Patent Publication Number: US-2020287224-A1

Title: Hydrogen generation using a fuel cell system with an rep

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/590,112, filed Nov. 22, 2017, the entire disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT OF GOVERNMENT RIGHTS 
     This invention was made with government support under DE-EE0006669 awarded by the Department of Energy. The government has certain rights in this invention. 
    
    
     BACKGROUND 
     The present application relates generally to the field of H 2  (“hydrogen”) generation by integrating a reforming-electrolyzer-purifier (“REP”) with a high-temperature fuel cell. Specifically, an REP may be used with the fuel cell to generate hydrogen. Examples of REPs and systems that include them are described in PCT Publication No. WO 2015/116964, which is assigned to the assignee of the present application. 
     An REP requires partially-reformed fuel in order to maintain an even temperature profile and heat balance within the REP during operation. For example, the presence of a small amount more CH 4  (“methane”) than is desired from the reforming process may have a substantial impact on providing a consistent temperature profile in the REP. However, reforming the fuel often requires specialized equipment in the fuel preparation, conversion, and reforming heat supply, which may increase cost and complexity of the system as well as operating cost for the additional equipment. It may, therefore, be advantageous to incorporate the desired feed preparation and partial reforming process into a fuel cell in a fuel cell system. 
     SUMMARY 
     One embodiment relates to a fuel cell system including a fuel cell having an anode and a cathode configured to output cathode exhaust. The fuel cell is configured to generate waste heat. The fuel cell system further includes a reformer configured to partially reform a feed gas using the waste heat and output a hydrogen-containing stream. The fuel cell system further includes an REP having an REP anode configured to receive a first portion of the hydrogen-containing stream and an REP cathode. 
     In one aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, a heat exchanger is configured to heat the feed gas using the waste heat and to output a heated feed gas. The waste heat is conveyed to the heat exchanger in the cathode exhaust. The reformer is configured to receive the heated feed gas. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell is configured to receive a remaining portion of the hydrogen-containing stream. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes an indirect reforming unit disposed on the anode. The indirect reforming unit is configured to further reform the hydrogen-containing stream and output a fuel turn gas. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, a first portion of the fuel turn gas is the first portion of the hydrogen-containing stream received by the REP anode, and the anode is configured to receive a remaining portion of the fuel turn gas. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the REP anode is configured to receive a portion of anode exhaust output from the anode. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the anode is configured to receive a remaining portion of the fuel turn gas output from the indirect reforming unit. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes an anode gas oxidizer (“AGO”) configured to receive anode exhaust from the anode and to oxidize the anode exhaust with air from an air supply, and a heat transfer element disposed in the AGO. The fuel cell system is configured to mix first portion of the fuel turn gas with water from a water supply to form a hydrated feed gas. The heat transfer element is configured to receive the hydrated feed gas and transfer heat from an oxidation reaction in the AGO to the hydrated feed gas. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the REP anode is configured to receive the hydrated feed gas from the heat transfer element. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes a second reformer disposed between the heat transfer element and the REP anode. The second reformer is configured to further reform the hydrated feed gas before introduction to the REP anode. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the reformer is configured to receive heat from an oxidation reaction in the AGO. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes an anode gas oxidizer (“AGO”) configured to receive anode exhaust from the anode. The AGO is configured to oxidize the anode exhaust with air from an air supply. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the AGO is configured to receive heated sweep gas from the REP cathode. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes a heat transfer element disposed in the AGO. The fuel cell system is configured to mix a first portion of the hydrogen-containing stream with water roan a water supply to form a hydrated feed gas. The heat transfer element is configured to receive the hydrated feed gas and to transfer heat from an oxidation reaction in the AGO to the hydrated feed gas. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the REP anode is configured to receive the hydrated feed gas from the heat transfer element. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes a second reformer disposed between the heat transfer element and the REP anode. The second reformer is configured to further reform the hydrated feed gas before introduction to the REP anode. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the reformer is configured to receive heat from an oxidation reaction in the AGO. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes a heat transfer element disposed in the AGO and configured to receive air from the air supply and transfer heat from an oxidation reaction in the AGO to the air passing through the heat transfer element. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the REP cathode is configured to receive the air passing through the heat transfer element as sweep gas. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the REP anode is configured to receive steam from a water supply. 
     Another embodiment relates to a method of operating a fuel cell system including providing a fuel cell having an anode and a cathode, providing a reformer, and providing an REP having an REP anode and an REP cathode. The method further includes generating waste heat from the fuel cell and heating the feed gas with the waste heat, forming heated feed gas. The method further includes partially reforming the heated feed gas in the reformer and outputting a hydrogen-containing stream and feeding a first portion of the hydrogen-containing stream to the REP anode. 
     In one aspect of the method, which is combinable with the above embodiments and aspects in any combination, the method further includes hydrating the first portion of the hydrogen-containing stream with steam from a water supply to form a hydrated feed gas. 
     In another aspect of the method, which is combinable with the above embodiments and aspects in any combination, the method further includes feeding a remaining portion of the hydrogen-containing stream to the anode and outputting anode exhaust from the anode. 
