WO1995031273A1

WO1995031273A1 - TWO-STAGE PROCESS FOR THE SELECTIVE REDUCTION OF NO¿x?

Info

Publication number: WO1995031273A1
Application number: PCT/US1994/010974
Authority: WO
Inventors: Leslie C. Hardison; George E. Addison
Original assignee: Wheelabrator Clean Air Systems Inc.
Priority date: 1994-05-12
Filing date: 1994-09-28
Publication date: 1995-11-23
Also published as: AU7921294A

Abstract

A process for the removal of nitrogen oxides from waste gases is provided using a two-stage catalytic reactor scheme where the molar ratio of ammonia to nitrogen oxides is greater than 1.3:1. The first stage uses an acid washed mordenite catalyst and the second stage uses a mordenite containing a metal selected from the group consisting of copper, manganese, iron, cobalt, chromium or a combination thereof.

Description

TWO-STAGE PROCESS FOR THE SELECTIVE REDUCTION OF NO_x

FIELD OF THE INVENTION

The present invention relates to a process for the elimination of both ammonia (NH₃) and nitrogen oxides (NO_x) from industrial waste gases.

Reduction of NH₃ and NO_x to low residual levels is now, more than ever, an important objective especially considering the many stringent air pollution regulations.

Nitrogen oxides are formed by, among other things, the combustion of fossil fuels. Specifically, the oxidation of the molecular nitrogen in the combustion air and the oxidation of chemically bonded nitrogen in the fuel. Nitrogen oxides also occur in flue or exhaust gases from nitric acid production plants. It is well known that nitrogen oxides are a big pollution source, and various catalysts, flow schemes and methods have previously been suggested for reducing the levels of emitted NO_x. As an example, U.S. Pat. No. 4,781 ,902 (Schoubye) discloses a two-stage reduction/oxidation process with a first stage reduction reaction using a V₂O_s catalyst and NH₃ injection followed by a second stage oxidation of excess NH₃ using vanadium and an alkali metal catalyst. Likewise, U.S. Pat. No. 4,912,776 (Alcorn) teaches a two stage NO_x reduction process.

The process of Alcorn oxidizes NO to N0₂ in the first stage, without NH₃ injection, using a nobel metal containing catalyst. Reduction is carried out in a second stage with NH₃ addition using a V₂0₅/Ti0₂ catalyst.

Zeolitic catalyst are also known for their ability to achieve high conversions of NO_x. Kittrel et al., U.S. Pat. No. 4,473,535, for example, teaches the use of a single bed of catalyst comprising copper exchanged mordenite. U.S. Pat. No. 4,847,058 (Odenbrand et al.) teaches a two stage process, where the irst stage uses a hydrogen form mordenite catalyst without NH₃ addition and the second stage reduces N0₂ using a vanadium oxide type catalyst with NH₃ addition. Although there exist a variety of different processes to eliminate NO_x, to the best of applicant's knowledge, none has provided for the removal of NO_x where the molar ratio of NH₃/NO_x is greater than 1/1 using a two stage catalyst scheme of an acid washed mordenite followed by a copper containing mordenite..

One object of the present invention, therefore, is to provide a new NO_x removal process to handle situations where the waste gas stream to be treated contains NH₃ and

NO_x in a molar ratio greater than 1 / 1.

A further object is to provide a combination of two catalyst stages that is not particularly sensitive to fluctuations in NH₃/NO_x levels. - 2 -

Yet another object of the present invention is to provide a two stage catalytic process that does not require injection and/or addition of NH₃ between catalyst stages.

Still another object is to provide a two stage process that allows high conversion of NO_x where ammonia addition is set at a fixed rate prior to the first stage to achieve NH₃/NO_x molar ratios of from about 1.3:1 to 2.0:1.

