U.S. patent application number 13/529188 was filed with the patent office on 2013-05-23 for use of ammonia as source of hydrogen fuel and as a getter for air-co2 in alkaline membrane fuel cells.
The applicant listed for this patent is Dario Dekel, Shimshon Gottesfeld, Ziv Gottesfeld, Miles Page. Invention is credited to Dario Dekel, Shimshon Gottesfeld, Ziv Gottesfeld, Miles Page.
Application Number | 20130130136 13/529188 |
Document ID | / |
Family ID | 47422941 |
Filed Date | 2013-05-23 |
United States Patent
Application |
20130130136 |
Kind Code |
A1 |
Page; Miles ; et
al. |
May 23, 2013 |
Use of Ammonia as Source of Hydrogen Fuel and as a Getter for
Air-CO2 in Alkaline Membrane Fuel Cells
Abstract
Embodiments of the invention provide an ammonia operated fuel
cell system including an alkaline membrane fuel cell (AMFC) having
an anode, and an ammonia thermal cracker including a combustion
chamber, the cracker being in gas communication with an ammonia
source, and configured to provide a supply of H.sub.2 directly to
the AMFC anode.
Inventors: |
Page; Miles; (US) ;
Dekel; Dario; (Zichron Yaakov, IL) ; Gottesfeld;
Ziv; (Gan Yoshiya, IL) ; Gottesfeld; Shimshon;
(US) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Page; Miles
Dekel; Dario
Gottesfeld; Ziv
Gottesfeld; Shimshon |
Zichron Yaakov
Gan Yoshiya |
|
US
IL
IL
US |
|
|
Family ID: |
47422941 |
Appl. No.: |
13/529188 |
Filed: |
June 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61500879 |
Jun 24, 2011 |
|
|
|
Current U.S.
Class: |
429/416 |
Current CPC
Class: |
H01M 8/222 20130101;
Y02E 60/36 20130101; H01M 8/0668 20130101; H01M 8/0263 20130101;
H01M 8/0606 20130101; Y02E 60/50 20130101; H01M 2008/1095 20130101;
H01M 8/06 20130101 |
Class at
Publication: |
429/416 |
International
Class: |
H01M 8/06 20060101
H01M008/06 |
Claims
1. An ammonia operated fuel cell system comprising: an alkaline
membrane fuel cell (AMFC) having an anode; an ammonia thermal
cracker including a combustion chamber, the cracker being in gas
communication with an ammonia source, and configured to provide a
supply of H.sub.2 directly to the AMFC anode.
2. The ammonia operated fuel cell system of claim 1 wherein exhaust
from the anode of the AMFC is fed into the combustion chamber of
the cracker.
3. The ammonia operated fuel cell system of claim 1 wherein the
cracker is configured to produce an output of substantially 75%
Hydrogen and 25% Nitrogen.
4. The ammonia operated fuel cell system of claim 1 further
comprising a flow channel configured to provide a substantially
uniform concentration of H.sub.2 to substantially all parts of an
active area of the AMFC.
5. The ammonia operated fuel cell system of claim 4 wherein the
flow channel includes a spiral in a first rotational sense between
an inlet and a center and a spiral in a second rotational sense
between the center and the outlet.
6. The ammonia operated fuel cell system of claim 1 wherein a
cathode of the AMFC is in gas communication with the ammonia
source.
7. The ammonia operated fuel cell system of claim 6 wherein an air
supply to the cathode of the AMFC is configured to receive a bleed
of ammonia from the ammonia source.
8. The ammonia operated fuel cell system of claim 1 wherein an
ammonia supply to a cathode of the AMFC is configured to getter
CO.sub.2 from the air supply.
9. The ammonia operated fuel cell system of claim 1 wherein an
exhaust from a cathode of the AMFC is fed into the combustion
chamber of the ammonia cracker.
10. The ammonia operated fuel cell system of claim 1 wherein the
cracker output also contains significant levels of ammonia
residues.
11. The ammonia operated fuel cell of claim 10 wherein the ammonia
residues concentration of the cracker output are from about 1 ppm
to about 20,000 ppm.
12. The ammonia operated fuel cell system of claim 1 where the
thermal cracker is heated substantially by combustion of ammonia.
Description
FIELD OF INVENTION
[0001] This invention relates to the field of Alkaline Membrane
Fuel Cells (AMFCs) and in particular to utilizing ammonia as a
source to generate electricity.
