Abstract:
A recuperative heat exchanger ( 36 ) is provided for use in a fuel processor ( 20 ), the heat exchanger ( 36 ) transferring heat from a fluid flow ( 34 ) at one stage of a fuel processing operation to the fluid flow ( 32 ) at another stage of the fuel processing operation. The heat exchanger ( 36 ) includes a housing ( 56 ) defining first and second axially extending, concentric annular passages in heat transfer relation to each other; a first convoluted fin ( 70 ) located in the first passage to direct the fluid flow therethrough; and a second convoluted fin ( 72 ) located in the second passage to direct the fluid flow therethrough.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to fuel processors, and in more particular applications to fuel processors for distributed hydrogen production.  
       BACKGROUND OF THE INVENTION  
       [0002]     It has been well-established that a critical key to the long-term success of fuel cell vehicles is the development of a hydrogen infrastructure. Fuel cell vehicles are projected by many to be the eventual replacement of, or at the very least supplement to, the internal combustion engine vehicle. This is driven primarily by the growing concerns over greenhouse gas and air pollutant emissions, long-term availability of fossil fuels, and energy supply security. The proton exchange membrane (PEM) fuel cells which are the focus of almost all the current efforts towards the development of commercially viable fuel cell vehicles require hydrogen as a fuel. Virtually all efforts towards the on-board production of hydrogen from more portable hydrocarbon fuels have been abandoned in recent years, and almost all fuel cell vehicle manufacturers are currently focusing on refueling the vehicles with high-purity liquid or gaseous hydrogen.  
         [0003]     The means by which hydrogen can be produced in large quantities are well understood. Steam reforming of methane is the primary means by which hydrogen is currently being produced on an industrial scale. Today, about half of the world production of hydrogen is used in oil refineries, mainly for the production of automotive fuels. Another 40% is consumed in the commercial production of ammonia. However, the annual production volume of hydrogen in the United States is comparable to only two days worth of gasoline consumption. Furthermore, hydrogen is currently being produced predominantly on a large industrial scale. For a successful transportation infrastructure, the hydrogen refueling network must be well-distributed. Hydrogen is, however, very problematic to distribute. Gaseous hydrogen has one of the lowest energy densities, making it difficult to transport in the amounts that would be required for a transportation fuel cell infrastructure. Distributing hydrogen in liquid form is also difficult—it requires very low temperatures (22K), and even in liquid form hydrogen has a low energy density. Because of these concerns, it can be reasonably concluded that a hydrogen infrastructure capable of supplying the refueling needs of fuel cell vehicles will need to rely on the distributed production of high-purity hydrogen.  
         [0004]     The widely distributed hydrogen production necessary for a transportation fuel cell infrastructure is much smaller than the typical refinery or ammonia-producing hydrogen production scale. Various means by which high-purity hydrogen can be economically produced at this small scale are currently being pursued. One such means of production is to apply, on a smaller scale, the well-understood methods of producing a hydrogen at the current large scales. The predominant method of producing a hydrogen at large scales is by steam reforming natural gas (methane) over a catalyst. The steam reforming reaction produces hydrogen and carbon monoxide as follows: 
 
CH 4 +H 2 O→3H 2 +CO 
 
         [0005]     The steam reforming reaction is highly endothermic, requiring 206 kJ of energy per mole of methane consumed. Some of the CO produced is converted to CO 2  via the associated water-gas shift reaction: 
 
CO+H 2 O→CO 2 +H 2  
 
         [0006]     This reaction is exothermic, and liberates 41 kJ of energy per mole of CO consumed. Steam reforming of methane is typically carried out at temperatures in the range of 700° C.-900° C. Since the reaction is endothermic, heat must be supplied to the reactor. This is typically accomplished by loading the catalyst into a series of tubes which are placed in a furnace. The hydrogen can be extracted from the steam reforming product gas (reformate) through various well-understood means, such as metal membrane or pressure swing adsorption (PSA).  
         [0007]     It has long been understood that in order to make steam reforming of natural gas feasible at the smaller scales required for a distributed production of hydrogen for fuel cell vehicles, a greater integration between the heat-producing combustor and the endothermic steam reforming reaction is needed. Attempts to build such systems have met with some success in the past, but the performance efficiency has always been limited by the ability to transfer the required heat into the steam reforming reaction without generating extremely high (&gt;1000° C.) metal temperatures.  
