An Overview of the Sustainable Aviation Fuel: LCA, TEA, and the Sustainability Analysis ^†

Abstract

This paper investigates how the present paths support massive manufacturing by evaluating the existing state of sustainable aviation fuel (SAF) technologies, examining technology readiness levels (TRL), fuel readiness levels (FRL), costs, economic conditions, emissions, etc. This assessment summarizes major conclusions about bio-jet replacements for conventional jet fuels. In order for SAF to play a sustainable role, a full life cycle emissions assessment, techno-economic analysis (TEA), and discussions about the sustainability of SAF materials are required. The life cycle assessment (LCA) discusses the capability of SAF in cutting down emissions, TEA argues for its economic viability, and the sustainable supply of SAF feedstock is a third critical factor determining the sustainability of the industry. With all the analyses, this overview provides recommendations for the sustainable development of the SAF industry and calls on industry stakeholders to enhance cooperation.

1. Introduction

The impacts of aviation fuel production and application on land deterioration, water deficiency, carbon releases, and the long-term prosperity of aviation attract considerable discussions. The intricacy of those associations between the industry and its surroundings emphasizes the necessity of thoroughly investigating the technological scenarios to guarantee that the aviation sector contributes positively to universal energy demands. There are several challenges to reducing emissions in the aviation industry, including the availability, affordability, and sustainability of alternative fuels. As modified to access approximately identical volumetric (35.06 MJ L⁻¹) and gravimetric densities (43.1 MJ kg⁻¹) to those of Jet A1, it is assured that SAF is the most compatible with the current infrastructure and aircraft among all the alternative jet fuels [1]. A complete LCA is necessary for an exhaustive overview of SAF networks.

Meanwhile, net zero emissions by 2050 and the increasing occurrence of flights require SAF to be produced at a large scale and sold at a competitive price. A techno-economic study might evaluate the economic viability of ecologically sound agriculture. Additionally, the amount of interest in the marketplace encourages elaborate raw material evaluation and selection, and the continuous enhancement of manufacturing. Ultimately, effective schemes ought to be established to counteract price fluctuations. The paper concentrates on the following issues:

• Introducing the certified SAF technologies;
• Compiling techno-economic analysis and life cycle assessment;
• Quantifying the contemporary production state and the prospective market;
• Pinpointing key investigations on the sustainability of SAF.

Section 2 examines the primary producing approaches and some explorations, Section 3 explains how LCA discrepancies among various sources affect the technologies’ comparative studies, Section 4 conveys a thorough techno-economic assessment (TEA), Section 5 explores sustainable SAF feedstocks, and Section 6 concludes investigations and identifies promising areas. Figure 1 is the mind map of this paper, presenting detailed research aspects in each section.

2. Producing Approaches

2.1. ASTM Standards and SAF Technologies

Three ASTM standards are deployed when manufacturing and testing alternative aviation fuels (Table 1).

ASTM D4054 specifies the testing criteria for the alternatives, evaluating their substance and performance. The testing is divided into four levels to ensure the alternatives are compatible with the engine structure and that the aircraft would be as safe, durable, and reliable after the SAF input [2].

ASTM D1655 sets the mandatory criteria for aviation fuels. In addition, the standard acts as a thorough reference for manufacturers when integrating biomass resources into petroleum refineries and incorporating SAF into current infrastructures [3].

ASTM D7566 acts as a substance and properties guideline for unnecessary additional fuels before mixing with ASTM D1655-approved fuels [4].

Table 1. 11 SAF technologies accredited by ASTM [5,6,7].

