Patent Publication Number: US-2022238898-A1

Title: High efficiency humidity management system for fuel cells and higher-temperature electrochemical systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/554,957, filed Aug. 29, 2019, which claims the benefit of and priority to U.S. Provisional Application No. 62/725,042, filed Aug. 30, 2018, the entire disclosures of which are hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     The present application relates generally to the field of humidity management systems for fuel cells and more specifically to a humidity transfer device for transferring steam between anode exhaust and fuel cell feed gas. 
     In a conventional fuel cell system, the fuel cell (e.g., solid oxide, molten carbonate, phosphoric acid, solid acid, etc.) generally requires receiving a feed gas with a specific and narrow range of humidity levels for proper operation of the system generally and/or the fuel cell more specifically. Further, prolonged operation of the system with a feed gas outside the desired humidity range may result in degradation of the fuel cell, thereby increasing maintenance costs and reducing the overall lifecycle of the system. 
     In one of these conventional systems, a separate liquid water supply from a commercial source (i.e., tap water) is used to meet humidification requirements for feed gases. Due to sensitivities of fuel cells to impurities, this water supply must be purified to protect equipment from scaling, alkalinity/acidity, conductivity of dissolved solids, and to produce high quality steam. The water is then vaporized using excess heat from system or from separate fuel combustion, which requires additional energy inputs. 
     During operation of a conventional fuel cell system, humidified anode exhaust is generally output from the system. In such a system, steam that has already been generated in the system is lost, reducing the overall efficiency of the system. Alternatively, in some systems, the anode exhaust may be cooled and compressed in a chiller to separate the steam as water and then the water is re-boiled for reuse as steam elsewhere in the system. However, the process of compressing and vaporizing the steam requires large energy inputs and may also reduce the overall efficiency of the system. Specifically, the equipment needed to separate water in a conventional system may include several pieces of bulky and parasitic equipment, which are prone to mechanical failure and demand frequent maintenance and repair. Further, this water recovery method generates significant quantities of discharge waste water output from the system, which not only wastes water but may require permits to operate, increasing the cost and complexity of installing and operating the system. 
     Accordingly, it may be advantageous to provide a fuel cell system with a humidity transfer device configured to provide feed gas with a reliable and adjustable level of humidity by recycling steam from anode exhaust. 
     SUMMARY 
     One embodiment relates to a humidity transfer assembly, including a pressure vessel and a humidity transfer device disposed in the pressure vessel. The humidity transfer device includes an enclosure, a first inlet line fluidly coupled to the enclosure and configured to supply anode exhaust thereto, a first outlet line fluidly coupled to the enclosure and configured to output anode exhaust therefrom, and a second inlet line fluidly coupled to the enclosure and configured to supply feed gas thereto. The humidity transfer device is configured to transfer steam from anode exhaust to feed gas and to output feed gas into the pressure vessel. 
     One aspect of the humidity transfer assembly relates to a pressure in the pressure vessel being approximately the same as a pressure of anode exhaust received in the enclosure. 
     Another aspect of the humidity transfer assembly relates to the enclosure being formed from plastic. 
     Another aspect of the humidity transfer assembly relates to water disposed in the pressure vessel and defining a water level, and a feed gas conduit extending from the enclosure and defining a conduit outlet disposed below the water level. 
     Another aspect of the humidity transfer assembly relates to an introduction of feed gas from the feed gas conduit into the water vaporizing a portion of the water. 
     Another aspect of the humidity transfer assembly relates to a water level controller configured to adjust a steam-to-carbon ratio for feed gas in the pressure vessel by controlling the water level in the pressure vessel. 
     Another aspect of the humidity transfer assembly relates to a feed gas outlet formed in the pressure vessel and configured to output humidified feed gas from the pressure vessel. 
     Another aspect of the humidity transfer assembly relates to the humidity transfer device being a shell-and-tube configuration. 
     Another aspect of the humidity transfer assembly relates to the humidity transfer device being a planar stack configuration. 
     Another aspect of the humidity transfer assembly relates to a polymer-electrolyte membrane disposed in the enclosure and separating the anode exhaust from the feed gas. 
     Another aspect of the humidity transfer assembly relates to the polymer-electrolyte membrane being formed from at least one of Nafion, Aquivion, or another hydrocarbon. 
     Another embodiment relates to a fuel cell system, including a fuel cell having an anode and a cathode and an electrochemical hydrogen separator having an anode and a cathode. The fuel cell system further includes a humidity transfer device configured to receive anode exhaust from the anode of the fuel cell and to receive cathode exhaust from the cathode of the electrochemical hydrogen separator. The humidity transfer device is configured to transfer steam from the anode exhaust to the cathode exhaust. 
