Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/54—Nitrogen compounds
  • B01D53/56—Nitrogen oxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/73—After-treatment of removed components

Definitions

• the invention relates to reducing the environmental damage caused by large-scale combustion, and provides both improved processes and improved compositions for achieving this, especially in the field of NO x reduction by selective noncatalytic reduction (SNCR) in the gas phase.
• SNCR selective noncatalytic reduction
• Carbonaceous materials including the typical hydrocarbon fuels such as coal, oil and gas, as well as refuse, are burned in increasing amounts each year.
• high temperatures e.g., 1000° to 1700oC
• nitrogen oxides (NO x ) formation is greatly increased because the high temperatures (e.g., 1000° to 1700oC) normally encountered in large scale combustors.
• NO x nitrogen oxides
• Prominent among the selective non-catalytic reduction (SNCR) processes are those disclosed for example by Lyon in U.S. Patent 3,900,554 and by Arand et al in U.S.
• urea can be added alone or in solution to
• Chawla et al describe a nozzle which enables injection of a two-phase mixture of air and aqueous NO x -reducing composition into an effluent at sonic velocity to achieve an improved distribution of particle sizes, but could also benefit from such an additive.
• Another object is to mitigate utilization and
• the invention provides a process for improving the reliability of known means, compositions and methods for reducing the concentration of pollutants in a gas stream by: preparing an aqueous solution comprising a pollutant-reducing agent and a water-dispersible polymer, and introducing the solution into the gas stream.
• the solution comprising the pollutant-reducing agent and the water-dispersible polymer is mixed with air and injected as a dispersion of liquid droplets in a gas stream at or near the sonic velocity of the mixture, into a passage containing a combustion effluent at a temperature effective for reducing a pollutant concentration.
• Figure 1 is a flow chart of a process scheme
• Figure 2 is a cross-sectional view of the detail of an exemplary nozzle for use in the invention.
• Effluents in need of treatment are produced by a variety of sources including large industrial and utility boilers, circulating fluidized bed boilers, and gas turbines. It will be understood, though, that although written in terms of the reduction of nitrogen oxides in the effluent from the combustion of a carbonaceous fuel, the invention is applicable in any high temperature environment having nitrogen or sulfur oxides, or other pollutant that can be treated by selective gas-phase reaction. Temperatures will typically be greater than about 800oC and can be as high as about 1100oC, even 1200oC and higher.
• the known processes involve preparing a treatment composition (typically as an aqueous solution), transporting the composition to a gas stream (e.g., a combustion
• the treatment composition is introduced into the effluent by an injector which generally comprises a conduit, sometimes fitted at the tip with a nozzle, extending into an effluent gas
• a portion of effluent or other gas is employed to help atomize and disperse the treatment composition.
• the spray or injection pattern is defined with precision, often with the aid of computer to assure good distribution and reaction.
• the present invention helps improve operation by providing a more predictably-defined pattern. Apparatus of varying degrees of sophistication are known for introducing NO x -reducing compositions into a high temperature environment. Some comprise coaxial, multi-tubular structures, such as those disclosed by
• Figure 1 shows a representative multistage treatment in a large combustor. This figure shows the final mixing and introduction of three individual active components. These are shown as being provided from individual sources A, B and C and stored in tanks 20, 30 and 40, which are associated with metering pumps 22, 32 and 42 as well as suitable conduits, and valves to enable mixing in the manner called for by a controller (not shown) or manual operation.
• the temperature of effluent at the point of injection, the concentration of active component in the solution, and the size of the droplets in the dispersion are selected to achieve reduction in nitrogen oxides or other pollutant levels in the effluent.
• One exemplary injection apparatus for use according to the invention is described in the above-identified disclosure of Chawla et al and is capable of supplying a pollutant-reducing agent comprised of one or more active components in a two-phase mixture of liquid and gaseous components.
• a nozzle of this type is shown in Figure 2 and will be described in greater detail below.
