The Effects of the Use of Algae and Jatropha Biofuels on Aircraft Engine Exhaust Emissions in Cruise Phase

Abstract

Due to environmental pressure and the prevailing political and economic situation in the world, alternatives to traditional fossil fuels are being sought. The use of bio-derived fuels may reduce the emission of pollutants present in jet engine exhausts. The presented research investigates the possibility of replacing the conventional fuel, which is kerosene, with plant-derived fuels from marine algae and jatropha. During the analysis, based on the available data, the emission indices of pollutants were computed, and then, for the adopted aircraft and route, the emissions for kerosene and alternative fuels were determined. A significant reduction in the emission of most analyzed compounds (even by 40% for CO) was achieved compared to the emission for kerosene. The obtained results are discussed in the conclusion section.

1. Introduction

It is estimated that the aviation sector is developing at an annual rate of 5% [1,2] and contributes to 5% of the net radiative forcing of climate [3] and 2.6% of global anthropogenic CO₂ emissions [4], which is predicted to reach 5% by the year 2050 [5].

Moreover, globally, air transport generates approximately 6% of nitrogen oxides and less than 1% of sulfur oxides [6]. Gases and particulate matter emitted by jet engines during an aircraft’s voyage accumulate in the atmosphere near the busiest airways, mainly in northern latitudes. These compounds, as a result of further photochemical reactions in the atmosphere, cause many unfavorable phenomena—acid rains, photochemical smog, an increase in the concentration of tropospheric ozone, the greenhouse effect, etc.

Moreover, in civil aviation, the increase in fuel consumption is the highest. This is directly related to the dynamics of the development of air transport on a global scale, which has resulted in the doubling of the number of flights in the last three decades [7,8].

The 21st century is characterized by increased efforts to implement the idea of sustainable development in aviation and to limit the negative impact on the environment; hence, there have been many initiatives and strategies, research and projects, innovative solutions and implementations.

The Committee on Aviation Environmental Protection (CAEP), a technical committee of the ICAO (International Civil Aviation Organization) Council, established in 1983, has a significant impact on the regulation of the environmental impact of aviation, in particular, on air quality, climate change, and noise emissions. The CAEP assists the council in formulating new policies and adopting standards and recommended practices (SARPs) related to noise and emissions from aircraft engines [9].

The most important legislative measures introduced since the 1990s include the adoption by the ICAO of standards for aviation emissions of NO_x, CO₂, PM, and nvPM, as well as the establishment of a new CO₂ Compensation and Reduction System for International Aviation (CORSIA), the main goal of which has been to maintain, since 2021, a net-zero increase in CO₂ emissions in civil international aviation, in relation to the emissions from 2019–2020 [9]. These regulations were included in Annex 16 of the ICAO Convention.

The most important initiatives of the European Union in this area include declarations of responsible aviation development, emphasizing economic, social, and environmental responsibility. The main goals are: increasing fuel efficiency; the stabilization of CO₂ emissions in the aviation sector (carbon-neutral growth); reducing CO₂ emissions in aviation by 50% by 2050 (compared to 2005); reducing the carbon footprint of freight air transport; and the development of biojet fuels [10].

In order to achieve these goals, in line with the idea of sustainable development, the European Commission initiated the Aviation Strategy for Europe, which was adopted in December 2015 [11]. It marked a milestone in the recovery of the European economy, strengthening its industrial base and its position as a world leader while reducing the negative environmental impact of air transport.

At the 77th IATA (International Air Transport Association) Annual General Meeting in Boston, USA, on 4 October 2021, IATA member airlines passed a resolution committing them to achieving net-zero carbon emissions from their operations by 2050 (Fly Net Zero) [12]. This pledge brings air transport in line with the objectives of the Paris Agreement to limit global warming to 1.5 °C. IATA member airlines and the wider aviation industry are collectively committed to ambitious emissions reduction goals. Sustainable aviation fuel (SAF) has been identified as one of the key elements in helping achieve these goals. Governmental support is essential in using sustainable aviation fuels to achieve the industry’s climate goals.

Medium-term mitigation for pollutants emissions from the aviation sector can potentially come from improved fuel efficiency.

