Internal-combustion engines – Combined devices – Generating plants
Reexamination Certificate
2001-09-29
2003-01-07
Argenbright, Tony M. (Department: 3747)
Internal-combustion engines
Combined devices
Generating plants
C123S002000, C429S010000, C429S010000, C429S006000, C429S006000
Reexamination Certificate
active
06502533
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESERCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
The proposed concept is related to conversion of liquid and gaseous hydrocarbon and alcohol fuels to product gas containing hydrogen, carbon monoxide, and traces of hydrocarbons that is useable in fuel cells. In particular, it relates to the unique capability of internal combustion engines (ICEs) operated with fuel in excess of the stoichiometric quantity to carry out this fuel conversion process.
The background of the invention includes processes and systems for supplying fuel to fuel cells, the use of internal combustion engines as chemical reactors, and power plants combining these elements.
Fuel cells are electrochemical systems that generate electrical current by chemically reacting a fuel gas and an oxidant gas on the surface of electrodes. Conventionally, the oxidant gas is oxygen or air, and the fuel gas is hydrogen or a mixture of hydrogen, carbon monoxide, and traces of hydrocarbons. The fuel gas may also contain non-fuel gases including nitrogen, water vapor and carbon dioxide. The specific fuel gas composition requirements depend on the type of fuel cell. Low temperature fuel cells, exemplified by proton exchange membrane (PEM) cells and alkaline fuel cells (AFC), can only utilize hydrogen as fuel, and contain precious metal catalysts that are poisoned by carbon monoxide. High temperature fuel cells, exemplified by solid oxide fuel cells (SOFC) and molten carbonate fuel cells (MCFC), do not contain precious metal catalysts, and utilize hydrogen, carbon monoxide, and traces of hydrocarbons as fuel. Most fuel cell types are adversely affected by sulfur compounds.
Pure hydrogen is the ideal fuel for all fuel cell types, but it is not widely available. Further, storage and transportation involves large, heavy and costly means such as compressed gas bottles. Practical fuel cell generators must therefore utilize commonly available and easily transported fuels including natural gas, liquefied petroleum gas (LPG), methanol, ethanol, gasoline and diesel fuel, and logistic fuel. These hydrocarbons and alcohols must be reformed to fuel gas suitable for the particular fuel cell application. In addition, these fuels often contain sulfur that must be removed. Conventional processes for desulfurizing and reforming liquid and gaseous fuels are well known in the art, and will only be summarized.
Fuel reforming is based on the endothermic reaction of hydrocarbon or alcohol fuel with steam and/or CO
2
, to form CO and H
2
. This can be done in two ways. The first is steam reforming. Steam reformers use high temperature catalyst filled tubes heated by burners fueled by fuel cell exhaust fuel and air streams. Steam is supplied by a waste heat boiler. Heat transferred across the tube wall drives the endothermic reaction. Such systems provide the highest hydrogen yield, but tend to be large, complex, and slow to start up and respond to load changes. Further, they require sulfur removal from the feedstock to avoid catalyst poisoning. The second is partial oxidation (POX) reforming. POX reformers and catalytic autothermal reformers eliminate high temperature heat exchangers by reacting a rich mixture of fuel and air to provide the reforming heat within the gas stream. Steam is added to the hot hydrogen and carbon monoxide to cool the stream and increase hydrogen yield. Non-catalytic POX reformers operate at temperatures around 1000° C. for gasoline and up to 1400° C. for heavy hydrocarbons, necessitating special heat-resistant materials. Autothermal reformers use a catalyst to operate at temperatures under 1000° C., and may be less costly. These systems are smaller, simpler and faster responding than steam reformers, and are preferred for applications such as vehicle propulsion. Even so, there is a delay before power is available in a cold start and the feedstock must be low in sulfur.
Generally heavier liquid hydrocarbons such as diesel fuel are the most difficult to reform, and have the greatest tendency to form soot rather than the desired product gas. Further, they are more likely to contain large amounts of sulfur. “Logistic” fuel is an extreme case. It is a low-grade, high sulfur diesel fuel that may be the only fuel available to the military in the field. While reciprocating and turbine ICEs operate directly on logistic fuel, fuel cell power plants require extensive fuel processing capability, resulting in additional size and weight.
The method of sulfur removal depends on both the reforming system and the type of fuel. If the reforming reaction uses a catalyst, then the sulfur is typically removed from the feedstock prior to reforming. Hydrodesulfurization is the classic means used for liquid hydrocarbons. Hydrogen separated from the product gas stream is reacted with the fuel over the catalyst to convert the sulfur compounds to hydrogen sulfide. The hydrogen sulfide is then removed by passing the stream through a zinc oxide bed. Activated charcoal filtration is sufficient to remove sulfur from natural gas before reforming. Non-catalytic POX reformers tolerate sulfur in the fuel, and convert it to hydrogen sulfide that can be removed from the product gas with a zinc oxide bed.
Since low temperature fuel cells can only utilize hydrogen and do not tolerate over 50 ppm CO, shift conversion and selective oxidation stages must be added to increase hydrogen and decrease CO levels. The situation is simpler for high temperature fuel cells. At 600° C. to 1000° C., CO and moderate quantities of hydrocarbons are reformed at the nickel anode surface using the steam, CO
2
and heat from the power generation reaction. The reforming process only needs to break down the heavy hydrocarbons into a mix of gasses that the SOFC can utilize directly or reform internally without soot formation. High-temperature fuel cell systems can therefore use the product gas from steam, autothermal and POX reformers directly.
Startup characteristics are often important in fuel cell power plants operating on hydrocarbon and alcohol. A certain amount of time is needed to start a reformer to generate hydrogen, and high temperature fuel cells require time to heat to operating temperature regardless of the availability of fuel. This delay necessitates an interim power source such as a battery or ICE for applications that require immediate response, such as vehicle propulsion or emergency power.
ICEs include turbine, reciprocating piston or other machines that compress air, heat the air by reacting fuel with the oxygen in the air, and expand the heated air to produce work. The theoretical amount of fuel required to consume the oxygen in the air is termed the stoichiometric quantity. Typically, the amount of fuel added is less than the stoichiometric quantity (a lean mixture), since this makes the most efficient and economical use of the fuel. Fuel in excess of the available oxygen (a rich mixture) is discharged in the exhaust and produces no useful work. The composition of excess hydrocarbon fuel, however, is changed by the combustion process. Rich mixture exhaust contains hydrogen, carbon monoxide, and small amounts of hydrocarbons in addition to nitrogen and water vapor. Oxides of nitrogen (NOX), typical pollutants produced by lean mixtures, are suppressed by the reducing environment created by the rich mixture. In addition, sulfur compounds are converted to hydrogen sulfide. The overall result of rich ICE operation with hydrocarbon fuel is shaft work and almost complete conversion of the excess fuel into product gas containing hydrogen and CO. One of the specific problems with a rich running ICE is the production of soot. The theoretical rich soot formation limit for fuel with a stoichiometric ratio of 14.65 is 5.5, but in a real piston ICE soot formation occurs at higher ratios.
Use of an air/fuel mixture richer than stoichiometric in an ICE is a known technique to produce combustible gas. U.S. Pat. No. 4,041,9
Argenbright Tony M.
Harris Katrina B.
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