More climate-friendly flying Less emissions thanks to biofuel

Status: 06/17/2021 6:04 p.m.

Contrails on the horizon show: An airplane was traveling here. But they also say something about the emissions that are released when flying. A new study shows: biofuel is more climate-friendly.

From Iris Völlnagel, SWR

You can see them most clearly in the early hours of the morning: Since more planes have started taking off from Frankfurt Airport, the contrails in the sky over Mainz have been thickening. They seem harmless, but climate researchers have long been concerned about them. Not because they are so-called chemtrails, as some conspiracy tellers suspect, but because they contribute to global warming. Because: Even before CO2, contrails cause the largest share of global warming through aviation. This is what researchers from the German Aerospace Center (DLR) found out together with NASA.

Contrails become “warming” clouds

While in flight, aircraft emit black carbon particles – so-called soot particles – because the fuel is not completely burned. In addition to soot, aircraft exhaust also contain water vapor. At high altitudes, where the air is very cold, the water condenses on the particles and freezes. At an altitude between 8,000 and 12,000 meters, ice crystals form, which we see as contrails. These ice crystals can last for several hours and form high clouds – so-called contrail cirrus. Depending on the position of the sun and the surface, they have a warming or cooling effect. Research shows that global warming effects predominate.

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Mixed fuel helps the climate

For years, scientists at the German Aerospace Center (DLR) have been researching, in collaboration with NASA, whether airplanes run on biofuel in a more environmentally friendly way. With their experiments, the scientists were able to prove that using a 50-50 mixture of kerosene and sustainable fuel can reduce the number of ice crystals in the contrails by half.

“We were able to clearly demonstrate that fewer soot particles in the exhaust gases through sustainable fuels result in fewer ice crystals in contrails,” says Christiane Voigt from the DLR Institute for Atmospheric Physics. “A smaller number of ice crystals reduces the additional energy input into the atmosphere caused by contrails. This significantly reduces the climate-warming effect of the contrail clouds.” The scientists assume a percentage of 20 to 30 percent.

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The scientists had already carried out the tests in 2018. For this purpose, the DLR research aircraft ATRA, an Airbus A320, flew from the US military base Ramstein in Rhineland-Palatinate with various fuel mixtures over Germany. It was followed with a one to two minute delay by NASA’s CD-8 research aircraft. With the help of measuring devices, the scientists from NASA, DLR and the Max Planck Institute for Chemistry were able to record data on emissions and contrails. They also tried different fuel blends – including a sustainable biofuel.

There are now individual attempts by the airlines to refuel their planes with biofuel. In May 2021, Air France announced its first long-haul flight from Paris to Montreal, Canada, using partially sustainable fuel made from cooking oil. During the test flight, there was kerosene and 16 percent biofuel in the tank. So far, a maximum of 50 percent biofuel has been allowed in aircraft, says Martin Kaltschmitt, head of the Institute for Environmental Technology and Energy Economics at the Technical University of Hamburg.

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The challenge of fuel development

The greatest challenge on the way to climate-friendly flying is the development of biofuel, say scientists. Biofuel, so-called Sustainable Aviation Fuel (SAF), is currently mainly produced from old vegetable and edible oils. In contrast to conventional kerosene based on mineral oil, biofuels are not subsidized and are much more expensive, also because they can only be produced in small quantities.

“Bio-based fuels will not be the only solution. With bio-based fuel, we don’t get the amount we need,” says Patrick Le Clercq, ECLIF project manager at the DLR Institute for Combustion Technology in Stuttgart. The future also lies with hydrogen, believes Le Clerq. In further experiments, the scientists from DLR and NASA now want to investigate how one hundred percent biofuel affects the atmosphere