Abstract:
A fuel processor for rapidly achieving operating temperature during startup. The fuel processor includes a reformer, a shift reactor, and a preferential oxidation reactor is provided for deriving hydrogen for use in creating electricity in a plurality of fuel cells. A first combustion heater system is coupled to at least one of the reformer, the shift reactor, and the preferential oxidation reactor to preheat the component during a rapid startup sequence. That is, the first combustion heater system is operable to produce thermal energy as a product of the combustion of air and fuel in the form of a first heated exhaust stream. This first heated exhaust stream is then used to heat the component directly or by using a heat exchanger type system. The first heated exhaust stream is also used by a second combustion device as a source of oxygen or diluent.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to fuel processors and, more particularly, relates to a fuel processor having a two-stage lean combustion system for rapid start of the fuel processor.  
         BACKGROUND OF THE INVENTION  
         [0002]    H 2 —O 2  fuel cells use hydrogen (H 2 ) as a fuel and oxygen (as air) as an oxidant. The hydrogen used in the fuel cell can be derived from the reformation of a hydrocarbon fuel (e.g. methanol or gasoline). For example, in a steam reformation process, a hydrocarbon fuel (such as methanol) and water (as steam) are ideally reacted in a catalytic reactor (a.k.a. “steam reformer”) to generate a reformate gas comprising primarily hydrogen and carbon monoxide.  
           [0003]    An exemplary steam reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh. For another example, in an autothermal reformation process, a hydrocarbon fuel (such as gasoline), air and steam are ideally reacted in a combine partial oxidation and steam reforming catalytic reactor (a.k.a. autothermal reformer) to generate a reformate gas containing hydrogen and carbon monoxide. An exemplary autothermal reformer is described in U.S. application Ser. No. 09/626,553 filed Jul. 27, 2000. The reformate exiting the reformer contains undesirably high concentrations of carbon monoxide most of which must be removed to prevent poisoning of the catalyst of the fuel cell&#39;s anode. In this regard, carbon monoxide (i.e., about 3-10 mole %) contained in the H 2 -rich reformate/effluent exiting the reformer must be reduced to very low nontoxic concentrations (i.e., less than about 20 ppm) to avoid poisoning of the anode.  
           [0004]    It is known that the carbon monoxide, CO, level of the reformate/effluent exiting a reformer can be reduced by utilizing a so-call “shift” reaction wherein water (i.e. steam) is added to the reformate/effluent exiting the reformer, in the presence of a suitable catalyst. This lowers the carbon monoxide content of the reformate according to the following ideal shift reaction:  
           CO+H 2 O→CO 2 +H 2 .  
           [0005]    Some (i.e., about 0.5 mole % or more) CO still survives the shift reaction. Hence, shift reactor effluent comprises hydrogen, carbon dioxide, water carbon monoxide, and nitrogen.  
           [0006]    The shift reaction is not enough to reduce the CO content of the reformate enough (i.e., to below about 20-200 ppm). Therefore, it is necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the shift reactor, and prior to supplying it to the fuel cell. It is known to further reduce the CO content of H 2 -rich reformate exiting the shift reactor by a so-called “PrOx” (i.e., preferential oxidation) reaction effected in a suitable PrOx reactor operated at temperatures which promote the preferential oxidation of the CO with air in the presence of the H 2 , but without consuming/oxidizing substantial quantities of the H 2  or triggering the so-called “reverse water gas shift” (RWGS) reaction. The PrOx and RWGS reactions are as follows:  
           CO+½ O 2 →CO 2  (PrOx)  
           CO 2 +H 2 →H 2  O+CO (RWGS)  
           [0007]    The PrOx process is described in a paper entitled “Methanol Fuel Processing for Low Temperature Fuel Cells” published in the Program and Abstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach, Calif., and in Vanderborgh et al U.S. Pat. No. 5,271,916, inter alia.  
           [0008]    Desirably, the O 2  required for the PrOx reaction will be about two times the stoichiometric amount required to react the CO in the reformate. If the amount of O 2  exceeds about two times the stoichiometric amount needed, excessive consumption of H 2  results. On the other hand, if the amount of O 2  is substantially less than about two times the stoichiometric amount needed, insufficient CO oxidation may occur and there is greater potential for the RWGS reaction to occur. Accordingly in practice, many practitioners use about  4  or more times the stoichiometric amount of O 2  than is theoretically required to react with the CO.  
           [0009]    PrOx reactors may be either (1) adiabatic (i.e. where the temperature of the reactor is allowed to rise during oxidation of the CO) or (2) isothermal (i.e. where the temperature the reactor is maintained substantially constant during oxidation of the CO). The adiabatic PrOx process is sometimes effected via a number of sequential stages, which progressively reduce the CO content in stages, and requires careful temperature control, because if the temperature rises too much, the RWGS reaction can occur which counter productively produces more CO. The isothermal process can effect the same CO reduction as the adiabatic process, but in fewer stages and without concern for the RWGS reaction if (1) the reactor temperature can be kept low enough, and (2) O 2  depletion near the end of the reactor can be avoided.  
