Characterization of Particle Emissions from GTM 400 Fueled with HEFA-SPK Blends

Abstract

As aviation is a rapidly growing sector, many actions must be taken to significantly reduce the emission of harmful gases such as CO₂, CO, HC, NOx, and particulate matter (PM). One accessible solution is the use of drop-in sustainable aviation fuels (SAFs), which do not require any changes in the engine or infrastructure construction. The aim of this research was to analyze changes in non-volatile particulate matter (nvPM) emissions for SAF blends compared to Jet A-1 using a miniature jet engine, as there is still limited research on particulate matter emissions from miniature engines, especially for SAFs. This study focuses on non-volatile particle emissions from HEFA-SPK fuel, with comprehensive analyses of particle number and particulate mass-emission indices, as well as number-based and volume-based particle-size distribution (PSD). The tests were conducted on the miniature GTM 400 engine, which was specially designed for SAF testing. The tested fuels were 30/70%v and 50/50%v blends of HEFA-SPK/Jet A-1, as well as neat Jet A-1 as a reference fuel. The results showed that the use of 50%v HEFA-SPK can reduce non-volatile particulate mass emissions up to 59% at low engine loads, and non-volatile particle number emissions by up to 56% at maximum thrust, compared to Jet A-1.

1. Introduction

The aviation sector needs to meet new emission regulations in the coming years, such as the European Fit for 55 package or international CORSIA (Carbon-Offsetting Reduction Scheme for International Aviation). New standards are being established for greenhouse gas emissions, particularly CO₂ emission, which are linked to the increased use of alternative aviation fuels. As the aviation sector is responsible for 2% of global CO₂ emissions, significant efforts must be made to reduce these levels [1]. Since CO₂ is not the only critical pollutant from aviation, other emissions, such as carbon monoxide, unburned hydrocarbons, sulfur oxides, nitrogen oxides, and particulate matter, also need to be reduced [2]. These pollutants affect air quality around airports and contribute to atmospheric phenomena in the upper layers of the troposphere, where the cruise phase occurs [3]. As aircraft operations at the airport level, and up to 1000 ft, have a significant impact on the environment and human health, it is crucial to reduce toxic exhaust gases and particulate matter in the vicinity of the airport. The longest phase of the LTO (Landing and take-off) cycle is the taxi phase, which can last up to 26 min, depending on the airport infrastructure and traffic [4,5]. Any reduction in pollutant emissions during the taxi phase can significantly improve air quality around airports.

Particulate matter (PM) has been widely researched in recent years due to its significant impact on air quality. Particles emitted by jet engines are mostly smaller than 80 nm and are formed through the nucleation mechanism [4,6]. The accumulation mechanism produces particles with diameters between 80 and 1000 nm, which contribute the most to the total particle mass emissions [4,6]. Jet engines emit both volatile particles (vPM) and non-volatile particles (nvPM). Non-volatile particles are generated within the combustion chamber of the jet-propulsion system and are predominantly composed of soot, along with various constituents of the exhaust gases that deposit onto the carbonaceous core. This process is well described in the literature. Volatile particles undergo condensation and agglomeration within the exhaust plume or subsequently disperse into the ambient atmosphere, where they react with toxic exhaust gas compounds such as sulphur oxides and nitrogen oxides or hydrocarbons [6]. The fuel composition plays a crucial role in PM formation, especially sulfur and aromatics content. The presence of aromatics leads to higher soot and black carbon emissions due to incomplete combustion and the formation of soot nuclei or uncombusted aromatic species [7,8]. Different types of aromatics (e.g., naphthalenes) also influence the magnitude of non-volatile PM emissions. Fuels with higher naphthalenic content result in higher black carbon mass emissions [9,10]. Reducing sulfur content in fuel can significantly decrease the number of volatile particles emitted [10,11].

