PROCESS AND COMPOSITION FOR NOx and SOx REDUCTION

Abstract: The invention presented relates to a process and composition for the reduction of nitrogen oxides and sulfur oxides in the effluent from the combustion of a carbonaceous fuel. The composition is a slurry which includes calcium hydroxide effective for the reduction of sulfur oxides in the effluent; a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent; and water. The process permits reductions of sulfur dioxide beyond those found when calcium hydroxide is used alone, without sacrificing nitrogen oxides reductions.

Description: -1- DESCRIPTION PROCESS AND COMPOSITION FOR NO χ AND SO χ REDUCTION Technical Field The present invention relates to a novel process and composition for achieving superior reductions in the levels of both nitrogen oxides (NO χ , where x is an integer, generally 1 or 2) , and sulfur oxides (SO χ , where x is an integer, generally 2 or 3) , in the effluent from the combustion of a carbonaceous fuel. When fossil fuels are used in suspension fired boilers, such as large utility boilers, temperatures above about 2000°F and typically above 2200°F are generated. At these temperatures, both nitrogen and sulfur present in the fuels can be oxidized to form acidic combustion gases. Such high temperatures also tend to cause the production of NO χ from nitrogen in the atmosphere, the temperatures being so high that free radicals of oxygen and nitrogen are formed and chemically combine as nitrogen oxides. These pollutants can also form in the effluent streams of other combustors, such as municipal waste combustors and industrial or process boilers or heaters. In fact, any combustion source in which the fuel contains sulfur will yield sulfur oxides and nitrogen oxides in the effluent. -2- Both nitrogen oxides and sulfur oxides are troublesome pollutants which are found in the combustion streams of boilers when fired as described above, and comprise major components of acid rain. It is further believed that nitrogen oxides 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." What is desired, therefore, is a composition effective for achieving both nitrogen oxides and sulfur oxides reductions in a combustion effluent via a single treatment agent introduced in an efficient and practical manner not heretofore seen. Background Art Compositions and processes for the reduction of NO χ and/or SO χ from combustion effluents have been studied extensively. Due to the increased importance given environmental matters 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 gas phase reactions have been disclosed. For instance, Lyon, in U.S. Patent 3,900,554 discusses the reduction of NO in a combustion effluent by the use of aqueous solutions of ammonia, optionally with a further reducing agent such as hydrogen, at temperatures which can range up to about 2100 β F. In what is generally considered to be an improvement on the Lyon disclosure, Arand, Muzio, and Sotter, in U.S. -3- Patent No. 4,208,386, disclose a process involving the introduction of aqueous solutions of urea for the reduction of nitrogen oxides in a selective, free-radical mediated gas phase process. In addition, Hofmann, Sprague, and Sun have taught the use of ammonium carba ate for the reduction of nitrogen oxides, in U.S. Patent 4,997,631. Other nitrogenous treatment agents useful for reducing NO χ 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. While such nitrogenous treatment agents are uniquely effective at reducing N0 χ in combustion effluents, there is no indication in the patent literature that they are at all effective at facilitating the reduction of sulfur oxides without further processing or additional agents. The SO χ reduction art is replete with examples of the use of compositions containing limestone (CaC0 3 ) , quick lime (CaO) , or slaked lime (Ca(OH 2 )) and other related compounds as a scrubbing medium for removing the sulfur oxides from a combustion effluent. Such processes are discussed, for example, 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 S0 2 in combustion effluents, there is no teaching that the activity of such agents and processes can be increased by use of a composition effective to reduce both SO χ and -4- NO χ in a single introduction. Indeed, the most recent prior art demonstrates the contrary. 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 NO χ and SO χ in a combustion effluent. The disclosed composition comprises a lime-urea hydrate which is used as a sorbent for scrubbing NO χ and SO χ from an effluent. No synergy is seen in the data. 