Performance and Exhaust Emissions of a Gas-Turbine Engine Fueled with Biojet/Jet A-1 Blends for the Development of Aviation Biofuel in Tropical Regions

Abstract

Biofuels as alternative fuels in today’s world are becoming increasingly important for the reduction of greenhouse gases. Here, we present and evaluate the potential of a new alternative fuel based on the conversion of medium-chain fatty acids to biojet (MBJ), which was produced from coconut oil using hydrotreated processes. MBJ is produced by using both deoxygenation and isomerization processes. Several blends of this type of biojet fuel with Jet A-1 were run in a gas-turbine engine (Rover 1S/60, ROTAX LTD., London, England) for the purpose of investigating engine performance and emissions. Performance results showed almost the same results as those of Jet A-1 fuel for these fuels in terms of thermal efficiency, brake-specific fuel consumption, turbine-inlet temperature, and exhaust-gas temperature. The results of exhaust-gas emissions also showed no significant effects on carbon monoxide, unburned hydrocarbon, and nitrogen oxides, while a decrease in smoke opacity was found when blending MBJ with Jet A-1. MBJ performed well in both performance and emissions tests when run in this engine. Thus, MBJ brings hope for the development of aviation biofuels in tropical regions that have an abundance of bioresources, but are limited in technology and investment capital.

1. Introduction

In recent years, the aviation industry has affirmed its intention to support sustainable development by working towards carbon-neutral growth, while maintaining increased traffic service followed by restrictions in carbon emissions as a first step to achieving a carbon-free future [1]. This requires policies such as a carbon tax to restrain global warming [2]. In the long term, however, the airline sector forecast that aircraft materials and efficiency improvements are not enough to improve flight technologies and combat the challenges of reducing greenhouse-gas (GHG) emissions. Consequently, the air-transport industry itself has focused on the development of biofuels as one of the key candidates for alternative fuels in order to meet rising expectations regarding environmental conservation. This is because the use of biofuels in aircraft is able to cut down lifecycle CO₂ emissions by up to 80% [3,4,5]. Using biojet fuels can also reduce other pollution emissions, such as CO, HC, and particulate matter (PM) [6,7,8,9,10,11].

Although using biofuels for aviation purposes is clearly a good option, application in practice depends on many factors, two of which are price and supply sources. Currently, aviation production processes approved by the American Society for Testing Materials (ASTM) are primarily those produced using hydroprocessing technologies. At present, six types of aviation fuels have been approved by the ASTM for blending with jet fuel for use in aircraft: Fischer-Tropsch Synthetic Paraffinic Kerosene, Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene, Fermented-Sugar-Synthesized Isoparaffins, Fischer-Tropsch Synthetic Paraffinic Kerosene and Aromatics, Alcohol-to-Jet Synthetic Paraffinic Kerosene, and Catalytic Hydrothermolysis Jet Fuel Kerosene [12]. The production processes involved in producing these aviation biofuels consist of two main steps: (1) hydrodeoxygenation, which eliminates oxygen atoms in biomass molecules to produce hydrocarbons; and (2) cracking and isomerization to break down carbon-chain lengths into jet-fuel range and producing branched hydrocarbons, respectively (Figure 1). Thus, advanced technologies are needed that are expensive, especially for the second step, because reactions take place at high pressure, and use noble metals and require the presence of hydrogen. As a result, it is not easy to utilize these approved processes in emerging countries such as Indonesia due to limited technology and investment capital. Therefore, a more feasible production process is needed to produce aviation biofuel in emerging tropical countries in general, and Indonesia in particular. Such a production process could use less complicated technology and cheaper methods by taking advantage of available bioresources in tropical regions where many types of plant oils are abundant. By choosing feedstocks consisting of plant oils predominantly containing medium fatty acid chain length (C8–C16), the hydrocracking process can be reduced or even omitted, as shown in Figure 1, which is also stated in Thong et al. [13]. This production process is currently being improved to manufacture a better product.

