Flushing the exhaust gas recirculation lines of a gas turbine

1. A method for the purging of an exhaust gas recirculation line of a gas turbine including a compressor, the method comprising:

operating the gas turbine at a purging speed;

discharging blow-off air from the compressor; and

introducing the blow-off air into the exhaust gas recirculation line for purging,

wherein the introduction of the blow-off air includes:

introducing the blow-off air into the exhaust gas recirculation line;

directing a first partial flow of the blow-off air in a direction of a second end of the exhaust gas recirculation line; and

directing a second partial flow of the blow-off air in a direction of a first end of the exhaust gas recirculation line.

2. The method as claimed in claim 1 , comprising:

discharging the blow-off air from the second end of the exhaust gas recirculation line.

3. The method as claimed in claim 1 , wherein the introduction of the blow-off air includes:

introducing the blow-off air at the second end of the exhaust gas recirculation line, which opens into an intake duct of the gas turbine;

flowing the blow-off air through the exhaust gas recirculation line opposite to a direction of a recirculation flow during normal operation of the gas turbine; and

discharging the blow-off air from the first end of the exhaust gas recirculation line.

4. The method as claimed in claim 3 , wherein the blow-off air, after flowing through the exhaust gas recirculation line, is directed from the first end of the exhaust gas recirculation line directly or indirectly into an exhaust stack.

5. The method as claimed in claim 1 , comprising:

controlling, by a control element, a ratio of the blow-off air which flows to the second end of the exhaust gas recirculation line to the blow-off air which flows to the first end of the exhaust gas recirculation line.

6. The method as claimed in claim 1 , comprising:

introducing the first partial flow of the blow-off air into the exhaust gas recirculation line for purging; and

feeding the second partial flow of the blow-off air via a second blow-off line to a heat recovery steam generator.

7. The method as claimed in claim 2 , wherein the introduction of the blow-off air includes:

introducing the blow-off air at the second end of the exhaust gas recirculation line, which opens into an intake duct of the gas turbine;

flowing the blow-off air through the exhaust gas recirculation line opposite to a direction of a recirculation flow during normal operation of the gas turbine; and

discharging the blow-off air from the first end of the exhaust gas recirculation line.

8. The method as claimed in claim 7 , wherein the blow-off air, after flowing through the exhaust gas recirculation line, is directed from the first end of the exhaust gas recirculation line directly or indirectly into an exhaust stack.

9. The method as claimed in claim 1 , wherein the gas turbine includes a generator,

wherein the operating of the gas turbine comprises operating the gas turbine at rotational speeds in a range of 20% to 50% of the nominal rotational speed of the gas turbine, and

wherein the generator is operated as a motor which drives the gas turbine and simultaneously purges the exhaust gas recirculation line with the blow-off air and a boiler with exhaust flow of the gas turbine.

10. The method as claimed in claim 1 , wherein the introducing of the blow-off air into the exhaust gas recirculation line comprises introducing the blow-off air into a section of the exhaust gas recirculation line which is at most 10 to 20% away from an end of the exhaust gas recirculation line such that the blow-off air is not introduced into the end of the exhaust gas recirculation line.

11. The method as claimed in claim 1 , comprising:

purging the exhaust gas recirculation line to a location in an exhaust gas duct downstream of a heat recovery steam generator (HRSG), which is downstream of the gas turbine; and

purging the HRSG with exit flow of the gas turbine.

12. The method as claimed in claim 1 , comprising:

operating the compressor at rotational speeds which are above standstill of the compressor and 20% below a nominal rotational speed of the gas turbine.

13. The method as claimed in claim 1 , comprising:

purging the blow-off air through the second end of the exhaust gas recirculation line into the compressor intake duct for a predetermined time; and

purging the exhaust gas recirculation line in the direction of the first end of the exhaust gas recirculation line.

14. The method as claimed in claim 13 , wherein the predetermined time is in a time range of several seconds to several minutes.

15. A gas turbine comprising:

a compressor;

an exhaust gas duct;

a compressor intake duct having a compressor intake flow;

an exhaust gas recirculation line connecting the exhaust gas duct of the gas turbine to the compressor intake duct for recirculation of exhaust gases into the compressor intake flow;

at least one purging line leading from a compressor blow-off point into the exhaust gas recirculation line; and

a control element arranged in the exhaust gas recirculation line between a mouth of the purging line into the exhaust gas recirculation line and a mouth of the exhaust gas recirculation line into the compressor intake duct to at least one of control and prevent back-purging of blow-off air into compressor inlet flow,

wherein the compressor is configured to be operated at rotational speeds which are above standstill of the compressor and 20% below a nominal rotational speed of the gas turbine, and

wherein the control element is configured to control a ratio of the blow-off air which flows to the compressor inlet flow at a second end of the exhaust gas recirculation line to the blow-off air which flows to a first end of the exhaust gas recirculation line.

16. The gas turbine as claimed in claim 15 , comprising:

a purge valve arranged in the purging line for controlling a purging mass flow with which the exhaust gas recirculation line is purged by blow-off air.

17. The gas turbine as claimed in claim 15 , wherein the purging line is connected to the compressor blow-off point, and

wherein the gas turbine comprises a first blow-off line connecting the compressor blow-off point to the exhaust gas duct of the gas turbine.

18. The gas turbine as claimed in claim 17 , comprising:

a first blow-off valve arranged in the first blow-off line.

19. The gas turbine as claimed in claim 15 , comprising:

a second blow-off line connecting a second compressor blow-off point to the exhaust gas duct of the gas turbine.

20. The gas turbine as claimed in claim 15 , comprising:

a recirculation fan having a drive and arranged in the exhaust gas recirculation line,

wherein the recirculation fan is arranged in a disconnectable manner to the drive so that during blow off, the recirculation fan is configured to freewheel opposite to a normal direction of rotation of the recirculation fan.

21. The gas turbine as claimed in claim 15 , comprising:

a recirculation fan having at least one of variable geometry guide vanes and impeller blades, and being arranged in the exhaust gas recirculation line,

wherein a direction of rotation of the recirculation fan is reversible so that during the blow off the recirculation fan is configured to provide at least one of reduced flow resistance and assistance to the purging line.

22. The gas turbine as claimed in claim 15 , comprising:

a generator,

wherein the gas turbine is configured to be operated at rotational speeds in a range of 20% to 50% of the nominal rotational speed of the gas turbine, and

wherein the generator is configured to be operated as a motor which drives the gas turbine and simultaneously purges the exhaust gas recirculation line with blow-off air and a boiler with exhaust flow of the gas turbine.

23. The gas turbine as claimed in claim 15 , wherein the at least one purging line is configured to introduce blow-off air into a section of the exhaust gas recirculation line which is at most 10 to 20% away from an end of the exhaust gas recirculation line such that the blow-off air is not introduced into the end of the exhaust gas recirculation line.

24. The gas turbine as claimed in claim 15 , wherein the exhaust gas duct is located downstream of a heat recovery steam generator (HRSG), which is downstream of the gas turbine, and

wherein the exhaust gas recirculation line is purged to a location in the exhaust gas duct, and the HRSG is purged with exit flow of the gas turbine.