DE2504027A1 - METHOD FOR REMOVING NITROGEN OXIDES FROM EXHAUST GAS - Google Patents

Description

Process for removing nitrogen oxides from exhaust gases

The invention relates to a method for the selective reduction of nitrogen oxides in exhaust gases, in addition to nitrogen oxides Contain sulfur oxides, in the presence of a solid catalyst. , ·

The invention relates in particular to a method for the catalytic reduction of nitrogen oxides of the general chemical formula NO, in which χ is 0.5, 1, 1.5, 2 or 2 _f 5, in exhaust gases,

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which in addition to nitrogen oxides also sulfur oxides of the general chemical formula SO, in which χ is 2 or 3.

Such exhaust gases occur in particular as flue gases or in In the form of pre-cleaned flue gases, but also fall in the form of other exhaust gases ah.

There are many ways to remove nitrogen oxides from exhaust gases Procedure known. A distinction is made between dry and wet processes. The dry processes include catalytic ones Oxidations, catalytic decomposition, catalytic reductions and adsorption processes. The catalytic reduction process are after nonselective and selective

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INSPECTED

Differentiated reduction processes. In the case of the non-selective In the reduction process, the oxygen contained in the exhaust gas also reacts with the reducing agent used. The invention lies in the field of selective reduction processes, the feature of which is that the reducing agent does not is oxidized by the oxygen in the exhaust gas. Ammonia and hydrogen sulfide are used as reducing agents.

It is known to use metals of the platinum group or metals or metal oxides of the first, fifth, sixth, seventh and eighth groups of the Periodic Table of the Elements as active components for catalysts for the selective reduction in the removal of nitrogen oxides using ammonia as a reducing agent can be used. However, it is also known that in catalytic reactions, in addition to nitrogen oxides, sulfur oxides present act as catalyst poisons, especially in exhaust gas purification. The service lives of the catalytic converters are very short for such exhaust gases. Flue gases with a content of more than 10 ppm SO can hardly be treated, and those with more than T ppm SO are difficult to treat on an economically sensible basis.

However, this contrasts with typical SO contents of 10 to 2000 ppm in common flue gases. Catalytic converters Reduction and removal of nitrogen oxides from flue gases, which cause high concentrations of catalyst poison in Form containing sulfur oxides are not known with industrially usable characteristics.

In view of this prior art, the invention is therefore based on the object of creating a method for the selective removal of nitrogen oxides from exhaust gases which contain sulfur oxides, which is economical and at a high rate

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technical efficiency works and in particular enables long catalyst life.

To solve this problem, a method of the type mentioned is proposed which, according to the invention, is characterized in that the reduction is carried out in the presence of a solid catalyst containing iron sulfate on a support using ammonia gas as the reducing agent at a temperature in the range of 250-550C and with a relative volume flow of the discharged exhaust gas (space velocity) based on the catalyst volume of 1000 to 50,000 h "~ ¹ .

According to a further embodiment of the invention the process is preferably carried out at a temperature in the range of 350 to 450 ⁰ C.

The process is therefore carried out with a catalyst, the active component of which is iron sulfate, namely ironClI) sulfate and / or iron (III) sulfate contains and on one usual carrier is applied. The reaction mechanism on which the process is based is Nitrogen oxides and the sulfur oxides present in the exhaust gas Oxygen not affected. The catalyst proves to be surprisingly insensitive to the poisoning effects of the sulfur oxides. The catalytic activity remains unchanged even over long reaction times. A poisoning effect from the sulfur oxides is not detectable.

The invention thus essentially relates to a dry process for the removal of nitrogen oxides from exhaust gases using a new combination of a special catalyst and a special reducing agent.

