Patent Publication Number: US-2011076573-A1

Title: Solid Oxide Type Fuel Cell and Operating Method Thereof

Description:
TECHNICAL FIELD  
     The present invention relates to a solid oxide type fuel cell, and more particularly to an operating method of a fuel cell at the time of actuation or shutdown. 
     BACKGROUND ART  
     The above solid oxide type fuel cells are under development as third generation fuel cells for power generation use, and at present three different types including cylindrical, monolithic and planar stacked types of such cells are known. Any of these solid oxide type fuel cells has a stacked structure in which a solid electrolyte layer consisting of an oxide ion conductor is sandwiched between an air electrode layer (cathode) and a fuel electrode layer (anode); in the case of the planar stacked type for instance, a plurality of power generating cells each consisting of such stacked structure and separators are alternately laid one over the other to form a stacked body and accommodated into a housing to constitute a module. 
     In a solid oxide type fuel cell, oxidant gas (oxygen) is supplied to the air electrode layer side and fuel gas (H 2 , CO, CH 4  or the like) is supplied to the fuel electrode layer side as reactive gases. Porous layers are used as both the air electrode layer and the fuel electrode layer to enable the reactive gases to reach the interface with the solid electrolyte layer. 
     In the power generating cell, the oxygen supplied to the air electrode layer side passes pores in the air electrode layer and reaches the vicinities of the interface with the solid electrolyte layer, where it receives electrons from the air electrode layer to be ionized into oxide ions (O 2− ). These oxide ions diffusively move within the solid electrolyte layer toward the fuel electrode layer. The oxide ions having reached the vicinity of the interface with the fuel electrode layer react therein with fuel gas to generate reaction products (including H 2 O and CO 2 ) and discharge electrons into the fuel electrode layer. 
     The electrons generated by such an electrode reaction can be taken out as electromotive force by an external load on a separate route. 
     Incidentally, when conventionally known fuel cells including such solid oxide type fuel cells are to be operated (caused to generate electricity), it is necessary to preheat the fuel cell stack (especially power generating cells) to the working temperature of each fuel cell. This is to activate the electrochemical reaction of the power generating cells (see Patent Document 1). 
     Conventionally, when a fuel cell stack is to be raised in temperature to prepare for the start of operation, a heating device such as an electric heater or a burner is arranged on the periphery of the fuel cell stack, and the stack surface is heated with radiant heat from the heating device. This preheating method using radiant heat, however, involves a problem that a long time is needed to raise the temperature and accordingly a long time (e.g. about eight hours) is taken before the power generating operation is started. Moreover, if the temperature is to be raised with an electric heater, power consumption is extremely high, inviting a problem of extra power consumption before power generation by the fuel cells starts. 
     Incidentally, in such fuel cells, the ambient atmosphere of the fuel electrode is kept in a reduction condition by supplying inert gas (e.g. nitrogen gas) containing a minute quantity of hydrogen to the power generating cells in the temperature rising/falling process at the time of actuation or shutdown; this process is known as purging (see Patent Document 2 for instance). 
     This is to prevent the fuel electrode from being oxidized by the oxygen remaining within the fuel cells in the high temperature state at the time of actuation/shutdown. Oxidization of the fuel electrode would extremely deteriorate the power generating performance. 
     Solid oxide type fuel cells in particular, because of their extremely high working temperature of 600 to 1000° C., are difficult to start or stop rapidly and take a long time to start or stop, resulting in the susceptibility of the fuel electrode to oxidation; especially in a seal-less structure having no gas leak-preventive seal in the outer circumferential part of power generating cells where external oxygen-containing gas can more easily flow into the cells, massive purging is required at the time of actuation or shutdown and, furthermore, the increase in the size of fuel cells entails a proportional increase in the quantity of nitrogen gas used. 
     Conventionally, purging is predominantly accomplished by providing dedicated gas cylinders separately, nitrogen gas and hydrogen for the purging purpose are supplied from them to the fuel gas supply route, and this involves a problem that the whole fuel cell system, including the gas cylinders and the nitrogen gas supply route, is enlarged in size with a consequence of complicating the maintenance work. 
     Patent Document 1: Japanese Patent Laid-Open No. 8-162137 
     Patent Document 2: Japanese Patent Laid-Open No. 2-244559 
     DISCLOSURE OF THE INVENTION  
     A first object of the present invention is to provide a solid oxide type fuel cell in which reductive reformed gas containing hydrogen is generated from the initial stage of actuation by heating a reformer and a water vapor generator in the fuel module with a heating device at the time of actuation, and is enabled to be actuated rapidly while being supplied with this purge gas on the fuel electrode side of the power generating cell and an operating method of this fuel cell, and further to utilize this operating method to totally dispense with nitrogen conventionally needed for preventing the fuel electrode from being oxidized. 
     A second object of the invention is to provide a fuel cell operating method which enables the temperature raising and lowering actions at the time of actuation and shutdown to be accomplished without requiring a separate purging gas supply line. 
     A third object of the invention is to provide a fuel cell power generating device and a shutdown method permitting the accomplishment of purging without requiring a separate purging gas supply line. 
     [First Mode for Implementing the Invention] 
     In order to achieve the first object stated above, a solid oxide type fuel cell in a first mode for implementing the invention is a solid oxide type fuel cell wherein a fuel cell assembly is formed by aggregating a plurality of power generating cells and accommodating the assembly into a housing and a power generating reaction is generated by supplying reformed gas into the fuel cell assembly during operation, characterized in that a reformer or a reformer and a water vapor generator are installed in the housing and a heating device for heating at least the reformer is arranged. 
