Relation of the Content of Sustainable Components (HEFAs) in Blends with Hydrotreated Straight-Run Kerosene to the Properties of Aviation Fuel

Abstract

An expected increase in the demand for aviation transport service will result in the deterioration of the environment and human health, respectively, due to extra greenhouse gas (GHG) emissions. Concerns from EU institutions about the issue have led to legislation initiatives and, later, to development of Regulation (EU) 2023/2405 for the reduction of GHG emissions via the substitution of fossil kerosene with an increasing share of sustainable components. Hydroprocessed esters and fatty acids (HEFAs) are the most commercially acceptable sustainable alternative but their influence on aviation fuel properties needs to be further evaluated in terms of all required and extended properties, as per ASTM D1655. The main properties, together with the rarely reported upon existent gum, water separation, corrosion, and the electrical conductivity of HEFAs and their blends with fossil kerosene were quantitatively evaluated in this study. For every increase of 10% (v/v) of HEFAs, the following fuel properties improve: the freezing point decreases by 1.3 °C, the smoke point increases by an average of 3 mm, and the specific net energy increases by 0.08 MJ/kg. The acidity of HEFAs are an order of magnitude higher than that of conventional aviation fuel and, thus, close to the limit. The existent gum of the studied SAF is higher than that of fossil kerosene due to, most probably, the presence of non-evaporated residual material.

1. Introduction

Aviation fuel traditionally originates from the kerosene fraction of crude oil, which is further processed in refinery hydrotreating or caustic washing units [1], mainly to reduce mercaptan sulfur and, thus, fulfil the specified limit. Additives like antioxidants, metal deactivators, and static dissipators enhance the properties of kerosene for its transformation into JET A-1 turbine fuel [2]. The aviation industry is experiencing a revival since the COVID-19 pandemic [3,4], leading to an increase in global demand for aviation fuel. According to the International Air Transport Association (IATA), this demand is expected to reach 6.46 million gallons per day (around 19.8 thousand tons/day). The International Energy Agency (IEA) predicts a slightly higher demand of 7.19 million gallons per day (around 22 thousand tons/day) in 2024 [5]. Looking ahead, this demand is projected to double by the year 2050 [3,6,7]. However, the increased consumption of aviation fuel will affect the environment and human health with increased greenhouse gas (GHG) emissions [8,9]. Air transport produced 914 million tons of CO₂ in 2019, which is 2.1% of the total human-produced emissions of CO_2, and 12% of the CO₂ emissions from the transport sector [4,8]. In order to counteract the negative influence of emissions, the EU and all its member states have adopted the motion to achieve climate neutrality by 2050 (European Green Deal), as the initial aim is to reduce EU emissions by at least 55% by 2030 (Fit for 55 package) compared to 1990 [10]. Decarbonization policies, in light of transport emissions, require a reduction of 90% by 2050 (compared to 1990 levels) [11]. The new EU legislation initiative RefuelEU allocates transport sector emission reduction goals for the aviation sector, with the vision of blending sustainable aviation fuels (SAFs) with conventional/mineral jet aviation fuel as the key potential concept for decarbonizing the air transport sector [12]. Conventional jet fuel GHG emissions are 90 g/CO_2eq/MJ and are more than twice as high as those of SAF-type hydroprocessed esters and fatty acids—HEFAs (max. 40–53 g/CO_2eq/MJ) [8,13]—as the GHG emissions of the latter are also recyclable. This initiative was updated in Regulation (EU) 2023/2405 of the European Parliament and of the Council, which determines a mandatory scheme on the use of SAFs. Together with the positive effect (reduction of the carbon footprint of the aviation sector) of SAFs, a concern arises about their availability and price, which is higher than that of fossil kerosene fuels [3,12,14]. SAFs are defined as eight different synthetic components described in ASTM D7566, with the commercial dominance [3,14] of the use of HEFAs for their growing availability and reduced costs [15] of advanced feedstock, mature production technology (deoxygenation and isomerization of waste vegetable oils and animal fats), and successful tests in real flights [1].

Initiative RefuelEU, its update in Regulation (EU) 2023/2405, and the period thereafter are accompanied by a detailed study on the production and effect of synthetic components [1,2,3,8,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33] and especially HEFAs [3,7,8,13,14,15,16,17,19,20,21,22,24,25,26,27,28,30,32,33,34,35] on their blends with fossil aviation fuel properties. Some of the publications [14,15,20,30,31,32] reveal the effects of chemical composition on jet fuel performance characteristics. The arene content [14,16] seems to have a critical influence on its physicochemical properties. Bio-jet fuels possess satisfactory low-temperature fluidity [33]—low freezing point [16,20,22,30,31] (due to high content of iso-paraffins)—and viscosity [17,20,30]. They are reported in the literature [14] to have favorable thermal stability but relatively poor oxidative stability due to the absence of heteroatom-containing compounds and oxidatively stable arenes. The satisfactory combustion property [14,16] of SAFs is characterized by a higher smoke point and a higher specific energy [17,20,22,30] (due to the absence of arenes) in comparison to fossil aviation fuel. Their incompatibility [14] with current fueling systems is defined by seal material swell and low lubricity—lack of arenes and polar compounds. SAFs exhibit good fuel volatility [20,22] determined by distillation [30] and flash point [17,30,31], and also satisfactory fuel density [17,20,22,30,31]—low arene content. Other properties like total acid number, water content, differential scanning calorimetry, surface tension [17], copper corrosion rating, and existent gum [31] are infrequently reported. Most of the references [16,17,22,31,33] describe some properties only for fossil fuels, or only for SAFs, and few references [14,30] report some of the target properties of fossil/SAF blends. Not a single reference that fully characterizes (as per all ASTM D1655 requirements) fossil/SAF blends can be found. Blend properties are of great importance, since [30] not all of the fuel characteristics are linearly dependent on SAF content and cannot be calculated on the basis of fossil and SAF value only.

