Process for Nitrogen Oxides Reduction and Minimization of the Production of Other Pollutants

1. Technical Field

The present invention relates to a process for the reduction of nitrogen oxides (NO.sub.x) in the effluent, especially the oxygen-rich effluent, from the combustion of a carbonaceous fuel while minimizing the production of other pollutants, such as ammonia (NH.sub.3) and/or carbon monoxide (CO).

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 in suspension fired boilers such as large utility boilers, temperatures above about 2000.degree. F. and typically about 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. Nitrogen oxides can form even in circulating fluidized bed boilers which operate at temperatures which typically range from 1300.degree. F. to 1700.degree. F.

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

Unfortunately, the temperatures within a suspension fired or circulating fluidized bed boiler render most common methods of reducing NO.sub.x concentrations, such as effluent scrubbing or catalyst grids, either uneconomical, infeasible, or both, particularly when combined with the need to minimize the production of other pollutants such as ammonia (NH.sub.3) and/or carbon monoxide (CO).

2. Background Art

Processes and compositions for the reduction of nitrogen oxides in an effluent from the combustion of a carbonaceous fuel have been developed extensively over recent years. With the increased attention to the health risks and environmental damage caused by agents such as smog and acid rain, it is expected that NO.sub.x reduction research will continue to be pursued.

In the past, most processes for the reduction of nitrogen oxides levels have concentrated on achieving maximum NO.sub.x reductions without addressing the problems raised by the production of other pollutants, such as ammonia and carbon monoxide. More recently, in a unique application of NO.sub.x reducing principles, Epperly, Peter-Hoblyn, Shulof and Sullivan, in U.S. patent application entitled "Multi-Stage Process for Reducing the Concentration of Pollutants in an Effluent" having Ser. No. 022,716, filed Mar. 6, 1987, disclose a method of achieving substantial NO.sub.x reductions without the production of a major amount of other pollutants through a multiple stage injection process. The disclosed process, though, is more concerned with maintaining a low level of other pollutants than with achieving a maximum amount of nitrogen oxides reduction because each injection is designed not to maximize NO.sub.x reduction but to minimize the production of other pollutants.

Although effective for reducing the level of nitrogen oxides in an effluent, the fact that the NO.sub.x reduction is not maximized at each injection indicates that further reductions are possible. Furthermore, even in the prior art disclosures which teach methods which may lead to a lessened production of other pollutants, most do so under relatively static conditions and do not account or compensate for changes in effluent conditions, such as effluent temperature variations as load varies, which are often encountered. There exists a present need, therefore, for a process which can achieve maximum nitrogen oxides reductions without the production of substantial amounts of other pollutants under practical effluent conditions.

The present invention meets this need and provides the ability to control NO.sub.x in concert with other pollutants under varying as well as constant load conditions in a manner and to a degree never before available. According to one aspect, the process comprises introducing (most commonly by injecting) a NO.sub.x reducing treatment agent into an effluent according to a NO.sub.x reducing treatment regimen under conditions such that the treatment agent is operating on the high temperature or right side of its nitrogen oxides reduction versus effluent temperature curve, especially on the high temperature or right side of the curve plateau.

An object of the present invention is to achieve significant reductions in nitrogen oxides levels without the production of substantial amounts of other pollutants by effecting a treatment regimen while monitoring the condition of the effluent and, when a change in effluent condition is observed, adjusting the treatment regimen by varying one or more treatment regimen parameters to effect an adjusted treatment regiment which operates on its nitrogen oxides reduction versus effluent temperature curve further to the right than did the originally-effected treatment regimen on its nitrogen oxides reduction versus effluent temperature curve.

Another object of the present invention is to achieve significant reductions in nitrogen oxides levels without the production of substantial amounts of other pollutants by determining the nitrogen oxides reduction versus effluent temperature curves for each of a plurality of treatment regimen and effecting the treatment regimen which will, under the effluent condition currently existing, operate furthest to the right on its curve than the others.

Still another object of the present invention is to achieve significant reductions in nitrogen oxides levels without the production of substantial amounts of other pollutants by effecting a treatment regimen and adjusting the position of introduction of the treatment regimen to cause the introduction to be performed at a different effluent temperature and thereby effect the treatment regimen to operate more towards the right side of the plateau of its nitrogen oxides reduction versus effluent temperature curve.

Yet another object of the present invention is to achieve significant reductions in nitrogen oxides levels without the production of substantial amounts of other pollutants by effecting a treatment regimen under conditions effective to reduce the effluent nitrogen oxides concentration and then varying one or more treatment regimen parameters to shift the treatment regimen nitrogen oxides reduction versus effluent temperature curve towards the right side of the curve plateau.

