Abstract:
A system for removing sulfur from a continuous reformate stream feeding a fuel cell stack. First and second sulfur traps are disposed in parallel between a hydrocarbon reformer and the fuel cell stack. The ends of the sulfur traps are connected to conventional four-way valves such that either trap may be selected for trapping sulfur from the reformate stream, while the other trap is undergoing regeneration by backflushing the accumulated adsorbed sulfur deposits. Thus, the sulfur traps may be used and stripped alternately, permitting continuous supply of desulfurized reformate to the fuel cell assembly. In a currently preferred embodiment, the hot cathode air exhaust is used to assist in stripping the out-of-service trap. In an alternative embodiment, two reformers are provided and the reformers are alternately regenerated along with their respective traps.

Description:
This invention was made with United States Government support under Government Contract/Purchase Order No. DE-FC26-02NT41246 awarded by DOE. The government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates to hydrocarbon reforming for supplying hydrogen-containing reformate fuels to fuel cells; more particularly, a system for removing sulfur from a reformate fuel stream; and most particularly, to an improved arrangement for continuously desulfurizing a reformate fuel stream. 
     BACKGROUND OF THE INVENTION 
     Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A well known class of fuel cells, referred to in the art as “solid-oxide” fuel cells (“SOFC”), includes a solid-oxide electrolyte layer through which oxygen anions migrate from a cathode to combine with hydrogen, forming water at the anode. In an SOFC, electrons flow through an external circuit between the electrodes, doing electrical work in a load in the circuit. 
     In the prior art, an SOFC is readily fueled by “reformate” gas, which is the effluent from a catalytic hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C. An SOFC can use fuel gas containing CO with the H 2 , the CO being oxidized to CO 2 . 
     The long term successful operation of an SOFC depends primarily on maintaining structural and chemical stability of the fuel cell components during steady state conditions, as well as transient operating conditions such as cold startups and emergency shut downs. Three types of reformer technologies are typically employed in conjunction with an SOFC (steam reformers, dry reformers, and partial oxidation reformers) to convert hydrocarbon fuel to hydrogen using water, carbon dioxide, and oxygen, respectively, with byproducts including carbon dioxide and carbon monoxide, accordingly. 
     Known hydrocarbon fuels for use in a reformer are, for example, gasoline, diesel, JP-8, Jet-A, and natural gas. A serious problem in the use of such fuels can be the presence of sulfur and sulfurous compounds. Ultra-low sulfur road fuels, being introduced in Europe and North America, have low levels of sulfur, with limits in the range of 10 to 50 parts per million (ppm) by weight. Some refinery streams and, for example, Fischer Tropsch synthetic diesel fuel are essentially sulfur-free—but when distributed in the fuel infrastructure it is very difficult to consistently deliver fuels with a sulfur level of less than 30 ppm. In some regions of the world, commercial hydrocarbon fuels contain elevated levels of sulfur, e.g., in an amount of about 300 to about 5,000 ppm by weight. It is likely that these high sulfur fuels will continue to be used in some parts of the world and in some industries (for example shipping and aviation) for long into the future. Fuel cell stacks can be particularly sensitive to sulfur—which tends to accumulate in the anode and cut power density and efficiency. Reformer catalysts and washcoat materials may also have some sensitivity to sulfur. In addition, endothermic reformer catalysts operating at low temperature tend to be particularly intolerant to sulfur, which can also adversely affect achievable reformer efficiency. In addition, sulfur can increase the propensity to form soot and other carbonaceous deposits. If coking or sooting occurs, due to a premature gas phase reaction before the fuel enters the reformer, within the reformer or as a post reaction in the system manifolding, the resulting particulate matter can enter the SOFC and degrade its efficiency and performance. Thus the long term successful operation of the fuel cell system is compromised by sulfur in the fuel. 
     Pending U.S. patent application, Ser. No. 09/781,687, filed Feb. 12, 2001, published Sep. 26, 2002 as US Patent Application Publication No. 2002/0136936 A1, the relevant disclosure of which is incorporated herein by reference, discloses a system and method for trapping impurities and particulate matter, and especially sulfur and sulfur-containing compounds, in energy conversion devices. The system comprises a regenerable trap including a trap element and, optionally, a filter element. The reforming system is fluidly coupled to the trapping system, which is positioned after the reforming system. 
     A drawback of the disclosed trappng system is that when the trap becomes loaded with trapped material, fuel cell operation must be suspended in order for the trap to be purged of the trapped material and thus regenerated. During such regeneration, the reformer is operated in a fashion to produce a gas suitable for removal of the trapped material (i.e., at high oxygen/carbon ratios) and the reformate gas is passed through the trap, reversing the adsorption process. The effluent from the trap is exhausted from the system via a control valve. A problem with this approach is that the fuel being reformed during regeneration is still contaminated with sulfur. Another problem is that the temperature at the reformer exit may be more than 900 C during start-up which can deteriorate the active materials in the sulfur trap. Yet another problem is that an extra heat exchanger must be used upstream of the reformer to cool recycled anode gas when the recycled gas is used to provide an oxidant for endothermic reforming. 
     What is needed in the art is a method and apparatus that permits continuous supply of desulfurized reformate to a fuel cell while simultaneously permitting regeneration of the sulfur strap, in an efficient configuration that protects the active materials in the sulfur trap from high temperature modes. 
     It is a principal object of the present invention to provide a continuous stream of sulfur-free reformate to a fuel cell for continuous operation thereof. 
     SUMMARY OF THE INVENTION 
     Briefly described, a system for removing sulfur from a continuous reformate stream comprises first and second regenerable sulfur traps disposed in parallel between a hydrocarbon reformer and a fuel cell assembly. The ends of the sulfur traps are connected to conventional four-way valves such that either trap may be selected for trapping sulfur from the reformate stream, while the other trap is undergoing regeneration by purging out the accumulated sulfur deposits. Thus, the sulfur traps may be loaded and purged alternately, permitting continuous supply of reformate to the fuel cell assembly. In a currently preferred embodiment, selected amounts of hot cathode air exhaust, hot anode gas exhaust and/or steam are used to control the temperature and oxygen concentration in the out-of-service trap, in order to assist in purging and thus regenerating the out-of-service trap. The timing of the adsorption/regeneration modes may be controlled so that regeneration occurs faster than adsorption to assure complete purging of sulfur before the trap is returned to its adsorption mode. In an alternate embodiment, a second reformer is disposed parallel to the first reformer and in series with the second regenerable sulfur trap so that the reformers may also be sequentially regenerated along with the associated sulfur traps. In a preferred embodiment, additional amounts of anode exhaust from the stack may be added to the stream between the regenerating trap and regenerating reformer to further reduce the amount of free oxygen flowing to the reformer to improve reformer regeneration. Alternatively, the amount of cathode exhaust flowing to the regenerating reformer from the regenerating sulfur trap may be reduced or completely switched off to control the temperature of and the oxygen concentration in the regenerating reformer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic drawing of a prior art system for desulfurizing a reformate stream, substantially as disclosed in the incorporated US Patent Application Publication reference, also showing an optional anode recycle loop for thermal reforming; 
         FIG. 2  is a schematic diagram of a first embodiment of an improved apparatus in accordance with the invention for desulfurizing a reformate stream while providing a continuous stream of desulfurized reformate to a fuel cell assembly; 
         FIG. 3  is a schematic diagram of a second embodiment of an improved apparatus in accordance with the invention; and 
         FIG. 4  is a graph showing the switching sequence of valves during the regeneration cycle of the second embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a prior art system  10  includes a fuel cell stack  12 , preferably a solid oxide fuel cell (SOFC) stack as is known in the art, although an apparatus in accordance with the invention is also useful for use with other types of fuel cell systems, for example, a molten carbonate fuel cell (MCFC) (not shown). A catalytic hydrocarbon reformer  14  receives a hydrocarbon fuel  16  and optionally air  18  and expels a reformate stream  20 . Fuel  16  is preferably selected from the group consisting of, but not limited to, conventional liquid fuels, such as gasoline, diesel, ethanol, methanol, kerosene, and others; conventional gaseous fuels, such as natural gas, propane, butane, and others; and alternative fuels, such as hydrogen, biofuels, dimethyl ether, and others; and synthetic fuels, such as synthetic fuels produced from methane, methanol, coal gasification or natural gas conversion to liquids, combinations comprising at least one of the foregoing methods, and the like; and combinations comprising at least one of the foregoing fuels. A sulfur-adsorptive, regenerable trap  22  containing suitable materials, preferably as disclosed in the incorporated reference or containing a high surface area, nanostructured sorbent of relatively low capacity, adsorptively retains sulfurous compounds passing through trap  22 , for example, hydrogen sulfide (H 2 S) and sulfur dioxide (SO 2 ) as may be present in stream  20 . 
