Patent Publication Number: US-2007122339-A1

Title: Methods and apparatus for hydrogen production

Description:
BACKGROUND OF THE INVENTION  
      This invention relates generally to hydrogen production, and more specifically to methods and apparatus for generating hydrogen that can accommodate a varying demand.  
      One of the challenges to realization of the hydrogen economy is development of a low-cost hydrogen production technology that can meet a varying demand for hydrogen (e.g., at a refueling station). During an initial phase of a hydrogen economy, variations in hydrogen demand will make the choice of reforming technology challenging.  
      Electrolysis of water to produce H 2  is clean, but highly energy intensive, typically requiring 50 kilowatt hours of power for every kilogram of hydrogen produced. Most electrolyzers utilize electrical power almost exclusively to split liquid water into H 2  and O 2 . At least one known recent electrolyzer design operates with steam. In the latter electrolyzer, less electric power is used because a portion of the energy required to split steam into H 2  and O 2  comes from the heat energy of the steam itself. Also, a byproduct of electrolysis is O 2 , which is typically vented to the atmosphere, as it is not economical to capture, store, and sell it.  
      Catalytic partial oxidation (CPO) is a promising reforming technology for H 2  production from natural gas (NG) or other carbon containing fuel including, but not limited to, ethanol, methanol, etc. The use of pure O 2  instead of air can advantageously result in compact reactors and can reduce or eliminate H 2  clean up systems. However, the use of pure O 2  dictates the use of an air separation unit (ASU), resulting in higher capital costs because the ASU is one of the most expensive units in contemporary H 2  or GTL (gas to liquid) plants. Also, there is excess heat available in CPO reforming that is not utilized in the reforming process when air is used as a source of O 2 , thus reducing the overall efficiency of CPO reforming.  
     BRIEF DESCRIPTION OF THE INVENTION  
      Therefore, the present invention, in one aspect, provides a method for producing hydrogen (H 2 ). The method includes utilizing an electrolyzer to produce H 2  gas from steam, mixing byproducts of the electrolyzer and hydrocarbon fuel, and utilizing a catalytic partial oxidation (CPO) reformer to produce CO and H 2  from the mixed byproducts and hydrocarbon fuel. The method further includes removing the CO and remaining steam from the produced CO and H 2 , to thereby produce additional H 2 .  
      In another aspect, the present invention provides a method for producing hydrogen (H 2 ) that includes utilizing an electrolyzer to produce H2 gas from steam, utilizing byproducts of the electrolyzer as input to a catalytic partial oxidation (CPO) reformer to produce more H 2 , and utilizing steam and heat byproducts from the CPO reformer as input to the electrolyzer.  
      In yet another aspect, the present invention provides an apparatus for producing hydrogen (H 2 ). The apparatus includes an electrolyzer configured to produce H 2  gas from steam and a catalytic partial oxidation (CPO) reformer. The CPO reformer is coupled to the electrolyzer and configured to utilize byproducts of the electrolyzer as input to produce more H 2 . The electrolyzer is coupled to the CPO reformer to utilize steam and heat byproducts from the CPO reformer as input to the electrolyzer.  
      It will be seen that configurations of the present invention can provide a single hydrogen production system generating H 2  at a variable scale, dependent upon demand. Also, a compact reformer can be used because O 2  is used instead of air as the oxidant and there is no N 2  dilution. The lack of N 2  dilution also results in easy and economical H 2  separation and purification, and in many configurations, no pressure swing adsorption unit (PSA) is required. Furthermore, in many configurations, high-pressure electrolysis eliminates the need for an expansive O 2 , air or a syngas compressor.  
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  is a diagrammatic representation of a configuration of the present invention.  
       FIG. 2  is a diagrammatic representation of another configuration of the present invention similar to the configuration shown in  FIG. 1  in which no external steam generation is needed.  
       FIG. 3  is a diagrammatic representation of yet another configuration of the present invention in which an amine-based solvent is used to absorb and remove CO 2  from the product stream.  
       FIG. 4  is a diagrammatic representation of yet another configuration of the present invention in which a pressure swing absorber (PSA) is used to separate H 2  from other reaction products.  
