Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines
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98 Fuels and Lubes: Chemistry and Treatment
asphaltene Content
Asphaltenes are complex, highly aromatic compounds with a high molecular
weight that usually contain sulphur, nitrogen and oxygen as well as metals
such as vanadium, nickel and iron. A high asphaltene content indicates that a
fuel may be difficult to ignite and will burn slowly, and may also contribute to
deposit formation in the combustion chamber and exhaust system, especially at
low engine loads.
If the HFO is unstable, the asphaltenes will precipitate from the fuel and
block filters and/or cause deposits in the fuel system, as well as lead to exces-
sive sludge formation in the fuel separator. Furthermore, when operating on
HFOs with high asphaltene content, good performance of the lubricating oil
should be emphasized. It is important that the lube oil is able to bind the com-
bustion residues containing asphaltenes from blow-by gases entering the crank-
case and thus prevent deposit formation on the engine component surfaces.
sulphur Content
This has no influence on combustion but high-sulphur levels can be dangerous
because of acid formation, mentioned earlier in this chapter. In recent years
there has been a tendency to equate sulphur content with cylinder liner wear,
but opinions differ on this matter (see also Chapter 3). Sulphur in the fuel may
cause cold corrosion and corrosive wear, especially during low load opera-
tion. It also contributes to deposit formation in the exhaust system, normally
together with vanadium and/or sodium in the form of sulphates. These deposits
can also cause high-temperature corrosion.
Sulphur content has come under scrutiny in recent years, with the IMO and
European Union proposing or setting limits to reduce emissions of SOx in the
exhaust gases. Among the options are to refine low-sulphur crude oils, equip
refineries with desulphurization units, fit shipboard scrubbers or trade with sul-
phur emissions (see also Chapter 3).
water Content
This is the amount of water in a given sample of the oil and is usually deter-
mined by centrifuging or distillation.
sediment Content
All HFOs contain a certain amount of sediment, which can be organic or
inorganic. The total sediment content (TSP analysis) describes the fuel’s
cleanliness (presence of sand, dust, dirt, catalyst fines and other solid/inor-
ganic contaminants); stability (resistance to breakdown and precipitation of
asphaltenes): and compatibility with another fuel quality.
Cloud point
The cloud point of an oil is the temperature at which crystallization of paraffin
wax begins to be observed when the oil is being cooled down.
pour point
This is the lowest temperature at which an oil remains fluid and thus is impor-
tant to know for onboard handling purposes. An alternative is the solidifying
point or the highest temperature at which the oil remains solid. It usually lies
some 3°C below the pour point. According to a major enginebuilder, the lowest
admissible temperature of the fuel should be about 5–10°C above the pour point
to secure easy pumping. The whole fuel handling system, including tanks and
pipes, should be heated to a temperature at least 10–15°C above the pour point.
Flash point
The flash point is defined as the lowest temperature at which an oil gives off
combustible vapours, or the point at which an air/oil vapour mixture can be
ignited by a flame or spark. A low flash point will not affect combustion, but the
fuel can be dangerous to handle and store; this is especially so if the pour point
is high, making it necessary to heat the HFO close to its flash point. Special
explosion-proof equipment and fuel separators can be used in extreme cases.
A high vapour pressure (often indicating a low flash point) can also cause
cavitation and gas pockets in the fuel pipes; these can be avoided by using an ele-
vated pressure in the fuel handling system. (It should be noted that some insur-
ance companies demand the use of fuels with a flash point higher than 60°C.)
specic gravity
This is normally expressed in kilograms per cubic metre or grams per cubic
centimetre at 15°C. As the density of the fuel depends upon the density of the
individual components, fuels can have identical densities but widely varying
individual component densities. Apart from being an indicator of the ‘heavi-
ness’ of a fuel, when measured by a hydrometer, the specific gravity can be
used to calculate the quantity of fuel by weight in a tank of given dimensions.
Typical values for standard fuels are given in Table 4.2.
Density mainly affects the fuel separation. Traditional centrifugal separa-
tors can remove water and, to some extent, solid particles from HFOs with
densities up to 991 kg/m
3
at 15°C. The latest separator designs can clean heavy
fuels with densities up to 1010 kg/m
3
at 15°C. It is important to ensure that the
correct separation flow rate and temperature are applied to achieve an efficient
reduction of water, catalyst fines, sodium and sediment from the fuel.
The following is the fuel oil specification of a leading low-speed diesel
engine manufacturer. The properties are considered the worst in each case that
can be burnt in the particular engine.
