CURRENT STATUS OF THE U.S. DOE/OCDO A-USC MATERIALS TECHNOLOGY RESEARCH AND DEVELOPMENT PROGRAM

J. ShingledeckerElectric Power Research Institute, Charlotte, NC USA

R. PurgertEnergy Industries of Ohio, Independence, OH USA

P. RawlsNational Energy Technology Laboratory, Pittsburgh, PA USA

ABSTRACT

The United States Department of Energy (U.S. DOE) Office of Fossil Energy and the Ohio Coal Development Office (OCDO) have been the primary supporters of a U.S. effort to develop the materials technology necessary to build and operate an advanced-ultrasupercritical (A-USC) steam boiler and turbine with steam temperatures up to 760°C (1400°F). The program is made-up of two consortia representing the U.S. boiler and steam turbine manufacturers (Alstom, Babcock & Wilcox, Foster Wheeler, Riley Power, and GE Energy) and national laboratories (Oak Ridge National Laboratory and the National Energy Technology Laboratory) led by the Energy Industries of Ohio with the Electric Power Research Institute (EPRI) serving as the program technical lead. Over 10 years, the program has conducted extensive laboratory testing, shop fabrication studies, field corrosion tests, and design studies. Based on the successful development and deployment of materials as part of this program, the Coal Utilization Research Council (CURC) and EPRI roadmap has identified the need for further development of A-USC technology as the cornerstone of a host of fossil energy systems and CO2 reduction strategies. This paper will present some of the key consortium successes and ongoing materials research in light of the next steps being developed to realize A-USC technology in the U.S. Key results include ASME Boiler and Pressure Vessel Code acceptance of Inconel 740/740H (CC2702), the operation of the world’s first 760°C (1400°F) steam corrosion test loop, and significant strides in turbine casting and forging activities. An example of how utilization of materials designed for 760°C (1400°F) can have advantages at 700°C (1300°F) will also be highlighted.

INTRODUCTION Need for A-USC

In 2010, coal provided 48% of the electric generation in the United States [1] and 40.6% of the electric generation in the world [2]. Coal is abundant with some studies estimating 150 years of world-wide reserve at current consumption rates [3]. Despite its use as major source of electricity, coal faces strong regulatory and economic challenges as the world adopts policies for reducing carbon consumption. As world-wide demand for electricity grows, it is clear that a robust portfolio of power generation options is needed to ensure reliable and environmentally responsible electricity. To meet the challenges of the 21st century, advancements in coal technology are needed. The Coal Utilization Research Council (CURC)-EPRI Coal Technology Combustion roadmap, Figure 1, identifies key technologies needed to ensure that coal remains an option for power generation in the future. A key aspect of this roadmap is the deployment of

Advances in Materials Technology for Fossil Power Plants Proceedings from the Seventh International Conference October 22–25, 2013, Waikoloa, Hawaii, USA

Copyright © 2014 Electric Power Research Institute, Inc. Distributed by ASM International®. All rights reserved.

D. Gandy, J. Shingledecker, editors

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higher efficiency pulverized coal combustion using Advanced Ultrasupercritical (A-USC) technology. For the world, the International Energy Agency (IEA) has proposed, in its High Efficiency Low Emission (HELE) roadmap for coal technology that coal generation from inefficient subcritical plants be replaced by higher efficiency USC and A-USC plants as a first step to carbon reduction, prior to commercial deployment of Carbon Capture and Storage (CCS) technologies [4]. Thus, there are strong environmental and economic drivers for the development and deployment of A-USC technology in the U.S. and worldwide.

