Patent Publication Number: US-11642646-B2

Title: Hydrogen generation systems

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/821,605, now U.S. Pat. No. 10,894,244, filed Mar. 17, 2020, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Hydrogen generation reactions convert hydrocarbons, such as methane, into hydrogen gas. Hydrogen gas can be used, e.g., as fuel for vehicles. 
     SUMMARY 
     We describe here systems for energy-efficient, low-emission production of hydrogen gas (H2) from hydrocarbons. The systems include a steam methane reactor (SMR) having a bayonet flow path in which incoming reactant fluid flowing along the flow path is heated by transfer of recovered heat from outgoing fluid flowing along the flow path. Catalytic foam and heat transfer foam disposed along the bayonet flow path catalyze a hydrogen generation reaction in the SMR and facilitate heat transfer to the incoming reactant fluid. Product fluid from the SMR is provided to a water gas shift (WGS) reactor. The fluid flows across one or more WGS catalysts and one or more heat transfer materials disposed along a reaction channel in the WGS reactor. The WGS catalysts and heat transfer material catalyze a hydrogen generation reaction in the WGS and facilitate removal of heat generated by the exothermic WGS hydrogen generation reaction. Cooling fluid heated by heat from the WGS hydrogen generation reaction can be provided as input into the SMR. The use of heat transfer among fluid streams in the SMR enables energy efficient production of hydrogen to be achieved. 
     In a general aspect, a system for production of hydrogen includes a steam methane reformer (SMR) including an outer tube, wherein a first end of the outer tube is closed; and an inner tube disposed in the outer tube, wherein a first end of the inner tube is open. An SMR flow channel is defined within the inner tube and an annular space is defined between the outer tube and the inner tube. The flow channel is in fluid communication with the annular space. The SMR includes a foam disposed in the annular space between the outer tube and the inner tube. The system includes a water gas shift (WGS) reactor including a reaction tube, wherein a WGS reaction channel is defined within the reaction tube, and wherein the WGS reaction channel is in fluid communication with the SMR flow channel; a heat transfer material disposed in the WGS reaction channel; and a WGS catalyst disposed in the WGS reaction channel. 
     Embodiments can include one or any combination of two or more of the following features. 
     An outlet of the SMR flow channel is in fluid communication with an inlet of the WGS reaction channel. 
     The WGS reactor includes a housing, the reaction tube of the WGS reactor being disposed in the housing, and wherein a cooling fluid channel is defined between the housing and the reaction tube of the WGS reactor. An outlet of the cooling fluid channel is in fluid communication with an inlet of the annular space of the SMR. An inlet of the WGS reaction channel and an outlet of the cooling fluid channel are disposed at a first end of the WGS reactor. The WGS reactor includes a flow controller configured to control a flow rate of cooling fluid through the cooling fluid channel. 
     The foam of the SMR includes an SMR catalyst. The SMR catalyst is disposed on the foam of the SMR. The SMR catalyst is configured to catalyze an SMR hydrogen generation reaction in which hydrogen and carbon monoxide are produced. The SMR includes an outer heat exchange foam disposed in the annular space between the outer tube and the inner tube, wherein a distance between the outer heat exchange foam and a second end of the outer tube is less than a distance between the foam assembly and the second end of the outer tube. 
     The SMR includes an inner heat exchange foam disposed in the SMR flow channel. 
     A bayonet flow path through the SMR is defined from an inlet at a second end of the outer tube, along the annular space between the outer tube and the inner tube toward the first end of the outer tube, along the SMR flow channel, and to an outlet at a second end of the inner tube. 
     The WGS catalyst includes a foam including a WGS catalyst material. The WGS catalyst material includes a foam substrate, wherein the WGS catalyst material is disposed on the foam substrate. 
     The WGS catalyst includes: a first WGS catalyst disposed in the WGS reaction channel and configured to catalyze a hydrogen generation reaction in a first temperature range; and a second WGS catalyst disposed in the WGS reaction channel and configured to catalyze the hydrogen generation reaction in a second temperature range lower than the first temperature range. The heat transfer material is disposed in the WGS reaction channel between the first WGS catalyst and the second WGS catalyst. 
     The heat transfer material disposed in the WGS reaction channel includes a foam. 
     The system includes a furnace, wherein a portion of the SMR is disposed in the furnace. The first end of the outer tube of the SMR is disposed in the furnace. The system includes an external heat transfer material disposed on an outer surface of the outer tube of the SMR. 
     In a general aspect, combinable with the previous aspect, a method for producing hydrogen includes flowing a first gas along a bayonet flow path of a steam methane reformer (SMR) to produce a first product, including flowing the first gas through a foam disposed along the bayonet flow path; providing the first product produced in the SMR to an input of a water gas shift (WGS) reaction channel defined within a reaction tube of a WGS reactor; and flowing a second gas including the first product through the WGS reaction channel to produce a second product. Flowing the second gas includes flowing the second gas across a heat transfer material disposed in the WGS reaction channel to reduce the temperature of the flowing second gas; and flowing the second gas across a WGS catalyst disposed in the reaction channel. 
     Embodiments can include one or any combination of two or more of the following features. 
     Flowing the first gas along the bayonet flow path of the SMR includes flowing the first gas from an annular space into an SMR flow channel, wherein the annular space is defined between an outer tube and an inner tube disposed within the outer tube and the SMR flow channel is defined within the inner tube. Flowing the first gas along the bayonet flow path of the SMR includes flowing the first gas from an inlet at a second end of the outer tube, along the annular space toward a first end of the outer tube, along the SMR flow channel defined within the inner tube, and to an outlet at a second end of the inner tube. The method includes heating the first gas flowing along the annular space with heat from the gas flowing along the flow channel defined within the inner tube. 
     Flowing the first gas through a foam disposed along the bayonet flow path includes flowing the gas through a catalytic foam. 
     The method includes flowing a cooling fluid through a cooling fluid flow pathway defined between a housing of the WGS reactor and the reaction tube of the WGS reactor. Contacting the flowing second gas to the heat transfer material disposed in the WGS reaction channel includes transferring heat from the flowing second gas to the cooling fluid. The method includes heating the cooling fluid to a temperature of between 100° C. and 300° C. The method includes providing heated cooling fluid from the cooling fluid flow pathway to an input of the bayonet flow path of the SMR. The method includes providing heated cooling fluid from the cooling fluid flow pathway to an input of the WGS reaction channel. The method includes adjusting a flow rate of the cooling fluid through the cooling fluid flow pathway based on a rate at which the first product is provided to the input of the WGS reaction channel. 
     The method includes providing the first product to the input of the WGS reaction channel at a temperature equal to or greater than a temperature at which the WGS catalyst structure catalyzes a hydrogen generation reaction. The method includes providing the first product to the input of the WGS reaction channel at a temperature of between 200° C. and 450° C. 
     Flowing the second gas across the WGS catalyst includes: flowing the second gas across a first WGS catalyst disposed in the WGS reaction channel, wherein the first WGS catalyst is configured to catalyze a hydrogen generation reaction in a first temperature range; and flowing the second gas across a second WGS catalyst disposed in the reaction channel, wherein the second WGS catalyst is configured to catalyze the hydrogen generation reaction in a second temperature range lower than the first temperature range. The method includes flowing the second gas across the heat transfer material after flowing the second gas across the first WGS catalyst. 
     Flowing the second gas to the heat transfer material includes reducing the temperature of the flowing second gas to a temperature at which the WGS catalyst is capable of catalyzing a hydrogen generation reaction. 
     The method includes flowing the first gas along the bayonet flow path of the SMR to produce carbon monoxide and hydrogen. Providing the first product to the input of the WGS reaction channel includes providing carbon monoxide to the input of the WGS reaction channel. 
     The method includes flowing the second gas along the WGS reaction channel to produce carbon dioxide and hydrogen. 
     In a general aspect, combinable with any of the previous aspects, a steam methane reformer (SMR) system includes an outer tube, wherein a first end of the outer tube is closed; an inner tube disposed in the outer tube, wherein a first end of the inner tube is open. A flow channel is defined within the inner tube and an annular space is defined between the outer tube and the inner tube, the flow channel being in fluid communication with the annular space. The SMR system includes a catalytic foam disposed in the annular space between the outer tube and the inner tube, the catalytic foam including a catalyst. 
     Embodiments can include one or any combination of two or more of the following features. 
     The catalytic foam includes a foam substrate, wherein the catalyst is disposed on the foam substrate. 
     The SMR system includes an outer heat exchange foam disposed in the annular space between the outer tube and the inner tube. A distance between the outer heat exchange foam and a second end of the outer tube is less than a distance between the catalytic foam and the second end of the outer tube. The e outer heat exchange foam has an annular shape. 
     The catalytic foam has an annular shape. 
     The SMR system includes an inner heat exchange foam disposed in the flow channel. 
     The catalytic foam contacts the inner tube. 
