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INTEGRATION OF MOLTEN CARBONATE FUEL CELLS FOR SYNTHESIS OF
NITROGEN COMPOUNDS
FIELD OF THE INVENTION
[0001] In various aspects, the invention is related to chemical production
processes
integrated with use of molten carbonate fuel cells.
BACKGROUND OF THE INVENTION
[0002] Molten carbonate fuel cells utilize hydrogen and/or other fuels to
generate
electricity. The hydrogen may be provided by reforming methane or other
reformable
fuels in a steam reformer that is upstream of the fuel cell or within the fuel
cell.
Reformable fuels can encompass hydrocarbonaceous materials that can be reacted
with
steam and/or oxygen at elevated temperature and/or pressure to produce a
gaseous
product that comprises hydrogen. Alternatively or additionally, fuel can be
reformed in
the anode cell in a molten carbonate fuel cell, which can be operated to
create
conditions that are suitable for reforming fuels in the anode. Alternately or
additionally,
the reforming can occur both externally and internally to the fuel cell.
[0003] Traditionally, molten carbonate fuel cells are operated to maximize
electricity
production per unit of fuel input, which may be referred to as the fuel cell's
electrical
efficiency. This maximization can be based on the fuel cell alone or in
conjunction with
another power generation system. In order to achieve increased electrical
production
and to manage the heat generation, fuel utilization within a fuel cell is
typically
maintained at 70% to 75%.
[0004] U.S. Published Patent Application 2011/0111315 describes a system and
process for operating fuel cell systems with substantial hydrogen content in
the anode
inlet stream. The technology in the '315 publication is concerned with
providing
enough fuel in the anode inlet so that sufficient fuel remains for the
oxidation reaction
as the fuel approaches the anode exit. To ensure adequate fuel, the '315
publication
provides fuel with a high concentration of H2. The H2 not utilized in the
oxidation
reaction is recycled to the anode for use in the next pass. On a single pass
basis, the H2
utilization may range from 10% to 30%. The '315 reference does not describe
significant reforming within the anode, instead relying primarily on external
reforming.
[0005] U.S. Published Patent Application 2005/0123810 describes a system and
method for co-production of hydrogen and electrical energy. The co-production
system
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comprises a fuel cell and a separation unit, which is configured to receive
the anode
exhaust stream and separate hydrogen. A portion of the anode exhaust is also
recycled
to the anode inlet. The operating ranges given in the '810 publication appear
to be
based on a solid oxide fuel cell. Molten carbonate fuel cells are described as
an
alternative.
[0006] U.S. Published Patent Application 2003/0008183 describes a system and
method for co-production of hydrogen and electrical power. A fuel cell is
mentioned as
a general type of chemical converter for converting a hydrocarbon-type fuel to
hydrogen. The fuel cell system also includes an external reformer and a high
temperature fuel cell. An embodiment of the fuel cell system is described that
has an
electrical efficiency of about 45% and a chemical production rate of about 25%
resulting in a system coproduction efficiency of about 70%. The '183
publication does
not appear to describe the electrical efficiency of the fuel cell in isolation
from the
system.
[0007] U.S. Patent 5,084,362 describes a system for integrating a fuel cell
with a
gasification system so that coal gas can be used as a fuel source for the
anode of the
fuel cell. Hydrogen generated by the fuel cell is used as an input for a
gasifier that is
used to generate methane from a coal gas (or other coal) input. The methane
from the
gasifier is then used as at least part of the input fuel to the fuel cell.
Thus, at least a
portion of the hydrogen generated by the fuel cell is indirectly recycled to
the fuel cell
anode inlet in the form of the methane generated by the gasifier.
[0008] An article in the Journal of Fuel Cell Science and Technology (G.
Manzolini
et. al., J. Fuel Cell Sci. and Tech., Vol. 9, Feb 2012) describes a power
generation
system that combines a combustion power generator with molten carbonate fuel
cells.
Various arrangements of fuel cells and operating parameters are described. The
combustion output from the combustion generator is used in part as the input
for the
cathode of the fuel cell. One goal of the simulations in the Manzolini article
is to use
the MCFC to separate CO2 from the power generator's exhaust. The simulation
described in the Manzolini article establishes a maximum outlet temperature of
660 C
and notes that the inlet temperature must be sufficiently cooler to account
for the
temperature increase across the fuel cell. The electrical efficiency (i.e.
electricity
generated/ fuel input) for the MCFC fuel cell in a base model case is 50%. The
3
electrical efficiency in a test model case, which is optimized for CO2
sequestration, is also
50%.
[0009] An article by Desideri et al. (Intl. J of Hydrogen Energy, Vol. 37,
2012)
describes a method for modeling the performance of a power generation system
using a
fuel cell for CO2 separation. Recirculation of anode exhaust to the anode
inlet and the
cathode exhaust to the cathode inlet are used to improve the performance of
the fuel cell.
The model parameters describe an MCFC electrical efficiency of 50.3%.
[0010] U.S. Patent 5,169,717 describes a method for integrating a molten
carbonate fuel
cell with a system for production of ammonia. The integrated system uses a
front end
different from the molten carbonate fuel cell to process the input hydrogen
and nitrogen
streams for production of ammonia.
SUMMARY OF THE INVENTION
[0011] In an aspect, a method for synthesizing nitrogen-containing compounds
is
provided. The method includes introducing a fuel stream comprising a
reformable fuel
into the anode of a molten carbonate fuel cell, an internal reforming element
associated
with the anode, or a combination thereof; introducing a cathode inlet stream
comprising
CO2 and 02 into the cathode of the fuel cell; generating electricity within
the molten
carbonate fuel cell; generating an anode exhaust comprising H2 and CO2;
separating CO2
from at least a portion of the anode exhaust to produce a first stream having
a CO2 content
greater than a CO2 content of the anode exhaust and a second gas stream having
an H2
content greater than an H2 content of the anode exhaust; and using at least a
portion of
the second gas stream in an ammonia synthesis process.
[0012] This application is related to the following PCT applications, filed on
even date
herewith: WO 2014/151182; WO 2014/151184; W02014/151184; WO 2014/151187;
W02014/151188; W02014/151189; W02014/151191;
W02014/151192;
W02014/151193; W02014/151194; W02014/151196;
W02014/151199;
W02014/151203; W02014/151203; W020141151207;
W02014/151210;
WO 2014/151214; WO 2014/151215; WO 2014/151216; WO
2014/151218;
WO 2014/151219; WO 2014/151224; and WO 2014/151225.
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BRIEF DESCRIPTION OF THE FIGURES
[0013] FIG. 1 schematically shows an example of a configuration for molten
carbonate
fuel cells and associated reforming and separation stages.
[0014] FIG. 2 schematically shows another example of a configuration for
molten
carbonate fuel cells and associated reforming and separation stages.
[0015] FIG. 3 schematically shows an example of the operation of a molten
carbonate
fuel cell.
[0016] FIG. 4 schematically shows an example of a combined cycle system for
generating electricity based on combustion of a carbon-based fuel.
[0017] FIG. 5 schematically shows an example of a combined cycle system for
generating electricity based on combustion of a carbon-based fuel.
[0018] FIG. 6 schematically shows an example of a configuration for
integrating molten
carbonate fuel cells with a process for synthesis of a nitrogen-containing
compound.
[0019] FIG. 7 schematically shows an example of a configuration for
integrating molten
carbonate fuel cells with a process for synthesis of a nitrogen-containing
compound.
[0020] FIGS. 8A-8C shows simulated results of flows in an integrated system
for
producing a nitrogen-containing compound.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Overview
[0021] In various aspects, the operation of molten carbonate fuel cells can be
integrated
with a variety of chemical and/or materials production processes, including
but not
limited to processes for synthesis of compounds in the presence of a catalyst,
such as an
ammonia synthesis catalyst. A production process can correspond to production
of an
output from the molten carbonate fuel cells, and/or a production process can
consume or
provide one or more fuel cell streams.
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Integration with Production of Nitrogen-Containing Intermediate and Final
Products
[0022] Ammonia can typically be made from H2 and N2 via the I Iaber-Bosch
process
at elevated temperature and pressure. Conventionally, the inputs can be a)
purified H2,
which can be made from a multi-step process that can typically require steam
methane
reforming, water gas shift, water removal, and trace carbon oxide conversion
to methane
via methanation; and b) purified N2, which can typically be derived from air
via pressure
swing adsorption. The process can be complex and energy intensive, and the
process
equipment can benefit strongly from economies of scale. An ammonia synthesis
process
utilizing molten carbonate fuel cells can provide one or more advantages
relative to a
conventional process, including but not limited to additional power
production, reduced
complexity, and/or better scalability. Additionally or alternately, an ammonia
synthesis
process utilizing molten carbonate fuel cells can provide a mechanism to
reduce CO2
production and/or generate CO2 for use in other processes.
[0023] In various aspects, the MCFC system can generate syngas as an output.
The
syngas can be largely free of any impurities such as sulfur that would need
removal,
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and the syngas can provide a source of H2 for the ammonia synthesis. The anode
exhaust can first be reacted in a water-gas shift reactor to maximize the
amount of H2
relative to CO. Water-gas shift is a well-known reaction, and typically can be
done at
"high" temperatures (from about 300 C to about 500 C) and "low" temperatures
(from
about 100 C to about 300 C) with the higher temperature catalyst giving faster
reaction
rates, but with higher exit CO content, followed by the low temperature
reactor to
further shift the syngas to higher H, concentrations. Following this, the gas
can
undergo separation via one or more processes to purify the H2. This can
involve, for
example, condensation of the water, removal of CO2, purification of the H2 and
then a
final methanation step at elevated pressure (typically about 15 barg to about
30 barg, or
about 1.5 MPag to about 3 MPag) to ensure that as many carbon oxides as
possible can
be eliminated. In conventional ammonia processes, the water, CO2, and methane
streams generated during purification of the H2 stream, as well as additional
off-gases
from the ammonia synthesis process, can represent waste streams of very low
value.
By contrast, in some aspects, the various "waste" gases can create streams
that can be
used in other parts of the MCFC ¨ Ammonia system, while potentially generating
still
other streams that can be useful in further processes. Lastly, the H2 stream
can be
compressed to ammonia synthesis conditions of about 60 barg (about 6 MPag) to
about
180 barg (about 18 MPag). Typical ammonia processes can be performed at about
350 C to about 500 C, such as at about 450 C or less, and can result in low
conversion
per pass (typically less than about 20%) and a large recycle stream.
[00241 As an example of integration of molten carbonate fuel cells with
ammonia
synthesis, the fuel stream to the anode inlet can correspond to fresh sources
of
reformable fuel and/or H2 along with (optionally but preferably) recycle off-
gas from
the ammonia synthesis process, which can contain Hz, CH4 (or other reformable
hydrocarbons), and/or CO. Ammonia processing, due to large recycle ratios and
the
presence of diluents (for example: the methane produced by methanation to
remove all
carbon oxides), can produce significant purge and waste streams. Most of these
streams, as long as they do not contain reactive oxidants such as oxygen, can
be
compatible with the fuel cell anode inlet. The anode inlet can additionally or
altnerately
comprise separation gases from hydrogen purification, as these gases can
typically
contain a mixture that comprises H2, CO, CO25 H20, and potentially other gases
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compatible with the anode. The anode exhaust can then be processed using a
water gas
shift reaction and H2 separation to form a high purity H2 stream. At least a
portion of
such an H2 stream can then be used as an input for an ammonia synthesis
process.
Optionally, in addition to performing separations on the high purity H2
stream, the H2
stream can be passed through a methanator prior to use for ammonia synthesis.
The
goal of the one or more separations and/or purifications can be to increase
the purity of
the H2 stream, so that at least a portion of an H2 stream with increased
purity can be
used as an input for the ammonia synthesis.
[0025] For the cathode inlet stream, CO2 and 02 can be provided from any
convenient source, such as a co-located external CO2 source (for example, a
gas-
turbine and/or boiler exhaust stream), recycled CO2 separated from the anode
exhaust,
recycled CO2 and/or 02 from the cathode exhaust, carbon containing streams
separated
as part of hydrogen purification, and/or CO2 separated from an output of the
ammonia
synthesis plant. Typically, a mixture of these streams may be used
advantageously, and
any residual fuel value in the streams can be used, e.g., to provide heat to
raise the
cathode inlet stream temperature up to the MCFC inlet temperature. For
example, fuel
streams that are off-gasses from separation and/or the ammonia process can be
mixed
with sufficient oxidant (air) to combust substantially all the residual fuel
components
while also providing sufficient oxygen to react with CO2 in the cathode to
form
carbonate ions. The cathode exhaust stream can have reduced concentrations of
both
CO2 and 02, as these gases can be reacted to form carbonate that can be
transported
into the anode stream. Because the MCFC can reduce the CO2 and 02 content of
the
cathode inlet stream, the cathode exhaust can have an enhanced nitrogen
concentration
on a dry basis in comparison to air. For systems that are designed to separate
CO2
effectively, the cathode exhaust may have CO2 concentrations below about 10%
or
below about 5% or below about 1% on a dry basis. The oxygen content may
additionally or alternately be below about 15% or below about 10% or below
about 5%
on a dry basis. The N2 concentration can typically exceed about 80% or about
85% or
can be greater than about 90% on a dry basis. After capture of the heating
value of this
stream (such as through steam generation for heat, heat exchange with other
process
streams, and/or additional electricity), the cathode exhaust can optionally
but
advantageously be used to form a high purity N2 stream for use in the ammonia
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synthesis. Any of the typical separation methods for generating pure nitrogen
can
operate more efficiently on this stream. Optionally, one or more separation
processes or
purification processes can be performed on the N2 stream in order to generate
an N2
stream of increased purity. At least a portion of the N2 having increased
purity can
then optionally but advantageously be used as the input for ammonia synthesis.
During
operation, the fuel cell can be operated to match the needs of the ammonia
synthesis,
such as selected lower or greater amounts of electrical production relative to
hydrogen
(and/or syngas) production.
[0026] Relative to conventional systems (such as described in U.S. Patent
5,169,717),
the above integration method can reduce or eliminate the need for a separate
front end
system for generating the purified H2 and N2 input streams. For example,
instead of
having a dedicated steam reformer and subsequent cleanup stages, the MCFC can
be
operated to reform sufficient amounts of reformable fuel to provide purified
H2 while
also generating electrical power. Typically this can be done by operating the
fuel cell
at lower fuel utilizations than typical. For example, the fuel utilization can
be below
about 70%, such as below about 60% or below about 50% or below about 40%. In
conventional MCFC operations, fuel utilizations of about 70-80% can be
typical, and
the residual syngas produced by the anode can be used as fuel to heat incoming
streams
to the cathode and/or anode. In conventional operations, it can also be
necessary to use
the anode exhaust stream to provide CO2 to the cathode after it is reacted
with air. By
contrast, in some aspects it is not necessary to use syngas from the anode
exhaust for
simple combustion and recycle. The ammonia synthesis process can provide a
number
of waste or purge streams which may be utilized, maximizing the amount of
syngas
available for ammonia synthesis. Similarly, as noted above, the cathode
exhaust from
the MCFC can provide a higher purity initial stream for forming the purified
N2 stream.
Concentrating the generation of input streams for ammonia synthesis in the
MCFC and
associated separation stages can reduce the equipment footprint as well as
providing
improved heat integration for the various processes.
[0027] Urea is another large chemical product that can be made by the reaction
of
ammonia with CO2. The basic process, developed in 1922, is also called the
Bosch¨
Meiser urea process after its discoverers. The various urea processes can be
characterized by the conditions under which urea formation takes place and the
way in
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which unconverted reactants are further processed. The process can consist of
two main
equilibrium reactions, with incomplete conversion of the reactants. The net
heat
balance for the reactions can be exothermic. The first equilibrium reaction
can be an
exothermic reaction of liquid ammonia with dry ice (solid CO2) to form
ammonium
carbam ate (H2N-COONR4):
2 NH3 + CO2 ''s¨H2N-COONH4
[0028] The second equilibrium reaction can be an endothermic decomposition of
ammonium carbamate into urea and water:
H2N-COONH4 vs--(NH2)2C0 + H20
[0029] The urea process can use liquefied ammonia and CO2 at high pressure as
process inputs. In prior art processes, carbon dioxide is typically provided
from an
external resource where it must be compressed to high pressure. In contrast,
the current
process, as shown in figure 6, can produce a high pressure liquefied carbon
dioxide
stream suitable for reaction with the liquid ammonia product from the ammonia
synthesis reaction.
[0030] In various aspects, urea production can be improved by providing one or
more
inputs (e.g., electric, heat, CO2, NH3, H20) and/or accepting one or more
outputs (e.g.,
H20, heat) from the MCFC while eliminating the need for a large number of
separate
systems. Additionally, as with most equilibrium processes involving
substantial
product removal and recycle, purge or waste streams can be generated. These
purge or
waste streams can be the result of side reactions and impurity buildup within
the
recycle loop. In a typical stand-alone plant, these streams can often be of
low value, and
potentially can require further purification, with additional processes and
equipment,
for recycle. By contrast, in various aspects, the purge or waste streams can
be used
advantageously and in a much simpler fashion. The anode inlet can consume any
reformable fuel and/or syngas composition. Streams diluted with materials that
can be
combusted, for example, nitrogen compounds such as ammonia, can be reacted
with air
to produce N2, water and heat which can be utilized as part of the cathode
inlet along
with any streams containing residual CO2, CO, and H2. As the MCFC system can
typically be operated at low pressure (below about 10 barg or about 1 MPag and
often
near-atmospheric conditions), there can be a reduced or minimized need to
recompress
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any of the purge or waste streams, as these process streams can be
sufficiently
pressurized for MCFC use.
[0031] Additionally, the urea process can be integrated into a combined system
with
an ammonia synthesis process. This integrated approach can reduce and/or
eliminate
many processes from the conventional approach, which can require an ammonia
plant
(steam reformer, water gas shift, pressure swing adsorption to produce H2 +
air
separation plant) plus a separate supply of cold CO2 (dry ice) typically made
remotely
and then transported to the plant. The current system can eliminate many of
these
processes and, as it can separate a CO2 stream at high pressure, can provide
the
necessary reactants at advantageous conditions. Specifically, rather than
transport CO2
as dry ice for use at a remote urea plant, carbon dioxide can be provided from
separation of a stream derived from the MCFC anode exhaust in liquefied form,
and
thus can easily be compressed to appropriate reaction pressures. This can
avoid
substantial energy inefficiencies in cooling, transport and recompression of
the CO2.
[0032] As described above, an MCFC can be integrated with an ammonia plant for
ammonia production while reducing or minimizing the amount of additional
equipment. Additionally or alternately, a separation can be performed on the
anode
exhaust from an MCFC system to provide a source of CO2. This source of CO2 can
then be further separated and/or purified so that at least a portion of the
CO2 can be
used for the urea synthesis process. For example, CO2 separation can be
performed
using a process comprising cryogenic separation. This can reduce or eliminate
the need
for separate production and/or transport of cold CO2. Further additionally or
alternately, the MCFC system can provide electric power and/or can provide or
consume heat by heat exchange with the MCFC inputs/output streams and/or by
heat
exchange with the separation systems.
[0033] FIG. 6 schematically shows an example of integration of molten
carbonate
fuel cells (such as an array of molten carbonate fuel cells) with a reaction
system for
performing ammonia synthesis and/or urea synthesis. In FIG. 6, molten
carbonate fuel
cell 810 can schematically represent one or more fuel cells (such as fuel cell
stacks or a
fuel cell array) along with associated reforming stages for the fuel cells.
The fuel cell
810 can receive an anode input stream 805, such as a reformable fuel stream,
and a
CO2-containing cathode input stream 809. In FIG. 6, anode input stream 805 can
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include an optional recycled portion of an off-gas 847 produced by the ammonia
synthesis process 840. In FIG. 6, cathode input stream 809 can include an
optional
recycled portion of CO2 829 separated from the anode and/or cathode output of
the fuel
cell 810 in separation stages 820. The anode output 815 from fuel cell 810 can
then be
passed through one or more separation stages 820, which can include CO2, H20,
and/or
H2 separation stages, optionally as well as water gas shift reaction stages,
in any
desired order, as described below and as further exemplified in FIGS. 1 and 2.
Separation stages can produce one or more streams corresponding to a CO2
output
stream 822, H20 output stream 824, and a high purity H2 output stream 826. The
separation stages can also produce an optional syngas output 825. A cathode
output 816
can be passed into one or more separation stages 820. Typically, the
separation stage(s)
used for the cathode output can be different from the separation stage(s) for
the anode
output, but the resulting streams from the separation can optionally be
combined, as
shown in FIG. 6. For example, CO2 can be separated from the cathode output 816
and
added to one or more CO2 output streams 822. The largest product separated
from the
cathode output 816 can be a high purity N, stream 841. The high purity H2
output
stream 826 and the high purity N2 stream 841 can be used as reactants for
ammonia
synthesis stage 840 to generate an ammonia output stream 845. Optionally, a
portion
of the ammonia output stream can be used as an input 851 for urea production
850,
along with CO2 stream(s) 822 from the separation stages 820, to generate a
urea output
855. Optionally, the input ammonia stream 851 for urea production 850 can be
from a
different source. Optionally, either the ammonia production stage 840 or the
urea
production stage 850 can be omitted from the configuration.
Additional Fuel Cell Operation Strategies
[0034] As an addition, complement, and/or alternative to the fuel cell
operating
strategies described herein, a molten carbonate fuel cell can be operated so
that the
amount of reforming can be selected relative to the amount of oxidation in
order to
achieve a desired thermal ratio for the fuel cell. As used herein, the
"thermal ratio" is
defined as the heat produced by exothermic reactions in a fuel cell assembly
divided by
the endothermic heat demand of reforming reactions occurring within the fuel
cell
assembly. Expressed mathematically, the thermal ratio (TH) = QEx/QEN, where
QEN is
the sum of heat produced by exothermic reactions and QEN is the sum of heat
consumed
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by the endothermic reactions occurring within the fuel cell. Note that the
heat produced
by the exothermic reactions corresponds to any heat due to reforming
reactions, water
gas shift reactions, and the electrochemical reactions in the cell. The heat
generated by
the electrochemical reactions can be calculated based on the ideal
electrochemical
potential of the fuel cell reaction across the electrolyte minus the actual
output voltage
of the fuel cell. For example, the ideal electrochemical potential of the
reaction in a
MCFC is believed to be about 1.04V based on the net reaction that occurs in
the cell.
