Abstract:
An integrated oxidation-reduction process whereby a thermal oxidation zone and a NOx reduction zone are incorporated into a single device. A thermal oxidation and catalytic reduction system having a multi-spiral, heat recuperative configuration and including a lean NOx catalytic section disposed in the low-temperature regions is disclosed, as is a corresponding method. The catalyst may be disposed on the oxidizer walls or on a matrix of porous inert media disposed in the spiral passages of the oxidizer. A film-injection technique to selectively provide reactants to the catalyst surface is also disclosed. The catalyst may be limited to a concave portion of a sidewall to diminish boundary layer separation of the reactants.

Description:
FIELD OF THE INVENTION 
     This invention relates to the reduction of nitrogen oxides and the thermal oxidation of organics, and more particularly, to a device and method for the reduction of nitrogen oxides and the thermal oxidation of organics in a net oxidizing environment of a gas and particulate matter stream from industrial and vehicle exhaust. 
     BACKGROUND OF THE INVENTION 
     Destruction or conversion of atmospheric pollutants in industrial gas streams and internal combustion engine exhaust streams has been a long-standing research and development goal. Such atmospheric pollutants include products of incomplete combustion, such as carbon monoxide and unburned hydrocarbons, oxides of nitrogen [“NOx”], and carbonaceous particulate matter [“PM”]. 
     Lean-burning engines, such as diesel engines and lean-burning gasoline or natural gas engines, often emit levels of pollutants above regulatory limits. In response to air quality regulations, vehicle manufacturers employ pollution control devices in internal combustion engine exhaust systems to reduce these emissions. Traditional gasoline engine pollution control devices employ a ceramic honeycomb monolith or a packed bed of pellets having a coating of a noble metal catalyst. Such devices catalyze the reactions of carbon monoxide and unburned hydrocarbons with oxygen, typically at approximately 260° C. to 427° C. (500° F. to 800° F.). Other devices employ catalysts that also catalyze the reaction of oxides of nitrogen. Unfortunately, two factors render such catalytic devices alone insufficient for treating vehicle engine exhaust (especially diesel engine) and similar industrial emissions. First, the catalytic devices are ineffective at destroying PM, which is present in engine gas streams, especially those from diesel engines. Second, the PM and other particulates deposit on the monolith, thereby preventing gaseous constituents from reaching the catalytic material, or possibly deactivating or poisoning the catalyst. In general, conventional three-way-catalysts fail to reduce NOx under lean-burn (that is, oxygen-rich) conditions common to many internal combustion engines. 
     Internal combustion engines are the subject of regulations limiting NOx emissions. The simultaneous emission limits for both particulate matter and NOx presents a unique problem because the two pollutants typically have an inverse relationship in engine exhaust. Internal combustion engines generally can be configured and tuned to produce an exhaust stream having low PM and high NOx concentrations or, alternatively, high PM and low NOx concentrations. Traditionally, engines that employ oxidation catalyst devices may be adjusted to minimize NOx formation because of the catalysts&#39; inability to reduce NOx. Such adjustments may compromise engine efficiency and performance. 
     Although not generally employed in reducing NOx emissions from internal combustion engines, various techniques exist for reducing NOx emissions from gas streams in other applications. One technique for reducing NOx emissions is selective catalytic reduction (SCR), which reduces NOx in the presence of a reducing agent, such as of ammonia (NH 3 ), over a catalyst. Typically, selective catalytic NOx reduction is employed with exhaust stream temperatures in the range of 288° C.-427° C. (550° F.-800° F.). SCR catalysts have the limitations discussed herein above. 
     Another approach for removing NOx is selective non-catalytic reduction (SNCR), which employs a chemical that selectively reacts, in the gas phase, with NOx in the presence of oxygen at a temperature greater than 621° C. (1150° F.). Chemical NOx reduction agents used in such processes include ammonia (NH 3 ), urea (NH 2 CONH 2 ), cyanuric acid (HNCO) 3 , iso-cyanate, hydrazine, ammonium sulfate, atomic nitrogen, melamine, methyl amines, and bi-urates. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a system and method or reducing pollutant emissions from internal combustion engines and industrial exhaust gas streams. Specifically, an object of the present invention is to provide a system and method for reducing NOx, and oxidizing PM and oxidizable constituents in an engine or industrial exhaust stream. It is an object to provide such a system and method in a single, compact device, especially one suitable for use in a mobile vehicle engine. 
     It is another object of the present invention to provide a device and method for the integrated, substantially-simultaneous oxidation and reduction, especially thermal oxidation of organics and catalytic reduction of NOx, of an exhaust gas stream. 
     It is a further object of the present invention to provide a system and method for reducing NOx emissions from an engine exhaust stream under lean-burn (that is, oxygen-rich) conditions. 