     In another aspect of the method, which is combinable with the above embodiments and aspects in any combination, the method further includes reforming the hydrated feed gas prior to introduction to the REP anode. 
     In another aspect of the method, which is combinable with the above embodiments and aspects in any combination, the method further includes oxidizing the anode exhaust in an AGO and transferring heat from the AGO to the hydrated feed gas. 
     In another aspect of the method, which is combinable with the above embodiments and aspects in any combination, the method further includes transferring heat from the AGO to air from an air supply and feeding the heated air to the REP cathode for use as sweep gas. 
     In another aspect of the method, which is combinable with the above embodiments and aspects in any combination, the sweep gas maintains a substantially uniform temperature across the REP cathode. 
     In another aspect of the method, which is combinable with the above embodiments and aspects in any combination, the method further includes receiving the sweep gas at the AGO and oxidizing the anode exhaust with the sweep gas. 
     In another aspect of the method, which is combinable with the above embodiments and aspects in any combination, the method further includes mixing a portion of the anode exhaust with the hydrated feed gas prior to introduction to the REP anode. 
     Another embodiment relates to a fuel cell system including a feed system having a water supply and a fuel supply. The feed system is configured to purify water from the water supply and fuel from the fuel supply, and to mix the water and fuel to form a hydrated feed gas. The fuel cell system further includes a fuel cell having an anode configured to receive a first portion of the hydrated feed gas, and a cathode. The fuel cell system further includes a reformer-electrolyzer-purifier (“REP”) having an REP anode configured to receive a second portion of the hydrated feed gas, and an REP cathode. 
     In one aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes a water treatment system configured to purify the water from the water supply. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes a first heat exchanger configured to receive the hydrated feed gas and to vaporize at least a portion of the water in the hydrated feed gas. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes a second heat exchanger configured to receive the hydrated feed gas and to vaporize water in the hydrated feed gas remaining after passing through the first heat exchanger. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes a reformer configured to reform a portion of the hydrated feed gas to hydrogen. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system further includes at least one of a hydrogen purification device or a hydrogen pressurization device configured to recycle at least a portion of hydrogen output from the REP anode to the feed system. 
     In another aspect of the fuel cell system, which is combinable with the above embodiments and aspects in any combination, the fuel cell system is configured to mix the hydrogen from the REP anode with the hydrated feed gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a fuel cell system, integrated with a reformer-electrolyzer-purifier (“REP”), according to an exemplary embodiment. 
         FIG. 2  is a schematic view of another embodiment of the fuel cell system, which feeds fuel turn gas to the REP after additional reforming. 
         FIG. 3  is a schematic view of another embodiment of the fuel cell system, which feeds fuel turn gas directly to the REP after it is mixed with fuel cell anode exhaust. This configuration may reduce manufacturing costs but increase operating costs. 
     
    
    
     DETAILED DESCRIPTION 
     A reformer-electrolyzer-purifier (“REP”) assembly includes at least one electrolyzer molten carbonate fuel cell and may include a plurality of electrolyzer fuel cells formed in a fuel cell stack, also referred to as an REP stack. The at least one electrolyzer fuel cell is a cell operated in reverse so as to electrolyze water to produce hydrogen, and at the same time to purify the hydrogen from the hydrocarbon reforming process by electrochemically removing CO 3  ions. CO 2  may be provided by reforming a hydrocarbon, such as methane. Removal of the CO 3  ions then drives the reforming reaction to completion. 
     Before undergoing the electrochemical reaction in a fuel cell, hydrocarbon fuels such as methane, coal gas, etc. are typically reformed to produce hydrogen for use in the anode of the fuel cell. In internally reforming fuel cells, a steam reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels without the need for expensive and complex reforming equipment. In addition, the endothermic reforming reaction can be used advantageously to help cool the fuel cell stack. Internally reforming fuel cells employing direct internal reforming and indirect internal reforming have been developed. 
     Direct internal reforming (“DIR”) is accomplished by placing a reforming catalyst (“DIR catalyst”) within the active anode compartment. This catalyst is exposed to the electrolyte of the fuel cell. 
     Indirect internal reforming (“IIR”) is accomplished by placing the reforming catalyst (“IIR catalyst”) in an isolated chamber within the fuel cell stack and routing the reformed gas from this chamber into the anode compartment of the fuel cell. 
     A REP stack generally includes a molten carbonate fuel cell (“MCFC”) stack and a system using an REP stack includes a power supply for supplying power to the REP stack for driving the electrolysis reactions. Although a system generally includes reforming, such as internal or external reforming, it is also contemplated that the REP and/or the system more generally may omit internal and/or external reforming, and may be used for electrolyzing a supply gas containing CO 2  and water and operating and purifying hydrogen without reforming. 
