Patent Publication Number: US-2023142906-A1

Title: Fuel cell system including catalyst ring anode tail gas oxidizer

Description:
FIELD 
     Aspects of the present invention relate to fuel cell systems, and more particularly, to fuel cell systems including a catalyst ring anode tail gas oxidizer (ATO). 
     BACKGROUND 
     Fuel cells, such as solid oxide fuel cells, are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. These fuel cells may operate using hydrogen and/or hydrocarbon fuels. There are classes of fuel cells, such as the solid oxide regenerative fuel cells, that also allow reversed operation, such that oxidized fuel can be reduced back to unoxidized fuel using electrical energy as an input. 
     SUMMARY 
     According to various embodiments, provided is a fuel cell system anode tail gas oxidizer (ATO) comprising: an inner ATO wall; an outer ATO wall; and a first catalyst ring disposed in a chamber formed between the inner ATO wall and the outer ATO wall, the first catalyst ring comprising: an inner wall; an outer wall; and a matrix disposed between the inner wall and the outer wall and loaded with an oxidation catalyst. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate example embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention. 
         FIG.  1    is a schematic of a fuel cell system, according to various embodiments of the present disclosure. 
         FIG.  2 A  is a sectional view showing components of the hot box of the system of  FIG.  1   ,  FIG.  2 B  shows an enlarged portion of the system of  FIG.  2 A ,  FIG.  2 C  is a three dimensional cut-away view of a central column of the system of  FIG.  2 A , and  FIG.  2 D  is a perspective view of an anode hub structure disposed below the central column of the system of  FIG.  2 A , according to various embodiments of the present disclosure. 
         FIGS.  3 A- 3 C  are sectional views showing fuel and air flow through the central column of the system of  FIG.  2 A , according to various embodiments of the present disclosure. 
         FIG.  4    is a partial perspective view of the central column of the system of  FIG.  2 A , according to various embodiments of the present disclosure. 
         FIG.  5 A  is a photograph showing an exemplary central column  400 , with the outer cylinder of the ATO removed,  FIG.  5 B  is a photograph showing a top perspective view of the catalyst ring of the ATO, and  FIG.  5 C  is a photograph showing a close up view of the top surface of a portion of the catalyst ring, according to various embodiments of the present disclosure. 
         FIG.  5 D  is a top view according to an alternative catalyst ring, according to various embodiments of the present disclosure. 
         FIG.  6    is a perspective view of a modified ATO, according to various embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims. 
     In a solid oxide fuel cell (SOFC) system, a fuel inlet stream may be humidified in order to facilitate fuel reformation reactions such as steam reformation and water-gas shift reactions. In addition, during system startup, shutdown, and power grid interruption events, water may be added to a fuel inlet stream in order to prevent coking of system components such as catalysts. Conventionally, such humidification is performed by vaporizing water in a steam generator containing corrugated tubing. Water flows through the corrugated tubing and is heated by the cathode recuperator heat exchanger exhaust stream which flows around the outside of the tubing. However, utilizing relatively low-temperature cathode recuperator exhaust stream generally requires substantial lengths of corrugated tubing, in order to absorb enough heat to vaporize the water. Further, the steam generator is relative large and bulky, which also adds to the system size, complexity and manufacturing costs. 
     In contrast, embodiments of the present disclosure provide a water injector configured to inject water directly into the anode exhaust recycle stream which provides heat to vaporize the water into steam and/or aerosolize the water into droplets small enough to be entrained in the anode exhaust stream. The anode exhaust recycle stream is recycled into the fuel inlet stream provided into the fuel cell stack, such that humidified fuel is provided to the fuel cells of the fuel cell stack. Thus, the prior art steam generator may be omitted to reduce system size, complexity and cost. In addition, the embodiment system may operate using relatively short, non-corrugated water conduit, which may improve system response times and reduce system size and cost. 
