WO2004015800A2

WO2004015800A2 - Reformate cooling by water injectioin

Info

Publication number: WO2004015800A2
Application number: PCT/US2003/025468
Authority: WO
Inventors: Brian Bowers; Jennifer W. Rumsey; Srinivasa Prabhu
Original assignee: Nuvera Fuel Cells, Inc.
Priority date: 2002-08-13
Filing date: 2003-08-13
Publication date: 2004-02-19
Also published as: AU2003262662A8; AU2003262662A1; WO2004015800A3

Abstract

This invention relates to simplified methods of operating a fuel reformer and a fuel cell stack in an integrated way. In particular, it relates to cooling of the hydrogen-rich reformate before it enters the stack to make it suitable for operation of the stack. Specifically, the reformate is cooled to a required temperature by the injection of water to cool the reformate by equilibration of the sensible heat of the reformate with the sensible heat of the water. The cooling water and any condensate are removed before the reformate enters the stack.

Description

REFORMATE COOLING BY WATER INJECTION

RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No. 60/403,119, filed August 13, 2002, the entire teachings of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The creation of electricity by fuel cells by the reaction of hydrogen with oxygen is of increasing interest because of its potential to create no emissions but water. However, in small or mobile applications, it can be inconvenient to supply pure hydrogen. Instead, a hydrocarbon or oxygenated hydrocarbon (e.g., an alcohol) fuel is reformed into hydrogen and carbon dioxide by reaction of the fuel with water. The resulting reformate is fed to the fuel cell for the production of electricity.

Polymer electrolyte membrane (PEM) fuel cells are a promising form of fuel cell for mobile and small installations. A principal advantage is their low temperature of operation, which renders them faster to start up and safer to operate. PEM fuel cells currently operate in the range below 100 deg. C, and most typically in the range of about 60 deg. C to 80 deg. C. In contrast, other types of fuel cells, such as molten carbonate and solid oxide fuel cells, operate at over 400 deg. C. They are suitable for fixed base utility operation, but are less practical for consumer use.

However, because of their low temperature of operation, PEM cells are readily poisoned by carbon monoxide (CO), a byproduct of the fuel reforming reaction. Secondary steps of fuel reforming utilize the "water gas shift", which converts CO and water into carbon dioxide and hydrogen. This typically significantly reduces the CO content, but cannot completely abolish it because of chemical equilibrium among the species of molecules. Hence, a terminal CO removal step is usually required.

Terrninal removal of CO is typically done by preferential oxidation (PrOx) of CO with controlled amounts of oxygen and a catalyst. Because of the thermodynamics, it is preferably to conduct this reaction at as low a temperature as possible, for example in the range of 150 to 250 deg. C. Hence, the refonnate is typically cooled before beginning the CO removal reaction. This typically precipitates water, which is provided in excess during the reforming and shifting steps to improve the hydrogen yield. The reformate, now saturated at about 100 to 200 deg. C, is then passed to the PrOx reactor for CO removal. This removal, when performed adiabatically, can raise the temperature of reformate by up to 100 deg. C. The resulting reformate stream may then be well over 200 deg. C, and i any event is normally over 100 deg. C even when the PrOx is cooled, in order to prevent condensation. Such high temperatures can damage currently available PEM membranes, for example by partially dehydrating them during operation, or by altering their degree of crystallinity. Since individual fuel cells are typically connected in series to form a fuel cell "stack", a unit containing dozens or hundreds of individual cells, damage of even a single PEM membrane can render hundreds of cells inoperative. hi current practice, the reformate is cooled by a conventional heat exchanger before entering the fuel cell stack ("stack"). This also precipitates additional water from the saturated refonnate stream. This solution can be awkward in small reformer/stack pairs, particularly in mobile operations, where space is at a premium and part count must be minimized.

SUMMARY OF THE INVENTION We have found that reformate can be cooled to the required temperature by the injection of liquid, particularly water, and even more particularly cold water, into the flowing refonnate stream just before entry into the fuel cell stack. The injected water cools the reformate primarily by having a cooler temperature than the reformate (equilibration of sensible heat), hi the process, the temperature of the reformate can be reduced to a useable temperature, such as 60 to 70 deg. C. All that is required is a source of water that is somewhat cooler than the temperature at which the stack is operated. The differential temperature required depends on details of the system and its operation, h general, at least 2 degrees C, frequently 3 or more degrees C, and often 5 degrees C or more of temperature difference between the cooled reformate and the stack operating temperature is employed. The volume of water required per volume of reformate depends on the temperature difference between the reformate and the water, and on the final temperature to be attained. Moreover, additional water will be precipitated by the reduction of temperature of the refonnate. Hence, recovery of the added water in addition to the precipitated water does not require any additional equipment. It should be noted that, in general, no net humidification of the water is obtained in normal operation, because the reformate is normally saturated with water at temperatures higher than those required for entry into the fuel cell stack. The amount of water required can be controlled by a temperature sensor at the inlet of the stack, so that water injection is increased or decreased to obtain the required temperature, or to place the temperature within an acceptable range. Alternatively or in addition, in a system controlled by a microprocessor, the amount of heat required to be removed can be calculated from other parameters, and the required amount of water is injected without requiring a sensor. A cooling zone, which can comprise a length of pipe, and/or other heat exchange mechanisms can also be used to supplement the cooling effect of the water injection.

An important advantage of the cooler of the invention is that it contributes essentially no pressure drop to the reformate flow path. This can be a critical requirement in low pressure and ambient pressure systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Fig. 1 is a schematic diagram of a reformate cooler according to one embodiment of the invention; and Fig. 2 is a schematic diagram of a preferential oxidation reactor, fuel cell stack, and reformate cooler according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the reformate cooler of the invention' is shown schematically in Figure 1. Reformate (R) leaves a preferential oxidation reactor (PrOx) and enters a passageway (P), which may be a pipe or other conduit. Water (W) is supplied from a source to an injector (1). The injector typically comprises at least one nozzle (N) to spray or to atomize the water into the reformate, thereby improving the efficiency of cooling. This maybe supplemented (or less preferably, replaced) with passive devices to promote mixing and equilibration between the water and the reformate, such as foams, beads, baffles and the like (not illustrated). The injected water, and any additional precipitated water, collects in the trap (T) and returns to a water reservoir (not shown) through a drain (D). Temperature feedback is optionally provided by a sensor (S) near the entrance to the fuel cell stack ("stack"). The amount of water to be injected can be calculated based on properties of the system other than reformate temperature, such as the rate of fuel consumption, or electricity output from the system, and the results of the computation can be used by a controller (C) to drive the injector.

Figure 2 shows the location of the reformate cooler in the fuel cell system, according to one embodiment of the invention. Reformate R enters a cooler/separator and is cooled towards the temperature of the stack by heat exchange with stack coolant. Precipitated water is recovered, and the reformate R enters the PrOx for CO removal. The temperature of the reformate upon exiting the PrOx reaction stage is typically greater than the stack operating temperature. As noted above, this is disadvantageous, since the higher-temperature reformate can damage the fuel cell membrane, such as by partially dehydrating the membrane, or changing its crystal structure.

In the embodiment of the invention shown in Fig. 2, the wann reformate enters a reformate cooler (P) prior to entering the fuel cell stack. Water W also enters the reformate cooler, and leaves the cooler, with condensate, via the drain D. The water mixes with the higher-temperature reformate, thus lowering the temperature of the reformate. The cooled reformate, now at or preferably below the stack temperature, Ts, then enters the stack. Heat is removed from the stack by a cooling loop C, which cools the reformate before the PrOx and is itself cooled in a radiator RAD, which may also cool water for the final reformate cooling.

In use, the reformate is typically at low to moderate pressure, up to about 4 bar (although optionally higher). For a fuel cell stack operating at about 70 deg. C, reformate is injected with a water spray sufficient to cool the reformate to about 2 to 5 degrees cooler than the stack operating temperature. (The cooling can be greater in some designs, for example where stack flooding is a concern.) The temperature is appropriate to avoid damage to the fuel cell stack, while the slight degree to which the incoming refonnate is cooler than the stack operating temperature prevents condensation of water from the refonnate onto components of the stack. The required cooling water will typically be from the system radiator or reservoir, or optionally as supplied water, typically at ambient temperature. Preferably, the temperature of the cooling water is lower than the stack operating temperature. A minimum useful temperature difference between the reformate cooling water and the stack operating temperature is in the range of about 150% to 200% of the required differential between the reformate temperature and the stack temperature. Cooler cooling water is prefened, if available in the system. i steady state operation, the amount of water to supply to the reformate cooler P can be calculated by the system controller from the amount of fuel used, or from other system parameters such as rate of electricity output, and can be interpolated during transients to different load levels. Alternatively, or in addition, a temperature sensor can be used to provide feedback to the system controller or to the device directly controlling water injection. The water injector has been described as a spray nozzle, but is not limited. Any method of mixing the flowing reformate with cooling water is potentially of use in the invention. Examples of mixers include sprayers, atomizers, nebulizers, homogenizers, injectors, and passage of the water and the reformate together through a static or active mixer. Alternatively, the flowing streams can simply be physically mixed, for example by injecting the refonnate into cool water and recovering it. Mixing enhancers, such as particulate media, meshes, foams, baffles, and similar known mixing means, can be used to speed heat transfer from the reformate to the cooling water. Although the cooler shown in Figs. 1 and 2 has a generally horizontal flow configuration, the cooler can be oriented vertically, or in any direction, depending on the details of the particular reformer and stack to be connected. When operated vertically, or in general other than horizontally, cooling water and reformate can flow in the same or in opposite directions in the passage. Cooling can be enhanced by exposing the passageway to the atmosphere, or to another source of cooling gas or liquid. For instance, additional cooling can be provided by passing a cool fluid over the exterior of the passageway. However, with the efficient and easily- implemented reformate cooler of the present invention, the complexity of typical heat exchangers can generally be avoided. It is anticipated that in the near future PEM stacks will be operated at higher temperatures, and eventually at temperatures above 100 deg. C. This method will still be useful for any cooling that the reformate might require, albeit in a different range of temperatures.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

CLALMSWhat is claimed is:

1. A method for cooling reformate before entering a fuel cell stack, the method comprising: producing reformate in a reactor; providing a passageway between the reactor and the stack; flowing refonnate from the reactor into the passageway; injecting liquid into the reformate; and collecting injected liquid and precipitated liquid in a trap.

2. The method of claim 1 wherein the liquid is water.

3. The method of claim 1 wherein producing reformate in a reactor comprises performing a preferential oxidation reaction.

4. The method of claim 1 wherein the amount of liquid injected is controlled to maintain the temperature of the reformate in a desired range.

5. The method of claim 4 wherein a system controller calculates the amount of liquid to be injected.

6. The method of claim 4 wherein a temperature sensor is used to provide feedback to the liquid injection device.

7. The method of claim 4 wherein the system controller calculates the amount of liquid to inject based on values of system parameters.

8. The method of claim 1 wherein additional cooling is provided by passing a cool fluid over the exterior of the passageway.

9. The method of claim 1 wherein the reformate is cooled to a temperature at least about 2 degrees C cooler than the operating temperature of the stack.

10. The method of claim 1 wherein the injection of the cooling liquid into the refonnate is accomplished by at least one of a sprayer, atomizer, nebulizer, homogenizer, injector, and an active or static mixer.

11. The method of claim 1 wherein heat transfer between the liquid and the reformate is enhanced by passive devices in the passageway.

12. A system for cooling refonnate before entrance into a fuel cell stack, comprising: a passageway for flowing reformate between a reactor and the stack; an injector device which injects liquid into the reformate while the reformate flows through the passageway; and a collector for removing injected liquid and precipitated liquid from the passageway.

13. The system of claim 12, wherein the liquid is water.

14. The system of claim 12, wherein the reactor comprises a preferential oxidation reactor.

15. The system of claim 12, further comprising a controller wliich controls the amount of liquid injected to maintain the temperature of the reformate in a desired range.

16. The system of claim 12, further comprising a temperature sensor used to provide feedback to the injector.

17. The system of claim 12, wherem the reformate is cooled to a temperature of at least about 2 deg. C cooler than the operating temperature of the stack.

18. The system of claim 12, wherein the injector device comprises at least one of a sprayer, atomizer, nebulizer, homogenizer, injector, and an active or static mixer.

19. A power system comprising: a fuel reformer that produces a reformate at a first temperature; a fuel cell that uses the refonnate to produce power, the fuel cell having an operating temperature that is less than the first temperature; and an injector device that injects a liquid into the reformate before the reformate enters the fuel cell to cool the reformate to a second temperature, the second temperature being less than or equal to the fuel cell operating temperature.

20. A system for cooling refonnate before entrance into a fuel cell stack, comprising: means for flowing reformate between a reactor and the stack; means for injecting liquid into the reformate before the reformate enters the stack; and means for removing injected liquid and precipitated liquid from the system.