     One aspect of the fuel cell system relates to a polymer-electrolyte membrane disposed in the humidity transfer device, the polymer-electrolyte membrane separating the cathode exhaust from the anode exhaust and passing steam therebetween. 
     Another aspect of the fuel cell system relates to the humidity transfer device being configured to output dehumidified anode exhaust to the anode of the electrochemical hydrogen separator. 
     Another embodiment relates to a fuel cell system, including an electrochemical hydrogen separator having an anode and a cathode and a humidity transfer device configured to receive anode exhaust from the anode and to receive cathode exhaust from the cathode. The humidity transfer device is configured to transfer steam from the anode exhaust to the cathode exhaust. 
     One aspect of the fuel cell system relates to a polymer-electrolyte membrane disposed in the humidity transfer device, the polymer-electrolyte membrane separating the cathode exhaust from the anode exhaust and passing steam therebetween. 
     Another aspect of the fuel cell system relates to a fuel cell having an anode and a cathode. The anode of the electrochemical hydrogen separator is configured to receive anode exhaust from the anode of the fuel cell. 
     Another aspect of the fuel cell system relates to an electrochemical hydrogen compressor having an anode and a cathode. The anode of the electrochemical hydrogen compressor is configured to receive humidified cathode exhaust from the humidity transfer device. 
     Another aspect of the fuel cell system relates to the cathode of the electrochemical hydrogen compressor being configured to output hydrogen. At least a portion of the hydrogen is mixed with the cathode exhaust output from the cathode of the electrochemical hydrogen separator. 
     Another aspect of the fuel cell system relates to the hydrogen being mixed with the cathode exhaust upstream from the humidity transfer device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a humidity transfer device with a shell-and-tube configuration, according to an exemplary embodiment. 
         FIG. 2  shows a cross-sectional view of a humidity transfer device with a planar stack configuration, according to an exemplary embodiment. 
         FIG. 3  shows a perspective view of the humidity transfer device of  FIG. 2 . 
         FIG. 4  shows a schematic of a pressure vessel assembly containing a humidity transfer device, according to an exemplary embodiment. 
         FIG. 5  shows a schematic of a humidity transfer device, according to another exemplary embodiment. 
         FIG. 6  is a schematic view of a fuel cell system incorporating vapor phase steam transfer, according to an exemplary embodiment. 
         FIG. 7  is a schematic view of a fuel cell system with a single-stage vapor phase steam transfer, according to an exemplary embodiment. 
         FIG. 8  is a schematic view of a fuel cell system with a two-stage vapor phase steam transfer, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In order to operate a fuel cell system, feed gas (i.e., feedstock) may be mixed with steam to form a humidified fuel. This humidified fuel may then be fed to an anode or a cathode of a fuel cell in the system for reaction in the fuel cell. The humidification of feed gas may require a device for transferring steam from another stream or supply to the feed gas. For example, as shown in the FIGURES, a humidity transfer device is shown according to various exemplary embodiments to transfer steam from anode exhaust to the feed gas for the fuel cell system. As will be discussed in further detail below, the anode exhaust may already humidified (e.g., mixed with steam) before the anode exhaust is introduced to the humidity transfer device. In contrast, prior to being introduced to the humidity transfer device, the feed gas either is not mixed with steam or is not mixed with enough steam for proper reaction in the system. The transfer of steam in the humidity transfer device from the anode exhaust to the feed gas reuses steam that would otherwise be output as waste from the system, thereby reducing or eliminating the need to separately generate steam to mix with and humidify the feed gas. 
     As described in this application, the humidity transfer device is a solid-state device configured to be incorporated into various electrochemical systems for steam and water management, recovery, and recycling. For example, the humidity transfer device may be incorporated into fuel cell systems that include chemical reactions involving steam (e.g., a steam-methane reformer (“SMR”)), and higher-temperature fuel cells (e.g., molten carbonate, polybenzimidazole, solid acid, solid oxide, or phosphoric acid, etc.) operating either as a conventional fuel cell or in an electrolysis mode. These fuel cells may include proton-conducting, carbonate ion-conducting, oxide ion-conducting, hydroxide ion-conducting, and mixed ion-conducting fuel cells or combinations of different types of fuel cells in a single system. 
