Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 180

Figure 11.2. Sulfur content in 56 samples of jet fuel from around the world. The number of samples
in each category is shown in parentheses. Average values are marked with a diamond on
a gray window representing minimum and maximum sample values. (a) Sulfur content by
geographical region; and (b) Sulfur content by jet fuel type. Adapted from Hadaller and
Johnson (2006).
synthetic Fischer-Tropsch fuel. Three of the samples fromAsia/Australia were extracted from oil
shale. In Figure 11.1, the average values are marked by a diamond on a gray window representing
the envelope of minimum and maximum values. Although a few samples of Jet A-1 exhibited
relatively high values of sulfur, most of the samples had sulfur content of about 500 parts per
million (ppm) or less, which is significantly below the specified limit of 3000 ppm (Chevron,
2006). Sulfur can be corrosive to the engine and it is of concern in emissions, but extremely low
levels of sulfur (below ∼ 100 ppm) have also been correlated with increased engine wear.
The WFSP study presented results for a range of other fuel parameters as well, some of which
are reproduced in Figure 11.3, and will be discussed in the following paragraphs. The units for
the parameters are identified by the symbols for mass [M], length [L], time [T], temperature [K]
and amperes [A].
Density (Figs. 11.3a and 11.4a) measured in ML − 3 , here in kilograms per cubic meter [kg/m 3 ]:
Fuel density represents the mass per unit volume of fuel. Density is a key parameter because
increased fuel weight means that more energy must be supplied to move the loaded aircraft, but
it is also correlated with other performance parameters discussed below, such as specific energy
(heat of combustion).
Fuel injectors meter their output of fuel by its volume, not its density. Consequently, when the
fuel is injected into the combustion chamber, the density of the fuel will govern the fuel/air ratio.
Thus, density is directly related to the thrust through the injected fuel volume and fuel reaction
properties (Fazal et al. , 2011).
Since the density of liquid fuel decreases linearly with temperature (Fig. 11.4a), the standards
in Table 11.1 require that the measurement be taken at a standardized reference temperature of
15 ◦ C. All of the fuel sampled in theWFSP met the minimum requirements for aviation fuel; none
of the samples exceeded the maximum value of 845 kg/m 2 (JP-5) and 840 kg/m 2 (Jet A, Jet A-1
and JP-8).
Kinematic viscosity at -20 ◦ C (Figs. 11.3b and 11.4b), measured in units of L 2 t − 1 , here
square millimeters per second [mm 2 /s]: Viscosity is a measure of fluid resistance to shear stress.
Consequently, lower viscosity fluids deform upon application of shear forces more readily, and
they pour more easily. In aircraft operation, this property is important in cold starts, reignition at
altitude, lubrication and combustion quality. Higher viscosity causes larger pressure drops across
the fuel lines, requiring the pumps to work harder to maintain a given flow rate. In addition,
higher viscosity fuels have an impact on combustion quality: Liquid fuel enters the combustion
chamber as an atomized spray. Higher viscosity fuels tend to cause larger droplets, and the spray
pattern does not penetrate as deeply into the chamber. Incomplete combustion can result, with