Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines
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88 Fuels and Lubes: Chemistry and Treatment
Originally, crude oil was distilled in an atmospheric distillation process
to derive the required grades in the required quantity. The vacuum distillation
process was developed to enable further refinement of atmospheric process res-
idue. Both atmospheric distillation and vacuum distillation are refinery proc-
esses based on the physical separation of crude oil components into distillate
fuels. Because these processes do not chemically alter the structure of the oil,
the original crude oil could be reproduced by mixing together all of the output
streams and components in the appropriate proportions.
These two simple distillation processes, however, do not produce the
amount of distillate oil products consistent with increasing worldwide demand.
Other, more complex refinery processes—known as secondary conversion
processes—are therefore required to satisfy demand for distillate fuel products,
taking some of the product streams from the distillation processes and altering
the chemical structure of the oil. This reduces the amount of residual fuel oil
and increases the amount of distillate fuel (Figure 4.1).
Among the secondary conversion processes are ‘cracking’ processes that
break the long molecules of the heavier fuel fractions into shorter molecules,
which can be more easily processed into fuel oil products in demand. There are
two basic types of cracking processes: thermal cracking and catalytic cracking.
Thermal cracking uses temperature, pressure and time to provoke a chemi-
cal reaction that alters the structure of the oil and can be applied to both distil-
late and residual fuels. Typical thermal cracking processes include visbreaking,
which significantly lowers the viscosity of a heavy residue to enable blending
with other fuels, and coking, which destroys the fuel oil, producing only distil-
late fuel and coke (a form of carbon).
Catalytic cracking processes also alter the chemical composition of residual
fuel oil, using a catalyst rather than high pressure (HP) to break down complex
Good Quality HFO
Long Residue
Crude
oil
Catalytic
Cracking
Atmospheric
Distillation
Vaccum
Distillation
Short Residue
Viscosity
Breaking
Very Heavy
Residue &
Visbroken
Residue
HFO
Heavy cycle
oil
Light cycle oil
Diluents, like Cutter Stock
FiguRe 4.1 simplied conguration of a typical complex fuel renery
hydrocarbons into simpler molecules. The catalyst is a substance that assists
the process of the chemical reaction but does not change its own properties.
FCC, the most common of these processes, can convert gas oil and resid-
ual oil into high-octane gasoline and diesel fuel. In this case the catalyst—in
the form of fine particles approximately 20–100 m in diameter—circulates
between a reactor and a regenerator in a fluidized bed process. Large catalytic
crackers contain around 500 tonnes of expensive catalyst.
Continual recycling of the catalyst causes its structure to break up, prima-
rily due to attrition, and some of the resulting small particles are carried over
into the fractionator. Although refiners attempt to minimize the loss of catalyst
from the catalytic cracking process, some carryover of the expensive catalytic
fines from the unit is inevitable.
The product drawn off from the bottom of the fractionator is called slurry
oil, also known as decant oil or FCC bottoms. Highly aromatic, the slurry oil
has a high density (generally around 1000 kg/m
3
at 15°C) and a low viscosity
(approximately 30–60 cSt at 50°C). It is an ideal blending component for resid-
ual fuels due to its high aromaticity, which contributes stability to the finished
fuel. It is from this refinery source that catalytic fines enter residual fuel.
pRobLems wiTh heavy FueLs
The problems are reflected in the effects on the engine in terms of wear and
tear and corrosion resulting from harmful components in the fuel. It is the duty
of the ship’s engineer to be aware of these harmful constituents, their effect on
the operation of the engines and the solutions available to counter the harmful
properties. The problems of present and future heavy residual fuels can be cat-
egorized as follows:
1. Storage and handling
2. Combustion quality and burnability
3. Contaminants, resulting in corrosion and/or damage to engine compo-
nents, for example: burnt-out exhaust valves.
storage problems
The problems of storage in tanks of bunker fuel result from the build-up of
sludge, leading to difficulties in handling. The reason for the increase in sludge
build-up is that heavy fuels are generally blended from a cracked heavy resid-
ual using a lighter cutter stock, resulting in a problem of incompatibility. This
occurs when the asphaltene or high molecular weight compound suspended
in the fuel is precipitated by the addition of the cutter stock or other diluents.
The sludge which settles in the bunker tanks or finds its way to the fuel lines
tends to overload the fuel separators with a resultant loss of burnable fuel, and
perhaps problems with fuel injectors and wear of the engine through abrasive
particles.
