U.S. patent application number 11/347829 was filed with the patent office on 2006-09-28 for integrated fuel cell system.
Invention is credited to David J. Edlund, William A. Pledger.
Application Number | 20060216562 11/347829 |
Document ID | / |
Family ID | 22703330 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060216562 |
Kind Code |
A1 |
Edlund; David J. ; et
al. |
September 28, 2006 |
Integrated fuel cell system
Abstract
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/or maintaining the purity of
the process water used in the system.
Inventors: |
Edlund; David J.; (Bend,
OR) ; Pledger; William A.; (Sisters, OR) |
Correspondence
Address: |
KOLISCH HARTWELL, P.C.
200 PACIFIC BUILDING
520 SW YAMHILL STREET
PORTLAND
OR
97204
US
|
Family ID: |
22703330 |
Appl. No.: |
11/347829 |
Filed: |
February 3, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11084382 |
Mar 18, 2005 |
6994927 |
|
|
11347829 |
Feb 3, 2006 |
|
|
|
10127030 |
Apr 19, 2002 |
6869707 |
|
|
11084382 |
Mar 18, 2005 |
|
|
|
09190917 |
Nov 12, 1998 |
6376113 |
|
|
10127030 |
Apr 19, 2002 |
|
|
|
Current U.S.
Class: |
429/410 ;
429/414; 429/416; 429/423; 429/434; 429/441 |
Current CPC
Class: |
C01B 2203/047 20130101;
C01B 2203/0816 20130101; C01B 2203/1076 20130101; H01M 8/0612
20130101; B01J 2219/00081 20130101; C01B 2203/1011 20130101; H01M
8/04022 20130101; B01J 2208/00504 20130101; C01B 2203/1223
20130101; C01B 2203/1604 20130101; B01J 8/0285 20130101; B01J
2219/00123 20130101; C01B 3/38 20130101; C01B 2203/0244 20130101;
C01B 2203/0475 20130101; C01B 2203/0827 20130101; C01B 2203/0844
20130101; C01B 2203/1288 20130101; C01B 2203/1676 20130101; C01B
2203/1217 20130101; H01M 8/0618 20130101; C01B 3/586 20130101; C01B
2203/0445 20130101; C01B 2203/1235 20130101; H01M 8/0631 20130101;
C01B 2203/044 20130101; C01B 2203/0811 20130101; C01B 2203/1058
20130101; C01B 2203/0822 20130101; C01B 2203/169 20130101; H01M
8/04044 20130101; H01M 2008/1095 20130101; B01J 2208/00088
20130101; C01B 3/323 20130101; H01M 8/04029 20130101; C01B
2203/0233 20130101; C01B 2203/0261 20130101; C01B 2203/1628
20130101; B01J 19/2475 20130101; C01B 3/501 20130101; C01B 2203/146
20130101; B01J 8/009 20130101; C01B 2203/1258 20130101; B01J
2208/00371 20130101; C01B 2203/0866 20130101; H01M 8/04156
20130101; B01J 8/0465 20130101; B01J 2219/00103 20130101; C01B
2203/0283 20130101; C01B 2203/085 20130101; C01B 2203/1041
20130101; C01B 2203/1695 20130101; Y02E 60/50 20130101; C01B
2203/041 20130101; Y02P 20/10 20151101; B01J 2219/00157 20130101;
C01B 2203/1657 20130101; C01B 2203/0877 20130101; B01J 8/067
20130101; B01J 2208/00132 20130101; B01J 2208/00256 20130101; H01M
8/04014 20130101; C01B 2203/0883 20130101; C01B 2203/1276 20130101;
C01B 2203/82 20130101; B01J 2219/00069 20130101; B01J 8/006
20130101; B01J 2219/00135 20130101; C01B 2203/1241 20130101; H01M
8/04291 20130101; B01J 8/0257 20130101; C01B 2203/066 20130101;
C01B 2203/142 20130101; C01B 2203/1282 20130101 |
Class at
Publication: |
429/026 ;
429/034 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Claims
1. A fuel cell system, comprising: a fuel cell stack adapted to
receive hydrogen gas and an oxidant and to produce an electric
current therefrom, wherein the fuel cell stack includes an anode
chamber to which the hydrogen gas is delivered and a cathode
chamber to which the oxidant is delivered, and further wherein the
fuel cell stack is adapted to produce a water stream that is
removed from the fuel cell stack; and a cooling assembly that is
adapted to receive and cool the water stream, wherein the cooling
assembly includes an ion-exchange bed through which at least a
portion of the water stream is passed to remove ions from the water
stream.
