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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
  • Y02C20/00—Capture or disposal of greenhouse gases
  • Y02C20/10—Capture or disposal of greenhouse gases of nitrous oxide (N2O)

Definitions

• the present invention relates to a process for the reduction of nitrogen oxides (NO x , where x is an integer, generally 1 or 2) in a combustion effluent by the use of the hydrolysis products of urea, which provides advantag ⁇ es over conventional NO x reducing processes.
• Nitroaceous fuels can be made to burn more complete ⁇ ly 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.
• sus ⁇ pension fired boilers such as large utility boilers, temperatures above about 2000°F and typically about 2200°F to about 3000°F are generated.
• Nitrogen oxides can form even in circulating fluidized bed boilers which operate at temperatures which typically range from 1300°F to 1700°F, as well as gas turbines and diesel engines .
• Nitrogen oxides are troublesome pollutants which are found in the combustion 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 photochemical smog formation, through a series of reactions in the presence of some hydrocarbons. Moreover, nitrogen oxides comprise a significant contrib ⁇ utor to acid rain and have been implicated as contribut- ing to the undesirable depletion of the ozone layer. They may also impact on the warming of the atmosphere commonly referred to as "the greenhouse effect”. In addition, some or all of these effects are believed to be attributable to nitrous oxide.
• SNCR processes which are temperature dependent, generally utilize a nitrogenous substance such as urea or ammonia, as well as non-nitrogenous substances included as “enhancers” for the nitrogenous substances, and pro ⁇ ceed in the gas phase by a complex series of free radi- cal-mediated chemical reactions.
• Such reactions involve various nitrogen, hydrogen, oxygen, and carbon-containing species and radicals.
• Urea and ammonia differ, in that they appear to be most effective at different tempera ⁇ tures.
• nitrous oxide N 2 0
• Nitrous oxide which is defined differently than NO x for regulatory purposes, is coming to be recognized as a pollutant, albeit second ⁇ ary to nitric oxide (NO) and nitrogen dioxide (N0 2 ) .
• Urea is generally considered the most desirable NO x reducing species because of its effectiveness and rela ⁇ tively broad temperature window, as well as its non-toxic and environmentally benign nature, when compared with ammonia. Urea, it is believed, breaks down in the efflu ⁇ ent into the amidozine radical (NH 2 -), which appears to be the moiety responsible for the reduction of NO x . However, urea can, under certain conditions, also break down into cyanic or isocyanic acid according to the following reac ⁇ tion formula
• cyanic or isocyanic acid produced can then pro- ceed further to form nitrous oxide and carbon monoxide or molecular nitrogen and carbon dioxide when combined with NO x according to the following set of reactions
• Urea can thermally decompose to biuret and isocyanic acid at temperatures between about 302°F and 440°F with a concomitant major weight loss. From there, the decompo- sition proceeds to cyanuric acid and isocyanic acid at temperatures of about 450°F to 620°F.
• urea hydrolyzes to products which are believed to include ammonia (NH 3 ), ammonium carbamate (NH 2 COONH 4 ) ("carbamate”), ammonium carbonate ((NH 4 ) 2 C0 3 ) (“carbonate”), and ammonium bicarbonate (NH 4 HC0 3 ) (“bi ⁇ carbonate”). Hydrolysis generally continues sequentially from carbamate, through carbonate and then to bicarbon ⁇ ate, each composition being more stable than the previous one.
• each of the noted hydrolysis products is individually commercially available, it is more desirable to produce them via urea hydrolysis. This is because the thusly formed hydrolysate has advantages over the indi- vidual hydrolysis products, even if combined in the same approximate ratios.
• One advantage is cost, since urea can be significantly less expensive than the individual hydrolysis products.
• a maximum solubility of about 25% for the hydrolysate (based on initial urea concentration) has been observed, which is superior to the solubility of bicarbonate, i.e., about 18%. This can be significant in terms of transportation costs and final treatment agent concentrations.
• the hydrolysate prepared comprises at least in part a single unique structure of carbonate and bicarbonate which is in a complex with carbamate (expressed as carba ⁇ mate • bicarbonate/carbonate) .
• carbamate expressed as carba ⁇ mate • bicarbonate/carbonate
• residual urea may also be pres ⁇ ent.
