## 5,229,090
Jul. 20, 1993 

# United States Patent [19] 

## Hofmann et al. 111111111111111111111111111111111111111111111111111111111111111111111111111 US005229090A 

### [11] 

## Patent Number:

[45] 

## Date of Patent:

9 Claims, 3 Drawing Sheets A system for the reduction ofnitrogen oxides in the effiuent from the combustion ofa carbonaceous fuel is presented. This system utilizes means for determining the lowest achievable nitrogen oxides concentration withrespect to a treatment agent to be introduced into the effiuent; means 'for determining whether the baseline nitrogen oxides concentration at the point ofintroduction is above the lowest achievable nitrogen oxides concentration; and means for introducing the treatment agent into the effiuent under conditions effective to reduce the effiuent nitrogen oxides concentration. 4,902,488 2/1990 Epperly et al. 423/235 4,997,631 3/1991 Hofmann et al.

. 5,057,293 10/1991 Epperly et al. 423/235

FOREIGN PATENT DOCUMENTS

W087/02025 10/1986 PCT Int'l Appl.

. W089/02780 8/1988 PCT Int'l Appl.

. OTHER PUBLICATIONS Fenimore, "Destr.uction ofNO by NH3 in Lean Burnt Gas," Combustion and Flame 37: 245-250 (1980). Sun, Hofmann, and Pachaly, "Post Combination NO x Reduction With Urea: Theory and Practice," 7th Annual International Pittsburgh Coal Conference, Sep. 10-14, 1990. 

### Primary Examiner-Gregory A. Heller 

### Attorney, Agent, orFirm-St. Onge StewardJohnston & Reens [54] PROCESS FOR NITROGEN OXIDES REDUCfION TO LOWEST ACHIEVABLE LEVEL [75] Inventors: John E. Hofmann, Naperville; William H. Sun, Aurora, both ofIll. [73] Assignee: NaIco Fuel Tech, Naperville, Ill. [21] Appl. No.: 725,183 [22] Filed: Jut 3, 1991 [51] Int. Cl.s COIB 17/00; COlB 21/00; B011 8/00 [52] U.S. Cl 423/235 [58] Field ofSearch 423/235, 235 D, 239, 423/239 A [56] References Cited
U.S. PATENT DOCUMENTS
3,900,554 8/1975 Lyon. 4,208,386 6/1980 Arand et al.

. 4,325,924 4/1982 Arand et al.

. 4,624,840 11/1986 Dean et al. 423/235 4,719,092 1/1988 Bowers. 4,751,065 6/1988 Bowers. 4,770,863 9/1988 Epperly et al.

. 4,777,024 10/1988 Epperly et al.

. 4,780,289 10/1988 Epperly et al.

. 4,803,059 2/1989 Sullivan et al.

. 4,844,878 7/1989 Epperly et al.

. 4,863,704 9/1989 Epperly et al.

. 4,863,705 9/1989 Epperly et al.

. 4,873,066 10/1989 Epperly et al.

. 4,877,591 10/1989 Epperly et al.

. 4,888,165 12/1989 Epperly et al.

. [57] ABSTRACf

# u.s. Patent July 20, 1993 Sheet 1 of3 

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# o oDISCLOSURE OF INVENTION

BRIEF DESCRIPTION OF THE DRAWINGS
The objects ofthis invention will be described and the present invention better understood and its advantage more apparent in view ofthe following detailed description, especially when read with reference to the appended drawings, wherein:

FIG. 1 is a graphical illustration ofthe critical nitrogen oxides concentration at three baseline nitrogen oxides levels (NOxi); 

### 2 4,927,612, all to Bowers, U.S. Pat. No. 4,803,059 to Sullivan and Epperly, U.S. Pat. Nos. 4,844,878 to Epperly, Sullivan, and Sprague, 4,863,705 to Epperly, Sullivan, and Sprague, 4,873,066 to Epperly, Sullivan, 5 and Sprague, 4,877,591 to Epperly and Sullivan, and 4,888,165 to Epperly and Sullivan. Each ofthese patents teaches the use ofcertain chemical "enhancers", useful themselves and which also shift the temperature range in which urea is most effective when they are used in conjunction with urea. Also disclosed are special techniques, such as cooling the effluent or highly diluting the treatment agent, whichpermitintroduction at higher temperatures.

Otherprocesses whichimproveupon thebasicArand
et al. urea process are disclosed by U.S. Pat. Nos. 4,777,024 to Epperly, Peter-Hoblyn, Shulof, Jr., and Sullivan and 4,780,289 to Epperly, O'Leary, and Sullivan. These processes each involve introducing certain
treatment agents using methods designed to maximize NOx reductions while minimizing the production or generation ofother pollutants such as ammonia or carbon monoxide In other words, these patents teach processes which impact on the overall efficiency of the disclosed nitrogen oxides reducing processes. Although potentially effective at higher temperatures, these patents do not disclose particular treatment agents or conditions useful for the reduction of NOx at temperatures above 2100· F.

An object ofthe present invention is to provide a process which can utilize the lowest achievable nitrogen oxides concentration for an effluent in order to achieve the reduction ofnitrogen oxides.

Another object ofthe present invention is the use of the lowest achievable nitrogen oxides concentration for the effluent and introduced treatment agent by determining the difference between the actual nitrogen oxides concentration and the lowest achievable nitrogen oxides concentration and introducing the treatment agent to reduce the nitrogen oxides concentration at least partially towards the lowest achievable nitrogen oxides concentration.

It is also an object ofthe present invention to determine the amount ofnitrogen oxides reductions possible in a given effluent location using a given treatment agent by determining the lowest achievable nitrogen 50 oxides concentration, measuring the actual nitrogen oxides concentration and then subtracting the lowest achievable concentration from the actual to determine the potential reduction.