     A thickness of the catalytic foam is equal to a width of the annular space. 
     The catalytic foam has a porosity of between 10 pores per inch (ppi) and 30 ppi. 
     A length of the catalytic foam along the inner tube is between 10 inches and 5 feet. 
     A length of the catalytic foam in an externally heated section of the outer tube is between 10% and 30% of a length of the outer tube. 
     The catalytic foam includes a metal foam. The catalytic foam includes nickel. 
     The catalytic foam includes silicon carbide. 
     A bayonet flow path through the SMR system is defined from an inlet at a second end of the outer tube, along the annular space between the outer tube and the inner tube toward the first end of the outer tube, along the flow channel, and to an outlet at a second end of the inner tube. 
     A ratio between a cross-sectional area of the flow channel and a cross-sectional area of the annular space is between 1 and 5. 
     The inner tube is coaxial with the outer tube. 
     A width of the annular space between the outer tube and the inner tube is between 0.2 inches and 4 inches. 
     A length of the outer tube is between 8 feet and 30 feet. 
     The SMR system includes an elongated baffle disposed in the flow channel. 
     The SMR system includes a heat transfer material disposed on an outer surface of the first end of the outer tube. The heat transfer material includes a fin disposed on the outer surface of the first end of the outer tube. The heat transfer material includes a baffle disposed on the outer surface of the first end of the outer tube. The heat transfer material includes a foam disposed on the outer surface of the first end of the outer tube. 
     In a general aspect, combinable with any of the previous aspects, a method for producing hydrogen in a steam methane reformer (SMR) system includes flowing a gas along a bayonet flow path of the SMR system. The bayonet flow path is defined by an annular space defined between an outer tube and an inner tube disposed in the outer tube, wherein a first end of the outer tube is closed and a first end of the inner tube is open; a flow channel defined within the inner tube, wherein the flow channel is in fluid communication with the annular space. Flowing the gas along the bayonet flow path includes flowing the gas through a catalytic foam disposed in the annular space between the outer tube and the inner tube. 
     Embodiments can include one or any combination of two or more of the following features. 
     Flowing the gas along the bayonet flow path includes flowing the gas through an outer heat exchange foam disposed in the annular space between the outer tube and the inner tube. 
     The method includes flowing the gas through the outer heat exchange foam before flowing the gas through the catalytic foam. 
     Flowing the gas along the bayonet flow path includes flowing the gas through an inner heat exchange foam disposed in the flow channel. 
     Flowing the gas along the bayonet flow path includes flowing the gas from the annular space into the flow channel. The method includes flowing the gas from the annular space at the first end of the outer tube into the flow channel at the first end of the inner tube. 
     The method includes heating the gas flowing in the annular space with heat from the gas flowing in the flow channel defined within the inner tube. 
     The method includes heating the gas in the annular space at the first end of the outer tube. 
     The method includes flowing the gas along at least a portion of the bayonet flow path in turbulent flow. 
     The method includes producing hydrogen from the gas flowing along the bayonet flow path. 
     In an aspect, combinable with any of the previous aspects, a water gas shift (WGS) reactor system includes a housing; a reaction tube disposed in the housing, wherein a reaction channel is defined within the reaction tube and a cooling fluid channel is defined between the housing and the reaction tube; a catalyst disposed in the reaction channel, the catalyst configured to catalyze a hydrogen generation reaction; and a heat transfer material disposed in the reaction channel. 
     Embodiments can include one or any combination of two or more of the following features. 
     The catalyst includes a first catalyst disposed in the reaction channel and configured to catalyze the hydrogen generation reaction in a first temperature range; and a second catalyst disposed in the reaction channel and configured to catalyze the hydrogen generation reaction in a second temperature range lower than the first temperature range. The heat transfer material is disposed in the reaction channel between the first catalyst and the second catalyst. The first catalyst is configured to catalyze the hydrogen generation reaction at a temperature of between 200° C. and 450° C. The second catalyst is configured to catalyze the hydrogen generation reaction at a temperature of between 180° C. and 350° C. 
     A distance between the heat transfer material and an inlet of the reaction channel is less than a distance between the catalyst structure and the inlet of the reaction channel. The catalyst includes a catalyst configured to catalyze the hydrogen generation reaction at a temperature of between 200° C. and 450° C. 
     The catalyst includes a foam including a catalyst material. The catalytic foam includes a foam substrate, wherein the catalyst material is disposed on the foam substrate. The foam has a porosity of between 5 pores per inch (ppi) and 30 ppi. 
     The catalyst includes catalyst pellets. 
     The heat transfer material includes a foam. The foam has a porosity of between 5 ppi and 30 ppi. 
     The heat transfer material includes a fin. 
     The WGS reactor system includes a cooling channel heat transfer material disposed in the cooling fluid channel. The cooling channel heat transfer material includes a foam. 
     The housing includes a cylindrical housing, and wherein the reaction tube is coaxial with the cylindrical housing. 
     The WGS reactor system includes an inner tube disposed in the reaction tube, wherein the reaction channel is defined by an annular space between the reaction tube and the inner tube, and wherein an inner cooling fluid channel is defined within the inner tube. 
     The WGS reactor system includes multiple reaction tubes disposed in the housing. 
     An inlet of the reaction channel and an outlet of the cooling fluid channel are disposed at a first end of the WGS reactor. 
     An inlet of the reaction channel is in fluid communication with an outlet of the cooling fluid channel. 
     An outlet of the cooling fluid channel is configured to be in fluid communication with an inlet of a steam methane reformer (SMR). 
     The WGS reactor system includes a flow controller configured to control a flow rate of cooling fluid through the cooling fluid channel. 
     In a general aspect, a method for producing hydrogen in a water gas shift (WGS) reactor includes flowing a cooling fluid through a cooling fluid channel defined between a housing of a WGS reactor and a reaction tube disposed in the housing; and flowing a gas including carbon monoxide and steam through a reaction channel defined within the reaction tube. Flowing the gas through the reaction channel includes flowing the gas across a heat transfer material disposed in the reaction channel to transfer heat from the flowing gas to the cooling fluid in the cooling fluid channel; and flowing the gas across a catalyst disposed in the reaction channel, the catalyst configured to catalyze a hydrogen generation reaction. 
     Embodiments can include one or any combination of two or more of the following features. 
     Flowing the gas across the heat transfer material includes reducing the temperature of the flowing gas to a temperature at which the catalyst structure catalyzes the hydrogen generation reaction. The method includes reducing the temperature of the flowing gas to between 200° C. and 450° C. 
     Flowing the gas across the catalyst includes flowing the gas across a first catalyst disposed in the reaction channel, wherein the first catalyst is configured to catalyze the hydrogen generation reaction in a first temperature range; and flowing the gas across a second catalyst disposed in the reaction channel, wherein the second catalyst is configured to catalyze the hydrogen generation reaction in a second temperature range lower than the first temperature range. The method includes receiving the gas into the reaction channel at a temperature within the first temperature range. The method includes receiving the gas into the reaction channel at a temperature of between 200° C. and 450° C. The method includes flowing the gas across the heat transfer material after flowing the gas across the first catalyst. Flowing the gas across the heat transfer material includes reducing the temperature of the flowing gas to within the second temperature range. The method includes reducing the temperature of the flowing gas to between 180° C. and 350° C. 
     The method includes flowing cooling fluid through an inner cooling fluid channel defined within an inner tube disposed in the reaction tube. 
     Flowing the gas through the reaction channel includes flowing the gas from a first end of the WGS reactor to a second end of the WGS reactor; and wherein flowing the cooling fluid through the cooling fluid channel includes flowing the cooling fluid from the second end of the WGS reactor to the first end of the WGS reactor. 
     The method includes adjusting a flow rate of the cooling fluid through the cooling fluid channel based on a flow rate of the gas through the reaction channel. 
     The method includes outputting the cooling fluid from the cooling fluid channel at a temperature of between 100° C. and 300° C. 
     The method includes providing steam from the cooling fluid channel to an input of the reaction channel. 
     The method includes providing steam from the cooling fluid channel to an input of a steam methane reformer. 
     The approaches described here can have one or more of the following advantages. The use of recuperated heat to heat and cool fluid streams to target temperatures enables the hydrogen generation process to be an energy efficient, low-emission process. The systems can be modular, e.g., enabling a target throughput to be achieved by change in system configuration or operation. The systems can be scalable for large-scale, energy efficient hydrogen generation. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram of a hydrogen generation system. 
         FIG.  2 A  is a cross sectional view of a steam methane reformer (SMR). 
         FIG.  2 B  is a cross-sectional view of the SMR of  FIG.  2 A  along the line A-A′. 
         FIG.  2 C  is a cross-sectional view of the SMR of  FIG.  2 A  along the line B-B′. 
         FIG.  3    is a diagram of a water gas shift (WGS) reactor. 
         FIG.  4    is a diagram of a WGS reactor. 
