Abstract:
A fuel processor and a fuel cell system which have a desulfurizer with multiple sensors, and a method of measuring an adsorbent state in of an adsorbent the desulfurizer. The desulfurizer determines the saturation state of the adsorbent using a signal difference between at least two sensors installed adjacent to an inlet and an outlet of the desulfurizer. The desulfurizer provides an accurate determination of the saturation of the adsorbent, and can be used to determine when the adsorbent should be changed. Two desulfurizers can be alternatively used to allow for a consistent fuel flow.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 2006-76371, filed on Aug. 11, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the present invention relate to a fuel processor that reforms a fuel source suitable for supply to a fuel cell stack, and more particularly, to a fuel processor having an improved method of measuring the state of an adsorbent in a desulfurizer, a fuel cell system including the fuel processor, and a method of operating the fuel cell system. 
     2. Description of the Related Art 
     A fuel cell is an electric generator that converts the chemical energy in a fuel into electrical energy through a chemical reaction, and a fuel cell can continuously generate electricity as long as the fuel is supplied.  FIG. 1  is a schematic drawing illustrating the energy transformation structure of a fuel cell. Referring to  FIG. 1 , when air that includes oxygen is supplied to a cathode  1  and a fuel containing hydrogen is supplied to an anode  3 , electricity is generated by a reverse electrolysis reaction as water and protons move through an electrolyte membrane  2 . However, a unit cell  10  does not generally produce a useful high voltage. Therefore, electricity is generated by a stack  20  (referring to  FIG. 2 ) in which a plurality of unit cells  10  are connected in series. 
     A hydrocarbon group containing material such as a natural gas can be used as a fuel source for supplying hydrogen to the stack  20 . Hydrogen is often extracted from a fuel source in a fuel processor  10 , as depicted in  FIG. 2 , in order to supply hydrogen to the stack  20 . 
     The fuel processor  10  includes a desulfurizer  11 , a reformer  12 , a burner  13 , a water supply pump  16 , first and second heat exchangers  14   a  and  14   b , and a CO removal unit  15 . The CO removal unit  15  consists of a CO shifter  15   a  and a CO remover  15   b . The hydrogen extraction process is performed in the reformer  12 . That is, hydrogen is generated in the reformer  12 , through a chemical reaction  1  indicated below between a hydrocarbon group containing gas, that acts as the fuel source, entering from a fuel tank  17 , and steam entering from a water tank  18 , by the action of a water supply pump  16 . The reformer  12  is heated by the burner  13 . 
     
       
                 
         
             
             
         
      
     
     However, at this time, CO and CO 2  are generated as byproducts. If a fuel containing CO, at a concentration of 10 ppm or more, is supplied to the stack  20 , the electrodes can be poisoned, resulting in greatly reducing the performance of the fuel cell. Therefore, the content of CO in an outlet of the reformer  12  is controlled to be 10 ppm or less, by installing the CO shifter  15   a  and the CO remover  15   b.    
     A chemical reaction  2  indicated below occurs in the CO shifter  15   a  and chemical reactions  3 ,  4 , and  5 , as indicated below, occur in the CO remover  15   b . The CO content in the fuel that has passed through the CO shifter  15   a  is 5,000 ppm or less and the CO content in the fuel that has passed through the CO remover  15   b  is 10 ppm or less. 
     
       
                 
         
             
             
         
      
     
