BACKGROUND OF THE INVENTION Field of the Invention The invention concerns a reformate stream cooler in which the surfaces of the cooler that come into contact with the reformate stream are coated with a catalyti- cally active material.

Description of the Related Art Gas generation systems comprising a reformer to generate a reformate stream, a reformate stream cooler, and a downstream shift stage to purify the reformate stream are used in fuel-cell-powered motor vehicles to provide the hydro- gen that is needed to operate the fuel cells. In addition to hydrogen, the reforming of carbon-containing fuels produces by-products such as carbon monoxide, which must only be present in very small quantities in proton-exchange membrane (PEM) fuel cells. Accordingly gas purification upstream of the fuel cell is required. Present technology for the reduction of the carbon monoxide concentration in hydrogen-rich streams include the water-gas shift reaction and the selective oxidation of carbon monoxide in fixed-bed reactors with suitable selective oxidation catalysts. However, at the necessarily low operating temperatures, the water-gas shift reaction is com- paratively slow, so that higher amounts of catalyst are required, leading to larger shift stages and/or increased costs due to the higher noble metal content. The selective oxidation units (for selective oxidation of carbon monoxide) are not able to handle high carbon monoxide concentrations (> 2%).

Moreover, as for example described in US 5,873, 951, the generation of hydro- gen from carbon-containing fuels brings with it the problem of sooting or coking (i. e. the formation of carbon black or carburization). This problem, also known as metal dusting corrosion, is encountered where hot carbon monoxide-containing gas cools on a metal surface and the carbon monoxide breaks down into carbon and carbon dioxide in the air-carbon reaction. In this manner carbides are formed in the metal structure, which leads to the degradation of the material structure. Metal dusting corrosion not only affects steel, but also nickel-based materials, for example. The intensity of the corrosion increases with carbon monoxide partial pressure and the molar carbon monoxide/carbon dioxide ratio at the metal surface.

Metal dusting corrosion can be prevented by carrying out the desired process outside of the critical temperature range for metal dusting corrosion, or by"bypass- ing"the critical temperature range as rapidly as possible. A common method of bypassing the critical temperature range is quenching the reformate stream by introducing water into the reformate stream between the reformer and the shift stage. The disadvantage of this method is that the thermal energy contained in the reformate stream can not then be utilized elsewhere, which leads to a significantly lower efficiency.

To provide a solution to the problem of efficiently reducing carbon monoxide in a compact system, EP 0 974 393 A2 provides a gas generation system comprising a reformer, a carbon monoxide shift reactor, and a catalytic burner. The publication describes that the fuel gas is conducted in counter-current flow, which allows effi- cient cooling of the carbon monoxide shift stage (which shifts the shift gas balance in the reformate stream towards a lower carbon monoxide concentration), in a more compact design.

However, there remains a need for an improved gas generation system for real-life applications that offers consistent high performance and reliability over the entire lifetime of the system, as well as a more compact design.

BRIEF SUMMARY OF THE INVENTION A gas generation system comprises a reformer to generate a hydrogen- containing reformate stream, a reformate stream cooler (or heat exchanger), and a shift stage downstream of the reformate cooler to purify the reformate stream. The surfaces of the cooler that come into contact with the reformate stream are coated with a material that contains at least one catalytically active constituent. The coating is selected such that it also protects against corrosion and sooting in the presence of oxidizing, reducing, and carbon-containing gases. By directly utilizing the coated reformate stream cooler as a catalytically active reactor unit, a water-gas shift reaction to reduce the carbon monoxide concentration takes place to some extent in the cooler prior to the reformate stream entering the actual shift stage. This enables the size of the subsequent shift stage (s) to be reduced, resulting in a more light- weight and compact gas generation system.

These and other aspects will be evident upon reference to the attached Fig- ures and following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow diagram for a conventional system for the reforming of carbon- containing fuels with subsequent gas purification and water injection.

Fig. 2 is a flow diagram for an embodiment of the present system for the reforming of carbon-containing fuels with gas purification and reformate stream cooling.

