WO1994009896A1

WO1994009896A1 - Process and composition for sorbent reduction of n2o

Info

Publication number: WO1994009896A1
Application number: PCT/US1993/010211
Authority: WO
Inventors: M. Linda Lin; John E. Hofmann; William H. Sun
Original assignee: Nalco Fuel Tech
Priority date: 1992-11-02
Filing date: 1993-10-25
Publication date: 1994-05-11
Also published as: TW240177B

Abstract

The invention presented relates to a process and composition for the reduction of nitrogen oxides in the effluent from the combustion of a carbonaceous fuel while avoiding the generation of nitrous oxide. The composition is a slurry which includes a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent, calcium hydroxide, and water. The process permits reductions of nitrogen oxides reductions while minimizing generation of nitrous oxide.

Description

DESCRIPTION

PROCESS AND COMPOSITION FOR SORBENT REDUCTION OF N₂0

Related Applications

This application is a continuation-in-part of co- pending application having Serial No. 07/800,588, filed in the names of Gullett, Hofmann, Peter-Hoblyn, and Val¬ entine on November 27, 1991, the disclosure of which is incorporated herein by reference.

Technical Field

The present invention relates to a novel process and composition for achieving superior reductions in the levels of nitrogen oxides (NOx, where x is an integer, generally 1 or 2) in the effluent from the combustion of a carbonaceous fuel while avoiding the generation of substantial amounts of nitrous oxide (N₂0) .

When fossil fuels are used in suspension fired boil¬ ers, such as large utility boilers, temperatures above about 2000 F and typically above 2200 F are generated. At these temperatures, nitrogen present in the fuels can be oxidized to form nitrogen oxides. Such high tempera- tures also tend to cause the production of NOx from ni¬ trogen in the atmosphere, the temperatures being so high that free radicals of oxygen and nitrogen are formed and chemically combine as nitrogen oxides. Nitrogen oxides can also form in the effluent streams of other com- bustors, such as municipal waste combustors and industri¬ al or process boilers or heaters. In fact, any combus¬ tion source in which the fuel contains nitrogen will yield nitrogen oxides in the effluent.

Nitrogen oxides are troublesome pollutants which are found in the combustion streams of boilers when fired as described above, and comprise a major component of acid rain. It is further believed that NOx can undergo a process known as photochemical smog formation, through a series of reactions in the presence of some hydrocarbons. They may also impact on the warming of the atmosphere commonly referred to as "the greenhouse effect."

Recently, many processes for the reduction of NOx in combustion effluents have been developed. They can be segregated into two basic categories: selective and non-selective. Among the selective processes, which are believed in the art to be the more desirable, there is a further division between^" selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) pro- cesses. Although SCR processes are believed to be capa¬ ble of achieving higher levels of nitrogen oxides reduc¬ tions, SNCR processes are often preferred because of their greater economy and flexibility.

SNCR processes, which are temperature dependent, generally utilize a nitrogenous substance such as urea or ammonia as well as non-nitrogenous substances. They proceed in the gas phase by a complex series of free radical-mediated chemical reactions involving various nitrogen, hydrogen, oxygen and carbon-containing species and radicals. Unfortunately, it has recently been found that many nitrogenous substances, when introduced into an effluent, can lead to the generation of nitrous oxide. Although there has not been a great deal of attention paid to this generation of nitrous oxide, the emission of N₂0 into the environment is potentially undesirable.

What is desired, therefore, is a composition effec¬ tive for achieving nitrogen oxides reductions in a com¬ bustion effluent while avoiding or minimizing the genera¬ tion of nitrous oxide.

Background Art

Compositions and processes for the reduction of NOx from combustion effluents have been studied extensively. Due to the increased importance given environmental mat¬ ters recently, it is expected that such research will continue to be extensively pursued.