Scientific works, e.g., [13,14,15], undertake research and analyses in the scope of possibilities of reducing the negative impact of air transport on the environment. For example, in [16], it was proposed to modify aircrafts’ trajectory in such a way that their fuel consumption and related pollutants emissions would be lower. On the other hand, in [17,18], the possibilities of modifying the design of a turbine engine were indicated in order to reduce emissions. Such modifications are aimed at performing the combustion reaction in the engine combustion chamber as close as possible to the complete combustion reaction, thus reducing the amount of toxic compounds in the exhausts of the turbine engine. There are also works presenting modifications to the aircraft structure in order to obtain better flight parameters with lower fuel consumption [13,19]. Such a modernized propulsion requires lower fuel consumption or energy to cover a given distance. As a result, both fuel consumption and emissions related to the distance traveled can be reduced.

Interesting studies are those that analyze the replacement of traditional fuels with alternative fuels such as biofuels [20], or mixtures of hydrocarbon fuels and hydrogen [21]. In particular, the use of hydrogen as fuel or admixtures to hydrocarbon fuel seem interesting from the point of view of increasing the heating value of the fuel and reducing CO₂ emissions [22]. In the case of bio-derived fuels, the most promising are studies on the use of fuels made from marine algae or jatropha [20].

Algae are very efficient at biomass production. Many of them use sunlight and nutrients to create biomass that contains key elements such as proteins, lipids, and carbohydrates, which may be processed and upgraded to a range of biofuels and products. An efficient algae biofuel production system requires resources such as adequate land and climate, sustainable water resources management, an additional supply of carbon dioxide (CO₂), and other nutrients (e.g., phosphorus and nitrogen) [23]. Algae can be an attractive feedstock in numerous locations, as their diversity allows for high potential biomass yields in various climates and environments. Depending on the strain, algae can grow using fresh, saline, or brackish water from surface water sources, groundwater, or seawater sources [23]. In addition, they can grow in water from second-use sources such as treated industrial wastewater, municipal, agricultural or aquaculture wastewater, as well as water produced from oil and gas drilling operations [23]. As algae do not pollute the ecosystem, they are perceived as a clean renewable energy source. Biofuels nowadays produced from algae biomass include bioethanol, biomethanol, biobutanol, biodiesel, biomethane, biohydrogen and bio-oil. The perceived benefits warrant greater attention from researchers to microalgae research. However, in spite of the many published results of annual studies on the production of biofuels from algae, advances in technology to compete with fossil fuels in terms of cost-effectiveness remain a major challenge. The major challenges in producing biofuel from algae are related to an efficient harvesting methodology and a cost of the pretreatment of biofuel that is higher than the cost of growing algae [24].

Another promising alternative to conventional jet fuel is obtained in the hydroprocessing of Jatropha curcas [25,26,27]. This plant is considered a potential source to provide a significant portion of future jet biofuel production due to its fast-growing ability, high energy content, and tolerance to extreme climates and poor soil conditions. According to [28], currently, the net jatropha jet biofuel (JBF) production cost varies from USD 0.6–1.6/kg, which is the lowest amongst other oil-based feedstocks including palm, corn, castor, rapeseed, soybean, pennycress, camelina, and algae. However, the minimum JBF cost is still slightly beyond the market price of Jet-A conventional fuel [28]. The lifecycle GHG emissions of jatropha JBF varies from 22 to 54 (gCO₂e/MJ), which accounts for up to a 75% reduction in emissions relative to Jet-A. The GHG emissions reduction using JBF from Jatropha is higher than palm and lower than algae, while it stays close to rapeseed, soybean, and salicornia feedstocks [28]. The key challenge for the jatropha JBF industry is to decrease the production cost to a competitive level with conventional Jet-A, and to preserve a high and a consistent cultivation yield [28].

In [20], the parameters of fuels produced from algae and jatropha are presented, such as lower heating values (LHV) and emission indices (EI) of individual pollutants present in the exhausts.

The LHVs of alternative fuels in relation to the LHV of kerosene (JetA1) and the densities of these fuels are shown in

Table 1. The lower heating values (LHVs) and density of fuels.

Fuel Type	LHV [MJ/kg]	Density [kg/m3]
JetA1	42.9	775
jatropha	39.5	839
algae	38.2	842

.