2. The fuel cell system of claim 1, wherein only a portion of the
water stream is passed through the ion-exchange bed.
3. The fuel cell system of claim 1, wherein all of the water stream
is passed through the ion-exchange bed.
4. The fuel cell system of claim 1, wherein the ion-exchange bed
includes an anion-exchange resin.
5. The fuel cell system of claim 1, wherein the ion-exchange bed
includes a cation-exchange resin.
6. The fuel cell system of claim 5, wherein the ion-exchange bed
includes both anion- and cation-exchange resins.
7. The fuel cell system of claim 1, further comprising a water
storage reservoir adapted to receive the water stream.
8. The fuel cell system of claim 1, further comprising means for
providing hydrogen gas for the fuel cell stack.
9. The fuel cell system of claim 1, further comprising means for
producing hydrogen gas for the fuel cell stack.
10. The fuel cell system of claim 9, further comprising means for
recovering water from the hydrogen gas.
11. A method for operating a fuel cell system, the method
comprising: producing hydrogen gas; combusting a fuel stream to
produce a heated combustion exhaust stream; heating at least a
portion of a fuel processor and a heat exchange assembly with the
heated combustion exhaust stream; producing hydrogen gas in the
fuel processor via a steam reforming reaction; delivering at least
a portion of the hydrogen gas to a fuel cell stack; heating a fluid
with the heat exchange assembly to produce a heated fluid stream;
and delivering at least a portion of the heated fluid stream to at
least one of a residential, commercial, or industrial
application.
12. The method of claim 11, wherein the fluid is air, and the
heated fluid stream is delivered to provide space heating to the
application.
13. The method of claim 11, wherein the fluid is water, and the
heated fluid stream is delivered to provide space heating to the at
least one application.
14. The method of claim 11, wherein the fluid is water, wherein the
application is a residential application, and further wherein the
heated fluid stream is delivered to provide domestic hot water to
the residential application.
15. The method of claim 11, wherein the fluid is water, wherein the
application is a commercial application, and further wherein the
heated fluid stream is delivered to provide process hot water to
the commercial application.
16. The method of claim 11, wherein the fluid is water, wherein the
application is an industrial application, and further wherein the
heated fluid stream is delivered to provide process hot water to
the industrial application.
17. The method of claim 11, wherein the fluid is at least one of
ethylene glycol and propylene glycol, and the heated fluid stream
is delivered to provide space heating to the at least one
application.
18. The method of claim 11, wherein the fuel includes hydrogen gas
produced by the fuel processor.
19. The method of claim 11, wherein the fuel includes an exhaust
stream from the fuel cell stack.
20. The method of claim 11, wherein the fuel cell stack is adapted
to produce a water stream that is removed from the fuel cell stack,
and further wherein the method includes removing impurities from
the water stream.
Description
RELATED APPLICATIONS
[0001] This application is a continuation patent application
claiming priority to similarly entitled U.S. patent application
Ser. No. 11/084,382, which was filed on Mar. 18, 2005, and issued
on Feb. 7, 2006 as U.S. Pat. No. 6,994,927, and which is a
continuation of U.S. patent application Ser. No. 10/127,030, now
U.S. Pat. No. 6,869,707, which was filed on Apr. 19, 2002, and
which is a continuation of U.S. patent application Ser. No.
09/190,917, now U.S. Pat. No. 6,376,113, which was filed on Nov.
12, 1998. The complete disclosures of the above-identified patent
applications are hereby incorporated by reference for all
purposes.
FIELD OF THE INVENTION
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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/or maintaining the purity of
the process water used in the system.
[0006] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a process flow diagram of a fuel-cell system in
which propane or natural gas is used as the fuel to heat the fuel
processor during a cold start-up.
[0008] FIG. 2 is a process flow diagram of a fuel-cell system in
which a liquid fuel is used to heat the fuel processor during a
cold start-up.
[0009] FIG. 3 is an embodiment of the invention in which stored
hydrogen is used to heat the fuel processor during a cold
start-up.
[0010] FIG. 4 is a process flow diagram of a fuel-cell system in
which hydrogen purged from the anode chamber of the fuel cell is
combusted to provide additional water for recovery and use.
[0011] FIG. 5 is a process flow diagram of a fuel cell system in
which hydrogen purged from the anode chamber of the fuel cell is
combusted to provide additional heat and water for recovery and
use.
[0012] FIG. 6 is an embodiment of the invention in which high-grade
heat is recovered from the fuel processor.