Density at 15°C maximum 991 kg/m
3
Kinetic Viscosity
at 50°C maximum 700 mm
2
/s (cst)
at 100°C maximum 55 mm
2
/s (cst)
properties of fuel oil 99
100 Fuels and Lubes: Chemistry and Treatment
Carbon residue (CCR) maximum 22 m/m (%)
sulphur maximum 5.0 m/m (%)
ash content maximum 0.2 m/m (%)
vanadium maximum 600 mg/kg (ppm)
sodium maximum 100 mg/kg (ppm)
aluminium maximum 30 mg/kg (ppm)
silicon maximum 50 mg/kg (ppm)
sediment (shF) maximum 0.10 m/m (%)
water content maximum 1.0 v/v (%)
Flash point minimum 60°C
pour point maximum 30°C
Combustion equations
Table 4.3 gives a list of the symbols designating the atoms and molecules of
the elements and compounds in liquid fuels. For the atomic weights given, the
suffix denotes the number of atoms accepted as constituting one molecule of
the substance.
Tables 4.4–4.6 summarize the equations of the chemical reactions taking
place during combustion. Nitrogen compounds are ignored. The weight of
oxygen needed is evaluated, from the appropriate equation, for every element
in the fuel, and the results are added together. The chemically correct quantity
of air required to burn the fuel exactly to produce carbon dioxide and water
vapour and unburnt nitrogen is approximately 15:1, termed the stoichiometric
ratio. The atmosphere may be assumed to contain 23.15 per cent of oxygen by
weight and 20.96 per cent by volume. At standard atmospheric pressure, 1 kg
of air occupies a volume of 0.83 m
3
at 20°C and 0.77 m
3
at 0°C. An average
diesel fuel should comprise in percentages, carbon 86–87; hydrogen 11.0–13.5;
sulphur 0.5–2; oxygen and nitrogen 0.5–1.0. The greatest energy output in
the combustion of fuels is from the hydrogen content, and fuels having a high
hydrogen:carbon ratio generally liberate greater quantities of heat. Typical
examples are given in Table 4.7.
Table 4.2 Typical values for standard fuels
Fuel sg (g/cm
3
) Flash point (°C) Lower Cv (kJ/kg)
gas oil 0.82–0.86 65–85 44 000–45 000
Diesel oil 0.85 65 44 000
heavy fuel 0.9–0.99 65 40 000–42 000
200 s Redwood no. 1–3500 s Redwood no. 1
Low-sulphur Fuels
Environmental concerns over damage to ecosystems resulting from sulphur
emissions have stimulated the creation of SOx emission control areas (SECAs).
An IMO framework proposed a global fuel sulphur content cap of 4.5 per cent
and a 1.5 per cent sulphur limit in certain SECAs, the latter posing commer-
cial and technical challenges for marine fuel producers, bunker suppliers, ship
operators and machinery designers. Future controls call for significantly lower
fuel sulphur content limits (see Chapter 3).
Low-sulphur fuel oil is generally produced by refining low-sulphur crudes.
Refineries may need to sweeten their crude slates or perhaps just tap off certain
sweeter grades from their internal production runs in order to yield the desired
low-sulphur fuels. Suppliers have the challenge of storing and marketing addi-
tional grades of fuel. Enginebuilders and lube oil formulators have to match
the total base number (TBN) (see Lubricating Oils, p. 117) of cylinder and
system lubricants to the new low-sulphur fuel grades. Summarizing the impact
of a low-sulphur fuel regime on ship owners and onboard staff, ExxonMobil
Marine Fuels highlights the following:
l More has to be paid for the fuel burned in SECAs.
l Procedures need to be established for masters and chief engineers to
know when and where they must burn specific fuels.
