Steam Generator for Advanced Ultra-Supercritical Power Plants 700 to 760C
Authors:P .S. Weitzel
名师教育Babcock & Wilcox
Power Generation Group, Inc.Barberton, Ohio, U.S.A.Prented to:
ASME 2011 Power Conference Date:
July 12-14, 2011Location:
Denver, Colorado, U.S.A.
Technical Paper
BR-1852
Steam Generator for Advanced Ultra-Supercritical Power Plants 700 to 760C
P.S. Weitzel
Babcock & Wilcox Power Generation Group, Inc.
Barberton, Ohio, U.S.A.
buyPrented at:
ASME 2011 Power Conference
Denver, Colorado, U.S.A.
July 12-14, 2011
BR-1852
Abstract
Advanced ultra-supercritical (A-USC) is a term ud to designate a coal-fired power plant design with the inlet steam temperature to the turbine at 700 to 760C (1292 to 1400F). Average metal temperatures of the final superheater and final reheater could run higher, at up to about 815C (1500F). Nickel-bad alloy materials are thus required. Increasing the efficiency of the Rankine reg西谷椰子
enerative-reheat steam cycle to improve the economics of electric power generation and to achieve lower cost of electricity has been a long sought after goal. Efficiency improvement is also a means for reducing the emission of carbon dioxide (CO2) and the cost of capture, as well as a means to reduce fuel consumption costs. In the United States (U.S.), European Union, India, China and Japan, industry support associations and private companies working to advance steam generator design technology have established programs for materials development of nickel-bad alloys needed for u above 700C (1292F). The worldwide abundance of more afford-able coal has driven economic growth. The challenge is to continue to improve the efficiency of coal-fired power generation technology, reprenting nearly 50% of the U.S. production, while maintaining economic electric power costs with plants that have favorable electric grid system operational characteristics for turndown and rate of load change respon.
The technical viability of A-USC is being demonstrated in the development programs of new alloys for u in the coal-fired environment where coal ash corrosion and steamside oxidation are the primary failure mechanisms. Identification of the creep rupture properties of alloys for higher temperature rvice under both laboratory and actual field conditions has been undertaken in a long-term program sponsored by the U.S. Department of Energy (DOE) and the Ohio Coal Development
Office (OCDO). Ultimately, the economic viability of A-USC power plants is predicated on the comparable lower levelized cost of electricity (LCOE) with carbon capture and questration (CCS) using either oxy-combustion or post-combustion capture. Using nickel alloy components will drive the design and configuration arrangement of the steam generator relative to the plant. A-USC acceptance depends on achieving the higher func-tional value and lowering the level of risks as this generation technology appears in a new form.
Introduction
Thermal efficiency improvement for economic gain has been an important engineering endeavor for over 250 years. The Newcomen steam engine appeared in 1750 and attained 0.5% efficiency [1]. 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 achieve 17% thermal ef-ficiency by 1834. American Electric Power’s (AEP) Philo Plant Unit 6 steam generator was the first commercial su-percritical unit in rvice 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, 4500 psi,
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1150F/1050F/1000F) and was supplied by The Babcock & Wilcox Company (B&W). In 1959, Philadelphia Elec-tric Company’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 (2,000,000 lb/h, 5000 psi, 1200F/1050F/1050F) and later operated at 32.4 MPa, 610C/554C/554C (4700 psi, 1130F/1030F/1030F) [2]. The net plant heat rate for Ed-dystone was 8534 Btu/kWh, a 39.99% higher heating value (HHV) net plant efficiency without environmental system auxiliary power. The units, using stainless steel materi-als, led the world toward commercial supercritical boilers. Nickel-bad alloys are currently being evaluated for ASME Code acceptance up to 760C (1400F) steam conditions.Once-through supercritical plants became valued to the U.S. market again in about 2000 and most new larger electric utility coal-fired plants have been supercritical with variable pressure operating mode. Two Babcock & Wilcox Power Generation Group, Inc. (B&W PGG) design efforts have been underway in this period. In one effort, the DOE and OCDO are spons
oring a materials development program by a consortium of boiler vendors that are eking qualification of ASME Code Section I alloys suitable for 760C (1400F) turbine throttle steam temperatures [3, 4]. The cond effort is an internal B&W PGG-funded program for A-USC boiler design and additional materials development. The highest design steam conditions for the two programs are 36.2 MPa, 735C/760C (5000 psi, 1356F/1400F) with a final feedwater temperature of 343C (649F). Incread efficiency reduces CO 2 emissions, the costs of carbon capture, water u, particulates, sulfur oxides (SO x ) and nitrogen oxides (NO x ) emissions, and fuel consumption. Rearch and development (R&D) programs are being conducted worldwide to advance the technology in 700C (1292F) steam generator design and materials development of the needed nickel-bad alloys.The abundance of more affordable coal worldwide has driven economic growth for about two centuries. Although there are diver opinions, rerves are estimated to last for
another 100 to 200 years. The main consideration is to u the resource as efficiently and economically as practical. Coal-fired power generation technology is responsible for nearly 50% of U.S. electricity production. New plants must have favorable electric grid system operational character-istics for turndown and rate-of-load-change respon. The A-USC development programs rve an important mission to improve the economics of electric power generation while reducing adver effects on the environment.
Rearch programs in both Europe (such as the THER-MIE AD700 program) and in the U.S. DOE Boiler Materi-als for Ultrasupercritical Coal Power Plants have t a goal to improve thermal efficiency and reduce carbon dioxide emission through application of materials with higher tem-perature capability up to 760C (1400F) [4]. The Electric Power Rearch Institute (EPRI) manages the project and the consortium includes the U.S. domestic boiler manufactur-ers B&W PGG, Alstom Power, Babcock Power, and Foster Wheeler. There is also a sponsored program for the devel-opment of A-USC steam turbine materials in which Alstom Power, General Electric and Siemens have participated. The effort of this 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. OCDO is also sponsoring this rearch.
Advanced cycles, with steam temperatures up to 760C (1400F), will increa the efficiency of coal-fired plants, before adding CCS, from an average of 36 to 39% efficiency (for the current domestic fleet considering the retirement and inactive status of subcritical units) to about 47% (HHV). This efficiency increa will enable coal-fired power plants to generate electricity at competitive rates while reducing CO 2 and other fuel-related emissions by as much as 17 to 22%. Steam temperatures and pressures up to 760C/35 MPa (1400F/5000 psi) are required. Combining CCS wit
h A-USC plants will provide lower cost of electricity generation with 90% carbon capture. A-USC coal-fired steam generators have the potential for comparable lower cost of electricity especially when combined with the requirements for CCS. As the result of a B&W PGG economic study applying B&W PGG/Air Liquide (AL) technology and starting with references [5, 6], the relative efficiency and levelized cost of electricity for A-USC with oxy-combustion CCS are shown to be lower in comparison to other technologies (Figures 2 and 3). In a future requiring carbon limits, the LCOE (cents per kilowatt-hour) is the lowest for A-USC with B&W PGG/AL oxy-combustion. The HHV efficiency of 39.4% for the current 600C (1112F) state-of-the-art plant is nearly regained by using A-USC with oxy-combustion which provides an efficiency of 38.9%.
In Figures 2 and 3, Cas 1, 3, 5 and 7 u 600C super-critical technology; Cas 2, 4 and 6 u A-USC 730C/760C technology; Ca 3 and Ca 4 u post-combustion CCS; Ca 5 and Ca 6 u oxy-combustion CCS per DOE study estimates; Ca 7 also us oxygen membrane technology; and the B&W PGG cas are 600C and 730C technology
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牛肉的切法with B&W PGG/AL oxy-combustion improvements.
Fig. 1 AEP Philo 6 universal pressure steam generator, B&W Contract UP-1.
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The net plant heat rate (NPHR) and corresponding net plant efficiency is the factor with high impact t
o the cost of electricity. The day-ahead pricing offer by a power plant to the independent system operator (ISO) is made considering fuel expen recovery. The competitive order for lection by the ISO of the plant’s offer curve and the ability to compete will be the measure of the success of any power generation technology.
Evaluation of the economic cost of the new candidate materials necessary for A-USC steam power plants shows that the potential thermal efficiency improvement appears viable and is within the expected margin for achieving an equal or better cost of electricity and significant reductions in emissions per kilowatt-hour [3]. Cost reductions due to smaller equipment in other areas of the plant, along with fuel savings, will help offt the higher capital cost of the A-USC boiler, including the required nickel-bad materials.
Current state-of-the-art steam generator
Figure 4 is an example of the current commercial state-of-the-art for USC units in the U.S. market (Ca 1 of Figures 2 and 3). USC will utilize main steam temperatures to about 605C (1121F), and hot reheat temperatures to about 621C (1150F). The most advanced cycle conditions for the current market in the U.S. are for the John W. Turk, Jr., AEP Hemp-stead project, scheduled for operation in 2012. B&W PGG is supplying the 690 MW (gross) boiler designed at 26.1
MPa, 602C/608C, 299C feedwater (3785 psi, 1116F/1126F, 570F feedwater).
The design features for Turk are more conventional for the B&W PGG Carolina (two-pass) boiler arrangement with multi-lead ribbed tube spiral wound lower furnace, mix tran-sition to the vertical tube upper furnace enclosure, two-pass arrangement pendant heating surface, and the two parallel path gas biasing horizontal convection pass with reheater, primary superheater and economizer banks. Stainless steel tubing is ud for the superheater and reheater. The high temperature headers and steam leads are 9Cr ferritic steel.
Materials development
Capable, qualified materials must be available to the industry to enable development of steam generators for A-USC steam conditions. Major components, such as in-furnace tubing for the waterwalls, superheater/reheater c-tions, headers, external piping, and other accessories require advancements in materials technology to allow outlet steam temperature increas to reach 760C (1400F). Experiences with projects such as the pioneering Philo and Eddystone supercritical plants and the problems with the stainless steel steam piping and superheater fireside corrosion provided a valuable precautionary lesson for A-USC development [7]. Industry organizations thus recognized th
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at a thorough pro-gram was required to develop new and improved materials and protection methods necessary for the high temperature steam conditions.
