Abstract:
A system and method for reducing concentrations of reactants in gases flowing through an exhaust system are disclosed. Briefly described, one embodiment comprises creating a turbulent flow of gases along a periphery region of an exhaust pipe, the turbulent flow created by the gases transported over a wire mesh residing in the periphery region; and mixing the gases in the periphery region with a reactant gas being transported in a flow-through region, the mixing caused by the turbulent flow of gases, such that concentration of the reactant gas is reduced as the reactant gas is transported through the exhaust system.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to fuel cells, and, more particularly, to exhaust systems.  
         [0003]     2. Description of the Related Art  
         [0004]     Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst, disposed at the interfaces between the electrolyte and the electrodes, typically induces the desired electrochemical reactions at the electrodes. The location of the electrocatalyst generally defines the electrochemically active area.  
         [0005]     One type of electrochemical fuel cell is the polymer electrolyte membrane (PEM) fuel cell. PEM fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two electrodes. Each electrode typically comprises a porous, electrically conductive substrate, such as carbon fiber paper or carbon cloth, which provides structural support to the membrane and serves as a fluid diffusion layer. The membrane is ion conductive (typically proton conductive), and acts both as a barrier for isolating the reactant streams from each other and as an electrical insulator between the two electrodes. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION®. The electrocatalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support).  
         [0006]     In a fuel cell, an MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. The plates typically act as current collectors and provide support for the MEA. In addition, the plates may have reactant channels formed therein and act as flow field plates providing access for the reactant fluid streams to the respective porous electrodes and providing for the removal of reaction products formed during operation of the fuel cell.  
         [0007]     In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given separator plate may serve as an anode flow field plate for one cell and the other side of the plate may serve as the cathode flow field plate for the adjacent cell. In this arrangement, the plates may be referred to as bipolar plates. Typically, a plurality of inlet ports, supply manifolds, exhaust manifolds and outlet ports are utilized to direct the reactant fluid to the reactant channels in the flow field plates. The supply and exhaust manifolds may be internal manifolds, which extend through aligned openings formed in the flow field plates and MEAs, or may comprise external or edge manifolds, attached to the edges of the flow field plates.  
         [0008]     A broad range of reactants can be used in PEM fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol (in a direct methanol fuel cell). The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.  
         [0009]     During normal operation of a PEM fuel cell, fuel is electrochemically oxidized on the anode side, typically resulting in the generation of protons, electrons and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the membrane, to electrochemically react with the oxidant on the cathode side. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant on the cathode side to generate water reaction product.  
         [0010]     The unused reactants are expelled out of the stack into the atmosphere via an exhaust system including a fuel exhaust pipe and an oxidant exhaust pipe. During fuel cell operation, especially during start-up, shut-down or restart, hydrogen may diffuse to the cathode side and consequently a mix of hydrogen and air will be present in the air exhaust pipe, sometimes with concentrations as high as 50/50 hydrogen/air, posing a significant flammability/detonability concern.  
         [0011]     There have been attempts to reduce the hydrogen concentration in the air exhaust pipe by dilution through adding more air, or by stack voltage bleeddown to actively consume the hydrogen remaining in the anode loop.  
         [0012]     Accordingly, although there have been advances in the field, there remains a need in the art for improving fuel cell exhaust systems. The present invention addresses these needs and provides further related advantages.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     A system and method for reducing concentrations of reactants in gases flowing through an exhaust system is disclosed. Briefly described, in one aspect, an embodiment may be summarized as a method comprising creating a turbulent flow of gases along a periphery region of an exhaust pipe, the turbulent flow created by the gases transported over a wire mesh residing in the periphery region; and mixing the gases in the periphery region with a reactant gas being transported in a flow-through region, the mixing caused by the turbulent flow of gases, such that concentration of the reactant gas is reduced as the reactant gas is transported through the exhaust system.  
         [0014]     In another aspect, an embodiment may be summarized as an exhaust pipe that transports an oxidant gas and hydrogen exhausted from a fuel cell from an inlet of the exhaust pipe to an outlet of the exhaust pipe; and a wire mesh disposed within the exhaust pipe around a periphery of the exhaust pipe, the wire mesh causing a turbulent flow in portions of the exhaust pipe such that the oxidant transported through the exhaust pipe mixes with the hydrogen in the exhaust pipe.  
