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U.S. RETROFITS FOR STEAM POWER PLANTS ARE BURGEONING DUE TO NUCLEAR EXTENSION PROGRAMS, ENVIRONMENTAL REGULATIONS AND LOW-COST GAS
Anumber of factors are driving U.S. power plant operators to consider retrofits and repowering as ways to meet long-term demands for reliable, low-cost electricity. With many nuclear plants having operated for more than 30 years, retrofits offer a way to extend a plant’s operational lifewhile boosting efficiency and ensuring safe, reliable operations. The situation is similar for ageing coal-fired plants, as reliability becomes amajor concern for operators of large plants (above 300 MW) that need to continue serving baseload demand.
More recent drivers of coal plant retrofits in the U.S include heightened regulatory pressure, low-cost natural gas and the need to increase the performance and reliability of the sustainable coal fleet. The problem plant operators now face is how to design a retrofit or repowering program that is economically feasible, maximizes return on investment and is economically responsive and viable relative to changingmarket dynamics.
If operators look first at nuclear or coal-fired plants, the first question is, “Do I really need a retrofit, or should I build a new plant to replace one at the end of its design life?” Permitting processes for new plants can be time-intensive and, while demand for electricity is expected to increase across the board, operators have to balance the timing of plant construction with the forecasted need for a new plant’s generating capacity.
Long-termrevenue expectations also factor into a plant owner’s consideration of a retrofit investment. Some degree of revenue certainty for a given plant, made possible through a power purchase agreement or other means,may justify the investment to enhance the plant’s efficiency, output, or both, in order to maximize its financial performance.
If retrofitting a steam plant is the most desirable option, the next major issue to address is minimizing the impact of the retrofit project on operations. In most instances, a steam turbine retrofit can be completed during a planned outage thereby avoiding any loss of generating time.
Many retrofit projects taking place today in the U.S. are geared towards improving the performance and reliability of an aging power plant fleet.With some nuclear plants reaching the end of their design life, retrofits can be viewed as a means of increasing longevity. The most common driver for retrofitting nuclear lowpressure (LP) turbinemodules is to install more rotors and mitigate the risk of stress corrosion cracking (SCC), a condition that predominantly impacts shrunk-on disc rotors and some mono-bloc designs.
Retrofitting the LP cylinder of a nuclearor coal-driven steam turbine, for example, can allowthe plant to operate safely for up to 20 additional years. Turbine and blading technology has come a longway in the 30 or more years since some U.S. steam plants went online. Blades can be re-designed to extract more energy from the steam flow, and can be machined from a single block of material for better mechanical integrity.
The exhaust area is the main performance parameter for LP modules, which is why the design of any nuclear LP retrofit starts with the selection of the appropriate last stage blade (L-0). OEM fleets from the 1970s and 1980s typically have undersized exhaust areas.
Today’s technology allows these areas to be increased, thereby minimizing losses in steamcapture.Although themaximumblade length of any retrofit solution is dictated by the existingLP outer casing,when retrofitting modern L-0 blades into existing casings, the blade length can often be increased above the length of the OEMblade.
Where boosting output is the main driver, power plant owners also will look beyond the last-stage blade to find opportunities for increased efficiency across the full steam path. Technologies, such as impulse (low reaction) and reaction blading can be selected or blended to deliver the most efficient steam path. In addition to extending a plant’s operating life, a range of technology upgrades, aided by computer-assisted design andmodelling systems, can lead to a jump in generating capacity.
Case in point: DominionVirginia Power retrofitted one high-pressure (HP) and two LP steamturbines atUnits 1 and 2 (960MW each) of its North Anna power plant, and Units 1 and 2 (850 MW each) at its Surry power station, both inVirginia. Soon to complete, the retrofit will boost the output by a combined total of 234 (MWnet winter).
Operators have approached retrofits on a component-by-component basis, a reality driven largely by historic procurement practices. Some plant owners are now considering the benefits of a broader, more holistic retrofit program that leverages upgrades across the steampath to achievemore significant increases in plant performance.
Many of the aforementioned advancements in turbine engineering and materials can be applied to a plant’s boiler, thereby increasing steam flow and temperatures at the steam turbine inlet. Boosting boiler output, channelling more steam into a more efficient turbine, and upgrading generator components delivers value to the operator in terms of net power output and overall operational cost efficiency. While an integrated retrofit is broader in scope than focusing solely on the turbine, the plant can recoup its investment by selling more electricity over a longer period of time without increasing fuel consumption.
Minnesota Power launched an integrated retrofit project at its Boswell Energy Centre (BEC) located in Cohasset,Minnesota. BEC is a coal-fired plant whose four units deliver just over 1,025MW.Units 1 and 2, eachwith an output of 70 MW, began operation in 1958 and 1960, respectively. Unit 3 has an output of 350 MW and began operation in 1973. Unit 4, which has an output of 535 MW, went online in 1980.
Minnesota Power installed air quality control equipment at Boswell 3 to curb environmental impact. The goal was to maintain net output. An Optimized Plant Retrofit study identified the additional capacity needed to drive the air quality control system and demonstrated how that increase could be achieved through an integrated retrofit focused on upgrading the unit’s boiler pressure parts, firing system, HP/IP turbine cylinders, installing a low-temperature superheater and a low NOx burner system with over-fire air. Result: Environmental compliance while increasing output.
In light of stricter environmental regulations and the realities of an ageing steam fleet, we expect to see continued growth in the U.S. steam retrofit market as economic realities drive plant owners tomake themost of existing investments.
Charles Athanasia is Vice President of AlstomThermal Services North America. For more information, visit www.alstom.com