     It is yet a further object of the present invention to provide a thermal oxidation and catalytic reduction system having a catalytic surface to reduce NOx under lean-burn conditions. 
     It is yet a further object of the present invention to provide film injection techniques to enhance contact between a reactant and a catalyst. 
     It is yet a further object of the present invention to provide techniques for the injection of supplementary fuel into a system for the thermal oxidation of organics and catalytic NOx reduction in such a manner that the performance of the NOx reduction catalyst is enhanced. 
     According to the present invention, a thermal oxidation and catalytic reduction system arranged in a compact, multi-spiral, recuperative configuration is provided that includes two interspaced, coiled sidewalls that form a spiral inlet passage and a spiral outlet passage, and a central chamber. A thermal oxidation zone, which is preferably disposed in the central chamber, may be located between the inlet and outlet (that is, entrance and exit) passages, which form a spiral, counter-current heat exchanger. A matrix of porous inert media may be disposed within each one of the spiral passages and in the central chamber. The oxidation reaction zone, which is in flow communication with the spiral inlet passage and spiral outlet passage, receives heat primarily by convection from the oxidized gases and loses heat primarily by radiation to the matrices, which are in intimate contact with the gas stream. 
     The thermal oxidation and catalytic reduction system utilizes a catalytic surface to reduce NOx. Preferably a lean-NOx catalyst is employed in the appropriate regions of the thermal oxidation and catalytic reduction system—that is, proximate the inlet (for conditions in which the inlet gas stream is within or belowthe range at which the particular catalyst may be effective) and the outlet. The catalytic surface may be disposed either on the sidewalls forming the spiral passages, on the media, or in a combination of the media and sidewalls. In the embodiment in which the catalytic surface is disposed on one or more sidewalls, the matrix may be omitted from the passage adjacent to the catalytic surface. 
     The matrices foster stable oxidation of the reacting gas at low temperatures (for example 788° C.-1093° C. (1450° F.-2000° F.)) within the reaction zone of a thermal oxidizer portion of the present system) compared with premixed flames. Thus, the system according to the present invention diminishes the formation of oxides of nitrogen. Further, the matrices provide a highly radiative environment and long residence times, which promote the destruction of gas phase organics, CO, and PM. Further, the geometry of the present invention provides regions that have temperature ranges that are well-suited for a wide variety of NOx reduction techniques. Specifically, the relatively smooth temperature profile of the gas stream within the spiral passages, compared with combustion processes using (for example) premixed flames, provides relatively long residence times within a wide range of temperatures to enable the present invention to employ a broad range of emission control techniques, especially those relating to NOx reduction. Further, the present invention, because of the stable oxidation conditions created therein, is well-suited to the wide variations in flow rate and temperature (approximately 70° C. to 600° C.) common to engine exhaust streams. 
     According to another aspect of the present invention, a thermal oxidizer is integrated with a lean-NOx catalyst that utilizes a reducing agent or reactant stream, which includes, among other constituents, hydrogen, hydrocarbons, and carbon monoxide, to chemically reduce NOx to diatomic nitrogen [“N 2 ”]. The integration of the oxidation process and the lean-NOx reduction process enables the reducing agent both to reduce NOx and to provide supplemental fuel to enhance the oxidation process, as well as providing a compact system. 
     In another aspect of the present invention, a thermal oxidation and catalytic reduction system is provided having a thermal oxidation zone, a catalytic surface disposed on the concave surfaces of the sidewalls, and film-injection means to supply reactants (especially hydrogen, hydrocarbons, and carbon monoxide) to the catalytic surface in greater concentration than if the reactants were premixed in the gas stream. The concave surfaces of the sidewalls are the preferred substrate for the catalytic surface to avoid boundary layer separation of the gas stream over the convex surface. The effectiveness and performance of the catalytic reduction of NOx may thus be enhanced. The film injection for the purpose of enhancing effectiveness of lean-NOx catalysts is preferably employed in the exterior portions (defined angularly) of the sidewalls. 
