Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 191

linked to the presence of certain polar molecules in the fuel (Refaat, 2009). Fuels with a bias
toward lower carbon number will burn more cleanly. Consequently, there is less deposition of
unburned hydrocarbons in the fuel system, which can improve engine combustion quality and
simplify maintenance.
For precise control of an intermittent combustion process, such as that of a piston-driven engine,
it is advantageous to have a narrower distribution of fuel rather than a wider one so that all of the
fuel components ignite almost simultaneously. A short, consistent ignition delay time allows for
more efficient operation of such engines, as well as cleaner burns with fewer emissions. Even
for turbine engines (which is based on a continuous combustion process), if the distribution of
components is too narrow, there may be difficulties in high-altitude relight (Blakey et al. , 2011).
In Figure 11.7, the composition of many fuel samples are shown as percentages of the fuel
composition by carbon number, and, where available, structural class. Despite the obvious differ-
ences in the fuel blends, all of these fuels meet jet fuel specifications with one exception. Note
that most of the graphs are presented in terms of mass fraction. However, the measurement for Jet
A in Figure 11.7a is given as a molar percentage, although the units are still dimensionless. Fuel
composition on a molar basis is useful for computational chemistry, because the equations for
chemical kinetics are naturally expressed in those units. The mole fraction is related to the mass
fraction through the atomic mass, which is expressed in grams per mole [g/mole]. Mass-based
carbon composition in a sample of JP-4 (the military equivalent of Jet B) are shown in Figures
11.7b,c. The shale-derived Jet B in Figure 11.7c is much richer in the C7-C9 range, relative to the
other petroleum-derived fuels, and is strongly influenced by an abundance of isoparaffins in the
fuel mixture. Based on the discussion of hydrocarbon composition effects on fuel characteristics
above, one should expect that these fuels would have good cold weather properties, as is required
for JP-4.
The Synthetic Paraffinic Kerosene (SPK) fuels in Figures 11.7d-i are manufactured from
natural gas (Fig. 11.7d-g) and coal (Fig. 11.7h,i), in a process which compresses the fuel from
the gaseous state into liquid. In order to meet the minimum density requirements and aromatics
content for jet fuel, they must be blended in a 50/50 mixture with conventional petroleum jet fuel
(Moses, 2008). The S-8 fuel (Fig. 11.7d) is a wide cut fuel and was used in a 50/50 blend with
JP-8 by the US Air Force. The composition of the GTL-1 (Fig. 11.7e) was tuned for automotive
diesel, not jet fuel, and it did not meet the freeze temperature requirements. (Note that this fuel is
dominated by n -paraffins.) The GTL-1 fuel was further processed to create GTL-2 (Fig. 11.7f),
which increased the isoparaffin content, although it also shifted and broadened the hydrocarbon
distribution. This manipulation permitted GTL-2 to meet freeze temperature requirements. The
Shell GTL fuel (Fig. 11.7g) is a natural gas-derived synthetic fuel that exhibits significantly
different composition from the prior fuels but still meets the requirements for jet fuel. The ITK
fuel (Fig. 11.7h) is a coal-derived product, which has been used at the OR Tambo International
Airport in Johannesburg since 1999. An example of the first fully synthetic F-T fuel is shown
in Figure 11.7i. It is manufactured from coal, and enhanced with synthetic aromatics in order
to meet jet fuel standards. This is the only synthetic fuel that does not need to be blended with
conventional fuels at this time.
One example of a Hydrogenated Esters and FattyAcids (HEFA, also called bio-SPK or HRJ) is
shown in Figure 11.7j, derived from oil extracted from jatropha and algae by Boeing. This fuel has
a relatively high percentage of isoparaffins, which would be expected to improve its cold-weather
properties while retaining the advantageous energy density of higher carbon content. Boeing has
also characterized other bio-derived HEFA, for which the green crude was comprised of oils
from camelina, jatropha and algae in varying proportions. All of these blends yielded similar
hydrocarbon composition in terms of carbon number distribution and the ratio of n -paraffins to
isoparaffins (Kinder and Rahmes, 2009).
While developing new aircraft designs and vetting new alternative fuels, we would like to
improve the combustion efficiency through smart design of combustors, inlets and nozzles and
to minimize pollutant output. Computational fluid dynamics has been used for decades to model