CA2135195A1 - Process for the catalytic reduction of nitrogen oxides - Google Patents

Abstract

The present invention relates to a process for the catalytic reduction of nitrogen oxides in a combustion effluent. The inventive process generally involves treating urea under conditions effective to form a urea hydrolysate; introducing the urea hydrolysate into the effluent (50), under conditions effective to form ammonia and produce an ammonia-containing effluent; and catalytically reducing the nitrogen oxides in the effluent by passing the ammonia-containing effluent through a catalysis zone (30) comprising a catalyst effective for the reduction of nitrogen oxides in the presence of ammonia.

Description

w093/~2047 2 1 3 ~ 1~,3 PCT/US93/03976 ¦ DESCRIPTION

PROCESS FOR T~E CATALYTIC RE3UCTION OF NITROGEN OXIDES

: Related Application - This application is a continuation-in-part of copending U.S. Patent Application entitled "Nitrogen Oxides Reduction Using a Urea Hydrolysate", having Serial No. 07/820,907, filed in the names of von Harpe, Pachaly, Lin, Diep, and Wegrzyn on January 16, 1992; and U.S.
Patent Application entitled "Catalytic~Non-Catalytic Combination Process for Nitrogen Oxides Reduction", having Serial No. 07/626,439, filed in the names of Luftglass, Sun, and Hofmann on December 12, 1990, the disclosures of each of which are incorporated herein by refexence, Technical Field .

The present invention relates to a process for reducing nitrogen oxides (NOx, where x is an integer, generally 1 or 2) in the effluent from the combustion of carbonaceous fuels and other organic matter by a catalyzed reaction. The process involves the catalytic . reduction of nitrogen oxides in an efficient, economical, .. ...
I and safe manner not heretofore seen.

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WO 93t22047 PCT/US~3/03976 ~ 1 3 S ~ 2-Carbonaceous fuels can be made to burn more completely, and with reduced emissions of carbon monoxide ~Co) and unburned hydrocarbons, when the oxygen concen-trations and air/fuel ratios employed are those which permit high flame temperatures. When fossil fuels are used in suspension fired boilers, such as large utility .
boilers, temperatures above about 2000~ and typically about 2200F to about 3000F are generated. Unfortunate-ly, such high temperatures, as well as hot spots of higher temperatures, tend to cause the production of thermal NOX, the temperatures being so high that free radicals of oxygen and nitrogen are formed and chemically combine as nitrogen oxides.

Nitrogen oxides can also be formed as the result of the oxidation of nitrogen containing species in the fuel, .
such as those found in heavy fuel oil, municipal solid waste, and coal. NOX can form even in circulating fluidized bed boilers, which operate at temperatures typicaIly ranging from 1300F to 1700F.

Nitrogen oxides are troublesome pollutants which are found in the combustion effluent streams of boilers and other combustion units when fired as described above, and comprise a major irritant in smog. It is .further believed that nitrogen oxides contribute to tropospheric ozone, an ac~nowledged threat to health, and can undergo a process known as photochemical smog formation through a series of reactions in the presence of sunlight and hydrocarbons.~ Moreover, nitrogen oxides comprise a significant contributor to acid rain, and have been implicated as contxibuting to the undesirable warming of the atmosphere, commonly referred to as the "greenhouse effect".

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W093/22047 2 1 ~ ~ PCT/~IS93/03976 ~ ecently, many processes for the reduction of NOx in combustion effluents have been developed. They can j generally be segregated into two basic categories:
~ selective and nonselective. Among the selective -~i processes, which are believed in the art to be the more . desirable, there is a further division between selective catalytic reduction (SCR) and selective non-catalytic ~t reduction ~SNCR) processes.
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- SCR processes generally involve passing the nitrogen ¦ oxides laden effluent across a cata~yst bed in the presence of ammonia, and are capable Qf achieving NOx reductions as high as 50% and even as high as 9~% or higher. Unfortunately, the use of an SCR process which requires the presence of ammonia can be undesirable because the toxicity and instability of ammonia, as well as the requlrement to maintain ammonia in a high vapor ~ pressure gas or liquid. The use of ammonia can cause ;~ severe problems during storage, handling, and transportation.

