Algae is touted as holding the greatest potential as a long-term replacement for kerosene, but cost issues, combined with insufficient investment, leave experts generally agreeing that its widespread use in aviation is at least a decade away.
The oil that can be extracted from algae is structurally close to conventional Jet A1 fuel, and algae can be cultivated without competing with food crops for land. The challenge is that, at this stage, only a handful of relatively small companies are putting the necessary resources into producing algae-derived biofuel.
"Algae is by far the path with the highest potential," says Odile Petillon, head of operations at EADS Innovation Works' Energy and Propulsion Technical Capability Centre. "It is the only solution if we look at the mid- to long term.
"We have the technology potential to improve what exists and we have lab work going on, but it's a small industry. We are really at the beginning. It will take time, but it's the only solution in the long term."
Michael Lakeman, regional director of biofuel strategy at Boeing Commercial Airplanes, says oil derived from algae "performs just as well as other plant oils, such as camelina or jatropha", but it has an added benefit.
"Certain forms of algae also have the potential to directly produce some of the molecules found in jet fuel," he says, "circumventing or reducing the need for many of the downstream processing steps. The biotechnology for these advanced pathways is still under development."
Petillon says algae-derived oil is so similar to kerosene structurally that there are only "a few things to change", including using hydrogen to remove any remaining oxygen and reducing the length of the hydrocarbon chain to improve its properties. These changes are made as part of a two-step process that Petillon is confident could be achieved in a single step.
EADS in 2010 carried out a test flight using a Diamond DA42 New Generation light twin with its Austro Engine AE300s burning biofuel derived purely from algae. The fuel was provided by Argentina-based microalgae producer Biocombustibles del Chubut (see p30).
Petillon describes the results as positive, and says the demonstration "proved that we could have enough fuel to do a 100% test".
However, she says it was "very difficult to find 3,000-4,000 litres [790-1,060USgal] of algae biofuel for the test". To put that into perspective, the global aviation industry uses 250 billion litres of kerosene a year.
"It's not only about getting the price acceptable," says Petillon. "Very large numbers of plants need to be set up by investors and I can't say at what pace investors will set up businesses."
To compete with kerosene, algae-based biofuel would need to cost "below 30¢ for 1kg", says Petillon, but current prices range from €10 ($14.1)/kg "up to several thousand euros". She adds: "With the current available technology, [algae-derived biofuel] is far from being economically affordable compared to oil. We need to see where we can reduce costs and come to something sustainable within 10 or 15 years."
Lakeman agrees, pointing out that the main drawback of algae is that the technology needed to produce large quantities is still too expensive to do so at a competitive cost. "There are many aspects of producing fuel from algae that contribute to the expense, but in large part the cost is driven by the energy needed to move around large volumes of water and in separating that water from the algal biomass as it is harvested," he says.
Boeing is collaborating with the Chinese Academy of Sciences and the Qinqdao Institute of Bioenergy and Bioprocess Technology to examine ways of reducing the cost.
"One significant focus area is to produce algal biomass more efficiently and more productively at lower cost," says Lakeman. "Even though the research projects are still at an early stage, promising results are already being delivered."
Biocombustibles del Chubut (BC) has claimed that it will be able to sell algae-derived jet fuel for $1,125/t. By comparison, the International Air Transport Association's jet fuel monitor shows that, as of 1 April, the cost of 1t of kerosene was $1,075.
BC president Marcelo Machin says: "At our facilities in Patagonia, we produced more than 1.2t of oil on a monthly basis, using only a surface of 350m² [3,770ft²]. This installation also includes some fermenters and photo-bioreactors. You can figure out on average from operation costs how we came to this figure."
EADS used algae-derived fuel from Biocombustibles del Chubut to conduct Diamond DA42 test flight in 2010
Machin believes cost reductions can be achieved by combining algae with other cultures, increasing the yield by pushing the growth rate of the algae and using renewable energy to power the process.
Another company with ambitious goals - and significant US government funding to help it - is California-based Sapphire Energy.
Sapphire produces what it calls "Green Crude", a drop-in transport fuel derived from algae that uses only sunlight and carbon dioxide as a feedstock. The company, which has been awarded more than $100 million by the US government under a programme to help the nation reduce its dependence on foreign oil, has carried out tests using a 50/50 blend of Green Crude and kerosene in co-operation with Continental Airlines and Japan Air Lines.
Sappire chief executive Jason Pyle says the company's approach is unique because it uses algae as a primary feedstock to produce jet fuel that is compatible with existing petroleum infrastructure. "Some companies use algae like you'd use yeast, but we are the only company to use algae as a primary feedstock," he says. "This is an important distinguishing feature."
Jet fuel produced by Sapphire is "chemically indistinguishable from petroleum-derived counterparts", says Pyle, which means that, if certificated, it could be distributed using existing pipelines and used without modifications to aircraft engines.
"The likelihood of infrastructure - fuelling, jet engines, and so on - altering to accommodate a new liquid fuel product is effectively zero," he adds. "But the product we make is 100% Jet A."
Testing the fuel with Continental and JAL as a 50/50 blend produced results that exceeded expectations, says Pyle. "The expectation was that it would perform identically to petroleum-derived fuel, but in three areas it performed better." The fuel burned cooler in the jet engine, had a slightly higher density, and its emissions contained fewer particulates, he says. "In every other capacity, it performed identically to Jet A."
Having come up with the process and carried out initial tests, the next major step for Sapphire before attempting to scale up production to commercial levels will be certification, which Pyle believes is most likely to be carried out by the US military.
Sapphire plans to build a site this year that will be capable of delivering certification quantities of Green Crude by "as early as 2012", says Pyle. The certification process could take as little as several months or longer than a year, and from there it will be "a matter of scaling up and customer demand".
Airline demand is unlikely to be strong until costs come down, which Pyle says is the long-term aim. "I believe fundamentally that we have to compete with existing petroleum prices over the long run," he adds.
How soon the aviation industry will be using algae-derived jet fuel on a large scale will therefore depend on airlines' willingness to pay a premium over petroleum - at first, at least - and enter into long-term contracts, says Pyle.
"All aviation fuel could be replaced [by algae-based fuel] in 10-15 years, but commercial airlines could be the last in line [as customers], meaning it could take 50-100 years."
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