Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2000-09-18
2003-04-22
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000, C429S006000
Reexamination Certificate
active
06551732
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention is directed to fuel cells and, in particular, to an increase in the efficiency of proton exchange membrane (PEM) fuel cells.
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel directly into electric energy in a continuous process. The overall fuel cell reaction typically involves the combination of hydrogen with oxygen to form water. For example, at 25 degrees Celsius and at 1 atmosphere pressure, the reaction
H
2
+1/2(O
2
)=H
2
O
takes place with a free energy change (&Dgr;G) of −56,69 kcal/mole. In a galvanic cell, this reaction produces a theoretical cell voltage of 1.23 volts.
The main types of fuel cells used today include proton exchange membrane (PEM) fuel cells, phosphoric acid fuel cells, alkaline fuel cells, solid oxide fuel cells, and molten carbonate fuel cells.
Fuel cells are limited by their need for pure hydrogen fuel. Most types of fuel cells are sensitive to even small amounts of impurities. A “reformer” is a known device in which a hydrocarbon fuel is mixed with steam, in the presence of a catalyst, to convert the fuel/steam mixture to hydrogen, carbon monoxide, carbon dioxide, water and impurities. Since most known reformers are sensitive to the presence of impurities, impurities such as sulphur are generally removed from the fuel before entering the reformer. Additional mechanisms are required to almost completely eliminate carbon monoxide (CO) and other potentially harmful impurities from the reformer product gas.
In simple form, a fuel cell consists of two electrodes, an anode and a cathode, separated from one another by an electrolyte or ion-conducting membrane. Oxygen is fed over the cathode and hydrogen is fed over the anode, generating electricity as well as heat and water. Fuel cells which use a reformer can use the hydrogen from any hydrocarbon fuel, including natural gas, methanol, or gasoline.
Fuel cells are environmentally friendly due to the near absence of emission of nitrogen oxides, sulfur oxides, hydrocarbons, carbon monoxide etc. Fuel cells have the potential of being thermodynamically more efficient, and therefore have the potential to reduce green house gas (carbon dioxide) emission. Fuel cells with proton exchange membrane (PEM) electrolyte have the benefit of being compact and being able to work at close to ambient temperature (and therefore suitable for quick start). Fuel cells with PEM electrolyte are considered a good potential source of power for future automobiles as well as for disaggregated electricity and heat production with virtually unlimited potential in developing countries undergoing electrification.
The PEM fuel cell technology has advanced to such a degree that a suitcase-sized fuel cell can provide enough power for a car. For example, Daimler-Benz unveiled its fuel cell vehicle, NECAR 3, in September, 1997. This car uses a Ballard PEM fuel cell engine and methanol fuel with a fuel processor and has a 400 kilometer range. The major automobile companies in the world are allocating a lot of resources on PEM fuel cell automobiles development with the hope that commercial production can begin in several years. DaimlerChrysler released NECAR 4 in 1999 and will bring out NECAR 5 in 2000.
Utility companies and machinery manufacturers are also investing in this area in an attempt to commercialize this technology for small power generators. Recently, at least one manufacturer announced that it had increased the power density of a PEM fuel cell to 1.31 kW/liter. The recent progress in PEM fuel cell caused a lot of attention in the media and the investment community.
As indicated above, PEM fuel cells run on hydrogen or hydrogen containing gases. Such gases have to be obtained by processing a conventional fuel through, for example, a steam reforming unit, a partial oxidation unit, or an auto-thermal reforming unit, as are well known. Recently, A. D. Little developed a selective oxidation process that reduces the CO content to below 10-30 ppm so that the hydrogen containing gas produced from such fuel processors can be used in PEM fuel cells without poisoning the fuel cell. Toyota Motor Corporation is now developing a new small methanol reformer which is approximately 600 millimeters long and 300 millimeters in diameter with a reforming section and a carbon monoxide oxidizing section, with a production rate of 600 liters of hydrogen per minute. Improvements have also been made in the more than 20 minutes startup time reported in 1997.
In a typical PEM fuel cell powered by a liquid fuel, the fuel is converted to hydrogen containing gas by a steam reformer, a partial oxidation reactor, or an autothermal reformer. The autothermal reforming process is more compact and efficient, and is likely to start up faster than the steam reformer. Therefore, the autothermal reformer has great potential for being used in fuel cell vehicles (as well as in other applications), especially those powered by hydrocarbons such as gasoline, propane, or natural gas.
In an autothermal reformer, the fuel, an oxygen containing gas, and water vapor are fed to react and form hydrogen, carbon monoxide, and carbon dioxide, with a small amount of one or more impurities, for example, methane. The partial oxidation reaction of the fuel is exothermic and supplies the heat for the endothermic steam reforming reaction. The reformate then is reduced in temperature and undergoes a water-gas shift reaction to react most carbon monoxide with water vapor into hydrogen and carbon dioxide. Then, the gas is further cooled to close to 90 degrees Celsius and goes through a selective oxidation process in which an oxygen containing gas is fed to selectively oxidize the carbon monoxide so that its concentration is less than 10-30 ppm when exiting from the selective oxidation unit. This nearly carbon monoxide free gas is then sent to the anode of the fuel cell.
On the cathode side of the fuel cell, air or another oxygen containing gas is fed through the cathode. At least a portion of the water formed in the fuel cell reaction is brought out by the cathode gas in the form of water vapor. In order to reduce the size, a fuel cell typically works at 2-3 bar, for example, by compressing the air or oxygen containing gas before it is used in the cathode and the fuel processor.
Due to the existence of inert gases in the anode gas, some of the anode gas has to be purged and burned with the some of the vent gas, i.e. effluent gas, from the cathode. The resultant hot gas can be used to supply the heat for steam reforming, or raise steam and/or heating of the feed streams to the fuel process. For example, U.S. Pat. Nos. 4,828,940 and 4,994,331 mention the use of cathode effluent and anode effluent as the oxidant and fuel for the burner that supplies heat for steam reforming as well as for preheating of the cathode effluent. The resultant hot gas can also be used to drive a gas turbine expander which may be mechanically connected with the air compressor. If only a portion of the cathode effluent is used for combustion of the anode purge gas, the remaining portion of the cathode effluent can be used to drive a gas turbine expander. Additional cooling of the fuel cell may be necessary and can be carried out by a cooling water system. The heat of the reformer effluent can be used to raise water vapor.
U.S. Pat. No. 5,360,679 teaches a hydrocarbon fueled fuel cell electric power generation system that producers electrical power from gaseous or liquid hydrocarbon fuels using a fuel cell stack employing ion exchange membranes. A reformer is used to produce a hydrogen-rich gas. The effluent from the fuel cell cathode is not used in the reformer to produce hydrogen for the fuel cell anode.
U.S. Pat. No. 5,976,724 generally teaches a fuel cell power plant that includes an autothermal reformer. An anode gas loop circulates between the anode section of the fuel cell and an an
Air Products and Chemicals Inc.
Chase Geoffrey L.
Maples John S.
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