To minimize the problems of sludging, the ship operator has a number of
options. He or she may ask the fuel supplier to perform stability checks on
problems with heavy fuels 89
90 Fuels and Lubes: Chemistry and Treatment
the fuel being provided. Bunkers of different origins should be kept segregated
wherever possible and water contamination kept to a minimum. Proper opera-
tion of the settling tanks and fuel-treatment plant is essential to prevent sludge
from entering the engine itself. A detergent-type chemical additive can be used
to reduce the formation of sludge in the bunker tanks (Table 4.1).
water in the Fuel
Water, whether fresh or saline, has always been a problem because it finds its
way into the fuel during transport and onboard the ship from various sources
including condensation in the fuel storage tanks. Free water can seriously dam-
age fuel-injection equipment, cause poor combustion and lead to excessive cyl-
inder liner wear. If it is sea water, it contains sodium, which will contribute to
corrosion when combined with vanadium and sulphur during combustion.
Water can normally be removed from the bunkers by proper operation of
separators and properly designed settling and daily service tanks. However,
where the specific gravity of the fuel is the same or greater than water, removal
of water is difficult—or indeed not possible—and for this reason the maximum
specific gravity of fuel supplied for ship’s bunkers has generally been set at 0.99.
burnability
The problems that are related to poor or incomplete combustion are many and
complex and can vary with individual engines and even cylinders. The most
significant problem, however, is the fouling of fuel injectors, exhaust ports
and passages, and the turbocharger gas side due to failure to burn the fuel
completely. Fuels that are blends of cracked residual are much higher in aro-
matics and have a high carbon-to-hydrogen ratio, which means they do not
burn as well.
Other problems arising from these heavier fuels are engine knocking, after-
burning, uneven burning, variation in ignition delay and a steeper ignition
pressure gradient. These factors contribute to increased fatigue of engine com-
ponents, excessive thermal loading, increased exhaust emissions, and critical
piston ring and liner wear. The long-term effects on the engine are a significant
increase in fuel consumption and component damage. The greatest fouling
and deposit build-up will occur when the engine is operated at reduced or very
low loads.
The fuel qualities used to indicate a fuel’s burnability are Conradson car-
bon residue, asphaltenes, cetane value and carbon-to-hydrogen ratio.
To ensure proper fuel atomization, effective use of the centrifugal separa-
tors, settling tanks and filters is essential, and the correct fuel viscosity must be
maintained by heating, adequate injection pressure and correct injection tim-
ing. The maintenance of correct running temperatures according to the engine
manufacturer’s recommendations is also important, particularly at low loads.
Additives which employ a reactive combustion catalyst can also be used to
reduce the products of incomplete combustion.
problems with heavy fuels 91
Table 4.1 eects of heavy fuels
properties present ho Future ho eect on engine
viscosity (Red 1 at 37°C
heating temperature)
3500 5200
increased fuel heating
required
pumping 50 65
Centrifuging 95 98
injection 110–120 115–130
Density at 15°C 0.98 0.99
water elimination
becomes more dicult
pour point (°C) 30 30
noxious element Carbon
residue (%) 6–12
15–22
Fouling risk of
components/increased
combustion delay
asphaltenes (%) 4–8 10–13 hard asphaltene
producing hard particles
soft asphaltene giving
sticky deposits at low
output
increased combustion
delay with defective
combustion and
pressure gradient
increases
Cetane number 30–55 25–40
high-pressure
gradients and starting
problems
sulphur (%) 2–4 5 wear of components due
to corrosion below dew
point of sulphuric acid
(about 150°C)
vanadium (ppm) 100–400 120–500 burning of exhaust valves
at about 500°C
sodium (ppm) 18–25 35–80 Lower temperature in case
of high na content
silicon and aluminium
(CCF slurries)
wear of liners, piston
grooves, rings, fuel pump
and injectors
92 Fuels and Lubes: Chemistry and Treatment
high-Temperature Corrosion
Vanadium is the major fuel constituent influencing high-temperature corro-
sion. It cannot be removed in the pre-treatment process, and it combines with
sodium and sulphur during the combustion process to form eutectic compounds
with melting points as low as 530°C. Such molten compounds are very corro-
sive and attack the protective oxide layers on steel, exposing it to corrosion.
