DESCRIPTION OF THE INVENTION

[0035] The invention provides a urea-based SCR process that can advantageously utilize the enthalpy of the flue gas, which can be supplemented if need be, to convert urea to ammonia. There are several embodiments which will be described in preferred forms. It is intended, however, that various features of the embodiments can be utilized in combination with embodiments other than those specifically detailing the features. Common elements and features of the drawings will have common reference numerals throughout the drawings.

[0036] This new process makes use of the easy handling feature of urea reagent and provides complete gasification and good mixing employing a side stream gas mass to provide thorough mixing required for high levels of NO _x reduction. In particularly advantageous embodiments, heat necessary for gasification is derived from the enthalpy of the combustion gases.

[0037] The process is effective with urea, but can utilize other NO _x -reducing reagents capable of generating a reactant gas containing ammonia upon heating. As will be clear from the following, when certain of these reagents are gasified, the reactant gas will also contain HNCO which reacts with water to convert to ammonia and carbon dioxide. It is an advantage of the invention that this can be easily achieved without prehydrolysis of the NO _x -reducing reagent which has the attendant risk of plugging nozzles and other equipment. By the term “gasification” we mean that substantially all of the urea is converted into a gas, leaving no significant dissolved or free solids or liquid to contact with and foul SCR catalysts.

[0038] The term “urea” is meant to include the reagents that are equivalent to urea in the sense that they form ammonia and HNCO when heated, whether or not they contain large amounts of the pure chemical urea in the form introduced into the combustion gases; however, the reagents that are equivalent to urea typically contain measurable quantities of urea in their commercial forms and thus comprise urea. Among the NO _x -reducing reagents that can be gasified are those that comprise a member selected from the group consisting of: ammelide; ammeline; ammonium carbonate; ammonium bicarbonate; ammonium carbamate; ammonium cyanate; ammonium salts of inorganic acids, including sulfuric acid and phosphoric acid; ammonium salts of organic acids, including formic and acetic acid; biuret; triuret, cyanuric acid; isocyanic acid; urea formaldehyde; melamine; tricyanourea and mixtures of any number of these. Yet other NO _x -reducing reagents are available that do not form HNCO, but decompose to a mixture of gases including hydrocarbons. Among this group are various amines and their salts (especially their carbonates), including guanidine, guanidine carbonate, methyl amine carbonate, ethyl amine carbonate, dimethyl amine carbonate, hexamethylamine; hexamethylamine carbonate; and byproduct wastes containing urea from a chemical process. Amines with higher alkyls can be employed to the extent that the hydrocarbon components released do not interfere with the NO _x -reduction reaction.

[0039] The term “urea” is thus meant to encompass urea in all of its commercial and equivalent forms. Typically, commercial forms of urea will consist essentially of urea, containing 95% or more urea by weight. This relatively pure form of urea is preferred and has several advantages in the process of the invention.

[0040] It is a further advantage of the invention, that while ammonia need not be utilized at all, the apparatus described herein improves the introduction of SCR reagents including ammonia and thus makes its use more practical even though the problems with its storage will not be fully resolved.

[0041] The urea solution is introduced at a rate relative to the NO _x concentration in said combined stream prior to passage through said NO _x -reducing catalyst effective to provide an NSR of from about 0.1 to about 2, depending on a number of factors, but more typically is within the range of from 0.5 to 1.1. The term “NSR” refers to the relative equivalents of nitrogen in the urea or other NO _x -reducing agent to the equivalents of nitrogen in the NO _x in the combustion gases to be treated.

[0042] The term “combustor” is meant in the broad sense to include all combustors which combust carbonaceous fuels to provide heat, e.g., for direct or indirect conversion to mechanical or electrical energy. These carbonaceous fuels can include the hydrocarbons normally used as fuels as well as combustible waste materials such as municipal solid waste, industrial process waste and the like. Burners and furnaces, as well as, internal combustion engines of the Otto, Diesel and turbine types, are included within the definition of the term combustor and can benefit from the invention. However, since the problems and advantages of successful achievement of reliable NO _x reduction on combustors utilizing ammonia as a reducing agent are so pronounced, the large-scale combustor is used throughout this description for purposes of example. Stationary and mobile combustors of all types are, however, contemplated. However, the current invention is not limited to combustor flue gases. Rather, any hot flue gas that can benefit from passing through an SCR reactor for NOx reduction can benefit from this invention.

