CA2174477C - Integrated catalytic/non-catalytic process for selective reduction of nitrous oxides - Google Patents

Abstract

In a combined SCR-SNCR process for reducing the NO
content of a stream of combustion products by NH3 SCR is employed for primary NO reduction. NH, is injected into the SNCR zone only when the NO content of the SCR
effluent exceeds a preselected design maximum value

Description

This application relates to processes for reducing nitrous oxide, NO, sometimes referred to as NOx, in combustion-zone effluent gas streams.
More particularly, the invention concerns a selective catalytic reduction (~~SCR~~) process which, under limited circumstances, employs pretreatment of a combustion zone effluent gas stream by a selective non-catalytic reduction (~~SNCR~~) process, i.e.; only when the NO content oz the SCR effluent exceeds a preselected maximum value.
In another respect the invention pertains to an integrated SCR-SCNR NO reduction process which minimizes the cost of chemical reducing agents.
Carbonaceous fuels can be made to burn more completely, with reduced emissions of carbon monoxide and unburned carbon and/or hydrocarbons, when the air/fuel ratio employed-causes a high flame temperature. When fossil fuels are used in suspension fired boilers e.g.
large utility boilers, flame temperatures above 2000°F, to 3000°F, are generated. Such high temperatures, as well as hot spots of higher _2_ temperatures, cause theproduction of thermal NO, the temperatures being so high that atomic oxygen and nitrogen are formed and chemically combine as nitrogen oxides. -Nitrogen oxides, i.e., NO or NO, ("NO"), can also be formed as the result of. oxidation of nitrogen-containing species in the fuel, e.g. those found in heavy fuel oil, municipal solid waste and coal. NO
derived from nitrogenous 'compounds contained in the. fuel canform even in circulating fluidized bed boilers which operate at temperatures that typically range from 1300°F
to 1700°F.
NO is a troublesome pollutant found in the combustion effluent stream of boilers and other combustion equipment. Nitrogen oxides contribute to tropospheric oaohe, a known threat to health, and can undergo a process known as photochemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons. Moreover, NO is a significant contributor to acid rain and has been implicated as contributing to the undesirable warming of the atmosphere, commonly referred to as the 'greenhouse effect!' .

Recently, various-processes for reducing NO in combustion effluents have been developed. They can generally be segregated into two categories: selective and non-selective. The selective processes include SCR
and SNCR processes: ' -SCR processes involve passing--the combustion zone effluent across or through a catalyst bed in the presence of NH3, to achieve NO reductions as high as 50%-95% or higher. SNCR processes involve introducing NO reducing agents e.g. ~.. NF-I3 into the effluent at higher temperatures than SCR processes, to-achieve NO reductions of up to 50% or greater.
SCR processes for reducing NO are well known and utilize a variety of catalytic agents. For instance, in European Patent Application WO 210,292, Eich cltz and Weiler,disclose the catalytic removal of nitrogen oxides using activated charcoal or activated coke, as a catalyst, with the addition of NH3' Kato et al., in U.S.
Patent No. 4,138,469, and Henke in U.S. Patent No.
4,393,0'31, disclose the catalytic reduction of NO with NH3, using platinum group metals and/or other metals e.cj.
titanium, copper, molybdenum, vanadium, tungsten, or oxides thereof to achieve the desired Catalytic reduction.

Another catalytic reduction process is disclosed by Canadian patent No. 1,100,292 to , which discloses the use of a platinum group metal, gold, and/or silver catalyst deposited on a refractory oxide. Mori et ai , in U.S. PatentNo_ 4,107,272 disclose the catalytic reduction of NO using oxysulfur, sulfate, or sulfite _ compounds of vanadium, chromium, manganese, iron, copper, and nickel with the addition of NH, gas.
In a mufti-phase catalytic system, Gincrer, in-U. S.
Patent No. 4,268,488, discloses treating a NO containing effluent to a first catalyse comprising a copper compound e.g. copper sulfate and a second catalyst comprising metal combinations e.g. sulfates of vanadium and iron or tungsten and iron on a carrier, in the presence of NH,.
- SNCR processes were also proposed to remove NO from combustion gas efflueht streams by injecting NH3 or an NH3 precursor in the presence of oxygen, without using catalysts. For example, such processes are disclosed in U.S. Patent No. 3,900,554 and in U.-S. Patents Nos.
4,777,024; 5,057,293; and 4,780,289.
In addition, combination SNCR-SCR processes have been proposed, e.g. the processes disclosed in U.S.
Patents Nos. 4,978,514; 5,139,'764; 4,286,467; 4,302,431 and 5,233,934.

