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AMC Chief Scientist: Alternative fuels, new source of 'cleaner jet fuel'

  • Published
  • By 1st Lt. Neil Samson
  • Air Mobility Command Public Affairs
SCOTT AIR FORCE BASE, Ill. --  Just as the character "Doc" in the "Back to the Future" movie loaded the flux-capacitor of the time-travel modified 1975 DeLorean with a banana peel and trash from Marty McFly's garbage can, Air Force scientists are looking for the next readily available, renewable clean-burning aircraft fuel.

Air Force and AMC scientists are currently making strides on alternative fuels certification for different aircraft fuel blends.

"Alternative fuel blends are derived from a biological source and not derived from petroleum or any foreign oil source," said Dr. Donald Erbschloe, AMC's Chief Scientist.

The first type of alternative fuel tested by the Air Force, used a Fischer Tropsch chemical process, resulting in a synthetic liquid fuel that is derived from coal or natural gas feedstocks.

According to the U.S. Energy Information Administration, the U.S. is the world's second largest coal producer with more than one billion tons of coal produced each year. If some of this capacity were to be used for FT fuel production, it could reduce the country's dependency on foreign sources of fuel.

"There wouldn't be a lack in the supply of coal because the United States is the Saudi Arabia of coal," said Erbschloe.

Erbschloe said a gas mixture known as "syngas" starts the FT process, combining carbon monoxide and hydrogen. These building blocks undergo a multi-stage chemical reaction, the end product of which is synthetic kerosene. This end-product differs from petroleum-based JP-8 (standard jet fuel) because it does not generate ample amounts of aromatics, a particular subset of hydrocarbons that are composed of ring-like structures, such as benzene.

"We need aromatics in our fuel so our engines won't leak," Erbschloe said. The presence of aromatics promotes swelling and elasticity in rubber and other seal materials and these effects are exploited in aircraft engines and fuel tanks. Since FT fuels are aromatic-poor, a 50/50 blend of JP-8 and FT fuels is employed to provide enough aromatic content to ensure proper seal performance.

Before alternative fuels can be used in everyday operations, they need to be certified for use on aircraft.

"The Air Force has developed a rigorous, risk-based approach to aircraft certification of alternative fuels," said Erbschloe. Certification is based on stringent testing in the laboratory, on the ground and through flight testing.

"The tremendous strides being made on the energy front are demonstrated through the certification of these artificial FT fuels," said Erbschloe.

As a result of this certification process, the JP-8/FT blend is now approved in the JP-8 specification and considered interchangeable with petroleum-based JP-8, said Ms. Elizabeth Christensen, an engineer in Air Force Materiel Command Alternative Fuels Certification Office.

"Developing the certification process for the 50/50 blend of JP-8 and FT gave us the building blocks for additional alternative fuel certification programs, most notably Hydroprocessed Renewable Jet (HRJ) fuel," said Christensen. HRJ is a class of biomass-based fuels that could lessen the environmental impact of Air Force operations, she said.

"The Air Force Research Laboratory have discovered some mixtures of organic molecules from biomass fuel blends appearing different from petroleum-based, so it is a question of whether it is functional or not," said Erbschloe. "Petroleum-based fuels tend to produce contaminates and smoke, but FT fuels tend to burn cleaner, thus smoke-lined trails behind aircraft may be things of the past with these new alternative fuel blends."

"Even though the final fuel burns cleaner, the upfront process of creating FT fuel is more carbon-dioxide intensive," adds Erbschloe. "Carbon dioxide is a greenhouse gas, and you produce about twice the amount of carbon dioxide as you would if you produce FT, and great care has to be exercised in the FT process and the impact of refining these FT fuels."

One HRJ blend is derived from the camelina plant, a non-food rotational crop. More than a year ago, an A-10 Thunderbolt II flew on a mixture of camelina biomass-derived fuel and conventional JP-8 fuel - the world's first fully biofuel-powered flight.

Another type of alternative fuel, a tallow-based biomass fuel blend, is derived from animal fats.

"The C-17 was flight tested more than a year ago on a tallow-based blend, and it flew just fine," said Erbschloe.

"Since FT and HRJ can be manufactured domestically, they represent a significant step toward reducing our reliance on foreign fuel sources to meet the Air Force's substantial aviation fuel requirements," added Christensen. "The overarching goal is to enable the fleet to use fuel from a wide variety of sources--petroleum, coal, natural gas, animal fat, plants, and potentially other feedstocks--to ease energy security and sustainability impacts."

"The reason to be excited about biofuels is because of sustainability," said Erbschloe. "It is about finding a feed stock we can continue to grow domestically, in a consistent and sufficient volume."