     Referring to  FIG. 1 , a fuel cell system  100  is shown according to an exemplary embodiment. The system  100  includes a fuel cell  102  (e.g., base load direct fuel cell (SureSource™ fuel cell) or solid oxide fuel cell (“SOFC”), having an anode  104  and a cathode  106 . While the fuel cell  102  is shown having one anode  104  and one cathode  106 , it should be understood that the fuel cell  102  may be configured as a fuel cell stack having a plurality of fuel cells stacked in series. The system  100  further includes an REP  108  (e.g., a second fuel cell), having an REP anode  110  and an REP cathode  112 . Similarly to the fuel cell  102 , while the REP  108  is shown having one anode  110  and one cathode  112 , it should be understood that the REP  108  may be configured as a cell stack having a plurality of cells stacked in series. 
     Fuel and water are fed to the system  100  as part of a feed system  113 , for operation of the fuel cell  102  to output power and of the REP  108  to output hydrogen. The feed system  113  includes a fuel supply  114  and a water supply  116 . The fuel fed to the system  100  from the fuel supply  114  may include natural gas, anaerobic digester gas (“ADG”), and/or other suitable fuel. The fuel is then desulfurized, such that it may be received in the fuel cell  102  without causing damage to or degradation of the fuel cell  102  due to sulfur buildup. The water fed to the system  100  from the water supply  116  is purified in a water treatment system (“WTS”)  118  (i.e., purifier). A first portion  120  of the water output from the WTS  118  is mixed with the desulfurized fuel, forming a hydrated (e.g., saturated) feed gas (i.e., fuel/water mixture). 
     The feed gas is then fed through a first heat exchanger  122 . Heat is transferred in the first heat exchanger  122  from cathode exhaust output by the cathode  106  of the fuel cell  102  to the feed gas, vaporizing substantially all of the water and increasing the temperature of the feed gas. The heated feed gas is then fed to a first reformer  124  (i.e., a preconverter), in which the heated feed gas is reformed (e.g., slightly reformed) by reacting the fuel with steam to produce hydrogen. For example, approximately 1-2% of methane in the heated feed gas is reformed to hydrogen. The reforming reaction in the first reformer  124  is endothermic and preheating the feed gas in the first heat exchanger  122  prior to feeding the feed gas to the first reformer  124  provides the heat needed for the conversion of some of the feed gas to hydrogen. According to some embodiments, a small amount of hydrogen may be required in the feed gas to prevent damage to indirect reforming cells  129  inside the fuel cell  102 . The first reformer  124  then outputs a slightly reformed feed gas (i.e., a hydrogen-containing stream) configured to be received at the fuel cell  102  and the REP  108 . 
     A first portion  126  of the slightly reformed feed gas is then fed through a second heat exchanger  128 . Heat is transferred in the second heat exchanger  128  from the cathode exhaust output by the cathode  106  to the first portion  126  of the reformed feed gas, increasing the temperature of the reformed feed gas. 
     Referring still to  FIG. 1 , after passing through the second heat exchanger  128 , the first portion  126  of the slightly reformed feed gas is then fed to the fuel cell  102  for reaction. Specifically, the slightly reformed feed gas is received in the fuel cell  102  at an indirect reforming unit  129  (e.g., fuel turn). The indirect reforming unit  129  is disposed on (e.g., directly on) the anode  104  of the fuel cell  102  and is configured to pass at least a portion of (e.g., all of) the feed gas received in the indirect reforming unit  129  to the anode  104 . Feed gas output from the indirect reforming unit  129 , which may be referred to as fuel turn gas or a hydrogen-containing stream (and output into a fuel turn manifold  130 ), is partially reformed even further in the indirect reforming unit  129  before it is fed to the anode  104  or the REP  108 . The partial reforming process in the indirect reforming unit  129  is endothermic, such that heat is passed from the anode  104  and/or the cathode  106  to the indirect reforming unit  129 , thereby cooling the fuel cell  102  during operation. 
     After reaction of the fuel turn gas in the anode  104 , the anode  104  outputs anode exhaust (a mixture of CO 2 , H 2 O, H 2  and CO), which is fed to an anode gas oxidizer (“AGO”)  132 . The AGO  132  further receives air from an air supply  134 , which is compressed and pumped into the AGO  132  through a compressor/blower  136 . The AGO  132  further receives heated air from the REP cathode  112 , which includes the CO 2  and oxygen from the REP  108 . The heated air may be used as sweep gas in the REP cathode  112  before being fed to the AGO  132 . The operation of the REP  108  transfers CO 2  and oxygen to the heated air used as sweep gas, thereby increases the voltage and performance (e.g., power output) of the fuel cell  102 . Furthermore, air sweeping of the REP cathode  112  also improves performance of the REP  108  by reducing the voltage and power required for operating the REP  108 . The air supplied to the REP  108  must be heated before being sent to the REP  108  in order to maintain a consistent thermal profile within the REP  108 . The anode exhaust is oxidized in the AGO  132  with the air from the air supply  134  and the sweep gas from the REP cathode  112 , and the AGO  132  outputs an oxidized feed gas to the cathode  106 . Additional air may then be mixed with the oxidized feed gas downstream from the AGO  132  and upstream from the cathode  106  in order to provide a desired temperature to the cathode  106 , which is less than an oxidizer outlet temperature of the AGO  132 . In the cathode  106 , CO 2  and oxygen are reacted and transferred to the anode  104  to produce power and the cathode  106  outputs a cathode exhaust. The cathode exhaust is then passed through the second heat exchanger  128 , where heat is transferred from the cathode exhaust to the reformed feed gas, as discussed above. The cathode exhaust is then passed through the first heat exchanger  122 , where heat is transferred from the cathode exhaust to the feed gas, as discussed above. After passing through the first and second heat exchangers  122 ,  128 , the cathode exhaust may be output from the system  100  or used in other portions of the system  100  for heat transfer or as a heat source. 
