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 of nitrogen oxides in the

effiuent from the combustion of a carbonaceous fuel is

presented. This system utilizes means for determining

the lowest achievable nitrogen oxides concentration

with respect to a treatment agent to be introduced into

the effiuent; means 'for determining whether the baseline nitrogen oxides concentration at the point of introduction 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 of NO by NH3 in Lean Burnt

Gas," Combustion and Flame 37: 245-250 (1980).

Sun, Hofmann, and Pachaly, "Post Combination NOx

Reduction With Urea: Theory and Practice," 7th Annual International Pittsburgh Coal Conference, Sep.

10-14, 1990.

Primary Examiner-Gregory A. Heller

Attorney, Agent, or Firm-St. Onge Steward Johnston &

Reens

[54] PROCESS FOR NITROGEN OXIDES

REDUCfION TO LOWEST ACHIEVABLE

LEVEL

[75] Inventors: John E. Hofmann, Naperville;

William H. Sun, Aurora, both of Ill.

[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 of Search 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 of 3 5,229,090

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DISCLOSURE OF INVENTION

BRIEF DESCRIPTION OF THE DRAWINGS

The objects of this invention will be described and

the present invention better understood and its advantage more apparent in view of the following detailed

description, especially when read with reference to the

appended drawings, wherein:

FIG. 1 is a graphical illustration of the 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 of these patents teaches the use of certain 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, which permit introduction at higher temperatures.

Other processes which improve upon the basic Arand

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 of other 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 of the 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 of nitrogen oxides.

Another object of the 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 of the present invention to determine the amount of nitrogen 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 of the process of the 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 of nitrogen 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 NOx reduction research will continue to be

pursued.

In the past, most processes for reducing nitrogen

oxides levels in oxygen rich effluents have concentrated 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.

in U.S. Pat. No. 4,208,386 which details the reduction of

NOx in combustion effluents by introducing urea at 65

temperatures between BOO· F. and 2000· F. The Arand

et al. process has been significantly improved by disclosures such as U.S Pat. Nos. 4,751,065, 4,719,092, and

The present invention relates to a system for reducing

nitrogen oxides (NOx , where x is a positive integer,

generally I of 2) 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.

Nitrogen oxides are troublesome pollutants which are

found in the combustion effluent streams of boilers and 15

comprise a major irritant in smog. It is further believed

that nitrogen oxides can undergo a process in the troposhere known as photo-chemical smog formation

through a series of reactions in the presence of sunlight

and hydrocarbons. Moreover, nitrogen oxides comprise 20

a significant contributor to acid rain and have been

implicated in the depletion of the ozone layer of the

stratosphere. Nitrogen oxides may also be a contributor

to the undesirable warming ofthe atmosphere known 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 "high temperature" applications, such as in situations

where the temperature of the 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 NOx when the temperatures are so high Due to boiler geometry and other

considerations, it is often possible to introduce treatment agents only in selected locations in a boiler Ifsuch

locations are at temperatures where commonly_used 40

nitrogen oxides processes were considered infeasible, it

was believed that significant NOx reductions could not

be achieved.

What is desired, therefore, is a system whereby nitrogen oxides reductions can be achieved even in situations 45

where they were thought infeasible, such as at temperatures above 2100· F., as well as a system whereby the

introduction of NOx reducing 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

such as milk or skimmed milk, amino acids, proteins and

5 monoethanolamine and various other compounds

which are disclosed as being effective at the reduction

of nitrogen oxides in an effiuent. Also contemplated are

mixtures of urea 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 NOx

reducing treatment agent is effective in accordance

with a nitrogen oxides reduction versus effiuent temperature curve, which is typically unique to the combustor

in which the effiuent is produced Each such curve has

a characteristic shape, with a left side which denotes the

effiuent temperatures where increasing temperature

corresponds to increasing NOx reductions, and a right

side which denotes the effiuent temperatures where

increasing temperature corresponds to decreasing NOx

reductions. Each nitrogen oxides reduction 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 of NOx reduction realized.

The curve plateau is that portion of the curve where

the highest 20% of nitrogen oxides reduction values are

found, when compared with values at other portions of

the curve, especially the highest 10% of NOx reduction

values. The curve plateau represents the effiuent temperature range in which the achievable NOx 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

curve plateau (i.e., on the left side ofthe NOx reduction

versus effiuent treatment curve), achievable NOxreductions increase with increasing effiuent temperature at

the point of introduction of the treatment agent. Contrariwise, at temperatures on the right side ofthe curve,

above the temperature range within the curve plateau,

achievable NOx reductions decrease with increasing

temperature at the point of introduction.