These and other objects which will be readily apparent to the skilled artisan can be accomplished by the application ofthe process ofthe present invention as described in more detail herein below. 

### 5,229,090 1

TECHNICAL FIELD

BACKGROUND ART PROCESS FOR NITROGEN OXIDES REDUCfION TO LOWEST ACHIEVABLE LEVEL

Processes and compositions for the reduction ofnitrogen oxides in combustion effluents have been developed extensively over recent years. With the increased attention to the health risks and environmental damage 55 caused by agents such as smog and acid rain, it is expected that NO x reduction research will continue to be pursued.

In the past, most processes for reducing nitrogen oxides levels in oxygenrich effluentshaveconcentrated 60

on achieving NOx reductions at temperatures below 2100· F. and especially below 2000· F. Most notable among these processes is that disclosed by Arand et 

### aI. inU.S. Pat. No. 4,208,386 which detailsthereductionof NO x in combustion effluents by introducing urea at 65 temperatures between 

### BOO· F. and 2000· F. TheArand et al. process has been significantly improvedby disclosures such as U.S Pat. Nos. 4,751,065, 4,719,092, and

The presentinventionrelates toasystemforreducing nitrogen oxides (NO x, where x is a positive integer, generally I of2) in the effluent from the combustion of

a carbonaceous fuel. The inventive system avoids the application of fruitless and counterproductive NOr 10 reducing treatment processes. It also enables the practitioner to achieve nitrogen oxides reductions in situations previously thought infeasible.

Nitrogenoxides aretroublesome pollutants whichare found in the combustion effluent streams ofboilers and 15 comprise a major irritant in smog. It is further believed thatnitrogen oxides can undergo aprocess in the troposhere known as photo-chemical smog formation through a series ofreactions in the presence ofsunlight and hydrocarbons. Moreover, nitrogen oxides comprise 20

a significant contributor to acid rain and have been implicated in the depletion of the ozone layer ofthe stratosphere. Nitrogen oxides may also be acontributor to theundesirable warming ofthe atmosphereknown as the "greenhouse effect". 25

Efforts have long been under way to develop selective, non-catalytic nitrogen oxides reduction (SNCR) processes because of the efficiency and economy of such processes Non-catalytic processes often have the drawback, though, of being extremely temperature 30 dependent and were long believed to be non-applicable in "hightemperature" applications, such as in situations where the temperature ofthe effluent being treated is above 2100· F. In such situations, it had been thought that only burner modifications (which are limited in 35 usefulness) could be used to reduce NOxwhen the temperatures are so high Due to boiler geometry and other considerations, it is often possible to introduce treatment agents only in selectedlocations in aboiler Ifsuch locations are at temperatures where commonly_used 40 nitrogen oxides processes were considered infeasible, it

was believed that significant NO xreductions could not be achieved.

Whatis desired, therefore, is a system whereby nitrogen oxidesreductions canbeachievedeven in situations 45 wherethey were thoughtinfeasible, such as attemperatures above 2100· F., as well as a system whereby the introduction ofNOxreducing treatment agents in counterproductive situations can be avoided

### 4
sugar, molasses, 5- or 6-membered heterocyclic hydrocarbons having at least one cyclic nitrogen such as pyridine and pyrolidine, hydroxy amino hydrocarbons suchas milkor skimmedmilk, amino acids, proteins and 5 monoethanolamine and various other compounds which are disclosed as being effective at the reduction ofnitrogen oxides in an effiuent. Also contemplated are mixtures ofurea or ammonia with the other disclosed compositions as enhancers therefor.

As taught by U.S. Pat. No. 4,780,289 to Epperly, O'Leary, and Sullivan (the disclosure of which is hereby incorporated by reference), each known NO x 

### reducing treatment agent is effective in accordance with anitrogen oxidesreduction versus effiuenttemperature curve, which is typically unique to the combustor in which the effiuent is produced Each such curve has

acharacteristic shape, withaleftsidewhichdenotes the effiuent temperatures where increasing temperature corresponds to increasing NO x 

### reductions, and a right side which denotes the effiuent temperatures where increasing temperature corresponds to decreasing NO x 

### reductions. Eachnitrogen oxidesreduction versus effiuent temperature curve also includes what can generally be referred to as a curve plateau where temperature increases do not correspond to a significant change in the amount ofNO x 

### reduction realized. The curve plateau is that portion ofthe curve where thehighest 20% ofnitrogen oxides reduction values are found, when compared with values at otherportions of the curve, especially the highest 10% ofNO x

### reduction values. The curve plateau represents the effiuent temperature range in which the achievable NO x

### reductions realized by introduction of the particular treatment agent are maximized.

As noted, when treatment agents are introduced at temperatures below the temperature range within the

curveplateau (i.e., onthe left side ofthe NOx

### reduction

versus effiuenttreatmentcurve), achievableNOxreductions increase with increasing effiuent temperature at the point ofintroduction ofthe treatment agent. Contrariwise, attemperatures onthe rightside ofthe curve, above the temperature range within the curve plateau, achievable NO x 

### reductions decrease with increasing temperature at the point ofintroduction.

It

### has been found that any introduction further to the rightthan aprevious introduction (i.e., atahighereffiuent temperature) will result in decreased concentration of secondary pollutants such as ammonia (NH3) and carbon monoxide (CO) In the case of nitrous oxide (N20), also considered a secondary pollutant, N20 levels increase until the point where peak NOx 

### reductions are achieved. The N20 levels then decrease rapidly. In fact, on the right side ofthe plateau, and on the right side of the curve as a whole, the emissions of secondary pollutants are generally minimized.