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 publication 0 373 351. It is indicated that, when lime is present at a level of at least 35% of the dispersion, a polysaccharide or ligninsulfonate dispersing agent is to be employed. DeMichele and Quattroni directly assert that their process performs better than dry lime, but make no direct comparison with independent introduction of a lime slurry and urea, respectively. In U.S. 5,058,514, Mozes, Mangal, and Thampi disclose a slurry injection process for S0 2 and NO χ reduction using calcium carbonate and a nitrogenous progenitor such as urea. No interaction was found. In fact, according to the inventors, the advantage of the use of their process is in the independent activity of the components - i.e., no loss of activity as compared with independent use of the components. Mozes and coworkers considered lack of interference to be significant and surprising. There is absolutely no teaching or suggestion that a synergy could be achieved using urea and calcium hydroxide. -5- Disclosure of the Invention The present invention relates to a composition and process for achieving superior reduction of SO χ in a combustion effluent while simultaneously reducing N0 χ . The composition is a slurry which comprises calcium hydroxide, a calcium based sorbent effective for the reduction of sulfur oxides in an effluent, and a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent. 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 sulfur oxides greater than can be achieved by the introduction of either component alone, without sacrificing NO χ reductions. Brief Description of the Drawings The invention will be better understood and its advantages will become more apparent when the following detailed description is read in conjunction with the accompanying drawings, in which FIGURE 1 is a graphical illustration of the results of Example I; and FIGURE 2 is a graphical illustration of the results of Example II. Detailed Description of the Preferred Embodiment As noted, the present invention relates to a composition for achieving simultaneous reduction of nitrogen oxides and sulfur oxides in a combustion effluent. The composition comprises calcium hydroxide, which is chosen for its effectiveness at the reduction of sulfur oxides in an effluent. The composition further comprises a nitrogenous treatment agent which has been selected for its effectiveness at NO χ reduction in combustion effluents. The inventive composition is introduced into a combustion effluent as a slurry. The sulfur oxides reductions achieved are found to be superior to those reductions expected from the use of calcium hydroxide alone. Although many calcium based sorbents for S0 2 reduction are known, including lime, calcium carbonate, calcium hydroxide, and mixtures thereof, it has been found that calcium hydroxide (which is intended to include slaked CaO) alone forms a synergistic combination with a nitrogenous treatment agent. The activity of other sorbents, such as calcium carbonate, has been found to be consistent with what was disclosed by Mozes, Mangal, and Thampi - no reduction in activity when used with urea, but no synergism. Generally, the sorbent must comprise 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 varied depending upon the amount of S0 2 in the effluent and the amount of the nitrogenous treatment agent in the composition. Preferably, the calcium hydroxide will be included in the composition at a molar ratio of calcium to baseline (that is, pre-treatment level of) S0 2 of about 1:2 to about 4:1, more preferably about 1:1 to about 3:1. In general, calcium hydroxide will comprise at least about 10% of the inventive slurry and preferably between about 15% and a b out 50%. When the slurry components are considered dry, i.e., without diluent, calcium hydroxide comprises up to about 97%, and more preferably about 65% to about 97% of the composition excluding diluent. Nitrogenous treatment agents useful for forming the composition of the present invention include those disclosed, for instance, in U.S. Patents 4,997,631, 5, 0 17,347, and 5,057,293, the disclosures of each of which are incorporated herein by reference. Such nitrogenous agents include urea, ammonia, hexamethylenetetramine (HMTA) , ammonium carbamate, and various ammonium salts. Preferred for use as the N 0χ reducing component of the invention composition is urea. The nitrogenous treatment agent is preferably present 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 stoichiometric ratio" refers to the ratio of the concentration of reducing-radicals such as NH χ radicals (NH χ 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 N Oχ reduction to N 2 ) to the concentration of nitrogen oxides in the effluent and can be expressed as [NH χ ]/[NO χ ]. Alternatively, the molar ratio of the treatment agent to the NO χ 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. -8- Advantageously, the nitrogenous treatment agent is included in an amount of about 3% to about 35% by weight of the total composition, excluding diluent (i.e., water) and about 1% to about 15% of the final slurry. When the urea is added in an aqueous solution, the solution water should not be taken into account in calculating the weight of urea being added. Generally, the weight ratio of sorbent to nitrogenous agent in the inventive slurry will be about 1.5:1 to about 33:1, more preferably about 2.7:1 to about 22:1. 