     The film-injection also provides cooling to the catalytic surface, thereby enabling the catalytic surface to be disposed further into the thermal oxidation and catalytic reduction system and yet operate within its optimum temperature range. The resulting augmented catalytic surface area may provide increased NOx reduction without increasing oxidizer size. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the thermal oxidation and catalytic reduction system according to an embodiment of the present invention; 
     FIGS. 2A through 2G are views of a section of the thermal oxidation and catalytic reduction system of FIG. 1 showing various arrangements of the catalytic surface; 
     FIG. 3 is a schematic view of the thermal oxidation and catalytic reduction system according to another aspect of the present invention; 
     FIG. 4 is a schematic view of a thermal oxidation and catalytic reduction system according to another aspect of the present invention; 
     FIG. 5 is a perspective view of a portion of a sidewall of the thermal oxidation and catalytic reduction system shown in FIG. 4 to illustrate another aspect of the present invention; 
     FIG. 6 is a schematic view of a thermal oxidation and catalytic reduction system according to another aspect of the present invention; 
     FIG. 7 is a typical temperature profile of the gas stream according to the embodiment of the present invention shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 to illustrate a first embodiment of the present invention, a thermal oxidation and catalytic reduction system  10   a  comprises a first coiled sidewall  18  that is interspaced with a second coiled sidewall  20  to form a spiral inlet passage  22  and a spiral outlet passage  24 . The first sidewall  18  forms an exterior portion  26  and an interior portion  28 . The second sidewall  20  forms an exterior portion  30  and an interior portion  32 . The general configuration of such a spiral device may be broadly referred to as as “swiss roll.” The terms “exterior portion” and “interior portion,” as used to define portions of the sidewalls herein and in the appended claims, refer to angular portions of the sidewalls such that the exterior portion generally has a greater radius of curvature than the interior portion. The term exterior portion is further defined below. 
     The first sidewall  18  and the second sidewall  20  each have a concave surface  34  and a concave surface  38 , respectively, which each face the center of thermal oxidation and catalytic reduction system  10   a , and a convex surface  36  and a convex surface  40 , respectively, which each face away from the center of the thermal oxidation and catalytic reduction system  10   a . A cylinder  44  is disposed near the center of thermal oxidation and catalytic reduction system  10   a  to form a central chamber  46  having a heating means  49  disposed therein. Alternatively, rather than being formed by a cylinder, a central chamber may be formed between spiral, spaced-apart, opposing ends of sidewalls, as shown in co-pending U.S. patent application Ser. No. 09/072,851, entitled “A Device for Thermally Processing a Gas Stream, and Method for Same,” which is incorporated herein by reference in its entirety. 
     Within the spiral inlet passage  22  is an inlet passage matrix  50  of porous inert media; within the spiral outlet passage  24  is an outlet passage matrix  52  of porous inert media; and within the central chamber  46  is a central chamber matrix  54  of porous inert media. Matrices  50 ,  52 , and  54  are shown in FIG. 1 in cut-away sections of the thermal oxidation and catalytic reduction system  10   a  for clarity. Specifically, matrix  50 , matrix  52 , and matrix  54  each comprise a porous bed of solid, heat-resistant media through which a gas stream  4  passes. The present invention broadly encompasses matrices  50 ,  52 , and  54  in an combination. For example, matrices  50  and  52  may be wholly omitted from their respective passages (that is, only the central chamber  46  has a matrix); matrix  50  may be omitted from all or a potion of spiral inlet passage  22 ; matrix  52  may be omitted from all or a portion of spiral outlet passage  24 ; matrices  50  and  52  may be partially omitted from their respective passages (that is, the central chamber  46  and portions of the passages  22  and  24  have matrices); matrix  54  may be omitted from a portion of central chamber  46  (that is, all or portions of spiral inlet passage  22  and/or spiral outlet passage  24 —including any combination thereof—have matrices  50  and  52 , respectively, and a portion of central chamber  46  has matrix  54 ). 
     The media of matrices  50 ,  52 , and  54  encompass a bed of any ceramic, metal, or other heat-resistant media, including: metal wool, balls, chunks, granules (preferably approximately 0.25″ to 1″″ diameter for the balls, chunks, or granules); saddles, preferably approximately 0.5″ to 1.5″ nominal size; pall rings; foam, preferably having a void fraction of approximately 90% and about ten to thirty pores per inch; and honeycomb. Metal wool or foam are preferred. Regardless of the type or material of the media, interstitial diameters of approximately 0.125″ to 1.0″ are preferred. 
     Although the Figures generally use balls to represent the media, the present invention encompasses any combination of the above or other types and sizes of media, whether used separately or in combination, and whether randomly or structurally arranged. Further, the media may include an engineered matrix portion that has two or more flow control portions. The materials of the media are chosen according to their heat transfer properties. The size, composition, and material selections are determined to obtain a desired overall heat transfer and catalytic reaction characteristic. U.S. patent application Ser. No. 08/921,815, entitled “Matrix Bed For Generating Non-Planar Reaction Wave Fronts and Method Thereof”, filed Sep. 2, 1997, and U.S. patent application Ser. No. 08/922,176, entitled “Method of Reducing Internal Combustion Engine Emissions, and System for Same,” filed Sep. 2, 1997, which are each incorporated herein by reference in their entireties, describe the engineered matrix and the media in greater detail. 
     Further, co-pending U.S. patent application Ser. No. 09/072,851 describes aspects and features of a spiral, thermal processing device, including, for example, devices and methods for controlling a thermal oxidizer, heating means  49 , means for providing inlet and outlet devices, and the like, that may be applied to the present invention. Heating means, and operating and control techniques are also described in U.S. patent application Ser. No. 08/922,176. 
