Design Features of Advanced Ultra Super Critical Plants, Part I

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Thermal efficiency improvement for economic gain has been an important power engineering endeavor for more than 250 years. The Newcomen steam engine appeared in 1750 and attained 0.5% efficiency. James Watt patented improvements in 1769 and achieved 2.7% efficiency by 1775, launching the Industrial Revolution. Watt limited operating pressure to about 0.034 MPa (5 psig). Richard Trevithick is credited with engine improvements that permitted increasing steam pressure to 1 MPa (145 psig), to achiev
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  Thermal efficiency improvement for economic gain has been an important power engineering endeavor for more than 250 years. TheNewcomen steam engine appeared in 1750 and attained 0.5% efficiency. James Watt patented improvements in 1769 and achieved2.7% efficiency by 1775, launching the Industrial Revolution. Watt limited operating pressure to about 0.034 MPa (5 psig). RichardTrevithick is credited with engine improvements that permitted increasing steam pressure to 1 MPa (145 psig), to achieve 17%thermal efficiency by 1834.American Electric Power’s (AEP) Philo Plant Unit 6 steam generator was the first commercial supercritical unit in service early in 1957(Figure 1). Philo 6, a double-reheat design, delivered 120 MW, operating at 85 kg/s, 31 MPa, 621C/565C/538C (675,000 lb/h, 4,500psi, 1,150F/1,050F/1,000F) and was supplied by The Babcock & Wilcox Co. (B&W). 1. AEP Philo 6 universal pressure steam generator, B&W Contract UP-1. Source: B&W PPGIn 1959, Philadelphia Electric Co.’s Eddystone steam generator, a dual-reheat design supplied by Combustion Engineering Inc.,initially delivered 325 MW at 252 kg/s, 34.5 MPa, 649C/565C/565C and later operated at 32.4 MPa, 610C/554C/554C. The net plantheat rate for Eddystone was 8,534 Btu/kWh, a 39.99% higher heating value (HHV) net plant efficiency without environmental systemauxiliary power.These units, using stainless steel materials, led the world toward commercial supercritical boilers. Nickel-based alloys are currentlybeing evaluated for ASME Code acceptance up to 760C (1,400F) steam conditions.Once-through supercritical plants became valued in the U.S. market again in about 2000, and most new larger electric utility coal-firedplants have been supercritical with variable-pressure operating mode. New plants must have favorable electric grid systemoperational characteristics for turndown and rate-of-load-change response. The A-USC development programs serve an importantmission to improve the economics of electric power generation while reducing adverse effects on the environment.Two Babcock & Wilcox Power Generation Group Inc. (B&W PGG) design teams have been steadily working on two developmentprograms. In one case, the U.S. Department of Energy (DOE) and the Ohio Coal Development Office (OCDO) are sponsoring amaterials development program by a consortium of boiler vendors that are seeking qualification of ASME Code Section I alloyssuitable for 760C turbine throttle steam temperatures. The second effort is an internal B&W PGG-funded program for A-USC boilerdesign and additional materials development.The highest design steam conditions for the two programs are 36.2 MPa, 735C/760C with a final feedwater temperature of 343C.Increased efficiency reduces carbon dioxide (CO 2 ) emissions, the costs of carbon capture, water use, particulates, sulfur oxides (SO x )and nitrogen oxides (NO x ) emissions, and fuel consumption.Research and development is being conducted worldwide to advance the technology in 700C steam generator design and materialsdevelopment of the needed nickel-based alloys. Research programs in both Europe (such as the THER-MIE AD700 program) and theU.S. (the DOE Boiler Materials for Ultrasupercritical Coal Power Plants) have set a goal to improve thermal efficiency and reduce CO 2 emission through the application of materials with higher temperature capability, up to 760C.The Electric Power Research Institute manages the U.S. project. The consortium membership includes the U.S. domestic boilermanufacturers B&W PGG, Alstom Power, Babcock Power, and Foster Wheeler. There is also a sponsored program for thedevelopment of A-USC steam turbine materials in which Alstom Power, General Electric, and Siemens have participated. The goal ofthe materials development consortium is to address the pre-competitive industry-wide data needs such as ASME Code allowable  stress and other properties to qualify the new materials. The OCDO is also sponsoring this research.Advanced cycles, with steam temperatures up to 760C, will increase the efficiency of coal-fired plants from an average of 36% to 39%(for the current domestic fleet, excluding older subcritical units slated for retirement) to about 47% (HHV). This efficiency increase willenable coal-fired power plants to generate electricity at competitive rates while reducing CO 2 and other fuel-related emissions by asmuch as 17% to 22%. Steam temperatures and pressures up to 760C/35 MPa are required to accomplish this goal.Combining carbon capture and sequestration (CCS) with A-USC plants will provide the lowest cost of electricity generation with 90%carbon capture than any other future CCS option. A B&W PGG economic study applying B&W PGG/Air Liquide (AL) technology andstarting with baseline coal combustion plant designs developed by the DOE, showed the relative efficiency and levelized cost ofelectricity (LCOE) for A-USC with oxy-combustion CCS to be lower in comparison with other technologies (Figures 2 and 3). Shouldcarbon-limited power generation be required in the future, the LCOE (cents per kilowatt-hour) is the lowest for A-USC with B&WPGG/AL oxy-combustion. The HHV efficiency of 39.4% for the current 600C (1,112F) state-of-the-art plant is nearly regained by usingA-USC with oxy-combustion, which provides an efficiency of 38.9%. 