Coconut oil mainly consists of triglycerides and fatty acids that have their chain lengths within jet-fuel range (C6–C18), and dominantly at C12 and C14, so coconut oil is well-suited for typical carbon chain-length distribution in aviation-fuel production. Therefore, coconut oil was chosen to use as feedstock for production in this study. Other potential plant oils for use as feedstocks in this production process are listed in

Table 1. Fatty acid profiles of suitable oils for use as feedstocks in medium-chain fatty acid to biojet (MBJ) production.

Oil	Fatty Acid Composition (wt%)
	8:0	10:0	12:0	14:0	16:0	18:0	18:1	18:2	18:3
Coconut oil [21]	5–9	4–6	42–53	17–20	7–11	2–3	5–9	1–4	0–0.2
Palm kernel [21]	3–5	3–6	48–55	12–19	3–8	4–8	15–21	0.5	-
Babassu [21,22]	4–6	6–8	44–48	15–20	5–11	3–6	10–16	1–3	-
Butia capitata [23]	8	11	39	8	5	2	21	5	-
Macauba [24]	3	3	33	10	10	2	33	4	-

. Palm kernel oil also has the potential to be used as feedstock due to it its fatty acid composition and current availability. However, the palm-oil industry also produces large amounts of biomass, which must be utilized by reuse and recycling so that it does not become an environmental problem. Currently, the use of fiber and shell alone as fuel for direct combustion to produce steam and electricity can supply more than what is required [14]. However, studies to increase the potential use of fiber and shell continue to be carried out in various ways such as producing biopellet fuel [15], fast pyrolysis [16], microwave-assisted pyrolysis [17], producing sugars via cellulose and hemicellulose recovery [18], polypropylene composites [19], or biocomposites [20], thus improving the sustainability of oil palm crop.

According to the ASTM D4054-19 standard [25], candidates for alternative fuels and additives for use in aviation gas-turbine engines are required to pass an approval process that includes several strict requirements and testing ranges of specification properties in full-scale engine testing. The gas-turbine-engine test is one of the most important steps in the approval process; therefore, an investigation into the effects of using biofuels in gas-turbine engines was conducted. The aim of this study is to evaluate the feasibility of MBJ as an alternative fuel for aviation purposes. This study focuses on analysis of the performance and exhaust-gas emissions of a 1S/60 Rover gas-turbine engine operating on Jet A-1 and J2, J5, J10, and J20 blends, which are mixtures of 2%, 5%, 10%, and 20% volumes of MBJ with Jet A-1, respectively.

2. Materials and Methods

2.1. Materials

Coconut oil used in this production process was produced by the home industry in West Java, Indonesia. It was pretreated with a step that included filtration, dewatering, degumming, and bleaching to eliminate contaminants. The fatty acid profile of coconut oil is shown in

Table 2. Fatty acid profile of coconut oil. Reprint with permission, 2020, Elsevier [26].

Fatty Acid Carbon Chain	<C8:0	C8:0	C10:0	C12:0	C14:0	C16:0	C18:0	C18:1	C18:2	C18:3
Composition (wt%)	0.5	7.8	6.7	47.5	18.1	8.8	2.6	6.2	1.6	0.2

.

Chemical compounds used in the production process, namely, NiMo catalyst, zeolite catalyst, nickel, molybdenum, silica alumina, and platinum were purchased from Sigma Aldrich.

2.2. Production Process

The production process to manufacture MBJ is the same as the production process presented by Thong et al. [13]. In order to use this production process, it was necessary to use feedstocks that consisted primarily of medium carbon chain lengths. As mentioned previously, by choosing plant oils that contain medium-chain length fatty acids, it is possible to avoid the hydrocracking step in the production process, as compared to the approved production process for aviation biofuel produced from plant oil and animal fats, namely, Hydroprocessed Esters and Fatty Acids (HEFA). In this study, coconut oil was chosen as the feedstock for producing biofuels. The fatty acid profile of the used coconut oil is presented in Table 2, which shows that the carbon chain length of fatty acids in coconut oil is mainly within jet fuel range (C8–C16), and dominantly at C12, which is the same as the average carbon chain length of jet fuels [27,28].