_5_ Prior combined SNCR-SCR processes sought to avoid handling NH3 and/or sought to reduce catalyst costs, focusing on SNCR as the primary NO reduction stage. SCR
waa relegated to a secondary role, i.e., to remove other - pollutants from the SNCR effluent and to minimize catalyst consumption.
However, we have determined that the use of SNCR as the primary stage for NO removal is not cost effective and results in substantially increased chemical consumption which more than offsets any savings from reduced catalyst consumption. Moreover, overfeeding the reducing chemicals to the SNCR stage to produce excess NH3 in the feed to the SCR stage, as in certain prior processes, causes poorchemical utilization and severe NO/NH3 stratification at the inlet to the SCR stage. This stratification greatly diminishes the effectiveness of SCR, with poor NO removal and high NH, breakthrough from the SCR stage.
We have now discovered that the overall economics of NO removal from combustion zone affluent gas streams can be significantly improved and the technical limitations caused by NO/NH3 stratification can be significantly reduced by an improved integrated SCR/SNCR process. Our process is an improvement on prior combination SNCR/SCR
processes. These prior processes included the steps of -~~~~~~i contacting the effluent gas stream in an SNCR zone with NH; to reduce part of the NO in the stream and then contacting the SNCR zone effluent in an SCR zone with NH3 and a NO reduction catalyst, to further reduce the NO
content of the gas stream. Our improvement in such prior combined SCR/SNCR processes comprises the steps of injecting NH3 into the gas stream upstream of the SCR zone to provide a mixed gas/ammonia stream, providing sufficient catalyst in the SCR zone to reduce the NO
content of the SCR zone effluent to a preselected maximum value at a design total NO throughput and at a design total gas stream throughput, and injecting NH3 into the SNCR zone only when the NO content of the SCR effluent exceeds this preselected maximum value.
As used herein, the term ~~preselected maximum value"
means the desired or target maximum limit of the concentration of NO in the SCR zone effluent. The desired maximum concentration ar target maximum concentration limit is commonly established by reference _ to maximum NO emissions standards set by regulations, laws or recommendations promulgated by cognizant governmental authorities, quasi-governmental authorities or-industry associations. Based on such laws, regulations and recommendations or other considerations, the designer and/or operator of combustion equipment preselects a specific target or desired maximum NO

