Flying green: The use of biofuels for aviation

Commercial and private aviation is growing rapidly around the world, but the increasing cost of fuel, and pressure to reduce emissions is leading airlines to look at more environmentally friendly sources of energy. Jeffrey Decker looks into the use of biofuels for aviation.

According to Airports Council International, if current trends continue, airline tickets sales will double by 2025, reaching 9 billion annually. Today, fuel accounts for about 30% of the airline industry’s operating costs, compared with about 10% five years ago. The cost of jet fuel was up to over $2 per gallon (1 US gallon = 3.785 litres) last year from $0.75 a gallon in 2001, forcing the aviation world to look at energy efficiency and new sources of fuel seriously.

In 2005, US airlines spent more than $33 billion on fuel, but governments are taking a hit, too. The largest fuel customer in the world is the US military, at 8 billion gallons per year (30 billion litres). The US Air Force spent $5 billion on fuel in 2006, and is currently leading tests of synthetic jet fuel converted from coal to liquid using an 80 year-old process called Fisher-Tropsch. While this method of fuel production may end up being cheaper, the fuel is only marginally cleaner than conventional jet fuel (as long as the primary energy source is still coal). These tests are getting strong attention and this synthetic fuel is being seen as the reliable successor to petroleum, causing renewable fuels to be pushed further down the road.

Not too far, though, particularly in nations which have signed the Kyoto protocol and have emissions reduction deadlines to make. CO2 emissions from planes and jets could surpass automobile emissions within six years, and urgent action is needed. Currently, aviation contributes 2%-3% of all fossil fuel-based carbon dioxide emissions. To help tackle this, the European Union has proposed including aviation in its carbon trading plan by 2011.

The Vanguard Squadron in South Dakota runs on 100% ethanol jeff decker

Two years ago, Giovanni Bisignani the CEO of the International Air Transport Association told a Geneva summit, ‘If we could save a minute on every flight each year, we would save $3.6 billion on total operating costs, including $700 million on fuel, and 4.2 million tonnes of CO2 emissions, not to mention other pollutants and greenhouse gasses. In Europe alone, IATA estimates that delays are costing 15 million minutes of unnecessary flight a year, which translates into $1.5 billion in wasted operating cost and more than 1 million tons of CO2 waste.’

Efficiency is a virtue and clean energy a priority, but the technologies for clean flight are still emerging. Hydrogen cells with sufficient output are still too large to be practical. Solar doesn’t produce enough power per unit area. Biofuels (particularly biodiesel or synthetic kerosene derived from biomass sources) show promise, but first must pass the strict performance and safety tests.

Testing ethanol in light aircraft

On 18 December the Federal Aviation Administration in the US released the findings of its first extensive test of ethanol in an aviation engine. ‘The engine produced an average increase in peak horsepower of 4.3 (2.3%) when operating on the AGE-85 (aviation grade ethanol) than it did when operating on standard avgas. However, peak power required 56% more fuel mass flow with the AGE-85 than with avgas,’ reads the report by David Atwood and Anatoliy Ivanov. ‘Using AGE-85 would increase fuel weight by 9% above that of 100LL (low lead)for an equivalent volume of fuel and would reduce operating range by 35% from 100LL.’

Decreased range due to the need for more fuel may be acceptable in light aircraft considering the benefits of cleaner emissions and domestic production. The report from the Propulsion and Fuel Systems Branch of FAA’s William J. Hughes Technical Centre points out that general aviation is exempt from the1990 Clean Air Act Amendment in the US and is a leading source of airborne lead, something which a switch to bioethanol would help to alleviate.

While the spark ignition aircraft engine endurance test didn’t study emissions, other studies have found lower levels of carbon monoxide and volatile organic compounds than with avgas. Although ethanol releases the same levels of carbon dioxide than normal fuel, not all of this is fossil CO2, as some of it will have been absorbed from the atmosphere during the plants life (assuming the ethanol is made from a biological source). Ethanol does however have slightly higher levels of NOx than standard fuel, but lower levels of SOx.

Plant holding synthetic kerosene in Tulsa syntroleum

The biggest concern with using ethanol in aviation is its uncertain effect on fuel systems. The FAA team tested a Lycoming IO360-C, with adjusted ignition timing and increased fuel mass flow using AGE 85, ‘a blend of at least 85% ethanol denatured with 2% gasoline, less than 1% biodiesel, and the rest pentane isomerate.’

Through 150 hours they pushed the engine to maximum temperatures to test wear, performance, materials compatibility, range, efficiency, oil dilution and deposit formation. ‘The engine experienced normal wear except for the exhaust valve face and stems, which showed a hammered effect,’ the report states.

The FAA has also funded research at Baylor University in Texas, where, after 26 years of testing Dr Max Shauck proclaims ethanol as ‘the best fuel there is.’