     Spiral inlet passage  22  and spiral outlet passage  24 , because of the oxidation reaction occurring preferably within the central chamber  46 , generally form an inlet passage low temperature region  58 , an outlet passage low-temperature region  60 , an inlet passage high-temperature region  62 , and an outlet passage high temperature region  64 . 
     According to one aspect of the present invention, a catalytic surface  70   a  may be disposed on first sidewall  18  in the inlet passage low-temperature region  58 , and another catalytic surface  70   b  may be disposed on second sidewall  20  in the outlet passage low-temperature region  60 . Catalytic surfaces  70   a  and  70   b  preferably are disposed proximate the exterior ends of the inlet passage  22  and/or the outlet passage  24 , respectively, at locations in which the catalytic surfaces  70   a  and  70   b  would contact gases near the optimum catalytic temperature range for the catalyst formulation. The term “low-temperature region,” as used herein and in the appended claims, refers to a portion of the thermal oxidation and catalytic, reduction system  10   a  that corresponds to a temperature range within which a lean-NOx catalyst may be operative to catalytically reduce NOx. It is understood that the “low-temperature region” may be proximate the inlet of the spiral inlet passage  22 , proximate the outlet of the spiral inlet passage  24 , or within either passage. Further, it is understood that the term “exterior portion,” when used to refer to portions of a sidewall, also generally refers to the region of the sidewall having a temperature range within which a lean-NOx catalyst may be operative to catalytically reduce NOx. 
     The temperature range of the low-temperature regions (that is, the regions within the device that the catalytic surface is disposed) may be chosen, in part, according to the optimum operating temperature range of the catalyst, the temperature at which the rate of creation of NOx from the fuel equals the rate at which NOx is reduced (which may set an upper effective temperature limit), mechanical properties of the catalytic material and its substrate, and like characteristics of the particular application. Catalytic surfaces  70   a  and  70   b  (as well as surface  70   c , which will be discussed below) preferably are formed of a catalyst material suitable for low-temperature reduction of NOx in lean, gasoline or diesel engine exhaust (or that from a similar industrial process). Although any suitable catalyst may be employed, a lean-NOx catalyst is preferred. An example of such a lean-NOx catalyst is one that incorporates ircn (II)-complex impregnated molecular sieves and is further treated with [Pd(NH 3 ) 4 ]CL 2 —as described in “Development of a Lean-NOx Catalyst Containing Metal-Ligand Complex Impregnated Molecular Sieves,” Paul, et al., SAE Technical Paper Series 962050 (1996). The molecular sieve may, for example, comprise the type MCM-41 available from Mobil Oil Corp. Such a catalyst has an optimum effectiveness when operating at a temperature of approximately 288° C. to 427° C. 
     FIGS. 2A through 2G show various configurations of catalytic surfaces  70   a  and  70   b  on a section of sidewall  18  and/or sidewall  20  in the low-temperature regions  58  and  60  of the spiral passages  22  and  24 . According to the embodiment of the invention shown in FIG. 1, the porous inert media of matrices  50  and  52  is absent from the passages  22  and  24  that contain catalytic surfaces  70   a  and  70   b . In FIGS. 2A through 2G, catalytic surfaces  70   a  and  70   b  are represented in a cross-hatched pattern for clarity. 
     FIG. 2A shows catalytic surface  70   a  disposed on first sidewall concave surface  34  and second sidewall convex surface  40 , and catalytic surface  70   b  disposed on second sidewall concave surface  38  and first sidewall convex surface  36 . Catalytic surface  70   a , therefore, is disposed on both the concave and convex surfaces of the low temperature region  58  of the spiral inlet passage  22 , and catalytic surface  70   b  is disposed on both the concave and convex surfaces of the low temperature region  60  of the spiral outlet passage  24 . FIG. 2B shows catalytic surface  70   b  disposed on first sidewall convex surface  36  and the second sidewall concave surface  38  so as to line opposing sides of spiral outer passage  24 . FIG. 2C shows catalytic surface  70   a  disposed on first sidewall concave surface  34  and second sidewall convex surface  40  so as to line opposing sides of spiral inlet passage  22 . FIG. 2D shows catalytic surface  70   a  disposed on first sidewall concave surface  34  so as to line the radially outward side of spiral inlet passage  22 . FIG. 2E shows catalytic surface  70   a  disposed on second sidewall convex surface  40  so as to line the radially inward side of spiral inlet passage  22 . FIG. 2F shows catalytic surface  70   b  disposed on the second sidewall concave surface  38  so as to line the radially outward side of spiral outlet passage  24 . FIG. 2G shows catalytic surface  70   b  disposed on the first sidewall convex surface  36  so as to line the radially inward side of spiral outlet passage  24 . 