         [0015]     These and other aspects of the invention will be evident upon reference to the following detailed description and attached drawings.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0016]     In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings.  
         [0017]      FIG. 1  is a block diagram of an embodiment of an oxidant exhaust system including a wire mesh residing in a fuel cell system.  
         [0018]      FIG. 2  illustrates a selected portion of the oxidant exhaust system of  FIG. 1 .  
         [0019]      FIG. 3A  is a longitudinal, cut-away view of a portion of the oxidant exhaust system of  FIG. 2  showing in greater detail the wire mesh residing in the exhaust pipe.  
         [0020]      FIG. 3B  is a cross-sectional view, at plane B-B′, of the oxidant exhaust system showing the wire mesh residing in the exhaust pipe.  
         [0021]      FIG. 3C  is a gas flow velocity diagram indicating relative gas flow velocities in the oxidant exhaust system at plane B-B′ of  FIG. 3A .  
         [0022]     FIGS.  4 A-D illustrate selected wire mesh fabrics that may be used for the wire mesh embodiments of  FIGS. 1-2  and  3 A-C.  
         [0023]      FIG. 5  illustrates an alternative embodiment of a oxidant exhaust system having one or more eductors disposed through the pipe wall.  
         [0024]      FIG. 6  illustrates an alternative embodiment of a oxidant exhaust system employing a catalyst to facilitate reaction of reactant gases passing through the exhaust pipe.  
         [0025]      FIG. 7  illustrates an alternative embodiment of a oxidant exhaust system having a plurality of wire mesh layers disposed within the exhaust pipe.  
         [0026]      FIG. 8  illustrates an alternative embodiment of a oxidant exhaust system employing an increased diameter portion to increase the volume of mixable gasses.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. One skilled in the art, however, will understand that the invention may be practiced without these details. In other instances, well-known structures associated with fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.  
         [0028]      FIG. 1  is a block diagram of an embodiment of a oxidant exhaust system  100  residing in a fuel cell system  102 . The oxidant exhaust system  100  dilutes the relative percentage of hydrogen, or other types of reactant gases, with other gases such as oxidant (air) as the gases are exhausted through an exhaust system by generating turbulence in the velocity flow pattern of exhaust gases. More specifically, the generated turbulence, described in greater detail hereinbelow, causes mixing of portions of hydrogen diffused to the cathode side with the exhausted air in the exhaust system. That is, the air as it is transported through the exhaust system, dilutes the concentration of the hydrogen to desirable levels before venting out to the atmosphere. An exemplary benefit of reducing the concentration of or diluting exhausted reactant gases is a reduction in the flammability of exhaust gases.  
         [0029]     The simplified fuel cell  104  residing in the fuel cell system  102  comprises an anode  106 , a cathode  108  and a membrane  110 . Fuel cell  104  may be any type of suitable fuel cell, such as a polymer electrolyte membrane (PEM) fuel cell or the like.  
         [0030]     A reactant, or fuel, is supplied to the anode  106 , via flow path  112 . In the exemplary embodiment, hydrogen may be used as the reactant. A valve and/or a pressure regulator (not shown) may be used to control flow and/or pressure of the reactant in the flow path  112 . Alternative embodiments of the oxidant exhaust system  100  may be configured to operate with fuel cells which use other modes of anode operation.  
         [0031]     An oxidant, such as oxygen or air, is supplied to the cathode  108 , via flow path  114 . In an exemplary embodiment, air or oxygen is used as the reactant.  
         [0032]     During the exhaust process, valves (not shown) of the fuel cell system  102  may be actuated to direct gases residing in the cathode  108  into the oxidant exhaust system  100 , generally denoted by the flow paths  116 . It is appreciated that any suitable configuration of flow paths  116  may be used to direct fluids from the cathode  108  into the oxidant exhaust system  100 , and accordingly, such paths need not be described in detail herein for understanding the principles of operation of the various embodiments of the oxidant exhaust system  100 .  
