Abstract:
A catalytic hydrogen combustor is disclosed that is adapted to combust substantially all the hydrogen in the fuel cell exhaust gas during both steady state operation and for short duration higher hydrogen concentration pulses. Hydrogen combustion is catalyzed in two zones, a first zone in which a lower level of catalyst activity is used to catalyze combustion of a higher concentration hydrogen pulse as it moves through the zone and a second zone in which a higher level of catalyst activity is used to catalyze combustion of a lower concentration of hydrogen that is characteristic of steady state fuel cell operation.

Description:
BACKGROUND 
       [0001]    Embodiments described herein concern catalytic combustion of hydrogen. One embodiment concerns catalytic combustion of hydrogen in the fuel cell exhaust of a fuel cell powered vehicle. 
         [0002]    Polymer Electrolyte Membrane (PEM) fuel cells emit exhaust gas during operation that is primarily air, water and hydrogen. The concentration of hydrogen in the exhaust from a PEM fuel cell during steady state operation is relatively low. A simple calculation using the commonly accepted hydrogen utilization, see for instance U.S. Pat. No. 6,569,549, and air utilization gives a concentration of hydrogen in the combined anode and cathode exhaust stream of about 1%. For short periods during a change in operation of the PEM fuel cell, the amount of hydrogen emitted can be significantly larger than is emitted during steady state operation. Hydrogen in fuel cell exhaust can create a risk of uncontrolled rapid hydrogen combustion. 
       SUMMARY 
       [0003]    Embodiments described herein relate to systems and methods for reducing hydrogen concentration in a stream of gas that may be exhaust from a fuel cell. One embodiment provides a catalytic hydrogen combustor comprising a monolith that forms a plurality of passageways that extend along a flow direction from an inlet to an outlet. A first coating containing hydrogen combustion catalyst is disposed on walls of the monolith passages in a first zone of the monolith that extends from the inlet to a transition location between the inlet and the outlet. A second coating containing hydrogen combustion catalyst is disposed on walls of the monolith passages in a second zone of the monolith that extends from the transition location to the outlet. The second coating is formulated to catalyze hydrogen combustion at a greater rate than the first coating. 
         [0004]    Another embodiment provides a method of reducing an amount of hydrogen in fuel cell exhaust for steady state operation exhaust flow that contains a low concentration of hydrogen and for exhaust flow containing a short duration pulse of higher concentration hydrogen. The method includes directing the flow of exhaust through a first catalytic combustion zone in which the exhaust is exposed to a first hydrogen combustion catalyst. Subsequently, the flow of exhaust is directed through a second catalytic combustion zone in which the exhaust is exposed to a second hydrogen combustion catalyst. The second catalytic combustion zone is configured and the second hydrogen combustion catalyst is formulated to catalyze combustion of substantially all hydrogen in a flow of fuel cell vehicle exhaust during steady state operation. The first catalytic combustion zone is configured and the first hydrogen combustion catalyst is formulated to catalyze combustion that at least substantially reduces the concentration of hydrogen in exhaust containing a pulse of higher concentration hydrogen. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is an oblique view of a catalytic hydrogen combustor. 
           [0006]      FIG. 2  is a partially cut away side view of the catalytic hydrogen combustor of  FIG. 1 . 
           [0007]      FIG. 3  illustrates the compositions of fuel cell exhaust during an operation transition and during steady state operation. 
           [0008]      FIG. 4  illustrates a catalytic hydrogen combustor having two zones of ceramic and catalyst coating. 
           [0009]      FIG. 5  illustrates the operating temperatures along the length of a catalytic hydrogen combustor during a steady state operating condition. 
           [0010]      FIG. 6  illustrates the operating temperatures along the length of a catalytic hydrogen combustor during a second steady state operating condition. 
           [0011]      FIG. 7  illustrates the surface temperatures at locations along the length of a catalytic hydrogen combustor during a time period that includes a hydrogen pulse and thereafter. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Embodiments described herein concern catalytic combustion of hydrogen and an apparatus that accomplishes that combustion. More particularly, the embodiments concern catalytic combustion of hydrogen in a gas stream in which the concentration of hydrogen is not constant. 
