Abstract:
Described herein is a highly heat integrated fuel processor assembly that can be used for hydrogen production from a fuel source. The assembly comprises a heat exchanger type integrated reformer/combustor sub-assembly  51  also including catalyst able to induce the reforming and the combustion reaction. The fuel processor also comprises a high temperature WGS reactor  52 , a low temperature WGS reactor  53  and a selective CO oxidation or methanation reactor  54  so that the train of reactors can maximize hydrogen production and minimize the CO concentration of the product. The fuel processor further comprises a series of steam generators and heat exchangers that enhance the heat integration of the fuel processor. The whole fuel processor assembly or sub-assemblies can be employed for highly efficient distributed hydrogen generation.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to fuel processors for distributed hydrogen production and more particular to fuel processors where hydrocarbons are reformed to produce hydrogen. 
       BACKGROUND OF THE INVENTION 
       [0002]    Growing concerns over greenhouse gas emissions and air pollution emanating from energy usage and over the long-term availability of fossil fuels and energy supply security drive the search for alternative energy sources and energy vectors. Hydrogen has emerged as the preferred new energy vector since it addresses all these concerns. It can be used in both internal combustion engines and fuel cells for both stationary and mobile applications of any size. Particularly, its usage in fuel cells to produce electricity or to co-generate heat and electricity represents the most environment friendly energy production process due to the absence of any pollutant emissions. Furthermore, hydrogen can be produced from abundant and local renewable energy sources such as biofuels, solar or wind providing for secure and sustainable energy availability. 
         [0003]    The critical questions for the successful implementation of hydrogen as an energy vector are its sourcing and distribution. Hydrogen has been produced at large scale for many decadesin refineries and chemical plants. Its successful introduction into the transportation and distributed energy production sectors, however, requires the establishment of sufficient refueling and distribution networks. Hydrogen transportation is very inefficient and expensive due to its low energy density in its usual form. Even when hydrogen is compressed or liquefied, its transportation requires specialized and bulky equipment that minimizes the amount that can be safely carried, increasing resource consumption and cost. This issue can become insurmountable in the early stages of the implementation when demand will be low and not able to justify costly infrastructure options such as pipeline networks. The only feasible option will then be distributed hydrogen production facilities. 
         [0004]    Numerous proposals for distributed hydrogen production facilities ranging in capacity from a few Nm 3 /h to a few hundred Nm 3 /h are in the research and development phases and a few have been already implemented. Even though such facilities are much smaller than the ones employed in the refineries and the chemical plants, they are based on the same process technologies and involve hydrogen production by the reformation of hydrocarbon fuels. These proposals take advantage of the well established distribution network of such fuels to address the raw material availability concerns. The fuels most commonly mentioned include natural gas, propane, butane (LPG) and ethanol as the representative of the biofuels. They can be reformed according to the reactions: 
         [0000]      CH 4 +H 2 O→CO+3H 2  ΔH=49.3 kcal/mol 
         [0000]      C 3 H 8 +3H 2 O—+3CO+7H 2  ΔH=119.0 kcal/mol 
         [0000]      C 4 H 10 +4H 2 O→4CO+9H 2  ΔH=155.3 kcal/mol 
         [0000]      C 2 H 5 OH+H 2 O→2CO+4H 2  ΔH=57.2 kcal/mol 
         [0005]    The reforming reactions are highly endothermic, as indicated by the heats of reactions (ΔH), requiring substantial amounts of heat input typically covered by an external heat supply. Since these reactions take place at temperatures in the range of 700-900° C., the demand for heat input is enlarged by the need to heat up the reactants. The technique typically employed is to place the catalyst containing tubes of the reactor inside a fired furnace which provides the necessary heat. This is a rather inefficient arrangement due to the severe heat transfer limitations that exist and the metallurgical limits that must be observed. A more efficient reactor configuration must be employed. 