       FIG.  1    is a schematic representation of a SOFC system  10 , according to various embodiments of the present disclosure. Referring to  FIG.  1   , the system  10  includes a hotbox  100  and various components disposed therein or adjacent thereto. The hot box  100  may contain fuel cell stacks  102 , such as a solid oxide fuel cell stacks containing alternating fuel cells and interconnects. One solid oxide fuel cell of the stack contains a ceramic electrolyte, such as yttria stabilized zirconia (YSZ), scandia stabilized zirconia (SSZ), scandia and ceria stabilized zirconia or scandia, yttria and ceria stabilized zirconia, an anode electrode, such as a nickel-YSZ, a nickel-SSZ or nickel-doped ceria cermet, and a cathode electrode, such as lanthanum strontium manganite (LSM). The interconnects may be metal alloy interconnects, such as chromium-iron alloy interconnects. The stacks  102  may be arranged over each other in a plurality of columns. 
     The hot box  100  may also contain an anode recuperator heat exchanger  110 , a cathode recuperator heat exchanger  120 , an anode tail gas oxidizer (ATO)  500 , an anode exhaust cooler heat exchanger  140 , a splitter  550 , a vortex generator  552 , and a water injector  160 . The system  10  may also include a catalytic partial oxidation (CPOx) reactor  200 , a mixer  210 , a CPOx blower  204  (e.g., air blower), a system blower  208  (e.g., air blower), and an anode recycle blower  212 , which may be disposed outside of the hotbox  100 . However, the present disclosure is not limited to any particular location for each of the components with respect to the hotbox  100 . 
     The CPOx reactor  200  receives a fuel inlet stream from a fuel inlet  300 , through fuel conduit  300 A. The fuel inlet  300  may be a fuel tank or a utility natural gas line including a valve to control an amount of fuel provided to the CPOx reactor  200 . The CPOx blower  204  may provide air to the CPOx reactor  202  during system start-up. The fuel and/or air may be provided to the mixer  210  by fuel conduit  300 B. Fuel (e.g., the fuel inlet stream  1721  described below with respect to  FIGS.  4 A- 4 C ) flows from the mixer  210  to the anode recuperator  110  through fuel conduit  300 C. The fuel is heated in the anode recuperator  110  by a portion of the fuel exhaust and the fuel then flows from the anode recuperator  110  to the stack  102  through fuel conduit  300 D. 
     The system blower  208  may be configured to provide an air stream (e.g., air inlet stream) to the anode exhaust cooler  140  through air conduit  302 A. Air flows from the anode exhaust cooler  140  to the cathode recuperator  120  through air conduit  302 B. The air is heated by the ATO exhaust in the cathode recuperator  120 . The air flows from the cathode recuperator  120  to the stack  102  through air conduit  302 C. 
     An anode exhaust stream (e.g., the fuel exhaust stream described below with respect to  FIGS.  3 A- 3 C ) generated in the stack  102  is provided to the anode recuperator  110  through anode exhaust conduit  308 A. The anode exhaust may contain unreacted fuel and may also be referred to herein as fuel exhaust. The anode exhaust may be provided from the anode recuperator  110  to the splitter  550  by anode exhaust conduit  308 B. A first portion of the anode exhaust may be provided from the splitter  550  to the anode exhaust cooler  140  through the water injector  160  and the anode exhaust conduit  308 C. A second portion of the anode exhaust is provided from the splitter  550  to the ATO  500  through the anode exhaust conduit  308 D. The first portion of the anode exhaust heats the air inlet stream in the anode exhaust cooler  140  and may then be provided from the anode exhaust cooler  140  to the mixer  210  through the anode exhaust conduit  308 E. The anode recycle blower  212  may be configured to move anode exhaust though anode exhaust conduit  308 E, as discussed below. 
