Lefebvre A.H., Ballal D.R. Gas Turbine Combustion: Alternative Fuels and Emissions
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450 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
and fuel injectors, restricting fuel ow and disrupting the spray pattern
inside the combustor. Such deleterious changes in fuel properties with time
are referred to as: the fuel storage stability and exposure to high temperature
in the engine is the fuel thermal stability.
Antioxidants, such as hindered phenols, may be added to the fuel to
improve storage stability. Antioxidants work by interrupting the chain
of oxidation reactions, preventing the formation of peroxides, soluble
gums, or insoluble particulates. Peroxides can attack elastomeric fuel sys-
tem parts, gums produce engine deposits, and particulates can plug fuel
lters.
Jet fuel is used to remove heat from engine oil, hydraulic uid, and
air-conditioning equipment. The resulting heating of fuel accelerates the
reactions that lead to gum and particulate formation. A threshold tempera-
ture (usually over 160°C) above which such reactions begin is a measure of
fuel “thermal stability.” An apparatus used to quantify thermal stability is
called a “Jet Fuel Thermal Oxidation Tester (JFTOT)” described in ASTM
D 3241.
Gum formation can be minimized by excluding olens, removing mercap-
tans and water, and eliminating trace metal particles from fuel.
10.3.2.3 Sediment
Sediment tends to be found in the heavier grades of residual fuel. It consists
of nely divided solids that are produced either from carbonaceous mate-
rials through the action of air on unstable components, or from inorganic
matter, including drilling mud, sand, metals, and chlorides from evaporated
brine droplets. Sediment gives rise to deposits in storage tanks, to blockage
in lters and ow lines, and to wear in the fuel injector. These problems are
minimized by ltration and careful handling.
10.3.2.4 Ash
Ash is comprised of various metallic substances that undergo chemical
change during combustion to form nonvolatile inorganic compounds of
varying melting points and differing corrosion properties. These com-
pounds tend to adhere to and foul the nozzle guide vanes and turbine blades
at temperatures around 925 K and tend to promote corrosion at higher tem-
peratures. The problem is extremely complex, and the extent of the damage
caused is very dependent on the temperature history of the overall combus-
tion process.
The ash content of distillate fuel is usually quite small and presents no
problems. In the heavier fuel oils, where the ash content may exceed 0.1%,
the problem is more serious. It may be alleviated by treatment during the
renery stage, but at a signicant penalty in cost and availability. Insoluble
ash can be removed by centrifuging.
Alternative Fuels 451
10.3.2.5 Water
Water can occur in jet fuel in three different forms: dissolved water, free
water, and fuel–water emulsion. Free water and/or fuel–water emulsion are
potentially hazardous and must be avoided.
10.3.2.5.1 Dissolved Water
A typical water-saturated kerosine-type fuel contains between 40 and 80
ppm of dissolved water at 21°C (70°F). Figure 10.2 shows the water solubility.
All distillate fuels contain dissolved water in amounts depending on their
composition, temperature, and storage history. For example, fuels with high
aromatic content can absorb more water into solution than parafnic types,
and more water will dissolve in warm than in cold fuel. Provided it remains
dissolved, water does not have any adverse effects on engine operation [4].
10.3.2.5.2 Free Water
Free water exists as a separate liquid phase. When water-saturated fuel cools,
free water separates out, in the form of numerous small droplets. These sus-
pended droplets (dispersed water) give the fuel a hazy appearance. At subzero
fuel temperatures, these water droplets form ice crystals and slush, which
can collect and build up on fuel lters to cause blockage and malfunctioning.
0.050
0.040
0.030
0.020
0.010
0.005
0.004
0.003
0.002
0.001
240
260
280 300 320
340
Temperature, K
Avgas
Jet A-1, Jet A,
JP5, Kerosine
Water solubility, volume percent
JP4, Jet B
Figure 10.2
Solubility of water in aviation fuels.
452 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
The problem of engine-lter icing can be overcome by heating the fuel. This,
however, incurs penalties in terms of extra weight and a small increase in
engine fuel consumption. The problem may also be overcome through the
use of fuel-soluble icing inhibitors, as described in the next section.
