Abstract:
An electrochemical system adapted to facilitate the direct injection of a variety of carbonaceous fuels or to perform steam electrolysis. The electrochemical system comprises either of three operating modes: a single stage fuel cell embodiment, a two stage fuel cell embodiment, and an electrolyzer embodiment. The system further includes a feed tube having appropriate seals introducing carbonaceous fuel or water directly into a fuel mixing chamber inside the electrochemical cell stack. One or more exit conduits allow the gas mixture to exit from the fuel mixing chamber.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to electrochemical systems, such as solid-oxide electrolyte fuel cells and fuel cell assemblies for directly converting chemical energy into electricity. More particularly, the present invention relates to a modified fuel cell system adapted to facilitate the direct injection of carbonaceous fuels.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Planar, or flat, solid oxide fuel cell stacks are well known in the industry. Generally, a fuel cell is an electrochemical device that combines a fuel, such as hydrogen, with oxygen to produce electric power, heat and water. The solid oxide fuel cell consists of an anode, a cathode and an electrolyte. The anode and cathode are porous, thus allowing gases to pass through them. The electrolyte, located between the anode and cathode, is permeable only to oxygen ions as they pass from the cathode to the anode. The passing of the oxygen ions through the electrolyte creates an excess of electrons on the anode side to complete an electrical circuit through an external load to the cathode side, which is electron deficient.  
           [0003]    A solid oxide fuel cell is very advantageous over conventional power generation systems. It is known in the industry that such devices are capable of delivering electric power with greater efficiency and lower emissions as compared to engine-generators.  
           [0004]    Known planar solid oxide fuel cell stacks utilize a forced flow of gases through their electrodes. Furthermore, they employ fuel and air flow designs so that all, or at least many, of the cells are fed the same fuel and air compositions. The stacks are capable of producing good, but not optimal efficiencies. Furthermore, the stacks tend to exhibit significant local flow differences amongst cells and within cells. This can lead to increased stack performance degradation and reduced stack efficiency. Further still, the stacks may require significant pressure drops, and therefore compression power, for the flowing gases.  
           [0005]    Known fuel cell systems for operation on hydrocarbon fuels require processing of the fuel prior to its introduction into a fuel cell stack or bundle. Such processing involves adding water, steam, spent fuel mixture, and/or air to the fuel and passing the resulting mixture over a hot catalyst bed. These fuel processors are vulnerable to solid carbon formation problems, necessitating either frequent cleaning or else operation with fairly high oxygen/carbon ratios which can reduce possible fuel cell efficiency. In addition, the catalysts used often cannot tolerate significant sulfur levels and thus require the fuel to be previously desulfurized.  
           [0006]    It is known that the ability to directly and continuously feed carbonaceous fuels (without prior addition of water or oxygen) to a high temperature fuel cell has significant and distinct advantages. For example, such a system significantly improves the possible efficiency of converting chemical energy to electrical energy. Additionally, such a system eliminates much of the costly auxiliary equipment needed in conventional complete fuel cell systems, such as fuel processors, some heat exchangers, water systems and the like. It is expected that such a system will facilitate the ability to use fuels which are normally difficult to employ with conventional fuel cell systems in a practical manner, such as diesel fuel and distillate heating oils. The present invention reduces the size, weight, complexity and cost of employing a complete fuel cell system  
           [0007]    U.S. Pat. No. 5,366,819 (Hartvigsen et al.) and U.S. Pat. No. 5,763,114 (Khandkar et al) disclose a fuel steam reformer located inside a furnace, which also houses stacks of solid oxide fuel cells. The fuel cell stacks furnish the required heat input to the reformer. The reformer contains a hot steam reforming catalyst bed which converts hydrocarbon fuel (desulfurized natural gas) and water into a hot fuel gas mixture suitable for feeding into the fuel cell stacks.  
