Patent Publication Number: US-2019190050-A1

Title: Solid oxide fuel cell system

Description:
This Application is a Divisional of application Ser. No. 15/344,647 filed Nov. 7, 2016, which claims priority to Japanese Patent Application No. 2015-227369 filed Nov. 20, 2015, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a solid oxide fuel cell system. 
     2. Description of the Related Art 
     A fuel cell cogeneration system has received attention as one type of distributed power generator. In particular, development of a solid oxide fuel cell (hereinafter called an “SOFC”) operating at high temperature with use of a solid oxide as an electrolyte has been progressed as a distributed power generator having high power generation efficiency. 
     In a solid oxide fuel cell system (hereinafter called an “SOFC” system) including the SOFC, exhaust gas containing substances to be purified, such as carbon monoxide, is discharged during power generation. Accordingly, the SOFC system including a purification catalyst (i.e., an exhaust gas purification catalyst) disposed in an exhaust passage is proposed to remove the substances that are to be purified (see, e.g., Japanese Unexamined Patent Application Publication No. 2015-18750). The SOFC system disclosed in Japanese Unexamined Patent Application Publication No. 2015-18750 includes a startup heater because of the necessity of heating the purification catalyst from ordinary temperature up to activating temperature. In Japanese Unexamined Patent Application Publication No. 2015-18750, particularly, an evaporator can also be heated together with the purification catalyst by the startup heater. 
     SUMMARY 
     One non-limiting and exemplary embodiment provides a solid oxide fuel cell system capable of suppressing reduction in durability of a purification catalyst that is included in a purifier. 
     In one general aspect, the techniques disclosed here feature a solid oxide fuel cell system including a reformer that reforms a power generation gas supplied to the reformer, and that produces hydrogen-containing gas as anode gas, a solid oxide fuel cell that generates, through an electrochemical reaction, electric power from the anode gas produced by the reformer and cathode gas containing oxygen, a combustor that burns anode off-gas and cathode off-gas both discharged from the solid oxide fuel cell, and that produces exhaust gas, a purifier that is heated by heat of the exhaust gas produced by the combustor, and that includes a purification catalyst to remove substances to be cleaned up, those substances being contained in the exhaust gas, and a controller, wherein temperature of the purifier is not lower than 150° C. and not higher than 250° C. during operation, and the controller raises the temperature of the purifier to 300° C. or higher for a predetermined time. 
     The solid oxide fuel cell system according to one aspect of the present disclosure has an advantageous effect of being able to suppress reduction in durability of the purification catalyst that is put in the purifier. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one example of a configuration of an SOFC system according to an embodiment of the present disclosure; 
         FIG. 2  is a graph depicting one example of activity states of a purification catalyst before SO 2  poisoning and after SO 2  poisoning; 
         FIG. 3  is a graph depicting one example of an activity state of the purification catalyst in the case of heating the purification catalyst to a predetermined temperature after the SO 2  poisoning; and 
         FIG. 4  illustrates one example of a configuration of an SOFC system according to Example 6 of the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     (Underlying Knowledge Forming Basis of the Present Disclosure) 
     The inventors have conducted intensive studies on the SOFC system. As a result, the inventors have gained the following knowledge. The SOFC system is operated at high temperature from 550° C. to 750° C., for example, and is able to effectively utilize heat generated during power generation. Stated in another way, in the SOFC system, anode off-gas discharged from an anode of the SOFC is ignited and is burnt together with cathode off-gas discharged from a cathode of the SOFC, thereby generating exhaust gas. In the SOFC system, a power generation system with high energy efficiency is realized by effectively utilizing heat of the exhaust gas. However, as the heat of the exhaust gas is utilized more efficiently, the exhaust gas at lower temperature is supplied to a purifier that is disposed in an exhaust path near its outlet. The heat of the exhaust gas can be utilized, for example, to preheat cathode gas supplied to the SOFC, to preheat a material for use in power generation, and to heat an evaporator. However, when the heat of the exhaust gas is effectively utilized in such a manner, temperature of the exhaust gas at timing of being supplied to the purifier is fairly lower than that at timing of being generated. 
     The inventors have found that, with deterioration of a purification catalyst contained in the purifier, activity of the purification catalyst at relatively low temperature reduces, and purification characteristics for substances to be cleaned up, such as carbon monoxide, degrade. The inventors have also found that, when the exhaust gas at temperature having lowered is supplied to the purifier, concentrations of the substances to be cleaned up, which are contained in the exhaust gas discharged to the outside of the SOFC system, increase with degradation of the purification characteristics of the purification catalyst. 
