Fuel cell heat recovering reformer and system

A fuel cell system includes a reformer for reforming fuel and generating hydrogen gas. A stack generates electricity through an electrochemical reaction between the hydrogen gas and oxygen. A fuel supply unit supplies fuel to the reformer. An air supply unit supplies air to the reformer and the stack. The reformer includes a reformation reactor unit for generating the hydrogen gas and a heat-insulating unit including a vacuum area covering the reformation reactor unit and recovering heat generated from the reformation unit.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0012967 filed on Feb. 26, 2004 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a fuel cell system, and more particularly, to the structure of a reformer of the fuel cell system.

BACKGROUND OF THE INVENTION

A fuel cell is generally known as an electricity generating system which directly converts chemical energy into electric energy through an electrochemical reaction between oxygen, or air containing the oxygen, and hydrogen contained in hydrocarbon-grouped materials such as methanol and natural gas. Specifically, the fuel cell has a feature that it can produce electricity through the electrochemical reaction between hydrogen and oxygen without combustion and provides heat as a byproduct thereof that can be used simultaneously.

Fuel cells are classified into a phosphate fuel cell working at a temperature of about 150° C. to 200° C., a molten carbonate fuel cell working at a high temperature of about 600° C. to 700° C., a solid oxide fuel cell working at a high temperature of 1,000° C. or more, and a polymer electrolyte membrane fuel cell (PEMFC) and an alkali fuel cell working at room temperature or a temperature of 100° C. or less, depending upon the kind of electrolyte used. These fuel cells work basically on the same principle, but are different from one another in kind of fuel, operating temperature, catalyst, and electrolyte.

The recently developed polymer electrolyte membrane fuel cell (PEMFC) has an excellent output characteristic, a low operating temperature, and a fast starting and response characteristic as compared with other fuel cells, and uses hydrogen obtained by reforming methanol, ethanol, natural gas, etc. Accordingly, the PEMFC has a wide range of applications such as a mobile power source for vehicles, a distributed power source for the home or buildings, and a small-sized power source for electronic devices.

The aforementioned PEMFC has a fuel cell main body (hereinafter, referred to as a stack), a fuel tank, and a fuel pump supplying fuel to the stack from the fuel tank, to constitute a typical system. Such a fuel cell further includes a reformer for reforming the fuel to generate hydrogen gas and supplying the hydrogen gas to the stack. Therefore, in the PEMFC the fuel is stored in the fuel tank is supplied to the reformer by means of pumping power of the fuel pump. The reformer then reforms the fuel and generates the hydrogen gas. The stack makes the hydrogen gas and oxygen electrochemically react with each other, thereby generating electric energy.

Alternatively, such a fuel cell can employ a direct methanol fuel cell (DMFC) scheme directly supplying liquid fuel containing hydrogen to the stack and generating electricity. The fuel cell employing the direct methanol fuel cell scheme does not require the reformer, unlike the PEMFC.

In the fuel cell system described above, the stack substantially generating the electricity has a stacked structure of several or several tens of unit cells having a membrane-electrode assembly (MEA) and a separator (or a bipolar plate). The MEA has a structure such that an anode electrode and a cathode electrode are bonded to each other with an electrolyte membrane therebetween. The separator simultaneously performs a function of a passage through which oxygen and hydrogen gas required for the reaction of the fuel cell are supplied, and a function of a conductor connecting the anode electrode and the cathode electrode of each MEA to each other in series.

Therefore, through the separator, hydrogen gas is supplied to the anode electrode and oxygen is supplied to the cathode electrode. An oxidation reaction of the hydrogen gas takes place in the anode electrode and a reduction reaction of oxygen takes place in the cathode electrode. Due to movement of electrons generated at that time, electricity, heat, and water can be collectively obtained.

The reformer described above is an apparatus which converts through a catalytic reformation reaction the liquid fuel containing hydrogen and water into the hydrogen gas required for generation of electricity by the stack, and, in addition, which removes noxious substances such as carbon monoxide which poisons the fuel cell and shortens its lifetime. The reformer includes a reforming section for reforming the fuel and generating the hydrogen gas, and a carbon-monoxide removing section for removing carbon monoxide from the hydrogen gas. The reforming section converts the fuel into reformed gas that is rich in hydrogen through a catalytic reaction such as steam reformation, partial oxidation, natural reaction, etc. The carbon-monoxide removing section removes carbon monoxide from the reformed gas using a catalytic reaction such as a water-gas shift reaction, an oxidation reaction, etc., or hydrogen purification with a separating membrane.

In the conventional reformer of a fuel cell system, the reforming section includes an exothermic reaction portion inducing a catalytic oxidation reaction between fuel and air and generating combustion heat, and an endothermic reaction portion accepting the combustion heat, inducing a catalytic reformation reaction of the fuel, and generating the hydrogen gas. Therefore, in the reforming section, the endothermic reaction portion accepts a predetermined reaction heat from the exothermic reaction portion and generates the reformed gas having rich in hydrogen from a mixed fuel of liquid fuel and water through the catalytic reformation reaction.

However, since the conventional reformer of a fuel cell system has a structure such that the reaction heat can easily leak outside, the reforming section, which needs a uniform temperature distribution, exhibits an uneven temperature distribution, such that reaction efficiency and thermal efficiency of the overall reformer deteriorates.

SUMMARY OF THE INVENTION

In accordance with the present invention a reformer of a fuel cell system capable of effectively recovering reaction heat for forming reformed gas, and a fuel cell system employing the reformer, is provided.

According to an aspect of the present invention, a reformer of a fuel cell system includes: a reformation reactor unit for reforming fuel and generating hydrogen gas, and a heat-insulating unit having a vacuum area covering the reformation reactor unit and recovering heat generated from the reformer reactor.

The heat-insulating unit may include an inner wall surrounding the reformation reactor unit, and an outer wall which is disposed with a gap from the inner wall to form the vacuum area.

The reformation reactor unit may include a first reactor for combusting liquid fuel and air through a catalytic oxidation reaction and generating heat, and a second reactor for absorbing the heat generated from the first reacting section, vaporizing mixed fuel of the liquid fuel and water, and generating reformed gas from the vaporized fuel through a catalytic reformation reaction.

The inner wall and the outer wall may be made of at least one material selected from stainless steel, ceramics, aluminum, and zirconium.

The reformer may further include at least one carbon-monoxide reducing section which is connected to the reformation reactor unit and reduces the concentration of carbon monoxide from the reformed gas generated from the reformation reactor unit.

A heat-insulating member may be disposed in the vacuum area, and the heat-insulating member may be made of glass fiber or porous ceramics.

At least one spacer for maintaining a gap may be disposed between the inner wall and the outer wall, and the spacer may be formed as a connection member provided to penetrate the inner wall and the outer wall.

According to another aspect of the present invention, there is also provided a fuel cell system including: a reformer for reforming fuel and generating hydrogen gas; a stack for generating electricity through an electrochemical reaction between the hydrogen gas and oxygen; a fuel supply unit for supplying the fuel to the reformer; and an air supply unit for supplying air to the reformer and the stack, wherein the reformer includes a reformation reactor unit for generating the hydrogen gas and a heat-insulating unit having a vacuum area covering the reformation reactor unit and recovering heat generated from the reformation reactor unit.

The fuel supply unit may include: a first tank for storing liquid fuel; a second tank for storing water; and a fuel pump connected to the first tank and the second tank.

The air supply unit may include an air pump for drawing in external air.

The fuel cell system may employ a PEMFC scheme.

DETAILED DESCRIPTION

An exemplary embodiment of a fuel cell system according to the present invention employs a PEMFC in which a reformed gas that is rich in hydrogen is generated by reforming fuel containing hydrogen and the chemical energy generated by allowing the reformed gas and oxygen to electrochemically react with each other is directly converted into electric energy.

Referring now toFIG. 1, fuel cell system100according to an embodiment of the present invention includes a reformer20which reforms fuel containing hydrogen to generate reformed gas that is rich in hydrogen. Stack10converts the chemical reaction energy between the reformed gas generated from the reformer20and oxygen into electric energy to generate electricity. Fuel supply unit30supplies the fuel to the reformer20. Air supply unit40supplies external air to the stack10and the reformer20.

In the fuel cell system100the principal fuel for generating electricity is water plus a hydrocarbon-grouped or an alcohol-grouped hydrogen fuel such as methanol, ethanol, natural gas, etc. For convenience, the combination of liquid fuel and water is defined as mixed fuel.

The fuel cell system100can generate electricity through an electrochemical reaction between the reformed gas and oxygen in the external air supplied from the air supply unit40.

Alternatively, the fuel cell system100may supply oxygen gas from storage tanks or cylinders and the reformed gas generated from the reformer20to the stack10and may generate electric energy through an electrochemical reaction therebetween.

In the description below, the exemplary embodiments will be explained using external air for the needed oxygen.

The fuel supply unit30includes a first tank31for storing liquid fuel containing hydrogen, that is, hydrocarbon-grouped or alcohol-grouped hydrogen fuel such as methanol, ethanol, natural gas, etc. Second tank32stores water. Fuel pump33is connected to the first tank31and the second tank32, respectively. The air supply unit40includes an air pump41for drawing the external air.

FIG. 2is a cross-sectional view illustrating a structure of the reformer shown inFIG. 1. Referring toFIGS. 1 and 2, the reformer20used in the fuel cell system100includes a reformation reactor unit21for generating the reformed gas rich in hydrogen from the mixed fuel through a predetermined catalytic reaction.

The reformation reactor unit21includes a first reactor22which combusts the liquid fuel with air to generate reaction heat having a predetermined temperature, and a second reactor23which, using the reaction heat, generates the reformed gas rich in hydrogen gas.

The first reactor22is an exothermic section for supplying heat required for generating the reformed gas, and combusts the liquid fuel and the external air through a catalytic oxidation reaction.

The first reactor22may be formed in a plate shape constituting a flow channel (not shown) enabling the fuel and the air to flow. A general catalytic oxidation layer (not shown) for promoting the catalytic oxidation reaction between the fuel and the air is formed in the flow channel. The first reactor22and the first tank31of the fuel supply unit30can be connected to each other through a first supply line51. The first reactor22and the air supply unit40can be connected to each other through a second supply line52. Discharge line59connected to the first reactor22discharges to the outside of the reformer20the combustion gas generated from the catalytic oxidation reaction and the remaining fuel and air not participating in the reaction.

The second reactor23absorbs the reaction heat generated from the first reactor22and evaporates the mixed fuel. The second reactor23generates the reformed gas rich in hydrogen from the evaporated fuel through a steam reformation (SR) catalytic reaction. The second reactor23may be formed in a plate shape constituting a flow channel (not shown) enabling the mixed fuel required for the catalytic reformation reaction to flow, and may be provided close to the first reactor22. A general catalytic layer (not shown) for promoting the catalytic reformation reaction of the mixed fuel is formed in the flow channel. The second reactor23can be connected to the first and second tanks31and32of the fuel supply unit30through a third supply line53.

According to the present invention, the first and second reactors22,23are not limited to a plate shape having the flow channel and disposed close to each other, but may be formed in a cylindrical reaction vessel shape. That is, the first reactor22and the second reactor23may be constructed such that: the first reactor22has a structure wherein a catalytic oxidation layer is formed is inside a reaction vessel, the second reactor23has a structure wherein a catalytic reformation layer is formed inside a reaction vessel, and the reaction vessel of the second reactor23is located at the inside of the reaction vessel of the first reactor22.

Further, in the reformer20according to an exemplary embodiment, at least one carbon-monoxide reducing section24for reducing the concentration of carbon monoxide from the reformed gas through a water-gas shift (WGS) catalytic reaction or a preferential CO oxidation (PROX) catalytic reaction may be disposed between the stack10and the reformation reactor unit21.

The carbon-monoxide reducing section24includes: a cylindrical reaction vessel25connected to the reformation reactor unit21, a typical water-gas shift catalyst layer (not shown), or a preferential CO oxidation catalyst layer (not shown) formed at the inside of the reaction vessel25.

The reaction vessel25has an inlet25aallowing the reformed gas supplied from the reformation reactor unit21to flow in the inner space of the reaction vessel25and an outlet25ballowing the reformed gas to be supplied to the stack, the reformed gas having the concentration of carbon monoxide reduced through a catalytic reaction caused by the catalytic layer in the inner space. The inlet25aof the reaction vessel25can be connected to the second reactor23through a fourth supply line54. The outlet25bof the reaction vessel25can be connected to the stack10through a fifth supply line55.

Referring now toFIGS. 1 and 3, the stack10used in the fuel cell system100includes multiple electricity generators11which, to generate electric energy, induce an oxidation/reduction reaction between the reformed gas reformed by the reformer20and the air.

