Jet fuel, a refined petroleum product, provides the necessary energy for modern aircraft, enabling global air travel and commerce. This specialized fuel is designed to perform reliably under the extreme conditions of flight. Understanding its origin involves tracing its path from ancient geological formations to complex industrial processes.
The Primary Source: Crude Oil
The vast majority of jet fuel originates from crude oil, a fossil fuel formed over millions of years from the remains of ancient marine organisms. Microscopic plants and animals, such as algae and zooplankton, absorbed energy from the sun and stored it as carbon molecules within their bodies. When these organisms died, their remains settled on ocean or lake bottoms, becoming buried under layers of sand, silt, and rock.
Over geological timescales, the immense pressure and heat from overlying sediments transformed this organic matter into a waxy substance called kerogen. With continued burial, increasing temperatures and pressures converted the kerogen into liquid and gaseous hydrocarbons, forming crude oil and natural gas deposits within porous rock formations. Crude oil is a complex blend of various hydrocarbons, alongside smaller amounts of compounds containing nitrogen, oxygen, sulfur, and metals. This rich chemical composition makes crude oil an efficient and energy-dense feedstock for a wide range of petroleum products, including jet fuel.
Transforming Crude Oil into Jet Fuel
The journey from crude oil to jet fuel takes place in a refinery through a series of sophisticated processes. The initial step is distillation, where crude oil is heated to high temperatures and introduced into tall columns. As the vapors rise, they cool and condense into different liquid fractions based on their boiling points. The kerosene fraction, which boils between 200 and 305 degrees Celsius, is collected as the primary component for jet fuel.
Following distillation, the kerosene fraction undergoes further processing and purification to meet stringent specifications for aviation. Hydrocracking uses hydrogen and catalysts to break down heavier hydrocarbon molecules within the kerosene fraction, increasing the yield of desired jet fuel components and improving their properties. Desulfurization, often achieved through hydrotreating, removes sulfur compounds by reacting them with hydrogen. This process also reduces other impurities like nitrogen and oxygen.
The treated kerosene is then blended with various additives to ensure it meets aviation standards. Key properties for jet fuel include its flash point, the lowest temperature at which its vapors ignite, and its freeze point, the temperature at which it begins to solidify. For instance, Jet A-1 has a minimum flash point of 38 degrees Celsius and a maximum freeze point of -47 degrees Celsius. These specifications ensure the fuel remains fluid and safe across the wide range of temperatures encountered during flight.
Emerging Sources: Sustainable Aviation Fuels
While crude oil remains the dominant source, the aviation industry is increasingly exploring sustainable aviation fuels (SAF) as an alternative. SAFs significantly reduce the lifecycle carbon emissions of air travel, offering reductions of up to 80% compared to conventional jet fuel. These fuels are considered “drop-in” replacements, meaning they can be blended with traditional jet fuel and used in existing aircraft and infrastructure without modifications.
Several pathways exist for producing SAF, each utilizing different renewable feedstocks. Hydroprocessed Esters and Fatty Acids (HEFA) is the most commercially available SAF today, made from sources such as used cooking oil, animal fats, and vegetable oils. This process involves hydrodeoxygenation to remove oxygen from the fats, followed by cracking and isomerization to produce jet-range hydrocarbons. Another pathway is Alcohol-to-Jet (AtJ), which converts low-carbon alcohols, like ethanol or isobutanol derived from agricultural waste or energy crops, into jet fuel through a series of chemical reactions.
Fischer-Tropsch (FT) synthesis is another versatile pathway that can convert a wide range of carbon-containing materials, including biomass and municipal solid waste, into a synthesis gas which is then converted into liquid hydrocarbons suitable for jet fuel. Despite their environmental benefits, SAFs face challenges, including higher production costs and limited production capacity. Scaling up SAF production also requires addressing concerns around feedstock availability and ensuring sustainable sourcing to avoid competition with food crops.