Abstract:
A catalytic reformer assembly comprising a mixing chamber wherein fuel and air are mixed. The wall of the mixing chamber tapers toward an outlet end. A catalyst bed formed in an annular shape surrounds the outlet end such that the walls of the mixing chamber shield the catalyst from direct exposure to fuel droplets injected into the mixing chamber. The fuel/air mixture flows out of the mixing chamber, then turns and counterflows through the catalyst bed outside the mixing chamber. Hot reformate from the catalyst bed flows in a reformate flow chamber extending along the outer surface of the walls of the mixing chamber, heating the wall surface within the mixing chamber for instantaneous evaporation of injected fuel. A plenum for incoming air surrounds the reformate flow chamber which is also heated thereby.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a catalytic hydrocarbon reformer for converting a hydrocarbon stream to a gaseous reformate fuel stream comprising hydrogen; more particularly, to a fast light-off catalytic reformer; and most particularly to a method and apparatus for rapid heating and vaporization of liquid hydrocarbon fuel and good mixing of vaporized fuel and air, especially during cold start-up of a hydrocarbon reformer. The present invention is useful for providing reformate rapidly after start-up to a fuel cell, especially a solid oxide fuel cell, or to an internal combustion engine or vehicle exhaust stream to improve emission reduction performance. 
       BACKGROUND OF THE INVENTION 
       [0002]    A catalytic hydrocarbon fuel reformer converts oxygen and a fuel comprising, for example, natural gas, light distillates, methanol, propane, naphtha, kerosene, gasoline, diesel fuel, bio-diesel or combinations thereof, into a hydrogen-rich reformate stream comprising a gaseous blend of hydrogen, carbon monoxide, and nitrogen, plus trace components. In a typical reforming process, the hydrocarbon fuel is percolated with oxygen in the form of air through a catalyst bed or beds contained within one or more reactor tubes mounted in a reformer vessel. The catalytic conversion process is typically carried out at elevated catalyst temperatures in the range of about 700° C. to about 1100° C. 
         [0003]    The produced hydrogen-rich reformate stream may be used, for example, as the fuel gas stream feeding the anode of an electrochemical fuel cell. Reformate is particularly well suited to fueling a solid-oxide fuel cell (SOFC) system because a purification step for removal of carbon monoxide is not required as in the case for a known proton exchange membrane (PEM) fuel cell systems. 
         [0004]    The reformate stream may also be used in spark-ignited (SI) or diesel engines. Reformate can be a desirable fuel or fuel-additive; the reformate stream also can be injected into the vehicle exhaust to provide benefits in reducing vehicle emissions. Hydrogen-fueled vehicles are of interest as low-emissions vehicles because hydrogen as a fuel or a fuel additive can significantly reduce air pollution and can be produced from a variety of fuels. Hydrogen permits a SI engine to run with very lean fuel-air mixtures that greatly reduce production of NOx. As a gasoline additive, small amounts of supplemental hydrogen-rich reformate may allow conventional gasoline-fueled internal combustion engines to reach nearly zero emissions levels. As a diesel fuel additive, supplemental reformate may enhance operation of premixed combustion in diesel engines. Reformate can be injected into the vehicle exhaust stream to improve NOx reduction and/or as a source of clean chemical energy for improved thermal management of exhaust components (for example, NOx traps, particulate filters and catalytic converters). 
         [0005]    Fuel/air mixture preparation constitutes a key factor in the reforming quality of catalytic reformers, and also the performance of porous media combustors. A problem in the prior art has been how to vaporize fuel completely and uniformly, especially at start-up when the apparatus is cold. A related problem is that injected fuel droplets may follow a line-of-sight path directly to the entry surface of the catalyst, resulting in extreme, localized fuel/air inhomogeneities. Inhomogeneous fuel/air mixtures can lead to decreased reforming efficiency and reduced catalyst durability through coke or soot formation on the catalyst and thermal degradation from local hot spots. Poor fuel vaporization can lead to fuel puddling, resulting in uncertainty in the stoichiometry of fuel mixture. Complete and rapid fuel vaporization well ahead of the catalyst is a key step to achieving a homogeneous gaseous fuel-air mixture and consequent efficient reformate generation. 