         FIG.  5    is a diagram of a WGS reactor. 
         FIG.  6    is a diagram of a hydrogen generation system. 
         FIG.  7    is a process flow chart. 
         FIG.  8    is a plot of the temperature differential between the inner and outer tubes of an SMR. 
         FIGS.  9 A and  9 B  are simulations of heat transfer in an SMR with and without a foam, respectively. 
     
    
    
     DETAILED DESCRIPTION 
     We describe here systems for energy-efficient, low-emission production of hydrogen gas (H2) from hydrocarbons. The systems include a steam methane reactor (SMR) having a bayonet flow path in which incoming reactant fluid flowing along the flow path is heated by transfer of recovered heat from outgoing fluid flowing along the flow path. Catalytic foam and heat transfer foam disposed along the bayonet flow path catalyze a hydrogen generation reaction in the SMR and facilitate heat transfer to the incoming reactant fluid. Product fluid from the SMR is provided to a water gas shift (WGS) reactor. The fluid flows across one or more WGS catalysts and one or more heat transfer materials disposed along a reaction channel in the WGS reactor. The WGS catalysts and heat transfer material catalyze a hydrogen generation reaction in the WGS and facilitate removal of heat generated by the exothermic WGS hydrogen generation reaction. Cooling fluid heated by heat from the WGS hydrogen generation reaction can be provided as input into the SMR. The use of heat transfer among fluid streams in the SMR enables energy efficient production of hydrogen to be achieved. 
     The hydrogen generation systems described here are modular and have a small footprint. The systems can be upgraded or turned down without significant downtime. Elements of the systems, such as tubes, manifolds, flanges, and catalysts, can be taken apart or replaced easily, enabling maintenance or operational adjustments with low downtime. 
     Referring to  FIG.  1   , a schematic diagram of a hydrogen generation system  100 , shown in an operational configuration, includes a steam methane reactor (SMR)  200  and a water gas shift (WGS) reactor  300  that together generate hydrogen gas (H2) from hydrocarbons, such as natural gas, biogas, methane, methanol, or other suitable hydrocarbons. Fluid  102 , including hydrocarbons and water vapor (steam) is input into the SMR and reacted in the presence of a catalytic foam. Recuperated heat from fluid flowing through the SMR  200  and externally applied heat raise the temperature of the reactants flowing through the SMR  200  to a temperature at which the SMR hydrogen generation reaction occurs. Heating of the reactants using residual heat from fluid flowing through the SMR reduces the heating load on an external heat source, thereby enabling energy efficient operation. Product gas  104  generated in the SMR includes hydrogen gas and carbon monoxide. 
     At least a portion of the product gas  104  (e.g., hydrogen and carbon monoxide), along with steam, is provided as fluid input into the WGS reactor  300 . For energy efficient operation, the product gas  104  is output from the SMR at a temperature appropriate for input into the WGS reactor  300 , thereby enabling active heating or cooling of the fluid input into the WGS reactor  300  to be avoided. The fluid input into the WGS reactor  300  flows along a reaction channel of the WGS reactor  300  and reacts in the presence of a WGS catalyst, such as a catalytic foam, to produce hydrogen and carbon dioxide. A heat exchange material, such as a foam, is disposed in the reaction channel and transfers excess heat generated by the exothermic WGS hydrogen generation reaction to a cooling fluid  108  flowing through the WGS reactor  300 . Cooling of the fluid in the reaction channel by heat transfer facilitated by the heat exchange material allows active cooling in the WGS reactor  300  to be avoided, enabling energy efficient operation. Product gas  110  generated in the WGS includes hydrogen gas and carbon dioxide. Heated cooling fluid  106 , in the form of steam, can be provided as part of the fluid  102  input into the SMR  200 . In some examples, additional steam is provided from an external water source, e.g., for system startup. 
     Referring to  FIGS.  2 A- 2 C , the SMR  200  includes two concentric tubes, an outer tube  202  and an inner tube  204  disposed coaxially in the outer tube  202 . A first end of the outer tube  202  at a first end  206  of the SMR  200  is closed and a first end of the inner tube  204  is open. An annular space  210  is defined between the outer tube  202  and the inner tube  204 . A flow channel  212  is defined within the inner tube  204  and is in fluid communication with the annular space  210 . An elongated baffle  213  is disposed along at least a portion of the length of the inner tube  204 . 
     Fluid (e.g., gas) flowing through the SMR  200  follows a bayonet flow path (indicated by arrows in  FIG.  2 A ) through the SMR  200  from an inlet  214  into the annular space  210  at a second end  216  of the SMR  200 , along the annular space  210  toward the first end of the outer tube  202  at the first end  206  of the SMR, into the flow channel  212 , along the flow channel  212  toward a second end of the inner tube  204  at the second end  216  of the SMR, and to an outlet  220  at the second end of the inner tube  204 . Reactants (e.g., hydrocarbons and water) are input into the bayonet flow path at the inlet  214 . A hydrogen generation reaction occurs toward the first end  206  of the outer tube in the presence of an SMR catalyst, generating products (e.g., hydrogen gas and carbon monoxide) that are output from the SMR  200  via the outlet  220 . An example hydrogen generation reaction that occurs in the SMR  200  is represented as follows:
 
CH 4 +H 2 O→3H 2 +CO.
 
     The hydrogen generation reaction is an endothermic reaction that occurs above a reaction temperature, such as between 600° C. and 1000° C. An external heat source  222  heats the fluid flowing along the annular space  210  at the first end  206  of the SMR  200  to at least the reaction temperature. The external heat source  222  can be driven by combustion (e.g., a gas-powered furnace), solar energy, or another appropriate energy source. 
     Fluid in the annular space  210  at the first end  206  of the SMR  200  is heated by the external heat source  222 . The heated fluid flows from the annular space  210  at the first end  206  of the SMR  200  into the flow channel  212 , entering the flow channel at high temperature. The bayonet flow path of the SMR, in which the outer and inner tubes  202 ,  204  (and hence the annular space  210  and flow channel  212 ) are concentric, provides a configuration in which heat from the high-temperature fluid flowing along the flow channel  212  can be transferred back to the lower-temperature fluid flowing along the annular space  210 . The inner tube  204  is designed to facilitate this heat transfer, e.g., the inner tube  204  can be formed of a material with high thermal conductivity, such as a metal or silicon carbide, and can have thin walls. The use of recuperated heat to raise the temperature of the fluid flowing along the annular space  210  lessens the load on the external heat source  222 , improving the energy efficiency of the hydrogen generation reaction. In addition, when the external heat source  222  is a combustion furnace, a reduced load on the furnace reduces hydrocarbon consumption of the external heat source  222 , thereby reducing emissions associated with the hydrogen generation reaction. 
     Referring specifically to  FIGS.  2 A and  2 B , a catalytic foam  230  is disposed in the annular space  210  between the outer tube  202  and the inner tube  204 . The catalytic foam  230  includes an SMR catalyst that catalyzes the hydrogen generation reaction (e.g., the generation of hydrogen and carbon monoxide from hydrocarbons and water). The hydrogen generation reaction occurs primarily in the portions of the bayonet flow path in which the catalytic foam  230  is disposed, and that are at a temperature at or above the reaction temperature. For instance, the hydrogen generation reaction occurs in portions of the bayonet flow path that are heated by the external heat source  222 , such as in a heated portion  221  of the annular space  210  toward the first end  206  of the SMR  200  and in an end space  223  at the first end  206  of the SMR  200 . The hydrogen generation reaction can also occur in regions outside the heated portion  221 , e.g., regions that are heated to or above the reaction temperature by heat transfer from fluid flowing along the flow channel  212  (discussed further below). 
     In some examples, the SMR catalyst is coated onto a foam substrate to form the catalytic foam  230 . In some examples, the SMR catalyst is integrated or impregnated into a foam substrate to form the catalytic foam  230 . The catalytic foam  230  is a porous structure through which one or more fluid flow paths are defined from an upstream side  232  to a downstream side  234  of the catalytic foam  230 . As fluid flows along the bayonet flow path through the SMR  200 , the fluid flows through the fluid flow paths of the catalytic foam  230  and the catalyst in the catalytic foam  230  catalyzes the hydrogen generation reaction in the flowing fluid. The porosity of the catalytic foam  230  provides a high surface area for contact between the catalytic foam  230  and the flowing fluid, which facilitates efficient catalysis of the hydrogen generation reaction. 
     The catalytic foam  230  includes a thermally conductive material such that the catalytic foam  230  also facilitates heat transfer to the fluid flowing through the catalytic foam  230  from the external heat source  222 , the fluid flowing along the flow channel  212  in the inner tube  202 , or both. Physical contact between the catalytic foam  230  and the outer tube  202  enables transfer of heat from the external heat source  222  to the fluid flowing through the catalytic foam. Physical contact between the catalytic foam  230  and the inner tube  204  enables transfer of heat from the fluid flowing along the flow channel  212  to the fluid flowing through the catalytic foam. The high surface area of the catalytic foam  230  facilitates heat transfer. The porosity of the catalytic foam  230  also can lead to turbulent fluid flow in at least a portion of the annular space  210 , further facilitating heat transfer to the fluid flowing along the annular space  210  and enhancing the energy efficiency of the hydrogen generation process. 