Exhaust valves and piston crowns are very susceptible to high-temperature
corrosion. One severe form creates mineral ash deposits on the valve seats,
which, with constant pounding, cause dents and eventually a small channel
through which the hot gases can pass. The compounds become heated and then
attack the metal of the valve seat.
As well as their capacity for creating corrosion, vanadium, sulphur and
sodium deposit out during combustion to foul the engine components and,
being abrasive, lead to increased liner and ring wear. The main defence against
high-temperature corrosion has been to reduce the running temperatures of
engine components, particularly exhaust valves, to levels below that at which
the vanadium compounds are melted. Intensively cooled cylinder covers, lin-
ers, and valves, as well as rotators fitted to valves, have considerably reduced
these problems. Special corrosion-resistant coatings such as stellite and plasma
materials have also been applied to valves.
Low-Temperature Corrosion
Sulphur is generally the cause of low-temperature corrosion. In the combustion
process, the sulphur in the fuel combines with oxygen to form sulphur dioxide
(SO
2
). Some of the sulphur dioxide further combines to form sulphur trioxide
(SO
3
). The sulphur trioxide formed during combustion reacts with moisture to
form sulphuric acid vapours, and where the metal temperatures are below the
acid dew point (160°C), these vapours condense as sulphuric acid, resulting in
corrosion.
The obvious method of reducing this problem is to maintain temperatures
in the engine above the acid dew point through good distribution and control of
the cooling water. There is always the danger that an increase in temperatures
to avoid low-temperature corrosion may lead to increased high-temperature
corrosion. Attack on cylinder liners and piston rings as a result of high-sulphur
content fuels has been effectively reduced by controlled temperature of the cyl-
inder liner walls and alkaline cylinder lubricating oils.
abrasive impurities
The normal abrasive impurities in fuel are ash and sediment compounds. Solid
metals, such as sodium, nickel, vanadium, calcium and silica, can result in
significant wear to fuel-injection equipment, cylinder liners, piston rings and
ring grooves. A comparatively new contaminant—metallic catalyst fines com-
prising very hard and abrasive alumina and silica particles—is cause for much
concern. These particles carry over in the catalytic cracking refinery process
and remain suspended in the residual bottom fuel for extended periods. It has
been known for brand new fuel pumps to be worn out in a matter of days to the
point where an engine fails to start through insufficient injection pressure, as a
result of catalyst fines in the fuel.
Catalytic fines formed through the break-up of the catalyst as it is recycled
through a cat-cracker plant (see p. 88) are variable in size, with a range from
sub-micron to approximately 30 m and occasionally up to 100 m. While con-
sidered to be spherical particles, this is not necessarily the case.
The composition of catfines varies, depending on the type of feedstock and
on whether the main unit of the cracker is intended to optimize gasoline (light)
or diesel (heavier) grades. Exact catalyst compositions are not quoted today,
but all of them contain some form of synthetic crystalline zeolite material. The
zeolite is dispersed in a matrix comprising an active matrix, clay and binder.
Both the zeolite and the active matrix are forms of aluminium oxide and sili-
con oxide. The ratio of silicon to aluminium in the catalyst varies considerably,
ranging from 0.65:1 to 2:1.
In the mid-1990s, Det Norske Veritas determined that the annual world-
wide average ratio of silicon to aluminium was 1.64:1, based on analyses of
fuel oil samples. Recent analyses indicate that the worldwide average ratio was
around 1.2:1 and that this is likely to fall because modern catalysts tend to con-
tain a higher concentration of aluminium.
The only effective method of combating abrasive particles is correct fuel
pre-treatment. Separator manufacturers recommend that the separators should
be operated in series (a purifier followed by a clarifier) at throughputs as low
as 20 per cent of the rated value (see Fuel Oil Treatment later in this chapter).
pRopeRTies oF FueL oiL
The quality of a fuel oil is generally determined by a number of specific param-
eters or proportions of metals or impurities in a given sample of the particular
fuel. Such parameters include viscosity, specific gravity, flash point, Conradson
carbon, asphaltenes content, sulphur content, water content, vanadium content
and sodium content. Two parameters of traditional importance have been the
calorific value and viscosity. Viscosity, once the best pointer to a fuel’s quality
or degree of heaviness, is now considered as being only partially a major qual-
ity criterion because of the possible effects of constituents of a fuel.