         [0006]    Fuel vaporization is especially challenging under cold start and warm-up conditions for a fuel reformer. In the prior art, it is known to vaporize injected fuel by preheating the incoming air stream to be mixed with the fuel, or by preheating a reformer surface for receiving a fuel spray. However, none of the prior art approaches is entirely successful in providing reliable, complete vaporization of injected liquid fuel under start-up conditions. 
         [0007]    During start-up in a typical prior art fast light-off reformer, fuel and air are mixed stoichiometrically (or nearly-stoichiometrically) and burned in the fuel/air mixing chamber, and the hot combustion products are passed through the catalyst bed. This combustion phase provides the initial energy required to light-off the reforming catalyst and heats the fuel/air mixing zone to assist in fuel vaporization. 
         [0008]    After a brief combustion period, typically about 2 to 20 seconds, combustion is quenched and a very rich fuel/air mixture is supplied to initiate reformate production. The atomized fuel mixes with the airflow within the volume defining the mixing zone prior to reacting within the catalyst. The energy generated during the reforming process (exothermic reaction) continues to heat the reformer, including a heat exchange section downstream of the reforming catalyst. Under warmed-up operation, the heat exchange section transfers heat from the hot reformate gas to the incoming airflow. This heat exchange provides energy to the mixing zone to assist fuel vaporization. 
         [0009]    After the end of combustion but while the reformer is warming up, a transitional heat deficit develops in heat energy available in the mixing chamber for fuel vaporization. This deficit arises because the heat energy stored in the mixing section of the reformer during the combustion stage is depleted during early reforming before the heat exchange section is sufficiently warm to provide substantial heat from the reforming process back into the incoming airflow. The extent and duration of this deficit is dependent upon a number of factors, including heat generated and stored during combustion, the thermal mass of the catalyst and heat exchange section, and heat transfer rates within the reformer assembly. The maximum temperature that the catalyst face can sustain without thermal degradation of the catalyst, which typically is about 1100-1200° C., limits the duration of combustion, which thus limits the amount of energy that may be stored and available for fuel vaporization during early reforming. 
         [0010]    What is needed in the art is a compact reformer arrangement that provides sufficient volume, residence time, and heat to accomplish good fuel/air mixing and heating following a combustion phase during warm up of a hydrocarbon catalytic reformer. 
         [0011]    It is a primary object of the invention to reduce or eliminate the transitional heat deficit experienced by prior art reformers during start-up of the reformer. 
       SUMMARY OF THE INVENTION 
       [0012]    A catalytic reformer assembly comprises a cylindrical mixing chamber wherein fuel and air are mixed, initially for a combustion phase to heat the reformer, and subsequently for supplying a fuel/air mixture to the catalyst bed for reforming during a catalytic reforming phase. The mixing chamber decreases in diameter toward its outlet end. The catalyst bed is formed in an annular shape and surrounds the mixing zone outlet end such that the walls of the mixing chamber shield the catalyst from line-of-sight exposure to fuel droplets injected at the entrance end of the mixing chamber. The fuel/air mixture flows out of the mixing chamber, then turns to flow back through the catalyst bed outside of and surrounding the end of the mixing chamber. Hot reformate from the catalyst bed flows in an annular flow chamber along the outer surface of the walls of the mixing chamber in a direction counter to the direction of materials flow through the mixing chamber, which walls are heated thereby, providing a hot surface in the mixing chamber for instantaneous evaporation of injected fuel. Preferably, the reformate flow chamber is annular and extends the full length of the mixing chamber. Preferably, a plenum for incoming air surrounds the reformate flow chamber and is also heated thereby. The fuel injector may be located either near the air entry to the mixing chamber, for co-flow of air and fuel therethrough, or near the mixture exit from the mixing chamber, for counter-flow injection of fuel into the mixing chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0014]      FIG. 1  is a schematic longitudinal cross-sectional view of a prior art catalytic hydrocarbon reformer assembly; 
           [0015]      FIG. 2  is a schematic longitudinal cross-sectional view of a first embodiment of a catalytic hydrocarbon reformer assembly in accordance with the invention; 
           [0016]      FIG. 3  is a schematic longitudinal cross-sectional view of a second embodiment of a catalytic hydrocarbon reformer assembly in accordance with the invention; and 
           [0017]      FIG. 4  is a schematic longitudinal cross-sectional view of a third embodiment of a catalytic hydrocarbon reformer assembly in accordance with the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring to  FIG. 1 , a prior art fast light-off catalytic reformer assembly  10  comprises a jacket  12  concentric with a cylindrical reactor  14  defining an annular heat-exchanging plenum  16  therebetween which is closed at both axial ends. Plenum  16  communicates with a reforming chamber  18  within reactor  14  via a plurality of openings  20  formed in the wall of reactor  14 . Air  22  for combustion and for reforming enters reformer assembly  10  via inlet duct  24  formed in the wall of jacket  12 . Fuel  26  is injected by a fuel injector  28  mounted in end  30  directly into reforming chamber  18  wherein the fuel mixes with air  22  entering from chamber  16  via openings  20 . An igniter  32  such as a spark plug or other ignition source is disposed through end  30  of reactor  14  into chamber  18 . Reforming catalyst  34  is disposed in reactor  14  downstream of the flow of mixture  36  through chamber  18 . Downstream of catalyst  34  is a heat exchanger  38 . Intake air  22  is passed through a first side of heat exchanger  38  and hot combustion or reformate gases  40  exiting catalyst  34  are passed through a second side, thus heating intake air  22 . 
