WO2009085641A1

WO2009085641A1 - Catalytic apparatus with up-stream wire mesh in-line mixer

Info

Publication number: WO2009085641A1
Application number: PCT/US2008/086482
Authority: WO
Inventors: Sivanandi Rajadurai
Original assignee: Acs Industries, Inc.
Priority date: 2007-12-19
Filing date: 2008-12-11
Publication date: 2009-07-09

Abstract

A compact, knitted, crimped wire mesh mixer (131) disposed in the exhaust system (103) of an internal combustion engine (101), between the nozzle (129) of a reductant injection unit and a selective catalytic reduction unit (109), significantly increases the uniformity of the reductant in the exhaust stream (135) by the time the stream reaches the catalysis unit (113). The wire mesh mixer (131) has a density that is less than or equal to 5% and a circumferential contact area between the mixer (131) and the conduit (107) of the exhaust system (103) to which the mixer (131) is attached that is less than or equal to 50%, e.g., the circumferential contact area is approximately 20%.

Description

CATALYTIC APPARATUS WITH UP-STREAM WIRE MESH IN-LINE MIXER

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit under 35 USC §119(e) of U.S. Provisional

Application No. 61/015,005 filed on December 19, 2007, the contents of which in its entirety is hereby incorporated by reference.

FIELD

[0002] The present invention relates to a method and apparatus for reducing pollution by improving the control of emissions of NO_x in flue gas generated from combustion.

DEFINITIONS

[0003] As used in the specification and the claims, the term "reductant" means a material that can reduce NO_x or a precursor of such a material. Non-limiting examples of reductants include ammonia, an ammonia precursor (e.g., urea), hydrocarbons (e.g., diesel fuel), and compatible mixtures of these and other reductants. For ease of presentation, the following discussion is often in terms of urea, the most commonly used reductant, it being understood that other reductants can be substituted for or combined with urea in the practice of the invention.

BACKGROUND

[0004] The United States' Clean Air Act Amendments enacted in 1990 regulate and reduce the amount of harmful pollutants released into the atmosphere. The Clean Air Act places strict guidelines on the amount of pollutants present in discharged flue gases from combustion sources and chemical plants. As a result, industries seek to implement efficient systems and methods for controlling the release of harmful pollutants in a cost-effective manner.

[0005] Nitrogen oxides (NO_x) and sulfur oxides (SO_x) are primary combustion pollutants targeted for reduction by the Clean Air Act. Various treatment processes and methods have been developed to reduce NO_x as a combustion by-product. NO_x reduction technologies employed for reducing the NO_x concentration in flue gas have been typically either selective catalytic reduction (SCR) or non-catalytic reduction (SNCR) using ammonia or urea to convert the NO_x to elemental nitrogen.

[0006] In the SNCR process, ammonia reduces NO_x present in the flue gas to nitrogen and water without the presence of a catalytic substance. The SNCR process is significantly limited, however, by the fact that it requires a very high operational temperature, ranging from 800°C to 1200⁰C. Accordingly, SNCR is not as desirable for diesel engine exhaust, and even at these high operational temperatures conversion of NO_x is limited to about 60% at most.

[0007] The SCR process requires the presence of a catalyst during the contact between the flue gas and any of various substances injected into the exhaust stream. These compounds include ammonia, ammonia precursors such as urea, and mixtures thereof. The SCR process is thought to likely be necessary to meet the 2010 NO_x legislation parameters. The process can be carried out at temperatures significantly lower than the SNCR process, typically at 200⁰C to 600°C. However, the SCR process is more costly than the SNCR process because it uses a catalyst and because the catalyst deteriorates with age and thus requires replacement every 3-5 years. Additionally, the catalyst can convert some of the SO₂ present in the flue gas to SO₃, which may cause additional pollution concerns. Lastly, in certain applications the catalyst bed may trap particulate matter, thereby blocking the catalyst surface and possibly leading to a flue-gas pressure drop because of plugged pore area. As a result, a particulate removal device such as an electrostatic precipitator may precede SCR. [0008] Current SCR systems and Lean NO_x trap systems use expensive injectors, flappers, atomizers, and helical or twisting static mixers, and require a relatively long distance between the injector and the front face of the catalyst in an attempt to achieve good mixing of the reductant with the exhaust stream. Nevertheless, NO_x reduction does not rise to the desired performance levels due to incomplete mixing in the short distance available along the undercarriage of a vehicle.

SUMMARY

[0009] In accordance with a first aspect, there is provided a method for reducing NOx pollutants in an exhaust stream (135) originating from combustion in the presence of air, said exhaust stream (135) being conducted in an exhaust pipe (107), said method comprising in order:

(a) spraying a reductant (133) into the exhaust stream (135) from an injector (129);

(b) conducting the stream (135) containing the reductant (133) through a mixer (131) which is attached to the exhaust pipe (107) and is effective to disperse the reductant (133) substantially throughout the stream (135), said mixer (131) comprising a crimped, knitted wire mesh having the following properties:

(i) a density that is less than or equal to 5%; and

(ii) a circumferential contact area that is less than or equal to 50%; and

(c) subjecting the exhaust stream (135) to selective catalytic reduction (109). [0010] In accordance with a second aspect, there is provided an exhaust system (103) for a combustion device (101) comprising: a conduit (107) directing exhaust (135) from said combustion device (101) to a selective catalytic reduction unit (109); an injector (129) supplied with a reductant solution (133) for dispensing said solution (133) into the conduit (107); and a crimped, knitted wire mesh mixer (131) disposed between the injector (129) and the unit (109), wherein the crimped, knitted wire mesh of the mixer (131) is characterized by:

(i) a density that is less than or equal to 5%; and

(ii) a circumferential contact area that is less than or equal to 50%.