     The meaning of the fuel cell assembly in this context includes a fuel cell stack in which a plurality of power generating cells and separators are alternately laid one over the other like a planar stacked type fuel cell or a plurality of power generating cells are aggregated and connected via connecting members (inter-connectors) into a bundle, like cylindrical fuel cells. 
     An electric heater or a combustive burner, for instance, can be used as the heating device. 
     Also, it is desirable to arrange on the periphery of the fuel cell assembly a combustion catalyst for burning reformed gas discharged from the fuel cell assembly or a heater for igniting reformed gas. 
     As the solid oxide type fuel cell, it is possible to use a solid oxide type fuel cell of seal-less structure wherein the residual gas not used in the power generating reaction is burned outside the fuel cell assembly. 
     An operating method of a solid oxide type fuel cell in the first mode for implementing the invention is an operating method of a solid oxide type fuel cell in which a fuel cell assembly is formed by aggregating a plurality of power generating cells and accommodating the assembly into a housing and a power generating reaction is generated by supplying reformed gas into the fuel cell assembly during operation, characterized in that a reformer, a water vapor generator and a heating device are provided; at least the reformer is heated with the heating device at the time of actuating the fuel cell; fuel gas and water vapor from the water vapor generator are supplied to the reformer which has been heated to generate reformed gas whose main component is hydrogen; and the reformed gas is supplied to the fuel cell assembly to raise the temperature of the fuel cell assembly while keeping the fuel electrode of the power generating cell in a reduction condition. 
     It is desirable in the operating method of the solid oxide type fuel cell for the reformed gas whose main component is hydrogen supplied to the fuel cell assembly to be discharged from the fuel cell assembly and burned to raise the temperature of the fuel cell assembly. 
     According to the first mode for implementing the invention, as a reformer and a water vapor generator equipped with a heating device are installed in a housing into which a fuel cell assembly is accommodated and these reformer and water vapor generator are heated with the heating device when the fuel cell is actuated, it is made possible to supply the reformed gas whose main component is hydrogen into the fuel cell assembly from immediately after the actuation and to keep the fuel electrode side of the power generating cell in a reduction condition without having to use nitrogen gas. Furthermore, as the fuel cell assembly can be heated in a short period of time by burning the residual reformed gas in the vicinity of the fuel cell assembly, the fuel cell is enabled to be rapidly actuated. 
     Moreover, once the combusting reaction is started, as the reformer and the water vapor generator are heated by that combustion heat, heating with the heating device as at the time of actuation is no longer necessary. This enables power consumption by an electric heater, for instance, used as the heating device, to be significantly reduced. 
     [Second Mode for Implementing the Invention] 
     In order to achieve the second object stated above, an operating method of a fuel cell in a second mode for implementing the invention is an operating method of a fuel cell whereby raw fuel is reformed with a reformer and the reformed gas is supplied to a power generating cell to perform power generating operation, characterized in that the fuel cell is raised in temperature at the time of starting operation while keeping the ambience of the fuel electrode in a reduction condition by generating with the reformer reductive gas containing hydrogen by partial oxidation reforming reaction or auto-thermal reforming reaction and supplying that gas to the fuel electrode of the power generating cell. 
     It is desirable, in the operating method of the fuel cell described above, for the unburned fuel gas discharged from the fuel cell to be burned with combusting means and the resultant combustion heat to be utilized for raising the temperature. 
     Another operating method of the fuel cell in the second mode for implementing the invention is an operating method of the fuel cell whereby a reformer is provided, raw fuel is reformed with this reformer and the reformed gas is supplied to a power generating cell to perform power generating operation, characterized in that the fuel cell is lowered in temperature at the time of shutdown while keeping the ambience of the fuel electrode in a reduction condition by generating with the reformer reductive gas containing hydrogen by partial oxidation reforming reaction or auto-thermal reforming reaction and supplying that gas to the fuel electrode of the power generating cell. 
     It is desirable for the ambience of the fuel electrode to be kept in a reduction condition until the temperature of the fuel cell at least is lowered to 300° C. or below. 
     As the fuel cell referred to above, it is possible to use, for example, a solid oxide type fuel cell of seal-less structure wherein the residual gas not used in the power generating reaction is burned outside the fuel cell assembly. 
     According to the second mode for implementing the invention, as a reforming reaction is caused to take place in the reformer by the heating which accompanies the partial oxidation reaction or the auto-thermal reaction at the time of actuation or shutdown to generate reductive gas containing hydrogen and the supply of this reductive gas to the fuel electrode enables the fuel electrode of the power generating cell to be kept in a reduction condition, oxidation of the power generating cell in the temperature rising or falling cycle and the consequent performance deterioration of the power generating cell can be prevented, and this feature contributes to elongating the service life. 
     In addition, since no purge requiring a large quantity of nitrogen gas as in the conventional practice is needed, the need to install a purging gas supply line including a nitrogen cylinder, for instance, is eliminated, resulting in simplified maintenance work. 
     Furthermore, where the unburned gas discharged from the fuel cell is burned by combusting means, the heat of combustion facilitates raising the temperature of the fuel cell including the reformer, and this serves to reduce the length of time required for actuation. 
     [Third Mode for Implementing the Invention] 
     In order to achieve the third object stated above, a fuel cell shutdown method in the third mode for implementing the invention is a fuel cell shutdown method whereby a power generating reaction is generated by supplying fuel gas to the fuel electrode layer side and oxidant gas to the air electrode layer side, characterized in that the stack temperature is lowered while keeping the fuel electrode layer side in a reduction condition by supplying water and hydrogen or hydrocarbon fuel to the fuel cell while reducing the flow rate thereof at the time of stopping power generation. 
