EP0643668A1

EP0643668A1 - PROCESS AND APPARATUS FOR REMOVING NO x? FROM EXHAUST GASES USING CYANURIC ACID--------------------------------------------------

Info

Publication number: EP0643668A1
Application number: EP92919814A
Authority: EP
Inventors: Ralph J. Slone; David F. May; Stephen W. Watson
Original assignee: Cummins Power Generation Inc
Current assignee: Cummins Power Generation Inc
Priority date: 1991-08-26
Filing date: 1992-08-26
Publication date: 1995-03-22
Also published as: EP0643668A4; WO1993003998A1; JPH07501260A

Abstract

A process and apparatus are disclosed in which cyanuric acid is utilized to remove nitrogen oxides (or ''NOx'') from exhaust gases or the like. In one embodiemnt, particles of cyanuric acid (14) are contacted with the exhaust gas under conditions in which the cyanuric acid decomposes and the NOx is reduced. A venturi (24) is utilized to controllably convey the particles of cyanuric acid into the exhaust gas (38). Supplemental fuel such as diesel or other fuels may be added to control reaction conditions such as temperature. In other embodiments, a liquid cyanuric acid slurry is formed, with the liquid comprising diesel fuel, water or other suitable liquid. The liquid cyanuric acid slurry may be heated and/or catalyzed prior to contact with the exhaust gas. In certain embodiments, the liquid-cyanuric acid slurry is heated and/or catalyzed in a manner so as to produce decomposition products including reactive species such as free radicals, which are subsequently contacted with the exhaust gas.

Description

PROCESS AND APPARATUS FOR REMOVING NO_χ FROM EXHAUST GASES USING CYANURIC ACID

Field of Invention

The present invention relates to the removal of nitrogen oxides or "NO^." from exhaust gases and the like, and more particularly to a process and apparatus utilizing particles of cyanuric acid or a liquid-cyanuric acid slurry.

Background of the Invention Recent emphasis on ecological concerns in the environment has spawned many efforts to solve the world's air pollution problems. Two major concerns are acid rain and photochemical smog. While many compounds contribute to these problems, N0_χ plays an important role, imposing a significant threat to the environment and human health.

^,,N0_χ" is a family of compounds of nitrogen and oxygen, primarily NO and N0₂. NO_χ comes from a variety of sources, most notably cars, trucks and industrial plants. Specifically, NO_χ is produced by high temperature combustion systems, metal cleaning processes, and the production of fertilizers, explosives, nitric acid, sulfuric acid and the like. In many urban environments, automobiles and diesel engine trucks are the major sources of N0_χ. NO is the stable oxide of nitrogen at combustion temperatures. Hence, it is more abundantly produced than N0₂. However, at ambient conditions, the equilibrium between NO and N0₂ favors N0₂. Therefore, the effective control of N0_χ concerns both the control and removal of both NO and N0₂ from exhaust gas streams from sources such as those mentioned above.

Many attempts have been made to control the generation or release of N0_χ. Many known strategies involve the control of combustion conditions. This can be accomplished by reducing the temperature and amount of oxygen present during the combustion process. Another strategy is a reburning process. In this process, NO_χ compounds are incinerated in a secondary combustion zone, using particular fuels which do not contain nitrogen. Another strategy is removal of N0_χ from the post-combustion gas or exhaust stream.

Several ways to remove NO_χ downstream from the combustion process are known. One such strategy is a scrubbing technique which takes advantage of the fact that N0₂ combines with water to form nitric acid. Nitric acid reacts with ammonia to yield the stable product ammonium nitrate. However, known scrubbing techniques do not remove NO. To overcome this obstacle, those skilled in the art have sought to oxidize NO to N0₂ and then apply the aqueous scrubbing process to remove the N0₂. NO can be oxidized to N0₂ using various organic compounds, such as aldehydes, alcohols, ketones, or organic acids in the presence of oxygen. However, the use and disposal of organic solvents presents a problem, and the process is relatively inefficient.

Another strategy for removing NO_χ from gas streams is the reduction of NO_χ to nitrogen and water. The prior art teaches catalytic and non-catalytic processes. In the non-catalytic processes, high temperatures typically are required. In the catalytic processes, problems are encountered when exposing the catalyst to the exhaust gas stream. The catalyst is subject to fouling, poisoning and disintegration. These shortcomings tend to make the catalytic processes taught by the prior art expensive, unreliable and potentially hazardous.

Recently, a non-catalytic method of NO_χ reduction involving exposure of a gas stream containing NO_χ to gaseous HNCO has been disclosed. HNCO, also known as isocyanic acid, is an unstable gas at ordinary temperatures and pressures, and thus is hard to handle and store. This problem has been addressed by generating HNCO from more stable, less toxic materials as it is used. One such material is cyanuric acid. Cyanuric acid decomposes when heated, forming HNCO. The gaseous HNCO is then injected into the gas stream where the HNCO is thermally decomposed and the NO_χ reduction reaction takes place, providing that the temperature is high enough to allow the reaction to proceed. In this isocyanic acid process, the conversion of cyanuric acid to HNCO and the NO_χ reduction take place at relatively high temperatures, such as 1200 to 2600°F (649 to 1427°C), and sometimes can require a catalyst.

The isocyanic acid process, however, has a significant practical problem that limits its applicability. This process requires an expensive and complicated system to meter and convert solid cyanuric acid into gaseous HNCO for subsequent injection into the exhaust. The complexity of the system required for these steps in general limits its ability to follow, for example, a power system's changing load or to operate under transient conditions, such as varying speed and/or load conditions. Practical treatment of the exhaust for NO_χ from fossil fueled power systems such as gas turbines and internal combustion engines typically requires inexpensive, simple, and low cost process(es). The failure of the isocyanic system to meet these criteria severely limits its commercial potential.

For example, the following system components, likely to be required in an isocyanic acid system, contribute to its relative complexity, high cost and lack of operating reliability: air lock required to isolate the cyanuric acid powder fed/metered from a screw feeder to a sublimator or vaporizer; a sublimation chamber required to convert the cyanuric acid powder to a gas along with associated components such as electric heaters or heat exchangers to supply heat from the exhaust to the sublimation chamber; and a stirrer or the like to distribute the cyanuric power metered into the sublimator; a cracker to crack the gasified cyanuric acid from the sublimator to HNCO for injection into the exhaust for reaction with NO_χ; and a complex, expensive control system required to operate and perform diagnostics on the above described system elements.

