Abstract:
A method for improving the efficiency of a hydrocarbon catalytic reformer and close-coupled fuel cell system by recycling a percentage of the anode exhaust syngas directly into the reformer in a range between about 20% and about 60%. Oxygen is supplied to the reformer at start-up. Under equilibrium conditions, oxygen required for reforming of hydrocarbon fuel is derived entirely from endothermic reforming of water and carbon dioxide in the recycled syngas. Recycling of anode syngas into the reformer increases fuel efficiency, adds excess water to the reformate to increase protection against anode coking, and protects the fuel cell stack against air- and water-borne contaminants. A method for producing an excess amount of syngas for exporting for other purposes is also provided.

Description:
This invention was made with Government support under DE-FC26-02NT41246 awarded by DOE. The Government has certain rights in this invention. 

   TECHNICAL FIELD 
   The present invention relates to high temperature fuel cells having a solid-oxide electrolytic layer separating an anode layer from a cathode layer; more particularly, to high temperature fuel cell systems comprising a plurality of individual fuel cells in a stack wherein fuel is provided by an associated catalytic hydrocarbon reformer; and most particularly, to such a fuel cell system wherein steady-state reforming is substantially endothermic and wherein anode tail gas is recycled through the reformer to improve system efficiency. 
   BACKGROUND OF THE INVENTION 
   Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a non-permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid-oxide fuel cell” (SOFC). It is further known to combine a plurality of such fuel cells into a manifolded structure referred to in the art as a “fuel cell stack” and to provide a partially-oxidized “reformate” fuel to the stack from a hydrocarbon catalytic reformer. 
   Prior art catalytic partial-oxidizing (POX) reformers typically are operated exothermically by using intake air to partially oxidize hydrocarbon fuel as may be represented by the following equation for a hydrocarbon and air:
 
C 7 H 12 +3.5(O 2 +3.77N 2 )→6H 2 +7CO+13.22N 2 +heat.  (Eq. 1)
 
Prior art reformers typically are operated slightly fuel-lean of stoichiometric to prevent coking of the anodes from decomposition of non-reformed hydrocarbon within the fuel cell stack. Thus some full combustion of hydrocarbon and reformate occurs within the reformer in addition to, and in competition with, the electrochemical combustion of the fuel cell process. Such full combustion is wasteful of fuel and creates additional heat which must be removed from the reformate and/or stack, typically by passing a superabundance of cooling air through the cathode side of the stack.
 
   It is known to produce a reformate containing hydrogen and carbon monoxide by endothermic steam reforming (SR) of hydrocarbon in the presence of water in the so-called “water gas” process, which may be represented by the following equation:
 
C 7 H 12 +7H 2 O+heat→13H 2 +7CO.  (Eq. 2)
 
Many known fuel cell systems use water in the reforming process, either recovered from the fuel cell exhaust or supplied to the system. In the case of recovered water, a large heat exchanger is required to condense the water, adding mass, cost, and parasitic losses to the system. In the case of supplied water, the water must be filtered and deionized, resulting in added cost, complexity, and maintenance requirements. In addition, for vehicular applications, the water must be stored, transported with the reformer, and periodically replenished.
 
   It is also known to produce a reformate containing hydrogen and carbon monoxide by endothermic reforming of hydrocarbon in the presence of carbon dioxide in the so-called “dry reforming” process, which may be represented by the following equation:
 
C 7 H 12 +7CO 2 +heat→6H 2 +14CO.  (Eq. 3)
 