Technologies	Certification	Feedstock	FRL	TRL	Blend Limit	Year
Fischer–Tropsch hydroprocessed synthesized paraffinic kerosene (FT)	ASTM D7566 Annex A1	Coal, natural gas, and biomass	6–7	6–8	50%	2009
Synthesized kerosene with aromatics derived by alkylation of light aromatics from non-petroleum sources (FT-SKA)	ASTM D7566 Annex A4	Coal, natural gas, and biomass	6–7	6–8	50%	2015
Synthesized paraffinic kerosene from hydroprocessed esters and fatty acids (HEFA)	ASTM D7566 Annex A2	Vegetable oils, animal fats, and used cooking oils	9	9	50%	2011
Synthesized iso-paraffins from hydroprocessed fermented sugars (SIP)	ASTM D7566 Annex A3	Biomass for sugar production	5–8	7–9	10%	2014
Alcohol to jet synthetic paraffinic kerosene (ATJ-SPK)	ASTM D7566 Annex A5	Ethanol, isobutanol, and isobutene from biomass	7–8	7–8	50% (Ethanol); 30% (Isobutanol)	2018; 2016
Co-hydroprocessing of esters and fatty acids in a conventional petroleum refinery (Co-processed biomass)	ASTM D1655 Annex A1	Vegetable oils, animal fats, and used cooking oils from biomass processed with petroleum	6–7	7–8	5%	2018
Catalytic hydro-thermolysis jet fuel (CHJ)	ASTM D7566 Annex A6	Vegetable oils, animal fats, and used cooking oils	6–7	4–6	50%	2020
Synthesized paraffinic kerosene from hydrocarbon-hydroprocessed esters and fatty acids (HC-HEFA-SPK)	ASTM D7566 Annex A7	Bio-oils from algae	6	5	10%	2020
Co-hydroprocessing of Fischer–Tropsch hydrocarbons in a conventional petroleum refinery (Co-processed FT)	ASTM D1655 Annex A1	Fischer–Tropsch hydrocarbons co-processed with petroleum	6–7	7–8	5%	2020
ATJ derivative starting with the mixed alcohols (ATJ-SKA)	ASTM D7566 Annex A8	C2–C5 alcohols from biomass	/	/	50%	2023
Co-processed HEFA	ASTM D1655 Annex A1	Fats, oils, and greases (FOG) co-processed with petroleum	/	/	10%	2018

Table 1. 11 SAF technologies accredited by ASTM [5,6,7].

Technologies	Certification	Feedstock	FRL	TRL	Blend Limit	Year
Fischer–Tropsch hydroprocessed synthesized paraffinic kerosene (FT)	ASTM D7566 Annex A1	Coal, natural gas, and biomass	6–7	6–8	50%	2009
Synthesized kerosene with aromatics derived by alkylation of light aromatics from non-petroleum sources (FT-SKA)	ASTM D7566 Annex A4	Coal, natural gas, and biomass	6–7	6–8	50%	2015
Synthesized paraffinic kerosene from hydroprocessed esters and fatty acids (HEFA)	ASTM D7566 Annex A2	Vegetable oils, animal fats, and used cooking oils	9	9	50%	2011
Synthesized iso-paraffins from hydroprocessed fermented sugars (SIP)	ASTM D7566 Annex A3	Biomass for sugar production	5–8	7–9	10%	2014
Alcohol to jet synthetic paraffinic kerosene (ATJ-SPK)	ASTM D7566 Annex A5	Ethanol, isobutanol, and isobutene from biomass	7–8	7–8	50% (Ethanol); 30% (Isobutanol)	2018; 2016
Co-hydroprocessing of esters and fatty acids in a conventional petroleum refinery (Co-processed biomass)	ASTM D1655 Annex A1	Vegetable oils, animal fats, and used cooking oils from biomass processed with petroleum	6–7	7–8	5%	2018
Catalytic hydro-thermolysis jet fuel (CHJ)	ASTM D7566 Annex A6	Vegetable oils, animal fats, and used cooking oils	6–7	4–6	50%	2020
Synthesized paraffinic kerosene from hydrocarbon-hydroprocessed esters and fatty acids (HC-HEFA-SPK)	ASTM D7566 Annex A7	Bio-oils from algae	6	5	10%	2020
Co-hydroprocessing of Fischer–Tropsch hydrocarbons in a conventional petroleum refinery (Co-processed FT)	ASTM D1655 Annex A1	Fischer–Tropsch hydrocarbons co-processed with petroleum	6–7	7–8	5%	2020
ATJ derivative starting with the mixed alcohols (ATJ-SKA)	ASTM D7566 Annex A8	C2–C5 alcohols from biomass	/	/	50%	2023
Co-processed HEFA	ASTM D1655 Annex A1	Fats, oils, and greases (FOG) co-processed with petroleum	/	/	10%	2018