Figure 1: CURC-EPRI Roadmap Combustion Timeline and Impact

National Programs The world-wide development of A-USC technology aimed at steam temperatures of 700°C (1300°F) and greater started initially around 1998 with a variety of European Projects [5]. In 2001, the U.S. Department of Energy in conjunction with the Ohio Coal Development Office (OCDO) and cost share from all the major U.S. boiler and turbine original equipment manufacturers (Alstom, B&W, Foster Wheel, Riley Power, GE), the Energy Industries of Ohio (EIO), and the Electric Power Research Institute (EPRI) with support from Oak Ridge National Laboratory (ORNL) and managed through the National Energy Technology Laboratory (NETL) began an ambitious pre-competitive research and development project that would lead to higher efficiency coal-fired power plants with reduced CO2 emissions [6,7]. Achieving major increases in coal-fired power plant efficiency requires an increase in steam conditions (temperature and pressure) and therefore demands the utilization of new materials and technologies to implement these new materials. Hence, the R&D program’s goal was to develop the materials technology necessary to achieve Advanced Ultrasupercritical steam (A-USC) power plant steam conditions up to temperatures and pressures of 760°C (1400°F) and 35 MPa (5000 psi), respectively which can reduce all emissions, including CO2, by 20% or greater compared to today’s U.S. fleet [8,9]. Beyond Europe and the United States, national programs now exist in Japan (initiated in 2008), China (2011), and India (2012) with announced plans for demonstration in both China and India around 2020 [10, 11, 12]. In the following sections, some of the successful developments of the U.S. Program are highlighted which shows that materials and fabrication technologies now exist to construct such a system.

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ALLOY SELECTION The consortium first defined the conditions required for the materials to operate at A-USC conditions up to 760°C (1400°F). Based on these preliminary design studies, nickel-based alloys were selected for detailed evaluation because only these alloys had the requisite strength required to operate at A-USC conditions. Current USC boilers operate at temperature of approximately 600°C which is the limit of the most advanced creep-strength enhanced ferritic (CSEF) steels, but for temperatures above 700°C (1300°F), nickel-based alloys are clearly required. Figure 1 shows the temperature required to produce creep-rupture in 100,000 hours for various alloys. With the aim of 760°C (1400°F), age-hardenable alloys (Inconel alloy 740 and Haynes 282) were considered beyond solution strengthened alloys such as 617. Using the criteria of 100,000 hour strength at 100MPa (14.5ksi), these temperature limits can be observed from the plot. Although strength is an important consideration, a number of other critical material properties, must be considered for materials operating at A-USC conditions. Much of the work highlighted in the following sections is related to the other important properties needed for boilers and turbines including: fabricability, weldability, weld performance, tensile and fatigue properties, notch sensitivity, and steam side oxidation and fireside corrosion resistance. Table 1 provides a list of some alloys which were studied initially by the consortium and eliminated from future testing. Table 2 provides a partial list of the key nickel-based alloys selected by the consortia for study along with comments on their applicability and limitations.

Figure 2: 100,000 hour creep-rupture strength as a function of temperature for alloy/alloy class.

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Table 1. Ferritic and Iron-based Austentic Alloys Selected for Early Evaluation and Some Important Findings (Status)

Alloy Component Comments SAVE12 Pipe Unstable microstructure, welding was challenging [13]

(eliminated from testing) Super 304H (CC2328)

SH/RH Met strength projections, may need coatings in some environments and shot-peening for oxidation resistance [14]

(only for tubing) HR6W SH/RH,

Pipe Did not meet strength projections [15] (stopped research, new

chemistry now available) *SH/RH: Superheater and Reheater Tubing

Table 2. Nickel-Based Alloys Under Evaluation Alloy Component Comments

Haynes 230 SH/RH, Pipe Successful welding trials, maximum size limitations for pipe may limit applicability

CCA617 SH/RH, Pipe Higher strength than 617 but not enough data to change ASME code stress values, not suitable for high sulfur coals, only

successful SMAW welds in nickel-based alloys, strain-age cracking concerns, low strength limits applicability for turbine

rotor Alloy 263 Castings,

Rotor Back-up cast alloy to 282, good castability and weldability, lower

strength but good ductility Inconel

740/740H SH/RH, Pipe Highest strength alloy in ASME B&PV code to enable A-USC up

to 760°C (1400°F), excellent fireside corrosion resistance, successful fabrication and welding, prime candidate for boiler

components, cannot be air cast for valves and shells Haynes 282 Castings,

Rotor Higher creep strength than 740, relatively insensitive to starting microstructural condition, good forging ‘window’ for rotor, can

be cast for valves and casings Waspalloy Rotor, Bolts,

Blades Back-up alloy with good turbine history Cannot be welded

reliability. Poor ductility Nimonic

105 Bolts, Blades Highest creep strength alloy. Only considered for bolting and

blading (non-welded components).

