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Flying is now cheaper, safer, faster and greener than it was 60 years ago but global emissions are expected to increase over the next 20 years as air traffic looks set to double. The aerospace industry has known for a long time that to become sustainable and environmentally friendly, how aircraft are fueled needs to change.
Aircraft engines are designed to burn a narrow range of fuels, in the region of 7 to 18 carbon atoms i.e. kerosene- type fuels. Utilizing fuels falling outside this range could weaken safety, efficiently and operability.
But such fuels are finite; alternatives are needed, but kerosene is such a good fuel that it is hard to replace. Neither solar power or electricity can’t meet aviation’s requirements and are more likely to be used in road transport or energy generation, while nuclear power is also unsuitable. Currently, aerospace giants are trying to make remaining resources go further, while research into substitutes continues.
Sustainable alternative jet fuels (SAJF) must meet current standards for jet fuels, either alone or blended with conventional fuels. They must be produced from non-petroleum sources of hydrocarbons and have lower carbon emissions than conventional petrol-based fuels over their entire lifecycle.
SAJF must be sustainable not only in terms of reducing net lifecycle carbon emissions relative to conventional jet fuels – which research reveals isn’t always possible - but also in terms of environmental, societal and economic factors. Bringing SAJFs to market at a competitive price is the elusive goal for many reasons, although a few are on the cusp of making the transition to the mainstream. If alternatives can be commercialized, then the aerospace industry has the potential to a significantly lower volume of carbon emissions in a more timely and aggressive manner than might be achieved with changes to the operation, infrastructure, and aircraft design.
But it isn’t feasible to simply switch from conventional jet fuel to another fuel – not without any major modifications at least – nor are there any which are commercially ready, but we could see some alternative being introduced over time.
Hydrogen emerged as the favored substitute; a greener fuel three times more efficient than oil. However, its four times bulkier, and is incredibly energy intensive to create, releasing lots of CO2 in the process. Furthermore, it’s unlikely to ever meet any safety standards, and the need to keep it cool could present a danger if the aircraft were to crash. In theory, hydrogen would be a great alternative fuel and proof of concept shows it could work, but not enough hydrogen can be produced in an environmentally friendly manner to make it practical.
Instead, research is now focusing on alternative alternatives, like biofuels made from sustainable sources like algae, logging waste and rubbish that don’t compete with food or water. Such biofuels could have an important role to play in conventional gasoline and diesel applications, with synthetic kerosenes produced from biomass likely to be blended with or completely substituting conventional kerosene. These drop-in fuels have aggregate properties basically equivalent to jet fuels and they are compatible with conventional jet fuels, existing aircraft, and existing fuel infrastructure.
Researchers from the University of California Berkeley have developed a new method to convert sugarcane biomass and ketones into a fuel of the correct molecular-weight range, which could help reduce greenhouse gas (GHG) emissions. The method can be tailored for aviation fuels and lubricants by changing production strategies and represents a viable alternative where no others currently exist.
Other alternatives include coal-to-liquid (CTL) fuels which are now fully certified to be used in a 50/50 blend with kerosene, and indeed is in South African airports. It could, however, take decades before production levels are high enough for it to be used worldwide.
Gas-to-liquid (GTL) fuels convert natural gases or gaseous hydrocarbons into gasoline or diesel. Although it has the same CO2 lifecycle as conventional jet fuel it releases no sulfate and fewer particles emissions. Furthermore, it can utilize burnt-off methane from oil rigs.
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