Method of operating a gas turbine power plant with steam injection

In a method of and an apparatus for operating a power station plant, essentially comprising a gas-turbine group (40, 41, 46), a waste-heat steam generator (8) and a downstream steam consumer, for example a steam turbine (1) having a generator (2), the exhaust gas of the gas-turbine group (40, 41, 46) releases heat to the water fed via a feed-water line (15) and directed in counterflow through the waste-heat steam generator (8). The steam generated is fed to the steam consumer (1) via at least one steam line (6). The water fed by the feed-water line (15) is directed through the waste-heat steam generator (8) in a once-through arrangement. The exhaust gases of the gas-turbine group (40, 41, 46) are directed through the waste-heat steam generator (8) at every operating instant. The steam generated in the waste-heat steam generator (8) is directed into the gas-turbine group (40, 41, 46) via an injection steam line (23) when the steam consumer (1) is shut off.

FIELD OF THE INVENTION 
The invention relates to a method of and an apparatus for operating a power 
station plant, comprising a gas-turbine group, a waste-heat steam 
generator and a downstream steam consumer, for example a steam turbine 
having a generator, the exhaust gas of the gas-turbine group releasing 
heat to the water fed via a feed-water line and directed in counterflow 
through the waste-heat steam generator, and the steam generated being fed 
to the steam consumer via at least one steam line. 
BACKGROUND 
Such methods of and apparatuses for operating a power station plant are 
known. In the waste-heat steam generator, heat energy is removed from the 
exhaust gases of the gas-turbine group and water is evaporated with this 
heat energy, and this steam is fed to a steam consumer. The steam consumer 
may be, for example, a steam turbine having a generator for generating 
electricity or an industrial plant. If the steam consumer is shut off, for 
example for maintenance, steam is no longer required. Therefore a bypass 
line leading to a stack is normally attached in such a way as to branch 
off from the exhaust-gas line arranged between gas turbine and waste-heat 
steam generator. Via a distribution valve, for example a flap valve, the 
exhaust-gas flow can be directed through the waste-heat steam generator or 
directly through the stack. It is thereby possible to operate the 
gas-turbine group in the open state, i.e. without producing steam which 
cannot be consumed. However, the exhaust gases are then released without 
heat recovery directly to the environment. This means a considerable loss 
of heat energy and considerable investment costs for the stack and the 
distribution valve. In addition, the distribution valve for the 
exhaust-gas flow can never be made completely tight; losses of the 
exhaust-gas flow of several percent may occur and thus losses in the 
output of the steam turbine. Furthermore, the maintenance costs for the 
distribution valve are very high, since this valve is a component 
subjected to high thermal stress. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the invention, in a method of and an apparatus 
for operating a power station plant of the type mentioned at the 
beginning, is to increase the availability and the efficiency of the power 
station plant. 
According to the invention, this is achieved when the water fed by the 
feed-water line is directed through the waste-heat steam generator in a 
once-through arrangement, when the exhaust gases of the gas-turbine group 
are directed through the waste-heat steam generator at every operating 
instant, and when the steam generated in the waste-heat steam generator is 
directed into the gas-turbine group via an injection steam line when the 
steam consumer is shut off. 
The advantages of the invention may be seen, inter alia, in the fact that a 
distribution valve in the exhaust-gas line is no longer required, since 
steam can also be used for energy recovery at every operating instant of 
the gas-turbine group. The construction costs are thereby reduced and the 
disadvantages of a distribution valve avoided. When the steam consumer is 
shut off, the energy of the exhaust gases is partly recovered by steam 
being directed into the gas-turbine group. The steam fed in there is 
replaced by demineralized water. The efficiency of the gas-turbine group 
is therefore increased. By the water being directed through the waste-heat 
steam generator in a once-through arrangement, the steam quantity and the 
steam temperature can be set by the water quantity in the feed-water line. 
It is therefore especially expedient if a measuring orifice is arranged in 
the injection steam line and a regulating valve is arranged in the 
feed-water line. The water quantity and thus the steam quantity which is 
directed into the gas-turbine group can be set by the regulating valve. 
BRIEF DESCRIPTION OF THE DRAWINGS 
A more complete appreciation of the invention and many of the attendant 
advantages thereof will be readily obtained as the same becomes better 
understood by reference to the following detailed description when 
considered in connection with the single drawing, wherein an exemplary 
embodiment of the invention is shown with reference to a schematic 
representation of a combined gas- and steam-turbine plant. 
Only the elements essential for understanding the invention are shown. 
Details of the plant which are not shown are, for example, the exact 
configuration of the waste-heat steam generator as well as the control of 
the water/steam cycle. The direction of flow of the working media is 
indicated by arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawing, the gas-turbine group shown comprises a 
compressor 40, a turbine 41 and a generator 46, which are connected via a 
shaft 42, as well as a combustion chamber 43. In the compressor 40, air is 
drawn in via an air feed 44 and compressed, and the compressed air is 
directed into the combustion chamber 43. Fuel 45 is fed there to the 
combustion air and the fuel/air mixture is burned. The resulting flue 
gases are directed into the turbine 41, where they are expanded and some 
of their energy is converted into rotary energy. This rotary energy is 
used to drive the generator 46 via the shaft 42. 
