WO2010072304A1 - Method and device for catalytic denitrification of gas flows from large combustion plants - Google Patents

Abstract

The invention relates to a method and to a device for catalytic denitrification of gas flows from large combustion plants, particularly of power plant exhaust gases. In order to achieve effective denitrification in an economically feasible manner, even for so-called "oxyfuel" plants using oxygen as a combustion gas, the invention proposes that the catalytic treatment is performed not in the zero-pressure gas flow, but in a gas flow compressed to a pressure of at least 2 bar.

Description

 description

Process and apparatus for the catalytic denitrification of gas streams

large combustion plants

The invention relates to a method for the catalytic removal of at least part of nitrogen oxides from a gas stream, in particular exhaust gas stream, a

Large combustion plant, in particular a power plant, and a device for carrying out the method.

In order to secure the energy supply of an economy, power plants, ie industrial installations for providing in particular electrical and partly additional thermal power, are indispensable. In such power plants primary energy is used, which is made available after appropriate conversion as useful energy. As a rule, gas streams are generated which can not be released to the environment without further purification steps. Especially in caloric power plants where fossil fuels, e.g. Coal, petroleum or natural gas, burned, usually incurred as flue gases referred to waste gas streams containing environmentally harmful components. The problem is especially the formation of nitrogen oxides in the combustion of fossil fuels. These nitrogen oxides can be produced in different ways, for example by oxidation of the nitrogen contained in the combustion air and / or in the fuels. Attempts have been made to reduce the formation of nitrogen oxides by primary measures. Thus, e.g. special combustion processes that work with air staging or fuel staging used. A reduction of the combustion temperature by an exhaust gas recirculation results in a reduced nitrogen oxide formation. For lignite-fired power plants, the previously applicable

Exhaust gas limits are usually met by primary measures. Coal-fired power plants, on the other hand, already require secondary measures today in order to reduce nitrogen oxide emissions. In practice, only the so-called selective non-catalytic reduction (SNCR) and the so-called selective catalytic reduction (SCR) have prevailed in practice. If particularly low nitrogen oxide levels in the exhaust gas are to be achieved, selective catalytic reduction (SCR) is the method of choice. In the selective catalytic reduction, a vanadium-titanium oxide catalyst with a typical working temperature of 300 to 450 ⁼ C is generally used. Rarer is a suitable for higher temperatures (350 to 600 ⁰ C) suitable zeolite catalyst. The reducing agents are ammonia or ammonia-forming derivatives such as urea. They are also special

Catalyst arrangements are known (e.g., Shell Lateral Flow Reactor) which can operate at lower temperatures (up to about 180 ° C). However, their application is limited in temperature down by the formation of ammonium salts. Higher sulfur levels in the exhaust gas cause higher operating temperatures to avoid salt formation.

The catalyst for denitrification of the exhaust gas stream, i. to reduce the nitrogen oxide content of the exhaust stream, can usually be used at three different positions in the exhaust stream of a power plant. These variants are defined in professional circles as follows:

1. "High Dust" arrangement: Here, the catalytic treatment is carried out as the first cleaning step for the flue gas before a dust separation or sulfur removal.

2. "Low Dust" Arrangement: The catalytic treatment takes place here after the dust separation, but before the desulfurization. In this case, a vanadium-titanium oxide catalyst or zeolite catalyst is used at a temperature between 300 and 400 ⁰ C usually.

3. "tail end" arrangement: The catalytic denitrification is carried out after the dust separation and the desulfurization. For this purpose, a vanadium-titanium oxide catalyst is usually used at a temperature between 300 and 35O ⁰ C.

The current methods for the catalytic denitrification of power plant exhaust gases are, for example, in "Catalytic Air Pollution Control", RM Heck, RJ. Farrauto, van Nostrand Reinhold (1995). An overview of these processes was also presented in the 40th Power Engineering Collaborative Event 2008; "Conflict of interest emissions and Energy efficiency ", Prof. M. Beckmann, Prof. U. Gampe, S. Grahl, S. Hellfrisch, TU Dresden, Institute of Energy Technology.