     This method is to maintain the reduction condition by generating water vapor by utilizing the thermal capacity of the fuel cell while continuing to supply small quantities of water and fuel gas even after the stop of power generation, and supplying a mixture of reformed gas and water vapor to the fuel electrode. 
     In the fuel cell shutdown method described above, it is desirable for the supply quantity of the hydrogen or hydrocarbon fuel to be so reduced that the cell voltage of the fuel cell is 0.5 V or above when the stack temperature is 300° C. 
     Although there is a fear that, if the supply of fuel gas is stopped at a stack temperature of not below 300° C., that the fuel electrode layer side is oxidized and deteriorates, the fuel electrode layer side can be kept in a reduction condition by decreasing gradually the supply of fuel gas so that the cell voltage may not fall below 0.5 V until the stack temperature drops to around 300° C. 
     It is also desirable in the fuel cell shutdown method described above for the supply quantity of water, when the supply of water to the fuel cell is to be stopped, to be reduced so that the water vapor temperature attributable to the water is not less than 200° C. 
     If the water vapor temperature were 200° C. or below when the supply of water is stopped, the temperature of the water vapor would drop rapidly to 100° C. to make continuous generation of water vapor difficult. As a result, liquid water would be supplied into the cell, and the cell might be deteriorated or cracked. For this reason, the water vapor temperature at the time of stopping the supply of water should be 200° C. or above. 
     A fuel cell power generating device in the third mode for implementing the invention is characterized in that it is provided with fuel cells which supply electric power according to the supply quantity of fuel gas and the supply quantity of oxidant gas, a fuel supply line which supplies fuel gas to the fuel cells, an oxidant gas supply line which supplies oxidant gas, a water supply line which supplies water, and a control unit which performs the shutdown control referred to above. 
     As the fuel cells referred to above, it is possible to use solid oxide type fuel cells of seal-less structure wherein the residual gas not used in the power generating reaction is discharged from the outer circumferential part of power generating cells. 
     In the third mode for implementing the invention, the fuel electrode layer can be kept in the reduction condition at the time of stopping power generation by supplying water and hydrogen or hydrocarbon fuel to the fuel cells while reducing the flow rates thereof, and oxidation of the power generating cells in the temperature rising or falling cycle and the consequent performance deterioration of the power generating cells can be prevented, thereby enabling the service life to be elongated. 
     In addition, since no purge requiring inert gas as in the conventional practice is needed, the need to install a purging gas supply line including an inert gas cylinder (e.g. a nitrogen cylinder) is eliminated, enabling the maintenance work to be simplified as well as the hardware itself to be reduced in size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the schematic internal configuration of a solid oxide type fuel cell in the first mode for implementation to which the invention is applied; 
         FIG. 2  is a diagram showing gas flows in the fuel cell stack during operation; 
         FIG. 3  is a diagram showing a state of installation of a heating device relative to a reformer and a water vapor generator; 
         FIG. 4  is a diagram showing another state of installation of the heating device relative to the reformer and the water vapor generator different from the state shown in  FIG. 3 ; 
         FIG. 5  is a diagram showing the schematic internal configuration of a solid oxide type fuel cell in the second mode for implementation to which the invention is applied; 
         FIG. 6  is a diagram showing the internal arrangement of a solid oxide type fuel cell in the second mode for implementation to which the invention is applied; 
         FIG. 7  is a diagram showing the configuration of the essential part of the fuel cell stack of  FIG. 5 ; 
         FIG. 8  is a diagram showing the schematic configuration of a fuel cell power generating device in the third mode for implementation to which the invention is applied; 
         FIG. 9  is a diagram showing the configuration of a solid oxide type fuel cell stack for use in the fuel cell power generating device of  FIG. 8 ; and 
         FIG. 10  is a diagram showing the shutdown control figure according to the invention. 
     
    
    
     DESCRIPTION OF SYMBOLS  
     
         
           1  Fuel cell assembly (fuel cell stack) 
           5  Power generating cell 
           8  Separator 
           20  Housing 
           21  Reformer 
           22  Water vapor generator 
           23  Combustion catalyst 
           24  Heating device (electric heater, combustion burner) 
           103  Fuel electrode 
           105  Power generating cell 
           121  Reformer 
           123 ,  124  Combusting means (combustion catalyst, auxiliary electric heater) 
           201  Fuel cell (fuel cell stack) 
           203  Fuel electrode layer 
           204  Air electrode layer 
           220  Control unit 
           230  Oxidant gas supply line 
           240  Fuel supply line 
           250  Water supply line 
       
    
     BEST MODES FOR CARRYING OUT THE INVENTION  
     First Embodiment  
     This first embodiment corresponds to the first mode for implementing the invention. 
     The first embodiment of the invention will be described below with reference to  FIG. 1  through  FIG. 4 . 
       FIG. 1  shows the schematic internal configuration of a solid oxide type fuel cell to which the invention is applied;  FIG. 2 , gas flows in the fuel cell stack during operation; and  FIG. 3  and  FIG. 4 , states of installation of a heating device relative to a reformer and a water vapor generator. 
     As shown in  FIG. 1 , the solid oxide type fuel cell (fuel cell module) of this embodiment has a housing  20  (drum) on whose internal wall an insulator (not shown) for heat retaining is arranged, a fuel cell stack  1  which causes a power generating reaction to occur is arranged at the center of this housing  20 , and a reformer  21 , a water vapor generator  22  and so forth are arranged on the periphery of this fuel cell stack  1 . 
     The reformer  21 , packed inside with a Ni (nickel)-based or Ru (ruthenium)-based reforming catalyst for use with hydrocarbon, can reform hydrocarbon fuel gas supplied from outside into fuel gas mainly consisting of hydrogen. 
