1. Introduction The Zhejiang Banshan Natural Gas Power Generation Project is the only large-scale power generation project in Zhejiang Province constructed in conjunction with the West-East Gas Pipeline Project. Due to the precious natural gas resources in western China, production costs are high, and the long transportation distances result in high pipeline costs. The estimated gate station price for natural gas reaching Hangzhou, Zhejiang, is 1.37 yuan/m³. The lower heating value of natural gas from the Xinjiang Tarim Basin via the West-East Gas Pipeline is 36345 kJ/m³, equivalent to a unit lower heating value gas price of 0.0377 yuan/MJ. With a standard coal price of 350 yuan/t, the unit heating value price is 0.012 yuan/MJ, only slightly less than one-third of that of natural gas. Therefore, natural gas prices are highly sensitive to electricity prices. The Zhejiang Banshan Natural Gas Power Generation Project utilizes three sets of 350 MW-class gas-steam combined cycle generator units, a technology currently considered mature. Due to the high gas price, the Banshan project focuses on improving the combined cycle efficiency to reduce electricity costs. A gas-steam combined cycle generator set has four main components: a gas turbine, a waste heat boiler, a steam turbine, and a generator. The configuration consists of one or more gas turbines and a corresponding number of waste heat boilers, forming single-shaft and multi-shaft configurations. The gas turbine is the key equipment in a gas-steam combined cycle generator set, and its selection has the greatest impact on the unit's performance and the power plant's economy. This project should select a typical large-capacity heavy-duty gas turbine for power generation. Currently, the representative models of mature and well-performing gas turbines are the E-class (including improved E-class models) and the F-class. Simultaneously, corresponding waste heat boilers and steam turbines will be configured. [b]2 Main Component Selection[/b] 2.1 Gas Turbine 2.1.1 "E" Class Gas Turbine The "E" class gas turbine is a general term for a 100 MW-class gas turbine, with a single unit capacity of 114.7–157 MW. Examples include 9E (GE), GT11N2 (ALSTOM), and V94.2 (SIEMENS). E-class gas turbines have turbine inlet gas temperatures of 1060–1204℃. Taking the GE 9E gas turbine as an example, their single-cycle efficiency is only 33.8%, and the combined-cycle efficiency is approximately 52.4%. These types of gas turbines have been in large-scale commercial operation for over a decade, primarily using petroleum products as fuel. The 300 MW E-class gas turbines used in the Zhenhai and Wenzhou Longwan gas turbine power plants in Zhejiang Province are all GE 9E gas turbines. 2.1.2 “F” Class Gas Turbines “F” class gas turbines are a general term for 250 MW-class gas turbines, widely used abroad. Single-unit capacities range from 255.6 to 270 MW, turbine inlet gas temperatures range from 1235 to 1400℃, single-cycle efficiencies range from 36.9% to 38.5%, and combined-cycle efficiencies range from 56.7% to 58%. Examples include PG9351FA (GE), V94.3A (SIEMENS), GT26 (ALSTOM), and M701F (Mitsubishi). 2.1.3 Compared to Class E, which has lower efficiency, a 300-350 MW combined cycle requires two 100 MW gas turbines to drive one 100 MW steam turbine (two-to-one). Due to the large number of gas turbines, the cost per kilowatt is high. Furthermore, the lower turbine inlet gas temperature results in lower efficiency in both single-cycle and combined-cycle operations, but also lower operating and maintenance costs. Class F has higher power output; a 350 MW combined cycle can be achieved using one 250 MW gas turbine driving one 100 MW steam turbine (one-to-one). However, the higher turbine inlet gas temperature leads to higher efficiency in both single-cycle and combined-cycle operations, but also higher operating and maintenance costs. The performance of the improved E-class gas turbine is between that of the E-class and F-class, with a power output of 150 MW. Its combined cycle efficiency is improved compared to the E-class, but a two-to-one configuration is still required to form a 350 MW combined cycle. Thanks to the manufacturer's efforts, the price has decreased, and the cost per kilowatt of the project is lower than that of the E-class. Examples include the 9EC, GT13E2, and V94.2A, all of which are improved E-class gas turbines. 