           [0008]    U.S. Pat. No. 5,741,605 (Gillett et al.) discloses the use of fuel reformers which use steam-laden spent fuel gas mixed with incoming hydrocarbon fuel and a reforming catalyst bed to produce a hot fuel gas mixture. Like the above patents, this configuration is also thermally integrated with fuel cell stacks and is external to the fuel cell assemblies themselves.  
           [0009]    Thus, there is an unsatisfied need to have a complete fuel cell system adapted for the direct injection of carbonaceous fuels into fuel cell stacks without operating problems resulting from solid carbon formation.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is an electrochemical system adapted to allow for the direct injection of carbonaceous fuels for employment therein. The present invention can be employed as either a single stage embodiment or as a two stage embodiment. The fuel cell stack is operated with an electrochemical fuel utilization that is high enough, such as at least 30%, to supply enough oxygen to the fuel mixture in order to prevent significant amounts of carbon to accumulate in the fuel cell system&#39;s fuel chamber.  
           [0011]    It is an object of the present invention to provide an electrochemical system adapted to facilitate the direct injection of carbonaceous fuels.  
           [0012]    It is another object of the present invention to provide an electrochemical system adapted to allow for the direct injection of carbonaceous fuels having a single stage embodiment and a two stage embodiment.  
           [0013]    It is yet another object of the present invention to provide an electrochemical system whereby the accumulation of carbon within the system is controlled to permit continuous power generation for thousands of hours without carbon removal being required.  
           [0014]    It is still yet another object of the present invention to permit steam electrolyzer operation using an unmixed steam feed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a cross section of two adjacent, identical cells, contained in a stack of such cells, of a single stage configuration of the present invention.  
         [0016]    [0016]FIG. 2 is a cross section of two adjacent, identical cells, contained in a stack of such cells, of a two stage configuration of the present invention.  
         [0017]    [0017]FIG. 3 is a cross section of two adjacent, identical cells, contained in a stack of such cells, of a steam electrolyzer of the present invention.  
         [0018]    [0018]FIG. 4 is a cross-section of an alternative embodiment of the present invention as shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The present invention is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details.  
         [0020]    Referring now to FIG. 1, a cross section showing a single hollow circular cell  10  contained in a stack  12  of like cells of the single stage configuration system of the present invention is shown. It is also noted that FIG. 1 shows two adjacent cells having like elements. For purposes of explanation, stack  12  is referred to as having just one cell  10 , however any numbers of cells  10  may be employed in stack  12 . A cylinder centerline  14  is also shown. Cells  10  surround a fuel mixing chamber  18 . An oxidizer chamber  38  surrounds stack  12  and provides a source of oxygen to the stack.  
         [0021]    Each cell  10  is separated from and electrically connected to adjacent cells by an electronically conductive separator disc  22   a, b . Each cell  10  contains only one separator disc  22   a , the second separator disc  22   b  being a separator of an adjacent cell. Inside each cell  10  is a solid oxide electrolyte disc  24 . A fuel electrode  26  abuts electrolyte disc  24  directly below electrolyte disc  24 . Fuel electrode  26  may advantageously be a sulfur tolerant fuel electrode, such as that described in U.S. Pat. No. 6,238,816 B1, the details of which are incorporated by reference herein. A fuel diffusion layer  28  is positioned between the fuel electrode  26  and separator  22   b . An oxygen electrode  32  abuts electrolyte disc  24  directly above electrolyte disc  24 . An oxygen diffusion layer  30  is positioned between the oxygen electrode  32  and separator  22   a . Both fuel diffusion layer  28  and oxygen diffusion layer  30  are highly porous and sufficiently thick so as to allow the requisite gases to diffuse through them with only moderate composition gradients. Layers  28  and  30  are also good electrical conductors. It is appreciated that fuel electrode  26  and fuel diffusion layer  28  could alternatively comprise the same material, thereby being a single structure, such as a fuel electrode-diffusion layer  310  (FIG. 4). Fuel electrode-diffusion layer  310  would serve the same purposes of both fuel electrode  26  and fuel diffusion layer  28 . Additionally, oxygen diffusion layer  30  and oxygen electrode  32  could be a single structure, such as an oxygen electrode-diffusion layer  320  (FIG. 4).  
