Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2001-12-06
2004-02-03
Casaregola, Louis J. (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S039520
Reexamination Certificate
active
06684643
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the operation of a gas turbine plant with CO
2
as the working medium.
BACKGROUND OF THE INVENTION
Gas turbine plants with internal combustion and a quasi-closed circuit for CO
2
as working medium represent examples of a very promising, environmentally friendly technique for energy recovery or conversion. Differing from conventional gas turbine plants for energy recovery, in which fossil fuels are used and large amounts of CO
2
are released, gas turbine plants with a quasi-closed CO
2
circuit make it possible to considerably reduce the CO
2
emissions, and also emissions of nitrogen oxides, caused by the combustion of carbon-containing fuels in atmospheric oxygen. Thus in a known manner, the flue gases arising from the combustion process are cooled and by recirculation are supplied anew to the intake region of the gas turbine plant, with subsequent internal combustion. Such a recirculation, principally of CO
2
which results from the combustion process, can only take place to the extent to which the atmospheric oxygen also present within the combustion process is used up. If the combustion process is fed with atmospheric oxygen, the flue gases arising in the combustion remain mixed with atmospheric nitrogen, so that the CO
2
emission problem can however be only marginally reduced, and especially in this case the CO
2
is mixed with nitrogen oxides in the resulting flue gases and can only be isolated from the circuit with greater difficulty.
In order to solve the said nitrogen problem with simultaneous environmentally friendly elimination of CO
2
, a gas turbine plant with a CO
2
process has been proposed, as schematically shown in FIG.
5
. The quasi-closed, CO
2
-charged gas turbine process shown in
FIG. 5
has a combustion chamber
2
in which fossil fuel, for example natural gas (CH
4
) via the supply duct
6
, is combusted with the exclusive addition of pure oxygen O
2
) through the supply duct
7
. Since exclusively pure oxygen O
2
is used as the oxidant, and no atmospheric oxygen is combusted as a result no nitrogen compounds enter into the further combustion cycle. The flue gases
21
emerging from the combustion chamber
2
drive a gas turbine
3
, which is connected by a shaft
19
to a generator
5
for current production. The flue gases
21
expanding within the gas turbine
3
emerge as exhaust gases
20
from the gas turbine
3
and, via an external cooling heat exchanger
13
, arrive directly in a compressor
18
in which they are compressed and, after exiting the compressor
18
, are fed to a condenser
4
. The compressor
18
is arranged on the common shaft
19
with the turbine
3
and also the generator
5
in the embodiment example shown in FIG.
5
. Before the exhaust gases compressed by the compressor
18
enter the condenser
4
, a recuperative withdrawal of heat takes place by means of a heat exchanger
14
, so that the conditions fall below the condensation point of CO
2
within the condenser
4
and the compressed and cooled CO
2
passes into the liquid state. Water can be optionally branched off at the condensation point by means of a control valve
10
. Uncondensed gas portions are removed from the circuit process via a control valve
9
from the condenser
4
which has a heat exchanger
12
, and furthermore a partial flow of the liquefied CO
2
is taken off via a control valve
8
. The degree of charging, and thereby the power of the circuit process, can be controlled by the regulated tapping of CO
2
from the circuit. From environmental standpoints, by separating the CO
2
from the process by condensation, that state of aggregation of this gas is produced in which the CO
2
arising can easily be disposed of under environmentally friendly conditions, especially as concerns the problem of greenhouse gases.
The liquefied main portion of CO
2
which has not been branched off is compressed by means of a pump
1
and again supplied to the combustion chamber
2
in a correspondingly preheated and compressed form, via a duct
17
after passing through diverse recuperator stages
14
,
15
and
16
.
In order to be able to operate the above-described quasi-closed CO
2
process with technically reasonable efficiencies, it is appropriate to ensure a complete condensation of the whole of the CO
2
. In order to be able to produce the liquid phase of CO
2
in the condenser
4
, pressure conditions of between 60 and 70 bar must prevail in the connecting duct between the compressor
18
and the condenser
4
. Such a high output pressure at the beginning of condensation of the CO
2
before entry into the condenser
4
leads, however, in the course of the compression by the pump
1
following the condenser
4
, to an upper circuit pressure of 250-300 bar. Such a high pressure level is however not permissible within the combustion chamber, in view of the very high combustion temperatures which prevail there.
A further problem in the operation of the said gas turbine plants is represented by the extremely high heat capacity of highly pressurized CO
2
, which likewise rises with increasing pressure conditions. Thus even the three recuperatively acting heat exchangers
14
,
15
and
16
shown in
FIG. 5
are not sufficient for the CO
2
flow, in order to heat the CO
2
to a corresponding preheat temperature before entry into the combustion chamber.
SUMMARY OF THE INVENTION
The invention therefore has as its object to further develop a process for the operation of a gas turbine plant with CO
2
as working medium, and also to develop a gas turbine plant of the said category related to this, so that the efficiency and the process parameters connected thereto are optimized within the quasi-closed CO
2
circuit. In particular, measures are to be found which aid in preventing an overloading of the combustion chamber as regards its operating conditions.
According to the invention, a process for the operation of a gas turbine plant with CO
2
as the working medium is disclosed. In at least one combustion chamber, hydrocarbons are combusted in a CO
2
atmosphere enriched with oxygen to flue gases. The flue gases largely comprise CO
2
and H
2
O and expand within a turbine stage following the at least one combustion chamber. The flue gases are then compressed in a compressor stage and also at least partially condensed in a following condenser such that at least a portion of the CO
2
and H
2
O is liquefied and partially drawn off together with uncondensed flue gas components. A main portion not drawn off of liquid CO
2
is compressed by means of a pump unit and preheated in at least one recuperator stage. The liquid CO
2
is then supplied to the combustion chamber and developed such that the compressed main portion of CO
2
is pre-expanded to a combustion pressure and, with the main portion CO
2
, is supplied for combustion to the combustion chamber.
By the measure according to the invention, of pre-expansion of the main portion CO
2
compressed by the pump unit and typically at a pressure level between 250 and 300 bar after the pump unit, the high pressure level of the CO
2
can be reduced to pressure values between 70 and 100 bar so that a safe and efficient operation of the combustion chamber is ensured.
The pressure reduction typically takes place using a turbine stage within the quasi-closed CO
2
circuit, following the pump unit and effecting an efficient decompression of the CO
2
gases before they enter the combustion chamber.
The additional turbine stage is preferably arranged in the CO
2
circuit immediately upstream of the combustion chamber and effects the desired pressure reduction there. Between the pump unit and the previously sketched arrangement of the turbine stage, there is likewise preferably provided a multi-stage, for example three-stage, recuperator which preheats the CO
2
gases compressed by the pump unit to a temperature desired for the combustion. Recuperator stages serve for this purpose and provide for a specific heat transfer from the expanded hot gases emerging from the turbine st
Alstom Technology LTD
Burns Doane Swecker & Mathis L.L.P.
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