     Cathode exhaust generated in the stack  102  flows to the ATO  500  through exhaust conduit  304 A. The vortex generator  552  may be disposed in exhaust conduit  304 A and may be configured to swirl the cathode exhaust. The anode exhaust conduit  308 D may be fluidly connected to the vortex generator  552  or to the cathode exhaust conduit  304 A or the ATO  500  downstream of the vortex generator  552 . The swirled cathode exhaust may mix with the second portion of the anode exhaust provided by the splitter  550  before being provided to the ATO  500 . The mixture may be oxidized in the ATO  500  to generate an ATO exhaust. The ATO exhaust flows from the ATO  500  to the cathode recuperator  120  through exhaust conduit  304 B. Exhaust flows from the cathode recuperator and out of the hotbox  100  through exhaust conduit  304 C. 
     Water flows from a water source  206 , such as a water tank or a water pipe, to the water injector  160  through water conduit  306 . The water injector  160  injects water directly into first portion of the anode exhaust provided in conduit  308 C. Heat from the first portion of the anode exhaust (also referred to as a recycled anode exhaust stream) provided in exhaust conduit  308 C vaporizes the water to generate steam. The steam mixes with the anode exhaust, and the resultant mixture is provided to the anode exhaust cooler  140 . The mixture is then provided from the anode exhaust cooler  140  to the mixer  210  through the anode exhaust conduit  308 E. The mixer  210  is configured to mix the steam and first portion of the anode exhaust with fresh fuel (i.e., fuel inlet stream). This humidified fuel mixture may then be heated in the anode recuperator  110  by the anode exhaust, before being provided to the stack  102 . The system  10  may also include one or more fuel reforming catalysts  112 ,  114 , and  116  located inside and/or downstream of the anode recuperator  100 . The reforming catalyst(s) reform the humidified fuel mixture before it is provided to the stack  102 . 
     The system  10  may further a system controller  225  configured to control various elements of the system  10 . The controller  225  may include a central processing unit configured to execute stored instructions. For example, the controller  225  may be configured to control fuel and/or air flow through the system  10 , according to fuel composition data. 
       FIG.  2 A  is a sectional view showing components of the hot box  100  of the system  10  of  FIG.  1   , and  FIG.  2 B  shows an enlarged portion of  FIG.  2 A .  FIG.  2 C  is a three dimensional cut-away view of a central column  400  of the system  10 , according to various embodiments of the present disclosure, and  FIG.  2 D  is a perspective view of an anode hub structure  600  disposed in a hot box base  101  on which the column  400  may be disposed. 
     Referring to  FIGS.  2 A- 2 D , the fuel cell stacks  102  may be disposed around the central column  400  in the hot box  100 . For example, the stacks  102  may be disposed in a ring configuration around the central column  400  and may be positioned on the hot box base  101 . The column  400  may include the anode recuperator  110 , the ATO  500 , and the anode exhaust cooler  140 . In particular, the anode recuperator  110  is disposed radially inward of the ATO  500 , and the anode exhaust cooler  140  is mounted over the anode recuperator  110  and the ATO  500 . In one embodiment, an oxidation catalyst  112  and/or the hydrogenation catalyst  114  may be located in the anode recuperator  110 . A reforming catalyst  116  may also be located at the bottom of the anode recuperator  110  as a steam methane reformation (SMR) insert. 
     The ATO  500  comprises an outer cylinder  502  that is positioned around inner ATO insulation  556 /outer wall of the anode recuperator  110 . Optionally, the insulation  556  may be enclosed by an ATO inner cylinder  504 . Thus, the insulation  556  may be located between the anode recuperator  110  and the ATO  500 . An ATO oxidation catalyst may be located in the space between the outer cylinder  502  and the ATO insulation  556 . A fuel inlet path bellows  854  may be located between the anode exhaust cooler  140  and the inner ATO cylinder  504 . An ATO thermocouple feed through  1601  extends through the anode exhaust cooler  140 , to the top of the ATO  500 . The temperature of the ATO  500  may thereby be monitored by inserting one or more thermocouples (not shown) through this feed through  1601 . 
     The anode hub structure  600  may be positioned under the anode recuperator  110  and ATO  500  and over the hot box base  101 . The anode hub structure  600  is covered by an ATO skirt  1603 . The vortex generator  552  and fuel exhaust splitter  550  are located over the anode recuperator  110  and ATO  500  and below the anode exhaust cooler  140 . An ATO glow plug  1602 , which initiates the oxidation of the stack fuel exhaust in the ATO during startup, may be located near the bottom of the ATO  500 . 
