Abstract:
A method of providing anode gas exhaust from a fuel cell stack and carbon dioxide capture by feeding reformed fuel and air into a fuel cell stack where gas exhaust is fed to a series of oxidation/reduction beds to provide exit streams a) of H 2 O and CO 2  which is fed to a condenser to recover CO 2 , and b) H 2 O and CO which is recirculated to the fuel cell stack.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to fuel cell systems utilizing metal based redox (reduction/oxidation) reaction components which treat and recirculate fuel cell spent fuel gas providing increased hydrogen content and capturing carbon dioxide. 
         [0003]    2. Description of Related Art 
         [0004]    Ceramic fuel cells are energy conversion devices that electrochemically combine carbon fuels and oxidant gases across an ionic conducting solid electrolyte and are disclosed in detail by Nguyen Q. Minh in  J. Am. Ceram. Soc.,  76 [3] 563-88 (1993) “Ceramic Fuel Cells.”  FIG. 1  shows general solid electrolyte fuel cell operation where solid electrolyte  1  is sandwiched between an anode  2 , which receives fuel/reformed fuel  3  and a cathode “air” electrode  4  which receives air/oxidant  5  to generate electrons (electricity)  6 . The reformed fuel has many impurities such as sulfur removed from it. An external load and direct current, exhaust gases and heat out are also shown. 
         [0005]    U.S. Pat. No. 4,729,931 (Grimble) taught that in a high temperature solid oxide fuel cell, air and a fuel are combined to form heat and electricity. Because fuels such as methane and alcohol can, under certain conditions, form carbon or soot at the very high temperatures at which these fuel cells operate, and carbon and soot can reduce the efficiency of the fuel cell, the fuels that can be used in the cell have generally been limited to carbon monoxide and hydrogen. The carbon monoxide and hydrogen can be obtained by reforming fuels such as methane, ethane, and alcohols. Reforming is a process in which the reformable fuel is combined with water and/or carbon dioxide to produce carbon monoxide and hydrogen. The reformed fuel is then used in the solid oxide fuel cell. Since reforming is an endothermic process, additional thermal energy must be supplied either by direct combustion or by heat transfer through the walls of a heat exchanger. 
         [0006]    Solid oxide system applications were discussed by W. L. Lundberg in  Proceedings of the  25 th    Intersociety Energy Conversion Engineering Conference,  Vol. 3; IECEC-90; Aug. 12-17, 1990 Reno Nevada; “System Applications of Tubular Solid Oxide Fuel Cells;” discussing desulfurizers, preheaters for the air stream, power conditioners and their association in a coal powered power plant. Other systems patents include, for example, U.S. Pat. Nos. 5,532,573; 5,573,867; 6,689,499B2; and 6,946,209B1 (Brown et al., Zafred et al., Gillett et al. and Israelson). 
         [0007]    A schematic of the fuel side of a conventional prior art solid fuel cell (SOFC) system  10  operating once-through on natural gas fuel (methane, ethane, possibly propane and butane, with nitrogen, carbon dioxide and sulfur compounds such as H 2 S) is shown in  FIG. 2(A) . It typically features a reformer  12  that transforms the incoming natural gas fuel  14  to a reformed fuel mixture of H 2  and CO  13  that can be converted electrochemically into electric power in a fuel cell stack  16  which contains a plurality of fuel cells. While it is desirable to utilize all the fuel using the thermodynamically efficient electrochemical process, practical considerations such as fuel flow maldistributions limit electrochemical fuel utilization (FU) in such systems to about 70%, as shown in  FIG. 2 ; Fuel-starvation of any area of the stack can result in damage to the SOFC stack  16 . 
         [0008]    Anode/spent fuel gas recirculation  18 , which decreases apparent in-stack fuel utilization, is commonly employed, as shown in  FIG. 2B , to decrease stack sensitivity to fuel flow mal-distribution. Recirculation pump is shown as  20 . Water fed to the reformer is shown as  22  in both figures. Anode gas recirculation with this system is detrimental to performance as it tends to decrease the net electrochemical (Nernst) potential by decreasing the inlet and average mole-fractions of fuel across the stack, thus system FU=70% but stack FU=only 50%. 
         [0009]    The variation of inlet, exit and average Nernst potentials as a function of the ratio of the recirculated anode off-gas volumetric flow to the fresh (reformed) fuel volumetric flow for a system, FU line  24 , of 70% is shown in  FIG. 3 , along with the variation of instack FU and cell DC efficiency (for a representative cell operating point). While the instack FU decreases as the recirculation flow is increased, the average Nernst across the stack decreases resulting in a loss of cell DC efficiency, and consequently, an undesirable overall loss in system performance. All other values (curves) are shown decreasing at increasing flow ratios. 
         [0010]    What is needed is a system where electrochemical fuel utilization (FU) is improved to the point of 85% to 100% and where carbon dioxide can be captured rather than being released to the atmosphere: 13.9% CO 2  in  FIG. 2A  and 13.9% in  FIG. 2B . It is a main object of this invention to provide a system where FU&gt;90% and essentially pure CO 2  can be captured. 