Future environmental laws will prescribe a further reduction in nitrogen oxide levels in power station flue gases. For example, e.g. in

Germany has set a limit of 100 mg / m ³ for power plants which will be operational from 2013 onwards. Such low values can only be achieved in practice with secondary measures.

However, all previously known catalyst arrangements have considerable disadvantages, in particular when used in coal power plants:

In the High Dusf arrangement, the catalyst life is greatly reduced due to the high catalyst poison and dust load of the flue gases, as well as dust buildup of the catalyst, salt formation problems, and sulfur dioxide formation Corrosion problems occur Due to the high particulate load, which leads to binding of NH3, increased ammonia consumption and slippage occur.

Dust levels are lower in the Low Dusf arrangement, but salt formation can lead to relocations, and the catalyst poisons that reduce catalyst life are still present, and the problem of corrosion also arises.

Although the catalyst loading is lower in the "tail end" arrangement, the entire exhaust gas flow must be warmed up again to the operating temperature of the catalytic converter, which is energetically unfavorable.

Overall, so far the "low dust" arrangement is preferred.

In general, the arrangement of the catalyst in the almost pressure-free region of the flue gas leads in all cases due to the pressure loss to increased energy consumption, which adversely affects the overall energy balance of the power plant. Recently, new power plant concepts have been proposed in which the fossil fuel, eg coal, is burned with an oxygen-rich combustion gas, in particular with technically pure oxygen or with oxygen-enriched air (oxygen combustion gas process). The oxygen content of this combustion gas is for example 95 to 99.9% by volume. The resulting flue gas contains mainly carbon dioxide with a share of about 70 to 85 vol%. The aim of these new concepts is to compress the carbon dioxide produced during combustion of the fossil fuels and concentrated in the flue gas in suitable deposits, in particular in certain rock layers or salt water bearing layers, and thus to limit carbon dioxide emissions to the atmosphere. This is intended to reduce the climate-damaging effect of greenhouse gases such as carbon dioxide. Such power plants are referred to in the art as so-called "oxyfuel" power plants.

Since the flue gases of these power plants in addition to carbon dioxide contain other environmentally harmful components, a flue gas cleaning is required as in conventional power plants to meet legal emission requirements for exhaust emissions to the atmosphere or requirements for carbon dioxide storage. In particular, when transporting the carbon dioxide-rich exhaust gas in pipelines for further use or

Storage and also when introducing into rock formations certain maximum limits for the nitrogen oxide content to be observed. In the previously known concepts, therefore, dedusting, denitrification and desulfurization of the flue gas take place in successive steps. Subsequent to this flue gas cleaning, the thus treated, carbon dioxide-rich exhaust gas is compressed to be supplied for further use or storage. The catalytic nitrogen oxide removal is thus also here in the unpressurized area before the compression of the exhaust gas.

Carbon dioxide-containing gas streams also accumulate in other large-scale combustion plants that are operated with fossil fuels. These include, for example, industrial furnaces, steam boilers and similar large-scale thermal plants for power and / or heat generation. It is conceivable that even such systems are operated with oxygen or oxygen-enriched air, so that carbon dioxide-rich exhaust gas streams are formed, from which the carbon dioxide is separated and a recovery or storage (eg by pressing in the ground) is supplied. In this case, it is also necessary to observe maximum limits for the nitric oxide content. Object of the present invention is to provide a method of the type mentioned above and an apparatus for performing the method so that in an economical manner an effective catalytic nitrogen oxide removal from the gas stream is made possible.

According to the invention, this object is achieved in that the catalytic removal of the nitrogen oxides is carried out in a gas stream compressed to a pressure of at least 2 bar.

With this procedure, there are significant technical and economic improvements over the previously used methods for catalytic denitrification of exhaust gas streams from large combustion plants. Due to the volume flow concentration caused by the pressure increase, significantly more compact catalyst units can be used. In addition, the heat produced during the compression of the exhaust gas flow can be used to heat the exhaust gas flow to the optimum operating temperature of the catalytic converter. This significantly improves the energy efficiency of the process.

