Abstract:
The present invention provides a hydrocarbon fuel reformer systems that improve the recovery and utilization of the water vapor and/or the heat energy within the reformer system. In a preferred embodiment, the present invention utilizes a desiccant matrix maintained in a water transfer assembly to collect, in a continuous manner, water from a process stream and transfer the water to a reactivation stream to return the water vapor to the reformer, with or without the use of a heat exchanger, and without requiring the collection and evaporation of liquid water.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/572,032, filed on May 18, 2004, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to reformer systems utilized for producing hydrogen from a hydrocarbon fuel, and more particularly to a reformer system having components configured to recover water vapor from a process gas stream and transfer the recovered water to a reactivation stream. Such reformer systems may be used to produce a hydrogen-rich reformate stream that becomes, in turn, the anode feed gas for a H 2 —O 2  fuel cell stack. 
     BACKGROUND OF THE INVENTION 
     H 2 —O 2  fuel cells separate the hydrogen (H 2 ) fuel and an oxidant, typically oxygen from air, with an electrolyte. Within the fuel cell, the hydrogen gas separates into electrons and hydrogen ions (protons) at the anode. The hydrogen ions pass through the electrolyte to the cathode with the electrons traveling through a power circuit (e.g., to a motor) and returning to the cathode, where they combine with the hydrogen ions and oxygen to form water. The reaction rates at the anode and cathode are generally enhanced by a catalyst. 
     There are several broad types of fuel cells, each incorporating a different electrolyte system, and each having advantages that may make them particularly suited to given commercial applications. One type is the proton exchange membrane (PEM) fuel cell, which employs a thin polymer membrane that is permeable to protons but not electrons. PEM fuel cells, in particular, are well suited for use in vehicles, because they can provide high power and weigh less than other fuel cell systems. 
     For many applications, it is desirable to use a readily available hydrocarbon fuel, such as methane (natural gas), methanol, gasoline, or diesel fuel, as the source of the hydrogen that will be feed into the fuel cell. Such fuels are relatively easy to store, and there is an existing commercial infrastructure for their supply. Due in part to the established production, storage and distribution infrastructure, liquid fuels such as gasoline are particularly suited for vehicular applications. However, hydrocarbon fuels must be dissociated to release hydrogen gas for use in the fuel cell. Power plant fuel processors for providing hydrogen contain one or more reactors or “reformers” wherein the fuel reacts with steam, and sometimes air, to yield reaction products comprising primarily hydrogen and carbon dioxide. 
     The use of hydrocarbon reformate fuel cell systems in cars and other vehicles presents special concerns. In addition to the desirability of using readily available liquid fuels, discussed above, the reformer and fuel cell systems must be relatively light in weight, and must be able to operate efficiently under a wide range of ambient conditions (e.g., under a range of temperatures and humidity conditions). They should also exhibit good cold-start performance to produce power quickly, and respond quickly to varying system demands to provide the necessary power quickly. Thus, it is desirable to minimize the need for external heating of the reactants being fed into the reformer. It is also desirable to minimize the amount of liquid water that must be supplied to or handled within the system, to reduce or avoid the need to replenish system water and to reduce the complications associated with operations at temperatures below 0° C. (32° F.). 
     Typically, there are several components in the reformate fuel cell system that require water, particularly including the reformer (e.g., a steam reformer or autothermal reformer) that requires steam as a reactant and some carbon monoxide clean-up reactors (e.g., a water gas shift or WGS reactor), as well as the fuel cell that requires humidification of the MEA in order to function properly. A common approach to enhancing water balance in fuel cell systems incorporates a series of condensing heat exchangers at various points in the system. For example, a condensing heat exchanger may be positioned downstream of the reformer to cool the reformate to a temperature at or below its dew point and thereby condense a portion of the water vapor. The condensate water is then separated from the gaseous reformate and stored in a reservoir until it is returned to the reformer where it is heated to create steam. Heat exchangers have also been used to cool the cathode exhaust stream and condense water vapor that can then be used to humidify the MEA. 