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 20% to about 70% solids by weight, preferably about 26% to about 50% solids by weight. As an example, assuming an effluent gas volume of 1,000,000 SCF/hour, and an S0 2 level of 1000 parts per million (ppm) and a NO χ level of 300 ppm (as may typically be found), a slurry of Ca(OH) 2 and urea in water can be introduced at a sorbent to S0 2 molar ratio of 2 (or 384.4 lbs/hour) and an NSR of 2 (46.75 lbs/hour) . This results in a ratio of sorbent to urea of 8.22. The thusly formed slurry is then introduced into the effluent under conditions effective to reduce the pollutant levels therein. Generally, the slurry is introduced into the effluent while it is at a temperature which varies between about 1500 β F and about 2200 β F. 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 desired 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, depending on how long it is expected to sit prior to introduction. The thusly formed slurry is then introduced, e.g., injected, into the effluent to be treated. By doing so, uniform mixing and sufficient penetration of the slurry for effectiveness can be ensured. The injection is designed to take place from nozzles positioned in preexisting ports to achieve uniform distribution of the slurry in cross-sectional planes at various levels in the effluent. Because of the need for uniform distribution of the slurry throughout the desired cross-sectional planes of the effluent for effective pollutant reduction, the nozzles must be chosen to be effective for this purpose. The skilled artisan will recognize that certain conventional 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 evaporate the diluent can delay effect of the treatment chemicals 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 preferably 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, molasses, sugar, 5- or 6-membered heterocyclic hydrocarbons having at least one cyclic nitrogen such as pyridine and 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 amount of about 0.5% to about 25% by weight when employed, function to lower the effluent temperatures at which the slurry is most effective. Such enhancers are disclosed in, for instance, U.S. 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 in the names of Epperly, Sullivan, and Sprague on September 22, 1988; International Patent Application entitled "Process for the Reduction of Nitrogen Oxides in an Effluent", having Publication No. WO 89/03242, filed in the names of Epperly, Sullivan, and Sprague on October -11- 14, 1988; U.S. Patent No. 4,830,839; U.S. Patent No. 4,770,863; and U.S. Patent No. 4,902,488; U.S. Patent No. 4,863,704, the disclosures of each of which are incorporated herein by reference. The use of the inventive composition for NO χ and SO χ reduction according to the process of the present invention 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 disclosures of each of which are incorporated herein by reference. For instance, in a first stage of such a process, N0 χ and SO χ are reduced using the composition as described above. In a second stage, the inventive composition (perhaps at different ratios) or a different treatment agent for either N0 χ or SO χ reduction can then be effected by the means described above. In the alternative, the first stage can comprise a urea or ammonia solution, and the second stage the inventive composition solution; or the first stage can comprise the composition and the second a urea, ammonia or hexa- methylenetetramine solution, etc. By doing so, the advantages of the use of the inventive composition are maximized. Along with the inventive composition, other components can be included, such as a complexing agent for copper. It has surprisingly been found that the use of the composition of the present invention creates a synergistic decrease in the level of sulfur oxides in the -12- effluent, as compared with those reductions which can be achieved by introduction of the separate components individually, without significantly affecting NO χ 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 otherwise indicated, all parts and percentages are by weight and are based on the weight of the product at the particular stage of processing indicated. EXAMPLE 1 The apparatus utilized is a pilot scale 14.7 kilowatt (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) and sulfur dioxide. The furnace is operated with tangential and axial air totalling 0.39 m 3 /minute (13.72 ft 3 /minute) STP, including excess air of 50%. Gas emissions are sampled and passed through heated sample lines to continuous emission monitors. Analysis of sulfur dioxide concentration is performed by an ultraviolet analyzer which follows particle traps and a heated sample line (about 350 β C) . Removal percentages are calculated by determining removal values at a sorbent to sulfur stoichiometric ratio between 1:1 and 3:1 and then curve fit by a regression technique and interpolated to removal at a sorbent to sulfur ratio of 2:1. Nitrogen -13- oxides are analyzed by a chemiluminescent method which reports N0 χ concentrations which do not include nitrogen dioxide; but earlier tests show that nitrogen