Combined cycle power plant

Details Combined cycle power plant Combined cycle power plant

1999-09-22

2001-08-14

Thorpe, Timothy S. (Department: 3746)

Power plants

Combustion products used as motive fluid

Multiple fluid-operated motors

Reexamination Certificate

active

06272841

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a combined cycle power plant including combined gas and steam turbine plants.
BACKGROUND ART
The combined cycle power plant is a power generation system which has combined gas and steam turbine plants. In this system, the gas turbine takes charge of a higher-temperature range of the thermal energy, while the steam turbine takes charge of the remaining lower-temperature range, thereby efficiently recovering and using the thermal energy. This is currently the most common power generation system.
In order to improve the efficiency, the development of the combined cycle power plant has been focused on how high the higher-temperature range of the gas turbine can be set.
To realize a higher-temperature range, a cooling system must be provided in consideration of the heat resistance of the turbine structure. Conventionally, air is used as a cooling medium in such a cooling system.
However, as long as air is used as the cooling medium, even if a desirable higher-temperature range can be achieved, the plant will inevitably suffer from (i) loss of power necessary for boosting the air (which was used for cooling) up to a specified pressure by using an internal air compressor, and (ii) lowering of the average gas temperature and thus the energy of the gas because the air used for cooling target portions is finally made flow through the passage (in the turbine) together with the high-temperature gas. As a result of the above effects (i) and (ii), it is very difficult to further improve the thermal efficiency.
In order to solve the above problem and to further improve the efficiency, another type of combined cycle power plant has been proposed, in which the steam is used as the cooling medium of the gas turbine instead of air as mentioned above.
Japanese Unexamined Patent Application, First Publication, No. Hei 5-163960 discloses an example thereof. The general structure of the disclosed power plant is shown in FIG.
2
.
That is, the combined cycle power plant
10
comprises (i) gas turbine plant
11
mainly including gas turbine
13
, air compressor
18
, and combustor
19
, (ii) exhaust heat recovery boiler
14
, mainly comprising high-pressure drum
20
, medium-pressure drum
21
, and low-pressure drum
22
, which uses the exhaust gas from the gas turbine plant
11
as a heat source, and (iii) steam turbine plant
12
mainly comprising high-pressure turbine
15
a
, medium-pressure turbine
15
b
, and low-pressure turbine
15
c
to which the steam is supplied from the exhaust heat recovery boiler
14
.
The cooling system employed here is the steam cooling system
50
in which the medium-pressure steam from the medium-pressure drum
21
of the exhaust heat recovery boiler
14
is introduced as the cooling steam via steam supply path
51
into steam cooling section
52
provided in a higher-temperature portion to be cooled in gas turbine
13
. That is, the above higher-temperature portion is cooled, and thus the above cooling steam is heated and obtains energy. The cooling steam is then supplied via steam recovery system
53
to the medium-pressure turbine
15
b
of the steam turbine plant
12
. That is, the steam can be efficiently recovered.
Here, steam section
60
is a backup section, and the backup steam can be supplied from the high-pressure drum
20
of the exhaust heat recovery boiler
14
via high-pressure steam line
42
. This backup section is used when the gas turbine
13
is activated.
When the gas turbine
13
is activated, the amount of steam generated in the boiler is insufficient due to a delay of the follow-up operation at the exhaust side. Therefore, the steam cooling system
50
cannot operate normally until the insufficiency is corrected, and that is why the backup system using the steam section
60
is necessary.
Although this publication Hei 5-163960 discloses the concept that steam is used as a cooling medium for gas turbine
13
, the disclosed system also has many problems when examined in detail.
For example, the backup operation using the steam section
60
is performed to remedy the insufficiency of the amount of steam generated in the boiler when the gas turbine
13
is activated, that is, this operation is not effective when the steam supply to the high-temperature portion (to be cooled) in the gas turbine
13
is cut off due to a problem occurring during the normal loading operation.
That is, the disclosure in Hei 5-163960 does not include the concept of the backup operation necessary for an abnormal phenomenon or condition during the normal loading operation. Therefore, when such an abnormal phenomenon occurs, the cooling medium has to be switched to air, or the gas turbine
13
has to be tripped.
If the cooling medium is switched from steam to air, the cooling ability is obviously insufficient, and thus a measure of reducing the thermal load, such as the running-back of the load of the gas turbine, is necessary.
It is an object of the present invention to solve the above problems in the conventional system, and to provide a combined cycle power plant which has a backup function operating during the normal operation as well as when the plant is activated, so as to reliably maintain the steam cooling operation when an abnormal phenomenon occurs.
DISCLOSURE OF INVENTION
To achieve the above object, the present invention provides a combined cycle power plant having a gas turbine plant and a steam turbine plant combined together, the power plant comprising:
an exhaust heat recovery boiler for generating steam for driving the steam turbine by using exhaust heat from the gas turbine; and
a steam cooling system for cooling a target high-temperature portion in the gas turbine by using steam, where superheated steam from the steam cooling system is recovered and used in the steam turbine, and
wherein the steam turbine plant comprises at least a high-pressure turbine and a low-pressure turbine, wherein an exhaust gas from the high-pressure turbine is introduced into the steam cooling system, and when the exhaust gas from the high-pressure turbine is unexpectedly insufficient, high or medium-pressure steam output from the exhaust heat recovery boiler is introduced into the steam cooling system.
Accordingly, even if the steam supply to the high-temperature portion to be cooled in the gas turbine plant is stopped due to an abnormal phenomenon, e.g., the high-pressure exhaust gas runs out or is reduced when the steam turbine is tripped during the normal loading operation, high or medium-pressure steam can be immediately supplied as backup steam instead of the high-pressure exhaust gas, thereby reliably continuing the operation of the gas turbine plant.
In the above combined cycle power plant, when the high-pressure steam output from the exhaust heat recovery boiler is introduced into the steam cooling system, saturated steam may be infused so as to control the temperature of the introduced steam.
That is, the backup steam may be saturated steam instead of using saturated water. In this case, it is possible to prevent mist from reaching the high-temperature portion to be cooled of the gas turbine plant, and to prevent problems such as generation of thermal shock and closure of the cooling path near the head of the relevant rotational portion.


REFERENCES:
patent: 5628179 (1997-05-01), Tomlinson
patent: 0 736 669 (1996-10-01), None
patent: 5-163960 (1993-06-01), None
patent: 9-4417 (1997-01-01), None
patent: 9-280010 (1997-10-01), None

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