DE2341242A1 - METHOD FOR TREATMENT OF EXHAUST GAS - Google Patents

Description

UEXKÜ'-L % STOUP «= RG PATENTANWÄLTE

2 HAMBURG 52

BESELERSTRASSE 4

DR. J.-D. FRHR. by UEXKÜLL DR. ULRICH GRAF STOLBERG 1 "} L") DIPL.-ING. JÜRGEN SUCHANTKE

Esso Research and _{(prio: 1 # September 19? 2} Engineering Company _us

P.O. Box 55

Linden, NJ / V.St.A. Hamburg, August 9, 1973

Process for the treatment of exhaust gases

The invention relates to a method for treating exhaust gases, and more particularly to a method for removing Nitrogen oxides from exhaust gases from stationary incinerators

Nitrogen oxides such as NO and NOp and in general ΝΟ _χ are known to be air polluting compounds. These compounds enter into photochemical reactions with hydrocarbons, which leads to the formation of smog. Well-known emission sites for nitrogen oxides are the exhaust gases from combustion engines, flue gases from nitric acid factories emerging from chimneys and stationary incineration plants such as power plants.

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Oßf & fNSPECTED

According to the invention a method for the treatment of Proposed exhaust gases to reduce the content of nitrogen oxides that is characterized in that

1. Ammonia is added to the exhaust gas and

2. the exhaust gas pretreated in this way under oxidizing conditions at a gas inlet temperature of about 316 to 510 ° C with a catalyst with a Content of copper oxide on a refractory support with a total surface area of at least

40 m / g is treated.

According to the invention, ammonia is added to the exhaust gas to reduce the content of nitrogen oxides and then under oxidizing conditions with a transition series metal catalyst, but none Precious metal, on a porous, refractory support at an inlet temperature of at least about 316C treated. The preferred metallic catalyst is copper oxide.

According to a preferred embodiment of the invention an exhaust gas from a stationary source, i.e. a flue gas, in a continuous one-step process

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drive to reduce the content of nitrogen oxides in Gas treated. Ammonia is added to the hot combustion gas stream and the gas stream pretreated in this way becomes with a supported on a support catalyst made of a transition metal for the selective reduction of Nitrogen oxides treated under oxidizing conditions.

Ammonia is added to the exhaust gas flow in amounts of at least 0.5 mol per mol of nitrogen oxides KO. Such people

Λ.

genes correspond to at least about 0.7 times the stoichiometrically required for the reduction of nitrogen oxides Amount, provided that the nitrogen oxides are about 90 Vol. Ji consist of NO and 10 Vol. Sf of N0 ". The reduction the nitrogen oxides can be represented by the following equations;

6 NO + H NH, -> 5 N _? + 6 H "0

3 NO ₂ + k NH > 7/2 N ₂ + 6 H

The preferred amount of ammonia added is at least about 0.6 mol per mol of NO; in particular, ammonia is approximately stoichiometric (0.73 mol of ammonia per mol of NO, if NO is 90 % from NO and 10 %

Λ. Χ

consists of NOp) up to about 5 moles of ammonia per mole of NO admitted. As soon as the stoichiometrically required ammonia

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amount is exceeded, practically an almost quantitative reduction of NO occurs, so that a

Ji

Increasing the amount of ammonia does not lead to significantly lower values for NO in the outflowing gas. On the other hand, when the catalysts of the invention are used, the catalytic oxidation of ammonia with nitrogen oxides avoided by oxygen in the exhaust gas or at least kept very low, so that an essential excess ammonia does not lead to large amounts of NO in the outflowing gas. ammonia can generally be in the outflowing gas, even if a substantial excess of ammonia is fed in can no longer be proven.

The exhaust gas containing ammonia is treated with the catalyst at gas inlet temperatures of about 316-510 ⁰ C and preferably from about 343 454 ° C.

In power plants, the exhaust gas is usually pretreated either between the economizer and the air preheater where the temperature is usually about 316-399 ° C, or it is pretreated flowing out of the air preheater at a temperature of usually about 149 ⁰ C gas. Catalysts for reducing NO, which are active at temperatures as low as ⁰ C, are quickly poisoned and have

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a limited service life, while catalysts with a longer service life have higher temperatures require. According to a preferred embodiment of the invention, ammonia is added to the exhaust gas stream, after the exhaust gas flow has passed through the flue gas preheater in a normal electricity company.