     The catalytic foam  230  has an annular shape. As shown in  FIGS.  2 A and  2 B , a thickness t c  of the annulus of the catalytic foam  230  (referred to simply as the thickness of the catalytic foam  230 ) is equal to the radial distance between the outer wall of the inner tube  204  and the inner wall of the outer tube  202  (referred to as the width of the annular space  210 ) such that the catalytic foam  230  is in physical contact with both the outer tube  202  and the inner tube  204 . Contact between the catalytic foam  230  and the outer and inner tubes  202 ,  204  enables heat transfer from the external heat source  222  and the fluid flowing along the flow channel  212  in the inner tube  202  to the fluid flowing through the catalytic foam  230 . In some examples, the thickness t c  of the catalytic foam  230  is less than the width of the annular space  210  and the catalytic foam  230  is in physical contact with only one of the tubes, such as with only the outer tube  204  or only the inner tube  204 . 
     The porosity of the catalytic foam  230  (e.g., pores per inch) and length of the catalytic foam  230  (referring to the length of the catalytic foam along the axis of the outer tube  202  from the upstream side  232  to the downstream side  234  of the catalytic foam  230 ) affect the surface area of the catalytic foam  230 , thus affecting the efficiency of catalysis and heat transfer. Increased porosity and length both increase the opportunity for contact between the flowing fluid and the catalytic foam  230 , thereby enhancing the efficiency of both catalysis and heat transfer. The length of the catalytic foam  230  also affect the drop in fluid pressure that occurs across the catalytic foam  230  as fluid flows through the catalytic foam  230 . Increased porosity and length both cause increased pressure drop across the catalytic foam  230 , which can slow fluid flow along the bayonet flow path, reducing throughput of the SMR  200 . The porosity and length of the catalytic foam  230  can be selected to achieve efficient catalysis and heat transfer with low pressure drop across the catalytic foam  230 . For instance, the catalytic foam  230  can have a porosity of between 10 pores per inch (ppi) and 30 ppi. In some examples, the catalytic foam  230  is entirely within the heated portion  221  of the outer tube  202  (as in the example of  FIG.  2 A ), e.g., the length l c  of the catalytic foam  230  is between 10% and 30% of the length of the heated portion  221  of the outer tube  202 , such as between 10 inches and 5 feet in length. In some examples, the catalytic foam  230  extends beyond the heated portion  221  of the outer tube  202  and can extend up to the entire length of the outer tube. In some examples, the porosity and length of the catalytic foam  230  can be selected such that a fluid pressure drop of less than 1 pound per square inch (psi) occurs across the catalytic foam  230 . 
     The catalytic foam  230  includes a material (e.g., the foam substrate) having a thermal conductivity sufficient to facilitate heat transfer to fluid flowing through the catalytic foam  230 , e.g., heat transfer from the fluid flowing along the flow channel  212 , heat transfer from the external heat source  222 , or both. The material of the catalytic foam  230  is non-reactive to the fluid (e.g., the reactants and products of the hydrogen generation reaction) flowing along the bayonet flow path of the SMR  200  in the temperature range at which the SMR  200  is operated. The material of the catalytic foam  230  can be thermally compatible with, e.g., have a similar thermal expansion coefficient as, the material of the outer tube  202 , the inner tube  204 , or both, e.g., to avoid delamination of the catalytic foam  230  from the tubes  202 ,  204 . For instance, the foam can be a metal foam, such as a nickel or stainless steel foam, or a silicon carbide foam; or another suitable material. 
     Referring to  FIG.  2 A , an outer heat exchange foam  250  formed of a thermally conductive material is disposed in the annular space  210  between the outer tube  202  and the inner tube  204 . A distance between the outer heat exchange foam  250  and the inlet  214  at the second end  216  of the outer tube  202  is less than a distance between the catalytic foam  230  and the inlet  214 , such that fluid flowing along the annular space  210  flows through the outer heat exchange foam  250  prior to flowing through the catalytic foam  230 . The outer heat exchange foam  250  is in physical contact with the inner tube  204  and facilitates heat transfer from the fluid flowing along the flow channel  212  to the fluid flowing through the outer heat exchange foam  250 . 
     Referring also to  FIG.  2 C , an inner heat exchange foam  252  formed of a thermally conductive material is disposed in the flow channel  212  defined within the inner tube  204 . The inner heat exchange foam  252  is in physical contact with the inner tube  204  and facilitates heat transfer from fluid flowing through the inner heat exchange foam  252  to the fluid flowing along the annular space  210 . The porosity of the outer and inner heat exchange foams  250 ,  252  provide a high surface area for contact between the foams  250 ,  252  and the fluid flowing through the respective foam, which facilitates efficient heat transfer. The porosity of the outer and inner heat exchange foams  250 ,  252  also can lead to turbulent fluid flow in at least a portion of the annular space  210  or the flow channel  212 , respectively, further facilitating heat transfer. In some examples, a catalytic foam can be disposed in the flow channel  212 , e.g., in addition to or instead of the inner heat exchange foam  252 . 
     The heat transfer enabled by the outer and inner heat exchange foams  250 ,  252  enables the fluid in the annular space  210  to be preheated before the fluid reaches the catalytic foam  230 , using excess heat recovered from the higher temperature fluid flowing along the flow channel  212 . The use of recuperated heat to preheat the fluid flowing along the annular space  210  can reduce the amount of heat provided by the external heat source  222  to heat the fluid flowing along the annular space  210  to the reaction temperature, thereby enhancing the efficiency of the SMR  200 . 
     The outer heat exchange foam  250  has an annular shape. A thickness of the annulus of the outer heat exchange foam  250  (referred to as the thickness of the outer heat exchange foam  250 ) is equal to the width of the annular space  210  such that the outer heat exchange foam  250  is in physical contact with both the outer tube  202  and the inner tube  204 . In some examples, the thickness of the outer heat exchange foam  250  is less than the width of the annular space  210  and the outer heat exchange foam  250  is in physical contact with only one of the tubes, such as with only the inner tube  204 . 
     The inner heat exchange foam  252  also has an annular shape. A thickness t i  of the annulus of the inner heat exchange foam  252  is equal to the radial distance between the inner tube  204  and the elongated baffle  213  such that the inner heat exchange foam  252  is in physical contact with the inner tube  204 . In some examples, the thickness t i  of the inner heat exchange foam  252  is less than the radial distance and the inner heat exchange foam  252  is in physical contact with the inner tube  204  but not with the elongated baffle  213 . In some examples, the elongated baffle  213  is not present, and the inner heat exchange foam  252  is annular or cylindrical, with a thickness that is equal to or less than the radius of the flow channel  212 . 
     The porosity and length of each of the outer heat exchange foam  250  and the inner heat exchange foam  252  can be selected to achieve efficient heat transfer with low pressure drop across the respective heat exchange foam  250 ,  252 . For instance, each of the heat exchange foams  250 ,  252  can have a porosity of between 10 pores per inch (ppi) and 30 ppi. The length of the outer heat exchange foam  250  can be as small as, e.g., 4 inches, and as long as the distance between the inlet  212  and the upstream side  232  of the catalytic foam  230 . The length of the inner heat exchange foam  252  can be as small as, e.g., 4 inches, and as long as the distance between the first end  208  of the inner tube  204  and the outlet  220  at the second end  218  of the inner tube  204 . In some examples, the porosity and length of the outer and inner heat exchange foams  250 ,  252  can be selected such that a pressure drop of less than 1 pound per square inch (psi) occurs across the each of the outer and inner heat exchange foams  250 ,  252 . In some examples, the outer heat exchange foam  250 , the inner heat exchange foam  252 , or both are not present. 
     The outer and inner heat exchange foams  250 ,  252  are formed of a material having a thermal conductivity sufficient to facilitate heat transfer to the fluid flowing along the annular space  210 . The material of the heat exchange foams  250 ,  252  is non-reactive to the fluid (e.g., the reactants and products of the hydrogen generation reaction) flowing along the bayonet flow path of the SMR  200  in the temperature range at which the SMR  200  is to be operated. The material of the outer and inner heat exchange foams  250 ,  252  can be thermally compatible with, e.g., have a similar thermal expansion coefficient as the inner tube  204 , e.g., to avoid delamination. For instance, the heat exchange foams  250 ,  252  can be metal foams, such as nickel or stainless steel foams; or silicon carbide foams; or another suitable material. 