The shape of the nitrogen oxides reduction versus effiuent temperature curve for any particulartreatment agentistheresultofthe reactionkineticsofthenitrogen oxides reducing reaction in the effiuent. It is believed that in most cases there are two competing reaction paths taking place In one ofthese, the treatment agent combines withhydroxyl radicals (OR) intheeffiuentto form nitrogen oxides (and thus increase NOx

### levels). In the competing path the treatment agent reacts with hydroxyl radicals and nitrogen oxides in the effiuent to form N2 (and thus decrease NOx 

### levels). These two reaction paths can be summarized with respect to urea
as the treatment agent by the following: 

### 5,229,090

BEST MODE FOR CARRYING OUT THE INVENTION 

## 3 FIG. 2is agraphical illustration ofthe effect ofbaseline nitrogen oxideslevels on the nitrogen oxidesreduction versus effiuent temperature curve; and

FIG. 3 is a graphical illustration of the results of Example I.



### The introduction of nitrogenous compositions, by whichismeantcompositionshavingatleastonecompo- 10 

### nentcontaining nitrogen as an elementthereof, for NO x 

### reduction comprises a selective, free radical-mediated process, often referred to as selective non-catalytic reduction (SNCR) Suitable nitrogenous compositions for use as a NOx-reducing treatment agent include cya- 15 nuric acid, ammonia such as disclosed by Lyon in U.S. Pat. No. 3,900,554, and ureasuch as disclosed by Arand etaI. in eitherofU.S. Pat. Nos. 4,208,386 and4,325,924, thedisclosures ofeachofwhichareincorporatedherein by reference. 20

Additional appropriate nitrogenous treatment agents and methods known as being effectivefor the reduction ofnitrogen oxides include those disclosed by International Patent Application entitled "Reduction ofNitrogen- and Carbon-Based Pollutants Through the Use of 25 UreaSolutions", having PublicationNo. WO 87/02025, filed in the name ofBowers on Oct. 3, 1986; U.S. Pat. No. 4,751,065 in the name of Bowers; U.S. Pat. No. 4,719,092, to Bowers; U.S Pat. No. 4,927,612, also to Bowers; U.S. Pat. No. 4,770,863 to Epperly and Sulli- 30
van; U.S. Pat. No. 4,888,165 to Epperly and Sullivan; U.S Pat. No. 4,877,591 to Epperly and Sullivan; U.S. Pat. No. 4,803,059 to Sullivan and Epperly; U.S. Pat. No. 4,863,705 to Epperly, Sullivan, and Sprague; U.S. Pat. No. 4,844,878 to Epperly, Sullivan, and Sprague; 35 U.S. Pat. No. 4,770,863 to Epperly and Sullivan; International Patent Application entitled "Composition for Introduction into a High Temperature Environment", having Publication No. WO 89/10182, filed in the

names ofEpperly, Sprague, and von Harpe on Apr. 28, 40 1989; U.S. Pat. No. 4,902,488 to Epperly, O'Leary, Sullivan, and Sprague; U.S. Pat. No. 4,863,704 to Epperly, Peter-Hoblyn, Shulof, Jr., Sullivan, and Sprague; U.S. Pat. No. 4,873,066 to Epperly, Sullivan, and Sprague; copending and commonly assigned U.S. pa- 45 tent application entitaled "Hybrid Process for Nitrogen OxidesReduction", having Ser. No. 07/395,810, filed in the names ofEpperly and Sprague on Aug. 18, 1989; U.S. Pat. No. 4,997,631 to Hofmann, Sprague, and Sun; and copending and commonly assigned U.S. patent 50 application entitled "Processforthe In-Line Hydrolysis ofUrea", having Ser. No. 07/561,154, filed in thenames ofvon Harpe and Pachaly on Aug. 1, 1990, the disclosuresofeach ofwhichare incorporatedhereinby reference. 55

Thesepatents and applications contemplatetheuseof treatment agents which comprise urea (or one or more of its hydrolysis products such as ammonium carbamate, ammonium carbonate, ammonium bicarbonate, ammoniaand mixtures or complexes thereof) orammo- 60 nia (or compounds which produce ammonia as a byproduct such as ammonium salts like ammonium formateand ammonium oxalate), as well as othercompositions such as hexamethylenetetramine (HMTA), oxygenated hydrocarbons such as ethylene glycol, ammo- 65 nium salts oforganic acids such as ammonium acetate and ammonium benzoate, heterocyclic hydrocarbons having at least one cyclic oxygen such as furfural,(I)

(II) 

