Assessing the future prospects of emerging technologies for shipping and aviation biofuels: A critical review

Renewable and Sustainable Energy Reviews 197 (2024) 114427 Available online 5 April 2024 1364-0321/© 2024 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Assessing the future prospects of emerging technologies for shipping and aviation biofuels: A critical review Ana Arias a , * , Chrysanthi-Elisabeth Nika b , Vasileia Vasilaki b , Gumersindo Feijoo a , Maria Teresa Moreira a , Evina Katsou b a CRETUS, Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, 15782, Santiago de Compostela, Spain b Department of Civil & Environmental Engineering, Uxbridge Campus, Institute of Environment, Health and Societies, Brunel University London, Middlesex, Uxbridge, UB8 3PH, UK ARTICLE INFO Keywords: Biofuels Sustainability Prospective LCA Certification Energy transition ABSTRACT There is an urgent need to switch from fossil to bio-based fuels in the transport sector, particularly in shipping and aviation. The growth of the world’s population has resulted in a significant impact on passenger transport, with a noticeable increase in greenhouse gas emissions, depletion of fossil resources and associated risks in all three pillars of sustainability. In this context, new policies, standards and targets have been developed to reduce this environmental damage, which is mainly caused by the use of fossil fuels. Therefore, the alternative of using biofuels seems to be the most appropriate solution, to the extent that important targets have been set and specific directives have been developed for the integration of biofuels in the maritime and aviation sectors. However, to demonstrate that switching to biofuels is indeed beneficial, it is necessary to evaluate new biofuel scenarios from a life-cycle perspective, with particular emphasis on analyses that provide information beyond the present, such as prospective life-cycle assessments. To this end, the focus of this review is on the current trends in the production of biofuels for the marine and aviation sectors, taking into account the main targets set, the existing regulations and directives on the subject, and an analysis of the type of technologies used for their production. It also addresses biofuel Life Cycle Assessment (LCA) scenarios and future LCA approaches, and how these analyses should be carried out to be effective. Finally, key policies, standards and certifications are analyzed. The trends and bottlenecks discussed in this review concerning the actual and future development of the biofuels sector could be used by policy makers and stakeholders to identify efforts that favor the integration of biofuels into the value chain. Furthermore, it could be concluded that the evaluation of the guidelines foreseen in the development of competitive scenarios based on emerging technologies, as well as the adoption of policies and restrictions on the use of fuels, are key conditions to establish the roadmap for the widespread implementation of biofuels. 1. Introduction The growth of population is directly affecting over the transport sector. The intensity and the “road transport” is causing important issues over the environment, given the emissions, and to communities’ health, as a more pollutant ambient is being faced. Regarding air pollutants, just focusing on transport sector, it is responsible of the 45% of the emissions of nitrogen oxides. 2% of sulfur oxides emissions, 13% of particular matter, 8.7% of non-methane volatile organic compounds and 1.2% of NH 3 emissions, according to the European Environment Agency. Besides, in 2021 a total of 0.84 Gt and 0.71 Gt of CO 2 has been emitted by shipping and aviation sectors, respectively, according to the International Energy Agency. These emissions are mostly the result of the use of fossil-based fuels, that entails, not only the release of harmful compounds in the use-phase, but also in the production one. Given this, there is a need on providing more efficient and less harmful primary resources to respond to the demands of population with respect to passenger transport daily routines and travels, and the use of bio-based fuels could imply important benefits, as those are less harmful and could also be more efficient. In this regard, the use of bio-based fuels, which entails less environmental damage throughout its life cycle, should be promote. Besides, the combination of eco-efficient trajectories together with the use of biofuels could add value in the pursuit for more sustainable transport * Corresponding author. E-mail address: [email protected] (A. Arias). Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser https://doi.org/10.1016/j.rser.2024.114427 Received 23 January 2023; Received in revised form 21 March 2024; Accepted 2 April 2024 Renewable and Sustainable Energy Reviews 197 (2024) 114427 2 [1]. In this context, the main targets for transport sector are the ones reported by the European Union (EU) Strategy, European Green Deal, EU Emissions Trading System, International strategies (International Marine Organization’s (IMO)) and ReFuelEU Initiative, and included below. 1. Reduce carbon intensity by at least 40% using low-carbon fuels by 2030, and 70% by 2050, using zero-carbon fuels (IMO strategy). 2. Establish control mechanisms for CO 2 emissions in terms of monitoring, reporting and verification (Regulation (EU) 2015/757 of the European Parliament and of the Council on the monitoring, reporting and verification of carbon dioxide emissions from maritime transport). 3. Encourage market-based carbon measures, such as a CO 2 tax (ReFuelEU Initiative, ‘Fit for 55’ package”). 