Sustainable aviation fuels (SAFs) are among the mid-term and long-term solutions for reducing greenhouse gas (GHG) emissions without requiring changes to airport infrastructure or aircraft engines design. Currently, certified SAFs are “drop-in” fuels, meaning they can be used in existing engines just like conventional aviation fuel. The most widely used SAF is Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene (HEFA-SPK), which was certified under ASTM D7566 in 2011 [12]. The maximum blending limit of HEFA-SPK with Jet A-1 is 50%. HEFA-SPK is primarily produced from waste and residues, such as used cooking oil (UCO), but it can also be derived from oily biomass sources such as jatropha or camelina. The range of feedstock suitable for HEFA-SPK production is broad [13]. According to recent studies, the environmental impact of HEFA-SPK production and use varies depending on the feedstock composition and production methods. As SAF emissions should be compared with conventional aviation fuel throughout the entire life cycle, the Life-Cycle Assessment (LCA) is a crucial parameter for SAFs. LCA values differ in CO₂ emission reduction depending on used feedstock. The LCA values for HEFA-SPK derived from used cooking oil, palm fatty acid distillate, tallow, jatropha, and camelina oil are about 20% of the Jet A-1 baseline value, which is equal to 89 gCO₂e/MJ. The LCA of HEFA-SPK derived from plant oils (soybean oil, rapeseed oil) is about 70% of the Jet A-1 baseline [13,14]. The combustion of fuel in aircraft engines is the final stage of the core LCA. Comparing only emissions from this stage, the reductions depend on used engine and its parameters but also on fuel composition and used feedstock. According to [15], 50%v of HEFA-SPK used in GTM 140 miniature jet engine reduced CO and CO₂ emission compared to Jet A-1. Studies made by Undavalli et al. [16] show that while a 50:50 blend of HEFA and Jet A-1 reduces NO_X, CO, and UHC emissions by approximately 40%, 18%, and 28% respectively, CO₂ emissions remain unchanged. Research [17] conducted on the Rolls-Royce Trent XWB-84 engine using 100% HEFA-SPK confirmed that NOx emissions are not significantly affected by fuel type. Instead, they are more sensitive to engine thrust settings and combustor inlet conditions [17].

To better understand emissions from sustainable fuels, this article focuses on the particulate matter emissions in terms of particle-size distribution and particle-emission indices from miniature jet engine GTM 400. While there are many studies on gaseous emissions from SAFs, both on miniature and full-scale engines, there is still very little research on particulate emissions from miniature jet engines, which are now increasingly used in laboratory testing of alternative fuels.

2. Materials and Methods

2.1. Test Engine and Fuels

The fuels examined in the study were mixtures containing 30% volume of HEFA-SPK and 70% volume of Jet A-1 (marked as 30%HEFA), and 50% volume of HEFA-SPK and 50% volume of Jet A-1 (marked as 50%HEFA). Additional tests were made on Jet A-1, a conventional aviation fuel, which served as the control fuel for comparison purposes. The primary feedstock utilized in the production of this particular HEFA-SPK fuel was predominantly cooking oil.

The tests were conducted on GTM 400, a miniature turbine engine with maximum thrust of 400N. The engine consists of one-stage radial compressor, annular combustion chamber, and one-stage axial turbine. The engine bench was specially designed for tests of alternative aviation fuels. Specific parameters of GTM 400 are presented in

Table 1. Specific parameters of GTM 400.

Parameter	Unit	Value
Thrust max.	N	400
Thrust min.	N	15
RPM max	rpm	85,000
RPM min	rpm	27,000
Compression ratio	-	3.3:1
Mass air flow rate	g/s	770
Exhaust gas temperature	°C	750
Fuel consumption	g/min	1200

.

2.2. Apparatus and Procedures

During the tests, the EEPS 3090 (Engine Exhaust Particulate Sizer™ spectrometer, TSI Incorporated, Shoreview, MN, USA) analyzer was used for nvPM concentration measurement. EEPS 3090 measures the discrete range of particle diameter from 5.6 nm to 560 nm [18]. Technical parameters of the EEPS 3090 analyzer are presented in

Table 2. Technical parameters of EEPS 3090 [18].