During operation of the MCFC, the cell will typically have an output voltage
less than
1.04 V due to various losses. For example, a common output/operating voltage
can be
about 0.7 V. The heat generated is equal to the electrochemical potential of
the cell
(i.e. ¨1.04V) minus the operating voltage. For example, the heat produced by
the
electrochemical reactions in the cell is ¨0.34 V when the output voltage of
¨0.7V.
Thus, in this scenario, the electrochemical reactions would produce ¨0.7 V of
electricity and ¨0.34 V of heat energy. In such an example, the ¨0.7 V of
electrical
energy is not included as part of Qhx. In other words, heat energy is not
electrical
energy.
[0035] In various aspects, a thermal ratio can be determined for any
convenient fuel
cell structure, such as a fuel cell stack, an individual fuel cell within a
fuel cell stack, a
fuel cell stack with an integrated reforming stage, a fuel cell stack with an
integrated
endothermic reaction stage, or a combination thereof. The thermal ratio may
also be
calculated for different units within a fuel cell stack, such as an assembly
of fuel cells
or fuel cell stacks. For example, the thermal ratio may be calculated for a
single anode
within a single fuel cell, an anode section within a fuel cell stack, or an
anode section
within a fuel cell stack along with integrated reforming stages and/or
integrated
endothermic reaction stage elements in sufficiently close proximity to the
anode section
to be integrated from a heat integration standpoint. As used herein, "an anode
section"
comprises anodes within a fuel cell stack that share a common inlet or outlet
manifold.
[0036] In various aspects of the invention, the operation of the fuel cells
can be
characterized based on a thermal ratio. Where fuel cells are operated to have
a desired
thermal ratio, a molten carbonate fuel cell can be operated to have a thermal
ratio of
about 1.5 or less, for example about 1.3 or less, or about 1.15 or less, or
about 1.0 or
less, or about 0.95 or less, or about 0.90 or less, or about 0.85 or less, or
about 0.80 or
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less, or about 0.75 or less. Additionally or alternately, the thermal ratio
can be at least
about 0.25, or at least about 0.35, or at least about 0.45, or at least about
0.50.
Additionally or alternately, in some aspects the fuel cell can be operated to
have a
temperature rise between anode input and anode output of about 40 C or less,
such as
about 20 C or less, or about 10 C or less. Further additionally or
alternately, the fuel
cell can be operated to have an anode outlet temperature that is from about 10
C lower
to about 10 C higher than the temperature of the anode inlet. Still further
additionally
or alternately, the fuel cell can be operated to have an anode inlet
temperature that is
greater than the anode outlet temperature, such as at least about 5 C greater,
or at least
about 10 C greater, or at least about 20 C greater, or at least about 25 C
greater. Yet
still further additionally or alternately, the fuel cell can be operated to
have an anode
inlet temperature that is greater than the anode outlet temperature by about
100 C or
less, such as by about 80 C or less, or about 60 C or less, or about 50 C or
less, or
about 40 C or less, or about 30 C or less, or about 20 C or less.
[0037] As an addition, complement, and/or alternative to the fuel cell
operating
strategies described herein, a molten carbonate fuel cell (such as a fuel cell
assembly)
can be operated with increased production of syngas (or hydrogen) while also
reducing
or minimizing the amount of CO2 exiting the fuel cell in the cathode exhaust
stream.
Syngas can be a valuable input for a variety of processes. In addition to
having fuel
value, syngas can be used as a raw material for forming other higher value
products,
such as by using syngas as an input for Fischer-Tropsch synthesis and/or
methanol
synthesis processes. One option for making syngas can be to reform a
hydrocarbon or
hydrocarbon-like fuel, such as methane or natural gas. For many types of
industrial
processes, a syngas having a ratio of H2 to CO of close to 2:1 (or even lower)
can often
be desirable. A water gas shift reaction can be used to reduce the H2 to CO
ratio in a
syngas if additional CO2 is available, such as is produced in the anodes.
[0038] One way of characterizing the overall benefit provided by integrating
syngas
generation with use of molten carbonate fuel cells can be based on a ratio of
the net
amount of syngas that exits the fuel cells in the anode exhaust relative to
the amount of
CO2 that exits the fuel cells in the cathode exhaust. This characterization
measures the
effectiveness of producing power with low emissions and high efficiency (both
electrical and chemical). In this description, the net amount of syngas in an
anode
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exhaust is defined as the combined number of moles of H2 and number of moles
of CO
present in the anode exhaust, offset by the amount of H2 and CO present in the
anode
inlet. Because the ratio is based on the net amount of syngas in the anode
exhaust,
simply passing excess H2 into the anode does not change the value of the
ratio.
However, H2 and/or CO generated due to reforming in the anode and/or in an
internal
reforming stage associated with the anode can lead to higher values of the
ratio.
Hydrogen oxidized in the anode can lower the ratio. It is noted that the water
gas shift
reaction can exchange H2 for CO, so the combined moles of H2 and CO represents
the
total potential syngas in the anode exhaust, regardless of the eventual
desired ratio of
H2 to CO in a syngas. The syngas content of the anode exhaust (H2 + CO) can
then be
compared with the CO2 content of the cathode exhaust. This can provide a type
of
efficiency value that can also account for the amount of carbon capture. This
can
equivalently be expressed as an equation as
Ratio of net syngas in anode exhaust to cathode CO2 = net moles of (I-1,) +
CO)ANoph
moles of (CO2)cmpioph
[0039] In various aspects, the ratio of net moles of syngas in the anode
exhaust to the
moles of CO2 in the cathode exhaust can be at least about 2.0, such as at
least about
3.0, or at least about 4.0, or at least about 5Ø In some aspects, the ratio
of net syngas
in the anode exhaust to the amount of CO2 in the cathode exhaust can be still
higher,
such as at least about 10.0, or at least about 15.0, or at least about 20Ø
Ratio values of
about 40.0 or less, such as about 30.0 or less, or about 20.0 or less, can
additionally or
alternately be achieved. In aspects where the amount of CO2 at the cathode
inlet is
about 6.0 volume % or less, such as about 5.0 volume % or less, ratio values
of at least
about 1.5 may be sufficient/realistic. Such molar ratio values of net syngas
in the
anode exhaust to the amount of CO2 in the cathode exhaust can be greater than
the
values for conventionally operated fuel cells.
[0040] As an addition, complement, and/or alternative to the fuel cell
operating
strategies described herein, a molten carbonate fuel cell (such as a fuel cell
assembly)
can be operated at a reduced fuel utilization value, such as a fuel
utilization of about
50% or less, while also having a high CO2 utilization value, such as at least
about 60%.
In this type of configuration, the molten carbonate fuel cell can be effective
for carbon
capture, as the CO2 utilization can advantageously be sufficiently high.
Rather than
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attempting to maximize electrical efficiency, in this type of configuration
the total
efficiency of the fuel cell can be improved or increased based on the combined
electrical and chemical efficiency. The chemical efficiency can be based on
withdrawal of a hydrogen and/or syngas stream from the anode exhaust as an
output for
use in other processes. Even though the electrical efficiency may be reduced
relative to
some conventional configurations, making use of the chemical energy output in
the
anode exhaust can allow for a desirable total efficiency for the fuel cell.
[0041] In various aspects, the fuel utilization in the fuel cell anode can be
about 50%
or less, such as about 40% or less, or about 30% or less, or about 25% or
less, or about
20% or less. In various aspects, in order to generate at least some electric
power, the
fuel utilization in the fuel cell can be at least about 5%, such as at least
about 10%, or at
least about 15%, or at least about 20%, or at least about 25%, or at least
about 30%.
Additionally or alternatively, the CO2 utilization can be at least about 60%,
such as at
least about 65%, or at least about 70%, or at least about 75%.
[0042] As an addition, complement, and/or alternative to the fuel cell
operating
strategies described herein, a molten carbonate fuel cell can be operated at
conditions
that increase or maximize syngas production, possibly at the detriment of
electricity
production and electrical efficiency. Instead of selecting the operating
conditions of a
fuel cell to improve or maximize the electrical efficiency of the fuel cell,
operating
conditions, possibly including an amount of reformable fuel passed into the
anode, can
be established to increase the chemical energy output of the fuel cell. These
operating
conditions can result in a lower electrical efficiency of the fuel cell.
Despite the
reduced electrical efficiency, optionally, but preferably, the operating
conditions can
lead to an increase in the total efficiency of the fuel cell, which is based
on the
combined electrical efficiency and chemical efficiency of the fuel cell. By
increasing
the ratio of reformable fuel introduced into the anode to the fuel that is
actually
electrochemically oxidized at the anode, the chemical energy content in the
anode
output can be increased.
[0043] In some aspects, the reformable hydrogen content of reformable fuel in
the
input stream delivered to the anode and/or to a reforming stage associated
with the
anode can be at least about 50% greater than the net amount of hydrogen
reacted at the
anode, such as at least about 75% greater or at least about 100% greater.
Additionally
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or alternately, the reformable hydrogen content of fuel in the input stream
delivered to
the anode and/or to a reforming stage associated with the anode can be at
least about
50% greater than the net amount of hydrogen reacted at the anode, such as at
least
about 75% greater or at least about 100% greater. In various aspects, a ratio
of the
reformable hydrogen content of the reformable fuel in the fuel stream relative
to an
amount of hydrogen reacted in the anode can be at least about 1.5 : 1, or at
least about
2.0 : 1, or at least about 2.5 : 1, or at least about 3.0 : 1. Additionally or
alternately, the
ratio of reformable hydrogen content of the reformable fuel in the fuel stream
relative
to the amount of hydrogen reacted in the anode can be about 20 : 1 or less,
such as
about 15 : 1 or less or about 10 : 1 or less. In one aspect, it is
contemplated that less
than 100% of the reformable hydrogen content in the anode inlet stream can be
converted to hydrogen. For example, at least about 80% of the reformable
hydrogen
content in an anode inlet stream can be converted to hydrogen in the anode
and/or in an
associated reforming stage(s), such as at least about 85%, or at least about
90%.
Additionally or alternately, the amount of reformable fuel delivered to the
anode can be
characterized based on the Lower Heating Value (LHV) of the reformable fuel
relative
to the LHV of the hydrogen oxidized in the anode. This can be referred to as a
reformable fuel surplus ratio. In various aspects, the reformable fuel surplus
ratio can
be at least about 2.0, such as at least about 2.5, or at least about 3.0, or
at least about
4Ø Additionally or alternately, the reformable fuel surplus ratio can be
about 25.0 or
less, such as about 20.0 or less, or about 15.0 or less, or about 10.0 or
less.
[0044] As an addition, complement, and/or alternative to the fuel cell
operating
strategies described herein, a molten carbonate fuel cell (such as a fuel cell
assembly)
can also be operated at conditions that can improve or optimize the combined
electrical
efficiency and chemical efficiency of the fuel cell. Instead of selecting
conventional
conditions for maximizing the electrical efficiency of a fuel cell, the
operating
conditions can allow for output of excess synthesis gas and/or hydrogen in the
anode
exhaust of the fuel cell. The synthesis gas and/or hydrogen can then be used
in a
variety of applications, including chemical synthesis processes and collection
of
hydrogen for use as a "clean" fuel. In aspects of the invention, electrical
efficiency can
be reduced to achieve a high overall efficiency, which includes a chemical
efficiency
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based on the chemical energy value of syngas and/or hydrogen produced relative
to the
energy value of the fuel input for the fuel cell.
[0045] In some aspects, the operation of the fuel cells can be characterized
based on
electrical efficiency. Where fuel cells are operated to have a low electrical
efficiency
(EE), a molten carbonate fuel cell can be operated to have an electrical
efficiency of
about 40% or less, for example, about 35% EE or less, about 30% EE or less,
about
25% EE or less, or about 20% EE or less, about 15% EE or less, or about 10% EE
or
less. Additionally or alternately, the EE can be at least about 5%, or at
least about 10%,
or at least about 15%, or at least about 20%. Further additionally or
alternately, the
operation of the fuel cells can be characterized based on total fuel cell
efficiency
(TFCE), such as a combined electrical efficiency and chemical efficiency of
the fuel
cell(s). Where fuel cells are operated to have a high total fuel cell
efficiency, a molten
carbonate fuel cell can be operated to have a TFCE (and/or combined electrical
efficiency and chemical efficiency) of about 55% or more, for example, about
60% or
more, or about 65% or more, or about 70% or more, or about 75% or more, or
about
80% or more, or about 85% or more. It is noted that for a total fuel cell
efficiency
and/or combined electrical efficiency and chemical efficiency, any additional
electricity
generated from use of excess heat generated by the fuel cell can be excluded
from the
efficiency calculation.
[0046] In various aspects of the invention, the operation of the fuel cells
can be
characterized based on a desired electrical efficiency of about 40% or less
and a desired
total fuel cell efficiency of about 55% or more. Where fuel cells are operated
to have a
desired electrical efficiency and a desired total fuel cell efficiency, a
molten carbonate
fuel cell can be operated to have an electrical efficiency of about 40% or
less with a
TFCE of about 55% or more, for example, about 35% EE or less with about a TFCE
of
60% or more, about 30% EE or less with about a TFCE of about 65% or more,
about
25% EE or less with about a 70% TFCE or more, or about 20% EE or less with
about a
TFCE of 75% or more, about 15% EE or less with about a TFCE of 80% or more, or
about 10% EE or less with about a TFCE of about 85% or more.
[0047] As an addition, complement, and/or alternative to the fuel cell
operating
strategies described herein, a molten carbonate fuel cell (such as a fuel cell
assembly)
can be operated at conditions that can provide increased power density. The
power
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density of a fuel cell corresponds to the actual operating voltage VA
multiplied by the
current density I. For a molten carbonate fuel cell operating at a voltage VA,
the fuel
cell also can tend to generate waste heat, the waste heat defined as (Vo ¨
VA)*I based
on the differential between VA and the ideal voltage Vo for a fuel cell
providing current
density I. A portion of this waste heat can be consumed by reforming of a
reformable
fuel within the anode of the fuel cell. The remaining portion of this waste
heat can be
absorbed by the surrounding fuel cell structures and gas flows, resulting in a
temperature differential across the fuel cell. Under conventional operating
conditions,
the power density of a fuel cell can be limited based on the amount of waste
heat that
the fuel cell can tolerate without compromising the integrity of the fuel
cell.
[0048] In various aspects, the amount of waste heat that a fuel cell can
tolerate can be
increased by performing an effective amount of an endothermic reaction within
the fuel
cell. One example of an endothermic reaction includes steam reforming of a
reformable fuel within a fuel cell anode and/or in an associated reforming
stage, such as
an integrated reforming stage in a fuel cell stack. By providing additional
reformable
fuel to the anode of the fuel cell (or to an integrated / associated reforming
stage),
additional reforming can be performed so that additional waste heat can be
consumed.
This can reduce the amount of temperature differential across the fuel cell,
thus
allowing the fuel cell to operate under an operating condition with an
increased amount
of waste heat. The loss of electrical efficiency can be offset by the creation
of an
additional product stream, such as syngas and/or fl,, that can be used for
various
purposes including additional electricity generation further expanding the
power range
of the system.
[0049] In various aspects, the amount of waste heat generated by a fuel cell,
(Vo ¨
VA)*1 as defined above, can be at least about 30 mW/cm2, such as at least
about 40
mW/cm2, or at least about 50 mW/cm2, or at least about 60 mW/cm2, or at least
about
70 mW/cm2, or at least about 80 mW/cm2, or at least about 100 mW/cm2, or at
least
about 120 mW/cm2, or at least about 140 mW/cm2, or at least about 160 mW/cm2,
or at
least about 180 mW/cm2. Additionally or alternately, the amount of waste heat
generated by a fuel cell can be less than about 250 mW/cm2, such as less than
about
200 mW/cm2, or less than about 180 mW/cm2, or less than about 165 mW/cm2, or
less
than about 150 mW/cm2.
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[0050] Although the amount of waste heat being generated can be relatively
high,
such waste heat may not necessarily represent operating a fuel cell with poor
efficiency.
Instead, the waste heat can be generated due to operating a fuel cell at an
increased
power density. Part of improving the power density of a fuel cell can include
operating
the fuel cell at a sufficiently high current density. In various aspects, the
current
density generated by the fuel cell can be at least about 150 mA/cm2, such as
at least
about 160 mA/cm2, or at least about 170 mA/cm2, or at least about 180 mA/cm2,
or at
least about 190 mA/cm2, or at least about 200 mA/cm2, or at least about 225
mA/cm2,
or at least about 250 mA/cm2. Additionally or alternately, the current density
generated
by the fuel cell can be about 500 mA/cm2 or less, such as 450 mA/cm2, or less,
or 400
mA/cm2, or less or 350 mA/cm2, or less or 300 mA/cm2 or less.
[0051] In various aspects, to allow a fuel cell to be operated with increased
power
generation and increased generation of waste heat, an effective amount of an
endothermic reaction (such as a reforming reaction) can be performed.
Alternatively,
other endothermic reactions unrelated to anode operations can be used to
utilize the
waste heat by interspersing "plates" or stages into the fuel cell array that
are in thermal
communication but not fluid communication. The effective amount of the
endothermic
reaction can be performed in an associated reforming stage, an integrated
reforming
stage, an integrated stack element for performing an endothermic reaction, or
a
combination thereof. The effective amount of the endothermic reaction can
correspond
to an amount sufficient to reduce the temperature rise from the fuel cell
inlet to the fuel
cell outlet to about 100 C or less, such as about 90 C or less, or about 80 C
or less, or
about 70 C or less, or about 60 C or less, or about 50 C or less, or about 40
C or less,
or about 30 C or less. Additionally or alternately, the effective amount of
the
endothermic reaction can correspond to an amount sufficient to cause a
temperature
decrease from the fuel cell inlet to the fuel cell outlet of about 100 C or
less, such as
about 90 C or less, or about 80 C or less, or about 70 C or less, or about 60
C or less,
or about 50 C or less, or about 40 C or less, or about 30 C or less, or about
20 C or
less, or about 10 C or less. A temperature decrease from the fuel cell inlet
to the fuel
cell outlet can occur when the effective amount of the endothermic reaction
exceeds the
waste heat generated. Additionally or alternately, this can correspond to
having the
endothermic reaction(s) (such as a combination of reforming and another
endothermic
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reaction) consume at least about 40% of the waste heat generated by the fuel
cell, such
as consuming at least about 50% of the waste heat, or at least about 60% of
the waste
heat, or at least about 75% of the waste heat. Further additionally or
alternately, the
endothermic reaction(s) can consume about 95% of the waste heat or less, such
as
about 90% of the waste heat or less, or about 85% of the waste heat or less.
[0052] As an addition, complement, and/or alternative to the fuel cell
operating
strategies described herein, a molten carbonate fuel cell (such as a fuel cell
assembly)
can be operated at conditions corresponding to a decreased operating voltage
and a low
fuel utilization. In various aspects, the fuel cell can be operated at a
voltage VA of less
than about 0.7 Volts, for example less than about 0.68 V, less than about 0.67
V, less
than about 0.66 V, or about 0.65 V or less. Additionally or alternatively, the
fuel cell
can be operated at a voltage VA of at least about 0.60, for example at least
about 0.61,
at least about 0.62, or at least about 0.63. In so doing, energy that would
otherwise
leave the fuel cell as electrical energy at high voltage can remain within the
cell as heat
as the voltage is lowered. This additional heat can allow for increased
endothermic
reactions to occur, for example increasing the CH4 conversion to syngas.
Definitions
[0053] Syngas: In this description, syngas is defined as mixture of H2 and CO
in any
ratio. Optionally, H20 and/or CO2 may be present in the syngas. Optionally,
inert
compounds (such as nitrogen) and residual reformable fuel compounds may be
present
in the syngas. If components other than H2 and CO are present in the syngas,
the
combined volume percentage of H2 and CO in the syngas can be at least 25 vol%
relative to the total volume of the syngas, such as at least 40 vol%, or at
least 50 vol%,
or at least 60 vol%. Additionally or alternately, the combined volume
percentage of H2
and CO in the syngas can be 100 vol% or less, such as 95 vol% or less or 90
vol% or
less.
[0054] Reformable fuel: A reformable fuel is defined as a fuel that contains
carbon-
hydrogen bonds that can be reformed to generate H2. Hydrocarbons are examples
of
reformable fuels, as are other hydrocarbonaceous compounds such as alcohols.
Although CO and H20 can participate in a water gas shift reaction to form
hydrogen,
CO is not considered a reformable fuel under this definition.
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[0055] Reformable hydrogen content: The reformable hydrogen content of a fuel
is
defined as the number of H2 molecules that can be derived from a fuel by
reforming the
fuel and then driving the water gas shift reaction to completion to maximize
H2
production. It is noted that H2 by definition has a reformable hydrogen
content of 1,
although H2 itself is not defined as a reformable fuel herein. Similarly, CO
has a
reformable hydrogen content of 1. Although CO is not strictly reformable,
driving the
water gas shift reaction to completion will result in exchange of a CO for an
H2. As
examples of reformable hydrogen content for reformable fuels, the reformable
hydrogen content of methane is 4 H2 molecules while the reformable hydrogen
content
of ethane is 7 H2 molecules. More generally, if a fuel has the composition
CxHyOz,
then the reformable hydrogen content of the fuel at 100% reforming and water-
gas shift
is n(H2 max reforming) = 2x + y/2 ¨ z. Based on this definition, fuel
utilization within
a cell can then be expressed as n(142 ox)/n(H2 max reforming). Of course, the
reformable hydrogen content of a mixture of components can be determined based
on
the reformable hydrogen content of the individual components. The reformable
hydrogen content of compounds that contain other heteroatoms, such as oxygen,
sulfur
or nitrogen, can also be calculated in a similar manner.
[0056] Oxidation Reaction: In this discussion, the oxidation reaction within
the
anode of a fuel cell is defined as the reaction corresponding to oxidation of
H2 by
reaction with CO2 to form H20 and CO2. It is noted that the reforming reaction
within the anode, where a compound containing a carbon-hydrogen bond is
converted
into H2 and CO or CO2, is excluded from this definition of the oxidation
reaction in the
anode. The water-gas shift reaction is similarly outside of this definition of
the
oxidation reaction. It is further noted that references to a combustion
reaction are
defined as references to reactions where H2 or a compound containing carbon-
hydrogen bond(s) are reacted with 02 to form H2O and carbon oxides in a non-
electrochemical burner, such as the combustion zone of a combustion-powered
generator.
[0057] Aspects of the invention can adjust anode fuel parameters to achieve a
desired
operating range for the fuel cell. Anode fuel parameters can be characterized
directly,
and/or in relation to other fuel cell processes in the form of one or more
ratios. For
example, the anode fuel parameters can be controlled to achieve one or more
ratios
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including a fuel utilization, a fuel cell heating value utilization, a fuel
surplus ratio, a
reformable fuel surplus ratio, a reformable hydrogen content fuel ratio, and
combinations thereof.
[0058] Fuel utilization: Fuel utilization is an option for characterizing
operation of
the anode based on the amount of oxidized fuel relative to the reformable
hydrogen
content of an input stream can be used to define a fuel utilization for a fuel
cell. In this
discussion, "fuel utilization" is defined as the ratio of the amount of
hydrogen oxidized
in the anode for production of electricity (as described above) versus the
reformable
hydrogen content of the anode input (including any associated reforming
stages).
Reformable hydrogen content has been defined above as the number of H2
molecules
that can be derived from a fuel by reforming the fuel and then driving the
water gas
shift reaction to completion to maximize H, production. For example, each
methane
introduced into an anode and exposed to steam reforming conditions results in
generation of the equivalent of 4 H2 molecules at max production. (Depending
on the
reforming and/or anode conditions, the reforming product can correspond to a
non-
water gas shifted product, where one or more of the H2 molecules is present
instead in
the faun of a CO molecule.) Thus, methane is defined as having a reformable
hydrogen content of 4 H2 molecules. As another example, under this definition
ethane
has a reformable hydrogen content of 7 H2 molecules.
[0059] The utilization of fuel in the anode can also be characterized by
defining a
heating value utilization based on a ratio of the Lower Heating Value of
hydrogen
oxidized in the anode due to the fuel cell anode reaction relative to the
Lower Heating
Value of all fuel delivered to the anode and/or a reforming stage associated
with the
anode. The "fuel cell heating value utilization" as used herein can be
computed using
the flow rates and Lower Heating Value (LHV) of the fuel components entering
and
leaving the fuel cell anode. As such, fuel cell heating value utilization can
be computed
as (LHV(anode_in) ¨ LHV(anode_out))/LHV(anode_in), where LHV(anode_in) and
LHV(anode_out) refer to the LHV of the fuel components (such as H2, CH4,
and/or
CO) in the anode inlet and outlet streams or flows, respectively. In this
definition, the
LHV of a stream or flow may be computed as a sum of values for each fuel
component
in the input and/or output stream. The contribution of each fuel component to
the sum
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can correspond to the fuel component's flow rate (e.g., mol/hr) multiplied by
the fuel
component's LHV (e.g., joules/mol).
[0060] Lower Heating Value: The lower heating value is defined as the enthalpy
of
combustion of a fuel component to vapor phase, fully oxidized products (i.e.,
vapor
phase CO2 and H20 product). For example, any CO2 present in an anode input
stream
does not contribute to the fuel content of the anode input, since CO2 is
already fully
oxidized. For this definition, the amount of oxidation occurring in the anode
due to the
anode fuel cell reaction is defined as oxidation of H2 in the anode as part of
the
electrochemical reaction in the anode, as defined above.
[0061] It is noted that, for the special case where the only fuel in the anode
input flow
is H2, the only reaction involving a fuel component that can take place in the
anode
represents the conversion of H2 into f120. In this special case, the fuel
utilization
simplifies to (H2-rate-in minus -rate-out)/H2-rate-in. In such a case, H2
would be
the only fuel component, and so the H2 LHV would cancel out of the equation.
In the
more general case, the anode feed may contain, for example, CH4, H2, and CO in
various amounts. Because these species can typically be present in different
amounts in
the anode outlet, the summation as described above can be needed to determine
the fuel
utilization.
[0062] Alternatively or in addition to fuel utilization, the utilization for
other reactants
in the fuel cell can be characterized. For example, the operation of a fuel
cell can
additionally or alternately be characterized with regard to "CO2 utilization"
and/or
"oxidant" utilization. The values for CO2 utilization and/or oxidant
utilization can be
specified in a similar manner.
[0063] Fuel surplus ratio: Still another way to characterize the reactions in
a molten
carbonate fuel cell is by defining a utilization based on a ratio of the Lower
Heating
Value of all fuel delivered to the anode and/or a reforming stage associated
with the
anode relative to the Lower Heating Value of hydrogen oxidized in the anode
due to the
fuel cell anode reaction. This quantity will be referred to as a fuel surplus
ratio. As
such the fuel surplus ratio can be computed as (LHV (anode_in)/ (LHV(anode_in)-
LHV(anode_out)) where LHV(anode_in) and LHV(atiode_out) refer to the LHV of
the
fuel components (such as H2, CH4, and/or CO) in the anode inlet and outlet
streams or
flows, respectively. In various aspects of the invention, a molten carbonate
fuel cell can
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be operated to have a fuel surplus ratio of at least about 1.0, such as at
least about 1.5,
or at least about 2.0, or at least about 2.5, or at least about 3.0, or at
least about 4Ø
Additionally or alternately, the fuel surplus ratio can be about 25.0 or less.
[0064] It is noted that not all of the reformable fuel in the input stream for
the anode
may be reformed. Preferably, at least about 90% of the reformable fuel in the
input
stream to the anode (and/or into an associated reforming stage) can be
reformed prior to
exiting the anode, such as at least about 95% or at least about 98%. In some
alternative
aspects, the amount of reformable fuel that is reformed can be from about 75%
to about
90%, such as at least about 80%.
[0065] The above definition for fuel surplus ratio provides a method for
characterizing the amount of reforming occurring within the anode and/or
reforming
stage(s) associated with a fuel cell relative to the amount of fuel consumed
in the fuel
cell anode for generation of electric power.
[0066] Optionally, the fuel surplus ratio can be modified to account for
situations
where fuel is recycled from the anode output to the anode input. When fuel
(such as
H2, CO, and/or unreformed or partially reformed hydrocarbons) is recycled from
anode
output to anode input, such recycled fuel components do not represent a
surplus amount
of reformable or reformed fuel that can be used for other purposes. Instead,
such
recycled fuel components merely indicate a desire to reduce fuel utilization
in a fuel
cell.
[0067] Reformable fuel surplus ratio: Calculating a reformable fuel surplus
ratio is
one option to account for such recycled fuel components is to narrow the
definition of
surplus fuel, so that only the LHV of reformable fuels is included in the
input stream to
the anode. As used herein the "reformable fuel surplus ratio" is defined as
the Lower
Heating Value of reformable fuel delivered to the anode and/or a reforming
stage
associated with the anode relative to the Lower Heating Value of hydrogen
oxidized in
the anode due to the fuel cell anode reaction. Under the definition for
reformable fuel
surplus ratio, the LHV of any H2 or CO in the anode input is excluded. Such an
LHV
of reformable fuel can still be measured by characterizing the actual
composition
entering a fuel cell anode, so no distinction between recycled components and
fresh
components needs to be made. Although some non-reformed or partially reformed
fuel
may also be recycled, in most aspects the majority of the fuel recycled to the
anode can
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correspond to reformed products such as H2 or CO. Expressed mathematically,
the
reformable fuel surplus ratio (RRFs) = LHV RF/ LHV OH, where LHV RF is the
Lower
Heating Value (LHV) of the reformable fuel and LHV OH is the Lower Heating
Value
(LHV) of the hydrogen oxidized in the anode. The LHV of the hydrogen oxidized
in
the anode may be calculated by subtracting the LHV of the anode outlet stream
from
the LHV of the anode inlet stream (e.g., LHV(anode_in)-LHV(anode_out)). In
various
aspects of the invention, a molten carbonate fuel cell can be operated to have
a
reformable fuel surplus ratio of at least about 0.25, such as at least about
0.5, or at least
about 1.0, or at least about 1.5, or at least about 2.0, or at least about
2.5, or at least
about 3.0, or at least about 4Ø Additionally or alternately, the reformable
fuel surplus
ratio can be about 25.0 or less. It is noted that this narrower definition
based on the
amount of reformable fuel delivered to the anode relative to the amount of
oxidation in
the anode can distinguish between two types of fuel cell operation methods
that have
low fuel utilization. Some fuel cells achieve low fuel utilization by
recycling a
substantial portion of the anode output back to the anode input. This recycle
can allow
any hydrogen in the anode input to be used again as an input to the anode.
This can
reduce the amount of reforming, as even though the fuel utilization is low for
a single
pass through the fuel cell, at least a portion of the unused fuel is recycled
for use in a
later pass. Thus, fuel cells with a wide variety of fuel utilization values
may have the
same ratio of reformable fuel delivered to the anode reforming stage(s) versus
hydrogen
oxidized in the anode reaction. In order to change the ratio of reformable
fuel delivered
to the anode reforming stages relative to the amount of oxidation in the
anode, either an
anode feed with a native content of non-reformable fuel needs to be
identified, or
unused fuel in the anode output needs to be withdrawn for other uses, or both.
[0068] Reformable hydrogen surplus ratio: Still another option for
characterizing
the operation of a fuel cell is based on a "reformable hydrogen surplus
ratio." The
reformable fuel surplus ratio defined above is defined based on the lower
heating value
of reformable fuel components. The reformable hydrogen surplus ratio is
defined as
the reformable hydrogen content of reformable fuel delivered to the anode
and/or a
reforming stage associated with the anode relative to the hydrogen reacted in
the anode
due to the fuel cell anode reaction. As such, the "reformable hydrogen surplus
ratio"
can be computed as (RFC(reformable_anode_in)/ (RFC(reformable_anode_in) -
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RFC(anode_out)), where RFC(reformable_anode_in) refers to the reformable
hydrogen
content of reformable fuels in the anode inlet streams or flows, while RFC
(anode_out)
refers to the reformable hydrogen content of the fuel components (such as Hz,
CH4,
and/or CO) in the anode inlet and outlet streams or flows. The RFC can be
expressed in
moles/s, moles/hr, or similar. An example of a method for operating a fuel
cell with a
large ratio of reformable fuel delivered to the anode reforming stage(s)
versus amount
of oxidation in the anode can be a method where excess reforming is performed
in
order to balance the generation and consumption of heat in the fuel cell.
Reforming a
reformable fuel to form H2 and CO is an endothermic process. This endothermic
reaction can be countered by the generation of electrical current in the fuel
cell, which
can also produce excess heat corresponding (roughly) to the difference between
the
amount of heat generated by the anode oxidation reaction and the carbonate
formation
reaction and the energy that exits the fuel cell in the form of electric
current. The
excess heat per mole of hydrogen involved in the anode oxidation reaction /
carbonate
formation reaction can be greater than the heat absorbed to generate a mole of
hydrogen
by reforming. As a result, a fuel cell operated under conventional conditions
can
exhibit a temperature increase from inlet to outlet. Instead of this type of
conventional
operation, the amount of fuel reformed in the reforming stages associated with
the
anode can be increased. For example, additional fuel can be reformed so that
the heat
generated by the exothermic fuel cell reactions can be (roughly) balanced by
the heat
consumed in reforming, or even the heat consumed by reforming can be beyond
the
excess heat generated by the fuel oxidation, resulting in a temperature drop
across the
fuel cell. This can result in a substantial excess of hydrogen relative to the
amount
needed for electrical power generation. As one example, a feed to the anode
inlet of a
fuel cell or an associated reforming stage can be substantially composed of
reformable
fuel, such as a substantially pure methane feed. During conventional operation
for
electric power generation using such a fuel, a molten carbonate fuel cell can
be
operated with a fuel utilization of about 75%. This means that about 75% (or
Y4) of the
fuel content delivered to the anode is used to form hydrogen that is then
reacted in the
anode with carbonate ions to form H20 and CO,. In conventional operation, the
remaining about 25% of the fuel content can be reformed to H2 within the fuel
cell (or
can pass through the fuel cell unreacted for any CO or H2 in the fuel), and
then
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27
combusted outside of the fuel cell to form H20 and CO, to provide heat for the
cathode
inlet to the fuel cell. The reformable hydrogen surplus ratio in this
situation can be
41(4-1) = 413.
[0069] Electrical efficiency: As used herein, the term "electrical efficiency"
("EE")
is defined as the electrochemical power produced by the fuel cell divided by
the rate of
Lower Heating Value ("LHV") of fuel input to the fuel cell. The fuel inputs to
the fuel
cell includes both fuel delivered to the anode as well as any fuel used to
maintain the
temperature of the fuel cell, such as fuel delivered to a burner associated
with a fuel
cell. In this description, the power produced by the fuel may be described in
terms of
LHV(el) fuel rate.
[0070] Electrochemical power: As used herein, the term "electrochemical power"
or
LHV(el) is the power generated by the circuit connecting the cathode to the
anode in
the fuel cell and the transfer of carbonate ions across the fuel cell's
electrolyte.
Electrochemical power excludes power produced or consumed by equipment
upstream
or downstream from the fuel cell. For example, electricity produced from heat
in a fuel
cell exhaust stream is not considered part of the electrochemical power.
Similarly,
power generated by a gas turbine or other equipment upstream of the fuel cell
is not
part of the electrochemical power generated. The "electrochemical power" does
not
take electrical power consumed during operation of the fuel cell into account,
or any
loss incurred by conversion of the direct current to alternating current. In
other words,
electrical power used to supply the fuel cell operation or otherwise operate
the fuel cell
is not subtracted from the direct current power produced by the fuel cell. As
used
herein, the power density is the current density multiplied by voltage. As
used herein,
the total fuel cell power is the power density multiplied by the fuel cell
area.
[0071] Fuel inputs: As used herein, the term "anode fuel input," designated as
LHV(anode_in), is the amount of fuel within the anode inlet stream. The term
"fuel
input", designated as LHV(in), is the total amount of fuel delivered to the
fuel cell,
including both the amount of fuel within the anode inlet stream and the amount
of fuel
used to maintain the temperature of the fuel cell. The fuel may include both
reformable
and nonreformable fuels, based on the definition of a reformable fuel provided
herein.
Fuel input is not the same as fuel utilization.
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[0072] Total fuel cell efficiency: As used herein, the term "total fuel cell
efficiency"
("TFCE") is defined as: the electrochemical power generated by the fuel cell,
plus the
rate of LHV of syngas produced by the fuel cell, divided by the rate of LHV of
fuel
input to the anode. In other words, TFCE = (LHV(el) + LHV(sg
net))/LHV(anode_in),
where LHV(anode_in) refers to rate at which the LHV of the fuel components
(such as
H2, CH4, and/or CO) delivered to the anode and LHV(sg net) refers to a rate at
which
syngas (H2, CO) is produced in the anode, which is the difference between
syngas input
to the anode and syngas output from the anode. LHV(el) describes the
electrochemical
power generation of the fuel cell. The total fuel cell efficiency excludes
heat generated
by the fuel cell that is put to beneficial use outside of the fuel cell. In
operation, heat
generated by the fuel cell may be put to beneficial use by downstream
equipment. For
example, the heat may be used to generate additional electricity or to heat
water. These
uses, when they occur apart from the fuel cell, are not part of the total fuel
cell
efficiency, as the term is used in this application. The total fuel cell
efficiency is for the
fuel cell operation only, and does not include power production, or
consumption,
upstream, or downstream, of the fuel cell.
[0073] Chemical efficiency: As used herein, the term "chemical efficiency", is
defined as the lower heating value of H2 and CO in the anode exhaust of the
fuel cell,
or LHV(sg out), divided by the fuel input, or LHV(in).
[0074] Neither the electrical efficiency nor the total system efficiency takes
the
efficiency of upstream or downstream processes into consideration. For
example, it
may be advantageous to use turbine exhaust as a source of CO2 for the fuel
cell
cathode. In this arrangement, the efficiency of the turbine is not considered
as part of
the electrical efficiency or the total fuel cell efficiency calculation.
Similarly, outputs
from the fuel cell may be recycled as inputs to the fuel cell. A recycle loop
is not
considered when calculating electrical efficiency or the total fuel cell
efficiency in
single pass mode.
[0075] Syngas produced: As used herein, the term "syngas produced" is the
difference between syngas input to the anode and syngas output from the anode.
Syngas may be used as an input, or fuel, for the anode, at least in part. For
example, a
system may include an anode recycle loop that returns syngas from the anode
exhaust
to the anode inlet where it is supplemented with natural gas or other suitable
fuel.
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Syngas produced LHV (sg net) = (LHV(sg out) - LHV(sg in)), where LHV(sg in)
and
LHV(sg out) refer to the LHV of the syngas in the anode inlet and syngas in
the anode
outlet streams or flows, respectively. It is noted that at least a portion of
the syngas
produced by the reforming reactions within an anode can typically be utilized
in the
anode to produce electricity. The hydrogen utilized to produce electricity is
not
included in the definition of "syngas produced" because it does not exit the
anode. As
used herein, the term "syngas ratio" is the LHV of the net syngas produced
divided by
the LHV of the fuel input to the anode or LHV (sg net)/LHV(anode in). Molar
flow
rates of syngas and fuel can be used instead of LHV to express a molar-based
syngas
ratio and a molar-based syngas produced.
[0076] Steam to carbon ratio (S/C): As used herein, the steam to carbon ratio
(S/C)
is the molar ratio of steam in a flow to reformable carbon in the flow. Carbon
in the
form of CO and CO2 are not included as reformable carbon in this definition.
The
steam to carbon ratio can be measured and/or controlled at different points in
the
system. For example, the composition of an anode inlet stream can be
manipulated to
achieve a S/C that is suitable for reforming in the anode. The S/C can be
given as the
molar flow rate of H20 divided by the product of the molar flow rate of fuel
multiplied
by the number of carbon atoms in the fuel, e.g. one for methane. Thus, S/C
=fino/(fcH4
X #C), where f1120 is the molar flow rate of water, where fcH4 is the molar
flow rate of
methane (or other fuel) and #C is the number of carbons in the fuel.
[0077] EGR ratio: Aspects of the invention can use a turbine in partnership
with a
fuel cell. The combined fuel cell and turbine system may include exhaust gas
recycle
("EGR"). In an EGR system, at least a portion of the exhaust gas generated by
the
turbine can be sent to a heat recovery generator. Another portion of the
exhaust gas can
be sent to the fuel cell. The EGR ratio describes the amount of exhaust gas
routed to
the fuel cell versus the total exhaust gas routed to either the fuel cell or
heat recovery
generator. As used herein, the "EGR ratio" is the flow rate for the fuel cell
bound
portion of the exhaust gas divided by the combined flow rate for the fuel cell
bound
portion and the recovery bound portion, which is sent to the heat recovery
generator.
[0078] In various aspects of the invention, a molten carbonate fuel cell
(MCFC) can
be used to facilitate separation of CO2 from a CO2-containing stream while
also
generating additional electrical power. The CO2 separation can be further
enhanced by
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taking advantage of synergies with the combustion-based power generator that
can
provide at least a portion of the input feed to the cathode portion of the
fuel cell.
[0079] Fuel Cell and Fuel Cell Components: In this discussion, a fuel cell can
correspond to a single cell, with an anode and a cathode separated by an
electrolyte.
The anode and cathode can receive input gas flows to facilitate the respective
anode
and cathode reactions for transporting charge across the electrolyte and
generating
electricity. A fuel cell stack can represent a plurality of cells in an
integrated unit.
Although a fuel cell stack can include multiple fuel cells, the fuel cells can
typically be
connected in parallel and can function (approximately) as if they collectively
represented a single fuel cell of a larger size. When an input flow is
delivered to the
anode or cathode of a fuel cell stack, the fuel stack can include flow
channels for
dividing the input flow between each of the cells in the stack and flow
channels for
combining the output flows from the individual cells. In this discussion, a
fuel cell
array can be used to refer to a plurality of fuel cells (such as a plurality
of fuel cell
stacks) that are arranged in series, in parallel, or in any other convenient
manner (e.g.,
in a combination of series and parallel). A fuel cell array can include one or
more
stages of fuel cells and/or fuel cell stacks, where the anode/cathode output
from a first
stage may serve as the anode/cathode input for a second stage. It is noted
that the
anodes in a fuel cell array do not have to be connected in the same way as the
cathodes
in the array. For convenience, the input to the first anode stage of a fuel
cell array may
be referred to as the anode input for the array, and the input to the first
cathode stage of
the fuel cell array may be referred to as the cathode input to the array.
Similarly, the
output from the final anode/cathode stage may be referred to as the
anode/cathode
output from the array.
[0080] It should be understood that reference to use of a fuel cell herein
typically
denotes a "fuel cell stack" composed of individual fuel cells, and more
generally refers
to use of one or more fuel cell stacks in fluid communication. Individual fuel
cell
elements (plates) can typically be "stacked" together in a rectangular array
called a
"fuel cell stack". This fuel cell stack can typically take a feed stream and
distribute
reactants among all of the individual fuel cell elements and can then collect
the
products from each of these elements. When viewed as a unit, the fuel cell
stack in
operation can be taken as a whole even though composed of many (often tens or
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31
hundreds) of individual fuel cell elements. These individual fuel cell
elements can
typically have similar voltages (as the reactant and product concentrations
are similar),
and the total power output can result from the summation of all of the
electrical
currents in all of the cell elements, when the elements are electrically
connected in
series. Stacks can also be arranged in a series arrangement to produce high
voltages. A
parallel arrangement can boost the current. If a sufficiently large volume
fuel cell stack
is available to process a given exhaust flow, the systems and methods
described herein
can be used with a single molten carbonate fuel cell stack. In other aspects
of the
invention, a plurality of fuel cell stacks may be desirable or needed for a
variety of
reasons.
[0081] For the purposes of this invention, unless otherwise specified, the
term "fuel
cell" should be understood to also refer to and/or is defined as including a
reference to a
fuel cell stack composed of set of one or more individual fuel cell elements
for which
there is a single input and output, as that is the manner in which fuel cells
are typically
employed in practice. Similarly, the term fuel cells (plural), unless
otherwise specified,
should be understood to also refer to and/or is defined as including a
plurality of
separate fuel cell stacks. In other words, all references within this
document, unless
specifically noted, can refer interchangeably to the operation of a fuel cell
stack as a
"fuel cell". For example, the volume of exhaust generated by a commercial
scale
combustion generator may be too large for processing by a fuel cell (i.e., a
single stack)
of conventional size. In order to process the full exhaust, a plurality of
fuel cells (i.e.,
two or more separate fuel cells or fuel cell stacks) can be arranged in
parallel, so that
each fuel cell can process (roughly) an equal portion of the combustion
exhaust.
Although multiple fuel cells can be used, each fuel cell can typically be
operated in a
generally similar manner, given its (roughly) equal portion of the combustion
exhaust.
[0082] "Internal reforming" and "external reforming": A fuel cell or fuel cell
stack may include one or more internal reforming sections. As used herein, the
term
"internal reforming" refers to fuel reforming occurring within the body of a
fuel cell, a
fuel cell stack, or otherwise within a fuel cell assembly. External reforming,
which is
often used in conjunction with a fuel cell, occurs in a separate piece of
equipment that
is located outside of the fuel cell stack. In other words, the body of the
external
reformer is not in direct physical contact with the body of a fuel cell or
fuel cell stack.
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In a typical set up, the output from the external reformer can be fed to the
anode inlet of
a fuel cell. Unless otherwise noted specifically, the reforming described
within this
application is internal reforming.
[0083] Internal reforming may occur within a fuel cell anode. Internal
reforming can
additionally or alternately occur within an internal reforming element
integrated within
a fuel cell assembly. The integrated reforming element may be located between
fuel
cell elements within a fuel cell stack. In other words, one of the trays in
the stack can be
a reforming section instead of a fuel cell element. In one aspect, the flow
arrangement
within a fuel cell stack directs fuel to the internal reforming elements and
then into the
anode portion of the fuel cells. Thus, from a flow perspective, the internal
reforming
elements and fuel cell elements can be arranged in series within the fuel cell
stack. As
used herein, the term "anode reforming" is fuel reforming that occurs within
an anode.
As used herein, the term "internal reforming" is reforming that occurs within
an
integrated reforming element and not in an anode section.
[0084] In some aspects, a reforming stage that is internal to a fuel cell
assembly can
be considered to be associated with the anode(s) in the fuel cell assembly. In
some
alternative aspects, for a reforming stage in a fuel cell stack that can be
associated with
an anode (such as associated with multiple anodes), a flow path can be
available so that
the output flow from the reforming stage is passed into at least one anode.
This can
correspond to having an initial section of a fuel cell plate not in contact
with the
electrolyte and instead can serve just as a reforming catalyst. Another option
for an
associated reforming stage can be to have a separate integrated reforming
stage as one
of the elements in a fuel cell stack, where the output from the integrated
reforming
stage can be returned to the input side of one or more of the fuel cells in
the fuel cell
stack.
[0085] From a heat integration standpoint, a characteristic height in a fuel
cell stack
can be the height of an individual fuel cell stack element. It is noted that
the separate
reforming stage and/or a separate endothermic reaction stage could have a
different
height in the stack than a fuel cell. In such a scenario, the height of a fuel
cell element
can be used as the characteristic height. In some aspects, an integrated
endothermic
reaction stage can be defined as a stage that is heat integrated with one or
more fuel
cells, so that the integrated endothermic reaction stage can use the heat from
the fuel
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cells as a heat source for the endothermic reaction. Such an integrated
endothermic
reaction stage can be defined as being positioned less than 5 times the height
of a stack
element from any fuel cells providing heat to the integrated stage. For
example, an
integrated endothermic reaction stage (such as a reforming stage) can be
positioned less
than 5 times the height of a stack element from any fuel cells that are heat
integrated,
such as less than 3 times the height of a stack element. In this discussion,
an integrated
reforming stage and/or integrated endothermic reaction stage that represent an
adjacent
stack element to a fuel cell element can be defined as being about one stack
element
height or less away from the adjacent fuel cell element.
[0086] In some aspects, a separate reforming stage that is heat integrated
with a fuel
cell element can correspond to a reforming stage associated with the fuel cell
element.
In such aspects, an integrated fuel cell element can provide at least a
portion of the heat
to the associated reforming stage, and the associated reforming stage can
provide at
least a portion of the reforming stage output to the integrated fuel cell as a
fuel stream.
In other aspects, a separate reforming stage can be integrated with a fuel
cell for heat
transfer without being associated with the fuel cell. In this type of
situation, the
separate reforming stage can receive heat from the fuel cell, but the decision
can be
made not to use the output of the reforming stage as an input to the fuel
cell. Instead,
the decision can be made to use the output of such a reforming stage for
another
purpose, such as directly adding the output to the anode exhaust stream,
and/or for
forming a separate output stream from the fuel cell assembly.
[0087] More generally, a separate stack element in a fuel cell stack can be
used to
perform any convenient type of endothermic reaction that can take advantage of
the
waste heat provided by integrated fuel cell stack elements. Instead of plates
suitable for
performing a reforming reaction on a hydrocarbon fuel stream, a separate stack
element
can have plates suitable for catalyzing another type of endothermic reaction.
A
manifold or other arrangement of inlet conduits in the fuel cell stack can be
used to
provide an appropriate input flow to each stack element. A similar manifold or
other
arrangement of outlet conduits can additionally or alternately be used to
withdraw the
output flows from each stack element. Optionally, the output flows from a
endothermic
reaction stage in a stack can be withdrawn from the fuel cell stack without
having the
output flow pass through a fuel cell anode. In such an optional aspect, the
products of
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the exothermic reaction can therefore exit from the fuel cell stack without
passing
through a fuel cell anode. Examples of other types of endothermic reactions
that can be
performed in stack elements in a fuel cell stack can include, without
limitation, ethanol
dehydration to form ethylene and ethane cracking.
[0088] Recycle: As defined herein, recycle of a portion of a fuel cell output
(such as
an anode exhaust or a stream separated or withdrawn from an anode exhaust) to
a fuel
cell inlet can correspond to a direct or indirect recycle stream. A direct
recycle of a
stream to a fuel cell inlet is defined as recycle of the stream without
passing through an
intermediate process, while an indirect recycle involves recycle after passing
a stream
through one or more intermediate processes. For example, if the anode exhaust
is
passed through a CO, separation stage prior to recycle, this is considered an
indirect
recycle of the anode exhaust. If a portion of the anode exhaust, such as an H,
stream
withdrawn from the anode exhaust, is passed into a gasifier for converting
coal into a
fuel suitable for introduction into the fuel cell, then that is also
considered an indirect
recycle.
Anode Inputs and Outputs
[0089] In various aspects of the invention, the MCFC array can be fed by a
fuel
received at the anode inlet that comprises, for example, both hydrogen and a
hydrocarbon such as methane (or alternatively a hydrocarbonaceous or
hydrocarbon-
like compound that may contain heteroatoms different from C and H). Most of
the
methane (or other hydrocarbonaceous or hydrocarbon-like compound) fed to the
anode
can typically be fresh methane. In this description, a fresh fuel such as
fresh methane
refers to a fuel that is not recycled from another fuel cell process. For
example,
methane recycled from the anode outlet stream back to the anode inlet may not
be
considered "fresh" methane, and can instead be described as reclaimed methane.
The
fuel source used can be shared with other components, such as a turbine that
uses a
portion of the fuel source to provide a CO2-containing stream for the cathode
input.
The fuel source input can include water in a proportion to the fuel
appropriate for
reforming the hydrocarbon (or hydrocarbon-like) compound in the reforming
section
that generates hydrogen. For example, if methane is the fuel input for
reforming to
generate H2, the molar ratio of water to fuel can be from about one to one to
about ten
to one, such as at least about two to one. A ratio of four to one or greater
is typical for
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external reforming, but lower values can be typical for internal reforming. To
the
degree that H2 is a portion of the fuel source, in some optional aspects no
additional
water may be needed in the fuel, as the oxidation of H2 at the anode can tend
to
produce H20 that can be used for reforming the fuel. The fuel source can also
optionally contain components incidental to the fuel source (e.g., a natural
gas feed can
contain some content of CO, as an additional component). For example, a
natural gas
feed can contain CO2, N2, and/or other inert (noble) gases as additional
components.
Optionally, in some aspects the fuel source may also contain CO, such as CO
from a
recycled portion of the anode exhaust. An additional or alternate potential
source for
CO in the fuel into a fuel cell assembly can be CO generated by steam
reforming of a
hydrocarbon fuel performed on the fuel prior to entering the fuel cell
assembly.
[0090] More generally, a variety of types of fuel streams may be suitable for
use as an
input stream for the anode of a molten carbonate fuel cell. Some fuel streams
can
correspond to streams containing hydrocarbons and/or hydrocarbon-like
compounds
that may also include heteroatoms different from C and H. In this discussion,
unless
otherwise specified, a reference to a fuel stream containing hydrocarbons for
an MCFC
anode is defined to include fuel streams containing such hydrocarbon-like
compounds.
Examples of hydrocarbon (including hydrocarbon-like) fuel streams include
natural
gas, streams containing Cl ¨ C4 carbon compounds (such as methane or ethane),
and
streams containing heavier C5+ hydrocarbons (including hydrocarbon-like
compounds), as well as combinations thereof Still other additional or
alternate
examples of potential fuel streams for use in an anode input can include
biogas-type
streams, such as methane produced from natural (biological) decomposition of
organic
material.
[0091] In some aspects, a molten carbonate fuel cell can be used to process an
input
fuel stream, such as a natural gas and/or hydrocarbon stream, with a low
energy content
due to the presence of diluent compounds. For example, some sources of methane
and/or natural gas are sources that can include substantial amounts of either
CO2 or
other inert molecules, such as nitrogen, argon, or helium. Due to the presence
of
elevated amounts of CO2 and/or inerts, the energy content of a fuel stream
based on the
source can be reduced. Using a low energy content fuel for a combustion
reaction
(such as for powering a combustion-powered turbine) can pose difficulties.
However, a
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molten carbonate fuel cell can generate power based on a low energy content
fuel
source with a reduced or minimal impact on the efficiency of the fuel cell.
The
presence of additional gas volume can require additional heat for raising the
temperature of the fuel to the temperature for reforming and/or the anode
reaction.
Additionally, due to the equilibrium nature of the water gas shift reaction
within a fuel
cell anode, the presence of additional CO2 can have an impact on the relative
amounts
of H2 and CO present in the anode output. However, the inert compounds
otherwise
can have only a minimal direct impact on the reforming and anode reactions.
The
amount of CO, and/or inert compounds in a fuel stream for a molten carbonate
fuel
cell, when present, can be at least about 1 vol%, such as at least about 2
vol%, or at
least about 5 vol%, or at least about 10 vol%, or at least about 15 vol%, or
at least
about 20 vol%, or at least about 25 vol%, or at least about 30 vol%, or at
least about 35
vol%, or at least about 40 vol%, or at least about 45 vol%, or at least about
50 vol %, or
at least about 75 vol%. Additionally or alternately, the amount of CO2 and/or
inert
compounds in a fuel stream for a molten carbonate fuel cell can be about 90
vol% or
less, such as about 75 vol% or less, or about 60 vol% or less, or about 50
vol% or less,
or about 40 vol% or less, or about 35 vol% or less.
[0092] Yet other examples of potential sources for an anode input stream can
correspond to refinery and/or other industrial process output streams. For
example,
coking is a common process in many refineries for converting heavier compounds
to
lower boiling ranges. Coking typically produces an off-gas containing a
variety of
compounds that are gases at room temperature, including CO and various Cl ¨ C4
hydrocarbons. This off-gas can be used as at least a portion of an anode input
stream.
Other refinery off-gas streams can additionally or alternately be suitable for
inclusion in
an anode input stream, such as light ends (Cl ¨ C4) generated during cracking
or other
refinery processes. Still other suitable refinery streams can additionally or
alternately
include refinery streams containing CO or CO2 that also contain H2 and/or
reformable
fuel compounds.
[0093] Still other potential sources for an anode input can additionally or
alternately
include streams with increased water content. For example, an ethanol output
stream
from an ethanol plant (or another type of fermentation process) can include a
substantial portion of H2 0 prior to final distillation. Such H20 can
typically cause
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only minimal impact on the operation of a fuel cell. Thus, a fermentation
mixture of
alcohol (or other fermentation product) and water can be used as at least a
portion of an
anode input stream.
[0094] Biogas, or digester gas, is another additional or alternate potential
source for
an anode input. Biogas may primarily comprise methane and CO2 and is typically
produced by the breakdown or digestion of organic matter. Anaerobic bacteria
may be
used to digest the organic matter and produce the biogas. Impurities, such as
sulfur-
containing compounds, may be removed from the biogas prior to use as an anode
input.
[0095] The output stream from an MCFC anode can include H20, CO2, CO, and H2.
Optionally, the anode output stream could also have unreacted fuel (such as H2
or CH4)
or inert compounds in the feed as additional output components. Instead of
using this
output stream as a fuel source to provide heat for a reforming reaction or as
a
combustion fuel for heating the cell, one or more separations can be performed
on the
anode output stream to separate the CO2 from the components with potential
value as
inputs to another process, such as H2 or CO. The H2 and/or CO can be used as a
syngas for chemical synthesis, as a source of hydrogen for chemical reaction,
and/or as
a fuel with reduced greenhouse gas emissions.
[0096] In various aspects, the composition of the output stream from the anode
can be
impacted by several factors. Factors that can influence the anode output
composition
can include the composition of the input stream to the anode, the amount of
current
generated by the fuel cell, and/or the temperature at the exit of the anode.
The
temperature of at the anode exit can be relevant due to the equilibrium nature
of the
water gas shift reaction. In a typical anode, at least one of the plates
forming the wall
of the anode can be suitable for catalyzing the water gas shift reaction. As a
result, if
a) the composition of the anode input stream is known, b) the extent of
reforming of
reformable fuel in the anode input stream is known, and c) the amount of
carbonate
transported from the cathode to anode (corresponding to the amount of
electrical
current generated) is known, the composition of the anode output can be
determined
based on the equilibrium constant for the water gas shift reaction.
Keg = [CO2] [142] / [CO] [F120]
[0097] In the above equation, Keg is the equilibrium constant for the reaction
at a
given temperature and pressure, and [X] is the partial pressure of component
X. Based
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on the water gas shift reaction, it can be noted that an increased CO2
concentration in
the anode input can tend to result in additional CO formation (at the expense
of H2)
while an increased H20 concentration can tend to result in additional H2
formation (at
the expense of CO).
[0098] To determine the composition at the anode output, the composition of
the
anode input can be used as a starting point. This composition can then be
modified to
reflect the extent of reforming of any reformable fuels that can occur within
the anode.
Such reforming can reduce the hydrocarbon content of the anode input in
exchange for
increased hydrogen and CO2. Next, based on the amount of electrical current
generated, the amount of H2 in the anode input can be reduced in exchange for
additional f120 and CO,. This composition can then be adjusted based on the
equilibrium constant for the water gas shift reaction to determine the exit
concentrations for H2, CO, CO), and H20.
[0099] Table 1 shows the anode exhaust composition at different fuel
utilizations for
a typical type of fuel. The anode exhaust composition can reflect the combined
result
of the anode reforming reaction, water gas shift reaction, and the anode
oxidation
reaction. The output composition values in Table 1 were calculated by assuming
an
anode input composition with an about 2 to 1 ratio of steam (f1/0) to carbon
(reformable fuel). The reformable fuel was assumed to be methane, which was
assumed to be 100% reformed to hydrogen. The initial CO2 and H2 concentrations
in
the anode input were assumed to be negligible, while the input N2
concentration was
about 0.5%. The fuel utilization Uf (as defined herein) was allowed to vary
from about
35% to about 70% as shown in the table. The exit temperature for the fuel cell
anode
was assumed to be about 650 C for purposes of determining the correct value
for the
equilibrium constant.
TABLE 1 - Anode Exhaust Composition
Uf 35% 40% 45% 50% 55% 60% 65% 70%
Anode Exhaust Composition
H20 %, wet 32.5% 34.1% 35.5% 36.7% 37.8% 38.9% 39.8% 40.5%
CO2 %, wet 26.7% 29.4% 32.0% 34.5% 36.9% 39.3% 41.5% 43.8%
H2 %, wet 29.4% 26.0% 22.9% 20.0% 17.3% 14.8% 12.5% 10.4%
CO %, wet 10.8% 10.0% 9.2% 8.4% 7.5% 6.7% 5.8% 4.9%
N2 %, wet 0.5% 0.5% 0.5% 0.4% 0.4% 0.4% 0.4% 0.4%
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CO2 %, dry 39.6%
44.6% 49.6% 54.5% 59.4% 64.2% 69.0% 73.7%
H2 %, dry 43.6%
39.4% 35.4% 31.5% 27.8% 24.2% 20.7% 17.5%
CO %, dry 16.1% 15.2% 14.3% 13.2%
12.1% 10.9% 9.7% 8.2%
N2 %, dry 0.7% 0.7% 0.7% 0.7% 0.7% 0.7% 0.7%
0.7%
H2/C0 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.1
(H2-0O2)/ 0.07 -0.09 -0.22 -0.34 -0.44 -0.53 -0.61
-0.69
(CO+CO2)
Table 1 shows anode output compositions for a particular set of conditions and
anode
input composition. More generally, in various aspects the anode output can
include
about 10 vol% to about 50 vol% H20. The amount of H20 can vary greatly, as H20
in
the anode can be produced by the anode oxidation reaction. If an excess of H20
beyond what is needed for reforming is introduced into the anode, the excess
H20 can
typically pass through largely unreacted, with the exception of H20 consumed
(or
generated) due to fuel reforming and the water gas shift reaction. The CO2
concentration in the anode output can also vary widely, such as from about 20
vol% to
about 50 vol% CO2. The amount of CO2 can be influenced by both the amount of
electrical current generated as well as the amount of CO2 in the anode input
flow. The
amount of H2 in the anode output can additionally or alternately be from about
10 vol%
H2 to about 50 vol% H2, depending on the fuel utilization in the anode. At the
anode
output, the amount of CO can be from about 5 vol% to about 20 vol%. It is
noted that
the amount of CO relative to the amount of H2 in the anode output for a given
fuel cell
can be determined in part by the equilibrium constant for the water gas shift
reaction at
the temperature and pressure present in the fuel cell. The anode output can
further
additionally or alternately include 5 vol% or less of various other
components, such as
N2, CH4 (or other unreacted carbon-containing fuels), and/or other components.
[00100] Optionally, one or more water gas shift reaction stages can be
included after
the anode output to convert CO and H20 in the anode output into CO2 and f12,
if
desired. The amount of H2 present in the anode output can be increased, for
example,
by using a water gas shift reactor at lower temperature to convert H2O and CO
present
in the anode output into H2 and CO2. Alternatively, the temperature can be
raised and
the water-gas shift reaction can be reversed, producing more CO and H2O from
H2 and
CO2. Water is an expected output of the reaction occurring at the anode, so
the anode
output can typically have an excess of H20 relative to the amount of CO
present in the
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anode output. Alternatively, H2 0 can be added to the stream after the anode
exit but
before the water gas shift reaction. CO can be present in the anode output due
to
incomplete carbon conversion during reforming and/or due to the equilibrium
balancing
reactions between H20, CO, H2, and CO2 (i.e., the water-gas shift equilibrium)
under
either reforming conditions or the conditions present during the anode
reaction. A
water gas shift reactor can be operated under conditions to drive the
equilibrium further
in the direction of forming CO2 and H2 at the expense of CO and H2O. Higher
temperatures can tend to favor the formation of CO and H20. Thus, one option
for
operating the water gas shift reactor can be to expose the anode output stream
to a
suitable catalyst, such as a catalyst including iron oxide, zinc oxide, copper
on zinc
oxide, or the like, at a suitable temperature, e.g., between about 190 C to
about 210 C.
Optionally, the water-gas shift reactor can include two stages for reducing
the CO
concentration in an anode output stream, with a first higher temperature stage
operated
at a temperature from at least about 300 C to about 375 C and a second lower
temperature stage operated at a temperature of about 225 C or less, such as
from about
180 C to about 210 C. In addition to increasing the amount of H2 present in
the anode
output, the water-gas shift reaction can additionally or alternately increase
the amount
of CO2 at the expense of CO. This can exchange difficult-to-remove carbon
monoxide
(CO) for carbon dioxide, which can be more readily removed by condensation
(e.g.,
cryogenic removal), chemical reaction (such as amine removal), and/or other
CO2
removal methods. Additionally or alternately, it may be desirable to increase
the CO
content present in the anode exhaust in order to achieve a desired ratio of H2
to CO.
[00101] After passing through the optional water gas shift reaction stage, the
anode
output can be passed through one or more separation stages for removal of
water and/or
CO2 from the anode output stream. For example, one or more CO2 output streams
can
be formed by performing CO2 separation on the anode output using one or more
methods individually or in combination. Such methods can be used to generate
CO2
output stream(s) having a CO2 content of 90 vol% or greater, such as at least
95% vol%
CO2, or at least 98 vol% CO2. Such methods can recover about at least about
70% of
the CO2 content of the anode output, such as at least about 80% of the CO2
content of
the anode output, or at least about 90%. Alternatively, in some aspects it may
be
desirable to recover only a portion of the CO2 within an anode output stream,
with the
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recovered portion of CO2 being about 33% to about 90% of the CO2 in the anode
output, such as at least about 40%, or at least about 50%. For example, it may
be
desirable to retain some CO2 in the anode output flow so that a desired
composition can
be achieved in a subsequent water gas shift stage. Suitable separation methods
may
comprise use of a physical solvent (e.g., SelexolTM or RectisolTm); amines or
other
bases (e.g., MEA or MDEA); refrigeration (e.g., cryogenic separation);
pressure swing
adsorption; vacuum swing adsorption; and combinations thereof A cryogenic CO2
separator can be an example of a suitable separator. As the anode output is
cooled, the
majority of the water in the anode output can be separated out as a condensed
(liquid)
phase. Further cooling and/or pressurizing of the water-depleted anode output
flow
can then separate high purity CO2, as the other remaining components in the
anode
output flow (such as H2, N2, CH4) do not tend to readily form condensed
phases. A
cryogenic CO2 separator can recover between about 33% and about 90% of the CO2
present in a flow, depending on the operating conditions.
[00102] Removal of water from the anode exhaust to form one or more water
output
streams can also be beneficial, whether prior to, during, or after performing
CO2
separation. The amount of water in the anode output can vary depending on
operating
conditions selected. For example, the steam-to-carbon ratio established at the
anode
inlet can affect the water content in the anode exhaust, with high steam-to-
carbon ratios
typically resulting in a large amount of water that can pass through the anode
unreacted
and/or reacted only due to the water gas shift equilibrium in the anode.
Depending on
the aspect, the water content in the anode exhaust can correspond to up to
about 30% or
more of the volume in the anode exhaust. Additionally or alternately, the
water content
can be about 80% or less of the volume of the anode exhaust. While such water
can be
removed by compression and/or cooling with resulting condensation, the removal
of
this water can require extra compressor power and/or heat exchange surface
area and
excessive cooling water. One beneficial way to remove a portion of this excess
water
can be based on use of an adsorbent bed that can capture the humidity from the
moist
anode effluent and can then be 'regenerated' using dry anode feed gas, in
order to
provide additional water for the anode feed. HVAC-style (heating, ventilation,
and air
conditioning) adsorption wheels design can be applicable, because anode
exhaust and
inlet can be similar in pressure, and minor leakage from one stream to the
other can
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have minimal impact on the overall process. In embodiments where CO2 removal
is
performed using a cryogenic process, removal of water prior to or during CO2
removal
may be desirable, including removal by triethyleneglycol (TEG) system and/or
desiccants. By contrast, if an amine wash is used for CO2 removal, water can
be
removed from the anode exhaust downstream from the CO2 removal stage.
[00103] Alternately or in addition to a CO2 output stream and/or a water
output stream,
the anode output can be used to form one or more product streams containing a
desired
chemical or fuel product. Such a product stream or streams can correspond to a
syngas
stream, a hydrogen stream, or both syngas product and hydrogen product
streams. For
example, a hydrogen product stream containing at least about 70 vol% H2, such
as at
least about 90 vol% H2 or at least about 95 vol% H2, can be formed.
Additionally or
alternately, a syngas stream containing at least about 70 vol% of H2 and CO
combined,
such as at least about 90 vol% of H2 and CO can be formed. The one or more
product
streams can have a gas volume corresponding to at least about 75% of the
combined H2
and CO gas volumes in the anode output, such as at least about 85% or at least
about
90% of the combined H2 and CO gas volumes. It is noted that the relative
amounts of
H2 and CO in the products streams may differ from the H2 to CO ratio in the
anode
output based on use of water gas shift reaction stages to convert between the
products.
[00104] In some aspects, it can be desirable to remove or separate a portion
of the H2
present in the anode output. For example, in some aspects the H2 to CO ratio
in the
anode exhaust can be at least about 3.0 : 1. By contrast, processes that make
use of
syngas, such as Fischer-Tropsch synthesis, may consume H2 and CO in a
different
ratio, such as a ratio that is closer to 2 : 1. One alternative can be to use
a water gas
shift reaction to modify the content of the anode output to have an H2 to CO
ratio
closer to a desired syngas composition. Another alternative can be to use a
membrane
separation to remove a portion of the H2 present in the anode output to
achieve a
desired ratio of H2 and CO, or still alternately to use a combination of
membrane
separation and water gas shift reactions. One advantage of using a membrane
separation to remove only a portion of the H2 in the anode output can be that
the
desired separation can be performed under relatively mild conditions. Since
one goal
can be to produce a retentate that still has a substantial H2 content, a
permeate of high
purity hydrogen can be generated by membrane separation without requiring
severe
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conditions. For example, rather than having a pressure on the permeate side of
the
membrane of about 100 kPaa or less (such as ambient pressure), the permeate
side can
be at an elevated pressure relative to ambient while still having sufficient
driving force
to perform the membrane separation. Additionally or alternately, a sweep gas
such as
methane can be used to provide a driving force for the membrane separation.
This can
reduce the purity of the H, permeate stream, but may be advantageous,
depending on
the desired use for the permeate stream.
[00105] In various aspects of the invention, at least a portion of the anode
exhaust
stream (preferably after separation of CO2 and/or H20) can be used as a feed
for a
process external to the fuel cell and associated reforming stages. In various
aspects, the
anode exhaust can have a ratio of H2 to CO of about 1.5 : 1 to about 10 : 1,
such as at
least about 3.0 : 1, or at least about 4.0 : 1, or at least about 5.0 : 1. A
syngas stream
can be generated or withdrawn from the anode exhaust. The anode exhaust gas,
optionally after separation of CO2 and/or H20, and optionally after performing
a water
gas shift reaction and/or a membrane separation to remove excess hydrogen, can
correspond to a stream containing substantial portions of H2 and/or CO. For a
stream
with a relatively low content of CO, such as a stream where the ratio of H2 to
CO is at
least about 3 : 1, the anode exhaust can be suitable for use as an H2 feed.
Examples of
processes that could benefit from an H2 feed can include, but are not limited
to,
refinery processes, an ammonia synthesis plant, or a turbine in a (different)
power
generation system, or combinations thereof. Depending on the application,
still lower
CO2 contents can be desirable. For a stream with an H2-to-CO ratio of less
than about
2.2 to 1 and greater than about 1.9 to 1, the stream can be suitable for use
as a syngas
feed. Examples of processes that could benefit from a syngas feed can include,
but are
not limited to, a gas-to-liquids plant (such as a plant using a Fischer-
Tropsch process
with a non-shifting catalyst) and/or a methanol synthesis plant. The amount of
the
anode exhaust used as a feed for an external process can be any convenient
amount.
Optionally, when a portion of the anode exhaust is used as a feed for an
external
process, a second portion of the anode exhaust can be recycled to the anode
input
and/or recycled to the combustion zone for a combustion-powered generator.
[00106] The input streams useful for different types of Fischer-Tropsch
synthesis
processes can provide an example of the different types of product streams
that may be
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desirable to generate from the anode output. For a Fischer-Tropsch synthesis
reaction
system that uses a shifting catalyst, such as an iron-based catalyst, the
desired input
stream to the reaction system can include CO2 in addition to H2 and CO. If a
sufficient
amount of CO2 is not present in the input stream, a Fischer-Tropsch catalyst
with water
gas shift activity can consume CO in order to generate additional CO,,
resulting in a
syngas that can be deficient in CO. For integration of such a Fischer-Tropsch
process
with an MCFC fuel cell, the separation stages for the anode output can be
operated to
retain a desired amount of CO2 (and optionally H20) in the syngas product. By
contrast, for a Fischer-Tropsch catalyst based on a non-shifting catalyst, any
CO2
present in a product stream could serve as an inert component in the Fischer-
Tropsch
reaction system.
[00107] In an aspect where the membrane is swept with a sweep gas such as a
methane
sweep gas, the methane sweep gas can correspond to a methane stream used as
the
anode fuel or in a different low pressure process, such as a boiler, furnace,
gas turbine,
or other fuel-consuming device. In such an aspect, low levels of CO2
permeation
across the membrane can have minimal consequence. Such CO2 that may permeate
across the membrane can have a minimal impact on the reactions within the
anode, and
such CO2 can remain contained in the anode product. Therefore, the CO2 (if
any) lost
across the membrane due to permeation does not need to be transferred again
across the
MCFC electrolyte. This can significantly reduce the separation selectivity
requirement
for the hydrogen permeation membrane. This can allow, for example, use of a
higher-
permeability membrane having a lower selectivity, which can enable use of a
lower
pressure and/or reduced membrane surface area. In such an aspect of the
invention, the
volume of the sweep gas can be a large multiple of the volume of hydrogen in
the
anode exhaust, which can allow the effective hydrogen concentration on the
permeate
side to be maintained close to zero. The hydrogen thus separated can be
incorporated
into the turbine-fed methane where it can enhance the turbine combustion
characteristics, as described above.
[00108] It is noted that excess H2 produced in the anode can represent a fuel
where the
greenhouse gases have already been separated. Any CO2 in the anode output can
be
readily separated from the anode output, such as by using an amine wash, a
cryogenic
CO2 separator, and/or a pressure or vacuum swing absorption process. Several
of the
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components of the anode output (H2, CO, CH4) are not easily removed, while CO2
and
H20 can usually be readily removed. Depending on the embodiment, at least
about 90
vol% of the CO2 in the anode output can be separated out to form a relatively
high
purity CO2 output stream. Thus, any CO2 generated in the anode can be
efficiently
separated out to form a high purity CO2 output stream. After separation, the
remaining
portion of the anode output can correspond primarily to components with
chemical
and/or fuel value, as well as reduced amounts of CO2 and/or H20. Since a
substantial
portion of the CO2 generated by the original fuel (prior to reforming) can
have been
separated out, the amount of CO2 generated by subsequent burning of the
remaining
portion of the anode output can be reduced. In particular, to the degree that
the fuel in
the remaining portion of the anode output is H2, no additional greenhouse
gases can
typically be formed by burning of this fuel.
[00109] The anode exhaust can be subjected to a variety of gas processing
options,
including water-gas shift and separation of the components from each other.
Two
general anode processing schemes are shown in FIGS. 1 and 2.
[00110] FIG. 1 schematically shows an example of a reaction system for
operating a
fuel cell array of molten carbonate fuel cells in conjunction with a chemical
synthesis
process. In FIG. 1, a fuel stream 105 is provided to a reforming stage (or
stages) 110
associated with the anode 127 of a fuel cell 120, such as a fuel cell that is
part of a fuel
cell stack in a fuel cell array. The reforming stage 110 associated with fuel
cell 120 can
be internal to a fuel cell assembly. In some optional aspects, an external
reforming
stage (not shown) can also be used to reform a portion of the reformable fuel
in an
input stream prior to passing the input stream into a fuel cell assembly. Fuel
stream 105
can preferably include a reformable fuel, such as methane, other hydrocarbons,
and/or
other hydrocarbon-like compounds such as organic compounds containing carbon-
hydrogen bonds. Fuel stream 105 can also optionally contain H2 and/or CO, such
as H2
and/or CO provided by optional anode recycle stream 185. It is noted that
anode
recycle stream 185 is optional, and that in many aspects no recycle stream is
provided
from the anode exhaust 125 back to anode 127, either directly or indirectly
via
combination with fuel stream 105 or reformed fuel stream 115. After reforming,
the
reformed fuel stream 115 can be passed into anode 127 of fuel cell 120. A CO2
and
02-containing stream 119 can also be passed into cathode 129. A flow of
carbonate
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ions 122, CO2, from the cathode portion 129 of the fuel cell can provide the
remaining reactant needed for the anode fuel cell reactions. Based on the
reactions in
the anode 127, the resulting anode exhaust 125 can include H20, C01, one or
more
components corresponding to incompletely reacted fuel (H2, CO, CH4, or other
components corresponding to a reformable fuel), and optionally one or more
additional
nonreactive components, such as N2 and/or other contaminants that are part of
fuel
stream 105. The anode exhaust 125 can then be passed into one or more
separation
stages. For example, a CO2 removal stage 140 can correspond to a cryogenic CO2
removal system, an amine wash stage for removal of acid gases such as CO2, or
another suitable type of CO2 separation stage for separating a CO2 output
stream 143
from the anode exhaust. Optionally, the anode exhaust can first be passed
through a
water gas shift reactor 130 to convert any CO present in the anode exhaust
(along with
some H2 0) into CO2 and H2 in an optionally water gas shifted anode exhaust
135.
Depending on the nature of the CO2 removal stage, a water condensation or
removal
stage 150 may be desirable to remove a water output stream 153 from the anode
exhaust. Though shown in FIG. 1 after the CO2 separation stage 140, it may
optionally
be located before the CO2 separation stage 140 instead. Additionally, an
optional
membrane separation stage 160 for separation of H2 can be used to generate a
high
purity permeate stream 163 of H2. The resulting retentate stream 166 can then
be used
as an input to a chemical synthesis process. Stream 166 could additionally or
alternately be shifted in a second water-gas shift reactor 131 to adjust the
H2, CO, and
CO2 content to a different ratio, producing an output stream 168 for further
use in a
chemical synthesis process. In FIG. 1, anode recycle stream 185 is shown as
being
withdrawn from the retentate stream 166, but the anode recycle stream 185
could
additionally or alternately be withdrawn from other convenient locations in or
between
the various separation stages. The separation stages and shift reactor(s)
could
additionally or alternately be configured in different orders, and/or in a
parallel
configuration. Finally, a stream with a reduced content of CO2 139 can be
generated as
an output from cathode 129. For the sake of simplicity, various stages of
compression
and heat addition/removal that might be useful in the process, as well as
steam addition
or removal, are not shown.
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[00111] As noted above, the various types of separations performed on the
anode
exhaust can be performed in any convenient order. FIG. 2 shows an example of
an
alternative order for performing separations on an anode exhaust. In FIG. 2,
anode
exhaust 125 can be initially passed into separation stage 260 for removing a
portion
263 of the hydrogen content from the anode exhaust 125. This can allow, for
example,
reduction of the H2 content of the anode exhaust to provide a retentate 266
with a ratio
of H2 to CO closer to 2 : 1. The ratio of H2 to CO can then be further
adjusted to
achieve a desired value in a water gas shift stage 230. The water gas shifted
output 235
can then pass through CO2 separation stage 240 and water removal stage 250 to
produce an output stream 275 suitable for use as an input to a desired
chemical
synthesis process. Optionally, output stream 275 could be exposed to an
additional
water gas shift stage (not shown). A portion of output stream 275 can
optionally be
recycled (not shown) to the anode input. Of course, still other combinations
and
sequencing of separation stages can be used to generate a stream based on the
anode
output that has a desired composition. For the sake of simplicity, various
stages of
compression and heat addition/removal that might be useful in the process, as
well as
steam addition or removal, are not shown.
Cathode Inputs and Outputs
[00112] Conventionally, a molten carbonate fuel cell can be operated based on
drawing
a desired load while consuming some portion of the fuel in the fuel stream
delivered to
the anode. The voltage of the fuel cell can then be determined by the load,
fuel input to
the anode, air and CO2 provided to the cathode, and the internal resistances
of the fuel
cell. The CO2 to the cathode can be conventionally provided in part by using
the anode
exhaust as at least a part of the cathode input stream. By contrast, the
present invention
can use separate/different sources for the anode input and cathode input. By
removing
any direct link between the composition of the anode input flow and the
cathode input
flow, additional options become available for operating the fuel cell, such as
to
generate excess synthesis gas, to improve capture of carbon dioxide, and/or to
improve
the total efficiency (electrical plus chemical power) of the fuel cell, among
others.
[00113] In a molten carbonate fuel cell, the transport of carbonate ions
across the
electrolyte in the fuel cell can provide a method for transporting CO2 from a
first flow
path to a second flow path, where the transport method can allow transport
from a
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lower concentration (the cathode) to a higher concentration (the anode), which
can thus
facilitate capture of CO2. Part of the selectivity of the fuel cell for CO,
separation can
be based on the electrochemical reactions allowing the cell to generate
electrical power.
For nonreactive species (such as N2) that effectively do not participate in
the
electrochemical reactions within the fuel cell, there can be an insignificant
amount of
reaction and transport from cathode to anode. By contrast, the potential
(voltage)
difference between the cathode and anode can provide a strong driving force
for
transport of carbonate ions across the fuel cell. As a result, the transport
of carbonate
ions in the molten carbonate fuel cell can allow CO2 to be transported from
the cathode
(lower CO2 concentration) to the anode (higher CO2 concentration) with
relatively high
selectivity. However, a challenge in using molten carbonate fuel cells for
carbon
dioxide removal can be that the fuel cells have limited ability to remove
carbon dioxide
from relatively dilute cathode feeds. The voltage and/or power generated by a
carbonate fuel cell can start to drop rapidly as the CO2 concentration falls
below about
2.0 vol%. As the CO2 concentration drops further, e.g., to below about 1.0
vol%, at
some point the voltage across the fuel cell can become low enough that little
or no
further transport of carbonate may occur and the fuel cell ceases to function.
Thus, at
least some CO2 is likely to be present in the exhaust gas from the cathode
stage of a
fuel cell under commercially viable operating conditions.
[00114] The amount of carbon dioxide delivered to the fuel cell cathode(s) can
be
determined based on the CO2 content of a source for the cathode inlet. One
example of
a suitable CO, -containing stream for use as a cathode input flow can be an
output or
exhaust flow from a combustion source. Examples of combustion sources include,
but
are not limited to, sources based on combustion of natural gas, combustion of
coal,
and/or combustion of other hydrocarbon-type fuels (including biologically
derived
fuels). Additional or alternate sources can include other types of boilers,
fired heaters,
furnaces, and/or other types of devices that burn carbon-containing fuels in
order to
heat another substance (such as water or air). To a first approximation, the
CO2
content of the output flow from a combustion source can be a minor portion of
the
flow. Even for a higher CO2 content exhaust flow, such as the output from a
coal-fired
combustion source, the CO2 content from most commercial coal-fired power
plants can
be about 15 vol% or less. More generally, the CO2 content of an output or
exhaust
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flow from a combustion source can be at least about 1.5 vol%, or at least
about 1.6
vol%, or at least about 1.7 vol%, or at least about 1.8 vol% , or at least
about 1.9 vol%,
or at least greater 2 vol%, or at least about 4 vol%, or at least about 5
vol%, or at least
about 6 vol%, or at least about 8 vol%. Additionally or alternately, the CO2
content of
an output or exhaust flow from a combustion source can be about 20 vol% or
less, such
as about 15 vol% or less, or about 12 vol% or less, or about 10 vol % or less,
or about 9
vol % or less, or about 8 vol % or less, or about 7 vol% or less, or about 6.5
vol% or
less, or about 6 vol% or less, or about 5.5 vol% or less, or about 5 vol% or
less, or
about 4.5 vol% or less. The concentrations given above are on a dry basis. It
is noted
that the lower CO2 content values can be present in the exhaust from some
natural gas
or methane combustion sources, such as generators that are part of a power
generation
system that may or may not include an exhaust gas recycle loop.
[00115] Other potential sources for a cathode input stream can additionally or
alternately include sources of bio-produced CO2. This can include, for
example, CO2
generated during processing of bio-derived compounds, such as CO2 generated
during
ethanol production. An additional or alternate example can include CO2
generated by
combustion of a bio-produced fuel, such as combustion of lignocellulose. Still
other
additional or alternate potential CO2 sources can correspond to output or
exhaust
streams from various industrial processes, such as CO2-containing streams
generated
by plants for manufacture of steel, cement, and/or paper.
[00116] Yet another additional or alternate potential source of CO, can be CO2-
containing streams from a fuel cell. The CO2-containing stream from a fuel
cell can
correspond to a cathode output stream from a different fuel cell, an anode
output stream
from a different fuel cell, a recycle stream from the cathode output to the
cathode input
of a fuel cell, and/or a recycle stream from an anode output to a cathode
input of a fuel
cell. For example, an MCFC operated in standalone mode under conventional
conditions can generate a cathode exhaust with a CO2 concentration of at least
about 5
vol%. Such a CO2-containing cathode exhaust could be used as a cathode input
for an
MCFC operated according to an aspect of the invention. More generally, other
types of
fuel cells that generate a CO2 output from the cathode exhaust can
additionally or
alternately be used, as well as other types of CO2-containing streams not
generated by a
"combustion" reaction and/or by a combustion-powered generator. Optionally but
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preferably, a CO2-containing stream from another fuel cell can be from another
molten
carbonate fuel cell. For example, for molten carbonate fuel cells connected in
series
with respect to the cathodes, the output from the cathode for a first molten
carbonate
fuel cell can be used as the input to the cathode for a second molten
carbonate fuel cell.
[00117] For various types of CO2-containing streams from sources other than
combustion sources, the CO, content of the stream can vary widely. The CO2
content
of an input stream to a cathode can contain at least about 2 vol% of CO,, such
as at
least about 4 vol%, or at least about 5 vol%, or at least about 6 vol%, or at
least about 8
vol%. Additionally or alternately, the CO2 content of an input stream to a
cathode can
be about 30 vol% or less, such as about 25 vol% or less, or about 20 vol% or
less, or
about 15 vol% or less, or about 10 vol% or less, or about 8 vol% or less, or
about 6
vol% or less, or about 4 vol% or less. For some still higher CO, content
streams, the
CO2 content can be greater than about 30 vol%, such as a stream substantially
composed of CO2 with only incidental amounts of other compounds. As an
example, a
gas-fired turbine without exhaust gas recycle can produce an exhaust stream
with a
CO2 content of approximately 4.2 vol%. With EGR, a gas-fired turbine can
produce an
exhaust stream with a CO2 content of about 6-8 vol%. Stoichiometric combustion
of
methane can produce an exhaust stream with a CO2 content of about 11 vol%.
Combustion of coal can produce an exhaust stream with a CO2 content of about
15-20
vol%. Fired heaters using refinery off-gas can produce an exhaust stream with
a CO2
content of about 12-15 vol%. A gas turbine operated on a low BTU gas without
any
EGR can produce an exhaust stream with a CO2 content of ¨12 vol%.
[00118] In addition to CO2, a cathode input stream must include 02 to provide
the
components necessary for the cathode reaction. Some cathode input streams can
be
based on having air as a component. For example, a combustion exhaust stream
can be
formed by combusting a hydrocarbon fuel in the presence of air. Such a
combustion
exhaust stream, or another type of cathode input stream having an oxygen
content
based on inclusion of air, can have an oxygen content of about 20 vol% or
less, such as
about 15 vol% or less, or about 10 vol% or less. Additionally or alternately,
the oxygen
content of the cathode input stream can be at least about 4 vol%, such as at
least about
6 vol%, or at least about 8 vol%. More generally, a cathode input stream can
have a
suitable content of oxygen for performing the cathode reaction. In some
aspects, this
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can correspond to an oxygen content of about 5 vol% to about 15 vol%, such as
from
about 7 vol% to about 9 vol%. For many types of cathode input streams, the
combined
amount of CO2 and 02 can correspond to less than about 21 vol% of the input
stream,
such as less than about 15 vol% of the stream or less than about 10 vol% of
the stream.
An air stream containing oxygen can be combined with a CO2 source that has low
oxygen content. For example, the exhaust stream generated by burning coal may
include a low oxygen content that can be mixed with air to form a cathode
inlet stream.
[00119] In addition to CO2 and 02, a cathode input stream can also be composed
of
inert/non-reactive species such as N2, H20, and other typical oxidant (air)
components.
For example, for a cathode input derived from an exhaust from a combustion
reaction,
if air is used as part of the oxidant source for the combustion reaction, the
exhaust gas
can include typical components of air such as N2, H20, and other compounds in
minor
amounts that are present in air. Depending on the nature of the fuel source
for the
combustion reaction, additional species present after combustion based on the
fuel
source may include one or more of H20, oxides of nitrogen (N0x) and/or sulfur
(S0x),
and other compounds either present in the fuel and/or that are partial or
complete
combustion products of compounds present in the fuel, such as CO. These
species may
be present in amounts that do not poison the cathode catalyst surfaces though
they may
reduce the overall cathode activity. Such reductions in performance may be
acceptable,
or species that interact with the cathode catalyst may be reduced to
acceptable levels by
known pollutant removal technologies.
[00120] The amount of 02 present in a cathode input stream (such as an input
cathode
stream based on a combustion exhaust) can advantageously be sufficient to
provide the
oxygen needed for the cathode reaction in the fuel cell. Thus, the volume
percentage of
02 can advantageously be at least 0.5 times the amount of CO2 in the exhaust.
Optionally, as necessary, additional air can be added to the cathode input to
provide
sufficient oxidant for the cathode reaction. When some form of air is used as
the
oxidant, the amount of N2 in the cathode exhaust can be at least about 78
vol%, e.g., at
least about 88 vol%, and/or about 95 vol% or less. In some aspects, the
cathode input
stream can additionally or alternately contain compounds that are generally
viewed as
contaminants, such as H2S or NH3. In other aspects, the cathode input stream
can be
cleaned to reduce or minimize the content of such contaminants.
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[00121] In addition to the reaction to form carbonate ions for transport
across the
electrolyte, the conditions in the cathode can also be suitable for conversion
of nitrogen
oxides into nitrate and/or nitrate ions. Hereinafter, only nitrate ions will
be referred to
for convenience. The resulting nitrate ions can also be transported across the
electrolyte for reaction in the anode. NOx concentrations in a cathode input
stream can
typically be on the order of ppm, so this nitrate transport reaction can have
a minimal
impact on the amount of carbonate transported across the electrolyte. However,
this
method of NOx removal can be beneficial for cathode input streams based on
combustion exhausts from gas turbines, as this can provide a mechanism for
reducing
NOx emissions. The conditions in the cathode can additionally or alternately
be
suitable for conversion of unburned hydrocarbons (in combination with 02 in
the
cathode input stream) to typical combustion products, such as CO2 and H20.
[00122] A suitable temperature for operation of an MCFC can be between about
450 C
and about 750 C, such as at least about 500 C, e.g., with an inlet temperature
of about
550 C and an outlet temperature of about 625 C. Prior to entering the cathode,
heat
can be added to or removed from the combustion exhaust, if desired, e.g., to
provide
heat for other processes, such as reforming the fuel input for the anode. For
example, if
the source for the cathode input stream is a combustion exhaust stream, the
combustion
exhaust stream may have a temperature greater than a desired temperature for
the
cathode inlet. In such an aspect, heat can be removed from the combustion
exhaust
prior to use as the cathode input stream. Alternatively, the combustion
exhaust could
be at very low temperature, for example after a wet gas scrubber on a coal-
fired boiler,
in which case the combustion exhaust can be below about 100 C. Alternatively,
the
combustion exhaust could be from the exhaust of a gas turbine operated in
combined
cycle mode, in which the gas can be cooled by raising steam to run a steam
turbine for
additional power generation. In this case, the gas can be below about 50 C.
Heat can
be added to a combustion exhaust that is cooler than desired.
Fuel Cell Arrangement
[00123] In various aspects, a configuration option for a fuel cell (such as a
fuel cell
array containing multiple fuel cell stacks) can be to divide the CO2 -
containing stream
between a plurality of fuel cells. Some types of sources for CO2-containing
streams
can generate large volumetric flow rates relative to the capacity of an
individual fuel
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cell. For example, the CO2-containing output stream from an industrial
combustion
source can typically correspond to a large flow volume relative to desirable
operating
conditions for a single MCFC of reasonable size. Instead of processing the
entire flow
in a single MCFC, the flow can be divided amongst a plurality of MCFC units,
usually
at least some of which can be in parallel, so that the flow rate in each unit
can be within
a desired flow range.
[00124] A second configuration option can be to utilize fuel cells in series
to
successively remove CO2 from a flow stream. Regardless of the number of
initial fuel
cells to which a CO2-containing stream can be distributed to in parallel, each
initial fuel
cell can be followed by one or more additional cells in series to further
remove
additional CO2. If the desired amount of CO2 in the cathode output is
sufficiently low,
attempting to remove CO2 from a cathode input stream down to the desired level
in a
single fuel cell or fuel cell stage could lead to a low and/or unpredictable
voltage output
for the fuel cell. Rather than attempting to remove CO2 to the desired level
in a single
fuel cell or fuel cell stage, CO2 can be removed in successive cells until a
desired level
can be achieved. For example, each cell in a series of fuel cells can be used
to remove
some percentage (e.g., about 50%) of the CO2 present in a fuel stream. In such
an
example, if three fuel cells are used in series, the CO2 concentration can be
reduced
(e.g., to about 15% or less of the original amount present, which can
correspond to
reducing the CO2 concentration from about 6% to about 1% or less over the
course of
three fuel cells in series).
[00125] In another configuration, the operating conditions can be selected in
early fuel
stages in series to provide a desired output voltage while the array of stages
can be
selected to achieve a desired level of carbon separation. As an example, an
array of
fuel cells can be used with three fuel cells in series. The first two fuel
cells in series
can be used to remove CO2 while maintaining a desired output voltage. The
final fuel
cell can then be operated to remove CO2 to a desired concentration but at a
lower
voltage.
[00126] In still another configuration, there can be separate connectivity for
the anodes
and cathodes in a fuel cell array. For example, if the fuel cell array
includes fuel
cathodes connected in series, the corresponding anodes can be connected in any
convenient manner, not necessarily matching up with the same arrangement as
their
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corresponding cathodes, for example. This can include, for instance,
connecting the
anodes in parallel, so that each anode receives the same type of fuel feed,
and/or
connecting the anodes in a reverse series, so that the highest fuel
concentration in the
anodes can correspond to those cathodes having the lowest CO2 concentration.
[00127] In yet another configuration, the amount of fuel delivered to one or
more
anode stages and/or the amount of CO2 delivered to one or more cathode stages
can be
controlled in order to improve the performance of the fuel cell array. For
example, a
fuel cell array can have a plurality of cathode stages connected in series. In
an array
that includes three cathode stages in series, this can mean that the output
from a first
cathode stage can correspond to the input for a second cathode stage, and the
output
from the second cathode stage can correspond to the input for a third cathode
stage. In
this type of configuration, the CO2 concentration can decrease with each
successive
cathode stage. To compensate for this reduced CO2 concentration, additional
hydrogen
and/or methane can be delivered to the anode stages corresponding to the later
cathode
stages. The additional hydrogen and/or methane in the anodes corresponding to
the
later cathode stages can at least partially offset the loss of voltage and/or
current caused
by the reduced CO2 concentration, which can increase the voltage and thus net
power
produced by the fuel cell. In another example, the cathodes in a fuel cell
array can be
connected partially in series and partially in parallel. In this type of
example, instead of
passing the entire combustion output into the cathodes in the first cathode
stage, at least
a portion of the combustion exhaust can be passed into a later cathode stage.
This can
provide an increased CO2 content in a later cathode stage. Still other options
for using
variable feeds to either anode stages or cathode stages can be used if
desired.
[00128] The cathode of a fuel cell can correspond to a plurality of cathodes
from an
array of fuel cells, as previously described. In some aspects, a fuel cell
array can be
operated to improve or maximize the amount of carbon transferred from the
cathode to
the anode. In such aspects, for the cathode output from the final cathode(s)
in an array
sequence (typically at least including a series arrangement, or else the final
cathode(s)
and the initial cathode(s) would be the same), the output composition can
include about
2.0 vol% or less of CO2 (e.g., about 1.5 vol% or less or about 1.2 vol% or
less) and/or
at least about 0.5 vol% of CO2, or at least about 1.0 vol%, or at least about
1.2 vol% or
at least about 1.5 vol%. Due to this limitation, the net efficiency of CO2
removal when
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using molten carbonate fuel cells can be dependent on the amount of CO2 in the
cathode input. For cathode input streams with CO2 contents of greater than
about 6
vol%, such as at least about 8%, the limitation on the amount of CO2 that can
be
removed is not severe. However, for a combustion reaction using natural gas as
a fuel
and with excess air, as is typically found in a gas turbine, the amount of CO2
in the
combustion exhaust may only correspond to a CO2 concentration at the cathode
input
of less than about 5 vol%. Use of exhaust gas recycle can allow the amount of
CO2 at
the cathode input to be increased to at least about 5 vol%, e.g., at least
about 6 vol%. If
EGR is increased when using natural gas as a fuel to produce a CO2
concentration
beyond about 6 vol%, then the flammability in the combustor can be decreased
and the
gas turbine may become unstable. However, when H2 is added to the fuel, the
flammability window can be significantly increased, allowing the amount of
exhaust
gas recycle to be increased further, so that concentrations of CO2 at the
cathode input
of at least about 7.5 vol% or at least about 8 vol% can be achieved. As an
example,
based on a removal limit of about 1.5 vol% at the cathode exhaust, increasing
the CO2
content at the cathode input from about 5.5 vol% to about 7.5 vol% can
correspond to a
¨10% increase in the amount of CO2 that can be captured using a fuel cell and
transported to the anode loop for eventual CO2 separation. The amount of 02 in
the
cathode output can additionally or alternately be reduced, typically in an
amount
proportional to the amount of CO2 removed, which can result in small
corresponding
increases in the amount(s) of the other (non-cathode-reactive) species at the
cathode
exit.
[00129] In other aspects, a fuel cell array can be operated to improve or
maximize the
energy output of the fuel cell, such as the total energy output, the electric
energy output,
the syngas chemical energy output, or a combination thereof For example,
molten
carbonate fuel cells can be operated with an excess of reformable fuel in a
variety of
situations, such as for generation of a syngas stream for use in chemical
synthesis plant
and/or for generation of a high purity hydrogen stream. The syngas stream
and/or
hydrogen stream can be used as a syngas source, a hydrogen source, as a clean
fuel
source, and/or for any other convenient application. In such aspects, the
amount of
CO2 in the cathode exhaust can be related to the amount of CO2 in the cathode
input
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stream and the CO, utilization at the desired operating conditions for
improving or
maximizing the fuel cell energy output.
[00130] Additionally or alternately, depending on the operating conditions, an
MCFC
can lower the CO2 content of a cathode exhaust stream to about 5.0 vol% or
less, e.g.,
about 4.0 vol% or less, or about 2.0 vol% or less, or about 1.5 vol% or less,
or about
1.2 vol% or less. Additionally or alternately, the CO2 content of the cathode
exhaust
stream can be at least about 0.9 vol%, such as at least about 1.0 vol%, or at
least about
1.2 vol%, or at least about 1.5 vol%.
Molten Carbonate Fuel Cell Operation
[00131] In some aspects, a fuel cell may be operated in a single pass or once-
through
mode. In single pass mode, reformed products in the anode exhaust are not
returned to
the anode inlet. Thus, recycling syngas, hydrogen, or some other product from
the
anode output directly to the anode inlet is not done in single pass operation.
More
generally, in single pass operation, reformed products in the anode exhaust
are also not
returned indirectly to the anode inlet, such as by using reformed products to
process a
fuel stream subsequently introduced into the anode inlet. Optionally, CO2 from
the
anode outlet can be recycled to the cathode inlet during operation of an MCFC
in single
pass mode. More generally, in some alternative aspects, recycling from the
anode
outlet to the cathode inlet may occur for an MCFC operating in single pass
mode. Heat
from the anode exhaust or output may additionally or alternately be recycled
in a single
pass mode. For example, the anode output flow may pass through a heat
exchanger
that cools the anode output and warms another stream, such as an input stream
for the
anode and/or the cathode. Recycling heat from anode to the fuel cell is
consistent with
use in single pass or once-through operation. Optionally but not preferably,
constituents of the anode output may be burned to provide heat to the fuel
cell during
single pass mode.
[00132] FIG. 3 shows a schematic example of the operation of an MCFC for
generation of electrical power. In FIG. 3, the anode portion of the fuel cell
can receive
fuel and steam (H20) as inputs, with outputs of water, CO2, and optionally
excess H2,
CH4 (or other hydrocarbons), and/or CO. The cathode portion of the fuel cell
can
receive CO2 and some oxidant (e.g., air/02) as inputs, with an output
corresponding to
a reduced amount of CO2 in 02-depleted oxidant (air). Within the fuel cell,
C032- ions
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formed in the cathode side can be transported across the electrolyte to
provide the
carbonate ions needed for the reactions occurring at the anode.
[00133] Several reactions can occur within a molten carbonate fuel cell such
as the
example fuel cell shown in FIG. 3. The reforming reactions can be optional,
and can be
reduced or eliminated if sufficient H, is provided directly to the anode. The
following
reactions are based on CH4, but similar reactions can occur when other fuels
are used in
the fuel cell.
(1) <anode reforming> CH4 + H20 => 3H2 + CO
(2) <water gas shift> CO + H20 => H2 + CO2
(3) < reforming and water gas shift combined> CH4 + 2H20 => 4H2 +
CO2
(4) <anode H, oxidation> H, + CO2 => H20 + CO2 + 2e-
(5) <cathode> 1/202 + CO2 + 2e => CO2
[00134] Reaction (1) represents the basic hydrocarbon reforming reaction to
generate
H2 for use in the anode of the fuel cell. The CO formed in reaction (1) can be
converted to H2 by the water-gas shift reaction (2). The combination of
reactions (1)
and (2) is shown as reaction (3). Reactions (1) and (2) can occur external to
the fuel
cell, and/or the reforming can be performed internal to the anode.
[00135] Reactions (4) and (5), at the anode and cathode respectively,
represent the
reactions that can result in electrical power generation within the fuel cell.
Reaction (4)
combines H2, either present in the feed or optionally generated by reactions
(1) and/or
(2), with carbonate ions to form H20, CO2, and electrons to the circuit.
Reaction (5)
combines 02, CO2, and electrons from the circuit to form carbonate ions. The
carbonate ions generated by reaction (5) can be transported across the
electrolyte of the
fuel cell to provide the carbonate ions needed for reaction (4). In
combination with the
transport of carbonate ions across the electrolyte, a closed current loop can
then be
formed by providing an electrical connection between the anode and cathode.
[00136] In various embodiments, a goal of operating the fuel cell can be to
improve the
total efficiency of the fuel cell and/or the total efficiency of the fuel cell
plus an
integrated chemical synthesis process. This is typically in contrast to
conventional
operation of a fuel cell, where the goal can be to operate the fuel cell with
high
electrical efficiency for using the fuel provided to the cell for generation
of electrical
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power. As defined above, total fuel cell efficiency may be determined by
dividing the
electric output of the fuel cell plus the lower heating value of the fuel cell
outputs by
the lower heating value of the input components for the fuel cell. In other
words,
TFCE = (LHV(el) + LHV(sg out))/LHV(in), where LHV(in) and LHV(sg out) refer to
the LHV of the fuel components (such as H2, CH4, and/or CO) delivered to the
fuel cell
and syngas (H2, CO and/or C01) in the anode outlet streams or flows,
respectively.
This can provide a measure of the electric energy plus chemical energy
generated by
the fuel cell and/or the integrated chemical process. It is noted that under
this definition
of total efficiency, heat energy used within the fuel cell and/or used within
the
integrated fuel cell / chemical synthesis system can contribute to total
efficiency.
However, any excess heat exchanged or otherwise withdrawn from the fuel cell
or
integrated fuel cell / chemical synthesis system is excluded from the
definition. Thus,
if excess heat from the fuel cell is used, for example, to generate steam for
electricity
generation by a steam turbine, such excess heat is excluded from the
definition of total
efficiency.
[00137] Several operational parameters may be manipulated to operate a fuel
cell with
excess reformable fuel. Some parameters can be similar to those currently
recommended for fuel cell operation. In some aspects, the cathode conditions
and
temperature inputs to the fuel cell can be similar to those recommended in the
literature. For example, the desired electrical efficiency and the desired
total fuel cell
efficiency may be achieved at a range of fuel cell operating temperatures
typical for
molten carbonate fuel cells. In typical operation, the temperature can
increase across
the fuel cell.
[00138] In other aspects, the operational parameters of the fuel cell can
deviate from
typical conditions so that the fuel cell is operated to allow a temperature
decrease from
the anode inlet to the anode outlet and/or from the cathode inlet to the
cathode outlet.
For example, the reforming reaction to convert a hydrocarbon into H2 and CO is
an
endothermic reaction. If a sufficient amount of reforming is performed in a
fuel cell
anode relative to the amount of oxidation of hydrogen to generate electrical
current, the
net heat balance in the fuel cell can be endothermic. This can cause a
temperature drop
between the inlets and outlets of a fuel cell. During endothermic operation,
the
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temperature drop in the fuel cell can be controlled so that the electrolyte in
the fuel cell
remains in a molten state.
[00139] Parameters that can be manipulated in a way so as to differ from those
currently recommended can include the amount of fuel provided to the anode,
the
composition of the fuel provided to the anode, and/or the separation and
capture of
syngas in the anode output without significant recycling of syngas from the
anode
exhaust to either the anode input or the cathode input. In some aspects, no
recycle of
syngas or hydrogen from the anode exhaust to either the anode input or the
cathode
input can be allowed to occur, either directly or indirectly. In additional or
alternative
aspects, a limited amount of recycle can occur. In such aspects, the amount of
recycle
from the anode exhaust to the anode input and/or the cathode input can be less
than
about 10 vol% of the anode exhaust, such as less than about 5 vol%, or less
than about
1 vol%.
[00140] Additionally or alternately, a goal of operating a fuel cell can be to
separate
CO2 from the output stream of a combustion reaction or another process that
produces
a CO2 output stream, in addition to allowing generation of electric power. In
such
aspects, the combustion reaction(s) can be used to power one or more
generators or
turbines, which can provide a majority of the power generated by the combined
generator/fuel cell system. Rather than operating the fuel cell to optimize
power
generation by the fuel cell, the system can instead be operated to improve the
capture of
carbon dioxide from the combustion-powered generator while reducing or
minimizing
the number of fuels cells required for capturing the carbon dioxide. Selecting
an
appropriate configuration for the input and output flows of the fuel cell, as
well as
selecting appropriate operating conditions for the fuel cell, can allow for a
desirable
combination of total efficiency and carbon capture.
[00141] In some embodiments, the fuel cells in a fuel cell array can be
arranged so that
only a single stage of fuel cells (such as fuel cell stacks) can be present.
In this type of
embodiment, the anode fuel utilization for the single stage can represent the
anode fuel
utilization for the array. Another option can be that a fuel cell array can
contain
multiple stages of anodes and multiple stages of cathodes, with each anode
stage having
a fuel utilization within the same range, such as each anode stage having a
fuel
utilization within 10% of a specified value, for example within 5% of a
specified value.
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Still another option can be that each anode stage can have a fuel utilization
equal to a
specified value or lower than the specified value by less than an amount, such
as having
each anode stage be not greater than a specified value by 10% or less, for
example, by
5% or less. As an illustrative example, a fuel cell array with a plurality of
anode stages
can have each anode stage be within about 10% of 50% fuel utilization, which
would
correspond to each anode stage having a fuel utilization between about 40% and
about
60%. As another example, a fuel cell array with a plurality of stages can have
each
anode stage be not greater than 60% anode fuel utilization with the maximum
deviation
being about 5% less, which would correspond to each anode stage having a fuel
utilization between about 55% to about 60%. In still another example, one or
more
stages of fuel cells in a fuel cell array can be operated at a fuel
utilization from about
30% to about 50%, such as operating a plurality of fuel cell stages in the
array at a fuel
utilization from about 30% to about 50%. More generally, any of the above
types of
ranges can be paired with any of the anode fuel utilization values specified
herein.
[00142] Still another additional or alternate option can include specifying a
fuel
utilization for less than all of the anode stages. For example, in some
aspects of the
invention fuel cells/stacks can be arranged at least partially in one or more
series
arrangements such that anode fuel utilization can be specified for the first
anode stage
in a series, the second anode stage in a series, the final anode stage in a
series, or any
other convenient anode stage in a series. As used herein, the "first" stage in
a series
corresponds to the stage (or set of stages, if the arrangement contains
parallel stages as
well) to which input is directly fed from the fuel source(s), with later
("second,"
"third," "final," etc.) stages representing the stages to which the output
from one or
more previous stages is fed, instead of directly from the respective fuel
source(s). In
situations where both output from previous stages and input directly from the
fuel
source(s) are co-fed into a stage, there can be a "first" (set of) stage(s)
and a "last" (set
of) stage(s), but other stages ("second," "third," etc.) can be more tricky
among which
to establish an order (e.g., in such cases, ordinal order can be determined by
concentration levels of one or more components in the composite input feed
composition, such as CO2 for instance, from highest concentration "first" to
lowest
concentration "last" with approximately similar compositional distinctions
representing
the same ordinal level.)
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[00143] Yet another additional or alternate option can be to specify the anode
fuel
utilization corresponding to a particular cathode stage (again, where fuel
cells/stacks
can be arranged at least partially in one or more series arrangements). As
noted above,
based on the direction of the flows within the anodes and cathodes, the first
cathode
stage may not correspond to (be across the same fuel cell membrane from) the
first
anode stage. Thus, in some aspects of the invention, the anode fuel
utilization can be
specified for the first cathode stage in a series, the second cathode stage in
a series, the
final cathode stage in a series, or any other convenient cathode stage in a
series.
[00144] Yet still another additional or alternate option can be to specify an
overall
average of fuel utilization over all fuel cells in a fuel cell array. In
various aspects, the
overall average of fuel utilization for a fuel cell array can be about 65% or
less, for
example, about 60% or less, about 55% or less, about 50% or less, or about 45%
or less
(additionally or alternately, the overall average fuel utilization for a fuel
cell array can
be at least about 25%, for example at least about 30%, at least about 35%, or
at least
about 40%). Such an average fuel utilization need not necessarily constrain
the fuel
utilization in any single stage, so long as the array of fuel cells meets the
desired fuel
utilization.
Applications for CO2 Output after Capture
[00145] In various aspects of the invention, the systems and methods described
above
can allow for production of carbon dioxide as a pressurized fluid. For
example, the
CO2 generated from a cryogenic separation stage can initially correspond to a
pressurized CO2 liquid with a purity of at least about 90%, e.g., at least
about 95%, at
least about 97%, at least about 98%, or at least about 99%. This pressurized
CO2
stream can be used, e.g., for injection into wells in order to further enhance
oil or gas
recovery such as in secondary oil recovery. When done in proximity to a
facility that
encompasses a gas turbine, the overall system may benefit from additional
synergies in
use of electrical/mechanical power and/or through heat integration with the
overall
system.
[00146] Alternatively, for systems dedicated to an enhanced oil recovery (E0R)
application (i.e., not comingled in a pipeline system with tight compositional
standards), the CO2 separation requirements may be substantially relaxed. The
EOR
application can be sensitive to the presence of 02, so 02 can be absent, in
some
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embodiments, from a CO2 stream intended for use in EOR. However, the EOR
application can tend to have a low sensitivity to dissolved CO, H2, and/or
CHa. Also,
pipelines that transport the CO2 can be sensitive to these impurities. Those
dissolved
gases can typically have only subtle impacts on the solubilizing ability of
CO2 used for
EOR. Injecting gases such as CO, H2, and/or CH4 as EOR gases can result in
some
loss of fuel value recovery, but such gases can be otherwise compatible with
EOR
applications.
[00147] Additionally or alternately, a potential use for CO2 as a pressurized
liquid can
be as a nutrient in biological processes such as algae growth/harvesting. The
use of
MCFCs for CO2 separation can ensure that most biologically significant
pollutants
could be reduced to acceptably low levels, resulting in a CO2-containing
stream having
only minor amounts of other "contaminant" gases (such as CO, H2,1\12, and the
like,
and combinations thereof) that are unlikely to substantially negatively affect
the growth
of photosynthetic organisms. This can be in stark contrast to the output
streams
generated by most industrial sources, which can often contain potentially
highly toxic
material such as heavy metals.
[00148] In this type of aspect of the invention, the CO2 stream generated by
separation
of CO2 in the anode loop can be used to produce biofuels and/or chemicals, as
well as
precursors thereof. Further additionally or alternately, CO2 may be produced
as a
dense fluid, allowing for much easier pumping and transport across distances,
e.g., to
large fields of photosynthetic organisms. Conventional emission sources can
emit hot
gas containing modest amounts of CO2 (e.g., about 4-15%) mixed with other
gases and
pollutants. These materials would normally need to be pumped as a dilute gas
to an
algae pond or biofuel "farm". By contrast, the MCFC system according to the
invention can produce a concentrated CO2 stream (-60-70% by volume on a dry
basis)
that can be concentrated further to 95%+ (for example 96%+, 97%+, 98%+, or
99%+)
and easily liquefied. This stream can then be transported easily and
efficiently over
long distances at relatively low cost and effectively distributed over a wide
area. In
these embodiments, residual heat from the combustion source/MCFC may be
integrated
into the overall system as well.
[00149] An alternative embodiment may apply where the CO2 source/MCFC and
biological/ chemical production sites are co-located. In that case, only
minimal
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compression may be necessary (i.e., to provide enough CO2 pressure to use in
the
biological production, e.g., from about 15 psig to about 150 psig). Several
novel
arrangements can be possible in such a case. Secondary reforming may
optionally be
applied to the anode exhaust to reduce CH4 content, and water-gas shift may
optionally
additionally or alternately be present to drive any remaining CO into CO, and
H2.
[00150] The components from an anode output stream and/or cathode output
stream
can be used for a variety of purposes. One option can be to use the anode
output as a
source of hydrogen, as described above. For an MCFC integrated with or co-
located
with a refinery, the hydrogen can be used as a hydrogen source for various
refinery
processes, such as hydroprocessing. Another option can be to additionally or
alternately use hydrogen as a fuel source where the CO2 from combustion has
already
been "captured." Such hydrogen can be used in a refinery or other industrial
setting as
a fuel for a boiler, furnace, and/or fired heater, and/or the hydrogen can be
used as a
feed for an electric power generator, such as a turbine. Hydrogen from an MCFC
fuel
cell can further additionally or alternately be used as an input stream for
other types of
fuel cells that require hydrogen as an input, possibly including vehicles
powered by
fuel cells. Still another option can be to additionally or alternately use
syngas
generated as an output from an MCFC fuel cell as a fermentation input.
[00151] Another option can be to additionally or alternately use syngas
generated from
the anode output. Of course, syngas can be used as a fuel, although a syngas
based fuel
can still lead to some CO2 production when burned as fuel. In other aspects, a
syngas
output stream can be used as an input for a chemical synthesis process. One
option can
be to additionally or alternately use syngas for a Fischer-Tropsch type
process, and/or
another process where larger hydrocarbon molecules are formed from the syngas
input.
Another option can be to additionally or alternately use syngas to form an
intermediate
product such as methanol. Methanol could be used as the final product, but in
other
aspects methanol generated from syngas can be used to generate larger
compounds,
such as gasoline, olefins, aromatics, and/or other products. It is noted that
a small
amount of CO2 can be acceptable in the syngas feed to a methanol synthesis
process,
and/or to a Fischer-Tropsch process utilizing a shifting catalyst.
Hydroformylation is
an additional or alternate example of still another synthesis process that can
make use
of a syngas input.
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[00152] It is noted that one variation on use of an MCFC to generate syngas
can be to
use MCFC fuel cells as part of a system for processing methane and/or natural
gas
withdrawn by an offshore oil platform or other production system that is a
considerable
distance from its ultimate market. Instead of attempting to transport the gas
phase
output from a well, or attempting to store the gas phase product for an
extended period,
the gas phase output from a well can be used as the input to an MCFC fuel cell
array.
This can lead to a variety of benefits. First, the electric power generated by
the fuel cell
array can be used as a power source for the platform. Additionally, the syngas
output
from the fuel cell array can be used as an input for a Fischer-Tropsch process
at the
production site. This can allow for formation of liquid hydrocarbon products
more
easily transported by pipeline, ship, or railcar from the production site to,
for example,
an on-shore facility or a larger terminal.
[00153] Still other integration options can additionally or alternately
include using the
cathode output as a source of higher purity, heated nitrogen. The cathode
input can
often include a large portion of air, which means a substantial portion of
nitrogen can
be included in the cathode input. The fuel cell can transport CO2 and 02 from
the
cathode across the electrolyte to the anode, and the cathode outlet can have
lower
concentrations of CO2 and 02, and thus a higher concentration of N2 than found
in air.
With subsequent removal of the residual 02 and CO2, this nitrogen output can
be used
as an input for production of ammonia or other nitrogen-containing chemicals,
such as
urea, ammonium nitrate, and/or nitric acid. It is noted that urea synthesis
could
additionally or alternately use CO2 separate from the anode output as an input
feed.
Integration Example: Applications for Integration with Combustion Turbines
[00154] In some aspects of the invention, a combustion source for generating
power
and exhausting a CO2-containing exhaust can be integrated with the operation
of
molten carbonate fuel cells. An example of a suitable combustion source is a
gas
turbine. Preferably, the gas turbine can combust natural gas, methane gas, or
another
hydrocarbon gas in a combined cycle mode integrated with steam generation and
heat
recovery for additional efficiency. Modern natural gas combined cycle
efficiencies are
about 60% for the largest and newest designs. The resulting CO2-containing
exhaust
gas stream can be produced at an elevated temperature compatible with the MCFC
operation, such as 300 C ¨ 700 C and preferably 500 C ¨ 650 C. The gas source
can
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optionally but preferably be cleaned of contaminants such as sulfur that can
poison the
MCFC before entering the turbine. Alternatively, the gas source can be a coal-
fired
generator, wherein the exhaust gas would typically be cleaned post-combustion
due to
the greater level of contaminants in the exhaust gas. In such an alternative,
some heat
exchange to/from the gas may be necessary to enable clean-up at lower
temperatures.
In additional or alternate embodiments, the source of the CO2-containing
exhaust gas
can be the output from a boiler, combustor, or other heat source that burns
carbon-rich
fuels. In other additional or alternate embodiments, the source of the CO2-
containing
exhaust gas can be bio-produced CO2 in combination with other sources.
[00155] For integration with a combustion source, some alternative
configurations for
processing of a fuel cell anode can be desirable. For example, an alternative
configuration can be to recycle at least a portion of the exhaust from a fuel
cell anode to
the input of a fuel cell anode. The output stream from an MCFC anode can
include
H20, CO2, optionally CO, and optionally but typically unreacted fuel (such as
H2 or
CH4) as the primary output components. Instead of using this output stream as
an
external fuel stream and/or an input stream for integration with another
process, one or
more separations can be performed on the anode output stream in order to
separate the
CO2 from the components with potential fuel value, such as H2 or CO. The
components with fuel value can then be recycled to the input of an anode.
[00156] This type of configuration can provide one or more benefits. First,
CO2 can
be separated from the anode output, such as by using a cryogenic CO2
separator.
Several of the components of the anode output (H2, CO, CH4) are not easily
condensable components, while CO, and H20 can be separated individually as
condensed phases. Depending on the embodiment, at least about 90 vol% of the
CO2
in the anode output can be separated to form a relatively high purity CO2
output stream.
Alternatively, in some aspects less CO2 can be removed from the anode output,
so that
about 50 vol% to about 90 vol% of the CO2 in the anode output can be separated
out,
such as about 80 vol% or less or about 70 vol% or less. After separation, the
remaining
portion of the anode output can correspond primarily to components with fuel
value, as
well as reduced amounts of CO2 and/or H20. This portion of the anode output
after
separation can be recycled for use as part of the anode input, along with
additional fuel.
In this type of configuration, even though the fuel utilization in a single
pass through
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the MCFC(s) may be low, the unused fuel can be advantageously recycled for
another
pass through the anode. As a result, the single-pass fuel utilization can be
at a reduced
level, while avoiding loss (exhaust) of unburned fuel to the environment.
[00157] Additionally or alternatively to recycling a portion of the anode
exhaust to the
anode input, another configuration option can be to use a portion of the anode
exhaust
as an input for a combustion reaction for a turbine or other combustion
device, such as
a boiler, furnace, and/or fired heater. The relative amounts of anode exhaust
recycled
to the anode input and/or as an input to the combustion device can be any
convenient or
desirable amount. If the anode exhaust is recycled to only one of the anode
input and
the combustion device, the amount of recycle can be any convenient amount,
such as
up to 100% of the portion of the anode exhaust remaining after any separation
to
remove CO2 and/or H20. When a portion of the anode exhaust is recycled to both
the
anode input and the combustion device, the total recycled amount by definition
can be
100% or less of the remaining portion of anode exhaust. Otherwise, any
convenient
split of the anode exhaust can be used. In various embodiments of the
invention, the
amount of recycle to the anode input can be at least about 10% of the anode
exhaust
remaining after separations, for example at least about 25%, at least about
40%, at least
about 50%, at least about 60%, at least about 75%, or at least about 90%.
Additionally
or alternately in those embodiments, the amount of recycle to the anode input
can be
about 90% or less of the anode exhaust remaining after separations, for
example about
75% or less, about 60% or less, about 50% or less, about 40% or less, about
25% or
less, or about 10% or less. Further additionally or alternately, in various
embodiments
of the invention, the amount of recycle to the combustion device can be at
least about
10% of the anode exhaust remaining after separations, for example at least
about 25%,
at least about 40%, at least about 50%, at least about 60%, at least about
75%, or at
least about 90%. Additionally or alternately in those embodiments, the amount
of
recycle to the combustion device can be about 90% or less of the anode exhaust
remaining after separations, for example about 75% or less, about 60% or less,
about
50% or less, about 40% or less, about 25% or less, or about 10% or less.
[00158] In still other alternative aspects of the invention, the fuel for a
combustion
device can additionally or alternately be a fuel with an elevated quantity of
components
that are inert and/or otherwise act as a diluent in the fuel. CO2 and N2 are
examples of
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components in a natural gas feed that can be relatively inert during a
combustion
reaction. When the amount of inert components in a fuel feed reaches a
sufficient
level, the performance of a turbine or other combustion source can be
impacted. The
impact can be due in part to the ability of the inert components to absorb
heat, which
can tend to quench the combustion reaction. Examples of fuel feeds with a
sufficient
level of inert components can include fuel feeds containing at least about 20
vol% CO2,
or fuel feeds containing at least about 40 vol% N2, or fuel feeds containing
combinations of CO2 and N2 that have sufficient inert heat capacity to provide
similar
quenching ability. (It is noted that CO2 has a greater heat capacity than N2,
and
therefore lower concentrations of CO2 can have a similar impact as higher
concentrations of N2. CO2 can also participate in the combustion reactions
more
readily than N2, and in doing so remove H2 from the combustion. This
consumption of
H2 can have a large impact on the combustion of the fuel, by reducing the
flame speed
and narrowing the flammability range of the air and fuel mixture.) More
generally, for
a fuel feed containing inert components that impact the flammability of the
fuel feed,
the inert components in the fuel feed can be at least about 20 vol%, such as
at least
about 40 vol%, or at least about 50 vol%, or at least about 60 vol%.
Preferably, the
amount of inert components in the fuel feed can be about 80 vol% or less.
[00159] When a sufficient amount of inert components are present in a fuel
feed, the
resulting fuel feed can be outside of the flammability window for the fuel
components
of the feed. In this type of situation, addition of H2 from a recycled portion
of the
anode exhaust to the combustion zone for the generator can expand the
flammability
window for the combination of fuel feed and H2, which can allow, for example,
a fuel
feed containing at least about 20 vol% CO2 or at least about 40% N2 (or other
combinations of CO2 and N2) to be successfully combusted.
[00160] Relative to a total volume of fuel feed and H2 delivered to a
combustion zone,
the amount of H, for expanding the flammability window can be at least about 5
vol%
of the total volume of fuel feed plus H2, such as at least about 10 vol%,
and/or about 25
vol% or less. Another option for characterizing the amount of H2 to add to
expand the
flammability window can be based on the amount of fuel components present in
the
fuel feed before H2 addition. Fuel components can correspond to methane,
natural gas,
other hydrocarbons, and/or other components conventionally viewed as fuel for
a
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combustion-powered turbine or other generator. The amount of H2 added to the
fuel
feed can correspond to at least about one third of the volume of fuel
components (1 : 3
ratio of H2 : fuel component) in the fuel feed, such as at least about half of
the volume
of the fuel components (1 : 2 ratio). Additionally or alternately, the amount
of H2
added to the fuel feed can be roughly equal to the volume of fuel components
in the
fuel feed (1: 1 ratio) or less. For example, for a feed containing about 30
vol% CH4,
about 10% N25 and about 60% CO2, a sufficient amount of anode exhaust can be
added
to the fuel feed to achieve about a 1 : 2 ratio of H2 to CH4. For an idealized
anode
exhaust that contained only H2, addition of H2 to achieve a 1 : 2 ratio would
result in a
feed containing about 26 vol% CH4, 13 vol% H2, 9 vol% N25 and 52 vol% CO2.
Exhaust Gas Recycle
[00161] Aside from providing exhaust gas to a fuel cell array for capture and
eventual
separation of the CO2, an additional or alternate potential use for exhaust
gas can
include recycle back to the combustion reaction to increase the CO,) content.
When
hydrogen is available for addition to the combustion reaction, such as
hydrogen from
the anode exhaust of the fuel cell array, further benefits can be gained from
using
recycled exhaust gas to increase the CO2 content within the combustion
reaction.
[00162] In various aspects of the invention, the exhaust gas recycle loop of a
power
generation system can receive a first portion of the exhaust gas from
combustion, while
the fuel cell array can receive a second portion. The amount of exhaust gas
from
combustion recycled to the combustion zone of the power generation system can
be any
convenient amount, such as at least about 15% (by volume), for example at
least about
25%, at least about 35%, at least about 45%, or at least about 50%.
Additionally or
alternately, the amount of combustion exhaust gas recirculated to the
combustion zone
can be about 65% (by volume) or less, e.g., about 60% or less, about 55% or
less, about
50% or less, or about 45% or less.
[00163] In one or more aspects of the invention, a mixture of an oxidant (such
as air
and/or oxygen-enriched air) and fuel can be combusted and (simultaneously)
mixed
with a stream of recycled exhaust gas. The stream of recycled exhaust gas,
which can
generally include products of combustion such as CO,, can be used as a diluent
to
control, adjust, or otherwise moderate the temperature of combustion and of
the exhaust
that can enter the succeeding expander. As a result of using oxygen-enriched
air, the
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recycled exhaust gas can have an increased CO2 content, thereby allowing the
expander
to operate at even higher expansion ratios for the same inlet and discharge
temperatures, thereby enabling significantly increased power production.
[00164] A gas turbine system can represent one example of a power generation
system
where recycled exhaust gas can be used to enhance the performance of the
system. The
gas turbine system can have a first/main compressor coupled to an expander via
a shaft.
The shaft can be any mechanical, electrical, or other power coupling, thereby
allowing
a portion of the mechanical energy generated by the expander to drive the main
compressor. The gas turbine system can also include a combustion chamber
configured
to combust a mixture of a fuel and an oxidant. In various aspects of the
invention, the
fuel can include any suitable hydrocarbon gas/liquid, such as syngas, natural
gas,
methane, ethane, propane, butane, naphtha diesel, kerosene, aviation fuel,
coal derived
fuel, bio-fuel, oxygenated hydrocarbon feedstock, or any combinations thereof.
The
oxidant can, in some embodiments, be derived from a second or inlet compressor
fluidly coupled to the combustion chamber and adapted to compress a feed
oxidant. In
one or more embodiments of the invention, the feed oxidant can include
atmospheric
air and/or enriched air. When the oxidant includes enriched air alone or a
mixture of
atmospheric air and enriched air, the enriched air can be compressed by the
inlet
compressor (in the mixture, either before or after being mixed with the
atmospheric
air). The enriched air and/or the air-enriched air mixture can have an overall
oxygen
concentration of at least about 25 volume %, e.g., at least about 30 volume %,
at least
about 35 volume %, at least about 40 volume %, at least about 45 volume %, or
at least
about 50 volume %. Additionally or alternately, the enriched air and/or the
air-
enriched air mixture can have an overall oxygen concentration of about 80
volume % or
less, such as about 70 volume % or less.
[00165] The enriched air can be derived from any one or more of several
sources. For
example, the enriched air can be derived from such separation technologies as
membrane separation, pressure swing adsorption, temperature swing adsorption,
nitrogen plant-byproduct streams, and/or combinations thereof. The enriched
air can
additionally or alternately be derived from an air separation unit (ASU), such
as a
cryogenic ASU, for producing nitrogen for pressure maintenance or other
purposes. In
certain embodiments of the invention, the reject stream from such an ASU can
be rich
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in oxygen, having an overall oxygen content from about 50 volume % to about 70
volume %, can be used as at least a portion of the enriched air and
subsequently diluted,
if needed, with unprocessed atmospheric air to obtain the desired oxygen
concentration.
[00166] In addition to the fuel and oxidant, the combustion chamber can
optionally
also receive a compressed recycle exhaust gas, such as an exhaust gas
recirculation
primarily having CO2 and nitrogen components. The compressed recycle exhaust
gas
can be derived from the main compressor, for instance, and adapted to help
facilitate
combustion of the oxidant and fuel, e.g., by moderating the temperature of the
combustion products. As can be appreciated, recirculating the exhaust gas can
serve to
increase CO2 concentration.
[00167] An exhaust gas directed to the inlet of the expander can be generated
as a
product of combustion reaction. The exhaust gas can have a heightened CO2
content
based, at least in part, on the introduction of recycled exhaust gas into the
combustion
reaction. As the exhaust gas expands through the expander, it can generate
mechanical
power to drive the main compressor, to drive an electrical generator, and/or
to power
other facilities.
[00168] The power generation system can, in many embodiments, also include an
exhaust gas recirculation (EGR) system. In one or more aspects of the
invention, the
EGR system can include a heat recovery steam generator (HRSG) and/or another
similar device fluidly coupled to a steam gas turbine. In at least one
embodiment, the
combination of the HRSG and the steam gas turbine can be characterized as a
power-
producing closed Rankine cycle. In combination with the gas turbine system,
the
HRSG and the steam gas turbine can form part of a combined-cycle power
generating
plant, such as a natural gas combined-cycle (NGCC) plant. The gaseous exhaust
can be
introduced to the HRSG in order to generate steam and a cooled exhaust gas.
The
HRSG can include various units for separating and/or condensing water out of
the
exhaust stream, transferring heat to form steam, and/or modifying the pressure
of
streams to a desired level. In certain embodiments, the steam can be sent to
the steam
gas turbine to generate additional electrical power.
[00169] After passing through the HRSG and optional removal of at least some
H20,
the CO2-containing exhaust stream can, in some embodiments, be recycled for
use as
an input to the combustion reaction. As noted above, the exhaust stream can be
71
compressed (or decompressed) to match the desired reaction pressure within the
vessel for
the combustion reaction.
Example of Integrated System
[00170] FIG. 4 schematically shows an example of an integrated system
including
introduction of both CO2-containing recycled exhaust gas and H2 or CO from the
fuel cell
anode exhaust into the combustion reaction for powering a turbine. In FIG. 4,
the turbine
can include a compressor 402, a shaft 404, an expander 406, and a combustion
zone 415.
An oxygen source 411 (such as air and/or oxygen-enriched air) can be combined
with
recycled exhaust gas 498 and compressed in compressor 402 prior to entering
combustion
zone 415. A fuel 412, such as CH4, and optionally a stream containing H2 or CO
487 can
be delivered to the combustion zone. The fuel and oxidant can be reacted in
zone 415 and
optionally but preferably passed through expander 406 to generate electric
power. The
exhaust gas from expander 406 can be used to folin two streams, e.g., a CO2-
containing
stream 422 (that can be used as an input feed for fuel cell array 425) and
another CO2-
containing stream 492 (that can be used as the input for a heat recovery and
steam generator
system 490, which can, for example, enable additional electricity to be
generated using
steam turbines 494). After passing through heat recovery system 490, including
optional
removal of a portion of H20 from the CO2-containing stream, the output stream
498 can be
recycled for compression in compressor 402 or a second compressor that is not
shown. The
proportion of the exhaust from expander 406 used for CO2-containing stream 492
can be
determined based on the desired amount of CO2 for addition to combustion zone
415.
[00171] As used herein, the EGR ratio is the flow rate for the fuel cell bound
portion of
the exhaust gas divided by the combined flow rate for the fuel cell bound
portion and the
recovery bound portion, which is sent to the heat recovery generator. For
example, the
EGR ratio for flows shown in FIG. 4 is the flow rate of stream 422 divided by
the combined
flow rate of streams 422 and 492.
[00172] The CO2-containing stream 422 can be passed into a cathode portion
(not
shown) of a molten carbonate fuel cell array 425. Based on the reactions
within fuel
cell array 425, CO2 can be separated from stream 422 and transported to the
anode
portion (not shown) of the fuel cell array 425. This can result in a cathode
output
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stream 424 depleted in CO2. The cathode output stream 424 can then be passed
into a
heat recovery (and optional steam generator) system 450 for generation of heat
exchange and/or additional generation of electricity using steam turbines 454
(which
may optionally be the same as the aforementioned steam turbines 494). After
passing
through heat recovery and steam generator system 450, the resulting flue gas
stream
456 can be exhausted to the environment and/or passed through another type of
carbon
capture technology, such as an amine scrubber.
