Ensuring the Climate and Environmental Integrity of Alternative Fuels

Abstract

Public and private investments in alternative fuelsmolecular energy carriers with potentially lower net greenhouse gas emissions than conventional fossil fuelsare rapidly expanding due to their perceived decarbonization potential. However, alternative fuels like hydrogen, ammonia, and methanol are not inherently “good” simply because they can be produced with low-carbon methods. Their climate and environmental impacts vary widely depending on how they are produced, handled, and usedfactors that most assessments fail to fully account for. This can result in poorly informed decision-making, with overestimated benefits of deployment choices. In this Perspective, we highlight potential environmental impacts from alternative fuel value chains and provide an actionable guide for their responsible deployment. Specifically, we provide a brief overview of alternative fuel attributes, assess climate and air quality implications across production, handling, and use, and discuss technological, political-economic, social, and other environmental considerations for responsible deployment. We recommend improvements to alternative fuel assessments and suggest safeguards to maximize their climate and environmental integrity, including best-in-class practices and enabling policies. Given the urgency of achieving global climate goals, it is critical to ensure that scaling up of alternative fuels adheres to sustainable practices from the outset.

Introduction

The energy transition from fossil fuels to alternative fuels is a key component of decarbonization strategies to achieve global climate goals. , Here, we define alternative fuels as molecular energy carriers with a potentially lower net greenhouse gas (GHG) emission intensity across their value chain, when compared to conventional fossil fuels. Alternative fuels include hydrogen (H₂), ammonia (NH₃), methanol, dimethyl ether (DME), oxymethylene ethers (OME _x ), ethanol, and synthetic forms of methane (CH₄), diesel, kerosene, and gasoline (where the latter are produced from nonfossil carbon sources).

Although electrification with renewable energy is often the most efficient mechanism to significantly reduce GHG emissions for many applications like short- and medium-haul transportation and most building heating needs, it is not a feasible decarbonization approach for all energy systems. Alternative fuels are a promising decarbonization option where electrification is limited, such as in regions with insufficient grid infrastructure or where renewable electricity deployment is challenging, and for energy-intensive applications. Such high-energy density applications include energy storage (i.e., storing excess clean energy for later use), long-haul shipping and aviation, and production of very-high-temperature heat (>1000 °C), as in steel and cement manufacturing. − Some alternative fuels can also serve as low-emission chemical feedstocks. While global investments in alternative fuel production doubled from 2021 to 2023, , limiting global warming to less than 2 °C above preindustrial levels would require a 10-fold increase in these investments by 2030.

However, alternative fuels are not inherently beneficial to the environment and communities. Although they have the potential to considerably reduce carbon dioxide (CO₂) emissions from their production and combustion compared to conventional fossil fuels, , their impacts extend beyond CO₂. − Each step in a fuel’s value chain can impact the climate and air quality through a variety of GHG and air pollutant emissions, as well as the health of communities and the environmentland, water, and biodiversitythrough unsustainable or socially inequitable practices. − Specific value chain impacts depend on how alternative fuels are produced (e.g., how resources are obtained and used; what energy source is used), handled (e.g., how they are transported and stored; how emissions are mitigated), and used (e.g., what are the energy losses and combustion emissions). Therefore, depending on specific value chains, shifts to alternative fuels could yield a range of benefits or harms to the climate and the environment relative to incumbent fossil fuels. ,,

Understanding the full suite of impacts is crucial for informed decision-making. However, a comprehensive system-level understanding of their environmental and social impacts across the entire value chain remains limited, and no integrated framework exists to evaluate these impacts collectively. For example, prior studies have examined specific aspects of alternative fuel systemse.g., life-cycle emissions of certain pollutants, − resource constraints, ,, and economic feasibility. ,,, And existing reviews have focused on specific technologies or technology sectors, − modeling practices (e.g., integrated assessment and energy system models), environmental impacts, , or political-economic challenges − for specific fuels.

This Perspective highlights the full scope of potential climate, air quality, and other environmental and social impacts of alternative fuel value chains, and offers actionable, science-based recommendations to guide future assessments and decision-making processes that support responsible deployment. First, we provide an overview of the alternative fuels receiving the most commercial attention and their feedstocks and production pathways. Next, we describe the climate and air quality implications of their various value chains and summarize technological, political, economic, social, and other environmental challenges and barriers to widespread deployment. In the final section, we recommend best-in-class practices and policy actions to ensure that alternative fuels deliver maximum climate and air quality benefits while preserving the environment and supporting people’s health and livelihoods.