Another major problem with water-contaminated fuels is that of bacterial
or fungal growth, leading to the corrosion of aluminum fuel tanks and fuel-
distributing systems [10]. This problem is especially acute if the fuel contains
water that is saline or brackish. In addition to providing the initial source of
bacteria, this extraneous water usually contains the mineral salts that accel-
erate their development and growth within the aircraft fuel tank and also
cause metal corrosion. The best approach to microbial contamination is pre-
vention, i.e., keeping the amount of free water in the fuel to a minimum.
When microorganisms reach problem levels, approved biocides have
been used. Even if biocides effectively stop biogrowth, it is still necessary to
remove the accumulated biomass to avoid lter plugging. Also biocides are
toxic, and water or fuel containing them must be disposed of appropriately.
10.3.2.5.3 Emulsion
This is a mixture in which very small droplets (less than 100 µm diameter)
of one liquid are dispersed in the continuous phase of another liquid. The
emulsied mixture is stabilized by surfactants that congregate on the sur-
face of the droplets, preventing them from coalescing. Such surfactants can
impair the ability of a lter/separator to remove free water from fuel; i.e.,
they “disarm the coalescer.”
10.3.2.6 Sodium
Trace quantities of sodium are found in crude oils, usually as sodium chlo-
ride, owing to the marine origin of petroleum. During combustion, sodium
chloride is converted to hydrogen chloride, which has a corrosive effect on
the hot metal sections downstream. It can also attack the chromium-rich
oxide layer on the surfaces of turbine blades, leading to a metal deterioration
known as “black plague” [1].
Sodium can be removed, along with potassium and water-soluble calcium
salts, by washing with water. The success of this operation depends on a
working difference between the relative density of the fuel and that of the
water. Fuels with relative densities greater than 0.97 may not be washable
unless they are rst blended with a compatible fuel of lower relative density
or the density of the water is increased.
10.3.2.7 Vanadium
At temperatures above about 922 K, the trace quantities of vanadium that
appear in some fuel oils produce molten compounds that solidify on contact
with turbine blades and give rise to fouling and corrosion. The corrosive
Alternative Fuels 453
effects of vanadium can be combated through the addition of a suitable mag-
nesium compound whose purpose is to raise the melting point of the ash
above the turbine inlet temperature. The concentrations of contaminants
present in various fuels of interest have been studied by Goodger [1]; his
ndings are shown in Figure 10.3. More detailed information is also con-
tained in References [10–12].
10.3.3 Additives
Additives are fuel-soluble chemicals in the parts per million concentration
range, which are blended with fuel to enhance fuel properties and handling,
Heating oil
Diesel fuel
Gas oil
Fuel
oils
Existent gum
(mg/100 ml)
Sulfur
(% mass)
Carbon residue
(% mass)
Sediment (% mass)
Ash (% mass)
Relative density
0.7
6
4
2
0
0.8 0.9
1.0
Free water
(% volume)
Avcat
Avtur
Avtag
Mogas
Avgas
Concentration, % mass, % volume, or mg/100 ml
Figure 10.3
Concentrations of contaminants in petroleum fuels.
454 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
of crucial importance to engine performance and efciency. Additives must
be nontoxic, fuel soluble, and thermally stable.
10.3.3.1 Gum Prevention
Additives that improve the storage and thermal stability of fuels are anti-
oxidants, metal deactivators, and catalytic passivators. Antioxidants inter-
rupt degradative oxidation processes or keep them from proceeding to an
undesirable extent. Metal deactivators depress the catalytic activity of metals
by converting them into organometallic compounds of reduced activity [3].
Catalytic passivators reduce the catalytic effect of metals by forming a lm
on the metal.
10.3.3.2 Corrosion Inhibition/Lubricity Improvers
The tanks and pipelines of the fuel distribution system are constructed pri-
marily of uncoated steel. Corrosion/rust inhibitors, generally, are hydro-
carbons whose solubility in fuel is combined with an afnity for metallic
surfaces. It is believed that they are absorbed on metallic surfaces to form
protective monomolecular lms on the metal. Such rust inhibitors include
amines, phosphates, sulfonates, and fatty acids.
Lubricity additives contain a polar group that adheres to metal surfaces
[13], forming a thin lm of the additive. This lm acts as a boundary lubri-
cant when two metal surfaces come in contact. Trace amounts of certain
oxygen-, nitrogen-, and sulfur-containing compounds can act as lubricity
improvers. Both corrosion and lubricity are surface processes, therefore cor-
rosion inhibitors also improve lubricity.
