JPH0295416A - Removal of nitrogen oxide from flue gas by selective reduction - Google Patents

Abstract

PURPOSE: To obtain a catalyst having activity and selectivity free from being affected even in the presence of sulfur dioxide by carrying a vanadium (III) compound on a SiO2 carrier and converting the vanadium (III) into V2 O5 . CONSTITUTION: In the production of the catalyst used for a ammonia catalytic reduction method of nitrogen oxide, the V (III) compound is applied on the SiO2 carrier material and is converted to V2 O5 . At the same time, titanium (III) is converted to titanium (IV). The resultant catalyst obtained in the manner exhibits excellent activity and selectivity for the reaction of nitrogen oxide with ammonia and the oxidation of ammonia for forming nitrogen molecule at a higher temp.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for selectively reducing nitrogen oxides in flue gas using ammonia, a method for producing a catalyst used in the method,
and catalysts.

Nitrogen oxides are released when fossil fuels are burned. These nitrogen oxides are produced, in part, by the reaction of molecular nitrogen and molecular oxygen from the air.

At high temperature combustion, these gas molecules can react to produce nitrogen oxides. When the flue gas is cooled and thermodynamic equilibrium is achieved, the nitrogen oxides separate again into molecular nitrogen and molecular oxygen. However, because the reaction to nitrogen and oxygen proceeds at an extremely slow rate, nitrogen oxides remain in the flue gas in amounts associated with thermodynamic equilibrium at high temperatures.

Another source of nitrogen oxides are nitrogen compounds present in fossil fuels. Nitrogen in these compounds reacts to produce nitrogen oxides. Nitrogen oxides can also be produced by radical reactions during combustion at relatively low temperatures.

To date, attempts have been made to catalytically accelerate the decomposition of nitrogen oxides to nitrogen and oxygen to achieve technically possible rates, but these attempts have been unsuccessful.

Therefore, in order to prevent the release of nitrogen oxides, these oxides must be reacted with reducing agents. When nitrogen oxides are reduced, carbon dioxide and/or water are produced in addition to molecular nitrogen. For example, if methane is used as a reducing agent, carbon dioxide and water are produced.

On the other hand, if hydrogen is used as the reducing agent, only water is produced in addition to molecular nitrogen. The reaction with the reducing agent must be carried out at such low temperatures that the nitrogen oxides are no longer thermodynamically stable. Although the reaction can be carried out in the homogeneous gas phase, it is preferable to use a catalyst. Thereby, the final temperature can be kept low.

However, a major drawback is that fossil fuel combustion is generally carried out using an excess of oxygen.

In the combustion of natural gas, the degree of excess oxygen may be relatively low. To this end, the flue gas produced during combustion of natural gas contains less than 1% by volume of oxygen. In the combustion of fuel oil and coal, relatively large excesses of air must be used. In these cases, the flue gas contains about 2-3% by volume oxygen. Therefore, the oxygen content of the flue gas is generally much higher than the nitrogen oxide content, ranging from 300 to 700 ppm.

A special case is that of nitric acid plants. Nitric acid is produced by the Ostwald process, in which ammonia is combusted with air over a platinum/rhodium network catalyst to produce nitrogen oxides. The nitrogen oxide thus produced reacts with molecular oxygen to form nitrogen dioxide in the homogeneous gas phase window. This compound reacts with water to produce nitric acid and nitrogen oxide. The nitrogen oxide is then oxidized again to produce nitrogen dioxide. When nitrogen dioxide is reacted with water again, nitrogen oxide is released again. In the end, even in the most modern equipment a gas mixture containing 350 ppm of nitrogen oxides is discarded. In older equipment, the nitrogen oxide content in the effluent gas ranges from 600 to 1600 ppm. 2000
This is an improvement compared to the situation decades ago, when gases containing ~3000 ppm of nitrogen oxides were being released. Also, because nitric acid plants use excess oxygen, a method is needed to remove nitrogen oxides from oxygen-containing gas mixtures.

The problem with the presence of molecular oxygen in nitrogen oxide-containing effluents is that the oxygen typically interacts preferentially with the added reducing agent before the less reactive nitrogen oxides are reduced. It means reacting.

