WO2006128426A1

WO2006128426A1 - Power station having hot co2 gas recycling and method for operating the same

Info

Publication number: WO2006128426A1
Application number: PCT/DE2006/000906
Authority: WO
Inventors: Ludger Blum; Christoph Schneiders; Ernst Riensche
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2005-05-31
Filing date: 2006-05-26
Publication date: 2006-12-07
Also published as: DE102005025345A1

Abstract

The invention relates to an improved power station having hot CO<SUB>2</SUB> gas recycling which recycles CO<SUB>2</SUB> from the combustion process at high temperature. For this, compared with the previously known power station designs, two improvements have been made. Firstly, the flue gas/air heat exchanger was matched to the requirements of the high temperature membrane and connection to the high temperature membrane designed in such a manner that the hot gas and the air now flow in countercurrent through these components. The hot flue gas flows in this case first through the membrane and then is cooled in the heat exchanger, the air in contrast is first heated in the heat exchanger and reaches the membrane with sufficiently high temperature. The gas temperatures in the membrane are virtually constant and the temperature differences between flue gas and air in the membrane are advantageously low. The abovementioned connection succeeds in producing a region within the hot gas circuit having considerably decreased temperature of approximately 500°C. By shifting position of the circulation fan to this region, the problem of previously absent implementation of a hot gas fan is solved.

Description

Power plant with CO2 hot gas recirculation and method for operating the same

The invention relates to a power plant, in particular an effective high-temperature membrane power plant with hot gas recirculation. The invention further relates to a method for operating such a power plant.

State of the art

As power plants with hot gas recycling, inter alia fossil-operated steam power plants or high-temperature fuel cells with anode gas or cathode gas recirculation are known. From the process engineering is also known that an at least partial recirculation of gas streams can lead to improved concepts, or in some cases is even mandatory. The return of gas streams is usually done with a blower or a compressor, which promote the gas back. In the prior art, such circular, however, are run fan only available up to an operating temperature of about 500 ⁰ C. In the case of very hot gases to be returned with temperatures above 500 ^° C., therefore, special solutions must be sought.

In the above-mentioned high-temperature fuel cells with anode gas or cathode gas recirculation, the hot recirculated gas usually encounters cold feed gas. This creates an area with lowered temperature in the gas cycle. In these systems, the problem of a particularly temperature resistant hot gas blower can be mitigated by positioning the blower in this colder main flow of the gas loop. Although now the fan promotes the incoming gas flow forward, the gas flow is nevertheless stimulated in the entire circuit and thus also promoted the recirculating gas. More difficult is the initial situation with another system concept on which this invention is based. Its basic concept has a necessarily hot recirculating gas flow. In this case, but also the feed gas is hot, so that there is no temperature reduction after mixing and thus no way to use in practice only up to 500 ⁰ C loadable blower.

Current status of power plant concepts with CO2 separation In the longer term, efforts are being made throughout the world to separate CO2 from power plants by developing suitable processes and thus to significantly reduce CO 2 emissions. The separation of CO2 from power plant processes can basically be achieved via 3 technology routes [I]:

l .a "post-combustion capture": separation of CO2 from the flue gas after combustion by means of suitable washes or, in the long term, by membrane systems

The disadvantage of this method is that high volume flows of flue gas with relatively low CO2 concentration must be cleaned. Membranes for the separation of CO2 will therefore have a high demand for membrane area.

l .b "Pre-combustion Capture": Separation of CO2 in an intermediate step after coal gasification (or natural gas reforming), but before the

Combustion with air

The various coal gasification processes developed so far are preferably operated with oxygen or enriched air (and steam) under pressure (about 20-30 bar). Therefore, the coal gas has two significant advantages in terms of CO 2 separation. On the one hand, the real volume flow (with little nitrogen and at high pressure) is about 100 times less than the flue gases of conventional steam power plants. This leads directly to high partial pressures of the main components CO and H2. After an additional CO conversion to CO2 and H ₂ by means of a steam feed (shift reactor) for the conditioning of the carbon dioxide for CO2 separation, two options are available:

• Separation of CO2 z. B. with a wash or

• Separation of a sufficient amount of H2 by means of a membrane leaving a CO2-rich gas in the retentate, suitable for liquefaction and landfill. In both options, the hydrogen is subsequently in a gas and

Steam turbine process (gas and steam process) (with Fb turbine, which is developed eg at SIEMENS) flows.

l .c "Oxyfuel process": Very simple CO ₂ separation after combustion with pure oxygen

This procedure has a decisive advantage. Combustion in pure oxygen produces only CO2 and water vapor as a combustion product, which can be separated from the CO2 in a very simple way by cooling the gas mixture by condensation.

