Multi-Stage Process for Reducing the Concentration of Pollutants in an Effluent Using an Ammonium Salt

The present invention relates to a process for reducing the concentration of pollutants, especially pollutants such as nitrogen oxides (NO.sub.x), in the effluent from the combustion of a carbonaceous fuel by injecting a first treatment agent into the effluent at a first temperature zone and injecting a second treatment agent into the effluent at a second temperature zone, wherein the first and second treatment agents are injected under conditions effective to reduce the effluent pollution index. At least one of the treatment agents comprises an ammonium salt of an organic acid having a carbon to nitrogen ratio of greater than 1:1. Preferably, the effluent is the oxygen-rich effluent from the combustion of a carbonaceous fuel.

Carbonaceous fuels can be made to burn more completely, and with reduced emissions of carbon monoxide and unburned hydrocarbons, when the oxygen concentrations and air/fuel ratios employed are those which permit high flame temperatures. When fossil fuels are used to fire large utility boilers, temperatures above about 2000.degree. F. and typically bout 2200.degree. F. to about 3000.degree. F. are generated. Unfortunately, such high temperatures, as well as hot spots of higher temperatures, tend to cause the production of thermal NO.sub.x, the temperatures being so high that free radicals of oxygen and nitrogen are formed and chemically combine as nitrogen oxides. Even in circulating fluidized bed boilers that operate at temperatures of 1300.degree. F. to 1600.degree. F., significant amounts of nitrogen oxides can be formed.

Nitrogen oxides are troublesome pollutants which are found in the combustion effluent streams of large utility and circulating fluidized bed boilers when fired as described above, and comprise a major irritant in smog. It is further believed that nitrogen oxides often undergo a process known as photo-chemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons. Moreover, nitrogen oxides are a significant contributor to acid rain.

Unfortunately, the high temperatures within boilers render most common methods of reducing NO.sub.x concentrations, such as effluent scrubbing or catalyst grids, uneconomical, infeasible, or both.

Many different processes and compositions have been proposed for chemically reducing nitrogen oxide levels in an effluent. These proposals call for adding chemicals, dry or in solution, directly to the effluent and achieve significant NO.sub.x reductions. However, none have been identified which add a number of different chemicals at defined, distinct temperature zones to achieve NO.sub.x reductions of greater than 50%, and preferably greater than 75%, with commercially practical residence times. Moreover, some of the techniques are capable of reducing NO.sub.x only at the expense of creating undesirable levels of other pollutants such as ammonia and/or carbon monoxide.

In U.S. Pat. No. 3,900,554, Lyon discloses reducing nitrogen monoxide (NO) in a combustion effluent by injecting ammonia, as a pure substance or in the form of ammonium formate, ammonium oxalate or ammonium carbonate, or their aqueous solutions, into the effluent for mixing with the nitrogen monoxide at a temperature within the range of 1600.degree. F. to 2000.degree. F. Lyon also suggests the use of reducing agents, such as hydrogen or various hydrocarbons, to permit the effective use of ammonia at effluent temperatures as low as 1300.degree. F. Although the patent suggests staged injection of the ammonia composition, there remains no teaching of the efficacy of injecting distinct compositions at different temperature zones to optimize NO.sub.x reduction without producing a substantial amount of other pollutants.

In U.S. Pat. No. 4,208,386 Arand et al. disclose that, for oxygen-rich effluents, the temperature of the effluent should be in the range of 1300.degree. F. to 2000.degree. F. for reducing the nitrogen oxides concentration using urea added dry or in aqueous solution. Alkanoic solvents are said to be reducing agents which, like hydrogen, carbon monoxide, etc., enable the effective operating temperature to be lowered to below 1600.degree. F. Disclosed again is the suggestion to inject in increments, but these incremental injections are of the same urea composition and must all be at positions meeting the same temperature and oxygen concentration conditions. The same holds true for U.S. Pat. No. 4,325,924 to Arand et al.

Although the prior art discloses injection of a composition for reducing nitrogen oxides at a number of spaced positions in, for instance, Bowers, in U.S. Pat. Nos. 4,751,065 and 4,719,092, each disclosure is related to the injection of the same composition at locations in which the same conditions, such as temperature and oxygen concentration, exist.