     According to an exemplary embodiment, a second portion  127  of the slightly reformed feed gas may be fed to the REP  108 . Specifically, a second portion  121  of the water from the WTS  118  is mixed with the second portion  127  of the reformed feed gas, forming a hydrated feed gas with a higher water content than the reformed feed gas. This additional water may be needed in the REP  108  to compensate for water that is consumed in the REP electrolysis reaction (H 2 O+CO 2 →H 2 +CO 3 ). If there is insufficient heat recovery in heat exchangers  142  and  146  (discussed in further detail below) to vaporize and heat the additional water needed, water may be shifted from the second portion  121  of the water to the first portion  120  of the water and vaporized with cathode exhaust in the heat exchanger  122 . In this configuration, in order to avoid excess steam from being sent to the fuel cell  102 , a portion of the desulfurized fuel (i.e., bypass fuel  115 ) may be bypassed around the pre-converter  124  and fuel takeoff to the REP  108  (e.g., where the second portion  127  of the slightly reformed feed gas is separated) in order to reduce the steam-to-fuel ratio to the normal levels fed to the fuel cell  102 . As shown in  FIG. 1 , the bypass fuel  115  is introduced downstream from where the first and second portions  126 ,  127  of the reformed feed gas are formed, such that the bypass fuel  115  is only mixed in the first portion  126  of the reformed feed gas. 
     The hydrated feed gas, combining the second portion  127  of the slightly reformed feed gas and the second portion  121  of the water is then received in the AGO  132  at a first heat transfer element  138 . Specifically, as the hydrated feed gas passes through the first heat transfer element  138 , heat is transferred from the oxidization reaction in the AGO  132  to the hydrated REP feed gas, increasing the temperature of the hydrated feed gas. The heated hydrated feed gas is then fed to a second reformer  140 , in which the hydrated feed gas is further reformed. According to an exemplary embodiment, the second reformer  140  may be located inside the AGO  132  to facilitate the transfer. The second reformer  140  outputs a reformed feed gas, which is fed to the REP anode  110  for reaction in the REP  108 . The amount of reforming in the feed gas to the REP  108  is controlled to a desired level by the amount of heat transferred in the first heat transfer element  138 . This balances the heat in the REP  108  and results in a smooth temperature profile in the REP  108 . The REP anode  110  outputs hydrogen (e.g., 95-98% purity) as REP anode exhaust. According to another exemplary embodiment, heat may also be transferred directly to the second reformer  140  from the AGO  132 . 
     The REP anode exhaust is then fed through a third heat exchanger  142 . Heat is transferred in the third heat exchanger  142  from the REP anode exhaust to the feed water, increasing the temperature of the second portion  121  of the water. The heat evaporates liquid water present in the second portion  121  and superheats the steam. The REP anode exhaust is then fed to a third reformer  144 , in which the REP anode exhaust is reformed (e.g., partially reformed), to remove all CO from an output stream. The output from the REP anode  110  contains CH 4 , CO 2 , and traces of CO, as well as hydrogen. Many devices that use hydrogen are sensitive to and/or cannot tolerate CO. However, by passing the stream output from the REP anode  110  across reforming catalysts at a lower temperature, all of the CO in the stream is converted to CH 4  and CO 2 . Because there is so little CO and CO 2  in the stream output from the REP anode  110 , only a minor portion of the hydrogen in the stream is consumed. It should be noted that if the hydrogen is being used in a system that is tolerant of CO, the stream may be output without passing through the third reformer  144 . Referring still to  FIG. 1 , the output stream is then fed through a fourth heat exchanger  146 . Heat is transferred in the fourth heat exchanger  146  from the REP anode exhaust to the second portion  121  of the water, further increasing the temperature of and partially vaporizing the water. As shown in  FIG. 1 , the second portion  121  of the water is heated in the fourth heat exchanger  146  and then the third heat exchanger  142  before being mixed with the second portion  127  of the slightly reformed feed gas to form the hydrated feed gas with higher water content. 
     At least a portion of the output stream may be exported from the system  100  for storage or other use, or may be used in the system  100  for other purposes. Some users of hydrogen may require a high-purity and/or high-pressure hydrogen. In these cases, a hydrogen purification and/or pressurization device  145  may be included in the system  100 . If the output stream is purified, the impurities (mainly CH 4 ) may be recycled to the feed system  113  via a recycling line  155  and mixed with the other fuel supplied to the fuel cell  102  and the REP  108 . 
