By Shaelyn Patzer
A steam-cooled A-USC corrosion test loop after installation. Photo courtesy: NETL
Energy demand across the world continues to rise. We face a critical challenge-how to meet the energy needs of a growing global population while also ensuring that electricity remains both cost-effective and environmentally sustainable?
While the general consensus of the energy science community has concluded that more power generation options must be created and pursued, it is untenable to hobble the portfolio of generation technologies. Traditional methods of power generation rely on fossil fuels and, in order to assure a robust energy future, it is vital to continue to pursue these resources.
One project undertaken by the U.S. Department of Energy (DOE) and the Ohio Coal Development Office (OCDO) seeks to do precisely that. Operating under a long-term collaborative partnership between Energy Industries of Ohio, the Electric Power Research Institute (EPRI), and a number of industry partners, the Advanced Ultrasupercritical (A-USC) Boiler Consortium set out to identify, evaluate, and qualify the construction materials needed to create critical components for the coal-fired boilers needed to produce the next generation of A-USC power plants.
Traditional fossil fuel technologies emit large quantities of CO2. In order to mitigate the cost of using these technologies, cleaner, more efficient versions of existent technologies must be developed. Coal, in particular, is currently one of the most widely used fuels across the globe for a number of reasons. The fuel is plentiful and available at low cost in many countries, including China, India, and the United States. As an energy source, coal reserves represent an important resource that can bolster economies and stimulate growth. However, in order for these benefits to be realized, coal technologies must be refined.
One promising method to keep coal a viable fuel option is the development of high-efficiency, A-USC coal-fired power plants. A-USC power plants will be more efficient than existing coal fired power plants and have many other benefits: they reduce the amount of CO2 that must be captured for the same amount of power generated by traditional plants, they enable fuel cost savings, and they are more economical.
By the 1950s, coal-fired power plants operated at a (then) cutting-edge, steam pressure of 2,400 psi and main steam temperatures of up to 538 °C. These plants are now called “subcritical.” Supercritical plants, which came to prominence in the 1960s, produce greater pressure and heat (3500 psi and temperatures up to 565 °C) and, subsequently, greater efficiencies. However, capabilities still must be improved. Increasing the pressure and temperature of a power plant is the most cost-effective method of bolstering plant efficiency, and the need for a new generation of ultrasupercritical power plants has been recognized.
A-USC power plants must achieve higher temperatures and pressures than ever before, and in order to do so, new, advanced materials must be created to enable the use of advanced steam cycles in coal-based power plants. These advanced cycles, with steam temperatures up to 760 °C, can increase the efficiency of coal-fired boilers from an average of 35 percent efficiency (current domestic fleet) up to 47% (HHV). This efficiency increase will enable coal-fired power plants to generate electricity at competitive rates (irrespective of fuel costs) while reducing CO2 and other fuel-related emissions by as much as 29%.
The development of advanced materials to meet the needs of A-USC power plants is critical and is recognized by both government and private industry. To achieve this goal, a project on materials for A-USC Coal-Fired Boilers was started. This research and development project was a multi-year effort to identify, evaluate, and qualify high-temperature materials technology for the construction of A-USC coal-fired boilers.
The success of this program will not only facilitate the development of ultrasupercritical power plants, but will fulfill several other objectives. First, the project team identified advanced materials that will make it possible to maintain a cost-competitive, environmentally acceptable coal-based electric generation option. Because high sulfur coals are plentiful in the United States, the materials being evaluated in this project would need to be resistant to the corrosive conditions caused by burning high-sulfur coals. The results of this project will enable American boiler manufacturers to successfully compete in world markets for building high-efficiency coal-fired power plants.