     The water vapor generator  22  is a heat exchanger for obtaining high temperature water vapor required for the reforming reaction, is arranged in a position opposing the central part of the fuel cell stack as shown in  FIG. 3  or  FIG. 4  for instance so that it can absorb more of exhaust heat from the fuel cell stack  1 , and guides high temperature water vapor to the reformer  21  via a water vapor guide pipe  17 . 
     The fuel cell stack  1  is configured by stacking in the longitudinal direction a large number of unit cells  10  each configured of power generating cells  5  on both faces of whose solid electrolyte layer  2  a fuel electrode layer  3  and an air electrode layer  4  are arranged, a fuel electrode current collector  6  arranged outside the fuel electrode layer  3 , an air electrode current collector  7  arranged outside the air electrode layer  4  and separators  8  arranged outside the current collectors  6  and  7 . 
     In the unit cells  10  structured as described above, the solid electrolyte layer  2  is configured of yttria-stabilized zirconia (YSZ) or the like; the fuel electrode layer  3 , of a metal such as Ni or Co or a cermet such as Ni—YSZ, C—YSZ; the air electrode layer  4 , of LaMnO 3 , LaCoO 3  or the like; the fuel electrode current collector  6 , of a spongy porous sintered metal plate of Ni-based alloy or the like; the air electrode current collector  7 , of a spongy porous sintered metal plate of Ag-based alloy or the like; and the separators  8 , of stainless steel or the like. 
     The separators  8  have a function to electrically connect the power generating cells  5  to one another and to supply reactive gas to the power generating cells  5 ; they have fuel gas passages  11  which let in fuel gas (reformed gas) from the outer circumferential faces of the separators  8  and discharge it from the substantially central parts  11   a  of the faces of the separators  8  opposite the fuel electrode current collector  6  and oxidant gas passages  12  which let in oxidant gas from the outer circumferential faces of the separators  8  and discharge it from substantially the centers  12   a  of the faces of the separators  8  opposite the air electrode current collector  7 . 
     Within the fuel cell stack  1  a fuel gas manifold  13  and an oxidant gas manifold  14  extending in the stacking direction are formed as shown in  FIG. 1 ; a fuel gas guide pipe  15  from the reformer  21  is connected to the fuel gas manifold  13 , and an oxidant gas guide pipe  16  guided from outside is connected to the oxidant gas manifold  14 . 
     This solid oxide type fuel cell is formed in a seal-less structure having no anti-gas leak seal on the outer circumferential part of the power generating cells  5 ; when in operation, as shown in  FIG. 2 , fuel gas (reformed gas) and oxidant gas (air) supplied from the substantially central parts of the separators  8  to the power generating cells  5  through the fuel gas passages  11  and the oxidant gas passages  12  are distributed evenly all over the fuel electrode layer  3  and the air electrode layer  4  while being diffused in the outer circumferential direction of the power generating cells  5  to cause a power generating reaction to take place, and surplus gas (exhaust gas) not consumed in power generating reaction is freely discharged from the outer circumferential part of the power generating cells  5  within the housing  20 . An exhaust port  20   a  for discharging the exhaust gas within the housing to the outside of the housing is provided in the top part of the housing  20 . 
     Incidentally in this embodiment, heating devices  24  are provided for the reformers  21  and the water vapor generators  22  so that the reformers  21  and the water vapor generators  22  can be heated by thermal conduction. Electric heaters or combustion burners are used as the heating devices  24 . 
       FIG. 3  and  FIG. 4  show how the heating devices  24  are fitted where electric heaters are used as the heating devices  24 . 
       FIG. 3  shows a case in which plate-shaped heaters are used; these plate-shaped heaters are fitted to the outer surface of the box-shaped reformers  21  and water vapor generators  22  opposite the side faces of the fuel cell stack  1 , and their direct heat warms the reformers  21  and the water vapor generators  22  from outside. 
       FIG. 4  shows a case in which pipe-shaped heaters are used; a plurality (two) of such heaters are arranged within each of the reformers  21  and each of the water vapor generators  22  in the longitudinal direction, and heat the reformers  21  and the water vapor generators  22  from inside. 
     Incidentally, in the cases illustrated in  FIG. 3  and  FIG. 4 , two reformers  21  and two water vapor generators  22  are provided respectively on the peripheries of the fuel cell stack  1  in pairs opposite each other with the fuel cell stack  1  in-between, and each of them is provided with a heating device  24 . 
     In two positions near the fuel cell stack  1 , combustion catalysts  23  are arranged in the stacking direction as combusting means for facilitating the combustion of surplus gas (reformed gas) discharged from the outer circumferential part of the fuel cell stack  1 . As these combustion catalysts  23 , for instance what uses thin plate-shaped honeycomb catalysts with Pt, Pd or the like carried by alumina carriers can be applied. 
     Further, though not shown, ignition heaters or igniters which ignite and burn discharged surplus gas, in place of the combustion catalysts  23 , can be arranged in suitable positions near the fuel cell stack as surplus gas combusting means. Also, these combustion catalysts  23  and ignition heaters or igniters can be used in combination. Any of these combusting means are very effective in quickly igniting and burning reformed gas discharged from the fuel cell stack  1  and heating the whole stack efficiently and uniformly. 
     Next, the operating method of the solid oxide type fuel cells configured as described above will be described. 
     At the time as the actuation of the fuel cells, the heating devices  24  are operated, the reformers  21  and the water vapor generators  22  are heated and raised in temperature by thermal conduction, fuel gas (hydrocarbon fuel), oxidant gas (air) and water (having gone through heat exchange outside to become high temperature water) are supplied from outside into the fuel cell module respectively through the fuel gas guide pipe  15 , the oxidant gas guide pipe  16  and a water supply pipe  18 . As the reformers  21  and the water vapor generators  22  are heated by the heating devices  24  from outside (in the case of  FIG. 3 ) or inside (in the case of  FIG. 4 ) by thermal conduction, the reformers  21  are raised to a sufficiently high temperature for reforming and the water vapor generators  22  begin to generate high temperature water vapor in only a few minutes. 
