Patent Application No. 08/626,780 entitled Method for Reducing Emissions of From a Diesel Engine By Urea Injection SCR, filed April 2, 1996, by J. D. Peter- Hoblyn, James M. Valentine and Barry N. Sprague.

Technical Field The invention relates to means and methods which enable the safe and reliabie reduction of nitrogen oxides (not) emissions while permitting a diesel or other lean-burn engine to operate efficiently with low particulate emissions. The need for fuel efficiency is a long-term environmental (carbon dioxide) as well as economic issue. NOX has short term environmental consequences.

While there is known to be a tradeoff between NOX and complete combustion, diesel and other lean-burn engines produce both NOX and particulates during normal operation. However, when primary measures (actions which affect the combustion process itself, e.g., exhaust gas recirculation and engine timing adjustments) are taken to reduce one, the other is usually increased. Thus, combustion conditions selected to reduce pollution from particulates and obtain good fuel economy tend to increase NOX, Current and proposed regulations challenge manufacturers to achieve good fuel economy and reduce particulates and NOX, Lean-burn engines will be necessary to achieve The fuel economy objective.

Diesel engines operate with an excess of air to fuel, as such are "lean- burn" engines, with significant levels of oxygen in the exhaust gases.

However, with high levels of oxygen in the exhaust, catalyst systems typically used for treating exhaust from spark ignition engines (which run with close to stoichiometric balance of air and fuel) are not effective in reducing NOX generated by diesel and other lean-burn engines.

It would be desirable to employ primary means to increase fuel economy and reduce particulate generation and to utilize SCR (selective catalytic reduction) as a secondary measure (after the pollutant is generated) to reduce NOx. Unfortunately, the use of SCR has to date depended on the use of ammonia which has safety problems associated with its storage and transport. Urea has not been practical for many SCR applications - particularly mobile NOX sources - due to the difficulty in converting it from a solid or an aqueous form to its active gaseous species, typically NH1 and HNCO radicals.

There is a current need for a safe, economical and effective answer to the problems associated with urea SCR, particularly for mobile diesel and other lean-burn engines.

Background Art Selective catalytic reduction utilizing ammonia has haa some degree of success as a secondary measure for stationary sources of NOX, but would be too dangerous for mobile use.

In U. S, Patent No, 3,900,554, Richard Lyon disclosed that ammonia can be used to reduce NO, in a noncatalytic system, now termea selective noncatalytic reduction (SNOW). This process cannot achieve the 90% and above reductions that are possible with catalysts. but has been considered an improvement over SCR in situations where high reductions are not critical.

The danger of dealing with ammonia, however, remains a problem.

Unfortunately, the process is not useful for diesel engines because the temperature of diesel exhaust does not reach that (above 1600°F) necessary for SNOR. Japanese published patent application 1-318716, Dec, 1989, discloses that ammonia can be employed with a diesel engine to effect SNCR, but as a practical matter it would have to be introduced into the cylinder and load would have to be fairly constant for it to have any significant, reliable effect.

In U. S, Patent No, 4,208,386, Arand, et al., disclosed that urea, like ammonia, could be employed for SNCR systems. However, the same temperature limitation exists. And, while the noted Japanese application identifies urea as an ammonia precursor, not only is diesel exhaust

temperature too low, but no means are identified to break down the urea and assure proper mixing. The primary purpose of the noted Javanese application is to replace SCR catalysts, but it does not disclose any effective means for achieving this, Even the use of an additive with urea or ammonia, which can extend the temperature range to about 1300°F, is not adequate for diesel engines.

Where SCR caTalysts are employed to limit the NO, emissions from diesel engines, one has to deal with either the dangers of ammonia or a risk of fouling the catalysTs under most conditions. The costs which could result would be prohibitive, even if regulatory approval could be obtained, knowing that shut down would reduce projected reliability. If an SCR system were to require frequent shutdowns, it may not be considered suitable technology. In this regard, also see R, J. Hulterman: "Selective Catalytic Reduction Of NO, from Diesel Engines Using Injection Of Urea": Ph. D. thesis, September, 1995. Hulterman describes a number of technical challenges including clogging of atomizers, decomposition problems and system dynamics, The limited attempts to use urea SCR for diesel engines have required the use of large pyrolization chambers or other devices following the point of urea introduction into the exhaust, as disclosed in U. S, Patent No. 5,431,893, to Hug. Equipment of this type is bulky and expensive, and is often not practical from an engineering standpoint, especially for road transport application. Moreover, the description calls for cooling the urea conduit to guard against hydrolysis - a result contrary to what applicants now find is desired. Also, see PCT publication WO 95/518,251, by J. D. Peter-Hoblyn.