It has been found that any introduction further to the

right than a previous introduction (i.e., at a higher effiuent 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 particular treatment

agent is the result ofthe reaction kinetics ofthe nitrogen

oxides reducing reaction in the effiuent. It is believed

that in most cases there are two competing reaction

paths taking place In one of these, the treatment agent

combines with hydroxyl radicals (OR) in the effiuent to

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. 2 is a graphical illustration of the effect of baseline nitrogen oxides levels on the nitrogen oxides reduction versus effiuent temperature curve; and

FIG. 3 is a graphical illustration of the results of

Example I.

The introduction of nitrogenous compositions, by

which is meant compositions having at least one compo- 10

nent containing nitrogen as an element thereof, for NOx

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 urea such as disclosed by Arand

et aI. in either of U.S. Pat. Nos. 4,208,386 and 4,325,924,

the disclosures of each of which are incorporated herein

by reference. 20

Additional appropriate nitrogenous treatment agents

and methods known as being effective for the reduction

of nitrogen oxides include those disclosed by International Patent Application entitled "Reduction of Nitrogen- and Carbon-Based Pollutants Through the Use of 25

Urea Solutions", having Publication No. WO 87/02025,

filed in the name of Bowers 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 of Epperly, 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

Oxides Reduction", having Ser. No. 07/395,810, filed in

the names of Epperly 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 "Process for the In-Line Hydrolysis

ofUrea", having Ser. No. 07/561,154, filed in the names

of von Harpe and Pachaly on Aug. 1, 1990, the disclosures of each of which are incorporated herein by reference. 55

These patents and applications contemplate the use of

treatment agents which comprise urea (or one or more

of its hydrolysis products such as ammonium carbamate, ammonium carbonate, ammonium bicarbonate,

ammonia and mixtures or complexes thereof) or ammo- 60

nia (or compounds which produce ammonia as a byproduct such as ammonium salts like ammonium formate and ammonium oxalate), as well as other compositions such as hexamethylenetetramine (HMTA), oxygenated hydrocarbons such as ethylene glycol, ammo- 65

nium salts of organic 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 similar reaction pathways corresponding to

pathways I and II, as known to the skilled artisan Be5 sides urea, this also applies, inter alia, to ammonia

At the left side of nitrogen oxides reduction versus (NH3), commonly employed for nitrogen oxides reduceffluent temperature curve, both pathways I and II are tions. The lowest achievable nitrogen oxides concentrarelatively slow due to the low temperature of the efflu- tion varies with the particular treatment agent to be

ent as well as the low concentration of hydroxyl radi- introduced as well as effluent conditions such as temcals at such low temperatures. Thus, the level of nitro- 10 perature, baseline concentration ofNOx , and concentragen oxides reduction is not very great and byproduct tion of CO, since each combination of treatment agent

(e.g. NH3 and CO emissions are high. Because the over- and effluent conditions has specific reaction kinetics

all rate of pathway II is faster than that of pathway I at associated therewith.

these temperatures, the NOx concentration will be re- Ideally, it would be desirable to determine the critical

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 of hydroxyl radicals), oxygen level,

increases while pathway I still lags behind. Because of baseline NOx level, and the ratio of carbon to hydrogen

this, the nitrogen oxides concentration is reduced to a in the fuel being burned Although the critical NOxlevel

greater extent but byproduct emissions remain signifi- 20 can be measured experimentally, the task of evaluating

cant because residence time is generally not adequate the effects of all the parameters is time consuming and

for the reactions to fully utilize injected treatment difficult because of limitations with experimental comagents. If increased residence time is available, both bustors. Instead, a computer model with a theoretical

increased NOx reductions and lower byproduct emis- kinetic mechanism describing the reduction of nitrogen

sions will be observed. 25 oxides by the desired treatment agent is advantageously

If the effluent temperature at the point of introduc- utilized.

tion is within the "plateau" region, the rate of pathway For instance, nitrogen oxides reduction by urea is

II is increased significantly and the rate of pathway of I believed to proceed through complex chemical reacis negligible. Because of this, emissions of byproducts 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 of comtion is at its maximum with resultant significant NOx 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 of nitrogen containing compounds including

pathway I becomes more significant and both pathways NOx (Miller, J. A., Bowman, C. T., "Mechanism and

I and II begin competing for the reagents available. 35 Modeling of Nitrogen Chemistry in Combustion", Fall ,

Since the rates of both pathways I and II are ex- Meeting of the 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 of the rates of pathways I and corporated in the current model:

II, provided that sufficient reagents are present. 40 1. H2+02=20H

Throughout the NOxreduction versus effluent tempera- 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 NOx concen- 4.0+H2=OH+H

tration This concentration is also referred to as critical 5. H+02+M=H02+M

NOx , 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 of NOxpresent in the effluent is at 9.20H=O+H20

or below this lowest achievable nitrogen oxides concen- 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 other words, unless the baseline NOxlevel, that 55 16. H+H02=H2+02

is the nitrogen oxides concentration in the effluent prior 17. 2H02=H202+02

to any treatment which is to be effected, is above the 18. H202+M=20H+M

critical NOx 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

tration increases sharply with temperature, especially at 23. NH+NO=N20+H

effluent temperatures above about 1900· F. When this 24. N20+H=N2+0H

"critical" NOxlevel is higher than baseline (or existing) 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 of nitrogen 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

with this type offlow are described by a set of nonlinear

IO ordinary differential equations. The number of equations depends on the number ofspecies being modelIed,

and the equations are interrelated through rates of the

reactions shown above. The resulting set of differential

equations are integrated simultaneously using a numeri15 cal integrator (Hindmarsh, A. C., "ODEPACK, A Systematized Collection of ODE Solvers", Scientific Computing, R. S. Stepleman et al. (eds ), Vol I of IMACS

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 of pilot scale combustors (Sun, W. H., Hofmann, J.

E., Pachaly, R., "Post Combustion NO...Reduction with

Urea: Theory and Practice", Presented at the Seventh

Annual International Pittsburgh Coal Conference, Sep30 tember, 1990). This demonstrated that the 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. The baseline NO... concentrations affects NO... reductions in two ways: (I) the

45 critical nitrogen oxides concentrations are higher with

higher baseline 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 of the NO... reduction curve for three baseline

NO... levels, as shown in FIG. 2, clearly shows the widening of the 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 of any nitrogenous treatment agent, even at effiuent temperatures of 1500· F. and below, the lowest achievable nitrogen oxides concentration at these lower temperatures is

relatively low, Le., on the order of about 5 parts per

million (ppm), depending on the other factors discussed

above Even these low levels can be 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 of gas-phase subroutines developed at the Sandia National Laboratories
0.8

2

4

8

12

20

2

4

8

12

NSR

2.5 2.0 1.5

10

TABLE I-continued

It can be seen that NOx reductions increased when

higher NOx baseline levels were present, and that NSR

Reduction (ppm)

NSR Needed for Maximum

Reduction

The amount of urea required to achieve the maximum

reduction decreases with the achievable reduction and

additional urea does not give higher reductions. In fact,

over-treatment can actually lead to an increase in the

level of NOx for those cases where injection temperatures are higher than 2000· F., and thus lessens the

achievable reduction.

. Generally, when the difference between the baseline

NOx concentration and the critical nitrogen oxides ~oncentration is expressed as a percentage ofthe total mtrogen oxides in the effiuent (Le., baseline, when the difference is a positive number), the NSR of the 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 system of the present invention can be combined

into a multi-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 of Epperly, Peter-Ho.blyn,

Shulof, Jr., and Sullivan on Aug. 12, 1988, the dISclosures of each ofwhich are incorporated herein by reference.

In a first stage of such a process, NOx is reduced at

higher temperatures, such as above about 1900· F. using

the inventive system as described above. In a second

stage, the NOxis reduced at lower temperatures acc~rding to the process disclosed in the patents and applIcations cited above. By doing so, the advantages of the

reduction of nitrogen 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 NOx Concentration 3 18 72 170

(ppm)

MaxiInum Achievable 197 182 128 30

9

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

1900· F. where the critical NOx 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 higher where the lowest

achievable nitrogen oxides concentration can be as high

as about 100 to 200 ppm or higher, the critical NOx level

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 at the location desired for introduction with the 15

desired treatment agent by suitable means as discussed

above The actual or baseline nitrogen 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 of the boiler system.

When this actual or baseline NOx level is higher than

the lowest achievable nitrogen oxides concentration,

NOx 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 of NOx at the existing effiuent conditions

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. If greater 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 of urea needed to achieve

the maximum reduction is determined by the difference

between the baseline nitrogen oxides level and the criti- 45 Example I

cal NOx concentration. For a constant baseline NOx An ethylene cracker is operated such that the effiuent

level, increasing the effective temperature increases the temperature at the point of the nitr?gen.oxides re~uccritical nitrogen oxides concentration and thus reduces tion treatment is 1922· F. The baselIne nitrogen OXides

the maximum achievable reduction at each temperature. level in the effiuent is varied by varying the heat input.