The contact mass consists of an active material such as copper oxide or ferric oxide in an essentially inert porous support material such as aluminum oxide, aluminum oxide / silicon oxide or Silicon oxide. The preferred catalyst has a content of copper oxide on aluminum oxide. Cheap Results are also obtained with iron oxide on either aluminum oxide, aluminum oxide / silicon oxide, or silicon oxide or obtained with a mixture of copper oxide and iron oxide on aluminum oxide. In each In the case, the carrier must have a total surface area of at least 40 m / g (determined by nitrogen absorption or by BET method).

Copper (preferably in the form of copper oxide) is the preferred active material; in the In general, however, the catalyst can also contain other non-noble transition metals in the form of the free Metals, metal oxides or other metal compounds

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fertilize included. In general, the active material of the catalyst in the reactor is in the form of Metal oxide entered. The active catalyst material can consist of one or more metals. Oxides of the transition metals which can be used according to the invention are, for example, the oxides of the non-noble Group IB metals such as copper, group VB such as vanadium, group VIB such as chromium, molybdenum or tungsten, group VIIB such as manganese or the ferrous metals of group VIII such as iron, cobalt and nickel. Mixtures of copper with one or more non-noble metals of groups VIB or VIII, which are usually initially in the form of oxides good results. For certain embodiments of the Invention can also be a catalyst made of vanadium potassium, as is usually used for the oxidation of SO "to SO, is used. The precious metals, such as platinum or palladium, are possible not used because they are caused by sulfur oxides, which are mostly present in the gases treated according to the invention, are poisoned, and because they do not have a sufficient service life and, moreover, are expensive. That Need catalyst metal or the catalyst metals not completely insensitive to sulfur poisoning

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However, it is necessary that they are also in the sulfated form as NO reduction catalysts are active. Most of those which can be used according to the invention Transition metal oxides are sulfated when exposed to sulfur dioxide and oxygen, and the ^ ü Sulphation usually occurs in the course of the process according to the invention. Even those in so-called sulfur-free ones Exhaust gas traces of sulfur that are still present are sufficient, to sulphate the catalysts after a certain period of time.

Customary, essentially inert, high-melting carrier materials can be used as carriers for the active materials such as aluminum oxide, aluminum oxide / silicon oxide or silicon oxide or similar compounds are used will. The supports are in a porous form with a total surface area (BET) of at least 40 m 2 / g before. Preferred supports are porous materials with a high total surface area such as, for example at least 80 and in particular at least about 100 m / g; The choice of the respective carrier material depends mainly on the choice of active material. For example, copper oxide is on aluminum oxide or alumina / silica is a more effective catalyst, while copper oxide on silica is much less effective. The contact between

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the exhaust gas and the catalyst is preferably passed through a plague bed of the gas Catalyst reached. The flow rate can be about 1000 V / V / h to about 100,000 V / V / h or more be. A small pressure drop is essential for power plants and in numerous other systems necessary because the incinerators operate essentially at atmospheric pressure or at only slightly increased Pressure are operated, so that an excessive loss of energy is avoided in this way. In power plants the pressure drop across the catalyst bed must be substantially no more than 45 torr and frequently be less. For specified flow rates, Particle sizes and shapes will be the combination of gas velocity and catalyst bed depth chosen so that there is a suitable low pressure drop of mostly below 45 Torr and frequently of about 9 Torr

Particularly favorable results in the removal of nitrogen oxides are achieved with the help of copper oxide on aluminum oxide at inlet temperatures of about 316 to 510 C, and preferably about 343 to 454 ° C. Copper oxide is therefore particularly suitable for power plants. Preferred flow rates when using copper oxide on aluminum oxide

