Biodiesel Production and Life Cycle Assessment: Status and Prospects

Abstract

Biodiesel synthesis, particularly through transesterification, is a mature technology in constant evolution and update. These innovative changes should be validated from different points of view: economic, social, and, especially, environmental perspectives. In this sense, life cycle assessment (LCA) is the perfect procedure to verify the sustainability of these advances. This brief review covered the present status and future prospects of life cycle assessment (LCA) applied to biodiesel production. For this purpose, the current energy scenario, along with the foundations of biodiesel production and LCA, has been explained, including current research about the specific application of LCA to biodiesel from various perspectives. As a result, LCA was proven to be a versatile tool that can be easily adapted to biodiesel production, which includes continuous innovative works that should be validated from an environmental perspective. However, the counterpart is the heterogeneity found in LCA studies in general, especially concerning functional units (from 1 MJ to 1 t of biodiesel, for instance) and boundary system selection, mainly due to the wide range of possibilities in biodiesel processing. This fact makes the comparison between works (and general recommendations) difficult, requiring additional research. Nevertheless, further studies will cover the existing gaps in LCA, contributing to completing the outlook on its application to biodiesel. Nevertheless, biodiesel production, compared to diesel, normally presents better environmental impacts in categories like global warming and ozone depletion potential.

1. Introduction

Currently, the environmental and energy scenario is continuously changing and complex, with green technologies as the cornerstone for the implementation of a sustainable energy mix in regions or countries. This way, there is a need to reduce greenhouse gas emissions, enhance energy security, and move towards sustainable development. Renewable energy sources, particularly biofuels, are becoming more significant in this transition. These usually include solar, wind, hydro, geothermal, marine, and bioenergy. There are several advantages related to renewable energy sources, like their abundance, wide distribution, and environmentally friendly nature. At the beginning of this century, these sources accounted for approximately 13–15% of global primary energy consumption, with biomass contributing the largest share [1].

The integration of green technologies into the national energy systems is needed to achieve climate goals and reduce dependence on fossil fuels. For instance, some studies point out the relevance of renewable energy sources in the energy mix of different countries in achieving sustainability goals and reducing greenhouse gas (GHG) emissions. Consequently, the electricity mix should minimize coal and gas usage while maximizing renewable energy sources [2].

Furthermore, international organizations, including the International Energy Agency (IEA) and the European Renewable Energy Council (EREC), have developed scenarios to project the future role of renewables. By 2040, renewables could supply up to 50% of global energy demand, with electricity generation from these sources exceeding 80% of total global electricity. Also, biofuels like biodiesel are expected to contribute significantly to decarbonizing the transport sector [1].

Apart from that, global energy demand is expected to increase by up to 50% in 2050, requiring Generation Expansion Planning to optimize generation sources, capacity, and location to minimize total cost while satisfying demand [3]. Indeed, there are several policies focused on this purpose, such as the European Green Deal, whose main objective is to reduce GHG emissions by 0% in 2050, aside from the improvement of efficiency in electricity production [4]. For a successful energy transition, technology development and deployment must be paired with strong policy enforcement, promoting societal commitment and stronger cooperation within the EU to achieve these climate targets without technological developments [5].

Consequently, these prospects and policies highlight the potential for renewable energy to become the cornerstone of a sustainable energy future. Regarding biofuels like biodiesel, their role in the future energy scenario is crucial, as they are derived from biomass and mainly used in the transportation sector. Their utilization could promote carbon neutrality (with the subsequent decrease in emissions), energy security (fostering lower energy dependence), and rural development (with job creation), among other advantages. Indeed, many countries around the world have focused on these practices, with representative examples. Specifically, countries or regions like the United States, Europe, China, or Brazil have invested in projects and policies where renewable energies can have a decisive effect [1], and some works highlight the relevance of strategic energy mix and energy generation planning according to different criteria [3].

In this sense, the implementation of biorefineries (where biofuel production is perfectly feasible) as the replacement for petrol-based industries is an interesting idea, with vegetable oils as a possible starting point to obtain different products such as biodiesel, glycerol or biolubricants, among others [6,7,8,9].

As a matter of fact, various research lines have dealt with this subject, concluding that biodiesel production may be a central process for the implementation of biorefineries or a supplementary option to pre-existing facilities. For example, the assessment of the sustainability of a biorefinery based on soybean, palm, and microalgae oils to produce biodiesel, green diesel, and propylene glycol was carried out, finding pros and cons for its industrial implementation [10]. Another paper consisted of the techno-economic analysis of a biorefinery based on biodiesel production and other chemicals like succinic acid, pointing out the relevance of glycerol as a key renewable building block to produce commodity chemicals [11]. Biorefineries applied to biodiesel production are also the basis for potential improvements by resorting to bioengineering or genetic engineering to enhance the sustainability and efficiency of the process [12]. As previously mentioned, biodiesel production can be a central process in a biorefinery, with the possible valorization of specific wastes for diverse purposes, like direct biodiesel production or its positive effect on the sustainability of the process. In the case of the former, different waste can be optimized in a biorefinery context, as explained in the literature for spent coffee grounds, which were successfully valorized through pre-treatments and esterification, obtaining high ester contents in the final product (nearly 90%) and proving the feasibility of this biorefinery [13]. For the latter option, the production of catalysts from natural sources or wastes is another promising contribution to the sustainability of the global process, like the synthesis of a green catalyst obtained from tangerine peel ashes. As a result, a high conversion of WCO to produce biodiesel was found [14].

However, as remarked in some cases in these works, there are challenges related to the implementation of biorefineries based on biofuels, mainly having to do with land use and food security. Biofuels are normally categorized into first, second, and third generations, depending on their origin (mainly from food crops, lignocellulosic biomass, and algae, respectively). In this manner, issues like land competition, land use change, or water and fertilizer use could arise depending on the raw material selected, which are interesting points in life cycle assessment. Different alternatives are proposed in this framework, like the use of marginal or degraded lands, the selection of non-edible crops, and the integration of crop rotation systems (which will be covered in this review in the following sections). In any case, the prospects of green technology strategies should be linked to policy support (promoting sustainable feedstock production, investing in infrastructure and logistics, promoting partnerships, and updating national energy and climate plans), technological breakthroughs, public acceptance, and global cooperation, with a general assessment of the environmental impact in this green transition [1].

Nonetheless, these challenges, along with innovative alternatives for the petroleum industry, will present new technologies that should be evaluated from an environmental point of view, including biodiesel production. Although this technology is mature enough, new advances like the use of heterogeneous catalysts or ultrasound and microwave-assisted processes require life cycle assessment to constitute a real alternative for pre-existing production methods.

Considering the above, the aim of this work is to review the status and prospects of LCA applied to biodiesel production, in order to carry out a critical analysis of current results and further needs in this field.

2. Biodiesel Production and Main Quality Parameters

2.1. Main Steps and Production Pathways

Typical biodiesel production is normally influenced by four main factors, summarized in Figure 1. It should be noted that these factors are interrelated, and the selection of a certain raw material can determine the subsequent pre-treatments, synthesis, and separation techniques. Equally, a certain process or technology is highly recommended for a certain kind of feedstock depending on its properties.

Regarding biodiesel production, there are multiple ways to carry out this process, depending on the nature of the raw material or the selected chemical route. As observed in Figure 2, the synthesis routes to produce biodiesel or diesel-like products (which can be named green diesel in the case of hydrogenation processes, where hydrogenated vegetable oils, HVO, are obtained) might be completely different. In this sense, products with similar characteristics are obtained, which can be perfectly used as a replacement for diesel oils. However, concerning pyrolysis and Fischer–Tropsch synthesis, the liquid phase obtained can present variable properties depending on the operating conditions, which should be carefully controlled to obtain fuels with similar properties to diesel [15,16]. On the other hand, transesterification and esterification obtain relatively similar products (fatty acid methyl esters, FAMEs, with equivalent and relatively predictable properties in general), which represent biodiesel in the strict sense. That is the reason why the interest of this review is mainly focused on the products obtained through transesterification and esterification, pointing out the existence of other alternative products for this purpose.

Equally, a wide range of feedstocks can be used, like triglycerides, free fatty acids, biomass in general (including oils, agricultural waste, etc.), or even synthesis gas (syngas, a mixture of CO and H₂, resulting for instance in steam reforming processes) [17].

Generally, if biodiesel production is strictly considered, vegetable oils (including a wide range of examples like cardoon, corn, jatropha, palm, rapeseed, safflower, soybean, and sunflower oils) are used as the main source of triglycerides (or free fatty acids) [18], including WCO, whose conversion to biodiesel could contribute to its sustainable management [19]. Also, biodiesel can be obtained from algae or animal fats, with positive results according to the literature [20,21]. Although this review can refer to biodiesel production from algae or animal fats in several cases, the focus will be on the use of vegetable oils and WCO.

As a result, the main raw materials used in biodiesel production can be classified as first-generation biodiesel (mainly obtained from vegetable oils and animal fats for food use), second-generation biodiesel (based on waste and by-products generated in food processing, especially WCO), and third-generation biodiesel (including microalgae and other biomolecules not competing with food production). The nature of these raw materials, according to this classification, will play an important role in LCA applied to biodiesel production, as observed in future sections, as their origin will present an environmental impact by itself, implying different collection or extraction strategies, for example.