         [0022]    A fuel electrode annular seal  34  surrounds fuel electrode  26  and fuel diffusion layer  28 . Seal  34  extends from separator  22   b  to electrolyte disc  24 . The upper end of seal  34  is substantially flush with electrolyte disc  24 . The lower end of seal  34  is substantially flush with separator  22   b . An oxygen electrode annular seal  36  is located inside oxygen electrode  32  and oxygen diffusion layer  30 . Seal  36  extends from electrolyte disc  24  to separator  22   a . The upper end of seal  36  is substantially flush with separator  22   a . The lower end of seal  36  is substantially flush with electrolyte disc  24 .  
         [0023]    Separators  22   a  and  22   b  can be made of any material common in the field, such as a high-temperature alloy which forms a thin protective oxide surface layer with good high-temperature electrical conductivity. Electrolyte disc  24  may be of yttria-stabilized zirconia, or any other suitable material. Fuel electrode  26  and fuel diffusion layer  28  can be of, for example, a doped ceria/nickel mixture. Nickel foam may be used for fuel diffusion layer  28  except in cells operating on fuel mixtures with very high oxygen potentials. Oxygen electrode  32  and diffusion layer  30  can be of, for example, strontium-doped lanthanum manganite. Seals  34  and  36  can be made from a suitable glass. A thin layer of ink, such as an ink made from a finely-divided electrode composition, may be applied on each side of separators  22   a, b . Ink is applied to improve the electrical contact between the components of cell  10 .  
         [0024]    Hot oxidizer manifold  38  contains an oxygen bearing gas mixture, which is typically comprised of nitrogen, oxygen, water vapor and carbon dioxide.  
         [0025]    An unsealed fuel flow layer  40 ,  42  is at each end of stack  12 . Unsealed fuel flow layers  40 ,  42  allow partially oxidized fuel gas to continually exit from stack  12 . Stack  12  is additionally clamped or situated between a first electrically conductive end plate  44  and a second electrically conductive end plate  46  via a spring-loaded clamping means (not shown) or any other method conventional in the art. A first unsealed fuel flow layer  40 , which is substantially annular in form, is directly below and abuts the bottom of first end plate  44 . The second unsealed fuel flow layer  42  is directly above and abuts the top of the second end plate  46 , both of which are substantially annular in form.  
         [0026]    Stack  12  still further includes a thermal insulation  48  at the base of stack  12  and below second end plate  46 . A fuel feed tube  50  is introduced into stack  12  through the center of stack  12  at its end having thermal insulation  48 , second unsealed fuel flow layer  42  and second end plate  46 , and is introduced between thermal insulation  48  and the annular second end plate  46  overlying thermal insulation  48 . Fuel feed tube  50  serves as a conduit for the introduction of a carbon-containing fuel, such as natural gas, diesel fuel and distillate oils, directly into the center of stack  12 . Fuel feed tube  50  is sealed by welding to second end plate  46  via a sealing tube  52  and disc  54 . Thermal insulation  48  is also present in the annulus between tubes  50  and  52 .  
         [0027]    Thermal insulation  48  can optionally further include additional insulation (not shown) and thermal insulation  48  serves to insulate the heated stack  12  and fuel feed tube  50  during operation. It is noted that fuel electrode  26 , fuel diffusion layer  28 , oxygen diffusion layer  30 , oxygen electrode  32 , unsealed fuel flow layer  40 , unsealed fuel flow layer  42  and thermal insulation  48  are porous and permit gas to flow through them. The remaining elements in FIG. 1 are substantially impervious.  