     The anode hub structure  600  is used to distribute fuel evenly from the central column to fuel cell stacks  102  disposed around the central column  400 . The anode flow hub structure  600  includes a grooved cast base  602  and a “spider” hub of fuel inlet conduits  300 D and outlet conduits  308 A. Each pair of conduits  300 D,  308 A connects to a fuel cell stack  102 . Anode side cylinders (e.g., anode recuperator  110  inner and outer cylinders and ATO outer cylinder  502 ) are then welded or brazed into the grooves in the base  602 , creating a uniform volume cross section for flow distribution as discussed below. 
     A lift base  1604  is located under the hot box base  101 , as illustrated in  FIG.  2 C . In an embodiment, the lift base  1604  includes two hollow arms with which the forks of a fork lift can be inserted to lift and move the system, such as to remove the system from a cabinet (not shown) for repair or servicing. 
     As shown by the arrows in  FIGS.  2 A and  2 B , air enters the top of the hot box  100  and then flows into the cathode recuperator  120  where it is heated by ATO exhaust (not shown) from the ATO  500 . The heated air then flows inside the cathode recuperator  120  through a first vent or opening  121 . The air then flows through the stacks  102  and reacts with fuel (i.e., fuel inlet stream) provided from the anode hub structure  600 . Air exhaust flows from the stacks  102 , through a second vent or opening  123 . The air exhaust then passes through vanes of the vortex generator  552  and is swirled before entering the ATO  500 . 
     The splitter  550  may direct the second portion of the fuel exhaust exiting the top of the anode recuperator  100  through openings (e.g., slits) in the splitter into the swirled air exhaust (e.g., in the vortex generator  552  or downstream of the vortex generator in conduit  304 A or in the ATO  500 ). At such the fuel and air exhaust may be mixed before entering the ATO  500 . 
       FIGS.  3 A and  3 B  are side cross-sectional views showing flow distribution through the central column  400 , and  3 C is top cross-sectional view taken through the anode recuperator  110 . Referring to  FIGS.  2 A,  2 B,  3 A, and  3 C , the anode recuperator  110  includes an inner cylinder  110 A, a corrugated plate  110 B, and an outer cylinder  110 C that may be coated with the ATO insulation  556 . Fuel from fuel conduit  300 C enters the top of the central column  400 . The fuel then bypasses the anode exhaust cooler  140  by flowing through its hollow core and then flows through the anode recuperator  110 , between the outer cylinder  110 C and the and the corrugated plate  110 B. The fuel then flows through the hub base  602  and conduits  300 D of the anode hub structure  600  shown in  FIG.  3 B , to the stacks  102 . 
     Referring to  FIGS.  2 A,  2 B,  2 C,  3 A, and  3 B , the fuel exhaust flows from the stacks  102  through conduits  308 A into the hub base  602 , and from the hub base  602  through the anode recuperator  110 , between in inner cylinder  110 A and the corrugated plate  110 B, and through conduit  308 B into the splitter  550 . The first portion of the fuel exhaust flows from the splitter  550  to the anode exhaust cooler  140  through conduit  308 C, while the second portion flows from the splitter  550  to the ATO  500  through conduit  308 D, as shown in FIG.  1 . Anode exhaust cooler inner core insulation  140 A may be located between the fuel conduit  300 C and bellows  852 /supporting cylinder  852 A located between the anode exhaust cooler  140  and the vortex generator  552 , as shown in  FIG.  3 A . This insulation minimizes heat transfer and loss from the first portion of the anode exhaust stream in conduit  308 C on the way to the anode exhaust cooler  140 . Insulation  140 A may also be located between conduit  300 C and the anode exhaust cooler  140  to avoid heat transfer between the fuel inlet stream in conduit  300 C and the streams in the anode exhaust cooler  140 . In other embodiments, insulation  140 A may be omitted from inside the cylindrical anode exhaust cooler  140 . 