The authors of [14] highlight a significant lack of information regarding certain properties of bio-jet fuel in the published literature, such as existent gum, water separation, corrosion, and electrical conductivity. While these properties are not typically focal points in fuel performance evaluations, they remain important. For instance, high levels of existent gum can indicate fuel contamination (higher boiling-point oils, particulate matter, or oxidation products [36]), potentially affecting storage stability. Consequently, there is a necessity, in the existing literature, for the characterization of these seldom-reported properties and, together with other extended properties, for the exposition of a full quantitative understanding of the influence of mineral JET A-1 and HEFAs on their blend quality. Together with the fulfilment of the technical requirements for high-quality products, additional political initiatives are needed [35] to overcome the existing challenges (inadequate financial support, feedstock competition, land constraints, costly scale-up) for HEFA promotion and, thus, to meet the 2050 sustainability targets in the aviation sector.

2. Materials and Methods

Two types of components—conventional (fossil-, mineral-, or crude oil-derived) and synthetic (HEFA-SPK, SAF, or biojet)—and their blends containing up to 42% (v/v) of the second component were evaluated in this study. Mineral component was produced in a crude oil refinery, situated in Burgas, Bulgaria and HEFAs were supplied from ENI Biorefinery that operates with a dedicated UOP Ecofining^TM technology. The property variations of mineral JET A-1 and biojet fuel characteristics are presented in

Table 1. Physicochemical properties of commercial batches of crude oil-derived JET A-1 turbine fuel for the period 2019–2023, mineral turbine fuel—JET A-1 used in this study—and commercially produced HEFA-SPK.

Property	Test Method	Requirements of DEF STAN 91/ASTM D1655	Property Variations of Commercial Batches of Mineral Turbine Fuel—JET A-1			Mineral Turbine Fuel—JET A-1 Used in This Study	HEFA-SPK
			Min.	Max.	Aver.
APPEARANCE
Visual		Clear, bright, and visually free from solid matter and un-dissolved water at ambient temperature	Clear, bright, and visually free from solid matter and un-dissolved water at ambient temperature
Color	ASTM D156-23	report	20	30	29.8	30	26
Particulate contamination, mg/L	ASTM D5452-23	max 1.0	0.08	0.8	0.20	0.20	NA
Particulate at point of manufacture. Cumulative channel particle counts	IP 565/14
≥4 μm (c)		max 19	14	18	15.9	15	17
≥6 μm (c)		max 17	4	17	13.9	14	16
≥14 μm (c)		max 14	8	13	10.2	10	12
≥21 μm (c)		report	5	12	8.2	7	10
≥25 μm (c)		report	0.7	11	6.9	6	9
≥30 μm (c)		max 13	0	9	5.7	5	8
COMPOSITION
Total acidity, mg KOH/g	ASTM D3242-17	max 0.015	0.001	0.003	0.0013	0.001	0.011
Total aromatics, % (v/v)	ASTM D6379-21e1	max 26.5	11.3	19.3	16.3	18.1	0.8
Sulfur total, % (m/m)	ASTM D5453-19a	max 0.30	0.0001	0.002	0.00028	0.0005	0.0007
Mercaptan sulfur, % (m/m)	ASTM D3227-23	max 0.0030	0	0.0003	0.000012	<0.0003	<0.0003
VOLATILITY
Distillation °C	ASTM D86-23
Initial boiling point		report	148	174	167	166	158
10% (v/v) distilled		max 205.0	178	191	186	184	167
50% (v/v) distilled		report	201	210	207	204	186
90% (v/v) distilled		report	226	236	232	232	251
End boiling point		max 300.0	236	248	242	244	266
Residue, % (v/v)		max 1.5	1.0	1.2	1.2	1.2	1.2
Loss, % (v/v)		max 1.5	0.1	1.5	0.9	0.8	1.4
Flash point, °C	ASTM D56-22	min 38.0	48.5	62.0	57.5	55.5	48.0
Density at 15 °C, kg/m3	ASTM D4052-22	min 775.0/ max 840.0	796.8	806.8	803.0	802.4	753.6
FLUIDITY
Freezing point, °C	ASTM D7153-22ae1	max minus 47.0	−50.5	−47.3	−48.7	−49.3	<−56.0
Viscosity at minus 20 °C, mm2/s	ASTM D445-21e2	max 8.0	4.2	5.1	4.8	4.548	3.838
COMBUSTION
Smoke point, mm	ASTM D1322-22	min 25.0	25.0			25.0	>42
Specific energy—net, MJ/kg	ASTM D4809	min 42.80	43.21	43.42	43.30	43.277	44.128
CORROSION
Corrosion. Copper strip (2 h +/− 5 min at 100 °C +/− 1 °C), class	ASTM D130-19	max 1	1a			1a	1b
THERMAL STABILITY (JFTOT) at 260 °C
Tube deposit rating (Annex A1-VTR), class	ASTM D3241-23a	Less than 3, no “Peacock” or “Abnormal” color deposits	0. no (P) or (A)			0. no (P) or (A)	0. no (P) or (A)
Filter pressure differential, mm Hg		max 25	0.00	2.00	0.15	0	0
CONTAMINANTS
Existent gum, mg/100 mL	IP 540/19	max 7	1			1	6
Microseparometer (MSEP), rating
MSEP without SDA	ASTM D3948-22	min 85					82
MSEP with SDA		min 70	92	99	97	97
CONDUCTIVITY
Electrical conductivity, pS/m	ASTM D2624-22	min 50/max 600	355	590	472	511	9
Total metabolically active microbial contamination, pg ATP/mL	ASTM D7687	max 10	2.6				<5.0 1

