Integrated fuel cell system

The invented system includes a fuel-cell system comprising a fuel cell that produces electrical power from air (oxygen) and hydrogen, and a fuel processor that produces hydrogen from a variety of feedstocks. One such fuel processor is a steam reformer which produces purified hydrogen from a carbon-containing feedstock and water. In the invented system, various mechanisms for implementing the cold start-up of the fuel processor are disclosed, as well as mechanisms for optimizing and/or harvesting the heat and water requirements of the system, and maintaining desired the feed ratios of feedstock to water in the fuel processor and purity of the process water used in the system.

FIELD OF THE INVENTION

This invention relates generally to the design and operation of a system for producing electrical power. More specifically, the invention relates to a system for producing electrical power with a fuel cell, especially a proton-exchange-membrane fuel cell (PEMFC).

BACKGROUND AND SUMMARY OF THE INVENTION

Purified hydrogen is an important fuel source for many energy conversion devices. For example, fuel cells use purified hydrogen and an oxidant to produce an electrical potential. A process known as steam reforming produces by chemical reaction hydrogen and certain byproducts or impurities. A subsequent purification process removes the undesirable impurities to provide hydrogen sufficiently purified for application to a fuel cell.

In a steam reforming process, one reacts steam and a carbon-containing compound over a catalyst. Examples of suitable carbon-containing compounds include, but are not limited to, alcohols (such as methanol or ethanol) and hydrocarbons (such as methane or gasoline or propane). Steam reforming requires an elevated operating temperature, e.g., between 250 degrees centigrade and 1300 degrees centigrade, and produces primarily hydrogen and carbon dioxide. Some trace quantities of unreacted reactants and trace quantities of byproducts such as carbon monoxide also result from steam reforming. When a steam reforming unit, or fuel processor, is started from a cold, inactive state, it must be preheated to at least a minimum operating temperature before the above reforming reaction will take place. A need exists for efficient and alternative methods for this preheating of a steam reforming unit. Efficient operation of the fuel processor also requires careful indexing and control of the ratios of water and carbon-containing feedstock. It is also necessary to maintain and control the purity of the water feeds used with the steam reforming unit and fuel cell.

The invented system includes a fuel-cell system comprising a fuel cell that produces electrical power from air (oxygen) and hydrogen, and a fuel processor that produces hydrogen from a variety of feedstocks. One such fuel processor is a steam reformer which produces purified hydrogen from a carbon-containing feedstock and water. In the invented system, various mechanisms for implementing the cold start-up of the fuel processor are disclosed, as well as mechanisms for optimizing and/or harvesting the heat and water requirements of the system, and maintaining desired the feed ratios of feedstock to water in the fuel processor and purity of the process water used in the system.

Many other features of the present invention will become manifest to those versed in the art upon making reference to the detailed description which follows and the accompanying sheets of drawings in which preferred embodiments incorporating the principles of this invention are disclosed as illustrative examples only.

DETAILED DESCRIPTION OF THE INVENTION

As shown inFIG. 1, the invention consists of a fuel-cell system comprising a fuel cell10that produces electrical power from air (oxygen) and hydrogen, and a fuel processor12that produces hydrogen from a variety of feedstocks. Generally, said fuel cell is a net producer of water, and said fuel processor12is a net consumer of water.

Fuel cell10is preferably a proton exchange membrane fuel cell (PEMFC) and may utilize internal humidification of air and/or hydrogen, including so called self-humidification, or external humidification of air and/or hydrogen. Fuel cell10produces byproduct water and byproduct heat in addition to electrical power.

Many feedstocks are suitable for producing hydrogen using fuel processor12including, but not limited to, carbon-containing compounds such as hydrocarbons, alcohols, and ethers. Ammonia is also a suitable feedstock. Fuel processor12preferably produces hydrogen by reacting the carbon-containing feedstock with water by a process commonly known as steam reforming. In this case fuel processor12consumes water in addition to consuming feedstock. It is within the scope of the present invention that other chemical methods for making hydrogen from a feedstock, such as partial oxidation and autothermal reforming, may also be used rather than steam reforming.

FIG. 1is a process flow diagram for one embodiment of a fuel cell system of this invention. The fuel cell10receives hydrogen produced by the fuel processor12. The fuel processor produces hydrogen by reacting, at high temperature, a feedstock from storage reservoir14and water from storage reservoir16. Pump20moves feedstock from reservoir14and delivers said feedstock to the fuel processor12. Likewise, pump21moves water from reservoir16and delivers said water as stream22to the fuel processor12. Pumps20and21deliver the feedstock and water to the fuel processor at a pressure ranging from ambient pressure to approximately 300 psig.

