Alternative
aviation fuels and their approval process contribute
to one of the biggest challenges in deployment of an enhanced volume
of aviation fuels. One of the approved fuel blending components is
hydroprocessed esters and fatty acids (HEFAs) for mixing with petroleum-derived
fuel (Jet A/A-1) up to 50 vol %. As the ASTM fuel specification (D7566)
is merely based on the performance expected, the final blend concentration
of HEFA in Jet A/A-1 can vary depending on the resultant fuel properties.
Currently, there is a lack of information on how these properties
are affected by the constituent chemical composition. The aim of this
study was to compare three HEFAs sourced from different feedstocks
(camelina, tallow, and mixed fat) and their blends with Jet A (10–60
vol %) based on the detailed chemical composition. The chemical composition
was obtained using two-dimensional gas chromatography with mass spectrometry
and flame ionization detector. The properties of interest were the
distillation profile, density, viscosity, flash point, freezing point,
and net heat of combustion. The key observation was that the distillation
profile had the main impact on the final fuel properties. Additionally,
the selection of the feedstock or the process conditions yielding
an end HEFA composition can adversely affect properties, such as viscosity
and/or freezing point.
The composition of Navy Jet fuel
JP-5 was determined using gas
chromatography(GC)-electron impact mass spectrometry and GC ×
GC/(flame ionization detection) to contain by mass 21% linear alkanes,
29% cycloalkanes, 32% isoalkanes, and 18% aromatic compounds. Various
quaternary mixtures of n-dodecane, n-butylcyclohexane, n-butylbenzene, and 2,2,4,4,6,8,8-heptamethylnonane
were prepared as possible surrogates for this jet fuel and analyzed
for density and viscosity (253 to 333) K, speed of sound (288 to 333)
K, surface tension (294 ± 1 K), and flashpoint. Deviation cutoffs
for “matching” the JP-5 based on previous studies and
fuel specification were ±1.7% for density, ±1% for speed
of sound, ±3.5% for bulk modulus, ±2.6% for viscosity, ±2.2%
for surface tension, and ±10% for flash point (minimum of 333
K). Seven quaternary mixtures met these cutoffs. Three had compositions
that were comparable to the JP-5 and would be good candidates for
engine testing. All mixture excess molar volumes showed a trend of
increasing to a maximum and then decreasing as the concentration of n-butylbenzene increased. Viscosity deviations ranged from
(−0.03 to −0.48) mPa·s.
One
challenge in the deployment of alternative aviation fuels is
the lengthy “fuel approval process”, which costs millions
of dollars and can take many years as the exact effect of these alternative
options on engine and framework is still an unknown. A candidate aviation
fuel needs to pass the tests as deemed necessary by the ASTM D4054
Standard Practice. The fuel manufacturer faces the risk of not receiving
the ASTM certification after significant financial and time investment,
which currently acts as a considerable hindrance to broadening the
alternative aviation fuel options in commercial and military aircraft.
Approval tests are based on the fuel properties and fuel performance
as there is currently a knowledge gap on fuel chemical composition–property
correlations. Therefore, the aim of this study was to accomplish the
first step in this target, i.e., to obtain a detailed chemical composition
of four approved blending components (FT-SPK, HEFA, SIP, and ATJ)
and their mixtures with Jet A using GC × GC-TOF/MS and GC ×
GC-FID. Infrared spectroscopy and principal components analysis were
utilized as additional techniques to demonstrate the differences among
the blending components and Jet A, further utilizing their infrared
spectral features. Moreover, the main physiochemical properties were
measured, such as distillation profile, density, viscosity, flash
point, freezing point, and net heat of combustion. Lastly, the impact
of the differences in chemical composition on these main fuel properties
was discussed.
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