Overview of Alternative Fuels

Alternative fuels are molecules manufactured using various feedstocks and energy sources through thermochemical, electrochemical, or biological processes. , These fuels differ in their physical, chemical, and biological/environmental properties, which influence their energy efficiency, storage and transportation requirements, fire risk, and potential environmental and health impacts, ultimately determining their suitability for specific end uses. Supporting Text 1.1 and Table S1 summarize these properties for a range of alternative fuels.

There are four dominant alternative fuel production pathways: (1) water- and renewable-based, commonly referred to as “green” and “electro” (“e–”); (2) fossil fuel-based with carbon capture and sequestration (CCS) technologies, commonly referred to as “blue”; (3) biomass-based, which we term “bio”; and (4) nonrenewable waste from recycled plastic materials, herein referred to as “plastic-based”. While other nonrenewable waste materials (e.g., rubbers and synthetic textiles) can be used for fuel production, they have different chemical compositions and processing requirements and are not addressed here. Notably, alternative fuels become conventional fuels when produced via carbon-intensive pathways using fossil fuels without CCS. , These GHG-intensive fuels, commonly called “gray fuels,” as well as byproducts of fossil fuel extraction or refining, such as liquefied petroleum gas (mainly composed of propane and butane), are not considered in this work. However, their emerging alternative variants derived from nonfossil fuel sources, like biopropane generated from biomass, are indirectly included. Figure , Supporting Text 1.2 and Figures S1 and S2 provide more detail on the production pathways of alternative fuels from initial feedstocks (e.g., water, fossil fuels with CCS, and biomass) and intermediate feedstocks (e.g., hydrogen, methanol, methane, and ethanol).

1.

Main alternative fuel production pathways. (A) Possible green, blue, and bio production pathways (green, blue, and violet, respectively, in this figure) for several alternative fuels. The initial feedstocks for these pathways are water, fossil fuels (with CCS), and biomass, respectively. Intermediate feedstocks for the green, blue, and bio pathways include hydrogen, methanol, methane, and ethanol, which derive from the initial feedstocks. N₂ from air and captured CO₂ from a variety of sources are used to produce N- and C-based fuels, respectively. The energy sources for these pathways include on-site renewables, fossil fuels, biomass, electricity grid, and process heat. Electrofuels are green fuels produced entirely using renewable electricity (see Figure S2). (B) Main direct production pathways for synthetic diesel, kerosene, and gasoline from various biomass types. (C) Main plastic-based fuel pathways. Pathways in panels B–C utilize various energy sources, including fossil fuels, biomass, electricity grid, and renewables. Standard conversion processes are summarized close to the arrows and in Table S2. Additional figure caption information can be found in the Supporting Information 2. ATR: autothermal reforming; A­(bio)­MR: autothermal (bio)­methane reforming; conv: conversion; DME: dimethyl ether; F-T: Fischer–Tropsch; FAME: fatty acid methyl ester; HEFA: hydroprocessed esters and fatty acids; H–B: Haber-Bosch; HCs: hydrocarbons; HVO: hydrotreated vegetable oil; OMEx: oxymethylene ethers; RWGS: renewable water gas shift reaction; sep.: separation; S­(bio)­MR: steam (bio)­methane reforming; SPK: synthetic paraffinic kerosene.

Because of the diversity of feedstocks and production pathways, there could be green, blue, bio, and/or plastic-based variants of the same fuel (Figure ; Table S2). For example, H₂ can be derived from water, natural gas with CCS, biomass, or plastic waste. Additionally, other production pathways are being developed, such as H₂ produced from (bio)­methane pyrolysis or extracted from geologic reservoirs. − Due to this variation in production pathways, the same fuel can have a large range of emissions and environmental impacts. We refer to emissions as the suite of (direct and indirect) GHGs and conventional air pollutants, although most attention in the literature has been focused on CO₂ emissions alone.

Alternative fuels are considered for various decarbonization applications in the transport, industry, and energy sectors (Figure S3) due to their high volumetric energy density (Figure S4 and Table S1), CO₂-free combustion, and other advantages. Currently, most green, blue, and plastic-based fuels are at early or pilot stages of commercial maturity (Figure S3), primarily due to scalability and cost barriers. Supporting Text 1.2–1.3 summarizes the uses and advantages of different alternative fuels and briefly describes their commercial maturity.