~1~~4'~"~
concentration limit value which the combustion facility is not to exceed and that "preselected maximum value" is then used in operating the facility according to the claimed process. This preselected maximum value can be the same as the maximum value set by the law, regulations, or recommendation or, to provide a safety margin, the preselected maximum value can be-less than the maximum value set by the law, regulation, or recommendation. Further, the preselected maximum value can be changed from time totime in response to changes in the laws, regulations or recommendations or in response to historical experience with the same or similar combustion systems. Prior to any such change however, the preselected numerical value.
The term "design total NO throughput" refers to the total NO flow rate-(e.g., pounds per hour) in the combustion zone effluent, as determined by material balance: calculations made by the combustion system designer. These calculations are based on specific "design" parameters, e.g. measured NO concentration in the combustion zone effluent, fuel composition, fuel flow rates, air flow rates, combustion efficiency, etc.
The term "design total gas stream throughput" means the total gas stream flow rate (e. g., pounds per hour, cubic foot per minute, etc.) of the combustion zone ' ~~~44~r _8_ effluent gas as determined by material balance calculations made by the combustion system designer, based on specific "design" parameters, s.g. fuel composition, fuel flow rates, ai r flow rates, combustion efficiency, etc.
The amount ofcatalyst provided in the SCR zone is sufficient, at design total gas stream throughput and at design NO throughput, to provide the necessary contact time under the ambient reaction conditions to achieve a - preselected maximum NO concentration in the SCR effluent at any condition of gas stream throughput and/or NO
throughput up to these design maximum rates. When total gas stream throughputs or NO throughputs to the SCR stage which are above these design maxima, the NO content of the SCR effluent will rise above the preselected maximum, because the catalyst quantity is insufficient-to accommodate throughputs above these maxima..
If the NO content of the SCR effluent exceeds the preselected maximum value, and only if this occurs, NH, is injected into the SNCR zone. NH; is injected into the SNCR zone at the minimum Normalized Stoichiometric Ratio mole ratio ("NSR") to reduce the NO throughput in the SCR
zone to below the design maximum throughput. (Aa used herein, "NSR" means the actual NH3:N0 mole ratio in the SNCR stage divided by the stoichiometric NH,:NO ratio in 21'744?7 this stage.) This minimizes or entirely prevents NHS
breakthrough from the SNCR zone.
Furthermore, NH3 is injected into the SNCR stage at the optimum temperature for maximum overall NH;
utilization. As will be explained below, there is an optimum temperature range for NH3 injection into the SNCR
stage which will maximize the overall NH3 utilization of the combined SCR/SNCR process. In a preferred embodiment of the inventipn, hydrogen (H2 is also injected into the SNCR stage and, optionally at the inlet to the SNCR
stage, to broaden the optimum temperature range for NH3 utilization in this stage.
In a further preferred embodiment of the invention, rapid mixing baffles are positioned in the gas stream upstream of the SNCR stages and the SCR stage, to further -reduce-the possibility of NH3/NO stratification by turbulent mixing of the components of the SNCR effluent and turbulent mixing of the NH3 and/or hydrogen injected into the SNCR effluent upstream of the SCR zone.
In the accompanying drawings:
Fig. 1 is a flow sheet which schematically depicts a typical installation;
Fig. 2 is a graph of maximum NOx reduction in % as abscissa and temperature in °F as ordinate;
Fig. 3 is a graph of NH3 ufilization factor as abscissa and temperature in °F
as ordinate;

-9 a-Fig. 4 and Fig. 5 are graphs of (NH;)i/(NO,~i mode ratio as abscissa and (NO)x removal, in percent as ordinate;
Fig. 6 is a dual graph with each of chemical utilization, percent and ammonia slip, in PPM as abscissa and normalized stoichiometry ratio as ordinate;
Fig. 7 is a graph of NOx reduction in % as abscissa and temperature in °F as ordinate;
Fig. 8 is a graph of NH3 slip, in PPM as abscissa and temperature in °F as ordinate;
Fig. 9 is a graph of Ei2/NH3 ratio for max.NOx reduction as abscissa and temperature in °F as ordinate; and Fig. 10 is a graph of NH3 utilization factor as abscissa and temperature in °F
as ordinate.
As seen in Fig. 1, a carbonaceous fuel 10 is mixed with air ~ ~'~ 4 4'~'~
-1~-I1 and burned in a combustion zone 12. The fuel 10 is typically a fossil fuel which is burned to fire electric utility and industrial steam boilers. The fuel 10 may include coal, fuel oil or natural gas. In addition to utility and industrial boilers, the invention has application in controlling NO emissions from fluidized bed boilers, fuel fired petrochemical furnaces, cement kilns, glass melting furnaces, hazardous waste incinerators and even in methanation plants and landfill gas removal applications.
The high temperature effluent gases 13 from the combustion zone are, optionally passed through rapid mixing baffles 14 to increase the chemical and thermal homogeneity of the combustion zone effluent 13. At total gas stream and total NO throughputs which are below the design throughputof tl2e system, the combustion zone effluent 13 passes through the SNCR zone-15 without injection of NH3 16 or hydrogen 17. When the gases have cooled to an appropriate temperature for SCR processing, NH3 18 and, optionally, hydrogen 19 are injected, preferably at or from the edges of the rapid mixing baffles 21, located just upstream of the SCR zone 22.