10.3.3.3 Anti-Icing
Anti-icing additives are soluble agents with a nonreversible hygroscopicity
that allows them to combine with moisture in the fuel, thereby preventing its
separation and the formation of ice crystals. A well-known anti-icing addi-
tive is di-ethylene glycol monomethyl ether (di-EGME). Fuel system icing
inhibitors (FSII) work by combining with any free water that forms and low-
ering the freeze point of the mixture so that no ice crystals are formed. If a
fuel containing FSII comes into contact with free water, the additive forms a
thick, gelatinous-type paste. For this reason, FSII is not added at a renery,
but added during fuel delivery to the military aircraft. Commercial carriers
do not use FSII, but have heaters on their main fuel lters to melt any ice
formed.
10.3.3.4 Antistatic–Static Dissipators
During the fueling of an aircraft, static electricity charges can build up to
such an extent that the ensuing spark discharge is capable of initiating res
Alternative Fuels 455
and explosions [14,15]. Charging rates are highest when low-conductivity
fuels are pumped at high ow rates, and for some time, the oil companies
have taken precautions to reduce the ignition hazard by imposing ow-rate
restrictions during product handling [15].
Another practical solution is to increase the electrical conductivity of
the fuel so that the charges can dissipate as rapidly as they are generated.
When the additive is used, the fuel conductivity must be between 50 and
450 CU at the point of delivery into the aircraft [4,8]. The only additive
currently approved for use is Stadis 450 (ASTM D 2624)—a proprietary
composition.
10.3.3.5 Metal Deactivators
When the fuel is contaminated with metals, like copper and zinc, its ther-
mal stability is degraded. Metal deactivators are chelating agents [3] that
form stable complexes with specic metal ions and inhibit their catalytic
activity, thereby mitigating fuel oxidation that causes deterioration in ther-
mal stability. Metal deactivators are not normally employed in commercial
aviation.
10.3.3.6 Antismoke
Considerable interest has been show in the use of fuel additives to reduce
exhaust smoke. The most successful of these are organometallic com-
pounds of barium, manganese, and iron [16–19]. Tests on all three com-
pounds by Hersh et al. [19] demonstrated very substantial reductions
in exhaust emissions index when they were added to No. 2 fuel oil for
an electric-utility gas turbine. The optimal injection rate was found to
be approximately 25 ppm by mass of metallic base element. The results
obtained with the manganese-based additive are shown in Figure 10.4,
which demonstrates that the additive reduces the exhaust-plume opacity
by reducing the carbonaceous emissions. From measurements carried out
at base-load operation, it was found that exhaust-particle size distribu-
tions do not change signicantly with additive use, the mean particle size
being 0.5 µm.
There is an obvious drawback to the use of metal-based fuel additives:
during the combustion process, they might form oxides that could deposit
on the turbine blades. Conceivably, they could also create an extra pollut-
ant in the engine exhaust gas, one that is less acceptable environmentally
than smoke. For these reasons, improving fuel–air mixing and other param-
eters in combustor design are really the satisfactory solution to the smoke
problem.
Synthetic alternative fuels offer the real opportunity of either completely
eliminating or signicantly reducing the aromatics content of fuel. As shown
later, this leads to a corresponding decrease in soot/smoke formation.
456 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
10.4 Fuel Properties
In a combustion chamber, the fuel must be injected, vaporized, and mixed
with air before combustion can occur. The extent to which these processes
are limiting to combustion depends, to a very great extent, on the physical
properties of the fuel.
10.4.1 relative Density
Fuel density is very important because it denes the energy content in the
fuel tank with a xed volume. For a military aircraft, fuel with a high volu-
metric energy content (MJ/L or Btu/gal) maximizes the energy stored in a
xed volume of the fuel tank, thus providing the longest ight range. For
commercial aircrafts that take on just enough fuel to reach their intended
destination safely, a less dense fuel with a higher gravimetric energy con-
tent (MJ/kg or Btu/lb) may minimize fuel weight. For example, for kerosine
(density = 0.81 g/mL at 15°C), the volumetric energy content is maximum
35.06 MJ/L; whereas for a wide-cut jet fuel (density = 0.762 g/mL at 15°C), the
gravimetric energy content is maximum at 43.54 MJ/kg.
