Lefebvre A.H., Ballal D.R. Gas Turbine Combustion: Alternative Fuels and Emissions
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470 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
Kurtovich and Hayes [32], tetraisopropyl titanate was found to be the most
promising. Although the results so far achieved are encouraging, no additive
for aircraft fuels has yet been found that, when used in small concentrations,
can effectively increase the SIT.
10.5.4 Limits of Flammability
Combustible gases and vapors are capable of burning in air only within
closely dened limits of concentration. Within these limits, a ame, once ini-
tiated, can spread any distance away from the source of ignition. It is custom-
ary to dene rich and lean ammability limits, which respectively represent
the maximum and minimum fuel concentrations that produce combustible
mixtures. The lean limit of ammability is closely related to the ash point.
For kerosine-type fuels at room temperature, the range of ammability lies
between fuel/air ratios of approximately 0.035–0.28 by mass, the exact value
depending on the size and shape of the vapor space in the fuel tank. Limiting
concentrations for a number of other fuels in air at standard pressure and
temperature are listed in Reference [1].
Barnett and Hibbard [28] have shown that lean-limit mixtures of all hydro-
carbons contain about the same heat of combustion per unit volume of fuel–
air mixture. On this basis, the fuel concentrations for lean-limit mixtures at
1 atm are estimated as
LL=
×
=
×
×
4350 18710
6
LSEMM
MJ/kgor
LSEMM
Btu/lb.
.
where L is the lean-limit concentration, percent by volume and MM is the
molecular mass.
The limits of ammability do not depend greatly on pressure, except at
very low pressures below 7 KPa. Generally, they tend to spread slightly with
an increase in pressure. The inuence of temperature is more pronounced.
At low temperatures, the fuel may release insufcient vapor, so that the
mixture strength lies below the lean limit of ammability. At high tempera-
tures, the production of excessive amounts of vapor could move the mixture
strength above the rich limit. Thus, ammable mixtures are produced only
within a certain range of fuel temperatures. For an average kerosine and a
typical wide-cut fuel (JP-4), the ammability temperature limits at sea level
are 254–288 K and 316–338 K, respectively.
Fuel vaporization increases at high altitudes; thus, at altitude, ammable
mixtures are formed at much lower temperatures than on the ground.
Of special interest in aviation is the possibility of forming ammable mix-
tures in the ullage space of fuel tanks at high altitude. Liquid hydrocarbon
fuels absorb atmospheric gases; the solubility of oxygen being greater than
that of nitrogen. At high altitudes, the dissolved oxygen in the fuel comes
out of solution and can raise the oxygen content of ullage air from 21 to as
Alternative Fuels 471
high as 33% ( = 10 K increase in ammability temperature). Also, fuel boiling
can promote a signicant loss of fuel through tank vents and produce vapor
lock in pipelines and fuel pump cavitation. Consideration is given to “ullage
purging” by nitrogen from a ground source prior to takeoff.
10.5.5 Smoke Point
The inuence of fuel type and composition on soot formation is discussed
in Chapter 9. Probably, the most widely used index of soot-forming tendency
is the smoke point (ASTM D 1322). It is determined experimentally by burning
the fuel under test in a special wick lamp and slowly increasing the height
of the ame until it begins to smoke. The maximum height of smokeless
ame in millimeters is the smoke point; the higher this is, the tendency of the
fuel to soot formation is lower. For many aircraft fuels, the height of smoke-
less ame is specied to be not less than 20 mm.
Although the smoke point is not a fundamental property of a fuel, it pro-
vides a very satisfactory indication of the propensity to form soot. Naegeli
and Moses [33] used a high-pressure research combustor, operating over a
wide range of burner inlet conditions, to determine the effect of fuel molecu-
lar mixture on soot formation. Their results showed an excellent correlation
of ame radiation with smoke point, as illustrated in Figure 10.18. In this
gure, the average index R is dened as
R =
Flameradiation of test fuel
Flameradiation of r
eeference (JetA)fuel
.
10
0.9
1.1
1.3
1.5
1.7
15
Smoke point, mm
Average index R
20 25
Figure 10.18
Effect of smoke point on average index of ame radiation. (From Naegeli, D.W. and Moses,
C.A., ASME Paper 80-GT-62, Gas Turbine Conference, New Orleans, LA, 1980.)
