Abstract:
A catalytic combuster having a mixing section upstream of a combustion section. The mixing section includes a multi-port injector for injecting a first reactant gas into the mixing section in a plurality of directions perpendicular the direction or flow of a second reactant gas.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to catalytic combusters for burning gases supplied thereto, and more particularly to a combuster having a unique mixing section for intimately mixing the gases before supplying them to the combustion catalyst.  
         BACKGROUND OF THE INVENTION  
         [0002]    Catalytic combusters (a.k.a. catalytic burners) for burning gases are known and generally comprise a gas inlet section, a gas mixing section, and a catalyst, downstream of the mixing section, where combustion occurs. Combusters are used with a variety of gaseous, hydrogenous fuels, and have many applications one of which is to heat the fuel processor, or other components, of a fuel cell system. Fuel-cell-system combusters burn the tailgases (effluents) from the anode and cathode compartments of the fuel cell, which tailgases comprise hydrogen and oxygen respectively. So-called “radial inlet”, prior art combusters have a mixing section like that shown in FIG. 1 which tend to (1) have a high pressure drop, and (2) produce a slow moving layer of well-mixed gases adjacent the perforated perimeter wall that defines the mixing chamber, (3) a more rapidly moving central region of poorly mixed gases, and (4) a relatively long resistance time for the gases in the mixing section. Because of the long resistance time in the mixing section, the heat that is generated in the combustion section, downstream of the mixing section, can be transmitted back into the mixing section where it can cause auto-ignition of the well-mixed gases (i.e. adjacent the perforated wall). This, in turn, can cause overheating or localized heating that can be detrimental to the combuster.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention is an improved catalytic combuster that has a mixing section upstream of the combustion section that quickly and thoroughly mixes the gases before they enter the catalytic combustion section. The improved combuster has an easily maintainable design that is readily assembleable/disassembleable, has a relatively low pressure drop, and has a low residence time (and hence reduced auto-ignition potential) for the gases in the mixing section, yet still provides excellent mixing of the gases in the mixing section. More specifically, the invention is directed to an improvement to the mixing section of the combuster that achieves intimate mixing of the gases throughout the mixing section in a short distance from the inlet end of the combuster. The improved mixing section comprises a housing having a wall that defines the mixing chamber, a first gas-permeable, upstream distributor (e.g. open cell foam, sintered metal or a perforated plate) at the entrance to the mixing chamber for admitting one of the gases into the mixing chamber in a first direction, and a a second gas distributor in the form of a multi-port injector extending into the mixing chamber and surrounded by the housing wall for admitting the other gas into the mixing chamber in a plurality of directions generally perpendicular to the first direction  
           [0004]    According to one embodiment of the invention, the multi-port injector comprises a distribution ring having a first, inner, ported (i.e. perforated), annular wall that defines a hole in the center of the distribution ring, and a second, outer, ported annular wall spaced radially outboard from the first ported wall. The inner and outer walls together define an annular cavity that receives the second gas and is adapted to inject the second gas into the mixing chamber in a radially inwardly second direction through the first permeable wall, and a radially outwardly third direction through the second perforated wall. At least one conduit communicates the inlet with the distribution ring. Preferably, multiple such conduits extend between the inlet and the ring. Each of the conduits has a mouth that receives the other gas from the inlet. The several mouths are arranged in a circle concentric with the hole in the distribution ring, with the mouths opening through a baffle plate that separates the inlet from the vestibule. A conical deflector, concentric with, and radially inboard of, the circle of mouths, directs the second gas from the inlet into the conduits that supply the distribution ring.  
           [0005]    According to another embodiment of the invention, the injector comprises a plurality of blind gas distribution tubes, each defined by a sidewall and extending into the mixing chamber downstream of the first gas distributor. The sidewalls each have a plurality of ports therein downstream of the first gas distributor for injecting the other gas into the mixing chamber in a direction generally perpendicular to the direction of flow of the first gas.  
           [0006]    An inlet section of the combuster supplies the other gas to the injector. A gas-permeable, homogenizing diffuser (preferably an open-cell foam) at the downstream end of the mixing chamber restricts the outflow of the gases from the mixing chamber to promote mixing upstream of the diffuser and to distribute the outflow substantially uniformly over the cross section of the combuster transverse the length of the combuster at the entrance to the combustion section.  
           [0007]    According to significant aspect of the invention, a plurality of openings are provided in the housing upstream of the distributor for supplying a first gas behind (i.e. upstream) the distributor. An annular plenum surrounds the openings (and preferably the downstream diffuser), and serves to supply the one gas to the openings in the housing behind the gas distributor. The gas flows in the plenum in a direction that is countercurrent to the direction of flow of other gas through the mixing section which helps to cool the mixing section and further suppress the possibility of auto-ignition. The combuster includes a vestibule located between the openings in the housing and the gas distributor to receive the one gas from the openings  
           [0008]    According to a preferred embodiment of the invention, the several sections (i.e. inlet, mixing and combustion) of the combuster are each separate and discrete units that are connected to the next adjacent unit by means of a quick-disconnect connection to provide convenient access to the innards of the combuster for readily maintaining the combuster or modifying it (e.g. during design development).  
           [0009]    Combusters made in accordance with the distributor ring embodiment of the invetion have demonstrated 80+% mixing of the gases in 70% of the cross sectional area (i.e. transverse the direction of flow through the combuster) of the mixing section. In contrast only about 45% of the cross sectional area of the radial mixers (see FIG. 1) contained 80+% mixed gases. Such improved mixing is achieved in combusters in accordance with the present invention that have demonstrated as much as 34% lower pressure drop, and shorter residence times than the prior art radial inlet combusters. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention will be better understood when considered in the light of the following detailed description of one specific embodiment thereof which is given hereafter in conjunction with the following drawings in which:  
         [0011]    [0011]FIG. 1 is a sectioned isometrical view of a “radial inlet” mixing section of a prior art combuster;  
         [0012]    [0012]FIG. 2 is a side sectional view of one embodiment of a combuster in accordance with the present invention; and  
         [0013]    [0013]FIG. 3 is an exploded, isometrical view of the mixing section of the combuster of FIG. 2.  
         [0014]    [0014]FIG. 4 is a side sectional view of another embodiment of a combuster in accordance with the present invention.  
         [0015]    [0015]FIG. 5 is an exploded, isometrical view of the mixing section of the combuster of FIG. 4. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]    [0016]FIG. 1 depicts the mixing section  2  of a prior art radial inlet combuster adapted to be connected, via a gas diffuing foam (not shown) to a combustion section (not shown) downstream of the mixing section  2 . The mixing section comprises a mixing chamber  4  defined by an annular, gas-distributing wall  6  having a plurality of apertures  8  varying in size (as shown) with the larger diameter apertures being upstream of the smaller diameter apertures. Gas (e.g. O 2 -containing cathode tailgas from an H 2 -O 2  fuel cell) enters an annular plenum  9  via a tangential inlet  10 , and passes through a porous sintered metal partition  12  that separates the plenum  9  from annular chamber  14  immediately behind the perforated wall  6 . The porous sintered metal partition  12  creates a backpressure in the plenum  9  that causes the plenum  9  to pressurize equally throughout such that the gas therein will pass through the porous sintered metal partition  12  into the chamber  14  substantially uniformly over the entire area of the partition  12 . Hence, the gas entering the chamber  14  will be distributed substantially uniformly throughout the chamber  14  from whence it passes into the mixing chamber  4  through the plurality of apertures  8  in the gas-distributing perforated wall  6 .  
         [0017]    A second gas (e.g H 2 -containing. anode tailgas from an H 2 -O 2  fuel cell) enters the mixing section  2  via the inlet  16  and fills the anode gas plenum  18 . The anode gas plenum  18  is separated from the mixing chamber  4  by means of a porous sintered metal plate  20  which serves to provide backpressure in the anode gas plenum  18  and to distribute the flow of gas substantially uniformly across the cross section (i.e. transverse the direction of flow) of the mixing chamber  4 . The first gas entering the mixing chamber  4  through the perforated cylindrical wall  6  mixes with the second gas entering the mixing chamber  4  through the porous sintered metal plate  20 , and the mixed gases proceed from the mixing chamber  4  into the combustion section (not shown) of the combuster. Unfortunately, the gas does not mix uniformly throughout the cross section of mixing chamber  4 . Rather, a high degree of mixing occurs in the region nearest the perforated wall  6 . Significantly less mixing occurs in the radial center of the mixing chamber  4  remote from the perforated wall  6 . When the poorly mixed gases reach the catalyst in the combustion section (not shown) an uneven temperature distribution occurs throughout the catalyst with hotter zones occurring near the center of the catalyst bed (i.e. where the gases are poorly mixed) than at the perimeter of the catalyst bed which receives the well mixed gases. Moreover, heat generated in the combustion section can propagate back into the mixing section and cause auto-ignition of the slow moving well-mixed gas adjacent the perforated wall  6 . Auto-ignition in the mixing chamber  4  causes a sharp temperature rise therein which is detrimental to the materials that comprise the mixing section, and particularly, to the sintered materials  20  and  12 .  
         [0018]    [0018]FIGS. 2 and 3 depict one embodiment of the present invention and comprises (1) a mixing section  22  having an inlet section  24  (i.e. for one of the gases) joined thereto by means of a quick disconnect connection  26 , and (2) a combustion section  28  joined thereto by a quick disconnect connection  30 . The combustion section  28  comprises a housing  32  containing a catalyst bed  34  (e.g. pellets, monolith, etc.) suitable for promoting the combustion of fuel cell anode tailgases  36  and cathode tailgases  38 . The combustion section  28  is detachably connected to the mixing section  22  by means of the quick disconnect  30  comprising a strap type clamp having a tension band  41  that squeezes a clamping ring  40  about the perimeters of the flanges  42  and  44  that engages and presses the flange  42  on the end of the housing  32  and the flange  44  at the aft end of the mixing section  22  together.  
         [0019]    The mixing section  22  comprises a housing  46  defining a mixing chamber  48  where the gases are mixed before they pass through an open cell foam homogenizer/diffuser  50  that serves to (a) further mix the gases, (b) distribute the gas flow substantially evenly over the cross section of the combuster, and (c) prevent propagation of any flame that might exist in the transitional region  52  (i.e. between the foam  50  and catalyst  34 ) back into the mixing chamber  48 . The homogenizing foam  50  will have pore sizes varying from about 10 pores per lineal inch (ppi) to about 80 (preferably about 20-40 ppi). The gases to be mixed are provided to the mixing chamber  48  (a) from two directions generally perpendicular to each other, and (b) into relatively narrow (i.e. compared to FIG. 1) mixing zones within the mixing chamber  48 . More specifically the cathode tailgas  38  is supplied to the mixing chamber  48  via a plenum  54  which is tangentially supplied with cathode tailgas  38  via inlet  56 . The plenum  54  surrounds both the mixing chamber  48  and the homogenizer  50 , and is defined by the annular enclosure  58 . The enclosure  58  has a first larger portion  60  that defines a large volume  62  of the plenum  54  that receives the cathode tailgas  38  and delivers it to a narrower, lower volume portion  64  of the plenum  54 . In the drawings, the cathode tailgas  38  flows generally from right to left in the plenum  54  which is countercurrent to the flow of anode tailgas  36  through the combuster. This countercurrent flow serves to cool the mixing section  48  while preheating the cathode tailgas  38  for improved combustion. The cathode tailgas  38  exits the plenum  54  via a plurality of holes  66  in the housing  46  which serve to communicate the plenum  54  with the vestibule  68  which, in turn, supplies the cathode tailgases  38  to the mixing section  48 .  
         [0020]    More specifically, the mixing section  48  is defined at least in part by a gas-permeable upstream distributor  70  which is herein depicted as a perforated plate, but which could just as well be an open cell metal or ceramic plate or the like. The distributor  70  has a skirt  71  that nests within the housing  46 , and is adapted to allow the gas in the vestibule  68  to flow into the mixing chamber  48  in an axial direction relative to the length of the combuster. At the same time, the anode tailgas  36  enters the combuster through the inlet section  24 . More specifically, the anode tailgas  36  enters inlet pipe  72  flows through the hollow cone  75  and engages the conical deflector or flow splitter  74  which is coaxial with the cone  75  and axially aligned with the pipe  72  along the center line of the combuster, and serves to direct the anode tailgas  36  into the mouths  76  of a plurality of conduits  78  which serve as gas delivery tubes to the mixing chamber  48 . The inlet section  24  is separated from the mixing section  22  by a baffle plate  80 . The conduits  78  extend from the baffle plate  80  through the vestibule  68  to the perforated plate  70  to conduct the anode tailgas  36  through the vestibule  68  without mixing it with the cathode tailgas  38  therein. Rather, the conduits  78  deliver the anode tailgases to a donut-like distribution ring  82  for dispensing the anode tailgases  36  into the mixing chamber  48  in a direction generally perpendicular to the direction of the flow of the cathode tailgases  38  through the perforated plate  70 . More specifically, the distribution ring  82  comprises a first multi-ported annular wall  84  defining a hole  86  in the center of the ring  82  and a second multi-ported annular wall  88  spaced radially outboard from the first wall  84 . The inner and outer walls  84  and  88  define an annular cavity  90  therebetween which is adapted to receive the anode tailgases  36  from the delivery conduits  78 , and thence to deliver the anode tailgases  36  into the mixing chamber  48  in a radial direction (i.e. inwardly into the hole  86  and outwardly into the annular space  92  surrounding the ring  82 ) for transverse flow mixing of the gases  36  and  38  in the mixing chamber  48 . Flanges  91  and  94  on the upstream end of the mixing section  22  and the downstream end of the inlet section  24  respectively are held together by tension clamp  96  having a tensioning screw  98  for tightening the annular band  100 , and thereby pressing the flanges  91  and  94  together by means of the locking ring  102 . Gaskets  104  and  106  sealingly engage the plate  80 . The combuster may be readily disassembled for modification and/or maintenance by simply releasing the tension clamps about the flanges  42 ,  44 ,  91  and  94 , and separating the inlet  24 , mixing section  22  and combustion section  28  one from the other.  
         [0021]    [0021]FIGS. 4 and 5 depict another embodiment of the invention and comprises (1) a mixing section  122 , (2) an inlet section  124  (i.e. for one of the gases) joined thereto by means of a quick disconnect connection  126 , and (3) a combustion section  128  joined thereto by a quick disconnect connection  130 . The combustion section  128  comprises a housing  132  containing a catalyst bed  134  (e.g. pellets, monolith, etc.) suitable for promoting the combustion of fuel cell anode tailgases  136  and cathode tailgases  138 . The combustion section  128  is detachably connected to the mixing section  122  by means of the quick disconnect  130  comprising a strap type clamp having a tension band  141  that squeezes a clamping band  140  about the perimeters of the flanges  142  and  144  and presses them together.  
         [0022]    The mixing section  122  comprises a housing having a wall  146  defming a mixing chamber  148  where the gases are mixed before they pass through an open cell foam homogenizer/diffuser  150  that serves to (a) further mix the gases, (b) distribute the gas flow substantially evenly over the cross section of the combuster, and (c) prevent propagation of any flame that might exist in the transitional region  152  (i.e. between the foam  150  and catalyst  134 ) back into the mixing chamber  148 . The homogenizing foam  150  will have pore sizes varying from about 10 pores per lineal inch (ppi) to about 80 (preferably about 20-40 ppi). The gases to be mixed are provided to the mixing chamber  148  (a) from two directions generally perpendicular to each other, and (b) into relatively narrow mixing zones within the mixing chamber  148 . More specifically the cathode tailgas  138  is supplied to the mixing chamber  148  via a plenum  154  which is tangentially supplied with cathode tailgas  138  via inlet  156 . The plenum  154  surrounds both the mixing chamber  148  and the homogenizer  150 , and is defined by the annular enclosure  158 . The enclosure  158  has a first larger portion  160  that defines a large volume  162  of the plenum  154  that receives the cathode tailgas  138  and delivers it to a narrower, lower volume portion  164  of the plenum  154 . In the drawings, the cathode tailgas  138  flows generally from right to left in the plenum  154  which is countercurrent to the flow of anode tailgas  136  through the combuster. This countercurrent flow serves to cool the mixing section  148  while preheating the cathode tailgas  138  for improved combustion. The cathode tailgas  138  exits the plenum  154  via a plurality of holes  166  in the wal  146  which serve to communicate the plenum  154  with the vestibule  168  which, in turn, supplies the cathode tailgases  138  to the mixing section  148 . The mixing section  148  is defined at least in part by a gas-permeable upstream distributor  170  which is herein depicted as a perforated plate, but which could just as well be an open cell metal or ceramic plate or the like. The distributor  170  has a skirt  171  that nests within the wall  146 , and is adapted to allow the gas in the vestibule  168  to flow into the mixing chamber  148  in an axial direction relative to the length of the combuster. At the same time, the anode tailgas  136  enters the combuster through the inlet section  124  via inlet  172  and the hollow cone  175 , and into the mouths  176  of a plurality of blind tubes  178  which serve as gas distribution and delivery injectors to the mixing chamber  148 . The inlet section  124  is separated from the mixing section  122  by a baffle plate  180 . The blind gas distribution tubes  178  extend from the baffle plate  180  through the vestibule  168  and the perforated plate  170  to conduct the anode tailgas  136  through the vestibule  168  without mixing it with the cathode tailgas  138  therein. Rather, the distribution tubes  178  dispense the anode tailgases  136  into the mixing chamber  148  via a plurality of ports  182  that extend radially (i.e. realtive to the length pf the tube  178 ) through the annular sidewalls  181  of the tubes  178 , and in a direction generally perpendicular to the direction of the flow of the cathode tailgases  138  through the perforated plate  170 .  
         [0023]    Flanges  91  and  94  on the upstream end of the mixing section  122  and the downstream end of the inlet section  24  respectively are held together by tension  196  having a tensioning screw  198  for tightening the annular band  100 , and thereby pressing the flanges  191  and  194  together by means of the locking ring  101 . Gaskets  105  and  107  sealingly engage the plate  180 . The combuster may be readily disassembled for modification and/or maintenance by simply releasing the tension clamps about the flanges  142 ,  144 ,  191  and  194 , and separating the inlet  124 , mixing section  122  and combustion section  128  one from the other.  
         [0024]    While this invention has been disclosed primarily in terms of the specific embodiment thereof, but is not limited thereto but rather only to the extent set forth hereafter in the claims which follow.