Abstract:
An SOFC fuel cell stack system including means for recycling a portion of the SOFC anode tailgas into the inlet of a hydrocarbon reformer supplying reformate to the stack. Recycle means includes a pump. A first heat exchanger ahead of the pump cools the tail gas via heat exchange with incoming cathode air, allowing use of an inexpensive pump. To facilitate endothermic or steam reforming of hydrocarbons, CO 2 , and water in the later portions of the reformer, heat is added back into the tailgas recycle by installing a second heat exchanger or an electric heater downstream of the pump. Air and fuel being supplied to the reformer also are preheated, and reforming air flow may be increased to increase exothermic activity in the early portions of the reformer to permit recycling of large percentages of tailgas such that greater overall reformer efficiencies are achievable.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to fuel cells; more particularly, to means for recycling a portion of the anode tail gas of a solid oxide fuel cell (SOFC) stack into a hydrocarbon reformer supplying reformate to the stack; and most particularly, to improved means for such recycling of anode tail gas to increase overall fuel and reforming efficiency. 
       BACKGROUND OF THE INVENTION 
       [0002]    Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate to combine with hydrogen atoms to produce electricity and water; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs). 
         [0003]    In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a partially oxidizing (CPOx) catalytic hydrocarbon reformer. 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, respectively, of the hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C. 
         [0004]    CPOx of hydrocarbon fuels benefits from simple design, robust behavior over the whole range of system operability, long durability and life, and low cost manufacturing. However, the CPOx fuel reforming process is inherently limited to relatively low reforming efficiencies in the 70-80% range. 
         [0005]    Adiabatic fuel reforming with recycle ingestion, combining exothermic and endothermic reforming sequentially within a single reformer, offers the advantage of recycling unused H 2  and CO within the stack anode tailgas back into the system through the reforming reactor, thus enriching the reformate. It is known in the art to recycle a portion of the tailgas from the stack anodes into the inlet to the reformer, which improves stack power density and system efficiency and reduces carbon precipitation and deposition in the system. A problem with this improvement is that a significant pressure drop occurs from the inlet of the reformer to the outlet of the SOFC stack. Thus, a high-pressure flow source is required in the system to return the anode tailgas to the reformer inlet, which source adds significantly to the cost of the system; and further, control may become unstable under low-flow conditions. 
         [0006]    For example, it is disclosed in co-pending application, US 2006/0263657 A1 (“the &#39;657 application”) to employ an aspirator or a gas pump to meter the recycled anode tailgas at a steady and controllable flow rate. However, the aspirator requires a high flow volume of an aspirating gas, which causes a parasitic system loss. 
         [0007]    The gas pump presents a different problem in that a gas pump which can withstand the exhaust temperatures (ca. 700-850° C.) of the stack tailgas must be formed of very expensive high-temperature materials. Therefore, for a presently-practical system employing a relatively inexpensive gas pump, the tailgas is passed through a heat exchanger wherein the temperature is reduced to about 150° C. 
         [0008]    Given that in most prior art cases fuel and air are at best moderately pre-heated (ca. 150° C.) and the recycle is ingested at a low temperature of about 150° C., the ingestion of cooled tailgas into an adiabatic reactor can have several negative effects:
       1. Highest efficiencies cannot be reached because the air flow to the reactor must be increased to prevent the onset of carbon formation and stack deactivation. The amount of air generally dictates the efficiency at the outlet of the reactor because the exothermic reactions of the molecular oxygen in the air with fuel create H 2 O and CO 2  rather than H 2  and CO. Maximum efficiencies are around 100% or just above with high recycle flow rates of 30% to 50%.   2. The necessary increase in air flow increases the amount of molecular nitrogen (N 2 ) flow to the system. Nitrogen is an inert to the system. As part of the fluid, however, it must be heated to reforming temperature without performing any useful work. This represents one of the major loss factors of the fuel cell. Further, large amounts of the nitrogen that was heated to reforming temperature returns with the recycle gas at a low temperature only to be heated again. Nitrogen flows of 50% of the total volumetric reformer flow are not unusual at high rates of recycle. Typical nitrogen flows at CPOx and endothermic reforming are around 35%.   3. The high content of nitrogen in the reformate flow acts as a diluent. This dilution of the reforming gases lowers the rate of diffusion of hydrogen and carbon monoxide to the stack anode. The effect is a decrease in power density of the stack which thus requires either a larger stack or leads to lower stack fuel utilization.   4. Last but not least, the ingestion of large amounts of recycle leads to a breakthrough of parent fuel and C2 or higher hydrocarbons. The increased amount of O 2  to the system together with the large amounts of ingested H 2 O and CO 2  keeps the reactor in thermodynamically stable reforming at reforming temperatures well above the carbon formation margin. However, large amounts of unreacted fuel entering the SOFC stack are acceptable only with methane and to some extent with natural gas. The stack cannot tolerate large amounts (5% or more at high recycle rates) of higher hydrocarbons or fuels such as diesel or JP8.