4. In absolute terms and with a time horizon, 50% reduction of total annual GHG emissions by 2050 (European Green Deal, EU Strategy, EU Emissions Trading System) [2,3]. These targets are the main driving force for the development of new emerging technologies for biofuel production. Reducing the carbon footprint in the transport sector towards to achieve zero-carbon emissions requires the adoption of sustainable technologies and the use of renewable resources [4,5]. The technology must evolve from low Technology Readiness Level (TRL) values to high values in order to prove the technological feasibility of the process on a large scale, which will provide insight into future potential, stability, risk and market penetration. The use of prospective methodologies is considered a suitable tool to develop the assessment of the carbon impact from a broad perspective [6,7]. Considering the need to provide a forward-looking perspective, assessments should take into account changes in the environment in which the technology will be applied at least 50 years from now, trying to envision the most accurate future scenario that the emerging technology will face [8,9]. The use of prospective assessments require the use of expert knowledge, projected estimations and databases [10–12]. To this end, it is necessary to know the existing situation considered as a reference scenario, to propose possible improvements but also the main bottlenecks that hinder the transition to biofuel production. In this regard, the main goal of this critical review is to evaluate the technology and the methodologies being developed in the field of biofuel production, as well as the identification of standards and certifications schemes on the use of biofuels in transport activities. On the other hand, the potential and market penetration of biofuels, the use of prospective life cycle assessment (LCA), social life cycle assessment (S-LCA) and techno-economic assessment (TEA) as appropriate environmental, social and techno-economic impact assessment are also evaluated. 2. Regulations on biofuels in the transport sector The EU has adopted the Renewable Energy Directive (REDII 2021–2030) as the core for achieving the decarbonization of transport sector, including the limitation on the use of energy crops for the subsequent biofuel production, the encouragement of the development of advanced biofuels and the defense on the maintenance of bio-based natural resources below the earth limits, to avoid its depletion and its environmental effects [13–16]. The commitment to the valorization of agricultural, livestock and forestry residues, the use of algae, non-food raw materials, as well as other non-useable waste streams, such as waste cooking oil, are the main approaches to focus on biofuel production alternatives. This Directive also introduces the main criteria that the biofuels may comply in order to ensure that its production and use is encompassed within the boundaries of sustainability development that, in general terms, refers to the fact that those biofuels does not entail an environmental load comparable or higher than that of fossil-based fuels. In this way, governments, stakeholders, policy makers and development organizations are betting on a proactive and anticipatory action, with the aim of avoiding the environmental damage caused by the massive and uncontrolled use of fossil resources for the production of fuels. In this context, other organizations have developed a series of guidelines, documentation and certification schemes focused on the biofuel sector, with the objective of supporting their development from a perspective that promotes sustainable production and use. One of the most recognized is the guidance biofuel certification document created by the International Sustainability & Carbon Certification (ISCC), mainly focused on wood-based biofuels [17]. It is considered a verification of compliance with social, environmental and traceability criteria for biofuels in accordance with the targets defined by the European regulations for transport fuels. This document compiles sustainability requirements [18,19], GHG emissions estimation [20] and traceability of biofuels value chain [21]. As it could be seen on Fig. 1, there are two transportation sectors (aviation and maritime) that, both at present and in the estimated projections, have the higher potential impact on the levels of emissions and environmental damage derived from the use of fossil fuels, because of this, a large number of regulations have been developed for these sectors. The American Society for Testing and Materials (ASTM) D7566 has approved the use of Sustainable Aviation Fuels (SAF) in jet engines for passenger and freight transport, given their good performance and reduced environmental loads compared to traditional fossil fuels [22], but with some restrictions. The use of biofuels in the fuel blends is subjected to the ASTM standard, in order to ensure the technical and safety conditions in flights. Using biofuels for aircraft, decarbonization of the aviation sector could be achieved, but still certain barriers could be detected when implementing bio-based fuels in the aviation sector. Some of them includes higher cost in comparison to fossil fuels, lack of guidelines for certification process and of policy framework and governmental funding for implementation [23–25]. Replacing 90% of conventional aviation fuels with biofuels is expected to reduce emissions by 53% by 2050 [26,27]. The Air Transport Action Group has published the Waypoint 2050 report, with the purpose of achieving net-zero emissions by 2050 in the aviation sector, following the guidelines set to achieve the goals of the European Green Deal and the SDGs [28,29]. The Fit For 55-package of the European Green Deal is a recent European Commission action plan (July 2021). The proposal aims to create a European mandate for the supply and implementation of SAF at most EU airports, encouraging the use of biofuels for air transportation. The main reason for the development of this initiative is to achieve the EU climate targets for the reduction of emissions and dependence on fossil resources. The main goal is that the percentage of SAF in the air transport departing from EU airports would increase gradually, reaching the target of 63% by 2050. According to ICAO (International Civil Aviation Organization), 57 airports distribute SAF worldwide, representing more than 440 000 commercial flights. Around 24 policies have been adopted or are being developed by international organizations and purchase agreements have been signed for some 34.9 billion liters of SAF [30]. Moreover, the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) is a scheme that favors the integration of biofuels in this transport sector. The scheme promotes the reduction of carbon dioxide emissions through the improvement of aviation technologies and operational conditions, as well as the development of national and/or regional regulatory initiatives [31]. On the other hand, in order to encourage the use of advanced shipping bio-fuels