     In the embodiments of the invention shown in FIG. 2A, catalyst material directly communicates with the gas stream  4  within both the spiral inlet passage  22  and the spiral outlet passage  24 . In the embodiments shown in FIGS. 2C,  2 D, and  2 E, catalytic surface  70   a  directly communicates with gas stream  4  within the spiral inlet passage  22 , but the spiral outlet passage  24  lacks direct contact with catalyst material (that is, these embodiments lack catalyst surface  70   b ). In the embodiments shown in FIGS. 2B,  2 F, and  2 G, catalytic surface  70   b  directly communicates with gas stream  4  within the spiral outlet passage  24 , but the spiral inlet passage  22  lacks direct contact with catalyst material (that is, these embodiment lacks catalyst surface  70   a ). The term “directly communicate,” as used herein and in the appended claims in conjunction with a specified passage and a surface, refers to direct contact between the specified passage and that surface, but excludes contact between the surface and the gas stream in the passages other than those expressly specified. 
     The length along sidewall  18  and/or  20  on which catalytic surface  70   a  and/or  70   b  are disposed will vary according to parameters associated with the particular application, including, for example, gas stream  4  inlet temperature and desired outlet temperature, gas flow rates, heat transfer characteristics of the gas, and heat transfer characteristics and configuration of the thermal oxidation and catalytic reduction system (including, for example, passage width, length, height, number of turns, and like geometric and mechanical parameters), and others, as will be understood by those familiar with such devices and applications. Further, film-injection techniques, as described herein, may enable catalytic surfaces  70   a  and/or  70   b  to maintain a temperature within the catalysts&#39; target operating range, even while the local gas stream  4  temperature is higher, because the film injection may provide cooling. The catalyst material that forms catalytic surfaces  70   a  and  70   b  is preferably electroplated, sputtered, or applied by a series of washcoat methods onto sidewalls  18  and  20 , which are preferably formed of metal, although other conventional methods of forming catalytic surfaces  70   a  and  70   b  on sidewalls  18  and  20  may be employed. 
     The device may be arranged such that a matrix of porous inert media is disposed on the inlet side (that is, upstream) of catalytic surface  70   a  in the spiral inlet passage  22 , and such that a matrix of porous inert media is disposed on the outlet side (that is, downstream) of catalytic surface  70   b  in the spiral outlet passage  24 . The design parameters of such matrices, which are not shown in the figures, may be determined by heat transfer, pressure drop, and gas characteristics (among other similar variables), as will be understood by those familiar with the particular use and with the devices and methods described herein. 
     Referring to FIG. 3 to illustrate another aspect of the present invention, a thermal oxidation and catalytic reduction system  10   b  is structurally similar to thermal oxidation and catalytic reduction system  10   a  (shown in FIG.  1 ), except for the inlet passage matrix  50 , the outlet passage matrix  52 , and catalytic surfaces. In the embodiment shown in FIG. 3, matrices  50  and  52  are disposed within spiral inlet passage  22  and spiral outlet passage  24  proximate the exterior portions  26  and  28  of sidewalls  18  and  20 , respectively (that is, within low temperature regions  58  and  60 ). A catalytic surface  70   c  is coated onto the media of matrices  50  and  52  to provide large contact surface area with the gas stream  4 . In addition to having both matrices coated with catalytic surface  70   c , the present invention encompasses having the catalytic surface  70   c  disposed only the spiral inlet matrix  50 , only on the spiral outlet matrix  52 , and any combination of matrix  50 , matrix  52  and portions of the sidewalls (that is, by employing catalytic surface  70   a  and/or  70   b  as shown in FIGS.  2 A through  2 G). Catalytic surface  70   c  may be formed on the surface of the matrices  50  and/or  52  by a series of washcoat methods if the media is ceramic or other non-conducting substrate, or by electroplating if the media is metal wool or other electrically conducting substrate. Sputtering may also be used. 
     Referring to FIGS. 1 and 3 to illustrate another aspect of the present invention, system  10   a ,  10   b , or  10   c  may employ multiple, sequentially-disposed catalytic surfaces, each of which are formed of a unique catalyst formulation. Specifically referring to FIG. 1, catalytic surface  70   a  comprises a first catalytic surface  70   a ′, a second catalytic surface  70   a ″, and a third catalytic surface  70   a ′″. Catalytic surfaces  70   a ′,  70   a ″, and  70   a ′″ are preferably disposed angularly adjacent such that surface  70   a ′ is disposed within spiral inlet passage  22  relatively upstream (that is, as defined by the path of gas stream  4 ) of surface  70   a ″, which, in turn, is upstream of surface  70   a ′″. First catalytic surface  70   a ′ may be formed of a lean-NOx catalyst formulation that has an effective temperature range that is optimized for the expected gas and surface temperatures proximate surface  70   a ′. Likewise surfaces  70   a ″ and  70   a ′″ may be formed of lean-NOx catalyst formulations having temperature ranges that are similarly optimized. For example, the catalyst forming surface  70   a ′ may be a lean-NOx catalyst that is optimized to catalyze the reduction of NOx over a temperature range of 250° C. to 350° C., and is disposed at a location within spiral inlet passage  22  such that surface  70   a ′ encounters temperatures approximately within that range. Surfaces  70   a ″ and  70   a ′″ may be optimized to catalyze the reduction of NOx over temperature ranges approximately of 350° C. to 450° C., and 450° C. to 550° C., respectively, and may be correspondingly and respectively disposed within spiral inlet passage  22  downstream of surface  70   a ′. Surfaces  70   a ′,  70   a ″, and  70   a ′″ may be contiguous or may be disposed with gaps therebetween. 