¹ value for 42% SAF in mineral JET A-1 blend.

. Analysis methods are also shown in Table 1 and are in accordance with DEF STAN 91/ASTM D1655 requirements due to the necessity for HEFA/mineral JET A-1 blends to satisfy them (ASTM D7566). Density was measured at 15 °C according to ASTM D4052 utilizing automated sample injection equipment. Distillation analysis was executed by both ASTM D86 and ASTM D2887 procedures, as the former method provides approximately one theoretical plate fractionation and the second is simulated distillation determined by gas chromatography (GC) of studied fractions in the boiling point range from 55.5 to 538 °C. ASTM D1322 test method provides an indication of smoke-producing properties of aviation turbine fuels in a diffusion flame. The instrument measures the smoke point from 0 mm to 50 mm. A high smoke point indicates a fuel of low smoke-producing tendency. Freezing points of studied aviation fuel and SAF were measured as per ASTM D7153 requirements. This test method covers the determination of the temperature (ranging from 20 °C to minus 80 °C) below which solid hydrocarbon crystals may form. Specific net energy or heat of combustion was determined by bomb calorimeter equipment, according to ASTM D4809. Thermal oxidation stability determination is described in ASTM D3241. This test method uses an instrument that subjects the test fuel to conditions occurring in gas turbine engine fuel systems. The fuel is pumped at a fixed volumetric flow rate through a heater, after which it enters a precision stainless-steel filter where fuel degradation products may become trapped. To test for existent gum content according to IP 540, a 50 mL sample was evaporated in an aluminum block bath for a specified period under controlled conditions of temperature and flow of air. Existent gum is the nonvolatile residue. Total metabolically active microbial contamination of both conventional and SAF components was measured according to ASTM D7687 procedure.

Table 2. Boiling range and normal paraffin distribution of mineral turbine fuel and SAF by gas chromatography—ASTM D2887 (SIMDIS).

	TURBINE FUEL-JET A-1	SAF
	From Tank Storage in European Refinery	HEFA-SPK
SIMDIS 2887
Recovered mass%	°C	°C
IBP	123	134
5	155	143
10	166	151
20	176	161
30	188	168
40	197	178
50	208	189
60	217	201
70	227	217
80	236	248
90	249	269
95	255	277
FBP	273	311
ASTM D 86 correlation (STP 577) distribution
Recovered Vol%	°C	°C
IBP	164	159
5	176	163
10	182	167
20	189	171
30	194	175
50	205	187
70	217	218
80	224	235
90	232	251
FBP	248	274

presents the SimDist and the ASTM D86 correlation (STP 577) distribution of both conventional turbine fuel and biojet in line with ASTM D7566.

3. Results and Discussion

The strict aviation safety and performance requirements for aircraft jet engines and the increased aviation fuel demand in post-COVID-19 times encourage refineries to spend a lot of effort to produce high-quality jet fuel. Since properties of both constituents in aviation turbine fuel (JET A-1)—conventional (ASTM D1655) and synthetic (ASTM D7566)—have to answer the requirements of ASTM D1655, the quality variation in the conventional part should be considered first, and then the contribution of the synthetic blending component—synthesized paraffinic kerosene from hydroprocessed esters and fatty acids (HEFA-SPK)—can be evaluated.

