Fuels synthesised from coal and natural gas can already power aircraft, but what industry needs is a bio-derived jet fuel that can address global warming
Two events last September brought attention to alternative fuels. The US Air Force began flight testing synthetic jet fuel in a Boeing B-52 bomber and Virgin Group chairman Sir Richard Branson announced plans to invest $3 billion over the next 10 years in renewable energy sources.
Motivations behind the events may have been different - the US military wants to reduce its dependence on foreign oil while Branson is taking aim at global warming - but both have a similar purpose: to spur development and production of alternatives to petroleum-based fuels.
With volatility in fuel prices inextricably linked to instability in oil-producing regions, commercial and military operators are finding common cause in the need to find alternative sources of aviation fuel that can be cheaper, and cleaner, than today's gasoline and kerosene.
Previous flurries of interest in synthetic fuels were responses to oil crises, and faded quickly when prices fell. But the era of cheap oil may be ending, and this time the twin imperatives of replacing foreign oil and tackling global warming may sustain interest in alternative jet fuels.
South Africa's Sasol pumps a 50% synthetic fuel blend at Johannesburg airport
Alternative fuels come in two flavours: those derived from non-renewable fossil sources such as coal and natural gas and generally described as synthetic fuel, or synfuel and those from renewable biological sources, such as crops, and generically termed biofuel. The US military is pursuing synfuel, because of the country's ample coal reserves, but the holy grail of aviation is a bio-jet fuel replacement for kerosene.
In mid-December, the Boeing B-52 began flying with a 50:50 blend of synthetic and standard JP-8 jet fuel in all eight engines, having flown first using the mix in one pair of Pratt & Whitney TF33s. Cold-weather testing of the synfuel blend will be completed by March. The USAF then plans to test the fuel in an afterburning fighter engine, says Paul Bollinger, special assistant for installations, environment and logistics.
Synfuel for the B-52 was produced from natural gas using the Fischer-Tropsch (F-T) process, invented in Germany and used during the Second World War, and now attracting interest because it can produce a cleaner jet fuel from coal, gas or biomass feedstock. But the Syntroleum plant in Tulsa, Oklahoma that produced the fuel was mothballed in September after supplying several thousand gallons to the USAF. "There wasn't any economic reason to keep it open," the company says.
Last May the US Department of Defense announced plans to buy up to 760 million litres (200 million USgal) of synthetic F-T kersoene for use in a 50% blend with JP-8 and JP-5 (used by the US Navy), but no contracts have been awarded. Bollinger expects the USAF to place its next order in March or April, but is not sure who will provide the fuel. "Shell may be interested, and there are firms in the USA who are producing the fuel on a pilot basis. Some may be up to a demonstration level by mid-year," he says.
The only commercial-scale producer is South Africa's Sasol, which produces 100,000 barrels a day of synthetic fuels from coal using a modernised F-T process. Sasol produces the only approved synthetic jet fuel, a 50% blend with conventional Jet A kerosene that is commercially available at Johannesburg's international airport.
Synthetic F-T kerosene is blended because, unlike conventional jet fuel, it contains no aromatic hydrocarbons. While partially responsible for the smoke and soot produced by gas turbines, aromatics also cause elastomeric engine seals to swell. "Without them, the seals will shrink and cause leakage," says Mike Farmery, Shell Aviation global fuel technical and quality manager. "There are also problems with lubricity."
Otherwise, synthetic F-T kerosene is a great turbine fuel, Farmery says, because it has better thermal stability, allowing engines to run hotter for improved fuel efficiency. The cleaner-burning synfuel also reduces emissions, producing no sulphur and fewer particulates, and extends engine life. But blending with jet fuel to overcome the seals issue negates the performance benefits, he says.
To produce synfuel, the feedstock is converted to carbon monoxide and hydrogen - a mix called syngas - then processed in an F-T reactor to produce heavy paraffinic waxes than can be refined with conventional petroleum methods. There are three types of F-T plant: coal-to-liquids (CTL), as operated by Sasol and planned in the USA gas-to-liquids (GTL), in large-scale operation for diesel production and biomass-to-liquids (BTL), still at the pilot stage.
Syntroleum's gas-to-liquids plant was idled after producing synfuel for USAF tests
CTL requires gasification of the coal, an expensive and energy consuming-step that produces carbon dioxide, which must be sequestered underground. GTL is simpler, and "well to wheel" CO2 emissions are similar to conventional refining. BTL promises big CO2 reductions, but gasification of biomass is tricky, says Farmery.
Synthesising fuel from non-renewable sources such as coal or natural gas is not considered sustainable in the long term, and does not address global warming. Research, therefore, is focused on developing a viable bio-jet fuel that is "carbon neutral" - produced from plants that take CO2 out of the atmosphere, so that burning the fuel adds no net greenhouse gases.
The ground transport industry is rapidly adopting biodiesel, produced from vegetable oils, recycled cooking oils, animal fats and even wood pulp. But the fuel has poor low-temperature properties, freezing in cold weather and at high altitude, and a breakthrough is needed before biofuel can replace jet fuel.
Hurdles to overcome
In an October 2006 report on alternative fuels, researchers from Boeing, NASA and MTU Aero Engines who examined bio-derived fuels concluded "significant technical and logistical hurdles need to be overcome. However the task is not insurmountable and no single issue makes biofuel unfit for aviation use."
To be a sustainable source of energy, enough crops need to be grown to meet the demand for biofuel from all sectors, and to avoid conflict with the need for crops as foodstuffs. To replace only the diesel demand of Germany with bio-diesel would require four times its available farmland area and replacement of every crop with rapeseed feedstock, NASA calculates.
© US Air Force
A USAF B-52 is now flying with 50:50 synfuel/JP 8 blend in all eight engines
Meeting commercial aviation's demand for biofuel would be a challenge, the report says. To supply just US airlines, even a 15% blend of bio-jet fuel would require more than 7.6 billion litres a year, and producing this much fuel from soybeans would require land about the size of Florida, the report calculates.
In a 2003 analysis of renewable aviation energy sources, the UK's Imperial College Centre for Energy Policy and Technology put the cost of producing biodiesel at between $33.50 and $52.60 per gigajoule (GJ) of energy. This includes conversion and distribution, but raising crops is 75% of the cost, says Dr Ausilio Bauen, one of the report's authors. The equivalent cost for kerosene in 2003 was $4.6/GJ.
Biodiesel is normally blended with conventional diesel - to a maximum of 20% to avoid issues with storage stability, as it breaks down and should be used within six months of manufacture. "It goes rancid," says Farmery. Biodiesel also degrades rubber gaskets and hoses, but these can be replaced with resistant materials.
Undoubtedly the greatest problem with biodiesel is its need for warm temperatures, Bauen says. Blended with kerosene, biodiesel raises the fuel's cloud point (CP) - the temperature at which micro-crystals form. "Even just 10% by weight biodiesel blend raises the CP from -51ºC [-60°F] to -29ºC," the report states.
Purdue University researchers are closing in on cold-resistant biodiesel, and Baylor University in Texas hopes to secure a grant to take biodiesel/kerosene blends back in the air. In the 1990s, the university's Institute for Air Science flew a Beech King Air for 60h with a Pratt & Whitney Canada PT6A running a 20:80 blend of biodiesel and kerosene and the other 100% Jet A.
The aircraft was flown to 25,000ft (7,600m) and "appeared to have no problems", says institute director Dr Max Shauck. The team tested blends as high as 50% on the ground. "We found that, in blends up to 50%, the performance was exactly the same as Jet A," he says. There was even an unexpected bonus: "The fuel bladders had become somewhat brittle over time and there was a rejuvenation."
A disadvantage of biodiesel for aviation is its oxygen content, which adds weight. "The trick is to get rid of the oxygen in biomass," says Farmery. "BTL can, but is expensive." Oxygen in vegetable oil can be removed through hydrotreatment, a process he believes is key to the US Defense Advanced Research Projects Agency's (DARPA) new BioFuels programme.
DARPA is looking for processes to produce a biofuel JP-8 surrogate affordably. Its goal is a 60% or better conversion efficiency (by energy content) from crop oil to bio-jet fuel, with a path to 90%. Industry has high hopes for the effort. Timothy Held, manager, advanced combustor engineering with General Electric, believes the outcome of the BioFuels programme "has a significant chance of becoming a viable aviation fuel".