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Biofuel: aviation alternative?

George Marsh

Obtaining more passenger/freight-miles for less fuel is a priority aim for aviation, given the pincer pressures of escalating fuel costs and environment, which are squeezing operators to desperation point. A more sustainable basis for fuelling aircraft is needed, and biofuel is a leading, if controversial, candidate.

The biofuel issue remains fraught with controversy. Recent reports – such as the Gallagher review in the UK – have added to a growing perception that certain biofuels could have worse environmental impacts than fossil fuels. Many, including Gallagher (chairman of the UK's Renewable Fuels Agency) say that while we ultimately need biofuels, demanding targets that are driving the market for unsustainable products should be reduced, allowing the development of biofuels to be 'slowed' and more research undertaken into how they can be produced sustainably (and perhaps more importantly, how we can ensure that they are being produced sustainably).

Against this backdrop, aviation is looking to biofuels as one possible answer to its growing fuel supply difficulties, in addition to introducing aircraft with better fuel consumption.

In recent times it has been shown that modern jet engines can run satisfactorily on fuels derived from live plants and other organic matter, rather than oil from plants fossilised millenia ago. A leading progenitor is the US Air Force which, as one of the world's largest users of fossil fuels, badly wants to reduce its dependency on oil with the cost and sourcing difficulties and finite nature that goes with it.

According to a US Air Force spokesperson, every US$10/barrel rise in the cost of conventional fuel costs the USAF US$600 million per year. The force is aiming to certificate its entire aircraft fleet to run on a 50:50 blend of synthetic jet fuel (synfuel) and conventional kerosene by 2011. Any synfuel must be a drop-in replacement for normal jet fuel, with similar density, freeze point and flash point characteristics to standard jet fuel. Energy content is important too, though unfortunately most synfuels are currently less energy dense than conventional fuel so that more fuel has to be carried for a given mission requirement. An advantage, though, is that synfuel burns more cleanly than conventional kerosene, with fewer noxious emissions.

In August last year, an 8-engined Boeing B-52H bomber became the US Air Force's first aircraft type to be approved to run on 50:50 synfuel/kerosene. This event was followed in December by a synfuel-powered flight made by a C-17 Globemaster military heavy-lifter. Flight testing taking place on a Rockwell B1 bomber is due to culminate in a supersonic trial. Engine maker Pratt and Whitney is this year evaluating synfuel on its engines used in the US Air Force's F15 and F22 fighters and the B1 bomber and expects formal certification this summer. Jeff Braun, director of the US Air Force's Alternative Fuels Certification Office says that, following certification of 50:50 blends for the entire US Air Force fleet by 2011, the next stage will involve increasing the proportion of synfuel in the mix to the point where engine performance starts to deteriorate, in order to establish where, if anywhere, that point is.

Paving the way for biofuels

Running the air force's entire fleet with a 50:50 blend will require 1.5 billion litres of synfuel per year by mid next decade. Initially, the US Air Force is using fuels synthesised from coal and natural gas, resources with which the USA is well endowed, and sourced from commercial suppliers able to field the necessary conversion processes.

Such use will, however, prepare the way for plant-based biofuels, as well as other biofuels derived from biomass sources including food and agricultural waste, lumber from timber and forestry, and animal fats etc.

The leading technology for converting a variety of carbon-based materials into liquid fuels is the Fischer-Tropsch (F-T) process. Invented in Germany in the 1920s, this versatile system first converts feedstock to a synthetic gaseous mix of mainly carbon, hydrogen and oxygen called syngas. This is then catalysed into liquid hydrocarbon, which is subsequently refined into a range of synfuels including jet fuel.

Significant drawbacks of the F-T system are its cost and carbon dioxide emissions. Gasifying the initial feedstock is energy intensive and therefore expensive. Converting coal or gas to liquid (CTL and GTL) hydrocarbons produces more CO2 even than emitted in refining crude oil (assuming that the CO2 is not sequestered and stored). While a military organisation might be prepared to accept the cost and ‘own goal’ in environmental terms for the sake of an assured fuel supply, this course is clearly not ideal.

Far better, it would seem, to use plant-derived biofuels along with wastes and other biomass, both as the feedstock and as the energy source for the process, thereby reducing CO2 emissions by 70 to 90%.

Yet, here too there are problems. The required technology is in its infancy so experience is limited. There are serious doubts about whether enough crops and biomass could be available to meet large-scale aviation biofuels demands. A particular objection, currently under the media spotlight, is that the large areas of crop monoculture that would be required might put pressure on the human food chain. Even if crops that have little to do with food, such as jatropha and wood, are used it can be argued that cultivation requires resources of land, water, fertiliser and manpower which could otherwise be devoted to food crop cultivation.

Aviation would probably also compete with land transport for biofuels. In fact substantial aviation demand could inflate prices for all three uses. (We already see rises in the prices of rapeseed and other oils, the price of palm oil in particular having doubled over the last three years).