472 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
It is of interest to note that all the fuels used in these tests had virtually
the same hydrogen content. Thus, a correlation for soot-forming tendency
based on hydrogen content alone could not account for the signicant dif-
ferences that are apparent in Figure 10.18. The differences are attributed by
Naegeli and Moses [33] to variations in the polycyclic aromatic content of the
fuel. The data suggest that the smoke point provides a better index of soot-
forming tendency than a simple hydrogen correlation.
10.5.5.1 Luminometer Number
This index is measured on a modied smoke lamp (ASTM D1740) that includes
thermocouples above and below the ame and a device for measuring ame
radiation in the green-yellow band of the visible spectrum [1]. At a standard-
ized level of radiation, the temperature rise, ΔT, across the ame is measured
for the test fuel and compared with the corresponding values for isooctane (a
parafn) and tetralin (an aromatic), which are assigned values of 100 and 0,
respectively. The luminometer number of the test fuel is then expressed as
Luminometernumber
test fuel tetralin
=
−
100
∆∆
∆
TT
TTT
isooctane tetralin
−∆
.
The concept of luminometer number was introduced in the late 1950s in
the hope that it would provide a more accurate description of the radiation
signature of a fuel than is given by the smoke point. These early expectations
were not fully realized in practice, and the apparatus and test method are no
longer in common use.
10.5.5.2 Smoke Volatility Index
Sometimes used in preference to smoke point, this index is:
SVIsmoke point(mm)
volume percentoftota
=+042.
(llfractions boiling below478 K).
Fuels with high aromatic and naphthalene contents produce more soot.
Since soot particles are harmful to combustor and turbine blades, both the
total aromatic content and the naphthalene content are tightly controlled.
10.5.6 Pressure and Temperature effects
In the selection of fuels for aircraft gas turbines, full account must be taken of
the very wide ranges of pressure and temperature to which the fuel may be sub-
jected. At an altitude of 12 km, the ambient air pressure is only a quarter of the
pressure at sea level. Under extreme conditions, air temperatures at altitude may
Alternative Fuels 473
be as low as 193 K, whereas very high fuel tank temperatures may be attained
when an aircraft on the ground is exposed to tropical sun. Therefore, it is neces-
sary to study the effects of pressure and temperature on fuel properties.
10.5.6.1 Subatmospheric Pressure
As the aircraft gains altitude, the air dissolved in the fuel begins to be
released and may escape through the tank vents, carrying fuel vapor along
with it. Additionally, “vapor lock” could occur. Also, when the fuel vapor
pressure equals the high-altitude ambient pressure, fuel boiling may result.
To minimize boiling, evaporation loss, and vapor lock, most civil aircraft
have pressurized fuel tanks.
10.5.6.2 Low Temperature
Reference has already been made to the emergence of ice crystals at fuel tem-
peratures below 273 K and the formation of wax crystals at the freezing point
of the fuel. The main problem arising with both types of crystal formation
is clogging of the fuel lters. Most commercial aircraft have heaters on their
main fuel lters to melt any ice that is collected. Currently, a FSII approved
for Jet A, Jet A-1 and JP-8 fuels is di-ethylene glycol monomethylether (di-
EGME), which works by combining with any free water that forms and low-
ering mixture freeze point.
10.5.6.3 High Temperature
During ight at high supersonic speed, the bulk liquid fuel in the tank expe-
riences a steady rise in temperature as the kinetic heat imparted to the air-
craft structure is transferred to a continually diminishing volume of fuel.
Other sources of heat include heat exchangers for engine lubricating oil and
hydraulic uid, and more heat is generated in the engine fuel pump. As a con-
sequence, the temperature of the fuel entering the combustion chamber may
reach 473 and 533 K at ight Mach numbers of 2.2 and 3.0, respectively [34].
When the fuel ow is reduced to start the descent, a steep rise in fuel tem-
perature occurs in the fuel system through soakage of heat from the engines.
This leads to fuel oxidation and deposition of gum and other insoluble solid
particles on heat exchanger elements, lters, and fuel nozzles.