### 5,229,090 

## 5

CO(NH2h and OH-NOx 

### 6 There is a lowest achievable NOx concentration for virtually all nitrogenous treatment agents, each of which have similarreaction pathways correspondingto pathways I and II, as known to the skilled artisan Be5 sides urea, this also applies, inter alia, to ammonia At the left side ofnitrogen oxides reduction versus (NH3), commonly employed for nitrogen oxides reduceffluent temperature curve, both pathways I and II are tions. The lowest achievablenitrogen oxides concentrarelatively slow due to the low temperature ofthe efflu- tion varies with the particular treatment agent to be
ent as well as the low concentration ofhydroxyl radi- introduced as well as effluent conditions such as temcals at such low temperatures. Thus, the level ofnitro- 10 perature, baselineconcentrationofNO x, and concentragen oxides reduction is not very great and byproduct tion ofCO, since each combination oftreatment agent (e.g. NH3 and CO emissions are high. Becausethe over- and effluent conditions has specific reaction kinetics all rate ofpathway II is faster than that ofpathway I at associated therewith. these temperatures, the NOx concentration will be re- Ideally, itwould be desirableto determinethecritical duced somewhat, if a sufficiently long reaction time 15 nitrogen oxides level, e.g. calculate it, as a function of (i.e., residence time) is available. temperature, carbon monoxide (which, in part, deterAs temperatures increase, the rate of pathway II mines the presence ofhydroxyl radicals), oxygen level, increases while pathway I still lags behind. Because of baseline NOxlevel, and the ratio ofcarbon to hydrogen this, the nitrogen oxides concentration is reduced to a inthe fuel beingburned Althoughthecritical NOxlevel greater extent but byproduct emissions remain signifi- 20 can be measured experimentally, the task ofevaluating
cant because residence time is generally not adequate the effects ofall the parameters is time consuming and for the reactions to fully utilize injected treatment difficult because oflimitations with experimental comagents. If increased residence time is available, both bustors. Instead, a computer model with a theoretical increased NO x reductions and lower byproduct emis- kinetic mechanism describing the reduction ofnitrogen sions will be observed. 25 oxides by the desiredtreatment agentis advantageously

If the effluent temperature at the point ofintroduc- utilized. tion is within the "plateau" region, the rate ofpathway For instance, nitrogen oxides reduction by urea is II is increased significantly and the rate ofpathway ofI believed to proceed through complex chemical reacis negligible. Because ofthis, emissions ofbyproducts tions with each step an elementary reaction These reacare less than those on the left side and chemical utiliza- 30 tions have been compiled by many in the field ofcomtion is at its maximum with resultant significant NO x bustion kinetics. Notably, J. A. Miller and C. T. Bowreductions. When the effluent introduction temperature man have researched and focused on high temperature is higher still, and on the right side, off the plateau, reactions ofnitrogen containing compounds including pathway Ibecomes more significantand bothpathways NO x (Miller, J. A., Bowman, C. T., "Mechanism and I and II begin competing for the reagents available. 35 Modeling ofNitrogen Chemistry in Combustion", Fall

,

Since the rates of both pathways I and II are ex- Meeting ofthe Western States SectionlThe Combustremely fast at such temperatures, the limitation for tion Institute, Dana Point, Calif., October, 1988). NOx reductions is not residence time or treatment Among the reactions reported, the following set is inagents used, but the ratio ofthe rates ofpathways I and corporated in the current model: II, provided that sufficient reagents are present. 40 1. H2+02=20H ThroughouttheNOxreduction versus effluenttempera- 2. OH+H2=H20+H ture curve for a given treatment agent, it is this ratio 3. O+OH =02+H which determines the lowest achievable NO x concen- 4.0+H2=OH+H tration This concentration is also referred to as critical 5. H+02+M=H02+M NO x, as first proposed by Fenimore in "Destruction of 45 6.0H+H02=H20+02 NO by NH3 in Lean Burnt Gas", Combustion and 7. H+H02=20H Flame, 37:245-250 (1980). 8. O+H02=02+0H

When the amount ofNOxpresent in the effluent is at 9.20H=O+H20

orbelowthis lowestachievablenitrogenoxidesconcen- 

### 10. 2H+ M = H2+ M tration, neither an increase in residence time nor treat- 50 11. 2H+ H2= 2H2
ment rate to provide an excess (e.g., beyond a 50% 12.2H+H20=H2+H20 excess) of the stoichiometric amount will lower the 13. H+OH+M=H2 O+M NOx concentration any further Indeed, this may sub- 14. H+O+M=OH+M stantially increase the generation of secondary pollut- 15. 20+M=02+M ants. In otherwords, unless thebaseline NOxlevel, that 55 16. H+H02=H2+02 is thenitrogen oxides concentrationin theeffluentprior 17. 2H02=H202+02 to any treatment which is to be effected, is above the 18. H202+M=20H+M critical NO x level, treatment agent introductions are 19. H202+H=H02+H2 fruitless at best. 20. H202+0H=H20+H02

It has been found that the concentration of NOx 60 21. NH+02=HNO+O which is the lowest achievable nitrogen oxides concen- 22. NH+02=NO+OH trationincreases sharplywithtemperature, especially at 23. NH+NO=N20+H effluent temperatures above about 1900· F. When this 24. N20+H=N2+0H "critical" NOxlevel is higherthanbaseline (orexisting) 25. N20+M=N2+0+M nitrogen oxides levels for any particular application, 65 26. N20+0=N2+02 chemical treatments will serve to actually increase the 27. N20+0=2NO concentration ofnitrogen oxides to this critical level. 28. N20+0H=N2+H02 Clearly, this is counterproductive.

## 5,229,090 

## 8 (Kee, R. J., Miller, J. A., 

### Jefferson, T. H., 

### "CHEMKIN A General Purpose, Problem-Independent, Transportable, Fortran Chemical Kinetics Code Package", Sandia Laboratories Report #SAND80-8003, Livermore, 5 Calif., 1980). This model describes an ideal plug flow, i.e., no temperature or species concentration gradient in radial direction and no back-mixing. Species concentrations withthis typeofflow aredescribedby asetofnonlinear IO ordinary differential equations. The number of equations depends on the numberofspeciesbeing modelIed, and the equations are interrelated through rates ofthe reactions shown above. The resulting set ofdifferential equations are integrated simultaneously using anumeri15 cal integrator (Hindmarsh, A. 

### C., "ODEPACK, A Systematized Collection ofODE Solvers", Scientific Computing, R. S. Stepleman et al. (eds ), Vol I ofIMACS Transactions on Scientific Computation, p. 55, North20 HolIand, Amsterdam; 1983) This integrator was developed at the Lawrence Livermore National Laboratory and has shown to be efficient in solving combustion problems.