In all oxyfuel processes, a portion of the flue gas is returned at temperatures below 1000 ⁰ C in order to limit the temperatures in the combustion chamber to 1500 ⁰ C. Combustion with pure oxygen without an air / nitrogen mixture or without recycled CO ₂ gas would regularly lead to combustion temperatures of well over 2000 ° C. This flue gas contains predominantly CO ₂ . The temperature level of the recirculated flue gas can be very different depending on the concept.

In principle, there are also different processes with regard to the production of oxygen: 2.a concept with air separation plant (LZA)

The oxygen can be obtained by means of low-temperature decomposition in a LZA, but with high energy expenditure. The flue gas returned by the blower has a low temperature [2]. This varies between 160 and 340 ⁰ C in the various published variants.

2.b concept with high temperature θ2 membrane

Highest O2 / N2 separation selectivity can be achieved by so-called "dense" membrane systems based on mixed conductors with simultaneous electron and oxygen ion conductivity Modified perovskites, such as those currently used as cathode materials for high-temperature fuel cells, are particularly suitable for this purpose is a high one

Operating temperature of at least approx. 800 ^° C required [3, Page 62].

2. c Concept with low temperature θ2 membrane

For the time being, alternative porous low-temperature θ2 membrane systems are still out of the question, since there is still a very high need for development here.

The starting point of this invention is the high-temperature membrane

Power plant concept of the collaborative project "OXYCOAL-AC" of the RWTH Aachen, where the CO2 from the combustion process is recirculated at high temperature to maintain a sufficiently high temperature in the membrane. [4] In co-current with the flue gas, the preheating of the air takes place first in a heat exchanger and then also inside the membrane. However, this concept currently has two disadvantages:

• The selected shading will lead to inhomogeneous temperature gradients within the membrane.

• In this arrangement, a circulation blower is to be operated at a very high temperature (850 ^° C.). The realization of this component is a serious problem and requires considerable research [4].

At moderate operating temperatures up to 500 ⁰ C fans can for the gas extraction o tion for high volume flow with only a slight increase in pressure up to 800 mbar are used. For pressure increases over 800 mbar compressors are used. Up to a pressure ratio of 7, gas streams are compressed in one stage without intermediate cooling.

5 In contrast, special fans, such as belt-driven centrifugal fans, would have to be used for hot gas production, which allow only a low pressure build-up due to the high operating temperatures and the resulting technical conditions. As a rule, hot gas blowers are simulated in contrast to normal blowers with a significantly poorer efficiency (60%).

Task and solution

The object of this invention is to provide an effective and improved compared to the prior art power plant with CO ₂ -Heißgasrückführung available that CO2 from the combustion process with high

Temperature returns, and which constructively has no problem with a hot gas blower.

Furthermore, it is the object of the invention to provide a method for operating the aforementioned improved power plant. 0

The objects of the invention are achieved by a power plant with CO2 hot gas recirculation and further features according to the main claim. as by a method for operating this power plant according to the independent claim. Advantageous embodiments of the power plant and the method for operating the same can be found in the respective dependent claims.

Subject of the invention

The subject of the invention is a power plant with CO2 hot gas recirculation with all the features of the main claim. The term power plants with both coal or gas-powered steam power plants, as well as, for example, high-temperature fuel cell power plants are included.

The power plant comprises a supply for fuel, a supply of oxidant and a reaction chamber. In a steam power plant, the reaction chamber corresponds to the burner chamber or the steam generator, while in a high-temperature fuel cell corresponds to the actual membrane-electrode unit in which the electrochemical reaction takes place. From the reaction chamber, CO2-containing hot flue gas, or anode and cathode exhaust gas is discharged.

