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

Definitions

• the present invention relates to improvements in the combustion of carbonaceous fuels, and more particularly to improvements in firing boilers with reduced emissions of carbon- and nitrogen-based pollutants.
• Carbonaceous fuels burn more completely, with reduced emissions of carbon monoxide and unburned hydrocarbons, at oxygen concentrations and combustion air/fuel ratios which permit optimized high flame temperatures.
• these temperatures are above 2000°F and typically from about 2200°F to 3000°F.
• thermal NO — the temperatures being so high that free radicals of nitrogen and oxygen are formed and chemically combine as nitrogen oxides (NO A).
• the flame temperature can be lowered to reduce NO A formation by the use of large excesses of air or fuel, or a hybrid of both processes known as staged combustion.
• these approaches create excessive carbon-based pollutants.
• Pacztowski discloses a more detailed, controlled two-stage catalytic system.
• the operating temperatures for the second catalytic stage where ammonia is utilized are preferably within the range of from 275°F to 900°F. This process and that of Jones et al unfortunately depend on the use of catalysts which create additional costs in terms of initial investment and servicing requirements.
• Lyon discloses a non-catalytic system for reducing nitrogen monoxide (NO) in a combustion effluent.
• Lyon discloses that ammonia and specified ammonia precursors, including ammonium carbonate also disclosed by Jones, et al , or their aqueous solutions, can be injected into the effluent, for mixing with the nitrogen monoxide at a temperature within the range of 1600°F to 2000°F.
• a reducing agent such as hydrogen gas or various hydrocarbons, can be mixed with the effluent to permit the reduction reaction to occur at temperatures as low as 1300°F, thereby assuring avoidance of high temperature oxidation of ammonia to nitrogen monoxide.
• Lyon points out that at temperatures above 2000°F, the use of ammonia was counterproductive — increasing NO rather than decreasing it.
• Williamson discloses the purification of acid gas-containing streams at low temperatures approaching ambient. Williamson discloses contacting the gas stream with an amine vapor in sufficient concentration such that its partial pressure is at least 5% of the total pressure of the gas stream. This system thus requires large amounts of the treating gas and requires equipment for separating that gas from the effluent upon completing the treatment.
• the present invention provides a process for reducing the concentration of nitrogen oxides in an oxygen-rich effluent from the combustion of a carbonaceous fuel.
• the process comprises injecting a solution comprising at least one additive compound selected from the group consisting of guanidine, guanidine carbonate, biguanide, guanylurea sulfate, melamine, dicyandiamide, calcium cyanamide, biuret, ljl'-azobisformamide, methylol urea, methylol urea-urea condensation product, dimethylol urea, methyl urea, dimethyl urea, and hexamethylene ⁇ tetramine, into said effluent.
• the temperature of the effluent at the point of injection, the concentration of the additive compound in the solution, and the size of the droplets in the dispersion, are selected to achieve reduction in nitrogen oxide levels in the effluent.
• a preferred embodiment of the invention provides for introducing an aqueous solution of hexamethylenetetramine and urea. Injection is preferably done at a plurality of spaced positions and at a uniform droplet size within the range of from about 10 to about 10,000 microns Sauter mean diameter. Variations of particle sizes within this broad range have been found effective to achieve uniform mixing of the additive compound with the effluent gas at a temperature in excess of 1300°F. For the purposes of this description, all temperatures herein are measured using an unshielded K-type thermocouple. Droplet sizes are determined with a Malvern 2200 instrument, utilizing a Franhofer diffraction, laser-based system. And, unless otherwise indicated, all parts and percentages are based on the weight of the composition at the particular point of reference.
• urea and materials such as ammonium carbonate, ammonium oxalate, ammonia, hydrazine, ammonium hydroxide, and various amines can be employed with the named additive compounds.
• Aqueous solutions are preferred according to the present invention due to their economy and can be employed with suitable effectiveness in most situations.
• the effective solutions will vary from saturated to dilute. While water will be an effective solvent for most applications, there are instances where other solvents may be advantageous in combination with water.