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are about 5000 to about 50,000 V / V / h. Although other compounds are known, the more effective is selective Are NO reduction catalysts; many of these materials

and in particular the noble metals, however, do not meet the other necessary conditions of one according to the invention to be used catalyst, which means that it is effective even at temperatures above 316 C even at long permanent exposure to sulfur oxides and oxygen, as these, as already mentioned, the metallic Converting catalysts from the oxide form to the sulfate form; in addition, they do not promote the oxidation of Ammonia and NO. Iron oxide on a suitable carrier

Jt '

with a high total surface area meets the requirements for a catalyst used according to the invention and is actually even more active than copper oxide. The preferred inlet temperatures for iron oxide catalysts are about 316 to ^ 5 ^ ⁰ C and the preferred flow rates are about 10,000 to 100,000 v / v / h. If necessary, a mixed contact compound containing iron oxide and copper oxide can also be used.

The catalysts can slowly lose activity over prolonged periods of time. This loss of activity occurs due to the deposition of small amounts of carbonaceous material and / or plug ash

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on the surface and / or a chemical change on the catalyst surface. If this condition occurs, The catalysts can be treated by treating them with air or exhaust gases with a higher than usual content of oxygen to remove the carbonaceous deposits and / or plug ash. It can also regenerate the catalysts by treatment with a reducing gas such as hydrogen, carbon monoxide or hydrocarbons such as methane, ethane, propane, butane, hexane, decane or mixtures thereof or with a reducing agent diluted with steam Gas take place, so that the catalyst is converted back into the active form. After prolonged exposure to sulfur oxide Exhaust gas, the catalysts are sulfated to a large extent. Either or both of the regeneration methods set forth can be carried out with an existing catalyst as required, generally only one occasional rather than constant regeneration is required. In those cases where the regeneration with a reducing Gas is required to reverse the long-term effects of sulfations, copper is a special one suitable active material.

The catalysts can be prepared in a manner known per se. The supports can have any shape, such as the shape of beads or substantially

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cylindrical extrudates and beyond, they can be of any size. However, preference is given to using catalysts in which the support particles are of such a size that the empty space of the catalyst bed is at least 0.50 and preferably at least 0.60. This void space can be achieved, for example, by using saddles such as the shapes or rings shown in U.S. Patents 2,639,909 and 3,060,503. The use of Katalysatorkörporn which result in a bed to a large space, resulting in a significantly lower pressure drop for a given flow velocity, it is therefore much easier to achieve a maximum pressure drop of ^l \ 5 Torr or less through the catalyst bed when said catalyst bed has a space of at least 0.50.

The impregnation of the carrier materials with the respective active material can be made in a manner known per se. For example, previously calcined support materials in a manner known per se with known impregnation solutions such as, for example, aqueous copper nitrate solutions are impregnated if a copper oxide surface layer is desired, This is followed by calcination in order to convert the salt used for impregnation into the corresponding oxides. The oxide can be reduced with hydrogen or some other reducing agent

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Gas can optionally be reduced to the free metal. By the impregnation process known per se Catalysts obtained in which the active material within the entire support mass in the pores of the support is dispersed.

If necessary, the active material can also be in the form of a relatively narrow band near the surface ' the carrier particles are applied in order to improve the contact between the active materials and the gas flowing through. (External surfaces in this case mean that these surfaces are close to the external shape of the particle and are not caused by the internal pore structure. In the case of using rings, a preferred shape of the carrier, the outer surface of the carrier includes both the inner cylindrical surface and the outer cylindrical surface Surface and ends.)

The band of impregnated material in this case extends inwardly from the outer surfaces to a depth of less than 0.875 cm and usually takes up no more than 70 % of the carrier volume and frequently about 20 to 40 % of the carrier volume. Zone impregnation or "surface impregnation" can be carried out by spraying the carrier particles with an aqueous impregnation solution and rotating them at the same time

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or by the methods described in US Pat. No. 2 fk6,936 or in any other suitable manner. The surface impregnation is preferably achieved by immersing the calcined carrier particles in a volatile polar water-soluble liquid, preferably a C ₁ - to C _1Q -primary alcohol such as n-pentanol or a Cg-oxo alcohol (a mixture of isomeric C 1 - primary alcohols), the carrier particles are then transferred from the water-miscible liquid into an aqueous impregnation solution such as copper nitrate and immersed in this aqueous solution for a predetermined time, which depends on the desired impregnation depth, and then dried and calcined. A particular advantage of the only superficially impregnated catalyst supports over supports in which the active materials are evenly distributed within the pores of the support is that a significantly better utilization of the active material is achieved.

