Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
1998-12-16
2001-09-25
Langel, Wayne (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S650000, C423S652000, C429S010000
Reexamination Certificate
active
06294149
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This application claims the priority of 197 55 813.5, filed Dec. 16, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for operating a system for the water vapor reforming of a hydrocarbon and to a reforming system which can be operated according to this process. The invention particularly relates to a process for operating a mobile system for the water vapor reforming of methanol in a fuel-cell-operated motor vehicle for providing the hydrogen required for the fuel cells and to a system which can be operated in this manner, as well as to an operating process of a corresponding fuel cell system. For reasons of simplicity, the term “hydrocarbon”, in addition to the actual hydrocarbons, will also include their derivatives, such as methanol.
In water vapor process for reforming hydrocarbons, a hydrogen separating stage having a selectively hydrogen permeable membrane separates the hydrogen generated by the reforming reaction in the warmed-up operation from the other constituents of the formed reformate gas. In addition to alternative methods, such as the CO conversion to carbon dioxide by way of a CO oxidation or the so-called CO shift reaction, this represents a method for obtaining a product gas which essentially consists of hydrogen, in which the CO concentration does not exceed a defined low threshold value. This is important, for example, when the product gas is used as the anode gas of a fuel cell system because the carbon monoxide acts as a catalyst poison. The hydrogen separating stage can be connected as a separate unit to the reforming reactor or can be integrated in the reforming reactor.
As known, the water vapor reforming reaction for reforming a hydrocarbon or hydrocarbon derivative, such as methanol, takes place endothermally and at a reaction temperature which is higher than the room temperature. During a cold start of such a system, the water vapor reforming reaction does not immediately provide hydrogen. The system components must first be brought to a corresponding heated operating temperature. However, particularly when the systems are used in motor vehicles, it is desirable to have driving power by the fuel cells available as soon as possible after the triggering of a starting operation of the vehicle and thus also of the reforming system. This requires, in turn, that the reforming system be capable of providing hydrogen as quickly as possible, at as low a cost as possible. Various special measures for the cold start of reforming systems have been suggested for this purpose.
It is known from French Patent Documents FR 1.417.757 and FR 1.417.758 to introduce during a cold start of a water vapor reforming system for methanol first a mixture of methanol and an oxidant into the reforming reactor in order to carry out a corresponding combustion reaction and thus heat the reactor. The oxidant feed is then terminated. The methanol/water vapor mixture to be reformed is fed and the water vapor reforming reaction is started. In the case of the system of French Patent Document FR 1.417.757, a heating space is in thermal contact with the reforming reaction space. In the heating space, residual gas from the reaction space which is not diffused through a separating membrane is non-catalytically burned with oxygen. An analogous measure is described in Japanese Published Patent Application JP 4-321502 (A).
German Patent Document DE 44 23 587 C2, discloses obtaining hydrogen optionally by means of an exothermal partial oxidation and/or an endothermal water vapor reforming of methanol in a reforming reactor filled with a suitable catalyst material, such as a Cu/ZnO material, depending on the control of the feeding of the individual reaction partners into the reactor and the temperature existing there. When the process is carried out appropriately, the two reactions will take place in parallel, in which case an autothermal reaction course can be set.
It is also known to use the anode-side exhaust gas of a fuel cell system fed with hydrogen by a reforming system directly or after an intermediate storage for heating the reforming reactor. See, for example, Japanese Published Patent Applications JP 4-338101 (A), JP 4-160003 (A) and JP 2-160602 (A). Japanese Patent Document JP 4-338101 (A), is used especially for starting the system, and Japanese Patent Documents JP 4-160003 (A) and JP 2-160602 (A), is additionally used for the reforming reaction in the reforming reaction space while heat is additionally generated by a partial oxidation reaction.
In the fuel cell system described in U.S. Pat. Document U.S. Pat. No. 5,248,566, the fuel cells are fed by hydrogen on the anode side. This hydrogen is generated by a partially oxidizing reforming reactor, the anode exhaust gas of the fuel cells is burned in an afterburner while feeding air. The resulting generated heat is used for heating the interior of a motor vehicle which is equipped with the fuel cell system.
Special cold starting measures were also suggested for systems for the water vapor reforming of a hydrocarbon without the use of a hydrogen separating stage. U.S. Pat. Documents U.S. Pat. No. 4,820,594 and U.S. Pat. No. 5,110,559 describe systems for water vapor reforming hydrocarbon in which a burner is integrated in the reforming reactor. The reforming reactor is in thermal contact with the reaction space of the reactor by way of a heat-conducting partition. During cold start, a combustible mixture is burned in this burner at an open flame. In U.S. Patent Document U.S. Pat. No. 5,110,559 the flame originates from the reforming reactor itself, the combustible hydrocarbon to be reformed being fed to the reaction space during the cold start. The hot combustion exhaust gases of the burner integrated in the reactor are guided into a CO shift converter connected in order to heat it and in this manner bring the system to the operating temperature more quickly.
A problem occurs, however, when the process of partial oxidation of the hydrocarbon, i.e., the POX process, is used in connection with a selectively hydrogen-separating member. A sufficiently high operating pressure, typically above 10 bar at the membrane, is required for achieving a sufficient hydrogen diffusion capacity. Simultaneously the POX process requires an oxygen-containing gas, such as air, which must therefore be compressed to the membrane operating pressure, which leads to correspondingly higher costs.
The present invention addresses the technical problem described above by providing a process and a system for the water vapor reforming wherein the system components reach their operating temperature as quickly as possible during a cold start at relatively low cost. Hydrogen can be provided correspondingly rapidly and can optionally be used in fuel cells. The present invention is also directed to a fuel cell system operating process of the initially mentioned type.
By means of the process according to the invention, during a cold start, the reforming system can be brought comparatively rapidly to its normal warmed-up operating condition without major expenditures. A heating operation is carried out wherein first the reforming reactor, which can be designed for POX operation as well as for water vapor reforming is operated at a relatively low pressure in a POX operation. The exothermal POX process generates heat, which, depending on the system construction, is transported via a direct solid-state heat conduction and/or by product gas generated during the partial oxidation as a heat carrier medium into the hydrogen separating stage and heats the membrane there. The product gas emerging from the reactor which, because of partial oxidation of the hydrocarbon, already contains hydrogen, is then transmitted from the hydrogen separating stage to the catalytic burner device and is catalytically burned there. Since the burner device is in thermal contact at least with an evaporator and the reforming reactor, these system components are also rapidly heated. Although supplement
Autenrieth Rainer
Boneberg Stefan
Poschmann Thomas
Schuebler Martin
Wieland Steffen
Crowell & Moring LLP
Langel Wayne
Xcellsis GmbH
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