[0013] FIG. 7 shows another embodiment of the invention comprising
a dual pump head in which a single motor is used to simultaneously
drive two pump heads that deliver both the feedstock and water to
the fuel processor.
[0014] FIG. 8 shows yet another embodiment of the invention adapted
to preheat either the feedstock or feed water prior to delivery to
the fuel processor by heat exchange with hot hydrogen exiting the
fuel processor.
[0015] FIG. 9 shows yet another embodiment of the invention
comprising one or more ion-exchange beds to maintain low electrical
conductivity of the process water.
[0016] FIG. 10 shows a process flow diagram for a fuel cell system
in which ion exchange and activated carbon beds are used to purify
feed water prior to injection into the fuel processor.
DETAILED DESCRIPTION OF THE INVENTION
[0017] As shown in FIG. 1, the invention consists of a fuel-cell
system comprising a fuel cell 10 that produces electrical power
from air (oxygen) and hydrogen, and a fuel processor 12 that
produces hydrogen from a variety of feedstocks. Generally, said
fuel cell is a net producer of water, and said fuel processor 12 is
a net consumer of water. Fuel cell 10 is 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 cell 10 produces byproduct water and byproduct heat
in addition to electrical power.
[0018] Many feedstocks are suitable for producing hydrogen using
fuel processor 12 including, but not limited to, carbon-containing
compounds such as hydrocarbons, alcohols, and ethers. Ammonia is
also a suitable feedstock. Fuel processor 12 preferably produces
hydrogen by reacting the carbon-containing feedstock with water by
a process commonly known as steam reforming. In this case fuel
processor 12 consumes 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.
[0019] FIG. 1 is a process flow diagram for one embodiment of a
fuel cell system of this invention. The fuel cell 10 receives
hydrogen produced by the fuel processor 12. The fuel processor
produces hydrogen by reacting, at high temperature, a feedstock
from storage reservoir 14 and water from storage reservoir 16. Pump
20 moves feedstock from reservoir 14 and delivers said feedstock to
the fuel processor 12. Likewise, pump 21 moves water from reservoir
16 and delivers said water as stream 22 to the fuel processor 12.
Pumps 20 and 21 deliver the feedstock and water to the fuel
processor at a pressure ranging from ambient pressure to
approximately 300 psig.
[0020] Hydrogen produced by the fuel processor is initially hot
because the fuel processor must operate at elevated temperatures of
250.degree. C. to 1300.degree. C. The product hydrogen stream 23
from the fuel processor is cooled using heat exchanger 24 and fan
26 to 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.degree. C. and approximately 80.degree. C., product
hydrogen is passed into the anode chamber 28 of the fuel cell
stack.
[0021] Air stream 29 is delivered to the cathode chamber 30 of said
fuel cell stack 10 by a blower 32. Alternatively, a compressor
could also be used in place of blower 32. 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.).
[0022] 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 pump 34 circulates hot coolant from the
fuel cell stack into heat exchanger 36. Fan 38 blows cool air over
the hot surfaces of heat exchanger 36, 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.
[0023] A pressure regulator 40 ensures that the pressure of the
hydrogen supplied to the anode chamber 28 of said fuel cell 10
remains 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 valve 42 serves 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 stream 44 may be vented directly to the
surroundings, as shown in FIG. 1, or it may be used for the purpose
of producing heat, or for other purposes. In some embodiments of
this invention hydrogen stream 23 may be flowed continuously in
excess through anode chamber 28, eliminating the need for said
purge valve 42. Since some liquid water may be entrained in said
purge hydrogen stream 44, an optional water knock-out may be placed
in purge stream 44 for the purpose of separating and collecting
said entrained liquid water.
[0024] Excess air is continuously flowed through the cathode
chamber 30. 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 chamber 30 as stream 52. Stream 52
contains substantial water, as both liquid and vapor, available for
recovery. Stream 52 is 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, stream 52 is first passed through a knock-out 54 that
separates liquid water from the oxygen-depleted air and water
vapor. Liquid water stream 56 flows out of said knock-out 54 and
the liquid water is collected within water reservoir 16. The
gas-phase stream 58 exiting knock-out 54 comprises the
oxygen-depleted air and water vapor.
[0025] Stream 58 is directed into fuel processor 12 for 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 stream 58 is to be used for combustion, there is
no primary reason to cool stream 58 or stream 52, other than to
assist with separation of liquid water within said knock-out
54.