The relative density d
r
(formerly called specic gravity) of a fuel is related
to its average boiling point and chemical composition. In general, it is high-
est for aromatics and lowest for parafns, with naphthenes and olens lying
in between. The relative density is fairly easy to determine, and it provides
0.6
0.4
0.2
0
0
40
80 120
Manganese concentration. ppm by mass
Emission index, g/kg fuel
Total
Ash
Mn
X
O
Y
SO
4
Carbon
Figure 10.4
Exhaust particulate emissions index vs. manganese concentration. (From Hersh, S., Carr, R.C.,
and Hurley, J.F., ASME Paper 78-GT-64, Gas Turbine Conference, London, 1978.)
Alternative Fuels 457
a useful indication of hydrogen/carbon ratio, caloric value, and tendency
toward carbon formation. Typical values for liquid petroleum fuels range
from about 0.72 for gasoline up to 0.97 for heavy fuel oil.
The relative density at T
1
/T
2
of a fuel is usually dened as the ratio of the
mass of a given volume of fuel at a temperature T
1
to the mass of an equal vol-
ume of pure water at a temperature T
2
. A reference temperature of T
1
= 289 K
is often used, with T
2
set at 277 K, where the density of water is highest. The
relationship between relative density and fuel temperature for some typical
aviation fuels is shown in Figure 10.5.
10.4.1.1 API Gravity
In the United States, fuel density is often specied in terms of the API grav-
ity. It is calculated with the formula
APIgravity
at
r
=
−
141 5
289
289
131 5
.
..
d
K
K
0.90
0.86
0.82
0.78
0.74
0.70
0.66
240
260
280
300
320
Fuel temperature, K
Relative density
Avgas
Avtag (JP4)
Avtur
Avcat (JP5)
Gas oil
Diesel oil
Figure 10.5
Relative densities of typical gas turbine fuels.
458 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
10.4.1.2 Molecular Mass
This fuel property is best determined experimentally. It can be estimated
with reasonable accuracy from the line in Figure 10.6, which conforms to the
relationship:
MM
APIgravity
=
11 280
11
,
()
.
.
10.4.2 Distillation range
The distillation range of a fuel is important because it largely determines the
physical and combustion characteristics of the fuel and has a dominating
inuence on availability. Availability is increased by widening the boiling
range, either by lowering the initial boiling point, or both. The ASTM D-86
distillation curves for various fuels are shown in Figure 10.7. Since practi-
cal fuels are a mixture of many compounds, each having its own boiling
point, these fuels have no single boiling point, but rather a distillation range
dened by the temperature vs. percentage fuel evaporated relationship.
10.4.3 Vapor Pressure
The vapor pressure of a liquid is the pressure exerted by the vapor above
its surface at a given temperature. A high vapor pressure is desirable from a
200
100
50
30
20
10
API gravity
30 50 100 200
Molecular mass, kg/kmol
300500
Figure 10.6
Relationship between API gravity and molecular mass.
Alternative Fuels 459
combustion viewpoint, because it ensures rapid evaporation of fuel in the pri-
mary combustion zone. On the other hand, low vapor pressure has advantages
in terms of reduced pressure in unvented fuel tanks, lower fuel losses due to
evaporation at high altitudes in vented tanks, and reduced re hazard.
The vapor pressures of some typical kerosine fuels are shown in Figure 10.8
as a function of temperature. Note that the vapor pressure increases steeply
with temperature. This is why fuels with low vapor pressure are essential for
high-supersonic ight speeds.
10.4.4 Flash Point
The ash point is dened as the lowest temperature at which a fuel gives off
sufcient vapor to form a ammable mixture with air. As might be expected,
the ash point is directly related to vapor pressure; the higher the vapor
pressure, the lower the ash point. In general, the ash point of kerosine
provides a measure of its ammability, whereas that of heavier oils is an
indication of their volatility. It is also useful for classifying fuels from the
viewpoint of re risk.
The signicance of the ash point in the selection of aircraft fuels has been
discussed by Johnson et al. [20]. The variation of ash point with relative
600
550
500
Temperature, K
450
400
350
300
0204060
Percentage evaporated
Avgas
JP4, Jet B
Jet A, A-1
Diesel No. 2
JP5
80 100
Figure 10.7
ASTM D-86 distillation curves for various fuels.