Hydrogen produced by the fuel processor is initially hot because the fuel processor must operate at elevated temperatures of 250° C. to 1300° C. The product hydrogen stream23from the fuel processor is cooled using heat exchanger24and fan26to blow cool ambient air over the hot heat exchanger surfaces. Once cooled to a temperature near to or lower than the operating temperature of the fuel cell, which typically is between approximately 0° C. and approximately 80° C., product hydrogen is passed into the anode chamber28of the fuel cell stack.

Air stream29is delivered to the cathode chamber30of said fuel cell stack10by a blower32. Alternatively, a compressor could also be used in place of blower32. An example of suitable blowers are centrifugal blowers because of their low noise during operation and low power requirements. However, centrifugal blowers are generally limited to relatively low delivery pressure, typically <2 psig. For higher delivery pressures, a linear compressor may be used. Linear compressors are based on an electromechanical (solenoid) drive that is characterized by relatively low power consumption and low noise. An example of a suitable linear compressor is Model Series 5200 sold by Thomas Compressors & Vacuum Pumps (Sheboygan, Wis.).

A coolant circulating loop is used to maintain the temperature of the fuel cell stack within acceptable limits, such as those described above. The coolant serves the purpose of cooling both the cathode and anode chambers of the fuel cell stack. To this end, coolant circulating pump34circulates hot coolant from the fuel cell stack into heat exchanger36. Fan38blows cool air over the hot surfaces of heat exchanger36, thereby reducing the temperature of the coolant. The coolant may be de-ionized water, distilled water, or other non-conducting and non-corrosive liquids including ethylene glycol and propylene glycol.

A pressure regulator40ensures that the pressure of the hydrogen supplied to the anode chamber28of said fuel cell10remains at an acceptable value. For most PEM fuel cells, this range of pressures is between ambient pressure to 4 atmospheres, with a pressure range between ambient pressure and approximately 1.5 atmospheres being preferred. Within the anode chamber of the fuel cell hydrogen is consumed and, at the same time, diluted with water vapor. Thus, a periodic purge of hydrogen-rich gas from the anode chamber is required. Purge valve42serves this purpose. The purge hydrogen represents a small amount of the total hydrogen supplied to the fuel cell, typically only about 1% to 10% of the total. The purge hydrogen stream44may be vented directly to the surroundings, as shown inFIG. 1, or it may be used for the purpose of producing heat, or for other purposes. In some embodiments of this invention hydrogen stream23may be flowed continuously in excess through anode chamber28, eliminating the need for said purge valve42. Since some liquid water may be entrained in said purge hydrogen stream44, an optional water knock-out may be placed in purge stream44for the purpose of separating and collecting said entrained liquid water.

Excess air is continuously flowed through the cathode chamber30. Typically the air flow rate is 200% to 300% of the stoichiometric requirement of oxygen to support the magnitude of electrical current produced by the fuel cell, although flow rates outside of this range may be used as well. Oxygen-depleted air is discharged from said cathode chamber30as stream52. Stream52contains substantial water, as both liquid and vapor, available for recovery. Stream52is typically saturated with water vapor, and as an example, approximately one third or more of the total water may be freely condensed to liquid water. In one embodiment of this invention, stream52is first passed through a knock-out54that separates liquid water from the oxygen-depleted air and water vapor. Liquid water stream56flows out of said knock-out54and the liquid water is collected within water reservoir16. The gas-phase stream58exiting knock-out54comprises the oxygen-depleted air and water vapor.

Stream58is directed into fuel processor12for the purpose of supporting combustion within said fuel processor to generate the required heat for satisfactory operation of the fuel processor (if the fuel processor is based on steam reforming), or to supply oxidant (oxygen) for partial-oxidation of the feedstock (if the fuel processor is based on partial oxidation or autothermal reforming). Since stream58is to be used for combustion, there is no primary reason to cool stream58or stream52, other than to assist with separation of liquid water within said knock-out54.

