Abstract:
A method for fueling a solid oxide fuel cell stack is provided. The method includes passing a first portion of hydrocarbon fuel through a catalytic hydrocarbon reformer to generate a first reformate. The first reformate is passed through a hydrocarbon cracker to generate a second reformate such that a portion of any non-reformed hydrocarbon fuel in the first reformate is converted to methane. The second reformate is supplied to the fuel cell stack.

Description:
RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS 
     The present application is a Continuation-In-Part of a pending U.S. patent application Ser. No. 11/231,703, filed Sep. 21, 2005 and published Mar. 22, 2007 as US Patent Application Publication No. US 2007/0065687 A1. 
    
    
     GOVERNMENT INTEREST 
     The present invention was supported in part by a U.S. Government contract, no. DE-FC2602NT41246. The United States Government may have rights in the present invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates to solid oxide fuel cell (SOFC) systems; more particularly, to such systems wherein a portion of the anode tail gas is recirculated directly into the reformer; and most particularly, to a system wherein reformate from a partial-oxidation hydrocarbon reformer, with unprocessed hydrocarbon fuel, is passed through a hydrocarbon cracker ahead of the fuel cell stack to permit internal reforming of small aliphatic residues such as methane within the fuel cell stack. 
     BACKGROUND OF THE INVENTION 
     SOFC systems are well known. An SOFC typically is fueled by “reformate” gas, which is the partially oxidized effluent from a catalytic partial oxidation (CPOx) hydrocarbon reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen (H 2 ). The CPOx reactions also release heat that serves to maintain the temperature of the reformer. A CPOx reformer is a very simple and easily controlled device with good transient behavior and dynamic range. A known disadvantage of a CPOx reformer is that it has a relatively low fuel-processing efficiency that limits overall system efficiency. 
     To improve stack power density and system efficiency and to reduce carbon precipitation and deposition in the system, it is known in the art to recycle a portion of the tail gas from the stack anodes through the reformer. The stack anode tail gas has a large amount of water vapor and CO 2  as well as unreacted H 2  and CO gases. When these gases are fed back to the reformer, endothermic “steam reforming” reactions can occur in the fuel reformer. Stack anode tail gas recycle is known to be enhanced by fuel reformer technology that can sustain its temperature in the presence of endothermic reactions. Such technology may consist of a heat exchanger construction wherein hot combustor effluent passes on one side of the heat exchanger (combustor side), and a mix of fuel, air, and recycle gas passes through the other side (reforming side). The reforming side is catalytically treated to allow for the preferred reactions to occur. This mechanization yields high fuel processing efficiencies that, in turn, yield high system efficiencies. 
     Disadvantages to this approach are complexity and potential durability issues with the heat exchanger/reformer device because of the higher temperatures required for endothermic reforming; the system complexity required to channel the combustor gases through the reformer; and the potential for carbon precipitation in the produced reformate which may have lower water vapor content by volume. 
     Where natural gas is the fuel, steam reforming with added water (no recycle) is a very common approach. In some cases, the natural gas fuel is pre-reformed to break-down higher hydrocarbons (heavier than methane) and this high-methane mix is fed directly to an SOFC stack. H 2 O is typically added to the reformate to allow steam reforming reactions to occur within the SOFC stack itself. This arrangement is known as “Internal Reforming” in the art. In this prior art approach, the heat required for endothermic reforming to occur is supplied by the electrochemical heat released in the SOFC stack, and not by heat exchange with the combustor gases. Internal endothermic reforming within the SOFC stack is very attractive for its high fuel processing efficiencies, but in the prior art it requires a supply of external water injection to the system. 
     There is a limitation, however, to the range of operation in a system with this fuel processing configuration. The system efficiency is quite high when a fraction, or all, of the fuel can be reformed internally to the stack. The problem is that the reforming process requires the stack to provide the necessary heat to support the endothermic reactions, and it is not capable of providing that heat below a certain system operating power. This means the efficiency of the system, when operating at low electric load, is that of a CPOx system and reaches the highest system efficiencies only when higher loads can support internal reforming. 
