As concerns about global warming intensify throughout the world, aviation is receiving a disproportionate level of scrutiny for its contribution to total global production of greenhouse gases.
Even though aviation emits only about one-ninth as much carbon dioxide (CO2) as do motor vehicles, its high-visibility nature as an activity, its rapid growth as an industry and the fact that aviation emits most of its CO2 and particulate emissions in the upper atmosphere has made it a particular target for environmentalists.
Elizabeth Barratt-Brown, a senior attorney with the National Resources Defense Council, told last week’s Eco-Aviation Conference in Washington, D.C. that in the United States, unless the industry achieves enormous efficiency increases, “by 2050 aviation emissions are expected to almost equal the amount from automobiles” because of aviation’s growth. The event, sponsored by Air Transport World magazine and Leeham Company, was the first aviation environmental forum to be held in the United States.
Luckily for Earth, perhaps, the soaring price of oil has made the search for sustainable, CO2-neutral alternatives an immediate economic imperative as well as an environmentally critical focus for many human commercial activities – with aviation foremost among them. Economic experts are now viewing high oil prices as a long-term fact of life rather than a short-term blip, and say aviation in its present form simply can’t live with the possibility of the price of a barrel of oil leveling at $200.
Research into fossil-fuel alternatives is snowballing. Eventually, a clean fuel such as hydrogen may be the answer for aviation – but the technologies that will allow it to be used safely and economically to power large aircraft are generally regarded as being 40 or more years away.
For aviation, it increasingly appears that biofuels – jet fuels made from plants or algae using any one of a variety of processes – represent by far the best medium-to-long-term hope for the economic and environmental survival of the industry. One of the main advantages of biofuels is that the plants used to make the fuels need lots of CO2 to grow, potentially making it possible for the aviation industry to achieve true carbon-neutrality.
“Boeing Commercial Airplanes and its partners are actively accelerating development of second-generation biofuels because they present an economically viable opportunity to sustainably power the world’s commercial aircraft fleet,” said Boeing in a recent briefing document entitled ‘What is the future of jet fuel?’
Aviation’s ‘proven track record’
Aviation’s “proven track record” in reducing its “carbon footprint” on a per-passenger basis already is excellent, with a 70 percent improvement in fuel-efficiency and CO2 emissions per passenger mile in the last 50 years, said Rolls-Royce senior environmental analyst Nuno Taborda.
“Aviation spends relatively more than any other industry on CO2 reduction,” he said. Others noted that during the last 30 years, the U.S. automobile industry did not improve the fuel-efficiency and CO2 emissions of its products at all.
But civil aviation is only just starting. “The IATA (International Air Transport Association) goal is for a 25 percent emissions reduction per passenger by 2020,” from an average of 4 kilograms of CO2 per 100 passenger kilometers to 3 kilograms, said Billy Glover, Boeing Commercial Airplanes’ managing director of environmental strategy. In the U.S., “the Air Transport Association goal is for 30 percent by 2025.” These goals do not include any positive effects from using sustainable biofuels which might be available by then, Glover added.
Various partnerships have been established to foster the development of alternative fuels and other ways to improve aviation’s environmental efficiency. It is one area on which Airbus and Boeing cooperate willingly. One leading forum is the Commercial Aviation Alternative Fuels Initiative (CAAFI), which includes partners from the aviation industry, fuel suppliers, universities, and various U.S. government agencies.
CAAFI has established a fuel-certification roadmap that envisages achieving certification of jet fuels made entirely from biomass-derived pure hydrogenated oils in 2013. CAAFI also has set several intermediate targets, beginning this year with the planned certification of a fuel made from a 50 percent blend of biomass-derived syngas and conventional jet fuel. (Syngas is a mixture of carbon monoxide and hydrogen and is created from feedstock by the Fischer-Tropsch process, which was discovered in 1923. Syngas can be processed into jet fuels.)
Finding the right biofuel feedstock
Key to the entire aviation biofuel issue is just what type of biomass is most suitable for fuel production. Several vital issues must be taken into account. First is the density and energy content of the fuel: It must take up a sufficiently small space that it can be carried in an aircraft and, similarly, a given volume of the fuel must produce enough energy so that an aircraft can carry enough in its tanks to complete its flight.
Second is the “carbon lifecycle” of the biofuel: that is, the net amount of CO2 produced during production and burning of the fuel, less the amount the biomass feedstock for the fuel absorbs while growing.
Third is the amount of sulfur and other particulates produced. Fourth is the hugely sensitive political issue of making sure the land and biomass used to make biofuel does not reduce the amount of food available to humanity and the Earth’s fauna.
These considerations immediately rule out “first-generation biofuels” such as ethanol produced from corn and soybeans. Not only does ethanol not contain enough energy per unit volume to be suitable as an aviation fuel, but growing enough corn or soybeans to power all the world’s airliners would require an area just about the size of the United States, according to Boeing. Nor does ethanol have suitable boiling and freezing points for aviation use.
Experts believe “second-generation biofuels” derived from the wood and nuts of plants such as Jatropha curcas (Barbados Nut) and babassu, which grow strongly in arid areas unsuitable as arable land and which (in jatropha’s case) are poisonous anyway, represent a good interim solution.
These Latin American plants, as well as other flora such as switchgrass and salt-water-tolerant plants known as halocytes (among them marsh grasses found in parts of the Middle East), could be grown for fuel production in non-arable areas suited to their particular growth requirements. Different parts of the world would grow different biofuel-producing plants, depending on their local climatic and soil conditions.
However, there is a problem: Although their oils offer much higher energy content and much better boiling/freezing-temperature characteristics than ethanol, these plants wouldn’t yield enough oil per hectare to be able to serve the aviation industry’s fuel requirements unless, again, very large areas were given over to their cultivation.
Algae a likely long-term answer
There is broad consensus throughout the industry that, longer-term, algae represent the optimum solution to aviation’s fuel needs. A number of basic problems need to be solved, such as ensuring enough light gets to every part of an algae tank to enable all the cells to grow properly; and drying algae cells sufficiently to enable the oil they contain to be extracted and cracked into jet fuel.
But Boeing and Airbus are confident these problems can be solved – and the benefits that algae offers as a “third-generation biofuel” are immense. Algae can produce an oil yield 15 times that of second-generation biofuel plants: The world’s entire airliner fleet could be powered from a cultivated area just the size of West Virginia, or Belgium, says Boeing.
Additionally, because algae can be grown in tanks anywhere, biofuel-producing algae farms could be sited next to facilities producing jet fuel from coal or natural gas using the Fischer-Tropsch process. These “coal-to-liquid” or “gas-to-liquid” processes generate large amounts of CO2 from fossil fuels, making them unsuitable as sustainable fuel sources. However, if the CO2 they generate is piped off and used to grow algae in nearby farms, the two forms of fuel production together could create an efficient, carbon-neutral symbiosis for jet fuel production.