Still referring toFIG. 1, fuel processor12is preferably a steam reformer with internal hydrogen purification. Examples of suitable steam reforming units are disclosed in pending U.S. patent applications Ser. Nos. 08/741,057 and 08/951,091, which are both entitled “Steam Reformer With Internal Hydrogen Purification” and the disclosures of which are hereby incorporated by reference. As described previously, the process of steam reforming involves the chemical reaction of a feedstock with water at elevated temperature, and is generally known to those skilled in the art. The operating temperature for steam-reforming is generally between approximately 250° C. and approximately 1300° C., and for most common alcohols and hydrocarbons (except methane) is in the range of approximately 250° C. and approximately 800° C. To initially heat fuel processor12during a cold start-up, a suitable fuel such as propane or natural gas is fed from a supply source60to the fuel processor. The fuel is combusted within the fuel processor12until the fuel processor is hot enough to begin steam reforming the feedstock. A throttle valve62regulates the flow of propane or natural gas fuel to the fuel processor during this cold start-up.

Combustion exhaust stream64exits the fuel processor as a hot gas stream laden with water vapor. The water vapor in said combustion exhaust stream64has essentially two sources: as a byproduct of burning the fuel, and as a component of air stream58. It is desirable to recover the water from combustion exhaust stream64and to recover heat from said exhaust stream64. Condenser66serves this purpose. Hot, moist exhaust stream64passes into said condenser66and is chilled using a cold fluid stream68. Streams with temperatures near or less than 20° C. have proven effective. Liquid water condenses and flows out of condenser66as liquid stream69, and is collected in water reservoir16.

Cold fluid stream68is warmed by the process of passing hot exhaust stream64through said condenser66. For example, cold outside air may serve as stream68and be heated for the purpose of space heating in a residential, commercial, or industrial application. Alternatively, cold water may serve as stream68and be heated for use as domestic or process hot water, or said hot water may be used for space heating or other heating applications. Yet another embodiment is that a cold fluid other than air or water including, but not limited to ethylene glycol and propylene glycol, serves as stream68.

Once fuel processor12has reached a suitable temperature for steam reforming the feedstock, feed water and feedstock are pumped into said fuel processor. For methanol, this temperature should be at least 250° C., with temperatures of at least 450° C. and preferably at least 600° C. being used for most hydrocarbon feedstocks. The steam reforming reaction produces a hydrogen-rich reformate gas mixture that is preferably purified within the fuel processor, such as disclosed in our above-identified pending applications, which are incorporated by reference. The pure product hydrogen stream23is passed to the fuel cell as previously described. The hydrogen-depleted stream75that is rejected by the hydrogen purifier is passed through throttle valve78to be used as fuel for combustion to heat said fuel processor12. At this time during the operation of fuel processor12there is no longer a need to supply propane or natural gas fuel that was used for the cold start-up, and that fuel supply is shut off.

FIG. 2is another embodiment of the present invention in which the fuel processor12is heated during a cold start-up by combustion of a liquid fuel, rather than propane or natural gas. The liquid fuel may be diesel, gasoline, kerosene, ethanol, methanol, jet fuel, or other combustible liquids. During a cold start-up, liquid fuel is removed from storage supply100using pump102. The discharged liquid fuel from pump102is admitted through a suitable nozzle or jet into the combustion region in fuel processor12where the fuel is mixed with air and burned to heat said fuel processor. The liquid fuel may be vaporized or atomized prior to injection into fuel processor12to facilitate combustion.

Yet another embodiment of the present invention related to cold startup of fuel processor12is shown in FIG.3. In this case cold start-up is accomplished by combustion of hydrogen fuel within fuel processor12. Hydrogen fuel is stored by within hydrogen storage vessel150by any known method. An example of a particularly well-suited method for storing hydrogen fuel is as a metal hydride. Said metal hydride then comprises a metal hydride storage bed serving as storage vessel150.

Metal hydrides exist in equilibrium with gaseous hydrogen (see F. A. Lewis, “The Palladium Hydrogen System” Academic Press, 1967; and “Hydrogen in Metals I: Basic Properties” edited by G. Alefeld and J Völkl, Springer-Verlag, 1978, the disclosures of which are hereby incorporated by reference). The equilibrium pressure of hydrogen gas over a given metal hydride is a function of the chemical composition of the metal hydride and the temperature of the system. Thus, it is possible to select a metal hydride chemical composition such that the equilibrium pressure of hydrogen over the metal hydride is between 0 psig (ambient pressure) and 10 psig at a temperature of about 15° C. to 22° C. Increasing the temperature of the metal hydride system increases the equilibrium pressure of hydrogen over the metal hydride.

Returning to FIG.3and for purposes of illustration, it is assumed that storage reservoir150contains a suitable quantity of a metal hydride, and is called a metal hydride bed. During a cold start-up, fuel hydrogen stream152is withdrawn from hydride storage bed150and, after passing through isolation valve154, is admitted into fuel processor12where said hydrogen fuel is combusted to heat the fuel processor. As fuel hydrogen is withdrawn from storage bed150, the pressure of gaseous hydrogen in said storage bed will begin to decrease and the bed will begin to cool in temperature (phenomena well known to those skilled in the art of hydrogen storage in metal hydride beds). To counteract these trends, warm combustion exhaust stream64is flowed through metal hydride storage bed150to heat said metal hydride bed. Then, the now cool exhaust exits the warmed metal hydride bed150as cool exhaust stream158. This allows the pressure of gaseous hydrogen to remain sufficiently high to discharge most of, to nearly all of, the hydrogen from said storage bed150.

Alternative embodiments of this invention would use other sources to heat metal hydride bed150including electrical resistance heaters and combustion of hydrogen or other fuel to directly heat storage bed150.

After completing cold start-up of fuel processor12and hydrogen is being produced by the fuel processor, isolation valve154is closed and hydride storage bed150is recharged with hydrogen so that it will be ready for the next cold start-up. Recharging of storage bed150is accomplished by taking a hydrogen slip stream160from purified product hydrogen stream23after said product hydrogen stream has been cooled by passing through heat exchanger24. During this hydrogen recharging operation, byproduct heat should be removed from hydride storage bed150, such as through any known mechanism. An optional isolation valve162is placed in hydrogen slip stream160to facilitate maintenance.

An advantage of this embodiment of the invention is that the fuel required for cold start-up of fuel processor12is clean burning hydrogen, acquired from a previous period of operating the system. Thus, it is not necessary to periodically resupply an auxiliary fuel such as propane or diesel for start-up purposes, nor is it necessary to have a large external storage reservoir for said auxiliary fuels.

FIG. 4presents yet another embodiment of the present invention in which purge hydrogen stream44is passed into combustor200for the purpose of generating additional water to be recovered ultimately by knock-out54and condenser66. Combustor200may be catalytic or non-catalytic. Air to support combustion of purge hydrogen stream44is supplied by the cathode exhaust stream52which is depleted, but not devoid, of oxygen as described previously. The single outlet from combustor200is exhaust stream202that is enriched in water (vapor and liquid) as a result of burning purge hydrogen stream44.

In yet another embodiment of the present invention, heat is recovered in addition to water recovery from combustion of purge hydrogen44.FIG. 5shows combustor200coupled to heat exchanger250for the purpose of recovering and using heat generated by combustion of purge hydrogen stream44within said combustor200. Heat exchanger250may be simply heat-conductive fins on the exterior of combustor200, or a heat exchange fluid may be passed between combustor200and heat exchanger250. Said heat exchange fluid may be circulated based on natural convection currents, or it may be forcibly circulated by a circulation pump. To utilize the recovered heat a suitable cold fluid stream is passed over hot heat exchanger250. One such suitable cold fluid stream is air, in which case fan252blows a cold air stream over heat exchanger250resulting in an increase in the temperature of said air stream. Other suitable cold fluids include, but are not limited to, water, ethylene glycol, propylene glycol, and both the feedstock and feed water to be fed to fuel processor12.

Useful heat can also be recovered from fuel processor12.FIG. 6shows this embodiment of the invention. Heat exchanger300extracts heat from the high-temperature combustion regions of fuel processor12. Pump302may be used to circulate a heat transfer fluid between fuel processor12and heat exchanger300, as shown inFIG. 6, or circulation of said heat transfer fluid may be based on naturally occurring convection currents. Alternatively, heat exchanger300may comprise a series of heat-conductive fins placed on the hot regions of the fuel processor. For purposes of heat recovery and use, a suitable cold fluid is passed over the hot heat exchanger300. Such a suitable cold fluid may be an air stream supplied by fan305. In this case said air stream is heated by passing over hot heat exchanger300. Other suitable cold fluid streams include, but are not limited to, water, ethylene glycol, and propylene glycol.