     What is needed in the art is a system mechanization and algorithm that incorporates the benefits of each prior art system configuration in an architecture that allows for full flexibility in fuel processing, incorporating CPOx, endothermic, and internal reforming depending upon the power load of the fuel cell system. 
     It is a principal object of the present invention to improve the fuel efficiency of a solid oxide fuel cell stack system over the full range of operating loads. 
     SUMMARY OF THE INVENTION 
     Briefly described, an SOFC stack system in accordance with the invention includes an endothermic reformer and fuel flow arrangement that permits optimized fuel reforming at all power load levels of the fuel cell stack between 0% and 100%. 
     A portion of the anode tail gas is recycled into a preparatory mixing chamber wherein the tail gas is combined with fresh air and fuel. The mixture is sent to a main reformer that is configured for endothermic reforming. Reformate from the main reformer, and during higher load operation, unprocessed fuel mixed with reformate, is sent through a hydrocarbon cracker that breaks any hydrocarbons in the reformate into methane before the reformate enters the stack. This invention allows for a reforming strategy that includes the following operation, or any blend thereof. The first mode is at 0% load, and there is no reforming in the stack, all reforming is done in the main reformer. The second mode is at 100% load when all of the fuel is internally reformed by the stack and none of the fuel is reformed in the main reformer. At loads between 0% and 100%, the reforming is a mixture of CPOx and endothermic reforming in the main reformer and internal reforming within the fuel cell stack. This strategy allows the system to take advantage of the highest fuel processing efficiencies available through the full range of stack operation. 
    
    
     
       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 flow diagram of a first prior art SOFC system without recycle of anode tail gas; 
         FIG. 2  is a schematic flow diagram of a second prior art SOFC system having recycle of anode tail gas into the fuel stream ahead of the reformer; 
         FIG. 3  is a schematic flow diagram of an SOFC system as disclosed in US Published Patent Application No. 2007/0065687 A1, the relevant disclosure of which is incorporated here, showing recycle of anode tail gas into the inlet to the SOFC stack; and 
         FIG. 4  is a schematic flow diagram of an improved, hybrid SOFC system in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a first prior art SOFC system  10  comprises an SOFC stack  12  having an anode inlet  14  for reformate  16  from a CPOx reformer  18 ; an anode tail gas outlet  20 ; an inlet  22  for heated cathode air  24  from a cathode air heat exchanger  26 ; and a cathode air outlet  28 . Anode tail gas  30  and spent cathode air  32  are fed to a burner  34 , the hot exhaust  35  from which is passed through heat exchanger  26  to heat the incoming cathode air  36 . The residual potential chemical energy (H 2  and CO) in the anode tail gas is not recovered as additional electrical output  38  of the stack but instead is partially recovered as heat energy in exchanger  26 . 
     Referring to  FIG. 2 , a second prior art SOFC system  110  comprises the elements just described for first prior art system  10 . However, in addition, a first portion  140  of anode tail gas  30  is diverted away from burner  34  to an anode tail gas cooler  142  and thence through an anode tail gas pump  144  which directs cooled portion  141  of the tail gas into an entrance to an air/fuel preparation chamber  148  ahead of endothermic reformer  118 . Second portion  143  of anode tail gas  30  is sent to burner  34  as in embodiment  10 , and the hot effluent  135  is sent to cathode air heat exchanger  26  via a prior heat exchanger in reformer  118 . Fortified reformate  116  is sent to stack anode inlet  14 . Thus, residual hydrocarbons in the anode tail gas are exposed to reforming for a second time, and heat is recovered in both the reformer and the cathode air heater. Elevated temperatures in the reformer are necessary to permit endothermic reforming. System  110  is known to improve significantly the fuel efficiency of an SOFC system, resulting in an increase in electrical output  138 . 