In general, the process of the invention, particularly in power plants, is carried out at essentially atmospheric conditions Pressure carried out.

When the sulfur oxide content of the combustion gases is so high that it is necessary to remove the nitrogen oxides and the sulfur oxides the sulfur oxides can be removed either before or after the nitrogen oxides are removed.

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- lit -

According to a preferred embodiment of the invention, the sulfur oxides are first removed from an exhaust gas or another flue gas stream with a content of oxygen, SO and NO; then ammonia is added to the desulphurized gas stream and the ammonia-containing gas stream is then treated with a catalyst to remove the nitrogen oxides. In this case, measures known per se can be taken to remove the sulfur oxides from the exhaust gases in the first stage. Processes involving a dry solid sorbent are preferred over wet processes using aqueous solutions, such as, for example, the processes of the US Bureau of Mines Report RI 5735 (1961), (alkalized aluminum oxide), US Pat. No. 3 ^ H 865 (alkalized with iron oxide and antimony oxide activated alumina) or GB-PS 1 089 716 (copper oxide on alumina), since the wet processes are carried out at temperatures below the boiling point of water at atmospheric pressure and therefore reheating of the exhaust gases is necessary in order to remove NO with suitable to enable long-life catalysts. The addition of ammonia to the desulphurized exhaust gas, which generally contains only about 10 % of the SOp contained in the inflowing gas, and the catalytic reduction of the nitrogen oxides are carried out in the manner described.

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The use of sulfur-resistant catalysts is important because of the amounts contained in the desulfurized exhaust gases Sulfur oxides are still high enough to poison sulfur-sensitive catalysts like precious metal catalysts.

According to a further embodiment of the invention, the nitrogen oxides are first reduced and then, in a second stage, the sulfur oxides are removed from an exhaust gas with an original content of oxygen, nitrogen oxides and sulfur oxides. In this case, ammonia is added to the exhaust gas flow and then treated with a non-noble metal catalyst for the selective reduction of NO by ammonia under oxidizing conditions. The sulfur oxides can be removed in a manner known per se. Either a wet process using aqueous adsorbent solutions or a dry process using solid sorbents can be carried out. When using a catalyst containing K ₂ OV ₃ O ₅ to remove Ν0 _χ , the sulfur oxide content of the exhaust gas is essentially or completely in the form of SO, which can be removed in a manner known per se.

The invention is explained in more detail below using the example.

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example

In this example, the selective reduction of nitrogen oxides by ammonia is operated on a continuous laboratory scale Procedure described.

The synthetic exhaust gas with a content of about 3 to 5
Oxygen, about 2700 ppm by volume, sulfur dioxide and about 990 to 1010 ppm by volume,
Nitrogen oxides (about 90 % NO and 10 % NOp) were mixed with ammonia in various experiments. The ammonia-treated
Gas mixture was in each experiment through a laboratory reactor
with a diameter of about 2.1 cm with a plague bed with a depth of about 7 * 5 cm, the plague bed using a commercially available copper oxide on ij-aluminum oxide beads
about 0.3 cm in diameter. The copper content of the catalyst was approximately 8 wt.% And the catalyst was completely impregnated. The flow rate was 5000 V / V / h. The gas inlet temperatures were 3 ^ 3 ° C or
399 ° C. The amounts of ammonia added were about 0.25 to 0.80 moles of ammonia per mole of NO; the molar ratio of NH,: NO
was kept constant in each experiment. From the following
Table I is the percent conversion at various
Molar ratios of NH, / NO and with the specified gas inlet

j X

to take leave temperatures.