           [0010]    One known isothermal reactor is essentially a catalyzed heat exchanger having a thermally conductive barrier or wall that separates the heat exchanger into (1) a first channel through which the H 2 -rich gas to be decontaminated (i.e. CO removed) passes, and (2) a second channel through which a coolant flows to maintain the temperature of the reactor substantially constant within a defined working range. The barrier wall has a catalyzed first surface confronting the first channel for promoting the CO+O 2  reaction and an uncatalyzed second surface confronting the second channel for contacting the coolant therein to extract heat from the catalyzed first surface through the barrier. The catalyzed surfaces of adjacent barriers oppose each other, and are closely spaced from each other, so as to define a narrow first channel through which the H 2 -rich gas moves.  
           [0011]    The reformation process of gasoline or other hydrocarbons operate at high temperatures (i.e. about 600-800° C.). The water gas shift reactor is active at temperatures of 250-450° C., The PrOx reaction is active at temperatures of 100-200° C. Thus, it is necessary that the reformer, the water gas shift (WGS) reactor, and the PrOx reactor are each heated to temperatures sufficient for the fuel processor to operate. During start-up, however, a conventional fuel processor is such that the heating of various components is staged. This approach can lead to undesirable lag time for bringing the system on line. Alternately, external electrical heat sources (i.e. heaters) may be employed to bring the components to proper operating temperatures. This approach requires an external source of electricity such as a battery.  
           [0012]    Accordingly, there exists a need in the relevant art to provide a fuel processor that is capable of heating the fuel processor components quickly to achieve these high operating temperatures for startup. Furthermore, there exists a need in the relevant art to provide a fuel processor that maximizes this heat input into the fuel processor while minimizing the tendency to form carbon. Still further, there exists a need in the relevant art to provide a fuel processor capable of heating the fuel processor while minimizing the use of electrical energy during startup and the reliance on catalytic reactions.  
         SUMMARY OF THE INVENTION  
         [0013]    According to the principles of the present invention, a fuel processor for rapidly achieving operating temperature during startup is provided having an advantageous construction. The fuel processor includes a reformer, a shift reactor, and a preferential oxidation reactor for deriving hydrogen for use in creating electricity in a plurality of H 2 —O 2  fuel cells. A first combustion heater system is coupled to at least one of the reformer, the shift reactor, and the preferential oxidation reactor to preheat the component(s) during a rapid startup sequence. That is, the first combustion heater system is operable to produce thermal energy as a product of the combustion of air and fuel in the form of a first heated exhaust stream. This first heated exhaust stream is then used to heat the component either directly or by using a heat exchanger type system.  
           [0014]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0016]    [0016]FIG. 1 is a schematic view illustrating a fuel processor according to a first embodiment of the present invention;  
         [0017]    [0017]FIG. 2 is a schematic illustration of a second combustion burner system;  
         [0018]    [0018]FIG. 3 is a schematic view illustrating a fuel processor according to a second embodiment of the present invention;  
         [0019]    [0019]FIG. 4 is a schematic view illustrating a fuel processor according to a third embodiment of the present invention; and  
         [0020]    [0020]FIG. 5 is a schematic view illustrating a fuel processor according to a fourth embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For example, the present invention is hereafter described in the context of a fuel cell fueled by reformed gasoline. However, it is to be understood that the principles embodied herein are equally applicable to fuel cells fueled by other reformable fuels.  
         [0022]    Referring to FIG. 1, a fuel processor, generally indicated as  10 , according to a first embodiment of the present invention is illustrated. Fuel processor  10  generally includes a first burner system  12 , an auto thermal reformer (reformer)  14 , a heat exchanger  16 , a water gas shift reactor/heat exchanger (WGS/HX)  18 , a preferential oxidation/vaporizer (PrOx/vaporizer) reactor  20 , a fuel cell stack  22 , a second burner system  24 , and a catalytic combustor (comb) reactor  26 .  
         [0023]    First burner system  12  and second burner system  24  are primarily used to heat the components of fuel processor  10  during a startup cycle to achieve rapidly and efficiently an optimal operational temperature within fuel processor  10 . First burner system  12  and second burner system  24  may burn various types of fuels, such as but not limited to hydrocarbons or hydrogen. Following such, fuel processor  10  is then capable of efficiently producing electrical energy through the combination of hydrogen and oxygen according to known fuel cell technology. First burner system  12  and second burner system  24  are each of either a premixed or diffusion-type burner that produces heat through internal combustion. The rate of heating is determined by the heating value of the fuel that is burned. The amount of fuel burned is dependent upon the air stream rate at near overall stoichiometric conditions. First burner system  12  and second burner system  24  could be thermal or catalytic in design.  