The isocyanic acid process has been modified by carrying out the NO_χ reduction in the presence of carbon monoxide (CO) . However, this process still operates at relatively high temperatures, such as 932 to 1472°F (500°C to 800°C), and often requires the use of a catalyst in the NO_χ-laden gas stream. The operating conditions for such a process, such as high temperatures and appropriate concentrations of CO, typically are not found in a diesel exhaust gas stream (or in other combustion systems) under some conditions of operation. Either intermittent performance must be tolerated or the exhaust stream must be heated to maintain a high temperature, and in any event a catalyst may be required. The system complexity again limits the applicability of such processes.

Direct injection of cyanuric acid into an exhaust stream has been considered. Japanese Laid-Open Application No. 54-28771 to Wada, et al. discloses the treatment of exhaust gases containing N0_χ by adding 100-

10,000 micron diameter granules (with a preferred diameter of 500 to 5,000 microns) of a compound selected from cyanuric acid, urea, ammonium sulphate, ammonium chloride, ammonium phosphate, ammonium carbonate and an organic compound having a nitrile group.

Wada discloses use of reducing agent granule size as a means to reduce temperature control requirements of the reaction zone. Wada theorized that, if the granule size was large enough, a low temperature region would be created around the granule as the granule dropped through the exhaust gas. Over time, the region around the granule is heated by the exhaust gas until it reaches the unspecified optimum temperature for reduction efficiency. The exhaust gas temperature is specified to be 600 - 1500°C, or 1200 - 1500°C.

Summary of the Invention The present invention provides a process and apparatus that are reduced in complexity over the isocyanic acid process previously described, which is a result in part of the elimination and/or replacement of several undesirable components, while also providing significant improvements and advantages over the process disclosed in Wada. In one embodiment of the present invention, direct injection of particles of cyanuric acid is utilized, which results in a more efficient NO_χ reduction process based on the specific mass of cyanuric acid required to reduce NO_χ (Lbs. cyanuric acid/Lb. NO_χ) . This performance benefit results at least in part from the generation of HNCO or other decomposition products from cyanuric acid in-situ, which minimizes their decomposition and/or oxidation by reducing the time they are exposed to the reactor's hot oxidizing gases before reaction with N0_χ. A venturi is utilized to controllably convey cyanuric acid particles into the NO_χ-containing exhaust stream. Control over the reaction conditions with supplemental fuel injection and oxidation, which also provide oxidation reaction products, as well as over parameters such as temperature, gas composition and reducing agent residence time, provide additional benefits, while yielding a low- cost yet effective NO_χ reduction process and apparatus, which may be used to reduce N0_χ produced by diesel or other internal combustion engines, boilers, turbines or other industrial processes that generate NO_χ-containing gas streams.

In another embodiment of the present invention, a liquid-cyanuric acid slurry is utilized to deliver cyanuric acid and/or decomposition products into the NO_χ- containing exhaust stream. The slurry may be formed by adding solid cyanuric acid to a suitable liquid such as water or a fuel such as diesel fuel. The liquid-cyanuric acid slurry may be heated prior to injection into the NO_χ- containing exhaust stream, which can serve to "dry" the cyanuric acid and thereby reduce the residence time within the reaction chamber required for optimum N0_χ reduction and/or reduce the size requirements of the reaction chamber. In the case of liquids such as water, the slurry may be heated or catalyzed in a manner so as to produce decomposition products including reactive species such as free radicals, which are subsequently injected into the NO_χ-containing exhaust stream and result in a reduction of the NOx, which may occur at a reduced temperature. Use of a liquid-cyanuric acid slurry can provide benefits such as improved metering consistency of the reducing agent into the exhaust stream, and less sensitivity to the physical quality and/or particle size of the cyanuric acid.

Accordingly, it is an object of the present invention to provide a process and apparatus for removing NO_χ from exhaust gases in which particles of cyanuric acid are contacted with the NO_χ-containing exhaust gas.

It is another object of the present invention to provide a process and apparatus for removing N0_χ from exhaust gases in which a liquid-cyanuric acid slurry is utilized to contact cyanuric acid and/or decomposition products with the Nθ_χ-containing exhaust gas.

It is yet another object of the present invention to provide a process and apparatus for removing N0_χ from exhaust gases in which a liquid-cyanuric acid slurry is heated and cyanuric acid and/or decomposition products are contacted with the NO_χ-containing exhaust gas.

Finally, it is an object of the present invention to provide a process and apparatus for removing N0_χ from exhaust gases in which a liquid-cyanuric acid slurry is heated and/or catalyzed to produce decomposition products, which are contacted with the NO_χ-containing exhaust gas.

Brief Description of the Drawings The above-mentioned features and objects as well as other features and objects of the present invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a diagram illustrating an embodiment of the present invention in which particles of cyanuric acid are contacted with the NO_χ-containing exhaust gas;

FIG. 2 is a diagram illustrating an embodiment of the present invention in which a liquid-cyanuric acid slurry is utilized to contact cyanuric acid and/or decomposition products with the NO_χ-containing exhaust gas; and

FIG. 3 is a diagram illustrating an embodiment of the present invention in which a liquid-cyanuric acid slurry is heated and/or catalyzed to produce decomposition products, which are contacted with the NO_χ-containing exhaust gas.

Detailed Description of the Invention With reference to the figures, preferred and alternative embodiments of the present invention will now be described. Although the present invention will now be described in terms of exhaust gases, it should be understood that the present invention may be useful with any gas stream from which NO_χ is to be removed. In addition, throughout the specification and claims, references to cyanuric acid are intended to include its tautomer, isocyanuric acid, since, for purposes of the present invention, the two forms are equivalent. Referring now to FIG. 1, an embodiment of the present invention utilizing particles of cyanuric acid will now be described.