Climate and Air Quality Impacts of Alternative Fuel Value Chains

While low-emission variants of alternative fuels generally reduce climate impacts as compared to conventional fossil fuels, the magnitude of their net climate impacts depends on their entire value chainhow these fuels are produced (production pathway), handled (transported and stored), and used (Figure ). These impacts are driven by emissions of both direct (e.g., CO₂, CH₄, nitrous oxide [N₂O]) and indirect (e.g., H₂, NH₃) GHGs at each stage of a fuel’s value chain. , The net climate impact, assessed across the entire value chain and relative to conventional fossil fuel use, determines how any alternative fuel choice will contribute to achieving climate goals. While typically evaluated through a life-cycle assessment (LCA), , there is a need for improved alignment between LCA outcomes and policy goals, particularly as it relates to temporal trade-offs that are not considered within traditional LCAs. For instance, using Global Warming Potentials (GWPs) with a single time frametypically 100 years despite significant effects from both short-lived and long-lived climate pollutantscan obscure the trade-offs between short- and long-term climate objectives. ,

2.

Climate and air quality impacts of main alternative fuels. The implications apply to all fuel variants unless indicated by the color (see legend). The pathways or variants assessed are indicated with colors left to the fuel name. See Supporting Information 2 for more detailed information on the variants considered for each fuel or group of fuels. CCS: carbon capture and storage; emis.: emissions; HC: hydrocarbons; HCl: hydrogen chloride; HM: heavy metals; LUC: land use change; NO _x : nitrogen oxides (NO + NO₂); N₂O: nitrous oxide; PM: particulate matter; PAH: polyaromatic hydrocarbons, PCBs: polychlorinated biphenyls.

Similarly, alternative fuels can impact communities through emissions of air pollutants along the value chain, including nitrogen oxides (NO _x ), sulfur oxides (SO _x ), volatile organic compounds (VOCs), and particulate matter (PM). Compared to climate impacts, air quality impacts are more localized, often disproportionately burdening frontline communities, and exacerbating existing inequities in air quality and public health.

Production: Impacts of Feedstock Sources

Fossil Fuels

Although alternative fuels can be produced with low-emission feedstocks, fossil fuels are currently the primary feedstock due to their cost-effectiveness and wide availability; when produced without CCS, these fuels are conventional rather than alternative. For example, 99% of today’s H₂ production relies on natural gas or coal without CCS. Minimizing the GHG intensity of alternative (blue) fuel production requires maturation and large-scale adoption of CCS, which demands increasing capture efficiency, ensuring long-term CO₂ storage, and addressing transport and storage infrastructure challenges. − Additionally, reducing methane emissions from natural gas supply chains is essential to reducing the GHG intensity of blue fuels.

CH₄, an intermediate fuel feedstock that is also the main component of natural gas and a byproduct of coal extraction, is a potent GHG with both direct and indirect warming effects (i.e., by absorbing infrared radiation and reacting with hydroxyl radicals in the atmosphere, which increases concentrations of tropospheric ozone and stratospheric water vapor). Per kilogram emitted, CH₄ from fossil sources has around 83 times the warming power of CO₂ over the 20 years following its release and 30 times the warming power over 100 years. Thus, unintended and operational CH₄ emissions from natural gas value chains, as well as from coal extraction, increase climate warming. , For example, a 2% CH₄ leakage rate by volume from natural gas supply and use increases the climate impact of blue H₂ production by approximately 92% over a 20 year horizon or 34% over a 100 year horizon, relative to an emission rate of 6.2 kg CO₂/kg H₂ assuming zero-CH₄ leakage and 85% CCS. Similar concerns around CH₄ leakage apply to biogas (a combination of CH₄ and CO₂) or “renewable natural gas” (RNG) (Supporting Text 1.4). Consequently, methane emissions monitoring and mitigation is critical to ensure the climate benefits of methane-based alternative fuels are realized.