• the temperature of the effluent will have an influence on the concentration of the solution. At temperatures of from about 1300°F to about 2000°F, the solution will tend to operate effectively at high concentration, e.g., from 25 to 40 weight percent. On the other hand, at temperatures in excess of 2000°F, the solution will tend toward very dilute solutions. At these high temperatures, the water may comprise greater than 80%, 85% or 90% by weight of the solution, with the additive compound comprising as low as from about 0.5 to about 10% by weight of the solution.
• hexamethylene- tetr-amine is utilized to a greater extent than urea, being almost totally consumed during its interaction with NO A under proper conditions. It is further surprising that, when used in combination with urea, hexamethylenetetramine actually increases the utilization of the urea in reducing NO .A. Thus, hexamethylenetetramine is an enhancer for urea utilization in addition to being a superior NO A reducing agent in its own right.
• n.amed additive compounds are employed as NO A. reduction optimization and economics dictate.
• hexamethylene ⁇ tetramine will be present in an amount of at least about 25% based on the combined weight of it and the other active NO A reducer such as urea.
• Weight ratios of hexamethylenetetramine to urea of from about 1:3 to 3:1 are exemplary.
• the solution of additive compound will be dispersed uniformly within the effluent gas stream at a point where the effluent is at a temperature above 1300°F, and preferably above 1500°F.
• Large industrial boilers of the type employed for utility power plants and other large facilities will typically be water jacketted and have access only at limited points. In the most typical situation, the boiler interior can be accessed only in the area of the flame and at an area above the fl-ame, where the temperatures at full load are typically within the range of from about 2200°F to about 2600°F. For boilers operating efficiently with gas, the temperature at this point of access will typically fall within the range of from about 2100°F to about
• the additive compound solutions according to the present invention are preferably injected at a number of spaced points from nozzles which are effective to uniformly form and disperse droplets of the solutions within the flowing effluent stream to achieve uniform mixing.
• the size of the droplets of solution will be within the range of from about 10 to about 10,000, and preferably within the range of from about 50 to 10,000 microns Sauter mean diameter. At temperatures below 2000°F, droplet sizes of less than 150 microns are quite effective, while at higher temperatures the droplets should be larger, preferably larger than 500 microns.
• the concentration of the additive compound or compounds within the effluent gas should be sufficient to provide a reduction in nitrogen oxide levels.
• the additive compound will be employed at ' a molar ratio of nitrogen in the additive compound to the baseline nitrogen oxide level of from about 1 to 10 to 2 to 1, and will more preferably be within the range of from about 1 to 4 to 3 to 2.
• HMTA hexamethylenetetramine
• a Babcock & Wilcox 110 megawatt utility boiler was fired with residual fuel oil through ten Peabody burners to achieve 80 megawatt output.
• a series of medium-to-course droplet forming atomizers were positioned to inject treatment solutions into the effluent which was at an average temperature of about 1600°F. The following runs were made:
• HMTA hexamethy ⁇ lenetetramine
• Example 2 The boiler referred to in Example 2 was fired to 108 megawatt output. Treatment solutions were injected into the effluent at an average effluent temperature of about 2100°F. The products of combustion contained a level of NO which equated to about 10 moles per hour. The following runs were made:
• HMTA hexamethylenetetramine
• Example 1 The procedure of Example 1 is repeated for the additive compounds and combinations of them set forth in Table 4 below. The temperatures of the effluent, concentration of solutions, and feed rates are changed from Example 1 as indicated in Table 4.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
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• Oil, Petroleum & Natural Gas (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Treating Waste Gases (AREA)
• Processing Of Solid Wastes (AREA)

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• 1986
  • 1986-10-03 AU AU64080/86A patent/AU6408086A/en not_active Abandoned
  • 1986-10-03 WO PCT/US1986/002090 patent/WO1987002023A1/en not_active Application Discontinuation
  • 1986-10-03 EP EP19860906208 patent/EP0237568A4/de active Pending

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