Through programs such as Transformative Power Generation, High-Performance Materials, and Advanced Coal Processing, the National Energy Technology Laboratory (NETL) is conducting research and development (R&D) to enable the nation’s fleet of coal and natural gas power plants to operate in a flexible, efficient, and reliable manner, while reducing operation and maintenance costs.
Although the country has a diverse energy portfolio, including nuclear power and a growing renewables sector, fossil power plants still supply a majority of the nation’s electricity. However, these facilities are aging. Most coal plants are 35 to 55 years old, with many even older. Additionally, the natural gas combined cycle (NGCC) fleet, which provides significant flexible capacity services to the grid, has many plants that are 15 to 20 years old, and some have been in operation for 25 to 35 years (Figure 1).
1. This graph shows the commercial online years of currently operating coal-fired and natural gas combined cycle (NGCC) units in the U.S. Source: National Energy Technology Laboratory
Both classes of assets will be required to operate more flexibly than when they were originally built, raising challenges with respect to materials reliability and efficiency. Forced outage rates have been increasing as these plants age and cycle more to balance load, driven by increased variable renewable generation on the grid.
“More and more, the grid is calling on fossil resources to provide energy in a way that requires plants to ramp up quickly and then ramp back down. These plants were never designed to operate that way,” NETL Technology Manager for High-Performance Materials research Briggs White explained. “When the plants aren’t dispatching, they aren’t in the money and are typically turned off or idled at minimum load. All this ramping causes damage to the plant and they don’t generate revenue.”
Addressing Reliability Through Superior Materials
NETL analyzed data from the North American Electric Reliability Corp. (NERC) to determine the key sources of forced outages in coal and gas plants. The data was sourced from the U.S. fleet of coal and NGCC plants from 2013 to 2017. The analysis found that within coal plants, boilers are a key cause of frequent failure. As much as 54% of the forced fleet outages were caused by issues with the boiler, with waterwall issues responsible for as much as 40% of boiler-related outages. In total, boiler plus steam turbine components within coal plants caused 70% of the forced outages. For gas plants, there are similar components in heat recovery steam generators (HRSGs), and steam turbines drive failures as well.
NETL assessed the specific failing components within these subsystems to determine a cause. It found that pipes and other surfaces were failing from a combination of fatigue failure from cycling, and erosion and corrosion from opportunity fuels and oxide spallation. Components made from more robust materials seemed to present solutions.
To address these challenges, the U.S. Department of Energy’s (DOE’s) High-Performance Materials program competitively awarded $10 million in funds to General Electric (GE) to support the development of materials to address thermal fatigue and corrosion, two of the leading causes of damage to steam cycle components inside power plants.
In one of the projects, NETL is working with Edison Welding Institute and Manufacturing Technology Inc. along with GE to improve the durability of dissimilar metal welds (DMWs) for boiler and HRSG applications. This will enhance erosion and oxidation resistance in high-temperature steam for high-pressure turbine blades inside the existing steam-powered fleet. The goal of this work is to reduce maintenance costs while enhancing the capability of coal plant cycling operations.
NETL and GE are also developing cost-effective weld overlay compositions for boiler tubing and multi-layered coatings that deliver improved resistance against erosion and oxidation in turbine blades under high-pressure steam conditions. Other goals of this improved materials project include increasing the time between outages for both boilers and turbines while also eliminating or significantly reducing the nickel content in weld overlay, which will mitigate costs. These advanced coatings could decrease power plant component cost, increase performance, and extend time between outages.
Beyond these two projects with GE, NETL is collaborating with partners to develop additional manufacturing technologies capable of robust DMWs, either through additive manufacturing of joints with graded compositions, solid-state processing, or advanced welding. In addition, NETL is investing in data analytics approaches for predicting when components will fail, informed by materials models and process data.
Addressing outages is a crucial priority for NETL, especially as units age. When looking at demographics, the existing fleet is dominated by older subcritical units that are performing increasing amounts of cycling, which results in increased equivalent forced outages (EFOR). Up to 68% of the current fleet, or 751 boilers, are in this subcritical category, and the EFOR rate increases with age. For example, a 100-MW coal plant can expect to see its EFOR rate go up to about 17 outages a year after it has been in operation for 40 years.
Embracing the latest computer technologies can also aid the development of advanced materials. For example, a NETL study demonstrated that machine learning (ML) and data analytics can be used in designing next-generation alloys to operate fossil fuel plants with greater efficiency while producing affordable electricity and lowering greenhouse gas emissions. Researchers at NETL’s Albany, Oregon, facility validated the application of ML analysis to enable more rapid and accurate design of high-entropy alloys (HEAs), critical materials for ultra-efficient power generation.