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Table I. % NO _x Molar ratio Gas inlet temperature converted NH, / NO in ⁰ C 35 0.25 343 45 0.33 343 50 0.33 399 52 0.38 343 55 0.4 399 75 0.5 399 95 0.67 343 95 0.67 343 100 0.73 343 100 0.8 343 100 0.8 399

As can be seen from Table I, there is a substantially linear relationship between the percentage of NO converted and the molar ratio of NH,: N0 when stoichiometric amounts of ammonia are used, less than 0.73 moles of ammonia per mole of NO, provided that NO consists of 90% NO and 10 % NO ". When stoichiometric or greater amounts of ammonia are used, the NO conversion is essentially quantitative. The table shows no clear differences between the conversion achievable at gas inlet temperatures of 343 and 399 ° C. In further experiments under similar conditions

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it was found that the NO conversions were considerable be lower when the gas inlet temperature is 316 ° C. The nitrogen oxide content in the outflowing gas was with a Dynasaiences air pollution monitor instrument- with the NX-130- "total oxides of nitrogen "sensor with scale markings at 55O ppm, 1500 ppm and 5OOO ppm (manufacturer Dynasciences Corp.) certainly.

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Claims (1)

Claims 1. A method for the treatment of exhaust gas to reduce the nitrogen oxide content, characterized in that (1) Ammonia is added to the exhaust gas and (2) the thus pretreated exhaust gas under oxidizing conditions at gas inlet temperatures of about 316 to 510 C with a catalyst containing copper oxide on a refractory support with a total surface area of at least ^ O m / g is treated. 2. The method according to claim 1, characterized in that the amount of ammonia added is at least about 0.5 mol each Moles of NO present in the exhaust gas. 3. The method according to claim 1 or 2, characterized in that a catalyst with a content of copper oxide Alumina is used. k. Process according to Claim 1 or 2, characterized in that a catalyst containing copper oxide and the oxide of at least one non-noble transition metal from group VI-B or the iron group of the periodic table is used. 409812/110 0 5. The method according to claim 1 or 2, characterized in that that a catalyst containing copper oxide and iron oxide is used. 6. The method according to claim 1 to 5, characterized in that the exhaust gas outlet temperature is about 343 to 454 ° C. 7. The method according to claim 1 to 6, characterized in that a fixed bed of a catalyst with a content of a transition metal oxide is used on a porous refractory support and that this bed has a void of at least 0.50. 8. The method according to claim 1 to 7, characterized in that the gas with a particulate catalyst with a Content of a non-noble transition metal oxide is treated on a porous support, the metal oxide being essentially completely in an outer zone adjoining the outer surfaces of the carrier particle an impregnation depth of no more than about 0.875 cm. 9. The method according to claim 8, characterized in that a catalyst is used in the form of rings. 0 9812/1100 10. The method according to claim 1 to 9, characterized in that the gas is treated with the catalyst in a fixed bed at a gas flow rate of at least 5000 V / V / h and that the pressure drop through the catalyst bed is not more than k5 Torr. 11. The method according to claim 1 to 10, characterized in that that the catalyst occasionally regenerates with an oxygen-containing gas and / or a reducing gas will. 12. A method for the treatment of nitrogen oxides, sulfur oxides and oxygen containing gas stream to reduce the Content of nitrogen oxides and sulfur oxides, characterized in that (1) Treating the gas stream with a solid sorbent to selectively remove sulfur oxide, (2) ammonia is added to the desulphurized gas stream and (3) the desulfurized gas stream under oxidizing conditions at gas inlet temperatures of about 316-510 ° C a solid non-noble transition metal catalyst for the selective reduction of nitrogen oxides is treated, with the catalyst contains at least one non- 409812/1100 noble transition metal oxide on a porous high-melting support with a total surface of at least ¹ IO m / g. 13. A method of treating a gas stream containing nitrogen oxides, sulfur oxides and oxygen for reduction the content of nitrogen oxides and sulfur oxides, characterized in that (1) ammonia added to the gas stream, (2) the gas stream is treated under oxidizing conditions at gas inlet temperatures of about 316 to 510 ⁰ C with a solid non-noble transition metal catalyst for the selective reduction of nitrogen oxides, the catalyst at least one non-noble transition metal oxide on a porous high-melting support with a total surface of at least ² JO m / g and that (3) the reduced nitrogen gas stream with a sorbent for selective removal of Sulfur oxides is treated. si: kö 409812/1100