Abstract:
A system for desulfurizing hydrocarbon fuel for a reformer and SOFC stack in an SOFC system. The system comprises a liquid phase desulfurizer for low-temperature desulfurization of an amount of liquid fuel ahead of reformer/stack startup and for continuous removal of large refractory sulfur-containing compounds from low-temperature fuel thereafter during operation of the reformer/stack,and gas phase desulfurizer for continuous high-temperature desulfurization of a stream of vaporized hydrocarbon fuel downstream of the liquid phase desulfurizer. The gas phase desulfurizer may be either upstream or downstream of the reformer.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to treatment of hydrocarbon fuels; more particularly, to means for removing sulfur from hydrocarbon fuels; and most particularly, to method and apparatus for removing sulfur from hydrocarbon fuels in a small scale continuous process such as is needed for supplying fuel to a fuel cell.  
       BACKGROUND OF THE INVENTION  
       [0002]     Sulfur is a naturally occurring constituent in petroleum and in most natural gas reserves. When sulfur-containing hydrocarbon fuels are used to power a solid oxide fuel cell (SOFC) stack, sulfur acts as a “poison” to the catalysts in the stack anodes themselves and also in the reformer catalyst used for converting the hydrocarbon fuels into reformate fuel for the fuel cell stack. Such poisoning decreases the activity of catalysts and can decrease the life of metallic parts due to increased corrosion at high temperatures. Therefore, removal of sulfur from hydrocarbon fuels intended for use in SOFCs is imperative to the successful operation of SOFC systems.  
         [0003]     Further, the emission of sulfur compounds from the combustion of fuels leads to environmental pollution in the form of acidic oxides of sulfur. Maximum fuel-sulfur content standards in the year 2006 are projected to be as follows: 
        Gasoline: 30 ppm by weight     Diesel fuel: &lt;15 ppm by weight     JP8 jet fuel: 50 ppm by weight     Natural gas: &lt;10 ppm by weight        
 
         [0008]     In the prior art, several different desulfurization technologies are known, for example, hydrodesulfurization and zinc oxide sorbents. Hydrodesulfurization technologies are currently applicable to large installations such as refineries, and due to their large size and system pressure requirements such technologies are not readily adaptable to mobile, relatively small fuel cell auxiliary power units (APUs) in transportation applications. Chemical scavengers such as zinc oxide are effective for desulfurization in natural gas pipelines, but waste products make them unattractive for mobile systems.  
         [0009]     Two promising technologies for fuel desulfurization in small scale, mobile fuel cell applications employ either a) gas phase sorbents based on metal oxides, or b) liquid phase sorbents based on zeolite materials.  
         [0010]     Gas phase sorbent technology can work well for a gaseous effluent that does not contain large refractory sulfur-containing organic molecules such as thiophenes, benzothiophenes, and the like. Such molecules tend either to clog gas phase sorbent systems or to slip through the sorbent. Further, such sorbents require elevated temperatures to be effective; thus, at startup of an SOFC system when the sorbents are initially cold there will be no desulfurization and so the system catalysts will be initially poisoned.  
         [0011]     Liquid phase sorbents based on zeolite materials can operate over a temperature range from about 0° C. to about 120° C. However, reaction rates for complete desulfurization, down to the levels required for SOFC stacks and reformer catalysts, are unacceptably low; up to six hours may be required.  
         [0012]     What is needed is a method and apparatus for continuously desulfurizing hydrocarbon fuel for an SOFC reformer and stack from startup through continuous operation at elevated temperature.  
         [0013]     It is a principal object of the present invention to adequately desulfurize fuel being supplied to an SOFC reformer and stack.  