         [0013]    PEM fuel cells emit exhaust gas that contains at least hydrogen, oxygen, water and nitrogen. A catalytic hydrogen combustor in a fuel cell exhaust should initiate combustion of hydrogen in fuel cell exhaust gas at a low temperature (ignition temperature) and should operate at a low temperature during the fuel cell operation. Catalyzed hydrogen combustion releases heat that raises the temperature of the exhaust gas, catalyst and surrounding structures. Overheating of a catalytic combustor can result in two undesirable consequences. High temperatures can sinter the catalytic component on the supporting monolith diminishing the effectiveness of the catalytic component to catalyze hydrogen combustion. Also, should temperatures reach the auto-ignition temperature of hydrogen, rapid and uncontrolled combustion of hydrogen could result. 
         [0014]      FIG. 1  illustrates a catalytic hydrogen combustor  10 . A monolith  12  forms passages  18  such that gas flows through the monolith  12  from an inlet end  22  of the catalytic hydrogen combustor to an outlet end  24  of the catalytic hydrogen combustor  10 . The monolith  12  comprises a generally cylindrical outer shell  14  that forms a cylinder that lies along an axis X along a direction from the inlet end  22  of the catalytic hydrogen combustor  10  to the outlet end  24 . The monolith  12  further comprises a honeycomb element  16  that is positioned within the outer shell  14  and that extends along the axis X. 
         [0015]    The honeycomb element  16  of the monolith  12  allows gas to flow through the honeycomb element  16  from the inlet end  22  to the outlet end  24  and presents surfaces that form passages through which gas flows. The honeycomb element  16  may form a series of connected chambers that extend from the inlet end  22  to the outlet end  24  and the chambers should be sized and configured such that at least substantially all of the gas passing through the honeycomb element  16  will pass over surfaces of the honeycomb element  16  sufficiently to allow a catalyst on the surface to catalyze combustion of hydrogen in the gas passing through the honeycomb element  16 . The configuration of the honeycomb element  16  is not limited to any configuration or configurations. The honeycomb element  16  shown by  FIG. 1  forms channels  18  that provide a path for gas to pass through the honeycomb element  16  in the direction of the axis X from the inlet end  22  to the outlet end  24 . 
         [0016]    The monolith  12  conducts heat and has sufficient mass and specific heat so that heat released by hydrogen combustion does not raise the temperature of the monolith  12  to an unacceptable level. The honeycomb  16  of the monolith  12  is formed of corrugated stainless steel foil that is approximately . 04  mm thick. The monolith  12  may be of the LS type supplied by Emitec USA. 
         [0017]    The honeycomb element  16  is coated by a ceramic and hydrogen combustion catalyst coating on the surfaces that define the channels  18 . The hydrogen combustion catalyst is platinum and the supporting ceramic is gamma alumina. As discussed below, the amount of heat created by catalyzed hydrogen combustion in a region of the monolith depends on the concentration of hydrogen in the fuel cell exhaust and the concentration of hydrogen combustion catalyst particles on the surface of the monolith. The structure of a monolith with a platinum and gamma aluminum coating is known to those of skill in the art and is described by U.S. Pat. No. 7,610,752, which is owned by the assignee of this application, and is incorporated by reference herein. 
         [0018]    Fuel cells that power vehicles operate for extended periods, minutes to hours at a steady state condition emitting exhaust gas of substantially constant composition. This steady state operation creates exhaust gas typically having a hydrogen concentration of less than one percent hydrogen in the combined anode and cathode fuel cell exhausts. During transitions in fuel cell load and operation, the fuel cell can emit a pulse of hydrogen into the exhaust gas that can result in the exhaust gas comprising ten to twenty percent hydrogen during for a period that lasts several seconds.  FIG. 3  illustrates the composition of fuel cell exhaust for a hydrogen pulse emission at the left of the graph followed by steady state composition at the right of the graph. Fuel cell exhaust gas contains water that affects reaction kinetics. The exhaust conditions near stoichiometry during the pulse but are not near stoichiometry during steady state operation. A catalytic hydrogen combustor catalyzes combustion of substantially all hydrogen in the exhaust gas that enters the monolith  12  for both steady state and pulse exhaust gas compositions and maintains the monolith  12  at an acceptably low temperature. 