That application calls for use of a diesel particulate trap, with urea being introduced into the exhaust gases before entering the trap. That disclosure

notes that it was possible for the trap to collect the urea which had not been fully dissociated before reaching the trap and hold it there, with the particulates, until all urea was reduced to gaseous form useful in an SCR reactor.

Urea hydrolysates have been identified as alternatives to urea in several contexts. See for example, U.S. Patent No. 4,997,631 to Hofmann, et al., PCT application WO 92/02291 to von Harpe, et al., and U. S. Patent No.

5,139,754, Hofmann, Sun and Luftglass, Also see U. S. Patent No. 5,281,403 to Jones and JP HEI 2-191,528 to Ebina. Each of these requires the use of added hydrolysis equipment and ends up producing ammonia to some extent. The disclosures of these documents are incorporated by reference. On-board storage of hydrolysates would also be undesirable.

The art as it now stands continues to look at ammonia as the most suitable chemical for SCR processes - just as it has for the past several decades - whether in gaseous form as traditionally employed or as an aqueous solution prepared by the complete hydrolysis of urea. The art so far has failed to meet the need for a system for effectively supplying urea, as an alternative to ammonia, to an SCR catalyst.

The storage and handling of ammonia in the manner of the prior art - whether as a gas, aqueous solution or a hydrolysate - is not only expensive, it does not eliminate the possibility of leakage and the associated health and safety problems. The art is awaiting the development of a process which would permit the use of urea in an SCR process simply, reliably, economically and safely for both man and catalyst.

Disclosure of Invention It is an object of the invention to provide a safe, reliable SCR system for a mobile diesel or other lean-burn engine.

It is another object of the invention to eliminate the safety problems associated with the storage and handling of ammonia for mobile uses.

It is another object of the invention to enable the use of urea with protection for SCR catalysts over a wide temperature window without causing deposits on the catalyst which would reduce its effectiveness and increase back pressure on the engine.

It is another object of the invention to enable the use of urea which enables virtually immediate gasification to avoid wetting of or solids deposition on the catalyst, It is a further object of the invention to enable the use of urea or like reagent in an SCR system in a manner that provides good mixing of the reagent with the exhaust gases.

It is yet another, more specific object of the invention to allow optimization of fuel consumption and particulate generation within a diesel or other lean-burn engine while reducing exhaust NOx concentrations by the use of a urea-SCR system, It is a yet further object of the invention to enable introduction under pressure of discrete, metered charges of urea or like reagent into the exhaust gases.

It is a yet further and more specific object of the invention to enable introduction under pressure of discrete, metered charges of urea or like reagent into the exhaust gases, without the need for a gaseous propellant.

It is still another object of the invention to enable the use of urea or like reagent in an SCR system in a manner which provides self-cleaning of injector means.

It is yet another specific object of the invention to provide a simple mecnanical aevice for accomplishing tne above objects and preferably to enable close coupling of the reagent injection means and the SCR catalyst.

These and other cbjects are achieved by the present invention which provides an improved process and apparatus for NOX reduction. The process, in one of its aspects, comprises: heating and pressurizing an aqueous solution of nitrogenous NOx-reducing reagent, and injecting heated and pressurized NOx-reducing reagent into exhaust gas at an exhaust gas temperature of from 200 to 650"C, upstream of an SCR reactor. Injectors of the pintle type are preferred.

Preferably a plurality of injectors are operated to inject the solution and are positioned in radial, skewed and/or longitudinally-spaced orientation on the exhaust pipe. A catalyst can be provided in the conduit for the heated reagent to facilitate hydrolysis. The heat can be supplied by heat exchange with the exhaust and/or by an auxiliary means, In one embodiment of the invention, a mixing device is provided to assure mixing and/or prevent contact of any ungasified reagent with the SCR reactor, Also, an oxidation catalyst can be provided downstream of the SCR reactor for the purpose of

eliminating ammonia or other gaseous byproducts which might otherwise pass through the system.

