In 1998 the Big
Three automakers pledged to put clean-burning cars on the road by 2001,
beating the Clean Air Act Amendments mandate by five years. They declared
that their gasoline-burning engines would emit 50% fewer nitrogen oxides
(NOx) and 70% fewer hydrocarbons, thanks to advanced catalytic
converters. Shortly after this low-emission vehicle concept was announced,
the U.S. Environmental Protection Agency (EPA) revealed its concern
that these reductions might not be achievable if high-sulfur gasoline
and diesel fuel continue to be used. The reason: Studies involving EPA
and the automobile and oil industries showed that fuel sulfur atoms
can bond with reactive sites on the catalyst surface, preventing catalyzed
reactions needed to break down NOx and hydrocarbons.
Because of its
concern that high-sulfur gasoline could decrease the effectiveness of
advanced catalytic converters, EPA mandates that by 2005 the nation's
largest oil refiners must reduce the sulfur content of gasoline by 90%,
from an average of 300 parts per million (ppm) to 30 ppm. EPA also calls
for an equally large reduction in diesel fuel's sulfur levels (to 15
ppm) by mid-2006.
Diesel fuel is
used mostly in trucks, but demand for it is expected to rise. Because
they are 40% more efficient than gasoline-powered vehicles and produce
less carbon dioxide, light-duty diesel vehicles, including sedans and
sport utility vehicles, are being developed for the Partnership for
a New Generation of Vehicles (PNGV). By combining the diesel engine
with an electric motor in a hybrid car, the PNGV goal of 80 miles per
gallon for a family-sized sedan might be met.
Advanced emissions
control devices are being designed for these diesel vehicles, to reduce
their emissions of NOx and particulate matter (PM) to regulated
levels. According to the EPA, the combination of advanced emissions
control devices and ultra-low-sulfur fuel in both gasoline and diesel
engines could greatly reduce air pollution, which has been blamed for
respiratory health problems, crop damage, acid rain, and low visibility.
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Photo
by Warren Gretz, Courtesy of DOE/NREL, modified by Jane Parrott.
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Enter ORNL
ORNL researchers
have conducted several studies related to the EPA decision on fuel sulfur.
For example, in a study for the Department of Energy (DOE), Jerry Hadder
of ORNL's Energy Division used a refinery model to determine the impacts
of low-sulfur gasoline production on petroleum refining operations and
investments. "We estimated that the total refining industry investment
to enable production of low-sulfur gasoline will be $5 to $10 billion,"
Hadder says. "Partly as a result of our study, DOE expressed concerns
about the high degree of technical uncertainty surrounding refinery
product quality and the potential consequences of rising costs for the
refining industry and gasoline consumers." EPA set a schedule for introducing
low-sulfur gasoline that was in line with DOE's recommendation to facilitate
a smoother, more certain implementation. Based on ORNL's refining cost
estimates, DOE recommended to EPA that ultra-low-sulfur diesel fuel
be phased in according to the market demand for light-duty diesel vehicles
with advanced emissions controls. If this recommendation is followed,
diesel fuel production and distribution costs could be reduced by $20
billion over a 10-year phase-in.
ORNL researchers
in the Chemical Technology and Engineering Technology divisions have
been involved in several projects concerning sulfur levels in automotive
fuels. In one effort, properties of sulfur-containing compounds in fuel
samples from refineries and gas stations are being measured. In another,
the effects of fuel sulfur on diesel emission controls are being determined
experimentally. In a research project on biodesulfurization, genetically
engineered microbes and a solvent have been tested to determine how
well they remove sulfur from diesel fuel.
Chemical
Properties of Sulfur-Containing Fuels
At DOE's new Physical
Properties Research Facility at ORNL (see New
User Facility Has Old (But Excellent) Instruments), vintage
instruments are being used for a DOE Fossil Energy Program project to
help refineries meet EPA goals for sulfur in fuels. Chemical Technology
Division researchers Bill Steele, Debra Bostick, Catherine Mattus, and
Tom Schmidt, director of the new user facility, are measuring a range
of thermochemical and thermophysical properties of sulfur-containing
compounds shown to exist in gasoline and diesel samples obtained from
refineries and gas stations nationwide. Results from these measurements
will aid in the design of either improvements in existing refining technology
or in the conceptual testing of new processes (e.g., advanced catalysts
and absorbers) for sulfur removal.
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Debra
Bostick measures properties of sulfur-containing compounds in
gasoline and diesel samples obtained from refineries.
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Petroleum refineries
extract hydrogen from natural gas to produce plastics (e.g., polyethylene)
and add the hydrogen to crude oil to make gasoline. To remove sulfur
during gasoline production, hydrogen is added under pressure along with
a catalyst to produce hydrogen sulfide, which is removed as a gas. To
produce the maximum amount of gasoline from crude oil, the process of
adding hydrogen must be run in a narrow temperature and pressure range.
The catalyst used effectively removes most sulfur-containing compounds.