This production process basically consists of two main steps: (a) dehydrogenation process, which eliminates the oxygen atom in the coconut-oil molecule to produce straight-chain paraffins (n-paraffins); and (b) isomerization process, which converts straight-chain into branched-chain paraffins (isoparaffins) as presented in Figure 1.

In the dehydrogenation process (Figure 2a), the coconut oil was placed inside a hydrogenation reactor. Hydrogen was heated to 200 °C and added to mix with the coconut oil. In the batch, coconut oil and H₂ were reacted with a sulfided NiMo catalyst, with nickel as a promotor, molybdenum as an active side, and silica alumina as catalyst support. The heterogeneous catalyst is produced by Pertamina with composition NiO = 0.1–4 wt% (Nippon Organic Colour & Chemical Co. Ltd., Kyoto, Japan), MoO₃ = 8–28 wt% (Nippon Organic Colour & Chemical Co. Ltd., Kyoto, Japan), and gamma alumina as the catalyst support (Catapal, B-Sassol, Sandton, South Africa). The hydrotreating catalyst had 100–250 m²/g specific area, and 0.6–6 wt% pore phosphorous content. The catalyst volume that was loaded into the middle section of reactor was 75 cm³. The hydrotreating catalyst was sulfided by dimethyl-disulfide (DMDS, MERCK, Darmstadt, Germany) up to a ratio of sulfur/catalyst of 8–10 wt%. The reaction was performed under 70 bar pressure, at a temperature of 370 °C, and weight hour sever velocity of 0.5 h⁻¹. The ratio of H₂ and hydrocarbons was 1000 NmL/mL. The products of this reaction were then passed through pressure separator units. Water and gases were eliminated from the hydrocarbons. Excess H₂ and produced gasses were drawn into a H₂ scrubber. Hydrocarbons were then placed into a low-pressure separator batch with the addition of N₂ to remove the H₂S. Lastly, the hydrocarbon product was delivered into a product tank. This product is called hydrotreated deoxygenation (HDO), which basically consists mostly of n-paraffins, so it has some disadvantages, especially a higher freezing point, as compared with the jet-fuel specification. Therefore, it was needed to treat HDO with an isomerization process, which aims to convert n-paraffins into isoparaffins, which have a much lower freezing point.

As shown in Figure 2b, the HDO was reacted at an isomerization reactor with the addition of preheating H₂ (150 °C) and the presence of a Pt–Pd/zeolite catalyst. The isomerization heterogenous catalyst was a commercial catalyst with the composition of platina (Pt) = 0.7 wt% and palladium (Pd) = 0.5 wt%. The reaction was performed at 30 bar pressure, a temperature of 300 °C, liquid hour sever velocity of 1 h⁻¹, and an H₂/hydrocarbon ratio of 200 mL/mL. The catalyst volume that was loaded into the middle section of reactor was 50 cm³. The catalyst was activated using hydrogen. Products of the isomerization process were treated similarly to the dehydrogenation process, as previously mentioned. The final hydrocarbon product was MBJ.

MBJ mainly consists of n-paraffins and isoparaffins with a carbon chain length within C8–C16, as shown in Figure 3, which is far different from the consistency of jet fuels that typically contain five hydrocarbon classes: n-paraffins, isoparaffins, cycloparaffins, aromatics, and olefins. This difference in composition may affect both the physical properties and the resulting combustion, which could affect engine performance and exhaust emissions because each hydrocarbon class has a different effect on combustion quality and physical properties [28,29,30].

ASTM D1655-20 is the widely used jet-fuel standard [31], with the typical properties of pure MBJ, blends of MBJ/Jet A-1, and Jet A-1. These were measured as shown in

Table 3. Typical properties of Jet A-1 and MBJ. Developed from [32].