     Hydrocarbon-based fuel gas is joined and mixed in the fuel gas guide pipe  15  with high temperature water vapor guided by the water vapor guide pipe  17  from the water vapor generators  22  and guided into the reformers  21 , and hydrocarbon-based fuel gas is reformed in the reformers  21  into fuel gas mainly consisting of hydrogen by the action of the reforming catalyst for use with hydrocarbon. This reforming reaction is an endothermic reaction, and the high temperature heat required for the reforming reaction (650 to 800° C.) is obtained by receiving direct heat from the appended heating devices  24 . 
     The reformed gas generated by the reformers  21  is guided to the fuel gas manifold  13  in the fuel cell stack  1  and introduced into the separators  8  through the manifold  13 . Together with air introduced through the oxidant gas manifold  14 , these reactive gases pass the inside of the separators  8  and flow to diffuse in the outer circumferential direction from the substantially central parts of the power generating cells  5 . This reformed gas is reductive gas containing hydrogen, and keeps the fuel electrode side of the power generating cells  5  in a reduction condition. 
     Surplus gas mainly consisting of hydrogen, which is not used in the power generating reaction, is discharged out as it is from the outer circumferential part of the fuel cell stack  1 , and promptly ignited and burned by the combusting means (combustion catalysts, ignition heaters or igniters). 
     In this way, according to the invention, reformed gas mainly consisting of hydrogen obtained by the reforming reaction can be supplied into the fuel cell stack from immediately after actuation, and moreover the fuel cell stack can be heated and raised in temperature in a short period of time by burning the reformed gas mainly consisting of hydrogen in the vicinity of the stack, thereby enabling the fuel cell to be rapidly actuated. 
     Incidentally, the time taken until the start of power generation (rated operation) can be reduced to three hours or less, much shorter than the usual eight hours. In addition, the introduction of nitrogen gas into the stack for preventing oxidation (nitrogen purge) conventionally required at the time of starting operation is made unnecessary, making the installation of a nitrogen gas cylinder or piping arrangement dispensable with a corresponding saving in cost. 
     Furthermore, once the combustion reaction is started, the nearby reformers  21  and water vapor generators  22  are heated by that combustion heat, and therefore the heating by the heating devices  24  used at the time of actuation becomes no longer necessary. 
     In this embodiment, the heating devices  24  need to be worked for only about  30  minutes after the start of operation. This results in a substantial reduction in power consumption compared with a case in which, for instance, electric heaters are used as the heating devices  24 . 
     Second Embodiment  
     This second embodiment corresponds to the second mode for implementing the invention. 
     The second embodiment of the invention will be described below with reference to  FIG. 5  through  FIG. 7 . 
       FIG. 5  shows the schematic internal configuration of a solid oxide type fuel cell to which the invention is applied;  FIG. 6  shows its internal arrangement; and  FIG. 7  shows the configuration of the essential part of the fuel cell stack. 
     As shown in  FIG. 5  and  FIG. 6 , the solid oxide type fuel cell of this embodiment has a housing  120  (drum) whose internal wall is covered with an insulator (not shown), and a fuel cell stack  101  to generate a power generating reaction is arranged at the center of the insulated housing  120 . 
     The fuel cell stack  101 , as shown in  FIG. 7 , has a structure in which a power generating cell  105  comprising a fuel electrode  103  and an air electrode  104  arranged on both sides of a solid electrolyte  102 , a fuel electrode current collector  106  outside the fuel electrode  103 , an air electrode current collector  107  outside the air electrode  104 , and separators  108  outside the current collectors  106  and  107  are stacked in many layers sequentially in the longitudinal direction. 
     The solid electrolyte  102  is configured of yttria-stabilized zirconia (YSZ) or the like; the fuel electrode  103 , of a metal such as Ni or a cermet such as Ni—YSZ; the air electrode  104 , of LaMnO 3 , LaCoO 3  or the like; the fuel electrode current collector  106 , of a spongy porous sintered metal plate of Ni or the like; the air electrode current collector  107 , of a spongy porous sintered metal plate of Ag or the like; and the separators  108 , of stainless steel or the like. 
     The separators  108  have a function to electrically connect the power generating cells  105  to one another and to supply reactive gas to the power generating cells  105 ; they have fuel gas passages  111  which let in fuel gas supplied from a fuel gas manifold  113  from the outer circumferential faces of the separators  108  and discharge it from the substantially central parts of the separators  108  opposite the fuel electrode current collector  106  and oxidant gas passages  112  which let in oxidant gas supplied from an oxidant gas manifold  114  from the outer circumferential faces of the separators  108  and discharge it from the substantially central parts of the faces of the separators  108  opposite the air electrode current collector  107 . 
     The fuel cell stack  101  is formed in a seal-less structure having no anti-gas leak seal on the outer circumferential part of the power generating cells  105 ; when in operation, as shown in  FIG. 7 , fuel gas and oxidant gas (air) supplied from the substantially central parts of the separators  108  to the power generating cells  105  through the fuel gas passages  111  and the oxidant gas passages  112  are distributed evenly all over the fuel electrode  103  and the air electrode  104  while being diffused in the outer circumferential direction of the power generating cells  105  to cause a power generating reaction to take place, and unburned gas not consumed in power generating reaction is freely discharged out (within the housing  120 ) of the outer circumferential part of the power generating cells  105 . Incidentally, discharged unburned gas is combusted on the periphery of the fuel cell stack  101 . 