Because of the diverse benefits offered by this project and the wide array of skill sets needed for success, many industry partners collaborated on the research and development efforts. The program was based on a plan developed between US-based boiler manufacturers and EPRI, and was supplemented by the recommendations of several DOE-organized workshops on the subject of advanced materials, and the DOE and the OCDO visions of clean coal generation technologies. The DOE, through NETL, supplied the majority of the project funding. The prime contractor was Energy Industries of Ohio (EIO), and EPRI managed provided technical direction and leadership. The domestic boiler-manufacturers involved- ALSTOM Power Inc. (Alstom), The Babcock & Wilcox Company (B&W), Foster Wheeler Development Corporation (FW), and Riley Power, Inc. (Riley)-all provided cost-sharing. OCDO also provided significant levels of funding. Oak Ridge National Laboratory, another DOE national laboratory, provided specialized lab-scale materials testing and characterization support to the project through funding mechanisms separate from the consortium.
Due to the large scope of the project, the project was divided into eight (8) technical tasks: conceptual design and economic analysis, mechanical properties, steamside oxidation and resistance, fireside corrosion, weldability, fabricability, coatings, and alterations to design codes. For each task, a consortium lead was assigned, but research was generally conducted by multiple organizations which took advantage of the unique expertise in each organization and ensured equitable sharing of work and results.
Design and Economics
While the creation of advanced materials for boiler operations is vital, the designs must also remain cost-effective in order to be commercially deployable. Design studies conducted under the consortium showed that, while the cost of an improved boiler would not be recouped through savings in fuel reduction, it is the most economically feasible option for CO2 reduction. In light of this, several boiler manufacturers proposed alternative designs to traditional boilers in order to boost the economic attractiveness of an A-USC boiler without figuring in the factor of carbon capture and storage.
Additionally, the task conceptually considered the use of advanced material boilers in oxy-combustion power generation-which involves burning fuel in the presence of nearly pure oxygen rather than air. It was concluded that the size of boilers used for oxy-combustion would not alter significantly from air firing and that convection pass tubing quantity would be reduced. Finally, research indicated that further design optimization of materials would be required to cope with the higher feedwater and membrane wall temperatures that will occur in future A-USC power plant designs.
Extensive long-term mechanical properties testing was conducted at Oak Ridge National Laboratory on the most promising candidate alloys for an A-USC boiler. It was quickly determined that materials with the requisite strength, ductility, and fatigue resistance to operate at high temperatures and pressures (760 °C and 5000 psi) do exist. Multiple alloys were initially selected to undergo testing based on their creep strength, including three (3) nickel-based alloys (Haynes 230, CCA 617, and Inconel 740) for superheater/reheater tubes and main steam piping; and an austenitic steel (Super 304H) for use in superheater/reheater tubes. Long-term creep testing, lasting between 20,000 to 95,000 hours to failure, was critical in assessing material stability and offered confidence in employing them in designs.
Novel testing approaches were developed to analyze potential materials. The data consequentially collected through these tests was used to support both component design by rule and design by analysis. The databases created from results also proved instrumental in improving ASME code rules for design of boilers. Finally, research conducted under this task led to development activities outside of the consortium-most notably, the extrusion of the world’s first large diameter, thick wall Inconel 740H pipe.
Steamside exfoliation is a concern in modern supercritical and ultrasupercritical (USC) boilers. A series of laboratory studies were conducted to evaluate the potential for short-term exfoliation or long-term overheating in A-USC boiler applications. B&W conducted detailed analysis of large lab test data sets of alloys to 10,000 hours.
Overall, no major concerns with steam oxidation of nickel-based alloys was found, but stainless steels and ferritic steels were found to be very sensitive to alloy chemistry, surface condition, and temperature. In addition to steam-side oxidation and exfoliation, a study on materials for duct work (typically carbon steel in today’s power plants) in oxy-combustion was conducted. This study showed that the potential for very low pH condensates must be addressed by design or alloy selection.
Testing conducted under this task evaluated the relative resistance of various advanced alloys to fireside corrosion over the full temperature range expected for an A-USC plant. Through a systematic study that covered a large number of potential materials (over 450 samples), a down selected group of materials was identified for further tests in air-cooled boiler corrosion probes and in a steam-cooled corrosion test loop. These tests, which were conducted in actual coal-fired utility boilers, successfully demonstrated material performance in a realistic fireside environment. The task was so successful in demonstrating that alloys and overlay solutions exist to achieve acceptably low corrosion rates in fireside corrosion, that fireside corrosion was not considered necessary to include in a follow-on A-USC component test facility.
