台灣未來超臨界電廠之發展 Supercritical Power Plants in Taiwan

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1 台灣未來超臨界電廠之發展 Supercritical Power Plants in Taiwan 莊嘉琛 * 1 蘇燈城 * 2 J.C. Sue, C.C. Chuang * 1 台北科技大學冷凍空調工程研究所 * 2 台北科技大學機電科技研究所 摘要由於目前燃料價格 ( 石油及煤碳 ) 飆漲, 興建高效率的超臨界機組為目前台灣電力開發之要務, 主要理由為高燃料價格 相對地需提高機組的效率, 以降低運轉成本 由於超臨界機組採用高效率之空氣污染防治設備使得空氣污染排放量降低 關鍵字 : 超臨界 串列式汽輪機 複列式汽輪機 鍋爐 汽輪機 ABSTRACT Due to the fuel cost increasing recently, the high efficiency of supercritical steam cycles have planned to erect in Taiwan area. The major reasons are higher fuel prices, the premium that competition has placed on operating efficiency, and the inverse relationship between thermal efficiency and the emission levels. Keyword : Supercritical, Tandem compound, Cross compound, Boiler, Turbine 1. Introduction The steam power plants commonly use coal, natural gas, heavy fuel oil, or nuclear energy as the source of heat. A few smaller plants use biogases; saw dust or other waste products as the heat source. The modern trend is toward supercritical units, which have the advantages of larger capacity, higher efficiency, and lower operating costs. The capacity of the current generation of supercritical units is 800 MW or higher. About 85% of the supercritical units scheduled to come of line after 2003 will be 800 MW or larger. The main steam design pressure is 24 MPa to 25 MPa and main steam/reheat steam design temperature is 570 to 590 for 60 Hz operation. The capital cost of an 800 MW supercritical unit is around US$ 930/kW (as of April 2004). The supercritical boiler is a once-through design which (with sliding pressure) means heating, evaporating, and superheating of the incoming feed water are completed within the single tube without installing any steam drum. The typical boiler-turbine island net efficiency operating in the supercritical regime is 42.5% (after the auxiliary power consumption is deducted) with high degree of reliability. The supercritical plant flow diagram is shown on Fig.1[3]. The net efficiency of turbine-island itself is 47%. The power consumption of yard systems such as Air Quality Control System (AQCS) and coal handling system, etc. is around 2.5% of steam turbine power output. The boiler minimum load condition and load swing operating condition is determined by mill & burner system performance. The supercritical plant efficiency is increased by 10% compared to the sub-critical units. The operating cost of supercritical plant saves a significant percentage compare to the conventional units

2 Fig MW Supercritical Plant Flow Diagram 1.1 Development of Supercritical Units in the World Up to now, there are over 500 supercritical units in commercial operation since the first one (unit capacity of 88 MW) was put into commercial operation in West Germany in In the Asian area, Japan made the technical transition from USA and built the first supercritical unit (600 MW) with very good commercial performance since its debut in Thereafter, the other countries like as Mainland China, Korea, and Indonesia decide to build supercritical units. In Taiwan, Mai-Liao Power Corp., a subsidiary of Formosa Group, built 7 supercritical units (600 MW), which boilers were manufactured by Mitsubishi Heavy Industries, Ltd. (5 units) and Combustion Engineering Inc. of USA (2 units). Those units were put into commercial operation in 1997 and since then, the development of fossil power plants in Taiwan was advanced to a new era. The initial reliability and availability of supercritical units were not as good as sub-critical ones. Nowadays, the reliability and availability of supercritical units reaches to or even exceeds the level of sub-critical units due to the improvements in material technology, development of more responsive control processes, and the accumulation of operation experience. The benefits supercritical units include lower heat rate, lower emission levels per MW produced and a fast startup/shutdown capability. 1.2 USA The first supercritical unit for commercial operation in the USA was Philo power plant with capacity of 125 MW (operating steam conditions: 31 MPa/621 /566 /538 ) in Thereafter, Eddystone power plant with capacity of 325 MW (operating steam conditions: 34.3 MPa/649 / 566 /566 ) was put into commercial operation in In the 1967~1976 period, 118 supercritical units were built and the maximum unit capacity was 1,300 MW. During this period, nuclear power plants were being built for use as base-load units. Due to the low availability for supercritical units; the installation of these units was slowed. In 1980s, the environmental concerns for greenhouse gases and ill-conceived radiation worries dominated, causing oil or gas fired combined cycle units to be substituted for coal and nuclear units to meet the rising demand for power. However, in the late 1990s, due to the improvement of the air pollution control equipment and the ability to deliver low cost from open pit mines by unit trains, construction of supercritical units was on the re-bound. The new supercritical units in the USA recently constructed are: (1) The third stage renew plan of Genesee power plant (1x450 MW) in State (2) Oak power plant (3x600 MW) in State (3) Council Bluffs power plant (1x900 MW) in State (4) Big Cajon power plant (1x675 MW) in State Up to now, there are 169 supercritical units in commercial operation, and almost 70% have a unit capacities of 500~800 MW. 1.3 Japan In 1961, the "Supercritical Pressure Fossil Generation Committee" conducted investigation and feasibility study on supercritical units for one year and submit a report to confirm the necessity and feasibility of development of supercritical units in Japan in In 1964, Japanese Govern proposed to study and manufacture supercritical units for all units with capacity of 450 MW and above. Based on the said conclusions of this report, supercritical units now share about 61% of total capacity of the Japanese power units built since then. Originally, the supercritical technology of Japan was from USA, and in order to have the capability of partial load operation and Daily Start-Stop (DSS) operation, the technology of sliding pressure from Europe was incorporated into the power plant design. After many times of Research & Development studies and improve- ments, the 5-2-2

3 thermal efficiency was increased and the availability was improved to match that of sub-critical units. Therefore, Japan becomes the leading country in the development of supercritical units with unit capacities as large as 1,000 MW. In Japan, major power generation equipment makers deem unit capacities of 700 MW and 1,000 MW as standard products. Main steam conditions of 24.2 MPa/600 are also taken as state-of-art operating conditions. Some makers study to raise main steam conditions to 34.5 MPa/620~650 and hope to increase the thermal efficiency another 10%. 1.4 Europe The development of supercritical units is concentrated in Germany and Italy. Germany is the first one to pragmatize the supercritical generation technology. The matured steam conditions of early units were 25 MPa /540 /540. In recent years, considering low fuel cost and strict environmental protection regulations, different low emission coal-fired technologies were studied. Finally, it was concluded that higher steam conditions could meet the stricter requirements. Therefore, supercritical units were reconsidered in 1990s. As of the year 2000, more than 10 supercritical units are in commercial operation and the unit capacities range from 400 to 1,000 MW. The sliding pressure design was incorporat- ed into the German supercritical units at an early stage of the design process. Most of the supercritical units were operated in the daily Start-Stop mode, and had a good control capability for load change. The major steam conditions of supercritical units are 26 MPa/540~580 / 560~ 600 in Germany for present stage. Italy is another European country develop- ing supercritical units. Up to now, there are 13 supercritical units in commercial operation (of which 5 units are coal-fired) with unit capacities of 600~660 MW and steam conditions of 24.2 MPa/538 /538. Nine units with unit capacity of 660 MW are planned to build in the near future. Another country, Denmark, imported supercritical technology from Germany, and has 3 supercritical units (unit capacities of 350~412 MW) in operation, all using seawater as the condenser cooling media. The thermal efficiency of the 412MW unit is 49%, making it the highest in the world. Its steam temperatures are 580 /580 /580 with double reheat and the sea water temperature averages Selection of Unit Capacity for Taiwan Area The current trend for new power plants installed in Japan, Korea and Mainland China, is for unit capacities to be larger than 800 MW. Units with capacity of 800 MW or above scheduled to be put into commercial operation after 2003 share about 86% of the new construction. Taking Korea Electric Power Co. (KEPCO) of Korea as an example, many standardized 500MW supercritical units were installed in the past as a way to unify construction and O&M management. Lately, due to the problems of seeking new plant sites and transmission line right-of-ways, same as what we have in Taiwan power market, reducing the power generation costs by increasing the capacity of each unit is being implemented. Therefore, KEPCO established a new standardized unit capacity of 800 MW after three-year investigation of the technology capability and productivity of local related heavy industry manufacturers. Most Japanese supercritical units in commercial operation range from 700 MW to 1,000 MW and those in Korea range from 500 MW to 800 MW. This is in contrast to Taiwan, where Taipower largest unit is a sub-critical 550 MW. Because the power generation costs can be reduced by increasing the unit capacity and in consideration of unit capacity development in both Japan and Korea, it is recommended to increase the unit capacity for new units here in Taiwan. Generally, there are four types of proven commercialized Steam Turbine Generator (STG) for large capacity supercritical units, that is, 700 MW (50/60Hz, as in Japan), 800 MW (60Hz, as in Korea), 900 MW (50Hz, as in Europe) and 1,000 MW (50/60Hz, as in Japan & 50Hz, as in Europe). Among which, only two classes, 800 MW and 1,000 MW, meet Taipower requirements in consideration of 60Hz cycle system and proven commercialized technology. Hence, it is recommended to select one type from the 800MW class or one type from the 1000MW class for new development project. The considerations for selection of unit capacity are commercial operation experiences, compatible technology development of STG, Taipower operating experiences, generation cost, unit arrangement, construction period, availability, reliability, and anticipated load demand, etc. (1) Commercial operation experiences: 5-2-3

