Abstract:
A non-thermal plasma reactor and method for preparing same for conversion of exhaust gas constituents is prepared from an extruded monolith comprising a plurality of conductive and exhaust channels separated by substantially planar dielectric barriers. Conductive material printed onto selected monolith channels form the conductive channels, which are connected along bus paths to form an alternating sequence of polarity, separated by exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged to exclude exhaust gases and to prevent electrical leakage. During operation, exhaust gas flows through exhaust channels and is treated by the high voltage alternating current flowing through the conductive channels. The substantially planar dielectric barriers provide a uniform electrical response throughout the exhaust channels. In a preferred embodiment, the monolith comprises a perimeter boundary wall of increased wall thickness to provide electrical insulation between the conductive channels and the housing and to further provide robust crush resistance when inserting the element into a reactor housing. The one-piece monolith is specifically designed for fabrication via extrusion. A minimal number of in-line structural ligaments are preferably extruded as part of the monolith for providing optimal structural support while minimizing backpressure losses.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. provisional application Ser. No. 60/141,394 filed Jun. 29, 1999 by David E. Nelson entitled “Plasma Reactor Design for Treating Auto Emissions —Durable and Low Cost,”. 
    
    
     TECHNICAL FIELD 
     This invention relates to reactors for chemical reduction of nitrogen oxide (NOx) emissions in the exhaust gases of automotive engines, particularly diesel and other engines operating with lean air fuel mixtures that produce relatively high emission of NOx. More particularly, the invention pertains to an improved non-thermal plasma reactor for use with diesel engines and the like and an improved method for preparing same. 
     BACKGROUND OF THE INVENTION 
     In recent years, non-thermal plasma generated in a packed bed reactor has been shown to be effective in reducing nitric oxides (NOx) produced by power plants and standby generators. These units usually have a reducing agent, such as urea, to enhance the conversion efficiency. The packed bed reactor consists essentially of a high voltage center electrode inserted into a cylinder of dielectric material, usually a form of glass or quartz. 
     An outside or ground electrode is formed by a coating of metal in various forms, including tape, flame spray, mesh, etc. The space between the center electrode and the inside diameter of the dielectric tube is filled or packed with small diameter glass beads. When high voltage alternating current is applied to the center electrode, the surfaces of the beads go into corona, producing a highly reactive and selective surface for inducing the desired reaction in the gas. 
     Unfortunately, the packed bed design with its loose beads and glass dielectric is impractical for use in the conditions found in a mobile emitter, such as a car or truck. The vibration and wide temperature swings of the vehicle system would damage the packed bed and the necessary temperature and vibration isolation needed to make it survive would not be cost effective. 
     A stacked plate reactor for use with diesel engines and other engines operating with lean air fuel mixtures is disclosed in commonly assigned U.S. patent application Ser. No. 09/465,073 entitled “Non-thermal Plasma Exhaust NOx Reactor,” which is hereby incorporated by reference herein in its entirety. Disclosed therein is a reactor element comprising high dielectric, nonporous, high temperature insulating means defining a group of relatively thin stacked cells forming gas passages and separated by the insulating means. Alternate ground and charge carrying electrodes in the insulating means on opposite sides of the cells are disposed close to, but electrically insulated from, the cells by the insulating means. The electrodes may be silver or platinum material coated onto alumina plates. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates to prevent arcing while providing a stable electrode spacing for a uniform electric field. The electrodes are coated onto alumina in a pattern that establishes a separation between the electrodes and the connectors of alternate electrodes suitable to prevent voltage leakage. 
     In commonly assigned U.S. Provisional Application Ser. No. 60/141,427 filed Jun. 29, 1999 entitled “Design and Method of Manufacturing a Plasma Reactor for Treating Auto Emissions —Stacked Shapes,” which is also hereby incorporated by reference herein in its entirety, a non-thermal plasma reactor element is prepared from formed building blocks of dielectric material. The formed shape defines an internal cell in the plasma reactor having an exhaust passage for flowing exhaust gas to be treated therethrough. Individual cells are provided with a conductive print disposed thereon to form electrodes and connectors. In a preferred embodiment, the conductive print comprises a continuous grid pattern having a cutout region disposed opposite the terminal connector for reducing potential voltage leaks. Multiple cells are stacked and connected together to form a multi-cell stack. 