       SUMMARY OF THE INVENTION  
       [0008]     Embodiments of the invention are disclosed herein in a highly integrated steam reforming fuel processor, which in combination with a Pressure Swing Adsorption (PSA) can deliver high purity hydrogen at a scale which is well-suited to the distributed production of hydrogen for a transportation fuel cell infrastructure. This fuel processor overcomes the heat transfer limitations of previous designs, and is thereby capable of achieving a high level of hydrogen product efficiency without extremely high metal temperatures.  
         [0009]     In accordance with one feature of the invention, a recuperative heat exchanger is provided for use in a fuel processor, the heat exchanger transferring heat from a fluid flow at one stage of a fuel processing operation to the fluid flow at another stage of the fuel processing operation.  
         [0010]     According to one feature of the invention, a fuel processing unit includes a steam reformer and a recuperative heat exchanger, the heat exchanger connected to the steam reformer to direct a fluid flow to the steam reformer, the fluid flow being a steam/fuel feed mix to be reformed in the steam reformer into a reformate, the steam reformer connected to the heat exchanger to direct the fluid flow back to the heat exchanger after it has been reformed into the reformate.  
         [0011]     In accordance with one feature, the heat exchanger includes a housing defining first and second axially extending, concentric annular passages in heat transfer relation to each other; a first convoluted fin located in the first passage to direct the fluid flow therethrough; and a second convoluted fin located in the second passage to direct the fluid flow therethrough.  
         [0012]     In one feature, the recuperative heat exchanger further includes a cylindrical water-gas shift reactor extending centrally through the housing at a location radially inward from the first and second passages.  
         [0013]     In accordance with one feature of the invention, the heat exchanger includes a cylindrical wall; a first convoluted fin bonded to a radially inwardly facing surface of the wall, the convolutions of the first convoluted fin extending axially to direct the fluid flow through the heat exchanger; and a second convoluted fin bonded to a radially outwardly facing surface of the wall, the convolutions of the second convoluted fin extending axially to direct the fluid flow through the heat exchanger.  
         [0014]     Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a diagrammatic representation of a fuel processing system embodying the invention;  
         [0016]      FIG. 2  is a perspective view of an integrated steam reformer/combustor assembly of the invention;  
         [0017]      FIG. 3  is a section view of one embodiment of an integrated fuel processing unit of the invention;  
         [0018]      FIG. 4  is a flow schematic showing the fluid flows through the fuel processing unit of  FIG. 3 ;  
         [0019]      FIG. 5  is a sectioned, perspective view from above of the integrated fuel processing unit of  FIG. 3 ;  
         [0020]      FIG. 6  is a temperature versus flow path graph showing the temperature profile for one embodiment of the steam reformer/combustor of  FIG. 2 ;  
         [0021]      FIG. 7  is a sectioned, perspective view from above of another embodiment of an integrated fuel processing unit of the invention;  
         [0022]      FIG. 8  is an enlarged section view of the portion encircled by line  8 - 8  in  FIG. 7  and highlighting selected components of the integrated fuel processing unit;  
         [0023]      FIG. 9  is an enlarged section view of the portion encircled by line  9 - 9  in  FIG. 7  and highlighting selected components of the integrated fuel processing unit;  
         [0024]      FIG. 10  is an enlarged section view of the portion encircled by line  10 - 10  in  FIG. 7  and highlighting selected components of the integrated fuel processing unit;  
         [0025]      FIG. 11  is a section view taken from line  11 - 11  in  FIG. 10 ;  
         [0026]      FIG. 12  is a view similar to  FIG. 10 , but highlighting other components of the integrated fuel processing unit;  
         [0027]      FIG. 13  is a view similar to  FIGS. 10 and 12 , but again highlighting yet other components of the integrated fuel processing unit;  
         [0028]      FIG. 14  is an enlarged view of the portion encircled by line  14 - 14  in  FIG. 7 ;  
         [0029]      FIG. 15  is a view taken from line  15 - 15  in  FIG. 14 ; and  
         [0030]      FIG. 16  is a view similar to  FIG. 8 , but highlighting other components of the fuel processing unit.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]     A system schematic of a highly integrated fuel processor  20  is shown in  FIG. 1 . In this system, the only source of fuel required for the combustor is the hydrogen-depleted off gas  21  from the Pressure Swing Absorption (PSA)  22 . The high-pressure side of the PSA  22  is preferably designed to operate at 100 psig, while the low-pressure side of the PSA is designed to operate at near-atmospheric pressure (˜1 psig). Heat recovered from a combustor exhaust  24  of combustor  25  is used in a vaporizer  29  to vaporize and superheat a water feed  26  for a steam reformer  28 , as well as to preheat a natural gas feed  30  for the steam reformer  28  in a fuel preheater  31 . Heat is recovered from a reformate stream  32  exiting the steam reformer  28  and is used to further preheat the now mixed steam-natural gas feed  34  in a recuperative heat exchanger  36 . A water-gas shift (WGS) reactor  38  is used to increase hydrogen production. Downstream of the water-gas shift reactor  38 , additional heat is recovered from the reformate stream  32  and is used to preheat a combustor feed  40  and the water feed  26  to the vaporizer  29 . The reformate stream  32  is cooled to a temperature suitable for the PSA  22 , and excess water is condensed out. The heat removed in this process is at a fairly low temperature, and is not recovered. The condensate  41  can be recovered and reused, if it is desirable to do so.  
         [0032]     Optimized hydrogen conversion is accomplished in this design through the use of an integrated steam reformer and combustor  42  which integrates the reformer  28  and the combustor  25  and which has highly effective heat transfer characteristics. The integrated steam reformer and combustor  42  will hereafter in this description be referred to as the SMR reactor  42 . As best seen in  FIG. 2 , the SMR reactor  42  is constructed as a metal cylinder  44  of a high-temperature alloy with a first convoluted fin structure  46  brazed to an inner surface  48  along the entire circumference of the cylinder  44 , and a second convoluted fin structure  50  brazed to an outer surface  52  along the entire circumference of the cylinder  44 . The inside fin  46  is wash-coated with a steam reforming catalyst, and the outside fin  50  is wash-coated with a catalyst capable of oxidizing both hydrogen and methane. The cylinder  44  is of sufficient thickness to be used as a portion of a pressure vessel  54 , shown in  FIG. 3 , which contains the high-pressure steam reformer feed  34  and reformate stream  32 , shown in  FIG. 4 . Cylindrical metal sleeves  56  and  58  (not shown in  FIG. 2 ) serve to channel the flow through the coated fin structures  46  and  50 , respectively.  
         [0033]      FIGS. 3 and 4  show one embodiment of the fully assembled fuel processor  20 . The fuel processor  20  consists of the cylindrical high-pressure vessel  54  situated within and coaxial to a low-pressure cylinder or vessel  60 . The water and natural gas feeds  26 , 30  for the steam reformer  28  are vaporized (in the case of the water) and preheated in a coiled tube  62  which is situated between the two cylinders  44 , 60 . The preheated feed  34  enters the pressure vessel  54  through a tube  64  at a top domed head  66  of the vessel  54  and passes into the recuperator  36 .  
         [0034]     The construction of the recuperator  36  is similar to the SMR reactor  42 , with a highly augmented fin structures  70  and  72  (such as a louvered or lanced-offset fin) brazed to both the outside and inside surfaces  76  and  78  of the cylinder  56 . The flows will again be channeled through these fin structures  70 , 72  by cylindrical metal sleeves  44  and  80 . The recuperator cylinder  56  and fins  70 , 72  are sized such that the annular area encompassing the fin  70  bonded to the outer surface  76  is the same as the annular area encompassing the steam reforming catalyst coated fin  46  in the SMR reactor  42 . The recuperator cylinder  56  extends past the ends of the fins  70 , 72  on one side by an amount approximately equal to the length of the SMR reactor  42 . This allows the recuperator cylinder  56  to function as the previously mentioned inner sleeve  56  for the SMR reactor  42 . Similarly, the SMR reactor cylinder  44  can extend past its fins  46 , 48  so that it functions as the outer sleeve  44  for the recuperator  36 .  
         [0035]     In the fully assembled fuel processor  20 , the steam reformer feed  34  flows through the outer fin  70  of the recuperator  36 , then passes through the inner (steam reforming catalyst coated) fin  46  of the SMR reactor  42 , where it is converted to a hydrogen rich reformate  32 . The reformate flow  32  is then baffled so that upon exiting the steam reforming fin  46  it turns upward and passes along the inner surface  78  of the extended recuperator cylinder  56  and flows up through the inner fin  72  of the recuperator  36 , where it transfers heat to the incoming steam reformer feed  34 .  