• Hydrolysis of a 10% aqueous urea solution was con ⁇ ducted under pressures sufficiently high to maintain the resulting hydrolysate in solution. Such pressures also facilitate hydrolysis. Hydrolysis was performed under pressures of at least about 500 pounds per square inch (psi), more preferably at least about 650 psi. If it was desired to maintain ammonia in solution, the pressure was to be at least about 750 psi. As the concentration of the initial urea solution is increased, the pressure was increased to achieve equivalent results.
• psi pounds per square inch
• Patent 4,208,386, Arand, Muzio, and Sotter improve on the Lyon process by teaching the intro ⁇ duction of urea for NO x reduction in oxygen-rich effluents at temperatures in the range of 1600°F to 2000°F, when urea is introduced into the effluent alone, and 1300°F to 1600°F when urea is introduced with an ancillary reducing material.
• Arand, with Muzio and Teixeria also teach the introduction of urea into fuel-rich combustion effluents to reduce nitrogen oxides at temperatures in excess of about 1900°F in U.S. Patent 4,325,924.
• the present invention relates to the reduction of nitrogen oxides using the hydrolysis products of urea in an SNCR reaction, which are effective at NO x reduction while avoiding the disadvantages of art-recognized SNCR processes.
• These hydrolysis products can be formed under reduced pressure conditions, and even after introduction of a urea solution into an effluent stream.
• the present invention relates to the forma ⁇ tion of the hydrolysis products of urea without the need for application of increased pressure. In doing so, the installation and maintenance of high pressure conduits or other equipment is avoided. In fact, the formation of the desired NO ⁇ -reducing moieties can occur after injec ⁇ tion of the "raw material" urea solution into the efflu ⁇ ent.
• the aqueous urea solution to be hydrolyzed further comprises a water soluble alkaline agent, such as potas ⁇ sium hydroxide (KOH) and/or sodium hydroxide (NaOH), or a water soluble salt of sodium, potassium, calcium or mag- nesium which, upon exposure to high temperatures, will decompose to form the respective hydroxide or oxide thereof, or mixtures thereof.
• a water soluble alkaline agent such as potas ⁇ sium hydroxide (KOH) and/or sodium hydroxide (NaOH), or a water soluble salt of sodium, potassium, calcium or mag- nesium which, upon exposure to high temperatures, will decompose to form the respective hydroxide or oxide thereof, or mixtures thereof.
• KOH potas ⁇ sium hydroxide
• NaOH sodium hydroxide
• Water solubility is believed to be important in maintaining the association between the alkaline agent and urea, even after water evapora ⁇ tion.
• the alkaline agent should be present at a molar ratio of alkaline agent to urea of about 0.01:1 to about 2:1, more preferably about 0.1:1 to about 1:1 (for instance, in the case of sodium hydroxide, it should be present in the solution at a weight ratio to urea of about 0.067:1 to about 0.67:1) .
• the alkaline agent By the inclusion of the alkaline agent, the need for the application of pressure during hydrolysis is reduced or eliminated. Accordingly, the alkaline agent-contain ⁇ ing aqueous urea solution can be introduced into the effluent prior to hydrolysis, with the same beneficial effects as if hydrolysis had been effected prior to entry into the effluent. Although the precise reason for this is not fully understood, it is believed that formation of the NO ⁇ -reducing moieties occurs immediately after water evaporation, when the droplets of solution have entered the effluent. Because the effect observed is that of the urea hydrolysis products, not urea itself, the postulated mechanism is believed likely.
• 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. Again, this may be insignificant since, at the temperature of the effluent, virtually complete hydrolysis is expected.
• the hydrolysis of urea can be 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 reductions in nitrogen oxides can be achieved with equivalent hydroly- sis conditions by the use of the catalysts.
• the catalyst metal is mixed into the urea solution prior to introduc ⁇ tion into the effluent.
• the urea solution should comprise sufficient urea to provide the desired level of hydrolysate for substantial reduction of nitrogen oxides under the efflu- e ⁇ t and load conditions existing.