2.1.4 Installation Scheme: For the Banshan project, three combined cycle units (each with a power output of 350 MW) are to be constructed. To compare the E-class and F-class gas turbines, a cost of electricity (COE) analysis is conducted on three representative models: ① Model 1: A "two-to-one" multi-shaft (three-shaft) E-class combined cycle unit is constructed using the typical GE 9E gas turbine, with a combined cycle efficiency of 52.4%. ② Model 2: A single-shaft F-class combined cycle unit constructed using the SIEMENS V94.3A gas turbine, with a combined cycle efficiency of 57.4%. ③ Model 3: A multi-shaft (tri-shaft) E-class improved combined cycle unit constructed using the ALSTOM GT13E2 gas turbine, a representative E-class improved model, with a combined cycle efficiency of 52.9%. For detailed model comparison, please refer to reference [1]. The results are as follows: ① When the unit cost per kilowatt of an E-class unit is 3600 yuan, E-class cannot compete with F-class. Even with a lower gas price (0.4 yuan/m³), the electricity cost of E-class is 316.9 yuan/MWh, while the electricity cost of F-class is 308 yuan/MWh. The difference widens further when gas prices increase. ② If the unit cost per kilowatt of the E-class unit is reduced to 3400 yuan, when the gas price is 0.562 yuan/m3, the electricity cost of E-class and F-class units is the same, at 340 yuan/MWh. When the gas price is less than 0.562 yuan/m3, the electricity cost of E-class is lower than that of F-class. ③ If the improved E-class unit is used, the unit cost per kilowatt is 3400 yuan. When the gas price is 0.705 yuan/m3, the electricity cost of the improved E-class unit is the same as that of F-class, at 368 yuan/MWh. When the gas price is less than 0.705 yuan/m3, the electricity cost of the improved E-class unit is lower than that of F-class. Therefore, it can be seen that the F-class unit scheme is used in this project. E-class (including the improved E-class) is suitable for areas with relatively low gas prices. 2.2 Waste Heat Boiler The waste heat boiler connects the gas and steam cycles. This project, like the Zhenhai and Wenzhou Longwan gas turbine power plants, selects a waste heat boiler without supplementary combustion. Both domestic and international gas-steam combined cycle power plants adopt a scheme of one gas turbine paired with one waste heat boiler. Therefore, the waste heat boiler equipped with an F-class gas turbine in this project will have a larger capacity than the waste heat boilers equipped with E-class gas turbines in Zhenhai and Wenzhou Longwan plants. Currently, most waste heat boilers have a single-pressure, non-reheat system on the steam side (such as Zhenhai and Wenzhou Longwan plants) [2]. After applying an F-class gas turbine, its exhaust gas temperature (i.e., the inlet flue gas temperature of the waste heat boiler) is above 540℃. At the same time, due to the use of natural gas, the outlet flue gas temperature of the waste heat boiler can be reduced to 80-90℃. Therefore, a three-pressure system with reheat is selected for the steam side of the waste heat boiler to improve the combined cycle efficiency. 2.3 Steam Turbine In a gas turbine power plant, multiple gas turbines or one gas turbine drive one steam turbine to form a group, i.e., the n+1 mode. n is the number of gas turbines, and 1 is the number of steam turbines. That is, in order to improve the efficiency of the steam turbine, a high-power steam turbine should be selected as much as possible. In Zhenhai and Wenzhou Longwan plants, n=2; in this project, n=1; the turbine power is 100 MW. For ease of operation and peak shaving, the turbines in the gas turbine power plant do not have regenerative exhaust. 2.4 Generator: Pay attention to generator capacity matching. To maximize the economic benefits of the combined cycle unit, the generator capacity should have a margin. The generator capacity selected for this project is 400 MW. [b]3 Configuration Scheme of Main Units in Combined Cycle Units[/b] There are three different configuration schemes for combined cycle units composed of F-class gas turbines and other main units: Scheme 1: One F-class GT + one HRSG + one ST form a single-shaft unit (GT and ST coaxial), with GT and ST driving the same G (referred to as F-class single-shaft). Scheme 2: One F-class GT + one HRSG + one ST form a dual-shaft unit (GT and ST separate shafts), with GT and ST driving their respective G units, for a total of 3 G units (referred to as F-class dual-shaft). Option 3: A three-shaft unit (each GT and ST is a separate shaft) consisting of 2 F-class GTs + 2 HRSGs + 1 ST. The 2 GTs and 1 ST drive their respective Gs, for a total of 3 Gs (referred to as F-class three-shaft). 3.1 Adaptability to Power Plant Capacity: Based on the power plant's construction capacity, when the number of F-class gas turbines is odd, single-shaft or dual-shaft combined cycle units are preferable; while when the number of F-class gas turbines is even, multi-shaft combined cycle units can be considered depending on the number of turbines and load conditions. The Zhejiang Banshan Natural Gas Power Generation Project has 3 F-class gas turbines, therefore Option 3, i.e., a 2+2+1 multi-shaft combined cycle unit, is not suitable. 