In the past, several useful processes for achieving nitrogen oxides reductions via selective non-catalytic gas phase reactions have been disclosed. For instance, Lyon, in U.S. Patent 3,900,554 discusses the reduction of nitrogen oxide (NO) in a combustion effluent by the use of aqueous solutions of ammonia, optionally with a fur¬ ther reducing agent such as hydrogen, at temperatures which can range up to about 2100 F. In what is generally considered to be an improvement on the Lyon disclosure, Arand, Muzio, and Sotter, in U.S. Patent No. 4,208,386, disclose a process involving the introduction of aqueous solutions of urea for the reduc- tion of nitrogen oxides in a selective, free-radical mediated gas phase process. Other nitrogenous treatment agents useful for reducing NOx in combustion effluents are disclosed, for instance, in U.S. Patent 5,017,347 to Epperly, Sullivan, and Sprague, and U.S. Patent 5,057,293 to Epperly, Peter-Hoblyn, Shulof, Jr., Sullivan, Sprague, and O'Leary.

In addition, Hofmann, Sprague, and Sun have taught the use of ammonium carbamate for the reduction of nitro¬ gen oxides, in U.S. Patent 4,997,631. Significantly, they found that the use of ammonium carbamate results in the substantially reduced generation of N₂0 when compared with other nitrogenous treatment agents. Still further reductions are desirable.

The use of compositions containing limestone (CaC0₃), quick lime (CaO), or slaked lime (Ca(OH₂)) and other related compounds as a scrubbing medium for removing the sulfur oxides (SOx) from a combustion effluent has been well documented. Such processes are discussed, for exam¬ ple, by Kuroda et al. in U.S. Patent 4,687,649 and Franke et al. in U.S. Patent 4,540,555. In addition, Torbov has shown that the use of calcium compounds in a slurry is superior to the use of such compounds dry, in U.S. Patent 4,555,996.

Although believed effective for reduction of S0₂ in combustion effluents, there is no teaching that such agents and processes have any activity for the reduction of nitrous oxide when used in combination with an agent for the reduction of NOx.

Thompson and Muzio, in U.S. Patents 4,731,233 and 4,795,586, disclose a process and composition which can, according to the patentees, simultaneously reduce NOx and SOx in a combustion effluent. The disclosed composition comprises a lime-urea hydrate which is used as a sorbent for scrubbing NOx and SOx from an effluent. No reduction in N₂0 is seen in the data, nor is the subject even ad- dressed. In addition, DeMichele and Quattroni discuss the use of a mixture of urea, hydrate lime and water in conjunction with an electrostatic precipitator to reduce nitric oxide and sulfur anhydrides, in European publica¬ tion 0 373 351. It is indicated that, when lime is pres- ent at a level of at least 35% of the dispersion, a poly- saccharide or ligninsulfonate dispersing agent is to be employed. DeMichele and Quattroni make no reference to the minimization of nitrous oxide generation.

In U.S. 5,058,514, Mozes, Mangal, and Thampi dis- close a slurry injection process for S0₂ and NOx reduction using calcium carbonate and a nitrogenous progenitor such as urea. No interaction to reduce nitrous oxide genera¬ tion was found. In fact, according to the inventors, the advantage of the use of their process is in the indepen- dent activity of the components - i.e., no loss of activ¬ ity as compared with independent use of the components.

Mozes and coworkers considered lack of interference to be significant and surprising. There is no teaching or suggestion that a synergy could be achieved using urea or other nitrogenous agents and calcium hydroxide so as to minimize the generation of nitrous oxide when reducing nitrogen oxides.

Disclosure of the Invention

The present invention relates to a composition and process for achieving reduction in a combustion effluent while substantially avoiding the generation of N₂0. The composition is a slurry which comprises a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent and a calcium based sorbent com- prising calcium hydroxide.

This composition can be injected into a combustion effluent as a slurry. It has been found that use of this composition can achieve reductions in nitrogen oxides, without generating significant amounts of nitrous oxide.