Property	Test Method	ASTM D1655	Jet A-1 (J0)	J2	J5	J10	J20	MBJ (J100)
Density at 15 °C, kg/m3	ASTM D4052	775–840	804.0	802.4	801.3	797.8	792.1	756.1
Viscosity at −20 °C, mm2/s	ASTM D445	Max 8.0	3.953	3.955	3.942	3.938	3.921	3.777
Low heating value, MJ/kg	ASTM D4809	Min 42.8	43.2	43.23	43.24	43.26	43.32	43.9
Smoke point, mm	ASTM D1322	Min 25.0	20.0	20.2	20.7	21.9	23.5	53.7
Naphthalene, % v/v	ASTM D1840	Max 3	2.0	-	-	-	-	-
Freezing point, °C	ASTM D2386	Max −47	−51.0	−51.1	−51.1	−51.2	−51.3	−53.0
Flash point, °C	ASTM D56	Min 38	41	40.7	40.5	40.4	40.1	36
Aromatics, % v/v	ASTM D1319	Max 25	19.5	-	-	-	-	-
Total sulfur, % m/m	ASTM D4294	Max 0.3	0.06	-	-	-	-	0.0037

, which also shows that MBJ could meet almost all properties required by the ASTM D1655 jet-fuel standard, except for density and flashpoint. These two properties were slightly lower than what the standard specifies, which may have been because MBJ consisted mainly of n-paraffins and isoparaffins, which have lower density and lower flashpoints when compared with those of other hydrocarbon classes, such as cycloparaffins and aromatics [27,28]. Table 3 also indicates that MBJ had significantly lower sulfur content, which means that using MBJ would reduce SOx emissions.

2.3. Experiment Setup

Figure 4 illustrates the experimental rig test used in this study. A stationary 1S/60 Rover gas-turbine engine was used for testing biofuel blends to investigate the effect on performance and emissions. The specifications of this engine, which was typical, are shown in

Table 4. Specifications of 1S/60 Rover gas-turbine engine. Adapted from [32].

Engine Type	1S/60 Type 08
Compressor	Single stage centrifugal
Turbine	Single stage axial
Combustion chamber	Reverse flow
Governed speed	46,000 rpm
Maximal fuel consumption	0.635 kg/kWh
Maximal air mass flow	0.603 kg/s
Average air/fuel ratio	50:1
Pressure ratio	2.8:1
Maximal continuous jet-pipe temperature	580 °C (1076 °F)
Maximal oil temperature	110 °C (230 °F)

. The engine was coupled with a transmission ratio of 15.33 to hydraulic dynamometer DFX, which was connected to spring balance and tachometer in order to adjust load and speed, respectively. An air meter was used to measure air inlet flow rate. Engine fuel consumption was measured using a burette tube (flask) that was connected to the fuel tank. The turbine, turbine-inlet temperature (TIT), and exhaust-gas temperature (EGT) were recorded using a thermometer. Air temperature and humidity were measured to rigorously inspect the surrounding environmental parameters between each condition testing. Exhaust gases, including CO, HC, and NOx emissions, were sampled and measured using an SPTC autocheck gas analyzer. Detailed information about the engine is provided in

Table 5. Technical data and uncertainty of experiment apparatuses. Adapted from [32].

Apparatus	Measurement	Range	Accuracy
Tachometer	Brake speed	0–5000 rpm	±50 rpm
Spring balance	Load	0–50 lb	±2 lb
TIT indicator	Turbine inlet temperature (TIT)	300–900 °C	±10 °C
EGT indicator	Exhaust gas temperature (EGT)	250–620 °C	±10 °C
Flask tube	Fuel measurement	0–2000 mL	±2 mL
Digital stopwatch	Time-measured fuel consumption	0–99 min	±0.01 s
Manometer	Pressure	−380–380 mm	±1 mm
SPTC autocheck gas analyzer	Unburnt hydrocarbon	0–60,000 ppm	±1 ppm
	Nitrogen oxide	0–5000 ppm	±1 ppm
	Carbon oxide	0–10%	±0.001%
	Carbon dioxide	0–20%	±0.01%

.