In the early 1990s, the U.S. Air Force initiated a program to develop a
fuel with improved thermal stability. It led to the introduction of an addi-
tive package [35] to improve the thermal stability of fuel by about 100°F
(60°C) from 325°F to 425°F. This additive became known as the “ + 100
(plus one hundred)” and the new fuel was called JP-8 + 100. This new fuel
is designed to alleviate the need to develop expensive specialty fuels, such
as JP-7 and JP-TS, for future advanced aircraft and will also decrease main-
tenance costs for current aircraft. The initial JP-8 + 100 additive package is
474 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
based on a detergent/dispersant, packaged with an anti-oxidant and MDA
at a concentration of 256 mg/L. This additive has been shown to produce a
50–95% reduction in deposits with a wide variety of JP-8/Jet A fuels.
10.6 Classification of Liquid Fuels
Table 10.1 lists the typical properties of conventional liquid fuels for gas tur-
bines. Light distillates such as gasoline and kerosine are used for aircraft,
and heavy distillates are used for industrial gas turbines.
TABLe 10.1
Typical Properties of Liquid Fuels
True Distillates Ash-Bearing Fuels
Fuel Type Kerosine No. 2 Distillate
Blended
Residuals and
Crudes
Heavy
Residuals
Relative density at 311 K 0.793 0.82–0.88 0.80–0.92 0.92–1.05
Viscosity at 311 K (m
2
/s)
1.4 × 10
−6
2 × 10
−6
–4 × 10
−6
2 × 10
−6
–10
−4
10
−4
–18 × 10
−4
Viscosity at 311 K (cSt) 1.4 2.0–4.0 2–100 100–1800
Flash point (K) 311–344 339–367 283–367 353–403
Flash point (°F) 100–160 150–200 50–200 175–265
Pour point (K) 228 253–273 263–318 263–308
Pour point (°F)
−50 −10–30
15–110 15–95
Lower specic energy
(MJ/kg)
42.8 42–43 42–43 40–41
Lower specic energy
(Btu/lb)
18,400 18,060–18,490 18,060–18,490 17,200–17,630
Sulfur (% by mass) 0.01–0.1 0.1–0.8 0.2–3 0.5–4
Nitrogen (% by mass) 0.002–0.01 0.005–0.06 0.06–0.2 0.05–0.9
Hydrogen (% by mass) 12.8–14.5 12.2–13.2 12.0–13.2 10–12.5
Ash (fuel as delivered)
(ppm)
1–5 2–50 25–200 100–1000
Ash (inhibited) (ppm) 25–250 100–7000
Trace metal contaminants
(untreated)
Sodium plus potassium
(ppm)
0–0.5 0–1 1–100 1–350
Vanadium (ppm) 0–0.1 0–0.1 0.1–80 5–400
Lead (ppm) 0–0.5 0–1 0–1 0–25
Calcium (ppm) 0–1 0–2 0–10 0–50
Source: Goodger, E. M., Transport Fuels Technology, Landfall Press, Norwich, UK, 2000. With
permission.
Alternative Fuels 475
Gasolines (including naphtha) are excellent fuels of high burning qual-
ity. However, their low viscosity may result in poor lubricity, while their
low ash point and high volatility may require special attention to safety.
Aviation gasolines have a typical distillation range of 300–495 K. They
include Avgas (UK military), JP-4 (US military), and Jet B (US civil) fuels.
The kerosines consist of rened hydrocarbons from the distillation of crude
petroleum, or blends of the latter with suitable cracked products. They
include the normal aviation kerosines, such as Jet A-1, Jet A, and Navy’s
high-ash-point JP-5.
Light to heavy distillates include No. 2-GT gas turbine fuel, No. 2 burner
fuel, diesel oil, and marine gas oil. Diesel fuels have additional requirements
of cetane number, and the primary fuel in this group is the 2-D diesel fuel.
Blended heavy distillates include 3-GT gas turbine fuel, 4-D diesel fuel, and
marine diesel fuel. Their vanadium content is less than 5 ppm, but their rela-
tively high wax content may necessitate heating for pumping and ltering.
These fuels lie between blended heavy distillates and heavy residuals. They
are produced by blending to provide specic maximum sulfur levels. Their
vanadium content is fairly high, normally between 5 and 100 ppm. Blended
residuals include light residual oil and light furnace oil.