DETAILED DESCRIPTION OF THE INVENTION In order to be able to reliably operate a gas generation system, for example in a fuel-cell-powered motor vehicle, the carbon monoxide concentration in the refor- mate stream has to be reduced. Reducing the carbon monoxide concentration also suppresses or prevents the reaction that is responsible for formation and deposition of carbon black, which leads to carburization and consequently to the embrittlement of components. Components that are at risk include pipes and channels exposed to the carbon monoxide-containing reformate stream, heat exchangers used to cool the reformate stream, shift stages, nozzles, and all of the components connected down- stream of the reformer, but upstream of the fuel cell. Moreover, some of the catalyst material in these components may be clogged by soot particulates, rendering it unavailable for the catalytic reaction. The transfer of heat and materials in heat exchangers and similar components is also inhibited by the deposits (sooting).

Reactors in gas generation system are typically constructed of high-temperature- resistant alloys, which provide a long service life. But in a carbon monoxide- containing reformate stream atmosphere, iron and nickel, which are constituents of such alloys, exhibit significant catalytic activity with respect to sooting.

Fig. 1 shows a conventional method of preventing metal dusting corrosion.

The critical temperature is bypassed by quenching the reformate stream 4 by intro- ducing water 5 into the reformate stream between a reformer 1 and a shift stage 3.

This process has the disadvantage that thermal energy contained in the reformate stream cannot be used to heat other reactant streams.

To solve this problem, the present gas generation system, which comprises a reformer 1 to generate a reformate stream 4, a reformate stream cooler 2, and a shift stage 3 connected downstream of cooler 2 for reformate stream purification, may be employed. The surfaces of cooler 2 that come into contact with the refor- mate stream 4 are coated with a soot-inhibiting material that is also catalytically active with respect to the water-gas shift reaction.

In order to vary the catalytic activity with respect to the water-gas shift reac- tion and/or the soot-inhibiting activity, the composition of the coating may vary along the flow path. In one embodiment, the coating contains at least two areas of different composition in terms of soot-inhibiting activity and/or catalytic activity with respect to the water-gas shift reaction.

In another embodiment, cooler 2 comprises a feed line and a discharge line, connecting reformer 1 with cooler 2 and cooler 2 with the downstream shift stage 3, respectively. The lines are coated, with a composition that exhibits greater soot- inhibiting activity and lesser catalytic activity with respect to the water-gas shift reaction than the coating inside cooler 2.

In another embodiment, cooler 2 and the feed line and the discharge line are coated with a material of the same composition, and cooler 2 is coated with an additional material, selected to exhibit high catalytic activity with respect to the water-gas shift reaction and lower soot-inhibiting activity than the common coating.

The following references relating to the exhaust gas/catalytic converter area of internal combustion engines provide information on chemical substances and/or chemical compounds that may be used to promote certain reactions, and/or to suppress, or even prevent, other chemical reactions : WO 00/33408, EP 414 573 B1, EP 427 493 A2, EP 637 461 Al, WO 93/01130, EP 305 119 B1, EP 428 753 B1, EP 21325 All EP 630 289 B1, EP 238 700 Bl, US 4,503, 162,"Promotion of the Water Gas Shift Reaction by Cesium Surface", Ind. Eng. Chem. Fundam. 1986,25, 36-42, "Reactant-promoted reaction mechanism for Water-Gas Shift Reaction on RH-doped Ce02", Journal of Catalysis 141,71-81 (1993),"Thermal stability of oxygen storage properties in a mixed CeO2-ZrO2 system", Applied Catalysis B: Environmental 16 (1998) 105-117.

If, for example, y-AI203 is used as catalyst support material, then the oxygen- ion-conducting support material is thermally stabilized against surface losses by the addition of oxides such as Zr02 and/or Ce02. On the other hand, the addition of a cerium compound to the catalytic material leads to an increased activity of the catalytic material for promoting the water-gas shift reaction and acting as an. oxygen storage unit.

Adding caesium compounds to the coating in the form of alkaline doping and/or adding further alkali oxides or alkaline earth oxides and/or metal oxides of the IIb subgroup also increases the activity of the coating with respect to the water-gas shift reaction. Caesium compounds that may be used include caesium oxides or other oxygen-containing compounds that are converted to oxides at high temperatures, such as carbonate, acetate, and nitrate.

Vanadium components that may be used in the production of the catalyst, in- dividually or in the form of mixtures, include various vanadium oxides or vanadium compounds that are converted to vanadium oxides when heated in contact with air.

Suitable vanadium oxides include V205, V307, V409, and V6O13, as V204, and oxides such as V203, V305, V407, V509, Vein, and V7013, with V205 being particularly preferred. Vanadium oxides of this type may be produced, for example, by the thermal decomposition of ammonium metavanadate (NH4VO3), by the heating of mixtures of V203 and V205, or by the reduction of V205 with sulphur oxide gas.

Suitable vanadium compounds that are converted to a vanadium oxide when heated in contact with air include ammonium metavanadate, vanadyl sulphate, vanadyl chloride, vanadyl dichloride dihydrate, vanadyl trichloride, other vanadyl halides, metavanadic acid, pyrovanadic acid, vanadium hydroxide, vanadyl acetylace- tonate, or vanadyl carboxylates, such as vanadyl oxalate, with ammonium metava- nadate being particularly preferred.

Oxides, such as V205 and Si02, act as protective agents against aging of the noble-metal catalyst in the coating as they are structural promoters, which-on account of their thermally stabilizing action-inhibit structural changes of the noble- metal catalyst during manufacturing of the coating and during operation of the gas generation system. The noble-metal catalyst may comprise at least a metal, a metal- containing compound, and/or a metal-containing alloy.

The catalytically active coating may be applied to cooler 2 using processes known in the art, such as dip-coating, spraying, doctor blade application, or other application processes. The coating, as well as being suitable to reduce or prevent the formation and deposition of carbon black and subsequently the carburization and embrittlement of the components, may also make it possible to suppress methana- tion that takes place as an undesired secondary reaction. By directly utilizing the coated cooler as a catalytically active reactor, a water-gas shift reaction to reduce the carbon monoxide concentration takes place to some extent in cooler 2 prior to the actual shift stage 3. This results in some hydrogen production occurring in cooler 2 in addition to the hydrogen production which occurs in the reformer. This makes it possible to reduce the size of the subsequent shift stage (s), which in turn results in a more lightweight and compact gas generation system. Moreover, the service life of the catalysts employed in the shift stages is increased.

The overall efficiency of the system increases, since the coating of cooler 2 makes it possible to extract a significant amount of heat from the reformate stream 4. The reaction temperatures in cooler 2 are typically in the range of 300 to 850°C, preferably between 400 and 700°C. A reformate stream cooler coated in this man- ner is able to adjust the thermodynamic equilibrium with respect to the water-gas shift reaction suitable for the discharge conditions, such as temperature, pressure, stoichiometric ratios (water/carbon monoxide ratios), while at the same time prevent- ing sooting.

The surfaces of cooler 2 that come into contact with the reformate stream 4 may be coated with at least one layer of a further base coating. This base coating is disposed between the wall of the cooler and the coating layer and comprises at least one metal, typically chromium, silicon, aluminum, magnesium, manganese, titanium, rare earths, and/or compounds and/or alloys of these substances. The base coating or"diffusion coating"may be applied to cooler 2 in a manner known in the art.

Various processes that are appropriate for corrosion-and wear-resistant high- temperature composite materials may be employed. Examples of possible processes include, but are not limited to, build-up welding, thermal spray processes, such as plasma spraying, vacuum plasma spraying, plasma-powder surfacing, high-velocity flame spraying, or laser coating. Other examples can be found in US 5,873, 951, US 6,139, 649, US 6,165, 286, and US 5,972, 429. In addition to the advantages already outlined above, the diffusion coating of the present system offers excellent corrosion protection as well as protection against sooting in oxidizing, reducing, and carbon- containing gases.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.