• the liquid component of a two-phase mixture in the case of Chawla et al , Burton or DeVita injectors. or a single-phase, liquid component in the case of other nozzles not requiring a gas typically comprises a solution having at least one pollutant-reducing agent effective in reducing NO x and/or SO x under the
• conditions of injection may contain enhancers to best match the pollutant-reducing agent to the effective temperature window, and also usually includes various additives to protect the system against blockage.
• the active pollutant-reducing agent e. g.
• NO x -reducing agent is desirably incorporated in the liquid phase.
• a NO x -reducing agent is introduced as an aqueous treatment solution, such as urea or other NH-containing composition in water with one or more enhancers.
• a aqueous treatment solution such as urea or other NH-containing composition in water with one or more enhancers.
• the liquid component can include suspended solids or an immiscible fluid material.
• the gaseous component can be similarly complex.
• Various NH-containing compositions in their pure and typical commercial forms, will generate effective gas-phase agents (e.g., the amidozine radical) when
• NH-containing compositions those selected from the group consisting of ammonia, ammonia precursors, urea, urea precursors, urea hydrolysis products, products of reaction of urea with itself or other compositions, related compositions, and mixtures of these.
• these compounds are ammonium carbonate, ammonium formate, ammonium citrate, ammonium acetate, ammonium oxalate, other ammonium salts
• organic and organic particularly of organic acids, ammonium hydroxide, various stable amines, guanidine, guanidine carbonate,biguanide, guanylurea sulfate, melamine, dicyanimide, calcium cyanamide, biuret,
• HMTA hexamethylenetetramine
• ammonia carbamates such as ammonium carbamate, ammonium carbonate, ammonium bicarbonate and other ammonia salts, various urea
• Enhancers are additive materials which modify the effectiveness of a pollutant-reducing agent in terms of its effective temperature window, its utilization
• enhancers are the above materials when used in suitable combination, oxygenated hydrocarbons, and mixtures of these.
• oxygenated hydrocarbons are ketones, aldehydes, alcohols including polyols, carboxylic acids, sugars, starch hydrolysates, hydrogenated starch
• the various treatment solutions are premixed to include the NO x -reducing composition and enhancer most appropriate for the range of temperatures expected to be encountered at its
• the concentration of the compositions at the points of introduction can be finally adjusted as desired by water from the common source shown.
• the solution in the storage vessels will also preferably contain the water-dispersible polymers according to the invention. It is possible, of course to meter these in on a real-time basis in the concentration required. It is an advantage of the invention that these water-dispersible polymers are highly and rapidly dispersible in water throughout their full range of effective concentrations.
• the water-dispersible polymers of the invention are those which have the ability to increase the surviv-ability of the droplets to a position farther downstream in the hot combustion gases than is achieved without the polymer. Significantly, the polymers increase the size of the droplets formed, reduce the number of very small droplets, and preferably, also narrow the distribution of droplet sizes formed upon injection.
• the preferred polymers provide both of these advantages.
• unsubstituted water soluble polymers including: vinyl polymers based on acrylic and lower (e.g. 1-4 carbon) alkylacrylic acids, acrylamide and lower alkylacryl-amides, styrene, esters of any of these, and mixtures of two or more of these; polyethers, including alkylene oxide polymers such as homo and copolymers of ethylene oxide and propylene oxide; and mixtures of these and polymers of like monomers which have high molecular weights, e.g. greater than 500,000.
• the polymers are highly polar due to their ionic character or the presence of polar ether or like groups.
• substituted polymers are those with sulfo or sulfoalkyl (e.g., sulfomethyl or sulfoethyl) substitutions to a portion of the acid or amide group of polymers of the above type.