Parameter	Value
Diameter range	5.6–560 nm
Number of measurement channels per decade	16
Resolution	10 Hz
Exhaust sample volume flow rate	0.6 m3/h
Compressed air volume flow rate	2.4 m3/h
Input sample temperature	10–52 °C

. The measuring probe was made of stainless steel and placed perpendicular to the engine outlet at a distance of 80 cm. EEPS 3090 measures a particle number concentration and particle-size distribution. To present particulate mass concentration or volume-based particle-size distribution, appropriate calculations assuming particle density have been made. The particles are treated as spheres with a uniform distribution of density based on a specific model. The density-change curve, which was established through empirical findings for particles from a jet engine, was considered [19]. The limitation of this solution is that the particles are considered spherical and their volume is calculated on this basis, which may lead to an underestimation of the particle-mass concentration or volume-based distribution.

The Number Particle Emission Index was calculated as a sum of particle number concentration for every measured diameter, multiplied by total mass flow, and divided by fuel flow, which allowed us to calculate number of particles per kg of used fuel. The mass particle-emission index was calculated analogously based on the mass concentration of particulate matter.

The tests were conducted for every selected fuel from 10% of maximum thrust (F) to 100% of maximum thrust, with step for every 10%. On every measurement point, the engine worked for 20 s, and the results are the average value of the 20 s measurements. Some of the results of engine parameters are presented in

Table 3. Selected results of engine parameters.

Fuel Type	RC [%]	Thrust [N]	Fuel Flow [kg/h]	T3 [°C]	P2 [hPa]	Total Mass Flow [kg/s]	RPM [1/min]
Jet A-1	10	22	11.91	966.29	174.85	0.186	32,305
	30	48.8	16.25	893.26	333.95	0.257	42,903
	50	88.9	21.45	892.20	569.13	0.337	53,624
	70	156.8	28.23	928.23	892.48	0.422	64,020
	100	321	43.39	1089.40	1554.24	0.555	78,475
30% HEFA	10	23	12.09	958.57	175	0.183	32,373
	30	47.5	16.38	893.25	329.65	0.253	42,770
	50	89.4	21.82	885.81	578.63	0.340	54,013
	70	152.2	28.31	914.81	890.15	0.422	63,995
	100	315	46.15	1067.97	1528.33	0.558	78,765
50% HEFA	10	22.0	12.12	942.30	176.58	0.179	32,342
	30	49.5	16.70	885.91	342.03	0.261	43,388
	50	89.9	21.63	879.79	563.40	0.334	53,446
	70	156.3	28.51	911.73	890.93	0.422	64,055
	100	323	47.16	1061.27	1554.75	0.550	79,150

. The ambient temperature during the tests was 20 °C, the atmospheric pressure was 1000.9 hPa, and the humidity was 75%.

3. Results

3.1. Particle-Number-Emission Indices

Analyses refer to non-volatile particulate matter. The number of particles emitted per unit of fuel mass is defined by the Particle-Number-Emission Index (EI_N). The changes in the Particle-Number-Emission Index depending on the engine thrust and burned fuel are shown in Figure 1. For the entire engine-operating range, the highest values were for conventional aviation fuel Jet A-1, with only two points of almost the same value for Jet A-1 and 30% HEFA for 70% (about 1.4 × 10¹⁶), 80% (1.01 × 10¹⁶) of the engine thrust. Fifty percent of the HEFA blend had the lowest values in almost the entire engine-operating range, except 20%, when the nvPM EI_N values were higher than for 30% HEFA but still lower than for Jet A-1. The maximum EI_N value for Jet A-1 was for 30% of the engine thrust, and it was equal to 2.13 × 10¹⁶ particles on kg of fuel, and for 30% HEFA and 50% HEFA, the maximum nvPM EI_N was for 40% of thrust and it was, respectively, 1.92 × 10¹⁶ and 1.85 × 10¹⁶ particles on kg of fuel. For the maximum engine load, the 50% HEFA blend showed a significant reduction in nvPM EI_N, with a value of 1.53 × 10¹⁵, while the Jet A-1 was 3.44 × 10¹⁵ particles/kg of fuel. The error bars shown in the graph correspond to one standard deviation from the mean of the particle number.

