Fuel processor reactor with integrated pre-reforming zone

Details Fuel processor reactor with integrated pre-reforming zone Fuel processor reactor with integrated pre-reforming zone

2001-03-09

2004-09-21

Johnson, Jerry D. (Department: 1764)

Gas: heating and illuminating

Apparatus for converting or treating hydrocarbon gas

C048S198300, C048S198600, C048S198700, C423S650000, C423S651000, C423S652000, C423S653000, C423S654000, C422S198000, C422S198000, C422S198000, C422S198000, C422S200000, C422S201000, C422S202000, C422S203000, C422S211000, C422S236000

Reexamination Certificate

active

06793698

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a compact apparatus for generating hydrogen. More particularly, this invention relates to a compact hydrogen generating apparatus suitable for use in conjunction with a fuel cell.
BACKGROUND OF THE INVENTION
Fuel cells convert the chemical energy of a fuel into usable electricity via a chemical reaction and without employing combustion as an intermediate step. Like batteries, fuel cells generate DC current by means of an anode and cathode separated by an ion-transmissive medium. The most common fuel cells are based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. At the anode, hydrogen atoms are split by a catalyst into hydrogen ions (protons) and electrons. The hydrogen ions then travel through the ion-transmissive medium to the cathode. At the same time, the electrons move through an external circuit to a load and then to the cathode. There, the oxygen, hydrogen ions and electrons combine to form water.
One benefit of fuel cells is that the hydrogen they require for operation can be obtained in various ways from renewable sources. Another benefit is that the end products of the fuel cell reaction typically are mostly carbon dioxide and water. Thus, fuel cells have several environmental advantages over internal combustion engines, and therefore have been the subject of much recent research.
Fuel cells operate most efficiently on pure hydrogen. But because hydrogen can be dangerous when stored in quantity and because hydrogen has a low volumetric density compared to fuels such as natural gas, methanol, gasoline or diesel fuel, hydrogen for use in fuel cells for stationary uses generally must be produced at a point near the fuel cell, rather than being produced, stored and distributed from a centralized refining facility. In order for fuel materials other than hydrogen to be utilized by fuel cells, generally a fuel processor must be used to release the hydrogen contained in them. Suitable fuel materials for on-site processing into hydrogen include but are not limited to methanol, ethanol, natural gas, propane, butane, gasoline and diesel fuels. Such fuels are conventionally easy to store and there is a nationwide infrastructure for supplying them.
Particularly when the fuel cell is of the proton exchange membrane (PEM) type, the hydrogen gas is delivered to the fuel cell as a “wet”, i.e. water-saturated, gas in order to avoid drying out the membrane. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. Among the earliest PEMs were sulfonated crosswirked polystyrenes. More recently, sulfonated fluorocarbon polymers have been employed. Such PEMs are described in G. E. Wnek et al., New Hydrocarbon Proton Exchange Membranes Based o Sulfonated Styrene-Ethylene/Butylene-Styrene Triblock Copolymers,
Electrochenical Society Proceedings
, Vol. 95-23 (1995), at pages 247 to 251.
Among the methods for producing hydrogen from a fuel material, probably the most common is synthesis gas production, achieved either via steam reforming or partial oxidation. Synthesis gas principally comprises carbon monoxide and hydrogen, but also can contain carbon dioxide and minor amounts of methane and nitrogen. In a conventional steam reforming process, a mixture of desulfurized hydrocarbon feedstock, such as natural gas, and steam are passed at high temperature and elevated pressure over a suitable reforming catalyst, such as a supported nickel catalyst, to facilitate the chemical reaction. When natural gas (methane) is the feedstock, the principal reaction is
CH
4
+H
2
O⇄CO+3H
2
The concentration of each constituent in the synthesis gas depends on the ratio of steam to hydrocarbon passing over the catalyst, and on the temperature and pressure at which the gases leave the catalyst. The steam reforming reaction is highly endothermic (&Dgr;H =kJ/mole) that generally requires a large excess of steam and a significant heat source to move the equilibrium to the right. Fuels typically are reformed at a temperature of from about 750° to about 950° C. (1400° to 1800° F.) and a pressure of from about 100 kPa to about 7 MPa. Generally, an auxiliary fuel source, which can be either a portion of the feed or the residual fuel exiting the anode, is burned to supply by heat transfer from the hot combustion gases the heat necessary for the steam reforming reaction.
Because current fuel cells require nearly pure hydrogen to function effectively, impurities (primarily carbon monoxide) in the reformer reaction products stream must be removed. Hence, the reformer reaction products themselves are usually further subjected to the reversible “water gas shift” reaction in which carbon dioxide and hydrogen are produced from carbon monoxide and steam according to the reaction
CO+H
2
O⇄CO
2
+H
2
Although the water gas shift reaction is somewhat exothermic, the steam reforming process overall remains highly endothermic.
Partial oxidation (POX) reforming also can be used to convert fuel materials into hydrogen; however, this process produces only about 75 percent as much hydrogen compared to steam reforming. The overall partial oxidation reaction for natural gas is
CH
4
+0.5O
2
⇄CO+2H
2
In a typical partial oxidation reformer, a fuel source and air are combined and ignited and then passed through a partial oxidation catalyst to be converted into carbon dioxide and hydrogen. Controlling the ratio of fuel source to oxygen provides a continuous and mildly exothermic reaction. Partial oxidation reforming typically occurs at a temperature of from about 6500 to about 1300° C. and a pressure of from about 1 to about 25 bar. Because the steam reforming reaction is endothermic and occurs only a high temperature, during a cold start of the reforming system, there generally is insufficient hydrogen for the fuel cells until the components of the reformer can be brought up to a sufficient operating temperature. Steam reformers generally have a poor transient response capability. Also, steam reforming processes generally work best on a comparatively large scale, where sophisticated and expensive techniques using volume-intensive equipment can be profitably employed to generate and recover heat. Steam reforming processes thus have not proved to be easily adaptable for use in small-volume, compact systems such as those destined for use in mobile vehicles.
Although partial oxidation reforming processes do not suffer from the drawbacks associated with steam reforming, nevertheless partial oxidation reformers have a different set of problems and thus do not necessarily represent a ready alternative for use in compact systems. For example, fuels produced by partial oxidation reforming contain only about 3545 percent hydrogen, compared to the approximately 70-80 percent hydrogen obtained in fuels produced by steam reforming. Also, the art associated with partial oxidation reformers is not as advanced compared to steam reformers, and it can prove sometimes difficult to find a suitable partial oxidation catalyst for a given feedstock. Thus, many designs based on modifications of steam reforming and partial oxidation processes continue to be proposed.
Systems are known in which certain reforming process components are integrated into a common module. For example, U.S. Pat. No. 5,516,344 discloses a reformer integrated with a shift converter connected downstream of the reformer. A burner associated with the unit combusts a supplied mixture, whereupon the reformer and shift converter are heated by the hot combustion gases.
U.S. Pat. No. 4,925,456 discloses a process and apparatus for producing synthesis gas that employs a plurality of double pipe heat exchangers for primary reforming in a combined primary and secondary reforming process. The primary reforming zone comprises at least one double-pipe heat exchanger-react

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