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The invention is described in more detail below on the basis of exemplary embodiments in conjunction with the drawings. Show it:

Fig. 1 in a graphical representation

percentage nitrogen oxide removal from the exhaust gas as a function of ■ reaction temperature with space velocity as a parameter;

Fig. 2 in a graphical representation

percentage nitrogen oxide removal from the exhaust gas as a function of the reaction temperature with the the weight of the support based on the percentage iron content of the catalyst as parameters and

Fig. 3 in a graphical representation

Percentage of nitrogen oxide removal from the exhaust gas as a function of the operating time for a catalytic converter, as used in the process and for a prior art catalyst under the same conditions.

In the context of this description, the term "space velocity ¹ ·, which is given in the unit h ~" means the hourly exhaust gas feed based on the catalyst volume used, ie the volume flow of the feed gas based on the volume of the catalyst.

As already mentioned at the beginning, H ₂ , CH ₄ , C ₂ H *, CO, NH ₃ and HUS are used as reducing agents for the catalytic reduction of

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Known nitrogen oxides. From one used for this purpose reducing agent it is required that, due to its thermodynamic characteristics, it must be able to Reduce NO to N ~ at relatively low temperatures and not to react with the oxygen present in the exhaust gas in addition to the nitrogen oxides. The reducing agent should only react selectively with the nitrogen oxides. Usual exhaust gases, in particular smoke gases, are contained in, the Usually at least 1 to 20% by volume of oxygen. When using reducing agents that also react with oxygen To remove nitrogen oxides from such exhaust gases, so large amounts of reaction heat are released that either systems with special devices for heat dissipation or multi-stage reactors must be used. This will not only complicate the process management, but also take the system costs, the energy requirement and in particular the reducing agent consumption increases. In this regard, ammonia gas is used as a reducing agent for the process of the invention selected.

A reduction catalyst that can be used for the process must have a high catalytic activity, a high selectivity and a long service life and in particular be as completely inert as possible to the sulfur oxides which usually act as catalyst poisons. It has been reported that catalysts composed of iron oxides deposited on a carrier effective catalysts for the reduction of nitrogen oxides by ammonia. They are said to be sulfur oxide free nitrogen oxide-containing reaction systems lead to satisfactory reduction efficiencies. Attempts by the applicant have shown, however, that these catalyst systems in Sulfur oxide-containing nitrogen oxide reduction systems not only have a relatively low level of activity, but very much can be deactivated quickly. They are therefore used as catalysts

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for the selective reduction of nitrogen oxides in common exhaust gases, especially smoke gases that contain large amounts of sulfur oxides are unusable.

The invention is based on the unexpected and surprising finding that the iron sulfates, which were previously unknown in connection with the removal of nitrogen oxides from exhaust gases, have a high activity, high selectivity and high efficiency as a selective reduction catalyst material for nitrogen oxide reduction during removal Exhaust gases in connection with the use of ammonia as a reducing agent, specifically in this combination. The temperature range to be maintained is 250 - 550, preferably 350 - 450 ⁰ C. The iron sulphates are completely inert to the catalyzer poison effect of the sulfur oxides «

As a catalyst carrier for the active catalyst material used in the context of the invention are known per se Refractory porous inorganic catalyst supports used, namely preferably aluminum oxide, silicon dioxide, silicon dioxide and refractory substances containing or consisting of these oxides, diatomaceous earth, Containing or consisting of activated carbon, zirconium oxide, zeolite, molecular sieves, silicon dioxide and magnesium oxide refractories, cordierite and mullite. In particular, alumina, silica and silica-alumina double oxides are used.

These carriers preferably have a specific surface area

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of at least 50 m / g. Their pore volume is preferably in the range from 0.2 to 1.5 ml / g. They also preferably have relatively large pore diameters and high breaking strength.