     In the housing  120 , in addition to the fuel cell stack  101 , reformers  121 , water vapor generators  122 , combustion catalysts  123 , auxiliary electric heaters  124 , a temperature raising burner  126  and so forth are arranged in respectively suitable positions around the stack  101 . As shown in  FIG. 6 , these members except the temperature raising burner  126  are arranged in pairs opposing each other with the fuel cell stack  101  at the center in-between. 
     The reformer  121 , packed inside with a hydrocarbon catalyst, reforms hydrocarbon fuel (raw fuel) introduced from outside into fuel gas mainly consisting of hydrogen. Further, this reformer  121  has a heater  125  (or an igniter) built into it for actuating partial oxidation, to be described afterwards. 
     A fuel gas supply pipe  115  and an air supply pipe  117  from outside are connected to the inlet side of the reformer  121 , and its outlet side is connected to the fuel gas manifold  113  in the fuel cell stack  101  via piping  109 . An oxidant gas supply pipe  116  from outside is connected to the oxidant gas manifold  114  in the fuel cell stack  101 . 
     At the time of actuation, a hydrocarbon fuel such as city gas or LPG is introduced into the fuel gas supply pipe  115 , and air is introduced into the air supply pipe  117  and the oxidant gas supply pipe  116 . 
     The water vapor generators  122  are heat exchangers deriving heat from the exhaust heat from the fuel cell stack  101  and intended for obtaining high temperature water vapor required for the reforming reaction; a water supply pipe  118  is connected to their inlet side, and the outlet side is connected to the fuel gas supply pipe  115  by a water vapor guide pipe  119 . During power generating operation, high temperature water vapor from these water vapor generators  122  is introduced into the fuel gas supply pipe  115  via the vapor guide pipe  119 . 
     The combustion catalysts  123  are provided as combusting means for unburned gas discharged from the fuel cell stack  101 , and are arranged in the stacking direction. As these combustion catalysts  123 , for instance what uses thin plate-shaped honeycomb catalysts with Pt (platinum), Pd (palladium) or the like carried by alumina carriers can be applied. 
     Further in this embodiment, in addition to these combustion catalysts  123 , the auxiliary electric heaters  124  or the like are also arranged as combusting means for unburned gas, and it is of course possible to use these combustion catalysts  123  and auxiliary electric heaters  124  in combination. At any rate, unburned gas is quickly ignited and burned by these combusting means in the vicinity of the stack, and the whole stack is heated efficiently and uniformly. 
     Next, one embodiment of the operating method of solid oxide type fuel cells according to the invention will be described. 
     In the solid oxide type fuel cell of the configuration described above, at the time of actuation (when the operation is started), a temperature raising burner  126  is ignited to start raising the temperature of the fuel cell stack  101 . The partial oxidation actuating heaters  125  built into the reformers  121  are then operated and, at the same time, hydrocarbon fuel and air are introduced into the reformers  121  respectively from the fuel gas supply pipe  115  and the air supply pipe  117 . The mixed gas of these hydrocarbon fuel and air is ignited by the partial oxidation actuating heater  125  to give rise to a partial oxidizing reaction in the reformers  121 . 
     In this partial oxidizing reaction, part of the hydrocarbon fuel is burned to generate nitrogen and water (water vapor) and, at the same time, a reforming reaction occurs among the resultant combustion heat, water vapor arising from the combustion and the remaining hydrocarbon fuel not involved in the combustion (namely the partial oxidizing reaction) to generate reductive gas deriving from hydrogen and nitrogen (partial oxidation reforming reaction). The reductive gas in the reformers  121  is supplied into the power generating cells  105  via the piping  109  and the fuel gas manifold  113 , thereby enabling the ambience of the fuel electrode to be kept in a reduction condition from immediately after actuation. 
     On the other hand, unburned gas discharged from the outer circumferential part of the fuel cell stack  101  of the seal-less structure is burned around the stack by the action of the combustion catalysts (the auxiliary electric heaters  124  are turned on in advance if necessary), and the resultant radiant heat warms and raises the temperature of the fuel cell stack  101  from the outer circumferential part. 
     As the stack temperature rises, the quantity of air supplied to the reformers  121  is gradually decreased and water vapor is supplied from the water vapor generators  122 ; when the stack temperature has risen to the working temperature of around 500° C., a shift to water vapor reforming by hydrocarbon fuel and water vapor takes place (auto-thermal reforming reaction). In this while, hydrogen-containing reductive gas continues to be generated by the reformers  121  and supplied to the power generating cells  105 , with the result that the reduction condition of the fuel electrode ambience is continuously maintained. 
     During the power generating operation after the temperature rise, a mixed gas of water vapor from the water vapor generators  122  and hydrocarbon fuel from the fuel gas supply pipe  115  is supplied to the reformers  121 , and hydrogen-rich reformed gas is generated by the water vapor reforming reaction in the reformers  121 ; the supply of the reformed gas to each of the power generating cells  105  causes a power generating reaction to take place in the fuel cell stack  101 . Incidentally, the supply of air to the reformers  121  by the air supply pipe  117  is stopped during power generating operation. 
     Although the supply of reformed gas to the fuel cell stack  101  is stopped when the temperature has fallen at the time of shutdown, the supply of hydrocarbon fuel is gradually reduced even during the temperature falling process to cause the partial oxidation reforming reaction or the auto-thermal reforming reaction to take place in the reformers  121 , and the continuation of these reactions at least until the stack temperature falls to 300° C. or below to supply reductive gas containing hydrogen to the power generating cells  105 , enabling the fuel electrode ambience to be maintained in the reduction condition. 