The US Air Force has used first-generation F-T fuel from Sasol, a South African company that produces synfuel from coal (CTL), and US-based Syntroleum which utilises a gas-to-liquid (GTL) variant of the process. Shell employs GTL technology at a major biofuel pilot plant in Malaysia that has provided well over a million litres of synthetic jet fuel for the US Air Force since it started tests in mid 2007. These and other companies are working towards producing fuels that can be approved as drop-in replacements for standard Jet A fuel, the type predominantly used by the military, in 50:50 blends initially and ultimately as full 100% substitutes.

Oklahoma-based Syntroleum, who produced the fuel used in tests leading up to the B-52 certification, concedes that the F-T process used with coal is expensive. However, working with Tyson Foods as a partner, it hopes to use a cheaper and greener method to produce biofuel from animal fats. This avoids the first stages of the process used when coal is the feedstock. Syntroleum says it could have a 5000 barrel per day biofuel plant running within two years.

Experience gained with CTL, GTL and other first-generation synthetic fuels will prepare the way for the subsequent use of biomass-to-liquid (BTL) produced fuels, where the final product that goes into the jet engine will be very similar.

Crop-based BTL has superior environmental credentials because CO2 absorbed during the growing period offsets emissions of the gas when the fuel is burned. Aircraft can certainly run on fuel derived from oils extracted from plants such as palm, coconut, rape and soy. Light aircraft in Brazil have been running for years on ethanol, an alcohol-based fuel obtained primarily from fermentation of sugar and corn.

Another conversion process is the esterification of plant-derived oils to produce fatty acid methyl esters (FAME). These can power the several light aircraft types that now have diesel engines, and may also be used, mixed in modest proportions with kerosene, in jet engines. Downsides at present, for both alcohol fuels and FAME, are freezing points that are too high and reduced energy content compared with standard fuel. Nevertheless, engine maker Snecma has successfully tested a 30% FAME blend in a CFM56 jet engine, the biofuel in this case being made from rapeseed and sunflower oils.

Civil aircraft drivers

Civil air transport operators, like the military, are keen to find a viable substitute for conventional jet fuel, and sustainability might come higher up their wish lists. When Virgin Atlantic made history on 24 February this year by flying, without passengers, a Boeing 747 from London Heathrow to Amsterdam with one of its four GE CF6 engines running on biofuel, the airline declared that the biofuel used is a truly sustainable type that does not compete with food or for fresh water resources.

Virgin Atlantic teamed with Boeing and engine maker General Electric Aviation (GE) for the programme. The biofuel used was a mix of babassu and coconut oils produced by Imperium Renewables. Air New Zealand intends, also in collaboration with Boeing, to make a flight with a B747 in which at least one of the aircraft's Rolls-Royce jet engines will be part powered by biofuel derived from jatropha, a fast growing non-food plant.

Algae biofuel

Alternatives to food plant-derived biofuels are the focus of research taking place around the world. Some are produced from waste and other organic feedstocks. Others are based on oils generated by micro-organisms, in particular algae – the organisms responsible for the blooms that grow on ocean surfaces and the scum that floats on ponds. The National Renewable Energy Laboratory (NREL) in the United States estimates that high lipid algae could produce up to three times more oil than soybeans on an equivalent area and resource basis. Algae would be grown in giant ponds and a Boeing paper has suggested that, with 10,000 gallons harvested per acre each year, some 320 billion litres (85bn US gallons) of biofuel could be produced on a landmass equivalent in size to the small US state of Maryland (12,400 square miles).

Virgin Atlantic would like to be able to use algae-based biofuel and may be first to do so as it is planning a demonstration later this year. Other airlines are bound to follow, if only to reinforce their green credentials. Continental Airlines, for instance, has announced that it is planning a biofuel demonstration flight with a CFM56-powered Boeing 737 for next year. It says it is working with fuel suppliers to identify sustainable fuel sources that do not impact food crops and water resources or contribute to deforestation.

A number of fuel companies are actively following the micro-organism route. As examples, Aquaflow Bionomics in New Zealand is pursuing algal-based fuels while Gevo Inc in Pasadena has a licence from University College Los Angles (UCLA) to genetically modify Escheridia Coli bacteria in order to produce an efficient biofuel synthesiser. Gevo produces advanced higher alcohol-based biofuels including iso butanol.

European airframer Airbus narrowly beat Virgin Atlantic/Boeing to the post in terms of a first synthetically fuelled flight by a commercial airliner, but in this case its synfuel was processed from gas. On 1 February one of its A380 ‘super jumbos’ took off from Filton, UK, with 11 tonnes of Shell GTL synfuel aboard, and made a three-hour flight to Toulouse, France, with one of its jet engines burning a 40% GTL blend.

Working with a team that included Qatar Airways, Qatar Petroleum, Shell and engine maker Rolls-Royce, Airbus had been preparing for the biofuel demonstration since last September. After the flight, Rolls-Royce reported that all four Trent 900 engines, including the one running on GTL, were operated to full power on take-off with no restrictions. Airbus has since shared its A380 GTL findings with the rest of the aviation community.