         [0033]      FIG. 2  illustrates a selected portion of the oxidant exhaust system  100  of  FIG. 1 . The oxidant exhaust system  100  comprises an oxidant exhaust pipe  202  and a wire mesh  204 . Oxidant exhaust pipe  202  may be any suitable gas channeling device or the like, and is illustrated for convenience as a tubular pipe. Any suitable material, such as, but not limited to, metal, plastic or ceramic, may be used for the oxidant exhaust pipe  202  by the various embodiments. For convenience, the cross section of the oxidant exhaust pipe  202  is illustrated as circular (see also  FIGS. 3B and 7 ). However, any suitable cross-section geometry may be used for the oxidant exhaust pipe  202  by the various embodiments.  
         [0034]     Wire mesh  204  resides in the oxidant exhaust pipe  202  and is oriented substantially adjacent to an inner surface  206  of the wall  208 . That is, the wire mesh  204  is disposed around the periphery of the oxidant exhaust pipe  202 . Because the wire mesh  204  is disposed within and around the periphery of the oxidant exhaust pipe  202 , a flow-through region  214  resides through a center portion of the oxidant exhaust pipe  202 , providing an unobstructed passage or flow path for transport of exhaust gases.  
         [0035]     Exhaust gases may be collected by a collection system (not shown), and then injected into inlet  210  of the oxidant exhaust pipe  202 . As the gases are transported through the oxidant exhaust pipe  202 , the wire mesh  204  causes turbulence in the gas flow, thereby mixing incoming gases with other gases residing in the oxidant exhaust pipe  202 . Gases are ultimately vented out of the oxidant exhaust system  100 , exiting the outlet  212  of the oxidant exhaust pipe  202 . Venting may be directly out into the atmosphere or into another device, depending upon the embodiment and/or the specific application of the oxidant exhaust system  100 .  
         [0036]      FIG. 3A  is a longitudinal, cut-away view of a portion of the oxidant exhaust system  100  of  FIG. 2  showing in greater detail the wire mesh  204  residing in the oxidant exhaust pipe  202 .  FIG. 3B  is a cross-sectional view, at plane B-B′, of the oxidant exhaust system  100  showing the wire mesh  204  residing in the oxidant exhaust pipe  202 .  FIG. 3C  is a gas flow velocity diagram indicating relative gas flow directions and/or velocities in the oxidant exhaust system  100  at plane B-B′ of  FIG. 3A .  
         [0037]     It is apparent from FIGS.  3 A-C that the wire mesh  204 , in this exemplary embodiment, is similar to a sleeve inserted inside the walls  208  of the oxidant exhaust pipe  202 . As gas moves from the inlet  210  to the outlet  212  ( FIG. 2 ), gas traveling along a periphery region  302  of the oxidant exhaust pipe  202  is in contact with the wire mesh  204 .  
         [0038]     Accordingly, functional forces between the moving gas and the surfaces of the wire mesh  204  causes a turbulent flow of the gas being transported along the periphery region  302  ( FIG. 3C ).  
         [0039]     The relative direction of turbulent gas flow along the plane B-B′ of the periphery region  302  of oxidant exhaust pipe  202  is random through the fabric/structure of the wire mesh  204 , as generally indicated by the directional arrows  306  ( FIG. 3C ). The relative velocity and direction of gas traveling along the flow-through region  304  is generally indicated by the directional arrows  308  ( FIG. 3C ). Thus, it is appreciated that gas flowing in the flow-through region  304  is transported in a substantially laminar fashion. The highest velocity of the exhaust gas occurs in the center of the flow-through region  304 . The slowest velocity of the exhaust gas occurs in or along the periphery region  302 .  
         [0040]     It is appreciated that the velocity of the gas in the periphery region  302  is less than the relative velocity of gases in the flow-through region  304 . Gases in the periphery region  302  are slowed by friction between the gas and the wire mesh  204 , and by friction between the gas and the inner surface  206 . Accordingly, gases initially moving into the flow-through region  304  may more quickly travel from the inlet  210  into the interior of the oxidant exhaust pipe  202 .  
