Biofuels, E-Fuels, and Waste-Derived Fuels: Advances, Challenges, and Future Directions

Abstract

The imperative to decarbonize global energy systems and enhance energy security necessitates a transition towards ecofuels, broadly classified as biofuels, waste-derived fuels, and electrofuels (e-Fuels). The primary goal of this review is to provide a holistic and comparative evaluation of these three pivotal ecofuel pillars under a unified framework, identifying their strategic niches in the energy transition by critically assessing their interconnected technical, economic, and policy challenges. It offers a comparative dissection of inherent resource constraints, spanning biomass availability, the immense scale of renewable electricity required for e-Fuels, sustainable carbon dioxide (CO₂) sourcing, and the complexities of utilizing non-biodegradable wastes, identifying that true feedstock sustainability and holistic lifecycle management are paramount, cross-cutting limitations for all pathways. This review critically highlights how the current global reliance on fossil fuels for electricity production (approx. 60%) and the upstream emissions embodied in renewable energy infrastructure challenge the climate neutrality claims of ecofuels, particularly e-Fuels, underscoring the necessity for comprehensive well-to-wheels (WtW) lifecycle assessments (LCAs) over simpler tank-to-wheels (TtW) approaches. This perspective is crucial as emerging regulations demand significant greenhouse gas (GHG) emission reductions (70–100%) compared to fossil fuels. Ultimately, this synthesis argues for a nuanced, technologically neutral deployment strategy, prioritizing specific ecofuels for hard-to-abate sectors, and underscores the urgent need for stable, long-term policies coupled with robust and transparent LCA methodologies to guide a truly sustainable energy transition.

1. Introduction

1.1. The Imperative for an Energy Transition

Fossil oil consumption is now widely recognized as unsustainable due to its finite reserves and its contribution to over 75% of anthropogenic CO₂ emissions annually, driving unprecedented environmental and climate degradation [1]. In recent years, climate change mitigation has become the dominant paradigm shaping fuel policy, with net-zero emissions targets prompting a paradigm shift from energy diversification to full-scale decarbonization [2,3]. Amid increasing geopolitical instability and supply chain disruptions, energy supply and security have re-emerged as critical global priorities, affecting nearly all countries that rely on energy imports for more than half of their domestic needs [4].

In response, governments and international bodies have enacted legislative instruments such as the European Union (EU) Green Deal and the United States (U.S.) Inflation Reduction Act mobilizing over USD 1 trillion in subsidies and investments to reduce fossil fuel dependency and accelerate renewable energy deployment [5,6]. The International Energy Agency (IEA) forecasts that from 2024 to 2030, renewable energy use will rise by over 60%, hence increasing the proportion of renewables in total final energy consumption to almost 20% by 2030 from 13% in 2023 (Figure 1) [7]. Assuming a steady growth rate, the share of renewables would be approximately 14% in 2025. This reflects an average annual growth rate of about 0.5% from 2010 to 2025. Despite this progress, fossil fuels still comprise nearly 81.5% of the global primary energy consumption, underscoring the inertia of incumbent energy infrastructures and fossil-based economic models [8]. Within the EU-27, an estimated additional annual GHG reduction of 134 MtCO₂-eq per year (around 55% reduction compared to 1990 levels) needs to be achieved by 2030, assuming 2024 as a starting point, to meet the Fit for 55 climate targets [9].

The EU remains significantly reliant on external energy sources, with net imports covering approximately 90% of its crude oil requirements annually [10]. In response, concerted measures are being implemented across EU industrial sectors to curtail this oil dependency, increasingly leveraging renewable and sustainable energy sources as mandated by strategic initiatives like the REPowerEU plan [11]. Within this context, the transport sector warrants particular attention as it continues to depend on liquid hydrocarbons for over 90% of its energy needs, representing a critical bottleneck in the energy transition [12]. Globally, the transport sector contributes substantially to anthropogenic climate change, accounting for approximately 25% of total energy-related CO₂ emissions, and has exhibited a faster emissions growth rate compared to many other sectors over recent decades [13]. Recognizing this challenge, Directive (EU) 2018/2001 sets ambitious targets, stipulating that renewable energy must constitute at least 14% of the final energy consumption in transport by 2030, with advanced biofuels contributing a minimum share of 3.5% [14]. To meet these goals, alternative energy carriers such as biofuels from diverse biomass feedstocks, waste-derived fuels, electrolytic hydrogen (H₂) generated from renewable power, and synthetic e-Fuels produced via CO₂ hydrogenation offer pathways to reduced lifecycle GHG emissions [15,16,17,18,19]. The pursuit and scaling of these alternative fuel technologies are underpinned by a significant global shift in capital allocation, with investment in clean energy markedly outpacing fossil fuel expenditures in recent years, a trend encompassing renewable power, grids, storage, and low-emission fuels, as visually detailed in Figure 2.

Simultaneously, the EU Green Deal places strong emphasis on accelerating the adoption of electric mobility, which inherently necessitates a dramatic scale-up of battery manufacturing capabilities within the region [5]. Indeed, global production capacity for Lithium-Ion Batteries (LIBs) has followed an exponential growth trajectory since 2009, with forecasts indicating potentially approximately tenfold expansion between 2020 and 2030 and approximately fivefold between 2022 and 2030 to satisfy burgeoning demand, primarily from the automotive sector [21,22]. This anticipated surge in LIB production and deployment, however, presents substantial environmental challenges, particularly concerning the sustainable sourcing of raw materials and the end-of-life management of large battery volumes [23,24,25]. The lifespan of an electric vehicle battery depends on the type of battery and the use of the electric vehicle and typically ranges from 3 to 10 years [26]. The fabrication of LIBs depends critically upon raw materials like lithium, graphite, nickel, and cobalt, many of which are classified as critical due to factors including high cost, geopolitical supply concentration, and projected demand exceeding current extraction rates [27,28,29]. Current recovery efficiencies and recycling practices for spent LIBs within the EU are often insufficient, with material recovery rates frequently falling below 50% for valuable components, hampered by technological limitations and logistical hurdles in collection and processing [30,31,32]. Moreover, the intensive mining operations required to extract virgin materials for LIBs, coupled with the risks associated with an improper disposal of end-of-life batteries, pose significant potential environmental hazards, such as habitat degradation and water resource contamination, along with attendant human health risks in affected communities [33,34,35]. Current battery technology, while advancing, faces limitations in energy density, restricting its applicability in long-distance aviation and maritime transport, where gravimetric and volumetric energy densities are critical [36,37]. These inherent limitations of current battery technology underscore the continued need for high-energy-density liquid and gaseous ecofuels, particularly for long-haul transport, aviation, and maritime sectors.

A “tank-to-wheels” (TtW) emissions analysis focuses solely on the emissions produced by a vehicle during its operation, ignoring the upstream emissions associated with electricity generation [38,39]. This approach can underestimate the overall environmental impact of vehicle electrification. The “clean energy” status of electricity as a transportation fuel is challenged by the global electric power generation mix, where fossil fuels still contribute significantly, accounting for approximately 60% of global electricity production, as seen in Figure 3 [40,41]. A closer coupling of the transport and power sectors is anticipated, requiring a comprehensive “well-to-wheels (WtW)” emissions assessment to accurately reflect the energy pathways and associated environmental burdens [42,43].

Currently, industrial sectors such as ammonia production and oil refining primarily use H₂ as a feedstock [44,45]. Hydrogen as an alternative fuel for transportation, especially long-distance road transport, etc., faces challenges related to high costs, environmental impacts from production, low volumetric energy density, and the need for extensive infrastructure development [46,47,48]. The chemical conversion of hydrogen to derivative compounds, such as ammonia or hydrocarbons, can enhance its energy density and improve storage capabilities [49]. The conversion offers advantages in energy storage and transportation due to higher energy densities and compatibility with existing infrastructure [50,51,52]. CO₂ plays a crucial role in the conversion of hydrogen to alternative fuel derivatives, serving as a carbon source in the production of synthetic fuels via processes like the Fischer–Tropsch (F-T) synthesis [53,54]. The infrastructure requirements for hydrogen, including production, distribution, and dispensing, represent a significant hurdle compared to the established infrastructure for conventional fossil fuels, necessitating substantial investment and technological advancements [55]. Global H₂ demand reached 97 Mt in 2023, currently constituting only a minor fraction of total global energy consumption, with low-emission H₂ production accounting for less than 1 Mt of this total [56,57]. As of late 2024, hydrogen production remains overwhelmingly dependent on unabated fossil fuels, particularly through steam methane reforming (SMR) of natural gas, while low-emission pathways, including renewable-powered electrolysis and fossil fuels integrated with carbon capture, utilization, and storage (CCUS), contribute less than 1% to the global supply [58]. Hydrogen derived from electrolysis powered solely by renewable energy sources qualifies as fully sustainable due to the absence of direct GHG emissions during its production process [59]. Strategically, hydrogen could be blended with going natural gas networks to specific volumetric limits, offering a potential pathway to reduce the carbon footprint of the gas grid, although this requires a careful assessment of infrastructure compatibility and safety standards [19].

Achieving the ambitious global target of net-zero emissions by 2050 mandates an accelerated ramp-up of low-emissions hydrogen output, projected to require approximately 50 Mt derived from electrolysis and over 15 Mt from CCUS-equipped fossil fuel facilities annually by 2030 [58]. Reflecting this imperative, the global pipeline for announced large-scale renewable and low-carbon hydrogen projects reached a potential investment value of USD 680 billion by 2024, although confirmed commitments through final investment decisions (FID) stood at a considerably lower USD 75 billion [60]. While definitive country-specific production capacities for 2025 remain dynamic, China currently leads in operational electrolyzer deployment, with significant project developments also underway across Europe, North America, and other regions aiming to establish substantial green hydrogen output [61,62].

Presently, the Sinopec Kuqa facility located in Xinjiang, China, represents the world’s largest operational green hydrogen plant, equipped with 260 MW of alkaline electrolyzer capacity targeting an annual output of 20,000 t, despite encountering initial operational hurdles related to variable renewable power input [63]. Identifying the subsequent largest facilities involves ambiguity due to diverse project maturation stages and differing metrics (e.g., electrolyzer capacity vs. actual output); however, prominent large-scale developments include the NEOM Green Hydrogen Project in Saudi Arabia, which targets 219,000 t of annual production from 4 GW of electrolysis capacity by 2026 [62,64]. Further illustrating the scale of ambition are multi-gigawatt projects planned for around 2030, such as the Western Green Energy Hub in Australia (50 GW renewables, targeting 3.5 Mt H₂/year) and the BrintØ (Denmark) and Aqua Ventus (Germany) projects, each proposing 10 GW of renewable capacity to produce approximately 1 Mt H₂/year, primarily leveraging vast offshore wind and solar resources [62,65,66]. Numerous other gigawatt-scale initiatives are progressing globally, signaling a clear trend towards large-scale production facilities anticipated to come online towards the end of the decade [60]. Several projects aiming for outputs exceeding 2 Mt/year, like the aforementioned Western Green Energy Hub, have been announced, predicated on harnessing extensive renewable energy potential, although specific technological choices, finalized offtake agreements, and precise operational timelines often remain under final development [62].

The realization of these large-scale hydrogen ambitions is intrinsically linked to the parallel expansion of renewable energy generation; for context, the global installed offshore wind capacity was approximately 81 GW by the end of 2024, a figure dwarfed by the hundreds of gigawatts of renewable capacity implicitly required by the multi-hundred-billion-dollar hydrogen project pipeline [60,67]. Europe’s strategy, in particular, focuses on utilizing its significant offshore wind potential to achieve green hydrogen production at the megaton-per-year scale [58,62]. Economically, the levelized cost of producing green hydrogen currently exceeds that of conventional gray hydrogen manufactured from natural gas without emissions abatement; however, substantial cost reductions are anticipated, with projections suggesting costs for offshore wind-derived hydrogen could decrease from approximately USD 7/kg in 2025 to potentially USD 1–2/kg by 2050, driven primarily by declining renewable electricity prices and electrolyzer capital expenditures [68,69,70,71]. These drivers, namely falling renewable electricity costs due to ongoing technological advancements and economies of scale in solar PV and wind power, coupled with anticipated reductions in electrolyzer capital expenditures resulting from manufacturing scale-up, improved design, and material innovation, are indeed critical. However, the certainty of achieving the lower end of the USD 1–2/kg H₂ target by 2050 is subject to several variables and inherent uncertainties. These include the actual pace of technological breakthroughs in both renewable generation and electrolysis efficiency, the consistency of supportive government policies and financial incentives to de-risk investments and stimulate demand [72,73,74], the timely development of necessary infrastructure for widespread renewable energy deployment and hydrogen transport [38,55], and global supply chain dynamics for critical materials used in electrolyzers and renewable energy technologies [24,28] (with the understanding that critical material supply chains are a noted challenge for energy transition technologies). Furthermore, sustained R&D efforts and successful scaling from pilot projects (such as the Sinopec Kuqa facility aiming for 20,000 t annually [63] or the NEOM Green Hydrogen Project targeting 219,000 t by 2026 [62,64]) to multi-gigawatt commercial operations (like those planned for around 2030 [60,71]) are crucial prerequisites for these cost reductions to materialize as projected. These anticipated reductions hinge primarily on continued declines in renewable electricity prices and electrolyzer capital expenditures, though achieving this cost target requires sustained innovation and significant scale-up.

1.2. The Role of Ecofuels and the Objectives of This Review

Beyond direct electrification and hydrogen utilization, ecofuels present alternative mobility solutions capable of leveraging existing distribution infrastructure, offering competitive energy densities, and often requiring only minimal modifications to current internal combustion engines (ICEs) [43]. These ecofuels provide distinct advantages over purely electric or hydrogen-based systems, particularly regarding the ease of transport and storage using established networks, superior gravimetric and volumetric energy density compared to batteries, and inherent compatibility with a vast fleet of existing vehicles [12,15]. An “ecofuel” is broadly defined as a fuel obtained from renewable, circular feedstocks, such as organic wastes, non-food biomass, or atmospheric CO₂, rather than finite fossil resources [16,17]. Fundamentally, the production of ecofuels involves harnessing contemporary carbon fixation processes, operating on timescales of years or decades, in stark contrast to the geological timescales spanning millions of years required for the formation of fossil fuels [18,19].

These characteristics position carbon-based ecofuels as potentially viable medium-term alternatives for decarbonizing the transport sector, especially for applications where electrification faces significant hurdles [12,38]. However, notable carbon-free exceptions are also under investigation, such as ammonia (NH₃), which garners interest due to its potential for direct use or as an effective hydrogen carrier, leveraging its relatively high hydrogen density and established handling protocols, despite challenges in its direct combustion characteristics [46,50,51]. Ecofuels derived from the conversion of carbonaceous feedstocks can be produced in various physical states, including gaseous (e.g., biomethane), liquid (e.g., ethanol, biodiesel, e-methanol), and solid forms (e.g., biochar), depending on the conversion pathway and intended application [16].

Among these, liquid ecofuels are generally favored for transportation due to their ease of handling, storage, and integration into the existing fuel infrastructure designed primarily for liquid hydrocarbons [12,43]. The advantageous properties of liquid fuels include simplified logistics, established safety protocols, and significantly higher volumetric energy densities compared to gaseous fuels like hydrogen, which require complex high-pressure or cryogenic storage systems [46,47]. When comparing common energy carriers, liquid ecofuels exhibit substantially higher gravimetric and volumetric energy densities than current battery technologies, although typically lower than conventional fossil fuels like gasoline or diesel [36,37]. This comparative analysis, visually represented in Figure 4, underscores the technical limitations related to energy density that currently constrain the widespread application of battery–electric solutions, particularly in energy-demanding sectors such as long-haul trucking, aviation, and maritime shipping [12,36].

Ecofuels encompass diverse energy carriers produced via three primary technological routes: biofuels, electrofuels (e-Fuels), and fuels derived from non-biomass waste streams [16]. Specifically, e-Fuels represent a category of synthetic fuels synthesized utilizing captured carbon oxides (CO₂ or CO) and H₂, the latter typically generated through electrolysis powered by renewable electricity resources like solar, wind, or nuclear power [18]. While hydrogen itself presents a promising zero-emission vector, its large-scale deployment as a direct transportation fuel is significantly hampered by the current lack of dedicated production, distribution, and refueling infrastructure [55]. Consequently, liquid e-Fuels derived from renewable hydrogen offer considerable advantages, including compatibility with existing liquid fuel storage and transport networks, adaptability for use in current ICEs, and favorable energy density characteristics compared to direct hydrogen storage [12,15]. Distinct from e-Fuels, biofuels are defined as energy products originating from renewable biomass or its residues, encompassing a range of substances such as biogas, biodiesel, bioethanol, and bio-hydrogen [16]. A third category involves fuels generated through the thermal treatment, often pyrolysis, of non-biodegradable waste materials, primarily plastics and other carbonaceous residues like end-of-life tires [75]. Pyrolysis, a thermochemical conversion process, demonstrates versatility by enabling the transformation of both biomass into products like syngas and biochar and also waste plastics into valuable low-molecular-weight hydrocarbon fractions resembling kerosene, gasoline, and diesel fuels [76,77,78,79,80].