• sulfo or sulfoalkyl e.g., sulfomethyl or sulfoethyl
• ionic polymers both anionic and cationic, including acrylic homo and
• the anionic polymers are preferred in solutions containing significant hardness, e.g.,
• acrylic polymers are anionic copolymers of acrylic acid and acrylamide, such as those available from Nalco Chemical Company (Naperville, Illinois) as 7177PULV, 7744, and 8173 and cationic polymers of dimethylaminoethyla ⁇ rylic acid and methychloride
• quat/acrylamide such as those from Nalco Chemical
• the anionic polymers should have an RSV (Reduced Specific Viscosity determined in 1N NaNO 3 at a concentration of 0.045 g of polymer as the sodium salt per 100 cc) of at least 10-13, and preferably at least 25.
• the cationic polymers should have an RSV of at least 10 as measured in 1N NaNO 3 by Ubbelohde size 75 Viscometer at 30oC.
• polyethers are high molecular weight (e.g. 100,000 to 10 million, preferably at least 1 million) ethylene oxide polymers, such as those available from Union Carbide, Specialty Chemicals Division (Danbury, Connecticut) under the designations Polyox WSR and Ucarfloc. Also effective are lower molecular weight polyethers available from Union Carbide as UCON food grade lubricants and
• polyethylene glycols available from Union Carbide as Carbowax polyethylene glycols.
• a preferred group of anionic polymers are alkali metal salts of copolymers of acrylic acid (AA) and acrylamide (AM) with a mole ratio of AA to AM in the range of 1:10 to 2:1 with RSV's of at least about 25.
• Latex forms of these polymers are effective but must be inverted in water to free the polymer for dispersion within the water.
• the latexes are typically polysoap stabilized water-in-oil emulsions with the polymer being dissolved in a suitable solvent such as LOPS (Low Odor Parafinic solvent).
• LOPS Low Odor Parafinic solvent
• Suitable polymers are “prism" polymers such as water soluble terpolymers containing the following repeating units in the following percentages:
• R is a lower alkylene group from 1-6 carbon atoms.
• R is preferably ethylene.
• M is H, NH 4 or
• terpolymer having a Reduced Specific Viscosity of at least 25.
• These terpolymers having the above described formula preferably contain between 10-30 mole percent by weight of (A), 20-30 mole percent of (B), and 30-70% of (C).
• These polymers, to give the best results, should preferably have a RSV (Reduced Specific Viscosity determined in 1N NaNO 3 at a concentration of 0.045 g of polymer as the sodium salt per 100 cc) of at least 30.
• RSV Reduced Specific Viscosity determined in 1N NaNO 3 at a concentration of 0.045 g of polymer as the sodium salt per 100 cc
• water-in-oil emulsions that contain 25 to 30% polymer. More detail as to the preparation of these polymers is given in U.S. Patent 3,284,393, RE 28,474 and RE 28,576. Also suitable are high molecular weight (RSV of at least 18) sulfonated polymers such as 2-acrylamido-2-methyl propane sulfonic acid (AMPS).
• RSV high molecular weight
• AMPS 2-acrylamido-2-methyl propane sulfonic acid
• Polymers such as ionic AA/AM copolymers and prism polymers have the further advantage of enabling greater flow rates.
• the high molecular weight polyethers have the added advantage that they can increase the velocity of injection.
• droplets typically in an amount of from about 10 ppm to about 1000 ppm, and more narrowly from about 50 ppm to about 500 ppm, based on the weight of the solution.
• composition be dispersed from a nozzle to provide
• droplets having an average droplet size within the range of from about 5 to about 10,000 microns Sauter mean diameter, and a relative span of droplet sizes of less than 2. Most preferably, the median droplet size will be within the range of from about 200 to 1000 microns.
• relative span refers to the distribution of droplet sizes ⁇ the lower the value, the more nearly the same they are. This term is defined more precisely in Example 1.
• the invention provides a greater degree of reliability to processes and apparatus which employ or are used in connection with aqueous solutions of the type described. This is
• a hardness-suppressing composition comprising at least one member selected from the group consisting of polymers, phosphonates, chelants, phosphates and mixtures of any two or more of these, in an amount effective to suppress hardness.
• a hardness-suppressing composition comprising at least one member selected from the group consisting of polymers, phosphonates, chelants, phosphates and mixtures of any two or more of these, in an amount effective to suppress hardness.