In general, there are different steps that are required in biodiesel production from these sources, which should be taken into account in LCA studies devoted to biodiesel synthesis. The main steps are as follows (included in

Table 1. Main steps in biodiesel production.

Step	Comments
Collection	Depending on the feedstock, there might be differences. In general, harvesting of vegetable oils is the most common option
Oil extraction	It can be carried out by mechanical or chemical processes, affecting LCA
Pre-treatment	Highly influenced by the nature of the feedstock (its acidity, the presence of impurities, etc.)
Synthesis	Equally, it depends on the properties of the feedstock. Transesterification with methanol is common
Separation/purification	Different factors should be taken into account, like the role of catalyst (homogeneous or heterogeneous) and water use

) [22]:

• Collection of samples, through harvesting seeds in the case of vegetable oils, or by collecting the corresponding waste where it is generated.
• Extraction. In this case, it can be mechanical (notably in the case of vegetable seeds) or chemical (using solvents, mainly through Soxhlet extraction). As observed in studies included in this work, this stage is critical in LCA due to energy consumption and the use of solvents.
• Pre-treatments, depending on the characteristics of the raw material previously collected. These pre-treatments can involve filtration or drying processes, among others. These additional steps normally have an influence (to a greater or lesser extent) on LCA.
• Synthesis is the key process to transform the corresponding oil into biodiesel. Again, depending on the raw material, different routes can be followed at this stage. Also, batch (economic and simple option) or continuous flow processing (more efficient in general terms) could have a relevant influence on the efficiency and environmental impact of the process.
• Separation or purification. Some factors can affect these actions, like the by-products obtained during the synthesis, which should be removed from the final product in order to achieve high purity levels (with the subsequent predictable properties of the fuel). In this sense, glycerol separation from biodiesel through gravity, centrifugation, or decantation is a common step, possibly requiring chemical treatment or salt addition if the emulsion is generated. Additionally, purification is an essential point in LCA, as steps like alcohol recovery (through evaporation or distillation), washing (to remove soap and impurities, including homogeneous catalysts), filtration (to remove heterogeneous catalysts), drying (by applying heat or drying agents like sodium sulfate) or distillation [1]. Energy consumption and the environmental impact of new reagents should be considered in the corresponding life cycle inventory (LCI). Also, the reutilization of glycerol, a typical by-product in this process, is fundamental, as its valorization (for pharmacy and energy purposes, among others [23]) could lead to crucial and positive changes in the resulting LCA. Likewise, the separation of the spent catalyst requires additional steps like filtration or washing treatments, which should be included in LCA.

The synthesis of fatty acid methyl esters (that is, FAMEs or biodiesel) can be mainly carried out through transesterification and hydrolysis combined with esterification, as observed in Figure 3 [24]. Transesterification involves the reaction of triglycerides with alcohol to produce fatty acid alkyl esters (biodiesel) and glycerol. The reaction is reversible, and typically acid or base catalysts are used. The general mechanism includes the formation of diglycerides and monoglycerides in intermediate steps. In base-catalyzed reactions, alkoxide ions attack the carbonyl carbon of triglycerides, generating a tetrahedral intermediate that rearranges to yield esters and diglycerides. Acid-catalyzed transesterification is usually slower and requires higher temperatures and longer reaction times, although its conversion is high [22].

It should be noted that, for these processes, ethanol could be used to obtain similar compounds (fatty acid ethyl esters, FAEEs). However, most studies resort to methanol mainly due to economic reasons. These chemical routes will be relevant in LCA (as explained later in this review), especially concerning the inventory analysis. This way, as observed in Figure 3a for transesterification and Figure 3b for hydrolysis and esterification, methanol is required to produce FAME and glycerol. Regarding methanol, different methanol/oil ratios are used in the literature, which is a decisive factor that contributes to LCA. As expected, high ratios would imply high inputs in LCA, and further steps for the recovery and reuse of unreacted methanol would have a negative result in LCA. The role of glycerol as a by-product or waste is essential for a positive LCA, requiring further studies, which will not be covered in this work. In any case, its use as an energy source for reforming processes to produce hydrogen, or its purification to obtain high-purity glycerol in pharmaceutical products, would enhance this process in a biorefinery context [23]. The selection of one chemical route or another will depend on the properties of the raw material, mainly its free fatty acid content [25].

As observed in Figure 3, one of the most important aspects in these chemical routes is the raw material, that is, fatty acids or lipids from natural sources. Further sections will explain the impact of raw material on LCA, but it is interesting to describe the main sources for biodiesel production at this point, which determines the category of biodiesel and the subsequent implications in LCA.

Thus, first-generation biodiesel is mainly obtained from vegetable oils, specifically from food crops like corn [26], soybean [27], palm [28], or rapeseed [29], as typically studied in the literature. This is usually a traditional and mature process. Yet, there are clear disadvantages like food competition or possible contribution to deforestation, as these crops require fertile soil. Second-generation biodiesel is based on different residues like organic waste, waste cooking oil or used cooking oil (WCO or UCO) [30,31], agricultural and forestry residues. Also, non-food crops can be used in this context, like cardoon [32,33], castor [34,35], jatropha [36,37], or safflower [38,39,40], offering good characteristics in the resulting biodiesel in general. In this sense, food competition could be avoided, as wastes derived from typical land use, along with crops that are easily adapted to semi-barren lands, represent more sustainable practices. Finally, third and fourth-generation biodiesel is focused on biofuel production from algae, microalgae, and bacteria, with some advantages like CO₂ capture. These issues are of great interest in LCA works focused on biodiesel production, having a great influence on the environmental impact of the process. However, feedstock selection, in addition to environmental criteria, normally depends on regional availability, economic aspects, and oil content, but the interest in non-edible and waste oils is increasing due to their lower costs and environmental impact.

Special mention should be given to WCO, an emerging waste with great potential in biodiesel production (apart from other products like bio-lubricants, bio-solvents, animal feed, or asphalt additives, for example) [31,41,42]. This versatility points out the relevance of WCO in many industries, together with its corresponding contribution to their LCAs, as the valorization of waste typically has a positive environmental effect.

This waste can be considered a low-cost, renewable, and widely available feedstock for sustainable energy generation, whose use enhances its valorization and waste disposal [43]. Indeed, it is a viable and sustainable feedstock for biodiesel generation, as observed in the literature, where the reduction in environmental burdens, offsetting the need for virgin vegetable oils, has been proven. However, WCO normally contains high levels of FFA and moisture, with possible soap generation through saponification and a decrease in biodiesel yield. Therefore, different pre-treatments are required for this waste, like filtration, washing, drying, acid esterification to reduce FFA, neutralization, or even the use of ion exchange resins. Consequently, LCA would be focused on these primary pre-treatments instead of land usage (which is nonexistent in this case). Again, LCA is necessary to understand a specific biodiesel production, where every detail can be decisive in the global environmental impact of the whole process. For the rest of the influencing factors (like transesterification and separation methodologies), which will be covered later in this document, WCO shows a similar behavior compared to other vegetable oils.

2.2. The Role of Catalysts in Biodiesel Production

Getting back to Figure 3, the production of biodiesel (from the previous stages to the chemical synthesis) implies a series of operational aspects that should be taken into account to make the process more efficient (with the subsequent positive economic impact) and sustainable (paying attention to the environmental impact). Regarding the latter, there are details that could be crucial in LCA, as explained in further sections.

Specifically, in the case of FAME generation, the role of catalysts is essential due to obvious reasons. The most common catalysts are homogeneous ones, with alkali (NaOH, KOH, or sodium methoxide) and acid-catalyzed (H₂SO₄ or HCl) as the most representative. It should be noted that the former are adequate for raw materials with low FFA content, to avoid soap formation and the subsequent decrease in yield, along with a more complicated separation process. Also, other processes like microwave-assisted transesterification could reduce reaction time [44]. These factors could present clear advantages in LCA, reducing the environmental impact of separation processes and energy consumption. In the case of acid catalysts, they are suitable for high-FFA oils, but these reactions normally require a higher alcohol/oil ratio and catalyst concentrations, which could imply a negative LCA. The combination of both kinds of catalysts could be possible, in particular when acid catalysts are used in a pre-treatment for alkali-catalyzed transesterification, obtaining higher yields and avoiding saponification.

More recently, new catalysts have arisen in biodiesel production, especially heterogeneous catalysts like lipases (that can catalyze the reaction under mild operating conditions), which are generally expensive and require further LCA tests, as their reusability and stability are variable and should be extended as much as possible to reduce their environmental impact. In this regard, challenges like catalyst leaching and deactivation over time should be solved, increasing reaction rates compared to homogeneous catalysts and optimizing the synthesis and reaction conditions [25,45]. One interesting aspect regarding LCA is the scalability of the process and the origin of these heterogeneous catalysts. Even though heterogeneous catalysts like CaO are normally selected, their synthesis should be considered in LCA, as calcination processes at high temperatures (up to 1000 °C), with CO₂ emissions, are required. In this sense, new mild conditions and natural resources are being investigated in the literature, like the use of bone waste (from fish, goat, or chicken, among others), crustacean or mollusk shells, or agricultural waste, among others [25,46]. Equally, nanocatalysts (a subclass of heterogeneous catalysts with high surface area, reusability, and catalytic activity) can be crucial in future research, with CaO, MgO, and their composites as clear examples of great performance in biodiesel production, low cost, and environmental friendliness. Again, they can be obtained from waste materials like eggshells and animal bones, contributing to their valorization and the sustainability of the corresponding synthesis [47].

