Mon 20 Mar 2017 – Using biofuels to help power jet engines reduces particle emissions in their exhaust by as much as 50 to 70 per cent and so can help reduce contrail formations that produce climate warming effects, say research scientists led by NASA. The findings, published in the journal Nature, follow a series of flight tests undertaken in 2013 and 2014 as part of the Alternative Fuel Effects on Contrails and Cruise Emissions Study, or ACCESS, in which NASA partnered with the German Aerospace Center (DLR) and the National Research Council Canada (NRC). The tests involved flying NASA’s workhorse DC-8 as high as 40,000 feet while its four engines burned a 50/50 blend of conventional jet fuel mixed with camelina-derived biofuel. A trio of research aircraft took turns to fly behind the aircraft at distances ranging from 300 feet to more than 20 miles to take measurements and study contrail formation.
Contrails, which are composed primarily of water in the form of ice crystals, are produced by aerosol emissions from the hot aircraft engine exhaust mixing with ice-supersaturated cold air typical at cruise altitudes.
“Soot emissions are a major driver of contrail properties and their formation,” said Bruce Anderson, ACCESS project scientist at NASA’s Langley Research Center in Hampton, Virginia. “As a result, the observed particle reductions we’ve measured during ACCESS should directly translate into reduced ice crystal concentrations in contrails, which in turn should help minimise their impact on Earth’s environment.”
Persistent contrails create long-lasting, and sometimes extensive, thin cirrus clouds that would not normally form in the atmosphere and trap outgoing longwave radiation, a climate warming effect known as radiative forcing.
Although there has been progress in determining the true impact of contrails from jet engine emissions, uncertainties remain over the extent. Scientists are sure though that they are responsible for a significant climate warming effect, with NASA going as far as to say the impact on the atmosphere has been larger than all the aviation-related carbon dioxide emissions since the first-ever powered flight.
The authors of the Nature report say modelling studies of the present and future effects of aviation on the climate require detailed information about the number of aerosol particles emitted per kilogram of fuel burned and the microphysical properties of those aerosols that are relevant for cloud formation. However, they point out, previous observational data at cruise altitudes are sparse for engines burning conventional fuels, and no data had previously been reported for biofuel use in-flight.
“This was the first time we have quantified the amount of soot particles emitted by jet engines while burning a 50/50 blend of biofuel in flight,” said Rich Moore, lead author.
The ACCESS programme had two phases, and in the first a heavily instrumented NASA HU-25 Guardian aircraft measured chemical components from the NASA DC-8’s exhaust. In the second phase, the trailing aircraft also included a Falcon 20-E5 jet owned by DLR and a CT-133 jet provided by NRC.
“Measurements in the wake of aircraft require highly experienced crew members and proven measuring equipment, which DLR has built up over many years,” said co-author Hans Schlager of the DLR Institute of Atmospheric Physics. “Since 2000, the DLR Falcon has been used in numerous measurement campaigns to investigate the emissions and contrails of commercial airliners.”
The observations from the programme quantify the impact of biofuel blending on aerosol emissions at cruise emissions and provide key microphysical parameters, which will be useful to assess the potential of biofuel use in aviation as a viable strategy to mitigate climate change, say the authors.
The researchers add they plan to continue their studies to understand and demonstrate the potential benefits of replacing current fuels in aircraft with biofuels, and NASA says it aims to employ biofuels on its proposed supersonic X-plane.
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