     Referring to  FIG. 1 , a humidity transfer device (“HTD”)  10  is shown according to an exemplary embodiment. The HTD  10  includes a shell-and-tube configuration including a shell  12  disposed annularly about a membrane  14 . A gap (i.e., a space) is defined between the shell  12  and the membrane  14  such that during operation of the HTD  10 , anode exhaust passes between the shell  12  and the membrane  14 . It should be understood that while  FIG. 1  shows the shell  12  and the membrane  14  having generally cylindrical shapes, the shell  12  and the membrane  14  may define other shapes, such that the shell  12  is disposed about the membrane  14 . The shell  12  may be formed from plastic or other suitable material and is configured to fully enclose the membrane  14  therein. The shell  12  defines a shell inlet  16  (i.e., a shell inlet baffle) configured to receive and feed humidified anode exhaust to an interior portion of the shell  12 . It should be understood that humidified anode exhaust refers to a mixture of anode exhaust and steam. Steam is transferred through the membrane  14  to feed gas, as will be described in further detail below, until substantially all of the steam is separated from the anode exhaust, forming a dry anode exhaust. The shell  12  further defines a shell outlet  18  (i.e., a shell outlet baffle) opposing the shell inlet  16  and configured to output the dry anode exhaust from the shell  12 . 
     Referring still to  FIG. 1 , the membrane  14  is a polymer-electrolyte membrane (“PEM”), which is configured to transfer steam therethrough without passing anode exhaust or feed gas (e.g., natural gas) therethrough. The membrane  14  may be formed from materials, such as Nafion, Aquivion, or various hydrocarbons. For use with higher-temperature fuel cells, it may be advantageous to provide a membrane  14  formed from a material rated to withstand an anticipated operational temperature of the fuel cell. In the configuration shown in  FIG. 1 , anode exhaust and feed gas may separately flow through the HTD  10  for transferring steam from the anode exhaust to the feed gas without mixing the two streams. The membrane  14  defines at least one passage  20  (i.e., channel, opening, tube, etc.) extending from an inlet end  22  to an outlet end  24  of the membrane  14 . While  FIG. 1  shows the inlet end  22  and the outlet end  24  to be at opposing ends of the membrane  14 , it should be understood that the inlet and outlet ends  22 ,  24  may be positioned at other locations in the HTD  10 , such that the at least one passage  20  remains fluidly separated from direct contact with the shell  12 . 
     As shown in  FIG. 1 , the at least one passage  20  includes a plurality of passages  20  extending between the inlet and outlet ends  22 ,  24 . According to an exemplary embodiment, the plurality of passages  20  may be defined in the membrane  14  in a parallel arrangement, such that each of the plurality of passages  20  begins at the inlet end  22  and ends at the outlet end  24 . According to another exemplary embodiment, the plurality of passages  20  may be defined in the membrane  14  in a series arrangement, such that each of the passages  20  defines a pass, forming a generally serpentine arrangement. 
     While  FIG. 1  shows anode exhaust passing between the shell  12  and the membrane  14  (e.g., external to the membrane  14 ) and feed gas passing through the passages  20 , it should be understood that the HTD  10  may be configured such that feed gas passes between the shell  12  and the membrane  14  and the anode exhaust passes through the passages  20 . According to another exemplary embodiment, the passages  20  may be formed in a parallel configuration, such that a first plurality of the passages  20  are fluidly separated from a second plurality of the passages  20 . In this configuration anode exhaust passes through the first plurality of passages  20  and feed gas passes through the second plurality of passages  20 , such that steam is transferred through the membrane  14  between each of the first and second pluralities of passages  20 . 
     Referring now to  FIG. 2 , an HTD  30  is shown according to another exemplary embodiment and operates in a substantially similar way as the HTD  10 . The HTD  30  includes a planar stack configuration. The HTD  30  defines an enclosure  32  (i.e., shell) and a substantially planar membrane  34  disposed within the enclosure  32 . The membrane  34  is a polymer-electrolyte membrane substantially the same as the membrane  14  described with respect to  FIG. 1 . The membrane  34  defines an upper surface  36  and an opposing lower surface  38 . The enclosure  32  includes a top wall  40  proximate and spaced apart from the upper surface  36  of the membrane  34 . Similarly, the enclosure  32  includes a bottom wall  42  opposing the top wall  40 , the bottom wall  42  being proximate and spaced apart from the lower surface  38  of the membrane  34 . The top wall  40 , bottom wall  42 , and membrane  34  may all be substantially parallel, although they may define other orientations according to other exemplary embodiments. 