Another alternative for catalysts in biodiesel production is the use of methanol under supercritical conditions, requiring high temperatures (around 350 °C) and high-pressure conditions to offset the absence of catalysts. Even though these are fast processes (in a few minutes) with nearly complete conversion, high energy and equipment costs are a challenge in LCA [22].

2.3. Influence of Operating Conditions on Biodiesel Production

In addition to the kind of catalyst selected, there are other variables that can affect the transesterification process and biodiesel production in general [24,43]:

• FFA content and moisture, which could have a negative impact on base-catalyzed transesterification (with soap formation and reducing the yield of the whole process). In this sense, oils with FFA content above 3% require pre-treatment, as observed in Figure 3b for hydrolysis and esterification.
• Alcohol/oil molar ratio. The stoichiometric ratio of this reaction is 3:1, but higher ratios are used to increase the conversion yield. As previously explained, this excess methanol could imply negative results in LCA. Moreover, it could be negative in glycerol separation.
• Temperature and reaction time. Higher temperatures and longer reaction times normally increase conversion. However, the nature of alcohol (its boiling point) should be considered. For a typical transesterification with methanol, reaction temperatures of 60 °C are recommended, with around 1% of catalysts, for one hour. Under these conditions, high conversions (above 90%) are achieved.

As observed in this review work, almost every factor in biodiesel production might present an environmental impact, requiring LCA. In general, according to the previous points, factors like reaction temperature or the amount of methanol could improve conversion and the absence of by-products (which could be positive in LCA), whereas the LCI could be negatively affected as electricity use and methanol addition would be increased. As in many cases, the intermediate approach could be convenient, and LCA is the perfect tool to compare and validate all these alternatives.

2.4. Quality of Biodiesel and Influence on LCA

Once biodiesel is produced and purified, a high-quality product is obtained, which can be perfectly used in Diesel engines as long as basic properties are within the standards established (e.g., EN 14214 or ASTM D6751 specifications, see Table 2). Even though these aspects can seem unconnected to LCA, they can play a major role, especially when it comes to engine performance. In this manner, these critical parameters can affect engine life, with the corresponding extension or reduction. As a consequence, aspects like emissions or even the LCA related to engine manufacturing can be influenced by the regular use of biodiesel in this type of engine. Thus, some relevant properties of biofuels for good engine performance are the following:

• Viscosity. In order to avoid incomplete combustion and poor atomization, or even clogging, viscosity values should be between 3.5 and 5 cSt at 40 °C, as established in some standards [48,49].
• Density. Compared to diesel fuels, it is slightly higher in the case of biodiesel (between 860 and 900 kg·m⁻³). This fact could imply higher fuel consumption, but normally lubricity is improved, which could reduce engine wear and contribute to its durability [50].
• Flash and Fire Points. They are commonly high (above the typical lower limits, established at 120–130 °C) in biodiesel compared to diesel, which implies safety during storage and handling, reducing the risk of accidental combustion.
• Cetane Number. This is an essential property for LCA, as it is related to the fuel’s ignition quality. The higher it is, the shorter the ignition delay is, with more efficient combustion and the subsequent decrease in emissions and environmental impact [18,51].
• Cloud and Pour Points. Equally important, these cold flow properties are normally higher compared to traditional diesel, which implies flow problems in cold climates [52]. In such a way, a reduction in the service life of the engine can take place, unless some additives are incorporated [53]. The influence of these additives should be taken into account in LCA to assess if their efficiency can offset the environmental impact of their production and use.
• Boiling Point. Biodiesel presents high boiling point values, contributing to more complete combustion whereas the risk of deposits in the combustion chamber might increase.
• Combustion Efficiency and Energy Content. These are key factors in LCA. On the one hand, due to the fact that biodiesel is an oxygenated fuel, more complete combustion can be found compared to diesel fuel, especially reducing unburned hydrocarbons, carbon monoxide, and particulate matter emissions [22,54]. The latter, including soot and smoke, are significantly reduced with biodiesel use due to the absence of aromatic compounds and the presence of oxygen in biodiesel, associated with more complete combustion and less soot formation. On the other hand, lower energy density is found in biodiesel, with a possible decrease in power of 5 to 10%. Consequently, higher amounts of biodiesel are required in this sense, and that is the reason why some studies adjust their functional unit (FU) to energy generated/consumed instead of specific amounts of biodiesel (like 1 t).
• Water Content and Presence of Alcohol. Drying is essential to avoid the presence of water to ensure fuel stability and prevent microbial growth during storage, whereas the presence of traces of alcohol (mainly methanol) could contribute to rubber seal degradation and corrosion in engines, particularly aluminum parts. Again, the role of purification steps is fundamental in LCA, for their efficiency and energy performance during the process and their consequences in the final engine [55,56].
• Oxidation Stability. One of the major drawbacks of biodiesel is its instability during storage, primarily due to oxidation, which occurs when unsaturated fatty acid chains in biodiesel react with oxygen. This leads to the formation of peroxides, aldehydes, ketones, and acids, which compromise fuel quality and damage engine components. Thus, during storage, external conditions like air, light, moisture, or temperature significantly impact biodiesel quality, accelerating degradation through an increase in acidity, viscosity, and sediments that affect the engine [57]. To avoid it, the addition of antioxidants (both natural [58,59] and synthetic like BHT, TBHQ [60], or PG, among others, with unequal results in LCA depending on different factors such as extraction or synthesis) and proper storage (including the use of sealed, opaque and dry containers) is recommended [22,41,61,62]. Alternatively, biodiesel stability could be improved through catalytic transfer hydrogenation using bimetallic catalysts (such as Zn-Cr bicarbonate and formate, for instance) and glycerol feed [63].
• Emissions. As previously explained, during combustion in engines, some pollutants are considerably reduced when biodiesel is used, up to 50% in CO, 70% in PM, and 90% in polycyclic aromatic hydrocarbons (PAHs). However, NO_x emissions commonly increase compared to petroleum diesel. This is mainly due to the higher flash and combustion points and oxygen content in biodiesel, which favor NO_x release in Diesel engines [1,18,64].
• Biodegradability. In general, biodiesel is more biodegradable than other petroleum fuels like gasoline, making accidental spills of the former less harmful to the environment, with a positive contribution to LCA [65].

Summing up, biodiesel normally presents advantages and challenges compared to traditional diesel. This way, aspects such as sustainability and the environmental benefits during its combustion have been widely covered in the literature. Apart from that, safety and lubricity are also interesting per se. However, other factors like cold flow properties and short oxidation stability require the use of additives or their blend with diesel (up to 15–20%, as it does not cause significant wear) [18]. All these issues should be covered in LCA, particularly when new components like additives or catalysts are added to the process to improve its final properties or efficiency of the process (covering both their effect on biodiesel and their own synthesis or origin). Previous studies have shown that additives can present a positive effect on biodiesel properties (like stability, combustion efficiency, and cold flow properties) and their engine performance, with low concentrations [66]. The improvement of biodiesel properties and the requirement of low concentrations are substantial to confer positive LCA in biodiesel production, especially if these additives come from natural sources.

Table 2. Specifications for biodiesel.

Property	ASTM D6751 [67]	EN 14214 [48]
Viscosity at 40 °C, cSt	1.9–6.0	3.5–5.0
Density at 15 °C, kg·m−3	Not specified	860–900
Cetane number	≥47	≥51
Flash point, °C	≥130	≥120
Water content, %	≤0.05	≤0.05
Acid number, mg KOH·g−1	≤0.5	≤0.5
Sulfur content, ppm	15	10

Table 2. Specifications for biodiesel.

Property	ASTM D6751 [67]	EN 14214 [48]
Viscosity at 40 °C, cSt	1.9–6.0	3.5–5.0
Density at 15 °C, kg·m−3	Not specified	860–900
Cetane number	≥47	≥51
Flash point, °C	≥130	≥120
Water content, %	≤0.05	≤0.05
Acid number, mg KOH·g−1	≤0.5	≤0.5
Sulfur content, ppm	15	10

As seen in Table 2, and according to multiple studies, biodiesel normally complies with most specifications (some of them for pure biodiesel, others for blends with diesel), although there might be some exceptions depending on the nature of the lipid, processing, etc. In any case, the use of additives is usually enough to comply with all the standards. The adjustment of these properties is essential to ensure a high-quality product, suitable for its use in Diesel engines.

Only the description of biodiesel processing, including its origin, operating conditions, and quality of the final product, would be enough to understand the relevance of LCA in this process. Nevertheless, there is another important aspect, like the fact that biodiesel technology is continuously changing, offering innovative solutions to different challenges [1]. Thus, non-catalyzed supercritical alcohol transesterification, enzyme-catalyzed processes, and heterogeneous catalysts seem to offer a promising outlook for biodiesel production, with the continuous optimization of the plant equipment (including the design and automation of pre-treatment units, reactors, separation units, washing and drying units or storage tanks) as a typical research line. Additionally, emerging technologies like microwave-assisted or ultrasound-assisted transesterification, reactive distillation, membrane reactors, or integrated biorefineries (where other co-products like bioethanol or biogas are generated) could contribute to reducing costs and increasing the yield of the process. In other words: These research areas are mainly devoted to solving real challenges, like (apart from the already mentioned barriers) the increase in oil yield from feedstock, the search for low costs and robust catalysts, and the implementation of these technologies on the industrial scale (especially for possible sustainable development of rural or developing areas).