     The desulfurizer  11  located at an inlet of the reformer  12  removes sulfur compounds (sulfur) contained in the fuel source. The sulfur compounds are absorbed while passing through the desulfurizer  11 , because sulfur is a fuel cell containment. Sulfur can easily poison the electrodes, if the sulfur compounds are present at 10 parts per billion (ppb), or more, when supplied to the stack  20 . 
     When the fuel processor  10  is operating, a fuel source such as a natural gas is supplied to the reformer  12 , through the desulfurizer  11 , from the fuel tank  17 . A portion of the fuel source is used as a fuel for igniting the burner  13 . Then steam, from the first and second heat exchangers  14   a  and  14   b , reacts with the desulfurized fuel source in the reformer  12  to generate hydrogen. The hydrogen produced is supplied to the stack  20 , after the CO content is reduced to 10 ppm, or less, while passing through the CO shifter  15   a , and the CO remover  15   b.    
     In the related art, as shown in  FIG. 3 , the desulfurizer  11  has a structure in which an adsorbent  11   b  of a zeolite material is filled into a transparent housing  11   a . In the conventional desulfurizer  11 , there is a risk of using the adsorbent  11   b  after it has exhausted its adsorption capability. In this case, a fuel having a high sulfur content may be supplied to the stack  20 . This problem can be solved by directly observing the adsorbent  11   b , with the naked eye, through the transparent housing  11   a . The observation often takes place whether the adsorbent  11   b  reaches a saturation state or not. This observation is conducted because the color of the adsorbent portion of the desulfurizer  11  changes when the adsorbent  11   b  adsorbs sulfur and the time to replace the adsorbent  11   b  can be determined by observing the change of color of the adsorbent  11   b , through the transparent housing  11   a . In general, the color gradually changes from the inlet toward the outlet of the fuel source. 
     However, in the direct observing method, if the replacement time is missed, the adsorbent  11   b  is still used after the adsorption capability of the adsorbent  11   b  is exhausted. Therefore, there is a high possibility of supplying a fuel containing a high sulfur content to the stack  20 . Also, some adsorbents, like zeolite, change different colors with respect to different sulfur compounds. Therefore, when different kinds of sulfur compounds are adsorbed, an appropriate time to replace cannot be determined. 
     As depicted in  FIG. 4 , a method of indicating the time to replace an adsorbent  11   c , by measuring the electrical resistance of the adsorbent  11   c  using a sensor  11   d , has been disclosed in Japanese Patent Publication No. 2004-178975. In this method the sensor  11   d , formed of zinc oxide, is installed at an outlet of the adsorbent  11   c . When the electrical resistance changes, the sensor  11   d  issues an alarm signal indicating that it is time to replace the adsorbent  11   c . That is, since the adsorption of the sulfur compounds begins at the inlet of the desulfurizer  11 , and gradually progresses toward the outlet of the desulfurizer  11 , if the resistance of the adsorbent  11   c  changes, due to the adsorption of sulfur compounds detected by the sensor  11   d , the sensor  11   d  is designed to recognize that the adsorbent  11   c  is saturated with the sulfur compounds, and to give an alarm to replace the adsorbent  11   c.    
     However, this method assumes that the sulfur compounds are sequentially and gradually adsorbed by the adsorbent  11   c , from the inlet towards the outlet of the desulfurizer  11 . However, in practice, the adsorption of the adsorbent  11  does not progress gradually. In practice, the degree of adsorption may vary even in the same surface, that is, in the X and Y directions. That is, the adsorbent  11   c  can be relatively rapidly saturated with the sulfur compounds in portions where the fuel gas can easily pass, thereby causing non-uniform adsorption. Accordingly, if the time to replace the adsorbent  11  is measured using only one sensor  11   d , and the sensor  11   d  gives an alarm to replace the adsorbent  11  when only the portion where the sensor  11   d  is located becomes saturated with the sulfur compounds, the time to replace may have already passed. Furthermore, since a low sulfur content, such as 10 ppb, must be detected in the desulfurizer  11 , the regional adsorption differences greatly reduce measuring reliability. 
     Accordingly, there is a need to develop a measuring method with high reliability by which a correct time to replace the adsorbent in a desulfurizer can be detected. 
     SUMMARY OF THE INVENTION 
     The aspects of present invention provide a fuel processor that allows a desulfurizer to appropriately indicate the time to replace an adsorbent by correctly and accurately measuring the adsorption state of sulfur compounds in the adsorbent, a fuel cell system comprising the fuel processor, and a method of operating the fuel cell system. 
     According to various aspects of the present invention, there is provided a fuel processor that comprises a desulfurizer in which at least one sulfur sensor is installed at each inlet and outlet of the desulfurizer, in order to determine the sulfur saturation state of an adsorbent, using a signal difference between a signal measured at the inlet and a signal measured at the outlet. Herein, sulfur and sulfur compounds are used interchangeably to indicate elemental sulfur and/or compounds containing sulfur atoms. 
     The fuel processor may further comprise a reformer that extracts hydrogen gas from a fuel source; a burner that heats the reformer to an appropriate temperature for the hydrogen extracting reaction; and a CO removing unit that removes CO produced during the hydrogen extraction reaction in the reformer. 
     The sulfur sensor may be installed in the adsorbent in the desulfurizer, or alternately may be installed in an inner space of a housing adjacent to the adsorbent in the desulfurizer. 
     A plurality of desulfurizers may be provided so as to be alternately used, and the sulfur sensor may be a type of sensor that detects at least one electrical, physical, and chemical characteristic variation. 
     According to aspects of the present invention, there is provided a method of operating a fuel cell system in which sulfur compounds contained in the fuel source are removed using a desulfurizer and hydrogen gas is extracted from the fuel source in order to supply to a stack, comprising: providing sulfur sensors respectively at an inlet and outlet of the desulfurizer; measuring signal variations according to time between the signals measured in each of the sulfur sensors; and determining the saturation state of an adsorbent in the desulfurizer using the signal variations. 
     The determining of the saturation of the adsorbent may be made when 
                   ⅆ          Δ   ⁢           ⁢   Y              ⅆ   t       &lt;   0     ,         
where dt is the variation of time and d(|ΔY|) is the variation of signal difference between the inlet sensor and the outlet sensor.
 