Accordingly, the invention described herein will provide adequate NO_x removal in a first stage and will scavenge excess NH₃ in a second stage, thus producing only a small NH₃ slip. Other objects and advantages of this invention will be readily apparent from reference to the accompanying drawing, summary of the invention and detailed description of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 schematically illustrates the reactor configuration of this invention containing two stages of catalyst and the process flow scheme.

SUMMARY OF THE INVENTION It has been surprisingly determined that high levels of NO_x conversion can be achieved in circumstances where the molar ratio of NH₃/NO_x in the process feed gas is greater than 1 :1. In situations where the NH₃/NO_x ratio is greater than 1:1 there is a risk that too much NH₃ will react with oxygen. This occurs when a single catalyst system is chosen, such as a metal containing zeolite which has a high activity for the oxidation of NH₃ with oxygen. This results in leaving an insufficient amount of NH₃ to give adequate NO_x reduction. If such a situation occurs, it is not possible to correct the problem by raising the temperature of the catalytic reaction because this will lead to even more ammonia being oxidized, and thus, increasing the emission level of NO_x. Likewise, reducing the catalyst reaction temperature will not solve the problem because the lower temperature will reduce the desired NO_x-NH₃ reaction. Accordingly, the only alternative is to add excess ammonia. The optimum ammonia addition rate would result in the ammonia just being consumed at the exit of the catalyst bed. However, this may be wasteful of ammonia because a substantial amount of NH₃ only will be oxidized with 0₂ rather than NO_x, and in any event requires very precise control of the ammonia rate. This is costly under the best circumstances, and may be impossible under conditions of varying NO_x content of the feed gas. Excess ammonia ultimately ends up in the reactor effluent as undesirable ammonia slip that must be either recovered or lost.

The present invention solves the problem of high NH₃/NO_x ratios by providing two stages of catalyst, each having a different composition, placed in series, in a single reactor. The first stage contains a non-metal containing mordenite, preferably a mordenite that has been acid-washed to maximize the H- mordenite content and the silica to alumina ratio. Preparation of the type of catalyst used in this invention is more fully described below. This type of catalyst gives very good NO_x reduction over a broad range of temperatures of from about 750°F to 900°F when there is a surplus of ammonia present. However, the oxidation of NH₃ with 0₂ using this catalyst is limited such that excess ammonia leaves the first stage unreacted.

The excess NH₃ from the first catalyst stage is handled by providing a second stage of catalyst comprising a metal containing mordenite catalyst, preferably containing a metal selected from the group consisting of copper, manganese, iron, cobalt, chromium, or a combination thereof. The second stage catalyst also reduces any unreacted NO_x from the first stage, but more importantly has a high activity for the oxidation of the excess NH₃ to produce a very low ammonia slip exiting the reactor. In many applications where it is necessary to reduce NO_x levels of waste gas streams there is insufficient ammonia, or none at all. Ammonia is needed for the NO_x-

NH₃ reduction reaction, utilizing NH₃ as a reducing agent according to the following reactions:

4 NH₃ + 4 NO + 0₂ - 4 N₂ + 6 H₂0 8 NH₃ + 6 N0₂ - 7 N₂ + 12 H₂0 In addition, the waste gas to be treated may have varying NO_x concentration or have varying gas flow rates, thus making it difficult for commercially available instrumentation to control the additional ammonia so as to give the optimal 1:1 molar ration of NH₃/NO_x. Addition of too much or too little ammonia may result in excessive ammonia slip or excessive NO_x emissions. The present invention solves the problem by setting a fixed rate of ammonia addition so that molar ratios of NH₃/NO_x arc allowed to vary from about 1.3:1 to about 2.0:1. Using these high ratios of NH₃/NO_x with the two stages of catalyst as described above results in NO_x removal efficiencies in excess of 99% without the problem of excessive ammonia slip. Accordingly, a broad embodiment of the present invention provides a process for eliminating nitrogen oxides from a waste gas stream comprising, in combination, the steps of; (b) adding ammonia to the waste gas stream to form a gas mixture having a molar ratio of ammonia to NO_x greater than 1 : 1.1 (a) preheating or cooling the waste gas stream containing NO_x to obtain a temperature of at least about 750°F (c) contacting the resulting gas mixture from step (b) with a first catalyst comprising a non-metal containing acid washed mordenite to substantially reduce the NO_x producing a first gaseous reaction product containing unreacted ammonia; (d) contacting the first gaseous reaction product, without adding additional ammonia, with a second catalyst comprising mordenite impregnated with a metal selected from the group consisting of copper, manganese, iron, cobalt, chromium or a combination thereof to reduce any unreacted NO_x present in the first gaseous reaction product and to oxidize the unreacted ammonia producing a second gaseous reaction product; and (e) removing the second gaseous reaction product from the process, wherein substantially all of the NO_x originally in the waste gas stream is converted to N₂ and water, and there is minimal ammonia's slip. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The instant invention is capable of processing a variety of gas streams contaminated with nitrogen oxides (NO_x). For instance, waste gas streams from power plants, gas turbines or the like or tail gas from nitric acid manufacture are but a few of the possible gas compositions that can be catalytically treated in the presence of ammonia in accordance with the two stage catalytic process of this invention. FIG. 1 schematically illustrates the flow scheme and catalyst bed configuration of a preferred reactor configuration. NO_x containing waste gas 11 is introduced to reactor configuration 10 at approximately 250°F and is heated via gas burner 12 to a temperature of at least 750°F, preferably between about 750°F and about 900°F. The waste gas may contain ammonia depending on the nature of the process from where the waste gas is obtained. The heated waste gas is then mixed with ammonia (NH₃) that is injected into reactor configuration 10 through sparger 13. The amount of ammonia injected is determined by the concentration of NO_x in the waste gas. Sufficient NH₃ is added to achieve a ratio of NH₃/NO_x greater than 1.0/1, preferably from 1.3:1 to

2.0: 1. The mixture of heated waste gas and injected NH₃ is contacted in an up-flow direction with a first catalyst bed 14. Substantially all of the NO_x in the waste gas is reduced through reaction with NH₃ in catalyst bed 14. The gaseous reaction product exiting catalyst bed 14 is immediately contacted with catalyst bed 15. Because excess NH₃ was added to the waste gas stream through sparger 13, no additional NH₃ is required before the gaseous reaction product is contacted with catalyst bed 15. Any NO_x not reacted in catalyst bed 14 is reduced to nitrogen and water and any excess NH₃ is oxidized by oxygen in catalyst bed 15 according to the following reaction:

4NH₃ + 30₂ -* 6H₂0 + 2N₂ The reaction products 16 existing catalyst bed 15 contain only minor amounts of NO_x and NH₃, low enough for release to the atmosphere without further treatment. In some cases, however, it may be desirable to further treat the reaction products exiting catalyst bed 15, for example, to recover the exothermic heat of reaction. Conversion levels of NO_x of at least 99% are possible using the two stage catalyst scheme of this invention. Reduction products 16 are removed from the top of the reactor.

A gas phase catalytic reaction occurs when the gas mixture contacts the first bed of catalyst. A reaction temperature in the range of from about 750^βF to about 900°F is preferable with a gas space velocity ranging from about 5000 hr"¹ to about 30,000 hr^"1. The reaction zone pressure is operated close to atmospheric pressure in most cases, but higher pressures may be accommodated to provide for effective power recovery from the waste gas. These reaction conditions apply equally to the catalytic reaction that occurs in the second catalyst bed 15.

