Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
A METHOD FOR OPERATING A GAS TURBINE AND A GAS TURBINE FOR
IMPLEMENTING THE METHOD
TECHNICAL FIELD
The present invention relates to the field of power plant technology. It
pertains to a
method for operating a (stationary) gas turbine, as well as a gas turbine for
implementing the method.
PRIOR ART
A gas turbine with reheating (reheat gas turbine) is known (see, for example,
the
US patent application US-A-5,577,378 or "State-of-the-art gas turbines ¨ a
brief
update," ABB Review 02/1997, Fig. 15, turbine type GT26), which combines
flexible operation with very low flue gas emission values.
The machinery architecture of the gas turbine of Type GT26 is unique and is
exceptionally well-suited to realizing a concept that is the subject matter of
the
present invention, because:
- even in the case of the compressor, there is a significant diversion
of
compressor air at intermediate compressor pressures,
- the concept of sequential combustion renders an increased stability
of
=
25. combustion possible in conjunction with reduced levels of excess
oxygen,
and
- a secondary air system is present, which renders it possible to
divert air
from the compressor, to cool it down, and to use the cooled air for cooling
the combustor and the turbine.
The principle of the known gas turbine with reheating is shown in Fig. 1. The
gas
turbine 11, which is a portion of a combined cycle power plant 10, comprises
two
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connected compressors, arranged behind one another on a commonly shared
shaft 15, namely a low pressure compressor 13 and a high pressure compressor
14, as well as two combustors, namely a high pressure combustor 18 and a
reheat
combustor 19, and the pertinent turbines, namely a high pressure turbine16 and
a
low pressure turbine 17. The shaft 15 drives a generator 12.
The manner in which the unit works is as follows: air is drawn in via an air
inlet 20
from the low pressure compressor 13, and is compressed initially to a level of
intermediate pressure (ca. 20 bar). The high pressure compressor 14 then
further
compresses the air to a level of high pressure (ca.32 bar). Cooling air is
diverted
at both the level of intermediate pressure and at the level of high pressure
and
cooled down in pertinent OTC coolers (OTC = Once Through Cooler) 23 and 24
and conducted further to the combustors 18 and 19 and turbines 16, 17 via
cooling
lines 25 and 26 for cooling purposes. The remaining air from the high pressure
compressor 14 is conducted to the high pressure combustor 18 and heated there
by the burning of a fuel, which is introduced via the fuel feedline 21. The
resultant
flue gas is then expanded in the downstream high pressure turbine 16 to an
intermediate level of pressure as it performs work. After expansion, the flue
gas is
reheated in the reheat combustor 19 by the burning of a fuel that is
introduced via
fuel feedline 22 before it is expanded in the downstream low pressure turbine
17,
performing additional work in the process.
The cooling air, which flows through the cooling lines 25, 26, is sprayed in
at
suitable points of the combustors 18, 19 and turbines 16, 17 to limit material
temperatures to a reasonable degree. The flue gas, which comes from the low
pressure turbine 17, is sent through a heat recovery steam generator 27 (HRSG)
in order to generate steam, which flows within a water-steam circuit through a
steam turbine 29 and performs additional work there. After flowing through the
heat recovery steam generator 27, the flue gas is finally released to the
outside
through a flue gas line 28. The OTC coolers 23, 24 are a portion of the water-
steam circuit; superheated steam is generated at their outlets.
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As a result of the two combustions in the combustors 18 and 19, which are
independent of each other and follow one another, great flexibility of
operation is
achieved; the combustor temperatures can be adjusted in such a way that the
maximum degree of efficiency is achieved within the existing limits. The low
flue gas
levels of the sequential combustion system are provided by the inherently low
emission levels, which can be achieved in the course of reheating (under
certain
conditions, the second combustion even leads to a consumption of N0x).
On the other hand, combined cycle power plants with single stage combustion in
the
gas turbines are known (see, for example, the US patent application US-A-
4,785,622
or US-B2-6,513,317), in which a coal gasification unit is integrated in order
to provide
the requisite fuel for the gas turbine in the form of syngas, which is
recovered from
coal. Such combined cycle power plants are designated IGCC (Integrated
Gasification Combined Cycle) plants.