Brief Description of the Drawings The invention will be better understood and its advantages more apparent from the following detailed description, especially when read in light of the accompanying drawings, wherein: Figure 1 is a schematic representation of one embodiment of the invention; Figure 2 is a representation of one arrangement of injectors on an exhaust conduit in close-coupled relation to an SCR catalyst, partially cut away to show a the spray pattern of the injectors; Figure 3 is a schematic representation, similar to that of Figure 1, but with the provision of a control system and an oxidation catalyst in the exhaust system to prevent inadvertent release of large amounts of ammonia or other by-product gases; and Figure 4 is a cross-section taken along line 4-4 in Figure 2 showing a spray pattern achieved by a skewed injector arrangement.

Detailed Description of a Preferred Embodiment In this description, the term "lean-burn engine" is meant to include engines that can be operated with at least a 1% excess of. The term "engine is meant in the broad sense to include all combustors which combust fuel to provide heat, e.g., for direct or indirect conversion to mechanical or electrical energy. Internal combustion engines of the Otto, Diesel and

turbine types, as well as burners and furnaces, are included and can benefit from the invention. However, since the problems and advantages of successful achievement of reliable NOy reduction on diesel engines are so pronounced, the diesel engine is used throughout this description for purposes of example. Stationary and mobile engines are contemplated.

The term "Diesel engine" is meant to include all compression-ignition engines, for both mobile (including marine) and stationary power plants and of the two-stroke per cycle, four-stroke per cycle and rotary types.

The term "hydrocarbon fuels is meant to include all of those fuels which form emulsions with aqueous NOx-reducing reagents such as urea, either with or without an added emulsifier. Gasoline, jet fuel, diesel fuel, and various other distillate fuels are included.

The term "distillate fuel" means all of those products prepared by the distillation of petroleum or petroleum fractions and residues.

The term ' petroieum" is meant in its usual sense to include all of those materials regardless of source normally included within the meaning of the term, including hydrocarbon materials, regardless of viscosity, that are recovered from fossil fuels.

The term "diesel fuel" means "distillate fuels" including diesel fuels meeting the ASTM definition for diesel fuels or others even though they are not wholly comprised of distillates and can comprise alcohols, ethers, organo-nitro compounds and the like (e.g., methanol, ethanol, diethyl ether, methyl ethyl ether, nitromethane). Also within the scope of this invention, are emulsions and liquid fuels derived from vegetable or mineral sources such as

corn, alfalfa, shale, and coal. These fuels may also contain other additives known to those skilled in the art, including dyes, cetane improvers, anti-oxidants such as 2,6-di-tertisry-butyl-4-methylphenol, corrosion inhibitors, rust inhibitors such as alkylated succinic acids and anhydrides, bacteriostatic agents, gum inhibitors, metal deactivators, upper cylinder lubricants, antiicing agents and the like.

The term "nitrogenous NOx-reducing reagent" is broad enough to include all of those nitrogenous materials known to reduce NOX in the presence of a NO reducing catalyst. Among the prererrea suitable NOy- reducing reagents are those that are normally liquid or solid at temperatures up to 50"C and do not readily hydrolyze to ammonia or other gaseous species under the conditions of storage, but can be broken down (with a catalyst if necessary) into the effective gaseous species (e.g., NH and HCNO radicals) at temperatures practical for diesel engines. e.g., from about 200 to about 650"C, including the following: urea, ammonium carbamate and the alkali metal and alkaline earth carbamate salts; urea hydrolysis products, including ammonium carbonate and ammonium biccrsonste; urea dimers and polymers, such as biuret; urea adducts and urea condensation products; amines, such as melamine, triethyl amine and ethanol amine; and the like.

The term "nitrogenous NOx-reducing reagent" applies to not only a reagent to which heat and pressure are applied according to the invention, but to the breakdown products of that reagent. It is desired that the NOX- reducing reagent not readily hydrolyze to ammonia or other gases under the conditions of storage, e.g., temperatures of -30 to 50"C, because the presence of gaseous species requires pressure storage, and it is not desirable to store fuel under pressure. See U. S. Patents No. 5,057,293 and 5,489,419 for

a more complete listing of effective nitrogenous NO,-reducing reagents, the disclosures of which are hereby incorporated by reference in their entireties.

