Typically talking, HRSGs are composed of 4 key elements: the economizer, the evaporator, the superheater, and the water preheater[clarification required]. The assorted elements of the unit are assembled with the intention to fulfill the operational wants of the unit. Please see the attachment for an instance of a Modular HRSG Basic Association (Basic Association).
Modular HRSGs could also be categorized in a wide range of methods, together with the course through which exhaust gases circulate and the variety of stress ranges obtainable. HRSGs are categorized into vertical and horizontal varieties primarily based on the course of the circulate of exhaust gases. Vertical HRSGs have exhaust fuel that flows vertically over vertical tubes, and horizontal HRSGs have exhaust fuel that flows vertically over horizontal tubes. HRSGs could also be divided into two teams primarily based on the stress ranges they function at: single stress and multi stress. Single stress HRSGs have only one steam drum and steam is created at a single stress degree, whereas multi stress HRSGs have two (double stress) or three (triple stress) steam drums and generate steam at a number of stress ranges. Due to this, three parts are utilized in triple stress HRSGs: the decrease stress (low stress) part, the intermediate stress (reheat/IP) part, and the upper stress (HP) part. Every portion is comprised of a steam drum and an evaporator part, each of that are accountable for the conversion of water to steam. After that, the steam flows by superheaters, which increase the temperature over the saturation level of the water.
It is not uncommon for the steam and water stress sections of an HRSG to expertise a wide range of deterioration processes, together with creep, thermal fatigue, creep-fatigue, mechanical fatigue, Move Accelerated Corrosion (FAC), corrosion, and corrosion fatigue, amongst others.
Some HRSGs have auxiliary or duct firing capabilities. These additional burners provide higher vitality to the HRSG, which leads to extra steam being produced and, because of this, a rise within the output of the steam turbine. Generally, duct firing produces extra electrical output for a less expensive preliminary funding. Because of this, it’s usually utilized in peaking actions.
Diverter valves may additionally be utilized in HRSGs to regulate the circulate of water coming into the HRSG. This permits the fuel turbine to proceed to operate even when there isn’t a want for steam or when the HRSG should be shut down.
Emissions controls may additionally be discovered within the HRSG, though this isn’t assured. Some could embody a Selective Catalytic Discount system to lower nitrogen oxides (that are a major contributor to the event of smog and acid rain) and/or a catalyst to extract carbon monoxide from the ambiance. Due to the presence of an SCR, the HRSG’s general configuration is considerably altered. Optimum efficiency of the NOx catalyst is achieved at temperatures of between 650 levels Fahrenheit (340 levels Celsius) and 750 levels Fahrenheit (400 levels Celsius). To do that, the evaporator a part of the HRSG will usually must be divided and the SCR will must be put in between the 2 sections. There have recently been a number of low-temperature NOx catalysts launched to the market, permitting 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 specific type of HRSG that doesn’t have any boiler drums. The consumption feedwater is routed in a steady course, moderately than being divided into parts for economizers, evaporators, and superheaters. Because of the nice diploma of flexibility supplied by this design, the sections are capable of broaden or shrink in response to the quantity of warmth load acquired from the fuel turbine. As a result of there aren’t any drums, the steam output will be modified shortly, and there are fewer variables to control, making it wonderful for cycle and base load functions. Utilizing the suitable materials choice, it’s potential to function an OTSG dry, which implies that the new exhaust gases could go by the tubes with none water working by them. A bypass stack and exhaust fuel diverter system, that are wanted to run a combustion turbine whereas a drum-type HRSG is out of service, are not essential.
Warmth restoration has the potential to be employed broadly in energy-related initiatives. The steam from the HRSG is used to energy desalination services within the Persian Gulf space, which has plenty of vitality. Universities are wonderful candidates for Human Assets Technique Group submissions. They might make use of a fuel turbine to generate energy with nice dependability for utilization on the campus. The HRSG could use the warmth recovered from the fuel turbine to generate steam or sizzling water to be used in district heating or cooling programs. As a way to permit their oil-fired boilers to be shut down when at sea, giant ocean tankers (such because the Emma Maersk) use warmth restoration know-how.