CO₂ Recycling

Carbon-based e-fuelsmethanol, DME, OME _x , CH₄, ethanol and synthetic diesel, kerosene, and gasolinerequire CO₂ as feedstock along with H₂ for their production. Sources of CO₂ include flue gases from power plants (using fossil fuels or biomass), waste CO₂ from industrial processes, biogenic sources (e.g., municipal solid waste-based biogas production and cellulosic ethanol fermentation), and air via direct air capture (DAC). , Capturing CO₂ from these sources for reuse as a feedstock is commonly known as CO₂ recycling. , However, it is challenging to account for the net reduction in CO₂ emissions for alternative fuels containing recycled CO₂. For example, using fossil-derived CO₂ from a power plant or industrial facility still contributes directionally to climate warming because fossil-based carbon from underground reservoirs will be eventually released into the atmosphere when the fuel is combusted. Potentially carbon-neutral CO₂ sources include DAC and biogenic wastes that release no more CO₂ than would be emitted naturally over time. , Moreover, the additional energy and infrastructure build-out required to scale DAC can offset part or all of its potential benefits. , Addressing these challenges is essential to accurately account for the full GHG footprint of CO₂ feedstocks used for carbon-based e-fuels.

Biomass

Differences in biomass sources for CO₂ recycling and biofuel production add further complexity to the climate assessment of alternative fuels. The accounting for net emissions is highly influenced by the spatial scale at which it is done and changes in the rate of harvesting, as many biomass systems involve moving activities across a landscape. For example, carbon stocks (i.e., the amount of carbon stored) within a harvested forest stand might drop precipitately but not be affected across the landscape, as harvesting generally happens using a rotational system with regrowth equal to or greater than the rate of harvest across the landscape. However, even when harvesting and regrowth are in balance, the climate impact can still be significant if biomass sourcing leads to land use change.

Sourcing biomass that diverts food crops or competes for resources such as land, water, and fertilizer can lead to direct and indirect land use change (LUC and ILUC, respectively). , Land use changes, whether direct or indirect, can trigger deforestation or other forms of ecosystem conversion that result in the release of stored CO₂ and CH₄ from soil and vegetation. ,, For example, the GHG emissions from sugar cane-derived bioethanol can be up to 60% higher than those of gasoline when the effects of deforestation are included. , In contrast, bioethanol from agricultural and forest residues could have 70–90% lower emissions than gasoline. , Moreover, displacing the use of waste biomass could indirectly contribute to GHG emissions (Supporting Text 1.4).

Plastics

Pyrolysis and gasification of plastics to make plastic-based fuels require multiple processing steps that generate various climate and air pollutants, including CO, CO₂, CH₄, VOCs, PM, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), heavy metals, hydrogen chloride, and sulfur dioxide. , The processes involved could result in higher emissions when compared to landfilling, depending on the plastic type, technology, and process conditions. For example, under typical pyrolysis and gasification conditions, GHG and PM emissions can be 3–8 and 8–40 times higher, respectively, than landfilling. Moreover, the combustion of waste plastic oils (plastic-based diesel) released higher levels of NO _x , CO, and HC than conventional diesel combusted in the same engine, though the smoke index was lower due to reduced aromatic content in pyrolysis oils and, thus, less soot formation. , Moreover, fossil-fuel-derived plastics carry a carbon burden due to the GHG emissions associated with the production and transportation of fossil fuels (Supporting Text 1.4).

Production and Handling: Impact of the Process Energy

Like feedstocks, the choice of energy source to power the processes involved in producing and handling alternative fuels influences a fuel’s net climate and environmental impact. Fossil fuel energy sources, like natural gas, coal, and crude oil, result in high life-cycle GHG emissions (∼400–1300 g CO₂e/kWh), in addition to air pollutant emissions. ,, Renewable energy sources, such as solar (thermal or photovoltaic), geothermal, and wind, have lower net GHG emissions than fossil fuels (2–300 g CO₂e/kWh, including the embedded energy for their manufacture, depending on the technology), , as well as minimal air pollution emissions. Bioenergy sources have a wide range of life-cycle GHGs (15–650 g CO₂e/kWh) and air pollutant emissions due to the diversity of bioenergy options and production methods (e.g., waste incineration, biomass gasification, and pyrolysis).

Accordingly, utilizing emissions-intensive energy sources, including an electricity grid with a heavy fossil fuel mix, in the production of alternative fuels can undercutand even negatetheir potential climate benefits. For instance, grid-based electrolytic H₂ production could generate up to twice the GHG emissions of fossil-fuel-based (gray) H₂ in regions with coal-heavy electricity grids, such as parts of the Western United States. By contrast, sourcing electricity from behind-the-meter clean energy resources or procuring additional/incremental carbon-free grid electricity with 100% hourly matching to electrolysis energy demand can maximize climate benefits. − However, with incomplete additionality, competition with electric grid decarbonization could potentially result in increased emissions from other grid-dependent uses. ,

Other impacts associated with process energy, such as the impact of long-distance transportation, are discussed in the Supporting Text 1.4.