     With respect to the REP cathode  112 , air may be fed from the air supply  134 , through a second heat transfer element  148  in the AGO  132  and received in the REP cathode  112 . Specifically, as the air passes through the second heat transfer element  148 , heat is transferred from the oxidization reaction in the AGO  132  to the air, increasing the temperature of the air. This preheated air introduced to the REP cathode  112  is used as sweep gas, which reduces the concentration of CO 2  and O 2  in the REP cathode  112 . This process results in a lower voltage across the REP  108  and power consumption. If CO 2  and O 2  is desired as a byproduct, the system  100  may also operate without sweep gas. However, it should be noted that the use of sweep gas helps to maintain a uniform temperature in the REP cathode  112 , thereby maximizing the life of the REP  108 . 
     As described above,  FIG. 1  depicts an embodiment that utilizes waste heat from the fuel cell  102  to heat (e.g., in the first heat exchanger  122 ) and enable partial reformation (e.g., in first reformer  124 ) of a portion of feed gas (i.e., the second portion  127  of the slightly reformed feed gas) to the REP  108 . This waste heat may be considered to be external to the fuel cell  102  because the waste heat is conveyed in the exhaust stream from the fuel cell cathode  106 . As described below, other embodiments may utilize waste heat internal to fuel cell  102  to enable partial reformation of at least a portion of the feed gas to the REP. For example, an indirect reforming unit  229  may utilize waste heat from the exothermic reactions within fuel cell  102 . Further,  FIG. 1  depicts an embodiment that utilizes anode exhaust from the fuel cell  102  (e.g., after it is oxidized in AGO  132 ) to heat (via the first heat transfer element  138 ) and enable additional partial reformation (e.g., in the second reformer  140 ) of a feed gas to the REP  108 . However, as described below, it should be understood that in other exemplary embodiments anode exhaust may be utilized in different ways. For example, according to other exemplary embodiments may anode exhaust may be mixed (i.e., blended) with a feed gas to an REP, to provide the REP feed gas with sufficient hydrogen content. 
     Referring now to  FIG. 2 , a fuel cell system  200  is shown according to an exemplary embodiment. In fuel cell system  200 , excess fuel is sent to a fuel-cell  202  and output from an indirect reforming unit  229  as excess fuel turn gas rather than feeding all of the fuel turn gas directly to the anode of a fuel cell, as shown in  FIG. 1 . The excess fuel provides additional cooling to the system  200 , allowing the system  200  to operate at a higher load. Further, the majority of reforming required to maintain a proper heat balance in an REP  208  is performed in indirect reforming unit  229 . This reforming may be performed without adding equipment (i.e., components) to the system  200 , thereby reducing cost and complexity of the system  200  capable of maintaining proper heat balance. With reference to  FIG. 2 , like reference numerals to  FIG. 1  denote similar elements. The system  200  includes a fuel cell  202 , having an anode  204  and a cathode  206 . The system  200  further includes an REP  208  (e.g., a second fuel cell), having an REP anode  210  and an REP cathode  212 . 
     Fuel and water are fed to the system  200  as part of a feed system  213 , for operation of the fuel cell  202  to output power and of the REP  208  to output hydrogen. The feed system  213  includes a fuel supply  214  and a water supply  216 . The fuel fed to the system  200  from the fuel supply  214  may include natural gas, anaerobic digester gas (“ADG”), and/or other suitable fuel. The fuel is then desulfurized, such that it may be received in the fuel cell  202  without causing damage to or degradation of the fuel cell  202  due to sulfur buildup. The water fed to the system  200  from the water supply  216  is purified in a WTS  218 . A first portion  220  of the water output from the WTS  218  is mixed with the desulfurized fuel, forming a hydrated (e.g., wet) fuel. The hydrated fuel is further mixed with hydrogen output from the REP  208  to form a feed gas. 
     The feed gas is then fed through a first heat exchanger  222 . Heat is transferred in the first heat exchanger  222  from cathode exhaust output by the cathode  206  of the fuel cell  202  to the feed gas, increasing the temperature of the feed gas and vaporizing substantially all of the water. The heated feed gas is then fed to a first reformer  224 , in which the heated feed gas is reformed (e.g., slightly reformed, partially reformed, etc.) by reacting the fuel with steam to produce hydrogen. The reforming reaction in the first reformer  224  is endothermic and preheating the feed gas in the first heat exchanger  222  prior to feeding the feed gas to the first reformer  224  provides the heat needed for the conversion of some of the feed gas to hydrogen. A small amount of hydrogen may be needed to prevent damage to the indirect reforming cells  229  inside the fuel cell  202 . The first reformer  224  then outputs a slightly reformed feed gas configured to be received at the fuel cell  202 . 
     The slightly reformed feed gas is then fed through a second heat exchanger  228 . Heat is transferred in the second heat exchanger  228  from the cathode exhaust output by the cathode  206  to the reformed feed gas, increasing the temperature of the reformed feed gas. After passing through the second heat exchanger  228 , the slightly reformed feed gas is then fed to the fuel cell  202  for reaction. Specifically, the slightly reformed feed gas is received in the fuel cell  202  at an indirect reforming unit  229  which outputs fuel turn gas. The indirect reforming unit  229  is disposed on (e.g., directly on) the anode  204  of the fuel cell  202  and transfers heat from the anode  204 . Feed gas output from the indirect reforming unit  229  forms fuel turn gas (which is output into a fuel turn manifold  230 ), which is partially reformed even further in the indirect reforming unit  229 . The partial reforming process in the indirect reforming unit  229  is endothermic, such that heat is passed from the anode  204  and/or the cathode  206  to the indirect reforming unit  229 , thereby cooling the fuel cell  202  during operation. At least a portion of the fuel turn gas from the indirect reforming unit  229  is passed through a fuel turn manifold  230  to the anode  204 . 