The urea flowrate needed in order to close this "gap" 50 Nitrogen oxides are reduced by introducing a l~%

between baseline and critical NOx levels, 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 of the concentration of theoreti- 55 ..;;T;.;.A.;;B;;.L~E~2;;...---------

cally available reducing radicals such as NH2 or NCO Baseline Final

to the concentration of nitrogen oxides in the effiuent. NOx (ppm) NOx (ppm)

Alternatively, the molar ratio ofthe treatment agent to 90 ;~

the nitrogen oxides concentration can be used in place 90 46

of NSR when the chemistry of reduction 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 of about 0.1 to about 5

times the amount by which the baseline nitrogen oxides

concentration exceeds the lowest achievable nitrogen

5 oxides concentrati<m when expressed as a percent ofthe

total effiuent nitrogen oxides concentration.

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

S. The process of claim 2 wherein said lowest achievable 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 NOx level is achieved.

Example II

A boiler fired with pulverized coal emits 250 ppm

nitrogen oxides. A 10% solution of urea is injected at

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

the critical NOx level of 100 ppm is reached with negligible emission of byproducts carbon monoxide, ammo- 10

nia, or nitrous oxide.

To reduce NOx emissions further, a 10% solution of

urea is injected at a location where the effiuent temperature is 1850· F. The urea injection rate is 50 ppm to

achieve an additional reduction of 25 ppm, to 75 ppm, 15

because chemical utilization is high at this temperature.

Byproduct emissions are low because of the low treatment rate.

Accordingly, employment of a process of this 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 of illustration only and are not to be construed as limiting the invention.

The above description is for the purpose of teaching 25

the person of ordinary 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 worker upon 30

reading the description. It is intended, however, that all .

such obvious modifications and variations be inclUded

within the scope of the present invention which is defined by the following claims.

We claim: 35

1. A process for reducing nitrogen oxides in the effiuent from the combustion of a 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 of ammonia and ammonium bicarbonate, one or more of the hydrolysis products of urea or

mixtures or complexes thereof, compounds which produce ammonia as a byproduct, ammonium formate,

ammonium oxalate, hexamethylenetetramine, ammo- 45

nium salts of organic acids, 5- or 6- membered heterocyclic hydrocarbons having at least one cyclic nitrogen,

hydroxy amino hydrocarbons, amino acids, proteins,

monoethanolamine, or mixtures thereof, the process

comprising: 50

(a) determining the lowest achievable nitrogen oxides

concentration for said treatment agent at the conditions existing in the effiuent at the point of introduction;

(b) determining the baseline nitrogen oxides concen- 55

tration at the point of introduction;

(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 of hydroxyl 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 of a 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 of ammonia and ammonium bicarbonate, one or more of the hydrolysis products of

urea or mixtures or complexes thereof, compounds which produce ammonia as byproduct,

ammonium formate, ammonium oxalate, hexamethylenetetramine, ammonium salts of organic

acids, 5- or 6-membered heterocyclic hydrocarbons having at least one cyclic nitrogen, hydroxy amino hydrocarbons, amino acids, proteins, monoethanolamine, or mixtures thereof at

the conditions existing in the effiuent at the point

ofintroduction, where the effiuentis at a temperature of at least about 1900· F.;

ii. determining the baseline nitrogen oxides concentration at the point of introduction;

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 of claim 8 wherein said nitrogenous

treatment agent of said second stage comprises urea,

ammonia, cyanuric acid, ammonium carbamate, ammonium carbonate, mixtures of ammonia and ammonium

bicarbonate, ammonium formate, or ammonium oxalate.

• • • • •

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 of claim 1 wherein the baseline nitro- 40

gen oxides concentration at the point of introduction 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 PATENT AND TRADEMARK OFFICE

CERTIFICATE OF CORRECTION

PATENT NO. :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 that said 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".

At column 1, line 66, delete ".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 entitaledll 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".

At column 5, line 53, insert --.-- immediately after "furtherll •

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 II reaction".

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 PATENT AND TRADEMARK 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 error appears 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·" . · At column 9, line 2, delete " " immediately after "F". · At column 9, line 6, delete " II immediately after llPll. · At column 9, line 17, insert --·-- immediately after "above".

At column 9, line 61, insert --·

-- immediately after "defined".

At column 9, line 66, delete " II . immediately after "P".

At column ~o, line 12, delete " " immediately after "F". · At column 10, line 38, delete " " immediately after "Pll. · At column 11, line 8 , delete " II . immediately after "F".

At column 11, line 38, "V" should read --P--.

At column 14, line 24, delete " " immediately after "F". · At column 14, line 37, delete " " immediately after liP". ·

Signed and Sealed this

Fifteenth Day ofMarch, 1994

Attest:

Attesting Officer

BRUCE LEHMAN

Commissioner of Patents and Trademarks