For the flue gas a circulation (hot gas recirculation) is provided which comprises at least one heat exchanger, a high-temperature membrane for separating O 2 from air and a fan. The fan is used for compression and gas conveying in the circulation. The blower arranged for the realization of the hot gas recirculation in a favorable position, or the compressor is characterized in that as a fan only up to 500 ⁰ C designed blower is used.

Compared to the power plant concept known from the literature

OXYCOAL-AC (400 MW), the invention differs by constructive measures for influencing the temperature in the gas cycle, in particular especially for the creation of a local temperature reduction for a favorable blower positioning. The object of the invention was achieved in particular by combining two measures.

A) Adaptation of the flue gas / air heat exchanger to the requirements of the high-temperature membrane

The shading of the high-temperature membrane with the associated heat exchanger was designed so that the membrane can be operated under optimal conditions (high temperature, low temperature gradients). The hot gas and the air flow through these components now in

Countercurrent. The hot flue gas flows through the first membrane and is then cooled in the heat exchanger, the air, however, is first heated in the heat exchanger and reaches the membrane with sufficiently high temperature, so that almost constant high gas temperatures are present in the membrane. Also the temperature differences between

Flue gas and air in the membrane are low, so that constructive problems due to thermal stresses are not expected.

B) Position shift of the circulation blower in the newly created colder area of the gas cycle

By this interconnection, it has also been possible to create an area within the hot gas cycle with a significantly lowered temperature of about 500 ⁰ C. By position shift of the circulation blower in this area (after membrane and heat exchanger and before the

Combustion chamber) is now the problem solved for hitherto lack of realization of a hot gas blower. There is only a slight disadvantage of the volume flow increased somewhat at this point in the cycle as a result of the added oxygen. While a hot gas blower for temperatures around 850 ⁰ C has a considerable research need, blowers at 500 ⁰ C can be represented in principle with existing technology.

Special description part

The subject matter of the invention will be explained in more detail below with reference to figures, without the subject matter of the invention being restricted thereby. 1: Oxycoal concept with O ₂ from LZA (920 MW): CO ₂ recycling at low temperature and combustion with O ₂ / CO ₂ FIG. 2: Power plant concept OXYCOAL-AC (400 MW): CO ₂ hot gas recirculation with hot gas blower, flue gas / air heat exchanger in DC Figure 3: Membrane power plant "Oxycoal-FZJ" (simulation with PRO / 11):

CO ₂ hot gas recirculation, countercurrent heat exchanger and circulation blower in the main stream at medium temperature

All three concepts are based on the oxyfuel process, in which combustion with pure oxygen as combustion products only delivers CO2 and water vapor, which can be easily separated from CO2 by condensing the gas mixture. A great potential for development is attributed to high-temperature O2 membranes, in particular due to energetic aspects. A prerequisite for this is that inexpensive membranes are available.

FIG. 1 illustrates a conceivable future power plant concept in which the oxygen used is first provided in an air separation plant (LZA). The flue gas leaving the combustion chamber or the steam generator has low design variants Temperatures between 160 and 340 ⁰ C on. There are no outstanding requirements for fans to be used here.

FIGS. 2 and 3 each show concepts in which the oxygen required directly from the compressed air via O 2

Membranes is separated, while the predominantly CO2 existing recirculated flue gas is used as purge gas. As a result, a low CO ₂ partial pressure on the permeate side is achieved, which is important for a promising driving force of the O ₂ permeation. The high-temperature membranes are arranged directly in the hot gas cycle.

According to the concept in FIG. 2, the hot gas is first slightly cooled in a DC heat exchanger, passes through the high-temperature membrane and is enriched in the combustion chamber with oxygen enriched. The temperature in the gas cycle varies only slightly from about 850 ⁰ C when leaving the combustion chamber to about 700 - 800 ⁰ C when re-entering the same. Regardless of whether the blower needed for the return would be arranged in the direction of flow directly behind the combustion chamber or between the heat exchanger and Hochtemperaturmemb- ran or between the high-temperature membrane and the combustion chamber, it would have to withstand temperatures above 700 ⁰ C in any case.