The aqueous solutions of these reagents can be employed up to the solubility limits of the particular reagent. Typically, in the case of urea, the aqueous solution of nitrogenous NOx-reducing reagent will contain from about 5 to about 65% reagent based on the weight of the solution. A narrower range is from about 25 to about 40%, e.g. about 35%. It is an advantage of the invention that hydrolysis of the reagent can begin under pressure, but the pressure can maintain the reagent in aqueous solution (liquid form) for effective metering by the injectors.

Reference is made to Figure 1 which illustrates in schematic form, one embodiment of the invention wherein the exhaust from a diesel engine 10 is treated to reduce NOx. The engine is equipped with an exhaust system having an exhaust passage, such as 20, leading to a catalytic reactor, such as SCR unit 30, effective for selective catalytic NOX reduction. The invention enables utilization of aqueous solutions of urea and like nitrogenous NOX- reducing reagents in place of ammonia for SCR NO reduction in a manner which avoids wetting or forming solid deposits on the catalyst. These advantages are achieved by pressurizing and heating the reagent such that the aqueous solution is flashed off substantially immediately upon injection and the urea in the reagent has been at least partially hydroiyzed prior to injecting the solution, which will then comprise the pro.ducts of the at least partial hydrolysis, into the exhaust system.

The invention enables the gasification of the reagent with no significant residence time in the exhaust gases and permits close-coupling of a reagent injector and an SCR catalyst (e.g., less than 1 meter, and preferably from about 0.05 to about 0.5 meters). The invention also provides

the ability to inject the reagent in discrete charges to facilitate high energy mixing and accurate dosage. Because the injection is done in a manner effective for causing immediate gasification of the fluid fed to the injector, and mixing of it with the exhaust gases. These features of the invention make it fully compatible with other emissions control and fuel economy technologies such as particulate traps, pass-through exhaust catalysts for reducing hydrocarbons, fuel catalysts for improving the operation of these devices and/or improving fuel economy, fuel emulsions, exhaust gas recirculation, engine timing modifications, and the like. In fact, SCR for NOX control in accordance with the Invention will enable Tne optimization of other technologies to control other emissions, Figure 1 shows a diesel engine 10 having an exhaust manifold/passage 12 directing the exhaust from the engine to an exhaust system 20 including a NOx-reducing SCR catalyst 30 before discharge of the combustion gases to the atmosphere. The diesel engine is supplied with fuel from tank 40 via line 42 and fuel injectors 44, 44',44", and 44"' The fuel tank includes diesel fuel and can be emulsified with water and/or an oxygenated hydrocarbon and/or contain a platinum group metal catalyst composition and/or an auxiliary catalyst composition as will be explained later. Combustion air from line 14 enters turbine 16, and is introduced into the cylinders of the diesel engine and compressed in normal fashion for a diesel engine within each cylinder.

For modern, high-performance diesel engines it is typical to employ a turbine to pressurize the combustion air and to utilize the energy from the exhaust gas to pressurize the combustion air prior to introduction into the cylinders. Turbine 16 is driven by turbine 17 positioned in exhaust line 12. The

diesel fuel is injected into the cylinders where it ignites in the presence of the air which has been heated due to compression within the cylinders, The arrangement of Figure 1 enables reducing the emissions of NOX from a diesel engine by causing essentially immediate gasification of the reagent upon injection. This facilitates a rapid response time necessary for good control and permits feeding reagent on an as-needed basis. An aqueous urea solution is introduced from tank 50, through line 52 into exhaust passage 12 by pump 54. Line 52 is shown to include a heater 56. Heating can also oe achieved by providing good neat exchange contact (either direct or indirect) with the exhaust system or the engine. For example, the line 52 can be wrapped around the exhaust conduit 30. The heating means 54 can be used as the sole or a supplementary heater, It is desired to heat the solution to at least 100"C, and preferably to within the range of from about 200 to about 500°C. Diesel exhaust can be employed as the source of heat where the exhaust gas temperatures are more than 300"C.

The pressure required for the aqueous solution of reagent in line 52 can be provided by a separate pump. The pressure will be sufficient, at least, to maintain the reagent solution in the liquid phase for accurate metering.