     In a currently preferred embodiment, trap  22  includes a filter element and a trap element. The filter element includes a particulate filter in the first chamber of the trapping system wherein the particulate filter includes a washcoat disposed on the filter material. 
     Various sensors such as, for example, temperature sensor  21  and/or pressure differential sensor  23  can be positioned in electrical communication with trap  22  to detect the sulfur level content of trap  22 , and to control and schedule the trap&#39;s regeneration based on those levels. Trap  22  can then be regenerated by adjusting the air-fuel ratio of the reformate, or by increasing the operating temperature of the trap, as known in the art. 
     When in the fuel cell operation mode, Desulfurized stream  24  is passed into the anode side  26  of fuel cell stack  12  where it reacts with oxygen provided from air  27  on the cathode side  28  to produce electricity as is well known in the art. Optionally, after being cooled by heat exchanger  35 , a portion  30  of anode exhaust  32  may be recirculated into reformer  14 , assisted via a high-temperature, pressurized pump  34 , to provide the oxidant for endothermic reforming; the balance  36  of anode exhaust  32  is disposed of in known fashion. Hot cathode exhaust air  38  is passed to atmosphere. Waste heat  40  from fuel cell stack  12  may be directed into reformer  14 , for example, by proximity thereto, to assist in endothermic reforming. 
     Desulfurizing trap  22  requires periodic regeneration as described in the incorporated reference. A three-way valve  42  downstream of trap  22 , after receiving a control signal from various monitoring sensors such as sensors  21 , 23 , permits the venting of desorbed sulfurous materials to a suitable destination  44  when regeneration is required and SOFC  12  may be taken offline. 
     Referring to  FIG. 2 , a first embodiment  110  in accordance with the invention, like prior art embodiment  10 , comprises a fuel cell stack  12 , having anode side  26  and cathode side  28 , and a reformer  14  for receiving fuel  16  and air  18 , and a portion of recycled anode gas  30 , as may be needed for generating a reformate stream  20 . The improvement in first embodiment  110  is the provision of first and second equivalent regenerable traps  122   a ,  122   b  arranged in parallel flow. Each of traps  122   a ,  122   b  may be constructed of a trap element, and optionally a filter element, as disclosed in the incorporated reference. A first four-way valve  160  and a second four-way valve  162  are connected across the respective entrances and exits of traps  122   a ,  122   b  as shown in  FIG. 2  such that reformate stream  20  may be directed as desired alternately through either trap  122   a  or trap  122   b  as desulfurized stream  24 . 
     The arrangement shown in  FIG. 2  permits reformate stream  20  and desulfurized stream  24  to be directed into fuel cell stack  12  continuously by the selection of either trap  122   a  or trap  122   b . Likewise, this arrangement permits the offline regeneration of the traps preferably in a direction counter to the flow of reformate, of whichever trap is not in service. As shown in a first operating mode in  FIG. 2 , trap  122   a  is selected for online reformate flow and trap  122   b  is offline. To change to a second and alternate operating mode, actuation of valves  160 ,  162  serves to bring trap  122   b  online and places trap  122   a  offline. 
     In the first operating mode, as shown in  FIG. 2 , all or a portion  146  of hot, oxygen-depleted cathode exhaust  38  may be sent to offline trap  122   b  via a backflush inlet  166  of valve  162  to permit reverse-flow regeneration of the offline trap to appropriate waste destination  44 . Other gases  148  may be supplied to valve  162  as desired, either with or instead of cathode exhaust portion  146 , for example a mixture or all or part of the cathode exhaust  38  and part of the anode exhaust  36  and optionally including steam  150  as a means to control temperature and oxygen concentration of gas  152  for trap regeneration. 
     In operation, the valves are switched periodically so that the just-regenerated trap now receives reformate and the saturated trap may be regenerated. The regeneration period of the storage and regeneration can be relatively short, for example, less than one minute for conditions wherein the temperatures of storage and regeneration are approximately equal, and several minutes if the temperatures are substantially different. The system is balanced so that offline regeneration occurs somewhat faster than online adsorption. In this way, the traps are completely purged of sulfur prior to being placed back online with the stack. This timing is easily achieved with choice of appropriate adsorbent materials, regenerating gases, and temperatures, as known in the art. Preferably, the proportions of gases  146 , 148 , during the regeneration, are adjusted so that when the trap is placed back online to the fuel cell stack, no oxygen is present in the stream. For example the flow of cathode exhaust  146  to valve  162 , containing amounts of oxygen, can be switched off and steam or anode exhaust contained in the regeneration gas  152  can remain flowing at the end of the regeneration cycle—such that no free oxygen reaches the fuel cell stack  12  in the fuel gas and, optionally, so that the surface of active materials in the traps  122   a/b  can be reduced. 