[0011] In accordance with a third aspect, there is provided an exhaust system (103) comprising:

(a) a pipe (107) having an internal wall;

(b) a reductant injector (129) within the pipe (107), said reductant injector (129) producing a spray pattern; and

(c) a knitted wire mesh mixer (131) within the pipe ( 107), said mixer (131) being located downstream of the reductant injector (129); wherein mixer (131) is positioned relative to the reductant injector (129) so that the spray pattern contacts the mixer (131) without contacting the internal wall of the pipe (107). [0012] In accordance with a fourth aspect (see FIG. 15), there is provided a mixer (131) for use in an exhaust system (103), said mixer (131) comprising knitted wire mesh which has a first portion and a second portion, the first portion being downstream from the second portion when the mixer is installed in the exhaust system (103), wherein the wire mesh of the first portion is coated with a ceramic material (e.g., TiO₂) capable of facilitating conversion of isocyanic acid to ammonia and the wire mesh of the second portion is free of the coating. [0013] In accordance with a fifth aspect, there is provided a method of making a wire mesh mixer (131) comprising:

(a) making at least one wire mesh tube;

(b) flattening the at least one wire mesh tube;

(c) crimping each side of the at least one wire mesh tube;

(d) forming a roll from the at least one crimped, flattened tube with the peaks of adjacent crimp patterns substantially sitting on one another (see FIG. 11);

(e) inserting the roll into a pipe; and

(f) brazing the resulting structure.

[0014] The figure numbers and reference numbers used in the above summaries of the various aspects of this disclosure are only for the convenience of the reader and are not intended to and should not be interpreted as limiting the scope of the invention. More generally, it is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention. [0015] Additional features and advantages of the invention are set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. It is to be understood that the various features of the invention disclosed in this specification and in the drawings can be used in any and all combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a schematic representation of a vehicle NO_x reduction system.

[0017] FIG. 2 is a cross-sectional view showing an injector, a wire mesh mixer, and a SCR catalyst installed in a vehicle exhaust system.

[0018] FIGS. 3 A and 3B are cross-sectional and perspective views, respectively, of a wire mesh mixer having flat faces. [0019] FIGS. 4A and 4B are cross-sectional and perspective views, respectively, of a wire mesh mixer having concave faces.

[0020] FIGS. 5 A and 5 B are cross-sectional and perspective views, respectively, of a wire mesh mixer having stepped faces.

[0021] FIGS. 6A and 6B are cross-sectional and perspective views, respectively, of a wire mesh mixer having slanted faces.

[0022] FIGS. 7A and 7B are cross-sectional and perspective views, respectively, of a wire mesh mixer having two sections.

[0023] FIG. 8 is a cross-sectional view illustrating the use of a wire mesh mixer having two sections, one downstream of the system's injector and the other upstream of the system's SCR catalyst.

[0024] FIG. 9 is a cross-sectional view of a wire mesh mixer oriented at an angle to the local centerline of an exhaust pipe.

[0025] FIG. 10 is a photograph of three wire mesh mixers having different crimp patterns.

[0026] FIGS. 1 IA and 1 IB are schematic drawings illustrating the structure of a wire mesh mixer prepared by rolling a crimped wire mesh tube. FIG. 1 IA is a side view of the rolled structure and FIG. 1 IB is a perspective view illustrating the interaction of the crimps on adjacent layers.

[0027] FIG. 12 is a photograph showing a wire mesh mixer attached to an exhaust pipe by brazing.

[0028] FIG. 13 is a photograph of a wire mesh mixer produced from wire mesh tubes of different widths.

[0029] FIG. 14 is a photograph of a wire mesh mixer having a non-uniform mesh density.

[0030] FIG. 15 is a photograph of a zone coated mixer.

[0031 ] FIG. 16 is a plot of calculated NO_x gamma values with and without a wire mesh mixer of the present application.

[0032] FIG. 17 shows the structure of the prior art mixers used in the comparison study of

FIG. 18.

[0033] FIG. 18 is a plot of experimental data comparing NO_x conversion of a wire mesh mixer of the present application (bars with slanted hashing) with NO_x conversion of two prior art mixers (brick filled bars and solid filled bars) under identical conditions.

[0034] The reference numbers used in the figures refer to the following: 101 engine

103 exhaust system

105 pre-oxidation catalyst

107 conduit/exhaust pipe

109 catalytic converter section

1 11 hydrolysis catalyst

1 13 SCR catalyst

115 oxidation catalyst

119 control unit

121 valve

123 tube

125 pump

127 reservoir

129 injector/injector nozzle

131 wire mesh mixer

133 reductant/reductant solution

135 stream of exhaust gas

137 retaining ring for wire mesh mixer

139 perforations in retaining ring

141 steps on face of wire mesh mixer

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] As discussed above, selective catalytic reduction (SCR) is used to reduce the amounts ofNO_x in a gas phase of a stream such as an engine exhaust. It employs a reductant that is added to the exhaust stream and a catalyst. At present, a water solution of urea ((NH₂)_I-C=O), e.g., a 32% water solution of urea, is commonly used as the reductant. When introduced into the exhaust gases, the water evaporates from the solution and forms small particles of urea. Because of the short distances in a vehicle exhaust system, small particles of urea may not be formed and thus some urea dissolved in water may be present in the exhaust stream as it moves downstream from the location of urea injection. Urea decomposes upon melting (133°C) and in the exhaust environment, especially in the presence of a catalyst, is converted by thermolysis at temperatures of 120⁰C and above into isocyanic acid (H-N=C=O) and ammonia. The isocyanic acid then undergoes hydrolysis at temperatures of 16O⁰C and above to ammonia and carbon dioxide. Finally, the ammonia reacts with NO_x species to reduce the NO_x and produce nitrogen gas.