       FIG.  3 B  also shows air flowing from the air conduit  302 A to the anode exhaust cooler  140  (where it is heated by the first portion of the anode exhaust) and then from the anode exhaust cooler  140  through conduit  302 B to the cathode recuperator  120 . The first portion of the anode exhaust is cooled in the anode exhaust cooler  140  by the air flowing through the anode exhaust cooler  140 . The cooled first portion of the anode exhaust is then provided from the anode exhaust cooler  140  to the anode recycle blower  212  shown in  FIG.  1   . 
     As will be described in more detail below and as shown in  FIGS.  2 A and  3 B , the anode exhaust exits the anode recuperator  110  and is provided into splitter  550  through conduit  308 B. The splitter  550  splits the anode exhaust into first and second anode exhaust portions (i.e., streams). The first stream is provided into the anode exhaust cooler  140  through conduit  308 C. The second stream is provided to the ATO  500  through conduit  308 D. 
     The relative amounts of anode exhaust provided to the ATO  500  and the anode exhaust cooler  140  is controlled by the anode recycle blower  212 . The higher the blower  212  speed, the larger portion of the anode exhaust is provided into conduit  308 C and a smaller portion of the anode exhaust is provided to the ATO  500  via conduit  308 D, and vice-versa. 
     The anode exhaust provided to the ATO  500  is not cooled in the anode exhaust cooler  140 . This allows higher temperature anode exhaust to be provided into the ATO  500  than if the anode exhaust were provided after flowing through the anode exhaust cooler  140 . For example, the anode exhaust provided into the ATO  500  from the splitter  550  may have a temperature of above 350° C., such as from about 350 to about 500° C., for example, from about 375 to about 425° C., or from about 390 to about 410° C. Furthermore, since a smaller amount of anode exhaust is provided into the anode exhaust cooler  140  (e.g., not 100% of the anode exhaust is provided into the anode exhaust cooler due to the splitting of the anode exhaust in splitter  550 ), the heat exchange area of the anode exhaust cooler  140  may be reduced. The anode exhaust provided to the ATO  500  may be oxidized by the stack cathode (i.e., air) exhaust and provided to the cathode recuperator  120  through conduit  304 B. 
       FIG.  4    is a sectional perspective view showing the water injector  160  and ATO  500  in the central column of  FIG.  2 A . Referring to  FIG.  4   , the splitter  550  comprises the horizontal slits shown in  FIG.  3 A . However, in other embodiments, the splitter  550  may comprise tubes that extend through the outer wall of the anode exhaust conduit  308 B rather than the slits. 
     The water injector  160  may include and injector ring  162  and a shroud  166 . The injector ring  162  may be disposed inside the anode exhaust conduit  308 C between the anode exhaust cooler  140  and the anode recuperator  110  and may be fluidly connected to the water conduit  306 . The injector ring  162  is a tube that extends around the fuel conduit  300 C. The injector ring  162  may include injection apertures (i.e., openings)  162 A configured to inject water directly into the first portion of the anode exhaust flowing in the conduit  308 C from the splitter  550  and anode recuperator  110 . The water may be vaporized by the hot first portion of the anode exhaust. The injection apertures  162 A may be configured to generate streams or droplets of water, which may be vaporized instantaneously or within seconds of emerging from the injector ring  162 . The injector ring  162  may also be sized to provide substantially uniform circumferential flow of water therein and to minimize a pressure drop in the anode exhaust flowing thereby. 
     The shroud  166  may be a cylinder which surrounds the injector ring  162 . The shroud  166  may be configured to segregate the water from the second portion of the anode exhaust flowing into the ATO  500  through the splitter  550 . In particular, the second portion of the anode exhaust flowing outside of the shroud  166  may be directed by the splitter  550  radially outward toward the anode exhaust conduit  308 D and the ATO  500 , while the first portion of the anode exhaust flowing inside of the shroud  166  is directed upward by the splitter  550  toward the injector ring  162  in the anode exhaust conduit  308 C. Accordingly, the shroud  166  may be configured to prevent or reduce the amount of water and/or the first portion of the anode exhaust that has been humidified by the injected water from being injected into the ATO  500  by the splitter  550 . In other words, the shroud  166  is configured such that substantially all of the water and the humidified first portion of the anode exhaust are directed towards the anode exhaust cooler  140 . 