     This is because the transition of the fuel electrode from the reduction condition in the high temperature ambience after the operation stop (300° C. or above) causes the fuel electrode  103  to be oxidized. Therefore, it is necessary at the time of temperature fall to keep the fuel electrode in the reduction condition until the stack temperature falls to 300° C. or below. 
     As described so far, the operating method according to the invention causes a reforming reaction to be generated in the reformers by the heating ensuing from the partial oxidizing reaction or the auto-thermal reaction during the temperature rise at the time of actuation and the temperature fall at the time of shutdown, and by supplying the fuel electrode with the reductive gas containing hydrogen generated by these reactions enables purging to keep the fuel electrodes  103  of the power generating cells  105  in the reduction condition; oxidation of the power generating cells  105  in the temperature rising or falling cycle and the consequent performance deterioration of the power generating cells  105  can be thereby prevented, and this feature contributes to elongating the service life. 
     In addition, since no purge requiring a large quantity of nitrogen gas as in the conventional practice is needed, the need to install a purging gas supply line including a nitrogen cylinder, for instance, is eliminated, resulting in simplified maintenance work, and the device itself is simplified and can be made more compact. 
     Furthermore, where the unburned gas discharged from the fuel cell stack  101  is burned by combusting means such as combustion catalysts, the heat of combustion facilitates raising the temperature of the fuel cell stack  101  including the reformers  121 , and this serves to reduce the length of time required for actuation. 
     In particular, solid oxide type fuel cells of the seal-less structure requires a large quantity of purging at the time of actuation or shutdown on account of such reasons as the high working temperature, the long time taken to start or stop and the susceptibility of the cells to inflow of oxygen-containing gas from outside, but the operating method according to the present invention which requires no purging with inert gas is extremely useful for such fuel cells. 
     Third Embodiment  
     This embodiment corresponds to the third mode for implementing the invention. 
     The third embodiment of the invention will be described below with reference to drawings. 
       FIG. 8  shows the schematic configuration of a fuel cell power generating device to which the invention is applied;  FIG. 9  shows the configuration of a fuel cell stack for use in this fuel cell power generating device; and  FIG. 10  shows the shutdown control figure according to the present invention. 
     The fuel cell power generating device of this embodiment, as shown in  FIG. 8 , is configured of a fuel cell module  210  in which a solid oxide type fuel cell  201  (fuel cell stack  201 ) which generates a D.C. output according to the supply quantity of fuel gas and the supply quantity of air, a fuel reformer  215  which reforms a mixture gas of fuel gas (e.g. methane gas or city gas) and water vapor into hydrogen-rich gas and supplies it to the fuel cell stack  201  and the like are accommodated into a thermally insulating housing; a fuel supply line  240  which is composed around the fuel cell module  210  of a fuel gas blower  221 , a desulfurizer  223  and various fuel gas supply pipes and introduces fuel gas into the fuel reformer  215 ; an air supply line  230  which is composed of an air blower, air supply pipes and the like and supplies oxidant gas (air) to the fuel cell stack  201 ; a water supply line  250  which is composed of a water supply pump  225  and a water supply pipe and introduces water (incidentally this water is turned into water vapor by a water vapor generator not shown within the fuel cell module) into the fuel cell module; an inverter  224  which converts the D.C. output from the fuel cell stack  201  into an A.C. output and supplies A.C. power Pa to an external load (not shown); and a control unit  220  which controls the flow rates in the air supply line  230 , the fuel supply line  240  and the water supply line  250  among other elements. 
     Further it is so disposed that various items of detection information sent by detectors (not shown) arranged in suitable positions in the fuel cell power generating device including cell voltage information V, stack temperature information T 1 , water vapor temperature information T 2  and output power information Pa are inputted to this control unit  220 . 
     The fuel cell stack  201  here is formed, as shown in  FIG. 9 , by stacking in the longitudinal direction a large number of unit cells  209  each configured of a power generating cell  205  on both faces of whose solid electrolyte layer  202  a fuel electrode layer  203  and an air electrode layer  204  are arranged; a fuel electrode current collector  206  arranged outside the fuel electrode layer  203 ; an air electrode current collector  207  arranged outside the air electrode layer  204 ; and separators  208  arranged outside the current collectors  206  and  207 . 
     Out of the constituent elements of the unit cell  209 , the solid electrolyte layer  202  is configured of yttria-stabilized zirconia (YSZ) or the like; the fuel electrode layer  203 , of a metal such as Ni or Co or a cermet such as Ni—YSZ or Co—YSZ; the air electrode layer  204 , of LaMnO 3 , LaCoO 3  or the like; the fuel electrode current collector  206 , of a spongy porous sintered metal plate of Ni-based alloy or the like; the air electrode current collector  207 , of a spongy porous sintered metal plate of Ag-based alloy or the like; and the separators  208 , of stainless steel or the like. 
     The separators  208  have a function to electrically connect the power generating cells  205  to one another and to supply reactive gas to the power generating cells  205 ; they have fuel gas passages  211  which let in fuel gas supplied from the outer circumferential faces of the separators  208  and discharge it from the substantially central parts  211   a  of the separators  208  opposite the fuel electrode current collector  206  and oxidant gas passages  212  which let in oxidant gas from the outer circumferential faces of the separators  208  and discharge it from the substantially central parts  212   a  of the faces of the separators  208  opposite the air electrode current collector  207 . 
     Within the fuel cell stack  201 , a fuel gas manifold  217  and an oxidant gas manifold  218  extending in the stacking direction are formed; reformed fuel gas circulates in the manifold  217 , externally supplied air circulates in the manifold  218 , and the gases are introduced from the manifolds  217  and  218  into the gas passages  211  and  212  of the separators  208  and are discharged from gas outlets  211   a  and  212   a  to be distributed and supplied to the electrodes of the power generating cells. A pair of end plates  208   a  and  208   b  formed of stainless steel or the like are disposed at both ends of the fuel cell stack  201 , and the power generated by the fuel cell stack  201  can be taken out via these end plates  208   a  and  208   b.    
