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Technology | Syntroleum Fuels | Specifications |
Diesel fuel is composed of a variety
of blending components of different hydrocarbon types.
Refiners use blending components to balance the key specifications
that produce the optimum diesel fuel for specific applications
and operating environments.
Some of the blending components are straight-run streams
that come directly from the crude oil in the primary distillation
process. Other blending components are hyrdrocracked streams
produced from heavy gas oils, thermally cracked distillates
typically produced from the delayed coking of refinery
residual streams, and light-cycle oils produced from fluid
catalytic cracker (FCC) units. Depending on the sulfur
content of the crude oil, the straight-run and processed
streams may require desulfurization before addition into
the final diesel-fuel blend.
Each blending component or refinery stream has different
characteristics depending on how the fuel is processed
at the refinery. To produce a finished diesel fuel that
meets the specifications for the area where the fuel will
be sold, careful attention to the volumetric blended of
these streams is required. In most cases, refiners carefully
balance the blending components of the final diesel blended
to ensure certain minimum specifications are met while
still maximizing the total volume of blending components.
As a result, individual specifications relative to the
final diesel blend vary significantly.
Several key diesel-fuel specifications contribute to optimum
performance, some benefiting the engine performance and
others the environment. To create a fuel for optimum diesel-engine
performance, refiners focus on the fuel’s cetane
number, density, heating value, low temperature properties
and stability. At the same time, refiners also control
the fuel’s sulfur, aromatic and distillation range
to create an environmentally sound fuel that meets mandated
regulations.
Depending on the application and operating conditions,
refiners carefully balance the key specifications to produce
diesel fuel for optimum performance that meets environmental
requirements.
Diesel fuels ignite upon injection into the combustion
chamber. For optimal performance and power, auto ignition
must occur with the minimum possible delay or ignition
lag.
Thus, refiners and marketers measure the fuel’s
cetane number to determine how readily the fuel ignites
in the engine. Fuels with big cetane numbers ignite more
readily, providing shorter ignition-delay periods.
A minimum cetane number of 40 is specified for diesel
fuels in the United States and 50 in Europe, as well as
most other parts of the world. Cetane numbers higher than
50 provide optimum operation and low particulate matter
(PM) emissions.
Syntroleum’s fuel has a cetane number greater than
74, which exceeds all current specifications for diesel
fuel, even those established by the Worldwide Fuel Charter.
Such a high cetane number allows Syntroleum’s fuel
to be effectively blended with lower-quality diesel fuel
components to produce higher-quality fuels with positive
economic benefits for the refining industry.
Increasing the fuel density increases the power output
of a diesel engine per unit volume of fuel consumed. Yet,
reducing the fuel density reduces nitrogen oxide (NOx)
and PM emissions.
Research shows that reductions in density as small as
5 percent can reduce PM emissions by as much as 20 percent
in older engines; more modern engines show further PM
reductions. Lower NOx emissions are observed with lower
peak pressures and temperatures associated with burning
low-density fuels. Therefore, current diesel-engine design
focuses on how to weigh positive (emission reductions)
factors associated with lower fuel densities between performances
(more power) associated with higher fuel densities.
Sulfur compounds are chemically bound to the various hydrocarbon
components of diesel fuel. The amount of sulfur contained
in diesel fuel is dependent on the quality of the crude
oil from which it was refined and the components used
in the final blend. Cracked diesel-fuel components tend
to have higher levels of more complex sulfur compounds
called dibenzothiphenes. Most sulfur compounds in diesel
blending streams can be reduced by treatment with hydrogen.
However, to do so, increases the cost of producing the
diesel and lowers the overall yields.
During combustion, any sulfur compounds contained in the
diesel fuel are converted and expelled as sulfur dioxide
– a regulated pollutant in most areas of the world.
Research shows that reducing the level of sulfur in diesel
fuel has a direct effect on reducing the amount of sulfur
dioxide emissions. Therefore, a maximum sulfur level is
specified for all diesel fuels.
The United States, Europe and some Asian countries limit
sulfur emissions for on-road diesel to 500 parts per million
(ppm). In less developed areas of the world the maximum
sulfur ranges form 2000 ppm to above 5000 ppm.
Synroleum’s fuel, which is produced from natural
gas, removes all traces of sulfur during the feed preparation
process, so Syntroleum diesel fuel contains no measurable
sulfur. Since it meets all other American Society for
Testing and Materials (ASTM) D-975 diesel fuel standards,
Syntroleum’s fuel is the cleanest “stand-alone”
diesel fuel available today.
In addition, the absence of sulfur allows Syntroleum fuel
to be blended with other diesel fuel components to produce
high quality, ultra-low sulfur diesel fuels with positive
economic benefits to the refinery.
Aromatic streams are added in various amounts to diesel
fuel to volumetrically increase yields up to the point
that their addition maximizes one or more of the diesel-fuel
specifications. Aromatics increase the density of the
fuel (and thus its heating value) and improve cold flow
properties. Yet, aromatics decrease the cetane number
to the diesel fuel and have been identified as contributors
of NOx and PM emissions, especially ploynuclear-aromatics.
The United States limits the total aromatics level to
35 percent by volume in most states and 10 percent by
volume for California Air Resources Board regulations.
Much of the debate on future fuel quality focuses on the
need to further reduce the level of aromatics in diesel.
