Sustainability Assessment of Alternative Energy Fuels for Aircrafts—A Life Cycle Analysis Approach

Abstract

Aviation is of crucial importance for the transportation sector and fundamental for the economy as it facilitates trade and private travel. Nonetheless, this sector is responsible for a great amount of global carbon dioxide emissions, exceeding 920 million tonnes annually. Alternative energy fuels (AEFs) can be considered as a promising solution to tackle this issue, with the potential to lower greenhouse gas emissions and reduce reliance on fossil fuels in the aviation industry. A life cycle analysis is performed considering an aircraft running on conventional jet fuel and various alternative fuels (biojet, methanol and DME), including hydrogen and ammonia. The comparative assessment investigates different fuel production pathways, including the following: JETA-1 and biojet fuels via hydrotreated esters and fatty acids (HEFAs), as well as hydrogen and ammonia employing water electrolysis using wind and solar photovoltaic collectors. The outputs of the assessment are quantified in terms of carbon dioxide equivalent emissions, acidification, eutrophication, eco-toxicity, human toxicity and carcinogens. The life cycle phases included the following: (i) the construction, maintenance and disposal of airports; (ii) the operation and maintenance of aircrafts; and (iii) the production, transportation and utilisation of aviation fuel in aircrafts. The results suggest that hydrogen is a more environmentally benign alternative compared to JETA-1, biojet fuel, methanol, DME and ammonia.

1. Introduction

The adverse impacts of the “climate crisis” have forced politicians to accelerate the implementation of measures to mitigate it. In this direction, Europe has committed to decarbonizing the economy through the European Green Deal (COM (2019) 640 final), which includes objectives to be realized by 2030. In order to reach the climate neutrality goals by 2050, it is necessary to develop green technologies, establish sustainable industries and reduce pollution. The global CO₂ (carbon dioxide) emissions in 2021 were approximately 37.12 Gt, of which 2.79 Gt CO₂eq alone was produced in the EU 27 [1]. Through measures taken in the EU, greenhouse gases (GHGs) have decreased by 31% between 1990 and 2020; however, projections of this trend indicate that the reduction in emissions is insufficient to meet the objectives set for 2030, i.e., a reduction of 55% compared to 1990 levels and carbon neutrality by 2050 [1]. Particularly, emissions from the transport sector have increased continuously (by 19% compared to 1990) against the trend of total emissions, exceeding the projected emission limit for 2030. In particular, CO₂ (carbon dioxide) emissions from transportation (including international aviation but excluding international maritime transport) remained 28% higher in 2017 than in 1990.

From the above, it is obvious that special efforts should be made specifically towards the reduction in CO₂ emissions in the transportation sector as it represents almost a quarter of Europe’s greenhouse gas emissions and is the main cause of air pollution in cities [1]. For this reason, EU member states are promoting the electrification of vehicles or hydrogen combined with fuel cells. However, as this option is only feasible in small vehicles, the use of alternative energy fuels (AEFs) is considered as a top priority in the most difficult sectors to decarbonize, such as the aviation sector.

According to flight tracking statistics for 2019, planes passing across civil platforms emitted 903 Mt. (megaton) CO₂ and 4 Mt. NOx, representing 89% of the total emissions of carbon dioxide and nitrous oxide [2]. In 2017, the International Air Transport Association (IATA) projected that by 2036, there will be 7.8 billion air travellers, almost twice as many as in 2016 [3]. The COVID-19 pandemic caused a 60% decrease in worldwide aviation traffic in 2020 compared to 2019 [4], although by the end of 2024, it is predicted to reach pre-COVID levels [5]. Significant efforts have been made over the past few decades to mitigate and decrease the environmental impact of aviation. These efforts include, inter alia, improving aircraft weight, airframe aerodynamics, engine cycle performance, etc. [6]. Recently, the European Parliament and the Council agreed on the ReFuelEU Aviation proposal—part of the Fit-for-55 package—which outlines that alternative fuels should comprise 70% of those in use by 2050 [7]. Many European countries have adopted the abovementioned; for instance, the French national low carbon strategy suggested that alternative jet fuels should be incorporated at a percentage of 2% in 2025 and up to 63% in 2050 [8]. Currently, the American Society for Testing Materiel (ASTM) has certified seven pathways through which alternative energy fuels (the majority of which are drop-in fuels) could be used in aircraft engines at different levels [4].