Furthermore, although the reduction of the concentration of nitrogen oxides in an effluent to as great an extent as possible is highly desirable, prior art systems for reducing NO.sub.x concentrations are limited, not only by the amount of NO.sub.x reduction that can be achieved utilizing them, but also by the amount of other pollutants, such as ammonia or carbon monoxide, generated as byproducts of the NO.sub.x -reducing process.

What is desired, therefore, is a process for substantially reducing the concentration of nitrogen oxides in an effluent while maintaining a suitably low level of other pollutants.

This invention relates to a process for reducing the concentration of a pollutant in the effluent from the combustion of a carbonaceous fuel. One of the objectives of the invention is to achieve the desired level of pollutant control, such as a significant reduction in nitrogen oxides concentration, while minimizing other harmful emissions such as ammonia and carbon monoxide, and maximizing the utilization of the chemicals employed.

More particularly, the present invention comprises a process which serially treats the effluent from the combustion of a carbonaceous fuel by injecting different treatment agents at different effluent temperatures. For example, a first treatment agent is injected into the effluent at a first temperature zone, a second treatment agent is injected into the effluent at a second temperature zone, and the process is repeated, if desired, to achieve the desired level of pollutant control. The term "treatment agent" as used in this description should be understood to refer to a composition comprising a reductant chemical, i.e., a pollution reducing chemical capable of reducing NO.sub.x, sulfur oxides (SO.sub.x) or other pollutants, and, preferably, a solvent. The composition of each treatment agent is formulated to be effective at reducing the concentration of the target pollutant, especially nitrogen oxides, in the effluent when injected into the effluent at the designated temperature zone. In the practice of the present invention, at least one of the injected treatment agents comprises an ammonium salt of an organic acid having a carbon to nitrogen ratio of greater than 1:1.

It has been found that nitrogen oxide reduction can be improved by increasing the amount of reductant chemical employed in the treatment agent. However, a point is reached where emissions of other pollutants such as ammonia are experienced. The emission of such other pollutants is undesirable. For instance, the emission of ammonia can lead to harmful deposits of ammonium bisulfate. Furthermore, carbon monoxide can also be produced. This limits the amount of nitrogen oxides control possible in any one treatment step. It has also been found that different chemical formulations are effective at reducing nitrogen oxides concentration at different temperatures.

Moreover, it is not possible to inject chemicals in every location in a boiler, because of design considerations. The injection must occur in a location where space is available inside the boiler for distribution of chemicals. Injection directly on heat exchange tubes could lead to harmful deposits and ineffective use of chemicals. As a practical matter, adequate space for injection may typically exist in a boiler at two to four locations, and these will be at different temperatures because of the heat transfer taking place.

In the practice of this invention, nitrogen oxides reduction is maximized by selecting the locations at which injection is possible, formulating treatment agents that are effective at reducing the nitrogen oxides level at the temperature at each location, injecting the chemicals at each location to maximize reduction while avoiding other emissions such as ammonia and carbon monoxide, and controlling the injection process as boiler load varies. For example, if boiler load drops from 100% to 50%, temperatures at each location may be lowered and changes in injections (amount, composition, or both) may be needed.

This invention can be used to achieve a given level of nitrogen oxides control and also to minimize the chemical cost of doing so. To accomplish this, use of the least expensive treatment agent is preferably maximized first, followed by the next least expensive treatment agent, etc., until the desired level of control is achieved.

Although this description is written in terms of the reduction of the concentration of nitrogen oxides in the effluent, the skilled artisan will recognize that the process of this invention may be equally applicable to the reduction of other pollutants which may be found in the effluent from the combustion of a carbonaceous fuel. Furthermore, although written in terms of utilization in a suspension-fired boiler, the description should be understood to be equally applicable to other types of units such as circulating fluidized bed boilers firing a variety of fuels including refuse.