         [0024]    Preferably, combustion heating occurs in two stages, namely within first burner system  12  and second burner system  24 , to minimize the initial temperature of the gases that are necessary to efficiently and quickly heat reformer  14 , WGS/HX  18 , PrOx/vaporizer reactor  20 , and a catalytic combustor  26  to operating temperature. That is, reformer  14 , WGS/HX  18 , PrOx/vaporizer reactor  20 , and catalytic combustor  26  are each susceptible to damage if exposed to excessive temperature. However, in order to heat these components to a predetermined operating temperature with a single burner, it is necessary that the output gases of the single burner be sufficiently heated initially to carry enough heat downstream to heat the remaining components. Therefore, the output gases of the single burner may pose a risk to upstream components since the temperature may be above that which the upstream component is capable of tolerating. Accordingly, it is preferable to employ a two stage heating system to effectively heat all components during start up without exposing such components to excessive temperature. Combustion in two stages also serves to reduce the maximum flame temperature, which reduces the production of undesirable NOx formation. Alternatively, combustion could occur in more than two stages for improved local control of the resultant heating. For example, an additional burner could be used to heat WGS/HX  18  and PrOx/vaporizer reactor  20  directly.  
         [0025]    As best seen in FIG. 1, fuel processor  10  is arranged such that first burner system  12  includes a first air inlet stream  28  and a first fuel inlet stream  30 . First air inlet stream  28  may be obtained as a direct feed from a system air compressor (not shown) or from the air feed  34  to fuel cell stack  22 . The use of air from the air feed  34  to fuel cell stack  22  may provide additional flow rates to achieve higher heating capacity, if required.  
         [0026]    The heated exhaust stream of first burner system  12 , generally indicated as  32 , exits first burner system  12  as a fuel lean combustion exhaust to heat the downstream components of fuel processor  10 . The particular temperature of first burner exhaust stream  32  is preferably sufficient to heat the catalyst within reformer  14  to its optimized operating temperature, typically in the range of about 600-800° C. for hydrocarbons, such as gasoline. To this end, first burner system  12  is preferably a premixed or diffusion-type and includes a high temperature zone for flame stability. It is contemplated that first burner exhaust stream  32  of first burner system  12  may be diluted with downstream air (not shown) to control the first burner exhaust stream to a temperature suitable to heat the catalyst within reformer  14 . This downstream air may be obtained by diverting a portion of first air inlet stream  28  within first burner system  12  or may be obtained by utilizing another air source (i.e. reformer air  36 ). However, first air inlet stream  28  is preferably obtained as a direct feed from a compressor or from temporarily bypassing fuel cell stack  22  and using air supply from a stack air inlet stream  34  to achieve a high flow rate of air. This arrangement, as illustrated in FIG. 1, minimizes the pressure drop through fuel processor  10  by preventing air from first air inlet stream  28  from passing through heat exchanger  16  during startup and, further, by preventing a reformer air inlet stream  36  or a reformer steam  38  from passing through first burner system  12  during normal operation.  
         [0027]    Accordingly, first burner exhaust stream  32  from first burner system  12  sequentially heats a reformer inlet zone  40 , reformer  14 , heat exchanger  16 , and a sulfur trap  42 . A bypass valve  44  is opened and a WGS valve  46  is closed such that first burner exhaust stream  32  bypasses WGS/HX  18  and flows to catalytic combustor  26  and second burner system  24 . However, it should be understood that bypass valve  44  and WGS valve  46  might be replaced with a single three-way valve (see FIG. 3). However, this two-valve arrangement enables bypass valve  44  to be located away from the high temperature of reformate gas stream  54 . Therefore, bypass valve  44  may be made of lower temperature, better sealing materials to eliminate any leaks of reformate to catalytic combustor  26 , which may lead to a loss to system efficiency.  
         [0028]    It is believed that a brief description of the remaining components and connections of fuel processor  10  is beneficial to adequately describe the startup procedures and components. Hence, with reference to a “normal” operation (e.g. after the system has started up and is running), reformer inlet zone  40  includes a reformer fuel inlet stream  48 , such as gasoline, and reformer air and inlet flow  50  from heat exchanger  16  to produce an inlet stream  52 . Inlet stream  52  enters reformer  14  and catalytically reacts the fuel from reformer fuel inlet stream  48  and air and water from reformer inlet flow  50  to form a H 2 -rich reformate gas stream  54 . Reformate gas stream  54  passes through heat exchanger  16 , which removes excess heat generated during the reformation cycle from reformate gas stream  54 . This heat is then used by heat exchanger  16  to heat a mixture of reformer air inlet stream  36  and reformer steam  38  to produce reformer inlet flow  50 . Reformate gas stream  54  then passes through sulfur trap  42  to remove sulfur and other hydrocarbons and upon exit mixes with a water flow  56  to control the temperature into WGS/HX  18  and further to humidify the effluent.  
         [0029]    During normal operation, WGS valve  46  is open such that the humidified reformate gas stream  54  passes therethrough to WGS/HX  18  and PrOx/vaporizer reactor  20 . As mentioned above, WGS/HX  18  is a water gas shift reactor and heat exchanger combination system. The heat exchanger portion of WGS/HX  18  is fluidly separate from the water gas shift reactor portion to enable efficient heating of the shift reactor catalyst during the startup procedure.  
