Dia•Com
is a leading international provider of innovative, cost-effective
molded diaphragm solutions critical to the operation of essential
systems and equipment in industrial, automotive, aerospace,
medical instrumentation, and food and water processing applications.
The company's reputation for excellence is based on superior
quality in the design, manufacture and application of its
high-performance, state-of-the-art, fabric-reinforced and
homogeneous elastomeric diaphragm seals.
| Gas-to-liquid (GTL) technology has been used for many years to synthesize hydrocarbons from natural gas. Recently, interest has grown in the production of GTL fuels and their emission reduction benefits. Several companies produce or have produced GTL fuels, including Shell, Sasol, ExxonMobil, and others.1
Many studies have examined the impact of GTL fuel on exhaust emissions from light- and heavy-duty vehicles and engines (summarized in Reference 2). In a majority of cases, GTL fuel produced a reduction in regulated emissions (hydrocarbons [HC], oxides of nitrogen [NOx], carbon monoxide [CO], particulate matter [PM]) compared to conventional diesel fuel.
Much of the emission data reported have come from
short-term studies, where the engine/vehicle has been
switched to GTL fuel from conventional diesel fuel for the
purposes of collecting emission test results. Upon
concluding the tests, the engine/vehicle is then switched
back to conventional diesel fuel. Thus, the long-term
effect of GTL fuel on engine systems has not been
adequately quantified.
FUEL PROPERTY TESTING
Previous studies of GTL diesel fuel do not always list
complete fuel properties or test methods.2 The fuel used
in this study was tested to determine physical and
chemical properties. In addition, researchers performed
elastomer compatibility testing to characterize the impact
of GTL fuel on fuel system elastomers.
FUEL PRODUCTION TECHNOLOGY – Shell Global
Solutions (US) Inc. provided the fuel that was used for
fuel property and emission testing, and on-road use.
The SMDS (Shell Middle Distillate Synthesis) process is
well documented, so only a brief description is offered
here.3 The process is illustrated in Figure 1. The process
was developed at Shell Research & Technology Centre
Amsterdam and is comprised essentially of three stages:
Manufacture of synthesis gas (hydrogen + carbon
monoxide–with a H2:CO ratio of approximately two) from
natural gas by non-catalytic auto-thermal partial
oxidation using, for example, the Shell Gasification
Process.
Wax synthesis from CO + H2 by Heavy Paraffin
Synthesis (HPS), followed by flash distillation to
separate light ends (e.g., liquefied petroleum gas).
Cracking of wax to distillates by Heavy Paraffin
Conversion (HPC), where the boiling range and quality
of the products can be adjusted to produce either
kerosene or atmospheric gas oil (diesel).
A recent modification to this process, designated as
SMDS-2 offers an improved HPS (Heavy Paraffin
Synthesis) catalyst, which will enable the manufacturers
to increase production capacity considerably. In addition,
adjusting the severity in the hydrocracking/isomerization
(HPC) stage allows control of the n- to iso- paraffin ratio
in the final product.
Figure 1. Schematic illustration of Shell SMDS process.
OO 2 2
Natural
Gas
CO + 2H 2
SSyynnththeessisis
H 2 O
– CH 2 –
HHyyddrrooggeennaattioionn
Naptha
Kerosene
Gasoil HHyyddrrooccrraacckkiningg
ASU
SGP HPS
HPC
CH4
Lube oil
feedstock
Syngas
manufacture Syngas
manufacture
Solvents
Waxes
FUEL PROPERTY TEST RESULTS - The fuel was
tested for a wide range of properties, such as
composition, energy content, cold flow properties, and
elastomer compatibility. All fuel property testing was
performed at Southwest Research Institute in San
Antonio, TX.
Except for elastomer compatibility, which will be
discussed separately, the test results are compiled in
Table 1. Where applicable, the ASTM D975 specification
is also listed.
The fuel composition was tested through elemental
testing and hydrocarbon determination. GTL fuel is
composed of carbon and hydrogen. The fuel H/C ratio is
about 2.1, about 16% greater than conventional diesel
fuel. The high H/C ratio is due to the near zero aromatic
content of the GTL diesel fuel.4 As with most GTL fuels,
the Shell GTL has near zero sulfur content.
The very low aromatic content and/or the high H/C ratio
of diesel fuel have been shown to reduce NOx and PM
emissions in previous studies.5,6,7 Thus, testing with GTL
fuel is likely to result in reductions in NOx and PM
emissions.
Fuel sulfur reductions also result in PM emission
reductions, though with diminishing returns as sulfur
content becomes very low.6 In newer technology
engines, the near zero sulfur content of GTL fuel may
prove more beneficial by enabling sulfur sensitive
emission control devices.
GTL fuels have reduced densities compared to
conventional diesel fuels,2 which have been shown to
reduce the PM emission in older technology engines.6,7
Low density fuels, such as GTL fuel, may alter the fuel
mass flow rates.7 Previous GTL fuel studies have not
noted adverse effects on engine operation as a result of
the lower fuel density.9,10,11
The cetane number of GTL fuels is most often reported
as >74, much higher than conventional diesel fuels.12
GTL fuel is composed almost wholly of paraffins. Nparaffins
are known to have very high cetane numbers,
while iso-paraffins have lower cetane numbers.13
Increasing cetane number has been linked to a decrease
or no change in NOx emissions. The effect appears to be
depend on engine model year, with a less prominent
effect on newer technology engines.5,6,7 |
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