Material-related failures in steam generators have oc-curred due to internal oxidation and corrosion, fireside corro-sion, oxidation and erosion, inadequate welding procedures and fabrication techniques, and inadequate material proper-ties data (long-term creep) [8]. A-USC proponents believe that by finding new materials and by adapting tho from
other applications, steam conditions up to 760C (1400F) are
Fig. 2 Comparison of net plant efficiency for CCS options.
Fig. 3 Comparison of levelized cost of electricity for CCS
options.
Fig. 4 Current state-of-the-art 600C USC.
possible and components designed for the conditions can achieve economical rvice life.
The DOE/OCDO Materials Development Program for A-USC technology includes primary task categories for conceptual design and economics, material properties test-ing, steam-side oxidation, fireside corrosion, welding and fabrication techniques, coating development and testing [8, 9]. Industry experience gained in the materials development program with new and better analysis methods, in finding the proper handling techniques throughout the procurement, fabrication, delivery and operating process, and achieving lengthy rvice exposure time of the materials has been es-ntial to the new product introduction.
The conceptual design ud by the DOE/OCDO was a current practice two-pass boiler arrangement. This t the temperature design window for the components. Test-ing conducted in the program for the thermal coefficients of expansion, hardness, toughness and other mechanical properties is important to the design and fabrication of
materials. In addition, welds, and weldments for both thick ctions and tubes were tested. To achieve 760C (1400F) steam temperatures, longer creep rupture strength testing at higher temperatures is very important. Some of the creep rupture tests have now achieved 30,000 hours [9]. To gain the benefit of the higher cost, high strength nickel-bad materials they must be ud to the optimum level of their strength capability.
Laboratory oxidation testing of plain and coated speci-mens at 650C (1202F), 750C (1382F) and 800C (1472F) at 0.1 and 1.7 MPa (14.5 and 246.6 psi) with some at exposure times of 10,000 hours, have produced some interesting re-sults [10]. Steam-side oxidation rates and weight loss were lower for materials with chromium content of more than 12% with ferritic steels and 19% Cr for iron-bad austenitic materials. Shot peening or blasting has been effective for USC (620C/1148F) steam generators. Surface cold work treatment of non nickel-bad materials ud above 700C (1292F) does not produce effective results [11, 12]. Fireside corrosion from attack by molten coal ash contain-ing elements such as sodium, sulfur and chlorine forming alkali sulfates, etc., thin the outside tube surfaces. Low NO x burners and unburned carbon may also contribute to corro-sion of the waterwalls, superheater and reheater [13]. With a dependence on the fuel type, the corrosion rates typically increa up to a maximum, at about 690 to 730C (1274 to 1350F), and then decrea (Figure 5). A-USC outlet steam temperatures at 700 to 760C (1292 to 1400F) will result in mean tube metal temperatures up to about 815C (1500F) and the experience above certain tube metal operating tem-peratures is that corrosion will be reduced [14]. Chromium content of the ba material and protection measures with cladding should provide adequate economic lifetimes. One consideration is to design the A-USC steam generator with the final superheater operating beyond the peak. Laboratory tests with Eastern, Midwestern and Western coal ash and in situ testing progra
ms expod various materi-als and coatings/claddings [8, 9, 15]. Western coal is a less aggressive fuel for A-USC. Higher chromium content in the ba material, or with coatings at a level of about 27%, will help to reduce the corrosion rates. Testing with conditions for oxy-combustion CCS are in progress [15]. Fabrication process were tested to acquire knowledge on handling the new alloys in process such as bending, machining, swaging and welding. Shop welding practices, particularly with dissimilar metal welds (DMW), were tested in many combinations of product forms and materials. Field welding procedures were evaluated when installing and repairing test ctions to determine procedural limitations [9, 16].
The extension to higher steam temperatures requires careful evaluation and lection of materials for the fur-nace enclosure at the conditions. The temperature pickup along the enclosure is steep. Ferritic alloys SA213T23 and SA213T92 with coatings are some of the materials being considered for higher sulfur fuel and low NOx combustion. B&W PGG-funded R&D trial panels that were fabricated using T23 and T92 to develop the shop and field practices for welding and repairs (Figure 6).
Design by ASME Section I
The design methods for ASME Section I have ud for-mulae that have undergone improvements in the history of the Code to meet high standards of safety and better calcu-lation techniques for the product form. Analysis to support new formulae, provide rules and knowledge on the new materials, and develop work process was supported by the DOE/OCDO Materials Development Program for A-USC technology.
The new ASME formula for pipe and tube thickness design was submitted and accepted for Section I, Appen-dix A-317. The new formula will tend to compute thinner requirements at high temperature. ASME A-317 minimum thickness, t:
Fig. 5 Typical effect of temperature on fireside corrosion rate.
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