As illustrated in FIG. 1, cyanuric acid powder 14 is placed in vessel 12 through opening 10. Cyanuric acid powder 14 provides a supply of cyanuric acid for the N0_χ removal process of the present invention. Screw feeder 16 controllably conveys cyanuric acid powder (now denoted by arrow 20) down conduit 18. Conduit 18 connects to venturi 24, which, along with compressed air 22 (or other appropriate transport gas) , fluidize the cyanuric acid and convey the cyanuric acid particles through conduit 26 into the NO_χ-containing exhaust gas, as indicated by cyanuric acid particles 36 in exhaust pipe 38. Venturi 24, compressed air 22 and conduit 26 provide a controllable mechanism to deliver cyanuric acid power 36 to the N0_χ containing exhaust gas (denoted as N0_χ exhaust 34 in FIG. 1) . The transport gas supplied to venturi 24 may be any suitable gas under pressure that when conveyed into the exhaust gas stream (along with the cyanuric acid and supplemental fuel) does not inhibit the NO_χ reduction reaction or produce undesirable reactions or species. The rate at which the cyanuric acid is supplied into the exhaust stream will vary depending upon the concentration of the NO_χ in the exhaust gas and the overall reaction conditions in the particular application. In the preferred embodiment, the cyanuric acid is supplied at a rate so as to provide approximately stoichiometric quantities of HNCO and N0_χ. Supplemental fuel supply 28 is connected to pump

30. Supplemental fuel is conveyed by pump 30 through conduit 32. Conduit 32 connects to exhaust pipe 38 at a point upstream from where conduit 26 connects to exhaust pipe 38. Pump 30 provides a controllable mechanism to provide appropriate amounts of supplemental fuel from supplemental fuel supply 28 for optimum NO_χ reduction. In the preferred embodiment, supplemental fuel is supplied at a rate to maintain the temperature in reaction chamber 40 at approximately 1310 °F. As further illustrated in FIG. 1, supplemental fuel, cyanuric acid particles and the NO_χ-containing exhaust gas are conveyed to in-line exhaust reaction chamber 40. Reaction chamber 40 is heated by the oxidation of the supplemental fuel, which provides heat and fuel-oxidation reaction products. Without being bound by theory, in reaction chamber 40, gasification and cracking of the cyanuric acid occurs, leading to reactions of the resultant HNCO and/or other decomposition products such as NCO with the N0_χ, resulting in a consumption of the decomposition products and a reduction of the N0_χ in the exhaust gas. Without being bound by theory, one possible reaction is (HNC0)₃ → 3NC0 + 3/2 H₂, with the NCO reacting with NO to form N₂ and C0₂. Exhaust gas with reduced N0_χ is discharged from outlet 44. While the precise chemistry is not fully known, it is believed that heat generated by the oxidation of the supplemental fuel, and possibly active species produced as reaction products of the oxidation of the supplemental fuel, promote the initiation and/or overall efficiency of the N0_χ reduction reaction(s) . Optimization of the process of this embodiment of the present invention for removing NO_χ from the exhaust gas is achieved through a controlled balancing of several factors. First, the residence time of the cyanuric acid particles in reaction chamber 40 is controlled so as to allow optimum conversion of the cyanuric acid to HNCO or other decomposition products such as NCO and subsequent reaction with NO_χ in the exhaust gas. Reactor temperature is controlled to minimize supplemental fuel consumption (by controlling the supply of supplemental f el to the minimum amount to maintain the reactor temperature in an optimum range for the N0_χ reaction) and N₂0 formation (N₂0 is known as a green-house gas and generally is favored at high temperatures) , while optimizing the conversion efficiency of cyanuric acid to HNCO or other decomposition products such as NCO. This embodiment of the present invention has achieved reaction of NO_χ with HNCO and/or other decomposition products to give over 95% N0_χ reduction efficiencies, while achieving low absolute levels of N0_χ under conditions substantially as set forth in Example No. 1.

In addition, the process of this embodiment of the present invention balances reaction parameters (including residence time and temperature) such that an acceptable range of cyanuric acid particle sizes may be utilized with the present invention. Thus, commercially available cyanuric powders can be used in this process of the present invention with optimized heat transfer for conversion of the cyanuric acid particles to HNCO or other decomposition products at acceptably low temperatures.

Further, the injection, decomposition, and oxidation of the supplemental fuel is believed to produce chemical species and heat that serve to drive, and lower the optimum temperature for, the reaction of HNCO and/or other decomposition products with NO_χ. Part of the improved efficiency of this embodiment is believed to result from the conditions produced in the reaction chamber, which allow prompt reaction of the HNCO and/or other decomposition products with the NO_χ since such decomposition products are generated in-situ, while minimizing the time available for oxidation by the exhaust gas. The design of the reactor can readily be optimized for fluid dynamics and exhaust back pressure to give the most desirable residence times, chemistry and mixing through control and/or optimization of length/diameter ratio(s) and total volume(s) for a specific exhaust flow. The cyanuric acid particles in reaction chamber 40 have a preferred residence time of 0.5 seconds, with a range of 0.1-1.0 seconds. With the temperatures and other parameters controlled as disclosed herein, such residence times minimize the temperature required for conversion of cyanuric acid to decomposition products, and for their subsequent reaction with NO_χ.

Minimizing the temperature in reaction chamber 40 avoids undesirable N₂0 formation and also improves process efficiency by reducing or eliminating the oxidation of cyanuric acid gas or the decomposition products during the conversion process. Without being bound by theory, the oxidation of cyanuric acid and HNCO occurs according to the following side reactions:

(HNCO)₃ or HNCO + 0₂ → NO + H₂0 + C0₂. Controlling the reaction temperature to a range of approximately 1000-1600 °F, with a reaction temperature of approximately 1310 °F preferred, has been found to allow an acceptable N0_χ reduction reaction, while minimizing undesirable N₂0 formation or reducing agent oxidation.

The above reaction chamber residence time(s) and temperature(s) have permitted the selection and use of practical, commercially available cyanuric acid powders. A preferred particle size of approximately 100 microns in diameter, with a range of approximately 50-200 microns, has been determined as providing acceptable results in the present invention. Through control of the reaction parameters as disclosed herein, the largest cyanuric acid particles can be injected and efficiently converted into HNCO and/or other decompositions products using a practical size reaction chamber, while maintaining acceptable temperatures in reaction chamber 40. The particle size(s) selected provide(s) sufficient heat transfer to allow full conversion of the cyanuric acid powder to decomposition products (at the cited residence times and temperatures) , while minimizing N₂0 formation, oxidation losses and solid particle emissions into the air. Cyanuric acid particles outside the selected range are believed to have a higher potential of being emitted into the air from the exhaust stream, since larger particles do not have sufficient heat transfer to be converted to a gas, and smaller particles may have a reduced residence time in reaction chamber 40.

