Compact Multiple tube steam reformer

A compact, multitube steam reformer converts a fuel into a reformate stream comprising hydrogen. In one embodiment, the reformer comprises a closed vessel and a burner disposed within the vessel. The burner comprises a start fuel manifold for receiving and distributing a start fuel stream, an oxidant manifold for receiving and distributing an oxidant stream, and a burner fuel manifold for receiving and distributing a burner fuel stream. The oxidant manifold comprises a plurality of oxidant distribution tubes, each having an inlet end and an outlet end, disposed in a separator member. The burner fuel manifold comprises a plurality of burner fuel distribution tubes, each having an inlet end and an outlet end. The burner fuel distribution tubes extend through the start fuel manifold and the oxidant manifold and are fluidly isolated therefrom. The outlet end of each of the burner fuel distribution tubes extends into the inlet end of a corresponding oxidant distribution tube, thereby forming a gap between the outer wall of the burner fuel distribution tube and the inner wall of the oxidant distribution tube. The start fuel manifold has one or more openings therein associated with at least a portion of the burner fuel distribution tubes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) As used in this description and in the appended claims, “anode exhaust” means the fluid exiting from the anode side of an operating fuel cell and “cathode exhaust” means the fluid stream exiting the cathode side of an operating fuel cell. Fuel means gaseous or liquid fuels comprising aliphatic hydrocarbons and oxygenated derivatives thereof, and may further comprise aromatic hydrocarbons and oxygenated derivatives thereof. Reformate means a gas stream comprising hydrogen produced from a fuel by a steam reformer. Oxidant means substantially pure oxygen, or a fluid stream comprising oxygen, such as air or cathode exhaust, for example. “Inverted bowl” means any structure having the overall shape of an inverted bowl and topological equivalents thereof (for example, an inverted open box or cylinder are topological equivalents of an inverted bowl), exclusive of any holes in the surface of the structure. FIG. 1 is a sectional side elevation view of an embodiment of the present steam reformer. FIGS. 2 - 4 are sectional side elevation views of various component assemblies of the steam reformer illustrated in FIG. 1 . As shown in FIG. 1 , reformer vessel 100 comprises shell 102 and header 104 . The interior walls (including the top and bottom) of vessel 100 have an insulating layer 106 . Disposed within vessel 100 are vaporizer 110 , and shell 120 containing reforming section 200 (see also FIGS. 2 and 3 ), and burner 300 (see also FIG. 4 ). Vaporizer 110 comprises a heat exchange coil, which may be a finned tube or corrugated tube helical coil, for example. In operation, a pressurized mixture comprising fuel and water is fed from vaporizer inlet 112 and is injected into vaporizer 110 at a pressure in the range of 0.35 to 35 barg, preferably in the range of 0.35 to 14 barg. The fuel/water mixture flows within vaporizer 110 and is heated by heat exchange with the burner exhaust stream exiting reforming section 200 to produce a process stream comprising vaporized fuel and steam. If desired, the fuel, water, or both may be preheated before introduction into vaporizer 110 to assist in vaporization. The reactant stream exiting vaporizer 110 is supplied via vaporizer outlet 114 to reforming section 200 . Referring to FIGS. 1 and 2 , the reactant stream exiting vaporizer outlet 114 is supplied to reactant plenum 202 , where it is distributed via feed tubes 204 to reformer tubes 206 . Reformer tubes 206 are disposed within burner tubes 208 fixed to burner tube plate 210 . As best shown in FIG. 3 , each reformer tube 206 comprises outer tube 212 and inner tube 214 . The upper end of outer tube 212 is fluidly sealed by end cap 216 . End cap 216 and outer tube 212 may form a flat head arrangement, as illustrated in FIGS. 1 and 2 , or they may form other arrangements such as an elliptical head, if desired, as illustrated in FIG. 3 . Seal 218 also fluidly seals the lower end of outer tube 212 in the region between outer tube 212 and feed tube 204 . Inner tube 214 is located substantially concentrically within outer tube 212 and feed tube 204 . It may be maintained in this position by stabilizer members, such as perforated plates 220 and 222 , respectively. Perforated plate 222 may be maintained in position by at least one foot 224 , which prevents plate 222 from sliding down inner tube 214 . Alternatively, or additionally, the stabilizer members may comprise fins 226 connected to inner tube 214 . Outer tube 212 , inner tube 214 , and perforated plates 220 and 222 define catalyst bed 228 . Catalyst bed 228 may comprise catalyst pellets for promoting the steam reforming reaction. If desired, catalyst bed 228 may extend beyond perforated plate 220 . Similarly, inner tube 214 may extend beyond perforated plate 220 . Further, inner tube 214 may also comprise a perforated cap on the upper end thereof, and/or the upper end of inner tube 214 may also be perforated, if desired. Reformer tubes 206 may