[0043] FIG. 1 is a schematic representation of one embodiment of the invention wherein a side stream is separated from the main effluent stream from a combustor and urea is injected into it at a temperature sufficient to fully decompose or otherwise gasify the urea to active gas species. A large-scale combustor 20 burns fuel with the resulting production of nitrogen oxides (NO _x ) that must be at least partially removed. The combustion gases are used to heat water in heat exchanger array 22 before the combustion gases are exhausted to the atmosphere by passage 23 and apparatus downstream. A mixing device 24 is optional following adding urea reagent to a side stream and combining the side stream with a main combustion gas stream as will be explained. The term “side stream” is used herein to refer to a stream of relatively small volume relative to the total volume of combustion gases to be treated by gasified urea and NO _x -reduction catalysts, 26 , 26 ′ and 26 ″. The side stream can be obtained by splitting off a side stream portion 28 of the full stream of combustion gases in passage 23 leaving principal stream 29 of combustion gases. The separation in various embodiments will be made before or after treatment. In addition, the side stream can be formed by drawing in a stream of air from sources external of the combustor.

[0044] Catalysts 26 , 26 ′ and 26 ″ are employed in an array forming a reactor and are SCR catalysts as known in the art for reducing NO _x utilizing ammonia or urea in various hydrolyzed, gasified, pyrolyzed and like forms. Among the suitable SCR catalysts are those capable of reducing the effluent nitrogen oxides concentration in the presence of ammonia. These include, for instance, activated carbon, charcoal or coke, zeolites, vanadium oxide, tungsten oxide, titanium oxide, iron oxide, copper oxide, manganese oxide, chromium oxide, noble metals such as platinum group metals like platinum, palladium, rhodium, and iridium, or mixtures of these. Other SCR catalyst materials conventional in the art and familiar to the skilled artisan can also be utilized. These SCR catalyst materials are typically mounted on a support such as a metal, ceramic, zeolite, or homogeneous monolith, although other art-known supports can also be used.

[0045] Among the useful SCR catalysts are those representative prior art processes described below. Selective catalytic reduction processes for reducing NO _x are well known and utilize a variety of catalytic agents. For instance, in European Patent Application WO 210,392, Eichholtz and Weiler discuss the catalytic removal of nitrogen oxides using activated charcoal or activated coke, with the addition of ammonia, as a catalyst. Kato, et al., in U.S. Pat. No. 4,138,469 and Henke in U.S. Pat. No. 4,393,031 disclose the catalytic reduction of NO _x using platinum group metals and/or other metals such as titanium, copper, molybdenum, vanadium, tungsten, or oxides thereof with the addition of ammonia to achieve the desired catalytic reduction. See also EP 487,886, which specifies a V ₂ O ₅ /WO ₃ /TiO ₂ catalyst with a working range of 220° to 280° C. Other catalysts based on platinum can have operating temperatures even lower, e.g., down to about 180° C.

[0046] Another catalytic reduction process is disclosed by Canadian Patent 1,100,292 to Knight, which relates to the use of a platinum group metal, gold, and/or silver catalyst deposited on a refractory oxide. Mori, et al., in U.S. Pat. No. 4,107,272, discuss the catalytic reduction of NO _x using oxysulfur, sulfate, or sulfite compounds of vanadium, chromium, manganese, iron, copper, and nickel with the addition of ammonia gas.

[0047] In a multi-phased catalytic system, Ginger, in U.S. Pat. No. 4,268,488, discloses exposing a nitrogen oxides containing effluent to a first catalyst comprising a copper compound such as copper sulfate and a second catalyst comprising metal combinations such as sulfates of vanadium and iron or tungsten and iron on a carrier in the presence of ammonia.

[0048] The effluent containing the reactant gas is most preferably passed over the SCR catalyst while the combustion gases including the gasified urea or other reagent are at a temperature of at least about 100° C. and typically between about 180° and about 650° C., preferably above at least about 250° C. In this manner, the active species present in the effluent due to gasification of the reagent solution most effectively facilitate the catalytic reduction of nitrogen oxides and condensation of water is controlled. The effluent will typically contain an excess of oxygen, e.g., up to about 15% of that required to fully oxidize the carbonaceous fuel. Use of the present invention with any of the above SCR catalysts (the disclosure of which are specifically incorporated by reference) reduces or eliminates the requirement for the transport, storage and handling of large amounts of ammonia or ammonium water.

[0049] In FIG. 1 , the main full stream of combustion gases in duct 23 is split to provide side stream 28 and a principal stream 29 of volume greater than the side stream. Urea, which decomposes at temperatures above 140° C., is injected from storage 30 via nozzle 32 with suitable valves 34 and controllers (not shown) into a flue gas stream 28 split off after a primary superheater or an economizer (shown generally as heat exchanger 22 ). To achieve the goal of gasification for a urea or a urea-related NO _x -reducing reagent, temperatures above about 300° C. are typically employed for gasification.