SUMMARY OF THE INVENTION
Some embodiments of the present disclosure proceed from the recognition that
due
to the use of gas turbines with reheating in an IGCC plant, the advantages of
this
type of gas turbine can be made usable for the plant in a particular manner.
According to an aspect of the present invention, there is provided a method
for
operating a gas turbine, the method comprising: drawing in and compressing air
with
the gas turbine; conducting compressed air to a combustor; combusting a syngas
that is generated from coal with said compressed air in said combustor to
generate
hot gases; expanding the hot gases that occur in the course of combustion in a
downstream turbine as said hot gases perform work; separating a portion of the
compressed air into oxygen and nitrogen; conducting said separated oxygen to
and
using said separated oxygen in a coal gasifier to produce syngas; conducting a
portion of said compressed air to said gas turbine to cool parts of the gas
turbine
exposed to hot gases; wherein said gas turbine comprises a gas turbine with
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reheating including two combustors and two turbines, wherein in a first
combustor of
said two combustors, said syngas is combusted using said compressed air and
the
resultant hot gases are expanded in a first turbine, and wherein in a second
combustor of said two combustors, said syngas is combusted using the gases
coming out of the first turbine and the resultant hot gases are expanded in a
second
turbine; cooling the gas turbine with the nitrogen that occurs in the
separation of the
air; wherein the gas turbine comprises a first compressor for compressing
drawn-in
air to an initial pressure stage and a second compressor for compressing the
air
further from the initial pressure stage to a second, higher pressure stage;
cooling said
first combustor and said first turbine directly with compressed air from said
first
compressor; separating a portion of the air coming from the first compressor
into
oxygen and nitrogen; and using the nitrogen that occurs in the course of said
separating to cool the second combustor and second turbine.
Some embodiments of the present invention are directed to a method for the
operation of a gas turbine that works in concert with a coal gasifier, which
is
characterized by an improved degree of efficiency, which can also be realized
to
particularly good effect using available components, as well as to create a
gas turbine
for implementing the method.
In one aspect, a gas turbine with reheating is used in a gas turbine unit that
works
with syngas from a coal gasifier, which comprises two combustors and two
turbines,
in which, in the first combustor, syngas is burned employing the compressed
air, and
the resultant hot gases are expanded in the first turbine, and in which syngas
is
burned in the second combustor, using the gases that come from the first
turbine,
and the resultant hot gases are expanded in the second turbine, and the
nitrogen
that occurs in the separation of the air is used to cool the gas turbine. The
solution
according to some embodiments has the following advantages:
- No OTC cooler is required, as a result of which the degree of efficiency is
increased.
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- Less cooling air is required, which is also to the good of the degree of
efficiency.
- The comparatively cold nitrogen from the air separation unit can be used
to
cool critical components, whereas the warmer air from the compressor can be
used
5 to cool less critical components; this, too, improves the unit's degree
of efficiency.
- The cooling described can be realized especially simply in the case of
gas
turbines with reheating of the known structural type, such as the type GT26
gas
turbine, for example, due to the specific secondary air system.
In one embodiment of the method, the gas turbine comprises a first compressor
for
the purpose of compressing intaken air to an initial pressure stage, and a
second
compressor to compress the air further from the initial pressure stage to a
second,
higher pressure stage, a portion of the air coming from the initial compressor
is
separated into oxygen and nitrogen, and the nitrogen that occurs in the course
of
this separation is used to cool the second combustor and second turbine.
In the process, in some embodiments, a portion of the compressed air that is
diverted from the initial compressor for the separation is diverted prior to
the
separation, and mixed with the nitrogen, which occurs in the course of the
separation
and is provided for cooling purposes. Particularly favorable circumstances
arise if
about 50% of the compressed air that is diverted for the separation, is
diverted from
the initial compressor prior to the separation and mixed with the nitrogen
that occurs
in the course of separation, which is also provided for cooling purposes.