     Referring now to  FIG. 3 , the enclosure  32  further includes opposing side walls  44  extending between the top wall  40  and the bottom wall  42 . As shown in  FIGS. 2 and 3 , a first passage  46  is defined between the membrane  34  and the top wall  40  and is configured to receive anode exhaust passing therethrough. Similarly, a second passage  48  is defined between the membrane  34  and the bottom wall  42  and is configured to receive feed gas passing therethrough. As shown in  FIG. 3 , the membrane  34  extends completely between the side walls  44 , fluidly separating the anode exhaust in the first passage  46  from the feed gas in the second passage  48 . Similarly as described above with respect to  FIG. 1 , in the HTD  30 , steam passes through the membrane  34  from the anode exhaust to the feed gas, while the anode exhaust and the feed gas remain separated. 
     While  FIGS. 2 and 3  show an enclosure  32  having only one membrane  34  and two passages  46 ,  48 , it should be understood that the enclosure  32  may include more than one membrane  34  disposed in a parallel stack within the enclosure  32 , such that additional passages are defined between each of the adjacent membranes  34 . The passages may alternate between passing anode exhaust and feed gas, such that each membrane  34  engages one of anode exhaust or feed gas on its upper surface  36  and engages the other of anode exhaust or feed gas on its lower surface  38 . 
     Referring again to  FIG. 2 , the enclosure  32  defines a first end  50  and a second end  52 . As shown in  FIG. 2 , the anode exhaust may define a stream flowing from the first end  50  to the second end  52  and the feed gas may define a stream flowing in an opposing direction, from the second end  52  to the first end  50 . According to other exemplary embodiments, the streams may flow in the same direction or the feed gas stream may flow orthogonally to the anode exhaust stream. While  FIGS. 1-3  show feed gas and anode exhaust passing through an HTD  10 ,  30  for steam transfer, it should be understood that other streams may be passed through the HTD  10 ,  30 . For example, such streams may include cathode exhaust, anode feed gas, cathode feed gas, etc. 
     As discussed above, a humidification process occurs in the HTD  10 ,  30  as steam is transferred from the anode exhaust to the feed gas. The amount of steam mixed with the feed gas is defined as a steam-to-carbon ratio (“S/C ratio”), measuring the molecular ratio of steam (H 2 O) relative to carbon in the feed gas (e.g., CH 4 ). During the humidification process, the S/C ratio increases as more steam is introduced to and mixed with the feed gas. For example, the volume flow rate of anode exhaust fed to the HTD  10 ,  30  may be increased to achieve a desired S/C ratio of the feed gas. Similarly, the humidity and therefore the S/C ratio of the anode exhaust may be increased prior to passing through the HTD  10 ,  30 . According to another exemplary embodiment, steam may be mixed directly with the feed gas apart from the anode exhaust to directly increase the S/C ratio of the feed gas. 
     Steam may continue to be introduced to and mixed with the feed gas until the S/C ratio reaches a desired level, preferably within a range for proper operation of the fuel cell system (e.g., with a S/C ratio between 1:1 and 5:1). While the feed gas may be mixed with steam for humidification, according to other exemplary embodiments, the feed gas may be mixed with liquid water and the mixture of feed gas and water is heated until at least a portion of the water shifts to steam. The feed gas and water mixture may be heated until a sufficient amount of steam is generated to provide a humidified fuel with the desired S/C ratio. 
     Referring now to  FIG. 4 , a pressure vessel assembly  60  is shown with an HTD  10  disposed therein, according to an exemplary embodiment. As shown in  FIG. 4 , the HTD  10  is a shell-and-tube configuration similar to the HTD  10  shown in  FIG. 1 . However, it should be understood that other HTDs (e.g., the HTD  30  shown in  FIGS. 2 and 3 ) may be used in the pressure vessel assembly  60  instead of or in addition to the HTD  10  as shown in  FIG. 4 . The pressure vessel assembly  60  includes a pressure vessel  62  that is generally hollow and defines a flange  64  extending laterally outward from an upper end  66  of the vessel  62  and an opening  68  formed at the upper end  66 . The pressure vessel assembly  60  further includes a lid  70  (i.e., cap) which is disposed on the vessel  62  at the upper end  66  and coupled to the flange  64  to secure the lid  70  in place on the vessel  62 . When the lid  70  is secured in place, the lid  70  sealingly engages the flange  64  and fully encloses the vessel  62 , such that the pressure vessel assembly  60  may be pressurized. 