BACKGROUND
[0002] Alkaline membrane fuel cells (AMFCs) generate electricity
using mostly Hydrogen fuel. In the past, the Hydrogen source
considered for AMFC fuel cell stacks has been a tank of compressed
Hydrogen gas connected to the AMFC. Compressed Hydrogen, however,
is not easily available in many parts of the world, and handling of
compressed H.sub.2 gas tanks can be demanding because of transport
regulations and because of the low weight % Hydrogen in such gas
tanks. Ammonia tanks, on the other hand, contain liquid ammonia
under pressure of several bars and consequently have significantly
higher weight % of Hydrogen versus compressed H.sub.2 gas. Use of
ammonia as source of Hydrogen can therefore provide an improvement
in Hydrogen fuel packaging and facilitates fuel supply in many
rural areas where ammonia is much more readily available than
compressed Hydrogen.
SUMMARY
[0003] In general, in an aspect, embodiments of the invention can
provide an ammonia operated fuel cell system including an alkaline
membrane fuel cell (AMFC) having an anode, and an ammonia thermal
cracker including a combustion chamber, the cracker being in gas
communication with an ammonia source, and configured to provide a
supply of H.sub.2 directly to the AMFC anode.
[0004] Implementations of the invention can include one or more of
the following features. Exhaust from the anode of the AMFC is fed
into the combustion chamber of the cracker. The cracker is
configured to produce an output of substantially 75% Hydrogen and
25% Nitrogen. The ammonia operated fuel cell further includes a
flow channel configured to provide a substantially uniform
concentration of H.sub.2 to substantially all parts of an active
area of the AMFC. The flow channel includes a spiral in a first
rotational sense between an inlet and a center and a spiral in a
second rotational sense between the center and the outlet. A
cathode of the AMFC is in gas communication with the ammonia
source. An air supply to the cathode of the AMFC is configured to
receive a bleed of ammonia from the ammonia source. An air supply
to a cathode of the AMFC is configured to getter CO.sub.2 from the
air supply. An exhaust from a cathode of the AMFC is fed into the
combustion chamber of the ammonia cracker.
[0005] Various aspects of the invention may provide one or more of
the following capabilities. Hydrogen can be provided to an AMFC
using bottled ammonia. An ammonia cracker can be configured to
provide Hydrogen to an AMFC using the bottled ammonia. To prevent
carbonation of the AMFC ionomer by CO.sub.2 entering the AMFC with
the air stream, gettering of the CO.sub.2 by ammonia can be
accomplished. AMFCs can be operated in area where there is a short
supply of neat Hydrogen. Up to a 5.times. extension of operation
time per tank fill-up can be accomplished due to the higher density
of liquefied ammonia versus that of compressed hydrogen, in a 200
bar cylinder.
[0006] These and other capabilities of the invention, along with
the invention itself, will be more fully understood after a review
of the following figures, detailed description, and claims.
BRIEF DESCRIPTION OF THE FIGURES
[0007] FIG. 1 shows a schematic diagram of an AMFC system with a
direct supply of H.sub.2.
[0008] FIG. 2 shows a schematic diagram of an AMFC system with a
supply of H.sub.2-rich gas mixture from an ammonia cracker.
[0009] FIG. 3 shows a schematic diagram of a flow channel.
[0010] FIG. 4 shows a schematic diagram of an AMFC system with a
direct bleed of ammonia into an air feed stream.
DETAILED DESCRIPTION
[0011] Embodiments of the invention provide techniques for
providing H.sub.2 to an AMFC using an ammonia cracker. Preferably,
the ammonia is provided to the AMFC system from bottles of liquid
ammonia contained under low pressure. Ammonia flows from the bottle
to the thermal cracker, which is configured to produce H.sub.2 that
is provided to the AMFC. The Hydrogen/Nitrogen mixture generated by
the cracker can be distributed along the active area of the AMFC
using a flow channel that is optimized (e.g., using a series of
counter-oriented spirals) to provide a substantially uniform
distribution of H.sub.2 to the electrode active area. The AMFC
system can also be configured such that exhaust from an anode
and/or cathode of the AMFC is directed to a combustion chamber of
the ammonia cracker. The AMFC system can be further configured such
that ammonia is bled into an air feed stream of the AMFC to
preferably reduce carbonation of the OH.sup.- conducting ionomer by
CO.sub.2 gettering. Other embodiments are within the scope of the
invention.
