Patent Publication Number: US-2023148377-A1

Title: Fuel cell system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to Korean Application No. 10-2021-0152589, filed Nov. 8, 2021, whose entire subject matter of which is hereby incorporated by reference. 
     BACKGROUND 
     1 Field 
     The present disclosure relates to a fuel cell system, and more particularly, to a fuel cell system using a liquid fuel. 
     2. Background 
     A fuel cell system is a power generation system that generates electrical energy by electrochemically reacting hydrogen (included in hydrocarbon-based substances such as methanol, ethanol, natural gas, etc.) with oxygen. 
     A fuel cell can adjust an amount of power generation by adjusting a flow rate of air supplied to a stack and an amount of gas reformed through a reformer. The gas that is supplied to a reformer may produce hydrogen by supplying and reforming a gas fuel. 
     The flow rate of the air that is supplied to the stack can be adjusted by a blower. However, the flow rate of air, which is adjusted by the blower, may be slightly limited, so there may be a problem in that when a power generation amount operation condition is excessive, a stable supply of air may be difficult, and the power generation efficiency may deteriorate. 
     When a burner is for heating a reformer, a density of reforming gas, and/or the like may be improved, and reaction efficiency of the reformer may be improved. 
     A fuel cell system using a liquid fuel is disclosed in Korean Patent No. 10-2116876, the subject matter of which is incorporated herein by reference. However, this document proposes a structure that directly supplies a liquid fuel to a fuel treatment apparatus that reforms a liquid fuel, such that heat supply (generated by a phase change of the liquid fuel) is not used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Arrangements and embodiments may be described in detail with reference to the following drawings in which like reference numerals refer to like elements and wherein: 
         FIG.  1    is a diagram showing a configuration of a fuel cell system according to an embodiment of the present disclosure; 
         FIG.  2    is a view illustrating a fuel treatment apparatus according to an embodiment of the present disclosure; 
         FIG.  3    is a view schematically showing a configuration for describing flow and heat exchange of a liquid fuel and a gas fuel in the fuel cell system according to an embodiment of the present disclosure; 
         FIG.  4    is a schematic view illustrating a first storage tank and relevant components according to an embodiment of the present disclosure; 
         FIG.  5    is a view illustrating flow of a fuel and gas in a preheating mode in  FIG.  3   ; 
         FIG.  6    is a view illustrating flow of a fuel and gas in a reforming mode in  FIG.  3   ; 
         FIG.  7    is a view illustrating flow of a fuel and gas in a power generation mode in  FIG.  3   ; 
         FIG.  8   a    is a schematic cross-sectional view illustrating a gas flow section of a fuel evaporator, and  FIG.  8   b    is a schematic cross-sectional view illustrating a fuel flow section of a fuel evaporator; 
         FIG.  9   a    is a schematic cross-sectional view illustrating a gas flow section of a fuel evaporator, and  FIG.  9   b    is a schematic cross-sectional view illustrating a fuel flow section of a fuel evaporator; 
         FIG.  10   a    is a schematic cross-sectional view illustrating a gas flow section of a fuel evaporator, and  FIG.  10   b    is a schematic cross-sectional view illustrating a fuel flow section of a fuel evaporator; 
         FIGS.  11   a  and  11   b    are views showing the state in which the shape of an internal pipe is changed in  FIGS.  10   a  and  10   b   , respectively; 
         FIG.  12    is a perspective view of a fuel evaporator according to a fourth embodiment of the present disclosure; 
         FIG.  13   a    is a schematic cross-sectional view illustrating a gas flow section of a fuel evaporator, and  FIG.  13   b    is a schematic cross-sectional view illustrating a fuel flow section of a fuel evaporator; and 
         FIGS.  14   a  and  14   b    are views showing the state in which the shape of an internal pipe is changed in  FIGS.  13   a  and  13   b   , respectively. 
     
    
    
     DETAILED DESCRIPTION 
     The entire configuration of a fuel cell system  1  according to a first embodiment of the present disclosure is described with reference to  FIGS.  1  to  2   . The fuel cell system  1  may include a fuel processing unit I, a power generation unit II, a cooling water circulation unit III, and/or a heat collection unit IV. The fuel processing unit I may include a fuel processor  10 , a fuel valve  30  that adjusts a flow of fuel gas that is supplied to the fuel processor  10 , and a first blower  71  that blows air to the fuel processor  10 , etc. 
     Referring to  FIG.  2   , the fuel processor  10  may include a desulfurizer  110 , a burner  120 , a vapor generator  130 , a reformer  140 , a first reactor  150  and/or a second reactor  160 . The fuel processor  10  may include at least one mixer. For example, the at least one mixer may include a first mixer  111  and/or a second mixer  112 . The above components of the fuel processor  10  are structural components. 
     The desulfurizer  110  can perform a desulfurizing process of removing a sulfur compound included in a fuel gas. For example, the desulfurizer  110  may have an adsorbent therein. A sulfur compound (included in the fuel gas passing through the desulfurizer  110 ) can be adsorbed to the adsorbent. The adsorbent may be a metal oxide, Zeolite, activated carbon, etc. The desulfurizer  110  may include a filter that removes foreign substances included in a fuel gas. 
     The burner  120  can supply heat to the reformer  140  to promote a reforming reaction in the reformer  140 . For example, a fuel gas discharged from the desulfurizer  110  and air flowing inside from the outside (of the fuel cell system) may be mixed in the first mixer  111  and then supplied to the burner  120 . The burner  120  can generate a combustion heat by burning the gas mixture of fuel gas and air. The internal temperature of the reformer  140  can be maintained at an appropriate temperature (e.g., 800° C.) by heat supplied from the burner  120 . 
     Exhaust gas produced by combustion of the gas mixture in the burner  120  can be discharged out of the fuel processor  10 . 
     The vapor generator  130  can discharge vapor by evaporating water. For example, the vapor generator  130  can evaporate water by absorbing heat from the exhaust gas produced by the burner  120  and from the first reactor  150  and/or the second reactor  160 . 
     The vapor generator  130  may be disposed adjacent to a pipeline through which exhaust gas discharged from the first reactor  150 , the second reactor  160 , and/or the burner  120  flows. 
     The reformer  140  can perform a reforming process of producing hydrogen gas from fuel gas with a sulfur compound removed, by using a catalyst. For example, a fuel gas discharged from the desulfurizer  110  and vapor discharged from the vapor generator  130  may be mixed in the second mixer  112  and then supplied to the reformer  140 . The fuel gas and the vapor supplied to the reformer  140  may perform a reforming reaction in the reformer  140 , such that hydrogen gas may be produced. 
