In 1995 three buildings of Malden Mills, the maker of Polartec climate control fabrics, burned down in a major industrial fire. But instead of shutting down operations and dismissing the employees of this major manufacturer in Lawrence, Massachusetts, Aaron Feuerstein, president and owner of Malden Mills, decided to rebuild the mill buildings and re-employ the workers. For this decision in the age of corporate "rightsizing" and relocation of plants to countries where labor is cheaper, he received international acclaim and awards for his humanitarian treatment of employees. President Clinton cited him for corporate responsibility.
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The
Malden Mills power plant in Lawrence, Massachusetts, has a Solar
Turbines gas turbine engine (Centaur 50S SoLoNOx Engine) that
has been improved partly because of materials research results
from ORNL.
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In his decision to build a highly efficient
cogeneration power plant to replace the inefficient one in which the
fire started, Feuerstein faced a problem. Under Environmental Protection
Agency (EPA) rules, a new power plant must meet stringent limits on
the exhaust gas concentrations of pollutants such as nitrogen oxides
(NOx) and carbon monoxide (CO). To help solve this problem, the Department
of Energy (DOE) decided to make Malden Mills a demonstration site for
a power plant featuring a gas turbine, the type of engine that in different
forms flies airline passengers across oceans and drives M-1 tanks across
battlefields. The gas turbine is evolving into the workhorse of electricity
production in the 21st century.
The old Malden Mills power plant was
to be replaced with a cogeneration plant that combines a natural gas-fired
turbine with a steam recovery boiler for the production of electricity
and heat. This combined heat and power (CHP) plant uses fuel 25 to 40%
more efficiently than do today's coal-fired plants and emits about 40%
less carbon dioxide, a greenhouse gas.
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This
type of gas turbine is used at Malden Mills to generate power.
It meets environmental regulations partly because of improvements
in the combustor liner resulting from ORNL materials research. |
A Centaur 50 gas turbine engine built by Solar
Turbines, Inc. was to be used in the power plant. The problem was that,
although this engine is highly efficient, it would discharge too much
NOx, violating EPA regulations. So, DOE provided funding
for technical support to help upgrade the Solar Turbines engine so that
it would meet emissions specifications. DOE asked material scientists
in ORNL's Metals and Ceramics (M&C) Division to help determine which
ceramic composite and protective coating would work best in liners of
the combustion chamber (combustor) for use in the rebuilt plant's gas
turbine.
After an intensive 24-month effort involving
Solar Turbines, ORNL, Pratt and Whitney, DOE's Argonne National Laboratory,
B. F. Goodrich, and Honeywell Advanced Composites, a turbine outfitted
with ceramic composite combustor liners was put into operation in August
1999. In a November dedication ceremony at Malden Mills, Secretary of
Energy Bill Richardson declared that this power generation unit has
the "lowest emissions of any industrialized heat and electric combined
facility in the United States—and possibly the world."
According to
DOE, natural gas turbines are expected to make up more than 80% of the
power-generating capacity to be added in the United States over the
next 10 to 15 years. Of the more than 200 new power plant projects announced
recently in the United States, 96% plan to use natural gas and most
will employ gas turbines. The global turbine market is also promising,
with estimates of worldwide power generation acquisitions approaching
$100 billion over the next 10 years.
How a Land-Based Gas Turbine Works
A turbine is a rotary engine that uses a continuous stream of fluid
to turn a shaft that drives machinery, such as the rotor of an electric
generator. In a steam turbine, high-pressure steam is forced through
turbine wheels to rotate a shaft driving a generator. Fossil fuel power
plants and nuclear power plants, which heat water to make steam, use
steam turbines to drive large electricity generators.
A gas turbine
generally consists of a compressor, combustor, and turbine. Part of
the turbine drives the compressor, which sucks in large quantities of
air, compresses it, and feeds the high-pressure air into the combustor.
There the air is mixed with a fuel, such as natural gas, kerosene, or
gas derived from coal. The mixture is burned, providing high-pressure
gases to drive the turbine.
In a CHP plant, the hot exhaust gases from
the gas turbine are used to generate steam in a heat recovery boiler
and the steam is used in the industrial process. As a result, less fuel
is needed. The CHP plant not only spins an electric generator but also
supplies heat, making it a cogeneration facility.
In another type of
power facility, the combined cycle gas turbine (CCGT) plant, the exhaust
heat from the gas turbine is used to produce steam for a power-producing
steam turbine. Combining the output of the gas and steam turbines generates
more electricity for the same amount of fuel. A CCGT may also have a
recuperator, or heat exchanger, which uses some of the energy in the
gas turbine's exhaust gases to preheat the air entering the combustor.
In this way, the energy efficiency of the CCGT is improved.