         [0030]    PrOx/vaporizer reactor  20  is a preferential oxidation reactor and a vaporizer system. The vaporizer portion of PrOx/vaporizer reactor  20  is used as a heat exchanger to remove excess heat from the preferential oxidation reaction and to produce reformer steam  86  and separate the reaction catalysts from the steam flow. WGS/HX  18  and PrOx/vaporizer reactor  20  are used to reduce CO-level therein to acceptable levels. The CO-depleted, H 2 -rich reformate stream  58  is then fed into the anode side of fuel cell stack  22 . Simultaneously, oxygen from stack air inlet stream  34  is fed into the cathode side of fuel cell stack  22 . The hydrogen from reformate stream  58  reacts with the oxygen from stack air inlet stream  34  across a membrane electrode assembly to produce electricity. Anode exhaust or stack effluent  60  from the anode side of fuel cell stack  22  includes a portion of hydrogen that is directed back to catalytic combustor  26  to provide heat. Cathode exhaust  62  from the cathode side of fuel cell stack  22  includes oxygen also for use in catalytic combustor  26 . The flow of cathode exhaust  62  to catalytic combustor  26  is controlled via a pair of control valves, namely a combustor air control valve  64  and a cathode exhaust back pressure valve  66 . The closing of cathode exhaust back pressure valve  66  produces a back pressure that forces air through combustor air control valve  64  for combustion in catalytic combustor  26 . The opening of cathode exhaust back pressure valve  66  permits flow to an exhaust  67 .  
         [0031]    During a startup cycle, bypass valve  44  is opened and WGS valve  46  is closed, thereby sending the lean gases of first burner system  12  indirectly to second burner system  24 . It should be understood that it is necessary to bypass WGS/HX  18  when lean combustion gases are flowing within fuel processor  10 , since the oxygen within the combustion gases may react with the CuZn catalyst that is typically used in water gas shift (WGS) reactors. However, if WGS/HX  18  includes a nonpyrophoric catalyst, bypass valve  44  and WGS valve  46  are not necessary and lean combustion gases may be permitted to flow along the normal operation path through fuel cell stack  22  to second burner system  24  to simplify fuel processor  10 . Generally, it is undesirable to allow dry air to flow through fuel cell stack  22  for extended periods of time due to the drying of the membranes typically used in PEM stacks. However, according to the principles of the present invention, if valves  44 ,  46  were not used, the resultant gas flow through fuel cell stack  22  is acceptable since it contains moisture, which is a product of the lean combustion within first burner system  12  and very low carbon monoxide levels to prevent “poisoning” of the catalysts. If desired, a bypass valve may be used to bypass fuel cell stack  22 .  
         [0032]    Second burner system  24  is used to indirectly heat catalytic combustor  26 , WGS/HX  18 , and PrOx/vaporizer reactor  20 . Second fuel inlet stream  68  is introduced downstream of catalytic combustor  26  and into second burner system  24  such that during the combustion process, most of the remaining oxygen is consumed. However, it should be noted that it is preferable to remain slightly fuel lean within second burner system  24  to insure that unburned hydrocarbons are not present in the heated exhaust stream  70 . Second burner system  24  is preferably a premixed or diffusion-type. More preferably, second burner system  24  is a premixed type when used with liquid fuel to reduce the amount of emissions produced by the flame.  
         [0033]    As best seen in FIG. 2, catalytic combustor  26  is indirectly heated. That is, under start conditions, combustor gas flow  88  to second burner system  24  is the product of lean combustion in first burner system  12  flowing through bypass valve  44 . Combustor gas flow  88  is indirectly heated across a liner  202 , which separates a flame  204  of second burner system  24  from combustor gas flow  88 . Second fuel inlet stream  68  is added to and mixes with combustor gas flow  88  after catalytic combustor  26 . For premix operation, second fuel inlet stream  68  is injected and mixes with the gas exiting the catalytic combustor  26  in a mixing chamber  206  before introduction into flame chamber  208 . For diffusion operation, there is no mixing in chamber  206  and second fuel inlet stream  68  is injected downstream of a flame holder  210  and directly into flame  204 . For liquid fuel operation, the premixed approach is preferred so as to reduce the amount of emissions from flame  204 . Flame holder  210  may be of any conventional type, such as but not limited to a swirler, perforated plate (as shown in FIG. 2), backward facing step, bluff body, or transverse jets. Flame  204  can be initiated by spark plug  212 .  
         [0034]    As best seen in FIGS. 1 and 2, a spray vaporization zone  72  is downstream from second burner system  24 , which employs a spray water stream  74  to reduce the gas temperature of exhaust stream  70  exiting second burner system  24  in the event the exit temperature of exhaust stream  70  is too high for a start vaporizer  80  or WGS/HX  18 . The temperature of exhaust stream  70  exiting second burner system  24  is reduced as it passes through spray vaporization zone  72  and start vaporizer  80  to produce exhaust stream  82 . Exhaust stream  82  then flows through the heat exchanger of WGS/HX  18  and a run vaporizer  96  to exhaust  67 .  