         [0036]     The water-gas shift (WGS) reactor  38  is a cylindrical catalyst-coated monolith  84  which is encased in insulation  86  and is located within the inner sleeve  80  of the recuperator  36 . The reformate flow  32  exits the recuperator  36  towards the top of the pressure vessel  54 , where it is forced to reverse direction due to a domed head  88  which separates the reformate  32  from the preheated steam reformer feed  34  entering the pressure vessel  54 . The reformate  32  flows down through the WGS monolith  84  at the center of the cylinders  80  and  90 . Upon exiting the WGS reactor  38 , the reformate flow  32  is diverted towards the walls  44 , 92  of the pressure vessel  54  and passes through a combustor preheater  94 .  
         [0037]     The construction of the combustor preheater  94  is very similar to that of the SMR reactor  42  and the recuperator  36 , with highly augmented fin structures  96 , 98  (such as a louvered fin) brazed to both the outside and inside surfaces  100 , 102  of the cylinder  92 . As was the case with the reactor  42 , the cylinder  92  serves as a part of the pressure vessel  54  and is welded to the SMR reactor cylinder  44 . The reformate  32  passes through the fin  98  on the inside surface  102  of the preheater  94 , and transfers heat to the combustor feed gases  40  which pass through the fin  96  on the outer surface  100  of the preheater  94  in a countercurrent direction. Upon exiting the fin  96 , the reformate  32  passes over a water preheater  104 , which consists of a coiled tube  106  through which the water  26  for the steam reformer  28  is flowing. It is expected that the reformate  32  is cooled in these preheaters  94 ,  104  to such an extent that some water is condensed out of the reformate  82 . Downstream of the water preheater  104 , the reformate  32  (and any condensate) reach the bottom dome  108  of the pressure vessel  54  and exit the vessel  54  to pass to a heat exchanger  110  which cools the reformate  32  down to a temperature appropriate for the PSA  22 . The heat removed from the reformate  32  in this heat exchanger  110  is considered to be waste heat, and can be discharged to the surrounding ambient.  
         [0038]     The hydrogen-depleted off gas  21  from the PSA, now at near-atmospheric pressure (˜1 psig), is mixed with the combustor air  112  to comprise the combustor feed  40 . This feed gas  40  passes into the low-pressure cylinder  60  and flows up through the combustor preheater  94  and into the fin  50  on the outer surface  52  of the SMR reactor  42 . The combustor feed  40  flows vertically up though this catalyst-coated fin  50 , counter-current to the flow  32  passing through the fin  46  on the inner surface  48  of the SMR reactor  42 . The hydrogen, methane, and carbon monoxide in the combustor feed  40  are catalytically combusted as the flow passes through the fin  50 . The heat generated is conducted through the cylindrical wall  44  of the SMR reactor  42  and feeds the endothermic steam reforming reaction occurring on the fin  46  attached to the inside surface  48  of the SMR reactor  42 .  
         [0039]     Upon exiting the fin  50  of the reactor  42 , the combustor exhaust gas  24  continues to flow upward through the annular region between the low-pressure and high-pressure cylinders  44 , 60 , passing over the water vaporizer  29  and natural gas preheater  31 . The water vaporizer  29  and natural gas preheater  31  consist of the coiled tube  62  which resides within the annular space between the cylinders  44 , 60 . The preheated liquid water  26  enters the coiled tube  62  at the bottom and flows upward, receiving heat from the high-temperature combustor exhaust  24  which is flowing over the tube  62 . As the water passes through the tube  62 , it is fully vaporized and then mildly superheated. The natural gas  30  enters the coiled tube  120  at some point along the length of the coil  62 , and mixes with the superheated steam. Both fluids are then further heated by the combustor exhaust in the remaining length of the coiled tube  62 , after which they are piped into the high-pressure vessel  54 . An alternative design (best seen in  FIGS. 7 and 9 ) would have the natural gas  30  preheated in a separate coiled tube downstream (with respect to the combustor exhaust flow  24 ) of the water coil, and the two fluids would be mixed after exiting their respective coiled heat exchangers.  
         [0040]     A large percentage of the combustion reaction typically occurs over a relatively small initial length of the catalyst region. It can be advantageous to force the combustion reaction to be more evenly distributed over the length of the reactor  42 . Since the combustion reaction is diffusion-limited, this can be achieved to some extent by having an initial region where the convoluted fin structure  50  is uninterrupted, thus providing a more laminar flow which minimizes diffusion, and an exit region in which the convoluted fin structure  50  is turbulated by the use of louvers, slits, lances, etc. to promote greater diffusion of the reactants for final cleanup of the methane, hydrogen, and carbon monoxide.  