The mixed combination SNCR zone effluent, NH3 and hydrogen are contacted in the SCR zone 22 with the NO reduction catalyst 23. The SCR zone effluent 24, with NO content at or below the design maximum, is then vented to the atmosphere through stack 25.
As previously noted our process employs SCR as the -primary stage for NO removal, because SCR provides essentially 100 percent utilization of injected NH3 and, thus, offers the highest NO removal with the minimum chemical consumption. The temperature of the combustion gas at which contact with the catalyst is effected in the SCR stage will suitably range from 300.°F to 1200°F.
SNCR is only used when the maximum NO removal capability of the SCR stage is reached and exceeded. The combustion gas temperature at which NH3 is injected in the SNCR stage will range between 1200°F and 2200°F.
Chemical utilization is maximized in the SNCR stage by minimizing the NSR in this stage and by using hydrogen to expand the temperature window for the selective reduction of NO with NH3. H~ injection is used for achieving maximum NO removal and high NH3 utilization in the SNCR
stage at low NSR. By selecting the Hz/NH3 mole ratio and the chemicals injection temperature, high NO removal and high total NH3 utilization can be simultaneously achieved in the SNCR stage as depicted in Figs. 2 and 3. H~ also functions to reduce NH3 breakthrough from the SNCR stage In the preferred embodiment of this invention, rapid mixing baffles 14, 20, and 21 are used in the integrated system prior-to the injection of chemicals, during the injection of chemicals, and after the injection of chemicals. The-purpose of using the baffles 14 prior to the injection of chemicals 16, 17, 18, 19 is to ensure uniform temperature and gaseous species distribution, thus maximizing the NO reduction-by the injected chemicals. Baffles 21 are used during the injection o~
chemicals to enhance chemical mixing with the flue gas.
Baffles, 20, installed downstream of the injection of chemicals, further reduce temperature and chemical species'stratification upstream of the SCR stage 22.
In a preferred embodiment of this invention, chemicals are injected in three locations within the integrated low NO system. The first and preferred location is upstream of the SCR system. NH3 only is injected in this location. The sole purpose for the injection of NH3 in this location is to effect the reduction of NO on the surface of the catalyst. The amount of NH3 injected is increased to maintain a stack NO
value and/or NH3 breakthrough below a certain threshold.