Each electricity generator11constitutes a unit cell for generating electricity, and includes an MEA12for oxidizing/reducing the reformed gas and oxygen in the air and separators16for supplying the reformed gas and the air to the MEA12.

In each electricity generator11, the separators16are disposed on both surfaces of the MEA12. The stack10is formed by continuously stacking electricity generators11. Input end plate13and output endplate13′ are disposed at the outermost sides of the stack10.

In the MEA12, a conventional MEA structure is employed, in which an electrolyte membrane is interposed between an anode electrode and a cathode electrode constituting respective side surfaces thereof. The anode electrode is supplied with the reformed gas through the separator16, and includes a catalyst layer for converting the reformed gas into electrons and hydrogen ions through an oxidation reaction and a gas diffusion layer (GDL) for smoothly moving the electrons and the hydrogen ions. The cathode electrode is supplied with the air through the separator16, and includes a catalyst layer converting oxygen in the air into electrons and oxygen ions through a reduction reaction and a gas diffusion layer smoothly moving the electrons and the oxygen ions. The electrolyte membrane is made of a solid polymer electrolyte with a thickness of 50 to 200 μm, and has an ion exchanging function of moving the hydrogen ions generated from the catalyst layer of the anode electrode to the catalyst layer of the cathode electrode.

The separators16are disposed at both sides of the MEA12and are in close contact with the anode electrode and the cathode electrode of the MEA12. A flow channel17for supplying the reformed gas to the anode electrode and supplying the air to the cathode electrode is formed in the surfaces of the separators16coming in close contact with the anode electrode and the cathode electrode of the MEA12.

The respective end plates13,13′ are disposed at the outermost sides of the stack10and have a function of closely compressing the plural electricity generators11. End plate13includes a first supply pipe13afor injecting the reformed gas formed from the reformer20into the flow channels17and a second supply pipe13bfor injecting the air into the flow channels17. End plate13′ includes a first discharge pipe13cfor discharging the hydrogen gas not participating in the reaction and finally remaining in the plural electricity generators11, and a second discharge pipe13dfor discharging the air not participating in the reaction and finally remaining in the plural electricity generators11. Here, the carbon-monoxide reducing section24can be connected to the first supply pipe13athrough the aforementioned fifth supply line55. The air supply unit40can be connected to the second supply line13bthrough a sixth supply line56.

When the fuel cell system100having the aforementioned structure operates, a part of the reaction heat generated from the first reactor22of the reformation reactor unit21may not be delivered to the second reactor23and may leak outside, so that unevenness in temperature can occur in the second reactor23and thus reaction efficiency or thermal efficiency deteriorates.

However, according to an exemplary embodiment of the present invention, the reformer includes a heat-insulating unit26for recovering heat so that it does not externally leak from the reformation reactor unit21.

Referring toFIG. 2, the heat-insulating unit26according to the exemplary embodiment includes an inner wall27surrounding the whole reformation reactor unit21, and an outer wall28which is disposed with a predetermined gap from the inner wall27and surrounds the inner wall27. As a result, a space is formed between the inner wall27and the outer wall28, the space constituting a vacuum area60implemented through a conventional vacuum process. The vacuum area60covers the reformation reactor unit21.

The inner wall27and the outer wall28may be made of at least one material selected from stainless steel such as Steel Use Stainless (SUS), heat-insulating metal such as aluminum and zirconium, and heat-insulating non-metal. The inner wall27and the outer wall28may be made of the same material or may be made of different materials. The inner wall27and the outer wall28may have the same thickness or may have different thicknesses.

The heat-insulating unit26includes first and second connection members29a,29bpenetrating the outer wall28and the inner wall27and connecting the first and second supply lines51,52to the first reactor22, and third and fourth connection members29c,29dpenetrating the outer wall28and the inner wall27and connecting the third and fourth supply lines53,54to the second reactor23.

A fifth connection member29e, penetrating the outer wall28and the inner wall27and connecting the discharge line59to the first reactor22, is formed in the heat-insulating unit26.

The first, second, third, fourth, and fifth connection members29a,29b,29c,29d,29eserve as spacers for penetrating the inner wall27and the outer wall28and maintaining the gap therebetween, as well as serving as connecting means for connecting the supply lines and the discharge line to the reformation reactor unit. The spacers are not limited to the above structure of the connection members, but may have a structure of a bead or rib shape or a protrusion shape supported by both the inner wall27and the outer wall28.

The operation of the fuel cell system according to the exemplary embodiments of the present invention having the aforementioned structures will be described in more detail.

First, the fuel pump33supplies the liquid fuel stored in the first tank31to the first reactor22through the first supply line51. The air pump41supplies the external air to the first reactor22through the second supply line52. Then, the first reactor22generates predetermined reaction heat through the catalytic oxidation reaction between the liquid fuel and the air. At this time, the first reactor22discharges the combustion gas and the remaining fuel and air having not participated in the reaction, to the outside of the reformer20through the discharge line59.

Also, the fuel pump33supplies the liquid fuel and the water stored in the first tank31and the second tank32, respectively, to the second reactor23through the third supply line53. The second reactor23is supplied with the reaction heat generated from the first reactor and maintains a preheated state at a predetermined temperature.

Then, in the second reactor23, a reaction of decomposing the mixed fuel (of the liquid fuel and the water) and a reaction of shifting carbon monoxide simultaneously take place through the steam reformation (SR) catalytic reaction, thereby generating the reformed gas containing carbon dioxide and hydrogen. At this time, the second reactor23cannot completely perform the carbon-monoxide shift reaction, thereby generating the reformed gas containing a minute amount of carbon monoxide as a byproduct.

In the course of undergoing the above processes, since the reaction heat generated from the reformation reactor unit21can be recovered by the inner wall27and the outer wall28as well as the vacuum area60covering the reformation reactor unit21, the reformer20can prevent loss of the reaction heat generated from the reformation reactor unit21, so that it is possible to enhance the reaction efficiency and the thermal efficiency of the reformation reactor unit21.

In order to further enhance the heat-insulating effect, a heat-insulating member may be provided in the vacuum area60. The heat-insulating member may be made of glass fiber62or porous ceramics64provided to surround the inner wall27, as shown inFIGS. 4 and 5.

Further, in order to enhance the heat-insulating effect, the inner surface of the outer wall28may be coated with a reflective layer or the inner surface may be formed to be rough, so as to reflect the reaction heat generated from the reformation reactor unit21.

The reformed gas is supplied to the reaction vessel25of the carbon-monoxide reducing unit24through the fourth supply line54. At this time, the reformed gas can be supplied to the reaction vessel25by means of the pumping power of the fuel pump33. The reaction vessel25then reduces the concentration of carbon monoxide from the reformed gas through the water-gas shift (WGS) catalytic reaction or the preferential OC oxidation (PROX) catalytic reaction.

Subsequently, the reformed gas is supplied to the first supply pipe13aof the stack10through the fifth supply line55. At this time, the reformed gas may be supplied to the first supply pipe13aof the stack10by means of the pumping power of the fuel pump33.

Simultaneously, the air pump41supplies the external air to the second supply pipe13bof the stack10through the sixth supply line56.

Therefore, when supplied with the reformed gas through the first supply pipe13aof the stack10and supplied with the external air through the second supply pipe13b, the stack10generates electricity, heat, and water in accordance with the following reactions:
H2→2H++2e−Anode reaction
½O2+2H++2e−→H2O   Cathode reaction
H2+½O2→H2O+current+heat   Total reaction

Referring to the above reactions, the reformed gas is supplied to the anode electrode of the MEA12through the separator16, and the air is supplied to the cathode electrode. When the reformed gas flows through the anode electrode, hydrogen is decomposed into electrons and protons (hydrogen ions) in the catalytic layer. When the protons pass through the electrolyte membrane, electrons, oxygen ions, and protons are synthesized to generate water with the help of a catalyst. The electrons generated from the anode electrode do not pass through the electrolyte membrane but are moved to the cathode electrode through an external circuit. Through these processes, electricity, water, and heat are generated.

In the fuel cell system according to the present invention, since the structure capable of recovering the reaction heat generated from the reformation reactor unit is provided, it is possible to enhance the reaction efficiency and the thermal efficiency of the reformer. Therefore, it is possible to further enhance performance and efficiency of the overall system.

Although the exemplary embodiments of the present invention have been described, the present invention is not limited to the above exemplary embodiments, but may be modified in various forms without departing from the scope of the appended claims, the detailed description, and the accompanying drawings of the present invention. Therefore, those skilled in the art would appreciate that such modifications belong within the scope of the present invention.