     The presence of the catalytic foam  230  and the outer and inner heat exchange foams  250 ,  252  along the bayonet flow path enables both high throughput through the SMR  200  and energy efficient operation of the SMR  200 . For instance, heating the fluid flowing along the annular space  210  with recuperated heat from the higher temperature fluid flowing along the flow channel  212  enables the reaction temperature to be reached with less input of heat from the external heat source  222 , providing for energy efficient SMR operation. In addition, by heating the fluid flowing along the annular space  210  with recuperated heat, the annular space  210  can be made relatively wide, such as between 0.2 inches and 4 inches, which can accommodate relatively high volume gas flow. 
     Referring to  FIG.  2 A , a heat transfer material  258  is disposed on an outer surface of the first end  206  of the outer tube  202  to facilitate heat transfer from the external heat source  222  to the fluid flowing along the bayonet flow path of the SMR  200 . In the example of  FIG.  2 A , the heat transfer material  258  is a fin; in some examples, the heat transfer material  258  can be a baffle, a foam, or another structure suitable for facilitating heat transfer. The heat transfer material  258  enhances the efficiency of heat transfer from the external heat source  222  to the fluid flowing along the annular space  210 , contributing to the energy efficient operation of the SMR by increasing the amount of heat produced by the external heat source  222  that is used to heat the fluid in the bayonet flow path. 
     The locations, lengths, and properties (e.g., porosity, thermal conductivity) of the catalytic foam  230  and the inner and outer heat exchange foams  250 ,  252  can be selected to achieve a desired temperature at one or more points along the bayonet flow path. For instance, the foam locations, lengths, and properties can be selected to achieve a target temperature at the catalytic foam  230  to facilitate a high efficiency hydrogen generation reaction. In some examples, fluid output from the SMR is provided to a WGS reactor to act as a reactant in a further hydrogen generation reaction, and the foam locations and lengths can be selected to achieve a target temperature of fluid output from the outlet  220  of the flow channel  212 , such as a target temperature for input into the WGS reactor. By outputting fluid from the SMR  200  at the target temperature for input into the WGS reactor, the use of external heat sources for preheating WGS reactor inputs can be reduced or eliminated, enhancing the overall efficiency of the system. 
     The catalytic foam  230  and outer and inner heat transfer foams  250 ,  252  can be removed from the SMR  200  and replaced, e.g., with foams of different characteristics (e.g., different porosity, length, thermal conductivity, or other characteristics). For instance, exchanging one or more of the foams can help a desired performance to be achieved, such as a target throughput or a target temperature of the output fluid from the SMR  200 . 
     In some examples, the length of the outer tube  202  is between 8 feet and 30 feet, e.g., for a modular hydrogen generation system. In some examples, the outer tube  202  can be longer, e.g., for an industrial plant scale hydrogen generation system. The width of the annular space can be between 0.2 inches and 4 inches. The ratio between a cross-sectional area of the flow channel  212  and a cross-sectional area of the annular space  210  (see  FIG.  2 B ) is greater than one, e.g., between 1 and 5, to accommodate the increase in moles of gas resulting from the hydrogen generation reaction. 
     In some examples, the catalytic foam  230 , outer heat transfer foam  250 , inner heat transfer foam  252 , or a combination of any two or more of them is a non-uniform structure, e.g., having a non-uniform porosity or a multimaterial composition. For instance, in locations at which fluid pressure drop across a foam is less important, the foam can be configured with smaller pores to enhance heat transfer. The foam can be a multimaterial foam, e.g., a foam having an outer shell of nickel for chemical compatibility with an inner shell of aluminum or copper for heat transfer efficiency. In some examples, the outer heat transfer foam  250 , the inner heat transfer foam  252 , or both can be replaced by a solid, cylindrical tube. 
     Heat transfer in the SMR (e.g., heat transfer from fluid flowing along the flow channel to fluid flowing along the annular space  210 ) is related to the pressure of the flowing fluid. Increased fluid pressure generally results in increased heat transfer. The walls of the inner and outer tubes  202 ,  204  for an SMR operating at high pressure are thicker than the walls of the inner and outer tubes  202 ,  204  for an SMR operating at lower pressure. The increased wall thickness can reduce heat transfer. SMR components (e.g., wall thickness for the inner and outer tubes) and operating parameters (e.g., fluid pressure) can be designed to balance such competing factors. 
     In the example of  FIGS.  2 A- 2 C , the SMR  200  includes a single set of tubes that includes the outer tube  202  and the inner tube  204 . In some examples, an SMR includes multiple sets of tubes, each set having an outer tube and an inner tube. The multiple sets of tubes can be operated in parallel for increased throughput and can be heated by a single external heat source  222  sized to generate sufficient heat for the multiple sets of tubes. 
     The products of the hydrogen generation reaction in the SMR  200 , including hydrogen gas and carbon monoxide, along with excess steam, are output from the SMR  200  via the outlet  220 . The SMR output is provided as input to a water gas shift (WGS) reactor, where carbon monoxide and water (e.g., steam) are reacted in the presence of a WGS catalyst to generate hydrogen gas and carbon dioxide. 
     The output from the SMR  200  is at a temperature sufficient for input into the WGS reactor. The WGS reactor includes one or more WGS catalysts, each of which operates in a respective temperature range, and the SMR output is at a temperature at or above the temperature range of the WGS catalyst such that external, active heating of the SMR output does not occur prior to input into the WGS reactor. The temperature of the SMR output is controllable by adjustment of parameters that affect heat transfer between the fluid flowing along the flow channel  212  and the fluid flowing along the annular space  210  of the SMR, e.g., characteristics of the outer heat transfer foam  250 , the inner heat transfer foam  252 , diameters and materials of the outer and inner tubes  202 ,  204 , flow rate of fluid along the bayonet flow path, or other factors. 
     Referring to  FIG.  3   , an example WGS reactor  300  includes a housing  302  and a reaction tube  304  disposed in the housing  302 . A reaction channel  306  is defined within the reaction tube  304 . For instance, the housing  302  and the reaction tube  304  both can be cylindrical tubes, with the reaction tube  304  coaxial with the cylindrical housing  302 . In the example of  FIG.  3   , the reaction channel  306  is an annular space defined between the reaction tube  304  and an inner tube  308  disposed in the reaction tube  304 . In some examples, the reaction channel  306  is cylindrical and no inner tube is disposed in the reaction tube  304 . 
     Reactant fluid, such as the fluid output from the SMR, enters into an inlet  305  of the reaction channel  306  at a first end  310  of the WGS reactor  300  and flows along the reaction channel  306 . A hydrogen generation reaction occurs along the reaction channel  306  in the presence of a WGS catalyst that is disposed in the reaction channel  306 . The hydrogen generation reaction generates products (e.g., hydrogen gas and carbon dioxide) that are output from the reaction channel  306  via an outlet  307  at a second end  312  of the WGS reactor. For instance, the inlet  305  of the reaction channel  306  at the first end  310  of the WGS reactor  300  is in fluid communication with the outlet  220  of the SMR  200  (see  FIG.  2 A ), and fluid output from the SMR is provided into the reaction channel  306  of the WGS  300 . An example of the WGS hydrogen generation reaction is represented as follows:
 
CO+H 2 O→H 2 +CO 2 .
 
     The hydrogen generation reaction in the WGS  300  is an exothermic reaction. Heat generated by the hydrogen generation reaction in the WGS  300  is removed by cooling fluid, such as water, flowing along a cooling fluid channel  314  defined between the housing  302  of the WGS and the reaction tube  304 . Cooling fluid can also flow through an inner cooling fluid channel  316  defined within the inner tube  308 . The cooling fluid enters into an inlet of each the cooling fluid channel  314  and the inner cooling fluid channel  316  at the second end  312  of the WGS reactor  300 , and exits from an outlet of each the cooling fluid channel  314  and the inner cooling fluid channel  316  at the first end  310  of the WGS reactor. The direction of flow of the fluid in the reaction channel  308  is from the first end  310  to the second end  312  of the WGS reactor  300 ; the direction of flow of the cooling flow is the opposite, from the second end  312  to the first end  310  of the WGS reactor  300 . As the cooling fluid flows along the cooling fluid channel  314  and the inner cooling fluid channel  316 , the cooling fluid is heated with heat from the fluid flowing along the reaction channel  308 . In some examples, the cooling fluid is liquid water at the inlets and is heated such that the cooling fluid is steam, or a mixture of liquid water and steam, at the outlets. 
     A WGS catalyst and a heat transfer material are disposed in the reaction channel  306  of the WGS reactor  300 . The configuration of the WGS catalyst and the heat transfer material can be adjusted, e.g., to achieve a target throughput or hydrogen generation efficiency, to achieve operation in a target temperature range, or to achieve another goal. For instance, the position of the WGS catalyst and the heat transfer material along the reaction channel  306  can be adjusted. The structure and extent of the WGS catalyst and the heat transfer material can be adjusted. In the example of  FIG.  3   , the WGS reactor  300  is configured as a two-catalyst system with a heat transfer material  334  disposed between two WGS catalysts  330 ,  332 . In the example of  FIG.  4   , the WGS reactor  300  is configured as a one-catalyst system, with a heat transfer material  434  and a single WGS catalyst  430 . Other configurations of WGS catalysts and heat transfer materials are also possible. 