NH3 or A2/NH3 are Injected in the SNCR-Stage 2 only if the stack NO threshold is not achieved or stack NH3 threshold is exceeded. It is expected that up to three discretely separate injection manifolds will be used.
Each of these manifolds may inject NH3 only, H, only, or a combination of the two chemical-s. NSR would range from 0 to 2, and H2/NH3 mole ratio would range from 0 to 5.
In the preferred embodiment of this invention, the NH3 injection would be-staged, thus reducing the NSR ratio for each stage and improving chemical utilization. The amount and location of Hz injection would depend on injection temperature and NH3-Ha mole ratio.
Prior processes used SNCR as the primary method for NO reduction. Certain of these processes increased the NSR in the SNCR-stage and used NH3 breakthrough from this stage to feed NH3 to the SCR stage. It was disclosed that higher NSR in the SNCR stage increases the NO removal in this stage and the excess NH, was used to effect further reduction of NO in the SCR stage. However, experience does not demonstrate the economic attractiveness of this approach.
The data in Fig. 4 are for low injection temperature conditions, and data in Fig. 5 are for high injection temperature conditions. The solid lines show the mole ratio and the corresponding NO removal obtained with the use of a nitrogenous reducing chemical in the SNCR stage.
The dotted lines depict the overall mole ratio required to achieve a specific overall NO removal with an integrated SNCR/SCR system.
Thus, increasing the NO removal achieved in the SNCR
stage increases the mole ratio required to achieve a specific overall integrated system NO removal. For an overall integrated system NO removal of 90 percent, the mole ratio is between two to four times the mole ratio for a stand alone SCR eystem.
The penalty in chemical consumption is less for low temperature injection conditions because of the reduced tendency for NH3 oxidation at low temperatures, as shown in Fig. 4.-- Nevertheless, chemical consumption can still more than double that used in our integrated SCR/SNCR
system, in which SCR is employed as the primary stage for NO removal Typical curvesillustrating NH3 utilization and breakthrough in an SNCR stage are depicted in Fig. 6.
The data show that increasing the NSR reduces chemical utilization and increases NH3 breakthrough. For this reason, in our process we limit the mole ratio in the SNCR stage to control NH3 breakthrough to below 20 percent of the initial NO concentration at the inlet to the SNCR
stage.
Reducing the mole ratio in the SNCR stage not only improves chemical utilization but to also reduces NH3 concentration as well as NH3/NO concentration stratification at the discharge of the SNCR stage.
In addition to the NSR, chemical utilization in the SNCR stage is greatly influenced by reaction temperature Fig. 7 depicts the change in NO reduction with temperature as generated by analytical modeling.. As discussed above, the NO reduction/temperature relationship has the shape of a bell curve with maximum NO removal obtained at a specific temperature.-Increasing or reducing the reaction temperature results in reducirig the NO removal. The injection of Ha with NH3 reduces the optimum reaction temperature and the magnitude of this temperature reduction depends upon the amount of H2 used. An increase in H,/NH3 mole ratio produces a larger reduction in optimum temperature as shown in Fig. 7.
NH3 breakthrough from the SNCR stage is also dependent upon reaction temperature.- The dependence of NH3 breakthrough on temperature is shown in Fig. 8. Fig.
8 shows that NH3 breakthrough rapidly increases with a reduction in temperature. Fig. 8 shows that, for a given temperature, injection of H2 with NH3 reduces NH;
breakthrough. For example, for a reaction temperature of 1500°F, increasing A2/NH3 mole ratio from 0.125 to 2.5 reduces NH3 breakthrough from-150 ppm to 40 ppm. Ha injection can thus be used to control NH3 breakthrough as well as to improve NH3 utilization in-the SNCR stage.
Maximum NO reduction as a function of temperature can be achieved by changing Ha/NH3 mole ratio. These data, determined by analytical modeling, are depicted in Fig. 9. The data show that at temperatures between 1800°F and 2000°F, little or no Ha injection is needed_ As the reaction temperature is reduced, Ha requirements to maintainoptimum NO reduction is increased.
As discussed above, our invention provides improved chemical utilization. This is achieved in the SNCR stage by reducing the NO removal requirement in this stage and, in turn, the required NSR. It is also achieved by improving total NH3 utilization, i.e., the sum of NH3 used for NO reduction and NH, breakthrough. In an integrated SCR/SNCR system, maximizing total NH3 utilization will ultimately result in the maximum efficiency of use of chemicals. NH3 utilization factor as a function of temperature is presented in Fig. 10. Fig. 10. shows that, depending on H,/NH3 mole- ratio, an optimum ~~~~4'~'~

temperature can be defined to achieve 100 percent total NH3 utilization. Such temperature, for example, is 1800°F
for H2/NH3 mole ratio of 0.0, and 1600°F for H2/NH3 mole ratio of 0_.125.
However, optimum temperature for maximum total NH3 utilization does not produce the maximum NO removal in the SNCR stage. As shown in Fig. 7, injecting NH3 at optimum temperature for maximum total NH3 utilization results in less than one-third the NO removal achieved by injecting NH3 at the optimum temperature for maximum NO
removal.
Preferably, Ha is injected to achieve maximum NO
reduction in the SNCR stage at all temperature conditions. The use of Ha maintains essentially a constant NO removal over a broad temperature range. The amount of H, used changes with temperature as shown in Fig. 9. H, injection is also used to maximize total NH3 utilization. While maintaining maximum NO removal in the SNCR stage, the use of Ha allows NH, utilization to range between 70 and 90 percent.
In addition to the chemical factors discussed above, the NO removal performance of integrated system is improved in our invention by reducing the stratification of temperature-and chemical species concentration within the flue gas. Flue gas temperature stratification exists in a utility boiler application for a variety of reasons.
Burners malfunction can result in extended flames and the creation of hot furnace regions in the convective section of the furnace.- Heat transfer surface fouling can lead to non-uniform heat absorption profile resulting in temperature atratifications. Many boilers are designed with divided furnaces to bypass portions ofthe flue gas around the reheat and/or superheat sections of the boiler, allowing the control of steam temperature with minimal use of stream attemperation. The use of flue gas bypass results in temperature stratification at the discharge of the economizer or at the inlet .to the SCR
system. Temperature--stratification in the SNCR stage influences NH3 reduction of NO (Fig. 7). Localized high temperatures due to severe stratification can result in the oxidation of NH3 to NO. Localized low temperatures result in high NH3 breakthrough.

