Mixing element for gas turbine units with flue gas recirculation

1. An intake section upstream of an inlet of a compressor of a gas turbine unit with flue gas recirculation, the intake section comprising:

at least one section of the intake section with a flow path defined by sidewalls in which airflow of intake air is flowing along an airflow direction;

a plurality of mixing ducts extending into the flow path from at least one sidewall;

each of said mixing ducts having an intake at said at least one sidewall for receiving recirculated flue gas of the gas turbine and having at least one outlet opening distanced from said sidewall for blowing recirculated flue gas out of the mixing duct into the airflow:

wherein at least two, or at least three, or at least four, or at least five of the mixing ducts are arranged in a row adjacent to each other, said row being aligned along the airflow direction, a most upstream mixing duct defining a leading edge of the row,

wherein the mixing ducts all have different lengths of extension into the flow path, and

wherein the upstream mixing duct of the row extends most into the flow path, and the mixing ducts arranged in the row are of successively decreasing length as a function of downstream position such that a most downstream mixing duct of the row extends least into the flow path, and wherein the lengths are regularly decreasing along the row.

2. The intake section according to claim 1 , wherein each mixing duct is attached to a single sidewall only and freely extends into the flow path, and wherein the at least one outlet opening is located in a tip region of the mixing duct.

3. The intake section according to claim 1 , wherein along a direction perpendicular to the airflow direction at least two, or at least three, or at least four rows of mixing ducts that are distanced from each other, in the flow path, and/or wherein along the airflow direction at least two or at least three rows are distanced from each other in the flow path.

4. The intake section according to claim 1 , wherein each mixing duct of the row of mixing ducts has an axis perpendicular to the airflow direction in the flow path, the row of mixing ducts extending into the flow path, wherein the airflow direction is parallel to the sidewalls.

5. The intake section according to claim 1 , wherein each mixing duct comprises four pairwise parallel walls defining a duct, and wherein the at least one outlet opening opens in a direction perpendicular to the direction of the airflow or in a direction parallel and concurrent to the direction of the airflow.

6. The intake section according to claim 1 , wherein the at least one outlet opening opens in a direction perpendicular to the direction of the airflow and perpendicular to an axis of the mixing duct, and wherein the at least one outlet opening is comprised of at least two outlet openings for each mixing duct to blow recirculated flue gas in opposite directions into the airflow in the flow path.

7. The intake section according to claim 6 , further comprising at least one rounded or straight guide vane located in the at least one outlet opening or in a region of the at least one outlet opening and wherein the guide vane is located at half height of a total height of the outlet opening in a direction along the axis of the mixing duct.

8. The intake section according to claim 1 , wherein the at least one outlet opening opens in a direction perpendicular to the direction of the airflow and parallel to the axis of the mixing duct, and wherein an impingement plate is located downstream, with respect to a flow direction of the flue gas, of the at least one outlet opening arranged perpendicular to the flow direction of the flue gas and parallel to the airflow direction.

9. The intake section according to claim 1 , wherein the mixing ducts are arranged in a region where the flow path is defined by four pairwise parallel sidewalls, and

wherein a silencer and/or a filter is arranged upstream and/or downstream of the row of mixing ducts, the silencer and/or the filter being arranged in the region, upstream of the region, and/or downstream of the region.

10. The intake section according to claim 1 , wherein the most upstream located mixing duct further comprises a rounded leading edge.

11. A gas turbine comprising an intake section according to claim 1 .

12. The intake section according to claim 1 , wherein the row has at least three mixing ducts.

13. The intake section according to claim 1 , wherein the row has at least four mixing ducts.

14. An intake section upstream of an inlet of a compressor of a gas turbine unit with flue gas recirculation, the intake section comprising:

at least one section of the intake section with a flow path defined by sidewalls in which airflow of intake air is flowing along an airflow direction;

a plurality of mixing ducts extending into the flow path from at least one sidewall;

each of said mixing ducts having an intake at said at least one sidewall for receiving recirculated flue gas of the gas turbine and;

wherein at least two, or at least three, or at least four, or at least five of the mixing ducts are arranged in a row, adjacent to each other, said row being aligned along the airflow direction, a most upstream mixing duct of the row defining a leading edge of the row,

wherein the mixing ducts of the row all have different lengths of extension into the flow path, and wherein the most upstream mixing duct of the row extends most into the flow path, the most downstream mixing duct extending least into the flow path, and the mixing ducts arranged in the row are between being of successively and regularly decreasing length as a function of downstream position such that a most downstream mixing duct of the row extends least into the flow path, wherein the trailing edge of the row is defined by an inclined trailing edge wall, and

wherein each lateral side of the row has triangular outlet openings open in a direction perpendicular to the direction of the airflow and perpendicular to axes of the mixing ducts for blowing recirculated flue gas out of the mixing duct into the airflow.

15. The intake section according to claim 14 , wherein the row has at least three mixing ducts.

16. The intake section according to claim 14 , wherein the row has at least four mixing ducts.

17. The intake section according to claim 14 , wherein the row has at least five mixing ducts.

18. The intake section according to claim 14 , wherein along a direction perpendicular to the airflow direction at least two, or at least three, or at least four rows of mixing ducts that are distanced from each other in the flow path.

19. A method for recirculating flue gas comprising

mixing fresh ambient air with recirculated flue gas via an intake section upstream of an inlet of a compressor of a gas turbine, the intake section comprising:

at least one section with a flow path defined by sidewalls in which fresh airflow of intake air is flowing along an airflow direction;

wherein at least two mixing ducts are arranged in a row adjacent to each other, said row being aligned along the airflow direction, a most upstream mixing duct defining a leading edge of the row,

wherein the mixing ducts in the row all have different lengths of extension into the flow path, and

wherein the most upstream mixing duct of the row extends most into the flow path and the mixing ducts arranged in the row are of successively decreasing length of extension into the flow path as a function of downstream position.

20. The method according to claim 19 , wherein flow control elements, controlled based on a mass flow measurement in a flue gas recirculation line and/or based on a measurement of a composition of the mixed intake air upstream of the compressor, and/or based on a measurement of combustion quality, provided in the mixing ducts or upstream of the mixing ducts are used for controlling mass flow of recirculated flue gas.