     In the two-catalyst configuration of the WGS reactor  300  shown in  FIG.  3   , a first WGS catalyst  330  and a second WGS catalyst  332  are disposed in the reaction channel  306 . The first WGS catalyst  330  catalyzes the WGS hydrogen generation reaction in a first temperature range, e.g., between 200° C. and 450° C. The first WGS catalyst  330  can be a high temperature WGS catalyst that catalyzes the WGS hydrogen generation reaction at temperatures of, e.g., between 310° C. and 450° C. The first WGS catalyst  330  can be a medium temperature WGS catalyst that catalyzes the WGS hydrogen generation reaction at temperatures of, e.g., between 200° C. and 350° C. The reactants are input into the reaction channel  306  at a temperature within the first temperature range such that the first WGS catalyst  330  can catalyze the hydrogen generation reaction in the gas flowing across the first WGS catalyst  330 . 
     The second WGS catalyst  332  is disposed further along the reaction channel  306  such that the distance between the first WGS catalyst  330  and the inlet  305  of the reaction channel  306  is less than the distance between the second WGS catalyst  332  and the inlet  305  of the reaction channel  306 . Gas flowing along the reaction channel  306  flows across the first WGS catalyst  330  before flowing across the second WGS catalyst  332 . The second WGS catalyst  332  catalyzes the WGS hydrogen generation reaction in a second temperature range that is lower than the first temperature range. For instance, the second WGS catalyst  332  catalyzes the WGS hydrogen generation reaction in a temperature range of, e.g., between 180° C. and 350° C. When the first WGS catalyst  330  is a high temperature WGS catalyst, the second WGS catalyst  332  can be a medium temperature WGS catalyst; or the second WGS catalyst  332  can be a low temperature WGS catalyst that catalyzes the WGS hydrogen generation reaction at temperatures of, e.g., between 180° C. and 250° C. When the first WGS catalyst  330  is a medium temperature WGS catalyst, the second catalyst  332  can be a low temperature WGS catalyst. 
     A heat transfer material  334  is disposed in the reaction channel  306  between the first WGS catalyst  330  and the second WGS catalyst  332 , with the distance between the heat transfer material  334  and the inlet  305  of the reaction channel  306  being less than the distance between the second WGS catalyst  332  and the inlet  305  of the reaction channel  306 . Fluid flowing along the reaction channel  306  first flows across the first WGS catalyst  330 , then across the heat transfer material  334 , and then across the second WGS catalyst  332 . The heat transfer material  334  is in physical contact with the reaction tube  304 , the inner tube  308 , or both. The heat transfer material  334  facilitates in-situ transfer of heat from the fluid flowing along the reaction channel  306  (e.g., heat generated by the exothermic hydrogen generation reaction that occurs at the first catalyst  330 ) to the cooling fluid flowing along the cooling fluid channel  314 , the inner cooling fluid channel  316 , or both. This heat transfer reduces the temperature of the gas flowing along the reaction channel to the temperature range at which the second WGS catalyst  332  can catalyze the hydrogen generation reaction. 
     In some examples, an input side heat transfer material (not shown) is disposed in the reaction channel  306  such that fluid received into the reaction channel  306  flows across the input side heat transfer material prior to flowing across the first catalyst  330 . This input side heat transfer material reduces the temperature of the fluid to the temperature range at which the first WGS catalyst  330  can catalyze the hydrogen generation reaction. For instance, when fluid from the SMR  200  ( FIG.  2   ) is provided as input into the WGS  300  at a temperature that is too high for the first WGS catalyst  330 , the input side heat transfer material reduces the temperature of the input fluid to the temperature range of the first WGS catalyst  330 . In some examples, an output side heat transfer material (not shown) is disposed in the reaction channel  306  such that fluid flows across the output side heat transfer material after flowing across the second catalyst  332 . This output side heat transfer material facilitates recovery of heat into the cooling fluid after completion of the WGS hydrogen generation reaction, enhancing the energy efficiency of the WGS reactor. 
     Heat transfer materials  336 ,  338  are disposed in the cooling fluid channel  314  and in the inner cooling fluid channel  316 , respectively. Cooling fluid flowing along the cooling fluid channel  314  and the inner cooling fluid channel  316  flows across the heat transfer materials  336 ,  338 , respectively. The heat transfer material  336  is in physical contact with the reaction tube  304  to facilitate transfer of heat from the fluid flowing along the reaction channel  306  to the cooling fluid flowing along the cooling fluid channel  314 . The heat transfer material  338  is in physical contact with the inner tube  308  to facilitate transfer of heat from the fluid flowing along the reaction channel  306  to the cooling fluid flowing along the inner cooling fluid channel  316 . 
     As the cooling flow flows along the cooling fluid channels  314 ,  316 , the cooling fluid is heated by heat transfer from the fluid flowing along the reaction channel. In some examples, the heated cooling fluid is provided as input to the SMR  200  or returned as input to the reaction channel  306  of the WGS  300 . For instance, the heated cooling fluid can be saturated water or two-phase water (liquid/steam) produced at a temperature and flow rate appropriate for input into the SMR. 
     In the configuration of the WGS reactor  300  shown in  FIG.  3   , the heat transfer materials  336 ,  338  are aligned with the heat transfer material  334 . In some examples, the heat transfer materials  336 ,  338  are not aligned with the heat transfer material  334 . The heat transfer materials  336 ,  338  can extend along some or all of the length of the cooling fluid channel  314  and inner cooling fluid channel  316 , respectively. In some examples, only one of the heat transfer materials  336 ,  338  is present, or neither of the heat transfer materials  336 ,  338  is present. 
     The catalyst arrangement in the WGS reactor  300  enables activation and reduction of a single catalyst without affecting the other catalyst. In general, the catalyst(s) in the WGS reactor  300  are activated by slowly flowing a reducing gas across the catalyst at a slightly elevated temperature to reduce the catalyst to a metallic, active form. In some examples, the WGS catalyst(s) are activated externally prior to connection of the WGS to the SMR. 
     Referring to  FIG.  4   , the WGS reactor  300  is configured as a single-catalyst system in which a single WGS catalyst  430  is disposed in the reaction channel  306  of the WGS reactor  300 . The WGS catalyst  430  catalyzes the WGS hydrogen generation reaction at temperatures of, e.g., between 200° C. and 450° C. The WGS catalyst  430  can be a high temperature WGS catalyst or a medium temperature WGS catalyst. 
     A heat transfer material  434  is disposed in the reaction channel  306  such that a distance between the heat transfer material  434  and the inlet  305  of the reaction channel  306  is less than the distance between the WGS catalyst  430  and the inlet  305  of the reaction channel  306 . Fluid flowing along the reaction channel  306  first flows across the heat transfer material  434  and then flows across the WGS catalyst  430 . The heat transfer material  434  is in physical contact with the reaction tube  304 , the inner tube  308 , or both, and facilitates the transfer of heat from the fluid received into the reaction channel  306  to the cooling fluid flowing along the cooling fluid channel  314 , the inner cooling fluid channel  316 , or both. This heat transfer reduces the temperature of the fluid to within a temperature range at which the WGS catalyst  430  can catalyze the WGS hydrogen generation reaction. For instance, when carbon monoxide output from the SMR  200  ( FIG.  2   ) is provided as input into the WGS  300  at a temperature that is too high for the WGS catalyst  430 , the heat transfer material  434  reduces the temperature of the input fluid to the temperature range of the catalyst  430 . 
     Heat transfer materials  436 ,  438  are disposed in the cooling fluid channel  314  and in the inner cooling fluid channel  316 , respectively, and facilitate heat transfer from the fluid flowing along the reaction channel  306  to the cooling fluid flowing along the cooling fluid channel  314  and the inner cooling fluid channel  316 . In the configuration of the WGS reactor  300  shown in  FIG.  4   , the heat transfer materials  436 ,  438  are aligned with the heat transfer material  434 . In some examples, the heat transfer materials  436 ,  438  are not aligned with the heat transfer material  434 . The heat transfer materials  436 ,  438  can extend along some or all of the length of the cooling fluid channel  314  and inner cooling fluid channel  316 , respectively. In some examples, only one of the heat transfer materials  436 ,  438  is present, or neither of the heat transfer materials  436 ,  438  is present. 