2~.'~44'~'~

Like any other chemical reaction system, gaseous species stratification in a NO reduction system is detrimental. As discussed above, NH3 utilization in the SNCR stage is greatly dependent upon NSR. The stratification of NO may result in "localized" high NSRa, leading to poor NH3 utilization: Reaction time of Ha with NH3 is in the order of milli-seconds-. For the injected Ha to be fully utilized it is important that other gaseous species (NO and NH3) are uniformly distributed.
Preferably, stratification is further minimized in the beat mode-of our system with the use of rapid mixing baffles, e.g., the °delta wings" developed by Balcke-Durr of-Germany and others-for a broad range of flue gas mixing applications. The rapid mixing baffles are positioned in the flow path ofthe flue gas. The size and shape of the baffles and their orientation with respect to the flue gas flow direction are selected to induce a large downstream recirculation field. The recirculation enhances gas mixing, thus reducing or almost totally eliminating temperature or gaseous species stratification.
In the best mode of our system, the baffles are installed upstream of the NH3 and Ha injection location to ensure uniformity of flue gas temperature and gaseous species distribution. Preferably, the rapid mixing . ~1~44~7 baffles are also used for the injection of chemicals.
The injection of chemicals downstream of the baffles thus provides for mixing the chemicals with the flue gas.
Finally, the baffles are used to mix the flue gas downstream of the SNCR stage to reduce or substantially eliminate temperature and chemical species stratification at the inlet to the SCR stage.
In the preferred practice of our invention, the NH3 is injected into the SNCR zone at multiple discretely separate injection locations to reduce the NSR for each injection location and improve chemical utilization. In the beat mode of our invention there are at least two of these discrete separate injection locations. Likewise, hydrogen is injected into the SNCR zone at multiple discretely separate injection locations. This expands the temperature range for ammonia NH3 reduction of NO, improves overall chemical utilization and reduces NH3 breakthrough from the SNCR stage. The NH3 and hydrogen can be injected separately into the SNCR stage or as mixtures in each injection location.
Thus, according to the preferred practice of our invention, the NSR in the SNCR stage-ranges between 0 and 2 and the H2:NH; mole ratio in the SNCR stage ranges between 0 and 5. Under these conditions NH3 breakthrough from the SNCR zone is maintained at or below 20 percent -zl-of the inlet NO concentration of the gases eritering the SNCR stage. The NH3:N0 mole ratio of the process is selected to maintain NH3 in the effluent from the SCR
stage below 20 ppm.

Claims (18)

1. ~In a combination SCR-SNCR process for reducing the NO
content of a gas stream effluent from a combustion system, said effluent gas stream containing combustion products, including NO, said process including contacting said gas stream in an SNCR zone with NH3 to reduce part of the NO in said stream, and contacting the SNCR zone effluent in an SCR zone with NH3 and a NO reduction catalyst, to further reduce the NO
content of said gas stream, the improvement comprising (a) injecting NH3 into said gas stream downstream of said SNCR zone to provide a mixed gas-amonia stream, the quantity of NH3 injection being just sufficient to effect the NO reduction of step b);

(b) providing sufficient catalyst in said SCR zone to reduce the NO content of the SCR zone effluent to a preselected maximum value at the design total NO throughput of said system and at the design total gas stream throughput of said system; and (c) injecting NH3 into said SNCR zone only when the NO
content of the SCR zone effluent exceeds said preselected maximum value.