Another useful embodiment of the present invention is shown in FIG.7. Dual-head pump350supplies both feedstock from reservoir14and feed water from reservoir16to fuel processor12. Dual-head pump350comprises two pump heads driven by a single drive motor such that both pump heads are driven at the same speed over the entire operating speed range of the pump motor. The pumping rate of each feedstock and feed water is determined by the displacement of each respective cavity in dual-head pump350. For example, to preserve a fixed ratio of feed water to feedstock, as is desirable for steam reforming, the dual-head pump may be a gear pump with a ratio of displacement volume of the two pump heads being 3:1. Thus, if the larger displacement pump head supplied feed water to the fuel processor, and the smaller displacement pump head supplied feedstock (e.g., a liquid hydrocarbon), then the flow rate of feed water would be three times greater than the flow rate of feedstock into the fuel processor. This ratio would be essentially constant over the entire range of delivery rates achievable with the dual-head pump since this ratio is fixed by the displacement volumes of each of the two pump heads and both pump heads are driven at the same speed by the same drive motor. Suitable types of dual-head pumps include, but are not limited to, gear pumps, piston pumps, diaphragm pumps, and peristaltic pumps.

Yet another embodiment of this invention utilizes the hot product hydrogen stream23as it exits fuel processor12to pre-heat feed water stream22prior to introduction of said feed water into the fuel processor. As shown inFIG. 8, feed water stream22enters a counter-current heat exchanger400. Hot product hydrogen stream23also flows into counter-current heat exchanger400. The feed water stream and the hydrogen stream are isolated from each other, but are in thermal contact such that the hot hydrogen stream is cooled during passage through heat exchanger400and the feed water stream is warmed during its passage through heat exchanger400. When the invented system ofFIG. 8is used, it is preferable that product hydrogen stream23is cooled to a temperature at or near the operating temperature of the fuel cell (typically between approximately 40° C. and approximately 60° C.).

Maintaining acceptable water purity in the cooling loop for fuel cell28is an important aspect of the successful operation of a PEMFC system. Often, to achieve this objective, fuel cell manufacturers specify stainless steel for all wetted surfaces of the PEMFC cooling loop. This leads to considerable expense, especially since stainless steel radiators (heat exchangers) are expensive and, by virtue of the relatively poor thermal conductivity of stainless steel, large in size.

FIG. 9shows an embodiment of this invention that overcomes the need to use stainless steel components throughout the cool loop of the fuel cell, thereby improving the performance of said cooling loop and decreasing its cost. This objective is achieved by placing an ion exchange bed450in the cooling loop so that cooling water passes through the ion exchange bed during operation of the system. Either all of the cooling water or a portion of the cooling water is passed through the ion exchange bed. Since the objective is to maintain low ionic (both cationic and anionic) concentrations in the cooling water, ion exchange bed450should comprise both cation-exchange resins and anion-exchange resins.

If a slip stream of cooling water is passed through ion exchange bed450, the flow rate of said slip stream is sized to maintain sufficiently low ionic concentration in the cooling water. Because the cooling water typically passes over electrically charged surfaces within the PEMFC, it is important that the cooling water have a high electrical resistance, but it is not essential that the cooling water be of ultra-high purity with respect to ionic and non-ionic content.

It is also important to maintain acceptable levels of purity in the feed water that is to be used within fuel processor12so that the steam-reforming catalysts within said fuel processor are not poisoned and rendered non-effective.FIG. 10shows activated carbon bed500and ion exchange bed502placed in feed water stream22for the purpose of purifying said feed water of ionic and organic contaminants. The now purified feed water stream510is then admitted into fuel processor12. Activated carbon bed500removes organic impurities from feed water stream22. Such organic impurities may originate from a variety of sources including, but not limited to, combustion byproducts that are exhausted from fuel processor12and carried in exhaust stream64to condenser66, and from there into condensed liquid water stream69. Ion exchange bed502comprises both cation-exchange resins and anion-exchange resins, thereby removing both cations and anions from feed water stream22. Ionic contamination of feed water stream22may originate from a variety of sources including, but not limited to, corrosion of metallic wetted surfaces in the combustion exhaust line carrying exhaust stream64, condenser66, the line carrying condensed liquid water stream69to water reservoir16, and water reservoir16. The incorporation of ion exchange bed502allows the use of materials that are not especially corrosion resistant, but exhibit good thermal conductivity and relatively low cost, for the aforementioned wetted parts of the system, thereby improving the performance of condenser66and reducing the cost of the system.