• the urea solution comprises up to about 50% urea by weight, more preferably about 5% to about 45% urea by weight. Most preferably, the solution comprises about 10% to about 25% urea by weight, with the appropriate amount of alkaline agent to provide the molar ratios discussed above.
• the aqueous solution from which the hydrolysate is to be formed can be introduced into the effluent by suitable introduction means under conditions effective to produce the desired NO x -reducing moieties and reduce the effluent nitrogen oxides concentration in a selective, non-cata ⁇ lytic, gas-phase process.
• suitable introduction means include injectors, such as those disclosed by Burton in U.S. Patent 4,842,834, or DeVita in U.S. Patent 4,915,036, the disclosures of each of which are incorpo ⁇ rated herein by reference.
• injec ⁇ tion means is an injection lance, especially a lance of the type disclosed by Peter-Hoblyn and Grimard in Inter ⁇ national Publication WO 91/00134, filed July 4, 1989, entitled "Lance-Type Injection Apparatus for Introducing Chemical Agents into Flue Gases", the disclosure of which is incorporated herein by reference.
• the solution is introduced into the efflu ⁇ ent to be treated for N0 X reduction to produce an amount of the urea hydrolysis products effective to elicit a reduction in the nitrogen oxides concentration in the effluent.
• the solution is introduced into the effluent in an amount sufficient to provide a molar ratio of the nitrogen contained in the solution to the baseline nitrogen oxides level (by which is meant the pre-treatment level of N0 X in the effluent) of about 1:5 to about 10:1. More preferably, the solution is intro- quizd into the effluent to provide a molar ratio of solu ⁇ tion nitrogen to baseline nitrogen oxides level of about 1:3 to about 5:1, most preferably about 1:2 to about 3:1.
• the alkaline agent-containing, aqueous urea solution is preferably injected into the effluent gas stream at a point where the effluent is at a temperature above about 1300°F, more preferably above about 1400°F.
• Large indus ⁇ trial and circulating fluidized bed boilers of the types employed for utility power plants and other large facili ⁇ ties will typically have access only at limited points.
• the boiler interior in the area above the flame operates at temperatures which at full load approach 2200°F, even 2300°F. After subse ⁇ quent heat exchange, the temperature will be lower, usu ⁇ ally in the range between about 1300°F and 2100°F. At these temperatures, the flexibility and broad temperature window of the hydrolysate can effectively accomplish substantial reduction of nitrogen oxides in the effluent without the drawbacks of prior art processes.
• the hydrolysate can be enhanced by other compositions such as hexamethylenetetramine (HMTA), oxy ⁇ genated hydrocarbons such as ethylene glycol, ammonium salts of organic acids such as ammonium acetate and ammo ⁇ nium 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.
• en ⁇ hancers 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 hydrolysate achieves its peak reductions of N0 X .
• the solution When the solution is introduced without a non-nitro- genous hydrocarbon enhancer, it is preferably introduced at an effluent temperature of about 1500°F to about 2100°F, more preferably about 1550°F to about 2100°F. When the solution also comprises one of the enhancers discussed above, it is preferably introduced at an efflu- ent temperature of about 1300°F to about 1700°F, more preferably about 1400°F to about 1600°F or higher. The usefulness of introduction of the solution at these ef ⁇ fluent temperatures can depend on the particular compo ⁇ nents of the treatment agent (i.e., solution) and other effluent conditions, such as the effluent oxygen level.
• the treatment agent i.e., solution
• other effluent conditions such as the effluent oxygen level.
• the effluent into which the urea solution of this in ⁇ vention is injected is preferably oxygen-rich, meaning that there is an excess of oxygen in the effluent.
• the excess of oxygen is greater than about 1% by volume. Most preferably, the excess of oxygen is in the range of about 1% to about 12% or greater by vol- ume.
• inventive urea solution for NO x reduc ⁇ tion can be a part of a multi-stage treatment regimen which will reduce effluent nitrogen oxides.
• Such processes are discussed in, for instance, U.S. Patents 4,777,024 and 5,057,923, the disclosures of each of which are incorpo ⁇ rated herein by reference.
• NO x is reduced using the hydro ⁇ lysate as described above.
• a urea or ammonia solution (without alkaline agent) can be intro ⁇ quiz.
• the first stage can comprise a urea or ammonia solution, and the second stage a hydro ⁇ lysate solution.