In some jurisdictions, in fact, ammonia is reguired to be transported and stored in the form of ammonium water, which in its most commonly utilized form is only about 25% ammonia and is sometimes as dilute as 10%
ammonia. The use of ammonium water for providing ammonia -~ to facilitate catalytic NOX reduction can be impractical because of the increased costs of storage and transporting such a highly dilute substance.
Furthermore, the added cost of evapor`ating the ammonium water to evolve ammonia gas is also a disad~antageous aspect of its use.

~ `SNCR processes, which are temperature dependent, -~ generally utilize a nitrogenous substance such as urea or .
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w093~22047 PCT/US93/03976 213~

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ammonia as well as non-nitrogenous substances. They i proceed in the gas phase by a complex series of free radical-mediated cnemical reactions involving various nitrogen, hydrogen, oxygen and carbon-conta-ning species -~ and radicals. Unfortunately, it has recently been found that many nitrogenous substances, when introduced into an effluent, can lead to the generation and emission of ammonia (referred to as ammonia breakthrough or ammonia slip), which is consideret a pollutant itself.

What is desired, therefore, is a process for the selective catalytic reduction of nitrogen oxides which is able to achieve high NOX reductions and which can control and utilize ammonia slip to facilitate the SCR
reaction and thereby reduce the need for the storage, handling, or transport of ammonia.

Back~round Art - Urea has been recognized as a desirable agent for the selective non-catalytic reduction of NOx at least since the disclosure by Arand, Muzio, and Sotter in U.S. Patent 4,208,386 and Arand, Muzio, and Teixeira in U.S. Patent 4,325,g24. In addition, the use of ammonium carbamate, one of the hydrolysis products of urea, for SNCR nitrogen oxides reduction has been disclosed by Hofmann, Sprague, and Sun in U.S. Patent 4,997,631.

Selective catalytic reduction processes for reducing N0x are well known and utilize a variety of catalytic agents. For instance, in European Patent Application WO
j 210,392, Eichholtz and Weiler discuss the catalytic - removal of nitrogen oxides using activated charcoal or activated coke, with the addition of ammonia, as a catalyst. Kato et al. in U.S. Patent 4,138,469 and Henke .

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~ _ W093/22047 2 1 3 5 1 9 S PCT/US93/03976 in U.S. Patent 4,393,031 disclose the catalytic reduction ` of NOX using platinum group metals and/or other metals , such as titanium, copper, molybdenum, vanadium, tungsten, or oxides thereof with the addition of ammonia to achieve --~ , the àesired catalytic reduction.
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Another catalytic reduction process is disclosed by Canadian Patent 1,100,292 to Knight which relates to the use of a platinum group metal, gold, and/or silver catalyst deposited on a refractory oxide. Mori et al. in U.S. Patent 4,107,272 discuss the catalytic reduction of NOX using oxysulfur, sulfate, or sulfite compounds of vanadium, ~hromium, manganese, iron, copper, and nickel with the addition of ammonia gas.

In a multi-phased catalytic system, Ginger, in U.S.
Patent 4,268,488, discloses exposing a nitrogen oxides containing effluent to a first catalyst comprising a copper compound such as copper sulfate and a second catalyst comprising metal com~inations such as sulfates of vanadium and iron or tungsten and iron on a carrier in the presence of ammonia.

Unfortunately, the requirement for catalytic reactions to be conducted in the presence of ammonia means that a system for supplying ammonia gas to the effluent in the area of the catalyst must be developed and installed. In order to achieve high levels of NOx reduction in an SCR scheme, the molar ratio of ammonia to nitrogen oxides should be about 1:1 or even higher. Such high levels of ammonia create the possibility of leakage, causing health and safety problems.
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o j Disclosure of Invention - ~ .
I The present invention relates to a process for the -j catalytic reduction of nitrogen oxides in a combustion effluent. The process involves treating urea under conditions effective to form a urea hydrolysate;
introducing the hydrolysate into the effluent under conditions effective to form ammonia; and catalytically , reducing the nitrogen oxides in the effluent by passing the ammonia containing effluent through a catalysis zone comprisinSl a catalyst effective f or the reduction of nitrogen oxides in the presence of ammonia.
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- ~y the use of the process of the present invention, the catalytic destruction of nitrogen oxides can be effected in the presence of ammonia while reducing or eliminating the need for the undesirable storage, handlingj and transport of ammonia or ammonium water.