Still another object of the present invention is to achieve significant reductions in nitrogen oxides levels without the production of substantial amounts of other pollutants by effecting a treatment regimen, determining the position on its nitrogen oxides reduction versus effluent temperature curve at which the treatment regimen is operating and varying one or more treatment regimen parameters so that the varied treatment regimen is operating on its nitrogen oxides reduction versus effluent temperature curve further to the right.

Another object of the present invention is to achieve significant reductions in nitrogen oxides levels without the production of substantial amounts of other pollutants by effecting a treatment regimen and varying one or more treatment regimen parameters to drive the reaction or series of reactions by which the treatment regimen reduces nitrogen oxides towards a reduction of the production of other pollutants while substantially maintaining the level of nitrogen oxides reductions.

Yet another object of the present invention is to achieve significant reductions in nitrogen oxides levels without the production of substantial amounts of other pollutants by effecting a treatment regimen while monitoring boiler operating load and varying one or more treatment regimen parameters when a significant change in boiler load is observed to effect an adjusted treatment regimen.

Another object of the present invention is to achieve signficant reductions in nitrogen oxides levels without the production of substantial amounts of other pollutants by effecting a treatment regimen under conditions where the treatment regimen is operating on its nitrogen oxides reduction versus effluent temperature curve at a position to the right of the curve plateau and adjusting one or more treatment regimen parameters to operate the adjusted treatment regimen towards its curve plateau.

Still another object of the present invention is to ascertain the condition of the effluent by effecting a treatment regimen, measuring the condition of the effluent and, by reference to the nitrogen oxides reduction versus effluent temperature curve, determine what the condition of the effluent was prior to the treatment regimen being effected.

These and other objects will be described and the present invention better understood and its advantages more apparent in view of the following detailed description, especially when read with reference to the appended drawings, wherein:

FIGS. 1 and 1a graphically represent the results of Example I;

FIG. 2 graphically represents the results of Examples IIa, IIb and IIc;

FIGS. 3a-c graphically represent the results of Examples IIIa, IIIb and IIIc;

FIGS. 3d-f graphically represent the results of Examples IIIa, IIIb and IIIc presented as described below; and

FIG. 4 graphically represents the results of Example IV.

As used in this description, the term:

"nitrogen oxides reduction versus effluent temperature curve" refers to a plot of the data points generated when a treatment regimen is effected by introducing a treatment agent into an effluent over a range of effluent temperatures and the nitrogen oxides reduction at each introduction temperature is measured (and usually expressed in terms of percent of baseline);

"curve plateau" refers to that region of a nitrogen oxides reduction versus effluent temperature curve where the NO.sub.x reduction is substantially maximized over a range of temperatures and preferably encompasses at least two data points (of course a skilled artisan will recognize that a curve plateau will not necessarily be flat due to "data scatter" and other practical data generation effects);

"high temperature side" or "right side" refer to any point on the subject nitrogen oxides reduction versus effluent temperature curve which represents the reduction achieved when a treatment regimen is effected at a higher temperature than the original temperature at which a treatment regiment was effected;

"treatment regimen" refers to the introduction (such as by injection) of a treatment agent into an effluent and the conditions under which the treatment agent is introduced, such as treatment agent components (by which is meant the ingredients or chemical formulation of the treatment agent), treatment agent dilution (by which is meant the concentration of treatment agent components when the treatment agent used comprises a solution), relative presence of treatment agent components (by which is meant the relative weight ratio or fractions of the components which form the chemical formulation which makes up the treatment agent), etc.;

"treatment agent" refers to a composition comprising at least one reductant chemical, i.e., a pollution reducing chemical capable of reducing NO.sub.x, sulfur oxides (SO.sub.x) or other pollutants by facilitating a reaction (the term "reaction" will be understood to refer to a single reaction or a series of reactions), and, preferably, with a solvent;

"effluent condition" or "condition of the effluent" refers to the existing state of any one or more parameters which can be used to characterize the effluent, such as temperature, nitrogen oxides level, ammonia level, carbon monoxide level, excess oxygen level, sulfur oxides level, etc.;

"normalized stoichiometric ratio" (NSR) refers to the ratio of the concentration of reducing-radicals such as NH.sub.x radicals (NH.sub.x radicals, with x being an integer, are believed to be the moiety contributed by the treatment agent which facilitates the series of reactions resulting in NO.sub.x breakdown) to the concentration of nitrogen oxides in the effluent and can be expressed as [NH.sub.x ]/[NO.sub.x ] (alternatively, the molar ratio of the treatment agent to the NO.sub.x concentration can be used in place of NSR when the chemistry of reduction is not well defined; the term NSR as used herein will also be understood to encompass molar ratio when appropriate);

"oxygenated hydrocarbon" refers to a hydrocarbon which contains oxygen or an oxygen-containing group;

