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.
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 (CO2), 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 650xc2x0 C. to about 875xc2x0 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.
2CH4+O22CO+4H2xe2x80x83xe2x80x83(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:
CH4+2O2CO2+2H2Oxe2x80x83xe2x80x83(III)
Partial oxidation is generally performed at high temperatures (1200-1650xc2x0 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 (xe2x80x9cshift reactorxe2x80x9d) 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+H2OCO2+H2xe2x80x83xe2x80x83(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 (xe2x80x9creformatexe2x80x9d). 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 a hydrogen separation membrane together into a xe2x80x9cmembrane reactorxe2x80x9d. Such an integral arrangement enhances both the reforming and separating functions. Hydrogen formed in the reforming reaction can be continually removed by the separation membrane, thereby creating equilibrium conditions in the reformer favoring hydrogen formation. The formation of hydrogen on one side of the membrane also assists to maintain a hydrogen partial pressure favoring separation. However, this approach has rarely been successful in practice. It greatly increases the complexity of design and also greatly increases the complexity of maintenance of the unit.
Accordingly, it would be desirable to have a hydrocarbon fuel reforming system of relatively simple design, capable of high hydrogen recovery rates, and of adequate reliability, size, weight and cost for use in various industrial applications, including fuel cell applications. Embodiments of the present system address one or more of these concerns.
A fuel processing system is provided comprising:
(a) a primary fuel processing reactor for converting a feed stream to a first reformate stream comprising hydrogen;
(b) a first hydrogen separator located downstream of the primary fuel processing reactor and fluidly connected thereto for receiving the first reformate stream, the first separator comprising a first membrane for separating the first reformate stream into a first hydrogen-rich stream and a first retentate stream; and
(c) a secondary fuel processing reactor fluidly connected to the first separator for receiving and converting the first retentate stream to a second reformate stream comprising hydrogen.
In a preferred embodiment of the present fuel processing system, the first hydrogen separator is fluidly connected to the secondary fuel processing reactor for receiving the second reformate stream in addition to or in combination with the first reformate stream. The second reformate stream may be introduced into the first reformate stream via a compressor or an ejector.
Alternatively, the fuel processing system may further comprise a second hydrogen separator located downstream of the secondary fuel processing reactor and fluidly connected thereto for receiving the second reformate stream, the second separator comprising a second membrane for separating the second reformate stream into a second hydrogen-rich stream and a second retentate stream.
In the present fuel processing system, the primary fuel processing reactor may comprise a steam reformer, partial oxidation reformer, catalytic partial oxidation reformer, autothermal reformer, or a plasma reformer, for example. The secondary fuel processing reactor may comprise any of the foregoing or may comprise a shift reactor. If the feed stream comprises synthesis gas or a reformate stream from a high-temperature reformer, both the primary and secondary fuel processing reactors may comprise shift reactors. In a preferred embodiment, the primary fuel processing reactor is a steam reformer and the secondary reformer is a steam reformer or a shift reactor.
The present fuel processing system may further comprise a fuel supply for supplying fuel to the primary fuel processing reactor, an oxidant supply for supplying oxidant to at least one of the primary and secondary fuel processing reactors, and/or a water supply for supplying water vapor to at least one of the primary and secondary fuel processing reactors. The fuel processing system may also further comprise a heating device for heating the second reformate stream to a temperature within a predetermined temperature range.
The first and second membranes found in the first and second hydrogen separators, respectively, may be independently selected from the group consisting of palladium membranes, palladium alloy membranes, platinum membranes, platinum alloy membranes, titanium alloy membranes, ceramic membranes, zeolite molecular sieve membranes, carbon molecular sieve membranes, inorganic poly-acid membranes, and composite membranes thereof. They may be supported, and may be constructed as plate-and-frame, spiral wound, or hollow fiber modules, if desired. The membranes of the first and second hydrogen separators may be the same or different.
The feed stream may comprise a fuel selected from the group consisting of gasoline, diesel, natural gas, ethane, butane, light distillates, dimethyl ether, methanol, ethanol, propane, naphtha, kerosene, and combinations thereof.
