During the Last Decade, Leading Industrial Countries Initiated a New Wave of Research and Development
of supercritical (SC) steam power plants. This renewed interest, accompanied by a jump from SC steam parameters to ultra-supercritical (USC) parameters, was driven mostly by the increase in fuel costs on the world market and by increased environmental regulations. New installations are under development in certain countries around the world, but wider market penetration is still limited in some instances by cost and reliability concerns.
 Units 17 and 18 steam at NIPSCO's supercritical Schahfer Generating Station. Photo courtesy of NIPSCO. |
Based on a survey of independent power producers (IPPs) presented at the Fifth Annual Clean Coal Technology Conference in January 1997, IPPs consider advanced-steam cycle technologies (i.e., a SC steam generator) commercially proven, but more costly and risky than conventional steam-plant technology. This high-risk perception exists despite the reality that 350 SC units operate commercially worldwide, with reliabilities as high as those of conventional steam plants. Furthermore, EPRI reports that key SC plant performance parameters--availability, maintainability, cycling and low-load operation (with sliding pressure capability)--are equal to or better than those of subcritical cycles after an initial startup period.1
Worldwide Status
As a leading country in developing and installing SC units in the 1960s and 1970s, the United States today has about 86 GW of installed SC capacity. This capacity has not changed since 1991, when the last SC plant (the 1,425 MW Zimmer 1 plant) came on line. Of 162 operating units (as of 1998), 121 units burn coal, 40 units burn natural gas and 1 burns oil. Design capacities range from 300 MW to 1,400 MW. With the exception of two plants, all are designed for steam conditions of 3,300 to 3,700 psig (most at 3,500 psig) and 1,000 to 1,100 F (most at 1,050 F).
Installations of SC plants in the United States peaked during the 1970s and fell precipitously in the early 1980s. No new SC installations are planned in the United States except for speculation about a 796-MW lignite-fired unit in Texas with SC parameters of 3,910 psig/1,010 F/1,000 F.
In the United States, the Department of Energy's Federal Energy Technology Center has been developing technology for the next-generation of high-efficiency, low-emission USC boilers, called Low Emissions Boiler System (LEBS). Designed for main steam conditions of 4,500 psig and 1,100 F with two reheats, each at 1,100 F, LEBS utilizes an advanced, low-NOx slagging combustion system and an optional copper oxide flue gas cleanup system for NOx and SO2.
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Several countries in Europe and Asia are intensively developing SC and USC power plants, including Japan, Denmark, Germany and Korea; China is increasing activity as well. Figure 1 shows expected growth in SC and USC installed capacity for countries outside the United States, emphasizing the significant development programs in Japan and Korea.
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Japan has demonstrated the fastest growth in installed SC fossil capacity from 1990 to 1998. It also has the largest development program in place for building new SC coal-fired plants, scheduled for 2000 to 2005. This growth is accompanied by a continuous growth in single-unit capacity and steam parameters. The typical size of a Japanese SC coal unit in the mid-1990s was in the range of 400 to 700 MW; recently commissioned units at the Haramashi and Shinchi power plants are 1,000 MW, and units planned to be commissioned beyond 2000 will reach 1,050 MW.
Shinchi power station exemplifies Japan's efforts to push power plant technology to its limits.2-3 With a measured thermal efficiency above 43 percent (LHV), Shinchi is one of the largest coal-fired power plants in the world using sliding-pressure technology, which avoids losses associated with throttling valves at low loads. High-pressure steam is maintained at 1,200 psig for loads up to 320 MW. As loads increase to 960 MW, high-pressure steam slides up to a maximum 3,700 psig.
Korea has an ambitious program for fossil power plant development, similar in design to the "progressive standardization" method applied in the former Soviet Union. Under this plan, Korea has built 16 "standard" 500 MW SC, once-through units between 1990 and 1999, designed to burn bituminous coal with an ash content up to 17 percent and a sulfur content up to 1 percent. These units are designed for a steam pressure of 3,600 psig and superheated and reheated steam temperatures of about 1,005 F. The boilers are equipped with progressive NOx control features such as low-NOx burners and close-coupled overfire air ports. This standard design has achieved: 1) substantial savings in engineering design and construction costs, 2) improved plant reliability and availability, 3) reduced operational and maintenance problems, and 4) interchangeability and local replacement of spare parts.
Danish power companies operate six SC power plants. All units are firing imported bituminous coal shipped to Denmark in large vessels at competitive prices. Fuel cost, stringent environmental standards, and access to cold sea water were factors in driving the Danish power sector toward higher steam parameters in SC and USC boilers. Unit 3 at Nordjyllands has a planned net efficiency of 47 percent (LHV). This unit belongs to a group of so-called USC convoy units consisting of natural gas-fired and coal-fired 400 MW boilers with identical steam conditions (4,500 psig/1,090/1,090 F).
Denmark's next approach for increasing net efficiency involves a conventional PC combustion unit operated in parallel with a gas turbine for condensate/feedwater preheating. The startup of the first proposed unit of this kind, Avedore 2 in Copenhagen, was originally planned for 1999, but recently has been postponed.4-5 The plant is a combined heat and power (CHP) unit that would meet a Danish requirement for burning biomass and could fire either coal or gas as the main fuel. A gas-fired turbine is used to generate additional power, and its exhaust is used to heat the feedwater to the boiler, thereby enabling the steam turbine to generate more power. Supercritical steam conditions with 4,350 psig pressure and 1,076 F/1,112 F temperature, and a cooling water temperature of 50 F, give the plant an efficiency of just over 48 percent (LHV). Boiler feedwater temperature is 590 F and condenser pressure is 2.9 psi.
Performance Improvement
Current SC/USC efficiencies for large power plants are in the range of 43 to 45 percent (LHV), while efficiencies up to 50 percent (LHV) are expected in new plants by 2010 to 2015. Increasing the full-load efficiency of conventional SC fossil fuel units is generally based on a step-by-step approach. Various measures can be used to increase thermal efficiency relative to current conventional SC practice (Figure 2).
Steam conditions: Increasing steam pressure and temperature from 3,630 psig/1,000 F to 4,350 psig/1,110 F can increase efficiency by nearly 2 percentage points. Two important issues related to increased steam pressure and temperature, however, must be addressed: 1) development of materials with high-temperature strength and corrosion resistance for boiler and turbine pressurized metal parts, and 2) slag formation and fouling of waterwall and superheater tubes.
Materials used for manufacturing the boiler and the high-temperature turbine parts must satisfy the following mechanical criteria: adequate creep strength at operating temperature; adequate resistance to high-temperature corrosion; adequate formability, weldability and joining; and adequate thermal conductivity.
Second reheat: A second reheat stage can boost efficiency another 0.8 to 1.0 percentage points.6 This concept has been in use since the 1960s on many subcritical units and at least five SC units. These systems, however, are still not cost-effective in most cases and tend to limit operational flexibility. There are only two existing coal-fired USC plants using double reheat design--Eddystone 1 & 2 in the United States (steam temperature 1,280/1,050/1050 F) and the Nordjyllands convoy units (steam temperature 1,080/1,076/1,076 F) in Denmark.
Condenser Pressure: Decreasing the condenser pressure from 9.42 psig to 4.35-3.62 psig can further increase efficiency by 1.5 to 2.0 percentage points. The availability of cold seawater for cooling is one of the reasons for the very high efficiencies achieved at Danish USC plants.
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Low temperature heat recovery: Every 10 F reduction in stack gas exit temperature (while recovering the heat involved) increases efficiency by 0.1 to 0.15 percent. Low-temperature corrosion concerns limit large temperature reductions, but the flue gas exit temperature can typically be maintained at 300 to 350 F depending of the fuel sulfur content.
The net efficiency trend for SC and USC units commissioned and planned to be commissioned between 1990 and 2000 is presented in Figure 3. The average net efficiency will increase up to 46 percent (LHV) by 2000, with higher numbers for units in Denmark (up to 48 percent LHV). In Japan and the Netherlands, this increase is more conservative, and will be even lower for German power plants, because almost all units under construction and planned for the near future are designed for lignite, which is of lower quality than coals used in Japan and Denmark.
Availability and Reliability
Studies of the relative reliability of coal-fired subcritical and supercritical plants have shown that conventional subcritical boilers have had better reliability during their first 10 years of operation.6 After 10 years, the average outage time caused by the pressure parts of SC units had leveled off at less than 500 hours/year (representing about 94 percent availability), comparable to figures for subcritical plants. Availability of older SC units is as good as subcritical units, but only when used for baseload duty. These data have been confirmed by long-term experience with SC boiler operation in the former Soviet Union and Italy.7-8 The average annual availability factor for all 300 MW units in the former Soviet Union from 1990 to 1995 was 95 to 97 percent, which is higher than SC power plant availability in the United States and Germany, where the best units have availability factors of 94 to 97 percent and average values lie between 75 and 85 percent. The capacity factor for the 300 MW Soviet units before the collapse of the USSR was also rather high, at 66 to 72 percent.
An important feature of steam boilers is their load-following capability, which includes two characteristics: 1) ability for fast startup from different conditions, and 2) ability to handle sharp changes in load. SC once-through boilers, because of the absence of a drum and other thick-walled parts, require 15 to 20 percent less time for cold startups than conventional boilers. Using full/partial flow separators, modern once-through SC boilers also are capable of very fast load changes, typically 3 to 4 percent per minute, and even 5 percent per minute when using an advanced control system.
References:
- "Assessment of Supercritical Power Plant Performance," EPRI Report No. CS-4968, 1986.
- "Flexibility, efficiency are hallmarks of latest coal-fired units," Power, April 1996, p. 43.
- Jones, K., "Shinchi Leads Way for Large Advanced Coal-Fired Units," Electric Power International, Third Quarter, 1997, p. 36.
- Moscatelli, K. and G. Sormani, "Avedøre No. 2 CHP Plant -- The Most Advanced Steam Turbine in the World," Ansaldo Energia, POWER-GEN Americas '97.
- "Parallel Powering for Avedøre No. 2 Power Plant," Modern Power Systems, Vol. 16:5. May 1996, pp. 31-36.
- Couch, G. "OECD Coal-Fired Power Generation -- Trends in the 1990s," IEA Coal Research, The Clean Coal Centre, 1997, 83 pp.
- Lysko, V., et. al., "New Generation of Steam-Turbine Power Units," All-Russia Thermal Institute, Teploenergetika, No. 7, 1996.
- Benaty, A., et. al., "Design, Construction And Operational Experience In Supercritical Boiler Of 660 MW Power Units," POWER-GEN Asia O95, Vol. 1, pp. 611-645.
Acknowledgements:
This article is adapted from a paper presented at the 24th International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, Fla., March 1999.
Authors
Victor A. Gorokhov, Ph.D., is a senior engineer/project manager with Science Application International Corp. He has more than 25 years' experience in power engineering and environmental protection. Gorokhov holds a Ph.D. in power engineering from the Ukrainian National Academy of Science, and an M.S. in power engineering from the Azerbaijan Institute of Oil and Chemistry, Baku, USSR.
Massood Ramezan, Ph.D., P.E., is principal engineer with Science Applications International Corp. (formerly with Burns and Roe Services Corp.). He has more than 20 years' experience in areas of energy and environmental control technologies. Ramezan holds B.S., M.S. and Ph.D. degrees in mechanical engineering from West Virginia University.
Lawrence A. Ruth, Ph.D., is senior management and technical advisor in the Office of Power Systems Product Management at the U.S. Department of Energy's Federal Energy Technology Center. He is responsible for coordinating the development of the Vision 21 program. Ruth holds a Ph.D. in chemical engineering from the City University of New York.
Soung S. Kim, Ph.D., is project manager of the Low Emission Boiler System (LEBS) project at the U.S. Department of Energy's Federal Energy Technology Center. Prior to this position, she was a supervisory engineer at the Westinghouse Electric Corp. Kim holds a Ph.D. degree in chemical engineering from Carnegie Mellon University.