Note: Descriptions are shown in the official language in which they were submitted.
-- 1 --
1 FIELD OF THE INVENTION
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.
11 BACKGROUND OF THE INVENTION
. . _. ,
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(~
1 D AILED DESCRIPTION OF THE INVENTION
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
- 6 -
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;
3V
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~
-- 4 --
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~
.
1TABLE I
_
2KINETIC MQDEL
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
11 --
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~
- 12 -
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.
Z3V
- ~3 -
1 TABLE II
2 Comparative Examples of ~O
3 Reduction at Isothermal Conditions
4 NO RemainingNH3Remaining
~ ~vppm) (vp~)
6 1000 250 37S
7 1050 250 375
~ 1100 247 372
9 1150 210 338
1~00 89 206
11 1250 54 122
12 1300 62 50
13 1350 101 7
14 1400 159 0
1450 227 0
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.
12 TABLE III
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
192000 45 87 5629 118 1 283 0
201000 41 64 68 7 140 0 289 0
21500 38 36 82 1 149 0 292 0
22250 39 12 91 0 154 0 293 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
- 16 -
1 TABLE IV
. .
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
11500 12 291 23 38 104 0 320
122S0 12 210 41 1 115 0 323
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~
- 17 -
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~
- 19
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~
- 20
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~
1 TABLE VIX
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.
9 NO NH3
10 Temp (K) Remaining,~ m ~ ~ n
11 1000 2~0 375
12 1050 250 375
13 1100 250 375
14 1150 238 364
1200 209 334
16 1250 16~ 227
17 1300 142 226
18 1350 141 159
19 1400 169 83
1450 227 20
21 1500 29S
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~
1 TABLE VIII
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