[0012] FIG. 1 shows an example of an AMFC stack 100 with a
compressed hydrogen tank 102 as the fuel source. This is a simple
AMFC system design due at least in part to the direct feed into the
stack 100 of the stored fuel 102 through a hydrogen pressure
reducer 104 and a hydrogen flow controller.
[0013] FIG. 2 illustrates an AMFC fed by an ammonia storage tank
200. The ammonia storage tank can feed ammonia gas through an
ammonia pressure reducer 202 and an ammonia flow controller to a
thermal cracker 206 which generates a 75% Hydrogen/25% Nitrogen gas
mixture to be used as the fuel feed stream for the stack 210. While
FIG. 2 shows release of the anode exhaust 208 to the outside
atmosphere as an option, the anode exhaust 208 in such system can
also be fed into the fuel inlet of the combustor component 212 of
the cracker. The combustor component 212 then uses the anode
exhaust gas as fuel for the combustor. With the major feed to the
combustor 212 being the tank-stored ammonia fuel, the combustor 212
can maintain the temperature required for the catalytic
decomposition of ammonia into its component elements, according
to:
2NH.sub.3+heat=N.sub.2+3H.sub.2 (1)
[0014] The use of ammonia instead of Hydrogen to fuel the AMFC
system can be beneficial because, for example, ammonia is typically
much more accessible over large areas of the world when compared
with compressed Hydrogen, particularly so in rural areas, and
because a tank of a given volume filled by ammonia contains
5.times. the energy versus that same tank filled by compressed
hydrogen at 200 bars. One important characteristic of ammonia,
which explains the reluctance to use it as hydrogen carrier for
mainstream fuel cells utilizing proton conducting membranes
(PEMFCs), is the ultra-high sensitivity of a PEMFC to even minute
traces of ammonia, that can react aggressively by a base/acid type
process to convert protons in the acidic ionomer to ammonium ions,
according to:
NH.sub.3+H.sup.+=NH.sub.4.sup.+ (2)
[0015] The conversion of protons to ammonium ions according to
Equation (2) can deactivate a PEMFC. Since the conversion occurs at
even sub-ppm levels of ammonia in the fuel feed, cracking ammonia
according to Equation (1) to generate a fuel feed for a PEMFC can
be done only at the expense of the inclusion in the feed stream of
a highly effective ammonia filtering system which will guarantee
100% removal of the smallest traces of ammonia resulting from
imperfect conversion of ammonia in the cracker to Hydrogen by the
process identified in Equation (1). In contrast, the ionic
functionality in the alkaline membrane used in the AMFC is itself a
base, OH--, and consequently no reaction of NH.sub.3 residues from
the cracker occurs with the ionomer in the AMFC.
[0016] This is not to say, however, that other elements of the AMFC
cell and hardware are guaranteed immunity in the presence of
ammonia residues in the fuel feed stream. However, surprisingly,
initial testing performed on a system constructed in accordance
with FIG. 2, showed that, up to significant cell current levels,
the performance of the AMFC operating on cracked ammonia was
substantially identical to the performance of the same AMFC
operating using neat hydrogen gas. Such initial testing suggests
that, at the ammonia levels of 1 ppm to 20,000 ppm (the level that
was present in the fuel feed stream during our test), there is no
significant performance loss incurred by the ammonia entering the
AMFC anode. This demonstrates that, in turn, no catalyst
deactivation or other material instability was caused by the entry
into the AMFC anode of ammonia at a level 100.times. higher than
that required to completely deactivate a PEMFC.
[0017] It is believed that this is a first report of a reduction to
practice of the system shown in FIG. 2, consisting of an AMFC stack
fed by thermally cracked ammonia, while confirming the viability of
operation with no filtration of ammonia residues upstream of the
stack fuel inlet by demonstrating performance comparable to that
obtained with a neat Hydrogen feed in operation on non-filtered,
thermally cracked ammonia with ammonia levels of the order of 1 ppm
to 20,000 ppm in the non-filtered anode feed stream.
[0018] Kordesch et al described in U.S. Pat. No. 6,936,363, use of
an ammonia thermal cracker as source of hydrogen for an Alkaline
Fuel Cell (AFC) which uses an aqueous solution of KOH as
electrolyte. An important relevant difference between the AMFC
(membrane electrolyte fuel cell) and an AFC (liquid electrolyte
fuel cell)), is that the total volume of water per unit area of the
electrode is several orders of magnitude smaller in the case of the
AMFC. Consequently, the concentration of ammonia in contact with
the electrode surface in the AMFC is much higher at some given
level of ammonia residue in the hydrogen feed stream. The teaching
provided here of a combination of an ammonia cracker with an AMFC,
therefore addresses a significantly taller challenge of immunity of
both the membrane material and the catalysts, to high local
concentrations of ammonia, particularly when the ammonia levels
allowed in the cracker exhaust are high, as preferred for smaller
cracker size, lower cracker cost and lower energy demand.