3.1. Conventional, Crude Oil-Derived Aviation Turbine Fuel JET A-1

The property range of mineral aviation fuel refers mainly to the type of crude oils being processed, the presence of kerosene fractions from conversion processes in the final JET A-1 fuel, and the refinery scheme for processing (caustic washing or hydrotreatment) the kerosene fraction. The aviation turbine fuel (produced by a refinery situated in Bulgaria) properties, disclosed in Table 1, show moderate variation as this refinery treats only straight-run kerosene fractions in a dedicated fixed-bed hydrotreating unit (HTU) to produce its JET A-1 product. The HTU operates at low severity, as the weighted average bed temperature—WABT—of the reactor is around 290 °C and the liquid hourly space velocity—LHSV—is of maximum 1.4 h⁻¹. Even at this low severity, the HTU is capable of decreasing sulfur content of the kerosene feed down to less than 10 ppm (w/w) with an average value of around 3 ppm (w/w). Mercaptan sulfur is reduced to 0–3 ppm (w/w) with an average value of around 0.1 ppm (w/w). The ultra-low level of total sulfur and mercaptans in hydrotreated kerosene reveals the opportunity to blend it with components containing higher sulfur species. Therefore, HTU kerosene products can be blended with about 29% 115 ppm (w/w) mercaptan sulfur containing straight-run heavy naphtha (fraction with a boiling rage of 100–180 °C, which is typically utilized as a catalytic reformer hydrotreater feed) and, thus, can increase the potential sources for JET A-1 production—Figure 1.

The density of the straight-run kerosene feed is lowered in the hydrotreating unit, with about 3.5 kg/m³, as the hydrotreated kerosene density does not exceed a maximum of 806.8 kg/m³ with average value of 803 kg/m³ and, thus, stays in the middle of the specification limits of 775 kg/m³ ÷ 840 kg/m³. This conventional base allows blending with lighter and heavier components. The total acidity of hydrotreated kerosene is significantly lower (average 0.0013 mg KOH/g) than the ASTM D1655 requirement and, thus, enables blending with more acidic components. The average value of the flash point of commercial batches of mineral turbine fuel—JET A-1—is 57.5 °C and opens room for blending components with a lower flash point—straight-run heavy naphtha (SRHN). According to the correlation in Figure 1c, an additional 12.5% of SRHN can be blended with hydrotreated kerosene with a 53.7 °C flash point and, at the same time, the flash point of the blend will be slightly above the required minimum of 38 °C. Even the total aromatic content of 11.3% (v/v) ÷ 19.3% (v/v) is below specification limits of maximum 26.5% (v/v). Related to this property, the smoke point is, at minimum, an acceptable value of 25 mm. To lower the propensity of aviation jet fuels to form soot, the addition of a low-aromatic, high-smoke-point component is needed. Crude oil-derived JET A-1 fuel is both thermally and chemically stable as the filter pressure differential is between 0 and 2 mm Hg and existent gum is 1 mg/100 mL. Viscosity and freezing point are the physical properties used to quantitatively characterize the fluidity of jet fuel [2]. Viscosity at minus 20 °C varies in a very narrow range from 4.2 mm²/s to 5.1 mm²/s and fall in the middle of the acceptable interval—up to 8 mm²/s. The middle value of viscosity provides a fine spray of fuel droplets that evaporates quickly as it mixes with air [2]. In this light, the mineral aviation turbine fuel from Table 1 can be blended with more or less viscous materials. The freezing point is near the upper limit of the specification with its −48.7 °C and calls for a low-freezing component; nevertheless, the freezing point is well above the temperature at which aviation fuel completely solidifies [2]. Other properties, presented in Table 1, of the conventional JET A-1 fuel are within the specification limits. The distillation profile characterizes the fuel’s tendency to vaporize as the profile of aviation fuel can be analyzed according both ASTM D86 and ASTM D2887—simulated distillation. The first one is detrimental, as it concerns the specification limitation in ASTM D7566. Table 2 presents the SimDist of conventional turbine fuel and the ASTM D 86 correlation (STP 577). The comparison of both distillations analyzed according to ASTM D86 from Table 1 with those calculated according to ASTM D2887 values in Table 2 reveals that the bias between the two are within the reproducibility (repr.) of ASTM D86. For the initial boiling point (IBP), the absolute difference (AD) is 1.4 °C against 9.1 °C repr.; for 10% (v/v) recovered, 2.1 °C/4.0 °C repr.; for 50% (v/v) recovered, 0.7 °C/3.0 °C repr.; for 90% (v/v) recovered, 0.2 °C/3.5 °C repr.; and the biggest deviation is for the final boiling point (FBP), 4.8 °C/7.1 °C repr. In regard to the distillation profile, ASTM D2887 analyses and the ASTM D86 values calculated from them are a versatile approach, providing lower sample quantity and shorter analysis time advantages. ASTM D2887 also gives very important information about the distribution of hydrocarbons by total carbon atom numbers (Cn—sum of normal and non-normal alkanes)—Figure 2.

It is worth mentioning that crude oil-derived aviation fuel is a narrow C₉–C₁₅ cut with a unimodal type of C_n distribution curve with a peak at 11 total carbon numbers and it is important to acknowledge that this is typical for straight-run fraction distribution types, as presented in [1,16]. The distribution type of n-alkanes in conventional aviation fuel by carbon number repeats the unimodal distribution of total carbon atom numbers with a maximum at undecane.

