Combined cycle power plant with improved efficiency

1. A system, comprising:

a heat recovery steam generator (HRSG) configured to recover heat from an exhaust gas, wherein the HRSG defines an exhaust flow path of the exhaust gas and comprises a plurality of heat exchangers at different positions within the exhaust flow path relative to a direction of flow along the exhaust flow path, the plurality of heat exchangers including a last heat exchanger relative to the direction of flow, wherein the last heat exchanger is configured to transfer heat from the exhaust gas into water to produce a heated water;

a steam turbine thermally connected to the HRSG;

a flash tank fluidly coupled to the last heat exchanger in the HRSG, wherein the flash tank is configured to receive the heated water from the last heat exchanger, and the flash tank is configured to flash the heated water to supply a flash steam to the steam turbine;

a first supply line interconnecting the flash tank and the steam turbine to enable flow of the flash steam from the flash tank to the steam turbine;

a return line interconnecting the steam turbine and the last heat exchanger in the HRSG, wherein the return line is configured to enable flow of the flash steam from the steam turbine toward the last heat exchanger in the HRSG; and

a second supply line interconnecting the flash tank and the return line to enable flash condensate to flow from the flash tank to the return line, wherein a junction between the second supply line and the return line is positioned upstream of the flash tank.

2. The system of claim 1 , wherein the heated water received by the flash tank is heated only by the exhaust gas in the HRSG.

3. The system of claim 1 , comprising a heated water conduit interconnecting the flash tank and the last heat exchanger.

4. The system of claim 1 , comprising a flash valve disposed between the last heat exchanger and the flash tank, wherein the flash valve is configured to control supply of the heated water to the flash tank.

5. The system of claim 1 , comprising a gas turbine configured to output the exhaust gas to the HRSG.

6. The system of claim 1 , comprising a condenser disposed along the return line downstream from the steam turbine.

7. The system of claim 6 , comprising a first pump disposed along the return line downstream from the condenser.

8. The system of claim 7 , comprising a second pump disposed along the second supply line downstream from the flash tank.

9. A system, comprising:

a heat recovery steam generator (HRSG) configured to recover heat from an exhaust gas, wherein the HRSG defines an exhaust flow path of the exhaust gas and comprises a plurality of heat exchangers at different positions within the exhaust flow path relative to a direction of flow along the exhaust flow path, the plurality of heat exchangers including a last heat exchanger relative to the direction of flow, wherein the last heat exchanger is configured to transfer heat from the exhaust gas into water to produce a heated water;

a feedwater preheating system thermally coupled to the last heat exchanger, wherein the feedwater preheating system is configured to control a temperature of the water at the last heat exchanger;

a steam turbine thermally connected to the HRSG;

a flash tank fluidly coupled to the last heat exchanger in the HRSG, wherein the flash tank is configured to receive the heated water from the last heat exchanger, and the flash tank is configured to flash the heated water to supply a flash steam to the steam turbine;

a first supply line interconnecting the flash tank and the steam turbine to enable flow of the flash steam from the flash tank to the steam turbine;

a return line interconnecting the steam turbine and the last heat exchanger in the HRSG, wherein the return line is configured to enable flow of the flash steam from the steam turbine toward the last heat exchanger in the HRSG; and

a second supply line interconnecting the flash tank and the feedwater preheating system to enable flash condensate to flow from the flash tank to the feedwater preheating system, wherein the feedwater preheating system is configured to receive thermal energy from the flash condensate to control the temperature of the water at the last heat exchanger.

10. The system of claim 9 , comprising a flash valve disposed between the last heat exchanger and the flash tank, wherein the flash valve is configured to control supply of the heated water to the flash tank.

11. The system of claim 9 , comprising a gas turbine configured to output the exhaust gas to the HRSG.

12. The system of claim 9 , comprising a condenser disposed along the return line downstream from the steam turbine.

13. The system of claim 12 , comprising a first pump disposed along the return line downstream from the condenser.

14. The system of claim 13 , comprising a second pump disposed along the second supply line downstream from the flash tank.

15. The system of claim 9 , wherein the flash tank is configured to receive the heated water from the last heat exchanger via the feedwater preheating system.

16. A system, comprising:

a heat recovery steam generator (HRSG) configured to recover heat from an exhaust gas, wherein the HRSG comprises a plurality of heat exchangers disposed within an exhaust flow path of the exhaust gas at different positions in a direction of flow along the exhaust flow path, and a last heat exchanger of the plurality of heat exchangers in the direction of flow is configured to transfer heat from the exhaust gas into water to produce a heated water;

a steam turbine thermally connected to the HRSG;

a flash tank fluidly coupled to the last heat exchanger in the HRSG, wherein the flash tank is configured to receive the heated water from the last heat exchanger, and the flash tank is configured to flash the heated water to supply a flash steam to the steam turbine;

a first supply line interconnecting the flash tank and the steam turbine to enable flow of the flash steam from the flash tank to the steam turbine;

a second supply line interconnecting the flash tank and a gas turbine air preheating system to enable flash condensate to flow from the flash tank to the gas turbine air preheating system;

a first return line interconnecting the steam turbine and the last heat exchanger in the HRSG, wherein the first return line is configured to enable flow of the flash steam from the steam turbine toward the last heat exchanger in the HRSG; and

a second return line interconnecting the gas turbine air preheating system and the first return line, wherein the second return line is configured to enable flow of the flash condensate from the gas turbine air preheating system to the first return line, and a junction between the second return line and the first return line is positioned upstream of the flash tank.

17. The system of claim 16 , comprising a conduit/valve assembly fluidly coupled to the second supply line, wherein the conduit/valve assembly is configured to selectively enable bypassing of part or all of the heated water supplied to the flash tank by the last heat exchanger, and the conduit/valve assembly is configured to enable flow of bypassed heated water to the second supply line.

18. The system of claim 17 , comprising a heated water conduit interconnecting the flash tank and the last heat exchanger, wherein the conduit/valve assembly is fluidly coupled to the heated water conduit.

19. The system of claim 16 , comprising a heat source fluidly coupled to the flash tank, wherein the heat source is configured to supply additional heated water to the flash tank.

20. The system of claim 16 , comprising a gas turbine configured to output the exhaust gas to the HRSG.