Heavy residual fuels represent what is left after all the various distillation
processes are complete. These fuels have high viscosity and high content of
asphaltenes, sulfur, vanadium, and sodium. However, after various treat-
ments and with heating to facilitate pumping and atomization, they can be
utilized in heavy-duty gas turbines.
10.6.1 Aircraft gas Turbine Fuels
The fuel specications for aircraft engines are more strict than those for all
other types of gas turbines. The main requirements due to the airframe,
engine fuel system, and combustion chamber are listed below.
10.6.1.1 Airframe
1. Low re risk. This implies low vapor pressure, low volatility, high
ash point, and high conductivity to minimize the buildup of static
electricity during fueling.
2. High heat content for maximum range and/or payload, i.e., high cal-
oric value on a weight or volume basis, depending on whether the
aircraft is “weight limited” or “volume limited.”
3. High thermal stability, to avoid lter plugging, sticking of control
valves, etc.
4. Low vapor pressure, to minimize evaporation losses at high altitude.
5. High specic heat, to provide effective heat absorption on high-
speed aircraft.
476 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
10.6.1.2 Engine Fuel System
1. Pumpability. That is, the fuel must remain liquid and ow freely to
the atomizer. This is essentially a requirement for low viscosity.
2. Freedom from lter clogging by ice or wax crystals. Ice formation is
eliminated by the use of additives or by fuel heating. Wax formation
is related to gum content and thermal stability.
3. Freedom from vapor locking. This is achieved by the use of low-
vapor-pressure fuels.
4. High lubricity for minimum pump wear. This is obtained through
the presence or addition of highly polar compounds. (See Section
10.3.3.2 dealing with lubricity.)
10.6.1.3 Combustion Chamber
1. Freedom from contaminants that cause blockage of small passages
in fuel nozzles.
2. Good atomization. Atomization quality is most strongly affected by
viscosity.
3. Rapid evaporation. Evaporation rates are dependent on fuel volatil-
ity, and on atomization quality that determines the surface area of
the atomized fuel. Maximum rates of evaporation are achieved with
fuels of low viscosity and high volatility.
4. Minimum carbon formation, low ame radiation and low coke
deposition.
10.6.2 Aircraft Fuel Specifications
Table 10.2 shows the typical aviation fuel properties of past and current air-
craft fuels. The most widely used aviation fuel specications are those issued
by the U.S. ASTM D1655 and U.S. MIL-DTL-83133E, and the UK Ministry of
Defense (MOD) DEF STAN 91-91. A number of other countries have similar
agencies (Canadian General Standards Board CGSB-3.22, Russian GOST 10227,
and International Air Transport Association (IATA) publication). Finally, the
U.S. military maintains specications of JP-8 fuel widely used by the Air
Force and JP-5 used by the Navy. Two kerosine-type commercial jet fuels are
widely used today: Jet A-1 (freeze point –51°C) on international ights and Jet
A (freeze point –45°C) on almost all domestic ights in the United States.
10.6.3 industrial gas Turbine Fuels
For industrial and marine applications, the choice of fuel may be gov-
erned by economy and availability. In practice, this usually means residual
Alternative Fuels 477
TABLe 10.2
Typical Aviation Fuel Properties
Property Avgas JP-4 JP-5 JP-7 JP-8 (Jet A/A-l) RP-1
Approximate formula
a
C
7
H
15
C
8.5
H
17
C
12
H
22
C
12
H
25
C
11
H
21
C
12
H
23
H/C ratio 2.09 2.00 1.92 2.07 1.91 1.95
Boiling range, °F (°C) 115–295 140–460 360–495 370–480 330–510 350–525
(46–145) (60–240) (180–260) (190–250) (165–265) (175–275)
Freeze point, °F (°C)
b
−80 (−62) −57 (−49) −47 (−44) JP-8/Jet A-l: −60 (−51) −55 (−48)
Jet A: −50 (−45)
Flash point, °F (°C)
−10 (−23)
147 (64) 140 (60) 127 (53) 134 (57)
Net heating value 18,700 18,500 18,875 18,550 18,650
Btu/lb (kJ/kg) (43,490) (43,025) (43,895) (43,140) (43,370)
Specic gravity, 16°C (60°F) 0.72 0.76 0.81 0.79 0.81 0.81
Critical temperature, °F (°C) 620 (325) 750 (400) 750 (400) 770 (410) 770 (410)
Critical pressure, psia (atm) 450 (30.5) 290 (19.5) 305 (20.5) 340 (23) 315 (21.5)
Average composition
Aromatics (vol%) 25 10 19 3 18 3
Naphthenes 29 34 32 35 58
Parafns 59 45 65 45 39
Olens 10 2 2 2
Sulfur (ppm) 370 470 60 490 20
Source: Edwards, J.T., AIAA Journal of Propulsion Power, 19, 1089–1107, 2003. With permission.