• single members of this group can be employed where effective, or two or more members of a single group can be employed, as well as mixtures of members from different groups.
• compositions are: one or more polymers; combinations of one or more polymers and one or more phosphonates; combinations of one or more polymers, one or more phosphonates, and one or more chelants; combinations of one or more polymers, one or more phosphonates, one or more chelants and one or more phosphates; combinations of one or more polymers, one or more phosphonates, and one or more phosphates; combinations of one or more polymers and one or more chelants; combinations of one or more polymers and one or more phosphates; combinations of one or more polymers, one or more chelants and one or more phosphates; one or more phosphonates; combinations of one or more
• phosphonates and one or more chelants are effective and can be employed for scale conditioning and control as well as corrosion control.
• certain phosphates are effective and can be employed for scale conditioning and control as well as corrosion control.
• the hardness-suppressing composition is preferably employed at a level effective to suppress hardness.
• a polymer and a phosphonate are employed in combination at a weight ratio of within the range of from 1:25 to 25:1, but the preferred ratio is 4-6:1. Ratios outside these ranges can be employed so long as at least a minimum effective amount of each component in the combination is employed.
• a concentrate is prepared for final dilution, it is desired to employ enough of the hardness-suppressing composition to be effective in the concentrate and all contemplated degrees of dilution.
• surfactants are also employed to aid in maintaining dispersion of solution components
• pH modifiers such as monoethanolamine are employed in amounts effective to achieve long-term stability, especially when subjected to temperature extremes of from freezing to 120oF.
• One preferred hardness-suppressing formulation comprises 83% deionized water, 2.5% of a 60% solution of 1-hydroxyethylidene- 1,1-diphosphonic acid (HEDP), 10% of a 63% active
• ethoxylate ethoxylate
• aroma enhancer 1% aroma enhancer
• monoethanolamine 1%
• Dowfax 3B2 anionic sulfonate surfactant alkylated diphenyl oxide disulfonates
• PBTC 2-phosphono-1,2,4-tricarboxybutane
• AMP AMP in place of the polyacrylic acid
• PBTC low molecular weight polyacrylic acid made in aqueous media with bisulfite/ persulfate catalyst and PBTC.
• the hardness-suppressing formulation is employed at a level of from about 0.1 to 5% (e.g.,
• the NO x -reducing composition improved by the invention will typically be prepared and shipped as a concentrate which is diluted for use.
• these concentrates will contain 25 to 65% urea, 10 to 1000 ppm of a water-dispersible polymer of the invention, and 0.05 to 1.0 % of an effective hardness-suppressing composition, more preferably from 40 to 55% urea, e.g., 50%, from 50 to 500 ppm of the water-dispersible polymer, and from 0.1 to 0.75%, e.g., 0.5%, of the hardness-suppressing composition.
• the urea and a separate additive package may be preblended to form this concentrate.
• This concentrate is typically diluted prior to injection as required to achieve a urea concentration effective under the injection conditions. Typically, dilution to
• concentrations of from 5 to 25% urea are effective.
• the pH of the aqueous solution is above 5, and generally is within the range of from 7 to 11, e.g., 8 to 10.
• concentration of the reducing agent injected into the effluent should be sufficient to obtain a reduction, and preferably ensure a minimization, of the nitrogen oxide concentration.
• the reducing agents are employed in total amounts sufficient to provide a molar ratio of reducing agent to baseline nitrogen oxide content (i.e., prior to treatment at that stage) of about 1:4 to about 5:1.
• the ratio is preferably within the range from 1:2 to 2:1, even more narrowly 2:3 to 6:5. In most situations this amount will be distributed jointly by a number of nozzles from a number of points as is illustrated in Figure 1.
• liquid droplets comprising NO x -reducing agents can be injected at well-controlled particle size distributions and velocities to make the active agents available near the point of injection as well as at the far side of an effluent passage without large numbers of droplets impinging obstructions or surviving beyond the desired temperature window for NO x reduction to a lower-temperature area where ammonia would be generated.