A fuel cell power generation system is also provided. In one embodiment the fuel cell power generation system comprises:
(a) a primary fuel processing reactor for converting a feed stream to a first reformate stream comprising hydrogen;
(b) a first hydrogen separator located downstream of the primary fuel processing reactor and fluidly connected thereto for receiving the first reformate stream, the first separator comprising a first membrane for separating the first reformate stream into a first hydrogen-rich stream and a first retentate stream;
(c) a secondary fuel processing reactor fluidly connected to the first separator for receiving and converting the first retentate stream to a second reformate stream comprising hydrogen; and
a fuel cell stack comprising at least one fuel cell fluidly connected to receive the first hydrogen-rich stream from the fuel processing system.
Another embodiment of the present fuel cell power generation system further comprises a second hydrogen separator located downstream of the secondary fuel processing reactor and fluidly connected thereto for receiving the second reformate stream, the second separator comprising a second membrane for separating the second reformate stream into a second hydrogen-rich stream and a second retentate stream, and the fuel cell stack is connected to receive both first and second hydrogen-rich streams from the fuel processing system. In either embodiment, the at least one fuel cell may be a solid polymer electrolyte fuel cell.
A fuel processing method is also provided, comprising the sequential steps:
(a) supplying a feed stream to a primary fuel processing reactor;
(b) processing the feed stream in the primary fuel processing reactor to produce a first reformate stream comprising hydrogen;
(c) supplying the first reformate stream to a hydrogen separator and separating the first reformate stream therein into a first hydrogen-rich stream and a first retentate stream;
(d) supplying the first retentate stream to a secondary fuel processing reactor and processing the first retentate stream therein to produce a second reformate stream comprising hydrogen; and
(e) supplying the second reformate stream to a hydrogen separator and separating the second reformate stream therein into a second hydrogen-rich stream and a second retentate stream.
In this method, the first and second reformate streams may be supplied to the same hydrogen separator, or the first reformate stream may be supplied to a first hydrogen separator in step (c), and the second reformate stream may be supplied to a second hydrogen separator in step (e).
In the present method, the primary fuel processing reactor may comprise a steam reformer, partial oxidation reformer, catalytic partial oxidation reformer, autothermal reformer, or plasma reformer, for example. The secondary fuel processing reactor may comprise any of the foregoing or may comprise a shift reactor. Where the feed stream comprises synthesis gas or a reformate stream from a high-temperature reformer, both the primary and secondary fuel processing reactors may be shift reactors. In a preferred embodiment, the primary fuel processing reactor is a steam reformer and the secondary reformer is a steam reformer or a shift reactor.
The first and second membranes found in the first and second hydrogen separators may be independently selected from the group consisting of palladium membranes, palladium alloy membranes, platinum membranes, platinum alloy membranes, titanium alloy membranes, ceramic membranes, zeolite molecular sieve membranes, carbon molecular sieve membranes, inorganic poly-acid membranes, and composite membranes thereof. They may be supported, and may be constructed as plate-and-frame, spiral wound, or hollow fiber modules, if desired. The first and second membranes may be the same or different from each other.
The feed stream may comprise a fuel selected from the group consisting of gasoline, diesel, natural gas, ethane, butane, light distillates, dimethyl ether, methanol, ethanol, propane, naphtha, kerosene, and combinations thereof.
The present method may further comprise supplying water vapor, oxidant, or both, to the primary fuel processing reactor, the secondary fuel processing reactor, or both, as desired. In addition, the method may further comprise heating the second reformate stream to a temperature within a predetermined temperature range upstream of the hydrogen separator.
Although these embodiments of the apparatus and methods are described herein as comprising two fuel processing reactors and one or two hydrogen separators, additional reactors and separators may be included. For example, a third fuel processing reactor may be located downstream of the second hydrogen separator and fluidly connected thereto for receiving and converting the second retentate stream to a third reformate stream comprising hydrogen.