     Similarly, catalytic surface  70   b  comprises a first catalytic surface  70   b ′, a second catalytic surface  70   b ″, and a third catalytic surface  70   b ′″. Catalytic surfaces  70   b ′,  70   b ″, and  70   b ′″ are preferably disposed angularly adjacent such that surface  70   b ′ is disposed within spiral outlet passage  22  relatively upstream of surface  70   b ″, which, in turn, is upstream of surface  70   b ′″. Surfaces  70   b ′,  70   b ″, and  70   b ′″ may be formed of lean-NOx catalyst formulations as described above with reference to surfaces  70   a ′,  70   a ″, and  70   a′″.    
     Referring specifically to FIG. 3, catalytic surface  70   c  comprises a first catalytic surface  70   c ′, a second catalytic surface  70   c ″, and a third catalytic surface  70   c ′″. Catalytic surfaces  70   c ′,  70   c ″, and  70   c ′″ are preferably disposed angularly adjacent such that surface  70   c ′ is disposed within matrix  50  relatively upstream (that is, as defined by the path of gas stream  4 ) of surface  70   c ″, which, in turn, is upstream of surface  70   c ′″. Catalytic surfaces  70   c ′,  70   c ″, and  70   c ′″ are also disposed angularly adjacent within spiral outlet passage  24  such that surface  70   c ′ is disposed within matrix  52  relatively upstream (that is, as defined by the path of gas stream  4 ) of surface  70   c ″, which, in turn, is upstream of surface  70   c ′″. Catalytic surfaces  70   c ′,  70   c ″, and  70   c ′″ may be formed of lean-NOx catalyst formations and disposed according to the descriptions referring to surfaces  70   a ′,  70   a ′″,  70   a ′″,  70   b ′,  70   b ″, and  70   b′″.    
     According to another aspect of the present invention, a means for injecting a film of reactants proximate the catalytic surfaces  70   a  and/or  70   b  is provided. Referring specifically to FIG.  4  and FIG. 5 to illustrate a first embodiment of the film-injection means, a channel  76   a  enables flow of a reactant stream  6   a  to communicate with catalytic surface  70   a , which is disposed on concave surface  34  of first wall  18 . A spiral channel plate  74   a  is disposed substantially parallel to first side wall  18  so as to form reactant channel  76   a  between an inside, concave surface  78   a  of channel plate  74   a  and the convex surface  36  of first side wall  18 . Preferably, reactant channel  76   a  is disposed along substantially the entire length of catalytic surface  70   a . Plural transpiration holes  80  are disposed through catalytic surface  70   a  and through first sidewall  18  such that the reactant stream  6   a  is in fluid communication with spiral inlet passage  22 . 
     Similarly, another film injection means may comprise a spiral channel plate  74   b  that creates a reactant channel  76   b . Spiral channel plate  74   b  is disposed substantially parallel to first sidewall  20  so as to form reactant channel  76   b  between an inside, concave surface  78   b  of channel plate  74   b  and the convex surface  40  of second side wall  20 . Preferably, reactant channel  76   b  is disposed along substantially the entire length of catalytic surface  70   b . Plural transpiration holes  80  are disposed through catalytic surface  70   b  and through second side wall  20  such that a reactant stream  6   b  is in fluid communication with the spiral outlet passage  24 . 