We began a typical laboratory procedure starting and warming up the engine for about 5 min. After that, rpm and load conditions were adjusted to the expected values. Rpm level was slowly and carefully set using a control desk throttle lever on the control panel, while the engine load was controlled by the dynamometer. When the engine had reached the expected conditions, it was kept running for about 2 min to obtain the required constant rpm and loads. Experimental data were then recorded during engine consumption of 2 L fuel stored in the burette tube. The time was counted by a stopwatch to measure fuel consumption. In one testing mode, experimental data on engine revolutions, loads, exhaust-gas emissions, fuel consumption, and other parameters (temperature and pressure) were recorded; this set of data was measured three times during the testing period. The average values used for comparison were taken from when the engine was run on Jet A-1 and blends. Engine performance and emissions were recorded in the idle condition, and three dynamometer speed levels (2500 rpm, 2800 rpm, and 3000 rpm) with 8 loads formed the 9 testing conditions. This procedure was similarly processed as when the engine ran on the fuels being tested.

3. Results and Discussion

Performance and exhaust-gas emission characteristics are of major interest regarding the potential of new biofuels. An often-mentioned incentive for using biofuels is their capacity for improving engine performance and exhaust-gas emissions as compared to those of fossil fuels. Various tests on a Rover 1S/60 gas-turbine engine were performed using five different fuel samples: J0 (Jet A-1), J2, J5, J10, and J20. Results were analyzed and are compared below.

3.1. Performance Parameters

Turbine-inlet temperature (TIT) and exhaust-gas temperature (EGT) are typical indicators for evaluating engine performance characteristics. Higher TIT generally leads to higher efficiency and power. This is because more heat release is generated when the engine burning the same amount of fuel [27], whereas EGT commonly relates to heat loss, which directly affects engine thermal efficiency [27,28].

Figure 5 and Figure 6 show the effects of brake power on TIT and EGT when the engine was fueled with Jet A-1 and blends. These figures show that there were no significant differences in TIT and EGT when the engine ran on Jet A-1 and blends. As a result, the overall combustion process of Jet A-1 was similar to that of the blends. Generally, the combustion temperature is basically affected by adiabatic temperature and air/fuel ratio [27]. Therefore, when the engine runs at the same load and rotation speed, the air mass flow rate of the gas-turbine engine are generally similar if the energy content of the fuels is almost the same. This was consistent with the low heating value (LHV) of Jet A-1 and MBJ, as shown in Table 3.

A comparison between brake-specific fuel consumption (BSFC), and the overall efficiency of a gas-turbine engine fueled with Jet A-1 and blends is shown in Figure 7 and Figure 8, respectively. These figures illustrate results regarding the effect of brake load on brake-specific fuel consumption and thermal efficiency. As can be seen from these figures, there is no difference between them, which might be due to the fact that the fuel blends have almost similar properties as Jet A-1, as shown in Table 3. To generate the same power, the engine consumes a certain amount of fuel, which is indicated as fuel flow rate. One of the most important properties that directly affects the generation of heat is the net heating value [27,28,29,30,31]. Therefore, the similarity in the BSFC between an engine running on Jet A-1 and fuel blends is primarily due to the almost-similar net heating value of these fuels. Table 3 shows that the difference in LHV between MBJ and Jet A1 is approximately 1.6%; thus, the difference between J20 and Jet A-1 is around 0.32%. Clearly, this difference is too small to state its effect on the BSFC of a gas turbine engine.

3.2. Exhaust-Gas Emissions

Figure 9 shows the correlation of CO emissions with the brake power of the tested fuel types. CO emissions increased with the increase in the percentage of MBJ in Jet A-1. CO emissions are a product of incomplete combustion [27,28,32]. As shown in Figure 9, CO emissions from the engine when burning Jet A-1 and blends were similar at the same tested power levels. The main factor related to fuel properties that affects combustion quality is viscosity. Thus, the similarity in CO emissions when the engine was running with fuel blends as compared with Jet A-1 may be explained by the fact that MJB viscosity is almost the same as that of Jet A-1. Indeed, the difference in viscosity between J20 and Jet A-1 was approximately 0.8%.