This task involved weld studies that made major strides in the development of thick section nickel-based alloys, including welding of age-hardenable alloys that had not been welded before. This was the first major project to demonstrate heavy section welding on age-hardenable nickel alloys. Over thirty combinations of materials, weld metals, and processes were qualified on seven materials. Additionally, early cracking problems in Inconel 740 welds were addressed through a focused R&D effort that resulted in an optimized 740H composition, which can be reliably and consistently welded up to 3.5 inch section thickness.
All of the involved boiler manufacturers conducted fabrication trials. Three types of typical shop procedures were studied: forming (bending of tubing, press forming of headers and piping, and swaging of tube ends); machining (weld grooves for tube circumferential seams, header and pipe longitudinal and circumferential seams, and tube-to-header joints); and welding (gas tungsten arc welding for tube-to-tube joints and submerged arc welding for header and pipe longitudinal and circumferential). From these tests, no show stoppers were identified for any alloys. It was determined that machining of nickel super alloys is slower than traditional alloys, but tooling exists for current equipment. Furthermore, unique recrystallization studies were beneficial in improving code rules and setting fabrication schedules.
To support the use of alloys with good mechanical strength, but with limitations in oxidation or fireside corrosion resistance, a literature review on coating was completed early in the project. Based on this review, samples were provided to research activities in Steamside Oxidation and Fireside Corrosion. In general, it was found that coatings and weld overlays can be beneficial, especially for mitigation of fireside corrosion in waterwall applications. However, internal coatings appeared less advantageous for steam-side oxidation because lower cost options such as shot-peening of stainless steel were found to be sufficient.
Under this task, the American Society of Mechanical Engineers’ boiler and pressure vessel code Committee was engaged to begin writing rules to accommodate new advanced materials, such as Inconel 740 and other age-hardenable precipitation strengthened nickel super alloys, for boilers and pressure piping.
A series of modeling studies was performed to address unique aspects of using nickel-based alloys in the design of an A-USC boiler compared to traditional boiler design using steel technology. Unique attributes include the potential for weak weld metals in creep and different thermal-physical properties which effect stresses due to thermal transients in components. Based on these tests, a number of code improvements were made, including a new alternative formula for sizing piping and tubing in creep regime, new nickel-based alloy forming rules, new nickel-based alloy weld strength reduction factors, and the first age-hardenable alloy code case for boiler components.
By the end of 2015, after 15 years of intense R&D efforts, the A-USC consortium has achieved its goals. The advanced materials needed to fabricate boilers capable of withstanding the extreme environments of the next generation of power plants have been developed-two key nickel super alloys are now commercially available. Inconel 740H has been approved by the ASME for use in fired boilers, unfired pressure vessels, and pressure piping up to 800 °C, and an application is in preparation to increase the maximum allowable use temperature to 825 °C. The other alloy, Haynes H282, is undergoing laboratory tests to develop the mechanical properties database needed for an ASME Code Case.
Although this marks the conclusion of the consortium’s efforts to design materials for boiler operation, the members of the consortium are committed to bringing the A-USC technology to fruition. The next step of development is testing of the A-USC components. In November of 2015, the Department of Energy and the National Energy Technology Laboratory approved a project geared to advance the newly-developed materials closer to commercial demonstration readiness.
A Component Test, or ComTest, is the next phase. Formed by a number of the organizations involved in the original A-USC consortium, the new partnership will carry over the expertise and strong working relationships of the previous stage into the new effort, and will conduct a 7 MW equivalent scale test of the key A-USC components for a Rankine steam cycle power plant (ComTest). If the project proves successful, the materials created by the consortium could be commercially deployed by 2030.
Shaelyn Patzer is a contract technical writer for the National Energy Technology Laboratory