Caloric value
The calorific value or heat of combustion of a fuel oil is a measure of the amount
of heat released during complete combustion of a unit mass of the fuel, expressed
in kilojoules per kilogram. Calorific value is usually determined by a calorim-
eter, but a theoretical value can be calculated from the following equation:
Calorific value (kcal/kg of fuel)
C
8100 34000
0
8
H
100
properties of fuel oil 93
94 Fuels and Lubes: Chemistry and Treatment
where C, H and O are percentages of these elements in 1 kg of fuel. Carbon
yields 8080 kcal/kg when completely burned, hydrogen 34 000 kcal/kg; oxy-
gen is assumed to be already attached to its proportion of hydrogen: that is, an
amount of hydrogen equal to one-eighth the weight of the oxygen is nullified.
The sulphur compounds are assumed to have their combustion heat nullified by
the oxy-nitrogen ones.
Another variant of the formula is as follows:
Calorific value (kcal/kg of fuel) H 7500 33800
0
8
C
C, H, O being fractions of a kilogram per kilogram of fuel.
To convert the kilocalories per kilogram values to SI units as widely used:
1 4 187kcal/kg kJ/kg= .
For example a fuel of 10 500 kcal/kg in SI units:
10500 kcal/kg kJ/kg kJ/kg= × =10500 4 187 44000.
The calorific value as determined by a bomb calorimeter is the gross or
‘higher’ value, which includes the latent heat of water vapour formed by the
combustion of the hydrogen. The net or ‘lower’ calorific value is that obtained
from subtracting this latent heat. The difference between the gross and net values
is usually about 600–700 kcal/kg, depending upon the hydrogen percentage.
A formula that can be used to calculate the net value is:
CV CV 25 ( ) kJ/kg
n g
= − +f w
where CV
n
net calorific value in kilojoules per kilogram
CV
g
gross calorific value in kilojoules per kilogram
f water content of the fuel in percentage by weight
w water percentage by weight generated by combustion of the hydrogen
in the fuel
Typical gross calorific values for different fuels are as follows:
Diesel oil 10 750 kcal/kg
gas oil 10 900 kcal/kg
boiler fuel 10 300 kcal/kg
When test data are not available, the specific gravity of a fuel will provide
an approximate guide to its calorific value:
specic gravity at 15°C 0.85 0.87 0.91 0.93
gross Cv (kcal/kg) 10 900 10 800 10 700 10 500
viscosity
The viscosity of an oil is a measure of its resistance to flow, which decreases
rapidly with increase in temperature. Heating is necessary to thin the heavy
fuels of high viscosity in current common use and ease their handling (Figure
4.2). Viscosity is not actually a direct measure of HFO quality, but it deter-
mines the complexity of the shipboard fuel heating and handling system. HFO
must be heated to reach the correct injection viscosity to ensure optimized
combustion and engine performance. Furthermore, HFOs are typically classed
and marketed according to their viscosity.
Over the years different units have been used for viscosity (Engler Degrees,
Saybolt Universal Seconds, Saybolt Furol Seconds and Redwood No. 1 Seconds).
The majority of marine fuels today are internationally traded on the basis of vis-
cosity measured in centistokes (1 cSt 1 mm
2
/s). When quoting a viscosity, it
must be accompanied by the temperature at which it is determined. Accepted
temperatures for the viscosity determination of marine fuels are 40°C for distil-
late fuels and 100°C for residuals. If a fuel contains an appreciable amount of
properties of fuel oil 95
FiguRe 4.2 a typical temperature/viscosity chart for heavy fuels
96 Fuels and Lubes: Chemistry and Treatment
water, testing for viscosity determination at 100°C becomes impossible and
therefore many laboratories may routinely test fuels at a lower temperature
(80–90°C) and calculate the viscosity at 100°C.
Due to the variability of residual fuel composition, Det Norske Veritas
Petroleum Services (DNVPS) warns that calculations of viscosity measured at
one temperature to that at another temperature may not be accurate. Calculated
results should therefore be treated with caution.
A complication noted by DNVPS is that fuel is traded on the basis of vis-
cosity at 50°C, but the two main standards (ISO/CIMAC) give limits at 100°C.
By reference to ISO 8217 a buyer could order a 180 cSt fuel at 50°C to comply
with ISO RME 25. If the fuel was tested at 100°C and the viscosity result was
25 cSt at 100°C, this could be equivalent (by calculation) to 225 cSt at 50°C.
Clearly, DNVPS explains, the fuel in this case would meet the ISO require-
ments for viscosity but would be 45 cSt above the 180 cSt ordered viscosity.