in a large-scale market value chain, and to regulate its prices, the European Commission has introduced the “Inducement Price for the Promotion of Renewable fuels” [32]. The goal of this proposal is to make the use of advanced biofuels for a market supply chain possible for shipping sector through price regulation. The assessments conducted to link the relationship between price and biofuel effectiveness have A. Arias et al. Renewable and Sustainable Energy Reviews 197 (2024) 114427 3 been developed using 100% biofuels for a transport distance of at least 21 600 nautical miles. With an economic approach, the Poseidon Principles have also been developed, with the objective of carrying out a methodology to integrate the environmental risk within the financial and investment decision for the shipping sector [33]. For example, the European Investment Bank has succeeded in developing a Green Shipping Guarantee (GSG) Program, with the aim of accelerating investments in more sustainable and green technologies for shipping companies [34,35]. One of the goals in technology improvement is to achieve energy efficiency in shipping activities. In this context, the SEEMP (Sheep Energy Efficiency Management Plan), a mechanism developed by the International Maritime Organization, introduces a set of guidelines for the development of ship energy efficiency, encompassing calculation methods for measuring energy efficiency (Annex 7) [36], carbon intensity and correction factors (Annexes 14 and 17) [37,38], carbon-based indicators (Annex 15) [39], and carbon intensity classification of ships (Annex 16) [40], among others. All of the above policies, standards, targets, etc. are a reflection of the necessity for the sector to meet the requirements and objectives established for biofuels. An urgent acceleration of the commercialization of biofuels with a high level of quality is a must to be achieved. In this regard, the implementation of appropriate policy definitions and assessments could be considered a key role in the biofuel production sector. In fact, there are currently policies about this, briefly described below. −10-Year Framework of Programs on Sustainable Consumption and Production (SCP) Patterns: a global initiative to increase international cooperation to enhance SCP. - OECD: Sustainable Materials Management and Green Claims: a policy to foster green and sustainable growth at both economic and demographic levels. - UNIDO/UNEP Program on Resource-Efficient Cleaner Production: based on the recognition of methodologies and procedures that bring benefits to adequately address global challenges on stabilizing and improving sustainable environmental, social and economic practices, among others [41]. 3. What are the actual perspectives on the use and commercialization of shipping and jet biofuels? The development of more sustainable fuels for shipping and jet transports the main efforts and challenges are divided in Ref. [42]. 1. Technical management (i.e. improvement of the technologies to be more efficient). 2. Operational conditions (i.e. combustion temperature). 3. Enhance the use of more environmentally-friendly fuels (i.e. advocate for avoiding the fossil-based fuels). 4. Alternative power sources (i.e. using renewable sources). 5. Establishment of carbon capture and storage techniques on ship and aviation boards. In this context, all the strategic development on the shipping and aviation sector should be in line with the objectives of reducing carbon emission in both short, medium and long term (Fig. 2) [43,44]. But huge efforts are needed to achieve a zero-carbon emission of both transport sector because, given the estimations of the International Energy Agency, aviation sector requires about 220 Mton/year of biofuel oil equivalents to fully decarbonized, while for the case of shipping sector, is a little bit higher, amounting to 240 Mton/year of oil equivalents [45]. According to Solakivi et al. (2022), the marine fuels could be divided in four main categories: non-sustainable conventional fuels, LNG/LPG and non-renewable hydrogen, sustainable but underdevelopment biofuels and renewable/advanced biofuels and e-fuels [45]. In this regard, the main characteristics of the alternative marine alternative fuels are depicted on Table 1. On the other hand, it should be taking into account that the marine transportation sector could go for further alternative Fig. 1. Contribution on transport sectors on (a) GHG emissions, (b) air pollutants and (c) progression on CO 2 eq. emissions and projections per sector. Database: European Environmental Agency, Eurostat. A. Arias et al. Renewable and Sustainable Energy Reviews 197 (2024) 114427 4 fuels to replace the fossil-based ones in comparison to aviation sector, as the engines and infrastructures of ships are more adequate to use lower-quality fuels [46]. But, indeed, for both sectors, the main challenges to be addressed are the reduction on the production costs, the need of a higher technological mature and the increase on the availability of infrastructure to the alternative fuels use and distribution [47, 48]. Given the assessment of the most researched and developed alternatives for maritime fuels, should be mentioned which are the ideal characteristics that should have in order to be effective for the engines and infrastructures. The report developed by Gray et al. (2021) have identified the following: high energy density (to avoid the high storage volumes), low emissions levels, reduced production and use costs (to be attractive to replace the conventional ones), scalability (to be used for both short and long transport distances) and compatibility with actual infrastructure [52]. In this context, the report developed by Xing et al. (2021) have analyze different issues related with the adequacy of the alternative marine fuels, classifying them in priority levels and strengths for its application [59]. The use of methanol, hydrogen and ammonia as maritime fuels are the most adequate form the point of view of environmental impacts, however the differences on engines and on the capacity of energy production should be taking into account. While methanol could be used to all types of transport, ammonia is not yet available to be used for deep sea routes, while compressed H 2 is only effective for domestic shipping. In order to avoid these disadvantages, it