Patent Publication Number: US-2005142398-A1

Title: Prevention of chromia-induced cathode poisoning in solid oxide fuel cells (SOFC)

Description:
BACKGROUND OF THE INVENTION  
      The invention relates to a fuel cell system. More particularly, the invention relates to integration of a moisture removal system with the oxidant inlet of a solid oxide fuel cell (SOFC) system.  
      Solid oxide fuel cells (SOFCs) are devices that produce energy, usually electricity, from a variety of fuels using an electrochemical reaction. Oxygen transfer through the electrolyte, necessary for efficient energy conversion, is greatly accelerated at temperatures above 700° C. The overall fuel to electric efficiency in SOFCs may be as high as 90% and is not limited by classical thermodynamics for heat engines (Carnot cycle). Due to their high exhaust gas temperature, SOFCs have the ability to cogenerate heat and electric power, with the balance in favor of electric power. Hybrid power generation systems integrating the SOFCs and Turbines can have very high overall system efficiencies.  
      SOFCs may be tubular or planar in assembly. The key components of an SOFC are: anode, cathode, electrolyte, interconnects, manifold and seals. The cathode is largely exposed to a hot, oxidant environment, and is generally called the air or oxygen electrode. The temperature of the cathode feed gas is usually about 400° C. or higher. Similarly, the anode is exposed to the fuel and is called the fuel electrode. The interconnects interface with the anode on the fuel side and with the cathode on the air side and are usually made using oxidation resistant, heat resistant materials such as lanthanum chromite, lanthanum strontium chromite, ferritic stainless steels and chromium base alloys. Ferritic stainless steels typically contain at least 20 wt % of chromium.  
      Highly oxidizing conditions prevail at the cathode at elevated temperatures and high oxygen partial pressures. These, along with humidity and atmospheric moisture may oxidize chromium present in interconnects to chromium oxides or hydroxide or oxyhydroxide that grow as cathode scales and can vaporize to poison or deactivate the cathode. Cathode scales may grow to a thickness of tens of microns after exposure for thousands of hours in the SOFC environment in an intermediate temperature range. Chromium hydroxide and oxyhydroxide are particularly volatile and may degrade the cathode. To enhance life expectancy and operational efficiency of the SOFC cathode degradation must be eliminated.  
      Current methods for minimizing cathode degradation in SOFCs are not adequately developed and limit the useful operating life of the SOFCs. The problem may be minimized or eliminated by frequent maintenance or cathode scale removal. This may result in cell stoppage and induce a significant energy penalty associated with the power generation cycle. Alternatively, non-chromium containing alloys and ceramic materials with non-volatile chromium have been employed in interconnects. However, these materials are expensive, brittle, weak under tensile forces, or have high resistive losses making them unsuitable for interconnects applications. Many SOFC stacks employ interconnects and components made from alloys containing chromium and few suitable replacement materials are available. The problem of high cathode degradation rates has not been solved. Therefore, what is needed is a method for preventing poisoning (or degradation) of chromium cathodes. What is also needed is a method for removal of moisture (or water vapor) from the fuel cell oxidant supply. What is also needed is the applicability of such method in a continuous manner without stoppage or interruption of fuel cell supply. What is also needed is a system to regenerate or recuperate a desiccant bed (or drying agent) employed in the method by using the hot exhaust gases (usually air; which would otherwise have been wasted) produced by the fuel cell. What is also needed is a system that minimizes the energy penalty associated with drying the inlet oxidant.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention addresses these and other needs by providing system configuration and methods for removing moisture from a fuel cell oxidant supply and a methods for moisture removal that may be applied continuously without stoppage or interruption in fuel cell supply. It also provides a method to regenerate the desiccant bed using hot exhaust gases produced in the SOFC thus minimizing the energy penalty.  
      Accordingly, one aspect of the invention is to provide a fuel cell system. The fuel cell system comprises: at least one fuel cell having at least one oxidant inlet; and a moisture removal system in flow communication with the at least one oxidant inlet to remove moisture from an oxidant provided to the oxidant inlet.  
      A second aspect of the invention is to provide a method for removing moisture in a fuel cell system. The method comprises: providing at least one oxidant flow to the fuel cell system; directing at least a portion of the oxidant flow through a moisture removal system; and removing at least a portion of moisture from the oxidant flow.  
