CA1244230A - Non-catalytic method for reducing the concentration of no in combustion effluents - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is a process for non-catalytically removing NO from combustion effluent streams at tem-peratures from about 1300°K to 1600°K by injecting ammonia into a combustion effluent stream wherein the amount of ammonia injected and its point of injection is determined by the solution of the set of simul-taneous equations derived from the kinetic model of Table I hereof.

Description

2 The present invention relates to a non-cata-

3 lytic method for reducing the concentration of WO in

4 combustion effluents by the injection of ammonia. More particularly, the amount of ammonia and the point of 6 injection is dekermined by the solution of the set of 7 simultaneous equations derived from the kinetic model 8 disclosed hereinO Particulax benefits of the present 9 invention occur when ammonia is injected into a 10 cooling zone.

. . _. , 12 Combustion effluents and waste products from 13 various installations are a major source of air pollu-14 tion when discharged into the atmosphere. One particu-15 larly troublesome pollutant found in many combustion 16 e~fluent streams is NO2, a major irritant in smog.
17 Furthermore, it is believed that NO2 undergoes a 18 series of reactions known as photo-chemical smog for-19 mation, in the presence of su~llight and hydrocarbons.
20 The major source o~ NO2 is NO whirh to a large degree 21 is generated at such stationary installations as gas 22 and oil-fired steam boilers for electric power plants, 23 process heaters, incinerators, coal fired utility 24 boilerst glass furnaces, cement kilns, and oil field 25 steam generators.

26 Various methods have been dPveloped for 27 reducing the concentration of nitrogen oxides in com-28 ~ustion ef1uents. One such method which was developed 29 was a non-catalytic thermal deNOx method disclosed in lZ~30 2 U.S. Patent No. 3,300,554 to I.yon. The process dis-3 closed in that patent teaches the reduction o N0 to 4 N2 by injecting ammonia.intojthe combustion effluent stream at a temperature fr~m about 975K to about 6 1375K in a cavity which isr.substantially isothermal.
7 That is, wherein the temperature of the gases passing 8 through the cavity are cooling at a rate of less than 9 about S0C. Since the issuance of U.S. 3,900~554, there has been a proliferation of patents and publi-ll cations rela~ing to the injection o ammonia into 12 combustion effluent streams ~or reducing the concen-13 tration of N0. It is the general consensus of the 14 literature that ammonia injection at temperatures greater than about 1375g would result in the genera-16 ~ion of NO from ammonia and consequently conventional 17 selective non-catalytic N0x reduct;on processes ~re 18 practiced by injecting ammonia at temperatures lower l9 than ~bout 1375K. Because of this te~perature 20 limitation, it is difficult and sometimes not possible 21 to apply conventional non-catalytic NOX reduction 22 processes. This is because during the operating cycle 23 of some boilers and heatersr the ~emperature range 24 required by conventional processes corresponds to positions in the boiler or heater where it is me~ha-26 nically inconvenien~ to inject ammonia. In at least 27 some of these instances, this inconvenience could be 28 overcome if the combustion effluent could be contacted 29 with ammonia at temperatures above about 1375R and still o~tain satisfactory reductions in the concen-31 tration of NO. Furthermore, at temperatures between 32 about 1300~ and 1375R prior art methods were not 33 always adequate to reduce the NO content o combustion 34 efluent s~reams to environmentally desirable levels whe{eas the ?resent invention provides s~ch a ~ethod.

