Comparison of Emission Properties of Sustainable Aviation Fuels and Conventional Aviation Fuels: A Review

Abstract

In order to achieve the International Air Transport Association’s (IATA) goal of achieving net-zero emissions in the aviation industry by 2050, there has been a growing emphasis globally on the technological development and practical application of sustainable aviation fuels (SAFs). Discrepancies in feedstock and production processes result in differences in composition between SAFs and traditional aviation fuels, ultimately affecting the emission performance of the two types of fuel. This paper discusses the impact of CO₂/NO_x/SO₂/CO/PM/UHC emissions from the aviation industry on the natural environment and human health by comparing the two types of fuel under the same conditions. Fuel combustion is a complex process in the combustor of an engine, which transfers chemical energy into heat energy. The completeness of combustion is related to the fuel properties, including spray, evaporation, and flammability. Therefore, engine performance is not only affected by fuel performance, but also interacts with engine structure and control laws. The CO₂ emissions of SAFs differ significantly from traditional aviation fuels from a lifecycle analysis perspective, and most SAFs can reduce CO₂ emissions by 41–89%. Compared with traditional aviation fuels, SAFs and blended fuels can significantly reduce SO₂ and PM emissions. Pure Fischer–Tropsch hydroprocessed synthesized paraffinic kerosine (FT-SPK) can reduce SO₂ and PM emissions by 92% and 70–95% respectively, owing to its extremely low sulfur and aromatic compound content. In contrast, the differences in NO_x emissions between the two types of fuel are not significant, as their generation mechanisms largely stem from thermal drive and turbulent flow in the combustor, with emissions performance being correlated to power output and flame temperature profile in engine testing. CO and UHC emissions are related to engine operating conditions and the physical/chemical properties of the SAFs, with no significant upward or downward trend. Therefore, SAFs have significant advantages over conventional aviation fuels in terms of CO₂, SO₂, and PM emissions, and can effectively reduce the hazards of aviation to the environment and human health.

1. Introduction

The reduction of carbon emissions is paramount for ensuring the sustainable development of the aviation industry [1,2]. According to the 77th IATA Annual General Meeting, IATA has committed itself to achieving Fly Net Zero by 2050. Among the various methods supporting sustainable development in the aviation industry, SAF technology contributes the most to carbon reduction. The Air Transport Action Group (ATAG) predicts, in its Waypoint 2050 report, that SAFs, aviation technology innovation, operational measures, and upgrades to technical infrastructure, as well as other measures (such as market mechanisms), will contribute 71%, 12%, 9%, and 8%, respectively, to the Fly Net Zero plan. Compared with other methods of reducing carbon emissions in the aviation industry, such as engine technology upgrades, operations and infrastructure measure improvements, there is significant room for progress in the development of SAF technology [3,4]. Technologies such as electric, hybrid, and hydrogen-powered aircraft face numerous technical barriers, including powerplant innovation, hydrogen storage and transportation, and airport infrastructure redevelopment, making widespread adoption by 2050 challenging. The advantage of SAFs lies in their ability to serve as a “drop-in” replacement, without requiring changes to existing engine and combustion chamber designs. SAF production in 2024 is expected to triple compared with 2023, reaching 1.5 million tons (0.53% of 2024 aviation fuel demand), with a projected increase to 62 million tons globally by 2030 [5]. Reports indicate that the global airline market will grow at a compound annual growth rate (CAGR) of 9.3% from 2022 to 2030, with China’s CAGR at 11.2% [6]. Given the aviation industry’s rapid growth and the significant volume of traditional aviation fuel usage, detailed understanding of the emission performance of SAFs is crucial to achieving Fly Net Zero.

Unlike conventional, petroleum-based aviation fuels, SAFs can be derived from energy crop biomass, waste oils, or solid agricultural residues [7,8,9]. These renewable materials can capture and sequester atmospheric CO₂ during their growth, creating a closed-loop carbon cycle upon combustion and thereby reducing net carbon emissions. From the perspectives of raw materials and preparation processes, nine bio-jet fuel production pathways have been certified according to ASTM D7566. Consequently, the composition of SAFs differs from traditional aviation fuels. For instance, the maximum mass fraction of aromatics in SAFs such as Fischer–Tropsch hydroprocessed synthesized paraffinic kerosene (FT-SPK), synthesized paraffinic kerosene hydroprocessed esters and fatty acids (HEFA SPK), synthesized iso-paraffins (SIP), alcohol-to-jet synthetic paraffinic kerosene (ATJ-SPK), and hydroprocessed hydrocarbons, esters, and fatty acids (HC-HEFAs) cannot exceed 0.5%, while the sulfur content must be less than 15 ppm [10]. In contrast, according to ASTM D1655, Jet A or Jet A-1 fuel allows for maximum aromatic volume fractions and total sulfur mass fractions of 26.5% and 0.3%, respectively [11]. The composition of the fuel not only closely correlates with combustion performance within the combustion chamber but also influences engine exhaust emissions [12,13,14]. This is a critical concern for engine manufacturers [15]. The low aromatic and sulfur content of SAF will reduce particulate matter and sulfur dioxide emissions, promoting cleaner combustion behavior in aviation engines [16]. In addition to compositional differences, SAFs and conventional aviation fuels exhibit some differences in physical and chemical properties. For example, according to ASTM D7566, the final boiling points of Jet A or Jet A-1 fuel and SIP fuel are limited to 300 °C and 255 °C, respectively. The viscosity ranges at 15 °C for Jet A or Jet A-1 fuel and FT-SPK fuel are 775–840 kg/m³ and 730–770 kg/m³, respectively. Differences in volatility, thermal stability, and viscosity result in SAFs exhibiting different atomization and combustion performance in existing aircraft engines compared with conventional aviation fuels. This can affect pollutant emissions such as CO, NO_x, and unburnt hydrocarbons (UHC) to some extent, although engine operating conditions have a greater influence.

Current research on SAF emissions has some shortcomings. Firstly, due to the high fuel consumption and costs during engine testing, there is limited research on fuel emissions performance at the engine level [17] and even at aircraft levels and reviews on engine-level pollutant emissions remain scarce. Secondly, compared with traditional aviation fuels, SAFs demonstrate significant advantages in emissions performance regarding SO₂ and PM, which is widely acknowledged. However, there is controversy regarding the differences in emissions performance for CO and NO_x between sustainable and petroleum-based aviation fuels. As CO and NO_x emissions are partially attributed to engine properties rather than solely to fuel [18], it is necessary to conduct synthesis and analysis. Thirdly, the effects of various aviation pollutants on human health are less addressed and should be given significant attention. Air pollution, while contributing to climate change, is also the greatest environmental threat to global public health, causing an estimated 7 million premature deaths annually, with 20% of global newborn deaths attributed to air pollution exposure. Air pollution exposure is linked to many major global diseases; for instance, it is associated with 20% of global ischemic heart disease deaths, and PM_2.5 air pollution can lead to stroke, heart disease, lung disease, lower respiratory diseases (e.g., pneumonia), and cancer [19]. Transportation is one of the largest sources of air pollution, with fossil fuel air pollution responsible for 18–21.5% of global deaths annually [20]. Studies indicate that excess mortality due to aviation emissions will be significantly higher in 2050 compared with 2006. However, considering improved fuel policies (widespread use of SAFs and technological and operational improvements), the number of deaths among adults aged 25 and older is projected to decrease by 72% in 2050 compared with a scenario with no fuel improvements [21].

According to simulation data on global commercial aviation emissions in 2020 and estimates of domestic aviation emissions in China from 2014 to 2019, the approximate order of pollutants from highest to lowest is CO₂, NO_X, CO, SO₂, PM and UHC (the specific order may vary between the two data sets) [22,23]. This paper will systematically evaluate the impacts of six aviation pollutants on both the ecological environment and human health in Section 2.1, Section 2.2, Section 2.3, Section 2.4, Section 2.5 and Section 2.6, while also comparing the performance of sustainable and traditional petroleum-based fuels in corresponding pollutant emissions (a small portion of the fuels mentioned in the article belong to biofuels rather than SAFs and are here for reference). The emission of CO₂ will be compared and elucidated using life cycle analysis (LCA), while the emissions of other pollutants will be contextualized at the engine level. A comprehensive comparison of emission performance between SAFs and traditional aviation fuels will be elaborated in Section 2.7. This paper may contribute to the understanding of SAF benefits on emissions performance.

