Note: Descriptions are shown in the official language in which they were submitted.
CA 02516300 2005-08-16
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HYDROGENRECYCLE FOR HIGH TEMPERATURE FUEL CELLS
FIELD
This application is related to adsorptive gas separation, and in particular
rotary pressure
swing adsorption (PSA), as well as fuel cell applications, and QuestAir
Technologies' related
patent applications, including Nos. 09/591,275, 09/808,715, 60/323,169, and
60/351,798, the
disclosures of which are incorporated herein by reference.
The present disclosure relates to high temperature fuel cell systems, such as
solid oxide
fuel cell (SOFC) and molten carbonate fuel cell (MCFC)systems exploiting gas
separation
devices in which a first gas mixture comprising components A (e.g. hydrogen)
and B (e.g.
carbon dioxide) is be separated so that a first product of the separation is
enriched in component
A, while component B is mixed with a third gas component C (e.g. air, oxygen-
enriched air or
oxygen-depleted air) contained in a displacement purge stream to form a second
gas mixture
including components B and C, and with provision to prevent cross
contamination of
component C into the first product containing comp~nent A~ or of component A
into the sec~nd
gas mixture containing component C. The invention may be applied to hydrogen
(as exemplary
gas component A) enrichment from high temperature fuel cell anode exhaust gas,
where dilute
carbon dioxide (as exemplary gas component B) is to be rejected to the
atmosphere by purging
with cathode exhaust oxygen-depleted air (as exemplary gas component C) for
example in
SOFC embodiments or to be transferrred to the cathode oxidant feed gas by
purging with feed
air or oxygen-enriched air (as another exemplary gas component C) in MCFC
ernbodirnents.
BACKGROUND
Fuel cells provide an environmentally friendly source of electrical current.
One
type of high temperature fuel cell used for generating electrical power,
particularly envisaged
for larger scale stationary power generation, is the molten carbonate fuel
cell (MCFC). The
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MCFC typically includes an anode channel for receiving a flow of hydrogen gas
(or a fuel gas
which may react in the anode channel to generate hydrogen such as by steam
reforming and
water gas shift reactions), a cathode channel for receiving a flow of oxygen
gas, and a porous
matrix containing a molten carbonate electrolyte which separates the anode
channel from the
cathode channel, Oxygen and carbon dioxide in the cathode channel react to
form carbonate
ions, which cross the electrolyte to react with hydrogen in the anode channel
to generate a flow
of electrons. As the hydrogen is consumed, carbon monoxide is shifted by steam
to generate
additional hydrogen. Carbon dioxide and water vapor are produced in the anode
channel by
oxidation of fuel components, and by reduction of carbonate ions from the
electrolyte. A
typical operating temperature of molten carbonate fuel cells is about
650°C.
Another type of high temperature fuel sell is the solid oxide fuel cell
(SOFC). 'The
SOFC typically includes an anode channel for receiving a flow of hydrogen gas
(or a fuel gas
which reacts in the anode channel to generate hydrogen by steam reforming and
water gas shift
reactions), a cathode channel for receiving a flow of oxygen gas, and a solid
electrolyte which is
a ceramic membrane conductive to oxygen ions and separates the anode channel
from the
cathode channel. Oxygen in the cathode channel dissociates to o:~ygen ions,
which cross the
electrolyte to react with hydrogen in the anode channel to generate a flow of
electrons. As the
hydrogen is consumed, carbon monoxide may be oxidized directly or may be
shifted by steam
to generate additional hydrogen. Carbon dioxide and water vapor are produced
in the anode
channel by oxidation of fuel components. Typical operating temperatures of
solid oxide fuel
cells range between about 500° to about 1000°C.
Except in the rare instance that hydrogen (e.g. recovered from refinery or
chemical
process off gases, or else generated from renewable energy by electrolysis of
water) is directly
available as fuel, hydrogen must be generated from fossil fuels by an
appropriate fuel
processing system. For stationary power generation, it is preferred to
generate hydrogen from
natural gas by steam reforming or partial oxidation to produce "syngas"
comprising a mixture of
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WO 2004/076017 PCT/CA2004/000289
hydrogen, carbon monoxide, carbon dioxide, steam and some unreacted methane.
As hydrogen
is consumed in the fuel cell anode channel, much of the carbon monoxide reacts
with steam by
water gas shift to generate more hydrogen and more carbon dioxide. Other
carbonaceous
feedstocks (e.g. heavier hydrocarbons, coal, or biomass) may also be reacted
with oxygen and
steam to generate syngas by partial oxidation, gasification or autothermal
reforming. The fuel
cell may also be operated on hydrogen or syngas that has been generated
externally.
A great advantage of MCFC and SOFC systems is that their high operating
temperature
facilitates close thermal integration between the fuel cell and the fuel
processing system. The
high temperature also allows the elimination of noble metal catalysts required
by lower
temperature fuel cells.
Prior art Tt~(CFC systems have limitations associated with their high
temperature
operation, and with their inherent need to supply carbon dioxide to the
eathode while removing
it from the anode. Frior art SOFC systems face even m~re challenging
temperature regimes,
and are disadvantaged by the degradation of cell voltages at very high
temperatures under
conventional operating conditions.
The lower heat of combustion of a fuel usefully defines the energy (enthalpy
change of
the reaction) that may be generated by oxidising that fuel. The
electrochemical energy that can
be generated by an ideal fuel cell is however the free energy change of the
reaction, which is
smaller than the enthalpy change. The difference between the enthalpy change
and the free
energy change is the product of the entropy change of the reaction multiplied
by the absolute
temperature. This difference widens at higher temperatures, so higher
temperature fuel cells
inherently convert a lower fraction of the fuel energy to electrical power at
high efftciency,
while a larger fraction of the fuel energy is available only as heat which
must be converted to
electrical power by a thermodynamic bottoming cycle (e.g. steam or gas turbine
plant) at lower
efficiency.
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WO 2004/076017 PCT/CA2004/000289
Accumulation of reforming reaction products (carbon dioxide and steam) on the
fuel
cell anode opposes the electrochemical reaction, so that the free energy is
reduced. Higher
partial pressure of oxygen and carbon dioxide over the cathode, and higher
partial pressure of
hydrogen over the anode, drive the reaction forward so that the free energy is
increased.
Unfortunately, the reaction depletes the oxygen and carbon dioxide in the
cathode channel and
depletes hydrogen in the anode channel while rapidly increasing the
backpressure of carbon
dioxide in the anode channel. Hence the free energy change is reduced,
directly reducing the
cell voltage of the fuel stack. This degrades the electrical efficiency of the
system, while
increasing the heat that must be converted at already lower efficiency by the
thermal bottoming
cycle.
The free energy change is simply the product of the elechomotive force ("E")
of the cell
and the charge transferred per mole by the reaction ("2F''), where the factor
of two reflects the
valency of the carbonate ion. The following Nernst relation for a MCFC
expresses the above
described sensitivity of the electromotive force to the partial pressures of
the electrochemical
reactants in the anode and cathode claannels, vrhere the standard
electromotive force ("E~") is
referred to all components at standard conditions and d~dith water as vapor.
E = E - ~T In ~H2O(nnode)'~C02(ernode)
o ~F ~If2(nnode)'P~2(cnthode)'PC~2(catdtode)
Prior art MCFC systems do not provide any satisfactory solution for this
problem which
compromises attainable overall efficiency. Despite repeated attempts to devise
an effective
technology and method to maximize reactant concentrations, and minimize
product
accumulation in both the anode and cathode circuits that would be compatible
with MCFC
operating conditions, no such attempt has been adequately successful.
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WO 2004/076017 PCT/CA2004/000289
The accepted method for supplying carbon dioxide to the MCFC cathode has been
to
burn a fraction of the anode exhaust gas (including unreacted hydrogen and
other fuel
components) to provide carbon dioxide mixed with steam and nitrogen to be
mixed with
additional air providing oxygen to the cathode. This approach has limitations.
Even more of
the original fuel value is unavailable for relatively efficient
electrochemical power generation,
in view of additional combustion whose heat can only be absorbed usefully by
the thermal
bottoming cycle. Also, the oxygen/nitrogen ratio of the cathode gas is even
more dilute than
ambient air, further reducing cell voltage and hence transferring more power
generation load
less efficiently onto the thermal bottoming plant.
The following Nernst relation for a SOFC expresses the sensitivity of the
electromotive
force t~ the partial pressures of the electrochemical reactants in the anode
and cathode channels,
with the simplifying assumption that CO is converted by the water gas shift
reachon. This
sensitivity is of course greatest at the highest working temperatures of SOFC.
1~ ~a = ~ _ ~~ l~ ~~."-~(cara~de)
~ 0.5
~~l?(rtaaode) ~~~2(cnfDiade)
Adsorption gas separation systems have been considered in the prior art for
manipulating partial pressures of reactants in the fuel cell, so as to achieve
higher fuel cell
voltage E.
According to prior lcnown adsorptive processes, for enriching a component A of
a feed
2,0 gas mixture containing components A and B, an adsorbent material over
which component B is
more readily adsorbed and component A is less readily adsorbed may be
provided. The
adsorbent material contacts flow channels in adsorbers or adsorbent beds. When
the gas
mixture is introduced at a feed pressure and temperature to a first end of the
adsorber during a
feed step of the process, component B is preferentially adsorbed and a first
product enriched in
25 component A may be delivered from the second end of the adsorber as it
becomes loaded with
5
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289 ,
component B. The adsorber may then be regenerated to desorb component B in
reverse flow so
that the process rnay be repeated cyclically.
Regeneration of adsorbent materials may be achieved by alternative strategies
including
pressure swing, displacement purge, thermal swing, or combinations thereof,
according to the
prior art. It has also been claimed that regeneration of a carbon adsorbent
loaded with carbon
dioxide may be achieved by applying an electric current in so-called electric
swing adsorption.
In existing pressure swing adsorption (PSA) systems or vacuum pressure swing
adsorption systems (VPSA), the total pressure of the gas contacting the
adsorber is reduced
(pressure swing) following the feed step, thus reducing the partial pressure
of component B
contacting the adsorbent, and desorbing component B to be exhausted by purging
with a reflux
fraction of already enriched component A. The total pressure of the gas
mixture in the adsorber
is elevated while the gas flow in the adsorber is directed from the first end
to the second end
thereof, while the total pressure is reduced in the regeneration step while
the gas flow in the
adsorber is directed from the second end back to the first end. As a result, a
"light" product (a
gas fraction depleted in the more readily adsorbed component and enriched in
the less readily
adsorbed component A) is delivered from the second end of the sdsorber, and a
"heavy" product
(a gas fraction enriched in the more strongly adsorbed component B) is
exhausted from the first
end of the adsorber.
Alternatively, the total pressure may be kept approximately constant in the
regeneration
step, while component B is desorbed by a third preferably less readily
adsorbed component C,
which was not part of the feed gas mixture, with component C introduced in
reverse flow from
the second end back to the first end of the adsorbers (displacement purge),
thus reducing the
partial pressure of component B contacting the adsorbent, and exhausting
displaced component
B from the first end of the adsorbers. As a result, a first or "light" product
(a gas fraction
depleted in the more readily adsorbed component B and enriched in the less
readily adsorbed
component A) is delivered from the second end of the adsorber, and a "heavy"
product (a gas
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CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
mixture including the more strongly adsorbed component B and the displacement
component C)
is exhausted from the first end of the adsorber.
Regeneration may also be achieved by cyclically raising the temperature
(temperature
swing) of the adsorbent so as to reduce the adsorptive affinity for all gas
species, resulting in
desorption of component B which can then be purged in reverse flow by a purge
stream either
as a reflux of previously enriched component A or by displacement purge with a
component C.
Thermal swing adsorption (TSA) requires bulk heating and cooling of the
adsorbent on a cyclic
basis, so is limited to relatively low cycle frequencies. The heating step may
be achieved by
heating the purge stream before admission to the second end of the adsorbers.
According to the prior art, pressure swing and displacement purge may be
combined, so
that a displacement purge regeneration step is achieved at a lower total
pressure than the feed
pressure. When relatively low cycle frequency necessary for operation of
thermal swing
adsorption processes may be acceptable, thermal swing may be combined with
pressure sv.ring
and/or displacement purge regeneration strategies. The distinction of
displacement purge
processes in the present eonte~at is that the displacement purge stream is
externally provided and
includes a component C that is not contained in the feed gas mixture to be
separated, unlilce
conventional PSA or TSA processes where the purge stream is obtained
internally as a fraction
of the feed gas mixture undergoing separation.
Previously, application of displacement purge processes has been limited by
compatibility of components A, B and C. Even within the context of an overall
separation
being achieved, some intimate mixing will take place due to axial dispersion
in the adsorbers,
fluid holdup in gas cavities, and leakage across fluid seals and valves. While
components B and
C must obviously be compatible as they will be mixed as an intended outcome of
the process,
cross-contamination between components B and C would also take place so as to
require
compatibility of those components as well.
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WO 2004/076017 PCT/CA2004/000289
PSA is widely applied in hydrogen purification (e.g. from syngas generated by
steam
reforming or gasification of a hydrocarbon feedstock, after water gas shifting
to minimize
carbon monoxide concentration), with components A and B representing carbon
dioxide and
hydrogen. In that application, displacement purge using air (or any oxygen-
containing gas with
oxygen appearing as a component C) would in the prior art have been
impracticable owing to
unacceptable hazards of cross-contamination between hydrogen and oxygen.
SIJll~lblARY ~F THE Dl~'C'LO'S'ZTRE
The present disclosure addresses some of the limitations of the prior art in
the
application of gas separation systems to high temperature fuel cell systems.
In an embodiment of the present disclosure, a high temperature fuel cell
electrical
generation system is provided that is adapted to enable selective generation
of electrical power,
and/or hydrogen fuel, and/or useable heat, allowing flexible operation of the
generation system
wherein the generation system may additionally incorporate means for
mitigation of
"greenhouse" gas and other environmentally deleterious gas emissions, and for
enhancing
overall efficiency of operation to increase sustainability of fuel resource
use. In such an
embodiment, the high temperature fuel cell may be either a MCFC or a S~FC.
According to a ftrst embodiment of the disclosed systems and processes, there
is
provided an electrical current generating system that includes at least one
fuel cell operating at a
temperature of at least about 250°C, a hydrogen gas separation system
and/or oxygen gas
delivery system that includes at least one device selected from a compressor
or vacuum pump,
and a drive system for the device that includes means fox recovering energy
from at least one of
the hydrogen gas separation system, oxygen gas delivery system, or heat of the
fuel cell.
According to a second embodiment of an electrical current generating system
according to the
present disclosure, that also includes a high temperature fuel cell, a gas
turbine system may be
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
coupled to the hydrogen gas separation system or oxygen gas delivery system,
wherein the gas.
turbine system may be powered by energy recovered from at least one of the
hydrogen gas
separation system, oxygen gas delivery system, or heat of the fuel cell. The
hydrogen gas
separation system or the oxygen gas delivery system may include an adsorption
module, such as
a pressure swing adsorption module. These generating systems are particularly
useful for use in
conjunction with molten carbonate fuel cells and solid oxide fuel cells.
The present disclosure is concerned with gas separation for application within
a high
temperature fuel cell system, and more particularly with adsorptive separation
of a first gas
mixture containing less readily adsorbed first component (or fraction) A and
more readily
adsorbed second component (or fraction) B, with adsorber regeneration achieved
by
displacement purge, preferably in combination with pressure swing or thermal
swing
regeneration techniques. The displacement purge stream includes a preferably
less readily
adsorbed third component (or fraction) C which may be mixed with component B
in the
regeneration step. A particular requirement for safe use of a gas separation
system for use in a
high temperature fuel cell system is to provide means to avoid or strictly
minimise any mixing
between components A and C in externally delivered or discharged gas streams.
This
requirement arises in fuel cell and other applications where components A and
C may be
mutually chemically reactive, as when component A is a combustible fuel and
component C is
an oxidant.
Thus, a first gas mixture including components A and B is be separated so that
a first
product of the separation is enriched in component A, while component B is
mixed with a third
gas component C contained in a displacement purge stream to form a second gas
mixture
including components A and B, and with provision to prevent cross
contamination of
component C into the first product containing component A, or of component A
into the second
gas mixture containing component C. It is desirable that such cross
contamination be avoided
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WO 2004/076017 PCT/CA2004/000289
or at least strictly minimised for safety or other reasons. Component C may be
a major or minor
constituent of the purge gas stream.
An exemplary apparatus embodiment according to the present disclosure includes
a co
operating set of N adsorbers, each adsorber having a flow path between first
and second ends of
the adsorber, and a flow path contacting an adsorbent material within the
adsorber, with
component B being more readily adsorbed relative to components A and C which
are less
readily adsorbed by the adsorbent material. The adsorbers may be subjected to
a cyclic
adsorption process with process steps as set forth below, with a cycle period
T. Further, the N
adsorbers may be arranged so as to sequentially undergo the steps of the cycle
sequentially in
staggered phase so that the process may proceed in a substantially continuous
fashion.
The process for each adsorber includes a feed step in which the first gas
mixture is admitted at a
first total pressure t~ the first end of the adsorber, while a first or
"light" product gas enriched in
component A is delivered from the second end of the adsorber as component B is
preferentially
adsorbed on the adsorbent contacting the flow channels) of the flow path
within the adsorber.
The process also includes a displacement purge step in which displacement
purge gas
Colltalnlng component C is admitted to the second end of the adsorber, while a
second gas
mixture (or "heavy" product gas) is delivered at a second total pressure from
the first end of the
adsorbers as component B desorbs from the adsorbent. The first and second
pressures may be
substantially similar, or the second pressure may be substantially less than
the first pressure so
as to utilize a pressure swing in the performance of the separation process.
Immediately prior to the displacement purge step, a first "buffer" step may be
performed in the presently disclosed process, in order to substantially remove
interstitial and
adsorbed component A accumulated in the adsorber from the previous feed step,
so as to avoid
contamination of the second gas mixture to be produced in the imminent
displacement purge
step by component A. Likewise, immediately following the displacement purge
step, a second
"buffer" step may be performed in the inventive process, in order to
substantially remove
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
interstitial and adsorbed component C accumulated in the adsorber from the
previous feed step,
so as to avoid contamination of the first product gas to be produced in the
following feed step
by component C.
The optional buffer steps of an aspect of the present invention may be
accomplished in
several ways, including applications of the displacement purge principle by
introducing a buffer
sweep stream, optionally assisted by reducing the total pressure relative to
the pressure during
the displacement purge step in the adsorber during the buffer steps.
Typically, each buffer step
will generate an exhaust stream, in which there may be some admixture of
components A and
C; and such buffer step exhaust streams may be subjected to further processing
(such as
combustion to eliminate any unreacted mixture of A and C) for disposal. Buffer
sweep gas to
achieve displacement purge in the buffer steps may be provided as any less
readily adsorbed gas
stream. The first buffer sweep gas for a first buffer step preferably should
not contain unbound
component A, and the second buffer sweep gas for a second buffer step
preferably should not
contain unbound component C. The first buffer sweep gas may be or may contain
displacement
purge gas 0~ntallllng component C. The second buffer sweep gas may be or lnay
contain first
gas mixture containing component A.
The buffer sweep gas for either buffer step may be selected to be an inert
gas, which
may be flue gas recycled from combustion of the buffer sweep gas under
combustion conditions
for each stream such that A is removed from sweep gas for a first buffer step,
and C is removed
from sweep gas for a second buffer step. For higher temperature applications,
steam may be
used as buffer sweep gas.
Reducing the total pressure (e.g. below the second pressure at which the
displacement
purge step is conducted) during the buffer steps may be desirable to assist
the removal of
components A or C to be purged, and also to avoid any leakage (external to the
adsorbers) of
components A or C between process steps preceding and following each buffer
step. With
reduced total pressure in a Brst buffer step, desorbing component B may assist
the purging of
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component A during that first buffer step. Hence, a minor pressure swing to
reduce the total
pressure during buffer steps, for example by a modest level of vacuum if the
second pressure is
substantially atmospheric, may be used to enhance the reliability of the
buffer steps,
independently of whether a larger pressure swing is applied to assist the
enrichment of
component A.
If the first pressure is much larger than the second pressure, the process may
include
additional steps as provided in well-known pressure swing adsorption processes
for the
depressurization of the adsorber after a feed step and before the first buffer
step, and for
repressurization of the adsorber after the second buffer step and before the
next feed step.
Depressurization steps may include co-currrent and/or countercurrrent blowdown
steps.
Repressurization steps may inelude backfill and feed pressurization steps.
Depressurization and
repressurization steps may be achieved by single or plural pressure
equalization steps performed
between out-of phase adsorbers by providing fluid communication between the
first or second
ends of adsorbers undergoing a pressure equalization step.
In the case that pressure swing is combined with displacement purge in the
present
invention, it may be understood for greatest generality that any of the steps
known for f°SA and
VPSA processes may be incorporated in the present process, which is
characterised by the first
and second buffer steps respectively just before and just after the
displacement purge step. If
desired, a purge step using light product gas or cocurrent blowdown gas, as
purge gas may be
conducted in addition to (and before or after) the displacement purge step.
In order to perform the buffer steps with minimal losses of components A and C
during
those steps, it may be desirable that components A and C (and any buffer sweep
component D)
be weakly adsorbed, and that the number N of adsorbers be large with each
adsorber thus
having a small inventory of adsorbent material, so that the buffer steps may
occupy only a small
fraction of the cycle period T.
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The apparatus of the present disclosure may include a first valve means
communicating
to the first end and a second valve means communicating to the second end of
each adsorber, so
as to perform in sequence for each adsorber the complete cycle of the feed
step, any optional
depressurization steps, a first optional buffer step, a displacement purge
step, a second optional
buffer step, and any optional repressurization steps.
Many potential directional valve configurations (e.g. as used in PSA systems)
may be
used; an exemplary embodiment of the present invention may include rotary
distributor valves
as the first and second valve means. In such an embodiment the N adsorbers may
be mounted
as an array in a rotor engaged in fluid sealing contact on first and second
valve faces with a
stator. The gas separation apparatus of the invention may then be referred to
as a rotary
adsorption module ('6RAM'e).
According to a second exemplary embodiment of the present disclosure, the
rotor of a
rotary adsorption module in a gas separation system for use in the disclosed
systems and
processes may include a number of flow paths for receiving adsorbent material
therein for
preferentially adsorbing a first gas component in response to increasing
pressure in the llow
paths relative to a second gas component. The gas separation system also may
include
compression machinery coupled to the rotary module for facilitating gas flow
through the flow
paths for separating the first gas component from the second gas component.
The stator may
include a first stator valve surface, a second stator valve surface, and
plurality of function
compartments opening into the stator valve surfaces. The function compartments
may include a
gas feed compartment, a light reflux exit compartment and a light reflux
return compartment.
In the above exemplary embodiment the rotary adsorption module may itself
operate at
an elevated working temperature. For example, the operating temperature of the
adsorbers may
range from approximately ambient temperature to an elevated temperature up to
about 450°C,
as may be facilitated by recuperative or regenerative heat exchange between
the feed gas
mixture and the displacement purge stream. The rotary adsorption module may be
operated to
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WO 2004/076017 PCT/CA2004/000289
support a temperature gradient along the length of the flow channels, so that
for example the
temperature at the first end of the adsorbers is higher than the temperature
at the second end of
the adsorbers. As used herein, "operating temperature of the adsorbers"
denotes the
temperature of a gas flowing through the adsorbers and/or the temperature of
the adsorber beds.
The systems and processes of the present disclosure may be applied to hydrogen
(component A) enrichment from syngas mixtures as the first gas mixture, where
dilute carbon
dioxide (component B) is to be rejected such as directly to the atmosphere,
and the
displacement purge stream containing oxygen (component C) may be air or
advantageously
nitrogen-enriched air. The adsorbent material may be selected from those known
in the art as
effective to separate carbon dioxide in the presence of significant levels of
water vapor,
particularly in applications where the separation is performed at elevated
temperature.
For operation near ambient temperature, suitable ads~rbents may include (but
are not
limited to) alurnina gel, activated carbons, carbon molecular sieves,
hydrophilic ~eolites (e.g.
type 13X zeolite and many other zeolites known in the art), other molecular
sieves, and more
preferably hydrophobic ~eolites (e.g. type ~ ~eolite or silicalite). If the
displacement purge
stream is itself humid, it may be necessary to use relatively hydrophobic
adsorbents such as
active carbons and ~eolites such as ~-zeolite or silicalite. Alternatively,
the adsorbent in the
rotary adsorption module may be chosen to be selective at an elevated
operating temperature
(e.g., about 250°C to about 400°C) for carbon dioxide in
preference to water vapor. Suitable
such adsorbents lmown in the art include allcali-promoted materials.
Illustrative alkali-promoted
materials include those containing cations of alkali metals such as Li, Na, K,
Cs, Rb, and/or
alkaline earth metals such as Ca, Sr, and Ba. The materials typically may be
provided as the
hydroxide, carbonate, bicarbonate, acetate, phosphate, nitrate or organic acid
salt compound of
the alkali or alkaline earth metals. Such compounds may be deposited on any
suitable substrate
such as alumina. Examples of specific materials for elevated temperature
operation may
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WO 2004/076017 PCT/CA2004/000289
include alumina impregnated with potassium carbonate and hydrotalcite promoted
with
potassium carbonate, as disclosed in the prior art.
For use in the inventive systems, the adsorbent material may be a conventional
granular
adsorbent, or may advantageously bean adsorbent material supported in a
parallel passage
monolith of high surface area, so that the process may be conducted at
relatively high cycle
frequency (e.g. cycle period of about 1 second to about 10 seconds) in a
compact apparatus
which contains only a small inventory of adsorbent and consequently of
components A and C
which may be mutually chemically reactive. An exemplary such supported
parallel passage
adsorbent material may be a formed adsorbent sheet structure, supported on a
support substrate
(known suitable support substrates include thin fibreglass , wire mesh,
expanded metal, and
carbon matrices). It may be particularly preferred that the adsorbent be
supported as a formed
sheet structure ("structured adsorbent) on thin metallic sheets (e.g. of a
stainless steel wire mesh
or expanded metal foil approximately 150 to 250 microns thick) with metallic
spacers (e.g. of a
similar wire mesh or foil) between the sheets so that the adsorbent laminate
may additionally
function as an effective flame arrestor structure to suppress any accidental
reaction between
mutually reactive components A and C that may occur as the result of any
mechanical or
structural failure of fluid sealing.
An exemplary embodiment of the presently disclosed adsorptive separation
module
includes the use of a parallel passage adsorbent structure with narrow spacers
(and
correspondingly narrow channels) relative to the adsorbent sheet thickness.
Preferably, the
channel voidage ratio (ratio of channel volume to the volume of the active
adsorbent plus
channels) may be in the approximate range of 20% to 35%, e.g. less than the
typical 35%
voidage volume of conventional granular adsorbent, which may not achieve
adequate selectivity
for COa separation from humid gas in this application. It is also noted that
conventional rotary
adsorber technology (as used for removal of strongly adsorbed water vapour or
volatile organic
compounds from air) is based on adsorber wheels with monolithic parallel
channel adsorbent
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
supports (using corrugated sheet adsorbent or honeycomb extrudates) whose
channel voidage
ratio may be of the order of 60% to 80%, so that such adsorbers would be
ineffective for
separation of relatively less strongly adsorbed gases such as CO2.
In another application of the presently disclosed systems and processes, anode
exhaust
gas from solid oxide fuel cells (SOFC) typically contains carbon dioxide and
steam with
unreacted fuel components including hydrogen and carbon monoxide. A SOFC power
plant
also has an available stream of nitrogen-enriched air as the cathode exhaust
stream, or from a
vacuum exhaust of an oxygen VPSA unit which may be used to deliver enriched
oxygen to the
cathode inlet for enhanced voltage efficiency and other benefits. In such a
SOFC system, it is
desirable to improve overall efficiency by recycling hydrogen to the fuel cell
anode inlet. In an
embodiment of the present disclosure, after water gas shifting to convert most
carbon monoxide
to hydrogen (component A) and carbon dioxide (component ~), and preferably
after partial
removal of water vapor, the SOFC anode exhaust gas may be introduced to a
rotary adsorption
module (as described above) as first gas mixture, while air or nitrogen-
enriched air may be used
as displacement purge gas. If the nitrogen-enriched air as displacement purge
is the exhaust
from an oxygen VPSA providing enriched oxygen to the SOFC cathode, a single
vacuum pump
may be used to draw the second gas mixture (comprising exhaust carbon dioxide
and oxygen
depleted air) from the second end of the rotary adsorption module, thus
providing a pressure
swing vacuum for both the oxygen VPSA and the hydrogen-enrichment rotary
adsorption
module.
Industrial H2 PSA is normally conducted at considerably elevated pressures (>
10 bars)
to achieve simultaneous high purity and high recovery (~ 80%-85%). The feed
gas mixture
must be supplied at elevated pressure in order to deliver hydrogen (component
A) at
substantially the feed pressure, while also delivering carbon dioxide
(component B) at an
exhaust partial pressure of approximately one bar. If the carbon dioxide is
being exhausted to
16
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
the atmosphere, this represents a major loss of energy due to lost free energy
of mixing as the
carbon dioxide is diluted to its ambient partial pressure of about 0.00035
bars.
The presently disclosed systems and processes exploit the fact that air
contains only trace
quantities of carbon dioxide to use air or nitrogen-enriched air as the
displacement purge stream
to strip carbon dioxide from a syngas stream at low pressure, and thus achieve
useful hydrogen
enrichment without requiring compression to elevated pressures. Free energy is
thus captured
from dilution of carbon dioxide which may be discharged directly into the
atmosphere.
In application to advanced power generation technologies such as high
temperature fuel
cells, it will be appreciated that overall efficiency can be unexpectedly
increased by the
disclosed systems and processes, which may be used to enable recycle of
enriched hydrogen to
the anode while diluting carbon dioxide into the atmosphere, thus capturing
extra free energy
beyond that normally credited to a combustion process with carbon dioxide
delivered at a
reference pressure of one bar. In the particular case where the high
temperature fuel cell is a
MCFC, the above principles of enrichment of anode exhaust gas in hydrogen
using a
displacement purge adsorption process, for recycle to the anode inlet are
directly applicable,
with the further benefit that the purge desorption gas stream enriched in
carbon dioxide may be
recycled to the cathode inlet to desireably increase the concentration of
carbon dioxide in the
cathode inlet gas relative to that of air, as opposed to discharged into the
atmosphere.
Optionally, the purge gas stream enriched in carbon dioxide may be further
treated prior to
2,0 supply to the cathode inlet, such as by combustion or other process.
Without the buffer steps and other features of the disclosed systems and
processes to prevent
cross-contamination between oxidant and fuel components including hydrogen,
use of air or
even nitrogen-enriched air to purge hydrogen enrichment adsorbers would not
usually be
contemplated in view of safety concerns.
17
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WO 2004/076017 PCT/CA2004/000289
According to an embodiment of the disclosure, there is provided an electrical
current
generating system that includes a high temperature fuel cell, and a H2
enrichment rotary
adsorption module coupled to the fuel cell.
Solid oxide and molten carbonate fuel cells may be designed to operate at a
range of
pressures, with working pressures between about 1 bar to 10 bars being common
in the
disclosed systems. The disclosed systems and processes particularly apply to
high temperature
SOFC and MCFC fuel cell power plants using a hydrocarbon fuel such as natural
gas.
According to an embodiment of the disclosure, before being admitted to the
fuel cell anode
channel inlet, the fuel may be mixed with hydrogen rich gas separated by a
first rotary
adsorption module from the anode exhaust gas, with the separation optionally
performed after
the anode exhaust gas has been subjected to post-reforming and/or water gas
shift reach~n steps
so as to elevate the hydrogen concentration therein while oxidizing carbon
monoxide to carbon
dioxide.
In the important case of natural gas as the hydrocarbon fuel, the anode feed
gas
desirably comprises a mixture including methane and s, large excess of
recycled hydr~gen. 'fhe
excess hydrogen inhibits soot deposition by the methane cracking reaction,
thus allowing safe
operation with a minimum amount of steam in the anode feed gas. 'The amount of
steam in the
anode feed gas may be very low or even substantially zero if the recycle
hydrogen concentration
is maintained at a high level (e.g. about 85 - 90% of the anode feed gas).
Benefits of minimum
steam concentration in the anode feed gas include:
1. high initial ratio of H2 to H20 elevates the Nernst potential to improve
voltage
efficiency and output.
2, methane acts as a chemical sink for fuel cell reaction H20 by steam
reforming,
thus helping maintain a high ratio of H2 to H20 along the anode channel.
18
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WO 2004/076017 PCT/CA2004/000289
3. methane conversion to CO and H2 is delayed along the anode channel as H20
is supplied by the fuel cell oxidation reaction, thus alleviating steep
temperature gradients that
would result from overly rapid endothermic steam reforming at the anode
entrance.
4. low steam concentration inhibits conversion of CH4 and CO to C02, thus
ensuring that the steam reforming reaction within the anode channel is most
highly endothermic
to take up fuel cell waste heat for improved overall heat balance.
By contrast, prior art internally reforming MCFC and SOFC fuel cells typically
operate
with a substantial steam/carbon ratio in the anode feed gas to suppress carbon
deposition, thus
depressing fuel cell voltage performance. This prior art approach typically
requires pre-
reforming of a substantial fraction of the fuel natural gas to avoid excessive
cooling at the anode
entrance and steep temperature gradients, that would result fr~m overly rapid
endothermic
steam reforming as the fuel enters the an~de channel.
Tlae anode exhaust gas typically c~ntains some unreacted methane as well as a
considerable fraction of carbon monoxide. The systems and processes of the
present disclosure
pr~vide ~ptionally that steam may be added to the anode exhaust gas which may
then admitted
at elevated temperature to an adiabatic post-reformer, simultaneously
perfornnng the
endothermic steam reforming reaction with the exothermic water gas shift
reaction so that
external heat exchange for the post-reformer may not be needed.
The foregoing features and advantages will become more apparent from the
following
detailed description of several embodiments that proceeds with reference to
the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
Certain embodiments are described below with reference to the following
figures:
FIG. 1 shows an axial section of a rotary adsorption module.
FIGS. 2 through 4 show transverse sections of the module of FIG. 1.
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FIGS. S through 10 show alternative buffer step purge configurations for the
module of
FIG. 1.
FIGS. 11 through 16 show simplified schematics of alternative SOFC power plant
embodiments using the rotary adsorption module for enrichment and recycling of
hydrogen
from the anode exhaust gas.
FIGS. 17 through 19 show simplified schematics of alternative MCFC power plant
embodiments using the rotary adsorption module with supplementary thermal
swing
regeneration for enrichment and recycling of hydrogen from the anode exhaust
gas.
l~~Z'AL~~~ I~E~°C'~dFTl~N ~~° ~~~L~~1L ~'Il~~~DI~I~I~ITS°
FIGS. 1-4~
A hydrogen-enrichment rotary adsorption module with displacement purge
regeneration
is described below in connection with FIGS. 1-4. As used herein, a "rotary
adsorption module"
includes, but is not limited to, either a device wherein an array of adsorbers
rotates relative to ~
fixed valve face or stator or a device wherein the valve face or stator
rotates relative to an array
of adsorbers. Illustrated embodiments have the adsorbers mounted in a rotor,
with the rotor in a
housing which is a stator with fixed valve faces.
FIG. 1 shows a rotary adsorption module 1, which includes a number "N" of
adsorbers
3 or adsorber channels 3 in adsorber housing body 4. Each adsorber has a first
end 5 and a
second end 6, with a flow path therebetween contacting an adsorbent over which
a gas
component B is more readily adsorbed relative to a component A and a component
C which are
less readily adsorbed. The adsorbers are deployed in an axisymmetric array
about axis 7 of the
adsorber housing body. The housing body 4 is in relative rotary motion about
axis 7 with first
and second functional bodies 8 and 9, being engaged across a first valve face
10 with the first
functional body 8 to which a first gas mixture containing components A and B
is supplied in a
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
first sector and from which a second gas mixture containing components B and C
is withdrawn
from a second sector, and across a second valve face 11 with the second
functional body 9 from
which a first or light product enriched in component A is withdrawn in a first
sector and to
which a displacement purge stream containing component C is supplied in a
second sector.
In embodiments as particularly depicted in FIGS. 1-5, the adsorber housing 4
rotates
and shall henceforth be referred to as the adsorber rotor 4, while the first
and second functional
bodies are stationary and together constitute a stator assembly 12 of the
module. The first
functional body shall henceforth be referred to as the first valve stator 8,
and the second
functional body shall henceforth be referred to as the second valve stator 9.
In other
embodiments, the adsorber housing 4 may be stationary, while the first and
second functional
bodies are rotary distributor valve rotors.
In the embodiment shown in FIGS. 1-4, the flow path through the adsorbers is
parallel
to axis 7, so that the flow direction is axial, while the first and second
valve faces are shown as
flat annular discs normal to axis 7. However, more generally the flow
direction in the adsorbers
may be axial or radial or a. combination thereof, and the first and second
valve faces may be any
figure of revolution centred on axis 7, sueh as planar, conical, cylindrical,
etc. The steps ~f the
process and the functional compartments to be defined will be in the same
angular relationship
regardless of a radial or axial flow direction in the adsorbers.
FIGS. 2-4 are cross-sections of module 1 in the planes defined by arrows 12'-
13', 14'-
15', and 16'-17'. Arrow 20 in each section shows the direction of rotation of
the rot~r 4.
FIG. 2 shows section 12'-13' across FIG.1, which crosses the adsorber rotor.
Here,
"N"=72. The adsorbers 3 are mounted between outer wall 21 and inner wall 22 of
adsorber
wheel 208. Each adsorber in the particular embodiment depicted comprises a
rectangular flat
pack 3 of adsorbent sheets 23, with spacers 24 between the sheets to define
flow channels here
in the axial direction. Separators 25 are provided between the adsorbers to
fill void space and
prevent leakage between the adsorbers. The packs 3 may be radially tapered to
improve the
21
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
volume packing of adsorbent. In alternative embodiments, the adsorbers may
comprise multiple
layers of adsorbent laminate oriented in a concentric spirally wrapped
configuration, or other
suitable monolithic structure, or alternatively may compose beaded or other
particulate
adsorbent arrangments.
As shown in FIG. 1, the adsorbers 3 may include a plurality of distinct zones
between
the first end 5 and the second end 6 of the flow channels, here showwas two
zones respectively
a first zone 26 adjacent the first end 5 and a second zone 28 adjacent the
second end 6. As an
alternative to distinct zones of adsorbents, the different adsorbents may be
provided in layers or
mixtures that include varying gradients of adsorbent concentrations along the
gas flow path.
The transition from one adsorbent to another may also be a blended mixture of
the two
adsorbents rather than a distinct transition. A further option is to provide a
mixture of the
different adsorbents that may or may not be homogeneous.
In the case of a H2 adsorption separator operating at ambient temperature up
to about
250°C, the first zone may contain an adsorbent or desiccant selected
for removing very strongly
adsorbed components of the feed gas mixture, such as water or methanol vapor,
and some
carbon dioddide. The second zone play contain an adsorbent typically selected
for bulk
separation of carbon dioxide.
In the case of a H2 PSA operating at about 250°C to about 500°C,
the first zone may
contain an adsorbent that preferentially adsorbs CO2 relative to water vapor
as described above.
The second zone may contain an adsorbent (e.g., zeolite, Cu(I)-containing
material, or Ag(I)-
containing material) that preferentially adsorbs CO relative to water vapor.
According to one
version, the C02-selective adsorbent and the CO-selective adsorbent may be
included or mixed
together in a single zone rather than in two distinct zones.
The adsorbent sheets typically comprise a reinforcement material (e.g., glass
fibre,
metal foil or metal or carbon mesh) to which the adsorbent material is
attached or impregnated.
Satisfactory adsorbent sheets have been made by coating a slurry of adsorbent
crystals with
22
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
binder constituents onto the reinforcement material, with successful examples
including
nonwoven fibreglass scrims, woven metal (wire mesh) or carbon-based fabrics,
and expanded
metal foils. Spacers are provided by printing or embossing the adsorbent sheet
with a raised
pattern, or by placing a fabricated spacer between adjacent pairs of adsorbent
sheets.
Alternative satisfactory spacers have been provided as woven metal (wire mesh)
screens, non-
woven fibreglass scrims, and metal foils with etched flow channels in a
photolithographic
pattern. Adsorbers of the layered adsorbent sheet material may be formed by
stacking flat or
curved sheets; or by forming a spiral roll, with the flow channels between the
sheets extending
from the first end of the adsorber to the second end thereof; to fill the
volume of the adsorber
housing of the desired shape. Examples of methods and structures with packed,
spirally wound
adsorbents are disclosed in commonly owned, co-pending U.S. provisional
application l~To.
60/285,527, filed April 20, 2001, and incorporated herein by reference.
Typical experimental sheet thicknesses have included the range of about 50-300
microns, with spacer heights in the range of 100 to 200 microns, and adsorber
flow channel
length in the range of 10 cm to approximately 20 cm. Alternative dimensions
and lengths have
also proven successful.
In other embodiments of the invention, the adsorbers may be provided as an
array of
spiral rolls of adsorbent sheet and spacers as described above, with the array
supported in a
rotor.
Alternatively, the adsorbers may be fornzed by winding a single spiral roll of
adsorbent
sheet around the rotor hub and filling the annulus to wall 21. Spacers between
adjacent
adsorbent sheet layers may be formed by longitudinal spacers or corrugations,
establishing axial
flow channels between the sheets and extending between the first end 5 and
second end 6, while
the spacers or corrugations prevent flow transverse to the flow channels or
between adjacent
flow channels. Consequently, each such flow channel may be isolated from
neighbouring flow
23
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
channels through the adsorbent mass, and serves as a small independent
adsorber. With this
approach, the number N of independent adsorbers may be extremely large.
Also alternatively, the adsorbers may be provided as flow channels in a
monolith, for
example a honeycomb cordierite extrudate with adsorbent washcoated onto the
cell walls of the
honeycomb, The rotor may be formed from a single extrudate section, or from an
array of such
sections supported on the rotor.
In all cases, the adsorbers and rotor may be assembled with co-operating fluid
sealing
means so that substantially all fluid flow between the first and second ends
of the adsorbers
passes through the flow channels in the adsorbers, so that bypass leakage is
avoided.
FIG. 3 shows the porting of rotor 4 in the first and second valve faces
respectively in
the planes defined by arrows 14~'-15', and 16'-1T. An ads~rber port 30
provides fluid
c~mmunication directly from the first or sec~nd end of each adsorber to
respectively the first or
second valve face. Each such port 30 may be equivalently provided by a number
of small ports
for each adsorber.
FIG. 4 shows a typical stator valve face 100 of the first stator ~ in the
first valve face 10
and iaa the plane defined by arrows 14'-15', similar to a valve face 101 of
the sec~nd stator ~ in
the second valve face 11 and in the plane defined by arrows 16'-17'. Arrow 20
indicates the
directi~n of rotation by the adsorber rotor, In the annular valve face between
circumferential
seals 106 and 107, the open area of first stator valve face 100 ported to
external conduits is
indicated by clear angular sectors 111-116, which are separated by radial
seals 118
corresponding to the first functional ports communicating directly to
functional compartments
identified by the same reference numerals 111-116. Sector 113 is used for the
first buffer step,
and sector 114 is used for the second buffer step. If pressure swing is used
to augment
displacement purge regeneration, a sector 115 may be provided for a
pressurization step and a
sector 116 may be provided for a depressurization step. Similarly, the open
area of second
stator valve face 101 (as shown in FIGS. 5, 7 and 9) ported to external
conduits is indicated by
24
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
clear angular sectors 121-126, which are also separated by radial seals 118
corresponding to the
first functional ports communicating directly to functional compartments
identified by the same
reference numerals 111-116. Typical radial seal 118 provides a transition for
an adsorber
between being open to adjacent sectors. A gradual opening may be provided by a
tapering
clearance channel between the slipper and the sealing face, so as to achieve
gentle pressure
equalisation of an adsorber being opened to a new compartment. Much wider
closed sectors
may be provided to substantially stop flow to or from one end of the adsorbers
when
pressurization or depressurization steps are being perfornied from the other
end.
Turning back to FIG. 1, in the first valve face 100 feed gas (the first gas
mixture
including components A and B) is supplied to first sector 111 as indicated by
arrow 125, while
heavy product (the second gas mixture including components B and C) is
exhausted from
second sector 112 as indicated by arrow 126. In the second valve face 101, the
first or light
product gas (enriched in component A) is delivered from first sector 211 as
indicated by arrow
127, while displacement purge gas (including component C) is supplied to
second sector 122 as
indicated by arrow 128.
The rotor is supported by bearing 160 with shaft seal 161 on rotor drive shaft
162 in the
first stator 8, which is integrally assembled with the first and second valve
stators. The adsorber
rotor is driven by m~tor 163 as an exemplary rotor drive means.
FIGS. 5 and 6
FIG. 5 shows the first and second stator valve faces 100 and 101 of an
embodiment
with displacement purge gas as the first buffer purge gas, and the feed or
first gas mixture as the
second buffer gas. In FIG. 5 and also FIGS. 7 and 9, the first and second
stator valve faces are
being viewed in one direction as indicated by section arrows 14'-17'so that
the first stator valve
face is being viewed from behind while the second valve face is being viewed
from in front.
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
FIG. 6 shows the flow pattern through the adsorbers, in a circumferential
section including the
angular range of 0° to 360° about axis 7.
The first buffer purge gas is admitted by valve 180 to sector 123 in the
second valve
face 101, and displaces gas from sector 113 in the first valve face to burner
182 with co-
operating heat recovery means 183. The second buffer purge gas is admitted by
valve 185 to
sector 114 in the first valve face 100, and displaces gas from sector 123 in
the second valve face
to burner 182 with co-operating heat recovery means 183. The heat xecovery
means may be a
heat exchanger to preheat oxidant and fuel streams being supplied to the fuel
cell, or a steam
generator, or an internal combustion engine, or a gas turbine, or a Stirling
engine.
FIGS. 7 and 8
FIG. 7 shows the first and second stator valve faces 100 and 101 of an
embodiment
with recycled flue gas as the first and second buffer purge gases, with this
flue gas obtained by
combustion of the buffer purge gases so that unbound component C is removed
form the first
buffer purge gas and unbound component ~ is removed from the second buffer
purge gas. FIG.
8 shows the flow pattern through the adsorbers, in ~ circumferential section
including the
angular range of 0° to 360° about axis 7.
The buffer gas streams are admitted to the first valve face 10, with the first
buffer
stream through sector 113 and the second buffer stream through sector 114. A
portion of tile
first buffer stream is recirculated from sector 113' bacle to sector 113,
after being displaced by
the initially entering displacement purge stream.
The first buffer stream is withdrawn from sector 123 by blower or vacuum pump
187,
and the second buffer stream is withdrawn from sector 124 by blower or vacuum
pump 187'.
The buffer streams are passed through burner 182 with co-operating heat
recovery means 183,
and then through condenser 189 to reject excess water through discharge
conduit 190.
Complete or partial separation of the first and second buffer streams may be
maintained through
26
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
burner 182 and condenser 189, as indicated by dashed partitions 191 and 192,
so that
combustion conditions on each side of partition 191 may be maintained
appropriately fuel rich
on the first buffer stream side in order to remove unbound component C from
the first buffer
purge gas, and lean on the second buffer stream side to remove unbound
component A from the
second buffer purge gas. Alternatively, the first and second buffer streams
may be mingled
through a single blower and vacuum pump 187, and through the burner and
condenser, by
maintaining closely stoichiometric combustion conditions in the burner so that
unbound
components A and C are both extinguished. The burner may be a catalytic
combustor in order
to achieve satisfactory and sufficiently complete combustion under all
conditions.
FIGS. 9 and 10
FIG. 9 shows the Erst and second stator valve faces 100 and 101 of an
embodiment
with combined pressure swing and displacement purge regeneration and with
recycled flue gas
as the first and second buffer purge gases. FIG. 10 shows the flow pattern
through the
adsorbers, in a circumferential section including the angular range of
0° to 360° about axis 7.
W the first stator valve face 100, sector 115 is used for a feed
pressurization step, with
feed gas mixture introduced through an orifice or pressure reduction
restrictor 193, while sector
116 is used for a countercurrent blowdown step for depressurization preceding
the first buffer
step. In the second stator valve face 101, sector 125 provides a
repressurization step by light
reflux (pressure equalization) through conduit 195 and restrictor 196 with
sector 126 which
provides the corresponding depressurization step. Sector 125' provides another
repressurization
step by light reflux (pressure equalization) through conduit 195' and
restrictor 196' with sector
126' which provides the corresponding depressurization step.
Extended closed sectors of valve face 100 are provided as wide radial seals
(e.g. 197,
197') opposite the light reflux sectors 125,125', 126 and 126' of face 101.
Similarly wide radial
seals (e.g. 198, 198') are provided in closed sectors of valve face 101
opposite the feed
27
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
pressurization sector 115 and the countercurrent blowdown sector 116 of face
100. It may also
be noted in FIG. 10 that the radial seals leading sectors 111, 115, 116, 125,
125', 126, and 126'
have tapered clearance gaps (e.g. 199) between the rotor face and the
respective seal entering
those sectors, so as to provide smooth pressurization and depressurization
transitions by flow
throttling in the tapered clearance as each adsorber comes into registration
with the
corresponding sector.
If desired, a purge step using light reflux of enriched component B may be
included in
addition to a displacement purge step including component C.
FIGS.11-16
The following embodiments illustrate application of the invention to solid
oxide fuel
cell power plants. S~FC stack 302 includes a solid oxide electrolyte membrane
310 interposed
between anode channel 312 and cathode channel 314. The anode channel has an
inlet 316 and
an outlet 318 connected by anode loop including anode exhaust conduit 319 and
anode return
conduit 319', while the cathode channel 314 ha,s an inlet 320 and an outlet
321. If the fuel is
natural gas, it is internally reformed within the anode channel 312, while a
suitable excess
concenhation of recycled hydrogen and preferably soave steam is maintained in
anode loop 319
so as to prevent carbon deposition.
A first rotary adsorption module 1 according to the invention receives water
gas shifted
and cooled anode exhaust gas from anode outlet 318, first recuperator 322,
water gas shift
reactor 324, second recuperator 326, and water removal condenser 328 as feed
gas mixture in
conduit 130. The water gas shift reactor would typically have an exit
temperature in the range
of about 200°C to about 400°C. ~ Excess water is discharged by
conduit 329. Hydrogen
enriched gas as the light product of the first rotary adsorption module 1 is
delivered by conduit
132 to recycle fan 330, after which the enriched hydrogen recycle stream is
joined by feed
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CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
natural gas supplied to the anode loop by infeed conduit 336. The anode
recycle gas is reheated
in recuperators 326 and 322 before being admitted to the anode inlet 316.
Feed air is admitted by infeed conduit 340 to air feed blower 341, and thence
by
recuperator 342 to cathode inlet 320. Vitiated (nitrogen-enriched) cathode
exhaust air is
discharged from cathode outlet to optional heat hat recovery heat exchanger
344, and back
through recuperator 342 by conduit 346 which communicates to displacement
purge inlet
conduit 133 of the first rotary adsorption module. Spent displacement purge
gas, including
vitiated air and carbon dioxide, is discharged by conduit 131 to exhaust 350.
In FIG. 11, the buffer purge streams are supplied from the feed gas mixture
and the
displacement purge gas as described for FIGS. 5 and 6 above. The buffer
exhaust is
schematically indicated by conduit 352 from module 1. The spent buffer purge
gas is burned in
burner 182 with heat recovery means 183, and discharged by flue conduit 355.
FIG. 12 illustrates a similar embodiment using recycled flue gas as the buffer
purge gas,
as described for FIGS. 7 and 8 above. The spent buffer purge gas is burned in
burner 182 with
heat recovery means 183, excess water is removed by condenser 189 and water
discharge
conduit 190, and the: flue gas is recycle to buffer inlet sectors in module 1
by scheanatically
depicted return conduit 360.
FIG. 13 shows two independent alternative features, including use of a second
rotary
adsorption module 380 to transfer a fraction of water vapor (remaining in the
anode exhaust
stream after the water gas shift step in reactor 324) from conduit 319 to
anode return conduit
319', and use of a VPSA unit to provide enriched oxygen to the cathode and
nitrogen-enriched
exhaust to be used under vacuum as displacement purge gas. In this example,
the buffer purge
gas is provided from the feed gas mixture in conduit 130 and the displacement
purge gas (which
will be the oxygen vitiated exhaust of the oxygen VPSA) in conduit 133.
The second rotary adsorption module 380 includes an adsorber rotor 381 engaged
at
first and second ends with rotary valve faces 382 and 383. It uses an
adsorbent With high
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CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
selectivity for water vapor relative to carbon dioxide and other anode gas
constituents at the
operating temperature, which may be that of the water gas shift reactor exit.
In FIG. 13, enriched oxygen is delivered from oxygen VPSA unit 400 and
optionally an
oxygen product compressor directly to the cathode channel inlet 321 by conduit
402 through
recuperators 326 and 322. The VPSA unit includes a rotary adsorber module 404
and a light
reflux loop 406 to achieve high oxygen purity. The nitrogen enriched exhaust
is delivered
under vacuum by conduit 408 communicating to conduit 133. Motor or engine 410
drives feed
blower 341 and a vacuum pump 412 which draws the nitrogen-enriched exhaust
with exhaust
carbon dioxide as spend displacement purge gas from conduit 131.
The vacuum pump discharge may be passed by discharge conduit 414 through a
catalytic burner 416 (co-operating with heat recovery means 183) to remove any
residual
combustible components. Exhaust cathode gas is conveyed from the cathode
channel exit 320
by conduit 440 to combustor 182, thus providing the buffer gas burner with
enriched oxidant as
well as heat recovery from the cathode channel.
In FIGS. 11-13, the first rotary adsorption module would operate just above
ambient
temperature, after condensation of excess water from the feed gas mixture.
FIGS. 14-16
illustrate embodiments in which the first rotary adsorption module would
operate at an elevated
temperature corresponding to that of the water shift reactor exit. The
adsorbent would be
selected for carbon dioxide selectivity and insensitivity to water vapor. An
example of a
suitable adsorbent is potassium promoted hydrotalcite. Carbon dioxide removal
at a relatively
elevated temperature will reduce the flows and heat exchange load in
recuperator 326.
In FIGS. 14-16, the light product stream (enriched hydrogen) in conduit 132 is
split into
a first and a second recycle streams, which are recombined in ejector 500.
Ejector 500 has a
nozzle 501, a suction inlet 502, a mixing zone 503, and a diffuser 504. The
first recycle stream
is conveyed directly to suction inlet 502 by conduit 510. The second recycle
stream is passed
through recuperator 326, condenser 328, recycle blower 330, and with feed from
infeed conduit
CA 02516300 2005-08-16
WO 2004/076017 PCT/CA2004/000289
336 back through recuperator 326 to ejector nozzle 501. It will be seen that
blower 330
provides the driving energy through nozzle 501 to operate the ejector 500.
In FIG. 14, a fraction of the water condensate in conduit 329 is conveyed
through
recuperatox 520 by conduit 522, and thence to steam generator coil 524 in the
water gas shift
reactor 324, and as steam to be used as the buffer purge via conduit 360. The
spent buffer purge
steam is carried by conduit 352 to burner 182.
In FIGS. 15 and 16, a portion of the steam from steam generator coil 524 is
recycled by
conduit 530 to the anode gas loop. In FIG. 15, conduit 530 conveys steam
through recuperator
322 to a post-refornzer reactor 540 which adiabatically converts a portion of
unreacted methane
and unreacted carbon monoxide in the anode exhaust gas from outlet 318. The
post-reformer
uses any of the lcnown catalysts effective for steam methane reforming or high
temperature
water gas shift. Alternatively, conduit 530 may simply introduce the steam to
the inlet of water
gas shift reactor 5249 without having passed through recuperator 322. This
steam addition, and
the consequent greater extent of water gas shift achieved, is highly desirable
to maximize
hydrogen recovery, to assist in water removal from the anode loop, and also to
elevate the
carbon dioaside con centration in conduit 130 for more effective removal. A
further important
advantage is the maximal removal of carbon monoxide from the anode return
conduit 319', as
highly desirable to reduce the risk of carbon deposition in the anode channel
and to facilitate
carbon-free operation with a minimal concentration of steam in the anode
inlet.
FIG. 16 shows the steam from conduit 530 being supplied to a pre-reformer 550
for
partially converting the feed fuel to syngas (hydrogen and carbon monoxide) as
well as
methanating part of that syngas, so that higher hydrocarbons are at least
partially converted to
methane in order to reduce the risk of soot deposition in the fuel cell anode
channel. The pre-
reformer also re-establishes the water gas shift equilibrium to minimize
carbon monoxide after
carbon dioxide removal in the ftrst rotary adsorption module, so as to widen
the margin of
safety against carbon deposition. The pre-reformer will use a steam reforming
catalyst that may
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WO 2004/076017 PCT/CA2004/000289
be selected for tolerance to feed impurities, and may operate at a relatively
low temperature in
the range of about 400°C to about 600°C so as to promote the
methanation reaction. The pre-
reformer may operate adiabatically, with at least partial heat balance between
the endothermic
steam reforming reaction and the exothermic methanation reaction. In the
present invention,
methane is a desirable component of the anode feed since it will act as a
scavenger (by steam
reforming) for water generated by the fuel cell reaction. Recycle hydrogen
from the first rotary
adsorption module may be passed through the pre-reformer with the feed fuel as
shown in FIG.
16, or alternatively may be bypassed directly to the anode inlet 316 without
passing through the
pre-reformer.
Steam must be added to the inlet of pre-reformer 550 at a sufficient
concentration for
steam reforming and coking suppression in the pre-reformer.
Water vap~r array be transferred acr~ss the anode loop in any of the
embodiments by a
second rotary adsorption module 380 (as shown in FIG. 13) operating as a
desiccant humidity
exchanger coupled between conduits 319 and 319'.
1~ A heat engine may be used as the thernlal b~ttoming system t~ recover waste
kae~t
available from heat rec~very means 183 ass~ciated with buffer gas combustion,
or waste heat
from the fuel cell stack heat exchanger 344, or waste heat from the exothermic
water gas shift
react~r. While some waste heat will be used to preheat reactants and to
overcome heat
exchanger losses, there may be useful scope for a supplemental heat engine, to
drive plant
auxiliaries such as fans, blowers and vacuum pumps, and possibly to deliver
supplemental
power for export.
As hydrogen enrichment and optional oxygen enrichment features of the present
invention serve to elevate the fuel cell voltage efficiency and stack power
output, it may be
preferred that all export power be exported by the fuel cell stack, and that
the heat engines)
therefore be used solely to drive plant auxiliaries.
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WO 2004/076017 PCT/CA2004/000289
Engine 410 may be a gas turbine using air as its worldng fluid (as in some
embodiments
of copending patent application "Energy Efficient Gas Separation for Fuel
Cells" filed October
26, 2001, whose entire disclosure is incorporated herein), heated by indirect
heat exchange with
the anode andfor cathode gases and by combustion of the heavy product exhaust
gas from the
first PSA unit and/or by combustion of supplemental natural gas fuel.
Alternatively, the anode gas mixture may be used directly as the working fluid
for a fuel
cell stack heat recovery thermodynamic cycle, e.g. by a recuperative gas
turbine. This is
particularly attractive when an oxygen VPSA unit is used to concentrate
enriched oxygen as the
cathode oxidant stream. The gas turbine may then serve as engine 410 to
operate the oxygen
separation unit, and would preferably be applied in turbocharger
configurations.
Engine 410 may be an internal combustion engine fuelled by spent buffer gas
containing hydrogen and other fuel values, and/or by supplemental natural gas
fuel.
Alternatively, engine 410 may be any type of ea~ternal combustion engine, e.g.
a steam engine
or a Stirling engine. Since the first rotary adsorption module generates
enriched hydrogen, the
heat recovery means may advantageously be a Stirling engine with hydrogen
working fluide
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WO 2004/076017 PCT/CA2004/000289
Figs. 17 -19
Figs. 17 - 19 show particular MCFC embodiments of the disclosure, in which
supplementary thermal swing regeneration is applied to assist in desorption of
COz from the
adsorbent material. In each of the embodiments illustrated in these figures,
air (rather than
cathode exhaust gas as in the case of the above SOFC embodiments) may be used
as a
displacement purge gas to remove COZ from the rotary adsorber module, and the
COZ-laden air
is advantageously recycled for use as oxidant in the molten carbonate fuel
cell cathode.
Somewhat oxygen-enriched air or other suitable purge gas stream may
alternatively be used
instead of air.
In the embodiments shown in Figs. 17, 18 and 19, a cooling coil 501 is shown
in water
condenser 328 for the purpose of cooling the anode exhaust stream prior to
entry into rotary
adsorption module 1, to enhance adsorption of CO2 by the adsorbent material
inside module 1.
A suitable displacement purge gas stream such as air or somewhat oxygen-
enriched air is
introduced from inlet recuperator 510 by blov,~er 515, and is preheated by the
final cooling of
cathode e~~haust gas in recuperator 520, so as to provide preheated purge gas
to assist in the
desorption of CO2 from the adsorbent material in rotary adsorption module 1.
Components are
identified in Figs. 17 - 19 by the same reference numerals as in preceding
Figs. 11 - 16
applicable to SOFC embodiments. Water vapor may also be transferred across the
anode loop
in any of the MCFC embodiments of the invention by a second rotary adsorption
module 380
(as shown in FTG. 18 and 19) operating as a desiccant humidity exchanger as
similarly shown
and described in the above SOFC embodiments.
In the above disclosed systems for both SOFC and MCFC fuel cells, the systems
may
optionally be modified to allow for at least a portion of the enriched H2
product gas stream
produced by the rotary adsorption module 1 to be exported from the high
temperature fuel cell
system for use as H2 fuel for other applications, such as PEM fuel cells. A
second gas
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WO 2004/076017 PCT/CA2004/000289
separation system, such as an additional displacement purge or pressure swing
adsorption
system may be included to further enrich the H2 concentration of the exported
H2-containing
gas stream, so as to purify the H2 fuel for use in other applications
requiring high purity H2.
Rotary displacement purge or pressure swing adsorption systems may
advantageously be used
for such purification purposes. Other separation systems suitable to purify
the export H2 stream
may also be used, such as membrane separation systems. In the case where a
pressure swing
adsorption system is used as a second separation system for purification of
the export H2
stream, a feed compressor may be required to compress the export H2 gas stream
to an
appropriate feed pressure for PSA separation, as is conventionally known in
the PSA field.
Such H2 export gas stream may optionally then be stored for further use as H2
fuel, such as by a
high pressure H2 storage system. In such applications therefore, the disclosed
inventive high
temperature fuel cell systems may be used to selectively produce electrical
current directly in
the high temperature fuel cell, H2 for export as fuel, andlor heat for
additional power generation
or other uses - and the relative production of these three "products" may be
varied and
determined by the control of the adsorptive separation systemfs) and recycled
gas streams used
in combination with the high temperature fuel cell.
It will be evident that there may be many other alternatives and variations of
the
disclosed systems and processes, particularly for application to molten
carbonate and solid
oxide fuel cell generation systems.
For SOFC and 1VICFC power plants, the disclosed systems and processes may
enhance
power generation performance by increasing the ratio of hydrogen to steam
partial pressure in
the anode.' Estimated efficiencies based on fuel lower heating value are in
the range of about
65% to about 75% for natural gas fuelled fuel cell power plants. The presently
disclosed
systems and processes also facilitate cogeneration of efficiently generated
electrical power,
purred hydrogen, and low-grade waste heat suitable for building heating or
domestic hot water
utilities.
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WO 2004/076017 PCT/CA2004/000289
Having illustrated and described the principles of the disclosure with
reference to
several embodiments, it should be apparent to those of ordinary skill in the
art that the
specifically disclosed embodiments may be modified in arrangement and detail
without
departing from such principles.
36
