Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 184

Figure 11.5. Hydrocarbon content in jet fuels by class from (a) the World Fuel Sampling Program, adapted
from Hadaller and Johnson (2006); and (b) all jet fuel deliveries to the US military in
2004, reproduced from Colket et al . (2007) with permission from the American Institute
of Aeronautics and Astronautics.
content in the final blended product. In most of the synthetic F-T fuels, up to 50% synthetic is
blended with conventional jet fuel, with all of the aromatics coming from the petroleum stream
(Moses, 2008).
The WFSP analyzed the hydrocarbon content of the sample fuels using a number of ASTM-
specified protocols, some of which gave somewhat conflicting results, particularly at low
concentrations. Figure 11.5a shows the composition of the jet fuel analyzed via ASTM D1319.
It is separated into categories of aromatics, saturates (alkanes in the form of n -, iso- and
cyclo-paraffins) and olefins (alkenes, or unsaturated hydrocarbons with one or more double
carbon-to-carbon bonds). Figure 11.5b shows a histogram of the aromatics content in all of the
deliveries of Jet-A to the US military in 2004. Note that each of the numbers on the abscissa
( x -axis), cover a 1.5% range in value, i.e., about 24% of the fuel deliveries had aromatics content
in the range of 20.5 ± 0.75%v/v. On average, the aromatics content of fuels was in the range of
20%, although it varied from below 10% to about 24%. All of these fuels met the standards for
aviation fuel, which indicates that, from a practical standpoint, the requirements allow substantial
flexibility in composition. However, there are will be some differences in performance between
the fuels with the highest and lowest aromatics content.
Napthenes (cycloalkanes) are other fuel components of interest. They are also based on carbon
rings, like aromatics, except that, rather than some double bonds, they have only single carbon-
to-carbon bonds, i.e., they are saturated cyclic hydrocarbons.
Hydrogen content measured in nondimensional units as a percentage of hydrogen by mass
to total fuel mass [%m/m]: Fuels with higher hydrogen content burn more cleanly and produce
more energy per unit mass. The disadvantage of high hydrogen content fuels is the relatively
lower energy content per unit volume. The WFSP found that hydrogen levels in jet fuel were in
the range of 14%m/m, and that synthetic Fisher-Tropf fuel had higher hydrogen content (Hadaller
and Johnson, 2006).
Lubricity commonlymeasured by wear scar diameter in units of L , millimeters [mm]: Lubricity
refers to a fluid's capacity to reduce friction betweenmoving parts. It is characterized bymeasuring
the wear on a fixed steel ball after a specific length of time after contact with a rotating cylindrical
ring that is partially immersed in fuel (Chevron, 2006). The depth of the wear scar increases with
decreasing lubricity. This property is important for jet fuels, as aircraft components rely on the
fuel itself to lubricate moving parts, e.g., in fuel pumps and control units. For a discussion of wear
scar testing on a wide range of samples, see Knothe (2008b).
Thermal stability commonly measured by the pressure drop, p, across a filter measured
in units of L , millimeters of mercury [mm Hg]: Specialized test equipment called the Jet Fuel