The research presented in this paper is aimed at verifying whether plant-derived fuels can reduce the emission of pollutants in exhausts. Based on previous research by the authors [16,29,30] and the parameters of plant-derived fuels, it was possible, based on a case study, to analyze the changes in emissions and fuel consumption of a passenger aircraft in its cruising phase. The landing and take-off cycle (LTO phase) was omitted in the research due to the fact that the cruising phase is the longest part of the flight, so the most fuel is consumed during it. Moreover, for the LTO range, it would be necessary to define the performance of the aircraft and its engines. In this phase, the engines work with a higher thermodynamic load, so the determined emission would be burdened with a greater calculation error than for the cruising range.

The cruising range adopted for the analysis is based on the actual trajectory of the aircraft, which was traced on the Flightradar24 platform.

Moreover, it was assumed that regardless of the fuel used, to simplify the calculations, the maximum take-off weight (MTOW) would be the same. In the case that it is necessary to take a larger amount of a given fuel onboard, compared to the initial variant (fueled with kerosene), it was assumed that a correspondingly smaller mass of cargo would be taken on board the aircraft.

Additionally, it was assumed that the geometry of the combustion chamber and the injector was maintained—the analysis did not take into account engine modifications. Due to the fact that turbine engines are multi-fuel engines, the geometry of the combustion chamber can be assumed the same for all variants of the fuel used. Due to similar fuel characteristics, the geometry of the fuel injector will also be maintained, which will change the size of fuel drops and their impact on the efficiency of the combustion chamber [31].

The route covering the polar regions was adopted to conduct the research. These regions were taken into consideration as they are an area of the world particularly vulnerable to climate change. The Arctic is warming at a rate of almost three times the global average. Since the 1970s, the average temperature of the Arctic has increased by 2.3 °C. Thus, without urgent action to cut greenhouse gas emissions, the world will continue to feel the effects of a warming Arctic: rising sea levels, changes in climate and precipitation patterns, increasing severe weather events, and loss of fish stocks, birds, and marine mammals.

2. The Research Object

The research object adopted for the analysis is the Airbus A340-200 aircraft. The aircraft is equipped with four Snecma CFM56-5C engines.

Table 2. Basic technical data of the Airbus A340-200 aircraft [32].

Seating Capacity	Length [m]	Wingspan [m]	Engines Type	Thrust [kN]	MTOW [kg]	Maximum Payload [kg]	Fuel Mass [kg]	Aircraft Range [km]	Maximum Speed [km/h]
210–250	59.39	60.3	4× Snecma CFM56-5C	4 × 140	207,000	51,000	110,400	12,400	914

shows the basic geometric data of the aircraft.

The picture in Figure 1 shows the Airbus A340 aircraft.

Figure 2 shows the dimensionless performance characteristics of the CFM56-5C engine, which were determined by the authors computationally in previous studies [16,30], and the method of determining the characteristics was validated on the WESTT CS/BV test bench. It describes the relationship between its thrust at a given altitude and flight speed. On this basis, it is possible to determine the fuel consumption of each aircraft engine for the thrust range corresponding to the cruising range and the corresponding specific fuel consumption.

3. Determination of the Effect of Fuel Change on Engine Performance and Emission Indices

To determine the emissions, the change in fuel consumption depending on the type of fuel used must be taken into account. As mentioned in the introduction, the LHVs of alternative fuels are smaller (see Table 1). Hence, it is necessary to determine how much fuel consumption has increased after using algae or jatropha fuel, assuming that the same temperature in front of the turbine and the same engine thrust are maintained as when kerosene is used. The values of thrust and temperature were adopted at the level of T₃ = 1100 K, K = 26,000 N on the basis of the earlier works of the authors [16,29,30]. The following formulas were used for the calculations [16,30]:

Specific fuel consumption:

$$SFC = \frac{C_h}{K},$$

(1)

Fuel consumption:

$$C_h = m_1^{\prime }·\tau ,$$

(2)

where:

Relative fuel consumption, $\tau$:

$$\tau = \frac{C_{um}\left(T_3{}^{*} - T_2{}^{*}\right)}{\mathsf{\eta }·\mathrm{LHV}}$$

(3)

As mentioned earlier, the LHVs of alternative fuels are smaller than that of kerosene. Hence, according to formula (3), in order to maintain the same temperature value in front of the turbine (T₃*), more fuel must be supplied to the combustion chamber.