Brief DescriPtion of the Drawin~s - The objec~s of this invention will be described and the present invention better understood and its advantages more apparent in view of the following detailed description, especially when read with reference to the appended drawing which is a schematic illustration of a boiler utilizing the present invention as described in the Example.

Best Mode for Carr~ing Out the Invention ;i As noted above, the present invention relates to the catalytic reduction of nitrogen oxides in a combustion effluent in the presence of ammonia by introducing into - the NOx-laden effluent a compound or compounds which , ......................... .
:,, _w093/22047 2 1 ~ 5 1~ ~ PCT/US93/03976 generate ammonia in the effluent. Advantageously, ~hese compounds comprise urea or the hydrolysis products of urea. After formation of ammonia in the effluent, the effluent is then passed across a catalyst in a catalysis zone for the reduction of NOX to molecular nitrogen (N2)-Under the proper conditions, urea hydrolyzes toproducts which are believed to include ammonia (NH3), ammonium carbamate (NH2COONH~) ("carbamate"), ammonium carbonate ((N~4)2CO3) ("carbonate"), and ammonium bicarbonate ~NH~HCO3) ("~icarbonate").
Hydrolysis generally c~ntinues sequentially from carbamate~ through carbonate and t~en to bicarbonate, each composition being more stable than urea.

Although each of the noted hydrolysis products is individually commercially available, it is more desirable to produce them via urea hydrolysis under the conditions detailed below. This is because the thusly formed hydrolysate has advantages over the individual hydrolysis products, even if combined in the same approximate ratios. A primary advantage is cost, since urea can be significantly less expensive than the individual hydrolysis products. Additionally, a maximum solubility of about 25% for the hydrolysate (based on initial urea concentration) has been observed, which is superior to the solubility of bicarbonate, i.e., about 18%. This can be significant in terms of transportation costs and final treatment agent concentrations.
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1 According to solubility and structural analyses, including high pressure li~uid chromatography (HPLCj ! using pnosphoric acid as solvent; carbon-13 nuclear ¦ magnetic resonance spectroscopy (NMR); thermal ;,;~
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PCT/~'S93/03976 w093/22047 213~135 gravimetric analysis (TGA); diff erential scanning calorimetry (DSC)i and measurement of "P" or "M"
alkalinity by acid titration, the hydrolysate prepared , according to this invention comprises at least in part a .-J single unique structure of carbonate and bicarbonate which is in a complex with carbamate (expressed as carbamate-~icarbonate/carbonate).
~i ¦ If the pressure exerted on the hydrolysate solution is suf f iciently high, ammonia also produced does not flash off, but remains in solution and remains available to contribute to the catalytic reduction of NOX. In - add_tion, depending on the conditions employed, residual urea may also be present.

Although a urea solution will hydrolyze under ambient conditions, typically less than 1% will do so, an insignificant amount in terms of facilitatins the catalytic reduction of nitrogen oxides. In f orming the inventive hydrolysate, temperature, pressure, concentration of the initial urea solution, and residence time are all important parameters, and must be balanced.
High pressure is particularly useful because the reaclion proceeds in the direction of smaller mole volumes du~ing the f ormation of carDamàte and carbonate. Higher temperature and longer residence times also result in higher levels of hydrolysis. However, under equivalent pressures, temperatures, and residence times, hydrolysis aecreases with increases in solution concentration.

Advanta~eously, hydrolysis of a 10% aqueous urea solution should be conducted under pressures sufficiently high to maintain the resulting hydrolysate in solution.
t Su~h pressures will also facilitate hydrolysis.
I Desirably, hydrolysis is performed under pressures of at , ~ , ,~

W O 93/22047 ~ 1 3 ~ ~ 9 ~ PC~r/US93/03976 least a~out 500 pounds per square inch (psi), more - prelerably at least about 650 psi. Is it is desireà to -, maintain ammonia in solution, the pressure should be at least about 7~0 psi. As the concentration of the initial urea solution is increased, the pressure is preferably I increased to achieve equivalent results.