Considering these reasons, the relevance of LCA in biodiesel production is considerable, constituting a noteworthy tool to validate the implementation of new technologies and the industrial scale-up of this specific process. The following section will deal with the most relevant concepts and foundations in LCA, which are essential to understanding the context of this review and the specific research included later.

3. Life Cycle Assessment

Life cycle assessment (LCA) is defined as a method for the analysis and assessment of environmental impacts of a product (also processes or services) throughout its life cycle, from raw material extraction to disposal [68]. Basically, it is a process used to evaluate the environmental burdens associated with products, processes, or services throughout their entire life cycle by identifying and quantifying materials and energy consumed, and waste released to the environment. As a result, the evaluation of a process, its comparison, and different proposals for its improvement are normally provided in LCA. This assessment can include the entire life of a product, process, or activity, which is commonly known as “cradle-to-grave”, one of the most popular approaches in LCA.

3.1. General Principles

The general principles of LCA are based on the ISO 14040 standard series [69], consisting of four stages [68,70] that will be briefly explained in the following subsections. This way, a homogeneous procedure for a correct LCA is desired, in order to make heterogeneous works about his subject comparable. However, every principle will be adapted to specific situations, as explained in specific papers later.

3.1.1. Goal and Scope Definition

Once the purpose of LCA is defined, the boundaries are established, including spatial and temporal scope, the functional unit (which permits the establishment of a basis to compare the environmental impact of a certain process or product), and the product system throughout its life cycle. This step is basic to avoid confusing results and misleading interpretations, recommending the establishment of clear boundaries according to the objectives of LCA, which can include (or exclude) steps of the global process. In that sense, different perspectives can arise, like the comparison of equivalent technologies or the comparison with traditional processes.

In general, the development of an LCA can be unmanageable at the practical level, and some assumptions are necessary to carry out feasible LCA studies. For example, some processes or flows can be excluded due to their negligible environmental impact, the effect of raw material can be considered negligible if the aim of a study is strictly to assess differences between processes (including catalysts), an average energy mix can be considered in a local scenario, etc. These assumptions should be made according to the objective of LCA, based on scientific criteria.

Thus, depending on the selected approach and assumptions, the scope might vary. As previously explained, a typical approach is “cradle-to-grave”, where all the stages of a process are included (production, transportation, and disposal, mainly). Nevertheless, other common approaches are: cradle-to-gate, where main considerations are strictly focused on production, from the extraction of raw materials to the final product; gate-to-gate, where only one part of the process is considered; cradle-to-cradle, which is similar to cradle-to-grave, but including recycling or reuse in the final process, supporting circular economy.

3.1.2. Inventory Analysis

It consists of the compilation of an inventory related to energy and material inputs and environmental releases, quantifying inputs and outputs associated with the product system that was established in the previous stage. For this purpose, the data sources should be reliable and relevant, obtained from databases, the existing literature, or even experimental data at the laboratory or industrial scale (which is recommended in some cases, in particular when innovative technologies are introduced). In this context, a life cycle inventory (LCI) is the tool to compile an inventory and quantify the inputs and outputs of the product system, considering the following: extraction of raw materials required; the processes involved in manufacturing; packaging, and distribution; the use and maintenance of the product during its service life; and the disposal, reuse or recycling of the product at the end of its life. Addressing how to allocate the different environmental burdens among multiple products or processes is necessary to carry out a suitable inventory analysis.

3.1.3. Impact Assessment

The evaluation of the environmental impacts related to the abovementioned inputs and outputs is included in this phase, implying a classification, characterization, normalization, and weighting of impact categories (see

Table 3. Main impact categories in LCA.

Impact Category	Abbreviation	Unit	Description
Acidification Potential	AP	mol H⁺-eq or kg SO2-eq	Assessment of the potential risk of acid rain
Abiotic Depletion Potential	ADP	MJ or kg Sb-eq	Quantifies the use of non-living natural resources like minerals and fossil fuels
Ecotoxicity	ECTOX	CTUe	Estimates the potential harm of chemical emissions to freshwater aquatic ecosystems
Eutrophication Potential	EP	kg P-eq, kg N-eq, etc.	Evaluation of the potential to cause nutrient enrichment in aquatic ecosystems
Global Warming Potential	GWP	kg CO2-eq	Evaluation of the potential contribution to global warming, for instance, through CH4 or CO2 emissions
Human Toxicity	HT	CTUh	Assesses potential harm to human health from chemical emissions, including cancer and non-cancer effects
Land Use	LU	m2 a crop-eq	Evaluates impacts on biodiversity and ecosystem services due to land occupation or transformation
Ozone Depletion	OD	kg CFC-11 equivalent	Evaluation of the potential to deplete ozone layer
Particulate Matter Formation	PMF	kg PM2.5-eq	Assessment of the potential impact of emissions that lead to the formation of fine particulate matter (especially PM2.5), causing respiratory and cardiovascular health issues in humans
Water Use	WU	m3 world-eq deprived	Measures freshwater consumption and its potential to cause water scarcity in affected regions

for more details). Although a complete LCA should include as many impact categories as possible, their correct prioritization (with possible discards) is important to reach interesting conclusions in the following step. Again, assumptions are decisive in discarding some categories.

In general, two levels of evaluation are used for these categories, that is, midpoint and endpoint. The former represents intermediate impact categories, related to the causes derived from the negative environmental effect. Consequently, midpoint levels are focused on physical and chemical processes and their effect on the environment. It is normally more specific and robust, but difficult to interpret and make good decisions if the stakeholder is not technical. On the other hand, the latter is related to final damage in different fields like human health, ecosystems, or natural resources, making it easier to communicate but with higher uncertainty.

3.1.4. Interpretation of the Results

Overall, clear and actionable insights to improve the environmental performance of the process are given at this point, recommending steps that can affect the goal and scope of the initial LCA. For this purpose, Life Cycle Impact Assessment (LCIA) evaluates the environmental impacts deduced from the corresponding inputs and outputs, with their corresponding classification, characterization, normalization, and weighting, where the relevance of the different impact categories is carried out. This facilitates the decision-making process to improve the environmental performance of a certain product, process, or service.

3.2. Other Aspects Related to LCA

For the correct development of an LCA, several programs are available, with a wide range of possibilities for the design of these studies, thanks to their adaptability to several databases and approaches. Furthermore, some processes and products can be modeled, with an automatic calculation of environmental impacts and carrying out of reports and comparisons when possible.

Table 4. Main software and databases used in LCA.

Software	Compatible Database	Description
Simapro	Ecoinvent, Agri-footprint, etc.	Very popular in research, with a detailed interface
GaBi	Ecoinvent, GaBi, etc.	Specialized in industrial applications
OpenLCA	Ecoinvent, ELCD, etc.	Open access and flexible
Umberto	Ecoinvent, GaBi, etc.	Focused on material flow

shows the most popular software and its main characteristics, perfectly applicable to most studies carried out in biodiesel production.

Thus, regarding the main database used in LCA, Ecoinvent is one of the most popular and complete sources, with data about industrial processes, agriculture, energy, etc. Also, GaBi is normally used in industry, with regional and specific data according to different sectors. There are other databases like ELCD or Agri-footprint, which are more specific but equally useful in specific conditions. Nevertheless, direct data from specific processes might be obtained from industries or research, and other software can be used to provide valuable information, especially in process design (like Aspen Plus, MATLAB, Unisim Design, etc.).

As a result, some methodologies (ReCiPe or Traci as the most popular) are followed to include the different evaluation levels, boundaries, or interpretation of results, many of them compatible with the abovementioned LCA software.

3.3. Specific Applications of LCA in Bioenergy

It should be noted that LCA offers a wide variety of applications, as it is easily adaptable to aspects related to different products, processes, or services. On the other hand, it is an interesting tool for the assessment of the real implementation of green technologies. LCA is widely used in many industries to assess their environmental performance, helping companies to identify opportunities for improvement and make decisions for a positive environmental impact.

Indeed, this tool is highly recommended for multidisciplinary tasks like product design, policy-making, or environmental management, among others. However, as it will be observed in this review for the specific case of LCA applied to biodiesel production, some challenges might arise, like the lack of data (and the corresponding uncertainty) to develop complete LCIs in certain research areas. Additionally, the requirement to unify criteria for methodological choices (mainly concerning the selection of the system boundaries or functional units) is another barrier.

These facts can lead to LCAs with a difficult comparison in similar fields, as in the case of biodiesel production, where a simple modification in raw materials could considerably change the system boundary, with the subsequent difficulty in comparing their environmental impact. In any case, further LCAs focused on specific and innovative approaches will gradually solve these challenges, contributing to the integration of new technologies and their maturity (by increasing their technology readiness level, TRLs). In addition, LCA applied to different regions (with the subsequent changes in the energy mix) would equally contribute to the expansion of knowledge.