     Commonly assigned U.S. Provisional Application Ser. No. 60/141,401 filed Jun. 29, 1999 entitled “Method of Manufacturing A Plasma Reactor For Treating Emissions —Durable and Low Cost,” which is hereby incorporated by reference herein in its entirety, and commonly assigned U.S. Provisional Application Ser. No. 60/141,403 filed Jun. 29, 1999 entitled “Design and Method Of Manufacture Of a Plasma Reactor With Curved Shape For Treating Auto Emissions,” which is also incorporated by reference herein in its entirety, disclose a reactor and method for preparing same, respectively. The reactor is characterized by a reactor element prepared from a curved, swept-shaped substrate specifically designed for fabrication via extrusion. The as-extruded curved substrate comprises a thick outer wall surrounding a plurality of channels separated by dielectric barriers. Selected channels are coated with a conductive material to form conductor channels. The prepared reactor element comprises multiple concentric exhaust channels, multiple concentric conductor channels having alternating polarity, each connected to its respective polarity via bus paths, in-line structural support ligaments for providing optimal structural support while preventing exhaust leakage, and thick outer walls providing high crush resistance and allowing robust mounting into the reactor housing. 
     While the above non-thermal plasma reactors meet some of the current needs and objectives, there remains a need in the art for an improved, durable, low cost non-thermal plasma reactor and improved method of manufacturing same. There further remains a need for a non-thermal plasma reactor that can be prepared with reduced manufacturing complexity, reduced number of components and reduced overall material cost. 
     SUMMARY OF THE INVENTION 
     A non-thermal plasma reactor for conversion of exhaust gas constituents comprises a reactor element prepared from an extruded monolith of dense dielectric material having substantially planar internal features. The monolith comprises a plurality of channels separated by substantially planar dielectric barriers and a perimeter boundary wall. Conductive material printed onto selected channels forms conductive channels that are connected along bus paths to form an alternating sequence of polarity, separated by exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and for preventing electrical leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged. During operation, exhaust gas flows through exhaust channels and is treated by high voltage alternating current flowing through the conductive channels. The substantially planar internal monolith features provide a uniform electrical response throughout the exhaust channels. 
     In a preferred embodiment, the monolith comprises a perimeter boundary wall of increased wall thickness to provide electrical insulation between the conductive channels and to further provide robust crush resistance when inserting the element into a reactor housing. 
     The present one-piece monolith is specifically designed for fabrication via extrusion. A minimal number of in-line structural ligaments are preferably extruded as part of the monolith for providing optimal structural support while minimizing backpressure losses. The present non-thermal plasma reactor provides the dual advantages of low cost and durable design. 
     The present method for preparing a non-thermal plasma reactor element comprises extruding a monolith comprising a perimeter boundary wall and a plurality of substantially planar dielectric barriers separating a plurality of channels for forming exhaust channels and conductive channels; selectively coating selected channels with a conductive material to form conductive channels; and applying a barrier coating to said conductive channels. Selective coating preferably comprises supplying a mask to the extruded monolith, coating the masked monolith with conductive material, with drying, and firing, as needed, to form the conductive channels. Masking may be repeated for applying a barrier coating to cover the conductive channels, with drying, and firing (as needed). Electrical connections are made, typically by attaching terminations to the bus paths with insulating connects, wrapping an insulator, such as an intumescent matt, around the monolith, inserting shielded wire through the reactor housing, and installing the monolith into a housing. 
     The method provides reduced fabrication complexity over prior manufacturing methods. For example, the method provides coating the monolith channels at the same time. Firing cycles occur with the entire monolith rather than as multiple pieces coated and fired separately. High durability is achieved via fabrication using material such as, but not limited to, dense cordierite, alumina, titania, mullite, plastic, and other high dielectric constant materials, or combinations thereof, the presence of structural ligaments, and thick perimeter monolith walls. Further, the enhanced control of monolith wall thickness achieves a uniform electrical response and stable plasma. The dielectric channels contain the conductors while providing resistance to voltage leakage while a dielectric coating prevents voltage leakage at channel ends. Overall cost is significantly reduced over currently known wire, tubular, and stacked plate designs, due to the low cost of the monolith substrate and the minimal secondary processing of the present method. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in the several Figures: 
         FIG. 1  provides a perspective view of an extruded monolith in accordance with the present invention. 