4 The commercial operation experiences of different unit capacities are stated in previous section. (2) Compatible technology development of STG: There are two types of STG layouts, Tandem-Compound (TC) type and Cross- Compound (CC) type. The TC type is predominates for large capacity STGs. The comparisons of both types of layout and the development trends of STG are described in I type of turbine. (3) Taipower operating experiences: Taipower has operating and maintenance experiences of over 60 years with the TC type sub-critical STG, but are without experiences with supercritical units. (4) Generation cost: According to the data of Table 1, it shows that based on the same fuel cost, the larger unit capacity will have the lower investment cost. When the unit capacity is 1,000 MW, the total investment cost is 91.87% compared to that of a 500 MW unit while an 800 MW will save 3.7% compared to that of 500 MW units. Table 1 Generation Cost Comparison between Different Unit Capacities [1] Unit Capacity(MW) Investment Cost(US$/kWh) O&M Cost (US$/kWh) Total (US$/kWh) (a) Plant design life is 25 years (b) Plant capacity factor is 80% (c) Interest rate is 4.5% Besides, Table 2 shows the equipment investment estimates. Although the cost of boiler island, turbine island, AQCS, civil, architecture and construction for 1,000 MW unit is about 25% higher than that of 800 MW one, however, the unit cost will be US$475 per kw for 1,000 MW unit compared to US$498/kW for 800 MW one. Thus, from the point of view of economy, it is feasible to enlarge unit capacity as required. Table 2 Comparison of Investment Cost for 800MW and 1000MW Coal-Fired [1] Unit Capacity (MW) Boilers US$104 x 10 6 US$124 x 10 6 Turbine, Auxiliary Equipment US$112 x 10 6 US$134 x 10 6 Civil, Installation US$158 x 10 6 US$187 x 10 6 Environmental Protection Equipment US$25 x 10 6 US$30 x 10 6 Unit Cost per kw US$498/kW US$475/kW Costs are for a power plant erected by local labor using local and worldwide procurement. (5) Units' availability In consideration of availability, from the information provided by Babcock-Hitachi that the availability is more than 90% for unit capacities from 500 MW to 1,000 MW, which is higher than average of 88.2% (as of 2001) of Taipower s existing sub-critical units. Therefore, from above analyses, they are concluded that: The larger the unit capacity is, the lower the investment cost will be. Based on the same total capacity for one site, the space required for larger capacity with fewer units is smaller than that for smaller capacity with more units. (e.g. 3 x 1,000 MW vs. 4 x 800 MW) The construction period difference between 800 MW and 1,000 MW units is 5.5 months. The availabilities for various unit capacities are almost same, but obviously higher than sub-critical ones. 2.1 Selection of Steam Conditions Since the higher steam temperature will reduce the plant heat rate and increase the plant thermal efficiency, all power plant designers /makers do their best to raise the main steam and reheat steam temperatures the turbine inlet in recent years. According to the study by the Alstom Company, for single reheat unit, the heat rate will be improved 1.25% for each 25 of the steam temperature is raised. Nevertheless, the materials for the turbine and boiler must be upgraded to withstand the higher temperatures at the same time

5 The cost for the material upgrade will be 10% more if the main steam and reheat steam temperatures are increased from 535 to 565. The optimum reheat steam pressure for modern power plant is 20% to 25% of the main steam pressure. Consideration of the erosion of blades in the LSB of steam turbine, the maximum moisture content at the turbine exhaust is not allowed to exceed 12%. Too low the reheat pressure (< 2.5%) results a negative efficiency improvement. In order to increase the thermal efficiency and reduce the heat rate, some makers design double reheat type, which heat rate is at least 1.5% less than the single reheat one of same unit capacity. However, the design and construction are more complicated, the difficulties of operation and control are increased, and the time and cost of maintenance are also increased 10% for double reheat unit. Hence, unless the fuel cost is relatively high, single reheat design is the best choice for the supercritical units. 2.2 Limitations on Physical Metallurgy According to the study report of Siemens, the limitations on physical metallurgy have high relation with the costs of material for specific steam condition. If the costs of major pressure parts and superheaters/reheaters are included, the best selection of boiler tube materials under different steam conditions needs to be evaluated together. A Ni-based alloy is under development for next generation of pulverized coal-fired power plant with steam conditions of 30 MPa, 700 and thermal efficiency of 50%. The pilot plant is scheduled to build after 2010 and its commercial operation started in Fig.2 shows the relationship of the steam temperature and efficiency improvement [3]. Efficiency improvement. 