This is done by adding enough reducing agent to convert nitrogen oxides into nitrogen,
This means that enough reducing agent must be added to completely react the excess oxygen before it can be converted to water and possibly carbon dioxide. Given a large flue gas, the non-selective reduction of nitrogen oxides is barely economically possible if the heat of reaction released during the reaction with molecular oxygen can be utilized. A huge amount of reducing agents will be involved. However, this is a fairly expensive expense and the thermal energy obtained is not always available.

Therefore, selective reduction of nitrogen oxides in oxygen-containing gas mixtures is required. The literature discourse advances to the effect that selective reduction is only possible with ammonia (C, M, van daBleek and P,
J. van den Berg, J. Chem.
.

Techn, Biotcchnol, Volume 30 (19
1980), pp. 467-475). In fact, it has been proven to some extent that selective reduction of nitrogen oxides is only possible with ammonia. There are numerous publications on the selective reduction of nitrogen oxides with ammonia, but they mostly concern the use of iron oxides or copper oxides applied to supports. The use of iron oxides as catalysts has been described, among other publications, by G. L. Bauerl.
E, S, C, Wu and Ken Nobe, In
g.

Eng, Chem, Prod, Dev, 17
Volume (1978), pp. 123-128, and by Yoshihiro Naruse, Takejio Gasawara, Toshihiko Hata, and Hisashi Kishitaka.

Ind, Eng, Chem, Prod, Res
, Dev, Vol. 19 (1980), pp. 57-61, pp. 62-65. The reaction on steel is described by Otto and M. 5helf.

J. Phys. Chem, Vol. 76 (1972), pp. 37-43. Also, T,
Selyama, M., Mizumoto, T., Ishihara and N. Yamazoe, Ing, Eng.

Chem, Prod, Res, Dev, 18
(1979), pages 279-283, describes reactions on steel applied to molecular sieves.

However, the use of ammonia as a reducing agent has a number of drawbacks that cannot be eliminated by the catalysts mentioned above. In fact, at low temperatures ammonium nitrate and ammonium nitrite are produced by the reaction of ammonia with nitrogen dioxide and water. Explosions of ammonium nitrite accumulated in the equipment have already caused tragic accidents. Therefore, selective reduction cannot be carried out even at very low temperatures. Mitsubishi's patent publication, German patent application no. 2727649, states:
Formulas are given for the lowest temperature at which the process will run without the formation of ammonium nitrite and ammonium nitrate. In general, it can be said that temperatures in excess of 300°C are necessary to prevent the formation of these explosive oxides.

If the reaction temperature is too high, ammonia will react with excess oxygen to form nitrogen oxides according to the following equation:

4NH+502→4NO+6H20 In this way, nitrogen oxides are generated again.

In this way, nitrogen oxide emissions are increased rather than decreased. Generally, the catalytic oxidation of ammonia to nitrogen oxides proceeds at temperatures above about 450°C. Therefore,
It is also harmful if the reaction temperature is too high.

Ultimately, ammonia emissions are at least as harmful as nitrogen oxide emissions. This means that the release of ammonia must be sufficiently prevented. This is a big problem. In most cases, the content of nitrogen oxides in the flue gas ranges from about 300 to 700 ppm. This means that the content of nitrogen oxides (particularly low contents) in a multimeter flue gas stream must be measured continuously and rapidly. In this case, the total addition of ammonia must be adapted to the determined nitrogen oxide content.

In general, one should try to keep large flue gas flows in phase to avoid excessive pressure drops. Mixing 300-700 ppm ammonia into a large phase stream of flue gas does not necessarily achieve a sufficient effect; the complete reaction of ammonia with small amounts of nitrogen oxides does not necessarily result in good mixing. I need. It would therefore be very attractive if it were possible to use a small excess of ammonia for flue gas purification by selective reduction using ammonia. A relatively high temperature is maintained in the last part of the catalyst bed. Therefore, excess ammonia must be selectively oxidized to nitrogen rather than to nitrogen oxides.

It is therefore an object of the present invention to provide a process, and a catalyst for use in the process, in which the reaction of nitrogen oxides with ammonia proceeds with high selectivity at temperatures above about 280°C. The catalyst must also be capable of selectively oxidizing ammonia to nitrogen with molecular oxygen at elevated temperatures.

