Ultrasupercritical Steam Turbines: Next-Generation Design and Materials

Home > Features > Ultrasupercritical Steam Turbines: Next-Generation Design and Materials

Ultrasupercritical Steam Turbines: Next-Generation Design and Materials

Generating plants fired with fossil fuels—coal, natural gas, and oil—supply about 70% of the world's demand for electricity. The need to further reduce environmental emissions from such plants is driving growing interest in higher-efficiency fossil fuel units, which produce proportionately less pollution per megawatthour. The most direct route to increasing efficiency is the evolutionary advance of increasing steam temperatures and pressures.

The ultrasupercritical fossil power plant is one option for high-efficiency, low-emissions electricity generation. Based on significantly higher steam temperatures and pressures beyond those traditionally employed for supercritical plants, the operating conditions of ultrasupercritical units put new demands on steam turbine design, particularly where the business climate demands flexible, reliable cycling operation of generating units.

Ultrasupercritical plants have been under development for some time in Japan; more recently, they have become a focus of development work in Europe, with increasing interest among the U.S. power industry as well. Ultrasupercritical units pose particular challenges for maintaining equipment reliability and flexible operation at more-advanced steam conditions.

A new EPRI report (1006844) reviews the operating demands on ultrasupercritical steam turbines from both a design and a materials perspective. Produced by Tony Armor, technical executive for EPRI's Global Coal Initiative, the report provides a worldwide survey of the current performance of existing supercriticals and an assessment of the technologies necessary for making new ultrasupercritical units successful. Armor drew extensively from the experience and insights of a network of consultants and contractors that participated in EPRI-led advanced supercritical design work in the 1980s.

Preparing for the next generation
Dramatic improvements in materials technology for boilers and steam turbines since the early 1980s, plus improved understanding of power plant water chemistry, have led to increasing numbers of new fossil power plants around the world that employ supercritical steam cycles. Many site-specific factors come into play in the selection of a supercritical versus a subcritical cycle, including the configured cycles' comparative expected reliability and availability.

"The reliability and availability of more recent supercritical units have matched or exceeded subcritical units in baseload operation, after early problems in first- and second-generation supercritical boilers and steam turbines were overcome," notes Armor.

The limited number of U.S. fossil plants built with subcritical cycles in the past 20 years has been mainly a result of relatively low fuel costs that eliminated justification for somewhat higher capital costs of higher-efficiency plants. But in some international markets where fuel cost is a higher fraction of the total cost, higher-efficiency cycles produce a lower cost of electricity with lower emissions, compared with a subcritical unit. This is particularly pertinent for an anticipated future in which emissions of carbon dioxide are constrained, for example, by international agreement.

More than 170 supercritical units are operating in the United States, about 60 in Japan, and about 60 in Europe. The greatest concentration of supercriticals is in Russia and in the former Eastern bloc countries, where more than 230 are in service.

According to Armor, "The world is now preparing for a major step forward in steam temperature and pressure for a new generation of ultrasupercritical units." The U.S. Department of Energy is already involved in preparing advanced boiler materials and designs for advanced supercriticals, he notes. "Attention to better design and materials concepts for the ultrasupercritical steam turbine is now needed," adds Armor.

Materials stressed by cycling
Cycling and startup needs for steam turbines to operate in a volatile electricity market puts an emphasis on the turbine's ability to handle fast loading and frequent load swings without significant loss of material life for critical components. Rotor cooling, turbine by-pass systems, on-line monitoring, stronger materials, and better control systems will all likely be needed.

"This is the crux of the challenge for ultrasupercriticals in the United States," says Armor. "We need these plants to have good cycling capability, but the materials used for ultrasupercriticals make them hard to cycle, and more prone to creep and fatigue damage. Ultrasupercriticals are particularly challenging units to cycle, and doing so calls for very careful design. Most ultrasupercriticals in Europe will be operated at baseload capacity, rather than cycled, for this reason."

Insights from analysis
Based on his review of current designs and performance of supercritical steam turbines operating in the United States, Europe, Russia, and Japan, EPRI's Armor identified additional, remaining design issues to be resolved for steam turbines and the need for stronger materials for turbine components. Insights include:

Better efficiencies and lower heat rates of ultrasupercritical units could demand optimized thermodynamic cycles (such as the addition of topping cycles, for example), improved turbine exhaust-flow schemes, and advanced feedwater heaters and condensers.
The need for high reliability and availability translates to requirements for stronger materials for rotor forgings, casings, steamlines, and valves. Specific methods to avoid hard particle erosion and operational losses in thermodynamic performance will be needed as well.
Ferritic materials will be replaced by nickel-based superalloys as steam conditions are increased. This changeover point is an issue still to be resolved.
Understanding maintenance needs of the ultrasupercritical steam turbine and its auxiliary systems is essential for long-term, reliable operation.