     The use of multiple heat exchangers increases the complexity of the resulting reformer system. For example, the water recovery efficiency of heat exchangers is reduced as the ambient temperature increases. Similarly, large radiators may be required to dissipate the heat of condensation. Moreover, the liquid condensate produced by the heat exchangers must be vaporized before being fed back into the reformer or fuel cell, thereby creating an additional energy load and decreasing the overall efficiency of the system. 
     Various methods for addressing the water balance within fuel cell systems have been described in the art. See, for example, German Patent Disclosure 42 01632, Strasser, published Jul. 29, 1993; U.S. Pat. No. 6,007,931, Fuller et al., issued Dec. 28, 1999; and U.S. Pat. No. 6,013,385, DuBose, issued Jan. 11, 2000. However, water management systems among those known in the art do not adequately address these needs, due to problems such as their inability to maintain true water balance over a wide range of operating conditions, mechanical complexity, reliability concerns, and increased system energy requirements. 
     SUMMARY OF THE INVENTION 
     The present invention provides several embodiments for a hydrocarbon fuel reformer system that improves the recovery and utilization of the water vapor and/or the heat energy within the reformer system while decreasing or eliminating the need for handling liquid water. In particular, the present invention utilizes a desiccant to collect water vapor from a process stream and transfer the water to a reactivation stream that is then fed back into the reformer as an input stream, with or without the use of an additional heat exchanger. Accordingly, the present invention provides a fuel reformer system comprising a reformer and at least one water transfer assembly, preferably coupled with a fuel cell, a heat exchanger, and a combustor for maintaining an overall water balance in the system under a range of operating conditions, thereby reducing energy requirements and component complexity, and enhancing reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram depicting a first embodiment of a hydrocarbon fuel reformer system according to the present invention connected to a fuel cell and showing basic process flow within the system; 
         FIG. 2  is a diagram illustrating the operation of a preferred embodiment of the water transfer assembly; 
         FIG. 3  is a diagram depicting a second embodiment of a hydrocarbon fuel reformer system according to the present invention connected to a fuel cell including a heat exchanger and providing an alternative process flow within the system; 
         FIG. 4  is a diagram depicting a third embodiment of a hydrocarbon fuel reformer system according to the present invention connected to a fuel cell similar to that in  FIG. 3 , but without the heat exchanger; 
         FIG. 5  is a diagram depicting a fourth embodiment of a hydrocarbon fuel reformer system according to the present invention connected to a fuel cell and providing for the use of two water transfer assemblies within the system; and 
         FIG. 6  is a diagram depicting a fifth embodiment of a hydrocarbon fuel reformer system according to the present invention connected to a fuel cell and providing for the use of both a water transfer device and a condensing heat exchanger. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a hydrocarbon fuel processor, i.e., a device that converts a hydrocarbon fuel into a hydrogen-rich reformate, preferably for use with a fuel cell. The reformate is the gaseous product or effluent comprising hydrogen that is produced by a reactor from a hydrocarbon fuel. As referred to herein, a “fuel cell” may be a single cell for the electrochemical production of electricity, but will more typically comprise a series of PEM fuel cells using hydrogen as the fuel and oxygen from the air as the oxidant. 
     The membrane in the PEM fuel cell is part of a membrane electrode assembly (MEA) having the anode on one face of the membrane, and the cathode on the opposite face. The membrane is typically made from an ion exchange resin such as a perfluoronated sulfonic acid. The MEA is sandwiched between a pair of electrically conductive elements that serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings for distribution of the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode catalysts. 
     The anode and cathode typically comprise finely divided catalytic particles, supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically precious metal particles, such as platinum. Such MEAs are, accordingly, relatively expensive to manufacture and require controlled operating conditions in order to prevent degradation of the membrane and catalysts. These conditions include proper water management and humidification, and control of catalyst fouling constituents, such as carbon monoxide. Typical PEM fuel cells and MEAs are described in U.S. Pat. No. 5,272,017, Swathirajan et al., issued Dec. 21, 1993, and U.S. Pat. No. 5,316,871, Swathirajan et al., issued May 31, 1994, the contents of which are incorporated herein by reference. 