2.2. A Brief Techno-Economic Comparison of SAF Technologies

Figure 2 presents a comparison of feedstocks and manufacturing procedures.

2.2.1. FT Versus HEFA

FT is the earliest SAF technology accredited by ASTM, via which the SAF manufacturing routes based on biomass (BtL) are established. The feedstocks for BtL vary from agricultural waste and wood residues to some microorganisms and MSW (municipal solid waste) [8]. The large-scale BtL (200MWth) produces SAF through bioconversion of gasified syngas to triglycerides (TAG). The expected baseline total capital investment (TCI) is 644 million USD, within the average range of 56 to 78 billion. The prices of FT have a relatively smaller discrepancy, ranging from 34 to 82 USD per gallon. That is because capital investment has a much greater impact on FT [9]. In comparison, the price of HEFA products fluctuates more widely (23–310 USD per gallon), enormously determined by the differentiated feedstock expenses. Meanwhile, it is much less influenced by energy costs suggesting that HEFA is the most effective SAF technology in terms of energy usage.

2.2.2. ATJ-SPK

The ATJ-SPK method generates alcohol from biomass waste, industrial wastewater, etc. Then, it employs and converts isobutanol and ethanol to synthetic paraffinic kerosene. The excessive selling price of alcohol intermediates is identified as the most significant hurdle for ATJ in improving the economic benefit [10].

2.2.3. SIP and CHJ

SIP fuels are restricted to a maximum 10% blending threshold because of their high viscosity. The blending degree could be lower in circumstances when viscosity exceeds allowable levels of the particular SIP fuel that is mixed. Catalytic hydro-thermolysis, also known as CH-SK or CHJ, represents the sixth category of fuel originating from fatty acids and triglyceride-based feedstocks. According to ASTM D7566 specifications, CHJ fuels necessitate a relatively stringent batch property requirement, with an aromatic content ranging from 8.4% to 21.2% by mass [11]. The deployment of SIP is now hampered by two major issues: the expensive feedstock cost and the huge energy consumption as well as the costliness of CHJ products due to the former.

2.2.4. Co-Processing Technologies

ASTM D1655 merely approves two co-processing routes [3]. Co-processed FT is the incorporation of FT into current petroleum refining operations. Thus, it retains the capability to process a variety of bio-oils and preserves the biodiesel properties at the same time. On the other hand, fats, oils, and greases could be co-processed with HEFA and then mixed with Jet-1. Co-processed HEFA is believed to be a co-processing route for merging sustainable materials and the HEFA process, and the product tends to be excluded from ‘drop-in’ aviation fuels [12].

The co-processing technologies intend to create SAF at a reasonable cost through utilizing the present refineries. In that way, it converts hydrocarbons from biomass or lipids into petroleum distillates. Following this, intermediates could deal with as much as 5% feedstock by volume. The technologies economic performance is often determined by the supporting facilities in the petroleum refineries.

2.3. Summary of the Primary Technologies

These technologies are ready for large-scale production, and substantial expenditures would be necessitated. Investment in infrastructure, such as modifying existing refineries and establishing up-to-date facilities in new manufacturing plants, is essential. Moreover, a transport network should be developed to distribute SAF products to airports world-wide [10].