       FIG. 5  is a diagrammatic representation of another configuration of the present invention similar to that illustrated of  FIG. 4 , but in which the PSA off gas is compressed and combusted in a microturbine to generate electricity to drive an H 2  compressor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Some configurations of the present invention provide a hydrogen generator that combines electrolysis and catalytic partial oxidation (CPO) reforming. A compact electrolyzer uses the excess steam from the reformer to produce H 2  and O 2 . The O 2  produced is mixed with a hydrocarbon fuel (which can be, by way of example and not by way of limitation, natural gas, methanol, ethanol, methane, ethane, propane, gasoline, or diesel, or a mixture thereof) and steam and sent to a CPO unit to produce syngas (CO+H 2 ). (As used herein, a “hydrocarbon fuel” is a combustible fuel having as products of its complete combustion only CO 2  and H 2 O, exclusive of any impurities.) This syngas is sent to a shift reactor to convert the CO and steam to CO 2  and H 2 . The shift reactor can be tuned to produce less than 0.5% CO in the outlet of the shift reactor. Any remaining CO will be converted to CO 2  using a small preferential oxidation (PrOx) catalyst with a small stream of O 2  coming out of the electrolyzer. A product stream containing H 2 , CO 2 , and steam is compressed. The H 2 O and CO 2  is condensed and removed as liquid during the compression and a pure H 2  product is compressed and stored at about 5000 to 10000 psi for refueling purposes.  
      Configurations of the present invention utilize byproducts of each technology (O 2  from electrolysis and excess heat and steam from CPO) to improve the efficiency of the combined system (e.g., 39% efficiency vs. 22% for electrolysis only, lower heating value basis). Some unit operations such as the air separation unit, air/O 2  compressor and pressure swing adsorption (PSA) can be eliminated to reduce the capital cost of the combined system. In some configurations, typically 30% of the full scale load of H 2  will be generated using the electrolyzer and the remaining H 2  will be generated using the CPO reformer.  
      In some configurations and referring to  FIG. 1 , an apparatus  100  for producing hydrogen (H 2 ) is provided. The apparatus includes an electrolyzer  102  configured to produce H 2  gas from steam. For example, electrolyzer  102  can utilize liquid water electrolysis supplying O 2 , a proton exchange membrane (PEM) stream electrolyzer supplying O 2  and using steam at about 100° C. to 300° C., or a solid oxide electrolyzer (SOEC) supplying O 2  and using steam at about 600° C. to 800° C. Also provided is a catalytic partial oxidation (CPO) reformer  104  that is coupled to electrolyzer  102  and configured to utilize byproducts of the electrolyzer as input such that apparatus  100  thus produces more H2. Also, electrolyzer  102  is coupled to CPO reformer  104  (albeit indirectly in the configuration illustrated in the Figure) to utilize steam and heat byproducts from CPO reformer  104  as input to electrolyzer  102 . In some configurations, electrolyzer  102  is configured to produce about 30% of the full-scale load of hydrogen produced by apparatus  100 . In this particular context, “about 30%” is intended to mean between 10% and 50%, inclusive. However, in some configurations, electrolyzer  102  is configured to produce between 10% and 80% of the full scale load of hydrogen. Because of the reuse of byproducts, apparatus  100  can be configured to operate at about 39% efficiency LHV. (“About 39%” in this particular context is intended to mean between 35% and 45% efficiency, inclusive, on an LHV basis.)  
      Some configurations of the present invention also include a multi-stage compression that condenses and removes H 2 O and CO 2  as liquid, and compresses and stores generated H 2  at 5000 to 10,000 PSI for refueling purposes.  
      Also in some configurations, apparatus  100  is further configured to mix byproducts of electrolyzer  102  with hydrocarbon fuel, utilize CPO reformer  104  to produce CO and H2 from the mixed byproducts and hydrocarbon fuel and utilize a shift converter (which, in apparatus  100 , comprises a high temperature shift converter  106  and a low temperature shift converter  108 ) to remove the CO and remaining steam from the produced CO and H 2  from CPO reformer  104 . A PrOx catalyst bed  110  is coupled to shift converter  108  and is configured to utilize a stream of O 2  to convert the CO to CO 2 , since CO requires much higher pressure to condense to a liquid for separating from the H 2  stream. A condenser  111  is used on the output stream of H 2 , H 2 O, and CO 2  to remove the H 2 O from the stream.  
      Some configurations of the present invention are also configured to mix O 2  and steam with the byproducts of electrolyzer  102 . This mixing is accomplished in apparatus  100  of  FIG. 1  by mixing air or O 2  with the output of PrOx catalyst bed  110  or the output of condenser  111 , feeding the mixture into a burner  112 , and feeding the output of burner  112  along with water into a steam generator  114 . The output of steam generator  114  includes steam, which is fed into electrolyzer  102 , which, in some configurations, is not only configured to generate about 30% of the full scale load of H 2 , but nay also be further configured to utilize excess steam and heat generated by CPO reformer  104 . In addition, part of the steam is generated by heat exchangers  116 ,  118 , and  120 . Reformer  104  outlet temperature is between about 700° C. and 1000° C., and low temperature shift reactor  108  outlet temperature is between about 150° C. and 300° C. Steam is generated by cooling the process syngas from reformer  104  outlet temperature to shift  118  and/or  120  outlet temperature. A multistage compressor  122  can be used to liquify CO 2  at each stage, resulting in an output of nearly pure hydrogen at a pressure of greater than 5000 PSI. Also advantageously in this configuration of apparatus  100  is that a portion of the O 2  produced by electrolyzer  102  goes to PrOx reactor  110  for oxidation of CO to CO 2 .  