Detailed Description of the Preferred Embodiment

As noted, the present invention relates to a compo¬ sition for achieving the reduction of nitrogen oxides in a combustion effluent without generating nitrous oxide. The composition comprises a nitrogenous treatment agent which has been selected for its effectiveness at NOx reduction in combustion effluents. The composition fur¬ ther comprises a calcium based sorbent comprising calcium hydroxide. The inventive composition is introduced into a combustion effluent as a slurry. The nitrogen oxides reductions achieved are found to be accompanied by re¬ duced generation of nitrous oxide when compared to the introduction of the NOx-reducing agent alone. Nitrogenous treatment agents useful for forming the composition of the present invention include those dis¬ closed, for instance, in U.S. Patents 4,997,631, 5,017,347, and 5,057,293, the disclosures of each of which are incorporated herein by reference. Such nitrog¬ enous agents include urea; ammonia; hexamethylenetetra- mine (HMTA) ; ammonium carbamate; cyanuric acid; and vari¬ ous ammonium salts. Preferred for use as the NOx reduc¬ ing component of the invention composition is urea.

Another preferred nitrogenous treatment agent com¬ prises one or more of the hydrolysis products of urea, which include ammonium carbamate ("carbamate"), ammonium carbonate ("carbonate"), ammonium bicarbonate ("bicarbon¬ ate"), and ammonia. In fact, under the proper hydrolysis conditions, these hydrolysis products form a hydrolysate which contains a single unique structure of carbonate and bicarbonate which is in a complex with carbamate (ex¬ pressed as carbamate bicarbonate/carbonate) . The prepa¬ ration and use of the hydrolysis products of urea for NOx reduction is described in International Patent Applica¬ tion entitled "Nitrogen Oxides Reduction Using A Urea Hydrolysate", having Publication No. WO 92/02450, filed in the names of von Harpe, Pachaly, Lin, Diep, and Wegrzyn on August 1, 1991, the disclosure of which is incorporated herein by reference.

If the pressure exerted on the hydrolysate solution is sufficiently high, ammonia also produced does not flash off, but remains in solution and remains available to contribute to the catalytic reduction of NOx. In addition, depending on the conditions employed, residual urea may also be present. Although a urea solution will hydrolyze under ambi¬ ent conditions, typically less than 1% will do so, an insignificant amount in terms of facilitating the cata¬ lytic reduction of nitrogen oxides. In forming the in- ventive hydrolysate, temperature, pressure, concentration of the initial urea solution, and residence time are all important parameters, and must be balanced. High pres¬ sure is particularly useful because the reaction proceeds in the direction of smaller mole volumes during the for- mation of carbamate and carbonate. Higher temperature and longer residence times also result in higher levels of hydrolysis. However, under equivalent pressures, temperatures, and residence times, hydrolysis decreases with increases in solution concentration.

Advantageously, hydrolysis of a 10% aqueous urea solution should be conducted under pressures sufficiently high to maintain the resulting hydrolysate in solution. Such pressures will also facilitate hydrolysis. Desir¬ ably, hydrolysis is performed under pressure of at least about 500 pounds per square inch (psi), more preferably at least about 650 psi. If it is desired to maintain ammonia in solution, the pressure should be at least about 750 psi. As the concentration of the initial urea solution is increased, the pressure is preferably in- creased to achieve equivalent results.

There is no true upper limit of pressure in terms of facilitating hydrolysis; rather, any upper limits com¬ prise practical as opposed to technical limits, since higher pressures, i.e., pressures above about 3000 psi, require vessels able to stand such pressures, which are generally more expensive and usually unnecessary. At the desired pressures, the temperatures and resi¬ dence times can be varied. Temperatures of only about 250 F will ensure the presence of some hydrolysate (e.g., no more than about 5%), whereas temperatures of about 600 F to 800 F will ensure that virtually all the urea has been converted to hydrolysate. Residence times can vary between about three minutes and about 15 minutes, preferably about five minutes to about 10 minutes. It will be recognized that the upper temperature and resi- dence time limits are less important since exceeding them will not result in lower levels of hydrolysis or a less effective hydrolysate, it is believed.

The temperature and residence time for urea hydroly¬ sis are related, and one (i.e., time) can be decreased as the other (i.e., temperature) is increased. For instance, hydrolysis at 400 F for 10 minutes may be gen¬ erally equivalent to hydrolysis at 500 F for five minutes and hydrolysis at 600 F for three minutes.

