Patent Application
20080187794
Catalytic reactors are commonly used in fuel cell systems for reacting components in a fluid stream and are typically sized to utilize the entire catalyst to keep costs and pressure drop to a minimum. As a result, such reactors are very sensitive to inlet composition. Uniform gas mixtures such as fuel/air mixtures aid in maintaining uniform operational temperature of catalyst and complete conversion of the gas mixture reactants.
Embodiment of the present invention describe a fuel cell system comprising a fuel cell stack, at least one catalytic reactor and at least one venturi, fluidly connected to said at least one catalytic reactor. A method of practicing an embodiment of the present invention comprises the steps of (a) providing a fuel cell system comprising: a fuel cell stack, at least one catalytic reactor, and at least one venturi, (b) mixing fuel and air in said at least one venturi, and (c) providing a fuel and air mixture from the at least one venturi to said at least one catalytic reactor.
Embodiments of the present invention describe fuel cell systems comprising a fuel cell stack, at least one catalytic reactor and at least one venturi. Preferably, the at least one venturi is fluidly connected to at least one catalytic reactor and functions as a mixer in a fuel cell system. More preferably, the system comprises two venturis which function as mixers for two different catalytic reactors. That is, it is more preferred to have one venturi mixer for each catalytic reactor. As such, for each additional catalytic reactor in a fuel cell system, an additional venturi mixer may be used.
In the embodiments, at least one venturi is configured to deliver a mixture of fluids to a catalytic reactor in a fuel cell system. The mixture of fluids, for example, comprises fresh fuel (natural gas and/or methane) and air, or anode exhaust and air, or anode exhaust and natural gas and air.
Preferably the fuel cell stack is a solid oxide fuel cell (SOFC) stack. A detailed description of a type of SOFC system is described, in U.S. patent application Ser. Nos. 11/491,487 (filed on Jul. 24, 2006) and 11/002,681 (filed on Dec. 3, 2004), both hereby incorporated by reference in their entirety.
“Catalytic reactor” as used herein describes an element in a fuel cell system capable of catalyzing a reaction between reactants conveyed thereto. These reactors typically comprise metal catalyst-containing tubes or other conduits. Catalytic reactors may be located at various places in a fuel cell system. Examples of catalytic reactors include, but are not limited to catalytic partial oxidation (CPOX) reactor and anode tailgas oxidation (ATO) reactor.
A CPOX reactor, for example, may be used in the start-up mode of a fuel cell system, to make the system independent of an external source of hydrogen. In this example, the CPOX unit produces hydrogen, water vapor, CO and CO2 from the air and fuel mixture.
An ATO reactor may comprise any catalytic burner or combustor in which air and a fuel (anode) exhaust streams, from the fuel cell stack are burned. The fuel exhaust stream may possibly be enriched with fresh fuel such as natural gas, propane, ethanol or other hydrocarbons. A non-limiting example of an ATO reactor is shown in U.S. application Ser. No. 11/656,563 hereby incorporated by reference in its entirety, where the ATO reactor is positioned between the outer cylinder and an inner cylinder of a fuel cell system.
The typical design of a venturi involves a conduit with a tapered middle section, although numerous other variations exist. In one example, inlets placed at the tapered region draw fluids into the conduit by virtue of the suction created at the tapered region. As such, a venturi is highly useful as a mixing device, among other things. Depending on the fuel cell system, mixing fluids in a venturi may or may not require the fuel to be at a higher pressure than the venturi outlet.
In one embodiment a venturi comprises a conduit comprising a throat region, an inner wall and an outer wall (i.e. sleeve), said throat region defined by a narrowing of the inner wall. An annular space is formed between the inner wall and the outer wall, said annular space radially circumventing the throat region. At least one inlet is located on said outer wall allowing external access to the annular space. At least one aperture is located on the inner wall connecting the annular space with the interior of the conduit.
Depending on the preferred design of a venturi, the cross-sectional profile of the annular space may be, for example, trapezoidal or rectangular as shown in
An example of a venturi 1 is shown in
An embodiment of the present invention addresses uniform fuel/air mixing and delivery to catalytic reactors used in fuel cell systems, such as SOFC systems. Here, the fuel mixer comprises a venturi. In this fuel mixer, gas with lower volumetric flow (typically fuel) is injected radially through multiple apertures 10 in the venturi sidewall into the higher volume flow stream (typically air.) This results in an evenly-distributed fuel/air mixture delivery to a catalytic reactor. Even distribution is in terms of both concentration and flow rate. The CFD results illustrated in
A venturi may be incorporated at various locations about a fuel cell system.
In one example, the CPOX reactor shown in
Thus in the start-up mode, the inlet fuel stream flows from fuel inlet 16 through valve 15 into venturi 1 where it is mixed with inlet air stream 2. The mixed fuel and air stream is provided to the CPOX reactor 11 via conduit 18. The CPOX reactor produces a hydrogen stream. The hydrogen stream flows into the fuel cell stack 21 thought valve 17. In steady state mode, the valves 15, 17 are switched to bypass the venturi 1 and CPOX reactor 11. In the steady state mode, the fuel cell stack 21 generates electricity from the fuel and air inlet streams provided from respective inlet conduits 19, 23. The stack provides a fuel or anode exhaust stream through conduit 104, and an air or cathode exhaust stream through conduit 102. The fuel exhaust stream is provided into the venturi 101 where it is mixed with an air inlet stream. The air inlet stream may comprise the stack cathode exhaust stream or a fresh air inlet stream. The air streams are controlled by one or more mixers 25. The mixed fuel exhaust and air stream is provided from venturi 101 into ATO reactor 13 via conduit 108 where they are burned and exhausted via conduit 27. The heat from the ATO reactor may be used to heat a reformer, a water vaporizer, fuel or air inlet streams or other system components.
The fluid streams preferably comprise gas phase fuel and air streams. In one alternative embodiment, fuel is supplied to the venturi in the form of a liquid, optionally under pressure. During mixing of the liquid fuel with the other fluids in the venturi, small droplets comprising fuel are formed which are subsequently dispensed from the venturi. The droplets then quickly evaporate to provide fuel in gaseous form. In this manner, fuel can be supplied to the system in liquid form. In a non-limiting example, liquid ethanol (or another alcohol fuel such as methanol) and heated air are supplied to the venturi. The venturi provides ethanol droplets mixed into the heated air stream into the CPOX reactor.
Numerous benefits flow from embodiments of the present invention. For instance, a low fuel side backpressure can be used due to the venturi effect (as compared to traditional injectors.) Also as noted, a more uniform mixing of fuel and air can be achieved. This in turn results in high fuel conversion rates due to evenly dispersed mixture, more uniform catalyst bed temperature, longer catalyst life due to elimination of hot spots, and a smaller catalytic bed.
The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The description was chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.