       SUMMARY OF THE INVENTION  
       [0014]     Briefly described, a system for desulfurizing hydrocarbon fuel for a reformer and an SOFC stack comprises a liquid phase sorbent for low-temperature desulfurization of an amount of liquid fuel ahead of reformer/stack startup and for continuous removal of large refractory sulfur-containing compounds from low-temperature fuel thereafter during operation of the reformer/stack, and a gas phase sorbent for continuous high-temperature desulfurization of a stream of vaporized hydrocarbon fuel downstream of the liquid phase sorbent and ahead of the reformer and the SOFC stack. The liquid and gas phase sorbents cooperating in sequence can reduce the sulfur content in fuel being passed continuously into the reformer to less than about 1.0 ppmv, and in reformate being passed into the stack, to less than 0.1 ppmv. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0016]      FIG. 1  is a schematic sequence of operations in a method and apparatus in accordance with the invention;  
         [0017]      FIG. 1   a  is a schematic sequence of operations in an alternate method and apparatus in accordance with the invention;  
         [0018]      FIG. 2  is a schematic drawing of an SOFC system equipped for continuous fuel desulfurization in accordance with the invention;  
         [0019]      FIG. 3  is a table showing volumes of sorbents arranged in accordance with the invention for continuous reduction of fuel sulfur content from 50 ppm by weight to less than 0.1 ppm by volume for a continuous fuel flow rate of 0.2 g/sec; and  
         [0020]      FIG. 4  is a table showing volumes of sorbents arranged in accordance with the invention for continuous reduction of fuel sulfur content from 50 ppm by weight to less than 1.0 ppm by volume.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     In a fuel desulfurizing process in accordance with the invention, liquid phase desulfurizing of sulfur-containing hydrocarbon fuel is combined with gas phase desulfurizing of partially desulfurized and vaporized fuel to yield a gas phase fuel suitable for reforming and a reformate suitable for use in an SOFC stack.  
         [0022]     Referring to  FIG. 1 , in a schematic flow diagram of a desulfurizing system  10  in accordance with the invention, a flow  12  of sulfur-containing hydrocarbon fuel is passed first through a low-temperature, liquid-phase desulfurizer  14 , for example, a copper-, silver-, cerium-ion exchanged zeolite sorbent with an alumina guard bed for removing large refractory sulfur-containing compounds in the liquid fuel. Such a zeolite is operative over a temperature range between about 0° C. and about 120° C. Partially desulfurized fuel  16  is then vaporized in a Fuel Delivery Unit (FDU)  18  in the presence of, for example, air and anode tail gas recycle, to form a gaseous fuel  20  which is passed through a gas-phase desulfurizer  22 , a hydrocarbon reformer  24  to produce a hydrogen-rich reformate fuel  26 , and is then sent to SOFC stack  30 . Alternately, gas phase desulfurizer  22  may be coupled to liquid-phase desulfurizer  14  and disposed in series with and down stream of reformer  24  at a point shown as  32  in  FIG. 1 .  
         [0023]     Referring to  FIG. 1   a , in a preferred embodiment  10 ′ , liquid-phase desulfurizer  14  may be coupled, in series, with a dual gas-phase desulfurizer  22 ′ having a coarse sorbent  23  and a polishing sorbent  25 . The need for “polishing” sorbent  25  is dependent on the sulfur tolerance of the SOFC anode and the reformer catalyst. The definition of a coarse” sorbent as used herein is a material which can reduce the level of sulfur to approximately 1 to 10 ppmv. A coarse gas phase sorbent can be, for example, a metal oxide such as zinc, copper, or manganese oxides, or a zeolite-like material such as, for example, zinc titanate, or calcium carbonate. The coarse gas-phase sorbent  23  may also be a separation membrane or a liquid material such as a liquid through which gas can be bubbled. A “polishing” sorbent is defined as a material which can reduce the level of sulfur down to sub parts per million levels.  
         [0024]     It is known that liquid phase desulfurization alone can provide fuel having extremely low levels of sulfur, on the order of 0.1 ppmw. The majority of the sulfur in the fuel is removed in a relatively short time of approximately 1 to 2 hours, in the temperature range of about −10° C. to about 80° C. Hence, the liquid phase sorbents operate within ambient temperature ranges. Since the liquid phase sorbents remove sulfur from the fuel while the fuel is sitting in the sorbent of liquid-phase desulfurizer  14  at ambient temperature and conditions, the liquid phase sorbent acts as a passive desulfurization system in the sense that no heating or pressurizing of the fuel or sorbent is necessary for desulfurization to occur.  