[0036] FIG. 1 depicts a representative urea SCR system. The exhaust gas from engine 101 is directed to an exhaust system 103 comprising a number of metal conduits and chambers making a continuous vessel housing various structures that perform operations on the exhaust gas stream. In the embodiment shown, the exhaust gas is directed first to a pre-oxidation catalyst 105 and then through a conduit 107 leading to a catalytic converter section 109, shown here as having (from upstream to downstream) a hydrolysis catalyst 111 , an SCR catalyst 113, and an oxidation catalyst 115, before being exhausted to the atmosphere (or a muffler). Typically, the catalyst is provided as a coating on or impregnation of a porous or tortuous monolithic catalyst support. The catalyst is secured in the metal housing by a support system.

[0037] A control unit 119 adjusts the flow of urea through a valve 121 in a tube 123 that passes into the conduit 107. A pump 125 connected to a reductant reservoir 127, e.g., a reservoir containing a water solution of urea, provides the flow force for the reductant. The end of tube 123 in conduit 107 is capped by a nozzle 129 designed to disperse the reductant throughout the flow stream. A wire mesh mixer 131 of the type described herein is situated in the conduit downstream of the nozzle. In FIG. 1, the wire mesh mixer is located upstream of the catalytic converter section 109. Alternatively, or in addition, mixer 131 can be positioned or further extended into the housing of the catalytic converter section and/or can be divided into multiple axially-spaced sections with at least one of the sections being located within the housing of the catalytic converter section to facilitate mixing into the larger cross-sectional area of that section (see discussion of FIG. 8 below for further details). [0038] Current SCR systems suffer from a number of problems. In addition to those referred to above, the ability of these systems to reduce NO_x is decreased due to poor mixing, incomplete urea decomposition, and catalyst fouling. In particular, the relatively short lengths of conventional exhaust systems do not provide sufficient residence time for mixing the urea with the hot exhaust gases or a hot third body substrate on which thermolysis can occur, so conversion is only about 20% at 330°C and about 50% at 440⁰C after initial contact and prior to reaching the catalytic converter (having an SCR catalyst). [0039] Low conversion is a particular problem for exhaust systems which operate at low temperatures. For example, small passenger car and truck engines can have exhaust temperatures in the 180-250⁰C range, as can larger engines when idling. Low exhaust temperatures can also exist when the environmental temperature is low. Indeed, in cold climates, urea can crystallize on the inside walls of a vehicle's exhaust system prior to the time the system reaches its operating temperature. Under hot operating conditions, the urea can undergo conversion to ammonia and isocyanic acid, resulting in overloading of the SCR catalyst, i.e., the presence of more ammonia at the SCR catalyst than needed to reduce NO_x present in the exhaust stream. Such a condition results in the release of unacceptable amounts of ammonia into the atmosphere, a process known as ammonia slip. In addition, low temperatures result in catalyst deactivation which, in turn, gives low NO_x conversion in, for example, the Bag 1 test of the Federal Test Cycle (FTP cycle 1975; cold start, first 505 seconds of emissions).

[0040] The SCR catalyst is also subject to fouling by crystallization of urea products remaining from incomplete urea decomposition. For example, under static conditions, a catalytic converter substrate, e.g., a cordierite substrate, can exhibit urea crystallization on the order of 20% at 18O⁰C.

[0041] In their most preferred embodiments, the SCR systems disclosed herein exhibit improved performance with regard to mixing, incomplete urea decomposition, and fouling. For example, with regard to fouling, under the same conditions where cordierite substrates exhibit approximately 20% urea crystallization, the wire mesh mixers disclosed herein exhibit crystallization of less than 2% (see Example 5). These low levels of crystallization are believed to be due to the mixer's small wire diameters which tend to minimize crystallization. Indeed, under the flow conditions of a typical exhaust system, the amount of urea crystallization exhibited by the wire mesh mixers drops essentially to zero. [0042] As to incomplete urea decomposition due to low temperatures, the wire mesh mixer can readily equilibrate within, for example, 97% of the temperature of the exhaust gas without major heat loss. In particular, because of its limited contact area with the wall of the exhaust system, the wire mesh does not conduct substantial amounts of heat to the wall. Moreover, the wire mesh tends to act as an inhomogeneous catalyst and thus urea which contacts the mesh tends to be converted to isocyanic acid and ammonia. Thus, even under cold conditions, the urea does not tend to crystallize on the wire mesh mixer, but rather is converted to isocyanic acid and ammonia. Also, at least to some extent, isocyanic acid which contacts the mesh tends to be converted to carbon dioxide and ammonia. Because of these effects, the wire mesh mixer is preferably located within the injector's spray cone. For example, the wire mesh mixer can be within about 3 inches of the injector. For a typical injector, such a spacing can avoid contact between urea and/or the urea/water solution with the walls of the exhaust system. More generally, the wire mesh mixer is preferably close enough to the injector so that the cone (spray pattern) of the water/urea solution produced by the injector does not contact the wall of the exhaust pipe before it contacts the wire mesh. In this way, the problem of crystallized urea accumulating on the walls of the exhaust system is avoided, as is the problem of ammonia slip resulting from the uncontrolled conversion of such urea to ammonia and isocyanic acid.

[0043] As to mixing, the wire mesh mixer provides a tortuous path for gas passing through the mesh leading to radial flow in addition to transverse flow. Also, as discussed in detail below (see the discussion of FIGS. 7 and 8), the mixer can be divided into more than one section, with at least a first section being located near to the injector and at least a second section being located near to the catalytic converter. In this way, the first section can ensure substantial conversion of the urea, e.g., it can function as a thermolyzing, pyrolyzing, and/or hydro lyzing unit, and the second section can ensure substantial uniformity of the exhaust stream when it enters the SCR catalyst. Of course, both sections will contribute to both the conversion and mixing functions of the system.

