Combined cycle with recirculation plant inlet oxygen concentration system

1. A power plant comprising a gas turbine unit adapted to feed flue gases into a diverter where they are divided into a recirculated flow, that is fed into a mixer together with fresh air to form a mixture that is fed to the gas turbine unit compressor inlet; and a discharged flow, wherein a monitoring system for the mixture oxygen content at the compressor inlet is provided, the monitoring system comprises:

at least one recirculated flow mass flow rate sensor,

at least one recirculated flow oxygen concentration sensor configured to measure the oxygen concentration in the recirculated flow that has been diverted by the diverter,

at least one mixture mass flow rate sensor, located downstream of the mixer, configured to measure the mass flow rate of the mixture entering the compressor, and

a control unit configured to process information detected by the at least one recirculated flow mass flow rate sensor, the at least one recirculated flow oxygen concentration sensor and the at least one mixture mass flow rate sensor, to determine an oxygen concentration upstream of the compressor inlet.

2. The power plant as claimed in claim 1 , wherein the at least one recirculated flow mass flow rate sensor is connected upstream of the mixer.

3. The power plant as claimed in claim 2 , wherein the at least one recirculated flow mass flow rate sensor is connected downstream of the diverter.

4. The power plant as claimed in claim 1 , wherein the at least one recirculated flow oxygen concentration sensor is connected upstream of the mixer.

5. The power plant as claimed in claim 4 , wherein a cooler is provided downstream of a boiler on a flow path for the recirculated flow, and the at least one recirculated flow oxygen concentration sensor is connected downstream of the cooler.

6. The power plant as claimed in claim 1 , wherein the at least one mixture mass flow rate sensor is connected at least partly upstream of the compressor.

7. The power plant as claimed in claim 1 , wherein the at least one mixture mass flow rate sensor provides an indirect measure of the mass flow rate.

8. The power plant as claimed in claim 7 , wherein the at least one mixture mass flow rate sensor comprises a temperature sensor, a pressure sensor and a guide vane position control.

9. The power plant as claimed in claim 8 , wherein the temperature sensor, the pressure sensor and the guide vane position control are connected to the control unit to supply it with information that is processed in connection with compressor maps to attain the mixture mass flow rate.

10. The power plant as claimed in claim 1 , wherein the discharged flow is fed into a CO 2 capture unit.

11. The power plant as claimed in claim 1 , wherein a gas turbine unit outlet is connected to a boiler that is in turn connected to the diverter.

12. A method of operating a power plant comprising a gas turbine unit adapted to feed flue gases into a diverter where they are divided into a recirculated flow, that is fed into a mixer together with fresh air to form a mixture that is fed to the gas turbine unit compressor inlet, and a discharged flow, wherein a monitoring system for a mixture oxygen content at the compressor inlet is provided, the method comprising the monitoring system:

detecting a recirculated flow mass flow rate through at least one recirculated flow mass flow rate sensor,

detecting, through at least one recirculated flow oxygen concentration sensor, a recirculated flow oxygen concentration in the recirculated flow that has been diverted by the diverter,

detecting, through at least one mixture mass flow rate sensor located downstream of the mixer, the mass flow rate of the mixture entering the compressor, and

processing information detected by the at least one recirculated flow mass flow rate sensor, the recirculated flow oxygen concentration sensor and the mixture mass flow rate sensor through a control unit, to determine an oxygen concentration upstream of the compressor inlet.

13. The method as claimed in claim 12 , wherein the discharged flow is fed into a CO 2 capture unit.

14. The method as claimed in claim 12 , wherein the flue gases from the gas turbine unit pass through a boiler of a steam turbine unit and are then fed into the diverter.