      A third aspect of the invention is to provide a solid oxide fuel cell system. The fuel cell system comprises: at least one fuel cell having at least one oxidant inlet; a moisture removal system in flow communication with the at least one oxidant inlet to remove moisture from an oxidant provided to the oxidant inlet; an anode; a cathode and an electrolyte.  
      A fourth aspect of the invention is to provide a method for removing moisture in a fuel cell system. The method comprises: connecting the moisture removal system to the fuel cell; allowing exchange of heat between moisture and drying species; allowing sufficient dwell time between the drying species and moisture; conveying dry cathode feed gas into the fuel cell and regenerating the moisture removing elements (desiccant) using hot exhaust air from the fuel cell along at least one hot air inlet.  
      These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Referring now to the figures wherein like elements are numbered alike:  
       FIG. 1  is a schematic view illustrating one embodiment of a fuel cell stack showing its components;  
       FIG. 2  is a schematic view illustrating an embodiment of a fuel cell system including a moisture removal system (along with desiccant regeneration system) that is a physical absorption system;  
       FIG. 3  is a schematic view illustrating an embodiment of a fuel cell system including a moisture removal system (along with desiccant regeneration system) that is a chemical absorption system;  
       FIG. 4  is a schematic view illustrating an embodiment of a vapor absorption refrigeration system for moisture removal; and  
       FIG. 5  is a schematic view illustrating an embodiment of a vapor compression refrigeration system for moisture removal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms.  
      Referring to the drawings in general and to  FIG. 1  in particular, it will be understood that the illustrations are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the invention thereto.  
      Fuel cells convert gaseous fuels (hydrogen, natural gas, gasified coal) via an electrochemical process directly into electricity. Their efficiencies are not limited by the Carnot cycle of a heat engine, and the pollutant output from fuel cells is many magnitudes lower than from conventional technologies. A fuel cell operates like a battery, but does not need to be recharged, and continuously produces power when supplied with fuel and oxidant.  
      A Solid Oxide Fuel Cell (SOFC), is regarded as an extremely efficient and versatile power-generating device. The current operating temperature of an SOFC is around 800° C. and developmental efforts to reduce the operating temperature are in progress. SOFCs are fuel flexible and can operate on multiple fuels, including carbon-based fuels. This results in potentially high overall fuel to electric efficiency of around 60% for simple cycles and higher efficiency for hybrid systems. Due to their high exhaust gas temperature they have the ability to cogenerate heat and electric power whereas hybrid systems maximize the electrical efficiency.  
      The high operating temperature of SOFC is mainly dictated by slow oxygen transfer rates through the electrolyte at lower temperatures. This factor, combined with the multi-component nature of the fuel cell and the required life expectancy of several years severely restricts the choice of materials for cell and manifold components. Each material used not only has to function optimally in its own right but has to be viewed in conjunction with the other cell components. The common requirements of all cell components (i. e. including, but not limited to, electrolyte, anode, cathode, interconnects, manifold and seals) are: 
          (i) chemical stability in fuel cell environments (partial pressure of oxygen exceeds 20 kPa on the cathode side and is less than 10 −17  on the anode side) and compatibility with other cell components;     (ii) phase and microstructural stability;     (iii) minimum thermal expansion mismatch between various cell components (laminated structure);     (iv) for structural components, reasonable strength and toughness at the cell operating temperature, as well as reasonable thermal shock resistance;     (v) low vapor pressure to avoid loss of material and     (vi) amenability to fabrication at competitive costs.        

      The cathode is a particularly important component of an SOFC. The atmosphere at the cathode is highly oxidizing. The common cathode material used in SOFC systems is Strontium (Sr) doped La-manganite (LSM), a p-type semiconductor. Doping LaMnO 3  with lower valent cations enhances the electronic conductivity. The extent and nature of the dopants dictates the electronic conductivity and the electrode reaction rates. A change of the morphology of the cathode layer with time, blocking of reaction sites or interfacial reaction between cathode and electrolyte during operation, all limit the life of SOFCs and need to be minimized. A number of other materials are also used, such as La—Sr-cobaltite, a material with much higher electronic conductivity, and in addition high ionic conductivity, but with the disadvantages (compared to LSM) of a high thermal expansion coefficient and lower stability due to interface reactions with the electrolyte.  