Consequently, the future of LCA applied to bioenergy is promising, as new and developing technologies will require environmental validation compared to pre-established processes. In this respect, different fields can be supported by future LCA applications, like lignocellulosic biofuels, alternatives for land usage or agricultural practices, biogenic carbon, etc. Specifically, for illustration only, a few works were devoted to these applications. In this way, several studies focused on LCA applied to biodiesel production have been carried out, like the treatment of waste like sewage sludge (where drying can be a challenge, offering alternatives like solar technologies to improve LCA) [71] or the valorization of spent coffee grounds in a biodiesel biorefinery [13], or using homogeneous and heterogeneous catalysts with high yield [72]. Regarding sewage sludge, a typical waste considered in LCA, its valorization for biodiesel production using deep eutectic solvents was assessed, requiring a reduction in methanol and energy consumption during the esterification/transesterification process [73]. Similar papers will be described in further sections, proving the scientific interest in this topic. In addition, the environmental impact of biofuel from microalgae through pyrolysis was assessed, pointing out biomass dehydration as one of the most challenging environmental burdens [74]. Even recently valorized energy vectors like hydrogen have been studied from this perspective, according to different sources [75]. On the other hand, alternative ethylene conversion from corn ethanol has been assessed by comparing it with fossil-derived production, with a considerable reduction in GHG [76].

Even though it is not the central topic of this review, it should be noted that LCA can present different approaches apart from the environmental impact. This way, economic assessment should be considered at this point. In general, four main economic costs are relevant to Life Cycle Costing (LCC): Acquisition costs (mainly including costs related to raw materials, facilities, or machinery), operating costs (regarding energy, maintenance, and manpower for biodiesel extraction and processing, among others), end-of-life costs (related to recycling and waste disposal) and external costs (having to do with indirect economic impacts like subsidies or environmental taxes). Thus, some aspects can be relevant in LCC applied to biodiesel production, like feedstock costs (which can imply up to 75% of total production cost, with WCO and animal fats as the cheapest sources), catalyst costs (those derived from waste material can reduce costs), kind of alcohol (in transesterification, methanol is preferred due to the lower cost and higher reactivity), plant capacity (normally, larger productions are recommended to reduce per-unit costs) and by-product value (for instance, valorization of glycerol can offset general production costs) [47].

Furthermore, social life cycle assessment (S-LCA, which considers measurable changes in well-being and health experienced by individuals or groups due to a change in a product system) is important, being the counterpart to typical LCA, where only environmental issues are considered [77]. With this regard, different aspects are normally covered in S-LCA, such as working conditions (including fair salaries, social welfare, or safety at work), human rights (abolishment of child labor or discrimination, among others), impact on local communities (especially concerning local development) and professional and business ethics. This way, specific studies about the relevance of s-LCA have been developed, for example, in Indonesian regions where palm oil was used for biodiesel production [78].

4. Bibliometric Analysis of LCA Applied to Biodiesel Production

As previously explained, LCA presents a wide range of possibilities, playing a major role in the feasible implementation of green technologies at an industrial level. Indeed, this fact has not escaped the notice of the scientific community, which considers LCA as an essential tool to validate and support multiple research projects. Specifically, those works related to biodiesel (both in production and general use) have normally resorted to LCA for this purpose. Consequently, as observed in Figure 4, there has been a considerable increase in the number of scientific publications combining both biodiesel and life cycle assessment, especially in the last two decades, with around 100 publications per year from 2021. According to these data, everything suggests that this trend will continue in the medium to long term.

Concerning the subject areas of these research works, Figure 5 shows the most representative fields of the journals where the selected papers were published. Both biodiesel and LCA are versatile subjects, which proves the variety and abundance of fields interested in this research line.

Specifically, the majority subject area was Environmental Science (covering 25% of the published papers), which can be explained by the environmental focus of LCA and the environmental benefits related to the implementation of biodiesel. Similarly, a subject area like Energy (24%) is relevant in LCA applied to biodiesel, due to the relevance of energy balance for a positive LCA and the role of biodiesel in the current energy scenario. Engineering is another interesting area (14%), as LCA is focused on the environmental feasibility of industrial facilities, with biodiesel production presenting a high TRL.

Apart from that, there are other subject areas where works focused on biodiesel and LCA were placed (with at least 2% of total publications). Among them, we can find the following: Chemical Engineering; Business, Management, and Accounting; Agricultural and Biological Sciences; Chemistry; Biochemistry, Genetics, and Molecular Sciences; Computer Sciences; or Social Sciences. Interestingly enough, there are economic and social subjects related to LCA, where Social Life Cycle Assessment (S-LCA) and economic analysis can be considered or even integrated into this kind of study.

Summing up, LCA applied to biodiesel has an increasing interest in the scientific community, mainly due to the multidisciplinary nature of this field, which reveals its great potential.

If the interrelation of the main keywords of these representative works is considered, the results are represented in Figure 6. In general, the publications devoted to LCA applied to biodiesel production (see Figure 6a) offered different points of view. For instance, various works deal with the environmental impact of biodiesel production from algae (see green cluster), whereas vegetable oil as raw material is another recurring subject (in particular, concerning palm or soybean oil, among others, as observed in blue cluster). Within this context, the role of waste management, like WCO, is important, as observed in the red cluster. Additionally, these groups of keywords are highly related to several central terms included in the purple cluster, like greenhouse gas effect, and crop and land use, among others, which are essential to understanding the inventory analysis, impact assessment, and their interpretation.

The main results when keywords of studies related to specific vegetable oil conversion to biodiesel were considered in Figure 6b. As observed, there are three main clusters: in blue, focused on processing, the main vegetable oil used as raw material (palm oil), and different approaches made in LCA; in red, one of the main characteristics of these papers is pointed out, like the comparison of modifications in biodiesel production (especially concerning land usage or cultivation, GHG and other emissions); in green, the possible effect of raw materials and its comparison with fossil fuel and its global warming potential (one resourceful approach in LCA applied to biodiesel production).

In this review, many of these concepts and interrelations will be covered in the section related to specific works about this issue (Section 5).

In order to develop this brief review, Scopus was investigated for all entries in the literature on the topics of “biodiesel” and “life cycle assessment” (including keywords such as “fatty acid methyl esters” and “LCA”) for the last 30 years, with special attention to the last 5-year period (2020–2025), as there has been a considerable increase in published papers, with the aim of updating information related to this field. The search, which was made from January to April 2025, returned 1266 results, from which up to 213 articles were considered for inclusion in this work, including information about 130 publications (mainly research) in this final paper.

5. Life Cycle Assessment Applied to Biodiesel Production: Main Factors to Consider

As explained in further subsections, the role of LCA in biodiesel production is essential to reinforce the contribution of this green technology to the energy transition. Indeed, specific examples have been covered in the literature, which will be equally explained in this section. As expected, biodiesel production, even though it is considered a technology with a high TRL, between 8 and 9 (especially concerning typical biodiesel production through homogeneous catalysis), normally presents innovative improvements, including the use of new heterogeneous catalysts or subcritical conditions (with lower TRLs in general). In this connection, LCA will be important to contribute to the validation or rejection of future applications at the industry level. However, to the best of our knowledge, the development of LCA studies is still in a preliminary stage, with great potential and room for improvement once these studies are unified and cover new research areas concerning biodiesel production.

Nevertheless, some common points can be inferred from these studies, covering many issues implied in the “cradle-to-grave” approach, as explained in the following subsections. It should be noted that, by covering these common topics, relevant information about the most influential factors in LCA in biodiesel production will arise.

5.1. Feedstock

Depending on the feedstock used for biodiesel production, there are different impact categories that could be affected to a greater or lesser extent. As a rule, the use of edible oils implies high land and water use, with the typical food versus fuel conflict, whereas this negative effect could be considerably reduced by using non-edible oils like Jatropha or Safflower (as these crops can be grown in marginal lands). Indeed, jatropha biodiesel production was studied, with its LCA pointing out that the use of fertilizers and combustion were the most relevant factors in environmental burden [80]. In the same vein, the effect of cultivar selection on LCA in rapeseed production was studied, finding that N-based fertilizers were critical contributors to environmental burdens (notably HT and GWP), recommending an adequate choice of cultivars depending on specific climate or pre-harvest conditions [81]. On the other hand, the use of waste (like WCO or animal fats) is an interesting option, reducing environmental burden and valorizing these wastes that, in any case, would require environmental management. Indeed, as found in the literature, food waste valorization (from restaurants) to produce products like bioethanol, biomethane, or bio-oil, among others, could be the cornerstone for the implementation of a biorefinery, with electricity consumption as the main contributor to the environmental burdens [82]. This way, WCO can be an economical biodiesel source, with a final product with similar properties compared to other biodiesel samples, but requiring some additional preparation steps like FFA neutralization and moisture removal. Also, the use of this kind of waste, along with different crops like cardoons or safflower (optimal for rotation crops or non-arable areas), could lead to positive LCA [83]. Another example of energy crops for rotation (for instance, with wheat) is camelina, which could present positive results in LCA applied to biodiesel production. Indeed, land use change and the food versus fuel debate would be avoided by using these crops, recommending, in any case, the addition of N-fertilizers in global production [84].