2. Aviation Fuel Emissions

2.1. LCA of CO₂ Emissions

In 2019, the aviation industry contributed approximately 3% of global CO₂ emissions. Without swift technological advancements and enhanced policies, CO₂ emissions from aviation could increase 2.5× from 2015 to 2050 [24]. However, projections suggest that, by mid-century, this figure could increase to 4.6–20.2%, due to a projected annual growth rate of 5% in aviation activities. Aviation and road transportation emit 560–867 g and 45–102 g of CO₂/ton-km, respectively, with the former being approximately ten times higher than the latter [25]. CO₂ emitted from aircraft engines during complete fuel combustion is released directly into the upper atmosphere, playing a significant role in climate change [26,27]. CO₂, as the most prominent greenhouse gas (GHG), possesses the ability to absorb and re-emit thermal radiation (infrared radiation) and has a long atmospheric lifetime [28]. The increasing concentrations of CO₂ and other GHGs in the atmosphere lead to the absorption and re-emission of more infrared radiation emitted from the Earth’s surface, resulting in an overall increase in the retention of heat in the Earth’s atmosphere [29,30]. This phenomenon contributes to global warming and associated climate change, such as sea level rise, extreme weather events, and disruption of ecosystems. In fact, both CO₂ and non-CO₂ emissions from the aviation industry collectively contribute to its impact on the global climate system, with their combined effects causing surface warming. Emissions of CO₂, water vapor, NO_x, and soot, along with contrail cirrus formation, result in net positive radiative forcing (RF), thereby inducing warming, whereas sulfate aerosol production results in negative RF, thus inducing cooling. Persistent contrail cirrus clouds interfere with the Earth’s incoming solar radiation and outgoing infrared radiation, with the net effect being climate warming. NO_x emissions cause warming through short-term ozone increase and cooling through long-term reductions in ozone, methane, and stratospheric water vapor [31]. Lee et al. have found that the net effective radiative forcing (ERF) from aviation in 2018 was +100.9 mW/m², with major contributions from contrail cirrus, CO₂, and NO_x at 57.4, 34.3, and 17.5 mW/m², respectively. The CO₂ term accounts for a net positive (warming) ERF, representing less than half (34%) of the aviation net ERF in 2018 [32]. In terms of human health, elevated levels of CO₂ near the airport can lead to respiratory issues such as shortness of breath, exacerbation of asthma symptoms, and decreased cognitive function. Prolonged exposure to high concentrations of exhaust emissions may also contribute to cardiovascular problems and increased susceptibility to respiratory infections [33].

As described in the introduction, SAF stands as one of the pivotal methods for reducing carbon emissions in the aviation industry, albeit typically not achieved by altering fuel composition to lower CO₂ generation during combustion. For instance, under comparable conditions, the CO₂ emission indices (EI-CO₂) for FT fuels derived from natural gas, coal, and JP-8 fuel are 3090, 3110, and 3160 g/kg, respectively, with marginal differences [34]. Conventionally, the comparison of CO₂ emissions between SAFs and traditional aviation fuels is based on life cycle analysis (LCA), a method assessing the environmental impacts on human and ecological health caused by a system [35,36]. The system boundary of LCA is depicted in Figure 1, illustrating all inputs and outputs of various fuels (ATJ, HEFA, SIP, Jet A-1) throughout each process or stage in the well-to-wake (WTWa) phase. LCA aims to scientifically and systematically quantify the resource consumption, energy usage, emissions output, and potential ecological impacts of SAFs throughout its entire lifecycle [37]. The lifecycle inventory can be divided into five categories: Firstly, raw material acquisition and processing, which involves recording and quantifying the energy use, water consumption, land use changes, and related GHG emissions associated with the cultivation/collection and preprocessing of raw materials (e.g., plant oils, algae, waste biomass) used in SAF production. Secondly, there is fuel production, which includes analysis of the direct and indirect energy requirements, chemical inputs, and resultant emissions (including but not limited to CO₂, NO_x, SO_x, and particulate matter) associated with conversion technologies such as hydrotreating and Fischer–Tropsch synthesis/Thirdly, transportation and storage considers the logistics chain from SAF production sites to airports, including the energy consumption and emissions during transportation. Fourthly, there is the usage phase. Although the combustion emissions of SAF in aircraft are typically similar to conventional jet fuel, the net lifecycle emissions can be significantly reduced due to differences in feedstock and production methods. This phase requires the assessment of direct emissions from flight operations. Finally, there is waste management and recycling, which involves analysis of the treatment methods and environmental impacts of byproducts or waste generated after SAF use [38,39].

Based on LCA findings, Staples et al. project that, by 2050, employing AJF could potentially reduce the life cycle greenhouse gases emission index (EI-GHGs) of aviation by up to 68.1%, with annual investments in AJF production capacity amounting to only about one-fifth of the global investment in pre-existing oil refining [40]. Moreover, research by Han et al. indicates that EI-GHGs of hydroprocessed renewable jet (HRJ), pyrolysis jet, and FTJ from corn stover could decrease by 41–63%, 68–76%, and 89%, respectively, compared with traditional aviation fuels. The GHG emissions of HRJ fuel in the WTWa phase are primarily influenced by fuel production activities, reflected in the consumption of H₂ and natural gas (NG), with the second source being related to the choice and utilization rate of fertilizers [41]. Jong et al. have analyzed the GHG emission savings of different SAFs compared with traditional aviation fuels, ranging from highest to lowest, as follows: FT (86–104%), HTL (77–80%), pyrolysis (54–75%), UCO-based HEFA (68%), and sugarcane- (71–75%) and corn stover-based ATJ (60–75%) [42]. FT process jet fuel could achieve 50–90 wt% GHG reduction in life cycle and the potential reduction on PM [43]. Furthermore, selecting suitable raw materials and conversion technologies, utilizing sustainable hydrogen sources, and implementing carbon capture and storage technologies can further reduce the greenhouse gas emissions of SAFs throughout the life cycle [44,45]. Cox et al. have conducted LCA analyses on biojet fuels derived from microalgae, pongamia, and sugarcane molasses. While the fossil energy ratio (FER) and GHG reduction effect of microalgae-based biofuel (FER = 1.0, 53% lower than aviation kerosene) are inferior to sugarcane molasses (FER = 1.7, 73% lower than aviation kerosene), the former demonstrates advantages in nutrient avoidance, reduced water usage, and lower land utilization rates, highlighting the superiority of third-generation biofuel feedstocks [46]. Figure 2 summarizes the lifecycle GHG emissions of various aviation fuels, with traditional aviation fuel emitting 90 g CO₂-eq/MJ. The CO₂-eq emissions of SAFs produced via different pathways vary. Specifically, the emissions for HEFA, CH, FT, hydroprocessed depolymerized cellulosic jet (HDCJ), direct sugar to hydrocarbons (DSHC), and ATJ are 3.1–40.0, 21.2–39.3, −1.6–18.2, −2.7–49.5, 22.0–80.2, and −27.0–117.5 g CO₂-eq/MJ, respectively. Additionally, SAFs produced using the same process but different feedstocks also exhibit varying CO₂ emissions. For instance, ATJ fuels made from sugar cane, corn grain, switchgrass, and poplar emit −27.0–19.7, 47.5–117.5, 11.7–89.8, and 32.0–73.0 g CO₂-eq/MJ, respectively [47].