      The electro-chemical performance of an SOFC cathode is greatly influenced by the materials characteristics of the cell interconnects. The interconnects interface with the anode on the fuel side and with the cathode on the air side and are usually made using oxidation resistant, low resistive loss, heat resistant materials such as ferritic stainless steels, chromium based alloys, lanthanum chromite, and lanthanum strontium chromite. Ferritic stainless steels typically contain about 26 wt % of chromium. In SOFC operation, highly oxidizing conditions prevail at the cathode at elevated temperatures and high oxygen partial pressures. These, along with humidity and atmospheric moisture may oxidize or hydrolyze chromium present in interconnects to chromium oxides or hydroxide or oxyhydroxide that deposit as cathode scales and poison or deactivate the cathode. Cathode scales may grow to a thickness of tens of microns after exposure for thousands of hours in the SOFC environment in an intermediate temperature range. Chromium hydroxide or oxyhydroxide are particularly volatile and may degrade the cathode. To enhance life expectancy and operational efficiency of the SOFC cathode degradation due to moisture must be eliminated. This must be done in a manner which minimizes the efficiency penalty.  
      The current invention discloses a method to dry the moist cathode feed gas without sacrificing the efficiency. This prevents oxidation and hydrolysis of chromium present in the interconnect material. Chromium oxides and hydroxides would normally deposit on the cathode and degrade it. Such cathode degradation is prevented by the current invention that increases the life expectancy of a fuel cell by preventing chromium oxides and hydroxide formation that poisons an SOFC cathode.  
      According to one embodiment of the present invention a fuel cell system  20  comprises at least one fuel cell  30  having at least one anode  40 , an electrolyte  60 , a cathode  80 , an interconnect  100  and a seal  105 , as generally shown in  FIG. 1 . The cathode  80  and the interconnect  100  are in intimate electrical contact via contact  90 . A fuel cell stack is obtained by repeated stacking of repeating unit  180  that comprises an anode  40 , electrolyte  60 , cathode  80 , cathode-interconnect contact  90  and interconnect  100 . The fuel cell is encased between extreme end plates  120 .  
      Interconnect  100  comprises, on the cathode side, a system for oxidant flow  140  that consists of oxidant inlets  145  that convey the cathode feed gas (or oxidant) to the cathode  80 ; and fuel flow conduits  160  to convey the fuel to the anode  40 . As generally shown in  FIG. 2 , heat exchangers  24  and  26  cool the moist, cathode feed gas to a low temperature. The cool, moist air is dried by passing through the moisture removal system  280  which dries the air. The hot, dry air is passed through the first heat exchanger  24  where it receives a portion of the heat from the incoming moist air  140 . Downstream of heat exchanger  24 , the hot dry air is conveyed to recuperator  28  where its temperature is substantially increased by absorbing heat from hot turbine exhaust gases  480  exiting from gas turbine  500 . The post-recuperator hot dry air  29  is input into the SOFC fuel cell system  20  via oxidant inlet  145 . The current produced on operating the fuel cell emerges from the SOFC along current direction  200 , as generally shown in  FIG. 1 . While the systems of  FIGS. 2, 3  and  4  are each shown as hybrid configurations, this is not a limitation of the invention. For example, the moisture removal system  220  can be configured to work with any stand-alone fuel cell system prone to chromia induced cathode poisoning.  
      Fuel cell system  20  is maintained in driving contact with combustor  600  and hot gases emerging therefrom are used to drive gas turbine  500  for power generation purposes. A portion of the exhaust gases  480  exiting gas turbine is conveyed to recuperator  28  to preheat the hot dry air  29  input into the fuel cell system  20  via oxidant inlet  145 . Another portion of the waste heat from the turbine exhaust  480 , the recuperator exhaust  35  is conveyed to the dryer  280  in the moisture removal system  220  so as to heat a desiccant bed and revitalize desiccant  460 .  