      Referring to  FIG. 2 , some configurations  200  of the apparatus do not require external steam generation. All the steam require for configuration  200  is generated in heat exchangers  116 ,  118 , and  120  around reformer  104  and shift reactors  106 ,  108 .  
      Referring to  FIG. 3 , some configurations  300  of the present invention utilize amine based solvent absorption system  325  columns  322 ,  324  to absorb and remove CO 2  from the product stream. One column  322  of absorption system  325  absorbs CO 2  and another column  324  is used for regeneration of solvents by heating. The H 2  coming out of absorption column  322  is further compressed by a hydrogen compressor  123  for storage.  
      Referring to  FIG. 4 , some configurations  400  of the present invention utilize a pressure swing absorber (PSA)  425  to separate H 2  from the remaining products instead of a PrOx, a multistage compressor, or an amine-based absorption system. A typical PSA generates greater H 2  with a purity greater than 99% with about 80% recovery. The product H 2  gas is further compressed to greater than 5000 PSI in a hydrogen compressor. PSA off gas containing H 2 , CO, CO 2  and some fuel can be burned in a burner  112  to generate energy to produce additional steam.  
      Referring to  FIG. 5 , in some configurations  500  of the present invention, the PSA off gas is compressed and combusted in a microturbine  525  to generate energy to produce electricity to drive H 2  compressor  123 .  
      It will thus be appreciated that configurations of the present invention can provide a single reforming system that generates H 2  at variable scales dependent upon demand. Also, configurations of the present invention can provide a compact reformer because there is no N 2  dilution, and can also provide simple H 2  purification. For example, in the case of fueling stations, 5000-10,000 psig of H 2  may be needed. If there is no N 2 , at an end of the reformer, there will be only H 2 , CO 2  and steam. Steam and CO 2  can be condensed while compressing H 2 , so PSA could be eliminated. Also, high-pressure electrolysis eliminates the need for an O 2  compressor.  
      In some configurations of the present invention and again referring to the Figure, a method for producing hydrogen (H 2 ) is provided. The method includes utilizing an electrolyzer  102  to produce H 2  gas from steam, mixing byproducts of electrolyzer  102  and hydrocarbon fuel, utilizing a catalytic partial oxidation (CPO) reformer  104  to produce CO and H 2  from the mixed byproducts and hydrocarbon fuel and converting the CO and remaining steam from the produced CO and H 2 , to thereby produce additional H 2  from apparatus  100 . Apparatus 100 can be referred to as a Water-Gas-Shift reactor.  
      In some configurations, removing the CO and remaining steam further comprises utilizing a PrO x  catalyst bed  110  with a small stream of O 2  produced by electrolyzer  102  to remove the CO. Removing the CO and remaining steam can comprise utilizing a shift reactor  108  to convert CO to CO 2 .  
      In some configurations of the present invention, the byproducts of the electrolyzer that are mixed with the hydrocarbon fuel comprise O 2  and steam. In some configurations, about 30% of the H 2  generated by apparatus  100  is generated by electrolyzer  102 .  
      In some configurations, excess steam and heat generated by CPO reformer  104  is utilized in electrolyzer  102 .  
      Also, in some configurations of the present invention, a method for producing hydrogen (H 2 ) is provided that includes utilizing an electrolyzer  102  to produce H 2  gas from steam, utilizing byproducts of electrolyzer  102  as input to a catalytic partial oxidation (CPO) reformer  104  to produce more H 2 , and utilizing steam and heat byproducts from CPO reformer  104  as input to electrolyzer  102 . In some of these configurations, about 30% of the hydrogen produced by apparatus  100  is generated by electrolyzer  102 . Also, about 39% efficiency is achieved in some configurations on an LHV basis. Some configurations further include condensing and removing H 2 O and CO 2  as liquid, and compressing and storing generated H 2  at 5000-10,000 PSI.  
      It will thus be appreciated that configurations of the present invention can provide a single reforming system generating H 2  at a variable scale, dependent upon demand by separate or simultaneous operation of the subsystems in the integrated apparatus. Also, a compact reformer can be used because there is no N 2  dilution. The lack of N 2  dilution also results in easy and economical H 2  purification, and in many configurations, no PSA is required. Furthermore, in many configurations, high-pressure electrolysis eliminates the need for an O 2  compressor. For non-H 2  fueling station applications, one can still use the PSA but can eliminate the air or syngas compressor required by the PSA.  
      While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.