   High temperature fuel cells inherently produce a combination of direct current electricity, waste heat, and syngas. The syngas, as used herein, is a mixture of unburned reformate, including hydrogen, carbon monoxide, and nitrogen, as well as combustion products such as carbon dioxide and water. In some prior art fuel cell systems, the syngas is burned in an afterburner, and the heat of combustion is partially recovered by heat exchange in heating incoming air for reforming or for the cathodes, or for both. In other prior art fuel cell systems, a portion of the anode syngas is recycled into the anode inlet to the fuel cell, in conjunction with fresh reformate, to improve the overall fuel efficiency of the fuel cell system. 
   What is needed in the art is a means for improving still further the fuel efficiency of a hydrocarbon reformer process. 
   What is also needed in the art is a means for improving the power density of a fuel cell stack. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Briefly described, a method and apparatus for operating a hydrocarbon catalytic reformer and close-coupled fuel cell system in accordance with the invention comprises recycling a percentage of anode syngas into the reformer, preferably in a range between about 20% and 60%. Although air must be supplied to the reformer at start-up, after the system reaches equilibrium operating conditions, some or all of the oxygen required for reforming of hydrocarbon fuel may be derived by endothermically reforming water and carbon dioxide in the syngas. Efficiency is improved over similar prior art fuel cell systems because the water is kept and used in the gas phase, thus obviating the need for a condenser. 
   Recycling of anode syngas into the reformer a) increases fuel efficiency by endothermic reforming of water and carbon dioxide in the syngas in accordance with Equation 2 above; b) adds excess water to the reformate to increase protection against anode coking; and c) provides another opportunity for anode consumption of residual hydrogen and carbon monoxide in the syngas. In addition, this “recycle reformate” provides a more concentrated fuel supply to the stack since little or no air is added to the reformer. Air added to the reforming process adds significant amounts of nitrogen which dilutes the resulting reformate. The higher concentration of fuel gasses in “recycle reformate” increases the power output of the fuel cell stack. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawing, in which 
       FIG. 1  is a schematic drawing of a high temperature fuel cell system in accordance with the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a high temperature fuel cell system  10  as may be suited to use as an auxiliary power unit (APU) in a vehicle  11  includes components known in the art of solid-oxide or molten carbonate fuel cell systems.  FIG. 1  is not a comprehensive diagram of all components required for operation but includes only those components novelly formed and/or arranged in accordance with the apparatus and method of the invention. Missing components will be readily inferred by those of ordinary skill in the art. 
   A hydrocarbon catalytic reformer  12  includes a heat exchanger/combustor  14 , preferably formed integrally therewith. A fuel cell stack  16  comprises preferably a plurality of individual fuel cell elements connected electrically in series as is known in the art. Stack  16  includes passageways for passage of reformate across the anode surfaces of the stack and passageways for passage of air across the cathode surfaces of the stack, as is well known in the prior art. A cathode air heat exchanger  22  includes an intake air side  24  and a combuster exhaust gas side  26 . A pump  28  is provided for recycling a portion  29   a ,  29   b  of the anode tail gas  31 , or syngas, into an inlet of reformer  12 . Optionally, stream portion  29   a  may be cooled as it enters pump  28  by optional heat exchanger  37 . The heat  27  absorbed from stream portion  29   a  can be used, as for example, for fuel vaporization, and for preheating of reformer inputs. An additional portion  33  of tail gas  31  may also be provided to exchanger/combustor  14 , and the balance  35  may be exhausted or diverted to other purposes. 
   Endothermic reforming with syngas recycle may be represented by the following equation,
 
C 7 H 12 +9H 2 O+10.5CO 2 +heat→10H 2 +10CO+5H 2 O+7.5CO 2   (Eq. 4)
 
Hydrogen and oxygen, combined to produce water in the electrochemical process of the fuel cell stack, are recovered by endothermic reforming and are used over again, thus greatly increasing the hydrocarbon fuel efficiency of the system. Further, the energy required for the water reforming is derived from the “waste” energy in the anode syngas which in prior art high temperature fuel cells is entirely discarded in the cathode cooling air and/or through the system exhaust.
 