Conventional
land-based gas turbines used for power generation are 33 to 40% efficient
when used in "simple cycle" mode—that is, without a recuperator or steam
generator. How can a CCGT both be made at least 60% efficient and meet
the goals of lower emissions and lower energy costs set in DOE's Advanced
Turbine Systems (ATS) Program? U.S. turbine manufacturers working in
the program concluded that turbine inlet temperatures must be over 1427°C
(2600°F) and that the amount of air typically bled from the compressor
to cool turbine components must be reduced. These two criteria, coupled
with the large size of the turbine engines involved, severely challenged
the gas turbine industry.
"For gas turbines to hold up under these sustained
temperature and pressure extremes, changes had to be made in the materials
used in them and in the ways they are manufactured," says Mike
Karnitz, manager of the gas turbine project in the M&C Division. "ORNL
materials research played a key role in identifying improvements in
turbine components to meet DOE goals."
How ORNL Helped
Gas turbines
use fuel more efficiently because they can be operated at higher temperatures
than can other power sources currently available. But high-temperature
operation can result in the formation of NOx from nitrogen in the fuel
and in the air. Large amounts of preheated air are pulled into the combustor,
and additional lower-temperature air is pumped through holes in the
metallic interior walls of the combustor to cool them as combustion
occurs inside. The pumped-in air mixing with the combustion gases creates
hot spots that trigger the formation of NOx through high-temperature
reactions between nitrogen and oxygen in the air. To reduce NOx formation,
one route is to replace the metal combustor liners with ceramic liners.
Because they can withstand more heat, ceramic liners should require
less cooling; therefore, they can be designed without cooling holes,
reducing the tendency to create hot spots. If less air is used and fewer
hot spots are present, less NOx is produced, making it possible for
the gas turbine to meet the air quality standard. In addition, reducing
the need for air cooling also saves energy, making the gas turbine more
efficient.
Selecting the best ceramic liner material
for the Solar Turbines' gas turbine for Malden Mills required two years
of testing. Among the primary candidate materials in this study were
numerous variations of silicon carbide (SiC)-based composites. To produce
these composites, continuous SiC-fibrous preforms were densified by
one of two processes, chemical vapor infiltration or Si-melt infiltration,
both of which create a relatively dense SiC matrix. The resulting continuous
fiber-reinforced ceramic composites (CFCCs) are strong and have an acceptable
fracture toughness and a noncatastrophic failure mode.
All SiC-based composite materials have a problem:
They degrade at elevated temperatures in a combustion environment. Although
SiC-based materials are relatively resistant to oxidation, significant
concentrations of water present in high-pressure combustion gases can
accelerate corrosion in these materials at temperatures typically encountered
in a gas turbine combustor (~1200°C). These problems are exacerbated
by the presence of boron-containing and other constituents introduced
during the composite fabrication process.
To understand and fully evaluate this water
vapor effect on different SiC-based composites, a team of M&C researchers
was assembled to examine this phenomenon and ultimately help select
the most stable CFCC material to use for combustor liners. Karren More,
Peter Tortorelli, Matt Ferber, and Jim Keiser brought to the team extensive
experience from previous work on microstructural characterization, high-temperature
corrosion, and mechanical reliability of ceramics and CFCCs. Particular
use was made of the "Keiser Rig," a unique high-temperature,
high-pressure exposure facility developed by Jim Keiser and Irv Federer
in the early 1990s to examine corrosion of candidate heat exchanger
materials.
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Karren
More and Larry Walker use the Hitachi HF2000 transmission electron
microscope to study fiber-matrix interfaces in continuous fiber-reinforced
ceramic composites (CFCCs). These composites are similar to the
CFCC combustor liners used in gas turbines for power production.
More and Walker also use the JEOL 733 electron microprobe to analyze
CFCCs after exposure to high-pressure water vapor in the ORNL
test furnace and after actual engine tests.
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Jim
Keiser and Mike Howell (on ladder) prepare to install samples
in the Keiser Rig. This ORNL test furnace is used to expose samples
of CFCC combustor liner materials and coatings to high-pressure
water vapor similar to the combustion gases in gas turbines.
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The Keiser Rig enabled the ORNL team to simulate
the high water-vapor pressures encountered in land-based gas turbines
such as Solar Turbines' engine. Microstructural characterization by
More of composites exposed in the Keiser Rig and the same materials
exposed in actual engine tests for comparable times revealed similar
modes of material degradation. The root cause of the CFCC microstructural
degradation in both laboratory and actual engine exposures was attributed
to high water-vapor pressures. Using the Keiser Rig, the ORNL researchers
screened the different CFCC materials, provided insight into the degradation
mechanisms for the different CFCC compositions, and reliably estimated
the degradation rates for each composition.