The still hot exhaust gases are fed directly to a waste-heat steam 
generator 8 via an exhaust-gas line 47. In this case, the hot exhaust 
gases are fed to the waste-heat steam generator 8 at every operating 
instant of the turbine 41, i.e. there is no bypass line for the exhaust 
gases. In the waste-heat steam generator 8, heat energy is removed from 
the exhaust gas and water is therefore evaporated. After the heat energy 
is released, the exhaust gas is passed into the open via a stack 48. The 
water is directed in counterflow in a once-through arrangement through the 
waste-heat steam generator 8. The superheated steam generated in the 
waste-heat steam generator 8 is fed via a steam line 6 to a steam turbine 
1. The steam is expanded in the steam turbine 1 to perform work and the 
energy gained is released to a generator 2 via a shaft 3. The exhaust 
steam issuing via an exhaust-steam line 4 is condensed in a condenser 5 
and held in intermediate storage in the associated hot well. The condensed 
water is discharged via a pump 10 and a condensate line 11 and directed 
into a deaerator dome 12. Demineralized water may additionally be fed into 
the deaerator dome via a water line 17. The deaerator dome 12 is mounted 
on a feed-water tank 13. The condensate is injected into the deaerator 
dome 12 via a distribution head (not shown). Bled steam from the steam 
turbine 1 passes into the deaerator dome 12 via a feed-steam line 16, as a 
result of which the condensate in the deaerator dome 12 is heated and 
deaerated. Inert and noncondensable gases can thus be separated from the 
condensate. The heated condensate is held in intermediate storage as feed 
water in the feed-water tank 13. The feed water is fed to the waste-heat 
steam generator 8 by a feed-water pump 14 and a feed-water line 15. If the 
steam turbine 1 is to be put out of operation quickly, a bypass steam line 
18 is normally provided for this purpose. A valve 7 in the steam line 6 is 
closed in the process and a valve 19 in the bypass steam line 18 is 
opened. The steam is thereby fed directly to the condenser 5. The bypass 
steam line 18 can likewise be utilized during start-up of the steam 
turbine 1 in order to prevent wet steam from passing into the steam 
turbine. 
An injection steam line 23 now branches off from the steam line 6 or, as 
shown, from the bypass steam line 18. Via the injection steam line 23, 
steam is injected into the combustion chamber 43 of the gas turbine or 
directly into the turbine 41. A throttle valve 22 and a measuring orifice 
21 are located in the injection steam line 23. A regulating valve 20 
arranged in the feed-water line 15 upstream of the waste-heat steam 
generator 8 is controlled via the measuring orifice 21. 
During normal operation of the power station plant with the steam turbine 
1, the valve 7 is open and the valve 19 as well as the throttle valve 22 
are closed. The temperature as well as the quantity of the steam in the 
steam line 6 is regulated via the regulating valve 20 by means of the 
steam state at the outlet from the waste-heat steam generator 8 in the 
steam line 6. If the temperature of the steam in the steam line 6 rises 
above a predetermined value, the regulating valve 20 is opened. The 
temperature of the steam in the steam line 6 falls due to the greater rate 
of flow of feed water through the waste-heat steam generator 8. If the 
temperature of the steam falls below a certain value, the regulating valve 
20 is slightly closed again and so forth. 
If the steam turbine 1 is now to be put out of operation, for example for 
maintenance, the valve 7 is closed and the throttle valve 22 opened. The 
steam quantity injected into the gas-turbine group is now regulated by 
means of the measuring orifice 21 and the regulating valve 20. If the 
steam quantity in the injection steam line 23 rises above a predetermined 
value, the regulating valve 20 is closed slightly. The steam quantity in 
the injection steam line 23 falls due to the smaller rate of flow of feed 
water through the waste-heat steam generator 8. If the steam quantity 
falls below a certain value, the regulating valve 20 is opened again and 
so forth. The steam injected into the gas-turbine group is naturally lost 
and must be replaced via the water line 17 by means of demineralized 
water. 
This method is made possible by the waste-heat steam generator 8, in which 
the water is directed in a once-through arrangement through the waste-heat 
steam generator 8. Owing to the fact that the regulating valve 20 can 
regulate the steam quantity during every operating state, the heat energy 
which is not utilized when less steam is required is simply discharged by 
means of a higher exhaust-gas temperature at the outlet from the 
waste-heat steam generator 8. 
The invention is of course not restricted to the exemplary embodiment shown 
and described. The bypass steam line may be omitted without affecting the 
invention. The throttle valve can also be opened during operation of the 
steam turbine in order to increase the output of the gas-turbine group. 
The steam turbine and the parts belonging to it may be replaced by any 
steam consumer. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.