"sugar" refers to a number of useful saccharide materials which are capable of decreasing the NO.sub.x concentration in an effluent under conditions as described herein, including non-reducing and reducing water soluble mono-saccharides and the reducing and non-reducing polysaccharides and their degradation products, such as pentoses including aldopentoses, methyl pentoses, keptopentoses like xylose and arabinose, deoxyaldoses like rhaminose, hexoses and reducing saccharides such as aldo hexoses like glucose, galactose and mannose, ketohexoses like fructose and sorbose, disaccharides like lactose and maltose, non-reducing disaccharides like sucrose and other polysaccharides such as dextrin and raffinose, hydrolyzed starches which contain as their constituents oligosaccharides, and water dispersible polysaccharides;

"amino acid" refers to any organic acid in which at least a portion of the nonacid hydrogen has been replaced by one or more amino groups and which therefore shows both basic and acidic properties;

"protein" refers to a polymeric compound comprising the polymerization or condensation product of amino acids;

"skimmed milk" refers to milk having some or all of the fat removed; and

"powdered milk" refers to non-fat dry milk, available commercially as Carnation Instant Non-Fat Dry Milk from Carnation Company of Los Angeles, Calif.

Appropriate treatment agents known as being effective at the reduction of nitrogen oxides include aqueous solutions of urea or ammonia, or gaseous ammonia, as disclosed by copending and commonly assigned U.S. patent application entitled "Reduction of Nitrogen- and Carbon-Based Pollutants Through the Use of Urea Solutions" having Ser. No. 784,826, filed in the name of Bowers on Oct. 4, 1985; copending and commonly assigned U.S. patent application entitled "Reduction of Nitrogen Based Pollutants Through the Use of Urea Solutions Containing Oxygenated Hydrocarbon Solvents" having Ser. No. 784,828, filed in the name of Bowers on Oct. 4, 1985 now U.S. Pat. No. 4,719,092; and U.S. Pat. No. 3,900,554 to Lyon, the disclosures of each of which are incorporated herein by reference.

The use of treatment agents which comprise other compositions such as hexamethylenetetramine (HMTA), ethylene glycol, furfural, hydrocarbons, sugar, milk or skimmed milk, amino acids, proteins and monoethanolamine are disclosed as being effective at the reduction of nitrogen oxides in an effluent in combination with aqueous solutions of urea or ammonia in several disclosures.

Included is copending and commonly assigned U.S. patent application entitled "Reduction of Nitrogen- and Carbon-Based Pollutants" having Ser. No. 906,671, filed in the name of Bowers on Sept. 6, 1986. Other disclosures which teach the use of such compositions include copending and commonly assigned U.S. patent application entitled "Process for the Reduction of Nitrogen Oxides in an Effluent" having Ser. No. 014,431, filed in the names of Epperly and Sullivan on Feb. 13, 1987; copending and commonly assigned U.S. patent application entitled "Process for the Reduction of Nitrogen Oxides in an Effluent Using a Heterocyclic Hydrocarbon" having Ser. No. 25,493, filed in the names of Epperly and Sullivan on Mar. 13, 1987; copending and commonly assigned U.S. patent application entitled "Process for the Reduction of Nitrogen Oxides in an Effluent Using Sugar" having Ser. No. 25,350, filed in the names of Epperly and Sullivan on Mar. 13, 1987; and copending and commonly assigned U.S. patent application entitled "Process for the Reduction of Nitrogen Oxides in an Effluent Using a Hydroxy Amino Hydrocarbon" having Ser. No. 039,013 filed in the names of Sullivan and Epperly on Apr. 15, 1987, the disclosures of each of which are incorporated herein by reference.

Other treatment agents which may be injected into an effluent to reduce the effluent nitrogen oxides level under certain conditions comprise hydrocarbons, such as an oxygenated hydrocarbon, a nitrogenated hydrocarbon like a hydroxy amino hydrocarbon or hydrocarbon peroxide, disclosed in copending and commonly assigned U.S. patent application entitled "Process for Reducing Nitrogen Oxides in an Effluent Using a Hydrocarbon or Hydrogen Peroxide" having Ser. No. 022,799, filed in the name of Sullivan on Mar. 6, 1987 the disclosure of which is incorporated herein by reference.

The nitrogen oxides reduction versus effluent temperature curve for a treatment regimen comprises a curve plateau, which, as described above, indicates where the NO.sub.x reduction elicited by the treatment regimen is maximized and that such maximum level is substantially maintained over a range of effluent temperatures. An exemplary nitrogen oxides reduction versus effluent temperature curve for a treatment regimen disclosed as being an effective nitrogen oxides reducing treatment regimen is reproduced as FIG. 1. FIG. 1 is the nitrogen oxides reduction versus effluent temperature curve for a treatment regimen comprising a treatment agent which comprises 10% by weight of urea, 4% by weight of hexamethylenetetramine and 10% by weight of furfural, which is injected into an effluent at the rate of 300 ml/hr. and an excess of oxygen in the effluent of 3.0% by volume.