         [0025]     In contrast, gas phase sorbents as used in gas-phaserdesulfurizers  22 ,  22 ′, require operation temperatures between about 300° C. and about 850° C., depending on the sorbent material used. By combining liquid phase and gas phase sorbent technologies together, the initial fuel can be desulfurized by the liquid-phase desulfurizer for startup. During operation of the fuel cell, the rate at which the fuel is used by the fuel cell increases such that full desulfurization by the liquid-phase desulfurizer can no longer occur. However, since the gas-phase desulfurizer will have been brought up to operation temperature after the fuel cell is in operation, the sorbents in the gas-phase desulfurizer will clean up what the liquid sorbent cannot. During system cool down, and when the system is at rest, the sorbents of the liquid-phase desulfurizer  14  work to fully desulfurize the fuel to be used for the next startup. Therefore, combining the use of liquid phase and gas phase sorbents result in an optimum continuous desulfurization system.  
         [0026]     In a presently preferred sequence of operations, as shown in  FIGS. 1 and 1   a , when SOFC  30  is not in service, fuel retained in liquid phase desulfurizer  14  continues to desulfurize passively over a period of up to several hours, down to a level at or below 0.1 ppmw. Desulfurizer  14  is provided with a fuel volume such that the SOFC system can be started up and operated on fully desulfurized fuel from desulfurizer  14  for a period of time adequate to warm the sorbent materials in gas-phase sulfurizer  22 ,  22 ′ to operating temperature. Then, as less-fully, desulfurized fuel begins passing through liquid phase desulfurizer  14 , the gas phase desulfurizer cooperates with the liquid phase desulfurizer to provide low-sulfur fuel at a sulfur content of about 1.0 ppmv continuously to reformer  24  and about 0.1 ppmv to fuel cell stack  30 .  
         [0027]     As, for example, in the case of requiring uninterrupted operation for 1000 hours without regenerating the sorbents and having a continuous fuel flow rate of 0.2 g/sec wherein the initial sulfur content is 50 ppmw or greater, the liter volumes of sorbents required to provide a continuous sulfur content of less than 0.1 ppmv (as just described) for gasoline, diesel fuel, and jet fuel are shown in  FIG. 3 . For a gasoline-powered fuel cell, approximately 30 total liters of sorbent is required, the great majority of which is for liquid sorbent  14 .  
         [0028]     Continuing development of reformer and fuel cell catalysts may result in less sulfur-poisoning sensitivity in future apparatus. Referring to  FIG. 4 , it is seen that if a future system can tolerate a sulfur level of 1.0 ppmv in the reformer and fuel cell, the volume of a sorbent system is reduced nearly ten-fold to little more than 3 liters. Note that the volumes of the sorbents needed would be much smaller if the regeneration cycle takes place every 10 to 25 hours, instead of every 1000 hours of operation. For example, with a 10 to 25 hour regeneration cycle, the volumes could be reduced by a factor of 10 to 4 times.  
         [0029]     In a currently preferred embodiment, liquid phase desulfurizer  14  is a copper-, silver-, cerium-ion exchanged zeolite sorbent with an alumina guard bed. Desulfurizer  14  is sized such that, beginning with fuel at 50 ppmw sulfur content, the output effluent of desulfurizer  14  is about 10 ppmw. Limiting the requirement of desulfurizer  14  to no less than 10 ppmw drastically reduces the volume of sorbent required as compared to prior art single-sorbent embodiments. Because the ion-exchange sorbent removes the large refractory sulfur-containing molecules, the rate and occurrence of plugging of the gas phase sorbents is greatly reduced. The surbent in coarse gas-phase desulfurizer  22  is preferably a packed column or a ceramic or metallic foam filter with the gas phase sorbent, such as zinc, copper, or manganese oxides, or a zeolite-like material such as, for example, zinc titanate, or calcium carbonate, applied to the surface. A polishing gas-phase sorbent desulfurizer  28  preferably includes, for example, a small grained, morphologically altered material such as zinc oxide, copper oxide or manganese oxide, as is known in the prior art.  
         [0030]     Referring to  FIG. 2 , an SOFC system  100  is shown, integrating the desulfurizing components  10  shown in  FIG. 1 .  
         [0031]     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.