2.3 Trend of Development of Steam Temperature and Pressure Nowadays, the main steam temperature for most power plants in commercial operation is 540 C ~580 C coupled with a reheat steam temperature 580 C ~620 C and is considered to be a mature design. Because of the improvement in metallurgy, main steam of the 620 C~640 C range is under development. The target of 700 C /720 C for throttle and reheat steam is expected to be reached in the next 15 years. The main development of main steam pressure is 23.5~24.5 MPa. As for the trend of future development, Japanese designers prefer to change the steam temperature rather than change pressure to maintain pressure at the level of about 24.5 MPa. European designers prefer to change steam temperature and pressure simultaneously and focus on increasing the turbine throttle pressure to 26.5 MPa (using material of special alloy steel for pressure parts), and even up to MPa. For increasing the thermal efficiency and reducing the consumption of fuel, major designers in Japan and Europe are developing power plants with steam conditions of 28.4 MPa/600 C/620 C. These advanced units are undergoing market surveys and cost analyses and are expected and launched for marketing in the next two years. The thermal efficiency of a coal-fired power plant is aimed to be over than 50%, and whether the plant with steam design conditions of 36.3 Mpa /700 C/720 C can become operational depends on the development progress and results of a suitable nickel based alloy. Fig.3 presents the advanced steam power plants development in the coming years. Fig. 2 Relationship of Steam Temperature and Fig. 3 Advanced Steam Power Plants 5-2-5

6 Nowadays, the steams design conditions of the most recent commercialized plants in 600MW to 1,000MW are as follows: (1) Main steam pressure: 23.5~26.5 MPa (2) Main steam temperature: 540~600 C (3) Reheat steam temperature: 540~610 C 2.4 Evaluation and Selection of Steam Temperature and Pressure As Taipower has no experience in operating coal-fired supercritical units and for overcoming the technical difficulties and risks in the switch from sub-critical to supercritical units, and after conducting study on similar overseas commercial units and FPC's (Formosa Petrochemical Corp.) IPPs (24.6 MPa/538 C/566 C) and Taipower consideration, the design conditions are concluded as follows: (1) Main steam pressure: 23.6 MPa gage (2) Main steam temperature: 566 C (3) Reheat steam temperature: 566 C (and 593 C for future project, if possible) (4) Condenser pressure: 7.45 kpa. (5) Design the condenser for 28 seawater with a maximum rise of 7 The plant operated at sliding pressure has 0 to 4% better efficiency than constant pressure at partial load conditions due to minimizing throttling loss of governing valves. Normally, the plant loading higher than 85% and lower than 35% is recommended to operate at constant pressure. The plant loading between 35% and 85% is suggested to operate at sliding pressure. 2.5 Evaluations and Selection of Turbine Type In large capacity fossil power plants, the design and selection of last stage blades (LSBs) of LP turbines are always a major consideration when deciding the final design condition for a new unit capacity. LSBs should be designed wide and long in order to work efficiently under the circumstance of expanded wet steam and prevent erosion of the blade surfaces by the wet steam. The LSBs are the longest blades in the turbine. These long LSBs must withstand very high centrifugal forces and vibrational stresses at the 60 cycle operating speed of 3,600 rpm. The LSBs have a critical speed that is affected by shape, contour, support, shrouding, loading and other factors that must be considered when selecting a set of LSBs for a turbine. LSB must be designed with lightweight, hard and high strength alloys for the low-pressure end of large capacity steam turbines. To avoid the aforementioned problems due to the limitation of physical metallurgy and manufacturing technology, the LP turbines requiring long LSBs are designed for operation at 1,800 rpm for 60 Hz units. (Same as that for nuclear power plants, which operate with very little superheat). Hence, steam turbines with high megawatt rating can be divided into a HP-IP section rotating at 3,600 rpm driving its own generator and a LP section rotating at 1,800 rpm driving a separate generator. This is known as cross compound (CC) generating unit. Usually the two turbines are side-by-side, allowing for a shorter but wider turbine pedestal and building. 