         FIG. 2  provides an alternate view of the monolith of  FIG. 1 , showing additional detail of the monolith face. 
         FIG. 3  provides a view of the present monolith after coating to form conductive channels. 
         FIG. 4  provides a view of the present monolith having selected channels plugged. 
         FIG. 5  provides a section view taken along the line  5 — 5  of  FIG. 4  of an embodiment of the present invention having an additional insulating barrier. 
         FIG. 6  provides a section view taken along the line  5 — 5  of  FIG. 4  of an embodiment of the present invention having an additional insulating barrier and multiple exhaust channels disposed between conductive channels. 
         FIG. 7  provides a section view taken along the line  5 — 5  of  FIG. 4  of an embodiment of the present invention having abbreviated conductive layers. 
         FIG. 8  provides a section view taken along the line  5 — 5  of  FIG. 4  of an embodiment of the present invention having alternating abbreviated conductive layers. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the several FIGURES,  FIG. 1  provides a perspective view of the present extruded monolith  10  for preparing the portion of the non-thermal plasma reactor where the conversion of exhaust gas is carried out (hereinafter referred to as the reactor “element”). The extruded monolith  10  has a frontal area  12  and a length  14 . Frontal area  12  may be rectangular, square, round, or any desired shape. Internal features of the extruded monolith  10  are substantially planar, as shown in  FIG. 2 . 
       FIG. 2  provides a view of additional detail of the as-extruded monolith  10  shown in  FIG. 1 . As-extruded monolith  10  comprises a plurality of channels  16  defined and separated by substantially planar dielectric barriers  18  and surrounded by perimeter boundary wall  20 . The extruded monolith  10  preferably further comprises structural support ligaments  22 , integrally formed as part of the extruded monolith  10 . 
     Channels  16  are bounded by common substantially planar dielectric barriers  18 . Perimeter boundary wall  20  of substantially increased wall thickness relative to barriers  18  forms the perimeter of the extruded monolith  10 . The increased thickness of perimeter boundary wall  20  enhances the reactor&#39;s ability to withstand localized stresses imparted from the retention system (not shown) during manufacture as well as from localized loads that may be imparted during system operation. The increased perimeter boundary wall  20  thickness also serves to impart robust electrical isolation between the reactor element and the reactor housing. The perimeter boundary wall  20  may have a thickness ranging from, for example, about 0.5 millimeters to about 5 millimeters. 
     The frontal area  12  and length  14  (best seen in  FIG. 1 ) of the extruded monolith  10  may each be selected so as to optimize fit, cost, and performance requirement variables of individual systems. Increasing the frontal area  12  decreases the space velocity of the exhaust gas and may significantly increase the conversion efficiency of the reactor. Increasing the length  14  may provide some effect; however, adjusting the length  14  generally has a less significant effect under typical operating conditions. Advantageously, an optimal combination of frontal area  12 , length  14 , and reactor size are selected in accordance with specific vehicle emissions criteria. 
     The space velocity of gas passing through a plasma reactor can be very high, with levels that may exceed 1 million inverse hours. The present invention achieves a low backpressure while taking advantage of the capability for a very high gas space velocity by providing a shape comprising a large frontal area  12  and a relatively short length  14 . In a preferred embodiment, the extruded monolith  10  comprises a frontal area  12  that is sufficiently large to achieve a low backpressure while the length  14  is adjusted to achieve the desired gas space velocity in accordance with the particular engine emission system. 
       FIG. 3  provides a view of the planar monolith  10  of  FIGS. 1 and 2 , after coating. Conductive material printed onto selected channels  16  form conductive channels  24  that are connected along bus paths  26 ,  28  to form an alternating sequence of polarity, separated by exhaust channels  30 . As shown in  FIG. 4 , conductive channels  24  and channels not selected for exhaust flow are preferably formed into plugged channels  32  at end portions of the monolith by plugging with a material suitable for excluding exhaust gases and for preventing electrical leakage between conductive channels  24 . Typically, exhaust channels  30 , disposed between opposite polarity conductive channels  24 , are left uncoated and unplugged. A catalytic coating may be disposed on exhaust channel  30  walls to promote reactions. 