Entire Value Chain: Impact of Energy Efficiency

The overall energy efficiency of alternative fuel value chains (i.e., the ratio of total energy a fuel yields relative to the energy needed to produce and deliver it) can vary substantially due to energy losses incurred at every value chain stage. These energy losses increase the energy input necessary to deliver a given amount of useable energy, which can contribute to increased GHG emissions and higher costs, particularly when compared to direct electrification with renewable energy. Direct electrification is generally more energy-efficient and less GHG-intensive than alternative fuels because it involves fewer transformations and associated energy losses. , For example, up to 90% of energy may be lost along the hydrogen and e-fuel value chain compared to <50% losses for direct electrification powering the same transportation application. ,

Consequently, less efficient alternative fuel pathways, particularly when used in applications that can otherwise transition to direct electrification (e.g., low-temperature heat and light-duty transportation), could result in higher net GHG emissions. , For energy-intensive applications where electrification is currently limited, such as long-haul shipping and aviation, the suitability of alternative fuels depends on trade-offs among fuel-to-power efficiency and emissions, energy density (Figure S4), cost, and compatibility with combustion and fuel-cell technologies.

Entire Value Chain: Climate Impact of Methane, Hydrogen, and Ammonia Emissions

Unintended and operational emissions of CH₄, H₂, and NH₃ across alternative fuel value chains reduce a fuel’s climate benefits, depending on the rate of those emissions. ,, Yet, these emissions are often overlooked or underestimated in LCAs, resulting in a significant underestimation of true climate impacts. ,,, As discussed above, methane emissions across (e−)­methane and other alternative fuel value chains and from its use as an intermediate feedstock can contribute to climate warming.

H₂ is an indirect GHG that warms the climate through its chemical oxidation with the hydroxyl radical in the atmosphere, which ultimately increases the concentrations of potent short-lived greenhouse gases (i.e., methane, tropospheric ozone, and stratospheric water vapor). , H₂’s climate impact is estimated to be 37 ± 15 times that of CO₂ over the 20 years following its release and 12 ± 3 times over 100 years. Currently, there is a lack of empirical data on the extent of H₂ emissions from existing infrastructure, with estimates varying by up to 100-fold, and real-world measurements just beginning. Recent studies show that minimizing H₂ emissions (<1%) maximizes the climate benefits of H₂ systems. ,, However, a 10% H₂ leak rate can reduce intended near-term climate benefits by up to 25% for several use cases.

Finally, ammonia is an air pollutant itself that undergoes natural chemical and biological transformations in the environment, resulting in the production of N₂O and other reactive nitrogen species. , Consequently, ammonia emissions across its value chain, including vented boil-off gas, leakage, spills, and engine slip, can contribute to climate warming and air quality deterioration. ,, If 0.4% of the nitrogen in ammonia fuel were transformed to N₂O (through natural and/or combustion processes), the climate benefits of switching from fossil fuels to low-emissions ammonia would be offset. ,, More research is needed to understand potential loss and environmental and atmospheric transformations of ammonia as an alternative fuel.

End Use: Impact of Fuel Combustion

The combustion of alternative fuels can emit CO₂ and air pollutants like NO _x , VOCs, CO, PM (including soot), and sulfur oxides (SO _x ) (Table S1). Emission rates depend on several factors, including a fuel’s hydrocarbon chain length, engine type, combustion conditions, and exhaust gas treatment technology (e.g., selective catalytic reduction [SCR]). Emissions of NO _x , VOCs, and other pollutants contribute to the formation of ground-level ozone and fine particulate matter (e.g., PM_2.5, aerosols), secondary pollutants that have adverse human health impacts, contribute to acid deposition, and affect radiative forcing. , Moreover, nitrogen dioxide (NO₂) is itself harmful to human health.

The combustion of all alternative fuels using ambient air produces NO _x (Table S1) due to the reaction of atmospheric nitrogen and oxygen at elevated temperatures (>1327 °C). NO _x production increases with combustion temperature; consequently, H₂ combustion can yield higher NO _x emissions compared to other fuels due to H₂’s elevated adiabatic flame temperature (∼2210 °C as opposed to 1950–1965 °C for methane, methanol, and ethanol in air). Additionally, the combustion of nitrogen-containing fuels like ammonia and refined fuels with nitrogen impurities (bio and plastic-based diesel/gasoline/kerosene) generates N₂O, a potent long-lived GHG (273 times more potent than CO₂ over 100 years following release) that also depletes protective stratospheric ozone in the atmosphere. Strategies to minimize these harmful combustion emissions are being explored.