In alternative embodiments, the cyanuric acid particles are ground in-situ before injection into the exhaust gas. In-situ grinding of the cyanuric acid powder extends the initial useful particle size range to on the order of 25,400-75,200 microns in diameter (before grinding) , which renders the acid easier to transport, while allowing appropriately sized particles to be fed into the injection venturi.

Control of the injection, decomposition, and oxidation of the supplemental fuel in reaction chamber 40 contributes to the efficiency and controllability of the NO_χ-reduction process of this embodiment of the present invention. Oxidation of the supplemental fuel generates heat, which provides a means for controlling the temperature in reaction chamber 40 to the optimum level for the particular cyanuric acid particle size, residence time and other reaction conditions for the particular application. In addition, it is believed that chemical species produced in reaction chamber 40 by the oxidation of the supplemental fuel assist in the initiation of the N0_χ reduction reaction(s) at acceptable temperatures and/or assist in driving the reaction(s) towards completion while avoiding unacceptable side reactions. Injection of the supplemental fuel in a manner to avoid complete mixing and/or atomization of the supplemental fuel before entering reaction chamber 40 is believed to create a stratified mixture, which oxidizes and/or cracks more slowly, and thereby producing active chemical species as fuel-oxidation by-products in a more controlled manner as opposed to rapid combustion that would create unacceptably high temperatures favoring N₂0 formation and decomposition product oxidation. While supplemental fuel may be delivered to reaction chamber 40 in a variety of ways, such as be spraying or high/low pressure injection, better results have been obtained by supplying the supplemental fuel through low pressure injection, which is believed to contribute to more stratified conditions in reaction chamber 40.

Supplemental fuel supply 28 may be any suitable fuel such as diesel fuel, natural gas, propane or methanol, and in the preferred embodiment is diesel fuel. In embodiments utilizing gaseous fuels such as natural gas, the fuel may be combusted in a burner (not shown) outside the exhaust, with the resulting hot combustion gases injected into the exhaust gas.

Reaction chamber 40 is of appropriate design and construction to produce good mixing, reduced flow velocity (thereby allowing a reasonably determinable increase in the residence times of the cyanuric acid particles) , and the reactive species produced by the oxidation of the supplemental fuel which facilitates the N0_χ reduction reaction(s) . The design of reaction chamber 40 to provide desirable residence times, along with the use of supplemental fuel oxidation, allow for a practical size reactor, which can be installed on power system exhausts or other industrial systems. The preferred length/diameter ratio and total volume for a specific flow is approximately 5.5 1/d at a ratio of reactor volume (158 ft³) of approximately 0.044 ft³ reactor volume/ft/minute exhaust flow (3600 dscf ) with a range of 4.0-6.5 1/d and .034-.054 ft³ reactor volume/ft³/minute exhaust flow (dscfm) . A reactor of such properties has been found to provide acceptable optimization of the 1/d ratio while permitting additional mechanical control of the gas velocity flowing through the reactor using butterfly valves or other flow control systems known in the art. Control of gas-flow velocity (and therefore residence times) can be achieved by a suitable feed-back control system using sensors and a preprogrammed microprocessor or the like to define the optimum reaction conditions for the particular application, including an appropriate balance of parameters such as temperature, cyanuric acid particle size, residence time and gas composition for the particular application. A suitable simplified feed-back control system, made possible by the simplicity of the direct injection system of this embodiment of the present invention, can be used for ready control of the supply of cyanuric acid powder needed to react with N0_χ in reaction chamber 40 as well as the temperature and reaction conditions in reaction chamber 40. As desired, the control system may also monitor the overall performance of the system, and conduct appropriate diagnostic checks to avoid system failures. With reference to FIGS. 2 and 3, embodiments of the present invention utilizing a liquid-cyanuric acid slurry will now be described.

As illustrated in FIG. 2, a liquid-cyanuric acid slurry 50 is produced in vessel 51. The liquid used to produce slurry 50 may be any suitable liquid for serving as a medium in which cyanuric acid may be controllably delivered from vessel 51 to. the NO_χ-containing exhaust gas, with or without heating and/or catalyzation (as discussed more fully below with reference to FIG. 3) . In preferred embodiments of the present invention, the liquid used to produce slurry 50 is water or a suitable fuel such as diesel fuel. Other liquids that may be utilized to produce slurry 50 are alcohols, organic acids and other liquids that do not adversely affect the NO_χ reduction reaction(s) or result in undesirable species. Cyanuric acid in granular or other solid form is added to the liquid in controlled amounts to form slurry 50.

Agitator 52 is positioned within vessel 51 to provide agitation or stirring of slurry 50. Agitator 52 can be any suitable device for agitating or stirring slurry 50, and in preferred embodiments consists of an electric motor driving a shaft on which is attached one or more multi-blade propellers. In preferred embodiments, slurry 50 is agitated on a substantially continuous basis. Production of an agitated slurry offers certain advantages, including decreased sensitivity to the physical quality and/or particle size of the input cyanuric acid. The agitation of the cyanuric acid in slurry 50 by agitator 52 effectively provides a conditioning of the cyanuric acid prior to subsequent processing and/or delivery into the NO_χ-containing exhaust gas.

Agitation in slurry 50 is believed to generate shear forces and/or collisions that physically break the cyanuric acid into small pieces or particles. Thus, the agitation conditions within vessel 51 enable the physical transformation of the cyanuric acid, which can be optimized for subsequent processing (i.e., such as reduced "drying time," discussed below). While the desired particle sizes in slurry 50 will depend upon the particular conditions, agitation so as to produce cyanuric acid particles up to a range of about 50 to 200 microns is believed to provide acceptable results. The concentration of the cyanuric acid in slurry 50 may be any suitable concentration, and in preferred embodiments is up to concentrations of 20 to 60% or even higher. The optimum concentration of cyanuric acid in slurry 50 will depend upon the particular operating parameters and exhaust gas characteristics and the like; it is noted, however, that higher concentrations of cyanuric acid may offer certain advantages in that the volume of vessel 51 and/or the "drying time" required for the cyanuric acid in subsequent stages may be reduced. Slurry 50 is controllably pumped from vessel 51 by pump 31. Pump 31 may be any suitable pump, and in preferred embodiments is a gear rotor pump driven by a variable speed electric motor. The rate at which slurry 50 is pumped from vessel 51 by pump 31 will depend upon the particular operating parameters and exhaust gas characteristics and the like. By way of example, the exhaust gas from a one megawatt power output diesel engine may be treated with a diesel fuel-cyanuric acid slurry (approximately 28% by weight cyanuric acid) pumped at a rate of approximately 90 pounds per hour.

Slurry 50 may be pumped by pump 31 directly into exhaust pipe 38, or optionally slurry 50 may be heated by heater 54. Heater 54 may be any suitable source of heat for heating slurry 50, and in preferred embodiments constitutes a heat exchanger deriving heat from exhaust pipe 38 or reaction chamber 40, or alternatively an external heat source powered by electrical or chemical fuel means. Because the cyanuric acid is "wet" when delivered from slurry 50, the cyanuric acid typically must be "dried" prior to decomposition for the N0_χ reduction reaction(s) . The drying of the cyanuric acid may be achieved in-situ by allowing for longer residence times of the cyanuric acid in reaction chamber 40, or alternatively the temperature within reaction chamber 40 may be appropriately increased. Because changes in reactor geometry or the like and increased reaction temperatures may be undesirable, heating such as by heater 54 prior to contacting with the exhaust gas may be utilized. Heating of slurry 50 by heater 54 serves to accelerate the "drying" and subsequent decomposition of the cyanuric acid. For example, heater 54 may heat slurry 50 to the point that slurry 50 "flashes" or decomposes very rapidly in the reaction chamber enabling low residence times for the N0_χ reduction reaction(s) . As discussed more fully below, depending upon the liquid used to form slurry 50, decomposition of the cyanuric acid may take place in reaction chamber 40, or, in alternative embodiments, thermal or catalytic decomposition of the cyanuric acid may be obtained prior to delivery into exhaust pipe 38 or reaction chamber 40.

The characteristics of heater 54 will depend upon the particular operating parameters and slurry and exhaust gas characteristics and the like. The liquid selected for slurry 50 is an important consideration. For example, when the liquid for slurry 50 is a fuel such as diesel fuel, slurry 50 should be heated only to modest levels prior to contacting with the exhaust gas, such as 200-400°F, in that heating to higher temperatures may result in carboning of the fuel. When the liquid for slurry 50 is a stable liquid such as water, however, slurry 50 may be heated to substantially higher temperatures. With appropriate liquids such as water, slurry 50 may be heated to the point that gasification of the liquid and/or the cyanuric acid occurs, such as up to 500-600°F. Alternatively, and as explained in more detail with reference to FIG. 3, with appropriate liquids such as water, slurry 50 may be heated to the point that decomposition products are produced from slurry 50 (such as up to 800-1200°F to produce HNCO, and up to 1200-1700°F or higher to decompose the HNCO) , with the decomposition products delivered into exhaust pipe 38 for reaction with NO_χ in the exhaust gas.

Supplemental fuel from supplemental fuel supply 28 is controllably conveyed by pump 30 to exhaust pipe 38. The presence and/or rate of supply of a supplemental fuel will depend upon the particular operating parameters and slurry and exhaust gas characteristics and the like. For example, if the liquid for slurry 50 is a fuel such as diesel fuel, little or no supplemental fuel may be required to maintain appropriate conditions for the NO_χ-reduction reaction(s) . In any event, the use of supplement fuel supply 28 to provide control reaction conditions such as temperature including candidate fuels have been discussed previously with respect to FIG. 1 and will not be further discussed here (it is noted, however, that with the decomposition product embodiments discussed below, the N0_χ reduction reactions may occur at a lower temperature and therefore require less or no supplemental fuel) .

Without being bound by theory, within reaction chamber 40, one or more reactions occur between the cyanuric acid and/or decomposition products such as NCO and the N0_χ, resulting in a reduction of the N0_χ in the exhaust gas. Temperatures within reaction chamber 40 for optimum NO_χ reduction with embodiments in which cyanuric acid is injected into the exhaust gas are similar to the embodiment of FIG. 1, although the temperature may optimally be increased somewhat in order to allow for sufficient "drying" and subsequent reaction of the cyanuric acid. Exhaust gas with reduced NO_χ is discharged from outlet 44.

In alternative embodiments, slurry 50 (with or without prior heating) is injected into exhaust pipe 38 or reaction chamber 40 by way of an atomizing nozzle (not shown) , which will serve to accelerate the evaporation of the liquid from the slurry and thus the subsequent decomposition and reaction in reaction chamber 40.

With reference to FIG. 3, other embodiments of the present invention now will be described. Embodiments discussed with reference to FIG. 3 are particularly useful with stable liquids such as water used to produce slurry 50.

Slurry 50 is produced in vessel 51 and is agitated by agitator 52 in a manner analogous to slurry 50 of FIG. 2. Pump 31 controllably conveys slurry 50 to heater 54 for heating. Heater 54 heats slurry 50 to produce decomposition products including reactive species such as free radicals. Heater 54 may heat slurry 50 up to about 500-600°F to gasify the cyanuric acid, and up to 800-1200°F to produce HNCO, and up to 1200-1700*F or higher to produce further decomposition products.

Decomposition products from heater 54 may be delivered from heater 54 into exhaust pipe 38 by conduit 56. Without being bound by theory, radicals such as NCO, H and NH₂ may be formed as decomposition products of the cyanuric acid, with, for example, the NCO radicals reacting with NO to form N₂ and C0₂, with such reactions able to occur at reduced temperatures of about 750-850°F.

In other embodiments, the output of heater 54, which may or may not contain decomposition products, is conveyed to catalyst chamber 58. Catalyst chamber 58 produces decomposition products, including reactive species such as free radicals. Decomposition products from catalyst chamber 58 are delivered into exhaust pipe 38. Again, without being bound by theory, radicals such as NCO, H and NH₂ may be formed as decomposition products of the cyanuric acid, with, for example, the NCO radicals reacting with NO to form N₂ and C0₂, with such reactions able to occur at reduced temperatures of about 750-850°F. Alternatively, heater 54 and catalyst chamber 58 may be combined so that heating and catalytic decomposition of the cyanuric acid occur substantially in a single step, although such may be achieved through a "staged process;" for example, the cyanuric acid may be gasified and/or cracked to produce HNCO at temperatures of about 800-1200°F, while catalytic decomposition may occur at temperatures of about 750-850°F. As needed, the temperature of the gaseous HNCO may be reduced in a conventional manner prior to contact with the decomposition catalyst.

Thus, with water or other appropriate liquids, slurry 50 may be heated to sufficient levels and/or catalyzed so as to result in decomposition of the cyanuric acid to produce decomposition products that may react with the N0_χ at reduced temperatures. Water, for example, has substantial thermal stability and does not produce hazardous by-products. In embodiments in which the cyanuric acid is thermally or catalytically decomposed to form free radicals, water has beneficial properties in that with water it is believed that no free oxygen is available to scavenge or quench the free radicals and thereby reduce the efficiency of the overall process.

Catalyst chamber 58 contains a suitable catalytic material for producing decomposition products useful for reducing N0_χ in an exhaust gas. By way of example, catalysts for use in catalyst chamber 58 may be zirconium, phosphorous and mixtures thereof, which may include zirconium and/or phosphorous in the plus four oxidation state, such as are disclosed in U.S. Patent No. 5,087,431 issued February 11, 1992 to Gardner-Chavis, et al. for "Catalytic Decomposition of Cyanuric Acid and Use of Product to Reduce Nitrogen Oxide Emissions. Other suitable catalysts are useful in the present invention, and other possible catalysts may include A1₂0₃, Ti0₂, cordierite, MgO, zeolites, V₂0₅, Pt, Pd, CeO, iron oxide, chromium oxide, NiO and combinations thereof. While the optimum temperature for catalytic decomposition will depend upon the particular catalysts, etc., catalytic decomposition temperatures of 750-850°F are believed to provide acceptable results.

The zirconium catalyst of the types which can be utilized in the present invention are commercially available and typically contain at least some zirconium in the plus four oxidation state. For example, the catalyst may be commercially available mixed-metal oxide catalysts which contain at least some zirconium or phosphorus in the plus four oxidation state. An example of a commercial zirconium-containing catalyst useful in the method of this invention is the zirconia catalyst ZR-0304T1/8 available from the Engelhard Corporation.

The catalyst utilized in the method of the present invention may be formed in any conventional manner such as tableting, pelleting, etc. , or the active catalyst material can be supported on a carrier. The carrier is generally inert and may include silica, alumina, clay, alumina-silica, silicon carbide, or even zirconia. The catalyst material may be deposited upon the carrier by techniques well known to those skilled in the art such as by depositing a solution containing the catalytic components on the carrier and thereafter drying and calcining the material. Utilizing these techniques, the catalytic components may be either coated and or impregnated in a carrier for use in catalyst chamber 58. Example No. 1

An embodiment of the present invention utilizing particles of cyanuric acid has been applied to a KTTA-50 G-3 heavy duty diesel engine manufactured by Cummins Engine Company, Inc. applied to a .95 megawatt generator set for power generation. Through application of the particulate cyanuric acid process disclosed herein, a 94.5% reduction in NO_χ was obtained, under conditions as substantially set forth below.

An embodiment of the present invention utilizing a liquid-cyanuric acid slurry, with diesel fuel serving as the liquid used to produce the slurry and also the supplemental fuel, has been applied to a similar engine/generator set as used in Example No. 1. Similar high levels of N0_χ reductions were obtained as described in Example Nos. 2, 3 and 4. ("CYA" is intended as a reference to cyanuric acid in the following examples.)

Example No. 4 Conditions: Engine load = 956 kW

Reactor inlet temp. = 993°F

Reactor outlet temp. = 1436°F wt% CYA in slurry = 29.80

Lbs CYA / gal fuel = 2.97

Engine fuel usage = 71.8 gal/hr Suppl. fuel usage = 13.2 gal/hr Cyanuric acid usage = 36.18 lbs/hr

ppm N0_χ in = 1380 ppm N0_χ out = 58 - 68

In summary, through the direct injection of cyanuric acid, or through the use of a liquid-cyanuric acid slurry, the present invention provides for simple, effective systems using essentially selective, non-catalytic reduction (SNR) process for the reduction of N0_χ in exhaust gases from combustion-power systems or other industrial processes. The direct injection of particles of cyanuric acid eliminates the need for components such as air locks, sublimation chambers (and associated stirrers) , cyanuric acid crackers, and systems to exchange heat from the exhaust for transfer to the sublimation chamber likely to be required in isocyanic systems. The use of a liquid-cyanuric acid slurry offers improved metering consistency and less sensitivity to cyanuric acid quality, and in some embodiments a lower NO_χ reduction reaction temperature. The reduced complexity of systems in accordance with the present invention significantly reduces system response time and enhances the transient operating capability, improves reliability-durability through reduction of components and complexity, and gives a major reduction in system costs over prior art isocyanic acid processes. Over 95% reduction of N0_χ has been demonstrated in the exhaust of large high speed heavy duty diesel engines. The present invention also provides a process and apparatus in which optimization and control of the parameters necessary to achieve efficient, maximum levels (up to 95% or more) of NO_χ reduction in exhaust gases can be readily achieved. This includes the parameters such as: reaction chamber design (residence time and mixing) ; reaction chamber temperature; cyanuric acid particle size or slurry concentration and/or delivery conditions; injection, decomposition, and controlled oxidation of supplemental fuel injected to maintain the reactor temperature and/or chemistry; optimized reaction chamber design for optimum fluid dynamics and back pressure and length/diameter vs. total volume/given flow; and a simple control system resulting from operational simplicity. While the present invention has been described in terms of preferred and alternative embodiments, it will be obvious to one skilled in the art that many alternations and modifications may be made without substantially departing from the spirit of the invention. Accordingly, it is intended that all such alternations and modifications be included in the spirit and scope of the invention as defined by the appended claims.

Claims

We claim:

1. A process for reducing nitrogen oxide in a gas comprising contacting the gas with particles of cyanuric acid, wherein the particles of cyanuric acid have diameters within the range of about 50 to 200 microns, wherein the residence time of the particles in the gas is within the range of about .1 to 1.0 seconds, wherein the temperature is sufficient for decomposition of the particles of cyanuric acid and reduction of the nitrogen oxide in the gas and is about or below 1600°F.

2. The process of claim 1, wherein the particles of cyanuric acid have diameters within the range of about 90 to 110 microns.

3. The process of claim 1, wherein the residence time of the particles in the gas is within the range of about .4 to .6 seconds.

4. The process of claim 1, wherein the particles of cyanuric acid are contacted with the gas at a temperature in the range of about 1000 to 1600°F.

5. The process of claim 1, wherein the particles of cyanuric acid are contacted with the gas at a temperature in the range of about 1270 to 1350°F.

6. The process of claim 1, wherein the particles of cyanuric acid are contacted with the gas at a temperature in the range of about 1290 to 1330°F.

7. The process of claim 1, wherein the particles of cyanuric acid are contacted with the gas in a reaction vessel, wherein the residence time of the particles of cyanuric acid in the gas is controlled by the flow of the gas through the reaction vessel.

8. The process of claim 7, wherein the reaction vessel has a length/diameter ratio within the range of about 4.0 to 6.5 1/d.

9. The process of claim 7, wherein the ratio of the reaction vessel volume to gas flow volume per minute is within the range of about .034 to .054 ft³ per ft^3/minute gas flow (dscfm) .

10. The process of claim 7, wherein the reaction vessel has a length/diameter ratio within the range of about 4.0 to 6.5 1/d and wherein the ratio of the reaction vessel volume to gas flow volume per minute is within the range of about .034 to .054 ft³ per ft^3/minute gas flow (dscfm) .

11. The process of claim 1, further comprising the step of oxidizing supplemental fuel, wherein the particles of cyanuric acid are contacted with the gas in the presence of reaction products produced by oxidation of the supplemental fuel.

12. The process of claim 11, wherein the supplemental fuel comprises diesel fuel, methanol, natural gas or propane.

13. The process of claim 11, wherein the supplemental fuel is in a stratified condition.

14. The process of claim 1, wherein the particles of cyanuric acid are contacted with the gas at a temperature sufficient to minimize the formation of N₂0.

15. The process of claim 1, further comprising the steps of: supplying compressed transport gas to a venturi; and supplying particles of cyanuric acid to the venturi, wherein the particles of cyanuric acid are propelled from the venturi into the gas containing the nitrogen oxide.

16. The process of claim 1, further comprising the step of grinding particles of cyanuric acid to produce particles of cyanuric acid that have diameters within the range of about 50 to 200 microns.

17. The process of claim 16, where the particles of cyanuric have a diameter in the range of about 25,400 to 75,200 microns before grinding.

18. A process for reducing nitrogen oxide in a gas comprising the steps of: introducing supplemental fuel into the gas; introducing particles of cyanuric acid into the gas; oxidizing the supplemental fuel; and reacting the cyanuric acid with the nitrogen oxide in the presence of at least certain of the reaction products produced by the oxidation of the supplemental fuel at a temperature sufficient for reduction of the nitrogen oxide in the gas.

19. The process of claim 18, wherein the particles of cyanuric acid have diameters within the range of about 90 to 110 microns.

20. The process of claim 18, wherein the particles of cyanuric acid have diameters of about 500 microns or less.

21. The process of claim 18, wherein the particles of cyanuric acid have diameters of about 200 microns or less.

22. The process of claim 18, wherein the particles of cyanuric acid are reacted with the nitrogen oxide at a temperature in the range of about 1000 to 1600°F.

23. The process of claim 18, wherein the particles of cyanuric acid are reacted with the nitrogen oxide at a temperature in the range of about 1270 to 1350°F.

24. The process of claim 18, wherein the particles of cyanuric acid are reacted with the nitrogen oxide at a temperature in the range of about 1290 to 1330°F.

25. The process of claim 18, wherein the supplemental fuel comprises diesel fuel, methanol, natural gas or propane.

26. The process of claim 18, wherein the supplemental fuel is in a stratified condition.

27. The process of claim 18, wherein the particles of cyanuric acid are reacted with the nitrogen oxide in a reaction vessel, wherein the residence time of the particles of cyanuric acid in the gas is controlled by the flow of the gas through the reaction vessel.

28. The process of claim 27, wherein the residence time of the particles in the gas is within the range of about .1 to 1.0 seconds.

29. The process of claim 27, wherein the residence time of the particles in the gas is within the range of about .4 to .6 seconds.

30. The process of claim 27, wherein the reaction vessel has a length/diameter ratio within the range of about 4.0 to 6.5 1/d.

31. The process of claim 27, wherein the ratio of the reaction vessel volume to gas flow volume per minute is within the range of about .034 to .054 ft³ per ft³minute gas flow (dscfm) .

32. The process of claim 27, wherein the reaction vessel has a length/diameter ratio within the range of about 4.0 to 6.5 1/d and wherein the ratio of the reaction vessel volume to gas flow volume per minute is within the range of about .034 to .054 ft³ per ft³minute gas flow (dscfm) .

33. An apparatus for reducing nitrogen oxide in a gas, comprising: a reaction vessel; means for supplying the gas to the reaction vessel; means for supplying supplemental fuel to the reaction vessel at a temperature sufficient for oxidation of the supplemental fuel in the reaction vessel; and means for supplying particles of cyanuric acid to the reaction vessel, wherein the cyanuric acid reacts with the nitrogen oxide in the presence of at least certain of the reaction products produced by the oxidation of the supplemental fuel at a temperature sufficient for reduction of the nitrogen oxide in the gas.

34. The apparatus of claim 33, wherein the means for supplying the particles of cyanuric acid to the reaction vessel comprises a venturi.

35. The apparatus of claim 33, wherein the means for supplying the particles of cyanuric acid to the reaction vessel comprises a supply of compressed transport gas and a venturi.

36. The apparatus of claim 33, wherein the particles of cyanuric acid have diameters within the range of about 90 to 110 microns.

37. The apparatus of claim 33, wherein the particles of cyanuric acid have diameters of about 500 microns or less.

38. The apparatus of claim 33, wherein the particles of cyanuric acid have diameters of about 200 microns or less.

39. The apparatus of claim 33, wherein the particles of cyanuric acid are reacted with the nitrogen oxide at a temperature in the range of about 1000 to 1600^βF.

40. The apparatus of claim 33, wherein the particles of cyanuric acid are reacted with the nitrogen oxide at a temperature in the range of about 1270 to 1350°F.