23(~

2 As is well-known, combustion is effected in 3 stationary combustion equipment such as boilers, fur-4 naces and incinerators in a section of the equipment

5 commonly referred to as a firebox. Generally, this is

6 accomplished by igniting a suitable fuel, in the pres-

7 ence of air, with one or more burners. Materials

8 other than conventional fuels can, however, be com-

9 busted in the firebox portions of the equipment which

10 is generally the case when combustion is effected in

11 an incinerator. In any event, the principal combus-

12 tion products are carbon dioxide and steam and those

13 products, along with the other combustion products

14 such as carbon monoxide and the various oxides of

15 nitrogen and sulfur, combined with any excess oxygen

16 and unconverted nitrogen to form what is referred to

17 herein as a co~bustion effluent. The combustion efflu-

18 ent will also contain a~out 0.1 to 15 volume percent

19 oxygen, preferably about 1 to 3 volume percent~

The temperature o the combustion effluent 21 is, then, a maximum at or near the point of combustion 22 an~ decreases axially (along the flow path) and radi-23 ally ~outwardly) as the effluent moves along its flow 24 path rom the point of combustion until it is, ulti~
25 ma~ely, emitted to the atmosphere or otherwise loses ~6 its identity as a combustion effluent. As previously 27 mentioned, the combustion effIuents, as they travel 28 through the combustion apparatus cool in stages. That 29 is, rapid cooling will occur when the combustion 30 effluent is in contact with heat exchange equipment, 31 such ~as heat transfer ~ubes. The prior art teaches 32 that NOx reduction was only possibla in the cavities 33 between then cooling zones as opposed to in the cool-lfh~ 30 1 ing ~ones themselves. By practice of the present 2 invention NOX can now be achiev~d at high temperatures 3 in or immediately before a cooling zoneO

4 The amount of ammonia used herein ranges 5 from about 0.5 to 10 moles, Preferably 1 to 3 moles of 6 ammonia per mole of NO to be removed.

7 The reaction may be carried out at pressures 8 from 0~1 atmospheres to 100 atmospheres. The veloci-9 ties of the combustion effluents as well as the mixing 10 of the ammonia in the post-combustion zone are regu-11 lated so that there is an effective residence time, in 12 a temperature range of about 975K to 1600~, to 13 enable the ammonia to remove N0x from the combustion 14 effluent stream. The residence time will range from lS about 0.001 to 10 seconds.

16 Although at temperatures above about 1375K
17 conventional non-catalytic deNOx processes are gener-18 ally inoperative, the inventors hereof have identified 19 a critical set of conditions whereby NOX may DOW be

20 practiced on a wider variety of combustion installa-

21 tions than heretofore ~hought possibleO In addi~ion,

22 practice of the present invention enables a more

23 effective non-catalytic deN0x operation at tempera-

24 tures above about 975K with particular advantages of

25 temperatures greater than about 1300K.

26 Because it is difficult to accurately simu-

27 late, on a laboratory scale, the temperature time

28 history of combustion effluents as they pass through a

29 tube bank in a boiler/heater, it is necessary to geo-

30 erate examples by means other than laboratory experi-

31 ments. Complex chemical reactions occur by a ~eries

32 of elementary reaction steps and if one knows the rate 23~

1 constants for such steps, a th~oretical kinetic 2 mechanism can be developed and verified through com-3 parison with experimental dataO An extensive block of 4 kinetic data was developed herein by use of apparatus 5 similar to the apparatus taught in U.S. 3~900,554 and 6 used to determine which elementary reactions would 7 likely be of significance during the reduction of NO
8 by NH3. For many of the rsactions, the rate constants 9 were well-lcnown accurately measured constants of 10 nature whereas for the remaining reactions the rate 11 constants were not accurately known and accordingly 12 were taken as adjustable parameters. That is, values 13 for the unknown rate constants were assumed, the reac-14 tion kinetics to be expected from these rate constants 15 were calculated and compared with the observed kine-16 tics~ Based on this comparison a new set of rate 17 constants was assu~ed, etc., until satisfactory agree-18 ment between calculation and experimentation were 19 finally obtained. As a result, the kinetic model 20 hereof and respective rate constants were developed by 21 the inventors hereof for accurately predicting the 22 conditions for the practice of the present invention.

23 In the practice of the present invention the 24 ef~luent stream to be treated is measured to determine the content of NO, 2, and H2O. These initial 26 conditions, as well as cooling rate measurements of 27 appropriate cooling ~oln~s~having a high temperature in 28 ~he range of about ~~ and 1600K are used in 29 conjunctin with the kinetic model hPreof with appro~
priate software to determine the amount of ammonia and 31 an injection point which will give NO reduction~
32 ~ppropriate software suitable for use herein would be

33 any computer program designed for numerical integra-

34 tion of chemical rate expressions. A non-limiting example of such software is CHEMKIN;

1 General-Purpose, Problem-Independent, Transportable, 2 Fortran Chemical Kinetics Code Package; R.J. Kee, J~A~
3 Miller, and T.H. JeEferson, an unlimited released 4 Sandia National Laboratory Report SAND80-8003 tl980~.
This report is also availab~e through the National 6 Technical Information Service, U.S. Department of 7 Commerce.

8 The following example is offered, not as an g illustration of -the subject invention but to demonstrate the validity of the kinetic model employed 11 herein. The model was used to calculate the NO
12 reduction to be expected for a 235 megawatt utility 13 boiler of the following chaxacteristics:

14 Flue Gas Flow Rate 2,000,000 lb/hr at 3-4~ 2 dry Flue Gas Temperature 880-1040C
16 NO Conc. 190-220 ppm at 3-4% 2 dry 17 Figure 1 (diamonds) contains actual performance data 18 on the above boiler at full load with a best fit curve 19 through the data. The circles in Figure 1 represent paper data generated by use of the kinetic model 21 hereof~ The Figure illustrates the surprisingly g~od 22 agreement of model data vs. actual data.

23 The good agreement between predicted and ob-24 served NO reduction illustrates that the kinetic model is reliable for calculating NO reduction~

26 The advantages of the kinetic comput~r model 27 hereof are substantial in that it permits one skilled 28 in the art to readily determine, by calculation, the 29 embodiment of the present invention which will yield optimum results for his particular circumstances. In 31 general, however, it may be said that the present lrh44!Z3~

1 In a~dition, co~lventional non-catalytic 2 thermal deNOx processes are further limited because 3 they teach the injection o~ ammonia into a constant 4 temperature, or isothermal, zone. This is limiting because in a conventional boiler or heater, operating 6 at constant load, combustion effluents typically leave 7 tne b~lrner flames at temperatures greater than about 8 1875~. As they travel through the boiler or heater 9 they cool in stages ~ not contin~ally. This staged cooling occurs because of the manner of heat removal 11 from the combustion effluents. Heat is usually removed 12 by heat transfer tubes which are arranged in banks 13 with substantial cavities between the banks. Conse-14 quently, combustion effluents are xapidly cooled while they flow through a tube bank, undergo very little 16 cool,ng as they pass through a cavity, rapidly cool 17 again ~hile passing through another tube bank, etc.
U.S. Patent No. 4,115,515 to Tenner et al, teaches that the injection apparatus should be installed in a cavity in such a manner that the ammonia contacts the combustion effluent stream as the effluents come into the cavity. Such a process has the effect that the reaction time, that is the time at constant temperature during which ammonia could reduce NOx, is the total time the combustion effluents spend passing through a cavity.
Unfortunately, in some boilers and heaters, this reaction time - though adequate to provide a useful NOx reduction - is not sufficient to provide as great a reduction in NOx concentration as may be enyironmentally desirable.
Therefore, there is still a need in the art for methods of practicing non-catalytic NOx reduction processes which will overcome, or substantially de-crease, the limitations o conventional practices.

4~i,23~

SUMMARY OF THE I.NVENTION

In accordance with the present invention there is provided a process for noncatalyticaLly reducin~ the concentration of combustion effluents containing NO and at least oil volume percent oxygen at temperatures from about 1300 K to 1600 K by injecting ammonia into the combustion effluent in a cooling zone wherein the combustion effluent is cooling at a rate of at least about 250 K psr second and wharein an effective amount of ammonia is used 90 that the concentratlon of NO is reduced.
In one preferred embodlmsnt of the present invention ammonia is injected at a point upstream from a desirable cooling zone and such an amount so that when the combustion effluent reaches the cooling zone it contains at least 0.4 moles of ammonla per mole of NO.

RRIEF DESCRIPTION OF THE FIGURES

The sole figure hereof shows actual performance data versus predicted performance data generated by use of the kinetic model disclosed hereln, for a 235 megawatt utility boiler.

h30 invention i9 an improved method Oe noncatalytic reduction of N0 with N~13, the improvement of tha present in~entlon relating to the tamperature at whlch the NH3 is contacted with the N0 containing combustion effluents. ~his contacting is done at temperat.ures from about 1300 K to about 1600 K at a point where the combustion effluen~s are cooling at a rata of at least 250 K/sec or at a point where enough ammoni.a is still present such that the volume ratio of ammonia to Nx is in the range of about 0.4 to 10 when it enters a cooling zone having a cooling rate of at least sbout 250 K/sec. Generally, the ammonia can be in~ected up to 0.04 seconds upstream from a cooling zone, preferably 0.02 seconds, and more preferably 0.01 seconds. The higher portion of the 1300 R to 1600 K temperature range relateY to higher initial N0 concentrations, lower 2 content of the combustion effluents, highar coollng rates and shorter delay times prior to cooling. To a somewhat lesser degree, the upper portion of the temperature range is also associated with higher H~0 content.

~,.'..~'~

.. .

2~

.

_ 3Rate constant K = ATn exp (-2/(1.98)T) 4 REACTION A n E
1. NH3+0=NH2+H2.246E+14 0.017071.
6 2. NH3-~0=NH2~0H.150E*~3 0,06040.
7 3. NH3~0H=NH2+H20 .326E+13 0.0 2120.
8 4. HNO+M=NO+H+M.186E+17 0.048680.
9 5. HNO~OH=NO+H20 .360E+14 0.0 O.
10 6. NH2~HNO=NH3~NO .175E~15 0.0 1000.
11 7. NH2~NO=NNH+OH .610E~20-2.46 1866.
12 8. NH2+02-HNO+OH.510E~14 0.030000.
13 9~ NNH~NH2=N2+N~3 .lOOE+14 0.0 O.
14 10. NH2+0-NH+OH.170E+14 0.0 1000 15 11. NH2+0H=NH~H20 .549E+11 ~.68 129Q.
16 12. NH2+H=NH+H2~500E+12 O~S2000.
17 13. NHfO2=NHO+0.300E+14 0.03400.
13 14. H~+OH=H20~H.220E+1~ 0.05150.
19 15. H+O~=OH~O.220E+15 0.016800 20 16. O~H2=OH+H.180E+11 1.08900.
21 17. H~H02=OH+OH.250E+15 0.01900, 22 18. O+H02=02+0H.~80E~15 0.01000.
23 19. OH~H~2=H2o+o2 .500E+14 0.0 1000.
24 20. OH~OH=O+H20.630E+13 0.01090.
2S 21. H02+NO=NO~+OH .343E+13 0.0 ~260.
26 22. H~N02-NO+OH.350E~15 O.Q1500.
27 23. 0-~N02=NO+02.lOOE~14 0.0 600.
28 2~. H~02+M=H02~M.150E~16 0~0 -995.
29 H~0/21**
30 Z5. NNH+M=N2~H~M.200E~15 0.030000.
31 26. N02~M=NO~O~M.llOE~17 0.066000.
32 27. NH3+M=NH2~H~M .480E+17 0.0 93929.
33 28. O+O+M=02+M.138E~19 -1.0 340.
34 29. NH2+NO=~2+H20 .910E~20-2.46 1866.

35 30. ~ OH=N2~H~0~3QOE~14 0.0 O.

36 31. NNH~0=~2+H~0.906E~12 0.0 O.

37 **i.e. A=21 x .15E+16 for H20 as "third body".
38Given this model, one having ordinary skill 39in the art can identify a corresponding set of simul-40taneous equations for solution.

3(31 1 Example of Use of_E~inetic Model 2 To illustrate the practice of the present 3 invention and its advantage over the prior art the 4 paper example below is presented.

A utility boiler is assumed with the follow-6 ing operating conditions, which operating conditions - 7 can be considered normal for such boilers: An excess 8 air level of 19~ of stoichiometric air while firing a 9 fuel oil of H to C mol ratio of 1.4. The boiler thus produces a flue gas containing 3.1% 2~ 12.3% CO, 11 10.6~ H2O and 74~N2. There will also be traces of NO, 12 the exact amount depending on the fuel's nitrogen 13 content and the manner in which the fuel is burned.
14 For this example the NO level will be assumed to be 250 vppm. Further, it is assumed there will be a trace 16 of the free radicals OH and O, the exact concentra-17 tions of these being determined by thermodynamic 18 equilibrium and therefore being a function of tempera-19 ture.

Co~bustion flue gas would exit the burners 21 at a very high temperature and cool as it passes 22 through the rest of the boiler. Typically, bollers 23 have a radiative section~ a large empty section 24 through which ~he flue gas passes, cooling by radia-tion as it travels, and a convection section, a sec-26 tion filled with banks o~ heat exchange tubes, the 27 flue gas cooliny by convection as it passes through : 2B these tube banks. As mentioned abover there are usu-29 ally cavities between the banks of tu~es. For purpos~s of this example l~t us assume that the boiler haQ one 31 cav i ty between tube b~lnk s whe r e i n th e t empe r a tu r e i ~

:

.

z3~

1 approximately constant and within the lOOO~K to 1500~K
2 range, the residence time o~ the flue gas within thi~
3 cavity being Ool seconds.

4 Now to illustrate the limitations of the prior art we will consider the application of the 6 prior art teachiny to the above case. According to the 7 prior art (U.S. 3,900,554 and 4,115,515) one would 8 inject NH3 as the flue ga~ enters the cavity. We 9 will assume that the amount of NH3 injection is 375 ppm which is well within the ranges taught by the 11 prior art~ Thu~ one has 0~1 seconds reaction ti~e in 12 the cavity for the NH3 to reduce the NO.

13 Table II below shows for cavity tempera~ures 14 from 1000K to 1500K the amounts of NO and NH3 cal-~ulated by the computer model to remain after ~.1 16 seconds reaction time. The prior ar~ teaches that at 17 temperatures substantially below about ll~S~ the 18 reduction of NO by NH3 is so slow as to be inoperable, 19 the calculations ~using the kinetic model hereof) agree with this teaching. The prior ar~ also teaches 21 that increasing the temperatuxe inc~eases the rate of 22 reaction but decreases the selecti~ity because a 23 greater proportion of the NH3 tends to oxidize to form 24 additional NO rather than to reduce the ~O, with the result that, while NO reduction is an operable process 26 at temperatures in thell45K to 1365K temperature 27 range, it is not operable at temperatures substanti-28 ally above 1365K. Indeed, for such excessive tempe-29 ratures the injec~ion of the NH3 hy prior art proce-sses may cause a net increase in NO. The calculated 31 results in Table II, especially the result at 1500K
32 are entirely consistent with such teachings.

- ~3 -2 Comparative Examples of ~O
3 Reduction at Isothermal Conditions 4 NO RemainingNH3Remaining ~ ~vppm) (vp~) ~ 1100 247 372 1~00 89 206 16 1500 2g5 0 17 Table II also illustrates the limitations of 18 the prior axt. The NO reduction varies with tempera-19 ture. For the specific conditions of this example the best reduction would occur near I250K, wherein only 21 about 54 vppm NO remained. Un~ortunately, this good 22 reduction would be accompanie~ by the emission to the 23 atmosphere of 122 vppm NH3. While N~3 emissions are 24 much less of an environmental concern than NO emis-sions~ they are ~till a concern.

26 Further, Table II shows that at 1300K ~O
27 may be reduced to 62 vppm with leftover NH3 of 50 28 vppm. Thus it was within the scope of the prior art to 29 minimize NH3 emissions by sacrificing some of the possible NO reduction.

31 Now to illustrate the practice of the pres-32 ent invention we assume the same boiler and conditions 33 as above. We also assume that the rate of cooling in a 34 tube bank upstream of the 125QK cavi~y is between 4000K/sec., and or ~50K/sec., these cooling rates ~ - -~2~30 1 covering the range ~f cooling ra~es normally used in commercial boilers. Further, we assume that the N}13 3 injection system is not located in the 1250K cavity 4 b~t at an upstream location where the flue gas temper-ature is either 1300K, 1350K, 1400K, or 1500K.
6 Under these assumptions, the NH3 would contact the NO
7 containing flue gas while it was cooling toward a 8 temperature of 1250K. There would be a reaction time 9 of however long it takes to cool to 1250K plu5 0.1 seconds at 1250K. Table III shows the results of such 11 calculations.

13 Initial 14 Temp. K - 1300 1350 1400 1500 vppm of = NO NH~ NO NH~ NO NH3 NO
16 Cooling ~ate 17 K/sec 184000 48 103 51 59 ~6 12 270 0 23 Comparison of Tables II and III reveals 24 that injecting NH3 into cooling flue gas in a tube bank at 13~0K with a 250Kfsec cooling rate would 26 provide more NO reduction than that achieved by iso-27 thermal injection at 1250R treduction to 39 vppm as 28 compared to 54 vppm~ while causing considerably less ~9 NH3 to be leftover (12 ppm as compared to 122 vppm).
In fact, injection at 13000K with any cooling rate 31 between ~50K/sec and 4000~/sec provide better 32 performance than isothermal injection.

33 In addition, one could inject NH3 at 1350K

34 and at relatively high cooling rates to obtain equiva-lent NO reduction to the prior art with appreciably ~2~3~

- 15 ~-1 less leftover NH3. Thus, in this instance the subject 2 invention provides an improvement over the prior art.
3 The fact that this is a valuable improvement becomes 4 clearer when one considers the trade off between NO
reduction and N~3 leftover. It was within the scope of 6 the prior art to improve the reduction of N0 by in 7 creasing the amount of NH3 injected into the 1ue gas, 8 this improvement being purchased at the expense of 9 having more NH3 leftover. The present invention adds more flexibility to this tradeoff and allows one to 11 achieve much better NO reduction for a given amount of 12 NH3 leftover. Table IV shows the results of calcula-13 tions similar to those in Table III but with the 14 amount of NH3 injected raised from 375 vppm to 750 vppm. Suppose, for the sake of an example comparing 16 the present invention with the prior art, that one had 17 a tube bank with a 500~K/sec cooling rate followed by 18 a cavity at 1250K and wanted the best NO reduction 19 possible with an NH3 leftover of 122 vppm or less. The prior art for this situation would only produce an NO
21 reduction down to 54 vppm. Table IV shows that if one 22 doubles the amount of NH3 injected, and injects at 23 1350K, one achieves reduction of NO down to 23 vppm, 24 a much better resultO Further, this much better result is obta;ned with an ~H3 leftover of only 38 vppm. As 26 can also be seen in Table IV, there are other combina~
27 tions of injection temperature and cooling rate which 28 also are an improvement over conventional techniques.

3iV

. .
2 Initial NH3 = 750 vppm 3 Initial 4 Temp K = 1300 1350 1400 1500 vppm of = NO NH~ N0 ~ NO ~ NO
6 Cooling Rate 7 K/sec 8~000 17 403 16 324 ~1 165 271 92000 15 381 15 ~51 36 43 300 101000 13 345 17 14~ 78 1 314 13 The above examples illustrate the advantage 14 of the present ;nvention over the prior art in retro-fitting the deNOx process to an existing installation.
16 That is, one takes the installation, for example a 17 boiler, as he finds it and as a consequence has no 18 control of the temperature in the cavities between 19 tube banks or the cooling rates within the tube banks.
Thus, in the prior art, a cavity is chosen for NH3 21 injection which is closest to optimum temperature. In 22 accordance with the present invention, one may now 23 install the NH3 injection system within a tube bank~
24 sinc the optimum temperature for NO reduction may very well occur within a tube bank.

26 While the rate of cooling in the tube bank 27 of an existing installation cannot be controlledt the 28 position of NH3 injection and the amount of NH3 29 injec~ed can be controlled. For example, it was illus-trated in the above examples that at a cooling rate of 31 4000K/sec there was one position for NH3 injection 32 and amount injected which gave optimum results. Of 33 course, for cooling rates betwee~ 4000K/sec and 34 250K/sec, there would be intermediate values which 3C~

1 would also give optimum re~sults. Thus, it is a pre-2 ferred embodiment of the present invention to adju~t 3 the position and the amount of NH3 injected to match 4 the cooling rate and thereby achieve optimum N0 reduc-tions.

6 Another limitation of the prior art which 7 was previously discussed was the upper temperature 8 limit of about 1375K. Under prior art practice, this 9 upper temperature limit was severe because, not only could NO not be reduced at temperatures above about 11 1375K, but additional N0 production usually resulted.

12 Table V below illustrates additional calcu-13 lations showing the extent to which the present inven-14 tion alleviates the problems associated with the heretofore upper temperature limit for ~H3 injection.
16 According to the prior art, an NH3 injection temperat-17 ure of 1500K would be inoperable. However, in 18 accordance with the present invention, such an injec-19 tion temperature is operable.

TABLE V
.
21 Initial NO = 250 vppm 22 Initial 23 Temperature, K 1500 1500 1500 160Q 1600 1600 24 Initial NH3, vppm 1500 3000 3000 lS00 3000 300U
25 Cooling Rate, 26 K/sec 10,000 4,000 10,000 lO,OOQ 4,000 lO,OOQ
27 Final N0 vppm 18 12 8 632 778 608 28 Final NH3 vppm 256 426 1658 0 0 0 29 The upper temperature limit at which NO
reduction can be achieved is a function of reaction 31 conditions. Table YI illustrates the effect of 32 changes in 2~ H20, and NO upon the exte~t of N0 Z3~

- 18 ~
1 production at very high temperatures. Combination.~ of 2 these extreme conditions could. allow NO reduction at 3 temperatures even above those shown.

. ~ 3~

1 c ~ o o o l ~0 ~ ~ ~
Z I o O ~ 0~
~ I o U~ O O
~ . .

~ I ~ ,, ,~"

~OD
o ~

~ ~ ~ ~ o ,~ ~ ~
~o o .
..

o C r~ 'I ~ ,~ .

~' O ~ O O ~D
4~ 0 ~ ~D O
o o I

o u ~ o o ,~ o o ~ ~ o o ~ o ~ .8 ~
r~Q) ~ g ~ ~ O ~ g U~
~ u~ o ~ o ~ dl ~ g ~

~1 ~ ~ ~ ~rs u: 1--a~ ~ o ~ ~ ~ ~ Ln w r~ o _I ~ ~

., 19 23~

1 While the above discussion relates to the 2 application of the present invention in retrofit situ-3 ations, it obviously may also be applied to the reduc-4 tion of N0 in new stationarX combustion equipment.
Suppose, for the purpose of discussion, that one is 6 building a new boiler of some kind and wishes to 7 achieve the best WO reduction possible within the 8 design constraints of that kind of boiler. Specii-g cally, we can a~s~sume that the boiler is intended to operate at very high flue gas velocity so that the ~1 time available for NO reduction is only 0.02 seconds~
12 Given this short reaction time the best possible NO
13 reduction consistent with acceptable NH3 leftover is 14 desired. To take an arbitrary but convenient value, we assume that the leftover NH3 must be less than 159 16 vppm~

17 Table VII shows calculations of what could 18 be achieved by the prior art in this situationO It is 19 evident that the best one can do within the limita-tions of the prior art is to operate near 1350K, this 21 giving one an NO reduction frorn 250 vppm to near 141 22 vppm. Of course this calculation is done assuming an 23 NH3 injection of 375 ppm and increasing the NH3 24 injection would improve NO reduc~ion. However it would also increase NH3 leftover and NH3 leftover is already 26 at the maximum permissible value. This reduction to 27 141 vppm i5 the best the prior art can achieve.