--~ There is no true upper limit of pressure in terms of -~ facilitating hydrolysis; rather, any upper limits ~l comprise practical as opposed to technical limits, since higher pressures, i.e., pressures above about 3000 psi, require vessels able to stand such pressures, which are generaliy more expensive and usually unnecessary.
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` At the desired pressures, the temperatures and residence times can be varied. Temperatures of only about 250F will ensure the presence of some hydrolysate ~e.g., no more than about 5%), whereas temperatures of about 600F to 700F will ensure that virtually all the urea has been converted to hydrolysate. Residence times can vary between about three minutes and about 15 minutes, preferably about five minutes to about lO
- minutes. Tt will be recognized that the upper temperature and residence time limits are less important since exceeding them will not result in lower levels of hydrolysis or a less effective hydrolysate, it is believed.

The temperature and residence time for urea hydrolysis are related, and one (i.e., time) can be decreased 2s the other (i.e., temperature) is increased.
~-~ For instance, hydrolysis at 400F for 10 minutes may be ~- generally equivalent to hydrolysis at 500F for five m~nutes and hydrolysis at 600F for three minutes.
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- W093/220~7 PCT/~S93/0397h 2 1 3 ~ a As noted, hydrolysis proceeds to consecutively form carbamate, carbonate, and bicarbonate. Although all three are present even under the least severe conditions, it is desired that the ratio of carbamate to bicarbon-ate/carbonate in the hydrolysate be about 10:1 to about ~-1:20, more preferably about 2:1 to about 1:10 for greatest effectiveness. This can be achieved by hydrolyzing at a fluid temperature of at least about 325F for about five minutes or longer.

Most preferably, the hydrolysis of urea is conducted in the presence of metal catalysts such as copper catalysts like copper nitrate, nickel catalysts like nickel sulfate, and iron catalysts~like iron (III) nitrate, with the copper and nickel catalysts preferred.
Since such catalysts enhance urea hydrolysis, greater hydrolysis levels can be achieved with equivalent hydrolysis conditions by the use of the ca~alysts. The catalyst metal is mixed into the urea solution prior to hydro}ysis. For instance between about five and about 1~, preferably about 10 parts per million (ppm) of catalyst (as metal) is mixed into a 10% urea solution, whereas about 2~ to aDout 60 ppm, preferably abou 50 ppm s mixed into a 25% urea solution.

In order to effectively supply sufficient urea hydrolysate to support the SCR process, it is desired that at least about 25% of the urea be hydrolyzed. More preferably, at least about 60% of the urea is hydrolyzed, ;~
most preferably at least about 80%.
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The level of hydrolysis achieved under any particular ; set of conditions can easily be determined, for instance, ~; Dy measuring the "P`' and ~M~ alkalinity of the hydrolyzed -; solution. For example, a 10% solution of urea has a pH -.,"~, ~:;,;!
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w093/22047 2 1 3 ~ PCT/~S93/03976 I , of 7.3 with 0 "~" alkalinity and 0.002% "M" al~alini~y as CaCO3. When hydrolyzea, the hydrolysate has a pH of 1 1 about 9.~, with up to 9% "P" alkalinity and 20.2% "M"
I alkalinity. 3y comparing the alkalinity of a hydrolyzed ¦~ solution with a theoretical maximum, using the fresh urea solution as a reference, the level of hydrolysis can be estimated.
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Hydrolysis level can also be determined using conductivity in the same manner as alkalinity can be used. The conductivity of a 10% urea solution is about 1.2 milli-mhos, whereas a 10% urea solution which nas been~completely hydrolyzed has a conductivity of about 120 milli-mhos.

The urea hydrolysis reaction is advantageously carried out "on site" (by which is meant within 1' ' convenient transport distance, such as about S0 miles, of the combustion source to be treated), a~d, most i~ advantageously, in-line because of the economy, ¦ ~ stabiIity, and relative safety of urea for transpor~ and storage. Further, the solubility of the hydrolysate is lower than that of urea, making urea more practical to transpor~.