has been reported that a possible solution is the use of propulsion technological systems, which should be adapted in function on the transport distance. Regarding the aviation sector, the requirements for the development of alternative fuels are stricter in comparison to that for maritime transport, as higher quality is required, with unique fuel properties required to be compiled. In this regard, for example, biodiesel could not be used as jet fuel, given its reduced energy density and high freezing point [60]. To this end, the main alternative for decarbonizing the aviation sector is with the use of SAF (Sustainable Aviation Fuels), defined as alternative fuels with similar properties to that of conventional ones but with reduced environmental footprint, thus being more sustainable. The advantage of the use of SAF relies on the fact that are classified as “drop-in” fuels, meaning that they could be used in the planes without needs of modifying the engines or infrastructures [61]. In this regard, the main companies manufacturing SAF are depicted on Table 2, including the type of feedstock, the production process methodology used by each of them, the production capacity and the applications. While on Table 3 some examples of airlines using SAF are depicted. As for shipping fuels, Gray et al. (2021) have also identified the idealities for jet-alternative-fuels, being the following the most outstanding: high energy density, high specific energy (to enhance the efficiency), high flash point (to be safe), low emissions levels, low viscosities and freezing points (to ensure the fuel quality at reduced temperatures), high thermal stability and good lubrication properties [52]. 3.1. Some advances on the use of bio-fuels W¨ artsil¨ a, a marine engine manufacturer, constructed a bio-methanol based engine for a ferry of the company Stena Germanica, the first of the world. This engine is based on a cylindric engine in which the biomethanol is injected and ignited using an unsignificant amount of fossil-based fuel [52]. On the other hand, regarding directly the bio-methanol, one of the biggest production facilities is located in Canada, Enerkem plat, being able of producing a total of 38 ML/year using as feedstock, municipal solid waste. Another company is BioMCN, with a lower capacity, 15 t/year, and using an alternative raw material, biogas. To this end, it could be observed that the degree of development of bio-methanol production could be encountered between a TRL6 to 8. Fig. 2. Goals for reducing the GHGs from shipping and aviation industries. A. Arias et al. Renewable and Sustainable Energy Reviews 197 (2024) 114427 5 The main problem to its further TRL is the bio-based feedstocks used for its production, its management and process stages required to the final bio-methanol production, at least for now, have a limit, that is a maximum capacity of 1300–2600 t/day. Some technological companies have also developed interesting process schemes to the production of alternative marine and jet sustainable fuels: Honeywell Co., an American company, uses vegetable oils and fats to obtain green diesel and Haldor TopsØe has constructed a hydrotreating-based technology to produce green diesel using raw grease raw materials. On the other hand, other companies such as SkyNGR, Project Solaris or Petrobras have adopted co-processing technologies in which the marine and jet bio-fuels are blended with petroleum-based traditional fuels. The rationale behind this is the attempt to achieve a more sustainable fuel give the reduced sulfur content that a mixing with a bio-based fuel could provide, thus also reducing the carbon-related emissions [55]. 4. What is the current stage of biofuel production and commercialization? According to the International Energy Agency (IEA), in 2020 the demand of biofuels amounts to 146.72 billion liters of biofuels and this Table 1 Main alternative fuels developed for replace fossil-based maritime fuels. Fuel alternative Emissions compared to conventional Advantages Disadvantages Other issues Future challenges Reference Ammonia Reduced level emissions of CO 2 , SO x and PM Cleaner energy C-neutral fuel Clean energy carrier Better store conditions than bio-H 2 (higher T) Could be used in internal combustion engines and fuel cells Higher level of NO x emissions 5-times more storage volume than conventional diesel Low heating value Shipping infrastructure and bunkering is not adequate for using ammonia as fuel Difficult storage Incomplete submission leads to NO x emissions Develop efficient ignition engines given its low auto-ignition [44,45, 49–51] H 2 Reduced level emissions of CO 2 , SO x and PM Available in abundance, higher efficiencies in fuel cells than fossil-based fuels Found in compound form, its energy potential is lost when it is refined, difficult storage, fuel cells are expensive, high flammability entailing potential risks Fuel cells require low maintenance Reduce the cost of fuel cell devices to enhance the use of as H 2 biofuel [45,50] Methanol High level of NO x , medium level of CO 2 Availability, not as expensive as other biofuels, engine simplicity, able to adapt existing engines Low energy content, low density and high viscosity than conventional fuels, 11 chips on services uses methanol as fuel, various feedstocks could be used for its production, including waste resources Most used form of marine fuel, with 160 ktons consumed annually Engine and fuel supply infrastructures adaptation [44,45, 51,52] Bio-LNG a Lower emission of CO 2, lower sulfur content and limited SO 2 emissions Modification of engine and fuel supply structure Lower cost than other biomassbased fuels, low energy density, higher storage volume The engines could be modified to reduce the emissions of NO x, most likely options to comply IMO f regulations Need to reduce the limitation of the availability of bunkering facilities [45,48, 50,51] HVO b Reduced emissions of NO x and CO 2 Slight modification of propulsion systems and engines, high energy density Its production could entail deforestation (feedstock) and is a expensive alternative fuel High quality fuel Expected growth in the next years [45,53, 54] FAME c Higher carbon footprint than other alternative fuels Slight modification of propulsion systems and engines Blend with conventional fuel to be effective, low energy content, low density and high