 
Thus the recycle percentage is limited in the prior art to about 20-25%.
       
 
         [0013]    What is needed in the art is a simplified method and apparatus for allowing endothermic reaction of high levels of tailgas recycle to increase the reformer efficiency. 
         [0014]    It is a principal object of the present invention to increase the overall fuel efficiency of a CPOx reformer coupled to an SOFC by recycling a relatively high percentage of anode tailgas into the reformer. 
       SUMMARY OF THE INVENTION 
       [0015]    Briefly described, an SOFC fuel cell stack system in accordance with the invention includes a path for recycling a portion of the SOFC anode tailgas into the inlet of an associated hydrocarbon reformer supplying reformate to the stack. The recycle path includes a controllable pump for varying the flow rate of tailgas. A first heat exchanger is provided ahead of the pump for cooling the tailgas via heat exchange with incoming cathode air, permitting use of an inexpensive gas pump as in the prior art. To facilitate endothermic or steam reforming of hydrocarbons, CO 2 , and water in the later portions of the reformer, heat must be increased in the reformer. Some heat may be added back into the tailgas recycle flow stream by installing a second, reversing heat exchanger or an electric heater downstream of the pump. Preferably, the fresh air and fuel being supplied to the reformer is also substantially preheated; and further, the air flow into the reformer is also increased to cause increased exothermic activity (catalytic oxidation and/or combustion) in the early (exothermic) portions of the reformer, thus increasing the amount of heat fed to the endothermic, latter portions of the reformer. The increase in heat supplied to the latter portions of the reformer permits recycling of much larger percentages of tailgas than is allowed by prior art systems, such that overall reformer efficiencies of 130% or greater may be achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is a schematic drawing of a first prior art embodiment of an SOFC stack system; 
           [0018]      FIG. 2  is a schematic drawing of a second prior art embodiment, taken in circle  2 , 3  in  FIG. 1 ; 
           [0019]      FIG. 3  is a schematic drawing of a third prior art embodiment, taken in circle  2 , 3  in  FIG. 1 ; and 
           [0020]      FIG. 4  is a schematic drawing of a first embodiment in accordance with the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Referring to  FIG. 1 , in a first prior art embodiment  10   a  of a solid oxide fuel cell stack assembly, substantially as shown as  FIG. 1  of the &#39;657 application, hydrocarbon fuel  13  is supplied from a fuel tank  12  via a control valve  14  to a vaporizing chamber  16 . Air  18  is supplied through a filter  20  and a cooling shroud  22  to a main air blower  24  powered by a motor  26 . Blower  24  supplies pressurized air to plenum  28 . A first air control valve  30  meters air  31  from plenum  28  into vaporizing chamber  16  where the air and fuel are mixed into a vapor mixture which passes into a start-up combustion chamber  32 . Chambers  16 , 32  are structurally part of a catalytic hydrocarbon reformer assembly  36 . During cold start-up of the SOFC system, the vapor mixture is ignited by a spark igniter  34  to provide a flow of hot gas through the reformer and the SOFC stack. During normal operation, the vapor mixture is not ignited but rather is passed into reformer  36  wherein the mixture is converted into reformate fuel containing hydrogen and carbon monoxide gases. Reformate is passed across the anode side  37  of SOFC stack  38 . Air is supplied from plenum  28  via a second air control valve  40  and a third air control valve  42 . Air flowing through third control valve  42  is passed through a cathode air heat exchanger  44  and is tempered as described below. The two air streams are joined  46  and passed to the cathode side  47  of SOFC stack  38 . Within stack  38 , oxygen from the air is combined with hydrogen and with carbon monoxide in an electrochemical reaction producing an electric potential across a stack anode lead  48  and a stack cathode lead  50 . Heated and depleted cathode air  51  is exhausted from stack  38  and passed to a combustor  52 . A first portion  53  of anode tailgas exhausted from stack  38  is also passed to combustor  52 , and the air/fuel mixture may be augmented by the addition of air from plenum  28  via a fourth control valve  54 . The tailgas is ignited by a spark igniter  56  in combustor  52 , and the hot exhaust gas  57  is passed through a heat exchanger side  58  of reformer assembly  36  to raise the temperature within the reformer. Exhaust gas  59  from reformer assembly  36  is passed through cathode air heat exchanger  44  to raise the temperature of incoming air and then is exhausted  60  to atmosphere. 