Catalyst beds 14 and 15 contain a solid paniculate type catalyst a ceramic honeycomb catalyst, or catalyst in any other physical from which provides a relatively large surface area and low gas pressure drop. The solid catalyst comprises substantially the zeolite mordenite, preferably a hydrogen form of synthetic mordenite. These synthetic mordenites can be obtained in both the sodium form and hydrogen form and at varied silica to alumina ratios. It is preferred that the mordenite be of hydrogen form and that the silica to alumina ratio be at least 16:1 , more preferably from 16:1 to 60:1. Zeolite pretreatment procedures commonly known in the art of manufacturing commercial grade catalysts can be employed to achieve the desired silica to alumina ratios. Most preferably the mordenite has undergone an acid washing procedure to maximize the H-mordenite content and the silica to alumina ratio. Typically, mordenite is synthesized by reacting an alumina silicate gel in a stirred autoclave. Synthetic mordenite is also commercially available from a number of sources including the Linde Division of Union Carbide and from the Norton Company. However, an alternative source of mordenite is available that yields a particularly unique type of synthetic mordenite for use in each of the catalyst beds of this invention. Such a mordenite is obtained by reacting volcanic glass with sodium hydroxide or sodium carbonate under autogenous pressure. Details of such a synthesis procedure are found in U.S. Pat. No. 4,935,217 (Simpson), the teachings of which are incorporated herein by reference. Following the teachings of the Simpson patent a solid mordenite catalyst is obtained that is strong enough such that the mordenite does not require the use of binder or support material. While the mordenite may be crystallized in the form of pellets, honeycomb or other suitable shape for use in a catalyst bed, such shapes may also be formed by utilizing mordenite in the form of a powder along with a suitable binder such as ethyl silicate. When this technique is used, the mordenite powder may be synthesized by any of several methods, such as the hydrothermal crystallization method of Simpson, or naturally occurring mordenite may be used.

As mentioned, the catalyst in bed 15 contains at least one metal component selected from the group consisting of copper, manganese, iron, cobalt, chromium or a combination thereof. This metal component can be incorporated in the catalyst in any suitable manner, such as impregnation, ion exchange, coprecipitation or congelation, provided that a uniform dispersion of the metal within the catalyst results. The active metal ingredients may be deposited on the surface of the mordenite crystals by decomposition of a water soluble metal salt such as Cu(NO_s)₂, leaving behind oxides, sulfides or other active metal compounds; or tV . metal component may be chemically bound to the mordenite by exchange of the mi ion for the hydrogen (H-mordenite) or sodium (Na-mordenite) using ion exchange icchniques familiar to those skilled in the art. The preferred method involves contacting the formed or solid paniculate catalyst with a soluble, decomposable compound of the metal, such as the nitrate salt of the metal, in an aqueous solution to impregnate the catalyst. The metal component may exist within the final catalyst as a compound such as an oxide, sulfide, halide, oxyhalide, etc., in chemical combination with one or more of the other ingredients of the catalyst or as an elemental metal. Best results are obtained when the metal exists as an oxide and is uniformly dispersed through out the catalyst. Generally, the metal component may be present in the final catalyst in any amount which is catalytically effective to achieve the desired conversion reactions mentioned above, but relatively small amounts arc preferred. In fact, the metal component generally will comprise from about 0.1 to about 10 wt. % of the final catalyst, calculated on an elemental basis. More preferably, the catalyst will comprise from about 0.5 to about 1.5 wt. % of the metal component.

EXAMPLE

Spherical catalyst particles containing 95+% mordenite converted from volcanic glass following the teachings of U.S. Pat. No. 4,935,217 were contacted with a 10% nitric acid wash solution. After rinsing, and drying, separate portions of the spheres were impregnated with the following 1M metal nitrate solutions: Cu(N0₃)₂ Fe(N0₃)₃-9H₂0

Cr(N0₃)₃-9H₂0 Mn(N0₃)₂

A 1.75 to 1 weight ratio of metal nitrate solution to acid washed mordenite was used for each catalyst preparation.