The ultimate aim of the present invention is to provide a catalyst whose activity and selectivity are not adversely affected by the presence of sulfur dioxide in the gas mixture.

So far, the best results have been obtained using catalysts based on vanadium oxide. These acidic catalysts are not adversely affected by scum such as sulfur dioxide and nitrogen oxides and exhibit reasonable selectivity. However, the activity and stability of this type of catalyst are not sufficient. Better results are obtained using vanadium oxide supported on titanium dioxide. Vanadium oxide-titanium dioxide catalyst is M. Inomata, A. Miyamoto, T. tJΩ, N. Kobayashi and Y. Murakami, Ind. Eng.
Chcm, Prod.

Rcs, Dcv, vol. 21 (1982), 42
Pages 4-426 and W, C, Wong and N.
By obc, Ind, Eng, Chcm.

Prod, Rcs, Dcv, 23 volumes (198
4), pages 5134-568.

However, titanium dioxide is not a suitable carrier. Processing this material into mechanically strong tablets is difficult. Also, the surface and hole structure is undesirable. Furthermore, the application of finely divided vanadium oxide to titanium oxide is intractable by hitherto known techniques.Finally, titanium oxide is a rather expensive support material.

As mentioned above, titanium oxide is not a suitable support for catalytically active components. Traditional supports, silicon dioxide and aluminum oxide, offer mechanical strength, accessible surfaces,
and has much better properties regarding hole structure.

In connection with the desired non-basic nature of the carrier (non-reactive with sulfur dioxide and nitrogen oxide), silicon dioxide is the most suitable of the carriers mentioned above. It has therefore been proposed to first prepare a homogeneous mixture of silicon dioxide and titanium dioxide and then apply it to the resulting mixed oxide to obtain vanadium pentoxide.

A homogeneous mixture is obtained by precipitating (hydrated) titanium dioxide from a homogeneous solution onto colloidal silicon dioxide (T, Cicada.

K., Fujimoto, T., Kunugi and 11. Written by Tominaga,
Ind.

Eng, CheOl, Prod, -Res, D
ev, Volume 20 (1981).

(pages 91-95). However, at the desired high reaction temperatures, the resulting catalysts give only low conversions of nitrogen oxide. When hydrated titanium dioxide precipitates from a homogeneous solution, it is unlikely that it will deposit as uniformly distributed fine particles on the surface of silicon dioxide.

On the contrary, it is expected that the (hydrated) titanium dioxide will precipitate in the form of small particle nodules between the silicon dioxide particles rather than on top of them. Therefore, the catalysts obtained by these authors cannot be distinguished from pure titanium dioxide.

The method of the present invention involves converting vanadium(III) compounds into S L
It is characterized in that a catalyst is used which is obtained by converting the v(III) compound into V2O5 after application to the O2 carrier material.

The present invention also relates to a method for producing a catalyst suitable for selectively reducing nitrogen oxides to molecular nitrogen based on vanadium oxide on a support, and to a catalyst suitable for removing nitrogen oxides from gases. It is something.

This method converts vanadium(III) compounds into S L O
2 to the carrier material and then apply the V(III) compound to the V2 carrier material.
It is characterized by changing to O5.

The catalyst of the present invention is comprised of a support material consisting essentially of 5102 loaded with V2O5.

Such a catalyst is obtained by the method for producing the catalyst described above. Surprisingly, it is found that such catalysts exhibit excellent activity and selectivity for the reaction of nitrogen oxide with ammonia and the oxidation of ammonia to produce molecular nitrogen at higher temperatures.

According to a preferred embodiment of the invention, vanadium(II)
Both the I) and titanium (Illr) compounds are applied to a silicon dioxide support. If it is desired to produce a catalyst comprising titanium dioxide, this material is preferably first applied to a silicon dioxide support and then the vanadium(III) oxide is applied. However, it is also possible to simultaneously support vanadium (I[[) and titanium (III) on a silicon dioxide carrier. When the components are applied, they are converted into vanadium (V) and titanium (IV) by oxidation.

The catalyst obtained according to the invention satisfies all of the above requirements.