         [0041]      FIG. 6  illustrates the expected typical temperature profile within the SMR reactor  42 . The SMR feed  32 , 34  is flowing from right to left, while the combustor feed  40  flows from left to right. The temperatures at the extreme tips of the fins  46  and  50 , as well as the temperature on either surface of the cylinder wall  44 , are also depicted. It can be seen in the graph that the heat transfer within the SMR reactor  42  is sufficient to keep the metal temperatures well below 1000° C. The ability to run the SMR flow  32 , 34  and the combustor flow  50  in a countercurrent direction without causing dangerously high metal temperatures results in a reformate exit temperature which is substantially higher than the reformate inlet temperature, thus maximizing the conversion of methane to hydrogen and minimizing the required effectiveness of the recuperator  36 .  
         [0042]     Since this design avoids the need for additional natural gas to be supplied to the combustor  25 , the control of the fuel processor  20  can be greatly simplified. Temperatures within the SMR reactor  42  can be controlled by adjusting the combustor air flow  112 , based on temperature feedback provided by a sensor (not shown) located on the outer sleeve  58  of the reactor  42  in the area where the peak exhaust gas temperature is expected. Further control is possible by incorporating an adjustable water bypass valve (not shown) of the water preheater  104 , so that the temperature of the water  26  being supplied to the vaporizer  29  can be adjusted by varying the percentage of water flow through the preheater  104 . Feedback from a temperature sensor located at the inlet  122  to the WGS reactor  38  could potentially be used as the control source for this valve.  
         [0043]     The high degree of thermal integration results in a volumetrically compact high-pressure vessel  54 , thus minimizing the wall thickness required for a vessel operating at the elevated temperatures and pressure required for the application. One preferred embodiment of the fuel processor  20  described in this application has a pressure vessel  54  which is 6 inches in diameter, with a total length of approximately 40 inches, and is expected to be capable of reforming 6.25 kg/hr of natural gas with a hydrogen production efficiency of 77.5% (the LHV of the hydrogen removed by the PSA, assuming 75% of the hydrogen in the reformate is removed, divided by the LHV of the natural gas feed), resulting in a hydrogen production rate of 1.87 kg/hr.  
         [0044]     Another embodiment of the fuel processing unit  20  is shown in  FIGS. 7-16 . This embodiment differs from that of  FIGS. 2-4  in that: 
        (a) the water preheater  104  has been moved from the integrated unit  20  to an external location and the combustor preheater  94  extends over the region previously occupied by the water preheater  104 ;     (b) the PSA off gas inlet and the reformate outlet, together with the region associated therewith, have been modified; and     (c) a separate coiled tube  130  for the natural gas preheater  31  has been added downstream from the coiled tube  62  for the water vaporizer  29 .        
 
         [0048]     As in the fuel processing unit  20  of  FIG. 3 , the fins  46 , 50  of the SMR reactor  42  are brazed to the cylinder  44  only and the fins  96 , 98  of the combustor preheater  94  are brazed to the cylinder  92  only, with the cylinders  44  and  92  being welded at their adjacent ends to form the cylindrical wall of the high-pressure vessel  54 . Further, as with the embodiment of the fuel processing unit  20  of  FIG. 3 , a cylindrical baffle or wall  132  is used to extend the inner boundary of the flow path for the combustor exhaust  24  through the portion of the coils  62 , 130  that extend beyond the top  66  of the pressure vessel  54 . This cylindrical baffle  132  is tack welded to the top  66  of the pressure vessel  54 , but does not contact the coils  62 , 130  or the low pressure vessel  60 . Accordingly, the high-pressure vessel  54  and the low-pressure vessel  60  are mechanically coupled in only two locations. The first, best seen in  FIG. 8 , is near the bottom of the pressure vessel  54  where an Air/PSA off-gas inlet structure  134  is welded to both the lower dome-shaped head  108  for the high-pressure vessel  54  and a lower dome-shaped head  136  for the low-pressure vessel  60  to form a rigid connection. The second location is near the top of the fuel processing unit  20 , as best seen in  FIG. 9 , where the coils  62 , 130  are welded at first locations  138  to the low-pressure vessel  60  and at a second location  140  to the dome-shaped top head  66  of the high pressure vessel  54  via a feed mix inlet tube structure  142 . The structure  142  includes a feed mix inlet manifold  143  that is connected to respective outlet ends  144  and  145  of the coils  62  and  130  by suitable fitting connections, and a downwardly extending mixing tube  146  with an internal mixing structure  147  (also shown in the illustrated embodiment is a central instrumentation tube  148  which may optionally be eliminated for production units). This second connection is far from rigid, being made through what are in effect two large springs (the coils  62 , 130 ). Accordingly, the high-pressure vessel  54  and the low-pressure vessel  60  are largely unconstrained in the axial direction relative to each other, and although the outer shell  58  of the vessel  60  will tend to run hotter, the differential thermal expansion should not generate significant stress.  
