LIST OF TABLES
Chapter 3: BACKGROUND
3.3 Description of the Fenchuganj Combined Cycle Power Plant
3.3.2 Heat Recovery Steam Generator
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While the gas turbine is running, filtered ambient air is drawn through the inlet plenum assembly, then compressed in the 17th-stage axial flow compressor. Compressed air from the compressor flows into the annular space surrounding the ten combustion chambers, from which it flows into the spaces between the outer combustion casings and the combustion liners and enters the combustion zone through metering holes in each of the combustion liners.
The fuel nozzles introduce the fuel into each of the ten combustion chambers, where it mixes with the combustion air and burns.
The hot gases from the combustion chambers expand into the ten separate transition pieces attached to the downstream end of the combustion chamber liners and flow from there to the machine's three-stage turbine section. Each stage consists of a row of fixed nozzles followed by a row of turbine buckets. In each nozzle row, the jet's kinetic energy is increased, with an associated pressure drop, and in each following row of moving buckets, a portion of the jet's kinetic energy is absorbed as useful work on the turbine rotor.
After passing through the 3rd-stage buckets, the exhaust gases are directed into the exhaust casing and diffuser, which contains a series of turning vanes to turn the gases from an axial direction to a radial direction, thereby minimizing exhaust hood losses. Then, the gases pass into the exhaust plenum and are introduced to the atmosphere through the exhaust stack.
Resultant shaft rotation turns the generator rotor to generate electrical power and drives the auxiliaries through the accessory gearbox.
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HP economizer 2
LP evaporator
HP economizer 1
The heating surface primarily consists of horizontal finned tubes joined together to form heating surface packages. After it leaves the last heating surface, the cooled exhaust gas flows through the HRSG exhaust gas stack. Diverter damper to be opened with 10%, 50% & 100%
sequentially exhaust flow.
There are three main types of HRSGs: unfired, supplementary fired, and fully fired. Unfired models use only the exhaust energy to produce steam. Supplementary and fully fired models use additional fuel to increase the production of steam.
Here, in this plant unfired model is used. Unfired model HRSG only uses the exhaust energy of the Gas Turbine to produce steam. The steam output from the HRSG is combined in a header system and directed to a steam turbine. Exhaust gas diverter dampers can divert flow to either the bypass stack for simple cycle operation or the HRSG for combined cycle operation.
The Specification of the HRSG are:
Name of the Manufacturer:
Doosan heavy Industry and Construction Co. Ltd.
Type: Unfired (only use the exhaust energy of Gas Turbine to produce steam).
Shape: Vertical.
Steam Generation Capacity: 78 t/hr.
The HRSG has two pressure levels (HP) and low pressure (LP), forced circulation, and vertical gas flow design. The High Pressure (HP) system steam generation system generates HP steam of specific quality, which means correct pressure and temperature, from the thermal energy contained in the GT exhaust gas. The steam is produced in the HRSG and fed to the HP primary steam system. Besides, it fulfills the following object:
Delivers feed water to the HP drum during start-up, shutdown, and power operation of the combined-cycle unit.
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Shuts of feed water supply during feedwater control malfunction in order to prevent overfeeding of the HRSG.
Supplies HP steam produced by the HRSG to the HP main steam system during regular operation.
Supplies HP feed water to the HRSG HP desuperheating system.
Maintains and safeguards the HP superheated steam temperature within the allowable main steam system limit during part-load operation at high ambient temperatures.
Figure 3.6: Heat recovery steam generator.
The HP Steam Generation system is located downstream of the exhaust gas inlet of the HRSG. The heating surface is fabricated mainly from finned tubes. The HP system is subdivided into the following sections, listed in the order in which exhaust gas flows through them;
HP Superheater
HP Evaporator
HP Economizer
The HP economizer recovers the remaining heat contained in the exhaust gas at the HP evaporator outlet. The HP evaporator generates steam through a forced circulation loop from and to the HP drum. The HP superheater heats the saturated steam from HP drum temperature up to the required conditions.
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The HP superheater and the HP economizer are cross counter flow heat exchangers, and the HP evaporator flow is cross to the exhaust gas flow.
The feedwater is supplied by HP feedwater pumps from the feed water tank to the HP economizers, where it is heated up to economizer outlet temperature and then delivered to the HP Drum, an HP feedwater control station is located downstream of the HP economizer. The location downstream of the economizer is chosen to avoid steaming in economizer.
Water is supplied by the HP evaporator circulation pump from the HP drum through downcomers to the HP evaporator's inlet header. Water partly evaporates in the HP evaporator, and the water/steam mixture is fed in tube risers from the outlet header back to the HP drum driven by the forced circulation. The HP drum has the following functions:
• ensure adequate mixing of feed water and boiler water.
• constitute a water reserve required for the circulation system
• allow water expansion during start-up
• ensure a thorough water and steam separation
The HP drum is installed outside of the casing thus is not heated by exhaust gas. The separation of water and steam is achieved using a water/steam separation system, which restricts the carryover of water to the superheater within limits. Saturated steam flows from the HP drum through connecting piping to the HP superheater. The HP main steam piping system receives HP steam from the HRSG and transfers it to the steam turbine.
The Low Pressure (LP) steam generation system generates LP steam of specific quality, which means correct pressure and temperature, from the thermal energy contained in the GT exhaust gas. The steam is produced in the HRSG and fed to the LP main steam system.
Besides, it fulfills the following object:
• Delivers feed water to the LP drum during start-up, shutdown, and power operation of the combined-cycle unit.
• Shuts of feed water supply during feedwater control malfunction in order to prevent overfeeding of the HRSG.
• Supplies LP saturated steam produced by the HRSG to the LP main steam system during regular operation.
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The LP Steam Generation system is located downstream of the exhaust gas inlet of the HRSG behind HP economizer 2. The heating surface is fabricated mainly from finned tubes. The LP system consists of an evaporator system only without an economizer and superheater suitable for producing saturated steam. The LP evaporator generates steam through a forced circulation loop from and to the LP drum.
The LP evaporator flow is cross to the exhaust gas flow. The feedwater is supplied by feedwater pumps from the feed water tank, then delivered to the LP drum; water is supplied by an LP evaporator circulation pump from the LP drum through downcomers to the inlet header of the LP evaporator.
The LP drum has the following functions:
• ensure adequate mixing of feed water and boiler water
• constitute a water reserve required for the circulation system
• allow water expansion during start-up
• ensure a thorough water and steam separation
The LP drum is installed outside of the casing thus is not heated by exhaust gas. Saturated steam flows from the LP drum through connecting piping to the LP steam system.
The HRSG exhaust gas system comprises the exhaust gas path's gas-tight component with HRSG exhaust gas inlet ducting, HRSG casing, and HRSG exhaust gas stack.
HRSG shall exhaust through a separate or a standard multi-flue exhaust stack, the height of which and the exhaust gas's exit temperature is determined to ensure adequate dispersion of the exhaust gases following the requirements of the Laws of Bangladesh and the Environmental Guidelines, as applicable.