundaries, biogas upgrading technology, CO₂ and energy source, and level of detail in the models.This variation was dominant in indicators like net reduction of climate impact, other environmental impacts, energy requirements, and capital and operational costs.For these indicators, a comparative approach was required between the alternatives from the information available in the literature.In some cases, the information was available in relation to the final product or the CO₂ source, which reduced the certainty of results.
The uncertainty evaluation was based on Section 2.2; detailed information can be found in the supplementary material.A low certainty was assigned when the literature review showed a high variety of results, poor information on alternatives with biomethane as sources of CO₂, and low stakeholders' knowledge regarding the alternative (one star).Conversely, a high certainty was defined for consistent results in the literature and information obtained from actual cases (three stars).
To provide some examples, the transport of CO₂ has high uncertainty because it depends on the location of potential users.Methane shows medium uncertainty because it can be used in the biogas upgrading plant, but it requires hydrogen sources.In that case, one of the materials, CO₂ or H 2 , needs transportation to the production site.The CO₂ for pH control can be integrated into the biogas or water treatment plants, replacing mineral acids [98].However, it could require a pressurized system for transport and storage to reach other markets [99].This characteristic increased the uncertainty in transport and other criteria like energy requirement, costs, and market size.Moreover, the technical readiness level received an intermediate uncertainty because its definition requires information for each system component [19], which could vary for each biogas plant and the technological route of the alternative.The information and references used for the assessment step are presented in the Supplementary Material.

Interpretation
The interpretation was made following the methodology in Section 2.2., which is a soft MCA with a qualitative interpretation to allow for a comparison between the alternatives and the criteria included in the assessment.
Fig. 4 summarizes the indicators with good, moderate, and poor performance for each alternative.For biogas actors, it could be natural to produce additional methane from CO₂, but there are some considerations regarding possible additional environmental impacts and the requirement of additional energy for hydrogen production.Therefore, when renewable energy sources are not possible, other alternatives for CO₂ utilization could be more relevant.Results show that alternatives of both direct use and conversion pathways are possible, like biomass production, pH Control, mineral carbonates, and methane, which have poor performance in only three aspects.On the contrary, fuels and bulk chemicals have few indicators with good performance, which suggests a low opportunity for implementation in connection with biogas upgrading facilities.Liquefied CO₂ displays a good and moderate performance in most indicators.Nevertheless, regulations and a saturated market are indicators that could show high competition and make the alternatives less attractive for biogas upgrading plants.
The results also show a high uncertainty, even among the alternatives with relatively better performance (Fig. 5).Indeed, around 50% of the indicators have low certainty due to the lack of information and documented cases of biomethane upgrading plants as the source of CO₂.Liquefied CO₂ and pH Control are alternatives with a low number of indicators with low certainty because they have been in the market regardless of the source of CO₂.Another alternative with a low number of indicators with low uncertainty is methane production, which could suggest a high potential for this alternative.Moreover, besides a lower performance, bulk chemicals and fuels show a high uncertainty.
Furthermore, Table 2 also provides some information regarding the studied criteria for assessment.For instance, the indicator of other environmental impacts has low certainty and performance for the alternatives.This indicator is relevant to avoid trade-offs with other environmental impacts, but it is still under study for both CCU and with biogas upgrading as a source of CO₂.Another criterion with poor performance is the delay of CO₂ emissions, a downside for many CCU alternatives in contrast with storage options like mineral carbonates.Nevertheless, reducing CO₂ emissions is possible for all the alternatives due to the use of CO₂ and the substitution of products.Moreover, technical indicators like energy requirement and CO₂ uptake also have low performance and certainty, which could benefit from continuous technological developments in the CCU field and energy efficiency.

Discussion
The MCA framework was adapted in this research to identify alternatives and criteria for evaluating a potential integration of biomethane systems and CCU technologies.The literature review prioritized a sample of relevant research related to carbon capture and utilization due to the low number of initiatives in the biogas sector.This approach offers a broad perspective of the alternatives and indicators used in previous research in the CCU field.Therefore, the review could omit some studies focusing on a single technological-related path like power-to-x or conversion of green hydrogen.Nevertheless, the results showed a representative number of alternatives using these technologies.