Higher concentrations of aromatic compounds in diesel
fuel increase flame temperatures during combustion, contributing
to NOx emissions. Decreasing total aromatic content in
diesel fuel from 30 to 10 percent reduces nitrogen dioxide
emissions by about 3 to 5 percent.
Syntroleum’s diesel fuel has no detectable aromatics
and as a result has a very high cetane number (greater
than 74). The absence of aromatics in Syntroleum diesel
produces lower hydrocarbons, HC, NOx and PM emissions
compared to standard diesel fuel.
The amount of paraffin, iso-paraffin and aromatic content
of diesel fuel affects the low-temperature properties
of the fuel, in turn affecting the operating performance
of the diesel engine
Yet, the precipitation of paraffin crystals and unsubstitued
aromatics in other diesel fuels will at low temperatures
clog the fuel filter and interrupt the fuel supply to
the diesel engine. Depending on the fuel properties, the
start of paraffin precipitation can be as high as 0 degrees
Celsius (C). Consequently, winter diesel fuels are specially
blended or treated with additives that inhibit the precipitation
of paraffin crystals to ensure problem-free operation
in cold weather. The added cold-temperature flow improvers
limit paraffin crystal growth, so the crystals remain
small enough to pass through the filter pores. As a result,
filterability is extended to lower temperatures. With
additives, today’s winter diesels are available
with cold-resistance guarantees to at least -22 degrees
C.
Refiners use additional measures to treat winter diesels,
filter heating and the addition of petroleum products
to the diesel fuel. The addition of regular gasoline to
diesel can also delay precipitation. However, the very
low cetane number of most gasolines, impairs the ignition
quality and considerably reduces the flash point of the
diesel fuel.
Studies prove that diesel fuels containing higher percentage
of iso-paraffin compounds result in a diesel fuel with
much better, low-temperature properties. As a result of
the manufacturing process, Syntroleum diesel contains
a high percentage of iso-paraffins, such that no additives
are required to improve the low-temperature properties.
This fuel performs well in the engine and eliminates the
need for costly additives.
The cloud point of a fuel is also affected by its distillation
characteristics. The maximum 90-percent distillation point
is limited to 315 degrees C in Canada, a country with
extremely cold winters. In sub-tropical areas it is limited
to 379 degrees C.
Testing has demonstrated that the cloud point, pour point,
and cold filter plugging point of Syntroleum diesel all
surpass the specifications and guidelines set forth in
American Society for Testing and Materials (ASTM) D-975
and basic diesel standard, EN 590.
The heating value or heat of combustion of diesel fuel
is the measured amount of available energy content from
a known quantity of fuel. The heating value is directly
proportional to the fuel density. Diesel fuels with higher
values result in higher power and increased fuel economy.
Two factors can be altered to change the heating value
of a fuel. These factors include (1) increasing the aromatics
content and (2) changing the distillation profile by raising
the initial boiling point and/or the end point. However,
these factors are limited by other fuel properties. Changing
the aromatic content is restricted by the minimum cetane-number
requirement, and adjustments to the distillation profile
are limited by the 90-percent distillation maximum-temperature
requirement.
The volatility characteristics of diesel fuel are expressed
in terms of the temperature at which successive portions
of the fuel are distilled from a sample of the fuel under
controlled heating in a standardized apparatus. One of
the most widely used methods is the American Society for
Testing and Materials D-86.
The distillation or boiling range of fuel depends on the
fuel’s chemical composition and, therefore, influences
other properties such as viscosity, flash point, auto
ignition temperature, cetane number and density.
The boiling range influences parameters that are important
for the operating behavior of the diesel fuel. Changing
the boiling range usually affects more than one fuel property.
For example, extension of the boiling range towards lower
temperatures leads to a fuel that has better low temperature
properties such as pour and cloud point, but the cetane
number is reduced. When the boiling range is moved toward
higher temperatures, refiners can include heavy compounds
in their final diesel blend, thereby increasing their
yield of diesel fuel. However, the heavier compounds in
this fuel could produce increased soot and cause injection
nozzle choking.
The back-end volatility of diesel fuel, expressed as the
90-percent or 95-percent distillation recovery temperature
(T90/T95), has some effect on emissions. When the volatility
is reduced, a slight increase in HC an CO emissions and
a small decrease in the NO2 emission is observed. Reducing
the volatility does not have an effect on PM emissions.
Given the small nature of these effects, diesel fuel volatility
is a minor factor in determining emission performance.
The storage and thermal stability of diesel fuel affect
injector deposits and clogged fuel filters from the formation
of gums and insoluble organic particulates.
In an effort to meet stricter emission standards and improve
combustion characteristics, thermal stresses are placed
on diesel fuel. This can cause the fuel to degrade and
form insoluble materials. Thus thermal stability plays
a larger role as engine manufacturers continue to design
fuel injectors that use diesel fuel as a coolant for high-pressure
injection systems.
As diesel fuels with very low levels of sulfur have been
shown to be more stable due to the sulfur removing process,
hydrotreating the fuel is an effective means to remove
sulfur and eliminate the building blocks of insoluble
organic particulates.
Syntroleum diesel is a fully saturated paraffinic hydrocarbon
with no measurable olefins, or aromatics and does not
exhibit any of the stability problems associated with
refined diesel fuels.
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