The discussion on endeavours towards reduction (and ultimate elimination) of pollutants from petrol and diesel engines in global road transport, is well aired and is gaining traction in the public mindset across nations; albeit there is a long road to travel before electric-powered vehicles become the majority zero-emission mode of transport.

While multiple advancements are under way in efforts to reduce emissions resulting from aircraft fuel combustion, there is less clarity in awareness of where things stand in the move away from carbon-based fuels in the aviation sector. In this article an investigation of developments made, and challenges facing the aviation sector in its drive to Net Zero 50, is explored.

Different solution approaches

While the battery-powered motor vehicle (e-car) is currently the principal replacement for the petrol-engine car, and will be for many decades to come, the replacement of the aircraft turbo-engine poses a more complex problem, requiring different solution approaches.

Advances have been made in electric design solutions for small taxiing aircraft; however, the powering of large aircraft necessitates an entirely different approach, principally through development of sustainable aviation fuels (SAF), which are complementary to conventional aviation fuel, but with reduced carbon footprint.

Carbon dioxide emissions, primarily from the combustion of fossil fuels, have risen dramatically since the start of the Industrial Revolution. Fossil fuels are formed through natural processes over millions of years.

The combustion of fossil-derived fuels to power aircraft turbine engines, where coal or natural gas are used as feedstocks, contributes to the creation of greenhouse gases in the Earth’s atmosphere.

Estimations of the contribution of overall emissions by the global aviation fleet range between 2.5% and 3.5%. The lower percentage value accounts for CO₂ emissions alone while the higher percentage includes vapour condensation trails (contrails) which contribute additionally to atmospheric warming.

In 2021 the International Energy Agency (IEA) published  its Net Zero by 2050: A Roadmap for the Global Energy Sector, which sets out a pathway to reach net zero emissions by 2050¹. In 2022, Global energy-related CO₂ emissions grew by more than 300 million tonnes, reaching a high of 37 billion tonnes (CO₂eq)².

Greatest challenge

Annual jet fuel consumption is approaching 100 billion gallons per year. The race to net zero is well under way and presents perhaps the greatest challenge yet to all energy sectors, including the world of aeronautical engineering.

In researching this topic:

• It was important to remind oneself of the composition of Earth’s atmosphere, the importance it plays in protecting planet Earth, and the dangers it faces in the absorption of daily emissions across the globe;
• To understand the nature and composition of aviation combustion fuel, a study is made of aviation fuel, including principal named types, their chemical composition, international jet fuel standards, and atmospheric flight emissions exhausted via gas turbine engines;
• A review of sustainable aviation fuels (SAF) is addressed, with explanation of the nature of SAF fuels and the expectation that such fuels may reduce global emissions of turbo-jet aircraft;
• A relatively recent concern relating to contrails is discussed, with review of commentary in respect of their contributions to the formation of greenhouse gases;
• This is followed by a discussion on rocket fuel and concerns of the atmospheric impacts arising during launch and passage through the atmosphere;
• A brief study is then made of small piston engine aircraft, with focus on emission of aviation gas to the atmosphere;
• The final section explores the commitment to Net Zero 50, with a focus of SAF as a percentage-mix drop-in fluid, to be followed as a 100% replacement fuel for kerosene, in the coming decades. Examples of innovative projects by leading players in the aviation industry offers some hope on the road to transformation.

Composition of Earth’s atmosphere

Earth’s atmosphere comprises approximately 78% nitrogen, 21% oxygen 1% of other gases including neon, hydrogen, and carbon dioxide. A balance in composition of elements is vital to sustaining a healthy atmosphere.

In the same way that glass traps heat in a greenhouse, the atmosphere traps heat next to the Earth. Gases inclusive of carbon dioxide (CO₂: 76%), methane (CH₄: 16%), nitrous oxide (NO₂: 6%), and a combined 2% of chlorofluorocarbon-12 (CCL₂F₂), hydrofluro carbon-23 (CHF₃), sulfurhexafluoride (SF₆) and nitrogen trifluoride (NF₃), trap extra energy from the Sun.