2.4. Technological Exploration

Algae, bio-based polycyclic alkanes, industrial waste gas, and lignin are the mainstream feedstocks explored for SAF. Biomass feedstocks must be deoxygenated and hydrogenated before they are converted. Photofermentation converts nitrogen from wastewater into ammonia and “biohydrogen” [13]:

N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi

(1)

Its constraint is the poor conversion efficiency of hydrogen. When the carbohydrate sources are replaced with volatile fatty acids, the biohydrogen output could increase because of the higher substrate conversion efficiency [14]. Acetic, lactic, propionic, and butyric acids are some representative alternatives. As for the acetic acid route, silver nanoparticles can function as catalysts during the metabolic process. They could increase the efficiency of acetic acid fermentation while decreasing ethanol synthesis [15].

3. The LCA of SAF

3.1. Methodology

An LCA is a technique that measures and assesses the ecological costs related to the usage of resources and waste disposal over a whole life cycle [16]. ‘From cradle to grave’ measurements are often adopted for LCA, which commences with the cultivation of biomass (micro-algae, super bamboo reeds, etc.) and recycling of waste (municipal and industrial waste, etc.). During the feedstock’s conversion and the fuel’s combustion, elements like GHG emissions, the consumption of resources (water, land, etc.), and the economic benefits are determined. The life cycle ends when SAF is disposed of in an environmentally friendly way. Evaluations of ecological influence are carried out for an array of actions, using diversified techniques. The quantification of emission indicators including CO₂, NO_x, and SO₂ decides the effects on the surroundings.

3.2. The LCA of SAF

The LCA of aviation fuels extends beyond carbon emissions when burning fossil fuels [17] to include the whole carbon footprint at all phases. This section adopts the GHG (greenhouse gasses) to measure LCA, which encompasses SAF production, sales, utilization, and disposal processes.

Table 2. The GHG(CO₂) emissions for technologies and a comparison with the conventional jet fuel.

Technologies Certified by ASTM D7566	GHG Emissions (gCO2eq MJ−1)	Multiples of Conventional Jet Fuel’s GHG Emissions
FT-SPK, FT-SPK/A	7.84	0.92
HEFA-SPK, HC-HEFA-SPK	65.22	0.27
SIP	42.02	0.53
ATJ	38.67	0.57
CHJ-SPK	20.58	0.77

lists the GHG emissions for different SAF technologies and compares the figures with that for the conventional jet fuel.

It should be emphasized that the commercially deployed HEFA processes predominantly utilize hydrogen sourced from fossil fuels for hydroprocessing, with a minimal contribution from green hydrogen. Technologies like FT and ATJ are capable of transforming a broader array of feedstocks into SAF. For example, Aemetis has secured a substantial agreement to utilize orchard residues in California for SAF production, while LanzaTech utilizes ethanol derived from off-gas of UK steel mills for SAF production [18]. Those partnerships play a significant role in diminishing GHG emissions throughout the entire life cycle of SAF.

4. The TEA of SAF

4.1. Methodology

A TEA quantifies, compares, and investigates producing approaches and identifies the best technical and economic performance. Incorporating commercial demands, the TEA generates the MJSP, indicating the economic feasibility and the sustainability of SAF technologies [16]. The TEA in SAF production usually involves cash flow analysis, sensitivity analysis, and technology assessment. Investigating the cost and profitability, cash flow analysis could assess the economic feasibility. Sensitivity analysis assists authorities and executives in comprehending essential parameters and mitigating risks. For instance, the cash flow analysis of FT technology indicates a baseline minimum jet sales price (MJSP) of 2 USD per liter, with the sensitivity analysis suggesting a price range of 1.54 to 2.53 USD per liter. Moreover, technology assessments examine technological approaches to identify the optimal path.

4.2. The TEA of SAF

Table 3. The average MJSP for technologies and a comparison with the conventional jet fuel.

Technologies Certified by ASTM D7566	MJSP Average (USD L−1)	Multiples of Conventional Jet Fuel’s MJSP
FT-SPK, FT-SPK/A	2.08	3.2
HEFA-SPK, HC-HEFA-SPK	1.12	1.2
SIP	3.99	7.0
ATJ	1.69	2.4
CHJ-SPK	1.30	1.6

employs the MJSP to evaluate the current economic dynamics of technologies, which signifies the breakeven selling price of SAF. The TEA for SAF manufacturing procedures regards the feedstock expenditure as a critical factor. Additionally, the expenses of the water, power, chemicals, and biocatalysts are taken into account.

Some SAF products, primarily derived from biomass, cost 1060–1133 USD/ton. That is comparatively affordable and competitive among all SAF products considering Jet A1 costs 670 USD/ton [19]. The economic feasibility up to 2040 is determined by 38% fiscal and 62% indirect measures like carbon taxes and SAF credits. After 2040, consumption-based initiatives are predicted to substantially raise demand for SAF and stimulate overall production, even more directly than the decrease in feedstocks [20].

Using Fischer–Tropsch synthesis, the variability in MJSP lies in the fluctuating costs of feedstocks. Tanzel et al. believe that the price of corn stover is approximately twice that of the second most expensive feedstock in FT evaluations. In contrast, SAF produced from bagasse, a residual product from sugarcane mills, results in a significantly lower MJSP [21]. In addition, a carbon credit of10 USD per ton of CO₂ release reduction helps lower the MJSP of SAF produced from FT technologies.

SIP products are more costly due to their high feedstock expense. Ranging from 2.78 to 4.8 USD per liter, the MJSP of SIP is generally higher than that of ATJ. The latter varies from under 1to over4 USD per liter. The lowest MJSP of ATJ is achieved by using corn to generate bioethanol in the US, which significantly diminishes the expenses associated with the feedstock and the manufacturing process. ATJ displays a broader disparity in the GHG emissions than that of SIP. The least amount of released CO₂ during an ATJ process is 17.5 gCO₂ MJ⁻¹, and the product accordingly has an MJSP (1.97 USD L⁻¹), 0.28 USD L⁻¹ higher than the average.

The unpredictable factors in a TEA include TRL and FRL, economic lifespan, ingredient prices, and technological investments. For instance, a price decline of 112 USD per ton in wheat grain (an ATJ feedstock) might reduce fuel production expenses by fourfold (447 USD per ton). Besides that, the carbon tax and other policy-related factors might narrow the cost difference between conventional fuels and SAF as well [22]. The production capacity is another changeable factor.

(cost of equipment)_x = (cost of equipment)_base·(capacity_x/capacity_base)^{scaling factor}

(2)

In 2027, the average capacity of SAF is predicted to rise 38-fold from that in 2010. Neste HEFA facilities in the Netherlands and Singapore initiated a roaring SAF capacity increase from 2010 to 2011. The Netherlands plants have continuously expanded ever since, and Singapore’s extension was put on hold until 2026 [23].

5. The Sustainability of SAF Feedstocks

To meet the sustainability criteria, SAF must:

• Reduce net greenhouse gas (GHG) emissions;
• Be produced with minimized freshwater;
• Not harm forestation, producing food, or diversified land use.

More specifically, the sustainability assessment of SAF refers to Jet-1 fuel standards. The concentration of aromatics indicates the GHG emissions, which is a critical parameter to determine the sustainability level. The standard for jet fuels is concluded in

Table 4. Sustainability measurements for jet fuel.