3.2 Comparison of Main Plant Layout and Feasibility of Overall Layout: Since Option 3 is not suitable based on the project's construction capacity, a comparison of the overall layout of Option 1 and Option 2 is made. Option 1's main plant layout consists of 3 single-shaft gas-steam combined cycle generator units arranged in parallel. In Scheme 1, the waste heat boiler, gas turbine, steam turbine, and generator are arranged on the same axis, with the gas turbine, steam turbine, and generator coaxial. Scheme 2, due to its dual-shaft arrangement, adds an extra generator to the plant. Comparing the two schemes, from the perspective of the main plant layout, Scheme 1 has a compact layout, shorter steam and water pipelines, convenient electrical wiring, and easier production management. The overall architectural design is better coordinated with the process and control design. Scheme 2 has a similar overall layout to Scheme 1, but due to the increased length of the main plant after the dual-shaft arrangement, the amount of steam and water pipeline work will increase to a certain extent. Moreover, the length of the entire power island area from the outer end of the waste heat boiler chimney to the outer end of the main transformer is increased by about 17 m compared to Scheme 1. From the perspective of overall layout, both schemes are feasible. 3.3 Start-up and Control Performance Scheme 1, the F-class single-shaft unit, can be divided into two types: generator mid-mounted and tail-mounted, with different start-up methods. The generator is positioned at the tail end of the steam turbine, i.e., the generator-tail-mounted scheme, using variable frequency start-up. Since the main units share a single shaft, power generation is impossible before grid connection after startup, and all three main units must operate at the same speed at all times. The generator is positioned between the gas turbine and the steam turbine, i.e., the generator-mid-mounted scheme, with a clutch between the turbine and generator. The gas turbine and generator can operate independently of the steam turbine, offering significant flexibility and clear advantages during startup. The gas turbine can start and increase load quickly, and can generate power first, allowing for an additional 30-40 minutes of power generation time per startup, improving economic efficiency. However, rotor removal from the generator is more difficult, requiring a large-tonnage crane or special equipment. Scheme 2, the F-class dual-shaft unit, has the same startup performance as the F-class single-shaft generator-mid-mounted unit. Scheme 2, the F-class triple-shaft unit, also offers flexible startup, but connecting the two waste heat boilers requires additional time and increases steam and water losses. Option 1 or 2, and Class F single-shaft or dual-shaft units, can achieve relatively flexible peak shaving and have high combined cycle efficiency. When the plant load rate is 70% to 100%, the three units operate simultaneously at the same load rate, and the efficiency at 70% load rate is 95% relative to 100% load rate. When the load rate is 60%, one unit is shut down, and the other two units each carry 90% load, with a relative efficiency of 96.3%. When the load rate is 40%, one unit is shut down, and the other two units each carry 60% load, with a relative efficiency of 91.3%. When the load rate is 30%, two units are shut down, and one unit carries 90% load, with a relative efficiency of 98.7%. 3.4 Coordination of Installed Capacity and Power Plant Functions Option 1 uses F-class single-shaft units, which can better adapt to the functions of power plants that handle peak shaving, mid-load, and partial base load. This is also in line with the role that the Zhejiang Banshan Natural Gas Power Generation Project should play in the power system in the future. Option 2 uses F-class dual-shaft units, which also have good adaptability to peak shaving, mid-load, and partial base load. However, compared with Option 1, its characteristic is that it is more suitable for natural gas combined cycle power generation projects that need to undertake heating tasks, because after the split-shaft arrangement, the entire system can better adapt to changes in heating load conditions. 3.5 Investment Cost Comparison Comparing the investment costs of power plants, Scheme 1, with its F-class single-shaft layout, connects the gas turbine, steam turbine, and generator in series. Each unit requires only one large-capacity generator, step-up transformer, and high-voltage feeder equipment. The gas turbine and steam turbine can share a single lubrication and control oil system. Scheme 2, with its F-class dual-shaft layout, increases the amount of piping work for the thermal system, and correspondingly, the costs of some process auxiliary systems and electrical systems will also increase. Its advantage is that future domestic production is more convenient; except for the gas turbine, the steam turbine and generator can be sourced domestically. If all main equipment (gas turbine, steam turbine, generator, waste heat boiler) is imported, Scheme 2 is slightly more expensive than Scheme 1. Calculations show that the cost per kilowatt for Scheme 1 is 3434 yuan, while the cost per kilowatt for Scheme 2 is 3460 yuan. 3.6 Overall Combined Cycle Performance: Compared to Scheme 1 (F-class single-shaft) and Scheme 2 (F-class dual-shaft), the overall combined cycle efficiency of Scheme 3 (F-class triple-shaft combined cycle) is basically the same. However, due to the use of a larger 200 MW-class steam turbine, the overall combined cycle efficiency of Scheme 3 (F-class triple-shaft combined cycle) is improved. For example, GE's F-class single-shaft combined cycle efficiency is 56.7%, while its F-class triple-shaft combined cycle efficiency is 57.1%. 3.7 Potential for Domestic Production and Maintenance Costs: For the F-class single-shaft scheme, the technical requirements for the shaft system are high due to connecting the gas turbine, steam turbine, and generator on a single shaft. The design coordination of the gas turbine, steam turbine, and generator is complex, requiring a foreign party to assume coordination responsibility. In addition to importing the gas turbine, it is also necessary to introduce domestic and foreign combined cycle production technology to produce the matching waste heat boiler, steam turbine, and generator domestically. For the F-class dual-shaft scheme, each turbine and gas turbine is connected to a generator. The thermal system is the same as the single-shaft scheme. Because the steam turbine and gas turbine are arranged separately, the responsibility and risk of ensuring the overall performance of a single-shaft F-class unit are avoided. Domestically, there is the capability to produce the main and auxiliary equipment (excluding the gas turbine), which is beneficial for future localization. From a maintenance perspective, the maintenance costs of F-class gas turbine combined cycle units are relatively high, and are mainly concentrated on the gas turbine. Therefore, if there is no significant progress in the localization of key components and the domestic availability of major spare parts for large-capacity F-class gas turbines, the maintenance costs of the main equipment for each scheme will not differ significantly. 3.8 Construction Convenience For the F-class single-shaft scheme, the four main units are arranged in a line, often using an independent island-type main plant, which occupies a small area and allows for a compact equipment layout. During power plant construction, each unit can operate independently without interference. Furthermore, future expansion is very convenient. For the F-class dual-shaft scheme, this arrangement is not advantageous compared to the F-class single-shaft scheme in terms of generator output lines, equipment, and steam pipeline layout. Therefore, from a construction convenience perspective, the F-class single-shaft scheme is preferred. 3.9 Comparison Results The F-class single-shaft combined cycle unit and the F-class dual-shaft combined cycle unit were analyzed from eight aspects. Based on all analyses, especially considering the convenience of main plant layout and operation management, as well as the current stage of investment cost comparison, the following conclusion can be drawn: the F-class single-shaft combined cycle unit scheme is suitable for the Zhejiang Banshan Natural Gas Power Generation Project. [b]4 Conclusion[/b] The Zhejiang Banshan Natural Gas Power Generation Project adopts three 350 MW-class gas-steam combined cycle generator units. The comparison conclusions are: ① Adopt: 250 MW-class F-class gas turbine; steam side is a three-pressure waste heat boiler with a reheat system. ② Adopt: 1 250 MW-class F-class gas turbine; 1 corresponding capacity waste heat boiler and 1 100 MW-class steam turbine configuration, using a single-shaft system (gas turbine, steam turbine and generator coaxial). [b]References[/b] ［1］ Tu Jin, Qian Haiping. Selection of Gas Turbine Units for Banshan Natural Gas Power Generation Project［J］. Zhejiang Electric Power, 2002, (5): 19-20, 40. [2] Tu Jin, Qian Haiping. Discussion on parameters of Wenzhou 300 MW gas-steam combined cycle generator unit [J]. Zhejiang Electric Power, 1999, (6): 20-23.