Several relations between the properties (their range is presented in Table 1) of commercial batches of mineral turbine fuel—JET A-1—produced by the European refinery can be derived. Distillation (ASTM D86) temperatures for the lowest boiling point components in mineral aviation fuel—both IBP and 10% distilled—are strongly correlated [33]; their squared correlation coefficient is R² > 0.87, with the lowest temperature being that at which the vapors above JET A-1 will ignite on the application of an ignition source—flash point. The straight line in Figure 3a describes the correlation between IBP and the flash point, the slope is 0.54 and indicates that every 10 °C increase in IBP results in a 5.4 °C increase in the flash point.

The slope of the straight line dependence between the 10% distilled and flash point is 0.92 and is pointed out in Figure 3b. Decreasing low boiling point components in aviation fuel will afford an additional degree of safety in the handling and transporting of fuel [2].

The net heat of combustion or net specific energy of conventional aviation fuel is known to be positively related to the hydrogen-to-carbon (H/C) ratio [2]. Since arenes (represented by total aromatics analysis) suffer from low H/C ratio due to the presence of double bonds, their increase in mineral jet fuel leads to a linear decrease in the net heat of combustion values, as is presented in Figure 3d. The correlation is strong due to the squared correlation coefficient value of 0.82 and, from Figure 3d, it can be calculated that the total aromatics content increase of 8% (v/v) (from 11.3% (v/v) to 19.3% (v/v)) is related to the decrease of net heat of combustion by 0.19 MJ/kg (from 43.42 MJ/kg to 43.23 MJ/kg). Combustion characteristics can also be predicted from the fuel density because this property is a function of fuel composition. The net heat of combustion can be calculated with a lower accuracy as R² = 0.76—Figure 3c—from the mineral jet fuel density than from its total aromatics. Density is also related (via its dependence on fuel composition) to the viscosity of the fuel and the strong correlation is presented in Figure 3e.

3.2. Synthetic Blending Component (HEFA-SPK) and the Effect over Its Blends with Conventional Aviation Turbine Fuel JET A-1

Physicochemical characteristics of the synthetic component used in this study, also called sustainable aviation fuel (SAF), synthesized paraffinic kerosene from hydroprocessed esters and fatty acids (HEFA-SPK), or simply biojet are shown in the right-hand column of Table 1. The evaluation of SAF’s influence over its blends with mineral jet was accomplished by preparing five blends with 2% (v/v), 6% (v/v), 20% (v/v), 34% (v/v), and 42% (v/v) of SAF, in accordance with the requirements of Regulation (EU) 2023/2405 on ensuring a level playing field for sustainable air transport. The boiling range and carbon number distributions of HEFAs, obtained by gas chromatography, are shared in Table 2 and Figure 2. The analyzed SAF appears to be a bit broader than conventional jet fuel, with mainly C₉–C₁₆ hydrocarbons and a low content of C₁₇–C₃₀ long-tail cut; the Cn distribution curve presented in [1,16] confirms the bimodal carbon number distribution, with the first peak at 10 total carbon numbers and the second at 15. Thus, the total carbon number distribution of biojet differs from that of the conventional normal unimodal distribution of total carbon atoms. The difference in the GC carbon number distribution explains the distinction in ASTM D86 distillation curves of both synthetic and mineral kerosene (Table 1). Biojet contains higher boiling-point components from 90% (v/v) distilled to the end of the distillation curve and lower boiling-point components in the beginning of the distillation curve.

Blending SAF with conventional aviation fuel is almost linear along the distillation profile, with the exception of IBP and the end (EBP) or final boiling point (FBP), as is shown in Figure 4. Increasing the SAF content results in lower IBP than the linearly calculated (Figure 4a) value, due to the higher content of low-boiling-point compounds (either higher content of low carbon number compounds—Figure 2a—or highest iso-alkane content) in the SAF, which heavily influence the blend’s IBP. Following the opposite trend, the effect of increased SAF content on the blend’s EBP—linearly calculated (Figure 4e) value—is below the measured one. The reason is the greater content of higher boiling-temperature components (presence of C₁₇–C₃₀ tail cut, according to Figure 2a) in SAF. The distillation properties are mainly influenced by the carbon chain length and the type (iso-, cyclo-, n-alkanes, arenes, and alkenes) of hydrocarbon. Within compounds of the same carbon number, boiling points follow the following order: iso-paraffin, n-paraffin, naphthene, and aromatic. Notably, the boiling point difference between iso-paraffin and aromatic hydrocarbons with the same carbon number is larger than the difference between compounds of the same class differing by one carbon number [2]. Like conventional aviation fuel, SAF’s physical distillation according to ASTM D86 values are very close to those of ASTM D2887 and are within the reproducibility of the first method, with the only exception being the final boiling point (FBP)—7.9 °C/7.1 °C repr. Similarly, for mineral kerosene, ASTM D2887 can be freely used for commercial applications, as it has greater benefits than ASTM D86.

Extending the hydrocarbon type composition from Figure 2a,b, only the arene content is included in the obligatory analyses for conventional and biojet fuels. The total aromatics content in the two fuels is shown in Table 1. The conventional aviation fuel contains 18.1% (v/v) total aromatics and the HEFAs are an almost arene-free component, with only 0.8% (v/v). The low total arene content in HEFAs is affects fuel performance, mainly positively (formation of low incipient soot/particulate matter emissions, high fuel efficiency, high H/C ratio, etc.), but also negatively (leaks in fuel systems due to seal shrinkage) [14,15,17,18]. Having in mind the latter, the ASTM D7566 set extended the requirements for minimum aromatics at 8–8.4% (v/v), depending on the analytical method employed.

