A380 flying over the Pyramids

Biofuels lead drive for sustainable flight

The aircraft industry is making huge strides on its quest to become more sustainable with new aircraft, innovative engines and alternative fuels, but can flying ever be green?

Air travel is something that modern travellers have taken for granted, but in the age of global warming it is coming under increasing pressure to clean up its act. The industry has been vehement in its defence of its environmental credentials, but despite that assertion it is investing heavily in new technologies that will continue to reduce its carbon emissions.These improvements broadly fall within three areas – fuel, engines and aircraft. Each new generation of aircraft and engine offers improved fuel efficiency, fewer emissions and improved noise performance, but it is in the area of fuels, or more accurately biofuels, that the chief breakthroughs are expected.

In 2011 it is predicted that 2.8 billion passengers will take to the skies, generating revenue of $457bn. The growth has been startling. A decade ago 1.8 billion people flew and the industry was worth $239bn. Air travel brings prosperity – over 33 million jobs and $1.5tn of GDP are supported by the air travel industry. It allows people to experience the whole world, but that freedom comes at a huge cost to the environment.
The International Air Transport Association (IATA) is leading the way as the industry tries to shed its reputation as environmentally unfriendly.Back in 2007 it adopted a vision for carbon-neutral growth on the path to building an emissions-free commercial plane by 2057. It has set a target for average improvement in fuel efficiency of 1.5 per cent per year to the end of the decade with a cap on net aviation CO2 emissions from 2020.

Air transportation, both passenger and freight, contributes 2 per cent of CO2 emissions, that figure is predicted to rise to 3 per cent by the middle of the century. Total emissions for 2010 increased by 3.5 per cent to 649 million tonnes CO2, compared with 627 million tonnes in 2009. This growth needs to be viewed against an increase in capacity of over 5 per cent, reducing the per passenger emissions by 1.7 per cent.

And it is from that per passenger metric that the industry draws its chief solace. Over the past 40 years the industry has reduced its fuel use and CO2 emissions per passenger kilometre by 70 per cent.

Biofuels

The most obvious way to cut CO2 emissions is to reduce fuel consumption. This can be done through improvements in aircraft technology as well as streamlining air traffic management. But the industry is putting its focus on developing alternative fuels. 

“The economics of our business have always driven fuel efficiency, which is the key to reducing emissions,” Giovanni Bisignani, director general and CEO of IATAS, said at the organisation’s AGM in early June. “But we were defensive, with no consensus or strategy. Once again, we had to take the lead.

“At our 2007 AGM in Vancouver I shocked you with a vision for a carbon-free future. The whole value chain then united and committed to targets, including cutting emissions in half by 2050. And we became a role model for other industries to follow. Already through IATA we saved over 76 million tonnes of CO2.

“More importantly, we discovered sustainable biofuels, which could reduce our emissions up to 80 per cent. Now we need big oil to scale up. They are green in their advertising but not in their actions. They prefer to pocket $1tr in profits than invest in green initiatives. They need targets to provide aviation biofuel at competitive prices.”

Biofuel taking to the skies

Alternative fuels have been shown to work. The next phase focuses on speeding up the use of alternative fuels commercially. The main requirements for sustainable alternative jet fuels are that they can be mixed with conventional jet fuel and can use the same supply infrastructure. They also need to meet the same specifications as conventional jet fuel, in particular resist cold (-47°C) and hot temperatures in the engine.

The final biofuel product must be standard jet fuel (Jet A/A1), whatever the refining process or feedstock used. In some instances, integration of the feedstock production with the refining processes, such as by taking the CO2 from the refinery to promote rapid growth of feedstocks like algae, which consume CO2 rather than release it, can further reduce the carbon footprint.

Airbus began the biojet era when it flew an A380 in 2008 with one engine powered by Fischer Tropsch Gas to Liquid (GTL) fuel. The following year, Qatar Airways flew an Airbus A340-600 with all four engines on a 50 per cent GTL mix, the first commercial passenger flight with alternative fuels.

Boeing was also testing biofuels. First when Virgin Atlantic flew a Boeing 747-400 with one engine operating on a 20 per cent biofuel mix of babassu oil and coconut oil and then Air New Zealand flew with one engine on 50 per cent jatropha-derived biofuel blended with 50 per cent kerosene.

These tests demonstrated that the use of biofuels from these sources as ‘drop-in’ fuels is technically sound and that no adaptation of aircraft was required. The next step was a long-term trial that was begun by Lufthansa in April. The German flag carrier began a six-month trial with an Airbus A321 on scheduled commercial flights on the Hamburg-Frankfurt-Hamburg route. Pending certification, one of the aircraft’s engines will use a 50-50 mix of biofuel and traditional kerosene.

The primary purpose of the project is to conduct a long-term trial to study the effect of biofuel on engine maintenance and engine life. During the six-month trial, Lufthansa will save around 1,500t of CO2 emissions.

