Abstract:
A catalytic reformer assembly and methods of operation, including fast start-up, are provided. The reformer assembly includes an electrically-conductive metallic vaporizer having a very high surface area. At start-up of the reformer, electric current is passed through the vaporizer to heat the material by resistance heating, providing a high-temperature, high-surface area environment for fuel vaporization. Preferably, the electric current is started a few seconds before starting fuel flow. The fuel is sprayed either onto or through the heated vaporizer, preferably before the fuel is mixed with incoming air to minimize convective cooling by the air and to reduce the pressure drop in the fuel flow. As the reformer warms up, energy from the reforming process heats the vaporizer via radiation and/or conduction such that electric power is needed only during the start-up phase. A control circuit regulates the amount and duration of electric power supplied to the vaporizer.

Description:
TECHNICAL FIELD 
     The present invention relates to a catalytic hydrocarbon reformer for converting a hydrocarbon stream to a gaseous reformate fuel stream comprising hydrogen; and more particularly, to a fast light-off catalytic reformer; and most particularly to a method and apparatus for rapid vaporization of liquid hydrocarbon fuel during cold start-up of a hydrocarbon reformer. The present invention is useful for rapidly providing reformate as a fuel to a fuel cell, especially a solid oxide fuel cell, or to an internal combustion engine or vehicle exhaust stream to improve emissions reduction performance. 
     BACKGROUND OF THE INVENTION 
     A catalytic hydrocarbon fuel reformer converts a fuel stream comprising, for example, natural gas, light distillates, methanol, propane, naphtha, kerosene, gasoline, diesel fuel, bio-diesel or combinations thereof, and air, into a hydrogen-rich reformate fuel stream comprising a gaseous blend of hydrogen, carbon monoxide, and nitrogen (ignoring trace components). In a typical reforming process, the raw hydrocarbon 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. 
     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 is the case for a proton exchange membrane (PEM) fuel cell system. 
     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). 
     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. 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 is a key step to achieving a homogeneous gaseous fuel-air mixture. 
     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. 
     What is needed is a method and apparatus for rapidly heating and vaporizing liquid hydrocarbon fuel injected into a reformer assembly, even when the overall assembly is in a cold start-up condition. 
     It is a primary object of the invention to prevent coking of the housing and catalyst of a hydrocarbon reformer, especially at start-up of the reformer. 
     SUMMARY OF THE INVENTION 
     A catalytic reformer assembly and methods of operation, including fast start-up, are provided. The reformer assembly includes a fuel vaporizer in the form of an electrically-conductive, metallic element having a very high surface area. At start-up of the reformer, electric current is passed through the element to heat it by resistance heating, providing a high-temperature, high-surface area environment for fuel vaporization. Preferably, the electric current is started before starting fuel flow to preheat the element. The fuel is sprayed either onto or through the heated metallic element, preferably before the fuel is mixed with incoming air to minimize convective cooling by the air and to reduce the pressure drop in the fuel flow. As the reformer warms up, energy from the reforming process heats the metallic element via radiation and/or conduction such that electric power is needed only during the start-up phase. A control circuit regulates the amount and duration of electric power supplied to the element. The invention contemplates that the heating element may remain energized after reforming has begun and/or may be continuously de-energized and re-energized as needed during the catalytic reforming. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an isometric view, partially in section, of a first prior art catalytic reformer assembly; 
         FIG. 2  is a schematic cross-sectional view of a second prior art catalytic hydrocarbon reformer assembly; 
         FIG. 3  is a schematic cross-sectional view of a first embodiment of a catalytic hydrocarbon reformer assembly in accordance with the invention; 
         FIG. 4  is a schematic cross-sectional view of a second embodiment of a catalytic hydrocarbon reformer assembly in accordance with the invention; and 
         FIG. 5  is a set of graphs showing the electrical characteristics of an exemplary electrically heated metallic fuel vaporizer in accordance with the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a first prior art fast light-off catalytic reformer assembly  01  includes a reactor  10  having an inlet  12  in a first end for receiving a flow of fuel  11  and a flow of air  13 , shown as combined fuel-air mixture  14 . Reactor  10  may be any shape, but preferably comprises a substantially cylindrical reactor tube. Reforming catalyst  16  is disposed within reactor  10 . A protective coating or firewall (not shown) may be disposed about catalyst  16 . 
     During operation, fuel-rich mixture  14  comprising air  13  and hydrocarbon fuel  11  such as natural gas, light distillates, methanol, propane, naphtha, kerosene, gasoline, diesel fuel, or combinations thereof, is converted by catalyst  16  to a hydrogen rich reformate fuel stream  18  that is discharged through outlet  20 . 