2.2. Aviation NO_X Emissions

The NO_x emissions in the aviation industry typically originate from three reaction mechanisms: thermal-NO_x, prompt-NO_x, and fuel-NO_x [48,49]. Thermal-NO_x is primarily generated in high-temperature regions above 1800 K, influenced primarily by temperature, excess air ratio, and residence time in the high-temperature zone [50,51]. Prompt-NO_x is minimally influenced by temperature. Its formation mechanism involves the decomposition of hydrocarbons during combustion, where CH radicals break the molecular bonds of nitrogen gas, generating the significant intermediate product HCN. Subsequently, HCN reacts with the radicals O and OH produced within the flame [52,53]. Fuel-NO_x is produced through the combustion of nitrogen elements present in the fuel [54]. NO_x emissions from aviation, along with CO₂ emissions and aviation-induced cloudiness, are major contributors to RF [55]. The climatic impact of NO_x takes complex forms. On one hand, it contributes to the production of ozone (O₃), a positive climate forcing agent that leads to warming [56,57]. The O₃ production efficiency from aviation NO_x emissions is approximately four times higher than that from surface NO_x emissions. On the other hand, O₃ photolysis generates excited oxygen atoms that react with water vapor to produce OH radicals, which are the primary sink for methane (CH₄) in the troposphere, thereby reducing methane concentration, a negative climate forcing agent that leads to cooling. The decrease in methane concentration, as a major precursor to O₃ formation, results in a reduction in O₃ concentration and stratospheric water vapor (SWV) levels. Despite the intricate interplay of reactions leading to uncertainties, it is generally believed that the net NO_x response from aircraft leads to a positive, warming RF [58]. According to Skowron et al.’s predictions, aircraft NO_x emissions in 2050 are expected to reach 2.17 Tg(N) year⁻¹ under scenarios of low air traffic growth and optimistic technological development. Conversely, under high air traffic growth and low technological development scenarios, emissions could reach 5.59 Tg(N) year⁻¹. This represents a significant increase from the 0.71 Tg(N) year⁻¹ recorded in 2006. They also suggest that the net NO_x RF of the aviation industry will vary greatly depending on background conditions. Reducing surface emissions and having a cleaner atmospheric background by 2050 could mitigate NO_x RF to some extent. For example, if anthropogenic surface emissions remain at current levels, the net NO_x RF under high air traffic growth in 2050 would be 17.5 mW m². This is 48% higher than under the 2050 representative concentration pathway (RCP) 2.6 scenario, which represents the cleanest background, and 28% higher than under the 2050 RCP 8.5 background conditions [59]. NO_x emissions in the aviation industry exhibit strong spatial variability, with aircraft emissions of NO_x at cruising altitudes above 2000 ft increasing total RF by approximately 2 ± 1 mW m⁻² through gas-phase photochemical reactions [60]. Short-lived NO_x gases cause sensitive changes in RF in the local regions of emission, but long-lived methane and carbon dioxide counterbalance regionally, resulting in globally averaged RF [61]. Besides CO₂, NO_x emissions from aviation dominate the pollutants’ emission quantities and environmental impacts, with increasing influence primarily concentrated around emission-dense airports, such as those in economic zones or tourist destinations [62].

The majority of NO_x emissions arise from thermal processes, such as engine testing typically controls power output, resulting in similar combustion temperatures. Consequently, differences in NO_x emissions between SAFs and traditional aviation fuels are often minimal [63,64]. Reksowardojo et al. [65] found no difference in NO_x emissions between medium-chain fatty acids to biojet (MBJ), produced via hydrogenation from coconut oil, and Jet A-1 fuel. This can be attributed to the nearly identical turbine-inlet temperatures (TIT) of different fuels burning in gas turbine engines at various power levels, suggesting similar combustion temperatures. Okai et al. [66], using a chemiluminescence detector (CLD) to measure NO_x concentrations, observed minimal differences in NO_x emissions between 100% aviation kerosene, FT-SPK fuel, and HEFA fuel, with no impact from variations in the proportion of SAFs in SAFs/aviation kerosene blend fuels. Wang et al. [67] reported NO emission levels between 2–2.5 ppm for HEFA-SPK and JP-8 fuels, as well as their mixtures, under colorless distributed combustion (CDC) conditions. This reduction of over 95% compared with traditional air combustion conditions was attributed to decreased thermal-NO_x generation due to lower peak temperatures. Differences in NO_x emissions between HEFA-SPK, JP-8, and four different blend ratios of mixed fuels were minimal under CDC conditions. Undavalli et al. [68] similarly found no significant trend in NO_x emissions for HEFA and Jet A-1 fuels, as well as their mixtures. However, at ratios of 20:80 and 50:50, mixed fuel NO_x emissions were slightly lower than those of pure fuels, supporting the 50:50 blend ratio as the drop-in proportion for HEFA fuels approved by ASTM D7566. These differences may be related to the adiabatic flame temperature of the mixed fuels. Boomadevi et al. [69] studied microalgae Spirulina biofuel, finding that, as the proportion of added Jet-A fuel increased, the variability in NO_x emissions remained low. However, with an increase in engine speed, NO_x emissions increased for all tested fuels. For instance, as the engine speed increased from 30,000 rpm to 80,000 rpm, NO_x emissions for Jet-A fuel and a 20% biofuel blend increased from 0.2 and 0.18 g per kilogram of fuel to 1.2 and 1.15 g/kg of fuel, respectively. Harper et al. [70] conducted tests on a small-scale (<250 kW) non-proprietary RQL combustion system, evaluating emissions from three conventional aviation fuels, three SAFs (two ATJ and one CHJ fuel), and three blended fuels at pressures of 1.0, 1.5, 2.0, and 2.5 bar. Their results show minimal differences in NO_x emissions among different fuels, followed by CO₂ and CO, with the greatest disparity observed in UHC emissions. Feser et al. [71] evaluated the performance and emissions of four different drop-in aviation biofuels, including UOP HEFA-SPK, Gevo ATJ, Amyris Farnesane, and SB-JP-8. NO_x emissions for all four fuels exhibited a similar trend with equivalence ratio variation, with the lowest and highest NO emissions occurring at equivalence ratios of around 0.6 and 1.0, respectively. The reason for this is that, when the equivalence ratio is less than 0.6, prompt NO has a greater impact as the equivalence ratio decreases. Conversely, when the equivalence ratio exceeds 0.6, combustion-induced temperature rise becomes the predominant factor in thermal-NO_x generation. Compared with the other three fuels, UOP HEFA-SPK demonstrated a significant increase in NO emissions for equivalence ratios greater than 0.6. If biofuels exhibit higher NO_x emissions, potential reasons include a high nitrogen content in the fuel composition or a low oxygen content, the latter of which can be addressed by incorporating pentanol or ethanol to increase oxygen content [72]. However, in the actual use of aviation fuel, oxygen content in the fuel is not permitted.

2.3. Aviation CO Emissions

In certain circumstances, such as insufficiently high temperatures, relative oxygen deficiency, and uneven fuel/air mixture, combustion of aviation fuel can produce the toxic gas CO [73]. CO exhibits approximately 250 times greater affinity to hemoglobin in the blood than oxygen (O₂), hence, upon inhalation, the colorless and odorless CO tightly binds with the iron in hemoglobin, leading to oxygen displacement. Blood carrying CO circulates throughout the body, depositing in the myocardium, skeletal muscles, nervous system, and brain, with myoglobin demonstrating similarly elevated affinity to CO compared with O₂ [74]. The most common symptoms following acute CO exposure include nausea, headache, dizziness, vomiting, altered consciousness, and subjective weakness. Severe poisoning symptoms encompass mental confusion, seizures, myocardial infarction, coma, and even death [75]. Auxiliary power units (APUs) and the main engines providing thrust are the primary sources of CO production on aircraft, with APUs predominantly utilized on the ground to supply the power needed to start the main engines, electrical systems, air conditioning, and compressed air. Elevated CO concentrations near airports or within cabins can lead to aerotoxic syndrome in individuals.