PROGRAM SUCESSES Inconel Alloy 740/740H Development and Code Case Acceptance Inconel ® Alloy 740 is an age-hardenable nickel-based alloy developed by Special Metals Corporation (Huntington, WV USA) for use as superheater and reheater tubing in A-USC power plants [16]. Due to its excellent high-temperature (creep) strength, see figure 2, and corrosion resistance, the consortium also evaluated its use for thicker components such as boiler piping and headers. This involved numerous welding and fabrication trials combined with long-term testing. Initial challenges were encountered when welding the alloy in sections up to 75mm (3”) in thickness. The consortium worked closely with the alloy designers and other research institutions to refine the alloy and weld metal composition. Revolutionary progress was made, and the alloy, Inconel 740H, is now considered weldable as shown in Figure 3 with numerous successful welded joints produced [17, 18].

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Figure 3. 75mm (3”) weldment in Inconel 740H Pipe (left) and Header mock-up (right) [17]

Materials used in the construction of fired pressure vessels must be designed to ASME B&PV Code Section I. While there are some materials which are allowed for Section I construction at 760°C (1400°F), they do not have the requisite strength (allowable stresses) needed to design and build an A-USC boiler. Thus, the consortium developed a comprehensive data package which included test data on multiple material heats and product forms containing long-term data [19]. Babcock & Wilcox championed the case with supporting data from the other project members. ORNL conducted the long-term testing and EPRI conducted the stress analysis. The approved Section I code case, Code Case 2702, contains fabrication rules, welding specifications (including weld strength reduction factors), stress allowables, and other key requirements. Users can now design and specify the alloy for use in Section I construction to a maximum use temperature of 800°C (1472°F). The alloy has also been adopted by ASME Section B31.1 for power piping. Creep-rupture testing has surpassed 45,000 hours with no drop-off in rupture strength or ductility. Research has included studies on notch sensitivity and microstructural development which suggest the alloy is suitable for long-term service at A-USC conditions [20, 21]. Fireside Corrosion Tests & Oxidation Behavior Concerns with fireside corrosion at A-USC temperatures and the wide variety of coals burned in the U.S., necessitated the consortium to do detailed evaluation of all the alloys’ fireside corrosion resistance. The aim was to determine the suitability of the alloys over the range of temperatures expected in superheaters and reheaters (SH./RH) in an A-USC plant and determine if higher chromium claddings or coatings would be required. Extensive laboratory tests have been conducted on a wide range of alloys, overlays, and coatings with varying sulfur contents [22], air cooled probes have been fabricated and testing for up to 16,000 hours in actual boiler environments [23], a steam-cooled corrosion test loop was operated in a high sulfur coal environment [24], and the world’s first 760°C (1400°F) steam cooled loop is now operating in a U.S. boiler. Figure 3 shows some results from one of the air-cooled probes. In this test, Inconel 740 performed as well as some of the high chromium weld overlays. Figure 4, shows the A-USC program’s steam loop which operates with a steam outlet of 760°C (1400°F) and contains a variety of materials and overlays. Current plans are to remove the loop in 2014 for destructive evaluation and comparison with other field and laboratory data. Work is also ongoing to evaluate oxycombustion environments. The results show there are alloys and weld overlays which can be used with confidence to enable an A-USC boiler with acceptable corrosion performance.

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Figure 4. Cleaned surface of ring samples (identification at the top) from an air cooled probes after 16,000 hours operation in a utility boiler. Note alloy 740 shows virtually no corrosion wastage with similar performance to high chromium weld overlays (52 and 72WO). Other

alloys, including HR6W show moderate attack.

Figure 5. Steam-cooled A-USC corrosion test loop after installation.

The steam cooled-corrosion loops provide limited information on steam-side oxidation and exfoliation. An extensive laboratory test program has been studying steam oxidation resistance for all A-USC alloys including the use of shot-peening on stainless steels. Parabolic rate constants, effect of pressure, weld oxidation, exfoliation and long-term (10,000 hour) exposures are all being examined in a comprehensive test program involving multiple laboratories [14]. Boiler Fabrication Trials and Heavy-Section Welding Extensive boiler fabrication studies have been completed which show it is possible to use typical boiler fabrication processes to construct an A-USC boiler using nickel-based alloys [25, 26, 27]. In some cases, flux processes were not possible for heavy section nickel-based piping and gas tungsten arc (GTAW) and gas metal arc (GMAW) welding was utilized. Hot wire GTAW (also known as hot wire TIG) to enable higher deposition rates has been successfully demonstrated on alloys including Inconel 740H in section thickness up to 75mm (3”) including narrow groove configurations to improve productivity (see Figure 3). Long-term weldments creep tests [30] are being conducted to establish weld strength reduction factors (WSRF) and ongoing work includes strain-age cracking studies. Figure 6 shows demonstration articles which include: forming,

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bending, welding (pipe and tube), machining, application of overlays, and dissimilar metal welding applied to A-USC alloys.

