Patent Application
20120014864
An autothermal reformer has a very short mixing and distributing section (such as between about 3% and about 30% of the length of the catalytic bed, which comprises a high surface area, monolithic, open cell foam, the catalytic bed comprising low-pressure drop unified foil or honeycomb. The short, open cell foam section is adequate to thoroughly mix and completely distribute the incoming steam and/or fuel gas and air across the entire cross section of the bed; the monolithic foil or honeycomb bed having low pressure drop and being effective in the reforming process.
Fuel cell power plants include fuel gas steam reformers which are operable to catalytically convert a fuel, such as natural gas or heavier hydrocarbons, into the primary constituents of hydrogen and carbon oxides. The conversion involves passing a mixture of the fuel gas and steam through a catalytic bed which is heated to a reforming temperature by heat transfer through metal walls. The required catalyst temperature varies depending upon the fuel being reformed. Catalysts used in hydrocarbon fuel reformers are typically nickel, and for low temperature methanol reformers, are typically copper or copper-zinc.
Autothermal reformers are often used when higher operating temperatures are required for the reforming process. In an autothermal reformer, the reaction gases are heated by burning excess fuel with air added within the reaction bed to the fuel and steam mixture so that the remaining fuel-steam mixture is raised to the temperature necessary for the fuel processing reaction. This eliminates the need for heat transfer through very high temperature metal walls. Typically, catalyst temperatures in an autothermal reformer are in the range of about 1,400° F. (760° C.) to about 1,800° F. (980° C.); however, for oxygenated fuels the reaction temperatures are in the range of about 500° F. (260° C.) to about 800° F. (430° C.). In order to transfer heat to the catalyst at the temperatures required for reformation of non-oxidized fuels, wall temperatures in a conventional reformer would be around 2,000° F. (1090° C.).
In U.S. Pat. No. 6,746,650, the catalyst bed structure is formed from a monolithic open cell foam core which is provided with a porous high surface area wash coat layer onto which the catalyst layer is deposited, as described in U.S. Pat. No. 6,140,266. The wash coat may be alumina, lanthanum-stabilized alumina, silica-alumina, silica, ceria, silicon carbide, or another high surface ceramic material. The choice of wash coat will depend on the operating parameters of the specific catalyst bed.
The interconnected open cells of the monolithic foam are impregnated with a catalyst either after the wash coat is deposited or simultaneously with the wash coat. The foam monolith may have an entry end portion which is coated with a catalyst consisting of lanthanum-promoted alumina, calcium oxide, and an iron oxide catalyst which can also be treated with a small amount of platinum, palladium or rhodium for improved low temperature fuel gas ignition. As an alternative configuration, the entry end may include a catalyst of platinum, palladium or rhodium without the iron oxide catalyst. The remainder of the foam monolith may be provided with a copper or copper-zinc catalyst, or with such noble metal catalysts such as platinum, palladium, rhodium, or the like, depending on the fuel. The open cell foam, once wash coated, provides the high surface area of catalysts needed to properly process the fuel gas. The open cell foam also provides an enhanced mixing and distribution gas flow pattern for gases passing through the monolith since the gases will flow both laterally and longitudinally through the structure. However, the tortuous passages in the ceramic foam result in high pressure drop of up to several psi.
Many fuel cell power plant designs operate at or near atmospheric pressure, making it difficult to accommodate the pressure drop in an autothermal reformer having the ceramic foam catalyst bed of the aforementioned patents.
A monolithic, open cell foam core not only mixes the incoming steam and/or air and fuel gases together very effectively, that is, achieving complete mixture in a very short longitudinal distance along the path of mass flow of the gases, the monolithic, open cell foam core also distributes the mixture laterally, over a very large cross sectional area within a short longitudinal distance. Stated alternatively, it is now known that the monolithic, open cell foam core promotes full mixture and cross sectional distribution of the gases in as little as several percent of the total length of an autothermal reformer.
An autothermal reformer suitable for use with fuel cell power plants operating at or near atmospheric pressures is achieved by having most of the autothermal reformer employ a catalyst bed having a very low pressure drop, such as a unitized foil or honeycomb catalyst support or bed, preceded by a very short section of monolithic, open cell foam which is sufficiently short that the pressure drop thereacross is insignificant, but the mixing and distribution of which is adequate for the reformation process within the low pressure catalyst bed. The length of the open cell foam is selected to be just enough for thorough mixing and complete spreading.
Other variations will become apparent in the light of the following detailed description of exemplary embodiments, as illustrated in the accompanying drawings.
In
In the illustrated embodiment, the support 120 is a unitized foil catalyst bed. This is formed from thin sheets of steel, one sheet being flat, and an adjacent corrugated sheet being affixed thereto. The sheets are given a wash coat of high surface area alumina, and then impregnated with a suitable catalyst. Alternatively, the catalyst may be applied in the wash coat with the alumina. The various catalysts which may be utilized are described hereinbefore and in the aforementioned patents. The flat sheet with the corrugated sheet attached thereto, with catalysts disposed over all of the surfaces of both sheets, are then rolled up so as to form a cylindrical bed (as seen in
Alternatively, a honeycomb support can be formed directly by extrusion of a ceramic “mud”, which is then fired at a high temperature to sinter the ceramic to a solid monolithic honeycomb support for the catalyst.
The catalyst beds 102, 120 are contained in an open-ended cylindrical inner housing 12. An intermediate cylindrical housing 16 surrounds the inner housing 12 and forms an inner annular gas flow path 18 for gas exiting from the catalyst bed 120. An outer cylindrical housing 20 forms the outermost wall of the reformer assembly 103. The outer cylindrical housing 20 combines with the intermediate cylindrical housing 16 to form an outer annular gas flow path chamber 22 in the assembly 103. The bottom of the assembly 103 is closed by a lower annular wall 24, and the upper end of the assembly 103 is closed by an upper annular wall 26 and a manifold 28. The inner housing 12 can be insulated to prevent heat loss to the stream in the annulus 18, and to keep housing 12 from reaching the high catalyst temperature.