be on the order of 2.5 to 12.5 cm (1 to 5 inch) in diameter, preferably about 2.5 to 5 cm (1 to 2 inch) in diameter. They may comprise standard, seamless, high-temperature alloy steel tubes for good dimensional control. Reformer tube spacing may be about 0.64 cm to 2.5 cm (0.25 to 1 inch) between outer tube walls, preferably about 0.76 to 1.0 cm (0.3 to 0.4 inch), to create a compact tube array. Burner tubes 208 are slightly larger than reformer tubes 206 to create a small heat transfer gap between burner tubes 208 and corresponding reformer tubes 206 . Burner tubes 208 may be about 0.64 cm to 1.3 cm (0.25 in to 0.5 inch) larger in outer diameter than the diameter of reformer tubes 206 , preferably about 0.64 cm larger in outer diameter. Burner tubes 208 may comprise dimensionally controlled standard, seamless, high-temperature alloy steel tubes. Burner tubes may be of any suitable thickness, and can be in the range of standard sizes of 0.90 mm, 1.3 mm, or 1.7 mm (0.035 inch, 0.049 inch, or 0.065 inch) wall thickness, to create nominal heat transfer gaps of 2.3 mm, 1.9 mm, or 1.5 mm (0.090 inch, 0.076 inch, or 0.060 inch), respectively. Burner tubes 208 may be attached to burner tube plate 210 by any suitable method to reduce or minimize any leakage paths of the burner gas around the heat transfer gaps between burner tubes 208 and reformer tubes 206 . For example, burner tubes 208 may be fully welded, brazed, or mechanically sealed to burner tube plate 210 . Alternatively, burner tubes 208 may be cut square, firmly seated and locally tack welded to burner tube plate 210 . The process stream is directed through the annular passage between feed tube 204 and inner tube 214 and is supplied to catalyst bed 228 . The process stream is flowed through catalyst bed 228 , where the process stream is converted into a hydrogen-rich reformate stream. The reformate stream exits catalyst bed 228 toward the top of reformer tube 206 and is directed down into inner tube 214 . The reformate exits inner tube 214 and enters collection plenum 230 (see FIGS. 1 and 2 ). Disposed within collection plenum 230 is bottom pan 232 , which is substantially in the shape of an inverted bowl. The reformate stream enters collection plenum 230 and is directed through openings 234 in bottom pan 232 and into reformate outlet tube 236 . Catalyst particles or fines entrained in the reformate stream are separated from the reformate stream due to the flow velocity of the stream as it changes direction from first being directed downwardly over bottom pan 232 and then being directed upwardly through openings 234 and then downwardly again into reformate outlet tube 236 . Thus, the quantity of catalyst exiting the present reformer is reduced, and fines are accumulated on the floor of collection plenum 230 and remain for the service life of the reformer. The reformate stream exits the reformer via reformate outlet tube 236 and may be provided to downstream equipment. Reforming section 200 further comprises insulating layers 238 and 240 , respectively. Insulating layer 240 may comprise a dense ceramic inner combustion liner. As best shown in FIG. 4 , burner 300 comprises oxidant manifold 310 , fuel manifold 320 , and start fuel manifold 350 . Oxidant manifold 310 is supplied with an oxidant stream via oxidant manifold inlet 312 . The oxidant is then directed into oxidant distribution tubes 314 , which are disposed within insulating layer 316 . Oxidant manifold inlet 312 , oxidant manifold 310 , and oxidant distribution tubes 314 may be suitably sized to ensure a substantially even distribution of the oxidant stream within each of oxidant distribution tubes 314 . Oxidant manifold 310 may further comprise a distribution plate 318 , comprising perforated plate, for example, to assist in providing a substantially uniform pressure drop across oxidant manifold 310 , if desired. Baffles or other means for providing a substantially uniform pressure drop may also be used, and are known to persons skilled in the technology involved here. During normal operation, burner fuel is supplied to fuel manifold 320 via fuel manifold inlet 322 . The burner fuel is then directed to fuel distribution tubes 324 . Fuel distribution tubes 324 extend through oxidant manifold 310 and into oxidant distribution tubes 314 . Fuel distribution tubes 324 may extend into oxidant distribution tubes 314 such that each pair of tubes co-terminate, as illustrated in FIG. 4 , or they may extend partly into oxidant distribution tubes 314 , if desired. An annular gap may be formed between each of fuel distribution tubes 324 and oxidant distribution tubes 314 . Fuel manifold inlet 322 , fuel manifold 320 , and fuel distribution tubes 324 may also be suitably sized to ensure a substantially equal pressure drop across each of fuel distribution tubes 314 and thus a substantially even distribution of the fuel stream within each of fuel distribution tubes 324 . Alternatively, the positions of the fuel and oxidant manifolds may be reversed, if desired. That is, the oxidant stream entering the oxidant manifold may be supplied to distribution tubes extending through the fuel manifold and into the fuel distribution tubes, and the burner fuel stream entering the fuel manifold may be