     The first reactor  150  can reduce carbon monoxide produced by a reforming reaction of the components included in the gas discharged from the reformer  140 . For example, carbon monoxide included in the gas discharged from the reformer  140  reacts with vapor in the first reactor  150 , such that carbon dioxide and hydrogen can be produced. The internal temperature of the first reactor  150  may be a temperature (e.g., 200° C.) that is lower than the internal temperature of the reformer  140  and higher than the room temperature. The first reactor  150  may be referred to as a shift reactor. The second reactor  160  can reduce remaining carbon monoxide of the components included in the gas discharged from the first reactor  150 . For example, preferential oxidation (PROX) may occur in which carbon monoxide included in the gas discharged from the first reactor  150  reacts with oxygen in the second reactor  160 . 
     Preferential oxidation may require a large amount of oxygen, air should be additionally supplied, but there may be a defect that the density of hydrogen that is supplied to a stack decreases because the hydrogen is made weaker by the additionally supplied air. Accordingly, in order to overcome this defect, selective methanation may be used such that carbon monoxide and hydrogen react. 
     The gas that is discharged from the fuel processor  10  (through the reformer  130 , the first reactor  150  and/or the second reactor  160 ) may be referred to as reforming gas. 
     Stacks  20   a  and  20   b  may generate electrical energy by applying electrochemical reaction to the reforming gas supplied from the fuel processor  10 . 
     The stacks  20   a  and  20   b  may be configured by stacking unit cells in which an electrochemical reaction occurs. 
     The unit cell may be composed of a membrane electrode assembly (MEA) in which an anode and a cathode are disposed with an electrolyte membrane therebetween, a separator, etc. Electricity may be generated by decomposition of hydrogen into hydrogen ions and electrons by a catalyst at the anode of a membrane electrode assembly, and water may be produced by combination of hydrogen ions and electrons at the cathode of a membrane electrode assembly. 
     The stacks  20   a  and  20   b  may include a stack heat exchanger that dissipates heat generated by an electrochemical reaction process. The stack heat exchanger may be a heat exchanger that uses water as a refrigerant. For example, cooling water that is supplied to a stack heat exchanger can absorb heat generated in an electrochemical reaction process, and the cooling water may be increased in temperature by the absorbed heat may be discharged out of the stack heat exchanger. 
     The fuel processing unit I may include a first storage tank  400  that stores a liquid fuel, a second storage tank  402  that supplies a gasified fuel to the reformer  140  of the fuel processor  10 , and fuel evaporators  410 ,  412 , and  414  in which a liquid fuel discharged from the first storage tank  400  exchanges with external air to become a gas fuel. The fuel evaporators  410 ,  412 , and  414  may include a first fuel evaporator  410  disposed in (or at) a first supply pipe, a second fuel evaporator  412  disposed in (or at) a second supply pipe, and a third fuel evaporator  414  disposed in (or at) a reforming gas discharge pipe  104   a . The configuration and connection relationships of the first storage tank  400 , the second storage tank  420 , and the fuel evaporators  410 ,  412 , and  414  may be described in detail below. In this disclosure, connections may be described between structural components. For example, the disclosure may reference a pipe which may be considered a channel, a duct, a connector, a path, a flow path and/or etc. 
     The fuel valve  30  may be disposed in a fuel supply pipe forming a fuel supply channel  101  through which fuel gas to be supplied to the fuel processor  10  flows. The flow rate of fuel gas to be supplied to the fuel processor  10  may be adjusted in correspondence to a degree of opening of the fuel valve  30 . For example, the fuel valve  30  can block the fuel supply channel  101  to stop supply of fuel gas to the fuel processor  10 . 
     A first fuel flow meter  51  may be disposed at (or in) the fuel supply pipe. The first fuel flow meter may detect the flow rate of fuel gas flowing in the fuel supply channel  101 . 
     The fuel processing unit I may include a first supply pipe forming a first supply channel  202  therein to supply external air to the fuel processor  10 , and the first blower  71  disposed in the first supply pipe for supplying external air to the fuel processor  10 . 
     The first blower  71  can blow air flowing from the outside to the fuel processor  10  through the first supply channel  202 . The first blower  71  can blow air flowing inside from the outside (of the fuel cell system) through an external air inflow channel  201  to the fuel processor  10 . 
     The air flowing through the first supply channel  202  and into the fuel processor can be supplied to the burner  120  (of the fuel processor  10 ). For example, the air flowing into the fuel processor  10  can be mixed with fuel gas, which is discharged from the desulfurizer  110 , in the first mixer  111  and then supplied to the burner  120 . 
     A first air filter  91  may be disposed in (or at) an external air inflow pipe forming the external air inflow channel  201 . The first air filter  91  may remove (or reduce) foreign substances such as dirt included in air. A first air-side check valve  81  may limit the flow direction of air. 
     The fuel processing unit I may include a first internal gas pipe forming a first internal gas channel  101  through which the fuel gas discharged from the desulfurizer  110  flows to the reformer  140 . The fuel processing unit I may also include a proportional control valve  31 , an internal fuel valve  32  that adjusts flow of fuel gas flowing into the reformer  140 , a second fuel flow meter  52  that detects the flow rate of fuel gas flowing through the internal gas channel  102 , a fuel-side check valve that limits the flow direction of fuel gas flowing through the internal gas channel  102 , and/or a sulfur detector  94 . 
     The proportional control valve  31  can adjust the flow rate, pressure, etc. of the fuel gas discharged from the desulfurizer  110  and flowing to the reformer  140  through internal/external feedback in an electrical control type. 
     The sulfur detector  94  can detect sulfur included in the fuel gas discharged from the desulfurizer  110 . The sulfur detector  94  may include an indicator that changes in color by reacting with a sulfur compound that is not removed by the absorbent of the desulfurizer  110 . The indicator may include phenolphthalein, a molybdenum compound, etc. 
     The fuel processing unit I may include a second internal gas pipe forming a second internal gas channel  103  that sends the fuel gas discharged from the desulfurizer  110  to the burner  120 . The burner  120  can use the fuel gas flowing inside through the second internal gas channel  103  for combustion. 
     The first internal gas channel  102  and the second internal gas channel  103  may communicate with each other. 
     The fuel processor  10  may be connected with a water supply pipe forming a water supply channel  303  in which water discharged from a water supply tank  13  may flow. a water pump  38  may generate flow of water flowing through the water supply channel  303 , a water supply valve  39  may adjust flow of water, and a water flow meter  54  may detect the flow rate of the water flowing through the water supply channel  303 . 
     The exhaust gas produced by the burner  120  (of the fuel processor  10 ) may be discharged from the fuel processor  10  through an exhaust gas discharge channel  210 . 
     The fuel processor  10  may be connected to a reforming gas discharge pipe forming a reforming gas discharge channel  104 . The reforming gas discharged from the fuel processor  10  can flow through the reforming gas discharge channel  104 . 
     The reforming gas discharge pipe may be connected to a reforming gas heat exchanger  21  which performs a heat exchange of reforming gas. A reforming gas valve  33  may adjust flow of the reforming gas flowing in the reforming gas discharge pipe to the reforming gas heat exchanger  21 . 