       SUMMARY 
       [0011]    The above needs are met and object accomplished by using a method of providing anode gas exhaust chemical recuperation from a fuel cell stack as well as carbon dioxide capture, comprising the steps: (a) feeding a fuel and optional water to a reformer to provide a reformed fuel stream consisting essentially of H 2 , CO and H 2 O; (b) feeding the reformed fuel as well as feeding air to a fuel cell stack containing a fuel electrode anode, and an air electrode with solid electrolyte between the electrodes, operating at a temperature over 600° C., preferably 600° C. to 850° C., to provide energy and anode gas exhaust containing at least H 2 , H 2 O and CO 2 ; (c) feeding the anode gas exhaust to a first oxidation/reduction bed to provide a first redox exit stream consisting essentially of H 2 O and CO 2 , which is split into a first redox exit stream and a second redox exit stream; (d) feeding the first redox exit stream to a condenser to provide separate CO 2  and H 2 O streams; (e) feeding the second redox exit stream to a second oxidation/reduction bed to form a final redox exit stream (recirculation redox exit stream) comprising at least 65 vol. % H 2 , which final redox exit stream is recirculated back into the reformed fuel in step (a). 
         [0012]    Additionally, a boiler can take a feed of water from the condenser in step (d) to provide steam which is then fed to the second oxidation/reduction bed to provide a final redox exit stream comprising at least 80 vol. % H 2 , which is recirculated back into the reformed fuel of step (a). In another embodiment based on use of a boiler, the two oxidation/reduction beds can be combined with anode gas exhaust from the fuel cell stack fed to a first portion which exits H 2 O and CO 2  into a condenser to recover CO 2  and exit water and with boiler water passing to a second portion which exits H 2  back into the reformed fuel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a better understanding of the invention, reference may be made to the Summary and preferred embodiments exemplary of the invention, shown in the accompanying drawings, in which: 
           [0014]      FIG. 1  is a schematic illustration of one type of SOFC operation; 
           [0015]      FIG. 2A  is a schematic flow diagram of a prior art reformer/SOFC system without anode off-gas/spent fuel recirculation, with approximate gas flow percents; 
           [0016]      FIG. 2B  is a schematic with anode off-gas/spent fuel recirculation with approximate gas flow percents; 
           [0017]      FIG. 3  is a graph of Nernst voltages, cell voltage, in-stack FU (fuel utilization=70%) and cell DC efficiency variation with anode off-gas recirculated flow for prior art system  FIG. 2B ; 
           [0018]      FIG. 4 , which best shows the invention, is a schematic flow diagram of the basic system of this invention utilizing two redox beds and a condenser to capture pure (greater than 98 vol. % CO 2 ) CO 2 ; 
           [0019]      FIG. 5  is a graph of Nernst voltages, cell voltage, in-stack FU (fuel utilization about 100%) and cell DC efficiency variation for the invention system of  FIG. 4 ; 
           [0020]      FIG. 6  is a schematic flow diagram of an optional system of this invention utilizing two redox beds and a condenser to capture pure (greater than 98 vol. % CO 2 ) CO 2  and recirculation of steam from a boiler to a second redox bed; and 
           [0021]      FIG. 7  is a schematic flow diagram of an optional system of this invention utilizing a boiler and combined oxidation reduction beds. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The proposed invention is shown schematically in  FIG. 4 . It shows the SOFC system  10 ′ of this invention, with reformer  32 , natural gas  34 , reformed fuel  13 , fuel cell stack  36 , anode/spent fuel recirculation  38  and  39 , at least one recirculation pump  40  and water  42  fed to the reformer. It utilizes anode gas recirculation in conjunction with metal/metal oxide 
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         [0000]    redox beds  44  (MO x →M) and  46  (M→[MO] x ), to extract H 2 O and CO 2  from the anode off-gas (recirculation gas)  38  and  39  from the stack  36  and return an H 2  and CO exit stream  48  back to the stack  36 . The H 2  and CO in the anode off-gas  39  (recirculation gas) reduces the metal oxide to metal in the first bed  44 , which is designed to completely utilize the H 2  and CO in the incoming recirculation gas  39 . A first portion of the resultant stream of H 2 O and CO 2    41  from the first bed  44  is sent to the second bed  46  and subsequently exhausted as gas  48  from the second bed  46 . A second portion  41 ′ of the H 2 O and CO 2  stream  41 , corresponding to the mass flow rate of fuel and water added to the system (to ensure consistent material balance and avoid system pressurization), is condensed in condenser  52  to yield a stream of essentially pure CO 2 ,  54 , which can be sequestered to enable nearly complete capture of the carbon present in the incoming fuel. Water  56  from the condenser is shown as  56  and can be recycled as stream  42  to reformer  32 . Q is shown as heat transfer. The present invention simplifies the CO 2  separation process without the need for expensive anode gas heat exchangers and complex CO 2  separation technologies. A variety of valves are shown as  50 . 
