Airbus and Shell recently made the first ever commercial flight using liquid fuel processed from gas when an A380 airliner flew from Filton in the UK to the airframer’s Toulouse, France headquarters. The flight marked the start of a program to evaluate the environmental impact of alternative fuels in the airline market.
One of the A380’s four Rolls-Royce Trent 900 engines was powered by a blend of Shell’s gas-to-liquid (GTL) fuel and standard jet-A. The other three engines burned jet-A. The aircraft’s segregated fuel tanks made it well suited for engine shutdown and relight tests.
For Airbus, the three-hour flight was the first step in its efforts to evaluate viable and sustainable alternative fuels. It believes that GTL fuel–which promises less impact on air quality and more efficient fuel burn–could be available at certain locations to make it a practical alternative fuel for commercial aviation in the short term.
The manufacturer believes that development of GTL will support future second generation bio-fuels, which are not presently available in sufficient commercial quantities. Airbus has committed itself to studying viable second generation bio-fuels when they become available.
With global prices of petroleum on the rise, scientists are striving to bring second-generation biofuels from the research stage to full production. First-generation biofuels–which airlines together with aircraft and engine manufacturers are close to flight-testing–cut overall carbon dioxide emissions because their feedstock plants absorb CO2, but they still have major environmental drawbacks. First-generation fuels can be seen as wasteful in that they use only part of the plant (cereal grains or beetroot, for example).
By contrast, second-generation biofuels are created using plants in their entirety, including straw, wood and so on. Also, the range of raw materials available to create second-generation fuels is larger and more types of plants can be used, as well as waste such as wood chips.
The greater efficiency of second-generation biofuels is seen as an answer to at least one major concern. Green groups have been challenging the use of huge land surfaces to produce fuel rather than food, which has already raised the price of some basic food, such as corn.
Using the entire plant is viewed as much better from the perspective of those who believe the primary role of agriculture as a food source for humans shouldn’t be compromised. Using vegetable waste is one way to solve the problem, but collecting waste–such as wood chips and straw– can be challenging. Forestry and farm waste is highly scattered, and building big plants to transform large quantities of waste would involve a lot of transportation to collect and consolidate it. Another alternative would have the biofuels process relying instead on small, local units.
The first generation of biofuels also has raised concerns about biodiversity. “Is it reasonable to replace rain forest with sugar cane or cereal fields?” a senior executive at a major U.S. airline recently asked rhetorically. Environmental experts see retaining biodiversity–the living fabric that covers the earth–as urgent an issue as global warming. Second-generation biofuels, if made from waste, do not affect biodiversity, but the problem remains if they are made from purpose-grown plants.
Three kinds of processes are under development. The first–biochemical–yields sugar and then ethanol but it is not well suited to aviation, according to Xavier Montagne, deputy scientific director of IFP, a French research institute on oil and energy. “Ethanol contains 33 percent less energy than today’s jet fuel,” Montagne, explained to AIN. Energy density is critical in aviation, where weight and volume are enemies of efficiency. In addition, ethanol’s flash point is too low. While jet fuel has a specification for 100 degrees F, the flash point for ethanol is just 59 to 64 degrees F.
The second process, called biomass to liquid (BtL), is thermochemical. Biomass is first transformed into a synthetic gas, then the Fischer-Tropsch (F-T) process converts the gas into liquid hydrocarbons. The final product can be used to make a fuel that is very close to (and, in some respects, even better than) current jet fuel.
“The first part of the process is the most challenging,” Montagne said. The second part, the F-T process, is well known now. However, further performance improvement is needed in F-T facilities, Montagne added.
In terms of greenhouse emissions, second-generation biofuels would perform appreciably better than those further along in development. First-gen biofuels can save up to 70 percent of CO2 emissions over oil-based fuels (measured through the so-called well-to-wheel approach), but savings of 90 percent can be achieved with BtL, according to Montagne.
Another second-gen biofuel is hydrotreated vegetable oil (HVO). For example, vegetable oil made from lipid algae, if hydrotreated, yields a fuel that is close to that obtained with BtL. Their main characteristics, including flash point, are consistent with aviation requirements. Their energy content is a bit lower than today’s jet-A fuel if measured per volume, but it is greater if measured per weight.
Last June, engine maker CFM International said it was evaluating alternative fuels made using biomass. But some still speculate whether the benefits of the second-generation biofuels are enough. “Should we spend millions on biofuel research and production, while IATA has assigned the industry the challenge to be carbon-free in 2050?” asked the same skeptical U.S. airline executive.