     The ATO  500  may surround the anode recuperator  110 , and the catalysts  112 ,  114  and  116  may be disposed inside the inner plenum which is surrounded by the anode recuperator  110 , similar to the configuration described in U.S. Pat. No. 9,287,572 B2, issued Mar. 15, 2016 and incorporated herein by reference in its entirety. 
     The ATO  500  may include a catalyst ring  510  disposed in an annular chamber formed between the outer cylinder  502  and the inner cylinder  504 . In particular, the catalyst ring  510  may be disposed at a distance from the splitter  550  that is sufficient for a majority of the oxidation of fuel exhaust to occur prior to the exhaust entering the catalyst ring  510 . In other words, the distance may be set such that un-catalyzed oxidation of the exhaust, such as the oxidation of hydrogen to form water and/or oxidation of carbon monoxide to form carbon dioxide, may be complete or more than 50% complete, before the exhaust enters the catalyst ring  510 . 
     The catalyst ring  510  may be configured to catalyze the oxidation of oxidizable species that remain in the catalyst exhaust after the un-catalyzed oxidation. For example, the catalyst ring  510  may include a catalyst or mixture of catalysts configured to catalyze the oxidation of carbon monoxide and/or fuel (e.g., hydrogen or hydrocarbon fuel, such as natural gas or methane) remaining in the exhaust. 
       FIG.  5 A  is a photograph showing an exemplary central column  400 , with the outer cylinder  502  of the ATO  500  removed,  FIG.  5 B  is a photograph showing a top perspective view of the catalyst ring  510  of the ATO  500 , and  FIG.  5 C  is a photograph showing a close up view of the top surface of a portion of the catalyst ring  510 , according to various embodiments of the present disclosure. 
     Referring to  FIGS.  5 A- 5 C , the catalyst ring  510  may include an outer wall  512 , an inner wall  514 , and a matrix  515  disposed there between. In some embodiments, the catalyst ring  510  may be formed of a high-temperature stable material, such as metals, for example stainless steel or Inconel (i.e., a high temperature nickel based alloy), or ceramic materials such as alumina, or the like. For example, the walls  512  may be metal and the matrix  515  may be ceramic coated with catalyst metal. In some embodiments, the outer wall  512  and the inner wall  514  may be cylindrical when viewed from the top. However other ring shapes, such as rectangular or hexagonal ring shapes may alternatively be used. The outer wall  512  may concentrically surrounding the inner wall  514 . The inner wall  514  may be attached to the inner cylinder  504  of the ATO  500 . The matrix  515  is attached to the inner wall  514  and the outer wall  512  by brazing or another suitable method. 
     The matrix  515  may have a honeycomb-type structure including channels  516 . The channels  516  may have any shape, so long as the channels  516  are configured to permit a fluid to flow through the catalyst ring  510 , from the top surface to an opposing bottom surface of the catalyst ring  510 . For example, the channels  516  may be straight or curved. In some embodiments, the channels  516  may extend in a direction that is substantially perpendicular to a plane of the top surface and/or bottom surface of the catalyst ring  510 . 
     In some embodiments, the channels  516  may be arranged in concentric rings surrounding the inner wall  514 . For example, the channels  516  may be arranged in at least 3, such as at least 5, at least 10, or at least 15 concentric rings. In other embodiments, the channels  516  may be disposed in an irregular arrangement. For example, the channels  516  may have any arrangement, so long as at least 3, such as at least 5, at least 10, or at least 15 channels  516  are disposed in a radial (i.e., horizontal) direction A (see  FIG.  5 C ), extending between the outer wall  512  and the inner wall  514 . The radial direction A may be perpendicular to the axial (i.e., vertical) direction of fluid (i.e., fuel and air exhaust) flow through the catalyst ring  510 . 