The curve plateau for FIG. 1 will be recognized as the nitrogen oxides reduction achieved by effecting the disclosed treatment regimen between effluent temperatures of 1530.degree. F. and 1680.degree. F. (the skilled artisan will recognize that due to normal experimental variations, the curve plateau, and indeed the nitrogen oxides reduction versus effluent temperature curve itself, for any given treatment regimen will show minor variations each time it is experimentally derived). This temperature range, it will be observed, provides the maximum nitrogen oxides reduction for this treatment regimen.

Merely maximizing the nitrogen oxides reduction, though, is not enough. Of concern is not only the nitrogen oxides level in the effluent, but also the level of other pollutants, such as ammonia and carbon monoxide which are often produced in the NO.sub.x reducing process. For instance, when NO.sub.x reduction is achieved by using treatment agent comprising urea or ammonia alone, ammonia is produced, whereas when NO.sub.x reduction is achieved by use of a treatment agent comprising urea or ammonia enhanced with a disclosed enhancer, or by use of a hydrocarbon treatment agent, ammonia and carbon monoxide are produced.

The presence of ammonia in the effluent should be avoided because, among other reasons, it can react with SO.sub.3.sup.= to form ammonium bisulfate which can foul heat exchange surfaces in a boiler. Moreover, ammonia has detrimental effects on ambient air quality, as has carbon monoxide. If the maximization of the nitrogen oxides level brings about the production of significant amounts of other pollutants, then such maximization is counterproductive. As discussed above, the prior art has attempted to rectify this by eliciting only that level of nitrogen oxides reduction as can be achieved without the production of other pollutants.

Surprisingly, a process for maximizing the nitrogen oxides reduction while minimizing the production of other pollutants has now been found. It has been determined that operation on the high temperature or right side of the nitrogen oxides reduction versus effluent temperature curve of a treatment regimen substantially reduces the production of other pollutants such as ammonia and carbon monoxide. In fact, it has been found that operation on a nitrogen oxides reduction versus effluent temperature curve plateau at any point further to the right of present operation will reduce the production of other pollutants while maintaining maximum nitrogen oxides reduction.

This surprising and advantageous result is aptly demonstrated by reference to FIGS. 1 and 1a, which graphically represent the results of Example I. As previously discussed, FIG. 1 reproduces the nitrogen oxides reduction versus effluent temperature curve for a treatment regimen which is effective at reducing the nitrogen oxides level in an effluent from the combustion of a carbonaceous fuel. FIG. 1a reproduces that same nitrogen oxides reduction versus effluent temperature curve and further has superimposed thereon the ammonia and carbon monoxide levels observed at each point on the curve. It can be seen that although NO.sub.x reduction is maximized throughout the curve plateau (i.e., injection in the effluent temperature range of about 1530.degree. F. to about 1680.degree. F.), performing the injection further to the right on the curve plateau (i.e., at higher temperatures in the plateau temperature range) leads to substantially reduced production of ammonia and carbon monoxide.

Operation further to the right on the curve can be achieved in one of two methods. First, the position on the curve at which the treatment regimen being used is being effected can be translated further to the right by effecting the treatment regimen at a higher effluent temperature. It will readily be observed by reference to FIGS. 1 and 1a that effecting the treatment regimen at a higher effluent temperature will translate the position of operation on the curve further to the right, thereby reducing the production of other pollutants while maintaining maximum nitrogen oxides reduction.

Effecting the treatment regimen at a higher effluent temperature can be accomplished by performing the treatment agent introduction at a location where the effluent temperature is higher, i.e., at a location upstream (or closer to the flame zone) from the original introduction location. This method for effecting the treatment regimen at a higher effluent temperature can oftimes be impractical because access to the boiler interior is often limited to specific points, due to water tubing, etc. Introduction at a location where the effluent temperature is at a desired level, therefore, is often not possible. Operation at a much higher effluent temperature can translate the position of operation on the curve too far to the right and off the plateau, thereby decreasing NO.sub.x reduction.

Altering the operating load of the boiler (i.e., fuel supply rate) will also cause a change in effluent temperature and, theoretically at least, be used to increase effluent temperature at the treatment agent introduction location by increasing operating load. In practice, though, altering the boiler operating load is not preferred because the effluent condition is altered in more than the temperature parameter, as will be discussed in more detail below. Nitrogen oxides level, as well as other parameters such as ammonia level and carbon monoxide level, are altered along with effluent temperature. Furthermore, the boiler operating load is usually maintained at a certain level to produce a specific, required output and is not available as a factor which can be altered to achieve NO.sub.x reduction.