     The WGS catalysts  330 ,  332 ,  430  of  FIGS.  3  and  4    can be pellets, beads, saddles, rings, or other structures formed of a catalyst material. The WGS catalysts  330 ,  332 ,  430  can be catalytic foams, foils, fins, or other structures that include a substrate and a catalyst material, e.g., with the catalyst material disposed on or integrated into the substrate. A catalytic foam is a porous structure through which one or more flow paths are defined. The porosity of the catalytic foam can be selected to achieve a high surface area, enabling efficient catalysis, as well as a low pressure drop across the catalytic foam, enabling efficient fluid flow along the reaction channel  306 . For instance, the catalytic foam can have a porosity of between 5 ppi and 30 ppi. The material of the catalytic foam is non-reactive to the fluid (e.g., the reactants and products of the WGS hydrogen generation reaction) flowing along the reaction channel  306  in the temperature range at which the WGS  300  is operated. For instance, the catalytic foam can be a metal foam, such as copper or aluminum, or a silicon carbide film, or another suitable material. In the two-catalyst configuration of  FIG.  3   , the first and second WGS catalysts  330 ,  332  both can have the same structure, or each of the first and second WGS catalysts  330 ,  332  can have a distinct structure. 
     The heat transfer materials  334 ,  336 ,  338 ,  434  are materials having a thermal conductivity sufficient to enable heat transfer from the fluid flowing along the reaction channel  306  to the cooling fluid flowing along the cooling fluid channel  314  or the inner cooling fluid channel  316  or both. The heat transfer materials  334 ,  434  disposed in the reaction channel  306  are non-reactive to the fluid (e.g., the reactants and products of the WGS hydrogen generation reaction) flowing along the reaction channel  306  in the temperature range at which the WGS  300  is operated. For instance, the heat transfer materials  334 ,  434  can be a metal, such as copper or aluminum, or silicon carbide, or another suitable material. 
     The heat transfer materials  334 ,  336 ,  338 ,  434  can be foams, fins, foils, rings, saddles, beads, or pellets, or other structures capable of heat transfer. In the example of a foam, the porosity and length of the foam can be selected to achieve a high surface area, enabling efficient heat transfer, as well as a low pressure drop across the foam, enabling efficient fluid flow along the reaction channel  306 . For instance, the heat transfer materials  334 ,  336 ,  338 ,  434  can be foams having a porosity of between 5 ppi and 30 ppi. 
     Referring to  FIGS.  3  and  4   , the flow rate of cooling fluid along the cooling fluid channels  314 ,  316  is controlled by a flow controller  340 . The flow rate can be selected or adjusted based on the temperature of the fluid input into the reaction channel  306 , the temperature of the cooling fluid input into the cooling fluid channels  314 ,  316 . The flow rate can be selected or adjusted based on a target output temperature of the fluid output from the reaction channel  306 , a target output temperature of the cooling fluid, or both. The flow rate can be selected or adjusted based on the catalyst configuration, the type of catalyst(s) (e.g., high-, medium-, or low-temperature WGS catalyst), or both. The flow rate can be selected or adjusted based on an actual or desired throughput. 
     Cooling of the fluid in the reaction channel  306  of the WGS reactor  300  enables the WGS hydrogen generation reaction to be carried out at high energy efficiency. The transfer of heat from the fluid in the reaction channel  306  to the cooling fluid cools the fluid in the reaction channel  306 , e.g., removing heat generated during the exothermic hydrogen generation reaction and reducing the temperature of the fluid to an appropriate temperature range for the WGS catalyst(s), with no energy-intensive active cooling of the fluid. Moreover, the heat transfer in the WGS reactor enables isothermal conditions to be achieved, improving the conversion efficiency of the WGS hydrogen generation reaction. 
     Referring to  FIG.  5   , a WGS reactor  500  includes multiple reaction tubes  504   a - 504   c  disposed in a housing  502 . A reaction channel  506   a - 506   c  is defined within each reaction tube  504   a - 504   c  (collectively referred to as reaction tubes  504 ). Reactant gas flows into the reaction channels  506   a - 506   c  (collectively referred to as reaction channels  506 ) at a first end  510  of the WGS reactor  500 , and product gas exits the reaction channels  506  at a second end  512  of the WGS reactor  500 . 
     A cooling fluid channel  514  is defined in the space between the housing  502  and the reaction tubes  504 . Cooling fluid enters into the cooling fluid channel  514  at the second end  512  of the WGS reactor and exits from the cooling fluid channel at the first end  510  of the WGS reactor  500 . 
     In the example of  FIG.  5   , the WGS reactor  500  is a single-catalyst system, with a single catalyst  522 , such a high temperature WGS catalyst or a medium temperature WGS catalyst, disposed in each reaction channel  506 . A heat transfer material  524  is disposed in each reaction channels  506  to facilitate heat transfer from the gas in the reaction channel  508  to the cooling fluid in the cooling fluid channel  514 . In some examples, the WGS reactor  500  including multiple reaction tubes can be configured as a two-catalyst system. 
     Referring to  FIG.  6   , the SMR  200  and WGS  300  are integrated into a system  600  for production of hydrogen gas (H2) from hydrocarbons. A combustion furnace  602  as the external heat source heating the first end of the SMR  200 . The system  600  also can be implemented with the WGS  500 , with an SMR including multiple sets of outer and inner tubes, or both. 
     The hydrogen generation reaction in the SMR  200  produces hydrogen gas (H2) and carbon monoxide (CO) from reactants including hydrocarbons and water vapor (steam) in the presence of a catalytic foam including an SMR catalyst. The hydrogen gas and carbon monoxide are output from the flow channel defined within the inner tube of the SMR onto an SMR product line  604  along with excess steam. The fluid (e.g., hydrogen gas, carbon monoxide, and steam) from the SMR  200  are provided as input to the reaction channel of the WGS  300 . The outlet of the SMR  200  is in fluid communication with the inlet of the WGS  300  via the SMR product line  604 . In some examples, additional steam is provided into the reaction channel of the WGS  300 , e.g., from a water storage  614  (discussed infra) or from a cooling fluid output line  620  from the WGS  300  (discussed infra) to achieve a target ratio of steam to carbon monoxide. 
     As discussed supra, the fluid flowing along the flow channel toward the outlet of the SMR  200  is cooled by heat transfer with the incoming fluid flowing along the annular space of the SMR. The temperature of the fluid at the outlet of the SMR is thus at least partially controllable by the extent of heat transfer with the fluid in the annular space. The heat transfer, and thus the outlet fluid temperature, is affected by the configuration of the SMR  200  (e.g., the position, length, porosity, or other characteristics of the catalytic foam and the heat exchange foams) and by the operation of the SMR (e.g., the flow rate of fluid along the bayonet flow path of the SMR  200 ). The configuration, operation, or both of the SMR  200  can be adjusted to achieve heat transfer such that the fluid output from the SMR  200  is at a temperature appropriate for input into the reaction channel of the WGS  300 . For instance, when the WGS  300  is configured with a high- or medium-temperature WGS catalyst toward the input of the reaction channel, the SMR  200  can be configured such that the carbon monoxide and steam arrive at the reaction channel of the WSG  300  with a temperature in the range at which the WGS catalyst is active. By making use of heat transfer within the SMR  200  to achieve a target temperature for fluid output from the SMR, external, active cooling devices are not used between the SMR  200  and the WGS  300 , and the role of external, active heating devices (e.g., the furnace  602 ) can be reduced, thus contributing to high energy efficiency of the system-level hydrogen generation process. 
     The hydrogen generation reaction in the WGS  300  produces hydrogen gas and carbon dioxide (CO2), which are output from the reaction channel of the WGS  300  onto a WGS product line  608  along with excess steam. The excess steam is removed from the fluid on the WGS product line  608  in a vapor liquid separator (VLS)  610 . The remaining hydrogen gas and carbon dioxide are sent downstream  611  for separation, with the carbon dioxide discarded (e.g., via a flue stack, discussed infra) and the hydrogen gas removed to a hydrogen storage, e.g., for use as fuel. The separated steam flows along a steam line  612  to a water storage  614 , which also stores water provided from an external water source  616 . The separated steam on the steam line  612 , the water from the external water source  616 , or both can be treated before storage in the water storage  614 . 
     Water from the water storage  614  is provided along a cooling fluid line  618  as cooling fluid input into the WGS  300 . The heated cooling fluid output from the WGS  300 , which is a mixture of liquid water and steam, flows along a cooling fluid output line  620 . The heated cooling fluid will ultimately be provided as an input reactant into the SMR  200 . The temperature of the heated cooling fluid output from the WGS  300  is affected by the configuration of the WGS  300  (e.g., the type, position, or other characteristics of the WGS catalyst and the heat transfer material(s)) and by the operation of the WGS  300  (e.g., the flow rate of fluid along the reaction channel and the flow rate of cooling fluid). The configuration, operation, or both of the WGS  300  can be adjusted such that the heated cooling fluid is output at a target temperature, such as a temperature sufficient for input into the SMR  200 . By heating the cooling fluid to a target temperature using recovered heat from the fluid in the WGS  300  reaction channel, external, active heating elements to heat the SMR input fluid are not used. In addition, external, active cooling are not used to remove heat from the exothermic WGS hydrogen generation reaction. The use of recovered heat to heat the SMR input fluid and the cooling of the exothermic WGS hydrogen generation reaction contributes to high system-level energy efficiency. 