     The method may further comprise controlling the measuring frequency of ΔY by controlling the dt. 
     A plurality of desulfurizers may be provided, and when one of the desulfurizers is determined as saturated with sulfur compounds, the other desulfurizer is operated, and the sulfur sensor may be a type of sensor that detects at least one electrical, physical, and chemical characteristic variation. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a schematic drawing illustrating the related art principle of electricity generation of a conventional fuel cell; 
         FIG. 2  is a block diagram showing a configuration of a related art fuel processor that processes a fuel that is to be supplied to a fuel cell; 
         FIG. 3  is a schematic drawing illustrating a structure of a related art desulfurizer employed in the fuel processor illustrated in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view illustrating another structure of a related art desulfurizer employed in the fuel processor illustrated in  FIG. 2 ; 
         FIG. 5  is a block diagram of a configuration of a fuel processor according to various embodiments of the present invention; 
         FIG. 6  is a schematic drawing illustrating a structure of a desulfurizer employed by the fuel processor illustrated in  FIG. 5 , according to various embodiments of the present invention; 
         FIGS. 7 through 9B  are graphs showing the variation of signals received from first and second sulfur sensors, which are employed by the desulfurizer illustrated in  FIG. 6 , according to time; 
         FIG. 10  is a schematic drawing showing an example of the operation of the desulfurizer illustrated in  FIG. 6 ; and 
         FIG. 11  is a schematic drawing illustrating the installation of a sulfur sensor in an inner space of a housing of the desulfurizer illustrated in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
     Aspects of the present invention will now be described more fully with reference to the accompanying drawings in which various embodiments of the invention are shown. Herein, the terms “sulfur” and “sulfur compounds” are used interchangeably. 
       FIG. 5  is a block diagram of a configuration of a fuel processor  100  according to various embodiments of the present invention. The basic structure, like in the related art, comprises a desulfurizer  110 , a reformer  120 , a burner  130 , and a CO removing unit  150  consisting of a CO shifter  151  and a CO remover  152 . When a fuel source such as natural gas is supplied from a fuel tank  170 , sulfur compounds contained in the fuel source are removed in the desulfurizer  110 . Afterward, hydrogen gas that is to be supplied to a stack  20  is extracted from a raw fuel by reacting the raw fuel with steam, in the reformer  120 . Water to make the steam can be supplied from a water tank  180 , using a pump  160 . A CO byproduct from the extraction reaction, is reduced to 10 ppm, or less, in the CO shifter  151  and the CO remover  152 . Reference numerals  141  and  142  are first and second heat exchangers for preheating water supplied to the reformer  120 . 
       FIG. 6  is a schematic drawing illustrating a structure of the desulfurizer  110  comprising an improved sensing mechanism. Referring to  FIG. 6 , an adsorbent  112 , for adsorbing sulfur compounds, is filled in a housing  111 , and first and second sulfur sensors  113   a  and  113   b , are respectively installed at an inlet  111   a  and an outlet  111   b , of the desulfurizer  110 . The desulfurizer  110  can also comprise a controller to analyze signals from the first and second sulfur sensors  113   a  and  113   b . This configuration allows the sulfur concentration to be measured not at a single point as in the related art, but at two points. In addition, it is within the scope of the present teachings to include additional sulfur sensors positioned in multiple locations along the housing  111 . Here, the adsorbent  112  can be any material that can adsorb sulfur compounds, for example, a zeolite or a zinc oxide. The first and second sulfur sensors  113   a  and  113   b  can be any type of sensor that can generate different signals in proportion to the variation of the sulfur concentration. The sensors  113   a  and  113   b  can measure the variation of sulfur concentration in terms of, for example, electrical resistance, electrical conductivity, ion conductivity, optical extinction, and/or mass. The purpose of the sensing mechanism is to detect the sulfur saturation state of the adsorbent  112  using the signal difference between the first and second sulfur sensors  113   a  and  113   b . The first and second sulfur sensors  113   a  and  113   b  can be any sensor as long as the signals of electrical, physical, or chemical characteristics emitted from both the first and second sulfur sensors  113   a  and  113   b  are the same, or can be mathematically related. 
       FIG. 7  is a graph showing the variation of a signal over time from the first sulfur sensor  113   a . The graph shows that initially, sulfur compounds are not detected due to an adsorbent being installed during time  0  to time b 1 . The sulfur compound concentration signal is shown to increase from 0, to a maximum saturation level, between times b 1  and s 1 . At time s 1 , the signal no longer varies, indicating that the adsorbent can no longer absorb any additional sulfur compounds. However, this graph indicates only the sulfur compound saturation at a point where the sulfur sensor  113   a  is installed. Accordingly, considering that the adsorption of the adsorbent  112  progresses from the inlet  111   a  toward the outlet  111   b  of the desulfurizer  110 , and considering that the adsorbent  112  filled in the inlet  111   a  side is saturated with the sulfur compounds, the adsorbent  112  filled in the outlet  111   b  side usually has adsorption capability. 
     In  FIG. 8 , Y 1  and Y 2  show variations of sulfur sensor signals according to time, respectively emitted from the first and second sulfur sensors  113   a  and  113   b . That is, at the beginning, sulfur compounds are not detected by the first and second sulfur sensors  113   a  and  113   b , due to the adsorbent  112  being newly filled during the time  0  to b 1 . When the adsorbent  112  begins to adsorb the sulfur compounds, the first sensor  113   a  detects the sulfur compounds and generates a signal corresponding to the amount of sulfur compounds present during the time b 1  to s 1 . Afterwards, the portion of adsorbent  112  at the position where the first sulfur sensor  113   a  is installed becomes saturated with sulfur compounds by the time s 1 . However, sulfur compounds are not detected by the second sulfur sensor  113   b  at this time. This indicates that the portion of the adsorbent  112 , at the position where the second sulfur sensor  113   b  is installed, still has adsorption capability. As the desulfurizing process continues, at the time period between times b 2  and s 2 , the second sulfur sensor  113   b  begins to detect the sulfur compounds, and generates a signal corresponding to the amount of sulfur compounds present. At the time s 2 , the adsorbent  112  is shown to be saturated with the sulfur compounds. 
     The adsorbent  112  may be replaced at the time b 2  when the detection of the sulfur compounds begins at sensor  113   b , rather than at the time s 2 , when the adsorbent  112  is determined to be saturated at outlet  111   b . The adsorbent  112  can be removed at time s 2  to insure that sulfur compounds are not directly fed to the stack  20 , due to the adsorption capability of the adsorbent  112  being exceeded. Accordingly, the desulfurizer  110  can be designed so that it gives an alarm to replace the adsorbent  112 , at the point when the second sulfur sensor  113   b  begins to detect the sulfur compounds. 
     According to some embodiments, b 2  represents a time when the slope of the difference of the two graphs becomes negative (−) if the difference |Y 1 −Y 2 | between the two graphs is as depicted in  FIG. 8B . In the  FIG. 8B , ΔY indicates an output signal difference between the first and second sulfur sensors  113   a  and  113   b , the appropriate time b 2  to replace the adsorbent  112  can be determined by the equation 
                 ⅆ          Δ   ⁢           ⁢   Y              ⅆ   t       &lt;   0         
where dt is time variation, and d(|ΔY|) is a signal variation between an inlet side sensor and an outlet side sensor. Accordingly, the differences between the two signals over time are measured in a controller  200  ( FIG. 6 ), by receiving the output signals from the first and second sulfur sensors  113   a  and  113   b . The controller can be a microprocessor or other suitable calculating/processing device. An alarm signal to replace the adsorbent  112  can be given at the point when the slope of the graph changes to a negative (−) value. Then, the user recognizes that it is time to replace the adsorbent  112  due to the alarm, and the desulfurizing process can be continued with a new adsorbent.
 