     While the fuel cell system  100  in  FIG. 1  showed all of the fuel turn gas being fed directly to the anode  104 , in the fuel cell system  200  shown in  FIG. 2 , a first portion  231  of the fuel turn gas is then output from the indirect reforming unit  229  through the fuel turn manifold  230  for use in the REP  208 . The remaining fuel turn gas is then fed to the anode  204  of the fuel cell  202  for reaction. The anode  204  then outputs anode exhaust, which is fed to an anode gas oxidizer (“AGO”)  232 . The AGO  232  further receives air from an air supply  234 , which is compressed and pumped into the AGO  232  through a compressor and/or blower  236 . The AGO  232  also receives heated air used as sweep gas in the REP cathode  212 . The anode exhaust is oxidized in the AGO  232  with the air from the air supply  234  and the sweep gas from the REP cathode  212 , and the AGO  232  outputs an oxidized feed gas. Additional air may then be mixed with the oxidized feed gas downstream from the AGO  232  and upstream from the cathode  206  in order to provide a desired temperature to the cathode  206 , which is less than an oxidizer outlet temperature of the AGO  232 . The oxidized feed gas and oxygen are then fed to the cathode  206 , where they are reacted and the cathode  206  outputs a cathode exhaust. The cathode exhaust is then passed through the second heat exchanger  228 , where heat is transferred from the cathode exhaust to the reformed feed gas, as discussed above. The cathode exhaust is then passed through the first heat exchanger  222 , where heat is transferred from the cathode exhaust to the feed gas, as discussed above. After passing through the first and second heat exchangers  222 ,  228 , the cathode exhaust may be output from the system  200  or used in other portions of the system  200  for heat transfer or as a heat source. 
     According to an exemplary embodiment, fuel turn gas is fed to the REP  208 . Specifically, a second portion  221  of the water from the WTS  218  is mixed with the first portion  231  of the fuel turn gas, forming a hydrated feed gas with a higher water content than the reformed feed gas. The hydrated feed gas is then received in the AGO  232  at a first heat transfer element  238 . Specifically, as the hydrated feed gas passes through the first heat transfer element  238 , heat is transferred from the oxidization reaction in the AGO  232  to the hydrated feed gas, increasing the temperature of the hydrated feed gas. The heated hydrated feed gas is then fed to a second reformer  240 , in which the hydrated feed gas is further reformed. The second reformer  240  outputs a reformed feed gas, which is fed to the REP anode  210  for reaction in the REP  208 . In this configuration, the level of reforming is controlled to improve the heat balance around the REP  208 . The REP anode  210  outputs hydrogen. 
     The REP anode exhaust is then fed through a third heat exchanger  242 . Heat is transferred in the third heat exchanger  242  from the REP anode exhaust to the feed water, increasing the temperature of the second portion  221  of the water. The heat evaporates liquid water present in the second portion  221  and may superheat the steam. The REP anode exhaust is then fed to a third reformer  244 , in which CO in the REP anode exhaust is removed by conversion to CH 4 . The output stream is then fed through a fourth heat exchanger  246 . Heat is transferred in the fourth heat exchanger  246  from the REP anode exhaust to the second portion  221  of the water, further increasing the temperature of and partially vaporizing the water. As shown in  FIG. 2 , the second portion  221  of the water is heated in the fourth heat exchanger  246  and then the third heat exchanger  242  before being mixed with the first portion  231  of the fuel turn gas to form the hydrated feed gas. 
     At least a portion of the output stream may be exported from the system  200  for storage or other use. As discussed with respect to  FIG. 1 , a hydrogen purification and/or pressurization device  245  may be included in the system  200 . If the output stream is purified, the impurities (mainly CH 4 ) may be recycled to the feed system  213  via a recycling line  255  and mixed with the other fuel supplied to the fuel-cell  202  and the REP  208 . 
     With respect to the REP cathode  212 , air may be fed from the air supply  234 , through a second heat transfer element  248  in the AGO  232  and received in the REP cathode  212 . Specifically, as the air passes through the second heat transfer element  248 , heat is transferred from the oxidization reaction in the AGO  232  to the air, increasing the temperature of the air. This preheated air introduced to the REP cathode  212  is used as sweep gas, which reduces the concentration of CO 2  and O 2  in the REP cathode  212 . This process results in a lower voltage across the REP  208  and power consumption. If CO 2  and O 2  is desired as a byproduct, the system  200  may also operate without sweep gas. However, it should be noted that the use of sweep gas helps to maintain a uniform temperature in the REP cathode  212 , thereby maximizing the life of the system  200 . 