The second method for operating further to the right on the curve is to vary one or more of the parameters of the treatment regimen being effected. For instance, the varied parameter can be the components of the treatment agent, the dilution of the treatment agent when in solution with a concommitant variation in treatment again introduction rate to maintain the NSR of the treatment regimen (as discussed above, the NSR refers also to the molar ratio of the treatment agent to the baseline nitrogen oxides level, where appropriate), the relative presence of treatment agent components, or combinations of any of the above. By varying treatment regimen parameters, the original nitrogen oxides reduction versus effluent temperature curve is replaced by the nitrogen oxides reduction versus effluent temperature curve for the varied treatment regimen. Selection of the parameter(s) to be varied and in what way they are varied can replace the original curve with a curve which is "shifted" to the left, thereby leading to operation on the shifted curve at a position further to the right.

This treatment regimen varying method for operating further to the right on the curve is illustrated by reference to FIG. 2, which graphically represents the results of Example II. FIG. 2 provides the nitrogen oxides reduction versus effluent temperature curve plateau for three treatment regimens which each comprise introducing a treatment agent into an effluent over a range of effluent temperatures and at an introduction rate of 300 ml/hr. and an excess of oxygen of 3.0% by volume. The treatment agent introduced for the first treatment regimen comprises an aqueous solution of 10% urea and 15% furfural; the treatment agent introduced for the second treatment regimen comprises an aqueous solution of 10% urea; and the treatment agent introduced for the third treatment regimen comprises an aqueous solution of 10% urea and 15% ethylene glycol. It will readily be observed that varying the treatment regimen by altering the treatment agent components between the three illustrated treatment agents can shift or alter the curve and thereby enable maintainance of the treatment regimen operating on the right side of the curve.

For example, if the treatment regimen being effected comprises a treatment agent which is an aqueous solution of 10% urea, and the effluent temperature at the treatment location is 1755.degree. F., thereby operating toward the left side of the treatment regimen nitrogen oxides reduction versus effluent temperature curve plateau, then varying the treatment regimen by varying the treatment agent components by injecting 15% furfural with the 10% urea replaces the original curve with a curve at which introduction at that effluent temperature operates further towards the right side of the curve plateau. Likewise, if the treatment regimen being effected comprises a treatment agent which is an aqueous solution of 10% urea and 15% furfural, and the effluent temperature at the point of introduction is 1665.degree. F., thereby operating near the midpoint of the treatment regimen nitrogen oxides reduction versus effluent temperature curve plateau, then varying the treatment regimen to vary the treatment agent to replace the 15% furfural with 15% ethylene glycol replaces the original curve with a curve at which introduction at that effluent temperature operates further towards the right side of the curve plateau.

Of course, the two methods for operating further to the right on the curve plateau disclosed according to the present invention are not mutually exclusive, but can in fact be combined. In other words, the effluent temperature can be varied along with one or more treatment regimen parameters.

Although it is possible to translate the performance of the treatment regimen further to the right on its curve plateau by effecting the treatment regimen at a higher temperature (i.e., by injecting at a location where the effluent temperature is higher), as discussed above it is more likely that the NO.sub.x reduction will be maximized while minimizing the production of other pollutants by the second of the two disclosed methods, i.e., by varying one or more treatment regimen parameters. The adjustment (or varying) of the treatment regimen is performed to drive the reaction by which the treatment agent facilitates the reduction of NO.sub.x towards the right side of the curve plateau.

As discussed above, varying one or more treatment regimen parameters serves to produce an adjusted (or new) treatment regimen which will have a different (or shifted) nitrogen oxides reduction versus effluent temperature curve compared to the original treatment regimen. By prior analysis of possible treatment regimens and knowledge of their particular nitrogen oxides reduction versus effluent temperature curves, one can determine the treatment regimen that will, when effected in place of the original treatment regimen, operate further to the right on its curve than the original treatment regimen operated on its curve. In fact, one can predict which will operate furthest to the right on the curve plateau under the particular conditions at which injection is desired (i.e., the effluent temperature at the location for injection).

Furthermore, if the effluent temperature at the injection location is known, prior knowledge of the nitrogen oxides reduction versus effluent temperature curves of available treatment regimens will allow initial selection of the treatment regimen which operates furthest to the right while remaining on the curve plateau, eliminating the need for varying any of the parameters.

For instance, as illustrated in FIGS. 3a-3c, which graphically reproduce the results of Examples IIIa-IIIc, the nitrogen oxides reduction versus effluent temperature curve for a plurality of treatment regimens, such as aqueous solutions comprising 10% by weight of urea and varying amounts of ethylene glycol which are introduced into an effluent over a range of effluent temperatures and at an introduction rate of 300 ml/hr and an excess of oxygen of 3.0% by volume, can be plotted. The data which comprises the plots can then be compared to determine which treatment regimen should be effected according to this invention for the effluent condition existing at the injection location.