Silicon dioxide N applied vanadium (1) and titanium (
III) can be prepared in a method known per se (Dutch Patent Application No. 8503090). In this case, the low valence metal ions are obtained electrochemically. Further, titanium (I[I) is also commercially available. Titanium(III) chloride is commercially available as is in conjunction with Ziegler-Natsuta polymerization. Vanadium(III) compounds are also commercially available, but on a relatively small scale.

Vanadium (I [I) and titanium (II) to silicon dioxide
Application of I) can be carried out by known methods.

However, these ions are preferably applied to the support by precipitation-precipitation from a homogeneous solution. This method is described, for example, in J. W. Geus, “Catalysis (III)
” (G, Poneelct, P, Gra
ngc and P, A. Jacobs (eds.) EIsc
vlar, amsterdam.

1983, pages 1-33.

The fact that the catalyst not only exhibits the required activity and selectivity in the desired temperature range (200-375° C.), but also allows the selective oxidation of ammonia to molecular nitrogen is surprising. The increased surface area per unit volume of the catalyst and the homogeneous mixture of active components would not have predicted that these catalysts would have such properties.

Preferably, the selective reduction is carried out in the presence of an excess amount of NH3 compared to the amount of nitrogen oxides to be removed. This excess amount is preferably from 0.5 to 40 mol%.

It is preferred to convert all remaining NHs to molecular nitrogen by the same catalyst at a higher temperature for reduction.

This can be done in the last part of the bed or in another downstream bed. Selective oxidation is carried out at a higher temperature than reduction, preferably 3
It is carried out at a temperature of 50 to 500°C and at least 50°C above the reduction temperature. If necessary, the activity of the catalyst of the invention can be further increased by adding other metal ions, such as copper ions, to the finely divided vanadium and/or titanium oxides applied to the support. . If such other ions are added, it is important that they are finely divided and uniformly distributed over the vanadium and/or titanium oxide carrier. The fact that the advantageous properties of these catalysts are not adversely affected by sulfur dioxide is also surprising.

The invention is briefly illustrated in the following examples, which should not be construed as limiting the invention in any way.

Example ■ V2O5 on silicon dioxide After dissolving ammonium metavanadate (Merck 9.a,) in deionized water, the pH of the solution was adjusted to 2 by adding formic acid. Complicated by the use of formic acid, e.g. H
Preventing undesired oxidation of V (III) by So 2- or CN- when 2SO4 is used. Formic acid does not cause these problems and furthermore has a sufficiently high dissociation constant and low p)11. :FJ adjustment can be done.

The resulting solution was placed in the cathode chamber of the electrolytic vessel.

The cathode used was a hollow lead pipe. A PT main electrode container was provided for the anode and separated from the cathode by a porous wall. 0.025A/with constant stirring
Apply a voltage so that the anode current density is crI,
The contents of the container were kept under nitrogen at all times. The valence of vanadium ions present in the solution was checked by potentiometric measurements. Furthermore, the discoloration of the solution was quantitatively and discontinuously tracked by UV/VIS spectroscopy. During reduction, the color of the solution ranges from deep red (V (V) ) to light blue (V (
■)), and while the reduction continues, V (III)
It turns green to represent. When the color turned deep purple due to V (II), the electrolysis was stopped. After turning off the current, this V (n
) is quantitatively oxidized to v (III). The resulting solution was transferred to a precipitation vessel under nitrogen. A stirrer is provided in this vessel with blades attached to the wall of the vessel to ensure complete homogenization of the solution. In the sedimentation container,
A water jacket heater is provided to ensure temperature regulation of the vessel without interference from electrical pulses;
Nitrogen was then continuously blown in to prevent premature oxidation. A suspension of carrier material (Degussa, Federal Republic of Germany, "Aeros i I", specific surface area 228 m, 7 g) acidified at pH 2.0 was added to the solution.The pH of the solution was adjusted to 0.1 mmol OH-/ The pH was raised by slowly injecting 5% ammonia solution at a rate of 1.0 min.The injection was stopped when the pH reached 6.