The hydrocarbon (HC) emissions of Jet A-1 and the fuel blends are presented in Figure 10. HC significantly dropped with greater brake power. Similar to CO, the presence of HC species in exhaust gases is a manifestation of incomplete combustion [27,28,32]. In the idle condition, HC emissions of the blends were significantly lower than that of Jet A-1. This might have been because of the lower viscosity and aromatics of MBJ. However, when increasing power, the difference in HC emissions of these fuels was insignificant. Indeed, increasing power led to an increase in fuel mass flow rate; thus, differences in viscosity and aromatics were minor when compared to the effects of the combustion rate, which increased with temperature increase, as shown in Figure 5.

A comparison of the effects of brake power on nitrogen oxide (NOx) emissions when the engine was run with blends and Jet A-1 is shown in Figure 11. There was no difference in NOx emissions between when the engine was run on fuel blends and Jet A-1. NOx emissions from a gas-turbine engine are strongly affected by combustion temperature, especially stoichiometric reaction temperature [27,28]. Figure 5 shows that the TIT of the engine when run with fuel blends and Jet A-1 was almost the same. TIT is the mean of the outlet combustion temperature, which can represent combustion temperature. Therefore, NOx emissions of the engine when run with fuel blends and Jet A-1 are expected to be almost the same.

The results of smoke-opacity measurements are shown in Figure 12. The smoke opacity of the engine when run with blends was lower than that of the engine when it was run with Jet A-1. The increase in the MBJ fraction in the blends led to a decrease in smoke opacity, potentially because of the lower content of aromatics in MBJ compared to that in Jet A-1, as shown in Table 3. Aromatics strongly affect the tendency to form soot particles in flames [30,33,34,35], and particulate matter and blackened smoke emissions [9,11,36,37,38]. In this case, lower smoke opacity is consistent with sooting tendency, as indicated by the smoke point of these fuels, which is presented in Table 3. MBJ has a higher smoke point than that of Jet A-1. The higher smoke point represents a lower tendency to form soot and, consequently, black-smoke emissions.

In summary, blending MBJ with Jet A-1 resulted in insignificant effects on engine performance parameters (TIT, EGT, BSFC, and overall efficiency) and CO, HC, and NOx emissions. Increasing the MBJ percentage in MBJ blends, on the other hand, led to a decrease in smoke opacity, which is a positive effect for aviation gas-turbine engines.

4. Conclusions

This study inspected the potential for MBJ to be approved as commercial aviation fuel. To achieve this aim, the study conducted testing on blends J2, J5, J10, and J20, which were mixtures of 2%, 5%, 10% and 20% vol. with Jet A-1, respectively; these correlated with Jet A-1 in performance and emission effects.

Results showed that blends of up to 20% vol. of MBJ with Jet A-1 had almost no difference in engine performance parameters, namely, turbine-inlet temperature, exhaust-gas temperature, brake-specific fuel consumption, overall efficiency, and exhaust-gas emissions (carbon monoxide, unburned hydrocarbons, and nitrogen oxides), while decreased smoke opacity was observed. These results are consistent with the similar physical properties of MBJ and Jet A-1. On the other hand, the lower content of aromatics in MBJ had a positive effect on smoke opacity. MBJ also consists of lower sulfur content and is expected to have lower sulfur oxide emissions. This study showed that blends of up to 20% vol. MBJ with Jet A-1 are feasible for use in aviation gas-turbine engines. Further testing on MBJ is still needed, such as for flight testing. However, this study offers practical suggestions for developing aviation biofuels in tropical countries that have an abundance of feedstock resources, but are limited in technology and investment capital.