As expected from the total aromatics in the two individual components, Figure 5b presents a linear (R² > 0.99) decrease of total aromatics with the increase of SAF content in its blends compared to conventional jet fuel. Every 10% (v/v) increase of SAF content in this blend leads to an almost 1.76% (v/v) decrease in aromatics. In other words, with the evaluated conventional and SAF components, it is not possible to answer the ASTM D7566’s minimum aromatic requirement if the SAF component is above 60% (v/v). In the case of conventional aviation fuel, aromatics are at the lower value (Table 1) of 11.3% (v/v); the maximum proportion of SAF in the blend in order to obtain a minimum of 8% (v/v) aromatics is no more than 19% (v/v). This means that the 2040 target of Regulation (EU) 2023/2405 for a minimum share of 34 % of SAF (of which 10% are synthetic aviation fuels) and the ASTM D7566 adoption of up to 50% (v/v) HEFA-SPK will not be achievable with the commercially produced mineral and bio components under study if additional aromatics components are not included in the blend.

Despite arenes (total aromatics), both mineral and biojet fuels contain n-alkane, iso-alkane, cycloalkane, and traces of alkene types of hydrocarbons [8,14,15,19]. Such a detailed composition (including hydrocarbon classes and carbon chain length) can be received by gas chromatography/mass spectrometry (GC/MS) determination. On one hand, chemical compositions of jet fuel defines its performance characteristics, but on the other hand, such a detailed composition is not required by specifications or standards and that is why the European producer of mineral jet fuel (like a typical refinery) does not possess dedicated GC/MS laboratory equipment. That is the reason for evaluating differences in chemical compositions and properties, respectively, of the studied components from Table 1 from the published literature on GC/MS data. What is more, the producer of HEFAs claims that the product properties and composition, respectively, can be controlled independently from the type of bio feedstock used [27,28]. References [17,18,20,21] reveal a comprehensive hydrocarbon composition of HEFAs and mineral aviation fuel. The differences in hydrocarbon classes are as follows: iso-alkane, 87–90.5% (m/m) for HEFAs against 31–39% (m/m) for conventional fuel; arenes (<1% (m/m)) and cycloalkanes (0.3–1.7% (m/m)), thus almost absent in HEFAs against 14–24% (m/m) arenes and 15–24% (m/m) cycloalkanes in fossil fuel; n-alkane, 8.5–13% (m/m) for HEFAs against 21–28% (m/m) for conventional fuel.

The low density, 753.6 kg/m³, of HEFAs is a function of the lack of arenes and cycloalkanes in its composition, although heavy hydrocarbons with 15 and more carbon atoms (Figure 2) in HEFA chain length are superior to those of fossil aviation fuel. Figure 4g presents the decrease of density of HEFA/mineral fuel blend with the increase of sustainable fuel. The linear dependence predicts that above 55% (v/v) SAF in the blend, it hardly stays within the required minimum of 775 kg/m³. When lower density fossil fuel (796.8 kg/m³ from Table 1) is blended, SAF content hardly reaches 47%.

The safe handling of aviation fuels is expressed by the flash point. The flash point versus increasing the SAF/fossil fuel blend ratio displays a strong (R² > 0.99) linear decreasing behavior, which is presented in Figure 4f. The lower flash point of SAF (48.0 °C) against that of mineral fuel (55.5 °C) is dominated by the presence of more volatile components in the lighter part of the distillation curve (Table 1 and Table 2 and Figure 2). Differences in hydrocarbon class additionally contribute to the lower flash point of SAF. For hydrocarbons of the same carbon number (let us consider 12), the lowest flash point is for iso-alkane (2-Methylundecane’s flash point is 41 °C) followed by n-alkane (n-Dodecane with a flash point of 71 °C) and the n-Hexylbenzene arene hydrocarbon with 83 °C. The sustainable component is rich in iso-alkane 87–90% (m/m) and arene-free.

The total acidity of HEFA-SPK is 0.011 mg KOH/g and is close to the maximum allowable acidity (Table 1). It appears that the acidity of the bio component is an order of magnitude higher than that of conventional aviation fuel. Blending biojet with the conventional fuel results (Figure 5a) in an increase of total acidity proportional to the increase in bio content, but remains below the standard requirement, even at the highest share of SAF in the aviation fuel blend.

Along with composition and acidity, the fluidity of the aviation fuel, blended with the bio component, is revealed in Figure 5. Both properties—freezing point and viscosity—quantitatively characterize the ability of the fuel to flow. The freezing point of compounds increases with the carbon number within each class, but it is greatly affected by the molecular shape. N-alkanes and unsubstituted arenes freeze at higher temperatures compared to other compounds with the same carbon number due to their geometry, which enables them to easily pack into a crystalline structure [2]. The presence of more low-boiling-point components, abundant in the lowest freezing point iso-alkanes—87–90% (m/m)—and the lack of unsubstituted arenes determines the best low-temperature properties of HEFAs (below −56 °C) in comparison to mineral aviation fuel (below −49.3 °C). Figure 5c shows the advantage of increasing the SAF/fossil fuel ratio over the resistance of the blend to solidification. With every 10% (v/v) increase of SAF, the blend freezing point decreases linearly by 1.3 °C. Thus, the addition of SAF allows the increase of the final boiling point of the kerosene fraction during the atmospheric distillation of crude oil and allows to secure greater availability and lower price, respectively [2].