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Although the activity of the catalyst with the iron concentration increases, this concentration is preferably in the range of 0.5-20 wt .-%, in particular in the Range from 1 to 10% by weight based on the weight of the carrier. These areas are characterized by the solubility of the Iron sulfates are conditional in the production of catalysts. ·

The catalyst can be prepared in a manner known per se. For example, the carrier powder is kneaded homogeneously with the powdery iron sulfates (iron (II) sulfate and iron (III) sulfate) in the presence of water and then shaped into spheres, cylinders, rings or other desired shaped bodies, be it by extrusion, be it it by tabletting or on the pelletizing pan. Alternatively, the initially formed carrier can be impregnated in a manner known per se with aqueous solutions of iron sulfates (iron (II) sulfate and iron (III) sulfate). The green catalyst thus obtained is subsequently at 100 - 600 ° C fired - 150 ⁰ C and then dried at 300th The moist catalyst must be dried and fired without being washed with water beforehand. -_, - ■ -

As the active component of that used in the method of the invention Catalyst are iron (II) sulfate, iron (III) sulfate, Ammonium iron sulfate, potassium iron sulfate and or «, or sodium iron sulfate.

The process can be carried out in a fixed bed reactor or in a fluidized bed reactor or in other types of reactor with a lower pressure drop, for example in a honeycomb line reactor will.

The exhaust gases or fumes produced with the method cisr invention can be treated and freed from nitrogen oxides,

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are in particular boiler and furnace flue gases, flue gases from incinerators, exhaust gases from combustion chambers and kilns, Waste gases from waste incineration plants and the waste gases from chemical processing plants. The common fuels used when burning Flue gases formed in heating systems usually contain around 10 to 10,000 ppm NO, 10 - 5000 ppm SO

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0.5-20% by volume O ₂ - and not more than 50% by volume H ₂ O.

The concentration of sulfur oxides in industry standard Fuel flue gases is depending on the sulfur content of the 100 - 3000 ppm SO (SO, and SO ^) of the fuel used. The concentration of nitrogen oxides not only depends the nitrogen content of the fuel, but also the combustion conditions, in particular the combustion temperature, away.

All of the above-described exhaust gases and flue gases can be made extremely economical with the method of the invention be freed from nitrogen oxides. Of course, sulfur oxide-free exhaust gases, in particular the exhaust gases, which arise in the nitric acid production, cleaned with the method of the invention with high efficiency will.

The reduction of nitrogen oxides in the presence of Oxygen occurring reactions can be described by the following equations:

NO + NH ₃ + 1/4 O ₂ » 3/2 H ₂ O + N ₂ ^N 2 (D NO ₂ + 2NH ₃ + 1/2 O ₂ - -> 3H ₂ O + 3/2 (2) ° 2 + 4 / 5NH. j} 4 / 5NO + 6 / 5H ₂ O (3)

In cases where oxygen is to be treated in addition to nitrogen oxides Gas mixture is present, it is therefore necessary

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that the oxidations correspond to the above reaction equations (1) and (2) can be performed selectively. For such a selectivity, the choice of reaction conditions is in particular the choice of the reaction temperature and the molar ratio ΝΗ -, / ΝΟ is of particular importance.

The reaction is preferably carried out in the temperature range of 250 550, especially from 350-450 ° C. At a The reaction temperature of below 250 C is the catalyst activity suppressed, so that neither adequate reaction speeds nor a sufficient degree of removal of nitrogen oxides from the gases to be treated can be achieved. At a reaction temperature above 550 ° C the iron sulfate deposited on the carrier becomes unstable and begins the. Oxidation according to that in equation (3) reaction mechanism described. .

The aforementioned reaction temperature range is closely related to the space velocity at which the reaction is carried out at the particular temperature. For a reaction temperature range of 250 ⁰ C - 550 ⁰ C space velocities of 1000 to - 50 000. set h, preferably 1000 - 3000 hr for the temperature range 250-350 ⁰ C, from 3000 to 25,000 h ^"1 for the temperature range 350-450 ° C and 25,000 to 50,000 h ^"1 for the temperature range 450-550 ⁰ C. the purposes of the invention are particularly preferred in the temperature range 350-450 ⁰ C is preferably so according to a further embodiment of the invention coupled to a space velocity range of 3000 - 25,000 h ~.