         [0019]      FIG. 4  illustrates the combustor  10  having a coating that includes a hydrogen combustion catalyst in two zones, a first zone  32  extending from the inlet end  22  to a transition location intermediate the inlet  22  and the outlet  24  and a second zone  34  extending from the outlet end  24  to the transition location to meet the first zone  32 . The first zone  32  extends greater than half the length of the combustor  10 . Within the first zone  32 , the monolith  12  is coated with a platinum and gamma alumina coating in which a low concentration of platinum is present. An example of such a coating for the first zone  32  is a platinum and gamma alumina coating in which the platinum is 0.33 weight percent of the gamma alumina and platinum coating. The second zone  34  extends from the first zone  32  the remainder of the length of the combustor  10  to the outlet end  24 . Within the second zone  34 , the monolith  12  is coated with a platinum and gamma alumina coating in which a higher concentration of platinum is present than is present in the first zone  32 . An example of such a coating for the second zone  34  is a platinum and gamma alumina coating in which the platinum catalyst is 2 weight percent of the gamma alumina and platinum coating. The platinum catalyst creates heat during catalytic combustion of hydrogen. The greater the density of catalyst, the greater the amount of heat that is created per unit area of coated surface. 
         [0020]      FIGS. 5 and 6  show the temperature within the combustor  10  along its length for two different steady state fuel cell exhaust gases.  FIG. 5  shows the temperatures of the combustor  10  for fuel cell exhaust at a relatively low inlet temperature and comprised of 0.75 percent hydrogen.  FIG. 6  shows the temperatures of the combustor  10  for fuel cell exhaust at a higher inlet temperature than is shown by  FIG. 5  and comprised of 0.5 percent hydrogen. As can be seen by the temperatures shown by  FIGS. 5 and 6 , little heat is created by catalytic combustion in the first zone  32  during steady state fuel cell operation that emits low hydrogen concentration exhaust gas. The measured temperatures in the first zone  32  do not differ significantly from the inlet gas temperature indicating that the measured temperature is approximately that of the exhaust gas with little if any heating due to catalytic combustion. However, the markedly increased temperatures in the second zone  34  shows significant catalytic combustion of that steady state exhaust in the second zone  34 . 
         [0021]      FIG. 7  shows temperature at locations along the combustor  10  from the inlet end  22  to the outlet end  24  for a period of time during which inlet exhaust gas included a hydrogen pulse that exceeded ten percent of the inlet exhaust gas. The curve  42  is at a location that is within the combustor  10  that is a distance from the inlet  22  that is one tenth the combustor length from the inlet end  22  to the outlet end  24 . The curve  44  is at a location that is within the combustor  10  that is a distance from the inlet  22  that is three tenths the combustor length. The curve  46  is at a location that is within the combustor  10  that is a distance from the inlet  22  that is four tenths the combustor length. The curve  48  is at a location that is within the combustor  10  that is a distance from the inlet  22  that is six tenths the combustor length. The curve  52  is at a location that is within the combustor  10  that is a distance from the inlet  22  that is eight tenths the combustor length. The curve  54  is at a location that is within the combustor  10  that is a distance from the inlet  22  that is nine tenths the combustor length. The locations of the temperatures shown by curves  42 ,  44 ,  46 , and  48  are within the first zone  32  of the combustor  10 . The locations of the temperatures shown by curves  52  and  54  are within the second zone  34  of the combustor  10 . 
         [0022]    As shown by  FIG. 7 , the catalyst coating in the first zone  32  is effective in catalyzing combustion of hydrogen in an exhaust gas having hydrogen concentration of the hydrogen pulse where the catalyst coating in the first zone  32  did not catalyze significant combustion of hydrogen for fuel cell exhaust gas having a hydrogen concentration based on steady state operation as shown by  FIGS. 5 and 6 . As is also shown by  FIG. 7 , the maximum temperature occurs progressively later at locations that are increasing distances from the inlet  22  as the heat wave travels through the combustor  10 . The maximum temperature in the combustor  10  decreases with distance from the inlet  22  indicating that the amount of hydrogen that is combusted decreases with distance from the inlet  22 . The temperature curves in  FIG. 7  increase to a maximum value and decrease prior to locations that are farther from the inlet  22  reaching the maximum temperature. The duration of the hydrogen pulse is less than the time for the heat wave to travel from the inlet end  22  to the outlet end  24  of the hydrogen combustor  10 . The decrease of maximum temperature extends into the second zone  34  despite the higher platinum catalyst density in the second zone  34 . Unlike the hydrogen combustion for the low hydrogen concentration steady state fuel cell exhaust, significant combustion of the hydrogen pulse occurs in the first zone  32  that significantly diminishes the amount of hydrogen in the exhaust gas that reaches the second zone  34  of the combustor  10 . 