• Figure 2 is the detail in cross section of an individual nozzle 160. Nozzles of this type are suitable for use in this invention, and discharge a two-phase mixture of gas and liquid at the characteristic sonic velocity of the mixture.
• the speed of sound of a mixture of the two components is approximately 20 to 30 meters per second, based on a volumetric proportion of gas of between 30 and 80 percent. Since the outflow velocity of the mixture is equal to its speed of sound, i.e., sonic velocity, the abrupt pressure drop from the mixing chamber produces an intensive division of the liquid phase into small droplets.
• the droplet diameter is effected essentially by this pressure drop and, as a result, is adjustable. It is an advantage of this invention that the water-dispersible polymer provides further adjustability in droplet size and enables narrowing the range of droplet sizes.
• the nozzle comprises a liquid conduit 161 surrounded at its end by an annular chamber 162, into which there opens a line 163 for the gas.
• the annular chamber 162 is connected to the interior of the conduit 161 by means of several transfer holes 164, so that the end region of the conduit 161 functions as a mixing chamber 165.
• the ratio of gas flow to liquid flow should be adjusted such that, shortly before exit from the mixing chamber 165 through outlet 166, the ratio is appropriate for the chosen pressure drop to enable injection of the mixture at its sonic velocity.
• a volumetric proportion of gas of between 30 and 80% is employed, i.e., in this case, the flow cross section occupied by the gas amounts to from 30 to 80% of the total flow cross section.
• the absolute pressure ahead of the mixing chamber will typically be in the range of between approximately 1.6 and 40 bar, depending on the desired droplet size and the phase exchange area dependent on this. Pressures on the order of magnitude of from about 2 to about 15, e. g. from about 3 to about 5, bar will be customary.
• Droplet sizes will be determined based on the requirements of temperature, effluent flow rate, spacing of downstream obstruction, NO x -reducing agent concentration, and desired effect. Typically, droplets will be formed with median diameters of from about 5 to about 10,000 microns. For many applications median diameters within the range of from 10 to 125 microns, e.g., less than about 50
• the preferred median droplet size will be within the range of from about 200 to about 1000 microns. It is also possible to be any suitable median droplet size. It is also possible to be any suitable median droplet size. It is also possible to be any suitable median droplet size. It is also possible to be any suitable median droplet size. It is also possible to be any suitable median droplet size. It is also possible to be any suitable median droplet size.
• the relative span of droplet sizes be less than 2, e.g., 1.25 to 1.5.
• the injection at sonic velocity with the water-dispersible polymer enables effective limitation of oversized droplets.
• the atomizing device shown in Figure 2 serves only to' illustrate one suitable form of nozzle. Depending on design and process requirements, the atomizing jet can also be designed and constructed differently. In particular, it is possible to incorporate either convergent or divergent sections of pipe at the end of the mixing chamber 165. Commercial nozzles of this type are
• NFTC 4.0 and NFTL-20-00-170 both available from Nalco Fuel Tech of Naperville, Illinois. Also, other
• atomization techniques can be employed including nozzles which operate without a separate atomizing fluid.
• a series of aqueous test solutions were prepared using the following water-dispersible polymers at dosages of 100 parts per million based on the weight of polymer in the active form (excluding the weight of carriers and diluents):
• the value D v0.5 is the volume median diameter as
• D v0.9 value indicates a droplet size at 90% of the
• the value identified as D 32 is the Sauter mean diameter.
• the value for relative span is calculated as the quotient of (D v0.9 - D v0.1 )/D v0.5 . All droplet sizes are in microns.
• Polyox 30 1.6 30 0.65 264 59 434 120 1.4
• Example 3 Three different types of polymers were tested using solutions of the indicated polymers at a concetration of 100 ppm based on the active polymer. The tests were run as indicated in Example 1.

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• 1992
  • 1992-10-13 US US07/959,705 patent/US5536482A/en not_active Expired - Fee Related
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