     The voltage from an individual fuel cell is only about 1 volt. Accordingly, to meet the higher power requirements of vehicles and other commercial applications, several cells are typically combined in series to increase the available voltage. This combination is typically arranged in a “stack” surrounded by an electrically insulating frame that has passages for directing the flow of the hydrogen and oxygen (air) reactants, and the water effluent. Because the reaction of oxygen and hydrogen also produces heat, the fuel cell stack must also be cooled. Arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, Meltser et al., issued Jun. 9, 1998; and U.S. Pat. No. 6,099,484, Rock, issued Aug. 8, 2000, the contents of which are incorporated herein by reference. 
     A “hydrocarbon fuel cell plant” is an integrated apparatus that comprises both a fuel cell and a hydrocarbon fuel processor for providing a hydrogen-containing feed stream to the fuel cell. Preferably, the hydrocarbon fuel processor converts one or more hydrocarbon fuels, using an oxidant and water, to create a hydrogen-containing reformate stream. The range of hydrocarbon fuels is not generally limited and may include gasoline, diesel fuel, natural gas, methane, butane, propane, methanol, ethanol, or mixtures thereof. For example, in a steam reformation process, a hydrocarbon fuel (such as methanol) and water (as steam) are ideally reacted in a catalytic reactor (commonly referred to as a “steam reformer”) to generate a reformate gas comprising primarily hydrogen and carbon monoxide. An exemplary steam reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh, the contents of which are incorporated herein by reference. For another example, in an autothermal reformation process, a hydrocarbon fuel (such as gasoline), air and steam are ideally reacted in a combined partial oxidation and steam reforming catalytic reactor (commonly referred to as an autothermal reformer or ATR) 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 contents of which are incorporated herein by reference. The reformate exiting the reformer, however, contains undesirably high concentrations of carbon monoxide, most of which must be removed to avoid poisoning the catalyst of the fuel cell&#39;s anode. 
     There are also reformer designs that can operate in a variety of modes depending on the demands placed on the reformer/fuel cell system, thereby improving the efficiency of the reformer system while maintaining an ability to provide a rapid cold-start response and to respond quickly to varying loads. One such quasi-autothermal reformer (QATR) is described in U.S. patent application Ser. No. 10/788,155, filed Feb. 26, 2004, the contents of which are incorporated herein by reference, and provides for modes of operation between those that can be obtained with a pure autothermal reformer (ATR) or pure steam reformer by integrating thermal and catalytic combustors with a steam reforming portion. However, the QATR, like the steam reformer and the ATR, tends to produce a reformate that contains undesirably high concentrations of carbon monoxide. 
     In this regard, the relatively high level of carbon monoxide (i.e., about 3-10 mole %) contained in the H 2 -containing reformate exiting the primary reformer reactor must be reduced to very low concentrations (e.g., less than 200 ppm, and typically less than about 20 ppm) to avoid poisoning the anode catalyst. Thus, reactors downstream of the primary reactor are typically utilized to lower the carbon monoxide concentration to tolerable levels. Such downstream reactors may include a water/gas shift (WGS) reactor and a preferential oxidizer (PrOx) reactor. The WGS reactor catalytically converts carbon dioxide and water to hydrogen and carbon dioxide. The PrOx reactor selectively oxidizes carbon monoxide to produce carbon dioxide, using oxygen from air as an oxidant. Control of air feed to the PrOx reactor is important to selectively oxidize carbon monoxide, while minimizing the oxidation of hydrogen to water. 
     In a preferred embodiment, the hydrocarbon fuel cell plant is suitable for use in a motor vehicle. In other preferred embodiments, the hydrocarbon fuel cell plant is suitable for use in stationary and typically larger applications, such as an emergency or supplemental power generator for home or commercial use. 