Skip Byrnes, the FAA Fuels Program Manager is not so convinced: ‘Frankly, we’re not as far along as Dr Shauck is in proposing that ethanol be the complete replacement,’ said Byrnes. ‘We just have not tested enough here to be confident saying that yet.’

In 1989 Shauck, along with his wife Grazia Zanin, had the confidence to fly a single-propeller prototype Velocity across the Atlantic Ocean. That flight brought the Harmon trophy and worldwide acclaim to Shauck, who’d already been an advocate of the fuel for more than ten years. He’d promoted the fuel for crop dusters in Brazil, where one third of the transportation fuel is now ethanol, and where in March 2005 a subsidiary of Embraer got the world’s first certification for the production of a 100% ethanol-powered airplane – the Ipanema.

As director of the Institute for Air Science at Baylor University in Waco, Texas, Shauck earned Supplemental Type Certification (STC) for 100% ethanol to power a Cessna 152. In 2000 he obtained STC for a series of crop dusting Piper Pawnee to fly on both Avgas and ethanol. That was the first dual fuel certification granted by the FAA.

Yet even Shauck concedes that ethanol won’t work in jets because its energy density and specific energy are too low. Range and payload would be greatly limited, and, at just 12ºC, its flash point would present significant safety dangers.

Design considerations negate those drawbacks in a piston craft, Shauck says. ‘What I do in my air shows is fly an airplane that’s been modified to take advantage of the higher octane,’ he explained.

A bonus is the doubling of time between engine overhaul. ‘There is less vibration when you use ethanol as a fuel. That’s a function of the larger range of flammability. All of the fuel is consumed in the initial spark,’ he says.

Michael Wagner has tested 5% ethanol blends for Rotax as head of the engine-maker’s aircraft centre, and says he plans to test higher content mixes. ‘In the European community there is a three-year mandate to increase alcohol in fuels, and of course we have to follow that,’ he says, adding that ethanol is easy to work with and it’s good on the engines. ‘Combustion is stable. It burns nicely. It’s clean. Performance is good. If you tune it right, the fuel/air mixture, you will not easily kill an engine with it. For the engine, it’s much better than avgas.’

Consistency is key, he added. An engine could be tuned for 100% or 10%, ‘But for all, I’m not so sure. That would require much more effort.’

The presence of oxygen in the blend is easily dealt with, he says. ‘We have to adjust the air:fuel ration on the carburetor setting, and the injector setting has to be checked. The detonator should be no problem.’

The only trouble with the fuel, he says, is its corrosive effect on some plastic and rubber components between the fuel tank and the engine. ‘Everything that’s in contact with the fuel: injectors, fuel pump, o-rings, sealants, gaskets, everything. It has to be resistant,’ Wagner advised.

That corrosion is one reason there was so much opposition to a congressional proposal to require petroleum fuels in the US to have a 10% mix of renewable fuel by 2010. The ‘Ten by Ten Act’ was a proposed amendment to the Clean Air Act that ignited fierce opposition this summer across general aviation sector in the US. Opponents hoped aviation could be exempted before Senator James Inhofe, a Republican from Oklahoma and a pilot, announced in July he was killing the bill in his committee.

The government heard the concerns, says Jon Yarian, spokesperson for lead author, former Republican Congressman Gil Gutknecht of Minnesota. ‘There is going to be some disruption, but obviously we feel that the experts and scientists in the industry can mitigate that risk and make this workable for everybody, including the aviation industry.’

Ian Walsh, Vice President and General Manager of Lycoming says engine companies do need to adapt to alternative fuels, ‘Not in the short term, but more in the long term. Domestically, I think it’s too early. Overseas I think it’s going to be a real issue.’

The debate baffles Steve Thompson, who, as a pilot for the Vanguard Squadron, uses 100% ethanol in his aerobatic plane’s Lycoming IO-320 engine. ‘We’ve been operating since 1993 on 100% ethanol,’ he says, adding, ‘Just 10% ethanol, virtually any airplane engine will burn that. It’s more hype than anything that’s making all that noise.’

The downside, he says, ‘is you’ve got to convert your fuel injector and get it to flow more. Then there’s the matter of getting the fuel itself,’ he says. The squadron is sponsored by the South Dakota Corn Council, which supplies locally-produced ethanol.

The council, Shauck and other supporters are quick to point out the reduced emissions of ethanol that would help meet looming emission deadlines around the world.

There are other drawbacks, points out John Heimlich, Vice President and Chief Economist of the Air Transport Association. ‘It freezes in pipelines. It’s one thing to have fuel, it’s another to get it into the airplane.’