2. ~The process of claim 1, further comprising injecting H2 into said SNCR zone when the NO content of the SCR zone effluent exceeds said preselected maximum value.

3. ~The process of claim 1, further comprising injecting H2 into said SNCR zone, when the NO content of the SCR zone effluent exceeds said preselected maximum value, for improving NO removal and for improving NH3 utilization.

4. ~The process of claim 1, further comprising injecting H2 into said SNCR zone, when the NO content of the SCR zone effluent exceeds said preselected maximum value, for broadening an optimum temperature range for NH3 utilization in said SNCR zone.

5. ~The process of any one of claims 2 to 4, wherein H2 is injected at an inlet to said SNCR zone.

6. ~The process of any one of claims 1 to 4, wherein a combustion gas temperature at which the NH3 is injected into said SNCR zone is between 1200°F and 2200°F.

7. ~The process of any one of claims 1 to 6, wherein a temperature of combustion gas at which contact with the catalyst is effected in the SCR zone is between 300°F and 1200°F.

8. ~The process of any one of claims 1 to 4, 6 and 7, wherein where the NO content of the SCR effluent exceeds the preselected maximum value, NH3 is injected into said SNCR
zone at a minimum Normalized Stoichiometric Ratio mole ratio to reduce the NO throughout in the SCR zone to below the design maximum throughout thereby minimizing or preventing NH3 breakthrough from said SNCR zone.

9. The process of claim 8, wherein NH3 breakthrough is below 20%, in terms of weight, of an initial NO
concentration at an inlet of said SNCR zone.

10. The process of claim 5, wherein a combustion gas temperature at which the NH3 is injected into said SNCR zone is between 1200°F and 2200°F.

11. The process of claim 10, wherein a temperature of combustion gas at which contact with the catalyst is effected in the SCR zone is between 300°F and 1200°F.

12. The process of claim 11, wherein where the NO content of the SCR effluent exceeds the preselected maximum value, NH3 is injected into said SNCR zone at a minimum Normalized Stoichiometric Ratio mole ratio to reduce the NO throughout in the SCR zone to below the design maximum throughout thereby minimizing or preventing NH3 breakthrough from said SNCR zone.

13. The process of claim 12, wherein NH3 breakthrough is below 20%, in terms of weight, of an initial NO
concentration at the inlet of said SNCR zone.

14. The process of any one of claims 1 to 13, wherein a Normalized Stoichiometric Ratio mole ratio is between 0 and 2 and a H2:NH3 mole ratio in said SNCR zone is between 0 and 5.

15. ~The process of any one of claims 1 to 14, wherein an amount of NH3 in the effluent from the SCR zone is below 20 ppm.

16. ~In a combined SCR-SNCR process for reducing the NO
content of a gas stream effluent from a combustion system, said effluent containing combustion products, including NO, said process including contacting said gas stream in an SNCR zone with NH3 to reduce part of the NO in said stream, and contacting the SNCR zone effluent in an SCR zone with NH3 and a NO reduction catalyst to further reduce the NO
content of said gas streams, the improvement comprising:
(a) injecting NH3 into said gas stream downstream of said SNCR zone and upstream of said SCR zone to provide a mixed gas-ammonia stream, the quantity of NH3 injected being just sufficient to effect the NO reduction in the SCR zone;
(b) providing sufficient catalyst in said SCR zone to reduce the NO content of the SCR zone effluent to a maximum value;
(c) passing said gas stream with an amount of NO that exceeds said maximum value to said SCR zone; and (d) injecting NH3 into said SNCR zone only when the NO
content of the SCR zone effluent exceeds said maximum.

17. The process according to claim 16, wherein the temperature of the combustion gas at which contact with the catalyst is effectd in the SCR zone is from about 300°F to 1200°F and the combustion gas temperature at which NH3 is injected in the SNCR zone is from about 1200°F to 2200°F.

18. The process according to any one of claims 1 to 17, wherein the catalyst in the SCR zone is selected from the group consisting of platinum group metals, titanium, copper, molybdenum, vanadium, tungsten, and oxides thereof.