Airbus aims to have a 50% blend of GTL fuel approved by next year, followed by 100% by 2013. Qatar, which has vast natural gas reserves, is building a major GTL plant and Qatar Airways plans revenue services with GTL from 2009. According to Sebastien Remy, head of the Airbus alternative fuels research programme, GTL fuel burns cleanly, is no worse than standard jet fuel for CO2 emissions, and has similar characteristics to future BTL fuel, making it a good precursor to genuine biofuel.

Remy expects that biofuels based on renewable non-food feedstocks such as cultivated algae will reach maturity by 2015 and that aircraft will be flying on advanced biofuels by 2020. He suggests that a quarter of all jet fuel could be ‘alternative’ by 2025. Airbus has been working with South Africa's Sasol on 100% synthetic fuel possibilities since 1999.

Meanwhile, flights powered 100% by biofuels have already occurred. US company Greenflight, formed three years ago to demonstrate the use of biofuel technology for aviation, has used fuel derived from recycled cooking oil to power a Vodochoky L-29 jet trainer. In three days of flight testing at Reno, Nevada, the trainer initially flew on a 25% blend of biodiesel with Jet A fuel, then transitioned to a 50:50 blend, and finally to 100% biofuel. Flying at 17,000 ft, where the temperature dropped to −4ºC, revealed no problems for the fuel, although it would have frozen to a gel had the temperature fallen to −10ºC. (Commercial jet fuel currently has to be able to operate down to −40ºC before it can be approved).

Nevada-based Biodiesel Solutions supplied the fuel for the tests. Greenflight has attracted support for its activities from the Institute of Air Science in Wacco, Texas, which conducts research into aviation fuel. Greenflight is expected to further highlight biofuel capability by flying its L-29 jet on a 10-leg crossing of the United States.


Overall, however, the outlook for aviation biofuels is uncertain. As we have noted, resources are being invested in technology and operational development and milestones are being reached. The next stage is to move towards widespread certification of viable 50% blends and then 100% substitutes, a process that the Commercial Aviation Alternative Fuels Initiative, a collaboration of interested parties, is supporting for the commercial sector. But controversy about the extent to which biofuels compete with food cultivation and the degree of carbon emissions reduction achieved by using them is clouding the political waters.

The International Monetary Fund, the charity Oxfam, and most recently the Gallagher report have suggested increasingly drastic figures for how much the food price inflation currently being experienced may be due to the present rush to biofuels. US food and agriculture officials, while conceding that biofuels are having some effect, argue that the true figure is lower. Oxfam holds that the subsidies provided by developed countries for biofuel crop cultivation look to poor countries like a tax on food that can tip their populations into starvation. In common with other campaigning organisations, Oxfam is pushing for an end to biofuel subsidies and further biofuel mandates.

The Committee on Environmental Audit of the UK House of Commons wants biofuel production to be limited to renewable feedstocks, having advocated in a report in January that UK and European support for the production of biofuels from conventional crops should cease because of detriment to food production. However, European Union energy commissioner Andris Piebalgs disagrees with this recommendation, saying that a range of biofuels will be needed if European targets for alternative fuels use are to be met.

On the emissions front, objectors to large-scale biofuel development argue that clearing new land for the extra production needed and the subsequent cultivation operations could release copious amounts of CO2 into the atmosphere. The situation is worse, they say, where virgin forest is cleared since the process is both energy intensive and destroys nature's means of sequestering CO2. To address the emissions concern the EU, which has a target of 10% of all transportation fuel to be based on renewable resources by 2020, says it does not count a source as sustainable unless it saves at least 35% in carbon emissions, cradle to grave, compared with conventional fuel usage.

According to officials, the EU currently uses just 2% of its cultivated grains for biofuels and hopes to accommodate production increases by bringing land previously set aside for non-cultivation back into crop growing use. In the UK, prime minister Gordon Brown wants to avoid supporting further increase in biofuel production greater than 5% unless the biofuels produced meet strict sustainability criteria. Whether any shift in biofuel production from the high-nutrient crops used today to less rich crops such as jatropha would count as sustainable is not clear at present.

Most of commercial aviation will probably do what is necessary to assuage public concerns about carbon footprint and the environment, initially by adopting blends of conventional jet fuel with first-generation biofuels where this can be done without detriment to engine performance, life and maintenance cost. It knows that it is safe to adopt solutions that are only partly ‘green’ because the public at large, although environmentally concerned, still has an insatiable appetite for flying. Consequently any transition to more sustainable second-generation biofuels is likely to be slow.

A degree of cynicism about recent events is epitomised by Jeff Gazzard of the Aviation Environment Federation who has referred to the Virgin Atlantic flight as “the airline spinning faster than its jet engines”, saying that the move brings us no nearer to knowing how 238 million tonnes of aviation fuel consumed right now can be made more sustainably.

Meanwhile, the military aviation community, especially in North America, will be less particular and will continue to build a fallback capability based on whatever CTL, GTL and BTL fuel alternatives are best suited to its immediate needs, against the possibility that a hostile producer nation could one day ‘turn off the taps’, critically exacerbating an already tight supply situation. For military organisations the priority is clear; above all, they have to achieve a secure fuel supply, and biofuels have a part to play.

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