When WCO was compared to first-generation biodiesel, a positive environmental footprint was found (with a 40% decrease), with transesterification presenting the highest impact, although improvement in transportation could lead to better results [85]. Regarding animal fats, biodiesel from mutton tallow was a feasible option according to LCA, requiring the reduction in methanol during transesterification [86]. Other waste, like grease trap waste, has been studied through LCA, pointing out the requirement for additional steps due to the heterogeneity of this waste and the subsequent lipid content and high sulfur content in general, finding different impacts like water disposal and solid wastes due to separation of lipids from grease trap waste. Nevertheless, a typical disposal of this waste would imply these steps, which could have a positive impact on LCA. As explained in microalgae, the yield in lipids is essential to make the process profitable and sustainable. In this study, a lower limit of 10% of lipids is required to present similar environmental metrics compared to typical soybean biodiesel and low-sulfur diesel production [87].

Finally, algae and microalgae could be a feasible option, as high productivity can be achieved with CO₂ capture. In this sense, the key role of the economic and environmental feasibility of this process is to increase lipid production and yield [88,89]. In parallel with vegetable oils, the relevance of the choice of microalgal species is an appealing approach, keeping high lipid and low protein contents with sustained growth rates [90], with some researchers noting the good environmental performance of Nannochloropsis sp. species [91]. In this regard, a comparison with canola biodiesel and ultra-low sulfur diesel was carried out, observing that GHG emissions were favorable for microalgae biodiesel, whereas costs presented room for improvement. For this reason, higher production rates and improvement in process efficiency (for instance, by reducing energy use in thermal dewatering [92]) will contribute to the productivity of microalgae in this context [93,94], as some impact categories like GWP depend on the yield of oil achieved and culture media recycling, with electricity consumed during cultivation as a key point [95]. Thus, one feasible option would be the use of photobioreactors might reduce electricity use and improve productivity, resulting in a more favorable LCA analysis [96]. Indeed, recent studies support the use of photothermal processes to produce biodiesel with 96.8% yield from microalgae lipids at room temperature, which is a clear advantage in LCA and environmental protection compared to traditional biodiesel production [97]. On the other hand, the combination of wet extraction with hexane, transesterification, and anaerobic digestion of residual biomass could generate biogas, which could be used in a combined heat and power unit for energy recovery, offering the best-performing scenario according to previous studies [98]. Following this philosophy, different scenarios for biodiesel production from microalgae were considered in LCA, finding that the best GHG emissions should include wastewater as nutrient input [99]. In other words, the implementation of a biorefinery based on algae could be advisable, combined with other processes like protein and succinic acid generation, which offered better results compared to their reference system (consisting of diesel, soy protein, and fossil-based succinic acid) [100].

5.2. Chemical Synthesis to Produce Biodiesel

One of the key processes in LCA. Involved in biodiesel production, it has been widely covered in the literature, with a special focus on energy input for heating and mixing, methanol use (whose production could imply environmental burdens, as observed in previous studies [101]) and catalyst production, recovery, reuse and its associated waste (as explained in the following subsection). Also, different by-products that have great potential are generated, like glycerol, implying new steps that could contribute to a positive impact on LCA [102].

Catalytic Performance

Normally, homogeneous catalysts (acid and base) have been a typical resource in FAME synthesis, but they require neutralization and generate wastewater with its corresponding treatment [24,103]. Nevertheless, the introduction of heterogeneous catalysts (acid, base, or ion-exchange resins), and biocatalysts (free or immobilized enzymes) could solve these problems, with durability and reusability as key aspects to reduce their environmental impact. High reusability (around 4 times, but increasing to up to 8 or 9 times depending on recent studies) could reduce the impact related to chemical synthesis of the catalyst, lowering energy and material inputs in catalyst production. In addition, the search for natural heterogeneous catalysts is a promising research line that could enhance their common use in biodiesel synthesis. Figure 7 shows the main advantages and disadvantages of every kind of catalyst according to the main factors that could affect LCA. In general, high conversion is advisable, which is normally achieved by using homogeneous catalysts, whose conversion is commonly above 90–95%. This is an advantage, as lower amounts of unreacted reagents will be found, assuring a predictable quality in final biodiesel. However, homogeneous catalysts usually have lower performance in other aspects, like the lack of recovery or reusability, waste generation (mainly water after purification), and their origin, which is not natural in many cases [25]. In conclusion, heterogeneous catalysts have great potential to contribute to the improvement of future LCAs, as many studies indicate the possibility of using natural sources for their production, with increasing reusability and conversion results. Apart from that, new magnetic catalysts could be easily recovered due to their properties, like thermal sensitivity, acid strength, or high specific surface area [104]. Specifically, a novel catalyst (ZnO-modified starfish-based catalyst) was used for biodiesel production, contributing to a highly efficient process [105]. In some contexts, even the nature or characteristics of raw material make it advisable to use heterogeneous catalysts, as in the case of jatropha oil with high acidity [80]. As a result, their catalytic performance is becoming more and more competitive and sustainable. However, some studies have pointed out the relevance of catalyst preparation in LCA, which presented the highest damage for all the impact categories in biodiesel production from waste date seed through esterification [106]. In these cases, reusability is critical to reduce the environmental impact of the corresponding catalyst.

5.3. Biodiesel Usage in Engines

The performance of biodiesel in internal combustion engines, particularly diesel engines, is a critical factor in determining its viability as a substitute for petroleum diesel [1]. In general, cleaner emissions related to biodiesel are found compared to traditional fuels, especially concerning CO₂ (whose net emission is considered zero, as the sources implied in biodiesel production tend to capture CO₂). In addition, lower emissions of particulate matter, sulfur compounds, and unburned hydrocarbons are related to biodiesel performance in Diesel engines. This way, in general, biodiesel can influence different aspects related to engine performance, including its durability, emissions, and consumption (see Figure 8).

As observed in this figure, fuel feeding in Diesel engines has an influence on LCA in biodiesel production, as higher consumption would imply an increase in biodiesel production, with the corresponding increase in the environmental impact in general. As previously explained, consumption is slightly higher in biodiesel, which is a challenge in this context. Nevertheless, the emissions are considerably lower when biodiesel is used, especially concerning CO and HC, offsetting the previous disadvantage and having a positive impact on LCA related to biodiesel production and, particularly, the use of Diesel engines. Finally, the durability of engines is highly influenced by biodiesel usage, mainly due to their viscosity and acidity, which should be suitable for these purposes to increase the service life and, consequently, reduce LCA in Diesel engines. The following subsections will cover these aspects in detail. It should be noted that not only do diesel engines require optimization when using biodiesel, but also other scenarios, like the shipping industry, could benefit from studies where combustion characteristics of biodiesel are optimized [107].

5.3.1. Engine Durability

While biodiesel offers clear environmental benefits, its impact on engine durability is a critical concern, especially for long-term use and higher blend diesel/biodiesel ratios [1]. Apart from that, there are other concerns that should be addressed in this case, like the formation of deposits on fuel injectors (coking), which can affect spray patterns, combustion efficiency, and global engine performance.

Also, the low oxidation stability of biodiesel can result in an increase in viscosity and acidity, provoking obstructions and corrosion in engines with the subsequent decrease in their service life [61]. Thus, aspects like fuel filter plugging, injector coking, or pump and seal wear should be considered, in particular, in cold climates where biodiesel requires additives to improve cold flow properties.

5.3.2. Emissions

Regarding engine combustion, as explained when the properties of biodiesel were covered, the oxygenated nature of biodiesel promotes more complete burning of the fuel, reducing CO, HC, and PM emissions, but increasing (due to high combustion temperatures) NO_x emissions [18]. Moreover, additives can be included to improve engine efficiency and decrease some emissions, as in the case of solketal or butyl diglycol [108], which are economic solutions that could contribute to the enhancement of LCA. Furthermore, the use of WCO in diesel blends offered a decrease in GHG emissions [109], with a subsequent positive impact on LCA.

5.3.3. Fuel Consumption

Finally, in terms of power output and torque, the performance of biodiesel and blends with diesel is similar and comparable to diesel, with slight variations. For instance, there is a slight reduction in power and torque due to the lower energy content of biodiesel (in general, 10% less than diesel). Subsequently, engines can exhibit a decrease of up to 5% in power, as oxygen content can offset the lower energy density. Also, more biodiesel is commonly required to produce the same power output, with an increase in fuel consumption.

To avoid these negative effects, the addition of antioxidants and good practices during storage (like the use of opaque containers in fresh areas) is advisable. Regarding UCO as raw material for biodiesel production, similar trends with no significant differences in combustion, emission, or performance characteristics are observed compared to other biodiesel samples (from virgin vegetable oils, for example), proving its suitability and valorization in this context [110].

5.4. Common Topics in LCA Applied to Biodiesel Production

Nevertheless, the existing works, although covering different aspects of biodiesel production (from raw materials to innovative processes), share common ground, especially concerning some aspects covered in the following subsections.

5.4.1. Aim and Scope

The aim of these LCAs is mainly limited to three options. First, a comparison with pre-established diesel production to assess the feasibility of the process from an environmental point of view. Second, a thorough analysis of certain biodiesel production, with the aim of offering improvement opportunities. Third, the study of the effect of changes (many of them innovative ones, like the implementation of a new catalyst) in specific stages with special relevance or impact on LCA.

5.4.2. Boundary System

The extension of the boundary might vary depending on these objectives. Thus, if a global comparison with diesel production is desired, extended boundaries (from well to wheel) are advisable, including land usage (or waste management) and the performance of biodiesel in diesel engines. On the other hand, more specific studies, where the effect of a certain change in a critical stage (for instance, conversion to biodiesel), can reduce these boundaries, establishing comparisons when necessary. In general, according to the present review, three main trends can be considered as observed in Figure 9. Concerning Scenario 1 (red circles), many studies have focused on the origin of the raw material for biodiesel production.