An ultrasupercritical test facility, retrofitted to an existing unit, is suggested as one option for testing new materials and designs. Originally proposed as part of EPRI-led advanced design work in the mid-1980s, the facility would have an advanced turbine-generator and new steam generator for testing turbine rotors, bolting, and valves as well as rotor cooling, seal development, very high pressure feed pump seals, and advanced feedwater heaters.

Russian intrigue
The greatest concentration of installed supercritical units is in the countries of the former Soviet Union, where 232 units are in operation, providing about 40% of all electricity needs in those countries. The units are designed at specific sizes, from 300 MW to 1200 MW. Technology for Russian machines evolved for more than 70 years largely independently of the rest of the developed world, and thus offers some intriguing possibilities for plant improvements, notes Armor. Boiler modifications for combustion of low-rank and high-ash coals, turbine designs with unusual back-end configurations, district heating applications of supercriticals, and water chemistry approaches using oxygenated treatments were all reviewed in the United States following a 1989 visit by EPRI staff to its then-Soviet counterparts in the power industry. Such ideas have been subsequently studied and, in some cases, deployed in the United States.

Unusual features of Russian supercritical designs, in some cases unique in the world, include:

Titanium blades for the last stages of 1200-MW low pressure rotors

Direct contact feedwater heaters to improve heat rate

Baumann exhausts with divided LP steam flow to improve efficiency

High pressure and intermediate pressure cylinder flanges with heating devices to minimize start-up times

Side-by-side condensers to lower steam velocity and increase condenser pressure

Supercritical units adapted for cogeneration

Annular boilers and spiral fireball designs for low-rank coals.

European initiatives
Advanced supercritical plants are currently being pursued by the European Commission, under the THERMIE project. Beginning in 1998, a group of 40 European companies began work to provide the technical support for development of an ultrasupercritical plant (steam conditions of 35 bar, 700C/720C/720C), with an overall plant efficiency of 55% (lower heating value). This plant is aimed at larger unit sizes and essentially baseload operation, curtailing the immediate need for special designs for rapid cycling.

Still, it is likely that optimized start-up systems and other cycling-focused approaches will be needed for profitable operation in a deregulated market, in order to take advantage of volatile electricity prices. Such innovations as a welded turbine rotor, comprised of small forgings welded together to reduce total mass and thermal stresses for more-flexible cycling operation, are likely to be deployed in the THERMIE ultrasupercritical demonstration plant.

Implied in the goals adopted for the THERMIE project is the ultimate application of nickel-based superalloys to replace ferritic alloys for high temperature components. These superalloys traditionally have been used for gas turbines, but at significantly smaller sizes than for steam turbines. The THERMIE project schedule lays out a substantial plan for development work leading to startup of the demonstration plant in 2012.

EPRI perspective

"Reliable application of ultrasupercritical steam turbine designs in new generating plants is a worldwide issue and will benefit from worldwide collaboration," emphasizes Armor. This applies to design issues as well as material issues, he adds in his report.

"EPRI will seek the best technology to address concerns that are raised in the report and welcomes the opportunity to work with government, equipment manufacturers, generating companies, and engineering consultants to prepare and execute a research and development plan to solve them," Armor adds.

EPRI members and target funders of the Global Coal Initiative may order Ultrasupercritical Steam Turbines: Design and Materials Issues for the Next Generation (1006844) by calling EPRI Customer Service, 800-313-3774, or download a copy by pressing here.

Photos:
Top: This 700-MW Toshiba turbine at the Kawagoe plant in Chubu prefecture, Japan, was one of the first supercritical units to use superclean low pressure turbine steels, originally developed by EPRI.

Second from top: The gas-fired Moss Landing plant of Duke Energy North America includes two large supercritical units that have been modified to operate under low-load conditions. This enables the units to operate more flexibly in increasingly volatile electricity markets.

For more information, contact Tony Armor, mailto:%20aarmor@epri.com, 650-855-2961.