         [0041]     As noted above, gas being transported through the flow-through region  304  flows in a substantially laminar flow pattern. At an interface region  312  between gases flowing in the periphery region  302  and gases flowing in the flow-through region  304 , the gases are mixed as a result of at least the created turbulent flow. Furthermore, velocity differences of gases in the interface region  312  (relative velocity differences between the gases of the flow-through region  304  and the periphery region  302 ) may contribute to the mixing. That is, the created turbulence and/or velocity differences mixes the gases transported in the periphery region  302  with other gases in and/or around the flow-through region  304 . Furthermore, it is appreciated that there is a mixing of the gases transported through the periphery region  302  itself.  
         [0042]     As noted above, during certain times during fuel cell operation, oxidant gases having a relatively high concentration of hydrogen, may be exhausted through the oxidant exhaust system  100 . For example, during start-up of the fuel cell  104  ( FIG. 1 ), a volume of oxidant gas having an undesirable concentration of hydrogen, may be introduced into the inlet  210  of the exhaust pipe. Other gases, such as inert gases or air, will already reside within the oxidant exhaust pipe  202 , such as in the periphery region  302 . As the mixture of hydrogen and air enters into the oxidant exhaust pipe  202 , the turbulence induced by the various embodiments of the oxidant exhaust system  100  will cause a mixing of the hydrogen with air already residing in the oxidant exhaust pipe  202 . That is, the hydrogen will become diluted, or less concentrated, as the air/hydrogen mixture is exhausted through the oxidant exhaust pipe  202 . When the reactant exits the oxidant exhaust pipe  202 , at outlet  212 , the concentration of the exiting gases will have a desirable level or concentration of hydrogen as a result of the mixing occurring along the length of the oxidant exhaust pipe  202  having the wire mesh  204  disposed therein.  
         [0043]     FIGS.  4 A-D illustrate selected exemplary wire mesh fabrics that may be used for the wire mesh  204  embodiments of  FIGS. 2-3 . Any suitable weave may be used, including the illustrated plain weave of  FIG. 4 A , the dutch weave of  FIG. 4B , the twilled weave of  FIG. 4C  and the twilled-dutch weave of  FIG. 4D . The weave patterns of FIGS.  4 A-D are intended to merely illustrate some of the possible weave patterns used by various embodiments of a oxidant exhaust system  100  ( FIG. 1 ). Other types of patterns and configurations may be used, such as an expanded metal mesh or perforated screen plate/mesh. Mesh patterns may be square, rectangular, diamond, hexagonal or other suitable geometric patterns. A crimped wire mesh may provide additional surface irregularities on the wire mesh  204  to further increase turbulence of gases passing through the periphery region  302  (FIGS.  3 B-C). Any suitable thickness of wire mesh may be used. If the mesh is made of metal, preferably aluminum, along with dilution the mesh reduces the gradient in flammable concentration along the pipe which has a strong mitigating effect on combustion flame speeds and pressures that can develop in the exhaust pipe during combustion.  
         [0044]      FIG. 5  illustrates an alternative embodiment of a oxidant exhaust system  100  having one or more eductors  502  disposed through the pipe wall  208 . As gases travel past the outlet  504 , thereby causing a vacuum at the outlet  504 , additional air is drawn into an inlet  506 . The air enters into the oxidant exhaust pipe  202 , further diluting the hydrogen passing by the eductors  502 . Other embodiments may draw other selected gases, such as inert nitrogen, through the eductors  502 . Eductors  502  may be placed at any desirable and/or convenient location along the length of the oxidant exhaust pipe  202 . Also, forced air may be introduced through the eductors to enhance the vacuum effect created at the outlet  504 . Furthermore, any suitable number of eductors  502  may be used by the various embodiments.  
         [0045]      FIG. 6  illustrates an alternative embodiment of a oxidant exhaust system  100  employing a catalyst layer  602  to facilitate reaction of reactant gases passing through the oxidant exhaust pipe  202 . As hydrogen comes in contact with the catalyst  602  and the oxidant gases the concentration of hydrogen in the oxidant exhaust pipe  202  is further reduced. The catalyst is coated on a perforated sheet to allow better gas circulation to the mesh.  
         [0046]     In an alternative embodiment, the catalyst layer  602  may be adjacent to and located between the wire mesh  204  and the wall  208 . Or, two similar catalyst layers  602  can be located on the pipe adjacent to the mesh, one between the mesh and the wall  208  and the other one on the other side of the mesh, towards the central portion of the pipe. In another embodiment, a catalyst coating may be applied to the wire mesh  204  and/or to the inner surface of the pipe wall  208 . Any type of catalyst structure and/or coating may be employed by the various embodiments of the oxidant exhaust system  100 .  
