The Soot Factor

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Next time you’re cruising at forty thousand feet in the air, think about how amazing it is that a few hundred tons of metal can whisk you between two distant cities in just a few hours. For the seasoned flier, air travel is so simple it almost feels like magic. Behind that magic, though, lie many technological innovations—one of the most important being the jet fuel that keeps the engines running.

Most aircraft engines today burn petroleum fuels that emit large volumes of carbon dioxide, the primary pollutant behind rising global temperatures. To reduce these emissions and make aviation more sustainable, biofuels may be a necessary replacement. Biofuels consume carbon dioxide from the atmosphere in production, balancing the amount they emit when burned. Because they have similar physical and chemical properties to petroleum fuels, biofuels could easily power existing jet engines. However, with thousands of possible biofuels competing for a single spot in the future of aviation, it’s hard to say which one to use. Thus, it’s necessary to consider a key piece of data: the soot factor.

Soot is the black residue left behind by burnt organic matter. When dispersed in the atmosphere, it absorbs solar energy and contributes to climate change alongside carbon dioxide. Under some circumstances, it can even induce the growth of high-altitude cirrus clouds that absorb solar radiation more strongly than carbon dioxide. When inhaled, soot can lead to the development of heart disease and certain cancers, adding to the public health risk of air pollution. To minimize the burden of soot emissions on the climate and human health, researchers must select biofuels that burn without releasing harmful amounts of soot.

In an effort to improve available data about soot emissions, the Pfefferle Lab Group at Yale developed a new method to measure a fuel’s “sooting tendency” and then examined two dozen biofuel candidates. Earlier techniques for calculating sooting tendency required researchers to burn large volumes of fuels to observe the complex properties of the flames. The Pfefferle group’s new method reduced the amount of fuel necessary to generate data. They opted to calculate sooting tendency by measuring the luminosity, or brightness, of the fuels’ flames when burning individual drops—the brighter the flame, the sootier the fuel.

The biofuel candidates subjected to this new test all fall under the category of terpenes, combustible chemicals found in organisms ranging from redwood trees to algae. Charles McEnally, a chemical engineering research scientist at the Pfefferle lab, explained that terpenes are of special interest because of their diversity.

“What’s interesting about terpenes is that the biochemistry that makes them is always the same, and the input molecule is always the same: its isoprene,” McEnally said. “Depending on exactly how the chemistry works, you can get an enormous number of different outputs. There are tens of thousands of terpenes that are known.”

Out of the twenty-four terpene biofuels that the Pfefferle group tested, seven were produced through a process known as hydrogenation, in which the chemical structure is modified to include more hydrogen atoms and fewer double bonds. These hydrogenated options outperformed their unmodified competitors for soot reduction, posting lower numbers on the sooting index that the Pfefferle group developed. Hydrogenation—as well as other chemical processes that are broadly referred to as “upgrading”—have the potential to further improve the properties of biofuel candidates. 

“We have all of organic chemistry at our disposal, so we’re no longer limited to the molecules that are in petroleum. Almost certainly, out of all of organic chemistry, there are other molecules that will make better fuels than the ones that happen to be in petroleum,” McEnally said.

 In their paper regarding terpene biofuels, the authors note that large-scale production of terpenes could shift toward bioreactors in the future. By genetically engineering microorganisms like E. coli to synthesize terpenes in bioreactors, the aviation industry could find a path to a simple and sustainable fuel source. 

The Pfefferle group’s measurements for terpenes add to an ever-growing set of data about biofuel candidates. Their simplified method for determining sooting tendency provides a starting point for further research. With the group’s work, a biofuel alternative to petroleum-based jet fuel may eventually be what takes you to the skies.