     In particular, with reference now to the drawing and to  FIG. 1 , a first embodiment of the present invention provides a hydrocarbon fuel cell plant  10  comprising a fuel processor  12  for reacting a hydrocarbon fuel feed stream  14  and an oxidant feed stream  10  to produce a hydrogen-containing reformate stream  18 . This first embodiment also includes a water transfer assembly  20  arranged to transfer water vapor from the reformate stream  18  exiting the fuel processor  12  to one of the input streams  16  being fed into the fuel processor. As illustrated in  FIG. 1 , the reformate stream  18 , after passing through the water transfer assembly  20 , is fed into to a fuel cell  22 . Also in this embodiment, the water vapor is preferably returned to the fuel processor  12  as part of the oxidant stream  16  supplied from an air source  24 , typically pressurized by a compressor  26 . The increase in temperature caused by compressing the air increases the effectiveness of the dry oxidant feed stream  28  as the reactivating stream in the water transfer assembly. The humidified oxidant stream feed  16  may be transferred directly to a fuel processor input or to an intermediate device which is, in turn, connected to a fuel processor input. 
     The water transfer assembly  20  preferably comprises a strong, non-shedding, non-toxic and non-corrosive desiccant matrix comprising a substrate with desiccant coating or embedded in the substrate. Further, the water transfer assembly  20  preferably comprises a moveable desiccant matrix that is moved in a manner to alternately expose portions of the desiccant matrix to a process stream containing water vapor, from which the desiccant absorbs water, and then exposed to a dry reactivation stream that desorbs water from the desiccant to humidify the reactivation stream and to prepare (reactivate) the desiccant for further absorption. As presently preferred, at least 85% of the water vapor for the combustion stream is transferred to the oxidant stream within the water transfer assembly. 
     One such water transfer assembly  20  is illustrated in  FIG. 2  and comprises a desiccant rotor  30  comprising a desiccant matrix  32  contained in a casing  34  that selectively and sequentially exposes portions of the desiccant matrix in either a process zone  36  or a reactivation zone  38 . A rotor drive  40  rotates the desiccant matrix  32  through the process zone  36  and reactivation zone  38 . The rotor drive  40  is operable to move the rotor  30  at a variable rate such that movement of the desiccant matrix  32  through the process zone  36  may be adjusted. In the process zone  38 , a high moisture process stream  42  is drawn through the desiccant matrix  32  in the process zone  36  and a portion of the water vapor is absorbed onto the desiccant matrix  32  and a dried process stream  44  is discharged and forms at least a portion of the oxidant feed stream  16 . 
     A portion of the desiccant matrix  32  with the absorbed water then rotates from the process zone  36  into the reactivation zone  38  where a dry reactivation stream  46  is drawn or forced through the desiccant matrix  32  causing a portion of the water to desorb, thereby drying (or reactivating) the desiccant matrix  32  and producing a wet reactivation stream  48 . A portion of the reactivated desiccant matrix  32  then rotates from the reactivation zone  38  into the process zone  36  where the cycle begins again. The rate of rotation of the desiccant rotor  30 , as well as the relative desiccant areas exposed in the process zone  36  and the reactivation zone  38  can be adjusted to accommodate variations in the process and reactivation streams  42 ,  46  and the desired water transfer rate. 
     Although in the preferred embodiment the water transfer process is continuous, a batch or other non-continuous method could also be used. Similarly, although  FIG. 2  illustrates a counter-flow for the process and reactivation streams, the effective segregation of the process and reactivation zones on the desiccant rotor by the casing render counter-flow and co-flow configurations equally viable, providing some additional design flexibility. 
     The desiccant matrix  32  area is characterized by an area substantially equal to the sum of the process zone area and the reactivation zone area. The ratio of the process zone area to the reactivation zone area are within the range of about 3:1 to 1:3. The desiccant matrix  32  preferably has a large surface area covered with both macropores and micropores. The actual desiccant can be embedded within or bonded to a high surface area-to-volume matrix structure, which may comprise a hexagonal “honeycomb” construction to define flow channels through the matrix. During the adsorption step, the process stream  42  is drawn or forced over the surfaces of the desiccant matrix  32  allowing a portion of the water vapor from the process stream  42  to attach to the desiccant as a quasi-liquid layer in a generally adiabatic process. Conversely, during the desorption step, a dry heated reactivation stream  46  is drawn or forced over the surfaces of the desiccant matrix  32 , thereby heating the desiccant and causing the water stored in the desiccant to desorb and enter the reactivation stream  48  as water vapor. 
