US20080023322A1

US20080023322A1 - Fuel processor

Info

Publication number: US20080023322A1
Application number: US11/494,127
Authority: US
Inventors: Robert A. Sinuc; Arne W. Ballantine; Michael D. Gasda
Original assignee: Honda Motor Co Ltd; Plug Power Inc
Current assignee: Honda Motor Co Ltd; Plug Power Inc
Priority date: 2006-07-27
Filing date: 2006-07-27
Publication date: 2008-01-31

Abstract

An electrochemical cell includes an anode, a cathode and a proton conductor. The anode includes a catalyst to reform a hydrocarbon to generate hydrogen at the anode. The proton conductor is in electrical contact with the anode and cathode to receive an applied voltage to cause protons to be transferred from the anode to the cathode to produce hydrogen at the cathode.

Description

BACKGROUND

The invention generally relates to a fuel processor.
A fuel cell is a type of electrochemical cell, which converts chemical energy directly into electrical energy. There are many different types of fuel cells, such as solid oxide, molten carbonate, phosphoric acid, methanol and proton exchange member (PEM) fuel cells.
As a more specific example, a PEM fuel cell includes a PEM membrane, which permits only protons to pass between an anode and a cathode of the fuel cell. A typical PEM fuel cell may employ polysulfonic-acid-based ionomers and operate in the 50° Celsius (C) to 75° temperature range. Another type of PEM fuel cell may employ a phosphoric-acid-based polybenziamidazole (PBI) membrane that operates in the 150° to 200° temperature range.
At the anode of the PEM fuel cell, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
H₂→2H⁺+2e⁻ at the anode of the cell, and Equation 1
O₂+4H⁺+4e⁻→2H₂O at the cathode of the cell. Equation 2
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.

SUMMARY

In an embodiment of the invention, an electrochemical cell includes an anode, a cathode and a proton conductor. The anode includes a catalyst to reform a hydrocarbon to generate hydrogen at the anode. The proton conductor is in electrical contact with the anode and cathode to receive an applied voltage to cause protons to be transferred from the anode to the cathode to produce hydrogen at the cathode.
In another embodiment of the invention, a system includes a voltage source to provide a voltage and a stack of solid proton conductor cells. The stack includes an anode chamber to receive a hydrocarbon flow and a cathode chamber. The stack is adapted to respond to the voltage to produce significantly pure hydrogen in the cathode chamber in response to the hydrocarbon flow and the voltage.
In yet another embodiment of the invention, a technique includes forming an electrochemical cell that has an anode and a cathode. A catalyst is provided at the anode to reform hydrocarbon and a solid proton conductor is provided between the anode and the cathode to transfer protons from the anode to the cathode to produce hydrogen at the cathode.
Advantages and other features of the invention will become apparent from the following drawing, description and claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a fuel processing system.

FIG. 2 is a schematic diagram of a fuel processing system according to an embodiment of the invention.

FIG. 3 is a schematic diagram of a cell structure to reform, compress and purify fuel according to an embodiment of the invention.

FIG. 4 is a schematic diagram of an electrochemical cell that performs reforming, compressing and purifying functions according to an embodiment of the invention.

FIG. 5 is a flow diagram depicting a technique to process fuel according to an embodiment of the invention.

FIG. 6 is a schematic diagram of a fuel cell system according to an embodiment of the invention.

DETAILED DESCRIPTION

A typical fuel cell produces electricity in response to reactants that are flowed through the fuel cell. For example, a proton exchange membrane (PEM) fuel cell receives hydrogen and oxygen and produces electricity in accordance with Equations 1 and 2 above in response to the reactants. The hydrogen for the PEM fuel cell may be provided by a hydrogen storage tank or alternatively, may be derived from a hydrocarbon, such as natural gas.
From an efficiency standpoint, it may be desirable to operate the fuel cell using substantially pure (99% by volume, for example) and compressed (150 pounds-force per square inch gauge (p.s.i.g.), for example), hydrogen. FIG. 1 depicts a fuel processing subsystem 10 that may be used for purposes of producing such as hydrogen flow for a fuel cell. The fuel processing subsystem 10 includes a reformer 20 that receives a hydrocarbon flow at its inlet and oxidizes the hydrocarbon flow to produce a reformate flow at its outlet 26. The reformate may contain, for example, approximately 50% hydrogen by volume. The fuel processing subsystem 10 may include a reformate compressor 30 which pressurizes the incoming flow to produce a pressurized stream of reformate at its outlet 32. For example, the reformate compressor 30 may produce a pressurized reformate stream of approximately 150 psig. The fuel processing subsystem 10 may also include a pressure swing adsorber (PSA) 34, which removes impurities from the reformate flow. It is noted that at the outlet 35 of the PSA 34, the flow to the fuel cell is not substantially pure hydrogen. In order for this to occur, the fuel processor 10 may also include, for example, a purifier such as a hydrogen pump. Alternatively, the reformate flow may be provided directly to the fuel cell.
The reformer 20 may use several different types of reforming processes for purposes of producing the reformate flow. For example, as depicted in FIG. 1, the reformer 20 may include an autothermal reactor (ATR) 24. Other types of reforming processes include catalytic partial oxidization (CPO) reforming and various steam reforming processes. As also depicted in FIG. 1, the reformer 20 may include a start-up burner 22 for purposes of adding thermal energy to the reformer 20 to facilitate its start-up.
The fuel processing subsystem 10 may be relatively inefficient and contain various moving parts. Furthermore, the size of the fuel processing subsystem 10 may be relatively large, as compared to the rest of the fuel cell system that receives the fuel stream from the fuel processing subsystem.
Referring to FIG. 2, for purposes of increasing the fuel processing efficiency, eliminating moving parts and lowering the overall fuel processing costs, a fuel processing subsystem 40 may be used in accordance with embodiments of the invention. Contrary to the fuel processing subsystem 10, the fuel processing subsystem 40 includes a combined reformer and hydrogen pump device 50 that converts an incoming hydrocarbon flow (received at inlet 54) into a substantially pure and pressurized (a pressure of about 150 psig) hydrogen flow at an outlet terminal 56 of the device 50. Thus, the device 50 reforms, purifies and compresses hydrogen gas in a single step; and, as further described below, the device 50 contains no moving parts. Additionally, the device 50 is compact in size, resulting in smaller downstream components; and lower cost materials may be used to form the fuel processing subsystem 40, as compared to conventional arrangements. Furthermore, the fuel processing subsystem 40 has a higher thermal efficiency for reforming and compressing.
As described below, in accordance with some embodiments of the invention, the combined reformer and hydrogen pump device 50 is generally an electrochemical cell hydrogen pump that contains a catalyst to reform the incoming hydrocarbon flow and contains a solid state proton conductor to directly extract hydrogen from the reformate to produce the purified and pressurized hydrogen.
More specifically, referring to FIG. 3, in accordance with some embodiments of the invention, the combined reformer and hydrogen pump device 50 may be formed from an electrochemical cell or stack of cells that have a cell structure 100. The cell structure 100 is part of an electrochemical cell and includes an anode 110 and a cathode 130. The anode 110 includes, for example, a gas diffusion layer (GDL) 114 that receives a hydrocarbon feedstock flow, which may contain, for example, oxygen and methane (for embodiments in which the incoming hydrocarbon flow is natural gas). The anode 110 includes a catalyst layer 120 to reform the hydrocarbon at the anode 110. More particularly, the reforming of the methane at the anode may be described by the following equation:
O₂, CH₄→H₂O+CO_← ^→H₂+CO₂ Equation 3
Thus, the above-described reformation produces hydrogen gas at the anode 110. A solid state proton conductor 146 of the cell structure 100 separates the anode 110 and the cathode 130. As an example, the solid state proton conductor 146 may be yttrium-doped barium cerate, in some embodiments of the invention. Because the structure 110 receives an applied voltage across the anode 110 and cathode 130 (applied by a voltage source 150), the structure 100 functions as an electrochemical hydrogen pump. In this regard, in response to the applied voltage, protons (supplied by the hydrogen gas as the anode 110) migrate from the anode 110 to the cathode 130 across the solid state proton conductor 146. This produces substantially pure (99% pure by volume, for example) hydrogen that diffuses through a GDL 136 at the cathode 130. Furthermore, this pumping action produces the relatively high pressure (150 p.s.i.g., for example) hydrogen at the cathode 130. Pumping hydrogen away from the anode 110 shifts the reaction in favor of further hydrogen production, which allows autothermal efficiency.
As also depicted in FIG. 3, steam diffuses through the proton conductor 146, which may cause coking on the anode 110, if not for measures that are further described below. As also depicted in FIG. 3, the solid state proton conductor 146 may conduct oxygen ions from the cathode 130 to the anode 110 that may lower the pumping efficiency. However, the oxygen ion conduction ratio may be tailored as a function of the cell temperature.
Referring to FIG. 4, in accordance with some embodiments of the invention, the cell structure 100 may be part of an electrochemical cell 160 that includes anode 164 and cathode 168 flow plates that are located on either side of the structure 100. The anode 164 and cathode 168 flow plates includes openings to form anode and cathode plenum passageways for purposes of communicating the reactants, product and exhaust from the electrochemical cell 160. More specifically, the electrochemical cell 160 receives an incoming hydrocarbon feedstock flow 162 (a flow containing methane and oxygen, for example) that flows through the anode flow channels 170 on the bottom surface of the anode flow plate 164 for purposes of communicating the feedstock flow to the cell structure 100. Flow channels 171 on the upper surface of the cathode flow plate 168 communicate the hydrogen from the solid state proton conductor 146, a flow that exits the cathode flow plate 168 via an opening that provides the hydrogen as a flow 169 into the cathode exhaust plenum passageway.
Referring to FIG. 5, a technique 200 may be used for purposes of reforming, compressing and purifying a hydrogen flow for use by a fuel cell-based system in accordance with some embodiments of the invention. Pursuant to the technique 200, hydrocarbon and oxygen are flowed to the anode chamber(s) of electrochemical cell(s), in accordance with block 202. Next, the hydrocarbon is reformed at the anode(s) of the cell(s) to produce hydrogen at the anode(s). Voltage(s) are applied to the cell(s), as depicted in block 206 and protons are conducted through proton conductor(s) of the cell(s) to produce a substantially pure hydrogen flow in the cathode chamber of the cell(s), as depicted in block 208.
Referring to FIG. 6, an electrochemical cell stack 310 may be formed from a stack of the electrochemical cells 160, in accordance with some embodiments of the invention. The electrochemical cell stack 310 may form an overall fuel processing subsystem 300 for a fuel cell-based subsystem 400. As depicted in FIG. 6, the fuel processing subsystem 300 includes a conduit 360 that supplies an incoming hydrocarbon flow and a conduit 362 that supplies an oxygen flow that is mixed with the hydrocarbon flow to form a feedstock for the electrochemical cell stack 310. The oxygen may be produced by an air blower 390 that flows through an oxygen enrichment subsystem 380. In this regard, an oxygen enrichment device, such as a thermal swing adsorber, pressure swing adsorber or oxygen ion pump, may be used to provide relatively pure oxygen for the reforming process. The hydrocarbon feedstock is communicated through a conduit 364 to water content control 370 in accordance with some embodiments of the invention.
More specifically, in order to prevent anode side coking, water may be added to the feedstock flow in accordance with some embodiments of the invention. As examples, the water content control device 370 may be a humidification device, such as a membrane humidifier, spray humidifier, boiler or other such device which captures heat from another portion of the fuel processor 300 and uses it to produce the required steam. For example, the exhaust from a pressure swing adsorber 350 that is downstream of the electrochemical cell stack 310 may be used for purposes of supplying the heat. In other embodiments of the invention, the water content control device 370 may be a humidification device that is obtained via direct injection of the steam which the waste product of the PSA 350. In other embodiments of the invention, anode coking may be prevented by using a wet gas, i.e., a gas that favors water production at the anode.
The flow leaves an outlet 322 of the water content control device 370 and passes through a venturi 320 in accordance with some embodiments of the invention. The venturi 320 is used for embodiments in which anode gas recirculation is used. In this regard, anode exhaust recirculation may be used when the mean free path of reforming reaction is longer than the length of the anode flow channels of the stack 310. By the use of exhaust gas recirculation, the average number of passes through the cells of the stack 310 may be increased until a sufficient fraction of the hydrocarbon is reformed. Thus, the incoming hydrocarbon feedstock flow passes through the main throat of the venturi 320 to an anode inlet 314 of the electrochemical cell stack 310. A venturi inlet 324 of the venturi 320 receives an anode exhaust flow (furnished by an anode exhaust outlet 318 of the electrochemical cell stack 310), and this flow is combined with the incoming flow to produce the flow to the anode inlet 314.
The incoming hydrocarbon feedstock flow is routed through the anode chamber of the electrochemical cell stack to produce hydrogen in the cathode chamber of the stack 310, and this hydrogen exits the stack 310 at the cathode outlet 316. In the context of this application, the “anode chamber” refers to the anode inlet and outlet plenum passageways as well as the anode flow chambers of the various cells of the electrochemical cell stack 310. In the context of this application, the phrase “cathode chamber” refers to the cathode inlet and outlet plenums as well as the cathode flow channels through the various cells of the electrochemical stack 310.
As depicted in FIG. 6, in accordance with some embodiments of the invention, the anode exhaust exits the recirculation loop in a conduit 330 and is provided to a burner 340 that furnishes a relatively low emission exhaust at its outlet 341. The substantially pure hydrogen flow from the cathode exhaust 316 may be routed through the PSA 350, which removes any remaining impurities before furnishing (at its outlet 352) the relatively pure hydrogen flow to the fuel cell-based subsystem 400.
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.