Recognizing the need for transport decarbonization, in 2023, the EU agreed on a landmark vision mandating that exclusively zero-emission vehicles, even ones running solely on CO₂-neutral fuels, can be licensed after 2035, effectively phasing out new fossil-fueled car sales [81]. This regulatory framework necessitates the establishment of a novel category of vehicles designed to operate solely on synthetic e-Fuels, signaling a potential pathway for preserving internal combustion (IC) technology using sustainable fuels [82]. Draft regulations further stipulate that qualifying e-Fuels must demonstrate substantial climate benefits, achieving GHG emission reductions potentially ranging from 70% to 100% on a lifecycle basis compared to fossil equivalents [83]. To evaluate the energy efficiency of fuel production pathways, the Fossil Energy Ratio (FER) given in Equation (1) [84] is frequently employed, described as the proportion of the usable energy content (E_out) of the end fuel output to the total fossil energy (E_fossil,in) consumed throughout its production process [85,86]. An FER value of unity signifies a theoretical break-even point where the energy output precisely matches the fossil energy input required for production [84]. Conventional fossil diesel, for instance, typically exhibits an FER of approximately 0.84, reflecting the significant fossil energy expenditures associated with crude oil extraction, transportation, and refining processes [87]. Conversely, an FER exceeding 1 indicates that the fuel production process yields more energy in the final product than the fossil energy invested, effectively leveraging the initial fossil input [88]. In idealized implementations utilizing entirely renewable energy inputs, such as totally renewable power-to-fuel scenarios, the FER approaches infinity as the requisite fossil energy input nears zero [89].

$$FER={\displaystyle \raisebox{1ex}{$E_{out}$}\!\left/ \!\raisebox{-1ex}{$E_{fossil,in}$}\right.}$$

(1)

While numerous reviews address discrete aspects of biofuels, e-Fuels, or waste-derived fuels, a critical gap remains in the literature in terms of a comprehensive study that holistically and comparatively evaluates these three pivotal ecofuel pillars under a unified framework. This study specifically dissects the interconnectedness of feedstock sustainability (extending beyond mere renewability), the practical implications of well-to-wheels lifecycle assessments (LCAs) versus TtW approaches, deep-seated infrastructural inertia, volatile policy landscapes, and the overarching economic viability challenges across all these fuel categories. By juxtaposing their respective potentials against these shared and unique hurdles, including those detailed for requisite technologies like large-scale electrolysis or advanced biomass conversion, this study aims to elucidate their most viable strategic niches and provide a more robust, evidence-based perspective for future research, policy formulation, and investment in truly sustainable energy pathways for the hard-to-abate sectors.

2. Methodology

This review provides a critical and holistic synthesis of the current ecofuel landscape. To ground this synthesis in the existing body of scientific work, a systematic literature search and bibliometric analysis were conducted. The data needed for the review and the keyword time analysis were collected by executing a topic search on the Web of Science (WOS) Core Collection database [90]. The search query used was TS = ((“biofuel” OR “e-fuel” OR “electrofuel” OR “waste-to-fuel”) AND (“decarbonization” OR “sustainability” OR “policy”)).

The search was limited to articles published between the years 2020 and 2025, yielding 1685 results. These results were further refined based on specific inclusion and exclusion criteria. Studies were included if they were peer-reviewed articles published in English. Sources were excluded if they were conference abstracts, editorials, or letters without sufficient technical data. According to the analysis results accessible via WOS, 687 (40.772%) of these articles fall under the “Energy Fuels” category and 588 (34.896%) under “Environmental Sciences”, with a total of 88 category records including “Green Sustainable Science Technology” and “Environmental Studies”.

To visualize the intellectual structure of this research field, the bibliographic data from the 1685 articles was exported from WOS and analyzed using VOSviewer 1.6.20 software [90,91]. A co-occurrence analysis of all keywords was performed to map the primary research themes and their interconnections, as shown in Figure 5.

The analysis reveals several distinct research clusters. The largest and most central cluster (green) is built around the core topics of “biofuel”, “life cycle assessment”, and “biodiesel”, connecting them strongly to “sustainability”, “decarbonization”, and “gasification.” A second prominent cluster (yellow) focuses on “bioethanol”, “lignocellulosic biomass”, and “agricultural residue”, indicating a strong research thrust in second-generation biofuels. A third, more recent cluster (blue) highlights the emerging field of “e-fuels”, directly linking “hydrogen”, “ammonia”, and “electrification” with “climate mitigation” and the broader “energy transition.” The temporal overlay, where older terms are blue and more recent terms are yellow, clearly indicates a research trend shifting from established topics like “biodiesel” and “transesterification” toward emerging concepts like “e-fuels”, “hydrogen”, and “decarbonization”, which are among the most recent keywords in the field.

3. Biofuels

Biofuels, derived from recently living organisms or their metabolic by-products, represent a diverse category of renewable energy carriers significant for decarbonizing sectors like transportation where direct electrification is challenging [71,92]. Despite their potential, the overall energy mix remains dominated by fossil fuels (92.5%), with renewables accounting for only 7.5%; within this renewable share, food and feed crop-based biofuels contribute 4.0%, while advanced biofuels and biogas represent 0.8%, as depicted in Figure 6 [72,93]. These fuels originate from various types of renewable biomass feedstocks, which are fundamentally organic materials containing stored solar energy, including agricultural crops, forestry residues, algae, and organic wastes such as manure or municipal solid waste [94].

Historically, wood and its derivatives have served as primary energy carriers for millennia and continue to be used today, often in pelletized forms for heating, although their combustion contributes significantly to particulate matter pollution in urban areas, posing respiratory health risks [95]. Wood’s widespread availability, recoverable energy content, and average energy density of approximately 15–20 MJ/kg (dry basis) make it a readily accessible biofuel; however, its combustion releases substantial quantities of CO₂, soot, smoke, and polycyclic aromatic hydrocarbons (PAHs), often exceeding emissions from cleaner fuels [96]. Similarly, dried animal dung serves as a primary fuel source for people in underdeveloped regions, particularly in resource-limited settings lacking access to modern energy, offering a renewable resource with an approximate energy content ranging from 10 to 15 MJ/kg, significantly lower than typical coal values of 20–30 MJ/kg [97]. The combustion of dried dung, especially indoors in poorly ventilated conditions, generates hazardous pollutants, including fine particulate matter, carbon monoxide (CO), dioxins, and chlorophenols, leading to severe health impacts such as respiratory infections, cardiovascular disease, and increased cancer risk [95].

3.1. First-Generation Biofuels

Biofuels are typically classified into generations based on their feedstock origin and the technological maturity of their production processes. First-generation (1-G) biofuels are characterized by their derivation from conventional agricultural feedstocks such as corn, sugarcane, sugar beet, wheat, palm oil, or soybean oil [98]. The primary economic considerations for 1-G biofuels revolve around achieving production costs competitive with fossil fuels, although factors like feedstock price volatility present significant challenges [99]. Governmental policies, including subsidies, mandates, and tax incentives, significantly influence the economic viability and market penetration of these biofuels [72]. A major point of contention is the direct competition between 1-G biofuel feedstock cultivation and global food supply systems, often termed the “food versus fuel” dilemma, which raises concerns about potential impacts on food prices and availability [100]. This includes concerns over indirect land use change (ILUC), where biofuel crop expansion displaces food production onto previously uncultivated land, potentially leading to deforestation and negating GHG savings [101]. Furthermore, the extensive land use requirements for cultivating 1-G feedstocks pose ethical and sustainability challenges, including risks of deforestation, biodiversity loss, and pressure on water resources [102].

The concept of energy balance, or Net Energy Balance (NEB), evaluates the amount of energy from the biofuel to the fossil energy input necessary for feedstock cultivation and conversion; for instance, soybean biodiesel yields approximately 93% more energy than is invested in its production [103]. A negative energy balance, where producing the biofuel consumes more fossil energy than the biofuel itself contains, fundamentally undermines its sustainability credentials and environmental benefits [104]. Consequently, the complete replacement of fossil fuels with 1-G biofuels on a large scale, such as within the EU, is considered unfeasible due to land constraints, sustainability issues, and limitations in net energy gain for some feedstocks [105]. While precise figures vary, the potential production capacity of 1-G biofuels like EU corn ethanol remains significantly smaller than the region’s total fossil fuel consumption in the transport sector, which still accounts for over 90% of energy use [12,106]. Despite these limitations, food crop-derived biofuels are currently helping to decarbonize the transportation sector by being blended with gasoline and diesel, contributing to meeting renewable energy targets, although their overall GHG reduction potential is debated and varies by feedstock [106,107]. From a practical perspective, their widespread use in blends can influence engine performance and durability. For instance, the lower energy density of ethanol (approximately 30% less than gasoline) leads to reduced fuel economy, while higher blends can cause material compatibility issues with gaskets and seals in the fuel systems of older, non-flex-fuel vehicles [108,109]. First-generation biofuels, primarily bioethanol and biodiesel, are still utilized globally, particularly in road transport applications across regions like the Americas, Europe, and Asia, often supported by blending mandates [110].

The general chemical structure of biofuels, which dictates their combustion properties and compatibility with engines, is primarily influenced by the nature of the biomass feedstock and the specific conversion process employed [111]. First-generation bioethanol production typically employs biochemical conversion technology, specifically the fermentation of sugars derived from starch or sucrose feedstocks [108]. Common feedstocks for 1-G bioethanol include corn, sugarcane, and sugar beet, with robust yeast strains, predominantly Saccharomyces cerevisiae, serving as the biological agents responsible for converting sugars into ethanol [112]. The primary chemical process used to produce 1-G biodiesel is transesterification [113]. This transesterification process involves reacting vegetable oils or animal fats (triglycerides) with a short-chain alcohol, typically methanol, in the presence of a catalyst (commonly KOH or NaOH), which breaks the fatty acid-glycerol bonds to yield fatty acid alkyl esters (biodiesel) and glycerol as a co-product [114]. Equation (2) illustrates transesterification, the core chemical reaction for producing 1-G biodiesel. It shows how triglycerides react with an alcohol (like methanol) to break down into biodiesel esters and glycerol, a fundamental process for converting oils and fats into diesel fuel substitutes.

$$\begin{gathered} Catalyst \\ Triglyceride+3R^{\prime }OH\rightleftharpoons 3RCOO{R}^{\prime }+C_3{H}_5{\left(OH\right)}_3 \end{gathered}$$

(2)

From the expressions in Equation (2) [115], Triglyceride represents vegetable oil or an animal fat (containing three fatty acid chains, R, attached to a glycerol backbone). R′OH represents a short-chain alcohol (typically methanol, CH₃OH, where R′ is CH₃). RCOOR’ represents fatty acid alkyl esters (biodiesel). C₃H₅(OH)₃ represents glycerol (glycerin). R represents long-chain alkyl groups originating from fatty acids.

3.2. Second-Generation Biofuels

Advanced or second-generation (2-G) biofuels are fundamentally distinguished from their 1-G counterparts by utilizing feedstocks not suitable for direct human consumption, thereby mitigating the “food versus fuel” conflict [94,98]. The guiding principle for sourcing these feedstocks is the avoidance of direct competition with established food production systems and the associated land use [102,103]. These advanced feedstocks encompass a diverse array of lignocellulosic biomass derived from agricultural residues (e.g., straw, corn stover) and forestry wastes (e.g., wood chips, sawdust), dedicated non-food energy crops, and various organic waste streams including municipal solid waste, food waste, and sewage sludge [116,117]. Materials originally derived from food products may only be classified as 2-G feedstocks when representing unavoidable residues or waste streams not diverted from the food chain or when they have become unsuitable for consumption [118]; for example, used cooking oil (UCO) is considered a 2-G feedstock, whereas the virgin vegetable oil from which it originated is classified as 1-G [119]. Food waste itself is a significant 2-G resource, although consistent data on per capita generation specific to biofuel potential (estimated at over 200 kg per capita per year for Europe) requires further consolidation [120,121]. Where dedicated energy crops are cultivated, preference is given to marginal or degraded lands unsuitable for conventional agriculture, aiming for reduced environmental impact and lower input requirements for resources like water and fertilizers compared to traditional food crops [122]. Examples of such non-food energy crops include perennial grasses like Miscanthus and switchgrass, short-rotation coppice species such as poplar and willow, and non-edible oilseed plants like Jatropha curcas, though optimal choices vary by region [123]. Figure 7 outlines the principal biochemical and thermochemical pathways for converting 2-G feedstocks into various biofuels. The arrows in the figure indicate the flow of materials or intermediates between the different processing stages. In the thermochemical route, biomass undergoes one of several high-temperature processes: (i) pyrolysis decomposes biomass into char, gas, and bio-oil. These products can be directly utilized or further processed for combined heat and power (CHP) generation; (ii) liquefaction converts biomass into bio-oil and gas, which are also suitable for CHP applications; (iii) gasification and co-gasification convert biomass into tar and syngas (a mixture primarily of CO and H₂). Syngas can be upgraded to liquid fuels such as bioethanol through Fischer–Tropsch catalysis or fermented biologically to produce the same. In the biochemical route, biomass is first subjected to pretreatment to disrupt its complex lignocellulosic structure. The resulting material is then hydrolyzed to release fermentable sugars, which are subsequently converted into biofuels (biogas, biohydrogen, and bioalcohols) via fermentation.

The transformation of these varied and often recalcitrant feedstocks into usable biofuels employs two main technological pathways: biochemical conversion, typically involving pre-treatment, enzymatic hydrolysis to release sugars, and subsequent fermentation; and thermochemical conversion, which includes processes like pyrolysis, gasification, and liquefaction followed by upgrading or synthesis steps [76,125].

3.2.1. Biochemical Route

The biochemical route to 2-G biofuel production is centered around the biorefinery concept, which integrates various conversion processes to sustainably produce fuels, power, and value-added chemicals from biomass [126]. Biorefineries are not solely restricted to biochemical methods; they often utilize a combination of mechanical, chemical, and thermochemical processes as well [126,127].

Biorefineries are commonly classified based on several factors, including the primary feedstock used (such as lignocellulosic, whole crop, or marine biomass), the key chemical intermediates produced (like the sugar platform from hydrolysis or the syngas platform from gasification), and the spectrum of final end-products, which can range from energy carriers to biobased chemicals and materials [128]. The diversity of biorefinery pathways is driven by feedstock availability, technological advancements, and market demands [126].

Effective conversion often requires feedstock preprocessing steps, such as mechanical shredding or milling, to improve handling efficiency [129,130,131]. For recalcitrant feedstocks like lignocellulosic biomass, additional pre-treatments are crucial to break down complex structures for biochemical conversion [129,130]. Established biochemical processes for 2-G feedstocks include anaerobic digestion for biogas production and fermentation to produce bioethanol [132,133].

3.2.2. Thermochemical Route

The thermochemical pathway utilizes heat to convert biomass into gaseous, liquid, and solid fractions through thermal decomposition [134,135]. Key processes include torrefaction, which uses mild temperatures (200–300 °C) to produce a solid [136,137], carbon-enriched bio-coal, pyrolysis, which operates at higher temperatures (300–800 °C) in an oxygen-free environment to favor the production of liquid pyrolytic oil (py-oil) [138], and gasification, which uses even higher temperatures (700–1200 °C) with a controlled amount of an oxidizing agent to maximize the yield of synthesis gas (syngas) [139].

The products from these processes consist of a solid fraction (biochar), a liquid fraction (bio-oil or py-oil), and a gaseous fraction (syngas) [134,135]. Syngas, composed mainly of CO and H₂, is a versatile intermediate for producing liquid fuels and chemicals, while bio-oil is a dense liquid energy carrier [140]. However, both raw products require upgrading [141,142]. Raw py-oil is typically unstable and corrosive, necessitating upgrading via processes like catalytic hydrodeoxygenation (HDO) to remove oxygen and improve its fuel properties [143,144]. Similarly, raw syngas must be cleaned of impurities and conditioned for downstream applications [145,146].