[0044] Preferably, when multiple sections are used, the first section is close enough to the injector so that the injector's cone of urea/water solution does not contact the wall of the exhaust pipe. Also, whether a single section or a multiple section mixer is used, the mixer preferably does not have a straight through path which would allow urea to pass through the mixer without contacting the wire mesh. It should be noted that when a multiple section mixer is used, the various sections can have the same or different properties depending on the particular application.

[0045] The wire mesh mixer can be employed in variety of geometries and configurations. FIGS. 2-8 show representative embodiments. In particular, FIG. 2 shows a basic system where nozzle/injector 129 introduces a reductant 133 into a stream 135 of exhaust gas. Nozzle 129 is located just upstream of wire mesh mixer 131 so that the spray cone of the injector reaches the mixer before contacting the inside surface of exhaust conduit 107. FIGS. 3 A and 3B show cross-sectional and perspective views of mixer 131 in conduit 107. As shown most clearly in FIG. 3B, the wire mesh of the mixer can be fitted into a retaining ring 137. As shown, the ring can have perforations 139 that can be made extending radially outward (similar to burrs) to frictionally engage the pipe wall during installation like little leaf springs.

[0046] FIGS. 4, 5, and 6 show variations of the mixer of FIG. 3 wherein the thickness of the mixer is not uniform. In particular, FIG. 4 shows a mixer that has a concave shape with the center of the mixer being thinner than the edges, FIG. 5 shows a mixer whose faces include steps 141, and FIG. 6 shows a mixer having slanted faces. Geometries of these and other types, e.g., combinations of the geometries of these figures, can be used to accommodate particular flow patterns from injector nozzle 129 and/or particular local geometries of the exhaust system.

[0047] FIGS. 7 and 8 show embodiments in which mixer 131 includes two axially- separated sections 131a and 131b. In FIG. 7, the two sections are located relatively near to one another, e.g., the gap between the mixers is on the order of the maximum thickness of the mixers, while in FIG. 8, they are separated by a substantial axial distance, e.g., the gap between the mixers is many times the maximum thickness of the mixers. In either case, the two sections preferably have different properties, but can have the same properties if desired. For example, in the FIG. 7 embodiment, the downstream section can be coated with a ceramic coating, such as TiO₂, to help in the conversion of isocyanic acid to ammonia, while the upstream section is uncoated so as not to increase its heat capacity and thus compromise the wire mesh's ability to catalyze the thermolysis of urea during cold start conditions. [0048] In practice, different exhaust system manufacturers have different systems and configurations for injection of a reductant into an exhaust stream. Center injection is preferred, but accurate placement of the nozzle in the center of the exhaust pipe requires rigor and is relatively expensive. Injection from one or more locations on the wall of the pipe gives an asymmetrical distribution of the injected fluid. Using spaced mixers can provide a better flow distribution, especially where the upstream mixer is tailored to the geometry of the injection spray and resulting particles, and the downstream mixer is tailored for impact pulverization of particles and thermolysis. Thus, the upstream mixer can be made to have a density gradient suitable for the injection spray geometry. For example, after brazing (see below), the mixer can be shaped (by compression molding) to have a thinner, denser area and a wider, less dense area, as shown in FIG. 6, and/or can be made with flow interrupting steps or ribs as shown in FIG. 5. Alternatively, the mixer can be shaped to have a concave face as in FIG. 4 or a convex face (not shown). Combinations of these approaches can be used. [0049] As a related alternative, the upstream section can be primarily concerned with distributing the urea/water solution and the urea particles, as well as the ammonia and isocyanic acid produced by the thermolysis of the urea, uniformly over the cross-section of the exhaust pipe (e.g., it can be thought of as a "homogenizing and thermolyzing unit"), while the downstream section is primarily concerned with achieving a velocity profile for the exhaust gas that is substantially uniform (e.g., it can be thought of as a "distributing unit"). [0050] This division of labor is particularly appropriate for the FIG. 8 embodiment, where the upstream section 131a is located close to the injector nozzle 129 and the downstream section 13 Ib is located close to the SCR catalyst 113 and within the portion of the exhaust system which houses the catalyst. The change in geometry, e.g., cross-sectional shape and/or size, from an exhaust pipe to a catalyst housing will generally result in a substantial perturbation of the velocity profile of the exhaust gas, e.g., it will generally result in high flow at the center and low flow at the edges for a symmetric system or low flow at the center and high flow at the edges for an asymmetric system. SCR catalysts work best when the flow velocities of the exhaust gas are substantially uniform across the catalyst's entrance face and centrally located. For example, the uniformity index (gamma) for the flow velocities is preferably close to 1.0 and 65% of the highest velocities should occupy 40% and 95% of the highest velocities should occupy 90% of the catalyst's front face. By locating mixer 13 Ib in the region of the transition from the exhaust pipe to the catalyst housing, both a substantially uniform velocity profile and centering of the flow at the entrance face to the catalyst can be readily achieved. In achieving their different functions, as discussed above, mixers 131a and 131b can employ the configurations of one or more of the geometries of FIGS. 3-6 and/or other geometries appropriate to the particular application of the invention. [0051] FIG. 9 shows an embodiment in which the wire mesh mixer is oriented at an angle θ to the local centerline of an exhaust pipe. As shown in FIG. 9, the mixer is angled with regard to the centerline in one plane. In practice, the mixer can be angled in more than one plane if desired. An angled configuration for the mixer can be especially useful in ensuring that the spray pattern from nozzle 129 does not contact the walls of the exhaust system before it contacts the wire mesh mixer. For example, as illustrated in FIG. 9, the slanted configuration is useful in cases where the injector nozzle is offset from the centerline of the exhaust pipe. Also, depending on the overall geometry of the exhaust system, an angled mixer can also be used to reduce the back pressure generated by the mixer.