   In operation, fuel  15   a  is controllably supplied from a source (not shown) to an inlet  30  of reformer  12 , as is known in the art. Fuel may comprise any conventional or alternative fuel as is known in the art, for example, gasoline, diesel, jet fuel, kerosene, propane, natural gas, carbon, biodiesel, ethanol and methanol. Air  17   a  is supplied from a source (not shown), such as a blower or other air pump, to intake air side  24  of heat exchanger  22  and thence to stack  16 . A portion of air  17   a  may be diverted selectively around heat exchanger  22  by control valve  32  to control the temperature of the air entering the fuel cell stack. 
   At start-up, fuel  15   b  and air  17   b  are also supplied to a reformer pre-heater  34  connected to an inlet  36  on reformer  12 . The air/fuel mixture in pre-heater  34  may be combusted therein, as by a spark igniter, or alternatively may be reformed therein upon an electrically-heated catalyst, to provide a hot exhaust for rapid warming of catalytic elements in reformer  12  to provide a rapid start-up of system  10 . At a time after start-up when such heating is no longer needed, the air flow and fuel flow to pre-heater  34  may be terminated. 
   Reformate  40  is supplied from reformer  12  to anodes in stack  16 . Syngas  31  (anode tail gas) is exhausted from stack  16  and is preferably assisted by inline pump  28 . First portion  29  of the exhausted syngas is recycled to an inlet of reformer  12 ; preferably, recycled portion  29  is between about 20% and about 60% of total syngas flow  31 . Second portion  33  of the exhausted syngas is recycled to an inlet of heat exchanger/combustor  14 . Heated cathode air  38  is exhausted from stack  16  and is provided to heat exchanger/combustor  14  wherein it is mixed with syngas portion  33  and combusted to provide heat for endothermic reforming of water and carbon dioxide with hydrocarbon fuel in reformer  12 . Spent air and combustion products  42  are exhausted from heat exchanger/combustor  14  and passed through exhaust side  26  of heat exchanger  22  wherein heat is abstracted by intake air  17   a  in inlet side  24 . Cooled exhaust is discharged to atmosphere  44 . 
   Optionally, additional fuel  15   c  may be controllably supplied to reformer  12  from a source (not shown) so that a greater portion of tailgas  31  may be exported for other purposes through exhaust  35 . 
   Under these or similar steady-state operating conditions, little or no outside air need be provided to reformer  12 . Sufficient heat is provided to the reformer from the sensible heat of the recycled tail gas plus combustion of syngas portion  33  to permit endothermic reforming of the input fuel and the water and carbon dioxide in the syngas. Most or all of the needed reforming oxygen is derived from the water and carbon dioxide. 
   The following benefits accrue to a fuel cell system in accordance with the invention: 
   1. The net fuel/electric efficiency of the system may be substantially increased over prior art high temperature fuel cell systems. Most of the system efficiency improvement is from higher reforming efficiency. Some of the improvement is from higher effective stack fuel utilization. 
   2. The power density of the stack is increased by increasing the concentration of reactants in the stack and by minimizing concentration polarization by less nitrogen dilution. 
   3. The system is allowed to operate with a higher margin of safety in terms of carbon formation in the reformer, reformate piping, or stack inlet. 
   4. The admission of water-borne contaminants on the fuel cell anodes is avoided or eliminated altogether, by eliminating the need for exogenous water and exogenous oxygen. All fuel cells tend to have sensitivities to trace contaminants which come into the system over time in the air, fuel, and/or water consumed in operation. The level of sensitivity depends in part upon the fuel cell technology and the operating temperature. While solid oxide fuel cells tend to have less sensitivity to contaminants than some other types, the accumulation of sulfur, metal oxides, salts, carbon, and other contaminants can lead to long term loss in performance. In endothermic reforming in accordance with the invention, the combustion water and oxygen are chemically pure, resulting from generation within the fuel cell system itself. 
   Using recycled anode exhaust as the steady-state oxidant for the system allows a near fully endothermic (using only recycle) reforming process. Depending upon the selected operating temperature for the stack and reformer, the efficiency of heat recovery in the final exhaust and the minimization of thermal losses to the walls, there may not always be a balance between heat required to preheat the reactants and do the endothermic chemistry with heat available through simple heat exchange from the cathode exhaust. Therefore, a portion of the anode exhaust which is not recycled into the reformer may be used as shown to supply combustive heat to the reformer to support the endothermic reforming process. 
   Reformate is a highly useful fuel which in itself can be exported for use on other apparatus, for combustion and/or exhaust after-treatment functions. It is possible to operate the reformer to produce excess reformate for these additional uses. To further improve the fuel cell system efficiency, the exported reformate may be taken from downstream of the stack, with reduced fuel utilization in the stack resulting in improved stack efficiency. If this export periodically is not necessary (e.g., vehicle engine or other reformate consumer is off) then the reformate volume can be reduced to just the amount required by the fuel cell stack with higher utilization. 
   While the invention has been described by reference to 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.