Expediently, the catalytic denitrification takes place at the end of a purification chain for the gas stream. Accordingly, the compression and catalytic treatment is preceded in particular by dedusting and desulfurization of the flue gas. As a result, the gas stream supplied to the catalyst is substantially free of dust and sulfur, which allows the use of catalysts which operate at low temperatures. This contributes to a further improvement of the heat integration.

Particular advantages arise in an application of the invention to large combustion plants, in particular power plants, according to the

Oxy-fuel gas processes work (so-called "oxyfuel" plants). In such systems, a compression of the carbon dioxide-rich exhaust stream is already provided in order to be able to use this or storage. This exhaust gas compression can be used in a technically elegant manner for the purpose of the invention. Accordingly, the catalytic treatment of the gas stream takes place in Range of this exhaust gas compression at elevated pressure. In this particularly preferred variant of the invention, therefore, the compressed gas stream is formed by a carbon dioxide-rich exhaust gas stream of a large combustion plant, are burned in the fossil fuels with a combustion gas having a higher oxygen content than air.

In this case, it is convenient to recycle a portion of the carbon dioxide-rich gas stream prior to compression of the gas stream for combustion. This causes on the one hand due to the reduction of the combustion temperature, a reduction of nitrogen oxide formation during combustion. On the other hand, the gas volume flow supplied to the catalyst is reduced, so that the catalyst unit can be made even more compact. Due to the gas recirculation, the nitrogen oxides are concentrated in the gas stream to the catalyst, which due to the i.A. positive reaction order regarding the nitrogen oxide concentrations further contributes to the reduction of the catalyst unit.

After the catalytic treatment, the compressed, carbon dioxide-rich and low-nitrogen gas stream may be sent for use or storage. For example, it can be pressed into rock layers of the subsurface without the risk of a negative influence on rock formations due to the action of nitrogen oxides. The resulting after carbon dioxide purification gas stream with reduced carbon dioxide content complies with the legal requirements regarding nitrogen oxide emissions after the catalytic Enstickung.

Preferably, a catalyst containing at least one mixed oxide is used for the catalytic removal of nitrogen oxides. Vanadium-titanium oxide is particularly suitable for this purpose. In this case, ammonia and / or ammonia-producing derivatives such as urea are preferably used as the reducing agent for the catalytic treatment. Alternatively, it is also possible to use a catalyst containing at least one noble metal, in particular platinum, which can already be used at temperatures of 100 to 200 ^° C. Particularly recommended is the use of a selective noble metal catalyst containing, for example, platinum on a mixed oxide support. In this case, hydrogen can be advantageously used as a reducing agent. With appropriately selected catalysts, the reaction can be accelerated by a partial conversion of nitrogen monoxide to nitrogen dioxide in advance become. In principle, the catalysts and reducing agents mentioned can also be combined.

The gas stream is preferably compressed to a pressure between 2 and 100 bar, particularly preferably between 5 and 50 bar, for the catalytic removal of nitrogen oxides.

In addition, the catalytic treatment is advantageously carried out at a temperature in the range of 100 to 350 ⁰ C, more preferably in the range of 120 to 250 ⁰ C. The heat required by using the heat of compression due to the compression of the gas stream, by heat exchange in countercurrent, by a Electric heater, an integration into the power plant (eg with steam) or by combinations of the mentioned methods are provided.

At the inlet of the gas stream for catalytic denitrification, the concentrations of nitrogen oxides are preferably 200 to 2500 mg / Nm ³ , more preferably 400 to 1000 mg / Nm ³ . The gas stream to be treated contains carbon dioxide as the main constituent (typically more than 70% by volume) and also oxygen (typically 2 to 10% by volume) as well as nitrogen and traces of other gases.