     The extruded monolith  10  defines one or more planar “cells,” each cell comprising a relatively tall (tall relative to the conductive channel  24 ) exhaust channel  30 , a dense dielectric barrier  18 , a relatively narrow conductive channel  24 , and another dense dielectric barrier  18 . One possible embodiment of the present reactor element, comprises a relatively tall exhaust channel  30  having a maximum dimension of about 0.5 to about 3 millimeters, a dense dielectric barrier  18  having a maximum dimension of about 0.25 to about 1.5 millimeters, a relatively narrow conductive channel  24  having a maximum dimension of about 0.1 to about 1.0 millimeters, and another dense dielectric barrier  18  having a maximum dimension of about 0.25 to about 1.5 millimeters. In one embodiment, equal sized conductive channels  24  are used to develop plasma. Alternate embodiments utilize defined templates (tooling) for custom extrusion of conductive channels  24  having varying dimensions. 
       FIG. 3  shows a six-cell monolith  10 . The number of cells utilized in a particular embodiment of the present non-thermal plasma reactor may vary depending upon the space velocity of the exhaust and the relative quality of the incoming exhaust gas. Generally, about 5 to about 200 cells are employed to achieve conversion of typical automotive type exhaust gas constituents. 
     The present method comprises forming the monolith  10 , selectively coating the formed monolith  10 , and firing and drying, as needed. In preparing the present reactor element from the extruded monolith  10 , thin conductive channels  24  are coated with a conductive media. Preferably, the conductive media is intimately disposed against the interior walls of the conductive channels  24  so as to enhance electron injection into the dielectric substrate comprising the monolith  10 . Preferably, selective coating and applying a barrier coating comprises applying a first mask to the monolith  10 ; applying a conductive coating material to form conductive channels  24 ; drying the coated monolith; firing the coated monolith  10 ; removing the first mask, if necessary; applying a second mask to the coated monolith  10 ; applying a barrier coating; allowing said barrier coating to dry; and firing the monolith. 
     The conductive channels  24  are connected alternately with first positive and then negative polarity electric field, repeatedly. In a preferred embodiment, the conductive channels  24  are connected using internal conducting paths to a common bus path for each polarity. For example, as shown in  FIG. 3 , on one side of the substrate  10 , the positive conductive channels  24  (indicated by plus signs “+”) extend slightly further than the negative thin conductive channels  24  (indicated by negative signs “−”) allowing the positive conductive channels  24  to connect with a positive bus path  26 . Similarly, on the opposite side of the monolith  10 , the negative conductive channels  24  extend slightly further than the positive conductive channels  24  allowing the negative conductive channels  24  to connect with a negative bus path  28 . The bus paths  26 ,  28  are connected to positive and negative termination pads  34  and  36 , respectively, located exterior to the monolith  10 . Wires connect the monolith  10  through a reactor housing wall to an electrical power source (not shown). The bus paths  26 ,  28  may be located on the face  12  of the monolith  10 . Alternately, three-dimensional conductive media may be used to dispose the bus paths  26 ,  28  along the path of the conductive channels  24 . In another embodiment, the bus paths  26 ,  28  may comprise a surface print disposed on a top region of the face  12 . 
     Optionally, the portion of the dielectric structural support ligaments  22  disposed within the conductive channels  24  may be coated with conductive media, if desired. When thus coated, the support ligaments  22  further serve to achieve a uniform plasma discharge by maintaining a substantially constant voltage along the horizontal and depth dimensions of the conductive channels  24 . Dielectric structural support ligaments  22  may be left uncoated, to reduce conductive material usage for applications that can tolerate increased voltage variation. Dielectric structural support ligaments  22  provided in exhaust channels  30  are not coated. 
     In certain applications, dielectric structural support ligaments  22  are employed to provide improved structural capability within bus paths  26 ,  28 . In these embodiments, a jumper may be utilized to assure a continuous current path between the conductive channels  24  and the termination pads  34 ,  36 . Alternatively, a portion of the structural ligaments  22  may be removed at one or both ends of the monolith  10  prior to coating the conductive channels  24  and bus paths  26 ,  28  so as to provide a current path through the support ligaments  22  while still allowing the ligaments  22  to provide structural support for most of the monolith  10 . 