dioxide concentrations are below 5% of the total nitrogen oxides concentration. All gas emission results are corrected to 0% 0 2 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 dry sorbent injection of calcium hydroxide, which is fed by a K-tron loss-in-weight twin screw feeder calibrated prior to and after each run; slurried calcium hydroxide at 30% solids metered into a Turbotak nozzle by a calibrated peristalic pump; and a sorbent/urea slurry where the urea as an aqueous solution is metered into the slurry by means of a calibrated dual syringe pump. Both the dry sorbent and slurry are injected through water cooled probes which inject coaxially to the process gas. The dry sorbent probe injects 15.7% of the total IFR air flow to effect sorbent conveyance and dispersion. 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 D 50 of 13 micrometers and a D 90 of 88 micrometers. Temperature at the location for injection is determined using a suction pyrometer and a type R thermocouple. The temperature at the point of the injection nozzle is -14- calculated by extrapolation of the temperature values from downstream ports. Sorbent injection is at a calcium to sulfur molar ratio corrected to 2:1. When urea is included in the slurry, it is present at an NSR of about 2. The results are illustrated in Figure 1, which shows that the presence of urea increases the ability of calcium hydroxide to remove sulfur dioxide from the gas. EXAMPLE 2 The process of Example 1 is repeated, except that nitrogen oxides reductions are measured. These results are illustrated in Figure 2 and show virtually no change in nitrogen oxides reductions caused by the presence of the calcium hydroxide sorbent. It is to be understood that the above examples are given by way of illustration only and are not to be construed 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 defined by the following claims.

Claims: -15- Claims 1. A process for reducing the concentration of pollutants in the effluent from the combustion of a carbonaceous fuel, the process comprising: a. forming a slurry comprising i. at least about 10% by weight of a calcium- based sorbent comprising calcium hydroxide effective for the reduction of sulfur oxides in the effluent; ii. about 1% to about 15% by weight of a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent; and iii. water; and b. introducing said slurry into the effluent under conditions effective to reduce the effluent concentration of sulfur dioxide and nitrogen oxides. 2. The process of claim 1, wherein the effluent temperature at the point of introduction of said slurry is between about 1500 β F and about 2200°F. 3. The process of claim 2, wherein calcium hydroxide is present in said slurry in an amount between about 15% and about 50% by weight. 4. The process of claim 3, wherein calcium hydroxide is present in an amount effective to provide a molar ratio of calcium to the baseline level of sulfur dioxide of about 1:2 to about 4:1. 5. The process of claim 1, wherein said nitrogenous treatment agents comprise urea, ammonia, hexamethylene- tetramine, ammonium carbamate, ammonium salts, or -16- ixtures 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 comprises 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 nitrogenous 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 pollutants. 11. The process of claim 10, wherein an additional stage of said multi-stage treatment regimen comprises the introduction of a solution of urea, ammonia, hexa- methylenetetramine, or mixtures thereof. 12. The process of claim 1, wherein the weight ratio of calcium hydroxide to nitrogenous treatment agent is about 1.5:1 to about 33:1. 13. A composition for the reduction of pollutants in the effluent from the combustion of a carbonaceous fuel, which comprises a slurry comprising: a. at least about 10% by weight of a calcium-based sorbent comprising calcium hydroxide effective for the reduction of sulfur oxides in the effluent; b. about 1% to about 15% by weight of a nitrogenous treatment agent effective for the reduction of nitrogen oxides in the effluent; and c. wate . 14. The composition of claim 13, wherein calcium hydroxide is present in said slurry in an amount between about 15% and about 50% by weight. 15. The composition of claim 14, wherein calcium hydroxide is present in an amount effective to provide a molar ratio of calcium to the baseline level of sulfur dioxide of about 1:2 to about 4:1. 16. The composition of claim 13, wherein said nitrogenous treatment agents comprise urea, ammonia, hexamethylene- tetra ine, ammonium salts, or mixtures thereof. 17. The composition of claim 16, wherein said nitrogenous 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 nitrogenous 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 1.5:1 to about 33:1.

Inventors: GULLETT BRIAN K, HOFMANN JOHN E, PETER-HOBLYN JEREMY D, VALENTINE JAMES M, GULLETT, BRIAN, K., HOFMANN, JOHN, E., PETER-HOBLYN, JEREMY, D., VALENTINE, JAMES, M.
Applicants: NALCO FUEL TECH
Application year: 1992
Main classification: B01J8
IPC codes: B01D53