     The HTD  10  may receive pressurized streams of gas. For example, the anode exhaust may be provided at approximately 70 psi. In an unpressurized setting, the HTD  10  may not be able to withstand high-pressure anode exhaust passing therethrough without causing damage to the HTD  10 . For example, if the shell  12  or enclosure  32  is formed from plastic, the pressure differential between the anode exhaust passing through the HTD  10  and the environment external to the HTD  10  may cause the shell  12  or enclosure  32  to rupture and leak anode exhaust. However, in the configuration shown in  FIG. 4 , the pressure vessel assembly  60  may be pressurized, such that the pressure inside the vessel  62  is close to the pressure of the anode exhaust, or within a pressure differential that will not cause damage to the HTD  10 . Advantageously, by pressurizing the pressure vessel assembly  60 , the shell  12  or enclosure  32  may be formed from less expensive and less resilient materials, thereby reducing the cost of the HTD  10 . 
     Referring still to  FIG. 4 , the pressure vessel assembly  60  includes a first inlet line  72  (i.e., conduit, passage, etc.) and a first outlet line  74  (i.e., conduit, passage, etc.) extending through the lid  70 . The first inlet line  72  is configured to transfer a stream (e.g., anode exhaust) from outside the pressure vessel assembly  60  to the HTD  10 . After steam is transferred in the HTD  10 , the first outlet line  74  is configured to transfer dry anode exhaust from the HTD  10  out of the pressure vessel assembly  60  for use elsewhere in the fuel cell system or export from the system. While  FIG. 4  shows the first inlet line  72  (i.e., anode exhaust inlet line) and the first outlet line  74  (i.e., anode exhaust outlet line) extending through the lid  70 , it should be understood that the first inlet line  72  and/or the first outlet line  74  may extend through the vessel  62  instead. 
     The pressure vessel assembly  60  further includes a second inlet line  76  (i.e., feed gas inlet line) extending through the lid  70 . The second inlet line  76  is configured to pass feed gas from a fuel supply to the HTD  10  for steam transfer, as described above, forming a partially-humidified feed gas (i.e., humidified feed gas). The partially-humidified feed gas is then output from the HTD  10  and into the vessel  62 . Water is provided to the vessel  62  for further mixing with the partially-humidified feed gas to form a humidified feed gas with a desired S/C ratio for use in a fuel cell system. The partially-humidified feed gas may be output from the HTD  10  from an opening in the HTD  10  or through a feed gas conduit  78 . For example, the feed gas conduit  78  may extend generally downward from the HTD  10  toward a lower end  67  of the vessel  62  toward the water. As shown in  FIG. 4 , the feed gas conduit  78  defines a conduit outlet  80  disposed below a water level  82 , although the conduit outlet  80  may be disposed above the water level  82  or in a pressure vessel  60  without water present according to other exemplary embodiments. When the conduit outlet  80  is disposed below the water level  82 , the injection of the partially-humidified feed gas generates gas pockets in the water and forms bubbles. The bubbling process may vaporize additional water into steam, increasing the S/C ratio of the feed gas mixed with the bubbled water. 
     The partially-humidified feed gas then passes from the HTD  10  or the feed gas conduit  78  into a mixing portion  84  of the vessel  62 , defined between the water level  82  and the lid  70 . As the partially-humidified feed gas passes through the mixing portion  84 , steam present in mixing portion  84  mixes with the partially-humidified feed gas, further increasing the S/C ratio to a desired level and forming a humidified feed gas. The humidified feed gas is then passed through a feed gas outlet  86  extending through the lid  70  for use in a fuel cell. According to other exemplary embodiments, the feed gas outlet  86  may extend through the vessel  62 . In either configuration, the feed gas outlet  86  may be disposed proximate the upper end  66  of the vessel  62  as the humidified feed gas has a high temperature and therefore rises in the vessel  62 . While  FIG. 4  shows the partially-humidified feed gas passing through the mixing portion  84 , according to another exemplary embodiment, the HTD  10  may be connected directly to the feed gas outlet  86 , such that steam is only transferred to the feed gas within the HTD  10  and not at other portions of the pressure vessel assembly  60 . In such a configuration, the vessel  62  may not be filled with water for generating additional steam. 
     As discussed above, it may be important to closely control the S/C ratio of the feed gas in the fuel cell system. However systems for controlling the S/C ratio in humidified feed gas often have a difficult time adjusting for changing humidification needs and increase the complexity of the control system in charge of operating the fuel cell system. For example, the required S/C ratio may change over the lifetime of the fuel cell system. Specifically, degradation of the fuel cell system over time may result in a need to increase or decrease the humidity level of the humidified fuel received at the fuel cell to compensate for these changes. According to another exemplary embodiment, different compositions of feed gas (e.g., natural gas, ADG, etc.) may require different S/C ratios for operation in a given fuel cell. If a fuel cell system is configured to operate with more than one composition of feed gas (either separately or mixed together), the system may require being able to generate a humidified feed gas with different S/C ratios based on the feed gas being supplied. 