As noted, hydrolysis proceeds to consecutively form carbamate, carbonate, and bicarbonate. Although all three are present even under the least severe conditions, it is desired that the ratio of carbamate to bicarbon¬ ate/carbonate in the hydrolysate be about 10:1 to about 1:20, more preferably about 2:1 to about 1:10 for great- est effectiveness. This can be achieved by hydrolyzing at a fluid temperature of at least about 325 F for about five minutes or longer.

Most preferably, the hydrolysis of urea is conducted in the presence of metal catalysts such as copper cata- lysts like copper nitrate, nickel catalysts like nickel sulfate, and iron catalysts like iron (III) nitrate, with the copper and nickel catalysts preferred. Since such catalysts enhance urea hydrolysis, greater hydrolysis levels can be achieved with equivalent hydrolysis condi- tions by the use of the catalysts. The catalyst metal is mixed into the urea solution prior to hydrolysis. For instance between about five and about 15, preferably about 10 parts per million (ppm) of catalyst (as metal) is mixed into a 10% urea solution, whereas about 20 to about 60 ppm, preferably about 50 ppm is mixed into a 25% urea solution.

The nitrogenous treatment agent is preferably pres¬ ent in a ratio of the nitrogen in the treatment agent to the baseline (i.e., pre-treatment) effluent nitrogen oxides level which can vary between about 0.5 and about 3.5. This ratio can be referred to as the "normalized stoichiometric ratio" or "NSR". "Normalized stoichiomet- ric ratio" refers to the ratio of the concentration of reducing-radicals such as NHx radicals to the concentra- tion of nitrogen oxides in the effluent and can be ex¬ pressed as [NHx]/[NOx]. NHx radicals, with x being an integer, are believed to be the moiety contributed by the nitrogenous treatment agent which facilitates the series of reactions resulting in NOx reduction to N₂. Alterna- tively, the molar ratio of the treatment agent to the NOx concentration can be used in place of NSR when the chem¬ istry of reduction is not well defined; the term NSR as used herein will also be understood to encompass molar ratio when appropriate.

Advantageously, the nitrogenous treatment agent is included in an amount of about 3% to about 40% by weight of the total composition, excluding diluent (i.e., wa- - li ¬ ter) . When the nitrogenous treatment agent is added in an aqueous solution (as is preferred), the solution water should not be taken into account in calculating the weight of nitrogenous treatment agent being added. In terms of the final slurry (including diluent), the ni¬ trogenous treatment agent should comprise about 1% to about 15% by weight.

Although many calcium based sorbents are known for SOx reduction, including lime, calcium carbonate, calcium hydroxide, and mixtures thereof, it has been found that calcium hydroxide (which is intended to include slaked CaO) forms a synergistic combination with a nitrogenous treatment agent, which results in the reduced generation of N₂0. Calcium hydroxide is, therefore, most preferred for use as the sorbent in the present invention.

Preferably, the sorbent comprises at least about 10% calcium hydroxide and is preferably at least about 40% and most preferably all calcium hydroxide. The amount of calcium sorbent in the inventive composition can be var- ied depending upon the amount of the nitrogenous treat¬ ment agent in the composition. In general, calcium hy¬ droxide will comprise at least about 10% of the inventive slurry and more preferably between about 15% and about 50%. When the slurry components are considered without diluent, calcium hydroxide should preferably comprise up to about 97%, and more preferably about 40% to about 97% of the composition. Besides the nitrogenous agent and the sorbent, the remainder of the composition comprises water or other diluent sufficient to form a slurry. Generally, the slurry comprises about 5% to about 70% solids by weight, preferably about 20% to about 50% sol¬ ids by weight. The relative amounts of sorbent and nitrogenous treatment agent will vary in proportion to the relative presence of SOx and NOx in the effluent. ^' For instance, at a 1:1 ratio of SOx (as S0₂) to NOx (based on parts per million - volume), the ratio of sorbent to nitrogen pres¬ ent is preferably about 1.5:1. When urea is the nitroge¬ nous agent, the slurry components without diluent will comprise about 75% calcium hydroxide, about 25% urea. When the ratio of SOx to NOx is 3:1, the sorbent to ni- trogen ratio is preferably about 5:1 (i.e., about 90%

CaOH₂, almost 9% urea). At a 6:1 ratio of SOx to NOx, the sorbent to nitrogen ratio should preferably be about 10:1 (about 95% calcium hydroxide, about 5% urea).

