Some fuel cell power plants include fuel gas selective oxidizers which are operable to reduce carbon monoxide to low levels in a reformed fuel gas, such as natural gas, before the gas is used as a fuel for fuel cell power plants. The procedure involves passing a mixture of the reformed fuel gas and gaseous oxygen through a catalytic bed which is capable of oxidizing carbon monoxide in an exothermic reaction. The reaction proceeds at controlled temperatures which are within a given range of about 360.degree. F. to about 170.degree. F. The temperature of the catalyst bed must be maintained above a particular threshold temperature which is between about 220.degree. F. to about 360.degree. F. at the entry stage of the catalyst bed, where the gases being treated are relatively rich in carbon monoxide, and will be reduced to lower temperatures of about 170.degree. F. to about 220.degree. F. at latter stages of the catalyst bed where the carbon monoxide content of the gas is lower. However, with good temperature control and heat transfer, the temperature can be as high as 240.degree. F. in the low temperature bed.
The catalysts typically used are platinum catalysts which are deposited on alumina granules. U.S. Pat. No. 5,330,727, granted Jul. 19, 1994 to J. C. Trocciola et al discloses a selective oxidizer assemblage which is proposed for use in a fuel cell power plant and describes the temperature regimes required to properly oxidize the carbon monoxide. The type of oxidizer shown in the aforesaid patent is conventionally referred to as a "shell and tube" heat exchanger.
The shell and tube fuel cell power plant selective oxidizers require a large amount of heat transfer surface area between the catalyst bed and the coolant in order to maintain the controlled temperatures needed to produce the degree of carbon monoxide oxidization required to operate the fuel cells properly. This need for large heat transfer surface area, when met by using catalyst-coated granules requires that the catalyst coated granules be diluted, which results in undesirably large and heavy oxidizer assemblies. For example, a 20 KW acid fuel cell power plant that includes a shell and tube oxidizer component requires a volume of about 4 cubic feet for the oxidizer. Higher power fuel cell power plants, such as 200 KW plants or larger, will require proportionately larger fuel gas oxidizers.
U.S. Pat. No. 5,853,674, granted Dec. 29, 1998 discloses a selective oxidizer assemblage which does not utilize catalyzed pellets, but rather uses a corrugated catalyst bed core which has catalyzed walls. The corrugated catalyst bed core forms parallel passages for the fuel being selectively oxidized, and also forms adjacent parallel coolant passages which are disposed in direct heat exchange relationship with the catalyst bed gas passages. This assemblage is lighter in weight and more compact than a selective oxidizer which uses catalyzed pellets and, because of the very high surface area of the corrugated core, provides very efficient heat transfer between the catalyzed bed passages and the coolant passages. The assemblage is formed from a sequence of essentially flat plates which are sandwiched around the corrugated passages, and the assemblage has a repeating pattern of catalyzed bed passages and non-catalyzed coolant passages. Gas flow reversal manifolds connect the catalyzed bed passages with the coolant passages. While the aforesaid flat plate assemblage makes a significant improvement in the reduction of weight and size, it does not fully meet the requirements and it does not sufficiently mix the flow pattern of the gases passing through it because of the inclusion of the corrugated gas flow passages. Thus the corrugated design provides a more desirable size and weight selective oxidizer assemblage, but the catalyzed pellet design provides a more extensive gas mixing flow pattern.
It would be desirable to provide a selective oxidizer which provides greater heat exchange capabilities in a smaller package. It would be highly desirable to provide a selective oxidizer assemblage for a process gas which is suitable for use in a fuel cell power plant, which selective oxidizer assemblage provides a gas mixing flow pattern of the catalyzed pellets and is compact and light in weight like the catalyzed wall selective oxidizer described above. It would be highly desirable to provide a process fuel gas selective oxidizer which is suitable for use in a fuel cell power plant, which selective oxidizer provides the necessary catalyzed and non-catalyzed coolant surface areas, but is compact, strong, and light in weight.