     The fuel cell stack  201  is formed in a seal-less structure in which an anti-gas leak seal is deliberately dispensed with on the outer circumferential part of the power generating cells  205 ; when in operation, surplus gas (high temperature exhaust gas) unconsumed in the power generation can be freely discharged from the outer circumferential part of the power generating cells  205  within the housing. Incidentally, the high temperature exhaust gas discharged within the inner space of the housing is discharged out of the module through an upper exhaust port. 
     Next, the shutdown control of the fuel cell power generating device of the above-described configuration will be described with reference to  FIG. 10 . This shutdown control is accomplished with the control unit  220  on the basis of various items of detection information (V, Pa, T 1 , T 2  and so on) sent by the detectors referred to above. 
     The shutdown control shown in  FIG. 10  is carried out in a state in which the flow rate of air supplied to the fuel cell stack  201  is maintained at a fixed level. In  FIG. 10 , the left vertical axis represents the supply quantity of fuel gas (methane) and the supply quantity of water, which is the source of water vapor, while the right vertical axis represents the stack temperature and the cell output and the horizontal axis represents the time elapsed. 
     As shown in  FIG. 10 , when a shutdown action is taken during a period of rated power generation (1 kW in output at the stack temperature of 750° C.), the cell output is lowered in about four hours from 1 kW to 0 W (output lowering period) while reducing the flow rates of methane and water supplied to the fuel cell module  210 , and after that, the stack temperature is lowered in about 15 hours from about 700° C. to 300° C. or below (temperature lowering period). The present invention features purge processing to avert the phenomenon of fuel electrode layer oxidation by keeping the fuel electrode layer side in a reduction condition in the high temperature ambience during the output lowering period and the temperature lowering period after the action to stop operation. 
     Thus, the shutdown control in this embodiment is intended to maintain, even when power generation is stopped, the reductiveness of the fuel electrode layer by continuing to supply small quantities of methane and water to the fuel cell module  210 , generating water vapor by utilizing the thermal capacity of the fuel cell module  210 , generating hydrogen by reforming reaction and supplying mixed gas with water vapor to the fuel electrode layer side. 
     Incidentally, in this shutdown control the flow rates of methane and water can be varied by regulating with the control unit  220  the actions of the fuel gas blower  221  (which may be a control valve) or the water supply pump  225  (which may be a control valve). 
     In this shutdown control, regarding the methane flow rate, it is necessary to reduce the supply quantity of methane so that the cell voltage V can be kept no lower than 0.5 V when the stack temperature T 1  is 300° C. 
     The reason is that stopping the supply of methane when the stack temperature T 1  is 300° C. or above, the fuel electrode layer which mainly consists of Ni may be oxidized by that heat to generate NiO, and such an oxidation-reduction reaction of the fuel electrode layer would seriously deteriorate the performance of the power generating cell. Therefore, until the stack temperature T 1  falls to around 300° C., the methane flow rate should be gradually reduced so that the cell voltage V may not fall to less than 0.5 V as shown in  FIG. 10 . This enables the fuel electrode layer side to be kept in the reduction condition. Incidentally, in controlling the methane flow rate, the stack voltage may be monitored instead of the cell voltage V. 
     Regarding the water flow rate, on the other hand, the supply quantity of water should be decreased so as to keep the water vapor temperature at or above 200° C. when water supply is stopped. The reason is that the temperature of water vapor will drop to 100° C. in a stroke if the water vapor temperature comes down to 200° C. or below, making it difficult to continuously generate water vapor, with the consequence that liquid water, instead of water vapor, is supplied into the cells, which might be thereby deteriorated or cracked. 
     As hitherto described, according to the present invention, the fuel electrode layer can be kept in the reduction condition at the time of stopping power generation in the high temperature ambience after the power generation stop by supplying water and fuel gas to the fuel cells while reducing the flow rates thereof, and oxidation of the power generating cells in the temperature rising or falling cycle and the consequent performance deterioration of the power generating cells can be prevented, thereby enabling the service life to be elongated. 
     In addition, since this shutdown method requires no purge with inert gas as in the conventional practice, the need to install a purging gas supply line including an inert gas cylinder (e.g. a nitrogen cylinder) is entirely eliminated, enabling the maintenance work to be simplified as well as the hardware itself to be reduced in size. 
     In particular in solid oxide type fuel cells, as the working temperature is as high as 600 to 1000° C., the thermal capacity of the module formed of a metal, ceramic or the like is large during the temperature lowering period, making it impossible to reduce the stack temperature in a short period of time, secure purging is required at the time of shutdown on account of such reasons as the sustained high temperature ambience and, in a seal-less structure, the susceptibility to inflow of external oxygen-containing gas (within the fuel cell module) into the cells due to the decrease of the battery pressure; the shutdown method according to the present invention which requires no purging with inert gas is extremely effective for such fuel cell working at a high temperature. 
     INDUSTRIAL APPLICABILITY  
     The present invention can provide a solid oxide type fuel cell in which reductive reformed gas containing hydrogen is generated from the initial stage of actuation by heating a reformer and a water vapor generator in the fuel module with a heating device at the time of actuation, and which is enabled to be actuated rapidly while being supplied with the purge gas on the fuel electrode side of the power generating cell together with an operating method of this fuel cell. 
     Also the present invention can provide an operating method of fuel cells which enables a temperature raising or lowering action at the time of actuation or shutdown without requiring a separate supply line for purging gas.