This anomaly could be easily removed if trading of marine fuels moved to vis-
cosity at 100°C.
The maximum admissible viscosity of the fuel that can be used in an instal-
lation depends on the onboard heating and fuel preparation outfit. As a guide,
the necessary pre-heating temperature for a given nominal viscosity can be
taken from the viscosity/temperature chart in the engine instruction manual.
Wärtsilä’s recommended viscosity range for its Sulzer low-speed engines is
13–17 cSt or 60–75 s Redwood No. 1. Details of the fuel handling/treatment
requirements of enginebuilders are provided on pp. 106–110.
Cetane number
The cetane number of a fuel is a measure of the ignition quality of the oil under
the conditions in a diesel engine. The higher the cetane number, the shorter the
time between fuel-injection and rapid pressure rise. The ignition quality of heavy
fuel oils varies considerably. Low ignition quality may cause trouble at engine
start-up and during low load operation, particularly if the engine is not suffi-
ciently pre-heated; it may also result in a long ignition delay and also cause a fast
pressure rise and very high maximum pressures. Deposits on the piston top and
exhaust valves, in the exhaust system and on the turbine nozzle ring and blades
can also be expected, with turbocharger fouling leading to reduced turbocharger
efficiency and higher thermal load on the engine. Modern diesel engines with a
higher compression ratio and optimized combustion process, however, are not so
sensitive to the ignition properties of HFOs compared with earlier designs.
A more usable pointer of ignition quality is the diesel index, expressed as:
Diesel index number
G A
100
where G specific gravity at 60°F on the API scale and A aniline point in
°F, which is the lowest temperature at which equal parts by volume of freshly
distilled aniline and the fuel oil are fully miscible.
Calculated Carbon aromaticity index (CCai)
The ignition quality of residual fuels is more difficult to predict than distillate
fuels because they comprise blends of many different components, but the igni-
tion quality of such fuels may be ranked by determining the calculated carbon
aromaticity index from density and viscosity measurements. It should be noted,
however, that the ignition performance of residual fuels is mainly related to
engine design and operational factors. Formulae and nomograms for CCAI
determination are published by major fuel suppliers and enginebuilders.
CCAI can be considered a rough tool for estimating the ignition properties
of a HFO, but more sophisticated devices, such as the fuel ignition analyser
(FIA), have been introduced in recent years for more accurate determination.
Conradson Carbon value
This is the measure of the percentage of carbon residue after evaporation of the fuel
in a closed space under control. The Conradson or coke value is a measure of the
carbon-forming propensity and thus an indication of the tendency to deposit car-
bon on fuel-injection nozzles and in the combustion chamber and exhaust system,
especially at low engine loads. The Ramsbottom method has largely replaced the
Conradson method of carbon residue testing, but it gives roughly the same results.
ash Content
The ash content is a measure of inorganic impurities in the fuel (typically sand,
nickel, aluminium, silicon, sodium and vanadium), which can cause different
kinds of problems. The most troublesome inorganic impurities are sodium and
vanadium, which form a mixture of sodium sulphate and vanadium pentoxide,
which melt and adhere to engine components, particularly exhaust valves.
Oxides of vanadium and sodium, mainly sodium vanadyl vanadates, are
formed during combustion and will mix or react with oxides and vanadates of
other ash constituents (e.g. nickel, calcium, silicon and sulphur). The sticking
temperature of the mixture may cause a deposit to be formed on the exhaust
valve, in the exhaust system or in the turbocharger. Highly corrosive in molten
salt, this deposit destroys the protective oxide layer (e.g. on an exhaust valve)
and leads to hot corrosion and a burnt valve.
Deposits and hot corrosion in the turbocharger, especially on the nozzle
ring and turbine blades, will reduce turbocharger efficiency. The gas exchange
will also be disturbed; less air flows through the engine and hence the thermal
load on the engine increases. Deposit formation rises with increased tempera-
tures and engine outputs.
Several measures are dictated to avoid these problems when running on
high-ash HFOs. It is important, for example, to operate an efficient fuel separa-
tion system, regularly clean the turbocharger and ensure strict quality control
of the bunkered fuel (to check that the amounts of ash and dangerous ash con-
stituents remain low). It is also essential to ensure clean air filters and charge
air coolers by regular cleaning based on pressure drop monitoring.
properties of fuel oil 97