41. The apparatus of claim 33, wherein the particles of cyanuric acid are reacted with the nitrogen oxide at a temperature in the range of about 1290 to 1330°F.

42. The apparatus of claim 33, wherein the particles of cyanuric acid are reacted with the nitrogen oxide at a temperature of about 1310°F.

43. The apparatus of claim 33, wherein the residence time of the particles of cyanuric acid in the gas is controlled by the flow of the gas through the reaction vessel.

44. The apparatus of claim 43, wherein the residence time of the particles in the gas is within the range of about .1 to 1.0 seconds.

45. The apparatus of claim 43, wherein the residence time of the particles in the gas is within the range of about .4 to .6 seconds.

46. The apparatus of claim 43, wherein the reaction vessel has a length/diameter ratio within the range of about 4.0 to 6.5 1/d.

47. The apparatus of claim 43, wherein the ratio of the reaction vessel volume to gas flow volume per minute is within the range of about .034 to .054 ft³ per ft³minute gas flow (dscfm) .

48. The apparatus of claim 43, wherein the reaction vessel has a length/diameter ratio within the range of about 4.0 to 6.5 1/d and wherein the ratio of the reaction vessel volume to gas flow volume per minute is within the range of about .034 to .054 ft³ per ft^3/minute gas flow (dscfm) .

49. The apparatus of claim 33, wherein the supplemental fuel comprises diesel fuel, methanol, natural gas or propane.

50. The apparatus of claim 33, wherein the supplemental fuel is in a stratified condition.

51. A process for reducing nitrogen oxide in a gas comprising the steps of: forming a liquid-cyanuric acid slurry; and contacting the gas with the liquid-cyanuric acid slurry at a temperature sufficient for reduction of the nitrogen oxide in the gas.

52. The process of claim 51, further comprising the step of heating the liquid-cyanuric acid slurry prior to contacting with the gas.

53. The process of claim 51, wherein the liquid comprises diesel fuel.

54. The process of claim 51, wherein the liquid comprises water.

55. The process of claim 51 , wherein the liquid comprises water, wherein the step of heating the liquid-cyanuric acid slurry produces decomposition products, and wherein the decomposition products are contacted with the gas at a temperature sufficient for reduction of the nitrogen oxide in the gas.

56. The process of claim 55, wherein the decomposition products include reactive species such as free radicals.

57. The process of claim 55, wherein the decomposition products include NCO.

58. The process of claim 55, wherein the decomposition products include HNCO.

59. The process of claim 54, further comprising the step of contacting the liquid-cyanuric acid slurry to a decomposition catalyst to form decomposition products, wherein the decomposition products are contacted with the gas at a temperature sufficient for reduction of the nitrogen oxide in the gas.

60. The process of claim 59, wherein the decomposition products include reactive species such as free radicals.

61. The process of claim 59, wherein the decomposition products include NCO.

62. The process of claim 59, wherein the decomposition products include HNCO.

63. The process of claim 59, wherein the catalyst comprises zirconium, phosphorous or mixtures thereof.

64. The process of claim 59, wherein the catalyst comprises zirconium in the plus four oxidation state, phosphorous in the plus four oxidation state or mixtures thereof.

65. The process of claim 51, wherein the step of forming a liquid-cyanuric acid slurry comprises the steps of: introducing cyanuric acid to the liquid; and agitating the liquid to produce a slurry.

66. The process of claim 51, further comprising the step of oxidizing supplemental fuel, wherein the liquid-cyanuric acid slurry is contacted with the gas in the presence of reaction products produced by oxidation of the supplemental fuel.

67. The process of claim 66, wherein the supplemental fuel comprises diesel fuel, methanol, natural gas or propane.

68. The process of claim 66, wherein the supplemental fuel is in a stratified condition.

69. An apparatus for reducing nitrogen oxide in a gas, comprising: a reaction vessel; a vessel containing a liquid-cyanuric acid slurry; and supplying means for supplying the liquid- cyanuric acid slurry to the reaction vessel, wherein the nitrogen oxide in the gas is reduced.

70. The apparatus of claim 69, further comprising agitator means for agitating the liquid- cyanuric acid slurry within the vessel.

71. The apparatus of claim 69, further comprising heating means for heating the liquid-cyanuric acid slurry prior to supplying of the liquid-cyanuric acid slurry to the reaction vessel.

72. The apparatus of claim 69, wherein the liquid comprises diesel fuel.

73. The apparatus of claim 69, wherein the liquid comprises water.

74. The apparatus of claim 71, wherein the liquid comprises water, wherein the heating means heats the liquid-cyanuric acid slurry to produce decomposition products, wherein the supplying means supplies the decomposition products to the reaction vessel, and wherein the decomposition products are contacted with the gas and the nitrogen oxide in the gas is reduced.

75. The apparatus of claim 74, wherein the decomposition products include reactive species such as free radicals.

76. The apparatus of claim 74, wherein the decomposition products include NCO.

77. The apparatus of claim 74, wherein the decomposition products include HNCO.

78. The apparatus of claim 71, further comprising a decomposition catalyst coupled to the heating means, wherein the liquid-cyanuric acid slurry contacts the decomposition catalyst to produce decomposition products, wherein the supplying means supplies the decomposition products to the reaction vessel, and wherein the decomposition products are contacted with the gas and the nitrogen oxide in the gas is reduced.

79. The apparatus of claim 74, wherein the decomposition products include reactive species such as free radicals.

80. The apparatus of claim 74, wherein the decomposition products include NCO.

81. The apparatus of claim 74, wherein the decomposition products include HNCO.

82. The apparatus of claim 78, wherein the catalyst comprises zirconium, phosphorous or mixtures thereof.

83. The apparatus of claim 78, wherein the catalyst comprises zirconium in the plus four oxidation state, phosphorous in the plus four oxidation state or mixtures thereof.

84. The apparatus of claim 69, further comprising means for supplying supplemental fuel to the reaction vessel at a temperature sufficient for oxidation of the supplemental fuel, wherein the nitrogen oxide in the gas is reduced in the presence of reaction products produced by oxidation of the supplemental fuel.

85. The apparatus of claim 84, wherein the supplemental fuel comprises diesel fuel, methanol, natural gas or propane.

86. The apparatus of claim 84, wherein the supplemental fuel is in a stratified condition.