The catalyst-supported support is thus filtered and carefully (
It was washed to remove NH4 salts, dried in air at 120°C for 24 hours, and then calcined at a temperature of 350°C for 72 hours. During the heat treatment oxidation of V (III) to V (V) takes place.

A typical example of a catalyst thus prepared had a V2O5 loading of 44% and a BET surface area of 80.7g. Electron microscopy showed that V205 particles were uniformly distributed on the silicon dioxide. This was confirmed by infrared diffuse reflectance spectroscopy. Removal of the infrared absorption properties of the hydroxyl group on silicon dioxide is
It was shown that the surface of silicon dioxide is sufficiently covered with vanadium (V) oxide. The average particle size of 7205 particles is 2nIII! Met.

In the catalytic reduction of CO, the activation energy is 92±2
It was found that KJ was 1 mol. This value is similar to that found for V2O5 applied to TiO or AΩO (7
5KJ1 mol) and unsupported or incompletely attached V
It is intermediate between the value for 2O5 (110 KJ 1 mol). This indicates that the adhesion between V2O5 and silicon dioxide in this technique is adequate.

The activity of this catalyst was investigated for selective catalytic reduction of NO by NHs. Up to 300°C, this reduction is fully selective. Above 300° C., the oxidation of ammonia begins, producing inter alia nitrous oxide N20 (see first two lines of the table below).

In Figure 1 the results are summarized as a function of temperature. The fact that ammonia is removed by reaction with molecular oxygen is responsible for the reduction of nitrogen oxide conversion at higher temperatures. Ammonia with 15N has been specifically used to investigate whether oxidation of ammonia at higher temperatures produces NO. The reaction products are analyzed using a mass spectrometer. The results given in the figure show that ammonia is substantially only reactive towards N20. The results in Figure 1 used relatively small amounts of catalyst and were therefore measured at high space velocities. Thus, for example at 300° C., the conversion of nitrogen oxide is incomplete. At lower space velocities, complete conversion can be easily achieved.

It turns out that it is quite possible to support the catalyst on pre-shaped carrier particles produced, for example, by the sol-gel process. V2O5 loading amount 2 types 1% and BET
A typical catalyst with a surface area of 140 TIi/g was found to exhibit substantially the same properties as the catalyst applied to powdered Aeros i I. This is very attractive because preformed silicon dioxide is available that has excellent mechanical properties. Supporting a catalyst on a pre-formed carrier can be achieved by, for example, vanadium (I[
It can be carried out by impregnating with the EDTA solution of I).

Examples ■ V in silicon dioxide T i 02 V205 aqueous medium
A solution of (III) ions was prepared as described in Example ■. Add a significant amount of T i (1!
3 solution (Merck, 15% T + C in 10% HCp,
Q3) was added.

After transferring the entire amount to the precipitation container described in Example ■,
Suspension of carrier material acidified to pH 2.0 (Aerosil 200, Degutsa, Federal Republic of Germany)
V, BET surface area 186 butts/g> was added. Add a solution of NaOH to 0.1 mmol OH while excluding air.
I)H was raised by gradual injection at a rate of 1/min. The injection was stopped after the pH reached 7.

The carrier supporting the catalyst is filtered, washed, and heated to 120°C in the atmosphere.
for 24 hours and then calcined for 24 hours at 350"C. A typical catalyst made in this way has a
(7) T i 02 and U 4.5% by weight (7) V2O
3 and had a BET surface area of 240rd/9. The activation energy for GO oxidation is 71±3KJ1
molar and was substantially equal to the activation energy for V2O5 applied to pure titanium dioxide.

Up to 350° C., this catalyst reacts with N by ammonia.

is sufficiently selective in the reduction of Last 2 in the table below
As shown in the row, the only products are molecular nitrogen and water. In FIG. 2, the results measured using this catalyst are summarized. Above about 350°C, the activity for No reduction gradually decreases. This is also caused by molecular oxygen now starting to react with ammonia. Less ammonia can be utilized for reaction with No. It is of particular importance that ammonia reacts with molecular oxygen and is oxidized by this catalyst essentially only to molecular nitrogen.