     The reforming gas discharge pipe may be connected with a bypass pipe forming a bypass channel  105  such that reforming gas discharged from the fuel processor  10  may flow back to the fuel processor  10 . The bypass pipe may be connected to the fuel processor  10 . The reforming gas flowing from the fuel processor  10  can be supplied to the burner  120  through the bypass channel  105 . The reforming gas supplied to the burner  120  through the bypass channel  105  may be used as a fuel for combustion in the burner  120 . A bypass valve  34  that adjusts flow of reforming gas flowing inside from the fuel processor  10  may be disposed in (or at) the bypass pipe. 
     The power generation unit II includes stacks  20   a  and  20   b , a reforming gas heat exchanger  21  for performing heat exchange based on the reforming gas discharged from the fuel processor  10 , an AOG heat exchanger  22  for performing heat exchange of the gas discharged from the stacks  20   a  and  20   b  (without reacting), and a humidifier  23  that supplies moisture to water (or liquid) that is to be supplied to the stacks  20   a  and  20   b.    
     A second blower  72  may be provided to provide (or blow) air to the stacks  20   a  and  20   b . The gas discharged (without reacting) from the stacks  20   a  and  20   b  may be referred to as anode off gas (AOG). In an example embodiment, the fuel cell system  1  may include two stacks  20   a  and  20   b , but the present disclosure is not limited thereto. 
     The reforming gas heat exchanger  21  may be connected to a reforming gas discharge pipe forming a reforming gas discharge channel  104  so that reforming gas discharged from the fuel processor  10  may flow. The reforming gas heat exchanger  21  may be connected to a cooling water supply pipe for forming a cooling water supply channel  304  in which water discharged from the water supply tank  13  may flow. The reforming gas heat exchanger  21  enables reforming gas flowing through the reforming gas discharge channel  104  to exchange heat with water flowing through the cooling water supply channel  304 . 
     A cooling water pump  43  and a cooling water flow meter  56  may be provided at the cooling water supply pipe. The cooling water pump  43  may send water stored in the water supply tank  13  to the reforming gas heat exchanger  21 . The cooling water flow meter  56  may detect the flow rate of water flowing through the cooling water supply channel  304 . 
     The reforming gas heat exchanger  21  may be connected to a stack gas supply pipe forming a stack gas supply channel  106 . Reforming gas discharged from the reforming gas heat exchanger  21  can flow to the stacks  20   a  and  20   b  through the stack gas supply channel  106 . 
     A reforming gas dehumidifier  61  may be disposed in (or at) the stack gas supply pipe to adjust the amount of moisture included in the reforming gas. Reforming gas flowing in the reforming gas dehumidifier  61  can be discharged from the reforming gas dehumidifier  61  after moisture is removed. 
     Condensate water produced in the reforming gas dehumidifier  61  can be discharged from the reforming gas dehumidifier  61  and can then flow to a first water collection channel  309 . A first water collection valve  44  that adjusts flow of water flowing through the first water collection channel  309  may be disposed in (or at) a first water collection pipe that forms the first water collection channel  309 . 
     The stacks  20   a  and  20   b  can generate electrical energy by electrochemical reaction to the reforming gas flowing through the stack gas supply channel  306 . In an example embodiment, when the fuel cell system  1  includes a plurality of stacks  20   a  and  20   b , reforming gas discharged (without reacting) from the first stack  20   a  can additionally generate an electrochemical reaction in the second stack  20   b.    
     The second blower  72  may be disposed between a second supply pipe that forms a second supply channel  203  and a stack-side air inflow pipe that forms a stack-side air inflow channel  204 . The second supply pipe may be disposed at a downstream side of a first air filter  91 . The second blower  72  can blow air flowing through the second supply channel  203 , through the stack-side air inflow channel  204  and to the stacks  20   a  and  20   b.    
     A second air-side check valve  82  that limits the flow direction of air flowing through the second supply channel  203  may be disposed in (or at) the second supply pipe. 
     An air flow meter  53  that detects the flow rate of air flowing through the stack-side air inflow channel  204  may be disposed in (or at) the stack-side air inflow pipe. 
     The humidifier  23  may supply moisture to air flowing through the stack-side air inflow channel  204 , and the humidifier  23  may discharge air including moisture to a stack-side air supply channel  205 . 
     A stack-side air supply valve  36  may be disposed in (or at) a stack-side air supply pipe that forms the stack-side air supply channel  205 . The stack-side air supply valve  36  may adjust flow of air to be supplied to the stacks  20   a ,  20   b.    
     The stack-side air supply pipe may be connected with individual supply pipes that form individual supply channels  206  and  207  corresponding to the stacks  20   a  and  20   b , respectively. Air flowing through the stack-side air supply channel  205  can be supplied to the stacks  20   a  and  20   b  through the individual supply channels  206  and  207 . 
     The plurality of stacks  20   a  and  20   b  may be connected to each other through a gas connection pipe that forms a gas connection channel  107 . The reforming gas discharged (without reacting) from the first stack  20   a  can flow into the second stack  20   b  through the gas connection channel  107 . 
     An additional dehumidifier  62  may be disposed in (or at) the gas connection pipe to remove (or reduce) water produced by condensation that is generated while the reforming gas flowing through the gas connection channel  107  flows through the first stack  20   a.    
     The water produced in the additional dehumidifier  62  can be discharged from the additional dehumidifier  62  and can flow to the second water collection channel  310 . A second water collection valve  45  that adjusts flow of water may be disposed in (or at) a second water collection pipe that forms the second water collection channel  310 . The second water collection pipe may be connected with the first water collection pipe. 
     Anode off gas (AOG) discharged (without reacting) in the stacks  20   a  and  20   b  can flow through the stack gas discharge channel  108 . 
     The AOG heat exchanger  22  may be connected to a stack gas discharge pipe that forms a stack gas discharge channel  108  to allow flowing of AOG gas discharged from the stacks  20   a  and  20   b.    
     The AOG heat exchanger  22  may be connected to a hot water supply pipe that forms a hot water supply channel  313  such that water discharged from the heat collection tank  15  may flow to the AOG heat exchanger  22 . The AOG heat exchanger may enable AOG flowing in the stack gas discharge channel  108  to exchange heat with water flowing in the hot water supply channel  313 . 
     A hot water pump  48  may send water from the heat collection tank  15  to the AOG heat exchanger  22 , and a hot water flow meter  55  may detect the flow rate of water flowing through the hot water supply channel  313 . The hot water pump  48  and the hot water flow meter  55  may be disposed in (or at) the hot water supply pipe. 
     The AOG heat exchanger  22  may be connected to an AOG supply pipe that forms an AOG supply channel  109 . The AOG heat exchanger  22  can discharge AOG exchange heat through the AOG supply channel  109 . The AOG discharged from the AOG heat exchanger  22  can flow to the fuel processor  10  through the AOG supply channel  109 . The AOG supplied to the fuel processor  10  through the AOG supply channel  109  can be used as a fuel for combustion in the burner  120 . 