         [0035]    Thermal energy from second burner system  24  is also utilized by initiating a start vaporizer water stream  78  though start vaporizer  80  to produce a start vaporizer steam flow  76 . Start vaporizer steam flow  76  flows across the backside of PrOx/vaporizer reactor  20  to heat PrOx/vaporizer reactor  20 , since the saturation temperature of start vaporizer steam flow  76  (134° C. at 3 atm.) complements the operating temperature of the catalyst within PrOx/vaporizer reactor  20 . It should be appreciated that utilizing the heat of vaporization can transfer significant thermal energy. Drains for eliminating condensed water may be incorporated to avoid the use of thermal energy to revaporize the condensed water. PrOx/vaporizer reactor  20  is of a heat exchanger type construction to separate the reaction catalyst from start vaporizer steam flow  76 , a PrOx inlet water flow  84 , and the resulting PrOx steam flow  86  that is generated.  
         [0036]    PrOx steam flow  86  provides additional heating of heat exchanger  16 , in addition to the direct heat provided to heat exchanger  16  by first burner system  12 . If during the startup cycle the temperature of a WGS/HX exhaust exit flow  98  exceeds the vaporization temperature (typically about 150° C.), run vaporizer  96  can generate additional steam  102 . That is, a run water  100  entering run vaporizer  96  is adjusted so that steam  102  is slightly superheated. Steam  102  joins with PrOx steam flow  86  to form reformer steam  38 , which flows to heat exchanger  16 . This process may be used to provide additional heating of heat exchanger  16 .  
         [0037]    During a rapid start up cycle of fuel processor  10 , full air flow is introduced in first air inlet stream  28  of first burner system  12 . Bypass valve  44  is opened and WGS valve  46  is closed so as to route a flow  90  to second burner system  24 . Ignition members  212 , such as spark plugs, within first burner system  12  and second burner system  24  are energized. Simultaneously, first fuel inlet stream  30  and second fuel inlet stream  68  are introduced to first burner system  12  and second burner system  24 , respectively, to start combustion. Alternate sequency may be appropriate depending on the mechanization hardware. Alternate scenarios could light off at reduced flow or lead with first burner system  12  or second burner system  24 . Confirmation of combustion within first burner system  12  and second burner system  24  is obtained by sensors such as flame ionization or temperature measurement of first burner exhaust stream  32  and at the exit of spray vaporization zone  72 , respectively. First fuel inlet stream  30  is controlled to produce the desired temperature for the catalyst within reformer  14  (typically about 600-800° C.) for gasoline type hydrocarbons.  
         [0038]    Second fuel inlet stream  68  is controlled to maintain near overall stoichiometric conditions to maximize the heat input to fuel processor  10  for rapid startup. That is, the total fuel flow, which equals the sum of first fuel inlet stream  30  and second fuel inlet stream  68 , reacts and consumes nearly all the oxygen provided by first air inlet stream  28  to maximize the combustion heat produced without resulting in unburned hydrocarbons.  
         [0039]    Spray water stream  74  is introduced within spray vaporization zone  72  to maintain the proper temperature of exhaust stream  82  through the vaporization of water so as not to exceed the temperature limits of the downstream components. That is, spray water stream  74  ensures that start vaporizer  80 , downstream from spray vaporization zone  72 , is not exposed to excessively high temperatures (i.e. greater than about 600° C.). Moreover, spray water stream  74  ensures that exhaust stream  82  are not excessively heated (typically less than about 300° C.) so as not to damage the CuZn type WGS catalyst. The control temperature may be altered with the usage of precious metal based catalysts. A reduction in gas temperature further occurs across start vaporizer  80  due to vaporization of start vaporizer water stream  78 . The quantity of start vaporizer water stream  78  is limited such that start vaporizer steam flow  76  is slightly superheated (typically about 150° C.). Further reduction in the quantity of start vaporizer water stream  78  is utilized to favor heating WGS/HX  18  rather than PrOx/vaporizer reactor  20 . The start operation is controlled as described above until fuel processor  10  is heated to a predetermined temperature for normal operation.  
         [0040]    Once the catalyst of PrOx/vaporizer reactor  20  and the catalyst of WGS/HX  18  are above their minimum operation temperatures (typically about 100° C. and about 220° C., respectively) and reformer steam  38  is flowing through heat exchanger  16 , fuel processor  10  is ready to begin normal operation. To determine whether such operating temperatures have been achieved, it is preferable to monitor and compare the temperature of PrOx steam flow  86  to the operating temperature of PrOx/vaporizer reactor  20  and the temperature of WGS/HX exhaust exit flow  98  to the operating temperature of WGS/HX  18 . The availability of steam is determined by monitoring the temperature of reformer air and steam flow  50 . For high sulfur fuels, it is preferably that sulfur be removed from the liquid fuel or sulfur trap  42  be at its operating temperature (typically about 300-500° C.) to ensure full capacity such that sulfur does not pass to the catalyst of WGS/HX  18  or other downstream catalysts, as such catalyst may be damaged by the presence of sulfur.  