Two characteristics of atmospheric gases determine the strength of their greenhouse effect. The first is their ability to absorb energy and radiate it (radiative efficiency). The second is the atmospheric lifetime, the length of time the gas stays in the atmosphere.

Carbon dioxide has the lowest global warming potential (GWP) among the listed greenhouse gases, however the excessively large, human-created, increase in the atmospheric concentration of CO₂, has contributed significantly to global warming. Likewise, methane is responsible for a large proportion of recent warming, despite having a GWP lower than several other greenhouse gases³.

A diagram of Earth’s atmospheric layers, curtesy of the University Corporation for Atmospheric Research – (UCAR) Centre for Science Education, is shown in figure 1.

Incorporating the troposphere, stratosphere, mesosphere, and thermosphere, the diagram provides a pictorial representation of pertinent features associating to aviation flight.

The troposphere extends from Earth’s surface to a height of approximately 50km. A graded altitude scale enables appreciation of important features, including Mount Everest as a reference feature, cloud formation types, average zone of commercial jet flight (10-14km), weather balloons, the ozone layer, and the boundary to the mesosphere. The ionosphere describes the region straddling the middle and upper atmosphere, which is ionised by ultraviolet light⁴.

Figure 1: Diagram of Atmospheric Layers: UCAR Centre for Science Education (UCAR SciEd). Randy Russell: University Corporation for Atmospheric Research (UCAR) https://scied.ucar.edu/

In the 1960s, aeronautical engineers adopted the Theodore Von Kármán line, located about 100km above mean sea level, as the defined boundary between the atmosphere and outer space.

The atmosphere becomes less dense with altitude and is too thin to support aeronautical lift beyond the Kármán line. Above the thermosphere is the exosphere, a vacuum of outer space. The thermosphere and exosphere are collectively known as the upper atmosphere⁵.

Commercial jet aeroplanes fly at altitudes between 10 and 14km, exhausting fumes for the duration of their individual flight schedules. Weather balloons may reach altitudes of 40km, approaching the upper stratosphere.

Flights of meteorological rockets

Flights of meteorological rockets may extend well into the mesosphere, reaching heights of 80km. Space craft and satellites reach the outer exosphere. The International Space Station (ISS) orbits in the thermosphere at approximately 400km⁶.

It is estimated that upwards to 100,000 flights take off per day across the world, ascending through the troposphere to the lower stratosphere. The number of domestic and global airline flights was estimated at 22 million in 2021⁷.

The aviation industry is facing arguably its greatest challenge to date, in its quest to achieve net zero aircraft emissions by the year 2050.

The International Panel on Climate Change (IPCC) has informed that net emissions must be reduced to zero as soon as possible to stabilise global temperatures. This implies that all man-made greenhouse-gas emissions must be removed from the atmosphere through reduction measures.

To achieve net zero and limit global warming to 1.5ᵒC (average global temperature above pre-industrial levels), it is necessary to remove and permanently store CO₂ from the atmosphere, by carbon dioxide removal (CDR)⁸.

Aviation hydrocarbon jet fuel: Make-up and standards

Petroleum (from the medieval Latin word petroleum meaning rock-oil) is a naturally occurring yellowish-black liquid mixture, consisting mostly of hydrocarbons, found in geological formations.

Crude oil is extracted by distillation, producing products including petrol (gasoline), diesel, kerosene, and jet fuel. Extraction, refining and burning of petroleum fuels, releases large quantities of greenhouse gases, thus contributing to climate change.

In the early 1920s, German scientists Franz Fischer and Hans Tropsch developed a then novel catalytic process, the FT-process, to convert coal into a synthetic fuel. They developed an indirect way to liquify coal in which solid coal is first transformed into a gas.

