Abstract:
A catalytic hydrocarbon reformer comprising a catalyst concentrically disposed within a reformer tube surrounded by an annular flow space for air entering a fuel-air mixing zone ahead of the catalyst. The catalyst is sustained by minimal insulative mounting material so that most of the side of the catalyst is exposed for radial radiative heat transfer to the reformer tube for cooling by air in the annular flow space. The forward portion of the mounting material preferably is formed of a thermally-conductive material to provide radial conductive cooling of the entry of the catalyst to prevent overheating during catalysis. The incoming air flow is protected from heat exchange with hot reformate exiting the catalyst, allowing for convective cooling of the catalyst side and greater cooling of the catalyst face, thus increasing the working life of the catalyst while providing for rapid startup of the reformer and associated fuel cell system.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to catalytic hydrocarbon reformers for providing hydrogen and carbon monoxide fuels; more particularly, to such reformers wherein incoming fuel and air are heated by extracting heat from formed reformate exiting the reformer; and most particularly, to a reformer wherein heating of incoming air is accomplished by extraction mostly of heat from the catalytic element itself. 
       BACKGROUND OF THE INVENTION 
       [0002]    Catalytic hydrocarbon reformers for converting air and hydrocarbon fuels into molecular hydrogen and carbon monoxide (reformate) as gaseous fuels for use in, for example, solid oxide fuel cells (SOFCs) are well known. 
         [0003]    Fuel-air mixture preparation in catalytic reformers is a key factor in fuel efficiency and reformer life. Inhomogeneous mixtures can lead to decreased reforming efficiency and reduced reformer catalyst durability through coke/soot formation on the catalyst and thermal degradation from local hot spots. 
         [0004]    Complete and rapid fuel vaporization is a key step to achieving a homogeneous fuel-air mixture. Fuel vaporization is especially challenging under reformer cold-start and warm-up conditions. In prior art technologies, such as vaporization via a preheated air stream, vaporization from a heated reformer surface, or vaporization via “cool flames”, the overall startup time to the beginning of electric generation by the associated fuel cell can be undesirably extended and overall system efficiency can be substantially reduced compared to under steady-state conditions. 
         [0005]    What is needed in the art is an improved heat transfer arrangement that provides greater and faster transfer of heat from a catalyst brick to the incoming air and/or fuel stream to cause more rapid and more complete vaporization of fuel earlier in the startup phase of reformer/fuel cell operation. 
         [0006]    It is a principal object of the present invention to improve fuel efficiency, to increase catalyst life, and to shorten start-up time in a catalytic hydrocarbon fuel reformer. 
       SUMMARY OF THE INVENTION 
       [0007]    Briefly described, the present invention comprises a catalyst brick concentrically disposed within a reformer tube which itself is disposed concentrically within an off-spaced housing, thereby defining an annular flow space between the reformer tube and the housing for air entering a fuel-air mixing zone ahead of the catalyst brick. The catalyst brick is sustained within the reformer tube by minimal mounting material; thus, most of the side of the catalyst brick is exposed for radial radiative heat transfer to the reformer tube and hence to cooling air in the annular flow space. Preferably, the forward portion of the mounting material is formed of a highly conductive material such as wire rope to provide a high level of radial conductive cooling of the entry portion of the catalyst brick which is known in the art to suffer degradation from localized overheating during catalysis. Preferably, the hot reformate exiting the catalyst sees reduced heat exchange with the incoming air flow compared to the prior art. Thus, incoming air can convectively cool the longitudinal side of the catalyst brick and allowing for reduced temperature of the face of the catalyst brick, thus increasing the working life of the catalyst brick while providing for more complete and more rapid fuel vaporization and hence more rapid startup of the reformer and associated fuel cell system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0009]      FIG. 1  is a schematic longitudinal cross-sectional view of a prior art catalytic hydrocarbon reformer; 
           [0010]      FIG. 2  is a schematic longitudinal cross-sectional view of a catalytic hydrocarbon reformer in accordance with the present invention; and 
           [0011]      FIG. 3  is a graph showing improved heating of the mixing zone of a reformer equipped in accordance with the present invention. 
