Developments and Issues in Renewable Ecofuels and Feedstocks

Abstract

Ecofuels and their feedstock come in three main product classes: electrofuels (e-Fuels), biofuels, and non-biowaste-derived fuels. Ecofuels originate from non-fossil sources, derived from circular raw materials such as non-food organic waste, renewable hydrogen, and captured CO₂ through a rapid process of carbon fixation. Proposed regulation drafts under discussion indicate that new fuels would need to reach a substantial degree of climate neutrality. The manufacture of all ecofuels, however, requires energy input to accomplish the conversion of the initial feedstock; their climate neutrality claims stem from the use of renewable electric energy and/or biomasses in the production process, but fossil fuels are still the main primary sources of global (and the EU’s) electric power, and most biofuels consumed in the EU transport mix are still crop-based, with potential conflicts with food and land use. Furthermore, entirely neglecting GHG emissions from renewable energy generation is scientifically debatable, as the impact of the energy (and the related GHG emissions) embedded in the materials used to build renewable energy facilities is small, but not nil. The paper reports ecofuel trends according to the above-mentioned originating technologies and discusses the issues related to their development.

1. Introduction

The early 20th century marked the beginning of the Oil Age, when hydrocarbons overtook coal and wood as the main energy source, with abundant, practical, and cheap fuel supply. After over a century of growing consumption, it is now obvious that fossil oil consumption is not sustainable, due to gradual depletion (estimates of the known reserves-to-consumption ratio indicate that fossil crude reserves could be depleted within the next 50 years) [1] and climate concerns linked to greenhouse gas (GHG) emissions. Initially, interest in alternative fuels was largely driven by the attempt to reduce traffic pollutant emissions, especially in urban areas, by means of clean fuel additives. More recently, priorities have specifically shifted to GHG reduction, with the Intergovernmental Panel on Climate Change (IPCC) calling for a 45% global CO₂ emission reduction by 2030, from 2010’s previous levels. Energy supply and security are also issues of the utmost concern, as the increasing cost of fossil fuels is an important factor behind global financial instability, and oil resource exploitation plays a key role in the development of international relations and the ensuing conflicts in the last century.

These reasons spurred the drive for fossil substitution, and binding regulations aimed at GHG reduction with less environmentally harmful renewable sources such as solar, wind, and ecofuels from physical feedstock were introduced [2]. As a consequence, the global share of primary energy consumption from renewables (excluding hydropower) reached 7.5% in 2022 (1% more than in 2021) with total energy consumption rising by 3% compared to 2019 pre-COVID-19 levels. However, fossil fuels’ share remained steady at 82% (Figure 1) [3]. Notwithstanding progress made, it is estimated that a 7% gap to the 2030 GHG target will still need to be filled in the EU-27 [4].

The European Union’s (EU) oil dependence amounts to more than 300 Mtoe/year. Reduction efforts involving all industrial sectors have intensified the study and development of renewable and sustainable alternative energy sources: much attention is focused on the transport sector, which relies on >95% liquid hydrocarbons and consumes >50% of their global production. GHGs from this sector account for >21% of overall global emissions, with a steeper increasing trend than the others [5]. In this direction, Directive (EU) 2018/2001 on the promotion of the use of energy from renewable sources [6] sets 2030 targets for renewables’ share in transportation at 14%, and an overall target of 3.5% in the form of advanced biofuels.

Alternatives to fossil fuels that could provide low CO₂ emissions throughout their production processes include hydrocarbons generated from renewable feedstock, electricity, and hydrogen from renewable sources. Recent initiatives such as the EU Green Deal [7] aim at promoting a rapid and generalized shift to electric-based mobility but would require a huge increase in the production of Lithium Ion Batteries (LIBs), which, so far, are the most efficient and common electric storage device. The total available LIB capacity is increasing exponentially: the storage capacity in 2009 was estimated to be 25.6 GWh, about 218 GWh in 2019, with future volume expectations exceeding 2500 GWh in 2030, corresponding to over 12.7 × 10⁹ tons of produced LIBs. This will also create a still unaddressed environmental issue as a vast amount of spent batteries will eventually have to be disposed of [8].

LIBs require scarce and expensive raw materials for production, such as Co, Ni, and Li; although spent LIBs are considered a highly valuable waste stream, actual recovery rates so far are quite low: in the EU, one of the most regulated markets, only approximately 12% LIBs are recovered after use [9]. Furthermore, aside from the economics involved, this approach is not without risks, as intensive mineral material mining for their production, and possible pollutants released when batteries are inappropriately disposed of, may cause decreasing environmental quality and human health problems [10].

Current battery technology is considered too heavy for aviation mobility and with too low an energy density for long-distance marine transport. Furthermore, as pointed out in relevant studies, electrification per se does not imply overall emission reduction: the emission approach based on “tank-to-wheels” analysis can lead to misleading conclusions [11,12]. Although electricity is addressed as the ultimate source of clean energy, fossil fuels are still the main primary source of the global electric power mix (Figure 2). The general consensus is that the transport and power sectors will increasingly integrate, and electricity production’s Carbon Intensity (CI) will become the major factor in assessing fuel cycle emissions under a “well-to-wheels” approach [13].

Hydrogen, used as feedstock in the chemical, refinery, and steel industries, can be both a fuel proper and a fuel “ingredient” for e-Fuels. H₂ is touted as the “ideal” fuel concerning pollution and GHG reduction, but faces similar problems to electric energy carriers, its use being challenging in aviation and long-distance shipping, as it requires the development of an entirely new distribution infrastructure. Its full value could be better exploited by conversion to derivatives that increase further its energy density, making long-distance transport and long-term storage cost-effective: by combining it with CO₂, hydrocarbons in virtually any molecular form or in combination with atmospheric N₂, ammonia as an efficient energy carrier can be produced.

As an energy carrier, H₂ needs primary sources for its industrial production, currently estimated at around 75 Mt/y as a pure gas and an additional 45 Mt/y as part of gas mixes, equivalent to 3% of the global energy demand. As of the end of 2021, about 47% of the global H₂ production was from steam reforming of CH₄ (blue hydrogen), 27% from coal, 22% from oil (as a by-product), and only around 4% from water electrolysis, of which only about 1% was from renewables [15]. Only the latter fraction (green hydrogen) can be considered fully sustainable, as it does not involve fossil energy contributions. Hydrogen can then be injected into natural gas (NG) grids. In doing so, hydrogen can displace natural gas, reducing GHG emissions and reliance on high-carbon fuels. In order to reach net-zero by 2050, green H₂ production would need to grow globally at the rate of 80–95% per year from current levels of ∼120 Mt/y to more than 300 Mt/y [16]. Based on publicly available information, several large RES-based H₂ projects existed or had been announced at the end of 2022. If fully operational, these could provide more than 100 Mt/y of green hydrogen, roughly a third of the needed 2050 amount estimated by the IEA to reach net-zero emissions. In 2023, the biggest green hydrogen producers worldwide were the USA (47 kt/y), Germany (27 kt/y), Canada (14 kt/y), Chile (2.6 kt/y), India (2.1 kt/y), and Spain (1.7 kt/y). Among the biggest planned facilities are the planned Fleur-de-lys Green Hydrogen Production Hub in Quebec (Canada) fed by an offshore 500 GW wind power plant, expected to provide 43 Mt/y with a yet undefined timeline; the Western Green Energy Hub (Australia) fueled by a 50 GW of wind and a solar facility, expected to start an estimated production of 3.5 Mt/y in 2027; Hydrogen City (Texas, USA), expected to deliver 3 Mt/y by 2026 fed by 60 GW of onshore wind and solar generation; The Hyrasia One facility (in Kuryk, Kazakhstan), expected to deliver 2 Mt/y of green hydrogen from 40 GW of wind and solar generation in the country’s southwestern steppes starting in 2030; and the Green Energy facility in Duqm (Oman), which will provide 1.8 Mt/y from 25 GW of wind and solar after 2026. Several other production facilities of a ≥2 Mt/y capacity were announced but without a specification of expected operativity or even an RES type. To put this information in its proper context, it should be noted that by the end of 2022, only 63.2 GW of offshore wind facilities had been installed globally. Additionally, multiple 1 Mt/y projects are being developed in Europe (Denmark, The Netherlands). In general, these facilities’ costs have not been disclosed [17].

An alternative to electric or hydrogen-based mobility could be provided by other “ecofuels” that, contrary to electricity and H₂, are fairly easy to transfer with existing infrastructure, have good energy density, and can be used in existing engine technology with minor modifications. Ecofuels originate from non-fossil sources, derived from circular raw materials such as non-food crops or organic waste, renewable hydrogen, and captured CO₂, through a rapid process of carbon fixation (in contrast to fossil fuels that took millions of years to form), although still originated from the same fundamental processes. Carbon-based ecofuels may therefore be the only medium-term feasible alternatives in the transportation sector, with a few exceptions, such as ammonia, which is considered a suitable C-free energy carrier with interesting properties (this is discussed in Section 2.4).

Different forms of ecofuels, gaseous, liquid, or solid, can be obtained by converting carbonaceous material. Liquid fuels are most practical in the transportation sector, because of their easily handled physical state and their ideal high gravimetric and volumetric energy densities, compared to gases or solids [18]. Figure 3 depicts the energy density and specific energy of common energy carriers, highlighting the technical energy “inferiority” of current battery technology.