Furthermore, the efficiency of the high-temperature membrane is reduced by the fact that the two gas streams, the supplied air and the hot gas have significantly different temperatures. Although the supplied air is preheated by a heat exchanger to about 600 - 700 ⁰ C, but at the same time the temperature of the flue gas to about 700 - 800 ⁰ C lowered. High-temperature membranes, however, only work from about 800 ⁰ C effectively. In addition, the large temperature gradients over the

Membrane adversely lead to mechanical stress and possibly cracks in the membrane. In contrast, two crucial changes have been made in the inventive power plant concept in FIG.

On the one hand, the interconnection of the high-temperature membrane and the associated heat exchanger has been changed. The hot gas and the air flow through these components no longer in the DC, but in countercurrent. The hot flue gas flows through the membrane first and is then cooled in the heat exchanger. The air is heated sufficiently in the heat exchanger before it flows through the membrane. This advantageously increases the temperature level at the membrane. In addition, the average temperature difference across the membrane is considerably reduced by this flow guidance, which leads to significantly lower design problems due to thermal stresses.

As a second process change, the blower, or the compressor, which is responsible for the circulation of the flue gas, removed from the hot side of the flue gas stream and placed at the point after the membrane and the heat exchanger, where the gas stream is significantly cooled. The compressor must compress a large volume flow with a small pressure ratio. The disadvantage of the increased by the added oxygen material flow is overcompensated by the sunken from about 850 ° C to about 500 ⁰ C temperature. While a hot gas blower for temperatures around 850 ⁰ C would still have a considerable chen research needs, are blowers, which are designed for an operating temperature of about 500 ⁰ C, in principle, already present. Literature cited in this application

[1] J. Ewers, W. Renzenbrink, F. Hannemann, G. Haupt, G. Zimmermann, Development of Combined Power Plant Concepts for CO 2 -free Power Generation, XXXVI. Power Plant Technical Colloquium, TU Dresden, 19.-20. Oct. 2004, entry no. V27.

[2] S. Hellfritsch et al, Progress in the further development of the oxyfuel process using the example of a lignite power plant, XXXVI. Power Plant Technical Colloquium, TU Dresden, 19.-20. Oct. 2004, entry no. V41. [3] Th. Melin, R. Rautenbach, Membrane Processes - Fundamentals of Modular and System Design, Springer-Verlag Berlin, 2nd ed., 2004. [4] U. Renz, Development of a Co2-emission-free coal combustion process for power generation in a combined project the RWTH Aachen, XXXVI. Power Plant Technical Colloquium, TU Dresden, 19-20 Oct. 2004, entry no. V42.

Claims

Patent claims

A power plant comprising a fuel supply (1), an oxidant supply (2), a reaction chamber (3), a gas circulation (4), at least one heat exchanger (5), and a high temperature membrane (6) for separating O 2 and N 2 and a blower arranged in the gas circuit (7), characterized in that a blower designed up to 500 ⁰ C fan is arranged.

2. Power plant according to the preceding claim 1, wherein the high-temperature membrane (6) for the separation of O 2 from air is designed as Gegenstromva- variant.

3. Power plant according to one of the preceding claims 1 to 2, wherein the heat exchanger (5) in the gas circulation between the high-temperature membrane (6) and the reaction chamber (3) is arranged.

4. Power plant according to one of the preceding claims 1 to 3 with a further heat exchanger (5) which is arranged in the flow direction of the supplied oxidant in front of the high-temperature membrane (6).

5. Power plant according to one of the preceding claims 1 to 4, wherein the fan between the heat exchanger (5) and the reaction chamber (3) is arranged.

6. A method of operating a power plant according to any one of claims 1 to 5, wherein the hot gas from the reaction chamber (3) is first cooled in a heat exchanger (5) and then passed through the blower before it again the combustion chamber (3). is supplied.

7. The method of claim 6, wherein the hot gas from the combustion chamber (3) before passing through the heat exchanger through the High-temperature membrane (6) is passed.

8. The method according to any one of claims 6 to 7, wherein the high-temperature membrane (6) is operated in countercurrent.