     In one embodiment shown in  FIG.  5 C , the matrix  515  may be formed from concentric cylindrical walls  517  (such as three or more concentric walls  517 ) separated from each other by cylindrical corrugated spacers  518 . In some embodiments, the cylindrical walls  517 , spacers  518 , and/or the outer and inner walls  512 ,  514  may be attached to one another by, for example, brazing or welding. The channels  516  may have a trapezoidal horizontal cross sectional shape, with the short and long parallel trapezoid sides alternating in the angular (i.e., clockwise or counter-clockwise) direction when viewed from the top. 
     In an alternative embodiment shown in  FIG.  5 D , the cylindrical walls  517  may be omitted from the matrix  515 . In this embodiment, the corrugated spacers  518  are attached to each other rather than to the pair of adjacent cylindrical walls  517 . In this embodiment, the channels  516  may have a hexagonal horizontal cross sectional shape direction when viewed from the top. The channels  516  form a close-packed hexagonal array when viewed from the top. 
     The matrix  515  may be loaded (i.e., having the surfaces of the channels coated) with an oxidation catalyst. In particular, the honeycomb structure of the matrix  515  may provide a high surface area for catalyst loading. Suitable oxidation catalysts may be configured to catalyze the oxidation carbon monoxide into carbon dioxide and/or oxidize any fuel remaining in the exhaust. For example, suitable oxidation catalyst may include catalyst metals such as platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), ruthenium (Ru), tantalum (Ta), nickel (Ni), copper (Cu), oxides thereof, alloys thereof, combinations thereof, or the like. In some embodiments, the oxidation catalyst may include palladium. The oxidation catalyst may be applied to the matrix  515  using any suitable process, such as by a washcoating process, for example. 
       FIG.  6    is a schematic view of an alternative ATO  500 A, according to various embodiments of the present disclosure. The ATO  500 A may be similar to the ATO  500 . Accordingly, on the differences there between will be described in detail. 
     Referring to  FIG.  6   , the ATO  500 A may include two or more catalyst rings  510 . For example, the ATO  500 A may include three catalyst rings  510  with the first ring located over the second ring, and the second ring located above the third ring, as shown in  FIG.  6   . However, the present disclosure is not limited to any particular number of catalyst rings  510 . For example, the number of catalyst rings  510  may be selected based on the composition of exhaust the ATO  500 A is configured to receive. 
     The catalyst rings  510  may be disposed between the outer cylinder  502  and the inner cylinder  504 , such that exhaust flowing through the ATO  500 A, (e.g., between the outer cylinder  502  and the inner cylinder  504 ) passes through each catalyst ring  510 . In some embodiments, the catalyst rings  510  may be disposed in a lower portion of the ATO  500 A, in order to permit non-catalyzed oxidation of the exhaust to be substantially complete, before the exhaust enters the catalyst rings  510 . The catalyst rings  510  and may be spaced apart from one another in the axial (i.e. vertical) direction, as shown in  FIG.  6   , or may directly contact one another. For example, the catalyst rings  510  may be spaced apart from one another, in an exhaust flow direction as shown by the exhaust flow arrows in  FIG.  6   , by a distance ranging from 0 to about 10 cm, such as from 0.5 to 5 cm, or from 1 to 2 cm. 
     In some embodiments, the catalyst rings  510  may be loaded with the same oxidation catalyst and/or may each have the same amount of catalyst loading. In other embodiments, the catalyst rings  510  may include different catalysts and/or may have different catalyst loading amounts. 
     The present inventors have determined that an ATO including a catalyst ring, as described herein may provide various unexpected benefits, as compared to conventional ATO designs. For example, the catalyst ring may provide an increased surface area for catalyst loading, which may increase the active area for oxidation, as compared to conventional designs. In addition, the catalyst ring may have a longer service life and may be manufactured at a lower cost, as compared to conventional designs. 
     The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.