According to a development of the inventive concept, the catalytic

Nitric oxide removal is not carried out in the intended for further use or storage, compressed, carbon dioxide-rich gas stream, but in a branched off from this after compression gas stream with reduced carbon dioxide content. This intended for delivery to the atmosphere gas stream is referred to in the art as "vent" gas. In this variant, the compression is followed by a particular cryogenic purification stage, in which the gas components, which can not be pressed, for example, are separated. In this way, a gas stream with increased carbon dioxide content, which is supplied for further use or storage, and a separated gas stream with reduced carbon dioxide content, which is released to the atmosphere. The latter gas stream has an increased nitrogen oxide content, so it must be cleaned before being released to the atmosphere. Since the gas stream is already compressed, in this process variant, the catalytic nitrogen oxide removal is advantageously carried out in this diverted gas stream. The invention further relates to a device for the catalytic removal of at least a portion of nitrogen oxides from a gas stream, in particular exhaust gas stream, a large combustion plant, in particular a power plant, with a catalyst device downstream of the large combustion plant in the gas flow direction.

On the device side, the stated object is achieved in that the catalyst device is preceded by a gas flow compression device.

Preferably, the gas stream compression device has at least two

Compressor stages, wherein the catalyst device is arranged in the gas flow direction after the first and before the last compressor stage. Particularly preferably, the catalyst device is arranged between the first and the second compressor stage, where typically a pressure of 2 to 5 bar prevails. It can also be e.g. be arranged at the end of a pre-compression in a range with a pressure of 10 to 30 bar.

The catalyst device advantageously has a honeycomb-shaped carrier structure with a catalytic coating which contains at least one mixed oxide, in particular vanadium-titanium oxide, and / or a noble metal, in particular platinum.

According to another variant of the invention, the catalyst device has a bed of a catalyst material containing at least one mixed oxide, in particular vanadium-titanium oxide, and / or a noble metal, in particular platinum.

The invention offers a whole series of advantages:

Due to the compression of the gas stream and optionally the arrangement of the catalytic nitrogen oxide removal after the return of a portion of the gas stream to the large combustion plant is leading to the catalytic nitrogen oxide removal

Volume flow reduced, whereby a smaller size of the apparatus is made possible.

Since the catalyst is exposed to a lower dust and poison load, its life is longer. The heat produced during the compression of the gas stream can usefully be used to achieve the temperatures required for nitrogen oxide reduction.

Due to the low residual sulfur dioxide content in the gas stream, the use of low-temperature catalysts is possible, which leads to a further energy saving.

Due to the high nitrogen oxide inlet partial pressure, the catalyst operates at an increased reaction rate.

Due to the fact that in the presence of oxygen at elevated pressures incipient - at least partial - gas phase reaction of NO to NO2 is suitable

Catalyst achieved an additional acceleration of the catalytic NOx reduction.

Because of the gas stream compression before the catalytic treatment, the pressure loss at the catalyst is less critical.

Because of the flue gas desulphurisation remaining in the low-pressure range, the gas stream conducted for compression and subsequent catalytic denitrification is largely free of sulfur, which avoids corrosion problems. Finally, there is still a significant advantage of the invention in that only gaseous products, in particular nitrogen and water vapor arise, so that no disposal problems occur.

The invention is suitable for all conceivable large combustion systems in which carbon dioxide-containing gas flows incurred. These include, for example, power plants powered by fossil fuels, industrial furnaces, steam boilers and similar large-scale thermal plants for power and / or heat generation. With particular advantage, the invention can be used in large combustion plants, which are supplied with technically pure oxygen or oxygen-enriched air as fuel gas and which consequently incurred exhaust gas streams with high carbon dioxide concentrations. In particular, the invention is suitable for so-called low-carbon coal-fired power plants, which are operated with oxygen as fuel gas (oxyfuel "power plants) and at where the carbon dioxide contained in the exhaust gas in a high concentration is separated and pressed in the underground ("C02-Caρture Technoiogy").