         [0022]    Referring still to  FIG. 1 , a second portion  62  of anode tail gas is provided via a recycle flow leg  63  to an inlet to vaporizing chamber  16  via a check valve  64  and a recycle pump  66  comprising an impeller, shaft, shaft bearing, and sealed impeller housing to physically pump the recycle gas. A controllable electric motor drive  68  powers the impeller via the impeller shaft. 
         [0023]    The anode tailgas at the inlet to the pump is at the stack operating temperature range of about 700° C. to 850° C., which can present a challenge with respect to providing bearings and seals suitable for use at these temperatures. Further, the motor must be thermally isolated from these temperatures within the pump. 
         [0024]    Referring to  FIG. 2 , in a second prior art embodiment  10   b,  an anode tailgas cooler in the form of a second heat exchanger  70  is provided in flow leg  63  ahead of primary cathode air heat exchanger  44  in the cathode air flow  72  from valve  42 . Anode tailgas portion  62  is passed through one side of exchanger  70 , and cathode air  72  is passed through the opposite side. Exchanger  70  reduces the temperature of tailgas  62  to an inlet temperature (tailgas  63 ) to pump  66  suitable for conventional technology (bearings, seals, motors). Further, this reduction in temperature improves the efficiency of the impeller within the pump by increasing the density of the pumped tailgas. 
         [0025]    The cathode air requirement of the system generally tracks with stack power, as does the flow rate of recycle gas  62 . This allows a single passive heat exchanger  70  to effectively cool the recycle gas under all operating conditions. Further, because of the high ratio of cathode air mass flow to recycle gas mass flow under all operating conditions, the recycle gas temperature at the pump inlet is fairly insensitive to changes in cathode air flow volume. Both of these are desirable characteristics of the invention. 
         [0026]    Under some conditions of off-peak system use, however, tailgas  62  may be cooled to a temperature below what is required for proper pump operations. In fact, because the recycled anode tailgas contains a high water content, it is possible that under such conditions water will be condensed in heat exchanger  70 . Because exchanger  70  is a passive device, no direct temperature control is possible. Referring to  FIG. 3 , in a third prior art embodiment  10 c an optional electric heater  74  having a low wattage capability can be installed in or around the line between heat exchanger  70  and check valve  64  and can be controlled to maintain the drybulb temperature of anode tailgas  63  above its dewpoint temperature (typically about 75° C.) when entering pump  66 , thus preventing sensible water from entering pump  66  and reformer  36 . 
         [0027]    Referring now to  FIG. 4 , a first improved embodiment  110   a  is shown in accordance with the invention. Embodiment  110   a  is largely the same as prior art embodiments  10   a,   10   b,   10   c  and all identical item numbers are shown as in  FIGS. 1-3 . The difference is that reformer  36  is not provided with a heat exchanger  58 ; rather a separate heat exchanger  158  is provided to heat incoming tailgas, having a first side for receiving the hot exhaust  157  from combustor  52  which then continues on as exhaust  59 , albeit at a somewhat lower temperature, to prior art heat exchanger  44  as in the prior art. The cool tailgas  63  ( FIGS. 2-3 ) from pump  66 , cooled by heat exchanger  70  prior to entering pump  66 , is directed through the second side of heat exchanger  158  to provide a heated tailgas stream  171  entering into mixing chamber  16  of reformer assembly  36 . 
