NON-CATALYTIC METHOD FOR REDUCING THE CONCENTRATION OF NITROGEN OXIDE IN COMBUSTION EFFLUENTS

Abstract: Process for non-catalytically removing NO from combustion effluent streams at temperatures from about 975<o>K to 1600<o>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 simultaneous equations derived from the kinetic model of Table I hereof.

Description: • Non-catalytic method for reducing the concβntration- .o f nitrogen oxide in combustion effluents. FIELD OF THE INVENTION The present invention relates to a non-cata- lytic method for reducing the concentration of NO in combustion effluents by the injection of ammonia. More particularly, the amount of ammonia and the point of injection is determined by the solution of the set of simultaneous equations derived from the kinetic model disclosed herein. Particular benefits of the present invention occur when ammonia is injected into a cooling zone. BACKGROUND OF THE INVENTION Combustion effluents and waste products from various installations are a major source of air pollu- tion when discharged into the atmosphere. One partiσu- larly troublesome pollutant found in many combustion effluent streams is θ2 > a major irritant in smog. Furthermore, it is believed that NO2 undergoes a series of reactions known as photo-chemical smog for- mation, in the presence of sunlight an hydrocarbons. The major source of NO2 is NO which to a large degree is generated at such stationary installations as gas and oil-fired steam boilers for electric power plants, process heaters, incinerators, coal fired utility boilers, glass furnaces, cement kilns, and oil field steam generators. Various methods have been developed for reducing the concentration of nitrogen oxides in com- bustion effluents. One such method which was developed was a non-catal tic thermal daNO x method disclosed in « - 2 - U.S. Patent No. 3,900,554 to Lyon which patent is incorporated herein by reference. The process dis- closed in that patent teaches the reduction of NO to N2 by injecting ammonia, into >the combustion effluent stream at a temperature fr m about 975°K to about 1375°K in a cavity which isr-substantially isothermal. That is, wherein the temperature of the gases passing through the cavity are cooling at a rate of less than about 50°C. Since the issuance of U.S. 3,900,554, there has been a proliferation of patents and publi- cations relating to the injection of ammonia into combustion effluent streams for reducing the concen- tration of NO. It is the general consensus of the literature that ammonia injection at temperatures greater than about 1375°K would result in the genera- tion of NO from ammonia and consequently conventional selective non-catalytic NO x reduction processes are practiced by injecting ammonia at temperatures lower- than about 1375°K. Because of this temperature limitation, it is difficult and sometimes not possible to apply conventional non-catalytic N0 X reduction processes. This is because during the operating cycle of some boilers and heaters, the temperature range required by conventional processes corresponds to positions in the boiler or heater where it is mecha- nically inconvenient to inject ammonia. In at least some of these instances, this inconvenience could be overcome if the combustion effluent could be contacted with ammonia at temperatures above about 1375°K and still obtain satisfactory reductions in the concen- tration of NO. Furthermore, at temperatures between about 1300°K and 1375°K prior art methods were not always adequate to reduce the NO content of combustion effluent streams to environmentally desirable levels whereas the oresent invention σrovides such a method. OMPI In addition, conventional non-catalytic thermal deNO x processes are further limited because they teach the injection of ammonia into a constant temperature, or isothermal, zone. This is limiting because in a conventional boiler or heater, operating at constant load, combustion effluents typically leave the burner flames at temperatures greater than about 1875°K. As they travel through the boiler or heater they cool in stages - not continually. This staged cooling occurs because of the manner of heat removal from the combustion effluents. Heat is usually removed by heat transfer tubes which are arranged in banks with substantial cavities between the banks. Conse- quently, combustion effluents are rapidly cooled while .they flow through a tube bank-, undergo very little cooling as they pass through a cavity, rapidly cool again while passing through another tube bank, etc. U.S. Patent No. 4,115,515 to Tenner et .al, also incor- porated herein by reference, teaches that the injec- tion 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 NO x , is the total time the combu- stion effluents spend passing through a cavity. Unfor- tunately, in some boilers and heaters, this reaction time - though adequate to provide a useful NO x redu- ction - is not sufficient to provide as great a reduc- tion in NO x concentration as may be environmentally desirable. Therefore, there is still a need in the art for methods of practicing non-catalytic NO x reduction processes which will overcome, or substantially de- crease, the limitations of conven ional practices. SUMMARY OF THE INVENTION In accordance with the present invention there is provided a process fo-r noncataly ically re- ducing the concentration of .combustion effluents con- taining NO and at least 0.1 volume percent oxygen at temperatures from about 975°K to 1600° by injecting ammonia into the combustion effluent. The amount of ammonia and the point of its injection is determined by the solution of the set of simultaneous equations derived from the kinetic model of Table I hereof. In one preferred embodiment of the present invention ammonia is injected at a point upstream from a cooling zone cooling at a rate of at least about 50°K and such an amount so that when the combustion effluent reaches the cooling zone it contains at least 0.4 moles of ammonia per mole of NO. In other preferred embodiments of the present invention, the ammonia is injected at a point where the stream is at a temperature from about 1300°K to 1600°K and is cooling at a rate greater than about 250°K/sec. BRIEF DESCRIPTION OF THE FIGURES The sole figure hereof shows actual perfor- manes data versus predicted performance data generated by use of the kinetic model disclosed herein, for a 235 megawatt utility boiler. DETAILED DESCRIPTION OF THE INVENTION As is well-known, combustion is effected in stationary combustion equipment such as boilers, fur- naces and incinerators in a section of the equipment commonly referred to as a firebox. Generally, this is accomplished by igniting a suitable fuel, in the pres- ence of air, with one or more burners. Materials other than conventional fuels can, however, be com- busted in the firebox portions of the equipment which is generally the case when combustion is effected in an incinerator. In any event, the principal combus- tion products are carbon dioxide and steam and those products, along with the other combustion products - such as carbon monoxide and the various oxides of nitrogen and sulfur, combined with any excess oxygen and unconverted nitrogen to form what is referred to herein as a combustion effluent. The combustion efflu- ent will also contain about 0.1 to 15 volume percent oxygen, preferably about 1 to 3 volume percent. The temperature of the corr.bustion effluent is, then, a maximum at or near the point of combustion and decreases axially (along the flow path) and radi- ally (outwardly) as the effluent moves along its flow path from the point of combustion until it is, ulti- mately, emitted to the atmosphere or otherwise loses its identity as a combustion effluent. As previously mentioned, the combustion effluents, as they travel through the combustion apparatus cool in stages. That is, rapid cooling will occur when the combustion effluent is in contact with heat exchange equipment, such as heat transfer tubes. The prior art teaches that N0 X reduction was only possible in the cavities between then cooling zones as opposed to in the cool- O PI ing zones themselves. By practice of the present "* invention NO x can now be achieved at high temperatures in or immediately before a cooling zone. The amount of ammonia used herein ranges from about 0.5 to 10 moles, preferably 1 to 3 moles of ammonia per mole of NO to be removed. The reaction may be carried out at pressures from 0.1 atmospheres to 100 atmospheres. The veloci- ties of the combustion effluents as well as the mixing of the ammonia in the post-combustion zone are regu- lated so that there is an effective residence time, in a temperature range of about 975°K to 1600° , to enable the ammonia to remove NO x from the combustion effluent stream. The residence time will range from about 0.001 to 10 seconds. Although at temperatures above about 1375°K conventional non-catalytic deNO x processes are gener¬ ally inoperative, the inventors hereof have identified a critical set of conditions whereby N0 X may now be practiced on a wider variety of combustion installa- tions than heretofore thought possible. In addition, practice of the present invention enables, a more effective non-catalytic deNO x operation at tempera- tures above about 975°K with particular advantages of temperatures greater than about 1300°K. Because it is difficult to accurately si u- late, on a laboratory scale, the temperature time history of combustion effluents as they pass through a tube bank in a boiler/heater, it is necessary to gen- erate examples by means other than laboratory experi- .ents. Complex chemical reactions occur by a series of elerrentary reaction steps and if one knows the rate constants for such steps, a theoretical kinetiσ mechanism can be developed and verified through com- parison with experimental data. An extensive block of kinetic data was developed herein by use of apparatus similar to the apparatus taught in U.S. 3,900,554 and used to determine which elementary reactions would likely be of significance during the reduction of NO by NH3. For many of the reactions, the rate constants were well-known accurately measured constants of nature whereas for the remaining reactions the rate constants were not accurately known and accordingly were taken as adjustable parameters. That is, values for the unknown rate constants were assumed, the reac- tion kinetics to be expected from these rate constants were calculated and compared with the observed kine- tics. Based on this comparison a new set of rate constants was assumed, etc., until satisfactory agree- ment between calculation and expe imentation were finally obtained. As a result, the kinetic model hereof and respective rate constants were developed by the inventors hereof for accurately predicting the conditions for the practice of the present invention. In the practice of the present invention the effluent stream to be treated is measured to determine the content of NO, O2, an H2O. These initial conditions, as well as cooling rate measurements of appropriate cooling zones having a high temperature in the range of about 975°K and 1600°K are used in conjunctin with the kinetic model hereof with appro- priate software to determine the amount of ammonia and an injection point which will give NO reduction. Appropriate software suitable for use herein would be any computer program designed for numerical integra- tion of chemical rate expressions. A non-limiting example of such software is CHΞMKIN; A O PI General-Purpose, Problem-Independent, Transportable, - Fortran Chemical Kinetics Code. ackage? R.J. Kee, J.A. Miller, and T.H. Jefferson, an unlimited released Sandia National Laboratory Report SAND80-8003 (1980). This report is also available through the National Technical Information Service, U.S. Department of Commerce. The following example is offered, not as an illustration of the subject invention but to demonstrate the validity of the kinetic model employed herein. The model was used to calculate the NO reduction to be expected for a 235 megawatt utility boiler of the following characteristics: Flue Gas Flow Rate 2,000,000 lb/hr at 3-4% O2 dry Flue Gas Temperature 880-1040°C NO Cone. 190-220 ppm at 3-4% O2 dry Figure 1 (diamonds) contains actual performance data on the above boiler at full load with a best fit curve through the data. The circles in Figure 1- ' represent caper data generated by use of the kinetic model hereof. The Figure illustrates the surprisingly good agreement of model data vs. actual data. The good agreement between predicted and ob- served NO reduction illustrates that the kinetic model is reliable for calculating NO reduction. The advantages of the kinetic computer model hereof are substantial in that it permits one skilled in the art to readily determine, by calculation, the embodiment of the present invention which will yield optimum results for his particular circumstances. In r. ral, however, it may be said that the present invention is an improved method of noncatalytic reduc- tion of NO with NH3, the improvement of the present invention relating to the temperature at which the NH3 is contacted with the NO containing combustion efflu- ents. This contacting is done at temperatures from about 975°K to about 1600°K at a point were the combustion effluents are cooling at a rate of at least 50°K/sec for temperatures of injection up to about 1300°K and cooling at a rate of at least about 250°K for injection temperatures from about 1300°K to 1600°K or at a point where enough ammonia is still present such that the volume ratio of ammonia to NO x is in the range of about 0.4 to 10 when it enters a cooling zone. Generally, the ammonia can be injected 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°K to 1600°K temperature range 1 relates to higher initial NO' concentrations, lower O2 content of the combustion effluents, higher cooling rates and shorter delay times prior to cool- ing. To a somewhat lesser degree, the upper portion of the temperature range is also associated with higher H2O content. O PI TABLE I K INET.IC MODEL Rate constant K = AT n exp (-E/(1.98)T) REACTION . A n E 1. H3+0= H2÷H2 .246E+14 0.0 17071. 2. NH3÷0=NH2+OH .150E+--3 0.0 6040. 3. H3+OH- H2+H2O .326E+13 0.0 2120. 4. HNO+M-NO÷H÷M .186E+17 0.0 48680. 5. HNO+OH=NO+H2θ .360E+14 0.0 0. 6. NH2+HNO-NH3+NO .175E+15 0.0 1000. 7. NH2÷NO=NNK÷OH .610E+20 -2.46 1866. 8. NH2+0 2 =HNO+OH .510E+14 0.0 30000. 9. H-NH2=N2÷ H3 .100E+14 0.0 0. 10. NH2÷0=KH+OH .170E+14 0.0 1000. 11. NH2+OH-NH+H2O .549E+11 0.68 1290. 12. NH2+H-NH+H2 .500E+12 0.5 2000. 13. NH+θ2=NHO+0 .300E+14 0.0 3400. 14. H2+OH=H2θ+H .220E+14 0.0 5150. 15. H+θ2=OH+0 .220E+15 0.0 16800. 16. 0+H2=OH÷H .180E+11 1.0 8900. 17. H+H0 2 =OH+OH .250E+15 0.0 1900. 18. 0+HO 2 =O2+OH .480E+15 0.0 1000. 19. OH÷Hθ2=H2θ+θ2 .500E+14 0.0 1000. 20. OH+OH=0+H2θ .630E+13 b.o 1090. 21. Hθ2+NO=Nθ2+OH .343E+13 0.0 -260. 22. H÷Nθ2=NO+OH .350E+15 0.0 1500. 23. 0+ θ2= O+θ2 .100E+14 0.0 600. 24. H+02+ =H02+M .150E+16 0.0 -995. H 2 0/21** - 25. NNH+M= 2÷K+M .200E+15 0.0 30000. 26. NO2+M-NO+O+M .110E+17 0.0 66000. 27. NH3+ =NH2+H+ .480E+17 0.0 93929. 28. 0+0+M=θ2+M .138E+19 -1.0 340. 29. NH 2 + O= 2+H2θ .910E+20 -2.46 1866. 30. NNH+OH=N2+H2θ .300E+14 0.0 0. 31. NNH+NO=N2+HNO .906E+12 0.0 0. **i.e. A=21 x .15E+16 for H 2 0 as "third body". Given this model, one having ordinary skill in the art can identify a corresponding set of sirnul- taneous eσuations for solution. Example of Use of Kinetic Model -To illustrate the practice of the present invention and its advantage over the prior art the paper example below is presented. A utility boiler is assumed with the follow- ing operating conditions, which operating conditions can be considered normal for such boilers: An excess air level of 19% of stoichiometric air while firing a fuel oil of H to C mol ratio of 1.4. The boiler thus produces a flue gas containing 3.1% 0 2 12.3% CO, 10.6% H 2 O and 74% 2 « There will also be traces of NO, the exact amount depending on the fuel's nitrogen content and the manner in which the fuel is burned. For this example the NO level will be assumed to be 250 vppm. Further, it is assumed there will be a trace of the free radicals OH and.O, the exact concentra- tions of these being determined by thermodynamic equilibrium and therefore being a function of tempera- ture. Combustion flue gas would exit the burners at a very high temperature and cool as it passes through the rest of the boiler. Typically, boilers have a radiative section, a large empty section through which the flue gas passes, cooling by radia- tion as it travels, and a convection section, a sec- tion filled with banks of heat exchange tubes, the flue gas cooling by convection as it passes through these tube banks. As mentioned above, there are usu- ally cavities between the banks of tubes. For purposes of this example let us assume that the boiler has one cavity between tube banks wherein the temperature is approximately constant and within the 1000°K to 1500°K- range, the residence time of the flue gas within this cavity being 0.1 seconds. Now to illustrate the limitations of the prior art we will consider the application of the prior art teaching to the above case. According to the prior art (U.S. 3,900,554 and 4,115,515) one would inject NH3 as the flue gas enters the cavity. We will assume that the amount of NH3 injection is 375 ppm which is well within the ranges taught by the prior art. Thus one has 0.1 seconds reaction time in the cavity for the NH3 to reduce the NO. Table II below shows for cavity temperatures from 1000°K to 1500°K the amounts of NO and NH3 cal- culated by the computer model to remain after 0.1 seconds reaction time. The prior art"teaches that at temperatures substantially below about 1145°K the reduction of NO by NH3 is so slow as to be inoperable, the calculations (using the kinetic model hereof) agree with this teaching. The prior art also teaches that increasing the temperature increases the rate of reaction but decreases the selectivity because a greater proportion of the NH3 tends to oxidize to form additional NO rather than to reduce the NO, with the result that, while NO reduction is an operable process at temperatures in thel!45°K to 1365°K temperature range, it is not operable at temperatures substanti- ally above 1365°K. Indeed, for such excessive te pe- ratures the injection of the NH3 by prior art proce- sses may cause a net increase in NO. The calculated results in Table II, especially the result at 1500°K are entirely consistent with such teachings. TABLE I I Comparative Examples of NO Reduction at Isothermal Conditions NO Remaining NH3Remaining Temp (°K) (vppm) (vppm) 1000 250 375 1050 250 375 1100 247 372 1150 210 338 1200 89 206 1250 54 122 1300 62 50 1350 101 7 1400 159 0 1450 227 0 1500 295 0 Table II also illustrates the limitations of the prior art.. The NO reduction varies with tempera- ture. For ' the specific conditions of this example the best reduction would occur near 1250°K, wherein only about 54 vppm NO remained. Unfortunately, this good reduction would be accompanied by the emission to the atmosphere of 122 vppm NH3. While NH3 emissions are much less of an environmental concern than NO emis- sions, they are still a concern. Further, Table II shows that at 1300°K NO may be reduced to 62 vppm with leftover NH3 of 50 vppm. Thus it was within the scope of the prior art to minimize NH3 emissions by sacrificing some of the possible N'O reduction. Now to illustrate the practice of the pres- ent invention we assume the same boiler and conditions as above. We also assume that the rate of cooling in a tube bank upstream of the 1250°K cavity is between 4G00 c ' sec . , and or 250°K/sec, these cooling rates OMPI covering the range of cooling rates normally used in commercial boilers. Further,, we assume that the NH3 injection system is not located in the 1250°K cavity but at an upstream location where the flue gas temper- ature is either 1300°K, 135θόκ, 1400°K, or 1500°K. Under these assumptions, the NH3 would contact the NO containing flue gas while it was cooling toward a temperature of 1250°K. There would be a reaction time of however long it takes to cool to 1250°K plus 0.1 seconds at 1250°K. Table III shows the results of such calculations. TABLE III , Initial Temp. °K = 1300 1350 1400 1500 vppm of = NO NH 3 NO NH-3 NO NH3 NO NH--, Cooling Rate °K/sec 4000 48 103 51 59 86 12 270 0 2000 45 87 56 29 118 1 283 0 1000 41 64 68 7 140 0 289 0 500 38 36 82 * 1 149 0 292 0 250 39 12 91 0 154 0 293 0 Comparison of Tables II and III reveals that injecting NH3 into cooling flue gas in a tube bank at 1300°K with a 250°K/sec cooling rate would provide more NO reduction than that achieved by iso- thermal injection at 1250°K (reduction to 39 vppm as compared to 54 vppm) while causing considerably less NH3 to be leftover (12 pprr. as compared to 122 vppm). In fact, injection at 1300°K with any cooling rate between 250°K/sec and 400C°K/sec provides better performance than isothermal injection. In addition, one could inject NH3 at 1350°K and at relatively high cooling rates to obtain equiva- lent NO reduction to the prior art with appreciably less leftover NH3. Thus, in this instance the subject invention provides an improvement over the prior art. The fact that this is a valuable improvement becomes clearer when one considers the trade off between NO reduction and NH3 leftover. It was within the scope of the prior art to improve the reduction of NO by in- creasing the amount of NH3 injected into the flue gas, this improvement being purchased at the expense of having more NH3 leftover. The present invention adds more flexibility to this tradeoff and allows one to achieve much better NO reduction for a given amount of NH3 leftover. Table IV shows the results of calcula- tions similar to those in Table III but with the amount of NH3 injected raised from 375 vppm to 750 vppm. Suppose, for the sake of an example comparing the present invention with the prior art, that one had a tube bank with a 500°K/sec cooling rate followed by a cavity at 1250°K and wanted the best NO reduction possible with an NH3 leftover of 122 vppm or less. The prior art for this situation would only produce an NO reduction down to 54 vppm. Table IV shows that if one doubles the amount of NH3 injected, and injects at 1350°K, one achieves reduction of NO down to 23 vppm, a much better result. Further, this much better result is obtained with an NH3 leftover of only 38 vppm. As can also be seen in Table IV, there are other combina- tions of injection temperature and cooling rate which also are an improvement over conventional techniques. O PI TABLE IV Initial NH3 = 750 vppm Initial Temp. °K = 1300 1350 1400 1500 vppm of = NO NH^ NO NH NO NH- NO NH^ Cooling Rate °K/sec 4000 17 403 16 324 21 165 271 2000 15 381 15 251 36 43 300 1000 13 345 17 149 78 1 314 500 12 291 23 38 104 0 320 250 12 210 41 1 115 0 323 The above examples illustrate the advantage of the present invention over the prior art in retro- " fitting the deNO x process to an existing installation. That is, one takes the installation, for example a boiler, as he finds it and as a consequence has no control of the temperature in the cavities between tube banks or the cooling rates within the tube banks. Thus, in the prior art, a cavity is chosen for NH3 injection which is closest to optimum temperature. In accordance with the present invention, one may now install the NH3 injection system within a tube bank since the optimum temperature for NO reduction may very well occur within a tube bank. While the rate of cooling in the tube bank of an existing installation cannot be controlled, the position of NH3 injection and the amount of NH3 injected can be controlled. For example, it was illus- trated in the above examples that at a cooling rate of 4000°K/sec there was one position for NH3 injection and amount injected which gave optimum results. Of course, for cooling rates between 4000°K/sec and 250°K sec, there would be interme iate values which would als o g i ve optimum results . Thus, it is a pre- ferred embodiment of the present i nvent i on to adj ust the pos i t i on and the amount of NH3 injected to match the cool ing rate and thereby achieve optimum NO reduc- tions . Another limitation of the prior art which was previously discussed was the upper temperature limit of about 1375°K. Under prior art practice, this upper temperature limit was severe because, not only could NO not be reduced at temperatures above about 1375°K, but additional NO production usually resulted. Table V below illustrates additional calcu- lations showing the extent to which the present inven- tion alleviates the problems associated with the heretofore upper temperature limit for NH3 injection. According to the prior art, an NH3 injection temperat- ure of 1500°K would be inoperable. However, in accordance with the present invention, such an injec- tion temperature is operable. TABLE V Initial NO = 250 vppm Initial Temperature, °K 1500 1500 1500 1600 1600 1600 Initial NH3, vppm 1500 3000 3000 1500 3000 3000 Cooling Rate, °K/sec 10,000 4,000 10,000 10,000 4,000 10,000 Final NO vppm 18 12 8 632 778 608 Final NH3 vppm 256 426 1658 0 0 0 The upper temperature limit at which NO reduction can be achieved is a function of reaction conditions. Table VI illustrates the effect of changes in O2, H2O, and NO upon the extent of NO O PI production at very high temperatures. Combinations of- these extreme conditions could allow NO reduction at temperatures even above those shown. TABLE VI Effect of Conditions on Upper Tβuperature Limit of NO Reduction 3 Tnitial Temperature, °K = 1500 1520 1540 1560 1580 1600 4 vppm of NO NH 3 NO NH 3 NO NH 3 NO NII3 NO NH3 NO NH3 5 ll no Case: Initial gas 8 1660 1232 11 652 58 49 374 608 6 composition = 250 pp 7 NO, 3000 ppm NU3, » 10.6% ll 2 0, 3.1% 0 2 , OH 9 nιx ' 10 at thermodynamic 10 equilibrium, 1.0 a n ] .1 pressure, and a cooling 12 rate of 10,000 θ /sec to 13 a final tαxiperature of 14 1250° and 0.1 sec re- 15 action time at 1250°K 16 Cruse 1: Same as H sp Case 5 2000 1688 1242 588 121 608 17 except 0 2 decreased l.o L% B Cane 2: Same as Base 11 1936 13 1582 17, 1114 38 538 181 70 537 0 19 Case except U2O in- 20 creased to 30% 21 Case 3: Same as Base 236 441 0 629 812 0 994 0 1183 0 22 Case except NO in- While the above discussion relates to the application of the present invention in retrofit situ- ations, it obviously may also be applied to the reduc- tion of NO in new stationary combustion equipment. Suppose, for the purpose of discussion, that one is building a new boiler of some kind and wishes to achieve the best NO reduction possible within the design constraints of that kind of boiler. Specifi- cally, we can assume that the boiler is intended to operate at very high flue gas velocity so that the time available for NO reduction is only 0.02 seconds. Given this short reaction time the best possible NO reduction consistent with acceptable NH3 leftover is desired. To take an arbitrary but convenient value, we assume that