Prior to calculating the critical NO

... concentrations, 25 the model was validated by comparing the results to that ofpilot scale combustors (Sun, W. H., 

### Hofmann, J. E., Pachaly, R., "PostCombustion NO

...Reduction with Urea: Theory and Practice", Presented at the Seventh Annual International Pittsburgh Coal Conference, Sep30 tember, 1990). This demonstrated thatthe trend and the shape of the experimental nitrogen oxides reduction

versus effiuent temperature graph were well modelled. The calculated reduced NO

...concentrations also agreed well with the experimental results. The critical NO

... 35 concentrations as a function of temperature at three baseline nitrogen oxides concentrations (NO

...i) are plotted, as shown in FIG. 1 As noted above, the critical nitrogen oxides concentration becomes significant for
most practical purposes at effiuent temperatures above
40 

### about 1000· C. (1832· F.). 

### The baseline nitrogen oxides concentrations also affect the critical NO

... concentrations, but primarily at temperatures above 1000· C. Thebaseline NO

...concentrations affects NO

... reductions in two ways: (I) the 45 critical nitrogen oxides concentrations are higher with higherbaseline nitrogen oxides, and (2) higher baseline NO

... allows nitrogen oxides to be reduced at higher temperatures. In other words, higher NO

... baselines 50 shift the right side of the nitrogen oxides reduction

versus temperature curve toward higher temperatures. A plot ofthe NO

... reduction curve for three baseline NO

...levels, as shown in FIG. 2, clearly shows the widening ofthe temperature window. 55 Unexpectedly, this lowest achievable nitrogen oxides concentration can be used to develop a methodology for NO

... reduction, especially at high temperatures where the critical NO

... level is 

### more than negligible. Although there is 

### a critical nitrogen oxides level 60 throughout the effective temperature range ofany nitrogenous treatment agent, even at effiuent temperaturesof1500· F. 

### andbelow, thelowestachievablenitrogen oxides concentration atthese lowertemperatures is relatively low, Le., on the order of about 5 parts per million (ppm), depending on the otherfactors discussed above Even these low levels canbe significant, though, depending on the desired nitrogen oxides concentration in the system. 

## 7 29. NH+OH=HNO+H 30. NH+OH=N+H20 31. NH+N=N2+H 32. NH+H=N+H2 33. NH+O=NO+H 34. NH2+0=HNO+H 35. NH2+0=NH+OH 36. NH2+0H=NH+H20 37. NH2+H=NH+H2 38. NH2+NO=NNH+OH 39. NH2+NO=N2+H20 40. NH3+0H=NH2+H20 41. NH3+H=NH2+H2 42. NH3+0=NH2+0H 43. NNH=N2+H 44. NNH+NO=N2+HNO 45. NNH+H=N2+H2 46. NNH+OH=N2+H20 47. NNH+NH2=N2+NH3 48. NNH+NH=N2+NH2 49. NNH+O=N20+H 50. HNO+M=H+NO+M 51. HNO+OH=NO+H20 52. HNO+H=H2+NO 53. HNO+NH2=NH3+NO 54. 2NHO=N20+H20 55. HNO+NO=N20+HO 56. N+NO=N2+0 57. N+02=NO+O 58. N+OH=NO+H 59. H02+NO=N02+0H 60. N02+H=NO+OH 61. NH2+NH=N2H2+H 62.2NH=N2+2H 63. NH2+N=N2+2H 64. N2H2+M=NNH+H+M 65. N2H2+H=NNH+H2 66. N2H2+0=NH2+NO 67. N2H2+0=NNH+OH 68. N2H2+0H=NNH+H20 69. N2H2+NO=N20+NH2 70. N2H2+NH=NNH+NH2 71. N2H2+NH2=NH3+NNH 72. 2NH2=N2H2+H2 73. NH2+02=HNO+OH 74. CO+O+M=C02+M 75. CO+OH=C02+H 76. CO+02=C02+0 77. 

### H02+CO=C02+0H 78. 2H+C02=H2+C02 79. NCO+H=NH+CO 80. NCO+O=NO+CO 81. NCO+N=N2+CO 82. NCO+OH=NO+CO+H 83. NCO+M=N+CO+M 84. NCO+NO=N20+CO 85. NCO+H2=HNCO+H 86. HNCO+M=NH+CO+M 87. H+HNCO=NH2+CO 88. NH+HNCO=NH2+NCO 89. NH2+HNCO=NH3+NCO 9O.0H+HNCO=NCO+H20 91. O+HNCO=NH+C02 92. CON2H.!=NH3+HNCO

The model developed assimilates reaction rates and 65 thermodynamic data using a library ofgas-phase subroutines developed at the Sandia National Laboratories0.8

2 4 8 12 20
2 4 8 12 NSR 1.5 2.0 2.5 

### 10

TABLE I-continued

It can be seen that NO x reductions increased when higher NOxbaseline levels were present, and that NSR Reduction (ppm) NSR Needed for Maximum Reduction

The amount ofurea required to achieve the maximum reduction decreases with the achievable reduction and additional ureadoes not givehigherreductions. In fact, over-treatment can actually lead to an increase in the level of NO x for those cases where injection temperatures are higher than 2000· F., and thus lessens the achievable reduction.

. Generally, when the difference between the baseline NOxconcentration and thecritical nitrogen oxides**~on** centrationis expressedas apercentageofthetotalmtrogen oxides in the effiuent (Le., baseline, when the difference is a positive number), the NSR ofthe treatment agent employed, whether urea,**ammo~i~,** or. another nitrogenous or non-nitrogenous compOSItIon, IS advantageously about 0.1 to about 5 times the difference. More advantageously, the NSR is about 0.5 to about 2 times the difference.