This is more clearly understood by reference to FIGS. 3d-3f, which graphically express the nitrogen oxides reduction, ammonia level and carbon monoxide level for each of the three treatment regimens represented by FIGS. 3a-3c, at three particular location of introduction effluent temperatures. If the effluent temperature at the introduction location is 1560.degree. F., the desired treatment regimen, therefore, is the regimen which comprises the treatment agent having 15% ethylene glycol, as illustrated in FIG. 3d. If the effluent temperature at the introduction location is 1650.degree. F., the desired treatment regimen, therefore, is the regimen which comprises the treatment agent having 10% ethylene glycol, as illustrated in FIG. 3e (the 15% ethylene glycol treatment regimen is not desired at 1650.degree. F. because, although FIG. 3e indicates ammonia and carbon monoxide levels are low, NO.sub.x reduction is also lower than the maximum, indicating that the 15% ethylene glycol treatment regimen is operating on its nitrogen oxides reduction versus effluent temperature curve to the right and off the curve plateau). If the effluent temperature at the introduction location is 1755.degree. F., the desired treatment regimen, therefore, is the regimen which comprises the treatment agent having 5% ethylene glycol, as illustrated in FIG. 3f, because both the 10% ethylene glycol and 15% ethylene glycol treatment regimens are operating to the right and off their curve plateau.

Another advantageous aspect of the present invention is in the situation where an effluent from the combustion of a carbonaceous fuel is required to have no more than a maximum level of another pollutant, such as ammonia and/or carbon monoxide. The process of this invention can be used to achieve the maximum possible NO.sub.x reduction, or a target level of NO.sub.x reduction, while maintaining the level of such other pollutants under such maximum level.

This can be accomplished using the normalized stoichiometric ratio (NSR) of the treatment regimen being effected. It will be recognized that as the NSR of a treatment regimen increases, the nitrogen oxides reduction elicited by that treatment regimen increases as does the production of other pollutants. This fact can be utilized by effecting a treatment regimen at a relatively low NSR, for instance at an NSR of about 1.0 to about 1.5, and at the right side of the treatment regimen curve plateau (the treatment regimen can be effected at the right side of its curve plateau by any of the methods for effecting a treatment agent at the right side of its curve plateau discussed herein). The NSR of the treatment agent can then be increased (by increasing the introduction rate of the treatment agent) until the maximum level of other pollutant is reached. If there is more than one other pollutant which has a maximum level, the NSR is increased until the first of such other pollutants reaches its maximum level. In this way, the highest possible NO.sub.x reduction can be achieved while maintaining the effluent in a condition which is below the maximum level for other pollutants.

For example, a treatment regimen which comprises an aqueous solution of 10% urea and 15% ethylene glycol introduced into an effluent at an introduction rate of 300 ml/hr. and an excess of oxygen of 3.0% has a nitrogen oxides reduction versus effluent temperature curve which is graphically reproduced as FIG. 3c, which graphically reproduces the results of Example IIIc. It will be observed by reference to FIG. 3c that this treatment regimen is operating towards the left side of its curve at an effluent temperature of 1555.degree. F., in the midsection of its curve at an effluent temperature of 1625.degree. F., and towards the right side of its curve at an effluent temperature of 1755.degree. F.

Increasing the treatment regimen NSR at these three points produces markedly different results. As illustrated in FIG. 4, which graphically reproduces the results of Example IV, increasing the NSR when the treatment regimen is towards the left side of its nitrogen oxides reduction versus effluent temperature curve results in a sharp increase in the amount of ammonia produced. Increasing the NSR when the treatment regimen is in the midsection of the curve results also in a relatively sharp increase in the production of ammonia. Increasing the NSR when the treatment regimen is on the right side of its curve results in a gradual increase in the level of ammonia produced. The NO.sub.x reduction can be much greater, therefore, when the treatment regimen is effected on the right side of its curve before a maximum level of ammonia is reached.

As illustrated in FIG. 4, for a treatment regimen which comprises injecting a treatment agent of 10% urea and 15% ethylene glycol, if the maximum allowable level of ammonia is 15 parts per million (ppm), an NSR of 5.47 is achievable when the treatment regimen is effected on the right side of its curve, whereas NSRs of no more than about 1.5 are achievable when the treatment regimen is effected on the midsection or on the left side of the curve.

Similarly, when a target level of nitrogen oxides is desired, the NSR can be increased until the nitrogen oxides reduction is sufficient to attain that level of nitrogen oxides, provided that a maximum level of other pollutants is not surpassed. In this way, if the treatment regimen is operating on the right side of its curve plateau, the target level of NO.sub.x is attained while a minimum of other pollutants are produced.

In another advantageous aspect of this invention, the process of this invention can be used to reduce NO.sub.x levels while minimizing the production of other pollutants through "load following." "Load following" refers to a process which involves adjusting the treatment regimen which is being effected in response to the operating load at which the boiler is being fired. When the boiler operating load changes, a change in effluent temperature occurs. Such a change in temperature of the effluent, it will be apprent, causes the point of operation on the nitrogen oxides reduction versus effluent temperature curve for the current treatment regimen to be translated either to the left, and hence away from minimization of other pollutants, or to the right, potentially off the curve plateau and onto the right side slope of the curve, and hence away from maximum nitrogen oxides reduction.