3.2. Particulate-Mass-Emission Indices

The mass of PM emitted per unit of fuel mass is defined by the Particle-Mass-Emission Index (EI_M). The changes in nvPM EI_M are shown in Figure 2. The use of HEFA blends resulted in a reduction of nvPM EI_M throughout the engine operating range from 10% to 50% of thrust compared to Jet A-1. For medium-engine loads, from 50% to 80% of thrust, the nvPM EI_M values were slightly higher for 30% HEFA than for Jet A-1, but for 50% HEFA, the nvPM EI_M values were lower than Jet A-1 in the entire engine-operation range. For engine fueled with conventional aviation fuel, an increase in nvPM EI_M was observed for low engine loads, from 88 mg/kg of fuel for 10% of thrust to 138 mg/kg of fuel for 30% of thrust, and after that point, the values dropped significantly to 3.5 mg/kg of fuel for maximum engine thrust. The highest value for 30% HEFA and 50% HEFA was appropriately 107 mg/kg of fuel and 114 mg/kg of fuel for 40% of engine thrust, and it decreased to 2.8 and 2.3 mg/kg of fuel for 100% of thrust. The error bars shown in the graph correspond to one standard deviation from the mean of the particle mass.

3.3. Particle-Size Distribution

The data measured by the EEPS were averaged for the same fuel-flow-rate settings and then converted into differential number-based (dEI_N/dlogD) and differential volume-based (dEI_V/dlogD) particle-size distributions. The EI_V is the particle-volume-emission index, and it is defined as the volume of particles emitted per unit of fuel mass. Figure 3 presents the number-based and volume-based particle-size distribution for the tested fuel blends for various engine settings. The error bars for these graphs are one standard deviation from the particle-number measurements, but this has not been plotted on Figure 3 so as not to impair the readability of the graphs. Particles produced via the process of nucleation are primarily responsible for the emission of nvPM for the entire engine load for all tested fuels. In most measurement points, the number-based PSDs exhibit a single-mode log-normal distribution.

PSDs indicate that, for low and medium engine loads, from 10 to 50% of engine thrust, the majority of the particles were in the range of 8 to 42 nm for Jet A-1 and from 8 to 40 nm for 30% HEFA and 50% HEFA. For 10% of engine thrust, the distribution of particles is different for Jet A-1 and for HEFA fuels. In the 10–50% range of engine thrust, the characteristic diameter for Jet A-1 is 16-19 nm, while for 30% HEFA and 50% HEFA, the characteristic diameter for 10% of thrust is 10 nm, and for 20–50% of thrust, it is 19 nm. For 60% of the engine thrust for all tested fuels, the majority of particles were in the range of 8–34 nm. For 70% of engine thrust, the majority of particles were in the range of 8–30 nm. Eighty percent of the distribution is single mode, and for 80% of engine thrust, the range of particles is from 7 to 22 nm for Jet A-1 and 50% HEFA and from 7 to 26 nm for 30% HEFA. For 90% and 100% of engine thrust, for all tested fuels, the particle-diameter range is 6–14 nm. There were no particles with a diameter larger than 52 nm in the particle-size distribution for all tested fuels. The increase in engine thrust affected the particle characteristic diameter towards smaller diameters, but it also affected the number of emitted particles. In most measurement points, the increase in engine thrust resulted in the reduction of particle numbers for every tested fuel. The addition of HEFA fuel in the blend with Jet A-1 resulted in a significant reduction of particle numbers in the entire engine-operation points. This implies that the utilization of HEFA fuel has a positive influence on particle emissions.

Volume-based particle-size distribution for all tested fuels showed that there is a slight change in particles with a diameter larger than 100 nm, and that these particles have a slight impact on the emitted volume of particles. This change is related to the large size of these particles, which affected the total volume, but the number of these particles is not seen in the number-based PSD. Without considering particles larger than 100 nm, the volume-based particle-size distribution has a single-mode character for all tested fuels. The characteristic diameter was between 10 and 52 nm for all measurement points, for all tested fuels. From 70% of thrust, the increase in engine thrust decreased significantly for EI_V, as well as for the Jet A-1 and HEFA blends.