         [0028]    Referring now to the operation of stack  12  and still referring to FIG. 1, stack  12  is generally preheated by a suitable preheating means (not shown) conventional in the art and preheated to a suitable temperature that is sufficiently hot, such as about 850° C. A gaseous or liquid carbonaceous fuel is introduced into stack  12  via fuel feed tube  50  at a sufficiently high flow rate so that the temperature of the carbonaceous fuel upon exit of stack  12  is low enough to prevent the formation of solid carbon or any other solid deposits to form or be deposited within fuel feed tube  50 . A typical maximum fuel feed temperature is about 400° C.; however the temperature is fuel-type dependent. Carbonaceous fuels suitable for use with this invention include natural gas, propane, gasoline, diesel fuel, kerosene, distillate heating oils, and other gaseous and distillate liquid hydrocarbons. Other suitable fuels include biogas, biodiesel, alcohols, and mixtures of gases or liquids containing carbonaceous compounds, including fuels from gasifiers. Generally, the suitable fuels must be essentially free from dissolved salts and particulates and contain limited levels of halogens and sulfur.  
         [0029]    Stack  12  is operated with an electrochemical fuel utilization high enough, such as at least 30%, so that enough oxygen is supplied to the fuel mixture in fuel chamber  18 , thus preventing a significant carbon accumulation in chamber  18 . It is appreciated that the minimum value depends upon the type of fuel used and stack operating temperature. It is also appreciated that carbon deposits between fuel feed tube  50  and sealing tube  52  will typically occur and such deposits are tolerable.  
         [0030]    Fuel feed tube  50  has a diameter and a spatial orientation such that a high fuel entry velocity and a sufficient mixing of the gas in fuel chamber  18  is achieved. It is noted that various operating conditions can be obtained by varying the flow of fuel through fuel feed tube  50  and stack current, thereby providing a relatively wide range of stack  12  power outputs and efficiencies.  
         [0031]    The partially oxidized fuel mixture exits stack  12  through unsealed fuel flow layers  40  and  42  whereby the partially oxidized fuel mixture encounters an oxidizing gas and is immediately completely oxidized. The best efficiencies are achieved when the electrochemical fuel utilization of stack  12  is close to about 90%. For example, at 900° C, the calculated maximum possible fuel cell efficiency of a single stage configuration on natural gas is over 60%, which is calculated by stack power/natural gas lower heating value.  
         [0032]    Hot oxidizer manifold  38  is continually supplied with air preheated by a heat exchange with the exhaust mixture which continually exits hot oxidizer manifold  38 . The temperature of stack  12  is maintained at a desired value, such as about between 800° C. and 900° C. and is maintained in this desired range by the combined cooling effects of incoming air, incoming fuel, and the endothermic chemical reactions (principally fuel molecules reacting with H 2 O and CO 2  gases) which occur in fuel mixing chamber  18  as well as in the cell layers  26  and  28 . These chemical reactions are possibly enhanced by the formation of a persistent cloud of extremely fine solid carbon particles within fuel mixing chamber  18 .  
         [0033]    It is noted that for purposes of explanation, the present invention is described and shown as being circular, however the system of the present invention may also be employed with electrochemical systems of any shape used in the art, such as polygonal or ovoid. In alternative embodiments, the center of cell  10  can be defined by any number of hollow cavities.  
         [0034]    Referring now to FIG. 2, an electrochemical fuel cell stack is shown having a two-stage embodiment of the present invention and is referred to as numeral  112 . Stack  112  includes the same components and elements in the same configuration as the single stage configuration as described above, including at least two cells  110 , a centerline  114 , a fuel mixing chamber  118 , separator discs  122   a, b , a fuel electrode  126 , a fuel diffusion layer  128 , a solid oxide electrolyte disc  124 , an oxygen diffusion layer  130 , an oxygen electrode  132 , a fuel electrode annular seal  134 , an oxygen electrode annular seal  136 , a hot oxidizer manifold  138 , an unsealed fuel flow layer  140 , end plates  144  and  146 , thermal insulation  148 , a fuel feed tube  150 , a sealing tube  152 , and a fuel feed tube sealing disc  154 .  