The concept of life cycle thinking is of major importance in the process of decision making [9]. LCA is an ISO-standardized methodology consisting of four mandatory steps. These steps are interdependent and iterative and include the following: goal and scope definition, life cycle inventory (LCI) analysis, life cycle impact assessment (LCIA) and life cycle interpretation [10,11]. The employment of an LCA allows for the assessment of the environmental impacts over the life cycle of a process. In this regard, the International Civil Aircraft Organization (ICAO) established the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) aiming to minimize aircraft greenhouse gas (GHG) emissions. Prussi et al. [12] assessed the life cycle GHG emissions of sustainable aviation fuels (SAFs) in the CORSIA system. Many studies have applied the LCA methodology to alternative fuel production pathways, mainly focusing on fuels used in the road transport sector [13,14,15]. For the aviation sector, a number of studies have performed LCAs on aircraft components (i.e., [16]), turbine engines [17], air transport fleets [18] and the production of SAFs (i.e., [19,20,21]).

Despite the fact that there are a number of studies investigating the usage of low-carbon-emission fuels in the aviation sector as well as the impacts of alternative production pathways of SAFs, there are a limited number of studies investigating the environmental impacts of alternative aviation fuels from raw material extraction and airport operations to fuel combustion in aircrafts. Therefore, the present study aims to perform a comparative assessment of different AEFs that can be used in the aviation sector from a life cycle analysis point of view. In this direction, this study tries to quantify the impacts—within the framework of a life cycle analysis—of combustion of JET A-1, methanol, dimethylethyl ether (DME), biojet fuels, hydrogen and ammonia in aircrafts. Furthermore, the environmental impacts of two production pathways (one via conventional sources and one via renewable energy sources) for the production of hydrogen and ammonia are also assessed, aiming to shed light on the transition towards the lowest possible environmental burden. The introduction points out the broad context and highlights the study’s relative purpose and its significance, Section 2 presents the systems under investigation as well as the methodology followed and Section 3 lists the results and discusses the main findings.

2. Materials and Methods

The methodological approach of this study is according to ISO 14040 standards consisting of the following: goal and scope definition (including the definition of the study objectives, system boundaries and functional unit); life cycle inventory (LCI), which quantifies the inputs to the system boundary and the outputs from the boundaries; life cycle impact assessment (LCIA), in which the environmental emissions are converted into life cycle environmental impacts; and improvements and interpretations. This step involves utilising the results as well as making necessary recommendations [10]. An LCA study includes various stages of a product’s or service’s life cycle, as well as the associated inputs and outputs. These stages can cover both initial stages of raw material extraction as well as stages regarding material processing, manufacturing, distribution, usage and disposal. The system boundaries (Figure 1) taken into consideration in this study include the following:

• Construction, maintenance and disposal of airports;
• Production, transportation and utilisation of aviation fuel;
• Manufacturing, operation and maintenance of aircrafts.

The functional unit utilised in this study is 1 tonne-km, which represents an airplane flight of 1 km with a load of goods equal to 1 tonne.

As fuels under study have different energy characteristics (

Table 1. Energy characteristics of the fuels under study [22,23,24].

Fuel	Specific Energy (MJ/kg)	Density at 15 °C (g/m3)	Energy Density (MJ/L)
JET A-1	43.2	0.808	34.9
Methanol	19.9	0.796	15.9
DME	31.7	0.735	21.1
BioJet—SAF	39	0.83	32.8
Liquid H2	120	0.071	8.4
Liquid Ammonia	18.6	0.73	13.6

), the same aircraft type is taken into consideration for fuel combustion and the same life cycle phases are assessed.

The present study uses the OpenLCA [25] software along with the Ecoinvent version 3.9.1 database [26], which includes life cycle inventory data on energy and material supply, resource extraction, construction materials, chemicals, metals, waste management services, transport services, etc. As a result, inventory data regarding airport construction/maintenance/operation and aircraft manufacturing/maintenance phases were obtained from the OpenLCA software database, while the rest of the data concerning remaining stages were developed from the literature data. Various methods of assessment have emerged over time to define the environmental flows of systems. In this work, LCA is carried out using the CML-IA technique (CML-IA V.4.8), which contains a set of impact classes and characterization methods for the assessment phase. The calculations were performed using CML-IA, with aggregation by weighted average and the elimination of zero data.

2.1. Airport

The stages of maintenance, construction and disposal are taken into consideration for the performance of the LCA. The inventory data for construction and maintenance of the airport take into consideration material consumption and energy expenditures related to the construction of sealed areas, such as aircraft parking lots, runways, etc., at airports [26]. As a reference airport, Frankfurt airport is considered. The foundation layer of the airport is 38 cm thick gravel with a life span of 100 years. The concrete floor is 24 cm with a concrete reinforcement of 1.9 kg steel/m² and a lifespan of 35 years. The distance for transport is 30 km. Transport services at the airport site are also taken into account. Additionally, land occupation and transformation are included in the inventory analysis. Building halls make up 70% of the built-up area, with the remaining 30% of buildings having an average height of 2.7 m and 4 floors. These assumptions are in accordance with previous studies [27]. Heat and electricity usage are part of the airport’s operating and maintenance life cycle. Natural gas is used to meet the airport’s heat requirements. The analysis also takes water usage into account. Current inventory analysis considers the airport’s energy infrastructure and energy consumption. It is pointed out that the transport services on the airport site are also taken into consideration. Furthermore, the inventory analysis also takes into consideration the land occupation and transformation.

2.2. Fuels

2.2.1. JETA-1

The present study considers Jet A-1 fuel kerosene, as it is utilised globally by the aviation industry. The majority of turbo engine aircrafts run on Jet A-1 fuel kerosene. The international standard AFQRJOS (Aviation Fuel Quality Requirements for Jointly Operated Systems) defines it as having a minimum freezing point of −47 °C [28].

Table 2. JET A-1 properties as indicated in previous studies.

Studies	[29]	[30]	[31]	[32]	[33]
Composition (% volume)	15–30% Aromatics	22.9% Aromatics (18.7% mono-aromatics/4.2% di-aromatics)	10–20% Aromatics	18% Aromatics 2.44% Bicyclic-aromatics	14.4% Aromatics
	70–85% Paraffins (iso, cyclo) and napthenes	77.1% Paraffins (n + iso) and naphtenes	50–65% Paraffins	22.15% n-paraffins 23.40% iso-paraffins 23.24% monocyclic-paraffins 9.25% bicyclic-paraffins 1.50% polycyclic-paraffins 13.46% n-, iso-alkylbenzenes 4.47% cyclo-alkylbenzenes	28.1% n-paraffin 38.8% iso-paraffin 1.2% Olefin 15.1% Naphtene
			20–30% Naphtene
			<5% Olefin

displays the composition of JET A-1 fossil fuel as presented in a number of previous studies.

The main stages of life cycle of JET A-1 include the following: i. all the refinery processes (including treatment of wastewater) associated with the production of bitumen, propane/butane, naphtha, kerosene, fuel oil and diesel; ii. transportation from the refinery to the end user; and iii. operation of petrol stations and storage tanks [26].

2.2.2. Dimethyl Ether

The simplest ether is dimethyl ether, sometimes referred to as methoxymethane, wood ether, dimethyl oxide or methyl ether. It is usually abbreviated as DME. At room temperature, it is a colourless, somewhat narcotic, non-toxic and extremely flammable gas; however, it can be handled as a liquid when lightly pressurized. Due to its high cetane number (compared to diesel), it is regarded as a promising fuel (characterized by high level of fuel ignitibility). As it lacks carbon-to-carbon bonds, the utilisation of DME as an alternative to diesel can alleviate the emissions of particulate matters and potentially eliminate the need for costly diesel particulate filters. DME can be produced directly from synthetic gas obtained by natural gas, coal or biomass. It can also be produced indirectly from methanol via a dehydration reaction. The production process in this study takes into consideration the dehydration of methanol via use of natural gas. The life cycle stages of the required raw materials, transportation and the final usage of dimethyl ether fuel in aircrafts are also taken into consideration [26].

2.2.3. Methanol

Currently, it is estimated that natural gas comprises approximately 65% of total methanol production [34]. There are also other available methanol production pathways mainly from renewable resources (i.e., forest waste, municipal solid wastes, wood, sewage and direct air capture). In the system under consideration, methanol is produced via steam reforming, obtaining syngas. There is no CO₂ use, and hydrogen is assumed to be burnt in a furnace. The system consists of five main units (reformer unit, SEWGS unit, membrane unit, methanol production unit and power plant unit). The life cycle of all chemicals (i.e., catalysts and sorbent), membranes, water and energy consumption, wastewater treatment and direct air emissions are taken into consideration in the inventory. In addition, emissions related to transportation and tank storage are included. The above-mentioned assumptions are aligned with those of previous studies [27]. Finally, exhaust and abrasion emissions are covered under the subject of particulate matter [26].

2.2.4. Sustainable Aviation Fuel—Biojet Fuel

Biojet fuel is a renewable, clean-burning biofuel with chemical properties similar to JETA-1. Sustainable aviation fuel (SAF) refers to a group of non-petroleum-based fuels (including renewably sourced fuel from non-biogenic precursors), which can include biofuels and synthetic fuels, developed to reduce GHGs in the aviation industry. Currently there are several pathways available for the production of SAF, including the following:

Sip or synthesized iso-paraffins from hydroprocessed fermented sugars: The method was approved in 2014 and involves sugar-based feedstock fermentation to produce hydrocarbons, which are subsequently converted into SAF. Typical feedstocks for this process include sugarcane and sugar beet.

HEFA or synthesized paraffinic kerosene from hydroprocessed esters and fatty acids: Approved in 2011, it converts lipids (oil and fat) into hydrocarbons for SAF production. Common feedstocks include waste oil, waste cooking oil, animal fat, soy oil and corn oil. This process has been extensively investigated and is fully commercially developed [35].

• Alcohol-to-Jet (ATJ) and Ethanol to-Jet (ETJ) processes: Approved in 2016 and 2018, respectively. The AtJ process converts alcohol feedstocks into a pure hydrocarbon fuel blending component by dehydration, oligomerization and hydroprocessing. Typical feedstocks for these processes are agricultural and forest residue, municipal solid waste (MSW) and energy crops like switchgrass.
• Fischer Tropsch Process: The process was approved in 2009, and it involves catalytic conversion of municipal waste and other feedstock (used in the AtJ/EtJ process to generate syngas) to liquid hydrocarbon.

In this study, the production of biojet fuel via the HEFA route is considered. This process includes, as illustrated in Figure 2, all processes (thermal hydrolysis, decarboxylation, hydrocracking and isomerization) to treat triglycerides from vegetable oil [36] derived from kernel residue. Fatty acid distillates (FADs) recovered from the stripping and deodorisation stages (5–10 wt%) during vegetable oil alkali refining contain unsaturated and saturated free fatty acids (C12 to C30) for increasing decarboxylation reactor yield [37]. Palm kernel oil is a promising feedstock for biojet fuel production, due to its lauric acid (C12H24O2) content, reaching an upper 90% conversion yield (A1 range hydrocarbons) and selectivity towards C10–C12 alkanes of approximately 58% [38].

FADs are made up of 93 wt% of free fatty acids (FFAs), while remaining components involve triglycerides, glycerides and unsaponifiable matters (vitamin E). The overall life cycle stages associated with biojet fuel produced through this method include the consumption of fuel in aircrafts. The process considered also includes glycerin as a coproduct.

2.2.5. Hydrogen

Hydrogen can be produced via thermal, electrical or biological processes. The most common fossil-fuel-based hydrogen production method is steam methane reformation [39] using methane and coal as feedstocks. The second most common method is splitting water using thermal or electrical energy. In this study, two routes are assessed for hydrogen production, including conventional methods as well as production via renewable energy sources [40]. The conventional technique considered is hydrogen production via steam methane reformer (Figure 3). The processes under consideration include natural gas production, steam reformation, water–gas shift reaction, hydrogen purification and cooling water delivery. Natural gas is processed and supplied to a facility, where it is compressed and heated before entering the steam reforming reactor, which operates at 850 °C and 25 bar and uses a commercial Ni-based catalyst. SMR is an endothermic process in which the hydrocarbon input reacts with steam to produce syngas mostly made up of H₂, CO₂ and CO.

As far as water electrolysis is concerned, in the case of hydrogen production via solar and wind energy, the composition is assumed to be:

• 50% mercury-based electrolysis cells;
• 25% diaphragm electrolysis cells;
• 25% membrane-based electrolysis cells.

The energy consumption is assumed to be 53 kWh/kgH₂. The above-mentioned are the European averages associated with water electrolysis [26]. The inventory data for all processes from raw materials to factory delivery are taken into consideration in the OpenLCA software.

2.2.6. Ammonia

Ammonia has the potential to be used as hydrogen carrier for on-site power generation via ammonia decomposition. Figure 4 illustrates the Haber process, which is the most common route for ammonia production [39]. The latter is an exothermic process that combines hydrogen and nitrogen in a 3:1 ratio to produce ammonia. The reaction is facilitated by a catalyst, and the optimal temperature ranges between 500 and 600 °C [41]. The impact of ammonia production depends on the methods used to produce hydrogen and nitrogen. The ammonia production route considered in the present study is via steam methane reforming, which is the most common process for hydrogen production. This is aligned with the European industrial ammonia production methods presented in OpenLCA software. The manufacturing process takes the following into consideration: emissions of natural gas (including air and auxiliary power), energy, transportation, infrastructure land use, waste, air and water. Transportation of raw materials, auxiliary materials and waste is included along with carbon dioxide (CO₂) produced. If the above-mentioned factors are not excessive, the production is considered stable. Discharge stream water is assumed to be discharged into rivers. This study also assesses renewable-energy-based ammonia production via solar and wind energy. Renewable-energy-based systems for the production of hydrogen and ammonia usually involve electrolysis, along with electricity supplied from solar photovoltaic (PV) collectors or wind turbines.

2.3. Aircraft

The aircraft weights 235 tonnes [42] and has 267 seats. Inventory data are sourced from 16 European Airbus production plants [26]. Inventory data include energy, water and material flows associated with the manufacturing process, as well as rail transportation of materials. Electricity consumption takes into consideration the required energy for obtaining deionized water. The manufacturing plant’s infrastructure is excluded. The transportation of different parts of the aircraft from varying production sites is also excluded from the study. The analysis excludes the stage of transportation of different parts of the aircraft from varying production sites The aircraft’s fuel consumption and related airborne emissions are critical for this study; thus, they are taken into consideration.

3. Life Cycle Impact Assessment

A life cycle impact assessment (LCIA) is implemented in order to demonstrate the impact on the environmental categories under investigation. Quantified mass and energy flows are allocated to impact categories using characterization factors [43]. Equation (1) is used to calculate the impact for a specific category (representing an environmental issue of concern):

ISc = ∑sMs × CFs

(1)

where:

• IS_c: Impact score for the environmental impact category studied c;
• Ms: Mass emitted or extracted from substance s;
• CFs Characterization factor of substance s for a specific impact category.

Emitted masses are multiplied by these factors and then are summed in each category to define an impact score (usually expressed in kg of equivalents of a reference substance). Global warming potential is a key performance indicator (KPI) for fuel production and is calculated in kg CO_2-equivalents This KPI helps in benchmarking the savings of GHG emissions when alternative fuels are compared to fossil fuels.

What is more, as GHGs released from the combustion of aircraft fuel include, inter alia, NOx, soot particles, and contrail cirrus clouds, the following impact categories were also assessed: global warming (GW); terrestrial acidification (TA); freshwater eutrophication potential (FEP); photochemical ozone formation (POF); particulate matter emissions (PM); freshwater ecotoxicity (FET); human carcinogenic toxicity (HCT); and land use occupation (LUO). The characterization and emissions factors specifically developed for aviation are used in the impact assessment.

The impact of human carcinogenic toxicity is associated with emission of toxic substances to humans in the environment and is expressed in terms of 1,4-dichlorobenzene DB equivalents. The impact of global warming represents the air emissions of greenhouse gases associated with climate change and is expressed in kg CO₂ eq. (equivalents). Freshwater eutrophication potential (FEP) is expressed in terms of kilograms of phosphate equivalents (kg PO4,3-eq), whereas photochemical ozone formation (POF) is related to the formation of ozone at the ground level of the troposphere caused by photochemical oxidation of volatile organic compounds (VOCs) and carbon monoxide (CO) in the presence of nitrogen oxides (NOx) and sunlight and is expressed in kgNOx equivalents. Particulate matter emissions indicate the potential for emissions and secondary formation of particulate matter (PM) and are expressed in kgPM. Freshwater ecotoxicity (FET) is related to the toxic effects of chemicals on an ecosystem, in this case, in freshwater ecosystems, causing biodiversity loss and/or species extinction and is expressed in 1,4-dichlorobenzene DB equivalents. Land use is related to the extraction of raw materials, production processes, agricultural land, area of industrial territory, landfill sites, incineration plant area, transport and use processes and is expressed in terms of m²a.

4. Results and Discussion

Based on the inventory data, the life cycle assessment of an aircraft running on conventional jet fuel and various alternative fuels is carried out. The assessment takes into consideration not only environmental but also performance parameters. The results are expressed as tonnes per km, standing for all aircraft classes regardless of the split between passengers and cargo payloads onboard. It is highlighted that the data were normalized, aiming to assess the magnitude of the impact of each category.

Figure 5 depicts the relative environmental impacts along the whole chain of the fuels under study (with conventional production pathways for hydrogen and ammonia). The fuel with the highest contribution to a specific environmental impact is marked; whereas the bars add up to 100%.

The overall life cycle GWP impacts indicate that despite the fact that biojet fuel produced by the HEFA process is a sustainable aviation fuel, the GWP impacts are comparatively high with a percentage of 27%, followed by methanol with 26%, DME with 16%, JET A-1 with 15%, ammonia with 14% and hydrogen with 2%. The GWP of biojet fuel is the highest, with 2.4 kgCO₂eq, followed by DME with 1.2 kgCO₂eq, JET A-1 with 1.08 kgCO₂eq, ammonia with 1.05 kgCO₂eq, methanol with 1 kgCO₂eq and hydrogen with 0.099 kgCO₂eq.

The high score of biojet fuel is related to the GHG emissions generated from the cultivation stage of oil seed crops (HEFAs), whereas 15% of the total GHG emissions are attributed to the conversion stage of hydroprocessing. Furthermore, the isomerization process is a carbon-intensive process. It is noted that the emission factors depend on the method of cultivation, the fertilizers used, the method of transport and the co-products. The production of ammonia—over its whole lifecycle- via the conventional production pathway accounts for 15% of GWP. This is associated with the steam methane reforming process, which is employed in conventional ammonia production in order to produce hydrogen. This process generates carbon emissions as byproducts, contributing to a high overall GWP.

Figure 6 illustrates the relative environmental impacts along the whole chain of liquid hydrogen and ammonia fuels. The conventional production route of hydrogen fuel via steam methane reforming contributes to 56% of the impact of global warming potential (GWP), whereas solar power production accounts for 27% and wind power production has the lowest contribution (17%). It is highlighted that the GWP of ammonia via SMR production is the highest, with 1.09 kgCO₂eq, whereas the GWP of hydrogen produced the conventional way is 0.099 kgCO₂eq. On the other hand, the GWP of ammonia produced using solar power is 0.52 kgCO₂eq and that of hydrogen produced using solar power is 0.049 kgCO₂eq. Finally, the GWP of ammonia produced using wind power reaches 0.34 kgCO₂eq and that of hydrogen reaches 0.03 kgCO₂eq.

The findings emphasize the need for investing in clean hydrogen production technologies in order to achieve lower life cycle environmental impacts. While the operational stage of hydrogen-powered aircrafts does not emit CO₂ emissions, it is critical to take into consideration the overall life cycle emissions. The same pattern also applies for the production of liquid ammonia. Wind-powered production technology contributes the least to the GWP, showcasing that the electricity generated from wind power for the electrolysis process is beneficial to the environmental impact; however, although electrolysis from renewable energy sources might reduce GHG emissions (as indicated by the GWP), other impact categories, such as terrestrial acidification (TA) and freshwater eutrophication potential (FEP), might increase.

The overall life cycle terrestrial acidification (TA) and freshwater eutrophication potential (FEP) impacts indicate that biojet fuel has a relative impact of 41% and 55%, respectively (Figure 5). Based on the above, biojet fuel seems to be associated with high percentages of sulfur oxides, phosphorus and phosphate emissions over its life cycle. Furthermore, while JETA-1 has a high life cycle TA impact (24%), its FEP impact is minimal (about 1%). DME has a high TA impact (21%) and a low FEP (15%). Thus, it is suggested to design and employ technologies that generate low phosphorous and phosphate emissions so as to achieve lower overall eutrophication potentials.

In regards to hydrogen and ammonia production (Figure 6), it is noticed that although conventional production via steam methane reforming contributes to the TA impact with percentages of 45% (hydrogen) and 44% (ammonia), the solar-based production route contributes significantly to the FEP impact with percentages of 59% (hydrogen) and 61% (ammonia). From this, it is obvious that despite the fact that the conventional production route leads to higher terrestrial acidification, the solar-based route results indicate significance impacts on freshwater eutrophication during its overall life cycle. Freshwater eutrophication is associated with phosphate and phosphorous emissions. Therefore, efforts should be directed towards reducing emissions coming from solar power pathways. Despite the fact that the contribution of natural resources to the global warming category through the electrolysis process is not significant, as natural resources reduce GHG emissions, their contribution to other impact categories (i.e., acidification and eutrophication potential) is significant.

The results of the impacts of the overall life cycle in terms of photochemical ozone formation (POF) and particulate matter (PM) indicate that biojet fuel has high impact scores in these categories, with percentages reaching 29% and 27%, respectively (Figure 5). On the other hand, even though the impact of JET A-1 on POF reaches 24%, its relative impact on PM is significantly lower (8%). It is also noticed that ammonia (produced via the conventional route) contributes with a percentage of 15% to the impact of POF and 43% to the impact of PM. This is associated with higher emissions of nitrogen oxides as well as non-methane volatile organic compounds (NMVOCs). The comparative life cycle analysis of hydrogen and ammonia (Figure 6) suggests that the conventional route of ammonia production contributes to their POF potential, with 33%. However, both conventional (38%) and solar-powered production (36%) contribute greatly to PM. In this way, it is evident that to attain a sustainable infrastructure that produces and utilises ammonia fuel through solar power production, it is crucial to decrease associated PM emissions.

In regards to the overall life cycle impacts of freshwater eutrophication potential (FEP) and human carcinogenic toxicity (HCT), it is noted (Figure 5) that JETA-1 as well as ammonia produced via the conventional pathway have a high level of impact, both with percentages of 27%. Based on this, it is recommended to find viable solutions in order to reduce or eliminate emissions associated with JETA-1 and ammonia life cycle pathways, including arsenic, barium, beryllium, cadmium, chromium, cobalt and lead. DME is found to contribute to HCT impact with a percentage of 51%, meaning that toxic emissions related to its life cycle should be reduced. Methanol contributes to HCT impact with 28%, whereas its relative contribution to FET is 14% (Figure 5).

Figure 6 indicates that the solar power production route for hydrogen and ammonia contributes significantly to the HCT impact with percentages reaching apx. 41%. The latter is related to the high amounts of life cycle emissions of compounds (i.e., cadmium, arsenic, barium, etc.). Conventionally powered production contributes with a percentage of 24%, indicating the need for the development of new manufacturing and solar cell production techniques. Finally, in regards to land use occupational (LUO) impact, it is noted that the solar-based production pathways (Figure 6) both contribute 41%, followed by the wind-based production pathways with percentages reaching appx. 35%.

Despite the fact that solar- and wind-based facilities are related to higher land use, they are generally employed in areas where large land areas are already available.

5. Conclusions

This study tries to shed light on the environmental impacts of an aircraft running on JETA-1 as well as on various alternative fuels (biojet fuel, methanol and DME), including hydrogen and ammonia, within a life cycle analysis. The comparative environmental impact assessment investigated different fuel production pathways including the following: the conventional refinery pathway for kerosene production; biojet fuel via hydrotreated esters and fatty acids (HEFAs); and hydrogen and ammonia, employing water electrolysis using wind and solar photovoltaic collectors.

The life cycle assessment (LCA) is credible as it adheres to international standards, constituting a valuable tool for the assessment of alternative energy fuels (AEFs). The LCA methodology can be customized to meet specific needs and criteria.

While adjustments are necessary, in order to comply with aviation fuel standards for alternative fuels, it is pointed out that long-term viability and environmental sustainability should also be achieved. In this way, they can be considered as viable solutions to the decarbonization of the aviation industry. The results of this study pinpoint innovations that need to take place in order to make the processes environmentally feasible from a life cycle point of view. To be more specific:

• Alternative energy fuels including hydrogen, methanol, DME and ammonia are environmentally friendly alternatives compared to kerosene.
• Amongst hydrocarbon fuels, biojet fuel produced via HEFAs has a high global warming potential, reaching 2.4 kgCO₂ equivalent per tonne-km.
• Both biojet fuel and JETA-1 exhibit higher POF emissions compared to other fuels. Biojet fuel, for example, has a life cycle POF of 0.0059 kgNOxeq/tkm, JETA-1 has a life cycle POF of 0.0049 kgNOxeq/tkm, and methanol has a POF of 0.001 kgNOxeq/tkm.
• Biojet fuel produced via HEFAs is associated with increased life cycle PM emissions reaching 0.00039 kgPM/tkm, whereas JET A-1 is associated with lower life cycle PM emissions (0.00012 kgPM/tkm).
• Despite the fact that ammonia and hydrogen are carbon-free fuels, they are associated with high emissions when they are produced via energy-intensive conventional processes like SMR.
• Hydrogen, even when produced via SMR, is associated with the lowest GHG emissions among the fuels under study, corresponding to 0.099 kg CO₂/tkm, whereas those of jet-fuelled aircrafts are 1.08 kg CO₂/tkm when their complete cycle is considered.
• The results pinpoint innovations that need to take place in order to make the processes environmentally feasible from a life cycle point of view. Nonetheless, it is imperative to further perform life cycle cost (LCC) analyses as well as to study the exergy of the systems in order to optimize the results in terms of economic and technical sustainability.