Under comparable conditions, SAFs exhibit only minor differences in CO emissions compared with traditional aviation fuels, far less than the disparities observed in SO₂ and PM emissions between the two. According to Yang et al.’s review, various conventional aviation fuels exhibit minor changes or even reductions (within 10%) in CO emissions when blended with different SAFs [76]. Discrepancies in the emission index of CO (EI-CO) across different fuels may stem from factors such as viscosity, carbon content, Sauter mean diameter (SMD), and final boiling point. Higher viscosity may result in a greater cone angle and premature flame extinguishment near the combustion chamber walls, leading to incomplete combustion and increased CO generation, while larger hydrocarbons can contribute to an increase in EI-CO. Lower SMD and final boiling point facilitate combustion on one hand but, on the other, prematurely remove the fuel from the primary zone by increasing the cone angle and reducing residence time, resulting in reaction quenching [77]. Schripp conducted emissions tests on neat Jet A-1, ReadiJet, and ATJ using a CFM56-5C4 engine, ensuring a stable environmental condition with minimal variability in parameters affected by environmental factors. There were no significant differences in CO and NO_x emissions among the three tested fuels. NO_x emissions from all fuels showed nearly linear growth with combustion chamber inlet temperature and thrust setting, whereas carbon monoxide emissions predominantly occurred at low power settings [78]. Reksowardojo et al. [65] investigated the correlation between CO emissions and brake power of Jet A-1 blended with MBJ fuels, finding that the difference in EI-CO between blended fuels and Jet A-1 was negligible. The similarity in CO emissions could be attributed to the nearly identical viscosity of MJB and Jet A-1, with a viscosity difference of approximately 0.8% for J20 (MBJ volume blending ratio at 20%) and Jet A-1. In Timko et al.’s study [79], during ground idle and 7% idle periods, neat natural gas-derived FT fuels exhibited around a 20% reduction in EI-CO compared with Jet A/Jet A-1, while 50% FT fuel fell within the range of Jet A/Jet A-1. Blends of fatty acid methyl ester (FAME) with jet fuel at 20:80 and 40:60 ratios showed a 5–10% decrease in EI-CO compared with Jet A/Jet A-1. Corporan et al. [63] found that the application of neat SAFs and blend fuels resulted in a reduction in EI-CO of 0–20% compared with JP-8/Jet A-1, regardless of engine power. They have suggested that the theoretically lower carbon content in neat SAFs could contribute to lowering EI-CO, although the actual reduction is limited. Additionally, favorable physical properties, such as lower density, may also contribute to the reduction in EI-CO for SAFs. Klingshirn et al. [64] similarly concluded that CO emissions from SAFs are comparable to JP-8. Neat HRJs exhibited approximately a 20% reduction in EI-CO during idle and cruise compared with JP-8, attributed to their lower total carbon content. In contrast, the variation in EI-CO emissions for blends of HRJ and JP-8 fuels was minimal or negligible. Currently, there is insufficient literature support regarding the CO emission characteristics of SAFs, constituting a significant research gap. Given that elevated CO emissions may indicate suboptimal engine combustion conditions and pose risks of CO accumulation in aircraft cabins due to ventilation system malfunctions, alongside the feasibility of detecting CO along with other combustion exhaust gases (such as CO₂ and NO_x) using the same analytical instruments without substantial additional modifications or expenses [78], future research efforts should be directed towards investigating the emission performance of SAFs.

2.4. Aviation SO₂ Emissions

The combustion of aviation fuels is linked to emissions in a complicated manner, with aircraft engine manufacturers consistently striving to develop machinery with high combustion efficiency and low pollution levels [80]. According to statistics, emissions from the aviation industry in 2019, including CO₂, CO, HC, NO_x, PM_2.5, and SO₂, amounted to 56,960,300.60 tons, 400,395.30 tons, 37,998.96 tons, 388,677.70 tons, 5,393,817.46 tons, and 68,719.92 tons, respectively, comprising a respective 98.42%, 0.7%, 0.07%, 0.68%, 0.01%, and 0.12% of the total emissions. Moreover, the average SO₂ emissions from 2017 to 2019 increased by 12.98% compared with 2014–2016, ranking second among all pollutants emitted, following only NO_x [23]. Figure 3 depicts the comprehensive emission trends of the six pollutants on China–foreign routes from 2014 to 2021 [81]. The figure illustrates a notable increase in the total emissions of six pollutants on China–foreign routes from 2014 to 2016, with the SO₂ emissions rising from 34,505 tons to 56,301 tons. The introduction of emission reduction measures, such as the CNG2020 strategy, led to a gradual decrease in the growth rates of the six pollutants, some even turning negative. From 2017 to 2019, the average growth rates of CO₂, HC, NO_x, SO₂, PM_2.5, and CO were 1.19%, −1.43%, 2.51%, 1.19%, 4.34%, and −0.23%, respectively. The outbreak and spread of the COVID-19 pandemic in 2020–2021 resulted in a sharp reduction in the number of China–foreign routes, bringing the SO₂ emissions to a low level. Most research on emissions has focused on CO₂, NO_x, and CO, with less attention given to SO₂ emissions [82]. However, SO₂ emissions from the aviation industry pose significant risks to human health and the natural environment. The sulfur element in SAF raw materials typically originates from the preprocessing process rather than the raw materials themselves [83]. Negligible sulfur elements and aromatic hydrocarbon content determine that the SO₂ emissions from SAF differ significantly from petroleum-based fuels [84,85].

The distinctive aspect of aviation pollutants lies in their accumulation directly within the upper troposphere and lower stratosphere during cruising altitudes (8–12 km), exerting complex and potent influences on Earth’s ecosystem [86]. While sulfur in aviation fuel can mitigate fuel pump wear to some extent, emissions of sulfur oxides are directly associated with this sulfur [87]. Approximately 98% of sulfur in fuel converts to SO₂, with the remainder appearing as SO₃ in exhaust gases [88,89]. Lower combustion temperatures favor the generation of SO₃ through reaction equilibrium, while metallic substances in the fuel or fuel system expedite the oxidation of SO₂ to SO₃ [90]. Oxides of sulfur combine with water to form sulfuric acid, which is absorbed on the surface of soot particles, creating volatile sulfate aerosol particles [91]. Under ice supersaturation conditions, these particles may serve as nuclei for ice crystals, leading to the formation of contrail cirrus clouds. Contrails enhance the uptake of sulfur oxides and subsequent conversion to sulfate aerosols, resulting in soot wet deposition and an eventual contribution to acid rain during deposition [92,93]. Contrail cirrus clouds alter the radiative properties and water budget of the upper troposphere, thereby affecting climate. Concurrently, acid rain inflicts severe damage on water sources, soil, vegetation, and other ecosystems.

Exposure to SO₂ in the atmosphere increases the risk of respiratory system diseases among individuals, including asthma attacks, bronchiolitis, upper airway infections, and pneumonia [94]. Even in environments with low concentrations of SO₂ (<200 ppb), asthmatic individuals exhibit sensitive and rapid airway constriction symptoms [95]. Short-term exposure to sulfur dioxide is positively correlated with respiratory system mortality and all-cause mortality rates [96]. Exposures to SO₂ exceeding 500 μg/m³ for 10 min can stimulate the respiratory tract of susceptible individuals, leading to symptoms such as acute bronchial asthma, pulmonary edema, and respiratory distress [97].

Predictably, the sulfur content in fuel correlates directly with the emissions of SO₂ after combustion. In the study by Corporan et al. [98], JP-8 and synthetic jet (synjet) fuel were blended at different volume ratios of 0%, 25%, 50%, and 75%. The mass fraction of sulfur elements in the fuel was 0.06, 0.05, 0.04, and 0.02, respectively. Combustion experiments with the T63 engine showed that the SO₂ emissions from the four blending ratios of JP-8/synjet fuel were approximately 34, 23, 15, and 7 ppm, respectively, with equivalence ratios of 1.00, 1.05, and 1.10 exhibiting similar emission quantities and trends. In the Alternative Aviation Fuel Experiment (AAFEX), researchers employed a sulfate particulate analyzer (SPA) in conjunction with a trace-level pulsed fluorescence SO₂ analyzer to measure SO₂ gas emissions from a DC-8 aircraft equipped with a CFM56 engine. The results indicate that, under APU load, the SO₂ emission indices (EI-SO₂) for JP-8 were 1.5–1.8 g kg⁻¹, while those for FT-2 were 0.13–0.15, suggesting a 92% reduction in aviation SO₂ emissions with SAF use, albeit with minimal differences in CO and NO_x emissions, as shown in Figure 4a. The SO₂ emissions from a 50:50 volume blend of FT-1 (Shell) and FT-2 (Sasol) fuels were approximately half that of pure JP-8, attributed to the high FSC of JP-8 at around 1200 ppm compared with negligible sulfur in neat FT fuels. Additionally, compared with JP-8, FT-2 exhibited nearly a 100% reduction in EI-particle-bound sulfate emissions and a five-fold reduction in EI-particle-bound organics. As shown in Figure 4b,c, EI-SO₂ remained relatively constant with increases in exhaust gas temperature (EGT) and power setting, while black carbon (BC) emissions decreased with rising EGT, with JP-8 exhibiting a greater reduction than FT-2. At varying EGT, the EI-SO₂ for FT-2 fuel ranges from 0.12–0.14 g/kg fuel, while for JP-8, it ranges from 1.51–1.58 g/kg fuel. EI-SO₂ shows a slight increase with increasing power. The EI-SO₂ for blended fuels falls between that of the JP-8 and FT fuels. Figure 4a–c correspond with [34,99,100], respectively. Yakovlieva et al. [101] incorporated fatty acids ethyl esters (FAEE) derived from rapeseed oil (RO) as bio-additives into Jet A-1 fuel at proportions of 10% and 20%. The sulfur contents in Jet A-1 and modified FAEE were 0.025% and 0.0085%, respectively. Traditional aviation fuel and blends containing 10% and 20% modified FAEE generated respective SO₂ pollution gases of 0.0367, 0.0158, and 0.0281 kg per landing-take-off (LTO) cycle, indicating that a 10% blend of modified FAEE effectively reduced SO₂ emissions by nearly 57% compared with pure Jet A-1. Cican et al. [102] examined the combustion parameters and emission characteristics of Jet A and Jet A/biodiesel (BD) blends in a JETCAT P80 microengine, simulating idle, cruise, and max stages by setting throttle positions at 18.7%, 30%, and 94%, respectively (BD belongs to biofuel rather than SAF, and is here for reference). SO₂ was detected using an MRU analyzer with a measurement range of 0–2000 ppm and an accuracy within ±10 ppm or 5% reading. As shown in Figure 4d, the EI-SO₂ during the cruise phase was the highest among the three stages, but a 10% BD blend effectively reduced EI-SO₂ in the exhaust gases during any stage, with EI-SO₂ dropping to approximately 10 ppm when the blend reached 20%. Moreover, increasing the BD content in the fuel also improved thrust by around 5% and reduced EI-NO_x by approximately 25%. This suggests that incorporating BD into aviation fuel does not require engine design modifications and does not significantly alter engine performance, making it a promising method for aviation fuel substitution. The sulfur content, methods/burning platform, and instrumentation of standard specification SAF and related fuels are shown in