     According to various embodiments of the present invention, the saturation state of the adsorbent  112  is measured using a signal difference between the plurality of sulfur sensors  113   a  and  113   b  respectively installed at the inlet  111   a  and outlet  111   b  of the desulfurizer  110 , and as a result, the time to replace the adsorbent  112  is determined. Therefore, the accuracy of the determination for replacing the adsorbent  112  is greatly improved as compared to the related art, in which only one sulfur sensor is used. For example, if a slight variation in the sulfur concentration is to be measured using only one sulfur sensor, the sensitivity of the sulfur sensor must be very high. In contrast, when a slight sulfur variation is measured using a difference between two sulfur sensors, the sulfur sensors do not need to be as sensitive to obtain a precise measurement. Also, since the concentration is measured at multiple locations, the possibility of causing a measuring error can be reduced as compared to measuring the concentration at one location. When the sulfur concentration measuring mechanism as described above is employed, a stable and continuous fuel processing operation can be performed since a correct replacing time of the adsorbent  112  can be detected. At this time, the frequency dt of obtaining a ΔY value, by receiving the signals from the first and second sulfur sensors  113   a  and  113   b , can be appropriately controlled as necessary by the controller  200 . 
     In some cases, as depicted in  FIG. 9A , the concentration of sulfur continuously increases at positions where the first and second sulfur sensors  113   a  and  113   b  are located, despite the saturation times having passed. In this case, a smoother concentration increasing value is generated prior to reaching complete saturation, after a sudden increase in the sulfur concentration. Also in this case, the replacing time b 2  is determined in the same manner as above. That is, the signal difference |ΔY| between the two signals |Y 1 −Y 2 | can be as depicted in  FIG. 9B . In this case, the b 2  point becomes the point when the slope becomes negative (−) as well. Accordingly, the replacing time b 2  of the adsorbent  112  can be given in the same manner as above. 
     When the appropriate time b 2  to replace the adsorbent  112  is given, the replacing work can be performed when the desulfurizing operation is stopped. However, as depicted in  FIG. 10 , multiple desulfurizers  110   a  and  110   b  can be provided. For example, if an adsorbent  112  in the desulfurizer  110   a  needs to be replaced, the adsorbent  112  in the desulfurizer  110   a  is replaced while the other desulfurizer  110   b  is in operation. The replacement can be performed either by using valves  114  controlled by the controller  200  or manually. In this manner, a fuel cell can continuously operate without stopping the fuel processing operation while replacing the adsorbent  112 . 
     According to various embodiments, the first and second sulfur sensors  113   a  and  113   b  can be installed in the adsorbent  112 . In other embodiments and as depicted in  FIG. 11 , the first and second sulfur sensors  113   a  and  113   b  can be installed in an inner space of the housing  111 , and detection patterns can be obtained. 
     As described above, the fuel processor according to the present invention provides the following advantages. 
     First, since the saturation state of an adsorbent is detected using the variation of signals between sulfur sensors respectively installed at an inlet and an outlet of a desulfurizer and the time to replace the adsorbent is determined using the result of detection, the determination of the time to replace is made more accurately as compared to the related art methods, in which the determination is made using only one sulfur sensor. 
     Second, since the concentration of sulfur is measured at multiple locations, the possibility of causing a measurement error is reduced as compared to when measurement is performed at only one location, thereby ensuring stable and smooth fuel processing. 
     Third, the fuel processing can be performed without stopping since multiple desulfurizers can be included in the system and the desulfurizers can be alternately operated. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.