[0019] Potential problems arise when switching from 100% hydrogen
feed of a system like that of FIG. 1 to 75% hydrogen mixed with 25%
inert gas (e.g., Nitrogen) like that of FIG. 2 one issue is how to
operate at high fuel utilization without loss of performance having
to do with the drop of Hydrogen concentration near the fuel outlet
well below the inlet level of 75%. This is fully expected as
Hydrogen is consumed along the flow channel while the inert
component of the anode gas mixture remains intact. Such continuous
dilution of the reactant Hydrogen gas is typically a smaller
problem in a PEMFC, because the anode process in the PEMFC does not
face the extra challenge of excess water buildup incurred in the
AMFC anode. In the AMFC, the anode is the water generating
electrode. Consequently, some given degree of hydrogen dilution by
inert gas will have a stronger mass transport consequences in the
water rich AMFC anode.
[0020] An innovative approach to meet this important challenge of
operating an AMFC with diluted Hydrogen gas at high performance
under high fuel utilization conditions has been discovered, relying
on a simple, one-pass mode for the fuel feed.
[0021] FIG. 3 shows an exemplary unique type flow field which
preferably provides a uniform Hydrogen concentration across the
active area of an AMFC cell, in spite of the large drop in hydrogen
concentration between inlet and outlet ports under high fuel
utilization conditions. The unique flow field 300 shown in FIG. 3
includes a spiral in a first direction 302 (e.g., a counter
clockwise-spiral direction) between an inlet and the center
followed by a second spiral in a second direction 304 (e.g.,
clockwise spiral direction) between the center and an outlet, with
the inlet and outlet ports preferably positioned at close
proximity. The result of such flow field design is that each
element of the active area is preferably fed by a pair of channels
of Hydrogen concentrations which, on average, are the same. For
example, each portion of the active area will preferably have next
to it a Hydrogen-rich and a Hydrogen-poor channel that offset each
other to feed the active area with a mixture that is preferably 50%
Hydrogen and 50% inactive gas. As a further example, near the
inlet, the two adjacent channels will preferably contain 75% and
25% Hydrogen, respectively. Thus, the active area near the inlet is
preferably fed by a 50% Hydrogen and 50% inactive gas mixture. At
another area, the breakdown could be 60% and 40% Hydrogen, which
again averages to 50% Hydrogen to that area. The exact values can
depend on the actual degree of fuel utilization. Preferably, the
configuration shown in FIG. 3 ensures a Hydrogen concentration
having a good uniformity (e.g., 50%) across the active area. This
is in contrast to a system that has a sharp drop from 75% to 25%
Hydrogen concentration between the inlet and outlet when an
ordinary flow field is used (i.e., a flow field without a "turning
around at the center.").
[0022] Next is described an innovative approach to the use of
ammonia as "CO2 getter" in a system of the type described in FIG.
2. Once ammonia is stored next to an AMFC fuel cell stack as the
source of the hydrogen fuel, was determined that the ammonia can
provide an additional function of special importance for the AMFC
CO.sub.2 immunity.
[0023] A previous patent application (i.e., Ser. No. 12/862,746,
filed Aug. 24, 2010, entitled "Systems and Methods of Security
Immunity to Air C02 in Alkaline Fuel Cells", the text of which is
incorporated by reference herein in its entirety) and assigned to
the assignee of the present application, describes a comprehensive
technique to prevent AMFC losses incurred by entry of air-CO.sub.2
into the AMFC cathode. The technique described in that patent
application is based on CO.sub.2 sequestration sub-units placed
upstream the cell and an in-situ, electrochemical decarbonation
taking place at the cell anode. While ammine or hydroxide active
materials were used for sequestration of CO.sub.2 in our previous
patent application, here ammonia preferably undergoes base-acid
reaction with CO.sub.2 to form ammonium carbonate:
NH.sub.3+H.sub.2O+CO.sub.2=(NH.sub.4.sup.+)(HCO.sub.3-) (3)
and/or
2NH.sub.3+H.sub.2O+CO.sub.2=(NH.sub.4.sup.+).sub.2(CO.sub.3.sup.=)
(4)
Capturing CO.sub.2 by ammonia in this way, can prevent carbonation
of the functional groups in the polymer by air CO.sub.2, according
to:
--(NR.sub.4.sup.+)(OH.sup.-)+CO.sub.2=--(NR.sub.4.sup.+)(HCO.sub.3-)
(5)
where --(NR.sub.4.sup.+)(OH.sup.-) is preferably a tetra-alkyl
ammonium hydroxide functional group of the AMFC ionomer and
--(NR.sub.4.sup.+)(HCO.sub.3.sup.-) is the carbonated (i.e.,
deactivated) form of such functional group. The process of Equation
(5) is known to degrade the AMFC performance significantly. Thus,
when ammonia is available next right to the stack, it could be used
for CO.sub.2 sequestration according to Equations (3) and (4),
thereby relieving the cell of air CO.sub.2 related performance
losses while avoiding the need of an additional unit and active
material for that purpose. Once the CO2 neutralization function of
the ammonia available next to a stack, according to Equations (3)
and (4), is considered, a remaining important question is how to
achieve high efficiency of this process, as reflected by the
probability of the reactions of Equations (3) and (4) much
exceeding the probability of ionomer carbonation by CO2 entering
through the air cathode, according to (5).
[0024] Ammonia can be naturally fed into the cell with the
Hydrogen/Nitrogen fuel mix (e.g., it is fed into the cell anode and
its level in the feed stream can be elevated to, for example, 0.1%
in the anode feed), to achieve CO.sub.2 sequestration. However,
this mode of ammonia supply to the cell is not likely to fulfill
the requirement of the probability of processes (3) or (4)
occurring being much higher than the probability of process (5)
occurring. The reason is that any CO.sub.2 molecule that enters
first into the cell cathode of an operating AMFC, encounters there
a current-sustained population of --(NR.sub.4.sup.+)(OH.sup.-)
groups, as the cathode process is generating continuously OH.sup.-
ions. Consequently, CO.sub.2 entering the cathode will typically
convert at high probability to carbonate according to process (5),
before being significantly exposed to any ammonia entering the cell
from its other (fuel) side.
[0025] An exemplary innovative approach to the resolution of this
difficulty is shown in FIG. 4. According to this scheme, some
ammonia may be bled 402 from the ammonia tank 404 into the air feed
stream 406 upstream the cathode inlet, so as to react with
air-CO.sub.2 according to Equations (3) and (4) before the air and
any of the CO.sub.2 in it reaches the cathode. The CO.sub.2 can be
fed upstream of a water exchanger used to humidify the incoming air
stream and the CO.sub.2 gettering process of Equation (3) and (4)
can then take place while the air stream is passing through the
water exchanger. To further increase the probability of CO.sub.2
gettering by the ammonia bled into the air feed stream 408, a tube
410 filled with inert high surface area substrate can be placed
between the water exchanger and the cathode inlet to preferably
increase the probability of the encounter between an ammonia/water
mix and the air-CO.sub.2. Other similar means for sustaining high
efficiency of the gettering process of Equations (3) and (4) by
providing proper inert substrate surface area and securing supply
of sufficient water for processes Equations (3) and (4) are
possible. In summary, bleeding of ammonia into the air feed stream
either upstream a water exchanger or a tube section upstream the
cathode inlet containing an inert, high surface area substrate,
will preferably ensure a high probability of gettering air-CO.sub.2
by ammonia and, thereby prevent carbonation of the AMFC
ionomer.
[0026] When bleeding ammonia into the air feed stream, as shown in
FIG. 4, the cathode exhaust stream 412 could contain ammonia at a
level of the order of, for example, 0.1% and, consequently, release
of such stream to the ambient air is likely to be unacceptable in
many locations. In a preferred system design, the cathode exhaust
in the system described in FIG. 4, rather than being released to
the ambient air is fed into the combustor component of the thermal
cracker. Routing the stack air exhaust into the combustor
preferably resolves this issue, as ammonia is fully combusted at
the temperature of the combustor in the presence of the large
excess of independent air feed (e.g., FIG. 4).
[0027] Other embodiments are within the scope and spirit of the
invention. For example, due to the nature of software, functions
described above can be implemented using software, hardware,
firmware, hardwiring, or combinations of any of these. Features
implementing functions may also be physically located at various
positions, including being distributed such that portions of
functions are implemented at different physical locations.
[0028] Further, while the description above refers to the
invention, the description may include more than one invention.
* * * * *