Claims

1. An electrochemical cell, comprising:

an anode comprising a catalyst to reform a hydrocarbon to generate hydrogen at the anode;

a cathode; and

a proton conductor in electrical contact with the anode and the cathode to receive an applied voltage to cause protons to be transferred from the anode to the cathode to produce hydrogen at the cathode.

2. The electrochemical cell of claim 1, wherein the proton conductor comprises a solid proton conductor.

3. The electrochemical cell of claim 1, wherein the proton conductor comprises yttrium-doped barium cerate.

4. The electrochemical cell of claim 1, wherein the anode is substantially flat.

5. The electrochemical cell of claim 1, wherein the anode is substantially tubular.

6. The electrochemical cell of claim 1, wherein

the hydrocarbon comprises methane, and

the catalyst is adapted to react the methane with oxygen to produce the first hydrogen.

7. A system comprising:

a voltage source to provide a voltage; and

a stack of solid state proton conducting cells comprising an anode chamber to receive a hydrocarbon flow and a cathode chamber, wherein the stack is adapted to respond to the voltage to produce significantly pure hydrogen in the cathode chamber in response to the hydrocarbon flow and the voltage.

8. The system of claim 7, wherein each of the cells comprise a catalyst to react the hydrocarbon flow with oxygen to produce hydrogen at an anode of the cell.

9. The system of claim 7, further comprising:

a burner to supply thermal energy to the stack during startup of the stack.

10. The system of claim 7, further comprising:

a first conduit to communicate oxygen to the anode chamber; and

a second conduit to communicate the hydrocarbon flow to the anode chamber.

11. The system of claim 7, wherein the stack comprises an outlet to the cathode chamber to supply said significantly pure hydrogen.

12. The system of claim 11, further comprising:

a purifier located downstream of the outlet to remove water from said significantly pure hydrogen.

13. The system of claim 7, wherein the stack comprises an outlet to the anode chamber to supply an anode exhaust, the system further comprising:

a burner to receive the anode exhaust.

14. The system of claim 7, wherein the stack comprises an inlet to the anode chamber and an outlet to the anode chamber to supply an anode exhaust, the system further comprising:

a feedback path to route at least some of the anode exhaust to the inlet.

15. The system of claim 7, wherein said substantially pure hydrogen comprises ninety nine percent hydrogen by volume.

16. A method comprising:

flowing a hydrocarbon into an electrochemical cell comprising an anode, a cathode and a solid proton conductor; and

inside the cell, reforming the hydrocarbon and conducting protons through the solid proton conductor to produce hydrogen at the cathode in response to the reforming.

17. The method of claim 16, further comprising flowing oxygen and the hydrocarbon into the anode of the cell.

18. The method of claim 16, wherein the proton conductor comprises yttrium-doped barium cerate.

19. A method comprising:

flowing a hydrocarbon and oxygen into electrochemical cells;

providing catalysts in the cells to reform the hydrocarbon flow to produce a first hydrogen flow; and

applying voltages to the cells to produce a second hydrogen flow having a substantially higher hydrogen content than the first hydrogen flow.

20. The method of claim 19, wherein the electrochemical cells are arranged in a stack.

21. The method of claim 19, further comprising:

providing solid proton conductors in the cells to produce the second hydrogen flow.

22. The method of claim 19, wherein the flowing the oxygen comprises enriching an air stream with oxygen to produce an oxygen enriched air stream and flowing the oxygen enriched air stream to the electrochemical cells.

23. The method of claim 19, wherein the flowing the hydrocarbon and oxygen into the electrochemical cells comprises flowing the hydrocarbon and oxygen into anode chambers of the electrochemical cells.

24. A method comprising:

forming an electrochemical cell having an anode and a cathode;

providing a catalyst at the anode to reform hydrocarbon; and

providing a solid proton conductor between the anode and cathode to transfer protons from the anode to the cathode to produce hydrogen in the cathode chamber.

25. The method of claim 24, further comprising:

forming the anode and cathode from substantially tubular members.

26. The method of claim 24, wherein forming the anode and cathode from substantially flat members.