Most preferably, the inventive slurry is formed by mixing a slurry of calcium hydroxide with a concentrated solution of the nitrogenous treatment agent, in the de¬ sired proportions. If the slurry is not to be introduced into the effluent immediately after it is prepared, it may be desirable to include suitable stabilizers etc., depending on how long it is expected to sit prior to introduction.

The thusly formed slurry is then introduced into the effluent under conditions effective to reduce the pollut¬ ant levels therein. Generally, the slurry is introduced into the effluent while it is at a temperature which varies between about 1500 F and about 2200 F.

By doing so, uniform mixing and sufficient penetra¬ tion of the slurry for effectiveness can be ensured. The injection is designed to take place from nozzles posi- tioned in preexisting ports to achieve uniform distribu¬ tion of the slurry in cross-sectional planes at various levels in the effluent. Along with the inventive compo¬ sition, other components can be included, if desired, such as a complexing agent for copper.

Because of the need for uniform distribution of the slurry throughout the desired cross-sectional planes of the effluent for effective pollutant reduction, the noz¬ zles must be chosen to be effective for this purpose. The skilled artisan will recognize that certain conven¬ tional nozzles would be effective for this purpose. Suitable individual nozzles for injection of the aqueous slurry include Turbotak® nozzles. Additional nozzles which may be suitable are described in International Patent Application entitled "Process and Apparatus for Minimizing Pollutant Concentrations in Combustion Gases", having Application No. PCT/EP91/00952, filed in the names of Chawla, von Bergmann, and Pachaly, on May 21, 1991 and West German Patent DE-26 27 880 C2, published 11 November 1982, the disclosures of each of which are incorporated herein by reference.

It is possible that, under certain circumstances, the amount of water or other diluent in the inventive slurry can have a "quenching" effect on the sorbent and nitrogenous agent. That is, the time it takes to evapo¬ rate the diluent can delay effect of the treatment chemi- cals until treatment is occurring at a lower temperature than initially desired. This may occur (although not necessarily) when the solids levels in the slurry are below about 38%, more likely below about 28% by weight.

When this occurs, the slurry can also comprise an "enhancer" such as oxygenated hydrocarbons like ethylene glycol, ammonium salts of organic acids such as ammonium acetate and ammonium benzoate, heterocyclic hydrocarbons having at least one cyclic oxygen such as furfural, mo¬ lasses, sugar, 5- or 6-membered heterocyclic hydrocarbons having at least one cyclic nitrogen such as pyridine and 5 pyrolidine, hydroxy amino hydrocarbons such as milk or skimmed milk, amino acids, proteins and monoethanolamine and various other compounds which are disclosed as being effective at reducing nitrogen oxides in an effluent. These enhancers, which are preferably present in an 10 amount of about 0.5% to about 25% by weight (excluding diluent) when employed, function to lower the effluent temperatures at which the slurry is most effective for nitrogen oxides reductions.

Such enhancers are disclosed in, for instance, U.S.

15 Patent No. 4,751,065; U.S. Patent No. 4,719,092; U.S. Patent No. 4,888,164; U.S. Patent No. 4,877,591; U.S. Patent No. 4,803,059; International Patent Application entitled "Process for the Reduction of Nitrogen Oxides in an Effluent", having Publication No. WO 89/02870, filed

20 in the names of Epperly, Sullivan, and Sprague on Septem¬ ber 22, 1988; International Patent Application entitled "Process for the Reduction of Nitrogen Oxides in an Ef¬ fluent", having Publication No. WO 89/03242, filed in the names of Epperly, Sullivan, and Sprague on October 14,

251988; U.S. Patent No. 4,830,839; U.S. Patent No.

4,770,863; U.S. Patent No. 4,902,488; and U.S. Patent No. 4,863,704, the disclosures of each of which are incorpo¬ rated herein by reference.

