Fuel processing system

Details Fuel processing system Fuel processing system

2000-07-19

2003-06-03

Langel, Wayne A. (Department: 1754)

Chemistry of inorganic compounds

Hydrogen or compound thereof

Elemental hydrogen

C423S650000, C423S652000, C423S655000, C423S656000, C429S010000, C429S010000

Reexamination Certificate

active

06572837

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods and apparatus for producing substantially pure hydrogen from a hydrocarbon fuel, and in particular, to an apparatus comprising a plurality of fuel processing reactors and hydrogen separation membrane units connected in series.
BACKGROUND OF THE INVENTION
The search for alternative power sources has focused attention on the use of electrochemical fuel cells to generate electrical power. Unlike conventional fossil fuel power sources, fuel cells are capable of generating electrical power from a fuel stream and an oxidant stream without producing substantial amounts of undesirable by-products, such as sulfides, nitrogen oxides and carbon monoxide. However, the commercial viability of fuel cell systems will benefit from the ability to efficiently and cleanly convert conventional hydrocarbon fuel sources, such as, for example, gasoline, diesel, natural gas, ethane, butane, light distillates, dimethyl ether, methanol, ethanol, propane, naphtha, kerosene, and combinations thereof, to a hydrogen-rich gas stream with increased reliability and decreased cost. The conversion of such fuel sources to a hydrogen-rich gas stream is also important for other industrial processes, as well. Several technologies are available for converting such fuels to hydrogen-rich gas streams.
Steam reformers convert hydrocarbons to reformate gas streams that contain hydrogen. Hydrocarbon feedstock and steam are reacted in reactors filled with catalyst (typically nickel-, copper- or noble metal-based), and hydrogen, carbon dioxide (CO
2
), and carbon monoxide (CO) are produced. For example, the following principal reactions occur in the steam reforming of methane (and natural gas):
The overall reaction (I) is highly endothermic, and is normally carried out at elevated catalyst temperatures in the range from about 650° C. to about 875° C. Such elevated temperatures are typically generated by the heat of combustion from a burner incorporated in the fuel processing reactor. Steam reforming is adversely affected by sulfur and/or other contaminants in the feedstock. Accordingly, fuel feed purification may be required prior to steam reforming.
Partial oxidation systems are based on substoichiometric combustion to achieve the temperatures necessary to reform the hydrocarbon feedstock. Feedstock and oxidant (oxygen or air, for example) are reacted to form hydrogen and CO. Taking methane as an example, the process is based mainly on the exothermic partial oxidation of the hydrocarbon.
2CH
4
+O
2
2CO+4H
2
  (II)
Other reactions may also occur, including endothermic cracking and/or pyrolysis, and endothermic reforming with carbon dioxide. Combustion of the feedstock, according to the following reaction, is minimized:
CH
4
+2O
2
CO
2
+2H
2
O  (III)
Partial oxidation is generally performed at high temperatures (1200-1650° C.). The heat required to drive the reactions is typically supplied by oxidizing a fraction of the fuel.
Catalytic partial oxidation systems employ catalysts to accelerate the reforming reactions at lower temperatures. The desirable result can be soot-free operation, since soot is a common problem with non-catalytic partial oxidation approaches, and improved conversion efficiencies from smaller and lighter equipment. However, common catalysts are susceptible to coking by feedstocks that are high in aromatic content at the low steam-to-carbon ratios typically employed.
Autothermal reforming is an approach that combines catalytic partial oxidation and steam reforming. A significant advantage of autothermal reforming technology is that the exothermic combustion reaction (II or III) is used to drive the endothermic reforming reaction (I).
More recently, a plasma reformer process has been developed that employs an electric arc to generate very high temperatures for reforming the fuel. The high temperature conditions avoid the need for catalysts.
In addition to the fuel processing step, other processing steps are generally performed to reduce the sulfur and/or CO content of the fuel gas to meet fuel cell requirements. Absorbent beds may be utilized to remove sulfur-containing compounds from the fuel gas, for example. A water gas shift reactor (“shift reactor”) is often employed to reduce the CO concentration in the fuel gas in order to avoid poisoning of the catalyst employed in the fuel cells and to produce additional hydrogen fuel. In the shift reactor, CO is combined with water in the presence of a catalyst to yield carbon dioxide and hydrogen according to the following reaction:
CO+H
2
O
CO
2
+H
2
  (IV)
In many instances, the reformate stream exiting the shift reactor is often passed through a selective oxidizer, to further reduce the concentration of CO present in the stream.
With respect to reliability and cost, conventional reformers have some disadvantages with respect to fuel cell use. For example, in vehicular applications in particular, conventional reformers tend to be quite large, which impacts material costs and undesirably increases the size and weight of the fuel cell power generation system, as a whole. Several approaches have been used in an effort to reduce the size and weight of hydrocarbon reforming systems without undesirable loss of performance.
For example, a conventional reformer can be followed by a hydrogen separation unit. A hydrogen separation unit employs a hydrogen-permeable membrane material to separate essentially pure hydrogen from the reformed fuel gas (“reformate”). Typically, these membranes are made of palladium or palladium alloy films supported by porous ceramic substrates, but may be made of other materials with high selectivity and high permeability for hydrogen. The reformer is typically operated at a relatively high pressure (for example, 20 to 35 barg) to provide a good hydrogen partial pressure on the high-pressure side of the palladium membrane. However, the amount of feedstock that can be reformed is limited by pressure-related equilibrium considerations. For example, the conversion of methanol to hydrogen is limited to about 92% at 35 barg. As well, the palladium membrane is limited as to the amount of hydrogen it can produce by such factors as temperature, the hydrogen partial pressure across the membrane, and equilibrium considerations. A typical reformer/palladium system produces in the range of 75% fuel efficiency at optimum conditions.
One approach to increasing the fuel efficiency of such a system is to recycle the retentate from the palladium unit back to the reformer and recover the unreformed fuel and/or hydrogen. However, such recycling systems suffer from several disadvantages. First, they generally include a compressor for the recycling loop, which introduces a parasitic load into the system and also increases its size, cost, and complexity. Second, recycling the retentate effectively dilutes the feedstock. The introduction of diluent gases increases the mass flows and thereby increases the size and cost of the reformer if hydrogen production capacity is to be maintained. Third, the retentate will be enriched in CO (and possibly hydrogen), as compared to the feedstock. As a result, the equilibrium conditions in the mixed feedstock/retentate will be less favorable with respect to hydrogen formation, by LeChatelier's principle. Accordingly, such recycling systems are less than optimal.
A similar approach employs staged hydrogen separation units downstream of the fuel processing reactor. The amount of hydrogen recoverable in a first palladium membrane unit, for example, is limited by the hydrogen partial pressure in the reformate stream and the hydrogen partial pressure differential across the membrane. A second palladium membrane unit is employed to recover some of the hydrogen in the retentate from the first unit. While equilibrium conditions in the second unit favor further hydrogen recovery, the hydrogen partial pressure of the retentate stream may be less than favorable.
Another approach is to incorporate a reformer and

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