Power plant and method for its operation

1. A method for operating a power plant including a gas turbine unit ( 1 ) adapted to feed flue gases ( 8 ) into a diverter ( 11 ) where the flue gases are divided into a recirculated flow ( 12 ) and a discharged flow ( 13 ), the recirculated flow ( 12 ) is fed into a mixer ( 16 ) together with fresh air ( 7 ) to form a mixture ( 6 ) that is fed to a compressor of the gas turbine unit ( 1 ), the gas turbine unit ( 1 ) having at least one combustion chamber ( 3 , 3 a , 3 b ) downstream of the compressor where a fuel ( 27 ) is burnt together with the mixture ( 6 ), the method comprising:

providing a C2+ or C2+ and H 2 content of the fuel to a control unit;

determining a flue gas recirculation ratio for the fuel based on the provided C2+ or C2+ and H 2 content; and

regulating online a mass flow rate of the recirculated flow ( 12 ), which is fed to the mixer, using the flue gas recirculation ratio,

wherein the power plant includes a cooler configured to produce water condensate located downstream of the diverter, and the flue gas recirculation ration is a ratio between the recirculated flow of flue gases downstream of the cooler and the flue gases at an outlet of the gas turbine.

2. The method according to claim 1 , wherein the recirculated flow ( 12 ) mass flow rate is increased when the fuel ( 27 ) C2+ or C2+ and H 2 content increases and is decreased when the fuel ( 27 ) C2+ or C2+ and H 2 content decreases.

3. The method according to claim 1 , wherein, together with the recirculated flow ( 12 ) mass flow rate, a temperature of the flue gas at a combustion chamber outlet is also regulated.

4. The method according to claim 3 , wherein the temperature at the combustion chamber outlet is increased and/or kept constant.

5. The method according to claim 1 , wherein the at least one combustion chamber includes a first combustion chamber ( 3 a ) and, downstream of the first combustion chamber: a high pressure turbine ( 4 a ), a second combustion chamber ( 3 b ) that is fed with flue gases partially expanded in the high pressure turbine ( 4 a ), and a low pressure turbine ( 4 b ) that is fed with flue gases generated in the second combustion chamber ( 3 b ), and

wherein the recirculated flow ( 12 ) mass flow rate is regulated in relation to a C2+ or C2+ and H2 content of fuel supplied into the second combustion chamber ( 3 b ).

6. The method according to claim 1 , wherein before the flue gases ( 8 ) are divided by the diverter ( 11 ), they are supplied into a boiler ( 9 ) of a steam turbine unit ( 10 ).

7. The method according to claim 1 , wherein the discharged flow ( 13 ) is fed into a CO 2 capture unit ( 21 ).

8. The method according to claim 1 , wherein the fuel ( 27 ) is natural gas.

9. A power plant comprising:

a gas turbine unit ( 1 ) adapted to feed flue gases ( 8 ) into a diverter ( 11 ) where the flue gases are divided into a recirculated flow ( 12 ) and a discharged flow ( 13 ), the recirculated flow ( 12 ) is fed into a mixer ( 16 ) together with fresh air ( 7 ) to form a mixture ( 6 ) that is fed to a compressor of the gas turbine unit ( 1 ), the gas turbine unit ( 1 ) comprises at least one combustion chamber ( 3 , 3 a , 3 b ) downstream of the compressor where a fuel ( 27 ) is burnt together with the mixture ( 6 ); and

a control unit ( 30 ) that is supplied with information regarding a C2+ or C2+ and H2 content of the fuel and is connected to at least the diverter ( 11 ) to drive the diverter and online regulate a mass flow rate of the recirculated flow ( 12 ), which is fed to the mixer, in relation to the fuel ( 27 ) C2+ or C2+ and H 2 content;

a cooler configured to produce water condensate located downstream of the diverter,

wherein the control unit ( 30 ) includes a computer implementing look-up tables ( 33 ) associating the fuel C2+ or C2+ and H 2 content (FCC) to a respective flue gas recirculation ratio (FGR) and regulates the recirculated flow based on the respective FGR, and

wherein the flue gas recirculation ratio is a ratio between the recirculated flow of flue gases downstream of the cooler and the flue gases at an outlet of the gas turbine.

10. The power plant according to claim 9 , wherein the at least one combustion chamber includes a first combustion chamber ( 3 a ) and, downstream of the first combustion chamber: a high pressure turbine ( 4 a ), a second combustion chamber ( 3 b ) that is fed with flue gases partially expanded in the high pressure turbine ( 4 a ), and a low pressure turbine ( 4 b ) that is fed with flue gases generated in the second combustion chamber ( 3 b ), and

wherein the control unit ( 30 ) regulates the recirculated flow ( 12 ) mass flow rate in relation to the C2+ or C2+ and H 2 content of the fuel ( 27 ) supplied into the second combustion chamber ( 3 b ).

11. The power plant according to claim 9 , wherein flue gases discharged from the gas turbine unit ( 1 ) are supplied into a boiler ( 9 ) of a stream turbine unit ( 10 ).

12. The power plant according to claim 9 , wherein the discharged flow ( 13 ) is supplied into a CO 2 capture unit ( 21 ).

13. The method according to claim 1 , wherein the fuel ( 27 ) C2+ or C2+ and H 2 content is continuously online measured and the recirculated flow ( 12 ) mass flow rate is correspondingly continuously online regulated.

14. The power plant according to claim 9 , comprising:

sensors ( 31 ) for continuously online measuring the fuel ( 27 ) C2+ or C2+ and H 2 content connected to the control unit ( 30 ), wherein the control unit ( 30 ) correspondingly continuously online regulate the recirculated flow ( 12 ) mass flow rate.

15. The method according to claim 1 , wherein the mixer is upstream of the gas turbine unit.

16. The power plant according to claim 9 , wherein the mixer is upstream of the gas turbine unit.