         [0035]    Stack  112  differs from stack  12  (FIG. 1) in that stack  112  includes only a single unsealed fuel flow layer  140  at an end of stack  112  and is flush with a first end plate  144  and is at the end of stack  112  that is opposite that of the point of introduction of fuel feed tube  150  into stack  112 . Stack  112  further includes a solid cylinder  156  comprised of any heat resistant material conventional in the art. Solid cylinder  156  is located above fuel chamber  118  and is flush with first end plate  144  so that the top of solid cylinder  156  abuts the bottom of first end plate  144 .  
         [0036]    Cells  110  that directly surround the fuel mixing chamber  118  provide an adequate amount of oxygen to achieve an electrochemical fuel oxidation of at least about 30%, which is the minimum value to prevent carbon accumulation problems, and which depends upon the type of fuel used and operating temperature. The fuel mixture flows through the annular fuel manifold  158 , whereby the plurality of cells  110  progressively further oxidize the fuel to a final cumulative electrochemical oxidation value, which can be as high as 100%. It is noted that this may even slightly exceed 100%, with a small percentage of free oxygen present. It is also noted that the cumulative oxidation value depends upon fuel feed rate, fuel type, stack electric current, and the number of cells in the stack  112 . The voltage of one or more cells near the exit layer  140  can be used for automatic closed-loop regulation of cumulative fuel electrochemical oxidation. It has been found that a two stage configuration of the present invention can achieve a higher average chemical potential or electromotive force due to the use of progressive oxidation of the fuel mixture as it flows through annular manifold  158 , together with the presence of a rich mixture in the fuel mixing chamber  118 . It has also been found that the two stage configuration of the present invention can advantageously operate at a higher overall electrochemical fuel utilization than a single stage system. For example, at 900° C., the calculated maximum possible fuel cell efficiency of a two stage configuration of the present embodiment of the present invention on natural gas is over 80%, calculated by the stack power/natural gas lower heating value.  
         [0037]    Turning now to FIG. 3, another embodiment of the present invention, a steam electrolyzer, is described and shown at numeral  200 . Steam electrolyzer  200  comprises at least one cell  210  arranged in a stack  212 . Of course steam electrolyzer  200  is described as having just a single cell for purposes of explanation, however any number of cells  210  may be employed in stack  212 . A centerline  214  is shown and an oxygen chamber  238 , or hot oxygen manifold, surrounds stack  212  for collecting the oxygen produced by stack  212 . Cells  210  surround a hydrogen/steam mixing chamber  218 .  
         [0038]    Each cell  210  is separated from and electrically connected to adjacent cells by an electronically conductive separator disc  222   a, b . Each cell  210  contains a single separator disc  222   a , the second separator disc  222   b  being a separator of an adjacent cell. Within each cell  210  is a solid oxide electrolyte disc  224 . A fuel electrode  226  abuts electrolyte disc  224  directly below electrolyte disc  224 . A fuel diffusion layer  228  is positioned between fuel electrode  226  and separator disc  222   b . An oxygen electrode  232  abuts electrolyte disc  224  directly above electrolyte disc  224 . An oxygen flow layer  230  is positioned between oxygen electrode  232  and separator  222   a . Both fuel diffusion layer  228  and oxygen flow layer  230  are highly porous. Layer  228  is sufficiently thick so as to allow the hydrogen and steam to diffuse through it with only moderate composition gradients. Layer  230  is sufficiently thick to minimize pressure drop from flowing oxygen. Layers  228  and  230  are also good electrical conductors.  
         [0039]    A fuel electrode annular seal  234  surrounds fuel electrode  226  and fuel diffusion layer  228 . Seal  234  extends from separator  222   b  to electrolyte disc  224 . The upper end of seal  234  is substantially flush with electrolyte disc  224 . The lower end of seal  234  is substantially flush with separator  222   b . An oxygen electrode annular seal  236  is located inside oxygen electrode  232  and oxygen flow layer  230 . Seal  236  extends from electrolyte disc  224  to separator  222   a . The upper end of seal  236  is substantially flush with separator  222   a . The lower end of seal  236  is substantially flush with electrolyte disc  224 .  
