Gas turbines with heat-recovery – steam generators (HRSGs) can be found in virtually every chemical process industries (CPI) plant. They can be operated in either the co-generation mode or the combined-cycle mode (Figure 1).
In the co-generation mode, steam produced from the HRSG is mainly used for process applications, whereas in the combined-cycle mode, power is generated via a steam turbine generator.
Gas turbines have several advantages as a power source: they can be started up quickly; they come in packaged modules, with power outputs ranging from 3 MW to 100 MW, that can be easily assembled and erected; they have high efficiencies of 25% to 35% (on a lower heating value [LHV] basis); and they require little or no cooling water. Recent developments include large-capacity units of up to 250 MW, with low emission characteristics (less than 10 ppmv NOx), as well as high combustion operating temperatures (in the range of 2,200°F), which results in efficiencies higher than 35%; the exhaust gas temperature is also higher, which helps to generate high-pressure/high-temperature superheated steam, making the Rankin cycle efficient.
The HRSG forms a major part of the steam system. In the combined-cycle mode, the efficiency of the combined gas turbine plus-HRSG system can reach 55-60% (LHV basis) with today's advanced machines, while in the generation mode, system efficiency can be as high as 75-85%.
The HRSG generates steam utilizing the energy in the exhaust from the gas turbine. However, some plants also have the capability of producing steam when the gas turbine is shutdown. This is done using a separate forced-draft fan along with a burner to generate hot gases, which are then used to generate steam. An
isolating damper system (also called a bypass damper) with seal air fans is required in these units to ensure that hot gases do not leak to the fan when the gas turbine is running and that maintenance can be performed on the gas turbine when the fresh air fan is operating. Bypass dampers are also used in some units to ensure that the gas flow to the HRSG can be modulated in order to match steam generation with steam demand. However, if fresh air firing is not used, an isolating damper is not required. Recent trends in HRSG design include multiple-pressure units for maximum energy recovery, the use of high temperature super heaters or reheaters incombined cycle plants, and auxiliary firing for efficient steam generation.
In addition, furnace firing is often employed in small capacity units when the exhaust gas is raised to temperatures of 2,400- 3,000°F to maximize steam generation and thus improve fuel utilization.
This article highlights some of the basic facts about gas turbine HRSGs.
This information can help plant engineers, consultants, and those planning co-generation projects make important decisions about the system and performance related aspects.
In the co-generation mode, steam produced from the HRSG is mainly used for process applications, whereas in the combined-cycle mode, power is generated via a steam turbine generator.
Gas turbines have several advantages as a power source: they can be started up quickly; they come in packaged modules, with power outputs ranging from 3 MW to 100 MW, that can be easily assembled and erected; they have high efficiencies of 25% to 35% (on a lower heating value [LHV] basis); and they require little or no cooling water. Recent developments include large-capacity units of up to 250 MW, with low emission characteristics (less than 10 ppmv NOx), as well as high combustion operating temperatures (in the range of 2,200°F), which results in efficiencies higher than 35%; the exhaust gas temperature is also higher, which helps to generate high-pressure/high-temperature superheated steam, making the Rankin cycle efficient.
The HRSG forms a major part of the steam system. In the combined-cycle mode, the efficiency of the combined gas turbine plus-HRSG system can reach 55-60% (LHV basis) with today's advanced machines, while in the generation mode, system efficiency can be as high as 75-85%.
The HRSG generates steam utilizing the energy in the exhaust from the gas turbine. However, some plants also have the capability of producing steam when the gas turbine is shutdown. This is done using a separate forced-draft fan along with a burner to generate hot gases, which are then used to generate steam. An
isolating damper system (also called a bypass damper) with seal air fans is required in these units to ensure that hot gases do not leak to the fan when the gas turbine is running and that maintenance can be performed on the gas turbine when the fresh air fan is operating. Bypass dampers are also used in some units to ensure that the gas flow to the HRSG can be modulated in order to match steam generation with steam demand. However, if fresh air firing is not used, an isolating damper is not required. Recent trends in HRSG design include multiple-pressure units for maximum energy recovery, the use of high temperature super heaters or reheaters incombined cycle plants, and auxiliary firing for efficient steam generation.
In addition, furnace firing is often employed in small capacity units when the exhaust gas is raised to temperatures of 2,400- 3,000°F to maximize steam generation and thus improve fuel utilization.
This article highlights some of the basic facts about gas turbine HRSGs.
This information can help plant engineers, consultants, and those planning co-generation projects make important decisions about the system and performance related aspects.