[0052] As can be seen from the foregoing, the combination of the injection spray geometry and the mixer geometry and density can be used to improve (a) the conversion of an ammonia precursor to ammonia and (b) the distribution of reactants and products throughout the gas stream. The use of a wire mesh mixer provides improvements in both areas while at the same time providing the flexibility to meet the diverse requirements of the numerous exhaust systems encountered in practice. Among their various advantages, wire mesh mixers can achieve thermolysis, mixing, and distribution without inserting a high thermal load in the exhaust stream which has been found to compromise system performance under conditions of low exhaust temperatures.

[0053] The wire mesh mixer is made by knitting a wire and then conforming the knit wire into a geometry desired for mixing. Examples of knitting wire to produce articles through which gases flow can be found in U.S. Patents Nos. 6,277,166 and 7,025,797, U.S. Patent Publications Nos. 2006/0037298 and 2007/0277490, and PCT Patent Publication No. WO 2008/143606, the disclosures of which are incorporated herein by reference. The wire mesh making up the wire mesh mixer can be composed of various materials and those materials can be subjected to various treatments (including coatings) either before or after being formed into a mesh. Examples of suitable materials include, without limitation, FeCr alloys and 304 and 31OS stainless steels. 304 SS is particularly well suited for this application. The wires making up the wire mesh can have various cross-sections, including, without limitation, round, hexagon, octagon, square, and flat.

[0054] Preferably, knitting produces a continuous tube or sock that can be compression molded into a desired geometry and/or rolled into a cylindrical shape to provide a disk-shaped or cylindrically-shaped mixer and then compressed, if desired. Preferably, the tube is crimped to promote interlocking of adjacent mesh layers or opposing faces can be folded over in a non-nesting configuration (e.g., arranging the crimp on one side orthogonal to the folded adjacent side). The knit density and the crimp structure, including the crimp's height, frequency, and angle, can be adjusted to change the porosity, and thereby the pressure drop, across the mixer, as well as the thermal load represented by the mixer. The knit density and the crimp structure also affects the contact area between the mixer's wire mesh and the wall of the exhaust pipe (or the retaining ring for the wire mesh when used), which, in turn, affects the amount of heat transfer to the environment exhibited by the mixer. [0055] The crimp structure also affects the radial compressive strength of a disk-shaped mixer, because the mixer must be compressed radially to fit within the exhaust system, and expansion after insertion will maintain the position of the mixer. Alternatively, as illustrated in the figures and discussed above, a retaining ring can be attached around the periphery of the mixer to facilitate its insertion and securement within the exhaust system. Interlocking of adjacent or abutting mesh is also important to prevent the mixer from telescoping out downstream due to the pressure drop across the mixer.

[0056] A variety of crimping geometries can be used including angled and herringbone (intersecting angled crimps). The tube can be provided with multiple crimps by changing crimp rolls or having a sufficiently large crimp roll to accommodate the entire length of the tube. Variable crimping can be used to produce a variable density mixer. For example, the portion of the tube disposed closer to the exhaust pipe axis can have a crimp height of 0.110 inch and the portion closer to the wall of the pipe can have a crimp height of 0.295 inch. The lower crimp height allows closer contact and a denser mixer. Instead of or in addition to varying crimp height to change the density, multiple crimps can be used to effect how closely the rolled layers lay abutting.

[0057] The density of the wire mesh mixer is determined by the crimping, as well as the wire mesh's needle count and courses per inch. To provide a low level of heat absorption during start up, the average density of the wire mesh mixer needs to be less than or equal to 5%. As known in the art, the average density (D) in percent of a wire mesh part can be calculated by: (1) determining the weight (W) of the part, (2) determining the volume (V) of the part, (3) determining the density (p) of the wire making up the wire mesh, and (4) calculating the average density from the equation: D=100*(W/(V*p)). Preferably, the average density is approximately 3%.

[0058] The crimping also determines the contact area between the wire mesh of the mixer and the conduit in which it is mounted (or the retaining ring when used). The contact area, in turn, determines the steady state heat conductivity from the mixer to the conduit. The contact area between the wire mesh and the wall of the pipe (or the retaining ring) needs to be less than or equal to 50% of the total surface area defined by thickness and circumference of the mesh, preferably, less than or equal to 25%. This contact area is referred to in the specification and in the claims as the "circumferential contact area." It is equal to 100 times the area of the wires at the outer circumference of the mesh divided by the product of the mesh's overall thickness times its circumference.

[0059] Because the wire making up the mesh is thin, its thickness normally can be ignored in determining the mesh's circumferential contact area. In such cases, the circumferential contact area can be determined from knowledge of the wavelength of the crimp pattern and the average length of the apices of the crimps when inserted in the pipe (or retaining ring). For example, a typical crimp wavelength is 11 millimeters and a typical apex length after insertion is 2 millimeters, leaving 9 millimeters out of contact with the wall of the pipe (or out of contact with the retaining ring for the wire mesh when used). The circumferential contact area is then: 100*(apex length/wavelength), or 18% for a 2 millimeter apex and a 11 millimeter wavelength.

[0060] As representative examples, FIG. 10 shows the circumferential surfaces of three wire mesh mixers having different crimp patterns, i.e., a herringbone pattern on the left and angled patterns at different crimp angles in the center and on the right, i.e., 10° for the center mixer and 30° for the right mixer. Each of these crimp patterns gives a density of less than 5% and a circumferential contact area less than 25%.

[0061] Once the crimping is completed, the crimped tube is rolled into a disk, as shown schematically in FIG. 11. In particular, FIG. 1 IA shows how the crimp pattern can give the mixer a high porosity (low density) by preventing nesting of the various turns of the rolled tube. FIG. 1 IB further illustrates how the peaks of the crimps can be offset and therefore cannot nest.

[0062] In addition to illustrating the effect of the crimp pattern on porosity, the outer layer of FIG. 1 IA also illustrates how the crimp pattern can reduce the circumferential contact area between the mixer and the exhaust pipe in which the mixer is mounted since the only possible points of contact are the peaks of the crimp pattern. As can be seen in this figure and in FIG. 10, the peaks are spaced apart and even when the mixer is compressed during insertion in an exhaust pipe or in a retaining ring, the peaks only constitute a fraction of the circumferential surface area of the mixer.

[0063] The rolled, crimped tube, a green article (i.e., unfinished) can be maintained in the desired configuration by gluing (such as by spraying with an acrylic polymer or an adhesive). The wire can be coated with a polymer (e.g., acrylic spray) before knitting. The green piece can also be fitted into a retaining ring 137 as shown in FIGS. 2-9 and discussed above. Alternatively, the exhaust pipe can be made with offset rings, e.g., ribs formed by crimping the pipe and extending radially inward (an indented pipe), with the mixer fitting between a pair of the rings.

[0064] The mixer can be brazed or sintered for additional strength, ease of assembly with the exhaust system, and to facilitate or secure the attachment of any retaining ring. Suitable brazes include BNi2 (NiCrSiB) and BNi7 (NiCrP). Brazing will burn off any adhesive and produce a robust mixer. Alternatively, a green, sintered, or brazed mixer can be inserted into a pipe with offset rings and having a brazing composition painted on the wall between the rings and the mixer brazed in place. Thus, a short length of exhaust pipe can be made with an integral mixer, and that length used in various exhaust system configurations. Brazing also strengthens the mixer against telescoping downstream.

[0065] FIGS. 12-15 are photographs of various wire mesh mixers constructed in accordance with the above procedures. Thus, FIG. 12 is a photograph showing a wire mesh mixer attached to an exhaust pipe by brazing; FIG. 13 is a photograph of a wire mesh mixer produced from wire mesh tubes of different widths; FIG. 14 is a photograph of a wire mesh mixer having a non-uniform mesh density; and FIG. 15 is a photograph of a wire mesh mixer a portion of which (the darker portion in this figure) has been coated with a ceramic material, e.g., TiO₂, to facilitate conversion of isocyanic acid to ammonia.

[0066] The coating of the wire mesh mixer of FIG. 15 can be performed by dipping the portion of the mixer to be coated into a solution containing TiO₂ and a binder, drying the mixer, and then sintering the resulting structure to form the TiO₂ coating. Because the coated portion of the mixer has a higher heat capacity than the uncoated portion, when installed in an exhaust pipe, the uncoated portion is preferably on the upstream side of the mixer which allows that portion to reach thermal equilibrium with the exhaust gas more quickly, thus shortening the time for full participation of the mixer in the conversion of a water/urea solution to ammonia.

[0067] By means of the foregoing, some and preferably all of the following advantages and improvements to existing SCR systems are achieved:

(a) uniform distribution of the exhaust gas and the reductant at the front face of the SCR substrate, e.g., a uniformity index (γ) greater than or equal to 0.98 and, preferably, greater than or equal to 0.99; and/or (b) effective thermolysis of urea into ammonia and isocyanic acid in a short distance thus protecting the SCR substrate from urea crystallization on its front face; and/or

(c) reduction in the length between the reductant injector and the SCR substrate; and/or

(d) improved NO_x reduction efficiency at any given NO_x/reductant ratio; and/or

(e) low back pressure; and/or

(f) minimal urea slip through the mixer to the SCR catalyst; and/or

(g) minimal urea crystallization on the mixer; and/or (h) minimal thermal fatigue issues.

[0068] Without intending to limit it in any manner, the invention will be further illustrated by the following examples.

Examples

EXAMPLE 1

[0069] This example compares NO_x conversion using ammonia gas injection with NO_x conversion using injection of a urea/water solution. A wire mesh mixer was used in the experiments in which a urea/water solution was injected, but not in the experiments where ammonia gas was injected. The amount of urea injected was selected to provide, upon complete conversion, an equimolar amount of ammonia to that injected in the ammonia injection experiment.

[0070] The mixer was made using 304 stainless steel, 0.14 in. OD wire, knitted into a tube and crimped with a 9 mm pitch, a 0.295 in. height, and a 30° angle. The crimped tube was folded and rolled into a disk, sprayed with acrylic, painted with BNi7 braze all over and then brazed, and finally compression rolled to the pipe ID with a final density of 3% (97% porosity). When inserted in an exhaust pipe, the circumferential contact area between the wire mesh of the mixer and the pipe's wall was 18%.

[0071] Sampling for NO_x conversion was performed at seven locations varying radially and circumferentially in relation to the exhaust pipe and at the exit. The results are shown in Table 1.

[0072] Although one might expect that direct ammonia injection would provide good results since it eliminates the inefficiency of conversion of an ammonia precursor, Table 1 shows unexpectedly that the present wire mesh mixer approach outperformed pure ammonia injection.

EXAMPLE 2

[0073] This example illustrates the ability of a wire mesh mixer to improve the distribution of a reductant at the surface of a SCR catalysis.

[0074] The experiment was performed using equipment designed to simulate a conventional engine exhaust equipped with a urea injection/SCR system. A reddish-colored liquid injected via the test equipment's urea injection port was used to simulate the introduction of a reductant into the system. The distributions of the liquid on a cordierite matrix with and without an upstream crimped, knitted wire mesh mixer were determined. [0075] Using a wire mesh mixer, specifically, a wire mesh mixer of the type described in Example 1, the test fluid dispersed to cover an area of approximately 30 sq. in. In contrast, without the mixer, less than one square inch of the cordierite matrix became reddish-colored. Accordingly, use of a crimped, wire mesh mixer resulted in an order of magnitude more area of gas flow containing the test fluid.

EXAMPLE 3

[0076] This example compares calculated ammonia (NH₃) distributions with and without a wire mesh mixer. The uniformity of the distributions was expressed in terms of gamma values, where γ was defined as:

yi. i

[0077] The results of this study are shown in FIG. 16. As can be seen in this figure, a γ value greater than 0.7 was achieved at about 60 mm from the front face of the mixer, and a value greater than 0.8 was achieved at about 150 mm. These short distances mean that the ammonia distribution at the face of a SCR catalyst will be substantially uniform in a typical vehicle exhaust system. The calculated values of FIG. 16 were confirmed experimentally by a 30% improvement in NO_x conversion through use of the wire mesh mixer.

EXAMPLE 4

[0078] This example compares the back pressure of conventional stamped mixers with the wire mesh mixers disclosed herein. [0079] As reported in Silvia Calvo et al, 2^nd International CTI Forum SCR System,

October 15-16, 2007, at page 20, the back pressure of a conventional stamped mixer with a 62 mm diameter at an exhaust flow rate of 450 kg/hour and an exhaust temperature of 298°K is

2.2 kPa. Under the same test conditions, a wire mesh mixer of the type described in Example

1 has a back pressure of only 1.7 kPa, i.e., a 23% reduction.

[0080] In another experiment using a 60 mm diameter and a 600 kg/hr gas flow rate, a 50.8 mm length of wire mesh mixer gave an 18 mbar pressure drop versus 25-40 mbar for a 25 mm length static mixer.

[0081] As this data shows, at comparable diameters, the wire mesh mixers disclosed herein have significantly less back pressure than conventional static mixers.

EXAMPLE 5

[0082] This example demonstrates that a wire mesh mixer is not a preferred surface for urea crystallization and that any crystallization that does occur can be easily reversed at temperatures below the operating temperature of a vehicle exhaust system. [0083] The experiment was performed by placing a wire mesh sample in a boiling solution of urea (30%). The sample was removed from the solution and dried. The densities of the mesh before and after the treatment were 0.50 gm/cc and 0.51 gm/cc, respectively. Heating of the wire mesh to 180°C completely removed the urea and returned the density to its original value.

EXAMPLE 6

[0084] This example compares the NO_x reduction achieved using a wire mesh mixer of the type disclosed herein compared to that achieved with prior art mixers. The experiments were performed using a diesel engine equipped with a urea- injection SCR unit. The engine was a 2007 model year, 305 hp ISB with standard DOC and DPF. Table 2 sets forth the seven static test mode conditions that were studied. The wire mesh mixer was of the type described in Example 1. Two conventional mixers having structures of the type shown in FIG. 17 were used for comparison.

[0085] The results are shown in FIG. 18. As can be seen in this figure, the wire mesh mixers disclosed herein exceeded the prior art mixers in NO_x conversion for each of the test modes, in most cases by well more than 10%.

[0086] From the foregoing, it can be seen that the present disclosure provides, among other things: (a) a SCR unit, e.g., a SCR unit for a diesel engine, which includes a knitted wire mesh static mixing device which is disposed between the location where a reductant is introduced into an exhaust stream and the location of an SCR catalyst; and/or

(b) a compact, knitted, and crimped wire mesh mixer that is disposed in the exhaust system of an internal combustion engine, between the location at which reductant injection occurs and the location of a selective catalytic reduction unit; and/or

(c) an exhaust system for an internal combustion engine that comprises a conduit connected to a selective catalytic reduction unit, an injector supplied with a reductant solution for dispensing said solution into the conduit, and a crimped, knitted wire mesh mixer disposed between the injector and the selective catalytic reduction unit; and/or

(d) a method for reducing gaseous pollutants in the exhaust of an internal combustion engine comprising spraying into a stream of said exhaust a reductant, then conducting the stream through a knitted wire mesh mixer effective to disperse the reductant substantially throughout the stream, and then optionally subjecting the exhaust stream to selective catalytic reduction.

[0087] As a result of the use of a knitted wire mesh mixer, the uniformity of the reductant in the exhaust stream is significantly increased by, for example, the time the stream reaches a catalysis unit. Other strategies for mixing use static mixers, or a spinning mixer (i.e., a static mixing device made rotatable). Static (and rotating) mixers have a plurality of angle and/or curved vanes to generate turbulence and thereby induce mixing. Compared to these other types of mixers, the knitted wire mesh mixers disclosed herein promote and allow significant radial flow, provide increased surface area for thermolysis and impact pulverization of ammonia precursor particles, and generate very little back pressure. [0088] A variety of modifications that do not depart from the scope and spirit of the invention will be evident to persons of ordinary skill in the art from the foregoing disclosure. For example, while the foregoing disclosure has been primarily directed to the use of a wire mesh mixer in a SCR application for a vehicle, SCR systems can be present in power plants, and the present mixers can be used in those operations as well. The turbulent and tortuous flow path of the mixer provides a higher retention time and contact time between the exhaust and the reductant for any combustion exhaust gas treatment operation. In general, the present devices and methods are useful in any environment in which there is combustion in the presence of air, thereby generating NO_x. Exemplary processes in which this invention can be used include catalytic cracking units (where heated gas is reacted), boilers (such as used for generating steam for steam turbines), gas turbines (such as where gas is combusted in thermal power stations), and the like. Accordingly, exemplary combustion devices with which this invention can be used include internal combustion engines, boilers, furnaces, turbines, and the like.

[0089] The following claims are intended to cover the specific embodiments set forth herein as well as modifications, variations, and equivalents of the foregoing and other types.

Table 1

Table 2

Static test mode conditions

Claims

What is claimed is:

1. A method for reducing NO_x pollutants in an exhaust stream originating from combustion in the presence of air, said exhaust stream being conducted in an exhaust pipe, said method comprising in order:

(a) spraying a reductant into the exhaust stream from an injector;

(b) conducting the stream containing the reductant through a mixer which is attached to the exhaust pipe and is effective to disperse the reductant substantially throughout the stream, said mixer comprising a crimped, knitted wire mesh having the following properties:

(i) a density that is less than or equal to 5%; and

(ii) a circumferential contact area that is less than or equal to 50%; and

(c) subjecting the exhaust stream to selective catalytic reduction.

2. The method of Claim 1 wherein the circumferential contact area is less than or equal to 25%.

3. The method of Claim 1 wherein the mixer produces a back pressure that is less than or equal to 2 kilopascals for an exhaust gas flow rate of 1200 kg/hour and an exhaust gas temperature of 55O⁰C.

4. The method of Claim 1 wherein the injector produces a spray pattern within the exhaust pipe and the spacing between the mixer's upstream face and the injector is sufficiently short so that the injector's spray pattern does not contact the inner surface of the exhaust pipe before contacting the upstream face of the mixer.

5. The method of Claim 1 wherein the mixer has upstream and downstream sections, the upstream section being proximal to the injector and the downstream section being proximal to the location at which the selective catalytic reduction occurs.

6. An exhaust system for a combustion device comprising: a conduit directing exhaust from said combustion device to a selective catalytic reduction unit; an injector supplied with a reductant solution for dispensing said solution into the conduit; and a crimped, knitted wire mesh mixer disposed between the injector and the unit, wherein the crimped, knitted wire mesh of the mixer is characterized by:

(i) a density that is less than or equal to 5%; and

(ii) a circumferential contact area that is less than or equal to 50%.

7. The exhaust system of Claim 6 wherein the circumferential contact area is less than or equal to 25%.

8. The exhaust system of Claim 6 wherein the system has at least one of the following characteristics:

(a) the mixer has a flat face;

(b) the mixer has a slanted face;

(c) the mixer has a face that is not flat;

(d) the mixer has a stepped face;

(e) the mixer has a curved face;

(f) the mixer has a concave face;

(g) the mixer has a non-uniform density;

(h) the mixer has a portion which is coated with a ceramic material;

(i) the mixer has first and second sections separated by a gap;

(J) the mixer has first and second sections separated by a gap and the sections have different densities; (k) the mixer is brazed;

(1) the mixer is brazed and shaped by compression molding; (m) the mixer is brazed to the conduit, (n) the mixer is sintered;

(o) the mixer's knitted wire mesh has a herringbone crimp; (p) the mixer is disposed in the conduit; (q) the mixer is disposed in the unit; and (r) the mixer is disposed in both the conduit and the unit;

9. The exhaust system of Claim 6 further comprising said combustion device, wherein said combustion device is an internal combustion engine.

10. The exhaust system of Claim 6 further comprising said combustion device, wherein said combustion device is a boiler, a furnace, or a gas turbine.

11. An exhaust system comprising:

(a) a pipe having an internal wall;

(b) a reductant injector within the pipe, said reductant injector producing a spray pattern; and (c) a knitted wire mesh mixer within the pipe, said mixer being located downstream of the reductant injector; wherein the mixer is positioned relative to the reductant injector so that the spray pattern contacts the mixer without contacting the internal wall of the pipe.

12. The exhaust system of Claim 11 wherein the pipe has a centerline and the mixer is oriented at an angle to the centerline.

13. A mixer for use in an exhaust system, said mixer comprising knitted wire mesh having a first portion and a second portion, the first portion being downstream from the second portion when the mixer is installed in the exhaust system, wherein the wire mesh of the first portion is coated with a ceramic material capable of facilitating conversion of isocyanic acid to ammonia and the wire mesh of the second portion is free of the coating.

14. The mixer of Claim 13 wherein the ceramic material comprises TiO₂.

15. A method of making a wire mesh mixer comprising:

(a) making at least one wire mesh tube;

(b) flattening the at least one wire mesh tube;

(c) crimping each side of the at least one wire mesh tube;

(d) forming a roll from the at least one crimped, flattened tube with the peaks of adjacent crimp patterns substantially sitting on one another;

(e) inserting the roll into a pipe; and

(f) brazing the resulting structure.

16. The method of Claim 15 wherein the brazing is between portions of the wire mesh which contact:

(i) other portions of the wire mesh;

(ii) portions of the wall of the pipe; or

(iii) both other portions of the wire mesh and portions of the wall of the pipe.

17. The method of Claim 15 wherein the roll is mounted in a retaining ring prior to step (f) and in step (f), the brazing is between the retaining ring and the wall of the pipe and between portions of the wire mesh which contact:

(i) other portions of the wire mesh; (ii) portions of the retaining ring; or (iii) both other portions of the wire mesh and portions of the retaining ring.

18. The method of Claim 15 wherein the crimping produces a herringbone crimp pattern.

19. The method of Claim 15 wherein the crimping produces an angled crimp pattern.