With the world's population growing and becoming increasingly wealthy, reliance on air travel has become of a fixture of life for many. But as passenger counts grow, airlines are needing ever-more fuel to keep their fleets in the sky.

Germany's Lufthansa group alone burns through 11 million liters of fuel each year - the equivalent of 1,000 full tanker trucks per day.

Given those fuel amounts, it's no surprise that the air-travel industry suffers from a negative environmental image. Still, according to the International Air Transport Association (IATA), the industry is only responsible for 2 percent of humans' global greenhouse gas emissions.

With oil prices rising and European emissions trading slated to begin in 2012, airlines will be faced with new expenses. Not only will they have to pay for the fuel the industry consumes, they'll also need to acquire certificates for each ton of CO2 emitted into the atmosphere.

IATA estimates 16 billion passengers will fly each year by 2050

Fuel costs account for 30 percent of airlines' expenses - part of the reason why the industry is searching for an alternative source of fuel.

Biofuel for jet engines

This month Lufthansa is starting a half-year of test of biofuel on domestic flights.

While biofuel has been tested successfully with jet engines before, the technology will now be put into practice. Individual test flights have taken place since 2008.

The modified Airbus A321 will fly between Hamburg and Frankfurt four times a day for half a year.

One of the plane's two engines will be fueled with kerosene, while the other will be fueled with a mixture of kerosene and biofuel.

Lufthansa plans to measure the CO2 emissions generated by the flights and examine the new fuel's effect on maintenance and engine lifespan.

Manfred Aigner, director of the Institute for Combustion Technology at the German Aerospace Center, says he's optimistic about the tests. Members of his institute will be studying the jet's engines during flight and at night when the plane is idled.

"Lufthansa consulted us when choosing a biofuel and mixing it," Aigner told Deutsche Welle. "In a soundproofed hall we run the engines as if the plane were flying. Then at regular intervals we measure the ways in which the engines change during use."

Aigner says the research has convinced him of one thing: Biofuel is more efficient than kerosene and emits less greenhouse gas when burned. Depending on how the biomass used to make the fuel is produced, results could range between 50 and 80 percent less CO2 emissions, Aigner estimates.

Scientists will study the Lufthansa plane's engines throughout the test

Lufthansa hopes it will save about 1,500 tons of CO2 emissions during the six month test.

Food or fuel?

Palm oil, rapeseed and animal fats form the basis of the fuel used in Lufthansa's biofuel test.

The airline demanded from its suppliers that its biofuels carry certification that their production neither damages rainforests nor moors, nor impacts food production.

According to Aigner, that sustainability requirement made it difficult to secure enough biofuel in advance of the test.

He added that biofuel is still difficult to acquire as a commodity. Airlines seeking to use the fuel are sure to find themselves competing with demand from automobiles and electricity providers.

"The question becomes: 'Who gets the biofuel first, and who gets what percentage of it?'" Aigner said.

While electricity providers can turn to the wind or the sun to gain energy, and automobiles can be propelled using hydrogen, airlines have limited other sources of fuel.

Kerosene for jet engines can only be replaced by liquefied coal, liquefied gas or biofuel.

Electricity isn't an option for air travel, as batteries are too weak and heavy to supply the energy needed for flight.

However, power generated by wind turbines could be used to produce methane gas, which could be liquefied and used for flight. The method would require careful attention to make sure little efficiency is lost in production, according to Aigner.

Environmentalists alarmed

While biofuels undoubtedly burn more cleanly than fossil fuels, not everyone agrees they're good for the environment or humans. Environmental organizations say they're not sold on the sustainability of even certified biofuels.

Massive Jatropha cultivation may cause problems for humans and nature

Reinhard Behrend, of Rainforest Rescue in Hamburg, says that with a global population of nearly seven billion humans, spare land for agricultural energy production simply isn't available.

"Production of enormous amounts of agricultural energy will be at the expense of humans - primarily in the tropics - and the natural environment," he told Deutsche Welle.

And Behrend is also critical of Lufthansa's plan to increase use of the oil-rich Jatropha plant for its biofuel.

While the airline contends that Jatropha can be planted in soil otherwise unfit for agriculture, Behrend says that's not the point.

"All investigations - by the German development service GIZ, for instance - have shown that if soil is poor, the plant doesn't produce enough oil to be economical," he said.

According to Greenpeace, Jatropha production would eventually suppress food supplies.

Author: Insa Wrede (gps)

Editor: Nathan Witkop