         [0019]    It will be seen that heat exchanger  38  is isolated from the combustion that initially occurs in the reforming chamber  18  prior to reforming. Consequently, until the heat exchanger  38  is sufficiently warmed-up by reformate  40 , energy to vaporize the fuel spray is available only from the heat stored in the walls of reactor  14  during the initial combustion phase; hence, a heat deficit for vaporization of injected fuel is seen, as described above, in the time between cessation of combustion and sufficient warmup of heat exchanger  38 . 
         [0020]    A reformer in accordance with the present invention improves upon this arrangement by providing a heat exchanging wall between the incoming air and the reformate that is directly preheated by the initial combustion phase and heated during the catalytic reforming phase by heated reformate. The improved arrangement thus eliminates the heat deficit in the prior art described above and thus maintains more energy during warm-up of the reactor to assist in vaporizing fuel droplets. 
         [0021]    Referring to  FIG. 2 , a first embodiment  110  of a fast light-off catalytic reformer assembly in accordance with the invention comprises a reactor  114  having preferably a generally cylindrical form and open at inlet end  113  and outlet end  115 . In an aspect of the present invention, reactor  114  is longitudinally tapered or funnel-shaped such that outlet end  115  is smaller in diameter than inlet end  113 . Reactor  114  defines a mixing chamber  118  as described below. 
         [0022]    Surrounding the smaller diameter portion  121  of reactor  114  is a ring-shaped reforming catalyst bed  134  for generating reformate  140  from a fuel/air mixture  136 . A reforming chamber  137  is defined by an outer longitudinal wall  139  and endwalls  141 , 143 . The inner wall of reforming chamber  137  is formed by reactor  114  such that the entire length of reactor  114  from inlet end  113  to outlet end  115  defines a partition for exchanging heat between reformate  140  and fuel/air materials within mixing chamber  118 . The entrance  145  to reforming chamber  137  is at outlet end  115  of mixing chamber  118 . The outlet  147  of reforming chamber  137  is a radial duct. 
         [0023]    It will be seen that, contrary to the flow arrangement in prior art reformer assembly  10 , the initial combustion phase within mixing chamber  118  preheats a heat exchanging surface between reforming chamber  137  and mixing chamber  118 . Further, prior art reformers do not provide direct heating of the mixing chamber by exposure of the reactor walls to hot reformate. 
         [0024]    A jacket  112  surrounds longitudinal wall  139  defining a plenum  116  for passage of air  122  from an inlet duct  124  in jacket endwall  150  into mixing chamber  118  via a radial passage  120 . A fuel injector  128  and ignition device  132  are disposed in jacket endwall  152 . 
         [0025]    It will be observed that inlet air  122  passes along the entire length L 1  of outer longitudinal wall  139  and endwalls  141 , 143  of reforming chamber  137 , thus offering the maximum possible heat exchange opportunity between hot reformate  140  and inlet air  122 . Further, prior art reformers do not provide direct heating of inlet air by exposure of air to hot reformate over the axial length L 2  of the mixing chamber. 