The amount of ammonia required for reduction depends on the NO and N0 ₂ concentration in the exhaust gas. If less than the stoichiometrically required amount of ammonia is supplied to reduce the nitrogen oxides, the degree of de-

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removal of nitrogen oxides from the gas mixture to be treated is low, as can be seen from equations (1) and (2). In the case of ammonia amounts above the amount stoichiometrically required to reduce the nitrogen oxides, on the other hand, however, the formation of NO also increases according to equation (3). This also lowers the degree of removal of nitrogen oxides from the gases to be treated. In addition, the gas leaving the reactor has to be post-treated to remove the excess ammonia. This increases the operating costs. For the reasons mentioned, the molar ratio NH ₃ / N0 is therefore preferably set in the range from 0.8 to 1.5, in particular in the range from 1.0 to 1.2.

The most important advantage that can be achieved by the method over the prior art is that the Process for removing the nitrogen oxides can be carried out over long operating times without any interruption, so in particular the catalyst maintains a long and undiminished activity and also in the presence of catalyst poisons, such as sulfur oxides and water vapor, not affected will. The problem of cleaning exhaust gases, which is regarded as difficult according to the state of the art, the nitrogen oxides, sulfur oxides and oxygen as well as water vapor contain, so can be economically, technically problem-free and with a high, long-term use according to the process of the invention stable efficiency.

Another advantage of the method of the invention is the high selectivity with which the nitrogen oxides are reduced. The ammonia gas used as a reducing agent is not oxidized by the oxygen contained in the exhaust gas flow. Because of these conditions, too, the process is surprisingly economical. In the preferred temperature range in particular, there is a high degree of denitration, that is to say the removal of oxidic nitrogen compounds

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of all kinds. These results are achieved even taking into account the optimum reaction temperature ranges within wide parameter ranges, in particular a wide reaction temperature range, achieved, so that to carry out a stable operation under optimal conditions no complex and precise regulation is required, but already a rather cheap coarse control of the parameters, in particular the temperature is sufficient. This moment too contributes to the economy of the process.

Most importantly, however, is that used in the method of the invention Catalyst completely stable and insensitive to the sulfur oxides feared as catalyst poisons, without to have to accept a reduction in the service life of the catalyst, the reactor for nitrogen oxide removal after the Invention can be used at any point in the flue gas cleaning or waste gas cleaning, both before even after desulfurization. For example, the efficiency of the process is not impaired in any way if this also only after the desulfurization of the to be treated Gases is switched. The efficiency is also due to a fluctuating degree of desulfurization in the upstream Desulfurization process not affected. In the same way, the process for nitrogen oxide removal can also be carried out before the desulphurisation process be switched. If a circuit for nitrogen oxide removal after desulphurisation is nevertheless preferred is only due to the easier adjustability and control of the reaction parameters.

Finally, the relative cheapness of the iron sulfate catalyst used in the process is one of the advantages of the Process to be expected. This catalytic converter is also designed to prevent industrial accidents during its manufacture or do not lead to harmful secondary contamination when used.

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The following examples are based on exhaust gases whose nitrogen oxide content (NO) 2u is approx. 90 - 95% by volume of NO,

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Remainder NO ₂ . It should therefore be emphasized at this point that the removal of NO ₂ from exhaust gases is technically much easier than the removal of NO. The degree of removal of nitrogen oxides from exhaust gases is therefore given in the following examples as the degree of removal of NO, that is to say nitrogen (II) oxide.

example 1

50 ml of an aqueous iron (II) sulfate solution are prepared by dissolving 16.6 g of FeSO ₄ «7H _{2 O in water.} This solution is applied dropwise to 15 g of a carrier material consisting _{of 7-Al 2} O _3. The carrier particles have a diameter of 1.5 mm and a length of about 3 to 5 mm. The mean specific surface area of the support

is 226 m / g, its pore volume is 0.85. ml / g. The iron (II) sulfate solution is dripped from a burette onto the carrier placed in a beaker. In this way, 14.2 ml of the iron sulfate solution are evenly absorbed by the carrier. The impregnated carrier is then dried for 3 hours at a temperature of 100-110 ° C. and then fired for a further 3 hours at 550 ° C. in a stream of air. % Fe applied - based on the weight of the carrier are in this way 6 wt..

A preheating zone and a catalyst zone are set in a reaction tube. The reaction tube used has a diameter of 20 mm and a length of 500 mm. 10 ml of the catalyst prepared in the manner described are filled into the reaction tube. The reaction temperatures and the temperature profile are in the electrical Oven set.

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The gas fed to the reactor consists of 500 ppm NO, 550 ppm NH ₃ , 1500 ppm SO ₂ , 3% by volume O ₂ , 10% by volume H ₃ O, the remainder being nitrogen. The feed takes place with a volume flow of 30 - 500 Nl / h. The space velocity is accordingly 3000 - 50,000 h ~ ¹ .

The degree of NO removal from the feed gas mixture is determined by measuring the NO concentration in the reactor leaving the reactor Gas determined.

The tests are carried out in the temperature range of 250 - 550 C.

Figure 1 shows the relationship between space velocity (RG) and nitrogen oxide removal, expressed as the percentage of NO removal, for various temperatures. The results shown in the graph show above all the high efficiency of the iron sulfate catalyst for nitrogen oxide removal. They also show that the present in the gas to be treated nitrogen oxides are removed to more than 80% when at a reaction temperature of 250 - a space velocity of 1000 3000 h 350 ⁰ C, at a reaction temperature door in the range of 350 450 ⁰ C, a space velocity of 3000 - 25,000 h ~ and at a reaction temperature in the range of 450 - 550 C a space velocity of 25,000 - 50,000 h ~ can be set. '^;

Example 2

Iron (H) sulfate catalysts containing 3.0, 6.0 and 12.0 % by weight of iron, based on the weight of the support, are prepared in the manner described in Example 1. The catalysts are placed in the same reactors and tested.

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The feed gas composed of 500 ppm NO, 550 ppm _NH3, 1500 ppm SO _2, 3 wt. ~% O _2, 10 wt .-% ^H 2 ° ^{"residual N} 2 * speed is set at 26 ~ 000 h.

The results shown in FIG. 2 are obtained. The curves shown show that the activity of the catalyst with the iron concentration on the support increases.

Example 3

Using 10 ml of the iron sulfate catalyst prepared according to Example 1 in the reactor described in Example 1 with a reaction at 400 ⁰ C, a volume flow of the gas mixture applied of 260 Nl / h corresponding to a space velocity of 26,000 h ~ The service life of the catalytic converter, i.e. the change in its catalytic activity over time, was checked during the removal of nitrogen oxides from exhaust gases. This led to the reactor gas mixture consists of 500 ppm of NO, at 550 ppm of NH ₃ to 1500 ppm of SO ₂ to 3 wt .-% of O _2, 10 wt -.% H ₂ O, remainder N _2.

The test is carried out for 1000 hours. As the results shown in Figure 3 show, the activity persists of the catalyst unchanged over the entire duration of the experiment. The catalyst thus also meets high industrial requirements.

Comparative example

50 ml of an aqueous iron (II) nitrate solution are prepared by dissolving 17.3 g of Fe (NO ₃ J ₂ * 6H ₂ O in water. The solution is added dropwise to 15 g of the same yr- Al ₂ O ₃ carrier dripped, the

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also for the catalysts described in Examples 1 to 3 is used.

14.2 ml of iron (II) nitrate solution are removed from the catalyst support absorbed. The so impregnated carrier is treated in the manner described in Example 1, so that in this way obtained iron oxide catalyst has an iron concentration of 6.0 wt .-%, based on the weight of the carrier.

With the comparative catalyst obtained in this way, the same Reactor and carried out the tests described in Examples 2 and 3 under the same conditions. The results are also shown in FIGS. The figures show that the iron oxide catalyst compared to the catalysts of the invention has a lower activity and a shorter service life. These results are obviously due to the chemical Attack by the sulfur oxides, against which the iron catalyst is not stable, leads to a decrease in the pore volume and to a reduction in the specific surface area of the Catalyst leads.

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Claims (15)

Claims 1. A method for the selective reduction of nitrogen oxides in exhaust gases, which contain nitrogen oxides and sulfur oxides, in the presence of a solid catalyst, characterized in that the reduction is carried out in the presence of a solid, iron sulfate on a carrier-containing catalyst using ammonia gas as a reducing agent a temperature in the range 250-550 ⁰ C and at a time related to the catalyst volume relative volume flow of the discontinued exhaust gas (space velocity) 1000-50000 h ~ performs. 2. The method according to claim 1, characterized in that the catalyst carrier used is aluminum oxide, Silica, containing or consisting of silica and alumina refractory material, diatomaceous earth. Activated carbon, zirconium oxide, Zeolite, a molecular sieve, a silicon dioxide and magnesium oxide containing or consisting of these oxides refractory material, cordierite or mullite is used. . 3. The method according to any one of claims 1 or 2, characterized in that there is a catalyst support with a specific surface area of 50 98 3 2/0760 at least 50 m / g and with a pore volume of 0.2 to 1.5 ml / g begins. 4. The method na cn one of claims 1 to 3, characterized in that a catalyst with an iron concentration of 0.5 to 20 wt .-%,
based on the weight of the carrier. 5. Katalygat-nr according to one of claims 1 to 3, characterized characterized in that a catalyst with an iron concentration of 1 to 10% by weight is obtained on the weight of the wearer. 6. The method according to one'der claims ί to 5, characterized in that a catalyst is used, which is obtained by impregnating the support with a
Iron sulphate solution, subsequent drying and firing
without washing with water after soaking 7. The method according to any one of claims 1 to 5, dad urch "characterized in that iaan uses a catalyst which is formed by impregnating a
Catalyst support with an aqueous iron (II) sulfate solution and then without washing with water
Drying and burning received. * 509832/0760 ^! , -, 8. The method according to any one of claims 1 to 7, characterized; that one uses a solid, iron sulfate-containing catalyst on a support, which by impregnating the support material with an aqueous solution of iron (II) sulfate and / or iron (III) sulfate and subsequent drying at a temperature of 100 to 150 ⁰ C without washing with water and subsequent firing at a temperature in the range from 300 to 600 ⁰ C is available. 9. The method according to any one of claims 1 to 8, characterized in that a space velocity of 1000 to 3000 h ^{-1 is} set ^{at a reaction temperature of 250 to 350 0 C.} 10. The method according to any one of claims 1 to 8, characterized in that a space velocity of 3000 to 25,000 h ~ is set at a reaction temperature of 350 to 450 C. 11. The method according to any one of claims 1 to 8, characterized in that a space velocity of 25,000 to 50,000 h ^{-1 is} set at a reaction temperature of 450 to 550 ⁰ C. 12. The method according to any one of claims 1 to 11, characterized 50 9 8 32/0760 characterized in that a molar ratio NH ₃ / NO of 0.8 to 1.5 is set. 13. The method according to any one of claims 1 to 11, in particular according to claim 9, characterized in that a molar ratio NHL / NO of 1.0 to 1.2 is set. 14. The method according to any one of claims 1 to 13, characterized characterized in that a catalyst is used, the active component of which is iron sulfate, ammonium iron sulfate, Potassium iron sulfate and / or sodium iron sulfate .is. - · - 15. Use of the method according to one of claims 1 to 14 for cleaning exhaust gases, in particular flame and smoke gases from combustion and heating systems, the 0.5 to 20 vol .-% oxygen, 10 - 5οόο vol-ppm sulfur oxides and contain 10-10 000 ppm nitrogen oxides. 509832/0760 Empty e.ite