       FIGS. 1 and 2  depict embodiments that each incorporate a second reformer  140 ,  240  to further partially reform feed gas to the REP. However, it should be understood that the second reformer  240  in  FIG. 2  may have lighter duty than second reformer  140  in  FIG. 1  because the feed gas entering second reformer  240  requires less reformation. In other words, the second reformer  240  in  FIG. 2  does not reform as much feed gas as the second reformer  140  in  FIG. 1  and therefore may have a smaller reformation capacity than the second reformer  140  in  FIG. 1 . By comparison, the feed gas entering second reformer  240  in  FIG. 2  contains more hydrogen than the feed gas entering second reformer  140  in  FIG. 1  because the feed gas in  FIG. 2  has undergone partial reformation in the indirect reforming unit  229 . As a result, the second reformer  240  may be smaller than the second reformer  140  for similar systems. 
     Referring now to  FIG. 3 , a fuel cell system  300  is shown according to an exemplary embodiment. In fuel cell system  300 , excess fuel is sent to a fuel cell  302  and output from an indirect reforming unit  329  as excess fuel turn gas and fed directly to the REP anode rather than feeding all of the fuel turn to the anode of a fuel cell, as shown in  FIG. 1 , or processing the fuel turn gas through reformer, as shown in  FIG. 2 . In this configuration, some of the anode exhaust gas which is essentially fully reformed, is mixed with the fuel turn gas to provide a total level of reforming in the combined gas feed to an REP  308  to the desired level to maintain a good heat balance in temperature profile in the REP  308 . With reference to  FIG. 3 , like reference numerals to  FIGS. 1 and 2  denote similar elements. The system  300  includes a fuel cell  302 , having an anode  304  and a cathode  306 . The system  300  further includes an REP  308  (e.g., a second fuel cell), having an REP anode  310  and an REP cathode  312 . 
     Fuel and water are fed to the system  300  as part of a feed system  313 , for operation of the fuel cell  302  to output power and of the REP  308  to output hydrogen. The feed system  313  includes a fuel supply  314  and a water supply  316 . The fuel fed to the system  300  from the fuel supply  314  may include natural gas, anaerobic digester gas (“ADG”), or other suitable fuel. The fuel is then desulfurized, such that it may be received in the fuel cell  302  without causing damage to or degradation of the fuel cell  302  due to sulfur buildup. The water fed to the system  300  from the water supply  316  and is purified in a WTS  318 . A first portion  320  of the water output from the WTS  318  is mixed with the desulfurized fuel, forming a hydrated (e.g., wet) fuel. The hydrated fuel may be further mixed with hydrogen output from the REP  308  to form a feed gas. 
     The feed gas is then fed through a first heat exchanger  322 . Heat is transferred in the first heat exchanger  322  from cathode exhaust output by the cathode  306  of the fuel cell  302  to the feed gas, increasing the temperature of the feed gas and vaporizing substantially all of the water. The heated feed gas is then fed to a first reformer  324 , in which the heated feed gas is reformed (e.g., slightly reformed, partially reformed, etc.) by reacting the fuel with steam to produce hydrogen. The reforming reaction in the first reformer  324  is endothermic and preheating the feed gas in the first heat exchanger  322  prior to feeding the feed gas to the first reformer  324  provides the heat needed for the conversion of some of the feed gas to hydrogen. A small amount of hydrogen may be needed to prevent damage to the indirect reforming cells  329  inside the fuel cell  302 . The first reformer  324  then outputs a reformed feed gas configured to be received at the fuel cell  302 . 
     The slightly reformed feed gas is then fed through a second heat exchanger  328 . Heat is transferred in the second heat exchanger  328  from the cathode exhaust output by the cathode  306  to the reformed feed gas, increasing the temperature of the reformed feed gas. After passing through the second heat exchanger  328 , the slightly reformed feed gas is then fed to the fuel cell  302  for reaction. Specifically, the slightly reformed feed gas is received in the fuel cell  302  at an indirect reforming unit  329  which outputs fuel turn gas. The indirect reforming unit  329  is disposed on (e.g., directly on) the anode  304  of the fuel cell  302  and a fuel turn manifold  330  is configured to pass at least a portion of the feed gas from the indirect reforming unit  329  to the anode  304 . Feed gas output from the indirect reforming unit  329 , forms fuel turn gas, which is partially reformed further in the indirect reforming unit  329 . The partial reforming process in the indirect reforming unit  329  is endothermic, such that heat is passed from the anode  304  and/or the cathode  306  to the indirect reforming unit  329 , thereby cooling the fuel cell  302  during operation. At least a portion of the fuel turn gas from the indirect reforming unit  329  is passed through the fuel turn manifold  330  to the anode  304 . 