The use of the inventive composition for NOx and SOx 30 reduction according to the process of the present inven¬ tion can be a part of a multi-stage treatment regimen which will reduce effluent pollutants. Such processes are discussed in, for instance, U.S. Patent 4,777,024 to Epperly, Peter-Hoblyn, Shulof, Jr., and Sullivan, as well as U.S. Patent 5,057,293 to Epperly, Peter-Hoblyn, Shulof, Jr., Sullivan, Sprague, and O'Leary, the disclo- sures of each of which are incorporated herein by refer¬ ence.

For instance, in a first stage of such a process, NOx is reduced using the slurry as described above. In a second stage, the inventive composition (perhaps at dif- ferent ratios) or a different treatment agent for NOx reduction (or the reduction of another pollutant like SOx, ammonia, or carbon monoxide), can then be effected. In the alternative, the first stage can comprise aqueous solutions of urea, one or more of the hydrolysis products of urea, a urea hydrolysate, or ammonia, and the second stage the inventive slurry. By doing so, the advantages of the use of the inventive slurry are maximized.

It has surprisingly been found that the use of the composition of the present invention creates a decrease in the level of nitrous oxide in the effluent, as com¬ pared with those levels which are found by introduction of the nitrogenous treatment agent alone, without signif¬ icantly affecting NOx reductions achieved.

The following examples are presented to further illustrate and explain the present invention and should not be viewed as limiting in any regard. Unless other¬ wise indicated, all parts and percentages are by weight and are based on the weight of the product at the partic¬ ular stage of processing indicated. EXAMPLE I

The apparatus utilized is a pilot sdale 14.7 kilo¬ watt (kw) (50,000 btu/hour) refractory-lined down-fired cylindrical furnace capable of firing natural gas or coal, which has an inner diameter of 15.2 centimeters and an overall length of about 4 meters. During natural gas firing, a gaseous combustion environment is simulated by doping the fuel with ammonia (which is oxidized to form nitrogen oxides). The furnace is operated with tangen- tial and axial air totalling 0.39 mVminute (13.72 ftVminute) STP, including excess air of 50%.

Gas emissions are sampled and passed through heated sample lines to continuous emission monitors. Nitrogen oxides are analyzed by a chemiluminescent method which reports NOx concentrations which do not include nitrogen dioxide; but earlier tests show that nitrogen dioxide concentrations are below 5% of the total nitrogen oxides concentration. N₂0 concentrations are monitored by bothon line gas chromatography (GC) and tunable diode laser infrared (TDIR) spectroscopy methods. The GC was used for analysis of grab samples taken before and during testing. The TDIR is used to monitor real time stack N₂0 emissions. The TDIR compares the infrared absorption of the gas sample to a known concentration of N₂0 span gas at the wavelength of N₂0. This method and apparatus were calibrated for this example over the 20 to 80 ppm range, with an accuracy of +/- 0.75 ppm. The two methods' re¬ sults were comparable. Tests conducted at six varying conditions showed a linear correlation coefficient ex- ceeding 0.99 between the two methods. All gas emission results are corrected to 0% 0₂ levels. Effluent baseline pollutant values are determined prior to testing while injecting deionized water in an amount equivalent to the treatment agents' to be injected. Testing is performed on slurried calcium hydroxide at 30% solids metered into a Turbotak nozzle by a calibrated peristaltic pump in a slurry with urea where the urea as an aqueous solution is metered into the slurry by means of a calibrated dual syringe pump; and a urea solution alone.

The slurry and urea solutions are injected through water cooled probes which inject coaxially to the process gas, which has a temperature of 1087 C (about 1988 F) . The Turbotak slurry probe uses air (18% of the total furnace air flow) to effect droplet atomization.

The droplet size distribution of the slurry exiting the Turbotak® nozzle is determined by use of a Munhall particle size analyzer which indicates that the slurry has a droplet size distribution with a D50 (that is a density wherein 50% of the droplets are below the indi- cated size) of 13 micrometers and a D90 of 88 microme¬ ters. Temperature at the location for injection is de¬ termined using a suction pyrometer and a type R thermo¬ couple. The temperature at the point of the injection nozzle is calculated by extrapolation of the temperature values from downstream ports. The slurry and urea solu¬ tion are injected so as to produce a specific NSR.