     A variety of desiccants may be used in the desiccant matrix including lithium chloride, metal silicate (silica gel) and advanced silica gels having increased numbers of available hydroxyl groups and micropores. Similarly, a variety of substrates may be used to support the desiccant including ceramic papers that can be treated to remove substantially all organic components and render them more suitable for high-temperature operations. The selection of the desiccant and substrate combination, as well as the sizing of the water transfer assembly, for a particular application will necessarily be guided by the operating parameters, performance and cost goals for the overall system. 
     A second embodiment is illustrated in  FIG. 3 . This second embodiment again provides a hydrocarbon fuel cell plant  110  comprising a fuel processor  112  for reacting a hydrocarbon fuel feed stream  114  and an oxidant feed stream  116  to produce a hydrogen-containing reformate stream  118 . In the second embodiment, the exhaust stream  120  from the fuel cell  122  is fed into a combustor  124  in order to consume any residual hydrogen or fuel vapor and produce a hot, wet combustor exhaust stream  126 . This combustor exhaust stream  126  is, in turn, fed into a heat exchanger  130  in which the exhaust stream  126  is cooled. An oxidant (air) stream  128  is also fed to the heat exchanger  130  in which the oxidant stream  128  is heated. The cooled combustor exhaust stream  126  and the heated oxidant stream  128  are then both fed into a water transfer assembly  134 . Water is adsorbed by the water transfer assembly  134  from the cooled combustor exhaust stream  126  to produce a dry exhaust stream  136 . Water is desorbed from the water transfer assembly  134  into the oxidant stream  128  to produce a wet oxidant stream  116  that is fed into the fuel processor  112 . The dry combustor exhaust stream  136  may then be discharged into the environment. 
     A third embodiment is illustrated in  FIG. 4 . This third embodiment again provides a hydrocarbon fuel cell plant  110  comprising a fuel processor  112  for reacting a hydrocarbon fuel feed stream  114  and an oxidant feed stream  116  to produce a hydrogen-containing reformate stream  118 . As in the second embodiment, the exhaust stream  120  from the fuel cell  122  is fed into a combustor  124  to consume any residual hydrogen or fuel vapor and produce a combustor exhaust stream  126 . In this third embodiment, the combustor exhaust stream  126  and an oxidant stream  128  are then both fed directly into a water transfer assembly  134 . The oxidant stream  128  is preferably heated, either as the result of compressing the air or by passage through a heat exchanger (not shown) before entering the water transfer assembly  134 . In the water transfer assembly  134 , water vapor from the combustor exhaust  126  is again transferred to the oxidant stream  128  to produce a wet oxidant stream  116  that is fed into the fuel processor  112 . 
     A fourth embodiment is illustrated in  FIG. 5 . This fourth embodiment again provides a hydrocarbon fuel cell plant  110  comprising a fuel processor  112  for reacting a hydrocarbon fuel feed stream  114  and an oxidant feed stream  116  to produce a hydrogen-containing reformate stream  118 . As in the second embodiment, the exhaust stream  120  from the fuel cell  122  is fed into a combustor  124  to consume any residual hydrogen or fuel vapor and produce a combustor exhaust stream  126 . In this embodiment, however, prior to entering the combustor  124 , at least a portion of the fuel cell exhaust stream  120 , preferably at least the cathode exhaust stream, is fed into a second water transfer assembly  138  so that a portion of the water vapor in the fuel cell exhaust  120  can be transferred to at least one of the fuel cell feed streams, preferably the cathode feed stream  140 . 
     The dehumidified fuel cell exhaust  120  is then fed into the combustor  124  to consume any residual hydrogen or fuel vapor and produce a combustor exhaust stream  126 . In this fourth embodiment, the combustor exhaust stream  126  and an oxidant stream  128  may then both be fed into a water transfer assembly  134 . The exhaust stream  126  and/or the oxidant stream  128  may pass directly into the water transfer assembly  134  or be conditioned by an intermediate heat exchanger as illustrated in  FIG. 3 . In the water transfer assembly  134 , water vapor from the combustor exhaust  126  is again transferred to the oxidant stream  128  to produce a wet oxidant stream  116  that is fed into the fuel processor  112 . 
