The definition of processes and system boundaries is a critical factor that leads to reliable results in LCA studies ^[1][64]. The expansion of system boundaries is usually found to be a limitation of LCA studies ^[2][9] or a suggestion for further research ^[3][34].

A highlighting point is the low number of gate-to-gate studies (four out of 38), representing an effort by sustainability researchers to expand system boundaries, especially with regard to upstream processes (cradle-to-gate). However, the literature still lacks complete LCA studies that consider downstream processes in their system boundary configurations (cradle-to-grave or cradle-to-cradle).

A limitation for expanding system boundaries in LCA studies is the limited availability of data about the downstream processes of CBE products ^[3][34]. In this sense, bio-based products, at their end of life, have many options to be reutilized, or, in the worst case, discarded. However, a few studies have considered these recirculation alternatives in their scope ^[4][5][24,47], mainly resulting from the difficulty of finding related data, which often leads to excluding some life cycle stages, such as the distribution, use, and the end of life of bio-based products ^[6][54].

The results of LCAs in a cradle-to-grave configuration seem more reliable than those using other configurations of system boundaries ^[7][52]. In this sense, the inclusion of all of the life cycle stages, especially at the end-of-life, is important because, since CBE products are named as circular, they should promote the reuse of their wastes, and this represents substantially less environmental impacts when compared to disposal in a landfill, for example ^[8][36]. Thus, assessing more CBE products from cradle-to-grave is highly suggested, because cradle-to-gate assessments need clarification about how wastes are recirculating. In contrast, cradle-to-grave assessments provide greater clarity about how products affect the environment throughout their life cycles.

In the literature reviewed, most LCA studies conducted midpoint assessments (34 out of 38), and only some studies assessed the endpoint (14 out of 38). Endpoint assessments are crucial for LCA because they use indicators that consider the areas of protection ^[9][76] where emissions are aggregated in indexes whose units are closely related to societal concerns ^[10][77]. This is different from midpoint assessments, whose indicators are ubicated in any place between the emission and the area of ^[9][76], and the use equivalences to be expressed, such as the acidification potential in terms of SO₂-eq ^[10][77].

The assessment of endpoints depends on the use of specific LCIA methods; the most used for this purpose are ReCiPe 2016 (endpoint categories) and Impact 2002+ (damage categories). Both methods were created for assessing endpoints globally and not for specific contexts (European or North American) ^[11][12][13][73,74,78], as are other popular LCIA methods of midpoint assessment. Moreover, both methods consider three common areas of protection:

• Human health: Expressed in disability-adjusted life years (DALY), this is a single measure that “combines the mortality and morbidity […] to estimate global disease burden and the effectiveness of health interventions” ^[14][79] (p. 10);
• Ecosystem quality: Expressed in potentially disappeared fraction (PDF) by m2/year (Impact 2002+) or simply species by year (ReCiPe 2016), it is the condition of an ecosystem compared to a reference state which can be from the past, present, or future (in a potential situation/condition) ^[15][80];
• Resource scarcity: Also known as resources (in Impact 2002+), it is expressed in megajoules (MJ), and refers to the consumption of non-renewable energy and minerals ^[13][78].

It is worthy of note that Impact 2002+ considers the use of climate change; however, its use is not precise because it is “the same category as the midpoint category Global Warming […] and is still expressed in kg CO2-eq” ^[13][78], so it is not aggregated with other midpoint categories, as with the other endpoints.

Furthermore, in any case, the number of endpoint indicators is smaller than midpoint indicators ^[9][76], and the complexity of calculating them is higher ^[10][77]. However, the assessment of endpoints is necessary for implementing the LCT in a CBE because bio-based products (as every product) and wastes directly impact human health ^[16][26], mainly due to the emissions and particulate matter they generate from potentially toxic substances along CBE processes of a biorefinery, for example ^[17][39].

Moreover, due to its close relation to agriculture, CBE has great potential to impact land use changes, so it can compromise ecosystems and jeopardize biodiversity. In this sense, Bartek et al. ^[18][62] argue that CBE can reduce damage to ecosystems and maintain flora and fauna, but only if sustainable agriculture is promoted and supervised in the first instance.