Ethanol is also getting the first investment of Virgin Fuels, a new company formed to explore alternative energies. A commitment last month from the Chairman of the Virgin Group means 10% of the conglomerate’s transportation profits will be spent on renewable energy technologies over the next ten years, a figure Chairman Sir Richard Branson expects to be $3 billion.

He made the announcement at the annual meeting of the Clinton Global Initiative in New York. ‘We must not be the generation responsible for irreversibly damaging the environment. We must hand it over to our children in as near pristine condition as we were lent it from our parents,’ he says.

The new Virgin Fuels, which announced its first move on 10 September 2006 with a $200 million investment in new California ethanol plants. CEO Shai Weiss is now on the board of Cilion, Inc., headquartered in Goshen, California. Cilion was formed in June and plans to convert locally-grown wheat into the unleaded petrol substitute. For now, the plant is producing ethanol for ground-based vehicles only.

Biodiesel for heavy aircraft

In a report presented at the 25th International Congress of the Aeronautical Sciences in September called ‘Alternative Fuels and their Potential Impact on Aviation,’ researchers from Boeing, NASA Glenn and MTU Aero Engines conclude that: ‘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.’

They write that the potential of the fuel is severely limited by the agricultural capacity of the world. ‘To replace only the diesel fuel demand of Germany (56.6 million tonnes in 2005) with biodiesel would require four times [Germany’s] land area and the replacement of every current crop with rapeseed (Europe’s favourite biodiesel feedstock).’ Commercial aviation consumes a lot of fuel – 13.6 billion US gallons of jet fuel in 2004 in the US alone. To offset that with ‘A 15% blend of bio-jet fuel would require 34 million acres (13.6 million hectares) of agricultural land, about the size of the state of Florida.’

Some crops yield 60 US gallons of biodiesel per acre (90 litres per hectare), but in general it takes ten times more crops to produce one gallon of biodiesel than it does to produce one gallon of ethanol. However the advantage of biodiesel is that it can be blended into both kerosene and avgas.

Rapeseed is the major crop currently used for biodiesel manufacture in Europe sxc photo library

Biodiesel is biodegradable, even with its poisonous catalyst methanol, and would ease spill concerns at airports. It does degrade rubber gaskets and hoses, but they can be replaced with resistant materials. Its carbon emissions are lower than in kerosene and diesel, but NOx emissions are still high. The fuel should be used within six months of manufacture and at no greater blend than 20% because it tends to break down.

In 2003 the Imperial College Centre for Energy Policy and Technology provided an exhaustive analysis of production costs in ‘The Potential for Renewable Energy Sources in Aviation.’ The authors concluded the cost of producing biodiesel would be between $33.50-$52.60/GJ. That figure includes distribution, necessary chemicals and the conversion process. Raising crops is 75% of the cost. The figure doesn’t include benefits from selling the waste flakes, which could have value to farmers and food producers. The 2003 rate for kerosene was $4.6/GJ (£2.9/GJ).

Costs may come down, said Dr Ausilio Bauen, one of the authors, especially as more attention and investment dollars go into renewable energy. ‘The study has received more attention this year than it did when it came out’, he said.

Without question the greatest problem with biodiesel is its need for warm temperatures, Bauen says, which don’t exist at high altitudes. In a blend, biodiesel raises the fuel’s cloud point – the temperature when micro crystals form. ‘Even just 10% by weight biodiesel blend raises the CP from -51°C to -29°C,’ the paper states. The United States Department of Agriculture is researching ‘winterizing’ biodiesel, but there’s been no breakthrough yet.

A whole range of biological feedstocks, from miscanthus to mixed prairie grass (pictured) could be used to make Fischer-Tropsch kerosene for aviation fuel cedar creek / university of minnesota

The Baylor University Institute for Air Science is hoping to secure a grant to take biodiesel and kerosene blends, or biojet, back into the air like they did in the late 90s. ‘We took an aircraft to 25,000 feet with one of the engines running with an 80/20 blend of biodiesel fuel and Jet A, and that appeared to have no problems,’ says Dr Shauck. They flew 60 hours with one Pratt and Whitney Pt6-20 powered by 100% kerosene and the other with the blend.

‘If we start those tests again we will definitely try 100%,’ Shauck adds.

‘We found that in blends up to 50%, the performance was the exact same as Jet A,’ he remembers. There was even an unexpected bonus. ‘The fuel bladders had become someone brittle over time, and there was a rejuvenating.’

He’s uncertain how to conquer the cold flow problems. One long-shot trick that’s been considered is heated fuel lines.  

With so much interest in alternative and renewable fuels, Baylor may soon find those grants. The Air Force is looking into a variety of feed stocks for fuel, including Biojet, said Mike Aimone, assistant deputy chief of staff for logistics, installations and mission support for the US Air Force. ‘It’s more than discussion. We actually have some research work with some universities on the topic, but they’re nowhere near ready to fly in a B-52. They’re many years off.’