         [0023]    The H 2 O and CO 2  stream  41  is directed to the second bed  46  where they oxidize the metal (M to MO x ). The resulting exit/exhaust stream of H 2  and CO  48  is recirculated back to the stack inlet at  62  to mix with the incoming reformed fuel  64  and can be utilized efficiently using the electrochemical process of the SOFC stack  36 . The beds  44  and  46  can be sized to support a high recirculation rate, of up to 3 recirculated flow/fresh fuel, as shown in  FIG. 4 , which can reduce the in-stack fuel utilization to low values, increasing its reliability to fuel flow mal-distributions, while still achieving an overall fuel utilization of nearly 100%. In  FIG. 4 , a stream  72  of about 70 vol. % to 80 vol. % H 2  and 20 vol. % to 30 vol. % CO, preferably about 75 vol. % H 2  and 25 vol. % CO can optionally be fed from first bed  44  to valve  50  feeding into the reformed fuel and stack  36 . 
         [0024]      FIG. 5  shows a plot of the variation of the inlet, exit and average Nernst potentials, the in-stack FU, and the cell DC efficiency as a function of the ratio of the recirculated anode H 2  and CO flow to the fresh fuel volumetric flow analogous to prior art  FIG. 3  for the proposed system; where  FIG. 5  shows a system FU line  60  of about 100% vs. a system FU of about 70% on  FIG. 3  for prior art systems. 
         [0025]    It is clear that even for moderate recirculation flows a cell DC efficiency increase of about an additional 30 percentage points can be realized. Part of the efficiency gain is directly due to the ability to increase overall system fuel utilization to about 100%, and the rest is due to the boost in average Nernst provided by the recirculated H 2 , CO flow. Although the proposed system introduces additional parasitic losses such as recirculation pumping loss, the overall system efficiency will be considerably higher than the conventional systems of  FIGS. 2A-2B . Further, as mentioned earlier, the reliability of the system to fuel-flow mal-distribution effects will be greatly enhanced with the system of this invention. Secondary advantages include a potential reduction in airflow required to cool the cells, as recirculation tends to make the cell temperature distribution along the cell more uniform. 
         [0026]    A variety of metals and metal oxide combination may be used for the beds  44  and  46  depending on the requirements to optimize the overall system. The beds contain a metal material selected from the group consisting of:
   Fe, Mn, Co, Cr, Al, Zr, Sc, Y, La, Ti, Hf, Ce, Nt, Cu, Nb, Ta, V, Mo, Pd, W, as well as their alloys and oxides, halides, sulfates, sulfites, and carbonates of these elements. Preferred materials are: Fe, Mn, Co, Cr, Al, Zr and their alloys and oxides. Fe and Fe oxides are most preferred.   
 
         [0028]    Since each bed  44  and  46  gets depleted, switching/reversing of gas flows via line  43  between the beds is necessary once the beds have reached their capacity, to ensure a continuous process. The frequency of switching will depend on the size of the bed. Reduction of the metal on bed  44  generally needs heat input while its oxidation in bed  46  generates heat Q. The beds are intended to be situated so that they can share the heat Q between themselves eliminating the needs for separate thermal management of the beds using heat exchangers. Optionally, the recirculation flow rate can be adapted to thermally manage the beds via sensible heat exchange. 
         [0029]      FIG. 6  shows yet another embodiment of the proposed system where an additional boiler  68  is used to generate steam  70  to oxidize the second bed  46  and essentially only H 2  gas  49  is recirculated back to the reformed fuel and then to the stack  36 . In  FIG. 6 , a stream of essentially all H 2    74  can optionally be fed from first bed  44  to valve  50  prior to entry into stack  36 . 
         [0030]    This invention is neither limited to solid oxide fuel cells (SOFCs) nor their operation on natural gas. Any fuel cells that either use H 2  or CO as their fuel can be adapted to use this system. Additionally, recirculation rates may be adjusted to ensure proper oxygen to carbon ratio to avoid carbon deposition. 
         [0031]    In an alternate embodiment shown in  FIG. 7 , the two beds of  FIGS. 4 and 6  are combined into a single chemical regenerator  80  where identifying numbers from  FIGS. 4 and 6  are repeated. This eliminates some streams of  FIGS. 4 and 6  but somewhat complicates heat transfer between previous beds  44  and  46 . In  FIG. 7 , reference can be made to previous text for system components and flow streams. 
         [0032]    The system of this invention utilizes a high efficiency fuel cell system, which can run reliably at high fuel utilizations with natural gas or any carbonaceous fuel, is presented. The system utilizes metal redox reactions to extract fuel from a fuel cell anode gas stream, which would otherwise be utilized inefficiently by direct combustion. The extracted fuel can be recirculated back to the fuel cell inlet at high rates not only to ensure a high inlet mole-fraction of fuel but also to increase the average Nernst potential across the cell. In theory, a 100% electrochemical utilization of the incoming fuel overall is possible whilst reducing in-stack fuel utilization values resulting in high system electrical efficiencies and enhanced reliability to fuel flow mal-distributions. Further, it also enables complete capture of CO 2  by condensing out the steam from the final anode side exhaust. The system can be easily adapted to existing fuel cell systems with minor modifications. 
         [0033]    While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.