Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 185

Thermal OxidationTester (JFTOT) exposes the fuel to a heated aluminumalloy tube in a controlled
way and passes it through a filter to collect any particulates that have formed. After the test is
complete, the pressure drop across the filter and a visual inspection of the aluminum tube for
discoloration are used to evaluate thermal stability. It may look unusual to measure a pressure
drop in terms of length when it is actually a force per unit area. The height of the fluid column is
related to the pressure though its density and gravitational acceleration ( g ), specifically p = ρgh .
By convention, standard atmospheric pressure at ground level is 760mmHg = 101.325 kPa. This
property represents the capacity for maintaining fuel properties due to thermal exposure. It is
particularly important for aviation fuels because high-performance engines use the fuel as a
mechanism for heat exchange, e.g., for cooling certain engine components or hydraulic fluid.
High temperatures can accelerate oxidation reactions in the fuel, leading to gum and particulate
formation. Antioxidants are used as additives to improve thermal stability.
Storage stability : refers to a fuel's capacity for retaining its essential properties while in storage.
This is essential for reserve stocks that may be stored for several years, but a more typical storage
time for aviation fuel is sixmonths. Fuel properties can be altered by oxidation processes over time,
which are a function of air exposure, elevated temperature, contaminants in the fuel or absorbed
from the container walls, and the presence of peroxides and antioxidants (Knothe, 2007).
Electrical conductivity measured in units of M − 1 L − 2 T 3 A 2 , picoSiemens per meter [pS/m]:
The rate at which electrical energy dissipates is proportional to the fuel's capacity to conduct
electricity. A fuel made up of pure hydrocarbons would not be electrically conductive, but trace
elements, specifically, the presence of polar molecules, could bestow the capability of holding
a charge. During pumping operations, the fuel can be in contact with a variety of materials in
hoses, pipes, fittings, and valves. Contact between dissimilar materials can cause the formation
of an electrical charge in the fuel. Given time, the static electricity would dissipate. However, if
the time constant for the dissipation is long, vaporized fuel could be at risk of burning should
it come into contact with a stray ignition source. Naturally derived petroleum products have
more contaminants than synthetic fuels and will tend to have a higher conductivity. Jet A-1 and
JP-8 require the incorporation of a static dissipater additive, which increases electrical conduc-
tivity, thereby decreasing the time required for dissipation and reducing the risk of unplanned
combustion.
Vapor pressure measured in units of ML − 1 T − 2 , kilopascals [kPa]: For a given substance at
thermodynamic equilibrium in a closed system, the liquid and gaseous phases (and solid phase, if
any is present) have no driving force to change phase, so they will remain in the same proportions
unless the system is perturbed. The proportionality is a function of temperature and pressure. At
a fixed temperature, liquids will remain stable at all pressures above the vapor pressure, but they
will boil when the pressure drops below the vapor pressure. Kerosene-type jet fuel, such as Jet
A, has a vapor pressure of about 1 kPa at 38 ◦ C. Wide-cut fuel, such as Jet B, is more volatile
and it has a higher vapor pressure. The specifications in CGSB-3-22 require that the maximum
permissible vapor pressure is 21 kPa at 38 ◦ C. Jet A carries no vapor pressure requirement.
Heat capacity , measured in L 2 T − 2 K − 1 , Joules per gram per degree C [J/(g ◦ C)]: Heat capacity
is not specified in the requirements, but it is a property that can affect engine efficiency. More
efficiency can be squeezed out of an engine if the fuel itself is used to cool engine components,
while it is simultaneously heated up en route to the combustor. If temperature is too high (over
480 ◦ C or so), thermal/catalytic cracking can occur. InWFSP, the average value of the heat capacity
at 0 ◦ C was 1.582 J/(g ◦ C) and showed a linear dependence on temperature.
Cetane number , a dimensionless term: The cetane number is related to the ignition delay
time between the injection of the fuel and the onset of ignition. It is particularly important for
piston-driven engines with periodic combustion spurts, for which precise control over combustion
timing is critical. Higher cetane number correlates to shorter ignition delay times, so especially for
automotive applications, a higher cetane number will result in cleaner, more complete combustion
(Refaat, 2009).
For additional reading on jet fuel standards and testing methods, see Chevron (2006), Knothe
(2008b); on jet fuel properties and composition, see Hadaller and Johnson (2006), Moses (2008,