Specifically in global scenarios and Scenario 1, land usage, raw material generation/collection, or waste collection, along with the corresponding extraction process, is important. Indeed, recent works have highlighted the relevance of these steps in the environmental impact and the subsequent results in LCA, for instance, when it comes to the environmental impact of the use of fertilizers [84]. There are many examples where these steps are relevant in LCA. For example, a study about biodiesel production from spent coffee ground (a waste with difficult management) through transesterification pointed out the high environmental burden related to oil extraction, drying process, and transesterification (in decreasing order according to their impact), with the former contributing to ecotoxicity and carcinogenic effect due to the use of electricity and chemical products [111]. Thus, for biodiesel production from oleaginous yeasts, cultivation of yeast and harvesting presented a high environmental impact that should be reduced to make the process more sustainable [112]. This kind of study can also be useful for the validation of new processes in certain regions, like in the Amazon, where the LCA of biodiesel production from palm oil was carried out (from agriculture and oil extraction to biodiesel production), showing a great opportunity for economic and social development of Pará and the Amazon region) [113].

Along with the product type, the region where the biodiesel plant is located is influential as well, as some studies have remarked for alternatives for petrodiesel (including rapeseed and soybean produced in different regions) [114], where the adaptation of the corresponding crop to pre-harvest conditions, land usage, transportation due to imports and the adaptation to local energy mix could play a vital role. The influence of land use changes in final LCA results was evident when soybean biodiesel production was considered from different countries apart from Argentina, reinforcing the abovementioned reasoning [115]. In this sense, LCA could be useful for the determination of suitable areas to cultivate for non-food purposes, as explained in the literature in the case of Mediterranean regions. As a result, biodiesel systems based on winter rape offered a better energy balance than traditional diesel systems, with better environmental performance (including less impact in abiotic depletion, photochemical oxidation, and global warming potential) [116].

Scenario 2 is equally interesting, as it considers the performance of biodiesel in engines, with the subsequent emissions and their impact. It is another important focus in this process, which normally reduces the attention to the initial stages, like in Scenario 1. In this respect, studies focused on waste might be adapted to this scenario, where extraction, production, and vehicle operation are commonly included in the boundary system, as observed in studies devoted to biodiesel production from grease trap waste [87]. Other studies have applied LCA to diesel/biodiesel emulsion fuels in engines and their corresponding emissions by adding carbon nanoparticles CNP, which could be another example of this scenario, obtaining valuable findings at different CNP concentrations (between 15 and 38 μM, LCA indicators were mitigated) [117]. Thus, depending on the objective of LCA, more restrictive scenarios could be useful to make good decisions that can contribute to the improvement of the environmental performance of processes related to biodiesel.

Finally, Scenario 3 is more restrictive, with a more limited boundary, where several assumptions are considered, like the negligible effect of land usage or performance in engines, due to the fact that these studies are focused on specific comparisons, commonly within the extraction and production process. Recent studies where the comparison of innovative catalysts was carried out could be included in this scenario, as the most relevant aspects will be related to transesterification and extraction processes [118].

Obviously, there are studies with extensive boundaries where all these steps are covered, offering an overview of the process, which can be interesting. For instance, a study about the use of different catalysts (CeO₂, La₂O_3, and Nd₂O₃) in biodiesel production from jatropha was considered from the plantation (including the use of fertilizers, pesticides, seed collection, etc.), transportation, oil extraction, hydrolysis, esterification and use (not including manual labor). Under these circumstances, La₂O₃ was the catalyst with the lowest environmental impact, with the whole process presenting a high impact due to electricity used in hydrolysis and oil extraction, with a smaller contribution to esterification [119].

However, due to factors like the lack of information regarding some specific categories or simply due to different assumptions or approximations, these boundaries can be reduced as previously explained.

Regardless of the scenario selected, one aspect to consider is the energy inputs for the processes implied in biodiesel production. In this sense, some works are focused on the reduction in energy consumption, whereas others are specifically adapted to the energy mix of the area of interest. In this regard, previous studies have highlighted the relevance of renewable energy plans in order to improve LCA results, as in the case of biodiesel production from WCO [120]. Also, in terms of circular economy, several studies pay attention to the reuse of energy generated in the process (mainly heat recovery). In general, when these steps are taken, LCA results are more attractive, proposing, by way of example, the implementation of renewable energy sources for this purpose. With this regard, previous studies have pointed out, for biodiesel production from soybean, jatropha and microalgae in China, the positive effect of solar energy and CO₂ capture (with the subsequent reduction in dependency on fossil fuels) in different impact categories (ADP, GWP and ODP) compared to fossil fuels, observing that jatropha and microalgae biodiesel were more competitive for all impacts (with lower levels of agricultural inputs per unit of oil produced) [121]. Equally, studies about LCA on biodiesel production from oleaginous yeasts recommended the replacement of electricity from fossil fuels with renewable sources like solar or hydroelectricity, along with recycling and reuse of solvents and waste heat, to reduce the environmental impact of the process [112]. Furthermore, other studies have proposed the alternative use of photovoltaic solar cells for electricity supply in biodiesel production from vegetable oil waste, decreasing the effect of freshwater consumption [122]. Finally, as observed in studies of jatropha biodiesel in rural marginal oil, the reuse and recycling of sub-products derived from biodiesel production enhanced the environmental sustainability of the process, using minimal water and fertilization resources [123].

5.4.3. Functional Unit

As explained in previous sections, the functional unit is important to allow easy comparison between LCA studies. In biodiesel production, the functional unit is highly influenced by the establishment of boundaries. Consequently, if a certain study is focused on production, FU is usually based on liters of produced biodiesel (for instance, 1 or 1000 L), whereas other studies could be interested in power generated by biodiesel (preferring 1000 MW in some cases). As a result, the FU helps these studies to emphasize their findings (focused on production or energy potential).

5.4.4. Inventory Analysis in LCA Applied to Biodiesel Production

Equally influenced by the process and boundary system, some interesting insights can be inferred from the studies covered in this review, like the negative impact of solvents in chemical extraction or the role of methanol and glycerol at this point. Thus, glycerol is an important by-product in biodiesel production, as considerable amounts are generated in this process. Thus, the direct use of glycerol (in reforming processes, for instance), its purification (for pharmaceutical use), or disposal (with the corresponding treatments and removal) are additional steps that should be considered in LCA. Regarding methanol, it is equally relevant in biodiesel production, as it is highly consumed, and its origin should be considered. Also, some additional processes like biolubricant production could contribute to the recovery of methanol, reducing its contribution to direct environmental impacts. As a consequence, in specific inventory analyses where glycerol or methanol take part, different impact categories could play a significant role, like energy use, climate change, human toxicity, or ecotoxicity, among others.

Moreover, other stages like the synthesis of biodiesel can considerably change LCIA, especially concerning the introduction of new and innovative catalysts whose origin and processing should be considered. Additionally, the performance of engines is relevant in this aspect, considering different factors such as particulate matter or emissions like NO_x.

5.4.5. Interpretation of Results

Finally, the interpretation of results is generally carried out by comparing with previous studies, particularly based on diesel processing, or the comparisons of modifications made in the process to assess their environmental impact. As a result, recommendations to improve the process are given, as explained in further sections with specific examples.

The abovementioned trends can be observed in specific works included in

Table 5. Examples of different articles focused on LCA applied to biodiesel production.