     Channels  76   a  and/or  76   b  may be formed by suitable means for producing such a channel, including dimpling channel plate  74   a  and/or  74   b  (as is described in co-pending application Ser. No. 08/922,176), by stamping channels into one or both of plates  74   a  and/or  74   b , by utilizing stand-offs or studs to space the plates apart, or other means, as will be understood by those familiar with such techniques. Similarly, sidewall  18  and/or  20  may be formed with dimples or channels to form channels  76   a  and/or  76   b . Reactant streams  6   a  and  6   b  comprise reducing agents that are effective for use with the catalyst material forming catalytic surfaces  70   a ,  70   b , and  70   c . Specifically, for a lean-NOx catalyst, reactant streams  6   a  and  6   b  may include hydrogen, hydrocarbons, and/or carbon monoxide. For example, a lean-NOx catalyst has demonstrated NOx reduction in a diesel engine exhaust stream by utilizing a 10 gram per hour stream of diatomic hydrogen [“H2”] for cars, and a 20 to 50 gram per hour stream of H2 for vans and trucks. Lean-NOx catalytic surfaces  70   a ,  70   b , and  70   c  may also utilize hydrogen, hydrocarbons, and carbon monoxide reactants already present in gas stream  4 , or created while gas stream  4  is within the thermal oxidation and catalytic reduction system  10   a ,  10   b , or  10   c , in addition to utilizing reactants supplied by streams  6   a  and  6   b . Further, the constituents and constituent concentrations of stream  6   a  may vary from those of stream  6   b  to optimize catalysis with the particular catalyst material used to form catalytic surfaces  70   a  and  70   b , respectively. Providing reactant streams  6   a  and  6   b  to the thermal oxidation and catalytic reduction system may be by conventional means. 
     Referring to FIG. 6 to illustrate another embodiment of the film injection means, injection ports  82   a  are disposed proximate the concave surface  34  of first side wall  18  to enable injection of reactant stream  6   a  into spiral inlet passage  22  along catalytic surface  70   a . Similarly, injection ports  82   b  may be disposed proximate the concave surface  38  of second sidewall  20  to enable injection of a reactant stream  6   b  into spiral inlet passage  24  along catalytic surface  70   b . Injection ports  82   a  and  82   b  may be formed in a shape that promotes downstream boundary layer stability (that is, that tends to keep the boundary layer attached to catalytic surfaces  70   a  and  70   b )—for example, an airfoil or similar tapered or non-bluff shape. 
     Although FIG. 6 shows three injection ports  82   a  and three injection ports  82   b , the number and location of injection ports  82   a  and  82   b , proximate their respective passages, will be determined according to the desired distribution of reactant stream  6   a  and  6   b , heat transfer characteristics of the thermal oxidation and catalytic reduction system, and like parameters, as will be understood by those familiar with such film injection, film cooling and thermal oxidizing and reducing techniques. The present invention also encompasses employing conventional film injection means, as will be understood by those familiar with such meals. Further, reactant channels  76   a  and  76   b , and/or injection ports  82   a  and  82   b , may be employed with the embodiment of the invention shown in FIG. 3 to supply reactant streams  6   a  and  6   b  to the catalytic surface  70   c . Reactant streams  6   a  and  6   b , and the corresponding devices  76   a ,  76   b ,  82   a , and  82   b  are omitted from FIG. 3 for clarity. 
     The method according to the present invention will be described in conjunction with the operation of the thermal oxidation and catalytic reduction system, using FIGS. 1,  2 , and  7  for illustration. FIG. 7 illustrates a typical temperature profile of gas stream  4  within the system by providing the relationship of gas stream  4  temperature versus distance x, which is measured spirally along the path of passages  22  and  24 , and, linearly across central chamber  46 . FIG. 7 includes curve A, which represents operation under partial load conditions, and curve B, which represents operation under full load conditions, preferably of an internal combustion engine. Gas stream  4  may be produced, however, by an internal combustion engine (which broadly includes spark and compression ignition engines), an industrial source (which broadly includes burners, turbine combustors, boilers, furnaces, chemical reactors, nitric acid digesters, and the like), or a similar process—preferably operating under oxygen-rich conditions. Gas stream  4  includes oxides of nitrogen, oxygen, and combustible constituents, including hydrocarbons, carbon monoxide, and PM. If stoichiometrically insufficient oxygen is present in gas stream  4 , which may occur especially where the present invention is employed with industrial exhaust gas streams, supplemental oxygen may be added according to known principles, and according to techniques described herein and conventional techniques. 
     Generally, gas stream  4  flows into spiral inlet passage  22  where it contacts catalytic surface  70   a  disposed of the first sidewall  18 , and after oxidation occurs within the thermal oxidation zone, gas stream  4  flows into spiral outlet passage  24  where it contacts catalytic surface  70   b  on the convex surface  40  of the second sidewall  20 . The catalytic reduction of the oxides of nitrogen is enhanced by the hydrogen, hydrocarbon, and carbon monoxide constituents of the gas streams  6   a  and  6   b , or by those already present in gas stream  4 . Therefore, the operation of the engine (for example, spark timing, injection timing, and valve timing) capable of supplying gas stream  4  may be adjusted to supply an optimum amount of such reactants to optimize NOx reduction. 