     An AOG dehumidifier  63  may adjust the amount of moisture included in AOG, and an AOG valve  35  may adjust flow of the AOG that is supplied to the fuel processor  10 . The AOG dehumidifier  63  and the ACG valve  35  may be disposed in (or at) the AOG supply pipe. AOG flowing in the AOG dehumidifier  63  can be discharged from the AOG dehumidifier  63  after moisture is removed. 
     Condensate water produced in the AOG dehumidifier  63  can be discharged from the AOG dehumidifier  63  and can then flow to a third water collection channel  311 . A third water collection valve  46  that adjusts flow of water flowing through the third water collection channel  311  may be disposed in a third water collection pipe that forms the third water collection channel  311 . The third water collection pipe may be connected with the first water collection pipe. 
     A stack-side air discharge pipe that forms the stack-side air discharge channel  211  may be connected to individual discharge pipes that form individual discharge channels  208  and  209  corresponding to the stacks  20   a  and  20   b , respectively. A stack-side air discharge valve  37  may be disposed in (or at) the stack-side air discharge pipe to adjust flow of air flowing through the air discharge channel  211 . 
     The air discharged from the stacks  20   a  and  20   b  can flow to the stack-side air discharge channel  211  through the individual discharge channels  208  and  209 . The air flowing through the stack-side air discharge channel  211  may include moisture produced by an electrochemical reaction that occurs in the stacks  20   a  and  20   b.    
     The stack-side air discharge pipe may be connected to the humidifier  23 . The humidifier  23  can supply moisture to the air flowing to the stacks  20   a  and  20   b  using the moisture included in the air supplied from the stack-side air discharge channel  211 . The air supplied to the humidifier  23  through the stack-side air discharge channel  211  can be discharged to a humidifier discharge channel  212  through the humidifier  23 . 
     The cooling water circulation unit III may include a water supply tank  13  that stores water (provided in the fuel cell system  1 ), a water pump  38  that sends water to the fuel processor  10 , a water supply valve  39  that adjusts flow of water that is to be supplied to the fuel processor  10 , a cooling water pump  44  that sends water to the reforming gas heat exchanger  21 , etc. 
     The heat collection unit IV may include a heat collection tank  15  that stores water to be used for heat exchange, and a heat collection pump  38  that sends the water stored in the heat collection tank  15  to outside of the heat collection tank  15 . 
     The water supply tank  13  may be connected to a water inflow pipe that forms a water inflow channel  301 . The water supply tank  13  can store water that is provided through the water inflow channel  301 . The water inflow pipe may be disposed with a first liquid filter  92  that removes foreign substances included in water supplied from the outside, and a water inflow valve  41  that adjusts flow of water flowing into the water supply tank  13 . 
     The water supply tank  13  may be connected to a water discharge pipe that forms a water discharge channel  302 . The water supply tank  13  can discharge at least a portion of the water stored in the water supply tank  13  to the outside of the fuel cell system through the water discharge channel  302 . A water discharge valve  42  may be disposed in (or at) the water discharge pipe to adjust flow of water that is discharged from the water supply tank  13 . 
     The water supply tank  13  may be connected to a water storage pipe that forms a water storage channel  308 . The water supply tank  13  can store water that flows through the water storage channel  308 . For example, the water discharged from the reforming gas dehumidifier  61 , the additional dehumidifier  62 , the AOG dehumidifier  63 , and/or the air dehumidifier  64  and flowing through the water collection channel  311  can flow into the water supply tank  13  through the water storage channel  308 . A second liquid filter  93  that removes foreign substances (included in the water that is collected to the water supply tank  13 ) may be disposed in (or at) the water supply pipe. 
     At least a portion of the water stored in the water supply tank  13  can flow to the reforming gas heat exchanger  21  based on the cooling water pump  43  and can exchange heat with reforming gas at the reforming gas heat exchanger  21 . The water discharged from the reforming gas heat exchanger  21  can flow through the stack water supply channel  305  and into the stacks  20   a  and  20   b.    
     The water flowing through the stack water supply channel  305  and into the stacks  20   a  and  20   b  can cool the stacks  20   a  and  20   b . The water flowing in the stacks  20   a  and  20   b  can flow through stack heat exchangers included in the stacks  20   a  and  20   b , and can absorb heat generated by an electrochemical reaction occurring in the stacks  20   a  and  20   b.    
     The plurality of stacks  20   a  and  20   b  may be connected by a water connection pipe that forms a water connection channel  306 . The water discharged from the first stack  20   a  can flow through the water connection channel  306  and into the second stack  20   b.    
     The water discharged from the stacks  20   a  and  20   b  can flow into the cooling water heat exchanger  24  through a stack water discharge channel  307 . The cooling water heat exchanger  24  enables the water discharged from the stacks  20   a  and  20   b  and the water discharged from the heat collection tank  15  to exchange heat. The water discharged from the stacks  20   a  and  20   b  can flow through the cooling water heat exchanger  24  and into the water storage channel  308 . 
     The water discharged from the heat collection tank  15  by the hot water pump  48  can flow into the AOG heat exchanger  22  through the hot water supply channel  313 . The water exchanging heat with AOG in the AOG heat exchanger  22  can be discharged to a first hot water circulation channel  314 . 
     The air heat exchanger  25  may be connected to a humidifier discharge pipe that forms a humidifier discharge channel  212  so that air discharged from the humidifier  23  may flow to the air heat exchanger  25 . The air heat exchanger  25  may be connected to the first hot water circulation channel  314  to which the water discharged from the AOG heat exchanger  22  may flow. The air heat exchanger  25  may enable the water flowing through the humidifier discharge channel  212  to exchange heat with the water flowing through the first hot water circulation circuit  314 . 
     The air having finished heat exchange in the air heat exchanger  25  can be discharged from the air heat exchanger  25  through the air discharge channel  213 . The air discharge pipe that forms the air discharge channel  213  may be connected with an exhaust gas discharge pipe forming the exhaust gas discharge channel  210 . Exhaust gas flowing through the exhaust gas discharge channel  210  may be mixed with air flowing through the air discharge channel  213 . 
     The air dehumidifier  64  may be disposed in (or at) the air discharge pipe. The air dehumidifier  64  may adjust the amount of moisture included in air that is to be discharged to the outside. Air flowing through the air dehumidifier  64  can be discharged from the air dehumidifier  64  after moisture is removed. 
     Condensate water produced in the air dehumidifier  64  can be discharged from the air dehumidifier  64  and can then flow to a fourth water collection channel  312 . A fourth water collection valve  47  that adjusts flow of water may be disposed in (or at) a fourth water collection pipe that forms the fourth water collection channel  312 . The fourth water collection pipe may be connected to the water storage pipe. 
     The air having finished heat exchange in the air heat exchanger  25  can be discharged from the air heat exchanger  25  through a second hot water circulation channel  315 . The water discharged from the air heat exchanger  25  can flow into the cooling water heat exchanger  24  through the second hot water circulation channel  315 . 