HEAs are alloys constructed with equal or nearly equal quantities of five or more metals, combining the strength and characteristics of the individual elements into a superior substance. Unlike conventional alloys, HEAs possess special properties, including outstanding wear resistance, exceptional high-temperature strength, structural stability, and superb corrosion and oxidation resistance, all characteristics needed to achieve maximum benefits in advanced power stations.
For instance, boilers and other components made from HEAs will enable power plants to burn less fuel and operate with greater efficiency at increased temperature and pressure levels. HEA components also are better equipped to withstand the stresses of frequent power plant startups and shutdowns required for integration of renewable energy sources into the grid.
NETL’s use of ML technology also eliminated the trial-and-error method to develop these advanced materials, speeding up development time while minimizing costs and expenditure of resources, making it much more feasible to deploy these alloys inside power plants.
Monitoring for Longevity
The DOE Office of Fossil Energy (FE) has supported more than 50 NETL-led R&D projects on cost-effective technologies expected to bring about near-term benefits for incorporation into commercial fossil plants. The projects are sponsored primarily by FE’s Transformational Power Generation program and the High-Performance Materials program, but also several other programs, and typically include project teams with academia and industry. These range from bench-scale testing to validation testing in actual fossil power plants.
The Transformative Power Generation program develops technologies to improve performance and extend the life of existing power plants. Program research also focuses on next-generation modular coal-fired power plants providing stable power generation with operational flexibility and high efficiency, as well as oxy-combustion and chemical looping combustion—technologies that provide options for coal-fired power generation in a carbon-constrained future.
Innovative technologies being researched include online sensors, instrumentation, algorithms, and failure mode detection systems to monitor key plant components and inform possible repairs. One such concept is condition-based monitoring, a maintenance philosophy that actively monitors the condition of equipment to predict failures and schedule maintenance to maximize availability and generating capacity while saving cost. R&D is also being performed on tools for predicting the life of critical equipment to ensure long-term safety and reliability of existing plants. For example, data analytics for coal plants for continuous controller characterization can grant operators optimized facility control, improved efficiency and economics, and improved control during load following, while detecting equipment deterioration so it can be addressed before costs mount.
Technology development efforts are focusing on rapid cycling and methods for responding to load changes. Advanced control methods for monitoring coal pulverizer operation, and controlling steam and gas temperatures at low loads are being field-tested to improve the performance and economics of existing power plants. The implementation of neural networks and sensor technologies enables intelligent control for optimal combustion system performance and can improve flexible operation capability.
Improving plant efficiency is critical to the economic viability of coal-fired power plants. Enhancements to steam cycle efficiency involve additional steam generation, pump and cooling tower upgrades, and advanced ultrasupercritical retrofit options that include increased steam temperature and use of advanced materials for enhanced heat transfer efficiency. Efficiently recycling resources, such as waste heat and water, also presents advantages for increasing plant efficiency and reducing costs.
Power plant testing of these technologies is focused on operating commercial plants and includes 21 different plants owned by 24 utilities in 12 states spanning the continental U.S. and Hawaii. Testing units range in size from 135 MW to 952 MW and include a range of coals: bituminous, subbituminous, and lignite. Boiler types include cyclone-fired, opposed-fired, and tangential-fired configurations, and span subcritical, supercritical, and ultrasupercritical boilers.
By developing the right tools and techniques alongside its partners, NETL is working toward a proactive approach in the day-to-day operations of the fleet. The discoveries made along the way, combined with changing energy trends, could also lay the groundwork for how power plants of the future are designed and built.
Perks of Partnering with NETL
NETL is uniquely positioned to bring new life and capital investment into this critical facet of the nation’s infrastructure and security, thereby sustaining local jobs and communities. This is due to the lab’s ability to engage in research that might be cost-prohibitive for industry to tackle alone, all while partnering with the private sector, universities, and other government agencies including other DOE national laboratories.
This ability to work cooperatively also helps members of industry who are front and center at plant operations. They can approach the lab with the problems they need addressed, allowing for a bottom-up process of research and problem-solving, rather than a top-down approach. By licensing its innovations, NETL is also committed to ensuring tools and techniques developed in-house make into the hands of those who will make the most use of them.
“Ultimately, our work at NETL is all about adapting with the times, and we do that by exercising our analytical capabilities and by listening to stakeholders. If need be, we adjust our program plans and activities accordingly,” White said. “We are always looking for project partners. If you are a utility, we want your input on what challenges you are seeing in the field. If you are a technology developer, we want to know what opportunities you are seeing in the market. By combining our experiences and expertise, we can find solutions that lower costs while improving performance and reducing environmental footprints. We can have the best of all worlds.”
—Briggs White is a technology manager for High Performance Materials, and Conor Griffith is a technical writer at the U.S. Department of Energy’s National Energy Technology Laboratory.