         [0040]    Separators  222   a  and  222   b  can be made of any material common in the field, such as a high-temperature alloy which forms a thin protective oxide surface layer with good high-temperature electrical conductivity. Electrolyte disc  224  may be of yttria-stabilized zirconia, or any other suitable material. Fuel electrode  226  and fuel diffusion layer  228  can be of, for example, a doped ceria/nickel mixture. Nickel foam may be used for fuel diffusion layer  228  . Oxygen electrode  232  and oxygen flow layer  230  can be of, for example, strontium-doped lanthanum manganite. Seals  234  and  236  can be made from a suitable glass. A thin layer of ink, such as an ink made from a finely-divided electrode composition, may be applied on each side of separators  222   a, b . Ink is applied to improve the electrical contact between the components of cell  210 .  
         [0041]    An exit tube  256  allows the hydrogen/steam mixture to continually exit stack  212 . Stack  212  is additionally clamped or situated between a first electrically conductive end plate  244  and a second electrically conductive end plate  246  via a spring-loaded clamping means (not shown) or any other method conventional in the art.  
         [0042]    Stack  212  still further includes a thermal insulation  248  at the base of stack  212  and below second end plate  246 . A water feed conduit  250  is introduced into stack  212  through the center of stack  212  at its end having thermal insulation  248  and second end plate  246 , and is introduced between thermal insulation  248  and second end plate  246  overlying thermal insulation  248 . Water feed conduit  250  serves as a conduit for the introduction of water, in either the liquid, vapor, or supercritical fluid state, directly into the center of stack  212 . Water feed conduit  250  is sealed by welding to second end plate  246  via a sealing tube  252  and a sealing disc  254 . Thermal insulation  248  is also present in the annulus between conduit  250  and tube  252 .  
         [0043]    Thermal insulation  248  can optionally further include additional insulation (not shown) and thermal insulation  248  serves to insulate the heated stack  212  and water feed conduit  250  during operation. It is noted that fuel electrode  226 , fuel diffusion layer  228 , oxygen flow layer  230 , oxygen electrode  232 , and thermal insulation  248  are porous and permit gas to flow through them. The remaining elements in FIG. 3 are substantially impervious to gasflow.  
         [0044]    Steam electrolyzer  200  may be contained within an insulated pressure vessel and operated at high pressures, even at pressures above the critical pressure of water. Such high pressure operation can eliminate the need for subsequent compression to yield high pressure hydrogen gas. For high pressure operation, the feed water is fed to tube  250  from a high pressure water pump (not shown).  
         [0045]    During operation, water is continually fed through water tube  250 , electric current is supplied to stack  212 , oxygen is continually withdrawn from chamber  238 , and a hydrogen/steam mixture is continually withdrawn through tube  256 . Stack  212  is operated with essentially zero pressure difference between chamber  218  and chamber  238  by regulation of the gas exit pressures. The incoming water mixes with the hydrogen steam mixture in mixing chamber  218 , resulting in the desired composition, for example about 10 to 20% steam. The preferred composition in chamber  218  contains sufficient steam for good diffusion in fuel diffusion layer  228  and for moderate cell electrolysis EMF but not an excessive amount, which will increase the size of external auxiliary equipment. The feed water may be preheated to any desired temperature before introduction to tube  250 .  
         [0046]    What has been described above are preferred aspects of the present invention. It is of course not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. It would be evident to one familiar with the art that the cells of the system of the present invention need not be identical. The object of the present invention may be performed with a system not having like cells, or cells of varying thicknesses in a single system or even comprising varying materials in a single system. Accordingly, the present invention is intended to embrace all such alterations, combinations, modifications, and variations that fall within the spirit and scope of the appended claims.