         [0047]      FIG. 7  illustrates an alternative embodiment of a oxidant exhaust system  100  having a plurality of wire mesh layers  204  disposed within the oxidant exhaust pipe  202 . Here, two layers of wire mesh  204  are illustrated, though any suitable number of layers may be used by the various embodiments. Different types of mesh sizes and/or patterns may be used. Or, a larger sheet of wire mesh may be rolled a plurality of times around itself to form the plurality of wire mesh layers  204 .  
         [0048]      FIG. 8  illustrates an alternative embodiment of a oxidant exhaust system  100  employing an increased diameter portion  800  to increase the volume of mixable gasses. The increased diameter portion  800  of the oxidant exhaust pipe  202  increases the volume of that portion of the oxidant exhaust system  100 . Gasses  802  entering through inlet  210  are initially transported through a through pipe portion  804 . Upon entry into the increased diameter portion  800 , gas velocity decreases. The slower moving gasses more readily mix with other gasses residing in the increased diameter portion  800 . In some embodiments, the outlet  212  comprises another smaller diameter through pipe portion  806 . The diameters of through pipe portions  804  and  806  may be different, depending upon the embodiment.  
         [0049]     For convenience, the transition between the through pipe portion  804  and the increased diameter portion  800 , and the transition between the increased diameter portion  800  and the through pipe portion  804 , are illustrated by the straight, vertically oriented transition wall  808 . In other embodiments, the transition wall  808  may be sloped, curvilinear or otherwise formed into any suitable shape.  
         [0050]     As noted above and illustrated in FIGS.  3 A-C, the wire mesh  204 , in the described exemplary embodiment, is similar to a sleeve inserted inside the walls  208  of the oxidant exhaust pipe  202 . Accordingly, wire mesh  204 , may be formed from a sheet of wire mesh rolled into a tube structure having a suitable diameter to facilitate insertion of the rolled wire mesh  204  into the oxidant exhaust pipe  202 . In one embodiment, the side ends of the wire mesh  204 , when rolled into a tubular form, could be secured with a fastener (not shown), secured with a suitable adhesive, or welded or soldered, such that the diameter of the rolled wire mesh  204  is fixed. Such an embodiment facilitates insertion into the oxidant exhaust pipe  202 . Accordingly, portions of the outside of the rolled wire mesh  204  may not be in contact with the inner surface  206 . Alternatively, the side ends of the wire mesh  204 , when rolled into a tubular form, may not be secured such that the wire mesh  204 , after insertion into the oxidant exhaust pipe  202 , expands to come into contact with relatively more or all of the inner surface  206 . In yet another embodiment, the diameter of the wire mesh  204  may be fixed such that the wire mesh must be forcibly inserted into the oxidant exhaust pipe  202  so that the wire mesh  204 , after insertion into the oxidant exhaust pipe  202 , is in frictional contact with some or all of the inner surface  206 .  
         [0051]     Turbulent flow within the oxidant exhaust pipe  202  of alternative embodiments of the oxidant exhaust system  100  may be generated by other structures of the wire mesh. For example, a helical coil of wire mesh may be disposed within the oxidant exhaust pipe  202 . Another embodiment may employ strips of wire mesh running the length, or running portions of the length, of the oxidant exhaust pipe  202 . Yet another embodiment may employ a plurality of rings of wire mesh across the length or portions of the length of the oxidant exhaust pipe  202 .  
         [0052]     For convenience of describing the various embodiments of a oxidant exhaust system  100 , various structures and features were separately described. Other embodiments may employ combinations of the above-described alternative embodiments. For example, but not limited to, one embodiment may employ the above-described catalyst  702  ( FIG. 6 ) and one or more eductors  604  ( FIG. 5 ).  
         [0053]     As used herein, the term “fluid” corresponds to gases and/or liquids. Accordingly, the terms “fluid” and the term “gas” (or the like) may be interchangeably used within the specification and/or claims.  
         [0054]     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.