Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 188

Table 11.2. Key fuel specifications for biofuels based on methyl esters for automotive applications.

Property FAME FAAE

Density at 15 ◦ C [kg/m 3 ] 860-900 -

Viscosity at 40 ◦ C [mm 2 /s] 3.5-5.0 1.9-6.0

< 0.20 or flash temperature < 130 ◦ C Methanol [%m/m] < 0.20

Sulfur [mg/kg] < 10 < 15

Total esters [%m/m] > 96.5 -

Monoglyceride [%m/m] < 0.8 -

Diglyceride [%m/m] < 0.2 -

Triglyceride [%m/m] < 0.2 -

Free glycerol [%m/m] < 0.02 < 0.02

Total glycerol [%m/m] 0.25 < 0.24

Total acidity [mg KOH/g] < 0.5 < 0.5

Specification EN 14214 ASTM D 6751

Typically, green crude has a higher proportion of heavier carbon components than jet fuel,
resulting in a fluid that is more viscous than diesel (Fig. 11.4b). Although vegetable oil can be
blended with conventional fuel for use in some piston-driven diesel engines without treatment, it
can form deposits, plugging up filters and injectors. Plant-derived fuels have very little sulfur or
aromatic content, so, in general, its emissions have less environmental impact.
A recent review of material compatibility (Fazal et al. , 2011) finds that, relative to petroleum-
based diesel, some biodiesel results in greater potential for plugged filters, sticking parts, and
corrosion of some metals. The corrosion process can also degrade fuel properties such as density
and viscosity. This study was primarily concerned with biodiesel for automotive applications,
but its findings could also apply to aviation. In any event, the disadvantages of biodiesel can
largely be mitigated by additives and blending with conventional fuels, with the exception of its
inherently lower energy density.
Biorefining processes such as transesterification are used to break down the vegetable oils to
alkyl esters to form FAAE. Vegetable oils, free fatty acids and FAAE can be used as precursors
for HEFA. In order to meet freeze point requirements and boost the specific energy, they must
undergo hydroprocessing to remove the oxygen, become saturated with hydrogen, and reduce
chain length (section 11.5.3). The composition of HEFA fuel will be essentially the same as
that of petroleum-derived fuels, but will have much less cycloparaffin content and only traces of
olefins, aromatics and sulfur. As a result, it will have the following:
• Advantages : better stability and blending properties due to absence of double bonds, oxygenated

molecules, and heteroatoms (here, primarily nitrogen and sulfur); cleaner burn, lower sulfur and
particulate emissions relative to petroleum jet; lower freeze point and higher specific energy
than FAAE/diesel blends, and

• Disadvantages : lower lubricity than biodiesel, so it will likely require additives or blending.

Since natural antioxidants may be lost during hydroprocessing, this fuel may also need additives
for stability.
For further background on biodiesel, see Blakey et al. (2011), Knothe (2010a,b); and on SPK
composition, properties, and standards, see Moser (2009).

11.3 FUEL PROPERTIES

11.3.1 Effect of composition on fuel properties
The hydrocarbon composition of any particular tank of jet fuel is dependent on the properties of
the crude oil, refining operations and the use of additives. Petroleum-based jet-grade fuels are