The results are set out in Table 1, and show that the presence of calcium hydroxide reduces the generation of N₂0.

EXAMPLE II

The process of Example I is repeated, except that the slurry is formed using a urea hydrolysate prepared by hydrolyzing urea at 375 F and 1500 psi for 10 minutes, in place of urea, and the solution injected is an aqueous solution of the hydrolysate in place of the urea solu- tion.

The results are set out in Table 2, and show that the presence of calcium hydroxide reduces the generation of N₂0,

It is to be understood that the above examples are given by way of illustration only and are not to be con¬ strued as limiting the invention.

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 de¬ fined by the following claims.

Claims

1. A process for reducing the concentration of nitrogen oxides in the effluent from the combustion of a carbona¬ ceous fuel, the process comprising: a. forming a slurry comprising i. a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent; ii. a calcium-based sorbent comprising calcium hydroxide; and iii. water; and b. introducing said slurry into the effluent under conditions effective to reduce the effluent concentration of nitrogen oxides without the generation of a substan¬ tial amount of nitrous oxide.

2. The process of claim 1, wherein the effluent temper¬ ature at the point of introduction of said slurry is between about 1500 F and about 2200 F.

3. The process of claim 2, wherein calcium hydroxide is present in said slurry in an amount at least about 10% by weight.

4. The process of claim 3, wherein calcium hydroxide is present in said slurry in an amount between about 10% and about 50% by weight.

5. The process of claim 1, wherein said nitrogenous treatment agents comprise urea, one or more of the hydro¬ lysis products of urea, a urea hydrolysate, ammonia, hexamethylenetetramine, cyanuric acid, ammonium carba- mate, ammonium slats., or mixtures thereof.

6. The process of claim 5, wherein said nitrogenous treatment agent is present in said slurry in an amount of about 1% to about 15% by weight.

7. The process of claim 6, wherein said nitrogenous treatment agent is present at an NSR between about 0.5 and about 3.5.

8. The process of claim 1, wherein said slurry compris¬ es about 20% to about 70% solids by weight.

9. The process of claim 1, wherein said slurry is formed by mixing a concentrated solution of the nitroge¬ nous treatment agent into a calcium hydroxide slurry, and immediately thereafter injecting the thusly formed slurry into the effluent.

10. The process of claim 1, which comprises at least one stage of a multi-stage treatment regimen effective to reduce effluent nitrogen oxides levels.

11. The process of claim 10, wherein an additional stage of said multi-stage treatment regimen comprises the in¬ troduction of a solution of urea, one or more of the hydrolysis products of urea, a urea hydrolysate, ammonia, hexamethylenetetramine, or mixtures thereof.

12. The process of claim 1, wherein the weight ratio of calcium hydroxide to nitrogenous treatment agent is about 0.5:1 to about 33:1.

13. A composition for the reduction of nitrogen oxides in the effluent from the combustion of a carbonaceous fuel, which comprises a slurry comprising: a. a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent; b. a calcium-based sorbent comprising calcium hydroxide; and c. water.

14. The composition of claim 13, wherein calcium hydrox¬ ide is present in said slurry in an amount of at least about 10% by weight.

15. The composition of claim 14, wherein calcium hydrox¬ ide is present in said slurry in an amount between about 15% and about 50% by weight.

16. The composition of claim 13, wherein said nitroge¬ nous treatment agents comprise urea, one or more of the hydrolysis products of urea, a urea hydrolysate, ammonia, hexamethylenetetramine, ammonium salts, or mixtures thereof.

17. The composition of claim 16, wherein said nitroge¬ nous treatment agent is present in said slurry in an amount of about 1% to about 15% by weight.

18. The composition of claim 17, wherein said nitroge¬ nous treatment agent is present at an NSR between about 0.5 and about 3.5.

19. The composition of claim 13, wherein said slurry comprises about 20% to about 70% solids by weight.

20. The composition of claim 13, wherein the weight ratio of calcium hydroxide to nitrogenous treatment agent is about 0.5:1 to about 33:1.