In the analyzed case, for the combustion chamber of the tested engine, the relative fuel consumption for kerosene, τ = 0.17, was calculated for the assumed combustion chamber efficiency of 99% [31]. The efficiency of the combustion chamber depends mainly on its geometry [31]. As mentioned in the assumptions, during the research conducted, the construction of the combustion chamber is the same regardless of the fuel consumed. The efficiency of the combustion chamber also depends on the size of the fuel drop [31] and the ease of evaporation of the fuel. Along with the increased density of alternative fuels, the efficiency of the combustion chamber will decrease if the injector geometry remains unchanged. According to [34], the difference in efficiency in favor of JetA1 will be 1%. If the same combustion temperature downstream of the combustion chamber is to be maintained for all the fuels covered by the analysis, then due to the lower LHV for alternative fuels, the relative fuel consumption will increase in relation to JetA1.

By applying the formulas (1–3) and the LHVs from Table 1 for different fuels, the change in fuel consumption by the engine is obtained. The results of these calculations are presented in

Table 3. Change in fuel consumption when using different fuels.

Fuel Type	Ch [kg/s]	Ch [kg/h]
JetA1	0.30	1095.45
jatropha	0.33	1189.74
algae	0.34	1230.23

.

As can be noticed, a decrease in the LHVs of the fuel is accompanied by an increase in the hourly fuel consumption—by 95 kg/h for jatropha fuel and 135 kg/h for algae fuel (both relative to kerosene).

The obtained values of hourly fuel consumption were used for further analyses and the determination of pollutant emissions in the exhaust.

4. The Flight Route Adopted to Determine Emissions in Cruise Phase

For the reasons mentioned in the introduction, the route between Paris and Vancouver was selected for the analysis. Figure 3 shows the flight route and its profile.

Based on the data retrieved from the FlightRadar24 platform, the time of the cruising phase was assumed to be 9 h and 36 min. The main parameters of the flight trajectory are shown in Figure 4.

This information was used for further analyses.

5. Determination of Pollutant Emissions

The application of alternative fuels is aimed at reducing the emission of pollutants present in exhaust gases. For a known engine operation range, the parameters of emission indices (EI) for the landing and take-off cycle (LTO) range can be found on the ICAO database [35]. To relate these emission indices to the cruising range, they must be reduced to cruising parameters. The methodology of dealing with this issue is presented in [16,29,30]. On this basis, the emission indices for CO₂, CO, HC, and NO_x were determined for the operating parameters (Ma = 0.8; H = 10,000 m). Based on the data retrieved from [20], it is possible to determine the change in the value of these emission indices after the use of alternative fuels. The EI values for kerosene and the two plant-derived fuels included in the calculations are presented in

Table 4. The emission indices for kerosene and two biofuels [20].

Fuel Type	EICO2 [kg/kg]	EINOx [kg/kg]	EIHC [kg/kg]	EICO [kg/kg]
JetA1	3.155	0.016	1.49 × 10−4	0.003
jatropha	2.672	0.013	4.17 × 10−6	0.006
algae	2.672	0.008	9.17 × 10−5	0.001

.

In order to determine the emission for the adopted route, a formula should be used that ties the emission indices of a given pollutant with the performance of the aircraft’s engines (thrust K and specific fuel consumption SFC, engine number l) and the duration of the cruise phase t (4) [36].

$$E = EI·K·SFC·l·t \left[\mathrm{kg}\right]$$

(4)

Table 5. Total emissions and fuel consumption on the route from Paris to Vancouver (cruise phase).

Fuel Type	ECO2 [kg]	ENOx [kg]	EHC [kg]	ECO [kg]	Ct [kg]
JetA1	130,411.60	661.36	6.16	115.74	41,334.89
algae	122,791.80	367.58	4.21	58.97	45,946.92
Difference [kg]	7619.79	293.78	1.95	56.77	−4612.03
Difference [%]	5.84	44.42	31.60	49.05	−11.16

and

Table 6. Total emissions and fuel consumption on the route from Paris to Vancouver (cruise phase).