Table 1. Information regarding the investigated SAF and fuels and the burning platforms and decline situation for SO₂ emissions in literature.

Ref.	Fuel	Sulfur Content (ppm)	Method/Burning Platform	SO2 Emissions
ASTM D7566 [10]	FT-SPK	<15	D5453/D2622	-
	HEFA-SPK	<15
	SIP	<2
	FT-SPK/A	<15
	ATJ-SPK	<15
	CHJ	<15
	HC-HEFA SPK	<15
ASTM D1655 [11]	Jet A or Jet A-1	<3000	D3227/IP 342	-
Corporan et al. (2007) [98]	JP-8	600	T63 Engine	SO2 emissions of JP8, 25% synjet, 50% synjet and 75% synjet were approximately 34, 23, 15, and 7 ppm, respectively.
	25% synjet	500
	50% synjet	400
	75% synjet	200
	synjet	<100
Bulzan et al. (2010) [99]	JP-8	1148	CFM56-2C1 Engine Garrett GTC 85-98CK (APU)	50:50, the emissions of SO2 from blended fuels are approximately half that of pure JP-8.
	FT1	19
	FT1 Blend	699
	FT2	22
	FT2 Blend	658
Yakovlieva et al. (2019) [101]	Jet A-1	250	Calculating	10% FAEE blend fuel reduced SO2 emissions by nearly 57%.
	FAEE of RO	85
Cican et al. (2019) [102]	Jet A	-	JETCAT P80 Microengine	With a 20% blend of BD, the EI-SO2 decreases to around 10 ppm.
	Biodiesel
Tran et al. (2020) [103]	Jet A-1	500	GE CF700-2D2 Engine	The increase in fuel sulfur content correlates with elevated emissions of volatile particulate matter.
	JP-5	200
	ATJ-SPK Blend	0
	ATJ-SPK Neat	0.96
Corporan et al. (2010) [104]	JP-8a	800	F117-PW-100 engine	Percentage of engine max thrust = 4, 33, 45, 54, 63%, JP-8/HRJ and JP-8a/HRJ/FT exhibit a reduction in EI-SO2 exceeding 40%.
	JP-8b	800
	JP-8b/HRJ	500
	JP-8a/HRJ/FT	500

. According to ASTM specifications, Jet A or Jet A-1, as representative conventional aviation fuels, require sulfur content to be below 3000 ppm, whereas the maximum specification value for sulfur content in the seven ASTM-certified SAFs listed in the table is 15 ppm, with sulfur content in SIP fuel limited to below 2 ppm. Blending SAFs significantly reduces sulfur content in conventional aviation fuels across various tests and decreases SO₂ emissions in engine tests, demonstrating the sustainability and environmental friendliness of SAFs.

2.5. Aviation PM Emissions

Compared with other sources of pollution in urban environments, aircraft engine PM emissions constitute a distinct pollution source [105]. Aviation PM emissions can be categorized into primary and secondary PM. Primary PM refers to solid and liquid particles emitted directly from the engine without undergoing chemical transformations in the atmosphere. Non-volatile particulate matter (nvPM) and volatile particulate matter (vPM) constitute the emissions of primary particulate matter (PM) in aviation activities [106]. BC, the primary component of nvPM, forms within aircraft engines under temperature and pressure conditions, where incompletely oxidized carbon atoms aggregate and condense into solid particles carried by airflow [107]. Additionally, metallic oxides (e.g., iron, copper, etc., originating from engine wear materials) and sulfates (combustion byproducts of sulfur-containing fuels) contribute directly to atmospheric particulate matter concentrations as they undergo minimal physical or chemical alterations in the atmosphere. Besides combustion processes, nvPM formation is influenced by factors such as engine design, combustion conditions, and aerodynamic effects. vPM comprises organic compound vapors, partial sulfides, and nitrides, which may initially exist in gaseous form during emissions but condense into solid or liquid particles during cooling, becoming part of particulate matter. vPM emissions occur at the engine exit, where hot combustion exhaust cools, sulfuric acid droplets act as representative condensation nuclei, and further condensation of water and organic substances occurs on the surface. During post-emission transport, the quantity and quality of particles undergo significant changes through processes like aggregation, condensation, and evaporation, forming vPM [108]. Secondary PM is not directly emitted but formed through complex chemical reactions in the atmosphere. When gaseous pollutants emitted by aircraft (such as nitrogen oxides (NO_x), volatile organic compounds (VOCs), etc.) react with other components in the atmosphere (such as oxygen, water vapor), new solid or liquid particles are formed. These reactions can generate sulfate, nitrate, organic aerosols, etc., which constitute the primary components of secondary PM, significantly impacting long-distance transport and long-term air quality.

The physicochemical composition of nanoparticles emitted by aircraft engines resembles diesel exhaust particles, which are classified as mutagenic and carcinogenic agents [109]. Studies investigating the impact of LTO emissions from 99 airports on public health have shown that premature mortality increased from an estimated 75 cases per year in 2005 to 460 cases per year by 2025, with a 6.1-fold increase, of which 2.1 times were attributed to PM_2.5-related emissions [110]. Emissions of sulfur dioxide and sulfate aerosols contribute to an increase in the concentration of fine particulate matter with a dry diameter of less than 2.5 µm (PM_2.5). Prolonged exposure to high concentrations of PM_2.5 increases the likelihood of respiratory and cardiovascular diseases, lung cancer, chronic physiological changes, and mortality [111,112,113,114]. PM_2.5 emissions during aircraft takeoff and landing significantly impact human health around airports, causing approximately 3600 premature deaths annually with standard aviation fuel containing approximately 600 ppm of sulfur. This accounts for 0.1% of global premature deaths, with the highest mortality rates attributed to aviation in Europe, eastern North America, and eastern China. Aviation-related PM_2.5 contributes approximately 1% to the total premature mortality attributed to PM_2.5 exposure. It is estimated that in 2017, approximately 67,000 deaths occurred in China due to environmental PM_2.5 and O₃ emissions, with the densely populated coastal regions of eastern China experiencing the greatest impact due to intensive aviation activities [115]. Compared with standard aviation fuel with a fuel sulfur content (FSC) of around 600 ppm, the use of ultra-low sulfur jet fuel (ULSJ) with an FSC of 15 ppm effectively reduces surface PM_2.5 concentrations, preventing approximately 620 deaths annually. Additionally, it increases the global average net radiative effect (RE) by reducing the aerosol cloud albedo effect (aCAE) by +7.0 mW m⁻² [87].