properties of fuel oil 101
Table 4.3 elements and compounds in liquid fuels and products of
combustion
name nature atomic
symbol
atomic
weight
molecular
symbol
molecular
weight
Carbon element C 12.00 C 12.00
hydrogen element h 1.008 h
2
2.016
oxygen element o 16.00 o
2
32.00
nitrogen element n 14.01 n
2
28.02
sulphur element s 32.07 s
2
64.14
Carbon
monoxide
Compound — —
Co 28.00
Carbon dioxide Compound — —
Co
2
44.00
sulphur dioxide Compound — — so
2
64.07
sulphur trioxide Compound — — so
3
80.07
sulphurous acid Compound — — h
2
so
3
82.086
sulphuric acid Compound — — h
2
so
4
98.086
water Compound — — h
2
o 18.016
102 Fuels and Lubes: Chemistry and Treatment
Table 4.4 Combustion equations
nature of Reaction equation gravimetric meaning
Carbon burned to carbon
dioxide
C o
2
Co
2
12 kg C 32 kg o 44 kg
Co
2
Carbon burned to carbon
monoxide
2C o
2
2(Co) 24 kg C 32 kg o 56 kg
Co
Carbon monoxide burned to
carbon dioxide
2(Co) o
2
2(Co
2
) 56 kg Co 32 kg
o 88 kg Co
2
hydrogen oxidized to steam 2h
2
o
2
2(h
2
o) 4 kg h 32 kg o 36 kg
steam (or water)
sulphur burned to sulphur
dioxide
s
2
2o
2
2(so
2
) 64 kg s 64 kg
o 128 kg so
2
sulphur dioxide burned to
sulphurous acid
so
2
h
2
o h
2
so
3
64 kg so
2
18 kg
h
2
o 82 kg h
2
so
3
sulphur dioxide burned to
sulphur trioxide
o
2
2(so
2
) 2(so
3
) 32 kg o 128 kg
so
2
160 kg so
3
sulphur trioxide and water
to form sulphuric acid
so
3
h
2
o h
2
so
4
80 kg so
3
18 kg
h
2
o 98 kg h
2
so
4
Table 4.5 volumetric meaning of equations
nature of Reaction volumetric Result
Carbon burned to carbon dioxide 1 vol C 1 vol o 1 vol Co
2
Carbon burned to carbon monoxide 2 vols C 1 vol o 2 vols Co
Carbon monoxide burned to carbon
dioxide
2 vols Co 1 vol o 2 vols Co
2
hydrogen oxidized to steam 2 vols h 1 vol o 2 vols h
2
o (steam
not water)
sulphur burned to sulphur dioxide 1 vol s 2 vols o 2 vols so
2
sulphur dioxide burned to sulphurous
acid
—
sulphur dioxide burned to sulphur
trioxide
—
sulphur trioxide and water to form
sulphuric acid
—
properties of fuel oil 103
Table 4.6 heat evolved by combustion
nature of Reaction Thermo-Chemical
equation
heat evolved per Kilogram
of element burned
(kcal/kg)
Carbon burned to carbon
dioxide
C o
2
Co
2
96 900 8 080
Carbon burned to carbon
monoxide
2C o
2
2(Co) 58 900 2 450
Carbon monoxide burned
to carbon dioxide
2(Co) o
2
2(Co
2
)
315 000
5 630 (per kg Co burned)
hydrogen oxidized to
steam
2h
2
o
2
2(h
2
o)
136 000
33 900
a
29 000
b
sulphur burned to sulphur
dioxide
s
2
2o
2
2(so
2
)
144 000
2 260
sulphur dioxide burned to
sulphurous acid
— —
sulphur dioxide burned to
sulphur trioxide
— —
sulphur trioxide and
water to form sulphuric
acid
— —
a
value if the latent heat of the steam formed is to be included
b
value if the latent heat of the steam formed is to be excluded
Table 4.7 hydrogen: carbon ratios of typical fuels
Fuel h:C Ratio Caloric value (kJ/kg)
methane 4:1 55 500
ethane 3:1 51 900
propane 2.7:1 50 400
Kerosene 1.9:1 43 300
heavy fuel 1.5:1 42 500
benzene 1:1 42 300
Coal 0.8:1 33 800
104 Fuels and Lubes: Chemistry and Treatment
l Better records must be kept, showing exactly what fuel is burned when
and where, and providing proof to authorities.
l Fuel must be tested on bunkering to ensure that it meets sulphur content
limits.
l Ship owners must ensure that operation teams and bunker buyers are
aware of the fleet’s needs for low-sulphur fuel in specific areas; and,
when chartering out a ship, the owner will need to negotiate a modifica-
tion to the charter party to ensure that the charterer is required to supply
low-sulphur fuel if trading to SECAs.
l Owners will need to consider modifying their existing tonnage to permit
better segregation of bunkers; and shipbuilders and designers will have to
create ships with much better means of segregating fuels and for switching
from one fuel grade to another. (Some newbuildings are already specified
with separate low-sulphur fuel tanks, and this is likely to become a stand-
ard design feature.) No modifications to engines are dictated, however, and
for ships trading exclusively in SECAs, it may be possible to use lube oil
in the main engine with a lower TBN and hence a lower cost.