a
For illustration of average carbon number, not designed to give accurate H/C ratios.
b
Typical.
478 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
oils or surplus gas, although sometimes a compromise is made (e.g., for the
automobile gas turbine, gas oil is preferred to residual fuel, to avoid the more
complex fuel system needed for the heavier fuel). Since industrial fuels must
necessarily be cheap, they are often impure. Also, industrial handling and
storage practices are not up to aeronautical standards, and contamination
of liquid fuel by water, salt, and sand is commonplace. Thus, to make the
heavier fuels suitable for reliable gas turbine use, it is usually necessary to
provide the following treatments [1].
1. Washing to remove trace metals, such as sodium, potassium, and
inorganic particulate matter
2. Inhibition of vanadium in the fuel by the addition of magnesium
compounds
3. Filtration to remove solid oxides, silicates, and other compounds that
could clog fuel pumps, ow dividers, and fuel nozzles
10.7 Classification of Gaseous Fuels
By far the most common gaseous fuel for industrial gas turbines is natural
gas. However, the diminishing supply of natural gas has led to increased
interest in other gaseous fuels, including by-products from industrial pro-
cesses, low-energy gas from coke or oil, and coke-oven gas. All gaseous fuels
are advantageous in terms of high thermal stability and clean (soot- and ash-
free) combustion. Table 10.3 lists the typical properties of common gaseous
fuels.
Natural gas consists mainly of methane, along with minor amounts of
other gaseous hydrocarbons, such as butane, ethane, and propane. Some
natural gases contain up to 15% of nitrogen and carbon dioxide. If the sul-
fur content is negligibly small, the gas is described as “sweet.” However, if
hydrogen sulde is present in signicant amounts, the gas is termed “sour”
and must be puried prior to combustion.
Coal gas is produced by the carbonization of bituminous coals in gas
retorts or coke ovens. Its composition and heating value vary with the type
of coal and the temperature of carbonization. Fuels of high heat content tend
to be rich in hydrogen and methane, with a nitrogen content of less than
11%. However, fuels of low heating value may contain as much as 55% nitro-
gen. The major impurity of concern is sulfur. Sulfur alone is not necessarily
harmful, but if trace metal compounds, particularly sodium and potassium,
are present along with sulfur, then turbine blade corrosion and erosion can
occur.
The coke-oven gas contains substantial amounts of hydrogen and meth-
ane and has a high energy density, in the 20–24 MJ/m
3
range. Scrubbing
Alternative Fuels 479
TABLe 10.3
Properties of Common Gaseous Fuels
Property Natural Gas Coal Gas (Low Btu) Coal Gas (High Btu) Coke-Oven Gas Blast-Furnace Gas Producer Gas
Energy density (Btu/ft
3
) 950–1150 110–165 500–700 525–650 90–100 120–140
Energy density (MJ/m
3
) 35–43 4.1–6.1 19–26 20–24 3.4–3.7 4.5–5.2
Relative density (air = 1.0)
0.58–0.72 0.80–0.92 0.41–0.48 0.40–0.45 0.95–1.05 0.86
Composition (vol%)
Methane 75–97 0.5–4.5 20–35 28–32 1.2
CH–other hydrocarbons 2–20 2–4 2–4
Hydrogen 12–16 40–55 50–55 1–4 16.5
Carbon monoxide 2–32 5–15 5–7 25–30 24
Nitrogen 1–16 30–55 4–11 1–6 55–60 50.8
Carbon dioxide 0.1 0.5–10 2–4 2–3 8–16 7.5