A. Ezzat, Pharos University, Alexandria, Egypt
Aviation jet fuel is one of the fastest-growing sources of greenhouse gas (GHG) emissions. It is expected that the aviation industry might contribute up to 22% of global carbon dioxide (CO2) emissions by 2050. Today, aviation fuel accounts for about 7% of global GHG emissions, including 940 megatons of CO2 emitted in 2023.
For this reason, the International Civil Aviation Organization (ICAO) has set ambitious goals to reduce CO2 emissions by half by 2050—compared to 2005 levels—by applying sustainability practices. The rising sustainability standards have led industry stakeholders and refining companies to collectively consider alternate pathways to reduce GHGs by utilizing renewable feedstocks, including different types of wastes, CO2 and renewable hydrogen (H2).
Aviation jet fuel performance. Principally, improved thermal efficiency in gas turbine engines is achieved through an increased compression ratio, which is associated with higher turbine fuel temperature, depending on the turbine’s characteristics.
Conversely, the changes in compression ratio can influence the rate of carbon formation, particularly with higher boiling point aromatic compounds. This leads to high thermal radiation, which can damage the thin combustion chamber. A deposition of CO2 on fuel nozzles may cause distortions of the atomizer spray pattern and reduce combustion efficiency.
In addition, aromatic content in turbine fuels is considered as one of the main sources of carbon particles, and therefore, has been controlled by specification limitations.
Excessive sulfur in fuel can also have an adverse effect on the carbon deposition under some conditions, so it is important to control sulfur content.
The main aviation jet turbine types and specifications. The two main products that are considered for jet fuels are naphtha and kerosene-based jet fuels.
These products are based on the following limiting specifications:
Freezing point (−40°F)
Flash point (110°F−150°F)
Distillation: Smoke point
Aromatics content.
Naphtha jet fuel:
Produced primarily for the military
A wide boiling range stock (extends through the gasoline and kerosene boiling ranges)
More volatile and has more safety problems in handling
Used in a national emergency when kerosene is not enough.
Kerosene jet fuel
Safe to handle
Commercialized
Narrow boiling range stock (350°F−550°F).
Sustainable aviation fuel (SAF). The introduction of the SAF concept is considered an alternative to traditional fossil jet fuel that is capable of reducing CO2 emissions in the aviation sector. SAF can be produced from two major sources:
Renewable or waste-derived, bio-based resources that meet sustainability criteria depending on their origins, such as renewable oils and fats
CO2 captured from different processes, combined with renewable H2 (produced via water electrolysis).
Synthetic jet fuels have shown a reduction in pollutants such as sulfur oxide (SOx), nitrogen oxide (NOx), particulate matter (PM) and sometimes CO2 emissions. Therefore, it is envisaged that using synthetic jet fuel blends might improve air quality. SAF specifications show some specification limitations in viscosity and flash point, so blending it with fossil aviation fuel should be considered.
Given concerns about biofuel specifications and the fossil feedstock volatility markets and their environmental impacts, jet fuel suppliers are considering more challenging blends to meet required jet fuel specifications. The future of SAF appears promising, driven by a commitment to ambitious decarbonization targets and continuous innovation for a more sustainable aviation industry.
In fact, some airlines have successfully tested a biofuel blend consisting of more than 20% SAF on commercial flights, leading to the modification of American Society for Testing and Materials (ASTM) D1655 Standard Specification for ATFs to permit up to 50 parts per million (ppm) (50 mg/kg) of fatty acid methyl ester (FAME) in jet fuel to allow higher cross-contamination from biofuel production.
Bio-jet fuel specifications. There are many different specialized types of bio-jet fuel, including those designed for passenger aircraft (e.g., JET A-1) through to military-grade fuels (e.g., JET F-34/JP-8). Each has different characteristics, including freezing points, viscosity and additives. TABLE 1 shows the specified properties of commercial products compared to ASTM specifications.
Since SAF specifications show some specification limitations in viscosity and flash point, blending them with fossil aviation fuel should be considered. This will lead to the production of sustained ATF without the need to redesign or upgrade equipment and jet engine components.
SAF synthesis. The application of SAF made from bio-residues and CO2 can achieve lifecycle emissions reductions of up to 80% compared to conventional jet fuel. In fact, the highly paraffinic nature of SAF produced through the Fischer-Tropsch process can provide benefits by reducing PM emissions. In developed countries, some legislations have set targets and obligations for overall SAF supply and specifications.
In addition, the recently adopted ReFuel Aviation regulations in airport terminals require aviation fuel suppliers to reduce the GHG emissions from aviation with mandated levels for SAF supplied in the terminal.
The adoption of SAF is encouraged in developed countries through tax credits, specifically under the U.S. Inflation Reduction Act (IRA) at the government level, in addition to specific state-level, low-carbon fuel standard credits, stimulating domestic production. Technology has been approved for SAF blends in aviation up to 50% with conventional refinery kerosene (viscosity standard Jet A1).
CO2 pathway for SAF. The pathway for SAF production involves the combination of CO2 with renewable H2. It includes the reverse water gas shift (RWGS) technology, carbon capture solutions, catalysts and adsorbent materials, along with its Fischer-Tropsch solution. The proposed process can be described as an integrated eFuel complex with near-full carbon recovery and optimized H2 consumption.
CO2 capture consists of extracting the CO2 produced by various industries. There are ongoing developments in carbon capture technologies, driven by direct air capture (DAC) and storage as a form of offset and the growing demand for synthetic fuels. Scaling up DAC, which is recognized as a key technology for the net-zero pathway, depends on the development of policies and regulations. Some studies have shown that DAC technologies are expected to capture about 980 MMt of CO2 in 2050.