The systemofthe presentinvention canbe combined into amulti-stage treatment regimen which will**reduc~** effiuent nitrogen oxides to a greater extent than preVIously contemplated. Such multi-stage processes are suggested in, for instance, U.S. Pat. No. 4,777,024 to Epperly, Peter-Hoblyn, Shulof, Jr., and Sullivan, as well as International Patent Application entitled "Multi-Stage Process for Reducing the Concentration of Pollutants in an Effiuent", having Publication No. WO 89/02780, fIled in the names ofEpperly, Peter-Ho.blyn, Shulof, Jr., and Sullivan on Aug. 12, 1988, the dISclosures ofeach ofwhich are incorporated hereinby reference.

In a first stage ofsuch a process, NO x is reduced at highertemperatures, such as above about 1900· F. using the inventive system as described above. In a second stage, theNOxis reducedatlowertemperatures**acc~rd** ing to the process disclosed in the patents and applIcations cited above. By doing so, the advantages ofthe reduction ofnitrogen oxides over a wider temperature range are obtained. 

## 5,229,090

Baseline NOx (ppm) 200 200 200 200 65 Injection Temperature ("F.) 1652 1832 2012 2192 Critical NOxConcentration 3 18 72 170 (ppm) 182 128 30 MaxiInum Achievable 197 

## 9 At higher temperatures, such as about 1800· F. or 1900· 

### F. where the critical NO x concentration can be somewhat greater, such as about 10 to 40 ppm, the lowest achievable nitrogen oxides concentration becomes more significant to the practitioner. At high 5 temperatures, Le., 2100· 

### F. and higherwherethelowest achievablenitrogen oxides concentration canbeas high

as about 100to 200ppmorhigher, thecritical NOxlevel is significant and most likely explains why reductions at such high temperatures were not before considered 10 feasible or even possible.

Surprisingly, though, the inventive system is able to accomplish such reductions by first determining the lowest achievable nitrogen oxides concentration in the effiuent atthe location desired forintroduction withthe 15 desired treatment agent by suitable means as discussed above The actual orbaselinenitrogen oxides concentration in the effiuent at that point is then determined by known means, such as by direct measurement of the effiuent or through calculation using the boiler operat- 20 ing conditions (such as boiler load) if there is sufficiently detailed knowledge ofthe boiler system.

When this actual orbaseline NO xlevel is higherthan the lowest achievable nitrogen oxides concentration, NO x reductions are possible and a treatment agent is 25 introduced so as to reduce the nitrogen oxides level in the effiuent at least partially and most preferably entirely to the lowest achievable NOx concentration.

This is most advantageously done by determining the difference between the baseline nitrogen oxides level 30 and the lowest achievable nitrogen oxides concentration (which difference represents the maximum achievable reduction ofNOxatthe existing effiuentconditions using the intended treatment agent) by means such as. a microprocessor, mathematic comparator or other SUIt- 35 able calculating means, and then introducing the treatment agent when the difference is a positive number in

a manner effective to reduce nitrogen oxides by that
amount or a lesser amount. Ifgreater NOx reductions

are attempted, the introduction will still not reduce 40 NOx levels below the critical nitrogen oxides concentration and, thus, be ineffective to that extent.

For instance, the amount ofurea needed to achieve the maximum reduction is determinedby the difference between the baselinenitrogen oxides level and the criti- 45 Example I cal NOx concentration. For a constant baseline NOx Anethylenecrackeris operated such thattheeffiuent level, increasing the effective temperature increases the temperature at the point ofthe nitr?gen.oxides**re~uc** critical nitrogen oxides concentration and thus reduces tion treatment is 1922· F. The baselIne nitrogen OXides themaximumachievablereduction ateachtemperature. level in the effiuent is varied by varying the heatinput. The urea flowrate needed in order to close this "gap" 50 Nitrogen oxides are reduced by introducing a

### l~% between baseline and critical NO xlevels, Le., the maxi- aqueous solution of urea into the effiuent at varymg
mum achievable reduction, as measured in normalized NSR's. The results are set out in Table 1 and graphistoichiometric ratio (NSR) decreases as the achievable cally reproduced as FIG. 3. maximum reduction decreases as shown in Table 1. [The NSR is the ratio ofthe concentration oftheoreti- 55**..;;T;.;.A.;;B;;.L~E~2;;...---------** cally available reducing radicals such as NH2 or NCO Baseline Final to the concentration ofnitrogen oxides in the effiuent. NOx (ppm) NOx(ppm) Alternatively, the molar ratio ofthe treatment agentto 90**;~** the nitrogen oxides concentration can be used in place 90 46 ofNSR when the chemistry ofreduction is not well 60 45 defined The term NSR as used herein will be under- 90 44 stood to encompass molar ratios when appropriate].**;~ ;~**  TABLE 1 73 47 73 46

## 5,229,090 

## 12 3. The process of claim 1 

### wherein said treatment agent is introduced at an NSR ofabout 0.1 to about 5 times the amountby which the baseline nitrogen oxides concentration exceeds the lowest achievable nitrogen 5 oxides concentrati<m when expressed as apercentofthe total effiuent nitrogen oxides concentration.