Ti content lower than required to sufficiently cover SiO2
Catalysts with O2 loading exhibit different behavior. A typical example is 6% T! 02 content and 9.5 deuterium% (7)
V 20 s-containing phase, average BE of 141 TIi/g
It has a surface area of T. The activation energy for CO oxidation for this catalyst is 86KJ1 mole, which is V2
It shows the interaction between O5 and SiO2. The reducing activity for No is quite low in this case and also at 400 °C
In this case, oxidation of NH3 to No also occurs.

The table below shows selective or non-selective NO reduction (1 and 2) and selective or non-selective NH3 oxidation <3 at different temperatures.
．． The relative proportions of the reaction products resulting from 4.5) are listed. In all examples the concentration was 300pp of NNO;
N83500ppm and 022%.

First catalyst (T I 0224%/V205 4.5
%) in the reaction mixture for a long time.
Not adversely affected by the presence of 0 ppm S02. Figure 3 represents the conversion of No as a function of time in the presence of sulfur dioxide.

[Brief explanation of the drawing]

Figures 1 and 2 are v2o5/SiO2 and V20, respectively.
5/T! 02/S! N using a catalyst of 02
FIG. 3 is a diagram showing the results when selective reduction of o is performed. FIG. 3 shows the conversion rate of No as a function of time in the presence of sulfur dioxide. Applicant: Herschau Chemie Bee, Bui. FIG, 25 feet

Claims (1)

[Claims] 1. A process for removing nitrogen oxides from a gas by selective conversion using ammonia in the presence of a catalyst based on vanadium oxide on a support,
II) After applying the compound to the SiO_2 support material, V(I
II) A process characterized in that it uses a catalyst obtained by converting the compound into V_2O_5. 2. The catalyst used is to first apply the Ti(III) compound to the SiO_2 support material, then apply the V(III) compound, and finally apply these respectively to the TiO_2 support material.
2. A method according to claim 1, characterized in that it is obtained by converting V_2O_5 into V_2O_5. 3. Process according to claim 2, characterized in that the Ti(III) compound is applied in such an amount that it completely or substantially completely covers the surface of the SiO_2 support material. 4. The catalyst is obtained by simultaneously applying a Ti(III) compound and a V(III) compound to a SiO_2 support material and then converting these compounds into TiO_2 and V_2O_5, respectively. The method of claim 1. 5. Using an excess amount of ammonia compared to the amount of nitrogen oxides to be removed and then selectively converting any unused ammonia to N_2 at higher temperatures. The method according to any one of Items 1 to 4. 6. The method according to claim 5, wherein the excess amount is 0.5 to 40 mol%. 7. Process according to any one of claims 1 to 6, characterized in that the selective reduction of nitrogen oxides is carried out at a temperature of 200 to 375°C. 8. Selective oxidation of ammonia occurs at a temperature of 350-500°C and at least 50°C below the temperature at which selective reduction occurs.
7. A method as claimed in claim 6, characterized in that it is carried out at a temperature of .degree. C. higher. 9. A method for producing a catalyst based on vanadium oxide on a support, suitable for the selective reduction of nitrogen oxides to molecular nitrogen, in which a vanadium(III) compound is applied to a SiO_2 support material. A method characterized in that the V(III) compound is converted into V_2O_5. 10. Claim No. 1, characterized in that first the Ti(III) compound is applied to the SiO_2 support material, then the V(III) compound is applied and finally these are converted into TiO_2 and V_2O_5, respectively. Method of Section 9. 11. Process according to claim 9 or 10, characterized in that the Ti(III) compound is applied in such an amount that it completely or substantially completely covers the surface of the SiO_2 support material. 12. Ti(III) compound and V(III) compound with SiO
12. Process according to any one of claims 9 to 11, characterized in that these compounds are simultaneously applied to the carrier material and then converted into TiO_2 and V_2O_5, respectively. 13. A catalyst suitable for the selective reduction of nitrogen oxides to molecular nitrogen, characterized in that it comprises a support material consisting essentially of SiO_2 applied with vanadium pentoxide, optionally together with titanium dioxide. catalyst. 14, providing a layer of titanium dioxide substantially completely covering the SiO_2 and depositing V_2O_5 on top of the layer of titanium dioxide.
14. Catalyst according to claim 13, characterized in that it comprises SiO_2 as support provided with.