     A fifth embodiment is illustrated in  FIG. 6 . This fifth embodiment again provides a hydrocarbon fuel cell plant  110  comprising a fuel processor  112  for reacting a hydrocarbon fuel feed stream  114  and an oxidant feed stream  116  to produce a hydrogen-containing reformate stream  118 . As in the second embodiment, the exhaust stream  120  from the fuel cell  122  is again fed into a combustor  124  to consume any residual hydrogen or fuel vapor and produce a combustor exhaust stream  126 . In this embodiment, however, prior to entering the combustor  124 , a portion of the fuel cell exhaust stream  120 , preferably at least the cathode exhaust stream, is fed into condensing heat exchanger  130 . 
     Within the condensing heat exchanger  130 , the fuel cell exhaust stream  120  is cooled to below its dew point, inducing the condensation of water vapor present in the fuel cell exhaust stream  120  to form a condensate consisting primarily of water. The condensate is first transferred to a water reservoir  142  and then to a humidifier  144  where it is used to add water vapor to a fuel cell feed stream, preferably the oxidant feed stream  140 . The dehumidified or dry fuel cell exhaust stream  146  is then fed into the combustor  124  to consume any residual hydrogen or fuel vapor and produce a combustor exhaust stream  126 . In this fifth embodiment, the combustor exhaust stream  126  and an oxidant stream  128  may then both be fed into a water transfer assembly  134 , with or without passing through an intermediate heat exchanger. In the water transfer assembly  134 , water vapor from the combustor exhaust  126  is again transferred to the oxidant stream  128  to produce a wet oxidant stream  116  that is fed into the fuel processor  112 . 
     In each of the embodiments of the present invention described above, the fuel processor  112  is configured to convert a hydrocarbon fuel to hydrogen for use in a fuel cell. A skilled practitioner will appreciate that the present invention can be utilized in a variety of fuel processing applications which produce a humidified reformate stream or wet process stream. Preferred fuel processors include steam reforming reactors, autothermal reactors, and quasi-autothermal reactors as generally described above. Although described and illustrated as a single unit, the fuel processor  112  may comprise one or more reactors including a primary reactor, a water/gas shift (WGS) reactor, and a preferential oxidation (PrOx) reactor, that act in concert to convert a hydrocarbon fuel stream in the presence of water/steam to produce a hydrogen-containing reformate. Similarly, each of the individual reactors may include one or more sections or reactor beds or comprise one of a variety of known designs. Therefore, the selection, configuration and arrangement of the reactors for use in combination with the present invention is expected to vary depending on the system requirements and application. 
     Preferably, the various aspects of the operation of the system are controlled using a suitable microprocessor, microcontroller, programmable processing unit, etc., which has central processing unit capable of executing a control program and data stored in a memory. The controller may be a dedicated controller specific to any of the components, or implemented in software stored in a main electronic control module. Further, although software based control programs are usable for controlling system components in various modes of operation as described above, it will also be understood that the control can also be implemented in part or whole by dedicated electronic circuitry. 
     The present invention also provides a water transfer device that transfers water vapor from a wet process stream to a dry reactivation stream. The water transfer assembly of this invention comprises a structure providing segregated flow paths through portions of a desiccant matrix for both the process gas stream and the reactivation gas stream. The desiccant matrix is preferably sized and configured to provide sufficient water transfer capacity for the particular application, taking into account the volume, temperature, and water content of both the process stream and reactivation stream being utilized and the range of ambient conditions under which the system is intended to operate. 
     The present invention provides a series of embodiments incorporating one or more water transfer assemblies that reduce or eliminate the need for handling liquid water within the hydrocarbon fuel cell plant, reduce the power demands associated with the condensation and evaporation of water within the plant, and increase the range of ambient conditions suitable for operation. 
     The examples and other embodiments described herein are exemplary and not intended to be limiting in describing the full scope of apparatuses, devices, components, materials, compositions and methods of this invention. Equivalent changes, modifications, rearrangement, and variations of specific embodiments, materials, compositions, components, and methods may be made with substantially similar results.