         [0041]    Normal, fuel rich operation may be accomplished via several methods. For instance, fuel rich reformer flow for normal operation can be established by starting reformer fuel inlet stream  48  and reformer air inlet stream  36 , and closing first air inlet stream  28  and first fuel inlet stream  30 . Preferably, this transition occurs rapidly so as to not linger at near stoichiometric conditions due to the excessively high associated reaction temperatures. Moreover, this transition should preferably occur quickly such that an atomic-oxygen-in-air-flow-to-carbon-in-fuel-flow ratio (oxygen-to-carbon ratio) of less than one is not encountered, which may produce undesirable carbon.  
         [0042]    Alternatively, normal, fuel rich operation may be established by initially fully closing first fuel inlet stream  30  and first air inlet stream  28  to first burner system  12 . Reformer air inlet stream  36  and reformer fuel inlet stream  48  is then initiated in a way to preferably avoid operation near stoichiometric conditions or oxygen-to-carbon ratio of less than one. However, operation in conditions where the oxygen-to-carbon ratio is less than one is permitted when steam flow is available. It is important to note that at least some steam flow is available after the start-up cycle from start vaporizer  80 , run vaporizer  96 , and PrOx/vaporizer reactor  20 , which have all been preheated during the start-up cycle.  
         [0043]    The change to fuel rich reformer operation is accompanied by the closing of second fuel inlet stream  68  and the addition of cathode exhaust  62  to catalytic combustor  26  to complete combustion before exhaust. To this end, catalytic combustor  26  must be kept sufficiently lean to maintain the catalyst temperature below its operating limit (typically about 750° C.). To this point, reformer fuel inlet stream  48  is below its full power setting to insure that the operation of catalytic combustor  26  is within acceptable temperature limits (less than about 750° C.).  
         [0044]    Once a stable flow through reformer  14  is established, WGS valve  46  is opened and bypass valve  44  is closed to direct reformate gas stream  54  through the WGS portion of WGS/HX  18 , the PrOx portion of PrOx/vaporizer reactor  20 , and fuel cell stack  22 . In conjunction with the changing of valve positions of bypass valve  44  and WGS valve  46 , the flow of a PrOx air inlet stream  92  and PrOx inlet water flow  84  is initiated into PrOx/vaporizer reactor  20 . As fuel cell stack  22  draws current, the hydrogen content of anode exhaust stream  60  is greatly reduced, which reduced the reaction temperature within catalytic combustor  26 . Hence, spray water stream  74 , if used, is controlled to produce the desired temperature until it can eventually be shut off. Start vaporizer water stream  78  to start vaporizer  80  is also shut off and the control of the exit temperature of WGS/HX  18  is regulated using combustor air control valve  64 , which controls the cathode exhaust stream  62  to catalytic combustor  26 .  
         [0045]    If not initiated previously, run water  100  is initiated to run vaporizer  96  once the temperature of WGS/HX exhaust exit flow  98  exceeds the water vaporization temperature (typically about 150° C.). The quantity of run water  100  is determined within the energy availability of WGS/HX exhaust exit flow  98  to fully vaporize and the quantity of water available or desired for operation. Run vaporizer steam flow  102  is available for reformer steam flow  38 . Fuel processor  10  is now in a normal operating mode for producing electricity.  
         [0046]    For normal operation, reformer air inlet stream  36  to reformer fuel inlet stream  48  ratio is set to provide the desired reformer reaction exit temperature (typically about 750° C.). The temperature of reformate gas stream  54  into the front end of WGS/HX  18  (typically about 250° C.) is controlled by the quantity of water flow  56  atomized into and vaporized by reformate gas stream  54 .  
         [0047]    PrOx air inlet stream  92  is set to provide the required carbon monoxide clean-up for fuel cell stack  22 . Similarly, PrOx inlet water flow  84  is set to remove the associated heat release from PrOx/vaporizer reactor  20 . PrOx inlet water flow  84  is also set to provide single phase PrOx steam flow  86  as indicated by temperature measurements of this stream. Alternate embodiments may adjust PrOx air and water flows to obtain optimum performance. For example excess PrOx air may be used to handle CO spikes, or two phase PrOx steam flow may be used to provide thermal balance. Run vaporizer steam flow  102  is used as a surplus to increase the fuel processor efficiency or to meet transient steam flow requirements of the system, but is not utilized for thermal balance. The increased steam  102  provided by run vaporizer  96  also reduces the carbon monoxide level from reformer  14 , which helps to minimize the exotherm within WGS/HX  18 . Steam  102  from run vaporizer  96  can also be used to moderate variations in reformer steam  38  or the molar steam flow to atomic carbon in fuel flow ratio (steam-to-carbon ratio). Further control of overall steam flow can be achieved through the use of PrOx/vaporizer reactor  20 . That is, if additional steam flow is required during transient operation, PrOx air inlet stream  92  can be increased to provide additional thermal energy by exothermic reaction with reformate gas stream  104  within PrOx/vaporizer reactor  20  to vaporize additional water delivered to PrOx/vaporizer reactor  20  via PrOx inlet water flow  84 .  