2. Comparing net plant efficiency for CCS options, we see that, on an equivalent basis, oxy-combustion offers the best plant efficiencywhen compared with other CO 2 capture options. For comparison, air-fired cases are the far left group, conventional carbon capturetechnologies are in the middle, and future carbon capture technology predictions are on the right. Data were taken from DOE/NETL2007-1291 “Pulverized Coal Oxy-combustion Power Plants,” Rev. 2; DOE/NETL 2007-1281 “Cost and Baseline for Fossil EnergyPlants,” Rev. 1; and B&W/AL Integration study using the warm recycle process with supercritical and ultrasupercritical steamconditions. Supercritical steam conditions are 241.3 MPa, 600C/620C; ultrasupercritical conditions are 27.6 MPa, 730C/760C.Source: Babcock & Wilcox Power Generation Group and Air Liquide  3. Note in this comparison of the levelized cost of electricity for carbon capture and storage options that the B&W/AL bars show thebest improvements in the LCOE when using oxy-combustion. All numbers are based on the same financial assumptions aboutburning the same bituminous coal and are estimated in 2007 dollars, not including owner’s costs. The Figure 2 data sources alsoapply to this figure. Source: Babcock & Wilcox Power Generation Group and Air LiquideIn Figures 2 and 3, Cases 1, 3, 5, and 7 use 600C supercritical technology; Cases 2, 4, and 6 use A-USC 730C/760C technology;Case 3 and Case 4 use post-combustion CCS; Case 5 and Case 6 use oxy-combustion CCS per DOE study estimates; Case 7 alsouses oxygen membrane technology; and the B&W PGG cases are 600C and 730C technology with B&W PGG/AL oxy-combustionimprovements.Evaluation of the economic cost of these new candidate materials necessary for A-USC steam power plants shows that the potentialthermal efficiency improvement appears viable and is within the expected margin for achieving an equal or better cost of electricityand significant reductions in emissions per kilowatt-hour. Cost reductions due to smaller equipment in other areas of the plant, alongwith fuel savings, will help offset the higher capital cost of the A-USC boiler, including the required nickel-based materials.Figure 4 is an example of the current commercial state of the art for USC units in the U.S. market (Case 1 of Figures 2 and 3). USCdesigns will utilize main steam temperatures to about 605C, and hot reheat temperatures to about 621C. The most advanced cycleconditions for the current market in the U.S. are for the John W. Turk, Jr. AEP Hempstead project, scheduled for operation in late2012. B&W PGG supplied the 690-MW (gross) boiler designed at 26.1MPa, 602C/608C, 299C feedwater inlet temperature.The design features for Turk are more conventional for the B&W PGG Carolina (two-pass) boiler arrangement with multi-lead ribbedtube spiral wound lower furnace, mix transition to the vertical tube upper furnace enclosure, two-pass arrangement pendant heatingsurface, and the two parallel path gas biasing horizontal convection pass with reheater, primary superheater, and economizer banks.Stainless steel tubing is used for the superheater and reheater. The high-temperature headers and steam leads are 9Cr ferritic steel.  4. Current state-of-the-art 600C ultrasupercritical design. Source: Babcock & Wilcox Power Generation GroupThe path forward to commercial A-USC plants is to determine whether further component testing at small scale in an electric utilitysetting is necessary or whether the industry is ready to move forward with a small commercial size demonstration at about 400 MW.The customer for the product of an operating electric generating unit is the transmission grid independent system operator that ismeeting the load demands at various system delivery locations by taking and transmitting power produced by many generating plants.This evolving and competitive market is served by units at plants that must bid a pricing offer one day ahead and that have variousoperating restrictions, cost structures, and characteristics that may or not align well with the power grid’s needs at each hour of theday. Low net plant heat rate, low fuel cost, wider load turndown range, and fast rate of response are valuable characteristics that willenable units to achieve positive net income. The owner/designer planning a new generating unit must factor these considerations intothe design.The tendency for supercritical units has been to select a larger size, greater than 400 MW and averaging about 750 MW, becauseeconomies of scale have normally resulted in the delivery of a lower cost per MW installed and a lower LCOE. The preferredcommercial size unit has been stated to be at 1,000 MW; Europe and China have been planning and constructing 1,000-MW units.The U.S. industry A-USC consortium is indicating that unit sizes between 350 MW and 1,000 MW would be possible. U.S. domesticA-USC plants will likely be teamed with CCS soon after the first commercial size demonstration.Design concepts for A-USC steam generators are being developed, starting with the current arrangements typical of present-dayapplication, and new and significantly different steam generator arrangements are to be expected, particularly when adapted to usethe advantages of oxy-combustion CCS.In the next two parts of this three-part report, we will explore the materials development process, the steam generator design, and theoverall cycle design of the A-USC power plant. The DOE and the Ohio Coal Development Office support for the A-USC Materials Development project is greatly appreciated. The efforts of industry-wide organizations have fostered an environment of cooperation in working toward the common pre-competitive needs for ASME Code materials development for A-USC.  —Paul S. Weitzelis a technical consultant in the New Product Development, Advanced Technology Design and Development,Technology Division of Babcock & Wilcox Power Generation Group Inc.Close Window
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