4. Discussion

The analyses have shown that the addition of HEFA fuel in the fuel blend results in particle reduction at almost all engine-operation points. To better compare the changes between the non-volatile Particle-Number-Emission Index and between the non-volatile Particulate-Mass-Emission Index, the percentage changes have been calculated related to Jet A-1. Also, the mean value for each type of fuel has been calculated for all thrust settings. The results are shown in

Table 4. Percentage reduction of non-volatile particle number in relation to Jet A-1 as a baseline with mean value.

F	10%	20%	30%	40%	50%	60%	70%	80%	90%	100%	Mean
Jet A-1	2.01 × 1016	2.00 × 1016	2.13 × 1016	2.12 × 1016	1.98 × 1016	1.76 × 1016	1.39 × 1016	1.01 × 1016	4.41 × 1015	3.44 × 1015	-
30% HEFA	−27%	−17%	−13%	−10%	−6%	−3%	+1%	0%	−3%	−24%	−10%
50% HEFA	−32%	−14%	−14%	−13%	−11%	−14%	−14%	−20%	−28%	−56%	−22%

and

Table 5. Percentage reduction of non-volatile particulate mass in relation to Jet A-1 as a baseline with mean value.

	10%	20%	30%	40%	50%	60%	70%	80%	90%	100%	Mean
Jet A-1	88.108	106.402	138.516	136.027	106.308	71.922	37.429	16.699	4.899	3.471	-
30% HEFA	−54%	−39%	−31%	−21%	−1%	+4%	+24%	+23%	−8%	−20%	−12%
50% HEFA	−60%	−37%	−29%	−17%	−5%	−8%	−11%	−14%	−17%	−34%	−23%

.

The addition of HEFA-SPK fuel in a blend with conventional aviation fuel reduces particle-number emission and particulate mass emission almost across the entire engine-operation range. The mean value of the nvPM EI_N reduction for 30% HEFA for all thrust levels is −10% compared to Jet A-1, and for 50% HEFA, it is −22% compared to Jet A-1. The biggest reduction is for 50% HEFA for the maximum engine thrust, which is even 56% fewer particles than for conventional aviation fuel.

For nvPM EI_M reduction, the mean value for 30% HEFA is −12%, and for 50% HEFA, it is −23% in the entire engine-operation range. For engine thrusts of 60% and more, 30% HEFA resulted in an increase in particulate-mass-emission index compared to Jet A-1 but also resulted in a significant reduction of nvPM mass for engine thrust below 40%. As the values of particulate mass for higher engine-operation points are very low, the percentage relation differs a lot. For 50% HEFA, the particulate mass emissions were reduced in the entire engine-operation range, from 60 to 5% compared to Jet A-1.

The presented results show that increasing the proportion of SAF in fuel leads to a reduction in both the number and mass of particulate matter in the exhaust gases. This relationship has been recorded earlier in studies performed on full-sized engines; for example, in studies made by Schripp et al. [20] and Rojo et al. [21]. According to other studies, Moore et al. [22] showed that the utilization of a 50% HEFA blend can reduce the PM number and mass by 50 to 70%. The tests were conducted on the full-scale engine CFM56-2-C1. The particle-size distribution shape is similar to the presented results, while the order of magnitude is also similar. The characteristic particle-diameter changes with increasing thrust towards smaller diameters, which were also presented in the research of Jasiński [23], were made on the Pratt & Whitney F100-PW-229 engine. Other studies conducted by Durdina et al. [24] on CFM56-7B showed that the utilization of 32% HEFA can reduce the non-volatile PM number by 60% for the idle phase and by 10% for the take-off phase, as well as non-volatile PM mass by 70% compared to conventional aviation fuel. Tests made by Chan et al. [25] on GE CF-700-2D-2 for 50% HEFA showed PM reduction from 40 to 60% compared to conventional aviation fuel. According to Jasiński and Przysowa [26], a blend containing 30% HEFA fuel can reduce the particulate-number index by 90% and the particulate-mass index by 75% compared to conventional fuels. The tests were made on the DGEN 380 engine [26]. The study presented in this article showed that, on a miniature jet engine such as the GTM 400, the nvPM-number-emission index can be reduced by up to 56%, and the nvPM-mass-emission index by up to 60%, which is significantly lower than the reductions reported in the cited study. That can point to the combustion characteristic of miniature jet engines as one of the possible reasons. In research conducted by Durdina et al. [27] on the Pratt & Whitney Canada PW545A turbofan engine fueled with conventional fuel and 30% v HEFA-SPK showed that the EI nvPM number has a similar course to the presented results, especially for the average engine setting values. For the abovementioned research, the results were corrected for particle losses. In this case, the reduction of EI nvPM mass was from about 32% to about 16%, and for the EI nvPM number, it was from about 20% to about 4% compared to conventional fuel [27]. Also, according to this study and previous studies [20,28], the maximum value of nvPM mass EI is usually in the range of 40% of the thrust until the moment of take-off. For the tested GTM 400, the maximum value of nvPM mass EI was also around 30–40% of the maximum thrust, but it decreased to 100% of the thrust. This may be due to the different characteristics of the engine, which is a miniature engine, so not all particle-emission parameters match the results for full-size engines. However, still comparing emissions for SAF and Jet A-1 on one engine, the nvPM mass and number EIs can be significantly reduced for sustainable fuels.