The process involves introducing metal catalysts at temperatures of 150–300 °C (302–572 °F), to set off a variety of chemical reactions resulting in liquid hydrocarbons, a liquid form of carbon monoxide blended with hydrogen, known as water gas (or syngas). This formed the platform for early developments which enabled hydrocarbons to be used as a transport fuel source, albeit a non-green source⁹.

The primary function of aviation turbine fuel is to power an aircraft. Kerosene and petrol are distillate derivatives of petroleum but have different chemical compositions. Kerosene is widely used to power jet engine aircraft: Jet-A and Jet-A1 are well known fuel standards. 

Kerosene is also used to power some rocket engines in a highly refined form called RP-1 (Rocket Propellant-1). Petrol is used to power piston engine aircraft. Such fuels are complex mixtures of hydrocarbons produced by distillation of crude oil.

Hydrocarbons, or alkanes, are organic compounds comprised almost entirely of hydrogen and carbon atoms. Hydrocarbons are found in nature in petroleum, natural gas and coal, or their hydrocarbon derivative forms.

Complex mixtures of hydrocarbons

Jet fuels are complex mixtures of hydrocarbons produced by distillation of crude oil. They contain hundreds of hydrocarbons as well as several additives. The actual composition of any given fuel depends on the source of the crude oil, refinery processes, and product specifications.

Regardless of source and production process, kerosenes and jet fuels primarily consist of C9 to C16 hydrocarbons and boil in the range 145-300°C.

The predominant components of jet fuels are branched and linear paraffins and naphthenes (cycloalkanes), which usually account for more than 70% of the components by volume. Aromatic hydrocarbons, such as alkylbenzenes and naphthalenes comprise up to 25% by volume¹⁰.

(Note: Detailed listings of complex straight-chain alkanes, with inclusion of associated isomers and their common names, sorted by number of carbon atoms, from 1 (Methane: CH₄) to 120 (n-isohectane: C₁₂₀H₂₄₂), are tabulated in¹¹.)

Aircraft emissions

There are two main sources of aircraft emissions, i) the jet engines; and ii) auxiliary power units (APU). Most jet fuel is burned in flight, hence most of the emissions occur at altitude. When hydrocarbons are completely combusted, the products are carbon dioxide (CO₂) and water vapour.

However, when jet fuel is burned, other emissions including sulphur oxides (SOx), nitrogen oxides (NOx), unburned hydrocarbons, and soot particulates, are formed.

These emissions result from both trace amounts of sulphur and nitrogen in the fuel, and engine design and operating conditions.

The International Civil Aviation Organisation (ICAO) has established limits for nitrogen oxide, carbon monoxide, unburned hydrocarbons, and smoke from commercial jet engines. Particulates and unburned hydrocarbons are the result of incomplete combustion. If present in high concentration these particulates are visible as soot or smoke coming out of the engine.

Particulates at ground level can contribute to haze and smog formation and can be harmful, if inhaled. Modern jet engines are designed to standards ensuring reduced particulate emissions¹².

Standards and certification

Standards to ensure jet fuels are fit for purpose are set and maintained by international organisations including: the American Society for Testing Materials (ASTM), the UK Civil Aviation Authority (CAA), and the International Civil Aviation Organisation (ICAO), which serves as the forum for cooperation in all fields of civil aviation among its 193 member states.

Jet engines and airplanes are certified by the Federal Aviation Authority (FAA) to operate on a fuel that is specified by the American Society for Testing and Measurements (ASTM).

International standards set requirements for criteria such as composition, volatility, fluidity, combustion, corrosion, thermal stability, contaminants, and additives, among others to ensure that the fuel is compatible. ASTM D1655, is the Standard Specification for Aviation Turbine Fuels¹³.