      In one embodiment of the fuel cell system  20  the moisture absorption system  220 , is a physical absorption system  280  as discussed above and generally shown in  FIG. 2 . The physical absorption system  280  comprises a desiccant or drying agent, such as but not limited to, indicating and non-indicating type silica gels, molecular sieves, alumino silicates, activated carbon or rice husks, with which physical absorption system  280  dries the incoming cathode feed gas  140  and is revitalized by the recuperator exhaust  35  exiting from recuperator  28 . The spent portion  700  of the recuperator exhaust  35  that is not used for desiccant regeneration or other purposes, is released into the environment  
      In another embodiment, the moisture absorption system  220  is a chemical absorption system  300  as generally shown in  FIG. 3 . As generally shown in  FIG. 3 , heat exchangers  24  and  26  cool the moist, cathode feed gas to a low temperature. The cold, moist air is dried by passing through the chemical absorption system  300  that dries the air. The cold, dry air is passed through the first heat exchanger  24  where it receives a portion of its heat form the incoming moist air  140 . Downstream of heat exchanger  24 , the hot dry air is conveyed to recuperator  28  where its temperature is substantially increased by absorbing heat from hot turbine exhaust gases  480  exiting from gas turbine  500 . The post-recuperator hot dry air  29  is input into the SOFC fuel cell system  20  via oxidant inlet  145 . The current produced on operating the fuel cell emerges from the SOFC along current direction  200 , as generally shown in  FIG. 1 . The chemical absorption system  300  comprises a chemical species known for its strong affinity for water (or moisture), such as but not limited to, sodium-sulfate, calcium chloride, calcium oxide, or a combination thereof. The chemical moieties dry the incoming cathode feed gas  140  and are revitalized by the recuperator exhaust  35  exiting the recuperator  28 . The spent portion  700  of the recuperator exhaust  35  that cannot be used for desiccant regeneration or other purposes, is released into the environment.  
      In another embodiment, the moisture system  220  is a condensation system  332 . In another embodiment, condensation system  320  comprises a vapor absorption refrigeration system  332  driven by waste heat from the turbine exit. The principles of vapor absorption refrigeration systems are known to the one well versed in the art. The air is dried by cooling the air to low enough temperature that causes condensation of the moisture which is removed from the system  340 . As generally shown in  FIG. 4 , heat exchanger  24  in this case, cools the moist, cathode feed gas to a low temperature. The cool, moist air is dried by passing through condensation system  320  that dries the air. The dry air is passed through the first heat exchanger  24  where it absorbs a portion of the heat from the incoming moist air  140 . Downstream of heat exchanger  24 , the dry air is conveyed to recuperator  28  where its temperature is substantially increased by absorbing heat from hot turbine exhaust gases  480  exiting from gas turbine  500 . The post-recuperator hot dry air  29  is input into the SOFC fuel cell system  20  via oxidant inlet  145 . The current produced on operating the fuel cell emerges from the SOFC along current direction  200 , as generally shown in  FIG. 1 .  
      The spent portion  700  of the exhaust gas  480  that cannot be used to power the vapor absorption refrigeration system  332 , in condensate removal or in other requirements, is released into the environment.  
      In another embodiment of the claimed invention, the refrigeration system  330  is a vapor compression refrigeration system  334  generally shown in  FIG. 5 .  
      In another embodiment of the present invention, a method for removing moisture in a fuel cell system  20  is disclosed. The method comprises: providing at least one oxidant flow  140  to the fuel cell system  20 , directing at least a portion of the oxidant flow  140  through a moisture removal system  220 ; and removing at least a portion of moisture from the oxidant flow  140 .  
      In a third embodiment of the present invention, a solid oxide fuel cell system  20  is disclosed. The said solid oxide fuel cell system  20  comprises at least one fuel cell  30  having at least one oxidant inlet  145 ; a moisture removal system  220  in flow communication with said at least one oxidant inlet  145  to remove moisture  240  from an oxidant  140  provided to said oxidant inlet  140 ; an anode  40 ; a cathode  80  and an electrolyte  60 .  
      In a fourth embodiment of the present invention a method for removing moisture  240  in a fuel cell system  20  is disclosed. The method comprises connecting the moisture removal system  220  to the fuel cell  30 , allowing exchange of heat between moisture  100  and drying species (or desiccant  460 ), allowing sufficient dwell time between the drying species and moisture  240 , conveying dry cathode feed gas  145  into the fuel cell  30  and regenerating the moisture removing elements (or desiccant  460 ) using hot air  480  along at least one hot air inlet  420 .  
      In all embodiments, the claimed invention minimizes the energy penalty associated with drying the inlet oxidant.  
      While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the invention. For example, while hybrid systems are depicted, simple systems are also encompassed within this invention. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present invention.