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The gas turbine is evolving into the workhorse of electricity production in the 21st century, and ORNL materials research is helping to boost its efficiency and reduce its emissions.

Turbine Renewal:
Shaping an Emerging
Gas-Fired Power Source

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.

Malden Mills power plant (jpeg, 26K)
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.

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.

Type of gas turbine used (jpeg, 23K)
Improved combustor liner (jpeg, 12K)
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.

Installing samples in the Keiser Rig (jpeg, 32K)
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.
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.

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.

Microstructural characterization (jpeg, 38K)
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.

"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."

Microstructual characterization (jpeg, 41K)
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.

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.

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.

Microturbine photo (jpeg, 13K) Microturbine schematic (jpeg, 22K)
Photograph and schematic of a microturbine, which could be used by a small business to generate power.

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.

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