Durdina et al. investigated the emission indices of nvPM mass and nvPM number from a CFM56-7B engine using a blend of Jet A-1 and 32% HEFA-SPK fuel. They found that blending fuels reduced the geometric mean diameter (GMD) and geometric standard deviation (GSD) of emissions at every point during ground idle and take-off, particularly with lower thrust levels, resulting in a relatively larger reduction in emissions. For instance, during ground idle, the SAF blend reduced nvPM number by 60%, while during take-off, the reduction was 10%. SAF blended fuel reduced the nvPM mass emission by 20% and the nvPM number emission by 25% over a standardized LTO cycle [116]. Schripp et al. conducted emissions tests on Jet A-1, a catalytic hydrothermolysis jet fuel (ReadiJet), and an ATJ fuel using a CFM56-5C4 engine. The hydrogen mass fractions in the three fuels were in the order of ATJ (15.5%) > Jet A-1 (14.1%) > ReadiJet (13.7%), while the aromatic content by volume was ReadiJet (20.9%) > Jet A-1 (15.6%) > ATJ (below 1%). ATJ fuel primarily consists of various branched aliphatic compounds such as iso-C₁₂H₂₆ and iso-C₁₆H₃₄. Aliphatic hydrocarbons, as opposed to aromatic compounds with the basic structure required for PM precursors, exhibit a lower tendency for PM formation. The PM formation tendency of hydrocarbons can be roughly ordered as follows: aromatics > cycloalkanes > branched alkanes > straight-chain alkanes. The results of this study confirm the above conclusions, showing a 70% reduction in particle mass for ATJ fuel compared with Jet A-1. ReadiJet fuel, with higher aromatic content and lower fuel hydrogen content, exhibited a significant increase in PM emissions [78]. Yu et al. [117] conducted a study on the PM emissions levels of the CF34-3A1 turbofan engine and the TPE331-6-252B turboprop engine. They found that, at different power settings, the EI-PM of the former was generally lower than that of the latter. However, both engines exhibited a minimum EI-PM at moderate power conditions, typically between 40–60% thrust. At high power settings, nvPM emissions such as black carbon or soot predominated, while with decreasing engine power, the mass fraction of vPM containing organic compounds and sulfur exceeded 50%. Kurzawska et al. [118] discovered that a 50:50 blend of ATJ/Jet A-1 fuel, under high engine load, could reduce total particle number and mass by approximately 18% and 53%, respectively, compared with Jet A-1. Chan et al. [119] found that, under all engine loads of the GE CF-700-2D-2 engine, Jet A-1 exhibited the highest EI-PM, with a moderate reduction of 7–25% for 100% CH-SKA fuel across the engine load range. For 50% HEFA-SPK and 100% FT-SPK fuels, EI-PM reductions were approximately 40–60% and 70–95%, respectively. The emission trends of BC were consistent across all fuels, but compared with Jet A-1, emissions from 100% CH-SKA, 50% HEFA-SPK, and 100% FT-SPK were reduced by 38–50%, 58–82%, and 70–95%, respectively. The EI-BC of Jet A-1 and three types of SAFs showed a strong correlation with changes in aromatic compounds and the H/C ratio in the fuels.

As widely recognized, the sulfur content in fuel has the greatest impact on the volatile aerosol fraction. Moore et al. [120] found that reducing FSC to near zero decreased the EI-aerosol number by 10–100 times, especially when the fuel flow rate was low (below 0.4 kg s⁻¹), resulting in significant reductions in EI-total number and EI-non-volatile particulate matter (EI-nvPM), albeit with diminished benefits at high thrust settings. Tran et al. [103] conducted particle number emissions tests using a GE CF700-2D2 engine and observed that the EI-volatile particulate matter for Jet A1 fuel was higher than that for JP-5, while the trend reversed for EI-nvPM. This discrepancy stemmed from Jet A1’s higher FSC and JP-5’s higher aromatic content. The ATJ-SPK blend, with aromatic and sulfur contents of 8% vol and 0 ppm, respectively, significantly reduced total particle number emissions by up to 97% compared with Jet A-1. Voigt et al.’s study indicates that particulate matter can modulate the quantity of cirrus ice crystals at current emission levels. They utilized an Airbus A320 equipped with two IAE-V2527-A5 engines to compare the emission performance of neat Jet A1 and semisynthetic jet fuel blends (FT-SPK or HEFA-SPK with Jet A-1). Semisynthetic jet fuels with high hydrogen content and low aromatic hydrocarbon content can lead to a 50% to 70% reduction in both soot and ice number concentrations, along with an increase in ice crystal size. The reduction in ice crystal number results in decreased energy deposition in the atmosphere, thereby reducing warming. Dicyclic naphthalene is a more effective precursor of soot than monocyclic aromatic or aliphatic hydrocarbon structures [121]. Lobo et al. [122] investigated aircraft engine emissions using neat FT fuel, used cooking oil (UCO)-derived HEFA alternative fuel, and blends of these with traditional aviation fuel, revealing significant reductions in EI-nvPM and sulfur oxides after blending with SAF. Moore et al. [123] conducted pollutant emission tests on a DC-8 aircraft equipped with four CFM56-2-C1 engines using three types of fuel: medium-sulfur-content Jet A fuel, low-sulfur-content Jet A fuel, and a blend of 50:50 HEFA and low-sulfur-content Jet A fuel. Compared with medium-sulfur-content Jet A fuel, low-sulfur-content Jet A fuel and the blend fuel reduced volatile fine particle number by approximately 9% and 52%, respectively; the blend fuel decreased non-volatile fine particle number by approximately 45%, while, unexpectedly, low-sulfur-content Jet A fuel increased non-volatile fine particle number by 30%. The application of the blend fuel reduced volatile particle volume, non-volatile particle volume, and total particle volume by approximately 50%. Additionally, blend fuel exhibited varying degrees of reductions in BC, CO and NO_x emissions, as shown in Figure 5. The NO_x emissions of three fuels exhibit minimal variation, consistent with previous findings. However, low-sulfur-content Jet A fuel and the blend fuel respectively reduce CO emissions by 33% and 22%, which can potentially be attributed to combustion heat.

Table 2. Information regarding the investigated SAF and fuels and the burning platforms and decline situation for PM emissions in literature.

Ref.	Fuel	Aromatics	Method/Burning Platform	PM Emissions
ASTM D7566 [10]	FT-SPK	<0.5 m%	D2425	-
	HEFA-SPK	<0.5 m%
	SIP	<0.5 m%
	FT-SPK/A	<20 m%
	ATJ-SPK	<0.5 m%
	CHJ	8.4–21.2 m%	D2425 or D6379/IP 436
	HC-HEFA SPK	<0.5 m%	D2425
ASTM D1655 [11]	Jet A or Jet A-1	<25/26.5 v%	D1319, etc.	-
Durdina et al. (2021) [116]	Jet A	18.1 v%	CFM56-7B26 engine	nvPM number decreased by 60% and 10% during ground idle and take-off phases, respectively. nvPM mass and nvPM number decreased by 20% and 25% over a standardized LTO cycle, respectively.
	32% HEFA-SPK blend	11.3 v%
Schripp et al. (2019) [78]	Jet A-1	15.6 v%	CFM56-5C4 engine	70% reduction in particle mass for ATJ fuel compared with Jet A-1.
	ReadiJet	20.9 v%
	ATJ	<1 v%
Kurzawska et al. (2021) [118]	Jet A-1	17.3 v%	GTM-120 engine	Total particle number and mass by approximately 18% and 53%, respectively
	50% ATJ-SPK blend	8.8 v%
Chan et al. (2016) [119]	Jet A-1	18.3 v%	GE CF-700-2D-2 turbofan engine	100% CH-SKA, 50% HEFA-SPK, and 100% FT-SPK respectively reduce EI-PM by 7–25%, 40–60%, and 70–95%.
	CH-SKA	17 v%
	FT-SPK	0.5 v%
	HEFA-SPK	10.4 v%
Tran et al. (2020) [103]	Jet A-1	18.5 v%	GE CF700-2D2 Engine	ATJ-SPK blend reduced total particle number emissions by up to 97% compared with Jet A-1
	JP-5	20.1 v%
	ATJ-SPK Blend	8 v%
	ATJ-SPK Neat	0 v%
Voigt et al. (2021) [121]	Jet A-1	18.8/17.2/18.6/ 16.5 v%	Airbus A320 equipped with two V2527-A5 engines	Semisynthetic jet fuels lead to a 50% to 70% reduction in both soot and ice number concentrations
	41% FT-SPK blend	11.4 v%
	49% HEFA-SPK blend	8.5 v%
	30% HEFA-SPK blend	9.5 v%
Lobo et al. (2015) [122]	Jet A-1	15.55 wt%	Garrett GTCP85 (APU)	50% UCO-HEFA blend fuel reduced nvPM number and mass emissions by approximately 35% and 60%, respectively.
	UCO-HEFA	1.91 wt%
Moore et al. (2017) [123]	Medium-sulfur-content Jet A fuel	21.1 ± 0.7 v%	DC-8 aircraft equipped with four CFM56-2-C1 engines	Compared with medium-sulfur-content Jet A fuel, the 50% HEFA blend fuel reduced the number of vPM and nvPM by 52% and 45%, respectively, while decreasing vPM volume, nvPM volume, and total PM volume by approximately 50%.
	Low-sulfur-content Jet A fuel	21.4 ± 1.4 v%
	50:50 HEFA: Low-sulfur-content Jet A fuel	12.9 ± 1.2 v%