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I When the inventive hydrolysate is formed in-line, as discussed herein~elow, the urea solution should comprise sufficient urea to provide the desired level of hydrolysate. Advantageously, the urea solution comprises up to about 50% urea by weight, more preferably abou- 5%
to about 45~ urea by weight. Most preferably, the -- , solution comprises about 10% to about 25~ urea by weight.
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~ ~- For efficiency, though, it may be desirable to j hydrolyze a more concentrated urea solution, i.e., about , , ~, '.

WO 93/~2047 . - PCT/l,1S93/03976 21 35 1 ~ 3 45% tO aDOUt 60% by weight, to minimize the heat requirements during hyQrolysis. This can most advan~ageously be done wben hydrolysis is performed "on site~, ;hat is, at cr near the location of the boile~ in which the hydrolysis products are to be introduced, but not in-line. The hydrolyzed solution can then be diluted tQ the appropriate level. On-site urea hydrolysis can be performed in a suitable vessel in which the urea solution can be raised to the desired pressures and temperatures.
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; -As noted, hydrolysis of urea can be performed ln-line, that is, while the urea solution is being supplied t~ an injec~or or other introduction means tO De ~ntroauced into a combustion effluent. This can be -- accomplished by passing the aqueous solution of urea through a supply conduit (referred to as a reaction conduit or tube) and applying heat and pressure.
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The conduit can be any of the conventional tu~es or pipes currently used to supply urea solutions to a combustion effluent in a NOX reducing apparatus or in operative connection therewith, provided the conduit r.
length and flow rates are sufficient to provide the desired residence times, as would be understood by the skilled artisan. Thus the process can ~e practiced without significantly effecting the efficiency of current urea-mediated NOX reducing processes since it does not require substantial alteration or retrofitting o r current lnstallations.

In tact, neat for the hydrolysis reaction can be provided by the effluent itself. As would be understood by the skilled artisan, a conventional heat exchanger can ~e installeà which transfers the desired amount of heat 'rom the e~luen~ -o the condui_ through whi~h the urea !
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~'0 93/22047 2 1 3 5 1 ~ 5 PC~r/US93/03976 ~3-solution is flowing, n oxder to avoid the need for an independent heat source.
-The urea hydrolysate can be lntroduced into the effluent by suitable introduction means under conditions effective to form ammonia in order to support a selective catalytic nitrogen oxides reducing process. Suitable . introduction means include injectors, such as those ~ disclosed by Burton in U.S. Patent 4,842,834, or DeVita in U.S. Patent 4,91~,036, the disclosures of each of which are incorporated herein by reference. One especially preferred type of injection means is an ir.jection lance, especially a lance of the type disclosed oy Peter-~oblyn and Grimard in International Publication No. Wo 91/00134, filed July 4, 1989, entitled "Lance-~ype Injec~ion Apparatus", the disclosure of which is ~ incorporated herein by reference.
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As noted, the heat exchanger for heating the urea solution to perform hydrolysis may be positioned in the effluent stream and, as a result of this, the heat required for facilitating the urea hydrolysis reaction is withdrawn from the effluent to avoid the need for an independent heat source. Such heat exchan~er preferably forms a unit together with the apparatus on which the injecto~ or othe- injection means are arranged within the effluent stream. The heat exchanger at the same time cools the injec~or means such as the injection lances, which is desirable at high effluent temperatures.
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The heat exchanger is pre~erably constructed in the I -orm of a jacket around the injection lance. The heated and hydrolyzed urea solution is passed from the heat e~changer directly into the com~ustion effluent or to a flash drum positioned outside the effluent, and the ~.' -~
~, ~ ~'093/22047 PCT/~S93/03976 ~13Sl.95 gaseous or liquid phase rormeà is separately supplied to ~` tne injectors of the injection lance, while pressurized air may optionally be added to the gaseous phase.
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Generally, the urea or hydrolyzed urea (preferably in solution, especially aqueous solution) of this invention is introduced into the effluent at an effluent temperature wherein the urea or the hydrolysate readily converts to ammonia in order to facilitate the catalytic reduction of nitrogen oxides. The effluent temperature at the point of introduction should also be chosen so as to avoid the utilization of the urea or urea hydrolysate, or the ammonia generated therefrom, to reduce NOX via an SNC~ pathway. Although some utilization in this manner is not disadvantageous, there is the possibility of some by-product formation ~such as carbon monoxide and nitsous oxide) which could become undesirable emissions themselves.