         [0048]    To increase or decrease the output from fuel processor  10 , reformer fuel inlet stream  48  is increased or decreased, respectively, and the commensurate changes in air and water flows throughout the system are made to maintain the system thermal balance and stoichiometry as described above. The commensurate changes in air and water flows may lead or lag behind the changes in fuel flow to achieve the optimal response time and control within the desired reactor operating regimes.  
         [0049]    It should be recognized that the present invention enables the unique capability to control the temperature of WGS/HX  18  and the ability to handle unloads of fuel cell stack  22 . The temperature of reformate  54  before entering WGS/HX  18  is controlled using water flow  56 . Primary control of the temperature of WGS/HX  18  is provided by exhaust stream  82 . The temperature of exhaust stream  82  can be adjusted by the amount of cathode exhaust  62  directed to catalytic combustor  26  by combustor air control valve  64 . According to the present embodiment, it is desirable that WGS/HX  18  operate at nearly constant temperature and within the operating limits thereof. Preferably, as described above, CuZn catalyst is used within WGS/HX  18  wherein temperatures above about 300° C. may damage the catalyst and temperatures below about 200° C. will greatly reduce the activity of the shift reaction. A challenge associated with this narrow temperature window involves the removal of heat generated by the exothermic reaction within WGS/HX  18 , specifically CO+H 2 O→CO 2 +H 2 +thermal energy. By utilizing exhaust stream  82 , which has a relatively high mass flow rate and thus a high heat capacity as compared to all of the available flow streams, the temperature rise (i.e. thermal energy) within WGS/HX  18  can be most effectively minimized. The ability to control the temperature of exhaust stream  82  is also important to prevent quenching of the water gas shift reaction with WGS/HX  18  (typically below about 220° C.), which may occur if the temperature of exhaust stream  82  is too low. Indirect exothermic reduction is achieved by maximizing the quantity of reformer steam  38  to reformer  14 , which lowers the carbon monoxide level entering WGS/HX  18 . Maximizing the quantity of run water  100  vaporized in run vaporizer  96  can also increase the flow of reformer steam  38 . Still further, increasing the flow of PrOx air inlet stream  92  can increase the steam generating capacity of PrOx/vaporizer reactor  20 . However, increased flow of PrOx air inlet stream  92  may decrease the efficiency of fuel processor  10 , which is not desirable for steady operation, but is effective to increase steam generation during transient operations to avoid reducing the steam-to-carbon ratio or to limit the exotherm within WGS/HX  18  to maintain the temperature of the WGS catalyst below damaging levels.  
         [0050]    At least three mechanisms are available for accommodating fuel cell stack  22  unloads. As used herein, the term unload refers to when the electric current draw from fuel cell stack  22  is reduced. Specifically, these mechanisms include 1) increasing cathode exhaust  62 , 2) initiating or increasing flow of start vaporizer water stream  78  to start vaporizer  80 , and 3) injecting spray water stream  74  into spray vaporization zone  72 . When fuel cell stack  22  unloads, the hydrogen content of anode exhaust stream  60  increases. This increase in hydrogen content will cause the temperature from catalytic combustor  26  to increase. Since it is necessary to control the temperature of WGS/HX  18 , it is necessary to limit or control the temperature of exhaust stream  70  from catalytic combustor  26 . Similarly, catalytic combustor  26  has an operating temperature limit of typically about 750° C. Exceeding this operating temperature limit may damage the catalyst material within combustor  26 .  
         [0051]    As a first measure, cathode exhaust  62  is controlled to cool the catalyst of catalytic combustor  26  below its temperature limit. This enables exhaust stream  82  to be used to cool WGS/HX  18 . If the temperature of catalytic combustor  26  exceed the desired temperature of WGS/HX  18  and all of the available cooling capacity of cathode exhaust  62  is used, the temperature of exhaust stream  82  is lowered by initiating or increasing flow of start vaporizer water stream  78  to start vaporizer  80 . This increases the steam-to-carbon ratio of fuel processor  10 . Transition of water from run vaporizer  96  to start vaporizer  80  may be appropriate to dissipate excess heat in exhaust stream  82 , if excess water is not available. However, direct spray water stream  74  may be used, particularly if the temperature into start vaporizer  80  exceeds its temperature limit (typically about 600° C.) or if an increased steam-to-carbon ratio in fuel processor  10  is not desired. It is important to note that spray water stream  74  into exhaust stream  70  is typically not recovered. Of course, as the load on fuel cell stack  22  decreases, the fuel flow and the overall fuel processor output is decreased to respond to the reduced power demand.  