2.6 Type of Turbine The TC turbine normally is utilized in an inland-based utility or for driving process equipment. Its high, medium and low-pressure turbines are connected line in (tandem compound or TC) by couplings for driving a generator or other machinery. Overall, this arrangement forms a relatively long, narrow assembly. In the case of a cross compound (CC) turbine installation, the high, and intermediate pressure turbines are in one or two casings which, coupled together, drive a generator operating at 3,600 rpm. Crossover lines deliver steam from the HP-IP exhaust to the one or two double flow LP turbines, which drive its own generator operating at 1,800 rpm. The LP turbine(s) are usually located alongside the HP-IP. This arrangement has the advantage of reducing the stresses in the LSBs or allowing much longer blades for the same stress levels. 2.7 Difference of TC and CC Type Turbine There is no any major difference of design for the high and medium pressure turbine between TC and CC units except following: (1) Low-pressure Turbine: Larger size LSBs are designed for CC turbines, thus requiring a larger diameter LP cylinder. In the contrast, the TC turbine is designed with high 5-2-6

7 speed LSBs, allowing its cylinder body to be more compact. (2) Control of Turbine: Because there are two separate shafts operating at different speeds in a CC turbine and steam goes through the crossover lines into the low-pressure turbine, additional Speed Matching Valves are required to avoid over speed in the event of a unit trip and for startup control. The interface of operation and control of the two separate shafts is more complicated. (3) Generator: The CC unit has a second generator and exciter unit due to the different outputs for the two turbines; separate isophase bus ducts and main transformers are also required. Furthermore the two generators must be properly synchronized for function. (4) Primary and Secondary Systems for Turbine Generators: that from the point of view of dimension, structure, O&M and initial equipment cost, the TC STG is a better choice than the CC STG. However in earlier decades, because of mechanical limitations, the CC turbine with an output above 600MW was the choice for most of utilities. After the 1980s, because of improvements in metallurgy and manufacturing technology, 34 and 36 inch long LSBs have successfully operated in TC turbines having a rating of 700~900 MW. 3. Conclusion Install 800MW supercritical unit with steam condition (24MPa/566 C/593 C) for first stage units. The unit capacity can be increased to 1000MW and steam temperature will be higher than 600 C after the supercritical units has been successfully operated in Taiwan. Fig.4 shows the supercritical technology trend for the near future. CC type turbine requires one set more than TC type does. (5) Startup speed: CC type turbine is 1~2 minutes later than TC type. (6) Civil and Foundation Dimension for STG Housing: According to information provided by Toshiba, civil and foundation housing dimensions for the TC units is 88% and 70% respectively compared those of CC type turbines. (7) Height of Overhead Service Crane for Turbine Generator: According to Mitsubishi Heavy Industries information, height of overhead service crane for CC. STG is about 1 meter higher than that of TC STG. Depending on plant arrangement, the crane span and capacity will be designed properly. (8) Cost: According to information provided by Toshiba and Alstom, initial equipment cost for the CC unit is 12~28% higher than that of TC unit and its O&M cost is 13% higher than for a TC unit. Besides, in consideration of initial equipment cost evaluation by TC 750MW as a base, the cost for 800MW, 900MW and 1000MW of TC type will be 6%, 17% and 26% higher respectively. The above explanation indicates very obviously Fig. 4 Supercritical Technology Trend Supercritical plant thermal efficiency is a function of a variety of factors such as coal quality, auxiliary power consumption (6% of power output), condenser pressure, and the assumed heating value of fuel (HHV vs. LHV), etc. The U.S. practice is to express efficiency in terms of HHV, while Europeans generally express efficiency based on LHV. Therefore Due to differences in these factors, European plants generally cite values of efficiency that are 3 to 5% higher than those cited by U.S. plants. Keep that in mind when attempting to make apples to apples comparisons of efficiencies.[2] 4. Acknowledgement Our company, Pacific Engineers & Constructors, Ltd., has performed the Supercritical Units Feasibility Study for Taipower. The supercritical unit capacity, boiler type, steam turbine type, steam pressure & temperature design conditions, AQCS, and balancing of plant, etc. are analyzed and evaluated in the feasibility study report. This paper 5-2-7

8 is to point out the key issues points for engineer reference. I deeply appreciate my advisor, Dr. Chia-Chin Chuang, who coaches my study on energy utilization field and evaluation of plant performance. 5. Reference 1. Changong Supercritical Unit Feasibility Report (Revised Edition), TPC (PECL), April Power Business and Technology for the Global Generation Industry, Vol.148, No.3, April European Supercritical Plant Development, Alstorn, Table & Figure List 1. Table 1 Generation Cost Comparison between Different Unit Capacities. 2. Table 2 Comparison of Investment Cost for 800MW and 1000MW Coal-Fired 3. Fig MW Supercritical Plant Flow Diagram 4. Fig. 2 Relationship of Steam Temperature and Efficiency improvement. 5. Fig. 3 Advanced Steam Power Plants 6. Fig. 4 Supercritical Technology Trend 5-2-8