Thermal efficiency improvement for economic gain has been an important engineering endeavor for more than 250 years. A few examples: The Newcomen steam engine appeared in 1750 and attained 0.5 percent efficiency . James Watt patented improvements in 1769 and achieved 2.7 percent efficiency by 1775, launching the Industrial Revolution. American Electric Power’s (AEP) Philo Plant Unit 6 steam generator was the first commercial supercritical unit in service early in 1957 (Figure 1). And in 1959, Philadelphia Electric 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 and later operated at 32.4 MPa . These units, using stainless steel materials, led the world toward commercial supercritical boilers.
Once-through supercritical plants became interesting to the U.S. market again around 2000 and most new electric utility coal-fired plants have been supercritical, with variable pressure operating mode. Two Babcock & Wilcox Power Generation Group Inc. (B&W PGG)
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design efforts have been underway in this period. In one effort, the DOE and OCDO are sponsoring a materials development program by a consortium of boiler vendors that are seeking qualification of ASME Code Section I alloys suitable for 760°C (1,400°F) turbine throttle steam temperatures [3, 4]. The second 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, 735°C/760°C (5,000 psi, 1,356°F/1,400°F) with a final feedwater temperature of 343°C (649°F).
Increased efficiency reduces CO2 emissions, the costs of carbon capture, water use, particulates, sulfur oxides (SOX) and nitrogen oxides (NOX) emissions and fuel consumption. Research and development programs are being conducted worldwide to advance the technology in 700°C (1,292°F) steam generator design and materials development of the needed nickel-based alloys.
The first project will be discussed in this paper.
Research programs in both Europe (such as the THERMIE AD700 program) and in the U.S. DOE Boiler Materials for Ultrasupercritical Coal Power Plants have set a goal to improve thermal efficiency and reduce carbon dioxide emission through application of materials with higher temperature capability up to 760°C (1,400°F) . The Electric Power Research Institute (EPRI) manages the project and the consortium includes the U.S. domestic boiler manufacturers B&W PGG, Alstom Power, Babcock Power and Foster Wheeler. There is also a sponsored program for the development 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 research.
Advanced cycles, with steam temperatures up to 760°C (1,400°F), will increase the efficiency of coal-fired plants, before adding CCS, from an average of 36-39 percent efficiency (for the current domestic fleet) to about 47 percent (HHV). This efficiency increase will enable coal-fired power plants to generate electricity at competitive rates while reducing CO2 and other fuel-related emissions by as much as 17-22 percent. Steam temperatures and pressures up to 760°C/35 MPa (1,400°F/5,000 psi) are required. Combining CCS with A-USC plants will provide lower cost of electricity generation with 90 percent 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 oxycombustion 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 AUSC with B&W PGG/AL oxy-combustion. The HHV efficiency of 39.4 percent for the current 600°C (1,112°F) state-of-the-art plant is nearly regained by using A-USC with oxycombustion, which provides an efficiency of 38.9 percent.