In addition, HFOs with a lower sulphur content have a slightly higher
energy value, underwriting a slightly improved specific fuel consumption.
Engine and exhaust system components will wear less and last longer since
sulphuric acid-based corrosion is reduced; and scrubbing exhaust gases for
inert gas generation is made simpler and cheaper.
Fuel additives
While it is common practice in automotive applications to include additives in
distillate fuel oils to enhance performance aspects, the use of additives in the
marine sector to improve combustion or secure other benefits is not yet wide-
spread. Many ship operators in the past have succumbed to the promises of
disreputable suppliers, the disappointing results leading to a general mistrust
by shipping of fuel additives. The potential advantages for operators from
using additives in both marine distillate and residual fuels are becoming more
appreciated, however, particularly as solutions for targeting specific problems.
Nevertheless, the cost of an additive on top of the fuel price can only be jus-
tified when the additive contributes to more reliable engine operation and a
reduction in the ship’s total operational costs.
The combustion phase is one of the areas where marine residual fuel addi-
tives can be used to advantage. Using an iron-based component such as fer-
rocene as an oxidation catalyst to accelerate main combustion and enhance
complete combustion of residual fuel oils is a proven technology. Chevron’s
Marine Combustion Improver is also iron based, but its chemistry is different
from conventional ferrocene technology and reportedly offers improved envi-
ronmental, health and safety properties. Available in liquid form for blend-
ing into the HFO, the combustion improver is formulated to enhance engine
operational efficiency and reliability by minimizing post-combustion fouling,
maintaining the cleanliness and efficiency of the exhaust system by controlling
and removing soot build-up and fostering a reduction in smoke emissions.
Iron-based combustion improvers are offered by Infineum to reduce exhaust
tract deposits and ensure that turbochargers maintain their efficiency, as well as
to reduce the risk and consequences of exhaust tract fires. Magnesium-based
additives can counter corrosion that may result from using high vanadium/
sodium content residual fuels. Biocide additives can minimize and control
the proliferation of fuel-borne bacteria and algae, together with the fuel filter-
plugging waste products that can result from the presence of such species.
biofuels
The production and use of biofuels has grown significantly in recent years, the
focus mainly on bioethanol blended into fossil motor gasoline (petrol) or used
directly, and biodiesel or fatty acid methyl ester diesel blended into fossil die-
sel or also used directly. While biofuels in land-based transport are becoming
commonplace, usage at sea is still under investigation but promises some tech-
nical merits. Marine engines are generally of lower speed and more tolerant to
burning alternative fuels than smaller, high speed automotive engines.
The fuel standard for marine applications, ISO 8217, currently relates
solely to fossil fuels and has no provision for biofuels. There are, however, a
number of available national standards for biodiesel in the automotive sector,
such as the European EN 14214. The marine bunker supply market would need
to work closely with regulators to develop appropriate international marine
standards.
Lloyd’s Register explains that for merchant shipping the way in which
biodiesels are supplied to the vessel must also be considered. Either the biodie-
sel is delivered pre-mixed to the required blend or the biofuel and diesel are
supplied separately for mixing onboard. Biodiesel blended prior to delivery to
the ship is affected by shelf life. Fuel ageing and oxidation can lead to high
acid number, high viscosity and the formation of gums and sediments. Fuel
management will therefore become more challenging. Supplies of biofuel and
diesel for onboard mixing enable the operator to dictate the exact blend of
biofuel depending on the conditions; but that would require new technology
installed onboard and extra complexity for the crew to handle.
Some operational problems can occur from using biodiesel, related to
the cold filter plugging point (CFPP) at which the fuel starts to turn to gel.
Considered to be the indication of low-temperature operability, the CFPP is in
a range between 0°C and 15°C for different types of biodiesels and can cause
problems with filter clogging, which can only be overcome by carefully moni-
toring the fuel tank temperature. Such a problem could well affect ships oper-
ating in cold climates, where additional tank heating coils might be required to
prevent it happening. Lloyd’s Register advises that careful choice of the biodie-
sel type can ensure that the problem is avoided.
properties of fuel oil 105
106 Fuels and Lubes: Chemistry and Treatment
Another challenge in using biodiesels is corrosion caused in the fuel-injection
and treatment systems as biodiesels are hygroscopic and maintain 1200–
1500 ppm water. It is essential therefore that the fuel is thoroughly conditioned
prior to injection, and also important that the fuel acid number is monitored to
ensure no rancid, acidic fuel is introduced into the injection system. A typi-
cal fuel-treatment system should incorporate separators to ensure water is
removed, as well as heaters at various stages to ensure the fuel is at the correct
temperature for introducing in the engine.