For the purposes of this description, the term "temperature zone" refers to a locale wherein, under steady state conditions, the effluent temperature is within a certain range; the term "ammonium salt of an organic acid having a carbon to nitrogen ratio of greater than 1:1" refers to salts which can be formed by the neutralization of ammonium hydroxide with an organic acid, preferably a carboxylic acid (i.e., an acid having one or more carboxyl (COOH) groups); if the acid has more than one carboxylate group, they may be partially or completely neutralized by ammonium hydroxide; the ratio of carbon to nitrogen in the salt is greater than 1:1, meaning that there is more than one carbon per each nitrogen in the compound, most preferably there are at least two carbons per each nitrogen in the compound; the term "oxygenated hydrocarbon" refers to a hydrocarbon which contains oxygen or an oxygen-containing group; the term "alcohol" refers to a hydrocarbon derivative in which one or more hydrogen atoms have been replaced by a hydroxy group.

The presence of pollutants in an effluent may be referred to as the "pollution index", which is used herein to denote an index which indicates the presence of all of the pollutants in the effluent. It will be understood that reducing the concentration of one pollutant, such as nitrogen oxides, in the effluent in a process which simultaneously leads to the generation of another pollutant does not lower the pollution index. The present invention accomplishes the reduction of nitrogen oxides while substantially avoiding the production of other pollutants such as ammonia or carbon monoxide, thus effecting a net lowering of the pollution index of the effluent, by a step-wise or multi-stage process wherein a plurality of treatment fluids are injected into the effluent at a plurality of temperature zones.

The use of the terms "first", "second" and "third" treatment zones in this description is meant to denote relative locations of the treatment zones. For instance, the second temperature zone can be any zone where the effluent temperature is lower than the effluent temperature of the first temperature zone. Similarly, the third temperature zone can be any zone where the effluent temperature is lower than the effluent temperature in the second temperature zone, etc. This description should not be read as indicating that any specific temperature zone for injection must in all cases be in a location where the effluent is in a specific temperature range (i.e., the first temperature zone does not in all cases have to be in a location where the effluent temperature is in he range of about 1700.degree. F. to about 2000.degree. F.).

The treatment agent to be injected at any particular temperature zone is preferably chosen to be most effective at the effluent temperatures existing within that zone. For instance, if the first available temperature zone for injection is in an upstream location comprising a temperature zone where the effluent temperature is in the range of about 1700.degree. F. to about 2000.degree. F., the treatment fluid can be chosen to be that which is most effective in that temperature range, such as an aqueous mixture of urea, as disclosed by Bowers in U.S. Pat. No. 4,719,092, or an aqueous mixture of ammonia, or gaseous ammonia itself, as disclosed by Lyons in U.S. Pat. No. 3,900,554, the disclosures of which are incorporated herein by reference. Although the mechanism by which ammonia or urea decrease the concentration of nitrogen oxides is not fully understood, it is believed that they function by facilitating a series of reactions involving NH.sub.x radicals (x being an integer) and NO.sub.x. The molar ratio of the concentration of NH.sub.x radicals to the concentration of NO.sub.x ([NH.sub.x ]/[NO.sub.x ]) is often referred to as the normalized stoichiometric ratio (NSR). If the geometry of the boiler permits, two injections can be made in an upstream location. The first can be further upstream in a temperature zone where the effluent temperature is about 1850.degree. F. to about 2000.degree. F. and the second at a location downstream from the first location in a temperature zone where the effluent temperature is about 1700.degree. F. to about 1850.degree. F. As indicated by the referenced disclosures, the urea or ammonia mixture can be more concentrated (e.g., about 20% to about 50% urea or ammonia by weight) in the lower temperature location and more dilute (e.g., about 5% to about 20% urea or ammonia by weight) in the higher temperature location.

It will be understood that the term "mixture" as used herein includes within its scope mixtures, solutions and dispersions, and the term "carrier" as used herein includes within its scope carriers, dispersants and solvents. Moreover, the terms "mixture", "dispersion" and "solution", and the terms "carrier", "dispersant" and "solvent", when used herein, will be understood to be interchangeable for the purposes of this disclosure.