Xccordinqly, introduction of the urea hydrolysate should be at an effluent temperature below about lS~0F, preferably below about 1350F. More preferably, the effluent temperature is between about 230F and about 1200r. When there is a su~stantial amount of urea present (i.e., when urea itself is being introduced or when there is less than about 80% hydrolysis), the effluent temperature should be at least about 3~0F.
Most preferably, the efCluent temperature at the point of introduction of the urea hydrolysate should be between about ~50F and a~out 950F.
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, J The hydrolysate may be at ambient temperature prior to introduction or, alternatively, it may be at or above its boiling point. In this way, the hydrolysate can flash off immediately upon introduction into the ,, - W O 93/22047 213 ~ 19 5 Pc~r/ US93/03976 effluen~, which mi~ht provide advantages in minimizlng air requirements for atomization, etc.

Advantageously, the urea or urea hydrolysate is introduced into the effluent in an amount sufficient to provide a molar ratio of the ammonia generated to the baseline nitrogen oxides level (by which is meant the pre-treatment level of NOX in the effluent) of about 1:10 to about 5:1. More preferably, treatment solution is introduced into the effluent to provide a molar ratio of ammonia to baseline nitrogen oxides of about 1:5 to about 3:1, most pre_erably about 1:2 to about 1:1.
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After introduction of the urea hydrolysate, the treated effluent, which now contains ammonia, is passed over a catalyst. The catalyst used is one capable of reducing the effluent nitrogen oxides concentration in the presence of ammonia. Such a catalyst comprises, for instance, activated carbon, charcoal or coke, vanadium oxide, tungsten oxide, titanium oxide, iron oxide, copper oxide, manganese oxide, chromium oxide, noble metals such as platinum group metals like platinum, palladium, rhodium, and iridium, or mixtures of these. Other catalyst materials conventional in the art and familiar to the skilled artisan can also be utilized. These catalyst materials are typically mounted on a support such as a ceramic substance or a zeolite although other art known supports can also be used.

The ammonia-containing effluent is most preferably passed over the catalyst while the effluent is at a temperature between about 230~ and about 950F. In this ;
manner, the ammonia present in the effluent by the introduction of the hydrolyzed urea solution most effectively facilitates the catalytic reduction of ,~

- W O 93/22047 PC~r/US93/03976 213~195 6 nitrogen oxides.

In a typical installation of the present invention as 7 illustrated in the attached figure, a boiler 100 comprises a flame zone 10 wherein oil 12 is com~usted to form an ef f luent 14. Effluent 14 travels from flame zone 10 through effluent conduit 20. A catalysis zone 30 is i disposed in effluent conduit 20 such that effluent 14 ~ passes therethrough while at a temperature of 500F. A
- catalyst 32, which comprises vanadia and titania loaded on a support is mounted in catalysis zone 30 to permit - effluent ;4 to pass thereover.
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An apparatus for the in-line hydrolysis of urea 40 as discussed above is disposed at effluent conduit 20 such that injection means 50 extends into effluent 14 at a point where the~temperature of effluent 14 is 700F.

A 25% aqueous solution of urea is passed through apparatus 40 and the resulting urea hydrolysis products injected into effluent 14 through injection means 50, where they degrade to ammonia. The ammonia travels to catalysis zone 30 where catalyst 32 reduces the nitrogen oxides to N2 in the presence of the thusly produced ammonia.

Vse of the present invention reduces or eliminates the requirement for the transport, storage and handling of large amounts of ammonia or ammonium water. Even where the inventive process does not provide all of the ammonia required for the catalytic reduction of nitrogen oxides, the reduction of the amount needed still provides significant advantages in terms of both safety and cost.
., The above description is for the purpose of teaching .
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-w093/22047 PCT/US93/03976 213513 ~ -the person of ordinary skill in t~e art how ~o practice the present invention and it is not intended to detail all of those obvious modifications and variations of it which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modi~ications and variations be included within the scope of the present invention which is defined by the following claims.