     Referring still to  FIG. 4 , the pressure vessel assembly  60  includes a water controller  88  disposed within and extending generally vertically in the vessel  62 . The water controller  88  measures the water level  82  in the vessel  62 . The percentage humidity in the mixing portion  84  of the vessel  62  may be determined based on the measured water level  82 . For example, if the water level  82  is measured before the water in the vessel  62  is heated, the amount of steam in the mixing portion  84  may be determined based on the drop in the water level  82  once the system is fully operating. According to another exemplary embodiment, the water controller  88  may directly measure the S/C ratio in the mixing portion  84  by measuring relative humidity above the water level  82 . 
     The water controller  88  may further include a heating element configured to heat the water in the vessel  62  to vaporize at least a portion of the water for generating steam in mixing portion  84  of the vessel  62 . According to another exemplary embodiment, the vessel  62  may be externally heated or heat may be transferred to the water in other ways to generate steam in the mixing portion  84 . The water controller  88  may further control the introduction of water from a water supply (e.g., tap) or other water source in the fuel cell system in order to provide makeup water if the water level  82  begins to drop. According to an exemplary embodiment, the water controller  88  may be automated to increase the steam generated until the S/C ratio in the humidified feed gas reaches a desired level for use in a specific fuel cell. For example, the water controller  88  may automatically adjust the humidity based on how long the fuel cell system has been operating to compensate for degradation of the system. Similarly, the water controller  88  may be automated to adjust the humidity in the mixing portion  84  based on a data input of a fuel type into the water controller  88 . 
     Referring now to  FIG. 5 , an HTD  110  is shown according to another exemplary embodiment. As will be discussed in further detail below, the HTD  110  is configured to provide feed gas at a sub-ambient humidification condition. During operation of a fuel cell, the feed gas to one or both of the anode or the cathode may be humidified with an HTD. It was found that the optimal dew point at the anode inlet is substantially lower than the corresponding dew point at the cathode inlet. For example, the anode inlet may have a dew point between approximately 10 and 20° C. and more particularly between approximately 15 and 20° C. At the dew point, the feed gas is fully humidified (i.e., saturated). With respect to the cathode, it was found that the cathode inlet had a dew point of greater than approximately 35° C. In this configuration, in order to achieve the same level of humidification of the feed gas in the cathode as in the anode, the feed gas and required water for the cathode must be heated to a much higher temperature than at the anode, which increases the energy required to complete the humidification process. With the lower dew point in the anode, it becomes important to precisely control the level of humidification in the feed gas. For example, too much humidity can lead to condensation forming in the feed gas, which can damage the fuel cell. 
     Referring still to  FIG. 5 , the HTD  110  includes an inlet  112  configured to receive feed gas from a feed gas source and an outlet  114  downstream from the inlet  112  and configured to output humidified feed gas therefrom. Downstream from the inlet  112 , the feed gas is split into a first conduit  116  and a second (i.e., bypass) conduit  118 . The first conduit  116  receives a first portion of the feed gas and passes the first portion to a vessel  120  filled at least partially with water. Specifically, the first conduit  116  defines a conduit outlet  122  disposed below a water level  124 , although the conduit outlet  122  may be disposed above the water level  124 . When the conduit outlet  122  is disposed below the water level  124  or the feed gas is output from the conduit outlet  122  proximate the water, the injection of the feed gas generates gas pockets in the water and forms bubbles. The bubbling process may vaporize additional water into steam, increasing the S/C ratio of the feed gas mixed with the bubbled water, even if the temperature of the feed gas is below the water vaporization temperature for the pressure level within the vessel  120 . A humidified first portion of feed gas is then output from the vessel  120  directly to the outlet  114 . 