     Ignition device  22  is disposed within reactor  10  to ignite fuel/air mixture  14  as desired. Heat generated by this reaction is used to provide fast light-off (i.e., rapid heating) of reforming catalyst  16  at start-up of the reformer. Ignition device  22  is disposed within the reactor  10  upstream of reforming catalyst  16 , i.e., between inlet  12  and reforming catalyst  16 . Ignition device  22  may be any device suitable for initiating exothermic reactions between fuel and air  14 , including, but not limited to, a catalytic or non-catalytic substrate, such as a wire or gauze as shown in  FIG. 1 , for receiving electric current from a voltage source; a spark plug; a glow plug; or any combination thereof. An associated control system  30  selects and maintains the appropriate fuel and air delivery rates and operates the ignition device  22  so as to achieve fast light off of the reforming catalyst  16  at start-up and to maintain catalyst  16  at a temperature sufficient to optimize reformate  18  yield. 
     Prior art reformer assembly  01  has no provision for preheating of either incoming fuel  11  or air  13  and thus is not optimally directed to capability for providing either fast light-off or steady-state operating conditions for generation of reformate  18 . 
     Referring to  FIG. 2 , a second prior art fast light-off catalytic fuel reformer  50  is seen to be adapted from reformer  01 , as shown in  FIG. 1 , and includes means for shortening the light-off induction period of the reformer. Components thereof having identical function are identically numbered, and those having similar or improved function are identically numbered with a prime indicator. New components bear new numbers. 
     In second prior art reformer  50 , inlet  12  is eliminated and that end of reactor  10  is blocked by end plate  52 . A jacket  54  is provided concentric with reactor  10  and defining an annular chamber  56  therebetween which is closed at both axial ends. Chamber  56  communicates with reforming chamber  58  within reactor  10  via a plurality of openings  60  formed in the wall of reactor  10 . Air  13  for combustion and for reforming enters reformer  50  via inlet duct  62  formed in the wall of jacket  54 . Combustion fuel  11  is injected by a fuel injector  66  mounted in end  52  directly into reforming chamber  58  during combustion mode where the fuel mixes with air  13  entering from chamber  56  via openings  60 . An igniter  22 ′, preferably a spark plug or other sparking device, is disposed through end  52  of reactor  50  into chamber  58 . Reforming catalyst  16  is disposed in reactor  10  downstream of the flow of mixture  14  through chamber  58 . Downstream of catalyst  16  is a heat exchanger  70 . Intake air  13  is passed through a first side of heat exchanger  70  and hot combustion or reformate gases  18 ′ exiting catalyst  16  are passed through a second side, thus heating intake air  13 . 
     Referring to  FIG. 3 , an improved reformer assembly  150  in accordance with the invention is structurally similar in many respects to prior art assembly  50  as shown in  FIG. 2 . Components thereof having identical function are identically numbered. New components bear new numbers in the 100 series. 
     End plate  52   a  closes the inlet end of reactor  10 . A jacket  54  is provided concentric with reactor  10  and defining an annular chamber  56  therebetween which is closed at both axial ends by end plates  52   a , 52   b . Chamber  56  communicates with reforming chamber  58  within reactor  10  via a plurality of openings  60  formed in the wall of reactor  10 . Air  13  for combustion and for reforming enters reformer  50  via inlet duct  62  formed in the wall of jacket  54 . Combustion fuel  11  is injected by a fuel injector  66  mounted in end  52   a  directly into reforming chamber  58  where the fuel mixes with air  13  entering from chamber  56  via openings  60 . An igniter  22 ′, preferably a spark plug or other sparking device, is disposed through a wall of reactor  10  into chamber  58 . Reforming catalyst  16  is disposed in reactor  10  downstream of the flow of mixture  14  through chamber  58 . Downstream of catalyst  16  is a heat exchanger  70 . Intake air  13  is passed through a first side of heat exchanger  70  and hot gases (either combustion products at start-up or reformate at steady state operation)  18 ′ exiting catalyst  16  are passed through a second side, thus heating intake air  13 . 
