Emission control system having a coated mixer for an internal combustion engine and method of use

An emission control system capable of remediating an exhaust from an internal combustion engine and having an exhaust passage, an oxidation catalyst, a reducing agent and a source of the reducing agent cooperating with an aperture in the exhaust passage. The aperture is disposed downstream of the oxidation catalyst. The system also includes a mixer arranged within the exhaust passage downstream of the aperture. The mixer includes a plurality of mixing elements. The mixer also includes a coating capable of hydrolyzing the reducing agent. At least a portion of the mixing elements which have a thermal conductivity greater than 8 W/m/° K. The system includes a selective catalytic reduction (SCR) catalyst disposed downstream from the mixer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One or more embodiments of this invention relate to an engine emission control system having a coated mixer and its method of use.

2. Background Art

Environmental regulations pertaining to the reduction of emissions from vehicular engines have been active in many countries. The oxides of nitrogen are of concern regarding emissions and include several compounds such as nitric oxide and nitrogen dioxide. These compounds are frequently referred to as NOxas defined by the United States Environmental Protection Agency.

NOxemissions from certain engines may be treated using a selective catalytic reduction (SCR) catalyst. The SCR system uses reducing agents, such as urea to yield satisfactory NOxconversion performance. Some recently designed delivery systems for the reducing agent involve delivery of a gas to the SCR.

In some systems the gas is generated by vaporizing a liquid reducing agent. Under these types of systems, vaporization of the liquid reducing agent to a gas can sometimes be incomplete. As a result, urea droplets may reach the face of an SCR catalyst. These droplets could lead to deposits of melamine and other solids resulting in diminished SCR performance. In addition, when the droplets or other materials are not evaporated and decomposed prior to contact with the SCR catalyst, part of the SCR catalyst must be used for this decomposition by hydrolysis. As a result, the SCR catalyst may be required to be relatively larger to achieve the same level of NOxconversion than if the droplets were not present.

What is needed is a system for thoroughly vaporizing and mixing the exhaust and the liquid reducing agent. The system should provide good effect during cold start engine NOxconversion operations while avoiding detrimental increases in back pressure on the emission control system.

SUMMARY OF THE INVENTION

An emission control system for remediating an exhaust from an internal combustion engine of a vehicle, having an exhaust passage for transporting exhaust gases form the engine. The system also includes an oxidation catalyst coupled to a portion of the exhaust passage and disposed downstream of the engine. The system has a source of a reducing agent cooperating with an aperture in the exhaust passage. The aperture is disposed downstream of the oxidation catalyst. A mixer is arranged within the exhaust passage downstream of the aperture. The mixer includes a plurality of mixing elements, at least a portion of which have a thermal conductivity greater than 8 W/m/° K when measured according to ASTM E 1225-04. In addition, there is a selective catalytic reduction (SCR) catalyst coupled to a wall of the exhaust passage and disposed downstream of the mixer. The mixer includes a coating disposed on at least a portion of the mixer. The coating is capable of hydrolyzing the reducing agent.

Another embodiment of the invention includes a metallic mixer having at least one mixing element. A coating is disposed on at least a portion of the mixing element. The coating includes a Lewis acid site. The coated mixing element has a thermal conductivity greater than 8 W/m/° K when measured according to ASTM E 1225-04.

Another embodiment of the invention includes a method for using a coated static mixer for an engine exhaust in an exhaust remediation system connected to an internal combustion engine. The method includes exposing a mixture of gaseous and liquid reducing agent from a source of a reducing agent. The mixer includes a mixing element having a coating capable of hydrolyzing the reducing agent. The coated mixing element has a thermal conductivity greater than 8 W/m/° K when measured according to ASTM E 1225-04. The source is disposed upstream of the inlet relative to the engine. The method also includes receiving the engine exhaust at the inlet to the mixer and transferring heat to the mixer from the engine exhaust. The liquid reducing agent is evaporated at the mixer and blended turbulently with the exhaust using the mixer element. The coating decomposes the reducing agent. The decomposed reducing agent and exhaust are released through an inlet of the mixer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Detailed embodiments of the present invention are disclosed herein. However, it should be understood that the disclosed embodiments are merely exemplary of the present invention, which may be embodied in various alternative forms. Moreover, the figures are not necessarily to scale, some features may be exaggerated or minimized to show details of particular components. Therefore, specific details disclosed herein are not to be interpreted as limiting, rather merely as a representative basis for the claims and/or for teaching one skilled in the art to variously employ the present invention.

Referring now toFIG. 1, an exemplary emission control system10is illustrated in a non-limiting view. Emission control system10receives an exhaust14from an engine12. Exhaust14enters the emission control system10at an intake16adjacent to the engine12. The exhaust14travels in an exhaust passage of pipe18having a longitudinal axis, a portion of pipe18connects intake16with an oxidation catalyst20. The oxidation catalyst20is part of the emission control system10which further includes a urea-based Selective Catalytic Reduction (SCR)22catalyst which is coupled downstream of the oxidation catalyst20. In the illustrated embodiment, the emission control system10further includes a diesel particulate filter24downstream of the SCR catalyst22. A reductant26, such as a reducing agent like aqueous urea, is stored in a storage vessel28and delivered to a reductant delivery system30by a conduit32. The delivery system30is coupled to a portion of the exhaust pipe18through an aperture upstream of the SCR catalyst22. The reductant26is metered out by the delivery system30, which comprises a pump34and a control valve36, where both the pump34and the valve36are controlled by a controller38.

In at least one embodiment, a coated mixer42is disposed intermediately between the control valve36and the SCR22catalyst. The coated mixer42hydrolyzes at least some of the reductant26and permits vaporization of residual droplets of the reductant26. Air and reductant26are injected into the exhaust pipe18as a fan spray40with at least some of the reductant26being vaporized and dispersed by the coated mixer42. The air and the reductant26form a resulting vapor which is then introduced into the exhaust gas14as a mixture and enters coated mixer42. Alternatively, any other means known to those skilled in the art to deliver reductant26to an exhaust gas after-treatment device may be used.

The emission control system10may be arranged on the underside of a vehicle chassis in any suitable manner. It should be appreciated that the exhaust passage18may include one or more bends or curves to accommodate a particular vehicle arrangement. Further still, it should be appreciated that in some embodiments, the emission control system10may include additional components not illustrated inFIG. 1or may omit certain components described herein. Elements of the emission control system10, particularly those in contact with the exhaust passage18may be in fluid communication with other portions of the emission control system10. Furthermore, it should be appreciated that the emission control system10may include two or more units of elements such as catalysts or control valves without departing from the spirit of the invention.

Any suitable oxidation catalyst20may be employed, such as a diesel oxidation catalyst. In at least one embodiment, the oxidation catalyst20is a precious metal catalyst, preferably one containing platinum, for conversion of hydrocarbons (HC), carbon monoxide (CO) and nitric oxide (NO) in exhaust14from engine12. The oxidation catalyst20may also be used to supply heat for fast warm up of the SCR22catalyst and the coated mixer42. The heat supply may be done, in part, by increasing the HC concentration in the exhaust gas14entering the oxidation catalyst20. In oxidation catalyst20, an exotherm is created when extra HC is reduced over the oxidation catalyst20. This can be accomplished through, for example, in-cylinder injection during either or both of a power or exhaust stroke of the engine (in a direct injection engine) or any of a number of other alternatives such as retarding injection timing, increasing exhaust gas recycling (EGR) and intake throttling, or any other means known to those skilled in the art to increase the HC concentration in the exhaust gas14. Alternatively, hydrocarbons may be injected directly into the exhaust gas14entering the oxidation catalyst20by employing any means known to those skilled in the art. In certain embodiments, HC from a fuel tank or a storage vessel may be delivered to oxidation catalyst20to generate extra heat to warm up SCR22catalyst and coated mixer42.

In at least one embodiment, the diesel particulate filter (DPF)24is coupled downstream of SCR22catalyst and is used to trap particulate matter (soot) generated during the drive cycle of the vehicle. Any suitable DPF24can be used. DPF24can be manufactured from a variety of materials including cordierite, silicon carbide, and other high temperature oxide ceramics.

The reductant26is intended to reduce NOxgenerated during combustion in engine12. Any suitable reductant can be used. As a non-limiting example, reductant26is a liquid reducing agent such as ammonium carbamate solution or urea. It is understood that other reducing agents known in the art may be used without departing from the spirit of this invention. Further, additives may be incorporated in the reducing agent, for example, to inhibit freezing during cold weather, without departing from the spirit of this invention.

The SCR catalyst22is intended to catalyze the reduction of nitrogen oxides with ammonia or other reductant26to form nitrogen and possibly water and/or other byproducts. Disposing the SCR22catalyst downstream allows the adsorption of ammonia, when used, and subsequent reaction with any NOxthat slips through the upstream system. This leads to net NOxconversion (NH3+NO→N2).

Any suitable SCR22catalyst can be used. In at least one embodiment, the SCR22catalyst includes an SCR catalyst composition, preferably, a base metal/zeolite and/or transition metal/zeolite formulation with optimum NOxconversion performance in the range of 200-500° C. SCR22catalyst composition may also include a catalyst support composition. Such support composition can provide a mechanism for holding and dispersing the active components of SCR22catalyst composition. Further, the support compositions may increase the surface area for adsorption of species being altered by action of the SCR22catalyst. Suitable supports include but are not limited to Al203, SiO2, TiO2, ZrO2, CeO2and combinations thereof. The support composition may be delivered to the SCR22catalyst composition via a molecular sieve, a sol-gel, or other process known to one of ordinary skill in the art. Further, the support composition can be provided as a powder and mixed with other components of the SCR catalyst composition in forming a slurry mixture to be applied by a washcoating process.

The SCR catalyst composition can also include a metal or metal-containing compound. Suitable non-limiting examples of metals include those from groups of metals known as the noble metal group (Ru, Rh, Pd, Os, Ir and Pt) and certain metals (Fe, Cu and Ag) from the group known as the late transition metal group.

The SCR catalyst composition may also include a composition of a cerium-containing oxide compound and a non-cerium-containing metal compound selected from the group consisting of alkali metal containing compounds, alkali-earth metal containing compounds, and combinations thereof. Suitable cerium-containing oxides include mixed oxide selected from the group consisting of Ce/Zr oxide, Ce/Pr oxide, Ce/Pr/Zr oxide, and combinations thereof.

The SCR catalyst composition may also include metal ions of a metal or metal-containing compound bonded to a conjugate base of an inorganic acid. The metal ions that are bonded to the conjugate base can be selected from the group consisting of alkali metal ions, alkali-earth metal ions, and combinations thereof. Suitable metal ions include, for example, ions derived from barium, lithium, sodium, potassium, cesium, magnesium, calcium, strontium and combinations thereof. Typically, the metal or metal-containing compound will alter the amount of chemical components in exhaust14(e.g., the NO, CO, SO2, hydrocarbons, etc.). Moreover, the conjugate base of the inorganic acid can be a conjugate base oxide of an inorganic acid. As used herein in at least one embodiment of the present invention, “conjugate base” means the ion formed when an acid loses one or more hydrogen ions (i.e., H+). As used herein in at least one embodiment of the present invention, “conjugate base oxide” means a conjugate base that has one or more bonds between oxygen and another element.

In at least one embodiment, the inorganic acid for which the base oxide is conjugate has a Kasuch that SCR catalyst composition provides enhanced ammonia adsorption and/or increases the number of acid sites on SCR22catalyst for absorbing ammonia. Typically, this organic acid has a Kavalue from 5.0×10−7to about 1. Another variation of inorganic acid for which the base oxide is conjugate has a Kavalue from about 5.0×10−5to about 1.0×10−1.

SCR22catalyst may be able to obtain good catalytic activity under relatively high temperature conditions of use, for example, 400° C. or higher. The SCR process may not be as efficient at lower temperatures. In addition, rapidly changing air to fuel ratios may mean that there is an excess of ammonia after the SCR process. As a consequence, it may be preferred to have the diesel particulate filter24downstream of the SCR22catalyst to oxidize ammonia that escapes from the SCR process.

Referring now toFIG. 2, a schematic cross-sectional view of the coated mixer42is illustrated along axis2-2ofFIG. 1. A non-limiting example of the mixer42is a static mixer. Static mixers are known to those skilled in the art. Static mixers are understood by those skilled in the art to be devices where fluid media are forced to mix themselves through a progression of divisions and recombinations. Typically, there are 2nlayerings of fluid media per n elements of mixing. Such devices are beneficial in harsh environments like the emission control system10because static mixers, typically, require no moving parts and, accordingly, maintenance and operating costs typically are extremely low. The energy for mixing, in the case of bringing a gaseous outflow into contact with the static mixer surfaces, is provided by the pressure under which the fluid flows through the static mixer. Typically, the pressure drop across the static mixer is low. It is understood that there may be a plurality of mixers disposed either longitudinally, laterally or both longitudinally and laterally.

The coated mixer42includes a peripheral wall50. A metallic honeycomb52connects to the interior side of peripheral wall50. Metallic honeycomb52includes a mixing element54. Mixing element54may include a flap or deflecting element for redirecting exhaust gas14(FIG. 1) and any spray40ultimately towards the inlet face of SCR22. Mixing element54increases the probability of the breakup of droplets of liquid spray as the droplets impact mixing element54. Mixing element54further assists to disperse the mixture of exhaust gas14and reductant26relatively uniformly across the inlet face of SCR22catalyst thereby increasing the quantity of SCR22usable to remediate NOxand avoiding saturation of any particular section by exhaust14and reductant26. These two factors may increase the relative efficiency of SCR22, thereby providing a more robust remediation system and/or providing an opportunity for cost savings. It is understood that mixer42may have one or more mixing elements54. A non-limiting example of the disposition of mixing element54includes multiple tabs inwardly directed from peripheral wall50. The tabs are longitudinally and radially offset from each other both radially and longitudinally relative to the longitudinal axis of the mixer.

It is understood that there are many types of turbulent static mixers that may be used without violating the spirit of the invention. Typically, mixer42is a gas-liquid static mixer having turbulent and/or radial flow characteristics. Turbulent flow for certain embodiments of mixer42has a Reynolds number greater than about 2300. Other Reynolds numbers may be appropriate for alternative mixer setups, such as 2000 or 3000. Non-limiting examples of gas-liquid static mixers include low pressure drop mixers, low-low pressure drop mixers, jet mixers, interfacial surface generator mixers, HEV, and SMX turbulent static mixers.

It is understood that the mixing elements54cannot completely block exhaust pipe18and must permit exhaust14to communicate between oxidation catalyst20and SCR22catalyst. Typically, it is desirable to increase overall back pressure of the emission control system10by less than 3%.

Referring now toFIG. 3, a longitudinal cross-sectional view transverse to axis3-3ofFIG. 1is schematically illustrated according to at least one embodiment of this invention. Peripheral wall50supports mixing element64. Mixing element64in this non-limiting example is a helical mixer56having at least one surface that is rotated at least 90°. Additional configurations of helical mixers may have rotations of at least 180°, 270°, 360° or 540° along a longitudinal axis without deviating from the spirit of the invention. However, it is to be understood that other types of mixing elements beyond those inFIGS. 2 and 3could be used.

Bonded to the surface of mixing element54(FIG. 2) or64(FIG. 3) is a coating58. The coating58is intended to react with (i.e., hydrolyze) reductant26to yield a chemical species capable of reacting with NOx, particularly when combined with the catalyst coating of SCR22catalyst. An example of the reductant26is urea. Urea may come in contact with the coating58either as a gas or a urea solution fog comprising both gas and liquid droplets. The coating58may have a suitable thickness, or weight per unit area for relatively low heat capacity (thermal mass) while providing an adequate number of chemically active surfaces for hydrolysis of reductant26. The range of coating weight on mixer42and/or mixing element54(FIG. 2) or64(FIG. 3) may range from 1-50 gm/m2. It should be understood that the range of coating weight may be independently selected from more than 1, 5 or 10 gm/m2to less than 5, 10, 50 or 100 gm/m2. However, in at least one embodiment, the coating58has a weight of 2-8 gm/m2. Coating58is in fluid contact with the exhaust gas14. Exhaust gas14, which contains reductant26, enters mixer42through an inlet60. The blended mixture of reductant26and exhaust gas14exits through an outlet62of mixer42after reductant26is decomposed when in contact with coating58. The blended mixture is transferred into SCR22catalyst.

Coating58may comprise active Lewis acid components such as metallic oxides, including alumina, titania, silica, zirconia, niobium pentoxide, tantalum pentoxide, and tungsten trioxide. The range of ratios of weight between alumina, when used, and other oxides may vary from less than 99:1, 95:5, 90:10, 75:25, or 50:50 to more than 40:60, 25:75, 10:90, 5:95, or 1:99. Typically, the ratio of alumina to other oxides may range from 99:1 to 25:75. The same ranges of ratios of weight may be used when using any of the two Lewis acid components. If three or more Lewis acid components are used in combination, the Lewis acid component having the largest portion typically may comprise at least 35 wt %. It should be understood that the Lewis acid component having the largest portion may comprise at least 25 wt % or at least 50 wt % without exceeding the spirit of the invention. When a mixed oxide, such as Fe203in combination with Fe304and/or Fe0, comprises a portion of the Lewis acid components, the mixed oxide is consolidated into a theoretical stoichiometry for oxides having the same metal. An example is Fe01.7for a mixture of Fe203, Fe304, and/or Fe0. The theoretical stoichiometry may be used for calculating relative ratios of Lewis acid components.

Further examples of Lewis acid components include a protonated zeolite or zeolite doped with transition metal dopants such as iron and copper, or lanthanide dopants such as cerium. For an iron-zeolite coating the range of content of iron may typically range from 0.1 to 15 wt %. However, the range may be independently chosen from 0.1 to 0.5, 1.0, or 1.25 wt % and less than 15, 10, 5, 3, 2.5 or 2 wt %. Iron may be doped into the zeolite by methods known in the art including ion exchange. The range of zeolite dopants may be 0.1 wt % to 30 wt %. More generally, the range of zeolite dopants may be selected independently from more than 0.1, 2, 5, or 10 wt % and less than 30, 20 or 15 wt %. The zeolite used may be any of the zeolites known in the art, such as ZSM, MFI and β-type zeolites.

Since the reaction of iron with water has a first dissociation constant pKaof 2.2 and a second pKaof 3.3, the pH of the solution will determine whether ion exchange doping occurs at a monovalent, divalent or trivalent condition. The resulting iron zeolites typically have pKavalues in the range of 3.5 to 4.5, depending upon the zeolite used. The pKaof the iron zeolite or, in general, any Lewis acid component may influence the number availability of hydrolysis sites for the component. The number and availability may vary the volume of hydrolysis catalyst required for exhaust14space velocity and the quantity of reductant26metered into the emission control system10.

It should be understood that solid ion exchange of zeolites with iron (II) chloride and iron (III) chloride may be necessary for complete exchange. The mixture may then be heated in streaming nitrogen at about 300° C. for two hours to complete the ion exchange. The exchange to zeolite may be washed with water and dried at 120° C. The mixture is then held at constant temperature of 300° C. for two hours in nitrogen. The zeolite catalyst may be applied to the mixing element54(FIG. 2) or64(FIG. 3) by suspending the powder and water with a binder. It should be understood that combinations of these compositions may be used in various embodiments of coating58without exceeding the spirit of this invention.

The coating58may be applied to mixing element54(FIG. 2),64(FIG. 3) and/or mixer50in a single pass or in multiple passes in thicknesses from 0.0005 inch to 0.1 inch. An example of multiple passes includes a washcoat layer followed by applied metal ions. Washcoat loadings of mixing element54(FIG. 2) or64(FIG. 3) can be provided in the range of 1-35 mg/in2of mixing element surface area. The washcoat of the coating58may have a surface area typically ranging from 10 to 400 m2/gm before exposure to exhaust14. Alternatively, ranges of surface area may include 25-300 m2/gm, or 50-150 m2/gm before exposure to exhaust14without violating the intent of the invention. A heat-treated washcoat of coating58may have a surface area ranging from 1-250 m2/gm, 10-200 m2/gm, or 50-100 m2/gm.

A washcoat of the coating58may be applied to mixing element54(FIG. 2) or64(FIG. 3) and then impregnated with additional metals from solution, if desired.

In certain embodiments, the coating58may be applied to mixing elements54(FIG. 2) or64(FIG. 3) by sol-gel techniques from heterometallic alkoxides. The use of sol-gel techniques appear to improve the distribution of alkali metals, alkali-earth, lanthanides and zirconia as opposed to impregnation methods. The particle size of the oxides of alkali metals, alkali-earth, and lanthanides remain small, in a range of about 2 to 10 nm, compared to that provided by any impregnation method. This provides a larger number of reaction sites for the hydrolysis reaction. More specific details of methods for preparing sol-gel coatings are provided by Narula and Nakouzi-Phillips in U.S. Pat. No. 6,153,272 which is incorporated here by reference. Still other ways of providing the invention for use will be apparent to those skilled in the art in view of the present disclosure.

It is beneficial for the mixing element54(FIG. 2) or64(FIG. 3) to heat up quickly during cold-start engine operations so that the residual droplets of liquid reducing agent may be vaporized and not be passed to the SCR as a liquid. The mixing element54(FIG. 2) or64(FIG. 3) may have a thermal conductivity greater than 8 watts per meter per degree Kelvin (W/m/° K). The mixing element54(FIG. 2) or64(FIG. 3) may also have thermal conductivity greater than 10 W/m/° K, 12 W/m/° K, 15 W/m/° K, 30 W/m/° K, 75 W/m/° K, or 100 W/m/° K when measured according to ASTM E 1225-04. It is advantageous if the coating58does not significantly decrease the thermal conductivity of mixing element54(FIG. 2) or64(FIG. 3) to less than 8 W/m/° K or an absolute decrease relative to the uncoated mixing element54(FIG. 2) or64(FIG. 3) of more than 10 W/m/° K. The absolute decrease in thermal conductivity associated with the coating may be more than 2, 3, or 5 W/m/° K without departing from the spirit of the invention.

The quickness with which the combined mixing element54(FIG. 2) or64(FIG. 3) and coating58become hydrolytically active may also reflect heat capacity (thermal mass) of the mixer50, mixing element54(FIG. 2) or64(FIG. 3), and coating58. A range of a ratio of the heat capacity of the mixer50to the heat capacity of the SCR22may be 0.001 to 0.95. The range may be independently chosen from 0.001, 0.005, 0.01, or 0.1 to 0.5, 0.75, 0.90 or 0.95 without departing from the spirit of the invention. A range of a ratio of the heat capacity of the coated mixing element54(FIG. 2) or64(FIG. 3) to the heat capacity of the SCR22may be 0.1 to 0.90. The range may be independently chosen from 0.01, 0.05, or 0.10 to 0.5, 0.75, 0.90, or 0.95 without departing from the spirit of the invention.

The improved effectiveness of emission control system10including mixer42with mixing elements54(FIG. 2) or64(FIG. 3) and coating58may be demonstrated in vehicles during testing using the standard test for cold-start emissions on light-duty vehicles, EPA FTP-75. The relative standard against which the present invention is compared is a 400-cell/in2ceramic honeycomb having an equivalent coating. Since the hydrolysis process may take place on the mixer instead of using additional SCR22catalyst volume, the estimated NOxconversion is up to 20% greater relative to a system not including the mixer42with coating58. SCR22derives advantages from the “freed up” volume that would otherwise be engaged in hydrolysis since the hydrolysis function has already occurred on the coating58.

Using mixer42, such as a coated static mixer, for remediating NOxfrom an internal combustion engine involves receiving reductant26, such as a mixture of gaseous and liquid reducing agent, from the source28of reductant agent26(FIG. 1). It is understood that the mixture can range from entirely gaseous to substantially liquid without violating the intent of this invention. The mixture of reductant26is received at the inlet60to mixer42. Mixer42is disposed downstream from inlet60and includes mixing element54(FIG. 2) or64(FIG. 3) having coating58. Coating58including the metal-zeolite composition capable of hydrolyzing the reducing agent. At substantially the same time as the mixture of reductant26arrives at the mixer, engine exhaust14(FIGS. 1 and 3) also arrives at inlet60to mixer42. It is understood that engine exhaust14may arrive at mixer42without any reductant26. Engine exhaust14transfers heat to mixer42which heats up rapidly due to its relatively low thermal mass, and its relatively high thermal conductivity, such as being greater than 9 W/m/° K or comparable to stainless steel at 15 w/m/° K. The warmed coated mixer42may evaporate the liquid reductant26and blend it turbulently with the exhaust14using the mixer element54(FIG. 2) or64(FIG. 3). As a consequence of reductant26coming in contact with coating58, reductant26is decomposed. Decomposed reductant26and exhaust14are released from mixer42through the outlet62of mixer42.