Typically, a heat recovery steam generator (HRSG) is an energy recovery heat exchanger that recovers heat from a hot gas stream, such as that produced by a combustion turbine or another waste gas stream. It generates steam that may be employed in a process (cogeneration) or used to power a steam turbine (thermal energy conversion) (combined cycle).
Generally speaking, HRSGs are composed of four key components: the economizer, the evaporator, the superheater, and the water preheater[clarification required]. The various components of the unit are assembled in order to satisfy the operational needs of the unit. Please see the attachment for an example of a Modular HRSG General Arrangement (General Arrangement).
Modular HRSGs may be classified in a variety of ways, including the direction in which exhaust gases flow and the number of pressure levels available. HRSGs are classified into vertical and horizontal varieties based on the direction of the flow of exhaust gases. Vertical HRSGs have exhaust gas that flows vertically over vertical tubes, and horizontal HRSGs have exhaust gas that flows vertically over horizontal tubes. HRSGs may be divided into two groups based on the pressure levels they operate at: single pressure and multi pressure. Single pressure HRSGs have just one steam drum and steam is created at a single pressure level, while multi pressure HRSGs have two (double pressure) or three (triple pressure) steam drums and generate steam at several pressure levels. Because of this, three portions are used in triple pressure HRSGs: the lower pressure (low pressure) section, the intermediate pressure (reheat/IP) section, and the higher pressure (HP) section. Each portion is comprised of a steam drum and an evaporator section, both of which are responsible for the conversion of water to steam. After that, the steam flows through superheaters, which boost the temperature over the saturation point of the water.
It is common for the steam and water pressure sections of an HRSG to experience a variety of deterioration processes, including creep, thermal fatigue, creep-fatigue, mechanical fatigue, Flow Accelerated Corrosion (FAC), corrosion, and corrosion fatigue, among others.
Some HRSGs have auxiliary or duct firing capabilities. These extra burners offer greater energy to the HRSG, which results in more steam being produced and, as a result, an increase in the output of the steam turbine. In general, duct firing produces more electrical output for a cheaper initial investment. As a result, it is often used in peaking activities.
Diverter valves may also be used in HRSGs to control the flow of water entering the HRSG. This enables the gas turbine to continue to function even when there is no need for steam or when the HRSG must be shut down.
Emissions controls may also be found in the HRSG, although this is not guaranteed. Some may include a Selective Catalytic Reduction system to decrease nitrogen oxides (which are a significant contributor to the development of smog and acid rain) and/or a catalyst to extract carbon monoxide from the atmosphere. Because of the presence of an SCR, the HRSG’s overall configuration is significantly altered. Optimal performance of the NOx catalyst is achieved at temperatures of between 650 degrees Fahrenheit (340 degrees Celsius) and 750 degrees Fahrenheit (400 degrees Celsius). To do this, the evaporator part of the HRSG will often need to be divided and the SCR will need to be installed between the two sections. There have lately been several low-temperature NOx catalysts introduced to the market, allowing the SCR to be positioned between the Evaporator and Economizer sections (350°F – 500°F (175-260°C)).
The once-through steam generator is a particular form of HRSG that does not have any boiler drums. The intake feedwater is routed in a continuous course, rather than being divided into portions for economizers, evaporators, and superheaters. Due to the great degree of flexibility provided by this design, the sections are able to expand or shrink in response to the amount of heat load received from the gas turbine. Because there are no drums, the steam output can be changed quickly, and there are fewer variables to regulate, making it excellent for cycle and base load applications. Using the right material selection, it is possible to operate an OTSG dry, which means that the hot exhaust gases may pass through the tubes without any water running through them. A bypass stack and exhaust gas diverter system, which are needed to run a combustion turbine while a drum-type HRSG is out of service, are no longer necessary.
Heat recovery has the potential to be employed widely in energy-related initiatives.
The steam from the HRSG is used to power desalination facilities in the Persian Gulf area, which has a lot of energy.
Universities are excellent candidates for Human Resources Strategy Group submissions. They may employ a gas turbine to generate power with great dependability for usage on the campus. The HRSG may use the heat recovered from the gas turbine to generate steam or hot water for use in district heating or cooling systems.
In order to allow their oil-fired boilers to be shut down when at sea, large ocean tankers (such as the Emma Maersk) use heat recovery technology.