Figure 6. A-USC Boiler Demonstration Articles Casting Developments and Scale-Up Steam turbine casing and valve bodies are traditionally made via casting. While previous European efforts had had some success with casting solid solution strengthened nickel-based alloys, the use of higher strength precipitation hardened alloys with high Ti and Al contents (which are very reactive in air) needed to be explored to see if conventional air/protective atmosphere casting techniques could be used. A research team from NETL-Albany and ORNL cast a large number of trial alloys and subject them to destructive evaluation and testing [28]. Based on successful results, scale up of the most promising alloys, Haynes 282 and Nimonic 263 was started with two different vendors. To date, static sand cast test blocks, step blocks, and centrifugal castings have been made and are being evaluated. Planning is now underway for larger pours ~2700kg (6,000lb) in valve geometries. Figure 7 shows a ~450kg (1000lb) finished weight step casting of cast Haynes 282. Modeling work includes casting simulations to determine suitability for pouring larger casing geometries and thermodynamic/diffusion modeling to develop appropriate homogenization heat-treatment cycles [29]. Weld repair studies, microstructural evaluation and mechanical property determination are all ongoing.

Figure 7. ~450kg (1,000 lb) finished weight step casting of Haynes 282

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Haynes 282 Forging

Scoping studies by the A-USC Turbine Consortium identified Haynes 282 as the most promising turbine rotor/disc alloy. The studies showed that creep behavior of Haynes 282 was relatively insensitive to starting microstructural condition and had adequate tensile and fatigue behavior for a 760°C (1400°F) rotor [30]. The current HP and IP A-USC turbine design being considered calls for a bolted rotor similar to an industrial gas turbine [31]. In such a design, the highest-temperature component is a forged disk. Prior to forging, ingots of Haynes 282, Figure 8, were produced via triple melting (VIM/ESR/VAR). A first ingot was cast with a weight of ~4500kg (10,000lbs) and sectioned to evaluate defects and effects of processing variations. A similarly sized second ingot was also cast and will be forged into a rotor disc for full property evaluation. Extensive mechanical property studies are planned for this full-scale component.

Figure 8. Upset and draw operation (left) and world’s first Haynes 282 triple melt ingot (right)

Economy of 760°C Materials at 700°C

The U.S. program’s temperature aim of 760°C (1400°F) steam is greater than the other programs worldwide focused on A-USC at 700°C (1300°F). This has lead to the extensive study of alloys Inconel 740H and Haynes 282. A critical advantage of these alloys is that they can be used more cost effectively than solid solution strengthened nickel-based alloys at 700°C (1300°F) due to their higher strength at lower temperatures with similar alloy cost. One specific study [32] compared alloy 617 to 740H for a prototypical A-USC main steam and hot reheat piping system and showed that by using 740H in lieu of 617, the number of pipes required would decrease, the length of each individual extruded pipe section would increase, the number of welds for the piping system would decrease, the pipe thickness would decrease, and the amount of welding (both filler material and welding time) would decrease. The combined effect of these results demonstrated that the utilization of 740H over 617 would result in a significant cost savings with regard to both construction and life management of an A-USC piping system at 700°C (1300°F). Figure 9 is a comparison of the piping system in that study. Of interest is that for the main steam pipe, two 617 pipes were required because the wall thickness of the 617 in a single pipe arrangement exceeded typical boiler fabrication sizes (thickness was greater than 100mm). Additionally, the processing characteristics for extrusion (the basis of this study) showed 740H could be extruded in large diameter thus only one reheat pipe was needed compared to two for the 617 system. Depending on alloy cost, the materials savings alone for using 740H in lieu of 617 was 5 to 20 million USD.