[0050] The urea solution is desirably maintained at a concentration suitable for storage and handling without precipitation or other problem. Concentrations of from about 5 to 70% can be employed with some degree of practicality, but concentrations of from about 15 to about 50% are more typical. It is an advantage of the invention that the amount of water in the urea solution can be varied alone or with steam added to suitably control the temperature of the gases in the side stream.

[0051] The temperature of the gases produced by gasifying reagents in this group should be maintained at a level that prevents their condensation. Typically, the temperature should be maintained at a temperature at least about 150° C., and preferably at least 200° C. A preferred temperature range for the gasification and for transfer of the gases produced by the noted group of reagents, is from about 300° to about 650° C. Ideally, the side stream 28 would decompose the urea into active species without need for further heating. This side stream (e.g., from 0.1 to 25% of the flue gas), typically less than 10% and usually less than 3%, e.g., from 0.1 to 2%, of the volume of the total combustion gases (flue gas), provides the required enthalpy for complete decomposition of urea and the sufficient momentum to mix the side stream 28 with the principal stream 29 while the principal stream 29 can be utilized for further heat exchange.

[0052] The vessel carrying the side stream 28 provides the required time and gas velocity for urea decomposition. After injection, a residence time from 1 to 10 seconds is typically provided to completely decompose urea and promote the reaction between HNCO and water vapor to form ammonia. Side stream gas velocity of 1 to 20 feet per second is maintained throughout the vessel to optimize vessel dimensions, achieve plug flow, enhance the urea droplet dispersion, evaporation, and decomposition into the side stream, and minimize droplet impingement on vessel walls. Internal channels and multiwalls may be preferred to achieve the optimum gas velocity and to minimize heat loss to outside environment. The optimum vessel design can be derived by using, among others, well-established design tools such as computational fluid-dynamics model.

[0053] The urea injection nozzle 32 introduces well-defined droplets. Both air assisted atomizer or a mechanical atomizer can be utilized. Droplet sizes less than 500 microns but typically less than 100 and preferably below 50 microns are desirable to rapidly evaporate and decompose urea droplets. Also in consideration of vessel size, small and slow droplets generated from, e.g., ultrasonic nozzles can be more desirable than large and fast droplets. If desired, steam can be introduced as necessary or desired. (See FIGS. 7 - 9 , in this regard.) This side stream 28 can then be directed to an injection grid 37 (or other suitable introduction device or apparatus such as a traditional ammonia injection grid) ahead of SCR reactor containing catalysts, e.g., 26 , 26 ′ and 26 ″. In this embodiment, a high temperature blower 36 is employed to provide a suitable injection pressure, e.g., about 1 psig or less, for the ammonia injection grid 37 and additionally provides mixing. Alternatively, a high temperature blower 36 can be located upstream of urea nozzle 32 instead of the depicted location.

[0054] A traditional ammonia injection grid 37 with densely located nozzles requires as low as 0.1% of the total combustor flue gas as the side stream. A static mixer 24 can be used if desired. Alternatively, injection grid 37 can comprise fewer and sparsely-placed nozzles or openings with a static mixer 24 located downstream to obtain a uniform distribution. This alternate design may reduce cost and maintenance associated with the injection grid. The mixing with the flue gas is facilitated due to an order of magnitude higher mass of side stream, e.g., 1 to 2% of the flue gas, compared to that injected through an ammonia injection grid (AIG) in a traditional ammonia SCR process. Thus, the current embodiment provides the flexibility to the type of injection grid depending on the application requirements.

[0055] It is an advantage of this and other embodiments of the invention that because relatively large volumes of side stream gases are mixed with the urea solution prior to introducing the gases into the SCR catalyst, an overt mixing procedure is not essential. It will be advantageous in many cases, especially where there is a high degree of fluctuation in gas volumes, to provide means for mixing the gases at one or more stages. Among the suitable mixing means are static mixers, cyclones, blowers and other process equipment that by design or effect mixes the gases.

[0056] It is another advantage of this embodiment of the invention that by utilizing the side stream comprised of combustion gases prior to full heat exchange, the enthalpy of the gases is utilized for gasification by direct heat exchange with the aqueous urea solution. Surprisingly, calculations will show that direct heat exchange in this manner using supplementary heat only as needed under low-load conditions—when the need for NO _x reduction is also low—will be much more efficient than employing supplementary heat in a cold stream to gasify urea. Advantageously, also, the addition of supplemental heat to the side stream can be an effective means to control the temperature in the side stream for consistent urea decomposition and SCR catalyst and maintain both temperatures within its effective temperature range.