The two main carbon capture pathways available are:
The post-combustion pathway that focuses on the treatment of CO2-containing flue gases at close to atmospheric pressure (e.g., chemical absorption and separation processes of CO2 from other gases)
The pre-combustion pathway, involving capturing CO2 before fuel combustion. In this case, the fuel is processed with steam or oxygen to create syngas.
The process principles are based on H2 production via water electrolysis and the RWGS reaction.
H2 and carbon monoxide (CO) undergo Fischer-Tropsch synthesis, followed by a hydrocarbon production slate to provide flexibility in the products generated, enabling the production of various paraffinic fuels, including SAF.
Fisher-Tropsch technology is based on the generated CO and H2, which is produced from synthesis gas, as illustrated in FIG. 1.
Steam reforming: This process is widely used to generate synthesis gas from different feedstocks. In its simplest form, methane is used as a feedstock, as shown in Eq. 1:
CH4 + H2O —> O + 3H2 (1)
Steam reforming is usually carried out in the presence of a catalyst—e.g., nickel dispersed in alumina in operating conditions involving temperatures of 850°C−940°C and a pressure of about 3 megapascals (MPa).
Partial oxidation of hydrocarbons: The chemical reactions involved in this approach require air separation units to remove the nitrogen from the air to yield an oxygen-based atmosphere for the reaction, as per Eq. 2:
CH4 + 1/2 O2 —> CO + 2H2 (2)
In this scheme, the combustion chamber is operating at high temperatures (1,200°C−1,500°C) without a catalyst. The process design should prevent competing reactions.
ATR: This process produces a nitrogen-diluted stream of syngas, using an autothermal reactor (ATR)a. Pretreated natural gas, steam and air are thoroughly mixed before entering the bed in the ATR. The reaction proceeds over a commercially available catalyst and yields syngas with a H2:CO ratio of approximately 2:1.
Low-temperature Fischer-Tropsch synthesis: This process is considered one of several technologies to polymerize the carbon and H2 components into long-chain molecules, including SAF, as shown in Eq. 3:
CO (gas) + 2 H2 (gas) —> (–CH2-)n (liquid) + CO2 (gas) + H2O (3)
The synthesis runs at low temperatures: 220°C−350°C, with a pressure of 2 Mp–3 Mpa
Catalysts (typically, cobalt and iron).
In fact, this process produces high-quality synthetic hydrocarbons in the range of SAF that are virtually free of sulfur and aromatics. The typical process operating conditions are temperatures of approximately 220°C−240°C and pressures of approximately 2 MP–2.5 MP, with a typical conversion of about 60%.
SAF applications and future trends. Passenger awareness of environmental sustainability is rising, compelling the aviation industry to explore and adopt low-emissions alternatives.
In developed countries, legislations set targets, sub targets and obligations for overall SAF supply stipulated in legal renewable energy directives for the development of clean energy. In addition, recently adopted ICAO regulations require the reduction of GHG emissions from aviation with mandated levels.
Conversely, some countries, such as the U.S., are encouraging SAF utilization through tax credits, specifically under the U.S. IRA at the federal level, in addition to specific state-level low-carbon fuel standard credits.
Modern technologies for SAF production via methanol from syngas are being applied. The resulting jet fuel received certification to qualify as a blending component and has been approved for aviation use in blends up to 50% with conventional refinery kerosene.
Other essential requirements involve securing access to renewable power derived from renewable sources at an acceptable cost. This power is expected to be available at a large scale and exhibit around-the-clock stability in the future. The optimal integration of all sustainability processes, including off-gas recycling, and synergies with existing assets such as chemical plants and refineries are being explored to capitalize on shared infrastructure and resources, thereby contributing to overall SAF cost-effectiveness.
Jet fuel blends are receiving attention for developed and commercial synthetic jet fuels. For example, the goal of the U.S. SAF flight test program is to qualify the fuel blend for fleet use. Fischer-Tropsch fuels are certified for use in the U.S. and international aviation fleets at up to 50% in a blend with conventional jet fuel.
The introduction of the single fuel concept is gaining more global interest, mainly to achieve sustainability by maximizing both aircraft and ground equipment interoperability by using a bio-jet fuel. To comply with ASTM specifications, three main additives are applied to improve the performance of aeronautical engines: antifreeze, antimicrobial agents and corrosion inhibitors. In addition, the additives should improve atomization and contribute to a higher fuel-air mixing rate, resulting from shorter spray tip penetration and a wider spray angle.
In fact, the high cetane number of the jet fuel would improve the cold starting properties of diesel engines, reducing the smoke during startup and from the exhaust, decreasing knocking and noise, increasing the fuel economy and improving overall engine durability.
The application of the single fuel concept (JP-8 fuel) in airports will lead to lower NOx and PM emissions and shifts the NOx-ppm trade-off favorably for almost all performance conditions.
Takeaways. SAF will be essential to achieving the aviation industry’s ambitious decarbonization targets by 2050. Regulations will play a large part in increasing the share of SAF used in aviation and defining the qualified products.
CO2 utilization and bio-SAF have great potential to supplement aviation turbine demands. Combining the production of bio-SAF and CO2-based industrial processes provides advantages regarding the most efficient use of carbon from different sources, with high efficiency and optimized costs. HP
NOTE
Syntroleum’s ATR