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Significant
materials recession (loss of liner thickness) was accompanied
by subsurface microstructural damage on the working surfaces of
a CFCC combustor liner. This outer liner ran in a Solar Turbines
Centaur 50S engine for >5000 hours at the Texaco, Bakersfield,
California test site without a protective environmental barrier
coating.
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"The corrosion reactions in the hot combustion
gas rapidly convert the silicon carbide–based materials to silicon dioxide,
silicate glasses, and volatile products," More says. "As a
result, there is a loss of sound liner thickness, making the liners
prone to premature failure. To protect the walls from this surface recession,
it was necessary to identify an oxide coating that could serve as an
effective environmental barrier to oxidation. Protective coatings for
the CFCC liners have been and continue to be evaluated at ORNL."
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The
microstructural degradation observed on the working surface of
a CFCC inner liner after 1000 h in an engine test at the Texaco
test site (left) was similar to the damage observed for a CFCC
sample co-processed with the liner exposed for 1000 h in the ORNL
Keiser Rig at a high water-vapor pressure (right). Comparison
of results from laboratory and engine exposures for several different
CFCCs demonstrated that the Keiser Rig provided a valid simulation
of the CFCC microstructural damage at a rate comparable to that
observed in actual combustor environments.
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The ORNL research, coupled with extensive
field testing by Solar Turbines and materials improvements and coating
development by the different materials manufacturers, laid the foundation
for the selection of the best CFCC composition and coating system for
the combustor liners used in the low-emission Centaur 50 engine installed
at Malden Mills. This collaboration allowed the demonstration project
at Malden Mills to proceed and meet the EPA limits for NOx and CO. The
liners have run without problems for more than 4000 hours.
The Appeal
of Gas Turbines
Rod Judkins, manager of ORNL's Fossil Energy Program,
says that coal-fired power plants provide 56% of the nation’s electricity
even though a new coal power plant has not been built by a U.S. electric
utility since the 1970s without a government subsidy. In the future,
he said, U.S. utilities are interested in building power plants that
use natural-gas-combined cycles.
"There are many reasons why utilities like
gas turbines for future power production," Judkins says. "Besides
being more efficient and producing less carbon dioxide than coal-fired
power plants, gas turbines cost $500 per kilowatt less, lowering the
cost of electricity by 10%. Also, gas turbine plants can be built faster,
and they come in a wide range of sizes, offering flexibility."
CCGTs are steadily becoming even more appealing. One reason is that
partnerships involving DOE, national laboratories, electric utilities,
natural gas companies, gas turbine manufacturers, and universities are
rapidly boosting the efficiency and reliability of natural-gas-combined
cycles while lowering their emissions.
The first U.S. application of a utility-sized
advanced CCGT engine to result from the efforts of DOE's ATS Program
was announced in December 1999. Sithe Energies of New York City, one
of the world's largest independent power producers, announced it was
building a power plant near Scribna, New York, that would incorporate
two 400-megawatt (MW) versions of the DOE-supported natural gas-fired
Frame 7H CCGT built by General Electric (GE) Power Systems. The first
of these units, about the size of a large locomotive, passed a critical
verification test in February 2000 and was to be shipped from GE's Greenville,
South Carolina, manufacturing facility to New York a few weeks later.
It is predicted
that these new CCGTs, which are currently in the testing stage, will
discharge half the NOx typical of existing utility-scale turbines. They
will also emit 20% less carbon dioxide than was produced by turbines
available only eight years ago.
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The new GE gas turbine produces power at 60% efficiency. |
These GE turbine engines and similar utility-scale
power systems being developed by Siemens-Westinghouse are now emerging
from DOE's ATS Program. An industrial-sized advanced gas turbine developed
under the ATS Program—the Solar Turbines Mercury 50—has already been
announced. This engine has an efficiency improvement of some 15% compared
with the state of the art before the ATS Program was initiated.
Improvements
in turbine designs, cooling systems, materials, and manufacturing
technologies achieved through the ATS Program by the gas turbine manufacturers
and the materials and component suppliers have made possible higher
turbine operating temperatures. As a result, advanced gas turbine technology
is ready to attain significantly improved power plant efficiency.
The
workhorse of electric utilities today is the highly centralized coal-fired
power plant steam turbine, which typically has a fuel efficiency of
less than 40%, although over 45% efficiency is claimed for advanced
steam plants in Europe. CCGTs designed for utilities under the ATS Program,
such as the new GE turbines, are exceeding 55% net efficiency (using
the definition of efficiency applied to coal-fired steam plants) or
60% efficiency in terms used for gas turbines. Because fuel represents
the largest single cost of running a power plant, a 10% increase in
efficiency can reduce operating costs by as much as $200 million over
the life of a typical gas-fired 400-MW combined cycle plant.