     A second portion (i.e., a bypass portion) of the feed gas passes from the inlet  112  through the second conduit  118  and to the outlet  114 . The second portion of the feed gas maintains the same level of humidification as the feed gas first received at the inlet  112 . Prior to being output from the outlet  114 , the humidified first portion of feed gas is mixed with the second portion of feed gas, such that the final mixture is at a sub-ambient level of humidification. A valve  126  (e.g., a needle valve) is disposed in the second conduit  118  and is configured to control the amount of feed gas that passes through each of the first and second conduits  116 ,  118 . For example, when the valve  126  is closed, all of the feed gas passes through the first conduit  116  and to the vessel  120 , where it is humidified. As the valve  126  is opened, feed gas begins to divert to the second conduit  118 . In particular, because the conduit outlet  122  is disposed below the water level  124 , the pressure at the conduit outlet  122  is greater than in the second conduit  118 , which forces the feed gas through the second conduit  118  rather than the first conduit. If the valve  126  is fully opened, substantially all of the feed gas would pass through the second conduit  118 , bypassing the first conduit  116  and vessel  120 , and the humidification level of the feed gas would remain unchanged. This configuration may be used when the feed gas supplied to the HTD  110  is already fully humidified. The valve  126  may be manually controlled or automatically controlled based on a humidification level measured at the outlet  114  or elsewhere in the HTD  110 . 
     Referring now to  FIGS. 6-8 , fuel cell systems incorporating an HTD for water recovery are shown according to various exemplary embodiments. It should be understood that a power plant may require a supply of high quality steam, such that the steam is substantially free from dissolved solids, chlorides, or other ions. Notably, the proliferation of these impurities in a fuel cell system may lead to degradation of the fuel cell system over the course of its service life. In order to remove the impurities, a power plant generally requires a supply of deionized water for high-quality steam generation. The deionization process often also requires the fuel cell system to include additional water purification equipment, which increase the capital investment in the overall fuel cell system as well as annual operating and maintenance costs. For example, conventional water purification systems may lead to higher operational costs because they generally require a high heat level. Furthermore, conventional water purification systems have a high rate of failure, leading to higher maintenance and replacement costs. 
     Conventional water management systems, which may include water purification systems, are large in size, and therefore add to the overall space required for installation of a fuel cell system. Specifically, the use of water management systems limits the locations that the fuel cell system can be installed due to the size requirements as well as a need for access to a water source. Furthermore, the conventional water management systems may have substantial carbon emissions as an output, which may limit installation of the fuel cell system based on local emissions standards. The use of these systems may also reduce operational efficiency for the fuel cell system because all of the additional required equipment (e.g., pumps, heaters, heat tracing, etc.) draw power and heat from the fuel cell system, resulting in substantial parasitic losses. 
     Use of conventional water management systems also often result in product gases from the fuel cell system that are wet (i.e., have a humidity level that is higher than a desired level), forming condensation. In this situation, the product gases may require drying, resulting in additional waste water output from the fuel cell system that needs to be recovered. This necessary waste water recovery may further increase the operational costs, for example, associated with removing water from the product gas as well as the capital costs for the fuel cell system to install the additional components required for drying the product gas. By using water recovery with an HTD, as shown in the systems in  FIGS. 6-8 , the costs associated with the water purification is reduced and/or eliminated as already-deionized water can then be reused in other portions of the system. 
     Referring to  FIG. 6 , a fuel cell system  140  is shown according to an exemplary embodiment. The system  140  includes a shift reactor  142  includes a shell  144  and a water-gas shift catalyst  146  disposed in the shell  144 . At least one passage  148  is formed between the shift catalyst  146  and the shell  144 . Anode exhaust from a fuel cell (e.g., a direct reforming fuel cell (“DFC”)) is fed through the shift catalyst  146  and reformed through a water-gas shift reaction in the shift catalyst  146 . The shifted anode exhaust is then output from the shift reactor  142 . It should be understood that while  FIG. 6  shows the system  140  with a shift reactor  142 , according to other exemplary embodiments, the system  140  may include a methanator in addition to or in place of the shift reactor  140 . 
     Due to the water-gas shift reaction, which adds water to the DFC anode exhaust in the shift catalyst  146 , the shifted anode exhaust may be wet (e.g., fully or oversaturated) and require the transfer of water out from the anode feed gas before being introduced to another fuel cell. As shown in  FIG. 6 , the shifted anode exhaust is fed through an HTD  150 . The HTD  150  may be substantially the same as any of the HTDs  10 ,  30 ,  110  discussed above in this application or any other suitable HTD. In the HTD  150 , water is transferred from the shifted anode exhaust to feed gas also passing through the HTD  150 , such that the HTD  150  outputs a dehumidified anode exhaust. 