"To get rid of the most
difficult sulfur compounds, the refinery must operate at a lower temperature
or higher hydrogen pressure, which will raise its fuel production costs,"
Schmidt says. "At a lower temperature, the reaction will go slower,
so more refining capacity must be added and more crude oil must be used
to produce the normal yield of gasoline. At a higher pressure, the wall
thickness of the refinery units must be doubled or tripled, which is
a difficult and expensive engineering problem."
ORNL is providing
petroleum companies such as Chevron, Phillips, Shell, and Texaco with
information on the properties of thiophene, benzyl thiophene, and dibenzyl
thiophene (model sulfur-containing compounds in crude oil) to aid in
the design of an improved refinery process for removing a higher fraction
of the sulfur in the oil. By using precise data from ORNL, the companies
will be less likely to overdesign their refinery equipment and raise
their costs more than necessary.
ORNL data help refineries
determine the temperatures and pressures required to reduce sulfur levels
in gasoline to the EPA limit. "We can tell them what percentage of sulfur
compounds will be removed by operation at a specific temperature or
pressure," Bill Steele says. "We might inform them that if they change
the temperature or pressure by a certain amount, the best they can get
is 50 ppm sulfur, which is short of the goal of 30 ppm."
Fuel Sulfur and Emissions
Control
What are the effects of
fuel sulfur on advanced emissions control systems for diesel engines?
Researchers in ORNL's Engineering Technology Division—John Storey,
Brian West, Norberto Domingo, Scott Sluder, Ralph McGill, and Ron Graves—are
directing and conducting research to help answer this question. This
work is part of the Diesel Emission Control-Sulfur Effects (DECSE) program,
a joint effort of DOE's Office of Transportation Technology, two national
laboratories, and manufacturers of diesel engines and emissions control
systems. Some of the work is being conducted at the Advanced Propulsion
Technology Center, another new DOE user facility at ORNL that will eventually
be located at the National Transportation Research Center.
One purpose of
the DECSE program is to conduct research to determine the impact of
fuel sulfur on exhaust emissions control systems designed to lower tail
pipe emissions of NOx and particulate matter (PM) from diesel
engines. The ultimate goal of the program is to determine the types
of diesel fuel, engines, and exhaust emissions control systems that,
working together, will enable diesel-powered vehicles to meet the stricter
new regulations.
EPA's new Tier
2 light-duty emissions standards will phase in from 2004 to 2009. Under
Tier 2, vehicle emissions are categorized into "bins," with NOx
emissions ranging from 0 to 0.2 grams per mile (g/mi), with a fleet
average of 0.07 g/mi. The PM standards for these bins range from 0 to
0.02 g/mi. Unlike current Tier 1 emissions standards, the same Tier
2 standards apply to all passenger vehicles regardless of engine type,
fuel type, or vehicle mass. Under the current Tier 1 standards, diesels
and larger vehicles have less stringent standards than do gasoline-powered
passenger cars. Current NOx emissions standards range from
0.4 to 1.5 g/mi, and PM emissions standards range from 0.08 to 0.12
g/mi.
"Sulfur from fuel
or lube oil can have detrimental effects on emissions and emission controls,"
West says. "Sulfur can poison catalysts such as NOx adsorbers
and contribute significantly to sulfate PM emissions. While lower sulfur
fuel is needed, the maximum tolerable level is still unknown. If we
can design emission controls that are less sensitive to sulfur, then
diesel engines have the potential for emissions that are on par with
gasoline engines, without having to mandate a zero-sulfur fuel." Current
diesel fuel is regulated to below 500 ppm sulfur, but EPA recently mandated
a 15 ppm sulfur cap on diesel fuel by 2006.
Using prototype
emissions control equipment, ORNL researchers recently demonstrated
that diesel vehicles have the potential to meet EPA Tier 2 emission
requirements. The researchers achieved 0.05 g/mi NOx, and
0.005 g/mi PM on the federal test procedure by fitting a light-duty
diesel vehicle with a prototype NOx adsorber and catalyzed
diesel particulate filter. These advanced emissions controls reduced
the vehicle's NOx and PM emissions by more than 90% from
its baseline levels when 3 ppm sulfur fuel was used. However, after
only 3000 miles of equivalent aging with 30 ppm sulfur fuel, the NOx
adsorber was severely poisoned, resulting in only 80% NOx
reduction. Under Tier 2, vehicles must be in emissions compliance for
120,000 to 150,000 miles, so more work is needed to solve this problem.
Fortunately, the diesel particulate filter appears to be less sensitive
to sulfur, although lower-sulfur fuel is still needed to prevent production
of sulfate PM.
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The
effects of low-sulfur fuel on a diesel engine's emissions control
system (top) were studied by ORNL researchers using this Mercedes
diesel car on a University of Tennessee chassis dynamometer (bottom).
ORNL's Scott Sluder is in the car.