     Specifically, gas stream  4  flows across the catalytic surface  70   a , as is represented in curve portions A 1  and B 1  in FIG. 7, and through matrix  50 , where heat is transferred from the matrix  50  and from the sidewalls to gas stream  4 , as is represented by the curve portions A 2  and B 2 . Gas stream  4  flows from the matrix  50  into the central chamber  46 , preferably where the combustible constituents, including PM and the un-reacted reducing agents, oxidize according to heat transfer and reaction principles described in co-pending application Ser. Nos. 08/922,176 and 09/072,851. The oxidation zone is represented in FIG. 7 as the relatively steeply-sloped curve portions A 3  and B 3 . Gas stream  4  then flows through matrix  52 , in which the gas stream  4  transfers heat to matrix  52  and sidewalls  36  and  38 , as is represented by the curve portions A 4  and B 4 . Gas stream  4  flows into contact with another catalytic surface  70   b  disposed along spiral outlet passage  24 , as is represented in curve portions A 5  and B 5 , whereby additional lean-NOx catalysis occurs, especially where reactant stream  6   b  is present. In FIG. 7, curve portions A 2 , B 2 , A 4 , and B 4  have a greater slope than corresponding curve portions A 1 , B 1 , A 5 , and B 5  to represent that matrices  50  and  52  increase the overall, local heat transfer coefficient therein. 
     The relatively smooth temperature profile of the curves A and B, compared with combustion processes corresponding to, for example, premixed flames, demonstrates that the present invention provides relatively long residence times of gas stream  4  within the temperature ranges corresponding to curve portions A 1 , B 1 , A 2 , B 2 , A 4 , B 4 , A 5 , and B 5 . The portions of the system that provide these temperature regions may be employed to optimize the NOx reduction process according to the methods discussed herein utilizing catalytic surfaces  70   a ,  70   b , and  70   c . Further, the long residence times at the wide range of temperatures enables the present invention to be employed with a wide variety of other emission control techniques that may require such residence times and temperatures. Further, the present invention, because of the stable oxidation conditions created therein, is well-suited to the wide variations in flow rate and temperature (approximately 70° C. to 600° C.) common to engine exhaust streams. 
     Under typical partial load operation of a lean-burn internal combustion engine, the temperature of gas stream  4  at the inlet of spiral inlet passage  22 , which is represented as point A 6 , may be approximately 200° C. Under typical full load operation of a lean-burn internal combustion engine, the temperature of gas stream  4  at the inlet of spiral inlet passage  22 , which is represented as point B 6 , may be approximately 400° C. The temperature at which the thermal oxidation reaction begins, which is represented as point A 7  and B 7 , may be approximately 788° C. to 825° C., although these temperatures are only exemplary, and may vary widely. 
     Referring to FIG. 3 to illustrate another method according to the present invention, the catalytic surface  70   c  may be disposed on inlet matrix  50  and catalytic surface  70   c  may be disposed on outlet matrix  52  to reduce NOx. In the embodiment shown in FIG. 3, gas stream  4  encounters catalytic surface  70   c  in the spiral inlet passage  22 , undergoes oxidation, and encounters catalytic surface  70   c  in the spiral outlet passage  24 . Reactant gas streams  6   a  and  6   b  may be injected into thermal oxidation and catalytic reduction system  10   c  proximate catalytic surface  70   c  using reactant channels  76   a  and  76   b  and/or injection ports  82   a  and  82   b , as generally described in FIGS. 4,  5 , and  6 . 
     Thermal oxidation and catalytic reduction system  10   c  employing catalytic surface  70   c  provides a temperature profile in which curves portion A 1  and A 2  would have substantially the same slope; likewise B 1  and B 2 ; A 4  and A 5 ; and, B 4  and B 5 . 
     Another method according to the present invention will be described using FIGS. 4 and 5, and it is understood that the present method preferably may be utilized in conjunction with the method described above relating to FIG.  1 . Because the lean-NOx-based catalytic reaction is enhanced by the presence of hydrogen, hydrocarbons and carbon monoxide, reactant gas stream  6   a  and  6   b  including these constituents may be injected into the spiral passages proximate catalytic surfaces  70   a  and  70   b  to enhance catalysis. Specifically, reactant gas streams  6   a  and  6   b  may be directed into reactant channels  76   a  and  76   b  and through transpiration holes  80 , which may be sized and arranged to provide desired film thickness, cooling characteristics, pressure drop of the reactant stream, local velocity of the reactant stream and gas stream  4 , and like parameters. Also, gas streams  6   a  and  6   b  may be injected though injection ports  82   a  and  82   b , which may be disposed along the desired sidewall. Preferably, injection ports  82   a  and  82   b  will preferably span substantially along the height of the catalytic surfaces  70   a  and  70   b , and inject streams  6   a  and  6   b  in a uniform profile at low Reynolds number. Regardless of the means used to inject the reactant streams  6   a  and  6   b , it is preferred that the Reynolds Number of the flow in the boundary region of catalytic surface  70   a  and  70   b  is below approximately 1,000 so as to produce a laminar boundary layer to provide a stratified flow. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention. Further, it is understood that the objects of the present invention are not exclusive, as other objects and advantages will be apparent to those skilled in the art.