ORNL's
contribution to the DOE-GE project was to manage the program in which
two companies—Howmet Corporation in Whitehall, Michigan, and PCC Airfoils
in Cleveland, Ohio—developed an improved manufacturing process for fabricating
single-crystal nickel-based superalloy turbine blades, or "airfoils."
These airfoils make the turbine spin when they are pushed by high-temperature
gas. Single-crystal components are preferred over conventionally used
materials because they are stronger at high temperatures. Thus, turbine
blades made of this material are required to withstand the higher-temperature
conditions needed to increase engine efficiency.
Single-crystal blades were first developed
for use in aircraft. Turbine blades used in current civil aircraft today
typically weigh up to 2.3 kg (5 lbs.), but blades for advanced land-based
gas turbines, which must also be grown as single crystals, can weigh
18.2 kg (40 lbs.). Howmet and PCC Airfoils are developing the manufacturing
technologies for these very large single-crystal airfoils. Five years
ago, single-crystal turbine airfoils of this size could not be produced.
Under the auspices of the ATS Program, efforts aimed at improving the
yield of these castings are continuing. ORNL's Mike Karnitz has provided
management support and technical oversight to this project.
Meeting Other Materials Challenges of Industrial Turbines
Bond coats.
In other ATS materials and manufacturing programs managed by Karnitz,
Siemens-Westinghouse and Pratt Whitney have improved thermal barrier
ceramic coatings for use on turbine airfoils to enable increases in
turbine rotor inlet temperatures needed to achieve ATS efficiency goals.
This effort has been strongly supported by a team of researchers in
ORNL's M&C Division that includes Matt Ferber, Allen Haynes, Michael
Lance, Karren More, Bruce Pint, Glen Romanowski, and Ian Wright. Thermal
barrier coatings (TBCs) have two layers. A ceramic top coat provides
insulation to help keep the single-crystal alloy blades from getting
so hot that they melt. The second layer is a metallic bond coat that
serves to "glue" the ceramic top coat to the metal blade and also provide
resistance to oxidation and corrosion. At the high temperatures of the
gas leaving the turbine combustor, the alloys used for the airfoils
can degrade rapidly through oxidation unless they have a protective
bond coat.
ORNL's TBC Program has two main goals, according to Wright.
"The first is focused on maximizing the ability of the metallic bond
coats to resist thermal oxidation," he says. "The second involves learning
how the complete TBC system degrades during service."
One of the keys to maximizing TBC lifetime is to minimize
the rate of growth of the oxide scale formed on the bond coat while
maximizing its adherence to the bond coat when exposed to the turbine
environment. ORNL efforts in this area involve using model alloy systems
to quantify improvements made possible through various alloy modifications.
A specially developed coating rig is used in the laboratory to explore
ways to incorporate these improvements into actual bond coatings. Collaboration
with the gas turbine industry participants in the ATS program results
in rapid assimilation of such developments and the examination of production-related
issues relative to modified TBCs.
An understanding of the actual
modes of degradation of TBCs is essential to the development of models
that can be used to predict service lifetimes. The ORNL effort in this
area involves the development and application of techniques to identify
and characterize the processes involved in TBC degradation. Crucial
insight into mechanisms at work is being gained through the application
of the M&C's Division's comprehensive suite of advanced surface analysis
techniques.
Turbine Materials Research at
ORNL: The Outlook
Microturbines are a new class of gas turbine of growing
commercial interest. They generate 75 kilowatts of electricity or less;
by comparison, larger industrial gas turbines generate 3 to 30 MW and
CCGTs produce power up to 400 MW. Currently, microturbines are about
the size of large refrigerator-freezers, and the number of manufacturers
is growing. One market for these microturbines may be commercial businesses
that want to use them to power their building appliances and provide
excess electricity they can sell back to their electric utility company.
Because the efficiency of current microturbines is just under 30%, DOE's
goal is to make advanced engines that are 35 to 45% efficient.
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Photograph
and schematic of a microturbine, which could be used by a small business to generate power.
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ORNL researchers are testing a wide range of materials
necessary to improve microturbine efficiency. They are studying ceramics
such as silicon nitride, which will be needed for the highest temperatures
anticipated in future gas turbines. They are evaluating foils of advanced,
heat-resistant metals, which will be needed for near-term use in turbine
recuperators.
As a world leader in materials research and
as DOE's largest energy research laboratory, ORNL is making a major
contribution toward ensuring that tomorrow's dominant source of electricity
will be low in emissions and high in efficiency.
Beginning
of Article
Turbine
Renewal:
