DD283364A5 - METHOD AND APPARATUS FOR REDUCING NITROGEN OXIDES IN COMBUSTION GASES - Google Patents

Abstract

The invention relates to methods and apparatus for a two-stage injection process for reducing NOx in combustion exhaust gases. NHi precursors (eg, liquid phase urea or ammonium hydroxide or the like) are injected into the exhaust at temperatures above 760C in a first injection zone to reduce NO to nitrogen. This step is followed by the injection of a peroxyl initiator, for example, a hydrocarbon (eg, methanol) into the exhaust gas at temperatures below 760C to oxidize the residual NO to NO2 in a second injection zone. Apparatus; Reduction of nitrogen oxides; two-stage injection process; NH, precursors; peroxyl}

Description

Field of application of the invention

The invention relates to techniques for removing nitrogen oxides from combustion exhaust gases. More particularly, the invention relates to techniques for reacting nitrogen monoxide (NO) to nitrogen (N ₂ ) and nitrogen dioxide (NO ₂ ) and to remove the nitrogen oxide from the exhaust gas before the exhaust gas exits to the atmosphere.

Characteristic of the known state of the art

Coal-fired power plant steam generators produce NJx and S0 ₂ emissions that cause acid rain. Extensive research has shown that the NOx laden portion of the acid rain components can cause extensive damage to trees and forests, so NOx control systems for coal fired steam generators may be required in many industrialized countries beyond Nox regulations currently required in Japan and West Germany. NOx control is also an important consideration for waste incinerators where there is currently no proven NOx control technique to achieve 70% or more removal of NOx.

Many existing coal-fired steam generators or solid waste incinerators already have wet scrubbers for controlling SO ₂ or HCI emissions. In particular, wet separators were built in the USA for coal-fired plants with an installed capacity of about 60,000 MWe (MWe = megawatt of electrics) and in western Germany at about 40,000 MWe. Many other countries are currently calling for the installation of wet scrubbers for coal-fired steam generators and solid waste incinerators.

The current industrial technology for NOx control consists mainly of the selective catalytic reduction (SCR), in which ammonia gas is injected into the exhaust gas and reacts with NOx over a catalyst at temperatures of about 371.11 "C and nitrogen and water vapor by-products Characteristic NO x reduction values are 80% The catalyst bed is normally designed to be sufficiently large enough to reduce ammonia leakage and to avoid contamination of the fly ash with ammonium salt deposits The term "ammonia leakage" or "ammonia -Slip "means the concentration of ammonia gas contained in the exhaust output from the NOx control process.The SCR technique costs vary depending on the condition of the equipment! Wiping about 60 dollars / kW to 120 dollars / kW The operating costs of SCR technology include the high cost of catalyst replacement about once every two years. The SCR technique is considered inapplicable to waste incinerators due to contamination and poisoning of the catalyst. The object of the present invention is to obtain equally high NO x removal values with very high capital and operating cost savings, without requiring the use of the SCR technique.

There are other NOx control methods using NHi precursors, e.g. As gaseous ammonia or liquid phase urea, which are injected into the exhaust gas at temperatures above 760 ° C to reduce NO to nitrogen. These processes are referred to as Selective Non-Catalytic Reduction (SNCR) and have the disadvantage that if enough precursor material is injected to achieve high NOx removal efficiency then unacceptable high ammonia leakage may occur. The ammonia slip combines with SO ₂ , SO ₃ , HCl and HF and forms ammonium salts at temperatures typically below 26O ⁰ C. When such salts condense solid particles form, which can lead to deposits in critical zones, e.g. B. in the air preheating plant in conventional steam generators. To avoid these problems, less NHi precursor is injected and the overall NOx reduction capability of the SNCR system is generally limited to between 30% and 60%. This level of performance does not compete with SCR systems and is unacceptably low.

Typical SNCR processes are described by RK Lyon, U.S. Patent No. 3,900,554 and AM Dean and others, U.S. Patent No. 4,624,840 (Exxon Thermal DeNOx process using gaseous ammonia) and by JK Arandt and others, U.S. Patent No. 4,208,386 and U.S. Patent No. 4,325,924 (EPRI / Fuel Tech NOxOUT process, using liquid phase urea). U.S. Patent Nos. 3,900,554, 4,624,840, 4,208,386, and 4,325,924 are incorporated herein by reference. Are both Exxon and the Fuel Tech process preferably only operate within a narrow temperature window, characteristically between 926.67 and 1037.78 ^{0 0} C C, but they can also be performed to the exhaust gas at somewhat lower temperatures with addition of hydrogen or hydrocarbons , Characteristically, in the Exxon process, hydrogen is added to methanol in the Fuel Tech process. It is believed that the actual NC reduction mechanism involves a large number of radical reactions. The most important is:

NH ₂ + NO = N ₂ + H ₂ O

When using a hydrogen or carbon hydrogen addition can reduce the normal temperature window (that is 982.22 ± 37.78 ° C), for example to 815.56 ± ⁰ C is 37.78 ⁰ C. In this case, contain a high ausieichend Komen addition of hydrogen to increase the Concentration of the hydroxyl (OH) radicals, which increases the concentration of NH ₂ radicals at low temperatures and reduces the ammonia leakage, after the reaction:

NH ₃ + OH = NH ₂ + H ₂ O

However, this technique for setting the optimum temperature for an SNCR process for reducing the values by the addition of hydrogen peroxide and hydrocarbons to below about 815.56 ⁰ C is limited. Below 760 ⁰ C, the effectiveness of NO reduction of the SNCR process is drastically reduced, to about 704.44 ° C with practically no more reduction.

As used herein, the term "temperature window" means the temperature range over which the SNCR processes are effective in reducing NO to nitrogen, and significantly, there is no NO reduction at very high temperatures.

As the temperature is reduced, the NO reduction efficiency increases until maximum at optimum temperature

Effectiveness of NO reduction is achieved. Further reductions in exhaust gas temperature very quickly cause increased ammonia leakage and reduced NO reduction efficiency until the lower temperature edge of the temperature window is reached. The term "optimum temperature" as used herein means the exhaust gas temperature for which the NO reduction efficiency of the SNCR process is a maximum. The term "SNCR process" means the application of simple or complete NHi precursors to the exhaust gas at temperatures within the temperature window, resulting in a selective, noncatalytic reduction of NO to nitrogen. The term "simple NHi precursors" refers to ammonia or other compounds, such as ammonium hydroxide or ammonium carbonate, or mixtures thereof, which release ammonia (NH ₃ ) upon thermal decompensation. The term "complex NHi precursors" refers to Urea (Nh ₂ I ₂ CO, cyanuric acid, biuret, triuret, ammelide, other amides, or mixtures thereof, which initially release NH ₂ radicals upon thermal decompensation.

Another type of NO x control method for exhaust gas in which NO is oxidized to NO ₂ is disclosed in US patent application entitled: PROCESS AND APPARATUS FOR REMOVING OXIDES OF NITROGEN AND SULFUR FROM COMBUSTION GASES (Method and apparatus for removing nitrogen oxides). and sulfur oxides from combustion gases), serial no. 734,393, filed by me on May 14, 1985, cited. In the process disclosed in the '393 method, reference is here as "Jones" process taken. The US patent application serial no. 734,393 is incorporated herein by reference. The Jones OxyÜationsverfahren operates between 426.67 ⁰ C and 760 ° C and uses a hydrocarbon (a peroxyl initiator), eg, methanol, dispersed in an air carrier to achieve high levels of oxidation of NO to NO _2. The preferred temperature window for this process is 426.67 ° C to 760 ⁰ C. This method is contains a different type of steam generator-injection process, the peroxyl radical (HO ₂₎ instead of the hydroxyl radical (OK), NO is oxidized to NO ₂ by the following reaction.:

NO + HO ₂ = NO ₂ + OH

It can be seen that a suitable temperature is about 760 ⁰ C as a dividing line between the NO reduction processes of the type SNCR (ie at 76O ⁰ C and higher) and the N0-oxidation Pro Jones Jones (ie at 76O ⁰ C and lower) acts.

Object of the invention

The present invention provides a means of achieving very high levels of NOx removal, on the order of 70% or more, especially in situations where coal-fired steam generators or waste incinerators already have wet scrubbers. The invention may also provide such NOx removal performance levels without the use of expensive catalytic NOx reduction processes.

Explanation of the essence of the invention

The object of the invention is to provide improved methods for reducing nitrogen oxides in combustion gases.

According to the invention, a method for the reduction of NOx in combustion exhaust gases is provided. The method includes the steps of injecting NHi precursors into an exhaust gas in a first injection zone, wherein the exhaust gas has a temperature above 76O ⁰ C to reduce NO to N ₂ / i. The second step is to inject a peroxyl initiator into the exhaust gas in a second zone to oxidize the remaining NO to N ₂ .

These and other features, aspects, and advantages of the present invention will become better understood when considered in light of the following detailed description, appended claims, and accompanying drawings.

Figure 1 is a schematic representation of a two-stage steam generator used to describe a preferred embodiment of a two-stage injection process carried out using the principles of the present invention.

Figure 2 is a schematic cross-sectional view of a preferred embodiment of an injector made using the principles of the present invention for the injection of NHi precursors and / or peroxyl initiators.

Figure 3 is a schematic representation of a preferred embodiment of an injection system for injecting dpi NHi precursors and / or peroxyl initiators, carried out using the principles of the present invention. Figure 4 illustrates a preferred flow pattern for the NHi precursors and peroxyl initiators initiated by injectors according to the present invention.

Figure 5 is a diagram illustrating the change of NO concentration as a function of mole ratio of injected urea in the exhaust gas to the NO concentration in the exhaust gas at S10 ° C and 96O ⁰ C.

Figure 6 is a diagram illustrating the change of CO in parts per million (ppm) as a function of the mole ratio of injected urea in the exhaust gas to the NOx concentration in exhaust gas at 96O ⁰ C.

Figure 7 is a graph illustrating the optimum temperature for the reduction of NO and the ammonia rflip with and without hydrocarbon when urea is injected according to the application of the principles of the present invention.

Figure 8 is a graph showing the percent conversion of NO to N ₂ and of NO to NO ₂ as a function of the molar ratio of methanol injected into the exhaust gas to the NOi concentration in the exhaust gas bsi at an injection temperature of 720 ° C.

Figure 9 is a diagram danteilt the change in CO concentration as a function of the mole ratio of fuel injected into the exhaust gas to the methanol concentration of NO in the exhaust gas at an injection temperature of 937.78 ^0C. Figure 10 is a diagram illustrating the ammonia slip in parts per million as a function of the mole ratio of fuel injected into the exhaust gas to methanol NOI concentration in the exhaust gas at 72o C ^0.

Figure 1 is a schematic view of a two-stage steam generator 10 useful in the practice of the present invention for reducing nitrogen oxides in an exhaust gas stream. The method most recently involves the reduction of NO to nitrogen by injecting NHi precursors (eg liquid phase urea or ammonium hydroxide) into the exhaust gas in a first injection zone 12 at a temperature above 76O ⁰ C and secondly the additional reduction of NO Nitrogen by oxidation of the residual NO to NO ₂ and the reduction of ammonia leakage by injecting a peroxyl initiator, eg. As hydrogen, Wassersioffperoxyd or hydrocarbon (eg., Methanol) in the exhaust gas in a second injection zone at temperatures below 760 ⁰ C. Additional NHi precursors include ammonia and complex compounds, for example. As cyanuric acid, biuret, triuret, ammelide or mixtures thereof. If the exhaust gas temperature in the first zone 12 falls below the optimum level, then hydrogen, hydrogen peroxide, or hydrocarbons (eg, methanol) may be injected along with the NHi precursor to lower the optimum temperature for NO reduction and the height to reduce the ammonia leakage. The location of injection of the NHi precursor may be at more than one level or point in the steam generator to compensate for temperature changes. As seen in Figure 1, an injection manifold 16 is supplied by a compressor 18 which supplies air to the injectors 20. Details of execution of the nozzle 20 can be seen from Figures 2 and 3, wherein the compressed air enters the nozzle 20 at the air inlet 22 and leaves the nozzle at the outlet 23. The NHi precursor enters the nozzle at the inlet 26, passes through the nozzle 30, mixes with the air in the mixing member 28 and exits at the outlet 24. It is recommended. To provide automatic control devices either the amount of hydrogen, hydrogen peroxide or hydrocarbon for co-injection or for the injection site or both on the basis of arranged in the first injection zone temperature sensors. Preferred injection materials are aqueous solutions of urea or methanol or both.

As previously mentioned, the optimum temperature for SNCR processes falls within a narrow temperature window. An overview of the state of the art shows that the optimum temperature 996 ⁰ C plus or minus about 23.89 ⁰ C or is between 954.44 ⁰ C and 1037.78 ° C. A possible reason for the degree of uncertainty is that the temperatures measured at these high values 10O ⁰ C to 65.56 ⁰ C due to the heat radiation losses from the sensor itself are too low, unless convective heat is supplied (by means of a suction Pyrometertechnik), the actual temperature can be measured. All temperatures quoted refer to the actual gas temperature, as measured with a suction pyrometer. According to the prior art by Arand and others (US-Pr'pnt no. 4,208,386) the temperature window for urea is explained in Example I, is shown clearly in, that the optimum temperature is about 996.11 ⁰ C is based on the reported in Table I NO reduction efficiency as a function of temperature. A surprising result which forms part of the present invention is that the optimum temperature for urea is clearly below 910 ° C, most likely 873.89 ° C. The optimum temperature given by Arand and others is clearly wrong. This surprising discovery requires a process that addresses the relatively narrow temperature window limitations previously thought to be applicable to SNCR processes. In one embodiment, a simple NHi precursor is injected into the exhaust at higher temperatures into an upstream portion of the SNCR process, followed by complex NHi precursors injected into the exhaust gas at lower temperatures into a downstream portion of the SNCR process become. In another embodiment of the simple NHi precursor at temperatures of about 1037.78 ° C is injected down to about 954.44 ° C, followed by complex NHi precursors from about 954.44 ⁰ C down to 843, 33 ° C are injected. The optimum temperature for the complex NHi precursor may be about 79.44 ⁰ are C lower than the optimum temperature for the simple NH, precursor, and thus provide a much wider temperature window than is set forth in the prior art. In another embodiment, an automatic control system could be used to inject simple or complex NHi precursors into the exhaust gas through one or more levels of injectors based on the input from the exhaust gas temperature sensors. Another embodiment suitable for automatic control would be to inject a mixture of simple and complex NHi precursors into the exhaust gas through one or more levels of injectors based on the exhaust gas temperature measured at each level and the ratio of simple and complex NHi precursors regulate that are delivered to a given level of injectors.

If there is more than one level with injectors in the SNCR process, then (1) in another preferred embodiment, additional dilution water would be added along with the NHi precursors. For example, aqueous urea or ammonia hydroxide, one would shift the location (s) toward cooler, downstream nozzles and / or increase the use of simple NHi precursors based on automatic control when the exhaust gas temperature (s) become too high (or) (2) reduce the dilution water injected with the NHi precursor, move the site (s) toward warmer, upstream nozzles, and / or increase the use of complex NHi precursors by automatic control When the exhaust gas temperature (s) become too low. Such an automatic control may be designed based on input from the exhaust temperature sensors, exhaust NO analyzers, or both.

In the present invention, suitable devices 7 for introducing injection chemicals are also indicated. Although many different techniques can be used, such as: As the direct injection of liquids and gases into the exhaust gas, it is recommended that the chemicals for injection with a carrier gas, eg. As air, steam, circulating air combustion gas or mixtures thereof premix. Pre-mixing takes place outside the steam generator in a suitable apparatus, and thus the resulting injection medium consists of premixed amounts of carrier gas and injection chemicals. A suitable apparatus for the preparation of the premixed injection medium contains a Trägergasventuridüse, z. Example, the above-mentioned nozzle 20, which atomizes and distributes the aqueous solutions of the injection chemicals in the carrier gas. For injecting high velocity into the exhaust, the injection medium is pressurized by an external source, e.g. For example, air or exhaust from the positive displacement compressor 18. If desired, one may use steam from a steam generator instead of air. By careful design of the nebulizer and dispensing nozzle 20, pressure losses in the premix stage can be minimized. The injection medium is then discharged into the exhaust gas or the air through the nozzle 20, which may be attached only to the walls 32 of the steam generator odei due to the cooling effect of the carrier gas the actual circumstances can be located inside the exhaust zone and at an internal injection Distributor line are attached. The actual design and location of the nozzles and the injection system may depend on the physical constraints imposed by the steam generator, the degree of NOx removal desired and other parameters, e.g. As the exhaust temperature, composition and speed distribution depend.

The two-stage steam generator injection method of the present invention provides the surprising and significant benefits of increasing overall NO conversion and reducing ammonia leakage as compared to a SNCR process which was carried out under similar conditions. Even if an SNCR process is operated under different conditions, it is not possible to achieve a greater degree of NO conversion with correspondingly low ammonia leakage than is possible with a 2-stage steam generator injection method of the present invention under the same conditions , Conversely, with the Jones method for NO.sub.2 / NO.sub.2 oxidation, one could be able to achieve fairly high NO conversion values, but even if a small amount of NO reduction to nitrogen were to occur in an upstream SNCR process, then For example, the total NO conversion efficiency of the two-stage process of the present invention is greater than if the Jones process were operated per se. The surprising benefits of significantly reduced ammonia leakage are not reported in the prior art. Another advantage of the two-stage steam generator injection process is that the residual NO converted to NO ₂ in the second stage can be removed by wet scrubbers or other techniques set forth in the prior art by Jones, thereby providing very high levels of NO x removal (ie 80% or more) can be achieved without the difficulty of correspondingly high levels of ammonia leakage, which can be reduced by the operating conditions. a SNCR process itself. These and other advantages of the present invention will be described in the following specific embodiments.

The two-stage injection process for steam generators for NOx control has found considerable interest for industrial use in coal-fired steam generators and solid waste incinerators. It has been found that the application to coal-fired steam generators is generally simpler than solid waste incinerators since it requires higher initial NOx levels, requiring slightly lower molar ratios of chemicals to NOx for the same levels of performance, and a lower carbon content in the Fly ash gives what results in a more effective use of methanol in the second stage. By way of example, a detailed description will be given of the use of the two-stage steam generator injection process in a solid waste incinerator to achieve 72% NOx removal without wet scrubber and 81% with wet scrubbers of the downstream exhaust gas.

embodiments Example I

The conditions in a current municipal 300 t / d solid waste incinerator were modeled in a combustion tunnel to determine the exact operation of the two-stage injection process for NOx control steam generators. These tests involved collecting collected fly ash from the electrostatic precipitator at the waste incinerator to determine the impact of the ash on the process chemistry. The ash entry rate was 3.0 grains / SCF (1 grain = 0.0648 g) or about 7000 mg / standard m ³ (Nm ³ ).

The initial composition of the exhaust gas prior to the injection process into the Zone 34 shown in Figure 1 steam generator (Zone A) is typically as follows (% by volume, dry basis):

NO 125 ppm HCI 840 ppm NOj 5 ppm SO ₂ 60 ppm NOx 130 ppm SO ₃ 3 ppm CO 30 ppm HF 15 ppm O ₂ 12% ash 7,000 mg / Nm ³ CO ₂ 10%

The water vapor content of the exhaust gas is characteristically 9% by volume.

In the first stage injection zone, an aqueous solution of urea or ammonia hydroxide is injected into the crossflowing exhaust gas using an apparatus (e.g., as described above and shown in Figures 1,2 and 3) for premixing the injection chemicals and the carrier gas, z. As air or steam. As shown in Figure 1, in this example, a rotary blower is used to generate compressed air as a carrier to the injectors via manifold 16. The amount of carrier gas is approximately 6% by mass of the total waste gas stream of the waste incineration plant. The injection chemical is mixed in the injection nozzle 20 with the carrier gas. The injector 2C passes through the steam generator wall 32 and includes the carrier gas and injection chemical ports 22, 26 for removing the injector 30 for maintenance outside the steam generator, device 30 (the chemical injector 30) for atomization, and / or Distribution and premixing of the injection chemical with the carrier gas in the venturi 40 of the nozzle 20 located above the outlet 24. The outlet 24 is designed to inject the premixed amounts of the carrier gas and injection chemical (NHi precursor or peroxy linitia). In a preferred embodiment, best shown in Figure 3, the injector operates at sonic or subsonic velocity, and tangentially-aligned injectors 42 are inserted on the sidewalls to create a tangential flow pattern and produce a very fine flow To promote all mixture and induce a gedralltes flow pattern in the mass flow of the exhaust gas. It is clear to provide double funnel-shaped nozzles for subsonic injection rates, or to provide an inner injection tube and multi-nozzle positioned above the exhaust section to promote improved mixing and / or, if necessary, better area coverage.

Although the present example describes the use of a dilute aqueous urea solution for the first stage of the two-stage injection process, another alternative would be to use an aqueous ammonium hydroxide solution. The ammonium hydroxide would immediately release NH ₃ vapor after heating in the exhaust zone, whereas after thermal decomposition of urea [chemical formula (NH ₂ hCO) released NH _{2 would} not occur until sufficient water evaporation from the droplets to urea supersaturation and / or crystallization into the drops leads. Therefore, by increasing the dilution or droplet size, the release of NH ₂ from the urea solution into the exhaust gas can be delayed to a certain extent, thereby forming a more intimate mixture with the

Exhaust gas and consequently a better area coverage to allow. This may be advantageous compared to ammonium hydroxide depending on the size of the steam generator, NOx control requirements, temperature level and velocity distribution errors, location of the injectors, and residence time constraints.

As shown in the graph of Figure 5, a molar ratio of urea to NO of 1.4 gave a 72% NO to nitrogen conversion at 91O ⁰ C from a starting level of 125 ppm NO. As also shown in Figure 5, there is an ammonia slip of 35ppm at a urea / NO mole ratio of 1.4. In Figure 6 it is shown that at an injection temperature of 96O ⁰ C and at a urea / NO molar ratio of 1.4, a corresponding increase in the CO emission of 13 ppm. At this point, the change in the exhaust gas composition is as follows:

Zone A, Ab). 1 125 ppm Zone B, Fig. 1 5 ppm After the injection Before the injection 130 ppm in the 1st stage in the 1st stage 30 ppm 35 ppm NO zero 5 ppm NO ₂ 40 ppm NOx 43 ppm CO NH ₃

When the temperature in the first-stage injection zone 12 deviates from optimum values, the removal efficiency of NOx is deteriorated unless corrective measures are taken. Among these measures would be the automatic control of the location in the steam generator where urea is injected based on the temperature sensor input and the use of more than one injection level as shown in Figure 1. Another alternative is the injection of a hydrocarbon with the NHi precursor, un ~. -jine to achieve a reduction in the optimum temperature. This effect is shown schematically in Figure 7, from which it can be seen that without CO injection of hydrocarbons, when the temperature falls below the optimum level, then the NH ₃ leakage drastically increases and the NO removal efficiency dramatically deteriorates , These two reversed effects can be compensated by co-injection of hydrocarbon at temperatures in the range of 76O ⁰ C to 982.22 ⁰ C. If CO emissions are a limiting factor, co-injection of hydrogen or hydrogen peroxide will also produce the same effect. In the preferred embodiment of this example, methanol is added to the aqueous urea solution for co-injection of hydrocarbon, and the amount of methanol is automatically controlled based on input signals from the temperature sensors.

Let us come for. For example, referring back to Figure 8, at a molar ratio of methanol to NO of 0.5 at the entrance to the second stage injection zone, a 24% conversion of NO to NO ₂ plus 5% conversion of NO results Nitrogen at 720 ⁰ C in the presence of 7000mg / Nm ³ waste incineration fly ash and 25ppm ammonia. It is known that fly ash contains unburned carbon, which somewhat compromises the effectiveness of NO conversion of the second stage. As shown in Figure 8, in the absence of ash (or when the carbon content of the ash is very low) with the same molar ratio of methanol to NO (about 0.5), about 45% conversion of NO to NO _{2 is achieved} plus about 7% conversion of NO to nitrogen at 72O ⁰ C in the presence of 25 ppm ammonia. Ammonia is present as a result of the ammonia slip from the first stage in the second stage.

The ammonia slip is reduced in the second stage injection zone as shown in Figure 10, with a 40% reduction included in current tunnel combustion tests, including a fly ash entry. It has been found that CO emissions increase by 28ppm in Figure 9, which indicates that the influence of fly ash results in a significant increase in CO emissions compared to ashless or low carbon ash conditions. At this point, the changes in the exhaust gas composition using a molar ratio of urea / NO in the first stage of 1.4 and a molar ratio of methanol to NO in the second stage of 0.5 are as follows:

Zone A, Fig. 1 Zone B, Fig. 1 Zone C, Fig. 1 Before injection After injection After injection in the in the in the I.Stufe I.step and 2nd stage before the 2nd stage NO 125 ppm 35 ppm 22 ppm NO ₂ 5 ppm 5 ppm 15 ppm NOx 130 ppm 40 ppm 37 ppm CO 30 ppm 43 ppm 71 ppm NH ₃ zero 35 ppm 21 ppm urea = 1.4 molar Methanol = 0.5 molar NO NO temperature = 1 67O ⁰ F Temperature = 1 330 ° F

At this point, the NOx removal is 72%. Assuming that 80% of the NO _{2 is} removed in a downstream wet scrubber or sputter dryer, the total NOx removal is 81%, from 130 ppm to 25 ppm. This effect level of NOx removal is directly comparable with the SCR processes with catalyst and is much higher than the NOx removal of 69% (that is, from 130ppm to 40 _r pm), which upon application of an SN CR process under the same conditions would be expected in a single-stage high-temperature injection zone.

Example II

Tests were conducted in a quartz-lined 0.5 MMBTU / hr pre-combustion tunnel at an exhaust gas flow rate of approximately 25 SCPM. Nitrogen oxide was introduced with the main combustion air, ammonia was injected with dilution air at the passage part and methanol 11 inches downstream using a carrier gas. The tests were carried out at an exhaust gas temperature of 648.89 ⁰ C at the methanol injection point. The exhaust gas was collected at the exit of the combustion tunnel by means of two water-cooled probes, one for the continuous gas analysis system and one for the ammonia collector. The residence time of the gas in the combustion tunnel was about one second. Ammonia was determined by passing the products of combustion through an impactor containing dilute sulfuric acid and analyzing the ammonia impingement solutions with an ion specific electrode. Ammonia concentrations were determined on the basis of the average of three-fold samples. The results are shown in Table 1:

Table 1 Feuerungsgeschwindigkeit : 0.5 MMBTU / h ppmv concentrations NH ₃ I % NOx Red. % NH ₃ -Red. Test BedingunoTi. Fuel: natural gas NOx 0 n / A n / A Burner system: 299 90 2% 0% Injection temperature: 293 0 22% n / A Methanol / NO Molar: 233 . NO NO ₂ 23.6% Results: 294 5 1. Basic conditions 288 5 quartz-lined combustion tunnel 41 29% 54% 2. NurNH ₃ injection proper 15 218 648.89 ⁰ C 212 3. Only methanol-one 2.2 sprit tongues 2% + (22% x 98%) = 4. Calculated per percentage terms NOx Red. 15 197 5. Actual percen! jale NOx Rej. under on turn of a two stage injection Zung

Although the temperature of 648.89 ° C is too low to allow a reasonable level of NO reduction to nitrogen with ammonia injection, actually about 2% has been achieved. With injection of only methanol in the Jones process, a reduction of NO to nitrogen of 22% was achieved. Summarizing these two independent results, one can expect the combined two-step process to yield a reduction of 23.6% NO to nitrogen by the practical application of the principles of the present invention. In fact, a reduction of NO to nitrogen of 29% was achieved, surpassing all expectations at 648.89 ° C. In the prior art, no injection method for a steam generator indicated that a 29% reduction of NO to nitrogen at ^r t dera low molar ratios or effect at such a low temperature. In addition, a 54% NH ₃ reduction was simultaneously observed. During the operation of the two-stage injection system, the NO feed was discontinued and the degree of reduction of NH ₃ dropped from 54% to only 10%. It is believed that the presence of NO _{2 is important} for high levels of F. attenuation of ammonia leakage.

Example III

Additional tests were performed in a 10 MMBTU / h crude oil fired steam generator. It burned crude oil with a sulfur content of 1.2% using a burner pressure tube with steam atomizer. Aqueous urea was injected into the upper furnace at a temperature of about 843.33 ° C. Methanol was injected as shown in Figure 1 between adjacent steam generator tubes into an area where the temperature of the cross-flow exhaust gas was 637.78 ° C. The molar ratio of urea to NOx was about 0.3 and the molar ratio of methanol to NOx was about 1.7. At the steam generator exit, exhaust samples were taken using an iced baffle and a continuous NO x NO chemiluminescence analyzer. The results are shown in Table 2.

Table 2 test conditions:

Firing rate: Fuel: Burner system: Injection temperature: Methanol / NO Molar: Urea / NO Molar:

10MMBTU / h

crude oil

Steam generator and burner with steam atomization 637.78 ⁰ C

Results:

NO

ppmv concentrations NO ₂ NOx

% N0x-Red.

1. Basic conditions

2. Only urea injection

3. Only methanol injection

4. Calculated percentage NOx Red.

5. Actual percentage NOx Red. using a two-stage injection

270 10 280

235 10 245

70 151 221

13% + (21% x 87%) = 30%

55

135

190

n / A

13%

21%

32%

Although 843.33 ⁰ C for SNCR processes are quite low yet and although even the urea injector provided a low exhaust gas coverage, a 13% reduction of NO would z achieved ;, ¹ nitrogen with urea in the first injection zone. In a methanol only injection in the second zone, a 21% reduction of NO to nitrogen was achieved. The expected percent NOx reduction based on calculations was 30%, the actual NOx percentage reduction using the two-stage injection method of this invention was 32%. The favorable result of a 32% conversion of NO to nitrogen appeared at 637.78 ° C, well below the lowest previously detected threshold temperature for the SNCR technology 704.44 ⁰ C. Again, in the prior art, no injection method for steam generator cited that achieve a 32% reduction of NO to nitrogen at temperatures below 637.78 ° C. The above descriptions of the preferred embodiments for the removal of NOx from exhaust streams are for illustrative purposes. It is not intended to limit the present invention to the specific embodiments described above as a result of manifest deviations from the art. The scope of the invention is defined in the following claims.