viscosity than conventional fuels, high cost It could lead to filter clogging and reduced fuel flow at medium temperatures Reduce its production complexity of removing the glycerol content [45,55, 56] e-fuels Reduced emissions and could be considered as “carbon capture systems” as are produced by combining CO 2 /N 2 and hydrogen Compatible with actual engines, fungible with conventional fuel, adequate energy density Low energy conversion efficiencies, high production costs in comparison with conventional and biomassbased fuels Safety fuels, aware on the feedstock availability for its production Increase the technology readiness level for its further development [51,57] Bioethanol Local emissions of CO 2 and NO x could be increased, depending on the engine used Biodegradable, lower production prices than other alternative fuels It can be corrosive for the engine materials Bio-ethanol fuel cells are a viable option for its implementation Need to adapt for its use on lower engines: increase cetane number and lubricating power [51] HDPO d Able to reduce the CO 2 emissions in more than 50% Large availability of biomass feedstock for its production, high energy density High production costs Technology is not mature yet Engines and new technologies are under construction [53,58] DME e Clean combustion with reduced emissions Heating value similar to diesel, compatible with diesel engines Lower density than conventional fuels, low viscosity and lubricity Similar properties as propane, so infrastructures to its distribution and use are yet available. Analyze the possibility of blending to increase its viscosity and lubricity [46] a LNG: liquified natural gas. b HVO: hydrotreated vegetable oil. c FAME: fatty acid methyl ester. d HDPO: Hydrotreated Pyrolysis Oil. e DME: Dimethyl ether. f IMO: International Maritime Organization. A. Arias et al. Renewable and Sustainable Energy Reviews 197 (2024) 114427 6 value increased to 155.43 billion liters in 2021 [62]. Bio-based fuels will be gradually introduced in aviation for outbound flights, considering first a 2% substitution of fossil-based fuel in 2025, and increasing the percentage by 5% (2030), 20% (2035), 32% (2040), 38% (2045) and 63% (2050) [63]. In this sense, at European level, the total number of facilities producing biofuels from different feedstocks is 339: 80.2% of them are already on a commercial scale, 16.2% on a pilot/demonstration scale and 3.5% are under R&D. Fig. 3 shows the total number of facilities per country and type of feedstock (agricultural {orange label}, forestry {dark green}, grasses and short-rotation coppice {light green}, waste residues {brown} and marine feedstocks {blue}). Fig. 4 provides a more holistic view on the current development of biofuel supply chains. Most of the supply chain is ruled by fossil-based fuels (93% share), the use of diesel and gasoline is dominant with percentage 60% and 24%, respectively. Concerning the biofuels, biodiesel is the one that stands out, followed by HVO (Hydrotreated Vegetable Oil) and bioethanol, with 20% and 13% share, respectively. Another important aspect regarding the biofuel value chain is about where are those facilities located for its production, in which the use of marginal areas is increasingly. The production of bioenergy, particular bio-based fuels, in marginal areas provides both opportunities and challenges in terms of sustainability [64]. Those areas are typically characterized to be low agricultural productive, thus offering a viable alternative for the growing of energy crops, thus avoiding the competition and potential impacts over food security, one of the targets of the Sustainable Development Goals” [65,66]. Biobased fuels obtained by the harvesting of marginal areas could contribute to the diversification of the energy resources and to the reduction on the depletion of fossil fuels and on the impacts over the environment, as those are lower massified [67]. However, in order to ensure sustainability, careful analysis of the use of marginal areas should be developed. Firstly, the selection of the type of crop used for biofuels production, those selected should have higher energy yields, low chemical and fertilization requirements and minimal negative environmental impacts [68,69]. Besides, the land management is also essential to ensure its long-term viability for biofuels production, for which crop rotation, agroforestry or soil conservation techniques could be effective [65]. Secondly, also economic and social factors should be assessed, local communities of those marginal areas should be involved in the decision-making progress in order to ensure a fair distribution of the benefits from the production of biofuels. On the other hand, the use of this marginal areas should also create employment opportunities and should promote rural development and economic growth [70,71]. As a general conclusion, it could be stated that the production of biofuels using marginal areas has the potential to promote more sustainable actions and to ensure food security in a higher level, but to achieve this, the address of environmental, social and economic factors is essential in order to provide a positive impact of biofuels projects in these regions. 5. Which are the types of biofuels being assessed? 95% of biodiesel (first-generation biofuel) is produced from edible oil-crops as feedstock. Food competition, large crop areas and water consumption are the main barriers [72]. The use of food crops to produce biofuels such as bioethanol leads to higher GHG emissions compared to bioethanol obtained from lignocellulosic feedstocks [73]. Biofuels obtained from non-food feedstocks are known as second-generation biofuels, which encompass the use of energy crops, waste streams from crops and agricultural activities, wood-derived waste resources and discarded cooking oil [73,74]. Second-generation biofuels are considered cleaner fuels than first-generation biofuels because of their lower environmental impact and higher energy efficiency, as well as cheaper feedstock supply [75]. However, higher investment is required due to the need of emerging technologies [76,77]. Finally, third-generation biofuels from the use of algae are considered the most promising for the future. Their production process is at an early stage of development; advantages include cheaper production processes than other