directed through openings between the fuel and oxidant distribution tubes, respectively. The fuel stream exiting each of fuel distribution tubes 324 mixes with the oxidant stream exiting corresponding oxidant distribution tubes 314 and is combusted in burner cavity 330 (shown in FIG. 1 ) to produce a hot burner exhaust stream. Combustion may be initiated by a conventional ignitor (not shown). Typically, the ignitor is only required to initiate start fuel combustion and thereafter combustion is self-sustaining as long as fuel and oxidant are supplied to burner 300 . Burner 300 may further comprise flame rod 340 for flame detection as part of a burner management system, if desired. The hot burner exhaust stream flows over reformer tubes 206 to maintain catalyst beds 228 at a suitable temperature for the steam reforming reactions. The burner exhaust stream may maintain the exit (top) portion of catalyst beds 228 at a temperature in the range of about 600° C. to about 800° C., preferably in the range of about 625° C. to about 775° C. The burner exhaust stream is directed between burner tubes 208 and reformer tubes 206 and exits burner cavity 330 . The burner exhaust stream then flows past vaporizer 110 , where it is cooled by heat exchange with the fuel/water mixture flowing through vaporizer 110 . The cooled burner exhaust stream is then discharged from vessel 100 via burner exhaust outlet 345 . The present steam reformer incorporates start burner elements into a single multi-element burner design. At start-up, oxidant is supplied to oxidant manifold 310 and start fuel from start fuel inlet 352 is supplied to start fuel manifold 350 . Start fuel is then directed through the gaps 354 between fuel distribution tubes 324 (as best shown in FIG. 4 ) and start fuel manifold 350 . The start fuel is then directed into oxidant distribution tubes 314 and mixes with the oxidant stream therein. The mixed stream exits oxidant distribution tubes 314 , is ignited by the ignitor (not shown), and is combusted in burner cavity 330 to produce a hot burner stream. The radial inflow of oxidant from oxidant manifold 310 into oxidant distribution tubes 314 promotes the entrainment of the start fuel into the oxidant stream. Thus, oxidant distribution tubes direct the start fuel therein, and prevent the start fuel from entering oxidant manifold 310 and creating a combustion/explosion hazard. Although FIG. 4 illustrates the present steam reformer having gaps 354 associated with each of fuel distribution tubes 324 , it is also possible to have gaps 354 associated with a portion of each of fuel distribution tubes 324 . Gaps 354 may comprise annular gaps between each of fuel distribution tubes 324 and start fuel manifold 350 . Alternatively, the communicating surfaces of fuel distribution tubes 324 and start fuel manifold 350 may form one or more discrete gaps 354 around the circumference of each fuel distribution tube 324 . As a further alternative, gaps 354 may comprise one or more discrete holes in start fuel manifold 350 distributed about the circumference of each of fuel distribution tubes 324 . Gaps 354 may be of any suitable dimensions and cross-section. A plurality of gaps 354 may be formed symmetrically or asymmetrically around each of fuel distribution tubes 324 (in this context, reference is made to radial symmetry or asymmetry). One or more discrete gaps 354 around the circumference of each fuel distribution tube 324 allow the local oxidant-to-fuel ratio to be controlled around the fuel distribution tube. This permits, for example, achieving a higher local oxidant-to-fuel ratio for improved flame holding characteristics, especially at high overall oxidant-to-fuel ratios at start-up or overall lean operating conditions. Ignition of the start fuel can be achieved with an electric ignition system, for example. The ignitor may be located within oxidant manifold 310 , within and extending from one of fuel distribution tubes 324 , or within burner cavity 330 . Where the positions of the fuel and oxidant manifolds are reversed (see discussion above), the gaps in start fuel manifold 350 alternatively comprise distribution tubes extending into the inlet ends of the oxidant distribution tubes. However, the arrangement discussed in the preceding paragraph is generally preferred. The hot burner exhaust stream heats reformer tubes 206 , as described. Start fuel is combusted until catalyst beds 228 reach a predetermined temperature, preferably within the desired operating temperature range of the steam reforming catalyst. Once a predetermined temperature has been reached in catalyst beds 228 , supply of start fuel to burner 300 may be interrupted and normal operation may be commenced, as described above. Alternatively, combustion of start fuel may be continued for some time after catalyst beds 228 have reached the desired temperature. For example, in fuel cell applications where the burner fuel for the present reformer is normally anode exhaust, start fuel may continue to be combusted until the associated fuel cell stack is capable of providing sufficient anode exhaust to sustain combustion in burner 300 , at which time supply of start fuel may be interrupted and normal operation initiated. The present steam reformer may be