3~

2 Comparative Examples of NO
3 reduction at Isothermal Conditions 4 Initial conditions NO a 250 vppm~ NH3 = 375 vppm, H2O = 10.6~, 2 = 3.1%, OH and O equal their 6 equilibrium values a nd the halance of the flue ga~ is 7 inert. Pressure = 1.0 atmospheres, Reaction Time =
8 0.02 seconds.

10 Temp (K) Remaining,~ m ~ ~ n 11 1000 2~0 375 16 1250 16~ 227 22 The present invention would show an improve-23 ment over the prior art. In ~able VIII below, results 24 are given for calculations in which it was a5sumed 2S that 750 vppm ~H3 was injected at 1400K in a fIue gas 26 at various cooling rates witb a final temperature o~
27 1300K. For a cooling rate of 2000K~sec. Table~
28 shows that a r~duction of NO to 59 vppm (as compared 29 with 141 vppm for the prior art) may be achieved with only 93 vppm NH3 leftover tas compared with 159 vppm 31 for the prior art). Thus, the practicP of the present 32 invention could achieve both better NO reduction:as 33 well as less NH3 leftoverO

~4~a23~

2 Example of subject invention: Initial con-3 ditions NO = 250 vppm, NH = 75.n vppm, H2O ~ 10.6~ O =
4 3.1~, OH and O equal their eq~ilibrium values and the balance of the flue gas is inert~ Pressure = 1.0 6 atmospheres, Reaction Time = 0.02 seconds at 1300K, 7 Initial Temperature = 1400K.
8 NO Nff3 ~ Cooling Rate ~ ni~
104000R/sec 54 232 . 112000K/sec 59 121000K/sec 82 9 13500K/sec 104 14250K/sec 115

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for reducing NO concentration of a combustion effluent wherein ammonia is injected into a flowing combustion effluent containing NO
and at least 0.1 volume percent oxygen when at least a portion of the effluent is at a temperature within the range of about 1300° K to 1600° K and wherein the combustion effluent passes through at least one cooling zone in which it is cooling at a rate of at least about 250° K per second, the improvement which comprises injecting ammonia into the combustion effluent at a point where the combustion effluent: (a) is at a temperature of about 1300° K to 1600° K and (b) is cooling at a rate of at least about 250° K per second, wherein the amount of ammonia injected is such that a reduction of NO in the combustion effluent is realized.

2. The process of claim 1 wherein about 0.4 to 10 moles of ammonia is injected per mole of NO of the combustion effluent.

3. The process of claim 2 wherein about 1 to 3 moles of ammonia is injected per mole of NO of the combustion effluent.

4. The process of claim 2 wherein the cooling zone in which the ammonia is injected is cooling at a rate of at least about 1000° K/sec.

5. The process of claim 4 wherein at least 0.25 volume percent of oxygen is present in the combustion effluent.

6. In a process for reducing NO concentration of a combustion effluent containing NO and at least 0.1 volume percent oxygen when at least a portion of the combustion effluent is at a temperature within the range of about 1300°
K to 1600°K and wherein the combustion effluent passes through at least one cooling zone in which it is cooling at a rate of at least about 250° K per second, the improvement which comprises injecting enough ammonia into the combustion effluent at a point upstream from a cooling zone having a cooling rate of at least about 250° R where the temperature of the combustion effluents is from about 1300° K to 1600° K such that when the combustion effluent reaches said cooling zone it contains at least 0.4 moles of ammonia per mole of NO.

7. The process of claim 6 wherein the combustion effluent contains from about 1 to 3 moles of ammonia per mole of NO when it reaches the cooling zone.

8. The process of claim 6 wherein at least 0.25 volume percent of oxygen is present in the combustion effluent.

9. The process of claim 6 wherein the ammonia is injected within 10.4 seconds upstream from the cooling zone.