Preferably,
the nitrogen that occurs in the course of separation is compressed prior to
mixing with
the compressed air that was diverted prior to separation.
In an embodiment of the gas turbine, a branching line is provided, which
branches off
from the inlet side of the air separation unit and discharges into the
nitrogen line at a
point provided, and in the nitrogen line, between the outlet of the air
separation unit
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and the prescribed discharge point of the branching line, a compressor is
provided to
compress the nitrogen.
Preferably, in some embodiments, the gas turbine exhibits two compressors,
connected behind one another, the air separation unit is attached on the side
of its
input, to the outlet of the first compressor, and the nitrogen line is led to
the second
combustor and to the second turbine.
In some embodiments, the air separation unit, particularly on the side of its
outlet,
exhibits an oxygen line to give off the oxygen that occurs in the course of
the
separation, which is led to a unit for the production of syngas by means of
gasifying
coal, and that a syngas input line transports the syngas that is generated
from the
syngas production unit to the combustors.
BRIEF EXPLANATION OF THE FIGURES
In what follows, the invention is to be explained in greater detail by virtue
of the
embodiment examples in conjunction with the drawings.
Fig. 1 shows the simplified schematic of a combined cycle power plant with
a
gas turbine with reheating or sequential combustion according to the prior
art,
respectively;
Fig. 2 shows the simplified schematic of an IGCC unit with a gas
turbine with
reheating or sequential combustion, respectively, as it is suitable for
realizing an
embodiment of the invention; and
Fig. 3 shows an embodiment example for cooling according to the
invention
using the nitrogen that is recovered in the separation of the air in a unit of
the type
depicted in Fig. 2.
DESCRIPTION OF EMBODIMENTS
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In Fig. 2, in a markedly simplified schematic, an IGCC unit with a gas turbine
with
re-heating, or sequential combustion, respectively, is shown, as it is
suitable for
realizing an embodiment of the invention. The combined cycle power plant 30
comprises a gas
turbine 11 with a low pressure compressor 13, a downstream high pressure
compressor, 14, a high pressure combustor 18 with a downstream high pressure
turbine 16 and a reheat combustor 19 with a downstream low pressure turbine
17.
The compressors 13, 14 and the turbines 16, 17 sit on a commonly shared shaft
15, by which a generator 12 is driven. The combustors 18 and 19 are supplied,
via
a syngas feed line 31, with syngas as fuel, which is produced by gasifying
coal
:coal feeding 33) in a coal gasifier 34. A cooling device 35 for the syngas, a
filtering device 36, and a CO2 separator 37 with a CO2 outlet 38 to release
the CO2
that is given off top the coal gasifier 34.
Oxygen (02), which is recovered in an air separation unit 32, and is added via
an
oxygen line 32a, is used to gasify coal in the coal gasifier 34. The air
separation
unit 32 receives compressed air from the outlet of the low pressure compressor
13. The nitrogen, (N2), which also occurs in the course of separation, is led
via a
nitrogen line 32b, for example, to the low pressure combustor 19.
For cooling the components of the combustors 18, 19 and turbines 16, 17 that
are
exposed to the hot gas, compressed cooling air is drawn off at the outlets of
both
compressors 13 and 14, cooled off in a topped OTC cooler 23 or 24,
respectively,
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and then led, via corresponding cooling lines 25 and 26, to those points that
are to
be cooled.
At the outlet of the low pressure turbine17, a heat recovery steam generator
27 is
provided, which, together with a connected steam turbine 29, is part of a
water-
steam circuit. The flue gas that escapes from the heat recovery steam
generator
27 is released to the outside by way of a flue gas line 28.