Fischer-Tropsch biofuels

By 2016 the US Air Force wants half of its fuel to come from the Fischer-Tropsch process, which converts carbon dioxide and hydrogen into liquid hydrocarbons using a catalyst under moderate pressure and temperatures. The process produces lubricants, petrol, diesel, jet fuel and almost any other oil-based product. Although the attention has been almost completely on using fossil fuels as the energy source, it is possible to use any carbon-based feedstock, and using any crop plant could create a carbon-dioxide cancellation cycle.

The 2003 Imperial College Report examines miscanthus, reed canary grass and herbaceous perennials and states, ‘No particular environmental problems are envisaged with the production of biomass derived FT kerosene, unless ligno-cellulosic FT plantations replace ecologically important land use, for e.g. native forests.’

Operating the air-fed or the more expensive oxygen-fed facilities is inexpensive after initial capital investments, though costs are highly dependent on electricity prices. Considering transportation, harvesting and other expenses, the report concludes FT can cost as little as $5.8/GJ, compared with the 2003 price of $4.6/GJ.

Assuming one-third of the United Kingdom’s arable land was available to produce FT feedstock, The UK’s annual production would be 51 PJ (1 x 1015 Joules) and the world could produce 22,800 PJ.

FT synthetic fuel has fewer particulates and is virtually sulphur-free. That’s an emissions advantage, but it also contributes to poor lubricity.

F-T makes progress, but coal to oil is still coal
On 15 December 2006 a B-52 flew from Andrews Air Force base with synthetic fuel in all eight engines for the first time. The 6.1 hour test was the fourth flight, and again the blend of 50% jet fuel performed the same as a B-52 on 100% standard fuel. The same success was reached in cold weather in February at Minot Air Force Base in North Dakota. The US Air Force will release its final report in June 2007 but is already searching for a commercial provider of the fuel.

Soon they’ll test it in a fighter jet in afterburner stage. The military and airlines are interested in fuel from the Fischer-Tropsch process because it’s made from any fossil fuel or carbon-based feedstock and because its emissions have less particulate matter than traditional petroleum fuels.

They have ‘several thousand gallons’ left to work with supplied by Syntroleum, a Tulsa, Oklahoma-based company that announced on 19 December a deal with Kuwait Foreign Petroleum Exploration Company to develop a 50,000 barrel per day gas-to-liquids (GTL) facility in Papua New Guinea. That plant will make diesel, and Director of Investor Relations Gary Gamino said aviation fuel production might surge, but ‘unless a serious player calls us with some deep pockets who can fund the effort, we’re plenty busy with current projects.’

The Tulsa facility that made the Air Force’s fuel was mothballed in September. ‘There wasn’t any economic reason to keep it open. We can’t afford to keep it running just for fun,’ Gamino said.

The leading candidate to provide the Air Force’s needs is Sasol, the South African firm that has been using Fischer-Tropsch to make several fuels for 50 years. ‘All the international flights that refuel at the South African airports make use of a synthetic jet fuel,’ said US Coal-to-Liquids Country Manager Renus Kelfkens. Each day they produce 100,000 barrels of fuel out of a total of 160,000 barrels of synthetic chemical products.

Currently the synthetic mix is 70% jet fuel, and they export none of it. ‘We’re exploring the size of the market beyond the South African environment,’ he said. A Sasol facility is about to begin production at a facility in Qatar.

Meanwhile, a major deal is warming up between Rentech and Peabody Energy, the largest coal producer in the US, to convert coal to oil at Rentech’s Colorado plant.

Jeffrey Decker is a freelance journalist based in the US

Fuels for aviation

Civilian aircraft are powered by a small selection of widely-available petroleum fuels. Military aircraft have the same selection, but many nations use additives to craft specific blends.

A small portion of the propeller airplane market is powered with diesel. Cirrus and Cessna, both leaders in the propeller market, are taking the first steps to releasing new diesel planes.

The piston engines of propeller aircraft run on a fuel called Avgas. It’s a high-octane fuel with lower volatility than petrol, which helps the fuel system in the cold temperatures of high altitudes. Though a replacement fuel is not yet ready, Avgas is being phased out of aviation because of its lead toxicity. 100LL can contain up 0.56 grammes of lead per litre.

Jet fuel
Jet engines on airliners or business jets are fuelled by different unleaded blends of kerosene. Jet A-1 is the most common. Jets in the US use Jet A, which is similar but freezes at -40ºC, seven degrees higher than Jet A. Jet B is the other commonly-used jet fuel and is used in only extremely cold weather. Jet Fuel can power diesel engines, both in the air and on the ground. It burns cleaner than Avgas and engine manufacturers are already offering propeller models that run on Jet fuel as a step towards emissions goals.

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