     As shown in  FIG. 4 , the present reactor and method preferably further comprises plugging each conductive channel  24  and bus path  26 ,  28  (if coated) to provide plugged channels  32  after the conductive channel walls are wet by the conductive media to prevent flow. Any suitable flow barrier material may be utilized for sealing the passages, including, but not limited to, extra conductive material, dielectric coating, mask, or a combination thereof. The end regions of the reactor may be coated with a dielectric encapsulent to inhibit voltage leakage from the ends of the reactor. This is particularly desirable when conductive material is used as a flow barrier in order to prevent voltage leakage while protecting the conductive material from oxidation. 
     The monolith  10  (which is now the reactor element) is formed into a non-thermal plasma reactor with wrapping or custom-formed insulation or support. In a preferred embodiment, an intumescent type insulating matt is disposed between the metal reactor housing and the monolith  10 . The matt support expands after installation and provides high holding force thus preventing the monolith  10  from shifting in the reactor housing. This is particularly advantageous for stabilizing the monolith  10  within the reactor housing during operation, when high operating temperatures cause housing expansion and thus increase the distance between the monolith  10  and the reactor housing. An insulating seal (not shown) is preferably provided in the area of the monolith-wire terminations  34 ,  36  over the connection to prevent voltage leaks through the matt support. The matt support may optionally comprise an insulator. 
     The wire system connects the monolith  10  to a power source through the reactor housing. The wire system may use a strain relief to achieve highly reliable connections with the monolith  10 . The matt further serves to protect the wire system from elevated temperatures. 
       FIGS. 5–8  provide side sectional views of the monolith  10 , taken along the line  5 — 5  of  FIG. 4 , showing various possible embodiments prepared in accordance with the present invention.  FIG. 5  shows an embodiment employing an insulating barrier  38  for preventing voltage leaks at ends of the monolith  10  having conductive media near the surface and for further sealing off conductive channels  24  to prevent exhaust flow and oxidation. 
     Additional embodiments for distributing conductive media and preventing voltage and exhaust leakage at substrate ends are shown in  FIGS. 6–8 . In  FIG. 6 , multiple exhaust channels  30  separated by dielectric barriers  18  are provided between conductive channels  24 . In the embodiment of  FIG. 6 , three exhaust passages  30  are provided between two conductive channels  24 . This configuration provides reduced backpressure for a given frontal area  12  and increases the amount of exhaust gas than can be treated. This embodiment provides the further advantage of reduced coating costs. The number of exhaust passages  30  disposed between conductive channels  24  is selected based upon the dielectric strength of the dielectric barrier  18  material and the maximum voltage capacity of the reactor power supply system. 
     In the embodiments of  FIGS. 7 and 8 , the optional insulating barrier  38  is not provided at the monolith  10  ends. The embodiment shown in  FIG. 7  contains the conductive media disposed in the conductive channels  24  at a distance from each end of the monolith  10 . For example, coating may be applied so as to dispose the conductive media from about 5 to about 15 millimeters from opposite ends of the monolith  10 . In  FIG. 8 , a staggered coating pattern is employed wherein the conductive media stops short at alternate ends of the monolith  10 . For example, in alternating conductive channels  24 , a section about 10 to about 30 millimeters from opposite end portions of conductive channels  24  are uncoated. 
     In an alternate embodiment, the present method comprises extruding a monolith having narrow conductive channels  24  at each parallel step level. Thick exhaust channels  30  are centered across each barrier  18  layer thickness and extend substantially across the full width of each layer  18 . Preparation comprises extruding a ceramic monolith substrate, coating the walls of the conductive channels  24  with conductive material, firing, and optionally cutting off a slight portion at each channel end to assure electrical isolation between layers. Robust electrical connections are then provided, preferably from the front and back regions of the extruded monolith  10 . 
     In yet another embodiment of the present method, a co-extruded ceramic-metal foil monolith is provided. The method comprises co-extruding a ceramic substrate with metallic foils providing a conductive channel that is integrally covered by dielectric material. A near continuous process may be employed such as, for example, firing the co-extruded ceramic monolith-metallic foil “log” using a continuous type kiln and separating the “log” into desired lengths with a traveling dicing saw. Robust electrical connections are provided along the side of the monolith by running the metallic foil outside the ceramic edges during extrusion. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.