biofuels, high feedstock availability and remarkable productivity, as algae -based feedstocks can produce 15–3000 times more oil for biodiesel production compared to first and second generation biofuels [78–80]. Despite the differences in first, second and third generation biofuels, evidence of lower environmental impacts compared to those of fossil origin has been reported (94 g CO 2 eq/MJ for petrol and diesel, Table 2 Some examples of companies producing SAF. Company Feedstock Technology Production annual capacity (million gallons) Some application Neste 100% renewable waste and residues (i.e cooking oil, fats and greases) NEXBTL™ technology, based on HEFA- technology (hydrodeoxygenation, isomerization and distillation) 515 Commercial airlines KLM, Lufhansa, Delta and American Airlines are using Neste SAF. Is available in 12 airports in the US and in 3 in Europe Gevo Inc. Inedible corn feedstock Alcohol-to-jet process using bio-ethanol and bio-isobutanol, produced using corn. 55, increasing to 100 in a five-year term Agreement with Delta Air Lines for supplying 75 million gallons of SAF yearly for the next seven years World Energy Inedible oils and waste HEFA-technology 230 per year Operating with United Airlines SkyNRG Agricultural residues HEFA-technology 33 40 airlines worldwide are using SkyNGR SAF, and this company has invested in a startup “Synkero” for producing e-fuel using green hydrogen and renewable energy Phillips 66 Waste oils HEFA-technology 290 It supplies SAF for British Airways Lanzajet Lignocellulosic (mainly waste wood and biomass) Ethanol-to-jet technology 100 Partnership of the “Marquis Industrial Complex” the first carbon-neutral industrial area from with SAF is supplied for Chicago O’Hare and Midway international airports Fulcrum Bioenergy Landfill waste Gasification/Fischer-Tropsch 7 The first US airline that has invested in a SAF company uses Fulcrum SAF. It has also agreements to operate with UK airlines in the future Red Rock Biofuels Wood-based residues (i.e. slash piles from forest clean-up usually burned in winter) Gasification/Fischer-Tropsch 6 Is a company included in the Cellulosic Fuel Purchase and Sale Agreement given its capacity of producing renewable diesel and SAF Shell Cooking oil, municipal waste and woody biomass Fischer-Tropsch 2 by 2025 Spanish Ryanair airlines have signed an agreement with Shell as SAF supplier. Also Luthansa, with a value amounting to 1.8 Mtons in the period 2024–2030 A. Arias et al. Renewable and Sustainable Energy Reviews 197 (2024) 114427 7 according to REDII) [76,81,82]. To this end, research and innovation in sustainable technologies must be the main focus of attention in future development. 6. Technological pathways to produce biofuels The technologies used classified as thermochemical processes are hydrothermal liquefaction, gasification, fast pyrolysis [83]. Gasification is based on partial oxidation generating solid and gaseous fuels, while pyrolysis requires absence of oxygen (no oxidation) and produces solid, liquid and gaseous fuels. Fast pyrolysis provides higher yields for liquid biofuels, and is characterized by shorter residence time and fast heating rates [84]. Hydrothermal liquefaction shows significant potential in biofuel production, given the ease and efficiency of the process (low reaction time, applicability to different feedstocks, high yields), producing a biocrude with low oxygen content and higher stability to be Table 3 Some examples of airlines using SAF. Company Prospects Companies providing SAF Approximate/ available quantities Other info Air France By 2030 incorporating at least 10% of SAF, to achieve 63% in 2050 Neste and DG Fuels 1.6 Mtons of SAF from 2023 to 2036 First flight using SAF launched in 2011 Air Canada Researching on hydrogen, electric and hybrid aircraft technologies Neste – Four commercial flights using SAF has been operated last year British Airways By 2030 incorporating at least 10% of SAF Phillips 66 & Lanzajet Multi-year agreement Co-partner of Speedbird project, aiming to increase SAF production in UK using alcohol-to-jet technolgies Finnair Half the net emissions by the end of 2025 and C- neutrality by 2045 Gevo 7 million gallons/year for 5-years starting from 2027 Agreement with other companies, such as Neste, as SAF suppliers Luthansa Half the C- emissions by 2030 OMV, Shell. 800 thousand tons from 2023 to 2030 with OMV and 1.8 Mtons with Shell between 2024 and 2030 Pioneer on testing SAF emissions in regular flights Malaysia Airlines By 2030 incorporating at least 10% of SAF, to achieve C- neutrality in 2050 Neste – First passenger flight using Neste’s SAF blended with jet fuel in 2022 TAP Air Portugal By 2030 incorporating at least 10% of SAF, to achieve 63% in 2050 Neste – The 1st flight using 39% of SAF launched in 2022 Swiss Half the C- emissions by 2030 Neste – Support on Synhelion company, that aims to provide SAF using solar energy sources United Airlines Reduce the GHG emission at 100% by 2050 Neste 52.5 million gallons over the next 3- years First US airline investing on SAF Qatar Airways Replace 10% of conventional jet-fuel with SAF by 2030 Gevo 25 million US gallons in the next 6-years First airline in the Middle East that has achieved the highest level of the IATA accreditation Iberia Airlines By 2030 incorporating at least 10% of SAF Gevo 6 million gallons First flight with SAF in 2011 and using biofuel produced from residuals in 2021. Table 3 (continued) Company Prospects Companies providing SAF Approximate/ available quantities Other info Ryanair Airlines By 2030 incorporating at least 12.5% of SAF Shell 120 million gallons between 2025 and 2030 Agreement to construct supply SAF infrastructures in more than 200 airports Norwegian Airlines 45% less CO 2 emissions by 2030 Neste – The aircrafts of Norwegian are capable on tank up to 50% SAF Vueling Airlines By 2030 incorporating at least 10% of SAF Cepsa, Repsol S.A. 800 thousand tones of SAF by 2030 with Cepsa, This airline has reduced its emissions in 60- ton CO 2 eq in three months with the use of SAF Virgin Atlantic Replace 10% of conventional jet-fuel with SAF by 2030 Gevo 70 million US gallons over the next 7 years Expected to be the 1st net zero transatlantic flight in 2030 from the UK Fig. 3. European facilities producing biofuels categorized by country and type of feedstock. Adapted from Data-Modelling