The manifold 28 includes an upper fuel/steam-inlet chamber 30 and a lower air-inlet chamber 32. The chambers 30 and 32 are separated by a plate 34 having a plurality of passages 36 formed therein (
The fuel/steam mixture enters the reformer assembly 103 via a heat exchange tube 44, as indicated by arrow A. The temperature of the incoming fuel/steam mixture is a minimum of about 300° F. (150° C.) to about 450° F. (230° C.) depending on the fuel vaporization requirement. The air stream enters the reformer assembly 3 via a heat exchange tube 46, as indicated by arrow B. The temperature of the entering air stream is similar to the temperature of the fuel/steam stream. The heat exchange tubes 44 and 46 can be finned tubes, or can be covered with a foam material similar to that shown in
The heat exchange tubes 44 and 46 both spiral around the cylindrical housing 16 through the chamber 22 so that heat is transferred to the fuel/steam mixture stream and to the air stream from the reformer exhaust stream, as is described in more detail hereinafter. Alternatively, the spiral tubes could be replaced by other heat exchanger mechanisms which are capable of transferring energy from the processed exhaust gas stream to the inlet streams. By the time that the fuel/steam mixture stream enters the inlet chamber 30, its temperature will have been raised to a temperature in the range of about 300° F. (150° C.) to about 1200° F. (480° C.), and by the time that the air stream enters the inlet chamber 32, its temperature will have been elevated to the same temperature range. The fuel/steam mixture exits the inlet chamber 30 via the tubes 40. The air stream (arrow B) enters the tubes 40 by way of the openings 42 and intermingles with the gas/steam mixture in the tubes 40. Thus, the proper admixture of air, steam and fuel will be formed in the tubes 40 and will flow therefrom into the inlet end 108 of the catalyst bed 102. The temperature of the air, steam and fuel admixture will be in the range of about 300° F. (150° C.) to about 1200° F. (480° C.), depending on the fuel being processed.
The catalyst bed 120 may be divided into different catalyst zones which include an inlet zone 48 and a subsequent zone 50. The inlet zone 48 is typically wash coated with alumina and then catalyzed with calcium oxide, iron oxide, and/or platinum, palladium or rhodium, so as to increase the temperature of the gas mixture by combustion or partial oxidation of the incoming fuel/steam/air mixture to a temperature in the range of about 700° F. (370° C.) to about 1900° F. (1040° C.), depending on whether the fuel is methanol or a heavy fuel such as gasoline.
The subsequent catalyst zone 50 is provided with an underlying wash coat of alumina which is overlain by a copper and/or zinc catalyst or noble metal catalyst. The catalyst zone 50 is operative to convert the fuel/steam and air mixture into a hydrogen-enriched fuel stream which may be further processed to be made suitable for use in a fuel cell power plant. The bed 120 is catalyzed in the following manner. A wash coated porous, high surface area alumina primer is applied to all outer and interstitial surfaces in the bed 120 which are to be catalyzed. The alumina wash coat can be applied to the bed 120 by dipping the bed 120 into a wash coat solution, or by spraying the wash coat solution onto the bed 120. The wash coated bed 120 is then heat treated so as to form the alumina layer on the core. The catalyst layer is then applied to the alumina surfaces of the bed 120. If so desired, the alumina coating and catalyzing steps can be performed concurrently. Similar steps may be used for other wash coating materials.
The reformed fuel stream exits from the catalyst bed 120 at a temperature of about 500° F. (260° C.) to about 600° F. (315° C.) for methanol and 1200° F. (480° C.) to 1500° F. (815° C.) for heavy hydrocarbon fuels, and then passes upwardly through the annulus 18 per arrows C. The exit gas from the reformer passes up through the annulus 18 in order to provide heat to the heat exchange tubes 44 and 46 of the incoming fuel/steam mixture and the incoming air stream. The reformed gas continues to supply heat to these tubes as it travels through the chamber 22, as indicated by arrows D, to an exit passage 52. The reformed gas continues to supply heat to the tubes 44, 46 as it travels through the chamber 22 to the exit passage 52. The reformed gas stream exits the assembly 103 through the passage 52 at a temperature of about 350° F. (175° C.) to about 1200° F. (480° C.), depending on the fuel, whereupon it is further cooled and then passes to a shift converter, a selective CO oxidizer, and the fuel cell power plant assembly.
As noted in
The monolithic, open cell foam catalyst bed may be coated with a reforming catalyst, similar to that used in the inlet zone 48, whenever fuels like gasoline, diesel or kerosene are being reformed, in order to prevent carbon formation on its surfaces (coking).
The short section of monolithic, open cell foam catalyst bed 102 may be used with a long, low pressure bed other than the illustrated monolithic foil, such as the aforementioned conventional, suitably catalyzed honeycomb.
The open cell foam 102 will have a length (in the flow direction) which is no greater than that necessary for the gases at the inlet to become thoroughly mixed, and for the mixture to be dispersed completely across the cross section of the unitized catalyst support 120. The length of the open cell foam 102 may be between about 3% and about 30% of the length of the unitized catalyst support 120, or more likely, it may be between about 5% and about 10% of the length of the support 120. The length of monolithic open cell foam required for adequate mixing and distribution may be determined empirically for any particular design of reformer, number of mixing tubes 40, and the velocity through those tubes, and any given fuel mixture.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US07/16511 | 7/20/2007 | WO | 00 | 1/19/2010 |