The combustion of long-chain hydrocarbons like synthetic kerosene (i.e., biokerosene, e-kerosene, Fischer–Tropsch kerosene, etc.), which is largely used as sustainable aviation fuel, can produce soot particles such as black carbon (BC), , especially under poor combustion conditions. BC absorbs solar radiation and converts it into heat, which is transferred to the atmosphere. , However, soot emissions from synthetic kerosene with lower aromatic content can be significantly lower compared to conventional kerosene. ,

The use of alternative fuelshydrogen, ammonia, and methanolin electrochemical fuel cells is briefly discussed in the Supporting Text 1.4.

Other Challenges to Responsible Deployment of Alternative Fuels

Implementation of low-emission alternative fuels faces technological, political, economic, social, and environmental challenges and barriers beyond those described above (Figure S5).

Technological Challenges

Technological maturity is needed for widespread deployment of alternative fuels to ensure reliability, high efficiency, and cost-effectiveness, and to minimize environmental externalities. However, except for certain biofuels, many alternative fuels are still in the experimental or pilot stage (Figure S3, Supporting Text 1.2), requiring additional investment in research, development, and demonstration. Additionally, ensuring a reliable supply of feedstock that causes limited net GHG emissions is challenging due to its limited availability and/or high cost. ,, Finally, the current expansion rate of renewable energy infrastructure is insufficient in most regions of the world to meet grid decarbonization targets, a gap that is further exacerbated by the additional electricity demand necessary to support the most beneficial green and e-fuel choices (Supporting Text 1.5).

Political and Economic Barriers

Political and economic factors can affect the adoption rate and overall sustainability of alternative fuels. While regulations and incentives have been established in some geographies to promote the adoption of low-emission alternative fuels (e.g., the Renewable Energy Directive in Europe) and offset their current higher cost compared to fossil fuels (e.g., the European Hydrogen Bank auctions), challenges lie ahead to ensure policy frameworks achieve emission reduction goals. Despite ongoing efforts to establish certification and standardization schemes for calculating emission impacts of alternative fuels (see examples in Supporting Information 1.6), transparent emissions accounting and global harmonization of methodologies are not assured. Similarly, regulatory frameworks and economic incentives, such as blending mandates and subsidies, vary significantly across regions and sectors, resulting in inconsistent implementation. Moreover, while subsidies can boost alternative fuel production and use, they often distort market dynamics, diverting resources from more efficient climate strategies. For example, in situations where electrification with renewable energy is not similarly subsidized, there is a risk alternative fuel production may displace electrification’s ability to provide larger and quicker rates of decarbonization.

Other Social and Environmental Challenges

The social and environmental impacts of alternative fuels are defined by fuel properties (i.e., flammability, toxicity, and combustion emissions), how natural resources are sourced and utilized, and where production facilities and other infrastructure (e.g., refueling stations) are located.

In addition to their combustion emissions that deteriorate air quality and harm public health, alternative fuels vary in their flammability and acute toxicity levels (Supporting Text 1.2 and Table S1), posing different risks to people and the environment. Special handling and management (e.g., safety guidelines and a properly trained workforce) are required to minimize potential safety and health risks across the value chain of many alternative fuels. Some production methods, such as plastic pyrolysis, pose chronic toxicity concerns to both workers and nearby residents as they can produce highly toxic byproducts, including dioxins, phthalates, and per- and poly fluoroalkyl substances (PFAS). ,

Some fuel production pathways are water-intensive, potentially impacting local communities and ecosystems. , While green fuels use water as initial feedstock, conventional biofuels often require substantial amounts of water for processing and conversion, significantly exceeding the water needs of other fuels like green H₂. , The water consumption intensity to make 1 kg of H₂ can vary from 20 to 130 L (0.17–1.1 L-water/MJ), depending on the source of the electricity used. Moreover, producing jet fuel HEFA (hydroprocessed esters and fatty acids) from crops requires around 150–570 L-water/MJ, depending on the feedstock, which can be up to 3 orders of magnitude higher than the estimated water consumption for hydrogen-derived jet fuels. DAC using low-temperature adsorption can coproduce water during sorbent regeneration, which, if coupled with electrolysis, could partially offset the freshwater demand of green H₂ production. In addition, water demands for process energy vary by type. Grid electricity that relies on thermal power plants requires substantial water for cooling, whereas photovoltaic and wind power have relatively low water usage. ,

Similarly, biomass used as a feedstock or energy source does not inherently provide environmental or societal benefits, as it can result in reduced terrestrial carbon stocks and divert land from food crops. This diversion results in land clearing elsewhere to meet food demands and competition for resources such as water and fertilizer, potentially affecting food and water supplies and prices. Examples of more beneficial biomass sources are given in the Recommendations section.

Technologies across the hydrogen value chain increasingly rely on fluoropolymers, a distinct subset of PFAS. These materials are used in proton exchange membranes built into some electrolyzers and fuel cells, as well as in seals and gaskets used in H₂ storage and transport systems. , These materials are critical to the function of hydrogen infrastructure due to their electric, chemical, and thermal stability, water repellency, flame retardancy, and low gas permeability. However, growing concerns about the toxicity, bioaccumulation, and pervasiveness of PFAS have resulted in limits on their use. Efforts to develop alternative fluorine-free materials for use in hydrogen infrastructure need to be accelerated, and systems to safely recover used fluoropolymers and other fluorine-containing waste products are essential.

The impact of infrastructure on local communities and land use changes are briefly mentioned in Supporting Text 1.7. Site-specific environmental impacts and social risks are generally not captured in conventional LCAs, underscoring the need for more integrative assessment frameworks to enable a comprehensive evaluation of alternative fuel value chains.

Recommendations

Maximizing the climate and air quality benefits of alternative fuels requires sourcing climate-beneficial feedstocks and clean energy, minimizing emissions of direct and indirect GHGs and air pollutants across their value chains, minimizing materials transport, and maximizing energy efficiency (Figure S6A). Ensuring the overall responsible deployment of alternative fuels further demands addressing a wide range of technological, political, economic, social, and environmental challenges and barriers (Figure S6B).

To achieve these goals, adherence to continuously improving best practices by alternative fuel producers and consumers is essential. This, in turn, requires robust policy frameworks with environmental and social regulations (e.g., the EU Chemical Strategy for Sustainability and the Safe and Sustainable by-Design framework), , recognized standards and certifications, and comprehensive emissions mitigation programs. Their design should be guided by a comprehensive system-level assessment methodology, informed by the latest science/research and development, with thorough and transparent emissions accounting and consideration of social equity (Supporting Text 1.8). Public investment should prioritize fuel pathways with high environmental and social benefits but low technological maturity (Supporting Text 1.9). Figure illustrates how these components fit together.

3.

Framework for maximizing the climate benefits and sustainability of alternative fuel value chains. Yellow highlights represent our recommendations and the stakeholders responsible for their implementation. Gray highlights identify the key components necessary to ensure the climate and environmental integrity of alternative fuel value chains, along with the stakeholders responsible for their development.

Best Practices for Maximizing Climate and Air Quality Benefits

Drawing on existing knowledge and strategies, we compile established and emerging best-in-class practices that warrant widespread adoption. This can also be considered an actionable guide for alternative fuel producers and consumers, as well as public and private investors.

• Prioritize renewable electricity use that minimizes competition with other essential decarbonization strategies (e.g., decarbonizing electricity grids), for example, by sourcing electricity from dedicated on-site clean energy installations or procuring additional clean electricity that matches hourly demand.

• Minimize climate footprint of fossil fuels used for feedstocks or process energy: e.g., producing hydrogen via steam methane reforming with >95% carbon capture efficiency, permanent CO₂ sequestration with storage designed for 1000+ years, and sourcing natural gas from suppliers that verify low (<0.2% volume-based) methane emissions pathways.

• Ensure carbon-containing fuel production pathways do not significantly increase net CO₂ emissions directly or indirectly by sourcing feedstocks and process energy with verified carbon neutrality through recognized standards and certifications.

• Utilize biomass feedstocks that do not negatively impact food/nutritional security or increase greenhouse gas emissions through direct or indirect land use change. Climate-beneficial biomass options that limit these effects include agricultural and forestry residues (crop stalks, straw, small-size wood materials not useable for other products like pulp, and woody mill wastes), sewage sludge, and waste cooking oils.

• Ensure that renewable gas or biogas does not increase net methane emissions to the atmosphere by sourcing biogas from facilities currently emitting it, such as wastewater treatment plants, landfills, and anaerobic digesters.

• Avoid using feedstocks that result in the emission of toxic air pollutants during fuel production (including pyrolysis and gasification) or utilization, such as plastics.