the leftover NH3 must be less than 159 vppm. Table VII shows calculations of what could be achieved by the prior art in this situation. It is evident that the best one can do within the limita- tions of the prior art is to operate near 1350°K, this giving one an NO reduction from 250 vppm to near 141 vppm. Of course this calculation is done assuming an NH3 injection of 375 ppm and increasing the NH3 injection would improve NO reduction. However it would also increase NH3 leftover and NH3 leftover is already at the maximum permissible value. This reduction to 141 vppm is the best the prior art can achieve. TABLE VII Comparative Examples of NO reduction at Isothermal Conditions Initial conditions NO = 250 vppm, NH3 = 375 vppm, H2O = 10.6%, O2 = 3.1%, OH and 0 equal their equilibrium values a nd the balance of the flue gas is inert. Pressure = 1.0 atmospheres. Reaction Time = 0.02 seconds. NO NH3 Temp (°K) Remaining, vppm Remaining, vppm 1000 250 375 1050 250 375 1100 250 375 1150 238 364 1200 209 334 1250 164 227 1300 142 226 1350 141 159 1400 169 .. .83 1450 227 20 1500 295 2 The present invention would show an improve- ent over the prior art. In Table VIII below, results are given for calculations in which it was assumed that 750 vppm NH3 ' was injected at 1400°K in a flue gas at various cooling rates with a final temperature of 1300°K. For a cooling rate of 2000°K/sec. Table VIII shows that a reduction of NO to 59 vppm (as compared with 141 vppm for the prior art) may be achieved with only 93 vppm NH3 leftover (as compared with 159 vppm for the prior art). Thus, the practice of the present invention could achieve both better NO reduction as well as less H3 leftover. O PI TABLE VII I Example of subject invention: Initial con- d iti ons NO = 250 vppm, NH = 750 vppm, H2O = 10 .6% 0 = 3.1%, OH and 0 equal their equilibrium values and the balance of the flue gas is inert. Pressure = 1.0 atmospheres . Reaction Time - 0.02 seconds at 1300°K, Initial Temperature = 1400°K. NO NH3 Cooling Rate Remaining, vppm Remaining, vppm 4000°K/sec 54 232 2000°K/sec 59 93 1000°K/sec 82 9 500°K/sec 104 0 250°K/sec 115 • 0

Claims: CLAIMS : 1. In a process for reducing NO concentra- tion 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 975°K to 1600°K and wherein the combustion effluent passes through at least one cooling zone, the improvement which comprises injecting ammonia into the combustion effluent at a point where the combustion effluent: (a) is at a temperature of about 975°K to 1600°K and (b) is cooling at a rate of at least aboαr 50°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 cool- ing zone in which the ammonia is injected is cooling at a rate of at least about 250°K/sec and the temperature of injection is in the range of about 1300° to 1600°K. 5. The process of claim 4 wherein at least 0.25 volume percent of oxygen is present in the com- bustion ef uent. -$JREΛ O PI 6. In a process for reducing NO concentra- tion 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° 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°K 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 in it contains at least 0.4 moles of ammonia per mole of NO. 7. The process of claim 6 wherein the co - bustion 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 com- bustion effluent. 9. The process of claim 6 wherein the ammonia is injected within 10.4 seconds upstream from the cooling zone. 10. A process for reducing NO concentration of a combustion effluent containing NO and at least 0.1 vol_me percent oxygen when at least a portion of the combustion effluent is at a temperature within the range of about 975°K to 1300°K, which process comprises ir. " rtinc ϊ-monia in an 3-c.:': and ai fjjREXj OM?I determi ned by the solut ion of the set of s imultaneous equations der ived from the kinet ic model of Table I hereof . 11. The process of claim 10 wherein the combustion effluent is in a range of about 1300°K to 1600°K and is passing through a cooling zone having a rate of cooling of at least about 250°K/sec at the point of ammonia injection. 12. The process of claim 11 wherein the combustion effluent contains from about 1 to 3 moles of ammonia per mole of NO when it reaches the cooling zone.

Inventors: DEAN ANTHONY M, DEGREGORIA ANTHONY J, HARDY JAMES E, HURST BOYD E, LYON RICHARD K, DEAN, ANTHONY, M., DEGREGORIA, ANTHONY, J., HARDY, JAMES, E., HURST, BOYD, E., LYON, RICHARD, K.
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1984
Main classification: B01D53
IPC codes: B01D53