Fuel Type	ECO2 [kg]	ENOx [kg]	EHC [kg]	ECO [kg]	Ct [kg]
JetA1	130,411.58	661.36	6.16	115.74	41,334.89
jatropha	118,750.53	558.61	0.19	267.98	44,434.74
Difference [kg]	11,661.04	102.75	5.97	−152.24	−3099.85
Difference [%]	8.94	15.54	96.99	−131.54	−7.50

show the emission of the main pollutants present in the exhausts and fuel consumption on the route from Paris to Vancouver. These data were related to the emission and fuel consumption of the conventional aviation fuel, which is kerosene.

As can be seen in Table 5 and Table 6, the use of algae fuel reduces the emissions of all analyzed pollutants, e.g., CO₂ emissions decrease by 7.6 tons (almost 6%), and NO_x emissions by 294 kg (44%), despite the increase in fuel consumption on the route by 11% (about 4.6 tons more compared to JetA1).

In the case of jatropha fuel, the reduction in CO₂ emissions was almost 11.7 tons (9%). The reduction in NO_x emission was 103 kg (15.5%). HC emissions were almost non-existent, while CO emissions increased by 152 kg (131.5%) compared to the reference variant (JetA1). The use of fuel from jatropha caused an increase in fuel consumption by about 3 tons (7.5%) for the route adopted for analysis.

6. Conclusions

The conducted analyses were aimed at verifying whether the use of particular alternative bio-derived fuels may lead to a substantial reduction in emissions in the cruise phase of the aircraft. These activities are in line with the current research trends for aviation.

The novelty of the research conducted, compared to the publication [20,34], is the reference of the measured emission indices (EI) of the CFM56-5C engine to the cruising range with the use of alternative fuels. Using the measured emission indices for CO₂, CO, HC, and NOx, the research methodology of the authors of the article allowed for the determination of the emission values of these pollutants for the cruising range.

The analyses conducted were based on the case of the Airbus A340-200 aircraft equipped with four Snecma CFM56-5C engines, covering the route between Paris and Vancouver airports. The cruising phase was assumed to be 9 h and 36 min, and the aircraft was characterized by the following operating parameters: Ma = 0.8; H = 10,000 m.

Two bio-derived fuels—from algae and jatropha—were analyzed as an alternative to the conventional fuel—kerosene JetA1.

The obtained results of pollutant emissions and fuel consumption were studied.

Based on the analysis, it was shown that the use of alternative fuels causes:

−

An increase in fuel consumption by 11.2% for algae fuel and by 7.5% for jatropha fuel;

−

A reduction inCO₂ emissions by 5.84% for algae fuel and by 8.9% for jatropha fuel;

−

A reduction in NO_x emissions by 44.4% for algae fuel and 15.5% for jatropha fuel;

−

A reduction in HC emissions by 31.6% for algae fuel and practically negligible HC emission for jatropha fuel;

−

A reduction in CO emissions by 49.1% for algae fuel and an increase in CO emissions by 131.5% for jatropha fuel.

In the case of changes in CO₂ emissions, the mass differences in those emissions were the biggest (from 7.6 to 11.7 tons), which is a significant value considering the fact that the obtained results are presented for only one case study (a particular aircraft and a particular route). Despite the large percentage differences in emission of the remaining pollutants considered, the mass changes in relation to the distance covered were not large (e.g., for NO_x they varied from 103 to 294 kg, and for other pollutants—CO and HC—they were negligible).

The research presented in this article shows that the use of bio-fuels—algae and jatropha fuels—can improve the environmental performance of the aircraft. However, the reduction in CO₂ and NO_x emissions for the cruising range is accompanied by an increase in fuel consumption. The use of the analyzed biofuels makes it necessary to take on board an additional 4.6 tons of algae fuel and 3 tons of jatropha fuel. In order to maintain the same MTOW, it is necessary to limit the mass of the cargo carried on board. This calls into question the economic aspect—is the use of such biofuels, despite a significant improvement in most environmental indicators, a profitable solution?

Another issue is the costs of production of biofuels. Microalgae is believed to be a promising feedstock for jet biofuel production. It grows rapidly, and its glycerides contents replicate the carbon structure of crude petroleum. However, processing microalgae into cost-effective fuels still faces many challenges, including the high costs of cultivation, harvesting, drying and oil-extraction units. Consequently, its oil is still extremely expensive compared to jatropha.

Further research should focus on laboratory tests of alternative fuels and their mixtures with kerosene or with hydrogen in order to obtain the same heating value as kerosene. Moreover, tests should be performed on the test bench.