     In addition, the inventors have focused attention to the fact that a small amount of SO 2  is contained as an impurity in cathode gas (e.g., air) supplied to the SOFC and hence SO 2  is contained in the exhaust gas as well. Moreover, the inventors have recognized a possibility that the purification catalyst may deteriorate due to SO 2  poisoning. The inventors have further found that, when the purification catalyst deteriorates due to SO 2  poisoning, the activity of the purification catalyst at relatively low temperature may disappear as described above, and hence durability of the purification catalyst may reduce. 
     On the basis of the findings described above, the inventors have conducted studies on the SOFC system capable of suppressing reduction in durability of the purification catalyst during operation (i.e., during power generation in the SOFC) and, as a result of the studies, have accomplished the SOFC system according to the present disclosure. In more detail, the following embodiments are proposed in the present disclosure. 
     A solid oxide fuel cell system according to a first aspect of the present disclosure includes a reformer that reforms a power generation gas supplied to the reformer, and that produces hydrogen-containing gas as anode gas, a solid oxide fuel cell that generates, through an electrochemical reaction, electric power from the anode gas produced by the reformer and cathode gas containing oxygen, a combustor that burns anode off-gas and cathode off-gas both discharged from the solid oxide fuel cell, and that produces exhaust gas, a purifier that is heated by heat of the exhaust gas produced by the combustor, and that includes a purification catalyst to remove substances to be cleaned up, those substances being contained in the exhaust gas, and a controller, wherein temperature of the purifier is not lower than 150° C. and not higher than 250° C. during operation, and the controller raises the temperature of the purifier to 300° C. or higher for a predetermined time. 
     With the features described above, since the controller raises the temperature of the purifier to 300° C. or higher for the predetermined time during the operation of the solid oxide fuel cell in an exemplary case, it is possible to recover activity of the purification catalyst, the activity having reduced due to sulfur poisoning. Therefore, the solid oxide fuel cell system according to the first aspect of the present disclosure has an advantageous effect that reduction in durability of the purification catalyst in the purifier can be suppressed. From the viewpoint of energy saving, the temperature of the purifier is preferably raised during power generation (e.g., during rated operation). 
     Here, the term “rated operation” implies that, in the solid oxide fuel cell system, the power generation by the solid oxide fuel cell is performed under rated operating conditions. The term “rated operating conditions” implies operating conditions specified to obtain a predetermined power generation output. 
     According to a second aspect of the present disclosure, in addition to the first aspect, the solid oxide fuel cell system may further include a cathode gas supplier that supplies the cathode gas to the solid oxide fuel cell, and the controller may raise the temperature of the purifier by controlling the cathode gas supplier to reduce a flow rate of the cathode gas supplied to the solid oxide fuel cell. 
     With the features described above, the controller can make control to reduce the flow rate of the cathode gas supplied from the cathode gas supplier. It is hence possible to reduce an amount of heat removed from the exhaust gas by the cathode gas, and to raise the temperature of the exhaust gas that is utilized to heat the purifier. As a result, in the solid oxide fuel cell system according to the second aspect of the present disclosure, the temperature of the purifier can be raised to 300° C. or higher for the predetermined time. 
     According to a third aspect of the present disclosure, in addition to the first aspect, the solid oxide fuel cell system may further include a cathode gas supplier that supplies the cathode gas to the solid oxide fuel cell, and the controller may raise the temperature of the purifier by controlling the cathode gas supplier to increase a flow rate of the cathode gas supplied to the solid oxide fuel cell. 
     With the features described above, the controller can make control to increase the flow rate of the cathode gas supplied from the cathode gas supplier. By increasing the flow rate of the cathode gas, an amount of heat removed from the exhaust gas by the cathode gas is increased, but the flow rate of the exhaust gas produced by the combustor can also be increased. Therefore, when an increase in an amount of heat specific to the exhaust gas at the increased flow rate is larger than the amount of heat removed from the exhaust gas by the cathode gas, the temperature of the exhaust gas supplied to the purifier can be raised eventually. As a result, in the solid oxide fuel cell system according to the third aspect of the present disclosure, the temperature of the purifier can be raised to 300° C. or higher for the predetermined time. 
     According to a fourth aspect of the present disclosure, in addition to the first aspect, the controller may raise the temperature of the purifier by performing control to reduce a power generation output of the solid oxide fuel cell. 
     With the feature described above, since the controller performs control to reduce the power generation output of the solid oxide fuel cell, a flow rate of the anode gas, which is contained in anode off-gas and has not been utilized for the power generation, can be increased. Thus, the flow rate of the anode gas, which has not been utilized for the power generation and is utilizable for combustion in the combustor, can be increased, and the temperature of the exhaust gas produced by the combustor can be raised. As a result, the temperature of the exhaust gas supplied to the purifier can be raised, and the temperature of the purifier can be raised to 300° C. or higher for the predetermined time. 
     According to a fifth aspect of the present disclosure, in addition to the first aspect, the solid oxide fuel cell system may further include a power generation gas supplier that supplies the power generation gas to the reformer, and the controller may raise the temperature of the purifier by controlling the power generation gas supplier to increase a flow rate of the power generation gas supplied to the reformer. 
     With the features described above, since the controller controls the power generation gas supplier to increase the flow rate of the power generation gas supplied to the reformer, a flow rate of the anode gas, which has not been utilized for the power generation and is utilizable for combustion in the combustor, can be increased. Hence the temperature of the exhaust gas produced by the combustor can be raised. As a result, the temperature of the exhaust gas supplied to the purifier can be raised, and the temperature of the purifier can be raised to 300° C. or higher for the predetermined time. 
     According to a sixth aspect of the present disclosure, in addition to the first aspect, the solid oxide fuel cell system may further include a reformation water supplier that supplies reformation water utilized to reform the power generation gas in the reformer, and an evaporator that evaporates the reformation water supplied from the reformation water supplier by utilizing heat of the exhaust gas, and that produces steam, and the controller may raise the temperature of the purifier by controlling the reformation water supplier to reduce a flow rate of the reformation water supplied to the evaporator. 
     With the features described above, since the controller controls the reformation water supplier to reduce the flow rate of the reformation water supplied to the evaporator, an amount of heat utilized in the evaporator to evaporate the reformation water can be reduced. As a result, the temperature of the exhaust gas can be raised, and the temperature of the purifier can be raised to 300° C. or higher for the predetermined time. 
     According to a seventh aspect of the present disclosure, in addition to the second aspect, the solid oxide fuel cell system may further include a temperature sensor that senses the temperature of the purifier, and a heater that heats the purifier, and the controller may control the heater to heat the purifier upon determining, on the basis of a result sensed by the temperature sensor, that the temperature of the purifier does not reach 300° C. when the temperature of the purifier is raised. 
     With the features described above, when it is determined, on the basis of a result sensed by the temperature sensor, that the temperature of the purifier does not reach 300° C., the purifier can be heated up to 300° C. with the provision of the heater. Thus, the temperature of the purifier can be reliably raised to 300° C. or higher. 
     Details of an embodiment will be described below with reference to the drawings. 
     Embodiment 
     (Configuration of SOFC System) 
     A configuration of an SOFC system  100  according to the embodiment is described below with reference to  FIG. 1 .  FIG. 1  illustrates one example of the configuration of the SOFC system  100  according to the embodiment of the present disclosure. 
     As illustrated in  FIG. 1 , the SOFC system  100  according to the embodiment includes an SOFC  1 , a power generation gas supplier  2 , a cathode gas supplier  3 , a reformation water supplier  4 , a reformer  5 , a combustor  6 , an air heat exchanger  7 , an evaporator  8 , and a controller  9 . 
     The reformer  5  reforms a power generation gas having been supplied to the reformer  5  and produces hydrogen-containing gas as anode gas. More specifically, the power generation gas supplier  2  supplies the power generation gas to the evaporator  8  through a material supply path  11 . Furthermore, the reformation water supplier  4  supplies water for use in the reformation to the evaporator  8  through a water supply path  13 . The power generation gas supplier  2  is constituted to be able to adjust a flow rate of the power generation gas supplied therefrom in accordance with a control command from the controller  9 . The reformation water supplier  4  is constituted to be able to adjust a flow rate of the reformation water supplied therefrom in accordance with a control command from the controller  9 . 
     The evaporator  8  evaporates the supplied reformation water and then supplies steam (water vapor) to the reformer  5  after mixing the steam with the power generation gas supplied to the reformer  5 . Upon a gas mixture of the steam and the power generation gas being supplied, the reformer  5  produces the hydrogen-containing gas (anode gas) through a steam reforming reaction. The reformer  5  supplies the produced anode gas to the SOFC  1  through an anode gas path  14 . 
     A reforming reaction carried out in the reformer  5  is not limited to the steam reforming reaction referred to above. As an alternative, though not specifically illustrated in  FIG. 1 , the system may be constituted to be able to further supply air to the reformer  5 , and the reformer  5  may produce hydrogen-containing gas by an auto-thermal method, for example, with use of the supplied air. 
     The power generation gas supplied to the SOFC system  100  may be a material containing an organic compound that is made up of at least carbon and hydrogen as constituent elements. Examples of the power generation gas include gases containing organic compounds made up of at least carbon and hydrogen, such as city gas, natural gas, LPG, and LNG each containing methane as a main component, hydrocarbons, and alcohols such as methanol. The reformer  5  produces the hydrogen-containing gas by causing the power generation gas to react with the steam (i.e., through the reforming reaction) with the aid of a Ru catalyst or a Ni catalyst, for example. 
     When a sulfur compound is added as an odorant to the power generation gas, or when the power generation gas contains a sulfur compound derived from the material, there is a possibility that the Ru catalyst or the Ni catalyst used in the reformer  5  may deteriorate due to poisoning, or that reduction in performance of the fuel cell may occur due to poisoning of an anode (fuel pole) of the fuel cell. To avoid such a possibility, when the power generation gas contains a sulfur compound, a desulfurizer (not illustrated) for removing the sulfur compound from the power generation gas may be further disposed in a stage upstream of the reformer  5 . 
     The SOFC  1  generates electric power through an electrochemical reaction from the anode gas produced by the reformer  5  and cathode gas containing oxygen. More specifically, the anode gas is supplied to the anode of the SOFC  1  from the reformer  5  through an anode gas path  14 . On the other hand, the cathode gas is supplied to a cathode of the SOFC  1  from the cathode gas supplier  3  through a cathode gas path  12 . The SOFC  1  generates electric power by employing the anode gas and the cathode gas both supplied to the SOFC  1 . The SOFC  1  discharges, to the combustor  6 , anode off-gas containing the anode gas that has not been used in the power generation, and cathode off-gas containing the cathode gas that has not been used in the power generation. The cathode gas supplier  3  is constituted to be able to adjust a flow rate of the cathode gas, which is supplied from the cathode gas supplier  3 , in accordance with a control command from the controller  9 . 
     The combustor  6  includes an ignitor (not illustrated) for igniting the anode off-gas discharged from the SOFC  1 , and produces exhaust gas by burning the anode off-gas together with the cathode off-gas discharged from the SOFC  1 . The exhaust gas generated by the combustor  6  flows through an exhaust path  15 , and it is discharged to the outside of the SOFC system through a purifier  16 . 
     The exhaust gas produced by the combustor  6  flows through the reformer  5 , the evaporator  8 , and the air heat exchanger  7  such that the reformer  5  and the evaporator  8  are heated to predetermined temperatures by heat of the exhaust gas before the exhaust gas reaches the purifier  16 . Furthermore, the exhaust gas is subjected to heat exchange in the air heat exchanger  7  with respect to the cathode gas in a stage before the cathode gas is supplied to the SOFC  1 . Hence the cathode gas can be preheated. 
     Moreover, in order to effectively utilize the heat of the exhaust gas, the SOFC system  100  is constituted, as illustrated in  FIG. 1 , in a state that the SOFC  1 , the reformer  5 , the combustor  6 , the air heat exchanger  7 , and the evaporator  8  are contained within a housing  10  covered with a heat insulating member, the housing  10  being called a hot module. 
     The purifier  16  includes a purification catalyst heated by the heat of the exhaust gas and acting to remove substances to be cleaned up, which are contained in the exhaust gas. The purifier  16  is disposed near an outlet of the exhaust path  15 . The exhaust gas of which heat has been partly consumed by the reformer  5 , the air heat exchanger  7 , the evaporator  8 , etc. flows through the purifier  16 . When the exhaust gas flows through the purifier  16 , the temperature of the purification catalyst is raised by the heat of the exhaust gas, while the substances to be cleaned up, which are contained in the exhaust gas, are removed by the purification catalyst. Accordingly, it can be said that the temperature of the purifier  16  (i.e., the temperature of the purification catalyst) is in a proportional relation to the temperature of the exhaust gas. In the SOFC system  100  according to this embodiment, the temperature of the purifier  16  (i.e., the temperature of the purification catalyst) during the power generation by the SOFC  1  under rated operating conditions is held within a temperature range of not lower than about 150° C. and not higher than about 250° C. although it varies depending on operating conditions, conditions of heat radiation and heat conduction from the housing  10 , and other various conditions. Examples of the substances to be cleaned up, which are contained in the exhaust gas, are carbon monoxide (CO), hydrocarbons (HC), and oxygen nitrides (NOx). 
     (Activity State of Purification Catalyst) 
     As described above, there is a possibility that, because the exhaust gas contains a small amount of sulfur dioxide (SO 2 ), the purification catalyst may deteriorate due to the SO 2  poisoning. The difference in activity state of the purification catalyst between before the SO 2  poisoning and after the SO 2  poisoning is described here with reference to  FIG. 2 .  FIG. 2  is a graph depicting one example of activity states of the purification catalyst before the SO 2  poisoning and after the SO 2  poisoning. In  FIG. 2 , the horizontal axis denotes temperature (° C.) of the purification catalyst, and the vertical axis denotes CO concentration (ppm) at the outlet of the exhaust path  15 . Thus,  FIG. 2  represents a variation of the CO concentration with respect to the catalyst temperature (i.e., temperature dependency of the CO concentration). 
     In experiments, a Pt/Al 2 O 3  catalyst held on a honeycomb of 400 CPSI (cells per square inch) was used as the purification catalyst. Exhaust gas having the space velocity (SV) of GHSV=60000 h −1  and containing 1000 ppm of CO, 3.6% of CO 2 , 7.7% of O 2 , 65.9% of N 2 , and 22.6% of H 2 O was introduced to flow through the SOFC system. A thermocouple was disposed upstream of the purification catalyst, and temperature dependency of the CO concentration was examined in the case of lowering the temperature of the purification catalyst to 200° C. or below. Thereafter, 13.5 ppm of SO 2  was supplied to the purifier  16  to continuously develop a reaction for 48 hours in a state of the temperature of the exhaust gas being held at 200° C. The supply of SO 2  was then stopped. The temperature dependency of the CO concentration was examined again in the case of lowering the temperature of the purification catalyst to 200° C. or below. 
     As illustrated in  FIG. 2 , before the SO 2  poisoning of the purification catalyst, the CO concentration at the outlet of the exhaust path  15  was maintained at about 0 ppm with respect to 1000 ppm of the CO concentration at an inlet of the exhaust path  15  over an entire temperature zone where the catalyst temperature was 200° C. or below. In other words, the activity of the purification catalyst was maintained. On the other hand, it was found that, after the SO 2  poisoning of the purification catalyst, the CO concentration abruptly increased and the activity of the purification catalyst disappeared at the temperature of the purification catalyst being 180° C. or below. 
     An activity state of the purification catalyst in the case of heating the purification catalyst to a predetermined temperature under conditions for the catalyst reaction after the SO 2  poisoning of the purification catalyst is described here with reference to  FIG. 3 . In the case described here, the predetermined temperature was set to 300° C. or lower, and the temperature of the purification catalyst was raised up to 350° C.  FIG. 3  is a graph depicting one example of an activity state of the purification catalyst in the case of heating the purification catalyst to the predetermined temperature after the SO 2  poisoning. 
       FIG. 3  represents a variation of the CO concentration with respect to the temperature of the purification catalyst (i.e., temperature dependency of the CO concentration), as in  FIG. 2 , with the horizontal axis denoting temperature (° C.) of the purification catalyst and the vertical axis denoting CO concentration (ppm) at the outlet of the exhaust path  15 . The temperature of the purification catalyst in the SOFC system  100  is raised by utilizing the heat of the exhaust gas that is supplied to the purifier  16 . Accordingly, the temperature of the exhaust gas supplied to the purifier  16  is to be increased in order to raise the temperature of the purification catalyst. Manners for raising the temperature of the exhaust gas will be described in detail later. 
     As illustrated in  FIG. 3 , the purification catalyst was heated after the SO 2  poisoning to the predetermined temperature of 300° C. or higher (350° C. here), and reduction in the temperature of the purification catalyst and change in the CO concentration at the outlet of the exhaust path  15  were observed. As seen from  FIG. 3 , it was found that, in the case of heating the purification catalyst to the predetermined temperature of 350° C. after the SO 2  poisoning, the CO concentration at the outlet of the exhaust path  15  abruptly increased and the activity of the purification catalyst disappeared at a time when the temperature of the purification catalyst lowered to 140° C. or below. On the other hand, it was found that, in the case of heating the purification catalyst to 200° C. after the SO 2  poisoning, namely in the case of holding the catalyst temperature at a level during the operation under the rated operating conditions, the CO concentration abruptly increased and the activity of the purification catalyst disappeared, as illustrated in  FIG. 3 , at a time when the temperature of the purification catalyst lowered to 180° C. or below. From the above observation results, it is understood that the activity of the purification catalyst can be maintained until reaching 140° C., as a result of heating the purification catalyst to the predetermined temperature of 300° C. or higher, particularly 350° C., after the SO 2  poisoning. In other words, it is recognized that the activity of the purification catalyst, which has reduced due to the SO 2  poisoning, can be regenerated (recovered) by heating the purification catalyst to the predetermined temperature (300° C. or higher, particularly 350° C.) after the SO 2  poisoning. 
     The timing of raising the temperature of the purification catalyst is not limited to a period during the rated operation, and it may be during an operation other than the rated operation. The temperature of the purification catalyst may be raised in a state where the operation is stopped. 
     On the basis of the above-described results, in the SOFC system  100  according to this embodiment, the purification catalyst in the purifier  16  is temporarily heated to the predetermined temperature for a predetermined time during the operation of the SOFC system  100  under the rated operating conditions such that the purification catalyst can be regenerated (recovered). More specifically, in the SOFC system  100 , the temperature of the exhaust gas is raised at predetermined timing in accordance with one of manners described in the following Examples 1 to 6, thereby heating the purification catalyst in the purifier  16  to the predetermined temperature. 
     The SOFC system  100  according to this embodiment may further include an indicator  24  that indicates an operation status. The indicator  24  is, e.g., a remote controller or a lamp disposed on a main body. When the purification catalyst in the purifier  16  is under heating to the predetermined temperature, the indicator  24  may indicate that an operation of raising the catalyst temperature is being performed. In a practical example, the indicator  24  indicates a message of “under warm-up” or “under clean-up”. 
     The SOFC system  100  according to this embodiment is described, by way of example, as being operated under the rated operating conditions given below. Thus, it is assumed that the flow rate of the power generation gas supplied from the power generation gas supplier  2  is 2.08 NLM, and that the flow rate of the cathode gas supplied from the cathode gas supplier  3  is 49 NLM. It is also assumed that a stoichiometric ratio (S/C) of steam to the power generation gas when the hydrogen-containing gas is produced through the steam reforming reaction is given by 2.5, and that a power generation output of the SOFC  1  is 700 W. It is further assumed that, as a result of setting a heat radiation amount from the housing  10 , a heat conduction area in the housing  10 , etc. to proper values, the temperature of the exhaust gas supplied to the purifier  16  during the power generation is held at 240° C. 
     Example 1 
     In the SOFC system  100  according to Example 1 of the embodiment, the purification catalyst in the purifier  16  may be temporarily heated to the predetermined temperature for the predetermined time by reducing the flow rate of the cathode gas supplied to the SOFC system  100  under the rated operating conditions described above. The predetermined time may be, for example, 30 minutes to 1 hour, and the predetermined temperature may be 300° C. or higher. 
     More specifically, in the SOFC system  100  according to Example 1, the controller  9  controls the cathode gas supplier  3  at predetermined timing during the operation under the rated operating conditions (i.e., during the power generation of the SOFC  1 ) to reduce the flow rate of the cathode gas, which is supplied to the SOFC  1 , for the purpose of raising the temperature of the exhaust gas and raising the temperature of the purifier  16 . Thus, in the SOFC system  100  according to Example 1, by reducing the flow rate of the supplied cathode gas to be smaller than that under the rated operating conditions, it is possible to reduce an amount of heat utilized in heat exchange between the cathode gas and the exhaust gas in the air heat exchanger  7 , for example, and to raise the temperature of the exhaust gas. As a result, the temperature of the purification catalyst in the purifier  16  can be raised to 300° C. or higher for the predetermined time. 
     Example 2 
     In the SOFC system  100  according to Example 2 of the embodiment, the purification catalyst in the purifier  16  may be temporarily heated to the predetermined temperature for the predetermined time by increasing the flow rate of the cathode gas supplied to the SOFC system  100  under the rated operating conditions described above. 
     More specifically, in the SOFC system  100  according to Example 2, the controller  9  controls the cathode gas supplier  3  at predetermined timing during the operation under the rated operating conditions to increase the flow rate of the cathode gas, which is supplied to the SOFC  1 , for the purpose of raising the temperature of the exhaust gas and raising the temperature of the purifier  16 . Thus, in the SOFC system  100  according to Example 2, by increasing the flow rate of the supplied cathode gas to be larger than that under the rated operating conditions, an amount of heat conducted to the cathode gas from the exhaust gas in the heat exchange between the cathode gas and the exhaust gas in the air heat exchanger  7  is increased. However, the flow rate of the exhaust gas produced by the combustor  6  can also be increased by increasing the flow rate of the supplied cathode gas. Therefore, when an increase in an amount of heat specific to the exhaust gas at the increased flow rate is larger than an amount of heat removed from the exhaust gas by the cathode gas, the temperature of the exhaust gas supplied to the purifier  16  can be raised eventually. As a result, the temperature of the purification catalyst in the purifier  16  can be raised to 300° C. or higher for the predetermined time. 
     Example 3 
     In the SOFC system  100  according to Example 3 of the embodiment, the purification catalyst in the purifier  16  may be temporarily heated to the predetermined temperature for the predetermined time by reducing the power generation output of the SOFC  1  under the rated operating conditions described above. 
     More specifically, in the SOFC system  100  according to Example 3, the controller  9  performs control to reduce the power generation output of the SOFC  1  at predetermined timing during the operation under the rated operating conditions for the purpose of raising the temperature of the exhaust gas and raising the temperature of the purifier  16 . On that occasion, only the power generation output is reduced while the flow rates of the supplied power generation gas and the supplied cathode gas are held at the same values as those under the rated operating conditions. Accordingly, a flow rate of the anode gas, which is contained in the anode off-gas and has not been utilized for the power generation, can be increased. Thus, by increasing the flow rate of the anode gas that has not been utilized for the power generation and that is utilizable for the combustion in the combustor  6 , the temperature of the exhaust gas produced by the combustor  6  can be raised. In other words, an amount of heat given to the exhaust gas is increased by reducing the fuel utilization (UF) in the power generation and by employing part of the fuel (anode gas), which is to be utilized in the power generation, for the combustion in the combustor  6 . The temperature of the exhaust gas can be hence raised to a value higher than that during the operation under the rated operating conditions. As a result, the temperature of the purification catalyst in the purifier  16  can be raised to 300° C. or higher for the predetermined time. 
     Example 4 
     In the SOFC system  100  according to Example 4 of the embodiment, the purification catalyst in the purifier  16  may be temporarily heated to the predetermined temperature for the predetermined time by increasing the flow rate of the power generation gas supplied to the reformer  5 . 
     More specifically, in the SOFC system  100  according to Example 4, the controller  9  controls the power generation gas supplier  2  to increase the flow rate of the power generation gas supplied to the reformer  5  at predetermined timing during the operation under the rated operating conditions for the purpose of raising the temperature of the exhaust gas and raising the temperature of the purifier  16 . With such control, a flow rate of the anode gas, which has not been utilized for the power generation and is utilizable for the combustion in the combustor  6 , can be increased. Accordingly, the temperature of the exhaust gas produced by the combustor  6  can be raised. Thus, a proportion of the power generation gas (anode gas) utilized for the power generation is reduced, while a proportion of the power generation gas (anode gas) utilized for the combustion in the combustor  6  is increased. In other words, as in Example 3, an amount of heat given to the exhaust gas is increased by reducing the fuel utilization (UF) in the power generation and by increasing part of the fuel (anode gas) supplied to be utilized for the power generation, the part being used for the combustion in the combustor  6 . The temperature of the exhaust gas can be hence raised to a value higher than that during the operation under the rated operating conditions. As a result, the temperature of the purification catalyst in the purifier  16  can be raised. 
     The inventors have found that, by increasing the flow rate (2.08 NLM) of the power generation gas under the rated operating conditions to 4.0 NLM, for example, the temperature of the exhaust gas can be raised to 800° C., and that the temperature of the purification catalyst in the purifier  16  can be raised to 350° C. or higher. 