A prominent application for upgraded syngas is its catalytic conversion into liquid hydrocarbon fuels, resembling gasoline and diesel, via the F-T synthesis [147,148]. Syngas can also be converted into other valuable products like bio-methanol and bio-dimethyl ether (bio-DME) [149,150,151].

3.3. Third-Generation Biofuels

Third-generation (3-G) biofuels are primarily derived from algal biomass, including microalgae and cyanobacteria, distinguished from second-generation feedstocks by characteristics such as high oil content, rapid growth rates, and, critically, the ability to be cultivated using non-arable land and non-potable water sources, thus minimizing direct competition with agriculture [152,153]. The demonstrated biofuel yield potential for algal cultivation significantly surpasses that of terrestrial crops used in first- or second-generation biofuels, with potential oil yields cited in ranges from approximately 12,000 up to 136,900 L/ha/yr under optimal conditions, compared to, for example, around 446 L/ha/yr for soy or 5950 L/ha/yr for palma oil [154]. Consequently, based on projected yields, it has been estimated that meeting the total U.S. liquid transportation fuel demand could theoretically require about 4% of the nation’s land area, potentially utilizing otherwise unproductive arid or coastal regions, although achieving even a 5% displacement of U.S. transport fuel with algal biofuels using current technologies could face sustainability challenges regarding resource inputs [155].

Cultivating algae requires significant resource inputs, notably water—though brackish, saline, or wastewater can often be utilized—and nutrients, particularly nitrogen and phosphorus, often supplied via fertilizers [156]. The reliance on fertilizers presents potential environmental implications, including GHG emissions associated with their industrial production and the risk of nutrient runoff leading to eutrophication of water bodies if not managed carefully [157,158]. However, algae’s adaptability to diverse environments, including cultivation in wastewater or brackish water on non-arable land, reduces pressure on fertile land and freshwater resources while offering co-benefits such as nutrient recycling from wastewater streams and environmental remediation through the uptake of pollutants [159,160,161]. Furthermore, algal cultivation intrinsically participates in carbon capture and utilization (CCU) by assimilating CO₂ during photosynthesis, potentially utilizing concentrated CO₂ streams from industrial point sources like power plant flue gas, thereby offering a route to valorize waste emissions [159,162].

Diverse processing pathways exist for converting microalgal biomass into biofuels, including biochemical methods like fermentation or anaerobic digestion, thermochemical methods such as pyrolysis or gasification to produce intermediates like bio-oil or syngas, and lipid extraction followed by transesterification to yield biodiesel [163,164]. Genetic engineering plays a potentially pivotal role in advancing algal biofuels, aiming to tailor microalgal strains for an enhanced production of specific fuel precursors or even a direct synthesis of molecules like biodiesel, butanol, gasoline components, methane, ethanol, or jet fuel [165]. Such genetic modifications define the transition towards fourth-generation concepts, targeting significantly improved biofuel yields per hectare, although specific quantitative projections remain under development [118,166]. While specific quantitative comparisons of CO₂-to-lipid conversion efficiencies between genetically modified strains (e.g., Nannochloropsis gaditana) and their wild-type counterparts require further consolidated data, research confirms the high lipid accumulation potential of species like N. gaditana (e.g., reaching 0.32 g/L lipid yield under specific enriched CO₂ lab conditions) and the focus of genetic engineering on optimizing these metabolic pathways [160,165,167].

3.4. Fourth-Generation Biofuels

Fourth-generation (4G) biofuels are distinguished by the application of advanced biotechnologies, particularly genetic and metabolic engineering, to optimize microorganisms such as bacteria, yeast, and microalgae for significantly enhanced biofuel production efficiency and yield, although the technology largely remains in the research and development phase [168,169,170]. The primary motivation driving 4G biofuel development stems from the imperative to overcome the inherent limitations of previous generations, including the food versus fuel debate associated with first-generation and land/resource constraints potentially impacting second- and third-generation feedstocks while pursuing enhanced environmental and economic sustainability [171]. Feedstocks characteristic for 4G processes include genetically engineered microorganisms capable of high productivity and, significantly, potentially utilizing atmospheric or industrial CO₂ as a direct carbon source, thereby circumventing competition for arable land and food resources [171,172,173]. Advanced conversion technologies employed typically involve sophisticated biochemical pathways within these engineered organisms, often targeting direct synthesis of fuel molecules and sometimes integrated with thermochemical or hybrid systems (e.g., utilizing syngas fermentation or hydrothermal liquefaction) to maximize efficiency and minimize environmental impact [168,174,175,176].

The principal objectives guiding the selection and development of 4G conversion techniques center on maximizing the efficiency of feedstock utilization, minimizing waste generation through highly integrated process designs, and ensuring overall process sustainability [168,174,177]. Beyond the primary fuel product, 4G biofuel processes often target the co-production of a diverse range of valuable compounds, such as bio-based materials, specialty chemicals, or pharmaceuticals, within an integrated biorefinery concept [177,178]. This integrated production of diverse co-products is anticipated to significantly enhance overall process sustainability and improve economic feasibility by creating multiple revenue streams and maximizing value extraction from the feedstock [177,178]. Fourth-generation biofuels are designed with the goal of achieving carbon neutrality or even carbon negativity, defined by the principle that the GHGs emitted during production and combustion are balanced or exceeded by the CO₂ captured and utilized during feedstock growth or synthesis. Specific strategies employed to ensure a significantly lower carbon footprint compared to conventional fossil fuels include the selection of feedstocks engineered for efficient CO₂ fixation, the direct utilization of captured atmospheric or industrial CO₂, and the integration of renewable energy sources to power the production process [171].

Integrated biorefineries play a crucial role in the 4G context, functioning to maximize value extraction from the biomass or CO₂ feedstock through the diversification of the product portfolio beyond just fuels [168,174,178]. Furthermore, this integrated biorefinery concept inherently contributes to waste reduction and enhanced sustainability by aiming for complete feedstock valorization and promoting closed-loop systems within a circular bioeconomy framework [177,178]. However, significant technical, logistical, and economic challenges must be overcome for 4G biofuels to transition from research to widespread commercial feasibility, including issues related to process scalability, feedstock consistency, and system integration [72,175,176]. Key obstacles currently hindering commercialization includes the need to improve production process productivity and robustness, achieve cost reductions through economies of scale and technological maturation, ensure consistent biofuel performance and quality meeting market specifications, and navigate complex regulatory landscapes and public perception [175,176,179]. Despite these substantial hurdles, considering the ongoing technological advancements and the urgent need for sustainable energy solutions, 4-G biofuels possess considerable long-term potential to contribute significantly to replacing conventional fossil fuels in an environmentally beneficial manner, provided current challenges in cost, scale, and efficiency can be effectively addressed [168,171,174,180].

3.5. Fifth-Generation Biofuels

Fifth-generation (5-G) biofuels represent a conceptual frontier beyond fourth-generation advancements and remain largely conceptual, often characterized by the integration of synthetic biology, potentially nanotechnology, and novel process designs like electro-biofuel or direct solar fuel production using highly engineered organisms to overcome fundamental efficiency, cost, and feedstock limitations inherent in previous generations, although a standardized definition remains emergent [170,172,181]. The core technological distinction for 5-G concepts frequently involves bespoke microorganisms engineered via synthetic biology, such as genetically modified algae or cyanobacteria, designed for direct and highly efficient conversion of fundamental inputs like CO₂, water, and renewable electricity or solar energy into target fuel molecules [182,183]. Anticipated advantages center on potentially achieving superior energy conversion efficiencies, utilizing ubiquitous feedstocks like CO₂ and sunlight thereby minimizing land and water resource competition, and offering pathways towards truly carbon-neutral or even carbon-negative fuel cycles [170]. This contrasts sharply with earlier biofuel generations, particularly 1-G, which faced significant resource-related criticism regarding competition with food production and extensive land and water use, sometimes leading to negative ILUC impacts like deforestation [170].

Currently, 5-G biofuel concepts are predominantly in the very early research and conceptual development phases, are currently at very low Technology Readiness Levels (TRLs), and face a long road to potential viability, with significant breakthroughs required in fundamental science and engineering merely to demonstrate technical feasibility, let alone to progress towards becoming practical alternatives to conventional fossil fuels [72,172,175,181]. Substantial technical and economic hurdles impede large-scale implementation, including the high costs associated with complex biotechnological processes, the challenge of developing stable and highly productive engineered microbial strains, achieving an efficient scalability of novel cultivation or bioreactor systems (such as electro-fermenters or advanced photobioreactors), and ensuring efficient downstream processing and fuel recovery [72,172,181,184]. While specific 5-G cultivation systems remain largely conceptual and largely theoretical, it is postulated that some principles from existing algae cultivation, involving large-scale open ponds or closed photobioreactors, might be adapted, likely alongside entirely novel bioreactor designs optimized for the highly specialized and often sensitive requirements of electrotrophic or phototrophic engineered organisms [183,185]. The final products envisioned for 5-G pathways are typically advanced liquid or gaseous fuels (e.g., customized alcohols, alkanes, hydrogen) synthesized directly by the engineered organisms, potentially secreted for easier recovery, with physical characteristics tailored to match or exceed conventional fuel standards [172,186].

Any future advancement from the current conceptual stage is entirely contingent upon extensive and high-risk fundamental research and development across numerous areas. These include significantly more advanced synthetic biology toolkits, breakthroughs in metabolic engineering for precise and efficient target molecule synthesis, entirely new reactor designs for these novel bioprocesses, highly efficient CO₂ capture and delivery systems, robust and predictive LCA methodologies for these nascent and theoretical pathways, and resolving formidable challenges related to the long-term genetic stability and ecological safety of potentially deploying highly engineered organisms [187,188,189]. Despite these profound and multifaceted challenges and the highly speculative nature of the field, some researchers suggest, from a distant theoretical perspective, that if these enormous scientific and engineering hurdles can be overcome and 5-G biofuels can be successfully developed (a possibility that would require significant breakthroughs cannot be overstated), synthetic biology and direct conversion pathways could be effective [100,181,190]. Such pathways could theoretically hold transformative potential for future sustainable energy production, perhaps offering routes to high-efficiency, low-impact fuels with minimal resource conflicts, though this remains a distant and uncertain vision.

The various generations of biofuels, from 1-G to 5-G, present distinct technological and infrastructural profiles when compared to fossil fuels, as summarized in

Table 1. Main advantages/benefits and main disadvantages/harms of each generation of biofuels compared to fossil fuels from technological/infrastructural perspective.

Fuel Source	Key Advantages/Benefits	Key Disadvantages/Harms
Fossil Fuels	- High volumetric and gravimetric energy density, ideal for transportation [191]. - Mature and extensive global infrastructure for extraction, refining, distribution, and utilization [191]. - Compatible with existing ICE technology [191]. - Diesel engines offer high torque and durability, suitable for heavy-duty applications [192]. - Diesel engines often have longer lifespans than gasoline engines [192].	- Infrastructure is capital-intensive and aging in some regions. - Combustion efficiency is not 100%, leading to energy loss as heat. - Gasoline engine components can be less durable than diesel [192].
1-G Biofuels	- Bioethanol and biodiesel can be blended with gasoline/diesel or used in modified/dedicated engines [171]. - Relatively mature production technologies (fermentation, transesterification) [193]. - Can utilize existing fuel distribution infrastructure to some extent (especially blends).	- Lower energy density than gasoline/diesel (e.g., ethanol has ~30% less), leading to reduced fuel economy/range and requiring more frequent refueling [194]. - Potential material compatibility issues (corrosion, elastomer degradation) with higher ethanol blends in older vehicles/infrastructure not designed for flex-fuels. - Scalability limited by arable land and water availability [99]. - Biodiesel can exhibit poor cold-flow properties (gelling at low temperatures), requiring additives.
2-G Biofuels	- Some 2-G biofuels (e.g., renewable diesel from BTL) are “drop-in” fuels, fully compatible with existing engines and infrastructure [195]. - Potential for higher overall energy yield per hectare if marginal lands are utilized effectively. - Ongoing R&D improving conversion efficiencies and reducing costs [99].	- Complex and technologically challenging conversion processes (e.g., pre-treatment of lignocellulose, enzyme production, gasification clean-up) [170]. - Scalability challenges: moving from pilot/demonstration to commercial scale is difficult and expensive [99]. - For non-drop-in fuels like cellulosic ethanol, the same issue of lower energy density compared to gasoline persists.
3-G Biofuels	- Potential for very high oil yields per hectare [196]. - Can produce a variety of fuels (biodiesel, bioethanol, bio-jet fuel, biogas) [196]. - Continuous harvesting possible in some systems. - Genetic engineering can tailor algae for higher lipid/carbohydrate content or stress tolerance [196].	- Technological challenges in cost-effective large-scale cultivation, harvesting (small cell size, dilute cultures), dewatering, and oil extraction [99]. - Strain stability and susceptibility to contamination/predation in open ponds [196]. - Energy balance can be negative if not optimized (more energy consumed than produced) [193]. - PBRs are costly and complex to scale up [196].
4-G Biofuels	- GE crops: Tailored biomass characteristics can simplify downstream processing. - Electrofuels: Can be designed as drop-in replacements for gasoline, diesel, jet fuel, compatible with existing infrastructure. Leverages advancements in electrolysis (AWE, PEM, SOEC) and CO2 capture technologies [197].	- GE crops: Long development and approval times for new GE varieties. - Electrofuels: Technologies are still maturing and require significant R&D for efficiency, durability, and cost reduction. Scalability challenges to meet large transport sector demands [197]. Storage of intermittent renewable electricity for continuous fuel production.
5-G Biofuels	- Engineered microorganisms: Advanced metabolic engineering and synthetic biology tools are rapidly developing, offering precise control over cellular pathways [198]. - Artificial photosynthesis: Multiple technological approaches being explored (PEC cells, molecular catalysts, PV-driven GDEs) [199]. Potential for direct solar-to-fuel conversion.	- Engineered microorganisms: Challenges in optimizing microbial pathways, achieving high product yields and titers, maintaining strain stability at industrial scale, and preventing contamination [200]. Metabolic burden on host cells. Low catalytic activity of key carbon-fixing enzymes. - Artificial photosynthesis: Major scientific and technological hurdles in catalyst design (efficiency, stability, cost), light harvesting, charge separation, product selectivity, and system integration. Difficulty in producing multi-electron reduced products beyond CO [199].

. Furthermore, each generation carries unique environmental advantages and disadvantages (

Table 2. Main advantages/benefits and main disadvantages/harms of each generation of biofuels compared to fossil fuels from an environmental perspective.