By replacing the current treatment regimen with a new treatment regimen, i.e., by adjusting one or more treatment regimen parameters such as dilution and introduction rate of the treatment agent, components of the treatment agent and relative presence of treatment agent components, the nitrogen oxides reduction versus effluent temperature curve is shifted (i.e., replaced with a new nitrogen oxides reduction versus effluent temperature curve) so that operation after the change is once again towards the right side of the curve plateau.

Furthermore, as discussed above, a change in boiler operating load leads to more than merely a change in effluent temperature. Frequently, a change in boiler load produces a change in the effluent with regard to NO.sub.x level. This becomes especially important when there is a maximum level of other pollutants which has to be met or a target level of nitrogen oxides reduction which has to be attained. The change in NO.sub.x level can be measured directly or, preferably, can be determined using a load-dependent boiler characterization factor.

For a given boiler and fuel, the characterization factor relates the NO.sub.x level and temperature of the effluent at given locations to boiler load, and it is determined experimentally. With this information, the treatment regimen being effected at a given location can be adjusted immediately upon change of boiler operating load as measured by fuel supply rate, for example. As a result, when load is reduced, the treatment regimen feed rate is reduced to achieve the NSR needed to attain target reductions at that load and the treatment agent components are varied as necessary to respond to the temperature change resulting from the change in operating load. If the feed rate of the treatment regimen were not reduced, the NSR would be excessive in view of the lower level of NO.sub.x and excessive NH.sub.3 and CO would be produced. Similarly, when boiler operating load is increased, the feed rate of the treatment regimen is increased to achieve the NSR needed at that load. Otherwise, the NSR would be low and NO.sub.x reduction would be inadequate. Again, the components of the treatment agent are adjusted to compensate for the effluent temperature change associated with the change in boiler operating load.

This characterization factor is dependent on boiler geometry, fuel type and boiler load and can be determined experimentally. Several other parameters such as number of burners in service affect the characterization factor, but those that are mentioned above are most important. By reference to the characterization factor for a given boiler and fuel, the nitrogen oxides level and temperature at a given location can be determined to a sufficient degree of certainty to permit the determination of how the treatment regimen should be adjusted to correct for translation on the nitrogen oxides reduction versus effluent temperature curve which occurs when the operating load is changed and for the change in NSR.

The preferred embodiment for maximizing nitrogen oxides reduction and controlling the production of other pollutants is by effecting a first treatment regimen which operates at the effluent temperatures currently existing on the right hand slope, off the curve plateau, of the treatment regimen's nitrogen oxides reduction versus effluent temperature curve. By knowledge of the effluent temperature and the curve for a plurality of treatment regimens, this can be simply accomplished using means described above. Once done, the treatment regimen can be adjusted, by means which will be apparent to the skilled artisan upon reading this disclosure, to bring the operation thereof "up" and onto the curve plateau. In other words, the curve can be shifted to do so. By this method, it can be assured that the treatment regimen being effected is operating as far to the right while still on its curve plateau as possible. The nitrogen oxides reduction is thus maximized while the production of other pollutants is thus controlled.

Another surprising aspect of this invention is in the use of a treatment regimen as a probe for effluent conditions. If the nitrogen oxides reduction versus effluent temperature curve (or, in fact, the ammonia or carbon monoxide production versus effluent temperature curves) for a treatment regimen is known, the effluent condition after that treatment regimen is effected will provide useful information about the effluent condition downstream from the location the treatment regimen is effected, it can even provide information on boiler operating load. For instance, if the nitrogen oxides level is relatively low, but the level of production of other pollutants is relatively high, then it can be assumed that the treatment regimen is operating on the left side of its curve plateau. Using this information, the effluent temperature can be determined with a reasonable degree of accuracy and, using the boiler characterization factor described above, the boiler load can be determined. Similarly, if the NO.sub.x, ammonia and carbon monoxide levels are all low, it can be assumed that the treatment regimen is operating on the right side slope, off the curve plateau, of its curve. Effluent temperature and boiler operating load can then be determined therefrom. The more intimate familiarity with the treatment regimen's curve, the more accurate the determinations can be.

It will be understood that there are significant temperature variations at any given boiler location, associated with effluent flow patterns, boiler firing conditions, wall effects, etc. A regimen effected at a boiler location and tailored to operate on the right side of its nitrogen oxides reduction versus effluent temperature curve when exposed to the median effluent temperature at such location will probably operate on the left side in a low temperature area within the location. The low temperature areas will result in production of ammonia and/or carbon monoxide, as apparent from a review of this disclosure. This effect can be minimized by minimizing treatment of the low temperature areas, such as by use of suitable injectors (familiar to the skilled artisan), by tailoring the treatment regimen to the low temperature areas, or by a multiple injection system utilizing both methods.