4. The process ofclaim 2 wherein said effiuent temperature is at least 2100· F.

S. The process ofclaim 2wherein said lowestachievable nitrogen oxide concentration is determined by the solution to the simultaneous nonlinear ordinary differential equations resulting from the flow model utilizing the following reaction set: 1. H2+02=20H 2.0H+H2=H20+H 3. O+OH =02+H 4. O+H2=OH+H 5. H+02+M=H02+M 6.0H+H02=H20+02 7. H+H02=20H 8. O+H02=02+0H 9.20H=O+H20 to.2H+M=H2+M 11. 2H+H2=2H2 l2.2H+H20=H2+H20 13. H+OH+M=H2 O+M 14. H+O+M=OH-f-M l5.20+M=02+M 16. H+H02=H2+02 l7.2H02=H202+02 18. H202+M=20H+M 19. H202+H=H02+H2 20. H202+0H=H20+H02 21. NH+02=HNO+O 22. NH+02=NO+OH 23. NH+NO=N20+H 24. N20+H=N2+0H 25. N20+M=N2+0+M 26. N20+0=N2+02 27. N20+0=2NO 28. N20+0H=N2+H02 29. NH+OH=HNO+H 30. NH+OH=N+H20 31. NH+N=N2+H 32. NH+H=N+H2 33. NH+O=NO+H 34. NH2+0=HNO+H 35. NH2+0=NH+OH 36. NH2+0H=NH+H20 37. NH2+H=NH+H2 38. NH2+NO=NNH+OH 39. NH2+NO=N2+H20 40. 

### NH3+0H=NH2+H20 41. NH3+H=NH2+H2 42. NH3+0=NH2+0H 43. NNH=N2+H 44. NNH+NO=N2+HNO 45. NNH+H=N2+H2 46. NNH+OH=N2+H20 47. NNH+NH2=N2+NH3 48. NNH+NH=N2+NH2 49. NNH+O=N20+H 50. HNO+M=H+NO+M 51. HNO+OH=NO+H20 52. HNO+H=H2+NO 53. HNO+NH2=NH3+NO 

## 11 had no significant effect on nitrogen oxides reductions when the critical NO x 

### level is achieved. Example II
A boiler fired with pulverized coal emits 250 ppm nitrogen oxides. A 10% solution ofurea isinjected at

a location where the effiuent temperature varies between 2000· F. and 2050· F. After about 0.2 seconds, the critical NOx

### level of 100ppm is reached with negligible emission ofbyproducts carbon monoxide, ammo- 10 

### nia, or nitrous oxide.

To reduce NO x 

### emissions further, a 10% solution of

ureaisinjected atalocation wheretheeffiuenttemperature is 1850· F. The urea injection rate is 50 ppm to achieve an additional reduction of25 ppm, to 75 ppm, 15 because chemical utilization is high at this temperature. Byproduct emissions are low because ofthe low treatment rate.

Accordingly, employment ofaprocess ofthis invention can produce high reductions of nitrogen oxides 20 with low byproduct emissions.

It 

### is to be understood that the above Examples are given by way ofillustration only and are not to be construed as limiting the invention.

The above description is for the purpose ofteaching 25 the person ofordinary skill in the art how to 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 workerupon 30 reading the description. Itisintended, however, that all

. such obvious modifications and variations be inclUded within the scope ofthe present invention which is defined by the following claims.

We claim: 35 1. 

### Aprocess forreducing nitrogen oxides in the effiuent from the combustion ofa carbonaceous fuel which is at a temperature of at least about 1900· V. using a nitrogenous treatment agent comprising urea, ammonia, cyanuric acid, ammonium·carbamate, ammonium car- 40 bonate, mixtures ofammonia and ammonium bicarbonate, one or more ofthe hydrolysis products ofurea or mixtures or complexes thereof, compounds which produce ammonia as a byproduct, ammonium formate, ammonium oxalate, hexamethylenetetramine, ammo- 45 nium salts oforganic acids, 5- or 6- memberedheterocyclic hydrocarbons having at least one cyclic nitrogen, hydroxy amino hydrocarbons, amino acids, proteins, monoethanolamine, or mixtures thereof, the process comprising: 50

(a) determining thelowestachievablenitrogen oxides

concentration forsaid treatmentagentattheconditions existing in the effiuent at the point ofintroduction; (b) 

### determining the baseline nitrogen oxides concen- 55

tration at the point ofintroduction; (c) determining the amount by which the baseline

nitrogen oxides concentration exceeds the lowest achievable nitrogen oxides concentration; and (d) introducing said treatment agent into the effiuent 60

under conditions effective to reduce the effiuent nitrogen oxides concentration when said determined amount is a positive number. 2. The process of claim 1 wherein the conditions existing in the effiuent used to determine said lowest 65 achievable nitrogen oxides concentration comprise effluent temperature and concentration ofhydroxyl radicals.

## 14 ides concentration to the lowest achievable nitrogen oxides concentration.

.

8. A multi-stage process for reducing the nitrogen oxides concentration in the oxygen-rich effiuent from 5 the combustion ofa carbonaceous fuel comprising: (a) a first stage which comprises

.

i. determining the lowest achievable nitrogen oxides concentration for a nitrogenous treatment agent comprising urea, ammonia, cyanuric acid, ammonium carbamate, ammonium carbonate, mixtures ofammonia and ammonium bicarbonate, one or more ofthe hydrolysis products of

urea or mixtures or complexes thereof, compounds which produce ammonia as byproduct, ammonium formate, ammonium oxalate, hexamethylenetetramine, ammonium salts oforganic acids, 5- or 6-membered heterocyclic hydrocarbons having at least one cyclic nitrogen, hydroxy amino hydrocarbons, amino acids, proteins, monoethanolamine, or mixtures thereof at theconditions existingin theeffiuentatthepoint ofintroduction, wheretheeffiuentis atatemperature ofat least about 1900· F.; ii. determiningthebaselinenitrogen oxides concentration at the point ofintroduction; iii. determining the amount by which the baseline

nitrogen oxides concentration exceeds the lowest achievable nitrogen oxides concentration; and iv. introducing said treatment agent into the effiuent

under conditions effective to reduce the effiuent nitrogen oxides concentration when said determined amount is a positive number; and b) a second stage which comprises introducing a

nitrogenous treatment agent into the effiuent at an effiuent temperature below about 1900· F. under conditions effective to further reduce nitrogen oxides in the effiuent. 9. The process ofclaim 8 wherein said nitrogenous
treatment agent of said second stage comprises urea, ammonia, cyanuric acid, ammonium carbamate, ammonium carbonate, mixtures ofammonia and ammonium bicarbonate, ammoniumformate, orammoniumoxalate. 