In the following, the invention will be explained in more detail with reference to embodiments schematically illustrated in the figures:

Show it

1 shows a comparison of block diagrams of a flue gas purification according to the prior art (A) with two variants of the invention (B and C)

2 shows a detailed view of a flue gas cleaning with catalytic denitrification in the compressed exhaust gas flow

1 shows a conventional flue gas cleaning according to the state of the art ("low-dust arrangement"), in which the exhaust gas stream (flue gas stream) of a combustion boiler K of a coal power plant is first fed to a dedusting unit by means of a filter unit F. catalytic reduction for removal of nitrogen oxide in a catalytic converter unit SCR After that, the flue gas of a flue gas desulfurization REA, for example in a

Laundry, subjected. Part of the purified exhaust gas is returned via return line Y to the combustion boiler V of the power plant.

In the present example, it is supposed to be a power plant operating on the basis of the oxygen-fired combustion process ("oxyfuel" power plant), where the coal is combusted with technically pure oxygen (oxygen content in the fuel gas> 95% by volume) A large proportion of them are pressed into rock layers of the subsurface in order to reduce carbon dioxide emissions to the atmosphere and the associated climate-damaging effects (greenhouse effect)

Purification stages treated exhaust gas compressed by a compressor V and first a cryogenic cleaning R supplied in which the non-compressible components are separated and withdrawn as gas stream G ("vent" gas). The remaining carbon dioxide-rich exhaust gas stream is finally passed, for example via pipelines and intermediate storage tank for pressing P. In this conventional flue gas cleaning, the selective catalytic reduction SCR thus takes place in the almost pressureless region before the exhaust gas compression.

In version B, a flue gas purification according to the invention is shown for comparison, in which the selective catalytic reduction SCR takes place in the exhaust gas flow after the compression V for comparison. In this case, as in the embodiment A, a power plant is assumed which operates according to the oxygen combustion gas process ("oxyfuel" power plant). The carbon dioxide-rich exhaust gas obtained in the combustion boiler K is first dedusted in a filter device F analogously to embodiment A. In contrast to the conventional arrangement, however, the flue gas desulfurization REA is now preferred according to the invention and carried out in the filter unit F already after the dedusting. A portion of the thus treated exhaust gas is returned to the combustion boiler K for lowering the combustion temperature and thus to reduce the formation of nitrogen oxide via the return line Y. The remaining exhaust gas flow is fed to a compressor V. The compressed exhaust gas stream is fed to the selective catalytic reduction of a catalyst unit SCR. The exhaust gas largely freed from nitrogen oxides is then in a cryogenic purification stage R in a gas stream G, which contains the non-compressible components and a carbon dioxide-rich and largely nitrogen oxide-free exhaust stream, which is provided for the compression P, divided. In contrast to the conventional flue gas cleaning according to embodiment A, the selective catalytic reduction in the catalytic converter unit SCR thus does not take place in the almost unpressurised region, but at an increased pressure after the compression of the exhaust gas flow in the compressor V.

In embodiment C, a variant of the invention is shown in which the selective catalytic reduction takes place in the branched gas stream G ("vent" gas). This variant differs from the embodiment B in that the compressed gas stream compressed in the compressor V is not supplied directly to the catalyst unit SCR, but is first treated in the cryogenic purification R. The non-compressible components are separated from the exhaust stream. The separated gas stream G contains a reduced proportion of carbon dioxide and an increased proportion of nitrogen oxides. For the substantial removal of the nitrogen oxides from the gas provided for release to the atmosphere gas stream G is in the catalyst unit SCR performed a selective catalytic reduction. The thus purified gas stream G can then be discharged to the environment. The remaining exhaust gas flow with increased carbon dioxide content is the compression P supplied.

FIG. 2 shows a detailed view of the compression and catalytic treatment of the exhaust gas flow. In this case, the arrangement corresponds to the embodiment B shown in the block diagram of FIG. 1. The carbon dioxide-rich flue gas RG prepurified in the dedusting and desulphurisation system (not shown) is compressed in the present embodiment in a compressor V in two stages to the desired pressure of 15 to 25 bar. In a downstream countercurrent heat exchanger W, the exhaust gas stream already preheated by the compression is further heated against the exhaust gas stream treated in the catalytic converter unit SCR, in order subsequently to be brought to the desired reaction temperature of 140 to 160 ^° C. in the following electric heater E. After the compression of the exhaust gas stream, hydrogen is supplied as the reducing agent RD. The selective catalytic reduction finally takes place in the catalyst unit SCR. For this purpose, a selective noble metal catalyst is used in the present embodiment, which has platinum on a mixed oxide carrier.