Appropriate temperature zones for injection according to the present invention may also be found downstream from the zones discussed above, where the effluent temperature is in the range of about 1300.degree. F. to about 1750.degree. F. Suitable treatment agents for injection into a temperature zone having such effluent temperatures comprise an ammonium salt of an organic acid having a carbon to nitrogen ratio of greater than 1:1. Advantageously, the organic acid comprises a carboxylic acid.

Particular treatment agents suitable for this temperature zone include compounds which are selected from the group consisting of ammonium acetate, ammonium and diammonium adipate, ammonium benzoate, ammonium binoxalate, ammonium caprylate, ammonium, diammonium and triammonium citrate, ammonium crotonate, ammonium and diammonium dodecanoate, ammonium and diammonium fumarate, ammonium heptanoate, ammonium linolenate, ammonium and diammonium malate, ammonium mono butyrate, ammonium oleate, ammonium and diammonium pthalate, ammonium propionate, ammonium salicylate, ammonium and diammonium succinate ammonium and diammonium tartarate, and ammonium, diammonium and triammonium trimellitate. The disclosed treatment agents may also include aqueous solutions of ammonia or urea and suitable enhancers such as hexamethylenetetramine (HMTA) and/or ethylene glycol.

The geometry of the boiler may also permit more than one temperature zone for injection within the effluent temperature range of about 1300.degree. F. to about 1750.degree. F. For example, an injection can be made at a location in a temperature zone where the effluent temperature is in the range of about 1550.degree. F. to about 1750.degree. F. A second location for injection can be in a temperature zone where the effluent temperature is in the range of about 1300.degree. F. to about l550.degree. F. The treatment agent injected in the second of the indicated temperature zones can be similar to that of the first or can be less dilute, or comprise a different enhancer concentration, etc., as would be familiar to the skilled artisan.

Another temperature zone in a boiler at which injection may be made is at the location where the effluent temperature is below about 1450.degree. F. As disclosed by copending and commonly assigned U.S. patent application entitled "Process for Reducing Nitrogen Oxides in an Effluent Using a Hydrocarbon or Hydrogen Peroxide", Ser. No. 22,799, filed in the name of Sullivan on March 6, 1987, the disclosure of which is incorporated herein by reference, a suitable treatment agent for injection into the effluent at such effluent temperatures comprises a hydrocarbon, such as ethylene glycol or furfural, or hydrogen peroxide. More than one temperature zone for injection of a treatment agent can also be located within the lower effluent temperature locations in the boiler.

In a preferred embodiment, the process comprises injecting a first treatment agent into the effluent at a first temperature zone. For instance, in a large suspension-fired utility boiler, the location of injection of the first treatment fluid can be upstream from the superheater, such that the effluent temperature in the first temperature zone is greater than about 1700.degree. F. The composition and amount of the first treatment agent can then be chosen to provide effective reduction of NO.sub.x concentration in an effluent which is at temperatures greater than about 1700.degree. F. while minimizing the production of ammonia. Suitable formulations for use as the first treatment agent are those comprising aqueous mixtures of urea or ammonia, or gaseous ammonia.

The terms "urea" and "ammonia" as used in this description includes the compounds themselves, as well as compounds equivalent in effect. Thus, unless otherwise specified, reference in this disclosure to urea or ammonia should not be taken as limiting to urea or ammonia themselves, but should extend to urea or ammonia and all of their equivalents. It is pointed out that the term equivalent is not limited to exact equivalents, and various equivalents will be optimally operable at some conditions which are different than those for other equivalents. Moreover, some equivalents may be more effective than others.

The urea or ammonia aqueous mixture functioning as the first treatment agent is preferably injected at a number of spaced positions within the first temperature zone from nozzles or other apparatus which are effective to uniformly form and disperse droplets of the mixture within the flowing effluent stream to achieve uniform mixing.

The rate of injection of the first treatment agent into the effluent at the first temperature zone is preferably that rate which achieves maximum NO.sub.x -concentration reduction up until the point of "ammonia breakthrough". "Ammonia breakthrough" is a term used in the art which refers to the point where a significant increase in the NH3 concentration with rate of injection is observed. The actual rate of injection of the first treatment agent is determined experimentally by "tuning" the rate of injection to achieve the conditions described above, because the actual rate will vary with effluent stream flow rate, as well as the particular temperature at that temperature zone, which can vary within the given range due to the load at which the boiler is fired. This applies equally to the rate of injection of all of the treatment agents considered herein.