FueL oiL TReaTmenT
Low-, medium- and high-speed diesel engines are designed to burn marine
diesel oil (ISO 8217, class DMB or BS 6843, class DMB) while low-speed
and many medium-speed engines can also accept commercially available HFO
with a viscosity up to 700 cSt (7000 s Redwood No. 1).
Centrifugal separation is widely accepted as the most effective means of
cleaning fuel oils before injection into the engine. Fuel density and viscosity
are key parameters for efficient separation: the greater the difference in density
between the particle and the fuel, the higher the separation efficiency. Because
both density and viscosity vary with temperature, however, separation tempera-
ture is the critical operating parameter. Separation efficiency is also a function
of the centrifuge’s fuel flow rate: the higher the flow rate, the more particles
are left in the oil and therefore the lower the efficiency. The flow rate is usually
constant and is based on the highest viscosity. Separation efficiency is further
influenced by differences in the characteristics of the fuel oil, such as polarity,
stability and contamination.
A correctly dimensioned separator system will ensure that catalytic fines
in the fuel are reduced to an acceptable level for injection and efficient engine
operation. In order to ensure effective and sufficient cleaning of the HFO
(removing water and solid contaminants), the fuel oil specific gravity at 15°C
should be below 0.991. Higher densities—up to 1.010—can be accepted if
modern centrifugal separators are installed, such as the systems available from
Alfa-Laval (Alcap), Westfalia (Unitrol) and Mitsubishi (E-Hidens II).
Alfa-Laval’s Alcap system comprises an FOPX separator, a WT 200 water
transducer and ancillary equipment including an EPC-400 control unit (Figure
4.3). Changes in water content are constantly monitored by the transducer,
which is connected to the clean oil outlet of the separator and linked to the con-
trol unit. Water, separated sludge and solid particles accumulate in the sludge
space at the separator bowl periphery. When separated sludge or water forces
the water towards the disc stack, minute traces of water start to escape with the
cleaned oil and are instantly detected by the transducer in the clean oil outlet.
The control unit reacts by triggering a sludge discharge or by allowing water
to drain off through a separate drain valve, thereby re-establishing optimum
separation efficiency.
A number of benefits are cited for the Alcap system over conventional fuel
cleaning systems. Continuous optimum separation efficiency is achieved since
water and sludge in the bowl are never permitted to enter the disc stack. Only
one Alcap FOPX separator is required, a single-stage unit achieving the same
and often higher separation efficiency than a traditional two-stage configura-
tion featuring two separators (a purifier followed by a clarifier).
Centrifugal separators can be operated in series or in parallel, the method
securing the best separation efficiency in theory and reality being the subject of
discussion over the years. With the introduction of modern centrifuges without
gravity discs, however, Alfa Laval now recommends that all available units in
an installation are operated in parallel because they operate correctly without
the need for constant adjustments, which earlier created problems. Operating
centrifuges in parallel rather than in series enables the throughput of each unit
to be reduced by a half, ensuring the longest possible residence time for the
fuel in the centrifuges and thus increasing separation efficiency.
Manual cleaning separators are not recommended for attended or unat-
tended machinery spaces. Centrifugal separators should be self-cleaning, either
with total discharge or with partial discharge. The nominal capacity of the sep-
arator should be as recommended by the supplier. Normally, two separators are
installed for HFO treatment, each with sufficient capacity to comply with the
fuel throughput recommendation.
The schematic representation of a HFO treatment system approved by
Wärtsilä is shown in Figure 4.4, the company highlighting the following points
in designing a particular installation:
Gravitational settling of water and sediment in modern fuel oils is an
extremely slow process due to the small density difference between the oil and
the sediment. To achieve the best settling results, the surface area of the settling
tank should be as large as possible, with low depth, because the settling process is
a function of the fuel surface area of the tank, the viscosity and density difference.
Fuel oil treatment 107
IPC/EPC
Transducer
Cleaned oil
Water
Oil
FOPX
Sludge and
water
MARST I
FiguRe 4.3 schematic diagram of alfa Laval’s alcap fuel-treatment system based on
its FopX centrifugal separator