     The heated cooling fluid output from the WGS  300  flows along the cooling fluid output line  620  to an accumulator  622 . The accumulator  622  also receives additional water from the water storage  614  along a water line  624 . Steam and water output from the accumulator  622  onto an accumulator output line  626  is heated in a heat exchanger  634  with heat from flue gases  636  from the combustion furnace  602 . Hydrocarbons provided via a hydrocarbon line  630  are heated in a heat exchanger  635  with heat from the flue gases  636 . The heated steam and hydrocarbons  632 ,  633 , respectively, are mixed in a mixer  628  and output onto an SMR input line  638 , which feeds the heated steam and hydrocarbons to the inlet of the outer tube of the SMR  200 . The use of recovered heat from the flue gases  636  to heat the mixture of steam and hydrocarbons to a temperature sufficient for input into the SMR contributes to high system-level energy efficiency. In this configuration, the outlet of the WGS cooling fluid flow channels is in fluid communication with the inlet of the SMR  200  such that the heated WGS cooling fluid ultimately is provided as a component of the fluid input into the SMR  200 . The flue gases  636 , after passing through the heat exchanger  634 , are discarded to a flue gas stack  640 . 
     Referring to  FIG.  7   , in operation of a hydrogen generation system including an SMR and a WGS reactor, a fluid (e.g., a gas) including reactants is provided as input ( 700 ) to an SMR. Specifically, the fluid is provided into an inlet of an annular space of the SMR at a second end of the SMR, with the annular space being defined between an outer tube and an inner tube of the SMR. The fluid provided to the inlet includes hydrocarbons, e.g., methane, natural gas, biogas, methanol, or other hydrocarbons. The fluid provided to the inlet also includes steam. 
     The fluid flows along a bayonet flow path of the SMR. Specifically, the fluid flows ( 702 ) along the annular space from the second end to the first end of the SMR. Along the annular space, the fluid flows through an outer heat exchange foam ( 704 ) that facilitates heat transfer from high-temperature fluid flowing along a flow channel defined in the inner tube of the SMR to the lower-temperature fluid flowing along the annular space. The outer heat exchange foam also can induce turbulent flow in the fluid flowing along the annular space, enhancing the heat transfer efficiency. 
     The fluid flowing along the annular space is heated ( 706 ) by an external heat source, such as a combustion furnace, toward the first end of the SMR. In the heated region of the SMR, the fluid flows through a catalytic foam ( 708 ), which catalyzes the SMR hydrogen generation to produce hydrogen gas and carbon monoxide from the hydrocarbon and steam reactants ( 710 ). The catalytic foam facilitates heat transfer to the gas flowing therethrough, e.g., heat transfer from higher-temperature product fluid flowing along the flow channel within the inner tube of the SMR and heat transfer from the external heat source. 
     The fluid, now at higher temperature and including hydrogen and carbon monoxide, flows ( 712 ) from the annular space into the flow channel at the first end of the SMR. The fluid in the flow channel flows from the first end of the SMR toward the second end of the SMR, opposite the direction of flow of fluid in the annular space. The fluid in the flow channel flows through an inner heat exchange foam ( 714 ) that facilitates heat transfer from high-temperature fluid flowing along the flow channel to the lower-temperature fluid flowing along the annular space. The inner heat exchange foam also can induce turbulent flow in the fluid flowing along the flow channel, enhancing the heat transfer efficiency. The presence of an elongated baffle in the inner tube also enhances heat transfer efficiency. 
     When the fluid flowing along the flow channel reaches the SMR outlet, the fluid (including hydrogen gas, carbon monoxide, and steam) is output from the SMR ( 716 ) at the second end of the SMR. The SMR output fluid is provided as input into a reaction channel of a WGS reactor ( 720 ). The heat transfer between fluid flowing along the annular space and fluid flowing along the flow channel in the SMR can result in the carbon monoxide being at a temperature sufficient for input into the WGS reactor, such as a temperature in or above a temperature range at which a WGS catalyst can catalyze the WGS hydrogen generation reaction. For instance, the fluid output from the SMR and provided as input into the reaction channel of the WGS reactor is at a temperature of between 200° C. and at least 450° C. 
     Fluid including carbon monoxide and steam flow along the reaction channel of the WGS reactor ( 722 ), flowing across one or more WGS catalysts and one or more heat transfer materials. Cooling fluid, such as water, flows along one or more cooling fluid channels ( 724 ). The direction of fluid flow along the reaction channel is opposite the direction of fluid flow along the cooling fluid channels. The flow rate of the cooling fluid can be adjusted ( 726 ), e.g., based on a flow rate of the fluid flow along the reaction channel (e.g., which is based on throughput of the SMR), based on a target output temperature for the cooling fluid, or based on a configuration or operation of the WGS. 
     In the example of  FIG.  7   , the WGS reactor is configured as a two-catalyst system, e.g., as shown in  FIG.  3   . The fluid in the reaction channel flows across a first WGS catalyst ( 728 ), e.g., a high-temperature or medium-temperature WGS catalyst. The first WGS catalyst catalyzes the WGS hydrogen generation reaction in a first temperature range ( 730 ), e.g., between 200° C. and 450° C., to produce hydrogen gas and carbon dioxide. The fluid in the reaction channel then flows across a heat transfer material disposed in the reaction channel ( 732 ). The heat transfer material reduces the temperature of the fluid to a second temperature range in which a second WGS catalyst operates by heat transfer to the cooling fluid flowing in the cooling fluid channel(s). The heat transfer raises the temperature of the cooling fluid, e.g., to between 100° C. and 300° C. The fluid in the reaction channel, now in the second temperature range, flows across a second WGS catalyst ( 734 ), e.g., a medium-temperature or low-temperature WGS catalyst. The second WGS catalyst catalyzes the WGS hydrogen generation reaction in a second temperature range ( 736 ) lower than the first temperature range, e.g., between 180° C. and 250° C. to produce hydrogen gas and carbon dioxide. 
     Fluid, including hydrogen gas, carbon dioxide, and excess steam, is output from the reaction channel of the WGS reactor ( 738 ). The excess steam is separated ( 740 ) and the separated steam, along with cooling fluid (e.g., a mixture of steam and liquid water) from the WGS reactor, are recycled ( 742 ) to be used, e.g., as input into the WGS reaction channel or as input into the SMR. 
     EXAMPLES 
     Simulations and experiments of heat transfer in an SMR were performed to evaluate the role of catalytic foam in transferring heat from the external heat source to the fluid flowing along the annular space of the SMR. 
     Referring to  FIG.  8   , foams of differing porosities were disposed in the annular space of an SMR. For each foam type, the SMR was heated to 400° C. and the temperature differential between the outer tube and the inner tube was measured by thermocouple. The temperature differential for each of three foams (10 ppi, 20 ppi, and 30 ppi), and the temperature differential for an empty annular space (no foam) is shown in  FIG.  8   . A lower temperature differential indicates temperature equilibration due to heat transfer. The measured temperature differential between outer and inner tubes was about 50° C. greater when no foam was used than when a foam was present, indicating lack of heat conduction without foam and effective heat conduction with foam. 
     Referring to  FIGS.  9 A and  9 B , the heat transfer characteristics of an SMR  150  were simulated to demonstrate the effect of foam on heat transfer from an external heat source into the SMR. The SMR  150  has an outer tube  152  and an inner tube  154 , with an annular space  160  defined between the outer tube  152  and the inner tube  154 , and a flow channel  162  defined within the inner tube  152 .  FIGS.  9 A and  9 B  show a cross section of only half of the SMR; the axis X-X′ is the axis along the center of the flow channel  162 . An external heat source  172  supplies heat to a heated portion  171  of the SMR. In  FIG.  9 A , a foam  180  is disposed in the annular space  160 . In  FIG.  9 B , no foam is present in the annular space ( FIG.  9 B ). Other parameters, including inlet fluid flow rate, inlet fluid temperature, annular width, and tube dimensions, were the same. The heat source  172  was simulated as a section of the outer tube  152  maintained at 875° C. As can be seen from  FIGS.  9 A and  9 B , with the foam  180  present in the annular space  130  ( FIG.  9 A ), the fluid in the annular space  160  reached a temperature of over 760° C., while in the SMR without foam ( FIG.  9 B ), the fluid in the annular space  160  reached a temperature of only 450° C. The foam  180  present in the annular space also resulted in increased temperature of the fluid in the flow channel  162  within the inner tube  152 , e.g., by heat transfer and by flow of heated fluid from the annular space  160  into the flow channel  162 . These results demonstrate the effective heat transfer provided by foam disposed in the annular space of an SMR. 
     Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims.