U.S. patent application number 10/527971 was filed with the patent office on 2006-06-15 for power generation apparatus.
Invention is credited to Arne Raheim, Arild Vik.
Application Number | 20060127714 10/527971 |
Document ID | / |
Family ID | 9944038 |
Filed Date | 2006-06-15 |
United States Patent
Application |
20060127714 |
Kind Code |
A1 |
Vik; Arild ; et al. |
June 15, 2006 |
Power generation apparatus
Abstract
A power generation apparatus comprises a fuel cell and a
reforming module, wherein the reforming module is adapted to reform
hydrocarbon fuel into hydrogen and other components, and to
separate said hydrogen from said other components. The apparatus is
arranged so that said hydrogen is fed from the reforming module to
the anode of the fuel cell. Carbon dioxide may be separated in the
reforming module. Hydrogen may be recycled from the anode outflow
back to the anode and/or tapped off. The apparatus may also contain
a desorption module for releasing carbon dioxide. The absorption
and release of carbon dioxide may be integrated and the carbon
dioxide absorbent and/or desorbent may be recycled. Components of
the apparatus may be thermally integrated. The apparatus may be
used to generate electricity and produce hydrogen.
Inventors: |
Vik; Arild; (Blomsterdalen,
NO) ; Raheim; Arne; (Hamar, NO) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD
SEVENTH FLOOR
LOS ANGELES
CA
90025-1030
US
|
Family ID: |
9944038 |
Appl. No.: |
10/527971 |
Filed: |
September 15, 2003 |
PCT Filed: |
September 15, 2003 |
PCT NO: |
PCT/GB03/03969 |
371 Date: |
October 13, 2005 |
Current U.S.
Class: |
429/411 ;
429/425; 429/513 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 8/0662 20130101; H01M 2008/1293 20130101; H01M 8/04164
20130101; H01M 8/04097 20130101; H01M 8/0625 20130101 |
Class at
Publication: |
429/019 |
International
Class: |
H01M 8/06 20060101
H01M008/06 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 13, 2002 |
GB |
0221304.9 |
Claims
1. A power generation apparatus comprising a fuel cell and a
reforming module, wherein the reforming module is adapted to reform
hydrocarbon fuel into hydrogen and other components, and to
separate said hydrogen from said other components, the apparatus
being arranged so that said hydrogen is fed from the reforming
module to the anode of the fuel cell.
2-31. (canceled)
Description
[0001] This invention relates to power generation apparatus
containing fuel cells and particularly, but not exclusively, to
apparatus which allow the co-production of hydrogen as well as
electricity.
[0002] There is an ever increasing need to produce power as
efficiently and cleanly as possible. Of particular concern is the
discharge of carbon dioxide into the atmosphere. This is widely
recognised to contribute to global warming and thus efforts are
made to reduce carbon dioxide emissions into the atmosphere. One
way of achieving this is of course to increase the efficiency with
which power is generated from fuel. Another potential way of
reducing carbon dioxide emissions into the atmosphere is to capture
and store the carbon dioxide produced by the power generation
process. This is problematic in conventional power generation
systems based on combustion in air, however, since the carbon
dioxide in the combustion products is mixed with a large amount of
nitrogen. The presence of nitrogen makes the capture and separation
of carbon dioxide significantly more expensive.
[0003] In recent years there has been a lot of interest in fuel
cells which are devices which are able to generate an electric
current and heat directly from fuel without combustion. The direct
generation of electric current means that the efficiency of such
devices is not limited by thermodynamic efficiency limits. However,
power generation systems based on fuel cells may still produce
carbon dioxide.
Fuel Cell Systems
[0004] Most fuel cells operate on gaseous fuel, usually hydrogen
(H.sub.2), methane (CH.sub.4) or carbon monoxide (CO), as well as
oxygen (O.sub.2). A fuel cell comprises an anode and a cathode
separated from each other by an electrolyte. Two types of
electrochemical reactions occur: oxidation half-reaction(s) at the
anode and reduction half-reaction(s) at the cathode. Typically,
hydrogen (which may have been produced in situ from natural gas or
other fuel in a process known as "reforming") undergoes
electrochemical reaction at the anode, oxygen (which may be
supplied in the form of air) undergoes electrochemical reaction at
the cathode and the net reaction provides water and generates
electrical power. Other components, such as methane or carbon
monoxide, may also be present in the inflow to the fuel cell,
particularly where the hydrogen is prepared by natural gas
steam-reforming or coal gasification. This means that as well as
water, there may be other products such as carbon dioxide.
[0005] There are several types of fuel cell, some of which are
described below.
[0006] PEM (Polymer Electrolyte Membrane or Proton Exchange
Membrane) cells operate at low temperatures (60-100.degree. C.).
The electrolyte is a solid, flexible polymer. Hydrogen cations pass
from the anode to the cathode. Platinum catalysts are used on both
the cathode and anode. Water is produced at the cathode.
[0007] PAFC (Phosphoric Acid Fuel Cells) operate at moderate
temperatures (150-200.degree. C.). The electrolyte is a phosphoric
acid matrix. Hydrogen cations pass from the anode to the cathode.
Platinum catalysts are used on both the cathode and anode. A small
amount of carbon monoxide in the hydrogen in-flow may be tolerated.
Water is produced at the cathode. The reactions for both PEM fuel
cells and PAFCs are:
[0008] At anode: 2H.sub.2.fwdarw.4H.sup.++4e.sup.-
[0009] At cathode: O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O
[0010] Net reaction: 2H.sub.2+O.sub.2.fwdarw.2H.sub.2O
[0011] MCFC (Molten Carbonate Fuel Cells) operate at high
temperatures (600-1000.degree. C.). The electrolyte is a matrix of
carbonates (e.g. Lithium, Sodium, Potassium and/or Magnesium
carbonates). Carbonate anions pass from the cathode to the anode,
and carbonate anions lost in this way are compensated for by
supplying carbon dioxide to the cathode. Carbon monoxide may also
be present in the hydrogen in-flow and used as fuel. Water is
produced at the anode. The reactions are:
[0012] At anode:
2H.sub.2+2CO.sub.3.sup.2-.fwdarw.2H.sub.2O+2CO.sub.2+4e.sup.-
[0013] (also, if CO present:
2CO+2CO.sub.3.sup.2.sup.-.fwdarw.4CO.sub.2+4e.sup.-)
[0014] At cathode:
O.sub.2+2CO.sub.2+4e.sup.-.fwdarw.2CO.sub.3.sup.2-
[0015] Net reaction: 2H.sub.2+O.sub.2.fwdarw.2H.sub.2O
[0016] (also, if CO present: 2CO+O.sub.2.fwdarw.2CO.sub.2)
[0017] SOFC (Solid Oxide Fuel Cells) also operate at high
temperatures (600-1000.degree. C.). The electrolyte is a solid
ceramic compound, e.g. zirconium oxides. Oxide ions pass from the
cathode to the anode. Carbon monoxide may again be used as fuel.
Water is produced at the anode. The reactions are:
[0018] At anode:
2H.sub.2.sup.+2O.sup.2-.fwdarw.2H.sub.2O+4e.sup.-
[0019] (also, if CO present:
2CO+2O.sup.2-.fwdarw.2CO.sub.2+4e.sup.-)
[0020] At cathode: O.sub.2+4e.sup.-.fwdarw.2O.sup.2-
[0021] Net reaction: 2H.sub.2+O.sub.2.fwdarw.2H.sub.2O
[0022] (also, if CO present: 2CO+O.sub.2.fwdarw.2CO.sub.2)
[0023] The two most promising types of fuel cell are the Solid
Oxide Fuel Cell (SOFC) typically operating at 600-1000.degree. C.,
and the Proton Exchange Membrane (PEM) fuel cell typically
operating at 80.degree. C.
[0024] The SOFC may operate on most gaseous hydrocarbon fuels or
fuels derived from the reforming of natural gas, diesel, gasoline
or the gasification of solid fuels. When carbonaceous fuels are
used, the product gases will contain carbon dioxide. In stationary
applications the carbon dioxide may be captured and sequestrated,
but this is more difficult to realise in mobile applications like
cars. Furthermore, the PEM fuel cell most commonly used for mobile
applications generally requires purified hydrogen for operation
below 150.degree. C.
[0025] There remains a need for highly efficient and clean power
and hydrogen generation systems to allow for a transition to a
sustainable, low pollution use of fossil fuel energy without
release of carbon dioxide to the atmosphere.
[0026] The separation of carbon dioxide may be realised by
different means. One possibility is to use membranes to separate
the different species, another is to absorb gases in liquids or
solids and desorb the gases separately using pressure swing or
temperature swing cycles.
[0027] Even though the efficiency of fuel cells is not limited
thermodynamically, practically it has proven difficult to achieve
efficiencies that approach the theoretical maximum. A number of
hybrid systems have utilised the excess heat generated by an SOFC
in a turbine or other machinery. However, these systems are
complex, and the total efficiency is limited by the thermodynamic
machinery.
[0028] One proposal is disclosed in U.S. Pat. No. 5,079,103. This
document recognizes that hydrogen may be present in the gases
exiting the anode of fuel cells such as MCFCs or SOFCs and, rather
than using this for low-quality power generation (e.g. by
combustion), seeks to separate it and carbon dioxide from the waste
gas and utilize it more efficiently. The hydrogen may be separated
from the waste gas by means of pressure swing absorption (PSA) and
recycled back to the anode. The carbon dioxide may also be
separated from the anode waste gas and, in the case of MCFCs,
channelled to the cathode, thereby reducing the requirement for
externally supplied carbon dioxide. The carbon dioxide may be
separated from the anode waste gas by a scrubbing step or by PSA.
Thus, this document discloses that recycling the hydrogen and
carbon dioxide in this way provides more energy than simply burning
the anode waste gas. The document also discloses, as anode feed
stream, natural gas which is internally reformed to hydrogen.
Because natural gas is used, desulphurization may be required and
recycling the hydrogen can assist with this. However, although this
document claims overall efficiencies of up to 70%, this is still
some way below the theoretical maximum. Furthermore, because carbon
dioxide removal is carried out on the outflow from the fuel cell,
large volumes of gas have to be cleaned.
[0029] US 2001/0010873 discloses an SOFC wherein a
hydrocarbon-containing fuel is introduced to a fuel cell and
converted therein to a synthesis gas (an endothermic reaction). The
synthesis gas then undergoes partial electrochemical reaction (an
exothermic reaction) thereby generating electricity. The
hydrocarbon-containing fuel is supplied in such an excess that no
additional cooling of the fuel cell is required, i.e. production of
the synthesis gas is sufficiently endothermic to counter-balance
the exothermic electrochemical reaction. This document teaches
using natural gas to which water has been added as the
hydrocarbon-containing fuel. The conversion of methane and water to
hydrogen and carbon dioxide occurs within the fuel cell before the
electrochemical reaction. The process of US 2001/0010873 suffers
from several problems. Firstly, it does not satisfactorily address
the problem of efficient use of fuel. Secondly, it does not provide
an efficient process for separation of the exhaust material.
Thirdly, in order to avoid coking, the amount of oxygen must be
kept low and this limits the electrochemical potential. This
results in below optimum performance with respect to electrical
efficiency and power density.
[0030] WO 02/15295 discloses a fuel cell generator in which the
depleted fuel from a first fuel cell chamber is further used in a
second fuel cell chamber to increase the fuel utilisation, to
produce an exhaust gas that contains essentially carbon dioxide and
water for further treatment so that carbon dioxide can be separated
and is not vented into the atmosphere. However, this system does
not utilise the carbon dioxide separation system to increase the
electrical efficiency further than what is obtained by the
increased fuel utilisation.
[0031] "SOFC Efficiency at non standard conditions",
Electrochemical Proceedings Volume 97-18, presents theoretical
considerations for SOFC systems with high efficiency, and suggests
the circulation of anode gas with condensation of water and
recycling of hydrogen as a means for obtaining high efficiencies in
hydrogen fuelled systems. Although improved electrical efficiencies
may be realised by this theoretical concept, this can only be done
by reducing the power density, since a very high cell potential is
required. This paper does not disclose systems which exhibit both
high efficiency and high power density.
[0032] U.S. Pat. No. 2,781,248, BE 881637 and other documents
disclose systems for the manufacture of hydrogen using calcium
oxide as a carbon dioxide absorbent.
[0033] Whilst the use of anode gas recycling to improve the
efficiency of a fuel cell system, and the use of calcium
oxide/calcium carbonate cycles for the manufacture of hydrogen are
known, there remains a need for improvements with respect to
electrical efficiency, power density, carbon dioxide separation and
parasitic losses.
The Current Invention
[0034] From a first aspect, the invention provides a power
generation apparatus comprising a fuel cell and a reforming module,
wherein the reforming module is adapted to reform hydrocarbon fuel
into hydrogen and other components, and to separate said hydrogen
from said other components, the apparatus being arranged so that
said hydrogen is fed from the reforming module to the anode of the
fuel cell.
[0035] Thus it will be seen that in accordance with the present
invention hydrogen is produced and separated prior to entering the
fuel cell. Separating the hydrogen before entering the fuel cell is
advantageous because it allows an increase in the electrochemical
potential (voltage) of the fuel. The presence of carbon dioxide or
water reduces the voltage. Furthermore, separation prior to the
fuel cell eases the processing and manipulation of the separated
components.
[0036] In the broadest aspect of the invention the
hydrogen-containing stream fed from the reforming module to the
anode may still contain some carbon dioxide. Preferably however it
contains no more than 10 mol % of carbon dioxide, preferably no
more than 5 mol % of carbon dioxide, more preferably no more than 1
mol % of carbon dioxide, more preferably no more than 0.1 mol % of
carbon dioxide, most preferably no or substantially no carbon
dioxide. This allows the voltage to be even higher, and makes the
components even easier to process and manipulate as desired,
compared to conventional systems. This is particularly useful where
efficient separation and/or sequestration of carbon dioxide is
desirable.
[0037] Preferably the hydrogen-containing stream fed to the anode
contains no or substantially no other components, apart from water.
More preferably, no or substantially no water is present.
[0038] References herein to water are intended to include water in
any state, i.e. liquid, vapour etc.
[0039] The other components from which the reforming module is
arranged to separate hydrogen will depend upon the fuel used and
the method of reforming. Suitable fuels include hydrocarbon fuels,
particularly those requiring heat for their reforming reaction.
Examples of suitable fuels are natural gas, methane, methanol,
diesel, gasoline, coal, biomass, gases from the gasification of
organic matter such as biomass or carbons/hydrocarbons, gases from
the biological decomposition of organic matter such as biomass or
carbons/hydrocarbons, and gas-hydrates. Any suitable method of
reforming may be used; one suitable method is steam reforming.
[0040] In the broadest aspect of the invention the
hydrogen-containing stream fed from the reforming module to the
anode may also contain some contaminants. Preferably however it
contains at least 90% hydrogen, more preferably at least 95%
hydrogen, more preferably at least 99% hydrogen, more preferably at
least 99.9% hydrogen, where the percentages specified are mol %.
Most preferably it contains no or substantially no other components
apart from hydrogen. This further enhances the voltage and eases
the processing and manipulation. This is because, in most types of
fuel cell, where hydrogen is the only anode inflow, the only anode
outflows will be hydrogen and, to the extent that electrochemical
reaction has occurred, water.
[0041] However, the presence of some nitrogen (for example, due to
it being present in natural gas fuel) may be tolerated since this
will not affect the voltage.
[0042] Where water is present in the anode outflow it may be
possible in accordance with the invention for it to remain at the
final stage of processing. Preferably, however, means are provided
for removing water from the anode outflow stream, for example a
condenser or a water absorption unit may be used. This further
allows an increase in the electrochemical potential (voltage) of
the fuel and also makes the resultant stream easier to manipulate
and utilise.
[0043] Hydrogen produced at the output of the fuel cell in
accordance with the invention may all be tapped off for use in a
separate process. This has clear benefits since purified hydrogen
is a valuable commodity in many applications. Preferably however
the apparatus of the present invention is arranged to, or to be
able to, recycle at least some hydrogen back to the inlet of the
fuel cell. An increased level of efficiency is obtainable by
recycling the hydrogen in the anode waste gas back to the anode, as
is described hereinabove, whilst still allowing the possibility of
some hydrogen being tapped off.
[0044] The amount of hydrogen recycled back to the anode compared
to the amount of hydrogen tapped off is preferably variable. This
means that the preferred apparatus has a large degree of
operational flexibility. The amount of hydrogen tapped off could,
for example, be varied in a wide range between zero and all or
substantially all of the hydrogen in the anode outflow, depending
on particular requirements. For example the ratio of hydrogen
recycled to hydrogen tapped off may be 100:0, 95:5, 90:10, 75:25,
50:50, 25:75, 10:90, 5:95 or 0:100.
[0045] Conventional SOFC systems have significant shortcomings. In
particular, where the cell voltage of a conventional SOFC is high,
the fuel utilisation is reduced because the electrochemical
potential of the fuel needs to be higher than the cell voltage, and
only a fraction of the fuel can be used. Conversely, where the cell
voltage of a conventional SOFC is low, the efficiency is low. In
contrast, some preferred embodiments of the current invention allow
high cell voltage, high efficiency and high fuel utilisation.
Alternative preferred embodiments of the current invention allow
low cell voltage, high efficiency (since in low cell voltage mode
generated heat may be used for the production of hydrogen) and high
fuel utilisation. In both types of embodiment the fuel utilisation
is high because the fuel is fully utilised for the generation of
electricity and/or hydrogen.
[0046] In one possible mode of operation according to the
invention, water is condensed from the anode exhaust stream, and
all the hydrogen in the resultant stream is recycled back to the
anode rather than tapped off.
[0047] In another possible mode of operation according to the
invention, water is condensed from the anode exhaust stream, and
all the hydrogen in the resultant stream is tapped off rather than
recycled back to the anode.
[0048] In a further possible mode of operation according to the
invention, water is condensed from the anode exhaust stream, some
of the hydrogen in the resultant stream is tapped off and some of
it is recycled back to the anode.
[0049] The apparatus may allow flexibility both in the short term
and in the long term. For example, if there is an immediate
requirement for more purified hydrogen, the amount of hydrogen
tapped off may be maximised simply by adjusting the means which
direct the hydrogen to be tapped off rather than recycled.
Adjustments can also be made to cope with differing loads on the
power generation system. For example greater efficiency may be
achieved at loads below the maximum by operating at a higher cell
voltage at the expense of power density. This is achieved when the
hydrogen recycling is controlled to achieve a high partial pressure
of hydrogen at the anode. This latter scenario is discussed in more
detail below and may be useful where the cost of fuel cells becomes
less of a consideration in the future, so that lower power density
(i.e. power per cell) is less problematic. It is worth noting,
however, that the use of higher hydrogen/water ratios, or operation
at higher cell voltages merely represents one of many ways of
operating an apparatus in accordance with the invention. Enhanced
efficiency may be achieved even when not operated in this way. For
example it may be chosen to operate the apparatus in a mode which
maximises power density (power produced per cell).
[0050] Any known process can be used for reforming the hydrocarbon
fuel. Similarly, any known process can be used for separating the
thereby produced hydrogen from other components present following
reforming.
[0051] One method which is compatible with the current invention
involves the reaction of methane with water. This produces hydrogen
and carbon monoxide (a reforming reaction). The carbon monoxide
then reacts with water to produce hydrogen and carbon dioxide (a
shift reaction). The carbon dioxide can be separated or absorbed by
any known method, e.g. in a scrubbing step, by pressure-swing
absorption (for example using an amine), or by reaction with a
further component. This allows the production of a separate stream
of carbon dioxide, or disposal of carbon dioxide, for example by
absorption into a disposable solid. For example, the carbon dioxide
can be reacted with a metal oxide, for example a group II metal
oxide, e.g. calcium oxide, to produce a metal carbonate (a
carbonation reaction). In such a case, the overall reaction of
reforming, shift and carbonation may be as follows:
CH.sub.4+2H.sub.2O+meO+4H.sub.2+meCO.sub.3 wherein me is a metal,
e.g. calcium. The carbon dioxide may subsequently be released from
the metal carbonate by heating it and this is known as a desorption
reaction, or where the metal is calcium, a calcination
reaction.
[0052] Absorption of carbon dioxide may also be achieved by
reaction with a metal hydroxide, e.g. a group II metal hydroxide,
e.g. calcium hydroxide.
[0053] As noted above, carbon dioxide may be released from a metal
carbonate in a desorption reaction. The apparatus of the current
invention therefore comprises, in some preferred embodiments, a
module which is adapted to allow desorption to take place.
[0054] Where provided for, the carbonation and desorption reactions
may occur in separate modules. For example, the reforming module
may be adapted so that reforming, shift and carbonation occur
therein to produce hydrogen and sequestered carbon dioxide (in the
form of a metal carbonate), and there may be a separate desorption
module adapted to release carbon dioxide from the metal carbonate.
This has the advantage that metal oxide from the desorption module
may be transferred to the reforming module, and metal carbonate
from the reforming module may be transferred to the desorption
module at an appropriate time. This reduces the need for a metal
carbonate to be bought in. It also allows the production of carbon
dioxide which is itself of value as a commodity, or as a component
to be supplied to the cathode of MCFC fuel cells. Of course, these
advantages are supplementary to the major environmental advantages
of carbon dioxide separation according to the invention.
[0055] In a further embodiment, the reforming modules and the
desorption module may be linked, so that the metal carbonate
produced by the former is desorbed by the latter. This is desirable
in terms of efficiency and ease of use.
[0056] According to a further embodiment means are provided for
switching flow to the reforming module to the desorption module
and/or for switching flow to the desorption module to the reforming
module. In this way the reforming module may also function as a
desorption module and/or the desorption module may also function as
a reforming module. For example, the flows to the reforming module
and desorption module may be switched for appropriate time periods.
This allows, for example, the carbon dioxide absorbed in the
reforming module during reforming to be subsequently released by
channelling the cathode outflow through the reforming module and/or
the carbon dioxide released from the desorption module to be
subsequently replaced by channelling the fuel input through the
desorption module. Other advantages of this system include the ease
of re-utilisation of the carbon dioxide absorbent.
[0057] Other possible alternatives for separating the hydrogen
produced in the reformer from the other components present include
the use of a hydrogen permeable membrane.
[0058] In the broadest scope of the invention the reforming module
and the fuel cell may be entirely separate from one another save
for the provision of hydrogen from the former to the latter. More
preferably though the reforming module is thermally integrated with
the fuel cell. This results in the net exothermic nature of the
electrochemical reactions being at least partially balanced with
the net endothermic nature of the non-electrochemical reactions
which occur in the reforming module. The integration may be
achieved through a close physical proximity between the elements or
through heat transfer means which could comprise a solid, liquid or
gas transfer medium.
[0059] Similarly, where there is a separate desorption module as
described previously it is preferably thermally integrated with the
fuel cell. This results in the net exothermic nature of the
electrochemical reactions being at least partially balanced with
the net endothermic nature of the non-electrochemical reactions
which occur in the desorption module. Again physical proximity or a
transfer medium may be used.
[0060] Preferably, therefore, the reforming module, the fuel cell
and the desorption module are thermally integrated with each other,
most preferably by heat transfer means between the fuel cell and
reforming module, and heat transfer means between the fuel cell and
desorption module. In a further envisaged embodiment, there are
also heat transfer means between the reforming module and the
desorption module.
[0061] In one preferred example, the heat transfer means comprise
heat transfer loops, e.g. loops which route the cathode exhaust via
the reforming and/or desorption module, loops which route the anode
exhaust via the reforming and/or desorption module. This is
particularly useful for utilising the high temperature "waste" heat
of SOFC systems. Where the anode or cathode exhausts are routed in
this way, in one embodiment the exhaust components are physically
separated from the reforming and/or desorption modules, thereby
interacting only in a thermal, as opposed to a chemical, manner. In
another embodiment, where the heat transfer loop is an anode
exhaust loop, the exhaust components are simply fed into the
reforming module such that they can interact chemically as well as
thermally in the reforming module. Thus, for example, the water
required for the reforming reaction is preferably taken directly or
indirectly from the anode exhaust loop.
[0062] As noted above, the anode exhaust may be recycled back to
the anode. The same applies to the cathode exhaust which may be
recycled back to the cathode via the reforming and/or desorption
modules. Such cathode flow recycling aids the removal of excess
heat from the fuel cell. The amount of cathode flow required to
remove the heat would normally be greater than the amount of flow
required for the electrochemical reaction. However, this allows the
air to be cycled several times, thereby reducing the need for air
from the outside and reducing the amount of air emitted. In one
example, the heated cathode exhaust is cooled by about 50 to
200.degree. C. in the calcination reactor and recycled to the stack
where it is heated by about 50 to 200.degree. C. before continuing
around the loop, a fraction of the air in the cathode loop being
replaced for the electrochemical reaction.
[0063] Efficient heat transfer in this way has the result that
neither the cooling effect of the non-electrochemical reactions,
nor the heating effect of the electrochemical reactions, are
wasted. Where the electrochemical reactions are more exothermic
than the non-electrochemical reactions are net endothermic,
preferably substantially all the cooling energy of the latter are
used to cool the electrochemical reactions. Conversely, where the
electrochemical reactions are less exothermic than the
non-electrochemical reactions are net endothermic, preferably
substantially all the energy of the former are used to drive the
non-electrochemical reactions.
[0064] In this way the preferred embodiment of the invention can
provide a power generation system which has an electrical
efficiency close to the theoretical, non-thermodynamic, maximum,
limited only by small thermal and pressure losses. At the same
time, the carbon dioxide separation process can be thermally
integrated with the power generation process.
[0065] In a further preferred embodiment, reactions which occur in
the reforming module (e.g. reforming, shift and carbonation) are
themselves thermally integrated with each other, either by virtue
of these reactions occurring in close vicinity to each other, or by
using additional heat transfer means where necessary. This further
enhances the efficiency of the apparatus.
[0066] The flexibility of the power generation apparatus has been
discussed above. This flexibility allows the apparatus to be
operated in accordance with further aspects of the invention.
[0067] As technology progresses, the unit cost of fuel cells is
reducing and in such modes of operation the current invention seeks
in part to take advantage of this. Conventionally, fuel cells are
operated at a cell voltage of about 0.7 volts. Operation at a
higher cell voltage has hitherto been avoided because this has been
associated with a lower power density, i.e. less power produced per
fuel cell. The electrical load which may be drawn from the fuel
cell exhibits an inverse relationship with the cell voltage.
[0068] In contrast, the applicant has appreciated that a lower
degree of fuel utilization per cycle, although associated with a
lower power density (power produced per cell), allows operation at
a higher cell voltage and this in turn gives greater efficiency.
Whilst this necessitates provision of more fuel cells, it means
that the fuel is used more efficiently to produce power.
[0069] Accordingly, from a further aspect, the invention provides a
method of operating a fuel cell, comprising recycling hydrogen from
the anode outflow back to the anode inlet, such that the fuel cell
has a cell operating voltage of at least 0.8 volts.
[0070] In accordance with this aspect of the invention therefore,
it is feasible to construct a power generation system which is not
designed to operate normally at its full capacity without suffering
any significant loss in efficiency. It allows however for surges in
demand for power to be met, again without significant loss of
efficiency. This is a much more practically useful prospect than a
system which only achieves its peak efficiency at full load--since
in practice a generator is unlikely to be operated at full load for
most of the time.
[0071] Preferably the method also comprises reforming hydrocarbon
fuel to hydrogen and other components, and separating said hydrogen
from said other components, in a separate reforming module, and
feeding said hydrogen from said reforming module to the anode of
the fuel cell. Other preferred features of this aspect are as
described for the apparatus above.
[0072] Preferably the minimum operating cell voltage is between 0.8
volts and 0.9 volts, more preferably between 0.82 volts and 0.87
volts, most preferably approximately 0.85 volts. This is the
voltage that is found to be most effective and practical for
efficient fuel utilisation. The actual voltage will depend on
specific demands of electricity and hydrogen, and will vary
depending on the specific application.
[0073] The cell voltage and partial pressure of hydrogen flowing
through the anode and recycled are of course related. Therefore,
when viewed from a yet further aspect the invention provides a
method of operating a fuel cell comprising recycling hydrogen from
the anode outflow back to the anode inlet, such that the molar
ratio of hydrogen to water in the anode outflow is greater than
0.5, more preferably greater than 1.0, more preferably greater than
10, more preferably greater than 25, more preferably greater than
40.
[0074] Preferably the method also comprises reforming hydrocarbon
fuel to hydrogen and other components, and separating said hydrogen
from said other components, in a separate reforming module, and
feeding said hydrogen from said reforming module to the anode of
the fuel cell. Other preferred features of this aspect are as
described for the apparatus above.
[0075] The applicant has appreciated that operating the apparatus
at a high cell voltage and/or a high partial pressure of hydrogen
at the anode is desirable in certain circumstances.
[0076] According to the Nernst equation, the fuel cell potential at
the outlet is reduced from the theoretical maximum voltage by an
amount that depends on the relative amounts of reactants and
products. As the ratio of hydrogen to water increases, a greater
fuel cell potential will be possible and the cell will be able to
operate at a higher cell voltage.
[0077] The fuel cell may be a PEM fuel cell, but is preferably a
PAFC fuel cell, or a high temperature fuel cell, most preferably a
molten carbonate fuel cell or solid oxide fuel cell (MCFC or SOFC).
These fuel cells are energy efficient in combination with the
improvements of the current invention and the SOFC system is the
most suited to the current invention.
[0078] In further aspects, the invention provides methods of
generating electrical power, optionally with co-production of
hydrogen, using the apparatus described above.
[0079] From a further aspect, the invention provides a power
generation apparatus comprising a fuel cell, a reforming module and
a desorption module, wherein the reforming module is adapted to
reform hydrocarbon fuel into hydrogen and carbon dioxide, to
separate said hydrogen from said carbon dioxide, and to absorb said
carbon dioxide by a carbonation reaction with a metal oxide to
produce a metal carbonate, and the desorption module is adapted to
allow the release of carbon dioxide from a metal carbonate, the
apparatus being arranged so that said hydrogen is fed from the
reforming module to the anode of the fuel cell. This provides an
integrated system with advantages as described above. Preferred
features of this aspect are as described above.
DETAILED EMBODIMENTS
[0080] Certain preferred embodiments of the invention will now be
described, by way of example only, with reference to the
accompanying schematic drawings in which:
[0081] FIG. 1 shows schematically an embodiment of a power
generation system in accordance with the present invention;
[0082] FIG. 2 shows schematically a second embodiment of a power
generation system in accordance with the present invention; and
[0083] FIG. 3 shows schematically a third embodiment of a power
generation system in accordance with the present invention.
DISCUSSION OF FIG. 1
[0084] Turning firstly to FIG. 1, there may be seen a schematic
representation of a power generation apparatus which generally
comprises a fuel cell module 23, a reforming module 21 and a
condenser unit 22.
[0085] The fuel cell module 23 comprises a cathode compartment 24
with inlet 11 and outlet 12, and an anode compartment 25 with inlet
3 and outlet 4. Between the anode portion 25 and the cathode
portion 24 is an electrolyte as is well known in the art.
[0086] The reforming module 21 is provided with a fuel inlet 1 and
a water inlet 13. Two outlets 2 and 7 are provided namely a
hydrogen outlet 2 and a carbon dioxide outlet 7. The hydrogen
outlet 2 from the reforming module 21 is fed to the anode inlet 3
of the fuel cell 23.
[0087] The anode outlet 4 of the fuel cell 23 is fed to the
condenser 22, provided with a water drain outlet 5 and a de-watered
gas outlet 6. Flow from the condenser gas outlet 6 is divided into
two channels 8 and 9 by a three-way valve 30. Flow from one of
these channels 8 is fed back to the anode inlet 3.
[0088] In operation, methane and water are fed into the reforming
module 21 by means of inlets 1 and 13 respectively. In the
reforming module 21, the methane fuel is reformed into carbon
dioxide and hydrogen as follows:
CH.sub.4+H.sub.2O.fwdarw.3H.sub.2+CO (reforming reaction)
CO+H.sub.2O.fwdarw.H.sub.2+CO.sub.2 (shift reaction)
[0089] It will of course be appreciated that the chemical equations
discussed in the present application relate to equilibria and that
the equilibrium positions will not necessarily be completely to the
right-hand (product) side of the equations. For example, the above
two reactions may occur only to a partial extent.
[0090] The hydrogen is separated from carbon dioxide by passing the
resultant mixture thereof through a hydrogen permeable membrane
which does not allow the carbon dioxide to pass. The hydrogen then
exits the reforming module 21 via the hydrogen outlet 2 whilst the
carbon dioxide exits through the other outlet 7 to be stored or
reused.
[0091] Hydrogen from the outlet 2 enters the anode inlet 3 of the
fuel cell. Air is supplied to the cathode via the air inlet 11 and
depleted air exits the cathode via the air outlet 12.
Electrochemical reaction occurs in the fuel cell to provide an
electrical current as is well known.
[0092] Water and unreacted hydrogen exit the anode outlet 4 to be
fed into the condenser 22. Condensed water leaves the condenser 22
via the drain outlet 5 (which may be used to replenish the supply
to the reformer inlet 13). The dewatered hydrogen exits through the
other outlet to the three-way valve 30. The valve 30 may be used to
determine what proportion of the hydrogen is recycled back to the
fuel cell through pipe 8 and therefore how much is tapped off
through pipe 9 for external use.
[0093] The high electrical efficiency and high total efficiency as
well as the flexibility with respect to operating flexibility will
be demonstrated by the following Example. For comparison,
efficiency and power density calculations are first performed for a
conventional state of the art SOFC system. A specific cell
resistance of Rc=0.25 ohm*cm2 and a fuel feed of Ff=1.25
mole/second of methane equivalent to 1 MJ/s are assumed for all the
systems.
Conventional SOFC System
[0094] A conventional SOFC system is typically operated under the
conditions shown below: TABLE-US-00001 Cell voltage Uc = 0.7 V
Operating temperature 1000 C. Fuel utilisation Fu = 85%
Water:hydrogen ratio at inlet 1:9 (minimum required to avoid soot
formation) Electrochemical potential at fuel cell inlet Ui = 1.00 V
Electrochemical potential at fuel cell outlet Uo = 0.76 V
Each mole of methane entering the system can free 8 electrons,
either directly or indirectly when converted to 4 hydrogen
molecules with two electrons each. Considering also the fuel
utilisation, the electrical output of the fuel cell will be
Electric output=8*F*Uc*Fu*methane feed rate Where F=Faradays
constant=96487 coulomb/mole The electrical efficiency is given by
Electrical efficiency=Electrical output/Energy consumed The energy
consumed is equal to the heat of formation of the methane entering
the system. The heat of formation (lower heating value) of methane
is LHV_CH4=802 kJ/mole, and the assumed feed rate is 1.25 mole/s.
With the given values: Electrical
efficiency=(8*96487*0.7*0.85*1.25)/(802000*1.25)=57% The average
electrochemical potential over the cell is Ua=0.5*(Ui+Uo), with the
given values Ua=0.5*(1.00V+0.76V)=0.88V The average power density
of the cell will be Wa=(Ua-Uc)*Uc/Rc, with the given values
Wa=(0.88V-0.7V)*0.7V/0.25 ohm*cm2=504 mW/cm2 The power density is
of significant importance, since the cost of the fuel cells is
inversely proportional to the power density.
EXAMPLE
A system corresponding to that shown schematically in FIG. 1 is
operated under the conditions shown below:
[0095] Fuel feed at fuel inlet 1: Ff=1.25 mole/s of methane [0096]
Cell voltage 0.7 V [0097] Operating temperature 1000 C [0098]
Water: hydrogen ratio at inlet (humidified hydrogen, zero carbon
content prohibits soot formation) 3:97 [0099] Electrochemical
potential at fuel cell inlet Ui=1.07V [0100] Electrochemical
potential at fuel cell outlet Uo=0.85V In this example, half of the
hydrogen entering the fuel cell anode chamber is converted
electrochemically, while the other half leaves the system as
produced hydrogen. Each mole of methane is converted to 4 moles of
hydrogen in the reformer, hence hydrogen is produced at a rate of
2.5 mole/s and converted electrochemically at a rate of 2.5 mole/s.
The net chemical energy consumed in the system is then given by the
difference in heating value of the methane fuel stream entering the
system and the produced hydrogen fuel stream leaving the system.
The heat of formation (lower heating value) of methane is
LHV_CH4=802 kJ/mole, the heat of formation (lower heating value) of
hydrogen is LHV_H2=242 kJ/mole and the assumed feed rate is 1.25
mole/s. Energy consumed=(802*1.25-0.5*242*4*1.25) kJ/s=397 kJ/s
Electric output=2*F*Uc*hydrogen converted The electrical efficiency
is given by Electrical efficiency=Electrical output/Energy consumed
With the given values: Electrical eff.=(2*96487*0.7*2.5)/(397)=85%
The average electrochemical potential over the cell is
Ua=0.5*(Ui+Uo), with the given values Ua=0.5*(1.07V+0.85V)=0.96V
The average power density of the cell will be Wa=(Ua-Uc)*Uc/Rc,
with the given values Wa=(0.96V-0.7V)*0.7V/0.25 ohm*cm2=728 mW/cm2
Compared to the conventional system, the electrical efficiency has
been improved from 57% to 85% and the power density increased from
504 mW/cm2 to 728 mW/cm2. This represents a substantial
improvement.