         [0006]    The products of the reforming reactions can yield substantial additional amounts of hydrogen by the water-gas-shift (WGS) reaction: 
         [0000]      CO+H 2 O→CO 2 +H 2  ΔH=−9.8 kcal/mol 
         [0007]    This reaction is typically carried out in two reactors: one high temperature (250-450° C.) that takes advantage of the higher reaction rates at higher temperatures and a low temperature (150-300° C.) on that takes advantage of the more favorable thermodynamic equilibrium and lowers the amount of CO present in the product stream to about 1%. When very low concentrations of CO are required, as when the product will feed a low temperature fuel cell, a selective CO oxidation or a methanation reaction takes place in a subsequent reactor that operates at low temperatures (120-250° C.) and lowers the CO amount to a few ppm. 
         [0008]    What is evident from the above is that production of hydrogen to feed a fuel cell requires a series of reactors that operate at vastly different temperature ranges. Heat management and optimization become, then, critical issues for distributed hydrogen generation systems and must be addressed with novel, highly heat integrated fuel processor configurations such as the ones of the present invention. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    The present invention relates to a fuel processor that produces a hydrogen rich stream suitable to feed low temperature fuel cells by the process known as steam reforming of hydrogen containing compounds. The fuel processor is comprised of four reactors and a multitude of heat exchangers so as to achieve a very high degree of heat integration and very high efficiency. To further increase efficiency, the reforming reactor is of a heat exchanger type comprised of a reformer/combustor assembly where the two sections are separated by a thin metal partition and are in thermal contact as to facilitate the efficient transfer of heat from the combustion to the reforming section. All four reactors and several of the heat exchangers can be placed inside a single shell, resulting in a very compact fuel processor well suited for distributed hydrogen generation. Combustion is mostly catalytic and takes place over a suitable catalyst. Steam reforming is a catalytic reaction and takes place over another suitable catalyst. 
         [0010]    In one aspect, the present invention relates to a fuel processor for producing hydrogen from a fuel source. The fuel processor comprises a heat integrated combustor/steam reformer assembly. A fuel and steam mixture is supplied to the reformer to be reformed and a fuel and air mixture is supplied to the combustor to be corn busted. The fuel processor also comprises a high temperature WGS reactor, a low temperature WGS reactor and a methanation reactor. The fuel processor further comprises a series of heat exchangers to exchange heat between different streams of the process. 
         [0011]    As a feature, the integrated combustor/steam reformer assembly includes a multitude of tubular sections defined by cylindrical walls separated from each other and supported on each end on plates machined as to allow the cylindrical walls to pass through them and to be in fluid connection with only one side of the plate. The inside wall of the tubular sections is coated with a catalyst that includes the desired reaction in the combustor feed. The outside wall of the tubular sections is coated with a catalyst that induces the desired reaction in the reformer feed. The assembly also includes an appropriately shaped reactor head that facilitates the introduction and distribution of the fuel and air mixture inside the tubular sections while it isolates the space defined between the plate and the reactor head from being in fluid connection with the surroundings. The assembly further includes an appropriately shaped reactor head that facilitates the collection and exit of the combustion products. The assembly space defined between the opposite plates and the external surfaces of the tubular sections is the reforming part of the assembly and is in fluid contact with other parts of the fuel processor allowing the introduction of the fuel and steam mixture in the reforming section and the removal of the products of the reforming reactions. 
         [0012]    As another feature, the combustor products are fed to a heat exchanger where they exchange heat with the reformer feed. The pre-heated feed is then fed to the reforming section. 
         [0013]    According to another feature, the products of the reforming reaction (reformate) exchange heat with the feed to the reformer in a heat exchanger placed after the exit of the reforming section. 
         [0014]    According to yet another feature, the reformate exchanges heat in a steam generator where steam is produced for the feed to the reformer. The reformate then enters the high temperature WGS reactor where most of the CO reacts and produces more hydrogen. 
         [0015]    According to yet another feature, the reformate exchanges heat in a steam generator where steam is produced for the feed to the reformer. The reformate then enters the low temperature WGS reactor where most of the remaining CO reacts and produces more hydrogen. 
         [0016]    According to yet another feature, the reformate exchanges heat with process water in a heat exchanger. The reformate then enters the CO selective oxidation reactor where most of the remaining CO reacts. 
         [0017]    According to yet another feature, the CO selective oxidation reactor is replaced by a methanation reactor where most of the remaining CO reacts. 