[00173] After transport of CO2 from the cathode side to the anode side of fuel
cell
array 425, the anode output 435 can optionally be passed into a water gas
shift reactor
470. Water gas shift reactor 470 can be used to generate additional H, and CO2
at the
expense of CO (and H20) present in the anode output 435. The output from the
optional water gas shift reactor 470 can then be passed into one or more
separation
stages 440, such as a cold box or a cryogenic separator. This can allow for
separation
of an H20 stream 447 and CO2 stream 449 from the remaining portion of the
anode
output. The remaining portion of the anode output 485 can include unreacted H2
generated by reforming but not consumed in fuel cell array 425. A first
portion 445 of
the H2-containing stream 485 can be recycled to the input for the anode(s) in
fuel cell
array 425. A second portion 487 of stream 485 can be used as an input for
combustion
zone 415. A third portion 465 can be used as is for another purpose and/or
treated for
subsequent further use. Although FIG. 4 and the description herein
schematically
details up to three portions, it is contemplated that only one of these three
portions can
be exploited, only two can be exploited, or all three can be exploited
according to the
invention.
[00174] In FIG. 4, the exhaust for the exhaust gas recycle loop is provided by
a first
heat recovery and steam generator system 490, while a second heat recovery and
steam
generator system 450 can be used to capture excess heat from the cathode
output of the
fuel cell array 425. FIG. 5 shows an alternative embodiment where the exhaust
gas
recycle loop is provided by the same heat recovery steam generator used for
processing
the fuel cell array output. In FIG. 5, recycled exhaust gas 598 is provided by
heat
recovery and steam generator system 550 as a portion of the flue gas stream
556. This
can eliminate the separate heat recovery and steam generator system associated
with the
turbine.
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[00175] In various embodiments of the invention, the process can be approached
as
starting with a combustion reaction for powering a turbine, an internal
combustion
engine, or another system where heat and/or pressure generated by a combustion
reaction can be converted into another form of power. The fuel for the
combustion
reaction can comprise or be hydrogen, a hydrocarbon, and/or any other compound
containing carbon that can be oxidized (combusted) to release energy. Except
for when
the fuel contains only hydrogen, the composition of the exhaust gas from the
combustion reaction can have a range of CO2 contents, depending on the nature
of the
reaction (e.g., from at least about 2 vol% to about 25 vol% or less). Thus, in
certain
embodiments where the fuel is carbonaceous, the CO2 content of the exhaust gas
can
be at least about 2 vol%, for example at least about 4 vol%, at least about 5
vol%, at
least about 6 vol%, at least about 8 vol%, or at least about 10 vol%.
Additionally or
alternately in such carbonaceous fuel embodiments, the CO2 content can be
about 25
vol% or less, for example about 20 vol% or less, about 15 vol% or less, about
10 vol%
or less, about 7 vol% or less, or about 5 vol% or less. Exhaust gases with
lower relative
CO2 contents (for carbonaceous fuels) can correspond to exhaust gases from
combustion reactions on fuels such as natural gas with lean (excess air)
combustion.
Higher relative CO2 content exhaust gases (for carbonaceous fuels) can
correspond to
optimized natural gas combustion reactions, such as those with exhaust gas
recycle,
and/or combustion of fuels such as coal.
[00176] In some aspects of the invention, the fuel for the combustion reaction
can
contain at least about 90 volume % of compounds containing five carbons or
less, e.g.,
at least about 95 volume %. In such aspects, the CO2 content of the exhaust
gas can be
at least about 4 vol%, for example at least about 5 vol%, at least about 6
vol%, at least
about 7 vol%, or at least about 7.5 vol%. Additionally or alternately, the CO2
content
of the exhaust gas can be about 13 vol% or less, e.g., about 12 vol% or less,
about 10
vol% or less, about 9 vol% or less, about 8 vol% or less, about 7 vol% or
less, or about
6 vol% or less. The CO2 content of the exhaust gas can represent a range of
values
depending on the configuration of the combustion-powered generator. Recycle of
an
exhaust gas can be beneficial for achieving a CO2 content of at least about 6
vol%,
while addition of hydrogen to the combustion reaction can allow for further
increases in
CO2 content to achieve a CO2 content of at least about 7.5 vol%.
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Alternative Configuration ¨ High Severity NOx Turbine
[00177] Gas turbines can be limited in their operation by several factors. One
typical
limitation can be that the maximum temperature in the combustion zone can be
controlled below certain limits to achieve sufficiently low concentrations of
nitrogen
oxides (NOx) in order to satisfy regulatory emission limits. Regulatory
emission limits
can require a combustion exhaust to have a NOx content of about 20 vppm or
less, and
possible 10 vppm or less, when the combustion exhaust is allowed to exit to
the
environment.
[00178] NOx formation in natural gas-fired combustion turbines can be a
function of
temperature and residence time. Reactions that result in formation of NOx can
be of
reduced and/or minimal importance below a flame temperature of about 1500 F,
but
NOx production can increase rapidly as the temperature increases beyond this
point. In
a gas turbine, initial combustion products can be mixed with extra air to cool
the
mixture to a temperature around 1200 F, and temperature can be limited by the
metallurgy of the expander blades. Early gas turbines typically executed the
combustion in diffusion flames that had stoichiometric zones with temperatures
well
above 1500 F, resulting in higher NOx concentrations. More recently, the
current
generation of 'Dry Low Nox' (DLN) burners can use special pre-mixed burners to
burn
natural gas at cooler lean (less fuel than stoichiometric) conditions. For
example, more
of the dilution air can be mixed in to the initial flame, and less can be
mixed in later to
bring the temperature down to the ¨1200 F turbine-expander inlet temperature.
The
disadvantages for DLN burners can include poor performance at turndown, higher
maintenance, narrow ranges of operation, and poor fuel flexibility. The latter
can be a
concern, as DLN burners can be more difficult to apply to fuels of varying
quality (or
difficult to apply at all to liquid fuels). For low BTU fuels, such as fuels
containing a
high content of CO2, DLN burners are typically not used and instead diffusion
burners
can be used. In addition, gas turbine efficiency can be increased by using a
higher
turbine-expander inlet temperature. However, because there can be a limited
amount of
dilution air, and this amount can decrease with increased turbine-expander
inlet
temperature, the DLN burner can become less effective at maintaining low NOx
as the
efficiency of the gas turbine improves.
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[00179] In various aspects of the invention, a system integrating a gas
turbine with a
fuel cell for carbon capture can allow use of higher combustion zone
temperatures
while reducing and/or minimizing additional NOx emissions, as well as enabling
DLN-
like NOx savings via use of turbine fuels that are not presently compatible
with DLN
burners. In such aspects, the turbine can be run at higher power (i.e., higher
temperature) resulting in higher NOx emissions, but also higher power output
and
potentially higher efficiency. In some aspects of the invention, the amount of
NOx in
the combustion exhaust can be at least about 20 vppm, such as at least about
30 vppm,
or at least about 40 vppm. Additionally or alternately, the amount of NOx in
the
combustion exhaust can be about 1000 vppm or less, such as about 500 vppm or
less,
or about 250 vppm or less, or about 150 vppm or less, or about 100 vppm or
less. In
order to reduce the NOx levels to levels required by regulation, the resulting
NOx can
be equilibrated via thermal NOx destruction (reduction of NOx levels to
equilibrium
levels in the exhaust stream) through one of several mechanisms, such as
simple
thermal destruction in the gas phase; catalyzed destruction from the nickel
cathode
catalyst in the fuel cell array; and/or assisted thermal destruction prior to
the fuel cell
by injection of small amounts of ammonia, urea, or other reductant. This can
be
assisted by introduction of hydrogen derived from the anode exhaust. Further
reduction
of NOx in the cathode of the fuel cell can be achieved via electrochemical
destruction
wherein the NOx can react at the cathode surface and can be destroyed. This
can result
in some nitrogen transport across the membrane electrolyte to the anode, where
it may
form ammonia or other reduced nitrogen compounds. With respect to NOx
reduction
methods involving an MCFC, the expected NOx reduction from a fuel cell / fuel
cell
array can be about 80% or less of the NOx in the input to the fuel cell
cathode, such as
about 70% or less, and/or at least about 5%. It is noted that sulfidic
corrosion can also
limit temperatures and affect turbine blade metallurgy in conventional
systems.
However, the sulfur restrictions of the MCFC system can typically require
reduced fuel
sulfur levels that reduce or minimize concerns related to sulfidic corrosion.
Operating
the MCFC array at low fuel utilization can further mitigate such concerns,
such as in
aspects where a portion of the fuel for the combustion reaction corresponds to
hydrogen
from the anode exhaust.
Example of Integration of MCFC and Production of N-Containing Compounds
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[00180] Ammonia synthesis through the Haber-Bosch process can require an N2/H2
feed that can be substantially free (e.g., <300 ppm) of any oxidizing species,
which can
include but arc not limited to CO, CO2, or 02. In a traditional process, the
feed to an
ammonia synthesis plant can be produced through a two stage natural gas steam
reforming and partial oxidation process to produce an 1812/1-12 mix low in 02.
The CO2
can be removed from the mixture using a bulk CO2 removal process, for example
a
solvent wash, then a methanation reactor where the remaining carbon oxides can
be
converted to methane and water. The gas feed can be dried and compressed to
ammonia synthesis operating pressure, such as about 80 barg to about 200 barg.
The
operating pressure can be dependent on the nature of the ammonia synthesis
process
being used. The synthesis reactor can achieve around 25% conversion of N2 to
NH3.
The reactor effluent can be chilled to low temperatures to condense out the
NH3
product. The remaining unreacted gases can be recycled back to the process
front end.
[00181] In an aspect, an MCFC Hz/power production system can be integrated
with a
nitrogen PSA (pressure swing adsorption) separator and/or integrated with an
ammonia
synthesis plant to produce ammonia from natural gas. The integrated process
can be a
lower CO2 emission alternative to the higher CO2 emissions reforming processes
for
ammonia synthesis. A flow diagram for an example of an integrated MCFC
Hz/power
production, nitrogen PSA, and ammonia synthesis process is depicted in Figure
7. In
the configuration shown in FIG. 7, natural gas 701 and steam 702 can be pre-
heated
and fed to the MCFC anode. The anode exhaust 703, a mixture of
H2/CO/CO2/water,
can be used to pre-heat 701 and 702 and then sent to water gas shift reactor
740 to
convert as much as possible/practical of the remaining CO to H2 and CO2. The
shifted
gas 704 can be dehydrated 705 and separated into a H2 stream 706 and CO2
stream
707. Stream 706 can be the hydrogen feed for the ammonia synthesis process.
The
nitrogen for ammonia synthesis can be produced through a swing adsorption
process
750, such as a pressure swing adsorption process. Air 708 can be compressed
and fed
into an adsorption column operating at elevated pressure. In the adsorption
column,
oxygen and other air impurities can be adsorbed by the sorbent, and a high
purity
nitrogen stream 709 can exit the column. The sorbent can be regenerated by
lowering
the pressure of the column and/or by increasing the temperature of the column.
This
can produce an oxygen enriched air stream 710. Streams 706 and 709 can be sent
to a
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methanation reactor 760 to remove any remaining carbon oxide and oxygen
impurities.
The H2/N2 mix 711 can be dried 712, compressed, mixed with the unconverted
gases,
pre-heated and sent to the ammonia synthesis plant as stream 713. The ammonia
synthesis plant at high temperature and high pressure can convert about 25% of
the
H2/N2 in 113 to NH3. Heat can be recovered from the ammonia product 714
through
pre-heating the feed and steam production. Stream 715 can be refrigerated 770
to
condense out NH3 716. Stream 718, a fraction of the CO2-containing stream 707,
can
be expanded in the refrigeration cycle and used as a refrigerant. The
unreacted gases
717 can be recycled back to the front end of the plant. The CO2 at atmospheric
pressure 719 can be mixed with enriched air stream 710 and heated via a burner
to
MCFC cathode inlet conditions, with the addition of methane (and air, if
needed) 720.
The heated gas can be mixed with a fraction of the exhaust 721 and fed to the
MCFC as
the cathode inlet. The remainder of the cathode exhaust 722 can be emitted to
the
atmosphere or can be sent for further treatment, if desired. The remaining
fraction of
the captured CO2 723 can be sold for use and/or can be sequestered, and/or the
CO2
can undergo additional subsequent processing, such as for use in synthesis of
organic
nitrogen-containing compounds. In addition to ammonia, this process can also
generate
about 200 MW of power that can be used through the process, for example to
operate
the gas compressors and pumps.
Additional Embodiments
[00182] Embodiment 1. A method for synthesizing nitrogen-containing compounds,
the method comprising: introducing a fuel stream comprising a reformable fuel
into an
anode of a molten carbonate fuel cell, an internal reforming element
associated with the
anode, or a combination thereof; introducing a cathode inlet stream comprising
CO2
and 02 into a cathode of the fuel cell; generating electricity within the
molten carbonate
fuel cell; generating an anode exhaust comprising H2 and CO2; separating CO2
from at
least a portion of the anode exhaust to produce a CO2-rich stream having a CO2
content
greater than a CO2 content of the anode exhaust and a CO2-depleted gas stream
having
an H2 content greater than an H2 content of the anode exhaust; and using at
least a
portion of the CO2-depleted gas stream in an ammonia synthesis process and/or
using
at least a portion of the CO2-rich stream in a second synthesis process for
forming an
organic nitrogen-containing compound (e.g., urea).
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[00183] Embodiment 2. The method of Embodiment 1, wherein using at least a
portion of the CO2-depleted gas stream comprises exposing the at least a
portion of the
CO2-depleted gas stream to a catalyst under effective ammonia synthesis
conditions so
as to form at least one ammonia-containing stream and one or more streams
comprising
gaseous or liquid products (which can include one or more streams comprising
gaseous
or liquid products include at least one stream comprising H2 and/or CH4), and
optionally recycling at least a portion of the one or more streams comprising
gaseous or
liquid products to form at least a portion of a cathode inlet stream.
[00184] Embodiment 3. The method of any of the above embodiments, further
comprising adjusting a composition of the anode exhaust, the at least a
portion of the
anode exhaust before CO2 is separated, the CO2-depleted gas stream, the at
least a
portion of the CO2-depleted gas stream before being used in the ammonia
synthesis
process, or a combination thereof.
[00185] Embodiment 4. The method of Embodiment 3, wherein adjusting the
composition comprises one or more of (i) performing a water gas shift process,
(ii)
performing a reverse water gas shift process, (iii) performing a separation to
reduce a
water content of the composition, and (iv) performing a separation to reduce a
CO2
content of the composition.
[00186] Embodiment 5. The method of any of the above embodiments, wherein the
at
least a portion of the CO2-depleted gas stream is formed by separating a H2-
concentrated stream from the CO2-depleted gas stream, the separated H2 -
concentrated
stream comprising at least about 90 vol% H2 (e.g., at least about 95 vol% H2,
at least
about 98 vol% H2, or at least about 99 vol% F12).
[00187] Embodiment 6. The method of any of the above embodiments, wherein the
anode exhaust has a molar ratio of H2 :CO of at least about 3.0:1 (e.g., at
least about
4.0:1), and optionally also about 10:1 or less.
[00188] Embodiment 7. The method of any of the above embodiments, further
comprising: withdrawing, from a cathode exhaust, a gas stream comprising N2;
and
using at least a portion of the withdrawn gas stream comprising N2 as a source
of N2 in
an ammonia synthesis process.
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[00189] Embodiment 8. The method of any of the above embodiments, wherein the
second synthesis process further comprises using ammonia from the ammonia
synthesis
process to form the organic nitrogen-containing compound.
[00190] Embodiment 9. The method of any of the above embodiments, wherein at
least about 90 vol% of the reformable fuel is methane.
[00191] Embodiment 10. The method of any of the above embodiments, wherein the
effective ammonia synthesis conditions comprise a pressure from about 6 MPag
to
about 18 MPag and a temperature from about 350 C to about 500 C.
[00192] Embodiment 11. The method of any of the above embodiments, wherein a
cathode inlet stream comprises exhaust from a combustion turbine.
[00193] Embodiment 12. The method of any of the above embodiments, wherein at
least a portion of the 02 in the cathode inlet stream is derived from an air
separation
step in which air is passed through a PSA apparatus to generate a nitrogen-
rich product
stream and an oxygen-rich off-gas stream, such that at least a portion of said
oxygen-
rich off-gas stream is sent to the cathode inlet, and such that at least a
portion of said
nitrogen-rich product stream is sent to the ammonia synthesis process.
[00194] Embodiment 13. The method of any of the above embodiments, further
comprising withdrawing, from a cathode exhaust, an N2-rich gas stream
comprising
N2; and using at least a portion of the N2-rich gas stream as a source of N2
in the
ammonia synthesis process (e.g., by exposing the at least a portion of the N2-
rich gas
stream to a synthesis catalyst under effective synthesis conditions).
[00195] Embodiment 14. The method of Embodiment 13, wherein using at least a
portion of the cathode exhaust stream as a source of N2 in an ammonia
synthesis
process comprises performing at least one of a separation process and a
purification
process on the N2-rich gas stream to increase the concentration of N2, and
then passing
at least a portion of the N2-rich gas stream into the ammonia synthesis
process with the
increased N2 concentration.
[00196] Embodiment 15. The method of any of the above Embodiments, further
comprising separating H20 from at least one of the anode exhaust, the CO2-rich
gas
stream, the CO2-depleted gas stream, and a cathode exhaust.
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[00197] Embodiment 16. The method of any of the above Embodiments, further
comprising exposing one or more of the CO2-rich stream, the CO2-depleted
stream,
and at least a portion of the anode exhaust stream to a water gas shift
catalyst.
[00198] Embodiment 17. The method of any of the above Embodiments, wherein the
cathode inlet stream comprises exhaust from a combustion turbine.
[00199] Embodiment 18. The method of any of the above Embodiments, wherein
less
than 10 \lot% of an anode exhaust is directly or indirectly recycled to the
anode or to
the cathode.
[00200] Embodiment 19. The method of any of the above Embodiments, wherein no
portion of the anode exhaust is directly or indirectly recycled to the anode.
[00201] Embodiment 20. The method of any of the above Embodiments, wherein no
portion of the anode exhaust is directly or indirectly recycled to the
cathode.
[00202] Embodiment 21. The method of any of the above Embodiments, wherein
less
than 10 vol% of H2 produced in the anode in a single pass is directly or
indirectly
recycled to the anode or to the cathode.
[00203] Embodiment 22. The method of any of the above Embodiments, the method
further comprising reforming the reformable fuel, wherein at least about 90%
of the
reformable fuel introduced into the anode, the reforming stage associated with
the
anode, or a combination thereof is reformed in a single pass through the
anode.
[00204] Embodiment 23. The method of any of the above Embodiments, wherein a
reformable hydrogen content of the reformable fuel introduced into the anode,
into the
reforming stage associated with the anode, or into a combination thereof is at
least
about 50% (e.g., at least about 75% or at least about 100%) greater than an
amount of
hydrogen reacted to generate electricity.
[00205] Embodiment 24. The method of any of the above Embodiments, wherein a
reformable fuel surplus ratio is at least about 2.0 (e.g., at least about 2.5
or at least
about 3.0).
[00206] Embodiment 25. The method of any of the above Embodiments, wherein a
CO2 utilization in the cathode is at least about 50% (e.g., at least about
60%).
[00207] Embodiment 26. The method of any of the above Embodiments, wherein an
electrical efficiency for the molten carbonate fuel cell is between about 10%
and about
40% (e.g., between about 10% and about 35%, between about 10% and about 30%,
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between about 10% and about 25%, or between about 10% and about 20%) and a
total
fuel cell efficiency for the molten carbonate fuel cell is at least about 55%
(e.g., at least
about 60%, at least about 65%, at least about 70%, at least about 75%, or at
least about
80%).
[00208] Embodiment 27. The method of any of the above Embodiments, wherein the
molten carbonate fuel cell is operated at a thermal ratio from about 0.25 to
about 1.5
(e.g., from about 0.25 to about 1.3, from about 0.25 to about 1.15, from about
0.25 to
about 1.0, from about 0.25 to about 0.85, or from about 0.25 to about 0.75).
[00209] Embodiment 28. The method of any of the above Embodiments, wherein a
ratio of net moles of syngas in the anode exhaust to moles of CO2 in a cathode
exhaust
is at least about 2.0 (e.g., at least about 3.0, at least about 4.0, at least
about 5.0, at least
about 10.0, or at least about 20.0), and optionally is about 40.0 or less
(e.g., about 30.0
or less or about 20.0 or less).
[00210] Embodiment 29. The method of any of the above Embodiments, wherein a
fuel utilization in the anode is about 50% or less (e.g., about 45% or less,
about 40% or
less, about 35% or less, about 30% or less, about 25% or less, or about 20% or
less) and
a CO2 utilization in the cathode is at least about 60% (e.g., at least about
65%, at least
about 70%, or at least about 75%).
[00211] Embodiment 30. The method of any of the above Embodiments, wherein the
molten carbonate fuel cell is operated at a first operating condition to
generate electrical
power and at least about 50 mW/cm2 (e.g., at least 100 mW/cm2) of waste heat,
the first
operating condition providing a current density of at least about 150 mAlem2,
and
wherein an effective amount of an endothermic reaction is performed to
maintain a
temperature differential between an anode inlet and an anode outlet of about
100 C or
less (e.g., about 80 C or less or about 60 C or less).
[00212] Embodiment 31. The method of embodiment 30, wherein performing the
endothermic reaction consumes at least about 40% (e.g., at least about 50%, at
least
about 60%, or at least about 75%) of the waste heat.
[00213] Embodiment 32. The method of any of the above embodiments, wherein the
molten carbonate fuel cell is operated at a voltage VA of less than about
0.68V (e.g.,
less than about 0.67V, less than about 0.66V, or about 0.65V or less), and
optionally of
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at least about 0.60V (e.g., at least about 0.61V, at least about 0.62V, or at
least about
0.63V).
[00214] Although the present invention has been described in terms of specific
embodiments, it is not necessarily so limited. Suitable
alterations/modifications for
operation under specific conditions should be apparent to those skilled in the
art. It is
therefore intended that the following claims be interpreted as covering all
such
alterations/modifications that fall within the true spirit/scope of the
invention.