• the use of the hydrolysate to reduce nitrogen oxides in a combustion effluent, especially when compared with the use of urea or ammonia, has been found to provide several important advantages.
• effluent tempera ⁇ tures i.e., below about 1700°F
• higher reductions of nitrogen oxides are observed with greater chemical utili ⁇ zation, and lower NSR requirements.
• the hydrolysate has a wider temperature window with lower ammonia slip at effluent temperatures greater than about 1600°F, and reduced generation of nitrous oxide and emission of car- bon monoxide.
• the kinetic flexibility of the hydrolysate is superior, with equivalent or better performance at shorter residence times.
• the hydrolysate com ⁇ prises virtually all volatiles, with no solids residue.
• the widened temperature window of the hydrolysate is believed to be due to the presence of different compo ⁇ nents (i.e., carbamate, carbonate, bicarbonate, ammonia, and residual urea), each of which have different reaction kinetics. Since the compositions are "released" for NO x reduction at different times, with ammonia and bicarbon ⁇ ate more kinetically reactive, followed in order of reac ⁇ tivity by carbonate, carbamate and urea, the effective temperature window is wider than any of the individual components.
• hydrolysate formed in a catalyzed hydrolysis reaction is more kinetically reactive than hydrolysate produced without a catalyst.
• Example I The apparatus employed is a combustor, called a "Flame Tube", which was designed to simulate conditions found in real-time industrial and utility boilers.
• the combustor has many refractory-lined sections. Total furnace volume is 10 cubic feet with about half of its volume forming a combustion chamber.
• the combustion chamber has an inner diameter of 15 inches and is a 48 inch long cylindrical section.
• the test section is main ⁇ tained at isothermal temperatures for chemical reactions. Combustion air and furnace draft are controlled by a variable speed ID fan. Typical firing conditions are as follows:
• Fuel No. 2 fuel oil
• a diagonistic system provides two main functions: (1) Flue gas analyses, and (2) Automatic data acquisition. Combustion gases are monitored for NO x , CO, 0 2 , N 2 0 and
• NH 3* A flue gas sample is drawn continuously from the furnace exit by a vacuum pump to the gas conditioning unit, followed by analyzers.
• the NO x analyzer used is a Model 10B chemiluminescent NO-NO x gas analyzer from Thermo Electron.
• the CO analyzer used is a Model 48 infrared CO Analyzer from Thermo Electron.
• the 0 2 is analyzed by a Model 326 Analyzer from Teledyne Analytical Instruments which utilizes a micro-fuel cell.
• a Perkin-Elmer Gas Chromatography Model 8410 equipped with an Electron Cap ⁇ ture Detector (ECD) is used to analyze N 2 0 via an automat ⁇ ic gas sampling valve.
• Ammonia measurements are per ⁇ formed by wet chemical methods. The procedure involves absorption of gaseous NH 3 in a given volume of acidic solution. The concentration of NH 3 is determined by means of direct potentiometry with an NH 4 + ion-select
• Effluent baseline pollutant values are determined prior to testing while injecting deionized water in an amount equivalent to treatment agents to be injected.
• Temperature at the location for injection is determined using a suction pyrometer and type R thermocouple.
• the temperature at the point of the injection nozzle is cal ⁇ culated by extrapolation of the temperature values from downstream points.
• the furnace is fired at a fuel feed rate of 1.6 gph using #2 oil and an excess 0 2 of 7%.
• the baseline NOx was determined to be about 225 ppm.
• Solution A 10% aqueous solution of urea without alkaline agent.
• Solution B 10% aqueous solution of urea containing potassium hydroxide at a 1:1 molar ratio.
• Solution C 10% aqueous solution of urea containing sodium hydroxide at a 1:1 molar ratio.
• Solution D 10% aqueous solution of urea containing monosodiumglutamate (C 5 H 8 NNa0 4 *H 2 0) at a 1:1 molar ratio, included as a control.
• the mixture of urea with the claimed alkaline agents has advantages in NOx reduction and/or the reduction of the production of secondary pollutants, N 2 0, NH 3 and CO over both a urea solution without alkaline agent or a urea solution having monosodiumglutamate. 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.

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