         [0052]    During a fuel processor shutdown, the electrical demand is reduced to a minimum predetermined level and fuel cell stack  22  is unloaded. When fuel cell stack  22  unloads, the H 2  content of anode exhaust stream  60  increases and, thus, increased flow of cathode exhaust  62  through combustor air control valve  64  is required to limit the catalyst temperature of catalytic combustor  26 . Bypass valve  44  is then opened and WGS valve  46  is closed to direct the reformate gas stream through bypass valve  44  to catalytic combustor  26 . PrOx air inlet stream  92  and PrOx inlet water flow  84  are also shut off. Still further, run water  100 , reformer air inlet stream  36 , and reformer fuel inlet stream  48  are shut off. If desired, continuing the flow of cathode exhaust  62  over the backside (or HX) of WGS/HX  18  can cool WGS/HX  18 . This continued flow may be desirable to reduce the temperature of the catalyst within WGS/HX  18  to lower its catalytic activity in the event it becomes exposed to air leaks after shutdown. However, WGS valve  46  and check valves (such as in anode exhaust  60 ) provide protection against air leaking into WGS/HX  18 .  
         [0053]    As best seen in FIGS.  3 - 5 , alternative embodiments of the present invention are illustrated, generally indicated as  10 ′,  10 ″ and  10 ′″; respectively. It should be noted that like reference numerals designate like or corresponding parts throughout the several views.  
         [0054]    Referring to FIGS.  3 - 5 , a direct air inlet  302  is provided to second burner system  24  to provide fresh combustion air (i.e. not exhaust gas from first burner system  12 ) to second burner system  24 . By providing direct combustion air to second burner system  24 , the operational stability of second burner system  24  may be improved. That is, the combustion air is accurately controllable and, thus, the stability of the flame within second burner system  24  is improved. By improving the stability and thus the temperature control of second burner system  24 , it may be possible to eliminate spray vaporization zone  72 .  
         [0055]    More particularly, as seen in FIGS.  3 - 5 , a bypass flow  90 ′,  90 ″,  90 ′″ from first burner system  12  is directed downstream of second burner system  24  and direct air inlet  302  is supplied to second burner system  24  for combustion. To start the fuel processor, direct air inlet  302  would be started and otherwise the startup procedure would be as previously described.  
         [0056]    With particular reference to FIG. 3, since catalytic combustor  26  is upstream of bypass flow  90 ′, the transition to normal operation is preferably modified to avoid sending reformate out exhaust  67 . Specifically, the transition to normal operation would include closing first fuel inlet stream  30  and first air inlet stream  28 . Reformer steam  38 , which is generated in part by start vaporizer  80 , purges reformer inlet zone  40 , reformer  14 , heat exchanger  16 , and sulfur trap  42  to remove air before opening the flow to WGSIHX  18 . WGS valve  46  is then opened and by-pass valve  44  is closed. Reformer air inlet stream  36  and reformer fuel inlet stream  48  are then started to begin fuel rich operation. Simultaneously, PrOx air inlet stream  92  and PrOx inlet water flow  84  would be started. Second fuel inlet stream  68  and direct air inlet  302  would then be stopped. Anode exhaust stream  60  and cathode exhaust  62  would then be reacted in combustor  26 . As fuel cell stack  22  draws current, the hydrogen content of anode exhaust stream  60  is greatly reduced and the reaction temperature in combustor  26  drops. Accordingly, spray water stream  74  and start vaporizer water stream  78  may be shut off. Thus, fuel processor  10 ′ is now in the normal operating mode.  
         [0057]    With particular reference to FIG. 4, it should briefly be noted that combustor  26  may be positioned downstream of second burner system  24  such that bypass flow  90 ″ may enter combustor  26  without flowing through second burner system  24 . Furthermore, with particular reference to FIG. 5, combustor  26  may be positioned along side second burner system  24 , thereby modifying bypass line  90 ′″. This arrangement has the advantage that combustor  26  is not exposed to exhaust stream  70  from second burner system  24  or spray water stream  74  from spray vaporization zone  72 .  
         [0058]    According to the principles of the present invention, a fuel processor is provided that is capable of heating the fuel processor components quickly to achieve proper operating temperatures for startup. Furthermore, the fuel processor of the present invention maximizes this heat input into the fuel processor while minimizing the tendency to form carbon. Still further, the fuel processor of the present invention provides a fuel processor capable of heating the fuel processor component while minimizing the use of electrical energy during startup and the reliance on catalytic reactions. It should be readily appreciated by those skilled in the art that the present invention enables the potential usage of inexpensive CuZn catalyst only without the need for an additional coolant loop. Moreover, the tight control of temperatures within the fuel processor, which is afforded by the present invention, enables the optimization of reactor size and catalyst usage resulting in reduced active metal costs. Still further, the present invention provides improved transient carbon monoxide concentration performance.  
         [0059]    The description of the invention is merely exemplary in nature and, thus, variations are not to be regarded as a departure from the spirit and scope of the invention.