Advantageously, in the situation where the temperature range within the first temperature zone is greater than about 1700.degree. F., and the first treatment agent is a mixture comprising urea or ammonia, the molar ratio of the nitrogen in the first treatment agent to the baseline nitrogen oxides level is about 1:5 to about 5:1, more preferably about 1:3 to about 3:1, and most preferably about 1:2 to about 2:1.

The temperature of the effluent will have an influence on the concentration of urea or ammonia in the mixture. At temperatures of between about 1700.degree. F. and about 1850.degree. F., the mixture will tend to operate most effectively at concentrations of about 20 to about 50 weight percent. Contrariwise, at temperatures of greater than about 1850.degree. F., the concentration of the mixture will typically be more dilute, such as about 5 to about 20 weight percent. Alternatively, when the effluent temperature is in the range of about 1700.degree. F. to about 1850.degree. F., the urea or ammonia mixture which comprises the first treatment agent may be enhanced by the addition of hexamethylenetetramine. Other enhancers which may be suitable for use include guanidine, guanidine carbonate, biguanidine, guanylurea sulfate, melamine, dicyandiamide, calcium cyanamide, biuret, 1,1,-azobisformamide, methylol urea, methylol urea-urea condensation product, dimethylol urea, methyl urea, dimethyl urea, and mixtures thereof. It is also understood that the first treatment agent can comprise gaseous ammonia. In addition, depending on boiler configuration, it is anticipated that at least two temperature zones (e.g., one at a location where the effluent temperature is about 1850.degree. F. to about 2000.degree. F. and another at a looation where the effluent temperature is about 1700.degree. F. to about 1850.degree. F.) may be possible and/or desired upstream from the superheater, as discussed above.

The process of this invention preferably further comprises injecting a second treatment agent into the effluent at a second treatment zone located downstream from the first temperature zone. For instance, in a large suspension-fired utility boiler, the second temperature zone can advantageously be at a location downstream from the superheater, where the temperature in the second temperature zone will typically be in the range of about 1300.degree. F. to about 1750.degree. F. However, as discussed above, the second temperature can be any defined zone having temperatures lower than the first temperature zone, e.g., it may be above or below the temperature of about 1300.degree. F. to about 1750.degree. F. so long as it is below that of the first temperature zone. The composition of the second treatment agent is then preferably chosen to achieve optimal nitrogen oxides reduction without ammonia breakthrough in this temperature zone. Advantageously, the second treatment agent comprises an ammonium salt of an organic acid having a carbon to nitrogen ratio of greater than 1:1. Preferably, the ammonium salt is one having the general formula ##STR1## wherein n is an integer, preferably from 1 to 4; R is alkyl, alkenyl, carboxyl, acyl, carbocyclic, aryl, amino, alkoxyalkyl, hydroxyalkyl, thiohydroxyalkyl, and combinations thereof; the ratio of carbon to nitrogen is greater than 1:1; and where when n is greater than 1, the ##STR2## groups are attached to the same or different carbon atoms.

Preferred ammonium salts for use as the second treatment agent include the ammonium salts of alkyl carboxylic acids such as heptanoic acid and mono butyric acid, alkenyl carboxylic acids such as linolenic acid and crotonic acid, and aryl carboxylic acids such as salicylic acid; the ammonium or diammonium salts of alkyl dicarboxylic acids such as succinic acid, adipic acid, dodecanoic acid, malic acid and tartaric acid, alkenyl dicarboxylic acids such as fumaric acid, and aryl dicarboxylic acids such as pthalic acid; and the ammonium, diammonium or triammonium salts of alkyl tricarboxylic acids such as citric acid, alkenyl tricarboxylic acids and aryl tricarboxylic acids such as trimellitic acid. Additionally, it will be recognized that aryl or other groups groups may have more than three carboxylic acid groups as substituents thereon, and that ammonium salts may form on four or more of such groups.

Most preferably, the ammonium salts include those ammonium salts of acetic acid (ammonium acetate), substituted and unsubstituted benzoic acid (ammonium benzoate), oxalic acid (crystallized) (ammonium binoxalate), caprylic acid (ammonium caprylate), citric acid (ammonium citrate), oleic acid (ammonium oleate) and propionic acid (ammonium propionate). It will further be recognized that the above-named salts may be substituted or unsubstituted. For instance, it will be recognized that ammonium benzoate may have other substituent groups at one or more of the ring carbons in place of a hydrogen group. Typical substituents include hydroxy, alkoxy, carboxy, alkyl, acyl, amino and alkenyl groups.

As used herein, the term "alkyl" refers to a straight or branched chain hydrocarbon containing no unsaturation, e.g., methyl, ethyl, isopropyl, 2-butyl, neopentyl, n-hexyl, etc.; the term "alkoxy" refers to a univalent radical composed of an alkyl group linked through an oxygen atom having its free valence bond therefrom, e.g., methoxy (--O--CH.sub.3), ethoxy (--O--CH.sub.2 --CH.sub.3), etc.; the term "acyl" refers to a carboxyl group in which the hydroxyl group has been replaced with another radical, such as alkyl, aryl, etc., e.g., acetyl ##STR3## the term "carbocyclic" refers to an organic radical whose `skeleton` is in the form of a closed ring of carbon atoms; the term "aryl" refers to an aromatic hydrocarbon group, e.g., phenyl, benzyl, naphthyl, etc.; the term "carboxyl" refers to a univalent radical composed of a carbonyl group and a hydroxyl group linked through a carbon atom having its free valence bond therefrom, e.g., ##STR4## the term "carbonyl" refers to a bivalent radical having the formula ##STR5## the term "hydroxyl" refers to the univalent group --OH which is characteristic of alcohols; the term "hydroxyalkyl" refers to an alkyl group having at least one hydrogen substituted with an hydroxyl group; the term "alkoxyalkyl" refers to an alkyl group having at least one hydrogen substituted with an alkoxy group; the term "thiohydroxyalkyl" refers to a hydroxyalkyl group wherein the oxygen of the hydroxyl group is replaced with sulfur; and the term "alkenyl" refers to an unsaturated radical of lower branched or unbranched alkyl groups it is derived from, having at least one double bond therein.

Advantageously, the second treatment agent of this invention is introduced into the effluent in solution or as a dispersion or mixture in a suitable carrier. Water is a preferred carrier due to the economy of aqueous solutions, dispersions and mixtures and the fact that they can be employed with suitable effectiveness in most situations. The effective mixture comprising the treatment agent of this invention will range from saturated to dilute. While water is an effective carrier for most applications, it will be recognized that there may be instances where other carriers may be advantageously used, either alone or in combination with water, as would be known to the skilled artisan.

The level of the ammonium salt of an organic acid having a carbon to nitrogen ratio of greater than 1:1 present in the second treatment agent is preferably in the range of about 0.5% to about 50% by weight, most preferably about 5% to about 35% by weight. Where urea or ammonia is employed in the second treatment agent, it should preferably be present in the mixture in the range of about 2% to about 65%, most preferably about 5% to about 30% by weight. The weight ratio of ammonium salt of an organic acid having a carbon to nitrogen ratio of greater than 1:1 to urea or ammonia, when urea or ammonia is used in the second treatment agent mixture, as discussed in more detail below, should advantageously be about 1:10 to about 4:1, more preferably about 1:5 to about 3:1. The most preferred weight ratio in the mixture is about 1:4 to about 2.5:1.

The temperature of the effluent at the point of introduction of the second treatment agent will have an influence on the concentration of the mixture. At lower temperatures (i.e., below about 1550.degree. F.), the mixture will tend to operate effectively at high concentration, e.g., about 10% to about 65% by weight treatment agent. On the other hand, at higher temperatures (i.e., in excess of about 1550.degree. F.), the mixture will tend more towards dilute. At these higher temperatures, water (or the solvent in case of non-aqueous solutions) may comprise greater than 80%, 85% or even 90% by weight of the mixture.

The second treatment agent for use under these conditions advantageously also comprises urea or ammonia and/or an enhancer. Suitable enhancers which may be used include hexamethylenetetramine (HMTA), an oxygenated hydrocarbon such as methanol, ethylene glycol, glycerol, sugar, furfural, furfural derivatives such as hydroxymethyl furfural, and mixtures of ortho-, meta-, and para-methyl phenols, such as cresylic acid; hydroxy amino hydrocarbons such as monoethanolamine; amino acids; and protein-containing compositions such as skimmed milk or powdered milk;. Other enhancers which may be suitable for use include guanidine, guanidine carbonate, biguanidine, guanylurea sulfate, melamine, dicyandiamide, calcium cyanamide, biuret, 1,1,-azobisformamide, methylol urea, methylol urea-urea condensation product, dimethylol urea, methyl urea, dimethyl urea, and mixtures thereof. The most preferred enhancers under these conditions are ethylene glycol, sugar and furfural.

The second treatment agent is injected into the effluent to provide a molar ratio of nitrogen in the agent to the baseline nitrogen oxides concentration existing at the second temperature zone suitable to maximize the reduction of NO.sub.x concentrations in the second temperature zone while minimizing the production of other pollutants, such as ammonia or carbon monoxide. Preferably, the mixture, when composed as described above, is injected so as to provide a molar ratio of nitrogen in the mixture to the baseline nitrogen oxides level of about 1:5 to about 5:1, more preferably about 1:3 to about 3:1 and most preferably about 1:2 to about 2:1. The enhancer is present in the agent in a weight ratio of enhancer to nitrogenated treatment agent components (ammonium salt, urea and/or ammonia) of, preferably, about 1:10 to about 5:1, more preferably about 1:5 to about 3:1. Most preferably, the weight ratio of enhancer to nitrogenated treatment agent components in the agent is about 1:4 to about 2.5:1.

Typically, the agent is prepared by dissolving a water-miscible enhancer in water at a concentration of about 5 to about 25 weight percent, more preferably about 10 to about 20 weight percent, and the desired amount of ammonium salt (and urea or ammonia, if used) mixed in. The resulting mixture is then injected into the effluent at a number of spaced positions within the second temperature zone from nozzles or other apparatus which are effective to uniformly form and disperse droplets of the solution within the flowing effluent stream to achieve uniform mixing. As discussed above, there can be at least two temperature zones, if desired and boiler configuration permits, within the indicated effluent temperature range with at least two treatment agents injected thereinto.

A more preferred embodiment of the present invention comprises injection of a third treatment agent into the effluent at a third temperature zone, wherein the third temperature zone is located sequentially downstream from the first and second temperature zones. For instance, in a suspension-fired utility boiler, the third temperature zone can advantageously be located after the economizer where the effluent temperature will be within the range of about 800.degree. F. to about 1450.degree. F. Under these conditions, the third treatment agent preferably comprises a hydrocarbon or hydrogen peroxide. The most preferred hydrocarbons suitable for use in the third treatment fluid under the indicated conditions are oxygenated hydrocarbons such as low molecular weight ketones, aldehydes, mono, di or polyhydric alcohols of aliphatic hydrocarbons and hydroxy amino hydrocarbons such as monoethanolamine and amino acetic acid (glycine). Ethylene glycol, methanol, furfural, sugar and glycerol are preferred oxygenated hydrocarbons for this purpose, with ethylene glycol and sugar being most preferred. Mixtures of polyols, such as those mixtures of low molecular weight polyols known as hydrogenated starch hydrosylates, can also be advantageously employed. Additional hydrocarbons which are suitable for use in the present invention include paraffinic, olefinic and aromatic hydrocarbons, including naphtha-based hydrocarbons, and mixtures thereof.

The hydrocarbon can be used alone in its pure form, in mixtures, in dispersions, or in solution, with water being the preferred carrier due to the economy of aqueous mixtures, although there may be instances where other carriers may be advantageously used, either alone or in combination with water, as would be known to the skilled artisan. The level of the hydrocarbon employed should be that level necessary to elicit optimal reductions in the concentration of nitrogen oxides in the effluent while also minimizing the presence of other pollutants, such as ammonia and carbon monoxide. Advantageously, the hydrocarbon is employed at a weight ratio of hydrocarbon to the baseline nitrogen oxides level existing at the third temperature zone of about 1:5 to about 5:1, most preferably about 1:2 to about 2:1. The exact amount of hydrocarbon employed may vary depending upon the overall economics of the process.