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Claims (20)

What is claimed is:

1. A process for the catalytic reduction of nitrogen oxides in a combustion effluent, comprising:
a) treating urea under conditions effective to form a urea hydrolysate;
b) introducing said urea hydrolysate into the effluent under conditions effective to form ammonia and produce an ammonia-containing effluent; and c) catalytically reducing the nitrogen oxides in the effluent by passing the ammonia-containing effluent through a catalysis zone comprising a catalyst effective for the reduction of nitrogen oxides in the presence of ammonia.

2. The process of Claim 1, wherein said urea hydrolysate is formed by subjecting an aqueous solution of urea to heat and pressure for a time sufficient to at least partially hydrolyze said urea to form a hydrolysate comprising a complex of ammonium carbamate with ammonium bicarbonate/ammonium carbonate, where the ratio of ammonium carbamate to ammonium bicarbonate/ammonium carbonate is about 10:1 to about 1:20.

3. The process of claim 2, wherein said pressure is at least about 500 psi.

4. The process of claim 3, wherein said aqueous urea solution is heated to at least 300°F for from about three minutes to about 14 minutes to at least partially hydrolyze said urea.

5. The process of claim 4, wherein said aqueous urea solution is hydrolyzed on site.

6. The process of claim 5, wherein said aqueous urea solution is hydrolyzed in-line.

7. The process of claim 2, wherein the urea solution is contracted with a catalyst for the hydrolysis of urea.

8. The process of claim 7, wherein said urea hydrolysis catalyst is selected from the group consisting of metals, metal oxides, metal salts, and mixtures thereof.

9. The process of claim 1, wherein said urea hydrolysate is introduced into the effluent at an effluent temperature between about 550°F and about 950°F.

10. The process of claim 1, wherein said urea hydrolysate is introduced into the effluent in an amount sufficient to provide a molar ratio of ammonia to nitrogen oxides of about 1:10 to about 5:1.

11. The process of claim 1, wherein the effluent is at a temperature between about 230°F and about 950°F when the ammonia-containing effluent passes through said catalysis zone.

12. A process for the catalytic reduction of nitrogen oxides in a combustion effluent, comprising:
a) introducing into the effluent a compound which generates ammonia under the conditions at which the effluent exists at the point of introduction in order to produce an ammonia-containing effluent; and b) catalytically reducing the nitrogen oxides in the effluent by passing the ammonia-containing effluent through a catalysis zone comprising a catalyst effective for the reduction of nitrogen oxides in the presence of ammonia.

13. The process of claim 12, wherein said compound which generates ammonia comprises a urea or a urea hydrolysate.

14. The process of claim 13, wherein said urea hydrolysate is formed by subjecting an aqueous solution of urea to heat and pressure for a time sufficient to at least partially hydrolyze said urea while flowing through said conduit to form a hydrolysate comprising a complex of ammonium carbamate with ammonium bicarbonate/ammonium carbonate, where the ratio of ammonium carbamate to ammonium bicarbonate/ammonium carbonate is about 10:1 to about 1:20.

15. The process of claim 14, wherein the urea solution is heated to a temperature of at least about 300°F for from about three minutes to about 15 minutes.

16. The process of claim 15, wherein the urea solution is subjected to a pressure of at least about 500 psi.

17. The process of claim 14, wherein the urea solution is contracted with a catalyst for the hydrolysis of urea.

18. The process of claim 17, wherein said urea hydrolysis catalyst is selected from the group consisting of metals, metal oxides, metal salts, and mixtures thereof.

19. The process of claim 14, wherein said urea hydrolysate is introduced into the effluent at an effluent temperature between about 550°F and about 950°F.

20. The process of claim 13, wherein said urea or urea hydrolysate is introduced into the effluent in an amount sufficient to provide a molar ratio of ammonia to nitrogen oxides of about 1:10 to about 5:1.