Fuel Source	Key Advantages/Benefits	Key Disadvantages/Harms
Fossil Fuels	- Modern gasoline vehicles have lower NOx and particulate matter (PM) emissions compared to older diesel engines [201]. - Diesel engines can have lower CO2 emissions per mile due to higher fuel efficiency [201].	- High GHG emissions [202]. - Air, water, and land pollution from combustion - Geopolitical price volatility and security concerns. - Non-renewable resource, finite reserves leading to depletion [203]. - Air pollution contributing to smog, acid rain, and respiratory/cardiovascular diseases [204]. - Water pollution from oil spills during transport and refinery discharges [203]. - Land disruption and habitat destruction from drilling activities [205]. - Emissions of CH4 and nitrous oxide (N2O) from tailpipes, and HFCs from A/C leakage [202]. - Upstream GHG emissions from refining and distribution [202].
1-G Biofuels	- Renewable resource base (crops, vegetable oils) [193]. - Can offer some GHG emission reductions compared to fossil fuels if LUC/ILUC is minimized (e.g., sugarcane ethanol on established land) [193]. - Biodegradable (especially biodiesel) [206]. - Bioethanol can act as a gasoline extender and smog-reducing agent [207].	- Significant GHG emissions if direct/indirect land use change (LUC/ILUC) occurs (e.g., deforestation for crop expansion) [193]. - Corn ethanol offers only modest GHG reductions due to fossil fuel inputs in production [207]. - High water consumption for crop irrigation and processing (e.g., corn ethanol) [207]. - Fertilizer and pesticide use leading to water pollution (eutrophication, runoff) and soil degradation [193]. - Monoculture cropping can reduce biodiversity [193].
2-G Biofuels	- Utilizes non-food feedstocks (agricultural/forest residues, dedicated energy crops on marginal land), reducing direct food competition [171]. - Potential for higher GHG emission reductions compared to 1-G biofuels, especially if residues are used and ILUC is avoided [193]. - Waste valorization (using residues) [193]. - Can be grown on marginal land, potentially improving soil carbon [100].	- Risk of ILUC if dedicated energy crops displace food production or lead to deforestation on other lands [193]. - Cultivation of energy crops can still require water and nutrients; potential for acidification/eutrophication if poorly managed [193]. - Removal of agricultural/forest residues must be sustainable to avoid soil degradation and biodiversity loss [208]. - Some BTL processes can still have notable lifecycle emissions depending on energy inputs and conversion efficiency [103].
3-G Biofuels	- High biomass productivity per unit area compared to terrestrial crops [196]. - Can be cultivated on non-arable land and in saline/brackish water or wastewater, minimizing competition for freshwater and fertile land [192]. - Efficient CO2 capture and utilization during growth Chen [196]. - Potential for wastewater treatment integration, reducing pollution [192]. - Biodegradable.	- Energy-intensive cultivation (e.g., mixing, pumping in PBRs) and harvesting/dewatering processes can negate GHG benefits if fossil energy is used [193]. - Nutrient requirements (nitrogen, phosphorus) can lead to eutrophication if not managed properly or sourced sustainably [196]. - Large water footprint for open pond systems due to evaporation, though saline water can be used [193]. - Potential biodiversity impacts if non-native algal strains escape into natural ecosystems or if large areas of coastal/aquatic habitats are converted [193]. - GHG emissions from producing fertilizers and materials for cultivation systems.
4-G Biofuels	- Genetically engineered (GE) crops: Potential for higher yields, improved stress tolerance, enhanced biomass composition for conversion, reduced input needs (water, fertilizer) [171]. - Electrofuels (e-Fuels): Potential for very low/zero lifecycle GHG emissions if produced using renewable electricity and direct air capture (DAC) or sustainable CO2 sources [197]. Can be “drop-in” fuels [197]. Utilize CO2 as a feedstock [197].	- GE crops: Risks of gene flow to wild relatives, impacts on non-target organisms, development of herbicide-resistant weeds/pesticide-resistant pests [192]. Potential for monoculture issues. - Electrofuels: High energy consumption for electrolysis and CO2 capture; overall efficiency depends heavily on renewable electricity source efficiency and carbon intensity [197]. Resource demands for catalysts and electrolyzer components. If fossil electricity is used, GHG emissions can be high.
5-G Biofuels	- Engineered microorganisms: Potential for highly efficient and specific conversion of various feedstocks (including waste CO2, lignocellulose) directly into desired fuel molecules or precursors using synthetic biology [198]. Could minimize by-products and processing steps. - Artificial photosynthesis (AP): Direct conversion of solar energy, water, and CO2 into fuels, mimicking natural photosynthesis but potentially with higher efficiency. Potential for closed-loop systems with minimal waste. Can produce stable, portable chemical fuels [199].	- Engineered microorganisms: Biosafety concerns regarding release of GMOs into the environment. Unintended ecological consequences. Potential for horizontal gene transfer. - Artificial photosynthesis: Current efficiencies are generally low for practical application, though some lab systems show promise (e.g., JCAP PV-GDE up to 19.1% solar-to-CO). Stability and durability of catalysts and materials under operational conditions are major issues. Resource demands for catalysts (some use noble metals) [199]. Environmental impact of producing system components.

) and varying socio-economic implications (

Table 3. Main advantages/benefits and main disadvantages/harms of each generation of biofuels compared to fossil fuels from a socio-economic perspective.

Fuel Source	Key Advantages/Benefits	Key Disadvantages/Harms
Fossil Fuels	- Historically low cost and cost-effective for energy production [203]. - Established industry provides significant employment and revenue [209]. - High energy return on investment (historically). Powered industrialization and modern societal development, improving living standards [209]. - Provides energy security for many nations (though often through imports) [209]. - Convenience and familiarity for consumers.	- Subject to price volatility based on geopolitical factors and market speculation [192]. - Significant explicit and implicit subsidies (globally USD 7 trillion in 2022, USD 5 trillion in unpriced environmental costs) distort markets and delay transition to cleaner alternatives [210]. - Externalized health and environmental costs not fully reflected in price [209]. - Finite resource, leading to eventual scarcity and increased extraction costs [203]. - Health impacts from air and water pollution lead to significant medical costs, lost productivity, and reduced quality of life, especially for communities near extraction/refining sites [211]. - Geopolitical tensions and conflicts over resource control [211]. - Occupational hazards in extraction (mining accidents, rig explosions) and refining, leading to injuries and fatalities [212]. - Displacement of communities for extraction projects.
1-G Biofuels	- Potential for job creation in agriculture and processing, rural development [193]. - Can enhance energy security by reducing reliance on imported oil [193]. - Diversification of agricultural markets. - Can support rural economies and provide income for farmers [193].	- Competition with food crops for land and resources, leading to increased food prices (“food vs. fuel” debate) [193]. - Production costs can be higher than fossil fuels, often requiring subsidies [194]. - Vulnerable to agricultural commodity price volatility. - Threat to food security, particularly for vulnerable populations, due to rising food prices and land competition [207]. - Land rights issues, potential for land grabbing and displacement of smallholders/indigenous communities for large-scale plantations [100]. - Poor working conditions and low wages in some feedstock production areas [100].
2-G Biofuels	- Potential for rural development and job creation in feedstock supply and biorefineries [171]. - Lower feedstock costs compared to 1-G (residues, non-food crops) [170]. - Potential for energy security by utilizing domestic biomass resources [193]. - Reduced direct competition with food production compared to 1-G [171]. - Potential for positive impacts on rural communities through new industries and job creation if developed sustainably [171].	- High capital costs for complex conversion technologies (e.g., enzymatic hydrolysis, gasification, FT synthesis) [193]. - Production costs often still higher than conventional fuels and 1-G biofuels, requiring subsidies or technological breakthroughs [193]. - Challenges in feedstock logistics (collection, transport, storage of bulky biomass) [170]. - Indirect food security impacts if large-scale energy crop cultivation displaces other land uses or affects resource availability (water, labor) [171]. - Land tenure issues and potential for displacement if marginal lands are not truly unused or if common lands are appropriated [100]. - Competition for biomass resources with other industries (e.g., paper, materials, bio-power) [208]. - Occupational health and safety risks in biomass harvesting and processing.
3-G Biofuels	- Potential for co-production of valuable products (e.g., animal feed, bioplastics, nutraceuticals) improving overall economics [196]. - Domestic production can enhance energy security [196]. - Does not directly compete with food crops for arable land or freshwater [192]. - Potential for job creation in new bio-based industries [196].	- High capital and operating costs for cultivation (especially closed photobioreactors (PBRs) and harvesting/processing [193]. - Current production costs significantly higher than fossil fuels and other biofuels, making it economically unviable without major breakthroughs or subsidies [193]. - Market for co-products is still developing. - Large land/water area requirements for significant fuel production, even if non-arable/saline, can still lead to resource competition or ecological impacts [193]. - Public acceptance and social license to operate for large-scale facilities. - Ethical concerns related to genetic engineering if used for strain improvement.
4-G Biofuels	- GE crops: Potential for increased farmer income if yields are higher and input costs lower. Potential to improve agricultural sustainability if designed for reduced inputs and grown on suitable land. - Electrofuels: Potential for energy security by using domestic renewable electricity. Job creation in new renewable energy and chemical industries. Market projected to grow significantly [197]. Could reduce reliance on geographically concentrated fossil fuel resources.	- GE crops: Costs of developing and accessing GE seeds. Market acceptance issues. Potential for corporate control over seed supply. Public acceptance and ethical concerns regarding GMOs (food safety, environmental impact, corporate control) [192]. Impact on smallholder farmers if access to GE technology is limited. - Electrofuels: Currently very high production costs (estimates range widely, can be 5–15 times fossil jet fuel), making them uncompetitive without significant cost reductions or strong policy support (e.g., carbon taxes, mandates) [197]. Requires massive investment in renewable electricity generation and CO2 capture infrastructure. Land use for massive renewable energy installations (solar, wind). Social acceptance of large-scale infrastructure. Ethical questions about resource allocation for energy-intensive processes if basic needs are unmet. Potential for exacerbating damage to human health and ecosystems despite lower carbon footprint, depending on externalities [213].
5-G Biofuels	- Engineered microorganisms: Potential for cost-effective production if high efficiency and productivity are achieved. Could utilize waste streams as feedstock. Potential to address waste management issues by converting waste into valuable fuels. - Artificial photosynthesis: If successful, could provide a very low-cost energy source using abundant inputs (sunlight, water, CO2). Could offer decentralized fuel production if systems become simple and robust.	- Engineered microorganisms: High R&D costs. Economic viability depends on achieving high conversion rates, product titers, and robust industrial strains, which is currently challenging [200]. Public perception and acceptance of highly engineered organisms. Potential for misuse (dual-use technologies) [214]. - Artificial photosynthesis: Currently far from economic viability due to low efficiency, high catalyst/material costs, and stability issues [199]. Scalability from lab to industrial production is a massive hurdle [199]. Social implications of large-scale deployment (land use for solar collection, resource allocation). Ethical considerations regarding equitable access to the technology and its benefits. Job displacement in traditional energy sectors [199].

), which must be considered for sustainable deployment.

3.6. Biofuel Types by Physical Condition

The fundamental principle governing biofuel production suitability lies in harnessing materials derived from recently living organisms or their metabolic by-products, effectively utilizing contemporary carbon fixation processes operating on much shorter timescales than geological fossil fuel formation [18,215]. Given the vast array of potential biomass sources, including agricultural crops, forestry residues, algae, and various organic wastes, biofuels exhibit significant diversity in their raw materials and are primarily categorized based on their physical state as liquids (e.g., bioethanol, biodiesel), gases (e.g., biogas, biohydrogen), or solids (e.g., wood pellets, biochar) [16,98]. The key physical and chemical properties of these biofuels, which dictate their storage, handling, and application, are summarized in

Table 4. Physical and chemical properties of representative biofuels.

Biofuel Type	Physical State	Chemical Formula/Typical Composition	Gravimetric Energy Density (MJ/kg)	Volumetric Energy Density (MJ/L)	Key Fuel Parameters	Ref.
Bioethanol	Liquid	C2H5OH	~27	~21	High octane number (>100 RON)	[109,211]
Biodiesel	Liquid	Fatty Acid Methyl Esters (FAME)	~38	~33	High cetane number (45–65)	[216,217]
Biogas	Gas	CH4: 50–75%, CO2: 25–50%	~18.7 (raw)	Varies with pressure	Varies with CH4 content	[217,218]
Wood Pellets	Solid	Lignocellulose	~18–20	N/A	Low ash, low moisture content	[219,220]

. The subsequent subsections will delve into these distinct physical categories, beginning with liquid biofuels.

3.6.1. Liquid Biofuels

Liquid biofuels are characterized as energy carriers existing in a liquid state at ambient conditions, predominantly derived from the processing of diverse biomass feedstocks [221]. Despite representing a relatively small fraction of the total global energy supply, examples such as bioethanol and biodiesel play a recognized role in energy statistics, particularly within the renewable energy portfolio targeted at decarbonizing the transport sector [222]. Liquid biofuels primarily serve as substitutes for conventional petroleum-derived fuels within the transportation sector [12,38], an area critically dependent on liquid hydrocarbons for over 90% of its energy needs [17] and responsible for approximately 25% of global energy-related CO₂ emissions [13]. While specific global production volumes fluctuate, bioethanol and biodiesel remain the most common liquid biofuels [98,113]; moreover, according to the latest IEA data, global biofuel production was approximately 176 billion liters in 2023, with projections indicating steady growth toward 2030 [7]. These fuels have distinct properties that determine their suitability for specific engines. For instance, biodiesel typically exhibits a high cetane number (45–65), similar to fossil diesel, ensuring good ignition quality [215,216]. Conversely, bioethanol has a high research octane number (RON) of over 100 RON, which allows for higher engine compression ratios and can improve thermal efficiency, though its energy density is lower than that of gasoline [109,215].

Chemically, biofuels often exhibit higher oxygen concentrations compared to traditional petroleum feedstocks, with typical values, particularly in less refined forms like py-oil, potentially ranging from 20 to 60 wt.%, though precise ranges vary significantly with fuel type and processing [141,142]. Furthermore, biofuels generally contain lower levels of sulfur and nitrogen compared to many petroleum-derived fuels, contributing to reduced emissions of sulfur oxides (SOx) and NOx upon combustion [223]. Stemming from characteristics like their relatively high energy density compared to alternatives like batteries [36] and gaseous fuels [46,47], along with their liquid state, these biofuels possess distinct advantages regarding ease of transportation and storage using existing infrastructure [12,15], simplified logistics, established safety protocols [46], and compatibility, often requiring minimal retrofitting for integration into current engine systems [43].

Within the liquid biofuel category, bioethanol and biodiesel are currently regarded as the most commercially significant types [98,113]. Major classifications for liquid biofuels often relate to their feedstock origin and production pathway, distinguishing between, for instance, triglyceride-based fuels derived primarily from oils and fats (1-G sources like palm oil or soybean oil [94,113]) and lignocellulosic-based fuels produced from non-food plant matter (2-G sources like agricultural residues or forestry wastes) [116,117,224]. Specific examples under the triglyceride-based category include biodiesel produced via transesterification of vegetable oils (e.g., palm, soybean) or animal fats [113,114]. Conversely, biofuels classified as lignocellulosic-based, sometimes referred to as “advanced” or “second-generation”, include cellulosic ethanol produced via biochemical routes [124] and synthetic diesel or gasoline fractions synthesized via F-T conversion of syngas derived from the gasification of agricultural residues or forestry waste [147,148].

Equation (3) [225] shows the ideal (stoichiometric) combustion of ethanol, a primary liquid biofuel, in air. It details the required oxygen for complete combustion and the resulting products (CO₂ and H₂O), which is fundamental for engine design, emissions analysis, and understanding the energy release process.

$$C_2{H}_{5}OH\left(\mathrm{l}\right)+3\left(O_2\left(\mathrm{g}\right)+3.76N_2\left(\mathrm{g}\right)\right)\to 2{CO}_2\left(\mathrm{g}\right)+3H_{2}O\left(\mathrm{g}\right)+\left(3\times 3.76\right)N_2\left(\mathrm{g}\right)$$

(3)

The expressions in the equation represent the following: C₂H₅OH (l) is liquid ethanol; O₂ (g) is gaseous oxygen; N₂ (g) is gaseous nitrogen (representing air, assumed inert); CO₂ (g) is gaseous carbon dioxide; H₂O (g) is gaseous water (steam); (l) is the liquid phase; (g) is the gaseous phase; 3.76 is the approximate molar ratio of N₂ to O₂ in air.

According to renewable energy roadmaps, established liquid biofuels like ethanol and biodiesel are projected to continue contributing to transportation energy demand towards 2030, complementing the mandated growth of advanced biofuels (e.g., EU target of 3.5% minimum share by 2030 [14]), though specific quantitative projections comparing their 2030 contribution to 2017 levels require further data. The suitability and necessity of employing liquid biofuels, particularly advanced types, are increasingly recognized for the shipping and aviation sectors [107], where electrification faces substantial energy density limitations [12,36]. These sectors account for a significant portion of total transportation energy demand, representing vital developing segments for biofuel deployment. Consequently, a strategic approach involving the deployment of advanced liquid biofuels (e.g., sustainable aviation fuel—SAF) is often recommended to address the challenging energy demands and decarbonization goals of the growing shipping and aviation industries [179]. Advanced liquid biofuels may offer supplementary advantages, potentially including reduced pollutant emissions compared to fossil fuels [43], which is significant for mitigating the climate impact of the transportation sector, although achieving cost-competitiveness relative to fossil fuels remains a key challenge influenced by processing costs and efficiency [171,175].

3.6.2. Gaseous Biofuels

Gaseous biofuels are generally characterized as low-density fuels due to their inherent physical state, which results in lower energy content per unit volume compared to liquid or solid fuels, impacting storage and transportation requirements [226]. Key factors contributing to the increased research and exploration of gaseous biofuels over the past decade include the sustained global focus on climate change mitigation [227], the re-emergence of energy security as a critical priority [228], the need to meet decarbonization targets [171], supportive government policies [229], and the pursuit of alternatives amid volatile fossil fuel prices [230]. The most significant types of gaseous biofuels currently recognized include biogas, biohydrogen, biomethane or bio-CNG, and green hydrogen produced using renewable electricity [231]. Biogas, for example, is primarily composed of methane (CH₄, typically 50–75%) and carbon dioxide (CO₂, 25–50%), with trace amounts of other gases, a composition which dictates its calorific value and suitability for being upgraded to biomethane for grid injection or vehicle use [217,218].