Physical properties for Jet A and Jet A-1 Fuels, include flashpoint, autoignition temperature, freezing point, maximum adiabatic burn temperature, density, specific energy, and energy density. Typical values for Jet A and Jet A-1 are shown in Table 1¹⁴. 

Sustainable aviation fuels

Fossil fuels are formed through natural processes over millions of years. The combustion of such fuels to power aircraft turbine engines, where coal or natural gas are used as feedstocks, contributes to the creation of greenhouse gases in Earth’s atmosphere. 

SAF is an alternative fuel made from non-petroleum feedstocks, with purpose of reducing emissions from air transportation. The composition of SAF is such that, when compared to conventional fuels, it has a higher hydrogen-to-carbon ratio and flashpoint, which results in lower emission pollution.

Production is by indirect synthesis of petroleum derivatives. Today, 100 years after Fisher and Tropsch first developed their catalytic solid to gas conversion process, the FT-process is again widely applied in the production of sustainable aviation fuels. The process breaks carbon material into its individual molecular elements, in a gas (synthesis) form.

Multiple ‘pathways’ are under development for production of SAF, some of which have been fully certified and are operational. ASTM D4054, Standard Practice for Evaluation of New Aviation Fuels and Fuels Additives, was developed to ensure safe and reliable operation of aircraft on alternative aviation fuels.

Any new (kerosene replacement) fuel must meet the conventional fuel ASTM specifications and be approved through the ASTM D4054 process, to be permissible for use in commercial flights. Upon successful completion of D4054 tests, the approval process for D7566, Standard Specification for Aviation Turbine Fuel Containing Synthesised Hydrocarbons, begins. Achieving certification may take from three to five years¹⁵.

As of July 2023, 11 conversion processes (PATHWAYS) for SAF production have been approved by ASTM and several other conversion processes are currently under evaluation¹⁶.

To be eligible within the ICAO Carbon Offsetting and Reduction Scheme (CORSIA), SAF must also meet a set of sustainability criteria, which are detailed on the CORSIA Eligible Fuels webpage¹⁶. All these processes use direct organic compounds feedstock; such as, coal, natural gas, biomass, vegetable oils, ethanol, and algae. SAF conversion technologies can transform a wide range of biomass and waste feedstocks into jet fuel¹⁷.

Table 2 provides a short overview detail of seven approved processes. Approved SAF processes are termed ‘drop-in’ fuels, meaning they can be blended directly with conventional Jet-A fuel. Currently approved SAF have drop-in ratios ranging from 10% to 50%. The aspiration is that ratios of up to 90% may be achievable in the decades to come.

A pathway different to those approved to date, is that of Electro-fuel (e-fuel), also termed power-to-liquid (PtL). It is a synthetic fuel that is carbon neutral, if it has been created using renewable energy only.

E-fuels are manufactured using captured carbon dioxide or carbon monoxide, together with hydrogen obtained from water split by electrolysis using sustainable (green) electricity sources such as wind, solar and nuclear power.

E-fuels, such as e-methanol (CH₃OH), or e-kerosene (C₁₂H₂₆C₁₅H₃₂,), are differentiated from biofuels as they are produced from renewable or decarbonised electricity. E-fuels offer similar performance to petrol and diesel but have a lower environmental impact due to their production process.

To be climate neutral, e-fuels must be produced from green electricity and have a closed CO₂ cycle when viewed from a holistic 'well-to-wheel' perspective.

Well-to-wheel (WtW) is a measurement used to accurately evaluate and compare vehicle emissions. It calculates the total climate impact of a product or service from its production to its use¹⁸.

In Figure 2, courtesy of ICAO Global Framework for Aviation Alternative Fuels (GRAAF), a block diagram of the Power-to-Liquid for Aviation concept, is shown.

The pathway does not depend on biomass and has no demand for arable land and limited demand for water. PtL has a more favourable GHG balance reduction to conventional fuels, with promise of upwards to 90% following further research and plant development^19,20.