         [0026]    Referring now to  FIG. 3 , a second embodiment  210  of a fast light-off catalytic reformer assembly in accordance with the invention is similar in many respects to first embodiment  110 , and common elements are so numbered. Different but analogous elements are number similarly but in the 200 series. The significant difference over first embodiment  110  is that the reforming chamber  237  is carried through the upstream reformer wall  252  to extend even further the heat exchange surface area of both the inner and outer walls  214 , 239  of the reforming chamber. Heated inlet air  122  enters mixing chamber  218  via a plurality, preferably six, of radial passages  220  extending from air plenum  216  through reforming chamber  237 . 
         [0027]    Referring now to  FIG. 4 , a third embodiment  310  of a fast light-off catalytic reformer assembly in accordance with the invention is similar in many respects to second embodiment  210 , and common elements are so numbered. Different but analogous elements are number similarly but in the 300 series. The significant difference over first and second embodiments  110 , 210  is that the reformate chamber  337  extends through the end of jacket  312  and completely surrounds and defines the mixing chamber  318 . Reformate  140  exits via an axial duct  347 , which is a packaging improvement for a reformer in an automotive system. Thus, reactor  314  comprises the inner of the reformate chamber, thereby exposing the outer surface of the entire reactor  314  to hot reformate gas. In one aspect of the invention, fuel injector  328  is moved to the downstream end  353  of the mixing chamber  318  such that fuel is injected into mixing chamber  318  in counterflow to air  122  moving through the chamber, thus improving turbulence and mixing. Likewise, igniter  332  is moved to end  353 . 
         [0028]    In one method in accordance with the invention for operating any of reformer assemblies  110 , 210 , 310  (using only the numbers of assembly  110  for simplicity), during start-up from a cold start, fuel spray is injected by fuel injector  128  into reactor  114  wherein the fuel is mixed with air  122  in a near-stoichiometric ratio, and ignited by igniter  132  to form hot exhaust gases which immediately begin to heat the walls of reactor  114 , catalyst bed  134 , and outer reformate wall  139 . 
         [0029]    After combustion has proceeded for a few seconds, ignition by ignitor  132  is terminated. Fuel flow is also terminated for a brief period to cause the preheat flame to be extinguished prior to commencing reforming. The fuel ratio is then made richer in fuel, and fuel/air mix  136  is passed into the reforming catalyst  134  to begin generation of reformate  140 . 
         [0030]    The present fast light-off catalytic reformer assembly and methods of operation rapidly produce high yields of reformate fuel without significant coking or hot-spotting of the reactor or reforming catalyst during start-up. 
         [0031]    The produced reformate  140  may be bottled in a vessel or used to fuel any number of systems operating partially or wholly on reformate fuel. Such power generation systems for reformer assembly  110  may include, but are not limited to, engines such as spark ignition engines, hybrid vehicles, diesel engines, fuel cells, particularly solid oxide fuel cells, or combinations thereof. The present fast light-off reformer and method may be variously integrated with such systems, as desired. For example, the present fast light-off reformer may be employed as an on-board reformer for a vehicle engine  400  operating wholly or partially on reformate, the engine having a fuel inlet in fluid communication with the reformer outlet for receiving reformate  140  therefrom. 
         [0032]    The present fast light-off reformer and methods are particularly suitable for use as an on-board reformer for quickly generating reformate  140  for initial start-up of a system. The present reformer and methods are particularly advantageous for hydrogen cold-start of an internal combustion engine, providing a supply of hydrogen-rich reformate which allows the engine exhaust to meet SULEV emissions levels immediately from cold-start. The present fast light-off reformer and methods are also particularly suitable for use as an on-board reformer for quickly generating reformate for use to improve premixed combustion in a diesel engine. A third application for with the present fast light-off reformer and methods are suitable comprises injecting the reformate into the vehicle exhaust stream to improve NOx reduction and/or as a source of clean chemical energy for improved thermal management of exhaust components (for example, NOx traps, particulate filters and catalytic converters). Vehicles wherein a fast light-off reformer is operated in accordance with the present invention may include automobiles, trucks, and other land vehicles, boats and ships, and aircraft including spacecraft. 
         [0033]    While the invention has been described by reference to certain preferred 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 disclosed embodiments, but that it have the full scope permitted by the language of the following claims.