# • • • • • 35 20

30 15 10

25 

## 5,229,090 

## 13 54.2NHO=N20+H20 55. HNO+NO=N20+HO 56. N+NO=N2+0 57. N+02=NO+O 58. N+OH=NO+H 59. H02+NO=N02+0H 60. N02+H=NO+OH 61. NH2+NH=N2H2+H 62.2NH=N2+2H 63. NH2+N=N2+2H 64. N2H2+M=NNH+H+M 65. N2H2+H=NNH+H2 66. N2H2+0=NH2+NO 67. N2H2+0=NNH+OH 68. N2H2+0H=NNH+H20 69. N2H2+NO=N20+NH2 70. N2H2+NH=NNH+NH2 71. N2H2+NH2=NH3+NNH 72. 2NH2= N2H2+ H2 73. NH2+02=HNO+OH 74. CO+O+M=C02+M 75. CO+OH=C02+H 76. CO+02=C02+0 77. H02+CO=C02+0H 78.2H+C02=H2+C02 79. NCO+H=NH+CO 80. NCO+O=NO+CO 81. NCO+N=N2+CO 82. NCO+OH=NO+CO+H 83. NCO+M=N+CO+M 84. NCO+NO=N20+CO 85. NCO+H2=HNCO+H 86. HNCO+M=NH+CO+M 87. H+HNCO=NH2+CO 88. NH+HNCO=NH2+NCO 89. NH2+HNCO=NH3+NCO 90.0H+HNCO=NCO+H20 91. O+HNCO=NH+C02 92. CON2H4=NH3+HNCO

6. The process ofclaim 1 wherein the baseline nitro- 40 gen oxides concentration at the point ofintroduction is determined by direct measurement.

7. The process of claim 1 wherein said treatment agent is introduced into the effiuent at a concentration and rate calculated to reduce the effiuent nitrogen ox- 45

50

55

60

65

## UNITED STATES PATENTAND TRADEMARKOFFICE 

# CERTIFICATE OF CORRECTION

PATENTNO. :5,229,090
DATED :July 20, 1993 INVENTOR(S) :John 

### E. Hofmann, et al. Page 1 of 2

It is certified that error appears in the above-indentified patent and thatsaid letters Patent is hereby corrected as shown below: 

### At column 1, line 29, insert

-- -- immediately after "processes".
At column 1, line 37, insert

-- -- immediately after "high".

At column 1, line 39, insert --.-- immediately after "boiler".
At column 1, line 47, delete "." immediately after "P".
At column 1, line 

## 49, insert --.-- immediately after "avoided".
At column 1, line 62, delete "." immediately after "P".
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## ".11 immediately after "P".
At column 2, line 22, insert --.-- immediately after "monoxide ll

• 

### At column 3, line 14, insert --.-- immediately after 

## "(SNCR)II.

At column 

## 3, line 39, 

# "wo 89/10182" should read --WO 89/10182--.
At column 3, line 46, lI

### entitaled ll 

### should read --entitled--.
At column 4, line 16, insert --.-- immediately after "produced".
At column 4, line 49, insert --.-- immediately after "(CO)".
At column 4, line 61, insert --.-- immediately after "place".
At column 5, line 44, insert --.-- immediately after "tration".
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• 

### At column 6, line 4, insert --.-- immediately after "artisan".
At column 6, line 19, insert

-- -- immediately after "burned".
At column 6, line 29, insert

-- -- immediately after 

## IIreaction".
At column 8, line 40, delete "." immediately after "C".
At column 8, line 40, delete "." immediately after "P".
At column 8, line 62, delete 

## ".11 immediately after "P".

## UNITED STATES PATENTANDTRADEMARK OFFICE 

# CERTIFICATE OF CORRECTION

PATENT NO.

DATED INVENTOR(S} : 

## 5,229,090 July 20, 1993
John E. Hofmann, et al. Page 2 of 2

It is certified that errorappears in theabove-indentified patent and that saidLetters Patent is hereby corrected as shown below:



### At column 8, 

### line 

## 66, insert


# --

# ·-- immediately after "above"

. At column 

## 9, line 

## 1, delete II II 

### immediately after 

## "F·"

. 

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# " " immediately after 

## "F".



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### immediately after 

## llPll.



# · At column 

## 9, line 

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# --

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### line 

## 61, insert 

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### line 

## 66, delete 

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### immediately after 

## "P".



# . At column**~o,** 

### line 

## 12, delete 

# " " immediately after 

## "F".



# · At column 

## 10, line 

## 38, delete 

## " 

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## "Pll.



# · At column 

## 11, line 

## 8 , delete 

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### immediately after 

## "F".



# . At column 

## 11, line 

## 38, 

## "V" should read 

## --P--.
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## 14, line 

## 24, delete 

## " 

# " immediately after 

## "F".



# · At column 

## 14, line 

## 37, delete 

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# ·
SignedandSealedthis

Fifteenth DayofMarch, 1994

Attest:

Attesting Officer BRUCE LEHMAN

Commissioner ofPatents and Trademarks