Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 189

mixtures of various hydrocarbons, including alkanes (paraffins) and cycloalkanes (naphthenes),
aromatics, and alkenes (olefins), as well as much smaller amounts of benzene and polycyclic
aromatic hydrocarbons, and trace amounts of sulfur and other materials. Note that, for the most
part, the standards described in Table 11.1 did not specify hydrocarbon composition. Instead,
the fuel properties are primarily specified in terms of performance. As a practical matter, the
jet fuel performance specifications (section 11.2.1) limits the carbon content of jet fuels to the
carbon numbers from C8 to C16 (Edwards, 2010), but the molecules can appear as straight,
branched or linked chains. Most of the fuel components are saturated, meaning that only single
carbon-to-carbon bonds appear in the mixture.
When crude oil is refined, it undergoes a distillation process to separate out the hydrocarbon
components by weight. As heat is applied, the lightest hydrocarbons will start to vaporize first and
will move up a distillation column, followed later by heavier hydrocarbons. Jet B is a wide-cut
fuel, which has a more diverse hydrocarbon blend with chain lengths from about C4 to C16.
The presence of the light carbon components gives Jet B its cold temperature properties. Jet
A-1 is required to have a lower freeze point than Jet A (Fig 11.3c), which makes it desirable for
operation in colder climates. However, it can be less expensive to refine oil to Jet-A standards,
since a broader temperature range can be used in the distilling process, permitting recovery of a
wider range of hydrocarbons.
The thermophysical properties of a pure hydrocarbon fluid are dependent on its carbon number
and its structural complexity. The fluid can be saturated (holding the maximumpossible number of
hydrogen atoms with only single bonds between all atoms), or unsaturated (replacing some of the
single carbon-to-carbon bonds with double bonds). The most abundant forms are that of simple,
straight-chain, saturated n -paraffins; branched, saturated isoparaffins; cyclic, saturated paraffins
(cycloalkanes/naphthenes); unsaturated olefins with at least one double bond; and unsaturated
cyclic aromatics (see Appendix A for a description of the underlying organic chemistry). Figure
11.6a shows that density increases with carbon number for saturated compounds. The densities of
n -paraffins and isoparaffins at 20 ◦ C are indistinguishable, but further structural complexity and
double bonds increase the density at a given carbon number. Discussion of density correlations
as a function of composition can be found inAlptekin and Canakci (2008), Alptekin and Canakci
(2009), Refaat (2009), and Saravanan and Nagarajan (2011).
Kinematic viscosity increases with chain length (Fig. 11.6b), degree of saturation, and branch-
ing (Knothe, 2005; Refaat, 2009). The boiling point increases with increasing carbon number
(Fig. 11.6c), and is dependent to a lesser extent on structural complexity. The specific energy
decreases with increasing carbon number and the presence of naphthenes and aromatics (Fig.
11.6d), while the opposite holds true for the energy density (Fig. 11.6e). Note that the n -paraffins
and isoparaffins share nearly the same dependence, while naphthenes are more strongly influ-
enced by carbon number fromC8 to C10. Since the energy density increases as the carbon number
increases, in a situation with a fixed volume, such as a completely filled fuel tank, more energy
can be released by a fuel with higher carbon number. High carbon numbers are advantageous in
this regard, although that must be balanced against the deleterious effects of increased viscosity
(Fig. 11.6b) and increased freeze temperature (Fig. 11.6f).
Lighter, low-carbon fuels have a lower freeze temperature than high-density high-carbon fuels
(Fig. 11.6f). This provides some insight as to the desirability of using a wide-cut blend for better
cold-weather performance, since a wide blend tends to have more hydrocarbons with low carbon
numbers. Also note that the only graph in Figure 11.6 in which n -paraffins and isoparaffins
separate out appreciably is in this property. The freeze temperature decreases with increased
structural complexity, such as increased branching and reduced level of saturation. Relative
to straight-chain, ladder-like alkanes, such molecules exhibit reduced packing efficiency, so
that lower temperatures are needed to reach an entropy level at which crystallization can occur
(Refaat, 2009). The freeze temperature of naphthenes from C8 to C10 increases more strongly
than the rate of increase for n -paraffins and isoparaffins. Considering the specific energy and
freeze temperature, increasing isoparaffin content will produce better cold weather properties,
without a substantial hit on the available energy content. If the concentration of isoparaffins can