A hydrocarbon, when utilized as the third treatment agent according to this invention, is preferably injected into the effluent at a number of spaced positions within the third temperature zone from nozzles or other apparatus which are effective to uniformly form and disperse droplets of the hydrocarbon, either alone or in a dispersion or solution as discussed above, within the flowing effluent stream to achieve uniform mixing. Depending on boiler configuration, there can be two zones of injection in the temperature range of about 800.degree. F. to about 1450.degree. F.

It will be recognized that the use of the terms "first", "second" and "third" herein is merely for the sake of convenient description. The actual numbering sequence will vary depending on the actual number of temperature zones chosen and the number of treatment agents injected in each situation. This number can vary depending on boiler geometry (as discussed above) and the particular pollutant level desired.

The effluent from the combustion of a carbonaceous fuel into which the treatment agents disclosed herein according to the present invention are injected is preferably oxygen-rich, meaning that there is an excess of oxygen in the effluent. Advantageously, the excess of oxygen is less than about 10% by volume. Preferably, the excess of oxygen is in the range of about 1% to about 10% by volume, more preferably about 1% to about 6% by volume.

In practicing the process of the present invention to maximize the reduction of the concentration of nitrogen oxides in the effluent, it is preferred to first "tune" the injection of the first treatment agent into the first temperature zone to optimize the injection (i.e., maximize NO.sub.x concentration reduction and minimize production of other pollutants). The injection of the second treatment agent into the second temperature zone is then "tuned", the injection of the third treatment agent into the third temperature zone (when a third treatment agent and third temperature zone are used) is advantageously "tuned" third, the injection of the fourth treatment agent into the fourth temperature zone (when a fourth treatment agent and fourth temperature zone are used) is preferably "tuned" fourth, etc., until the desired number of injections or level of pollutants is reached.

Tuning of the injections can advantageously be accomplished according to techniques disclosed by Epperly, O,Leary and Sullivan in their copending and commonly assigned U.S. patent application Ser. No. 50,198 entitled "Process for Nitrogen Oxides Reduction and Minimization of the Production of Other Pollutants", filed May 14, 1987, now U.S. Pat. No. 4,780,289; and Epperly, O,Leary, Sullivan and Sprague in their copending and commonly assigned U.S. patent application entitled "Process for Nitrogen Oxides Reduction With Minimization of the Production of Other Pollutants", having Ser. No. 207,389 filed on even date herewith.

The identity of other pollutants which comprise the limiting emissions can vary from boiler to boiler, situation to situation, or temperature zone to temperature zone. For instance, at temperature zones where the effluent temperature is relatively high, the limiting emission can be ammonia, whereas at temperature zones where the effluent temperature is relatively low, the limiting emission can be carbon monoxide. Furthermore, it may not be necessary in each case to "tune" the injection at each temperature zone. Rather, it may be desirable to achieve maximum possible reduction at earlier temperature zones irrespective of the production of other emissions, provided that the level of such other emissions can be reduced at later, or the last, temperature zones. In other words, it is the pollution index after the final injection that is most significant, not the pollution index at intermediate levels.

Alternatively, to obtain a target level of NO.sub.x reduction while minimizing chemical cost, maximum use of the least expensive of the treatment agents without significant production of other pollutants is first established. The use of the next least expensive treatment agent is maximized next, and this process is repeated until the desired target level is reached.

It will be further understood that when economics, boiler load, target NO.sub.x levels or other considerations dictate, what was the second temperature zone in one situation can become the first temperature zone in another, and what was the third temperature zone in one situation can become the second temperature zone in another, etc. Moreover, the difference between any two consecutive treatment agents may be the dilution of the mixtures which comprise the treatment agents.

The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the present invention, and it is not intended to detail all of those obvious modifications and variations of it which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the present invention which is defined by the following claims.