Biofuels and coal-to-liquids are poised to succeed kerosene in jets, but that does not mean they will not adversely impact the environment

All the alternative fuels in line to power jets have their downside, but a 21 January report from the UK House of Commons ripped open debate on whether biofuels are sustainable.

Biofuels in jets could have Aviation Fuels Committee approval by 2013, making it the runner up as a viable alternative to petroleum-based kerosene, with both fuels depending on a clean supply chain to cut overall pollution.

Considering the "life cycle" of fuels is necessary to judge their total environmental impact. The 21 January report from the Committee on Environmental Audit of the UK House of Commons does, and pulls no punches in its damning critique of plant-based fuels.

"The government and EU's neglect of biomass and other more effective policies to reduce emissions in favour of biofuels is misguided," the report says. "In general biofuels produced from conventional crops should no longer receive support from the government." It concludes that previous estimates overlook emissions like those of agricultural machinery and nitrous oxide emissions from fertiliser.

European Union Energy commissioner Andris Piebalgs strongly disagrees with the report, and the UK government last week was handed down an EU target to meet 15% of its energy needs from renewable sources by 2020 - with strong dependence on biofuels.

Preserving the environment is one aim of a biofuels project by Virgin Atlantic, which is working with Boeing and General Electric to fly a 747-400 from London Heathrow to Amsterdam in February. The date of that flight is not yet announced, nor are specifics on the biofuel that will power it.

Supply chain

That fuel, as well as biofuels developed by the University of North Dakota, Honeywell's UOP and others, all meet the rigorous demands of jet fuel, including density, freeze point and flash point. The challenge now is to establish a supply chain that produces fewer greenhouse-causing gasses and fewer toxins than petroleum, and not overstressing farmland.

relative co2 emissions compared to jet fuel

Finding the right source plant is key. Eyes are on the nut of Brazilian palm trees called babassu, canola oil and jatropha. According to the US Department of Energy's Renewable Energy Laboratory, algae could produce 150-300 times more oil than a crop of soybeans. They calculate 320 billion litres (85 billion USgal) of jetfuel could be produced on a landmass equivalent to the size of the small US state of Maryland.

As for emissions, biojet could yield 60-80% less CO2 . Emissions testing is ongoing for the new fuels, and collecting available data on several alternative fuels is the focus of Project 17 of the Partnership for AiR Transportation Noise and Emissions Reduction (PARTNER).

"There are enough studies to point us in certain directions, but I don't think there are sufficient measurements and knowledge to be sure of everything," says James Hileman, Massachusetts Institute of Technology research engineer. "Some things that we don't know well are things like the amount of soot or primary particulate matter that's created from combustion of some of these alternative fuels," he adds.

"That's true of the base fuel itself, Jet A. We need to better characterise the production of pollutants from Jet A and how those pollutants react in the environment," Hileman says. That's the focus of PARTNER Project 20.

The first successor to petroleum is Fischer-Tropsch fuels. A 50/50 blend with kerosene has been in use for nine years at Johannesburg airport and Sasol of South Africa is on the verge of Aviation Fuels Committee approval for a 100% synthetic fuel from its coal-to-liquids facilities.

"All qualification testing for a fully synthetic product was completed successfully," reports Johan Botha, general manager of product applications for Sasol of South Africa. "Paperwork by specification authorities is now in the process of completion with finalisation expected in the first quarter of 2008," he says.

FT plants are being built across the globe to make fuels for land and air. They're reliable but not especially green. Vivian Stockman of the Ohio Valley Environmental Coalition wishes coal-to-liquids was not on the fast track to widespread use. She points out that the National Coal Association predicts mining activity will double. "The coal field communities, we believe, cannot bear a doubling of mining where people already suffer from poison water, poison air and annihilated ecosystems," she says, "All this is coming from mountaintop removal, coal mining and the underground injection of coal slurry."

Environmentalists also worry about drawdown from the five barrels of water needed to produce a barrel of fuel.

Coal and natural gas are the primary, but not the only, feedstocks at the start of the FT process. Waste from agricultural and lumber operations are cheaper and the plants just finished absorbing CO2 .

"The technological challenge that people are going through now is to get the same BTU output from organic agricultural matter, whether it's banana peels or corn stalks or switch grass, algae - whatever," says William Anderson, US Air Force assistant secretary. "We believe that a middle term end state in the US will have feed stock of coal with a biomass element to it."

That mix is essential to a FT life-cycle with less CO2 released than with petroleum. When heat and catalytic reactions convert feedstocks to syngas, and the FT process turns syngas into liquid fuel, a potent stream of CO is discharged. Carbon capture and storage (CCS) into geological formations underground can bring the CO down to levels on par with petroleum. CCS is new, but in 2005 the Intergovernmental Panel on Climate Change concluded that after 100 years, 99% of that CO has a 90% chance of staying underground. After 20,000 years the chances for properly-performed CCS to hold that CO is about 66%.

Underground storage

Storing the CO2 released from an organic feedstock mix brings the CO2 output to practically zero. Without underground storage, the CO2 vented from the FT process is almost twice that of petroleum jet fuel processing and use. When the synthetic itself is burned in a jet engine, CO2 emissions are only about 2% less than when kerosene is burned, but oxides of nitrogen and sulphur are virtually zero and particulates emitted have been recorded at 90% less than kerosene emissions.

Emissions data on synthetic fuels is still uncertain. The USAF and others are collecting test data, but over nine years of use Sasol has not measured emissions. Other fuels have roles in the future, but while alcohol-based methanol and ethanol have uses in General Aviation, their mass and volumetric heats of combustion do not meet the needs of jet aircraft.

Liquid forms of methane and hydrogen have potential in jets, but the extremely cold liquids will need new engine and aircraft designs, as well as safe production, handling, storage and distribution.

Methane, the main component of natural gas, burns cleanly and is being researched by NASA as a potential rocket fuel. Huge pockets lie under the ocean floor and permafrost and could be tapped, especially if global warming threatens to release them as potent greenhouse gasses.

Nuclear option

Hydrogen yields ample power, but of all the fuels being considered, it requires the most volume and will only be used in unmanned air vehicles for years to come. Its emissions are almost exclusively water vapour, which is clean but does contribute to greenhouse-causing cloud formations. Splitting the hydrogen from water molecules is energy intensive and creates emissions of its own. If suitable permanent storage is available for radioactive waste, hydrogen isolated by nuclear energy will be among the cleanest aviation fuels.

Even compressed air is being looked at. Hunt Aviation of Nevada is proposing helium gas to lift its Gravityplane to cruising altitude where wings would fold for gliding. Turbines on the "fuel-less" aircraft would store wind energy for steering and VTOL landings, and the craft would also pull energy from the heat difference of high and low altitudes. Whether it is viable or can find investors remains to be seen.

Mass of fuel vs volume per unit energy

Solar cells

Plenty of support has the Solar Impulse under construction in Switzerland before its non-stop flight around the world. Future leaps in solar cell technology may have practical applications for light aircraft.

At Airventure in Oshkosh last summer, LSA manufacturer Sonex Aircraft revealed plans to manufacture a battery-powered two-seater that flies for an hour. At the same show Electric Aircraft took orders for its ElectraFlyer trike, which powers an 18hp (8kW) motor with two 18kg (40lb) lithium polymer batteries (which combined cost $7,500.) "It's almost eerily quiet," says company owner Randall Fishman. It is green, too, with complete system costs starting at $16,885 and flights lasting an hour and a half.

Source: Flight International