[0026] Still referring to FIG. 1, fuel processor 12 is 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.degree. C. and approximately 1300.degree. C., and
for most common alcohols and hydrocarbons (except methane) is in
the range of approximately 250.degree. C. and approximately
800.degree. C. To initially heat fuel processor 12 during a cold
start-up, a suitable fuel such as propane or natural gas is fed
from a supply source 60 to the fuel processor. The fuel is
combusted within the fuel processor 12 until the fuel processor is
hot enough to begin steam reforming the feedstock. A throttle valve
62 regulates the flow of propane or natural gas fuel to the fuel
processor during this cold start-up.
[0027] Combustion exhaust stream 64 exits the fuel processor as a
hot gas stream laden with water vapor. The water vapor in said
combustion exhaust stream 64 has essentially two sources: as a
byproduct of burning the fuel, and as a component of air stream 58.
It is desirable to recover the water from combustion exhaust stream
64 and to recover heat from said exhaust stream 64. Condenser 66
serves this purpose. Hot, moist exhaust stream 64 passes into said
condenser 66 and is chilled using a cold fluid stream 68. Streams
with temperatures near or less than 20.degree. C. have proven
effective. Liquid water condenses and flows out of condenser 66 as
liquid stream 69, and is collected in water reservoir 16.
[0028] Cold fluid stream 68 is warmed by the process of passing hot
exhaust stream 64 through said condenser 66. For example, cold
outside air may serve as stream 68 and be heated for the purpose of
space heating in a residential, commercial, or industrial
application. Alternatively, cold water may serve as stream 68 and
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 stream 68.
[0029] Once fuel processor 12 has 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.degree. C., with temperatures of at least
450.degree. C. and preferably at least 600.degree. 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 stream 23 is passed to the
fuel cell as previously described. The hydrogen-depleted stream 75
that is rejected by the hydrogen purifier is passed through
throttle valve 78 to be used as fuel for combustion to heat said
fuel processor 12. At this time during the operation of fuel
processor 12 there 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.
[0030] FIG. 2 is another embodiment of the present invention in
which the fuel processor 12 is 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 supply 100 using pump
102. The discharged liquid fuel from pump 102 is admitted through a
suitable nozzle or jet into the combustion region in fuel processor
12 where 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 processor 12 to facilitate combustion.
[0031] Yet another embodiment of the present invention related to
cold start-up of fuel processor 12 is shown in FIG. 3. In this case
cold start-up is accomplished by combustion of hydrogen fuel within
fuel processor 12. Hydrogen fuel is stored by within hydrogen
storage vessel 150 by 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 vessel 150.
[0032] 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 Volkl, 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.degree. C. to
22.degree. C. Increasing the temperature of the metal hydride
system increases the equilibrium pressure of hydrogen over the
metal hydride.
[0033] Returning to FIG. 3 and for purposes of illustration, it is
assumed that storage reservoir 150 contains a suitable quantity of
a metal hydride, and is called a metal hydride bed. During a cold
start-up, fuel hydrogen stream 152 is withdrawn from hydride
storage bed 150 and, after passing through isolation valve 154, is
admitted into fuel processor 12 where said hydrogen fuel is
combusted to heat the fuel processor. As fuel hydrogen is withdrawn
from storage bed 150, 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 stream 64 is flowed through metal
hydride storage bed 150 to heat said metal hydride bed. Then, the
now cool exhaust exits the warmed metal hydride bed 150 as cool
exhaust stream 158. This allows the pressure of gaseous hydrogen to
remain sufficiently high to discharge most of, to nearly all of,
the hydrogen from said storage bed 150.
[0034] Alternative embodiments of this invention would use other
sources to heat metal hydride bed 150 including electrical
resistance heaters and combustion of hydrogen or other fuel to
directly heat storage bed 150.
[0035] After completing cold start-up of fuel processor 12 and
hydrogen is being produced by the fuel processor, isolation valve
154 is closed and hydride storage bed 150 is recharged with
hydrogen so that it will be ready for the next cold start-up.
Recharging of storage bed 150 is accomplished by taking a hydrogen
slip stream 160 from purified product hydrogen stream 23 after said
product hydrogen stream has been cooled by passing through heat
exchanger 24. During this hydrogen recharging operation, byproduct
heat should be removed from hydride storage bed 150, such as
through any known mechanism. An optional isolation valve 162 is
placed in hydrogen slip stream 160 to facilitate maintenance.
[0036] An advantage of this embodiment of the invention is that the
fuel required for cold start-up of fuel processor 12 is 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.
