Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 186

2009), and Moses and Roets (2009); and on thermal, oxidative and storage stability, see Jain and
Sharma (2010), and Knothe (2007).

11.2.2 Alternative jet fuel specifications
For the near term, alternative fuels must be “drop-in” fuels, which can be used directly in current
aircraft turbine engines, usually by blending jet-grade petroleum and bio-jet fuel. Note that there
are standards for aviation gasoline (Avgas) for small piston-driven aircraft. Since that represents
a small percentage of aviation fuel, we will focus on the heavier aviation fuel designed for use
in turbine-powered aircraft. A variety of alternative jet fuels have been thoroughly tested on the
ground and have been used in demonstration flights by commercial airlines as well as the US
military. See Hendricks et al. (2011), and Kinder and Rahmes (2009) for some of this background
and Edwards (2007) for a description of the historical evolution of aviation fuel standards. The
challenge lies in making these fuels commercially viable, environmentally friendly, and broadly
available. The basic fuel types for alternative fuels for aviation are:

• SPK : Synthetic Paraffinic Kerosene derived from natural gas, such as Syntroleum's S-8,

Shell's GTL, and Sasol's GTL-1 and GTL-2. (GTL stands for Gas To Liquid, a reference
to its Fischer-Tropsch (F-T) manufacturing process.) SPK fuels are functionally similar to
Jet A, but they have negligible aromatics content.

• IPK : Synthetic paraffinic kerosene derived from coal, with properties that are comparable

to SPK.

• FSJF : Fully Synthetic Jet Fuel, a synthetic paraffinic kerosene developed by Sasol, which

consists of coal-derived fuel with synthetic aromatics. This is the only alternative fuel at this
time which does not require blending with conventional fuels (Moses and Roets, 2009).

• HRJ : Hydrotreated Renewable Jet fuel, which is derived from renewable fuel sources. Veg-

etable oils are one such source, but they require hydrotreatment to condition the oil to jet-fuel
quality. This fuel has also been called bio-SPK to underscore its functional similarities to SPK.
In 2011, it was re-named HEFA for Hydrogenated Esters and Fatty Acids. This fuel was cer-
tified by ASTM in July 2011 for use as jet fuel in a 50/50 blend with conventional Jet A fuel
under alternative fuel specification D7566.

An analysis by Hilemann et al. (2010) concludes that these fuels would reduce the net fuel
energy consumed by the aviation industry, improving overall fleet-wide energy efficiency. With
the exception of HEFA/HRJ, these fuels are derived from coal and natural gas. They use a variant
of the F-T process, which is a liquefaction technique. In 2009, F-T processing was the first
manufacturing method to be approved by the ASTM under its alternative jet fuel specification
ASTM D 7566. The SPKs were the first approved synthetic blending component under this
standard in 2009, and revisions were made in 2011 to also include HEFA.
“Neat” fuels are not mixed or diluted with other fuels. (For pure biofuels, the designation
“B100” may also be used, in which the “100” indicates that 100% of the fuel is bio-derived.)
One study found that an array of SPK and IPK F-T fuels met all jet fuel specifications except
for density (Moses, 2008). To mitigate this property, a 50/50 blend of F-T and conventional fuel
was certified for commercial aviation. It has been widely tested on the ground and in civilian and
military aircraft.
The composition of the F-T synthetic fuels can vary in hydrocarbon content, but they tend to
peak at a slightly lower carbon number than petroleum-based fuels. They also have little aromatics
content. The primary advantages and disadvantages of such synthetic fuels relative to conventional
fuels are:

• Advantages: cleaner burn; reduced carbon monoxide (CO), sulfuric gases (SO x ) and particulate

emissions; better thermal stability; potentially carbon-neutral; and
• Disadvantages: lower energy density, poor lubricity, higher freeze point, higher viscosity,
carbon capture and sequestration is required to be considered sustainable.