Program/Database/Methodology	Description	FU	Ref.
Traci 2.1	Attributional approach following the industrial implementation from laboratory scale for biodiesel production from spent ground coffee Comparison of different extraction processes	1 kg of biodiesel	[111]
ReCiPe	Biodiesel from waste date seed through esterification by using a magnetic catalyst (Fe3O4 nanoparticles), including raw material transportation, oil extraction, catalyst preparation and reuse, and esterification for biodiesel production (cradle-to-gate attributional approach)	1000 kg of biodiesel	[106]
Simapro, Ecoinvent	Biodiesel production from solaris seed tobacco, including tobacco seed production, oil extraction, and biodiesel production	1 kg of biodiesel from solaris tobacco	[101]
ReCiPe, Ecoinvent in Open LCA Software	Comparison of jatropha and rapeseed biodiesels in India, from cradle to grave	1 MJ of energy based on LHV of biodiesel	[124]
Ecoinvent	Biodiesel production from jatropha according to different catalysts and calcination processes	1000 MJ of biodiesel	[119]
Gabi-LCA software	Biodiesel production from mixed vegetable oil waste according to an attributional approach. Comparison with landfilling practices using solar energy	1 t of biodiesel	[122]
Buscar	LCA and LCC of biodiesel production from jatropha oil. LCA was complete, from planting to use in cars	1 t of biodiesel	[80]
ReCiPe, Simapro	LCA of jatropha biodiesel in rural marginal soil with three scenarios: minimal resources (MR), MR + use of sub-products, and additional use of bio-fertilizers and irrigation. The boundaries included raw material production, oil extraction, and biodiesel production	1 GJ of biodiesel energy	[123]
SimaPro	LCA from cradle to gate, including cultivation, pre-treatment, and transesterification of biodiesel production, with a comparison between conventional and room temperature processes	1 t of biodiesel	[125]
Gabi 6.3, Ecoinvent	Review of LCA applied to beef tallow and biodiesel production, with an increasing interest in Brazil	1 MJ of biodiesel	[126]
Aspen Plus, Ecoinvent	Biodiesel production from mutton tallow transesterification, including rendering, transesterification, thermal energy, transport, chemical usage, and electricity	1 MJ of biodiesel	[86]
ReCiPe	Life cycle inventory based on 5 biodiesel plants, with palm oil as the main raw material. This work addressed the effect of land usage, fertilization, and biogas in extraction mills	1 MJ of biodiesel	[127]
Lasen integrating Excel spreadsheets, Ecoinvent	Comparison with different transportation and biodiesel pathways modeled up to the exportation port	1 km driven in a 28-t truck in Switzerland	[115]
Simapro, Ecoinvent	Biodiesel from oleaginous yeasts and bio-crude production to estimate the environmental impact of the resulting biorefinery	300 L of biodiesel	[112]
ReCiPe, Ecoinvent	WCO for biodiesel production, compared with first and third-generation biofuels, including transportation, biodiesel production (including containers)	1 t of biodiesel	[85]
ReCiPe, Simapro, Ecoinvent	Energy valorization of greasy wastewater sludge through biodiesel synthesis, using choline chloride-based deep eutectic solvents with P-toluenesulfonic acid and oxalic acid as green catalysts. Comparison with two-step esterification/transesterification with H2SO4 and CaO catalysts and following an attributional approach	1 MJ of biodiesel from sewage sludge	[73]
Monte Carlo simulation	LCA of biodiesel produced from grease trap waste, including pre-treatment, fuel production, and vehicle operation	1 MJ of biodiesel	[87]
LCA Software, EDIP 2003	Comparison of biodiesel production from microalgae with conventional fossil-derived diesel derived to a filling station in the UK and combusted in a typical car engine, including cultivation, dewatering, oil extraction, refining, transport to biorefinery, esterification, transport to fill station, and combustion in a car	1 t of biodiesel	[95]
LCA software, Ecoinvent	Photothermal process to produce biodiesel from microalgae at room temperature, including microalgae growth, dewatering, and bio-oil production	1 MJ of biodiesel	[97]
Simapro	LCA applied to palm kernel shell catalysts to replace traditional catalysts for biodiesel production from palm cooking oil, including collection, transportation, extraction, preparation of catalyst, and transesterification with methanol	1 t of biodiesel	[118]
Simapro, Ecoinvent	LCA applied to diesel/biodiesel emulsion fuels in engines and their corresponding performance when carbon nanoparticles are added. Includes biodiesel production, nanoparticle production, fuel blend preparation, and engine test.	1 kg of fuel blend	[117]

:

As inferred from this table (where the version of software were not included to homogenize the content), there are various examples with clear differences in biodiesel production. Thus, different raw materials (from typical vegetable oils to waste like WCO, together with algae and microalgae) were found, which condition the rest of the processing to synthesize biodiesel. Also, the introduction of innovative improvements like new catalysts or technologies is typical in these cases. That is the reason why most studies have to adapt their LCA to these circumstances, which are unique in many cases. As a result, different criteria were followed in some critical respects, like system boundaries or FU selection. Regarding the latter, typical FUs are based on weight (1 t of biodiesel) or energy (1 MJ of biodiesel), the latter preferable when a comparison with fossil fuels or other renewable energies is carried out. Another aspect to consider is the order of magnitude of FU, which might vary (e.g., from 1 kg to 1 t of biodiesel). In general, high FU values are used for general processing, whereas the use of small units is considered when specific processes are implied, with lower yields compared to traditional biodiesel production (as in the case of some studies about microalgae) or low amounts of biodiesel required (for example, if engine tests are carried out).

One of the main characteristics of LCA studies in general, even though they are devoted to specific cases like biodiesel production, is the heterogeneous background for the development of these studies, along with the different criteria followed to carry out LCA in every specific context. Similar objections were found in LCA studies on bioenergy technologies, pointing out the incompatibility of some results due to variable FU definitions, incomprehensiveness of impact categories, or the lack of uncertainty in sensitivity analysis [128]. In this same work, useful recommendations are made that would be perfectly applicable and adapted to this case:

• Use of functional units of caloric value of end products, which makes it easier to compare with other forms of energy (fossil fuels or other renewable sources). It is especially relevant in biodiesel, as factors like density, combustion efficiency, and heating value could alter comparisons of FU based on weight. It should be noted that, between a FU of 1 MJ and 1 t of biodiesel, there is a considerable difference (as biodiesel normally has around 40 MJ per kilogram), making the comparison difficult in some cases, especially due to rounding effects.
• Selection of cradle-to-grave system boundaries in order to provide robust LCI in a wider range and complete existing results, as observed in the literature for biodiesel production from palm oil in Indonesia [129]. Thus, if the effect of feedstock is not considered because a certain LCA is a strict comparison within biodiesel production, some impact categories like land use will not be decisive at this point.
• Careful and reasonable assumptions according to the aim and scope.
• Complete system boundaries and impact categories (including GHG, energy, land use change, eutrophication, acidification, loss of biodiversity, and water depletion). As previous studies have pointed out, complete boundaries (mainly including agricultural and industrial processes) can provide a clear picture of the sustainability of biofuel utilization [130]. In our case, highly detailed LCAs would be recommended as recent studies cover an innovative approach that might be unknown in the current environmental scenario.
• It is preferable to use site-specific data instead of the software’s built-in database. In such a way, conversion of biodiesel production, its final yield (and those corresponding to glycerol, for instance), etc., are real data (obtained at laboratory or industrial scale) that can contribute to a more realistic LCA.

Nevertheless, the contribution of these works is valuable, as they assess the sustainability of various processes for biodiesel production under specific circumstances, which can be difficult to compare in many cases. In other words, and showing the example of boundary systems, they are different because every situation is unique, remarking the relevance of such technical studies and the versatility of LCA. These contributions are important to support a global LCA overview focused on biodiesel, which is an almost unmanageable subject by itself.

6. Conclusions

The main findings inferred from this review were the following:

• LCA applied to biodiesel has great interest, as the use of this tool is essential to support the sustainable implementation of this technology, from economic, environmental, and social points of view. Many works have covered a wide range of scenarios like different regions or countries, variable raw materials, and specific situations like energy supply or modified processes.
• Specifically, most studies are focused on LCA applied to detailed biodiesel production. Thus, every case has its own particularities, from specific waste, pre-treatments, secondary processes, or the use of different energy supplies. This fact points out the versatility of LCA, easily adapted to each process through the establishment of clear aims, boundaries, functional units, and inventory analysis, among others. Even though biodiesel production has high TRLs (especially transesterification and esterification with homogeneous catalysts, between 8 and 9), the industrial implementation of other innovative processes is still under development, requiring evaluation through LCA.
• Generally, the studies that were covered in this work carried out three modalities of LCA: (1) global LCA from well to wheel, to compare with traditional diesel of mineral origin; (2) similar to the previous step, with the main focus on establishing specific recommendations to improve the process; (3) more specific LCA studies (with limited boundaries and several assumptions) to compare changes made in one point of the process (like the use of various catalysts, extraction methods, energy sources, etc.).
• LCA can be applied to innovative aspects of biodiesel production, being decisive in assessing the feasibility of new trends at an industrial scale. On the other hand, the implementation of biodiesel production within a biorefinery context should be equally analyzed through LCA, as different processes could enhance the environmental impact of this kind of facility.
• Concerning biodiesel production from vegetable oils, different crops have been covered, especially jatropha and palm. The most influential steps in LCA were land usage (where the use of N-fertilizers and crop productivity play an important role) and the extraction process. For the latter, the main impact was due to energy consumption and the use of solvents in Soxhlet extraction, which can cause negative carcinogenic and ecotoxic effects. WCO is relevant at this point, as its combination with other innovative options (like heterogeneous catalysis) is an appropriate choice for the valorization of this waste.
• Other raw materials, like algae and animal fats, present challenges like the low efficiency in oil extraction from the corresponding source. Nevertheless, their implementation in biodiesel production is feasible and has great potential, as a reduction in some impact categories was found in general (for instance, biodiesel production from microalgae typically reduces GHG due to CO₂ capture).
• With this regard, the transformation of the raw material to biodiesel (mainly through transesterification and esterification) did not have a great impact on LCA compared to the aforementioned factors, especially when it comes to energy and pollution (possibly due to the high conversion and efficiency of the process in general).
• However, typical challenges related to LCA studies focused on specific cases might arise, like the requirement for standardizing LCA methods, metrics, and tools to ensure consistency and comparability across studies. In this context, it is highly advisable to carry out a complete LCA, considering complete system boundaries and unified functional units (1 MJ of biodiesel for energy purposes and 1 t of biodiesel for comparisons regarding production).
• As a result, even though these studies were heterogeneous, they present some points in common, like the relevance of energy supply to improve different aspects of LCA (with solar energy being highly recommended). Also, the efficiency of the process (in particular, during extraction and conversion to obtain biodiesel) could contribute to better results in LCA.
• Finally, more and complete LCA studies are required to fill the current research gap, especially concerning the use of innovative heterogeneous catalysts or production methods like the use of ultrasound or microwave-assisted transesterification.