It will further be understood that although operation on a nitrogen oxides reduction versus effluent temperature curve plateau is preferred, in some situations operating on the right side slope, off the plateau, can be desired, in order to maintain the production of other pollutants at a low level while still achieving significant reduction of nitrogen oxides.

The following examples further illustrate and explain the invention by detailing the operation of a treatment regimen using "right side of the curve" methods.

The burner used is a burner having an effluent flue conduit, known as a combustion tunnel, approximately 209 inches in length and having an internal diameter of 8 inches and walls 2 inches thick. The burner has a flame area adjacent the effluent entry port and flue gas monitors adjacent the effluent exit port to measure the concentration of compositions such as nitrogen oxides, sulfur oxides, ammonia, carbon monoxide, carbon dioxide, percent excess oxygen and other compounds of interest which may be present in the effluent. The effluent flue conduit additionally has thermocouple ports for temperature measurement at various locations. The temperature of the effluent into which the treatment agents are injected is measured at the location of injection utilizing a K-type thermocouple. Atomizing injectors described in copending U.S. patent application entitled "Process and Apparatus for Reducing the Concentration of Pollutants in an Effluent" having Ser. No. 009,696, filed in the name of Burton on Feb. 2, 1987, the disclosure of which is incorporated herein by reference, are positioned through ports in the effluent flue conduit in order to introduce and distribute the treatment agents into the effluent stream. The agents are injected into the effluent at a rate of 300 ml/hr. The burner fuel is a Number 2 fuel oil, and the burner is fired at a rate of 9.6 lbs/hr with an excess of oxygen of 3.0% by volume.

A baseline nitrogen oxides concentration reading is taken prior to beginning each run to calculate the injection ratio of treatment agent to baseline nitrogen oxides and the NSR, and a final nitrogen oxides reading is taken during and downstream from injection of the treatment agents to calculate the reduction in the nitrogen oxides concentration in the effluent elicited by each of the treatment agents injected. Moreover, an ammonia and carbon monoxide reading is taken during and downstream from injection of the treatment agents to calculate the production of other pollutants.

Aqueous solutions comprising 10% by weight of urea, 4% by weight of hexamethylenetetramine, 10% by weight of furfural and 0.1% by weight of a commercially available surfactant are injected into the effluent at the indicated temperatures. The results are set out in Table 1 and reproduced graphically in FIGS. 1 and 1a.

The procedure followed is that of Example I except that the boiler is fired at a rate of 9.6 lbs/hr to 12.0 lbs/hr to achieve the effluent temperatures.

The treatment agent injected is an aqueous solution which comprises 10% by weight of urea, 15% by weight of furfural, and 0.1% by weight of a commercially available surfactant. The injection temperature, % excess oxygen, NSR, baseline NO.sub.x, final NO.sub.x and % reduction of NO.sub.x for each run is set out in Table 2a and reproduced graphically in FIG. 2.

The procedure of Example IIa is repeated except that the treatment agent which is injected is an aqueous solution comprising 10% by weight of urea and 0.1% by weight of a commercially available surfactant. The results are set out in Table 2b and reproduced graphically in FIG. 2.

The procedure of Example IIa is repeated except that the treatment agent injected is an aqueous solution comprising 10% by weight of urea, 15% by weight of ethylene glycol and 0.1% by weight of a commercially available surfactant. The results are set out in Table 2c and reproduced graphically in FIG. 2.

The procedure of Example I is followed except that the boiler is fired at a rate of 9.6 lbs/hr. to 10.8 lbs/hr. The treatment agent injected comprises an aqueous solution of 10% by weight of urea, 5% by weight of ethylene glycol and 0.1% by weight of a commercially available surfactant. The results are set out in Table 3a and reproduced graphically in FIG. 3a.

The procedure of Example IIIa is followed except that the treatment agent injected comprises an aqueous solution of 10% by weight of urea, 10% by weight of ethylene glycol and 0.1% by weight of a commercially available surfactant. The results are set out in Table 3b and reproduced graphically in FIG. 3b.

The procedure of Example IIIb is followed except that the treatment agent injected comprises an aqueous solution of 10% by weight of urea, 15% by weight of ethylene glycol and 0.1% by weight of a commercially available surfactant. The results are set out in Table 3c and graphically reproduced in FIG. 3c.

A treatment agent comprising an aqueous solution of 10% by weight of urea, 15% by weight of ethylene glycol and 0.1% by weight of a commercially available surfactant is injected into the effluent combustion tunnel described in Example I at a range of NSRs and the production of ammonia measured. The normalized stoichiometric ratio (NSR) for each injection and the results are set out in Table 4 and graphically reproduced in FIG. 4. For convenience, the temperatures indicated in FIG. 4 represent the approximate average of the temperatures of each data point for the respective curve.

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.