	Component	Range of Value
Composition	n-alkanes	26–13 wt%
	Iso-alkanes	37–19 wt%
	Monocyclic alkanes	19–30 wt%
	Bicyclic alkanes	3–17 wt%
	Sulfur	0.30 wt% (max)
	Aromatics	25 vol% (max)
Properties	Hydrogen to carbon ratio (H/C)	2.01–1.90
	Specific energy (MJ kg−1)	43.2–42.9
	Density @ 15 °C (kg m−3)	775.0–840.0
	Energy density (MJ L−1)	33.7–35.5
	Net heat of combustion	42.8
	Average molecular weight (g mol−1)	152–166
	Viscosity (cSt)	3.5–6.5
	Flashpoint (°C)	42–60 (Min.38)
	Freezing point (°C)	−47

[22].

Comparing SAF, Jet-1, and hydrogen fuel, conventional jet fuels and SAF have a substantially greater energy density than hydrogen fuel. Jet-1 is the most advantageous to manufacture and operate, followed by SAF, able to be substituted in without considerable alteration to up-to-date aircraft and equipment. BtL and hydrogen fuel could be created from biomass waste and effluent. The industrial technique produces minimum pollution to the environment. In terms of the sustainability of SAF feedstocks, the latter generation performs better. Microalgae, as the third generation, are appealing due to their capability of capturing CO₂ and producing TAG (triacylglycerol) and other neutral lipids. They are not a threat to food, as they survive in containers, wastewater, and saltwater [24]. The selection of microalgae strains has a strong influence on lipid production and quality. First, the lipids must be extracted before they can be converted into products. Second, one approach of transesterification, gasification, pyrolysis, and hydrotreating transforms the lipids into SAF-related products. For instance, the transesterification converts the lipids to biodiesel, followed by decarboxylation, deoxygenation, and finally isomerization. The process effectively reduces the freezing point to the standard of Jet-1 [24] and the remaining biomass could generate gas, oil, and ethanol [17].

6. Conclusions

Sustainable aviation fuel is an appropriate option because its application involves minimal changes to the aircraft. As the technology and apparatus evolve, the principal challenge now lies in scaling up production and achieving a competitive average price. Alternative feedstocks are cultivated, including algae, bio-based polycyclic alkanes, industrial waste gas, MSW, lignin, and so on. Moreover, cutting-edge conversion methods are under investigation to enhance efficiency and decrease expenses. Those refer to the improvement of catalysts, optimization of processes, bioconversion, pyrolysis, and gasification. Diversifying feedstocks and technological advancements are crucial for minimizing environmental repercussions and ensuring a continuous supply. Moreover, mass production of SAF requires alliances to address the economic and logistical hurdles. Expanding SAF production necessitates cooperation among various parties, such as feedstock providers, fuel manufacturers, and airlines.

This paper commences with certified technologies and some explorations, covering technologies that are most promising for large-scale applications. Despite future uncertainties in the feedstock prices, energy supply efficiency, etc. (see Section 2.2 and Section 4.2 for details), the processes are as comprehensive as possible so that the TEA made in this paper is relatively accurate. The LCA in this paper refers to the International Organization for Standardization (ISO)’s standardization. It adopts the ‘from cradle to grave’ model and measures GHG emissions in every stage between feedstock procurement and the ultimate fuel products. Cash flow analysis, sensitivity analysis, and technology assessment are a few of the elements that are methodically combined in TEA. The information aggregated from different technologies is imperative in assisting the producers and policy makers in eliminating operational costs and raising the feasibility of SAF. Future studies could build on this investigation and focus on examining the LCA of other alternative fuels to discuss commercial values and cultivation potential. Subsequently, this overview explores a third determinant issue for SAF—the sustainable supply of feedstocks. The selection of materials decides the adoption of producing technologies, acting as a directive resource for the advancement of certification standards and the formation of policy schemes [16]. In summary, this paper provides an overview of the four aspects of SAF, namely technologies, LCA, TEA, and the feedstock sustainability from the interactions between technical and economic dimensions. It aims to propose industrial orientations in the long run, under the condition of minimizing the impact on the environment. To achieve green aviation, joint efforts shall be promoted to exchange expertise and fund research and technological advancement more willingly and more effectively.