     The state of the traps  122   a ,  122   b  can be continuously monitored by differential pressure, temperature, and inlet and exhaust gas composition sensors, such as sensors shown in  FIG. 1  as  21 , 23 , together with predetermined control algorithms. 
     Referring now to  FIG. 3 , a second embodiment  210  in accordance with the invention includes first and second traps  222   a , 222   b , first and second four-way valves  260 , 262 , and a fuel cell stack  12 . The novel feature of embodiment  210  is that two alternate reformers  214   a , 214   b  are also provided in parallel and are included in the changeable pathway between the four-way valves  260 , 262 . Thus, not only the traps but also the reformer catalysts and catalyst substrates may be backflushed of contaminants during regeneration mode. 
     In the first operating mode as shown in  FIG. 3 , all or a portion  246  of hot, oxygen-depleted gas from cathode exhaust  38  may be sent to the offline trap and reformer via a backflush inlet  266  of valve  262  to permit reverse-flow regeneration of the offline trap and reformer to appropriate waste destination  44 . Other gases  248  may be supplied to valve  262  as desired, either with or instead of cathode exhaust portion  246 , for example a mixture of all or part of the cathode exhaust  38  and part of the anode exhaust  36  and optionally including steam  250  as a means to control the oxygen concentration of gas  252  for trap and reformer regeneration. This can be useful for systems using fully endothermic reforming at relatively low temperatures, because substantial storage of sulfur on the catalyst is a known source of deterioration that is desirably mitigated. Preferably, the amount of free oxygen flowing from the regenerating trap and into the regenerating reformer may be reduced by introducing additional amounts  36   a  of anode exhaust  36  via three-way valve  270 . 
     To prevent residual oxygen from migrating to the anode, from the regenerating cycle, near the end of the regeneration cycle, and before valves  260 , 262  switch to reverse the regeneration/adsorption modes, the flow of cathode exhaust portion  246  to valve  262  can be switched off and steam and/or anode exhaust can remain flowing to the leg being regenerated. Alternately, to consume any residual oxygen, the amount of anode exhaust  36   a  being introduced to the reformer via valve  270  may be adjusted to achieve a stoichiometric or richer fuel/air ratio entering regenerating reformer  214   b  near the end of the regenerating cycle. The timing of either introducing additional amounts of gas  36   a  or switching off the flow of exhaust portion  246  is best shown in  FIG. 4 . In  FIG. 4 , line  280  represents the period of time of the full cycle (t) over which either leg completes its reforming and regeneration cycle. During time interval  282 , reformer  214   a  is in its regeneration period or cycle; during time interval  284 , reformer  214   b  is in its regeneration period or cycle. As shown, near the end  286  of their respective cycles, purge phase  288  begins during which additional amounts of gas  36   a  are introduced into the respective reformer or the flow of cathode exhaust portion  246  is switched off. Preferably, the purge phase continues beyond the end of the regeneration cycle to minimize the amount of oxygen present in the reformer when reforming again begins. 
     The order and strategic placement of components in the first and second embodiments ( FIGS. 2 and 3 ) to match or nearly match their optimal temperature of operation allows the components to operate at appropriate temperatures without the need for heat exchanger  35  and the use of a lower temperature recycle pump  134 , as used in prior art system  10 , thus offering a substantial reduction in weight, cost and complexity. For example, by placing reformer  14  downstream of and in thermal proximity of the stack and stack exhaust outlets, as shown, optimal, incrementally decreasing operating temperatures for the inlet and outlet of the stack of 650° C. and 850° C.; for the inlet and outlet of the reformer of 800° C. and 700° C.; for the inlet and outlet of the regenerable traps of 650° C. and 600° C.; and for the inlet to the pump, of 550° C. can be achieved. 
     Embodiment  110  is especially useful with low-sulfur fuels such as natural gas and low-sulfur gasoline. Embodiment  210  is especially useful with heavier fuels and high-sulfur fuels such as diesel fuels, JP8, or current jet fuel. This is because it is practical to make a robust endothermic reformer with light, low-sulfur fuels, but heavier and high-sulfur fuels tend to create problems with coking and contamination of the reforming catalysts; thus a periodic and frequent regeneration of the reformer catalyst is attractive. 
     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.