Ecofuels come from three main production technologies: electrofuels (e-Fuels), biofuels, and non-biomass, waste-derived fuels. e-Fuels are a class of synthetic fuels manufactured using captured CO₂ or CO, and H₂ obtained from renewable electricity sources (RESs) such as wind, solar, and nuclear (green hydrogen). While the current lack of compatible infrastructure is an important structural limit to the large-scale utilization of H₂ as a fuel, liquid e-Fuels derived from it are attractive for multiple reasons, including compatibility with existing storage and transportation infrastructure, adaptability to existing internal combustion engines, low sulfur content, and mixability with conventional fossil fuels.

“Biofuels”, on the other hand, comprise products derived from (renewable) biomasses or their residuals, which include biogas, biodiesel, bioethanol, biomethanol, biobutanol, bioethers (bio-DME or DiMethylEhter; bio-ETBE or EthylTerButylEther; bio-MTBE or MethylTerButylEther), and bio-hydrogen [19].

Third class, non-biomass waste-derived fuels are generated by thermal treatment (pyrolysis or other) of non-biodegradable plastic and other carbon-based waste materials (e.g., tires) [20,21]. The process of pyrolysis, used to process biomass to produce biofuels like syngas, py-oil, and biochar [22], can in fact be used to convert high molecular weight waste plastics into lower molecular weight hydrocarbons like gasoline, kerosene, and diesel.

In 2023, the EU adopted a policy proposal that will allow only zero-emission vehicles (including those that run “exclusively on CO₂ neutral fuels”) to be registered after 2035, de-facto banning the sale of cars with engines running on fossil fuels after that date. This proposal, still spurring intense debate among member states, commits the creation of a new vehicle category of combustion engines, which can only be filled with synthetic e-Fuels. Regulation drafts under discussion indicate that e-Fuels would need to reach a substantial degree of carbon neutrality, namely through a reduction in GHG emissions in the range of 70–100% [23].

As with fossil fuels, however, the manufacture of all synthetic fuels requires an energy input to accomplish the conversion of the initial feedstock: the fossil energy ratio (FER), defined as the ratio of the energy output of the final ecofuel product to the fossil energy required to produce it, can be used to measure this balance. A FER score of one means that no energy loss occurs in the conversion of fossil energy to another fuel. It should be noted that fossil diesel’s FER is only about 0.84 since energy is used to extract crude oil and produce diesel from it [24]. If the FER > 1, the fuel actually provides some leveraging of the fossil energy required to make it available for transportation; if a fuel is “completely” renewable, its FER approaches “infinity”, having no fossil energy requirements for its production, as in fully renewable power-to-fuel (P2F) schemes [25].

This paper aims to give a critical overview of the overall galaxy of ecofuels, electrofuels (e-Fuels), biofuels, and non-biomass waste-derived fuels, analyzing their production from feedstock to the final product, with a discussion of the issues, sometimes controversial, that affect their development.

2. Electrofuels

e-Fuels allow the conversion and storage of renewable energy from various sources into a liquid fuel through a process known as power-to-fuel, which is, in principle, carbon neutral. The transformation of renewable energy into more convenient energy carrier forms, liquid (e.g., methanol, Dimethylether-DME, etc.) or gaseous (e.g., CH₄, H₂) (respectively, power-to-liquid P2L and power-to-gas P2G schemes), offers a way to buffer RES intermittency and thereby alleviate one of the main limits to their large scale deployment. Electricity converted into synthetic, easy-to-store, and simple-to-transport fuels could replace conventional fuels and be a carbon-neutral alternative that enables the continued use of existing investments in internal combustion engine (ICE) vehicles and infrastructure. e-Fuels generated from wind and solar electric production could help achieve better exploitation of these sources’ variability by absorbing excess electricity when available since their output is highly dependent on meteorological conditions that may greatly vary over the short and long term, and are not well predictable in the long period. Additionally, the CO₂ required for fuel synthesis could be obtained from recycled/sequestered carbon, such as that captured from large fossil-burning power plants, or perhaps from the air itself [26,27,28]. It has been argued, however, that it is currently not technically feasible to produce e-Fuels in a fully climate-neutral fashion, due to the present methodology adopted for emissions calculation [29].

The e-Fuel production process is based on initial H₂ production and CO₂ capture with various technologies, summarized in

Table 1. Summary of current mainstream technologies for H₂ production and CO₂ capture.

Process Technology	Description	Pros	Cons	Ref.
H2 Production (* indicates from fossil feedstock)
Coal gasification *	Steam and O2 reacted with coal	Simple emissions control, low cost	Produces exhaust pollutants, use of fossil source	[31]
Steam reforming *	Endothermic catalytic reaction of steam and CH4	Mature technology	Use of fossil sources, cost linked to NG price	[32]
Plasma reforming *	Similar to steam reforming but with use of plasma heat from electric discharge	No catalyst required, smaller reactors	High electricity requirement, less consolidated technology	[33]
Partial oxidation *	Exothermic non-catalytic reaction with steam and O2	Fast reaction times, compact reactors, low cost	Use of fossil sources	[32]
Autothermal reforming	Combination of steam reforming and partial oxidation	Faster reaction and cheaper than steam reforming. Compact design	Use of fossil sources, still limited diffusion. Requires pure O2 or air separation unit-	[32]
Methane pyrolysis	Catalyzed, high-temperature CH4 cracking without oxygen	No CO2 generation, produces solid char residue, low cost	Produces tar residue which could plug the reactor. Less consolidated technology	[34]
Dark fermentation	Wet biomass fermented anaerobically in dark conditions	CO2 neutral, simple waste recycling technology	High cost, relatively low specific yield	[35]
Photofermentation	Wet biomass fermented anaerobically under light	CO2 neutral, simple waste recycling technology	High cost, relatively low specific yield	[36]
Biomass gasification	Dry biomass in abiotic conditions under controlled O2 and heat	CO2 neutral, simple waste recycling technology	Needs feedstock pretreatment, varying H2 yield due to feedstock, production of tar	[37]
Thermochemical water splitting	High temperature (800–900 °C) sequential H2O splitting	Suitable for large-scale production using sunlight or waste heat	Requires H2 distribution infrastructure due to large volumes, high cost, current viability uncertain, single-step conversion possible at T > 2500 °C	[38]
Photoelectrochemical water splitting	H2O splitting by irradiation-driven semiconductors in an electrolyte solution	Low-temperature and cost-effective electrode materials using unlimited solar energy	Overall high reactor costs and low solar conversion efficiency (˂3%)	[38]
Water electrolysis (various methods)	H2O direct splitting with electric energy input	Mature technology that can be integrated with renewable power sources	Use of corrosive electrolyte and costly proton exchange membranes (in some methods), slow startup, high costs linked to electric energy prices	[39]
Wastewater electrolysis	Water splitting in an organic-rich solution by Microbial Electrochemical Processes	Exploits the chemical energy of organics in solution, reducing required energy input by about 75%	Requires expensive proto-exchange membranes, experimental technology	[40]
Geological H2  extraction	Extraction of free H2 naturally present in geological media	Extraction with existing oil and gas drilling technology, most economical method	Geological occurrence not well understood	[41]
CO2 capture
Sorption	Applicable to combustion flue gases, or syngas prior to combustion; CO2 and H2 (from syngas), N2 (from flue gases) captured by solvents, membranes, and adsorbers	Mature technology (depending on industrial sector)	Low capture efficiency with combustion gases	[42]
Criogenic	CO2 captured by direct phase change (gas to liquid/solid) with N2, SOx, and NOx	High CO2 capture	High CAPEX	[43]
Oxyfuel combustion	Sorption with pressure/temperature swings. Applicable to oxygen combustion flue gases, O2 supplied by air separation unit, CO2 and steam are recovered with solvents and membranes	High CO2 capture	High CAPEX	[44]
Chemical looping	Sorption with pressure/temperature swings, applicable to flue gases, O2 extracted internally from solid state carrier by redox reactions	Cost-effective alternative to oxycombustion	Less mature technology	[45]
Fuel cells	CO2 is recovered by selective ionic transport together with H2O and H2	High capture efficiency, relatively cheap	Less mature technology	[46]
Direct Air Capture	Applicable to air without combustion, CO2 captured by membrane separation and/or sorption	Applicable anywhere without the need for point-source emissions	High CAPEX	[47]

. In the second step, i.e., Fischer–Tropsch (FT) synthesis, H₂ is combined with atmospheric (or captured) CO₂ and converted into a liquid energy carrier under a high pressure and temperature using catalysts. Other possible production routes are illustrated in Figure 4. Low-carbon fuels produced by reacting CO₂ with H₂ range from C1 to C3 gaseous compounds (CH₄, C₂H₆, and C₃H₈) to other high-value liquid fuels (e-gasoline, e-diesel, e-heating oil, e-kerosene, e-methanol, and e-jet fuel). One-step processes can convert CO₂ directly to liquid fuels; two-step processes convert CO₂/H₂ to CO/H₂O (reverse water gas shift), followed by a second reaction that combines CO with H₂ to obtain the desired final product. The production process also generates commercially valuable byproducts, like high-purity O₂ and heat. The major limitations in terms of process selectivity (efficiency of conversion to liquids) or the liquid formation rate are linked to the applied catalysts; therefore, their appropriate selection is key to process performance [30].

The main e-Fuels of interest in Europe are e-hydrogen, e-methanol, e-diesel, e-ammonia, e-LNG (Liquified Natural Gas), and e-kerosene. Cost-wise, the differences between these e-Fuels are relatively small, as they are all dependent on electricity prices; e-LNG, e-methanol, and e-diesel are also sensitive to CO₂ availability and cost. e-hydrogen and e-ammonia are the most economical to produce since they do not contain CO₂ as an additional ingredient, but e-hydrogen entails higher distribution and powertrain production costs [14].

2.1. e-Hydrogen

e-hydrogen (green hydrogen) is obtained from H₂O electrolysis using green electricity. The H₂O molecule is electrocatalytically split into H₂ and O₂ by an electrolyzer, e.g., a Polymer Electrolyte Membrane (PEM) electrolyzer, with potential efficiencies of up to 64% [48]. Electricity costs contribute significantly to overall production costs, therefore periods of low tariffs (e.g., excess production and low demand) are exploited to minimize cost, requiring buffering and storage in the case of intermittent production [49]. e-hydrogen production could find proper justification in P2G strategies based on renewable sources, or in the case of inelastic excess production from nonrenewable sources.

2.2. e-Methanol

Most methanol (CH₃OH) is traditionally produced from syngas, by an exothermal reaction of CO and H₂ at a high temperature and pressure; e-methanol is produced through the same process using green hydrogen (0.189 kg H₂ per kg CH₃OH), captured CO₂ (1.373 kg/kg methanol), and renewable electricity [50], with the recovery of the generated heat. Approximately 65% of methanol production cost is due to renewable H₂ cost [51,52]. CO₂ circularity is achieved by capture from biomass or direct air capture (DAC).

2.3. C8–C18 Liquid e-Fuels

Liquid hydrocarbons with carbon chains from C8 to C18, which include e-diesel and e-kerosene, are also produced from green-H₂ and CO₂ via the FT process, a heterogeneously catalyzed process that converts CO₂/H₂ and/or syngas (CO/H₂) to liquid hydrocarbons with the desired carbon chain length, with an efficiency of about 69% [53]. Just like e-methanol synthesis, H₂ and CO₂ are fed into a reactor at a high temperature and pressure, where a highly exothermic reaction occurs. The generated heat is supplied to an endothermic reaction in an RWGS reactor: a key factor influencing FT synthesis efficiency is the heat transfer between reactors. Alternative to the FT process are methanol-to-diesel and methanol-to-kerosene synthesis pathways (also applicable to e-products) [54].

Co- and Fe-based catalysts are used commercially in FT processes; other relevant catalysts include metal oxides (e.g., zeolite and aluminum oxide) with high specific surfaces; recent advancements in FT synthesis catalysis combine Fe-based catalysts with transition metal promoters (e.g., Fe–Mn–K). More active precious metal-based (e.g., nickel, ruthenium) catalysts are not practically applied in industrial applications due to their high cost.

2.4. e-Ammonia

Most industrial ammonia production occurs by NG steam reforming, followed by water gas shift and CO₂ separation to obtain pure H₂, which is then reacted with N in the Haber–Bosch process to form NH₄. Over 50% of global H₂ production is used in NH₄ synthesis which, in turn, is principally (≈95%) used in fertilizer production. NH₃ is also considered a next-generation carbon-free fuel since, as an energy carrier, it contains 22.5 MJ/kg, which is about half of typical hydrocarbon fuels but higher than methanol’s, and about 15–20% of hydrogen’s energy (by weight); however, its volumetric energy density (12.7 MJ/L), a paramount property for transport applications, is higher than the latter (in any form), as shown in Figure 3.

NH₃ is a versatile fuel that can be fed directly to high-temperature solid oxide fuel cells, cracked for use in low-temperature fuel cells, and partially cracked for combustion in turbines and internal combustion engines. As a fuel in thermal engines, NH₃ does not release CO₂ but generates NOx, which can be abated with adequate exhaust postprocessing. While H₂ remains the cleanest energy vector, NH₃ is overall more economical to produce, store, and deliver within existing industrial infrastructure for mass production and distribution. While ammonia, as a transportation fuel, was virtually unknown till a decade ago, innovation in combustion technology has made substantial progress possible [55], developing commercially available 4-stroke ammonia engine solutions for the marine sector that offer significant advantages over diesel motors for sustainable shipping operations [56]. Furthermore, recently a major Sino-Japanese car engines manufacturing venture announced the development of an internal combustion ammonia-fueled automotive engine for passenger cars, claiming 90% CO₂ emission reduction compared to traditional combustion engines [57].

Ammonia, industrially produced by means of the conventional Haber–Bosch process from atmospheric N₂ and H_2, is a “not green” process, as hydrogen is most commonly extracted by steam reforming from CH₄. The process requires a significant amount of energy (37–45 MJ/kg of atmospheric N fixed) and produces short of 2% of all global CO₂ emissions, approximately 90% of which is from the steam reforming process. It is estimated that between 71% and 92% of the CO₂ from methane reforming could be captured for reuse.

Feedstock for e-ammonia (e-NH₃) consists of green/blue H₂ and N₂ from air separation, which are fed into a renewables-driven Haber–Bosch reactor, with an approximate yield of 70% (green hydrogen is produced by water electrolysis using renewable electricity, blue hydrogen derives from steam methane reforming in which carbon emissions are captured and stored) [58]. In addition, distributed green ammonia technologies for the direct production of NH₃ from H₂O and N₂, completely bypassing the Haber–Bosch process, are being developed: recognized technologies include electrochemical, photochemical, biological, chemical looping, plasma-induced, and metallocomplex N₂ fixation, currently operating at basic research level (TRL 1–4) [58]. Possible ammonia production pathways and applications are schematically illustrated in Figure 5.

2.5. e-LNG

e-LNG, mainly consisting of CH₄, is produced from green H₂ and CO₂ via Sabatier’s reaction [59], and the catalytic reduction in CO₂ to produce methane can be expressed as follows:

CO_2(g) + 4H_2(g) → CH_4(g) + 2H₂O_(l)

The reaction is exothermic (+165 kJ/mol) and is typically operated at temperatures between 150 and 550 °C, depending on the specific catalyst: Ni is generally used for cost containment, but other metal catalysts are also suitable (in order of specific activity Ru > Fe > Ni > Co > Rh > Pd > Pt > Ir) [60]. The process could effectively close the carbon loop if the CO₂ is captured from industrial emissions or DAC. The process reaches a power-to-chemical efficiency of ≈42% when DAC is used and increases up to 51% when anthropic CO₂ point sources (e.g., industrial emissions) are exploited: a recent study indicated that the availability of the latter could lower production costs by over 40% over DAC-based processes [61]. The methane thus produced is then liquefied.

2.6. Other e-Fuels

Besides the above-mentioned e-Fuels, other synthetic fuels are being developed: these include oxygenated fuels such as DME (CH₃OCH₃) and Oxymethylene ethers (OME: CH₃O-(CH₂O)_n-CH₃, with physical properties depending on the formaldehyde chain length, n ≥ 1) that are conventionally produced from methanol. These are inherently ”cleaner” energy carriers as a consequence of their chemical composition that causes lower specific CO₂ emissions, and significant SOx, NOx, and PM emission reduction (per unit energy) compared to fossil diesel: some OME well-to-wheel emissions can be as low as 15% of the latter’s [62]. If methanol is sourced from a power-to-fuel platform (e-methanol), e-DME and e-OMEs are produced from it.

Another category of promising e-Fuels consists of Liquid Organic Hydrogen Carriers (LOHCs), an umbrella definition for a variety of organic liquids, including formic acid (HCOOH), benzyl toluene (C₁₄H₁₄), perhydro-benzyltoluene (C₁₄H₂₀), and N-Ethylcarbazole (C₁₄H₁₃N) that allow embedded H₂ storage at ambient conditions. This technology is based on hydrogenation (for H₂ storage) and dehydrogenation (for H₂ release) of specific organic molecules (generally liquids, like toluene-cyclohexane, C₁₃H₂₀, dibenzyltoluene, C₂₁H₂₀, etc.). Its advantages over H₂ liquefaction/compression include its inexpensive quality, its secure and easily manageable substances, the potential for improved long-term energy storage (without boil-off losses), and on-demand delivery of stored H₂ in an easily transportable form [63].

2.7. e-Fuels Issues

e-Fuels are expected to make a climate-neutral contribution in all sectors where conventional fuels are currently used (e.g., transportation or residential/industrial heating). Their climate neutrality claims stem from the use of renewable electric energy in the production process; however, the current EU energy mix is still dominated by fossil sources and, furthermore, entirely neglecting GHG emissions from RES generation is scientifically debatable, as the impact of the energy (and related GHG emissions) embedded in the materials used to build renewable energy facilities is small, but not nil. In fact, Scarlat et al. estimated the Carbon Intensity of electricity generated solely from photovoltaic and wind sources at 11.4 and 3.1 gCO_2eq/MJ, respectively, compared to that of the EU’s present-day “Grid Mix” of 86.1 gCO_2eq/MJ [29]. While not entirely emission-free, therefore, e-Fuels could alleviate global GHG emissions issues.

To accelerate e-Fuel adoption, further innovation in their production, vehicle technology, and infrastructure should occur, overcoming several barriers, including the following:

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Economic and technical: e-Fuels are costly to produce, and the price of renewable electricity futures is difficult to predict; furthermore, their optimal production routes are not yet fully established from a technical point of view. At present, not enough CO₂ from circular sources is commercially available, making the future evolution of CO₂ feedstock prices uncertain. CO₂ sequestration/capture efforts should be increased. Depreciation issues of existing assets, targeted on fossil fuels, will affect the economic positioning of e-Fuels: new infrastructure and the development of optimized engines and fuel cells will be needed;

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Organizational: System perspectives and stakeholders’ cooperation, still largely lacking, are necessary. e-Fuel production is highly dependent on RES availability; however, the current grid energy mix is still largely dependent on fossil sources. Several EU countries at the moment are highly dependent on electricity imports for normal operation, and even conventional electric capacity (from mixed sources) surplus is scarce. Current policies on battery vehicles and hydrogen mobility are not designed nor suitable for long-distance transport applications;

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Regulatory: Fossil fuels and CO₂ emissions are still relatively inexpensive, preventing companies and consumers from choosing more sustainable alternatives. The high time-variability of fossil energy is an obstacle to the adoption of alternative sources. The constant evolution of tax regimes for vehicles and fuels makes the outlook in the sector uncertain: long-term financial and fiscal aspects must be established. e-Fuels are not yet universally certified, while the transport industry relies on global standards. There is not a sufficiently uniform policy at the global level for achieving sustainability in the transport sector.

3. Biofuels

Biofuels, i.e., hydrocarbon fuels produced from organic (living or once living) matter in a short period of time, are another option for decarbonizing the transport sector. Biofuels are ultimately derived from the starch, sugar, cellulose, and animal/ vegetable lipids provided by renewable biomass feedstock: any biomaterial used as fuel, including wood, sawdust, leaves, and dried excreta, constitute biofuels. Wood and its derivatives (chips, sawdust, and charcoal) were the most important energy carriers used by mankind for millennia and, still recently, they are often used in pelletized form for domestic/industrial uses, constituting a source of particulate pollution in urban areas. Wood could be considered the ideal biofuel, as it is often readily available, and is actually more energy efficient than many other energy carriers, with about 70% of recoverable energy content (approx. 10 MJ/kg, on average); its downside is pollution, as it releases more CO₂, in addition to soot, smoke, and polycyclic aromatic hydrocarbons (PAHs), than other fuels [64].

Globally, more than 2 billion people still burn dried dung as a cheap fuel in areas where other fuel sources are scarce: dung is certainly renewable, and contains approximately 12 MJ/kg, about one-third of coal’s energy content. On the other hand, when burned, it generates pollutants, including dioxins and chlorophenols, that constitute a major health hazard in indoor conditions.

More practical biofuels have been developed through technology, differentiated as being first, second, and third generation according to their original feedstock provenance: food crops (corn, wheat, soybeans, vegetable oils, and sugar cane) were the most common first-generation (1-G) biofuel feedstock. In addition, 1-G biofuels (ethanol and biodiesel) are expensive to produce, with competitiveness deriving mostly from governmental incentives or regulatory obligations; they compete directly with food supply and arable land use and originate a series of ethical and sustainability issues. Some feedstock crops, such as soybean, even present an overall negative energy balance, i.e., they require more energy to produce than they ultimately contain [65]. Most importantly, full fossil fuel replacement by 1-G biofuels immediately proves unfeasible: for example, if all US-grown corn were used to produce ethanol, around 150 × 10⁹ liters could be produced annually, largely below the US national consumption of 530 × 10⁹ liters of gasoline (according to 2015 figures). Although food crop-based biofuels have a limited role in the decarbonization of the transport sector, they are still present used in road and rail transport in some countries (e.g., Europe).

The general structure of biofuels depends on the specific source from which each one is derived, and from the conversion technology adopted. Concerning process technology, 1-G biofuels consist of bioethanol obtained through the fermentation of C6 sugars, mostly from sugarcane or corn, using natural or genetically modified yeast strains. The other main 1-G biofuel is biodiesel, produced by the transesterification of oils from seeds and plants, a process consisting of breaking the bonds between long-chain fatty acids and glycerol (C₃H₈O₃), replacing them with externally supplied methanol (CH₃OH).

3.1. Second-Generation Biofuels

Second-generation (2-G), or advanced biofuels, differentiate significantly from 1-G biofuels since they are based on forest or waste cellulosic and lignocellulosic biomasses (extensively used to produce bioethanol), or on other waste biomasses, including food waste and other biological wastes, such as excess biological sewage sludge [66,67]. To qualify as 2-G, a fuel’s feedstock must no longer be suitable for human consumption. Food crops can still be considered 2-G if they have already fulfilled their food purpose: for example, while “virgin” vegetable oil is a 1-G feedstock, it becomes 2-G after it is no longer fit for food or cooking uses; similarly, food waste, estimated in about 10% of the food made available to consumers in Europe (approximately 130 kg/person-y) is also considered a 2-G feedstock [68]. In general, 2-G feedstock should not interfere in any way with food production; thus, crops should preferably be grown on marginal land: the implicit assumption is that they should have a minimal footprint and environmental impact, and not require large amounts of water or fertilizers: switchgrass, myscanthus, indiangrass, and jatropha, depending on location, are among the most popular nonfood feedstock crops [68].

Processes for the production of 2-G biofuels include two different approaches: thermochemical and biochemical.

3.1.1. Thermochemical Approach to 2-G Biofuels

Under the thermochemical approach, biomass is heated in a reducing or oxygen-poor atmosphere, leading to its conversion into three fractions: solid, known as biochar, liquid (referred to as pyrolytic oil, py-oil, or bio-oil), and gas, a mixture of H₂, CO, CO₂, CH₄, H₂O and other hydrocarbons, known as syngas. Torrefaction at 250 to 350 °C and pyrolysis, at 350–900 °C occur in the absence of oxygen; gasification is carried out in an oxygen-limited atmosphere at temperatures between 700 and 1200 °C [69,70,71]. From the fuel generation point of view, py-oil and syngas are the most interesting products, with properties similar to those of other liquid and gaseous energy carriers, although their use as transportation fuels usually requires further transformation due to their high water and impurities content and corrosive nature. The four most promising py-oil postprocessing processes are hydrodeoxygenation (to reduce oxygen content), catalytic cracking, steam reforming, and emulsification with diesel [72].

Syngas, mostly composed of C1 compounds and H₂, could be used directly as a fuel, or as a starting material for the production of transportation fuels through the use of catalysts to create additional C=C bonds, e.g., via the FT process. A simple method for industrial synfuel production from syngas is through sustainable methanol generated from CO and H₂ under the effect of reducing catalysts. Bio-methanol is also the basis for the production of a new generation of oxygenated fuels such as bio-DME.

3.1.2. Biochemical Approach

The biochemical approach is represented by so-called biorefineries, defined as systems focusing on the sustainable conversion of biomass into a spectrum of products and energy, including liquid or gaseous transportation biofuels [73]. Biorefineries in the current mainstream definition combine various individual processes, including thermochemical ones. Many different biorefinery classifications exist, depending on original feedstock, the main ones being the following [74]:

• Lignocellulosic biorefinery, based on natural dry raw materials, such as cellulose-containing biomasses, and wastes;
• Green biorefinery, based on natural wet biomasses, such as green grass, alfalfa, clover, or immature cereal;
• Two-platform concept biorefinery, based on sugar and syngas platforms;
• Conventional biorefinery, based on existing industries, such as sugar and starch;
• Marine biorefinery, based on marine biomass;
• Liquid-phase catalytic processing biorefinery, based on the production of functionalized hydrocarbons from intermediates derived from biomass;
• Forest-based biorefinery, based on integrated biomass and other feedstocks (including energy) processing for production of pulp, (paper) fibers, chemicals, and energy;
• Brown biorefinery, based on wet biomass from waste processing operations, such as municipal or industrial sewage.

Biorefineries can also be differentiated according to their final products, which can be fuels (bioethanol, biodiesel, bio-FT fuels, biogas/biomethane, bioethanol, bio-hydrogen, etc.) or raw secondary materials. In truth, the combination of the number of feedstocks and processes results in a very complex web of possible pathways and outputs that continuously expand according to the evolution of applied technologies [72]. Figure 6 is an attempt to represent these interconnections between feedstocks, processes, and platforms.

Most feedstocks require some kind of mechanical preprocessing prior to biorefination: in the case of lignocellulosic or biological solids feedstock, multiple pre-processing steps may be required to make treatment more efficient, including biochemical proper (enzymatic breakdown) [75], physico-chemical pre-processing (steam explosion; microwave and pulse electric field irradiation) [76,77]. Several processes for the production of 2-G biofuels have been developed: fermentation by means of genetically modified microorganisms is popular for feedstocks such as landfill gas and municipal waste.

3.2. Third-Generation Biofuels

3-G biofuels refer to algae-derived fuels: when it became apparent that algae were capable of much faster growth and advantageous yields than other feedstock, they were classified separately from 2-G feedstock. Algal cultivation has shown production potential for the production of diverse fuels of up to 15,000 L/ha biofuel, 10 times the best output ever generated by 2-G feedstock through different processes [78]. Projections based on the successful development of genetically modified microalgae (sometimes indicated as 4-G feedstock) indicate possible yield targets of 35,000 L/ha. This means that, for example, less than 0.5% of the US land area could generate enough biofuel to meet the country’s needs. Algae and microalgae constitute a wide range of autotrophic organisms that, in principle, require very large amounts of water and nutrients (N and P) to grow; this would imply a high use of already scarce water resources and GHG emissions from the production of N&P fertilizers [79]. However, since algae can grow almost anywhere, even on marginal waters such as wastewater or brackish water, with secondary benefits such as environmental remediation and sewage treatment opportunities [80], they are actually easier to cultivate than any other traditional biofuel crop, with no farmland conversion needs. Furthermore, as they use CO₂ as a carbon source, they can convert emissions from a wide array of sources, including flue gases from power plants, industrial facilities, etc., mitigating their impact, or capturing atmospheric CO₂ [28].

Recent progress in the development of new varieties of genetically modified organisms (e.g., Nannocholoropsis gaditana) has shown that they are capable of converting 55% of the uptaken CO₂ into lipids (in laboratory settings) against a typical conversion of about 20% by other species [81,82]. Microalgae-extracted lipids can be directly refined into liquid hydrocarbon molecules; harvested biomass can be fermented for butanol production and, importantly, could be genetically manipulated to produce biofuels including biodiesel, butanol, gasoline, methane, ethanol, and jet fuel [68].

3.3. Biofuel Types

3.3.1. Gaseous Biofuels: Biogas, Biomethane, Syngas, and Bio-Hydrogen

Since the adoption of Directive (EU) 2018/2001 [6], gaseous fuels from biomasses are technically no longer included in the official definition of biofuels; however, they play a significant role in the area of alternative ecofuels and as such they will be discussed herein.

Biogas is a mixture of combustible gases obtained from fermentation (anaerobic digestion, AD) of various biomasses, including municipal solid waste (MSW) landfills and MSW organic fraction [83], wastewater and sewage sludge [84,85], and zootechnical and agro-food waste biomasses [86]. Continental Europe is a major global producer of biogas, with 2021 volumes rising to 14.9 million Mtoe, 1.7% more than in 2020: Germany is the largest producer (7.5 Mtoe), followed by Italy (2.1 Mtoe), and France (1.4 Mtoe) [2]. The share of renewable energy in the EU in 2022 was 23%; biogas and bio-methane contributed to it 10.1% [87,88]. Biogas composition varies depending on feedstock and processes (

Table 2. Typical biogas composition.

Component Gas	Content (v/v)
Methane	40–75%
Carbon dioxide	15–60%
Hydrogen	Traces
Nitrogen	0–5%
Moisture	1–5%
Hydrogen sulfide	0–5000 [ppm]
Ammonia	0–500 [ppm]

), with a 60% CH₄ content by volume, and its lower heating value (LHV) is approximately 21.5 MJ/Nm³ (5.97 kWh/m³), with some serious end-use limitations [89].

Traditionally, biogas is converted onsite into electrical and thermal energy by combined heat and power (CHP) generators. Depending on type and size, the combined electricity and heat conversion efficiency is between 18 and 90% [90]. Biogas cannot be used as a transportation fuel or for industrial uses but, due to its easy production method, its local use can significantly benefit households and small communities in rural and decentralized areas of developing countries, providing a sustainable source of fuel much less polluting than wood or dung cakes [64].

Biogas, converted to the commercial NG standard (EN 16723) [91,92], is commonly indicated as “biomethane”, i.e., a mixture containing a minimum of 96% CH₄ and less than 2% CO₂ (by volume). Biomethanation takes place by CO₂ scrubbing and/or converting it to CH₄ by catalytic addition of H₂ [84]. Compared to the original biogas’s calorific content (21–39 MJ/Nm³), biomethane can thus contain up to 51 MJ/Nm³, and can be transported through existing NG networks: its uses include thermal combustion, fuel cells, compressed and liquifed natural gas (bio-CNG, bio-LNG, respectively), or transformation into alcohols, such as methanol or ethanol, after conversion to syngas (Figure 7). It has been estimated that full utilization of the global biomethane generation potential could cover about 20% of the current NG demand [93].

Syngas, a gas product of pyrolysis or gasification of waste biomasses, consists mainly of a mixture of H₂, CO, and CO₂. In addition to an energy carrier, it can be intermediate in biomass to fuel conversion: its primary use is the production of methanol (e-methanol, bio-methanol) and e-diesel (through the FT process). Biogas from landfills and AD could also serve as feedstock for biodiesel since it is not derived from fossil fuels. Syngas may directly power fuel cells by capturing the contained H₂ [94].

Direct bio-hydrogen production from biomass photo-fermentation (PHF) and dark-fermentation (DF) can be achieved: PHF is an anaerobic biological process carried out under solar irradiation by photosynthetic non-sulfur bacteria (e.g., oscillaria, Rhodospirillum rubrum, Rhodopseudomonas spheroids O.U001, and Rhodopseudomonas palustris) that use glucose and organic acids as substrates [95]. PHF has several drawbacks, including the required reactor footprint area for irradiation and slow process rates, significantly lower than theoretical expectations based on process kinetics. DF could be considered an incomplete evolution of AD, occurring when bacteria degrade carbohydrates to H₂, VFAs, and CO₂ under dark, anoxic conditions. Unlike AD, where methanogens decompose VFAs and H₂ into CH₄ and CO₂, in DF the growth of methanogens is inhibited to achieve final H₂ production. Despite considerable efforts, H₂ production by DF remains confined at the experimental level due to low process yield and production rates: although the stoichiometric potential is 12 mol H₂/mol glucose, the highest reported yields are still much lower, in want of more efficient catalysts and reactor engineering [35].

3.3.2. Liquid Biofuels

The 2018 revision of the Renewable Energy Directive 2018/2001 (RED II) [6] distinguished three main categories of liquid biofuels according to feedstock or technology:

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Those produced from food and feed crops (Article 26), for example, biodiesel from oil from rapeseed, sunflower, palm, and soy, or bioethanol from corn, wheat, sugar beet, barley, and rye;

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“advanced biofuels” from wastes, residues, and co-products (Part A, Annex IX), for example, algae, biomass fraction of municipal waste, straw, palm oil mill effluent, non-food cellulosic, or ligno-cellulosic material, using advanced technologies;

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Biofuels from wastes, residues, and co-products (Part B, Annex IX) such as used cooking oil and animal fats not fit for human food or animal feed that can be processed using mature technologies.

Advanced biofuel potential is still hindered by higher costs and low technological maturity, compared to crop-based biofuels: in 2022, only two commercial plants (TRL 9) and nine first-of-a-kind plants (TRL 8) were active in the EU for the production of advanced biofuels, with a combined production capacity of approximately 10⁹ L/y [96].

RED II defines sustainability and the GHG emission criteria that liquid biofuels used in transport must comply with; in particular, it introduces a sustainability concept for forestry feedstocks, as well as applicable GHG criteria. The latter require an emission reduction of ≥ 50% to 65% from biofuels, compared to fossil fuels, depending on the starting operation date of their producing facility (prior to 2015 or after 2021, respectively). A cap on the contribution of energy from crop-based biofuels to EU transport targets exists (≤7%), as well as on biofuels from used cooking oil and animal fats (1.7%), but direct import (and use) of such fuels is not limited. In 2021, most biofuels in the EU transport mix were crop-based, as indicated in Figure 8 [97].

In the EU, biodiesel and bioethanol use is limited to blends with fossil fuels. As far as biofuel uses, Europe favors biodiesel, as its most common motor fuel is its fossil counterpart, and as a whole, the EU is a net importer of fossil diesel and a gasoline exporter. Bioethers (ether biofuels) like DME, DBE (dibutylether), MTBE (methyl-teritiarybutylether), DEE (diethylether), TAME (ter-amylmethylether), ETBE (ethylterbutylether), and TAEE (ter-amylethylether) have been widely used as fuel additives since the 1970s to replace lead as a fuel component; however, they are no longer used in the USA. Some of these, such as DBE and OME, have recently been indicated among the most promising alternatives to fossil fuels for future diesel engines.

Biodiesel is a mixture of fatty acid methylesters (FAMEs) produced from edible or nonedible oils by transesterification: oil/fat are reacted with short-chain alcohol (usually methanol) in a 10:1 ratio, in the presence of a catalyst (e.g., NaOH or KOH) to form biodiesel and glycerol (also in 10:1 ratio). The latter (C₃H₈O₃) is a coproduct used in the manufacture of pharmaceuticals and cosmetics. Feedstock affects biodiesel production for as much as 75% of the final cost. More than 350 oil-bearing crops have been identified as promising feedstock for its manufacturing:

Table 3. Main feedstock for biodiesel production and their oil content [68].

Type of Feedstock	Crop	Oil Content %
Edible	Soybean	15–20
	Rapeseed	38–46
	Sunflower	25–35
	Peanut oil	45–55
	Coconut	63–65
	Palm	30–60
Nonedible	Jatropha seed	35–40
	Pongamia Pinnata	27–39
	Neem oil	20–30
	Castor	53
Other sources	Rubber seed	40–50
	Sea mango	54
	Cotton seed	18–25
	Microalgae	30–70

summarizes some of the main ones and their oil content.

Green diesel (bio-hydrogenated diesel) is a second-generation biofuel, derived from the same feedstock and hence similar to biodiesel, produced by hydrocracking with H₂ at a high pressure and temperature. The process breaks down the larger hydrocarbon chains of vegetable oils into the shorter chains of diesel fuel, without glycerol production. The efficiency of green diesel production via deoxygenation pathways depends on the active sites on the metal catalysts: these are preferably noble metals (Pd, Pt, and Rh) for higher efficiency, but also bimetallic catalysts, metal oxide compound catalysts (sulfide, carbide, phosphide, and nitride), or non-noble metals (Ni, Mo, Mn, Cu, W, Co, and Zn) are used for cost-effectiveness. Additionally, 3 gen and 4 gen biodiesel can be produced from algae, with the latter involving the metabolism of genetically modified algae to achieve a higher lipid yield [98].

The world renewable biofuel market is currently dominated by bioethanol (CH₃CH₂OH), produced by alcoholic fermentation of sugars, e.g., from sugarcane or corn (1-G). As a 2-G biofuel, it is produced from lignocellulosic feedstock fermentation after milling, pretreatment, hydrolysis, and detoxification. Although its energy equivalent is just 40% of mainstream fossil fuels, it burns cleaner due to its oxygen content; therefore, it is frequently used in blended gasoline mixes (E15, E85, where the number indicates the ethanol fraction), but finds applications also in fuel cells. From an economic perspective, the EU’s bioethanol production traditionally relied on feedstock that is noncompetitive respective to, for example, Brazilian sugarcane, which demonstrates 4–7 times higher specific energy productivity.

Biobutanol (CH₃(CH₂)₃OH), with isomers isobutanol, butan-2-ol, and tert-butanol, is less popular than ethanol and biodiesel; however, it is gaining attention due to its superior properties that could overcome some drawbacks of other low carbon alcohols used as fuels/additives: its higher heating value (almost double volumetric energy density than ethanol, close to gasoline’s) and its higher heat of evaporation (compared to ethanol) are beneficial for increasing fuel yield (in terms of specific mileage traveled), reducing the combustion temperature and NOx formation. Its higher flash point indicates that butanol is potentially safer for transportation and use at high temperatures. With a higher C-number, it is easier to blend with gasoline and diesel (intersolubility) due to its nonpolar long hydrocarbon chains and lower hygroscopicity; since it contains more oxygen than biodiesel, it could reduce engine emissions and protect engine components from wear due to its higher viscosity, lubricity, and noncorrosive nature.

All butanol isomers can be produced from common sucrose-based feedstock and other biomasses such as barley, straw, bagasse, corn core, and lignocellulose. Each feedstock has a different industrial biobutanol production process: 1 gen feedstock undergoes simple fermentation, and 2 gen can follow a hydrolysis/fermentation or gasification process followed by fermentation or catalytic reaction [99].

Microalgae are the ideal feedstock for 3-G biobutanol production via fermentation or lipid extraction. Algal biomass must be harvested, pretreated to release embedded monosaccharides, and fed into a fermentation process. Microalgae-based butanol production via the ABE fermentation process is achieved by bacterium C. acetobutilicum that converts residual solid matter in an unfiltered medium to butanol, with a higher yield compared to filtered hydrolysate (21.96 mg/g versus 10.03 mg/g) [100]. The main obstacle to widespread 3-G biobutanol production is the still high cost (capital and operating) for microalgae cultivation (approx. 400 €/t) [101].

Bioethers are produced from the dehydration of bio-based alcohols; their combustion and emission characteristics are superior to other biofuels. Bioethers have been long used as fuel additives (e.g., MTBE) to improve engine performance, but they have also been investigated as alternative fuels: DBE and OME are second-generation biofuels proposed as alternatives or in admixtures with diesel. They can be produced from biomass pulping and fermentation or partial oxidation of lignocellulosic biomass: in the former, cellulose, hemicellulose, and lignin that makeup lignocellulose are separated and fermented, and DBE is produced by the dehydration of the resulting butanol. Gasification involves the conversion of lignocellulosic biomass into syngas, then to alcohols that are dehydrated to ethers using suitable catalysts [102]. The gasification/syngas route can convert all carbon to fuel, while the fermentation route can only convert sugars, therefore the achievable yield of bioethers is higher via the former route.

3.4. Biofuel Issues

As with e-Fuels, biofuels are expected to make climate-neutral contributions in sectors where conventional fuels are currently used. However, the total amount of bioenergy that can be sustainably produced is uncertain for several reasons: first and foremost, the availability of feedstock and their yield. In addition, 1-G feedstock is inadequate from this point of view; 2-G biofuel production is still limited due to competition from other industrial sectors aiming at the same feedstocks; and great expectations lie in 3-G and 4-G bioenergy, pending progress on the successful genetic modification of microalgae.

Since biomass contains more C than H₂, adding extra hydrogen in gasification-based biofuels could increase fuel yield; using this energy with subsequent carbon capture-and-reuse to produce e-Fuels would increase the exploited amount of energy from biogenic carbon before it reenters its cycle, and improve bioenergy use in several sectors.

Improved national biomass policies are necessary to maximize the efficient use of available biomass in order to deliver GHG savings and allow for fair competition between various biomass users; however, no such policies exist at the moment.

To accelerate the adoption and innovation in biofuel production, feedstock availability, and infrastructure development, overcoming several barriers will be necessary. These include the following:

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Economic and technical: Biofuels still compete with other sectors for raw materials (e.g., the food sector, but also the cosmetic, pharmaceutical, bio-plastic, and heat production sectors). This affects the availability and market prices of these materials. Biofuels were supposed to increase energy independence, but in practice, they often distorted feedstock material markets: in the early 2000s, the EU-25’s biofuel consumption was 90% covered by domestic feedstock and just 10% by imports; two decades later, dependence on import has vastly increased due to rising biomass demand for biofuels. The EU-27’s biofuel consumption from used cooking oil feedstock increased from 0.09 Mtoe to 2.53 Mtoe (+2700%) between 2011 and 2020, with more than half of the used oil now imported from outside the EU at increasing prices. According to the IEA, “biodiesel, renewable diesel and biojet fuel producers are headed for a feedstock supply crunch during 2022–2027, if current trends do not change” [103];

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Organizational: The EU has adopted various strategies for transport and biofuels over the years; however, the specific strategy for biofuels has never been updated or revised since 2006, while the complex operating framework of the biofuel industry has evolved significantly since. Additionally, there is no clear indication of the EU policy on biofuels after the 2030 horizon, which may discourage research, development, and investments in the sector;

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Regulatory: Some transportation sectors have long-term decarbonization objectives, but no roadmap concerning their achievement. The constant evolution of tax regimes for vehicles and fuels makes the outlook in the sector uncertain: for example, sustainable aviation fuel (SAF) production is supported under long-term fiscal provisions in the United States, but no EU-level roadmap yet exists on production speed-up and support. Different feedstocks are even treated differently under various regulatory targets, increasing their implementation complexity; the same feedstock may be differently classified across EU member states. On the GHG emission calculation aspect, savings from biofuel use are determined according to official formulas that may not be fully reliable or consistent. For example, the EU approach does not factor in the risk of indirect land use change for crop-based biofuels, leading to a possible overestimation of GHG emission target achievement by over 60% [97]. Overestimations of emission reduction may also come from the use of default values for feedstock types, irrespective of origin: as an example, CO₂ emission factors from used cooking oil transport and distribution originating from Germany or France were calculated with the same coefficients used for used cooking oil imported from China [98]. Better-defined procedures and goals are therefore needed.

4. Non-Biomass Waste-Derived Fuels

The world is facing an increasing pollution problem from non-biodegradable materials, in the form, among others, of huge amounts of plastic and tire waste. The vast consumption of plastics has been rising steadily everywhere because of the advantages deriving from their use flexibility, low cost, and durability. A drastic increase during the recent COVID-19 pandemic was observed, due to the increase in the use of disposable food packaging and medical/health devices [104]. Dismissed worn tires are one the biggest and most problematic sources of solid waste because of their number, size, and volume. Every year, millions of tons of plastic and tires are discarded improperly, burned, or buried all over the world, representing a serious threat to the environment. Among the management techniques of plastic and tire waste (PTW), energy recovery through conversion into fuel is of great interest. PTW can generate alternative fuels for internal combustion engines since its initial raw materials are obtained from petrochemical polymeric products containing mainly carbon and hydrogen atoms [105,106].

The pyrolysis process is one of the most appropriate technologies to convert polymeric waste into fuel, although gasification and hydrothermal liquefaction have also been used [107]. Its main advantage is that it can convert both thermoplastic and thermoset materials into high-quality oils and chemicals without pre-treatment of mixed, unwashed, and unsorted waste, and without releasing toxic substances into the atmosphere. Waste plastic oil (WPO) has fuel characteristics similar to those of diesel fuel, depending on feedstock type (polystyrene-PS, polypropylene-PP, high-density polyethylene-HDPE, low-density polyethylene-LDPE, polyvinyl chloride-PVC, polyethylene terephthalate-PET, and other mixed plastics) and pyrolysis process parameters (temperature, pressure, residence time, heating rate, and type of catalyst). Benzene-rich products are obtained from PS and PET, and aliphatic hydrocarbon-based waxes are obtained from HDPE, LDPE, and PP [108].

Pyrolysis of mixed waste plastics yields on average 45–50% oil, 35–40% gas, and 10–20% char, depending on the specific technology, although some reports indicated an oil recovery of >80% (by weight) from pyrolysis of sorted LDPE with a zeolite catalyst (higher than wood biomass’s yield) [109]. LDPE pyrolysis at 300 °C yielded cleaner, S-free long-chain hydrocarbons (C8–C29) with properties similar to conventional diesel, with a prevalence of alkenes and aromatics, 56% gasoline fractions (C6–C12), 26% gasoline and kerosene (C13–C18), and 10% fuel oil (C18–C23). Liquid oils derived from PP and HDPE were reported to have density and API classification in the diesel range (C12–C16 hydrocarbons), meeting premium fuels specifications, while other parameters (ash and water content, heat value, flash point, and cetane number) were in line with conventional diesel and gasoline specifications [110]. On the other hand, waste PET and PVC result in low oil yield and fuels, with the lowest calorific value being less than 30 MJ/kg due to the presence of benzoic acid (in PET) and chlorine (in PVC) that deteriorate their characteristics [108].

Waste tire pyrolysis, on the other hand, differs from that of other polymeric materials due to tires’ complex physical–chemical properties. Tires’ C content is high, usually more than 80% (wt), with a heating value of 30–40 MJ/kg, higher than many other solid fuels. Tire-derived fuel (TDF), consisting of shredded waste tires, is used as an alternative solid fuel in combination with coal in the cement industry, since in controlled conditions it produces lower emissions than coal itself. Waste tire pyrolysis generally aims at the maximization of the liquid-phase product yield, because of the valuable chemicals that can be recovered; generated py-gas, on the other hand, has a high calorific value that could fully meet the process’s energy requirements. The solid residue (char, also called pyrolytic carbon black) is a mesoporous material with an average heating value of 30 MJ/kg, with its composition and properties dependent on pyrolysis conditions and the waste tire composition, with a high C and S content (up to 90% and 2% wt, respectively) [21].

Tires pyrolysis proceeds, after mechanical pretreatment, through a series of chemical reactions including polymer degradation first, followed by complex secondary intermediate reactions [21]. Catalysts are used to improve product yield and quality in tire pyrolysis, promoting heavy hydrocarbons cracking into small ones and helping minimize S and O compound formation. The most studied catalysts include microporous acid zeolites and MgO and CaO basic compounds; however, tires’ higher ash contents (>5% by weight) could affect catalysts’ active site efficiency during the process [111]. Higher carbon and fixed carbon content lead to an increase in char yield during waste tire pyrolysis: the oil yield can be between 40 and 50% (by weight) of the final products, depending on process parameters [112].

Tire pyrolysis oil, also called bunker oil, is a thick viscous liquid, dark brown to black, with a strong odor, that could become semi-solid at low temperatures. It is a highly complex mixture of saturated and unsaturated linear or cyclic C7–C20 hydrocarbons with critical properties similar to those of some fossil fuels, and a heating value of 40–43 MJ/kg. Its average composition has been reported as C ≈ 83%, H ≈ 6.6%, O ≈ 8.6%, N ≈ 0.3%, and S ≈ 1% (all figures by weight) [113]. A higher S content and lower flash point compared to fossil fuels are its main disadvantages, preventing its direct use in ICE: further distillation and removal of undesirable chemicals like S and N in existing refinery facilities can be required for this use. Several desulphurization methods have been described in the literature, e.g., the addition of alkaline additives, distillation, oxidation with hydrogen peroxide in the presence of acidic catalysts, or hydrorefining. Tire py-oil value as a secondary raw material in the Circular Economy, however, is high and lies in the production of high-value carbon products, such as carbon black, in substitution of the one from fossil fuel feedstock, which make up 60% of its manufacturing costs, and the recovery of important chemicals such as benzene, toluene, xylene, and dipentene, that are used in the production of many industrial and pharmaceutical products.

The gaseous product fraction from tire pyrolysis can range from little to >10% (wt) of the final products. It has a generally high heating value, up to 42 MJ/kg, and is composed of H₂ (≈26%), CH₄ (≈24%), CO_2, and CO (≈4%), olefins, paraffins and small amounts of S (≈4%) and N compounds [114]. The relatively high H₂S concentration, however, may cause over-the-limit SO₂ emissions from its combustion, with high costs of flue gas cleaning.

Gasification, either conventional or hydrothermal, is frequently used to generate H₂-rich syngas from waste tires; in addition to syngas, the process can also produce small quantities of biochar (hydrochar) and bio-oils. Syngas and char yields of 55% and 28% (wt), respectively, were observed at a process temperature of 786 °C. Gas components include CH₄, C₂H₄, C₃H₆, C₂H₆, CO, CO_2, and H₂, with the relative concentration depending on the process operating temperature.

The gasification of tires involves a primary reaction in which decomposition into heavy and light hydrocarbons and solid char occurs. Secondary reactions cause cracking of heavy hydrocarbons, reforming of light and heavy hydrocarbons, and, finally, gasification of char material to increase gas yield. Catalysts such as commercial Ni catalysts and natural minerals such as dolomite and olivine are often used in the process to increase the H₂ content of the final gas. Tire gasification includes two main technologies: conventional gasification and plasma gasification. In the former, the heat of the organic fraction decomposition is generated from the tires’ partial oxidation using air or oxygen; in plasma technology, thermal plasma is used to decompose tires into syngas and vitrified slag at higher process temperatures [115,116].

In conventional gasification, syngas with a higher hydrogen content of up to 57% and an LHV of 30–37 MJ/kg were observed in steam gasification, compared to a H₂ content of 15–20% and an LHV of 4–7 MJ/kg for air gasification; plasma gasification yielded the highest hydrogen content of 99% using CaO as a catalyst [117].

Hydrothermal liquefaction (HTL) is similar to pyrolysis in that they are both solid-to-liquid thermochemical processes; however, HTL is performed at lower temperatures (200–400 °C) for 5–120 min and higher pressures (5–25 MPa); in addition, HTL has no need for energy-intensive preliminary feedstock drying. HTL main products include biocrude, bio-oil, aqueous liquids, gases, and hydrochar, whose distribution and properties are dependent (as in pyrolysis and gasification) on process conditions and catalysts type/dosage; temperature is the most influential factor affecting yield and quality of liquid products [107]. HTL is a promising technology for high-quality liquid fuel production from waste tires: the resulting oil has been reported to be better than that produced from pyrolysis, with lower oxygen content (and consequently higher LHV), and high alkane concentrations. Most experimental waste tire HTL tests involved co-liquefaction with biomasses: liquid oil produced from the co-processing of waste tire and willow leaves in a 20/80 weight ratio at 400 °C showed an LHV of 45.52 MJ/kg, with a low O₂ content of 1.79% and an alkane concentration of 56.24% [118]. Co-liquefaction of waste tire and lignite 50/50 feedstock at 400 °C and 30% catalyst (Fe₂O₃) addition showed higher conversion rates than liquefaction of lignite alone, indicating a positive synergistic interaction between the two feedstocks [119].

In addition to original liquid and gas fractions, char from tire pyrolysis, gasification, and HTL can be used directly as a raw material or converted into an energy carrier through further gasification.

5. Discussion and Future Perspectives

Notwithstanding progress made in the sector, complete fossil fuel replacement at present is problematic, since there is far more fuel demand than could be provided by all ecofuel streams combined. At the moment, biofuel prices are higher than their fossil fuel counterparts, therefore their production and supply are driven by policy and mixed requirement regulations, rather than markets. It is usually argued that without regulatory mandates, both biofuel production and food prices would decrease: a study estimated that without specific policies, the global biofuel demand would be 25% lower for ethanol and 32% lower for biodiesel [120]. However, trickle-down effects on agricultural commodities prices would be somewhat limited, and food biomass would not necessarily become much cheaper: according to a study, ethanol and biodiesel feedstock commodity (wheat, grains, and vegetable oils) prices would only moderately decrease (≤8%), being linked to other prevalent volatility factors, and thus regulatory relaxation would not necessarily lead to an increase in global food use, affordability, and security [120].

The real issue in biofuel production is feedstock availability, rather than its cost. An answer to biofuel production augmentation lies in the use of existing sources, such as waste streams (2-G), or potential ones, such as microalgae (3-G and 4-G), that are abundant and renewable: biofuels have already been made from animal offal, sawdust, food and forestry waste, and spent vegetable oils. These sources, however, are relatively limited. A much more abundant one is a type of common waste cellulose (a long-chain polymer formed from sugar molecules) that could allow biofuels to take over a significant fraction of energy production. In fact, 1-G ethanol made from grain requires vast amounts of water, energy, fertilizers, pesticides, and other inputs to grow the crops. After harvesting, only the kernels are used; stalks, leaves, and husks are all discarded, often burned in the field, accelerating CO₂ release compared with natural decomposition processes that occur after integration into the soil. These are all cellulose-containing waste biomasses: it is estimated that the amount of cellulosic waste available annually in the United States would be sufficient to replace around one-third of transportation fuel in the country. The use of marginal land to grow cellulose-rich nonfood crops (such as various grasses) could increase its availability further, and the exploitation of urban waste cellulose such as cardboard, paper, and packaging waste, which accounts for around 40% of municipal waste, could allow ethanol to become the dominant transportation fuel.

Currently, technology to produce ethanol from cellulose exists, but its processing into sugar also requires significant amounts of energy and specialized enzymes. Integrated production methods, however, could lead to significant (>20%) cost reduction due to the inexpensive nature of this feedstock [121,122,123,124].

Algae, on the other hand, have the potential to yield much higher specific amounts of biofuel compared to other biomasses, due to their higher concentration of lipids/fatty acids [125].

A recent study on e-Fuel sustainability for global climate mitigation argued that these are not, in most cases, cost-effective options for the transportation sector, due to the fact that they are more expensive than other current options, and their production is highly energy-intensive. When carbon emissions are not a limiting factor, all other available options largely out-compete e-Fuels; even with a limited expendable C budget, it would be more sustainable to remove and store carbon rather than use it for e-Fuels production, that eventually re-emit it into the atmosphere [126].

The current EU approach, based on the tank-to-wheels approach, considers only tailpipe emissions: electric vehicles are thus considered zero-emission, while conversely the GHG and Circular Economy benefits of ecofuels are not properly accounted for; a more sustainable approach adopting a technology-neutral, well-to-wheels approach, in which the entire emissions of the fuel/energy vector production cycle is factored in, could reflect more accurately any GHG emission advantages provided by all ecofuels classes.

Additionally, the existing EU’s emissions trading system (ETS) set up a cap-and-trade mechanism under which the exchange of emissions allows the meeting of reduction obligations; however, transport sector operators are not permitted to use allowances for sustainable ecofuels, which could help reduce their current price gap with fossil fuels. In 2020, the highest CO₂ exchange price was 35 €/t; at the beginning of 2023 it was around 100 €/t. Still, these figures are significantly lower than the cost of CO₂ emissions reduction through ecofuels. As part of the 2023 revision of the ETS Directive, a new system (ETS2) was created to cover and address CO₂ emissions from sectors not included in the previous ETS, including road transport [127]. The new ETS2 will become fully operational in 2027; therefore, its practical effects on ecofuels markets are still to be seen.

Most important, efforts to curtail GHG emissions from transportation fuels should not be partial to a specific technology, as technologies are subject to periodical revolutionary innovations. The EU is currently placing emphasis on the need for the complete substitution of combustion engine passenger cars with electric vehicles (EV) within the next 10 years; in order to achieve this objective, member states are using up-front price subsidies, value-added tax (VAT) exemptions, no or low licensing fees, and other incentives, which are among the most impactful criteria in the customers’ decision-making process when choosing a new vehicle [128]. CO₂ emissions for the production of EVs, however, outbalance those of traditional combustion vehicles; the excess should in principle be compensated by the consumption of RES-only electricity during the vehicle’s lifetime [129]. EV charging with the current predominantly fossil energy mixed electricity, however, makes the environmental impact of EVs worse than those of ICEs. According to the European Council, in 2022 only 9 EU member states (out of 27) had an electric energy mix consisting of >50% renewable sources [130]. A rapid transformation of the electric energy sources mix would therefore be necessary to actually achieve the as-planned decarbonization of the transportation sector. Furthermore, full electrification will have a dramatic impact on energy demand: a recent study for the city of Surat (India) estimated that demand for public EV charging could increase by more than eightfold in the next 5 years (358.05 MWh/day in 2030, compared to 42.4 MWh/day in 2025), requiring a revolution in grid planning due to the resulting imbalanced demand [131]. Controlled charging strategies should also be an essential component of transport electrification: simulations have highlighted a significant deterioration of grid power quality following an EV penetration level of just 25%, ultimately resulting in greater heating losses in distribution lines, transformers, and connected loads [132].

A final consideration is of order concerning renewability and sustainability concepts: the terms “renewable” and “sustainable” are often interchanged in lay language, however, their meanings are quite different: renewable energy is produced using natural resources that are constantly replaced, and therefore should in principle never run out. However, even renewable sources, if used up faster than they regenerate, will eventually be depleted. For example, corn for biofuel generation is not sustainable per se, due to the limited amounts that could be cultivated compared to biofuel demand. Eolic and solar energy will never run out, but their sustainability is linked to the needed infrastructure for their exploitation (e.g., wind turbines or solar panels materials’ recyclability).

Sustainable refers to energy that meets present needs without compromising those of future generations, which can be the case of some fossil energy carriers (e.g., coal) that may be considered sustainable since existing reserves are so large that, discounting GHG emission issues, they could last for centuries if used with parsimony. Not everything renewable is sustainable, and vice-versa, not everything which is sustainable is necessarily renewable. Ecofuel policies should take this latter aspect into account.

Future Perspectives

In order to achieve significant fossil source substitution with ecofuels, a strategy based on the simultaneous exploitation of multiple sources and feedstocks should be pursued. As the electrical grid energy mix renewables’ share increases gradually, the prospects of e-Fuel production will become more sustainable, pending that CO₂ sequestration/capture technology can be substantially improved, as this is a limiting process component. CO₂ utilization for e-Fuels is not an alternative to large-scale storage for emission reduction, but it can support this goal and would help create secure CO₂ supply chains. The cost and effectiveness of CO₂ capture technology are steadily decreasing and improving. DAC technology has received significant attention in recent years, although there are still many issues regarding it [133].

The development of biofuels from waste or marginal biomasses can be an essential component of a sustainable ecofuel strategy. In addition to the exploitation of traditional waste biomasses for 2-G biofuel production, great expectations are placed on new bioengineered microalgae species as they can achieve greater specific lipid production. However, the size and buoyancy of microalgal cells already present significant challenges, as their efficient harvesting is critical for optimizing output yield and financial sustainability. Conventional techniques like filtration or sedimentation face significant limitations due to cells’ small size and require significant energy inputs. Improved energy-efficient methods for microalgae harvesting are being investigated: nanomaterials showed potential to revolutionize microalgae harvesting via magnetically induced improved separation [134].

New recently announced technologies, such as ammonia-fueled engines, could revolutionize the present emission situation, achieving near-zero emissions (i.e., >80% reduction in GHG emissions compared to low-sulfur fuels). Pilot diesel-type ammonia combustion marine engines can increase thermal efficiency by 15.8%, and reduce unburned ammonia and N₂O emissions by 89.3% and 91.2%, respectively, compared to older generation ammonia engines, and reduce carbon footprint and GHG emissions by 97% and 94%, respectively, compared to current diesel engines [135]. According to very recent industrial claims (yet to be validated by published scientific studies) [57], similar progress has now been introduced in passenger car engine production. This would likely change the GHG emission framework reference in the transportation sector and the need for a revision of regulations on their mitigation.

6. Conclusions

The current drive for the adoption of ecofuels in the transportation sector is driven by three crucial issues: ecological (pollution reduction and climatic concerns), economic (fossil resources depletion and raising energy costs), and strategic issues (energy security and consumption sustainability). At the same time, efforts to achieve fossil substitution are hindered by serious limitations: the first is the huge amounts of fossil fuel currently used, far more than can be totally replaced at the moment by any type of energy carrier under present technology. The second consists of the economics of ecofuel production, which has not independently reached positive economic leverage over conventional energy carriers due to technological reasons. The third lies in the sometimes hasty politically (ideologically?) oriented decisions that are announced without sustainable long-term strategies, and rashly implemented without considering possible alternatives. The possibility to harvest energy into different carriers from a multitude of locally accessible renewable and waste sources should become a key strategy for resilient, sustainable global energy management. It is, in fact, difficult to accurately predict technological evolution, as demonstrated by recently announced breakthrough progress on non-conventional ICE technology, and to revive abandoned industrial sectors.

Rather than putting all the chips on a single bet (i.e., mobility electrification) to eliminate ICE-based mobility altogether, decisional authorities should consider multiple-fold, resilient strategy approaches that could include gradual fossil fuels substitution with various types of locally sustainable ecofuels and develop technology-neutral strategic energy transition pathways of the main transport sectors towards decarbonization beyond 2030. A long-term transition to ecofuels from RESs and the biogenic or direct capture CO₂, or from bio and non-bio waste feedstocks, would require large-scale infrastructure in the proper locations. For all ecofuel types, feedstock availability is the main bottleneck, causing a substantial upstream value shift in the production chain. To overcome this, needed actions to increase ecofuel sustainability against traditional energy carriers could include providing a level playing field for the ecofuel sector; safeguarding the sustainable production of ecofuels with the uniform categorization of feedstock for advanced ecofuels and long-term biomass management policies, avoiding regulation inconsistencies, and adopting well-to-wheels emission accounting policies with a holistic view of all factors and processes that contribute to GHG emissions.