illustrates the aromatics content, burning platform/methods, and instrumentation of standard specification SAF and related fuels. As per ASTM regulations, Jet A or Jet A-1, representing conventional aviation fuels, require an aromatics content below 25/26.5 vol%. Among the seven ASTM-certified SAFs listed, five have maximum aromatics content specifications of 0.5 mass%, while FT-SPK/A and CHJ fuels have an aromatics content limited to below 20 and 21.2 mass%, respectively. Aromatics content in neat SAFs and their blends is notably lower than traditional aviation fuels, leading to reduced PM emissions in engine testing, thus affirming the sustainability and environmental friendliness of SAFs.

2.6. Aviation UHC Emissions

The formation mechanisms of UHC in jet fuel combustion within aircraft engines typically involve the following aspects. Firstly, incomplete combustion, where, under certain engine operating conditions, such as low-speed flight, takeoff, landing, or when there is insufficient oxygen supply in the combustion chamber, fuel may not fully mix with oxygen or complete the oxidation reaction within the limited time, resulting in the partial combustion of hydrocarbons and the formation of UHC. In real combustion processes, rich fuel conditions can lead to the production of CO, aromatics, UHC, and soot, while lean conditions can release oxygen-rich substances like ketones, peroxides, and NO_x [124]. Secondly, local low-temperature zones, where the combustion chamber may have regions of lower temperature, especially at the flame front edges or where fuel spray contacts the cold wall surface. Low temperatures can inhibit combustion reactions, causing some fuel molecules to exit with the exhaust before oxidizing. Thirdly, there is poor fuel atomization, where, if there are issues with the fuel nozzle or improper fuel pressure, the fuel may not atomize adequately into fine droplets, with larger droplets less likely to evaporate quickly and mix with air, increasing the likelihood of incomplete combustion. Finally, there is short residence time. Modern jet engines aim for high efficiency with compact combustion chambers, resulting in very short fuel residence times. This can prevent complete combustion of some fuel before it is expelled [125,126,127,128,129,130].

UHCs are a subset of volatile organic compounds (VOCs). A significant portion of VOC emissions from aircraft engines consists of UHCs [131,132]. These compounds undergo photochemical reactions with nitrogen oxides under sunlight, producing ozone and other secondary pollutants such as peroxyacetyl nitrates (PANs), which are primary contributors to photochemical smog and significantly impact air quality [133,134]. Although most UHCs have a short atmospheric lifespan, their involvement in various chemical reactions can lead to the formation of GHGs, like methane and carbon dioxide, indirectly contributing to global warming [124,135]. Over 8 million people fly on commercial airplanes daily, with approximately 5% suffering from respiratory diseases. Therefore, it is necessary to maintain high air quality onboard to protect the health of both crew and passengers. Formaldehyde, benzene, tetrachloroethylene, trichloromethane, 1,2-dichloroethane, and naphthalene are the six primary VOCs in the current cabin environment, posing risks to crew members that exceed the acceptable levels recommended by the US EPA [136]. Xylene in the cabin is significantly associated with irritation of the eyes, nose, and throat. Aldehydes, potential oxidation products of ozone, can also irritate human senses. Common symptoms among crew members and pilots include dry eyes and fatigue [137]. These VOCs may impact perceived indoor air quality and lead to passenger complaints. [138]. In air monitoring studies at Zurich Airport, a significant presence of reactive C2–C3 olefins and isoprene was found in engine exhaust. Isoprene was not detected in refueling emissions, and the benzene to toluene ratio in exhaust (1.7) was also higher than in kerosene refueling emissions (well below 1) [139]. Exposure to UHCs and VOCs poses health risks to humans. Inhalation of UHCs can irritate the eyes, nose, and throat, cause coughing, wheezing, and other respiratory issues, and increase the risk of chronic bronchitis and emphysema [140,141]. Long-term exposure to high VOC concentrations may raise the risk of cardiovascular diseases, including heart disease and hypertension. Polycyclic aromatic hydrocarbons (PAHs), which are highly toxic and potentially carcinogenic, can increase the risk of cancers such as skin and lung cancer through inhalation or skin contact [142,143,144]. High VOC concentrations can also affect the central nervous system, leading to headaches, dizziness, fatigue, and, in severe cases, impairing cognitive functions and behavior [145,146,147].

The comparison of UHC emissions between SAFs and traditional aviation fuels is controversial. Some scholars and engine manufacturers assert that both have similar UHC emission indices (EI-UHC), while others believe SAFs exhibit either lower or higher EI-UHC. The divergence in opinions arises from the interplay between engine conditions and fuel properties influencing UHC emissions. Undavalli et al. utilized a GTCP85 aircraft APU combustion platform and measured UHC emissions using the Signal 3000HM FID hydrocarbon analyzer. They discovered no significant trend in CO, UHC, and NO_x emissions across 18 different blends of HEFA and Jet A-1 fuels, although there was an overall decline in UHC emissions compared with Jet A-1. The engine operating conditions yielding increasing UHC emissions were, in order: main engine start (MES), environmental control system (ECS), and no load (NL). In MES mode, the engine operates at minimum capacity with an air to fuel ratio (AFR) of approximately 50. The low pressures and combustor temperatures in MES mode contribute to lower UHC emissions [68]. Liu et al. found that, in the premixed pre-evaporated Bunsen burner (PPBB) tests, blends of hydrothermal-condensation-hydrotreating jet (HCHJ) biofuel with RP-3 significantly reduced PM_2.5 and UHC emissions. In tests with the ZF850 jet engine, varying thrust from 48% to 97% showed no clear trend in EI-UHC for 0%, 5%, and 10% HCHJ blends with RP-3. Increasing AFR at low power reduced EI-UHC but prolonged combustion time due to lower temperatures. At high power, decreasing AFR increased EI-UHC due to higher fuel consumption, though higher combustion temperatures accelerated reactions. EI-UHC emissions were lowest in the 55–90% thrust range, with emission peaks at 48% and 95% thrust [148]. Badami et al. conducted studies on the SR-30 turbojet workbench using Jet-A, a synthetic gas to liquid (GTL) fuel, and a blend of 30% Jatropha methyl ester (JME) with 70% Jet-A. Similar to CO emissions, EI-UHC decreased with increasing angular speed. At 50,000–60,000 rpm, EI-UHC ranked from highest to lowest as GTL fuel, Jet-A, and the 30% JME blend. Above 70,000 rpm, the order shifted to Jet-A, GTL fuel, and the 30% JME blend, with differences in EI-UHC among the fuels diminishing as speed increased [149].

In addition to engine operating conditions, EI-UHC is also related to the properties of aviation fuel itself. Sundararaj et al.’s findings indicate that increasing the concentration of Camelina in blended fuels reduces EI-UHC, primarily due to the increase in primary zone temperature with higher Camelina concentration. Conversely, EI-UHC increases with higher concentrations of Jatropha in the blend, as Jatropha contains more carbon compounds and aromatics, leading to higher UHC emissions at low temperatures. Changes in the viscosity and density of fuel blends can alter spray characteristics, resulting in improper mixing in the combustion zone and subsequent UHC emissions. Moreover, EI-UHC is influenced by factors such as fuel residence time, evaporation time, and adiabatic flame temperature [77]. Ali et al.’s tests on the Olympus E-start HP turbojet engine show that for a 50% throttle valve position, 50% cotton methyl ester (CTME) blend fuel and 50% corn methyl ester (CRME) blend fuel reduce EI-UHC by 36.5% and 30.8%, respectively, compared with JetA-1. Additionally, EI-UHC decreases with increasing blend ratios of biodiesel, explained by the higher oxygen content in biofuels. The higher EI-UHC of JetA-1 may be due to its higher distillation points, leading to incomplete evaporation and combustion of the final fuel fraction [150]. Rehman et al. conducted emission tests on diesel and two different Jatropha biodiesel blends (15%, 25%) in an IS/60 Rovers gas turbine. The EI-UHC of both blends was lower than that of diesel and decreased with higher blend ratios, attributed to the higher fuel oxygen content in biodiesel. They also note that engine load/brake power had a greater impact on EI-UHC than biodiesel blending [151].