         [0023]    An embodiment of a catalytic hydrogen combustor may comprise a monolith that forms a plurality of passageways that extend along a flow direction from an inlet to an outlet, a first coating containing hydrogen combustion catalyst may be disposed on surfaces of the monolith that are adjacent to passageways in a first zone of the monolith between the inlet and a transition location between the inlet and the outlet, a second coating containing hydrogen combustion catalyst may be disposed on surfaces of the monolith that are adjacent to passageways in a second zone of the monolith between the transition location and the outlet and the second coating formulated to catalyze hydrogen combustion at a greater rate than the first coating. 
         [0024]    The first coating of the embodiment of a catalytic hydrogen combustor may comprise ceramic and a hydrogen combustion catalyst and the second coating may comprise ceramic and a hydrogen combustion catalyst. The ceramic of the first coating may be gamma alumina and the hydrogen combustion catalyst of the first coating may be platinum. The ceramic of the second coating may be gamma alumina and the hydrogen combustion catalyst of the second coating may be platinum. The weight percent of platinum in the second coating may be greater than the weight percent of platinum in the first coating. The weight percent of platinum in the second coating may be approximately 2 and the weight percent of platinum in the first coating may be approximately 0.33. 
         [0025]    An embodiment of a catalytic hydrogen combustor may comprise a monolith that forms a plurality of passageways that extend along a flow direction from an inlet to an outlet and the monolith may comprise a honeycomb element and an outer shell that surrounds the honeycomb element from the inlet to the outlet. A first coating containing hydrogen combustion catalyst may be disposed on surfaces of the monolith that are adjacent to passageways in a first zone of the monolith between the inlet and a transition location between the inlet and the outlet, a second coating containing hydrogen combustion catalyst may be disposed on surfaces of the monolith that are adjacent to passageways in a second zone of the monolith between the transition location and the outlet and the second coating may be formulated to catalyze hydrogen combustion at a greater rate than the first coating. The first coating of the embodiment of a catalytic hydrogen combustor may comprise ceramic and a hydrogen combustion catalyst and the second coating may comprise ceramic and a hydrogen combustion catalyst. The ceramic of the first coating may be gamma alumina and the hydrogen combustion catalyst of the first coating may be platinum and the wherein the ceramic of the second coating may be gamma alumina and the hydrogen combustion catalyst of the second coating may be platinum. The weight percent of platinum in the second coating may be greater than the weight percent of platinum in the first coating. The weight percent of platinum in the second coating may be greater than the weight percent of platinum in the first coating. The weight percent of platinum in the second coating may be approximately 2 and the weight percent of platinum in the first coating may be approximately 0.33. 
         [0026]    A method of reducing the amount of hydrogen in fuel cell exhaust for steady state operation exhaust flow that contains a low concentration of hydrogen and for exhaust flow containing a short duration pulse of higher concentration hydrogen may comprise directing the flow of fuel cell exhaust through a first catalytic combustion zone in which the exhaust is exposed to a first hydrogen combustion catalyst, subsequently directing the flow of fuel cell exhaust through a second catalytic combustion zone in which the exhaust is exposed to a second hydrogen combustion catalyst, the second catalytic combustion zone may be configured and the second hydrogen combustion catalyst may be formulated to catalyze combustion of substantially all hydrogen in a flow of fuel cell vehicle exhaust during steady state operation, and the first catalytic combustion zone may be configured and the first hydrogen combustion catalyst may be formulated to catalyze combustion that at least substantially reduces the concentration of hydrogen in exhaust containing a pulse of higher concentration hydrogen. The concentration of hydrogen in a pulse of higher concentration hydrogen may be reduced by the first catalytic combustion zone to an amount that is substantially the same as the concentration of hydrogen in fuel cell exhaust during steady state operation. The first hydrogen combustion catalyst and the second hydrogen combustion catalyst may each comprise a coating of platinum and gamma alumina and the weight percent of platinum of the first hydrogen combustion catalyst may be less than the weight percent of platinum of the second hydrogen combustion catalyst. The first hydrogen combustion catalyst may comprise a coating of platinum and gamma alumina having a weight percent of platinum of approximately 0.33. The second hydrogen combustion catalyst may comprise a coating of platinum and gamma alumina having a weight percent of platinum of approximately 2.