Studies made for other kinds of sustainable aviation fuel, such as ATJ (Alcohol to Jet) or FT-SPK (Fischer–Tropsch Synthetic Paraffinic Kerosene), also show a significant reduction in PM emissions, both in terms of particle number and particle mass [19,29]. According to the studies, the reduction of PM emissions for SAFs is comparable to Jet A-1 results from the lower aromatic content of SAFs. Non-volatile PM emission characteristics can vary depending on the engine type used in the experiment and also operation settings, fuel composition, and feedstock used for SAF production, but overall, the values of nvPM EI_M are much lower for SAFs than for Jet A-1 for low engine loads, and the difference in nvPM EI_M between SAFs and Jet A-1 decreases with an increase in engine thrust [6,30,31,32,33].

5. Conclusions

To reduce the impact of the aviation sector on the environment, it is crucial to develop new solutions that help to significantly reduce GHG and particulate matter emissions. One such solution is alternative aviation fuels, which will be used on a wider scale in the future due to, among other reasons, new European Union regulations. The presented research analyzed the HEFA-SPK fuel blends with Jet A-1 according to particulate matter emissions and led to the main conclusion: the addition of HEFA-SPK fuel in the fuel blend reduces the nvPM emission almost in the entire engine-operation range. As described in the article, 50% of HEFA-SPK in the fuel blend can significantly reduce particulate matter emission, from 11 to 56% for non-volatile particle number emission and from 5 to 60% for particulate mass compared to Jet A-1. As mentioned in the introduction, a very important phase of the LTO cycle is taxi, as it has a significant impact on the environment in the area of the airport and lasts the longest. Thrust settings for taxi are about 7%, so as presented in the article, the use of HEFA-SPK fuel can, up to 60%, reduce non-volatile particulate-mass emission from the taxi phase compared to conventional aviation fuel. As PM emission is divided into volatile and non-volatile PM, it would be useful to focus on a comprehensive analysis of volatile fraction of PM in future researches; for example, by using a Volatile-Particle Remover (VPR), which allows the separation of the non-volatile particle fraction from the volatile components and is used, among others, in ICAO CAEP tests. Also, the relation between the chemical composition and physicochemical parameters of SAF and PM emissions could help us better understand the mechanisms of PM formation from SAF fuels, and it could also reduce PM emissions depending on the chemical composition of new SAF. Also, expanding the scope of raw materials, which are used for SAF production, is very important, as many new raw materials are wastes and residues, so there is no need for plant cultivation. Moreover, when comparing sustainable aviation fuel and conventional fuel, it is crucial to consider the life-cycle emissions of these fuels, so the wastes and residues as the feedstock for SAFs can significantly reduce total life-cycle emissions from SAFs. Considering this, sustainable aviation fuels can be much more advanced in the future in terms of sustainability.