     While the fuel cell system  200  in  FIG. 2  showed the first portion  231  of the fuel turn gas being fed through the first heat transfer element  238  and the second reformer  240  before being fed to the REP anode  210 , in  FIG. 3 , a first portion  331  of the fuel turn gas is output from the indirect reforming unit  329  for use directly in the REP  308  without further intervening processing other than mixing with anode exhaust gas from the fuel cell. The remaining fuel turn gas is then fed to the anode  304  of the fuel cell  302  for reaction. The anode  304  then outputs anode exhaust, which is fed to an anode gas oxidizer (“AGO”)  332  and the REP  308 . The AGO  332  further receives air from an air supply  334 , which is compressed and pumped into the AGO  332  through a compressor and/or blower  336 . The anode exhaust is oxidized in the AGO  332  with the air from the air supply  334 , and the AGO  332  outputs an oxidized feed gas. Additional air may then be mixed with the oxidized feed gas downstream from the AGO  332  and upstream from the cathode  306  in order to provide a desired ratio of oxidized feed gas and oxygen for reaction in the cathode  306 . Because fuel turn gas output from the fuel turn manifold  330  has not been reformed to the level needed to provide proper heat balance to the REP  308 , a portion of the anode exhaust is mixed with this fuel turn gas. Anode exhaust may be added as required to balance heat in the REP  308  and provide a desired thermal profile in the REP  308 . 
     According to an exemplary embodiment, a first portion  350  of the oxidized feed gas is fed to the REP cathode  312  for sweeping the REP  308  cathode and diluting the CO 2  and O 2  from the REP  308 . It should also be noted that oxidized feed gas may be used for sweeping the REP  108 ,  208 , discussed above. Referring still to  FIG. 3 , the REP cathode  312  then outputs an REP cathode exhaust, which is mixed with a remaining second portion  351  of the oxidized feed gas and mixed with additional air from the air supply  334 , as discussed above. The mixture of air, oxidized feed gas, and REP cathode exhaust are then fed to the cathode  306 , where they are reacted and the cathode  306  outputs a cathode exhaust. The CO 2  and O 2  from the REP  308  increases the voltage across the fuel cell  302  and power output from the fuel-cell  302 . The cathode exhaust is then passed through the second heat exchanger  328 , where heat is transferred from the cathode exhaust to the reformed feed gas, as discussed above. The cathode exhaust is then passed through the first heat exchanger  322 , where heat is transferred from the cathode exhaust to the feed gas, as discussed above. After passing through the first and second heat exchangers  322 ,  328 , the cathode exhaust may be output from the system  300  or used in other portions of the system  300  for heat transfer or as a fuel source (e.g., in other fuel cells). 
     According to an exemplary embodiment, fuel turn gas is fed to the REP  308 . Specifically, a second portion  321  of the water from the WTS  318  is mixed with the first portion  331  of the fuel turn gas, forming a hydrated feed gas with higher water content. The hydrated feed gas is then fed directly to the REP anode  310  for reaction in the REP  308 . The REP anode  310  outputs REP anode exhaust, which includes hydrogen. As shown in  FIG. 3 , a portion of the anode exhaust may also be mixed with the hydrated feed gas for introduction to the REP anode  310  and the remaining portion of the anode exhaust is fed to the AGO  332 . 
     The REP anode exhaust is then fed through a third heat exchanger  342 . Heat is transferred in the third heat exchanger  342  from the REP anode exhaust to the feed gas, increasing the temperature of the second portion  321  of the steam. For example, the heat may evaporate any liquid water present in the second portion  321  or may superheat the steam. The REP anode exhaust is then fed to a third reformer  344 , in which the REP anode exhaust is reformed (e.g., partially reformed), to further produce an output stream including hydrogen. The output stream is then fed through a fourth heat exchanger  346 . Heat is transferred in the fourth heat exchanger  346  from the REP anode exhaust to the second portion  321  of the steam, further increasing the temperature of the steam. As shown in  FIG. 3 , the second portion  321  of the steam is preheated in the fourth heat exchanger  346  and then the third heat exchanger  342  before being mixed with the first portion  331  of the fuel turn gas to form the hydrated feed gas. 
     At least a portion of the output stream may be exported from the system  300  for storage or other use, or may be used in the system  300  for other purposes. Further, as discussed above, a portion of the outlet stream may be mixed with the hydrated fuel prior to feeding the hydrated fuel to the first reformer  324 . 
     According to certain embodiments described in this application, a portion of partially reformed fuel is taken from the fuel turn manifold  230 ,  330  and sent to the REP. This results in certain benefits compared to (a) sending partially reformed fuel from an external indirect reformer; or (b) sending partially reformed/partially spent fuel from anode exhaust. 
     In the case of (a), an external indirect reformer requires heat to drive the reforming process. However, the indirect internal reformer used in certain embodiments of this disclosure utilizes waste heat of adjacent fuel cell packages (e.g., adjacent to the anode) to drive the reforming process. It is more efficient to reform within the indirect internal reformer than the external indirect reformer. Of course, the external indirect reformer is still useful as a pre-reformer, as can be seen in  FIG. 1-3 . 
     In the case of (b), the anode exhaust has a lower amount of hydrogen relative to the carbon in the gas when compared to fuel turn gas, which has not been electrochemically reacted within the fuel cell and requires a relatively higher power input to the REP per unit (e.g., kg) of hydrogen produced. 
     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, values of parameters, mounting arrangements, use of materials, orientations, manufacturing processes, 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.