platform of resource economics (European Commission). A. Arias et al. Renewable and Sustainable Energy Reviews 197 (2024) 114427 8 used as fuel [85–87]. One of the main bottlenecks in the implementation of biofuel production using a hydrothermal process, based on a gasification and Fischer–Tropsch synthesis, are the investment and operational costs [88], in addition to the energy requirements [89]. Hydrotreatment is applied on a commercial scale for biojet production, which can be classified as HEFA (hydrotreated esters or fatty acids) or HVO. According to the ASTM D7566 standard, the process scheme known as HEFA-SPK (synthetic paraffinic kerosene) produces more than 5 billion liters of HEFA biofuel worldwide [90]. This process has two hydro-processing stages in which a combination of deoxygenation and decarboxylation reactions of lipid-based feedstocks takes place [91–93]. The main bottleneck for commercialization of the HEPA-SPK biojet fuel is the selling price, which could amount to 825–2000 $/ton, in comparison to that of fossil-based jet fuel: 329 $/ton [94]. However, the advantage of using HEFA-SPK biojet fuels mainly relies on the reduction of the carbon intensity in GHG emissions: 13.9 g CO 2 eq./MJ when using waste cooking oil as feedstock, 17.2 g CO 2 eq./MJ for corn oil and 22.5 g CO 2 eq./MJ using tallow [90]. According to Gao et al. (2022) the biochemical alternative for biofuel production includes five main stages: anaerobic digestion of lignocellulosic biomass, cleaning of the biogas produced in the digestion stage (for the removal of water, sulfur and nitrogen compounds), followed by the production of syngas with a reforming unit, with subsequent emission of flue gases, which pass through the Fisher-Trop reactor to obtain a mixed product that is separated into biodiesel, light gases recycled to the reforming stages and water [95,96]. The production of third-generation biofuels from microalgae requires a pretreatment step followed by a hydrolysis step, which can be chemical (under acidic conditions) or biological (using enzymes), rendering sugar recovery yields of up to 90% (Hemalatha et al., 2019; De-Farias-Silva et al., 2018). Although bio-based technologies for biofuel production demonstrate potential feasibility in terms of production yield, it is foreseeable that in a future scenario the integration of biofuel production into conventional fuel facilities will be the most viable alternative. This has been the approach followed by Ketabchi et al. (2019), a hybrid refinery using willow and algae residues as feedstock for ABE (Acetone-Butanol- Ethanol) fermentation. The butanol undergoes a pyrolysis stage, in which bio-oil is obtained, and then a gasification process of the gaseous stream is carried out for the production of syngas. After purification of the syngas to remove sulfur and nitrogen compounds (mainly H 2 S and NH 3 ), the biofuel is obtained from the Fischer–Tropsch reaction [97]. The alcohol-to-jet technology is also framed as a fermentation procedure, using lignocellulosic biomass, crops and sugar-based feedstocks to produce bioethanol, and also other alcohols, but in a smaller proportion. The liquid stream is then dehydrated, oligomerized and hydrogenated to obtain the final biofuel, which is usually applied in the fuel blend [98]. Table 4 summarizes the main European industrial facilities that produce biofuel using different production schemes. It also provides information on the TRL, the main feedstocks used and the production capacity of the facilities. This provides an overview of the technological maturity of biofuel production in Europe, as well as the degree of development and commercialization of biofuels. 7. Assessment of the sustainability of biofuel technologies and value chains The integration and evaluation of sustainability aspects (economic viability, social equity and environmental protection) must be taken into account in the biofuel production strategy. In this regard, the use of Life Cycle Assessment (LCA) methodology is considered as a long-standing methodology to assess the environmental loads of processes and/or products within their life cycle. It is based on ISO 14040:2006 standard and it has been used as a methodology to assess the environmental impact loads in biofuel production, as shown on Table 5, that provides examples of the most recent sustainability reports on biofuels. In order to get those, SCOPUS database has been used, considering as searching keywords “LCA”, “sustainability” and “biofuels”, and also reducing the time frame to the most recent years, from 2010 to the present. The selection of the articles has been made according to the information given (type of biofuel, inventory analysis, environmental profiles, among others), the type of technology (mostly considering emerging technologies for the production of biofuels) and the LCA calculation method used for the assessment, achieving the total articles presented on Table 5 afterwards a critical analysis of them based on the abstract, methods and main conclusions. On the other hand, only the articles on English language have been selected.” The main drawback encountered when assessing the articles reported on Table 5 is based on the fact that most of them are barely based on the evaluation of the environmental loads considering different feedstocks for the production of biofuels using different technologies, but there is a lack on the development of techno-economic assessments to evaluate the degree of profitability of the technology and process scheme being assessed. When talking about the prosperity and future framework of biofuels, ensuring that are beneficial under an economic perspective is a key factor for its integration on the value chain. On the other hand, neither a comparison with conventional fuels is being developed in most of the cases, with the exception of [110], in which report it has been concluded that the production of liquid biofuels using pine sawdust by gasification technology is not as productive to be competitive with the commercial prices. On the other hand, other authors have noticed that one way of increasing the competitiveness of biofuels on the value chain is by the use of renewable energy, as it is one of the main costs and also constraints when talking about sustainability [111,112]. Environmental and economic assessments in the development