Testing to gain regulatory approval for alternative aviation fuels is picking up momentum, but bio jet fuels have yet to prove themselves to the authorities
If hype was fuel, kerosene would be obsolete. But qualifying a new aviation fuel takes time, and for bio-derived jet fuel the work is only just beginning.
Flight tests early next year of a General Electric CF6-80C2-powered Boeing 747-400 operated by Sir Richard Bransons Virgin Atlantic Airways will be the highest-profile bio-jet fuel demonstration yet, but is seen more as a catalyst than a validation.
Approved biofuels are still several years away, and the near-term focus for aviation is on synthetic fuels that can be drop-in replacements for jet fuel. The principal driver for this work is the US military, which wants to reduce its dependence on imported oil.
Commercial airlines will be able to use synthetic fuels, but they will not answer public concerns over aviations contribution to global warming. That will have to wait on development and approval of next-generation biofuels and there are unanswered questions about their impact on the environment.
There are any number of different biofuels out there, but they all fall down when you focus on a drop-in replacement able to meet the performance specifications for Jet A, because the specs say it has to be a petroleum-based source, says GE Aviation fuels specialist Tim Held. The challenge is to persuade regulators to amend the current standards to allow different sources of fuels meeting the existing specifications. Longer term, aviation biofuels could require new standards.
Leading the way is the Commercial Aviation Alternative Fuels Initiative, sponsored by the US Federal Aviation Administration and industry associations representing manufacturers, airlines and airports, and established to evaluate and promote alternative fuels for civil aviation.
According to Rob Midgley, Shells technical manager, aviation fuels, CAAFI is tasked with assessing fuels and making recommendations to standards body ASTM, which is responsible for the commercial specifications.
The protocol for ASTM approval of an alternative fuel will involve assessment of its properties against the specification, followed by component and engine testing where required and leading to approval by each engine manufacturer. According to the roadmap drawn up by CAAFI, this will begin with synthetic jet fuel produced using the Fischer-Tropsch (F-T) process (see sidebar).
F-T fuels will be first because there is a precedent for their approval and because the process has been proved to produce a drop-in replacement for jet fuel. The specification has already been amended to include a synthetic jet fuel produced from coal by South Africas Sasol using the F-T process. The approval is limited to a 50% blend with kerosene, but Sasol is working towards approval of a 100% synthetic substitute for Jet A.
US Air Force trials of synthetic JP-8 jet fuel produced from natural gas by Syntroleum have led to qualification of a 50% blend in the Boeing B-52, and an order for larger quantities of gas-to-liquid F-T fuel from Shell for testing in the Boeing C-17 airlifter and Rockwell B-1 bomber. This will support plans to qualify all USAF aircraft by 2011 to use a synthetic fuel blend produced domestically from coal when it becomes available.
Recognising this work, Midgley says CAAFI and ASTM are working towards approval for blends of up to 50% XTL synthetic jet fuel produced from coal, gas or biomass using the F-T process. Such fuels are attractive because the process produces iso-paraffinic hydrocarbons that have the cold-flow characteristics required for jet fuel while burning cleaner than crude oil-based fuel.
But coal and gas are fossil fuels, the F-T process is expensive in terms of energy and converting coal to liquid generates disastrous amounts of carbon dioxide, says Midgley. Biomass-to-liquid F-T can reduce overall CO2 production by more than 90%, but is in its infancy and also expensive. The issue around the biofuel options is they are generally more expensive than crude oil, he says.
Biodiesels, or fatty acid methyl esters (FAME), produced from vegetable oil are increasingly used in ground transport, but poor thermal stability makes them unsuitable as an aviation fuel. Palm oil freezes at 0°C, which makes it hopeless as a jet fuel," says Midgley. Biodiesel’s energy density is also lower than kerosene, which would reduce aircraft range. But blends of biodiesel with Jet A are being tested, including a 30% FAME blend tested by Snecma in a CFM56 turbofan.
Affordability and sustainability of the feedstock are potential issues with biofuels. The four common sources of vegetable oils soy, sunflower, palm and rape supply both the food chain and biodiesel for ground transport and adding aviation demand could drive up prices. "Two to three years ago palm oil cost $400/t; today it’s more than $800/t while jet fuel costs $700/t,"
says Midgley. And the risk that demand could result in monocultures covering vast areas of land is an "enormous problem", he says, adding: "Monocultures cultivate diseases as much as crops."
The Virgin/Boeing/GE team working towards the biofuel demonstration is tight-lipped on its source, but some unconfirmed reports favour a fuel produced from wild algae, or pond scum, by New Zealand firm Aquaflow Bionomic.
Algae as well as cellulosic materials like woodpulp are among the higher-risk sources being studied under the US Defense Advanced Research Projects Agency’s BioFuels programme to produce a JP-8 substitute. But there is nearer-term interest in lower-grade weed crops "that do not compete with the food chain", says programme manager Douglas Kirkpatrick. European industry, meanwhile, is looking for funding to begin alternative fuels testing next year under the Alpha-Bird programme.
The testing required for approval of alternative fuels is only just beginning to catch up with the hype. NASA plans to test several types, beginning with synthetic F-T fuel supplied by Shell. Flame tube testing will be used to screen a wide range of fuels and select viable candidates for combustor rig testing, says Daniel Balzon, associate principal investigator for combustion at NASA Glenn Research Center in Ohio.
NASA plans to form partnerships with engine manufacturers to conduct full engine tests of new fuels, and the research agency also hopes to conduct ground tests using a NASA aircraft. One issue with some of the fuels is their availability in sufficient quantity, Balzon says, bench-scale testing requiring 150-200 litres (40-50USgal) of fuel, flame tube testing 1,000-2,000 litres and "thousands of gallons" needed for engine runs.
"Getting enough biofuel is an issue, and we may have to wait," he says.