         [0049]     As with the embodiment of the fuel processing unit  20  of  FIG. 3 , the fins  70 ,  72  of the recuperator  36  are brazed to the recuperator cylinder  56  only, not to the high-pressure vessel  54  or the adjacent inboard surfaces. As best seen in  FIG. 10 , the high-pressure vessel  54  and recuperator cylinder  56  are mechanically coupled at the top end only, where both are welded to a feed mix distribution ring  150  that serves to mount the dome head  88  and cylinder  56  to the high-pressure vessel  54 . As best seen in  FIG. 11  the feed mix distribution ring  150  includes a plurality of angularly spaced holes  152  which allow the steam reformer feed  34  to pass through to the recuperator  36  and the SMR reactor  42 . Because the components  54 , 56  are connected at only one end, they are free to move independently in response to differential thermal expansion, and accordingly should not generate significant stress as a result thereof.  
         [0050]     Preferably, the inboard cylinder  80  does not contact the fins  70 ,  72  or the cylinder  78 , but rather is connected to the inside of the cylinder  44  of the pressure vessel  54  via a flanged, ring-shaped baffle  154  that is welded to both of the cylinders  44  and  80 , as best seen in  FIG. 12 . Because the inboard cylinder  80  and the high-pressure vessel  54  are only connected at one location, they are free to move independently of one other in response to differential thermal expansion and therefore should not generate significant stresses.  
         [0051]     As with the embodiment of the fuel processor  20  of  FIG. 3 , the cylindrical wall  90  of the WGS reactor  38  is connected to the inboard cylinder  80  by a pair of flat, ring-shaped baffles  156 ,  158  welded to both ends of the WGS cylinder  90 , as best seen in  FIG. 13 . The baffle  158  located at the bottom is also welded to the inside of the inboard cylinder  80 . Because the annulus between the WGS cylinder  90  and the inboard cylinder  80  is not a flow channel, the annulus does not require a gas-tight seal at both ends. Because the upper ring-shaped baffle  156  is free to move relative to the inboard cylinder  80 , the cylinders  80  and  90  can move relative to each other in response to differential thermal expansion and therefore should not generate significant stresses.  
         [0052]     As with the embodiment of the fuel processing unit  20  of  FIG. 3 , the central volume of the combustor preheater region is occupied by a cylindrical enclosure  160  that is preferably filled with insulation, as best seen in  FIG. 14 . A castellated disc  162  is welded to the bottom of the enclosure  160  and to the inside of the cylinder  92  of the pressure vessel  54 . As best seen in  FIG. 15 , angularly spaced notches  164  are provided in the perimeter of the disc  162  in order to allow passage of the reformate flow  32 . Preferably, the enclosure  160  is not attached to the fins  98 . Again, because the enclosure  160  and the pressure vessel  154  are connected only at one end, they can move independently in response to differential thermal expansion and accordingly should not generate significant stresses as a result thereof.  
         [0053]     As best seen in  FIG. 16 , the reformate flow  32  exits the fuel processing unit via a small diameter tube  166  welded to the lower head  108  of the pressure vessel  54 . Combustion air  112  enters at the bottom of the unit  20  via a large diameter tube  168  that is preferably concentric with the pressure vessel  54 . A PSA offgas inlet structure  134  includes a tube  172  of the same diameter as the tube  168 , a PSA off gas inlet tube  174 , and a combustor air flow/PSA offgas injector  176  which includes a plurality of circumferentially spaced holes  178  that inject the PSA offgas into the airflow  112 . While the reformate outlet tube  166  and the offgas inlet structure  134  are rigidly connected to each other at two locations, allowing for stresses to develop with differential thermal expansion, there is little temperature difference between the PSA off gas flow  21  and the reformate flow  32 . Accordingly, no significant stresses are to be expected.