• Limit the use of alternative fuels to high-energy-density applications (such as long-haul shipping and aviation) and in situations where electrification is not feasible or in geographies where renewable electricity or grid infrastructure is not easily developed.

• Mitigate unintentional and operational emissions (e.g., venting and purging) of hydrogen, methane/natural gas/biogas, and ammonia during their production, handling, and usewhether as a feedstock or energy sourcethrough the implementation of comprehensive emissions mitigation programs.

• Minimize emissions of PM, NO _x , N₂O, and other air pollutants during fuel production and use (e.g., combustion) by adopting cleaner production methods, optimizing fuel efficiency through advanced engine technologies, and implementing emission control and mitigation strategies, among other measures.

Elements of a Comprehensive System-Level Assessment Methodology

A comprehensive system-level assessment methodology of alternative fuel value chains should capture the full suite of potential climate, air quality, ecosystem, energy efficiency, and opportunity cost impacts. This requires incorporating factors typically beyond traditional LCAs. Specific elements to include are

• Emissions of all direct and indirect greenhouse gases (e.g., CO₂, CH₄, H₂, N₂O) throughout the value chain evaluated over both short- and long-term time scales (e.g., GWP20 and GWP100) to account for near-term warming effects of short-lived climate pollutants.

• Emissions of air pollutantsincluding PM, NO _x , VOCs, NH₃, SO _x and other hazardous pollutantsthat degrade local and regional air quality and can also impact ecosystems.

• A “cradle to grave” system boundary for all fuel types that includes emissions from infrastructure, production processes, energy sources, materials (including feedstock and products) transport, unintended and operational emissions, storage, and end use.

• For individual feedstocks and energy sources (e.g., renewable energy, corn), an evaluation of the opportunity costs that considers whether allocation to other energy or sustainability initiatives (e.g., grid electrification, commodity chemicals production) could yield greater overall benefits.

• For each end use, a comprehensive consideration of alternatives, such as direct electrification or other fuel pathways to select the option that maximizes sustainability benefits and minimizes associated risks.

The potential for alternative fuels to effectively contribute to decarbonization depends on the sustainability and efficiency of their production, handling, and end use. Establishing and adhering to comprehensive sustainability frameworks, as presented in Figure , can maximize the climate and environmental integrity of alternative fuels while minimizing the risk of fuel choices that lock in unintended consequences for the climate, communities, and the environment.

Biography

Sofia Esquivel-Elizondo is a Low-Carbon Energy Scientist with Environmental Defense Fund (EDF) Europe. She studies the climate and other environmental impacts of alternative fuels, with a focus on hydrogen-derived fuels and biofuels. Her work, bridging science and policy, fosters collaboration with academia, industry, and NGOs to support international efforts for the responsible deployment of low-emission fuel pathways – especially with regards to international shipping and climate policy. Dr. Esquivel’s scientific background spans biofuel and biomolecule production from wastes, microbial nitrogen and hydrogen cycling, and the environmental impacts of alternative fuel value chains. More recently, she has published studies on hydrogen leakage and potential reactive nitrogen emissions from ammonia as a marine fuel. Dr. Esquivel received a B.Sc. in Chemical Engineering from Universidad de las Américas Puebla, an M.Sc. in Bioprocesses from Instituto Politécnico Nacional, and a Ph.D. in Environmental Engineering from Arizona State University, where she conducted her doctoral research at the Biodesign Institute. She completed postdoctoral research at the Max Planck Institute for Biology, before joining EDF.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.5c07609.

• Description of alternative fuels properties, production pathways, uses, and other value chain impacts; additional figure captions; supplementary figures of fuel pathways, uses, volumetric energy density, and technological, political, economic, social, and environmental challenges; supplementary figure with general recommendations for climate integrity and responsible deployment; and supporting table of fuel properties (PDF)

• Summary table of main production pathways and conversion processes of alternative fuels (XLSX)

Conceptualization: S.E.-E., I.O., R.A.A., B.T., and S.P.H. Investigation and Writing-Original Draft: S.E.-E. with I. O.’s and R.A.A.’s feedback. Data Visualization: S.E-E and I.R. Writing-Review & Editing: S.E.-E., I.O., R.A.A., B.T., I.R. and S.P.H.

This research was conducted with support from the ClimateWorks Foundation. Results reflect the authors’ views and not necessarily those of the supporting party.

The authors declare no competing financial interest.