“Our project is designed to research the long-term alternatives to conventional jet fuel,” Professor Dr Johann-Dietrich Wörner, chairman of the executive board of the German Aerospace Centre (DLR), says. “The object is to gather data on pollutants from biofuel in comparison with conventional kerosene over a longer period. The measured pollution pattern related to diverse stresses in flight and the composition of the exhaust gases will allow us to draw conclusions about the compatibility of biofuel and about the maintenance needs of aircraft engines. Since, above all, we expect a significant reduction in soot particles.”

Sustainable aircraft fuels

Production of the bio-synthetic kerosene used by Lufthansa rests on the basis of pure biomass (Biomass to Liquid- BTL). The producer is Neste Oil, a fuel-refining and marketing company from Finland. The company has years of experience in biofuel production and has cooperated with Lufthansa for many years.

The next step is to get the same authorisation for biofuel synthesised by hydrogenation of bio oils and fats and for incremental increases in the proportion of alternative fuel that can be blended for flight.

The biggest challenge today that remains is the availability of sustainable feedstock. Any solution has to work on current and future aircraft without the need for any modifications, so that it can be mixed with existing standard jet-fuel.

Fuel made from biomass is a good option from a CO2 emissions perspective, because when it is consumed, it releases the CO2 it absorbed when growing, thereby helping neutralise overall emissions. Promising options include algae, woodchip waste, camelina and halophytes such as salicornia (plants growing in salt water). In the longer-term, waste produce and yeast are some options being looked at.

At the recent Paris Airshow, Airbus stated that if there is adequate feedstock to produce commercial quantities of alternative fuel, and with support from government, it believes that by 2030, up to a third of aviation fuel could be alternative to standard jet fuel.

Engine improvements

When it comes to the engines there are two main directions that need to improve to increase efficiency. One is thermal efficiency, which is turning the energy in the fuel to shaft power. That would include a combination of factors including new materials, improved design and better arrangements. “Frankly, gas turbines have always been the most ferocious consumers of advanced materials technology, and I expect that to continue,” Alan Epstein, vice-president of technology and environment at Pratt & Whitney, says. “We have hundreds and hundreds of materials engineers here, and if we didn’t believe we got major value from them they wouldn’t be here.”

The other area is converting that shaft power into pushing the aeroplane; propulsive efficiency. “We have actually figured out how to do that with something called a gear turbo fan,” Epstein explains. “We make a big deal about the gear and the fan, but we don’t really talk too much about the thermal efficiency side of things, but that ‘stuff under the hood’, as it were, is just as important.”

In most jet engines the fan that pulls air into the engine is directly linked to the compressor that squeezes that air down for combustion, so they have to turn at the same rate. By placing gears between the compressor and the fan, decoupling the two, the geared turbo fan engine allows for a more efficient arrangement: a big, slow fan shoving air into a small, fast turbine. The result is a shorter, lighter engine that can produce the same amount of power as a larger conventional turbofan, while burning 12-15 per cent less fuel and emitting 35 per cent less carbon dioxide. Pratt & Whitney finished ground- and air-testing the engine this year, and the first of them will go on the Bombardier C-Series jet starting in 2013.

But beyond that Epstein is predicting that the diameter of the engines will grow. “We are now at 12 bypass ratio, but I can see 15-18 within a decade and then up to 25 looking forward,” he says. “At 25 you cannot fit the engine underneath the wing; so you cannot have a twin engine airplane with 25 bypass ratio engines beneath the wing because the landing gear would be too high.”

Aircraft

The environment is part of top-level requirements for the design of any new aircraft. One major strand of the research and technology efforts is therefore to investigate, test, validate and optimise the most advanced technologies, design features, configurations and architectures. This will lead to aircraft generating fewer emissions and less noise, while carrying a maximum payload over the mission range.

In addition to optimising propulsion systems and overall aerodynamic efficiency, the continuous and progressive introduction of advanced materials and new manufacturing processes also reduces the weight of an aircraft and, therefore, its fuel consumption and corresponding engine emissions.

The A380 is the first commercial aircraft to incorporate as much as 25 per cent composites. The carbon-fibre reinforced plastic composite centre wing box has saved up to 1.5t. As a result, with less than 3l per passenger per 100km, the A380 has very low fuel burn.

Reducing noise is equally important. The industry is working on nacelle designs, acoustic treatments and engine technologies aiming to reduce noise, hand-in-hand with engine manufacturers. One such innovation is Airbus’s zero-splice inlet technology for nacelles, to reduce fan noise.

The future

Historically, the aviation industry has been growing by around 5 per cent a year but efficiencies mean that emissions have grown by less than this, around 3 per cent per year.

However, the industry consensus is that a growing carbon footprint is unacceptable for any industry. But with fleet renewal to more efficient aircraft and the growing adoption of biofuels, the industry is convinced that it is on the path to a sustainable future. *

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