     Example 5 
     In the SOFC system  100  according to Example 5 of the embodiment, the purification catalyst in the purifier  16  may be temporarily heated to the predetermined temperature for the predetermined time by reducing an amount of steam produced from the evaporator  8 , and reducing an S/C value. Here, “reducing an S/C” is to reduce the flow rate of the reformation water supplied to the evaporator  8 . 
     More specifically, in the SOFC system  100  according to Example 5, the controller  9  controls the reformation water supplier  4  to reduce the flow rate of the reformation water supplied to the evaporator  8  at predetermined timing during the operation under the rated operating conditions for the purpose of raising the temperature of the exhaust gas and raising the temperature of the purifier  16 . With such control, since the S/C value can be reduced, namely an amount of the supplied reformation water can be reduced, an amount of heat utilized to evaporate the reformation water can be reduced. The temperature of the exhaust gas can be hence raised to a value higher than that during the operation under the rated operating conditions. As a result, the temperature of the purification catalyst in the purifier  16  can be raised. 
     The inventors have found that, by reducing the S/C value (S/C=2.5) under the rated operating conditions to S/C=2.2, the temperature of the exhaust gas supplied to the purifier  16  can be raised to 350° C. or higher. 
     Example 6 
     In Examples 1 to 3 among above-described Examples 1 to 5, the temperature of the exhaust gas supplied to the purifier  16  cannot be raised to 350° C. in some cases depending on the flow rate of the cathode gas, which is to be increased or reduced, and on the magnitude of the power generation output, which is to be reduced. It has been found that, for example, when the flow rate of the cathode gas is reduced from 49 NLM under the rated operating conditions to 10 NLM as in the SOFC system  100  according to Example 1 of the embodiment, the temperature of the exhaust gas can be raised to about 270° C., but it cannot be raised until reaching 350° C. It has also been found that, when the flow rate of the cathode gas is increased from 49 NLM under the rated operating conditions to 70 NLM as in the SOFC system  100  according to Example 2 of the embodiment, the temperature of the exhaust gas can be raised to about 290° C., but it cannot be raised until reaching 350° C. It has been further found that, when the power generation output of the SOFC  1  is reduced from 700 W under the rated operating conditions to 200 W as in the SOFC system  100  according to Example 3 of the embodiment, the temperature of the exhaust gas can be raised to about 250° C., but it cannot be raised until reaching 350° C. 
     Taking the above point into consideration, as illustrated in  FIG. 4 , the SOFC system  100  according to Example 6 of the embodiment may further include, in addition to the configuration of the SOFC system  100  according to the embodiment illustrated in  FIG. 1 , a temperature sensor  21  for sensing the temperature of the purifier  16 , and a heater  22  for heating the purifier  16 .  FIG. 4  illustrates one example of a configuration of the SOFC system  100  according to Example 6 of the embodiment of the present disclosure. 
     The SOFC system  100  according to Example 6 of the embodiment has the same configuration as the SOFC system  100  according to the embodiment illustrated in  FIG. 1  except for further including the temperature sensor  21  and the heater  22 . In view of the above point, the same members in the SOFC system  100  according to Example 6 of the embodiment as those in the SOFC system  100  according to Example 1 illustrated in  FIG. 1  are denoted by the same reference symbols, and description of the configuration of the SOFC system  100  according to Example 6 is omitted. A thermocouple, for example, can be used as the temperature sensor  21 . 
     It is assumed here that the temperature of the purifier  16  is raised, for example, by increasing or reducing the flow rate of the cathode gas supplied to the cathode gas supplier  3 , or by reducing the power generation output of the SOFC  1  in accordance with the control command from the controller  9 . When, under the above assumption, the controller  9  determines, on the basis of a result sensed by the temperature sensor  21  disposed in the purifier  16 , that the temperature of the purifier  16  does not reach, e.g., 350° C., the controller  9  instructs the heater  22  to start up heating of the purifier  16  until the temperature of the purifier  16  reaches 350° C. Thus, in the SOFC system  100  according to Example 6 of the embodiment, when the temperature of the purifier  16  is insufficient, the purifier  16  can be heated by the heater  22  such that the temperature of the purifier  16  is raised to 350° C. 
     The present disclosure can be widely utilized in a solid oxide fuel cell system including a purification catalyst acting to remove substances to be cleaned up, which are contained in exhaust gas.