Table 2
Assessment of alternatives and uncertainty evaluation, with + ++ : green; + +: yellow; + : red.Biogas systems offer highly pure and biogenic CO₂ that principally benefits the climate performance of all alternatives.Moreover, the possibility of substituting fossil-based products could reduce the system's emissions even more.Some approaches still highlight the importance of carbon capture as an option to storing CO₂ emissions and delay the climate impact.This approach reduces the importance of short-lived products that still rely on fossil carbon during production.CCU instead enhances the carbon cycle; this aspect is intensified in biogas systems with the natural capture of CO₂ in biomass growth.Differences in scale, geographical location, and high purity of CO₂ are relevant while assessing the alternatives.For instance, CO₂ uptake potential is a mentioned indicator in the literature, but it only considers the theoretical amount of CO₂ that can be used for the alternative.This research proposes considering the scale of sources and technologies for the assessment.Moreover, local demand for CO₂ can be a decisive factor in how it can be used [98].
Previous MCA studies' main objectives are to identify the alternatives of CCU with the most potential for future technical development (e. g., [12,13]).The integration of users and producers and the stakeholder perspective is rarely mentioned in the literature, which is vital considering that some CCU alternatives have already reached a high level of development.The inclusion of filtering criteria was also incorporated in this study.For instance, the dependence on fossil fuels was a relevant filtering indicator for the available technologies that could be incorporated into the vision of maintaining the renewable and green aspect of biogas solutions systems [10].Another filtering criterion for CCU technologies is the high level of maturity for mid or short-term implementation, which has also been considered in the literature (e.g., [13,87,100,101]).This means that many alternatives with lower TRL were omitted from the assessment and are still under development.In the future, some of those alternatives could be relevant for a potential integration with biomethane systems, like the production of proteins or fuels from algae [61,102] or formic acid [14].
The MCA presented in this study can be adapted to case-specific conditions to be used as decision support.For instance, the guidelines for techno-economic assessment recommend considering each component of the model to define the lower TRL level of the system [19].The CO₂ uptake and climate impact reduction could also benefit from  specific case studies.A specific location could provide information on the demand for CO₂ or sources of renewable energy in the area.For instance, hydrogen can be a bottleneck for many CCU options regarding climate impact, energy requirement, costs, and regulations, but its real impact is only possible to assess in a specific case.Moreover, the information on the case can also provide enough data to incorporate case-specific indicators like profitability and public acceptance.A more detailed model can also help to identify quantitative scales for assessing the indicator's good, moderate and poor performance.
The literature review suggests that a high number of studies focus only on the climate impact of the alternatives.Even though the debate and incentives for reducing climate impact are vital, it is still only one of the potential environmental impacts being considered.It is important to include other possible environmental impacts to avoid trade-offs by creating additional environmental impacts in the biogas system or compared to the product CCU substitutes.On the contrary, other positive environmental impacts could be reported.For instance, using CO₂ in greenhouses boosts plants' possibility of reducing environmental impacts due to less mineral fertilizers.Moreover, CCU could create impacts due to the requirement of additional materials and energy.
Furthermore, this research considers that anaerobic digestion produces biomethane as the main product and CO₂ as a by-product.Only the utilization of the CO₂ fraction from biogas upgrading was studied to have comparable alternatives.However, both methane and CO₂ can be used to produce syngas for alternatives like methanol or Fischer-Tropsch fuels and their derivates (e.g., [73,74]).The use of the carbon content of biogas could increase the yield of the product, potentially affecting the results in indicators like transport, costs, environmental impacts, and energy use.

Conclusions
CCU is a relevant process to mitigate the climate crisis that will affect future generations.There is high technological potential for using CO₂ as raw material for valuable products, yet only a few implemented cases exist.Alternatives of CCU from renewable sources of CO₂ , like biogas upgrading, are still underexploited, although a nearly pure stream of CO₂ has already been obtained.This research intended to identify CCU alternatives and provide an understanding to policymakers, practitioners, and researchers of what is required to implement relevant alternatives.
The MCA framework includes relevant indicators for integrating users and producers of CO₂.Results show that no alternative performs well in all the indicators and that there is still high uncertainty per indicator and in the alternatives.Nevertheless, it guides decisions regarding additional efforts for implementation when one indicator is impossible to meet.For example, methanation is a natural alternative for CCU in biogas upgrading that can increase the productivity of the biogas plant.Methanation requires renewable hydrogen sources, which might not be available in all cases and at all times, and thus other alternatives like biomass production, pH control, and mineral carbonation could look more attractive.
The high uncertainty in most indicators suggests that research is still required to assess biogenic sources of CO₂, like biogas.The use of soft MCA with uncertainty analysis helps to identify learning opportunities and direct efforts for implementing CCU that aggregated scores cannot provide.It can be the case of indicators with high and low certainty.For instance, in the case of liquid CO₂, the certainty that potential standards and regulations can hinder the application of CCU.Therefore, some research on solutions or directives could help to overcome this barrier and turn green CO₂ into a competitive alternative as a substitute for fossil based CO₂.
Moreover, innovative routes for bulk chemicals and fuels other than methane show poor performance and high uncertainty with CO₂ from biogas upgrading.In practice, poor performance could mean low feasibility in the short term.Possible changes could be required in the system boundaries, like a variation of carbon sources, such as the integration of other CO₂ sources or the methane of biogas, that was beyond the scope of this research.

Fig. 3 .
Fig. 3. Options for carbon dioxide use mentioned in the literature and in workshops (W).The number of mentions in the literature is shown in parentheses.

Fig. 4 .
Fig. 4. Number of indicators with low, medium, and high performance per alternative.