[0057] FIG. 2 illustrates an embodiment similar to that of FIG. 1 , but provides heater 38 to enable increasing the temperature of the side stream 28 sufficiently to assure breakdown of the urea as needed. This is especially useful when output is low for a boiler. It is an advantage of this arrangement that when heat is required, the amount required is far less than would be needed to heat either the entire effluent or simply the urea. A high temperature blower 36 , located downstream of urea nozzle 32 , can be located upstream of heater 38 instead of the depicted location. A heater 38 shown as a burner can be replaced with a steam coil heater, heat exchanger or other means to transfer heat to the side stream 28 .

[0058] FIG. 3 is a schematic representation of another embodiment of the invention wherein side stream 28 is separated from the main effluent stream from a combustor and heated as needed prior to injecting urea into it. The two streams ( 23 and 28 ) are combined and passed through a cyclone 40 to effect particle separation and complete mixing. A high temperature blower 36 , located downstream of urea nozzle 32 , can be located upstream of heater 38 . A heater 38 can be replaced with a steam coil heater, heat exchanger or other means to transfer heat to the side stream 28 .

[0059] FIG. 4 is a schematic representation of another embodiment of the invention wherein side stream 28 is separated from the main effluent stream 23 from combustor 20 and is passed through a cyclone 40 (or other particle separating device or apparatus) prior to heating as needed by heater 38 and injecting urea into it via injector 32 . A high temperature blower 36 , located downstream of urea nozzle 32 , can be located upstream of heater 38 or a cyclone separator 40 . A heater 38 can be replaced with a steam coil heater, heat exchanger or other means to transfer heat to the side stream 28 .

[0060] FIG. 5 is a schematic representation of an embodiment of the invention similar to that in FIG. 4 , wherein side stream 28 is separated from the main effluent stream 23 from a combustor 22 is heated and urea is injected into it just prior to or in cyclone 40 (or other particle separating device or apparatus). The resulting treated stream is passed via blower 36 though an injection grid 37 (or other suitable introduction device or apparatus) ahead of the SCR reactor. Also, an optional static mixer 39 is illustrated. A high temperature blower 36 , located downstream of cyclone 40 , can be located upstream of heater 38 . A heater 38 can be replaced with a steam coil heater, heat exchanger or other means to transfer heat to the side stream 28 .

[0061] FIG. 6 is a schematic representation of another embodiment of the invention wherein a stream of air is forced into duct 128 and heated, and urea is injected into it via injector 32 . The resulting stream is then passed through a mixer and injection grid as it is combined with the effluent stream from a combustor and passed through an SCR reactor. This embodiment shows heat exchanger 45 and burner 38 , but either or both can be employed as needed. Other means to transfer heat to the side stream 28 can replace the heat exchanger 45 or a burner 38 .

[0062] This embodiment is useful in situations where the configuration of combustor 20 does not easily permit construction of a side stream of combustion gases and, therefore, requires additional heat. This additional heat can be lessened by using the preheated combustion air commonly available in utility boilers.

[0063] FIG. 7 is a schematic representation of another embodiment of the invention similar to FIG. 6 , wherein steam is introduced by means 50 as the heat source.

[0064] FIG. 8 is a schematic representation of another embodiment of the invention similar to FIG. 7 , wherein the steam source 50 is located following introduction of the urea.

[0065] FIG. 9 is a schematic representation of another embodiment of the invention similar to FIG. 6 , wherein a side stream 228 is formed from combustion gases following treatment in the SCR catalyst reactor. This embodiment has the advantage that the gases have considerable heat value, especially if withdrawn prior to using them to preheat the combustion air.

[0066] FIG. 10 is a schematic representation of another embodiment of the invention similar to FIG. 9 , wherein a side stream 328 is formed from combustion gases following treatment in the SCR catalyst reactor and downstream particulate collection device 60 such as an electrostatic precipitator, bag filter, or a cyclonic separator. While gases have less heat value than the previous representation, this scheme offers an advantage of being substantially particulate free when applied on solid or liquid fired combustors. Low particulates minimize maintenance requirements.

[0067] In FIG 11 , a modification of urea injection from FIG. 6 is represented. Instead of aqueous urea injection, a finely ground, pulverized or micronized solid urea is injected using pneumatic carrier air via line 31 and nozzle 232 from line 234 . This solid urea injection can be adapted into all previous representations. Without water, solid urea has the advantage of lower heating requirements.

[0068] The above description is intended to enable the person skilled in the art to practice the invention. It is not intended to detail all of the possible modifications and variations that will become apparent to the skilled worker upon reading the description. It is intended, however, that all such modifications and variations be included within the scope of the invention that is seen in the above description and otherwise defined by the following claims. The claims are meant to cover the indicated elements and steps in any arrangement or sequence which is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.