A pilot plant activity test was used to determine N0_X conversion and slip NH₃ concentration using a synthetic waste gas stream comprising 18% C0₂, 1900 ppm NO, 10% 0₂ and the balance N₂. NH₃ was mixed with the synthetic waste gas to achieve a mole ratio of NH₃/NO_x of 0.9 to 1.1 to 1.0. Each of the above mentioned metal impregnated mordenite catalysts were evaluated along with an acid wash only mordenite catalyst (i.e., no added metals).

Although the testing apparatus could accommodate only one catalyst bed at a time, the results shown below in Table A, clearly show the advantages of using a two stage catalyst reactor scheme, the first stage containing an acid washed mordenite only catalyst followed by a second stage containing a metal containing mordenite catalyst. For example, the acid washed mordenite only catalyst converted 94.1% of the NO_x, however, it left an unacceptable NH₃ slip of 170 ppm. The Cu/mordenite catalyst, on the other hand, converted 97.8% of the NO_x and yielded a very low NH₃ slip of 1 ppm. Taking advantage of the different activities of each of the catalysts and using the above-mentioned two stage catalyst configuration would result in NO_x conversions of greater than 99% and an NH₃ slip of less than 50 ppm. Such results yield NO_x and NH₃ levels which are acceptable under the current air pollution regulations.

TABLE A

NO_x NH₃ SLIP

CATALYST CONVERSION, % CONCEN., ppm TEMP. OF

Acid Washed Mordenite 94.1 170 788 (no metals)

Cr/Mordenite 87.3 3 671

Mn /Mordenite 91.9 6 671

Cu/Mordenite 97.8 1 585

Fe/Mordenite 97.8 30 758 The present in vention has been described in terms of certain preferred embodiments. Of course, numerous other embodiments not specifically described may fall within the spirit or scope of the followi ng claims.

Claims

WE CLAIM AS OUR INVENTION:

1. A two stage catalytic process for eliminating nitrogen oxides from a waste gas stream comprising, in combination, the steps of;

(a) preheating or cooling as required a waste gas stream containing NO_x to obtain a temperature of at least about 750°F;

(b) adding ammonia to the heated waste gas stream to form a gas mixture having a molar ratio of ammonia to NO_x greater than about 1.1 /1;

(c) contacting the resulting gas mixture from step (b) with a first stage catalyst comprising comprising a non-metal containing mordenite to substantially reduce the NO_x by selection reaction with ammonia producing a first gaseous reaction product containing unreacted ammonia;

(d) contacting the f irst gaseous reaction product, without adding additional ammonia, with a second stage catalyst comprising mordenite and a metal selected from the group consisting of copper, manganese, iron, cobalt, chromium or a combination thereof to reduce any unreacted NO_x present in the first gaseous reaction product and to oxidize the unreacted ammonia producing a second gaseous reaction product; and

(e) removing the second gaseous reaction product from the process, wherein substantially all of the NO_x and NH₃ originally in the waste gas stream are converted to N₂ and water.

2. The process of claim 1 , wherein the ammonia is added by sparger to the heated waste gas stream immediately prior to contact with the non-metal containing mordenite catalyst at a constant rate to achieve ammonia to NO_x molar ratios from about 1.1 / 1 to about 2.0: 1.

3. The process of claim 1 , wherein the mordenite of the first stage catalyst is obtained by reacting volcanic glass with a basic compound under autogenous pressure.

4. The process of claim 1 , wherein the mordenite of the second stage catalyst is obtained by reacting volcanic glass with a basic compound under autogenous pressure.

5. The process of claim 1 , wherein the mordenite of the first and second stage catalysts is obtained by reacting volcanic glass with a basic compound under autogenous pressure.

6. The process of claim 1 , wherein the waste gas comprises an industrial process effluent in which the ratio of ammonia to NO_x is substantially greater than

1.1 /1 and no additional ammon ia is added.

7. The process of claim 1 wherein the ammonia addition is held at a constant rate such that as the concentration of NO_x in the waste gas varies, the ratio of NH₃/NO_x is always greater than 1.1 / 1 , without using ammonia and NO_x concentration monitoring equipment.