         [0018]    According to yet another feature, the reformate exchanges heat with process water in a heat exchanger before it exists the fuel processor. 
         [0019]    According to yet another feature, the fuel processor comprises a separator vessel where water condensed from the reformate is separated from the gaseous part of the reformate and is returned to the process. 
         [0020]    In another aspect of the present invention, the fuel processor comprises a heat exchanger where heat is exchanged between the combustor products and the fuel that is fed to the reformer. 
         [0021]    According to another feature, the fuel processor comprises a heat exchanger where heat is exchanged between the combustor products and process water to produce steam for the feed to the reformer. 
         [0022]    According to yet another feature, the fuel processor comprises a heat exchanger where heat is exchanged between the combustor products and the air that is fed to the combustor. 
         [0023]    According to yet another feature, the fuel processor comprises a heat exchanger where heat is exchanged between the combustor products and process water. 
         [0024]    According to yet another feature, the fuel processor comprises a separator vessel where water condensed from the combustor products is separated from the gaseous part of the products and is returned to the process. 
         [0025]    These and other features and advantages of the present invention will become apparent from the following description of the invention and the associated drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  illustrates the fuel processing system embodying the invention. 
           [0027]      FIG. 2  illustrates the integrated reformer/combustor assembly of the invention. 
           [0028]      FIG. 3A  is a flow schematic showing the fluid flows through the fuel processor according to one embodiment of the heat integrated fuel processor of the invention. 
           [0029]      FIG. 3B  is a flow schematic showing the fluid flows through the fuel processor according to another embodiment of the heat integrated fuel processor of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    The present invention is described in detail with reference to a few preferred embodiments illustrated in the accompanying drawings. The description presents numerous specific details included to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention can be practiced without some or all of these specific details. On the other hand, well known process steps, procedures and structures are not described in detail as to not unnecessarily obscure the present invention. 
         [0031]      FIG. 1  illustrates the heat integrated fuel processor  100  according to one embodiment of the present invention. The fuel processor assembly includes a flow passage  112  where a fuel and steam mixture entering at a temperature 120-400° C. is supplied to heat exchanger  42  where it is preheated to 300-700° C. by the reformate exiting the reformer/combustor assembly  51 . The preheated fuel and steam mixture is transferred through flow passage  14  to heat exchanger  41  where it is further preheated to 600-900° C. by the products of the combustor. The said preheated fuel and steam mixture enters the reforming section of the reformer/combustor assembly  51  where the desired reactions are induced by a catalyst. The reformer products exit assembly  51  at 600-850° C. and transfer part of their heat to the fuel steam mixture in heat exchanger  51  where they are cooled down to 400-700° C. The reformer products are farther cooled down to 280-400° C. by providing the necessary heat for steam generation in steam generator  43 . 
         [0032]    The reformate exiting steam generator  43  enters the high temperature WGS reactor  52  where most of the CO contained in the stream is converted to CO 2  by the water-gas-shift reaction. 
         [0033]    The WGS reaction is exothermic, so the products exit reactor  52  at 300-500° C. They are cooled down to 150-300° C. by providing the necessary heat for steam generation in steam generator  44 . 
         [0034]    The high temperature WGS products exiting steam generator  44  enter the low temperature WGS reactor  53  where most of the CO remaining in the stream is converted to CO 2  by the water-gas-shift reaction. The WGS reaction is exothermic, so the products exit reactor  53  at 160-350° C. They are cooled down to 100-200° C. in heat exchanger  45  where they exchange heat with process water providing hot process water. 
         [0035]    The low temperature WGS products exiting heat exchanger  45  enter the CO selective oxidation reactor  54  where most of the CO remaining in the stream is combusted to CO 2 . The selective oxidation reaction is exothermic, so the products exit reactor  54  at 120-250° C. They are cooled down to 60-80° C. in heat exchanger  46  where they exchange heat with process water providing hot process water. 
         [0036]    In another embodiment of the present invention, the selective CO oxidation reactor  54  is replaced with a methanation reactor where most of the CO contained in the stream exiting the low temperature WGS reactor is converted to CH 4  by the methanation reaction. 
