The technology have also been used on a few offshore installations for over 10 years. Most offshore installations are designed to generate power from open-cycle gas turbines which offer reduced capital costs, size and weight per MW installed , but with compromised energy efficiency and fuel costs per unit output. Combined-cycle system operation is suitable for stable load applications, but less suitable for offshore applications with variable or declining load profiles.
Additionally, the waste heat recovery unit WHRU can replace the gas turbine silencer, thereby mitigating some of the space and weight constraints. Residual heat may be used instead of fired heaters, thereby improving the overall system efficiency.
As such, the use of combined-cycle power technology is dependent on the power and heat demand of the installation. Combined-cycle technology is most cost-effective for larger plants. On an installation where the heat demand is large, the waste heat from the WHRU will normally be used for other heating applications, and hence there will be little residual heat left for power generation.
Retrofitting gas turbine generator technology to convert from simple, open-cycle systems to combined-cycle operation is complex and costly; hence this is not common in offshore installations. The additional topside weight and space necessary to incorporate a steam turbine, as well as the need for additional personnel on the platform to manage the steam system operations, makes a combined-cycle retrofit a challenging project.
A combined-cycle power system typically consists of the following equipment: gas turbines GTs ; waste heat recovery units for steam generation WHRU-SG ; steam turbines STs ; condensers; and other auxiliary equipment. The figure below illustrates a combined-cycle power system using a gas turbine generator with waste heat recovery and steam turbine generator. Figure 1: Combined cycle power system using a gas turbine generator with waste heat recovery and steam turbine generator. For a more detailed description of this technology in typical onshore applications, please refer to:.
The following are technologies that provide similar benefits high efficiency power generation and may be considered as alternatives to a combined-cycle system:. Figure 2 illustrates high-level applicability of technologies based on the demand for power and heat. Figure 2: The applicability of technologies based on the demand for power and heat.
Because gas turbines have low efficiency in simple cycle operation, the output produced by the steam turbine accounts for about half of the CCGT plant output.
The economizer is a heat exchanger that preheats the water to approach the saturation temperature boiling point , which is supplied to a thick-walled steam drum. The drum is located adjacent to finned evaporator tubes that circulate heated water. As the hot exhaust gases flow past the evaporator tubes, heat is absorbed causing the creation of steam in the tubes. The steam-water mixture in the tubes enters the steam drum where steam is separated from the hot water using moisture separators and cyclones.
The separated water is recirculated to the evaporator tubes. Steam drums also serve storage and water treatment functions. An alternative design to steam drums is a once-through HRSG, which replaces the steam drum with thin-walled components that are better suited to handle changes in exhaust gas temperatures and steam pressures during frequent starts and stops. In some designs, duct burners are used to add heat to the exhaust gas stream and boost steam production; they can be used to produce steam even if there is insufficient exhaust gas flow.
Saturated steam from the steam drums or once-through system is sent to the superheater to produce dry steam which is required for the steam turbine. The superheated steam produced by the HRSG is supply to the steam turbine where it expands through the turbine blades, imparting rotation to the turbine shaft. The energy delivered to the generator drive shaft is converted into electricity. After exiting the steam turbine, the steam is sent to a condenser which routes the condensed water back to the HRSG.
As the HRSGs are located directly downstream of the gas turbines, changes in temperature and pressure of the exhaust gases cause thermal and mechanical stress.
When CCGT power plants are used for load-following operation, characterized by frequent starts and stops or operating at part-load to meet fluctuating electric demand, this cycling can cause thermal stress and eventual damage in some components of the HRSG. The HP steam drum and superheater headers are more prone to reduced mechanical life because they are subjected to the highest exhaust gas temperatures.
Important design and operating considerations are the gas and steam temperatures that the module materials can withstand; mechanical stability for turbulent exhaust flow; corrosion of HRSG tubes; and steam pressures that may necessitate thicker-walled drums. The HRSG takes longer to warm up from cold conditions than from hot conditions. As a result, the amount of time elapsed since last shutdown influences startup time. When gas turbines are ramped to load quickly, the temperature and flow in the HRSG may not yet have achieved conditions to produce steam, which causes metal overheating since there is no cooling steam flow.
Steam conditions acceptable for the steam turbine are dictated by thermal limits of the rotor, blade, and casing design. As these systems operate efficiently over a narrow range of gas temperatures, they are often installed between evaporator modules.
Flexicycle Power Plant based on combustion engines. Bypass valves are used to control the admission of steam to the steam turbine when an engine set is not operating. Flexicycle power plants combine the advantages of high efficiency in simple cycle and the modularity of multiple engines supplying the steam turbine. The steam turbine can be run with only 25 percent of the engines at full load, or 50 percent of the engines at half load.
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