         [0028]    It will be seen by those of ordinary skill in the art that embodiment  110   a  is only one exemplary embodiment for raising the temperature of the cooled anode tailgas  63  between pump  66  and reformer  36 . Obviously, other means (not shown) for providing such heating can be heat exchange with the waste heat in anode tailgas stream  53  and/or cathode tailgas  51 , or electrical heating in known fashion. The operative concept in accordance with the invention, which anticipates all such embodiments, is that the temperature of cooled anode tailgas flow  63  is raised before the tailgas is entered into the reformer. 
         [0029]    A noted above, increased exothermic activity in the early portions of reformer  36  is also highly desirable. Thus, additional heat exchange or electrical heating (not shown) is preferably provided to either or both of air flow  31  and fuel flow  13  prior to their entry into mixing chamber  16 . 
         [0030]    Further, the volume of air flow  31  preferably is increased. In order to use large amounts of recycle, on the order of 40% to 60%, a substantial amount of air must be added to the mixture of fuel and recycle to raise the endothermic reforming temperature through exothermic reactions inside the reforming catalyst. This decreases somewhat the reforming and system efficiency but allows operation of the reformer in a thermodynamically stable fashion which avoids the formation of large amounts of carbon and the destruction of the stack. However, another side effect can be the breakthrough of fuel and higher hydrocarbons (if present in the parent fuel) into the fuel cell stack, as well as substantial dilution of the reformate due to the increase in N 2  through the increase in air flow. Water and CO 2  are present in the reformate, but the reforming temperatures are so low that the time to convert the residual hydrocarbons to H 2  and CO is not sufficient. Thus, it is necessary to preheat the recycle as described above, or still better, to also preheat all the reactants prior to injection into the reforming reactor (not shown) as known by those skilled in the art. Preheat of the reactants allows a significant decrease in the amount of air otherwise required to maintain the reforming process within thermodynamically stable boundaries. At high pre-heat of the fuel/air/recycle mixture, the amount of air can even be lower than the air requirement without any recycle addition. The pre-heat, which utilizes exhaust heat, together with the decrease in air volume at high recycle rates of 40% to 60% can deliver reforming efficiencies of greater than 130% and therefore very high system efficiencies. Adiabatic reformer  36  can be a replaceable, inexpensive ceramic catalyst in contrast to a purely endothermic reformer which is very expensive and cannot be serviced or replaced and must therefore be designed to last much longer. 
         [0031]    As noted above, breakthrough of fuel and higher hydrocarbons into the SOFC stack can be ruinous to the fuel cell anode. Accordingly, a way to solve the problem of hydrocarbon breakthrough is to add a boost endothermic catalyst  76  in series after adiabatic reactor  36  and ahead of fuel cell stack  38 . The boost endothermic reactor is a simple heat change endothermic reactor, as is known in the art and need not be shown or described here, that adds heat to convert any remaining hydrocarbon fuel into H 2  and CO in the presence of large amounts of water and carbon dioxide. Luckily, the present process is not starved of either H 2 O or CO 2 . The boost catalyst may be a rather small device because the gases do not need to be preheated to reforming temperature (already ca. 800° C. from adiabatic reactor  36 ) and a high space velocity is sufficient to reform the remainder of the fuel and higher hydrocarbons. The required heat addition is small and approximately 0.5 kW for a 5 kW system. Another benefit is based on the fact that the boost catalyst can be optimized for endothermic/steam reforming only because the reactor will never be challenged with start-up combustion, air, and molecular oxygen, or CPOx and exothermic high temperature reactions. The boost catalyst system does not require a secondary fuel injection system and is therefore inexpensive and easy to implement even as a retrofit, and can be optimized for long durability and life. The boost reactor system can operate with any hydrocarbon fuel, gaseous or liquid, of any type, e.g., gasoline, diesel, JP8, and the like. 
         [0032]    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.