Typically, pressures of 200 to 2000 psi can be utilized and are well within the capabilities of the type of lines, pumps and injectors utilized for fuel injection.

The use of pressure along with the heat has the added advantage of facilitating the hydrolysis of the urea or other reagent in a single phase system without requiring storage means because it can be done on an as needed basis. As noted, a catalyst can be employed in line 52 for aiding the hydrolysis of the urea. Injection of discrete charges of reagent facilitates accurate dosing and the use of injectors that resist fouling.

It is preferred, but not absolutely essential that the aqueous solution be maintained as a single phase and that bubbles be prevented from forming in the line 52. Exceptions can be made depending on the type of pump and injectors employed. Constant-pressure, solenoid-operated injectors can be operated at rates as high as 50 and even 100 cycles per second and the frequency or the mark space ratio (on-off pulse width ratio) can be varied to control the injection rate. Injectors that lift to begin injection at a particular threshold pressure can be fed by a positive displacement pump or via solenoid actuated valves.

Optionally, a static mixer 32 (or an uncatalyzed support or a diesel particulate trap) can be positioned between the gasification chamber and the NOx-reduction catalyst. It is, however, an advantage of the invention that the immediate gasification of the reagent solution and the high energy of introduction, permit close coupling of the injectors and the catalyst. It is also an advantage of the invention that engine designers can focus on fuel economy and low particulate emissions while relying on the SCR of the invention to control NOx The arrangement illustrated in Figure 1 shows injector 60 to be located just upstream of the outlet vanes 17 of a turbocharger. This is one of the preferred orientations, another being centrally within exhaust passage 12 (not shown). Figure 3 shows injection following the turbocharger. The injector means or the line 52 may contain one or more catalysts capable of aiding hydrolysis of the urea.

Figure 2 shows. partially cut away, one arrangement of a plurality of injectors. Injectors of the pintle type are preferred. These are well known for use in diesel engines to supply. A pintle-type injector has a plunger with a

projection at the terminal end which extends into a single injector opening in the closed position. This has advantages for control and to help maintain the injector hole free and open. Figure 2 shows a plurality of injectors 60 being fed aqueous reagent solution from a ring-shaped manifold 62, which in turn is fed from line 52 and heater 56. The individual injectors are shown threaded into mounting ring 64 and can be operated to simultaneously or sequentially inject the solution. They are shown oriented radially, skewed slightly (such as at an angle a, e.g., up to 60° or so) and are spaced radially around the exhaust pipe 20 in close-coupled relation to the catalyst 30.

Figure 4 is a cross-secTion taken along line 4-4 in Figure 2 showing a spray pattern achieved by a skewed injector arrangement. The skewed orientation can enhance mixing which, due to the high energy introduction offered by the invention without the use of air or other propellant, is already very good. They can also be in longitudinally-spaced orientation on the exhaust pipe.

Figure 3 illustrates a control system of a type useful to maintain the proper level of reagent introduction (i.e. dosage). The controller can also time the injections to occur at staggered times in a predetermined sequence designed to smooth out the rate of introduction despite the use of pulsed injectors. It is an advantage of the invention that the introduction of the reagent in discrete charges by injection, facilitates control of dosage in response to feed-forward control, with trim as to feedback parameters if desired. It is an advantage of the invention that the ability to closely control reagent dosing, facilitates the use of controllers with adaptive learning capabilities. It is also an advantage that the usually-occurring spikes or discontinuities in NOX levels can be better tracked with injection of reagent in the proper concentrations.

The aqueous reagent solution can be fed into the exhaust in response to a teed-forward controller in response to a number of measured parameters, including: fuel flow, throttle setting, engine speed, rack setting, intake air temperature, barometric pressure, intake air humidity, exhaust gas temperature and/or other parameters effective for particular engines. In addition, to the extent that sensors are available, trim or feed back control can be provided based on residual gas species following the catalyst, e.g., the level of NOx, HC or CO. For example, reference to Figure 3 shows a control system including flow meter 72 which can sense the fuel flow and generate an operation signal representative of fuel flow. Sensors are also shown to determine gas species in the exhaust (76) and the temperature of the exhaust (78) prior to the catalyst 30. The operation signal representative of fuel flow, exhaust gas temperature and residual gas species are received by a controller 74 and compared to stored values, The controller can then generate one or more control signals based on the comparisons. The control signal(s) is then sent to metering pump 54 or other suitable device for metering the correct amount of urea to line 52 or alternatively to injector nozzle controllers. If desired, feedback control can be employed to trim the system in response to residual levels of ammonia, other gas species, or any other measurable engine or exhaust gas property.

Figure 3 also shows a catalyst 34 downstream of the SCR catalyst 30 for the purpose of eliminating ammonia which might otherwise pass through the system and provide an objectionable odor. Among the catalysts suitable for this purpose are oxidation catalysts, which can have the added advantage of reducing hydrocarbons or particulate species which might remain in the exhaust. If desired, the SCR catalyst can be preceded by an in-line mixing device 32, or an uncatalyzed support material or other trap to pick up any ungasified water or reagent or particulate materials. The uncatalyzed

support material, if employed, can have a volume of from about 5 to about 50% of that of the SCR catalyst. The system of the invention is fully compatible with other emission control and fuel economy technologies such as particulate traps, pass-through exhaust catalysts for reducing hydrocarbons, particulates and carbon monoxide, fuel catalysts for improving the operation of these devices and/or improving fuel economy, fuel emulsions, exhaust gas recirculation, engine timing modifications, and the like.

The active species formed by the hydrolysis and gasification of the urea or other reagent are introduced into the exhaust gases in an amount sufficient to provide the degree of NOX reduction desired. The desired amount can be dictated by regulation, engine design requirements or other criteria. Typically, a molar ratio of the active species to the baseline nitrogen oxides level (by which is meant the pre-treatment level of NOX in the effluent) of at least about 0.3:1 will be employed. More narrowly, the reagent is supplied to provide a molar ratio of active species to baseline nitrogen oxides of about 0.5:1 to about 1:1. The reagent levels or target NOx concentrations in the exhaust can be preprogrammed into the controller 74 based on tested values for given fuel flows and related parameters, or sensors and related controls can be provided to provide real-time readouts. A sensor means might be provided to correct preprogrammed values by feedback control.

According to one embodiment of the invention, the fuel and the aqueous reagent are supplied to the fuel storage tank associated with the engine as an emulsion. Prior to introduction into the engine, the fuel and aqueous solution can be separated into their component parts by fuel water separators of the type known in the art for separating small amounts of water from diesel fuel. It will simply be necessary to use a suitable device, e.g., a filter or centrifuge, sized for the volumes of water employed. See copending,

commonly assigned U. S. Patent Application No. ~~~~~~ (attorney's docket number 2937-P1020A), filed November 19 1996 by J. D. Peter-Hoblyn, J. M.

Valentine and Theodore Tarabulski, the disclosure of which is incorporated by reference.

Among the catalysts suitable for aiding the hydrolysis of the urea (i.e., the hydrolysis catalysts) are ones which comprise a material selected from the group consisting of phosphoric acid and acid phosphates, alkali metal hydroxides and carbonates, such as sodium hydroxide, potassium hydroxide, sodium carbonaTe, potassium carbonate, alkali metal silicates, alkaline earth metal hydroxides and oxides, aluminum hydroxide and oxides, and mixtures of two or more of these. See also U.S. Patent No 4,997,631 to Hofmann, et al., PCT application WO 92/02291 to von Harpe, et al., U. S. Patent No.

5,139,754, Hofmann, Sun and Luftglass, U. S. Patent No. 5,281,403 to Jones and JP HEI 2-191,528 to Ebina for a further listing of catalysts and techniques.

Catalysts which comprise water-soluble materials can be added to or blended with the urea in tank 50 or otherwise prior to being introduced into the line 52 or other suitable conduit.

An important effect of injecting heated aqueous reagent solutions under high pressure gasification is an essentially immediate gasification and breakdown of any remaining urea hydrolysis products and remaining urea as active species such that there is a greatly-reduced risk of catalyst wetting, fouling or inactivation. The evaporation of the water and the gasification of the reagent solution upon introduction into the gasification chamber have the advantage that no atomizing air is required and no droplets of water will contact the catalyst - thereby greatly diminishing the threat of reagent deposition and/or catalyst deactivation. As a precaution against the possibility that some urea will not be hydrolyzed and might reach the

catalyst, a device 32 can be provided to catch and, if desired, break down the residues with a pyrolysis catalyst, such as in the form of a coating in the device 32, so that they do not enter the SCR catalyst.

The urea is typically supplied as an aqueous solution containing from 25 to 50% urea by weight. It can be stored in tank 50 in this form or the urea can be stored dry in a canister, with water passed through as needed to prepare a solution which is near saturation (to minimize water storage and use) or to any concentration suitable for the vehicle. It will be desired in many circumstances to provide heaters for the water and/or urea solution storage to prevent freezing or to reduce reaction time in the gasification chamber. Likewise, it may be useful to employ antifreeze materials.

The SCR catalyst used is one 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.

Among the useful SCR catalysts are those described in the representative prior art processes below. Selective catalytic reduction processes for reducing NOX 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. Patent 4,138,469 and Henke in U.S. Patent 4,393,031 disclose the catalytic reduction of NOX 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.

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 awl. in U.S.

Patent 4,107.272 discuss the catalytic reduction of NOX using oxysulfur, sulfate, or sulfite compounds of vanadium, chromium, manganese, iron, copper, and nickel with the addition of ammonia gas.

In a multi-phased catalytic system, Ginger, in U.S. Patent 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.

The effluent containing the gasified reagent is most preferably passed over the SCR catalyst while the effluent is at a temperature between about 100"C and about 500"C, preferably at least 300"C. In this manner, the active species present in the effluent due to hydrolysis and gasification of the reagent solution most effectively facilitates the catalytic reduction of nitrogen oxides. The effluent will preferably contain an excess of oxygen. 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.

Because the invention is compatible with other emission-reducing and fuel economy technologies, a number of hybrid processes become available to the engine designer, vehicle producer and retrofit market. For example, the fuel can be catalyzed with a suitable platinum group metal additive and/or auxiliary catalyst composition selected from the group consisting of compounds of sodium, lithium, potassium, calcium, magnesium, cerium, iron, copper, manganese, and mixtures. Among the compounds are any of those disclosed for example in prior U.S. Patent Nos. 4,892.562 and 4,891,050 to Bowers and Sprague. 5,034,020 to Epperly and Sprague, 5,215,652 to Epperly, Sprague, Kelso and Bowers, and 5,266,083 to Peter-Hoblyn, Epperly, Kelso and Sprague. WO 90/07561 to Epperly, Sprague, Kelso and Bowers, and U. S. Patent Application Serial No. 08/597, filed January 31, 1996, by Peter- Hoblyn, Valentine and Sprague, hereby incorporated by reference. Where the application permits. a blend of these compounds can be used with one or more other platinum group metal compounds such as soaps, acetyl acetonates, alcoholates, t3-diketonates. and sulfonates, e.g., of the type which will be described in more detail below.

The platinum group metal catalyst and/or other catalyst can be added in any manner effective for its intended purpose, such as by adding it to the fuel in bulk storage, to the fuel in a tank associated with the engine, or by continuous or intermittent addition, such as by a suitable metering device, into: the fuel line leading to the engine, or in the form of a vapor, gas or aerosol into the air intake, the exhaust gases before the trap, exhaust gases after the trap but before recirculation to the engine. or a mixing chamber or equivalent means wherein the exhaust gases are mixed with incoming air.

When employed, particularly in combination with particulate traps, platinum group metal catalyst compositions are preferably employed at concentrations of less than 1 part by weight of platinum group metal per million parts by volume fuel (ppm). For the purposes of this description, all "parts per million" figures are on a weight to volume basis, i.e., grams/miiiion cubic centimeters (which can also be expressed as milligrams/liter), and percentages are given by weight, unless otherwise indicated. Auxiliary catalysts are employed at levels effective for their intended purpose, preferably at levels of from 1 to 100 ppm of the fuel utilized, e.g., 10 to 60 ppm.

The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the present invention, and it is not intended to detail all of those obvious modifications and variations of it which will become apparent to the skilled worker upon reading this description. It is intended, however, that all such obvious modifications and variations be included within the scope of the present invention which is defined by the following claims. The claims cover the indicated components and steps in all arrangements and sequences which are effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.