Example: Selective noble metal catalyst with hydrogen as reducing agent

Pt mixed oxide catalyst approx. 400 ppm NOx, 2.5% O2, 10% N2 in CO2 T approx. 150 ⁰ C p approx. 25 barg

GHSV approx. 100 000 h-1 U-NOx approx. 90%

For comparison, classic DeNOx applications with vanadium / titanium oxide catalysts are included in the near-zero pressure range

Comparable reactions at a space velocity of 3000 - 10 000 h-1 and temperatures of 300 - 450 ⁰ C.

Claims

claims 1. A method for the catalytic removal of at least a portion of nitrogen oxides from a gas stream, in particular exhaust stream, a large combustion plant, in particular a power plant, characterized in that the catalytic removal of the nitrogen oxides is carried out in a compressed to a pressure of at least 2 bar gas stream. 2. The method according to claim 1, characterized in that the compressed gas stream is formed by a carbon dioxide-rich exhaust stream of a large combustion plant in which fossil fuels are burned with a combustion gas having a higher oxygen content than air. 3. The method according to claim 2, characterized in that a portion of the carbon dioxide-rich gas stream is recirculated to the combustion prior to the compression of the gas stream. 4. The method according to claim 2 or 3, characterized in that the compressed carbon dioxide-rich gas stream after the catalytic nitrogen oxide removal of a use or storage is supplied. 5. The method according to any one of claims 1 to 4, characterized in that the catalytic nitrogen oxide removal at least one mixed oxide, in particular vanadium-titanium oxide containing catalyst is used. 6. The method according to any one of claims 1 to 4, characterized in that for the catalytic removal of nitric oxide, a catalyst containing at least one noble metal, in particular platinum, is used. 7. The method according to any one of claims 1 to 6, characterized in that the gas stream to a pressure between 2 and 100 bar, in particular between 5 and 50 bar, is compressed. 8. The method according to any one of claims 3 to 7, characterized in that the catalytic nitrogen oxide removal at a temperature of 100 to 350, ° = _C] in particular 120 to 250 ⁰ C, is performed. 9. The method according to any one of claims 1 to 8, characterized in that the catalytic nitrogen oxide removal, a reducing agent, in particular ammonia and / or urea and / or hydrogen and / or carbon monoxide and / or a hydrocarbon is used. 10. The method according to any one of claims 2 to 9, characterized in that the compressed carbon dioxide-rich gas stream is fed to a purification stage, from which a gas stream with increased carbon dioxide content is withdrawn, which is supplied to the use or storage, and a gas stream with reduced carbon dioxide content is withdrawn , which is discharged to the atmosphere, wherein the catalytic nitrogen oxide removal in the gas stream with reduced Carbon dioxide content is carried out. 11. An apparatus for the catalytic removal of at least a portion of nitrogen oxides from a gas stream, in particular exhaust gas stream, a large combustion plant, in particular a power plant, with a large combustion plant in Gas flow direction downstream catalyst device, characterized in that the catalyst device is connected upstream of a gas flow compression device. 12. The device according to claim 11, characterized in that the Gas stream compression device has at least two compressor stages, wherein the catalyst device is arranged in the exhaust gas flow direction after the first and before the last compressor stage. 13. The apparatus according to claim 11 or 12, characterized in that the Catalyst device has a honeycomb-shaped support structure with a catalytic coating containing at least one mixed oxide, in particular vanadium-titanium oxide, and / or a noble metal, in particular platinum. 14. The apparatus of claim 11 or 12, characterized in that the catalyst device comprises a bed of a catalyst material containing at least one mixed oxide, in particular vanadium-titanium oxide, and / or a noble metal, in particular platinum.