A common algae commercially grown to make fish food holds promise as a source for both biodiesel and jet fuel, according to a new study.
The researchers, led by Greg O’Neil of Western Washington University and Chris Reddy of Woods Hole Oceanographic Institution, exploited an unusual and untapped class of chemical compounds in the algae to synthesise two different fuel products, in parallel, from a single algae.
“It’s far from a cost-competitive product at this stage, but it’s an interesting new strategy for making renewable fuel from algae,” said O’Neil, the study’s lead author.
Algae contain fatty acids that can be converted into fatty acid methyl esters, or FAMEs, the molecules in biodiesel.
For their study, O’Neil, Reddy and colleagues targeted a specific algal species called Isochrysis for two reasons: First, because growers have already demonstrated they can produce it in large batches to make fish food. Second, because it is among only a handful of algal species around the globe that produce fats called alkenones.
These compounds are composed of long chains with 37 to 39 carbon atoms, which the researchers believed held potential as a fuel source.
Biofuel prospectors have dismissed Isochrysis because its oil is a dark, sludgy solid at room temperature, rather than a clear liquid that looks like cooking oil.
The sludge is a result of the alkenones in Isochrysis — precisely what makes it a unique source of two distinct fuels, researchers said.
Alkenones are well known to oceanographers because they have a unique ability to change their structure in response to water temperature, providing oceanographers with a biomarker to extrapolate past sea surface temperatures.
Reddy and O’Neil began their collaboration first by making biodiesel from the FAMEs in Isochrysis.
Then they had to devise a method to separate the FAMEs and alkenones in order to achieve a free-flowing fuel.
The method added steps to the overall biodiesel process, but it supplied a superior quality biodiesel, as well as “an alkenone-rich fraction as a potential secondary product stream,” the researchers said.
“The alkenones themselves, with long chains of 37 to 39 carbons, are much too big to be used for jet fuel,” said O’Neil.
But the researchers used a chemical reaction called olefin metathesis. The process cleaved carbon-carbon double bonds in the alkenones, breaking the long chains into pieces with only 8 to 13 carbons.
“Those are small enough to use for jet fuel,” O’Neil said.
The study was published in the journal Energy & Fuels.