DISCUSSION OF FIG. 2
[0101] In FIG. 2, there may be seen a schematic representation of a
power generation apparatus according to a second embodiment of the
invention.
[0102] The power generation apparatus represented in FIG. 2 differs
from the apparatus illustrated in FIG. 1 in that instead of a
hydrogen permeable membrane, the reforming module 21a is adapted to
absorb carbon dioxide. This is subsequently desorbed in a
desorption module 21b, which therefore has a carbon dioxide outlet
7a.
[0103] The desorption module 21b includes a conduit 14 through it
which is connected to the cathode inlet and outlets 11,12
respectively. The exhaust gas flow exiting the cathode at the
outlet 12 may be routed via the conduit 14 through desorption
module 21b back to cathode inlet 11. This allows the heat of the
cathode exhaust gases to be used in the endothermic desorption
reaction occurring in the desorption module 21b. Not only does this
obviate the need to supply heat for the desorption module 21b, but
it reduces the need to cool the fuel cell 23.
[0104] It will be appreciated that whilst recycling of the cathode
gases via the conduit 14 is shown, this is not essential in order
to be able to realise the thermal integration set out above.
[0105] The reforming reaction takes place as in the first
embodiment: CH.sub.4+H.sub.2O.fwdarw.3H.sub.2+CO (reforming
reaction) CO+H.sub.2O.fwdarw.H.sub.2+CO.sub.2 (shift reaction)
[0106] Calcium oxide is then used to absorb the carbon dioxide to
produce calcium carbonate: CaO+CO.sub.2.fwdarw.CaCO.sub.3
(carbonation reaction) resulting in the following overall reaction:
CH.sub.4+2H.sub.2O+CaO.fwdarw.4H.sub.2+CaCO.sub.3
[0107] (integrated reforming & carbonation)
Utilizing heat from the fuel cell 23 via the conduit 14, carbon
dioxide is then desorbed from the carbonate according to
CaCO.sub.3+heat.fwdarw.CO+CO.sub.2 (desorption reaction) and the
calcium oxide is then recycled in the process. The desorption
reaction is referred to as a calcination reaction.
[0108] In this embodiment, the exothermic carbonation reaction is
thermally coupled to the endothermic reforming reaction by virtue
of both being carried out in the reforming module 21a. The
equilibrium of the overall reaction (integrated reforming and
calcination) gives 95+% (dry basis) hydrogen at standard
(approximately 500.degree. C.) reforming temperatures. The
exothermic electrochemical reaction is coupled to the endothermic
calcination reaction such that the calcination reaction is almost
complete at the high operating temperature of the SOFC.
[0109] The reforming module is thermally integrated with the fuel
cell (not illustrated). This is advantageous as the fuel cell
provides the heat necessary for the endothermic reforming reaction.
In one example of an integrated reforming/carbonation reactor,
approximately 221 kJ/mol is used for reforming, 174 kJ/mol is
released by the carbonation reaction, 38 kJ/mol is released by the
shift reaction, and a small amount of heat from the fuel cell is
provided to the reforming module.
[0110] The above processes allow a particularly efficient carbon
dioxide absorption and desorption process in accordance with the
principles of the present invention.
[0111] Modules 21a and 21b may be separate, as illustrated, which
requires the calcium oxide in the reforming module 21a and the
calcium carbonate in the desorption module 21b to be replenished
periodically. Alternatively, they may be linked, so that the metal
carbonate produced by the reforming module 21a is desorbed by the
desorption module 21b.
[0112] The use of calcium is only illustrative and other metals may
be appropriate instead.
DISCUSSION OF FIG. 3
[0113] In FIG. 3, there may be seen a schematic representation of a
power generation apparatus according to a third embodiment. The
power generation apparatus represented in FIG. 3 differs from the
apparatus illustrated in FIG. 2 in that the reforming module 21a is
adapted to absorb carbon dioxide by reaction with a metal hydroxide
as well as a metal oxide ("metal" is denoted below as "me"). In
addition, rather than using a condenser in the hydrogen recycle
loop, a water absorption unit 26, which does not have a water
outlet, is used.
[0114] In this embodiment the reforming and carbon dioxide
absorption reactions are as follows.
me(OH).sub.2+CH.sub.4.fwdarw.meO+CO+3H.sub.2
CO+me(OH).sub.2.fwdarw.H.sub.2+meCO.sub.3
[0115] The water may be absorbed by reaction with a metal oxide:
meO+H.sub.2O+me(OH).sub.2
[0116] Carbon dioxide may be desorbed by the usual process:
meCO.sub.3.fwdarw.meO+CO.sub.2
[0117] Accordingly, this embodiment allows the recycling of
reagents because the metal oxide produced in the desorption step
may be used in the water absorption step. This produces metal
hydroxide which may be used in the reforming and absorption
reactions. This in turn produces metal carbonate which reacts to
form metal oxide in the desorption step, thereby completing the
cycle. This embodiment exhibits high efficiency and prolongs the
life of the absorbent and desorbent.
[0118] Whilst this embodiment does not require a steamer or
condenser, a dryer may be used to remove excess water. This dryer
may take the form of a separate water desorption module, through
which the hot cathode outflow is channelled (not illustrated).
* * * * *