       
    
    
       [0012]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring to  FIG. 1 , a typical prior art catalytic hydrocarbon reformer  10 , such as used for supplying reformate to a SOFC system, is shown. Reformer  10  comprises a catalytic element  12 , typically a porous or channeled monolithic element known in the art as a “brick”, for converting a mixture  14  of hydrocarbons and oxygen to a reformate gas  16  containing molecular hydrogen and carbon monoxide. Reformate typically is used as a fuel in the SOFC  18  to generate electricity and heat. Fuel  20  is injected by a fuel injector  21  into a mixing zone  22  where it mixes with incoming air  24  to form mixture  14 . Catalytic element  12  is mounted within a typically cylindrical reformer tube  26  from which element  12  is sustained and centered by a material  28  that completely surrounds the reforming catalyst. An outer housing  30  defines an annular space  32  for passage of incoming air  24 . An igniter  34  extends into mixing zone  22  for ignition of mixture  14  during cold startup as described below. 
         [0014]    In prior art reformer  10 , heat transfer from catalyst brick  12  to the reformer walls  26  is severely restricted by mat material  28 . Reformer warm-up for fuel-air mixture preparation is chiefly achieved via counter-flow heat exchange between the hot reformate gas  16  exiting the reformer and the incoming air flow  24 . Radiation heat exchange from the front face  13  catalyst brick  12  into the fuel-air mixture  14  in mixing zone  22  is possible, and tests show that it can significantly increase the reformer heat-up rate. However, such heating requires exposing a high temperature catalyst surface directly to the incoming fuel-air mixture, which can lead to undesirable pre-ignition and gas phase combustion in the reformer mixing zone during reforming. Such gas phase combustion under reforming conditions is unacceptable because it leads to poor reforming efficiency and production of sooty deposits in the reformer. 
         [0015]    In operation, during initial start-up, fuel and air are burned in the mixing zone by ignition the mixture with igniter  34 . This combustion phase provides the initial energy required to light-off the reforming catalyst and heats the fuel-air mixing zone  22  to assist fuel vaporization. 
         [0016]    After a predetermined warm-up period, combustion is quenched and a very rich fuel-air mixture is supplied to initiate reformate production. The atomized fuel evaporates and mixes with the airflow within mixing zone  22  prior to reacting within catalyst  12 . The energy generated during the reforming process (exothermic reaction) continues to heat the reformer, including the heat exchange section  36  downstream of the reforming catalyst. Under warmed up operation, heat exchange section  36  transfers heat from the hot reformate gas  16  to incoming air flow  24 . This heat transfer provides energy to the mixing zone to assist fuel vaporization. 
         [0017]    After the end of combustion but before the reformer is warmed up, a deficit in heat energy for fuel vaporization develops. This deficit arises because the heat energy stored in the mixing section  22  of the reformer during the combustion process is depleted by reforming before the heat exchange section  36  warms up sufficiently to provide substantial heat from the reforming process. The extent and duration of this deficit is dependent on a number of factors including the heat generated and stored during combustion, the thermal mass of the catalyst and heat exchange section and the heat transfer rates within the reformer. The maximum temperature that the brick face  13  can sustain (approximately 1100° C. to 1200° C.) is a practical limit to prevent thermal degradation of the catalyst, which limits combustion duration and thus controls the amount of stored energy available for fuel vaporization during early reforming. 
         [0018]    Providing a compact reformer that provides sufficient volume, residence time, and heat to accomplish good fuel air mixture preparation is a significant challenge. Especially during warm-up, the heat deficit described above results in incomplete fuel vaporization. Consequent fuel puddling leads to excursions in fuel-air composition and volume that impact reformer efficiency and durability. A second challenge is thermal management of the reforming catalyst to prevent excessively high temperatures in the catalyst brick that can rapidly degrade catalyst function and shorten durability. 