[0037] FIG. 4 presents yet another embodiment of the present
invention in which purge hydrogen stream 44 is passed into
combustor 200 for the purpose of generating additional water to be
recovered ultimately by knock-out 54 and condenser 66. Combustor
200 may be catalytic or non-catalytic. Air to support combustion of
purge hydrogen stream 44 is supplied by the cathode exhaust stream
52 which is depleted, but not devoid, of oxygen as described
previously. The single outlet from combustor 200 is exhaust stream
202 that is enriched in water (vapor and liquid) as a result of
burning purge hydrogen stream 44.
[0038] In yet another embodiment of the present invention, heat is
recovered in addition to water recovery from combustion of purge
hydrogen 44. FIG. 5 shows combustor 200 coupled to heat exchanger
250 for the purpose of recovering and using heat generated by
combustion of purge hydrogen stream 44 within said combustor 200.
Heat exchanger 250 may be simply heat-conductive fins on the
exterior of combustor 200, or a heat exchange fluid may be passed
between combustor 200 and heat exchanger 250. 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
exchanger 250. One such suitable cold fluid stream is air, in which
case fan 252 blows a cold air stream over heat exchanger 250
resulting 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 processor 12.
[0039] Useful heat can also be recovered from fuel processor 12.
FIG. 6 shows this embodiment of the invention. Heat exchanger 300
extracts heat from the high-temperature combustion regions of fuel
processor 12. Pump 302 may be used to circulate a heat transfer
fluid between fuel processor 12 and heat exchanger 300, as shown in
FIG. 6, or circulation of said heat transfer fluid may be based on
naturally occurring convection currents. Alternatively, heat
exchanger 300 may 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
exchanger 300. Such a suitable cold fluid may be an air stream
supplied by fan 305. In this case said air stream is heated by
passing over hot heat exchanger 300. Other suitable cold fluid
streams include, but are not limited to, water, ethylene glycol,
and propylene glycol.
[0040] Another useful embodiment of the present invention is shown
in FIG. 7. Dual-head pump 350 supplies both feedstock from
reservoir 14 and feed water from reservoir 16 to fuel processor 12.
Dual-head pump 350 comprises 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 pump 350. 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.
[0041] Yet another embodiment of this invention utilizes the hot
product hydrogen stream 23 as it exits fuel processor 12 to
pre-heat feed water stream 22 prior to introduction of said feed
water into the fuel processor. As shown in FIG. 8, feed water
stream 22 enters a counter-current heat exchanger 400. Hot product
hydrogen stream 23 also flows into counter-current heat exchanger
400. 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 exchanger 400
and the feed water stream is warmed during its passage through heat
exchanger 400. When the invented system of FIG. 8 is used, it is
preferable that product hydrogen stream 23 is cooled to a
temperature at or near the operating temperature of the fuel cell
(typically between approximately 40.degree. C. and approximately
60.degree. C.).
[0042] Maintaining acceptable water purity in the cooling loop for
fuel cell 28 is 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.
[0043] FIG. 9 shows 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 bed 450 in 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 bed 450 should comprise both
cation-exchange resins and anion-exchange resins.
[0044] If a slip stream of cooling water is passed through ion
exchange bed 450, 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.
[0045] It is also important to maintain acceptable levels of purity
in the feed water that is to be used within fuel processor 12 so
that the steam-reforming catalysts within said fuel processor are
not poisoned and rendered non-effective. FIG. 10 shows activated
carbon bed 500 and ion exchange bed 502 placed in feed water stream
22 for the purpose of purifying said feed water of ionic and
organic contaminants. The now purified feed water stream 510 is
then admitted into fuel processor 12. Activated carbon bed 500
removes organic impurities from feed water stream 22. Such organic
impurities may originate from a variety of sources including, but
not limited to, combustion byproducts that are exhausted from fuel
processor 12 and carried in exhaust stream 64 to condenser 66, and
from there into condensed liquid water stream 69. Ion exchange bed
502 comprises both cation-exchange resins and anion-exchange
resins, thereby removing both cations and anions from feed water
stream 22. Ionic contamination of feed water stream 22 may
originate from a variety of sources including, but not limited to,
corrosion of metallic wetted surfaces in the combustion exhaust
line carrying exhaust stream 64, condenser 66, the line carrying
condensed liquid water stream 69 to water reservoir 16, and water
reservoir 16. The incorporation of ion exchange bed 502 allows 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 condenser 66 and reducing the cost of the
system.
[0046] The foregoing description of the preferred embodiments 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 many modifications and
variations are possible in light of the above teaching. The
embodiments were chosen and described in order to best explain the
principles of the invention and its practical application to
thereby enable others skilled in the art to best utilize the
invention in various embodiments and with various modifications as
are suited to the particular use contemplated. It is intended that
the scope of the invention be defined by the claims appended
hereto.
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