Figure 2: Power to Liquid for Aviation Infographic, ICAO Environmental Protection/GRAAF.

https://www.icao.int/environmental-protection/GFAAF/Pages/Project.aspx?ProjectID=46 

(In part 2, we will look at space flight launch emissions; aviation gasoline (avgas); and the commitment to net zero.)

Author: Eugene D Coyle, Technological University Dublin, with the support of Mohamed Al Siyabi, Abdullah Al Shibli, Abid Ali Khan, Military Technological College Oman. 

References

1) IEA International Energy Agency.Net Zero Road Map – A Global Pathway to Keep the 1.5C in Reach. https://www.iea.org/reports/net-zero-roadmap-a-global-pathway-to-keep-the-15-0c-goal-in-reach

2) IEA International Energy Agency. CO2 Emissions 2022. https://iea.blob.core.windows.net/assets/3c8fa115-35c4-4474-b237-1b00424c8844/CO2Emissionsin2022.pdf 

3) C2ES Centre for Climate and Energy Solutions. Main Greenhouse Gases: https://www.c2es.org/content/international-emissions/# 

4) IPCC International Panel on Climate Change AR6 Synthesis Report: Climate Change 2023. (IPCC). https://www.ipcc.ch/report/sixth-assessment-report-cycle/ 

5) Theodore Von Karman (1881-1963):  Hungarian born mathematician, aerospace engineer and physicist. The defined threshold of outer space is named in his honour.

6) Palmer Pail L The Atmosphere – A Very Short Introduction. Oxford University Press. ISBN 978 780-0-19-872203-8 

7) IATA International Aviation Training Authority. Global Airline Flight Numbers. https://www.iata.org/en/pressroom/2022-releases/2022-01-25-02/ 

8) ICAO International Civil Aviation Organisation. Future of Aviation. https://www.icao.int/environmental-protection/Documents/ScientificUnderstanding/EnvReport2016-WhitePaper_LAQ.pdf

9) de Klerk A. (2013). 'Fischer–Tropsch Process'. Kirk-Othmer Encyclopaedia of Chemical Technology. Weinheim: Wiley-VCH. pp. 1–20

10) Edwards James T. Properties of Aviation Fuels – Jet Fuels. National Research Council (US). Defence Technical Information Centre Fuel and Research Branch, Turbine Energy Division.

11) Wiki. List of straight-chain alkanes. https://en.wikipedia.org/wiki/List_of_straight-chain_alkanes 

12) Chevron Products Company. Aviation Fuels : Technical Review. Aviation-tech-review.pdf.

13) ASTM D1655: Standard Specification for Aviation Turbine Fuels. American Society for Testing and Materials ASTM International.  https://www.astm.org/d1655-22.html 

14) Physical Properties for Jet A and Jet A-1. [Table1] ASTM D1655-22: Standard Specification for Aviation Turbine Fuels.

15) US Energy Department. Sustainable Aviation Fuel – Review of Technical Pathways. Office of Energy Efficiency and Renewable Energy. energy.gov/eere/bioenergy

16) ICAO SAF Conversion. International Civil Aviation Organisation. SAF Conversion Processes. https://www.icao.int/environmental-protection/GFAAF/Pages/Conversion-processes.aspx 

17) ICAO CORSIA. Corsia Eligibility Criteria. ICAO https://www.icao.int/environmental-protection/CORSIA/Pages/CORSIA-Eligible-Fuels.aspx 

18) ENGIE. Green Mobility. E-fuels, what are they? https://www.engie.com/en/news/green-mobility

19) ICAO Power-to Liquid for Aviation. https://www.icao.int/environmental-protection/GFAAF/Pages/Project.aspx?ProjectID=46 

20) Airbus P&W SAF+ Airbus, Pratt & Witney, SAF+: Consortium Collaboration on Developing Next Generation Sustainable Fuels. https://airwaysmag.com/consortium-to-develop-e-saf-canada.