     The novel improvement in reformer assembly  150  consists in a fuel vaporizer  172  disposed within reactor  10  transversely of the flow path of fuel  11  being injected into reactor  10 . Vaporizer  172  preferably comprises an electrically-conductive metallic material in the form of a foam or spun/woven fibers to present a very large surface area for receiving and vaporizing liquid fuel spray from fuel injector  66 . As desired, and especially at reformer start-up, an electric circuit  174  is controllably imposed across vaporizer  172  which is electrically insulated from reactor  10 . The material from which vaporizer  172  is formed is selected to have a moderate ohmic resistance such that the vaporizer is resistively heated very quickly to a desired elevated operating temperature sufficient to continuously vaporize injected fuel for as long as is desired. The material must also be chemically inert at the operating environment of the reactor. Presently preferred materials include nickel and nickel alloys, although it is believed that other inert alloys can be made available which have still higher resistivity and thus even more rapid heating to even higher temperatures; and all such materials are fully comprehended by the invention. 
     After reformer assembly  150  is sufficiently warmed to begin fuel reforming, the heat thrown off by the exothermic reforming process can keep vaporizer  172  hot enough by radiation and conduction to continue vaporizing without requiring continued electric resistive heating. 
     Referring to  FIG. 4 , a second embodiment  250  of an improved catalytic hydrocarbon reformer assembly in accordance with the invention is similar in most respects to first embodiment  150 . However, the vaporizer  272  is disposed in an axial, spaced relationship with longitudinal axis A along reactor ( 10 ), and is energized by circuit  274 . Vaporizer  272  is also disposed in cylindrical form longitudinally along (and insulated and inbound from) the walls of reactor  10  rather than being disposed across the reactor as in first embodiment  150 ; and igniter  22  is returned to a prior art position in end plate  52   a.    
     This arrangement has at least two advantages. First, vaporizer  272  may be placed in direct contact  276  with catalyst bed  16 , resulting in a rapid transfer of heat by conduction from the catalyst bed to the vaporizer (whereas embodiment  150  must rely predominantly on radiative heating of vaporizer  172 ). Second, the cylinder of vaporizer  272  presents a very large macro-surface area for impingement of liquid fuel  11  and also shields the wall of reactor  10  from direct exposure to the fuel spray, which is known in the prior art to cause coking of the reactor. 
     A minor disadvantage of the arrangement shown in  FIG. 4  is that the heating load on the vaporizer is increased because the vaporizer is now fully exposed to both incoming air  13  and mixture  14 , both of which are cooling forces. 
     Referring to  FIG. 5 , electrical operation curves as a function of time from start-up are shown for an exemplary nickel foam vaporizer in accordance with the invention. Curve  302  shows a voltage increase applied across vaporizer  172 , 272  over 5 seconds. Curve  304  shows the corresponding temperature rise as calculated from electrical resistance of the vaporizer. Curve  306  shows the applied current, and curve  308  shows the resulting applied power. Finally, curve  310  shows the actual measured temperature rise of vaporizer  172 , 272 ; it is seen that the vaporizer can reach a temperature of at least 200° C. within 6 seconds and can maintain this temperature thereafter. 
     Reformer assembly  150 , 250  may be operated in any of several ways, depending upon a specific application or upon the operational status of the reformer. 
     In a first method in accordance with the invention, during start-up from a cold start, fuel  11  is spray injected by fuel injector  66  into vaporizer  172 , 272  wherein the fuel is instantly vaporized by contact with the hot material of the vaporizer. The vaporized fuel passes into reactor  10 , is mixed with air  13  in a near-stoichiometric ratio, and ignited by igniter  22 ′ to form hot exhaust gases  18 ′ which immediately begin to heat the first side of heat exchanger  70 . Preferably, circuit  174 , 274  is energized for a few seconds prior to commencing injection of fuel to preheat the vaporizer to vaporization temperature. 
     In one aspect of the invention, after combustion has proceeded for a few seconds, ignition by igniter  22 , 22 ′ is terminated, the fuel ratio is made richer in fuel, and the unburned fuel/air mix  14  is passed into the reforming catalyst  16 . Fuel flow is also terminated for a brief period to cause the preheat flame to be extinguished prior to commencing injection leading to the richer fuel mixture. Once catalytic reforming has begun, vaporizers  172 , 272  may be de-energized or allowed to remain energized depending upon the needs of the reformer. Also, the vaporizer may be controllably energized and de-energized during operation of the catalytic reformer. 
     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. The produced reformate  18 ′ 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  150  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 operating wholly or partially on reformate, the engine having a fuel inlet in fluid communication with the reformer outlet for receiving reformate  118  therefrom. 
     The present fast light-off reformer and methods are particularly suitable for use as an on-board reformer for quickly generating reformate  118  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. 
     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.