Claims (15)

1. A method for reducing NOx in combustion exhaust gases, characterized in that a) NHi precursors are introduced into an exhaust gas in a first injection zone, wherein the exhaust gas has a temperature above about 760 ⁰ C and whereby NO is reduced to N ₂ , and b is a peroxyl initiator is injected into the exhaust gas in a second zone), the exhaust gas has a temperature below about 760 ⁰ C and with the remaining NO to NO ₂ is oxidized. 2. The method according to claim 1, characterized in that an additional stage for the removal of NO _{2 is} contained, which exits from the second injection zone by the NO ₂ -containing gas stream passes a disordered after the second injection zone Naßabscheider. 3. The method according to claim 1, characterized in that in addition the step for removing NO _{2 is} contained, which exits from the second injection zone by introducing the NO ₂ -containing gas into a spray dryer. 4. The method according to claim 1, characterized in that the NHi precursors are selected from a group consisting of ammonia, urea, cyanuric acid, biuret, triuret, ammelide or mixtures thereof. 5. The method according to claim 1, characterized in that hydrogen, hydrogen peroxide or hydrocarbons are introduced into the first stage of the injection zone together with the NHi precursors. 6. The method according to claim 1, characterized in that the Pooxylinitiator is selected from a group consisting of propane, benzene, ethane, ethylene, η-butane, n-octane, methane, hydrogen, methanol, isobutane, pentane, acetylene, methyl alcohol , Ethyl alcohol, acetone, glacial acetic acid, ethyl ether, propyl alcohol, methyl ethyl ketone, propylene, toluene, formaldehyde, camphor, ether and glycol or mixtures thereof. 7. The method according to claim 1, characterized in that simple NHi precursors are injected into an upstream part at a relatively high exhaust gas temperature and complex NHi precursors in a lower part at relatively low exhaust gas temperatures, the optimum temperature in the lower part about 79.4 ⁰ C lower than the temperature of the upper part .. 8. The method according to claim 7, characterized in that the simple NHi precursors are injected into an upstream part, which has an exhaust gas at a temperature of about 954.4 ⁰ C to about 1037.3 ⁰ C and characterized in that the complex NHi precursors are injected into a part situated below, which has an exhaust gas at a temperature of 843.3 ° C to 954.4 ⁰ C. 9. The method of claim 1, characterized in that a mixture of simple and complex NHi precursors are injected into the exhaust gas through one or more levels of injection nozzles, wherein the ratio of delivered at a given level simple or complex NHi precursors automatically on Based on the exhaust gas temperature (s), the signal (s) of the exhaust gas NO analyzer or based on both regulated. 10. The method according to claim 9, characterized in that the simple NHi precursor is an aqueous solution of ammonium hydroxide and the complex precursor is an aqueous urea solution. 11. The method according to claim 9, characterized in that hydrogen, hydrogen peroxide or hydrocarbon are injected together with the complex NHi precursor in the exhaust gas in one or more levels of injection nozzles. 12. An apparatus for injecting either NHi precursors or peroxyl initiators, mixed with carrier gas, into an exhaust gas stream into a steam generator, characterized in that the apparatus comprises devices arranged outside the steam generator for mixing the NHi precursors and the peroxyl initiators with carrier gas are. 13. An apparatus according to claim 12, characterized in that the mixture of such NHi precursors or peroxyl initiators and the carrier gas is injected into the transversely flowing exhaust gas by arranged on the walls of the steam generator wall. 14. An apparatus according to claim 12, characterized in that the mixture of such NHi precursors or peroxyl initiators and the carrier gas is injected into the transversely flowing exhaust gas through an inner injection composite pipe and nozzles. 15. An apparatus according to claim 13, characterized in that the wall nozzles are tangentially aligned and give the exhaust gas a Tangentialdrall. For this 9 pages drawings