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The durability
of these emissions reduction systems has not yet been shown. But the
results indicate that efficient diesel engines operating on ultra-low-sulfur
fuel have the potential to power future cars and trucks, while emitting
no more pollution than their gasoline counterparts.
"Some people believe
that the new Tier 2 regulations will 'outlaw' the diesel engine in the
light-duty sector," West says. "Although much more research is needed
to make these systems commercially viable, the results of our laboratory
experiments are promising, suggesting that diesel engines can have emissions
comparable to those of gasoline engines, but with much better fuel economy."
Sulfur-Removing
Biotech Process
A novel chemical-biological
process for removing sulfur from gasoline and diesel fuel has been developed
by Abhijeet Borole, Catherine Cheng, Eric Kaufman, and Brian H. Davison,
all of ORNL's Chemical Technology Division, in collaboration with Petro
Star, a small refinery in Anchorage, Alaska, and Travis/Peterson Consulting,
Inc., also of Anchorage. Petro Star, which funded ORNL's biodesulfurization
research, has developed a chemical process that adds oxygen to sulfur
compounds in the diesel fuel to allow their selective removal using
a solvent. As a result of this oxidation-extraction process, a desulfurized
fuel and a high-sulfur extract are produced. The extract is about 10%
of the original fuel and has recoverable hydrocarbons, the fuel's energy
source. During the research project, this extract was sent to the ORNL
group, whose job was to use bacteria to remove the sulfur from the extract
and recover its fuel value.
"Biological processes
may not be fast enough alone to give commercially economic rates of
desulfurization," Borole says. "So, Petro Star's chemical process is
being studied as an initial step because it is believed that oxidized
sulfur species are more soluble in water where the bacteria reside,
making it easier for the microbes to act on the sulfur. Our studies
have indicated that this is the case."
For the research
project, the ORNL group used genetically engineered Pseudomonas
bacteria obtained from a Spanish molecular biology organization (Consejo
Superior de Investigaciones Cientificas of Madrid). Each microbe carries
an inserted gene from another bacterium that breaks the carbon-sulfur
bond by adding two oxygen atoms, converting each sulfur species to a
sulfate (SO4-2). The Pseudomonas bacteria
were selected to carry this gene for two reasons. They have a high tolerance
of the oil in which the sulfur compounds and hydrocarbons are present.
In addition, because they can potentially produce biosurfactants, they
can increase the contact between the oil and the water in which they
reside. This increased mixing reduces mass transfer limitations and
speeds up the biological oxidation reaction by which dibenzothiophene
sulfone (an organic sulfur compound in diesel fuel) is converted to
sulfate and sulfur-free hydrocarbons. In addition, the newly formed
sulfate is soluble in the water phase, which naturally separates from
the oil. The hydrocarbon-rich oil that remains can be used as a fuel.
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Abhijeet
Borole checks the operation of a bioreactor that uses suspended
bacteria to remove sulfur from petroleum feedstocks.
(Photo by Curtis Boles.)
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"We measured the
amount of energy the bacteria require for biodesulfurization, and we
studied how well and how fast they remove sulfur from the diesel extract,"
Borole says. "It was not as fast as we'd like. We need to do more research
to determine the best way for the bacteria to get more energy for desulfurization
through metabolism of carbon sources. We need to find ways to increase
the reaction rate to speed up the biological process for sulfur separation."
Borole notes that biodesulfurization
is a potentially economical process because it can be performed at room
temperature. Conventional refinery techniques for sulfur removal, which
add hydrogen to crude oil, gasoline, and diesel to produce hydrogen
sulfide gas, require temperatures as high as 300°C. "Biodesulfurization,"
Borole says, "may ultimately cost less because it will use much less
energy."
The ORNL group is now
doing research on modifying bacterial enzymes to make them more stable
in an oil environment so they can react more readily with polyaromatic
hydrocarbon (PAH) compounds, and later sulfur-containing PAHs, to upgrade
crude oil. For example, the appropriate enzyme could make oil flow more
easily at the production site or refinery. "We are proposing to use
directed evolution to produce an enzyme that is not only stable in oil
but also highly versatile," Borole says. "It would attack hundreds of
sulfur compounds, not just a few dozen, in crude oil, gasoline, and
diesel fuel."
The high concentration
of sulfur in transportation fuels and the ability of sulfur to poison
catalysts used in emissions controls are threats to the environment.
It is hoped that research at ORNL and elsewhere will find economic ways
to solve the sulfur problem.
Beginning
of Article
Related Web
sites
Advanced Propulsion Technology Center
ORNL's
Energy Division
ORNL's
Chemical Technology Division
ORNL's
Engineering Technology Division
ORNL's
Physical Properties Research Facility
DOE's Fossil
Energy Program
DOE's Diesel
Emission Control-Sulfur Effects (DECSE) Program
DOE's National Petroleum
Technology Office
UT Center for Transportation
Research
DOE's Office
of Energy Efficiency and Renewable Energy
Partnership
for a New Generation of Vehicles