In a test report by engine and aircraft manufacturers [152], GE found that adding Bio-SPK to conventional aviation fuel slightly reduces NO_x emissions (1–5%) but increases CO and UHC emissions by 5–9% and 20–45%, respectively. The increase in CO and UHC emissions is primarily due to changes in the carbon-to-hydrogen ratio of the fuel, which lowers peak flame temperature, as well as changes in spray quality and flame position. Rolls Royce’s tests in the annular rig indicated that CO, NO_x, and UHC emissions from Syntroleum Fischer–Tropsch fully synthetic aviation kerosene and JP-8 fuel are similar. Pratt and Whitney’s tests on a Pratt and Whitney Canada small turbofan engine showed no significant differences in UHC, CO, and NO_x emissions between Jet A-1, 100% Neste oil, and their 50:50 blend.

Table 3. Information regarding the investigated SAF and fuels and for burning platforms and changes for UHC emissions in literature.

Ref.	Fuel	Method/Burning Platform	UHC Emissions
Undavalli et al. (2022) [68]	Jet A-1	GTCP85 aircraft APU	With the increase in HEFA proportion, there is no clear trend of change; the UHC emissions of HEFA-blended fuel slightly decrease.
	18 different ratios of HEFA and Jet A-1 blend fuels
Liu et al. (2023) [148]	RP-3	ZF850 jet engine	The impact of HCHJ doping on the EI-UHC of RP-3 shows no clear trend.
	5% and 10% HCHJ blend fuels
Badami et al. (2014) [149]	Jet-A	SR-30 turbojet workbench	GTL fuel > Jet-A > 30% JME blend fuel (50,000–60,000 rpm).
	GTL fuel
	30% JME blend fuel
Sundararaj et al. (2019) [77]	Jet A-1	High enthalpy rig with combustor	EI-UHC decreases with the increase of Camelina or the decrease of Jatropha in the fuel mixture.
	Biofuel (Camelina)
	Biofuel (Jatropha)
Ali et al. (2017) [150]	Jet A-1	Olympus E-start HP turbojet engine	50% CTME blend fuel and 50% CRME blend fuel respectively reduced EI-UHC by 36.5% and 30.8%.
	50% CTME blend fuel
	50% CRME blend fuel
Rehman et al. (2011) [151]	Diesel	IS/60 Rovers gas turbine	Diesel > 15% Jatropha biodiesel > 25% Jatropha biodiesel
	15% Jatropha biodiesel
	25% Jatropha biodiesel
GE [152]	Conventional aviation fuel	GE engine	The emissions of UHC increase by 20–45%.
	Bio-SPK
Rolls Royce [152]	JP-8	Annular rig	Not much difference.
	FT fuel
Pratt and Whitney [152]	Jet A-1	a Pratt and Whitney Canada small turbofan engine	Not much difference.
	Neste oil
	50% Neste oil blend fuel

, below, summarizes information and comparative conclusions on UHC emissions from these studies. The UHC emission performance of fuel is determined by both engine conditions and fuel properties. EI-UHC is predominantly influenced by engine conditions, with the main point of contention being the composition and properties of SAFs. The closer SAFs are to conventional aviation fuels, the smaller the differences in EI-UHC. Otherwise, the physical and chemical properties of SAFs will complexly affect the entire process of atomization, combustion, and emissions, leading to either lower or higher EI-UHC.

2.7. Comprehensive Comparison of Emissions Performance between SAFs and Conventional Aviation Fuels

This study conducts a comparative analysis of CO₂/NO_x/SO₂/CO/PM/UHC emissions between traditional aviation fuels and SAFs. Overall, SAFs exhibit significantly lower emissions of SO₂ and PM from aviation engine exhaust compared with traditional fuels. SAFs offer no obvious advantages in CO₂, CO, NO_x and UHC emissions compared with conventional petroleum-based aviation fuels during engine combustion. However, from an LCA perspective, SAFs demonstrate a significant advantage in CO₂ emissions, thus positioning them as a key strategy for addressing carbon emissions challenges in the aviation industry. CO, NO_x and UHC emissions are determined by both the nature of the fuel and the operating conditions of the engine, which would explain the controversy over the strength of the two fuels in terms of the emissions of these pollutants. The comprehensive comparison of emission performance between SAFs and traditional aviation fuels is presented below.

From a combustion and emission perspective, the difference in CO₂ generation between SAFs and traditional aviation fuels is negligible. However, from an LCA perspective, SAFs derived from different processes and feedstocks, such as FT, HEFA, ATJ, and pyrolysis, can significantly reduce GHG emissions in the aviation industry by 41–89%. Due to the direct correlation between SO₂ emissions and sulfur content in fuels, SAFs have extremely low sulfur content, enabling pure FT fuel to reduce SO₂ emissions from pure JP-8 by 92%. Blending various types of SAFs with traditional aviation fuels can also reduce SO₂ emissions to varying degrees, depending on the blending ratio. The aviation industry’s PM emissions are closely related to sulfur and aromatic compound content in fuels, showcasing the advantages of SAFs. Compared with Jet A-1, fuels such as 100% CH-SKA, 50% HEFA-SPK, and 100% FT-SPK can reduce EI-PM by 7–25%, 40–60%, and 70–95%, respectively, while ATJ-SPK significantly reduces total particle number emissions by up to 97%. In contrast, NO_x emissions mostly originate from thermal drive, as engine tests control power output, maintaining similar combustion temperatures. Hence, the difference in NO_x emissions between SAFs and traditional aviation fuels is minimal. Similarly, under identical conditions, SAFs and traditional aviation fuels exhibit minor differences in CO emissions, with variations in EI-CO among different fuels possibly stemming from viscosity, carbon content, SMD, and final boiling point. EI-UHC is influenced by both engine operating conditions and fuel properties. When the composition and properties of SAFs are similar to conventional aviation fuels, the difference in EI-UHC is minimal. SAFs with lower aromatic hydrocarbon content, and lower distillation points are advantageous for reducing UHC emissions. However, if some properties of the SAFs, such as viscosity, density, residence time, etc., do not align with the design of the engine’s atomization and combustion systems, then incomplete fuel combustion result, with a concomitant tendency for increased UHC emissions.

3. Conclusions

SAFs represent a primary tool for achieving net-zero emissions in the aviation industry by 2050. Their combustion performance must be capable of seamlessly replacing traditional aviation fuels, considering both usability and safety. Equally important is the requirement for SAFs to mitigate the adverse effects of aviation emissions on ecological environments and human health while reducing GHG emissions.

From an LCA perspective SAFs have much lower CO₂ emissions than conventional aviation fuels. In an aeroengine, fuel combustion is a complex process in the combustor of the engine, which transfers chemical energy into heat energy. The completeness of combustion is related to a fuel’s composition and properties, including spray, evaporation, and flammability. Therefore, engine performance is not only affected by fuel performance, but also interacts with engine structure and control laws. The SO₂ and PM emissions from SAFs have a clear advantage due to less sulfur and aromatic hydrocarbon content. CO/UHC emission parameters are controlled by both the engine and the fuel properties, while NO_x emission parameters are mainly influenced by the engine. To sum up, SAFs offer substantial advantages in CO₂, SO₂, and PM emissions compared with conventional petroleum-based aviation fuels.

There are technical gaps in comparing the emission performance of SAFs and traditional aviation fuels. For instance, there is a need to intensify research on the emission performance of SAFs at the engine and even aircraft level. Comprehensive studies on the emissions performance of ASTM-approved and non-approved SAFs are lacking. The variability in SAF compositions due to different feedstocks and production processes complicates uniform analysis. Differences in experimental application engines, detection equipment, and human error prevent cross-comparison of results. Moreover, there is a paucity of research on the CO emission performance of SAFs.

In the future, more research should be directed towards the physicochemical properties, atomization and combustion performance of SAFs and their components, as well as their performance in engines and aircraft levels. The development and application of SAF technology will be promoted to accelerate the process of realizing net-zero emissions in the aviation industry.