     The system  140  includes an electrochemical hydrogen separator (“EHS”)  152 , which is a fuel cell operating in reverse to generate hydrogen. The EHS  152  includes an anode  154  and a cathode  156 . The anode  154  is configured to electrochemically react the dehumidified anode exhaust from the HTD  150  and output purified hydrogen from the cathode  156 . The anode  154  then outputs EHS anode exhaust, which is fed back to the shift reactor  142 . Specifically, the EHS anode exhaust is fed through the at least one passage  148  in the shift reactor  142  and reacts with the shift catalyst  146  to reform. The EHS anode exhaust may then output to an anode gas oxidizer (not shown) or elsewhere in the system  140  for use in the DFC or other fuel cell. In  FIG. 6 , the EHS anode exhaust and the DFC anode exhaust are shown passing through the shift reactor  142  in opposing directions. However, it should be understood that the EHS anode exhaust and the DFC anode exhaust may pass through the shift reactor  142  in other directions relative to each other. 
     Referring to  FIG. 7 , a fuel cell system  160  is shown according to an exemplary embodiment. The system  160  is a single-stage system, which takes exhaust gas directly from a DFC or other fuel cell for humidity transfer. For example, the system  160  includes a DFC  162  having an anode  164  and a cathode  166 . The anode  164  receives anode feed gas, reacts the anode feed gas, and outputs DFC anode exhaust. The DFC anode exhaust is then fed to an HTD  168 . The system  160  further includes an EHS  170  having an anode  172  and a cathode  174 . The cathode  174  receives and reacts feed gas and outputs EHS cathode exhaust. The EHS cathode exhaust is then fed to the HTD  168 . The HTD  168  may be substantially the same as any of the HTDs  10 ,  30 ,  110  discussed above in this application or any other suitable HTD. In the HTD  168 , water is transferred from the DFC anode exhaust to the EHS cathode exhaust in order to dehumidify the DFC anode exhaust. The dehumidified anode exhaust is then fed to the anode  172  of the EHS  170  for reaction. 
     Referring to  FIG. 8 , a fuel cell system  180  is shown according to an exemplary embodiment. The system  180  is a dual-stage system, which takes exhaust gas from a DFC or other fuel cell, through an EHS, for humidity transfer. For example, the system  180  includes a DFC  182  having an anode  184  and a cathode  186 . The anode  184  receives anode feed gas, reacts the anode feed gas, and outputs DFC anode exhaust. The system  180  further includes a EHS  188 , having an anode  190  and a cathode  192 . The DFC anode exhaust is fed, with or without being processed, to the anode  190  of the EHS  188 , where it is reacted and forms EHS anode exhaust. The EHS anode exhaust is then fed to an HTD  194 . The HTD  194  may be substantially the same as any of the HTDs  10 ,  30 ,  110  discussed above in this application or any other suitable HTD. The cathode  192  of the EHS  188  receives feed gas, which is reacted in the cathode  192 . The cathode  192  then outputs EHS cathode exhaust, which is fed to the HTD  194 . In the HTD  194 , water is transferred from the EHS anode exhaust to the EHS cathode exhaust, lowering the humidification level of the EHS anode exhaust. Dehumidified anode exhaust and wet cathode exhaust are then separately output from the HTD  194 . 
     Referring still to  FIG. 8 , the fuel cell system  180  includes an electrochemical hydrogen condenser (“EHC”)  196  having an anode  198  and a cathode  200 . The anode  198  receives and reacts the wet cathode exhaust from the HTD  194  and the cathode  200  condenses and outputs hydrogen. The hydrogen may be exported from or stored or used elsewhere in the system  180 . According to an exemplary embodiment, at least a portion of the hydrogen compressed by the cathode  200  may be mixed with the EHS cathode exhaust before it is fed to the anode  198  of the EHC  196 .  FIG. 8  shows the hydrogen being mixed with the EHS cathode exhaust upstream from the HTD  194 , but it should be understood that according to other exemplary embodiments, the hydrogen may be mixed with the wet cathode exhaust downstream from the HTD  194  before being fed to the anode  198  of the EHC  196 . 
     As utilized herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of this disclosure as recited in the appended claims. 
     It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples). 
     The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. 
     References herein to the position of elements (e.g., “top,” “bottom,” “above,” “below,” etc.) are merely used to describe the orientation of various elements in the FIGURES. It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure. 
     It is to be understood that although the present invention has been described with regard to preferred embodiments thereof, various other embodiments and variants may occur to those skilled in the art, which are within the scope and spirit of the invention, and such other embodiments and variants are intended to be covered by corresponding claims. Those skilled in the art will readily appreciate that many modifications are possible (e.g., variations in sizes, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. For example, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present disclosure.