Use of aviation biofuels was relegated until recently to testing on the ground (in labs and on test stands) and a few in-flight demonstrations. The landmark adoption by ASTM of revisions to the D7566-11 standard in July 2011 allowed for commercial flight applications without the need for special permissions.
Just as a new flight test or series of flight tests seldom failed to make news in the aviation industry, so too are commercial flights now permitted under the ASTM biofuel standard. For example, United Airlines made news Nov. 7 with the first U.S. commercial flight using biofuels. San Francisco-based Solazyme supplied algae oil that was refined into jet fuel by Honeywell’s UOP division near Houston. The blend used for the Boeing 737-800 flight was 40% Solazyme’s Solajet and 60% petroleum-derived commercial jet fuel (Jet-A).
Under the ASTM standard, up to 50% bio-derived synthetic blending components can be added to conventional jet fuel. These renewable fuel components, called hydroprocessed esters and fatty acids (HEFA), are identical to hydrocarbons found in jet fuel, but come from vegetable oil-containing feedstocks such as algae, camelina, and jatropha, or from animal fats called tallow.
Aviation fuel producers, distributors, airport fuel farms, and airlines in the global aviation community will use the standard to verify fuel quality and performance by testing to D7566 specifications. The updated standard includes new, specific requirements for the bio-derived synthetic fuel, including thermal stability, distillation control, and trace material amounts.
In all, about 25 “figures of merit” are assessed for each biofuel candidate under D7566-11, according to Mike Epstein, a degreed chemical engineer who serves as the leader of alternative fuels at GE Aviation. Flash point, freeze point, and energy density are the figures of merit most important to GE, he said, “although every last one matters.”
GE also has its own criteria for use of a particular biofuel. It has received lots from at least a dozen suppliers and is working with additional ones “large and small,” said Epstein. So new is the biofuels industry that “for some of those companies, it’s a challenge to make a thousand gallons, which is just a beginning point.” To put things into perspective, he said, the amount of jet fuel consumed annually around the world is about 65 billion gal.
Testing of biofuels begins at the beaker level, according to Epstein. If a sample meets the company’s “fit-for-purpose testing,” then it goes to component evaluations such as spray pattern tests. A biofuel that passes component testing then finds itself on an engine test stand. The next step is flight testing.
Epstein noted that GE has worked with several airframers on flight testing, including a recently completed series of tests using a camelina-based biofuel on an Embraer 170 jet under a broad range of flight conditions. One of the two GE CF34-8E engines used the ASTM-maximum 50% mix of HEFA with Jet-A.
“The fuel performed as expected and is consistent with Jet-A,” said Epstein.
Boeing says biofuels have proven to perform even better than conventional jet fuel.
Overall, Epstein said, GE is seeing little difference between conventional fuels and biofuels. Which makes sense, he said, because the biofuels are designed as drop-in replacements for conventional jet fuel. “While we’ve seen small differences with all the fuels we’ve tested, they’re all within tolerance of jet specification. The bio-derived and Fischer-Tropsch fuels run a little bit cooler—I’m talking only a degree of EGT [exhaust gas temperature].”
In discussing GE’s biofuels efforts, Epstein said aviation industry players are not looking at biofuels for a competitive advantage. “In fact, if anything, we’ve really tried to standardize processes in terms of…the fuel’s evaluation,” he said. “We tend to work through professional societies like ASTM and the FAA and the U.S. Department of Defense, as well as our industrial partners.” He noted that the fuel supplied to airlines at airports is generally owned by a consortium of airlines or by the airport itself. “So, quite literally, everybody is pulling from the same tank. The fuel has to be fungible.”
The flight test program with Embraer is a good example of the cooperative nature of the business. The fuel came from the U.S. Air Force, which has an extensive biofuels testing program of its own and is able to secure large amounts, according to Epstein. In this case, it supplied the biofuel to several aircraft and engine makers for testing.
In written responses to questions from AE, Jon Hart, Environment Strategy Manager – Civil Aerospace, Rolls-Royce, said: “The vast majority of Rolls-Royce alternative fuel testing is carried out in laboratory and rig tests. Rolls-Royce’s preferred strategy is to support testing that is capable of generating rigorous scientific data—such as lab, rig, and ground engine testing—as opposed to flight demonstration, which yields limited useful data.”