Viscosity is another aspect of aviation fuel fluidity. High viscosity influences spray pattern and droplet size, which crucially affect the engine’s re-light capability in flight [2,22]. Due to this consideration, an upper kinematic viscosity limit of maximum 8.0 mm²/s in jet fuel is fixed in specifications. In this line, the lower kinematic viscosity of SAF (3.8 mm²/s) than that of the fossil component (4.5 mm²/s) is an advantage. Viscosity is related to both the carbon number and to the hydrocarbon class of fuel components. The difference in viscosity between the bio and mineral part in studied blends can be explained by the greater cycloalkane content in the fossil fuel (15–24% (m/m) versus 0.3–1.7% (m/m) in SAF) constituent, which generally has slightly higher viscosity than alkanes or arenes [2]. Figure 5d presents a strong (R² > 0.95) exponentially decreasing behavior of the viscosity of the blends with the increase of the SAF/fossil component ratio.

Another advantage of the studied bio component performance properties are their improved fuel combustion quality compared to conventional aviation fuel, as shown in Figure 6. Emissions of carbon particles responsible for the visible smoke or soot and quantitatively presented by smoke point are influenced by both the engine design and fuel composition [2]. The SAF from Table 1 is characterized by a high smoke point of more than 42 mm, the value of which is above the detection limit of ASTM D 1322. This high value is determined by the lack of arenes in HEFAs—total aromatics of 0.8% (v/v)—and thus, the low carbonaceous particles forming propensity. Since the smoke point of the fossil aviation fuel, used for the blends in Figure 6a, is at the lower limit (25 mm) of the requirements of DEF STAN 91/ASTM D1655, the addition of HEFAs improves its combustion characteristics in terms of smoke point. The high squared correlation coefficient (R² > 0.96) reveals a strong exponential dependence of the HEFA/fossil fuel blend’s smoke point on the content of the bio component. Assuming that Figure 6a’s trend is valid for blends with greater than 40% (v/v) SAF content, from the equation in Figure 6a, the smoke point can be calculated for the neat HEFA component—65.2 mm.

Since space for fuel tanks in most airplanes is limited, having in mind the intention for longer flight range, and reduced total weight and fuel consumption, respectively [2,15], the bigger (44.128 MJ/kg versus 43.277 MJ/kg for fossil fuel) amount of gravimetrically expressed specific net energy of HEFAs is of great importance. The low arene hydrocarbon composition of the high H-to-C ratio of HEFA molecules determines its ascendant specific net energy [14]. The SAF content linearly enhances the SAF/fossil fuel blend’s specific net energy, as every additional 10% (v/v) of bio constituent increases the blend’s energy by 0.08 MJ/kg—Figure 6b.

The stability of aviation fuels can be divided into thermal (exposure to high engine operating temperatures) and storage (deterioration with time and air) [2]. The thermal stability of both evaluated components from Table 1 is excellent, as there is no filter pressure differential when analyzing them according to ASTM D324. The tube deposit rating is also within standard limits. However, the storage stability of HEFAs is a point of concern, as existent gum is 6 mg/100 mL and is very close to the upper limit (7 mg/100 mL) of DEF STAN 91/ASTM D1655. The issue is intensified by the low precision of the property measurement, as is pointed out in IP 540—repeatability of 6 mg/100 mL and reproducibility of 7 mg/100 mL. Thus, this study answers the appeal of [14] to shed some light on storage stability (rarely reported property) of biojet fuel. According to IP 540, existent gum evaporates in the air (Table 1 results) and the remaining residue/impurities are represented by higher boiling oils or particulate matter. That is verified by the presence of high boiling C₁₇–C₃₀ cut in HEFAs that is missing in the conventional component. Some specialists [36] assume that existent gum can also be an oxidation product. The latter case is also a possible cause, since the analysis of HEFAs is conducted after 2 months and a half of HEFA sample shipment. In order to evaluate the alteration of this property with time, HEFAs were tested for existent gum after storing the sample for another 3 months in a half-full (availability of air) metal container at room temperature. No further increase of existent gum was recorded and the value was high but constant at 6 mg/100 mL. Consequently, a non-evaporated residual material seems to be the primary cause for the high value of this property. Nevertheless, this finding of a detailed evaluation of oxidation stability and shelf life of HEFAs should be further evaluated in order to guarantee a reliable fuel supplying system and engine performance. Unlike the linear dependence [18] of existent gum content from the Farnesane/mineral aviation fuel ratio, the increase of SAF in its blends with JET A-1 fuel can be described by the complicated curve presented in Figure 7a. From 0 to 20% (v/v) SAF, the shape of the curve is a straight line parallel to the abscissa; from 20% (v/v) to 42% (v/v) SAF, it is also a straight line but with a sharply increasing slope, and it goes through a maximum at about 80% (v/v) before reaching the 6 mg/100 mL value at 100% (v/v) SAF. Nevertheless, all measured values of the blends up to 42% SAF lie below the upper limit and satisfy the standard requirements.

Limited information is published [14] in the existing literature about another of HEFAs’ properties—water separation characteristics (measured by microseparometer (MSEP) rating), which can be deteriorated by the presence of surfactants (detergents from equipment cleaning, some compounds from refinery, additives like static dissipater (SDA), etc.) in the fuel. This property is significant as well, since the inability of aviation fuel to separate from water increases its freezing point, provokes corrosion and even the growth of microorganisms [2]. Table 1 reveals that SAF’s MSEP rating is 82 and is lower than the rating of 97 for fossil fuel, which contains 5 mg/l SDA. The rating of SAF/conventional fuel blends presented in Figure 7b shows a complicated dependence on the SAF content. The MSEP is registered as a steep slope of up to 20% (v/v) SAF, followed by an almost straight line parallel to the abscissa at 82–84% blends.

Table 1, along with Figure 7c, shows that SAF possesses poor electrical conductivity of 9 pS/m and is almost equal to hydrotreated kerosene values of 0 ÷ 2 pS/m, as impurities (good conductors of electrical charge) in kerosene are transformed in the refinery hydrotreating unit. Fossil aviation fuel’s ability to dissipate charges is defined as 511 pS/m because of the availability of SDA in this fuel. The electrical conductivity of blends is revealed in Figure 7c. An increased SAF content above 20% (v/v) leads to a sharp drop of this property and, with the help of the derived equation of linear dependence, it can be easily calculated that a blend with up to 93% (v/v) SAF will stay within the specification limit of a minimum of 50 pS/m. The ability to blend such a large share of HEFAs is secured by the high value of electrical conductivity of the mineral component. Designing SAF/mineral jet blends properties requires a special attention to mineral component SDA dosage.

HEFAs are tested for an extra (not included in DEF STAN 91/ASTM D1655 nor in ASTM D7566) property—total metabolically active microbial contamination. It is known that, in addition to water, microorganisms need certain elemental nutrients from which phosphorus [2] is missing in crude oil-derived kerosene but is present in bio feeds. Nevertheless, the registered value of <5.0 pg ATP/mL for the blend of 42% SAF in mineral JET A-1 is below the limit of the maximum of 10.0 pg ATP/mL and indicates a lack of microbial contamination. In other words, HEFAs are sterile due to the severity of the deoxygenation and isomerization phase of the UOP Ecofining^TM technological unit, which has produced the HEFAs used in this study.

After processing and analyzing data from all SAF/fossil jet fuel blends properties, it was found that density correlates with major properties. This finding is presented in Figure 8—both the type of the curve (graphical description of dependence) and quantitative assessment (equation of the line which best fits analysis results). As expected, the increase of density is related to the linear increase of the flash point, kinematic viscosity, freezing point, and total aromatics. Following the opposite trend are the behavior of specific energy and the exponential decrease of the smoke point.

Another four interrelations between bio/mineral fuel properties are shown in Figure 9. The flash point can be calculated from the freezing point and distillation by the linear equations in Figure 9a,b. The smoke point in Figure 9c increases with the increase of total aromatics and confirms the exponential type of dependence graph shown in [1]. The decreasing arene hydrocarbon content (HEFAs are an almost arene-free component with high specific energy) increases specific net energy in a manner pointed out in Figure 9d.

4. Conclusions

HEFAs, fossil aviation fuel, and their blends were tested for compliance with ASTM D7566/DEF STAN 91 and ASTM D1655 requirements. It was established that the microbial contamination-free sustainable component improves most of the blends’ major properties; increases of 10% (v/v) of HEFAs had the following influence: decrease freezing point by 1.3 °C; decrease total aromatics by 1.8% (v/v); increase smoke point by an average of 3 mm; increase specific net energy by 0.08 MJ/kg. Attention should be paid to the acidity and existent gum of HEFAs. These property values are very close to ASTM D7566’s maximum allowable values, but nevertheless, even the most bio-saturated blend satisfies ASTM D1655 requirements. The existent gum value is high but constant with time at 6 mg/100 mL due to, most probably, the presence of non-evaporated residual material. HEFAs also deteriorate water separation characteristics and the electrical conductivity of blends.

Quantitative interrelations between bio/mineral fuel properties, derived in this study, reveal that an increase in density is related to a linear increase of the flash point, kinematic viscosity, freezing point, and total aromatics. Following the opposite trend is the behavior of specific energy and the exponential decrease of smoke point. The flash point can be calculated from the freezing point and distillation. The total aromatics increase defines the smoke point’s exponential increase and the specific net energy’s decrease.

Depending on the conventional component properties, the potential content of HEFAs in their blend would vary. Concerning total aromatics, a concentration range of HEFA-SPK from 60% (v/v) to 19% (v/v) is possible, while densities of mineral components from the European refinery limit SAF content to 47–55% (v/v).