In such a configuration of the unit, according to Fig. 3, the position of the
cooling is
now changed. In the combined cycle power plant 40 of Fig. 3, now, as before,
the
high pressure combustor 18 and the high pressure turbine16 are cooled by
compressed air, which is diverted at the outlet of the high pressure
compressor 14
and then cooled down in an OTC cooler 24. The cooling of the reheat combustor
19 and the low pressure turbine 17, now takes place in a different manner,
however. To this end, at the outlet of the low pressure compressor 13, 50% of
the
diverted compressed air is separated into oxygen and nitrogen in the air
separation unit 32. The other 50% are led past the air separation unit 32 in a
branching line 39. The oxygen, which is drawn off from the air separation unit
32
via oxygen line 32a is, as is shown in Fig. 2, used to gasify the coal. The
relatively
cool nitrogen that is produced is led through the nitrogen line 32b to a
compressor
41 and after compression, mixed with the 50% of the air from the branching
line
39. After mixing, the gas temperature is about 300-400 C, so that cooling the
cooling air that is extracted at the low pressure compressor 13 is not
necessary.
The resultant mixture is then used to cool the hot components of the reheat
combustor 19 and the low pressure turbine 17.
The advantages of this type of cooling are:
- No OTC cooler is needed, as a result of which the degree of
efficiency is
increased.
- Less cooling air is needed, which also benefits the degree of efficiency.
- The comparatively cold nitrogen from the air separation unit can be
used to
cool critical components, whereas the warmer air from the compressor can
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be used to cool less critical components; this, too, improves the unit's
degree of efficiency.
- The cooling described can be realized particularly simply in the
case of gas
turbines with reheating of the known type of construction, such as, for
example, the type GT26 gas turbine, because of the specific secondary air
system.
A prerequisite for the realization of this concept is that in the gas
turbine's two
combustors, undiluted coal gas can be used. The main technical challenges
associated with the combustion of such undiluted coal gas in the combustor of
a
gas turbine are:
- The achievement of low emission levels,
- Sufficient distance from the limits of flashbacks and pulsations,
- Maintaining operational flexibility in the event of changes in the
quality of
the coal gas as well as the possibility of support with other fuels (natural
gas or oil), and
- The drawing off and feeding in of cooling air into the areas of the
heating
gas channel in the combustor and in the turbine.
In the case of IGCC units, from conception onward, these challenges can be
overcome particularly well by means of a gas turbine with reheating for the
following reasons:
1. The inherent advantage associated with reheating with respect to NOx can
also
be transferred to syngas if the combustion temperatures in both combustors are
selected so as to be optimal, especially with a moderated temperature increase
in
the initial stage (high pressure combustor 18).
2. The stability of combustion and the operational flexibility in the case of
the gas
turbine with reheating are greater than in the case of a comparable gas
turbine
with single stage combustion. The operational limits are typically set by the
extinguishing and flashback of the flame and/or emission levels for any given
flame temperature, which gives rise to a permitted range of fuel qualities and
fuel
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reactivity levels. In the gas turbine with reheating, this operational limit
is clearly
increased because two combustion systems render operation in conjunction with
two independent flame temperatures possible, e.g. with a lower temperature in
the
initial stage and a higher temperature in the second stage, with slight
disadvantages with respect to NOx.
3. The requirements with respect to gas pressure can be minimized if the fuel
gas
is injected undiluted (without nitrogen) into the initial and the second
combustion
systems, which typically work with pressures in the range of > 30 bar, or
between
15 and 20 bar, respectively.
4. The concept of the extraction of cooling air, which is subsequently cooled
down
and fed into the machine again, lends itself particularly well to the use of
nitrogen
as a cooling medium.
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List of Reference Signs
10,30,40 combined cycle power plant
11 gas turbine
12 generator
13 low pressure compressor
14 high pressure compressor
shaft (gas turbine)
16 high pressure turbine
17 low pressure turbine
18 high pressure combustor
19 reheat combustor
air inlet
21,22 fuel feedline
23,24 OTC cooler
25,26 cooling line
27 heat recovery steam generator
28 flue gas line
29 steam turbine (steam cycle)
31 syngas feed line
32 air separation unit
32a oxygen line
32b nitrogen line
33 coal feeding
34 coal gasifier
35 cooling device
36 filtering device
37 CO2 separator
38 _ CO2 outlet
39 branching line
41 compressor