The production of gaseous biofuels from biowaste materials primarily employs thermochemical conversion processes such as gasification, pyrolysis, and hydrothermal liquefaction [135]. These gaseous fuels are typically utilized for energy generation in applications such as combined heat and power (CHP) systems, electricity generation in power plants, direct heating, and as fuel for transportation or industrial processes [232]. Specific advantages associated with using gaseous biofuels include potentially cleaner combustion leading to reduced pollutant emissions compared to some conventional fuels, versatility in application, and the ability to valorize diverse organic waste streams, although factors like process complexity, oxidant requirements (e.g., in gasification), and gas cleaning needs must be managed [233,234,235].

3.6.3. Solid Biofuels

Solid biofuels are fundamentally defined as fuels derived from solid organic, non-fossil materials of biological origin, encompassing biomass sourced from plants, animals, or the biogenic fraction of municipal waste [232,236]. According to standard energy statistics, the term “solid biofuel” represents an aggregate product category that includes charcoal, fuelwood, wood residues and by-products, black liquor, bagasse, animal waste, other vegetal materials and residuals, and the renewable fraction of industrial waste [237,238]. Suitable feedstocks for producing solid biofuels range from unprocessed forms like fuelwood directly obtained from forestry or agricultural residues to minimally processed materials such as wood chips derived from milling or chipping and further processed forms created through densification, like pellets and briquettes, or thermochemical upgrading via torrefaction or hydrothermal carbonization to produce biochar or hydrochar [239]. The quality of these solid fuels for combustion is largely determined by key properties such as their heating value (e.g., 18–20 MJ/kg for wood pellets), along with low ash and moisture content, which collectively affect combustion efficiency, operational costs, and emissions [219,220]. Consequently, solid biofuels are often categorized based on their origin (e.g., woody, herbaceous, or fruit biomass) or their processing level and form (e.g., firewood, chips, pellets, or briquettes) [240,241]. The key potential applications for these biomass resources, presented in

Table 5. Some examples of solid biofuel feedstocks [239,243,244,245,246].

Solid Wastes	Lignocellulosic Biomass
	Energy Crops	Forest Wastes	Agricultural Wastes
Dried animal manure Food waste Municipal solid waste Plastics Poultry waste Processed paper Wastewater sludge	Energy cane grass Energy cane leaf Energy cane stem Grass leaf Grass stem Hybrid Pennisetum Miscanthus Switchgrass Triarrhena lutarioriparia	Black locust Eucalyptus Firewood Fruit bunch Hardwood Hybrid poplar Pine Sawdust Softwood Spruce Willow chips Wood branches Wood chips	Barley straw Corn cobs and stover Fruit pits Fruit tree Grapeseed Grapevine pruning residues Olive pulp Rice husk and straw Sorghum straw Straw Sugarcane peel and bagasse Sunflower husks and stalks Wheat straw

, involve direct combustion to produce heat (for applications such as cooking, domestic hot water, and industrial processes) and to generate electricity [242].

Common utilization pathways for biomass designated as solid biofuel involve direct combustion, thermochemical conversion (including pyrolysis and gasification to produce other fuel types or intermediates), and potentially biochemical conversion routes depending on the specific biomass composition and intended product [247,248,249].

3.7. Challenges of Biofuel

While biofuels offer significant potential within the broader portfolio of ecofuels, their development, deployment, and long-term sustainability face numerous interconnected challenges spanning technical, economic, regulatory, and feedstock dimensions. Similarly to e-Fuels, biofuels are primarily anticipated to play a critical role in decarbonizing sectors where direct electrification is challenging, such as aviation, maritime shipping, and heavy-duty road transport, thereby contributing towards climate neutrality goals by replacing conventional fossil fuels. However, considerable uncertainty surrounds the total achievable potential for sustainable bioenergy production, largely driven by factors including biomass availability, logistical complexities, feedstock quality variability, conversion technology limitations, land use impacts, water consumption, and overall economic viability [72,250,251].

From a feedstock perspective, the limitations of earlier biofuel generations, discussed in Section 3.1, Section 3.2 and Section 3.3, persist [252]. First-generation feedstocks, primarily food crops, face significant constraints regarding sustainable production potential due to direct and ILUC impacts, competition with food supply, and associated biodiversity loss [227,253]. While 2-G biofuels utilize lignocellulosic biomass and waste residues, thereby mitigating ILUC concerns to some extent (as detailed in Section 3.2), their production scale is currently restricted by the technological challenges and costs associated with complex pre-treatment and conversion processes, as well as issues in scaling up production from demonstration plants [72]. Realizing the potential of 3-G (algae-based) and 4-G (genetically engineered organisms) bioenergy necessitates critical advancements in harvesting techniques (especially for microalgae), downstream processing efficiency, cost reduction, and managing public acceptance and safety concerns related to genetic modification [170,173].

Technological innovations aim to enhance biofuel yields and process efficiencies. For instance, the addition of hydrogen during the gasification of biomass shows potential for enhancing biofuel yields, potentially producing around 40–190 kg of hydrogen per ton of dry biomass, and offers integration opportunities with other renewable technologies like electrolysis [254]. Furthermore, coupling bioenergy production with carbon capture and utilization (CCU) or storage (CCS)—known as BECCU and BECCS—can significantly improve the overall energy exploitation from biogenic carbon and enhance bioenergy’s climate mitigation potential across various sectors [255,256]. BECCS, in particular, offers a pathway to negative emissions by storing the captured biogenic CO₂, while BECCU allows the captured CO₂ to be combined with renewable hydrogen for producing additional fuels (e.g., e-Fuels), potentially offering a less costly CO₂ source than direct air capture for future fuel synthesis needs [257]. Integrating BECCS/BECCU is thus pivotal not only for potentially achieving negative emissions but also for maximizing the overall carbon efficiency and resource potential of biofuel pathways within the broader energy system.

Overcoming these technical and feedstock challenges requires a supportive and stable policy environment, yet significant barriers exist in this domain. Enhanced national biomass policies are essential for optimizing biomass utilization hierarchies, maximizing verifiable GHG savings, and ensuring equitable competition among different biomass users [258]. However, comprehensive national policies designed to achieve these aims effectively are not universally established, and existing frameworks often lack long-term stability [72]. In the EU, for example, the specific biofuel strategy has evolved significantly since its inception (see, e.g., Commission of the European Communities, 2006), but the lack of a clear, stable, long-term policy outlook beyond 2030, coupled with frequently shifting priorities regarding preferred biofuel types, has negatively impacted investment security and hindered research, development, and deployment activities, particularly for advanced biofuels [72].

Regulatory uncertainty further compounds these issues. Many transportation sectors have long-term decarbonization goals but lack clearly defined, binding roadmaps for achieving them, creating ambiguity for fuel developers and investors [72]. Frequent changes in tax structures for vehicles and fuels add another layer of uncertainty. Policy support frameworks also differ significantly between major markets; for instance, the US employs substantial long-term fiscal incentives (“carrots”) like tax credits under the Inflation Reduction Act to stimulate SAF production, whereas the EU relies more heavily on mandates and blending obligations (“sticks”) like those in the Fit for 55 packages, with less long-term fiscal certainty [259]. First-generation biofuels, primarily bioethanol and biodiesel, are still utilized globally, particularly in road transport applications across regions like the Americas, Europe, and Asia, often supported by blending mandates [110]. This global dynamic is also reflected in feedstock markets, where policy incentives in one region can drive global trade; for example, the EU’s consumption of used cooking oil (UCO) has significantly increased its reliance on imports, primarily from Asia, raising concerns about feedstock costs and traceability [10,260,261]. Navigating the deployment of ecofuels requires understanding the complex and sometimes conflicting policy landscape, with major initiatives in the EU and US employing different approaches, summarized in

Table 6. Key policy mechanisms for ecofuels (EU and US).

Policy/Initiative	Jurisdiction	Key Targets/Goals	Primary Mechanisms	Ref.
EU Green Deal	EU	- Reduce fossil fuel dependency. - Accelerate renewable energy deployment. - Accelerate electric mobility adoption.	- Mobilizing investments and subsidies.	[5]
US Inflation Reduction Act (IRA)	US	- Reduce fossil fuel dependency. - Accelerate renewable energy deployment.	- Mobilizing investments and subsidies. - Substantial long-term fiscal incentives (tax credits) for production (e.g., SAF).	[6]
Fit for 55 Package	EU	- Achieve 55% GHG reduction by 2030 (vs. 1990 levels).	- Mandates and blending obligations (“sticks”).	[73]
REPowerEU Plan	EU	- Curtail oil dependency. - Increase leverage of renewable/sustainable sources.	- Strategic initiatives and mandates.	[11]
Directive (EU) 2018/2001 (RED II) (Biofuel/Renewable Transport Policy)	EU	- Renewables ≥ 14% of transport energy by 2030. - Advanced biofuels ≥ 3.5% share by 2030. - Evolved significantly over time.	- Mandated minimum shares and targets. - Although the new binding renewable energy target for 2030 is set at least 42.5%, the lack of a clear and stable biofuels or renewable transport policy outlook after 2030 is affecting investments.	[14]
Zero-Emission Vehicle Mandate (2035)	EU	- After 2035, including vehicles using CO2-neutral fuels, only zero-emission vehicles can be licensed. At least 30 million zero-emission vehicles by 2030.	- Phasing out new fossil-fueled car sales. - Incentives (tax benefits, purchase subsidies).	[262]
E-Fuel Standards (Draft/Delegated Act)	EU	- Qualifying e-Fuels must achieve 70–100% GHG emission reductions (lifecycle) vs. fossil fuels.	- Defining requirements for a new vehicle category operating solely on e-Fuels.	[263]
EU Emissions Trading System (ETS)	EU	- Provide market incentives for low-emission mobility investments.	- Carbon pricing (ETS price exceeded EUR 100/tCO2 in February 2023).	[264]
EU ETS2	EU	- Extend carbon pricing to new sectors.	- ETS2 (from 2027) covers road transport and building fuel combustion.	[265]

.

Inconsistencies within regulatory frameworks create additional hurdles. Different biofuel feedstocks often receive inconsistent treatment under various targets and regulations, adding implementation complexity [72]. Furthermore, the potential exists for the identical biofuel raw material to be classified variously across various EU member states, hindering market harmonization [104]. Significant challenges also pertain to the reliability and consistency of official methodologies for calculating GHG emission savings. Current EU approaches are often criticized for potentially overestimating savings, particularly due to the inadequate handling of ILUC emissions for crop-based biofuels and the use of default emission factors that may not accurately reflect the GHG footprint associated with feedstock geographical origin and specific supply chain logistics [104,266,267]. Addressing these inconsistencies and inaccuracies through refined regulations and calculation methods, ensuring greater transparency and data coherence, is crucial for validating the climate benefits of biofuels [115].

Economic and feedstock acquisition challenges remain prominent. Intense competition exists for biomass raw materials, not only among different biofuel producers but also with other sectors like food, feed, biochemicals, and material production, impacting market availability and driving up prices [213]. Contrary to initial expectations that biofuel adoption would significantly enhance energy independence, the dynamics of feedstock markets, particularly in regions like the EU, have led to increased reliance on imports [72]. Over the past two decades, the EU’s sourcing pattern has shifted towards greater import dependency [10]. Specifically, the consumption of UCO for biofuels surged between 2011 and 2020, driven partly by policy incentives like double counting, but this has significantly increased the reliance on imports, primarily from Asia, raising concerns about feedstock costs, traceability, and potential fraud [260,261]. While EU domestic UCO collection potential is estimated around 1.5–1.7 Mt/yr, consumption has been significantly higher (e.g., ~2.85 Mt UCOME in 2019), with imports filling the gap [268]. Looking ahead, the IEA highlights that while announcements for new renewable diesel (hydrogenated vegetable oil—HVO) and bio jet fuel (sustainable aviation fuel—SAF) facilities are increasing, actual production capacity growth is struggling to keep pace with ambitious targets, indicating that feedstock availability will remain a critical constraining factor for producers in the near term (2030), assuming current trends persist [72,269,270]. Specific quantitative projections for feedstock availability in this period remain uncertain [271].

In summary, while biofuels are integral to future energy scenarios, accelerating their sustainable adoption requires overcoming substantial hurdles related to policy stability, regulatory clarity, GHG accounting accuracy, technological maturity for advanced generations, and ensuring sustainable, economically viable feedstock supply chains amidst growing inter-sectoral competition.

4. Electrofuels (E-Fuels)

Electrofuels (e-Fuels) represent a class of synthetic fuels produced via power-to-fuel processes which enable the conversion of electrical energy, ideally from renewable sources, into chemical energy stored in liquid or gaseous fuels [272,273,274]. The fundamental principle behind their potential climate neutrality lies in utilizing CO₂ captured from atmospheric, biogenic, or industrial point sources, thereby creating a closed carbon loop when the CO₂ is released during fuel combustion [275,276]. Power-to-gas (P2G) and power-to-liquid (P2L) schemes specifically facilitate this transformation into convenient energy carriers, offering a crucial solution for managing the intermittency and surplus generation associated with large-scale wind and solar power deployment [277]. The production of e-Fuels can thus enhance the utilization of variable renewable energy by absorbing surplus electricity that might otherwise be curtailed, contributing to grid stability [278]. Synthetic fuels derived from electricity offer significant advantages due to their high energy density, ease of storage, and transportability using existing infrastructure, positioning them as potential drop-in replacements for fossil fuels in current internal combustion engine (ICE) vehicles [272]. The synthesis of these diverse e-Fuels follows several key pathways, all originating from renewable electricity and captured CO₂, as illustrated in Figure 8.

The production pathway for e-Fuels typically involves two primary initial stages: the generation of hydrogen, usually through water electrolysis powered by renewable electricity, and the capture of CO₂ [279]. Viable CO₂ sources include emissions from industrial processes (e.g., cement plants), biogenic sources (e.g., biogas or bioethanol production), or DAC technologies [280,281]. Subsequently, the captured CO₂ is reacted with the produced H₂ to synthesize fuels. For instance, the F-T synthesis converts syngas (a mixture of H₂ and CO, often derived from CO₂ via the reverse water–gas shift reaction) into liquid hydrocarbons using metal catalysts, typically iron (Fe) or cobalt (Co), at elevated temperatures (e.g., 200–350 °C) and pressures (e.g., 10–30 atm) [282,283,284]. Reacting CO₂ with H₂ can yield a range of gaseous and liquid low-carbon fuels, including methane, methanol, dimethyl ether (DME), and higher-value liquid e-Fuels like synthetic gasoline, diesel, and jet fuel [285].

The conversion of CO₂ to liquid fuels can occur via one-step (direct CO₂ hydrogenation) or two-step processes [286,287]; the latter typically involve an intermediate reaction such as the reverse water–gas shift (RWGS) expressed by Equation (4) to produce CO, which is then hydrogenated in a subsequent step like F-T synthesis [288,289]. This equation shows the reversible reaction used to produce CO from CO₂, a crucial intermediate step for synthesizing e-Fuels.

$${CO}_2+H_2\rightleftharpoons CO+H_{2}O$$

(4)

Besides the primary fuel product, e-Fuel production can generate commercially valuable by-products, including heat from exothermic reactions, oxygen from water electrolysis, and potentially waxes or other hydrocarbon fractions depending on the synthesis route and conditions [290]. However, achieving fully climate-neutral e-Fuel production faces challenges, partly stemming from reliance on fossil fuels in the current energy mix and complexities in LCA methodologies that must account for all upstream emissions [291]. Furthermore, process limitations related to catalyst performance significantly affect the selectivity towards desired liquid products and the overall rate of fuel formation, making catalyst selection and development crucial for improving the efficiency and economic viability of e-Fuel production [292].

The primary e-Fuels currently under consideration, including in Europe, are e-hydrogen, e-methane (the primary component of e-LNG), e-methanol, e-ammonia, and synthetic hydrocarbons like e-diesel and e-kerosene, which are pivotal for decarbonizing sectors where direct electrification is challenging [18,83,279]. These e-Fuels offer high energy density and compatibility with existing infrastructure, positioning them as potential drop-in replacements for fossil fuels [272]. The fundamental principle of their climate neutrality relies on utilizing CO₂ captured from atmospheric, biogenic, or industrial point sources, aiming for a closed carbon loop when renewable energy powers their entire production and utilization lifecycle [275,276]. However, achieving full climate neutrality is contingent on accounting for all upstream emissions, including those from infrastructure and any non-renewable energy used [291].

The production pathway for all e-Fuels commences with the generation of hydrogen, predominantly green e-hydrogen, which is produced via water electrolysis powered exclusively by renewable electricity sources such as solar or wind energy [18,290]. During this electrocatalytic process, H₂O is split into H₂ and O₂, as shown in Equation (5) [280]. The production of e-hydrogen is strategically justifiable when utilizing surplus renewable electricity that might otherwise be curtailed, thereby enhancing grid stability and maximizing the use of variable energy sources [277,278]. From a production cost perspective, e-hydrogen is often the most economical foundational e-fuel due to its direct production pathway, avoiding subsequent energy-intensive synthesis steps [279]. However, e-hydrogen faces significant economic disadvantages related to the high costs of storage and transportation infrastructure stemming from its low volumetric energy density [46,47,55]. The most substantial contributor to e-hydrogen production costs is the price of renewable electricity; these costs can be minimized by optimizing electrolyzer operation in conjunction with fluctuating renewable power supplies and through anticipated reductions in electrolyzer capital expenditures [68,69,70,71].

$$\begin{gathered} Electrolyzer \\ 2H_{2}O\left(l\right)+ElectricalEnergy 2H_2\left(g\right)+O_2\left(g\right) \end{gathered}$$

(5)

For carbon-containing e-Fuels, captured CO₂ is reacted with green hydrogen. E-methanol, for example, is synthesized by reacting green hydrogen with captured CO₂, with renewable electricity being crucial for producing the hydrogen and potentially for powering the direct CO₂ hydrogenation process [285]. The cost of this green hydrogen is usually the biggest part of the cost of making e-methanol. CO₂ circularity for e-methanol is achieved by using CO₂ from sustainable sources, which is re-released upon combustion, ideally forming a closed loop [275,276].

Liquid hydrocarbon e-Fuels with carbon chains typically ranging between C8 and C18, such as e-kerosene and e-diesel, are primarily generated by converting green hydrogen and captured CO₂ through F-T synthesis [272,290]. The F-T process converts syngas, which is a mixture of H₂ and CO, into liquid hydrocarbons, where CO is usually derived from CO₂ via the RWGS reaction (presented in Equation (1)) [288,289]. This synthesis employs metal catalysts, typically Fe or Co, at elevated temperatures and pressures [282,283,284,290]. Fe- and Co-based catalysts are favored for these procedures given their established activity, selectivity, and cost-effectiveness [282,284]. The F-T reaction is highly exothermic, and the generated heat is typically integrated into the plant to improve overall process efficiency, for instance, by preheating reactants or generating steam [290]. A critical factor for the efficiency of F-T synthesis coupled with an RWGS reactor is the precise management of the H₂/CO ratio in the syngas and effective process integration [288,289]. Alternative pathways to e-diesel and e-kerosene include the hydrocracking of heavier waxes produced during F-T synthesis [290] or potentially routes via e-methanol as an intermediate [285]. Catalyst selection and development remain crucial for improving the efficiency, selectivity towards desired liquid products, and economic viability of all e-fuel production routes [292].

Among the array of e-Fuels, e-ammonia is gaining prominence as a carbon-free energy vector with significant potential for decarbonizing various sectors. Conventionally, industrial ammonia synthesis relies on the Haber–Bosch (H-B) process given in Equation (6) [293], which catalytically combines atmospheric N₂ with H₂ at high temperatures (400–500 °C) and pressures (150–300 bar), followed by condensation and separation steps to recover liquid ammonia [294]. The primary global application of this industrially synthesized ammonia is in the production of nitrogenous fertilizers, a sector that consumes a substantial portion of global hydrogen production [295], with hydrogen itself being a key feedstock in ammonia production and oil refining, and global hydrogen demand reached 97 Mt in 2023 [56]. The conventional H-B process, predominantly utilizing hydrogen derived from SMR of natural gas, is not considered a “green” process due to its significant reliance on fossil fuels and associated GHG emissions [293,296]. This conventional process is energy-intensive, typically consuming 28–36 GJ of energy per ton of ammonia, and contributes approximately 1.8% to global CO₂ emissions, primarily originating from the SMR stage used for hydrogen production [296,297,298]. However, there is potential for capturing and reusing a significant portion of the CO₂ generated during the methane reforming stage of conventional ammonia production, which could mitigate some of its carbon footprint [299].

$$\begin{gathered} Catalyst,T,P \\ {N}_2\left(g\right)+3H_2\left(g\right)\rightleftharpoons 2N{H}_3\left(g\right) ({\Delta H}_{298K}^{°}=-91.4\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}) \end{gathered}$$

(6)

Equation (6) represents the H-B process, the industrial method for synthesizing ammonia. It highlights the exothermic, reversible reaction between nitrogen and hydrogen, which is critical for understanding both the conventional (gray/brown) and sustainable (blue/green) production of ammonia, a key e-Fuel candidate. N₂ (g) is gaseous nitrogen, H₂ (g) is gaseous hydrogen, NH₃ (g) is gaseous ammonia, ΔH°_298K is the standard enthalpy change of reaction at 298 K, and Catalyst, T, P indicate the requirement of a catalyst, high temperature, and high pressure [300].

In the context of next-generation carbon-free fuels, ammonia exhibits a liquid volumetric energy density of approximately 12.7 MJ/L [301], which is lower than that of methanol (15.6 MJ/L) and gasoline (32 MJ/L) [302] but significantly higher than that of liquid hydrogen (8.5 MJ/L) [301]. Thus, ammonia offers advantages for storage and transport compared to pure hydrogen, although its gravimetric energy density (18.6 MJ/kg) is lower than that of hydrocarbon and hydrogen (120 MJ/kg) [303]. Ammonia’s versatility allows it to be utilized as a fuel in various energy conversion systems, including direct use in ICEs and gas turbines for power generation or decomposition back into hydrogen and nitrogen for use in fuel cells [304]. Despite its benefits, the primary emissions concern when using ammonia directly as a fuel in thermal engines is the potential formation of NOx and unburnt ammonia slip, which can be mitigated through advanced combustion control strategies, SCR systems, and catalytic converters [305]. From an economic and infrastructural standpoint, ammonia offers advantages over hydrogen as an energy vector due to its higher volumetric energy density, established large-scale production, existing global storage and distribution networks, and less demanding liquefaction requirements (10 bar or 25 °C) compared to hydrogen’s cryogenic storage needs [296,306].

E-ammonia, whose production route is presented in Figure 9, is typically separated from air and “green” or “blue” hydrogen as primary feedstocks, differing from conventional synthesis by employing hydrogen produced with significantly lower or zero lifecycle GHG emissions while often still utilizing the H-B reactor for the synthesis step, achieving high conversion yields [307,308]. Green hydrogen is produced via water electrolysis powered solely by renewable energy sources, resulting in virtually zero GHG emissions during its production [309], whereas blue hydrogen is derived from fossil fuels, typically natural gas, with the associated CO₂ emissions captured and stored or utilized (CCUS) [310]. Recent advancements in combustion technology, particularly in the marine sector, have enabled the development of ammonia-fueled two-stroke and four-stroke engines, with major manufacturers demonstrating successful operations and targeting commercial availability, offering a pathway for sustainable shipping with significantly reduced CO₂ emissions compared to conventional diesel engines [311]. While developments in the automotive sector for direct ammonia use in passenger cars remain highly speculative and are significantly less mature than in the maritime sector, with current claims often stemming from unvalidated industrial research, proponents highlight potential but yet undemonstrated environmental benefits such as zero CO₂ exhaust emissions and reduced reliance on critical battery materials, contingent upon overcoming formidable technical, safety, and emission (e.g., NOx and ammonia slip) challenges [304]. Furthermore, alternative, distributed green ammonia production technologies aiming to bypass the traditional Haber–Bosch process, such as electrochemical synthesis at ambient conditions, direct photocatalytic nitrogen fixation, and plasma-assisted synthesis, are under active development, though most are currently at lower Technology Readiness Levels (TRL 3–5) [312].

Challenges of E-Fuels

e-Fuels are poised to play a critical role in achieving climate neutrality, particularly in hard-to-electrify sectors such as aviation, maritime shipping, and heavy-duty transport, where their compatibility with existing infrastructure and high energy densities offers distinct advantages over alternatives like batteries or gaseous hydrogen [11,12]. The theoretical climate neutrality of e-Fuels hinges on the exclusive use of renewable electricity in both hydrogen production and downstream synthesis processes, facilitating a closed-loop carbon cycle when the CO₂ used in fuel synthesis is re-released during combustion [12,313]. However, it is scientifically contentious to disregard the upstream GHG emissions associated with the construction, operation, and maintenance of renewable electricity infrastructure, such as photovoltaic panels and wind turbines, particularly during LCAs [1,3]. For instance, the carbon intensities of electricity from photovoltaics and onshore wind are estimated at approximately 20–80 gCO₂-eq/kWh and 10–30 gCO₂-eq/kWh, respectively, which, while substantially lower than the EU grid average of around 230–280 gCO₂-eq/kWh, depending on fuel content, is not negligible and must be integrated into WtW emission analyses [3,9,314,315]. Even when upstream emissions are included, e-Fuels can contribute meaningfully to GHG mitigation, with potential lifecycle reductions of 70–100% compared to fossil fuels, depending on the specific pathway and renewable energy share [11,12]. Furthermore, the choice of LCA methodology can significantly influence results, highlighting the need for transparent and standardized assessment protocols to ensure fair comparisons and validate climate neutrality claims.

Nonetheless, significant technical and economic barriers persist. The current low commercial availability of circular CO₂ sources (those derived from biogenic or industrial processes with minimal upstream emissions) restricts the scalability and cost-effectiveness of e-Fuel production, increasing reliance on more expensive options such as DAC. Accordingly, intensifying CO₂ capture and sequestration initiatives is essential to secure adequate feedstock and stabilize input pricing. Simultaneously, the unpredictability of renewable electricity prices and supply undermines economic feasibility, as e-Fuel synthesis is highly sensitive to fluctuations in electricity costs and availability [12]. Moreover, the lack of optimized technical routes, particularly concerning catalyst efficiency, product selectivity, and process integration, remains a key challenge limiting conversion efficiency and economic viability [1].

On a systemic level, the transition to e-Fuels demands extensive infrastructure developments, including new engine designs capable of handling diverse fuel chemistries, upgraded storage systems, and retrofitted distribution networks [11,12]. Organizational reforms, such as coordinated sectoral strategies, harmonized standards, and international certification schemes, are equally critical to enable widespread adoption [313]. However, the absence of a universal policy framework governing e-Fuel sustainability, coupled with highly variable national vehicle taxation regimes and fossil fuel subsidies, introduces financial uncertainty and deters long-term investment [3,10]. The entrenched global dependence on low-cost fossil fuels and imported electricity, particularly within the EU, further constrains the market entry of e-Fuels by rendering them economically uncompetitive in the near term [8,12].

5. Waste-Derived Fuels

The escalating global accumulation of non-biodegradable materials, particularly post-consumer plastics and end-of-life tires, presents a formidable environmental challenge, necessitating innovative waste management strategies and the development of circular economy pathways [75]. The global environmental challenge of pollution is significantly exacerbated by the accumulation of non-biodegradable materials, with plastics, including single-use items like bottles and bags, being major contributors that threaten ecosystems and human health [316]. The widespread and increasing global consumption of plastics is driven by their versatility, cost-effectiveness, durability, and lightweight nature, making them indispensable in numerous applications, particularly packaging, amidst rising global consumption patterns [317]. The COVID-19 pandemic triggered a significant surge in plastic consumption due to the heightened demand for single-use personal protective equipment, such as masks and gloves, and an increased reliance on delivery services and online shopping [318,319]. Discarded tires represent a particularly significant and problematic category of solid waste due to their high durability, non-biodegradable nature, large volumes that consume valuable landfill space, and their potential to pose fire hazards and create breeding grounds for disease vectors. Common improper disposal methods for plastic and tire waste globally include landfilling, illegal dumping, and open burning, leading to severe environmental consequences such as soil and water contamination, air pollution from toxic emissions, and habitat destruction [320].

5.1. Thermochemical Conversion of Plastic and Tire Waste

In managing plastic and tire waste, thermochemical conversion processes, particularly pyrolysis, are garnering significant interest for their potential to transform these problematic wastes into valuable fuels and chemical feedstocks [75]. The fundamental chemical basis allowing plastic and tire waste to be viable feedstocks for alternative fuels lies in their composition of long-chain polymers, which can be thermally depolymerized into smaller hydrocarbon molecules analogous to those found in conventional fuels, often possessing high calorific values [321,322]. Pyrolysis is considered a highly suitable thermochemical conversion technology for transforming polymeric waste into fuel, with other notable methods including gasification and, for energy recovery, incineration [75,323]. The primary operational and environmental advantages of using pyrolysis for converting polymeric waste include significant waste volume reduction, the ability to process mixed and contaminated plastics, and the generation of valuable products like oils and syngas, potentially with lower CO₂ emissions compared to traditional disposal methods like incineration [324,325]. Equation (7) outlines the pyrolysis process, a thermochemical method for converting complex polymeric wastes (plastics, tires) into simpler, more valuable products [75]. It shows the thermal degradation in an oxygen-free environment, yielding liquid oil, gas, and solid char, which is key to waste-to-fuel technologies.

$$\begin{gathered} \Delta ,InertAtm. \\ PolymerWaste\left(C_x{H}_y{O}_z\dots \right) \to PyrolysisOil\left(C_a{H}_b{O}_c\dots \right)+Syngas\left(CO,H_2,{CH}_4\right)+Char \end{gathered}$$

(7)

where C_xH_yO_z represents the complex chemical structure of plastic or tire waste, Δ indicates heat application, Inert Atm. indicates an inert (oxygen-free) atmosphere, C_aH_bO_c... represents the mixture of compounds in liquid pyrolytic oil, CO, H₂, CH₄... represents the main components of the gaseous product (syngas), and C represents the solid carbon residue (char) [75].

Waste plastic oil (WPO) can exhibit fuel properties comparable to conventional diesel or gasoline, depending significantly on the plastic feedstock composition and pyrolysis process parameters [326,327]. The pyrolysis of polystyrene typically yields styrene monomers and other aromatic compounds, whereas polyethylene terephthalate mainly produces benzoic acid, CO, CO₂, and a higher char yield, while polyolefins like high-density polyethylene, low-density polyethylene, and polypropylene primarily generate mixtures of paraffinic and olefinic hydrocarbons [328,329]. Typical product yield ranges from mixed waste plastic pyrolysis can vary significantly; for instance, liquid oil yields can be approximately 48–75 wt.%, and these are heavily influenced by feedstock composition (with polyolefins favoring higher oil yields), process temperature, residence time, reactor design, and the application of catalysts, as well as pre-treatment methods like sorting to remove problematic plastics like PVC and PET [330,331]. The pyrolysis of low-density polyethylene (LDPE) at around 300–350 °C, particularly with catalysts, can yield liquid fuels primarily composed of aliphatic hydrocarbons, with specific properties like viscosity and calorific value varying; for instance, one study using a TiO₂ catalyst at 350 °C reported a fuel composed of approximately 49.41% gasoline-like fractions [332,333]. Liquid oils derived from the pyrolysis of polypropylene and high-density polyethylene often exhibit properties that align more closely with diesel fuel specifications, such as calorific values around 45–47 MJ/kg and flash points between 40 and 60 °C, although their cetane numbers may vary and they typically have low ash and water content [326,334]. Waste polyethylene terephthalate (PET) and polyvinyl chloride (PVC) generally result in lower oil yields and fuel quality due to their inherent chemical structures: PET’s oxygen content leads to the formation of oxygenated compounds and higher char, while PVC releases corrosive hydrogen chloride gas, both of which diminish the yield and heating value of the desired hydrocarbon oil fraction [335,336,337].

5.2. Waste Tire Pyrolysis

The pyrolysis of waste tires differs fundamentally from that of many other polymeric materials owing to the complicated heterogeneous composition of tires. This composition consists of natural and synthetic rubbers, carbon black, steel, and various additives. This results in a product slate typically richer in aromatic compounds, sulfur, and char compared to the pyrolysis of simpler, more homogenous plastics [338]. Waste tires typically possess a high carbon content, often exceeding 80% on a dry basis, and a high heating value in the range of 30–40 MJ/kg, which is comparable to or even higher than that of traditional solid fuels like coal [339,340]. Tire-derived fuel (TDF) is commonly utilized as an alternative solid fuel in energy-intensive industrial applications such as cement kilns, where its co-processing with traditional fuels like coal is rationalized by its high calorific value, its potential to reduce net GHG emissions compared to conventional fossil fuels under optimized conditions, and its role in diverting a problematic waste stream from landfills [339].

The primary objective of waste tire pyrolysis is often the optimization of liquid oil yield, typically ranging from 40 to 55 wt.%. This is particularly valuable due to its potential as an alternative fuel or a source of chemical feedstocks. The simultaneously produced pyrolytic gas can be utilized to supply energy for the pyrolysis process, thereby enhancing its overall energy efficiency and economic viability [341]. The solid residue, or char, from waste tire pyrolysis is a porous material with a notable heating value, typically characterized by a high carbon content (often >80%) and a significant, yet variable, sulfur content (e.g., 1–3 wt.%), with its specific properties like surface area and elemental distribution being heavily influenced by pyrolysis conditions and the composition of the waste tire feedstock [342]. Following initial mechanical pre-treatment, the chemical transformation of tires during pyrolysis generally involves thermal decomposition in an inert atmosphere at elevated temperatures (typically 400–600 °C), where complex polymeric structures break down into volatile vapors that subsequently condense into liquid oil and non-condensable gases, leaving behind a solid char residue [338,343,344].

Catalysts in waste tire pyrolysis play a crucial role by lowering activation energy, potentially enabling lower process temperatures and shorter residence times, and influencing product selectivity towards more valuable shorter-chain molecules and facilitating heteroatom removal, thus improving pyrolytic oil yield and quality [345]. Common catalyst classes investigated include zeolites and metal oxides, with their general advantages and limitations summarized in

Table 7. Advantages and limitations of some types of catalysts in waste tire pyrolysis.

Catalyst Type	Advantages	Potential Deactivation	Refs.
Zeolites (ZSM-5, HY)	High aromatics/BTX selectivity, shape selectivity	Deactivation risk, limited kinetics	[346,347,348]
Metal Oxides	Lower activation energy, higher oil yield	Lower aromatics than zeolites	[345,348,349]
Noble Metals	Superior BTX/p-cymene selectivity	Cost, potential deactivation	[350]
Biochar/Tire Char	Cost-effective, boosts certain hydrocarbons, syngas	Less effect on aromatics (biochar)	[351,352,353,354]
Industrial Wastes	Reduce energy input, lower cost	Variable catalytic activity	[349]

. A significant challenge remains catalyst deactivation due to the high sulfur content and inorganic additives in tires, which can impede sustained efficiency.

The initial fixed carbon content of tires significantly influences the char yield during pyrolysis, with higher fixed carbon generally leading to more solid residue, while the liquid oil yield typically ranges from 25 wt.% to over 50 wt.%, being highly dependent on process parameters such as temperature, heating rate, and residence time [341].

Tire pyrolysis oil (TPO), sometimes referred to as bunker oil, is typically a dark-brown, viscous liquid with a pungent odor; its physical properties can necessitate preheating for handling and atomization [355]. TPO consists of a complex mixture of hydrocarbons (spanning C5–C50), including significant aromatic compounds like benzene, toluene, xylene, and limonene. It possesses a high heating value (approximately 40–44 MJ/kg) [355,356], comparable to some fossil fuels. Its approximate average elemental composition is 83–87 wt.% carbon, 7–10 wt.% hydrogen, 0.5–2.0 wt.% sulfur, and 0.2–0.8 wt.% nitrogen, with a low oxygen content [355,357]. Key disadvantages hindering direct TPO use in internal combustion engines include its high sulfur content (0.5–2.0 wt.%), leading to SOx emissions and engine wear, alongside other issues like viscosity and impurities that necessitate post-treatment. Key upgrading steps are therefore essential, prominently including desulfurization to reduce its high sulfur content, for which various chemical and catalytic methods are being explored, alongside other refining processes like distillation and filtration [358,359].

Beyond its fuel potential, tire pyrolysis oil holds significance as a secondary feedstock within the circular economy, offering a source for recovering valuable chemicals such as benzene, toluene, xylene, and limonene, which can serve as building blocks for the petrochemical industry and the production of high-value carbon products [360]. The typical yield for the gaseous product portion obtained from tire pyrolysis generally ranges from 5 wt.% to 15 wt.% of the initial feedstock, though this can vary based on process temperature and other operational parameters [341]. The gaseous fraction from tire pyrolysis possesses a significant lower heating value, around 37 MJ/kg, and is primarily composed of hydrocarbons (C1–C4 alkanes and alkenes like CH₄, C₂H₆, C₃H₈, and C₂H₄), H₂, CO, and CO₂, with trace amounts of sulfur compounds such as hydrogen sulfide (H₂S) [361]. A significant challenge with the direct combustion of tire pyrolysis gas is the presence of sulfur compounds, primarily H₂S, which can lead to the emission of SOx upon combustion, contributing to air pollution and acid rain, and also causing equipment corrosion, thereby incurring additional environmental and economic costs for gas cleaning [362,363].

5.3. Tire Gasification

Gasification, in its conventional forms (such as fixed-bed, fluidized-bed, or plasma-assisted) and potentially hydrothermal variations, is commonly employed to generate hydrogen-rich syngas from waste tires, with co-products typically including a solid char or slag and potentially some liquid tars or oils [364,365]. While specific syngas and char yield data for waste tire gasification at a precise temperature of 1273 K are not readily available in the reviewed literature, gasification studies generally indicate that increasing temperature tends to increase syngas yield and decrease char yield, though optimal conditions and precise yields vary with reactor type and other parameters [364]. Syngas produced from tire gasification typically comprises H₂, CO, CO₂, and CH₄, with their relative concentrations being primarily influenced by the gasification temperature and the type of gasifying agent used, such as air, steam, or oxygen [366,367]. During the primary reaction stage of tire gasification, the shredded tire material undergoes rapid heating and devolatilization, leading to the thermal decomposition of its organic constituents into volatile gases, liquid tars, and a solid carbonaceous char [364]. Following the initial decomposition, secondary reactions such as thermal cracking of tars, steam and CO₂ reforming of hydrocarbons, char gasification, and water–gas shift reactions occur at higher temperatures, generally increasing the overall syngas yield and modifying its composition, particularly enhancing its H₂ and CO content [368].

Catalysts such as natural minerals like dolomite and calcite, as well as commercial options including nickel-based catalysts and alkali metal compounds like CaO or MgO-based materials, are often used in tire gasification to promote tar cracking, enhance char conversion, and modify the syngas composition, primarily to increase hydrogen yield and improve gas quality [366,369]. The two principal technological approaches to waste tire gasification are conventional thermal gasification, which includes various reactor designs like fixed beds, fluidized beds, and entrained flow systems, and plasma gasification, which utilizes extremely high temperatures generated by plasma torches [364,370]. Conventional gasification technologies typically generate heat for organic fraction decomposition through partial combustion or indirect heating at temperatures generally below 1200 °C, producing a solid char by-product, whereas plasma gasification utilizes electrically generated plasma torches to achieve much higher temperatures (often exceeding 2000 °C and up to 14,000 °C in the plasma jet itself), resulting in a vitrified, inert slag as the solid residue [364,371,372]. In conventional tire gasification, using steam as a gasifying agent typically results in higher hydrogen content and a greater lower heating value (LHV) of the produced syngas compared to air gasification due to minimized nitrogen dilution, while the application of plasma technology, especially when combined with steam and catalysts like CaO, can further enhance hydrogen concentrations significantly [364,373].

5.4. Hydrothermal Liquefaction of Tires

Both hydrothermal liquefaction (HTL) and pyrolysis are thermochemical solid-to-liquid conversion pathways, but HTL operates at moderate temperatures (typically 200–400 °C) and high pressures (5–25 MPa) in the presence of water, thus accommodating wet feedstocks without prior drying, whereas pyrolysis generally employs higher temperatures (300–800 °C) at near-atmospheric pressure and typically requires dried feedstock [374,375]. The primary products generated from the hydrothermal liquefaction of waste materials are a liquid bio-crude oil, an aqueous phase containing water-soluble organics, a solid hydrochar, and a gaseous fraction, with temperature typically being the most influential process parameter determining their relative yields and quality, alongside residence time and catalyst presence [375,376]. Hydrothermal liquefaction is considered a promising technology for waste tires because it can process them effectively, yielding a bio-crude oil that typically exhibits a higher heating value and potentially higher alkane concentrations and typically lower oxygen content compared to oils from tire pyrolysis [374]. Co-processing waste tires with various biomass feedstocks is a commonly explored approach in experimental hydrothermal liquefaction. Specific quantitative data for the co-HTL of tires with willow leaves is not detailed in the provided literature. HTL oils from biomass generally feature higher heating values (around 30–36 MJ/kg) and lower oxygen content compared to pyrolysis oils, though detailed alkane concentrations vary with feedstock and conditions [377,378]. In the co-liquefaction of lignite and waste tire raw material, for instance, in supercritical water at 400 °C with an 80% tire content, total conversions up to 67% and oil yields of 50% have been achieved. The use of catalysts like Fe₂O₃ in other solvent systems has also shown high conversion rates. This suggests potential synergistic interactions where components from tire degradation may act as hydrogen donors or exhibit catalytic effects aiding lignite conversion [378,379].

5.5. Valorization of Char

The char produced from the pyrolysis, gasification, and hydrothermal liquefaction of waste tires has potential applications as a solid fuel, a precursor for activated carbon production due to its inherent porosity, a reinforcing filler in composite materials, a soil amendment, or even as a low-cost catalyst or catalyst support in other chemical processes [380,381].

6. Discussion

The imperative to transition from an energy system predominantly reliant on fossil fuels necessitates a rigorous comparative evaluation of alternative ecofuel pathways, encompassing biofuels, e-Fuels, and waste-derived fuels [5]. While each pathway offers potential routes to decarbonization, their viability, scalability, and ultimate contribution to a truly sustainable energy future are shaped by a complex interplay of resource demands, technological readiness, economic realities, policy landscapes, and comprehensive environmental impacts. This discussion critically compares these ecofuel options across key thematic areas, guided by the crucial distinction that while many ecofuels are renewable (i.e., derived from resources that naturally replenish on a human timescale), their sustainability (i.e., meeting present needs without compromising future generations’ ability to meet theirs, encompassing environmental, social, and economic viability) is not inherent and requires careful lifecycle management.

A central part of this evaluation involves an integrated comparison that highlights the distinct trade-offs and strategic complementarities of each ecofuel pathway. The land and food system pressures associated with some biofuels, for example, represent a stark trade-off against their advantage of leveraging existing fuel infrastructure. E-Fuels, in contrast, avoid this land use conflict but introduce a different dependency on the massive build-out of renewable electricity and sustainable CO₂ capture, which carries its own significant cost and resource implications. Waste-derived fuels present a third paradigm, offering a circular economy solution to pollution but facing technological hurdles related to feedstock variability and product purification. Recognizing these trade-offs reveals their strategic complementarities, particularly for decarbonizing hard-to-abate sectors like aviation and maritime transport, where direct electrification is not viable. In these areas, a portfolio approach is necessary, where advanced biofuels might serve near-term needs while synthetic e-fuels, despite their current high costs, represent a more scalable long-term solution for achieving deep decarbonization.

6.1. Resource and Feedstock Constraints: The Sustainability Test

A fundamental differentiator among ecofuels is their resource base, which dictates scalability and potential conflicts with other essential needs. Biofuels exhibit a wide spectrum: 1-G biofuels, derived from food crops [100], face significant sustainability challenges due to direct competition with food supply, leading to concerns about food prices and security, and extensive land and water requirements [104,105,382]. The risk of ILUC, where feedstock cultivation displaces other land uses leading to deforestation or peatland drainage, can negate their GHG benefits [111,248]. Second-generation biofuels aim to mitigate these issues by utilizing non-food lignocellulosic biomass such as agricultural/forestry residues and various organic wastes [128] or by cultivating dedicated energy crops on marginal lands [383] not suitable for conventional agriculture. While theoretically abundant [384,385,386], large-scale exploitation faces challenges in efficient and cost-effective collection, transportation, and pre-treatment of these often bulky and heterogeneous feedstocks, alongside growing competition with other bio-based industries [249,268]. Third-generation biofuels from microalgae offer very high theoretical oil yields per hectare and can be cultivated on non-arable land using non-potable or saline water, thus minimizing direct competition with agriculture [157,162]. However, they require significant nutrient inputs (nitrogen, phosphorus), potentially leading to a reliance on fertilizers with their own GHG footprint and risks of water body eutrophication if not managed carefully, and they face substantial harvesting challenges [156,161].

E-Fuels shift the primary resource burden from biomass to vast quantities of renewable electricity and sustainably captured CO₂ [276,277]. The paramount constraint is the massive scale-up required for renewable power generation (solar, wind), especially as fossil fuels still account for approximately 60% of global electricity production [31,387], and the renewable energy infrastructure itself has an embodied energy and material footprint [335,336,387]. CO₂ sourcing is another critical factor; while biogenic (e.g., from biogas or bioethanol plants) and industrial point sources (e.g., cement plants) [283] are available, achieving the necessary scale for widespread e-fuel production may necessitate reliance on more energy-intensive and currently expensive DAC technologies [338,360,362]. Water for electrolysis to produce green hydrogen is also a significant input [300,301].

Waste-derived fuels offer a unique proposition by utilizing problematic non-biodegradable waste streams, particularly post-consumer plastics and end-of-life tires [346], thereby addressing a significant environmental pollution issue while generating energy carriers [75]. The feedstock is often abundant locally and can have a low or even negative cost (due to tipping fees) [388]. However, the inherent heterogeneity, potential contamination (e.g., PVC in plastics leading to HCl emissions [366], sulfur in tires leading to SOx precursors [375]), and inconsistent supply can pose significant technological challenges for conversion processes and product quality control.

Ultimately, no pathway is without resource constraints. Biofuels grapple with land use sustainability and competition; e-Fuels with the monumental scale-up of genuinely low-carbon electricity and sustainable CO₂; and waste-derived fuels with the logistics and technical challenges of variable feedstocks. Intense competition for these fundamental resources (sustainable biomass, truly renewable electricity, captured CO₂) is anticipated, making holistic resource management and hierarchical use critical for a sustainable transition [268]. The “sustainability” of feedstock sourcing is paramount, far exceeding mere “renewability”.

6.2. Techno-Economic Viability, Scalability, and Policy Levers

The economic landscape of ecofuels is complex and heavily influenced by policy, as they generally struggle to compete with the established cost-effectiveness of fossil fuels. First-generation technologies like ethanol fermentation and biodiesel transesterification are mature [116,118]. However, their production costs often exceed those of fossil fuels, necessitating ongoing policy support through mandates and subsidies [102,389]. Second- generation biofuels face even higher capital costs [390] due to complex pre-treatment and conversion processes, though cost reductions are anticipated with technological maturation and integrated biorefinery concepts valorizing co-products [252,391]. Third-generation (algal [392]) and later-generation biofuels remain largely in R&D, with current production costs significantly higher than other options [179,180,187].

E-Fuels currently represent the most expensive ecofuel category, with production costs estimated to be 2.5 to 4 times higher than conventional jet fuel [391,393]. This is primarily due to the high cost of green hydrogen (projected at ~USD 7/kg in 2025, with targets of USD 1–2/kg by 2050 contingent on major breakthroughs) and CO₂ capture, especially via DAC [340,341,394]. Significant cost reductions depend on steeply falling renewable electricity prices, electrolyzer CAPEX reductions, and economies of scale [395].

Waste-derived fuels can benefit from very low or even negative feedstock costs, as municipalities or industries may pay tipping fees for waste disposal. However, the capital costs of pyrolysis, gasification, or HTL plants can be substantial, and the resulting oils or syngas often require significant and costly upgrading (e.g., desulfurization, hydrotreatment [396,397]) to meet fuel specifications or chemical feedstock purity requirements [75,354].

Policy levers are therefore critical drivers for all ecofuels [398,399]. Mandates, such as the EU’s Renewable Energy Directive (RED II) [14] targets (e.g., 14% renewable energy in transport by 2030, with a 3.5% sub-target for advanced biofuels), and financial incentives, like those in the US Inflation Reduction Act [6] and the EU Green Deal [5], are essential for market creation and deployment. However, a significant barrier is policy instability and a lack of clear, long-term vision beyond current target dates (e.g., post-2030 in the EU), which deters large-scale investment, particularly in advanced technologies with long development times. Carbon pricing mechanisms, like the EU Emissions Trading System (ETS) [264] and the upcoming ETS2 [400] for transport and buildings (from 2027), aim to internalize the external costs of fossil fuels and make ecofuels more competitive, but current carbon prices may not yet reflect the full marginal abatement cost for many ecofuel pathways.

6.3. Infrastructural Challenges and Systemic Integration

The ease of integration into existing energy systems and the scale of the new infrastructure required vary significantly among ecofuel types. Liquid biofuels like bioethanol (in blends like E10), biodiesel, and HVO, as well as liquid e-Fuels (e-gasoline, e-diesel, e-kerosene), are often designed as “drop-in” or near drop-in fuels [57,64]. This allows them to leverage the extensive existing infrastructure for the storage, transportation, and distribution of conventional liquid fossil fuels, which is a major advantage for near-to-medium-term deployment. However, higher ethanol blends may require modifications to vehicle engines and fueling infrastructure. Hydrogen and ammonia (as e-Fuels or carriers) face substantial infrastructural challenges. Hydrogen, due to its low volumetric energy density, requires specialized high-pressure or cryogenic storage and transport, and a completely new refueling network for widespread use in transport [37,55]. Ammonia, while easier to store and transport than hydrogen, also requires dedicated infrastructure and handling protocols, and its use in engines necessitates new designs and emission control systems [298]. Beyond the fuel itself, the production of e-Fuels necessitates a monumental build-out of new infrastructure for renewable electricity generation [25,382] (far exceeding current capacities), potentially large-scale CO₂ capture facilities and pipeline networks, and advanced electrolyzer plants [19,66]. Waste-derived fuel systems require localized or centralized waste collection and sorting infrastructure, as well as dedicated conversion plants (pyrolysis, gasification, HTL) [355,367]. While the fuel products might integrate into existing refinery streams after upgrading, the initial processing infrastructure is specific.

Policy support for infrastructure development is thus as crucial as for fuel production itself. The transition will likely involve a mix: leveraging existing assets where possible for drop-in fuels while strategically developing new networks for promising but more disruptive energy carriers like hydrogen, especially for sectors where they offer unique advantages. The overall energy demand increase from widespread transport electrification or e-fuel production also implies significant grid reinforcement and smart management strategies.

6.4. Sustainability Beyond Renewability: LCA, Policy Coherence, and the Role of Ecofuels

The ultimate goal is not just renewable energy, but sustainable energy, demanding a holistic assessment of all impacts. The distinction that “renewable” sources are not inherently “sustainable” is vital [177,390,401]; for example, corn ethanol [402] is renewable but its large-scale production faces sustainability questions regarding land use, food competition, and water consumption. Similarly, while solar and wind energy are inexhaustible, the sustainability of their exploitation depends on factors like critical mineral sourcing for infrastructure, manufacturing energy, lifespan, and recyclability [386].

A critical tool for evaluating true sustainability is comprehensive LCA. The current EU TTW emissions accounting approach tends to favor electric vehicles and hydrogen fuel cell vehicles by not fully accounting for upstream emissions from electricity generation or the manufacturing of vehicles and batteries [390]. This can undervalue the potential GHG benefits of certain ecofuels produced with genuinely low-carbon pathways [247,387,401]. A comprehensive WtW or full LCA methodology is essential to more accurately reflect and compare the GHG emission advantages and overall environmental footprint across all ecofuel classes and against incumbent fossil fuels [38,42,403]. When evaluating climate impact, a comprehensive WtW perspective is crucial, highlighting significant differences between fuel types;

Table 8. Qualitative comparison of estimated lifecycle GHG emissions (WtW).

Energy Pathway	Estimated Lifecycle GHG Emissions (WtW)	Key Factors and Notes
Fossil Fuels	Baseline (High)	- The primary source of anthropogenic CO2 emissions.
1-G Biofuels (e.g., Corn)	Modest Reduction (Potentially High w/LUC)	- Corn ethanol offers only modest reductions due to fossil fuel inputs. - Significant risk of high emissions from ILUC. - Overall GHG reduction potential is debated.
1-G Biofuels (Low-LUC, e.g., Sugarcane)	Some Reduction	- GHG benefits depend heavily on avoiding LUC/ILUC.
2-G Biofuels (e.g., Cellulosic)	Potential for Higher Reduction	- Utilizes non-food feedstocks, reducing direct food competition. - Potential for higher GHG emission reductions compared to 1-G, especially if ILUC is avoided. - Residue removal must be sustainable to avoid soil degradation.
3-G Biofuels (Algae)	Variable	- Energy-intensive cultivation/harvesting can negate GHG benefits if fossil energy is used. - Nutrient inputs (fertilizers) are associated with GHG emissions.
Waste-Derived Fuels (Plastics/Tires)	Variable	- Potential for lower emissions compared to incineration. - Depends heavily on process efficiency and energy inputs.
E-Fuels (100% Renewable Source)	Very Low/Near Zero	- Potential for 70–100% GHG reduction compared to fossil fuels. - Relies on 100% renewable electricity and sustainable CO2. - Upstream emissions from renewable infrastructure (PV: 20–80; wind: 10–30 gCO2-eq/kWh) must be considered.
E-Fuels (Grid Mix/Fossil Source)	Variable (Potentially High)	- Overall emissions depend heavily on the carbon intensity of electricity used. - If fossil electricity is used, emissions can be high. - A total of ~60% of global electricity is currently from fossil fuels.
Electric Vehicle (100% Renewable Grid)	Very Low (Operational)	- Higher emissions during battery production. - Relies on a truly low-carbon grid. - Several EU countries exceed 50% renewable electricity.
Electric Vehicle (Grid Mix)	Variable (Potentially High)	- Can be worse than ICE vehicles if the grid is high-carbon (e.g., coal-based). - Upstream emissions from electricity generation must be included.

will be enhanced in the manuscript to present illustrative lifecycle GHG emission ranges (e.g., in gCO₂-eq/MJ) for key pathways to provide a better sense of scale for these comparisons. Such WtW analyses reveal, for example, that (i) fossil fuels establish a high baseline of GHG emissions; (ii) 1-G biofuels like corn ethanol offer only modest GHG reductions due to fossil fuel inputs in their production and significant risks of high emissions from ILUC; (iii) 2-G biofuels from residues or dedicated energy crops on marginal land hold potential for higher GHG emission reductions, provided ILUC is avoided and biomass harvesting is sustainable; (iv) e-Fuels can achieve very low or near-zero lifecycle GHG emissions (potentially 70–100% reduction vs. fossil fuels) only if they are produced using 100% renewable electricity and sustainably sourced CO₂ and when accounting for the lifecycle emissions of the renewable infrastructure itself (e.g., PV: 20–80 gCO₂-eq/kWh; wind: 10–30 gCO₂-eq/kWh). If produced with current grid electricity mixes (globally ~60% fossil fuels), their GHG benefits are severely undermined or negated; (v) waste-derived fuels can offer GHG benefits compared to traditional waste disposal methods like landfilling or incineration, especially if conversion is efficient, but a full LCA is needed for each specific pathway.

Policy coherence is essential to support these truly sustainable options. Instruments like the EU ETS and the new ETS2 (covering road transport and building fuels from 2027) are intended to provide market incentives for low-emission solutions by reflecting carbon costs. Linking WtW LCA results more directly to these policy instruments and ensuring robust carbon accounting are crucial for driving investment towards the most genuinely climate-beneficial ecofuels. Technological neutrality in policy is often advocated to allow a diverse range of solutions to emerge based on their sustainability merits and application suitability. Ultimately, a diversified strategy focusing on the most sustainable ecofuels for specific hard-to-abate sectors, alongside continued advancements in direct electrification where feasible, will be necessary for a successful and comprehensive energy transition.

7. Future Perspectives

7.1. Strategic Deployment for Hard-to-Abate Sectors

Achieving significant fossil fuel substitution necessitates a multifaceted strategy that integrates the development and deployment of biofuels, e-Fuels, and waste-derived fuels, prioritizing sectors where direct electrification presents substantial challenges, such as aviation and maritime transport. Emerging technologies such as ammonia-fueled engines offer the potential for substantial decarbonization in sectors like maritime shipping by utilizing a carbon-free fuel, aligning with stringent emission reduction targets like the International Maritime Organization’s goal for net-zero GHG emissions from international shipping close to 2050, although effective mitigation of NOx and unburnt ammonia slip is crucial [404]. Pilot diesel-type ammonia combustion marine engines have demonstrated successful operation with significantly reduced CO₂ emissions compared to conventional diesel engines. Ongoing developments are focused on optimizing combustion to improve thermal efficiency while minimizing unburnt ammonia slip and N₂O formation through advanced control strategies and aftertreatment systems.

7.2. The Future of Sustainable Feedstocks

The development of second-generation biofuels from non-food lignocellulosic waste, agricultural residues, forestry wastes, and energy crops cultivated on marginal lands plays a crucial role in a sustainable ecofuel strategy by minimizing direct competition with food production and reducing indirect land use change impacts. Vast quantities of currently underutilized lignocellulosic materials from agricultural residues like corn stover and wheat straw, forestry residues, and the organic fraction of municipal solid waste hold significant promise for scaling up second-generation biofuel production due to their widespread availability and potential to avoid food-fuel competition. Furthermore, cultivated microalgae (often considered a third-generation feedstock) hold significant promise due to their high oil content, rapid growth rates, and ability to be cultivated on non-arable land using non-potable water, thereby avoiding direct competition with agriculture. However, a critical bottleneck for the economic viability of microalgae-based biofuel production remains the cost-effective and energy-efficient harvesting of algal biomass from dilute cultures, which can represent almost a quarter of total production costs. Significant future advancements in scalable, low-energy harvesting technologies are therefore essential to unlock the full potential of microalgae as a sustainable biofuel feedstock, and this remains an intensive area of research and development. Ongoing research explores various innovative approaches, including advanced flocculation, flotation, and filtration techniques, to address this challenge.

7.3. Critical Pathways for E-Fuel Scalability

The sustainability of e-Fuel production, a cornerstone of this strategy, is significantly enhanced as the share of renewables in the electrical grid increases, provided that the entire production lifecycle, including hydrogen generation and CO₂ capture, is powered by these low-carbon electricity sources to ensure a genuinely closed carbon loop. Currently, a primary limiting factor in the sustainability of e-Fuel production is the reliance on an electricity grid still substantially powered by fossil fuels, which account for approximately 60% of global electricity production, alongside the upstream emissions associated with the manufacturing and deployment of renewable energy infrastructure itself; this necessitates a more comprehensive WtW LCA and an accelerated transition to fully renewable power generation. In this context, CO₂ utilization for e-Fuels creates a pathway for a circular carbon economy by reusing captured CO₂, whereas large-scale CO₂ storage aims for permanent sequestration of emissions. Both are considered complementary strategies within broader emission reduction goals. e-Fuel production potentially offers an alternative to direct geological storage for certain CO₂ streams. Beyond contributing to a closed carbon loop for climate neutrality, CO₂ utilization for e-Fuels can offer economic benefits by valorizing a waste product into high-value energy carriers, enhancing energy security by diversifying fuel sources, and providing a means to store surplus renewable energy. While the cost of CO₂ capture technologies is gradually declining and their effectiveness for point sources can be high (e.g., 90–95%), the current commercial availability of low-cost, circular CO₂ sources remains a constraint for e-Fuel production, often necessitating reliance on more expensive options like DAC [41]. DAC technology is operational in several pilot and demonstration plants, with larger facilities planned. But it currently faces substantial challenges regarding high energy consumption, significant capital costs, and the need for breakthroughs in sorbent materials and process efficiency to become economically viable for large-scale e-Fuel feedstock supply.

7.4. Long-Term Vision: Emerging Carbon-Free Fuel Technologies

While its application in the maritime sector is advancing, the exploration of ammonia engine technology in passenger cars remains at a very early, largely conceptual stage, primarily driven by unvalidated industrial research, and faces immense hurdles. Proponents highlight theoretical benefits such as zero CO₂ tailpipe emissions and reduced reliance on critical battery materials, but these are entirely dependent on surmounting substantial and currently unresolved technical (including effective mitigation of NOx and unburnt ammonia slip), safety, and infrastructural challenges. If, in the distant and uncertain future, these significant advances in ammonia engine technology for passenger cars were to be robustly validated, all critical safety and emission concerns effectively mitigated, and the necessary infrastructure established, it would then likely necessitate a significant re-evaluation of the transportation sector’s GHG emission framework and associated regulations, potentially creating pathways for ICEs using carbon-free fuels to contribute to decarbonization targets alongside vehicle electrification.

8. Conclusions

The global imperative to mitigate climate change, driven by the stark recognition that fossil fuels contribute over 75% of anthropogenic CO₂ emissions annually, coupled with the strategic need to enhance energy security amidst geopolitical instability and the economic impetus from legislative instruments like the EU Green Deal and the US Inflation Reduction Act mobilizing over USD 1 trillion, collectively propels the critical transition towards ecofuels in the transportation sector. This sector remains a significant challenge, accounting for approximately 25% of total energy-related CO₂ emissions globally and relying on liquid hydrocarbons for over 90% of its energy needs. However, the primary and most formidable obstacle to the wholesale replacement of fossil fuels, whose dominance began with large-scale oil extraction in the mid-19th century, remains their currently superior energy density and the deeply entrenched, cost-effective global infrastructure built around them.

Achieving widespread adoption of biofuels, e-Fuels, and waste-derived fuels is presently constrained by significant economic and systemic hurdles. These include the high production costs associated with advanced biofuel conversion and the particularly energy-intensive synthesis of e-Fuels, which can be 2.5 to 4 times more expensive than fossil jet fuels. Green hydrogen, a cornerstone for e-Fuels, faces projected costs of around USD 7/kg in 2025, though these are hoped to fall to USD 1–2/kg by 2050. The current reliance on an electricity grid where fossil fuels still generate approximately 60% of global output significantly impacts the lifecycle emissions of e-Fuels. Crucially, the lack of stable, long-term policy frameworks beyond targets like the EU’s 2030 goals (14% renewable energy share in transport and a minimum of 3.5% for advanced biofuels) injects significant investment uncertainty. Progress hinges on establishing robust support mechanisms, fostering substantial investment in R&D, and universally implementing comprehensive “WtW” or LCA methodologies. This is essential to accurately validate climate benefits, especially when draft standards for new fuels demand 70–100% GHG emission reductions compared to fossil fuels, and to ensure fair competition among sustainable options beyond current EU ETS carbon prices, which exceeded EUR 100/tCO₂ in February 2023 but may not cover full abatement costs. The upcoming ETS2 in 2027 aims to address some of these gaps for road transport and buildings. To navigate this complex transition and effectively substitute fossil fuels, critical future directions towards achieving ambitious net-zero emissions targets, such as those set for 2050, must include the following:

• Implementing a multifaceted strategy that champions technological neutrality while directing advanced biofuels and e-Fuels towards hard-to-abate sectors like aviation and maritime transport. These sectors are critical as direct electrification faces substantial energy density limitations.
• Moving beyond simple renewability to ensure holistic, long-term environmental, social, and economic viability across the entire value chain. This involves accelerating the research, development, and deployment of second-generation biofuels from lignocellulosic wastes and third-generation options like algae (with potential oil yields up to 136,900 L/ha/yr) alongside waste-to-fuel technologies to minimize resource conflicts and the ILUC impacts associated with some 1-G biofuels. Ensuring traceability for feedstocks like UCO, where EU consumption has significantly outpaced domestic collection (estimated at 1.5–1.7 Mt/yr versus ~2.85 Mt UCOME in 2019), is also key.
• Significantly boosting investment in R&D to reduce costs and improve efficiencies in e-fuel production pathways. This includes green hydrogen, where announced large-scale projects reached a potential investment value of USD 680 billion by 2024 against confirmed FIDs of USD 75 billion, and aiming for outputs like 50 Mt from electrolysis and 15 Mt from CCUS-equipped facilities by 2030 to meet net-zero targets. Sustainable CO₂ sourcing, moving beyond current reliance on DAC, which is still maturing despite pilot plants, is paramount. Exploring and supporting emerging carbon-free fuel options, such as ammonia, which shows promise for sectors like maritime shipping with new engines becoming available, will also be vital.
• Focusing concerted global efforts on securing sustainable and diverse feedstock availability, which has been identified as the most significant limiting factor across all ecofuel pathways. Simultaneously, strategic planning and investment are required for the massive scale-up of supporting infrastructure. This includes vastly expanded renewable electricity generation, considering the global installed offshore wind capacity was approximately 81 GW by the end of 2024, dwarfed by projected needs, grid reinforcement, and networks for CO₂ capture (with 90–95% effectiveness at point sources), transport, and utilization or storage.
• Developing and maintaining clear, stable, and internationally harmonized policy environments that provide long-term investment security beyond current targets like the EU’s 2035 phase-out of new fossil-fueled car sales. These policies must support innovation and create a level playing field for all genuinely sustainable ecofuel solutions.

Only through a dedicated and holistic implementation of these interconnected future directions can the ambitious but necessary goal of a truly sustainable energy transition be achieved, fundamentally reshaping our reliance on fossil fuels and ensuring long-term environmental protection and energy security for generations to come.