employed to reform any suitable fuel, particularly paraffinic naphthas and lighter fuels. In stationary fuel cell electric power generation system applications, for example, natural gas (methane) and propane are suitable fuels. The multitube design of the present steam reformer may be preferable to single tube reformer designs for several reasons. For example, for a given size of reformer, smaller reformer tubes may be used. Compared to larger reformer tubes, the thermal expansion of smaller tubes is significantly less, which may reduce catalyst crushing due to thermal cycling. As another example, the use of smaller tubes results in a smaller catalyst bed cross-sectional width relative to comparable single reformer tube designs. Smaller bed width permits lower reformer tube wall temperatures because of increased heat transfer efficiency. As a result, the reformer tubes may be made thinner. Thinner reformer tubes may be lighter and less costly to manufacture than thicker reformer tubes. Further, for a given size of reformer, multiple smaller reformer tubes have a greater surface area than a single larger reformer tube. The greater surface area results in a lower average heat flux, which also permits lower reformer tube wall temperatures, and thus may also permit the use of thinner reformer tubes. As a further example, the present reformer may also permit the use of less catalyst for a given reformer output. Typically, the rate of the steam reforming reaction, and hence the output of the reformer, is limited by the heat transfer rate from the burner to the catalyst bed. Compared to single reformer tube designs, the greater surface area and smaller catalyst bed width of multiple tubes provide for an increased heat transfer rate, and therefore, an increased reformate output for a given amount of catalyst. Thus, the present reformer may achieve a comparable output using less catalyst, compared to single reformer tube designs. This may also translate into cost and weight savings with the present reformer. As another example, multiple reformer tubes may be shorter than a single reformer tube in a reformer of comparable output. Also, in the present steam reformer, fuel is directed to the reformer tubes in a parallel flow arrangement. Shorter tubes and parallel flow of fuel may permit a lower pressure drop across the catalyst beds. As a result, fluidization of the catalyst beds may be reduced or eliminated. In fuel processing system applications, lower-pressure operation of the steam reformer may also increase system efficiency by reducing the parasitic load associated with pressurizing the fuel stream. The burner of the present steam reformer combines start-up and normal operation elements in a single unit having a simple design. In fuel cell-related applications, the burner may normally operate on anode and cathode exhaust, for example, whereas the start fuel is preferably the same as the fuel supplied to the reforming section of the reformer. In fuel cell electric power generation applications, for example, the burner of the present steam reformer is capable of operating on natural gas and air (start-up mode), a reformate and air (transition or “hot standby” mode), and the fuel cell anode and cathode exhaust (normal operation mode). The fuel and oxidant manifolds and associated distribution tubes may use a shell-and-tube construction, for example, for low-cost manufacturing. The burner may comprise an array of distribution tubes, such as a hexagonal array, for example. The reformer section of the present steam reformer also employs shell-and-tube construction that is amenable to low-cost, high-volume manufacturing. The reformer tubes may be arranged in an array having a high packing density, such as a hexagonal array, for example, in order to reduce the size and cost of the reformer. The reformer tubes of the present steam reformer also employ a simplified bayonet regenerative design. Other bayonet regenerative reformer tube designs typically employ three nested tubes, which can be expensive and also has the potential to create problems due to shifting of the tubes during thermal cycling. The present steam reformer comprises bayonet regenerative reformer tubes having a pair of nested reformer tubes. This design may decrease the construction costs associated with the reformer tubes, as well as simplifying thermal cycling-related concerns. The bottom pan of the present reformer also provides a simple and cost-effective system for catalyst fines collection. In addition, the present steam reformer is scalable from about 1 kW to multi-megawatt designs. It is capable of low pressure and high pressure reformer operation, and may be incorporated into fuel processing systems suitable for applications such as fuel cell electric power generation systems and merchant hydrogen production. Finally, as mentioned above, current steam reformers designed for fuel cell applications have typical start-up times of from about one to four hours. The present steam reformer may have a start-up time of as little as 10 to 20 minutes. While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated that the appended claims cover such modifications as incorporate those features which come within the scope of the invention.