         [0019]    Referring now to  FIG. 2 , compared to prior art reformer  10 , improved reformer  110  can eliminate the heat deficit by using direct radiation and/or conduction of heat from catalyst brick  12  itself, rather than from the reformate as in the prior art, which enables more rapid coupling of energy from the exothermic reforming process back into mixing zone  22  for good mixture preparation. Improved reformer  110  also provides a direct path for cooling catalyst brick face  13  to prevent thermal degradation of the catalyst. Overall, more heat must be rejected from the exothermic partial oxidation reforming process than is necessary for good mixture preparation; therefore, the invention further provides convective cooling of the reformer mixing zone walls to reject the extra heat and to prevent degradation and failure of the reformer wall materials from excessive temperatures that this direct coupling can create. 
         [0020]      FIG. 2  includes all aspects of the present invention. To highlight the changes compared to prior art reformer  10  shown in  FIG. 1 , the schematic overall shape of the reformer is unchanged. It is important to note that the invention is not restricted to this reformer arrangement, and novel thermal management techniques are generally applicable to a wide variety of reformer shapes and arrangements employing exothermic reactions. 
         [0021]    Improved reformer  110  allows direct conduction and/or radiation from catalyst brick  12  into mixing zone  22  to provide the heat required for good fuel-air mixture preparation. Compared to prior art reformer  10 , a portion of material  28  is removed, leaving material  128  extending over only a first portion of the side of catalyst brick  12 . Open air space  140  extends over a second portion of the side of catalyst brick  12  between catalyst brick  12  and reformer tube  26  to enable radiative coupling between the catalyst brick and the reformer tube. The amount of insulating mat material  128  compared to the longitudinal extent of air space  140  is chosen to provide the appropriate level of heat transfer from catalyst brick  12  to the reformer tube  26  to maintain the appropriate catalyst temperature. 
         [0022]    Further, to enable conductive coupling of the perimeter of catalyst face  13  with reformer tube  26 , a highly conducting material  142  (for example a wire rope) replaces a forward portion of material  128 . Though  FIG. 2  shows facilitation of both conduction and radiation heat transfer from brick  12 , conduction-only or radiation-only arrangements are also possible within the scope of the present invention by including only conductive material  142  or only air space  140 , respectively. 
         [0023]    Referring to  FIG. 3 , heat-up rate of reformer mixing zone  22  as a function of time from a cold start is shown for the prior art reformer  10  (curve  170 ); for an improved reformer  110  having only wire rope  142  (curve  172 ); and for an improved reformer  110  having only radial radiation cooling via air space  140  (curve  174 ). Both improved embodiments show increased heat up rates of the reformer mixing zone that result in improved reforming efficiency early in the warm-up and reforming process. 
         [0024]    Referring again to  FIGS. 1 and 2 , a further improvement for thermal management of the catalyst brick in accordance with the present invention is shown. In the prior art reformer  10 , the incoming air flow  24  gains heat chiefly from heat exchange with hot reformate  16  in heat exchange section  36 , which was heretofore thought to be an advantage. By heating the incoming air in this fashion, however, some energy already rejected by the catalyst is recycled back into the front of the reforming catalyst, which impedes the net heat rejection required by the reformer for exothermic partial-oxidation reforming without overheating of the catalyst. In the present invention, air flow  24  gains substantially less heat from the hot reformate in section  36 . Although this feature is shown schematically in  FIG. 2  by insulation  38  between the reformate gas and the incoming air flow, preferably air flow  24  is simply not conducted through this section of the reformer. More preferably, the reformer is shortened to allow for a more compact reformer design by significantly reducing the length of section  36 . Thus, air flow  24  is protected from exposure to reformate heat and is more capable (lower temperature) of convectively cooling the reformer tube wall that is directly coupled to the reforming catalyst by air space  140 . The purpose of this convective cooling is both to cool the periphery of the catalyst brick and to enable heat transfer from the catalyst to the reformer mixing zone. Calculations show that the maximum tube wall temperature is reduced by more than 200° C. by such convective cooling without exposure to reformate heat in accordance with the present invention. 
         [0025]    In summary, the present invention improves reformer thermal management compared to the prior art. Direct coupling of heat transfer from the catalyst brick rather than the reformate greatly reduces thermal lag as compared to a prior art reformer having a reformate-air heat exchanger. Further, using direct transfer of energy from the catalyst brick by convective cooling by the incoming air enables the air to be an efficient means of net heat rejection necessary from the reformer. 
         [0026]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.