     Referring to  FIG. 3 , SOFC system  210  as disclosed in co-pending application Ser. No. 11/231,703 is substantially the same as that of prior art embodiment  110  except that anode tail gas  241  from pump  244  is directed via pump  244  to the anode inlet  14  of stack  12 , bypassing reformer  18 , where the anode tail gas joins with reformate  16  from reformer  18  to form a feed stream  216 . The burner effluent  235  bypasses reformer  18 . Because reformer  18  is a CPOx reformer, in addition to the primary, independently controlled fuel flow  169  supplying fuel  170  to reformer  18 , a secondary, independently controlled fuel flow  269  is provided for supplying secondary fuel  270  into anode tail gas portion  240  to optimize the mixture feed stream  216  provided to stack anode inlet  12 . Preferably, the tail gas/secondary fuel mixture is passed through a clean-up catalyst  280  to reduce longer chain hydrocarbons to methane, H 2 , and CO. 
     Primary fuel reformer  18 , which is a simple and robust CPOx technology reformer, supplies between 0% and 100% of the reformate to the SOFC stack, with typical values between 30% and 70%. At 100%, there is no secondary fuel injection  270  to the recycle feed stream  216  and no internal reforming in the stack (0% internal reforming). At 0%, there is no CPOx reformate  16  to the stack and all of the secondary fuel  270  from flow control  269  is internally reformed (100% internal reforming). This blended strategy, referred to herein as “Light Internal Reforming”, generally results in a reformate feed stream  216  to the stack that has a high concentration of H 2  and H 2 O, as well as moderate amounts of CO and CO 2 , and a small amount (0-30%) of methane gas (CH 4 ). This arrangement allows for endothermic reforming within the stack itself for high fuel processing efficiencies and high electric output  238 . Further, this arrangement allows for reduced internal reforming load (&lt;100%) on the stack which can improve durability. In addition, the CPOx reformer primary fuel processing serves the needs of the system during the start-up phase when the stacks are not operational but are warming-up, as well as under transient conditions where less internal reforming may be desirable. 
     A problem with system  210  is that internal reforming is only available at higher electric loads. Thus, system  210  is relatively inefficient under low load conditions. Further, the benefits of passing the anode recycle through the reformer, as in system  110 , are not available. Thus, system  210  cannot enjoy use of anode recycle in endothermic reforming at any load condition. 
     What is needed is a flexible system that allows for endothermic reforming in the main reformer at low load conditions, internal reforming in the fuel cell stack at higher load conditions, and a hybrid mixture of endothermic and internal reforming at intermediate load conditions. 
     Referring to  FIG. 4 , a hybrid, flexible SOFC system  310  in accordance with the invention comprises most of the elements just described for second prior art system  110  which need not be repeated here. The following elements, however, are of special interest in system  310 . 
     First portion  140  of anode tail gas  30  is diverted ahead of burner  34  to anode tail gas cooler  142  and thence through anode tail gas pump  144  which directs cooled portion  141  into an air/fuel preparation chamber  148  ahead of endothermic reformer  18 . Second portion  143  of anode tail gas  30  is sent to burner  34 , and the hot effluent  135  is sent to cathode air heat exchanger  26  via a heat exchanger  137  in reformer  18 . Fortified reformate  116 , including secondary fuel input from  370  is sent to stack anode inlet  14  via a hydrocarbon cracker  360  to ensure that any residual hydrocarbon molecules in reformate  116  are small enough, and preferably are only methane, to be internally reformed within SOFC stack  12 . Hydrocarbon cracker  360  may be of any type as are well known in the art for breaking long-chain aliphatic compounds into short-chain aliphatic compounds. Thus, under relatively low stack load conditions, reformate  316  entering stack  12  has high concentrations of H 2  and H 2 O, moderate amounts of CO and CO 2 , and a small amount of methane gas (CH 4 ); whereas, under relatively high stack load conditions, reformate  316  entering stack  12  has a high concentration of H 2 O, relatively little H 2  and CO, and a large amount of methane gas (CH 4 ). 
     Reformer  18  may be fueled by a wide range of hydrocarbon fuels including but not limited to gasoline, JP-8, diesel, LPG, and natural gas. Further, the addition of hydrocarbon cracker  360  permits operation of stack  12  at any ratio of endothermic reforming to internal reforming between 100% endothermic and 100% internal, depending upon load  138 . 
     In operation, controller  362  senses demanded load  138  and commands actuators  364  that control at least air flow  366  and fuel flows  368 , 370  with programmed responses to provide the optimal endothermic/internal reforming ratio. 
     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.