A national R&D program has been under way to develop materials technology for constructing boilers and turbines capable of operating at advanced ultrasupercritical steam conditions in pulverized coal plants. The large-scale, multiyear, joint government/industry project seeks to increase the efficiency of power plants by increasing their steam conditions up to 1,400F (760C) at 5,000 psi (35 MPa). The ongoing project has already identified the materials and processes for successful operation at these higher steam conditions.
One method of keeping coal a viable generating fuel option in the face of increasing pressure to reduce carbon dioxide (CO2) emissions may be to develop advanced, high-efficiency, ultrasupercritical coal-fired power plants. The high efficiency of these plants enables fuel cost savings, lowers balance-of-plant costs (due to less coal pulverization, fuel transportation, ash handling, cooling water, and many other requirements), and reduces the amount of CO2 needing to be captured for a given amount of electricity generation.
Most coal-fired plants built in the 1950s and early 1960s operated with steam at a pressure of approximately 2,400 psi (16.5 MPa) and temperatures up to 1,000F (538C). Defined as “subcritical,” these plants achieved efficiencies of about 35% higher heating value (HHV) (see table).
 |
| Steam conditions and plant efficiency of coal-fired power plants. Source: EPRI |
Supercritical plants, which came into widespread use during the 1960s, operate at 3,600 psig and temperatures up to 1,050F, and achieve efficiencies of 38%.
Since the late 1970s, research has been under way to increase the live and reheat steam temperatures of coal plants, which is the most cost-effective means of improving plant efficiency. Recently, nearly two dozen so-called ultrasupercritical (USC) plants have been commissioned worldwide with main steam conditions of 3,400 to 4,200 psi and 1,080F to 1,112F. USC plants are expected to achieve a thermal efficiency (based on fuel HHV) of more than 42% (Figure 1). In the U.S., the boiler outlet steam conditions of American Electric Power’s 600-MW John W. Turk, Jr. Power Plant, now under construction in western Arkansas, are 3,625 psia and 1,115F/1,130F (see “Designing an Ultrasupercritical Steam Turbine,” July 2009). Turk will be the first USC plant built in the U.S.
 |
| 1. Technology push. The progressive increase in steam conditions pushes the metallurgical limits of today’s alloys. Source: Alstom Power |
The capability of plants to operate at higher temperature and pressure is dependent upon their materials of construction, and the key limiting factor of these materials is their inherent creep strength. Creep is the time-dependent deformation of materials, which generally takes place in a material subjected to sufficient stress at a temperature above half its melting temperature. To address this issue, the main enabling technology in designing USC plants has been the development of high-temperature materials with greater creep strength, including creep-strength-enhanced ferritic steels, advanced austenitic alloys, and nickel-based alloys.
Aggressive research and development (R&D) programs for alloy development and evaluation in Europe, Japan, and the U.S. have identified ferritic steels capable of meeting the duty requirements of USC plants to approximately 1,150F. Several European projects have completed research, or are nearing completion of research, to achieve steam conditions of about 1,290F and 5,500 psi with the help of nickel-based alloys. These projects include the Advanced (700C) PF Power Plant Project (AD700) and the Component Test Facility for a 700C Power Plant (COMTES 700).
U.S. DOE/OCDO Ultrasupercritical Project
In the U.S., the DOE/OCDO Advanced Materials for Ultrasupercritical Coal-Fired Boilers project, an approximately $50 million R&D program, is under way to build on these capabilities. The program aims to identify, evaluate, and qualify high-temperature materials technology for the construction of coal-fired boilers and turbines.
The U.S Department of Energy (DOE), through the National Energy Technology Laboratory (NETL), is the majority funder for the project, with significant cofunding coming from the Ohio Coal Development Office (OCDO). Energy Industries of Ohio is managing the program, and the Electric Power Research Institute (EPRI) is providing overall technical direction and coordination. The consortium has also included the best talents of all the major U.S. boiler manufacturers (Alstom Power, Riley Power, Babcock and Wilcox, and Foster Wheeler), steam turbine manufacturers (GE, Alstom, and Siemens), and Oak Ridge National Laboratory. Each of the industrial participants and EPRI is also contributing 20% of its own costs.
The U.S. goal to develop material for USC plants differs from the European goal in several respects. The U.S. program is aiming to achieve higher steam temperatures than the European program (1,400F versus 1,290F) for a so-called advanced ultrasupercritical (A-USC) plant, which necessitates the evaluation of different alloys. Also, because coals used in the U.S. typically contain higher sulfur and moisture than those used in Europe, the U.S. program is seeking alloys with corrosion resistance or those to be used with coatings or surface modifications. In addition, the U.S. program must also obtain American Society of Mechanical Engineers (ASME) code acceptance of boiler materials.
Email
Comments
Print
Reuse