     The cooling water heat exchanger  24  enables the water flowing through the stack water discharge channel  307  to exchange heat with the water flowing through the second hot water circulation channel  315 . 
     The exhaust heat exchanger  26  may be connected to an exhaust gas discharge pipe that forms the exhaust gas discharge channel  210  in which exhaust gas flows. The exhaust heat exchanger  26  may be connected to a third hot water circulation pipe that forms a third hot water circulation channel  316  so that water discharged from the cooling water heat exchanger  24  may flow to the exhaust heat exchanger  26 . The exhaust heat exchanger  26  enables the water flowing through the exhaust gas discharge channel  210  to exchange heat with the water flowing through the third hot water circulation channel  316 . 
     The exhaust gas having finished heat exchange in (or at) the exhaust heat exchanger  26  can be discharged to the exhaust channel  213  and the exhaust gas flowing through the exhaust channel  213  can be discharged to the outside. 
     The water having finished heat exchange in (or at) the exhaust heat exchanger  26  can be discharged to the hot water collection channel  317 , and the water flowing through the hot water circulation channel  317  can flow into the heat collection tank  15 . 
     A configuration related to flow of liquid gas and gaseous gas flowing to the reformer  140  may be described with reference to  FIGS.  3  and  4   . 
     The fuel cell system  1  may include the first storage tank  400  that stores liquid gas (or liquid fuel), the second storage tank  402  that supplies a gasified fuel to the reformer  140 , and fuel evaporators  410 ,  412 , and  414  in which a liquid fuel discharged from the first storage tank  400  exchanges with external air so as to be evaporated. 
     The fuel evaporators  410 ,  412 , and  414  include a first fuel evaporator  410  disposed in (or at) a first supply pipe, a second fuel evaporator  412  disposed in (or at) a second supply pipe, and a third fuel evaporator  414  disposed in (or at) a reforming gas discharge pipe  104   a.    
     The fuel cell system  1  may include a first liquid gas supply pipe  420  connecting the first storage tank  400  and the first fuel evaporator  410 , a second liquid gas supply pipe  422  connecting the first storage tank  400  and the second fuel evaporator  412 , and a third liquid gas supply pipe  424  connecting the first storage tank  400  and the third fuel evaporator  414 . 
     The fuel cell system  1  may include a first gaseous gas supply pipe  430  connecting the first storage tank  400  and the second storage tank  402 , a second gaseous gas supply pipe  432  connecting the second fuel evaporator  412  and the second storage tank  402 , and a third gaseous gas supply pipe  434  connecting the third fuel evaporator  414  and the second storage tank  402 . 
     The fuel cell system  1  may include a plurality of expansion valves  440 ,  442 , and  444  configured to provide a liquid fuel to at least one of the first fuel evaporator  410 , the second fuel evaporator  412 , and the third fuel evaporator  414 . The fuel cell system  1  may include the first expansion valve  440  disposed in (or at) the first liquid gas supply pipe and opening/closing an internal channel of the first liquid gas supply pipe or adjusting the amount of opening of the internal channel. The fuel cell system may include the second expansion valve  442  disposed in (or at) the second liquid gas supply pipe  422  and opening/closing an internal channel of the second liquid gas supply pipe  422  or adjusting the amount of opening of the internal channel. The fuel cell system may include the third expansion valve  444  disposed in (or at) the third liquid gas supply pipe  424  and opening/closing an internal channel of the third liquid gas supply pipe  424  or adjusting the amount of opening of the internal channel. 
     The fuel cell system  1  may include a liquid gas common pipe  426  connecting the fuel processor  10  with the first liquid gas supply pipe  420 , the second liquid gas supply pipe  422 , or the third liquid gas supply pipe  424 , and a common pipe valve  446  for opening/closing the liquid gas common pipe  426 . 
     The first storage tank  400  may store a liquid fuel. The first storage tank  400  may be a pressure tank type to store a fuel in a liquid state. The first storage tank  400  may be composed of a dual-structure tank and an insulator. 
       FIG.  4    shows a re-liquefier  450  that re-liquefies a fuel discharged from the first storage tank  400  and a pump  454 , disposed between the first storage tank  400  and the fuel evaporators  410 ,  412 , and  414 , that supplies a fuel discharged from the re-liquefier  450  to the fuel evaporators  410 ,  412 , and  414 . Expansion valves  440 ,  442 , and  444  that expand a liquid fuel flowing to the fuel evaporators  410 ,  412 , and  414  may be disposed between the first storage tank  400  and the fuel evaporators  410 ,  412 , and  414 . 
     The re-liquefier  450  can re-liquefy a refrigerant flowing to a separate heat pump and a gas fuel discharged and evaporated from the first storage tank  400 . The re-liquefier  450  can re-liquefy a gas fuel through evaporation of a refrigerant. 
     The first storage tank  400  and the re-liquefier  450  may be connected to a first pipe  456   a  through which a liquid-state fuel flows from the first storage tank  400 , and a second pipe  456   b  through which a gas-state fuel flows from the first storage tank  400 . The first pipe  456   a  is connected to a lower portion of the first storage tank  400 , so the liquefied gas stored in the first storage tank  400  can flow. 
     The second pipe  456   b  is connected to an upper portion of the first storage tank  400 , so the gaseous gas stored in the first storage tank  400  can flow. A compressor  452  may be disposed in (or at) the second pipe  456   b  to compress the gaseous gas discharged from the first storage tank  400 . 
     The re-liquefier  450  can mix the liquid fuel flowing through the first pipe  456   a  and the gas fuel flowing through the second pipe  456   b , and can cool and discharge the mixture as a liquid fuel. The pump  454  may be disposed such that the liquid fuel passing through the re-liquefier  450  may be provided to the fuel evaporators  410 ,  412 , and  414 . 
     A third pipe  456   c  is disposed between the re-liquefier  450  and the fuel evaporators  410 ,  412 , and  414 , so the liquid fuel discharged from the re-liquefier  450  can be supplied to the fuel evaporators  410 ,  412 , and  414 . The pump  454  may be disposed in (or at) the third pipe  456   c . The expansion valves  440 ,  442 , and  444  that expand a liquid fuel flowing to the fuel evaporators  410 ,  412 , and  414  may be disposed in (or at) the third pipe  456   c . A fourth pipe  456   d  may diverge from the third pipe  456   c  and can supply a liquid fuel in the pipe to the first storage tank. 
     The first storage tank  400  can temporarily store a gas fuel flowing from the fuel evaporators  410 ,  412 , and  414 . The gas fuel stored in the first storage tank  400  can be supplied to the reformer  140  through the fuel supply channel  101 . 
     The liquid gas common pipe  426  may be connected to the first liquid gas supply pipe  420 , the second liquid gas supply pipe  422 , and the third liquid gas supply pipe  424 . A filter  428  for preventing inflow of foreign substances, a common pipe valve  446  that adjusts flow of the liquid gas discharged from the first storage tank  400  and blocks high-pressure gas when the system is not used and in an emergency, and a pressure sensor  429  that senses the pressure of liquid gas may be disposed in the liquid gas common pipe  426 . 
     &lt;Operation Mode&gt; 
     The operation of the fuel cell system  1  may be described with reference to  FIGS.  5  to  7   . 
     The fuel cell system  1  can operate in a preheating mode WM that preheats the system, a reforming mode RM that secures the production amount of reforming gas, and a power generation mode PM that generates electricity through the stacks  20   a  and  20   b.    
     In the preheating mode WM, the first blower  71  is operated such that external air can be supplied to the reformer  140 . In the preheating mode WM, the fuel processor  10  may be preheated by generating combustion heat by burning a gas mixture of fuel gas and air through the burner  120  (of the fuel processor  10 ). 
     Referring to  FIG.  5   , in the preheating mode WM, the common pipe valve  446  is opened and the degree of opening of the first expansion valve  440  is increased, so a liquid fuel is supplied to the first fuel evaporator  410 . A liquid fuel changes in phase into a gas fuel in the first fuel evaporator  410 , and air flowing inside (from the outside) and flowing therein can be supplied with the temperature reduced to the burner  120 . The gas fuel discharged from the first fuel evaporator  410  flows to the second storage tank  402 . 
     When gas is produced up to a predetermined pressure in the second storage tank  420  through the operation, the gaseous gas of the second storage tank  402  may be supplied to the fuel processor  10 . Reforming gas may be produced in the fuel processor  10  by reforming the gas fuel discharged from the second storage tank  402 . 
     As the burner  120  is operated, both the temperature and pressure of air are increased at the outlet (in comparison to the inlet) of the first blower  71 , so high-temperature/high-pressure air flows into the first fuel evaporator  410 . The air flowing in the first fuel evaporator  410  may supply (or provide) heat for gasifying the liquid fuel discharged from the first storage tank  400 . Thereafter, the air discharged from the first fuel evaporator  410  may be supplied to the burner  120  in a low-temperature and high-pressure state. The density of the low-temperature high-pressure air may be low, so more air can be supplied into the same volume of the fuel processor  10 . More gas mixture can flow inside in combustion through mixing with gas in the fuel processor  10 , so the combustion time to a target temperature can be reduced. 
     When low-temperature high-pressure air and low-temperature gas fuel is supplied to the burner of the fuel processor  10  through the first fuel evaporator  410 , more gas mixture can be supplied to the burner. Since more gas mixture is supplied than disadvantageous arrangements, the preheating operation time may be reduced. 
     In the reforming mode RM, the amount of reformed hydrogen may be secured by supplying reforming gas produced through the reformer  140  back to the reformer  140 . In the reforming mode RM, high-temperature reforming gas may be produced from a gas fuel in the reformer  140 , and reforming gas produced through the bypass valve  34  may be fully used for combustion before the power generation mode PM. 
     Referring to  FIG.  6   , in the reforming mode RM, the degree of opening of each of the first expansion valve  440  and the third expansion valve  444  may be secured. Accordingly, the liquid fuel discharged from the first storage tank  400  can flow to the first fuel evaporator  410  and the third fuel evaporator  414 . In this example, the amount of the liquid fuel flowing to the third fuel evaporator  414  may be increased to more than the amount of the liquid fuel flowing to the first fuel evaporator  410  by increasing the degree of opening of the third expansion valve  444 . In the reforming mode RM, a size of the internal channel of the third liquid gas supply pipe  424  opened by the third expansion valve  444  may be larger than a size of the internal channel of the first liquid gas supply pipe  420  opened by the first expansion valve  440 . The temperature of the reforming gas flowing through the third fuel evaporator  414  may be higher than the temperature of the air flowing through the first fuel evaporator  410 . A liquid fuel may change the phase in the third fuel evaporator  414  in comparison to the first fuel evaporator  410 , so the amount of a fuel liquid flowing to the third fuel evaporator  414  may be increased. 
     In the operation in the reforming mode RM, the reformer  140  may produce reforming gas using the gaseous gas stored in the second storage tank  402 . Since the temperature of the reforming gas discharged from the reformer  140  reaches about 90 degrees or higher, a liquid fuel may be supplied to the third fuel evaporator  414  to use the waste heat of the reforming gas discharged from the reformer  140 . 
     The liquid gas supplied to the first fuel evaporator  410  and the third fuel evaporator  414  may be evaporated by exchanging heat with high-temperature reforming gas or the air flowing through the first supply pipe  202   a  and is then supplied to the second storage tank  402 . When a gas fuel is produced through the first fuel evaporator  410  and the third fuel evaporator  414 , gas can be produced more than when only the first fuel evaporator  410  is used. As described above, as the production amount of gas increases, the reformer reactor may produce reforming gas while consuming more gas and gets ready for entering the power generation mode operation. 
     High-temperature reforming gas discharged from the reformer  140  can be changed into low-temperature reforming gas by exchanging heat with a liquid fuel through the third fuel evaporator  414 . The low-temperature reforming gas may be supplied to the burner  120  through the bypass valve  34  and used to improve combustion efficiency. The low-temperature reforming gas may also decrease in density due to a temperature change, so it can be supplied more to the burner  120 . Reforming gas containing a large amount of hydrogen may also be burned in the burner  120 , so combustion reaction and efficiency can be improved. 
     In the power generation mode PM, electricity can be generated through an electrochemical reaction of oxygen and hydrogen in the stacks  20   a  and  20   b  by the reforming gas discharged from the reformer  140  and the air flowing inside from the outside. 
     In the power generation mode PM, as the first blower  71  is operated, external air may flow to the fuel processor  10 , and as the second blower  72  is operated, external air can be supplied to the stacks  20   a  and  20   b.    
     As the reformer  140  is operated, reforming gas discharged from the reformer  140  can be supplied to the stacks  20   a  and  20   b.    
     Referring to  FIG.  7   , when the system is operated in the power generation mode Pm, the liquid fuel discharged from the first storage tank  400  can be supplied to each of the first fuel evaporator  410 , the second fuel evaporator  412 , and the third fuel evaporator  414 . 
     In the power generation mode PM, all of the first expansion valve  440 , the second expansion valve  442 , and the third expansion valve  444  may be opened. In the power generation mode, the degree of amount may be set such that the amount of the liquid fuel flowing through the third expansion valve  444  is larger than the amount of the liquid fuel flowing through the first expansion valve  440  or the second expansion valve  442 . In the power generation mode, a size of the internal channel of the third liquid gas supply pipe  424  opened by the third expansion valve  444  may be larger than a size of the internal channel of the first liquid gas supply pipe  420  opened by the first expansion valve  440  or a size of the internal channel of the second liquid gas supply pipe  422  opened by the second expansion valve  442 . 
     When the system is operated in the power generation mode PM, the second blower  72  may be operated, which increases both the temperature and pressure of the air discharged from the second blower  72  in comparison to the air flowing into the second blower  72 , similar to the operation of the first blower  71 . 
     The second fuel evaporator  412  disposed in (or at) the second supply pipe  203   a  may exchange heat with the liquid fuel discharged from the first storage tank  400 , so low-temperature and high-pressure air is discharged from the first fuel evaporator  410 . The low-temperature high-pressure air discharged from the second fuel evaporator  412  is supplied to the stacks  20   a  and  20   b , reacts with reforming gas to generate power, and is then discharged. 
     The reforming gas produced in the reformer  140  may be supplied to the stacks  20   a  and  20   b  through the third fuel evaporator  414 , and non-reacting hydrogen (AOG) discharged without being used for power generation of the stacks  20   a  and  20   b  may be supplied back to the burner  120  and used to improve combustion efficiency. The reforming gas that is supplied to the stacks  20   a  and  20   b  may decrease in temperature through the third fuel evaporator  414 , so more reforming gas can be supplied into a predetermined volume, and accordingly reaction efficiency may be improved for power generation. 
     As described above, the temperature of the air and reforming gas that are supplied to the stacks  20   a  and  20   b  may decrease, so the density is greatly decreased in comparison to high temperature. Since the reaction area of air and reforming gas in the stacks  20   a  and  20   b  may be very limited, more reaction can be induced when density is low. This may decrease the density of the air and the reforming gas, so as to improve power generation efficiency of the stacks. 
     Various embodiments of the fuel evaporators  410 ,  412 , and  414  (i.e., the first fuel evaporator  410 , the second fuel evaporator  412 , or the third fuel evaporator  414 ) may be described with reference to  FIGS.  8   a    to  14   b.    
     The fuel evaporators  410 ,  412 , and  414  may include a housing that forms the external shape, and a fuel flow section through which a liquid fuel flows and a gas flow section through which air or reforming gas flows are formed in the housing. 
     Pluralities of fuel flow sections and gas flow sections may be alternately arranged. A protrusion that can increase a contact area of a liquid fuel or air may be formed in each of the pipe forming the fuel flow section and the pipe forming the gas flow section. The fuel flow section and the gas flow section may be formed in various types. 
     Referring to  FIGS.  8   a  and  8   b   , a fuel evaporator according to a first embodiment may have a structure in which a fuel flow section  470   a  and a gas flow section  462   a  are formed in a housing  460   a  having a cylindrical shape. The gas flow section  462   a  may be composed of a plurality of straight small-diameter pipes  463   a . The fuel flow section  470   a  can exchange heat with the gas flow section  462   a  through flow bending up and down in the spaces between the small-diameter pipes  463   a . A first inlet end  464   a  and a first outlet end  466   a  of the gas flow section  462   a  may be open in a direction parallel with the plurality of small-diameter pipes. A second inlet end  472   a  and a second outlet end  474   a  of the fuel flow section  470   a  may be open in a direction perpendicular to the first inlet end  464   a  and the first outlet end  466   a  of the gas flow section  462   a . The fuel flow section  470   a  may include a guider  471   a  formed in a direction perpendicular to the small-diameter pipes  463   a  of the gas flow section  462   a  and increasing a flow area for heat exchange by making a fuel flow up and down. 
     Referring to  FIGS.  9   a  and  9   b   , the housing  460   b  of a fuel evaporator according to a second embodiment may have a structure in which a U-shaped pipe is disposed therein. 
     A gas flow section  462   b  may include a plurality of small-diameter pipes  463   b  formed up and down and a U-shaped bending pipe  463   b   1  bending over the small-diameter pipes  463   b . A first inlet end  464   b  and a first outlet end  466   b  of the gas flow section  462   b  are open in a direction opposite to the direction in which the bending pipe  463   b   1  is disposed. 
     A fuel flow section  470   b  may have a structure flowing through a guider flowing left and right between the plurality of small-diameter pipes  463   b . A second inlet end  472   b  and a second outlet end  474   b  of the fuel flow section  470   b  may have a structure protruding in the circumferential direction of the housing  460   b.    
     Referring to  FIGS.  10   a  and  10   b   , a fuel evaporator according to a third embodiment may have a structure in which a housing  460   c  has a cylindrical shape and a fuel flow section  470   c  and a gas flow section  462   c  are formed therein. 
     The gas flow section  462   c  may be composed of a plurality of straight small-diameter pipes. The fuel flow section  470   c  can exchange heat with the gas flow section  462   c  through flow of spaces between the plurality of small-diameter pipes. A first inlet end  464   c  and a first outlet end  466   c  of the gas flow section  462   c  may be open in a direction parallel with the plurality of small-diameter pipes. A first inlet end  464   c  and a first outlet end  466   c  of the fuel flow section  470   c  may be open in a direction perpendicular to a second inlet end  472   c  and a second outlet end  474   c  of the gas flow section  462   c.    
     In the evaporator according to the third embodiment, the inside or outside of the pipe of the fuel flow section  470   c  or the gas flow section  462   c , as shown in  FIGS.  10   a  to  10   b   , may be formed in a flat shape, and as shown in  FIGS.  11   a  to  11   b   , may be formed in a shape having protrusions. 
     Referring to  FIGS.  12  and  14     b , a fuel evaporator according to the fourth embodiment may use a plate-type heat exchanger. The fuel evaporator according to the fourth embodiment may have a structure in which a housing  460   d  has a plate shape and a fuel flow section  470   d  and a gas flow section  462   d  are formed therein. 
     A first inlet end  464  and a first outlet end  466   d  of the gas flow section  462   d  are diagonally spaced apart from each other on a side of the housing  460   d . Referring to  FIGS.  13   a  and  14   a   , the gas flow section  462   d  has a plurality of channels connecting the first inlet end  464   d  and the first outlet end  466   d  in the housing  460   d . The gas flow section  462   d  has a plurality of channels formed in a direction perpendicular to the first inlet end  464   d  and the first outlet end  466   d . The plurality of channels formed in the gas flow section  462   d  may be formed such that the inside of the pipe is flat, as shown in  FIG.  13   a   , or may be formed such that protrusions are formed in the pipe, as shown in  FIG.  14     a.    
     A second inlet end  472   d  and a second outlet end  474   d  of the fuel flow section  470   d  are diagonally spaced apart from each other on a side of the housing  460   d . Referring to  FIGS.  13   b  and  14   b   , the gas flow section  470   d  has a plurality of channels connecting the second inlet end  472   d  and the second outlet end  474   d  in the housing  460   d . The fuel flow section  470   d  has a plurality of channels formed in a direction perpendicular to the second inlet end  472   d  and the second outlet end  474   d . The plurality of channels formed in the fuel flow section  470   d  may be formed such that the inside of the pipe is flat, as shown in  FIG.  13   b   , or may be formed such that protrusions are formed in the pipe, as shown in  FIG.  14     b.    
     An object of the present disclosure is to provide a fuel cell system that increases the density of air that is supplied to a burner of a reformer using a fuel that is supplied to the reformer. 
     An object of the present disclosure is to provide a fuel cell system that increases the density of reforming gas that is discharged from a reformer and supplied to a burner using a fuel that is supplied to the reformer. 
     An object of the present disclosure is to provide a fuel cell system that increases the density of air that is supplied to a stack using a fuel that is supplied to a reformer. 
     A fuel cell system of the present disclosure includes: a reformer performing a reforming process of producing hydrogen gas from a gasified fuel; a burner supplying heat to the reformer; a stack generating electrical energy by generating an electrochemical reaction using reforming gas and air discharged from the reformer; a first supply pipe supplying external air to the burner; and a second supply pipe supplying external air to the stack. 
     In order to achieve the objects, the fuel cell system may include: a first storage tank storing a liquid fuel; a second storage tank supplying a gasified fuel to the reformer; and a fuel evaporator making a liquid fuel discharged from the first storage tank exchange heat with air flowing through the first supply pipe or air flowing through the second supply pipe, and sending a gasified gaseous fuel to the second storage tank, in which it is possible to increase the density of air supplied to the stack or air supplied to the burner using coldness and heat of a liquid fuel. 
     The fuel evaporator may include: a first fuel evaporator configured to enable a liquid fuel discharged from the first storage tank and air flowing through the first supply pipe to exchange heat; and a second fuel evaporator configured to enable a liquid fuel discharged from the first storage tank and air flowing through the second supply pipe exchange heat, so it is possible to cool air supplied to the stack or air supplied to the burner. 
     The fuel cell system may include: a first liquid gas supply pipe connecting the first storage tank and the first fuel evaporator; a second liquid gas supply pipe connecting the first storage tank and the second fuel evaporator; a first expansion valve disposed in the first liquid gas supply pipe and opening/closing an internal channel of the first liquid gas supply pipe or adjusting the degree of opening of the internal channel; and a second expansion valve disposed in the second liquid gas supply pipe and opening/closing an internal channel of the second liquid gas supply pipe or adjusting the degree of opening of the internal channel, so it is possible to adjust flow of a fuel liquid in accordance with operation modes. 
     The first expansion valve may expand the internal channel of the first liquid gas supply pipe and the second expansion valve may expand the internal channel of the second liquid gas supply pipe in a preheating mode that preheats the reformer, so it is possible to supply a liquid fuel to a first fuel evaporator disposed in a first supply pipe through which air flows in the preheating mode. 
     The first expansion valve may increase the degree of opening of the internal channel of the first liquid gas supply pipe and the second expansion valve may increase the degree of opening of the internal channel of the second liquid gas supply pipe in a power generation mode that generates electricity using the stack, so it is possible to cool both air supplied to the stack and air supplied to the burner. 
     The fuel cells system may include a reforming gas discharge pipe configured to send reforming gas discharged from the reformer to the burner or the stack, and the liquid gas evaporator includes a third liquid gas evaporator disposed in the reforming gas discharge pipe and enabling reforming gas discharged from the reformer and a liquid fuel to exchange heat, so it is possible to cool high-pressure reforming gas discharged from the reformer. 
     The fuel cell system may include: a third liquid gas supply pipe connecting the first storage tank and the third fuel evaporator; and a third expansion valve disposed in the third liquid gas supply pipe and opening/closing an internal channel of the third liquid gas supply pipe or adjusting the degree of opening of the internal channel, so it is possible to supply a liquid fuel to the third liquid gas evaporator in accordance with operation modes. 
     The first expansion valve may increase the degree of opening of the internal channel of the first liquid gas supply pipe and the third expansion valve may increase the degree of opening of the internal channel of the third liquid gas supply pipe in a reforming mode that increases the amount of hydrogen included in reforming gas discharged from the reformer, so it is possible to gasify a liquid fuel using gas discharged from the reformer and increase density of reforming gas discharged from the reformer. 
     The degree of opening of the third expansion valve is larger than the degree of opening of the first expansion valve in the reforming mode, so it is possible to effectively change the phase of a liquid fuel. 
     The first expansion valve may increase the degree of opening of the internal channel of the first liquid gas supply pipe, the second expansion valve may increase the degree of opening of the internal channel of the second liquid gas supply pipe, and the third expansion valve may increase the degree of opening of the internal channel of the third liquid gas supply pipe in a power generation mode that generates electricity using the stack, so it is possible to gasify a liquid fuel through three fuel evaporator. 
     The degree of opening of the third expansion valve is larger than the degree of opening of the first expansion valve or the second expansion valve in the power generation mode, so it is possible to effectively change the phase of a liquid fuel. 
     The fuel cell system may include: a liquid gas common pipe connecting the fuel apparatus, the first liquid gas supply pipe, the second liquid gas supply pipe, and the third liquid gas supply pipe; and a common pipe valve opening/closing the liquid gas common pipe, so it is possible to stably keep a liquid fuel stored in the first storage tank. 
     The fuel cell system may include: a first blower disposed in the first supply pipe and supplying external air to the first supply pipe; and a second blower disposed in the second supply pipe and supplying external air to the second supply pipe, in which when the first blower is operated, the common pipe valve is opened, so it is possible to discharge the liquid fuel stored in the first storage tank when the fuel cell system is operated. 
     The fuel evaporator may include: a housing forming an external shape; a fuel flow section disposed in the housing and formed such that a liquid fuel flows; and a gas flow section disposed in the housing and formed such that air or reforming gas flows. 
     A plurality of protrusions is formed in each of a pipe forming the fuel flow section and a pipe forming the gas flow section, so it is possible to increase a heat exchange area. 
     According to the fuel cell system of the present disclosure, one or more effects can be achieved as follows. 
     First, there is an advantage of increasing efficiency of a reforming reaction by increasing the density of air that is supplied to a burner that heats a reformer using coldness and heat generated by a phase change of a liquid fuel. 
     Second, there is an advantage of increasing reaction efficiency for generating electricity in a stack by increasing the density of air that is supplied to the stack using coldness and heat generated by a phase change of a liquid fuel. 
     Third, there is an advantage of increasing efficiency of a reforming reaction by increasing the density of reforming gas that is discharged from a reformer and supplied to a burner that heats the reformer using coldness and heat generated by a phase change of a liquid fuel. 
     The effects of the present disclosure are not limited to those described above and other effects not stated herein may be made apparent to those skilled in the art from claims. 
     It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer can be directly on another element or layer or intervening elements or layers. In contrast, when an element is referred to as being “directly on” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “lower”, “upper” and the like, may be used herein for ease of description to describe the relationship of one element or feature to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “lower” relative to other elements or features would then be oriented “upper” relative to the other elements or features. Thus, the exemplary term “lower” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Embodiments of the disclosure are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the disclosure. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the disclosure should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to affect such feature, structure, or characteristic in connection with other ones of the embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.