Since its publication in 2003, Ecoinvent has become the most widely used database for background LCI data ^[19][81] in CBE and other sectors. In this study, 66% of LCA studies (25 out of 38) used the Ecoinvent database to extract data for the LCI construction, demonstrating the predominance of the use of this database. It is likely explained by the significant number of datasets. Ecoinvent contains datasets to assess more than 18,000 activities on industrial and agricultural processes, considering the natural resources, emissions, effluents, products, and wastes produced ^[20][82].

Nevertheless, some studies reported a limitation regarding using the Ecoinvent database because of the generic character of the data ^[3][21][34,70] or the lack of datasets for specific products they were assessing ^[22][37]. The problem is that LCA results may be either overrated or underrated, reflecting, for example, the results of GHG emissions or overestimated savings ^[4][24]. These limitations occasionally led the authors to use data from literature sources or personal communications ^[23][60], because they find them better for context-specific LCA assessments.

As a solution to this problem, the development and use of national/regional databases stands out. This has been encouraged since the early 2000s ^[24][83]; from that time, some countries, namely Europe, the United States, Australia and Japan, have pioneered the creation of databases for LCA, but the difference in technological levels and production processes between these countries and the rest of the world have led to the creation of national databases incorporating the vision of the stakeholders ^[25][84].

When viewed in this light, using national/regional databases for LCI analysis may result in more realistic environmental impact assessments ^[6][54] because they are adapted to the reality of each country. This is unlike global databases that cover many processes using generic data ^[3][34], and thus add uncertainty to the results ^[21][70]. Nonetheless, the development of databases represents a cooperative endeavour of sustainability researchers, practitioners, and government agencies that claim to support these initiatives ^[25][84]. Public and private policies are closely related with regard to developing LCI databases and implementing an LCT for a CBE.

Currently, the literature on LCT for a CBE is concentrated in Europe (Figure 13), especially in western Europe, confirming the results of previous reviews ^[2][26][8,9]. This fact is explained by European policies, which have both bioeconomy and circular economy at their core ^[27][29]. Hence, it is possible to recognize the importance of having public policies to promote a CBE to encourage the development of CBE projects and the sustainability assessment of those projects. However, in addition to public policies, the LCT for a CBE depends primarily on the effort of private organizations; thus, they have to be aware of the importance of reporting the sustainability impacts of their products ^[28][14].

Moreover, the bioeconomy depends on factors such as the time/season of the year and the region/country ^[21][70], which must be considered when defining public and private policies, because they can define the success or failure of CBE projects. In this sense, a factor in the failure of CBE projects is the lack of consideration for the seasonality of agricultural production and, thus, agricultural wastes, which are biomass feedstock. For this reason, it is necessary to think of more versatile production systems for a CBE, which can treat various types of biomasses. For example, Khounani et al. ^[29][46] considered the seasonality of olive production (as the primary biomass for biodiesel production) and included municipal solid waste as a secondary biomass to surpass the limitations of the quantity of feedstock in out-of-season conditions and to ensure continuous production.

Furthermore, agriculture is one of the most widespread economic sectors worldwide ^[30][85]. Thus, each country has specific agricultural characteristics (crops and techniques). Therefore, when defining policies to encourage a CBE, these countries’ specific agricultural characteristics must be taken into account because they are a factor in the success of CBE projects. The intensive cultivation of traditional crops generates great quantities of agricultural waste that, in a CBE, must be exploited to develop high-value-added products ^[31][5].

Other factors are the economic development of the countries and their necessity to decrease resource use. In this sense, many years ago, western Europe recognized the reduction of resource use and environmental impacts ^[21][70]; these countries enjoy developed economies ^[2][9]. These two factors triggered the bioeconomy and the realization of CBE projects and private and public policies for a CBE.

In general, all countries should search for a reduction of sustainability impact. Thus, with the basis of European policies, countries from other world regions can implement or enhance their policies. In this light, countries from the Global South usually enjoy a significant quantity of natural resources, and their economies are based on primary activities (agriculture, mining, and others). However, exploiting resources and land needs to be revisited ^[21][70]. Thus, CBE, through the use of bio-based wastes in biorefinery processes for producing biofuels and value-added materials ^[32][28], may represent a solution to relevant problems, especially for developing countries, such as food and energy security, employment, and reducing emissions of pollutants and motivating a greener energy matrix ^[33][61].

The call for implementing a bioeconomy and CBE is even more urgent in the least developed countries. For example, in sub-Saharan Africa, where many of the least developed countries are grouped, satisfying the basic needs of communities is a great challenge for governments. These countries might be the group that benefits the most from (bio)economic growth ^[21][70]. Basic needs fulfillment includes food and energy security that a CBE can directly satisfy ^[34][58]. In this regard, it is imperative to develop and offer win-win CBE solutions for Africa in order to optimize economic growth ^[23][60], the reduction of environmental impacts, and improve the population’s quality of life. Thus, public policies are highly relevant to encourage CBE at all levels, from community-based projects, to the improving of sustainable livelihoods ^[35][59], to regional and continental collaboration in Africa ^[23][60].

In the literature reviewed, 33% of application studies (13 out of 39) considered economic issues. Thus, the low number of economic assessments reflects an urgent need to incorporate sustainability’s economic dimension in the LCT for a CBE. Meanwhile, the insertion of social issues is even more urgent because only 8% of studies (three out of 39) included social issues in their scope. In this regard, despite the already recognized need for adding socio-economic aspects in bioeconomy LCT studies ^[36][68], several studies revealed the difficulty of incorporating economic and social issues in LCT studies for a CBE ^{[28][35][37][38][39][40]}[14,35,44,53,57,59].

To think of a wholly sustainable CBE, in addition to the environmental dimension, including the economic and social dimensions is imperative. Economically, the development of complete LCC assessments of CBE products is advised. In addition to initial capital (CAPEX) and operational costs (OPEX), these LCCs must consider environmental and social costs/revenues from the main hotspots (impact categories) ^[40][41][43,57].

A correct and complete economic assessment (using LCC) helps to reduce the uncertainties about the feasibility of CBE projects. In a general sense, implementing CBE projects demands significant CAPEX and OPEX, which puts profitability into question ^[42][40]. However, when organizations prioritize profitability maximization over minimizing environmental (or social) impacts, they will tend to increase ^[5][47]. Thus, it is crucial to reach a point of balance between profitability and environmental and social impacts. For this reason, some strategic factors must be carefully considered when implementing a CBE project, such as the localization of production plants or the points of feedstock generation to reduce transport costs that may result in better economic feasibility ^[29][46].

Hence, it is important to monetarize all the impacts that CBE products entail throughout their life cycles, even the costs of degradation/benefit of areas of protection and other economic sectors. For example, Mainardis et al. ^[41][43] estimated the costs resulting from the reduction of tourism activities in a coastal town due to the implantation of a biorefinery of seagrass wrack; meanwhile, Vance et al. ^[38][44] recognized the necessity of the monetization of ecosystem degradation due to the extraction of seagrass wrack because it provides shelter and food to coastal species.

Regarding the social dimension, the realization of complete SLCA is highly recommended. The SLCA is a powerful tool to assess the social dimension of sustainability ^[43][69]. The application of SLCA is necessary for a better comprehension of the social hotspots of CBE projects and the engagement of stakeholders, which is another factor of success for CBE initiatives ^[38][44] because it determines their social acceptance ^[33][61].

In this sense, CBE has the potential to enhance the quality of life of communities ^[34][58] by offering decent work to their citizens ^[44][25], ensuring their food and energy security ^[33][61]. However, this also requires public policies to incentivize CBE and their social responsibility, which SLCA should assess.

Finally, the inclusion of environmental issues has been increased in LCT studies for a CBE in recent years; on the contrary, economic and social elements remain widely disregarded ^[45][15]. The inclusion of LCC and SLCA in LCT studies is necessary for providing a complete view of the entire impact of CBE ^[40][57] and offering consolidated suggestions to decision-makers ^[35][59]. Therefore, including economic and social issues is essential for a real contribution of CBE to the sustainability of a bioeconomy and better stakeholder management and engagement in CBE activities.