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Figure 9. Comparison of a 617 and 740H piping system for a 700°C (1300°F) steam plant. The 617 system requires two main steam and two hot-reheat pipes to avoid wall thickness in excess of

75mm and to allow for extrusion. In contrast only one main steam pipe and one reheat pipe is needed for the 740H system due to its excellent strength and good fabrication characteristics.

CONCLUSIONS AND NEXT STEPS

Revolutionary progress has been made in advancing the materials technology to enable a 760°C (1400°F) A-USC power plant. These successes have only been made possible through a unique cost-sharing U.S. Consortium of manufacturers and research organizations dedicated achieving to the aggressive goals set forth by the U.S. DOE. The U.S. DOE/OCDO A-USC Consortium has had many successes including: demonstration of welding and fabrication of nickel-based alloys for an A-USC plant, extensive successful fireside corrosion testing including in-plant operation of the world’s first 760°C (1400°F) steam cooled corrosion test loop, development of casting techniques for high-alloy age-hardneable alloys, new materials for rotor forgings, and code acceptance of Inconel 740H which gives manufacturers, for the first time, an alloy with the requisite strength for a 760°C (1400°F) plant. Current research includes scale-up, demonstration, testing, and weld repair of castings, testing and evaluation of a Haynes 282 forged disc, long-term creep testing in excess of 45,000 hours to ensure long-term reliability, field corrosion studies, and economic evaluations. A new project to build a 760°C (1400°F) component test facility is now underway. This facility will be unique in that it will test heavy section components 60°C (100°F) hotter than any other test facility in the world, will incorporate cyclic operation of welds and valves, and will include a turbine test. Figure 9 shows a schematic of this planned facility. The facility plan is an outcome of workshops held with utility stakeholders who desire to see a demonstration before construction of an A-USC plant. In summary, the materials technology to design, build, and operate an A-USC power plant now exists or the research is nearing completion. Current research and development is ongoing and planned to improve the economy of building such a plant and to ensure its reliable operation.

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Figure 10. General arrangement of the planned 760°C (1400°F) component test facility

ACKNOWLEDGMENTS The authors are greatly indebted the members of the A-USC consortium who have undertaken this endeavor. First and foremost appreciation is given to Prof. R (Vis) Viswanathan for his leadership of the project prior to his retirement. Special thanks are given to the many members of the consortium past and present including but not limited to: H. Hendrix (EPRI), R. Ganta, J. Pschirer, and J. Marrion (ALSTOM Power), J. Tanzosh (B&W), H. Hack (Foster Wheeler), D. Saha and J. Breznak (GE), P. Torterelli (ORNL), J. Hawk and P. Jablonski (NETL-Albany), and B. Vitalis (Riley). The support and guidance of our sponsors is also greatly acknowledged: B. Romanosky (NETL), R. Conrad (DOE), and Ohio Office of Development. REFERENCES [1] “The CURC-EPRI Coal Technology Roadmap. August 2012: Update.” Available at:

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LEGAL NOTICE/DISCLAIMER This report was prepared by J. Shingledecker (EPRI) pursuant to a Grant partially funded by the U.S. Department of Energy (DOE) under Instrument Number DE-FG26-0 1 NT 41175 and the Ohio Coal Development Office/Ohio Department of Development (OCDO) under Grant Agreement Number CDO/D-OO-20 (now D-05-02A). NO WARRANTY OR REPRESENTATION, EXPRESS OR IMPLIED, IS MADE WITH RESPECT TO THE ACCURACY, COMPLETENESS, AND/OR USEFULNESS OF INFORMATION CONTAINED IN THIS REPORT. FURTHER, NO WARRANTY OR REPRESENTATION, EXPRESS OR IMPLIED, IS MADE THAT THE USE OF ANY INFORMATION, APPARATUS, METHOD, OR PROCESS DISCLOSED IN THIS REPORT WILL NOT INFRINGE UPON PRIVATELY OWNED RIGHTS. FINALLY, NO LIABILITY IS ASSUMED WITH RESPECT TO THE USE OF, OR FOR DAMAGES RESULTING FROM THE USE OF, ANY INFORMATION, APPARATUS, METHOD OR PROCESS DISCLOSED IN THIS REPORT

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