Method for operating a power plant

Details Method for operating a power plant Method for operating a power plant

2001-10-15

2002-12-31

Nguyen, Hoang (Department: 3748)

Power plants

Motive fluid energized by externally applied heat

Process of power production or system operation

C060S039010, C060S039182

Reexamination Certificate

active

06499300

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method for operating a power plant with a closed or quasi closed CO
2
process, wherein the power plant includes at least one compressor, at least one turbine, at least one generator, at least one recuperator and at least one burner unit with at least one oxygen separation device and at least one burner, wherein the burner unit is supplied with oxygen-containing gas, compressed and heated CO
2
medium from the CO
2
process, and fuel or a fuel mixture. The invention also relates to a power plant having at least the above-described features.
BACKGROUND OF THE INVENTION
A conventional power plant as well as a method for operating such a power plant is disclosed in WO 98/55208. The disclosed power plant includes a turbine in its CO
2
cycle that drives a compressor and a generator via a common shaft. Also provided are a recuperator or heat exchanger as well as a burner unit, whereby this burner unit in the disclosed power plant comprises a first burner, an oxygen separation device, and a second burner. Oxygen-containing gas, for example, compressed ambient air, a compressed, heated CO
2
medium of the CO
2
process, and fuel are fed to the burner unit. The oxygen separation device contains oxygen separation means that remove oxygen from the oxygen-containing gas and feed it to the CO
2
medium. The oxygen separation device, therefore, on the one hand, reduces the oxygen content of the oxygen-containing gas, and on the other hand enriches the CO
2
medium with oxygen. The oxygen-enriched CO
2
medium is burned in the second burner following the oxygen separation device together with the fuel, producing hot CO
2
medium. This hot CO
2
medium is fed to the turbine and expanded there. The expanded CO
2
medium is then fed to the recuperator, where it cools down. Then the expanded and cooled CO
2
medium is fed to the compressor and is then heated in the recuperator. The heated and compressed CO
2
medium is then again fed to the inlet side of the oxygen separation device. This results in a closed or quasi closed CO
2
process, from which the damaging CO
2
can be removed with relatively little expenditure and without risk to the environment. Accordingly, the CO
2
emissions associated with the burning of fossil fuels can be significantly reduced in such a power plant.
The core idea of a power plant with CO
2
process is that pure oxygen is added as an oxidant to the CO
2
medium. The combustion process with molecular oxygen produces a waste gas consisting essentially only of CO
2
and H
2
O, which significantly simplifies after treatment of this CO
2
medium, for example, in order to remove the H
2
O or CO
2
.
Since oxygen, which is produced in refrigerated plants, is very expensive, new technologies have been developed for producing oxygen. In this context, oxygen separation devices that are provided with a membrane that conducts oxygen ions and electrons, so-called MCM membranes (mixed conducting membranes), play an important role. Such an MCM membrane is provided with a retention side, on which the oxygen-containing gas is located, and a pass-through side, on which the gas to be enriched, for example CO
2
, is located. The MCM membrane transports oxygen ions from the retention side to the pass-through side and causes an electron transport from the pass-through side to the retention side. This causes oxygen to be removed from the gas on the retention side and to be fed to the gas on the passthrough side. In order to increase the efficiency of such an MCM membrane, it is advantageous to set a relatively high flow speed on the pass-through side so that the oxygen concentration on the pass-through side is as low as possible. It is advantageous for a long useful life of the MCM membrane to perform the following process steps independently from each other in separate units: Heating of the oxygen-containing gas, heating of the CO
2
medium, transport of the oxygen from the oxygen-containing gas to the CO
2
medium, and combustion of the oxygen-enriched CO
2
medium with fuel. The functional separation of these procedures makes it possible to optimize the individual process steps separately, whereby, in particular, the useful life of the MCM membrane can be increased. It is particularly important for the efficiency of the system that the oxygen separation device or its MCM membrane has an optimum operating temperature that is relatively high. To enable the MCM membrane to reach its high operating temperature, the CO
2
medium is preheated in the first burner preceding the oxygen separation device in a conventional power plant. The oxygen-containing gas is also preheated in a third burner preceding the oxygen separation device.
WO 98/55394 discloses a power plant in which the oxygen separation device is constructed as a so-called membrane reactor in which the enrichment of the CO
2
medium with oxygen and the combustion with the fuel take place more or less simultaneously. Such a membrane reactor essentially corresponds to an oxygen separation device with MCM membrane, which is, however, operated at substantially higher temperatures.
EP 0 953 748 A1 discloses a power plant in which the CO
2
medium is enriched with oxygen that is not produced with an oxygen separation device working with an MCM membrane, but with a cryotechnology plant.
Another power plant with CO
2
process in which the oxygen enrichment takes place with oxygen produced with cryotechnology is known from EP 0 939 199 A1.
U.S. Pat. No. 5,976,223 discloses a device for producing carbon dioxide and oxygen that works with two oxygen separation devices that each are equipped with an MCM membrane. The first oxygen separation device, which functions as a membrane reactor, is supplied with oxygen-containing gas that has been compressed and heated on the retention side. On the retention side, a gaseous fuel is supplied that reacts with the supplied oxygen and forms carbon dioxide. The oxygen-containing gas with reduced oxygen content is heated by the exothermic reaction that takes place during this process. The oxygen-containing gas heated in this manner is then fed to the second oxygen separation device on its retention side. The desired oxygen then accumulates on the pass-through side of this second oxygen separation device.
Other methods and devices provided with oxygen separation devices working with MCM membranes are known, for example, from EP 0 882 486 A1 and U.S. Pat. No. 5,865,878.
SUMMARY OF THE INVENTION
The present invention provides a power plant and a method of operating a power plant in which an optimum operating temperature for an oxygen separation device has been improved.
According to an embodiment of the invention, a portion of the CO
2
medium enriched with oxygen is burned in an additional burner following the oxygen separation device, and the oxygen enriched CO
2
medium is mixed with the CO
2
medium that has not yet been enriched with oxygen before it reaches the oxygen separation device. This measure makes it possible to increase the temperature of the CO
2
medium fed into the oxygen separation device to such an extent that an optimum operating temperature can be set or adjusted for the oxygen separation device. In particular, this makes it possible to compensate for heat losses. By optimizing the operating temperature of the oxygen separation device, its efficiency and therefore also the efficiency of the power plant, is improved.
In another aspect of an embodiment of the invention, the burner unit may be provided with a heat exchanger, through which again on one side the CO
2
medium flows upstream from the oxygen separation device, and on the other side the CO
2
medium flows downstream from the oxygen separation device, whereby hot CO
2
medium produced by the combustion in the additional burner is mixed downstream from the heat exchanger with the CO
2
medium heated in the heat exchanger. The CO
2
medium fed into the oxygen separation device is therefore heated primarily in the heat exchanger, whereby little external energy is required. The addition

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