         [0037]    The fuel processor assembly also includes a flow passage  124  where a fuel and air mixture is supplied to the combustion section of the integrated reformer/combustor assembly  51 . The fuel is combusted over a catalyst that induces the desired reaction in the combustor feed. The combustor products exit through flow passage  25  and feed heat exchanger  41  where they exchange heat with the feed to the reformer. They, then, exit the fuel processor through flow passage  126 . 
         [0038]    In one embodiment of the present invention, reactors  51 ,  52 ,  53  and  54  and heat exchangers  41 ,  42 ,  45  and  46  and steam generators  43  and  44  arranged as shown in  FIG. 1  can be housed in a single shell forming a compact and very efficient unit. A cylindrical shell 60 cm high and 30 cm in diameter is sufficient to house a unit with a hydrogen production capacity of 15 Nm 3 /h. 
         [0039]    In another embodiment of the present invention, heat exchanger  45  and  46  and reactor  54  can be placed in a second, separate shell to allow for greater flexibility in packaging the fuel processor as for example for mobile applications. 
         [0040]    In yet another embodiment of the present invention, the fuel processor can produce hydrogen for a higher temperature fuel cell that can tolerate CO concentrations of approximately 1%. In this embodiment, reactor  54  and heat exchanger  46  are completely removed from the fuel processor while all other parts are assembled in the manner described previously. 
         [0041]    In yet another embodiment of the present invention, the fuel processor can produce hydrogen for a higher temperature fuel cell that can tolerate CO concentrations of approximately 3-4% or the fuel processor can be connected to a hydrogen purification system such as a Pressure Swing Adsorption (PSA) unit. In this embodiment, reactors  54  and  53  and heat exchangers  45  and  46  are completely removed from the fuel processor while all other parts are assembled in the manner described previously. 
         [0042]      FIG. 2  presents in more detail one embodiment of the integrated reformer/combustor assembly of the invention. The assembly  51  comprises a multitude of tubular sections  120  separated from each other and supported on each end on tube sheets  131  and  132  machined as to allow the cylindrical walls to pass through them and to be in fluid connection with only one side of the sheet. The inside wall of the tubular sections is coated with a catalyst  122  that induces the desired reaction in the combustor feed. The total space inside the tubular sections  120  defines the combustion zone  115  where the majority of the combustion reactions take place. The assembly also includes an appropriately shaped reactor head  142  connected to tubesheet  132  and having a flow passage  124  so that it facilitates the introduction and distribution of the fuel and air mixture  24  inside the tubular sections  120  while it isolates the space defined between the plate  132  and the reactor head  142  from being in fluid connection with the surroundings. The assembly further includes a flow passage  141  that facilitates the collection of the combustion products  26  and directs them to heat exchanger  41  through the flue gas return line  25 . 
         [0043]    The outside wall of the tubular sections  120  is coated with a catalyst  121  that induces the desired reaction in the reformer feed  130  coming from heat exchanger  41  and directed by the distributor plate  151 . The products of the reforming reactions are collected by collector plate  152  and are driven to heat exchanger  42 . The assembly space defined between the opposite tube sheets  131  and  132  and between the distributor plate  151  and the collector plate  152  and the external surfaces of the tubular sections is the reforming zone  114  of the assembly where the reforming reactions take place. In the preferred embodiment of the present invention, the reforming reactions take place on the catalyst film  121  coating the tubular sections  120 . The advantage of the present invention is the high degree of heat integration between the reformer and the combustor since heat is only transported across the wall of tubular section  120  minimizing heat transfer resistances and maximizing heat utilization. 
         [0044]    In another embodiment, the reforming zone  114  can be filled with catalyst that induces the desired reaction in the reformer feed  130 . 
         [0045]    Since the tubes  120  and tube sheet  132  become very hot during operation, combustion can be initiated on the front surface of tube sheet  132  and back propagate through reactor head  142  and, possibly, through flow passage  124  if the fuel and air are pre-mixed. To avoid such a potentially very dangerous situation, the air and fuel can be kept separated until they enter the tubes  120  where combustion is desired. Air  135  enter the reactor head  142  through flow passage  124 , gets distributed and uniformly enters the tubes  120  through tube sheet  132 . Fuel  136  enters through a manifold  180  passing through flow passage  142  and placed adjacent to tube sheet  132  and is distributed to each tube through appropriately sized and shaped tips  181 . Adjusting the relative flows of air and fuel, combustion can be moved inside the tubes. 