of technology provide additional criteria in the decision-making process. Firstly, because of the need to control and reduce emissions released into the environment, taking into account the policies and restrictions imposed, and secondly, regarding the economic pillar, the technology must be able to achieve reasonable performance and productivity Fig. 4. Total quantities of fuel supply in the 27 EU-member states in 2020. Data obtained from: ETC CM Report 2022/02 Greenhouse gas intensities of transport fuels in the EU in 2020. Acronyms: LNG (Liquefied Natural Gas), CNG (Compressed Natural Gas), LPG (Liquid Petroleum Gas), HVO (Hydrotreated Vegetable Oil) and ETBE (Ethyl tert-butyl-ether). A. Arias et al. Renewable and Sustainable Energy Reviews 197 (2024) 114427 9 Table 4 European biofuel production facilities including the TRL, technology used, type of feedstock and production capacity. Facility Location TRL Technology Feedstock Output (amount) Center of Biorefining Technologies Denmark 4–5 Hydrothermal liquefaction Agricultural residues: sewage sludge, manure, industrial wastes, lignocellulosics Bio-oil (30 t/y) Advanced Biofuel Solutions Ltd United Kingdom 8 Gasification Organic residues and waste streams SNG (1500 t/y) Hydrogen (500 t/y) ALTACA ENERGY Turkey 6–7 Fast pyrolysis Various biomass sources Bio-oil (20 000 m 3 /y) AquaGreen ApS Denmark 9 Fast pyrolysis Organic residues, sludge Syngas and biochar (N/A) ArcelorMittal Belgium 8 Fermentation Waste gases Bioethanol (62 000 t/y) ARD France 4–5 Fermentation Lignocellulosic Bioethanol (100 m 3 /y) Audi AG Germany 8 Methanation Waste gases SNG (300 m 3 /h) AustroCel Hallein Austria 8 Fermentation Lignocellulosics: spent sulfite liquor (SSL) Bioethanol (30 000 t/y) BEST Austria 6–7 Gasification with FT- synthesis Biogenic resides and lignocellulosic waste FT liquids (58 m 3 /y) BioGasol Denmark 6–7 Fermentation Straw, grasses, garden waste Bioethanol (4000 t/y) Biojet AS Norway 6–7 Gasification Forest residues SAF (N/A) BioMCN Netherlands 8 Gasification Lignocellulosic: wood chips Bio-methanol (413 000 t/y) Bio SNG Guessing Austria 6–7 Gasification Lignocellulosics SNG (576 t/y) BioTfuel – consortium France 4–5 Gasification Forest waste, straw, dedicated crops FT liquids (60 t/y) Biozin Norway 8 Fast pyrolysis Forest and sawmill residues Pyrolysis oil (100 000 t/y) Borregaard AS Norway 6–7 Fermentation Sugarcane bagasse, straw, wood, energy crops Bioethanol (110 t/y) 9 Fermentation Spent sulfite liquor (SSL) Bioethanol (15 800 t/y) BP Spain 9 Hydrotreatment Oil crops, oils and fats SAF (N/A) BTG-BtL Netherlands 4–5 Fast pyrolysis N/A Pyrolysis oil (1000 t/y) 8 Fast pyrolysis Wood pellet processing waste Pyrolysis oil (3200 kg/h) Butamax United Kingdom 8 Fermentation Sugar and starch crops: corn Bio-butanol (240 500 t/y) Butamax Advanced Biofuels LLC United Kingdom 6–7 Fermentation Agricultural residues Bio-butanol (15 t/y) Cepsa Spain 6–7 Hydrotreatment Organic residues and waste streams HVO (50 000 t/y) Chempolis Ltd. Finland 6–7 Fermentation Non-wood and non-food lignocellulosics Bioethanol (5000 t/y) CHOREN Fuel Friberg GmbH & Co. KG Germany 6–7 Gasification Dry wood chips and residual forestry wood FT liquids (13500 t/y) 8 Gasification Dry wood chips and recycled wood FT liquids (200000 t/y) Clariant Germany 6–7 Fermentation Wheat straw, corn stover, miscanthus, sugarcane bagasse Bioethanol (1000 t/y) Conoco Philipps Ireland 6–7 Hydrotreatment Vegetable oils HVO (40000 t/y) Cutec Germany 4–5 Gasification Straw, wood, silage, organic residues FT liquids (0.2 t/y) Domsjoe Fabriker Sweden 9 Fermentation Spent sulfite liquor (SSL) Bioethanol (19000 t/y) DTU Chemical Engineering Denmark 4–5 Gasification Organic residues and waste streams Clean syngas (N/A) Ekobenz Poland 8 Hydrotreatment a Sugar and starch crops Gasoline-type fuels (22500 t/y) Energochemica Slovakia 8 Fermentation Dedicated crops and agri-food residues Bioethanol (55000 t/y) Enerkem SA Spain 8 Gasification Organic residues and waste streams Bio-methanol (265000 t/y) ENI Italy 8 Hydrotreatment Oil crops, oils and fats HVO (750000 t/y) Italy 9 Hydrotreatment Oil crops, oils and fats SAF (10000 t/y) Italy 9 Hydrotreatment Oil crops, oils and fats HVO (500000 t/y) Italy 8 Hydrotreatment Soybean oil, used cooking oil, animal fats and waste vegetable oil HVO (360000 t/y) Enviral Slovakia 9 Fermentation Lignocellulosics Bioethanol (50000 t/y) E.ON Gasification Development AB Sweden 9 Gasification Wood biomass SNG (200 MW) Eta Bio Bulgaria 8 Fermentation Wheat straw Bioethanol (50000 t/y) Fintoil Finland 8 Hydrotreatment Crude tall oil HVO (100000 m 3 /y) FlexJET Consortium United Kingdom 6–7 Hydrotreatment Oil crops, oils and fats SAF (1200 t/y) FLITE Netherlands 8 Alcohol-to-jet Sugar and starch crops SAF (30000 t/y) Fraunhofer Umsicht Germany 4–5 Fast pyrolysis Sewage sludge Bio-oil (450 m 3 /y) GIDARA Energy B.V. Netherlands 9 Gasification Organic residues and waste streams Bio-methanol (87500 t/y) Global Bioenergies Germany 6–7 Fermentation Cane sugar, beet sugar and starch Bio-isobutene (100 t/y) Goteborg Energi AB Sweden 8 Gasification Forest residues SNG (50000 t/y) Green Fuel Nordic Finland 9 Fast pyrolysis Forest residues: sawdust, crown trunks Pyrolysis oil (24000 t/y) HCS Group and Gevo Germany 6–7 Alcohol-to-jet Biomass SAF (60000 t/y) IBN-One France 8 Fermentation Cane sugar, beet sugar and starch Bio-isobutene (50000 t/y) INA Croatia 8 Fermentation Miscanthus, wheat straw Bioethanol (55000 t/y) Joint Venture of Air Liquide Netherlands 8 Gasification Organic residues and waste streams SAF (60000 t/y) Karlsruhe Institute of Technology Germany 6–7 Gasification Straw DME (608 t/y) Gasoline-type fuels (360 t/ y) LanzaTech UK United Kingdom 8 Alcohol-to-jet Organic residues and waste streams SAF (100000 m 3 /h) Neste Netherlands 9 Hydrotreatment Oil crops, oils and fats SAF (500000 t/y) Finland 8 Hydrotreatment Palm il, rapeseed oil and animal fat HVO (190000 t/y) Singapore 9 Hydrotreatment Oil crops, oils and fats HVO (1300000 t/y) Finland 6–7 Hydrotreatment Tall oil pitch HVO (40000 t/y) PNK ORLEN Poland 9 Hydrotreatment Used cooking oil (UCO) HVO (300000 t/y) Preem Sweden 9 Hydrotreatment Oil crops, oils and fats HVO (367000 m 3 /y) Sweden 8 Hydrotreatment Tall oil HVO (800000 t/y) (continued on next page) A. 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