         [0046]      FIG. 3A  presents a flow schematic for the fluid flows in one embodiment of the present invention. The fluid flows in the fuel processor  100  are the same as those presented in  FIG. 1 . The unit is farther heat integrated by employing a multi heat exchanger assembly  200  which utilizes the enthalpy of the flue gas stream to heat different process streams. The flue gas  26  exiting the reformer/combustor assembly  51  feeds the series of heat exchangers  71 ,  72 ,  73  and  74 . Heat exchanger  71  receives as the cold stream the feed stream  10  and outputs the evaporated and preheated feed stream  12 . Heat exchanger  72  receives de-ionized water  11  as the cold stream and outputs steam  13 . Streams  12  and  13  are combined with streams  35  and  36  coming from steam generators  43  and  44  respectively. The combined stream is the feed to the reformer stream  14  which is fed to heat exchanger  42  to get further preheated. 
         [0047]    Heat exchanger  73  receives air  21  as the cold stream and outputs preheated air  22 . Preheated air  22  is combined with fuel  23  and supplies the feed to the combustor. Fuel  23  may be the same fuel being reformed or any other suitable fuel. In one embodiment of the present invention, fuel  23  comprises the anode gas exiting the fuel cell when the fuel processor is coupled to a fuel cell for the production of heat and power. In another embodiment of the present invention, fuel  23  comprises the tail gas of the PSA or similar unit when the fuel processor is coupled to such a unit for the production of high purity hydrogen. 
         [0048]    Heat exchanger  74  receives cold process water  65  as the cold stream and outputs hot process water  66 . This is combined with hot process water streams  63  and  64  exiting heat exchangers  45  and  46  respectively. The combined stream  69  provides hot process water at temperatures of 50-80° C. and constitutes the useable heat production of the CHP unit. A properly designed heat exchanger assembly  200  can receive flue gas at temperatures of 500-900° C. and output the flue gas at temperatures below 50° C. 
         [0049]    In another embodiment of the present invention, heat exchangers  46  and  74  receive ambient or cold air as the cold stream and output hot air for heating purposes. 
         [0050]    In yet another embodiment of the present invention, when the heat output of the fuel processor can not be utilized, heat exchangers  46  and  74  are omitted. 
         [0051]      FIG. 3B  presents a flow schematic for the fluid flows in another embodiment of the present invention where water recirculation is used to decrease the water demand of the fuel processor. The steam reforming employed as the preferred hydrogen production reaction requires substantial amounts of water to be supplied along with the fuel. The benefit is that a large portion of the hydrogen is produced from the water, i.e. water acts as fuel in this process. This, however, places significant demands on the water supply to the unit and may limit its applicability to areas where water constraints exist. To overcome this, part of the water exiting the fuel processor is collected, re-circulated and re-used in the fuel processor. 
         [0052]    When the reformate  19  is cooled to below 100° C. in heat exchanger  46 , part of the water present in the reformate is condensed as to establish a thermodynamic equilibrium. This condensed water is separated in the aerated separator  81 . Additional water  91  may be fed to the separator to enhance the separation and to provide the total amount of water required to form streams  32  and  33  that feed the steam generators  42  and  44 . 
         [0053]    When the flue gas  26  is cooled to below 100° C. in heat exchanger  74 , part of the water present in the flue gas is condensed as to establish a thermodynamic equilibrium. This condensed water is separated in the aerated separator  82 . Additional water  92  may be fed to the separator to enhance the separation and to provide the total amount of water required to form stream  11  that feeds steam generator  72 . 
         [0054]    While this invention has been described in terms of several preferred embodiments, there are alterations, permutations and equivalents that fall within the scope of the present invention and have been omitted for brevity. It is therefore intended that the scope of the present invention should be determined with reference to appended claims.